I want to create a hard etch mask for etching Si anisotropically using KOH. The etched structures need to be quite deep, several 100 um.
I understand that the two common hard etch mask materials are Si3N4 and SiO2. Currently I have access to RIE, but only Ar and O2 gas tanks, so I can't dry etch the silicon nitride, which seems to be the most common method. We've tried sputtered Cr as a mask, but the adhesion to Si is less than ideal. The best success we've had is a complicated process: sputter Cr onto Si3N4 coated Si, create a photoresist mask, etch the Cr, strip the resist, etch the Si3N4 with BOE, strip the Cr and finally etch the Si with KOH. I'm looking for something simpler ideally.
Hi Ian,
I understand your concern, but the most straightforward thing to do in your case is to acquire CF4 or SF6. The mixtures of CF4+ O2 and SF6+O2 both are excellent enchants of Si3N4. If it's not feasible, you can try oxygen plasma cleaning before sputtering of Cr, which may improve the adhesion. You can also deposit Ti instead of Cr if you have a target available.
Thank you both very much, I really appreciate it. I will investigate the possibility of acquiring SF6 or CF4, although I am not sure the exhaust system is set up to handle the byproducts. We do have a Ti target, so that is certainly an option.
I tried etching 105 um Si in 1:1 KOH:DI water @80C. Took 105 mins. In my experience just the LPCVD Nitride was enough as a mask.
However, LPCVD at my cleanroom was super busy so I worked around to use Gold as a hard mask (e-beam). No Nitride at all. It worked surprisingly well.
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