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