The paper presents the results of developing a technology for direct laser writing of grayscale photomasks on thin (30-50 nm) chromium films. The contrast of grayscale microimages reaches 10, which is comparable to the characteristics of LDW-glass. The technique is based on dependence of chromium oxide layer thickness formed at laser heating on exposure dose. The oxide layer is then used as a analog mask for underlaying Cr film. The written grayscale masks are used to fabricate multilevel diffractive optical elements by the method of grayscale lithography. The photoresist layer was exposed by contact printing through the grayscale mask. Diffractive optical elements with a sawtooth profile depth in the range of 1-4 μm are made. The angle of the backward slope of the diffractive zones was 60 degrees. The article discusses examples of the use of chromium grayscale masks for fabrication of various 3D structures.
As applied to a maskless laser lithography writing system Heidelberg DWL 66+ and the photoresist-chromium technology, we have found the tolerances on fabricating the binary-phase Dammann gratings having 7x7 useful diffraction orders and a structure separable in X and Y components and illuminated by a beam with a wavelength of 633 nm. It has been found that the obtained tolerances guarantee the production of Dammann gratings with a total diffraction efficiency close to the theoretical one and a root-mean-square error of departure of the intensity of useful diffraction orders of less than 10%.
Bifocal diffractive-refractive intraocular lenses (BDRIOL) are used in ophthalmology to replace natural lenses that are clouded by cataracts. In Russia, the REPER-NN enterprise developed a BDRIOL manufacture technology on the base of a liquid photopolymer frontal polymerization placed between two transparent fused silica moulds. One of them has a spherical shape, the other has a flat surface with a diffractive optical element (DOE). In our BDRIOL design the diffractive structure depth changes so that the energy distribution over the foci weakly depends on the pupil diameter. Zeroth and first orders efficiencies should almost the same. Measurement of this distribution is complicated due to the diffractive profile depth corresponds to the operation of a polymer lens in the eye and is significantly greater than required for working in air. We have proposed to use temporal planarization of the diffractive mould to measure energy distribution over foci. The diffractive structure is filled with a material that provides approximately the same module of refractive index difference with fused silica as between the polymer and vitreous body of the eye. This difference is adjusted by the temperature and time selection at the photoresist baking. The sign inversion of the refractive index difference in comparison with the operating conditions in human eye changes the DOE focus to positive. The paper discussed specific photoresist treatment, refractive index measurement and preliminary results of the developed method.
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