Vacuum-ultraviolet Blazed Silicon Gratings Anisotropically Wet-etched By A Native-oxide Mask

Blazed gratings with asymmetric sawtooth profiles possess the desirable property of concentrating most of the diffracted light into a single nonzero order.However, accurate control of the sawtooth groove profile is difficult when the groove density is high and the blaze angle is only a few degrees,especially for gratings used at vacuurn ultraviolet and soft x-ray wavelengths.Traditional blazed gratings can be fabricated by diamond scribing with a ruling engine or by ion etching combined with interference lithography.Gratings fabricated by these methods suffer from a number of defects,including high surface roughness and poor groove profile,which lead to large amount of stray light and low diffraction efficiencies.Owing to the anisotropic character of single crystal silicon in the etchants of alkaline ions such as KOH solution,gratings with a controllable blaze angle and smooth blaze facets can be fabricated by anisotropic etching of an off-cut(111) silicon wafer under a mask of thermal-oxidation-grown SiO₂ or deposited Si₃N₄.A systemic research was carried out on fabrication of blazed gratings in the vacuum ultraviolet wavelength range on silicon crystal silicon substrates by anisotropic wet etching.According to the severe demands,the qualities of good single crystalline precise cutting,fine polishing and good cleaning were tested.Well-qualified silicon wafers with an off-cut angle of 5°were obtained.Using thermal-oxidation-grown SiO₂ layer as the mask of anisotropic etdhing,we studied the under-cutting of the wet etching of SiO₂ layer theoretically and experimentally.The character of the transferring of photoresist mask into the SiO₂ layer was summarized,the influence of remainder photoresist was observed and the preparatory sample of grating was obtained.Straight and clean photoresist mask with small line-to-period ratio are required and the thickness of the SiO₂ layer should be as thin as possible.Photoresist becomes viscous when heated above its glass transition temperature, so the line of photoresist mask can be smoothened.In order to avoid the influence of gravity,we heated the photoresist mask with the substrate back side up to maintain the thickness of photoresist lines.To reduce line edge roughness and clean the remainder photoresist,between the grating lines,we performed photoresist ashing by O₂ plasma reactive ion etching,which can etch the photoresist lines isotropically.As a result,straight,smooth and clean photoresist masks with small line-to-period ratio were obtained.The desirable blazed grating had a blaze angle of 5°,groove density of 1200 g/mm and a blaze wavelength of about 140 nm.On the surface of a silicon substrate, the thickness of native oxide layer generally ranges from 1 to 2 nm.First we analysed the feasibility of using the native oxide layer as the mask of anisotropic etching,then realized it experimentally.Then we present a simple and convenient method of controlling the profile of a blazed grating that consists of using patterned native oxide layer as the mask of anisotropic etching to obtain near-ideal grooves of sawtooth.With the blazed grating profile well controlled by this technique,a 1200g/mm blazed grating was fabricated,which had a blaze angle of 5.0°and smooth blaze facets of surface roughness about 0.2 nm rms.Efficiency measurements were made at the Spectral Radiation Standard and Metrology Beamline of the National Synchrotron Radiation Laboratory of China.It was found that the most of the incident radiation was diffracted into the -1 order.The grating sample was measured to blaze at the wavelength of 135 nm,where the peak measured absolute efficiency was 53.7%and the corresponding calculated groove diffraction efficiency was 83.2%in s polarization.