Proceedings of the
9th International Conference of Asian Society for Precision Engineering and Nanotechnology (ASPEN2022)
15 – 18 November 2022, Singapore

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

Yizhao Guan1, Shuzo Masui2, Shotaro Kadoya3, Masaki Michihata1 and Satoru Takahashi3,a

1Department of Precision Engineering, the University of Tokyo, 7-3-1 Hongo, Bunkyo City, Tokyo, 113-8654, Japan

2Laboratory for Future Interdisciplinary Research of Science and Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan

3Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, 4-6-1 Komaba, Meguro City, Tokyo 153-8904 , Japan


The subwavelength microgrooves have been widely applied to functional surfaces with microstructures. Inspection and measurement of microgroove depth are highly demanded to ensure the correct functions of microstructures. Traditional optical methods such as interferometry are non-invasive and have high throughput but are limited by the diffraction limit. It is hard to obtain depth-related information for microgroove which has a width smaller than the diffraction limit.

In this research, we conducted numerical simulation by RCWA and found that the TM polarized light can retrieve the depth information under oblique illumination for diffraction-limited microgroove, while TE polarized light has less depth dependency. We propose a dark-field method to retrieve phase information on periodic microgrooves surface by interfering TE and TM polarized light, naming as a self-interferometry method. The phase difference between the two polarization states is almost linearly related to microgroove depth. In this method, interference light has the same optical path so that measurement is stable against environmental disturbances. Moreover, the dark-field method ensures that strong direct reflection on the top surface would not blur depth-related phase information. An optical system was proposed to retrieve the phase difference between two polarization states.

Through numerical experiments, quantitative depth measurement is found available by the self-interferometry for periodic microgrooves that have width smaller than half of the wavelength. The depth measurement considered to be impossible in traditional interferometry has been achieved by the proposed self-interferometry.

Keywords: Optical depth measurement, Dark-field, Microgroove, polarization phase difference

PDF Download