COMPARISON OF ARSENIC AND BORON-DIFFUSION IN POLYCRYSTALLINE SINGLE-CRYSTAL SILICON SYSTEMS

Comparative studies of As and B diffusion in polysilicon-on-single-crystal silicon systems have been performed by cross-sectional transmission electron microscopy and secondary ion mass spectrometry. Arsenic and BF2 implanted in 300 nm polysilicon deposited by low-pressure chemical vapor deposition were diffused into the underlying silicon substrate by rapid thermal annealing or furnace annealing. Arsenic diffusion profiles are continuous across the polysilicon/single-crystal silicon interface except for a peak at the interface and show a gradual increase toward the interface within the polysilicon layer due to an inhomogeneous distribution of grain sizes in As-implanted polysilicon. On the other hand, indiffusion of B gives a discontinous doping profile at the interface due to accumulation of B-defect complexes created by dissolution of defect clusters in polysilicon. At low anneal temperatures, the B profiles in single-crystal silicon are shallower than the As profiles. This is because most of the B in polysilicon films is immobile during annealing near the peak region and because of low B segregation at grain boundaries, thereby reducing the impact of grain boundaries in terms of high diffusivity. For high thermal budget anneals, B diffusion into the substrate is greater than As diffusion because of higher B diffusivities in single-crystal silicon. At high anneal temperatures, the native oxide at the interface breaks up and causes the polysilicon layer to align epitaxially with respect to the underlying substrate. Time to breakup of the interfacial oxide depends on dopant species and anneal temperature. Oxide breakup takes longer for As-doped samples than for B-doped samples.