The results of the development of an algorithm for monitoring long-term instability of scanning laser writing systems in the manufacture of diffractive optical elements are presented. To implement the proposed algorithm, special microgratings of two types (2D calibration diffractive marks (2D-CDM) and 2D diffractive sensor elements (2D-DSE)) are built into the working structure of the element, and control is carried out by analyzing the diffraction efficiency of these structures. It is shown that to control the long-term instability of the coordinates of the diffraction zones formed on the substrate, as well as the height and duty cycle of the written structures, it is sufficient to measure 5 diffraction orders for 2D-CDM and for 2D-DSE. An algorithm for writing embedded 2D microgratings and analyzing the measured values of their diffraction efficiency is presented to obtain the corresponding data on errors in writing the working structure of the DOE.
The results of thermochemical laser writing on bi-layer Si/Ti, Si/Zr, Si/Hf films and single-layer Cr film are presented. Silicon capping layer on top of Ti, Zr, Hf metal films protects them from oxidation in atmosphere and increases optical absorption. It has been shown that the thermochemical effect leads to a significant increase in the reflectance in the exposed areas and makes it possible to produce grayscale structures on mentioned-above materials. Bi-layer Si/Ti, Si/Zr, Si/Hf films make it possible to form directly reflective patterns without etching. In addition, there is a significant increase in the optically measured profile depth between the exposed areas and the original bi-layer film. This effect can be used for one-stage technology for direct laser writing of reflective amplitude-phase diffractive structures on these bilayer materials. Writing on Cr film allows to get larger reflection and transmission change but requires etching.
The paper presents experimental results on the development of thermochemical laser technology for writing on a dual-layer a-Si/Cr film. This technology shows the possibility of manufacturing a transmitting photomask by removing the unexposed a-Si surface layer in the first selective etchant and subsequently etching the chromium layer below it in the second selective etchant. The areas of the chromium film covered with a silicide mask remain untouched during these operations. It has been shown that using a silicon cover antireflection coating (at a wavelength of 405 nm) can significantly increase the resolution of thermochemical laser writing technology. Moreover, the investigation reveals an expansion of the power range of effective laser writing during thermochemical modification of an a-Si/Cr film compared to laser writing on a Cr film, enabling smoother control of the duty cycle of the formed 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%.
Diffractive Transmission Sphere (DTS) can be successfully used for interferometric testing. Unfortunately, phenomena of annular rings occur on interferogram in a case of small period of diffractive patter. This effect reduces quality of measurements and became critical for apertures close to f/1. This work is devoted to computer simulation of such effects and development of technique for their partial compensation. It was proposed to do this compensation by modifying duty cycle. Computer simulation on the base of the rigorous coupled wave analysis was performed. Experimental DTS with was fabricated and tested.
Conformal correctors for high-power solid-state YAG: Nd3 + lasers are used for compensation of density variations inside active media. These correctors decrease laser beam aberrations and allows better focusing of laser light. Fabrication was done by direct laser writing on photoresist. Method of specular spectrometric reflectometry to control the shape of correctors at the stage of relief in a photoresist is considered. The combination of the methods used significantly increases the productivity of the production of correctors, combined with a reduction in the cost of the process.
Computer-generated holograms (CGH) due to their wide capabilities to transform the wavefront of light beams are used to solve various problems including interferometric measurements spherical and aspherical wavefronts. The errors of the wavefront formed by CGH are primarily related with errors in the hologram structure. One of main reasons for the errors in the CGH fabrication by the laser writing technology is the positioning errors of the laser writing system. Earlier methods based on the analysis of the parameters of embedded microstructures were developed to define these errors. In these methods, separate microstructures are used to determine the error value for each coordinate of the beam scanning system. This paper presents the testing method based on writing and diffractometric analysis of embedded small 2D marks containing two pairs of linear gratings. In each pair, one linear grating is shifted relative to the other by given amount. The gratings in one pair are rotated 90 degrees relative to the gratings in the other pair. Each 2D-mark is formed in two stages. One grating in each pair is quickly formed before CGH writing and the second grating is formed directly in the process of the writing. Defining the deviation of the shifts from the given values for each grating pair allows one to estimate the positioning errors along both coordinates by one mark. When the 2D-mark is illuminated with a probe laser beam, the shift leads to a redistribution of the diffraction efficiency between the diffraction orders. The analysis of the obtained diffraction patterns from series of the 2D-marks makes it possible to control the errors of the CGH structure as function of time for whole writing process.
The paper describes methods for manufacturing of diffractive optical elements by means of only "dry" processes starting from direct laser writing on titanium-containing films. According to first approach, direct laser writing onto thin Ti film forms surface oxide mask. Reactive ion etching removes non-oxidized Ti film and develops "latent" oxidized image. Subsequent thermal annealing of the oxidized Ti structure in air makes the mask more stable for following reactive ion etching of fused silica substrate to ensure proper phase depth of the binary diffractive structure. This makes it possible to avoid liquid etching, which reduces the yield and accuracy. The phase structure of the diffractive elements manufactured using the described method consists of the grooves etched in the fused silica substrate between ridges covered by TiO2 between them. We found out also that covering the Ti film by very thin Si layer helps to increase laser energy absorption at direct writing and creates quite resistant masking layer TiSi2 for the reactive ion etching. Preliminary estimates show that dual layer Si/Ti films can be used to create amplitude reflective DOEs. Possible application area for the developed methods is manufacturing of the diffractive optical elements used for precision generation of reference wavefronts in interferometric measurements of spherical and aspherical surfaces.
Controlling an accuracy of fabricating computer-generated holograms (CGH) is actual task. Such holograms are usually used for generating reference wavefront for interferometric testing of aspherical surfaces. The influence of external factors on the positioning systems of the writing system occurs during fabricating the holograms and leads to microstructure errors that affect the quality of the wavefront formed by such elements. Fabrication errors of CGH affect the accuracy and reliability of interferometric measurements. Controlling these errors allows determining the quality of the manufactured element and evaluating the accuracy of the wavefront which it forms. This paper presents the experimental results of using CGH error testing methods for laser writing systems that operate in a polar or cartesian coordinate system. These methods are based on writing of series of embedded small marks with gratings having 2-5 μm period and following measurement of light intensities in curtain diffraction orders. These marks consist of two parts, one of which is quickly formed before the fabrication of CGH and the second one during writing the pattern of the main CGH. The shift between the first and second segments of the mark makes it possible to determine the CGH writing errors caused by external influences on the positioning system for both circular and X-Y laser writing systems. To determine writing errors can be use simple optical diffractometer.
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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