Direct laser thermochemical writing on titanium films for rasterized images creation

Creating greyscale photomasks is an uncommon technical problem which in some cases can be solved by rasterization. At the same time, existing works on the direct laser thermochemical recording show the possibility of forming local areas of transparency as a result of oxidation of thin metal films, but the final contrast of the transmittance coefficient of the resulting structure turns out to be difficult to predict due to the complexity of the web of influencing factors. In the present work, we propose an experimental approach to combining the methods of greyscale thermochemical recording and rasterizing by creating structures with controlled transparency on titanium films which can form the basis for recording topologies of rasterized photomasks. The samples used in this study were thin (20–40 nm) titanium films which were treated using the Minimarker-2 technological complex based on a fiber ytterbium laser. Direct recording with a scanning focused beam was performed using a built-in system of galvanometric scanners. The optical and geometric characteristics of the recorded structures were analyzed using an optical microscope. The experimentally determined recording modes were confirmed by semi-analytical temperature modeling. It is shown that the formation of contrast structures occurs in the ranges of power densities about 15–140 MW/m² when scanning at speeds from 0.1 to 1 mm/s, and the change in the contrast of the structures is achieved at power densities of about 50–90 MW/m². The contrast of the transmission coefficient of the recorded structures relative to the initial value of the film transparency is controlled to vary from 1 to 40 %. In a number of regimes, the formation of periodic structures with a period of about 0.71 μm was revealed, leading to diffraction effects observed in reflected light. The paper presents theoretically modeled and experimentally confirmed modes of recording structures under the influence of nanosecond radiation. It is shown that varying the parameters of the effect allows localizing oxidation regions, which leads to a change in the contrast of the transmitted light and allows creating halftone rasterized images with specified values of grayscale in the transmitted light. The practical significance of the obtained results is demonstrated by the example of recording an optical element such as a halftone rasterized photomask with a specified geometry and contrast values.

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