Arsenic distribution in bilayers of TiSi2 on polycrystalline silicon during heat treatment

The diffusion behaviour of arsenic in bilayers of TiSi₂ on polycrystalline silicon (poly-Si) was investigated by the He⁺ backscattering technique and sheet resistance measurements. The TiSi₂ films were sputtered from a stoichiometric compound target onto poly-Si prepared by low pressure chemical vapour deposition. X-ray diffractometry was used to identify the phases present after the samples had been annealed at various temperatures. The as-deposited silicide layers were found to be amorphous and to recrystallize into orthorhombic TiSi₂ (C54 structure) on heat treatment at and above 800 °C. Arsenic was either implanted into the already formed TiSi₂ films or directly implanted into the poly-Si layer prior to silicide deposition. In the former case the arsenic was observed to redistribute solely within the silicide film without any significant out-diffusion into the underlying poly-Si layer on subsequent heat treatment. The arsenic initially present in the poly-Si layer, however, was found to migrate into the silicide film at temperatures in the vicinity of 900 °C. This feature makes arsenic incompatible with a particular metal/oxide/semiconductor process, where it is desired to carry out poly-Si drain-source doping by a single ion implantation step if TiSi₂/poly-Si is used as the gate level interconnection material. It is concluded that the behaviour of arsenic in a bilayer of TiSi₂/poly-Si is completely different from that reported for the WSi₂/poly-Si system.