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

Abstract

A kinetic model based on (i) the collision theory of heterogeneous unimolecular elementary reactions, (ii) statistical physics, and (iii) the concept of competitive adsorption is proposed for both p-type and n-type doping in silicon epitaxy by chemical vapor deposition (CVD). It takes into account both homogeneous and heterogeneous reactions, which involve the precursors (silane and dopant precursor) 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.