Experimental and numerical investigations of a hydrogen-assisted laser-induced materials transfer procedure

We present investigations of the mechanisms of a laser-induced transfer technique, which can be used for the spatially selective deposition of materials such as Si. This transfer is effected by irradiating the backside of a hydrogenated amorphous silicon film, deposited on a transparent substrate with an excimer laser pulse. The resulting release and accumulation of hydrogen at the film/substrate interface propels the silicon onto an adjacent receptor wafer. Time-resolved infrared transmission measurements indicate that the amorphous film is melted by the laser pulse and breaks into droplets during ejection. These droplets travel towards the receptor substrate and coalesce upon arrival. The transfer velocity increases as a function of fluence, the rate of increase dropping noticeably around the full melt threshold of the film. At this fluence, the transfer velocity reaches values of around 1000 m/s for typical films. Atomic force microscopy reveals that films transferred below the full melt threshold only partially cover the receptor substrate, while uniform, well-adhering films, which can be smoothed by subsequent laser irradiation, are obtained above it. Transfer of hydrogen-free Si films, on the other hand, does not occur until much higher fluences. The dynamics of the process have been simulated using a semiquantitative numerical model. In this model, hydrogen released from the melt front is instantaneously accumulated at the interface with an initial kinetic energy given by the melting temperature of Si and the enthalpy of solution. The resulting pressure accelerates the Si film, the dynamics of which are modeled using Newtonian mechanics, and the gas cools adiabatically as its kinetic energy is converted to the film's momentum. The results of the calculations are in good agreement with the experimental data. (C) 2000 American Institute of Physics. [S0021-8979(00)07904-4].