PROTOCOL FOR PHOTOLITHOGRAPHY USING MYLAR MASKS qb3. 1 PROTOCOL FOR PHOTOLITHOGRAPHY USING MYLAR
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PROTOCOL FOR PHOTOLITHOGRAPHY USING MYLAR MASKS (IN BNC)
The following procedure describes steps to perform photolithography in the Biomolecular Nanotechnology Center (BNC) in Stanley Hall at the University of California, Berkeley. Specific SU-8 photoresist protocols can found on Microchem’s website. Refer to their manuals for specifics like spin speed, development time etc.
1. Before you enter the clean room in BNC, be sure to gown up. A typical gowning process includes putting on a full body coverall (bunny suit), hair cap, shoe covers, gloves and safety glasses. Be sure to sign in the log book before you enter the clean room.
2. Basic equipment to perform SU-8 photolithography include: Spin-coater for spinning photo resist (PR) on the wafer, hot plates for baking steps, UV-exposure setup for exposing the PR, mask contact aligners for multiple mask processes and an organic chemical hood for development and washing.
3. Go into the door adjacent to the clean room door and switch on the vacuum pump and nitrogen supply for the UV-exposure contact aligner system.
4. Go into the clean room and plug in the vacuum pump for the PR spin-coater. Switch on the spin-coater and set the temperature of the hot plate on the spinner to the required temperature. (Note: The hot plate on the spin-coater takes a long time to equilibrate and is also not very uniform. If stand-alone hot plates are available, you should use them.) Switch on the UV-exposure system and the UV- lamp. The UV lamp needs to warm up for at least 30 minutes before using it to expose your SU-8. [Note: Sign in the log book before using the UV light. After turning this UV light off, it should not be turned on again for 30 minutes.]
5. Go to the organic hood inside the clean room and setup two wafer-sized dishes for the PR development. One dish will contain the SU-8 developer and the other will be used for IPA wash. LABEL all the dishes, no unlabelled chemical should be in the hood. Pour out chemicals carefully inside the hood from bottles kept under the hood. Washing should be done in the sink. (Note: Only use the left hand partition of the sink as the right side is not connected to a waste outlet!!!!). Take out the PR bottle from the PR cabinet. It is wise to aliquot out PR into smaller bottles to reduce contamination or spoilage of the 500 mL stock bottles.
6. Program the spinner to the required speed of rotation using the front console. Click on PROG, enter the program number you wish to run (generally 9 or 8), browse through the program and edit the values of parameters to the ones required by the SU-8 manual for the required thickness.
7. Take out the silicon wafer from the wafer box. Handle it carefully using flat edged tweezers. The shiny side of the wafer is where the SU-8 structures would be formed. Clean the surface of the wafer with a N2 gun carefully.
8. Make sure there is aluminum foil lining the inner walls of the spin-coater before proceeding to spin PR. Place the wafer on top of the spin-coater chuck. Center it. Click on RUN and enter the program number to run. The spinner will first test the centering by rotating the wafer at a slow speed. You can visually inspect the wafer centering and alter it. You can retest centering by pressing 0.
9. When satisfied with the centering, use a dropper to dispense about 2-3 ml of PR onto the wafer. Close the hood of the spinner and press START to commence with the spinning as soon as you dispense the PR. (Remember: Start spinning as soon as the PR is dispensed in order to achieve uniform thickness!!!)
10. After the spinning is done, carefully take out the wafer using tweezers. Place it on
the hotplate (already preheated) at the temperature and for the time specified in the SU-8 manual. Remember to cover the face of the wafer when you are doing this soft bake. You can use an inverted Petri dish, a wafer cover or aluminum foil (shown below). Note: For thick SU-8 (>20 microns), it is advisable to bring down the temperature of the wafer slowly. You can achieve this by switching the hot plate off and let the temperature fall down to room temperature.
11. While the wafer is being cooled, setup the UV-exposure system. The UV-lamp should have been warmed up for at least 30 minutes. Measure the UV-power using a photometer. Using this value and the suggested exposure energy given in the manual, calculate the recommended time of exposure. For chrome masks use 1.5 times this value (Actual Exposure Time for Chrome Masks = 1.5 x Calculated Exposure Time from Microchem’s Manual); for Mylar masks use 3 times this value (Actual Exposure Time for Mylar Masks = 3 x Calculated Exposure Time from Microchem’s Manual). Setup the exposure time on the front console of the machine.
12. Take out the Mylar mask and identify the side with the ink. This is the side that should face the wafer. (A trick to figure the ink side is to take a blade or any sharp object and carefully scratch on the surface. DO NOT CUT THE MYLAR! ) Spread out a drop of water on the opposite side (non-ink side) to stick it onto the glass slab attached to the mask frame. Wipe out the excess water. (Note: It is interesting to note that for alignment purposes, it is best to keep the alignment markers on the left and right side. This is because the microscope is set up for this arrangement and so you will avoid unnecessary problems of alignment.)
13. Place the wafer on the chuck. Start N2 purge to keep the surface of the wafer
clean. Be sure to keep the flat edge of the wafer over the big hole on the chuck to your right, to allow for nitrogen flow.
14. Switch on the Substrate Hold lever to lock the wafer in place.
15. Bring down the mask frame and switch on the Mask Frame Hold lever to hold
the mask frame down.
16. Align the mask (using translation and rotation levers on the sides of the mask frame) so that all the features on the mask are centered on the actual wafer.
17. Switch off N2 Purge and switch on Contact. DO NOT change the contact pressure. You should be able to see interference patterns through transparent regions of the mask. Slide the mask frame under the UV lamp, turn on the UV source and look away!
18. After the programmed time, the UV-lamp will stop. Slide out the mask frame. Switch off CONTACT, switch on N2 Purge and switch off Mask Frame Hold. Switch off Substrate Hold and take the wafer off the chuck carefully using tweezers. Put the wafer on the hot plate and perform a Post Exposure Bake (PEB) for the specified time and temperature. You should be able to see your features appear on the wafer during this bake. (Note: For thick >20 micron SU-8 features, bring down the temperature gradually after the bake)
19. Develop the wafer in SU-8 developer in the chemical hood using Microchem’s suggested times. Stir the wafer and the solution periodically during development. DO NOT remove parts of the wafer from the developer during development; this will cause non-uniform development across the surface of your wafer.
20. Rinse the wafer with IPA for 10 seconds over the IPA waste dish and then rinse it with water for 10 seconds over the left partition of the sink. Dry the wafer completely using the N2 gun.
21. If this was the only mask, do a hard bake of the wafer at 100 ºC for an hour. If you have other masks, hard bake the wafer at 100 ºC for 10-15 min.
22. Use tape to cover the alignment marks before spinning another layer of SU-8. DO NOT put the adhesive ON the alignment marks. Use a smaller piece of tape to cover a small portion of the adhesive layer in the middle of a piece of tape (the tape will resemble a bandaid: not sticky in the middle, sticky on the perimeter). Make sure the non-sticky center is the part covering the alignment marks, otherwise the alignment marks could be pealed off when you remove the tape. Do not use excess tape. Too much tape on the feature surface will result in non- uniform SU-8 spreading near the features; too much tape below the wafer can lead to reduced vacuum holding the wafer to the spin chuck.
23. Perform steps 8-16.
24. To align the mask, first do a coarse alignment using naked eyes to put the two alignment marks over each other. Turn on the light for the alignment microscope. Pull the microscope over the mask frame. Adjust the two eye pieces to put them directly over the alignment marks. Use the focus knob on top of the microscope to first focus the right eye piece. Then use the focus knob to the left of the microscope to focus the left eye piece. Use the split view knob to the right of the microscope to fit both views in your field of view. Use the translational and rotational knobs to align the alignment mark on the wafer to the one on the mask.
25. Perform steps 17-22.
26. Repeat steps 23-27 for all the masks you need to expose the wafer to.
27. After all SU-8 layers have been developed, do a hard bake at 100 ºC for an hour.
28. Your mold is now ready! Be sure to silanize it before pouring PDMS on it.
a. Edge Bead Removal: When PR is spun on top of the wafer, the centrifugal force pushes the PR out from the center. At the edge of the wafer, two competing forces act on the PR. The centrifugal force pushing the PR off the wa