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At low fluences, the [Sr]/[Ti] ratio of laser deposited SrTiO3 films appears to be a function of the laser fluence. The deviation from stoichiometry is remarkably constant in time. From an analysis of both the composition of the film and the irradiated target, we deduce a volume-diffusion-assisted preferential ablation process. At high fluences (above 1.3 J/cm2), stoichiometric SrTiO3 films are obtained. This is not due to a change in ablation mechanism, but follows from the fact that at 1.3 J/cm2 the calculated diffusion length of Sr within the irradiated target, becomes of the order of the ablation rate per shot.

Using projection optics we made a detailed study of the interaction of a spatially uniform 248 nm excimer laser beam and a 99% dense YBa2Cu3O6+x target. Below a threshold fluence of 1 J/cm2 the roughness of the irradiated target increases dramatically due to non-stoichiometric ablation. The overall target surface composition becomes increasingly Y rich and Cu poor, while the opposite is found for the corresponding ablated thin films. Above the threshold fluence the composition of the ablated target surface is conserved. As a result of the energy homogeneity of the laser beam obtained by means of projection optics, the optimization of the deposition parameters has been improved leading to the reproducible fabrication of flat, stoichiometric YBa2Cu3O7 films with Tc0’s over 91 K.

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