Wenhao Li

Merit function to design holographic gratings for moderate-resolution monochromators

A merit function is proposed and applied to design holographic concave gratings for moderate-resolution monochromators. To justify the validity of the merit function, imaging properties of gratings used for the coma-correction Seya-Namioka monochromator, designed by the present authors, Noda, and Takahashi, are compared through ray tracing and their aberration-correction mechanisms are also analyzed. The capability of the merit function is well demonstrated in the design of holographic gratings for another two moderate-resolution monochromators with different requirements. All the results obtained show that the merit function is not only straight and effective but also manages to balance various aberrations of the concave holographic grating very well.

Moiré alignment algorithm for an aberration-corrected holographic grating exposure system and error analysis.

To meet the required manufacturing accuracy of high-quality aberration-corrected holographic gratings, we propose a moiré alignment algorithm for the exposure system of holographic gratings. A model holographic grating exposure system is built with multiple degrees of freedom based on optical path function theory. The whole process algorithm is then derived, including the fourth-order orthogonal polynomial of the holographic gratings, fitted aberration coefficients, and an optimized Levenberg-Marquardt algorithm for the exposure systems recording parameters. Finally, the simulated alignment and error analysis of a 2400 gr/mm aberration-corrected holographic gratings exposure system are presented. The proposed moiré alignment algorithm for such exposure systems can effectively improve the alignment accuracy, ensuring better holographic grating aberration correction ability.

Precision measurement of X-axis stage mirror profile in scanning beam interference lithography by three-probe system based on bidirectional integration model

A profile of an X-axis stage mirror results in a phase error of gratings in Scanning Beam Interference Lithography. Traditional methods of measuring the profile require extra probes and another large stage mirror on Y-axis, or requires other operations such as rotating measured object to adjust the zero-adjustment errors. This paper introduces a three-probe system removing the need for Y-axis optical path structure and proposes a bidirectional integration model to solve the problem of zero-adjustment error, simplifying the optical path structure and the measurement process. This method is confirmed by theoretical analysis and experimental results, which is better than traditional methods and can also be used in other application fields of three-point method.

Establishment and experimental verification of the photoresist model considering interface slip between photoresist and concave spherical substrate

A thickness distribution model of photoresist spin-coating on concave spherical substrate (CSS) has been developed via both theoretical studies and experimental verification. The stress of photoresist on rotating CSS is analyzed and the boundary conditions of hydrodynamic equation are presented under the non-lubricating condition. Moreover, a multivariable polynomial equation of photoresist-layer thickness distribution is derived by analyzing and deducing the flow equation where the evaporation rate, substrate topography, interface slip between liquid and CSS, and the variation of rotational speed and photoresist parameters are considered in detail. Importantly, the photoresist-layer thickness at various CSS rotational speeds and liquid concentrations can be obtained according to the theoretical equation. The required photoresist viscosity and concentration parameters of different photoresist coating thickness under a certain coating speeds can be also solved through this equation. It is noted that the calculated theoretical values are well consistent with the experimental results which were measured with various CSS rotational speeds and liquid concentrations at steady state. Therefore, both our experimental results and theoretical analysis provide the guidance for photoresist dilution and pave the way for potential improvements and microfabrication applications in the future.

Application of photoresist melting method to the fabrication of holographic grating

A new technique of reducing surface roughness of photoresist grating is presented. In this paper, photoresist melting method, which is low-cost, short-period and easy realization, is presented. In addition, the influence of photoresist melting on the groove profile of the photoresist grating is investigated. Experimental results show that the surface roughness reduces due to surface tension, when developed photoresist is heated to be melting state by this method. In experiment, when the melted photoresist grating is etched by ion beam, the surface relief grating possessing preferable groove profile is obtained on K9 glass substrate, then the surface is evaporated by aluminum using vacuum evaporation, holographic grating with preferable groove profile is successively fabricated.