MEMS Variable Capacitor Research Articles

Design of a Flexible Metamaterial RF MEMS Circuit for Multi-Purpose/ a Tunable Bass-Band Filter

This paper represents a new and original design of a Metamaterial circuit based on matrix of resonator cells using variable analog MEMS capacitors ‘varactors’, periodically distributed in some way to fulfill a multiple applications circuit, whose one is a tunable pass band-filter. The cells are connected together with lumped elements computed using hybrid circuit EM simulation and allow precise computation of the frequency shift for every changing of the capacitive load. The design is extremely dynamic and can be used for numerous applications due to the agility in controlling the varactors. Those later, are actuated in some manner to achieve a tunable 2-pole pass-band filter, its center frequency is analogically and then digitally tuned from 11 to 12.6 GHz. Return losses less than 15 dB and insertion losses better than 1 dB over the whole operation band. The circuit has been conceived on a 525 µm-thick quartz substrate and it occupies a total area of 1.48×0.79 cm2. Finally, we discuss the challenging and opportunities in the field of Metamaterials based on RF MEMS.

Development of an SU-8 MEMS process with two metal electrodes using amorphous silicon as a sacrificial material

This work presents an SU-8 surface micromachining process using amorphous silicon as a sacrificial material, which also incorporates two metal layers for electrical excitation. SU-8 is a photo-patternable polymer that is used as a structural layer for MEMS and microfluidic applications due to its mechanical properties, biocompatibility and low cost. Amorphous silicon is used as a sacrificial layer in MEMS applications because it can be deposited in large thicknesses, and can be released in a dry method using XeF2, which alleviates release-based stiction problems related to MEMS applications. In this work, an SU-8 MEMS process was developed using α-Si as a sacrificial layer. Two conductive metal electrodes were integrated in this process to allow out-of-plane electrostatic actuation for applications like MEMS switches and variable capacitors. In order to facilitate more flexibility for MEMS designers, the process can fabricate dimples that can be conductive or nonconductive. Additionally, this SU-8 process can fabricate SU-8 MEMS structures of a single layer of two different thicknesses. Process parameters were optimized for two sets of thicknesses: thin (5–10 µm) and thick (130 µm). The process was tested fabricating MEMS switches, capacitors and thermal actuators.

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 puzzle-like fractal microelectromechanial system variable capacitor suppressing pull-in

This Letter introduces an electrostatically actuated fractal MEMS variable capacitor that, by utilising the substrate, extends the tuning range (TR) beyond the theoretical limit of 1.5 as dictated by the pull-in phenomenon. The backbone concept behind the fractal varactor is to create a suspended movable plate possessing a specific fractal geometry, and to simultaneously create a bottom fixed plate complementary in shape to the top plate. Thus, when the top plate is actuated, it moves towards the bottom plate and fills the void present within the bottom plate without touching it akin to how puzzle pieces are assembled. Further, a reasonable horizontal separation is maintained between both the plates to avoid shorting. The electrostatic forces come from the capacitance formed between the top plate and bottom plate, and from the capacitance formed between the top plate and the doped substrate. The variable capacitor was fabricated in the PolyMUMPS process and provided a TR of 4.1 at 6 V, and its resonant frequency was in excess of 40 GHz.

Two-cavity MEMS variable capacitor for power harvesting

Novel 2 × 2 mm2 MEMS capacitive plates with two cavities (two capacitors) have been designed, modeled and fabricated for power harvesting by utilizing residual mechanical vibration in the environment using the electrostatic mechanism. The device is unique in the use of an innovative two-cavity design and electroplated nickel as the main structural material. When the capacitance increases for one capacitor, it will decrease for the other. This allows us to use both up and down directions to generate energy. The two-cavity design has achieved higher average power than conventional single-cavity devices under a wide range of vibration frequencies and amplitudes based on the time-domain simulations using Matlab. The movable plate was designed to vibrate without deformation and with resonance frequencies of around 900 Hz and lower using COMSOL finite element tool. The prototype two-cavity MEMS variable capacitor has been successfully fabricated using surface micromachining. The initial testing to investigate the electrical dynamic behavior and power generation from the fabricated devices was also implemented.

Gain flattening technique for optical wireless front-end receiver considering various large window photodetectors

A novel gain flattening technique for an optical wireless front-end receiver structure involving a bootstrap transimpedance amplifier (BTA) integrated with a MEMS variable feedback capacitor has been demonstrated. The MEMS varicap replaces a fixed capacitance as the feedback element in the front end system to optimize the performance of the BTA in terms of its frequency response. The implementation of the MEMS device with a BTA optical front-end receiver was verified using CoventorWare ARCHITECT. The simulation results showed that the approach can significantly flatten the peaking gain by up to 14dB, when considering a system with various photodetector capacitances, ranging in value from 100pF to 1nF.

Wideband tunable Love wave filter using electrostatically actuated MEMS variable capacitors integrated on lithium niobate

A tunable bandpass filters with low insertion loss, sharp cut-off characteristics, wide bandwidth and wide tunability is strongly expected for future reconfigurable and cognitive wireless systems. In this study, a wideband tunable filter with the center frequency of 1.08GHz was fabricated by directly integrating Love wave resonators and electrostatically actuated MEMS variable capacitors on a 15° Y LiNbO3 wafer. The Love wave resonators have an extremely large electromechanical coupling coefficient (k2∼30%), providing wide bandwidth and wide tuning range. The 3dB bandwidth was tuned from 146MHz to 130MHz by applying 15V to one of the MEMS variable capacitors.

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.

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.

Modeling and modification of the parallel plate variable MEMS capacitors considering deformation issue

For electrostatically actuated parallel plate variable capacitors in conventional theory, pull-in occurs when the deflection of the movable plate is one-third of the original air gap. However, the electrostatic force is nonlinear, only the center of the movable plate nearly fully reached their maximum displacement when pull in occurs. In this paper, formulas for calculating the capacitance of two plates with an angle are presented. We model the deformation of the movable plate and analyze the mechanical behavior of the capacitors. We fabricated the parallel plate variable MEMS capacitors and used WYKO NT1100 optical surface profiler to measure the displacement related with deformation. The results show that the theoretical deviations of the capacitance and pull-in voltage in conventional theory and in this work are more than 2.6% and 1.3%. The model presented in this paper can be used for the design, analysis and optimization of electrostatically actuated parallel plate variable MEMS capacitors.

A Capacitive Sensing Scheme for Control of Movable Element with Complementary Metal–Oxide–Semiconductor Microelectoromechanical-Systems Device

In this paper, we describe a new capacitive-sensing scheme that detects the displacement of a movable structure for the control of a microelectoromechanical-systems (MEMS) device stacked on a complementary metal–oxide–semiconductor (CMOS) LSI. The problem is that the small capacitance of the air gap between the movable element and a sensor plate needs to be detected in spite of the large parasitic capacitance caused by LSI interconnections. The solution is a sensing circuit that features a shield plate and a ramp detection circuit. The shield plate separates the sensed capacitance from the parasitic capacitance. A ramp detection circuit enhances the detection sensitivity using a gradient signal generated by the parasitic capacitance. To check the effectiveness of the scheme, a sensing circuit and a MEMS variable capacitor were fabricated in a 0.6-µm CMOS process and a MEMS process. This scheme enhanced the sensitivity, which is the ratio of the output voltage to the sensed capacitance, by a factor of six. These results demonstrate that this scheme is suitable for the control of a CMOS-MEMS device.

On the Selection of the Number of Bits to Control a Dynamic Digital MEMS Reflectarray

The benefit of low loss exhibited in utilizing MEMS variable capacitors in a reconfigurable reflectarray is undermined by the inconsistency of the fabricated devices. A digitally controlled reflectarray with MEMS capacitive switches is proposed. This paper aims to determine an appropriate number of phase shifting bits for such implementation. Using array theory, the influence of the number of bits on the beam pointing performance is evaluated. A common feed horn is used to illuminate the reflectarrays. In addition, broadside plane wave illumination is also considered for comparison with existing works in phased arrays. Though at certain scan angles the results are different from those for traditional phased arrays, in general the results are found to be similar. This information lays the groundwork for designing a cost-effective digital MEMS reflectarray cell.

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.

Design and synthesis of wide tuning range variable comb drive MEMS capacitors

PurposeTo present a general synthesis method in the design of variable comb drive MEMS capacitors which can provide specified capacitance versus position profiles while give large tuning range.Design/methodology/approachBy carefully choosing design parameters and constraint conditions, the design process is implemented as the solution of a constraint optimization problem. An electric field analysis software based on the finite element method and an optimization software based on the evolutionary stochastic search are chosen to work together to implement the system.FindingsThe results verify that the shape of a variable MEMS capacitor has a great influence on the capacitance versus distance profile and demonstrates that a specific geometry of the MEMS capacitor can be found to match a desired capacitance‐distance profile.Research limitations/implicationsThe analytical expression for the capacitance formed between the fixed and the movable fingers is somewhat inaccurate. The results are not compared to measurement because no device has been fabricated.Practical implicationsA very successful numerical simulation example and guidance for MEMS designers to develop MEMS devices with variable electrode shapes.Originality/valueThis paper proposes a systematic way for the design of MEMS tunable capacitors with variable shape. The approach can be used to design MEMS devices with variable electrode shapes to satisfy specific requirements.

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.

MEMS Variable Capacitor Using Cross Membranes for RF Band

This paper describes a novel design of MEMS variable capacitor with high quality factor and wide tuning range. The proposed MEMS variable capacitor is composed of two cross movable electrodes and one dielectric layer between them. The fabrication process does not require any complex processes, such as wafer transfer, wafer backside etching, special planarization or supercritical point drying. The capacitor has tuning range of 330% at 5 V bias voltage, and an overall quality factor of 52 at 2 pF and 2.4 GHz.

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.