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I am trying to create a gold electrode layer on a ZnO substrate using photolithography, but I am having a hard time finding previous examples of photoresist on ZnO with the purpose to preserve the ZnO substrate. Here, they suggest TMAH as the developer, while in this paper, they use TMAH to etch Ga-doped ZnO. I suspect this is a result of variation in the relative TMAH/HO2 concentrations, which is not disclosed in the first source.

What are developers that do NOT etch ZnO? What photoresists can I use with these developers? Similarly, what are compatible removers?

JRE
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Selewirre
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1 Answers1

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Acetone/DMSO/NMP and the like are pretty good removers for usual novolak based resists. So is oxygen plasma, but that might also oxidize surrounding materials.

For most novolak based resists, you'll need an alkaline developer. If ZnO succumbs to that, then you have 2 options:

  1. reduce concentration until it doesn't etch ZnO. You'll have to try this. TMAH can be better or worse than inorganic solutions.

  2. deposit or spincoat a protective layer underneath the novolak, that is not harmed by alkaline solvents. Develop novolak. Etch through the protective layer with a second agent, that does not harm ZnO.

Upon slight further reading, it looks like ZnO is stable neither in acids nor bases. So you could spincoat a very thin polymeric layer underneath the photoresist, crosslink it, develop the mask above, then use oxygen plasma to etch through the crosslinked layer. The plasma will also remove a corresponding thickness from the photoresist, so that should be thick enough.

And a separate option would be of course, to prepare your gold electrodes not via a lift-off process, but by etching itself. I.e. deposit gold, make a negative mask on top of gold, and use dry etching to remove gold.

tobalt
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  • That's clever. Is method #2 commonly used in other situations? – Spehro Pefhany Nov 07 '23 at 07:40
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    @SpehroPefhany yes a LOT. It is a common technique to deposit a "hard mask" layer via a novolak step. The "hard mask" can be various things, e.g. metals, oxides, polymers. The hard mask is then used for further wet or dry etching, before it is stripped. wiki: https://en.wikipedia.org/wiki/Hardmask – tobalt Nov 07 '23 at 07:41