Research On Ion Beam Etching For Large-aperture Diffraction Gratings And Relative Problems

The diffractiive optical components are widely used in many applications,including IR Imaging Systems , spectrometer and other modern optical system. Withthe development of the laser technique, the using of large aperture diffractive opticalcomponents which are a important part of the laser are more and more abroad. It ismade by lithography and ion beam etching. The reasearch of how to get betteruniformity of the diffraction efficiency in whole aperture, high diffraction efficiencyand high laser damage threshold is very important problem. How to improve theperformance of the large aperture diffractive optical elements by ion beam etchingprocess is a important research too.The main contents of this paper is the research of ion beam etching technology,which focuses on the etching uniformity and pulse compression gratings diffractionefficiency measurements and others. Recently, with the continuous development ofhigh power laser, the aperture of diffractive optical element is also need to beimproved and a more hard requirements of ion beam etching is needed. How toimprove the uniformity and diffraction efficiency of the large aperture opticalcomponents is urgently needed. Until now, a important progress of the manufacturefor large aperture diffraction optical components has been made, and high powerlaser used these optical components has been built. The object of the ion beametching of the large aperture diffractive optical elements is supported by of the 863national project funding, The author's work is done under the guidance of the teachergroup. The main work is shown following:1. Improve the uniformity of ion beam etchingFor large-aperture diffraction optical components, the increased size broughtgreater challenges to the entire area uniformity of the components. The wholemanufacture process has a very high requirement on uniformity too. The ion sourcewe used has no good uniformity, and the sample plate is scanning on the minor axisonly. So, ion beam current uniformity along the major axis is important parametersfor the type of ion source usce in this paper.The relationship between ion beam current density uniformity and gas flowdistribution, the ion source parameter and the shape of ion beam channel is studiedin this paper. This paper mainly focuses on the relationship between ion beam current density uniformity and the shape of the beam channel. The modificationinclude the measurement of longitudinal beam density and orientation, themeasurement of transverse beam density distribution curve and integration,modification of longitudinal beam density distribution.First, a whole beam channel was made; Second, production of a separategraphite beam channel; third, moving the sample in the long axis to get a much moregood etching uniformity.2. Etching of the Multilayer Dielectric Gratings for Pulse CompressorThe etching of the Multilayer Dielectric Gratings for Pulse Compressor consiststwo parts: the judgments of initial photoresist mask and the modification; the graphictransfers during the ion beam etching.First, the photoresist mask is produced by holographic exposure, and someresidual photoresist is existed in the bottom of groove. So we need to judge thesituation of the mask, and to modify the the shape of the mask by plasma ashing. Theexperiment of plasma ashing used in this paper is done on the photoresist descumsystem for by large diffraction grating which is developed by our team. We use theoptical microscope to check the residual photoresist film both before and afterashingt. Atomic Force Microscopy is used to detect the height and duty cycle of thephotoresist mask. Sample with mall size can also uses Schottky Field EmissionScanning Electron Microscope to see the section and received the results of theashing .Through a large number of statistical results, a qualitative judgments is received,which called mask a high degree of > 300nm, duty cycle > 0.3, the whole area of thediffraction efficiency > 80% and uniform. This paper used the existed measuringinstruments, designed a new system to get more accurate measurement of thediffraction efficiency. This optical system eliminate the stray light such as the lightwith wavelength 808nm, reduce the TM polarization in the laser , eliminate theeffect of light source fluctuations and also reduce the effect of photodetector fatigueduring a long time irradiation.Second, In order to reduce the Difficulty of photoresist mask processing, we useCHF3which has a good Etching selectivity to photoresist and the SiO2film as thework gas. A qualitative judgments for graphic transfer of ion beam etching isreceived through the picture before and after etching which is get from the SchottkyField Emission Scanning Electron Microscope.