Microelectromechanical Switch Research Articles

Analysis of fluid-structure interaction for predicting resonant frequencies and quality factors of a microcantilever on a squeeze-film

The dynamic behavior of a flexible microcantilever vibrating above a substrate such as the cantilever-based structure of a radio frequency microelectromechanical switch is strongly affected by the motion of gas between the microcantilever and the substrate. In this paper, parameter studies to investigate the effects of geometry, material properties of a microcantilever, and ambient pressure on the resonant frequencies and quality factors of the microcantilever are described. A Reynolds-equation-based continuum model is used to derive the gas force reflecting the gas rarefaction effect due to the size effect of MEMS packed at low ambient pressure. The frequency response function of an inertially-excited microcantilever is developed using the mode superposition method. One interaction parameter, including information on geometry, material properties of a microcantilever, and ambient pressure, is defined to investigate the fluidstructure interaction in a microcantilever vibrating on a squeeze-film. The quality factors of the first three bending modes increase with the decreasing interaction parameter. The changing trend of associated resonant frequencies depends on the order of the bending mode.

Impact of adsorbed organic monolayers on vacuum electron tunneling contributions to electrical resistance at an asperity contact

Electrical contact resistance measurements are reported for RF micro-electromechanical switches situated within an ultrahigh vacuum system equipped with in situ oxygen plasma cleaning capabilities. Measurements were performed on fused (permanently adhered) switches with Au/Au contacts and functioning switches with Au/RuO2 contacts in both the presence and absence of adsorbed monolayers of pentane and dodecane. For switches adhered in the closed position, adsorption occurs only in regions external to direct contact. For functioning switches, however, it can occur either within or exterior to the contact. The data are analyzed within the framework of two distinct geometries, to explore how the presence of adsorbed molecules in regions close to the contact may impact vacuum tunneling contributions to the experimentally measured resistance: (1) The resistance associated with direct contact in parallel with a vacuum tunneling path, which upon uptake of the monolayer is replaced by the molecular resistance and (2) a series connection of the direct contact resistance with the molecular layer after adsorption occurs, with the vacuum tunneling path assumed to be negligible. In all cases, the experimental results quantitatively favor scenario (1), whereby uptake of the molecular layer effectively shuts down the vacuum tunneling path, in this case approximately 30 Ω in the absence of an adsorbed film. The methods described herein thus constitute a new and original approach to documenting vacuum tunneling levels in regions of close proximity.

A Design of Thermo-Electrostatic Actuated Microelectromechanical Switch with Low Voltage and Fabrication Efforts

Requirement of voltage up-converters due to high pull-in voltage is one of the main problems by merely electrostatic actuated Microelectromechanical system-based switches. Thermally actuated switches are another alternatives but with very high power dissipation. In this paper a low voltage switch is demonstrated, which uses a combined thermo-electrostatic actuator. The switch can be integrated with standard CMOS circuits without any up-converters. Thermally power dissipation for the switch is lower than just thermal actuators. The switching time is about 70µs and the maximal temperature of thermal actuator is lower than 150oC which cannot cause any longtime damage. Isolation and Insertion Loss quantities have been calculated to -25dB and -0.65dB at 20GHz from HFSS results respectively.

Electrical Contact Resistance and Device Lifetime Measurements of Au-RuO2-Based RF MEMS Exposed to Hydrocarbons in Vacuum and Nitrogen Environments

Electrical Contact Resistance (ECR) measurements are reported for RF micro-electromechanical switches with Au-RuO2 contacts, situated within an ultrahigh vacuum system equipped with in situ oxygen plasma cleaning capabilities. Two studies are reported, each involving a comparison of the ECR in vacuum and nitrogen environments for measurements performed immediately after cleaning. The first study reports measurements of initial resistance (resistance measured upon first time closure) versus pressure as dodecane gas is admitted to the chamber. A significant increase is observed at pressures in vacuum as low as 10−5 torr, (P/Psat < 10−4) consistent with earlier reports involving repetitive cycling of macroscopic switches in partial pressures of hydrocarbons in nitrogen. Somewhat unexpectedly, however, the resistance only doubles, even for pressures sufficiently high as to result in full monolayer condensation. In a second study, switch lifetimes in vacuum (10−8–10−9 torr) and nitrogen gas environments are compared, for switches operated immediately afterward, or alternatively left open for a number of days before operation. Although it was expected that vacuum would reduce and/or prevent contamination of the electrical contact surfaces, no enhancement or extension of lifetime was observed: Continuous operation of a switch in a nitrogen environment immediately after plasma cleaning was in fact the only procedure observed to indefinitely prolong device lifetime. The results suggest that (1) Hydrocarbon reaction products, but not mobile physisorbed hydrocarbons themselves, are responsible for increasing ECR by orders of magnitude and (2) Repetitive cycling motion of a clean switch in nitrogen inhibits formation of physisorbed hydrocarbon contaminants on the contacts, while vacuum levels far superior to 10−9 torr are required to prevent contamination.

Determination of long time discharge current in microelectromechanical system capacitive switches

An improved method to study the long term discharge current through the dielectric film in microelectromechanical system capacitive switches is presented. The method allows the detection of currents in the sub-fempto-Ampere range by monitoring the decay of the bias for minimum capacitance in the pull-up state. The method has been applied for time interval in excess of 10 000 s. Finally, it is shown that in carefully designed devices the method allows the calculation of the current that arises from the charge fluctuation decay.

Floating electrode microelectromechanical system capacitive switches: A different actuation mechanism

The paper investigates the actuation mechanism in floating electrode microelectromechanical system capacitive switches. It is demonstrated that in the pull-in state, the device operation turns from voltage to current controlled actuation. The current arises from Poole-Frenkel mechanism in the dielectric film and Fowler-Nordheim in the bridge-floating electrode air gap. The pull-out voltage seems to arise from the abrupt decrease of Fowler-Nordheim electric field intensity. This mechanism seems to be responsible for the very small difference with respect to the pull-in voltage.

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.

High-speed optical beam alignment based on a motion control technique for three-dimensional microelectromechanical systems optical switch modules

For optical axis alignment when light deflection control of a MEMS mirror is done at high speed, we propose a method based on identifying the quadratic surface shape. The method uses information about the optical power monitored when the mirror is inclined to a small angle. It is shown that the search for maximum optical power is possible by using this method. We also designed a control system with a band-stop filter for resonance frequency to decrease residual vibration generated when the mirror is moved at high speed. We confirm that the searchable level of optical power can be improved.

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.

Uncertainty propagation in a multiscale model of nanocrystalline plasticity

We characterize how uncertainties propagate across spatial and temporal scales in a physics-based model of nanocrystalline plasticity of fcc metals. Our model combines molecular dynamics (MD) simulations to characterize atomic-level processes that govern dislocation-based-plastic deformation with a phase field approach to dislocation dynamics (PFDD) that describes how an ensemble of dislocations evolve and interact to determine the mechanical response of the material. We apply this approach to a nanocrystalline Ni specimen of interest in micro-electromechanical (MEMS) switches. Our approach enables us to quantify how internal stresses that result from the fabrication process affect the properties of dislocations (using MD) and how these properties, in turn, affect the yield stress of the metallic membrane (using the PFMM model). Our predictions show that, for a nanocrystalline sample with small grain size (4 nm), a variation in residual stress of 20 MPa (typical in today's microfabrication techniques) would result in a variation on the critical resolved shear yield stress of approximately 15 MPa, a very small fraction of the nominal value of approximately 9 GPa.

A hybrid model for the charging process of the amorphous SiO2 film in radio frequency microelectromechanical system capacitive switches**Project supported by the National Natural Science Foundation of China (Grant No. 60676043).

Charging is one of the most important reliability issues in radio frequency microelectro- mechanical systems (RF MEMS) capacitive switches since it makes the actuation voltage unstable. This paper proposes a hybrid model to describe the transient dielectric charging and discharging process in the defect-rich amorphous SiO2 RF MEMS capacitive switches and verifies experimentally. The hybrid model contains two parts according to two different charging mechanisms of the amorphous SiO2, which are the polarisation and charge injection. The models for polarisation and for charge injection are established, respectively. Analysis and experimental results show that polarisation is always effective, while the charge injection has a threshold electric field to the amorphous SiO2 film. Under different control voltage conditions, the hybrid model can accurately describe the experimental data.

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.

A pure single-walled carbon nanotube thin film based three-terminal microelectromechanical switch

The electrical and physical properties of pure single-walled carbon nanotube thin films deposited through a layer-by-layer-self-assembly process are discussed. The film thickness was proportional to the number of dipping cycles. The film resistivity was estimated as 2.19×10−3 Ω cm after thermal treatment processes were performed. The estimated specific contact resistance to gold electrodes was 6.33×10−9 Ω m2 from contact chain measurements. The fabricated three-terminal microelectromechanical switch using these films functioned as a beam for multiple switching cycles with a 4.5 V pull-in voltage. This switch is believed to be a promising device for low power digital logic applications.

Novel multicontact radio frequency microelectromechanical system switch in high-power–handling applications

We report a novel multicontact radio frequency (RF) microelectromechanical system (MEMS) switch with mechanical independent switch elements and microspring contacts. The consistent contact arrangement and the robust contact design can effectively increase the contact area, reduce the current density, and therefore improve the power-/current-handling capability. The working mechanism of the switch with microspring contact is investigated by CoventorWare® simulation tools. The switch, fabricated by the Cu-Ni dual-metallic-sacrificial-layer surface micromachining, is actuated at 55 V for characterization. The closing time is 11 μs, and the opening time is 13.5 μs. The isolation is -30.9 dB at 2 GHz and -11.5 dB at 20 GHz; the insertion loss is -0.12 dB at 2 GHz and -0.22 dB at 20 GHz. The contact metal is Pt-Au, and the measured switch resistance drops from 48 to 1.2 Ω when the actuation voltage increases from 40 to 65 V. The switch element handles a current of 300 mA at 0.1 Hz. The switch is an excellent candidate for microwave applications requiring high-power handling.

Thermal switch design by using complementary metal–oxide semiconductor MEMS fabrication process

The present study focuses on implementing a complementary metal-oxide semiconductor (CMOS) microelectromechanical system thermal switch by using the commercially available Taiwan Semiconductor Manufacturing Company (TSMC) 0.35 μm two-poly four-metal CMOS process. There are two novel designs: first, the soft contact structure and post-processing fabrication; second, a new design of thermal actuator. To create the soft contact structure, residual stress effect has been utilised to make different bending curvatures. According to the experiments, the layer metal-1 has the largest residual stress effect that can achieve the largest deflection in the z-axis. Because the residual stress of the layer metal-1 is negative, the structure will bend down after release, hence providing larger contact area, which has been set up to obtain the lowest contact miss ability. In the post-processing fabrication, 0.3 μm thickness gold will be patterned at the contact tips. Since gold, rather than aluminium, has no oxidation issue, it has more reliability in preventing the problem of oxidation than aluminium. In the new thermal actuator design, the authors designed a novel folded-flexure with the electrothermal excitation to turn the switch on or off. In the prototype, the device size is 500 × 400 μm and the gap between two contact pads is 9 μm in off-state. Depending on the simulation results, the switch can work stably at 3 V, and the working temperature and operating bandwidth are individually 20-200°C.

Microwave modelling of radio frequency microelectromechanical rotary switches

In this study, both full 3D electromagnetic and equivalent circuit modelling of a radio frequency microelectromechanical systems (RF MEMS) single-pole eight-throw rotary switch are reported for the first time. Excellent agreement has been achieved between measurements and modelled performance for frequencies up to 12 GHz. In order to validate the effectiveness of the modelling, a 2-bit true time delay phase shifter having a centre frequency of 2 GHz has been demonstrated. The traditional topology of the switched delay line was adopted with two rotary switches employed for signal routing. This phase shifter represents the first ever application of any RF MEMS rotary switch. It is shown that the switch model can accurately predict the measured performance of the phase shifter, without the need for adding new circuit elements or tweaking of model parameters. The low measured root mean square deviations of insertion phase from the predicted responses increase as 1.3°, 1.9°, 2.5° and 3.4° when the phase states are stepped through 0°, 90°, 180° and 270°, respectively.

Effect of etch holes on the capacitance and pull-in voltage in MEMS tunable capacitors

Microelectromechanical systems (MEMS) tunable capacitors, switches or actuators are widely applied in wireless communication systems. In the fabrication process etch holes are used to release the sacrificial layer with relatively large structures, which obviously affects the performance of devices. However, most researchers neglect this effect during their designing of the capacitors, switches or actuators. This article presents the theoretical calculation of the capacitance of tunable capacitors with etch holes, and analyses the deviation of the capacitance and pull-in voltage with different parameters such as the length of the plates w, the length of the etch holes w h, the air gap between the two plates d, and the number of the etch holes. To validate the theory in this article, a tunable capacitor was fabricated by surface micromachined technology. The theoretical results compare well with the experimental results.

Dielectric charging characterization on microelectromechanical switches by discharge current transient

The discharge current method is used to quantify the dielectric reliability of microelectromechanical switches. Two dielectrics, silicon oxide and silicon nitride, were used to extract the charge trapped, the emissivity and the dielectric relaxation time by means of a circuital model with a current source simulating the charge trap. A figure of merit is proposed to better quantify the dielectric reliability issues.

The Effect of Silicon Nitride Stoichiometry on Charging Mechanisms in RF-MEMS Capacitive Switches

This paper discusses the mechanisms responsible for charging of plasma enhanced chemical vapor deposition (PECVD) silicon nitride films used in the fabrication of RF microelectromechanical (MEMS) switches. Nitride films deposited at different temperatures are characterized in order to better understand the effect of deposition conditions on material stoichiometry and stress. Both RF MEMS switches and metal-semiconductor-metal capacitors with PECVD silicon nitride as the dielectric layer were fabricated and their charging mechanisms were examined. Measurements indicate that charging arises from the formation of a defect band where charge transport occurs through a Poole-Frenkel-like effect. The calculated activation energy exhibits direct relation to material stoichiometry, and therefore to the nitride bandgap. Finally, it is viewed that lower temperature nitride is less prone to dielectric charging.

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.