Spatial thickness variations in laser-deposited thin films

We have theoretically studied spatial thickness variations in ‘on-axis’ and ‘off-axis’ laser deposited thin films. During ‘on-axis’ deposition, the substrate-target axes are coincidental to each other, while in the ‘off-axis’ mode, the axis of the rotating substrate is off-set by a fixed distance from the target axis. The ‘on-axis’ laser-deposited films show a wide range of thickness variations ranging from cos2 θ to cos14 θ thickness distribution, where θ corresponds to the angle subtended by the normal at any point on the substrate and the radial vector joining the target and the substrate. The thickness variations are controlled by a number of laser and other deposition variables including laser wavelength, pulse energy density, substrate-target distance and shape and size of the laser-irradiated spot. Calculations show that the laser-irradiated spot size is the most important parameter which controls the spatial thickness variations. It was found that for large laser irradiated spot diameters (>8 mm), the film uniformity was found to decrease with decreasing spot diameters, but this trend reversed at smaller irradiated spot sizes. Depending on the laser irradiated conditions and substrate-target geometry, the thickness variations have been found to vary from (cos θ)2.5 to (cos θ)12. Theoretical investigations were also conducted to understand the effect of processing variables (substrate-target distance, plume directionality, off-set distance, etc.) on the thickness uniformity during ‘off-axis’ laser-deposition conditions. The maximum uniformity diameter (for ± 5% uniformity) was found to be always less than substrate-target distance for directed laser plumes (>cos10 θ distribution).