**1. Introduction**

Thin-film systems are used in a variety of applications ranging from micro-electro-mechanicalsystems (MEMS) to artificial joints. Poor adhesion of the film material to the substrate leads to delamination or other modes of coating failure, and is an important factor of quality control in

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the manufacturing stage. However, detection of poor adhesion is not easy. In particular, when the film material is poorly adhered but not causing a structural abnormality, detection is very difficult. Static methods such as acoustic imaging microscopy or X-ray diffractometry cannot beusedto identify theproblem.Dynamic analysis capable of characterizing the elastic behavior of the interface is essential.

Among dynamic techniques to evaluate the film adhesion strength, ultrasonic techniques [1–5] are the prevailing methods. In these methods, ultrasonic waves are excited in the substrate and film materials, and abnormality is detected from the propagation characteristics of the ultrasonic wave. The recent trend indicates that the film thickness is reduced for better performance of the thin-film system. This forces the ultrasonic wavelength to be shorter, hence the frequency to be higher. Reduction in the wavelength works well for the purpose of detecting defects or other nonuniform issues in the interface, as the spatial resolution is increased. However, for characterization of elasticity of the film-substrate interface, an increase in the frequency makes the analysis difficult. This is because normally, poor adhesion has lower elastic modulus than the healthy adhesion. Consequently, the frequency is too high to oscillate the poor adhesion effectively, and the signal representing the poor adhesion tends to be small. In other words, the detection system tries to probe the oscillatory behavior caused by the poor adhesion at a frequency on the blue side of the spectrum, as schematically illustrated in **Figure 1**. The transmissibility (the transfer function) of a mechanical oscillator decreases with a quadratic dependence on the frequency (*f* −2) on the high frequency side of the resonance.

**Figure 1.** Resonance curves with different resonant frequencies.

Considering the above situation, we have devised an optical interferometric system to characterize the adhesion of thin films to their substrate [6]. A Michelson interferometer is used to analyze harmonic response of thin-film specimens when they are oscillated with an acoustic transducer. The film surface displacement resulting from the acoustic oscillation is detected as relative optical path changes behind the beam splitter. With the assumption that the filmsubstrate interface has a lower elastic modulus than the film or the substrate material and by choosing the acoustic frequency appropriately, it is possible to characterize the elastic behavior of the interfaces. For several thin-film specimens, resonance-like behaviors of the film-substrate interface have been found [7].

In the course of this research, we have learned much about practical issues of Michelson interferometers. The operation principle of a Michelson interferometer is straightforward. However, in reality, its application to engineering is not as simple as it sounds. Especially, when the interferometer is used in air, environmental disturbance can easily affect the measurement. It is always possible to place the entire interferometric paths in a vacuum, but it causes extra costs and handling procedures. In many engineering applications, it is not favorable.

In this paper, after various findings from the above research being discussed, a method is proposed to reduce environmental disturbance that compromises the optical phase signal representing the oscillation at the acoustic frequency. In this method, a known optical-path variation is introduced in a direction lateral to the interferometer axis so that the Initial Phase Difference is visualized as a pattern of mutually parallel dark and bright stripes (known as a carrier-fringe pattern [8]), and the data is processed in the spatial-frequency domain. With this configuration, a change in the relative optical path shifts the fringe locations in a plane normal to the interferometric axis. This allows us to perform two-dimensional analysis on the image plane of the imaging device, and thereby find a weakly-adhered spot. The method is especially useful when the frame rate of the imaging device is significantly lower than the acoustic frequency, as is normally in this case. Under this condition, the relative phase change at the acoustic frequency is detected as the corresponding reduction in the fringe contrast that can be related to the height of the main peak in the Fourier spectrum. Since the spatial frequency corresponding to the main peak is determined by the spacing of the carrier fringe pattern, the peak value does not depend on slow shift of the entire fringe pattern due to an environmental disturbance.
