Dear all,
Currently, I am manufacturing 2D-based semiconductors through semiconductor process. However, if metal is deposited on SiO2 or WSe2 after the photolithography process and lift-off is performed with acetone, some parts metals fall off. This problem was partially seen in Ni case, and the problem becomes serious when Ag is used. At first, it was thought that a problem occurred in the process of shining UV, so it was made by completely contacting the mask on the substrate, but it was not solved. Metal deposition was performed at a high vacuum of -6 or -7, and the substrate was cleaned again and PR or Developer was used as a new one.
Still, it hasn't been solved.
If you have experienced or solved the same problem, please help me.
Substrate : Sio2 OR Sio2/WSe2 (PMMA trasfer)
Resist : Positive PR, az5214E, soft bake 120'c for 10min after develop
Metal Deposition : Ag or Ni using E-beam Evaporation, 50~60nm,
0.1A/sec for rate
Lift off in acetone with ultra sonication
Have a nice day everyone looking at the questions. Thank you.
2023.03.01
I am attaching the picture for someone's understanding.
That's the result I got a while ago.
Thank you :)
I could imagine, that the issue is linked to the use of Acetone. Have you thought about using NMP instead of Acetone?
One potential solution to the lift-off problem when depositing metal on SiO2 or WSe2 after the photolithography process is to use a two-layered resist with an undercut. This involves first depositing a thick layer of positive resist (such as AZ9260) followed by a thin layer of negative resist (such as ma-N2403) on top. The negative resist is then patterned and etched to create an undercut profile. The positive resist is then patterned on top, and the metal deposition and lift-off process can be carried out as usual. The undercut profile allows the metal to adhere more strongly to the substrate, preventing it from falling off during lift-off. Additionally, increasing the thickness of the metal layer and using longer sonication times during lift-off can also help improve the adhesion of the metal to the substrate. Additionally, it may be worth trying a different lift-off solvent such as N-methyl-2-pyrrolidone (NMP) instead of acetone, as this can sometimes improve lift-off performance. Finally, make sure that the metal deposition rate is not too high, as this can also contribute to poor adhesion and lift-off issues.
Deposit Au instead of Ag or Ni. The thickenss should be 1 micron not 50 nm.
Clean the surface before deposition with different solvents then clean with ultrasounds in boiled water for 30 minutes.
The problem of lift-off is less for Ni because Ni is used as an adhesion layer. So before depositing Ag, you need to use Ti/Cr/Ni for good adhesion.
You have two options, either:
use an LOR layer (double layer PR) to obtain an undercut profile, keep in mind that the thickness of the bottom LOR layer must greater the thickness of the deposited metal (≈80-100nm in your case).
OR
use a negative PR to have a side wall similar to an undercut shape (this, however, would require a photomask replacement if you are doing photolithography)
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