Mass-spectroscopy and modeling of capacitive coupled hydrogen plasmas
Gaman, Cezar (2011) Mass-spectroscopy and modeling of capacitive coupled hydrogen plasmas. PhD thesis, Dublin City University.
Full text available as:
This work presents the characterization of a radio-frequency, capacitively coupled, symmetric, hydrogen plasma. Both steady-state operation and the time-prole of the afterglow when RF power is terminated are investigated. Fluxes of the hydrogen ions, H+ , H+2, H+3, at the grounded electrode are measured with an energy-resolved mass spectrometer. Spatial proles of the electron density are measured using a hairpin probe. Particle-in-cell simulations including a complex hydrogen chemistry are performed which enable direct comparison to the experiment.
In the steady-state operation, the electron density increases with both power and pressure, and the ion flux magnitudes and energy distributions are found to vary with power. The H+3 ion flux decreases with power and pressure, whereas the H+ and H+2 ion fluxes increase with power and pressure, with approximately equal fluxes at the highest pressure/power combination of 30.0 Pa and 750V peak-to-peak. In conjunction with the PIC results, it is determined that the H+3 ion remains the dominant ion in the plasma for all investigated parameter space, and that the strong variation in ion flux magnitudes and energy-distributions are due to fast-ion induced chemistry occurring in the sheath at the grounded electrode. A simple theoretical model is developed in order to estimate the electron temperature at the sheath edge if the IEDFs and electron density are known.
Investigations of the afterglow include time-evolution of the H+3 ion energy distribution, spatio-temporal proles of the electron density, and particle-in-cell simulations. The measured H+3 ion flux energy distribution persists substantially longer into the afterglow than is seen in the PIC simulations. This unusual result is explained in the hypothesis of super elastic collision of vibrationally excited hydrogenmolecule with an electron resulting in energy transfer to the electron.
The mechanics such super-elastic collisions are not included in the PIC simulation, and this is consistent with the discrepancy between the simulation and the experiment. Electron density measurements show a substantial increase in the density, as much as a factor of four, sharply rising immediately after the RF voltage is switched off. Small density rises, of order 10%, are seen in the simulation. An analysis showing the validity of the measurements, and two hypothesis to explain the density rise are presented. A method for determining the electron temperature time-prole in the afterglow is introduced.
Archive Staff Only: edit this record