The interaction of a spatially uniform 248 nm excimer laser beam with targets of various oxides (YBa ₂ Cu ₃ O _{7 - δ} , Y ₂ BaCuO ₅ , Y ₂ O ₃ and SrTiO ₃ ) is investigated. We identify two different mechanisms for non-stoichiometric ablation. Evidence is found that in YBa ₂ Cu ₃ O _{7 - δ} and Y ₂ BaCuO ₅ the non-stoichiometric ablation at low fluences is due to a phase transformation of the target surface. This is a gradual process, only observable after prolonged irradiation of the target. For congruently melting SrTiO ₃ such a phase transformation is neither expected nor observed. We find, however, that at low fluences within 100 laser shots the target surface composition equilibrates to a stable, Ti-rich composition. In this case, the mechanism we propose is preferential ablation, which is accompanied by bulk diffusion to compensate the induced surface depletion.

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Projection optics is a means to reduce the spatial energy density variations on the target during laser ablation. Using projection optics to study the interaction between a spatially uniform 248 nm excimer laser beam and a YBa2Cu3O6+x target we observe a well-defined threshold fluence of 1.0 J/cm2. Below this threshold non-stoichiometric ablation results in an dramatic change in target morphology and in a decay of the ablation rate. Above this threshold the stoichiometry of the target is conserved. The roughness of the deposited films increases along with the increase in roughness of the target above the threshold. Thus it is clear that in order to obtain flat stoichiometric films one should use a narrow fluence window just above the threshold.@en

Using projection optics we made a detailed study of the interaction of a spatially uniform 248 nm excimer laser beam and a 99% dense YBa 2Cu3O6+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.@en