Modeling of the expansion of laser-evaporated matter in argon, helium and nitrogen and the condensation of clusters

Manufacturing with short pulse lasers (pulse width between 10 and 100 ns) is becoming an established technology in micro structuring, drilling and laser vapour deposition. The material is evaporated in a few nanoseconds and expands into the surrounding atmosphere. In the material vapour behind the contact front supersaturation and the subsequent condensation process are calculated. The formation of clusters leads to an attenuation of the laser beam due to Mie scattering and Mie absorption for wavelengths in the UV range. The propagation of the laser-induced plasma vapour plume in different ambient gas atmospheres and the influence of the gas species on the condensation process is calculated with a finite-difference-scheme including the Euler equations, the front-tracking method and the equations of state. The cluster generation and growth is calculated quasi-stationary by models given by Lifshitz, Sloyozov, Frenkel, Raizer and Zel'dovich. Above a critical cluster radius the growth of a grain is favoured and the clusters increase unhindered. The results show that the velocity of the shock front is much higher in helium than in argon and nitrogen due to the higher speed of sound of helium. The temperature jump at the shock front is, however, fur argon much higher than for helium. The different temperatures close to the contact front due to different ambient gas species influence the supersaturation within the vapour and, therefore, the cluster growth. For ablating aluminum in nitrogen atmosphere cluster radii between 1 nm and 5 nm depending on the chosen parameters are calculated. (C) 1998 Elsevier Science B.V.