RF 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.

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.

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.

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.

Ku-band RF MEMS tunable comb line band reject filter on coplanar transmission line

A band reject filter with tuning capability is presented on a CPW transmission line on silicon substrate using comb line and RF MEMS variable capacitor, enabling compatibility with planar IC technology. A conventional CPW on a substrate consists of a central strip conductor with semi-infinite ground planes on either side. A comb line is etched on the signal line of the CPW and the MEMS bridge capacitor is put on the same line in shunt. Tunability of the filter is achieved by putting the MEMS bridge in either up or down state. The rejection at the centre frequency of stop bands are around −40.24dB for down state and −38.21dB for up state of the bridge. A low insertion loss, as low as −0.68dB, is obtained in the pass band. The proposed device structure is simulated using ANSOFT HFSS v13® for RF analysis and COVENTORWARE (2008)® for mechanical and electromechanical characterization, both static and transient analysis.

Low voltage RF MEMS variable capacitor with linear C-V response

An RF MEMS variable capacitor, fabricated in the PolyMUMPS process and tuned electrostatically, possessing a linear capacitance-voltage response is reported. The measured quality factor of the device was 17 at 1 GHz, while the tuning range was 1.2:1 and was achieved at an actuation DC voltage of 8 V only. Further, the linear regression coefficient was 0.98. The variable capacitor was created such that it has both vertical and horizontal capacitances present. As the top suspended plate moves towards the bottom fixed plate, the vertical capacitance increases whereas the horizontal capacitance decreases simultaneously such that the sum of the two capacitances yields a linear capacitance-voltage relation.

Design and Simulation of RF-MEMS Tunable Capacitors Using Multiple Voltage Sources

A novel tunable RF-MEMS variable capacitor(varactor) is presented. The Flip -Flop structure of the design makes it possible to either increase or decrease the capacitance. Dimensions, sizes and the materials used in the device are chosen so that it can be manufactured in Urmia MEMS Lab. Silicon surface micromachining thechnology is used for the structure, which is CMOS compatible. Electrostatic actuation method is selected because of it's low power consumption so it can be used in low-power wireless applications. The value of the capacitor is changed by varying the gap between the two plates. Tuning voltage is in the range of 0-2 volts. The capacitor has the initial value of 0.43pF, maximum value of 0.92pF and minimum value of 0.3pF. Actuation and varactor plates are separate from each other. The structure is simple and can be fabricated by employing only 3 masks.

Piezoresistive Position Sensing for the Detection of Hysteresis and Dielectric Charging in CMOS-MEMS Variable Capacitors

μA position sensing scheme for the detection of hysteresis and dielectric charging is presented. The sensing mechanism is based upon piezoresistance and is applied to an RF MEMS variable capacitor fabricated using a standard 0.35 μm CMOS process with MEMS postprocessing. The position sensor, based upon the piezoresistive property of the CMOS polysilicon layer, proves capable of detecting the effects of hysteresis and dielectric charging in the fabricated device. Potential applications of the sensing scheme include the mitigation of the effects of hysteresis and dielectric charging on MEMS variable capacitors through closed-loop control.

Tunable dual-frequency RF MEMS rectangular slot ring antenna

This paper presents the design, fabrication, and measurement of a novel, tunable, dual frequency rectangular slot antenna using RF MEMS technology. The antenna tuning is achieved by electrostatically actuated RF MEMS variable capacitors on a stub. Two different capacitor structures are implemented on the antenna: cantilever type and fixed–fixed beam type. The latter one is not sensitive to the stress on the structural layer and its measurement results agree well with the simulation results. Each antenna is fabricated monolithically using an in-house surface micromachining RF MEMS process on a 500-μm thick glass substrate and occupies an area of about 21 mm × 18 mm. The measurement results show that the antenna resonant frequency can be tuned in a 1 GHz range in the X-band.

Sandwich Structure Type RF-MEMS Variable Capacitor with Low Voltage Controllability and Wide Tuning Range

A sandwich structure type RF-MEMS variable capacitor is proposed, that consists of a movable middle plate, and fixed top and bottom plates having different areas. Simulation results show that the proposed capacitor can operate at a control voltage of less than 3.2V; it achieves a tuning range of 4.8:1 (capacitance: 630-130fF) in the range of 0 to 3.2V and at a frequency of 7.5GHz.

Germanium resistors for RF MEMS based microsystems

This paper introduces the use of germanium as resistive material in RF MicroElectroMechanical (MEMS) devices. Integrated resistors are indeed highly required into RF MEMS components, in order to prevent any RF signal leakage in the bias lines and also to be compatible with ICs. Germanium material presents strong advantages compared to others. It is widely used in microtechnologies, notably as an important semi-conductor in SiGe transistors as well as sacrificial or structural layers and also mask layer in various processes (Si micromachining especially). But it also presents a very high resistivity value. This property is particularly interesting in the elaboration of integrated resistors for RF components, as it assures miniaturized resistors in total agreement with electromagnetic requirements. Its compatibility as resistive material in MEMS has been carried out. Its integration in an entire MEMS process has been fruitfully achieved and led to the successful demonstration and validation of integrated Ge resistors into serial RF MEMS variable capacitors or switches, without any RF perturbations.

An intelligent bipolar actuation method with high stiction immunity for RF MEMS capacitive switches and variable capacitors

We propose an intelligent bipolar actuation (IBA) method for electrostatic actuators, which can suppress stiction induced by dielectric charging. The high stiction immunity is achieved by flipping the bias voltage polarity depending on the pull-out voltage, thereby restricting the amount of charge stored in the dielectric film.

Fabrication of RF MEMS variable capacitors by deep X-ray lithography and electroplating

Radio frequency micro electro-mechanical systems (RF MEMS) vertical cantilever variable capacitors fabricated using deep X-ray lithography and electroplating are presented. Polymethylmethacrylate (PMMA) layers of 100 μm and 150 μm have been patterned and electroplated with 70 μm and 100 μm thick nickel. A 3 μm thick titanium layer was used as plating base as well as etch time-controlled sacrificial layer for the release of the cantilever beam. The parallel plate layout includes narrow gaps and cantilever beams with an aspect ratio in nickel of up to 60 for 1 mm long features. Auxiliary structures support the beams and gaps during the processing. Room temperature electroplating significantly reduces the risk of deformations compared to the standard process temperature of 52°C. The capacitors operate in the 1–5 GHz range, and demonstrate good RF performance, with quality factors on the order of 170 at 1 GHz for a 1 pF capacitance.

Transient dynamics of a MEMS variable capacitor driven with a Dickson charge pump

In the actuation of electrostatically controlled capacitive MEMS devices, drive voltages over the supply voltage are commonly required. In order to obtain such voltages, charge pumps are often used. This paper presents and discusses two methods to drive capacitive MEMS devices using IC-compatible Dickson charge pumps designed to minimize power consumption. Drive circuits include voltage shaping to minimize charge injection in order to provide better reliability. The main design constraints to integrate them in state of the art IC technologies are identified and quantified. The required charge pumps are also optimized to provide an efficient use of its total capacitance. The transient dynamics of charge pumps connected to a RF MEMS variable capacitor are also analysed and discussed. Finally, the proposed methods are tested and validated through experimental results.

A mechanically reliable digital-type single crystalline silicon (SCS) RF MEMS variable capacitor

This paper reports the first demonstration of a single crystalline silicon(SCS)-based vertical gap-tuning MEMS variable capacitor with high mechanical reliability using the SiOG (silicon-on-glass) process. The proposed variable capacitor was fabricated by silicon-based micromachining technology, and its mechanical and electrical performances were tested. By adopting SCS, which has nearly zero stresses and superior thermal characteristics as a structural material, we can improve the manufacturability, reproducibility and mechanical reliability of the fabricated devices compared with a conventional metallic variable capacitor. The variable capacitor described in this paper is fabricated on a coplanar waveguide (CPW) transmission line and the top electrode in the driving part is incorporated with a SCS actuating membrane, resulting in a highly rigid structure without structural deformations. The designed SCS variable capacitor is actuated by an electrostatic force, and the total chip size is 1.05 mm × 0.72 mm. The measured pull-in voltage was 37 V, and the RF characteristics of the fabricated SCS variable capacitor from dc to 40 GHz were measured. With and without an applied dc bias voltage, the measured S11 and S21 were changed from −15.6 to −5.1 dB, and from −0.49 to −2.3 dB at 30 GHz, respectively. The measured capacitance at the up and the down states was 30 and 140 fF, respectively, which corresponds to the capacitance ratio of 4.67. The mechanical lifetime of the fabricated variable capacitor was also measured. RF performance degradation and stiction problems were not observed, even after 108 cycles of repeated actuations.