Hi Roger,
I think it not very easy to say if adding He will make the surface
smoother or not. The reason is that generally, if you etch some with lower
etching rate, the surface will be smoother. So, if increase He will
increase etching speed, which was showed in my privious experiments
(assume that this observation is right), the fast-etching effect of adding
He will compansate the surface-smoothing effect. I think that's why my
samples etched with different He concentration seems to be similar in
surface quality.
However, I still don't understand why adding He will increase the etching
speed.

Peng Yao
DOEs lab
Electrical Engineering Dept.
Univeristy of delaware
Newark D.E 19716