I was wondering if anyone had advice on removing a nickel hard mask - which seems to have become resistant to my standard Nickel etch. The sample is GaN with a hard Nickel mask, which has undergone AR-Cl ICP etching. Normally I would remove the nickel using a dilute (5-10%) Nitric acid etch, however it seems like it has become resistant to this etch.
I have also tried dilute Aqua Regia, a dilute Nitric + Acetic + Sulphuric acid mix, and eventually a concentrated Aqua Regia* (* NOT A GOOD IDEA - I got excited when it changed colour and started bubbling, but on inspection it etched everything EXCEPT the residue!)
My current theory is that during the ICP etch the Nickel has reacted possibly with the chlorine to produce NiCl. I have read that this can happen around 200C - which is a little above what the ICP set temperature is at 150C - however presumably the sample can heat up above this.
I was wondering if anyone has had similar issues - and/or if there were any wet etches I could try which would etch NiCl (should this be it).
Since you tried acids without success, I would recommend that you try a basic (and complexing) environment instead.
Your trouble may be with a nickel compound, although I suspect it's not (or not just) chloride, but rather hydroxide/oxide or hydroxychloride/oxychloride. With this in mind, and considering the attached abstract, I would try the following:
1. Make an ammonia/ammonium chloride solution with pH 9.5-10, e.g. 4 parts water : 2 parts concentrated ammonia : 1 part NH4Cl (by weight). Try it by itself, with gentle heating if needed, and look for blue or blue-green color next to your substrate. If no color, probably nothing substantial is happening. 2. To try to dissolve the actual metal, try to add a small amount of hydrogen peroxide and see what happens. You should do it inside a secondary container (in a hood of course), just in case the peroxide decomposes uncomfortably fast. Keep the hydrogen peroxide to 3% or less of the total formulation, i.e. at most one part in 10 of concentrated (30%) H2O2.
Good luck!
Seemingly of some possible interest:
https://www.researchgate.net/post/How-may-I-remove-the-nickel-oxides-on-Ni-without-corroding-the-Ni-substrate
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