A CONSIDERATION OF ADSORPTION PROCESSES IN THE CVD OF POLYSILICON

Although the growth mechanism of the CVD of polysilicon from silane ha s been extensively studied for some years, the chemistry of the gas-so lid interactions is still not fully understood. In particular, there h as been very little consideration of possible bonding modes of homogen eous silane species to silicon surfaces. In fact, most models of silic on growth from silane assume a simple, single site adsorption. In this paper we consider some models based on the interaction of SiH4 and Si H2 with dangling bonds on silicon surfaces. Initially, possible struct ures of silicon surfaces are described and then the adsorption and sub sequent heterogeneous decomposition of silane species are examined. Fr om 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 mo st likely number of sites for adsorption of silicon containing species will be two. We then show that after the adsorption step, SiH2 probab ly plays an important role in forming Si-Si bonds by rotational or tra nslational steps with the loss of hydrogen. Finally, a more quantitati ve analysis is given to allow a comparison to be made between theory a nd experiment. The analysis confirms that the most likely route for in teraction 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.