Resonance Fatigue Testing of Cantilever Specimens Prepared from Thin Films
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1139-GG03-17
Resonance Fatigue Testing of Cantilever Specimens Prepared from Thin Films Kwangsik Kwak, Masaaki Otsu and Kazuki Takashima Department of Materials Science and Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto, Japan ABSTRACT Fatigue properties of thin film materials are extremely important to design durable and reliable microelectromechanical systems (MEMS) devices. However, it is rather difficult to apply conventional fatigue testing method of bulk materials to thin films. Therefore, a fatigue testing method fitted to thin film materials is required. In this investigation, we have developed a fatigue testing method that uses a resonance of cantilever type specimen prepared from thin films. Cantilever beam specimens with dimensions of 1(W) x 3(L) x 0.01(t) mm3 were prepared from Ni-P amorphous alloy thin films and gold foils. In addition, cantilever beam specimens with dimension of 3(L) x 0.3(W) x 0.005(t) mm3 were also prepared from single crystalline silicon thin films. These specimens were fixed to a holder that is connected to an audio speaker used as an actuator, and were resonated in bending mode. In order to check the validity of this testing method, Young’s moduli of these specimens were measured from resonant frequencies. The average Young’s modulus of Ni-P was 108 GPa and that of gold foil specimen was 63 GPa, and these values were comparable to those measured by other techniques. This indicates that the resonance occurred theoretically-predicted manner and this method is valid for measuring the fatigue properties of thin films. Resonant fatigue tests were carried out for these specimens by changing amplitude range of resonance, and S-N curves were successfully obtained. INTRODUCTION The evaluation of mechanical properties including elastic modulus, tensile strength, fracture toughness and fatigue life is necessary to design microelectromechanical systems (MEMS) devices. In particular, fatigue properties of thin film materials are important to design durable and reliable MEMS devices. Fatigue tests of thin film materials have been carried out using proportionally down sized dog-born type specimen or cantilever bending specimen just same as ordinary-sized bulk materials [1]. In these testing methods, cyclic loading frequency is usually up to 10Hz, and it takes much time to obtain fatigue strength after 109~1010 cycles, which are required for designing MEMS devices. Therefore, On-chip resonating fatigue testing methods at a frequency of several tens of kHz have been developed to reduce testing time at high frequency fatigue region [3-6]. In this testing structure, however, a comb-driven actuator and a specimen are prepared concurrently by photo lithography process. This indicates that this technique can be only applicable for thin films on substrates. For a cantilever specimen, if the length is much larger than the thickness, bending resonance will easily occur by applying vibration. During the resonance vibration, cyclic stress is applied at the fixed end of the cantilever. This ind
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