Probabilistic Weibull behavior and mechanical properties of MEMS brittle materials

The objective of this work is to present a brief overview of a probabilistic design methodology for brittle structures, review the literature for evidence of probabilistic behavior in the mechanical properties of MEMS (especially strength), and to investigate whether evidence exists that a probabilistic Weibull effect exists at the structural microscale. Since many MEMS devices are fabricated from brittle materials, that raises the question whether these miniature structures behave similar to bulk ceramics. For bulk ceramics, the term Weibull effect is used to indicate that significant scatter in fracture strength exists, hence requiring probabilistic rather than deterministic treatment. In addition, the material's strength behavior can be described in terms of the Weakest Link Theory (WLT) leading to strength dependence on the component's size (average strength decreases as size increases), and geometry/loading configuration (stress distribution). Test methods used to assess the mechanical properties of MEMS, especially strength, are reviewed. Four materials commonly used to fabricate MEMS devices are reviewed in this report. These materials are polysilicon, single crystal silicon (SCS), silicon nitride, and silicon carbide.     

Characterization of strength properties of thin polycrystalline silicon films for MEMS applications

The aim of this work is to characterize the strength properties of polycrystalline silicon (polysilicon) with the use of tensile and bending test specimens. The strength of thin polysilicon films with different geometry, size and stress concentrations has been measured and correlated with the effective size of the specimen and its stress distribution. The test results are evaluated using a probabilistic strength approach based on the weakest link theory with the use of STAU software. The use of statistic methods of strength prediction of polysilicon test structures with a complex geometry and loading based on test values for standard material tests specimen has been evaluated.     

A new approach for handling and transferring of thin semiconductor materials

 Based on the fracture mechanics analysis of crack propagation, the phenomenon of subcritical crack growth was utilized for a controlled debonding of directly wafer-bonded interfaces. The approach allowed the well-defined separation of bonded wafers although the bond strength was high due to thermal annealing. The achieved splitting velocity depended on the wafer material, the wafer thickness ratio, the bonding process parameters, and the environmental conditions during cleaving. In combination with wafer bonding, the method can be used for a temporary stiffening and handling of thin and brittle wafers during fabrication, even if the wafers are exposed to high process temperatures. The approach can also be applied to fabricate micromechanical systems (MEMS). Received: 12 July 2001/Accepted: 26 February 2002 This paper was presented at the Conference of Micro System Technologies 2001 in March 2001.     

Mechanical properties of glass frit bonded micro packages

Frit glass bonding is a widely used technology for encapsulation of surface micro-machined structures like inertial sensors or gyroscopes on wafer level. Since for sensors in automotive applications, a lifetime of 15–20 years has to be guaranteed for, a reliable lifetime prediction is necessary. Different material parameters have to be known for a lifetime estimation based on stress corrosion cracking, which determines the long-term strength behaviour of most bonded interfaces of microsystems. Parameters needed for lifetime prediction have to describe the material’s resistance against crack propagation (fracture toughness K _IC), the stress situation in a micro package and the long-term strength behaviour. Results for fracture toughness investigations presented in this paper were determined by the micro chevron test. The stress situation in a micro package was calculated by a thermo-mechanical Finite Element Analysis. Furthermore the residual stress in the glass layer and the linear thermal expansion coefficient were determined by a crack width measurement in an environmental scanning electron microscope.     

Fatigue of directly wafer-bonded silicon under static and cyclic loading

 Fatigue tests were performed to investigate the reliability properties of wafer-bonded single crystalline silicon exposed to static or cyclic mechanical loading. A distinct decrease of strength with increasing load duration or cycle number was found which limited the lifetime of mechanically stressed wafer-bonded components. The occurrence of fatigue is related to siloxane bonds in the bonded interface between the silicon wafers. It was shown that fatigue is absent either if siloxane bonds are not present in the bonded interface or if local areas of pure silicon bonds were formed between the silicon wafers. As a consequence of this fatigue phenomenon, loaded wafer-bonded silicon sensors and actuators may suddenly fail during application. Therefore, appropriate fracture mechanical techniques were developed to predict either the time-to-failure or the cycles-to-failure for a given loading situation. These concepts allow for the optimization of the device layout with respect to long-term reliability, which can reduce the development time and costs.     

A test structure for characterization of the interface energy of anodically bonded silicon-glass wafers

In this paper a test structure is introduced, which allows the evaluation of the quality of an anodic bond interface in terms of surface energy. It is based on the creation of small non-bonded areas in the vicinity of small steps in the bond interface. Using finite element analysis simulations it was possible to calculate the surface energy of the monitored bonding processes. The test structure was used to investigate the influence of anodic bonding parameters (temperature and voltage) on the surface energy.     

Quality and mechanical reliability assessment of wafer-bonded micromechanical components

Strength tests and fracture mechanics models for Silicon wafer-bonded components are presented which can be applied during the development of bonding technologies, for the yield improvement and failure analysis] as well as for the reliability assessment of micromechanical sensors and actuators. Special attention is given to the influences of atomic bonding strength, the interface voids and the notches caused by etching steps prior to bonding on the fracture limit. If wafer-bonded interfaces are exposed to a mechanical loading for an extended time, e.g. in the order of months or years, stress corrosion effects decrease the bonding strength. As a consequence, stressed sensors and actuators fabricated by wafer bonding can suddenly fail during application after a load-dependent lifetime. Based on an appropriate fracture mechanics model, the time-to-failure data could be theoretically predicted.     

Identification of mechanical defects in MEMS using dynamic measurements for application in production monitoring

Investigations for non-destructive characterization of MEMS (Micro-Electro-Mechanical-Systems) are presented that can be applied in production monitoring in early stages. Different aspects and experimental results are shown for quadratic and circular silicon membrane structures with artificial structural defects. The quadratic membranes were manufactured with three variations of notches at the edges. The circular membranes had residues on the backside of the membrane resulting from the etching process. The dynamic properties of the structures were measured non-destructively by scanning laser-Doppler vibrometry. The consequences of the generated defects were investigated using the resonant frequencies and mode shapes of the membrane structures in comparison to the dynamic properties of accurate membranes. The results show that the generated defects lead to a variation of the dynamic properties depending on size and position of the defect.     

High-cycle fatigue and strengthening in polycrystalline silicon

The reliability of MEMS-based sensors for automotive applications critically depends on their high-cycle load-bearing capacity. To investigate the influence of high-frequency cyclic loading on mechanical properties of polycrystalline silicon, a statistically based investigation with the use of novel rotational specimens (90 kHz) has been conduced. Either fatigue fracture (above 3 GPa) or strengthening of the material at cyclic loading below 3 GPa has been observed.