Sonic,

1) Cracks like this are usually due to underexposure.

If possible, do an exposure matrix on a single wafer (expose different parts
of the wafer to different times, and develop all at once).

If you don't have the tools for that, I'd recommend just doubling your
exposure time, and walking it down until you find one that leaves your
features looking good without the cracks.

2) Also, are you sure you've actually got a 20 micron thick layer? The
asymptote on the spin curve for SU-8 50 is around 40-45 microns, as far as I
remember -- you can see up to 3000 RPM at
http://microchem.com/products/pdf/SU8_50-100.pdf

3) Your bake times may be a bit off. Again, data sheet is above, though note
that the times for a 20 micron thick layers on are a different data sheet
(for a more diluted SU-8 solution). The 45 minute cool down may be a bit
unnecessary, for this thickness, though I doubt it's causing your problem.

4) Finally, if the cracks don't go away, and your process can tolerate it,
just heat the SU-8 to 150 C for one to two minutes. They won't all go away,
but the largest ones will vanish, and there's very little reflow.

- Kevin

*********************************
Kevin Paul Nichols
MESA+ Institute for Nanotechnology
Mesoscale Chemical Systems
Meander 151
University of Twente
Postbus 217
7500 AE Enschede
The Netherlands

Office: +31 (0)53 489 26 31
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On 2/18/08 11:45 PM, "oygm"  wrote:

> 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 I¹ve 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?