Metallic Microelectromechanical Systems Research Articles

Bulk-like ferroelectricity and magnetoelectric response of low-temperature solution-processed BiFeO3–PbTiO3 films on Ni for metallic MEMS

Simple and cost-effective procedures for the direct integration of ferroelectric perovskite oxides into Ni structures are necessary to realize related multifunctional metallic microelectromechanical systems, such as dual-source energy harvesters. This is especially difficult in the case of lead-containing morphotropic phase boundary materials for high piezoelectric response because the two components are thermodynamically incompatible and the formation of NiOx or perovskite oxide reduction takes place depending on the processing conditions. We show here that low-temperature solution processing is an effective means to kinetically limit nickel oxidation, capable of providing BiFeO3–PbTiO3 films on Ni plates at only 500 °C. Bulk-like ferroelectric properties and a distinctive magnetoelectric response were attained. This perovskite system, not explored before on Ni, has a much larger switchable polarization than the widely studied Pb(Zr,Ti)O3, and it is shown here to present an excellent downscaling behavior of ferroelectric properties until the verge of the nanoscale.

Investigation of the Influence of Sublayer Thickness on Pairing of Metallic MEMS Shutter Blades

For the 3D self-assembly of microstructures, the net intrinsic stress within a hybrid metal layer stack is utilised to study the dynamics of the pairing of metallic MEMS shutter blades with dimensions of 150 × 400 µm2, representing a highly nonlinear Casimir system. The study focuses on two main geometries: (i) unpaired (freestanding) and (ii) paired metallic Micro-Electro-Mechanical Systems (MEMS) shutter blades. The hybrid metal stack comprises three metal layers that are under intrinsic stress, which enables the curling of the freestanding shutter blades. The top aluminum layer thickness is varied systematically, creating tailored stress in the shutter blades, resulting in the freestanding blades curling differently, as well as geometrical changes in the paired shutter blades. Concerning the thickness variations, a large technological processing window has been identified, during which the pairing of the neighbouring shutter blades occurs (the top aluminum layer thickness ranges between 70 and 88 nm). Circles and ellipses have been fitted to micrographs obtained by scanning electron microscopy (SEM) and Confocal Laser Scanning Microscopy (CLSM). We observed a trend of the reduction of the radius of curvature RoC (ranging between 17 and 79 µm) for the unpaired shutters, and eccentricity (0.36 < ε < 0.67) occurs throughout the system for the paired shutters. Concerning the specific shape of the overlapping region of the two blades of the paired shutters, three pairing scenarios (I, II, and III) have been identified, classified, evaluated, and reported. The overlapping length lo between the paired shutter blades ranges between 31 and 8 µm. These scenarios also reveal nonlinearities in the pairing process.

Fatigue Degradation Properties of LIGA Ni Films Using Kilohertz Microresonators

A micrometer-scale ultrasonic fatigue characterization technique is introduced to investigate the fatigue degradation properties of LIGA Ni structural films in the very high cycle fatigue regime. Kilohertz microresonators fabricated with the MetalMUMPs process were driven at resonance (~ 8 kHz) in controlled environments (30 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C , 50% relative humidity (RH); 80 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, 5% RH; and 80 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, 90% RH) to perform fatigue tests on 20-μm -thick Ni notched beams subjected to fully reversed bending over a wide range of stress amplitudes. It is shown that the fatigue degradation occurring at the notch, observed with scanning electron microscopy, can be quantified with the measured evolution of the microresonators' resonant frequency. The fatigue damage consists of extrusion formation and microcrack nucleation and does not propagate to form fatal cracks due to the extreme stress gradients at the notch. The resonant frequency evolution for fatigue tests performed at 80 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, 90% RH suggests that stress-assisted oxidation in the presence of humid environments may accentuate fatigue damage formation. This experimental technique brings critical information regarding the long-term fatigue degradation properties of metallic microelectromechanical systems (MEMS) devices. In particular, this technique can allow controlled studies of the effects of fatigue localization and large stress gradients typically encountered in MEMS components.

A method for probing the effects of conformal nanoscale coatings on fatigue crack initiation in electroplated Ni films

This paper describes an experimental technique to identify robust nanoscale coatings for improving the long-term reliability of metallic microelectromechanical systems. More specifically, the influence of nanoscale alumina coatings on the fatigue crack initiation process in 20μm thick electrodeposited Ni films was investigated in a mild (30°C, 50% RH) and harsh (80°C, 90% RH) environment. Atomic-layer-deposited alumina layers, with thicknesses of 5 and 25nm, were coated on Ni fatigue micro-resonators, and the fatigue degradation behavior in the very high cycle fatigue regime was compared to that of uncoated structures. Evidence based on post-test scanning electron microscopy and resonant frequency evolution plots shows that the coatings do not prevent the formation of fatigue extrusions and micro-cracks. However, their formation is likely delayed for the 25nm thick alumina-coated Ni films.

Electro-Thermal MEMS Switch With Latching Mechanism: Design and Characterization

This paper presents the design, fabrication, and characterization of a proprietary metallic microelectromechanical systems switch with unique latching and switching mechanisms that are driven electro-thermally by a set of V-shaped actuators (VSA) and modified Guckel U-shaped actuators. These distinctive actuators are specifically designed and uniquely integrated to provide desired large in-plane traveling distance and force for the switch contact and latching mechanisms. Due to the structural symmetry of the VSA, the out-of-plane displacements are minimized in the entire system. The latching mechanism reduces the total power consumption of the switch while providing a large mechanical contact force for a reliable switching function. This switch with latching capability only consumes power at the time of changing states. MetalMUMPs is employed to fabricate the device, in which electroplated nickel is used as the structural material. Using the metallic structure allows a low operating voltage. At 1.0 V with an electrical power of ~ 0.67 W, each nickel 4-arm VSA generates ~ 13.5-μm displacement and ~ 8-mN force.

Three‐dimensional modelling of intergranular fatigue failure of fine grain polycrystalline metallic MEMS devices

ABSTRACTThis paper presents a 3D finite element model to investigate intergranular fatigue damage of microelectromechanical systems (MEMS) devices and to account for the effects of topological randomness of material microstructure on fatigue lives. The topology of MEMS material grain structures is modelled using randomly generated 3D Voronoi tessellations. Continuum Damage Mechanics is used to model progressive material degradation due to fatigue. A new 3D micro‐grain debonding procedure is developed to consider both intergranular crack initiation and propagation stages. The fatigue damage model is then used to investigate the effects of microstructure randomness on the variability in fatigue life of cantilever MEMS devices. Three different types of randomness are considered: (1) topological disorder due to random shapes and sizes of the material grains, (2) variation in material properties considering a normally (Gaussian) distributed elastic modulus and (3) material defects or internal voids. The stress–life results obtained are in good agreement with experimental data. The progression of damage and the overall crack pattern obtained from the microcantilever beam model are consistent with empirical observations.

Measuring time-dependent deformations in metallic MEMS

The reliability of metallic microelectromechanical systems (MEMS) depends on time-dependent deformation such as creep. Key to this process is the interaction between microstructural length scales and dimensional length scales, so-called size-effects. As a first critical step towards studying these size-effects in time-dependent deformation, a purely mechanical experimental methodology has been developed, which is presented here. The methodology entails the application of a constant deflection to a μm-sized free-standing aluminum-alloy cantilever beam for a prolonged period of time. After this load is removed, the deformation evolution is immediately recorded by acquiring surface topographies through confocal optical profilometry. Image correlation and an algorithm based on elastic beam theory are applied to the full-field beam profiles to correct drift and improve limited optical profilometry precision, yielding the tip deflection as function of time with a precision of 7% of the surface roughness. A proof-of-principle measurement reveals a remarkable time-dependent deflection recovery. Assumptions and errors of the methodology are analyzed. Finally, it is concluded that the methodology is most suitable for the investigation of creep due to the simplicity of specimen handling, preparation and setup design, while maximizing long term stability and deformation precision.

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.

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)

Planarization techniques for vertically integrated metallic MEMS on silicon foundry circuits

Various micromachining techniques exist to realize integrated microelectromechanical systems (MEMS), which include sensors, signal processing and/or driving circuits, and/or actuators in one small die. Post-processing techniques performed on foundry-fabricated circuits (e.g., MOSIS) are attractive since such an approach eliminates the need for an in-house integrated circuit fabrication line to produce integrated MEMS. A method based on the combination of metallic (e.g., electroplating) micromachining techniques with multichip module deposited (MCM-D) processes is a possible candidate to realize vertically-stacked integrated MEMS using the post-processing of integrated circuits (post-IC) approach. In order to realize such devices, planarization of the surface of foundry-fabricated circuit chips or wafers is often required. In such planarization layers, mechanical and chemical stability, as well as adhesion between the circuit-containing substrate and the micromachined devices, should be addressed. A PI/BCB/PI sandwich interlayer system, which utilizes both advantages of DuPont polyimide PI 2611 and Dow benzocyclobutene (BCB) Cyclotene 3022 series, was developed as a planarization interlayer for vertically integrated MEMS. The PI/BCB/PI interlayer system shows an over 95% degree of planarization (DOP) as well as passes the Method 107G Thermal Shock from the military standard MIL-STD-202F. A interlayer system was also developed as an alternative to the PI/BCB/PI system.