A consideration of adsorption processes in the CVD of polysilicon

Abstract
Although the growth mechanism of the CVD of polysilicon from silane has been extensively studied for some years, the chemistry of the gas-solid interactions is still not fully understood. In particular, there has been very little consideration of possible bonding modes of homogeneous silane species to silicon surfaces. In fact, most models of silicon growth from silane assume a simple, single site adsorption. In this paper we consider some models based on the interaction of SiH₄ and SiH₂ with dangling bonds on silicon surfaces. Initially, possible structures of silicon surfaces are described and then the adsorption and subsequent heterogeneous decomposition of silane species are examined. From this analysis it is concluded that it is rather unlikely that only a single site will be involved in any adsorption step, and that the most likely number of sites for adsorption of silicon containing species will be two. We then show that after the adsorption step, SiH₂ probably plays an important role in forming Si-Si bonds by rotational or translational steps with the loss of hydrogen. Finally, a more quantitative analysis is given to allow a comparison to be made between theory and experiment. The analysis confirms that the most likely route for interaction of gaseous species with the surface involves two adsorption sites and that subsequent surface dissociation of the adsorbed species leads to incorporation of silicon atoms into the crystal lattice.