CHARACTERISTICS OF BORON-DIFFUSION IN POLYSILICON SILICON SYSTEMS WITH A THIN SI-B LAYER AS DIFFUSION SOURCE

Abstract

A new material, Si-B layer, as boron diffusion source for polysilicon/silicon systems, has been investigated. The Si-B layer was deposited on polysilicon in an ultrahigh vacuum chemical vapor deposition (UHV/CVD) system at 550 degrees C. The characteristics of boron diffusion in Si-B layer/polysilicon/silicon systems have been investigated by using secondary ion mass spectroscopy (SIMS) and cross-sectional transmission electron microscopy (XTEM). To remove the Si-B layer after the drive-in step, the Si-B layer was oxidized completely during thermal drive-in stage and removed with a diluted hydrofluoric acid. The effects of thermal oxidation of Si-B layer on boron diffusion profiles and polysilicon structures were analyzed. It was found that the boron profiles within the polysilicon are slightly dependent on the oxidation of Si-B layer. Moreover, the polysilicon grain size for Si-B layer source were enlarged, as compared with conventional BF2+-implanted polysilicon source. It is attributed to the effects of the gettering of oxygen impurity by the Si-B layer and secondary grain growth during Si-B layer oxidation. In addition, the boron diffusion profiles in the silicon substrate for Si-B layer source exhibited a more shallow junction depth and less sensitivity to the thermal budget, as compared with BF2+-implanted polysilicon source. This is considered to be the effect of the smaller surface concentration, C-s, in the silicon substrate for Si-B layer source.