I am using SU-8 2015 for my microfluidic channel structures. The substrate is 1mm thick glass wafer.
Issue : SU-8 flows off the substrate during the developing process. The adhesion is poor apparently.
Solution/s to this problem are needed.
The protocol I am following is as follows.
1) Cleaning and dehydration bake of the wafer.
2) Spin coating of SU8 of desired thickness.
3) Soft baking process. 2 mins at 65C and 5 mins at 95C.
4) Exposure using MA-6 Mask Aligner.
5) Post Exposure baking. 2 min at 65C and 5-10 min at 95C.
6) Developing in solution for 1min.
7) Rinsing with IPA and drying with Nitrogen gas.
Manufacturers of photoresist usually can deliver also primers which improving adhesion on surfaces like glass.
Hello Ahmad,
As Daniel pointed out, there are several adhesion promoters available to purchase (such as 'TI PRIME', 'HMDS' and 'OmniCoat').
Your dehydration bake - what temperature and duration are you using? I usually use 200oC on a hot plate for 15 minutes or longer.
You could also try a surface activation step such as Piranha (H2SO4 & H202), UV-ozone or oxygen plasma (if using an adhesion promoter, do this first).
The rest of your process steps look good.
Hope you manage to find a solution.
Jules
hi,
Th protocol that you described is usually followed for most of the resists and also the the problem. Normally when we pre-post bake the substrate with resist, it decides whether resist will stay or flow away while developing/etching,etc. Hence baking is the most important step while working with resists. With different substrates, baking timings and temperature varies drastically. you cannot use the same parameters for to different substrates. Just play with temperature-time parameters, it should provide you with the desired results. I solved the problem in similar way while using glass as the substrate and photoresists.
This is the problem because of poor adhesion. I tried to increase the adhesion between substrate and SU-8 by simply treating the glass substrate with oxygen plasma before coating the SU-8.
Yes, I found that the simplest way is a mild O2 plasma. The standard procedure is like your description. However, you can also improve adhesion by starting baking from lower temperature (say 30 degree C) for some minutes (depending on the SU-8 thickness).
Dear Usman,
I have used SU-8 2035 on 1mm glass substrate with cleaning with soap solution, followed by extensive DI wash and Nitrogen drying. I extended the bake times and got good adhesion of the resist. May be you can try that.
dear Usman,
our experience on glass substrates is that, due to very poor thermal conductivity, all baking times have to be dramatically extended with respect to silicon substrates. I would suggest to try and double all bake times to start with.
regards
Alvise
Similar to the post from A. Bagolini, be sure that you have adjusted your exposure time correctly. The microchem data sheet will provide a general factor for how long the exposure should be on various substrates compared to silicon. Glass should be ~1.5X the exposure time as you would do for silicon substrates.
http://microchem.com/pdf/SU-82000DataSheet2000_5thru2015Ver4.pdf
Usman,
I would definitely recommend Piranha cleaning ( 1part H2O2, 3 parts H2SO4) step for SU8 processing, as SU8 likes cleaner surfaces, followed by dehydration bake at higher temperatures, around 200C. As the thermal conductivity of the glass is lower than silicon, you will need longer soft bake. And also try ramping up your hot plate from 65C to 95C and ramp down to 65C. Ramping temperatures up and down helps in reducing stress in the film. SU8 film peels during developing due to low UV exposure. Try increasing your exposure times. Try doing sector exposure to figure out which one works best fro your process.
Happy processing :)
I agree with Jules Lloyd Hammond and Suhasini Gattu, I obtained this issues before and resolved them by cleaning the glass wafers by Piranha for 30 min and then dehydration bake at 200°C for 15-20 min.
the dehydration bake should be done before the SU-8 spinning. Or you can keep you cleaned & dehydrated wafers in vacuum drying oven or forced-air oven
Regards,
One trick to improve adhesion on glass is to first coat the whole glass wafer with a uniform and thin (0,5 or 1µm) layer of SU8. Overexpose it and overbake it and the next thick layer of SU8 will attach firmly on it.
Or you could buy the other formulation, the SU8 3000. It adheres very well to SiO2, metals ect
You might have issue with cleaning of Substrate. you make sure that the substrate you r using is properly cleaned before pouring the SU8 on it.
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