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--- photolithography [2026/08/13 16:18] – gus
+++ photolithography [2026/08/27 12:37] (current) – [General Photolithography Procedures] joey
@@ Line 81: / Line 81: @@
 ===Photoresist removal===
+Prepare a bath of acetone (for single layer resist) or PG Remover (for bilayer resist) in a large crystallizing dish. The dishes labelled '50x100' will do for five or fewer chips. Ensure that the dish is filled approximately half way with removal solvent; Not using enough solvent can result in a high concentration photoresist solution, which can leave residue behind on the chips.
+Place the chips in the bath and cover with a watch glass. Liftoff can take as little as 30 minutes or as long as several hours depending on the process and patterning. When checking the liftoff status, gently swirling the bath can help remove loose metal from the surface of the chips. If needed, it can also be helpful to place the dish in the sonicator for 5 minutes to remove stubborn metal and photoresist.
+When liftoff is visibly complete, prepare a rinse bath of IPA (for single layer resist) or DDI water (for bilayer resist) in similar crystallization dishes. With bilayer resist, prepare three baths to be used consecutively; PG Remover is very stubborn and will not come off with the first rinse. With a wash bottle of the appropriate solvent (i.e. acetone or PG Remover) in hand, use tweezers to fish chips out one at a time. Do not let the chips dry in the air. Unless your chips have fragile materials (e.g. 2D flakes), aggressively spray them with fresh solvent using the wash bottle, spraying the whole chip over several times. This step has been shown to be important in removing lasting photoresist residue on the metal. When finished spraying, immediately place the chip in the rinse bath. If removing bilayer resist, repeat this process by spraying water and moving the chip from water bath to water bath. Once clean, these chips should become hydrophobic (i.e. water will bead on the surface).
+Finally, remove the chip from the rinse bath and (again, assuming there are no fragile materials on your chip) blow-dry the chip using N2. Examine your lithography under a microscope.
 Matt has documented that hot PG remover leaves less PR residue than any other method we have tried.
-We are also currently working on developing a good process for cleaning up chips after lithography using AFM cleaning or O2 inductive plasma (barrel ash).
+This liftoff procedure can still leave behind ~5 nm of photoresist residue on the substrate. If your device's tolerances allow for it, this residue can be cleaned using the O2 inductive plasma cleaner. To do this, first unseal the cleaner by turning the pump off. Place your chips on a blank glass slide and using tweezers place the slide in the center of the chamber. Reseal the chamber by replacing the lid and turning the pump back on. If closed, open the primary valve on the O2 tank.
+When the chamber pressure reaches ~150 mTorr, open the O2 valve directly next to the plasma cleaner. This should flood the chamber with O2, and the pressure should spike and then settle around 5-7 Torr. Ensure the RF dial is set to "off" and turn the RF power switch on. Quickly close the O2 valve and watch the pressure drop. When the pressure reaches ~1 Torr, turn the RF dial to "high". As the pressure continues to drop, you should see the O2 plasma appear and get brighter. 30 seconds after seeing the first dim plasma, turn the RF dial to "off" and flip the RF power switch off. You can now re-open the chamber to remove your samples. Be sure to leave the chamber under vacuum when you leave.
+Photoresist residue or other lithography defects can be optically invisible; it is recommended that you AFM your lithography as necessary.
+We are also currently working on developing a good process for cleaning up chips after lithography using AFM cleaning.