How to dissolve Sapphire?

[kelvin490]

I got a Gallium Nitride crystal grown on Sapphire (alumina - Al2O3). I want to remove the 300 micron thick sapphire without damaging the Gallium Nitride film. How to remove sapphire by chemical or physical method? Any suggestions are appreciated.

 

[Mapes]

Hi kelvin490, welcome to PF. Unfortunately, alumina is quite stable. According to some wet etching resources I've collected, HF is required to wet etch it effectively. I suspect that HF will also wipe out the gallium nitride, but you might check this (of course, follow all safety precautions: fume hood, face shield, nitrile gloves, handy calcium gluconate gel as antidote).

Is it possible to mask the gallium nitride with something (e.g., photoresist) and ion mill the alumina layer away?

 

[Mk]

calcium gluconate

Hey Mapes, I'm not familiar with the field. What is calcium gluconate gel for? Treating acid burns?

 

[MATLABdude]

Hey Mapes, I'm not familiar with the field. What is calcium gluconate gel for? Treating acid burns?

Hydrofluoric acid is actually a pretty weak acid (so you don't feel an immediate burning sensation). However, it will leach the calcium out of you (bones, as well as the ions that allow your muscles to actuate). I'm told it's excruciatingly painful, and possibly fatal if left untreated. The other nasty thing is that non-dissociated HF is readily absorbed through the skin, so it's not enough to just wash it off. Note that this is hydrofluoric acid HF(aq), and not due purely to the action of the fluoride ion (F-)!

While fluorides (in moderate to high doses) are poisonous, a small amount (mg or few per mL) in helpful in preventing cavities. All sorts of FUD is out there that conflates these two unrelated issues (joined only by the term fluoride). A little bit of knowledge is a dangerous thing, as they say.

In any case, smearing on calcium gluconate is a remedy if you've had exposure to HF (or buffered oxide etch--BOE--which contains HF). Since it's so calcium rich, and owing to the osmotic gradient, the HF comes out of the body, and reacts with the calcium gluconate instead. If you have a really large spill, or you waited too long, I think they inject / drip a calcium chloride solution into you, to hopefully react with the HF in your body. You may have some precipitated calcium fluoride in your body (until it gets filtered out by your kidneys / liver) but that's better than the alternative.

Whenever I use HF, I don protective equipment (facemask, chemical apron and gloves) and wash down everything with calcium chloride solution when done.

 

[Mapes]

(What he said.)

I think MATLABdude's user name is deceptive; he sounds like an solid experimentalist

 

[kelvin490]

Hi kelvin490, welcome to PF. Unfortunately, alumina is quite stable. According to some wet etching resources I've collected, HF is required to wet etch it effectively. I suspect that HF will also wipe out the gallium nitride, but you might check this (of course, follow all safety precautions: fume hood, face shield, nitrile gloves, handy calcium gluconate gel as antidote).

Is it possible to mask the gallium nitride with something (e.g., photoresist) and ion mill the alumina layer away?

Thank you for your reply. I have also heard that some people use sodium hydroxide to dissolve alumina that form on aluminum. Is it also possible to dissolve alumina of 300 micron thick?

I have also think of ion milling, but it seems take a very long time.

 

[Mapes]

Thank you for your reply. I have also heard that some people use sodium hydroxide to dissolve alumina that form on aluminum. Is it also possible to dissolve alumina of 300 micron thick?

It's surely worth looking into. Your post reminded me of a couple additional resources: Kurt Williams' etch rate papers http://www.mindspring.com/~zipf-williams/papers/etch/JMEMS_EtchRates_1.pdf" [Broken]. Potassium hydroxide is shown here to etch alumina, and sodium hydroxide would likely also work, as the key is to provide hydroxide ions. Since silicon nitride is quite resistant to KOH and NaOH, it's possible that gallium nitride is too. Good luck!

 

[MATLABdude]

(What he said.)

I think MATLABdude's user name is deceptive; he sounds like an solid experimentalist

I'm not at all a 'solid' experimentalist. If I were, I'd have been done by now! :tongue:

But the username comes from a different time, when I was a lowly undergrad summer student hacking away in MATLAB, trying to code something I didn't quite understand. While that's well and good as a programmer (and pretty forgiving, as learning curves / danger go), working with toxic / corrosive chemicals and dangerous equipment is an entirely different proposition--so, to the OP and Mk, make sure you know what you're dealing with--and how to do it properly--when you work with this stuff (if you don't already)!