Surface Reactions in Discharge and CVD Deposition of Silane
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SURFACE REACTIONS IN DISCHARGE AND CVD DEPOSITION OF SILANE
Alan Gallagher Joint Institute for Laboratory Astrophysics, University of Colorado and National Bureau of Standards, Boulder, Colorado 80309-0440
ABSTRACT Glow discharge deposition of hydrogenated amorphous silicon films involves; (A) the electron collisions which produce the reactive species, (B) the gas reactions these species undergo while diffusing or drifting to the surfaces, and (C) the surface reactions involved in film growth and gas processing. I will first describe our knowledge of the electron and gas reactions in these discharges, then of the surface reactions, and finally I will offer some conjectures regarding the influence of these different surface reactions and bombardments upon film properties.
THE GAS DISCHARGE At the pressures and electrode spacings normally used for glow discharge deposition the cathode sheath thickness is a large fraction of the electrode spacing, and most of the gas processing occurs in the sheath. (In an rf discharge this sheath occurs at both electrodes.) All rf and dc sheath calculations show the existence of a large group of nearly-thermal low-energy electrons, plus a highly non-thermal high-energy tail [1-3]. These high-energy electrons drive the gas ionization, and as the silane dissociation cross sections rise very rapidly at 8 eV [4], they also cause most silane dissociation. This dissociation results from excitation of silane electronic states in the 8-12 eV region. These have a great deal of excess energy relative to SiHn radical dissociation products, and as a result there is little opportunity for separating H atoms to form H2 during the dissociation process. The primary products are thus expected to be SiH3 +H, SiH2 +2H, SiH3++H, and Si" 2 ++2H (or m2), and this is consistent with Melton's observations of methane dissociation by 100 eV electrons [51, and Lampe's study of silane photodissociation [6]. Following the initial dissociation and ionization, the dissociation products H and SiH2 + react with SiNk to produce more SiT 3 . The overall effect of this dissociation and ionization by high-energy sheath electrons, followed by the ion-molecule and H reactions, is to produce primarily SiH3 radicals, in excess of 90% of all SiHn radicals. At typical discharge pressures (0.1-1 Torr) a large fraction of the other radicals (SiH2 , SiH, and Si) react with SiH4 before reaching the surface. Si" 3 , on the other hand, has no exothermic reactions and only one energy-neutral reaction with SiH4 , and our data as well as typical reaction chemistry indicate that this reaction is extremely slow. Thus, at the surface the dominance of the SiH3 radical becomes even more pronounced. The result is that the film deposition process is initially a result of SiH3 reactions with the surface. This is confirmed by radical density measurements we have made at the surface of pure silane and silane/argon discharges [7]. In the silane/argon mixture, where we deliberately maintained a low silane pressure to minimize
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