Vishwanath,

Here are a few tips to get you going.

1) With regards to your streaks:  They are probably due to particulates or
air bubbles.  I suggest blowing possible particulates out of your syringe
prior to filling it with the applicable SU-8.  Secondly, try to make sure
there are as few air bubbles as possible in your syringe before applying
the resist to the wafer.

2) To improve the uniformity of your wafer rotation, you can have clips
machined to fit the backside of your spin chuck.  Use 4 clips placed at 90
degree angles to center the wafer prior to spinning. This will help to
insure a uniform spread when the resist is properly applied to the center
of the wafer prior to spinning.

3) Use more SU-8. If the amount of resist applied is not enough to reach
the edge of the wafer and coat the substrate evenly then you should add
more SU-8.  The excess resist will simply be sheared off of the wafer if
you are spinning at sufficient rpm.

4) With regards to SU-8 2100: your right, its not easy.  Spinning thick
layers of SU-8 can take ALOT of resist.  Also, the uniformity of your coat
is not set in stone until after expsorure.  Even after proper baking, the
resist tends to reflow and level itself when the substrate is placed on a
non-level surface. The reflow is slow, but definite.  Always place SU-8
coated substates on level sufaces prior to exposure.

5) Finally, you mentioned something that SU-8 process engineers see time
and time again.  Waves present after the spin.  Most of these are
allieviated during the prebake.


Sincerely,

John D. Williams
PhD Candidate
Department of Mehcanical Engineering
Louisiana State University
2508 CEBA
Baton Rouge, LA 70893
daltonw@lsu.edu


Today's Topics:

1. Photoresist. (krishna)
2. Re: Grey Scale Mask (Yuebin Ning)
3. Re: mask material for wet etch glass (Yuebin Ning)
4. wet-etching of silver (Helen Li)
5. Re: cleaving of ultra-thin Si wafers (Kenneth Smith)
6. RE: cleaving of ultra-thin Si wafers (Tan Joo Lett)
7. Reg: Spin coating of Su-8 2025 (Vishwanath Somashekar)


------------------------------

Message: 7
Date: Mon, 21 Apr 2003 22:32:50 -0500 (CDT)
From: Vishwanath Somashekar 
Subject: [mems-talk] Reg: Spin coating of Su-8 2025
To: mems-talk@memsnet.org
Message-ID: <50322221310311101@webmail.iastate.edu>

Hi,
I am a grad student at Iowa State and have been involved very recently with
the
fabrication of channels for experiments in microfluidics. I take a 100mm
silicon wafer, clean it with ethyl alcohol and then rinse with water and
prebake it for some time. Then, I put the wafer on the spin coating
machine.
The diameter of the chuck on which this wafer rests is slightly smaller
than
that of wafer. so I am not able to place the wafer such that the center of
the
wafer and the center of the chuck match. As a result of this, when i run
the
spin coating, I can see kinda wobbling movement at low speed which tends to
vanish at high speed. The way i dispose of the resist is take a syringe and
measure about 4ml of SU-8 2025 and dipense off at the center. Then I ramp
at
100rpm/sec for like 5 sec and then will ramp to the desired rpm based on
the
thickness of the coating required. The problems that I am faced with are
1) after the spin coating, I see like streaks of su-8 strecthing from the
center and the coating is not uniform and its kinda wavy and sometimes, the
coating is not on the entire surface.
2) we also use su-8 2100 which is really viscous and when we spin coat, it
coating is really pathetic. Can you suggest me some technique to improve.
As I
am new to this fabrication, I might not have described my problem
technically.
I apologize for that.

Thanks very much in advance.

vishwa


Vishwanath Somashekar
3219, Ross Rd.,
Ames
IA 50014
Ph: 515-292-1766(Res)

****************************************************************************
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Drink deep, or taste not the Pierian Spring
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