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Sensing & Measurement

A novel approach to detecting subnanometer in-plane motion

Combining two types of interferometry reveals in-plane motion with subnanometer resolution.
14 February 2007, SPIE Newsroom. DOI: 10.1117/2.1200701.0582

Optical and noncontact types of displacement-measuring techniques have become increasingly important and have attracted great attention over the past two decades.1–4 Of all optical techniques, laser interferometry and grating interferometry—best known through their use in laser-based optical encoders—have played an important role in industry and scientific research, for example, in semiconductor manufacturing and precision machining. Heterodyne interferometry provides high resolution but suffers from surrounding noise. The laser encoder is a laser interferometer that uses a grating to generate interference signals proportional to the grating's displacement. Because the laser encoder uses the grating as the measurement scale, it is less prone than traditional laser interferometers to influence by environmental disturbances. On the other hand, the resolution of the laser encoder is less than optimal for many applications.

To achieve subnanometer resolution positioning, we combined grating interferometry and optical heterodyne interferometry. Figure 1 shows the system configuration. It includes a heterodyne light source, a moving grating as a measuring scale, and a lock-in amplifier for phase measurement. The optical phase variations ϕi, which stem from the grating movement Δi, are simultaneously recorded into the p- and s-polarized light waves and measured by an optical heterodyne interferometer. The measured displacement Δi can be expressed as ϕid/8π. Here d is the grating pitch, and the suffix i denotes the x and y axes in Cartesian coordinates, respectively. Our approach incorporates the merits of heterodyne interferometry and the laser encoder at the same time: the system offers high resolution and is robust in the presence of noise. The experimental results demonstrate that our system can measure both small and long displacement with subnanometer resolution. By isolating the measurement system, we can reduce low-frequency noise. When only high-frequency noises are considered, our method can achieve measurement resolution of about 0.2nm.5

Figure 1. The system configuration of our method combines optical gratin and heterodyne interferometry to allow displacement measurements with subnanometer resolution. G: Grating. M: Mirror. PBS: Polarization beam splitter. AN: Analyzer. D: Photodetector. PM: Phase meter. HP5529A: Hewlett-Packard 5529A dynamic calibration system.

Figure 2 demonstrates 1D experimental results, in which we drove the piezoelectric motorized actuator one step with a low speed of 2pulse/s. The total measured displacement is about 8nm. The movement of the stage, measured by a Hewlett-Packard 5529A (HP5529A) dynamic calibration system, is about 10nm. Because the measurement resolution of the HP5529A is only about 10nm, the system could not read out the detailed motion and only gave decimal scale data.

Figure 2. Measurement result for one-step displacement. The enlarged plot presents the overshot and the underdamped oscillation.

This technology was developed by the Center for Measurement Standards of the Industrial Technology Research Institute and the Institute of Opto Mechatronics, National Central University, both in Taiwan. In-plane stage modules of different precision levels, which are equipped with our miniaturized subnanometer motion-sensing module, are now available. They meet both economic and precision requirements for various industrial and scientific applications.

In conclusion, we propose a new technique for subnanometer in-plane motion sensing, based on heterodyne interferometry and grating interferometry, that can reduce noise and achieve subnanometer resolution. In further papers, we will present the results of measuring multiaxis motion using a similar approach.

Lee Ju-Yi
Institute of Opto-Mechatronics, National Central University
Optical Metrology Laboratory, Taiwan
Institute of Opto-Mechatronics, National Central University

Ju-Yi Lee received his PhD degree from the Instit ute of Electro-Optical Engineering at National Chiao Tung University of Taiwan in 1999. Prior to joinin g National Central University as an assistant professor (since 2004), he was a researcher at the Industr ial Technology Research Institute of Taiwan. His research interests are optical metrology and sensors.

Wu Chyan-Chyi, Hsu Cheng-Chih
Industrial Technology Research Institute
Center for Measurement Standards, Taiwan

Chyan-Chyi Wu received his PhD degree in mechanical engineering at National Taiwan University in 2001. He is currently a researcher at the Industrial Technology Research Institute of Taiwan. His research interests are diffractive optics, nanometrology, and microsensors and actuators.

Cheng-Chih Hsu received his PhD degree from the Institute of Electro-Optical Engineering at National Chiao Tung University of Taiwan in 2003. He is now a researcher at the Industrial Technology Research Institute of Taiwan. His current research activities are optical metrology, nondestructive testing, and optical measurement in medical diagnostics.