I used bi layer PMMA for patterning my device on SiO2, I patterned it using Raith 150 Two (Electron beam lithography tool), but after deposition of 80 nm Au I tried to liftoff using acetone but t\it has not liftoff. Can you please tel me the methods to do bi layer PMMA liftoff?
- Check your exposure parameters and the resulting structures
- How thick is your PMMA bilayer?
- Do you bake your resist too much?
- Can you put the structure into ultrasonication?
Hi Johannes I checked with resulting structure I was able to obtain expected structure. EBL parameters are
EHT=23KV beam current=30.05 pA
EHT=20KV beam current=36.36pA
Both the cases Dose factors are 150 to 350 at a step of 10
My PMMA bilayer is 300 nm thick. My PMMA coating procedure is as follows
1. Pre bake at 200c
2. PMMA 495 2% 4000rpm 40 sec
3. Bake at 180 c for 5 min
4. PMMA 950 2% 4000 rpm 40 sec
5. Bake at 180 c for 5 min
I did ultrasonication for 1 hr, still not a single part has lifted off.
We use heated acetone in our center, ~50-60 C (be careful not to heat the solvent excessively), and have observed that the time depends on the metal you deposit. Nickel tends to come out in 1-2 minutes with a couple micron features (even without heating), and gold takes~10 minutes before it comes off.
Maybe the deposition rate was too high and gold was very hot after covering your sample surface. In this case PMMA could be completely caked and can not be dissolved in any solvent. Can you tell us your deposition conditions?
Thank you all for your valuable advice. Is it because of baking time? The deposition rate was 0.5A/s only. I think this was not a high deposition rate. I tried with heated acetone (at 60 C) also but it was not working. I will try with plasma ashing.
Ok, as mentioned NMP might be worth a try. I would try to avoid plasma ashing.
Can you check that you have an undercut in your resist bilayer? (e.g. exposing lines, breaking the substrate and checking in SEM.) If not, a higher EHT acceleration voltage could help.
It might be also possible that your PMMA is too crosslinked that it gets removed. You can try then to reduce the baking time and - if the Au deposition is the reason - try reducing the Au deposition rate.
The deposition rate 0.5A/s should be OK. We use 2-3A/s, pressure 10-7 Torr, distance between evaporation source (gold) and substrate is 35 cm.
Which type of deposition technique do you use (thermal evaporation or sputtering)?
The sample holder should not rotates during deposition.
Methyl formate is more effective than acetone and also works for lightly radiation-crosslinked PMMA. See the method I posted on this site, "Method for PMMA dissolution in the presence of epoxy-based dielectric". However, it is as flammable as acetone and even more volatile (b.p. 31.5C) so proceed with caution.
Methylene chloride is also worth a try, if your institution allows you to use it.
Hello,
It looks like your dose factor of e-beam lithography is very large and the beam current is not very high. What is your dose? If the exposure amount is too high, PMMA may get overexposed and it will be very hard to be removed. Thanks.
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