Giancarlo,

You may wish to examine the bake process used as well.  You do not give any
description of the process used, so if I mention anything here that you are
already doing, please do not feel offended.

Because of the relatively thick layer of resin that is being deposited, the
method for soft bake can be fairly critical.  Bear in mind that, depending on
the bake method used, additional difficulties can occur.

For instance, baking in a standard convection style oven present a problem when
working with thick films.  In an oven, generally, the top surface of the resist
is the first cured.  The will then create a diffusion barrier to prevent the
transport of additional solvent to the surface of the resist.  This will usually
require a much longer than normal bake time to ensure that all of the solvent is
removed.

Baking on a hotplate is the more ideal situation, but again you have to be
careful. If the wet resist encounters too large a change in temperature, the
material at the bottom of the resist film (in contact with the silicon) may
flash directly into vapor, causing bubbles (perhaps tiny ones not visible to the
eye) to form, since the thickness of the layer above this is too thick for the
solvent to escape directly.

The best method would be to perform a ramped proximity bake on a hot plate, so
that the initial heat-up step is controlled.  You may also want to increase the
bake, rather than decreasing it, to ensure that more of the residual solvent is
removed.


Best Regards,
Chad Brubaker

EV Group       invent * innovate * implement
Technology - Tel: (602) 437-9492, Fax: (602)437-9435 e-mail:
[email protected], www.EVGroup.com

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 -----Original Message-----
From:   Luigi Corti [mailto:[email protected]]
Sent:   Monday, August 11, 2003 8:16 AM
To:     General MEMS discussion
Subject:        [mems-talk] AZ4620 bubbles in the process

Hi everybody,
I have been working with the photoresist AZ4620. I do want to get 8um thick and
my unexpose area 10um-wide lines, which give a large exposed area.
My problem is that I always have bubbles after the exposure. I did split my
exposure time in several small periods with delays of 30sec in between. Even
though I still get those bubbles. I addition, I checked after the first second
of exposure and most of the bubbles are there by this time.  I start to thinking
about the baking time, and reduce it without success.
If anyone of you had work successfully with AZ4620, please can help with ideas
to solve this problems or with a working recipe. I really appreciate any input.
thanks

Giancarlo Corti,
Graduate Student,  Mechanical Engineering
University of Idaho, POBox: 0902
Moscow, ID  83844-0902
Phone: (208) 885-4996
email:  [email protected]
        [email protected]
http://www.uidaho.edu/~cort4258




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