Zhengkun Liu

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.

Soft X-ray holographic grating beam splitter including a double frequency grating for interferometer pre-alignment

Grating beam splitters have been fabricated for soft X-ray Mach- Zehnder interferometer using holographic interference lithography. The grating beam splitter consists of two gratings, one works at X-ray laser wavelength of 13.9 nm with the spatial frequency of 1000 lines/mm as the operation grating, the other works at visible wavelength of 632.8 nm for pre-aligning the X-ray interferometer with the spatial frequency of 22 lines/mm as the pre-alignment grating. The two gratings lie vertically on the same substrate. The main feature of the beam splitter is the use of low-spatial- frequency beat grating of a holographic double frequency grating as the pre-alignment grating of the X-ray interferometer. The grating line parallelism between the two gratings can be judged by observing the diffraction patterns of the pre-alignment grating directly.

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).

Chemical mechanical polishing to improve the efficiency uniformity of beam sampling grating

In order to improve the efficiency uniformity of large-aperture beam sampling gratings (BSGs), a conventional chemical mechanical polishing (CMP) process of fused silica by CeO₂ slurry is proposed to modify their groove profiles. With the proposed CMP process, the efficiency uniformity of several BSGs with an aperture of 430  mm×430  mm has been successfully controlled within an rms of 5%. The proposed CMP process is an effective method to improve the efficiency uniformity of large-aperture BSGs. Using the proposed CMP process, the requirement of the uniformity of the holographic ion beam etching process can be released in the realization of large-aperture BSGs.

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.

Dual-reflector configuration in varied line-space grating displacement sensor

A method to improve the accuracy of the wavelength encoding varied line-space grating displacement sensor is presented. Based on the detailed analysis of the measured displacement errors from the single-mirror configuration sensor, a dual-reflector configuration is used to replace the previous configuration, and greatly decreases its errors. Experiments are conducted in order to make comparison of the two configurations. The results show that the measured displacement error of the sensor with dual-reflector configuration is lower than 0.03 mm in full scale (0 to 50 mm), only about 10% of the sensor with single-mirror configuration.

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.

Structural color analysis of the photoresist grating influenced by the light source and its parameters

The basic principle and method to generate structural color from the photoresist grating of the multilayer dielectric diffraction grating are introduced. Rigorous coupled-wave analysis is used for computation with the open software RCWA-1D. The relationships between the characteristics of the photoresist and the structural color are explained. This paper discusses the effect of light source characteristics on the duty cycle color resolution, indicating that TE polarization is better than TM polarization, and the FWHM should be sufficiently large with an optimized value for the incident angle.

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.