Capacitive RF Microelectromechanical Systems Research Articles

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.

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.

Design of Novel Capacitive RF MEMS Shunt Switch with Aluminum Nitride (AlN) Dielectric

In this paper, design of novel capacitive RF microelectromechanical systems (MEMS) shunt switch with butterfly structured beam is presented to reduce the actuation voltage. With aluminum nitride (AlN) dielectric layer the RF characteristics of designed switch renders the excellent performance over 5GHz to 60GHz frequency. The reported capacitance ratio of switch maximum 125 at 3 um air gap. The realized switch exhibit low actuation voltages, in the range of 3-10 volts, an insertion loss of 0.5dB, and return loss of greater than 17dB over operating frequency range 5-60GHz, and an isolation loss greater than 20dB over operating frequency range 10-60 GHZ. The characterization of proposed switch actuation voltage has been compared with straight and meander type supported beams and at a different air gap. Improved isolation loss of aluminium dielectric layered based switch has been compared with silicon dioxide (SiO2) and silicon nitride (Si3N4) dielectric layers. The electromechanical analysis and electromagnetic analysis simulations ware carried out by 3D MEMS commercial software package, Intellisuite and full wave electromagnetic simulator, Ansoft HFSS respectively for designed switches.

RF MEMS Capacitive Switches for Wide Temperature Range Applications Using a Standard Thin-Film Process

In this paper, the design, fabrication, and measurements of a capacitive RF microelectromechanical systems (RF MEMS) switch with very low sensitivity to thermal stress is presented. The switch is built by thin-film technology (0.8-μm Au) and shows <;50-mV/°C variation in the pull-down voltage at 25 °C-125 °C. The effects of the residual biaxial stress and stress gradients are studied in detail and the final switch design is very tolerant to a wide range of stress. The switch exhibits excellent RF and mechanical performances, and a capacitance ratio of about 51-57 (Cu = 54-66 fF, Cd = 3.1 - 3.4 pF) is reported. The mechanical resonant frequency and quality factor are fο = 105 - 115 kHz and Qm ≈ 8, respectively, with a measured switching time of about 3-3.8 μs. The applications areas are in low-loss RF MEMS, phase shifters, and reconfigurable networks.

Capacitive RF MEMS Switches With Tantalum-Based Materials

In this paper, shunt capacitive RF microelectromechanical systems (MEMS) switches are developed in III-V technology using tantalum nitride (TaN) and tantalum pentoxide (Ta <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> ) for the actuation lines and the dielectric layers, respectively. A compositional, structural, and electrical characterization of the TaN and Ta <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> films is preliminarily performed, demonstrating that they are valid alternatives to the conventional materials used in III-V technology for RF MEMS switches. Specifically, it is found that the TaN film resistivity can be tuned from 0.01 to 30 Ω · cm by changing the deposition parameters. On the other hand, dielectric Ta <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> films show a low leakage current density of few nanoamperes per square centimeter for E ~ 1 MV/cm, a high breakdown field of 4 MV/cm, and a high dielectric constant of 32. The realized switches show good actuation voltages, in the range of 15-20 V, an insertion loss better than -0.8 dB up to 30 GHz, and an isolation of ~-40 dB at the resonant frequency, which is, according to bridge length, between 15 and 30 GHz. A comparison between the measured S-parameter values and the results of a circuit simulation is also presented and discussed, providing useful information on the operation of the fabricated switches.

A systematic reliability investigation of the dielectric charging process in electrostatically actuated MEMS based on Kelvin probe force microscopy

This paper presents a comprehensive investigation for the dielectric charging problem in electrostatically actuated microelectromechanical system (MEMS) devices. The approach is based on Kelvin probe force microscopy (KPFM) and targets, in this specific paper, thin PECVD silicon nitride films for electrostatic capacitive RF MEMS switches. KPFM has been employed in order to mimic the potential induced at the dielectric surface due to charge injection through asperities. The effect of dielectric thickness has been investigated through depositing SiNx films with different thicknesses. Then, in order to simulate the different scenarios of dielectric charging in real MEMS switches, SiNx films have been deposited over thermally grown oxide, evaporated gold and electroplated gold layers. Also, the effect of the deposition conditions has been investigated through depositing dielectric films using low and high frequency PECVD methods. The investigation reveals that thin dielectric films have larger relaxation times compared to thick ones when the same injection bias is applied, independently of the substrate nature. For the same SiNx film thickness, the decay time constant is found to be smaller in dielectric films deposited over metallic layers compared to the ones deposited over silicon substrates. Finally, the material stoichiometry is found to affect the surface potential distribution as well as the relaxation time constant.

Charge Carriers Injection/Extraction at the Metal-Polymer Interface and Its Influence in the Capacitive Microelectromechanical Systems-Switches Actuation Voltage

Opposite results concerning the sign of the parasitic charge accumulated at the metal dielectric contact in RF microelectromechanical systems (MEMS) capacitive switches are found in the literature. The mechanism concerning charge injection/extraction at the metal-dielectric contact and its influence on the pull-in voltage needs to be further clarified. A model-switch, for which only one dimension is in the microns range, is used to study the behaviour of a capacitive RF MEMS switch. The aim is to analyze how the electric charge is injected/extracted into or from the dielectric material under the applied field and to obtain realistic data to understand how this parasitic charge influences the pull-in voltage Vpi and the pull-off voltage Vpo. A triangle voltage is employed to measure Vpi and Vpo, by measuring the isothermal charging/discharging currents. Our results demonstrate that Vpi is strongly dependent on the injected/extracted charge on the free surface of the dielectric. The charge injected/extracted at the bottom side of the dielectric has no influence on the actuation voltage. The charge injected/extracted on the free surface of the dielectric determines an increase of the modulus of Vpi and, eventually, the switch can fail to actuate. An estimation of the charge stored into the material was obtained (i) by measuring the charging current and the discharging current and (ii) from the value of the Vpi. The parasitic charge necessary to keep the bridge stick to the insulator is 5.3 x 10(-4) C m(-2) for our experimental conditions. The modification of the Vpi determined by the stored charge in the dielectric is analyzed. An increase of the relative dielectric permittivity by a factor of 2 produces a decrease of the actuation voltage of 10%. A variation of 30% in the elastic constant determines a variation of about 20% in the Vpi. A voltage threshold for charge injection/extraction was not observed.

Capacitive RF MEMS Switches Fabricated in Standard 0.35-$\mu{\hbox{m}}$ CMOS Technology

The objective of this paper is to investigate the integration of capacitive type RF microelectromechanical systems (MEMS) switches in a standard CMOS technology. A maskless monolithic integration process dedicated to electrostatically actuated capacitive type RF MEMS switches is developed and optimized. The fabricated switches consist of composite metal-dielectric warped membranes. The warped-plate structure is used to increase the capacitance ratio of the switch. The switches are fabricated using the interconnect metal and dielectric layers available in a standard 0.35-μm CMOS process. Measurement results for the first switch show an insertion loss less than 0.98 dB, a return loss below 13 dB up to 20 GHz in the up-state, and a down-state isolation of 12.4-17.9 dB from 10 to 20 GHz. The capacitance ratio is enhanced up to 91:1 using the warped-plate structure. A second cascaded switch consisting of two shunt capacitive switches and a slow-wave high-impedance transmission line section is designed and fabricated for high-isolation applications. The measured insertion loss for this switch is less than 1.41 dB up to 20 GHz, the return loss is below 19 dB, and the isolation is 19-40 dB across the frequency band from 10 to 20 GHz. The proposed RF MEMS switches can be used in millimeter-wave CMOS RF front-ends where multiband functionality and reconfigurability is required.

High-$Q$ RF-MEMS 4–6-GHz Tunable Evanescent-Mode Cavity Filter

This paper presents a miniature high-Q tunable evanescent-mode cavity filter using planar capacitive RF microelectromechanical system (MEMS) switch networks and with a frequency coverage of 4.07-5.58 GHz. The two-pole filter, with an internal volume of 1.5 cm3, results in an insertion loss of 4.91-3.18- and a 1-dB bandwidth of 17.8-41.1 MHz, respectively, and an ultimate rejection of > 80 dB. RF-MEMS switches with digital/analog tuning capabilities were used in the tunable networks so as to align the two poles together and result in a near-ideal frequency response. The measured Qu of the filter is 300-500 over the tuning range, which is the best reported Q using RF-MEMS technology. The filter can withstand an acceleration of 55-110 g without affecting its frequency response. The topology can be extended to a multiple-pole design with the use of several RF-MEMS tuning networks inside the evanescent-mode cavity. To our knowledge, these results represent the state-of-the-art in RF-MEMS tunable filters.

A Self-Aligned Fabrication Process for Capacitive Fixed-Fixed Beam RF MEMS Components

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A self-aligned fabrication process for capacitive fixed-fixed beam RF microelectromechanical system (MEMS) components is disclosed. It enables the scaling of the critical dimensions and reduces the number of processing steps by 40% compared with a conventional RF MEMS fabrication process. RF MEMS varactors with beam lengths of 30 <formula formulatype="inline"><tex>${\rm \mu}\hbox{m}$</tex></formula> are demonstrated by using the self-aligned fabrication process, and the performance of a four-by-four RF MEMS varactor bank is discussed as well. At 20 GHz, the measured capacitance values range between 180.5 and 199.2 fF. The measured capacitance ratio is 1.15 when a driving voltage of 35 V is applied, and the measured loaded <formula formulatype="inline"> <tex>${Q}$</tex></formula> -factor ranges between 14.5 and 10.8. The measured cold-switched power handling is 200 mW. <formula formulatype="inline"><tex>$\hfill$</tex> </formula>[2008-0018] </para>

RF MEMS asymmetric capacitive switch with high‐isolation at selected low‐microwave frequency

AbstractAn asymmetric RF microelectromechanical systems (MEMS) capacitive switch with a novel architecture employing mode switching from a coplanar waveguide (CPW) mode to a coupled slotline (CSL) mode is reported for selected frequency within the 2–10 GHz low microwave frequency range. An additional metal‐insulator‐metal (MIM) capacitor is introduced on one end of the switch membrane as a tuning capacitor. The signal line of CSL is used as a tuning inductor. These two tuning elements together can improve the isolation of a regular capacitive RF MEMS switch by about 20 dB at the induced additional low resonant frequency ranging from 2 to 10 GHz. The measurements of the fabricated asymmetric switch demonstrated excellent isolation improvements. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 702–706, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22235

Reliability modeling of capacitive RF MEMS

The kinetic of dielectric charging in capacitive RF microelectromechanical systems (RF MEMS) is investigated using an original method of stress and monitoring. This effect is investigated through a new parameter: the shift rate of the actuation voltages. We demonstrate that this lifetime parameter has to be considered as a function of the applied voltage normalized by the contact quality between the bridge and the dielectric. We also demonstrate that this phenomenon is related to Frenkel-Poole conduction, which takes place into the dielectric. We finally propose a model that describes the dielectric charging kinetic in capacitive RF MEMS. This model is used to extract a figure-of-merit of capacitive switches lifetime.

Temperature study of the dielectric polarization effects of capacitive RF MEMS switches

This paper investigates both theoretically and experimentally the dielectric charging effects of capacitive RF microelectromechanical system switches with silicon nitride as dielectric layer. Dielectric charging caused by charge injection under voltage stress was observed. The amphoteric nature of traps and its effect on the switch operation were confirmed under both positive and negative control voltages. It has been confirmed that charging is a complicated process, which can be better described through the stretched exponential relaxation. This mechanism is thermally activated with an activation energy being calculated from the temperature dependence of the capacitance transient response. The charging mechanism, which is responsible for the pull-out voltage and the device failure, is also responsible for the temperature-induced shift of the capacitance minimum bias.