Degradation of Fracture and Fatigue Properties of MEMS Structures Under Cyclic Loading
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Degradation of Fracture and Fatigue Properties of MEMS Structures under Cyclic Loading Jong-jin Kim and Dongil Kwon School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea ABSTRACT The fracture and fatigue properties of LIGA nickel MEMS structures were evaluated by microtensile and fatigue test methods. A microtensile/fatigue device was developed and specimens with feature size ten micrometers were used. The fatigue property was derived from displacement amplitude – the number of cycles to failure curve by applying a dynamic load with a piezoelectric actuator. The tensile/fracture properties after various cyclic loading were also measured. Both fatigue and tensile test results showed a cyclic softening phenomenon. INTRODUCTION It is well known that mechanical properties of thin films differ from those of the bulk materials [1]. Information on the mechanical properties of thin films has become indispensable in the design of microelectromechanical systems (MEMS), where thin films are structural as well as electrical materials. In particular, MEMS such as gyroscopes, optical switches or micro-mirrors are subjected to cyclic stresses and often function under constant displacement conditions. Therefore, an understanding of the behavior of cyclically loaded films is essential in designing a new product and assessing its reliability. Much attention has been given to tensile properties of microsamples, but limited research has been performed on the fatigue resistance of micrometerscale materials [2-5], despite its importance in evaluating the lifetime and reliability of microdevices. In recent years, interest in LIGA (Lithographie, Galvanformung, Abformung) nickel MEMS structures has proliferated rapidly because of their high damage tolerance. The LIGA process was developed to overcome large sustained forces or torques that cannot be sustained in thin, two-dimensional components. The mechanical properties of LIGA nickel MEMS structures have been studied [6-12], but few reports on their fatigue behavior are available in the literature. Although stress-life behavior has been measured [13,14] and the underlying fatigue mechanisms have been investigated [15,16], no attempts have apparently been made to investigate low cycle fatigue properties and degradation of mechanical properties under cyclic loading. In this study, equipment with piezoelectric actuators and laser-speckle interferometry was
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used to evaluate the effect of cyclic loading on the mechanical properties of LIGA nickel MEMS structures. EXPERIMENTAL DETAILS Ni specimens 20 ㎛ thick, 10 ㎛ wide and 600 ㎛ long were fabricated by a surface micromachining process including nickel electroplating with a LIGA-like or UV-LIGA technique. A 50 nm Cu seed layer was deposited on a Corning glass 7740 wafer; then thick photoresist (PMER-LA900) was spun to a thickness of 25 ㎛ and prebaked at 110 for 360 seconds. To form a mold for nickel electroplating, the thick photoresist was exposed to UV light at 18 mW for 1
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