Analysis of Scattering-type Scanning Near-field Optical Microscopy for Residual-strain Measurements Chia-Chi Liao and *Yu-Lung Lo Department of Mechanical Engineering National Cheng Kung University No.1, Ta-Hsueh Road, Tainan 701, Taiwan ROC *Corresponding author: loyl@mail.ncku.edu.tw ABSTRACT An analytical model for residual-strain measurement based on the Scattering-type scanning near-field optical microscopy (s-SNOM) has been developed in this study. A-SNOM has a capability for inspection properties of materials in nanometer-scale and with resolution up to 10 nm. However, the scattering signals in s-SNOM are highly complex and contaminated by the background noise critically. To overcome the problem, we have proposed a mathematical model to improve the near-field signals by eliminating the background noise in heterodyne detection. According to the mathematical model, the study will discuss the signal in s-SNOM in detail, analyze the spectrum of measurements, and explore more methods to get better signal. Then, the mathematical model will be combined with other modified near-field ones to construct a novel near-field analytical model to fit the experimental data on phonon-polariton as possible. Based on the new analytical model, the dielectric constants of materials can be obtained more precisely, and the residual stress and strain relative to the variation of dielectric constants of SiC which most often utilized in micro- and nano-electromechanical system (MEMS and NEMS) can be determined more distinctly. 1. Introduction In order to overcome the restriction in resolution for optical microscopy, the aperture scanning near-field optical microscopy (SNOM) which has been developed [1]. In SNOM, the illumination light is confined by the metallic aperture to enhance near-field effect and achieve a nano-measurement in chemical, structural, and conduction properties. However, the resolution of SNOM is still restricted by the size of the tapered metal-coated optical fiber aperture and the waveguide cut-off effect [2]. The very small aperture of SNOM also severely restricts the light throughput, and the light intensity can not be simply increased to compensate the loss because of the risk of damaging due to heating effect [3]. Accordingly, an alternative SNOM was proposed. Scattering-type scanning near-field optical microscopy (s-SNOM, also called apertureless-SNOM) replaces the optical fiber with a sharp vibrating tip. In this configuration, the incident light illuminates the tip to get a small scatter and induce a local enhancement of the electric field between the tip and the specimen in near-field. The enhancement due to the near-field interaction depends on the dipole effect and is similar as the sphere with a nanometer-scale and makes an optical resolution at the sub-10 nm scale [4-6]. Comparing to the conventional SNOM, s-SNOM can obtain the measurement with better resolution but has some problems to get over. For the scattering detection, the signal of s-SNOM is the form as the interference between the near-field electric field and the background ones. Hence, the desired signal in s-SNOM will be affected by the background noise seriously. As a result, it is necessary to develop techniques for suppressing the background–scattering noise from the detected signal and improving the precision and reliability of the measured results. In s-SNOM, the signal often be obtained by lock-in amplifier and acquired by signal modulated process, such as heterodyne detection [7-8] or homodyne one [8-9]. Various formulas have been proposed to describe the characteristics of both techniques [10-12]. By analyzing the tip enhancement and modulated background components, the signal in s-SNOM can be improved. However, these studies not only neglect the near-field enhancement, but also utilized models overly complex. Recently, the current group proposed an analytical investigation into the modulation of s-SNOM homodyne and heterodyne signals [13-14]. This analytical model can Proceedings of the SEM Annual Conference June 7-10, 2010 Indianapolis, Indiana USA ©2010 Society for Experimental Mechanics Inc. 167 T. Proulx (ed.), MEMS and Nanotechnology, Volume 2, Conference Proceedings of the Society for Experimental Mechanics Series 2, DOI 10.1007/978-1-4419-8825-6_24, © The Society for Experimental Mechanics, Inc. 2011
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