Microelectromechanical resonators for radio frequency communication applications

Over the past few years, microelectromechanical system (MEMS) based on-chip resonators have shown significant potential for sensing and high frequency signal processing applications. This is due to their excellent features like small size, large frequency-quality factor product, low power consumption, low cost batch fabrication, and integrability with CMOS IC technology. Radio frequency communication circuits like reference oscillators, filters, and mixers based on such MEMS resonators can be utilized for meeting the increasing count of RF components likely to be demanded by the next generation multi-band/multi-mode wireless devices. MEMS resonators can provide a feasible alternative to the present-day well-established quartz crystal technology that is riddled with major drawbacks like relatively large size, high cost, and low compatibility with IC chips. This article presents a survey of the developments in this field of resonant MEMS structures with detailed enumeration on the various micromechanical resonator types, modes of vibration, equivalent mechanical and electrical models, materials and technologies used for fabrication, and the application of the resonators for implementing oscillators and filters. These are followed by a discussion on the challenges for RF MEMS technology in comparison to quartz crystal technology; like high precision, stability, reliability, need for hermetic packaging etc., which remain to be addressed for enabling the inclusion of micromechanical resonators into tomorrow??s highly integrated communication systems.     

A new approach for the design of low velocity coupling for ring shape anchored contour mode disk resonators

The design of RF MEMS filters is a very important research area in the RF MEMS resonator field especially for UHF frequencies. Having very high Q and compatibility with the state-of-the-art CMOS technology, RF MEMS resonators could construct channel select filters in RF transceivers front-end and surmount the need for IF stages by direct conversion. But, there is a problem for coupling of high stiff contour mode disk resonators, because there is no nodal region at the perimeter of this kind of resonators for establishing the low velocity coupling without applying asymmetry on the resonance performance of the resonators. This paper introduces a pioneering technique for low velocity coupling of these resonators without any asymmetry. Analytical design approach and FEM analysis are provided in this paper and our discussions are verified by various simulations. The resulting filter could be designed to obtain 0.004?% of bandwidth with reasonable sizes which is completely enough for channel selection in low GSM standard.     

Design of wide range MEMS tunable capacitor for RF applications

The RF applications like voltage controlled oscillators, tunable filters, resonators etc., requires tunable capacitors in their designs. This paper presents the design of wide range MEMS tunable capacitors for RF applications. This design consists of an air suspended bottom plate and a fixed top plate. The top fixed plate and the suspended bottom plate form the tunable capacitor. The capacitance range of this tunable capacitor is from 69.172 to 138.344?nF. This range is wider compared with the conventional MEMS tunable capacitors of tuning ranges in pico Farads. The fabrication process is similar to that of the existing standard integrated circuit fabrication processes, which makes this design suitable for integrated RF applications.     

MEMS based humidity sensor using Si cantilever beam for harsh environmental conditions

The RF applications like voltage controlled oscillators, tunable filters, resonators etc., requires tunable capacitors in their designs. This paper presents the design of wide range MEMS tunable capacitors for RF applications. This design consists of an air suspended bottom plate and a fixed top plate. The top fixed plate and the suspended bottom plate form the tunable capacitor. The capacitance range of this tunable capacitor is from 69.172 to 138.344 nF. This range is wider compared with the conventional MEMS tunable capacitors of tuning ranges in pico Farads. The fabrication process is similar to that of the existing standard integrated circuit fabrication processes, which makes this design suitable for integrated RF applications.     

Analysis of Frequency Locking in Optically Driven MEMS Resonators

Thin, planar, radio frequency microelectromechanical systems (MEMS) resonators have been shown to self-oscillate in the absence of external forcing when illuminated by a direct current (dc) laser of sufficient amplitude. In the presence of external forcing of sufficient strength and close enough in frequency to that of the unforced oscillation, the device will become frequency locked, or entrained, by the forcing. In other words, it will vibrate at the frequency of the external forcing. Experimental results demonstrating entrainment for a disk-shaped oscillator under optical and mechanical excitation are reviewed. A thermomechanical model of the system is developed and its predictions explored to explain and predict the entrainment phenomenon. The validity of the model is demonstrated by the good agreement between the predicted and experimental results. The model equations could also be used to analyze MEMS limit-cycle oscillators designed to achieve specific performance objectives     

Comparative analysis of a variety of high-Q capacitively transduced bulk-mode microelectromechanical resonator geometries

Microelectromechanical system (MEMS) based on-chip resonators offer great potential for sensing and high frequency signal processing applications due to their exceptional features like small size, large frequency-quality factor product, integrability with CMOS ICs, low power consumption etc. Capacitively transduced MEMS resonators are in general favored than their piezoelectrically transduced counterparts. Also among the former variety of microresonators, bulk acoustic mode of vibration is the preferred option for realizing high frequency of operation. So this study focuses on the design, simulation and optimization of some new as well as previously reported geometries of the particular variety of bulk-mode micromachined resonators based on capacitive transduction. A low motional resistance has been attempted for these resonators, which can make them ideal for use in radio frequency communication circuits like reference oscillators and filters.     

Perturbed Sierpinski carpet resonator and its usage in compact dual and triple band bandpass filters

In this paper, perturbed Sierpinski carpet fractal shaped resonator is characterized and applied in the design of multiband band pass filters (BPFs). The route to achieving compact multimode resonators starting from the Sierpinski carpet fractal shaped resonator is detailed. The proposed resonators are used to demonstrate two topologies of dual band bandpass filters with passband center frequencies at 3.5 GHz as well as 5.5 GHz, respectively. Both the designs exhibit nearly identical passband bandwidth. In the second design, it is observed that the second passband gets slightly shifted towards 5.85 GHz. However, with edge loading this shift is nullified. A scaled down version of the same resonator is designed and three such resonators are interdigitally coupled in conjunction with complementary split ring resonators to exhibit a novel technique of achieving compact triband bandpass filter. All designs have acceptable passband insertion loss in the respective bands. Accessories are used to improve passband selectivity in all designs and also a wide stop band till 10 GHz is obtained. Fabrication prototypes for all the variants are realized with simulated and measured results in good agreement. © 2016 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:418–425, 2016.     

Enhanced waveguide bandpass filters using S‐shaped resonators

This article presents a new solution for stopband performance improvement of rectangular waveguide bandpass filters using S‐shaped resonator loaded waveguide configurations at microwave and millimeter‐wave frequencies. The proposed filter structure is compact in size when comparing with the standard E‐plane counterpart. Compactness is achieved by taking advantage of the properties of slow wave effect in half wavelength resonators. Periodicity is readily imposed upon cascading the S‐shaped resonators within the rectangular waveguide. The structure is simple and compatible with E‐plane technology. This type of bandpass filters can be easily realized with a single metallo‐dielectric insert within a standard rectangular waveguide. Simulation and experimental results are presented to validate the argument along with some design guidelines. © 2009 Wiley Periodicals, Inc. Int J RF and Microwave CAE 2009.     

Electroplated nickel based micro-machined disk resonators for high frequency applications

Microelectromechanical system (MEMS) based resonators can be used for filtering and frequency synthesis applications in many subcomponents of radio frequency wireless integrated circuits due to their small size, high level of frequency selectivity, low cost batch fabrication, ease of integration with CMOS circuits. Electroplated nickel is an attractive low cost material for CMOS compatible MEMS due to their low deposition temperatures. Among the different modes of vibration, radial-contour mode resonators are preferred for high frequency applications because they offer higher effective stiffness. Two different types of electroplated nickel based radial-contour bulk-mode circular disk resonator geometries which depend on capacitive actuation and readout technique is presented in this work. Material, mechanical and electrical characterizations were performed on these structures to show their functionality.     

MEMS switches controlled multi-split ring resonator as a tunable metamaterial component

Based on the multisplit-ring resonator (MSRR) with MEMS switches, a tunable metamaterial component is proposed in this paper to realize multiband applications. Numerical simulations are carried out to verify the tunable capacity of the proposed structure. The simulated results show that the resonance frequency of the metamaterial component shifts to higher frequencies when the MEMS switches are at different states, and depends strongly on the place, state and microbeam height of MEMS switches. Moreover, the large tunable range can be obtained by controlling the up state or down state of MEMS switches, while the small tunable range can be obtained by controlling microbeam height of MEMS switches. That is, such controlling ways can realize both rough and minor tunable metamaterial component. The tunable method proposed in this paper is of great practical values in designing tunable metamaterial and negative refractive index material.     

High-Q UHF micromechanical radial-contour mode disk resonators

A micromechanical, laterally vibrating disk resonator, fabricated via a technology combining polysilicon surface-micromachining and metal electroplating to attain submicron lateral capacitive gaps, has been demonstrated at frequencies as high as 829 MHz and with Q's as high as 23 000 at 193 MHz. Furthermore, the resonators have been demonstrated operating in the first three radial contour modes, allowing a significant frequency increase without scaling the device, and a 193 MHz resonator has been shown operating at atmospheric pressure with a Q of 8,880, evidence that vacuum packaging is not necessary for many applications. These results represent an important step toward reaching the frequencies required by the RF front-ends in wireless transceivers. The geometric dimensions necessary to reach a given frequency are larger for this contour-mode than for the flexural-modes used by previous resonators. This, coupled with its unprecedented Q value, makes this disk resonator a choice candidate for use in the IF and RF stages of future miniaturized transceivers. Finally, a number of measurement techniques are demonstrated, including two electromechanical mixing techniques, and evaluated for their ability to measure the performance of sub-optimal (e.g., insufficiently small capacitive gap, limited dc-bias), high-frequency, high-Q micromechanical resonators under conditions where parasitic effects could otherwise mask motional output currents. [1051].     

Accurate Simulation of RF MEMS VCO performance including phase noise

A new coupled circuit and electrostatic/mechanical simulator (COSMO) is presented for the design of low phase noise radio frequency (RF) microelectromechanical systems (MEMS) voltage-controlled oscillators (VCOs). The numerical solution of device level equations is used to accurately compute the capacitance of a MEMS capacitor. This coupled with a circuit simulator facilitates the simulation of circuits incorporating MEMS capacitors. In addition, the noise from the MEMS capacitor is combined with a nonlinear circuit-level noise analysis to determine the phase noise of RF MEMS VCO. Simulations of two different MEMS VCO architectures show good agreement with experimentally observed behavior.     

Modeling of the intrinsic stress effect on the resonant frequency of NEMS resonators integrated by beams with variable cross-section

Nano-electro-mechanical systems (NEMS) resonators integrated by a double clamped beam with variable cross-section are used in several applications such as chemical and biological detectors, high-frequency filters, and signal processing. The structure of these resonators can experience intrinsic stresses produced during their fabrication process. We present an analytical model to estimate the first bending resonant frequency of NEMS resonators based on a double clamped beam with three cross-sections, which considers the intrinsic stress effect on the resonant structure. This model is obtained using the Rayleigh and Macaulay methods, as well as the Euler–Bernoulli beam theory. We applied the analytical model to a silicon carbide (SiC) resonator of 186 nm thickness reported in the literature. This resonator has a total length ranking from 80 to 258 μm and is subjected to a tensile intrinsic stress close to 110 MPa. Results from this model show good agreement with experimental results. The analytic frequencies have a maximum relative difference less than 6.3% respect to the measured frequencies. The tensile intrinsic stress on the resonant structure causes a significantly increase on its bending resonant frequency. The proposed model provides an insight into the study of the intrinsic stress influence on the resonant frequency of this nanostructure. In addition, this model can estimate the frequency shift due to the variations of the resonator geometrical parameters.     

Reducing anchor loss In thin-film aluminum nitride-on-diamond contour mode MEMS resonators with support tethers based on phononic crystal strip and reflector

In this paper, we report on (1) the proposed support tether configuration based on phononic crystal (PnC) strip to reduce the anchor loss in the thin-film piezoelectric-on-diamond (TPoD) contour mode microelectromechanical systems (MEMS) resonators, namely the \(100^0\) oriented thin-film aluminum nitride (AlN)-on-diamond wine glass (WG) and width shear (WS) mode MEMS resonators, (2) impacts of the geometrical dimensions on the band gaps of the proposed PnC structure, (3) evaluation of the performance of the Q improvement between the proposed support tether configuration and the one based on the reflector presented by B P Harrington and R Abdolvand. The designed resonators operate at approximately 115 and 156 MHz corresponding with the WG and WS modes. The band gap width covering these two operating frequencies is 30.2 and 20.6 MHz, respectively. The maximum Q of the WG mode resonator with five-unit cell PnC strip based support tethers achieves up to 398.5 % over that of the same resonator with reflector based support tethers. Similar to the WS mode one with three-unit cell PnC strip based support tethers, this Q is up to 591.1 %. The average Q of the WG mode resonator with PnC strip based support tethers over the maximum Q of the same one with different reflectors is enhanced about 964.4, 952.9, 364.2 and 4599.5 %. For the WS mode resonator, these Q values are up to 2388.8, 1830.4, 528.2 and 2384.5 %. The finite element (FE) analysis in COMSOL Multiphysics software (COMSOL) is utilized to simulate the proposed PnC strip as well as the WG and WS mode resonators.     

Novel SIS resonators waveguide filters

A novel stepped‐impedance slot (SIS) resonator waveguide band‐pass filter with quarter‐wave couplings is investigated. SIS resonators have been studied in order to increase the stopband width between the fundamental and spurious harmonic resonant frequencies. They have been employed in the design of waveguide band‐pass filters with good harmonic suppression. © 2005 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2005.     

A High-Q Widely Tunable Gigahertz Electromagnetic Cavity Resonator

This paper describes the design, fabrication and testing of a quasi-static electromagnetic cavity resonator fabricated using potassium-hydroxide (KOH) etching, shallow reactive-ion etching (RIE), metalization and wafer bonding. The resonator is distinguished by its simultaneous high-Q near 200, and wide high-frequency tuning range, 2.5-4.0 GHz for the experimental resonator presented here. When combined with an integrated actuator, it should be suitable for use in electronically tunable radio-frequency (RF) bandpass filters and oscillators. The experimental resonator, however, is tuned with an external piezoelectric actuator for simplicity     

Design of a multi-frequency resonator for UHF multiband communication applications

A new structure for RF MEMS resonators is demonstrated in UHF range. The presented resonator has the ability to work in multiple resonance modes by its specific anchor design. Also, its special electrodes are designed so that the desired mode at any time could be excited by embedding simple MOS switches. Since the resonance frequency of different modes could be adjusted by the inner radius of the resonator, the structure could be designed for wide range of arbitrary frequency differences between modes. Specific electrodes and anchors design guarantees low insertion loss and high quality factor in all modes. Modal and harmonic analysis verify our approach.     

Design and development of a MEMS butterfly resonator using synchronizing beam and out of plane actuation

This paper presents beam modeling techniques for maximizing mechanical sensitivity of a butterfly resonator for gyroscopic applications. We investigate the geometric aspects of synchronizing beam that connects the wings of a butterfly resonator. Our results show that geometric variation in the synchronizing beam can have a large effect on the frequency split and sensitivity of the device. The model simulation shows a sensitivity of \( 10^{ - 12} \)\( (m/^\circ /s) \) for a frequency split of 10 Hz resulting from the optimized synchronized beam. Out of plane actuation was developed to drive and sense the resonators displacement. Fabricated butterfly resonators were tested, and the experimental results show a frequency split of 305 Hz and 400 Hz while the model illustrated a split of 195 Hz and 330 Hz respectively. The design and analysis presented in this paper can further aid the development of MEMS butterfly resonators for inertial sensing applications.

     

Analysis and suppression of spurious modes of the ring shape anchored RF MEMS contour mode disk resonator

One of the most encumbering issues in RF MEMS resonators is spurious modes. The problem of spurious modes becomes more critical, when the ring type resonators are used. In the ring shape anchored contour mode disk resonator, for achieving a low serial resistance, the inner radius of the disk must be increased. This causes the spurious modes to become too close to the desired mode and degrade the operation of the resonator. In this work, spurious modes of before-mentioned device are introduced and their characteristics are evaluated by exact analytical approach. Based on those analytical approaches, we introduce two techniques for spurious mode suppression. The first technique is based on exciting the desired mode by proper electrode engineering and hence is an electrical approach. The second technique is reconfiguring of the anchor from a continuous ring to crossed ring segments and locating the segments on the phase discriminating lines to increase the insertion loss for spurious modes and decrease the losses for the fundamental mode. The final harmonic analysis verifies that the proposed techniques result a resonator with a pure frequency spectrum and spurious modes excluded over a very wide frequency range.     

Rotationally symmetric FDTD for wideband performance prediction of TM01 DR filters

The generalized perfectly matched layer (GPML) coupled with rotationally symmetric (RS)‐FDTD method has been utilized to extract the S‐parameters for several probe‐coupled TM₀₁ dielectric resonator (DR) filters to directly obtain the theoretical wideband spurious performance. The computationally efficient (RS)‐FDTD method has also been used to obtain accurate filter parameters for TE₀₁ and TM₀₁ dielectric resonators loaded in cylindrical cavities. The RS‐FDTD method combined with digital filtering and the Matrix Pencil technique are used to analyze the resonant frequencies, inter‐resonator coupling, and external Q values. When perturbation theory is used with RS‐FDTD, accurate values of unloaded Q are obtained. © 2002 Wiley Periodicals, Inc. Int J RF and Microwave CAE 12: 259–271, 2002.