A survey of Mechanical failure and design for Reliability of MEMS

In this paper, several experimental mechanical investigation techniques are presented to evaluate the reliability of micro-electro-mechanical systems (MEMS). Microsystems in recent years have spread in many everyday devices. We find micro-scale sensors and actuators in automotive, biomedical and aerospace applications where are demanded very strict performance requirements. Electromechanical non-linear coupling is often a crucial problem both in design and also for the reliability of the system. Mechanism of failure and failure modes has to be taken into account in order to evaluate the reliability of the final system. Focusing on device failure, it emerges that mechanical damage is the most significant source. In this paper a survey of recent advance in mechanical testing of MEMS is presented including: Mechanical fatigue, mechanical strength and plasticity, surface and contact failure and creep. Different design of testing specimens is discussed to identify the material properties and failure modes behavior in order to obtain design rules and strategies

Fault Simulation of Surface-micromachined MEMS Accelerometer

© ASEE 2009Surface-micromachined MEMS accelerometers have been used in many applications, such as automobile airbag deployment systems and aerospace inertial navigation. Due to the movable parts involved and their diversity in device structure and working principles, MEMS devices are vulnerable to much more defect sources compared to their VLSI counterparts. Typical defect sources for MEMS devices include point stiction, etch variation, broken-beam, etc. Such defects may greatly lower the fabrication yield and degrade the device reliability. It is important to understand the MEMS failure mechanisms and see how various defects will affect the device behavior. In this paper, point-stiction defect in a surface-micromachined MEMS comb accelerometer is investigated. ANSYS simulation is used to see how the influence of the point-stiction defect on device behavior depends on the locations of the defect. ANSYS model for the defect-free device is developed and simulated. After that, point-stiction defects are injected to simulate the faulty device behavior. Simulation results demonstrate that depending on the location of the defects, the influence on the device behavior may be trivial, parametric or fatal. The fault simulation of MEMS accelerometer is helpful in finding an effective testing strategy for MEMS devices. It may also offer some hints on how to further improve the yield and reliability of MEMS

The size-dependent electromechanical instability of double-sided and paddle-type actuators in centrifugal and Casimir force fields

The present research is devoted to theoretical study of the pull-in performance of double-sided and paddle-type NEMS actuators fabricated from cylindrical nanowire operating in the Casimir regime and in the presence of the centrifugal force. D'Alembert's principle was used to transform the angular velocity into an equivalent static, centrifugal force. Using the couple stress theory, the constitutive equations of the actuators were derived. The equivalent boundary condition technique was applied to obtain the governing equation of the paddle-type actuator. Three distinct approaches, the Duan-Adomian Method (DAM), Finite Difference Method (FDM), and Lumped Parameter Model (LPM), were applied to solve the equation of motion of these two actuators. This study demonstrates the influence of various parameters, i.e., the Casimir force, geometric characteristics, and the angular speed, on the pull-in performance. (C) 2017 Sharif University of Technology. All rights reserved

Automatic Control and Fault Diagnosis of MEMS Lateral Comb Resonators

Recent advancements in microfabrication of Micro Electro Mechanical Systems have made MEMS an important part of many applications such as safety and sensor/control devices. Miniature structure of MEMS makes them very sensitive to the environmental and operating conditions. In addition, fault in the device might change the parameters and result in unwanted behavioral variations. Therefore, imperfect device structure, fault and operating point dependencies suggest for active control of MEMS.;This research is focused on two main areas of control and fault diagnosis of MEMS devices. In the control part, the application of adaptive controllers is introduced for trajectory control of the device under health and fault conditions. Fault in different forms in the structure of the device are modeled and foundry manufactured for experimental verifications. Pull-in voltage effect in the MEMS Lateral Comb Resonators are investigated and controlled by variable structure controllers. Reliability of operation is enhanced by active control of the device under fault conditions.;The second part of this research is focused on the fault diagnosis of the MEMS devices. Fault is introduced and investigated for better understanding of the system behavioral changes. Modeling of the device in different operating conditions suggests for the multiple-model adaptive estimation (MMAE) fault diagnosis technique. Application of Kalman filters in MMAE is investigated and the performance of the fault diagnosis is compared with other techniques such as self-tuning and auto self-tuning techniques. According to the varying parameters of the system, online parameter identification systems are required to monitor the parameter variations and model the system accurately. Self-tuning banks are applied and combined with MMAE to provide accurate fault diagnosis systems. Different parameter identification techniques result in different system performances. In this regard, this research investigates the application of Recursive Least Square with Forgetting Factor. Different techniques for tuning of forgetting factor value are introduced and their results are compared for better performance. The organization of this dissertation is as follows:;Chapter I introduces the structure of the MEMS Lateral Comb Resonator; Chapter II introduces the application of control techniques and displacement feedback approach. Chapter III investigates the control approach and experimental results. In chapter IV, a new controller is introduced and designed for the MEMS trajectory controls. Chapter V is about the fault and different techniques of fault diagnosis in MEMS LCRs. Chapter 6 is the future work suggested through the current results and observations. Each chapter contains a section to summarize the concluding remarks

Effect of the centrifugal force on the electromechanical instability of U-shaped and double-sided sensors made of cylindrical nanowires

The U-shaped and double-sided nanostructures are promising for developing miniature angular speed sensors. While the electromechanical instability of conventional beam-type nanostructures has been extensively addressed in the literature, few researchers have investigated this phenomenon in the double-sided and U-shaped sensors. In this regard, the present work demonstrates the effect of the centrifugal force on the pull-in performance of the double-sided and U-shaped sensors fabricated from cylindrical nanowire and operated in the van der Waals (vdW) regime. Based on the modified couple stress theory, the size-dependent constitutive equations of the sensors are derived. The governing equations are solved by two different approaches, i.e. the analytic Duan–Adomian method and the numerical differential quadrature method. The influences of the vdW and centrifugal forces, geometric parameters and the size phenomenon on the pull-in parameters are demonstrated

Modeling creep and anelasticity in particle strengthened alloys with strain gradient crystal plasticity

Modeling creep and anelasticity in particle strengthened alloys with strain gradient crystal plasticity For small material volumes, size effects, e.g. due to the interface constraints or heterogeneous strain ¿elds, may signi¿cantly affect the mechanical behavior of metals such that a deformation mechanism that is less important for the response in bulk form may become decisive for the performance of the material. Such second order effects were observed experimentally in the last two decades and form engineering challenges for the development and production of high-end modern technology. For example, creep and anelasticity observed in metallic thin ¿lm components of capacitive RF-MEMS switches may lead to time dependent deviations from the design speci¿cations of the device. The characterization and understanding of the mechanical behavior of the material is indispensable to overcome the reliability issues of these switches which hinder their full commercialization. In this thesis, a numerical framework is presented for modeling the time dependent mechanical behavior of thin ¿lms made of particle strengthened fcc alloys as an extension of a previously developed strain gradient crystal plasticity (SGCP) model (here referred to as Evers-Bayley type model) for pure fcc metals. A physically based ¿ow rule for crystallographic slip is developed based on the dislocation-dislocation and dislocation-particle interaction mechanisms. The extended SGCP framework is intrinsically able to capture the effect of an inhomogeneous distribution of geometrically necessary dislocation densities on the material behavior via the formulation of a back stress incorporating a material length scale. In chapter 2, the physically based Evers-Bayley type model and a thermodynamically consistent strain gradient theory of crystal plasticity by Gurtin are compared by deriving micro-stresses for the Gurtin type formulation based on the energetic back stresses of the Evers-Bayley type models, incorporating dislocation-dislocation interactions. It is shown that the defect energy function for a micro-stress that con¬tains the physical description of the interaction between dislocations of different slip systems has a more complicated form than those suggested in literature and is possibly non-convex. It is also shown that similar boundary conditions can be de¿ned for the Evers-Bayley type and Gurtin type models despite their differ¬ent additional ¿eld equations within the ¿nite element context. Thereafter, in chapter 3, the SGCP model is employed in electromechanical ¿nite element simulations of bending of polycrystalline thin beams made of a pure metal and a two phase alloy with a focus on the description of anelastic material behavior. Sim¬ulation results obtained with the SGCP model show a macroscopic strain recovery over time following the load removal. However, a detailed analysis demonstrates that the anelastic relaxation time and strength have no solid physical basis. A comparison of the results with experimental data implies that a single deformation mechanism may not be adequate for capturing the material response. Moreover, the slip law falls short in describing the behavior of a particle enhanced material. Subsequently, an extension of the SGCP model for a more realistic description of the time dependent mechanical behavior of two phase alloys, i.e. creep and anelasticity, is given in chapter 4 and its appli¬cation in multiphysical simulations of a capacitive RF-MEMS switch is presented in chapter 5. A new constitutive rule for crystallographic slip is developed by considering dislocation-dislocation interactions and three distinct dislocation-particle interactions: i) the Orowan process, ii) the Friedel process and iii) the climb of edge dislocations over particles. The new constitutive rule is obtained by the combination of separate slip laws for each type of interaction and is built based on the physically well-founded Orowan type rate equation. A ¿ow rule for the slip rate of mobile dislocations governed by dislocation-dislocation interactions is written by taking into account the jerky and continuous glide regimes of dislocations. Slip laws corresponding to the Orowan and Friedel processes are constructed by considering thermally activated dislocation motion. The climb of edge dislocations is described via a thermal detachment model. Results of ¿nite element simulations of bending of a single crystalline thin beam and a micro-clamp experiment with the extended SGCP model show that creep and anelastic behavior of a metallic thin ¿lm can be predicted with the extended SGCP framework. The amounts of the plastic deformation, anelastic recovery strength and associated relaxation times strongly rely on particle properties, the diffusional rate and the magnitude of internal stresses. The results of the simulations of the micro-clamp experiment imply that inhomoge-neous material diffusion may play an important role in the anelastic behavior of polycrystalline thin ¿lms. The results also suggest that the internal stress formulation of the extended SGCP may need to be extended by considering additional sources of internal stresses. The extended SGCP framework is applied to analyse the behavior of a capacitive RF-MEMS switch in multiphysical simulations. The electrodes of the switch are considered to be made of a metal thin ¿lm with incoherent second phases and have a polycrystalline structure with columnar grains through the thickness and passivated surfaces. The variation of the gap between the electrodes over time is analyzed. First, the in¿uences of particle size, volume fraction, surface constraints and ¿lm thickness on the performance of the switch after a loading and unloading cycle are studied. Then, the effects of cyclic loading and the duration of the unloaded state between sequential cy¬cles are investigated. The results show that the residual changes in the gap and the amount and rate of time dependent recovery after the load removal are highly sensitive to the microstructure and the ¿lm thickness. The smallest amounts of permanent deformation and anelastic recovery are obtained with an upper elec¬trode made of a relatively thin ¿lm which has a surface passivation and involves small incoherent particles with a relatively large volume fraction. Furthermore, the simulations revealed that the maximum residual change of the gap measured after completion of the unloading stage of each cycle saturates within a few cycles. A shorter duration of the unloaded state between successive loading-unloading cycles leads to a larger maximum residual gap change. Due to the decreasing gap, the pull-in voltage also decreases within a few cycles and shows a tendency to level off to a certain value. However, the release voltage does not seem to be as sensitive to the residual deformations as the pull-in voltage. Finally, in chapter 6, the conclusions and recommendations for a future work are given

NSF/ESF Workshop on Smart Structures and Advanced Sensors, Santorini Island, Greece, June 26-28, 2005: Structural Actuation and Adaptation Working Group

This document is a result of discussions that took place during the workshop. It describes current state of research and development (R&D) in the areas of structural actuation and adaptation in the context of smart structures and advanced sensors (SS&AS), and provides an outlook to guide future R&D efforts to develop technologies needed to build SS&AS. The discussions took place among the members of the Structural Actuation and Adaptation Working Group, as well as in general sessions including all four working groups. Participants included members of academia, industry, and government from the US and Europe, and representatives from China, Japan, and Korea

MEMS Accelerometers

Micro-electro-mechanical system (MEMS) devices are widely used for inertia, pressure, and ultrasound sensing applications. Research on integrated MEMS technology has undergone extensive development driven by the requirements of a compact footprint, low cost, and increased functionality. Accelerometers are among the most widely used sensors implemented in MEMS technology. MEMS accelerometers are showing a growing presence in almost all industries ranging from automotive to medical. A traditional MEMS accelerometer employs a proof mass suspended to springs, which displaces in response to an external acceleration. A single proof mass can be used for one- or multi-axis sensing. A variety of transduction mechanisms have been used to detect the displacement. They include capacitive, piezoelectric, thermal, tunneling, and optical mechanisms. Capacitive accelerometers are widely used due to their DC measurement interface, thermal stability, reliability, and low cost. However, they are sensitive to electromagnetic field interferences and have poor performance for high-end applications (e.g., precise attitude control for the satellite). Over the past three decades, steady progress has been made in the area of optical accelerometers for high-performance and high-sensitivity applications but several challenges are still to be tackled by researchers and engineers to fully realize opto-mechanical accelerometers, such as chip-scale integration, scaling, low bandwidth, etc