WEBVTT 00:00:00.000 --> 00:00:03.720 align:middle line:84% JUD HOYT: We have some administrative things today. 00:00:03.720 --> 00:00:05.490 align:middle line:84% First of all, there are two handouts-- 00:00:05.490 --> 00:00:06.872 align:middle line:90% 34 and 35. 00:00:06.872 --> 00:00:08.039 align:middle line:90% We've got the lecture notes. 00:00:08.039 --> 00:00:11.040 align:middle line:84% And I'm handing out the solutions to homework number 4. 00:00:11.040 --> 00:00:14.350 align:middle line:84% And homework number 4 is going back. 00:00:14.350 --> 00:00:15.980 align:middle line:84% So you can pick up homework number 4. 00:00:15.980 --> 00:00:19.180 align:middle line:84% It's in the orange folder in the back there. 00:00:19.180 --> 00:00:23.980 align:middle line:84% And some reminders or announcements-- 00:00:23.980 --> 00:00:26.210 align:middle line:84% these will appear on the website today. 00:00:26.210 --> 00:00:31.420 align:middle line:84% So just to remind you, your written reports are due here on 00:00:31.420 --> 00:00:35.520 align:middle line:90% Thursday, December 9, in class. 00:00:35.520 --> 00:00:38.430 align:middle line:84% I have come up with a schedule for the presenters, 00:00:38.430 --> 00:00:41.310 align:middle line:84% for the people doing oral presentations, that's 00:00:41.310 --> 00:00:42.720 align:middle line:90% shown here. 00:00:42.720 --> 00:00:45.300 align:middle line:84% We have two lecture periods that we've 00:00:45.300 --> 00:00:47.280 align:middle line:84% dedicated to oral presentations-- 00:00:47.280 --> 00:00:49.480 align:middle line:84% the last two lectures of this term. 00:00:49.480 --> 00:00:55.650 align:middle line:84% So on December 7, we have four speakers. 00:00:55.650 --> 00:00:59.790 align:middle line:84% And these are the four speakers I have chosen for December 7. 00:00:59.790 --> 00:01:02.610 align:middle line:84% And your presentation time should 00:01:02.610 --> 00:01:04.170 align:middle line:84% be-- of your actual presentation-- 00:01:04.170 --> 00:01:06.510 align:middle line:90% should be about 16 minutes. 00:01:06.510 --> 00:01:08.520 align:middle line:84% And then beyond that, we'll leave 00:01:08.520 --> 00:01:11.410 align:middle line:84% three minutes for questions and answers period. 00:01:11.410 --> 00:01:14.250 align:middle line:84% So it's a short Q&A. And that's for each talk. 00:01:14.250 --> 00:01:17.100 align:middle line:84% So I'll make each talk about 20 minutes. 00:01:17.100 --> 00:01:19.180 align:middle line:84% And we'll have four people in one period. 00:01:19.180 --> 00:01:21.430 align:middle line:90% So that should work out fine. 00:01:21.430 --> 00:01:23.160 align:middle line:84% And the same thing on December 9. 00:01:23.160 --> 00:01:27.240 align:middle line:84% These are the four speakers that I have chosen for December 9 00:01:27.240 --> 00:01:30.990 align:middle line:84% and pretty much approved all the topics. 00:01:30.990 --> 00:01:33.780 align:middle line:84% If you're not sure your topics are approved, go on the website. 00:01:33.780 --> 00:01:37.210 align:middle line:84% And I have an indication whether it's been approved or changed. 00:01:37.210 --> 00:01:39.900 align:middle line:90% 00:01:39.900 --> 00:01:41.327 align:middle line:90% If you need help-- 00:01:41.327 --> 00:01:42.410 align:middle line:90% you're going to be graded. 00:01:42.410 --> 00:01:44.660 align:middle line:84% For those of you who are doing the oral presentations, 00:01:44.660 --> 00:01:46.462 align:middle line:90% you're graded on two things-- 00:01:46.462 --> 00:01:48.420 align:middle line:84% same as the written, but a slightly different-- 00:01:48.420 --> 00:01:50.850 align:middle line:84% is you're graded on the presentation 00:01:50.850 --> 00:01:53.280 align:middle line:84% itself, which includes your oral presentation as well 00:01:53.280 --> 00:01:54.480 align:middle line:90% as your slides. 00:01:54.480 --> 00:01:57.330 align:middle line:84% So you need to provide handouts, the same way that I give out 00:01:57.330 --> 00:01:58.770 align:middle line:90% handouts every lecture-- 00:01:58.770 --> 00:02:01.840 align:middle line:84% not as long because you're only speaking for 16 minutes. 00:02:01.840 --> 00:02:05.070 align:middle line:84% But if you need any help making Xerox copies-- 00:02:05.070 --> 00:02:06.687 align:middle line:84% you don't have access to a copier-- 00:02:06.687 --> 00:02:08.729 align:middle line:84% you don't need to pay to make those Xerox copies. 00:02:08.729 --> 00:02:14.250 align:middle line:84% My assistant, if you send her the PDF file, or the PowerPoint 00:02:14.250 --> 00:02:16.530 align:middle line:84% file preferably, or if you bring her 00:02:16.530 --> 00:02:19.470 align:middle line:84% a hard copy of your presentation, 00:02:19.470 --> 00:02:24.155 align:middle line:84% she'll make enough Xerox copies for the rest of the class. 00:02:24.155 --> 00:02:26.280 align:middle line:84% You need to give her some time, though, to do that. 00:02:26.280 --> 00:02:31.310 align:middle line:84% So make sure you get her that the day before, so she has time. 00:02:31.310 --> 00:02:34.790 align:middle line:84% And her hours are 8:00 AM to 6:00 PM-- 00:02:34.790 --> 00:02:37.980 align:middle line:90% 2:00 PM, 8:00 to 2:00 so. 00:02:37.980 --> 00:02:39.980 align:middle line:84% Make sure you get her that the day before if you 00:02:39.980 --> 00:02:42.497 align:middle line:90% want help making Xerox copies. 00:02:42.497 --> 00:02:44.580 align:middle line:84% And the second component of your grade, of course, 00:02:44.580 --> 00:02:46.640 align:middle line:90% is on the technical content. 00:02:46.640 --> 00:02:48.920 align:middle line:84% So one part of your grade is on the presentation. 00:02:48.920 --> 00:02:51.350 align:middle line:84% And that includes the quality of your graphics 00:02:51.350 --> 00:02:52.820 align:middle line:84% and the quality of your handouts. 00:02:52.820 --> 00:02:56.180 align:middle line:84% And the second part is on the actual technical content, 00:02:56.180 --> 00:02:58.262 align:middle line:84% the depth, and to see that you read 00:02:58.262 --> 00:03:00.470 align:middle line:84% adequate number of references and that you understand 00:03:00.470 --> 00:03:02.580 align:middle line:84% the material and that you can present it. 00:03:02.580 --> 00:03:04.072 align:middle line:90% Same thing for written reports-- 00:03:04.072 --> 00:03:06.530 align:middle line:84% the only difference is you won't be graded on your speaking 00:03:06.530 --> 00:03:07.760 align:middle line:90% because you won't be speaking. 00:03:07.760 --> 00:03:09.177 align:middle line:84% But for the written report, you'll 00:03:09.177 --> 00:03:11.150 align:middle line:84% be graded both on technical content, 00:03:11.150 --> 00:03:13.010 align:middle line:90% as well as on the presentation. 00:03:13.010 --> 00:03:15.200 align:middle line:84% How nicely did you lay out the report? 00:03:15.200 --> 00:03:17.000 align:middle line:90% How well is it sectioned? 00:03:17.000 --> 00:03:19.190 align:middle line:84% Are the references clear and easy to read? 00:03:19.190 --> 00:03:20.498 align:middle line:90% Do you have good graphs? 00:03:20.498 --> 00:03:22.040 align:middle line:84% Or are the graphs impossible to read? 00:03:22.040 --> 00:03:25.740 align:middle line:84% Things like that-- so both aspects will be part of it. 00:03:25.740 --> 00:03:27.990 align:middle line:84% And all of this information will be posted on the web. 00:03:27.990 --> 00:03:29.480 align:middle line:84% But this gives you an idea of when 00:03:29.480 --> 00:03:31.110 align:middle line:90% you need to prepare yourself. 00:03:31.110 --> 00:03:36.230 align:middle line:84% So yeah, this is where we are right now on November 23. 00:03:36.230 --> 00:03:39.050 align:middle line:84% The Thanksgiving holidays coming up on Thursday. 00:03:39.050 --> 00:03:40.370 align:middle line:90% We won't have a class Thursday. 00:03:40.370 --> 00:03:45.300 align:middle line:84% Today, we're finishing up chapter 10 on etching. 00:03:45.300 --> 00:03:46.470 align:middle line:90% And then we'll move on. 00:03:46.470 --> 00:03:49.380 align:middle line:84% We have two more formal lectures that I'll be giving. 00:03:49.380 --> 00:03:51.900 align:middle line:84% They'll be one on silicides and contacts and novel gate 00:03:51.900 --> 00:03:55.150 align:middle line:84% materials, and then on growth and processing 00:03:55.150 --> 00:03:57.990 align:middle line:84% of strained silicon and silicon germanium on December 2. 00:03:57.990 --> 00:04:01.140 align:middle line:84% And then you folks will start speaking on the 7th 00:04:01.140 --> 00:04:03.030 align:middle line:90% and finish up on the 9th. 00:04:03.030 --> 00:04:05.550 align:middle line:84% And your written reports, again, are due in class 00:04:05.550 --> 00:04:08.940 align:middle line:84% on that final Thursday, December 9. 00:04:08.940 --> 00:04:09.440 align:middle line:90% OK. 00:04:09.440 --> 00:04:12.890 align:middle line:84% Anybody have any questions or anything about the final report 00:04:12.890 --> 00:04:14.920 align:middle line:90% or the schedule? 00:04:14.920 --> 00:04:19.200 align:middle line:84% Send me an email if you have any questions about it. 00:04:19.200 --> 00:04:20.279 align:middle line:90% OK. 00:04:20.279 --> 00:04:23.520 align:middle line:90% So let's go on to today's notes. 00:04:23.520 --> 00:04:28.083 align:middle line:90% 00:04:28.083 --> 00:04:29.500 align:middle line:84% Each chapter now, you notice we're 00:04:29.500 --> 00:04:32.050 align:middle line:84% going through a little more rapidly as we finish up. 00:04:32.050 --> 00:04:34.750 align:middle line:84% This will be the last formal chapter 00:04:34.750 --> 00:04:36.740 align:middle line:84% of reading that you'll have in this class. 00:04:36.740 --> 00:04:39.880 align:middle line:84% There'll be a little bit on silicides you'll have to read. 00:04:39.880 --> 00:04:43.460 align:middle line:84% And then the last lecture, there is nothing written up on it. 00:04:43.460 --> 00:04:44.960 align:middle line:90% So you just come to lecture. 00:04:44.960 --> 00:04:47.950 align:middle line:84% So hopefully, you're finishing up reading chapter 10 00:04:47.950 --> 00:04:50.890 align:middle line:90% on etching. 00:04:50.890 --> 00:04:54.370 align:middle line:84% What I'm showing here on the first slide of handout number 34 00:04:54.370 --> 00:04:57.280 align:middle line:84% is kind of a summary of what we talked about last time, 00:04:57.280 --> 00:05:00.970 align:middle line:84% as far as plasma etching and the various mechanisms. 00:05:00.970 --> 00:05:03.250 align:middle line:84% There are certain characteristics 00:05:03.250 --> 00:05:05.480 align:middle line:90% which we said were important-- 00:05:05.480 --> 00:05:09.070 align:middle line:84% for example, the pressure of the chamber in which you're etching, 00:05:09.070 --> 00:05:11.440 align:middle line:90% the energy of the species. 00:05:11.440 --> 00:05:14.020 align:middle line:84% And these end up affecting things 00:05:14.020 --> 00:05:16.750 align:middle line:90% like selectivity and isotropy. 00:05:16.750 --> 00:05:18.835 align:middle line:84% At the top of the list of the type of etching 00:05:18.835 --> 00:05:20.710 align:middle line:84% are the most physical etching processes here, 00:05:20.710 --> 00:05:22.780 align:middle line:84% like sputter etching or ion beam milling. 00:05:22.780 --> 00:05:24.460 align:middle line:84% It's pretty much purely physical. 00:05:24.460 --> 00:05:26.895 align:middle line:84% There's not a lot of chemical reactions going on. 00:05:26.895 --> 00:05:28.270 align:middle line:84% If you go to the very bottom type 00:05:28.270 --> 00:05:30.520 align:middle line:84% of etching, which is wet chemical etching, 00:05:30.520 --> 00:05:31.480 align:middle line:90% it's entirely wet. 00:05:31.480 --> 00:05:33.370 align:middle line:84% There is no physical bombardment. 00:05:33.370 --> 00:05:36.340 align:middle line:84% And everything in between has a certain component 00:05:36.340 --> 00:05:41.610 align:middle line:84% of both chemical processes and physical processes going on. 00:05:41.610 --> 00:05:44.780 align:middle line:84% And these arrows point to the direction in which 00:05:44.780 --> 00:05:46.490 align:middle line:90% the quantity increases. 00:05:46.490 --> 00:05:50.360 align:middle line:84% So anisotropy, which is-- so the etch will 00:05:50.360 --> 00:05:53.000 align:middle line:84% be more vertical than it is horizontal-- increases as you 00:05:53.000 --> 00:05:55.520 align:middle line:84% go from wet chemical, which is very isotropic-- 00:05:55.520 --> 00:05:57.950 align:middle line:84% or if you go to sputter etching, it's very anisotropic. 00:05:57.950 --> 00:06:00.260 align:middle line:84% So anisotropy increases in this direction. 00:06:00.260 --> 00:06:04.250 align:middle line:84% Selectivity is very poor, or very low, for sputter etching. 00:06:04.250 --> 00:06:06.050 align:middle line:90% You can sputter anything. 00:06:06.050 --> 00:06:07.633 align:middle line:84% And then the sputter rates are not 00:06:07.633 --> 00:06:09.050 align:middle line:84% very different among the elements. 00:06:09.050 --> 00:06:11.150 align:middle line:84% Selectivity increases going down. 00:06:11.150 --> 00:06:14.005 align:middle line:84% So reactive ion etching is less selective. 00:06:14.005 --> 00:06:15.380 align:middle line:84% Plasma etching is more selective. 00:06:15.380 --> 00:06:17.820 align:middle line:84% And wet chemical etching tends to be the most selective. 00:06:17.820 --> 00:06:20.030 align:middle line:84% Selectivity increases in this direction. 00:06:20.030 --> 00:06:22.580 align:middle line:84% In general, the energy of the process 00:06:22.580 --> 00:06:25.010 align:middle line:84% increases going from the bottom to the top. 00:06:25.010 --> 00:06:28.700 align:middle line:84% So there's more energetics involved in sputter etching. 00:06:28.700 --> 00:06:31.700 align:middle line:84% The particles can come down with hundreds of electron volts 00:06:31.700 --> 00:06:35.000 align:middle line:84% and knock off atoms on the surface. 00:06:35.000 --> 00:06:38.510 align:middle line:84% So these processes tend to be more energetic. 00:06:38.510 --> 00:06:41.720 align:middle line:84% And the pressure of the system, here, 00:06:41.720 --> 00:06:45.740 align:middle line:84% tends to be lowest here, and then increasing 00:06:45.740 --> 00:06:50.120 align:middle line:84% as you go from here-- this type of sputter etching-- to, say, 00:06:50.120 --> 00:06:51.885 align:middle line:90% plasma etching. 00:06:51.885 --> 00:06:53.760 align:middle line:84% So that's just what we talked about last time 00:06:53.760 --> 00:06:54.990 align:middle line:90% by way of mechanisms. 00:06:54.990 --> 00:06:58.210 align:middle line:84% Today, I wanted to give some specific examples. 00:06:58.210 --> 00:07:01.290 align:middle line:84% And I want to emphasize gate etching issues. 00:07:01.290 --> 00:07:05.280 align:middle line:84% And I'll talk a little bit about modeling, the mechanics of how 00:07:05.280 --> 00:07:08.460 align:middle line:90% people model etch processes. 00:07:08.460 --> 00:07:12.900 align:middle line:84% On slide 2, this is a chart-- or it's half of a chart 00:07:12.900 --> 00:07:15.480 align:middle line:84% that I took directly from chapter 10 in your text. 00:07:15.480 --> 00:07:18.990 align:middle line:84% And what it shows is a series of common materials, 00:07:18.990 --> 00:07:21.930 align:middle line:84% and some typical etchants that are used, 00:07:21.930 --> 00:07:25.590 align:middle line:84% and then some comments about each of these etchants. 00:07:25.590 --> 00:07:30.030 align:middle line:84% For example, a very common way of etching polysilicon 00:07:30.030 --> 00:07:34.757 align:middle line:84% in a generic way is to use SF6 or CF4. 00:07:34.757 --> 00:07:36.840 align:middle line:84% What are the characteristics of that type of etch? 00:07:36.840 --> 00:07:42.000 align:middle line:84% Well, it tends to be isotropic or near isotropic and not very 00:07:42.000 --> 00:07:44.580 align:middle line:90% good selectivity over SiO2. 00:07:44.580 --> 00:07:47.440 align:middle line:84% So hit once you hit the SiO2, you continue etching. 00:07:47.440 --> 00:07:49.530 align:middle line:84% So this is a general purpose etch 00:07:49.530 --> 00:07:51.480 align:middle line:84% you might use if you're trying to strip off 00:07:51.480 --> 00:07:54.690 align:middle line:84% some poly off the back of the wafer or something-- probably 00:07:54.690 --> 00:07:58.052 align:middle line:84% not something you might use if you need anisotropy 00:07:58.052 --> 00:07:59.010 align:middle line:90% or something like that. 00:07:59.010 --> 00:08:01.950 align:middle line:90% 00:08:01.950 --> 00:08:07.660 align:middle line:84% A more anisotropic etch, but also not very selective to SiO2, 00:08:07.660 --> 00:08:11.260 align:middle line:84% would be a mixture of CF4 and hydrogen, or ChF3, 00:08:11.260 --> 00:08:14.650 align:middle line:90% some of these freon types. 00:08:14.650 --> 00:08:17.170 align:middle line:84% You can mix CF4 with oxygen. I think 00:08:17.170 --> 00:08:19.930 align:middle line:84% we showed last time an example of the chemistry of this. 00:08:19.930 --> 00:08:23.650 align:middle line:84% It's isotropic generally, reasonably isotropic. 00:08:23.650 --> 00:08:26.580 align:middle line:90% But it's more selective to SiO2. 00:08:26.580 --> 00:08:28.330 align:middle line:84% Again, a lot of times, you're etching poly 00:08:28.330 --> 00:08:30.160 align:middle line:84% on top of oxide or some dielectric. 00:08:30.160 --> 00:08:34.450 align:middle line:84% And so the selectivity of the etch, of the polysilicon etch, 00:08:34.450 --> 00:08:38.640 align:middle line:90% to the SiO2 is very important. 00:08:38.640 --> 00:08:42.059 align:middle line:84% The most commonly used etchant gases today for polysilicon-- 00:08:42.059 --> 00:08:44.835 align:middle line:84% and we'll talk about this when we talk about gate etching-- are 00:08:44.835 --> 00:08:48.090 align:middle line:84% HBr and chlorine or some combination of HBr, chlorine, 00:08:48.090 --> 00:08:51.270 align:middle line:84% and oxygen. These tend to be very anisotropic. 00:08:51.270 --> 00:08:53.760 align:middle line:84% So you can get very well-defined sidewalls. 00:08:53.760 --> 00:08:58.320 align:middle line:84% And most of them are reasonably selective compared to SiO2. 00:08:58.320 --> 00:09:01.770 align:middle line:84% With HBr and O2 combinations, you can get up to 100 to 1 00:09:01.770 --> 00:09:06.030 align:middle line:84% selectivity, depending on the etcher. 00:09:06.030 --> 00:09:08.075 align:middle line:84% If you want to etch single crystal silicon, 00:09:08.075 --> 00:09:09.450 align:middle line:84% you use exactly the same etchants 00:09:09.450 --> 00:09:12.150 align:middle line:84% you used for polycrystalline silicon. 00:09:12.150 --> 00:09:17.140 align:middle line:84% In general, there's not a big difference between the two. 00:09:17.140 --> 00:09:20.500 align:middle line:84% If you need to etch oxide, you can etch SF6. 00:09:20.500 --> 00:09:22.510 align:middle line:84% Interesting, any of the fluorinated species 00:09:22.510 --> 00:09:24.700 align:middle line:84% will help you with etching oxide. 00:09:24.700 --> 00:09:27.040 align:middle line:84% Notice that SF6 was also listed as etching poly. 00:09:27.040 --> 00:09:31.630 align:middle line:84% So that means it's not a very selective etch. 00:09:31.630 --> 00:09:35.140 align:middle line:84% And isotropic etches include a variety of the freons, 00:09:35.140 --> 00:09:38.620 align:middle line:90% like C2F6 and C3F8. 00:09:38.620 --> 00:09:40.870 align:middle line:84% They are somewhat selective over silicon. 00:09:40.870 --> 00:09:42.140 align:middle line:90% When do we etch oxide? 00:09:42.140 --> 00:09:44.110 align:middle line:84% In a Mohs process, for example, we're 00:09:44.110 --> 00:09:46.660 align:middle line:84% often etching oxide when we're opening up contact holes. 00:09:46.660 --> 00:09:49.060 align:middle line:84% So we have an interlayer dielectric 00:09:49.060 --> 00:09:51.640 align:middle line:84% of some sort, maybe 1,000 or 2,000 angstroms 00:09:51.640 --> 00:09:53.653 align:middle line:90% we need to get through it. 00:09:53.653 --> 00:09:55.570 align:middle line:84% With reasonably good control of the sidewalls, 00:09:55.570 --> 00:09:56.890 align:middle line:84% we also need to stop on the silicon. 00:09:56.890 --> 00:09:58.890 align:middle line:84% You don't want to etch away your source strains. 00:09:58.890 --> 00:10:03.440 align:middle line:84% So anisotropy and selectivity are also very important. 00:10:03.440 --> 00:10:07.970 align:middle line:84% Silicon nitride-- there are some isotropic etches, 00:10:07.970 --> 00:10:11.270 align:middle line:90% like CF4 and O2. 00:10:11.270 --> 00:10:15.620 align:middle line:90% An anisotropic CF4 and H2-- 00:10:15.620 --> 00:10:17.450 align:middle line:84% this one is selective over silicon. 00:10:17.450 --> 00:10:20.870 align:middle line:84% So it'll stop on silicon, but not over SiO2. 00:10:20.870 --> 00:10:24.600 align:middle line:84% And sometimes, you have stacks consisting of oxide and nitride. 00:10:24.600 --> 00:10:28.170 align:middle line:84% And you need to etch one of the two selectivity-- selectively. 00:10:28.170 --> 00:10:30.360 align:middle line:90% So it gets a little tricky. 00:10:30.360 --> 00:10:35.740 align:middle line:84% You need to pick the etchant and the reactor carefully. 00:10:35.740 --> 00:10:37.190 align:middle line:90% What are some other materials? 00:10:37.190 --> 00:10:39.227 align:middle line:84% So those are the more common ones. 00:10:39.227 --> 00:10:40.810 align:middle line:84% Maybe a little less common, but things 00:10:40.810 --> 00:10:43.300 align:middle line:84% that you do have to etch often in plasmas 00:10:43.300 --> 00:10:45.040 align:middle line:84% are, for example, aluminum wiring. 00:10:45.040 --> 00:10:47.140 align:middle line:84% Or aluminum contacts need to be etched. 00:10:47.140 --> 00:10:50.290 align:middle line:84% It's very common to use chlorinated species, 00:10:50.290 --> 00:10:53.830 align:middle line:84% like chlorine or CHCl3, or a mixture 00:10:53.830 --> 00:11:00.150 align:middle line:84% of chlorine and nitrogen. Chlorine itself 00:11:00.150 --> 00:11:03.030 align:middle line:90% can be somewhat isotropic. 00:11:03.030 --> 00:11:08.190 align:middle line:84% And some of these other gases are more anisotropic. 00:11:08.190 --> 00:11:13.090 align:middle line:84% Tungsten can be etched in SF6 or chlorine. 00:11:13.090 --> 00:11:15.630 align:middle line:84% So you notice a lot of the metals-- these are all metals-- 00:11:15.630 --> 00:11:19.770 align:middle line:84% are attacked by chlorine, which isn't too unusual 00:11:19.770 --> 00:11:22.470 align:middle line:84% if you know a common wet metal etchant is 00:11:22.470 --> 00:11:25.500 align:middle line:90% hydrogen chloride, HCL. 00:11:25.500 --> 00:11:28.560 align:middle line:84% So again, etching titanium or ti-nitride, 00:11:28.560 --> 00:11:32.220 align:middle line:84% these are very common etchants-- chlorine or mixtures of chlorine 00:11:32.220 --> 00:11:34.960 align:middle line:90% with hydrocarbons. 00:11:34.960 --> 00:11:35.535 align:middle line:90% Photoresist. 00:11:35.535 --> 00:11:36.910 align:middle line:84% An important thing, we need to be 00:11:36.910 --> 00:11:43.060 align:middle line:84% able to not etch or pattern so much, as you typically 00:11:43.060 --> 00:11:45.070 align:middle line:90% want to strip it. 00:11:45.070 --> 00:11:49.120 align:middle line:84% So an etching step would include an oxygen plasma, 00:11:49.120 --> 00:11:52.240 align:middle line:84% which reacts with the hydrocarbons 00:11:52.240 --> 00:11:53.620 align:middle line:90% and removes the photoresist. 00:11:53.620 --> 00:11:56.720 align:middle line:84% And the nice thing about O2 plasma is it's very selective. 00:11:56.720 --> 00:11:59.530 align:middle line:84% It doesn't etch too much of any-- it doesn't etch nitride, 00:11:59.530 --> 00:12:00.700 align:middle line:90% doesn't etch oxide. 00:12:00.700 --> 00:12:03.372 align:middle line:84% It will etch a little bit of silicon by oxidation. 00:12:03.372 --> 00:12:05.830 align:middle line:84% That is, it'll oxidize a little bit of the surface silicon. 00:12:05.830 --> 00:12:08.380 align:middle line:84% And then if you happen to do an HF dip, that'll get removed. 00:12:08.380 --> 00:12:10.540 align:middle line:84% So it has a small amount of silicon removal 00:12:10.540 --> 00:12:13.480 align:middle line:90% by an oxidation process. 00:12:13.480 --> 00:12:16.740 align:middle line:84% So again, on page three, that's the second half of the chart 00:12:16.740 --> 00:12:19.470 align:middle line:84% I took directly from chapter 10 in your textbook 00:12:19.470 --> 00:12:22.890 align:middle line:90% of some common etchants. 00:12:22.890 --> 00:12:26.610 align:middle line:84% Now, I want to go here on slide 4. 00:12:26.610 --> 00:12:28.830 align:middle line:84% I wanted to go through some etching 00:12:28.830 --> 00:12:32.520 align:middle line:84% challenges in moss frontend processing from the last five 00:12:32.520 --> 00:12:33.568 align:middle line:90% years or so. 00:12:33.568 --> 00:12:35.110 align:middle line:84% And I'll talk about a couple of them. 00:12:35.110 --> 00:12:37.650 align:middle line:84% One is, for example, we've talked 00:12:37.650 --> 00:12:42.070 align:middle line:84% about in the past DRAM, or dynamic random access memory. 00:12:42.070 --> 00:12:44.760 align:middle line:84% There are different ways of storing the charge 00:12:44.760 --> 00:12:46.200 align:middle line:90% in that memory. 00:12:46.200 --> 00:12:49.230 align:middle line:84% You need to make a capacitor of some sort on which you 00:12:49.230 --> 00:12:49.830 align:middle line:90% store charge. 00:12:49.830 --> 00:12:50.790 align:middle line:90% And that is your bit. 00:12:50.790 --> 00:12:52.860 align:middle line:84% Whether there's charge there or not charged 00:12:52.860 --> 00:12:55.950 align:middle line:84% there tells you whether you have a one or a zero. 00:12:55.950 --> 00:12:58.902 align:middle line:84% This capacitor you might think, well, let's just put a capacitor 00:12:58.902 --> 00:12:59.610 align:middle line:90% on the substrate. 00:12:59.610 --> 00:13:01.318 align:middle line:84% We all know how to make an MLS capacitor. 00:13:01.318 --> 00:13:04.290 align:middle line:84% But the problem is to make very high density DRAMs. 00:13:04.290 --> 00:13:07.440 align:middle line:84% You can't just make a capacitor flat on the silicon substrate. 00:13:07.440 --> 00:13:09.570 align:middle line:84% You can, but then it takes up too much area. 00:13:09.570 --> 00:13:11.940 align:middle line:84% So people have all different types of ways. 00:13:11.940 --> 00:13:14.520 align:middle line:84% People are thinking of ways of stacking the capacitor on top 00:13:14.520 --> 00:13:15.810 align:middle line:90% of the transistor. 00:13:15.810 --> 00:13:18.640 align:middle line:84% That's one method, of stacked capacitor topology. 00:13:18.640 --> 00:13:20.160 align:middle line:84% Another method is take the capacitor 00:13:20.160 --> 00:13:22.260 align:middle line:84% and bury it down in the substrate somewhere 00:13:22.260 --> 00:13:25.750 align:middle line:84% so you can get a smaller area for your DRAM cell. 00:13:25.750 --> 00:13:28.150 align:middle line:84% You can pack more memory on a given chip, 00:13:28.150 --> 00:13:33.110 align:middle line:84% and then you'll have much higher performance. 00:13:33.110 --> 00:13:35.440 align:middle line:84% So this is an example where you might 00:13:35.440 --> 00:13:38.680 align:middle line:84% need to etch down into a silicon substrate 00:13:38.680 --> 00:13:41.630 align:middle line:84% to make a trench prior to making a capacitor. 00:13:41.630 --> 00:13:44.020 align:middle line:84% This is an example where you have etched a trench that 00:13:44.020 --> 00:13:47.500 align:middle line:84% looks like this, lined it with oxide, 00:13:47.500 --> 00:13:50.770 align:middle line:84% and then filled it up with polysilicon to make one 00:13:50.770 --> 00:13:53.800 align:middle line:90% of the plates of the electrode. 00:13:53.800 --> 00:13:56.680 align:middle line:84% The other plate of the electrode could be the surrounding silicon 00:13:56.680 --> 00:13:58.990 align:middle line:90% material. 00:13:58.990 --> 00:14:01.057 align:middle line:84% And I took this out of an IBM journal of research 00:14:01.057 --> 00:14:02.890 align:middle line:84% and development just off the web if you want 00:14:02.890 --> 00:14:04.240 align:middle line:90% to look at that in more detail. 00:14:04.240 --> 00:14:06.940 align:middle line:84% So we need to make very high aspect ratio 00:14:06.940 --> 00:14:08.950 align:middle line:90% etches for these trenches. 00:14:08.950 --> 00:14:10.658 align:middle line:84% And we also need to care about the damage 00:14:10.658 --> 00:14:12.492 align:middle line:84% that we do to the silicon because it's going 00:14:12.492 --> 00:14:13.730 align:middle line:90% to be part of a capacitor. 00:14:13.730 --> 00:14:16.450 align:middle line:84% So we want it to have smooth surfaces and things 00:14:16.450 --> 00:14:18.190 align:middle line:84% so that we don't break down the oxide 00:14:18.190 --> 00:14:24.050 align:middle line:84% and we get a good electrical performance of the capacitor. 00:14:24.050 --> 00:14:25.040 align:middle line:90% In fact, let's see. 00:14:25.040 --> 00:14:27.200 align:middle line:84% Let's take a look here at page five. 00:14:27.200 --> 00:14:32.570 align:middle line:84% I took this from the 2003 ITRS table 73. 00:14:32.570 --> 00:14:36.050 align:middle line:84% This is just for DRAM trench capacitors. 00:14:36.050 --> 00:14:38.120 align:middle line:84% What are some of the technology requirements 00:14:38.120 --> 00:14:41.690 align:middle line:84% we're looking at in the long term here, going from 2010 00:14:41.690 --> 00:14:43.970 align:middle line:90% all the way out to 2020 or 2018? 00:14:43.970 --> 00:14:48.560 align:middle line:84% What do people expect they might need? 00:14:48.560 --> 00:14:52.040 align:middle line:84% Well, here there are some indications 00:14:52.040 --> 00:14:53.660 align:middle line:90% here of characteristics. 00:14:53.660 --> 00:14:55.650 align:middle line:84% They have things called the trench structure, 00:14:55.650 --> 00:14:56.630 align:middle line:90% how it will be shaped-- 00:14:56.630 --> 00:14:59.600 align:middle line:84% maybe more like an upside down bottle-- 00:14:59.600 --> 00:15:03.410 align:middle line:84% the circumference of the trench, the trench 00:15:03.410 --> 00:15:08.540 align:middle line:84% surface roughening factor, the effective oxide 00:15:08.540 --> 00:15:12.290 align:middle line:84% thickness that people will be using in that trench capacitor. 00:15:12.290 --> 00:15:14.810 align:middle line:84% Just to give you an idea, in nanometers here, 00:15:14.810 --> 00:15:17.090 align:middle line:84% we're talking about several nanometers, maybe two 00:15:17.090 --> 00:15:20.510 align:middle line:84% nanometers, scaling even thinner and thinner. 00:15:20.510 --> 00:15:24.470 align:middle line:90% The trench depth in microns. 00:15:24.470 --> 00:15:27.620 align:middle line:84% This would be a 30 for a 35 femto farad capacitor, 00:15:27.620 --> 00:15:28.580 align:middle line:90% just as an example. 00:15:28.580 --> 00:15:32.150 align:middle line:84% How deep will we have to etch, reasonably 00:15:32.150 --> 00:15:34.760 align:middle line:84% deep-- somewhere between 5 to 7 microns is what 00:15:34.760 --> 00:15:37.730 align:middle line:90% people are talking about doing. 00:15:37.730 --> 00:15:38.932 align:middle line:90% Aspect ratio. 00:15:38.932 --> 00:15:40.640 align:middle line:84% So remember, we talked about aspect ratio 00:15:40.640 --> 00:15:45.500 align:middle line:84% as being an important attribute of our capabilities for etching. 00:15:45.500 --> 00:15:49.220 align:middle line:84% Well, so here is the trench depth divided by its width. 00:15:49.220 --> 00:15:51.275 align:middle line:84% We're talking about very high aspect 00:15:51.275 --> 00:15:52.400 align:middle line:90% ratios that are increasing. 00:15:52.400 --> 00:15:53.608 align:middle line:90% And they're also quite large. 00:15:53.608 --> 00:15:55.910 align:middle line:84% In 2010, which is a ways off, we're 00:15:55.910 --> 00:15:59.060 align:middle line:84% reaching aspect ratios on the order of 100, and beyond that, 00:15:59.060 --> 00:16:00.290 align:middle line:90% going even higher. 00:16:00.290 --> 00:16:04.160 align:middle line:84% And you notice beyond 100 the cells are all 00:16:04.160 --> 00:16:08.270 align:middle line:84% marked red, indicating in that range, 00:16:08.270 --> 00:16:10.790 align:middle line:84% people don't really know yet how to etch such 00:16:10.790 --> 00:16:14.152 align:middle line:90% deep aspect ratio, 100 to 1. 00:16:14.152 --> 00:16:16.610 align:middle line:84% So new technology is going to have to be developed in order 00:16:16.610 --> 00:16:18.840 align:middle line:90% to do that. 00:16:18.840 --> 00:16:21.690 align:middle line:84% They have some indications here also of what the upper electrode 00:16:21.690 --> 00:16:23.130 align:middle line:90% might be made of. 00:16:23.130 --> 00:16:25.050 align:middle line:90% It seems to be metal. 00:16:25.050 --> 00:16:29.472 align:middle line:84% The dielectric material, people are talking about using high Ks. 00:16:29.472 --> 00:16:31.680 align:middle line:84% Again, you're trying to store a fair amount of charge 00:16:31.680 --> 00:16:33.000 align:middle line:90% in a small space. 00:16:33.000 --> 00:16:35.910 align:middle line:84% You can do that with a higher dielectric constant. 00:16:35.910 --> 00:16:39.960 align:middle line:84% The bottom electrode could be a combination 00:16:39.960 --> 00:16:43.090 align:middle line:90% of silicon and metals, as well. 00:16:43.090 --> 00:16:45.450 align:middle line:84% So just to give you an idea of what kind of trenches 00:16:45.450 --> 00:16:46.440 align:middle line:90% one needs to etch. 00:16:46.440 --> 00:16:48.870 align:middle line:84% And this increase in the aspect ratio 00:16:48.870 --> 00:16:54.240 align:middle line:84% is pretty amazing in terms of what technology requirements are 00:16:54.240 --> 00:17:00.330 align:middle line:84% going to be imposed on advanced etching techniques. 00:17:00.330 --> 00:17:02.260 align:middle line:90% OK, so that's one example. 00:17:02.260 --> 00:17:05.339 align:middle line:84% If I move on to slide 6, we not only 00:17:05.339 --> 00:17:08.130 align:middle line:84% have to etch silicon into the single crystal material, 00:17:08.130 --> 00:17:11.160 align:middle line:84% but we also have to etch silicon when we're etching the gate. 00:17:11.160 --> 00:17:14.099 align:middle line:84% And in fact, etching the gate is becoming 00:17:14.099 --> 00:17:15.540 align:middle line:90% more and more complicated. 00:17:15.540 --> 00:17:18.329 align:middle line:84% This is an example on slide 6 of etching 00:17:18.329 --> 00:17:20.550 align:middle line:84% what we call a compound gate, or a gate 00:17:20.550 --> 00:17:25.109 align:middle line:84% that already has a gate stack, or already has several materials 00:17:25.109 --> 00:17:27.359 align:middle line:90% that it consists of. 00:17:27.359 --> 00:17:29.730 align:middle line:84% And I apologize this is not a very good SEM. 00:17:29.730 --> 00:17:33.840 align:middle line:84% I took this off the website, again, from that IBM article. 00:17:33.840 --> 00:17:36.870 align:middle line:84% And what it is on the very top, there 00:17:36.870 --> 00:17:38.970 align:middle line:84% is photoresist that has been etched-- that has 00:17:38.970 --> 00:17:40.770 align:middle line:90% been patterned, I should say. 00:17:40.770 --> 00:17:43.410 align:middle line:84% Then there is a layer of tungsten silicide 00:17:43.410 --> 00:17:45.130 align:middle line:90% that has been etched. 00:17:45.130 --> 00:17:48.000 align:middle line:84% Then there's a layer below it of polycrystalline silicon that's 00:17:48.000 --> 00:17:50.700 align:middle line:84% been etched down to, say, the oxide. 00:17:50.700 --> 00:17:52.530 align:middle line:90% It's down to the gate oxide. 00:17:52.530 --> 00:17:55.050 align:middle line:84% And when presented with something like this, 00:17:55.050 --> 00:17:57.750 align:middle line:84% you have to think about using two different types. 00:17:57.750 --> 00:18:01.540 align:middle line:84% After you pattern your photoresist and you develop it 00:18:01.540 --> 00:18:03.332 align:middle line:84% and you're ready to etch it, underneath you 00:18:03.332 --> 00:18:04.790 align:middle line:84% have two different types of layers. 00:18:04.790 --> 00:18:06.190 align:middle line:84% You need to etch a silicide layer 00:18:06.190 --> 00:18:09.527 align:middle line:84% in this example, which was preexisting as a uniform layer. 00:18:09.527 --> 00:18:10.360 align:middle line:90% It has to be etched. 00:18:10.360 --> 00:18:12.320 align:middle line:84% And then you need to stop on the silicon 00:18:12.320 --> 00:18:17.110 align:middle line:84% and then switch gases to etch the polysilicon underneath. 00:18:17.110 --> 00:18:18.700 align:middle line:90% Now, this gets a little tricky. 00:18:18.700 --> 00:18:19.770 align:middle line:84% So you have two different etch gases. 00:18:19.770 --> 00:18:20.895 align:middle line:90% That doesn't sound too bad. 00:18:20.895 --> 00:18:23.640 align:middle line:84% You have an edge gas to etch the tungsten 00:18:23.640 --> 00:18:25.650 align:middle line:84% silicide and another series of gases 00:18:25.650 --> 00:18:28.380 align:middle line:90% you use to etch the polysilicon. 00:18:28.380 --> 00:18:31.200 align:middle line:84% One problem, though, you can come across, for example, 00:18:31.200 --> 00:18:33.780 align:middle line:84% tungsten silicides may not be super pure. 00:18:33.780 --> 00:18:38.280 align:middle line:84% It may have some oxides incorporated into it. 00:18:38.280 --> 00:18:43.140 align:middle line:84% And these oxides won't etch if the etchant 00:18:43.140 --> 00:18:44.740 align:middle line:90% is highly selective to oxide. 00:18:44.740 --> 00:18:46.530 align:middle line:84% So if you're using a single etch gas, 00:18:46.530 --> 00:18:48.300 align:middle line:84% then it's very selective to oxide, 00:18:48.300 --> 00:18:50.300 align:middle line:84% say is what you might want to do for the poly. 00:18:50.300 --> 00:18:52.860 align:middle line:84% That same etch gas might not do very well 00:18:52.860 --> 00:18:54.360 align:middle line:90% in the tungsten silicide. 00:18:54.360 --> 00:18:58.740 align:middle line:84% And what happens when you use an etching gas that 00:18:58.740 --> 00:19:02.370 align:middle line:84% is very selective to oxide and you don't do a breakthrough, 00:19:02.370 --> 00:19:05.017 align:middle line:84% if you have any native oxide on top of that structure 00:19:05.017 --> 00:19:06.600 align:middle line:84% and you don't break through it, you'll 00:19:06.600 --> 00:19:09.720 align:middle line:84% end up with something called RIE grass. 00:19:09.720 --> 00:19:12.660 align:middle line:84% And it's called grass because it looks like grass in the SEM. 00:19:12.660 --> 00:19:19.720 align:middle line:84% On the next slide, I'll show you a picture of grass on slide 7. 00:19:19.720 --> 00:19:23.920 align:middle line:84% This is an example of an extreme case of RIE grass. 00:19:23.920 --> 00:19:29.290 align:middle line:84% And you have a flat substrate down to which you have etched. 00:19:29.290 --> 00:19:32.380 align:middle line:84% And sometimes it's called micro masking. 00:19:32.380 --> 00:19:34.180 align:middle line:84% What happened, an etchant was used 00:19:34.180 --> 00:19:36.220 align:middle line:84% to etch this gate, whatever this material is. 00:19:36.220 --> 00:19:41.320 align:middle line:84% An etchant was used that had a very high selectivity of oxide. 00:19:41.320 --> 00:19:45.940 align:middle line:84% And there were apparently small oxide inclusions 00:19:45.940 --> 00:19:49.540 align:middle line:84% at the surface of the wafer when the etch was started 00:19:49.540 --> 00:19:51.220 align:middle line:90% that didn't get fully removed. 00:19:51.220 --> 00:19:53.470 align:middle line:84% And so those small oxide areas will 00:19:53.470 --> 00:19:55.180 align:middle line:90% act like little micro masks. 00:19:55.180 --> 00:19:57.190 align:middle line:84% They were not intentionally put there, 00:19:57.190 --> 00:20:00.880 align:middle line:84% but there's just a little bit of oxide here and there. 00:20:00.880 --> 00:20:02.440 align:middle line:84% So a breakthrough etch wasn't used. 00:20:02.440 --> 00:20:04.360 align:middle line:84% As a result, if the etching you're using 00:20:04.360 --> 00:20:06.460 align:middle line:84% is highly selective to oxide, it'll never etch it. 00:20:06.460 --> 00:20:09.160 align:middle line:84% And instead, you end up with little posts which look 00:20:09.160 --> 00:20:11.590 align:middle line:90% like little blades of grass. 00:20:11.590 --> 00:20:14.000 align:middle line:84% The solution to this is, well, you can say, 00:20:14.000 --> 00:20:16.310 align:middle line:84% well, don't use an etch that's so selective to oxide. 00:20:16.310 --> 00:20:20.300 align:middle line:84% But if you want to stop on SiO2, you wouldn't want to do that. 00:20:20.300 --> 00:20:22.820 align:middle line:84% The solution is the very first part of your etch step 00:20:22.820 --> 00:20:24.313 align:middle line:84% should be a breakthrough etch where 00:20:24.313 --> 00:20:26.480 align:middle line:84% you make sure you break through all the native oxide 00:20:26.480 --> 00:20:28.460 align:middle line:90% and then you continue on down. 00:20:28.460 --> 00:20:31.190 align:middle line:84% So it gets a little tricky when you 00:20:31.190 --> 00:20:34.640 align:middle line:84% start etching multiple materials or when 00:20:34.640 --> 00:20:36.860 align:middle line:84% you have to think about the fact that there 00:20:36.860 --> 00:20:39.960 align:middle line:84% may be materials on the wafer you don't know about. 00:20:39.960 --> 00:20:41.520 align:middle line:84% Native silicon dioxide may be there, 00:20:41.520 --> 00:20:42.853 align:middle line:90% even if you didn't put it there. 00:20:42.853 --> 00:20:44.830 align:middle line:84% It grows there automatically on the wafer. 00:20:44.830 --> 00:20:47.250 align:middle line:84% So you have to remember that you do get it off the surface 00:20:47.250 --> 00:20:49.380 align:middle line:84% before you use a highly selective etch. 00:20:49.380 --> 00:20:53.380 align:middle line:90% 00:20:53.380 --> 00:20:55.390 align:middle line:84% Slide 8, what I've done is I've listed 00:20:55.390 --> 00:20:58.660 align:middle line:84% some what I call etching challenges in MLS 00:20:58.660 --> 00:21:03.460 align:middle line:84% frontend processing as they apply to gate etching and logic 00:21:03.460 --> 00:21:05.170 align:middle line:90% devices in particular. 00:21:05.170 --> 00:21:08.090 align:middle line:84% This on the left-hand side is a TEM micrograph. 00:21:08.090 --> 00:21:10.720 align:middle line:84% I think I showed this the very beginning and several times 00:21:10.720 --> 00:21:12.520 align:middle line:84% throughout the course just to give you 00:21:12.520 --> 00:21:16.030 align:middle line:84% an idea of what a device five years 00:21:16.030 --> 00:21:18.010 align:middle line:84% ago or so might have looked like. 00:21:18.010 --> 00:21:21.490 align:middle line:84% This is a fairly large technology by today's standards, 00:21:21.490 --> 00:21:23.560 align:middle line:84% but I think it's a quarter micron 00:21:23.560 --> 00:21:26.800 align:middle line:84% or something like that channel-link technology. 00:21:26.800 --> 00:21:29.590 align:middle line:84% Here is a polycrystalline silicon gate. 00:21:29.590 --> 00:21:31.540 align:middle line:84% There's a thin layer underneath, which 00:21:31.540 --> 00:21:34.930 align:middle line:84% is the gate oxide separates the gate from the channel. 00:21:34.930 --> 00:21:38.830 align:middle line:84% And on top there is a layer, which is silicide. 00:21:38.830 --> 00:21:41.440 align:middle line:84% Now, that was put there afterwards, in this case. 00:21:41.440 --> 00:21:43.210 align:middle line:90% So the gate was first etched. 00:21:43.210 --> 00:21:45.970 align:middle line:84% And then metal was put down and then was 00:21:45.970 --> 00:21:47.800 align:middle line:84% reacted with the gate and the source drain 00:21:47.800 --> 00:21:49.540 align:middle line:84% in a silicide or a salicide process. 00:21:49.540 --> 00:21:51.353 align:middle line:84% We'll talk about that next lecture, 00:21:51.353 --> 00:21:52.520 align:middle line:90% when we talk about silicide. 00:21:52.520 --> 00:21:54.920 align:middle line:84% So you didn't have to etch through this. 00:21:54.920 --> 00:21:57.180 align:middle line:90% This was reacted later on. 00:21:57.180 --> 00:21:58.370 align:middle line:90% But what do we care about? 00:21:58.370 --> 00:22:00.230 align:middle line:84% Well, in this picture, first we need 00:22:00.230 --> 00:22:02.750 align:middle line:84% to control the gate length or the critical dimension, 00:22:02.750 --> 00:22:04.310 align:middle line:90% sometimes called the CD. 00:22:04.310 --> 00:22:08.420 align:middle line:84% The gate length is defined from this point to this point. 00:22:08.420 --> 00:22:11.960 align:middle line:84% So this width or length, so to speak, of the polysilicon 00:22:11.960 --> 00:22:16.820 align:middle line:84% needs to be controlled exactly on all areas of the chip 00:22:16.820 --> 00:22:18.890 align:middle line:90% and all areas of the wafer. 00:22:18.890 --> 00:22:20.210 align:middle line:90% The sidewall profile. 00:22:20.210 --> 00:22:21.180 align:middle line:90% What do I mean by that? 00:22:21.180 --> 00:22:23.930 align:middle line:84% Well, this profile, going from the very top down here, 00:22:23.930 --> 00:22:25.160 align:middle line:90% has to be controlled. 00:22:25.160 --> 00:22:27.710 align:middle line:90% You usually want it vertical. 00:22:27.710 --> 00:22:30.320 align:middle line:84% You don't usually want it splayed out like this. 00:22:30.320 --> 00:22:32.080 align:middle line:84% You don't want it retrograded like this. 00:22:32.080 --> 00:22:34.580 align:middle line:84% You don't want it straight with a little slit at the bottom. 00:22:34.580 --> 00:22:36.950 align:middle line:84% You want it perfectly vertical and stop. 00:22:36.950 --> 00:22:40.910 align:middle line:84% That sounds kind of trivial, but it's not easy, 00:22:40.910 --> 00:22:43.670 align:middle line:90% for various reasons. 00:22:43.670 --> 00:22:44.510 align:middle line:90% Selectivity. 00:22:44.510 --> 00:22:46.697 align:middle line:84% We care about, as we're etching this polysilicon, 00:22:46.697 --> 00:22:49.280 align:middle line:84% we don't want to etch away the resist that would be masking it 00:22:49.280 --> 00:22:51.920 align:middle line:84% on top or whatever mask material we're using, if we're 00:22:51.920 --> 00:22:53.330 align:middle line:90% using a hard mask of SiO2. 00:22:53.330 --> 00:22:55.940 align:middle line:84% So selectivity to the etch material, 00:22:55.940 --> 00:22:58.530 align:middle line:84% to the masking material, is very important. 00:22:58.530 --> 00:23:00.140 align:middle line:90% That's one type of selectivity. 00:23:00.140 --> 00:23:01.880 align:middle line:84% Selectivity to the gate dielectric. 00:23:01.880 --> 00:23:05.720 align:middle line:84% Down here in this is a dielectric material, probably 00:23:05.720 --> 00:23:08.660 align:middle line:84% a high K. That's probably 1 to 2 nanometers thick. 00:23:08.660 --> 00:23:11.300 align:middle line:84% We need to stop instantly, immediately, 00:23:11.300 --> 00:23:13.430 align:middle line:84% as soon as we hit it, automatically, 00:23:13.430 --> 00:23:18.210 align:middle line:84% and not go through it, and not over etch too much. 00:23:18.210 --> 00:23:22.280 align:middle line:84% So this is a real issue, getting chemical selectivity 00:23:22.280 --> 00:23:25.750 align:middle line:84% at the same time as getting good sidewall profile. 00:23:25.750 --> 00:23:28.210 align:middle line:84% I just mentioned the fact that Native oxide is often 00:23:28.210 --> 00:23:29.470 align:middle line:90% present on the polysilicon. 00:23:29.470 --> 00:23:31.268 align:middle line:84% We have to use a breakthrough etch 00:23:31.268 --> 00:23:32.560 align:middle line:90% as the initial step of etching. 00:23:32.560 --> 00:23:37.600 align:middle line:84% Otherwise we'll have all kinds of grass problems. 00:23:37.600 --> 00:23:40.990 align:middle line:84% There can be impact of different dopants. 00:23:40.990 --> 00:23:42.920 align:middle line:84% If the gate is doped prior to etching, 00:23:42.920 --> 00:23:46.150 align:middle line:84% which it can be if you put down in situ doped polysilicon, 00:23:46.150 --> 00:23:49.630 align:middle line:84% the etch rate for boron or heavily phosphorus doped poly 00:23:49.630 --> 00:23:51.370 align:middle line:90% may be different from-- 00:23:51.370 --> 00:23:54.130 align:middle line:84% and they may have different etch characteristics 00:23:54.130 --> 00:23:55.150 align:middle line:90% than that of undoped. 00:23:55.150 --> 00:23:56.990 align:middle line:84% So you need to take that into account. 00:23:56.990 --> 00:24:00.370 align:middle line:84% And we also mentioned at the very beginning 00:24:00.370 --> 00:24:02.410 align:middle line:84% of the last lecture, gate dielectric damage 00:24:02.410 --> 00:24:03.790 align:middle line:90% due to the antenna effect. 00:24:03.790 --> 00:24:06.670 align:middle line:84% What is that was you have ions coming down, 00:24:06.670 --> 00:24:07.900 align:middle line:90% being collected by the gate. 00:24:07.900 --> 00:24:08.950 align:middle line:90% Where do they go? 00:24:08.950 --> 00:24:11.308 align:middle line:84% Well, they don't go through the field oxide 00:24:11.308 --> 00:24:12.350 align:middle line:90% because that's too thick. 00:24:12.350 --> 00:24:14.800 align:middle line:84% They get shuttled down the gate and try to-- 00:24:14.800 --> 00:24:16.795 align:middle line:84% the current tends to go through the gate oxide, 00:24:16.795 --> 00:24:19.270 align:middle line:84% but to be careful that you don't put too much 00:24:19.270 --> 00:24:21.470 align:middle line:84% current through your gate oxide and damage it. 00:24:21.470 --> 00:24:24.340 align:middle line:84% So those are some basic issues when 00:24:24.340 --> 00:24:27.830 align:middle line:90% it comes to poly gate etching. 00:24:27.830 --> 00:24:32.240 align:middle line:84% Here's an here's an example on slide 9 of a gate etcher. 00:24:32.240 --> 00:24:34.920 align:middle line:84% It's a little bit older technology now, but at the time 00:24:34.920 --> 00:24:37.400 align:middle line:84% it was designed by a company called Lamb to be 00:24:37.400 --> 00:24:40.820 align:middle line:90% dedicated to gate etching only. 00:24:40.820 --> 00:24:43.580 align:middle line:90% It's called the TCP 9400. 00:24:43.580 --> 00:24:48.380 align:middle line:84% It's relatively low pressure, low bias, high density plasma, 00:24:48.380 --> 00:24:49.970 align:middle line:90% has a mechanical clamp check. 00:24:49.970 --> 00:24:52.490 align:middle line:84% Remember, we talked about the importance of controlling 00:24:52.490 --> 00:24:53.930 align:middle line:90% the temperature of the etch. 00:24:53.930 --> 00:24:57.060 align:middle line:84% To cool the wafer, it's got helium on the backside. 00:24:57.060 --> 00:24:59.990 align:middle line:84% This is an example of a wafer that's frontend processing only, 00:24:59.990 --> 00:25:04.340 align:middle line:84% so no metals are allowed to go into the tool. 00:25:04.340 --> 00:25:08.450 align:middle line:84% And you can get an etch rate of what's called the main etch. 00:25:08.450 --> 00:25:09.720 align:middle line:90% It's pretty fast. 00:25:09.720 --> 00:25:12.470 align:middle line:84% In fact, higher than you probably need in most etches 00:25:12.470 --> 00:25:13.877 align:middle line:90% today. 00:25:13.877 --> 00:25:16.460 align:middle line:84% The height of the poly today is probably only 1,000 angstroms. 00:25:16.460 --> 00:25:21.170 align:middle line:84% And this goes at 320 nanometers per minute. 00:25:21.170 --> 00:25:24.920 align:middle line:84% So quite efficient etching for poly gates. 00:25:24.920 --> 00:25:28.230 align:middle line:84% And it can give you the kind of profile shape and selectivity 00:25:28.230 --> 00:25:30.620 align:middle line:90% that you need. 00:25:30.620 --> 00:25:34.370 align:middle line:84% If we're going to slide 10, this is just an example of a recipe. 00:25:34.370 --> 00:25:38.510 align:middle line:84% If we were to use that Lamb, TCP 9400 gate etcher-- 00:25:38.510 --> 00:25:40.573 align:middle line:84% I took this off the Stanford website. 00:25:40.573 --> 00:25:41.990 align:middle line:84% This is an actual recipe that they 00:25:41.990 --> 00:25:45.770 align:middle line:84% have published showing the individual steps that you would 00:25:45.770 --> 00:25:48.080 align:middle line:84% need to use, just to give you an idea of some 00:25:48.080 --> 00:25:50.810 align:middle line:84% of the complexity of this etch process. 00:25:50.810 --> 00:25:52.970 align:middle line:84% Now, each step-- they're numbered here 1 00:25:52.970 --> 00:25:55.400 align:middle line:84% through 9-- corresponds to a different time 00:25:55.400 --> 00:26:00.410 align:middle line:84% sequence, a different step in the time sequence for the etch. 00:26:00.410 --> 00:26:03.620 align:middle line:84% So the very first step labeled number 1, 00:26:03.620 --> 00:26:09.020 align:middle line:84% you're adjusting the gap which is a parameter inside the tool. 00:26:09.020 --> 00:26:12.290 align:middle line:84% The pressure is being stabilized at 13 millitorr 00:26:12.290 --> 00:26:14.360 align:middle line:90% automatically by the computer. 00:26:14.360 --> 00:26:19.580 align:middle line:84% And you're setting up to flow some of your initial gases. 00:26:19.580 --> 00:26:22.040 align:middle line:90% You're flowing C2F6. 00:26:22.040 --> 00:26:25.850 align:middle line:84% And once you've stabilized after 20 seconds, 00:26:25.850 --> 00:26:28.150 align:middle line:90% you go on to the next step. 00:26:28.150 --> 00:26:30.880 align:middle line:84% OK, so the next step is step number 2. 00:26:30.880 --> 00:26:33.250 align:middle line:84% You're clamping the wafer mechanically to the chuck 00:26:33.250 --> 00:26:37.210 align:middle line:84% to get good control of the temperature. 00:26:37.210 --> 00:26:43.130 align:middle line:84% And you have certain characteristics in that step. 00:26:43.130 --> 00:26:45.400 align:middle line:90% So that lasts about 20 seconds. 00:26:45.400 --> 00:26:47.500 align:middle line:90% Step 3 is the first etch. 00:26:47.500 --> 00:26:48.910 align:middle line:84% Remember we mentioned when you're 00:26:48.910 --> 00:26:50.710 align:middle line:84% using a gas that's highly selective, 00:26:50.710 --> 00:26:53.350 align:middle line:84% you need to first start out with a breakthrough etch 00:26:53.350 --> 00:26:55.150 align:middle line:90% to get through the native oxide. 00:26:55.150 --> 00:26:59.080 align:middle line:84% Here you're etching for about ten seconds. 00:26:59.080 --> 00:27:01.390 align:middle line:90% You etch just based on time. 00:27:01.390 --> 00:27:05.575 align:middle line:84% You're flowing C2F6 all together with-- 00:27:05.575 --> 00:27:07.450 align:middle line:84% I guess that's the only thing you're flowing. 00:27:07.450 --> 00:27:11.860 align:middle line:84% And you've got RF power on the top and bottom, 250 00:27:11.860 --> 00:27:13.390 align:middle line:90% and 40 watts. 00:27:13.390 --> 00:27:14.950 align:middle line:84% And your pressure is 13 millitorr. 00:27:14.950 --> 00:27:17.680 align:middle line:84% So this should break through any native oxide 00:27:17.680 --> 00:27:20.170 align:middle line:90% for about ten seconds. 00:27:20.170 --> 00:27:21.910 align:middle line:90% And then the next step is 4. 00:27:21.910 --> 00:27:24.490 align:middle line:84% You need to stabilize the gas flows before the main etch. 00:27:24.490 --> 00:27:27.670 align:middle line:84% The main etch is the etch that's going to etch the polysilicon. 00:27:27.670 --> 00:27:29.620 align:middle line:90% So you turn on different gases. 00:27:29.620 --> 00:27:31.750 align:middle line:90% You turn off the C2F6. 00:27:31.750 --> 00:27:35.723 align:middle line:84% You turn on the chlorine, a mixture of chlorine and oxygen. 00:27:35.723 --> 00:27:37.390 align:middle line:84% That's what we call the main etch that's 00:27:37.390 --> 00:27:39.182 align:middle line:84% going to do most of the work in getting you 00:27:39.182 --> 00:27:41.382 align:middle line:84% through the polycrystalline silicon. 00:27:41.382 --> 00:27:43.090 align:middle line:84% So you're just stabilizing at this point. 00:27:43.090 --> 00:27:46.450 align:middle line:84% You notice there's no power on the top and bottom electrodes. 00:27:46.450 --> 00:27:48.790 align:middle line:84% So you're not actually-- the plasma is not on. 00:27:48.790 --> 00:27:50.890 align:middle line:90% So you're not lighting a plasma. 00:27:50.890 --> 00:27:53.500 align:middle line:84% You're just stabilizing the flows through the chamber. 00:27:53.500 --> 00:27:55.000 align:middle line:90% And that takes a while. 00:27:55.000 --> 00:27:57.400 align:middle line:84% And this condition in this particular case, 00:27:57.400 --> 00:27:59.650 align:middle line:84% stabilization step is about 20 seconds. 00:27:59.650 --> 00:28:03.130 align:middle line:84% Just gives you time to get the old gases purged out 00:28:03.130 --> 00:28:06.400 align:middle line:84% from the prior etch, which was the breakthrough etch, 00:28:06.400 --> 00:28:09.850 align:middle line:84% and get the new ones purged in and establish 00:28:09.850 --> 00:28:11.620 align:middle line:90% your pressure of 10 millitorr. 00:28:11.620 --> 00:28:13.420 align:middle line:84% Then you actually do the main etch. 00:28:13.420 --> 00:28:16.240 align:middle line:84% So here you turn the plasma on by turning the power 00:28:16.240 --> 00:28:17.800 align:middle line:84% onto the top and bottom electrodes. 00:28:17.800 --> 00:28:20.170 align:middle line:84% So the plasma is now lit, and you're actually 00:28:20.170 --> 00:28:23.170 align:middle line:84% etching here for a time of, in this particular example, 00:28:23.170 --> 00:28:24.070 align:middle line:90% 60 seconds. 00:28:24.070 --> 00:28:25.120 align:middle line:90% So you etch by time. 00:28:25.120 --> 00:28:27.610 align:middle line:90% So it's a 60-second etch. 00:28:27.610 --> 00:28:31.970 align:middle line:84% And then you move to the last stabilization now. 00:28:31.970 --> 00:28:34.130 align:middle line:84% And you're stabilizing to get ready for what's 00:28:34.130 --> 00:28:35.105 align:middle line:90% called the over etch. 00:28:35.105 --> 00:28:37.730 align:middle line:84% So there's a breakthrough etch to get through the native oxide. 00:28:37.730 --> 00:28:40.310 align:middle line:84% There's a main etch, which does most of the work. 00:28:40.310 --> 00:28:42.490 align:middle line:84% Now, the over etch is the amount of time 00:28:42.490 --> 00:28:44.990 align:middle line:84% you need, remember, where you've pretty much hit the bottom, 00:28:44.990 --> 00:28:47.820 align:middle line:84% but you may have stringers or other things. 00:28:47.820 --> 00:28:51.500 align:middle line:84% So the over etch typically has to have even greater selectivity 00:28:51.500 --> 00:28:52.430 align:middle line:90% than the main etch. 00:28:52.430 --> 00:28:55.220 align:middle line:84% And so the over etch in this example, 00:28:55.220 --> 00:28:56.940 align:middle line:84% you notice they removed the chlorine. 00:28:56.940 --> 00:29:00.080 align:middle line:84% So the chlorine flow, which was 40 CCM, has been removed. 00:29:00.080 --> 00:29:02.690 align:middle line:90% And you just have HBR and O2. 00:29:02.690 --> 00:29:04.970 align:middle line:84% This gives you a much higher selectivity. 00:29:04.970 --> 00:29:07.040 align:middle line:84% Maybe not as good of an etch rate, 00:29:07.040 --> 00:29:11.000 align:middle line:84% but it does have very high selectivity so that you will not 00:29:11.000 --> 00:29:12.080 align:middle line:90% break through that oxide. 00:29:12.080 --> 00:29:14.720 align:middle line:84% And then you do the over etch in this step 00:29:14.720 --> 00:29:17.720 align:middle line:84% where you light the plasma for 30 seconds using 00:29:17.720 --> 00:29:20.920 align:middle line:90% just a mixture of HBR and O2. 00:29:20.920 --> 00:29:22.480 align:middle line:90% You then unclamp the wafer. 00:29:22.480 --> 00:29:23.710 align:middle line:90% The plasma goes off. 00:29:23.710 --> 00:29:25.910 align:middle line:84% After those 30 seconds, you unclamp the wafer. 00:29:25.910 --> 00:29:28.820 align:middle line:84% And then you end and you're ready to unload. 00:29:28.820 --> 00:29:32.690 align:middle line:90% So it's all computer controlled. 00:29:32.690 --> 00:29:34.850 align:middle line:84% In this example of just etching polysilicon, 00:29:34.850 --> 00:29:37.070 align:middle line:84% you have three types of etches a breakthrough a main 00:29:37.070 --> 00:29:40.650 align:middle line:84% and an over etch and stabilization steps in between. 00:29:40.650 --> 00:29:42.793 align:middle line:90% And this is an example. 00:29:42.793 --> 00:29:44.210 align:middle line:84% And then you come out and you look 00:29:44.210 --> 00:29:46.002 align:middle line:84% at your etch rate and your sidewall profile 00:29:46.002 --> 00:29:47.480 align:middle line:90% and you see how it looks. 00:29:47.480 --> 00:29:52.460 align:middle line:84% Most etches, as we said, end up leaving a sidewall polymer 00:29:52.460 --> 00:29:55.670 align:middle line:84% that's either like a hydrocarbon based, 00:29:55.670 --> 00:30:00.230 align:middle line:84% or it might be a glass based, like an oxide-based material. 00:30:00.230 --> 00:30:01.430 align:middle line:90% That is what's responsible. 00:30:01.430 --> 00:30:03.890 align:middle line:84% Remember, it's the inhibitor layer that keeps you 00:30:03.890 --> 00:30:05.060 align:middle line:90% from etching sideways. 00:30:05.060 --> 00:30:07.460 align:middle line:84% That's what gives you such perfectly vertical side walls. 00:30:07.460 --> 00:30:09.170 align:middle line:84% You must have that inhibitor layer. 00:30:09.170 --> 00:30:11.810 align:middle line:84% The question is, what do you do with that inhibitor layer? 00:30:11.810 --> 00:30:14.030 align:middle line:84% Now that it's on your wafer, every single gate 00:30:14.030 --> 00:30:16.310 align:middle line:84% has some goop, whatever it might be, 00:30:16.310 --> 00:30:20.130 align:middle line:84% sidewall junk, polymer, whatever you want to call it. 00:30:20.130 --> 00:30:23.090 align:middle line:84% You need to do a post-etch cleanup process after that 00:30:23.090 --> 00:30:25.610 align:middle line:84% because that inhibitor layer is there. 00:30:25.610 --> 00:30:27.710 align:middle line:84% If you go into a TEM or something, 00:30:27.710 --> 00:30:29.760 align:middle line:84% you can see it, what you'll see is-- 00:30:29.760 --> 00:30:31.320 align:middle line:90% or maybe even an SEM. 00:30:31.320 --> 00:30:33.390 align:middle line:84% You'll see your polysilicon gate, 00:30:33.390 --> 00:30:35.430 align:middle line:84% and you'll see some material maybe 00:30:35.430 --> 00:30:38.730 align:middle line:84% that looks like this, some maybe amorphous-looking material. 00:30:38.730 --> 00:30:40.020 align:middle line:90% Very thin. 00:30:40.020 --> 00:30:43.860 align:middle line:84% It might only be 20 angstroms, something like that. 00:30:43.860 --> 00:30:44.850 align:middle line:90% Very thin. 00:30:44.850 --> 00:30:47.100 align:middle line:84% Thickness may vary from top to bottom. 00:30:47.100 --> 00:30:47.820 align:middle line:90% Whatever. 00:30:47.820 --> 00:30:50.790 align:middle line:90% But that's an inhibitor layer. 00:30:50.790 --> 00:30:52.470 align:middle line:84% That is what gave you that sidewall. 00:30:52.470 --> 00:30:55.230 align:middle line:84% But typically it's removed by either an H up dip 00:30:55.230 --> 00:30:57.720 align:middle line:90% or H combined with piranha. 00:30:57.720 --> 00:31:00.780 align:middle line:84% If it's some kind of hydrocarbon, 00:31:00.780 --> 00:31:02.910 align:middle line:90% you need the extra piranha. 00:31:02.910 --> 00:31:05.460 align:middle line:84% These are typically not things you want to leave on there. 00:31:05.460 --> 00:31:10.040 align:middle line:84% They can introduce contaminants in subsequent steps. 00:31:10.040 --> 00:31:12.770 align:middle line:84% So you have to do a cleanup process. 00:31:12.770 --> 00:31:15.230 align:middle line:84% OK, so that's just an example of using a dedicated gate 00:31:15.230 --> 00:31:18.890 align:middle line:84% etcher, how you would etch a polysilicon gate. 00:31:18.890 --> 00:31:21.830 align:middle line:84% Let's just talk about a couple of the characteristics people 00:31:21.830 --> 00:31:22.640 align:middle line:90% often see. 00:31:22.640 --> 00:31:27.230 align:middle line:84% And if you do gate etching, this is what you will be up against. 00:31:27.230 --> 00:31:29.210 align:middle line:84% Here's an example of what's called 00:31:29.210 --> 00:31:32.115 align:middle line:84% notching at the bottom of the polysilicon gate. 00:31:32.115 --> 00:31:33.740 align:middle line:84% It's probably a little bit hard to see. 00:31:33.740 --> 00:31:35.670 align:middle line:84% This is not the greatest micrograph. 00:31:35.670 --> 00:31:37.460 align:middle line:90% These are SEM cross-sections. 00:31:37.460 --> 00:31:39.590 align:middle line:84% These line widths from here to here 00:31:39.590 --> 00:31:43.170 align:middle line:84% is normally 0.2 microns from this point to that point. 00:31:43.170 --> 00:31:45.890 align:middle line:90% So it's a polysilicon line. 00:31:45.890 --> 00:31:49.410 align:middle line:84% And this is using sort of a standard flow rate. 00:31:49.410 --> 00:31:51.320 align:middle line:84% And here's where they modified the flow rate, 00:31:51.320 --> 00:31:52.710 align:middle line:90% making a little bit higher. 00:31:52.710 --> 00:31:55.380 align:middle line:84% And if you notice, the gate looks sort of straight, 00:31:55.380 --> 00:31:56.420 align:middle line:90% and then it goes down. 00:31:56.420 --> 00:32:00.240 align:middle line:84% It kind of goes in at the bottom like a little notch. 00:32:00.240 --> 00:32:03.560 align:middle line:84% This is considered to be undesirable in many cases. 00:32:03.560 --> 00:32:07.170 align:middle line:84% You typically want the gate sidewall to go straight down. 00:32:07.170 --> 00:32:11.010 align:middle line:84% Why would you not want the gate to be notched? 00:32:11.010 --> 00:32:14.920 align:middle line:84% Well, there are some examples of what it might do. 00:32:14.920 --> 00:32:16.940 align:middle line:84% Let's say your gate did go in like this. 00:32:16.940 --> 00:32:17.940 align:middle line:90% So you have it vertical. 00:32:17.940 --> 00:32:18.900 align:middle line:90% And then it kind of-- 00:32:18.900 --> 00:32:20.400 align:middle line:90% I'm going to make it worse. 00:32:20.400 --> 00:32:22.120 align:middle line:84% I'll make the notch look really bad. 00:32:22.120 --> 00:32:28.060 align:middle line:84% So it might look like that down here and something like that. 00:32:28.060 --> 00:32:30.030 align:middle line:84% Well, the problem with notching like that 00:32:30.030 --> 00:32:33.648 align:middle line:84% is when you go to do your iron implant, 00:32:33.648 --> 00:32:34.690 align:middle line:90% depending on what angle-- 00:32:34.690 --> 00:32:36.460 align:middle line:84% I'm going to implant the source and drain. 00:32:36.460 --> 00:32:40.420 align:middle line:84% Depending on what angle and how bad the notching is, 00:32:40.420 --> 00:32:44.080 align:middle line:84% your ion implant might end up looking like this. 00:32:44.080 --> 00:32:46.980 align:middle line:84% So there might be a little region on either side 00:32:46.980 --> 00:32:49.380 align:middle line:84% where the source and drain extensions don't actually 00:32:49.380 --> 00:32:52.140 align:middle line:84% get implanted and where there is no gate. 00:32:52.140 --> 00:32:54.960 align:middle line:84% Now you say, well, I just do a high enough temperature anneal, 00:32:54.960 --> 00:32:56.910 align:middle line:90% and then this arsenic will move. 00:32:56.910 --> 00:32:58.290 align:middle line:90% And it'll probably overlap. 00:32:58.290 --> 00:33:00.160 align:middle line:90% You'll probably be OK. 00:33:00.160 --> 00:33:01.440 align:middle line:90% But what if it doesn't? 00:33:01.440 --> 00:33:03.120 align:middle line:90% You can't go on probably. 00:33:03.120 --> 00:33:06.460 align:middle line:84% And if that notch is not well controlled, 00:33:06.460 --> 00:33:08.580 align:middle line:84% you can end up with all kinds of high series 00:33:08.580 --> 00:33:09.820 align:middle line:90% resistance in that region. 00:33:09.820 --> 00:33:12.230 align:middle line:84% So people typically don't want a notch because it ends up 00:33:12.230 --> 00:33:14.730 align:middle line:84% affecting what happens to the ion implant, because remember, 00:33:14.730 --> 00:33:16.770 align:middle line:84% this poly that you're forming, this gate 00:33:16.770 --> 00:33:19.140 align:middle line:84% is the mask for your next step, which 00:33:19.140 --> 00:33:23.750 align:middle line:84% is going to be implant the source and drain extensions. 00:33:23.750 --> 00:33:26.083 align:middle line:84% So this is an example of how they got rid of the notch. 00:33:26.083 --> 00:33:28.250 align:middle line:84% This notch is not as bad or doesn't look as dramatic 00:33:28.250 --> 00:33:29.900 align:middle line:90% as the one I drew on the board. 00:33:29.900 --> 00:33:33.467 align:middle line:84% They got rid of it by just changing a few things. 00:33:33.467 --> 00:33:35.300 align:middle line:84% They changed the pressure and the flow rate. 00:33:35.300 --> 00:33:37.675 align:middle line:84% And they're able to get a more vertical type of sidewall. 00:33:37.675 --> 00:33:39.350 align:middle line:84% So if you have to do polysilicon gates, 00:33:39.350 --> 00:33:41.962 align:middle line:84% you'll find yourself twiddling a lot of the knobs-- 00:33:41.962 --> 00:33:43.670 align:middle line:84% the pressures, the flow rates, and things 00:33:43.670 --> 00:33:47.045 align:middle line:84% like that-- until you get a shape that you're happy with. 00:33:47.045 --> 00:33:51.150 align:middle line:90% 00:33:51.150 --> 00:33:53.100 align:middle line:84% Here on slide 12 is another article 00:33:53.100 --> 00:33:54.600 align:middle line:90% I took out of literature. 00:33:54.600 --> 00:33:55.590 align:middle line:90% It's an older paper. 00:33:55.590 --> 00:33:57.270 align:middle line:84% It's from Applied Physics Letters. 00:33:57.270 --> 00:34:00.690 align:middle line:84% And what they're making a point here 00:34:00.690 --> 00:34:03.930 align:middle line:84% has to do with something called gas residence time in the plasma 00:34:03.930 --> 00:34:04.890 align:middle line:90% etcher. 00:34:04.890 --> 00:34:07.920 align:middle line:84% This residence time concept is also important 00:34:07.920 --> 00:34:10.510 align:middle line:90% if you're growing thin films. 00:34:10.510 --> 00:34:12.630 align:middle line:84% So it also is important in reactors and things. 00:34:12.630 --> 00:34:15.449 align:middle line:84% What it is has to do is the amount of time 00:34:15.449 --> 00:34:18.420 align:middle line:84% here in seconds or fraction of a second. 00:34:18.420 --> 00:34:22.260 align:middle line:84% Effectively, that gas, the volume of gas, 00:34:22.260 --> 00:34:26.429 align:middle line:84% occupies the plasma etcher itself. 00:34:26.429 --> 00:34:29.070 align:middle line:84% And you can see that residence time increases. 00:34:29.070 --> 00:34:31.562 align:middle line:84% So it goes up with each one of these lines 00:34:31.562 --> 00:34:33.270 align:middle line:84% as they go to higher and higher pressure, 00:34:33.270 --> 00:34:35.400 align:middle line:84% or it decreases as they go to lower flows-- 00:34:35.400 --> 00:34:38.130 align:middle line:90% 00:34:38.130 --> 00:34:39.600 align:middle line:90% as I go to lower pressures. 00:34:39.600 --> 00:34:43.630 align:middle line:84% And as I increase the flow rate, it goes down. 00:34:43.630 --> 00:34:45.420 align:middle line:84% So you're flowing through faster. 00:34:45.420 --> 00:34:48.989 align:middle line:84% Now, why would we care about the gas residence time? 00:34:48.989 --> 00:34:51.550 align:middle line:84% Well, we do care about it because the amount of time 00:34:51.550 --> 00:34:53.864 align:middle line:84% the fresh gas is sort of resident 00:34:53.864 --> 00:34:55.239 align:middle line:84% has to do with the amount of time 00:34:55.239 --> 00:34:58.090 align:middle line:84% the gas is available to actually do etching. 00:34:58.090 --> 00:35:01.390 align:middle line:84% And if you have a very long residence time, 00:35:01.390 --> 00:35:03.040 align:middle line:90% byproducts can develop. 00:35:03.040 --> 00:35:04.602 align:middle line:84% Actually, after all, these reactions 00:35:04.602 --> 00:35:05.560 align:middle line:90% are chemical reactions. 00:35:05.560 --> 00:35:07.600 align:middle line:90% They evolve byproducts. 00:35:07.600 --> 00:35:09.730 align:middle line:84% Byproducts can then sit and develop there 00:35:09.730 --> 00:35:11.630 align:middle line:84% for a period of fractions of a second. 00:35:11.630 --> 00:35:13.300 align:middle line:84% And those byproducts can then end up 00:35:13.300 --> 00:35:15.820 align:middle line:84% affecting the etch or the sidewall polymer formation 00:35:15.820 --> 00:35:16.910 align:middle line:90% or things like that. 00:35:16.910 --> 00:35:20.320 align:middle line:84% So residence time ends up being an important parameter. 00:35:20.320 --> 00:35:23.110 align:middle line:84% The way people control it is, well, they either up 00:35:23.110 --> 00:35:25.390 align:middle line:84% the flow rate, the total gas flow rate, 00:35:25.390 --> 00:35:30.220 align:middle line:84% or they change the pressure in some ways in the tool. 00:35:30.220 --> 00:35:33.620 align:middle line:90% 00:35:33.620 --> 00:35:37.690 align:middle line:84% Here's just an example from that same paper, an example of what 00:35:37.690 --> 00:35:40.480 align:middle line:84% happens as a function of the silicon etch rate 00:35:40.480 --> 00:35:43.310 align:middle line:90% as a function of flow rate. 00:35:43.310 --> 00:35:46.310 align:middle line:84% So here is etch rate in nanometers per minute. 00:35:46.310 --> 00:35:49.480 align:middle line:84% And this is a particular silicon etch. 00:35:49.480 --> 00:35:52.100 align:middle line:84% What they're using is chlorine, very common. 00:35:52.100 --> 00:35:57.100 align:middle line:84% And this was the so-called their present etching technique 00:35:57.100 --> 00:36:02.550 align:middle line:84% they had using a high gas flow rate and a conventional etching 00:36:02.550 --> 00:36:03.050 align:middle line:90% technique. 00:36:03.050 --> 00:36:06.730 align:middle line:84% So it also, when you change the residence time, increasing it 00:36:06.730 --> 00:36:11.210 align:middle line:84% is going to change the etch rate. 00:36:11.210 --> 00:36:13.570 align:middle line:84% So in this case, a shorter gas residence 00:36:13.570 --> 00:36:16.540 align:middle line:90% time increases the etch rate. 00:36:16.540 --> 00:36:18.310 align:middle line:84% I'm sorry, increase the flow means 00:36:18.310 --> 00:36:21.190 align:middle line:84% the amount of time any volume of gas 00:36:21.190 --> 00:36:23.290 align:middle line:90% is in the reactor is shorter. 00:36:23.290 --> 00:36:24.070 align:middle line:90% And why? 00:36:24.070 --> 00:36:26.620 align:middle line:84% Because it increases the availability of neutral etchant 00:36:26.620 --> 00:36:27.230 align:middle line:90% species. 00:36:27.230 --> 00:36:31.600 align:middle line:84% So you have the unreacted etchant coming in faster. 00:36:31.600 --> 00:36:34.300 align:middle line:84% And it also reduces the concentration of the reaction 00:36:34.300 --> 00:36:37.060 align:middle line:84% byproducts, because again, these are all chemical reactions. 00:36:37.060 --> 00:36:39.730 align:middle line:84% You want for a chemical reaction to go faster, 00:36:39.730 --> 00:36:44.140 align:middle line:84% you put in the reactants faster, and you remove the byproducts 00:36:44.140 --> 00:36:47.740 align:middle line:90% rapidly, as well. 00:36:47.740 --> 00:36:50.640 align:middle line:84% So not only does it affect etch rate, 00:36:50.640 --> 00:36:52.830 align:middle line:84% but changing things like pressure 00:36:52.830 --> 00:36:55.890 align:middle line:84% also affect the shape of what you get. 00:36:55.890 --> 00:37:01.500 align:middle line:84% Here's an example on slide 14 from that same article of three 00:37:01.500 --> 00:37:03.300 align:middle line:90% different pressures. 00:37:03.300 --> 00:37:08.790 align:middle line:84% Here we're at 10 tour to try to define a polysilicon gate. 00:37:08.790 --> 00:37:11.700 align:middle line:84% I'm sorry, 10 millitorr, 1 millitorr, and 0.1. 00:37:11.700 --> 00:37:13.590 align:middle line:84% So we're going down here each case 00:37:13.590 --> 00:37:16.050 align:middle line:84% by an order of magnitude and pressure. 00:37:16.050 --> 00:37:18.570 align:middle line:84% We're using lower pressure at high flows. 00:37:18.570 --> 00:37:20.910 align:middle line:84% You notice we get more anisotropic etching 00:37:20.910 --> 00:37:25.920 align:middle line:84% and less notching, at least when using this particular chlorine 00:37:25.920 --> 00:37:26.790 align:middle line:90% polysilicon etch. 00:37:26.790 --> 00:37:33.162 align:middle line:84% So here at 10 millitorr, you get a fairly isotropic etch. 00:37:33.162 --> 00:37:34.620 align:middle line:84% It's not totally isotropic, but you 00:37:34.620 --> 00:37:37.098 align:middle line:84% notice it's etching sideways, as well as vertically. 00:37:37.098 --> 00:37:37.890 align:middle line:90% And how can I tell? 00:37:37.890 --> 00:37:39.360 align:middle line:90% Because this is my photoresist. 00:37:39.360 --> 00:37:41.530 align:middle line:84% And it's undercut on the left and right side. 00:37:41.530 --> 00:37:45.930 align:middle line:84% So the line is a lot narrower than what had been designed. 00:37:45.930 --> 00:37:50.340 align:middle line:84% There's a little bit better anisotropy at a lower pressure 00:37:50.340 --> 00:37:52.590 align:middle line:84% at one millitorr, but we still have-- now we've 00:37:52.590 --> 00:37:55.380 align:middle line:84% developed a little notch at the bottom, which is, again, 00:37:55.380 --> 00:37:57.000 align:middle line:84% that's not considered to be good. 00:37:57.000 --> 00:38:01.260 align:middle line:84% Finally, we have the lowest residence time, or the lowest 00:38:01.260 --> 00:38:03.510 align:middle line:90% pressure here, at 0.1 millitorr. 00:38:03.510 --> 00:38:06.270 align:middle line:84% This paper demonstrates very vertical sidewalls 00:38:06.270 --> 00:38:07.440 align:middle line:90% and no undercutting. 00:38:07.440 --> 00:38:12.300 align:middle line:84% So this would be considered a very good quality etch. 00:38:12.300 --> 00:38:15.030 align:middle line:84% You may be able to make one device with this etch. 00:38:15.030 --> 00:38:16.530 align:middle line:84% I'm not saying this etch won't work, 00:38:16.530 --> 00:38:19.290 align:middle line:84% but from a manufacturing point of view, how do I control-- 00:38:19.290 --> 00:38:20.610 align:middle line:90% how do I control my line width? 00:38:20.610 --> 00:38:23.920 align:middle line:84% In my critical dimension, it's not well controlled at all. 00:38:23.920 --> 00:38:26.620 align:middle line:84% So this would not be considered a manufacturable type 00:38:26.620 --> 00:38:27.120 align:middle line:90% of process. 00:38:27.120 --> 00:38:31.360 align:middle line:90% 00:38:31.360 --> 00:38:32.800 align:middle line:90% There's another consideration. 00:38:32.800 --> 00:38:35.290 align:middle line:90% So we talked a lot about-- 00:38:35.290 --> 00:38:38.485 align:middle line:84% we care about the etch rate and the shape of the sidewall 00:38:38.485 --> 00:38:40.860 align:middle line:84% because that affects how we're going to implant it and do 00:38:40.860 --> 00:38:42.660 align:middle line:90% other things. 00:38:42.660 --> 00:38:45.870 align:middle line:84% A huge consideration anytime you are etching features-- 00:38:45.870 --> 00:38:48.300 align:middle line:84% this is not just true of a polysilicon gate, 00:38:48.300 --> 00:38:51.970 align:middle line:84% but I'm giving the example for a polysilicon gate, 00:38:51.970 --> 00:38:53.310 align:middle line:90% so gate etch is the example-- 00:38:53.310 --> 00:38:56.270 align:middle line:90% is the topography. 00:38:56.270 --> 00:38:57.880 align:middle line:90% And so what's the problem? 00:38:57.880 --> 00:39:00.230 align:middle line:84% Well, highly anisotropic etching. 00:39:00.230 --> 00:39:01.660 align:middle line:90% So etching, there's only down. 00:39:01.660 --> 00:39:02.680 align:middle line:90% Only vertical. 00:39:02.680 --> 00:39:04.120 align:middle line:90% Doesn't etch sideways. 00:39:04.120 --> 00:39:06.350 align:middle line:84% It's required to achieve good control-- 00:39:06.350 --> 00:39:09.430 align:middle line:84% we just said that-- over the gate length and the shape. 00:39:09.430 --> 00:39:13.630 align:middle line:84% But unless you have a long over etch 00:39:13.630 --> 00:39:16.000 align:middle line:84% that you do at the end of the step, 00:39:16.000 --> 00:39:18.640 align:middle line:84% you can get stringers left behind. 00:39:18.640 --> 00:39:21.260 align:middle line:84% And let's just show an example here. 00:39:21.260 --> 00:39:23.050 align:middle line:84% So on the left, I'm showing where we've 00:39:23.050 --> 00:39:25.540 align:middle line:90% etched through polysilicon. 00:39:25.540 --> 00:39:28.150 align:middle line:84% There was no topography on the original part of the wafer. 00:39:28.150 --> 00:39:29.180 align:middle line:90% It was just flat. 00:39:29.180 --> 00:39:31.690 align:middle line:84% So I have photoresist and I've etched polysilicon 00:39:31.690 --> 00:39:32.710 align:middle line:90% anisotropically. 00:39:32.710 --> 00:39:34.750 align:middle line:90% Everything looks very good. 00:39:34.750 --> 00:39:36.430 align:middle line:84% Now, what's happened on the right? 00:39:36.430 --> 00:39:39.320 align:middle line:84% Well, on the right-hand side, I had some initial topography. 00:39:39.320 --> 00:39:42.640 align:middle line:84% So I had a step that looked like this. 00:39:42.640 --> 00:39:46.420 align:middle line:84% So as a result of having that step, the polysilicon overlayer, 00:39:46.420 --> 00:39:48.850 align:middle line:84% which is shown by this sort of stippled region, 00:39:48.850 --> 00:39:50.950 align:middle line:84% the polysilicon went over everything. 00:39:50.950 --> 00:39:53.260 align:middle line:84% So the polysilicon in this region 00:39:53.260 --> 00:39:56.870 align:middle line:84% is actually thicker than it is in the flat portion. 00:39:56.870 --> 00:39:59.140 align:middle line:84% So by the time I cleared the gate 00:39:59.140 --> 00:40:01.570 align:middle line:84% and I just hit this oxide right here, 00:40:01.570 --> 00:40:05.410 align:middle line:84% I have left behind a stringer because I haven't etched 00:40:05.410 --> 00:40:07.632 align:middle line:84% long enough because the polysilicon thickness was 00:40:07.632 --> 00:40:08.590 align:middle line:90% thicker in this region. 00:40:08.590 --> 00:40:10.510 align:middle line:84% And remember, I'm only etching vertically. 00:40:10.510 --> 00:40:13.070 align:middle line:90% It's not going sideways. 00:40:13.070 --> 00:40:15.640 align:middle line:84% So this stringer is considered to be undesirable 00:40:15.640 --> 00:40:17.770 align:middle line:84% because it's now an extra gate electrode, 00:40:17.770 --> 00:40:21.280 align:middle line:84% or it's an extra thing you didn't want in your circuit. 00:40:21.280 --> 00:40:22.540 align:middle line:90% How do I get rid of it? 00:40:22.540 --> 00:40:26.830 align:middle line:84% Well, you say, just keep on etching. 00:40:26.830 --> 00:40:28.000 align:middle line:90% Go to do an over etch. 00:40:28.000 --> 00:40:31.090 align:middle line:84% So even though I've finished etching the poly gate 00:40:31.090 --> 00:40:33.550 align:middle line:84% and I'm done in this region, I'm not done over here. 00:40:33.550 --> 00:40:35.470 align:middle line:84% So you keep it in and you keep banging 00:40:35.470 --> 00:40:41.140 align:middle line:84% on the top of that stringer until it finally goes down to 0. 00:40:41.140 --> 00:40:45.520 align:middle line:84% So you need a very long over etch whenever you have surface 00:40:45.520 --> 00:40:48.580 align:middle line:84% topography in order to get rid of stringers. 00:40:48.580 --> 00:40:50.620 align:middle line:84% What's the disadvantage of a long ever etch? 00:40:50.620 --> 00:40:52.870 align:middle line:84% Well, you might break through your oxide 00:40:52.870 --> 00:40:54.670 align:middle line:84% because you're still beating on that oxide. 00:40:54.670 --> 00:40:56.552 align:middle line:84% So unless you have really good selectivity, 00:40:56.552 --> 00:40:58.510 align:middle line:84% as you're beating the stringer down and getting 00:40:58.510 --> 00:41:00.910 align:middle line:84% rid of the stringer, you could be destroying the oxide. 00:41:00.910 --> 00:41:02.260 align:middle line:90% What's the other bad thing? 00:41:02.260 --> 00:41:07.150 align:middle line:84% Well, your over etch may not be perfectly anisotropic. 00:41:07.150 --> 00:41:09.580 align:middle line:84% It may be etching sideways, a little bit on the gate. 00:41:09.580 --> 00:41:12.140 align:middle line:84% So unless it's perfectly anisotropic, you'll have that. 00:41:12.140 --> 00:41:14.380 align:middle line:84% So there's always a balance with over etch. 00:41:14.380 --> 00:41:17.140 align:middle line:84% So this is why you will find people say, 00:41:17.140 --> 00:41:19.420 align:middle line:84% if you want to get the finest patterns, 00:41:19.420 --> 00:41:21.760 align:middle line:84% your wafers should be completely flat. 00:41:21.760 --> 00:41:23.770 align:middle line:84% You should have no topography because if you 00:41:23.770 --> 00:41:25.790 align:middle line:84% have a completely flat wafer, you never 00:41:25.790 --> 00:41:27.040 align:middle line:90% have to worry about stringers. 00:41:27.040 --> 00:41:28.030 align:middle line:90% You etch down. 00:41:28.030 --> 00:41:29.570 align:middle line:90% You stop on the layer below. 00:41:29.570 --> 00:41:30.520 align:middle line:90% And you stop. 00:41:30.520 --> 00:41:31.300 align:middle line:90% Period. 00:41:31.300 --> 00:41:33.490 align:middle line:84% Your over etch in that case is only dictated 00:41:33.490 --> 00:41:36.640 align:middle line:84% by the non-uniformity in the film across the wafer, 00:41:36.640 --> 00:41:39.080 align:middle line:84% as we talked about before around uniforming etch rate. 00:41:39.080 --> 00:41:41.810 align:middle line:84% Now we have another requirement on over etch. 00:41:41.810 --> 00:41:44.240 align:middle line:84% I have to not only take into account non-uniformity 00:41:44.240 --> 00:41:47.000 align:middle line:84% across the wafer, but if I have topography 00:41:47.000 --> 00:41:52.430 align:middle line:84% and I have an anisotropic etch, I have to get rid of stringers. 00:41:52.430 --> 00:41:55.730 align:middle line:84% So you will see the topography on modern devices, 00:41:55.730 --> 00:41:57.800 align:middle line:84% as they get smaller and smaller, everything has 00:41:57.800 --> 00:41:58.967 align:middle line:90% to be smoother and smoother. 00:41:58.967 --> 00:42:01.820 align:middle line:84% That's why people went away from locos. 00:42:01.820 --> 00:42:04.222 align:middle line:84% Locos creates this big hump on the way. 00:42:04.222 --> 00:42:05.180 align:middle line:90% People don't like that. 00:42:05.180 --> 00:42:08.300 align:middle line:84% We do shallow trench, which creates a perfectly flat wafer. 00:42:08.300 --> 00:42:10.910 align:middle line:84% That way I can pattern a very, very fine line 00:42:10.910 --> 00:42:15.120 align:middle line:84% and I don't have to worry about stringers. 00:42:15.120 --> 00:42:17.670 align:middle line:84% Slide 16 is from a different text. 00:42:17.670 --> 00:42:19.730 align:middle line:84% It has another example of stringer formation. 00:42:19.730 --> 00:42:23.950 align:middle line:84% And maybe this cross-section might be easier for you to see. 00:42:23.950 --> 00:42:27.510 align:middle line:84% So here's an example where I have a field oxide. 00:42:27.510 --> 00:42:31.020 align:middle line:84% So the field oxide has a certain step height to it, shown here. 00:42:31.020 --> 00:42:34.170 align:middle line:84% And I have-- and of course, when you deposit the polysilicon, 00:42:34.170 --> 00:42:36.850 align:middle line:84% it gets deposited everywhere on the wafer. 00:42:36.850 --> 00:42:39.030 align:middle line:84% So this cross-hatched region is the poly. 00:42:39.030 --> 00:42:40.510 align:middle line:90% Then I want a pattern. 00:42:40.510 --> 00:42:42.540 align:middle line:84% And this is the photoresist after it's 00:42:42.540 --> 00:42:44.670 align:middle line:90% been developed and patterned. 00:42:44.670 --> 00:42:45.540 align:middle line:90% OK. 00:42:45.540 --> 00:42:49.080 align:middle line:84% Now, step B here, I put it into the etcher 00:42:49.080 --> 00:42:52.110 align:middle line:84% and I use the photoresist as a mask to etch the poly. 00:42:52.110 --> 00:42:53.250 align:middle line:90% So this looks good. 00:42:53.250 --> 00:42:56.700 align:middle line:84% I have very nice sidewalls and I stop on the oxide. 00:42:56.700 --> 00:42:57.760 align:middle line:90% But what happens? 00:42:57.760 --> 00:43:00.330 align:middle line:84% Well, in this region right here, look 00:43:00.330 --> 00:43:02.490 align:middle line:84% at the thickness of this gate material 00:43:02.490 --> 00:43:04.050 align:middle line:90% where it goes over the step. 00:43:04.050 --> 00:43:07.620 align:middle line:84% The thickness from this point of the top to the very bottom 00:43:07.620 --> 00:43:11.020 align:middle line:84% is almost twice what the thickness is in the flat region. 00:43:11.020 --> 00:43:13.830 align:middle line:84% So as a result, once I've cleared the flats, 00:43:13.830 --> 00:43:16.450 align:middle line:84% I still have this little notch of material 00:43:16.450 --> 00:43:18.370 align:middle line:84% that's un etched that's sitting there 00:43:18.370 --> 00:43:20.620 align:middle line:84% because my etch only etches vertically. 00:43:20.620 --> 00:43:22.900 align:middle line:84% If the etch had etched sideways, then you 00:43:22.900 --> 00:43:24.580 align:middle line:84% wouldn't have to worry about that notch. 00:43:24.580 --> 00:43:30.100 align:middle line:84% So an isotropic etch doesn't tend to give you stringers. 00:43:30.100 --> 00:43:31.850 align:middle line:84% Stringer formation is not such a big deal. 00:43:31.850 --> 00:43:33.680 align:middle line:90% But if it's anisotropic, it is. 00:43:33.680 --> 00:43:37.390 align:middle line:84% So what we do is you continue in step C with an over etch. 00:43:37.390 --> 00:43:39.220 align:middle line:84% You continue to beat down on this, 00:43:39.220 --> 00:43:43.570 align:middle line:84% and you introduce an extra 50% of an over etch 00:43:43.570 --> 00:43:45.070 align:middle line:90% to get rid of the stringers. 00:43:45.070 --> 00:43:46.900 align:middle line:84% If you leave the stringers in place, 00:43:46.900 --> 00:43:48.430 align:middle line:84% you'll have shorts on your devices 00:43:48.430 --> 00:43:50.740 align:middle line:84% because then you have this poly going 00:43:50.740 --> 00:43:52.330 align:middle line:90% all the way around the field. 00:43:52.330 --> 00:43:54.460 align:middle line:84% You can short the source drain to the gate, 00:43:54.460 --> 00:43:56.390 align:middle line:84% and you can get all kinds of problems. 00:43:56.390 --> 00:43:59.890 align:middle line:84% Now, this is drawn by a cartoon, or by an illustrator, 00:43:59.890 --> 00:44:00.640 align:middle line:90% so it's very easy. 00:44:00.640 --> 00:44:03.430 align:middle line:84% It doesn't show any detrimental effect of the over etch. 00:44:03.430 --> 00:44:06.790 align:middle line:84% In practice, over etching to remove stringers almost 00:44:06.790 --> 00:44:08.780 align:middle line:84% always has a little detrimental effect. 00:44:08.780 --> 00:44:10.570 align:middle line:84% And so the name of the game is trying 00:44:10.570 --> 00:44:13.600 align:middle line:84% to control your topography so you minimize your over etch 00:44:13.600 --> 00:44:17.015 align:middle line:84% so you don't have to destroy the structure by over etching 00:44:17.015 --> 00:44:17.515 align:middle line:90% too much. 00:44:17.515 --> 00:44:22.120 align:middle line:90% 00:44:22.120 --> 00:44:24.300 align:middle line:90% So let's look at slide 17. 00:44:24.300 --> 00:44:28.620 align:middle line:84% This is an example on that paper, that article from IBM, 00:44:28.620 --> 00:44:31.800 align:middle line:84% on the effects of topography on gate conductor 00:44:31.800 --> 00:44:33.615 align:middle line:90% yield with respect to shorting. 00:44:33.615 --> 00:44:35.490 align:middle line:84% As I mentioned, if you end up with stringers, 00:44:35.490 --> 00:44:38.260 align:middle line:84% the gate can be shorted to the source and drain. 00:44:38.260 --> 00:44:39.930 align:middle line:90% So here's an example. 00:44:39.930 --> 00:44:42.480 align:middle line:84% And they etched here 21 wafers or so-- 00:44:42.480 --> 00:44:43.500 align:middle line:90% 20 wafers. 00:44:43.500 --> 00:44:45.138 align:middle line:90% And the percent yield. 00:44:45.138 --> 00:44:46.680 align:middle line:84% So if you have a high yield, it means 00:44:46.680 --> 00:44:49.290 align:middle line:84% you don't have gate to source drain shorts. 00:44:49.290 --> 00:44:51.000 align:middle line:84% If you have a low yield, it means 00:44:51.000 --> 00:44:53.850 align:middle line:84% you have shorts between the gate and the source drain. 00:44:53.850 --> 00:44:55.870 align:middle line:90% So this wafer had no topography. 00:44:55.870 --> 00:44:58.380 align:middle line:84% So they etched the gates on perfectly flat wafers. 00:44:58.380 --> 00:44:59.620 align:middle line:90% And the yield was very high. 00:44:59.620 --> 00:45:02.820 align:middle line:84% It was like 90% for these five wafers. 00:45:02.820 --> 00:45:05.790 align:middle line:84% Now they introduced-- in the field oxide or somewhere, 00:45:05.790 --> 00:45:08.130 align:middle line:84% they introduced a 50-nanometer step. 00:45:08.130 --> 00:45:11.160 align:middle line:84% You can see the yield goes down a little bit. 00:45:11.160 --> 00:45:12.870 align:middle line:90% Not too bad, though. 00:45:12.870 --> 00:45:14.370 align:middle line:90% So they have more shorts. 00:45:14.370 --> 00:45:17.430 align:middle line:84% And here if you introduce a 100-nanometer step 00:45:17.430 --> 00:45:20.750 align:middle line:84% on this particular gate etch, you get a yield of only 30%. 00:45:20.750 --> 00:45:22.690 align:middle line:90% So 70% of devices are shorted. 00:45:22.690 --> 00:45:25.520 align:middle line:84% So this means you're getting a lot of stringers forming. 00:45:25.520 --> 00:45:28.360 align:middle line:84% So you can look for stringers visually in the microscope 00:45:28.360 --> 00:45:32.140 align:middle line:84% with a TEM or an SEM, or you can look at them with special test 00:45:32.140 --> 00:45:34.610 align:middle line:84% structures where you look at your shorting and your yield. 00:45:34.610 --> 00:45:36.490 align:middle line:84% So this is an example where you would 00:45:36.490 --> 00:45:39.670 align:middle line:84% say for this particular gate etch process 00:45:39.670 --> 00:45:43.402 align:middle line:84% that the 100-nanometer step is too much. 00:45:43.402 --> 00:45:44.860 align:middle line:84% You need to reduce your step height 00:45:44.860 --> 00:45:46.557 align:middle line:84% and not produce such an abrupt step. 00:45:46.557 --> 00:45:48.640 align:middle line:84% Otherwise you need to increase your over etch time 00:45:48.640 --> 00:45:51.230 align:middle line:90% or do something. 00:45:51.230 --> 00:45:53.260 align:middle line:84% So stringers are really a big issue, 00:45:53.260 --> 00:45:56.470 align:middle line:84% and it's always a trade-off between their removal 00:45:56.470 --> 00:46:01.890 align:middle line:90% and messing up the structure. 00:46:01.890 --> 00:46:05.460 align:middle line:84% Here on slide 18, I just showed you some-- 00:46:05.460 --> 00:46:09.660 align:middle line:84% I've shown some ways where we can calculate over 00:46:09.660 --> 00:46:12.300 align:middle line:84% etched requirements based on the topography one 00:46:12.300 --> 00:46:14.800 align:middle line:90% might have during gate etching. 00:46:14.800 --> 00:46:18.150 align:middle line:84% So here's an example on this particular figure where 00:46:18.150 --> 00:46:20.887 align:middle line:90% I'm showing you a field oxide. 00:46:20.887 --> 00:46:22.470 align:middle line:84% Remember we said the field oxide might 00:46:22.470 --> 00:46:24.120 align:middle line:84% have a certain height, which we're 00:46:24.120 --> 00:46:29.460 align:middle line:84% calling T sub FOX, the thickness of the field oxide. 00:46:29.460 --> 00:46:32.580 align:middle line:84% And it has a certain angle that it's been etched. 00:46:32.580 --> 00:46:34.885 align:middle line:84% So it's not perfectly straight up and down. 00:46:34.885 --> 00:46:37.260 align:middle line:84% It has a certain angle, theta, that it makes with respect 00:46:37.260 --> 00:46:38.560 align:middle line:90% to the substrate. 00:46:38.560 --> 00:46:40.020 align:middle line:90% So that's your field oxide. 00:46:40.020 --> 00:46:43.080 align:middle line:84% And this black region on top is meant 00:46:43.080 --> 00:46:44.580 align:middle line:90% to represent the polysilicon. 00:46:44.580 --> 00:46:47.520 align:middle line:84% So remember, polysilicon is always deposited everywhere 00:46:47.520 --> 00:46:49.818 align:middle line:90% across the chip. 00:46:49.818 --> 00:46:51.860 align:middle line:84% So the black region is the thickness of the poly. 00:46:51.860 --> 00:46:55.340 align:middle line:84% Now, you notice that on the sloped portion, 00:46:55.340 --> 00:46:57.560 align:middle line:84% the thickness of the poly from here to here 00:46:57.560 --> 00:47:02.390 align:middle line:84% is given by T poly times the cosine theta. 00:47:02.390 --> 00:47:06.320 align:middle line:84% So it's thicker here than it is here. 00:47:06.320 --> 00:47:08.940 align:middle line:90% It's just by geometry. 00:47:08.940 --> 00:47:11.640 align:middle line:84% So the thicker portion of the poly 00:47:11.640 --> 00:47:13.620 align:middle line:84% here means that you need to increase in order 00:47:13.620 --> 00:47:15.150 align:middle line:90% to get clear all the poly. 00:47:15.150 --> 00:47:17.550 align:middle line:84% And to avoid forming a stringer down here, 00:47:17.550 --> 00:47:19.600 align:middle line:84% you need to increase the etch time. 00:47:19.600 --> 00:47:21.780 align:middle line:84% And in fact, this formula gives you 00:47:21.780 --> 00:47:25.180 align:middle line:84% here, for this particular topography, 00:47:25.180 --> 00:47:31.050 align:middle line:84% gives you an example calculation of the over etch here in percent 00:47:31.050 --> 00:47:33.930 align:middle line:84% that you would need for this topography. 00:47:33.930 --> 00:47:36.420 align:middle line:84% And it's 1 over cosine theta, that quantity, 00:47:36.420 --> 00:47:38.920 align:middle line:90% minus 1, times 100%. 00:47:38.920 --> 00:47:42.040 align:middle line:84% So if you have a sidewall angle of 45 degrees-- 00:47:42.040 --> 00:47:44.280 align:middle line:84% so this particular example says 45 degrees-- 00:47:44.280 --> 00:47:49.260 align:middle line:84% just because of this, we need about a 40% poly over etch time 00:47:49.260 --> 00:47:50.610 align:middle line:90% to avoid stringers. 00:47:50.610 --> 00:47:52.150 align:middle line:90% So what do people do? 00:47:52.150 --> 00:47:53.910 align:middle line:84% Well, either you have no topography, 00:47:53.910 --> 00:47:56.400 align:middle line:84% you keep it very flat, you bury your field oxide, 00:47:56.400 --> 00:47:58.410 align:middle line:84% and that's called a shallow trench. 00:47:58.410 --> 00:48:00.960 align:middle line:84% Or if you are going to have a little bit of topography, 00:48:00.960 --> 00:48:03.090 align:middle line:84% you make it-- you kind of ramp the angles 00:48:03.090 --> 00:48:04.800 align:middle line:90% and you make very shallow ramps. 00:48:04.800 --> 00:48:06.990 align:middle line:84% You don't have anything really abrupt. 00:48:06.990 --> 00:48:09.640 align:middle line:84% So depending on this angle, you have 00:48:09.640 --> 00:48:11.170 align:middle line:90% to adjust your over etch time. 00:48:11.170 --> 00:48:12.940 align:middle line:84% Otherwise you'll end up with stringers. 00:48:12.940 --> 00:48:17.810 align:middle line:90% 00:48:17.810 --> 00:48:20.690 align:middle line:84% Sometimes, by the way, here on slide 19, 00:48:20.690 --> 00:48:22.880 align:middle line:84% sometimes I've been calling these things stringers. 00:48:22.880 --> 00:48:24.830 align:middle line:84% Stringers is the derogatory term. 00:48:24.830 --> 00:48:27.080 align:middle line:84% Stringer usually means something that was left behind 00:48:27.080 --> 00:48:29.450 align:middle line:90% that you didn't want there. 00:48:29.450 --> 00:48:32.210 align:middle line:84% Sometimes a stringer is something we want. 00:48:32.210 --> 00:48:35.840 align:middle line:84% In fact, when we want it, we give it a positive name. 00:48:35.840 --> 00:48:37.700 align:middle line:90% We call it a sidewall spacer. 00:48:37.700 --> 00:48:41.480 align:middle line:84% So we often use anisotropic etches 00:48:41.480 --> 00:48:45.530 align:middle line:84% without an over etch to get a sidewall spacer intentionally. 00:48:45.530 --> 00:48:48.200 align:middle line:84% Here's an example of an intentional production 00:48:48.200 --> 00:48:49.550 align:middle line:90% of a stringer. 00:48:49.550 --> 00:48:51.080 align:middle line:90% We have a substrate. 00:48:51.080 --> 00:48:53.300 align:middle line:90% We put one film on. 00:48:53.300 --> 00:48:55.760 align:middle line:84% It could be polysilicon that we have now patterned 00:48:55.760 --> 00:48:57.660 align:middle line:90% with a very sharp step. 00:48:57.660 --> 00:49:01.400 align:middle line:84% We put a second film on where we adjust the height-- 00:49:01.400 --> 00:49:03.560 align:middle line:84% or the thickness, I should say-- of the second film 00:49:03.560 --> 00:49:06.860 align:middle line:84% to be about, oh, just the right thickness 00:49:06.860 --> 00:49:10.790 align:middle line:84% so that after I've done a purely vertical or purely anisotropic 00:49:10.790 --> 00:49:14.450 align:middle line:84% etch, when I'm done, I have a sidewall spacer forming. 00:49:14.450 --> 00:49:16.980 align:middle line:90% So this material is left behind. 00:49:16.980 --> 00:49:20.310 align:middle line:84% The nice thing about it is self-aligned. 00:49:20.310 --> 00:49:24.000 align:middle line:84% So I didn't have to put another mask down and do 00:49:24.000 --> 00:49:25.680 align:middle line:84% photolithography to pattern this. 00:49:25.680 --> 00:49:29.400 align:middle line:84% All you have to do is have a step in your film, number one. 00:49:29.400 --> 00:49:31.110 align:middle line:90% You put another film down. 00:49:31.110 --> 00:49:33.210 align:middle line:84% You clear the whole thing with a vertical etch. 00:49:33.210 --> 00:49:33.840 align:middle line:90% And bingo. 00:49:33.840 --> 00:49:36.120 align:middle line:84% Right on the sidewall, self-aligned to it, 00:49:36.120 --> 00:49:38.440 align:middle line:90% is a spacer layer. 00:49:38.440 --> 00:49:40.590 align:middle line:84% In fact, that's exactly how spacer layers 00:49:40.590 --> 00:49:42.750 align:middle line:90% are formed in modern MOSFETs. 00:49:42.750 --> 00:49:44.610 align:middle line:84% The poly gate is etched vertically. 00:49:44.610 --> 00:49:48.060 align:middle line:84% A certain thickness of nitride or oxide is put down. 00:49:48.060 --> 00:49:50.040 align:middle line:84% It's etched anisotropically, and you end up 00:49:50.040 --> 00:49:51.360 align:middle line:90% with a sidewall spacer. 00:49:51.360 --> 00:49:55.122 align:middle line:84% So stringers, so to speak, are not necessarily bad. 00:49:55.122 --> 00:49:57.330 align:middle line:84% Usually the word stringer means something left behind 00:49:57.330 --> 00:49:58.240 align:middle line:90% that you don't want. 00:49:58.240 --> 00:50:00.510 align:middle line:84% But if you do want it, you call it a sidewall spacer. 00:50:00.510 --> 00:50:02.880 align:middle line:84% And the width of the spacer, which is from this point 00:50:02.880 --> 00:50:06.150 align:middle line:84% here to that point there that's sort of the lateral dimension, 00:50:06.150 --> 00:50:10.440 align:middle line:84% is a function of how long you did etch and how much over etch 00:50:10.440 --> 00:50:13.140 align:middle line:84% you did, as well as the thickness 00:50:13.140 --> 00:50:15.510 align:middle line:84% of film two relative to the thickness of film 00:50:15.510 --> 00:50:16.960 align:middle line:90% one in this example. 00:50:16.960 --> 00:50:22.280 align:middle line:84% So you adjust all of those to create different width spacers. 00:50:22.280 --> 00:50:27.642 align:middle line:84% That's a very critical process in modern technology. 00:50:27.642 --> 00:50:29.350 align:middle line:84% Now, this slide-- actually, I apologize-- 00:50:29.350 --> 00:50:30.350 align:middle line:90% is not in your handouts. 00:50:30.350 --> 00:50:31.300 align:middle line:90% It's on page 20. 00:50:31.300 --> 00:50:32.720 align:middle line:90% There's two slides here. 00:50:32.720 --> 00:50:37.100 align:middle line:84% The next three slides are not in your handout. 00:50:37.100 --> 00:50:37.600 align:middle line:90% This one is. 00:50:37.600 --> 00:50:39.830 align:middle line:84% It's the next two after that are not. 00:50:39.830 --> 00:50:41.680 align:middle line:84% And if you need the up-to-date handouts, 00:50:41.680 --> 00:50:43.210 align:middle line:90% they're all posted on the web. 00:50:43.210 --> 00:50:45.260 align:middle line:84% So you can get the PDFs yourself. 00:50:45.260 --> 00:50:48.160 align:middle line:84% I wanted to list here on slide 20 00:50:48.160 --> 00:50:50.980 align:middle line:84% a couple of new issues for gate etching and patterning 00:50:50.980 --> 00:50:52.520 align:middle line:84% that have come up in the last, say, 00:50:52.520 --> 00:50:54.062 align:middle line:84% three to five years that you're going 00:50:54.062 --> 00:50:56.650 align:middle line:90% to hear more and more about. 00:50:56.650 --> 00:50:59.470 align:middle line:84% In the year 2000, this is what people were concerned about. 00:50:59.470 --> 00:51:01.630 align:middle line:84% They were concerned about etching poly, 00:51:01.630 --> 00:51:04.360 align:middle line:84% making it vertical, maintaining a critical dimension 00:51:04.360 --> 00:51:06.800 align:middle line:90% of, say, 0.18 nanometers-- 00:51:06.800 --> 00:51:08.440 align:middle line:90% 0.18 microns, rather. 00:51:08.440 --> 00:51:10.750 align:middle line:84% And they were etching through only polysilicon 00:51:10.750 --> 00:51:13.780 align:middle line:84% and putting a silicide strap on top 00:51:13.780 --> 00:51:16.430 align:middle line:84% after the gate etch had been completed. 00:51:16.430 --> 00:51:20.800 align:middle line:84% So that was what should call the more traditional or conventional 00:51:20.800 --> 00:51:22.390 align:middle line:90% technology. 00:51:22.390 --> 00:51:24.070 align:middle line:84% What are people doing these days? 00:51:24.070 --> 00:51:27.770 align:middle line:84% Well, people these days are not just doing this. 00:51:27.770 --> 00:51:29.800 align:middle line:84% For one thing, the gates are a lot narrower. 00:51:29.800 --> 00:51:31.270 align:middle line:90% Look at today's gates. 00:51:31.270 --> 00:51:36.970 align:middle line:84% People are, instead of doing something like 200 nanometers, 00:51:36.970 --> 00:51:39.260 align:middle line:84% we're down to 70 or 80 nanometers. 00:51:39.260 --> 00:51:41.800 align:middle line:84% So the dimensions have shrunk quite a bit. 00:51:41.800 --> 00:51:44.860 align:middle line:84% And people are doing what they call fully silicided gates. 00:51:44.860 --> 00:51:48.040 align:middle line:84% So instead of just reacting the polysilicon just 00:51:48.040 --> 00:51:50.080 align:middle line:84% at the very top of the gate, people 00:51:50.080 --> 00:51:52.150 align:middle line:84% are actually putting down enough metal 00:51:52.150 --> 00:51:54.190 align:middle line:84% and reacting it at the right temperature 00:51:54.190 --> 00:51:57.020 align:middle line:84% that the metal reacts all the way to the very bottom. 00:51:57.020 --> 00:52:00.680 align:middle line:84% In this case, on the bottom would be nickel silicide. 00:52:00.680 --> 00:52:04.497 align:middle line:84% This was formed by depositing nickel and reacting it at 450 00:52:04.497 --> 00:52:05.580 align:middle line:90% all the way to the bottom. 00:52:05.580 --> 00:52:08.840 align:middle line:84% So there's no polysilicon left because these days, 00:52:08.840 --> 00:52:10.760 align:middle line:84% people don't want polysilicon gates. 00:52:10.760 --> 00:52:12.950 align:middle line:84% Polysilicon is a great material to etch. 00:52:12.950 --> 00:52:16.790 align:middle line:84% It's highly compatible chemically with oxide. 00:52:16.790 --> 00:52:20.000 align:middle line:84% But it has gate depletion problems. 00:52:20.000 --> 00:52:21.770 align:middle line:84% It doesn't have enough carriers in it. 00:52:21.770 --> 00:52:24.200 align:middle line:84% Polysilicon, what's the highest you can dope polysilicon? 00:52:24.200 --> 00:52:28.200 align:middle line:84% Well, maybe 1 to 5 times 10 to the 20. 00:52:28.200 --> 00:52:31.890 align:middle line:84% So that limits your resistivity and it 00:52:31.890 --> 00:52:33.690 align:middle line:90% limits your gate depletion. 00:52:33.690 --> 00:52:35.010 align:middle line:90% People want to use metals. 00:52:35.010 --> 00:52:38.800 align:middle line:84% What's the concentration of electrons in a metal? 00:52:38.800 --> 00:52:40.760 align:middle line:84% Well, 10 to the 22 or 10 to the 23. 00:52:40.760 --> 00:52:41.260 align:middle line:90% It's huge. 00:52:41.260 --> 00:52:42.843 align:middle line:84% It's three orders of magnitude higher. 00:52:42.843 --> 00:52:45.310 align:middle line:84% So we need more electrons or more holes, 00:52:45.310 --> 00:52:47.560 align:middle line:84% more carriers in our gates these days. 00:52:47.560 --> 00:52:50.860 align:middle line:84% The way we do it is we take poly and you convert it completely 00:52:50.860 --> 00:52:51.683 align:middle line:90% to silicide. 00:52:51.683 --> 00:52:53.350 align:middle line:84% This is called a fully suicided process. 00:52:53.350 --> 00:52:54.640 align:middle line:90% This is only in research. 00:52:54.640 --> 00:52:57.010 align:middle line:84% There's a lot of issues with doing this. 00:52:57.010 --> 00:52:58.480 align:middle line:84% Here's an example where they tried 00:52:58.480 --> 00:53:02.800 align:middle line:84% to do the siliciding with cobalt in the upper micrograph, upper 00:53:02.800 --> 00:53:03.940 align:middle line:90% SCM micrograph here. 00:53:03.940 --> 00:53:05.170 align:middle line:90% It didn't work too well. 00:53:05.170 --> 00:53:07.017 align:middle line:84% As you can see, the cobalt went down in. 00:53:07.017 --> 00:53:08.350 align:middle line:90% It didn't make it to the bottom. 00:53:08.350 --> 00:53:11.140 align:middle line:84% In fact, what happens the silicon got sucked up. 00:53:11.140 --> 00:53:13.448 align:middle line:84% And the next time we'll talk about silicide reactions. 00:53:13.448 --> 00:53:15.490 align:middle line:84% The silicon was actually moving, and you ended up 00:53:15.490 --> 00:53:16.550 align:middle line:90% with a big void. 00:53:16.550 --> 00:53:18.290 align:middle line:84% So you had no gate in this example. 00:53:18.290 --> 00:53:21.040 align:middle line:84% So this particular paper was just showing-- 00:53:21.040 --> 00:53:25.810 align:middle line:84% I took from a few months ago at the VLSI conference in Hawaii. 00:53:25.810 --> 00:53:28.000 align:middle line:84% Nickel silicide works much better 00:53:28.000 --> 00:53:30.550 align:middle line:84% for fully siliciding than does cobalt silicide, which 00:53:30.550 --> 00:53:32.950 align:middle line:90% leaves behind voids. 00:53:32.950 --> 00:53:34.190 align:middle line:90% So there's is an example. 00:53:34.190 --> 00:53:36.790 align:middle line:84% Not only are we etching polysilicon gates these days. 00:53:36.790 --> 00:53:38.685 align:middle line:84% We may end up siliciding them completely. 00:53:38.685 --> 00:53:40.310 align:middle line:84% And that's what people are considering. 00:53:40.310 --> 00:53:43.640 align:middle line:84% There's another type of process which I'm picturing here 00:53:43.640 --> 00:53:46.620 align:middle line:84% on the lower left called damascene replacement gate 00:53:46.620 --> 00:53:47.120 align:middle line:90% process. 00:53:47.120 --> 00:53:51.420 align:middle line:84% And here's an example of a gate where I have ti-nitride. 00:53:51.420 --> 00:53:54.050 align:middle line:84% So this material-- well, it's got tungsten on the top. 00:53:54.050 --> 00:53:55.220 align:middle line:90% That's the bright material. 00:53:55.220 --> 00:53:59.240 align:middle line:84% And this sort of gray material is a thin layer of ti-nitride. 00:53:59.240 --> 00:54:01.830 align:middle line:84% And the gate dielectric is hafnium dioxide, 00:54:01.830 --> 00:54:02.760 align:middle line:90% which is down here. 00:54:02.760 --> 00:54:04.880 align:middle line:84% This is on a silicon germanium type 00:54:04.880 --> 00:54:06.710 align:middle line:90% of high mobility transistor. 00:54:06.710 --> 00:54:09.450 align:middle line:84% That was also published about six months ago. 00:54:09.450 --> 00:54:11.510 align:middle line:84% Doesn't look anything like a polysilicon gate. 00:54:11.510 --> 00:54:14.900 align:middle line:84% So not only are we fully siliciding the gates, 00:54:14.900 --> 00:54:16.910 align:middle line:84% but sometimes we're actually replacing 00:54:16.910 --> 00:54:21.170 align:middle line:84% the polysilicon with another material, with a metal material. 00:54:21.170 --> 00:54:22.940 align:middle line:84% And ti-nitride is a very popular one. 00:54:22.940 --> 00:54:26.120 align:middle line:84% So gate etching is evolving from not only having 00:54:26.120 --> 00:54:29.090 align:middle line:84% to etch poly and stop on dielectric, 00:54:29.090 --> 00:54:31.010 align:middle line:84% but now we may have to etch metals. 00:54:31.010 --> 00:54:34.340 align:middle line:84% So lots of new technologies are being developed 00:54:34.340 --> 00:54:36.350 align:middle line:84% over the last two or three years and will 00:54:36.350 --> 00:54:41.190 align:middle line:84% continue to be developed to push the technology forward. 00:54:41.190 --> 00:54:43.970 align:middle line:84% So it's these next two slides, I apologize, not in your handouts, 00:54:43.970 --> 00:54:44.810 align:middle line:90% 21 and 22. 00:54:44.810 --> 00:54:48.428 align:middle line:84% But they are on the website, so you can download the PDF file. 00:54:48.428 --> 00:54:49.220 align:middle line:90% How did they make-- 00:54:49.220 --> 00:54:51.560 align:middle line:84% I just wanted to show you this lower left. 00:54:51.560 --> 00:54:54.740 align:middle line:84% How does one make a gate that looks anything like this, just 00:54:54.740 --> 00:54:57.020 align:middle line:90% to give you an idea? 00:54:57.020 --> 00:55:00.110 align:middle line:90% Well, this is the way you do it. 00:55:00.110 --> 00:55:05.360 align:middle line:84% Again, this particular example I took from IEDM of 2002. 00:55:05.360 --> 00:55:11.130 align:middle line:84% People use the ordinary process to make a polysilicon gate. 00:55:11.130 --> 00:55:12.470 align:middle line:90% So you put down poly. 00:55:12.470 --> 00:55:13.188 align:middle line:90% You etch it. 00:55:13.188 --> 00:55:14.480 align:middle line:90% You make your sidewall spacers. 00:55:14.480 --> 00:55:16.980 align:middle line:84% So that's just like what we've learned about in this course. 00:55:16.980 --> 00:55:19.490 align:middle line:84% And here is a very short channel MOSFET. 00:55:19.490 --> 00:55:21.140 align:middle line:84% Here's a very long channel MOSFET. 00:55:21.140 --> 00:55:23.272 align:middle line:84% Now, at the end of defining your MOSFETs, 00:55:23.272 --> 00:55:25.730 align:middle line:84% you put down this gray material, and you put it everywhere, 00:55:25.730 --> 00:55:26.600 align:middle line:90% very thick. 00:55:26.600 --> 00:55:27.950 align:middle line:90% So you put down a dielectric. 00:55:27.950 --> 00:55:29.690 align:middle line:90% And then you CNP it down. 00:55:29.690 --> 00:55:33.570 align:middle line:84% So you have dielectric that's been CNP'd and flattened. 00:55:33.570 --> 00:55:35.700 align:middle line:84% It's been flattened down just so it stops, 00:55:35.700 --> 00:55:39.450 align:middle line:84% the polishing stops, just around this height. 00:55:39.450 --> 00:55:44.460 align:middle line:84% Now what you do is the polysilicon is then chemically 00:55:44.460 --> 00:55:46.060 align:middle line:90% etched and removed. 00:55:46.060 --> 00:55:50.430 align:middle line:84% So you etch out with an etchant that digs out the poly. 00:55:50.430 --> 00:55:53.250 align:middle line:84% And if you want, you can even dig out the gate oxide 00:55:53.250 --> 00:55:54.690 align:middle line:90% and replace it with a high K. 00:55:54.690 --> 00:55:58.480 align:middle line:84% So this is what's called a replacement gate process. 00:55:58.480 --> 00:56:00.810 align:middle line:90% So actually, I use poly. 00:56:00.810 --> 00:56:03.930 align:middle line:84% To etch your poly, you put down sidewall spacers. 00:56:03.930 --> 00:56:06.090 align:middle line:84% You go through the high-temperature anneal. 00:56:06.090 --> 00:56:07.800 align:middle line:84% Now, why would anyone want to do this? 00:56:07.800 --> 00:56:09.480 align:middle line:84% You put down poly, you pattern it, 00:56:09.480 --> 00:56:11.820 align:middle line:84% and then later on you're going to remove it. 00:56:11.820 --> 00:56:12.810 align:middle line:90% Sounds kind of crazy. 00:56:12.810 --> 00:56:14.340 align:middle line:84% Why didn't you just put the metal down to begin with? 00:56:14.340 --> 00:56:16.380 align:middle line:84% But does anybody have any ideas on why 00:56:16.380 --> 00:56:18.742 align:middle line:84% you might want to use poly and then stick 00:56:18.742 --> 00:56:19.950 align:middle line:90% the metal in at the very end? 00:56:19.950 --> 00:56:23.610 align:middle line:90% 00:56:23.610 --> 00:56:25.380 align:middle line:84% Well, there's a couple of reasons. 00:56:25.380 --> 00:56:28.640 align:middle line:84% One reason is we know how to etch poly perfectly. 00:56:28.640 --> 00:56:30.818 align:middle line:84% We spent the last 20 years getting perfectly 00:56:30.818 --> 00:56:31.610 align:middle line:90% vertical sidewalls. 00:56:31.610 --> 00:56:32.860 align:middle line:90% So people know how to etch it. 00:56:32.860 --> 00:56:33.630 align:middle line:90% That's one reason. 00:56:33.630 --> 00:56:36.668 align:middle line:84% The second reason is poly can go up to very high temperatures. 00:56:36.668 --> 00:56:38.210 align:middle line:84% What do I have to do in this process? 00:56:38.210 --> 00:56:39.753 align:middle line:84% After you put the gate down, you have 00:56:39.753 --> 00:56:42.170 align:middle line:84% to implant the source drains, and you have to anneal them, 00:56:42.170 --> 00:56:46.460 align:middle line:84% typically at 1,000 degrees these days for 10 seconds, or 1,050. 00:56:46.460 --> 00:56:49.520 align:middle line:84% There aren't too many metals you can pop up to 1,050 00:56:49.520 --> 00:56:51.500 align:middle line:90% and not either melt them-- 00:56:51.500 --> 00:56:52.640 align:middle line:90% most metals will melt-- 00:56:52.640 --> 00:56:56.173 align:middle line:84% or if you don't melt them, they could undergo dramatic phase 00:56:56.173 --> 00:56:56.840 align:middle line:90% transformations. 00:56:56.840 --> 00:56:58.730 align:middle line:84% Some of them could diffuse into the wafer 00:56:58.730 --> 00:57:00.480 align:middle line:84% and cause problems with deep levels. 00:57:00.480 --> 00:57:02.540 align:middle line:84% So metals are not considered very 00:57:02.540 --> 00:57:03.990 align:middle line:84% good to have at high temperatures. 00:57:03.990 --> 00:57:07.910 align:middle line:84% So this is a way of using poly as a dummy. 00:57:07.910 --> 00:57:09.565 align:middle line:84% We know exactly how to pattern it. 00:57:09.565 --> 00:57:10.940 align:middle line:84% The only problem with it, it just 00:57:10.940 --> 00:57:11.870 align:middle line:90% doesn't have enough carriers. 00:57:11.870 --> 00:57:13.320 align:middle line:84% Well, we'll put it in the process. 00:57:13.320 --> 00:57:15.237 align:middle line:84% We won't change the process till the very end. 00:57:15.237 --> 00:57:16.375 align:middle line:90% We'll etch it out. 00:57:16.375 --> 00:57:17.750 align:middle line:84% I don't know how manufacturable-- 00:57:17.750 --> 00:57:19.792 align:middle line:84% I don't believe anyone is manufacturing products. 00:57:19.792 --> 00:57:23.130 align:middle line:84% This was just published in research about two years ago. 00:57:23.130 --> 00:57:24.480 align:middle line:90% It is kind of a neat idea. 00:57:24.480 --> 00:57:27.110 align:middle line:84% So after you remove this and you open this hole, 00:57:27.110 --> 00:57:28.790 align:middle line:84% well, then you have an open hole. 00:57:28.790 --> 00:57:32.990 align:middle line:84% You can put down your high K, your hafnium dioxide. 00:57:32.990 --> 00:57:35.750 align:middle line:84% You can put down your ti-nitride by sputtering, 00:57:35.750 --> 00:57:38.270 align:middle line:84% and a tungsten cap, and then maybe 00:57:38.270 --> 00:57:43.000 align:middle line:84% some capping layer of dielectric and CNP the whole thing down. 00:57:43.000 --> 00:57:46.110 align:middle line:84% So that's how they got-- in that last picture, that's how 00:57:46.110 --> 00:57:48.630 align:middle line:90% they got that replacement gate. 00:57:48.630 --> 00:57:51.870 align:middle line:84% And in fact, slide 22 was also not in your handout. 00:57:51.870 --> 00:57:54.270 align:middle line:84% But this is what it looks like in TDM 00:57:54.270 --> 00:57:57.270 align:middle line:84% a cross-section in a 50-nanometer MOSFET where they 00:57:57.270 --> 00:57:59.340 align:middle line:84% remove the poly, they dug it out, 00:57:59.340 --> 00:58:03.160 align:middle line:84% they put in this very dark black layer of hafnium dioxide. 00:58:03.160 --> 00:58:05.880 align:middle line:90% So that is your gate dielectric. 00:58:05.880 --> 00:58:08.490 align:middle line:84% The ti-nitride is next, a very thin layer. 00:58:08.490 --> 00:58:10.710 align:middle line:90% That is your gate material. 00:58:10.710 --> 00:58:13.000 align:middle line:84% They then have tungsten that they fill it up with. 00:58:13.000 --> 00:58:15.070 align:middle line:84% So they have the tungsten material, 00:58:15.070 --> 00:58:18.810 align:middle line:84% which is a little bit more environmentally friendly. 00:58:18.810 --> 00:58:21.450 align:middle line:84% Or it's an easier material to work with to fill up the plug. 00:58:21.450 --> 00:58:26.020 align:middle line:84% So that's an example where the poly was completely removed. 00:58:26.020 --> 00:58:28.380 align:middle line:84% So new types of things have to be done 00:58:28.380 --> 00:58:32.920 align:middle line:90% with forming gates these days. 00:58:32.920 --> 00:58:35.050 align:middle line:90% OK, let me move on here. 00:58:35.050 --> 00:58:38.080 align:middle line:84% Now on slide 23, I'll move away from the gate etching 00:58:38.080 --> 00:58:40.660 align:middle line:84% and talk a little bit, spend a few minutes, 00:58:40.660 --> 00:58:44.580 align:middle line:90% about modeling and simulation. 00:58:44.580 --> 00:58:46.890 align:middle line:84% There's a lot of similarity between the models 00:58:46.890 --> 00:58:48.640 align:middle line:84% that we introduced for deposition 00:58:48.640 --> 00:58:50.310 align:middle line:84% in the last couple of lectures that 00:58:50.310 --> 00:58:53.820 align:middle line:84% were in chapter 9 and the etching models. 00:58:53.820 --> 00:58:54.630 align:middle line:90% And why is that? 00:58:54.630 --> 00:58:57.290 align:middle line:84% Well, both deposition and etching 00:58:57.290 --> 00:59:01.880 align:middle line:84% use incoming chemical or neutral species and ion fluxes. 00:59:01.880 --> 00:59:04.520 align:middle line:84% And they have a lot of very similar processes. 00:59:04.520 --> 00:59:06.440 align:middle line:84% The difference is when you're depositing, 00:59:06.440 --> 00:59:07.760 align:middle line:90% the species come down. 00:59:07.760 --> 00:59:09.980 align:middle line:84% They move around and they stick, and they 00:59:09.980 --> 00:59:11.420 align:middle line:90% react in some way that deposit. 00:59:11.420 --> 00:59:13.460 align:middle line:84% When you're etching, they come down, they move around. 00:59:13.460 --> 00:59:15.460 align:middle line:84% They have a chemical reaction, but they actually 00:59:15.460 --> 00:59:16.290 align:middle line:90% remove material. 00:59:16.290 --> 00:59:18.320 align:middle line:84% So one can be considered mathematically 00:59:18.320 --> 00:59:19.550 align:middle line:90% the inverse of the other. 00:59:19.550 --> 00:59:22.220 align:middle line:84% But because in math, you can just 00:59:22.220 --> 00:59:25.320 align:middle line:84% change the sign from positive to negative in a computer program, 00:59:25.320 --> 00:59:31.410 align:middle line:84% you can use very much similar methodologies to model etching. 00:59:31.410 --> 00:59:36.140 align:middle line:84% So here's an example of things that might take place. 00:59:36.140 --> 00:59:38.720 align:middle line:84% During etching you have ionic species coming down. 00:59:38.720 --> 00:59:42.120 align:middle line:84% You have things coming down and being sputtered away. 00:59:42.120 --> 00:59:46.370 align:middle line:84% And you may have things coming down 00:59:46.370 --> 00:59:47.750 align:middle line:90% and being emitted or desorbed. 00:59:47.750 --> 00:59:52.100 align:middle line:84% So a lot of these fluxes, the whole flux equation, 00:59:52.100 --> 00:59:56.070 align:middle line:84% is very much analogous between deposition and etching. 00:59:56.070 --> 00:59:59.140 align:middle line:90% 00:59:59.140 --> 01:00:03.880 align:middle line:84% Here on slide 24, just like in the case of deposition, 01:00:03.880 --> 01:00:06.970 align:middle line:84% the etch rate is proportional to the net flux arriving 01:00:06.970 --> 01:00:07.882 align:middle line:90% at a point. 01:00:07.882 --> 01:00:09.340 align:middle line:84% Remember we said the deposition was 01:00:09.340 --> 01:00:12.640 align:middle line:84% proportional to the net flux, the flux in minus the flux out? 01:00:12.640 --> 01:00:15.590 align:middle line:84% Well, the etch rate is often proportional to the net flux. 01:00:15.590 --> 01:00:17.380 align:middle line:84% So we have the same kind of situation. 01:00:17.380 --> 01:00:20.710 align:middle line:84% You have a wafer, which has a certain patterned surface. 01:00:20.710 --> 01:00:24.820 align:middle line:84% You form a virtual dashed plane above it, 01:00:24.820 --> 01:00:27.070 align:middle line:84% and you look at the distribution, the angular 01:00:27.070 --> 01:00:30.250 align:middle line:84% distribution of the species arriving just 01:00:30.250 --> 01:00:32.050 align:middle line:84% above the surface on that dashed plane. 01:00:32.050 --> 01:00:34.870 align:middle line:90% 01:00:34.870 --> 01:00:37.050 align:middle line:84% If you have a chemical etching species that 01:00:37.050 --> 01:00:41.220 align:middle line:84% is something that's neutral that is not ionized, 01:00:41.220 --> 01:00:44.740 align:middle line:84% you assume you have an isotropic arrival distribution. 01:00:44.740 --> 01:00:47.740 align:middle line:84% So you put a cosine theta to the n, 01:00:47.740 --> 01:00:50.070 align:middle line:84% where n equals 1, for a chemical species. 01:00:50.070 --> 01:00:53.160 align:middle line:84% If you have an ionic species, it can be accelerated 01:00:53.160 --> 01:00:54.390 align:middle line:90% towards the surface. 01:00:54.390 --> 01:00:58.050 align:middle line:84% It can arrive very much with a vertical orientation. 01:00:58.050 --> 01:01:01.180 align:middle line:84% And here we prefer etching-- we put in cosine theta to the n 01:01:01.180 --> 01:01:04.920 align:middle line:84% where n could vary between 10 and 100, or something like that. 01:01:04.920 --> 01:01:07.530 align:middle line:84% Remember, the higher the value of n, much more vertical 01:01:07.530 --> 01:01:10.800 align:middle line:90% the distribution. 01:01:10.800 --> 01:01:12.480 align:middle line:84% In the deposition case, remember we 01:01:12.480 --> 01:01:15.300 align:middle line:84% had the concept of a sticking coefficient 01:01:15.300 --> 01:01:18.650 align:middle line:84% where we use very much that same concept. 01:01:18.650 --> 01:01:21.510 align:middle line:84% When we're etching we say an ionized species usually 01:01:21.510 --> 01:01:26.430 align:middle line:84% sticks where it lands and it's going to react and etch, 01:01:26.430 --> 01:01:29.940 align:middle line:84% while a reactive neutral species may have a low SC value. 01:01:29.940 --> 01:01:33.270 align:middle line:84% It might bounce around before it finally reacts and etches off 01:01:33.270 --> 01:01:36.270 align:middle line:90% the surface. 01:01:36.270 --> 01:01:39.180 align:middle line:84% As far as the physical component of etching like sputtering, 01:01:39.180 --> 01:01:41.400 align:middle line:84% the sputtering yield has the same angular dependence 01:01:41.400 --> 01:01:42.960 align:middle line:84% that we used in the deposition case. 01:01:42.960 --> 01:01:46.500 align:middle line:84% So again, very similar process going on when we include 01:01:46.500 --> 01:01:47.190 align:middle line:90% sputtering. 01:01:47.190 --> 01:01:51.920 align:middle line:90% 01:01:51.920 --> 01:01:55.100 align:middle line:84% So in slide 25, I'm showing what's 01:01:55.100 --> 01:01:58.010 align:middle line:90% called the linear etch model. 01:01:58.010 --> 01:02:02.030 align:middle line:84% There have been some very specific models developed. 01:02:02.030 --> 01:02:04.430 align:middle line:84% Here I just want to give a general purpose 01:02:04.430 --> 01:02:09.000 align:middle line:84% etch model, a couple of them, which can be applied broadly. 01:02:09.000 --> 01:02:11.310 align:middle line:84% The so-called linear model assumes 01:02:11.310 --> 01:02:13.920 align:middle line:84% that there are chemical and physical components that 01:02:13.920 --> 01:02:15.130 align:middle line:90% act independently. 01:02:15.130 --> 01:02:18.090 align:middle line:90% So they're not linked. 01:02:18.090 --> 01:02:19.810 align:middle line:84% They're independent of each other. 01:02:19.810 --> 01:02:23.430 align:middle line:84% And in this case, we have an etch rate that 01:02:23.430 --> 01:02:26.790 align:middle line:84% is given by just a sum of two terms, one term that 01:02:26.790 --> 01:02:29.160 align:middle line:84% has to do with the chemical flux, 01:02:29.160 --> 01:02:31.020 align:middle line:84% F sub C. That's the chemical flux 01:02:31.020 --> 01:02:32.490 align:middle line:90% onto the surface at any point. 01:02:32.490 --> 01:02:34.530 align:middle line:84% And another term that is proportional to F 01:02:34.530 --> 01:02:37.050 align:middle line:90% sub I, the ionic flux. 01:02:37.050 --> 01:02:42.480 align:middle line:84% In front of each one of these fluxes is a K factor, KI and KF. 01:02:42.480 --> 01:02:44.130 align:middle line:84% These are relative rate constants 01:02:44.130 --> 01:02:45.870 align:middle line:90% for the two components. 01:02:45.870 --> 01:02:48.940 align:middle line:84% Usually you use these as fitting parameters. 01:02:48.940 --> 01:02:52.920 align:middle line:84% If I have a large KI, I have a lot of ionic component, 01:02:52.920 --> 01:02:54.930 align:middle line:84% whereas if I have a large KF, there'll 01:02:54.930 --> 01:02:59.180 align:middle line:90% be more of a chemical component. 01:02:59.180 --> 01:03:01.910 align:middle line:84% What is that-- the physical component can 01:03:01.910 --> 01:03:03.710 align:middle line:84% be due to purely physical sputtering, 01:03:03.710 --> 01:03:05.930 align:middle line:84% for example, like we talked about before. 01:03:05.930 --> 01:03:10.070 align:middle line:84% Or it actually can be some ion enhanced mechanism in the regime 01:03:10.070 --> 01:03:12.720 align:middle line:84% where the chemical flux is not limiting the ion etching. 01:03:12.720 --> 01:03:16.580 align:middle line:84% So this is a linear etch model with the independent, chemical, 01:03:16.580 --> 01:03:18.200 align:middle line:90% and physical components. 01:03:18.200 --> 01:03:22.620 align:middle line:90% 01:03:22.620 --> 01:03:24.780 align:middle line:84% Here's just an example of some simulations. 01:03:24.780 --> 01:03:27.330 align:middle line:84% Remember, speedy SM was a topography simulator, 01:03:27.330 --> 01:03:30.410 align:middle line:84% does both deposition and etching. 01:03:30.410 --> 01:03:33.470 align:middle line:84% Here on slide 26., I have some speedy simulations 01:03:33.470 --> 01:03:35.210 align:middle line:90% using the linear etch model. 01:03:35.210 --> 01:03:39.290 align:middle line:84% And we have A, B, and C. Well, what we're doing-- 01:03:39.290 --> 01:03:41.060 align:middle line:84% each one of these contours, again, 01:03:41.060 --> 01:03:44.790 align:middle line:84% represents the shape of the etched surface at a given time. 01:03:44.790 --> 01:03:47.000 align:middle line:84% So we're looking at snapshots in time. 01:03:47.000 --> 01:03:49.493 align:middle line:84% Here I have an etched mask that looks like this. 01:03:49.493 --> 01:03:51.410 align:middle line:84% So this is my masking material on the surface. 01:03:51.410 --> 01:03:54.800 align:middle line:84% And I'm etching into the silicon, or into the film below. 01:03:54.800 --> 01:03:59.900 align:middle line:84% Here I have n equals 1 for my chemical flux angular 01:03:59.900 --> 01:04:00.810 align:middle line:90% distribution. 01:04:00.810 --> 01:04:06.200 align:middle line:84% So it's going to give you fairly isotropic type of etches here. 01:04:06.200 --> 01:04:09.230 align:middle line:84% Etching laterally, this dimension, just about the same 01:04:09.230 --> 01:04:10.250 align:middle line:90% as you etch vertically. 01:04:10.250 --> 01:04:12.530 align:middle line:84% So there's an isotropic type of etch. 01:04:12.530 --> 01:04:18.710 align:middle line:84% Here's where you have n equals 80 for the ionic flux angular 01:04:18.710 --> 01:04:19.490 align:middle line:90% distribution. 01:04:19.490 --> 01:04:22.520 align:middle line:84% And you end up with very vertical etching. 01:04:22.520 --> 01:04:24.830 align:middle line:84% Basically everything's coming straight down. 01:04:24.830 --> 01:04:27.390 align:middle line:84% And etching, there's not much lateral etching. 01:04:27.390 --> 01:04:29.000 align:middle line:90% So this is all chemical etching. 01:04:29.000 --> 01:04:30.380 align:middle line:90% The ion flux is zero. 01:04:30.380 --> 01:04:32.450 align:middle line:84% All physical etching in this case. 01:04:32.450 --> 01:04:34.490 align:middle line:90% And here's a 50/50. 01:04:34.490 --> 01:04:38.217 align:middle line:84% I have half chemical and half physical fluxes of both. 01:04:38.217 --> 01:04:39.800 align:middle line:84% And you can see it etches a little bit 01:04:39.800 --> 01:04:42.470 align:middle line:84% laterally and a certain amount vertically. 01:04:42.470 --> 01:04:45.500 align:middle line:84% So you can generate a variety of etch profiles using 01:04:45.500 --> 01:04:49.910 align:middle line:84% a relatively simple model with only maybe three or four 01:04:49.910 --> 01:04:50.933 align:middle line:90% parameters. 01:04:50.933 --> 01:04:52.850 align:middle line:84% And then you can fit this to what you actually 01:04:52.850 --> 01:04:55.025 align:middle line:90% see in your actual etches. 01:04:55.025 --> 01:04:57.970 align:middle line:90% 01:04:57.970 --> 01:05:00.710 align:middle line:84% The second type of model, which is quite different 01:05:00.710 --> 01:05:03.010 align:middle line:84% that's shown here in slide 27, is 01:05:03.010 --> 01:05:09.130 align:middle line:84% called a saturation dash absorption type of model 01:05:09.130 --> 01:05:10.670 align:middle line:90% because the fact-- 01:05:10.670 --> 01:05:13.900 align:middle line:84% very often, this is used when the chemical 01:05:13.900 --> 01:05:17.680 align:middle line:84% or the neutral etching and the ion etching or the physical etch 01:05:17.680 --> 01:05:19.240 align:middle line:90% components are coupled. 01:05:19.240 --> 01:05:21.860 align:middle line:84% So when one of these affects the other, 01:05:21.860 --> 01:05:24.400 align:middle line:84% when you have a synergistic effect, which we know 01:05:24.400 --> 01:05:26.350 align:middle line:84% is often the case, especially when 01:05:26.350 --> 01:05:30.070 align:middle line:84% you have side wall inhibitor layers or something, 01:05:30.070 --> 01:05:31.730 align:middle line:84% you need to use this type of model. 01:05:31.730 --> 01:05:33.530 align:middle line:90% So here's an example. 01:05:33.530 --> 01:05:35.920 align:middle line:84% Let's say you need an ion flux to remove 01:05:35.920 --> 01:05:40.760 align:middle line:84% an inhibitor layer that is formed by chemical etching. 01:05:40.760 --> 01:05:43.630 align:middle line:84% So instead of a linear model where these two just added up 01:05:43.630 --> 01:05:47.290 align:middle line:84% linearly, these add sort of resistors in parallel, 01:05:47.290 --> 01:05:48.920 align:middle line:84% so to speak, or capacitors in series 01:05:48.920 --> 01:05:50.170 align:middle line:90% if you're electrical engineer. 01:05:50.170 --> 01:05:54.490 align:middle line:84% The etch rate is just given by the inverse, 1 over, 01:05:54.490 --> 01:05:59.060 align:middle line:84% the sum of 1 over KI FI plus 1 over SC FC. 01:05:59.060 --> 01:06:05.100 align:middle line:84% So there's 1 over a chemical flux and 1 over an ionic flux. 01:06:05.100 --> 01:06:07.830 align:middle line:84% If you plot this equation, what does it look like? 01:06:07.830 --> 01:06:11.370 align:middle line:84% Here's the overall etch rate as a function of-- now 01:06:11.370 --> 01:06:15.660 align:middle line:84% I'm plotting as a function of the ion flux times K sub I. 01:06:15.660 --> 01:06:17.410 align:middle line:84% So it's a function of this quantity. 01:06:17.410 --> 01:06:19.560 align:middle line:90% So it goes up. 01:06:19.560 --> 01:06:24.090 align:middle line:84% And the parameter here is the chemical flux, SC. 01:06:24.090 --> 01:06:28.650 align:middle line:84% So if either flux is 0, so if SC is 0, I get 0 etch rate, 01:06:28.650 --> 01:06:30.420 align:middle line:84% regardless of what my ion flux is. 01:06:30.420 --> 01:06:35.290 align:middle line:84% You need both the chemical and the ionic flux. 01:06:35.290 --> 01:06:37.410 align:middle line:84% So if either flux is zero, the overall trade 01:06:37.410 --> 01:06:40.080 align:middle line:84% is 0 because both of them are required to etch the material. 01:06:40.080 --> 01:06:43.500 align:middle line:84% And you end up with these sort of saturating type of curves. 01:06:43.500 --> 01:06:46.110 align:middle line:84% Here's for a chemical flux of 0.5. 01:06:46.110 --> 01:06:50.220 align:middle line:84% As I increase the ion flux, you increase the etch rate 01:06:50.220 --> 01:06:52.140 align:middle line:84% until at some point it sort of saturates 01:06:52.140 --> 01:06:54.510 align:middle line:84% and it's no longer a function of ion flux. 01:06:54.510 --> 01:06:57.567 align:middle line:84% And it's limited, then, by the chemical process. 01:06:57.567 --> 01:06:59.400 align:middle line:84% So you have a series of desaturating curves. 01:06:59.400 --> 01:07:01.400 align:middle line:84% It sort of looks like transistor characteristics 01:07:01.400 --> 01:07:03.060 align:middle line:84% because you have two things controlling 01:07:03.060 --> 01:07:05.980 align:middle line:84% it, both the chemical flux and the ionic flux. 01:07:05.980 --> 01:07:11.180 align:middle line:90% 01:07:11.180 --> 01:07:14.390 align:middle line:84% So basically, this is that same plot I just showed. 01:07:14.390 --> 01:07:17.120 align:middle line:84% The etch rate tends to saturate when one component gets too 01:07:17.120 --> 01:07:18.450 align:middle line:90% large relative to the other. 01:07:18.450 --> 01:07:22.160 align:middle line:84% So you're always rate limited by the slower of the two series 01:07:22.160 --> 01:07:25.520 align:middle line:84% processes, either the chemical etch rate or the ionic etch 01:07:25.520 --> 01:07:26.120 align:middle line:90% rate. 01:07:26.120 --> 01:07:28.070 align:middle line:84% This is a very generalized approach. 01:07:28.070 --> 01:07:30.050 align:middle line:84% Again, it can be broadly applicable, 01:07:30.050 --> 01:07:35.090 align:middle line:84% and you can use it to fit a lot of different etch models, again, 01:07:35.090 --> 01:07:37.640 align:middle line:84% with relatively small number of parameters, 01:07:37.640 --> 01:07:39.260 align:middle line:84% maybe three to four parameters. . 01:07:39.260 --> 01:07:42.960 align:middle line:90% 01:07:42.960 --> 01:07:47.130 align:middle line:84% On slide 29, I'm showing a speedy simulation, again, 01:07:47.130 --> 01:07:50.520 align:middle line:84% where we have equal chemical and ionic components. 01:07:50.520 --> 01:07:54.070 align:middle line:84% And you notice what you get is reasonably anisotropic etching. 01:07:54.070 --> 01:07:57.840 align:middle line:84% There's a little bit of lateral etching here down at the bottom. 01:07:57.840 --> 01:07:59.340 align:middle line:90% And why is that? 01:07:59.340 --> 01:08:01.950 align:middle line:84% Well, because in order to continue etching, 01:08:01.950 --> 01:08:06.000 align:middle line:84% you need an ion flux to remove the inhibitor layer 01:08:06.000 --> 01:08:07.080 align:middle line:90% or whatever it is. 01:08:07.080 --> 01:08:11.430 align:middle line:84% And that ion flux tends to be very vertical because we have 01:08:11.430 --> 01:08:13.360 align:middle line:90% a large n value in this case. 01:08:13.360 --> 01:08:16.080 align:middle line:84% And so the side wall inhibitor is not removed. 01:08:16.080 --> 01:08:18.397 align:middle line:84% The inhibitor layer is not removed on the side walls, 01:08:18.397 --> 01:08:20.189 align:middle line:84% but it's only removed on the flat surfaces. 01:08:20.189 --> 01:08:22.380 align:middle line:84% If you change this n direction, you'll 01:08:22.380 --> 01:08:24.600 align:middle line:84% start to see a little more lateral etching-- 01:08:24.600 --> 01:08:27.029 align:middle line:84% if you change the n number, rather, 01:08:27.029 --> 01:08:31.380 align:middle line:84% which the n will determine the angle of the cosine theta 01:08:31.380 --> 01:08:32.790 align:middle line:90% to the n dependence. 01:08:32.790 --> 01:08:37.870 align:middle line:84% And then you'll get a little bit more lateral etching. 01:08:37.870 --> 01:08:41.850 align:middle line:84% So there's an example for equal chemical and ion components. 01:08:41.850 --> 01:08:45.220 align:middle line:90% 01:08:45.220 --> 01:08:48.939 align:middle line:84% OK, so I just want to summarize what we've done in chapter 10 01:08:48.939 --> 01:08:50.553 align:middle line:90% on plasma etching. 01:08:50.553 --> 01:08:51.970 align:middle line:84% There's a lot of important issues. 01:08:51.970 --> 01:08:53.387 align:middle line:84% Some of the key ones we've covered 01:08:53.387 --> 01:08:56.229 align:middle line:84% are the selectivity with respect to the layer below you, 01:08:56.229 --> 01:08:59.920 align:middle line:84% plus the selectivity with the layer above you, or the mask. 01:08:59.920 --> 01:09:01.779 align:middle line:90% The directionality of the etch. 01:09:01.779 --> 01:09:04.840 align:middle line:84% How much do you etch vertically versus laterally? 01:09:04.840 --> 01:09:08.248 align:middle line:84% And the shape of the profile on the sidewall. 01:09:08.248 --> 01:09:10.040 align:middle line:84% There tend to be two components of etching. 01:09:10.040 --> 01:09:13.450 align:middle line:84% There's a chemical one that is reactive neutral species 01:09:13.450 --> 01:09:14.962 align:middle line:90% or free radicals. 01:09:14.962 --> 01:09:16.420 align:middle line:84% There's a physical component, which 01:09:16.420 --> 01:09:18.970 align:middle line:84% is just due to the ions being accelerated towards the surface 01:09:18.970 --> 01:09:24.229 align:middle line:84% and reacting or sputtering off components from the wafer. 01:09:24.229 --> 01:09:26.000 align:middle line:84% These can be completely independent. 01:09:26.000 --> 01:09:27.550 align:middle line:84% And then we use the linear model, 01:09:27.550 --> 01:09:33.160 align:middle line:84% or they can act in a series in a synergistic manner in what we 01:09:33.160 --> 01:09:36.310 align:middle line:84% would use the last model that we showed. 01:09:36.310 --> 01:09:39.040 align:middle line:84% Topography on the wafer can really 01:09:39.040 --> 01:09:42.020 align:middle line:84% increase the selectivity requirements. 01:09:42.020 --> 01:09:45.050 align:middle line:84% It makes the patterning of fine features and stopping 01:09:45.050 --> 01:09:48.140 align:middle line:84% on thin layers very difficult. Of course, 01:09:48.140 --> 01:09:50.210 align:middle line:84% lithography is impacted, as well, 01:09:50.210 --> 01:09:53.330 align:middle line:84% by the presence of topography because of the depth of focus. 01:09:53.330 --> 01:09:57.120 align:middle line:84% But all modern processes seek to minimize topography. 01:09:57.120 --> 01:10:02.000 align:middle line:84% So people have moved away from field oxides that go up 01:10:02.000 --> 01:10:03.423 align:middle line:90% or that go down. 01:10:03.423 --> 01:10:05.840 align:middle line:84% We want fill oxide that's completely flat with the surface 01:10:05.840 --> 01:10:08.360 align:middle line:84% so you see shallow trench isolation. 01:10:08.360 --> 01:10:11.390 align:middle line:90% 01:10:11.390 --> 01:10:13.250 align:middle line:84% Special high-density plasma etchers 01:10:13.250 --> 01:10:15.650 align:middle line:84% have been developed to use chemicals 01:10:15.650 --> 01:10:20.000 align:middle line:84% like HBR, chlorine and oxygen combined together, specifically 01:10:20.000 --> 01:10:23.360 align:middle line:84% for gate etching to control the selectivity so you can stop 01:10:23.360 --> 01:10:25.310 align:middle line:84% on the gate dielectric and to control 01:10:25.310 --> 01:10:27.080 align:middle line:84% the shape of the sidewall profile. 01:10:27.080 --> 01:10:29.090 align:middle line:84% At the same time, you minimize damage 01:10:29.090 --> 01:10:33.690 align:middle line:84% to the underlying structures and the underlying gate oxide. 01:10:33.690 --> 01:10:36.330 align:middle line:84% The models that I've talked about here for etching 01:10:36.330 --> 01:10:40.590 align:middle line:84% are fairly what I would call empirical. 01:10:40.590 --> 01:10:43.290 align:middle line:84% If you want more predictive or more physically accurate 01:10:43.290 --> 01:10:45.475 align:middle line:90% models of plasma etching-- 01:10:45.475 --> 01:10:47.100 align:middle line:84% Professor Sarwin used to teach a class. 01:10:47.100 --> 01:10:49.080 align:middle line:84% I'm not sure he's presently teaching it-- 01:10:49.080 --> 01:10:50.670 align:middle line:84% really on all the details of plasma 01:10:50.670 --> 01:10:53.070 align:middle line:84% etching as an entire field in and of itself. 01:10:53.070 --> 01:10:55.770 align:middle line:84% We've only been able to give it two lectures and one 01:10:55.770 --> 01:10:57.820 align:middle line:90% chapter of your textbook. 01:10:57.820 --> 01:11:02.640 align:middle line:84% So these models, while helpful to people that we have now, 01:11:02.640 --> 01:11:05.700 align:middle line:84% people to use topography simulators like speedy, 01:11:05.700 --> 01:11:09.152 align:middle line:84% it's not going to give you the same level of chemical 01:11:09.152 --> 01:11:11.610 align:middle line:84% understanding if you had a full-blown model of the etching. 01:11:11.610 --> 01:11:13.080 align:middle line:90% Again, it's fairly empirical. 01:11:13.080 --> 01:11:14.883 align:middle line:90% We're fitting curves in speedy. 01:11:14.883 --> 01:11:16.050 align:middle line:90% And we're fitting the shape. 01:11:16.050 --> 01:11:18.660 align:middle line:84% But it gives you some idea of what's going on. 01:11:18.660 --> 01:11:22.320 align:middle line:84% If you want more details, there are other more detailed classes 01:11:22.320 --> 01:11:24.120 align:middle line:90% you can take. 01:11:24.120 --> 01:11:25.710 align:middle line:84% OK, before we finish up, let me just 01:11:25.710 --> 01:11:27.540 align:middle line:84% remind people if you came in late 01:11:27.540 --> 01:11:30.600 align:middle line:90% that we have set the schedule. 01:11:30.600 --> 01:11:32.772 align:middle line:90% After today is Thanksgiving. 01:11:32.772 --> 01:11:33.480 align:middle line:90% That's a holiday. 01:11:33.480 --> 01:11:34.410 align:middle line:90% There's no class. 01:11:34.410 --> 01:11:37.320 align:middle line:84% Next week we'll talk about silicides and novel gate 01:11:37.320 --> 01:11:38.010 align:middle line:90% materials. 01:11:38.010 --> 01:11:40.020 align:middle line:84% After that we'll talk about strained silicon. 01:11:40.020 --> 01:11:41.830 align:middle line:90% And then you guys start talking. 01:11:41.830 --> 01:11:45.060 align:middle line:84% I've picked the first four speakers here for the reports. 01:11:45.060 --> 01:11:46.710 align:middle line:84% You have to speak for 16 minutes, 01:11:46.710 --> 01:11:50.310 align:middle line:84% and then I have three minutes for questions and answers 01:11:50.310 --> 01:11:52.290 align:middle line:90% on the seventh and on the ninth. 01:11:52.290 --> 01:11:54.210 align:middle line:84% These are the people speaking on the ninth. 01:11:54.210 --> 01:11:55.660 align:middle line:84% If you're doing a written report, 01:11:55.660 --> 01:11:58.140 align:middle line:84% you must bring it to class on Thursday, December 9. 01:11:58.140 --> 01:11:59.940 align:middle line:84% That's the last you can hand it in. 01:11:59.940 --> 01:12:03.810 align:middle line:84% And this will be posted, this list, on the website. 01:12:03.810 --> 01:12:05.855 align:middle line:90% OK, thanks. 01:12:05.855 --> 01:12:08.000 align:middle line:90%