This review article presents recent advancements in the design and fabrication of thin-film (<3 μm) lead zirconate titanate (PZT) microelectromechanical system (MEMS) devices. The article covers techniques for optimizing highly (001)/(100) oriented chemical solution deposited PZT films to achieve improved piezoelectric coefficients. These PZT films combined with surface and bulk micromachining techniques are fabricated into actuators and transducers for radio frequency (RF) switches, nanomechanical logic, resonators, and power transformers for use in communication systems and phased-array radar. In addition, the large relative displacements generated by PZT thin films have been used to demonstrate mechanical mobility in MEMS devices, including insect-inspired flight actuators and ultrasonic traveling wave motors. In conjunction with actuation, PZT films are being developed for feedback sensors for the integrated control of insect-inspired robots.

PZT-Based Piezoelectric MEMS Technology

Latching RF MEMS Switches of Single-Pole-Single-Throw (SPST), Single-Pole-Double-Throw (SPDT) and Single-Pole-Triple-Throw (SP3T) types are proposed. The switches are built of a 20 $\mu{\rm m}$ thick nickel layer covered with a plated 2 $\mu{\rm m}$ gold on the top and side walls of nickel layer eliminating any potential warping due to thermal mismatch. The switches are actuated using a high stroke latching thermal actuator that exhibits a 32 $\mu{\rm m}$ displacement with a dc power of 250 mW under ambient pressure at 25 $^{\circ} {\rm C}$ (298 $^{\circ} {\rm K}$ ) and 153 mW under vacuum at $-193^{\circ} {\rm C}$ (80 $^{\circ} {\rm K}$ ). The measured velocity for the actuator is 2.3 $\mu{\rm m}/{\rm msec}$ . The RF measurement of the SPST switch is done under ambient and vacuum and over a wide range of temperatures from 25 $^{\circ} {\rm C}$ (298 $^{\circ} {\rm K}$ ) to $-193^{\circ} {\rm C}$ (80 $^{\circ} {\rm K}$ ). The switches survived a temperature cyclic test from 25 $^{\circ} {\rm C}$ (298 $^{\circ} {\rm K}$ ) to 85 $^{\circ} {\rm C}$ (358 $^{\circ} {\rm K}$ ) with a total variation of 0.2 dB in insertion loss. The switch demonstrates a worst IP3 of 60.14 dBm and high power handling capabilities that exceeds 30 Watt, only tested for an hour in 1 cycle. The RF performance of the SPST, SPDT and SP3T exhibits a worst insertion loss of 0.8 dB, 1.2 dB and 1.5 dB up to 40 GHz, 40 GHz and 18 GHz, respectively.

High Power Latching RF MEMS Switches

Vanadium dioxide (VO2) is a unique phase change material (PCM) that possesses a metal-to-insulator transition property. Pristine VO2 has a negative temperature coefficient of resistance, and it undergoes an insulator-to-metal phase change at a transition temperature of 68 °C. Such a property makes the VO2 thin-film-based variable resistor (varistor) a good candidate in reconfigurable electronics to be integrated with different RF devices such as inductors, varactors, and antennas. Series single-pole single-throw (SPST) switches with integrated VO2 thin films were designed, fabricated, and tested. The overall size of the device is $380~\mu \text{m}\times 600~\mu \text{m}$ . The SPST switches were fabricated on a sapphire substrate with integrated heating coil to control VO2 phase change. During the test, when VO2 thin film changed from insulator at room temperature to metallic state (low-resistive phase) at 80 °C, the insertion loss of the SPST switch was <3 dB at 10 GHz. In addition, the isolation of the SPST improved to better than 30 dB when the temperature dropped to 20 °C. These tunable characteristics of the RF switch provide evidence for VO2 as a useful PCM for the broad range of applications in reconfigurable electronics.

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Vanadium Oxide Thin-Film Variable Resistor-Based RF Switches

A new lateral electro-thermally actuated latching RF MEMS switch is presented in this paper. In contrast to conventional electrostatic or electro-thermal MEMS switches which require an actuation voltage to hold the switch in on or off state, this switch has a true mechanical latching design in such a way that there is no power required to hold the switch on or off. The switch structure only consumes power while transition between states. In order to satisfy low voltage operation, electro-thermal actuators are chose as the drive and latching actuators of the switch. The required actuation voltage for drive and latching actuators is 6 V. The switch consumes a power of <99 mW while transition from off to on state and also consumes a power of 46 mW for 0.3 ms in transition from off to on states. FEM simulations show that the return loss of the switch is below ?10 dB up to 140 GHz and is below ?20 dB up to 40 GHz. The insertion loss of the switch is less than ?1 dB up to 150 GHz. The switch isolation when it is off is below ?20 dB up to 160 GHz. The switch has potential applications in low voltage, low power and high performance RF tuning and switching applications.

Design and simulation of a novel electro-thermally actuated lateral RF MEMS latching switch for low power applications

This article presents a monolithically integrated reflective-type phase shifter (RTPS) utilizing silicon-on-insulator (SOI) radio frequency (RF) microelectromechanical systems (MEMS). The analog phase shifter employs a hybrid coupler and two identical reflective loads optimized to achieve a large phase shift range. The hybrid coupler is designed using two CPW-based couplers connected in a folded tandem configuration to achieve a compact size design. Various reflective load topologies are studied for optimum phase shift range and phase linearity over the bandwidth of interest. Measurement results demonstrate a continuous 120° tunable range from 26 to 30 GHz. The mmWave phase shifter exhibits a low insertion loss of 5.35 dB ± 0.6 dB at 28 GHz. The fabricated phase shifter has an overall device footprint of 4.0 $\text {mm}\times 2.6$ mm. All the components of the phase shifter module are co-fabricated in the 20 $\mu \text{m}$ device layer of a SOI wafer, which provides the flexibility of monolithic integration with other RF modules in phased array antenna systems. Contactless thermally actuated MEMS varactors are used in the reflective loads which do not suffer from the conventional contact-based reliability issues.

Thermally Actuated SOI RF MEMS-Based Fully Integrated Passive Reflective-Type Analog Phase Shifter for mmWave Applications

Here, a new thermally actuated latching wideband RF microelectromechanical systems (MEMS) switch is presented. The switch employs two thermal actuators connected to two thin metal arms which serve as signal lines of coplanar waveguide switch. The actuators pull the thin arms sequentially, and latch the switch. The switch can be actuated on and off by using either short voltage or current pulses. Using a dielectric bridge (nitride) as an interface between the actuators and the thin arms, the RF circuitry is separated from DC actuators, allowing wide-band operation. The switch demonstrates an excellent wideband RF performance with an insertion loss of better than 0.3 dB up to 20 GHz and better than 0.8 dB up to 40 GHz. The return loss and isolation of the switch is better than 20 dB for the entire frequency band. The switch also has a very satisfactory repeatability with better than 0.1-dB variation in insertion loss and less than 1-dB variation in return loss and isolation at 30-dB level up to 6000 times switching cycles. The switch has been also successfully tested for RF power handling capability up to 40 dBm. The proposed switch has very simple RF structure which makes it an ideal candidate to be integrated in the form of more complex circuitry. An application of the proposed switch for a band selection network which is used in multiband transceivers has been presented here.

M. Lisi

Papers

Thermally Actuated Latching RF MEMS Switch and Its Characteristics