Resonant Bending Fatigue Tests on Thin Films

Kwangsik Kwak ,Masaaki Otsu ,Kazuki Takashima

doi:10.18494/sam.2010.627

Abstract

The fatigue properties of thin-fi lm materials are extremely important in the design of durable and reliable micro-electromechanical systems (MEMS). However, it is rather diffi cult to apply the conventional fatigue testing method for bulk materials to thin fi lms as the specimen size is extremely small. Therefore, a fatigue testing method suitable for thin-fi lm materials is required. We have developed a fatigue testing method that uses the resonance of a cantilever-type specimen prepared from thin fi lms. Cantilever beam specimens with dimensions of 3(L) × 1(W) × 0.01(t) mm3 were prepared from Ni-P amorphous alloy thin fi lms and gold foils. In addition, cantilever beam specimens with dimensions of 3(L) × 0.3(W) × 0.005(t) mm3 were prepared from single-crystal silicon thin fi lms. These specimens were fi xed to a holder that was connected to an audio speaker (actuator) and were resonated in the bending mode. The Young’s moduli measured from the resonant frequencies of Ni-P and gold foil were 116 and 72 GPa, respectively. These values were comparable to those measured by other techniques, indicating that the specimens resonated in a theoretically predictable manner and that our method is valid. Resonant fatigue tests were carried out for these specimens by changing the amplitude range of resonance, and S-N curves were successfully obtained for Ni-P amorphous alloy thin fi lms, gold foils, and single-crystal silicon thin fi lms.

Highlights

In micro-electromechanical systems (MEMS), micromechanical components are used in the construction of moving parts, including microbeams and membranes
The evaluation of mechanical properties including the elastic modulus, tensile strength, fracture toughness, and fatigue life of such structural thin films is, necessary to enable the design of reliable MEMS devices
Methods, the cyclic loading frequency is usually up to 10 Hz, and it takes much time to obtain a sufficient fatigue strength after 109–1010 cycles, which are required for designing MEMS devices

Summary

Introduction

In micro-electromechanical systems (MEMS), micromechanical components are used in the construction of moving parts, including microbeams and membranes. On-chip resonating fatigue testing methods at a frequency of several tens of kHz have been developed to reduce testing time in the high-frequency fatigue region.[2,3,4,5] In this testing structure, a comb-driven actuator and a specimen are prepared concurrently by a photolithography process. This indicates that this technique is applicable to only thin films on substrates. A fatigue testing method for thin films by bending resonance has been developed and fatigue tests on Ni-P amorphous alloy thin films, gold foils, and single-crystal silicon films have been performed

Materials and specimens

Results and Discussion

Conclusions