Microelectromechanical Systems Capacitive Switches Research Articles

Linearity and RF Power Handling on Capacitive RF MEMS Switches

Linearity characteristics of capacitive tuners under large radio frequency (RF) power have become an important issue for today’s cellphone industry. The use of these devices as antenna tuners has become critically important due to the RF spectrum opening in the low-frequency range. Capacitive tuners connected directly to the antenna to change the RF voltage/current pattern and tune lower frequencies are subjected to suffer large voltage swings, especially in the isolation state or minimum capacitance. In this article, we analyze the effects of the large RF power or equivalent rms voltage to capacitive micro-electromechanical systems (MEMS) switches. The theoretical analysis combined with Coventor MEMS+ simulations will allow us to study the capacitance change and MEMS mechanical frequency response. This article evaluates two types of measurement setups to investigate the RF power effects on nominal capacitance and mechanical frequency response. Measurements up to 6 W were completed with two capacitive MEMS tuner products, confirming the modeling predictions and demonstrating the robustness of this technology under large RF power or voltage swings.

Design of a novel structure capacitive RF MEMS switch to improve performance parameters

This study reports the design and analysis of novel step structure RF micro-electromechanical system (MEMS) switch for low pull-in voltage, low insertion loss and high isolation by using uniform single meander. The central beam of the membrane is designed with 0.5 µm lower than the side beams to form a step-down structure which reduces the pull-in voltage. Stress analysis, electromechancial, switching time, quality factor and RF analysis have done to understand the behavioural characteristics of the proposed step-down switch. The analysis has been carried out for different beam and dielectric materials among them switch with gold material exhibits low pull-in voltage of 4.7 V, low insertion loss <1 dB and high isolation of −38.3 dB at 28.2 GHz for silicon nitride. The switch also shows good quality factor of 0.95 for gold material along with high capacitance ratio of 132. The upstate capacitance of 56.8 pF contributes low return loss and made the switch to transmit the signal up to 26.2 GHz and provides 7.2 pF of downstate capacitance to produce high isolation at 26.2 GHz which is efficiently used for K-band satellite applications.

Pull-in-voltage and RF analysis of MEMS based high performance capacitive shunt switch

This paper presents design of a RF MEMS (Radio Frequency Micro Electro Mechanical system) capacitive shunt switch to study the switch performance which depends on various indices. To achieve high performance in the MEMS switch, the different component materials of the switch were chosen very carefully. Modeling equations were also derived for designed fixed-fixed beam of the switch considering formed capacitances in both upstate and downstate positions. The FEM simulation and accurate analytical results were obtained to study the effect of capacitance on the RF performance of the switch. The optimum dimensions were chosen for achieving a low pull in voltage as approximately 19 V for the designed switch. Static analysis confirms the optimum dimensions of the switch that provide several advantages such as high isolation, low insertion loss and low pull in voltage in the switch circuit. RF analysis study concludes that for the optimized switch provide return loss, insertion loss and isolation as −43 dB, −0.05 dB, −12 dB respectively.

Fabrication and characterisation of RF MEMS capacitive switches tuned for X and Ku bands

Microelectromechanical systems (MEMS) capacitive switches discussed in this paper employ electrostatic actuation to perform switching. Capacitive switches employ inductive tuning for excellent switching characteristics in X and Ku bands. Employing inductive tuning is found to increase the switch beam inductance by a few tens of pico-henry. This enhances the Q factor and enables tuning of isolation over a narrow band of frequencies. Beam inductance can be extracted from the simulated isolation characteristics of the switch by curve fitting. This paper presents design, fabrication and characterisation of inductive tuned MEMS capacitive switches tuned for X and Ku bands. The devices are fabricated on high resistive (10 KΩ) silicon substrate by a five mask process. The characterisation of the fabricated devices are conducted using Cascade probe station and high frequency Power network analyser. Characterisation results show an actuation voltage of 18.5 volts. The insertion - loss and isolation are better than 0.5 dB and -40 dB respectively in the 8-18 GHz band.

Fabrication and characterisation of RF MEMS capacitive switches tuned for X and Ku bands

Microelectromechanical systems (MEMS) capacitive switches discussed in this paper employ electrostatic actuation to perform switching. Capacitive switches employ inductive tuning for excellent switching characteristics in X and Ku bands. Employing inductive tuning is found to increase the switch beam inductance by a few tens of pico-henry. This enhances the Q factor and enables tuning of isolation over a narrow band of frequencies. Beam inductance can be extracted from the simulated isolation characteristics of the switch by curve fitting. This paper presents design, fabrication and characterisation of inductive tuned MEMS capacitive switches tuned for X and Ku bands. The devices are fabricated on high resistive (10 KΩ) silicon substrate by a five mask process. The characterisation of the fabricated devices are conducted using Cascade probe station and high frequency Power network analyser. Characterisation results show an actuation voltage of 18.5 volts. The insertion - loss and isolation are better than 0.5 dB and -40 dB respectively in the 8-18 GHz band.

Low insertion loss and high isolation capacitive RF MEMS switch with low pull-in voltage

Micro electromechanical system (MEMS) shunt capacitive switches behave chiefly as a capacitor at both up and down states. Therefore, input-output matching for these switches is affected by varying the frequency. Although increasing the amount of capacitance at the up state reduces the actuation voltage, it deteriorates the RF parameters. This paper proposes a remedy for this problem in the form of two short high-impedance transmission lines (SHITLs) which are included at both ends of the MEMS switch. The SHITLs are implemented through adoption of a discontinued CPW transmission line. With this implementation, the switch can be modelled as a T circuit, neutralizing a capacitance behavior of MEMS switch at the up state. The paper also proposes an optimised fabrication process to realize a flat and planar bridge for the suggested RF MEMS switch. The measurement recorded by SEM and AFM shows that the proposed method significantly improves the planarity of the membrane. The RF and mechanical parameters of the fabricated switch are evaluated by Vector Network Analyser and Laser Doppler Vibrometer. At the up state, the switch has a return loss less than -20 dB in the entire frequency band (C-K). The isolation at the down state is better than 10 dB for the lower frequencies and increases to values better than 18 dB for the higher frequencies. The measured pull-in voltage is almost 20 V and the mechanical resonance frequency is 164 kHz.

Ka‐band RF MEMS capacitive switch with low loss, high isolation, long‐term reliability and high power handling based on GaAs MMIC technology

This study presents a Ka-band radio frequency (RF) micro electro mechanical systems (MEMS) capacitive switch with low loss, high isolation, long-term reliability and high power handling based on GaAs microwave monolithic integrated circuit (MMIC) technology. In this design, a T-matching structure is employed to realise impendence matching at ‘up-state’ by modifying the dimensions of centre signal line. Thus, the reflection loss and insertion loss are improved effectively. Measurement results show that in ‘up-state’ position, the input reflection coefficient (S11) is less than −20.4 dB with the forward transmission coefficient (S21) of better than −0.27 dB at Ka-band (27–40 GHz). At ‘down-state’, the switch is designed in a state of self-resonance to obtain high isolation. The measured isolation is better than 20 dB over Ka-band and can reach 35 dB at its self-resonant frequency of 35 GHz. The measured actuation voltage is about 36 V. The measured lifetime is at least 5.76 × 107 cycles. The measured power handling capability can reach up to 39 dBm. The proposed compact RF MEMS capacitive switch possesses excellent performances in terms of low insertion loss, high isolation, long lifetime and high power handling capability.

Dielectric Charging Asymmetry in SiN Films Used in RF MEMS Capacitive Switches

This paper presents a systematic investigation of the effect of stressing bias polarity to the discharge process of plasma enhanced chemical vapor deposition SiN films that have been fabricated under different deposition conditions. Dielectric charging asymmetry has been observed on the utilized films of metal-insulator-metal capacitors and microelectromechanical systems capacitive switches, since charging is enhanced when negative polarization bias is applied to the top electrode. In addition, it is shown that deposition conditions play a key issue role on the degree of this asymmetry. Finally, a physical model which assumes that hopping dominates the discharging process is used in order to gain a better understanding of the processes that are responsible for the appearance of asymmetric dielectric charging and the results correlate with the material’s electrical properties.

Design and experimental validation of a restoring force enhanced RF MEMS capacitive switch with stiction-recovery electrodes

This paper presents an approach for restoring force enhancement to radio frequency (RF) micro-electro-mechanical systems (MEMS) switch with stiction-recovery actuation mechanism. It is based on additional anti-stiction electrodes which are inserted to the coplanar waveguide (CPW) between the signal line and ground planes of a RF MEMS switch. If the movable membrane sticks to the substrate, electrostatic force, as extra restoring force, generated by the bias voltage between the anti-stiction electrodes can promise restoring action. The designed device with the proposed approach is fabricated. The restoring force of the fabricated device had reached 75, 177 and 520 μN by the bias voltages of 0, 10 and 20 V, respectively.

21.69⁻24.36 GHz MEMS Tunable Band-Pass Filter.

The K-band microelectromechanical systems (MEMS) tunable band-pass filter, with a wide-frequency tunable range and miniature size, is able to fulfill the requirements of the multiband satellite communication systems. A novel 21.69–24.36 GHz MEMS tunable band-pass filter is designed, analyzed, fabricated and measured. This paper also designs and analyzes an inductively tuned slow-wave resonator, which consists of the MEMS capacitive switch, the MEMS capacitor and the short metal line. The proposed filter has four different work states by changing the capacitance values of the MEMS switches. Measured results demonstrate that, for all four states, the insertion loss is 2.81, 3.27, 3.65 and 4.03 dB at 24.36, 23.2, 22.24 and 21.69 GHz, respectively. The actuation voltage is 0, 20, 16 and 26 V, respectively. The 3 dB bandwidth of the tunable filter is 5.4%, 6.2%, 5.7% and 5.9%, respectively. This study contributes to the design of miniature millimeter tunable filters with a wide-frequency tunable range.

Analysis of Pull-in-Voltage and Figure-of-Merit of Capacitive MEMS Switch

Theoretical and graphical analysis of pull-in-voltage and figure of merit for a fixed-fixed capacitive Micro Electromechanical Systems (MEMS) switch is presented in this paper. MEMS switch consists of a thin electrode called bridge suspended over a central line and both ends of the bridge are fixed at the ground planes of a coplanar waveguide (CPW) structure. A thin layer of dielectric material is deposited between the bridge and centre conductor to avoid stiction and provide low impedance path between the electrodes. When an actuation voltage is applied between the electrodes, the metal bridge acquires pull in effect as it crosses one third of distance between them. In this study, we describe behavior of pull-in voltage and figure of merit (or capacitance ratio) of capacitive MEMS switch for five different dielectric materials. The effects of dielectric thicknesses are also considered to calculate the values of pull-in-voltage and capacitance ratio. This work shows that a reduced pull-in-voltage with increase in capacitance ratio can be achieved by using dielectric material of high dielectric constant above the central line of CPW.

Design and characterization of a tunable patch antenna loaded with capacitive MEMS switch using CSRRs structure on the patch

In this paper, the design and characterization of a tunable patch antenna loaded with complementary split ring resonators (CSRRs) on the patch have been realized. To achieve tunable resonant frequency, bandwidth and radiation pattern, the antenna is further loaded with coplanar waveguide (CPW) on which Microelectromechanical System (MEMS) capacitive switches are placed periodically. The tunable property is achieved, when the switches moves from up state with the capacitive gap 1.5μm to down state having capacitive gap of 1μm. A parametric analysis has been presented to check the sensitivity of the antenna in terms of S11 parameter by varying different parameters of the MEMS switches and CSRRs. This work, strives to improve the degree of reconfigurability with increase in the number of switches. The value of actuation voltage to move switch from up to down state is 10.4V, which is very low over the other design. The switches exhibit fundamental frequency 14.6kHz, switching time 28.59μs, and capacitance ratio 15.27. Simulation has been carried out in Ansoft HFSS v. 13 and the distinct characterization property of the tunable antenna is shown through simulation.

Miniatured Phase Shifter

Objective: RF MEMS has emerged as a technology with immense potential for wireless communication and defence applications. In this work, a 5 bit RF MEMS switched line phase shifter is presented using cantilever switch achieving low actuation voltage. Methodology: In this work 5 different phase shifters are designed to get 5 bit phase shift at a particular frequency 10 GHz. The phase shifters consist of stubs like structure designed in the ground of CPW and MEMS capacitive series switches. Solid-state phase shifters based on p-i-n diodes or Field-Effect Transistor (FET) switches are used because they provide a good planar solution at a variety of microwave frequency ranges. Micro Electro Mechanical Systems (MEMS) phase shifters are progress today are mainly have established designs because of which the solid-state switch is replaced by a MEMS switch. Findings: By analysing theoretically and practically the results showed that a novel RF MEMS switch was capable of liberating less than 0.1dB insertion loss and more than 19 dB isolation between input and output ports at 20 GHz .This RF MEMS switch shows a 5V actuation voltage. Switching time of RF MEMS switches is measured using a theoretical approach. The switches demonstrated maximum switching time of 0.1μs. By using this switch a 5-bit miniaturized phase shifter is designed i.e. 180° bit, 90°, 45°, 22.25° and 11.5° bits were achieved using the switched line type phase shifters at 10 GHz. A novel switched line type phase shifter loaded with multiple switch pairs was developed and was capable of liberating a multiple phase shift angles within single design

A High Power Stress-Gradient Resilient RF MEMS Capacitive Switch

A high power-handling radio frequency (RF) microelectromechanical systems capacitive switch with low sensitivity to both in-plane stress and stress gradients is presented. The novel switch consists of four cantilever beams, which are tied together using an optimized center joint to minimize the effects of biaxial stress and stress gradients. Dimples with a thickness of 0.3 μm are used to result in an air dielectric in the down-state position, which greatly improves the reliability, while still maintaining a measured capacitance ratio of 5. The switch is capable of handling an RF power >12 W under hot switching conditions, as well as having low sensitivity to temperature variations (~20 mV/°C for V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> ). Application areas are in high power reconfigurable filters, matching networks, and tunable antennas for modern communication systems.

Identification of the transient stress-induced leakage current in silicon dioxide films for use in microelectromechanical systems capacitive switches

Dielectric charging at low electric fields is characterized on radio-frequency microelectromechanical systems (RF MEMS) capacitive switches. The dielectric under investigation is silicon dioxide deposited by plasma enhanced chemical vapor deposition. The switch membrane is fabricated using a metal alloy which is shown to be mechanically robust. In the absence of mechanical degradation, these capacitive switches are appropriate test structures for the study of dielectric charging in MEMS devices. Monitoring the shift and recovery of device capacitance-voltage characteristics revealed the presence of a charging mechanism which takes place across the bottom metal-dielectric interface. Current measurements on metal-insulator-metal devices confirmed the presence of interfacial charging and discharging transient currents. The field- and temperature-dependence of these currents is the same as the well-known transient stress-induced leakage current (SILC) observed in flash memory devices. A simple model was created based on established transient SILC theory which accurately fits the measured data and reveals that charge exchange at the bottom metal-dielectric interface is responsible for charging currents and pull-in voltage changes in these MEMS devices.

Effect of Environmental Humidity on Dielectric Charging Effect in RF MEMS Capacitive Switches Based on $C$– $V$ Properties

A capacitance-voltage (C- V) model is developed for RF microelectromechanical systems (MEMS) switches at upstate and downstate. The transient capacitance response of the RF MEMS switches at different switch states was measured for different humidity levels. By using the C -V model as well as the voltage shift dependent of trapped charges, the transient trapped charges at different switch states and humidity levels are obtained. Charging models at different switch states are explored in detail. It is shown that the injected charges increase linearly with humidity levels and the internal polarization increases with increasing humidity at downstate. The speed of charge injection at 80% relative humidity (RH) is about ten times faster than that at 20% RH. A measurement of pull-in voltage shifts by C- V sweep cycles at 20% and 80 % RH gives a reasonable evidence. The present model is useful to understand the pull-in voltage shift of the RF MEMS switch.

Intelligent Bipolar Control of MEMS Capacitive Switches

Closed-loop control of microelectromechanical systems (MEMS) capacitive switches was demonstrated by using an intelligent CMOS circuit. The control was based on fine tuning the bias magnitude of the switches according to the difference between sensed and targeted capacitances. Innovative designs were used to allow the CMOS circuit to sense low capacitance and to handle high voltage. The CMOS die of 3 × 1.5 mm2 was dominated by input/output and voltage regulation/protection circuits; the actual capacitance sense/control circuit was smaller than 0.1 mm . The entire circuit consumed 0.7 mW of power during active sense/control, which could be significantly reduced with less frequent sense/control and advanced CMOS technology. With a maximum actuation voltage of ±40 V and a target capacitance of 0.5 pF, a control accuracy of ±2.5% was demonstrated, which could be improved to ±1% with reduced parasitics through monolithic integration. Intelligence was programmed to alternate the bias sign when its magnitude required to maintain the targeted capacitance drifted significantly due to charging of the switch dielectric. Such intelligent control could also be used to compensate for process variation, material creep, ambient temperature change, and RF power loading, which would make MEMS capacitive switches not only more reliable, but also more robust.

Fast RF-CV Characterization Through High-Speed 1-port S-Parameter Measurements

We present a fast radio frequency-capacitance-voltage (RF-CV) method to measure the CV relation of an electronic device. The approach is more accurate, much faster, and more cost effective compared to the existing off-the-shelf solutions. Capacitances are determined using a single-frequency 1-port S-parameter setup constructed from discrete components. We introduce a new way to correct for nonlinearities of the used components, which greatly increases the accuracy with which the phase and magnitude of the reflected signal is measured. The measurement technique is validated on an RF microelectromechanical systems capacitive switch and a barium-strontium-titanate tunable capacitor. Complete CV curves are measured in less than a millisecond, with a measurement accuracy well below 1%.

A Physics-Based Predictive Modeling Framework for Dielectric Charging and Creep in RF MEMS Capacitive Switches and Varactors

In this paper, we develop a physics-based theoretical modeling framework to predict the device lifetime defined by the dominant degradation mechanisms of RF microelectromechanical systems (MEMS) capacitive switches (i.e., dielectric charging) and varactors (i.e., creep), respectively. Our model predicts the parametric degradation of performance metrics of RF MEMS capacitive switches and varactors, such as pull-in/pull-out voltages, pull-in time, impact velocity, and capacitance both for dc and ac bias. Specifically, for dielectric charging, the framework couples an experimentally validated theoretical model of time-dependent charge injection into the bulk traps with the Euler-Bernoulli equation for beam mechanics to predict the effect of dynamic charge injection on the performance of a capacitive switch. For creep, we generalize the Euler-Bernoulli equation to include a spring-dashpot model of viscoelasticity to predict the time-dependent capacitance change of a varactor due to creep. The new model will contribute to the reliability aware design and optimization of the capacitive MEMS switches and varactors.

Direct measurements and numerical simulations of gas charging in microelectromechanical system capacitive switches

Gas breakdown in microelectromechanical system capacitive switches is demonstrated using high resolution current measurements and by particle-in-cell/Monte Carlo collision (PIC/MCC) simulations. Measurements show an electric current through a 3 μm air gap increasing exponentially with voltage, starting at 60 V. PIC/MCC simulations with Fowler-Nordheim [Proc. R. Soc. London, Ser. A 119, 173 (1928)] field emission reveal self-sustained discharges with significant ion enhancement and a positive space charge. The effective ion-enhanced field emission coefficient increases with voltage up to about 0.3 with an electron avalanche occurring at 159 V. The measurements and simulations demonstrate a charging mechanism for microswitches consistent with earlier observations of gas pressure and composition effects on lifetime.