A Kinetic Model for Boron and Phosphorus Doping in Silicon Epitaxy by CVD

Abstract

A kinetic model based on ͑i͒ the collision theory of heterogeneous unimolecular elementary reactions, ͑ii͒ statistical physics, and ͑iii͒ the concept of competitive adsorption is proposed for both p-type and n-type doping in silicon epitaxy by chemical vapor deposition ͑CVD͒. It takes into account both homogeneous and heterogeneous reactions, which involve the precursors ͑silane and dopant precursor͒ and their homogeneous decomposition products, and four types of surface sites, hydrogen-terminated silicon and dopant sites and hydrogen-free silicon and dopant sites. The model provides analytical equations to describe dopant concentration and silicon growth rate as a function of deposition conditions, including temperature and partial pressure of dopant precursor. At low temperatures, the enhancement in growth rate with diborane is attributed to enhanced hydrogen desorption from boron sites, which act as catalytic sites for silicon growth. The relationship between boron concentration and diborane partial pressure is more complicated than a simple linear one. The suppression in growth rate with phosphine is modeled by a blocking factor that represents the extent of the "poisoning" effect of phosphorous on the surface. Homogeneous decomposition of phosphine accounts for phosphorous doping behavior at high phosphine partial pressures. The model agrees well with the experimental data.Chemical vapor deposition ͑CVD͒ and its derivatives such as ultrahigh vacuum CVD, metallorganic CVD, and atomic layer deposition are widely used in the semiconductor industry for forming epitaxial films of doped semiconductors, p-type or n-type. The critical importance of CVD prompts efforts to understand its kinetics, thermodynamics, and reaction mechanism. The basic process in CVD involves gaseous precursors that undergo decomposition reactions over a substrate and deposit a film of a material contained in the precursors. In Si epitaxy by thermal CVD, several precursors have been used as the Si source, silane ͑SiH 4 ͒, dichlorosilane ͑SiH 2 Cl 2 ͒, trichlorosilane ͑SiHCl 3 ͒, tetrachlorosilane ͑SiCl 4 ͒, and disilane ͑Si 2 H 6 ͒. The most commonly used precursor for p-type doping is diborane ͑B 2 H 6 ͒, whereas n-type dopant precursors include phosphine ͑PH 3 ͒ and arsine ͑AsH 3 ͒. Several groups have investigated the incorporation of B in Si epitaxy from various precursors. 1-5 At low temperatures, an increase in Si growth rate with B 2 H 6 partial pressure has been reported. 2,4 Glass et al. 6 attributed the growth rate enhancement at low temperatures to increased H 2 desorption due to B incorporation. Eversteyn and Put 7 observed an increase in polycrystalline Si growth rate by a factor of 2 at 680°C and a decrease in activation energy of SiH 4 decomposition in the presence of B 2 H 6 . Farrow 8 observed growth rate enhancement at temperatures from 700 to 1100°C, but with very high B 2 H 6 /SiH 4 ratios. The experiments conducted by Shiozawa et al. 9 indicated that the growth rate and B concentration varied as the square root of B 2 ...

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