Regan,
I know this is somiwhat old, but I was quite busy the last two weeks
finishing up my thesis. (Incidently, I covered the matter you are asking
about.)
(1) Even when your solutions would etch perfectly flat, the wafer has an
inherent wedge-shape, sometimes called parallelity. This value should be
specified by the manufacturer. E.g. Virginia semiconductor specifies a
parallelity better than 2um across the 2" wafers we frequently order.
Therfore, etching such thin membranes with a timed etch-stop is not a
good idea to begin with, I think.
(2) You use KOH at a relatively low concentration, and at a high
temperature. Diffusion effects are important under such conditions, and
that is why stirring changes your results. If you use a 45wt.% aqueous
KOH solution, which I frequently did, your results will be insensitive
to stirring: the reaction is rate-limited. (I used this solution up to
70C). Stirring might still be helpful for hydrogen bubble detachment,
but I could not see any difference. In any case, there is a transition
temperature above which the reation is diffusion-limited. Increasing the
solution concentration pushes this transition temperature up.
The problem is usually the thick oxide you will need for such a
concentrated KOH solution, but silicon nitride works very well.
All other anisotropic etchants which I used (TMAH, EDP), were sensitive
to stirring under any condition. The highly concentrated KOH is unique
in this sense.
I hope this makes sense to you.
[email protected] wrote:
>
> Dear colleagues,
>
> I am working on etching Si in KOH until 5 to 10um is left without
> using P++ doping.But I have observed a 7um etching depth
> difference between the OF side and it's opposite side. And an
> almost 20um difference in the maximum and minimum etching depth
> measured within the wafer.
>
> I tried a bubbler within the etching bath but the resulting
> etching distribution was almost the same as before.
>
> I am wondering if there are other ways of obtaining a good KOH
> etching distribution.Any idea and suggestions will be greatly
> appreciated.
>
> Regan Nayve
> [email protected]
>
> ****************************************
> Etching condition
> KOH : 15% 90C
> Temp control : $B!^ (B1C
> Etching bath : Stainless(with top cover)
> Bath dimension: L20 W23 H23 (cm)
>
--
==============================================================
Alexander D. Hoelke, Graduate Student Electrical Engineering
University of Cincinnati, Center for Microelectronic
Sensors and MEMS (CMSM) Cincinnati, Ohio 45221
Phone 513-556-4774 (work) 972-470-9735(home)
==============================================================
