# München 2019 – wissenschaftliches Programm

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# P: Fachverband Plasmaphysik

## P 6: Low Pressure Plasmas I

### P 6.3: Vortrag

### Montag, 18. März 2019, 17:15–17:30, HS 20

**Determination of the EEDF by a Langmuir probe AC technique in low pressure ICPs** — •Adrian Heiler^{1,2}, Roland Friedl^{1}, and Ursel Fantz^{1,2} — ^{1}AG Experimentelle Plasmaphysik, Universität Augsburg, 86135 Augsburg — ^{2}Max-Planck-Institut für Plasmaphysik, Boltzmannstr. 2, 85748 Garching

The electron energy distribution function (EEDF) in low pressure plasmas is typically evaluated by using the second derivative *d*^{2}*I*/*d**V*^{2} of a Langmuir probe characteristic (Druyvesteyn formula). Since measured *I*-*V* characteristics are inherently noisy, two-time numerical differentiation requires data smoothing techniques which can lead to a loss of information especially in the low energy range of the resulting EEDF. Therefore, an AC probe method was adopted to measure *d*^{2}*I*/*d**V*^{2} directly. This is done by superimposing a sinusoidal AC voltage of 13 kHz and amplitude in the range of 1 V on the probe DC bias and Fourier analysis of the resulting probe current. With this technique the EEDFs are accessible with high accuracy in the low energy range compared to EEDFs determined via numerical differentiation.

The system is applied to several gases (Ar, H_{2}, D_{2}, N_{2}) at an ICP discharge (planar coil, 2 MHz RF frequency, up to 2 kW power). Parameters like the modulation amplitude and number of applied sine oscillations per voltage step of the DC ramp were carefully chosen by systematic parameter variations. The shape of the EEDF for low electron energies, especially important in molecular gases (vibrational excitation processes), is investigated in particular and compared to simulations performed with the Boltzmann solver BOLSIG+.