You probably have reflections from the SiO - Si interface. try using anti
relection coating (Brewer Science). if it's not good enough, use SU8 as a
resist (still need the ARC)
Shay

Original Message:
-----------------
From: Matteo Dainese matteo@imit.kth.se
Date: Wed, 30 Jun 2004 09:51:21 +0200 (CEST)
To: mems-talk@memsnet.org
Subject: [mems-talk] photolithography problem


Hello,
I would be happy if anyone can give me some
suggestions on this problem:
My purpose is to etch silica waveguides, which
means etch approximately 6 microns of doped SiO2,
and it is important that the sidewalls after etching
are very straight, ideally 90 degrees, because the waveguide
cross section should be a square.
To obtain this it is important that the photoresist mask
has also very straight sidewalls, but so far I cannot
make better than about 80 to 85 degrees, which is not enough.
I use Microposit SPR 220 3.0 with a thickness
of about 3 microns and the recipe (trying to
follow data sheets indications) is:

soft bake 115 deg 90sec
exposure (KarlSuss mask aligner, low vac contact) 270mJ/cm2
resting interval 30min
post expo bake 115deg 90sec
develop in Microposit CD-26 developer 40sec (in an automatic machine)

I have tried to vary the exposure energy, but without much success.
Anyway the geometry is transferred well, so I don't think I'm
overexposing, but the sidewalls are not straight...
Can someone tell me which is the most critical step
of the lithography process that affects the slope of the sidewalls?

Thank you for the help
Regards
Matteo

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