High-Cycle Fatigue of Polycrystalline Silicon Thin Films in Laboratory Air

When subjected to alternating stresses, most materials degrade, e.g., suffer premature failure, due to a phenomenon known as fatigue. It is generally accepted that in brittle materials, such as ceramics, cyclic fatigue can only take place where there is some degree of toughening, implying that premature fatigue failure would not be expected in polycrystalline silicon where such toughening is absent. However, the fatigue failure of polysilicon is reported in the present work, based on tests on thirteen thin-film (2 μm thick) specimens cycled to failure in laboratory air (~25oC, 30-50% relative humidity), where damage accumulation and failure of the notched cantilever beams were monitored electrically during the test. Specimen lives ranged from about 10 seconds to 34 days (5 x 10 to 1 x 10 cycles) with the stress amplitude at failure being reduced to ~50% of the low-cycle strength for lives in excess of 10 cycles. INTRODUCTION Bolstered by the success of sensor applications, manufacturers of microelectromechanical systems (MEMS) are developing micromechanical components made of silicon-based structural films in actuator, power and other “safety-critical” and “high performance” applications. However, these safety-critical structures are often subjected to aggressive mechanical and chemical environments without sufficient understanding of the behavior of the material under such conditions; this is especially pertinent as the dimensions of the material components may be far smaller, i.e., micron-scale and below, than has been conventionally tested in mechanical property evaluations. Consequently, in order to ensure performance and reliability, design approaches must be employed that account for the time and cycle-dependent degradation of the material at the size-scales of interest. Micromechanical components are routinely subjected to cyclic stresses at kiloand megahertz frequencies, accumulating large numbers of stress cycles in relatively short periods of time. It is important to note that when cyclic stresses are applied, most materials degrade and can suffer premature failure due to the process of fatigue. Fatigue is the most commonly experienced form of structural failure, yet surprisingly is one of the least understood. The most well known form of fatigue, that of the cyclic fatigue of metals, is generally associated with the generation and motion of dislocations and the accumulation of plastic deformation; these processes can lead to the creation and advancement of a nucleated or pre-existing crack by alternately blunting and sharpening the crack tip (striation formation). However, the corresponding mechanisms of fatigue in brittle materials, such as the structural films commonly used in MEMS, are quite different. Due to their high Peierls forces, brittle materials such as ceramics and single crystal silicon have very limited dislocation mobility at low homologous temperature, making the possibility of cyclic fatigue failure far less obvious. However, premature cyclic fatigue can occur in brittle materials, e.g., polycrystalline ceramics and ordered intermetallics, but by a conceptually different mechanism.[1,2] Such failures are generally associated with kinematically irreversible deformations; specifically, crack-tip shielding mechanisms that operate primarily in Mat. Res. Soc. Symp. Proc. Vol. 657 © 2001 Materials Research Society