On the use of double probes in RF discharges

The paper deals with the interpretation of double-probe characteristics in the presence of RF voltages. The theory analyses the equations of the double probe with the insertion of electric currents modulated by the radio frequency and an exact solution for the thin-sheath approximation is derived. The case of an ion curve strongly dependent on the voltage is also considered and leads to a new method of RF compensation and to a derivation which, together with the experimental results, gives the electron temperature. Measurements which support the above theory are also presented. The electron temperature obtained from the double-probe characteristic of a thin probe is found to agree well with the electron temperature obtained from single-probe analysis. The plasma density is obtained by the comparison of the ion curves derived from the double-probe characteristic with the radial motion theory. This method does not need a return electrode inside the discharge vessel.