Microelectromechanical Capacitive Switches Research Articles

Material selection methodology for radio frequency (RF) microelectromechanical (MEMS) capacitive shunt switch

This paper describes the process of selecting the most optimum Radio Frequency Micro- electro- mechanical-systems (RF-MEMS) switch design using Ashby’s methodology. The switches are compared on the basis of parameters like actuation voltage, insertion loss, isolation and switching time using material selection charts. The chart shows that a low-voltage metal-to-metal contact shunt capacitive RF-MEMS having a bridge structure with Si-GaAs substrate, electroplated gold contacts and silicon nitride dielectric layer, is the most optimum of all the switches considered.

MEMS switching of contour-mode aluminum nitride resonators for switchable and reconfigurable radio frequency filters

Switching of transducer coupling in aluminum nitride contour-mode resonators provides an enabling technology for future tunable and reconfigurable filters for multi-function RF systems. By using microelectromechanical capacitive switches to realize the transducer electrode fingers, coupling between the metal electrode finger and the piezoelectric material is modulated to change the response of the device. On/off switched width extensional resonators with an area of <0.2 mm2 demonstrate a Q of 2000, K2 of 0.72, and >24 dB switching ratio at a resonator center frequency of 635 MHz. Other device examples include a 63 MHz resonator with switchable impedance and a 470 MHz resonator with 127 kHz of fine center frequency tuning accomplished by mass loading of the resonator with the MEMS switches.

Dielectric Charging Characterization of Triboelectric Effects in MEMS Switches

Dielectric charging is one of the key topics concerning microelectromechanical capacitive switch reliability; however, very little is known about how charge is generated on an exposed dielectric surface when brought in contact with another dielectric surface under high electric field. In this paper, we analyze and propose a model of surface charge generation of a microelectromechanical device produced by triboelectric effects under high electric field. A simplified theory has been developed that can be used either for electron or ion transfer, where a closed-form analytical solution is achieved under certain conditions. Measurements were performed during continuous hold-down stresses under different voltages and temperatures. The results based on pull-in voltage shift were fit using a MATLAB program where surface dielectric properties were extracted. The results show a qualitative agreement with the developed theory, and the extracted parameters can help to understand and predict the product reliability, as well as define a fabrication process screening procedure. [2015-0268]

Vertical, capacitive microelectromechanical switches produced via direct writing of copper wires.

Three-dimensional (3D) direct writing based on the meniscus-confined electrodeposition of copper metal wires was used in this study to develop vertical capacitive microelectromechanical switches. Vertical microelectromechanical switches reduce the form factor and increase the area density of such devices in integrated circuits. We studied the electromechanical characteristics of such vertical switches by exploring the dependence of switching voltage on various device structures, particularly with regard to the length, wire diameter, and the distance between the two wires. A simple model was found to match the experimental measurements made in this study. We found that the electrodeposited copper microwires exhibit a good elastic modulus close to that of bulk copper. By optimizing the 3D structure of the electrodes, a volatile electromechanical switch with a sub-5 V switching voltage was demonstrated in a vertical microscale switch with a gap distance as small as 100 nm created with a pair of copper wires with diameters of ~1 μm and heights of 25 μm. This study establishes an innovative approach to construct microelectromechanical systems with arbitrary 3D microwire structures for various applications, including the demonstrated volatile and nonvolatile microswitches.

Electrothermal actuation of vanadium dioxide for tunable capacitors and microwave filters with integrated microheaters

We report the fabrication, modeling and characterization of novel microwave tunable capacitors based on the metal-insulator transition (MIT) of Vanadium Dioxide (VO2) as a future replacement solution for microelectromechanical capacitive switches. We present the advantages of VO2-based capacitors over alternative technologies for microwave reconfigurable electronics in terms of ease of integration, design and performance at high frequency. We show the potential of the proposed devices for RF reconfigurable electronics by fabricating a tunable bandstop filter (22.5–19.8GHz) with insertion loss <2dB up to 40GHz. We propose an alternative method for actuation of the VO2 active regions by employing integrated microheaters and we study their effect on the filter performance by electrothermal and electromagnetic simulations.

A simple electrical test method to isolate viscoelasticity and creep in capacitive microelectromechanical switches

A bipolar hold-down voltage was used to study mechanical degradation in radio-frequency microelectromechanical capacitive shunt switches. The bipolar signal was used to prevent the occurrence of dielectric charging and to isolate mechanical effects. The characteristics of material stress relaxation and recovery were monitored by recording the change of the pull-in voltage of a device. The creep effect in movable components was saturated by repeated actuation to the pulled-in position, while comparison with a theoretical model confirmed the presence of linear viscoelasticity in the devices.

Experimental isolation of degradation mechanisms in capacitive microelectromechanical switches

DC and bipolar voltage stresses are used to isolate mechanical degradation of the movable electrode from charging mechanism in microelectromechanical capacitive switches. Switches with different metals as the movable electrode were investigated. In titanium switches, a shift in the pull-in voltages is observed after dc stressing whereas no shift occurs after the bipolar stressing, which is to be expected from charging theory. On switches with similar dielectric but made of aluminium, the narrowing effect occurs regardless if dc or bipolar stressing is used, which indicates the mechanical degradation as the mechanism responsible.

Effect of the substrate on RF power-handling capability of micro-electromechanical capacitive switches

RF power-handling capacity of micro-electromechanical capacitive switches was modeled by multiphysics finite-element analysis. Coupled electromechanical, electrothermal and thermomechanical models were constructed by using commercially available software. Simulations by using the models were validated by small- and large-signal RF measurements. The measurements showed that the power-handling capacity of the switches depended on not only the properties of the membrane (the movable part of the switches), but also the substrate properties and the circuit environment. These effects were captured in a compacted model, which could be used to guide the improvement of the power-handling capacity of the switches.

Strategies for dynamic soft-landing in capacitive microelectromechanical switches

Electromechanical dielectric degradation associated with the hard landing of movable electrode is a technology-inhibiting reliability concern for capacitive RF-MEMS switches. In this letter, we propose two schemes for dynamic soft-landing that obviate the need for external feedback circuitry. Instead, the proposed resistive and capacitive braking schemes can reduce impact velocity significantly without compromising other performance characteristics like pull-in voltage and pull-in time. Resistive braking is achieved by inserting a resistance in series with the voltage source whereas capacitive braking requires patterning of the electrode or the dielectric. Our results have important implications to the design and optimization of reliability aware electrostatically actuated MEMS switches.

Dielectric charging in capacitive microelectromechanical system switches with silicon nitride

The paper attempts to elaborate on the basic problem of dielectric charging in insulating films for microelectromechanical capacitive switches, the dependence of the film electrical properties on material stoichiometry and the uncertainty of whether the implementation of a leaky dielectric would reduce the charging effects. Silicon nitride films with stoichiometries (N/Si) ranging from 0.36 to 0.85 were assessed in metal-insulator-metal capacitors using the thermally stimulated depolarization current method and by obtaining the current-voltage characteristics. Capacitive switches were also fabricated with the same dielectric films. Results from both devices revealed an enhanced charging in the case of silicon-rich films.

Comment on “Comparison of air breakdown and substrate injection as mechanisms to induce dielectric charging in microelectromechanical switches” [Appl. Phys. Lett. 92, 043502 (2008)

The purpose of this comment is to provide additional insight into the reliability of microelectromechanical capacitive switches (MEMSs) investigated by Molinero and Castañer [Appl. Phys. Lett. 92, 043502 (2008)]. We show that the presence or absence of ambient humidity determines whether the shift in the capacitance-voltage (C-V) curve of oxide-based MEMS occurs as a result of voltage stress. In humid air, negative and positive shifts in the C-V curve are observed after negative and positive bias stress. In dry air no such shifts in the C-V curve are seen. These shifts are similar to those reported on oxide-based switches by Molinero and Castañer [Appl. Phys. Lett. 92, 043502 (2008)] where they show shifts occurring in room ambient pressure, but not in vacuum. This indicates that not only air pressure but also air humidity can be responsible for shifts in MEMS.

On the Trade-Off Between Quality Factor and Tuning Ratio in Tunable High-Frequency Capacitors

A benchmark of tunable and switchable devices at microwave frequencies is presented on the basis of physical limitations to show their potential for reconfigurable cellular applications. Performance limitations are outlined for each given technology focusing on the quality factor ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> ) and tuning ratio (eta) as figures of merit. The state of the art in terms of these figures of merit of several tunable and switchable technologies is visualized and discussed. If the performance of these criteria is not met, the application will not be feasible. The quality factor can typically be traded off for tuning ratio. The benchmark of tunable capacitor technologies shows that transistor-switched capacitors, varactor diodes, and ferroelectric varactors perform well at 2 GHz for tuning ratios below 3, with an advantage for GaAs varactor diodes. Planar microelectromechanical capacitive switches have the potential to outperform all other technologies at tuning ratios higher than 8. Capacitors based on tunable dielectrics have the highest miniaturization potential, whereas semiconductor devices benefit from the existing manufacturing infrastructure.

CMOS Compatible RF MEMS Switches with Serpentine Springs

This paper describes a serpentine flexure spring design and fabrication process development for radio frequency microelectromechanical (RF MEMS) capacitive switches with coplanar waveguide (CPW) lines. Sputtered tungsten is employed as the CPW line conductor instead of Au, a non-Si compatible material. The bridge membrane is fabricated from Al. The materials and fabrication process can be integrated with CMOS and SOI technology to reduce cost. Results show the MEMS switch has excellent performance with insertion loss 0.3dB, return loss -27dB at 30GHz and high isolation -30dB at 40GHz. The process developed promises to simplify the design and fabrication of RF MEMS on silicon.

A study of capacitance-voltage curve narrowing effect in capacitive microelectromechanical switches

In this letter, we report on the capacitance-voltage (C-V) curve narrowing effect, which occurs in the oxide-based microelectromechanical switches that are subjected to dc bias stress for a prolonged period of time. The narrowing effect for the noncontact dc bias stress condition is shown, which proves that membrane-to-dielectric contact is not needed for narrowing to occur. It is also shown that neither mechanical degradation nor charge trapping due to dielectric conduction or air ionization is solely responsible for the C-V instabilities reported in the literature.

Using metal-insulator-semiconductor structures to investigate dielectrics charging in capacitive microelectromechanical switches

Capacitance-voltage characteristics measured on metal-insulator-semiconductor structures are used to investigate dielectrics charging in the capacitive microelectromechanical switches. An analytical model based on this method has been established. Effects of electrical stress on charge accumulation in silicon-rich nitride were experimentally investigated. The charging parameters of the silicon-rich nitride were extracted by the exponential curve fitting method. The experimental results show that three main physical processes dominate the charging behavior of the silicon-rich nitride. It is expected that the accumulated charges can be eliminated by using an offset square-wave voltage to actuate the switches.

Efficient multiphysics modeling of microelectromechanical switches

Circuit simulations driven modeling approach with embedded multi-physics of capacitive radio frequency microelectromechanical systems switches is presented in this paper. Finite element method simulations in electromagnetic, electromagnetic-thermal and thermal-stress domains are sequentially performed to model and capture the behavioral phenomenology inherent to the selected device. Computed effects are then utilized to modify the original electromagnetic model, giving rise to the updated solutions with included electromagnetic-thermal-stress- electromagnetic physics. To enable circuit level design with the multi- physics models of capacitive microelectromechanical switches, an artificial neural network technique is utilized. Compared to the finite element method simulations, the developed neural network models are about 1500 times faster while maintaining smaller than 0.5% relative difference. Finally, these models are integrated in a circuit simulator, where the sensitivity study and optimization are performed to further demonstrate the advantages of the proposed modeling approach.

Dielectric charge measurements in capacitive microelectromechanical switches

Measurements of transient currents from the discharge of microelectromechanical capacitive switches after charging the dielectric with an ion gun are reported. It is shown that both the charge sign and the charge amount can be independently measured and that the values can be related to the pull-in shift as measured from C-V characteristics. Several capacitive switches of different sizes were fabricated and subjected to charging and discharging experiments. The analysis of the transients shows that several time constants are governing the charge dynamics, one of them is related to the resistivity-permittivity product of the dielectric.

Noncontact measurement of charge induced voltage shift in capacitive MEM-switches

The use of a modulated microwave signal to directly measure the voltage shift induced by charge in the dielectric layer of a capacitive microelectromechanical (MEM) switch is presented. This method does not require the metal bridge to contact the dielectric layer and is thus much less intrusive than previously reported measurements. The technique is a useful tool for understanding charge build up and dissipation in capacitive MEM switches.

Measurements of charging in capacitive microelectromechanical switches

Charge build up in capacitive microelectromechanical switches is measured during continuous cycling. Under unipolar cycling, charge rapidly builds up in the dielectric causing the switch control voltages to shift. However, under bipolar cycling, there is no net accumulated charge, but instead a decrease in the magnitude of the control voltages.

A 3-D integral equation-based approach to the analysis of real-life MMICs application to microelectromechanical systems

In this paper, we introduce a three-dimensional method-of-moments approach, suitable for the analysis of real-life monolithic circuits for microwave/millimeter waves. It shares the flexibility and the efficiency of the currently available spectral-domain commercial simulators, while allowing all metallizations to have finite thickness and finite conductivity and the ability to handle dielectric discontinuities. The method is successfully applied to several structures, like metal-insulator-metal capacitor, spiral inductors, and microelectromechanical capacitive switches in the 1-50-GHz frequency range.