Reliability Of Microelectromechanical Systems Research Articles

Numerical Simulation of Shock Resistant Microsystems (MEMS)

The mechanical response of shock-loaded microelectromechanical systems (MEMS) is simulated to formulate guidelines for the design of dynamically reliable MEMS. MEMS are modeled as microstructures supported on elastic substrates, and the shock loads are represented as pulses of acceleration applied by the package on the substrate over a finite time duration. For typical MEMS and shock loads, the response of the substrate is closely approximated by rigid-body motion. Results indicate that modeling the shock force as a quasi-static force for MEMS with low-natural frequencies may lead to erroneous results. A criterion is obtained to distinguish between the dynamic and quasi-static responses of the MEMS.

Condition assessment and fault prognostics of microelectromechanical systems

Microelectromechanical systems (MEMS) are used in different applications such as automotive, biomedical, aerospace and communication technologies. They create new functionalities and contribute to miniaturize the systems and reduce their costs. However, the reliability of MEMS is one of their major concerns. They suffer from different failure mechanisms which impact their performance, reduce their lifetime and their availability. It is then necessary to monitor their behavior and assess their health state to take appropriate decision such as control reconfiguration and maintenance. These tasks can be done by using Prognostic and Health Management (PHM) approaches. This paper addresses a condition assessment and fault prognostic method for MEMS. The paper starts with a short review about MEMS and presents some challenges identified and which need to be raised to implement PHM methods. The purpose is to highlight the intrinsic constraints of MEMS from PHM point of view. The proposed method is based on a global model combining both nominal behavior model and degradation model to assess the health state of MEMS and predict their remaining useful life. The method is applied on a microgripper, with different degradation models, to show its effectiveness.

The effect of water on the mechanical properties of native oxide coated silicon structure in MEMS

The influence of a humid environment on the reliability of microelectromechanical systems (MEMS) needs to be considered in applications such as chemical and biological sensors. We have examined the effects of water on the mechanical properties of silicon thin-films covered by a surface native oxide layer using reactive molecular dynamics simulations. The results of quasi-static tensions show that the silicon fracture strength of 11.2GPa in the presence of water is lower than the fracture strength of 16.3GPa in dry environment. Moreover, the effect of water on the tension properties of silicon is extremely rapid (within 2ns). In both dry and liquid water environments, the cracks initiate from the silicon structure rather than from the surface oxide layer.

Effect of Space Radiation on the Leakage Current of MEMS Insulators

The effect of space radiation on the reliability of microelectromechanical systems (MEMS) devices is an important consideration for future upper atmosphere and space applications. MEMS capacitors with insulator materials of silicon nitride (Si3N4), silicon oxide (SiO2), and ultrananocrystalline diamond (UNCD) were selected for radiation and leakage current studies. Leakage current was used as a measure of insulator performance and reliability, and is suggested here as a method to detect charge trapping, which also affects reliability. UNCD capacitors were orders of magnitude leakier than Si3N4 and SiO2, with Si3N4 being leakier than SiO2. SiO2 devices exhibited unstable leakage current with accumulated electric field stress, and were not utilized in radiation studies. Si3N4 capacitors exhibited leakage current decay (with a time constant of 190 s) under constant voltage stress above 2 MV/cm due to charge injection from the electrodes and trapping in the insulator. Si3N4 and UNCD capacitors were more sensitive to ionizing gamma radiation than to displacement damage from fast neutrons. Both Si3N4 and UNCD devices survived total doses of radiation representative of 20-100 years in the Van Allen radiation belts with 4 mm Al equivalent shielding. Capacitor equivalent circuit and resistor capacitor (RC) circuit charging models are developed to explain leakage current behavior of Si3N4 capacitors subjected to constant voltage stress and/or irradiation. In situ monitoring of Si3N4 capacitors placed next to the nuclear reactor core did not yield any single event effects at electric field strength of 1 MV/cm with a fast neutron fluence of 2×1012 n/cm2. Si3N4 MEMS capacitors appear best suited for upper atmosphere and space applications with their relatively low leakage current (low power consumption) and apparent radiation hardness.

High‐power MEMS switch enabled by carbon‐nanotube contact and shape‐memory‐alloy actuator

AbstractA forest of vertically aligned carbon nanotubes (CNTs) is integrated as an electrical contact material with a high‐power, normally‐open switch based on micro‐electro‐mechanical systems (MEMS) technology. A shape‐memory‐alloy (SMA) cantilever is thermally actuated to enable switching between the movable CNT forest and the copper electrode formed on the SMA. The out‐of‐plane SMA actuator provides high forces to enable distributed contacts with the CNT forest, achieving low contact resistances and high ON/OFF resistance ratios. The ON state of the switch shows contact resistances as low as 35 Ω with a dependence on the operating current. The device operation is performed with over 5‐W input powers. Long‐term operation with more than 1 × 106 switching cycles is demonstrated. The results indicate that a combination of the CNT‐based contact and the SMA actuator may be a promising path to realizing reliable MEMS contact switches for high‐power applications.

MEMS Reliability Review

Microelectromechanical systems (MEMS) represents a technology that integrates miniaturized mechanical and electromechanical components (i.e., sensors and actuators) that are made using microfabrication techniques. MEMS devices have become an essential component in a wide range of applications, ranging from medical and military to consumer electronics. As MEMS technology is implemented in a growing range of areas, the reliability of MEMS devices is a concern. Understanding the failure mechanisms is a prerequisite for quantifying and improving the reliability of MEMS devices. This paper reviews the common failure mechanisms in MEMS, including mechanical fracture, fatigue, creep, stiction, wear, electrical short and open, contamination, their effects on devices' performance, inspection techniques, and approaches to mitigate those failures through structure optimization and material selection.

Silica-Encapsulated Nanoparticle Films as Surface Modifications for MEMS

In an effort to improve the reliability of microelectromechanical systems (MEMS), silica thin films deposited by chemical vapor deposition were used to encapsulate gold nanoparticle coatings. These composite coatings were shown to provide extremely durable films that significantly reduce the adhesion energy of silicon-based microcantilever beams. The results discussed suggest that encapsulating nanoparticle films with a durable silica thin film may lead to improved MEMS reliability.

Reliability of Suspended Bridges on Superconducting Microstrip Filters Using MEMS Switches

This work proposes to use capacitive micro-electro-mechanical systems (MEMS) switches built on a superconducting microstrip hairpin filter to investigate the reliability of MEMS for long term survivability. This device is made of a YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> thin film deposited on a 20 mm × 20 mm LaAlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate by pulsed laser deposition and BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> by RF magnetron sputtering, which is utilized as a dielectric insulation layer at the switching points of contact. The major concern for capacitive MEMS switches is stiction between the gold suspended bridge membrane (top layer) and the dielectric material (bottom layer). The main failure mode results from charge build-up at the bottom layer which in turn depends on the actuation voltage. The actuation voltage measured at room and cryogenic temperature is used to derive and calculate the Young's modulus formula which takes into consideration the device geometry, residual stress and mechanical properties of the device. Modified Young's modulus equation will be validated through reliability data of membrane actuation and failure mode. This equation will in turn be used in modeling other RF MEMS devices operating at cryogenic temperatures.

On MEMS Reliability and Failure Mechanisms

Microelectromechanical systems (MEMS) are a fast‐growing field in microelectronics. MEMS are commonly used as actuators and sensors with a wide variety of applications in health care, automotives, and the military. The MEMS production cycle can be classified as three basic steps: (1) design process, (2) manufacturing process, and (3) operating cycle. Several studies have been conducted for steps (1) and (2); however, information regarding operational failure modes in MEMS is lacking. This paper discusses reliability in the context of MEMS functionality. It also presents a brief review of the most relevant failure mechanisms for MEMS.

High Resolution MEMS Accelerometers to Compare Running Mechanics between Trained and Untrained Runners

We have previously reported the validity and reliability of microelectromechanical system (MEMS) high resolution accelerometers (HRA) relative to VO2 in highly trained and untrained individuals during walking and running. PURPOSE: To determine if MEMS HRA could identify differences in mechanics of running between trained (T) and (UT) runners during treadmill locomotion. METHODS: Procedures were approved by the Eastern Michigan University CHHS Human Subjects Review Committee and subjects gave informed consent. 18 runners, [9 T (21.4±1.7yr, 65.5±5.7 kg, 70.1±6.2 ml/kg/min) and 9 UT (31.6±1.5yr, 69.8±3.7 kg, 49.2±4.2 ml/kg/min)] performed 2 incremental VO2max trials while wearing HRA (Microstrain, VT). Aggregate acceleration for vertical (VT), anterior/posterior (AP), medial/lateral (ML), axes were recorded and Euclidean resultants were computed (RES). Trials consisted of 2 min stages starting at 2kph and increasing 2kph until exhaustion. Group comparisons were made from 8-16 kph, as all subjects completed these stages. Axial root mean square (RMS), RMS relative to speed (EC), and ratio of RMS/RES (RA) were compared between T and UT to determine if differences in running mechanics could be identified between the two groups. Expired gasses were collected using an Oxycon Mobile (Viasys, CA). VO2 and RER, were used to determine O2 cost (O2C) and energy expenditure (EE). RESULTS: Regression of RES was strongly related to VO2, but the regression trend for T was significantly different than UT (p<.001) across running speeds. T exhibited lower RMS and EC than UT in all axes (p <.003). Although MLRA was also lower for T vs UT, VTRA was higher (p <.001). All of these differences while running were despite higher VO2, O2 cost, and lower RER in T vs. UT, indicating greater fat utilization in T. There was no significant difference in energy expenditure between groups. CONCLUSIONS: These results demonstrate that T runners accelerate less in all axes and relative to all speeds than UT, yet allocate a greater proportion of their RES acceleration in the VT axis. These differences in acceleration parameters are not necessarily reflected in lower O2C or EE of running in T vs UT. Therefore, it is unclear if these differences confer a performance advantage in T runners over UT, and the importance of these differences is yet to be determined.

On the mechanics of microsystems

This work is focused on the mechanical design of MEMS (micro electromechanical systems) and to the most relevant mechanically-coupled fields of interest. The paper represents an overview on some of the research topics investigated by the author in previous studies and provides a good background for the optimization of the mechanics of microsystems. The multiphysics problem is discussed, with reference to the structural-fluidic and the electromechanical coupling and their implications on the dynamic response. The stress and/or strain gradients in the material caused by building processes are introduced, as well as the MEMS reliability related to mechanical fatigue of cyclic loaded components.

Temperature effects on the mechanical reliability of MEMS structures: experimental study on creep and thermal fatigue

The reliability of micro electro-mechanical systems (MEMS) became a fundamental topic of investigation as a result of their widespread application in many day-to-day life devices. From the structural viewpoint the most relevant sources of lifetime reduction are mechanical fatigue and high temperatures. The typical working conditions of MEMS include repetitive cyclic loads and high operative frequencies; examples are represented by some of the most common applications: RF devices (switches, varactors, resonators, filters), inertial sensors, optical components, etc. These operative conditions activate that processes commonly associated to fatigue, like crack initiation and propagation and the self-heating of the material accompanied to the progressive degradation of mechanical properties. The fatigue associated to MEMS structures was investigated by the authors in some previous studies [1] providing experimental results in good agreement with the literature. High temperatures are responsible of additional mechanisms of material degradation leading to a reduction of components lifetime and devices reliability. The most diffused scenarios associate high temperature to: a) static loads and b) alternate loads. In the first case creep effect appears: the temperature is responsible of progressively increasing deflections of the structure under the applied load; the deformation is usually plastic (and so, permanent) and large. In the second case the cause of material damaging is actually mechanical fatigue, but the presence of high levels of temperature modifies the S-N curve obtained at ambient temperature and generally determines faster failures at lower load levels. This work is focused on the reliability of metallic MEMS structures subjected to static and alternate loads in presence of increasing levels of temperature. Some works in the literature were focused on the study of creep in MEMS that appears when static loads are associated to medium and high temperatures: Modlinski et al. [2] characterized the bridge of a capacitive switch made by an Al alloy, Larsen et al. [3] presented the design for a test structure devoted to investigation on creep for electroplated nickel, Zhang and Dunn [4] studied creep on thin gold-polysilicon bilayer films subjected to thermal loading. It was observed that long-term deformation of metal parts of microdevices may play a key role in the lifetime of a device, making creep a very important issue in the reliability of MEMS. Apart effects associated with diffusion, all plastic deformation occurs as a consequence of the migration of atomic planes in the crystal lattice of the material (namely dislocations); this effect is deeply emphasized by the coexistence of increasing temperature and stress. If alternate loads act on the component, the effect of temperature produces an acceleration of material damaging due to mechanical fatigue process; local plasticized area can be detected on the surface. © Owned by the authors, published by EDP Sciences, 2010 DOI:10.1051/epjconf/20100606001 EPJ Web of Conferences 6, 06001 (2010)

Resonant Bending Fatigue Tests on Thin Films

The fatigue properties of thin-fi lm materials are extremely important in the design of durable and reliable micro-electromechanical systems (MEMS). However, it is rather diffi cult to apply the conventional fatigue testing method for bulk materials to thin fi lms as the specimen size is extremely small. Therefore, a fatigue testing method suitable for thin-fi lm materials is required. We have developed a fatigue testing method that uses the resonance of a cantilever-type specimen prepared from thin fi lms. Cantilever beam specimens with dimensions of 3(L) × 1(W) × 0.01(t) mm3 were prepared from Ni-P amorphous alloy thin fi lms and gold foils. In addition, cantilever beam specimens with dimensions of 3(L) × 0.3(W) × 0.005(t) mm3 were prepared from single-crystal silicon thin fi lms. These specimens were fi xed to a holder that was connected to an audio speaker (actuator) and were resonated in the bending mode. The Young’s moduli measured from the resonant frequencies of Ni-P and gold foil were 116 and 72 GPa, respectively. These values were comparable to those measured by other techniques, indicating that the specimens resonated in a theoretically predictable manner and that our method is valid. Resonant fatigue tests were carried out for these specimens by changing the amplitude range of resonance, and S-N curves were successfully obtained for Ni-P amorphous alloy thin fi lms, gold foils, and single-crystal silicon thin fi lms.

Size-effects in time-dependent mechanics in metallic MEMS

Reliability of microelectromechanical systems (MEMS) depends a.o. on time-dependent deformation such as creep and fatigue [1]. It is known from literature that this behavior is affected by size-effects: the interaction between microstructural length scales and dimensional length scales [2,3]. Not much research has focused on characterizing size-effects in time-dependent material behavior, specifically for free-standing thin films [3]. This study investigates size-effects caused by grain statistics in timedependent deformation in µm-sized free-standing aluminum cantilever beams. A numeric-experimental method is used to determine material parameters. The experiment entails applying a constant deflection to the micro-beams for a prolonged period. The deflection is achieved with 50 nm resolution via a micro-clamp. The beams are then released. Immediately the deformation evolution is recorded by acquiring surface height profiles with a confocal optical profiler. Image correlation of the full-field beam profiles is applied to correct for specimen drift and tilt. The experiment yields the tip deflection as function of time with ~3 nm precision. In the numerical part, this data is combined with a finite element model based on a standard-solid material model. In this way material parameters describing time-dependent behavior are extracted. The time constant for the deflection evolution is determined within 20%, as verified by predicting a different experiment. Figure 1 shows the model and the numeric prediction of an experiment.

A partition of unity‐based multiscale approach for modelling fracture in piezoelectric ceramics

AbstractThe development of models for a priori assessment of the reliability of micro electromechanical systems is of crucial importance for the further development of such devices. In this contribution a partition of unity‐based cohesive zone finite element model is employed to mimic crack nucleation and propagation in a piezoelectric continuum. A multiscale framework to appropriately represent the influence of the microstructure on the response of a miniaturized component is proposed. It is illustrated that using the proposed multiscale method a representative volume element exists. Numerical simulations are performed to demonstrate the constitutive homogenization framework. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Influence of adhesive rough surface contact on microswitches

Stiction is a major failure mode in microelectromechanical systems (MEMS). Undesirable stiction, which results from contact between surfaces, threatens the reliability of MEMS severely as it breaks the actuation function of MEMS switches, for example. Although it may be possible to avoid stiction by increasing restoring forces using high spring constants, it follows that the actuation voltage has also to be increased significantly, which reduces the efficiency. In our research, an electrostatic-structural analysis is performed to estimate the proper design range of the equivalent spring constant, which is the main factor of restoring force in MEMS switches. The upper limit of equivalent spring constant is evaluated based on the initial gap width, the dielectric thickness, and the expected actuation voltage. The lower limit is assessed on the value of adhesive forces between the two contacting rough surfaces. The MEMS devices studied here are assumed to work in a dry environment. In these operating conditions only the van der Waals forces have to be considered for adhesion. A statistical model is used to simulate the rough surface, and the Maugis’s model is combined with Kim’s expansion to calculate adhesive forces. In the resulting model, the critical value of the spring stiffness depends on the material and surface properties, such as the elastic modulus, surface energy, and surface roughness. The aim of this research is to propose simple rules for design purposes.

Contact damage of tetrahedral amorphous carbon thin films on silicon substrates

We have investigated the fracture behavior of tetrahedral amorphous carbon films, with thicknesses 0.15 (ultrathin), 0.5 (thin), and 1.2 (thick) microns on silicon substrates. To that end, the systems were progressively loaded into a nanoindenter using a spherical tip, and surface and cross sections were subsequently examined using a focused ion beam miller at different loads. A transition was found as a function of film thickness: for ultrathin and thin films, cracking (radial and lateral) initiated in the silicon substrate and followed a similar path in the films. Thicker films, on the other hand, provided a higher level of protection to the substrate, and cracking (lateral and radial at the interface) was constrained to the film. The damage modes and the transition obtained differ from those that occur in thick coatings. Lateral cracks are highly dangerous, leading to delamination of thick films and to spallation when thinner films are used. The results have implications concerning the mechanical reliability of microelectromechanical systems.

The effect of residual stress on adhesion-induced instability in micro-electromechanical systems

Adhesion and residual stress play a critical role in the performance and reliability of MEMS (micro-electromechanical systems). The effects of residual stress on the pull-in instability of the micromechanical structure were examined using thin plate theory and energy criterion. It is shown that the critical gap between the film and the substrate is a function of the ratio of residual stress to bending rigidity and the adhesion energy. Such instability is mitigated by the presence of in-plane tensile residual stress in the film. But, it is aggravated by the presence of compressive residual stress. In addition, the effect of compressive residual stress is much more significant than tensile residual stress.

Simultaneous Quality and Reliability Optimization for Microengines Subject to Degradation

Micro-electro-mechanical systems (MEMS) represent an exciting new technology, but to achieve more widespread usage and wider adoption within more industrial applications, they must be highly reliable, and manufactured to stringent quality standards. Many challenging manufacturing issues are of concern during the fabrication of MEMS, such as precise dimensional inspection, reliability modeling, burn-in scheduling, avoiding stiction, and maintenance strategies. However, only limited mathematical tools for improving MEMS reliability, quality, and productivity are currently available. This paper proposes a mathematical model to jointly determine inspection & preventive replacement policies for surface-micromachined microengines subject to wear degradation, which is a major failure mechanism for certain MEMS devices. The optimal specification limits for inspection, and the replacement interval are determined by simultaneously optimizing MEMS quality and reliability. The proposed model can be used as a tool for decision-makers in MEMS manufacturing to make sound economical and operational decisions on reliability, quality, and productivity. While illustrated considering one specific microengine design, the proposed model can be applied to a broader range of MEMS devices that experience wear degradation between rubbing surfaces.

Effect of Charge Injection Due to ESD on the Operation of MEMS

The relative effect of charge injection due to human-body model electrostatic discharge (ESD) on the operation of capacitive microelectromechanical systems (MEMS) structures is studied. The influence on the operating characteristics as a function of feature size is analyzed; for small gaps (< 5 mum), the modified Paschen's law applies. A force factor is introduced to compare forces due to the control voltage and trapped charge due to ESD or triboelectrification and assess possible failure due to stiction. The results show that the relative effect of injected charge due to ESD increases inversely as the square of the gap separation for floating targets and inversely as the square of the plate area for grounded targets. For comparison purposes, the relative effect of charge injection due to triboelectrification is shown to be independent of device scaling. An electric-field model for the air gap and dielectric layer is introduced to assess the reliability of MEMS due to trapped charge in the dielectric.