## 1 Introduction  
Thin-film coating is used in a wide variety of applications ranging from micro-electro-mechanical systems (MEMS) to surface treatment of mechanical systems for wear-resistance enhancement. The recent trend indicates that reduced film thickness in a number of applications leads to better performance. In some cases, films of a few tens of nanometers in thickness are exposed to vigorous rubbing motion. With this trend, the quality control of the film-adhesion strength and endurance has become more important than ever. The reduction in the film thickness often means that a minute defect or imperfection at the film-substrate interface can cause significant coating damage, ending the life of the entire system. A number of destructive and nondestructive methods have been developed to characterize the film-substrate adhesion. Those classified as destructive methods [1; 2; 3; 4] assess the ultimate strength of the adhesion via measurement of the critical force that the coating can tolerate. Prevailing techniques classified as nondestructive methods use ultrasonic waves to probe the film-substrate interface [5; 6; 7; 8; 9]. An acoustic signal from an ultrasonic transducer is transmitted through the interface for the detection of abnormalities, such as delamination or defects. By measuring the velocity of the acoustic wave, it is possible to characterize the elasticity of the the interface. The destructive methods are useful for a number of applications. However, they are not suitable for the evaluation of the endurance or fatigue characteristics. It is rare that the surface of a thin-film system is subject to external force comparable to the ultimate stress. The coating is usually damaged by repetitive, low-level stresses. Similarly, the ultrasonic methods have limitations in practicability as the acoustic frequency used by most methods is unrealistically higher than the frequency range that a thin-film system normally experiences. As will be elaborated in the next section, the film-substrate interface can respond harmonically when driven at certain frequency ranges. Harmonic systems break most easily by repetitive use at or near the natural (resonant) frequency. This is true for a macroscopic system, as represented by the famous collapse of the Tacoma Narrows Bridge, or a micro-, nano-scale system, as represented by the fracture of a cantilever in MEMS. It is important to evaluate the adhesion strength using probing frequencies within a frequency range that thin-film systems are subject to and to examine if the interface exhibits resonance-like behaviors. The use of a proper frequency is important for a good signal-to-noise ratio, as well. If the probing frequency is orders of magnitude higher than the resonant frequency, the diagnostic system has poor sensitivity. The reduction in film thickness raises the ultrasonic frequency even higher. It is possible that the diagnostic system overlooks the most important frequency range. Considering the above issues, we have devised an opto-acoustic method to characterize the film-substrate interface in a range of audible frequencies. In this method, the specimen is oscillated with an acoustic transducer perpendicular to the film surface, and the resultant differential displacement of the film surface to the substrate is read out optically. By sweeping the driving frequency of the transducer, it is possible to explore for resonance-like behavior of the interface. Using this method, we studied the effect of a pre-coating surface treatment. The treatment, known as the oxygen-plasma bombardment technique, is widely used for thin-film coating on silicon substrates. According to the theory, plasma bombardment knocks off hydro-carbons and thereby strengthens the chemical bonding between the film and substrate materials. Using various film materials coated on the same silicon substrate with and without the surface treatment, we conducted experiments under various conditions and collected a great amount of data.