Observations of low cycle fatigue of Al thin films for MEMS applications

This paper addresses tension-tension (R=0) low cycle fatigue behavior of thin film Al structures released from a substrate. Al beam specimens 2 mu m thick, 50 mu m wide and 500 mu m long (gauge length) were micromachined over a window in a Si substrate. Cyclic load-displacement behavior was investigated using a multiple step test method, in which each specimen was cycled with several blocks of constant maximum displacement amplitude. Load, displacement and temperature were recorded continuously. A load drop and a corresponding stabilized state were observed for each displacement block. Monotonic tension tests to failure were also performed for comparison. Evidence of cyclic creep was observed. SEM images revealed fatigue markings on cyclically loaded specimens but not on monotonically loaded beams.