I am facing problem of lift off. After pattern writing by EBL and metal deposition, stripping off PMMA I faced some problem. Either metal surface stayed on SiO2 wafer or it disappear completely.What is the solution?
Hello Sehtab Hossain ,
Are you able to provide some information on yoru current protocol? Do you perform any dehydration bake or steps to improve adhesion?
Jules
The wafer is silicon with 200nm SiO2 on top. The wafer is cleaned with Acetone,IPA, DI water then with Piranha solution and dried with Nitrogen. PMMA is coated with 4000 RPM and baked for 2 minutes. After EBL writing, the sample is developed with MIBK for 2 minutes and IPA for 35 seconds and washed with DI and Cr of 10 nm thickness is deposited with thermal evaporation. To strip of PMMA , acetone is used with ultrasonic agitation. here two problems arises:
1. Cr surface remain in the middle of the wafer where the pattern is written and I am not getting Lift off pattern.
2.All the surface washed away and i am not getting pattern
There could be many possibility
It seems
metal surface stayed on SiO2 wafer: This tells me, more likely the thickness of your resist is lower with respect to the thickness of deposited metal!
or it disappear completely : You might have many residues on the contact area avoiding the proper adhesion! try to do descum if it is not damaging process in your fabrication! And do a gentle lift off if you have nanoscale fabrication!
Dear Sedighe Salimian ,
PMMA thickness was 80 nm where Cr thickness is 10 nm.
When i should do the descum process? After MIBK development or after PMMA stripping?
For the case of a gentle lift off, would you please explain the procedure a little bit?
1-Pretreat your sample at 180oc for 5 min before exposure in order to increase the adhision of layer
2-Bake pmma at 180oc for 5 min or 120 for 15 min
3- Perform descum after development before metalization
low power like 30 w 1 min.
In case you only deposit 10 nm of Cr, there should not be any problem with the thickness of pmma. Make sure your exposure dose and development time is enough.
Immerse your chip overnight in ACE
OR
50 TO 60 OC 1 hr/ 1.30.
if you have large scale pattern metal layer easily peels off.
Very small scratch in borders before putting the sample in ACE always helps.
Good luck
Sedighe Salimian ,
After stripping PMMA, I am getting lift off Cr in scratched area but not in patterned area.
What is your parameter for exposure?
PMMA type?
Spinning ?
Baking?
Dose?
current?
Do you check developed pattern before metal deposition?
perform dose array on a large area in order to to figure the calibrated parameters in your case. Check the large area pattern under microscope after development. You should clearly see the SiO2 substrate on h developed part. If it works on large pattern it should work for your special design as well.
EBL parameter: 400 pA, 1.3 nC/cm dose
495MW PMMA A-2,
4000 RPM for 1 minute, thickness 80nm
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