Microelectromechanical Systems Filters Research Articles

A mechanically coupled MEMS filter with high-Q width extensional mode resonators

This work presents a novel radio frequency (RF) narrowband Si micro-electro-mechanical systems (MEMS) filter based on capacitively transduced slotted width extensional mode (WEM) resonators. The flexibility of the plate leads to multiple modes near the target frequency. The high Q-factor resonators of around 100 000 enable narrow bandwidth filters with small size and simplified design. The 1-wavelength and 2-wavelength WEMs were first developed as a pair of coupled modes to form a passband. To reduce bandwidth, two plates are coupled with a λ-length coupling beam. The 79.69 MHz coupled plate filter (CPF) achieved a narrow bandwidth of 8.8 kHz, corresponding to a tiny 0.011%. The CPF exhibits an impressive 34.84 dB stopband rejection and 7.82 dB insertion loss with near-zero passband ripple. In summary, the RF MEMS filter presented in this work shows promising potential for application in RF transceiver front-ends.

Design and Fabrication of a MEMS Bandpass Filter with Different Center Frequency of 8.5-12 GHz.

The design simulation and fabrication results of a bandpass filter based on micro-electro-mechanical system (MEMS) switches are presented in this paper. The MEMS filter element consists of a MEMS capacitance switch and two resonant rings that are fixed onto coplanar waveguide lines through anchor points. The micromachine characteristics of the filter could be optimized to change the center frequency from 8.5 to 12 GHz by improving the geometrical parameters; other electrical parameters of the filter, such as stopband rejection, insertion loss, and return loss at each center frequency, were simulated and calculated. In order to evaluate the MEMS filter design methodology, a filter working at 10.5 GHz fabricated with an aluminum top electrode was used, and it displayed a low insertion loss of 1.12 dB and a high stopband rejection of 28.3 dB. Compared with the simulation results, these proposed filter showed better electrical performance. Our results demonstrated that the filter with the integrated RF MEMS switch not only provides the benefit of reduced size compared with a traditional filter, but also improves stopband rejection, insertion loss, and return loss.

Identification of MEMS Geometric Uncertainties through Homogenization

Fabrication imperfections strongly influence the functioning of Micro-Electro-Mechanical Systems (MEMS) if not taken into account during the design process. They must be indeed identified or precisely predicted to guarantee a proper compensation during the calibration phase or directly in operation. In this work, we propose an efficient approach for the identification of geometric uncertainties of MEMS, exploiting the asymptotic homogenization technique. In particular, the proposed strategy is experimentally validated on a MEMS filter, a device constituted by a complex periodic geometry, which would require high computational costs if simulated through full-order models. The complex periodic structure is replaced by an equivalent homogeneous medium, allowing a fast optimization procedure to identify imperfections by comparing a simplified analytical model with the experimental data available for the MEMS filter. The actual over-etch, obtained after the release phase, and the electrode offset of a fabricated MEMS filter are effectively identified through the proposed strategy.

A novel low-loss four-bit bandpass filter using RF MEMS switches

This paper details the design and simulation of a novel low-loss four-bit reconfigurable bandpass filter that integrates microelectromechanical system (MEMS) switches and comb resonators. A T-shaped reconfigurable resonator is reconfigured in a ‘one resonator, multiple MEMS switches’ configuration and used to gate the load capacitances of comb resonators so that a multiple-frequency filtering function is realized within the 7–16 GHz frequency range. In addition, the insertion loss of the filter is less than 1.99 dB, the out-of-band rejection is more than 18.30 dB, and the group delay is less than 0.25 ns. On the other hand, the size of this novel filter is only 4.4 mm × 2.5 mm × 0.4 mm. Our results indicate that this MEMS reconfigurable filter, which can switch 16 central frequency bands through eight switches, achieves a low insertion loss compared to those of traditional MEMS filters. In addition, the advantages of small size are obtained while achieving high integration.

Bending characteristics of radio frequency microelectromechanical system low‐pass filter based on flexible substrate

The aim of this paper is to investigate the bending characteristics of a low-pass RF MEMS (radio frequency microelectromechanical system) filter based on a flexible substrate. The proposed filter is achieved by complementary split ring resonator and MEMS bridge which are fabricated on a flexible LCP (liquid crystal polymer) substrate. The bending characteristics of the filter are revealed by theory, simulation and measurement. The experimental results demonstrate that the return loss of the filter deteriorates and the cut-off frequency of the filter decreases with the curvature of the flexible substrate increasing. The cut-off frequency is 38.5 GHz and the return loss is −22.7 dB at 10 GHz with the flat substrate. When the curvature of the substrate increases to 28.57 m–1, the cut-off frequency decreases to 14.8 GHz, and the return loss deteriorates to −5.7 dB at 10 GHz.

Thermal Conductivity of Aluminum Scandium Nitride for 5G Mobile Applications and Beyond.

Radio frequency (RF) microelectromechanical systems (MEMS) based on Al1-xScxN are replacing AlN-based devices because of their higher achievable bandwidths, suitable for the fifth-generation (5G) mobile network. However, overheating of Al1-xScxN film bulk acoustic resonators (FBARs) used in RF MEMS filters limits power handling and thus the phone's ability to operate in an increasingly congested RF environment while maintaining its maximum data transmission rate. In this work, the ramifications of tailoring of the piezoelectric response and microstructure of Al1-xScxN films on the thermal transport have been studied. The thermal conductivity of Al1-xScxN films (3-8 W m-1 K-1) grown by reactive sputter deposition was found to be orders of magnitude lower than that for c-axis-textured AlN films due to alloying effects. The film thickness dependence of the thermal conductivity suggests that higher frequency FBAR structures may suffer from limited power handling due to exacerbated overheating concerns. The reduction of the abnormally oriented grain (AOG) density was found to have a modest effect on the measured thermal conductivity. However, the use of low AOG density films resulted in lower insertion loss and thus less power dissipated within the resonator, which will lead to an overall enhancement of the device thermal performance.

Distributed strain and temperature sensing over 100 km using tunable-wavelength OTDR based on MEMS filters.

The possibility of distributed wide-range strain and temperature measurements in a 100 km long optical fiber using tunable-wavelength low-coherence optical time-domain reflectometer (OTDR) is demonstrated. The specified distance range is provided by employing two narrowband microelectromechanical system (MEMS) spectral filters tuned synchronously as well as by taking advantage of Raman amplification and amplification by remotely pumped erbium-doped fiber segments built into the fiber under test. With the time of a single measurement of 10 min and the spatial resolution of about 1 m, the measurement range reached 1000 µɛ in strain units, which is equivalent to the temperature range of 110°C.

Novel narrowband radio frequency microelectromechanical systems filters

This work presents three new narrowband radio frequency(RF) microelectromechanical systems (MEMS) filters made from mechanical or electrically coupled resonators with a quality factor Q of over 10 000. A low-stiffness rectangular-frame mechanical coupling beam (RFCB) is designed, and a filter with an RFCB is successfully developed with a center frequency of 75.5 MHz and a percent bandwidth (BW) of 0.4%. In addition, a tunable narrowband filter is achieved with a mechanically coupled four-disk linear structure filter. The center frequencies are 74.47 ± 0.54 MHz and the percent BW range is 0.48%–2.2%. In order to further simplify the architecture of the filter, a filter based on a single whisper gallery mode disk resonator is demonstrated, which has a center frequency of 79.42 MHz and a BW of only 0.25%. The three different RF MEMS filters have potential applications in wireless communication systems.

AlN MEMS filters with extremely high bandwidth widening capability

This paper presents radio frequency (RF) microelectromechanical system (MEMS) filters with extremely high bandwidth widening capability. The proposed filtering topologies include hybrid configurations consisting of piezoelectric MEMS resonators and surface-mounted lumped elements. The MEMS resonators set the center frequency and provide electromechanical coupling to construct the filters, while the lumped-element-based matching networks help widen the bandwidth (BW) and enhance the out-of-band rejection. Aluminum nitride (AlN) S0 Lamb wave resonators are then applied to the proposed filtering topologies. AlN S0 first- and second-order wideband filters are studied and have shown prominent performance. Finally, the AlN S0 first-order wideband filter is experimentally implemented and characterized. The demonstrated first-order filter shows a large fractional bandwidth (FBW) of 5.6% (achieved with a resonator coupling of 0.94%) and a low insertion loss (IL) of 1.84 dB. The extracted bandwidth widening factor (BWF) is 6, which is approximately 12 times higher than those of the current ladder or lattice filtering topologies. This impressive bandwidth widening capability holds great potential for satisfying the stringent BW requirements of bands n77, n78, and n79 of 5G new radio (NR) and will overcome an outstanding technology hurdle in placing 5G NR into the marketplace.

Design and Characterization Methodology for Efficient Wide Range Tunable MEMS Filters.

Here, we demonstrate the advantages of the laser Doppler vibrometer (LDV) over conventional techniques (the network analyzer), as well as the techniques to create an application-based microelectromechanical systems (MEMS) filter and how to use it efficiently (i.e., tuning the tuning-capability and avoiding both failure and stiction). LDV enables crucial measurements that are impossible with the network analyzer, such as higher mode detection (highly sensitive biosensor application) and resonance measurement for very small devices (fast prototyping). Accordingly, LDV was used to characterize the frequency tuning range and resonance frequency at different modes of the MEMS filters built for this study. This wide range frequency tuning mechanism is based simply on Joule heating from the embedded heaters and relatively high thermal stress with respect to the temperature of a fixed-fixed beam. However, we demonstrate that another limitation of this method is the resulting high thermal stress, which can burn the devices. Further improvement was achieved and shown for the first time in this study, such that the tuning capability was increased by 32% through an increase in the applied DC bias voltage (25 V to 35 V) between the two adjacent beams. This important finding eliminates the need for the extra Joule heating at the wider frequency tuning range. Another possible failure is through stiction and requirement of structure optimization: We propose a simple and easy technique of low frequency square wave signal application that can successfully separate the beams and eliminates the need for the more sophisticated and complicated methods given in the literature. The above findings necessitate a design methodology, and so we also provide an application-based design.

Harmonically-Driven Snapping of a Micromachined Bistable Mechanism With Ultra-Small Actuation Stroke

In this paper, we study the snapping action of a micromachined bistable mechanism harmonically driven with an ultra-small actuation stroke, which is realized through the mode amplification in a two-degrees-of-freedom (2-DOF) vibration system using an on-chip electrostatic actuator. The micromachined bistable mechanism is based on a curved beam configuration. The dynamic response of the device is theoretically predicted with the harmonic balance method. Further, to demonstrate this proposed study, a microelectromechanical systems (MEMS) prototype device is designed, fabricated, and tested. In experiments, the forward and backward state switching actions of the prototype device are realized with actuation stroke smaller than $0.4~\mu \text{m}$ , while the traveling distance between the two states is about $18~\mu \text{m}$ . To the best of our knowledge, we demonstrate for the first time the state switching of a cured beam bistable mechanism using harmonic driving in a 2-DOF vibration system. This paper has potential in applications including MEMS switches and MEMS filters. [2017-0221]

Wavelength-selective orbital-angular-momentum beam generation using MEMS tunable Fabry-Perot filter.

We demonstrate an on-chip device capable of wavelength-selective generation of vortex beams, which is realized by a spiral phase plate integrated onto a microelectromechanical system (MEMS) tunable filter. This vortex MEMS filter, being capable of functioning simultaneously in both wavelength and orbital-angular-momentum (OAM) domains at the 1550nm wavelength regime, is considered as a compact, robust, and cost-effective solution for simultaneous OAM- and wavelength-division multiplexed optical communications. The experimental OAM spectra for azimuthal orders 1, 2, and 3 show an OAM state purity >92% across a wavelength range of more than 30nm.

Lamb Wave AlN Micromechanical Filters Integrated With On-chip Capacitors for RF Front-End Architectures

This paper reports on the implementation of an ultraminiature 140 MHz narrowband filter based on aluminum nitride Lamb wave resonators. Monolithically integrated with a pair of on-chip capacitors and cascaded with a pair of inductors, the filter is well matched to 50 ohm, showing a remarkably high performance. A low pass-band insertion loss of 2.78 dB and steep filter skirts are achieved. The form factor of the monolithic microelectromechanical systems filter is more than ten times smaller than its surface acoustic wave counterpart in the intermediate frequency band and it involves much simpler matching circuits.

Drive-level dependence of doubly rotated langasite resonators with different configurations

The miniaturization of crystal resonators and filters toward the micro electromechanical systems (MEMS) and nano-structured scales demands improvement of nonlinear piezoelectricity theory and a better understanding of the nonlinear behavior of new crystal materials. The nonlinearities affect the quality factor and acoustic behavior of MEMS and nano-structured resonators and filters. Among these nonlinear effects, drive-level dependence (DLD), which describes the instability of the resonator frequency resulting from voltage level and/or power density, is a potentially significant problem for miniaturized resonators. Langasite, a promising new piezoelectric material, is of current interest for a variety of applications because of its good temperature behavior, good piezoelectric coupling, low acoustic loss, and high Q-factor. It has been recently used to make high-temperature MEMS. In this paper, we report experimental measurements of the DLD of langasite resonators with different resonator configurations (plano-plano, single bevel, and double bevel). The results show that the resonator configuration affects the DLD of the langasite resonator. The DLD measurement results for langasite are compared with literature values for quartz, langaniste, and langatate, and with additional new measurements for a GaPO4 resonator of type R-30 (-11.1° rotated Y-cut). Uncertainty analysis for the measured drive-level sensitivity is performed as well.

G-band rectangular waveguide filter fabricated using deep reactive ion etching and bonding processes

A G-band microelectromechanical system (MEMS) rectangular waveguide iris filter is designed and fabricated. The effects of the metallised layer and iris thickness, and roughness, on filter main performances are investigated. The prototypes were fabricated using MEMS manufacturing techniques. The key technique problems including deep etching, electroplating and bonding are researched and settled. The measured insertion loss can get to be 1.5–2.0 dB, the central frequency is 174 GHz, the bandwidth is 9.6 GHz, and the isolation out of the bandpass is larger than 15 dB. The test results show that the radio frequency MEMS filter meets practical requirements, which proves that it is a successful example for fabricating such rectangular waveguide devices at one to several hundred gigahertz frequencies using such presented processes.

A High-Performance Continuously Tunable MEMS Bandpass Filter at 1 GHz

This paper reports a continuously tunable lumped bandpass filter implemented in a third-order coupled resonator configuration. The filter is fabricated on a Borosilicate glass substrate using a surface micromachining technology that offers hightunable passive components. Continuous electrostatic tuning is achieved using three tunable capacitor banks, each consisting of one continuously tunable capacitor and three switched capacitors with pull-in voltage of less than 40 V. The center frequency of the filter is tuned from 1 GHz down to 600 MHz while maintaining a 3-dB bandwidth of 13%-14% and insertion loss of less than 4 dB. The maximum group delay is less than 10 ns across the entire tuning range. The temperature stability of the center frequency from -50°C to 50°C is better than 2%. The measured tuning speed of the filter is better than 80 s, and the is better than 20 dBm, which are in good agreement with simulations. The filter occupies a small size of less than 1.5 cm × 1.1 cm. The implemented filter shows the highest performance amongst the fully integrated microelectromechanical systems filters operating at sub-gigahertz range.

Fully Integrated High-Isolation Low-Loss Digitally Controlled MEMS Filters

In this paper, the design and packaging of fully integrated digitally controlled microelectromechanical system (MEMS) filters with low cost, high performance, and compact size are introduced. The four-channel switched filters operating in the frequency range of 11-20 GHz have been developed using MEMS technologies and four five-pole edge-coupled bandpass filters, which are modified from the traditional edge-coupled bandpass filter in the input and output feeders to achieve a better stopband performance. Microwave filters, MEMS circuits, and related packaging are investigated and demonstrated theoretically and experientially.

Microelectromechanical bandpass filters based on cyclic coupling architectures

Bandpass filters based on resonant microelectromechanical systems (MEMS) have the unique advantage of being able to leverage the benefits of classical mechanical filters, including their high quality factors of resonance, while simultaneously addressing the challenges associated with manufacturing cost and size, and enabling integrated fabrication with other on-chip components. While prior research has demonstrated the benefits of microelectromechanical filters composed of both isolated and coupled microresonators, the optimality of existing filter architectures, and their associated performance metrics, is yet to be fully determined. To this end, the current effort seeks to investigate the relative utility of micromechanical filter designs which exploit a nontraditional filter architecture founded upon cyclic, elastic coupling. Specifically, the work seeks to characterize the pertinent performance metrics and robustness characteristics associated with these systems, and to benchmark the acquired results against conventional, open-chain filter designs. The work ultimately demonstrates that MEMS filters based upon cyclic coupling architectures may be beneficially leveraged in certain filter implementations to improve overall system performance.

Design of Spring Coupling for High-$Q$ High-Frequency MEMS Filters for Wireless Applications

A second-order microelectromechanical systems (MEMS) filter with high selectivity and sharp rolloff is required in wireless transceivers used in dense wireless sensor networks (WSNs). These sensors are expected to replace existing wired sensors used in industrial-plant management and environmental monitoring. These filters, together with MEMS-based oscillators and mixers, are expected to replace off-chip components and enable the development of a single-chip transceiver. Such a transceiver will leverage the integrated MEMS components' characteristics to operate at lower power and, hence, longer battery life, making autonomous WSNs more feasible in a wider range of applications. As a result, this paper presents the design and optimization of the coupling beam of wineglass-mode micromechanical disk filters using simulated annealing. The filter under consideration consists of two identical wineglass-mode disk resonators, mechanically coupled by a flexural-mode beam. The coupled two-resonator system exhibits two mechanical-resonance modes with closely spaced frequencies that define the filter passband. A constraint is added on the beam length to eliminate the effect of the coupling-beam mass on the filter's resonant frequency. A new process flow is proposed to realize self-aligned overhanging coupling beams designed in this paper.

Performance comparison of Pb(Zr0.52Ti0.48)O3-only and Pb(Zr0.52Ti0.48)O3-on-silicon resonators

This paper provides a quantitative comparison and explores the design space of lead zirconium titanate (PZT)–only and PZT-on-silicon length-extensional mode resonators for incorporation into radio frequency microelectromechanical system filters and oscillators. We experimentally measured the correlation of motional impedance (RX) and quality factor (Q) with the resonators’ silicon layer thickness (tSi). For identical lateral dimensions and PZT-layer thicknesses (tPZT), the PZT-on-silicon resonator has higher resonant frequency (fC), higher Q (5100 versus 140), lower RX (51 Ω versus 205 Ω), and better linearity [third-order input intercept point (IIP3) of +43.7 dBm versus +23.3 dBm]. In contrast, the PZT-only resonator demonstrated much wider frequency tuning range (5.1% versus 0.2%).