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Description
The development of micro- and nanooptics requires improved methods for forming diffractive optical elements with linear dimensions comparable to or significantly smaller than the wavelength. The resolution of traditional optical lithography systems is limited by the diffraction limit.
It is shown that the diffraction limitation can be overcome by using a nonlinear exposure characteristic of the recording medium, optimizing the spatial distribution of the laser beam intensity, and using materials with a threshold mechanism for photostructural transformations.
It was proved that the improvement in resolution is possible with picosecond and femtosecond pulses. For pulse durations ranging from 10 fs to 100 ps, excitation and relaxation processes of charge carriers predominate, whereas for pulse durations exceeding 100 ps, the influence of structural changes and thermal diffusion increases. Reducing the pulse duration allows localization of the energy impact and reduces the thermal influence on adjacent areas of the structure. In the literature reviewed, the formation of quantum dots in a polymer nanocomposite matrix ensured the recording of lines 65 nm wide, which is approximately 1/8 of the laser wavelength. The use of AgInSbTe thermal resist allowed the production of features approximately 130 nm in size, corresponding to approximately 1/3 of the wavelength of the recording laser.
The experimental part of the study was aimed at the formation of code sequences for a modulation disk using direct laser recording. Recording was performed with a 405 nm laser on a glass substrate coated with a photoresist layer. The substrate rotated at a frequency of 7 rps. The linear velocity in the recording zone was approximately 0.8 m/s. The focused laser beam moved stepwise along the radius of the substrate. An automatic focusing system with a piezoelectric lens drive was used to compensate for axial deviations. The runout of the substrates used did not exceed 10 μm.
The quality of the formed elements was assessed based on the surface profile, microrelief, and width of the transition region between the chromium-coated area and the open surface of the glass substrate. The studies were conducted using scanning tunneling microscopy. For samples produced by contact lithography, the width of the transition region was 4.5 μm. Using direct laser writing, this value was reduced to 2.0 μm. Thus, the transition width was reduced by 2.5 μm, or approximately 55.6%, and the resolution of the modulation disk pattern increased by 2.25 times. Structural components of DOEs with a characteristic relief depth of 0.3–0.4 μm were formed on chalcogenide semiconductor films.
Keywords: direct laser writing, diffractive optical element, modulation disk, submicron structure, chalcogenide photoresist, spatial resolution, Bessel beam.