Plasma Diagnostics for Science and Technology

Plasma Rendering

Significance of Accurate EEDF Measurements

In most gas discharge plasmas the EEDF is not Maxwellian, and the assumption of Maxwellian EEDF leads to significant errors in calculations of the plasma parameters and rates of plasma-chemical processes inferred from the classical Langmuir probe diagnostics.

The basic plasma parameters, the plasma density N and the effective electron temperature are calculated as corresponding integrals of the EEDF. Accurate calculation of these parameters is possible only by obtaining undistorted EEDF from the plasma probe measurements.

Examples of typical EEDFs in RF capacitive and inductive plasmas

chart1 chart2 The Maxwellian EEPF on semi-log scale makes a straight line.

EEDF measurements yield meaningful results only when they contain accurate information about the majority of electrons in both elastic and inelastic energy ranges. Low energy electrons, EEDF "head", comprise the majority of the electron population and define the plasma density while the high energy "tail" defines inelastic processes (excitation and ionization).

Learn more about EEDF measurements: Probe Diagnostics Tutorial

The EEDF is found as the second derivative of the probe characteristic I(V), therefore prone to the error augmentation intrinsic to any differentiation procedure. That puts stringent technical requirements on the instrument for EEDF measurements.
Such instrument should precisely acquire the probe characteristic I(V) over wide dynamic range with high energy resolution so that the derivative, d2I/dV2 has the voltage (energy) gap between its zero point and peak no more than (0.3-0.5)Te, and the high energy "tail" extending beyond the inelastic threshold (ε > ε*) undistorted by noise.
Our MFPA instrument insures EEDF accuracy, and for all practical purposes eliminates instrument related distortions.

For more details read: Examples of EEPF measurements with our instruments in basic research and plasma processing.