A study on WSi2 thin films, formed by the reaction of tungsten with solid or liquid silicon, by rapid thermal annealing

Abstract

WSi 2 thin films have been grown on oxidised silicon substrates by the thermal induced reaction of a sputter-deposited poly-Si/poly-W layer. The heat treatment of the samples was performed in a rapid thermal, large area process, using a line electron beam as a heat source. A completely different growth behaviour of WSi 2 is observed below and above 1414°C, i.e. the melting point of silicon. Processing at temperatures below 1414°C leads to WSi 2 formation by a solid state reaction, leaving behind unreacted residues of the initial layers. Processing at temperatures above 1414°C leads to a reaction of the molten silicon with the solid tungsten. In this reaction, all tungsten is consumed. Simultaneously, excess silicon crystallises with a preferential (111) orientation. The dependence of the WSi 2 layer thickness on the process temperature can be explained by a model which takes into account the diffusion and reaction kinetics as well as oxygen contamination because these strongly influence the growth process and thus the thin film characteristics. The samples have been analysed using X-ray diffraction, transmission electron microscopy, secondary ion mass spectroscopy, scanning electron microscopy and electrical measurements. Tungsten silicide thin films, fabricated by the rapid thermal annealing process can, for example, be used as a backside contact of thin film solar cells.