Juergen,
My guess is that the stress may primarily be due to the SU-8 material. Cured
SU-8 has a TCE of ~50ppm/C, while nickel has a TCE of 13.3ppm/C (silicon is at
~3ppm/C).
Since the process for SU-8 typically "fixes" its relaxed point at ~95C (both due
to the original soft bake, and the following Post Exposure crosslink bake), it
will shrink ~3500ppm to room temperature, while the Nickel will only shrink
931ppm, and silicon will only shrink 210 ppm. This results in a high level of
tensile stress in the SU-8 film.
Once the SU-8 is cross-linked, it is highly impervious to change (at least,
until it reaches its decomposition temperature), so it will probably not do any
good to perform thermal cycling at this point.
A few things that can be done to help minimize this:
1 - During the soft bake process, give the material a chance to "relax" at a
lower temperature. SU-8 has a glass transition temperature (Tg) of 55C. If,
after the 95C bake that is required for soft bake (to drive off the solvents),
you place the wafer for an extended period of time in an ~60 C bake (oven is
fine), this will allow the film to re-flow to "relax" at 60C. Then, the amount
of stress will be based on an ~1700 ppm reduction (half the stress).
2 - Once exposed, SU-8 can actually cross-link at room temperature - its just a
question of time (and reaction kinetics). As in most other chemical reactions,
the rate of reaction will tend to accelerate with increased temperature (thus,
SU-8 will cross-link in a matter of minutes at 95C). As before, if you perform
the PEB at 60C for an extended period of time, the stress induced by thermal
expansion will be minimized.
3 - If you are really adventurous, you could try to "balance" the stress between
the two films - i.e., if you have the capability of adjusting the temperature
during the nickel deposition (even just a little), you may be able to choose a
process temperature for the SU-8, and another for the nickel deposition (which
is, again, the point at which the nickel film will be "relaxed"), that will
cause the stress to balance when cooled to room temperature.
Best Regards,
Chad Brubaker
EV Group invent * innovate * implement
Technology - Tel: 480.727.9635, Fax: 480.727.9700 e-mail:
[email protected], www.EVGroup.com
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-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of [email protected]
Sent: Monday, December 13, 2004 6:33 AM
To: [email protected]
Subject: [mems-talk] stresses in Ni-SU8 laminates
Dear all,
I made a device comprising a layer of electroformed Ni (20um) and a
layer of SU8 (30um). After releasing the device (20mm x 1.5mm)
it shows significant bending of several mm from one side to the other.
Could anyone of you give me some advice how to reduce the bending
(which probably comes from internal stress in the laminate).
Would thermal cycling relieve the stress and reduce the bending?
Any help is appreciated.
Thanks and Merry Christmas,
Juergen
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