Pulsed-laser evaporation technique for deposition of thin films: Physics and theoretical model.

Abstract

We have studied in detail the physical phenomena involved in the interaction of high-powered nanosecond excimer-laser pulses with bulk targets resulting in evaporation, plasma formation, and subsequent deposition of thin films. A theoretical model for simulating these laser-plasma--solid interactions has been developed. In this model, the laser-generated plasma is treated as an ideal gas at high pressure and temperature, which is initially confined in small dimensions, and is suddenly allowed to expand in vacuum. The three-dimensional expansion of this plasma gives rise to the characteristic spatial thickness and compositional variations observed in laser-deposited thin films of multicomponent systems. The forward-directed nature of the laser evaporation process has been found to result from anisotropic expansion velocities of the atomic species which are controlled by the dimensions of the expanding plasma.Based on the nature of interaction of the laser beam with the target and the evaporated material, the pulsed-laser evaporation (PLE) process can be classified into three separate regimes: (i) interaction of the laser beam with the bulk target, (ii) plasma formation, heating, and initial three-dimensional isothermal expansion, and (iii) adiabatic expansion and deposition of thin films. The first two processes occur during the time interval of the laser pulse, while the last process initiates after the laser pulse terminates. Under PLE conditions, the evaporation of the target is assumed to be thermal in nature, while the plasma expansion dynamics is nonthermal as a result of interaction of the laser beam with the evaporated material. The equations of compressible gas dynamics are set up to simulate the expansion of the plasma in the last two regimes. The solution of the gas-dynamics equations shows that the expansion velocities of the plasma are related to its initial dimensions and temperature, and the atomic weight of the species. Detailed simulations analyzing the salient features of the laser-deposition process have been carried out. The effects of various beam and substrate parameters including pulse energy density, substrate-target distance, irradiated spot size, and atomic mass of the species have been theoretically analyzed. This model predicts most of the characteristic experimental features of the laser evaporation and deposition of thin films. These characteristic features include (a) the effect of pulse energy density on atomic velocities, (b) the forward-directed nature of the deposit and its dependence on energy density, (c) spatial compositional variations in multicomponent thin films as a function of energy density, (d) dependence of the atomic velocities with atomic weights of various species in multicomponent films, (e) athermal non-Maxwellian-type velocity distribution of the atomic and molecular species, and (f) thickness and compositional variations as a function of substrate-target distance and irradiated spot size.