I have optimized beam current, dose, voltage for intrinsic Si and I got the pattern. But when I use same parameters for P-type Si, I couldn't get good pattern. I see some layer (may be PMMA) on top of the pattern and some places I don't see pattern.
It is always good to optimize your exposure based on the exact type of substrate that you will use eventually. Since EBL generates tiny features, a little bit of difference might become significant.
Can you discribe this picture in more detail? I dont think that doses will vary greatly for different types of doping of Silicon substrate, beacuse the average atomic weight approximately the same. But you can get the problems with adhesion. If you give us more information, may be we can help you.
Is it an optical image? It seems that the resolution of you pattern is not so extreme. Intrinsic Si and doped Si have a different conductivity, some charge effect can slightly change the dose but I would suggest to check also pre and post-processing parameters.
For lower patterns by e-beam lithography sharpness of the patterns depends on nature of the substrate used to write patterns. Lower will be the back-scattered electrons sharper will be the patterns. So optimize your writing parameters (dose, current, spot size etc.) for different substrate.
you should take care of focusing of the sample, astigmatism and sample to beam distance.
Thank you all for your valuable replies.
I tried varying different process parameters, but still I got the same problem!
Then I checked the PMMA, it is EXPIRED ! :(
I'm going to try with new PMMA, I hope this clears the problem. I found in some papers that expired PMMA can cause problems which looks like dose problems!
Оh, i dont think that your problems connected with expired date. I have small bottle with old pmma which expired 1 year ago and it still ok. Can you tell me what the bright and dark regions on your image?
some suggestions
1. Clean the substrate carefully (sometimes, PMMA is not fully attached to the wafer if the substrate surface is not clean enough.)
2. Do the dose matrix test to make sure the exact EBL parameters on this substrate.
# we have no problem on EBL with P-type Si and expired PMMA (my PMMA is stored in the cleanroom.)
Dark region is PMMA and Bright region is SiO2/si.
Yes, PMMA is not that old, it should work still..
I'm using RIE (CF4+O2) for etching SiO2. May be CF4 hardening the PMMA?
Therefore, the picture shows the RIE results?
After EBL, did you check the patten?
Thank you all for your valuable suggestions :)
Dr.Chung: Yes, now I got the pattern!
It seems like problem with RIE (CF4+O2). I think PMMA is hardened during RIE and couldn't remove with acetone (even with Piranha!). So, I did O2 plasma etching after RIE. Its working great, I found good pattern.
Thank you all.
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