2022年2022年激光直写技术 .pdf

Laser writing systems and technologies for fabrication of binary and continuous-relief diffractive optical elements A.G. Poleshchuk*, V. P. Korolkov Institute of Automation and Electrometry SB RAS, Novosibirsk, 630090, RussiaABSTRACT Precision laser-writing systems operated in polar coordinates and direct writing technologies for fabrication of diffractive optical elements and computer generated holograms have been described. These systems can manufacture continuous-relief and binary microstructures with minimum feature sizes of less than 0.6 m and laser beam positioning accuracy of 0.05 m over 300-mm substrates. Hardware and software of the system permit to write on different types of photosensitive and thermal recording materials. Several examples of fabricated diffractive elements have been presented. Keywords : diffractive optical elements, direct laser writing, microlens arrays, aspheric testing, computer-generated holograms.1. INTRODUCTION At present, one of the most promising trends in development of optics is a wide application of computer-generated holograms (CGH) or diffractive optical elements (DOE). These elements are thin relief phase structures and have been already widely used in optical computing devices, optical memory, displays, sensors, etc. Their potentialities depend in many respects on the fabrication technology which should ensure desired technical parameters. Until recently, DOEs were fabricated by means of image pattern generators specialized for tasks of semiconductor industry. They expose resist film covering a masking layer by generating of images in the shape of rectangles, trapezes, and straight lines oriented along two orthogonal axes x-y of the scanning system. Application of these systems for fabrication of most types of DOEs leads to light scattering at corners of microelements and, consequently, additional noises and distortions in the image are generated. There are some types of DOEs for whose description the polar coordinate system is more preferred than the Cartesian one. For example, CGHs designed to test and certify aspheric wavefronts of the primary mirrors in modern telescopes must have the circular diffractive structure fabricated with an accuracy of noworse than 0.1 m at minimal period of less than 1 m, and overall sizes of several hundred millimeters1. The flatness of the optical substrate for the CGHs must be no worse than /20. Such elements can be currently fabricated with the desired precision and quality only by means of writers with circular scanning of focused beam (either laser or electron beam) or by diamond turning2. In the present paper we consider only circular laser writing systems (CLWS) and some of the proposed methods for fabrication of the diffractive elements. First CLWS was created in the middle 1970s3 in order to fabricate axially symmetric computer-generated holograms to test aspheric optics. This work stimulated development of various types of CLWSs and fabrication technologies4,5,6,7specialized for DOEs. Development of the methods for DOE fabrication in the polar coordinate system began at the Institute of Automation and Electrometry of the Siberian Branch of the Russian Academy of Sciences (IA&E SB RAS) in second half of the seventies8. The main effort was focused on creation of high-precision CLWSs and development of the thermochemical technology for resistless manufacturing masks in thin chromium films9. At present, CLWSs developed by the IA&E SB RAS together with the Technological Design Institute SB RAS are used for DOE fabrication in Russia and in some other countries (Italy, Germany, and China)10, 11,12.Our diffractive elements were applied for certification of 6.5-m and 8.4-m main mirrors of telescopes made in the Steward Observatory Mirror Lab of Arizona University (USA) 1,13. Our efforts were also directed on the development of new methods for manufacturing of continuous-relief DOEs 14. *poleshchuk iae.nsk.su; tel: 007-383-3333-091, Fax: 007-383-3333-863; http:/www.iae.nsk.suIntl. Conf. on Lasers, Applications, and Technologies 2007: Laser-assisted Micro- and Nanotechnologies, edited by Vladislav Panchenko, Oleg Louchev, Sergei Malyshev, Proc. of SPIE Vol. 6732, 67320X, (2007) 0277-786X/07/$18 doi: 10.1117/12.751930Proc. of SPIE Vol. 6732 67320X-1Downloaded from SPIE Digital Library on 30 Oct 2010 to 61.175.193.50. Terms of Use: http:/spiedl.org/terms名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 1 页，共 10 页 - - - - - - - - - In the present paper the latest results of the development of circular laser writing systems and fabrication technologies for them have been reviewed. The practical results of DOE fabrication in IA&E SB RAS have been presented. 2. SYSTEM AND TECHNOLOGIES FOR DOE FABRICATION There are two ways for application of laser writing systems in DOE fabrication (as shown in Fig.2). The first way is a mask fabrication and then photolithographic processing. The masks can be amplitude or grayscale. This way has a good preference for the mass production of DOEs with average quality and specifications. The second way is a direct laser writing of binary or continuous-relief (blazed) profiles. This way is more preferable for fabrication of unique DOEs with excellent optical specifications, e.g. for wavefront shaping. Bellow we will more concentrate on the second way application of laser writing systems for direct fabrication of high-precision DOEs. Fig. 1. Methods for fabrication of binary and blazed DOEs. 3. SPECIALIZED LASER WRITING SYSTEM Fig. 2 shows a simplified functional diagram of the developed CLWS. We will consider briefly its principal units for better comprehension of the writing methods. Rotation unit. The substrate with the deposited recording layer is fixed on the faceplate of the air bearing spindle by a vacuum chuck. The angular encoder and the system of frequency multiplication and phase-lock control form clock pulse train ensuring a certain number of clock pulses per revolution (usually in range of 2-3 millions). The pulses are used to synchronize the laser writing beam modulation with the substrate rotation. Radial displacement unit. The air bearing linear table and the linear dc motor provide displacement of the optical writing head. The laser interferometer controls displacement of the linear table with a resolution of about 0.6 nm. Accuracy of positioning at the given coordinate is 20 nm rms. Writing beam power control. Two acousto-optic modulators AOM1 and AOM2 control the beam power of the argon laser operating at a wavelength of 457 or 514 nm. Using the two modulators allows us to extend the dynamic range of beam power control and separate functions of the beam modulation: ?compensation of changing linear scanning speed of the writing beam with radial coordinate (AOM1); ?modulation according designed microstructure (AOM2). Specialized laser writing system Masks fabricationDirect writing Binary mask Grayscale mask Halftone mask Binary DOE Blazed DOE Blazed DOE Blazed DOE Binary DOE Proc. of SPIE Vol. 6732 67320X-2Downloaded from SPIE Digital Library on 30 Oct 2010 to 61.175.193.50. Terms of Use: http:/spiedl.org/terms名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 2 页，共 10 页 - - - - - - - - - AOM1 included in the feedback loop from a signal of the photodetector PD1 suppresses also fluctuations of laser radia-tion power. AOM2 carries out binary or analog modulation (12 bits) depending on type of DOE. The vector pattern generator placed in the control computer controls both modulators by forming an analog signal for VHF drivers. The vector pattern generator is synchronized with the spindle rotation by clock pulses and pulse of beginning mark of revolution .Optical writing head. The optical head is displaced vertically on air bearings and focuses the collimated laser beam in 0.6 m spot. The autofocus system holds the objective focal plane (N.A. =0.65) on the substrate surface during writing. Photodetector PD 2 is used to measure intensity of a beam reflected by recording material and to test an influence of the beam on the recording material. Photodetector PD3 is intended for calibration of the whole modulation channel. Air bearingFace-plateLinear motorOptical headLinear tableSubstrateSpindleAngular encoderAOM1AOM2Patterngenerator12 bitsPD2Controlling computerAr+ laserVHF driver 2VHF driver 1PD3Fig. 2. Circular laser writing system. (a) (b) Fig. 3. Circular laser writing systems CLWS-300IAE (a) and CLWS-200S (b). Pictures of the laser writing systems CLWS-300IAE and CLWS-200S developed at IAE are shown in Fig.3. Main specifications and performance of the developed writing systems are summarized in Table 1. Proc. of SPIE Vol. 6732 67320X-3Downloaded from SPIE Digital Library on 30 Oct 2010 to 61.175.193.50. Terms of Use: http:/spiedl.org/terms名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 3 页，共 10 页 - - - - - - - - - Table 1. Performance and Specifications of the laser writing systems. Parameters CLWS -300IAE CLWS-200S Maximum diameter of the writing field 300 mm 200 mm Substrate thickness 1.5 30 mm 1.5 20 mm Recording spot diameter 0.6 m 0.6 m Rotation speed 300800 rpm 300800 rpm Absolute accuracy of radial coordinate positioning (rms) 0.1 m 0.05 m Accuracy of angular coordinate measurement (rms) 1 arc sec 1 arc sec Recording wavelength 457514-nm, Ar laser 457514-nm, Ar laser Typical writing time for writing field with 90 mm diameter 2 h 2 h Dimensions/Weight 1.5 x 1 x 1.4 m3/12t 1. x 0.7 x 1.1 m3/0.6t 4. METHODS FOR FABRICATION OF BINARY AND CONTINUOUS RELIEF DOEs 4.1. Thermochemical method for fabrication of binary DOEs The sequence of operations for binary phase DOE fabrication is shown in Fig. 4a. CLWS write binary amplitude microstructures by direct writing on chromium films deposited on optical substrate (I). Heating by a laser radiation (II) results in formation of a thin oxide layer on the film surface and in a change of the chemical properties through the film thickness15. This thermochemical effect allows us to generate directly patterns with spatial resolutions better than 1500 mm-1onto bare chromium films. The writing beam power is set to produce a track with the prescribed width and a sufficient optical density. After writing the mask the unexposed areas of the chromium film are removed in a liquid alkaline etchant15 (III). The thermochemical method of producing diffractive structures in chromium films is optimal for fabricating highly precise DOEs on nonstandard substrates since it does not require technological steps of resist spinning and development. The chromium pattern (III) can be used directly as a binary amplitude hologram or applied as a mask for replicating the elements. Additionally, chromium pattern can be used as a protective mask for reactive-ion etching of the fused silica substrate (IV). After removing (V) masking layer the binary phase DOE is ready. (c) Resist filmIII III IVVLaser radiation Ion beam Cr film III III IV V Laser radiation Ion beam Removing the Cr filmDevelopment (a) (b) (d) Fig. 4. Methods for fabrication of binary (a) and continuous relief (b) DOEs and typical examples (c, d) of DOE surface. Proc. of SPIE Vol. 6732 67320X-4Downloaded from SPIE Digital Library on 30 Oct 2010 to 61.175.193.50. Terms of Use: http:/spiedl.org/terms名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 4 页，共 10 页 - - - - - - - - - 4.2. Peculiarities of direct laser writing on photoresist for circular laser writing systems The direct laser writing technique for the circular systems differs from typical process of laser writing on photoresist16,17,18, which consists of following main steps: photoresist spinning, backing, laser exposure, and development. We added uniform preexposure19 by broad UV beam to work in the practically linear part of the characteristic curve of photoresist (depth H as function of exposure dose E). The preexposure with energy dose Epre also permits to use more effectively the beam power modulation range and to reduce considerably profile roughness due to laser power fluctuations20 and instability of interscan distance. The reduction coefficient can be estimated by the following equation for the total exposure dose: )1()(prelaslaslaslaslaspretotalEEEEEEEE+?+?+=, where Elas and ?Elas are, correspondingly, averaging laser exposure dose and fluctuation of the laser exposure dose. Hence the relative laser exposure fluctuations ?Elas/Elas are reduced by factor Kf : Kf=(Elas+Epre)/ElasFor a profile depth of 1 m the value Elas is approximately equal to Epre. Therefore Kf is equal to 2 for the maximum profile depth and increases with decreasing profile depth (with decreasing Elas). Fig. 5. Two modes of controlling the laser beam power: pulse mode (a) and continuous mode (a). At direct writing on photoresist the optical modulation channel must compensate a change of linear scanning speed in range of 1:10000 and provide range of 1:100 for multi-level writing. Total dynamic range 1:1000000 can not be achieved even with two optical modulators. Besides that testing a recording material near rotation center is quite complicated because the beam scanning speed considerably changes even on size of test structure. This problem can be solved by using of pulse-writing mode combining an amplitude modulation with pulse-frequency modulation (Fig. 5) that is typical for x-y writers. The pulse exposure mode is used at low linear speed of writing beam (near rotation center). Angular period of pulses is calculated in condition that overlapping value of the adjacent light spots is constant: = /2 r,where r - the radius of a written circular track, - constantlinear distance between points of switching on the adjacent pulses. The constant overlapping at short pulse duration ensures independence of exposure dose on radial coordinate. When scanning speed is high and laser beam considerably shifts for time , it is necessary to change the exposure mode to continuous one without pulse-frequency modulation. Continuous mode requires less writing beam power. The ratio Pcw/Ppulse between power levels for pulse Ppulse and continuous Pcw modes can be estimated as ?Intensity?Intensity = /2 rProc. of SPIE Vol. 6732 67320X-5Downloaded from SPIE Digital Library on 30 Oct 2010 to 61.175.193.50. Terms of Use: http:/spiedl.org/terms名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 5 页，共 10 页 - - - - - - - - - ,/?TPPpulsecwwhere T period of pulses at the boundary between area exposed with different modes. This expression is approximate because light intensity can not be completely switched off at pulse exposure mode and there is always some beam shift for time . The pulse duration is defined on the one hand by optical modulator, and on the other hand by laser power which should be enough to expose photoresist at short pulses. 4.3. Direct laser writing of gray-scal