Ni etching using nitric acid

I etched Ni film (200nm) coated on diamond substrate using 30% nitric acid. It worked well and I was able to remove the Ni in 1 minute. A few things I noted that I'm hoping to understand why they happened and if I could do something about it.


1. flakes of Ni were left of the diamond. The current experiment was an etch test and part of the Ni was coated by photo resist. I don't want to etch any longer because I already have significant under etching in the patterned area. Would changing the concentration of HNO3 help? My primary Etchant is 70% HNO3 which I diluted to 30% because from what I've read 30% HNO3 etches Ni better than 70% (that seems counterintuitive to me so maybe it's wrong).
2. The area with the Ni flakes seem to have etched the diamond. The areas are all 360nm deep holes instead of being 200nm islands which surprised me. I know diamond has solid solubility in Ni but that only happens at temperatures above 800C. The etching was done at room temperature. Is it possible to get such high temperature on the sample surface during etching? Is there any way to avoid it?


Thank you for your help.


