Hi, there:

When using SiO2 as the mask layer in KOH or TMAH etching, Thermal SiO2 is way
better than sputtering SiO2. But I dont think it can sustain for a 500um silicon
etching. I have done wet etching up to 400um with 2um thermal SiO2 as mask layer
and 35% KOH as etchant (forgot about the temperature, but it's around 65C). Dont
know much about the TMAH additive xiaoguang just mentioned, but sounds like
something worth looking at.

As for the case with metal as mask layer, I remember reading paper that says it
served well for deep etching. However, I think you need to closely inspect your
sputtering metal layer to make sure there's no pinholes or anything in that
nature.

good luck

Guocheng Shao

--- On Tue, 4/21/09, l j  wrote:


From: l j 
Subject: [mems-talk] KOH and TMAH etch masks other than SiN for long Si etch
To: mems-talk@memsnet.org
Date: Tuesday, April 21, 2009, 9:49 AM


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.