Hello,
I am having trouble sputter coating Mg on glass substrate. The adhesion is terrible during lift-off and I lose significant yield. I tested the same process with a silicon wafer, and the results were extraordinary but terrible with glass.
I know I can use an adhesion layer like titanium, but at this stage, I want pure magnesium structures.
Is there anyone with experience regarding this problem?
I have no direct experience with magnesium deposition and I got that you do not want a metallic adhesion layer, but can you consider to coat the glass beforehand with some other oxide? Al2O3 or TiO2 are excellent adhesion layers.
The other thing, glass substrates are not as clean as silicon ones. Try plasma and piranha cleaning and also if possible remove water from the surface by heating the substrate in vacuum prior of the deposition if the chamber allows for it.
I don't have experience with Mg, but I do have some experience with similar materials on glass. I suspect your issue might be an interface reaction. If we consider the glass as mostly SiO2, one question is whether Mg + SiO2 is more or less stable than MgO + Si. That is, which direction does the reaction
SiO2 + Mg 2MgO + Si
want to go? If you look at the free energies of formation [1,2] for MgO and Si, this reaction ver much wants to go to the right. That means at the glass-Mg interface, it is reasonable to expect that the first monolayers of Mg will 'steal' some oxygen from the glass, leaving behind Si. In other system I've looked at, like TiN [3], this same problem leads to a bad interface and poor adhesion.
If you can't tolerate an adhesion layer, then the simple solution might be to first deposit a bit of MgO (e.g., by first sputtering Mg with a partial pressure of oxygen) and grow the Mg on that. If the MgO is smooth, 1-2nm should be enough to keep the Mg metal from reacting with the glass.
I also second the cleaning ideas of Marco Asa , I agree that glass substrates out of the box are often not clean enough. I would usually first rinse with ammonia to get rid of organics on the surface, and then rinse and vapor clean with isopropyl or ethyl alcohol and finally clean in an oxygen discharge in the deposition system. I don't have experience with piranha etching of glass. My only reservation there would be that is awfully aggressive stuff, so don't use it without reading about safe procedures first.
[1] SiO2 https://webbook.nist.gov/cgi/cbook.cgi?ID=C14808607&Mask=2
[2] MgO https://webbook.nist.gov/cgi/inchi?ID=C1309484&Mask=2
[3] https://www.researchgate.net/publication/40506967_Titanium_nitride_thin_films_by_the_electron_shower_process
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