Study On The Swing Etching Technique Of Curved Gratings With High Diffraction Efficiency

The hyperspectral imaging spectrometer?HIS?is a new kind of spectral instrument which combines spatial imaging technology with spectral acquisition technique to obtain both spatial image information and spectral features of the object simultaneously.With the continuous progress of space optical remote sensing technology since the 1980s,the hyperspectral imaging spectrometer has played a very important role in geological exploration,environmental monitoring,biochemical analysis,clinical medicine and spaceborne observation technology.The Offner concentric optical system has become a prevalent configuration adopted in hyperspectral imaging spectrometers for the advantages of large field-of-view,compact structure,high spectral resolution and low distortion of spectral line.This requires that the convex grating,as the key component,must have the characteristics of large relative aperture,high diffraction efficiency,high groove density accuracy and low stray light,so as to meet the requirements for portability and high resolution of spaceborne imaging spectrometer.As the most common manufacture method of gratings,holographic ion beam etching has been widely used to fabricate planar gratings.However,there are still some difficulties in fabricating convex gratings with a small blaze angle and high diffraction efficiency.The curvature of the convex substrate leads to the inconsistency of the blaze angles and unetched areas on grating surface.In addition,reaction-bonded silicon carbide?RB-SiC?is very suitable for space spectrometer grating substrates because of its excellent chemical and mechanical properties.However,the effects of its material properties on the surface quality and diffraction properties of gratings are not studied.In view of this,a series of studies have been carried out on the ion source and stage system of the ion beam etching machine for curved gratings,the fabrication process of convex blazed grating by swing etching and the characteristics of silicon modified silicon carbide gratings.First,the system composition and working principles of the curved blazed grating ion beam etcher are introduced.The relationship between the uniformity of the ion source and the working parameters is studied,and the method to improve the uniformity of the ion beam is proposed.Second,according to the practical requirements of the curved blazed grating ion beam etcher,the movement trajectory of the three-dimensional stage is calculated theoretically.This paper presents an arc fitting algorithm for the motion control of the three-dimensional stage,which realizes the curve fitting motion required by the etching process.The experiment for measuring the trajectory of the stage with several working parameters is carried out,and the ideal trajectory is compared with the measured trajectory.The experimental results verify the feasibility of the algorithm for motion control of the stage,which meets the requirements for stability,accuracy and disturbance resisting capability of the etcher.Third,a swing etching method to fabricate convex blazed gratings is proposed.The quantitative mathematical relationship between the substrate curvature,grating aperture and the diffraction efficiency of convex gratings is studied.The diffraction efficiency of convex gratings under different parameters is calculated.The limitation of conventional translational ion beam etching method and the necessity of swing etching method are proved.A geometric model is established to simulate the evolution of grating grooves in swing etching process.The effects of the swing speed and beam slit width on the groove evolution are analyzed,and the optimized etching process parameters are calculated.Finally,a convex blazed grating with 45.5 gr/mm groove density,67 mm aperture,156.88 mm radius of curvature and 2.2°blaze angle have been fabricated by swing etching method.The peak diffraction efficiency of the convex blazed grating reaches 90%in the wavelength range of 900nm-2500nm.The effectiveness and feasibility of the swing etching method are validated.Fourth,the effect of the removal depths of silicon modification layers on silicon carbide grating and silicon carbide mirror is studied.The removal depths of 6-14?m is the optimization result for Si-modified RB-SiC as mirror substrates,and the removal depths of 9-12?m is the optimization result for Si-modified RB-SiC as grating substrates.Gratings for the atmospheric CO₂ spectrometer whose diffraction efficiency and stray light are 90.5%⁹4%and 5.30×10^-75.45×10^-7 are fabricated on Si modified RB-SiC substrates.In addition,the number and scale of high reflection points on the grating surface can be used as a basis for judging the removal depth of Si modified layer on silicon carbide grating substrates.These experimental results and rules founded have certain guiding significance for the use of Si modified RB-SiC as microstructural substrates.