Fatigue of Single Crystal Silicon
Since the first reporting of silicon fatigue in 1992, MEMS devices have been designed to operate at stress levels far below the fracture toughness of silicon. The mechanism behind microscale silicon fatigue is controversial until this day, however every proposed mechanism can agree on one thing: environmental factors play a large role fatigue.
Our lab has developed a process called 'epi-seal encapsulation' that allows for the packaging of MEMS devices in an extremely clean (oxygen-free, humidity-free, and organic-free) environment. It is our goal to show the absence of silicon fatigue when devices are packaged in this inert environment, allowing for greater flexibility (higher stress levels) in the design guidelines for MEMS devices experiencing high cyclic loading.
Wedge-shaped resonators were fabricated as fatigue test structures (as seen below), and were packaged with the 'epi-seal encapsulation' process. The resonators were subjected to fully-reversed cyclic loadings using an oscillator that implements automatic gain control (AGC). The stress is calculated at the notch based on a combination of the current output at sense electrode and an FEA simulation. So far the resonators have been subjected up to 7.5 GPa of stress with no sign of fatigue-like behavior at lifetimes of over 50 billion cycles (approximately 10 days).
V.A. Hong, S. Yoneoka, M.W. Messana, A.B. Graham, J.C. Salvia, T.T. Branchflower, E.J. Ng, and T.W. Kenny, "High-stress fatigue experiments on single crystal silicon in an oxygen-free environment," Solid-State Sensors, Actuators, and Microsystems Workshop, Hilton Head 2012, pp. 453-456, 2012.
S. Yoneoka, Y.Q. Qu, S. Wang, M.W. Messana, A.B. Graham, J.C. Salvia, B. Kim, R. Melamud, G. Bahl, and T.W. Kenny, "High-cyclic fatigue experiments of single crystal silicon in an oxygen-free environment," 2010 IEEE 23rd International Conference on Micro Electro Mechanical Systems (MEMS), pp. 224-227, 2010.