You should try thermally grown SiO2 for TMAH etch. 1um SiO2 should
withstand 7-8hours etching in 25% TMAH at 80C.
I also remember that there are additives to TMAH that can even lower
the SiO2 etch rate. Do some google search
>From my vague memory, Al can also be used with the right additive.
Best
Leo
On Mon, Apr 20, 2009 at 9:49 PM, l j wrote:
> Hello MEMS community,
>
> I am looking for input on problems with etch masking for long Si anisotropic
etch.
> Recently I found myself needing to do anisotropic ~500 micron etch of Si (100)
wafers, both sides polished. I do not have silicon nitride available as an
option.
>
> I have studied recipes for etch and masks. Seems like the prescription for
mask is CVD silicon nitride. For other mask options, recommendations seem to get
less certain and more sketchy.
> Pulling together references I could find, I have tried KOH 50% 80C and TMAH
25% 80C. The etch itself works as expected on the Si (100).
>
> For masking I have tried Ti 500A / Au 2500A sputtered at 300C; Cr 500A / Au
2500A sputtered at 300C; SiO2 500A sputtered at 300C. Wafers were solvent
cleaned and oxygen plasma cleaned prior to metal deposition.
>
> For the metal masks, after an hour or two in etch, it appears the adhesion
layer Ti or Cr is being attacked, and with another hour or two, the Au is
almost floating off the Si, intact but without the Ti or Cr.
>
> For the SiO2 mask in the TMAH, it appears the TMAH got through the SiO2 mask
within an hour.
> So, are Ti or Cr and Au known to work, or not, as etch mask for long KOH or
TMAH etch? Is there a solid reference in the literature? The behavior I saw
reminds me of a description of anodic dissolution (M. Datta, “Anodic dissolution
of metals at high rates,” IBM Journal of.. Research and Development, vol. 37,
pp. 207–226, 1993.) I also found in memsnet a reference to the metal and Si
combination resulting in a "primary cell" which removes the adhesion layer, but
was a little sketch on details.
>
> Is sputtered SiO2 not dense enough to mask TMAH? Si/SiO2 selectivity of TMAH
is supposed to be very high.
>
> Look forward to any insights.
> Thank you
> K.C.
>
--
Xiaoguang "Leo" Liu
Birck Nanotechnology Center,
Purdue University,
1205 W.State Street, West Lafayette, IN, 47906 USA
liu79@purdue.edu