Qing,
     The total answer is to use image reversal.  I have experience with up to 30
micron thick resist, I am sure and will do tests with you to establish that 50
microns is O.K.  Think positive resist.  Reverse polarity mask.  Do the first
exposure which establishes the slope of the exposed area.  Then reverse, a warm
ammonia process that neutralizes the exposed resist.  Then flood expose and the
amount of flood expose determines the slope of the finished image.  Think of
going from a negative angle, through a vertical wall to a positive overhang just
by controlling the flood exposure.  Contact me directly for a number of
technical papers and how to do free tests.  Bill Moffat

-----Original Message-----
From: Qing Yao [mailto:qingyao@uiuc.edu]
Sent: Monday, February 16, 2004 9:19 AM
To: MEMS-talk
Subject: [mems-talk] thick photoresist development


Hi,

I have difficulty in patterning thick photoresist. The following is what I did:

1> coat a 4" Si wafer with 5 layers of AZ4620 using a spinner. The thickness of
each layer is about 10 microns and the total thickness of PR is about 50
microns. Bake the wafer at 110 degree centigrade for 1 min. after the deposition
of each single layer.

2> print the image pattern on a transparency (emulsion down) using a 5080-dpi
laser printer. put the transparency mask on top of the PR layer and put a 4"
quartz plate on top of the mask. Then do Fusion Flood exposure. the dose is
about 3300 mJ/cm2. Lots of bubbles are generated during the exposure process.

3> use 1:4 AZ400K to develop the PR for about 9 minutes.

I used both optical microscope and surface profilometer to measure the side
wall. The slope angle is about 50 degrees.

However, I need the slope angle to be close to 90 degrees (at least larger than
80 degrees) so that it is almost perpendicular to the substrate. What can I do
to achieve it? Would you please give me any suggestions?

by the way, the thick photoresist layer is sort of mold (for electroless plating
of Ni on Si substrate).




Best Regards,


Qing Yao
___________________
M&IE @ UIUC
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