Search

This work is focused on the study of electrical and optical properties of a helium glow discharge during the treatment of polyethylene terephthalate films for the modification of their surface properties. The experimental results reveal that the change of the polymer surface properties, due to the bombardment by energetic plasma species, is accompanied by a profound change of the electrical characteristics of the discharge and a spatially uniform quenching of certain emitting metastable species. The threshold-like evolution of these phenomena with increasing rf voltage indicates that they are likely to be attributed to ions accelerated in the polymer-covered electrode sheath.

An investigation of the relation between silane electron impact dissociation and deposition rates of microcrystalline silicon thin films, has been performed in highly diluted SiH4in H2discharges, by applying a combination of experimental measurements and modeling of the process. A wide range of frequencies (13.56 MHz - 50 MHz), power densities (11 mW/cm2- 162 mW/cm2) and silane partial pressures (2% - 6%) at two total SiH4/H2pressures of 0.5 Torr and 1 Torr has been studied. In the lower pressure, independent of all other discharge parameters, SiH4primary dissociation has been found to be responsible for about 70% of the total silane consumption in the discharge, while a fraction of 12% of the initially produced silicon hydrides are incorporated into the growing film. The increase of pressure leads to a drop of the contribution of the SiH4primary dissociation to the total silane consumption and to an increase of the deposition efficiency of the initially produced radicals to a value of 26%. This result is attributed to the production of additional, two silicon atom precursors via secondary gas-phase reactions.

The effect of the double layers formation on the gas-phase composition of SiH4/H2discharges and the deposition rate of microcrystalline silicon thin films has been investigated by applying mass spectrometric and film growth measurements. Spatially Resolved Optical Emission Spectroscopy has been used for the detection of the appearance of this additional electron heating mechanism that in the present conditions results from the variation of either the discharge power or the total gas pressure. The experimental results have been in al1 cases combined to a gas phase and surface simulator of SiH4/H2discharges allowing thus the discussion for the rather limited effect of this mechanism on the deposition process of µc-Si:H thin films.

The effect of various process parameters on the deposition rate of microcrystalline hydrogenated silicon is presented. The various pathways leading to high deposition rates involve the optimization of a combination of parameters, while maintaining the crystalline character of the film and avoiding the presence of particles. The deposition rate increases in conditions that enhance and at the same time “cool down” the electron population. Such conditions are: moderately higher frequencies, higher pressures, and the presence of larger molecules (like disilane) even in small quantities. There is an optimum pressure, determined by primary dissociation, for a certain silane fraction. In addition, silane fraction and power density must also increase up to the limit of transition to amorphous growth and before attachment becomes too important. In all of these cases, there is a need for optimizing the distance of the deposition substrate to the source of radical generation, especially at higher pressures, to further increase the already important contribution of higher silicon radicals. It is shown that similar deposition rates can be obtained via radical fluxes with very different compositions. In every case, there is a need for sufficient H atom fluxes to ensure crystalline growth.

The usefulness of optical diagnostics for the investigation of gas discharges is outlined through the presentation of the application of Laser Induced Fluorescence, Spatially Resolved Optical Emission and Laser Scattering, in PE-CVD of a-Si:H. Spatial profiles of SiH and SiH radicals and particle scattered light are presented in various discharge conditions. Their information content is analyzed and the possibility of correlation with discharge and film characteristics is presented.