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.
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 development and research of new devices and systems of diffractive and integrated optics, which based on elements with thin-layer micro- and nanostructures, requires the improvement of the technological base. The most massive and lowcost is planar optical elements, on the surface of which diffractive and raster computer-synthesized micro- and nanostructures are formed, as well as structures based on synthesized metamaterials. In recent years, off-axis and axisymmetric computer-synthesized holograms for control and alignment of optical systems, microstructured optical elements with 3D microrelief for complex transformations of wavefronts and intensity distributions of light beams (micro-optics), integrated-optical passive and active circuits have been greatly developed. In this paper describes the principles of operation of two different scanning laser nanolithography system developed operating in a writing and polar coordinate system. Development and research work these lithographic systems were conducted at the Institute of Automation and Electrometry of the SB RAS for many years. The areas of applicability of these systems are described, their differences and technical limitations are considered. The emphasis is made on fundamentally similar units of installations of this class and the prerequisites for their unification are considered. Methods for increasing their resolution, speed and accuracy of writing are proposed, prospects and directions of their development are analyzed. The results of writing test optical elements on metal films using the described methods are demonstrated.
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.
A completely “dry” one-step method for the formation of reflective phase diffractive optical elements (DOE) using direct laser writing on thin Zr film without subsequent liquid or plasma treatment has been demonstrated. The method is based on effect of nanostructure formation during the thermochemical oxidation of the zirconium film deposited on a fused silica substrate. Direct laser writing of diffractive structures is performed by scanning a laser beam with a wavelength of 405- 532 nm, focused into a spot with a diameter of 400-700 nm. When line-by-line scanning with a step of 200-500 nm, the formation of nanogratings from tracks with a width of 70-100 nm and with a period equal to the scanning step was observed. It was experimentally established that lines in the form of cracks or deformations arise along the contour of the spatial distribution of temperature induced by the laser beam heating. Cracks or deformations occur under the influence of thermomechanical stresses at the boundary between the metal film and the oxide line, the thickness of which increases sharply during the oxidation of the metal. The formed nanogratings significantly change the optical properties of the film surface. Measurements by a white light interferometer show the presence of a relief with a depth of -200 to +500 nm, although when oxidizing a 100-nm thick Zr film, the increase in thickness cannot exceed 50-60 nm. Binary diffractive structures with a period of 0.9 - 10 μm, the diffraction efficiency of which exceeded 30%, were made by the new method. The one-step method can be useful for the manufacture of computer-generated holograms used in testing of aspherical surfaces.
The algorithm for estimating the linewidth for metal oxide grating embedded in the grooves of metal grating by analyzing diffractometric measurements in transmitted and reflected light are considered. To calculate the diffraction efficiency under of rigorous electromagnetic theory, the developed algorithm uses the functions of the GD-Calc toolkit. The estimation of the linewidth from the diffraction efficiency analysis for 1μm period of the grating formed on a Ti film has demonstrated discrepancy near 5% with result obtained from analysis microimages.
The accuracy of the asphere testing using computer-generated holograms (CGH) is determined by the precision of the diffractive pattern fabrication. In this paper, two methods for quality control of CGHs are presented. These methods are based on writing small marks, which have microgratings with 2-5 µm period. These marks consist of two parts, one of which is formed before the fabrication of CGH. The second one is embedded during the writing process of the main diffractive pattern. The shift between the first and second parts of the mark makes it possible to determine the drift of the positioning system for both coordinates.
The important effects, techniques, and factors are considered that aim to increase the spatial resolution of a scanning direct laser writing of diffractive structures on thin films of transition metals from titanium group (Ti, Zr, and Hf). The writing process is based on metal oxidation under the thermal action of a tightly focused laser beam. Scanning speed of the laser beam and film thickness were varied to get a regime of through oxidation (TO) of the metal film under laser heating. It results in strong increase of the film transmission in exposed area. TO ensures a strong threshold due to feedback connected with decreasing of laser power absorption near center of focused gaussian laser spot. To the best of our knowledge, the direct laser writing of amplitude diffractive structures on Zr and Hf films were performed for the first time. The new regime of direct laser writing on thin Zr films was revealed. It allows forming tracks with width of 100 nm and less at laser spot diameter of 700 nm and laser wavelength of 532 nm. In this work, the spectral dependence of the refractive index and extinction coefficient of hafnium films was first experimentally determined in the wavelength range of 250–1100 nm.
Methodology of local characterization of continuous-relief diffractive optical elements has been discussed. The local
profile depth can be evaluated using "approximated depth" defined without taking a profile near diffractive zone boundaries into account. Several methods to estimate the approximated depth have been offered.
KEYWORDS: Transmittance, Photoresist materials, Global system for mobile communications, Diffractive optical elements, Binary data, Modulation, Calibration, Manufacturing, Optical modulators, Photometry
The photolithography using gray-scale masks (GSM) with multilevel transmittance is now one of promising ways for manufacturing of high efficiency diffractive optical elements and microoptics. Such masks can be most effectively fabricated by laser or electron-beam writers on materials with a transmittance changing under influence of high-energy beams. The basic requirements for adaptation of existing and developed scanning laser writers are formulated. These systems create an image by continuous movement of a writing beam along one coordinate and overlapping of adjacent written tracks along another coordinate. Several problems must be solved at the GSM manufacturing: the calibration of the influence of the laser beam on a recording material without transferring the gray-scale structure into photoresist; the transmittance at the current exposed pixel depends on surrounding structures generated before recording of the current track and a character of the laser beam power modulation; essential increasing of the computed data in comparison with binary elements. The offered solutions are based on the results of investigations of the materials with variable transmittance (LDW-glass, a-Si film) and takes into account the specificity of diffractive blazed microstructures. The reduction of data amount for fabrication of multi-level DOEs is effectively performed using offered vector-gradient data format, which is based on piecewise-linear approximation of phase profile. The presented approaches to adaptation of laser writers are realized in software and hardware, and they allow to solve the basic problems of manufacturing GSMs.
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