Recent interest in high-speed scanning probe microscopy for high-throughput applications including video-rate atomic force microscopy and probe-based nanofabrication has sparked attention on the development of high-bandwidth flexure-guided nanopositioning systems (nanopositioners). Such nanopositioners are designed to move samples with sub-nanometer resolution with positioning bandwidth in the kilohertz range. State-of-the-art designs incorporate uniquely designed flexure mechanisms driven by compact and stiff piezoelectric actuators. This paper surveys key advances in mechanical design and control of dynamic effects and nonlinearities, in the context of high-speed nanopositioning. Future challenges and research topics are also discussed.

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Invited review article: high-speed flexure-guided nanopositioning: mechanical design and control issues.

An in-plane memory device based on multidomain BiFeO3 thin films is reported. Highly conductive domain walls appear only during the application of a read-out field, a non-destructive process that reduces accumulation of mobile defects on the walls.

/pdf/temporary-formation-of-highly-conducting-domain-walls-for-555q392l5d.pdf

Temporary formation of highly conducting domain walls for non-destructive read-out of ferroelectric domain-wall resistance switching memories.

This paper presents an mN-level contact and restoring force RF microelectromechanical systems metal-contact switch exhibiting high reliability, high linearity, and high power handing for dc-40-GHz applications. The device, which is insensitive to stress and temperature effects, achieves 1.2-1.5 mN of contact force (per contact) from 80 to 90 V and 1.0 mN of restoring force (per contact). The up-state capacitance is 8 fF, resulting in an isolation of 46, 31, and 14 dB at 1, 6, and 40 GHz, respectively. Measured results show switch resistances of 1-2 Ω and a reliability of >; 100 million cycles at 2-5 W under cold switching at 100 mW under hot-switching conditions, in an unpackaged and standard laboratory environment. Furthermore, the device was tested under prolonged hold-down conditions and demonstrated excellent RF power handling (>;10 W) and dc current handling (>;1 A) capability.

A High-Reliability High-Linearity High-Power RF MEMS Metal-Contact Switch for DC–40-GHz Applications

We present lifetime limitations and failure analysis of many packaged RF MEMS ohmic contacting switches with Au–Au, Au–Ir, and Au–Pt contact materials operating with 100 μN of contact force per contact in hermetically sealed glass wall packages. All metals were tested using the same switch design in a controlled environment to provide a comparison between the performance of the different materials and their corresponding failure mechanisms. The switch lifetimes of the different contact materials varied from several hundred cycles to 200 million cycles with different mechanisms causing failures for different contact materials. Switches with Au–Au contacts failed due to adhesion when thoroughly cleaned while switches with dissimilar metal contacts (Au–Ir and Au–Pt) operated without adhesion failures but failed due to carbon accumulation on the contacts even in a clean, packaged environment as a result of the catalytic behavior of the contact materials. Switch lifetimes correlated inversely with catalytic behavior of the contact metals. The data suggests the path to increase switch lifetime is to use favorable catalytic materials as contacts, design switches with higher contact forces to break through any residual contamination, and use cleaner, probably smaller, packages. (Some figures may appear in colour only in the online journal)

https://iopscience.iop.org/article/10.1088/0960-1317/22/10/105005/pdf

Lifetime limitations of ohmic, contacting RF MEMS switches with Au, Pt and Ir contact materials due to accumulation of ‘friction polymer’ on the contacts

This paper presents the electrical contact behaviors of gold-to-gold thin-film contacts under high current conditions and describes the major factors that influence these contact behaviors for radio frequency microelectromechanical system switches. The fundamental phenomena in the contact resistance versus contact force curve were investigated with a contact measurement setup, which we devised. Based on the experimental results, the contact resistance behavior with increasing contact force can be divided into three regions: first, an unstable contact regime before the minimum contact force is reached; second, a stable contact regime where the plastic deformation of the asperities explains the behavior and, finally, a saturated regime where the thickness effect of the film on a hard substrate becomes evident. The minimum contact force required for a stable contact depended on the roughness of the surface and the current flow through the contact. The smooth surface of the Au film and the metal softening caused by a high current flow of 100 mA had the effect of lowering the minimum contact force. After the minimum contact force is exceeded, the contact resistance initially follows the asperity plastic deformation behavior, where the contact resistance is related to the resistivity and hardness of the contact material. In the higher contact force regime, the decrease in the contact resistance becomes almost negligible. With increasing contact force, the effective contact area is limited by the effect of the film thickness on a hard substrate. From these results, we found the principal factors influencing the contact behaviors of the Au-to-Au contacts to be the resistivity, hardness and softening temperature of the contact materials and the design factors such as the surface roughness, the film thickness and the substrate.

https://iopscience.iop.org/article/10.1088/0960-1317/18/10/105010/pdf

Investigation of the electrical contact behaviors in Au-to-Au thin-film contacts for RF MEMS switches

It is well-recognized that MEMS switches, compared to their more traditional solid state counterparts, have several
important advantages for wireless communications. These include superior linearity, low insertion loss and high
isolation. Indeed, many potential applications have been investigated such as Tx/Rx antenna switching, frequency band
selection, tunable matching networks for PA and antenna, tunable filters, and antenna reconfiguration. However, none of
these applications have been materialized in high volume products to a large extent because of reliability concerns,
particularly those related to the metal contacts. The subject of the metal contact in a switch was studied extensively in
the history of developing miniaturized switches, such as the reed switches for telecommunication applications. While
such studies are highly relevant, they do not address the issues encountered in the sub 100μN, low contact force regime
in which most MEMS switches operate. At such low forces, the contact resistance is extremely sensitive to even a trace
amount of contamination on the contact surfaces. Significant work was done to develop wafer cleaning processes and
storage techniques for maintaining the cleanliness. To preserve contact cleanliness over the switch service lifetime,
several hermetic packaging technologies were developed and their effectiveness in protecting the contacts from
contamination was examined. The contact reliability is also very much influenced by the contact metal selection. When
pure Au, a relatively soft metal, was used as the contact material, significant stiction problems occurred when clean
switches were cycled in an N 2 environment. In addition, various mechanical damages occurred after extended switching
cycling tests. Harder metals, while more resistant to deformation and stiction, are more sensitive to chemical reactions,
particularly oxidation. They also lead to higher contact resistance because of their lower electrical conductivity and
smaller real contact areas at a given contact force. Contact reliability issues could also be tackled by improving
mechanical designs. A novel collapsing switch capable of generating large contact forces (>300μN) was shown to be
less vulnerable to contamination and stiction.

/pdf/metal-contact-reliability-of-rf-mems-switches-4frsf7s2yv.pdf

Metal contact reliability of RF MEMS switches

Ultra-high density MEMS probe memory device

This paper describes a unique method of encapsulating MEMS switches at the wafer level using a thin-film “microshell” lid and a novel micro-embossing, or “stamping” technique to seal the lid. After fabrication of the MEMS switch and subsequent formation of the microshell, the switches are released through gold tunnels that allow the penetration of a chemical etchant. In a controlled ambient, a “stamp” wafer is aligned to the device wafer, and the wafers are thermally compressed together. This process applies pressure across each tunnel to fuse the gold, thereby sealing the microshell packages. By sealing and passivating the switches at the wafer level, the wafers can be exposed to backend processing, packaging, and assembly steps such as dicing without damaging the sensitive MEMS devices. Furthermore, the size, cost, and complexity of the packaged system are significantly reduced compared to standard wafer bonding processes.Copyright © 2007 by ASME

Tsung-Kuan A. Chou

Papers

Metal contact reliability of RF MEMS switches

Ultra-high density MEMS probe memory device

A Stamp-Sealed Microshell Package for RF MEMS Switches