Study On The Dynamics Of Plasma Expansion In Nanosecond Pulsed Laser Deposition

Pulsed laser deposition (PLD), developed in the 1980's, is one of the new thin film deposition techniques which is most attractive and used widely today, and promises well development and application.The expansion of plasma flow into a vacuum is an important issue in the PLD since the quality of deposited films is critically dependent on the range and profile of kinetic energy and density of the produced plasma. There are great errors between experiments and the literatures which are used widely based on globe conservations of the mass and the momentum, and usually neglect the effects of ionization. To resolve this problem, a new dynamics model of plasma expansion in ns pulsed laser deposition has been developed under the condition the ionization fraction has not been neglected based on the local conservations of the mass and the momentum. The results show that:(1). As a new dynamic source, the ionization accelerates the expansion of plasma in all directions, and can not be neglected in ns pulsed laser deposition.(2). The self-similarity of plasma velocities is the result of plasma expansion, not an assumption employed in many literatures, whether in isothermal expansion or adiabatic expansion though the form of self-similarity varies with the direction of expansion of plasma.(3). The produced plasma will travel away from target surface in a high speed after the cessation of pulsed lased, which agrees with experiments.(4). There are some main factors to affect the velocity distribution of plasma, including the temperature of plasma, the average mass of particles, the ionization fraction and the ratio of the specific heat capacities at constant pressure and volume.By finite difference method, the evolution of plasma is studied for 45ns pulsed laser deposition of diamond-like thin film. We have discussed the evolution of plasma edge and the evolution of plasma density at the centre of target surface on the interaction of pulsed laser with target. Besides, the comparisons of our model with previous model and experiments have been preformed. Some significative results are obtained: during the pulsed laser ablation, the plasma density near target surface varies in a narrow range, and can be considered to be constant. Moreover, the ionization accelerates the expansion of plasma in all directions and causes the augment of all velocities in comparison with other model, and our results are fit with related experiments better than other model.