Electrostatically actuated failure of microfabricated polysilicon fracture mechanics specimens† By H. Kahn 1 , R. Ballarini 2 , R. L. Mullen 2 and A. H. Heuer 1 1 Department of Materials Science and Engineering, 2 Department of Civil Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA Received 4 November 1998; accepted 18 February 1999 Polysiliconfracturemechanicsspecimenshavebeenfabricatedusingstandardmicro- electromechanical systems (MEMS) processing techniques, and thus have character- istic dimensions comparable with typical MEMS devices. These specimens are fully integrated with simultaneously fabricated electrostatic actuators, which are capa- ble of providing sufficient force to ensure catastrophic crack propagation from blunt notches produced using micromachining. Thus, the entire fracture experiment takes place on-chip, without any external loading source. Fracture has been initiated using both monotonic and cyclic resonance loading. A reduction in the nominal toughness under cyclic loading is attributed to subcritical growth of sharp cracks from the micromachined notches in the fracture mechanics specimens. Fatigue fracture has been observed in specimens subjected to as many as 10 9 cycles, and environmental corrosion is implicated in at least some aspects of the fatigue. Keywords: polysilicon; fracture; fatigue; microfracture; electrostatic actuation; microelectromechanical systems (MEMS) 1. Introduction Polysiliconisthemostcommonlyusedstructuralmaterialforsurfacemicromachined microelectromechanical systems (MEMS) devices. (Surface micromachining refers to the production of complex pseudo-two-dimensional structures containing fine details onto a substrate by an additive process (such as chemical vapour deposition (CVD)) involving masks generated using photolithographic technology. This additive process is distinguished from bulk micromachining, which is a subtractive process involving controlled etching of fine details into a substrate using a photolithographically gen- erated etch-resistant mask. Many MEMS devices are fabricated using both surface and bulk micromachining. See Madou (1997) for further details on MEMS process- ing.) However, the ability to predict the long-term reliability of these structures is quite primitive; the fatigue and fracture behaviour of this material are not known on the relevant size scale. (Typical dimensions are of the order of micrometres.) In addition, traditional microfabrication processes induce unique surface modifications, † The authors dedicate this paper to the memory of Dr Haian Luo. Proc. R. Soc. Lond. A (1999) 455, 3807–3823 3807 c  1999 The Royal Society on March 22, 2016 http://rspa.royalsocietypublishing.org/ Downloaded from