A Review of Micro-Contact Physics for Microelectromechanical Systems (MEMS) Metal Contact Switches

Innovations in relevant micro-contact areas are highlighted, these include, design, contact resistance modeling, contact materials, performance and reliability. For each area the basic theory and relevant innovations are explored. A brief comparison of actuation methods is provided to show why electrostatic actuation is most commonly used by radio frequency microelectromechanical systems designers. An examination of the important characteristics of the contact interface such as modeling and material choice is discussed. Micro-contact resistance models based on plastic, elastic-plastic and elastic deformations are reviewed. Much of the modeling for metal contact micro-switches centers around contact area and surface roughness. Surface roughness and its effect on contact area is stressed when considering micro-contact resistance modeling. Finite element models and various approaches for describing surface roughness are compared. Different contact materials to include gold, gold alloys, carbon nanotubes, composite gold-carbon nanotubes, ruthenium, ruthenium oxide, as well as tungsten have been shown to enhance contact performance and reliability with distinct trade offs for each. Finally, a review of physical and electrical failure modes witnessed by researchers are detailed and examined

CMOS Compatible Thin-Film ALD Tungsten Nanoelectromechanical Devices

This research focuses on the development of a novel, low-temperature, CMOS compatible, atomic-layer-deposition (ALD) enabled NEMS fabrication process for the development of ALD Tungsten (WALD) NEMS devices. The devices are intended for use in CMOS/NEMS hybrid systems, and NEMS based micro-processors/controllers capable of reliable operation in harsh environments not accessible to standard CMOS technologies. The majority of NEMS switches/devices to date have been based on carbon-nano-tube (CNT) designs. The devices consume little power during actuation, and as expected, have demonstrated actuation voltages much smaller than MEMS switches. Unfortunately, NEMS CNT switches are not typically CMOS integrable due to the high temperatures required for their growth, and their fabrication typically results in extremely low and unpredictable yields. Thin-film NEMS devices offer great advantages over reported CNT devices for several reasons, including: higher fabrication yields, low-temperature (CMOS compatible) deposition techniques like ALD, and increased control over design parameters/device performance metrics, i.e., device geometry. Furthermore, top-down, thin-film, nano-fabrication techniques are better capable of producing complicated device geometries than CNT based processes, enabling the design and development of multi-terminal switches well-suited for low-power hybrid NEMS/CMOS systems as well as electromechanical transistors and logic devices for use in temperature/radiation hard computing architectures. In this work several novel, low-temperature, CMOS compatible fabrication technologies, employing WALD as a structural layer for MEMS or NEMS devices, were developed. The technologies developed are top-down nano-scale fabrication processes based on traditional micro-machining techniques commonly used in the fabrication of MEMS devices. Using these processes a variety of novel WALD NEMS devices have been successfully fabricated and characterized. Using two different WALD fabrication technologies two generations of 2-terminal WALD NEMS switches have been developed. These devices have functional gap heights of 30-50 nm, and actuation voltages typically ranging from 3-5 Volts. Via the extension of a two terminal WALD technology novel 3-terminal WALD NEMS devices were developed. These devices have actuation voltages ranging from 1.5-3 Volts, reliabilities in excess of 2 million cycles, and have been designed to be the fundamental building blocks for WALD NEMS complementary inverters. Through the development of these devices several advancements in the modeling and design of thin-_lm NEMS devices were achieved. A new model was developed to better characterize pre-actuation currents commonly measured for NEMS switches with nano-scale gate-to-source gap heights. The developed model is an extension of the standard field-emission model and considers the electromechanical response, and electric field effects specific to thin-film NEMS switches. Finally, a multi-physics FEM/FD based model was developed to simulate the dynamic behavior of 2 or 3-terminal electrostatically actuated devices whose electrostatic domains have an aspect ratio on the order of 103. The model uses a faux-Lagrangian finite difference method to solve Laplaces equation in a quasi-statatically deforming domain. This model allows for the numerical characterization and design of thin- _lm NEMS devices not feasible using typical non-specialized BEM/FEM based software. Using this model several novel and feasible designs for fixed-fixed 3-terminal WALD NEMS switches capable for the construction of complementary inverters were discovered

Review: Electrostatically actuated nanobeam-based nanoelectromechanical switches – materials solutions and operational conditions

Funding Information: This work was supported by the Latvian Council of Science (project No. 549/2012) and the University of Latvia project No. AAP2016/B043 and No. ZD2010/AZ19. Publisher Copyright: © 2018 Jasulaneca et al.; licensee Beilstein-Institut. License and terms: see end of document.This review summarizes relevant research in the field of electrostatically actuated nanobeam-based nanoelectromechanical (NEM) switches. The main switch architectures and structural elements are briefly described and compared. Investigation methods that allow for exploring coupled electromechanical interactions as well as studies of mechanically or electrically induced effects are covered. An examination of the complex nanocontact behaviour during various stages of the switching cycle is provided. The choice of the switching element and the electrode is addressed from the materials perspective, detailing the benefits and drawbacks for each. An overview of experimentally demonstrated NEM switching devices is provided, and together with their operational parameters, the reliability issues and impact of the operating environment are discussed. Finally, the most common NEM switch failure modes and the physical mechanisms behind them are reviewed and solutions proposed.publishersversionPeer reviewe

Lifetime Extension of RF MEMS Direct Contact Switches in Hot-Switching Operations by Ball-Grid-Array (BGA) Dimple Design

Direct contact RF microelectromechanical systems switches have demonstrated excellent ultrawideband performance from dc to 100 GHz. However, they are prone to failures due to contact adhesion and arcing, particularly for pure-gold/pure-gold contacts. In this letter, we present a new contact design employing ball grid array (BGA) dimples that limit the effective contact area to a few tens of nanometers in diameter. We experimentally show the performance of the BGA dimple with pure-gold/pure-gold contacts and demonstrate RF power handling greater than 1 W during hot switching in excess of 100 million cycles.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87268/4/Saitou10.pd

Novel Test Fixture for Characterizing Microcontacts: Performance and Reliability

Engineers have attempted to improve reliability and lifecycle performance using novel micro-contact metals, unique mechanical designs and packaging. Contact resistance can evolve over the lifetime of the micro-switch by increasing until failure. This work shows the fabrication of micro-contact support structures and test fixture which allow for micro-contact testing, with an emphasis on the fixture\u27s design to allow the determination and analysis of the appropriate failure mode. The other effort of this investigation is the development of a micro-contact test fixture which can measure contact force and resistance directly and perform initial micro-contact characterization, and two forms of lifecycle testing for micro-contacts at rates up to 3kHz. In this work, two different designs of micro-contact structures are fabricated and tested, with each providing advantages for studying micro-contact physics. After fabrication was refined, three functioning fixed-fixed Au micro-contact support structures with contact radii of 4, 6, and 10 µm and two functioning fixed-fixed Ag micro-contacts were tested using the µN force sensor at cycle rates up to 3 kHz. Comparing the PolyMUMPs micro-contact support structure to the fixed-fixed micro-contact support structure, it was determined that the fixed-fixed micro-contact support structure is the best structure for studying the evolution of micro-contact resistance

Hybrid Micro-Electro-Mechanical Tunable Filter

While advantages such as good thermal stability and processing-chemical compatibilities exist for common monolithic-integrated micro-electro-mechanically tunable filters (MEM-TF) and MEM-tunable vertical cavity surface emitting lasers (MT-VCSEL), they often require full processing to determine device characteristics. Alternatively, the MEM actuators and the optical parts may be fabricated separately, then subsequently bonded. This hybrid approach potentially increases design flexibility. Since hybrid techniques allow integration of heterogeneous material systems, best of breed compound optoelectronic devices may be customized to enable materials groups to be optimized for tasks they are best suited. Thus, as a first step toward a hybrid (AlxGa1-xAs-polySi) MT-VCSEL, this dissertation reports the design, fabrication, and demonstration of an electrostatically actuated hybrid MEM-TF. A 250x250-µm2, 4.92-µm-thick, Al0.4Ga0.6As-GaAs distributed Bragg reflector was successfully flip-bonded to a polySi piston electrostatic actuator using SU-8 photoresist as bonding adhesive. The device demonstrated 53nm (936.5 - 989.5nm) of resonant wavelength tuning over the actuation voltage range of 0 to 10 V

Indium Phosphide Based Optical Waveguide MEMS for Communications and Sensing

Indium phosphide (InP) is extensively used for integrated waveguide and photonic devices due to its suitability as a substrate for direct bandgap materials (e.g. In1-XGaXAsYP1-Y) operating at the lambda=1550 nm communications wavelength. However, little work has been reported on InP optical waveguide micro-electro-mechanical systems (MEMS). In this work, InP cantilever and doubly-clamped beams were micromachined on an In0.53Ga0.47As "sacrificial layer" on (100) InP substrates. Young's modulus was measured using nanoindentation and microbeam-bending. Intrinsic stress and material uniformity (stress gradient) were obtained by measuring the profile of doubly-clamped and cantilever beams using confocal microscopy. The study resulted in a Young's modulus of 80.4-106.5 GPa (crystal orientation-dependent). Although InP was grown lattice-matched to the substrate, arsenic from the underlying In0.53Ga0.47As sacrificial layer resulted in intrinsic compressive stress. Adding trace amounts of gallium to the InP layer during epitaxial growth induced tensile stress to offset the effect of arsenic. The materials characterization was extended to develop optical waveguide switches and sensors. In the first device, two parallel waveguides were actuated to vary the spacing between them. By modulating the gap using electrostatic pull-in actuation, the optical coupling strength was controlled via the evanescent field. Low voltage switching (<10 V), high speed (4 us), low crosstalk (-47 dB), and low-loss (<10 %) were achieved. Variable coupling over a 17.4 dB dynamic range was also demonstrated. The second device utilized a single movable input waveguide, which was actuated via electrostatic comb-drives to end-couple with one of several output waveguides. Low voltage switching (<7 V), 140 us switching speed (2 ms settling time), low crosstalk (-26 dB), and low-loss (<3.2 dB) were demonstrated. Sensing techniques based on mass-loading were developed using end-coupled cantilever waveguides. Here, the mechanical resonance frequency was measured by actuating the cantilever and measuring the end-coupled optical power at the output waveguide. A proof-of-concept experiment utilized a focused-ion-beam to mill the cantilever tip and resulted in a measurable resonance shift with mass-sensitivity delta_m/delta_f=5.1 fg/Hz. The cantilever waveguide devices and measurement techniques enable accurate resonance detection in mass-based cantilever sensors and also enable single-chip sensors with on-chip optical detection to be realized

Effects of electrical leakage currents on MEMS reliability and performance

Electrostatically driven MEMS devices commonly operate with electric fields as high at 108 V/m applied across the dielectric between electrodes. Even with the best mechanical design, the electrical design of these devices has a large impact both on performance (e.g., speed and stability) and on reliability (e.g., corrosion and dielectric or gas breakdown). In this paper, we discuss the reliability and performance implications of leakage currents in the bulk and on the surface of the dielectric insulating the drive (or sense) electrodes from one another. Anodic oxidation of poly-silicon electrodes can occur very rapidly in samples that are not hermetically packaged. The accelerating factors are presented along with an efficient early-warning scheme. The relationship between leakage currents and the accumulation of quasistatic charge in dielectrics are discussed, along with several techniques to mitigate charging and the associated drift in electrostatically actuated or sensed MEMS devices. Two key parameters are shown to be the electrode geometry and the conductivity of the dielectric. Electrical breakdown in submicron gaps is presented as a function of packaging gas and electrode spacing. We discuss the tradeoffs involved in choosing gap geometries and dielectric properties that balance performance and reliability

MEMS for Photonic Integrated Circuits

The field of microelectromechanical Systems (MEMS) for photonic integrated circuits (PICs) is reviewed. This field leverages mechanics at the nanometer to micrometer scale to improve existing components and introduce novel functionalities in PICs. This review covers the MEMS actuation principles and the mechanical tuning mechanisms for integrated photonics. The state of the art of MEMS tunable components in PICs is quantitatively reviewed and critically assessed with respect to suitability for large-scale integration in existing PIC technology platforms. MEMS provide a powerful approach to overcome current limitations in PIC technologies and to enable a new design dimension with a wide range of applications

Insulator Charging in RF MEMS Capacitive Switches

While capacitive radio frequency microelectromechanical (RF MEM) switches are poised to provide a low cost, high isolation, low power alternative to current RF switch technologies, there are still reliability issues limiting switch lifetime. Previous research identified insulator charging as a primary cause of switch failure. Changes in switch pull-in and release voltages were measured to provide insight into the mechanisms responsible for charging and switch failure. A spatial and temporal dependent model was developed to describe silicon nitride\u27s time-dependent charging as a function of applied bias. This model was verified by applying constant biases to metal-silicon nitride-silicon capacitors and tracking flatband voltage shifts. This knowledge of silicon nitride was then applied to MEM switches. Using novel waveforms and exploiting differences in actuation characteristics allowed the determination of charging characteristics and the investigation of switch failure. Results show tunneling is responsible for changes in the pull-in voltages-this includes a super-saturation effect explained by a steady-state trap charge and discharge condition. A program that models switch actuation was enhanced to include the time-dependent tunneling model. Finally, it was discovered insulator charging cannot explain permanent switch failure; instead, stiction from a contaminant on the insulator surface is likely the cause