I have been trying to resolve this issue for a long time now trying different strategies and still do not have a fix for this.
The problem:
I have a Si(p++)/SiO2 substrate with a 2D heterostructure consisting of hBN and TMDC. After exposure, I usually develop without a post-exposure baking step. I see cracks appearing only after the development process especially in resist where h-BN is under it. These cracks mostly appear starting from the edges of the pattern and propagate upto 50 microns in distance. I do not see any origin of cracks from the part where the resist is in direct contact with SiO2.
Things I have tried :
1. Different resist materials (PMMA, PMMA with copolymer EL11, ZEP520A). All resist have the same problem.
2. Using rounded corners in my pattern to avoid sharp features. The minimum feature size in my patterns is about 2 µm.
3. Post exposure bake.
4. I am already using low acceleration voltages of 10kV with 30µm aperture.
Can anyone help me with this?
have you tried to write a single rectangular lead only? I am think about overdose at the edges. What is your writing method, digital (dots) or analogue (lines)? You can try digital.
I agree with Muhammad, over-exposure can be a problem.
Do you have any images? Sometimes it can really help to diagnose.
You have already moved to lower accelerating voltages. Can you add a VERY thin metal layer on top of the photoresist to reduce charging effects? What are the other details of your exposure (dose/current)?
Furthermore; are you using any software for proximity effect correction?
You may try a more gentle development method using different (concentration) developer and/or development condition. Sometimes different pattern (size) can also have an influence.
Its pmma adhesion problem. Try avoid sharp corners in your design and do cold development in H2O:IPA mixure after the ebeam.
We had the same problem on PMMA/MMA/few-layer MoS2, especially the structure which has 1-2 um gaps in between the electrodes. As mentioned above we used cold IPA, and it worked.
Fahrettin Sarcan Hello, I am going to try H2O:IPA(7:3) developer, could you tell me the development time? The ebl resist I use is PMMA950/MMA EL9.
Thank you!
Dear Lin, nowadays, we are using Methyl isobutyl ketone and rinsing it in IPA. The developing time depends on the applied dose. But we use 35 sec in cold (14C) MIBK. If you are going to use cold IPA it takes longer (around 2 mins)
Fahrettin Sarcan Thank you for your answer, we've been using MIBK/IPA to develop samples and we found cracks in the area where EBL resist on the top of hBN. You recommended cold IPA and I give it a try, it works!
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