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--- photolithography [2022/09/13 11:30] – dublin
+++ photolithography [2024/03/19 14:27] (current) – dublin
@@ Line 15: / Line 15: @@
 (2)	Set spinner to 4000 RPM (ramp rate = 1000 R/s).
-(3)	Cleanly pipette P20 and drop onto chip; wait about 30 s. for solution to spread. Spin for 45 s.
+(3)	Cleanly pipette P20 and drop onto chip; wait about 30 s. for solution to spread. Spin for 45 s. NOTE: According to the manufacturer, adhesion promoter is not required for LOR. Skip this step if you're using LOR.
 (4)	Cleanly pipette LOR onto center of chip. Immediately spin for 45 s.
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 (7)	Place chip on hotplate at 115 C for 90 s (hard bake). Cool for 1 minute.
+(8) Edge bead removal: The edges of the chip will have built-up photoresist that is many times thicker than the rest of the chip. Edge bead removal is particularly important for fine features on small substrates, as edge beads prevent the chip from fully contacting the mask.
+Try putting a tiny amount of Remover PG on the tip of a swab and wiping it on a betawipe, then swab your edges at a 45 deg. angle to remove edge beads.
 (II) Exposure
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 Note: warning light will blink for about 1 minute. If it blinks longer the bulb needs to be replaced.
-(3)	Carefully insert mask, with purple (chrome) side facing sample. Add chip and align properly. Exposure time is 8-10 s. ~~Update Dublin 2022 ECE 418 aligner is no longer in use. The other aligner has less than half the light power, around 10 mW/cm^2, requiring twice the exposure time (ECE418: 3-4 s. New aligner, 8-10 s.)
+(3)	Carefully insert mask, with purple side facing sample. Add chip and align properly. Exposure time is 10-12 s.
-        If using LOR, expose for 10 seconds
+~~Update Dublin 2022: ECE 418 aligner is no longer in use. The other aligner has less than half the light power, around 6 mW/cm^2, requiring over twice the exposure time (ECE418: 3-4 s. New aligner, 10-12 s.)
 (4)	After all exposures, remove mask. Turn off bulb but leave N2 on until system is cool (30 minutes). If necessary, clean mask with acetone/IPA.
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 -Note S1813 is a positive photo-resist so exposure breaks bonds, allowing it to be washed away with developer
-(1)	Place chip in AZ300 bath for 20-45 s., constantly agitating. (Update Dublin 2022: 90s is consistently way too long for development. I find that 20s is almost always enough. Note that with LOR, you need to modify development time to [[fine_tuning]] your undercut thickness.)
+(1)	Place chip in AZ300 bath for 80-120 s., constantly agitating. (Update Dublin 2023: Note that with LOR, you need to modify development time to [[fine_tuning]] your undercut thickness.)
 (2)	Remove from developer bath and put into water for 60 s while agitating. This stops the development so the exact time isn't imperative.
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 === S1813 Recipe comments ===
-August 2022:
+May 2023:
-Development time for LOR: I found that using this recipe, but with 10 second exposure at 8 mJ/cm^2 and 15 seconds development gave a ~0.5 um undercut. - Dublin
+Development time for LOR: Baking LOR at 180C, 12 second exposure at 6 mJ/cm^2, and 100 seconds development gave a ~0.3 um undercut. - Dublin
 ===SU-8 Photoresist===
-  * SU-8 is a polymer like negative photoresist.
+  * SU-8 is a polymer-like negative photoresist.
   * We use SU-8 in liquid gated GFETs as a passivation layer to protect metal leads from the gate electrolyte.
   * Read the SU-8 process guide for SU-8 2002 [[https://kayakuam.com/wp-content/uploads/2019/09/SU-82000DataSheet2000_5thru2015Ver4-2.pdf|here]].