A plasma reactor has a dual frequency plasma RF bias power supply
furnishing RF bias power comprising first and second frequency
components, f(1), f(2), respectively, and an RF power path having an
input end coupled to the plasma RF bias power supply and an output end
coupled to the wafer support pedestal, and sensor circuits providing
measurement signals representing first and second frequency components of
a measured voltage and first and second frequency components of a
measured current near the input end of the RF power path. The reactor
further includes a processor for providing first and second frequency
components of a wafer voltage signal as, respectively, a first sum of the
first frequency components of the measured voltage and measured current
multiplied by first and second coefficients respectively, and a second
sum of the second frequency components of the measured voltage and
measured current multiplied by third and fourth coefficients,
respectively. A processor produces a D.C. wafer voltage by combining D.C.
components of the first and second frequency components of the wafer
voltage with an intermodulation correction factor that is the product of
the D.C. components of the first and second components of the wafer
voltage raised to a selected power and multiplied by a selected
coefficient.