Hi Robert, I used a similar setup to anisotropically wet etch Si. I also found that clear crystals formed upon cooling the solution when the etchant had consumed a large quantity of silicon (on the order of grams per liter of etchant solution); these crystals would dissolve when the etchant was heated again. Although I have never characterized them directly, I suspect they are some kind of water-soluble silicate compound. It is well established that etching of silicon in the presence of hydroxyl-containing solutions proceeds according to: Si + 2(OH-) + 2H2O → SiO2((OH)2)^2- + 2H2 I imagine that the water-soluble dihydroxyl silica product could form a complex with the positively charged tetramethylammonium ion (two (TMA)^1+ ions to one SiO2((OH)2)^2- ion) and crystallizes out when the solution becomes supersaturated (either through cooling or an increase in concentration). I believe I have data on the silicon etch rate, and possibly oxide selectivity, as a function of dissolved silicon content in a notebook at work. I'll check tomorrow when I get in. Best regards, Brian Stahl -- Brian C. Stahl Graduate Student Researcher UCSB Materials Research Laboratory brian.stahl@gmail.com / bstahl@mrl.ucsb.edu Cell: (805) 748-5839 Office: ò_Ó MRL 3117A On Mon, Jun 21, 2010 at 3:41 AM, Robert MacDonald < robm@shearwaterscientific.com> wrote: > 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 > _______________________________________________ > Hosted by the MEMS and Nanotechnology Exchange, the country's leading > provider of MEMS and Nanotechnology design and fabrication services. > Visit us at http://www.mems-exchange.org > > Want to advertise to this community? See http://www.memsnet.org > > To unsubscribe: > http://mail.mems-exchange.org/mailman/listinfo/mems-talk >