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1、Micromechanical Fatigue Testing by J.A. Connally and S.B. Brown ABSTRACT-This paper describes the design, modeling, and experimental test results of a single crystal silicon micromechani- cal device developed to evaluate fracture and fatigue of silicon based micromechanical devices. The structure is
2、 a cantilever beam, 300 microns long, with a large silicon plate and gold inertial mass at the free end. Torquing and sensing electrodes extend over the plate, and with associated electronics, drive the structure at resonance. Fatigue crack propagation is measured by detecting the shift in the natur
3、al frequency caused by the extension of a preexisting crack introduced near the fixed end of the cantilever. Experimental data are presented demonstrating time-dependent crack growth in silicon. Crack extensions of 10 to 300 nm have been measured with a resolution of approximately 2.5 nm, and crack
4、tip velocities as low as 2.1 10-14 m/s. It is postulated that static fatigue of the native surface silica layer is the mechanism for crack growth. The methodology established here is generic in concept, permitting sensitive measurement of crack growth in larger fatigue specimens as well. Introductio
5、n Micromechanicai fabrication techniques have revolutionized the ability of designers to miniaturize electromechanicai systems for use in compact low cost sensors, actuators, and transducers. Significant commercial potential is envisioned with this technol- ogy, and therefore, much effort is focused
6、 on developing new fabrication processes, exploring alternative device designs and applications, and engineering new electronic systems. However, little attention is given to investigating failure processes and structural reliability at a basic level unless failure modes become apparent. The commerc
7、ial use of micromechanical devices will lead to a greater emphasis on structural reliability as questions arise dealing with the ability of a particular device to meet performance specifications under various operating conditions. For example, micromechanicai devices are being fabricated for use in
8、automobiles, aircraft, and satellites where the intended lifetime can easily exceed 10 years.17 Other applications include biomedical instruments where reliability is the primary design criterion. More sophisticated packaging of electronic devices is similarly increasing the complexity of material i
9、nterconnects, with accompanying uncertainty about the interconnect reliability. A poorly characterized aspect of all of these structures is the time-dependent propagation of cracks within the structures. It is not possible to fabricate these structures without defects or stress concentrations. Therm
10、al or mechanical cycling coupled with incompatible mechanical and thermal properties will inevitably cause stress states that can cause either a crack to nucleate or a pre-existing crack to grow. Some small-scale structures addition- ally are designed to function within severe environments, where ch
11、emical processes can create or accelerate crack growth. These environments do not have to be exotic, for the presence of water J.A. Connally is associated with Cummins Engine Company, 1900 McKinley Avenue, Columbus, IN 47202. S.B. Brown is Associate Professor, Massachusetts Institute of Technology,
12、Room 8-106, 77 Massachusetts Avenue, Cambridge, MA 02139. Original manuscript submitted: April 30,1992. Final manuscript received: Septem- ber 23,1992. can easily produce static fatigue in silica and, as discussed below, in silicon. 11.16 It is not apparent that failure modes obtained on a macroscop
13、ic scale extend to the scale of smaller structures. The mechanisms that govern large scale failure on standard macroscopic, labora- tory specimens may not necessarily be those that govern the failure of smaller structures. At some scale continuum assump- tions will no longer be valid. Surface coatin
14、gs, microstructurai and surface features, and interface thicknesses now become large relative to structures dimensions. Our initial work, for example, suggests that static fatigue on this scale does not duplicate mac- roscopic static fatigue; fatigue limits and dependence on stress intensity may be
15、different. Our results suggest that water-in- duced, slow crack growth may occur in silicon devices by static fatigue of the silica layer that forms immediately when silicon is exposed to oxygen. Previous work on static fatigue of silicon has been inconclu- sive. Chen and Knapp performed stress corr
16、osion experiments with single-crystal silicon bars that were precracked with a Knoop microhardness tester and statically loaded in four-point bending? The surface of the beam was wetted with various liquids including distilled water, and the time to fracture re- corded. The loading was on the order of 95 percent of the static fracture load. The beams were monitored for a period of up to two weeks. None failed, and Chen
