Investigation of an rf Plasma with Symmetrical and Asymmetrical Electrostatic Probes

A capacitively coupled plasma facility and the techniques implemented to solve problems associated with the use of electrostatic probes in rf discharges are described. Supporting experimental results are also presented. A new technique devised to minimize the rf voltage at the plasma-probe junction is explained, and a floating optically coupled capacitive probe arrangement, for accurate measurement of this voltage, is discussed. Description is made of a feedback network which is used to stabilize rf potentials and plasma parameters over periods of several hours. The calibration of a focused X-band microwave interferometer indicates that the presence of the Pyrex plasma container has no noticeable effect on the electron density deduced from phase-shift measurements. At a pressure of 0.7 Torr in argon, space-resolved electron temperatures, deduced from symmetrical and asymmetrical double-probe I-V characteristics, indicate that the electron temperature is radially constant in the plasma column, and has a value which is 25% higher than that predicted by using the von Engel and Steenbeck theory for a similar positive column. The radially integrated charge density profile, deduced from the probe ion current using the collisionless Laframboise theory, agrees within 15% with that estimated from measurements made with a microwave interferometer. Collision-dominated probe theories are also considered and found to be less applicable.