I agree that you should play around with the baking parameters, but not
necessarily the PEB.  PEB is required at a basic level to cross link.
Hard bake is the step at which you really change the degree of
crosslinking.  In some ways PEB can be thought of as a hard bake, but it
is generally better to use the recommended PEB time/temperature, and
then if further crosslinking is desired or necessary, play with the hard
bake.  The data I have from MCC shows that after PEB, the material is
roughly 95% converted by DSC.  After a hard bake of 250 C for 5 hours,
that goes to 100.  There is also a formula for plateau modulus that
depends directly upon temperature and crosslink density times a constant
divided by the molecular weight of the crosslinks, which can/will be a
comparatively small number depending upon thickness of the material.

So the thinner the material, hotter, longer you bake during/after PEB,
the more brittle the material will become.  As to why it increased with
time, not a clue unless it sat at an elevated temperature.


Best Regards,
Dustin Warren, PE

EV Group
invent * innovate * implement
Application Engineer - Direct: +1 (480) 305 2447, Main: +1 (480) 305
2400 Fax: +1 (480) 305 2401
Cell: +1 (480) 274 3894
E-Mail: D.Warren@EVGroup.com, Web: www.EVGroup.com

-----Original Message-----
From: Gareth Jenkins [mailto:gjenkins971@googlemail.com]
Sent: Thursday, May 07, 2009 5:35 AM
To: General MEMS discussion
Subject: Re: [mems-talk] Brittle SU-8

I imagine you could play with the baking parameters. Reducing the
soft-bake
should retain more solvent and reducing the PEB should lessen the extent
of
cross-linking. However, I am not sure what happens over longer periods
of
time (i.e. will it always reach a similar state after natural solvent
evaporation and cross-linking).

Perhaps you could try storing your devices to minimise long term
effects.
They could be immersed in a liquid to reduce further solvent out-gassing
and
stored in a fridge to reduce further cross-linking.


On Thu, May 7, 2009 at 02:27, Wong Wai Chi  wrote:

> Hi, everybody,
>
> I used SU-8 as devices material. I used PMMA as sacrificial material
and
> release by using acetone, i can get good material properties after
release,
> however, it become brittle after one week. did anybody face the same
> problem? how to solve it?
>
> thank
> WaiChi