Is this a mould for making the channels? Where is the scale bar and also the thickness of the film? Sharp edges of the patterns have been developed in good condition, therefore , it can not be of the UV rays distribution

1) It can be due to low quality transparency photo mask , 2) It maybe that the post exposure development time has not been enough, 3) It maybe that your wafer has not been fully clean, 4) it maybe that your SU8 film thickness has been unifom

Hello Mr. Alvankarian

thank you. Sorry I just put up an unedited photo. This is Su8-2025 with 40 um thickness, and it is a mould for future PDMS chips. I also believe it is not a development issue since I have encountered those, and in those cases the edges dont become sharp, as you also mentioned. The photo mask is a transparency with hight quality and not a glass chrome mask, but it looks uniform under the microscope. I also clean my substrate with piranha before storing them in DI water. Someone proposed that bubbles could be forming because either i pour the su* too fast or its a problem with the SU8, so I will check that, I will also make higher resolution masks and see what happens. thank you. please let me know if anything else comes to mind.

Hello,

This may be the effect of some granulation on the printed film (but I don't really believe it : check with other PR photolithography). I would rather think to bubbles in your SU-8 PR : they may come from the PR (bubbles in the PR container, bubbles incorporated due to the pipetting) or from the baking of your SU-8. Which are your baking parameters ? If you use a single-step quick bake (or a too high baking temperature), solvent may not have the time to outgas from the SU-8 and creates bubbles - refer to the manufacturer guidelines to find the proper baking program. Some baking plates may have a bad temperature regulation which causes oveheating during PR bakes.

Note that I think you did everything well, but I an just tring to guess every mistake in the process that could cause such a problem.

Best regards.

Hi Durand

Thanks for taking the time to read and write me comments. its actually very helpful to brainstorm these points, because the mistake is somewhere along the protocol.

I directly pour SU8 onto my glass substrate, there could be bubbles and i will check that. Since I use glass as substrate I bake differently from Silicon wafers:

Su8 2025 spin coat at 2000rpm (40 microns)

pre bake: 3 min @ 67 C

20 min @ 97 C

expose

post bake 3 min @ 67 C

30 min @ 97 C

15 min @ 67 C

the hot plates are quiet new,

Dear Setareh

Which type of wafer (Pyrex or Silicon) you used? Before you coating, you should hydration your wafer also. For Pyrex I put on hotplate for 20min at 150 degree. The time for pre bake you also can prolong with 10 for 67degree.

Good luck

Anh

Hi Tien, I use glass slides, its a non conventional method but previous colleagues did get this issue. I also use transparencies for exposure instead of the glass-chrome or gold mask.

I use glycerol for index matching between the mask and the glass slide. thanks for the tip

Hi Aniruddh. I do dehydrate for 1 min @ 200 degrees for dehydration. I will chek the surface after each step, its a good suggestion. thanks

Cracks can appear sometimes on the surface of SU8 in the case of unsufficient exposure. These cracks can be cured by a post development hardbake at 130°C. In your case I can't tell if the blisters are located on top or at the bottom of the SU8 layer, but maybe a hardbake can smooth the resist.

Hi benoit

So the hard bake is after developping? in this case i should not see these spots if I chek right after developing right?

A hard bake (slowly ramp up to 130°C, 5 min and then slow cool down) can help to remove some cracks that can appear after development. I don't know if it could work in your case (or make it worst if it is trapped bubbles).

ok i will try, I thought 130 might be too high and might polymerize non exposed parts of Su8. but thanks its worth trying

just to clarify, the hard bake is the last operation, it is made after the development that consists in dissolving all the SU8 that has not been reticulated by UV and post exposure bake.

Ok got it. Thanks

Just as an update, the bubbles seem to fform because the prebake is not long enough for glass substrate, as i increased pre bake to 10min @ 67 and 30 min @ 97 degrees celcius the bubbles were appearing less and less.

In our lab we do our baking steps in an oven.

We allways start using an oven at room temperature.

First step let the oven reach 90 degrees, hold for 30 min, then let it cool for roughly 30 min. This allows for a coffee break.

Expose

Then into an oven again, let it reach 90 degrees, hold for 30 min, then cool down for roughly 2 hours

Develop

Last step is also using an oven, let it reach roughly 200 degrees, hold for roughly for three hours, then let it cool down to RT. This step is normally done over night.

Our protocol takes a lot of time compared to using hot plates, but we still use it since the results are good.

Such holes also can originate from underexposed SU-8. With plastic masks exposure time should be at least 1.5x the expose of glass/chrome masks. Overexposure is usually not too much of an issue for SU-8. A few tries with even higher exposure times might solve the problem.

Hi! What thickness do you use? You may leave the deposed SU8 for a certain time to allow gas to disappear before the soft bake (pre exposure bake).

hi evelyn, this is 40 microns, Su8 2025. I ve increased my prebake times and it seems to help a little.

Can I kindly ask to also check my question at:

https://www.researchgate.net/post/Why_the_thickness_of_SU8_that_I_am_achieving_is_far_from_the_data_sheet

Thank you very much indeed.