Christian,
The primary reason for the variable exposure time is because SU-8 has been
processed in so many different thicknesses, and on so many different surfaces,
that it is unlikely to be able to find a specific dose for one set of
parameters. In your case, you are doing 260 um on silicon. Since I've never
done exactly that thickness (and maybe nobody on this board has) I can't say
exactly what the best dose is. However, based on my experiences with similar
thicknesses (230 um, 300 um), I can say that the dose should be somewhere around
1000mJ/cm2. This would be as measured at I-line, since SU-8 is transparent to
g- and h-line, and should not be exposed to anything below I-line (<350nm).
A few additional guidelines when working with SU-8 2100:
1. I'd guess that you are spinning at ~1000 rpm. If so, you may not be getting
as good a surface uniformity as you'd like. The best way to deal with this is
to extend the soft bake time. Extended time at 95C will allow the SU-8 to reflow
(Tg = 55C); if the hotplate is level enough, it will greatly improve the
uniformity. Just make sure not to cool down too rapidly, as this can induce
stress in the film. If you do not have a controlled way to cool this down (some
sort of controlled cooling rate), your best bet would be to add in a third bake
step, which bakes for another few minutes at 65C.
2. Be careful with your mask design - try to avoid any sharp interior corners.
The thermal expansion of SU-8 is ~50 ppm/C, which is nearly twenty times that of
silicon. Since the point of neutral stress will be set by the soft bake, at
room temperature the su-8 will be under tensile stress (i.e., it will be
"stretched" over the silicon). Any sharp corners will be focus points for that
stress, and will most likely result in cracking.
3. With the exposure, bear in mind that SU-8 is negative acting, and is very
high transparency. What this means is A) Be careful to avoid underexposure - if
the film is underexposed, the foot of the feature will dissolve during develop
and B) overexposure is not as critical a factor as in many resist films. The
photoactive compound in SU-8 (which is chemically amplified) does not behave as
in typical amplified resists; it is not prone to migration within the unexposed
film over time. Instead, it fixes in place, effectively almost "freezing" the
exposure illumination.
4. Make sure to be careful with the PEB. Again, the "neutral" stress
temperature will be at whatever temperature the initial crosslink takes place
at. This is one reason why MCC recommends an initial 65C bake at the beginning
of the PEB - this sets the "neutral" temperature nearly as low as possible. You
also do not want to cool down too fast at this point, as it will thermally shock
the film, perhaps increasing the instance of cracks.
At this point, since I do not know the atuer of the difficulties you are
encountering, I cannot comment further, but hopefully, this helps a little bit.
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
This message and any attachments contain confidential or privileged information,
which is intended for the named addressee(s) only. If you have received it in
error, please notify the sender immediately and then delete this e-mail. Please
note that unauthorized review, copying, disclosing, distributing or otherwise
making use of the information is strictly prohibited.
-----Original Message-----
From: [email protected] [mailto:mems-talk-
[email protected]] On Behalf Of [email protected]
Sent: Thursday, May 06, 2004 7:27 PM
To: General MEMS discussion
Subject: [mems-talk] Processing of 260micron SU8 layer.
Hello All,
I was wondering whether anyone can help me. I am currently processing 260micron
SU8-2100 layers on Silicon substrates. I have tried the suggested procedure by
microchem but did not get the results I wanted. I have also gotten procedures
from
other groups in the literature but many are quite different to each other. I am
especially
concerned about the exposure times which varies a lot from different groups.
Has
anyone had experience in processing layers this thick with good results and
could share
their procedure or experiences.
Any input will be greatly appreciated.
Thank you.
Christian Antonio
Research Student
Industrial Research Institute Swinburne,
Australia.
_______________________________________________
[email protected] mailing list: to unsubscribe or change your list
options, visit http://mail.mems-exchange.org/mailman/listinfo/mems-talk
Hosted by the MEMS Exchange, providers of MEMS processing services.
Visit us at http://www.memsnet.org/
