Piezoelectric Actuators and Motors: Materials, Designs, and Applications

Microelectromechanical systems (MEMS) represents a technology that integrates miniaturized mechanical and electromechanical components (i.e., sensors and actuators) that are made using microfabrication techniques. MEMS devices have become an essential component in a wide range of applications, ranging from medical and military to consumer electronics. As MEMS technology is implemented in a growing range of areas, the reliability of MEMS devices is a concern. Understanding the failure mechanisms is a prerequisite for quantifying and improving the reliability of MEMS devices. This paper reviews the common failure mechanisms in MEMS, including mechanical fracture, fatigue, creep, stiction, wear, electrical short and open, contamination, their effects on devices' performance, inspection techniques, and approaches to mitigate those failures through structure optimization and material selection.

MEMS Reliability Review

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

This work reports on piezoelectric aluminum nitride (AlN) based dual-beam RF MEMS switches that have been monolithically integrated with AlN contour-mode resonators. The dual-beam switch design presented in this paper intrinsically compensates for the residual stress in the deposited films, requires a low actuation voltage (5 to 20 V) and facilitates active pull-off to open the switch and exhibits fast switching times (1 to 2 µs). This work also presents the combined response (cascaded S parameters) of a resonator and a switch that were co-fabricated on the same substrate. The response shows that the resonator can be effectively turned on and off by the switch. A post-CMOS compatible process was used for the co-fabrication of both the switches and the resonators. The single-chip RF solution presented constitutes an unprecedented step forward towards the realization of compact, low-loss and integrated multi-frequency RF front-ends.

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Dual-Beam Actuation of Piezoelectric AlN RF MEMS Switches Monolithically Integrated with AlN Contour-Mode Resonators

Surface science, MEMS and NEMS: Progress and opportunities for surface science research performed on, or by, microdevices

This paper presents test and characterization of various thin film contact materials for reliable high power RF- MEMS switches. We selected Au, Pt, Ir, and AuPt alloys for contact materials and fundamentally studied on contact phenomena and reliability of similar or dissimilar contacts using a contact measurement apparatus at high current condition. We also investigated the electrical contact behavior of the MEMS switches. From these studies, Au-to-Pt, Pt-to-Pt and Au-to-Ir contact showed reliable characteristics for the high power RF-MEMS switches.

Contact materials and reliability for high power RF-MEMS switches

In this paper, a fully wafer-level packaged RF MEMS switch has been demonstrated, which has low operation voltage, using a piezoelectric actuator. The piezoelectric actuator was designed to operate at low actuation voltage for application to advanced mobile handsets. The dc contact type RF switch was packaged using the wafer-level bonding process. The CPW transmission lines and piezoelectric actuators have been fabricated on separate wafers and assembled together by the wafer-level eutectic bonding process. A gold and tin composite was used for eutectic bonding at a low temperature of 300 °C. Via holes interconnecting the electrical contact pads through the wafer were filled completely with electroplated copper. The fully wafer-level packaged RF MEMS switch showed an insertion loss of 0.63 dB and an isolation of 26.4 dB at 5 GHz. The actuation voltage of the switch was 5 V. The resonant frequency of the piezoelectric actuator was 38.4 kHz and the spring constant of the actuator was calculated to be 9.6 N m−1. The size of the packaged SPST (single-pole single-through) switch was 1.2 mm × 1.2 mm including the packaging sealing rim. The effect of the proposed package structure on the RF performance was characterized with a device having CPW through lines and vertical feed lines excluding the RF switches. The measured packaging loss was 0.2 dB and the return loss was 33.6 dB at 5 GHz.

https://iopscience.iop.org/article/10.1088/0960-1317/16/11/005/pdf

A fully wafer-level packaged RF MEMS switch with low actuation voltage using a piezoelectric actuator

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

This paper presents the design, fabrication, and characterization of shunt type ohmic RF MEMS switches actuated by piezoelectricity. Since the proposed RF MEMS switches are actuated by piezoelectricity, they can be operated at an extremely low operation voltage of 5 V with a negligible power consumption of less than 0.1 μW. The proposed switches are primarily composed of piezoelectric cantilever actuators, a contact electrode and coplanar wave transmission lines suspended over the substrate. Unlike existing series type switches, the RF signal line of the shunt type ohmic switches is directly connected from the RF input port to the RF output port without any discontinuity, and their contact electrode is interconnected to all grounds. In this work, we consider three different types of RF MEMS switches in order to find the best geometry. The RF MEMS switch with one piezoelectric cantilever (Type-A) shows very poor isolation characteristic although it has better insertion loss than the others. The other two RF switches, which employ four piezoelectric cantilevers (Type-B and Type-C), differ in terms of the configuration of the cantilevers and the centered contact electrode. One has a centered contact electrode of square shape (Type-B), while the contact electrode of the other switch has a diamond shape (Type-C). They show relatively good isolation characteristics even at a very high frequency of 20 GHz, since the use of four piezoelectric cantilevers can generate a sufficiently large contacting force between the contact electrode and the RF signal line. The two fabricated RF MEMS switches have isolations of −23.0 and −24.0 dB, respectively, at an operation voltage of 5 V and a frequency of 20 GHz. The power-handling capability of the proposed RF MEMS switches increases to 1.4 W when an Iridium thin film coating is applied on the gold contact electrode.

Development of shunt type ohmic RF MEMS switches actuated by piezoelectric cantilever

The evaluation test of various thin film contact materials for reliable radio frequency (RF) microelectromechanical systems (MEMS) switches is presented. Given that two of the most important performance criteria of RF MEMS switches are a low contact resistance and high reliability, the purpose of this study is to search for feasible contact materials and combinations by measuring the contact resistance and hot switching reliability under high current condition. We selected gold, platinum, and iridium for the contact materials and compared the contact resistance and failure point for various similar or dissimilar contacts; Au/Au, Pt/Pt, Ir/Ir, Au/Pt, and Au/Ir using a contact measurement apparatus. Also, we investigated the insertion loss and power handling capability of Au/Au and Au/Ir in the RF MEMS switches. From these studies, it was found that dissimilar contacts such as Au/Pt or Au/Ir are an effective means of enhancing the reliability for high power RF MEMS switches. This evaluation method represents an important step toward the development of reliable metal contacts.

https://iopscience.iop.org/article/10.1143/JJAP.47.6558/pdf

Yong-Hee Park

Papers

Contact materials and reliability for high power RF-MEMS switches

A fully wafer-level packaged RF MEMS switch with low actuation voltage using a piezoelectric actuator

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

Development of shunt type ohmic RF MEMS switches actuated by piezoelectric cantilever

Investigation of Similar and Dissimilar Metal Contacts for Reliable Radio Frequency Micorelectromechanical Switches