Projects in Microscale Engineering for the Life Sciences

Labs

This page presents information on some of the primary techniques used in the lab portion of this class. All photographs are courtesy of Dr. Aranyosi.

Casting PDMS (PDF)

Casting Masters (PDF)

Coulter Counter (PDF)

Plasma Bonding (PDF)

Device Fabrication (PDF)

Filling Devices (PDF)

Reservoirs (PDF)

The base layer for the masters is a silicon wafer. Silicon wafers are used because they are extremely flat, so the pattern drawn in the mask will be replicated faithfully on the wafer. This image shows several silicon wafers stored vertically to minimize dust. This image shows Daniel opening the plasma cleaner. This device surrounds the wafers with an oxygen plasma, which is used to clean the surface and remove any impurities. Removing the wafers from the plasma cleaner. At this point they are molecularly clean, and should never be touched, even with gloved hands. Daniel holding the tray which has the clean wafers. A closer view of the clean wafers. This is about as clean as anything ever gets, folks! Notice how each wafer has a flat side — this side is used later to help position the wafer in the aligner. Dino inspecting the mask. The fab facility contains a video microscope to allow inspection of masks and of the wafers after fabrication. Notice that the mask is still in its sleeve — the sleeves containing the masks are only opened inside the fab, to reduce the chance that the mask gets dusty. SU-8 is used to make the master. The number (2035) indicates the type and viscosity of the SU-8. The viscosity is one of the factors that determines how thick of a layer you get. The foil-covered thing in the middle of this image is the spinner. A silicon wafer is centered on the spinner and held in place by vacuum. A glob of SU-8 is poured onto the wafer, as shown here. The wafer is then spun quickly to distribute the SU-8 evenly over the surface. In this and the next image, the brightness inside the spinner has been enhanced to make the features visible. The wafer here has a glob of SU-8 on the center. After spinning, most of the SU-8 ends up on the walls of the spinner, which is why they are covered in foil. A thin layer remains on the wafer. The speed at which the spinner is run helps determine the final thickness of SU-8 — the faster the spinning, the thinner the layer. For a 40 µm layer, the wafer is spun at about 10,000 RPM. After spinning, the wafer is transferred to a hot plate for a process called a soft bake. This process gets rid of the solvent that is mixed in with the SU-8 to keep it from hardening in the bottle. It also helps the SU-8 adhere to the wafer. To keep the hot plates clean, the SU-8 coated wafer is placed in the center of a larger wafer. This process ensures a good thermal contact between the hot plate and the wafer. The wafer stack is placed on the hot plate and allowed to bake for several minutes (depending on the thickness of the layer). It is then moved to the second plate, which is at a higher temperature. This image shows the two wafers on the hot plate. At this point there is no pattern visible on the wafer. The two hot plates are maintained at 66 and 97 degrees C, respectively. Inside the clean room the mask can be removed from its plastic sleeve. Here Daniel is showing students how one side of the mask contains the emulsion (i.e., has been printed on). The mask is held in place on the aligner, as shown here. To maximize the fidelity with which the mask pattern is transferred to the SU-8, the wafer is brought into direct contact with the mask. For this purpose, it is important to place the emulsion side down, so that the mask pattern is not separated from the wafer by the thickness of the mylar sheet. The wafer is placed on the disc on the right side of this image. This tray then slides under the mask. A joystick (far side of the aligner) is used to align the wafer and mask under microscopic observation. One of four in a series of images showing how the wafer is placed on the aligner. The flat side of the wafer faces the front of the aligner. The white plastic piece is flipped into place to allow precise placement of the wafer. Two of four in a series of images showing how the wafer is placed on the aligner. The flat side of the wafer faces the front of the aligner. The white plastic piece is flipped into place to allow precise placement of the wafer. Three of four in a series of images showing how the wafer is placed on the aligner. The flat side of the wafer faces the front of the aligner. The white plastic piece is flipped into place to allow precise placement of the wafer. Four of four in a series of images showing how the wafer is placed on the aligner. The flat side of the wafer faces the front of the aligner. The white plastic piece is flipped into place to allow precise placement of the wafer. Once the mask and wafer are in place and aligned, a UV light is turned on to expose the wafer. The light shines through the mask, so only the regions corresponding to transparent parts of the mask are exposed. Notice that everyone is wearing goggles to protect their eyes from harmful UV rays. A closer view of the exposure process. Here you can actually see the mask being hit with UV light. A view of the aligner/exposer from the other side. The paddle in the foreground is used to align the mask and wafer. The eyepieces are used during the alignment, but are faced away during the exposure. This is done because the eyepieces focus the light, so you could get an even more intense dose of UV by looking through them! After exposure the wafer is placed back on the hot plates for a process called a hard bake. Following that, they are immersed in SU-8 developer. This chemical removes the SU-8 that has not polymerized (i.e., has not been exposed to UV light). After this step the pattern on the wafer becomes visible. After a few minutes in the SU-8 developer, the pattern of the mask starts to emerge on the wafer. This pattern is actually formed from SU-8 that has remained on the wafter after the rest was washed away. When the wafer is done developing, the excess developer needs to be removed. Here, the developer is being blown off with nitrogen gas. This close-up of the previous image shows that the center of the wafer is dry, while the outer edge is still wet. This image shows another wafer being dried. This is a close-up of the previous image. The pattern of developer moving away from the center of the wafer shows that the visible structures are actually raised above the surface of the wafer. This image shows a device that has completely dried. It is now ready to be inspected with the microscope. Here, the team is inspecting one of the wafers. This particular wafer was made with the mask the wrong way up, so the features were less distinct than they should have been. This is an image of another one of the class devices. The features here are clearly well-demarcated. For reference, the thin channels leading off the edges of the screen have a width of 100 µm. Another of the class devices. This is the same design shown two photos above, but this one came from a master that was exposed with the mask the right way up. Here you can see a clear delineation between the 30 µm-wide and the 40 µm-wide parts of the three center channels. This wafer was exposed and developed properly, but for some reason the SU-8 did not adhere to the silicon wafer properly. As a result several of the designs are missing parts, some designs are missing entirely, and some are crooked. Making a new master from the same mask solved this problem. Yet another device designed by students in the class. This one came out beautifully, and all of the features were clearly resolved here. Daniel inspecting another part of the previous master. Here you can see the whole video microscope apparatus, with the wafer on the microscope stage. It is important to do this inspection in the clean room to avoid getting dust on the master.