High-cycle fatigue of single-crystal silicon thin films

When subjected to alternating stresses, most materials degrade, e.g., suffe r premature failure, due to a phenomenon known as fatigue. It is generally accepted that in brittle materials, such as ceramics, fatigue can only take place in toughened solids, i.e., premature fatigue failure would not be ex pected in materials such as single crystal silicon. The results of this stu dy, however, appear to be at odds with the current understanding of brittle material fatigue. Twelve thin-film (similar to 20 mum thick) single crysta l silicon specimens were tested to failure in a controlled air environment (30 +/- 0.1 degreesC, 50 +/- 2% relative humidity). Damage accumulation and failure of the notched cantilever beams were monitored electrically during the "fatigue life" test. Specimen lives ranged from about 10 s to 48 days, or 1 x 10(6) to 1 X 10(11) cycles before failure over stress amplitudes ra nging from approximately 4 to 10 GPa. A variety of mechanisms are discussed in light of the fatigue life data and fracture surface evaluation.