Keqiang Qiu

Algorithms for finely adjusting etch depths to improve the diffraction efficiency uniformity of large-aperture BSG

Beam sampling gratings (BSGs) employed in high-power laser systems usually have large aperture so that the adequate uniformity of diffraction efficiency is difficult to obtain. We proposed a deterministic method using controllable non-uniform etch to improve the efficiency uniformity of large-aperture BSGs. During the ion beam etching (IBE) process, etch depths are finely adjusted by the dynamic leaf. The motion trajectory of the dynamic leaf is calculated using the fine adjustment algorithm. Simulations are conducted on the basis of a typical example. The simulation predictions show that the cumulative error is 0.067 nm and about 99.1% of depth differences are in the range of the required etch depth tolerance, which suggests that the diffraction efficiency uniformity of BSG is expected to be effectively improved and thus can meet the requirement of a RMS of 5%. As a cost-effective solution, it also has a broad prospect in many optical fabrication fields, especially for the fabrication of large optics.

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.

New leak assembly based on fluidic nanochannels

Fluidic nanochannels with a characteristic dimension of ∼280 nm were fabricated and designed as a leak assembly, where the nanochannels were formed on silicon wafers and enclosed with Pyrex® glass. The geometric dimensions were characterized by scanning electron microscopy, and the gas flow conductance of He and other heavy gases (N2, O2, and Ar) was measured, and its uncertainty estimated, by the difference method. The results indicated that the measured flow conductance values were 45% less than the calculated flow conductance values. For helium, molecular flow was shown to occur at pressures ranging from vacuum to atmospheric pressure. As a consequence of the well-defined geometry, the prediction of flow conductance could be achieved for various gas species.

Design and fabrication of sine-top broadband gold-coated gratings

Abstract. Fabrication and testing results of sine-top, high-efficiency, broadband gold-coated gratings (BGCG) for high-power laser pulse compression applications are reported. These gratings differ from conventional metal-on-photoresist pulse compression gratings in that the gratings patterns are generated by directly etching the quartz substrate. The groove depth and duty cycle of the photoresist mask was controlled by changing photoresist thickness and adjusting exposure and development times, respectively. The duty cycle of the photoresist mask was further corrected by oxygen plasma etching. Using this method, high efficiency, sine-top, BGCG with line densities of 1740  lines/mm was achieved. The average diffraction efficiency at the-1st order was 89.2% and the peak value was 90% for TM polarized light as the wavelength increases from 750 to 850 nm.

Fabrication of conductance-controllable standard leak elements on anodic aluminum oxide using a selective coating method

To fabricate conductance-controllable standard leak elements, a selective coating method was developed to achieve a specific open area on anodic aluminum oxide. Aluminum oxide with an optimized thickness was deposited onto anodic aluminum oxide by ion beam sputtering, acting as a cover layer to seal the redundant pores in anodic aluminum oxide. Two leak elements with different uncovered areas were fabricated, and the values of their conductance were measured. The results indicate that the conductance of the leak elements can be controlled by the size of the uncovered area. The diameter of the tubes in the anodic aluminum oxide is approximately 70 nm, and gases such as helium flowing through the tubes exhibit molecular flow from vacuum to atmospheric pressures. Hence, the values of conductance of the leak elements are predictable for noncondensable gas species.

Variable-spot ion beam figuring

Abstract This paper introduces a new scheme of ion beam figuring (IBF), or rather variable-spot IBF, which is conducted at a constant scanning velocity with variable-spot ion beam collimated by a variable diaphragm. It aims at improving the reachability and adaptation of the figuring process within the limits of machine dynamics by varying the ion beam spot size instead of the scanning velocity. In contrast to the dwell time algorithm in the conventional IBF, the variable-spot IBF adopts a new algorithm, which consists of the scan path programming and the trajectory optimization using pattern search. In this algorithm, instead of the dwell time, a new concept, integral etching time, is proposed to interpret the process of variable-spot IBF. We conducted simulations to verify its feasibility and practicality. The simulation results indicate the variable-spot IBF is a promising alternative to the conventional approach.

Finely control groove-depth variations of large-area diffraction gratings

We proposed a technique for conducting on-the-fly fine adjustment of etch depths with sub-nanometer precision during the course of ion beam etching (IBE). Simulations were performed to evaluate the etch-depth control precision. The simulation prediction shows that the precision of fine control of etch depths is at the level of 0.1nm. The preliminary experiment was conducted. The early result and the simulation prediction are in agreement with each other, which indicates that this approach is feasible for finely controlling groove-depth variations of large-area diffraction gratings.

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.

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.

Fine-tuning the etch depth profile via dynamic shielding of ion beam

Abstract We introduce a method for finely adjusting the etch depth profile by dynamic shielding in the course of ion beam etching (IBE), which is crucial for the ultra-precision fabrication of large optics. We study the physical process of dynamic shielding and propose a parametric modeling method to quantitatively analyze the shielding effect on etch depths, or rather the shielding rate, where a piecewise Gaussian model is adopted to fit the shielding rate profile. Two experiments were conducted. The experimental result of parametric modeling of shielding rate profiles shows that the shielding rate profile is significantly influenced by the rotary angle of the leaf. The result of the experiment on fine-tuning the etch depth profile shows good agreement with the simulated result, which preliminarily verifies the feasibility of our method.