Kinetics of Decomposition of Silane (Diluted in Argon) in a Low Pressure Glow Discharge

Abstract

The high temperature electrons in a plasma are able to excite as well as to ionize neutral atoms and molecules. At sufficiently low pressures, the inelastic collisions between electrons and molecules are generally considered to be the most effective mechanism for molecular dissociation in the positive column of a glow discharge. It is therefore expected to obtain, at constant current density, a first-order reaction with respect to the molar fraction of the reagent, if all other reactions are negligible. In the case of silane diluted in argon investigated in this work, the first-order rate equation In Xsim -------kt is not observed; in this equation k is the first-order rate constant, t the gas transit time in the discharge, and XSiH4 the mole fraction of silane at time t. The k value increases with time during decomposition. This is attributed to an increase of the electronic temperature of the gaseous mixture during its transformation.A bibliographical analysis of the achieved researches on electric discharge chemistry shows that very often one is mainly concerned with the aspects of empiricism and reactor stability (1, 2). Some attempts to explain the chemical reaction mechanisms and to establish a general theory were not successful because the investigated systems were too complex. On the other hand, the physical aspects of the electric discharges are not simple and have a large influence on the behavior of the chemical reactions.Therefore, it is necessary (i) to study the influence of the physical parameters of the electric discharge on the total kinetics of an irreversible gaseous decomposition; (it) to work at low pressure and low current density in order to dissociate the thermal and electric phenomena; (iii)to use low pressures in order to limit the number of parallel reactions; (iv) to work with the best known discharges, often studied in an inert medium (rare gases); (v) to study the total kinetics of gases with a simple chemical dissociation mechanism (irreversible decomposition, few synthesis products in low concentration, etc. If the kinetics may be overcome under the previous conditions, it will be possible to investigate more complicated systems (with side reactions, opposing reactions, etc.).The purpose of this work it not to explain the decomposition mechanisms of a gas, but to examine the influences of the physical parameters of the discharge on the kinetics of gaseous decomposition. This paper only presents the experimental results on the silane kinetics. Choice of the Experimental ConditionsGases.--Silane, SiI~, has a simple structure and irreversibly decomposes to form an amorphous thin film on the walls of the reactor (3); the only gaseous reaction products found in our electric discharges are hydrogen with traces of disilane, Si2H6. Furthermore, silane diluted in an inert gas is widely used in microelectronic device technology and, more precisely, in its reactive plasma applications (3) for deposition of thin films on silicon substrates. These applications make use of an rf induced discharg...

Amorphous thin films for solar-cell applications. Final report, September 11, 1978-September 10, 1979

“…The existence of secondary, tertiary, and higher ions in a silane dis- The ion-molecule chemistry of silane has been studied previously by . + several groups [41][42][43]. They find the principal primary ions as S1H 2 and .…”

Analysis of Electron Transport Coefficients in SiH4 Gas Using Boltzmann Equation in the Presence of Applied Electric Field

“…Torr -1 measured by (Shimozuma et a1., 1983) using steady-state Townsend method. For the theoretical approach, (Ohmori et a1., 1986) who (Nolet, 1975).…”

Methods of deposition of hydrogenated amorphous silicon for device applications

“…Mass-spectrometric research on silane decomposition kinetics has been performed for flowing [298,[302][303][304][305][306] and static discharges [197,307]. In a DC discharge of silane it is found that the reaction rate for the depletion of silane is a linear function of the DC current in the discharge, which allows to determine a first-order reaction mechanism in electron density and temperature [302,304].…”