Laser sputtering. Part III. The mechanism of the sputtering of metals low energy densities

Abstract

A series of metals has been bombarded with 12 ns photon pulses at a wavelength of 248 nm. For an energy density near threshold of 2.3 ± 0.3 J cm2, Al, Au and Pt showed surface topography indicative of melting, whereas Mo, Re and W showed topography indicative of remaining solid. Provided the heating efficiencies are comparable with these metals, the results suggest that the surface temperatures were 2500 ± 400 K. To investigate the heating efficiencies, it is shown that the thermal-diffusion time constant is much less than the laser pulse length and that the relevant diffusion solution is therefore of the type ΔT ∞ (1 - R) ( M AρC) 1 2, where ΔT is temperature increase. R is reflectivity, M is molecular weight, K is thermal conductivity, p is density, and C is heat capacity. Considering the values of (1 -R)( M KρC) 1 2 for the six metals studied suggests that it was indeed correct to compare all but Al and, to some extent, Mo. This requires that the temperature estimate be altered to 2750 ± 700 K. Consequences of the present temperature estimates include the possibility, for an energy density of 2.3 ± 0.3 J cm2, of definitively excluding thermal sputtering and of confirming hydrodynamical sputtering as the cause of the craters found on Al and Au. © 1985.