This is a very interesting observation.  Plasma etch processes that
utilize a combination of physical and chemical mechanisms, in my
experience, usually etch faster at the edge of the wafer.  I used to
think that this was due to a chemical loading effect which resulted in a
lower concentration of reactants at the center of the wafer.  In the
situation you mention the etching mechanism is presumably purely
physical, so loading shouldn't be an issue.  The only other explanation
I can think of is that the ion flux in the etcher is altered by the
presence of the wafer, perhaps due to charging of the SiO2.

Roger Shile

-----Original Message-----

I am testing a capacitive RF etching chamber. And when used
30sccm argon gas to sputter with a power of 100W, my wafers
(coated fairly with ~70nm fairly uniform SiO2) came out with
a bizzare pattern. The 3d view of the etched samples resemble
a hill, with its edges etched the most, and the center the
least. This was without using any photoresist or mask. I
would greatly appreciate any plausible explainations.

Xiaoyu