Huoyao Chen

Alignment method combining interference lithography with anisotropic wet etch technique for fabrication of high aspect ratio silicon gratings.

A method was developed for aligning interference fringes generated in interference lithography to the vertical {111} planes of <110> oriented silicon wafers. The alignment error is 0.036°. This high precision method makes it possible to combine interference lithography with anisotropic wet etch technique for the fabrication of high aspect ratio silicon gratings with extremely smooth sidewalls over a large sample area. With this alignment method, 320 nm and 2 μm period silicon gratings have been successfully fabricated. The highest aspect ratio is up to 100. The sample area is about 50 mm × 60 mm. The roughness (root mean square) of the sidewall is about 0.267 nm.

Investigation on the properties of a laminar grating as a soft x-ray beam splitter

Laminar-type gratings as soft x-ray beam splitters for interferometry are presented. Gold-coated grating beam splitters with 1000 lines/mm are designed for grazing incidence operation at 13.9 nm. They are routinely fabricated using electron beam lithography and ion etching techniques. The laminar grating is measured to have almost equal absolute efficiencies of about 20% in the zeroth and -1st orders, which enables a fringe visibility up to 0.99 in the interferometer. The discrepancy of the grating profiles between the optimized theoretical and the experimental results is analyzed according to the comparison of the optimized simulation results and the measurement realization of the grating efficiencies. By a precise control of the grating profile, the grating efficiency in the -1st order and the fringe visibility could be improved to 25% and 1, respectively.

The method for measuring the groove density of variable-line-space gratings with elimination of the eccentricity effect

To eliminate the eccentricity effect, a new method for measuring the groove density of a variable-line-space grating was adapted. Based on grating equation, groove density is calculated by measuring the internal angles between zeroth-order and first-order diffracted light for two different wavelengths with the same angle of incidence. The measurement system mainly includes two laser sources, a phase plate, plane mirror, and charge coupled device. The measurement results of a variable-line-space grating demonstrate that the experiment data agree well with theoretical values, and the value of measurement error (ΔN/N) is less than 2.72 × 10(-4).

Optimum design of a holographic parallel flat-field grating

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.

Reducing Rowland ghosts in diffraction gratings by dynamic exposure near-field holography

Near-field holography (NFH) combined with electron beam lithography (EBL)-written phase masks is a promising method for the rapid realization of diffraction gratings with high resolution and high accuracy in line density distribution. We demonstrate a dynamic exposure method in which the grating substrate is shifted during pattern transfer. This reduces the effects of stitching errors, resulting in the decreased intensity of the optical stray light (i.e., Rowland ghosts). We demonstrate the intensity suppression of ghosts by 60%. This illustrates the potential for dynamic NFH to suppress undesirable periodic patterns from phase masks and alleviate the stitching errors induced by EBL.

Optical position sensor based on a digital wavelength-encoding grating ruler

Abstract. A wavelength-encoding optical position sensor was designed in this study. The critical component of the sensor is its innovative digital encoding grating ruler (DEGR), which is a substrate on which several blazed grating units with different line densities are arranged parallel to one another following a certain order. Two types of multi-DEGR were designed. We obtained over 100,000 codes that significantly assisted in designing long-range and high-resolution position sensors by optimizing the coding algorithm. The wavelength signals generated by the multi-DEGR were demodulated using concave grating and several photosensitive elements. A 100-mm multi-DEGR with 1000 codes was successfully fabricated using the combined methods of direct laser writing and holographic technology. We described the principle of the sensor in detail and established the entire sensor system. A bench test was conducted to test the signal response of the sensor. Bench test results exhibited 100% accuracy of the signal response of the optical sensor and an excellent temperature performance within −55°C and 75°C.

Anti-reflective sub-wavelength structures at a wavelength of 441.6 nm for phase masks of near-field lithography

With the development of micro- & nanofabrication technology, micro- & nanostructures have been widely used in many fields, including spectroscopy, coding, sensor, subwavelength element, etc. With phase masks realized by a combination of electron beam lithography (EBL), near field lithography (NFH) has great potential to fabricate versatile nanostructures, because it combines the advantages of both lithographic methods. Currently, subwavelength structures attract much attention due to their various functions, such as antireflection, polarization beam splitter and filter. In this presentation, aiming at reducing the interface reflection of a fused silica mask of NFH at a wavelength of 441.6 nm and incidence angles of either 0° or 32°. First, we will compare the difference of antireflection property of one-dimensional (1D) and two-dimensional (2D) subwavelength structures with line density of 3600 lines/mm by simulation. Then, the optimized 1D and 2D subwavelength structures with 3600 lines/mm will be fabricated by using EBL-NFH method. Finally, the antireflection property of these 1D and 2D subwavelength structures will be characterized at the wavelength of 441.6 nm.

Design of soft x-ray varied-line-spacing grating based on electron beam lithography-near field lithography

Soft x-ray varied line spacing grating (VLSG), which is a vital optical element for laser plasma diagnosis and spectrometry analysis, is conventionally fabricated by holographic lithography or mechanical ruling. In order to overcome the issues of the above fabrication methods, a method based on electron beam lithography-near field lithography (EBL-NFH) is proposed to make good use of the flexibility of EBL and the high throughput of NFH. In this paper, we showed a newly designed soft x-ray VLSG with a central groove density of 3600 lines/mm, which is to be realized based on EBL-NFH. First, the optimization of the spatial distribution of line density and groove profile of the VLSG was shown. As an important element in NFH, a fused silica mask plays a key role during NFH in order to obtain a required line density of VLSG. Therefore, second, the transfer relationship of spatial distribution of line densities between fused silica mask and resist grating was investigated in different exposure modes during NFH. We proposed a formulation about the transfer of line density to design of the groove density distribution of a fused silica grating mask. Finally, the spatial distribution of line densities between the fused silica mask, which is to be fabrication by using EBL, was demonstrated.

Improving the spectral resolution of soft x-ray flat-field spectrometer with multi-area gratings

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.

Design of multilayer grating in VUV spectrum by rigorous coupled-wave method

The rigorous coupled-wave method (RCWM) is extended to calculate the diffraction efficiency of multilayer grating in spectrum with strong absorption. This paper gives a detailed algorithm analysis of the RCWM, an enhanced, numerically stable transmittance matrix approach applied to homogenous layered media is generalized for multilayer grating structure. In order to calculate in the lossy medium, the wave vector of electric field is redefined. This method is programmed to design multilayer grating which is expected to use on Seya-Namioka synchrotron monochromator in vacuum ultraviolet (VUV) spectrum. The influence on diffraction efficiency caused by grating profile and multilayer stack is analyzed theoretically. The diffraction efficiencies of well-designed multilayer gratings are all greater than 10% in 50-110nm region, and they are about two to three times higher than that of the single metallic layer grating which is being used on Seya-Namioka monochromator now.