Hi Brad,

The exposure dose I am shooting for is around 300mJ/cm^2 on 2.5 micron thick
resist, I am controlling by time not by actual dose, but I try to be consistent
in terms of exposure time and how long the lamp has been on, etc. I am using lab
hotplates, nothing made for resist processings (vacuum chucks etc.). Temperature
uniformity once the plate is at steady state isn't bad, ±3°F over my active
area, but I haven't checked with substrate in place. The substrate is also
0.060" glass, so it's very thick in comparison to a Si wafer. The %TMAH is the
standard 0.24N stuff, I believe it works out to be 2.4% but I'd need to check
the data sheet, Microposit 319. Do you suggest using a diluted developer? I
didn't have much luck when I tried 3:1 dilution in getting much development at
all.

I can run a bunch of samples at the same exposure and vary PAB/PEB to see how it
turns out. I am used to s18xx resists, and all that the baking is doing is
getting rid of casting solvent, exposure is most important, just varies a bit
depending on how much residual solvent there is. Do you happen to know if the
PEB is used to finish crosslinking, or for diffusion purposes as well (I
couldn't get a clear answer from the sales guy I spoke with)? I have had samples
where the image appeared to be reversed even. Mind you, this was just shaking
down the resist and trying to get a feel for it quickly, I have an OK procedure
now but I have found that it is really hard to get consistent results, but the
PEB and PAB haven't been my concern, I was playing with exposure dose. So maybe
I will try again, varying PEB. Is 150°C OK for PEB? I have been doing 4 minutes
@100°C for PAB and 10 minutes @150°C for PEB.

Thanks,
David Casale

-----Original Message-----
From: Brad Cantos [mailto:[email protected]]
Sent: Monday, May 10, 2010 7:37 PM
To: General MEMS discussion
Cc: David Casale
Subject: Re: [mems-talk] Futurrex negative lift-off resists

David,
Can you say a bit more about the process and equipment that you are using?  What
is your exposure dose (mJ/cm2)?  Are you using hot plates that are made for
photoresist processing or lab hot plates?  Have you characterized the
temperature uniformity of the hot plate?  of the substrate on the hot plate?
When you say full strength TMAH developer, what is the %-age of the TMAH in
developer, for example, 2.25%?

What I have found in general is that the PEB is a the most critical parameter,
and PAB perhaps just a bit less so, but also important.  If there is a non-
uniformity in the hot plate temperature, or if the surface is not machined flat
and you are not using a vacuum to hold the substrate, the substrate temperature
will vary and this will cause process non-uniformity.  Similarly, if your
substrate has poor thermal characteristics, additional attention must be paid to
be sure the resist-coated surface reaches the required temperature.  You mention
a 7x7" area, so if your substrate is not round, you may be getting resist on the
back side, again causing poor thermal contact with the hot plate and non-uniform
temperature transfer.  I think the process latitude should not be that narrow.

Brad