Micro–Nano Step Height Measurement Using FMCW Self-Mixing Interferometry

The micro-nano step height measurement plays a pivotal role in high-precision manufacturing industries, such as MEMS, microelectronics, and photovoltaic cells. Optical interferometry is a widely used noncontact measurement method known for its characteristic of high resolution. In this work, we propose to use laser diode (LD)-based frequency-modulated continuous wave (FMCW) self-mixing interferometry (SMI) to measure micro-nano step height. Since the capability of optical frequency modulation for LD, the system shows simple structure without the need of extra optical or electronic components. The step height information is obtained from the fractional part of initial phase of FMCW SMI signal. In order to obtain the accurate initial phase, all-phase fast Fourier transform is applied, leveraging its characteristic of initial phase invariance. We first introduced the basic theory and measurement principle. Then, simulation and experiment were performed to verify the proposed method. After that, the factors influencing the measurement results were discussed. The preliminary experimental results show that the step height can be measured with deviation less than 20 nm with reference of the integrated position sensor within the piezo transducer, and the standard uncertainty of the measured mean value is less than 5 nm, which can be further improved by optimizing the experimental conditions. It provides a useful solution for micro-nano step measurement with good potential applications in high-precision manufacturing, with the advantages of compact structure, low implementation cost, easy operation, and high resolution.

I. Introduction

The micro-nano step height measurement is a critical aspect of high-precision manufacturing, essential for the development and quality control of various advanced technologies in industries, such as MEMS, microelectronics, and photovoltaic cells, where the accuracy of micro and nanoscale features directly impacts the performance and reliability of the final products. Different instruments and methods have been developed for micro-nano step height measurement, such as scanning probe microscope, scanning electron microscope, and stylus profilometer [1]. As compared with these instruments, optical interferometry shows its features of relative low cost and easy operation while maintaining good precision in engineering applications.