Patent ID: 8175827

Claim:
A computer-implemented method for determining a potential φ p of a plasma, comprising: inserting a probe into the plasma, the probe having a radius r 0 much larger than a Debye length λ D of the plasma, the probe further being connected to a network analyzer; applying an rf signal having an ac voltage and a frequency ω to the probe and sweeping through a predetermined range of dc bias voltages V p , the amplitude of each applied dc bias voltage with respect to chamber ground being much greater than the amplitude of the applied ac voltage of the rf signal and the frequency ω of the rf signal being much less than an unperturbed plasma frequency ω p 0 (ω<ω p 0 ); measuring, with the network analyzer, an ac impedance Z ac of the plasma throughout the dc voltage bias range for a series of fixed frequencies in the range noted above, the ac impedance Z ac including a real part Re(Z ac ) and an imaginary part Im(Z ac ); inputting the values of dc bias voltages V p and the corresponding values of Re(Z ac ) into an appropriately programmed computer; taking, at the computer, a first derivative of Re(Z ac ) with respect to dc bias voltage V p ; and determining, at the computer, the value of dc bias voltage V p for which ⅆ Re ⁡ ( Z a ⁢ ⁢ c ) ⅆ V p = 0 within an error tolerance; wherein the value of V p for which ⅆ Re ⁡ ( Z a ⁢ ⁢ c ) ⅆ V p = 0 equals the plasma potential φ p .