Molecular Dynamics Simulation Of Fatigue Failure Mechanism Of Monocrystalline Silicon At Microscale

Silicon is the most widely used material for integrated circuit(IC) and microelectromechanical systems(MEMS) which is developed based on the IC technology. The properties of silicon used in IC are its electrical properties, while the properties used in MEMS mainly are its mechanical properties. So with the development of MEMS, the silicon’s mechanical properties neglected in IC previously have got more and more attention.Fatigue is the main failure mode of mechanical products and the research focus in mechanical engineering and reliability engineering. The fatigue behavior of silicon films was first reported in Science by Connally and Brown in 1992, while the fatigue had not been found in the macro sample. Although a lot of research have been conduced to reveal the fatigue mechanism of silicon in microscale, the fatigue factors remain unclear and the fatigue mechanism is still under debate.The existing researches were mainly based on the experimental fatigue results to reveal the fatigue mechanism of silicon, in a pattern called ―from the outside to the inside‖. The current controversy indicates this researching pattern has its inherent limitations. In contrast, this study has try a different research pattern, which is called ―from the inside to the outside‖, to reveal the fatigue mechanism of silicon by molecular dynamics simulation in atomic scale. The fracture strength and fatigue behavior under constant stress and under cyclic stressing were studied in detail. The main contents and conclusions of this study are as follows:1. Influence of humidity on the fracture strength of silicon structure and its surface oxide layerWe examine the influence of liquid water on the tensile properties of amorphous silica(a-Si O2) using reactive molecular dynamics simulation. The results of the quasi-static tension show that liquid water reduces the tensile strength of a-Si O2 significantly. The tensile strength of dry a-Si O2 is 9.4 GPa while the tensile strength of a-Si O2 in the presence of liquid water is only 4.7 GPa. The strain-stress curve of dry a-Si O2 indicates that the stiffness of the a-Si O2 structure becomes stable with the increase of strain. On the other hand, the stiffness of the a-Si O2 with liquid water is gradually reduced with the increase of tensile strain. Moreover, the strain-stress curve of a-Si O2 in a high strain range in the presence of liquid water is similar to the yielding phenomenon of plastic metal. We propose that the stress-enhanced hydrolysis releases part of the stress for the rupture of the Si—O bonds, so that the stiffness of the a-Si O2 in the presence of liquid water decreases with the increase of strain.We have also examined the effects of water on the mechanical properties of silicon thin-films covered by surface native oxide layer. The results of quasi-static tensions show that the silicon fracture strength of 11.2 GPa in the presence of water is lower than the fracture strength of 16.3 GPa in dry environment. Moreover, in both dry and liquid water environments, the cracks initiate from the silicon structure rather than from the surface oxide layer.2. The fatigue mechanism of of silicon structure and its surface oxide layer under constant stressWe have examined the stress corrosion process of strained α-quartz(a crystalline form of silica) in liquid water. Significant crack propagations were observed in the quartz in liquid water under three different strains which were smaller than the fracture strength. The crack velocities in liquid water were all larger than the experimental crack velocity of bulk samples. The strain distributions of crack tips indicated that the crack propagates along the direction of maximum strain, and the strain is released after crack propagation. Mixed-pattern cracking, including stress corrosion cracking and purely stress-induced cracking similar to brittle fracture, occurs when the applied strain increases.The fatigue behavior of the silicon microstructures covered surface oxide layer in liquid water under static bending stress. The results show that the surface oxide layer was subjected to crack propagation induced by stress corrosion. Under most stress conditions, the cracks were terminated at the interface surface oxide layer and the silicon structure. Only on the condition of great strain the crack become unstable and propagated into the silicon structure, resulted in the static fatigue fracture of the silicon.3. Simulation of the oxidation of silicon under static stress at room temperatureThe results show that the thickness of the oxide layer under tensile stress is greater than the thickness under zero stress. Stress analysis showed that no residual stress was observed in the surface oxide layer under tensile stress and the zero stress. This result provides a good explanation of the contradictory results in static fatigue experiments.4. The fatigue behavior of of silicon microstructure under cyclic stressa) No crack formation and crack propagation was observed in single crystal silicon in vacuum conditions under 100 cyclic stresses.b) The results show that, the cyclic stresses have enhanced the oxidation of silicon in pure oxygen and the cyclic stress thickening of the oxide layer was observed for the first time. The results also show that the stress ratio R has a significant impact on the oxidation under cyclic stress. The enhanced effect on oxidation of the cyclic stress with negative ratio R is more significant than the stress relative to the negative cycles of oxidation stress significantly more than the cyclic stress with non-negative ratio R. These results explain the current controversy of the fatigue mechanisms of silicon micro-structure.c) The diffusion of the cyclic stress thicken oxide layer was along the cleavage plane of silicon, which weakens strength of the silicon micro-structure. The fatigue mechanism of monocrystalline silicon was discovered for the first time in the simulation. The comparative simulations indicate that the water has enhanced the diffusion of oxygen in the silica, which enhance the oxidation process under cyclic stress.d) A new fatigue mechanism of silicon microstructures was proposed. The fatigue behavior of silicon micro-structure consists of two distinct processes: the fatigue of the native oxide layer and the fatigue of the cyclic stress enhanced thicken oxide layer. The fatigue mechanism of native oxide layer is the stress corrosion cracking of silica under tensile stress in moist environment. The fatigue of the thicken oxide layer was induced by the cyclic stress enhanced oxidation and humid environments enhanced diffusion.5. Experimental verifications of the influence of environment on the tensile strength and static fatigue of silicon micro-structureAn externally-actuated tensile system was designed and the tensile specimens were fabricated to compare the tensile strength and static fatigue behavior in air and in liquid water. The results show that the tensile strength of silicon in liquid water was lower than the tensile strength in air, which had verified the simulation. The silicon micro-structures in air and in liquid water both subjected to static fatigue, verified the proposed static fatigue mechanism.In summary, with the support of the National Natural Science Foundation of China, this study has examined the fatigue mechanism of silicon micro-structure by molecular dynamics simulations. The study has revealed the fatigue mechanisms of silicon micro-structure and influencing factors and has provided guidelines for the design of fatigue-free MEMS.