The FDTD Analysis of Near-Field Response for Microgroove Structure With Standing Wave Illumination for the Realization of Coherent Structured Illumination Microscopy

Abstract

Microstructures are widely used in the manufacture of functional surfaces. An optical-based super-resolution, non-invasive method is preferred for the inspection of surfaces with massive microstructures. The Structured Illumination Microscopy (SIM) uses standing-wave illumination to reach optical super-resolution. Recently, coherent SIM is being studied. It can obtain not only the super-resolved intensity distribution but also the phase and amplitude distribution of the sample surface beyond the diffraction limit. By analysis of the phase-depth dependency, the depth measurement for microgroove structures with coherent SIM is expected. FDTD analysis is applied for observing the near-field response of microgroove under the standing-wave illumination. The near-field phase shows depth dependency in this analysis. Moreover, the effects from microgroove width, the incident angle, and the relative position between the standing-wave peak and center of the microgroove are investigated. It is found the near-field phase change can measure depth until 200 nm (aspect ratio 1) with an error of up to 20.4 nm in the case that the microgroove width is smaller than half of the wavelength.

Optical Depth Measurement for Microgrooves: A Self-interferometry Method based on Near-field Polarization Analysis

“…3 Phase-depth relationship obtained from RCWA simulation for periodic microgrooves with pitch size of 0.42 . The result is consistent with previous found theory for TM and TE case [5] [6]. (ASPEN 2022) 15-18 November 2022, Singapore.…”

“…To investigate how this result is consistent with previous research, a phase-depth relation is plotted in figure 3, taking pitch size 0.42 as an example. Theoretically, the reflected near-field phase increases linearly with depth and the phase-depth relation should almost follow perpendicular interferometry even under oblique incidence, for the TM case [5].…”

“…In previous research, the interaction of diffraction limited microgrooves (also called the subwavelength gratings) with polarized light has been studied and the polarization-dependent response has been observed [4]. Studies for light interaction with diffraction limited microgrooves have been conducted to study the reflected phase for depth measurement [ 5 ] . It was found that for diffraction-limited microgroove, the TM polarized light can retrieve the depth information even under oblique illumination.…”

“…In this research, we firstly conducted Rigorous coupled-wave analysis (RCWA) to confirm the polarization characteristic of microgroove interaction with oblique incidence with shape variation of microgrooves. In the previous study, the Finite-difference time-domain (FDTD) has been applied for discrete shape variation [5]. Since RCWA is much faster, the phase-depth relation of diffraction limited microgrooves is comprehensively studied with continuous shape variation.…”