A self-consistent fluid model for radio-frequency discharges in SiH4–H2 compared to experiments

Abstract

A one-dimensional fluid model for radio-frequency glow discharges is presented which describes silane/hydrogen discharges that are used for the deposition of amorphous silicon (a-Si:H). The model is used to investigate the relation between the external settings (such as pressure, gas inlet, applied power, and frequency) and the resulting composition of the gas and the deposition rate. In the model, discharge quantities such as the electric field, densities, and fluxes of the particles are calculated self-consistently. Look-up tables of the rates of the electron impact collisions as a function of the average electron energy are obtained by solving the Boltzmann equation in a two term approximation for a sequence of values of the reduced electric field. These tables are updated as the composition of the background neutral gas evolves under the influence of chemical reactions and pumping. Pumping configuration and gas inlet are taken into account by adding source terms in the density balance equations. The effect of pumping is represented by an average residence time. The gas inlet is represented by uniformly distributed particle sources. Also the radial transport of neutrals from the discharge volume into the discharge-free volume is important. As the fluid model is one dimensional, this radial transport is taken into account by an additional source term in the density balance equations. Plasma–wall interaction of the radicals (i.e., the growth of a-Si:H) is included through the use of sticking coefficients. A sensitivity study has been used to find a minimum set of different particles and reactions needed to describe the discharge adequately and to reduce the computational effort. This study has also been used to identify the most important plasma-chemical processes and resulted in a minimum set of 24 species, 15 electron-neutral reactions, and 22 chemical reactions. In order to verify the model, including the chemistry used, the results are compared with data from experiments. The partial pressures of silane, hydrogen, disilane, and the growth rate of amorphous silicon are compared for various combinations of the operating pressure (10–50 Pa), the power (2.5–10 W), and the frequency (13.56–65 MHz). The model shows good agreement with the experimental data in the dust free α regime. Discharges in the γ′ regime, where dust has a significant influence, could not be used to validate the model.

Rf discharge dissociative mode in NF3and SiH4

“…We evaluate the dissociation degree from the SiH + 4 peak intensity analysing the tests of the gas mixture taken downstream. However the SiH 4 molecules may be lost in the discharge not only because of the dissociation process but also due to a number of chemical reactions [35,37,38], e.g. :…”

“…It induced the appearance of numerous experimental and theoretical papers devoted to studying the properties of rf discharge in this gas (see, i.e. [31][32][33][34][35][36][37][38][39][40]). …”

Chemical sputtering by H2+ and H3+ ions during silicon deposition

“…The 2-dimensional model of the cylindrically symmetric reactor, with the spatial dimensions r and z, was built and described by Nienhuis et al 25 In the analysis presented here, the fluid model is used to compute an extensive set of time varying plasma parameters in the discharge. The following parameters have been modelled: electric potential, electron energy distribution function, electron density, and radical and ion densities (both positive and negative ions) as well as their fluxes to the electrodes.…”

“…The model of Nienhuis et al 25 was extended in this research with hydrogen chemistry, such as the production of…”

Electron drift velocity in silane in strong electric fields determined from rf breakdown curves

“…Therefore a large number of papers are devoted both to experimental studying and to simulating discharge characteristics in SiH 4 (see, e.g., [11][12][13][14][15][16][17]). In order to use fluid simulations it is necessary to know the electron transport parameters of the gas under study.…”