Two-step laser recrystallization of poly-Si for effective control of grain boundaries

Abstract

An excimer laser annealing method is proposed to increase the grain size and control the microstructure of polycrystalline silicon thin film. The proposed method is based on lateral grain growth during laser annealing. The lateral grain growth results when the lateral temperature gradient is larger than the vertical temperature gradient in molten a-Si. We designed a laser-masking window to maximize the lateral growth effect and affect the location of grain boundaries. The masking window transmits the laser beam selectively on a-Si film, so that the lateral temperature gradient is formed between molten a-Si region and the cooler unmelted a-Si region. After the first laser irradiation, a second laser irradiation without masking window is carried out to recrystallize the residual a-Si regions. Employing the proposed method, we obtained up to 1 μm grain size with 80 nm thick a-Si film at 200°C substrate temperature and also observed non-random grain boundaries by transmission electron microscopy (TEM). From observation with laser energy variations, we found that the lateral growth takes place when the laser energy is greater than the full melt threshold.