Proximity-effect-related reduction of the minimum element size in thermochemical laser writing

This study is intended to identify patterns in reducing the minimum size of writable elements in a structure as a function of the distance between elements when they are produced successively by direct thermochemical recording on a thin metal film using direct laser thermochemical writing, as a result of natural thermo-optical effects. Subject of study. The change in writable-element size (the proximity effect) studied here occurs because reduced absorption at the periphery of the beam as it passes through films on which elements have previously been written results in containment of the energy. Methods. This effect was studied by obtaining a solution to the quasi-steady-state heat conduction equation in closed form while taking into account the time-dependent, nonuniform spatial distribution of optical properties within the sample, using titanium films oxidized by exposure to laser light in air as an example. The model results are compared against available experimental studies of the optical and geometric properties of structures written on a 10 nm titanium film by a focused near-infrared cw laser beam. Main results. This research led to a means of predicting the widths of successively written tracks as a function of track separation. The upper threshold track separation values where the effect begins and the lower threshold values below which the new track no longer appears were determined. Both the theoretical and experimental results reveal that the resolution of thermochemical laser writing on thin metal films that form optically transparent oxides can be increased. Practical significance. The theoretical model proposed in this paper can be used to determine the laser exposure parameters required to produce various high-resolution photonic components, such as diffractive optical elements, half-tone photomasks, etc.