Thermal Effects on the Hydrogen Passivation of Silicon Wafers During Diode Laser Annealing

摘要

This paper presents a study of the laser induced thermal effects on the hydrogen passivation of silicon wafers. Thermal treatment, due to laser annealing, can be responsible for the release of atomic hydrogen from the dielectric layers, the diffusion of hydrogen into the silicon wafers and the passivation of defects. The whole process results in a reduction of active defects, and hence an increase of the minority carrier lifetime of the silicon wafers. Understanding of the mechanisms of heat transfer during the laser annealing is essential for obtaining an optimal hydrogenation effect. In this paper, a numerical model for evaluation of temporal and spatial temperature distributions in a silicon wafer during laser annealing is developed. The model is developed using the open-source C++ framework known as OpenFOAM. A continuous wave (CW) diode laser of 808nm wavelength is used for laser annealing of the silicon wafers containing dielectric layers of hydrogenated silicon nitride. The simulations are carried out at the experimental conditions and the impact of key laser annealing parameters, such as scanning speeds, laser power densities and substrates initial temperatures, are carefully investigated. The simulated results are then correlated with the experimental results of photoluminescene (PL) imaging and injection level dependent effective lifetime data. Our results show that the localized hydrogen passivation of the silicon wafers using laser annealing depends on the peak temperature and duration of heat. Additionally, it is found that the preheating of the wafer has a substantial effect on the reduction of thermal stress, while leading to a good degree of localized hydrogen passivation of the wafers. A correlation is also developed for the maximum spot temperature which will serve as a guideline for selecting optimum operating conditions.