Application Of A Langmuir Probe Ac Technique For Reliable Access To The Low Energy Range Of Electron Energy Distribution Functions In Low Pressure Plasmas

The electron energy distribution function (EEDF) in low pressure plasmas is typically evaluated by using the second derivative d2I=dV2 of a Langmuir probe I- V characteristic (Druyvesteyn formula). Since measured probe characteristics are inherently noisy, two- time numerical differentiation requires data smoothing techniques. This leads to a dependence on the employed filtering technique, and information particularly in the region near the plasma potential can easily get lost. As an alternative to numerical differentiation of noisy probe data, a well-known AC probe technique is adopted to measure d2I=dV2 directly. This is done by superimposing a sinusoidal AC voltage of 13 kHz on the probe DC bias and performing a Fourier analysis of the current response. Parameters such as the modulation amplitude (up to 1.5 V) and the number of applied sine oscillations per voltage step of the DC ramp are carefully chosen by systematic parameter variations. The AC system is successfully benchmarked in argon and applied to hydrogen plasmas at a laboratory inductively coupled plasma experiment (4- 10 Pa gas pressure, 300-1000W RF power). It is shown that the EEDF is reliably accessible with high accuracy and stability in the low energy range. Hence, a trustworthy determination of basic plasma parameters by integration of the EEDF can be provided.