Composition, microstructure, and properties of crystalline molybdenum silicide thin films produced by annealing of amorphous Mo/Si multilayers

Abstract

Amorphous Mo/Si multilayers were prepared by alternately sputtering Mo and Si onto silicon single-crystal substrates covered with SiO2 and onto substrates covered with polycrystalline Si. The multilayer thickness was about 200 nm and the composition modulation period was about 0.8 nm. The Mo/Si atomic ratio averaged over the multilayer was about 0.6. These specimens were isochronally annealed for 1 h at temperatures up to 1000 °C. In order to analyze annealing-induced variations in composition, microstructure, resistivity, and internal stresses, the specimens were investigated by x-ray diffractometry, Rutherford backscattering spectroscopy, and resistometry. Although the multilayer remained amorphous during annealing at temperatures below 350 °C, interdiffusion of Mo and Si occurred. Above 350 °C the layered structure disappeared and crystalline phases, viz., hexagonal MoSi2, Mo5Si3, and tetragonal MoSi2, appeared successively for increasing temperatures. According to the (equilibrium) phase diagram both hexagonal MoSi2 and crystalline Mo5Si3 were expected to occur simultaneously. It appeared, however, that hexagonal MoSi2 formed first, probably because of difficult nucleation of crystalline Mo5Si3. The hexagonal MoSi2 nucleated homogeneously, whereas Mo5Si3 nucleated heterogeneously. In MoSix layers on SiO2 the Mo5Si3 grew at the outer surface and at the MoSix/substrate interface. In MoSix layers on polycrystalline Si the Mo5Si3 reacted with Si to form hexagonal MoSi2 at temperatures above 700 °C. Finally, the hexagonal MoSi2 phase transformed into tetragonal MoSi2. The resistivity of the MoSix layer decreased distinctly as soon as hexagonal MoSi2 was formed and an even larger decrease occurred when hexagonal MoSi2 transformed into tetragonal MoSi2. The latter resistivity decrease was accompanied by a considerable improvement of overall crystalline perfection of the MoSix layer. The lowest resistivity (58 μΩ cm) was obtained after annealing at 1000 °C. The internal stress in the MoSix layer can be explained by the difference in thermal contraction between the MoSix layer and the Si substrate. After annealing at 1000 °C the internal stress equaled about 2.0 GPa.