It is possible to realise a front-etch as well, to avoid the problems
encoutered with back-side etching.

I have realised this with Al structures sitting on a SiO2 oxide layer
deposited on an Si wafer. As explained in [1-3], the front-side of the wafer
may be covered with a patterned resist material (AZ 9260) in order to
achieve an anisotropic etch of the SiO2 oxide. This first step is very
important as the resulting patterned oxide will act as an etch mask for the
second step: an isotropic etch of the bulk silicon. In the design I had
prepared "openings" running parallel to the devices (and through which the
underetching is performed) periodically leaving gaps or bridges. These
bridges of oxide will join the device to the wafer once the device has been
underetched. The vertical etch rate of this process (down into the Si handle
wafer) is approximately double the undercut etch rate (in one direction).
Please note that although my final design had polyimide covering Al as well,
this has been achieved succesfully with simpler devices as well for test
purposes and with several opening widths.

I hope this helps.
*
[1] M. Hautefeuille et al., "MultiMEMS sensor development", **Microelectronics
Reliability* , Volume
49, Issue 6,
June 2009, Pages 621-626.

*[2] T. Healy et al, “Silicon Fibre Technology Development for Wearable and
Ambient Electronics Applications,” Frontiers in Electronics Book, pp.
713-721, World Scientific Publishing Co. Pte. Ltd., June 2005, ISBN
978-981-256-884-7.
[3] S. Frederico, C. Hibert, R. Fritschi, P. Fluckiger, P. Renaud, A.M.
Ionescu, “Silicon Sacrificial Layer Dry Etching (SSLDE) for free-standing RF
MEMS architectures”, Sixteenth IEEE Annual International Conference on Micro
Electro Mechanical Systems, MEMS'03, Kyoto, Japan, 19-23 Jan. 2003.*

--
Mathieu Hautefeuille - Centro UNAMems, UNAM, México



On Thu, Aug 20, 2009 at 2:31 AM, tarun mudgal wrote:

> Dear all,
>
> I want to fabricate a diaphragm, in that regard i need to etch silicon
> anisotropically having aluminum metal pads already patternized on the
> silicon wafer.
>
> Now, since aluminium also gets etch in TMAH or KOH therefore I am facing a
> stalemate.
>
> If anybody has any ideas or suggestions it would be very helpful.
>
> Thanks
>
> Tarun Mudgal