Langmuir probe measurements at incomplete rf{compensation

Low-pressure radio-frequency (rf) discharges are widely used in material processing. Internal parameters of such discharges may be determined by Langmuir probes. Due to the varying plasma potential in rf-discharges only time-averaged probe characteristics may be measured. The usual evaluation of such disturbed characteristics gives incorrect plasma parameters. Often the rf-compensation of the probe used to avoid distortions of the characteristic is incomplete and a certain rf-influence remains. It is shown that the distortion of a probe characteristic because of an rf-signal across the probe sheath may be described in general by the convolution of the undistorted characteristic with an apparatus function which is determined by the shape of the signal. Using this concept of convolution, the effect of rf-fluctuations on the probe characteristic and its derivatives may easily be explained and methods for the approximated determination of plasma parameters from characteristics distorted by rf-influence may be derived. The methods are demonstrated by testing calculations and experimental examples.