Mechanical Properties of Thermally Crystallized Boron-Doped Silicon Thin Films

Abstract

AbstractP-type polycrystalline silicon films on silicon wafers were obtained by annealing at 575 °C boron-doped amorphous hydrogenated silicon (a-Si:H) films. During the anneal, the internal stress of the film changed from compression to tension. The crystallization kinetics became faster when increasing the boron concentration. The hardness and elastic modulus of each film were determined by nanoindentation. The elastic modulus increased systematically upon crystallization. Wafer curvature monitoring during the thermal cycle allowed us to derive the thermal expansion coefficient of a-Si:H for different boron doping levels. Above a critical temperature of 320°C, the internal stress of the a-Si:H films rapidly changed toward a tensile state, independent of the boron concentration. Analysis of the hydrogen-bonding configurations by Fourier transform infrared spectroscopy indicated that this rapid stress change was due to hydrogen out-diffusion. The evolution of the internal stress with time was followed during the 575°C crystallization isothermal plateau. The circular blistering and spalling observed upon annealing in some cases of low doping levels was correlated with the presence of microvoids and with the internal stress of the a-Si:H film.