Research problem-photoresist thickness

[intelwanderer]

Greetings PF.In my lab, we are having trouble making correct photoresist films(over 100 microns, SU-8 photoresist). They typically are not hardening enough before exposure. We want to solve this problem quickly as this is starting to interfere with the project.

I'm trying to find solutions to the topic on the web, but I would also appreciate some feedback from anybody who has done microfabrication. Any tips?Thanks in advance. I hope this is the right section to put it in-if it isn't, my apologies.

 

[phyzguy]

I have some experience in this area. Do you mean the photoresist is 100 um thick? I looked up the SU-8 datasheet (at microchem.com/pdf/SU-82000DataSheet2000_5thru2015Ver4.pdf), and it gives information on how to do the soft bake, and specifically says to use a hot plate and not to use a convection oven or a skin will form. What type of soft bake are you doing (type of bake, temperature, time)?

 

[intelwanderer]

Yes, 100 microns. We just bake it on a hot plate. Typically, we use a 65 degree celsius plate for around 5-6 minutes, and then a 95 degree plate for 45 minutes, but because of the film problems, we've been playing around and trying other stuff(an hour instead of 45 minutes, 105 degrees instead of 95, etc, etc...)
I think it has more to do with the photoresist application than the baking, but I could be wrong-I'm just a clueless undergrad. ;)

I recognize that paper. That is what we used to get the original values, hence why I think it's something else.

 

[phyzguy]

The data sheet says 95 degrees for 4-5 minutes up to 40 microns thick, and doesn't mention a 65 degree step first. Perhaps you're baking it too long, or perhaps the 65 degree step is the problem. I would try to do as close to what the data sheet says as possible - the guys who make the stuff usually know what they are doing. Perhaps skip the 65 degree step and try 95 degrees for 6-8 minutes?

How does the problem you are seeing manifest itself?

 

[intelwanderer]

I'll forward this to my grad student. It's been showing up for a while.

Apparently, because the photoresist isn't hardening enough before the photolithography. I'll tell you more when I get more back from him... I need to ask more questions.

 

[c43apb]

What is your exposure time and UV lamp intensity? 45 minute pre-bake sounds extreme. It's been a while since I have worked with SU-8, so I can't tell you the exact procedure I used, but I am certain each step of the procedure was in the 1-3 min range. The procedure outlined in the data sheet is a good starting point but you will most likely have to modify it for your own system.