William,
Thanks for your reply.
The chamber cleanliness does affect the DC bias. But the affection will not be
this much (from 90 to 0), I guess.

I didn't pay attention to the carrier wafers. I have lots of them. You are
right. I need pay attention to it. But I use the same Photoresist to hold the
samples. I wonder if the type of wafer will affect the bias when the wafers are
coverd by 6 um PR.

Yes, the machine has the cooling water in the lower electrode. I will check the
resistivity.

Thanks again for your useful information.

Xiaodong


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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.