Hi

I would agree with the comments posted, in particular the exposure
dose. Although I more familiar with thicker layers (>100um)
insufficient exposure dose will lead to differential stresses between
the upper an lower portions of the layer particularly if you are not
filtering deep UV during exposure. The datasheets estimates of
exposure energy are quite low and are probably optimised for best
resolution rather than adhesion etc. It is quite difficult to
overexpose the resist so don't worry about big increases in energy.
If you are exposing large areas then you will always end up with a
highly stressed film. If you can, include stress relief structures to
reduce this (i.e. mask off any regions you can in the design).
As for the expiry date, as long as it has been stored reasonably well
it shouldn't cause a problem. I have used SU-8 which is several years
past it expiry date without problems. As Michael Larsson mentioned, it
may have lost some solvent but this would result in a thicker layer.
Reducing solvent evaporation during softbake seems like a good idea
too.
Cheers

Gareth


2008/2/18 oygm :
> Hi, everybody
> I tried to coat a 20um su8 mask on glass. Su8-50 is used in this process.
After developing, cracks are observed everywhere on su8. These cracks are run
through the entire su8 layer. It is really wired since I used the same protocol
and get succeed last year.
>
> Here is the procedure I've followed:
> Plasma cleaning―bakes 20mins @ 200c
> Spin coating
> Soft bake 3mins @65c, 15mins @95c, cool down to RT in 45mins
> Expose 250mj/cm2
> Post expose bake 1mins @65c, 4mins @95c, cool down to RT in 45mins
> Develop
>
> Also I noticed that the su8 which I used was expired last July. Is the
expiration the explanation for cracking? Or the incorrect exposure dose? Or
something else?
>
> Thanks
>
> --sonic
>