Rob,

I don't know for sure but I can relate some similar experiences and maybe it
will help.

In EDP etching crystals will form after etching large amounts of silicon and
then cooling the bath. This does not occur when the EDP is allows to cool
without etching silicon.

In KOH, if you calculate the amount of OH you need to etch the number of grams
of silicon you will see that you use up the solution and drop the concentration
of the solution during the etch. When I was etching 50 wafers in a bath or using
an etch solution for more than one etch, this was an issue. If the concentration
drops below a certain limit (different for KOH, TMAH, etc...) the etch surface
gets rough and deposits will form in the corners of cavities of other surfaces.

So, my suggestion is to calculate the drop in concentration during your etch or
etches and see if the crystals occur at a certain concentration. I used 25% TMAH
at 90 degrees C for 25 wafers in a bath and 250 um of back-thinning without
problems. 4 gallons per etch and don't re-use.

Dan Chilcott

-----Original Message-----
From: mems-talk-bounces+dan.chilcott=itt.com@memsnet.org [mailto:mems-talk-
bounces+dan.chilcott=itt.com@memsnet.org] On Behalf Of Robert MacDonald
Sent: Monday, June 21, 2010 6:42 AM
To: mems-talk@memsnet.org
Subject: [mems-talk] TMAH Bath Maintenance

I have a TMAH bath I use for Si Etching. I run it at 18 percent by weight
concentration, and a temperature of 90 deg C. It has a cooled lid to prevent
evaporation, and I use a hydrometer to keep the density constant. I find that
over time crystals start to form in the bath.

Does anyone know what these crystals are?

Does anyone know if this effects the etch rate, or selectivity to silicon
dioxide?

Thanks,
Rob MacDonald
Shearwater Scientific
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This message has been reviewed by Chilcott, Dan - GS and determined free of
export controlled data. 6/22/2010 9:16:58 AM
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