High-Tc thin films prepared by laser ablation: material distribution and droplet problem

The lateral material distribution of laser-deposited YBa₂Cu₃O_7−δ films and the density of droplets coming from the target were studied by varying the laser pulse energy, the laser spot size and the target-to-substrate distance. Silicon wafers at ambient temperature were used as substrates to guarantee a large sticking coefficient of the particles. The deposition rate is found to depend linearly on the laser energy density E and quadratically on the spot size S at the target, whereas the droplet density is slightly dependent on E and increases linearly with 1/S, yielding a threshold energy of 0.9 J cm⁻². With a laser spot size of 7.15 mm² and a laser energy density of 1.2 J cm⁻², we were able to reduce the number of droplets to one to two per 500 μm² for a high quality high film with a typical thickness of 100 nm.