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PROFESSOR: Well, today
I'm going to give

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the fourth and final
case study of the course.

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And everything I'm
going to talk about

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is my thesis research
in one form or another.

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So this is work that's
definitely in progress.

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And in a lot of cases,
we haven't really

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drawn conclusions yet.

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So it'll be quite
rough and ready.

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But I hope it'll be informative
and raise some questions

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for you.

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The focus of my
thesis research is--

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well, broadly speaking,
finding approaches

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to manufacturing microfluidic
devices using polymers.

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And for many of you,
I think microfluidics

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will be something you know
at least a little about.

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Essentially, microchips that
manipulate very small volumes

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of fluids to perform various
experimental, or diagnostic, or

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other engineering tasks.

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And we were interested
in developing processes

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and choosing materials that will
help us make these things very

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cheaply so that they can be
used at the point of care

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in the case of diagnostic
devices, and so forth.

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Now, one particular
process that's

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of great interest for
this is the imprinting

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of thermoplastic
polymeric layers.

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Hot embossing is what
we're going to call it.

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And for a start, let
me say why polymers

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are interesting for making
these sorts of devices.

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I think I see
three main reasons.

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Firstly, they're cheap compared
to silicone or glass, which

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are other materials
that you might think

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of as being obvious
candidates to fabricate

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these microscopic fluidic
channels, and valves,

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and pumps, and so forth.

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So polymers are cheap.

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Second thing is, they're
often transparent.

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And that's really important.

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When you have a biological
sample inside this device

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and you're trying to
look at how it behaves

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in response to certain
stimuli, either you're

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exposing it to a dye
that will make cells

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with certain
properties fluoresce.

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You need to see
that fluorescence.

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Or you're prodding
a cell somehow

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to see whether its
stiffness gives you

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information about
its disease state,

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something along these lines.

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Anyway, but transparency
is an important property.

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And thirdly, a lot of plastics
are mechanically tough,

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which makes them highly
suitable to use in the field

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at the point of care,
in doctors' surgeries

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and so forth.

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So these are the three reasons
why polymers are interesting.

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And imprinting them is
interesting as a process,

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because that itself
has the potential

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to be quick and cheap.

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You can imagine a
cycle imprinting

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a thermoplastic polymer
occurring in around a minute.

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And you can choose what kind
of size workpiece you machine.

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You could machine
anything from the size

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of a single microfluidic
chip, all the way up

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to continuous reels
of polymeric film.

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And indeed, the reel to
reel printing approach

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is something that's been
used in industry already

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for quite rudimentary devices.

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3M has a patent on
hot embossing reels

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of thermoplastic polymers.

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And so it's a pretty
flexible process.

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And microfluidics
aren't the only sorts

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of things you might
think of making with it.

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A lot of optical devices
could be made this way.

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You could think of imprinting
refractive elements.

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Or actually 3M's
patent is so that they

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can imprint the reflective
films that are laminated

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onto the front of road signs.

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Imprinted into those films are
many parallel, V-shaped grooves

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that act as corner
[? key ?] reflectors

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so that the street signs reflect
light incident from any angle.

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And so those sorts of
optical applications

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are also pretty crucial.

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But anyway, let me just
describe how the process works.

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What I'm illustrating
here is not

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a role to role printing
process, but something

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where the workpiece is
about the size of a wafer.

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So tens of millimeters
in diameter.

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And first thing we
need is a hard stamp,

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a tool that's been
microfabricated

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using some other process.

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This tool might have
been etched from silicon.

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It might have been
electroplated with nickel.

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It might itself be a polymer,
but one that doesn't soften

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at the processing temperatures.

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Elastomers like PDMS,
polydimethylsiloxane

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are actually quite attractive
as stamp materials.

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So we have this stamp.

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We have two heated
plates to heat

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the stamp and the
workpiece, and a means

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of applying a load normal to
the surface of the workpiece.

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Usually what happens
is the material

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is heated above what's
called its glass transition

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temperature.

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And for anyone who isn't
familiar with this,

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thermoplastics essentially
are composed of an entangled

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network of polymer molecules.

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And there are one
or more temperatures

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for a given polymer at which
those entanglements loosen.

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And the chains can
slide past one another.

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And the stiffness and
the effective viscosity

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of the material around
that temperature

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falls by several
orders of magnitude.

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So above that temperature, it's
highly flexible, easily formed.

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Below that temperature, it's
much stiffer and useable

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as a device.

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So we go above the glass
transition temperature

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to soften it.

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How far we go above is an
important question to answer.

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That's a crucial
parameter to choose.

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We then apply a load.

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That load is going to be
ramped up over some time

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that we have a choice about.

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That might be an
important parameter.

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We hold that load for some time
to allow the material to flow,

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to fill cavities in the stamp.

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And while the load
is still applied,

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we cool down usually,
cool the material

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to below its glass transition
temperature, and finally,

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remove the load.

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And often, that removal step has
a lot of technical challenges

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involved in it,
because of differential

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thermal contraction of
the workpiece in the tool.

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So that gives you
some idea that there

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are at least four parameters
that we need to decide upon,

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temperature, a load,
and two times associated

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with that load.

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There are probably
more parameters

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that we're interested in.

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Now, at this point,
I should also

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mention a rather
similar process that has

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quite different applications.

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And that is thermal nano
imprint lithography.

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This is, essentially,
hot embossing,

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but done on very thin layers of
polymer, thinner than a micron.

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And these layers have been spun
onto a much harder surface,

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usually a silicon wafer.

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And the beauty of
this process is

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that it allows sub
micron features

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to be transferred to the wafer.

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This is, I think, a
really exciting process.

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It has the potential to
revolutionize lithography

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and semiconductor manufacturing,
because the resolution

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with which features
can be transferred

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is not limited by the
wavelength of light.

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Usually you would pattern
a photo resist layer

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with some kind of projection,
optical projection system.

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And we've really got
to the stage where

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critical dimensions
of transistors

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are making that a big challenge,
making the optical lithography

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systems cost tens or hundreds
of millions of dollars.

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So in this case, we have
this spun on polymer layer.

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It's dissolved in an organic
solvent, spun on the solvent.

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It evaporates.

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And then the spun on
layer is a thermoplastic.

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It can be softened.

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You press the the
nano fabricated stamp

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into the wafer.

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And ideally, squeeze
all the material

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from below the individual
features of the stamp.

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Now, in reality,
that's not possible.

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And there is a small
residual polymer

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layer that's left there.

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So you can imagine, once we've
removed the stamp or the mold,

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we have some extremely
thin regions of polymer.

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And then imagine that we want to
pattern, to etch away material

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in the wafer underneath.

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The plasma or the
etching solution

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to which the wafer is
exposed will quite readily

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break through that
residual polymer layer

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and remove the material.

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So these processes are similar.

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In many ways, they actually
often use the same polymers.

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But the boundary conditions, the
mechanics are rather different.

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And so I'll talk a little
bit about nano imprint,

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but mostly the
results that I'm going

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to show you are to do with
micro hot embossing, where

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the workpiece is substantially
thicker than the diameters

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or the depths of the features
that are being patterned.

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AUDIENCE: [INAUDIBLE]
about creating the mold,

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during the mold removal process,
do you have the mold walls

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getting [? dilated? ?] Because
the pattern is getting taken

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off and it might have a
[? dimension ?] [INAUDIBLE]??

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PROFESSOR: Yes, that was
an excellent question.

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You've hit on one of the
most important challenges

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of [INAUDIBLE] embossing.

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And [INAUDIBLE] was
asking, are there problems

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with dimensional stability
of the walls, the side

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walls during de-molding?

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And I'll show you some
results that relate to that.

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So yes, good point.

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Anyhow, as I already
mentioned, there

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are several parameters
to do with the process

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that we need to pick,
temperatures, load times,

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loading durations.

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The range that those
parameters can take

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will be limited by the
capabilities of the machine we

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00:11:52,250 --> 00:11:54,473
have, the physical
properties of the polymer

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00:11:54,473 --> 00:11:56,390
we're trying to pattern,
what temperature does

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it start to burn, and so forth.

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00:12:00,990 --> 00:12:02,750
What about the
material underneath?

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00:12:02,750 --> 00:12:04,820
If we're doing nano
imprint lithography,

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00:12:04,820 --> 00:12:06,440
are there temperature
constraints

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00:12:06,440 --> 00:12:09,366
that the wafer underneath
has to observe?

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00:12:09,366 --> 00:12:16,710
So that's what
constrains the process.

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00:12:16,710 --> 00:12:21,710
And, of course, we might also
be making engineering decisions

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about what type of
polymer we're going to use

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to achieve a certain end.

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And indeed, we might have some
flexibility in the pattern

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that we are going to emboss.

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We might think about designing
a pattern with an eye

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to manufacturability, something
that will be easier to imprint

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00:12:39,260 --> 00:12:40,610
with less variability.

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And that is one of the questions
that I'm trying to answer.

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00:12:45,170 --> 00:12:45,986
Mohammed.

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AUDIENCE: [INAUDIBLE] how
do you [INAUDIBLE] the stamp

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00:12:48,236 --> 00:12:49,495
[? control? ?]

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00:12:49,495 --> 00:12:50,870
PROFESSOR: Right,
well, the stamp

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can be made in a variety
of ways that are usually

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00:12:59,360 --> 00:13:02,040
traditional microfabrication.

235
00:13:02,040 --> 00:13:06,770
So one particularly
easy way of doing

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00:13:06,770 --> 00:13:10,160
it would be to use a process
called deep reactive ion

237
00:13:10,160 --> 00:13:11,420
etching.

238
00:13:11,420 --> 00:13:14,330
And here I'm talking
about micro embossing,

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00:13:14,330 --> 00:13:17,480
where the features are several
microns to many microns.

240
00:13:17,480 --> 00:13:21,860
And essentially, this is just
etching very deep trenches

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00:13:21,860 --> 00:13:26,210
into a silicon wafer
using a plasma process.

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00:13:26,210 --> 00:13:28,400
I could go into great
detail about how that works,

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but essentially, it's a
fluorine based chemistry.

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00:13:32,780 --> 00:13:36,950
The precursor gas is SF6.

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00:13:36,950 --> 00:13:41,780
And you build up these
very deep, narrow trenches

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00:13:41,780 --> 00:13:47,620
in the silicon wafer
by a series of steps.

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00:13:47,620 --> 00:13:51,930
So in fact, you would
start with the silicon

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00:13:51,930 --> 00:13:55,050
with a photoresist
mask or something.

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You'd do a brief fluorine based
plasma etch that would remove

250
00:14:01,450 --> 00:14:03,550
small amounts of material.

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00:14:03,550 --> 00:14:06,790
That's roughly isotropic.

252
00:14:06,790 --> 00:14:08,560
So if you want
vertical sidewalls,

253
00:14:08,560 --> 00:14:09,940
you've got to do something else.

254
00:14:09,940 --> 00:14:12,670
And the something
else is depositing

255
00:14:12,670 --> 00:14:14,800
a polymeric passivation layer.

256
00:14:14,800 --> 00:14:17,800
So you change the
gas in the chamber

257
00:14:17,800 --> 00:14:23,080
to C4F8, which creates
what's essentially

258
00:14:23,080 --> 00:14:26,680
a teflon coating that passivates
the sidewall of the trench

259
00:14:26,680 --> 00:14:27,580
that's developing.

260
00:14:27,580 --> 00:14:33,130
Then you go back to the
SF6, build up a new notch,

261
00:14:33,130 --> 00:14:36,160
and do this many times,
alternate many times.

262
00:14:36,160 --> 00:14:41,740
So whatever way you
pattern the stamp.

263
00:14:41,740 --> 00:14:46,900
Silicon actually, as we will see
later, for de-molding reasons,

264
00:14:46,900 --> 00:14:49,570
is actually a lousy material
to use for the stamp.

265
00:14:49,570 --> 00:14:54,700
So then you might go one step
further, electroplate nickel

266
00:14:54,700 --> 00:14:57,670
into that etched wafer,
peel off the nickel,

267
00:14:57,670 --> 00:14:59,320
and use that as the stamp.

268
00:14:59,320 --> 00:15:00,490
Much tougher.

269
00:15:00,490 --> 00:15:04,160
Or you might use
more novel materials,

270
00:15:04,160 --> 00:15:06,070
things like metallic
glasses, which

271
00:15:06,070 --> 00:15:11,110
are alloys with an
amorphous structure that are

272
00:15:11,110 --> 00:15:13,030
both very hard and very tough.

273
00:15:13,030 --> 00:15:15,800
They would be great
materials for a stamp.

274
00:15:15,800 --> 00:15:17,800
AUDIENCE: So what
you mentioned--

275
00:15:17,800 --> 00:15:20,860
you said that you're not
limited to wavelengths

276
00:15:20,860 --> 00:15:22,790
of the light in this process.

277
00:15:22,790 --> 00:15:26,440
But however to make the stamp,
then you are limited to--

278
00:15:26,440 --> 00:15:27,240
PROFESSOR: Yeah.

279
00:15:27,240 --> 00:15:27,790
AUDIENCE: [INAUDIBLE]

280
00:15:27,790 --> 00:15:28,800
PROFESSOR: Well, yes.

281
00:15:28,800 --> 00:15:29,850
AUDIENCE: [INAUDIBLE]
limitation exists.

282
00:15:29,850 --> 00:15:31,517
PROFESSOR: So what I
just described here

283
00:15:31,517 --> 00:15:34,000
was for micro embossing.

284
00:15:34,000 --> 00:15:36,160
I'm sort of describing
two processes

285
00:15:36,160 --> 00:15:38,860
in parallel that have
great similarities,

286
00:15:38,860 --> 00:15:40,580
but not in terms of scale.

287
00:15:40,580 --> 00:15:45,280
So this would be for features
that are a micron or larger.

288
00:15:45,280 --> 00:15:48,820
If you want to make
sub micron features,

289
00:15:48,820 --> 00:15:49,960
then you're quite right.

290
00:15:49,960 --> 00:15:53,000
You need a process that will
let you make a stamp with sub

291
00:15:53,000 --> 00:15:57,490
micron features, and
electron beam lithography

292
00:15:57,490 --> 00:16:01,480
is a great candidate for that
where you actually would--

293
00:16:01,480 --> 00:16:04,120
you would start
with a silicon wafer

294
00:16:04,120 --> 00:16:10,660
with a 100 nanometer
thick layer of a radiation

295
00:16:10,660 --> 00:16:11,890
sensitive resist.

296
00:16:11,890 --> 00:16:15,580
Then you would just actually
steer a focused electron beam

297
00:16:15,580 --> 00:16:17,830
across the surface, scan
it across the surface

298
00:16:17,830 --> 00:16:19,840
in the pattern that you wanted.

299
00:16:19,840 --> 00:16:24,850
And that will give you 5
nanometer resolution or better.

300
00:16:24,850 --> 00:16:27,730
The trouble is, it takes ages
because it's a serial process.

301
00:16:27,730 --> 00:16:30,310
You're scanning the
electrons over the surface.

302
00:16:30,310 --> 00:16:34,150
So once you've made the stamp,
you've paid however many $1,000

303
00:16:34,150 --> 00:16:34,860
for the stamp.

304
00:16:34,860 --> 00:16:38,380
You can stamp it in in a
minute into as many substrates

305
00:16:38,380 --> 00:16:38,950
as you like.

306
00:16:38,950 --> 00:16:43,540
And that's really the
reason for nano imprint.

307
00:16:43,540 --> 00:16:47,110
OK, so keep both of
those processes in mind.

308
00:16:47,110 --> 00:16:50,620
But a lot of the assertions
I'm going to make from now on

309
00:16:50,620 --> 00:16:55,390
are all about the
micron scale embossing.

310
00:16:55,390 --> 00:16:58,738
So as I said, there are
lots of different parameters

311
00:16:58,738 --> 00:17:00,280
we can choose to do
with the process,

312
00:17:00,280 --> 00:17:02,320
to do with the material
we're embossing,

313
00:17:02,320 --> 00:17:04,839
to do with the pattern
that we're embossing.

314
00:17:04,839 --> 00:17:08,230
And the overall
mission that I suppose

315
00:17:08,230 --> 00:17:12,310
we have is to try to provide
tools for people who are using

316
00:17:12,310 --> 00:17:16,630
this process to
achieve the desired

317
00:17:16,630 --> 00:17:25,779
microstructure with as little
time, energy, cost as possible.

318
00:17:25,779 --> 00:17:28,750
So if you're processing
a certain material,

319
00:17:28,750 --> 00:17:33,580
can you pick a pattern that will
take 30 seconds to replicate

320
00:17:33,580 --> 00:17:39,460
instead of a minute at
the available processing

321
00:17:39,460 --> 00:17:41,150
conditions?

322
00:17:41,150 --> 00:17:45,460
So that presupposes,
of course, that we

323
00:17:45,460 --> 00:17:50,680
have some specification on what
satisfactory replication is.

324
00:17:50,680 --> 00:17:52,300
The most obvious
specification would

325
00:17:52,300 --> 00:17:54,700
be that every
cavity in the stamp

326
00:17:54,700 --> 00:17:57,430
has been filled with polymer,
even the narrowest ones.

327
00:17:57,430 --> 00:18:01,210
That would be a good
specification to use.

328
00:18:01,210 --> 00:18:03,310
Another good
specification that's

329
00:18:03,310 --> 00:18:05,920
really important for
nano imprint lithography

330
00:18:05,920 --> 00:18:10,930
is to make the thickness of
those residual polymer layers

331
00:18:10,930 --> 00:18:12,850
very uniform.

332
00:18:12,850 --> 00:18:16,420
And that's important
because it does

333
00:18:16,420 --> 00:18:20,590
take some time for
the etching process

334
00:18:20,590 --> 00:18:22,510
to break through
that residual layer.

335
00:18:25,040 --> 00:18:30,640
And if the residual layer
varies in thickness,

336
00:18:30,640 --> 00:18:32,950
the time taken to break
through it will be different.

337
00:18:32,950 --> 00:18:35,770
And then you'll get different
properties of the underlying

338
00:18:35,770 --> 00:18:36,470
structure.

339
00:18:36,470 --> 00:18:42,850
So let's say you've imprinted
this 100 nanometer thick

340
00:18:42,850 --> 00:18:45,070
polymer layer.

341
00:18:45,070 --> 00:18:50,710
And on this side, the
residual layer is really thin.

342
00:18:50,710 --> 00:18:53,110
And on this side
it's twice as thick.

343
00:18:53,110 --> 00:18:58,480
You then expose this
with a fluorine plasma.

344
00:18:58,480 --> 00:19:02,058
You're trying to etch the
silicon underneath here.

345
00:19:02,058 --> 00:19:03,850
You're going to end up
with a deeper trench

346
00:19:03,850 --> 00:19:05,590
here than you are here
because of the time

347
00:19:05,590 --> 00:19:06,757
it took to get through that.

348
00:19:06,757 --> 00:19:11,590
So coming up with ways
of designing the stamp so

349
00:19:11,590 --> 00:19:15,520
that the residual layer ends
up being as uniform as possible

350
00:19:15,520 --> 00:19:21,880
is a really pressing challenge
actually for this lithography

351
00:19:21,880 --> 00:19:22,900
technique to be adopted.

352
00:19:25,930 --> 00:19:30,570
Anyhow, so what we're all about
is simplified modeling tools

353
00:19:30,570 --> 00:19:32,990
that can be used as
part of the design loop

354
00:19:32,990 --> 00:19:36,030
that are computationally
efficient enough

355
00:19:36,030 --> 00:19:41,940
to give results in a
reasonable amount of time,

356
00:19:41,940 --> 00:19:46,420
and that represent reality well.

357
00:19:46,420 --> 00:19:50,010
So that implies that we need
some way of characterizing

358
00:19:50,010 --> 00:19:52,410
the tool that we're using,
the materials that we're

359
00:19:52,410 --> 00:19:55,560
using efficiently so that
we can get information

360
00:19:55,560 --> 00:19:57,510
about the physical
properties of these things

361
00:19:57,510 --> 00:20:00,750
with a minimum of fuss.

362
00:20:00,750 --> 00:20:05,580
Now, I'm just going to
split the problem up

363
00:20:05,580 --> 00:20:08,670
into three notional
length scales

364
00:20:08,670 --> 00:20:12,220
so that we can have a clearer
picture of what's going on.

365
00:20:12,220 --> 00:20:17,100
And at the largest
length scale you

366
00:20:17,100 --> 00:20:20,910
can think of variations
in the quality

367
00:20:20,910 --> 00:20:23,550
of the embossed
features from one side

368
00:20:23,550 --> 00:20:25,420
of the substrate to the other.

369
00:20:25,420 --> 00:20:29,340
And that might occur because
the two heated plates that

370
00:20:29,340 --> 00:20:31,380
are compressing the
stamp and the substrate

371
00:20:31,380 --> 00:20:32,580
are not perfectly parallel.

372
00:20:32,580 --> 00:20:34,920
So the pressure's
greater on one side.

373
00:20:34,920 --> 00:20:40,590
It might be because one of
the plates is bowed or wavy.

374
00:20:40,590 --> 00:20:43,530
Or it might be to do
simply with the mechanics

375
00:20:43,530 --> 00:20:45,280
of the substrate itself.

376
00:20:45,280 --> 00:20:49,410
If that substrate is behaving
as a viscous fluid, say,

377
00:20:49,410 --> 00:20:53,070
then as you compress that
substrate, that flat, polymer

378
00:20:53,070 --> 00:20:54,570
substrate, you're
going to expect

379
00:20:54,570 --> 00:20:58,890
to see, in fact, a parabolic
distribution of pressure

380
00:20:58,890 --> 00:20:59,820
across it.

381
00:20:59,820 --> 00:21:05,300
So there are various
things to consider there.

382
00:21:05,300 --> 00:21:08,860
And then at what I'm going
to call the device scale,

383
00:21:08,860 --> 00:21:11,800
there are these
pattern dependencies

384
00:21:11,800 --> 00:21:15,610
where the arrangement
of features on the stamp

385
00:21:15,610 --> 00:21:20,590
can have a profound effect on
how well defined the features

386
00:21:20,590 --> 00:21:21,640
end up being.

387
00:21:21,640 --> 00:21:29,020
And what I've sketched here,
which is really just a sketch,

388
00:21:29,020 --> 00:21:32,140
it suggests if
there are features

389
00:21:32,140 --> 00:21:34,970
that are slightly more
closely packed together,

390
00:21:34,970 --> 00:21:38,620
then they may fill earlier,
or maybe sometimes later

391
00:21:38,620 --> 00:21:40,660
than regions that are
less densely packed.

392
00:21:40,660 --> 00:21:42,760
And if that's the
case, then maybe you

393
00:21:42,760 --> 00:21:45,340
want to put some
design rules in place

394
00:21:45,340 --> 00:21:49,660
that will constrain the
variation of density

395
00:21:49,660 --> 00:21:52,600
of features on the stamp.

396
00:21:52,600 --> 00:21:56,290
Finally, we've got at the
smallest scale effects

397
00:21:56,290 --> 00:21:58,070
to do with individual features.

398
00:21:58,070 --> 00:22:00,670
So if an individual
gap is too small, say,

399
00:22:00,670 --> 00:22:03,100
that's going to be very hard
to force polymer into it.

400
00:22:03,100 --> 00:22:05,890
And that could be a showstopper.

401
00:22:05,890 --> 00:22:09,400
So we've got these three
different length scales.

402
00:22:09,400 --> 00:22:13,000
I've alluded to these
sorts of problems,

403
00:22:13,000 --> 00:22:18,730
bowing of the patterns, non
parallelism of the patterns.

404
00:22:18,730 --> 00:22:20,920
They relate to the
substrate scale effects.

405
00:22:24,110 --> 00:22:31,310
This shows one particular
substrate that we embossed.

406
00:22:31,310 --> 00:22:35,990
This is a square of
PMMA, which is acrylic,

407
00:22:35,990 --> 00:22:39,060
which is about 100
millimeters square.

408
00:22:39,060 --> 00:22:43,190
We embossed with a wafer that
had been uniformly patterned

409
00:22:43,190 --> 00:22:48,380
with some silicon posts,
roughly 100 microns in diameter.

410
00:22:48,380 --> 00:22:51,230
And we just use this
as a probe to look

411
00:22:51,230 --> 00:22:56,480
at our embossing machine and
see how uniform a pattern could

412
00:22:56,480 --> 00:22:57,230
be produced.

413
00:22:57,230 --> 00:23:00,560
Now, aside from the fact that
huge chunks of the silicon

414
00:23:00,560 --> 00:23:02,447
wafer broke off
during de-molding--

415
00:23:02,447 --> 00:23:05,030
and that's a good illustration
of some of the problems that we

416
00:23:05,030 --> 00:23:06,140
face--

417
00:23:06,140 --> 00:23:12,530
we were able to look at the
topography of the pattern

418
00:23:12,530 --> 00:23:14,600
at different locations
on the wafer.

419
00:23:14,600 --> 00:23:17,900
We did that using
an interferometer,

420
00:23:17,900 --> 00:23:21,410
scanning from above,
measuring the topography.

421
00:23:21,410 --> 00:23:25,220
And so we were able
to see various things,

422
00:23:25,220 --> 00:23:29,720
that the emboss depth was higher
on this side than on this side.

423
00:23:29,720 --> 00:23:34,760
Then near the edge of the
wafer the embossed features

424
00:23:34,760 --> 00:23:37,260
tailed off in height.

425
00:23:37,260 --> 00:23:38,780
And so these are
the sorts of things

426
00:23:38,780 --> 00:23:43,460
that we need to come up
with ways of predicting

427
00:23:43,460 --> 00:23:47,270
so that we can counteract them.

428
00:23:47,270 --> 00:23:54,530
Anyway, what I'm going
to concentrate on

429
00:23:54,530 --> 00:23:57,500
for the rest of this
talk, in fact, is pattern

430
00:23:57,500 --> 00:24:04,520
dependent non uniformity,
which is, in many ways,

431
00:24:04,520 --> 00:24:07,670
the most challenging
to deal with.

432
00:24:07,670 --> 00:24:09,590
Effects to do with
the edge of the wafer

433
00:24:09,590 --> 00:24:13,430
become important when you're
trying to eke out one or two

434
00:24:13,430 --> 00:24:14,420
extra devices.

435
00:24:14,420 --> 00:24:16,780
You're trying to save a
little bit of material.

436
00:24:16,780 --> 00:24:20,990
But if the actual
design of the device--

437
00:24:20,990 --> 00:24:22,880
there may be many
devices across a wafer.

438
00:24:22,880 --> 00:24:25,670
If the design of the
individual device is faulty

439
00:24:25,670 --> 00:24:27,320
and it will never
replicate properly,

440
00:24:27,320 --> 00:24:28,990
then you're in real trouble.

441
00:24:28,990 --> 00:24:30,950
So trying to get
an understanding

442
00:24:30,950 --> 00:24:34,580
of what patterns will form
well and what patterns will not

443
00:24:34,580 --> 00:24:36,440
is crucial.

444
00:24:36,440 --> 00:24:43,760
And so although we are
working towards a unified way

445
00:24:43,760 --> 00:24:45,260
of dealing with
this, I'm just going

446
00:24:45,260 --> 00:24:50,000
to talk mostly about the
pattern dependencies now.

447
00:24:50,000 --> 00:24:56,780
So I've mentioned all of
these different questions that

448
00:24:56,780 --> 00:25:01,040
need to be answered, factors
that need to be chosen when

449
00:25:01,040 --> 00:25:03,080
you're designing a new process.

450
00:25:03,080 --> 00:25:07,430
And some of them are going
to be continuous variables,

451
00:25:07,430 --> 00:25:09,560
and some of them will
be discrete choices.

452
00:25:09,560 --> 00:25:15,530
And it's not absolutely
clear which will be which.

453
00:25:15,530 --> 00:25:17,210
I've really split
the decisions up

454
00:25:17,210 --> 00:25:19,453
into three categories,
the decisions

455
00:25:19,453 --> 00:25:20,870
you have to make
about the pattern

456
00:25:20,870 --> 00:25:24,290
you're going to emboss, what
are the shapes of the features?

457
00:25:24,290 --> 00:25:29,070
Rectangles, circles,
triangles, how big they are,

458
00:25:29,070 --> 00:25:32,270
and how they are
oriented on the stamp?

459
00:25:32,270 --> 00:25:34,850
And that can be
important when you're

460
00:25:34,850 --> 00:25:37,970
starting to think about
radial thermal contraction

461
00:25:37,970 --> 00:25:41,090
of the parts, or about
anisotropic material

462
00:25:41,090 --> 00:25:41,710
properties.

463
00:25:41,710 --> 00:25:47,150
So there you've got-- you know,
size is a continuous variable,

464
00:25:47,150 --> 00:25:49,460
and the feature
shape you might think

465
00:25:49,460 --> 00:25:52,130
of as being a discrete choice.

466
00:25:52,130 --> 00:25:56,370
When it comes to the
substrate itself,

467
00:25:56,370 --> 00:25:58,770
then you've got to
decide on the material.

468
00:25:58,770 --> 00:26:02,710
Various materials soften
at different temperatures.

469
00:26:02,710 --> 00:26:08,430
Some of them exhibit a range
of temperatures over which they

470
00:26:08,430 --> 00:26:10,380
behave rather like a rubber.

471
00:26:10,380 --> 00:26:13,470
Others don't really have
much of a rubbery region.

472
00:26:13,470 --> 00:26:15,330
They just flow
very easily as soon

473
00:26:15,330 --> 00:26:17,560
as you're above the glass
transition temperature.

474
00:26:17,560 --> 00:26:20,490
So that's a decision
that has to be made.

475
00:26:20,490 --> 00:26:23,700
And in micro
embossing, often you're

476
00:26:23,700 --> 00:26:30,120
picking between a variety of
off the shelf substrates, PMMA,

477
00:26:30,120 --> 00:26:33,510
plexiglass, polycarbonate,
materials like this,

478
00:26:33,510 --> 00:26:36,540
where these are technical
grade materials.

479
00:26:36,540 --> 00:26:38,740
They're molecular weight.

480
00:26:38,740 --> 00:26:41,430
In other words, the average
length of the polymer chains

481
00:26:41,430 --> 00:26:44,770
is not necessarily well
documented or well defined.

482
00:26:44,770 --> 00:26:46,530
So in that sense,
your material choice

483
00:26:46,530 --> 00:26:49,030
might be a discrete decision.

484
00:26:49,030 --> 00:26:51,870
On the other hand, if
you're spinning the polymer

485
00:26:51,870 --> 00:26:54,180
onto the wafer, and
you're dissolving it,

486
00:26:54,180 --> 00:26:59,310
then you do have the option of
choosing the molecular weight,

487
00:26:59,310 --> 00:27:02,460
and therefore, defining the
viscosity as essentially

488
00:27:02,460 --> 00:27:03,850
a continuous variable.

489
00:27:03,850 --> 00:27:07,140
So those are things that might
need to be thought about.

490
00:27:07,140 --> 00:27:08,940
And, of course,
there's the thickness.

491
00:27:08,940 --> 00:27:13,560
Either how thickly do
you spin this resistor?

492
00:27:13,560 --> 00:27:17,280
And that, a lot of
research in the field

493
00:27:17,280 --> 00:27:20,910
demonstrates the
optimal thickness

494
00:27:20,910 --> 00:27:23,250
is a strong function
of what pattern you're

495
00:27:23,250 --> 00:27:27,160
trying to emboss, how many
voids there are in the stamp.

496
00:27:27,160 --> 00:27:29,520
So the thickness of
that might be important,

497
00:27:29,520 --> 00:27:34,000
and also the thickness
in micro embossing

498
00:27:34,000 --> 00:27:37,980
where the substrate is
a millimeter thick, say.

499
00:27:37,980 --> 00:27:41,210
That's also a decision
that has to be made.

500
00:27:41,210 --> 00:27:43,662
So and the third
category, of course,

501
00:27:43,662 --> 00:27:45,870
is the process parameters,
the temperature, pressure,

502
00:27:45,870 --> 00:27:49,350
hold time, and so forth.

503
00:27:52,370 --> 00:27:56,570
Now, there's an awful
lot of variables there.

504
00:27:56,570 --> 00:28:00,580
And if one were to just take
a purely empirical approach

505
00:28:00,580 --> 00:28:05,710
and try doing a full factorial
or a fractional factorial

506
00:28:05,710 --> 00:28:08,230
exploration of the
space, it would

507
00:28:08,230 --> 00:28:11,470
start to look pretty
much like a nightmare.

508
00:28:11,470 --> 00:28:15,820
And luckily, we don't have to
stumble around in the dark,

509
00:28:15,820 --> 00:28:19,450
because we do already
have some reasonably

510
00:28:19,450 --> 00:28:22,810
good physical intuition about
how these materials behave.

511
00:28:22,810 --> 00:28:27,070
And, of course, there's
a huge research field

512
00:28:27,070 --> 00:28:30,320
to do with characterizing
these bulk materials,

513
00:28:30,320 --> 00:28:33,490
building theoretical
models of how they behave,

514
00:28:33,490 --> 00:28:36,610
fitting data to those models.

515
00:28:36,610 --> 00:28:40,990
And I'm sure mechanical
engineers here

516
00:28:40,990 --> 00:28:48,940
will be familiar with this sort
of way of describing polymers

517
00:28:48,940 --> 00:28:55,880
in a simplified manner,
where you build up a model.

518
00:28:55,880 --> 00:28:58,180
In this case, this is
a one dimensional model

519
00:28:58,180 --> 00:29:03,580
that sums up the viscoelastic
behavior of the polymer using

520
00:29:03,580 --> 00:29:07,660
a combination of springs,
elastic, nondissipative

521
00:29:07,660 --> 00:29:08,170
elements.

522
00:29:08,170 --> 00:29:13,300
And these elements here are
dashpots, which are essentially

523
00:29:13,300 --> 00:29:14,890
viscous components and damp.

524
00:29:14,890 --> 00:29:17,500
Yes, exactly, they
dissipate energy.

525
00:29:17,500 --> 00:29:24,310
So the various stiffnesses
and dissipation factors

526
00:29:24,310 --> 00:29:29,620
for the springs and dashpots
may be non-linear functions

527
00:29:29,620 --> 00:29:30,610
of temperature.

528
00:29:30,610 --> 00:29:33,170
They may be non-linear
functions of strain rate,

529
00:29:33,170 --> 00:29:35,350
and, in fact, almost always are.

530
00:29:35,350 --> 00:29:39,190
And so there are
many theses that

531
00:29:39,190 --> 00:29:43,240
have been written trying to
characterize these materials.

532
00:29:43,240 --> 00:29:49,180
And building on those
experimental approaches

533
00:29:49,180 --> 00:29:50,980
are simulation models.

534
00:29:50,980 --> 00:29:56,060
There are finite element
models of these polymers that

535
00:29:56,060 --> 00:29:58,420
have been lovingly built up.

536
00:29:58,420 --> 00:30:03,725
And so with enough
information about the polymer

537
00:30:03,725 --> 00:30:06,100
with a detailed model of the
stamp that you were thinking

538
00:30:06,100 --> 00:30:09,040
of embossing, and
knowing something

539
00:30:09,040 --> 00:30:12,430
about the dimensional tolerances
of the machine you have,

540
00:30:12,430 --> 00:30:18,610
you could do a simulation
and perhaps predict

541
00:30:18,610 --> 00:30:20,470
what was going to happen.

542
00:30:20,470 --> 00:30:22,450
The trouble is, it would
probably take years.

543
00:30:22,450 --> 00:30:28,600
Because when you start
having microscale patterns

544
00:30:28,600 --> 00:30:34,030
that you're trying to emboss,
the computational burden

545
00:30:34,030 --> 00:30:38,680
of using these full nonlinear,
finite, deformation polymer

546
00:30:38,680 --> 00:30:40,510
models becomes immense.

547
00:30:40,510 --> 00:30:44,320
And that's great if you have
one particular device that

548
00:30:44,320 --> 00:30:46,090
absolutely must be perfect.

549
00:30:46,090 --> 00:30:49,840
However, what we're interested
in doing in our research

550
00:30:49,840 --> 00:30:51,580
is finding some sort
of middle ground

551
00:30:51,580 --> 00:30:55,360
between the empirical
approach and the really

552
00:30:55,360 --> 00:30:59,950
rigorous, thorough,
theoretical approach

553
00:30:59,950 --> 00:31:03,650
that's based on a highly
controlled set of experiments.

554
00:31:03,650 --> 00:31:07,720
And so the idea is to find
approximate descriptions

555
00:31:07,720 --> 00:31:12,720
for the material that
are efficient to run,

556
00:31:12,720 --> 00:31:17,570
efficient to simulate patterns,
wafers that have thousands

557
00:31:17,570 --> 00:31:20,100
of features across them.

558
00:31:20,100 --> 00:31:27,710
So let's start to think
about how we can build out

559
00:31:27,710 --> 00:31:29,600
the bare bones of a
model that are going

560
00:31:29,600 --> 00:31:34,430
to inform the experiments that
we do about this embossing

561
00:31:34,430 --> 00:31:35,700
process.

562
00:31:35,700 --> 00:31:42,620
Now, what I've done here-- and
I won't go into laborious detail

563
00:31:42,620 --> 00:31:46,670
about, but essentially, this
is just building intuition

564
00:31:46,670 --> 00:31:48,560
about one particular polymer.

565
00:31:48,560 --> 00:31:52,490
This is a model for
polymethylmethacrylate,

566
00:31:52,490 --> 00:31:54,710
plexiglass, acrylic,
call it what

567
00:31:54,710 --> 00:31:59,060
you will, which a student
at MIT has recently

568
00:31:59,060 --> 00:32:02,900
written a thesis about, doing
compression experiments of it,

569
00:32:02,900 --> 00:32:07,340
building up a highly
faithful model of it.

570
00:32:07,340 --> 00:32:10,730
But there's a one dimensional
implementation of that model

571
00:32:10,730 --> 00:32:12,020
that we can run.

572
00:32:12,020 --> 00:32:15,000
We can look at what happens
when you give it various load

573
00:32:15,000 --> 00:32:20,180
profiles over time and look at
the stresses and the strains

574
00:32:20,180 --> 00:32:26,120
in each of those components
of the model over time.

575
00:32:26,120 --> 00:32:30,880
So what I've done
here is I've just

576
00:32:30,880 --> 00:32:34,030
done two what are essentially
thought experiments.

577
00:32:34,030 --> 00:32:40,150
The left hand column
of graphs shows

578
00:32:40,150 --> 00:32:44,380
what happens if the material
is heated above its glass

579
00:32:44,380 --> 00:32:48,910
transition temperature
and then given

580
00:32:48,910 --> 00:32:53,140
a 1 and 1/2 megapascal
compressive load which

581
00:32:53,140 --> 00:32:56,350
is held for 10 seconds.

582
00:32:56,350 --> 00:32:59,950
That load is maintained while
the temperature is reduced

583
00:32:59,950 --> 00:33:03,190
below the glass transmission.

584
00:33:03,190 --> 00:33:06,880
The glass transition here
is about 105 degrees C.

585
00:33:06,880 --> 00:33:11,870
So then we looked at how
the model responded to that.

586
00:33:11,870 --> 00:33:14,710
There's a compressive
strain, of course.

587
00:33:14,710 --> 00:33:17,320
There are stresses built
up in the left hand

588
00:33:17,320 --> 00:33:20,380
branch of that model, this
spring and this dashpot.

589
00:33:20,380 --> 00:33:23,360
And there are stresses in
the right hand side as well.

590
00:33:23,360 --> 00:33:27,820
And essentially what this shows
is that most of the deformation

591
00:33:27,820 --> 00:33:30,790
is frozen in place,
frozen by this

592
00:33:30,790 --> 00:33:32,480
cooling below glass transition.

593
00:33:32,480 --> 00:33:35,260
So when the load is removed,
this amount of strain

594
00:33:35,260 --> 00:33:38,310
remains, this P amount here.

595
00:33:38,310 --> 00:33:43,690
However, if you don't cool down
before you remove the load,

596
00:33:43,690 --> 00:33:45,610
and that's what is
shown on the right,

597
00:33:45,610 --> 00:33:47,830
then we see this recovery.

598
00:33:47,830 --> 00:33:50,050
We see the material
springing back,

599
00:33:50,050 --> 00:33:52,300
almost to its original shape.

600
00:33:52,300 --> 00:33:56,380
And so there's a very small
residual compressive strain

601
00:33:56,380 --> 00:33:57,310
in that case.

602
00:33:57,310 --> 00:34:01,450
And just looking at a
distance, from a distance

603
00:34:01,450 --> 00:34:04,060
at this simulation,
which is done

604
00:34:04,060 --> 00:34:09,400
at a particular temperature,
140 degrees C, essentially

605
00:34:09,400 --> 00:34:13,449
the material is behaving in a
springlike way predominantly.

606
00:34:13,449 --> 00:34:15,820
Most of the deformation
is recovered

607
00:34:15,820 --> 00:34:19,929
without cooling it to freeze
that deformation in place.

608
00:34:19,929 --> 00:34:24,760
And you could run similar
thought experiments

609
00:34:24,760 --> 00:34:27,159
at different temperatures,
different strain

610
00:34:27,159 --> 00:34:32,020
rates with the model and get an
impression of how much of that

611
00:34:32,020 --> 00:34:37,060
deformation is recovered
when the temperature is

612
00:34:37,060 --> 00:34:41,440
kept at its elevated level
when you remove the load.

613
00:34:41,440 --> 00:34:45,295
And so loosely speaking,
if most of the deformation

614
00:34:45,295 --> 00:34:47,170
is recovered at
high temperature,

615
00:34:47,170 --> 00:34:50,620
we would describe the material
as being rubbery over here.

616
00:34:50,620 --> 00:34:53,320
This is a plot
essentially of the ratio

617
00:34:53,320 --> 00:34:58,180
of that strain to
that strain, P over Q,

618
00:34:58,180 --> 00:35:03,490
as a function of temperature
and peak compressive stress.

619
00:35:03,490 --> 00:35:05,830
If that ratio is large,
we call it rubbery.

620
00:35:05,830 --> 00:35:08,030
If the ratio is
small, in other words,

621
00:35:08,030 --> 00:35:10,750
if there's been a lot
of plastic deformation

622
00:35:10,750 --> 00:35:12,970
that doesn't spring
back upon unloading,

623
00:35:12,970 --> 00:35:16,320
then we would sort of
classify the material

624
00:35:16,320 --> 00:35:18,100
as being in a glassy state.

625
00:35:18,100 --> 00:35:21,700
And sometimes glassy
is useful to you.

626
00:35:21,700 --> 00:35:23,500
Sometimes rubbery is preferable.

627
00:35:23,500 --> 00:35:31,000
So in any case, this sort
of intuition building

628
00:35:31,000 --> 00:35:35,050
is helpful to us
in that it begins

629
00:35:35,050 --> 00:35:39,490
to give us a starting point
for a physical model that

630
00:35:39,490 --> 00:35:42,580
will let us choose what
experiments we're going to do,

631
00:35:42,580 --> 00:35:45,670
and will let us build quick,
efficient simulations.

632
00:35:45,670 --> 00:35:51,760
And so for that particular
temperature that I looked at

633
00:35:51,760 --> 00:35:55,000
in detail, if you wanted
the simplest possible model

634
00:35:55,000 --> 00:35:56,500
of the polymer, you
might just think

635
00:35:56,500 --> 00:36:00,820
of making the model a
linear, elastic model, where

636
00:36:00,820 --> 00:36:03,350
the Young's modulus was a
function of temperature.

637
00:36:03,350 --> 00:36:06,040
And there are many
shortcomings of that.

638
00:36:06,040 --> 00:36:07,910
If you hold the load
for long enough,

639
00:36:07,910 --> 00:36:11,950
then there will of course be
plastic flow that's permanent.

640
00:36:11,950 --> 00:36:16,180
And the material is
not linear elastic.

641
00:36:16,180 --> 00:36:19,240
It changes its stiffness
as the strain increases.

642
00:36:19,240 --> 00:36:24,080
But we might as well start
simple and see if it works.

643
00:36:24,080 --> 00:36:26,660
It gives us information
about what types of features

644
00:36:26,660 --> 00:36:28,280
replicate and what do not.

645
00:36:28,280 --> 00:36:32,550
So that's what we
initially started with.

646
00:36:32,550 --> 00:36:36,950
And the idea is that
increasing the temperature

647
00:36:36,950 --> 00:36:42,000
reduces that modulus by three
or four orders of magnitude.

648
00:36:42,000 --> 00:36:44,510
We form the shape,
and then we cool down

649
00:36:44,510 --> 00:36:48,660
to freeze that
topography in place.

650
00:36:48,660 --> 00:36:54,020
And what this is
going to let us do

651
00:36:54,020 --> 00:36:59,120
is predict, for an
arbitrary stamp design,

652
00:36:59,120 --> 00:37:03,500
what the shape of the
embossed part will be,

653
00:37:03,500 --> 00:37:08,660
what its surface topography
will be for a chosen temperature

654
00:37:08,660 --> 00:37:11,780
and pressure for embossing.

655
00:37:11,780 --> 00:37:19,460
And here's the computational
approach that we decided upon.

656
00:37:19,460 --> 00:37:23,710
And this is really quick to run.

657
00:37:23,710 --> 00:37:27,490
The idea is that
we want to describe

658
00:37:27,490 --> 00:37:33,310
how the surface of the material
responds to a point load.

659
00:37:33,310 --> 00:37:37,720
And if the material
were linear elastic,

660
00:37:37,720 --> 00:37:41,440
and if it were
substantially thicker

661
00:37:41,440 --> 00:37:47,270
than the dimensions of
the features of interest,

662
00:37:47,270 --> 00:37:50,800
then the topography in
response to a point load

663
00:37:50,800 --> 00:37:56,470
would effectively go as one
over the radial distance

664
00:37:56,470 --> 00:37:58,490
from the point where
the load is applied.

665
00:37:58,490 --> 00:38:03,820
And that's a standard
contact mechanics result.

666
00:38:03,820 --> 00:38:07,630
But we're talking
about computing

667
00:38:07,630 --> 00:38:10,237
approximate topographies.

668
00:38:10,237 --> 00:38:12,570
And this is going to be done
in a discretized way, where

669
00:38:12,570 --> 00:38:15,960
we split the surface
of the polymer

670
00:38:15,960 --> 00:38:22,500
into a series of square elements
so we can translate this point

671
00:38:22,500 --> 00:38:25,380
load response into the
response of the surface

672
00:38:25,380 --> 00:38:30,420
to unit pressure,
applied over one element

673
00:38:30,420 --> 00:38:32,130
of our discretized surface.

674
00:38:32,130 --> 00:38:35,100
So this is the plane of
the surface of the wafer,

675
00:38:35,100 --> 00:38:37,380
the surface of the polymer.

676
00:38:37,380 --> 00:38:41,490
And we are trying to express
how far that part of the surface

677
00:38:41,490 --> 00:38:44,460
goes down when you apply
unit pressure here.

678
00:38:44,460 --> 00:38:46,410
And that's the answer.

679
00:38:49,280 --> 00:38:55,220
So we tried this with some
very simple geometries,

680
00:38:55,220 --> 00:38:59,750
lots of parallel channels that
we had etched into silicon.

681
00:38:59,750 --> 00:39:05,120
They were about 20 microns
deep, these silicon channels,

682
00:39:05,120 --> 00:39:12,080
and on the order of
120 microns in pitch.

683
00:39:12,080 --> 00:39:18,570
There are many channels
going across the screen.

684
00:39:18,570 --> 00:39:21,770
But what we show
here is on the left,

685
00:39:21,770 --> 00:39:26,720
scanning electron
micrographs of cross sections

686
00:39:26,720 --> 00:39:29,390
through those embossed
structures, where

687
00:39:29,390 --> 00:39:31,670
a variety of different
loads have been applied.

688
00:39:31,670 --> 00:39:35,660
And the loads were held
for less than a minute.

689
00:39:35,660 --> 00:39:38,480
So it's our working
assumption here

690
00:39:38,480 --> 00:39:42,050
that all the
deformation was to do

691
00:39:42,050 --> 00:39:44,390
with the rubbery
behavior of the material,

692
00:39:44,390 --> 00:39:48,150
and there wasn't really enough
time for plastic flow to occur.

693
00:39:48,150 --> 00:39:51,740
So we're just using our
zero [INAUDIBLE] model

694
00:39:51,740 --> 00:39:56,390
to figure out what the Young's
modulus of the material

695
00:39:56,390 --> 00:39:57,140
was essentially.

696
00:39:57,140 --> 00:40:01,670
There's one parameter that we
fit in doing this simulation.

697
00:40:01,670 --> 00:40:03,500
And that is the Young's modulus.

698
00:40:03,500 --> 00:40:06,900
Here are simulations
done using that model.

699
00:40:06,900 --> 00:40:11,180
So we're roughly
capturing the shape.

700
00:40:11,180 --> 00:40:13,100
And it seems to
all stack up if we

701
00:40:13,100 --> 00:40:19,610
choose this Young's modulus, 5
mega pascals at 130 degrees C.

702
00:40:19,610 --> 00:40:20,840
So that's all very well.

703
00:40:20,840 --> 00:40:23,840
But having these
parallel channels

704
00:40:23,840 --> 00:40:25,550
doesn't give us too
much information

705
00:40:25,550 --> 00:40:26,880
for a given experiment.

706
00:40:26,880 --> 00:40:31,190
So we have to think
quite carefully

707
00:40:31,190 --> 00:40:34,550
about what type of
characterization patterns

708
00:40:34,550 --> 00:40:36,830
to use in these experiments.

709
00:40:36,830 --> 00:40:39,590
We're trying to think
of ways of doing

710
00:40:39,590 --> 00:40:41,270
a minimal number
of experiments that

711
00:40:41,270 --> 00:40:43,910
will give us as much physical
information as possible.

712
00:40:43,910 --> 00:40:47,150
And what we came
up with was this.

713
00:40:47,150 --> 00:40:54,380
Here is a plan of the surface of
a silicon stamp that we etched.

714
00:40:54,380 --> 00:41:02,120
And it's a square array of
various patches of features.

715
00:41:02,120 --> 00:41:04,100
Some of them are
parallel channels.

716
00:41:04,100 --> 00:41:06,830
Some of them are square holes.

717
00:41:06,830 --> 00:41:10,320
And we have parallel channels
running in both directions.

718
00:41:10,320 --> 00:41:14,150
So for every line to
space ratio of channels,

719
00:41:14,150 --> 00:41:16,670
there's one set running
horizontally somewhere,

720
00:41:16,670 --> 00:41:18,200
one set running vertically.

721
00:41:18,200 --> 00:41:25,040
And they were arranged in a
random order across the stamp.

722
00:41:27,550 --> 00:41:30,970
Now, there's one
twist to this, which

723
00:41:30,970 --> 00:41:37,390
is that because we do
our simulation using

724
00:41:37,390 --> 00:41:41,200
a discrete Fourier transform,
a fast Fourier transform,

725
00:41:41,200 --> 00:41:47,320
the representation of the
space that we're simulating

726
00:41:47,320 --> 00:41:51,920
is assumed to be
periodic in space.

727
00:41:51,920 --> 00:41:57,815
So we accommodate that by
actually making the pattern

728
00:41:57,815 --> 00:41:58,315
periodic.

729
00:42:04,740 --> 00:42:10,720
What I show here is one
replicate of the pattern.

730
00:42:10,720 --> 00:42:14,640
And this is about 4
millimeters in diameter.

731
00:42:14,640 --> 00:42:18,520
But, in fact, what
we do is we have--

732
00:42:18,520 --> 00:42:25,512
I suppose on the stamp
we have a 3x3 array where

733
00:42:25,512 --> 00:42:29,190
what I've shown on the screen
is contained in this region.

734
00:42:29,190 --> 00:42:32,760
So it's pseudo periodic
from the point of view

735
00:42:32,760 --> 00:42:34,170
of the material surrounding it.

736
00:42:34,170 --> 00:42:35,760
The material's a
millimeter thick.

737
00:42:35,760 --> 00:42:37,440
This is 4 millimeters.

738
00:42:37,440 --> 00:42:41,730
And so what's going
on at the edge here

739
00:42:41,730 --> 00:42:44,250
looks exactly the
same to the material

740
00:42:44,250 --> 00:42:47,310
as this material sees over here.

741
00:42:47,310 --> 00:42:51,390
And that makes the simulation
match up with the experiments

742
00:42:51,390 --> 00:42:51,910
nicely.

743
00:42:51,910 --> 00:42:54,570
So we etch this stamp.

744
00:42:54,570 --> 00:42:57,240
We choose some
embossing conditions,

745
00:42:57,240 --> 00:43:00,420
press it into the
substrate, and use

746
00:43:00,420 --> 00:43:03,300
white light scanning
interferometry

747
00:43:03,300 --> 00:43:05,520
to measure the
surface topography.

748
00:43:05,520 --> 00:43:07,830
The smallest features
in this particular stamp

749
00:43:07,830 --> 00:43:12,060
are 5 micron in diameter.

750
00:43:12,060 --> 00:43:14,070
What we're also
doing now actually

751
00:43:14,070 --> 00:43:16,860
is a set of experiments where
we've scaled down this pattern

752
00:43:16,860 --> 00:43:19,950
100-fold to make a nano
imprint lithography

753
00:43:19,950 --> 00:43:21,750
stamp of the same pattern.

754
00:43:21,750 --> 00:43:27,630
And so we're trying to tie all
the mechanics together and see

755
00:43:27,630 --> 00:43:31,980
if similar effects
occur at the nanoscale.

756
00:43:31,980 --> 00:43:38,400
Anyhow, in the bottom left
here is a composite map

757
00:43:38,400 --> 00:43:42,780
of the surface, where the red
colors show that the higher

758
00:43:42,780 --> 00:43:47,190
topography is where more
material has penetrated further

759
00:43:47,190 --> 00:43:50,400
into the stamp cavities.

760
00:43:50,400 --> 00:43:54,540
And on the right here
are eight cross-sections

761
00:43:54,540 --> 00:43:59,400
through that topography,
labeled one, two, eight.

762
00:43:59,400 --> 00:44:01,950
Experimental data is shown
in black, underneath.

763
00:44:01,950 --> 00:44:06,480
And the prediction of
the linear elastic model,

764
00:44:06,480 --> 00:44:10,500
with only one fitted
parameter, the Young's modulus,

765
00:44:10,500 --> 00:44:11,410
is shown in red.

766
00:44:11,410 --> 00:44:15,810
And you can see that there's
remarkable correspondence

767
00:44:15,810 --> 00:44:17,580
between the two.

768
00:44:17,580 --> 00:44:23,730
We get both a prediction of
how far on average material

769
00:44:23,730 --> 00:44:27,240
penetrates cavities
of a given diameter.

770
00:44:27,240 --> 00:44:31,440
And also, we do capture these
interactions of patterns

771
00:44:31,440 --> 00:44:32,470
of different densities.

772
00:44:32,470 --> 00:44:37,180
You can see, for
example, let's say--

773
00:44:37,180 --> 00:44:39,840
well, I guess this
is a good example.

774
00:44:39,840 --> 00:44:45,900
Here you're seeing many
instances of a given trench

775
00:44:45,900 --> 00:44:51,340
diameter, but trenches that are
closer to different patterns

776
00:44:51,340 --> 00:44:54,010
feel less far than those that
are in the center of the patch.

777
00:44:54,010 --> 00:44:57,310
And this is all
captured reasonably

778
00:44:57,310 --> 00:45:03,240
well by this rubbery
model of the polymer.

779
00:45:03,240 --> 00:45:06,800
So looks like we've
got the basis here

780
00:45:06,800 --> 00:45:12,800
for doing efficient simulations.

781
00:45:12,800 --> 00:45:16,250
You can run that simulation
in 20 seconds in Matlab,

782
00:45:16,250 --> 00:45:19,430
and it's giving you,
I think, probably

783
00:45:19,430 --> 00:45:23,060
95% of the information a full
finite element simulation

784
00:45:23,060 --> 00:45:27,450
would give you, as far as
topography is concerned.

785
00:45:27,450 --> 00:45:33,320
So we can also abstract
our experimental results

786
00:45:33,320 --> 00:45:36,690
and view them in simpler ways.

787
00:45:36,690 --> 00:45:40,070
And one way of doing
that is to look

788
00:45:40,070 --> 00:45:44,060
within each region of
features and measure

789
00:45:44,060 --> 00:45:47,660
the peak penetration of
material into the cavity.

790
00:45:47,660 --> 00:45:50,720
So we just measure
the range of heights

791
00:45:50,720 --> 00:45:55,400
within the central 80% of the
area of each patch of features.

792
00:45:55,400 --> 00:45:58,760
And then plot that
peak penetration

793
00:45:58,760 --> 00:46:02,030
against some parameter.

794
00:46:02,030 --> 00:46:04,740
In this case, I plotted
the pattern density,

795
00:46:04,740 --> 00:46:09,320
which is the ratio of the cavity
width to the cavity pitch.

796
00:46:09,320 --> 00:46:12,200
And the different
colors in this graph

797
00:46:12,200 --> 00:46:15,260
represent different
feature pitches.

798
00:46:15,260 --> 00:46:20,690
So the pitch being the distance
from one cavity to the next.

799
00:46:20,690 --> 00:46:28,220
And what I show is the elastic
model matching up pretty well

800
00:46:28,220 --> 00:46:30,530
with the experimental results.

801
00:46:33,500 --> 00:46:37,250
Of course, you might
think that this

802
00:46:37,250 --> 00:46:40,310
is leading towards the
possibility of non dimensional

803
00:46:40,310 --> 00:46:43,940
groups to describe
features and how they fill,

804
00:46:43,940 --> 00:46:48,170
and putting down design
rules that would tell you

805
00:46:48,170 --> 00:46:49,970
how to scale the
embossing pressure

806
00:46:49,970 --> 00:46:52,530
as your pattern's scaled down,
and these sorts of things.

807
00:46:52,530 --> 00:46:56,660
So this is a great way
of viewing the results

808
00:46:56,660 --> 00:46:59,430
and getting
intuition about them.

809
00:46:59,430 --> 00:47:02,630
Of course, as you increase the
pressure and material starts

810
00:47:02,630 --> 00:47:06,620
to touch the tops of
the stamp cavities,

811
00:47:06,620 --> 00:47:09,740
this straight line
breaks down, of course.

812
00:47:09,740 --> 00:47:12,710
And the red symbols
show what happens

813
00:47:12,710 --> 00:47:16,820
when the pressure is increased
to a stage where the larger

814
00:47:16,820 --> 00:47:17,570
cavities fill.

815
00:47:20,090 --> 00:47:23,720
What you can also do is use
a series of these experiments

816
00:47:23,720 --> 00:47:27,830
to get material properties
as a function of temperature

817
00:47:27,830 --> 00:47:29,330
or of some other parameter.

818
00:47:29,330 --> 00:47:31,250
And that's exactly
what we did here,

819
00:47:31,250 --> 00:47:34,580
by doing a series
of embossing tests

820
00:47:34,580 --> 00:47:36,080
at different temperatures.

821
00:47:36,080 --> 00:47:40,910
And the gradient of that
line penetration to pressure

822
00:47:40,910 --> 00:47:44,210
will be inversely proportional
to the effective elastic

823
00:47:44,210 --> 00:47:46,050
modulus of the material.

824
00:47:46,050 --> 00:47:47,540
So we did that.

825
00:47:47,540 --> 00:47:51,440
We got out these values
for the Young's modulus

826
00:47:51,440 --> 00:47:52,580
against temperature.

827
00:47:52,580 --> 00:47:55,700
And luckily, they
match up really

828
00:47:55,700 --> 00:48:00,620
nicely with a model
for Young's modulus

829
00:48:00,620 --> 00:48:04,250
that was derived from bulk
compression experiments, that

830
00:48:04,250 --> 00:48:06,290
was just done in a
big and strong load

831
00:48:06,290 --> 00:48:09,140
frame using bulk materials.

832
00:48:09,140 --> 00:48:16,790
So you could start to see how,
with careful choice of test

833
00:48:16,790 --> 00:48:20,060
embossing patterns, we can
get a lot of information out

834
00:48:20,060 --> 00:48:25,310
of a new material quite
quickly, scale information,

835
00:48:25,310 --> 00:48:28,670
strain rate information perhaps,
temperature information.

836
00:48:28,670 --> 00:48:31,700
And really, one of the
big challenges here

837
00:48:31,700 --> 00:48:35,780
is deciding what are the
embossing tests you're

838
00:48:35,780 --> 00:48:38,510
going to do, what combination
of temperatures and pressures

839
00:48:38,510 --> 00:48:41,840
are you going to go for first
that will give you the most

840
00:48:41,840 --> 00:48:44,180
information?

841
00:48:44,180 --> 00:48:49,850
Now, I said before that
this rubbery model is really

842
00:48:49,850 --> 00:48:54,500
only useful if the whole
time is comparatively short.

843
00:48:54,500 --> 00:48:59,700
And often that's the case.

844
00:48:59,700 --> 00:49:02,290
You want the cycle time to
be as short as possible.

845
00:49:02,290 --> 00:49:06,320
So if you can get the
material into a state

846
00:49:06,320 --> 00:49:08,180
where you press
hard enough, you get

847
00:49:08,180 --> 00:49:10,440
the deformation almost
instantaneously,

848
00:49:10,440 --> 00:49:11,810
then that's great.

849
00:49:11,810 --> 00:49:14,090
But in a lot of cases,
you can't do that,

850
00:49:14,090 --> 00:49:16,350
and you do get
this plastic flow.

851
00:49:16,350 --> 00:49:20,300
So what I show here
is the evolution

852
00:49:20,300 --> 00:49:23,060
of an embossed
topography over time

853
00:49:23,060 --> 00:49:27,110
as we hold the load
at high temperature.

854
00:49:27,110 --> 00:49:29,420
The black symbols
down here show what

855
00:49:29,420 --> 00:49:32,090
happens if you start
cooling the material

856
00:49:32,090 --> 00:49:34,200
down as soon as the
load reaches its peak.

857
00:49:34,200 --> 00:49:38,060
So the material is
bowing into the cavities,

858
00:49:38,060 --> 00:49:39,720
but hasn't gone very far.

859
00:49:39,720 --> 00:49:43,820
And then right up here,
you've waited for 10 minutes.

860
00:49:43,820 --> 00:49:46,490
And all but the
smallest features

861
00:49:46,490 --> 00:49:49,700
have at least touched
the tops of the cavities.

862
00:49:49,700 --> 00:49:54,210
Of course, this y-axis is
showing the peak penetration.

863
00:49:54,210 --> 00:49:56,660
So it's the distance
from the top

864
00:49:56,660 --> 00:49:59,780
of the feature to the bottom.

865
00:49:59,780 --> 00:50:02,420
And it tells you nothing
about whether the corners

866
00:50:02,420 --> 00:50:04,070
of the features
have been filled.

867
00:50:04,070 --> 00:50:08,720
But this is a reasonably
useful measurement.

868
00:50:08,720 --> 00:50:12,740
Now, the question is, can
we adapt our simple model

869
00:50:12,740 --> 00:50:15,650
to capture this
viscoelastic behavior

870
00:50:15,650 --> 00:50:19,310
without increasing the
computational burden very much?

871
00:50:19,310 --> 00:50:23,120
And well, the next thing
you might think of doing

872
00:50:23,120 --> 00:50:26,900
is adding a linear
dashpot into the system.

873
00:50:26,900 --> 00:50:29,438
And maybe it's linear.

874
00:50:29,438 --> 00:50:30,980
That would be the
first thing to try.

875
00:50:30,980 --> 00:50:34,910
Maybe you do need to capture the
idea of a yield stress as well.

876
00:50:34,910 --> 00:50:38,330
And maybe you need to capture
the idea of a strain rate

877
00:50:38,330 --> 00:50:42,350
dependence, which
in PMMA is such

878
00:50:42,350 --> 00:50:46,250
that the yield stress
increases substantially

879
00:50:46,250 --> 00:50:49,930
with strain rates above about
10 to the minus 2 per second.

880
00:50:49,930 --> 00:50:57,320
So, in fact, we just tried
adding in a linear dashpot.

881
00:50:57,320 --> 00:51:06,720
So what this does, in effect,
is takes the point load

882
00:51:06,720 --> 00:51:10,350
response associated
with purely rubbery,

883
00:51:10,350 --> 00:51:14,850
purely linear elastic behavior,
which would be like this,

884
00:51:14,850 --> 00:51:20,130
and scales it by a factor
that's related to the time

885
00:51:20,130 --> 00:51:21,600
the load is applied.

886
00:51:21,600 --> 00:51:25,950
And all that's saying is
imagine this was viscoelastic.

887
00:51:25,950 --> 00:51:30,060
You applied a point load you get
some instantaneous deformation.

888
00:51:30,060 --> 00:51:36,210
But over time, I'm saying this
surface is just scaling down.

889
00:51:36,210 --> 00:51:40,140
All the points are staying
in the same ratio of height,

890
00:51:40,140 --> 00:51:42,090
but it's just scaling down.

891
00:51:42,090 --> 00:51:49,230
And we assume that
that scale factor

892
00:51:49,230 --> 00:51:51,360
was proportional
to the hold time,

893
00:51:51,360 --> 00:51:52,770
but you might not assume that.

894
00:51:52,770 --> 00:51:58,170
You might say, well,
what if there's actually

895
00:51:58,170 --> 00:52:03,600
a sort of limiting strain
where the polymer network is

896
00:52:03,600 --> 00:52:07,200
as stretched out as it is it
can be for a particular load?

897
00:52:07,200 --> 00:52:13,080
And in that case, you would
have I guess strain against time

898
00:52:13,080 --> 00:52:17,280
would asymptotically approach
some limiting value that

899
00:52:17,280 --> 00:52:18,750
was related to that spring.

900
00:52:18,750 --> 00:52:21,070
And that would also be
quite easy to compute.

901
00:52:21,070 --> 00:52:25,540
You compute a topography at
the start of the whole thing,

902
00:52:25,540 --> 00:52:28,770
a topography that would be
associated with infinite time,

903
00:52:28,770 --> 00:52:32,460
and then maybe interpolate
using an exponential function

904
00:52:32,460 --> 00:52:33,200
of hold time.

905
00:52:33,200 --> 00:52:37,200
But anyway, we just went
with a linear scaling.

906
00:52:37,200 --> 00:52:41,010
And it seems to work
remarkably well.

907
00:52:41,010 --> 00:52:43,890
For these particular
conditions, 110 degrees C,

908
00:52:43,890 --> 00:52:47,710
which is a few degrees
above glass transition.

909
00:52:47,710 --> 00:52:55,230
So we're definitely in a
region where plastic flow

910
00:52:55,230 --> 00:52:58,200
is significant, is important.

911
00:52:58,200 --> 00:53:04,050
And what we see, these are
the top experimental data,

912
00:53:04,050 --> 00:53:06,600
3D plot of the
measurements taken

913
00:53:06,600 --> 00:53:11,070
using optical interferometry
on the left for less than a

914
00:53:11,070 --> 00:53:13,800
minute holding time,
loading duration,

915
00:53:13,800 --> 00:53:17,100
and on the right when we left
that load in place for 10

916
00:53:17,100 --> 00:53:18,310
minutes.

917
00:53:18,310 --> 00:53:21,540
So you can see that obviously
over time the narrower features

918
00:53:21,540 --> 00:53:22,950
start to fill.

919
00:53:22,950 --> 00:53:27,450
And the simulation, to
a reasonable extent,

920
00:53:27,450 --> 00:53:29,200
has captured that.

921
00:53:29,200 --> 00:53:31,590
And in fact, the shortcoming
of the simulation

922
00:53:31,590 --> 00:53:34,740
is that it underestimates
how quickly the narrower

923
00:53:34,740 --> 00:53:38,340
features fill for the
parameters that we fit.

924
00:53:38,340 --> 00:53:44,250
And so that is definitely
an imperfect model,

925
00:53:44,250 --> 00:53:48,420
but it's pretty useful,
I think, for getting

926
00:53:48,420 --> 00:53:53,070
a first cut of a simulation.

927
00:53:53,070 --> 00:54:01,470
So, of course, now that we've
got this characterization idea,

928
00:54:01,470 --> 00:54:03,480
we can apply that to
different materials.

929
00:54:03,480 --> 00:54:08,430
And we did it with this
cyclic olefin polymer, ZEONOR,

930
00:54:08,430 --> 00:54:11,220
which is a thermoplastic.

931
00:54:11,220 --> 00:54:13,650
This one softens
around 135 degrees

932
00:54:13,650 --> 00:54:20,940
C. It has quite a few
advantages over PMMA.

933
00:54:20,940 --> 00:54:22,560
Although, it's
more expensive, it

934
00:54:22,560 --> 00:54:24,720
doesn't absorb water
as readily, which

935
00:54:24,720 --> 00:54:29,070
is pretty relevant to
microfluidic devices.

936
00:54:29,070 --> 00:54:33,270
It transmits light to shorter
wavelengths, which is relevant

937
00:54:33,270 --> 00:54:37,050
when you're exciting
fluorescent tags

938
00:54:37,050 --> 00:54:39,780
inside the chip with UV light.

939
00:54:39,780 --> 00:54:43,650
And so it's useful to
be able to characterize

940
00:54:43,650 --> 00:54:45,480
these new materials
as they come along,

941
00:54:45,480 --> 00:54:48,360
and when you don't necessarily
have an unlimited supply

942
00:54:48,360 --> 00:54:49,240
of the material.

943
00:54:49,240 --> 00:54:53,970
So again, this is a
plot of peak penetration

944
00:54:53,970 --> 00:54:55,650
against cavity diameter.

945
00:54:55,650 --> 00:55:00,150
And although most
of the data sit

946
00:55:00,150 --> 00:55:04,200
on a roughly straight
line, when you have cavity

947
00:55:04,200 --> 00:55:06,520
diameter being a large
proportion of the pitch,

948
00:55:06,520 --> 00:55:08,145
in other words, there
are very narrow--

949
00:55:10,800 --> 00:55:12,030
hello.

950
00:55:12,030 --> 00:55:19,020
There are very narrow walls
of the stamp material pressing

951
00:55:19,020 --> 00:55:25,230
into the polymer, that's
when the penetration does

952
00:55:25,230 --> 00:55:28,030
seem to deviate from
this straight line.

953
00:55:28,030 --> 00:55:38,240
So anyway, now, I've shown
you results from a few

954
00:55:38,240 --> 00:55:42,620
carefully chosen
operating points.

955
00:55:42,620 --> 00:55:45,020
And I showed a set
of results where

956
00:55:45,020 --> 00:55:46,610
only temperature was varied.

957
00:55:46,610 --> 00:55:50,600
And I showed some
for ZEONOR that

958
00:55:50,600 --> 00:55:54,980
was only one temperature,
one hold time, one load.

959
00:55:54,980 --> 00:55:58,610
And we need to start thinking
about what combination

960
00:55:58,610 --> 00:56:01,070
of operating parameters
would we want

961
00:56:01,070 --> 00:56:05,270
to deploy if we knew
nothing about a material

962
00:56:05,270 --> 00:56:07,940
except for its approximate
glass transition temperature?

963
00:56:07,940 --> 00:56:12,110
What would we want
to vary first?

964
00:56:12,110 --> 00:56:15,260
And so you might
think about doing

965
00:56:15,260 --> 00:56:18,350
some sort of fractional
factorial experiments

966
00:56:18,350 --> 00:56:23,750
where your variables of
interest were those connected

967
00:56:23,750 --> 00:56:26,270
with the process, the
temperature, the load, the hold

968
00:56:26,270 --> 00:56:29,030
time, the time over which
the load is applied.

969
00:56:29,030 --> 00:56:35,120
And that, indeed, is what I
did with a third material,

970
00:56:35,120 --> 00:56:40,610
another brand of cyclic
olefin polymer called TOPAS.

971
00:56:40,610 --> 00:56:48,380
And this is a material that
softens around 85 degrees C.

972
00:56:48,380 --> 00:56:53,450
And so we did a 2 to the 4
minus 1 fractional factorial

973
00:56:53,450 --> 00:56:58,550
where the four parameters
were embossing temperature,

974
00:56:58,550 --> 00:57:03,590
peak embossing force,
hold time, and the time

975
00:57:03,590 --> 00:57:07,550
over which the load was
ramped up to its peak.

976
00:57:07,550 --> 00:57:13,730
And the astute among you will
notice that these variables are

977
00:57:13,730 --> 00:57:18,110
not going to be independent,
because the average load

978
00:57:18,110 --> 00:57:21,540
over the loading time will
contribute to the hold time.

979
00:57:21,540 --> 00:57:27,800
So it's a rough and
ready set of experiments.

980
00:57:27,800 --> 00:57:32,090
But nevertheless, it gives you
some idea of what we might do.

981
00:57:32,090 --> 00:57:40,160
And here we have, again,
cavity penetration results

982
00:57:40,160 --> 00:57:44,150
as a function of cavity width
for different feature pitches.

983
00:57:44,150 --> 00:57:49,460
The top plot is for features
of a pitch of 100 microns.

984
00:57:49,460 --> 00:57:51,260
And this is for 50 and 25.

985
00:57:51,260 --> 00:57:55,580
And you see that, again, we
have this nice trend where

986
00:57:55,580 --> 00:58:01,970
the peak penetration increases
up to a maximum value.

987
00:58:01,970 --> 00:58:06,470
That maximum value is the height
of the cavities in the stamp.

988
00:58:06,470 --> 00:58:12,080
And you'll also notice that
I've put these little black dots

989
00:58:12,080 --> 00:58:13,350
on the graph as well.

990
00:58:13,350 --> 00:58:18,180
And those are the measured
heights of the stamp cavities.

991
00:58:18,180 --> 00:58:21,590
So you can see that actually the
heights of the stamp cavities

992
00:58:21,590 --> 00:58:25,880
fall off for the
narrower cavities.

993
00:58:25,880 --> 00:58:33,680
And that's associated with the
etching process that's used.

994
00:58:33,680 --> 00:58:35,660
The actual plasma
etch that makes

995
00:58:35,660 --> 00:58:38,210
the trenches in
the silicon stamp

996
00:58:38,210 --> 00:58:40,220
has a harder time etching
the narrower trenches.

997
00:58:40,220 --> 00:58:43,310
But it's a good sanity
check that we're actually

998
00:58:43,310 --> 00:58:47,450
measuring the penetration of
polymer into the cavities.

999
00:58:47,450 --> 00:58:53,930
Now, here are some
3D plots of results.

1000
00:58:53,930 --> 00:58:59,700
This in the top left, this
is essentially our standard,

1001
00:58:59,700 --> 00:59:03,050
our center point run
set of parameters

1002
00:59:03,050 --> 00:59:04,640
where for a 4
minute hold time you

1003
00:59:04,640 --> 00:59:07,098
get a reasonably good filling
of most of the feature sizes,

1004
00:59:07,098 --> 00:59:08,220
but it's not perfect.

1005
00:59:08,220 --> 00:59:12,900
So we can get an idea
of process variability.

1006
00:59:12,900 --> 00:59:15,380
This is a set of
parameters that was really

1007
00:59:15,380 --> 00:59:18,710
no use at all, 100 newtons.

1008
00:59:18,710 --> 00:59:26,180
We haven't even leveled out the
non-parallelism in the machine.

1009
00:59:26,180 --> 00:59:29,540
And one side of the substrate
was contacted with the stamp,

1010
00:59:29,540 --> 00:59:30,510
and the other was not.

1011
00:59:30,510 --> 00:59:32,660
So there's a lot of
missing data here.

1012
00:59:32,660 --> 00:59:36,560
And at 100 C, 900
Newtons, 8 minutes,

1013
00:59:36,560 --> 00:59:38,480
we pretty much got
complete filling

1014
00:59:38,480 --> 00:59:39,590
of most of the features.

1015
00:59:39,590 --> 00:59:48,350
So what we also
did, I should have

1016
00:59:48,350 --> 00:59:51,380
mentioned it randomized
the order of the samples

1017
00:59:51,380 --> 00:59:54,330
and interspersed these
center point runs.

1018
00:59:54,330 --> 00:59:54,830
Mohammed.

1019
00:59:54,830 --> 00:59:57,140
AUDIENCE: [INAUDIBLE] with
feature sizes [INAUDIBLE]

1020
00:59:57,140 --> 01:00:00,680
small, that forces
[INAUDIBLE] [? 100 ?] Newtons,

1021
01:00:00,680 --> 01:00:02,210
does it damage the stamp?

1022
01:00:04,760 --> 01:00:07,750
PROFESSOR: In these experiments
we haven't damaged the stamp.

1023
01:00:11,750 --> 01:00:14,540
Actually, the
compressive stresses

1024
01:00:14,540 --> 01:00:19,400
experienced by the silicon
are nowhere near enough.

1025
01:00:19,400 --> 01:00:24,470
I mean, to break the
tensile strength of silicon

1026
01:00:24,470 --> 01:00:26,120
is over 100 mega pascals.

1027
01:00:26,120 --> 01:00:29,180
But actually where
the danger arises when

1028
01:00:29,180 --> 01:00:30,440
you're cooling the substrate.

1029
01:00:30,440 --> 01:00:32,240
And we'll get onto that.

1030
01:00:32,240 --> 01:00:36,710
But it's really
the lateral forces

1031
01:00:36,710 --> 01:00:40,190
applied to the protruding stamp
features that are dangerous,

1032
01:00:40,190 --> 01:00:43,820
because they create
moments on the features,

1033
01:00:43,820 --> 01:00:46,542
and can cause cracks at
the base of the feature

1034
01:00:46,542 --> 01:00:47,250
as it propagates.

1035
01:00:47,250 --> 01:00:50,370
So good question.

1036
01:00:50,370 --> 01:00:53,190
All right, so anyway,
interspersed center point runs.

1037
01:00:53,190 --> 01:00:56,250
And then what we've done
to provide some basis

1038
01:00:56,250 --> 01:00:59,070
for a test of significance
of the effects

1039
01:00:59,070 --> 01:01:03,750
is to take the average of
these peak penetrations.

1040
01:01:03,750 --> 01:01:06,090
So we've just taken the
mean over all feature

1041
01:01:06,090 --> 01:01:07,950
sizes, and orientations,
and everything,

1042
01:01:07,950 --> 01:01:11,260
and put it down here.

1043
01:01:11,260 --> 01:01:13,560
And so you can do an ANOVA.

1044
01:01:13,560 --> 01:01:15,060
And you can look
at the significance

1045
01:01:15,060 --> 01:01:17,300
of various effects.

1046
01:01:17,300 --> 01:01:23,830
And actually we get
the impression--

1047
01:01:23,830 --> 01:01:26,040
I mean, certainly temperature,
force, and hold time

1048
01:01:26,040 --> 01:01:26,730
is significant.

1049
01:01:26,730 --> 01:01:30,270
That is exactly
what we expected.

1050
01:01:30,270 --> 01:01:38,070
The question down here is, well,
at the 5% level, loading rate,

1051
01:01:38,070 --> 01:01:40,740
the time over which
that load is ramped up

1052
01:01:40,740 --> 01:01:44,230
is not appearing quite
to be significant.

1053
01:01:44,230 --> 01:01:49,800
But you know, we
don't know, because we

1054
01:01:49,800 --> 01:01:54,390
didn't make a very
judicious choice of aliasing

1055
01:01:54,390 --> 01:01:55,980
arrangements.

1056
01:01:55,980 --> 01:02:00,330
We don't know whether that's
really at the 7% level

1057
01:02:00,330 --> 01:02:04,920
significant that loading
time is a relevant factor,

1058
01:02:04,920 --> 01:02:07,560
or whether it's the interaction
of temperature, force, and hold

1059
01:02:07,560 --> 01:02:08,460
time.

1060
01:02:08,460 --> 01:02:10,500
Just thinking about
this intuitively,

1061
01:02:10,500 --> 01:02:13,020
you do expect the
interactions to be important.

1062
01:02:13,020 --> 01:02:16,710
You expect the product
of temperature and force

1063
01:02:16,710 --> 01:02:21,510
to be relevant, or some
non-linear combination

1064
01:02:21,510 --> 01:02:26,970
of the two parameters,
just because a temperature

1065
01:02:26,970 --> 01:02:30,930
imparts a strain rate
for a given force.

1066
01:02:30,930 --> 01:02:33,930
So increasing the force will
increase the strain rate

1067
01:02:33,930 --> 01:02:34,700
and so forth.

1068
01:02:34,700 --> 01:02:35,385
So anyway.

1069
01:02:38,530 --> 01:02:42,670
And also, as we probably
would have expected,

1070
01:02:42,670 --> 01:02:45,040
there's significant
curvature in the results.

1071
01:02:45,040 --> 01:02:51,070
And harking back to that graph
of modulus against temperature,

1072
01:02:51,070 --> 01:02:54,850
that's exactly what
we would expect.

1073
01:02:54,850 --> 01:02:57,940
Actually, I should have
highlighted when I showed you

1074
01:02:57,940 --> 01:03:05,740
this graph that there are trade
offs to be made in picking

1075
01:03:05,740 --> 01:03:08,140
these operating parameters.

1076
01:03:08,140 --> 01:03:11,290
On the one hand, you
might say I don't

1077
01:03:11,290 --> 01:03:14,170
want to heat the material
any hotter than is necessary.

1078
01:03:14,170 --> 01:03:17,820
I'd rather just press
as hard as I need to,

1079
01:03:17,820 --> 01:03:21,970
and maybe operate
at 120, 115 degrees

1080
01:03:21,970 --> 01:03:24,520
C where I can
deform the material.

1081
01:03:27,160 --> 01:03:31,150
And then because I'm not at
a very high temperature, when

1082
01:03:31,150 --> 01:03:33,250
I cool down, there isn't
going to be very much

1083
01:03:33,250 --> 01:03:35,890
differential thermal contraction
of the stamp on the substrate.

1084
01:03:35,890 --> 01:03:37,700
So I won't have big
residual stresses,

1085
01:03:37,700 --> 01:03:39,200
and I won't risk
breaking the stamp.

1086
01:03:39,200 --> 01:03:42,580
Now, that would be a really
logical thing to say.

1087
01:03:42,580 --> 01:03:46,000
And from a process
control perspective,

1088
01:03:46,000 --> 01:03:49,300
however, you can see that this
may not be a very good place

1089
01:03:49,300 --> 01:03:51,790
to operate, where
modulus is highly

1090
01:03:51,790 --> 01:03:53,440
sensitive to temperature.

1091
01:03:53,440 --> 01:04:00,190
And our personal experience
with lab apparatus-- and this

1092
01:04:00,190 --> 01:04:01,720
is probably different
in industry.

1093
01:04:01,720 --> 01:04:06,020
But controlling the temperature
is fairly difficult.

1094
01:04:06,020 --> 01:04:09,612
I mean, this is quite
an optimistic error bar,

1095
01:04:09,612 --> 01:04:11,070
in fact, to put on
the temperature.

1096
01:04:11,070 --> 01:04:17,570
So that's, I think, a really
relevant consideration

1097
01:04:17,570 --> 01:04:21,130
for choosing hot embossing
parameters, how sensitive we

1098
01:04:21,130 --> 01:04:24,598
are to temperature.

1099
01:04:24,598 --> 01:04:26,556
AUDIENCE: [? Given ?]
the temperature readouts,

1100
01:04:26,556 --> 01:04:28,900
plus minus 1 degree?

1101
01:04:28,900 --> 01:04:31,000
PROFESSOR: That's
what I estimated

1102
01:04:31,000 --> 01:04:33,460
for that particular
machine, yeah.

1103
01:04:33,460 --> 01:04:37,510
And I think that there
are also big challenges

1104
01:04:37,510 --> 01:04:39,760
to do with temperature
uniformity across [INAUDIBLE]..

1105
01:04:39,760 --> 01:04:43,850
Yeah, that's really something
that people are looking at.

1106
01:04:43,850 --> 01:04:50,950
So, in fact, the side of the
industry that's more advanced

1107
01:04:50,950 --> 01:04:53,170
is really the nano
imprint lithography side,

1108
01:04:53,170 --> 01:04:55,440
more so than the
micro embossing side.

1109
01:04:55,440 --> 01:05:00,490
And in that case, it's
more attractive to go

1110
01:05:00,490 --> 01:05:03,790
to very high temperatures, to
make the material behave more

1111
01:05:03,790 --> 01:05:07,240
like a viscous fluid than
like a rubbery material

1112
01:05:07,240 --> 01:05:08,860
or a viscoelastic.

1113
01:05:08,860 --> 01:05:13,840
So then temperature variation
isn't so much of an issue.

1114
01:05:13,840 --> 01:05:17,050
So you're right down in
the 4 megapascal range.

1115
01:05:17,050 --> 01:05:21,590
So but yes, good
point, absolutely.

1116
01:05:21,590 --> 01:05:26,320
Now, yes, there's one
really interesting thing

1117
01:05:26,320 --> 01:05:27,760
about these results.

1118
01:05:27,760 --> 01:05:33,520
And I mentioned that I had
two sets of each feature size.

1119
01:05:33,520 --> 01:05:36,400
And one was oriented vertically
on the stamp, and one

1120
01:05:36,400 --> 01:05:37,610
horizontally.

1121
01:05:37,610 --> 01:05:41,320
And I just put those on
there on the off chance

1122
01:05:41,320 --> 01:05:43,450
that there was anisotropy
in the materials.

1123
01:05:43,450 --> 01:05:46,180
And it turns out, with
this TOPAS sample, which

1124
01:05:46,180 --> 01:05:48,010
is a gift from the
supplier, there

1125
01:05:48,010 --> 01:05:50,380
is really strong anisotropy.

1126
01:05:50,380 --> 01:05:56,440
And so you can see that
the filled in symbols

1127
01:05:56,440 --> 01:06:02,710
are for an arbitrarily defined
orientation, 90 degrees,

1128
01:06:02,710 --> 01:06:04,850
and the open symbols
for 0 degrees.

1129
01:06:04,850 --> 01:06:07,360
So turning the feature
through 90 degrees

1130
01:06:07,360 --> 01:06:11,800
gives you this really big
difference in penetration.

1131
01:06:11,800 --> 01:06:18,025
And these results are
based on five replicates.

1132
01:06:18,025 --> 01:06:21,130
You can see the error bars
there at one standard deviation

1133
01:06:21,130 --> 01:06:23,950
of the five replicate results.

1134
01:06:23,950 --> 01:06:28,000
And there's definitely a
significant anisotropy.

1135
01:06:28,000 --> 01:06:35,050
Whether that's to do with
residual stress in the cast

1136
01:06:35,050 --> 01:06:37,930
sheet, alignment of
the polymer chains

1137
01:06:37,930 --> 01:06:43,390
because of some feature
of its processing, I mean,

1138
01:06:43,390 --> 01:06:45,750
you can start to see that
if materials like this

1139
01:06:45,750 --> 01:06:49,230
have such a big
orientation dependence,

1140
01:06:49,230 --> 01:06:54,100
and that is so dependent on
how they've been processed,

1141
01:06:54,100 --> 01:06:56,420
you can't necessarily know
the whole processing history

1142
01:06:56,420 --> 01:06:57,420
of what you're supplied.

1143
01:06:57,420 --> 01:07:01,140
It will be useful to
have these quick checks

1144
01:07:01,140 --> 01:07:04,440
of the properties of materials
that are given to you.

1145
01:07:04,440 --> 01:07:07,800
And you wouldn't be able to
get that information just

1146
01:07:07,800 --> 01:07:10,770
by a bulk compression
experiment.

1147
01:07:10,770 --> 01:07:14,430
So these carefully designed test
patterns are really important.

1148
01:07:14,430 --> 01:07:17,370
I should say that this is
anisotropy in the material,

1149
01:07:17,370 --> 01:07:18,160
not in the stamp.

1150
01:07:18,160 --> 01:07:24,000
Because if we rotate the
stamp relative to the sample,

1151
01:07:24,000 --> 01:07:25,140
the results reverse.

1152
01:07:25,140 --> 01:07:28,770
So it's definitely
in the material.

1153
01:07:28,770 --> 01:07:34,310
Anyhow, that was for TOPAS.

1154
01:07:34,310 --> 01:07:37,190
And I showed you a
few slides ago what

1155
01:07:37,190 --> 01:07:41,630
happens when hold
time is relevant,

1156
01:07:41,630 --> 01:07:43,040
when you've got plastic flow.

1157
01:07:43,040 --> 01:07:48,800
And we saw that the introduction
of this linear dashpot,

1158
01:07:48,800 --> 01:07:51,740
the scaling of the point
load response function

1159
01:07:51,740 --> 01:07:55,250
was a reasonably good
way of capturing that.

1160
01:07:55,250 --> 01:07:58,520
What we're working on
now is this ability

1161
01:07:58,520 --> 01:08:01,872
to capture yield
stress to iron out

1162
01:08:01,872 --> 01:08:03,830
some of the shortcomings
of the model, the fact

1163
01:08:03,830 --> 01:08:06,680
that those narrower features
actually filled more

1164
01:08:06,680 --> 01:08:09,460
than the model predicted.

1165
01:08:09,460 --> 01:08:14,520
And the other thing
that we're trying to do

1166
01:08:14,520 --> 01:08:18,060
is extend this simulation
approach to thin substrates

1167
01:08:18,060 --> 01:08:20,220
for the nano imprint
lithography case,

1168
01:08:20,220 --> 01:08:23,250
where this type of point
load response function

1169
01:08:23,250 --> 01:08:24,960
isn't correct.

1170
01:08:24,960 --> 01:08:27,600
The features are the
same size roughly

1171
01:08:27,600 --> 01:08:29,399
as the thickness
of the substrate.

1172
01:08:29,399 --> 01:08:32,819
You have to start thinking about
lateral transported material.

1173
01:08:32,819 --> 01:08:36,149
And that's quite
an exciting topic.

1174
01:08:36,149 --> 01:08:40,170
Now, we had a couple of
good questions about damage

1175
01:08:40,170 --> 01:08:42,960
to the stamp and
de-molding issues.

1176
01:08:42,960 --> 01:08:49,770
And this is perhaps
the biggest impediment

1177
01:08:49,770 --> 01:08:53,250
to the use of imprinting,
a microscale structure.

1178
01:08:53,250 --> 01:08:56,460
There's not that much of a
problem for nano imprint.

1179
01:08:56,460 --> 01:08:59,460
But microscale
structures, it's crucial.

1180
01:08:59,460 --> 01:09:04,890
And here are some examples of
problems that have occurred

1181
01:09:04,890 --> 01:09:06,340
during cooling and de-molding.

1182
01:09:06,340 --> 01:09:11,850
This is an SEM picture
of a PMMA component

1183
01:09:11,850 --> 01:09:15,390
that has been embossed
with a hexagonal post.

1184
01:09:15,390 --> 01:09:18,460
This was made from
silicon, in fact.

1185
01:09:18,460 --> 01:09:25,439
And if you imagine
the substrate being

1186
01:09:25,439 --> 01:09:31,020
mostly on this side of the
feature, cooled under load,

1187
01:09:31,020 --> 01:09:35,520
the polymer having at least
10 times the thermal expansion

1188
01:09:35,520 --> 01:09:40,890
coefficient as silicon,
it has pushed itself

1189
01:09:40,890 --> 01:09:42,970
against the sides
of the features.

1190
01:09:42,970 --> 01:09:46,950
And when we de-mold, we get
these really substantial

1191
01:09:46,950 --> 01:09:48,029
ridges.

1192
01:09:48,029 --> 01:09:52,350
I think this was embossed
at 130 degree C, 20 degrees

1193
01:09:52,350 --> 01:09:55,140
above the glass transition.

1194
01:09:55,140 --> 01:09:59,850
And so what we
haven't yet worked out

1195
01:09:59,850 --> 01:10:05,220
is whether these
contact stresses

1196
01:10:05,220 --> 01:10:07,020
are large enough
actually to push

1197
01:10:07,020 --> 01:10:15,170
the stamp off the substrate,
or whether it's just

1198
01:10:15,170 --> 01:10:19,760
elastic potential energy
stored in the polymer

1199
01:10:19,760 --> 01:10:22,650
that causes this ridge to
spring up when you peel

1200
01:10:22,650 --> 01:10:26,600
the stamp off the substrate.

1201
01:10:26,600 --> 01:10:28,020
Here are some
optical micrographs

1202
01:10:28,020 --> 01:10:32,240
showing a similar
problem, a triangular hole

1203
01:10:32,240 --> 01:10:35,180
embossed into a
polymer substrate.

1204
01:10:35,180 --> 01:10:36,920
The material is
contracted from right

1205
01:10:36,920 --> 01:10:39,200
to left relative to
the stamp feature.

1206
01:10:39,200 --> 01:10:42,170
And in fact, sometimes
we see these shards

1207
01:10:42,170 --> 01:10:48,860
of polymer being sheared
off, and again, a hole where

1208
01:10:48,860 --> 01:10:53,540
there's been a few tens of
microns relative contraction.

1209
01:10:53,540 --> 01:10:56,630
AUDIENCE: I'm curious, is there
any sort of like lubricant

1210
01:10:56,630 --> 01:11:00,490
that you can use to
increase the load?

1211
01:11:00,490 --> 01:11:04,760
PROFESSOR: There
are products you

1212
01:11:04,760 --> 01:11:07,490
can get that you can
spray onto the mold,

1213
01:11:07,490 --> 01:11:10,070
or, indeed, treatments that
can be given to the mold

1214
01:11:10,070 --> 01:11:11,900
to reduce the
coefficient of friction

1215
01:11:11,900 --> 01:11:16,370
between the mold
and the substrate.

1216
01:11:16,370 --> 01:11:23,150
That won't reduce the
magnitude of the lateral force.

1217
01:11:23,150 --> 01:11:26,270
That's to do with Young's
modulus and the coefficient

1218
01:11:26,270 --> 01:11:27,470
of thermal expansion.

1219
01:11:27,470 --> 01:11:32,120
But yes, it will
potentially make it easier

1220
01:11:32,120 --> 01:11:37,130
once that lateral force is
exerted to pull the stamp out.

1221
01:11:37,130 --> 01:11:40,700
However, it's not-- the problem
isn't all to do with friction.

1222
01:11:40,700 --> 01:11:45,230
Sometimes you have-- either
your stamp is imperfectly made,

1223
01:11:45,230 --> 01:11:48,330
and there's a negative draft.

1224
01:11:48,330 --> 01:11:51,920
I mean, I exaggerate, but if
that were the stamp and that

1225
01:11:51,920 --> 01:11:53,810
were the polymer,
that's a problem

1226
01:11:53,810 --> 01:11:55,610
that people run
into quite a lot.

1227
01:11:55,610 --> 01:11:57,740
And then if the
stamp has been made

1228
01:11:57,740 --> 01:11:59,420
by deep, reactive
ion etching where

1229
01:11:59,420 --> 01:12:02,810
there's this cyclic
etching process,

1230
01:12:02,810 --> 01:12:06,710
and there are these little,
sub micron scallops,

1231
01:12:06,710 --> 01:12:10,040
you're susceptible to
mechanical locking.

1232
01:12:10,040 --> 01:12:13,298
And there's plastic
deformation of the polymer

1233
01:12:13,298 --> 01:12:14,090
as you pull it off.

1234
01:12:14,090 --> 01:12:16,700
So it's a good point.

1235
01:12:16,700 --> 01:12:19,790
Yes, and actually, one thing
that does work quite well

1236
01:12:19,790 --> 01:12:26,390
is to leave the Teflon
polymeric coating on the silicon

1237
01:12:26,390 --> 01:12:27,260
after etching.

1238
01:12:27,260 --> 01:12:29,360
And that does do quite a lot.

1239
01:12:29,360 --> 01:12:31,040
It has a very limited
lifetime, though.

1240
01:12:33,920 --> 01:12:37,310
Anyway, the point
of saying all this

1241
01:12:37,310 --> 01:12:41,270
is, as I alluded to
a few minutes ago,

1242
01:12:41,270 --> 01:12:46,460
you might think of
reducing the temperature

1243
01:12:46,460 --> 01:12:48,680
swing to a small
a value as you can

1244
01:12:48,680 --> 01:12:53,640
so that amount of thermal
contraction is restricted.

1245
01:12:53,640 --> 01:12:57,680
You might even think of
removing the load slightly

1246
01:12:57,680 --> 01:13:00,190
above the glass
transition temperature.

1247
01:13:00,190 --> 01:13:03,470
And if you're in a
region where some

1248
01:13:03,470 --> 01:13:06,020
of the deformation, or
most of the deformation

1249
01:13:06,020 --> 01:13:09,470
is actually plastic,
which is true in a region

1250
01:13:09,470 --> 01:13:12,710
about 10 degrees above
glass transition for PMMA--

1251
01:13:12,710 --> 01:13:17,540
if you can exploit the
material properties to avoid

1252
01:13:17,540 --> 01:13:19,700
having a big temperature
swing, then that

1253
01:13:19,700 --> 01:13:21,320
could potentially
be very exciting.

1254
01:13:21,320 --> 01:13:24,230
And together with
Matt Dirks, who's

1255
01:13:24,230 --> 01:13:27,560
in the lab for manufacturing
and productivity,

1256
01:13:27,560 --> 01:13:30,890
we looked into this a
couple of years ago,

1257
01:13:30,890 --> 01:13:37,670
and found that if we control the
temperature really carefully,

1258
01:13:37,670 --> 01:13:39,880
then it was promising.

1259
01:13:39,880 --> 01:13:44,750
If you de-mold at 50 degrees C,
so well below glass transition,

1260
01:13:44,750 --> 01:13:50,870
and then pull the stamp off--
this is a cross-section through

1261
01:13:50,870 --> 01:13:52,190
the part--

1262
01:13:52,190 --> 01:13:54,710
then you get this
horrendous ridge

1263
01:13:54,710 --> 01:13:57,440
on the side of the
part nearer the edge,

1264
01:13:57,440 --> 01:14:00,320
the side of the feature
nearer the edge of the part.

1265
01:14:00,320 --> 01:14:05,570
If you de-mold, say, well
above glass transition, 120,

1266
01:14:05,570 --> 01:14:07,520
you get this blue line.

1267
01:14:07,520 --> 01:14:11,180
So you get some bowing of
the base of the feature where

1268
01:14:11,180 --> 01:14:13,640
material has sprung back.

1269
01:14:13,640 --> 01:14:16,100
And you get splaying
of the sidewalls.

1270
01:14:16,100 --> 01:14:19,550
And this was repeatable
over many samples.

1271
01:14:19,550 --> 01:14:22,520
However, if we can hit
110 degrees C on the nose,

1272
01:14:22,520 --> 01:14:23,840
we get a nice, flat feature.

1273
01:14:23,840 --> 01:14:25,040
We get nice, flat sidewalls.

1274
01:14:25,040 --> 01:14:27,240
And we get no ridge at all.

1275
01:14:27,240 --> 01:14:29,450
And that would be great.

1276
01:14:29,450 --> 01:14:31,850
But again, that's the thing.

1277
01:14:31,850 --> 01:14:35,240
Can you, in a
production setting,

1278
01:14:35,240 --> 01:14:38,270
make it 110, and not 115 or 120?

1279
01:14:38,270 --> 01:14:42,650
And that, I think, is
probably possible to achieve,

1280
01:14:42,650 --> 01:14:45,818
but would increase the
cost of the apparatus.

1281
01:14:45,818 --> 01:14:51,600
So this is definitely
something to be aware of.

1282
01:14:51,600 --> 01:14:55,880
Another thing that
is of importance

1283
01:14:55,880 --> 01:14:58,280
is that the risk
of the whole part

1284
01:14:58,280 --> 01:15:01,260
bowing if the de-molding
temperature is too high.

1285
01:15:01,260 --> 01:15:04,380
We've observed
problems with that.

1286
01:15:04,380 --> 01:15:07,280
So that needs to be
thought of as well.

1287
01:15:07,280 --> 01:15:11,600
Here we can see
also that if you're

1288
01:15:11,600 --> 01:15:15,950
going to remove the load at
or above the glass transition

1289
01:15:15,950 --> 01:15:19,370
temperature, you have
to pull out the stamp,

1290
01:15:19,370 --> 01:15:20,930
cool it down jolly quickly.

1291
01:15:20,930 --> 01:15:25,310
Because otherwise the part
will start to contract back

1292
01:15:25,310 --> 01:15:27,900
to its original flat surface.

1293
01:15:27,900 --> 01:15:30,110
And so you leave
it for 10 minutes,

1294
01:15:30,110 --> 01:15:32,120
things start to
look pretty poor.

1295
01:15:35,750 --> 01:15:42,860
OK, that is an overview of
about half of my thesis.

1296
01:15:42,860 --> 01:15:45,650
And since we're at half past
9:00, I think we should stop.

1297
01:15:45,650 --> 01:15:51,170
But are there any
questions from anyone?

1298
01:15:51,170 --> 01:15:56,873
If not, then I guess the
floor is open for questions

1299
01:15:56,873 --> 01:15:57,665
about the projects.

1300
01:16:00,990 --> 01:16:03,240
PROFESSOR: [INAUDIBLE]
mention one thing [INAUDIBLE]..

1301
01:16:06,580 --> 01:16:11,290
I think later today, hopefully,
or by tomorrow at the latest,

1302
01:16:11,290 --> 01:16:14,770
I'll post the tentative
schedule for what

1303
01:16:14,770 --> 01:16:19,940
groups will be presenting on
each of Tuesday and Thursday.

1304
01:16:19,940 --> 01:16:22,640
It will be in the
normal class period.

1305
01:16:22,640 --> 01:16:24,647
So we'll plan for that.

1306
01:16:24,647 --> 01:16:26,230
And I'll have some
other instructions,

1307
01:16:26,230 --> 01:16:32,170
like it'll be best, if possible,
to send those presentations

1308
01:16:32,170 --> 01:16:33,430
to me a little bit early.

1309
01:16:33,430 --> 01:16:36,940
I see we do have a USB port.

1310
01:16:36,940 --> 01:16:42,560
So it's possible we can add up
the presentations on the fly.

1311
01:16:42,560 --> 01:16:45,190
I don't know quite how well
that works out in Singapore.

1312
01:16:45,190 --> 01:16:47,000
We'll have to figure
that out as well.

1313
01:16:47,000 --> 01:16:48,310
But be alert.

1314
01:16:48,310 --> 01:16:51,790
Some of you, based
on random draw,

1315
01:16:51,790 --> 01:16:54,280
will be doing
presentations on Tuesday.

1316
01:16:54,280 --> 01:16:57,550
And some groups will be
doing that on Thursday.

1317
01:16:57,550 --> 01:16:58,450
Are there questions?

1318
01:16:58,450 --> 01:16:59,950
AUDIENCE: Is there
a rough guideline

1319
01:16:59,950 --> 01:17:02,240
for how long the IEEE
letter should be?

1320
01:17:02,240 --> 01:17:04,375
PROFESSOR: Oh, the IEEE paper?

1321
01:17:04,375 --> 01:17:07,060
AUDIENCE: Yeah.

1322
01:17:07,060 --> 01:17:08,830
PROFESSOR: I think typically--

1323
01:17:08,830 --> 01:17:11,980
whatever it takes to
do a reasonable job.

1324
01:17:11,980 --> 01:17:14,230
I think something
typically on the order

1325
01:17:14,230 --> 01:17:18,250
of six, seven, eight
pages is, I think,

1326
01:17:18,250 --> 01:17:20,330
what we often saw in the past.

1327
01:17:20,330 --> 01:17:22,370
Sometimes it might be
a little bit shorter,

1328
01:17:22,370 --> 01:17:24,760
depending on how much
background you have.

1329
01:17:24,760 --> 01:17:28,060
So I don't have a
strict page guideline.

1330
01:17:31,440 --> 01:17:31,940
Good.

1331
01:17:31,940 --> 01:17:36,340
Any questions in
Singapore on that?

1332
01:17:36,340 --> 01:17:37,367
OK, great.

1333
01:17:37,367 --> 01:17:38,200
AUDIENCE: Professor.

1334
01:17:38,200 --> 01:17:38,825
PROFESSOR: Yes.

1335
01:17:38,825 --> 01:17:42,050
AUDIENCE: We have a
question for our project.

1336
01:17:42,050 --> 01:17:45,970
We actually have two questions.

1337
01:17:45,970 --> 01:17:51,990
So one question is that we
have two outputs in our--

1338
01:17:51,990 --> 01:17:53,220
can you hear me?

1339
01:17:53,220 --> 01:17:56,220
PROFESSOR: Yeah,
speak up a little bit.

1340
01:17:56,220 --> 01:18:01,080
AUDIENCE: OK, so one question
is that we have two outputs,

1341
01:18:01,080 --> 01:18:03,600
and then we are
measuring the data.

1342
01:18:03,600 --> 01:18:07,470
But these two outputs, they
are negatively correlated.

1343
01:18:07,470 --> 01:18:11,250
And our objective is
to maximize one output

1344
01:18:11,250 --> 01:18:13,320
and minimize the other output.

1345
01:18:13,320 --> 01:18:15,195
So we understand that
we can do it separately

1346
01:18:15,195 --> 01:18:17,970
in our optimization.

1347
01:18:17,970 --> 01:18:19,935
But when we want
to sort of create

1348
01:18:19,935 --> 01:18:24,510
a combined an objective
function and optimize these two

1349
01:18:24,510 --> 01:18:28,920
together, as they come from
the same set of inputs, how can

1350
01:18:28,920 --> 01:18:30,363
we do that?

1351
01:18:30,363 --> 01:18:32,280
PROFESSOR: Yeah, so the
classic approach there

1352
01:18:32,280 --> 01:18:33,990
is it's a trade off.

1353
01:18:33,990 --> 01:18:34,590
Right?

1354
01:18:34,590 --> 01:18:35,890
One or the other.

1355
01:18:35,890 --> 01:18:40,260
And what you typically
do is apply a weight

1356
01:18:40,260 --> 01:18:42,760
to the two alternatives.

1357
01:18:42,760 --> 01:18:44,370
So you have a different
numeric weight

1358
01:18:44,370 --> 01:18:49,920
based on your engineering
importance, or the customer

1359
01:18:49,920 --> 01:18:53,200
importance, or whatever
of the two effects.

1360
01:18:53,200 --> 01:18:56,880
So you have one function
that combines, say,

1361
01:18:56,880 --> 01:18:59,760
squared deviations
in the two outputs,

1362
01:18:59,760 --> 01:19:01,755
but with a different
weight on the two outputs.

1363
01:19:04,290 --> 01:19:06,170
AUDIENCE: OK, I see.

1364
01:19:19,130 --> 01:19:20,106
PROFESSOR: OK?

1365
01:19:20,106 --> 01:19:22,567
You're all set?

1366
01:19:22,567 --> 01:19:24,400
AUDIENCE: We have another
question actually.

1367
01:19:24,400 --> 01:19:26,212
PROFESSOR: Oh, OK.

1368
01:19:26,212 --> 01:19:27,670
AUDIENCE: I have
one more question.

1369
01:19:27,670 --> 01:19:31,510
It's regarding the
email you sent us.

1370
01:19:31,510 --> 01:19:35,560
So we were told that we can
explore one factor [INAUDIBLE]

1371
01:19:35,560 --> 01:19:37,430
strategy.

1372
01:19:37,430 --> 01:19:42,370
We are not quite sure what
strategy you are referring to.

1373
01:19:42,370 --> 01:19:46,510
Is it studying the output
and one input at a time?

1374
01:19:46,510 --> 01:19:49,210
Or is it more like
stepwise regression?

1375
01:19:49,210 --> 01:19:54,520
PROFESSOR: No, I'm referring
to the optimization approach,

1376
01:19:54,520 --> 01:19:59,860
the One Factor At a Time, OFAT,
that Professor Dan [? Frey ?]

1377
01:19:59,860 --> 01:20:04,220
talked about in his lecture.

1378
01:20:04,220 --> 01:20:06,400
So where you
explore the corners.

1379
01:20:06,400 --> 01:20:09,520
You look and see if
you've improved it or not,

1380
01:20:09,520 --> 01:20:11,710
to decide if you keep that move.

1381
01:20:11,710 --> 01:20:13,720
And then once you
do make a move,

1382
01:20:13,720 --> 01:20:16,750
then you randomly
decide which next corner

1383
01:20:16,750 --> 01:20:19,270
to experimentally try.

1384
01:20:19,270 --> 01:20:24,550
So you've actually got the
data already done from a DOE.

1385
01:20:24,550 --> 01:20:27,010
But now you can pretend
you were actually

1386
01:20:27,010 --> 01:20:31,480
running the experiment just one
experimental point at a time

1387
01:20:31,480 --> 01:20:33,640
using the OFAT approach.

1388
01:20:33,640 --> 01:20:36,010
And I think that
would be interesting

1389
01:20:36,010 --> 01:20:41,330
for online optimization to
compare also to response

1390
01:20:41,330 --> 01:20:45,070
surface modeling optimization.

1391
01:20:45,070 --> 01:20:46,420
AUDIENCE: OK.

1392
01:20:46,420 --> 01:20:47,560
All right, thanks.

1393
01:20:47,560 --> 01:20:49,390
PROFESSOR: Great, all right.

1394
01:20:49,390 --> 01:20:51,360
See you guys later.