A master is used as a mold for PDMS microchannels and valves. Fabrication of an AZ 4620 master requires multiples steps. First the wafer is cleaned using a piranha etch. Next, in a clean room environment, AZ 4620 is coated twice and patterned using a positive photomask (twice for 20 microns, once for 10 microns). The patterned wafer is then developed, and its features verified under a microscope.
Wafer cleaning is accomplished by means of a piranha etch. See Piranha for materials and safety procedure. For this purpose, we'll need about 40 ml to fully immerse the wafer in the cylindrical glass dish. Be sure to manipulate the wafer with caution, as it is very fragile. Use the wafer tweezers, and keep your other hand underneath in case it slips. Once pirnaha solution is prepared, let the wafer sit fully immersed for 30 min. After, carefully remove it: rinse well with distilled water, then submerge in a distilled water bath (2nd glass dish) for 5 min. Since patterning requires a dust-free environment, you may now proceed to the cleanroom. Replace the water twice more, every 5 min. Finish with a nitrogen blow dry.
In the clean room, use a small dish to soak the wafer in AZ 400K developer for 2 min, then rinse with distilled water and blow dry with nitrogen.
Place both hot plates on the counter without aluminum foil. It is important that heating be uniform throughout the wafer. Make sure the aluminum-wrapped cover is also around, as it will be used to cap off the wafer as it bakes. Place the wafer on a hot plate for 30 min at 200°C. Allow for 5 min of cooling afterward by placing it covered with aluminum in the upside-down lid.
Use the aluminum foil to protect the top of the spin coater, and facilitate later cleaning. Turn on the power, vacuum and nitrogen feed. About 1 ml of HMDS.
5s @ 500 RPM, ACL = 075
Each layer will deposit 10 micrometer thick photoresist, so for 20 micrometers you'll need the 2nd layer as well. Set the program to R, or so as to have the following:
4s @ 500 RPM, ACL = 075
Place the wafer on the spin coater using the alignment tool, and vacuum. Pour enough AZ photoresist to cover most of the wafer, then close lid and run program.
Heat as described, then allow for 5 min cooling.
Set the program to S, or so as to have the following:
4s @ 500 RPM, ACL = 075
Insert wafer and add photoresist as before.
Same heating pattern. This time let it cool and sit 1 hr before exposing.
Place the wafer at the center of the support, and activate sample vacuum. Use the alignment XY to position the mask as the center of the wafer, and slide everything under the lamp for exposure. After UV, let sit 2 min.
Since development requires access to microscopes, it is best to move out of the cleanroom at this stage. Immerse the wafer in undiluted AZ400K developer (1:4) for 5 min. Rinse with distilled water, then carefully blow dry with nitrogen. Verify that development is complete by observing features under a microscope. If not, develop further.
In the clean room, bake as described.