ION ENERGY-DISTRIBUTIONS ON SURFACES EXPOSED TO PLASMAS - AN EXPERIMENTAL AND THEORETICAL INVESTIGATION

The plasma sheath is a space region between a plasma and the surface of a solid next to the plasma. The plasma sheath influences the plasma assisted deposition processes of thin films. It is also a determining factor for particle extraction as a plasma diagnostic tool. Ions were extracted from a plasma, produced by a dc hollow cathode discharge. The extracted ions were mass analyzed and simultaneously their energy distributions were measured. During each experiment, the discharge parameters were kept constant, whereas the sheath voltage and therefore the thickness of the sheath in front of the extraction orifice was varied. In this way the influence of the sheath on the composition of the ion flux and on the energy distributions of the various ion species could be observed. The gases used were hydrogen, hydrogen-argon, and hydrogen-silan mixtures in the pressure range of 0.08-1 mbar. The ion current densities were about 1 mA/cm2 and the sheath voltages were varied from floating potential to 160 V. A numerical model was developed to calculate the potential distribution in the sheath and the energy distributions of the various ion species. These calculations were done by an iterative method to obtain a self consistent solution. By comparing the results of the calculations with the measured ion energy distributions and ion intensities, conclusions about collision processes of ions and neutrals could be made. The investigations show that due to collision processes, the composition of the ion flux passing the sheath is considerably changed and the energy distributions of the various ion species can be explained by a few simple collision processes.