Hi Sonia,

The problem of sticking to your mask may not necesarily be to do with
baking. If you pre-bake too long, you could increase the stress in
your layer as it is less compliant during latter processing. What mode
of contact are you using on your mask aligner? Pressure, vacuum,
proximity...? Try using pressure/proximity to see if this helps.
Another thing you could try is increasing the separation gap between
the sample and mask on your mask aligner. There should be a needle
valve to do this. It may be that it is configured for thin resist
layers.

Regarding undeveloped regions - try attaching a filter to block out
spurious UV wavelengths, thus only exposing your SU-8 to the
wavelength band to which it is sensitive (see datasheet). For thick
SU-8, it is quite common to use such a filter.

Let us know how you get on... and good luck!

Regards,

Michael

>> Dear All,
>
> I am having quite a lot of difficulty developping the recipe for 150microns
> thick layers of SU8 2075 on fused silica substrated sputtered with gold
> (4inch wafers). My main problems are:
>
> (a) Sticking of the wafer to the mask.
> (b) Some non-developped stuff stack to my patterns and I cannot get rid of
> that only if I reduce the dose and then, I begin havng some initiation of
> adhesion problems.
>
>  My process is as follows:
>
> 1. Spin Omnicoat: I spin 5 layers of Omnicoat to try to get a thick layer so
> that it is easier to remove the SU8 after my process. Each layer is spin at
> 3000rpm (as described in the Omnicoat data sheet) and baked for 30sec at
> 200C. After the 5 layers are spun, the total stack is baked for 1min at
> 200C.
>
> 2. Spin of SU8 2075: I spin it to a 150microns thickness. The coatings look
> normally pretty nice and uniform.
>
> 3. Soft-bake: I place my samples on the hotplate at 40C and ramp it up
> slowly to 95C. Keep it at 95C for 45min and then ramp it back down to 40C..
> The ramps are the key for us to avoid inducing cracking and lifting of the
> SU8 during developing.
>
> 4. EBR using acetone at 500rpm/sec for 70sec
>
> 5. Rest time: 3h
>
> 6. Exposure: 30sec (12mW/cm2). This dose is lower than the recommneded one
> but when we went to higher doses I was getting non developped stuff on the
> corners of my patterns and I could not get rid of that). As the wafers are
> gold coated, I thought that maybe this dose should be enough? I have here my
> first problem: THE WAFER STICKS NORMALLY TO THE MASK!
>
> 7. PEB: using the same ramps as for the soft-bake, I ramp it up to 95C, keep
> it for 25min and ramp it back down. Let it rest for 5 min or so to let it go
> down to RT before development.
>
> 8. Development: 8min in PGMEA. Rinsing in IPA.
>
> Anybody can detect something wrong in my process??
>
> Any suggestion will be greatly appreciated!
>
> Best regards,
>
> Sonia