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.

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A review of micro-contact physics for microelectromechanical systems (MEMS) metal contact switches

A soft-landing actuation waveform was designed to reduce the bounce of a single-pole single-throw (SPST) ohmic radio frequency (RF) microelectromechanical systems (MEMS) switch during actuation. The waveform consisted of an actuation voltage pulse, a coast time, and a hold voltage. The actuation voltage pulse had a short duration relative to the transition time of the switch and imparted the kinetic energy necessary to close the switch. After the actuation pulse was stopped, damping and restoring forces slowed the switch to near-zero velocity as it approached the closed position. This is referred to as the coast time. The hold voltage was applied upon reaching closure to keep the switch from opening. An ideal waveform would close the switch with near zero impact velocity. The switch dynamics resulting from an ideal waveform were modeled using finite element methods and measured using laser Doppler vibrometry. The ideal waveform closed the switch with an impact velocity of less than 3 cm/s without rebound. Variations in the soft-landing waveform closed the switch with impact velocities of 12.5 cm/s with rebound amplitudes ranging from 75 to 150 nm compared to impact velocities of 22.5 cm/s and rebound amplitudes of 150 to 200 nm for a step waveform. The ideal waveform closed the switch faster than a simple step voltage actuation because there was no rebound and it reduced the impact force imparted on the contacting surfaces upon closure

A soft landing waveform for actuation of a single-pole single- throw ohmic RF MEMS Switch.

The need for more energy-efficient solid-state switches beyond complementary metal-oxide-semiconductor (CMOS) transistors has become a major concern as the power consumption of electronic integrated circuits (ICs) steadily increases with technology scaling. Nano-Electro-Mechanical (NEM) relays control current flow by nanometer-scale motion to make or break physical contact between electrodes, and offer advantages over transistors for low-power digital logic applications: virtually zero leakage current for negligible static power consumption; the ability to operate with very small voltage signals for low dynamic power consumption; and robustness against harsh environments such as extreme temperatures. Therefore, NEM logic switches (relays) have been investigated by several research groups during the past decade. Circuit simulations calibrated to experimental data indicate that scaled relay technology can overcome the energy-efficiency limit of CMOS technology. This paper reviews recent progress toward this goal, providing an overview of the different relay designs and experimental results achieved by various research groups, as well as of relay-based IC design principles. Remaining challenges for realizing the promise of nano-mechanical computing, and ongoing efforts to address these, are discussed.

https://www.mdpi.com/2072-666X/6/8/1046/pdf

Nanoelectromechanical Switches for Low-Power Digital Computing

A micro contact model for electrical contact resistance prediction between roughness surface and carbon fiber paper

Electrical contact resistance testing was performed by hot-switching a simulated gold-platinum metal microelectromechanical systems contact. The experimental objective was to determine the sensitivity of the contact resistance degradation to current level and environment. The contact resistance increased sharply after 100 hot-switched cycles in air. Hot-switching at a reduced current and in nitrogen atmosphere curtailed contact resistance degradation by several orders of magnitude. The mechanism responsible for the resistance degradation was found to be arc-induced decomposition of adsorbed surface contaminants.

Electrical contact resistance degradation of a hot-switched simulated metal MEMS contact.

This paper presents the micro-contact performance comparison between contact pairs of Au/Au and composite contact pairs Au/Au-CNT. The Au/Au-CNT micro-contact's planar lower contact interface is an Au-CNT composite film with encapsulated CNTs. Micro-contact performance is affected by factors such as applied micro-contact force, temperature, current density, etc. At the micro-contact interface, asperities provide localized points for current flow. Increased temperature at these localized points may soften the contact metal and lead to bridge transfer. Prior work revealed that an Au/CNT contact pair performed poorly compared to an Au/Au contact pair, with two orders of magnitude difference in contact resistance. To maintain micro- contact performance and to reduce thermal effects, a Au/Au-CNT micro-contact was designed and fabricated. This design allows the micro-contact interface to remain Au/Au with the embedded CNTs acting as a thermal conduction conduit below the lower Au contact interface. The upper micro-contact support structure is an Au micromachined fixed-fixed beam with hemisphere-shaped upper contact geometry. The micro-contacts were studied under repeated cycles using an external, calibrated load, resulted in repeatable resistance of approximately 1Ω for nearly 40 million cycles. This research revealed that including a CNT composite film in the lower contact extend the operating life and lower contact resistance as compared to similarly constructed micro- contacts.

Micro-Contact Performance Characterization of Carbon Nanotube (CNT)-Au Composite Micro-Contacts

Microelectromechanical systems (MEMS) switches offer much lower power consumption, much better isolation, and lower insertion loss compared to conventional field-effect transistors and PIN diodes however, the MEMS switch reliability is a major obstacle for large-volume commercial applications [1] To enhance reliability, circuit designers need simple and accurate behavioral models of embedded switches in CAD tools to enable system-level simulations [2] Where Macro-switch researchers assess electric contact performance based on the type of load that is being switched, in MEMS literature, micro-switch performance and reliability is characterized by testing the devices under “hot-switched” or “cold-switched” load conditions; simple models are developed from the “hot” and “cold” characterizations By applying macro-switch performance characterization techniques, ie examining reliability based on the type of load that is being switched, clear characterizations of “hot” switching and “cold” switching external resistive, capacitive, and inductive loads are produced External resistive loads were found to act as current limiters and should be suitable under certain criteria for reducing current density through the contact area and thus limiting device failure to mechanical failure modes Alternatively, external capacitive loads increased current density under “hot” switching conditions at the moment the micro-switch closes; which increases the risk for material transfer and device failure Under DC conditions, the inductive loads had little effect in either “hot” or “cold” switching environments

Characterizing External Resistive, Inductive and Capacitive Loads for Micro-Switches

This paper reports the contact resistance evolution results of thin film, sputtered gold with encapsulated Ag colloids, micro-contacts dynamically tested up to 3kHz. The upper contact support structure consists of a sputtered gold surface micromachined, fixed-fixed beam designed with sufficient restoring force to overcome adhesion. The hemisphere-upper and planar-lower contacts are mated with a calibrated, external load resulting in approximately 100μN's of contact force. Contact resistance is measured, in-situ, using a cross-bar configuration and the entire apparatus is isolated from external vibration and housed in an enclosure to minimize contamination due to ambient environment. Additionally, contact cycling and data collection are automated using a computer and LabVIEW. The results showed that Au-Au micro-contact had a final resistance of 14Ωs after 10 million cycles and 81Ωs after 8 million cycles with Au-Ag micro-contacts.

Contact Resistance Evolution of Au-Au Micro-Contacts with Encapsulated Ag Colloids

: 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's 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 &#956;m and two functioning fixed-fixed Ag micro-contacts were tested using the &#956;N 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.

/pdf/novel-test-fixture-for-characterizing-microcontacts-5ds4janh8r.pdf

Benjamin Toler

Papers

A review of micro-contact physics for microelectromechanical systems (MEMS) metal contact switches

Micro-Contact Performance Characterization of Carbon Nanotube (CNT)-Au Composite Micro-Contacts

Characterizing External Resistive, Inductive and Capacitive Loads for Micro-Switches

Contact Resistance Evolution of Au-Au Micro-Contacts with Encapsulated Ag Colloids

Novel Test Fixture for Characterizing Microcontacts: Performance and Reliability