Fatigue (Cyclic Deformation) in Semiconductors

WeakeningFatigue (cyclic deformation) of a material caused by cyclic loading resulting in structural damage leading to catastrophic failure is known as fatigue. The majority of failures of engineering materials are caused by fatigue. Brittle material at room temperature-such as silicon-would not be expected to be susceptible to fatigue under applied cyclic stresses. Under applied stress brittle material fail almost completely, fracture at the site of the highest stress concentration which is generally the site of a flaw. The flaws are invariably processing-induced and randomly distributed, leading to scatter in the measured strength, which makes it difficult to predict a stress level near the fracture strength. Localized structural damage manifests itself in cracks. Once a crack has initiated, each loading cycle will grow the crack a small amount, even when repeated alternating or cyclic stresses are of an intensity considerably below the normal strength. The stresses could be due to vibration or thermal cycling. Fatigue damage is caused by the action of cyclic stress. No deformation occurs when a sample is exposed to fatigue because it’s brittle nature. The danger in fatigue that it occurs without warning at stress levels considerably below the yield stress. Over the years much experience has been accumulated by exploring the possible reasons for this occurrence and test have been suggested to evaluate the propensity for failure of machine elements which are exposed to pulsating or vibrational stresses. Nevertheless, the difficulty in predicting fatigue failure produces a wide spread of statistical results and often a deviation of ~ 50% from the average value is observed. In contrast other mechanical tests, do not deviate from an average value more than ~ 2–3%. This explains why many test specimens are used in fatigue experiments to reach a meaningful average value, below which the probability for fatigue fracture is quite low. A commonly used test, having repeated stress with reverse loading against the number of cycles in order to evaluate the endurance of a specimen is the S–N plot, where S represent stress and N stands for the number of cycles. It is not only alternating stress that may cause fatigue failure, but also the duration of exposure, as in material such as glass, that may fracture after long-term exposure to stress without undergoing any plastic deformation. The term for such behavior is ‘static fatigue’. Thermal fatigue is the term assigned to material failure caused by repeated changes in stress doe to the rise and fall of thermal gradients for various reasons, involving restrictions in thermal expansion or contraction.