Andrea,

Depending on what your tolerances for perpendicularity and roughness are,
you might want to use a different etchant.  If you're very good at aligning
to the crystal face, (<0.1 degrees), then if I remember correctly KOH 40%,
70C will give you the flattest, smoothest most perpendicular sidewalls.  KOH
is very selective and will form more vertical sidewalls, but slight
misalignment of the litho with the with the crystal axis will cause step
defects in the sidewall.  TMAH is less selective and will polish out the
defects and give you smoother sidewalls, but the geometry may not be exactly
perpendicular.

Once you can get the correct etch angle on your trenches, oxygen
percipitates and bubbles will be your next problems.  Annealing at 1300C,
using surfactants and agitation will fix that.

Gabriel



---------- Forwarded message ----------
> From: "Andrea Mazzolari" 
> To: "General MEMS discussion" 
> Date: Sat, 8 Jul 2006 15:50:34 +0200
> Subject: [mems-talk] Etch through Si wafer with TMAH
> Hi all. I need to etch through a (110) silicon wafer 500um thick. I'm
> planning to use TMAH. Will walls be perfectly vertical ? If not, which
> etchant could i use ? In case of tmah etch will under etch responsible for
> not vertical walls ?
> Based on what i know, walls will not be perfectly vertical, this sould be
> due to under etch... i'm i wrong ?
> Could someone suggest me some article on this argument ?
>
> Thanks!
> Andrea
>
>