Experimental Analysis for the Low-Temperature Growth of Poly-Si Films by Using Double Excimer Laser

Abstract

This paper is to investigate a double splitting excimer laser technique for making poly Si films. In this research, a KrF excimer laser of 248 nm in wavelength is used to irradiate a-Si films of 0.1 m in thickness on glass substrate to produce poly-Si ones. The control parameters are laser intensity (200~500 mJ/cm2), laser pulse number (1~2 shots) and delay time between two shots (one nanosecond). Average grain sizes from SEM photos are used to analyze the effects of these parameters. Firstly, in the excimer laser experiment, different laser fluences are utilized to study the effect on the microstructure of the silicon film. Purely from the viewpoint of heat transfer, the Si film obtains more energy has the slower cooling or solidification rate, which results in the larger grain. From the experimental results, it can be found that the grain size increases until the laser fluence increases up to the critical value of complete melting, which limits the grain growth method of energy increase. In this work, a double-splitting-laser method is proposed. In the method, a laser pulse from an excimer laser is divided into two pulses by a beam splitter. The cyclic optical path is used to control the delay time of the second pulse. Optical mirrors and optical attenuators are utilized to adjust the energy density of these two laser pulses. The delay time between these two pulses is changeable and controlled in the order of nanosecond. The second pulse is applied when the Si film is solidifying after the irradiation of the first one. This could enhance the solidification time and enlarge the grain size of the poly-Si film.