Boron Diffusion and Activation during Heat Treatment in Heavily Doped Polysilicon Thin Films for P+ Metal-Oxide-Semiconductor Transistors Gates

Abstract

In this work, we present a study of heavily doped polycrystalline silicon which is used as a transistor gate in standard complementary metal-oxide-semiconductor (CMOS) technology. Our aim is to study the redistribution and activation of boron during thermal annealing in thin silicon films implanted and deposited by low-pressure chemical vapor deposition (LPCVD) using a disilane (Si2H6) gaseous source. The boron concentration is monitored by secondary ion mass spectrometry (SIMS). The resistivity measurements by the four-probe method show that the films become conductors after annealing. Carrier mobility and active doping fraction are obtained by Hall effect measurements. To investigate the SIMS profiles, we have proposed a model based on the one-dimensional numerical resolution of Fick's laws. This model takes into account phenomena due to effects of very heavy doping such as that of clusters. The boron diffusion coefficient and its activation percentage are deduced from the adjustment of simulated profiles with SIMS experimental ones. We have used SUPREM IV software in order to estimate the boron diffusion coefficients in these films and compare them to our results. A small gap between the profiles simulated by our model and by SUPREM IV has been observed, in particular in the region where the boron limit solubility is exceeded. Nevertheless, the diffusion coefficient values obtained by the two methods are of the same order of magnitude.