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=====Photolithography=====
We are using shared equipment in the OSU Semi-cleanrom. You can look through the big windows to see the impressive equipment. The lab is run by Chris Tasker, chris@eecs.oregonstate.edu. To use equipment in this lab you must be trained by someone in our group and then certified. Rick Presley is our main contact for certification, presley@engr.orst.edu.

The most important source of processing information is always the photoresist manufacture's data sheet (see the T: drive). For more technical advice you can also call from the photoresist company, [[http://www.microchem.com/|MicroChem]], (the company that distributes Shipley products). We have worked hard to get good/reliable recipes - some of the trials are [[fine tuning|documented]].

(Moved some [[old_photolith_recipes]])

==General Photolithography Procedures (in OSU cleanroom) Updated 2022 ==
(I) Spin-coating
-For bilayer processing (metal deposition) complete all steps below
-For single layer processing (graphene) skip steps 3 – 5

(1)	Place chip on hotplate at 115 C for about 3 minutes to remove any residual water. Cool chip on cooling plate for 1 minute.

(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. 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.

(5)	Place chip on hotplate at 190 C for 4 minutes (hard bake). Cool for 1 minute.
(Note that this affects the undercut: higher bake temp leads to lower undercut etch rate.)

(6)	Cleanly pipette S1813 onto center of chip. Immediately spin for 30 s.

(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

-Using Aligner in Owen 433 (The ECE-418 aligner broke)

(1)	Check N2 and compressed air pressure (gauges near door; expect about 40 psi and 80 psi, respectively). 

(2)	Turn on N2, compressed air, and bulb. Set power to 350 W, constant power. Let bulb warm up for 15 minutes. 
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 side facing sample. Add chip and align properly. Exposure time is 10-12 s. 

~~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.
  
(III) Development

-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 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. 
(3)	Dry with N2. 


=== S1813 Recipe comments ===

May 2023:
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.
  * 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]].
  * [[http://memscyclopedia.org/su8.html|This site]] has experimental information useful for trouble-shooting SU-8 processes.

===Cross contamination=== 
  * Be extremely careful when using CD-26 & MF-351 in the same lab. One drop of MF-351 in a gallon of CD-26 ruins the whole gallon! This problem was so bad that Shipley built a separate facility just to keep these away from each other.

===Photoresist removal===
Matt has documented that hot PG remover leaves less PR residue than any other method we have tried.
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