Patent ID: 8192576

Claim:
In a plasma chamber for processing a semiconductor wafer, circuitry for monitoring and adjusting a pulsed RF bias voltage signal to be applied to a chuck in the plasma chamber, the chuck being configured to mount the wafer for processing, the circuitry comprising: an RF bias voltage detector for detecting individual pulses of the pulsed RF bias voltage signal applied to the chuck; a timing circuit for determining a time for sampling each of the individual detected pulses, wherein the time for sampling corresponds to a time when a characteristic peak-to-peak voltage value occurs in each individual detected pulse, the characteristic peak-to-peak voltage representing a maximum peak-to-peak voltage of an envelope of each individual detected pulse; a sample and hold circuit triggered at the sampling time for sampling each of the individual detected pulses to determine and hold a voltage value representing the characteristic peak-to-peak voltage value of each individual detected pulse, wherein the sample and hold circuit is configured to provide a feedback signal based on the characteristic peak-to-peak voltage value of at least one of the detected pulses; and a feedback circuit for adjusting the voltage of the pulsed RF bias voltage signal applied to the chuck according to a difference between the feedback signal and a desired voltage value of the RF bias voltage signal to hold electrical characteristics of the plasma steady during processing to compensate for a variation of an impedance presented to the chuck.