The authors report a new process combining interference lithography with potassium hydroxide (KOH) anisotropic etch
technique for fabrication of high aspect ratio silicon gratings on (110) oriented silicon wafers. This new process has the
ability in fabricating high aspect ratio silicon gratings with extremely smooth sidewalls over a large sample area. An
alignment method was developed to align interference fringes to the vertical (111) planes of (110) oriented wafers. In
addition, a room temperature etch process with 50 wt % KOH solution was chosen to finally get an etch anisotropy of 188.
Better etch uniformity was achieved by adding a surfactant to the aqueous KOH to promote the release of hydrogen bubbles.
To increase latitude in KOH etching process, deposition of aluminum under a sloped angle with respect to the grating
structures was utilized to obtain a high duty cycle nitride mask. To prevent the collapse of high aspect ratio grating
structures caused by surface tension, a liquid carbon dioxide supercritical point dryer was used in the drying process. The
authors successfully fabricated 320nm period gratings with aspect ratio up to 100 on 5-μm-thick silicon membranes on
(110) oriented silicon-on-insulator wafers. The sample area is about 50 mm × 60 mm. The roughness (root mean square)
of the sidewall is about 0.267 nm.
To widen the working waveband, a new holographic parallel flat-field grating (HPFG) with two sub-gratings lying parallel on the same substrate is designed. Grating parameters of the two gratings, one for 2~5 nm and the other for 5~30 nm, are optimized based on the aberration theory of concave grating. The radius tolerances of curvature of the substrate are also analyzed. Ray-traced spectral images indicate that errors cased by ±1% deviation of radius can be offset by shifting the detector position within 2.5 mm. Finally, we analyze the spectral image-focusing properties. Theoretical spectral resolution of this new HPFG is pretty much the same as that of existing holographic flat-field grating. The simulation results demonstrate that our work probably can be used in the compact spectrometers with a broad spectral region and moderately high resolution.
The ideal focal curve for the soft x-ray flat-field spectrometer is a straight line, but the real one is not, thus the inconformity of aberrations between different wavelengths in the working waveband is inevitable. In order to further reduce aberrations and improve spectral resolutions, multi-area gratings (divided perpendicular the direction of grating grooves) are devised. Firstly, the grating is divided into three areas, and the spectral aberrations for these areas are analyzed by means of ray tracing. Then, diffraction efficiencies for the areas with worse aberrations should be reduced to lower the proportion regional aberrations contributing to the overall aberration, therefore better spectral image could be obtained and the spectral resolution would be improved. Theoretical analysis demonstrates that: using multi-area grating, the spectral resolutions at wavelength of 0.8 and 1.1nm are increased from 123 and 333 to 401 and 671, respectively. At the same time, the spectral resolutions at other wavelengths are not reduced.