Question

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%.
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?

Answer 1

Adjusting the concentration of HNO3 or the etching process parameters may help avoid the remaining Ni flakes on the diamond substrate. High temperatures necessary for Ni solubility into diamond are not expected during room temperature processes, suggesting an alternative cause for the deeper etch observed.

Step-by-step explanation:

The student has observed that etching a Ni film from a diamond substrate using 30% nitric acid (HNO3) left Ni flakes and etched the diamond more than expected. The observation that 30% HNO3 etches Ni more effectively than 70% is likely due to the differences in reactivity and the production of different nitrogen oxides during the reaction with the metal, which depends on the acid concentration. At high concentrations, nitric acid behaves like a strong oxidizing agent and can create various nitrogen oxides and nitrates, potentially leading to over-etching or different etching patterns due to the stronger oxidizing environment.

To address the issue of Ni flakes remaining on the diamond, it might help to optimize the etching process by adjusting the concentration of HNO3 or changing the etching parameters such as temperature and time. However, if there is under-etching in the patterned area, further modifications might be needed to balance the etch rate and minimize damage to the diamond substrate.

The etching of the diamond substrate to a depth of 360nm is unexpected at room temperature, as the solid solubility of Ni into diamond requires temperatures above 800°C. It's unlikely that such high temperatures would be reached during the etching process at room temperature. One possible explanation for the deeper etch could be mechanical damage from the Ni flakes or chemical interaction with other agents present during the etching process, which might have affected the diamond.