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

We present investigations of the mechanisms of a laser-induced transfer tec hnique, which can be used for the spatially selective deposition of materia ls such as Si. This transfer is effected by irradiating the backside of a h ydrogenated amorphous silicon film, deposited on a transparent substrate wi th an excimer laser pulse. The resulting release and accumulation of hydrog en at the film/substrate interface propels the silicon onto an adjacent rec eptor wafer. Time-resolved infrared transmission measurements indicate that the amorphous film is melted by the laser pulse and breaks into droplets d uring ejection. These droplets travel towards the receptor substrate and co alesce upon arrival. The transfer velocity increases as a function of fluen ce, the rate of increase dropping noticeably around the full melt threshold of the film. At this fluence, the transfer velocity reaches values of arou nd 1000 m/s for typical films. Atomic force microscopy reveals that films t ransferred below the full melt threshold only partially cover the receptor substrate, while uniform, well-adhering films, which can be smoothed by sub sequent laser irradiation, are obtained above it. Transfer of hydrogen-free Si films, on the other hand, does not occur until much higher fluences. Th e dynamics of the process have been simulated using a semiquantitative nume rical model. In this model, hydrogen released from the melt front is instan taneously accumulated at the interface with an initial kinetic energy given by the melting temperature of Si and the enthalpy of solution. The resulti ng pressure accelerates the Si film, the dynamics of which are modeled usin g Newtonian mechanics, and the gas cools adiabatically as its kinetic energ y is converted to the film's momentum. The results of the calculations are in good agreement with the experimental data. (C) 2000 American Institute o f Physics. [S0021-8979(00)07904-4].