München 2019 – wissenschaftliches Programm
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 Heiler1,2, Roland Friedl1, and Ursel Fantz1,2 — 1AG Experimentelle Plasmaphysik, Universität Augsburg, 86135 Augsburg — 2Max-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 d2I/dV2 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 d2I/dV2 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, H2, D2, N2) 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+.