Soft X-ray Varied-line-spacing Gratings Fabricated By Dynamic Near-field Holography Using A Phase Mask~#

Soft X-ray flat-field spectrometers,which have characteristics of aberration?flat focus field and compact structure,are being increasingly applied to synchrotron radiation,high power laser,et al.Soft X-ray varied-line-spacing grating(VLSG)is the core element of flat-field spectrometers.With the performance improvement of various scientific facilities,the demand for high quality soft X-ray VLSGs(high central groove density,high accuracy of grating groove distribution and low stray light)becomes increasingly urgent.Conventional grating fabrication methods:mechanical ruling and holographic lithography have their own advantages and disadvantages for fabricating soft X-ray VLSGs.To meet the need for high precision VLSGs,a fabrication method for soft X-ray VLSGs,i.e.,holographic lithography(near field holography,NFH)using electron beam lithography(EBL)-written phase mask(phase mask)is proposed in this paper.The proposed method can combine the advantages and overcome the problems of the traditional fabrication methods.In this paper,the key issues of fabricating soft X-ray VLSGs by NFH are systematically studied,and the main works are shown as follows:1.Soft X-ray flat field grating with a central groove density of 3600 lines/mm for the spectral range of 0.3-0.6 nm is designed.The exposure parameters of the NFH were optimized for the VLSG fabrication and the corresponding phase mask was designed.The main error factors of the NFH were analyzed and the fabrication errors of the EBL,could be reduced by reverse matching of different exposure parameters of the NFH Finally,the NFH processes for phase mask fabrication are proposed to reduce the production cost.2.Dynamic NFH is proposed to suppress the parasitic structure caused by stitching errors in phase masks.A reference grating is set in dynamic NFH to form moire fringes to monitor the motion,and its theoretical monitoring accuracy can reach nanometer level.Compared with Static NFH,dynamic NFH can suppress the influence of the stitching errors of the phase mask,which leads to photoresist gratings with uniform height and duty cycle,and diffraction efficiency,reduced the stray light,and partially-suppressed Rowland ghosts.3.VLSGs with a central groove density of?3600 lines/mm were fabricated on concave substrates using the proposed method.Typical depth and duty cycle of the fabricated gratings are 3.0±0.5 nm and 0.32±0.10,respectively.Compared with the samples fabricated using static NFH,the samples by dynamic NFH show smoother and more straight lines,more uniform groove structure parameters and diffraction efficiency,and the influence of the phase mask parasitic structure perpendicular to the grating vector direction was significantly suppressed.The measured results of diffraction efficiency are consistent with the simulation at 1-6 nm.4.A method for testing the groove density distribution of VLSGs in plane substrate is proposed.The experimental setup system is established and its feasibility is verified.This system eliminates the influence of size broadening due to the VLSGs,and is especially suitable for the measurement of grating groove density with large groove density change rate.The experimental results show that the maximum groove density deviation between the fabricated and designed VLSGs is within±2 lines/mm,and the relative measurement error of the grating line density is within 5×10-5,smaller than the theoretical relative error of 7×10-5.The effect of the fabrication errors of the NFH on imaging is analyzed,and the image error can be compensated in a certain degree by adjusting spectrometer parameters.