Fabrication Of High-efficiency Freestanding Blazed Transmission Grating For X-ray Spectrum Analysis

The freestanding blazed transmission grating combines the advantages of the light splitting effect of transmission grating and the non-zero order blazed characteristics of blazed reflection grating.It has a wide range of potential applications in laser inertial confinement nuclear fusion,astrophysics,quantum electronics,and optoelectronics.At present,the main fabrication processes of freestanding blazed transmission gratings include monocrystalline silicon anisotropic wet etching process and deep reactive ion etching process.The former results in a smaller effective area due to the broadening of the bottom of the stiffener,while the latter results in a larger light scattering consumption due to the roughness of the side wall surface,and the diffraction efficiency of the fabricated grating is not high.Therefore,a new fabrication technique should be developed to meet the requirements of large effective diffraction area and smooth sidewalls.A new fabrication process combined with monocrystalline silicon metal-assisted chemical etching technique and hot pressing method was proposed in this thesis.The main contents include:(1)The freestanding blazed transmission grating was designed and analyzed based on scalar diffraction theory.The diffraction efficiency of the grating was simulated and analyzed in MATLAB based on rigorous coupled wave analysis.The solution process is divided into four steps:firstly,the solution area is divided into three areas,which are the incident area,the grating area,and the transmission area;secondly,the expression of the electromagnetic field in each area is obtained by Fourier series expansion;thirdly,the Fourier series expressions of the dielectric constant and permeability constant in the grating area are substituted into the Maxwell equations to derive the coupled wave equations;fourthly,Simultaneous electromagnetic field boundary conditions and the coupled wave equations are used to solve the amplitude and diffraction efficiency of each order.The simulation results show that the blazed transmission grating has a high diffraction efficiency and is not sensitive to polarization at the period of 1μm,the duty cycle of 0.2,the incident angle of 4.5°,and the groove depth of 10μm.The diffraction efficiency is as high as 45%,which meets the expected design requirement.(2)The monocrystalline silicon metal-assisted chemical etching technique has been systematically studied,including etchant,catalytic metal,and the process parameters.Through theoretical analysis and comparative experiments,when the catalytic metal is gold,the type of silicon wafer is(100),and the molar ratio of the etching solution HF:H2O2:H2O is 4.8:0.1:50,the roughness RMS of the grating sidewall is about 0.8nm and the verticality of the grating line is about 89°,which meets the requirements for the roughness and verticality of the blazed transmission grating.(3)A hot pressing process was introduced to increase the duty cycle of photoresist grating mask,whose parameters such as temperature,load and pressure was systematically studied.The hot pressing process is divided into six steps:firstly,placing the grating sample on a hot stage;secondly,covering with PDMS and preheating;thirdly,rolling the PDMS with a glass rod;fourthly,covering with a thin paper sheet and a glass substrate;fifthly,applying a load and heating;sixthly,removing the thin paper sheet and the glass substrate and lifting the PDMS.Through comparative experiments and theoretical analysis,when the temperature is 170℃,the applied load is 400kPa,and the optimized PDMS is adopted,the duty cycle of the photoresist grating after hot pressing is significantly increased,nearly twice that of the original sample,and the line surface is flat and uniform,which meets the requirements of the photoresist mask.(4)The whole fabrication process of the freestanding blazed transmission grating with a period of 1μm has been studied in detail,including the fabrication of grating sample units and freestanding structure masks,the fabrication of metal grating masks,the removal of back substrates,metal-assisted chemical etching and drying.A freestanding blazed transmission grating sample with a period of 1μm was obtained.It was composed of four 5mm × 5mm boxes,whose line height was 10μm and roughness of the side wall was about 0.8nm.The bottom of the stiffener was not widened,and the effective area was up to 70%.As the experimental results show,compared with the traditional fabrication process,the grating fabricated by the new process has a smoother sidewall and a higher effective area,which has laid a solid foundation for the accurate diagnosis of various X-ray light sources.