The fracture and fatigue behaviors of polysilicon thin film structures with arbitrary shapes were formulated by taking the stress distribution into account. The parameters appearing in both Weibull distribution and Paris’ law that describe the static strength and fatigue behaviors, respectively, are estimated using the maximum likelihood method on the basis of the results of tensile static fracture and fatigue tests performed on two types of polysilicon thin film specimens with different shapes fabricated using the same conditions. The difference between the fatigue lifetime distributions between the two types was well explained by applying the formula with a unique set of parameters. These results suggest that the fracture and fatigue behaviors of polysilicon thin films have a unique characteristics regardless of stress distributions.
This paper presents a scheme for predicting the strength of MEMS structures patterned into arbitrary shapes by deep reactive ion etching of a silicon wafer. The scheme is based on the inhomogeneous defect distribution on the etched surfaces. Single-crystal silicon specimens with different shapes were subjected to four-point bending tests with monotonically increasing load. The distributions of the fracture strength were described using two-parameter Weibull statistics, where the two parameters are defined as functions of the etching depth representing the inhomogeneity of the damage on the etched surface in the direction perpendicular to the wafer. In order to estimate the distribution of the local strength determined by the local level of damage, the etched surfaces of specimens without a notch were tested in three different bending directions corresponding to three different stress distributions, and three different strength levels were given due to surface roughness conditions caused by the non-uniformity of the etching process. The estimated values of the parameters were used to estimate the fracture strength of four types of notched specimens with different notch tip radii. The results of comparison between the distributions of predicted strengths and experimental data showed that the fracture strength of arbitrarily shaped structures is predictable on the basis of the information obtained from specimens without notch, by taking into account the characteristics of etched surface, i.e. the inhomogeneous damage.
This paper presents an evaluation of strength distribution and fatigue behavior of polycrystalline silicon thin film specimens patterned by etching into arbitrary shapes. The static strength distribution of specimens is described by a two-parameter Weibull distribution applied to local points along the etched surface. The fatigue lifetime is formulated also locally as a crack extension process, starting from initial cracks which represent the etching damage and thus determine the strength distribution, by applying the well-known Paris law. The parameters in the Weibull distribution and in Paris' law were fit to the results of tensile static strength and fatigue tests performed on the specimens with three different shapes fabricated under the same conditions. Parameter distribution ranges were analyzed using inferential statistics. It was found that the fracture and fatigue behaviors of the specimens can be described by using a unique set of parameters despite the different stress distribution. This means that the local characteristics of etching damage and subsequent damage accumulation under fatigue loading was independent of the shapes and thus of the stress distributions of specimens.
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