Some ideas RE DC-bias from our experience:
Chamber cleanliness:  Deposition onto top electrode sidewalls of chamber
can lower the DC bias.
Carrier wafer: You mention size of carrier wafer, but material *type* of
carrier wafer should be consistent--it effects the bias.
Do you have cooling water in the lower electrode?  If so, check the
resistivity of the water, if possible.  It should be de-ionized.  We
once put city water in as cooling water, and found that the DC bias
dropped significantly.  We currently use a recirculating chiller with
filters.

Hope this helps,
Bill

-----Original Message-----
From: xiaodong wang
Subject: [mems-talk] Questions about DC-bias in dry etching

Dear all,

I have questions about DC-bias in dry etching.