[labnetwork] Photomask sticks to SU8

Sat Feb 14 10:08:45 EST 2026

I've worked with SU8 for quite a while and have run into this issue in the past. As mentioned below, if your SU8 is sticking to the mask, it is likely that it hasn't been baked long enough.

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I have found that the bake times listed in the vendor sheet are "guidelines".
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Always bake longer than prescribed in the sheet.
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When fully baked, SU8 should have the consistency of hard candy. Your tweezers should not dent the film.
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I have never run into a scenario where a film has been "over baked".
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Ramp from room temp, up to 65 C, dwell, then ramp up to 95 C. Turn off the hotplate and allow to cool to room temp. This will keep film stress to a minimum.
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Thicker SU8 films (100 um+) can be challenging to bake. The surface of the SU8 will tend to skin over from heat and air flow over the wafer, preventing the underlying solvent from leaving the film. Cover the wafer with a beaker or petri dish to maintain a solvent environment during the bake. Prop one side of the dish up with glass slides to prevent any condensation from dripping on the film. This allows the film to bake from the wafer surface upward through the film.
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For film thicknesses of 500 um+ I typically ramp to temperature and allow the film to bake overnight, switching off the hotplate in the morning for slow cooling.
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With thicker SU8 films, I have seen the wafer begin to bow at around 60C. In these cases, I will reduce the bake temp to 55C and extend the bake time.

Cleaning your photomask.

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In many cases, SU8 that has not been fully cured will stick to the mask in the exposure areas. This will lead to repeated sticking issues.
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Exposed SU8 can be removed from the mask by soaking in either Acetone or Remover PG. It can take several hours to swell the material.
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Gently clean stuck on SU8 with a cleanroom swab, taking care not to damage the Cr layer on the mask plate.
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Once cleaned, quickly move to a bath of IPA, then rinse with DI water.

Tom Pennell
Cornell NanoScale Facility
________________________________
From: labnetwork <labnetwork-bounces at mtl.mit.edu> on behalf of Demis D. John <demis at ucsb.edu>
Sent: Friday, February 13, 2026 9:38 PM
To: Robert Nidetz <nidetz at umich.edu>
Cc: labnetwork at mtl.mit.edu <labnetwork at mtl.mit.edu>; Phillip Bauer <Phillip.Bauer at formfactor.com>
Subject: Re: [labnetwork] Photomask sticks to SU8

To add to this - once SU8 has been fully cured, it is very difficult to remove and doesn’t appear to dissolve in most solvents.  Hence why you need to use piranha in this case.  SU8 on your mask plate will have been exposed many times and possibly is very old (perhaps similar to curing).

-- Demis (contact info<https://wiki.nanotech.ucsb.edu/wiki/Demis_D._John>)
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On Fri, Feb 13, 2026 at 16:46 Robert Nidetz <nidetz at umich.edu<mailto:nidetz at umich.edu>> wrote:
Definitely try the developer first. If it has not been cross linked by the exposure then it will clean up easy. But if it’s been exposed, you’ll need piranha solution.

To prevent sticking in the future, make sure your tweezers cannot leave an impression after the soft bake once the wafer has cooled to room temperature. If they leave an impression, you’ll need to soft bake more.

Good luck!

Robert Nidetz, PhD (he/him/his)
University of Michigan
LNF User Liaison and Domain Expert
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On Thu, Feb 12, 2026 at 7:36 PM Phillip Bauer <Phillip.Bauer at formfactor.com<mailto:Phillip.Bauer at formfactor.com>> wrote:
Hello

I am somewhat new to SU-8 working with Positive resists throughout my career. I agree with data sheets are just guideline, for processing.  I saw a note from another University to run a 65 deg post bake after the 95 deg soft bake.  My question is about mask cleaning.  Is Sulfuric/Peroxide (Piranha ~ 60-90C) really needed.  I would think the PGMEA or PGMEA/Acetone followed by IPA would be an effective clean.  Just wanted to poll the audience.

<https://www.google.com/maps/search/4350+Innovation+Drive+%0D%0A+%0D%0AFarmers+Branch,+Texas+75244?entry=gmail&source=g>

Thank you,

Phillip Bauer

Principal Photolithography Process Engineer

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phillip.bauer at formfactor.com<mailto:phillip.bauer at formfactor.com>

M: 682 521 0874

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