By "dry skin" effect, I assume you mean wrinkles. Since they only occur in
the exposed area, this indicates insufficient crosslinking of the exposed
resist.
Either the post-exposure bake is not enough or your exposure dose is
insufficient (probably the exposure dose is the issue in my experience).
As for expiry date - 1 month is nothing. As long as it is stored OK it
should last well beyond the stated date.


On 12 January 2013 22:36, Bill Chow  wrote:

> Hi,
>
> I am working on a double layer structure with bottom layer 5um and top
> layer at 15um. I have used SU-8 2002 and SU-8 2010, however they have
> passed the expiry date for about a month (Would it be a problem?). In the
> last month, I was successed to make a beautiful sample out without any
> problem. However, Last week I tried to fabricate one more sample (due to
> misalignment of the structure in the first sample). I found there is a
> terrible problem arise. After I fabricated the first layer and spinned the
> 2nd layer, I put the sample on the 65C hot plate for soft bake. However,
> the sample shows a “dry skin” like structure between the interface of the
> first and second layer of SU-8 just after a few seconds on the hotplate. I
> don’t know why a previously worked process could failed after a few
> weeks.... I have try to use the same bottle of SU-8 2010 and let the sample
> to cool down to room temperature before the second layer softbake, however
> the problem remains. A very interest point is that, only the exposed area
> have the “dry skin” appearance, for the unexposed area it don’t have any
> problem. I hope someone could help me in this tough situation. Thanks.
>
> First layer 5um:
> Spin 3” Si wafer with SU-8 2002 at 500rpm for 5s, then 1000rpm for 30s.
> Softbake the sample  mailto:1min@65C then 3 min@95C (hotplate)
> Expose it without waiting for 25s with a total of 125mJ/cm^-2
> Postbake it  without waiting for 1min@65C and 1min@95C (hotplate)
>
> Second layer 15um:
> Spin the second layer without waiting with SU-8 2010 at 500rpm for 5s,
> then 2000rpm for 30s.
> Softbake the sample 2min@65C then 5min@95C (hotplate)
> Expose it without waiting for 45s with a total of 225mJ/cm^-2
> Postbake it without waiting for 1min@65C and 2mailto:2min@95C (hotplate)
>
> Regards,
> Bill
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