MEMS Variable Capacitor Research Articles

The Design, Simulation, and Parametric Optimization of an RF MEMS Variable Capacitor with an S-Shaped Beam

This study presents the design and simulation of an RF MEMS variable capacitor with a high tuning ratio and high linearity factor of capacitance–voltage response. An electrostatic torsion actuator with planar and non-planar structures is presented to obtain the high tuning ratio by avoiding the occurrence of pull-in point. In the proposed design, the capacitor plate is connected to the electrostatic actuators by using the s-shaped beam. The proposed design shows a 138% tuning ratio with the planar structure of the actuator and 167% tuning ratio by implementing the non-planar structure. A linearity factor of 99% is attained by adjusting the rates at which the capacitor plate rises as the actuation voltage increases and the rate at which the capacitance decreases as the plate rises. Parametric optimization of the design is performed by utilizing the finite element method (FEM) analysis and high-frequency structural simulator (HFSS) analysis to obtain an optimized high-tuning ratio RF MEMS varactor at low actuation voltage. S-parameters of the design are presented on HFSS, with a 50 ohm coplanar waveguide (CPW) serving as the transmission line. The proposed RF MEMS varactor can be utilized in tunable RF devices.

Enhancing Linearity in Parallel-Plate MEMS Varactors through Repulsive Actuation

This paper presents a new MEMS varactor that uses repulsive actuation to achieve an ultra-linear capacitance-to-voltage response. The approach proposed involves actuating the moveable electrode away from the fixed electrode, instead of the conventional closing-the-gap direction. This increasing-gap movement reduces the capacitance as the actuation voltage increases. The MEMS variable capacitor is fabricated using PolyMUMPs technology and exhibits an excellent linearity factor of 99.7% in capacitance-to-voltage response, and a capacitance tuning ratio of 11× was achieved. The proposed strategy will enable the development of high-performance MEMS-based tunable devices for various applications.

압전 액추에이터를 이용한 저전압 구동 MEMS 가변 캐패시터

In this work, low voltage MEMS variable capacitor was presented. Compare with other RF MEMS technologies, piezoelectric MEMS devices offer certain benefits such as low actuation voltage and fast moving, etc. The proposed structure consists of dual piezoelectric actuation bridges of in-plane polarized lead zirconate titanate (PZT) film to improve actuation voltage and reliability of devices. The PZT thin film, deposited on Au structural layer using RF magnetron sputtering method, was poled and driven with interdigitated electrodes(IDEs) to exploit dSUB33/SUB mode actuation. The in-plane polarized PZT films make it possible to develope dSUB33/SUB mode bending devices which lead to about two times improvement in strain performance. The fabricated variable capacitor shows that the tuning range is varied 100.9 to 111.2 nF with control voltage ranging from 0 to 1 V.

A new wide tunability MEMS based variable capacitor using two separate electro-static vertical comb drive actuators

This paper presents a new MEMS variable capacitor to achieve high stable region and extremely wide tunability. The idea is based on increasing the stable region in the gap between the plates of the capacitor. It is done by combining the functionality of two different vertical comb drive actuator sets to takeover a unity air gap variation. The design of the structure is carried out so that the performance of these two actuator sets is fully independent of one another and without any destructive or damaging effect on each other. The advantage of this scheme is that adding the second mechanism of actuation does not change the overall structure thickness compared with when the structure uses a single actuation mechanism. Therefore, the tunability increases sharply. Aluminum is the structural material used for the designs. Comb actuators are widely used as MEMS motors due to their long range of linear motion, low power consumption and ease of fabrication. A full review of electrostatic actuator portion is done. The structure is calculated using MATLAB software. To verify, the calculated results are compared with simulated results using Intellisuite software. The natural frequency is 1.173 KHz. According to calculation and simulation results the achieved minimum tuning range is 2300%.

Symmetry and asymmetry from MEMS variable capacitor by nonlinear micro stoppers

Mechanical stoppers in MEMS capacitive systems can dramatically affect electrical performances and result in complicated mechanical dynamic responses. This paper introduces electromechanical coupling nonlinear dynamic responses in MEMS variable dual-capacitor with an effect of nonlinear and asymmetrical stoppers. We found that the capacitance in the electrical circuit system related to the first-order derivative of the output voltage on a load resistor, and the variable dual-capacitor was strongly affected by the coupling of up and down superposition instantaneous electrostatic force and limited space by the length of nonlinear stoppers. The numerical calculation results and the experimental results in our analysis based on our system had a good agreement, and the numerical simulation results presented rich nonlinear impacts dynamic responses through the imposed voltage and the height of stoppers in MEMS variable dual-capacitive device. The device in operation cannot reach the 0.6 time's initial gap due to small forcing amplitude (1.026 g). However, we observed that the movable plate and stoppers (across the 0.6 time's initial gap) had fierce impacts due to big forcing amplitude (4 g) on to the device. With asymmetric stopper each impact, we also concluded that the movable plate would experience attenuations of the displacement until the moment to the next impacts. Moreover, the height of stoppers can not only result in complicated dynamic motion of the movable plate, but also can modulate a voltage of the fixed plate with its asymmetry structure.

The mechanical modeling of a special variable MEMS capacitor

In this paper, a special variable MEMS capacitor is offered to application in the telecommunication equipment especially SIW-based structures. The variable MEMS capacitor is designed with special serpentine arms around the membrane. By using these arms, the air damping and the stiffness of the capacitor are decreased. To evaluate the performance of the proposed structure, the mechanical model of it is extracted. Then the mathematical equations relevant to components such as spring constant, resonance (inherent) frequency, and especially the pull-in voltage are expressed. The static linear analysis and energy method is used to obtain the spring constant of the proposed structure. Also, the behavior of the proposed design is analyzed and simulated by applying the Intellsuite MEMS tool. The proposed design is optimized based on the dimension of the transmission line. Thus, the implementation of the MEMS capacitor is done using a gold layer of 6 μm and a 3 μm air gap. The simulation results show that, in zero bias, the natural frequency of the capacitor is 575 Hz. Also, the pull-in voltage is 1.34 V. Finally, the calculated and simulated results are compared and show that the proposed mechanical model for the proposed structure confirms its performance.

Design and simulation of a variable MEMS capacitor for tunable HMSIW resonator

Design and simulation of a variable MEMS capacitor for tunable HMSIW resonator

A new MEMS based variable capacitor using electrostatic vertical comb drive actuator and auxiliary cantilever beams

We have proposed a new wide tunable MEMS variable capacitor. In the proposed structure, an electrostatic vertical comb drive actuator is used to extend the tuning range. Moreover, the auxiliary cantilever-beams are used in the electrostatic comb drive actuator to delay the front sticking (Pull in) and increase the tunability. The effect of lateral gap distance between the fingers in the capacitance tunability is investigated. Not only a full review of electrostatic actuator portion is done but also the electric fields related to lateral gap changes are simulated by COMSOL software and its results are compared with theoretical results as well. The structure is calculated using MATLAB software. To verify, the calculated results are compared with simulated results using Intellisuite software. According to calculation and simulation results the achieved tuning range is 285%.

Optimization of a MEMS variable capacitor with high linearity and large tuning ratio

A novel micro-electro-mechanical system variable capacitor with a nonplanar top plate and a side leverage electrode structure is proposed to achieve a large tuning ratio and excellent linearity. A theoretical analysis of the structure is performed, including both a mechanical analysis and an electrical analysis. The effects of the structural parameters on the linearity were analyzed and a design method was derived to optimize the linearity. A simulation using CoventorWare was carried out to verify the capacitor performance with particular emphasis on the capacitance versus voltage (C–V) response. The results show a high linearity factor of 99.92% between 0.077 and 0.238 pF (the linear C–V response range) that corresponds to a linear capacitance tuning ratio of 309%.

A Novel Technique for Design of Ultra High Tunable Electrostatic Parallel Plate RF MEMS Variable Capacitor

This paper introduces a novel method for designing of low actuation voltage, high tuning ratio electrostatic parallel plate RF MEMS variable capacitors. It is feasible to achieve ultra-high tuning ratios way beyond 1.5:1 barrier, imposed by pull-in effect, by the proposed method. The proposed method is based on spring strengthening of the structure just before the unstable region. Spring strengthening could be realized by embedding some dimples on the spring arms with the precise height. These dimples shorten the spring length when achieved to the substrate. By the proposed method, as high tuning ratios as 7.5:1 is attainable by only considering four dimple sets. The required actuation voltage for this high tuning ratio is 14.33 V which is simply achievable on-chip by charge pump circuits. Brownian noise effect is also discussed and mechanical natural frequency of the structure is calculated.

Bulk silicon micromachined suspended fixed-end SiO2 film capacitor for passive high-pass RC filter

We reported a suspended fixed-end 5 μm thin SiO2 film beam and plate MEMS capacitor for passive high-pass RC filter. This suspended structure was fabricated using bulk silicon two-sided alignment micromachining technology. The dynamic model included both a first order derivative of a high-pass filter electrical system and MEMS variable capacitors driven by electrostatic force. The room temperature experiment showed that the MEMS high-pass filter circuit can convert a rectangular wave input signal into high frequency spikes at its output. Finite element calculating software COMSOL and numerical simulation MATLAB were both used to evaluate the motion modal and the output voltage. Compared with the commercial static capacitor in a high-pass filter, we could also find that the coupling of the electrostatic force and dynamic motion of the electrode plate in the designed single device and dual-device could show an asymmetric offset voltage electrical response.

Process Integration of Electroformed MEMS Variable Capacitors for Magnetostriction Measurements

Electrochemically deposited CoFe and CoFeB films are being studied for use as wireless remote sensors. Coded signals can be achieved through passive devices and interrogated to produce electronic signatures that are nearly impossible to replicate or counterfeit. A MEMS variable capacitor is being fabricated from a novel electroformed process with CoFe and CoFeB magnetostrictive films. This approach to MEMS fabrication is being used to incorporate electroplated ferric alloys into a variable capacitor, to study Joule magnetostriction. Such alloys are capable of a large change in material shape when subjected to a magnetic field. Integrating these films with a variable capacitor MEMS device allows for a unique approach to provide accurate microstrain measurements in low magnetic fields up to saturation, to provide feedback for the optimization without the use of expensive commercial setups. This process is unique to MEMS device fabrication and is differentiated by the use of a planar layered approach. Maintaining planarity by manipulating oxides to form electroplating molds, as well as provide a sacrificial material, results in an electrochemically deposited cantilever structure with an integrated magnetostrictive sensing element. This presentation will discuss the fabrication process of these devices. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000

Modeling and Characterization of Electroformed MEMS Variable Capacitors for Cobalt Iron Magnetostriction Measurements

Electroplated CoFe alloys demonstrating Joule magnetostriction (i.e., a large change in material shape induced by an applied magnetic field) have been recently developed for the creation of microfabricated magnetoelastic resonators for tagging applications. This development requires a measurement technique for evaluating film magnetostriction performance to provide feedback for optimization of the electrodeposition process. This technique must be suitable for electrodeposited films in terms of thickness and ease of processing, have the required sensitivity and dynamic range for accurate microstrain measurements in low magnetic fields up to saturation, and be cost competitive with commercial instruments. A new MEMS variable capacitor has been developed for this purpose that incorporates electroformed mechanical and electrical features as well as the electrodeposited film under test. This capacitor has a modeled 3 pF variable range covering microstrains up to 200 ppm. This presentation will cover the design, modeling, and characterization of these capacitors for magnetostriction measurements of CoFeB and CoFe electrodeposited films. This will also include a discussion of a custom printed circuit board for extracting capacitance signals during the magnetic field ramp inside of a superconducting quantum interference device (SQUID) magnetometer.

Low-voltage, High-tuning Range MEMS Variable Capacitor Using Closed-loop Control

A low-voltage, high-tuning range variable MEMS capacitor using parallel-plates electrostatic microactuators operated in close-loop is introduced in this paper. The structures were fabricated using an in-house 2-masks process on SOI wafers and the use of a closed-loop controller increases the stable displacements of the structure beyond the pull-in limitation along the full available gap (the limitation are the mechanical stoppers). Additionally, the structure has two pairs of actuators, in opposing directions, enabling bi-directional displacement leading to extended tuning range. The fabricated capacitors have a designed tuning range close to 17. A lower actuation voltage is also a main benefit for future integration on low-power devices (measured pull-in voltage of approximately 3.25V). The devices were characterized and a capacitance variation of 2.3pF was experimentally verified.

A new MEMS based variable capacitor with wide tunability, high linearity and low actuation voltage

We have proposed a new RF MEMS variable capacitor to achieve high linearity, wide tunability and low actuation voltage. The idea is based on increasing the linear region in the gap between the plates of the capacitor. It is done by adding a fixed-fixed beam below the fixed-free beam. The fixed-free beam is one plate of the capacitor. The voltage is applied to the fixed-free beam. In the vicinity of the pull-in voltage, the fixed-free beam losses its equivalent stiffness. The fixed-fixed beam is located in the vicinity of pull in situation of the fixed-free beam. This condition increases the equivalent stiffness of the fixed-free beam and allows the beam to continue moving down linearly and consequently increases the maximum capacitance of the structure. Geometrical and material property effects of the second beam on the linearity, tunability and voltage are investigated. The governing nonlinear equation for static deflection of the beam based on the Euler–Bernoulli beam theory has been presented.

Resonant frequency characterization of MEMS based energy harvesters by harmonic sampling analysis method

This paper presents a new and accurate experimental method based on harmonic distortion analysis to determine the resonant frequency of MEMS devices to be used as energy scavengers or more generally in widespread MEMS-based applications. This technique uses the mechanical–electrical analogy of MEMS variable capacitor acting as a low-pass filter to give access to both resonant frequency and damping factor of the mechanical system through the determination of the filter parameters as the cut-off frequency. Resonant frequencies ranging from 0.8kHz to 5kHz of electrostatic actuated MEMS-based harvesters have been measured by this technique with an uncertainty as low as a few parts in 103 in a good agreement with measurements carried out by using Deep Level Transient Spectroscopy system.

DESIGN, SIMULATION AND BIFURCATION ANALYSIS OF A NOVEL MICROMACHINED TUNABLE CAPACITOR WITH EXTENDED TUNABILITY

In this paper, a novel RF MEMS variable capacitor has been presented. The applied techniques for increasing the tunability of the capacitor are the increasing of the maximum capacitance and decreasing of the minimum capacitance. The proposed structure is a simple cantilever Euler–Bernoulli micro-beam suspended between two conductive plates, in which the lower plate is considered as stationary reference electrode. In this structure, two pedestals are located in both tips of the cantilever beam. In the capacitive micro-structures, increasing the applied voltage decreases the equivalent stiffness of the structure and leads the system to an unstable condition (pull-in phenomenon). By deflecting the beam toward the upper (lower) plate the minimum (maximum) capacitance decreases (increases) and tunability increases consequently. The located pedestals increase and decrease the maximum and minimum capacitance respectively. The results show that the proposed structure increases the tunability of cantilever beam significantly. Furthermore, bifurcation behavior of movable electrode has been investigated.

(Invited) Tunable RF MEMS and Its Applications

With fast development of mobile communication technology, the increasingly complex communicating environment and the crowded spectrum have raised stringent requirements on the RF front-end performance. In this paper, the influence of human body on the communication performance is analyzed. In particular, it is shown that the antenna input impedance faces huge variation in the frame of body-centric devices. Consequently, automatic impedance matching appears to be crucial to improve the performance withstand the inconstant environment. The investigation of key tunable components of RF MEMS is therefore presented. A variable capacitor with symmetric balanced structure is designed to achieve a large capacitance tuning ratio and a remarkable linearity of the C-V response. The proposed RF MEMS variable capacitor was modeled and simulated. The results show a linearity factor of 99.82% in C-V response and a large tuning ratio of 480% in a low actuation voltage range from 0V to 16.3V.

Voltage-Controlled $C{-}V$ Response Tuning in a Parallel Plate MEMS Variable Capacitor

In a conventional parallel plate MEMS variable capacitor, the capacitance versus voltage response (C-V response) has been deterministic. In this work, the C-V response is tuned versatilely through the application of a control voltage to an additional electrode in order that the initial gap between the parallel capacitor plates is set by the control voltage. Then, the capacitor plates are lifted (capacitance decreases) as the actuation voltage applied to the levering actuator increases. In this manner, the shape of the C-V response can be controlled even after the device is fabricated. At a zero control voltage, the fabricated MEMS variable capacitor exhibited a convex shape in the C-V response (i.e., the capacitance decreases slowly in the low actuation voltage region and rapidly in the high actuation voltage region). When 3 V was applied to the control voltage, the capacitor exhibited an almost linear C-V response with a linearity factor of 0.999. At 5 V of control voltage, the C-V response changed to a relatively concave shape (i.e., the capacitance decreases rapidly in the low actuation voltage region and slowly in the high actuation voltage region). The capacitance tuning ratio of the fabricated device exceeded 120% at all control voltages. The proposed C-V response tuning capability is vital and amenable to various circuit demands.

A novel high tuning ratio MEMS cantilever variable capacitor

This paper presents a novel cantilever-type MEMS variable capacitor with high tuning ratio. In previous works, the cantilever is used as a switch but in our design, it is applied as a variable capacitor. For increasing the maximum achievable capacitance of the cantilever, the suspended capacitive plate should be moved as parallel with fixed plate. The parallel movement can be obtained using the novel structure which utilized two additional lateral beams. Also to overcome the pull-in phenomenon in new structure, the different membrane thickness technique is used. The novelty of our design is adding the lateral beams to make parallel movement of the suspended plate to increase the variable capacitor of the cantilever. The new device is designed on a thick silicon substrate with a thin poly silicon membrane. The results show the tuning range of tunable capacitor with initial capacitance of 560.1 fF can be improved from 6:1 for conventional cantilever to 22.5:1 for the new cantilever. In other words the capacitance tuning range increased three times.