STRUCTURAL CHARACTERIZATION AND THERMAL-STABILITY OF W/SI MULTILAYERS

Tungsten/silicon multilayers with tungsten layers of a thickness of 1-2 nm were prepared by means of electron beam deposition. Their structure and thermal stability under rapid thermal annealing were investigated by a combination of x-ray diffraction techniques and cross-sectional transmission electron microscopy. The crystallization behavior was found to depend on the interdiffusion and mixing at the tungsten/silicon interfaces during deposition as well as during annealing. The as-deposited tungsten/silicon multilayers were amorphous and remained stable after annealing at 250-degrees-C/40 s. Interdiffusion and crystallization occurred after annealing all samples from 500-degrees-C/40 s up to 1000-degrees-C/20 s. By performing the same heat treatment in the tungsten/silicon multilayers, the formation of body-centered cubic W was observed with a layer thickness ratio delta(W)/delta(Si) = 1, whereas tetragonal WSi2 was detected in tungsten/silicon multilayers with a layer thickness ratio of delta(W)/delta(Si) less-than-or-equal-to 0.25. This dependence of the crystallization products on the layer thickness ratio delta(W)/delta(Si) originates from the different phenomena of interdiffusion and mixing at the tungsten/silicon interfaces. The possible formation of bcc tungsten as a first stage of crystallization of tungsten-silicon amorphous phase, rich in tungsten, is discussed.