Gas phase and surface kinetics in plasma enhanced chemical vapor deposition of microcrystalline silicon: The combined effect of rf power and hydrogen dilution

Abstract

A gas phase and surface simulator of highly diluted silane in hydrogen rf discharges used for the deposition of microcrystalline silicon has been developed. The model uses the spatial density distribution of SiH (X 2Π) radicals measured using laser induced fluorescence and the total silane consumption for estimating the primary electron induced silane dissociation, thus avoiding fluid or statistical approaches commonly used for the prediction of electron impact rate coefficients. A critical analysis is made for the relative importance of all the parameters involved either in the gas phase chemistry or in the surface processes. The model results are compared to experimental data concerning disilane production and film growth rate over a wide range of rf power densities in 2% and 6% SiH4 in H2 discharges. The good agreement between experimental and model results allows for the extension of the discussion to the composition of the radical flux reaching the substrate, the relative contribution of each of the radicals to the film growth, and the most probable mechanism of microcrystalls formation under typical conditions of low and high microcrystalline silicon deposition rate.

Exploration of the deposition limits of microcrystalline silicon

“…Figure 9 is a perfect example of the wide variety of situations leading to films with similar structure and optoelectronic properties. The figure is based on the results of a numerical simulation that is in close agreement with measurable experimental quantities like the deposition rate and the effluent composition [16]. These four films are deposited under quite different conditions, by a different flux of precursors leading to different growth-dominant species in each case.…”

“…In general, the limits of µc-Si:H growth, for a certain deposition temperature, depend on the rate of arrival of silicon precursors vs. the rate of their accommodation in the growing film network, and this is thought to be mediated by a high flux of hydrogen atoms [14,16,[28][29][30]. The flux of silicon precursors will in turn depend on their space-dependent rate of generation as well as on their gas-phase reactivity vs. transport to the deposition surface.…”

“…VHF is associated with very high power losses in the feed lines, as well as with deposition non-uniformities owing to standing and evanescent waves on large-area electrodes [36]. Figure 1 presents the effect of frequency, varied in a range where large-area uniformity is still not a problem, on both the measured deposition rate [22] and the calculated electron impact dissociation rate of silane [16]. As frequency increases, the constant power dissipation induces a drop of the electric field, an increase of the electron density and an enhancement of the power consumed by electrons.…”

“…These results can be further discussed in terms of reactivity vs. transport of the main radicals, by means of numerical modeling based on experimental measurements of radical generation in space, silane consumption, and deposition rate [16].…”

“…Still, the two processes are only partially alike, mostly because of the high hydrogen dilution of the silicon precursor gas and the quite different operation regimes used for depositing µc-Si:H. In fact, highly diluted silane discharges are in many ways different from the thoroughly studied pure silane or pure hydrogen discharges, and, thus, the accumulated extensive related literature on the deposition of a-Si:H is not particularly enlightening [16]. Nevertheless, the need for studying SiH 4 /H 2 discharges does not rise from pure scientific interest only; the increase of the film growth rate is a critical technical issue that needs to be solved rapidly while being attentive to the associated quality issues.…”