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

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

Novel Test Fixture for Characterizing MEMS Switch Microcontact Reliability and Performance

In microelectromechanical systems (MEMS) switches, the microcontact is crucial in determining reliability and performance. In the past, actual MEMS devices and atomic force microscopes (AFM)/scanning probe microscopes (SPM)/nanoindentation-based test fixtures have been used to collect relevant microcontact data. In this work, we designed a unique microcontact support structure for improved post-mortem analysis. The effects of contact closure timing on various switching conditions (e.g., cold-switching and hot-switching) was investigated with respect to the test signal. Mechanical contact closing time was found to be approximately 1 us for the contact force ranging from 10–900 μN. On the other hand, for the 1 V and 10 mA circuit condition, electrical contact closing time was about 0.2 ms. The test fixture will be used to characterize contact resistance and force performance and reliability associated with wide range of contact materials and geometries that will facilitate reliable, robust microswitch designs for future direct current (DC) and radio frequency (RF) applications

Étude du contact électromécanique Au−Ru/AlCu pour les MEMS RF ohmiques : modélisation, intégration et caractérisation

Aujourd'hui les systèmes de télécommunication sont le plus souvent construits à partir (i) d'électronique à l'état solide utilisant la technologie des semi-conducteurs telles les diodes PIN et les transistors FET ou (ii) de relais électromagnétiques. Chacune de ces technologies offre un compromis entre la fréquence d'opération, la linéarité, la capacité à transporter la puissance RF, les pertes d'insertion, l'isolation, le temps de commutation, la consommation électrique, la fiabilité, l'intégration à grande échelle, la masse et le coût. Par ailleurs, le besoin d'opérer à hautes fréquences est plus marqué avec l'arrivée de la 5G. Les MEMS RF sont à priori adaptés pour répondre aux nombreux défis des nouvelles générations de systèmes de télécommunication, cependant le principal obstacle reste leur fiabilité. Cette thèse s'inscrit dans un contexte d'amélioration des performances et de la fiabilité des micro-commutateurs MEMS RF à contact ohmique. L'encapsulation des MEMS RF aux niveau du wafer est nécessaire pour leur intégration dans des systèmes plus complets telles les matrices de commutations et les antennes configurables. En plus de permettre un contrôle de l'environnement directe du MEMS, la solution d'encapsulation ne doit pas altérer les performances RF du composant. La fiabilité du contact électromécanique est l'une des limitations majeures intrinsèques des MEMS RF ohmiques. Le contact Au-Ru/AlCu est proposé comme une configuration compatible avec un procédé MEMS intégrant une étape d'encapsulation au niveau du wafer par collage eutectique Al-Ge. L'utilisation d'un contact Au-Ru permet de réduire les forces d'adhésion et le transfert de matière en comparaison de l'utilisation d'une paire de contact symétrique de métaux nobles. Par ailleurs, l'empilement Ru/AlCu assure une résistivité totale proche de celle de l'AlCu tout en bénéficiant des propriétés avantageuses du Ru à l'interface de contact. Une étude fondamentale du contact électromécanique rugueux pour les applications MEMS RF a été conduite, et une méthodologie pour estimer la résistance électrique de contact en tenant compte de l'effet de la rugosité des surfaces a été développée. La fabrication d'un MEMS RF, sur une ligne de prototypage industriel 200 mm, encapsulé au niveau du wafer et intégrant le contact Au-Ru/AlCu a été réalisée avec succès. La solution proposée démontre un fort potentiel pour la fabrication d'une nouvelle génération de MEMS RF avec des performances accrues en comparaison des dispositifs actuellement sur le marché

Performance Comparison of Phase Change Materials and Metal-Insulator Transition Materials for Direct Current and Radio Frequency Switching Applications

Advanced understanding of the physics makes phase change materials (PCM) and metal-insulator transition (MIT) materials great candidates for direct current (DC) and radio frequency (RF) switching applications. In the literature, germanium telluride (GeTe), a PCM, and vanadium dioxide (VO2), an MIT material have been widely investigated for DC and RF switching applications due to their remarkable contrast in their OFF/ON state resistivity values. In this review, innovations in design, fabrication, and characterization associated with these PCM and MIT material-based RF switches, have been highlighted and critically reviewed from the early stage to the most recent works. We initially report on the growth of PCM and MIT materials and then discuss their DC characteristics. Afterwards, novel design approaches and notable fabrication processes; utilized to improve switching performance; are discussed and reviewed. Finally, a brief vis-á-vis comparison of resistivity, insertion loss, isolation loss, power consumption, RF power handling capability, switching speed, and reliability is provided to compare their performance to radio frequency microelectromechanical systems (RF MEMS) switches; which helps to demonstrate the current state-of-the-art, as well as insight into their potential in future applications

Investigation into Contact Resistance and Damage of Metal Contacts Used in RF-MEMS Switches

This research examines the physical and electrical processes involved in lifecycle failure of Microelectromechanical (MEMS) Radio-Frequency (RF) cantilever beam ohmic contact switches. Failures of these switches generally occur at the contact, but complete details of performance of microcontacts are difficult to measure and have not been previously reported. This study investigated the mechanics of microcontact behavior by designing and constructing a novel experimental setup. Three representative contact materials of varying microstructure (Au, Au5%Ru, Au4%V2O5) were tested and parameters of contact during cycling were measured. The Au4%V2O5, a dispersion strengthened material developed at Lehigh University, showed the most promise of the materials tested with the longest-life contact lasting more than 15.5 x 106 cycles. Evidence of time-dependent deformation and contact heating during cycling was noted in all materials tested. Material hardness was not proportional to contact lifetime or adhesive forces measured during testing. Surfaces of post-cycling contact surfaces were evaluated and failures were categorized by ductile or brittle separation characteristics. Separation characteristics were correlated by contact lifetime

Bulk Foil Pt-Rh Micro-relays for High Power RF and Other Applications.

This work explores the potential of bulk foil metal alloys on micromachined relays for high power DC and RF applications. Platinum-rhodium (Pt-Rh) is of particular interest because it is both chemically inert and mechanically robust. The contributions include the investigation of design and manufacturing options, addressing issues such as the geometry of electrostatically actuated cantilevers, the integration of heat sinks, the integration of encapsulation, batch mode fabrication, and other aspects. In one part of the investigation, DC micro-relay test structures using Pt-Rh contacts were benchmarked against the ones using stainless steel (SS316L) contacts. Devices with 6.5 mm2 footprint were directly assembled on the printed circuit boards (PCB). Devices also included microfabricated on-device heat sinks subjected to a heat management using forced cooling to dissipate contact heat. Fabricated micro-relays exhibited 1.5 Ω and 1.25 Ω on-state resistances for SS316L and Pt-Rh contacts, respectively. In hot switching high power tests, Pt-Rh and SS316L micro-relays operated up to 1.8 A and 2.6 A, respectively. In another part of the investigation, RF micro-relays with Pt-Rh contacts were designed and fabricated. Test structures with 6.4 mm2 footprint had 90 V pull-in voltage. The micro-relays had down-state insertion loss and up-state isolation better than-0.2 dB and -25 dB up to 5 GHz, respectively. Unpackaged micro-relays exhibited RF power handling up to 18.5 W hot switching in ambient air. The third part of this investigation was directed at batch mode manufacturing and packaging of micro-relays directly on PCB substrates. For this, 4x1 device arrays were designed, fabricated, and encapsulated. Subsurface metal layers on the PCB were used to transfer the signal into and out of the sealed encapsulation. The footprint of packaged test structures was 8.4 mm2. The contact resistance and the pull-in voltage for the fabricated devices were 78 V and 1.1 Ω for an actuation voltage of 115 V, respectively. The packaged devices operated in atmospheric pressure nitrogen and exhibited down-state insertion loss and up-state isolation better than -0.25 dB, and -15 dB, respectively for up to 5 GHz. Packaged devices operated up to 20 W hot switching RF power.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/89788/1/ozkeskin_1.pd

Experimental Validation of External Load Effects on Micro-Contact Performance and Reliability

This paper presents a follow-on study previously presented at the Holm Conference. In the previous work, it was theorized that micro-switch performance and reliability was directly related to the type of external load that was connected. In particular, unintended capacitive loads may discharge at unpredictable times during switch operation and severely degrade or destroy micro-contact surfaces while properly configured loads may actually enhance performance. The severity of this potential vulnerability can be mitigated by purposely including specific circuit elements in various load configurations. This current study is to experimentally investigate and analyze this phenomenon. Using microelectromechanical systems (MEMS) based devices, we have the ability to efficiently and inexpensively fabricate large numbers of identical micro-contact pairs and then connect them to external loads of interest. Using this approach, it was demonstrated that both performance and reliability can be drastically affected by loading. In all cases tested, series inductance and parallel capacitance resulted in premature failure of the micro-contacts tested. Various protective configurations were also tested and all such devices lasted to the targeted 10M cycles of operation with little sign of imminent failure

Improvements to Micro-Contact Performance and Reliability

Microelectromechanical Systems (MEMS) based devices, and specifically microswitches, continue to offer many advantages over competing technologies. To realize the benefits of micro-switches, improvements must be made to address performance and reliability shortfalls which have long been an issue with this application. To improve the performance of these devices, the micro-contacts used in this technology must be understood to allow for design improvements, and offer a means for testing to validate this technology and determine when such improvements are ready for operational environments. To build devices which are more robust and capable of continued operation after billions of cycles requires that improved fabrication techniques be identified and perfected to allow for more sophisticated designs to be tested