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 (∼25°C, 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 × 10⁵ to 1 × 10¹¹ cycles) with the stress amplitude at failure being reduced to ∼50% of the low-cycle strength for lives in excess of 10⁹ cycles.