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
[email protected] / [email protected]
Cell: (805) 748-5839
Office: ò_Ó MRL 3117A
On Mon, Jun 21, 2010 at 3:41 AM, Robert MacDonald <
[email protected]> 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
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