So this is negative resist (because you have a post exposure bake). It is
common for undercut appearing in negative resist. It is because the PEB
temeperature or time is not enough and use hotplate instead of convection
oven(oven T and time parameters can't be directly used for hotplate,
hotplate time should be much shorter).
----- Original Message -----
From: "Leidong Mao" 
To: 
Sent: Tuesday, January 13, 2004 7:04 PM
Subject: [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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