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

E-mail: croux@radiall.fr