Hi,
I'm doing a process that uses a thin layer of SU-8 as an insulator. I was getting short circuits so I made a simple metal-SU8-metal structure (Figure 1 ). My SU-8 tests included SU8-2000.5, 2001, and 2002 with thickness varying from 0.7 um to 4.5 um. I also tested two different e-beam evaporated thin films, gold and chromium, around 100 nm for each layer.
All of them seemed to be shorted. I have attached some pictures showing the layers under the microscope (Figure 2-5). I spotted some potential pinholes or bubbles ONLY in SU-8 layers on top of the bottom electrode. The SU-8 layer alone on Si and Au/Cr layers alone seem rather clean. I wonder why this is happening and if you have any suggestions?
Dear Amirhossein Omidvar,
SU8 is not good to use as an insulator since it can't stand high temperature when evaporated metal layer on the top of it.
Why not choose different types of materials as the insulator such as PMMA, PVA or some inorganic materials? They are well studied as insulators.
We generally find that tiny amounts of water contamination in a process can result in pin-holes in the final coating. If the liquid in the coating is not water soluble at all this can result. If air or gas is used in the process at all, ensure that air or gas supply is completely H2O free.
Hi Amirhossein Omidvar
I agree with the points raised by Xuhua Wang and Robb Engle It might be that 'high' substrate temperatures during Au/Cr coating might exacerbating H20 contamination effects. Perhaps a very simple way to reduce substrate heating would be to increase your deposition time with a moveable shutter, depositing your film in a series of short bursts with periods of substrate cooling in between.
Dear Xuhua Wang
The sole purpose of SU8 in my work is not electrical insulation, but it serves as a suspended membrane as well, and considering the complexity of my multi-layered structures and compatibility issues, SU8 is the best candidate.
Dear Robb Engle
I tried to bake the sample before and after metal deposition but still, I see those spots. If it is related to the photoresist, I wonder why these pinholes are more pronounced at the SU8-metal interface?
Dear Martin Goodchild
I deposited only 10 nm of Ti and 20 nm of Au this time, but still, I see the pinholes.
Is there a reference for H2O contamination that I can read more about?
I did an experiment where I cleaned the wafer with PG remover at the beginning, and with O2 plasma etch before and after metal deposition but still no success.
I appreciate all your comments.