Dear Christophe,

Is the material highly doped ?  This is common on heavily doped silicon.
You can use FTIR on a highly etched wafer to get quantitative
measurements of O2 and C.

Ken Smith
Kmbh Associates
4968 Charter Road
Rocklin, CA  95765  U  S  A
510-714-5055 Efax- 510 217 4421 or 561 658 6136

High Purity Float Zone and Specialty CZ Silicon  for Power, IR and
Mirror Optics, Optoelectronics, MEMS, SOI, and other Semiconductor
applications. Service in SOI, Polishing SSP and DSP.

CROUX@RADIALL.FR wrote:
>
> I'm working on micromachinig MEMS in silicon. My problem is about a defect
> which appears after wet etching of Si: a lot of annular rings are easily
> and distingly visible on wafer after etching. These rings are very
> numerous, and concentrics to the center of the wafer.
>      I tried to process few wafers together coming from different
> suppliers, and the problem doesn't appear systematicaly, so I don't think
> it comes from my process.
>      I use KOH to etch silicon, after a thermal grown masking  film of SiO2
> .
>           Has anybody met this problem ?
> Do you have any information about diffusion of polluting like Oxygene or
> Carbon, and about the way to identify
> (qualitative) and measure concentration of them ?
> Is there any stress in  wafers that can explain such rings in the material?
>
>           Thanks a lot.
>
> Christophe Roux