CMU MEMS Laboratory Publication Abstract


in Proceedings of SPIE Conference 4180: MEMS Reliability for Critical Applications (SPIE 4180), pp. 108-116, September 17-20, 1999, Santa Clara, CA, USA.
Mechanical effects of fatigue and charge on CMOS MEMS
K. M. Frederick and G. Fedder
CMOS MEMS devices, fabricated from up to 13 layers of materials to create independent conducting paths, are subject to incremental fracture at high stress and to charging effects. This paper expands on preliminary research, which has revealed several stages of change in CMOS MEMS physical properties as they are exposed to resonant motion. Cracks are first induced inside the stiffest layers, often silicon dioxide, in laterally resonant test structures with cyclic stress of 620 MPa. Prior to cracking, the aluminum top layer of the structure can also deform, which affects the electrical integrity of the conductor. Measured frequency responses of folded-flexure resonators demonstrate a nonlinear Duffing effect, producing a mushroom-shaped resonant peak. Cyclic stress of 70 MPa at the maximal stress points was insufficient to induce significant mechanical fracture in folded flexure resonators after 5 billion cycles, however an onset of change in stiffness was detected. Devices with a fixed DC actuation voltage experienced a change in the electrostatic force attributed to charge accumulation in polymer and oxide layers. The force decayed with an approximate one-hour time constant while resonant frequency and quality factor remained constant.
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Full paper not available from outside CMU

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