Improvements to floating double probe for time-resolved measurements in pulsed rf plasmas

A double probe is particularly well suited as a pulsed radio-frequency discharge diagnostic because it imposes minimal perturbation on the plasma, can follow the variations in the plasma potential easily, and requires no fixed potential reference from the plasma reactor walls. As pulsed discharges gain more attention in the semiconductor processing industry the double probe may be a more widely applicable diagnostic than the single probe, especially in plasma reactors which have insulating or semi-insulating walls. In order to use a double probe to make fast time-resolved measurements of electron temperature and ion density in a pulsed discharge, care must be taken to eliminate stray capacitance in the measurement circuitry. This difficulty was overcome by using optically isolated circuitry to achieve an ultimate time resolution of better than 500 ns. There must also be a reliable method for cleaning the probe. This was achieved by adding a third, reference electrode to serve as a needed ground reference. The probe was tested in an inductively coupled discharge reactor with a pulsed argon plasma. (C) 1998 American Institute of Physics.[S0034-6748(98)03103-7].