>
> This is probably unrelated, but I'm running into some etching problems
> with KOH and (100) wafers that sound similar.  We're stumped with
> this, so if anyone could supply any information, that would be a big
> help.
>
> We're etching a series of uniform V-grooves in a (100) wafer with
> 6000A of SiO2 thermally grown on the surface.  As far as I know the
> wafers are test-grade from Silicon Quest here in the US.
>
> The pattern is a series of 3 micron wide lines spaced on 25 micron
> centers.  The mask is aligned as carefully as possible with the <111>
> axis, but alignment problems could be a factor.  The mask specs say
> it's good down to 0.10 microns, but the manufacturer says their tests
> showed it good down to about 0.07 microns.  The mask generates lines
> 22 microns across and about 19 microns deep.
>
> We're using a 55%/wt KOH solution at a variety of temperatures and
> etch times.  The one I've used most recently is 80 deg C for 25
> minutes.
>
> When we put the resulting wafer in an SEM, we get very fine grooves
> with quite flat walls and nice sharp bottoms, but occasionally we get
> line breaks.  The (111) plane walls seem to jump by about half a
> micron, go for about a hundred microns or so, then jump back.  It
> would appear to be undercutting of the SiO2, but I can't figure out
> what would cause it.
>
> If anyone could suggest what parts of our process we might look at to
> fix this problem, I'd truly appreciate it.
>
> Thanks,
>
> Tom Benedict                          McDonald Observatory at Mount Locke
> tom@astro.as.utexas.edu               Department of Astronomy
> Tel: 512-471-3337                     University of Texas at Austin
> Fax: 512-471-6016                     Austin, Texas  78712
>

You are right, this sounds very familiar to the (110) problems.
It takes only a tiny spot in the (111) plane to be etched. This could be
caused by any imperfection etc. in the crystal. From there, it spreads in all
directions, along the (111) plane now a bit deeper than the damaged one. Its
borders are surrounded by a step. So, if you etch 25 um deep, and the step
etches definitely faster than the (100), lets say twice as fast, I can
easily see that you see it about 100 um long.

I guess that zone refined crystalls would help.
>
>