The white areas you see indicate an under crosslinking condition.
Your PEB is fine, but your exposure dose is too low. Microchem recommends
400-500 mJ/cm2 for 50 um SU-8. Your maximum dose is 334 mJ/cm2. I recommend that
you double your exposeure dose (For the aligner I use, the recommended exposure
doses are too low). The sufficiently crosslinked SU-8 may very well peel off, as
SU-8 does not adhere well to glass.
If you still have undercutting after having increased the exposure dose, it
could be due to too much shortwave UV in the aligner. Your aligner must have a
filter that cuts away UV below 350 nm.
Jacques Jonsmann
________________________________
Fra: [email protected] på vegne af Leidong Mao
Sendt: on 14-01-2004 01:04
Til: [email protected]
Emne: [mems-talk] SU-8 undercut issue.
Dear all,
I am trying to fabricate 50um deep channels using SU-8 photoresist on
glass slides. The procedures are:
1. Piranha etch glass slides for 15mins
2. Bake glass slides at 200C for 10mins
3. Spin on 50um SU-8 resist, 0-5 sec ramp to 500rpm, keep it for 5 sec, then
ramp to 2000rpm in 5sec, keep it for 30sec)
4. Soft bake, 65C for 6mins, 90C for 23mins,
5. Cool for 5mins
6. Interval exposure, 8.35mW/cm^2 intensity, exposure for
11.5s+2s(interval)+11.5s.I also tried 15s+2s+15s and 20s+2s+20s.
7.PEB, 65C for 1min, 95C for 5min.
8. Cool for 5mins
9.Develop for 6mins, rinse with methanol, N2 blow dry.
Now the problems are: The 11.5+2+11.5 observed by naked eye white area along the
channels, which may be undercuts. Under microscope, there are two channel edges
for different microscope focus, I dont know how to determine if they are
undercuts or overcuts. I increased the exposure dose trying to eliminate those
"undercuts", for the 15+2+15 one, no white area along the channles, but still
"undercuts" under microscope, for the 20+2+20 one, things are worst. The resist
along the channels looks like going to peel off (that is the kind of "white
area" I mentioned).
All in all, I dont what went wrong. Are the exposure dose too much in all cases?
By the way, my SU-8 50 photoresist from microchem is going to expire on
Feb,2004. That is pretty close, could that be possibly the reason?
Any inputs here will be appreciated, thanks in advance!
best,
leidong
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