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