I am trying to fabricate some gold microwave transmission lines. I use a simple lithography-evaporation-liftoff technique to do so. My substrate is sapphire, I use about 10nm of Ti as an adhesion layer deposited by e-beam evaporation, and 400nm of Au by thermal evaporation. The process is done at about 4*10^-6 torr.
Immediately after deposition the lines appear to be clean and smooth as intended. However, after a short while these blisters begin to appear across the surface. I've noticed cleaning the samples in water accelerates this process.
I've also done this exact same process with thinner films of Au, ~170nm, and not encountered the blisters.
My background is mostly in electronics and optics so I have a limited knowledge of materials science. From what I've gathered this process is known as delamination blistering and is a result of Hydrogen contamination and increased stress of the thicker Au films.
I've attached a microscope image of these blisters. Please note the very large structure is just a piece of dirt I used as a reference.
If anyone has any advice as to how I can avoid these blisters developing on my lines I would appreciate it.
Edit: I forgot to mention in that image the width of the line is about 300um, it is tapering over the length so it is difficult to be exact.
Do you have a cleaning process before spinning your negative resist? I am not familiar with sapphire but maybe some sort of oxide removal directly before loading into deposition chamber. Common process is to use an O2 plasma to remove residual organics then a low strength acid etch before loading.
FYI I observed the same 'blisters' after metallisation on AlN/sapphire layer. We believe that it was a cleanliness issue.
So as suggested by Eric Nallon, try a cleaner process before deposition. I'll also recommend to change the photoresist.
Please check the cleaning procedure. Additionally, you can enhance the gold adhesion by including a thin layer (1 or 2 nm) of Ti or Cr before depositing the gold layer.
Hello,
I deposit by sputtering Au and seen that it is best not to clean chemically the substrates, only argon plasma cleaning. A chromium layer improves the adhesion and prevents peeling gold.
Hi,
try to check also the possibility of contamination of gold during the evaporation. I hade same problems and I improved the process just by changing the crucible of gold. Are you using e-beam evaporation?
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