Holographic Ultraviolet Transmission Blazed Grating

Relying on the space station’s construction,China began to develop a large-scale multicolor imaging and slitless spectroscopy survey,namely the China Space Station Telescope,abbreviated as CSST.As one of the five modules of the CSST,the Sky Survey Module uses 24 high diffraction efficiency transmission gratings as dispersive devices to disperse light in the 255 nm to 1000 nm band before the focal plane.In the replica grating solution,the strong absorption of existing materials in the UV band(255 nm to 420 nm)significantly reduces the throughput efficiency and magnitude range of telescope surveys.In response to this problem,new methods need to be found to develop high-efficiency transmission grating technology in the UV band.The grating team at the University of Science and Technology of China(USTC)has been engaged in technical research on large-aperture diffraction gratings for inertial confinement fusion,which has developed holographic-ion beam etching grating technology.This paper explores the development of UV transmission gratings for CSST using holographic-ion-beam etching technology,including the design,fabrication and detection methods of such gratings.The causes and solutions of the critical problems encountered in the process are also analyzed and investigated.The main results include the following aspects.1.A 333 lines/mm UV transmission blazed grating for 255 to 420 nm was designed.Based on the idea of the segment advancement algorithm,the evolution of groove shape under ion beam etching is simulated.The relationship between ion beam incident angle and blazing angle is established accordingly,and how the mask parameters are designed to complete the etching of the blazed grating.The groove shape obtained after actual etching may deviate from the theoretical blazed grating.Combined with the groove evolution process,four types of irregular groove shapes are discussed:the height to width ratio of the actual mask is small or large,and the critical point between the ion beam incident direction and the initial profile is located at the bottom or sidewall of groove.2.A mixture of CHF3 and O2 is proposed to replace the conventional single gas etching to achieve online removal of redeposition,which improves the blazing angle control accuracy and makes the groove shape evolution more in line with the theoretical expectation.Conventional singular gas etching of CHF3 produces redeposition that accumulates in the shadow region where the ion beam is obscured by the homogeneous mask,causing the final blazing angle to deviate from the theoretical expectation,the groove shape to appear as a step toward the anti-blazing facet as well as increasing the roughness of the blazing surface.The redeposition composition was analyzed by TOF SIMS and was determined to be fluorocarbon organics and successfully removed by adding O2.3.A method to improve the uniformity of large-area gratings using two-dimensional scanning is proposed.The inconsistency of the ion beam currents and dispersion in the onedimensional scanning leads to non-uniformity of the grating groove shape over a large area,which can degrade the luminous flux of the blazed grating and affect the quality of the dispersive element.The method of two-dimensional scanning makes the blazing angle polar difference maintained within 1.1°,which greatly improves the consistency of groove parameters.The average diffraction efficiency of the blazed grating can reach 66%,the diffraction wavefront is 0.169λ,and the size of the effective area of the grating is 99.2×61.0 mm,which meets the technical requirements of CSST.4.As it is difficult to control the blazing angle and the anti-blazing angle by holographic-ion beam etching separately,ion beam etching(subtractive process)or coating(additive process)is proposed to improve the anti-blazing angle to make a high diffraction efficiency blazed grating.Theoretically,the diffraction efficiency of the blazed grating is analyzed in the range of anti-blazing angle in the optimal case,which is adjusted by the additive and subtractive processes,respectively.The experimental results show that both peak and average diffraction efficiency can be improved by 2%to 5%.