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).