High-Q Aluminum Nitride RF MEMS Lamb Wave Resonators and Narrowband Filters

High-Q Aluminum Nitride RF MEMS Lamb Wave Resonators and Narrowband Filters
Author: TING-TA YEN
Publisher:
Total Pages: 194
Release: 2012
Genre:
ISBN:
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The increasing demands for higher performance, advanced wireless and mobile communication systems have continuously driven device innovations and system improvements. In order to reduce power consumption and integration complexity, radio frequency (RF) microelectromechanical systems (MEMS) resonators and filters have been considered as direct replacements for off-chip passive components. In this dissertation, a new radio architecture for direct channel selection is explored. The primary elements in this new architecture include a multitude of closely-spaced narrowband filters (i.e., a filter bank) and an array of low-loss RF switches. This work addresses a number of issues related to this modern channel-select RF front end and explores the potential of utilizing piezoelectric aluminum nitride (AlN) resonator technology to fulfill these technical challenges. Characteristic studies of acoustic waves propagating in a piezoelectric thin film suggest the use of high-phase-velocity Lamb wave mode vibration for higher frequency applications. The lowest-order symmetric modes (S0 modes) can be efficiently excited, via the d31 (e31) piezoelectric coefficient, by utilizing interdigital transducer (IDT) electrodes, enabling co-fabrication of devices operating from tens of megahertz up to a few gigahertz on the same chip. An AlN "overhang" fine frequency selection technique is experimentally studied, allowing precise relative frequency control of an array of Lamb wave resonators (LWR) to 0.1%. Experimental results suggest the resonance frequency of Lamb wave resonators can be linearly adjusted by up to 5% with no significant effects on other resonator parameters. The first high temperature testing of AlN Lamb wave resonators above 600°C verifies its potential of being used in a harsh environment sensing telemetry. With a correct AlN/SiO2 thickness ratio, the first-order temperature coefficient of frequency (TCF) of a LWR can be reduced from -25 ppm/K to 3.9 ppm/K. In addition, increasing the input power level from -15 dBm to 10 dBm causes no bifurcation instability or frequency hysteresis on AlN Lamb wave resonators and only 0.05% frequency drift is recorded, showing an excellent power handling capability. A number of different resonator topologies are studied and demonstrated in this work as possible candidates for the filter bank. Mechanically-coupled filters utilize quarter-wavelength coupling beams to eliminate the mass-loading effect to adjoining resonators, and the bandwidths are determined by the equivalent stiffness of the coupling beam and the resonator itself. Numbers of identical resonators are mechanically-coupled as a filter with center frequency at 710 MHz and 0.4% fractional bandwidth (FBW). Furthermore, by introducing AlN overhang selection technique, an array of electrically self-coupled filters are fabricated with evenly-spaced center frequencies around 735 MHz and 500 kHz bandwidths (0.07% FBW). An array of ladder filters with center frequencies around 440 MHz and 2 MHz bandwidths (0.5% FBW) are also demonstrated, without post-process trimming. These closely and evenly spaced AlN Lamb wave filters demonstrate the potential to realize a purely mechanical, high performance, yet low-power RF front-end system. To further improve filter performance, capacitive-piezoelectric Lamb wave resonators, featuring sub-micron air gaps between piezoelectric structural layer and electrodes, are demonstrated with the aim of reducing interface energy dissipation. Quality factors of these capacitive-piezo Lamb wave resonators are measured over 5,000 at 940 MHz, posting the highest reported Q for single AlN resonators using d31 (e31) transduction. The Q * f products above 4.7×10^12 exceed those of commercialized FBAR and SAW resonators. Although the motional impedance of these devices inevitably rises to 1 kilo-ohm; when electrodes are separated from the AlN, this value is still much lower than conventional electrostatic resonators and can be easily terminated with on-chip matching networks. While designing the surface micromachining fabrication process dedicated to these capacitive-piezo devices, a thorough AlN etch rate table including commonly encountered cleaning and wet/dry etch steps is established. Although a large part of this dissertation concerns Lamb wave resonators, the last part of this dissertation focuses on a special corrugated cantilever beam design to improve conversion efficacy of a piezoelectric energy harvester. These vibration-sensitive piezoelectric AlN energy harvesters utilize corrugated cross-section cantilevers to achieve the same energy conversion effectiveness as that in a bimorph beam design, yet using a simple fabrication process similar to that of a unimorph beam. Due to the opposite signs of strains, the generated electric fields above and below the neutral plane have opposite polarities, and the generated energy can be extracted separately without the common cancellation issues encountered in a single piezoelectric beam design. This approach provides superior performance while simultaneously simplifying the fabrication process. A prototype multi-fold device resonating at 853 Hz with output power of 0.17 microwatt under a 1 G acceleration is recorded. Based on superb material properties and the 600°C thermal testing performed on RF resonators, these AlN energy harvesters offer a promising solution to scavenge vibration energies from harsh environments for advanced microsensor systems.

Temperature-Compensated and High-Q Piezoelectric Aluminum Nitride Lamb Wave Resonators for Timing and Frequency Control Applications

Temperature-Compensated and High-Q Piezoelectric Aluminum Nitride Lamb Wave Resonators for Timing and Frequency Control Applications
Author: Chih-Ming Lin
Publisher:
Total Pages: 384
Release: 2013
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ISBN:
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The explosive development of wireless and mobile communication systems has lead to rapid technology innovation in component performance, complementary metal-oxide semiconductor (CMOS) compatible fabrication techniques, and system improvement to satisfy requirements for faster signal processing, cost efficiency, chip miniaturization, and low power consumption. The demands for the high-performance communication systems whose fundamentals are precise timing and frequency control have driven the current research interests to develop advanced reference oscillators and radio frequency (RF) bandpass filters. In turn a promising microelectromechanical systems (MEMS) resonator technology is required to achieve the ultimate goal. That is, micromechanical vibrating resonators with high quality factor (Q) and good frequency-temperature stability at high series resonance frequency (fs) are the required fundamental components for a high-performance wireless communication system. Recently, Lamb wave mode propagating in piezoelectric thin plates has attracted great attention for designs of the electroacoustic resonators since it combines the advantages of bulk acoustic wave (BAW) and surface acoustic wave (SAW): high phase velocity and multiple frequency excitation by an interdigital transducer (IDT). More specifically, the Lamb wave resonator (LWR) based on an aluminum nitride (AlN) thin film has attracted many attentions because it can offer the high resonance frequency, small temperature-induced frequency drift, low motional resistance, and CMOS compatibility. The lowest-order symmetric (S0) Lamb wave mode propagation in the AlN thin plate is particularly preferred because it exhibits a phase velocity close to 10,000 m/s, a low dispersive phase velocity characteristic, and a moderate electromechanical coupling coefficient. However, the uncompensated AlN LWR shows a first-order temperature coefficient of frequency (TCF) of approximately -25 ppm/C. This level of the temperature stability is unsuitable for any timing application. In addition, the Q of the AlN LWR is degraded to several hundred while the IDT finger width is downscaled to a nanometer scale to raise the resonance frequency up to a few GHz. This dissertation presents comprehensive analytical and experimental results on a new class of temperature-compensated and high-Q piezoelectric AlN LWRs. The temperature compensation of the AlN LWR using the S0 Lamb wave mode is achieved by adding a layer of silicon dioxide (SiO2) with an appropriate thickness ratio to the AlN thin film, and the AlN/SiO2 LWRs can achieve a low first-order TCF at room temperature. Based on the multilayer plate composed of a 1-um-thick AlN film and a 0.83-um-thick SiO2 layer, a temperature-compensated LWR operating at a series resonance frequency of 711 MHz exhibits a zero first-order TCF and a small second-order TCF of -21.5 ppb/C^2 at its turnover temperature, 18.05 C. The temperature dependence of fractional frequency variation is less than 250 parts per million (ppm) over a wide temperature range from -55 to 125 C. In addition to the temperature compensation at room temperature, the thermal compensation of the AlN LWRs is experimentally demonstrated at high temperatures. By varying the normalized AlN and SiO2 thicknesses to the wavelength, the turnover temperature can be designed at high temperatures and the AlN LWRs are temperature-compensated at 214, 430, and 542 C, respectively. The temperature-compensated AlN/SiO2 LWRs are promising for a lot of applications including thermally stable oscillators, bandpass filters, and sensors at room temperature as well as high temperatures. The influences of the bottom electrode upon the characteristics of the LWRs utilizing the S0 Lamb wave mode in the AlN thin plate are theoretically and experimentally studied. Employment of a floating bottom electrode for the LWR reduces the static capacitance in the AlN membrane and accordingly enhances the effective coupling coefficient. The floating bottom electrode simultaneously offers a large coupling coefficient and a simple fabrication process than the grounded bottom electrode but the transduction efficiency is not sacrificed. In contrast to those with the bottom electrode, an AlN LWR with no bottom electrode shows a high Q of around 3,000 since it gets rid of the electrical loss in the metal-to-resonator interface. In addition, it exhibits better power handling capacity than those with the bottom electrode since less thermal nonlinearity induced by the self-heating exists in the resonators. In order to boost the Q, a new class of the AlN LWRs using suspended convex edges is introduced in this dissertation for the first time. The suspended convex edges can efficiently reflect the Lamb waves back towards the transducer as well as confine the mechanical energy in the resonant body. Accordingly the mechanical energy dissipation through the support tethers is significantly minimized and the Q can be markedly enhanced. More specifically, the measured frequency response of a 491.8-MHz LWR with suspended biconvex edges yields a Q of 3,280 which represents a 2.6x enhancement in Q over a 517.9-MHz LWR based on the same AlN thin plate but with the suspended flat edges. The suspended convex edges can efficiently confine mechanical energy in the LWR and reduce the energy dissipation through the support tethers without increasing the motional impedance of the resonator. In addition, the radius of curvature of the suspended convex edges and the AlN thickness normalized to the wavelength can be further optimized to simultaneously obtain high Q, low motional impedance, and large effective coupling coefficient. To further enhance the Q of the LWR, a composite plate including an AlN thin film and an epitaxial cubic silicon carbide (3C-SiC) layer is introduced to enable high-Q and high-frequency micromechanical resonators utilizing high-order Lamb wave modes. The use of the epitaxial 3C-SiC layer is attractive as SiC crystals have been theoretically proven to have an exceptionally large fs and Q product due to its low acoustic loss characteristic at microwave frequencies. In addition, AlN and 3C-SiC have well-matched mechanical and electrical properties, making them a suitable material stack for the electroacoustic resonators. The epitaxial 3C-SiC layer not only provides the micromechanical resonators with a low acoustic loss layer to boost their Q but also enhances the electromechanical coupling coefficients of some high-order Lamb waves in the AlN/3C-SiC composite plate. A micromachined electroacoustic resonator utilizing the third quasi-symmetric (QS3) Lamb wave mode in the AlN/3C-SiC composite plate exhibits a Q of 5,510 at 2.92 GHz, resulting in the highest fs and Q product, 1.61x10^13 Hz, among suspended piezoelectric thin film resonators to date.

High-Q AlN Contour Mode Resonators with Unattached, Voltage-Actuated Electrodes

High-Q AlN Contour Mode Resonators with Unattached, Voltage-Actuated Electrodes
Author: Robert A. Schneider
Publisher:
Total Pages: 219
Release: 2015
Genre:
ISBN:
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High-Q narrowband filters at ultra-high frequencies hold promise for reducing noise and suppressing interferers in wireless transceivers, yet research efforts confront a daunting challenge. So far, no existing resonator technology can provide the simultaneous high-Q, high electromechanical coupling (k_{eff}^2), frequency tunability, low motional resistance (R_x), stopband rejection, self-switchability, frequency accuracy, and power handling desired to select individual channels or small portions of a band over a wide RF range. Indeed, each technology provides only a subset of the desired properties. Recently introduced "capacitive-piezoelectric" resonators, i.e., piezoelectric resonators with non-contacting transduction electrodes, known for achieving very good Q's, have recently emerged (in the early 2010's) as a contender among existing technologies to address the needs of RF narrowband selection. Several reports of such devices, made from aluminum nitride (AlN), have demonstrated improved Q's over attached electrode counterparts at frequencies up to 1.2 GHz, albeit with reduced transduction efficiency due to the added capacitive gaps. Fabrication challenges, while still allowing for a glimpse of the promise of this technology, have, until now, hindered attempts at more complex devices than just simple resonators with improved Q's. This thesis project demonstrates several key improvements to capacitive-piezo technology, which, taken together, further bolster its case for deployment for frequency control applications. First, new fabrication techniques improve yields, reliability, and performance. Second, design modifications now allow k_{eff}^2's on par even with attached-electrode contour-mode devices, while most importantly, achieving unprecedented Q-factors for AlN. Third, a new electrode-collapsed based resonance-quenching capability allows ON/OFF switching of resonators and filters, such as would be useful for a bank of parallel filters. Fourth, an integrated voltage-controlled gap-reduction-based frequency tuning mechanism permits wide frequency tuning of devices and thus much improved frequency accuracy. Gap actuation also allows for the decoupling of filters in the OFF state. And fifth, switchable and tunable capacitive-piezo narrow-band filters are demonstrated for the first time. This thesis is divided into eight parts. In the first chapter, context is provided to demonstrate the purpose of this work. RF channel selection is introduced and a survey of currently available technology is presented. The second chapter explains key operating principles for MEMS resonators so a novice reader can be better equipped to fully understand the design choices made in later chapters. Chapter 3, on high-performance capacitive-piezo disk resonators, introduces the fundamental device of this thesis, providing examples of performance and design optimization, experimental results, simulation methods, and modeling. Chapter 4 introduces capacitive-piezoelectric disk arrays as a method to increase the area and thereby reduce the motional resistance of the unit disk resonator. Chapter 5 discusses voltage controlled gap actuation of the capacitive piezoelectric transducer's top electrode, which enables voltage controlled frequency tuning and on/off switching. Chapter 6 takes a thorough look at the fabrication technology needed to make capacitive-piezo devices, including lessons learned on how to avoid certain pitfalls. Chapter 7, on filters, contains both theory and measurement results of filters. Chapter 8 concludes the thesis by summarizing the key achievements of Chapters 3 through 7, highlighting key areas needing further development, and discussing implications of this technology for the future.

3D and Circuit Integration of MEMS

3D and Circuit Integration of MEMS
Author: Masayoshi Esashi
Publisher: John Wiley & Sons
Total Pages: 528
Release: 2021-03-16
Genre: Technology & Engineering
ISBN: 3527823255
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Explore heterogeneous circuit integration and the packaging needed for practical applications of microsystems MEMS and system integration are important building blocks for the “More-Than-Moore” paradigm described in the International Technology Roadmap for Semiconductors. And, in 3D and Circuit Integration of MEMS, distinguished editor Dr. Masayoshi Esashi delivers a comprehensive and systematic exploration of the technologies for microsystem packaging and heterogeneous integration. The book focuses on the silicon MEMS that have been used extensively and the technologies surrounding system integration. You’ll learn about topics as varied as bulk micromachining, surface micromachining, CMOS-MEMS, wafer interconnection, wafer bonding, and sealing. Highly relevant for researchers involved in microsystem technologies, the book is also ideal for anyone working in the microsystems industry. It demonstrates the key technologies that will assist researchers and professionals deal with current and future application bottlenecks. Readers will also benefit from the inclusion of: A thorough introduction to enhanced bulk micromachining on MIS process, including pressure sensor fabrication and the extension of MIS process for various advanced MEMS devices An exploration of epitaxial poly Si surface micromachining, including process condition of epi-poly Si, and MEMS devices using epi-poly Si Practical discussions of Poly SiGe surface micromachining, including SiGe deposition and LP CVD polycrystalline SiGe A concise treatment of heterogeneously integrated aluminum nitride MEMS resonators and filters Perfect for materials scientists, electronics engineers, and electrical and mechanical engineers, 3D and Circuit Integration of MEMS will also earn a place in the libraries of semiconductor physicists seeking a one-stop reference for circuit integration and the practical application of microsystems.

High-Q MEMS Capacitive-Gap Resonators for RF Channel Selection

High-Q MEMS Capacitive-Gap Resonators for RF Channel Selection
Author: Lingqi Wu
Publisher:
Total Pages: 124
Release: 2015
Genre:
ISBN:
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On chip capacitive-gap transduced micromechanical resonators constructed via MEMS technology have achieved very high Q’s at both VHF and UHF range, making them very attractive as on-chip frequency selecting elements for filters in wireless communication applications. Still, there are applications, such as software-defined cognitive radio, that demand even higher Q’s at RF to enable low-loss selection of single channels (rather than bands of them) to reduce the power consumption of succeeding electronic stages down to levels more appropriate for battery-powered handhelds. This dissertation focuses on improving the performance of MEMS capacitive-gap resonators to the degree which can be used to build the aforementioned RF channel-select filters. It first aims to enhance quality factor of MEMS capacitive-gap resonators by suppressing vibration energy loss via device substrate, which will lead to low insertion loss in RF channel selection. Then, in order to reduce an RF front-end filter’s bandwidth and termination resistance, it explores the method of building micromechanical resonator array composites that include large number of mechanically coupled resonators. Finally, the dissertation presents an experimentally demonstrated RF narrowband filter built upon mechanically coupled high-Q resonator array composites. Pursuant to further increasing Q at UHF for low insertion loss RF channel select application, the thesis develops an equivalent circuit model of a radial contour mode disk resonator that can analytically predict anchor loss dominated Q. Indicated by this improved equivalent circuit model, this work “hollows” the stems supporting all-polysilicon micromechanical disk resonators to effectively squeeze the energy conduit between the disk structure and the substrate, thereby suppressing energy loss and maximizing Q. By using the same fabrication process flow from the conventional all-polysilicon devices, the use of hollow stem support enhances Q with minimal increase in fabrication complexity. Measurements confirm Q enhancements of 2.6× for contour modes at 154 MHz and 2.9× for wine glass modes around 112 MHz over values previously achieved by full stem all-polysilicon disk resonators with identical dimensions. Measured Q’s as high as 56,061 at 329 MHz and 93,231 at 178 MHz for whispering gallery modes further attest to the efficacy of this approach. This dissertation also employs mechanically coupled disk array composites to increase resonator stiffness and lower motional resistance, which are both highly desired for RF front-end channel-select filters. By using half-wavelength coupling beams and proper electrode phasing design, measurements confirm that a 215-MHz 50-resonator disk array achieves 46.5× Q-normalized Rx reduction, with no observation of other undesired vibration modes. Notably, as indicated by the newly developed negative-capacitance equivalent circuit model, such array composite also shows enhanced frequency stability against dc-bias voltage fluctuations because of its large electrode-to-resonator overlap capacitance. Finally, the thesis demonstrates a 75MHz 3rd order 210 kHz bandwidth (0.3%) filter with a sharp roll-off of 20dB shape factor of 1.46. This filter employs three high-Q disk array composites connected by quarter-wavelength rotational coupling beams to achieve a weak coupling for narrowband selection. Each array composite itself includes seven flexural disk resonators coupled by strong quasi-zero length beams to enforce desired response. By using electromechanical analogies, the equivalent electrical circuit model of this filter can accurately capture the device’s response and provide insights for filter designers. Most importantly, the accuracy of the described equivalent circuit model in predicting quality factor, frequency stability, and filter response encourages the design of even more complex micromechanical circuits to come, for example, as would be needed in an all-mechanical RF front-end.

Piezoelectric MEMS Resonators

Piezoelectric MEMS Resonators
Author: Harmeet Bhugra
Publisher: Springer
Total Pages: 423
Release: 2017-01-09
Genre: Technology & Engineering
ISBN: 3319286889
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This book introduces piezoelectric microelectromechanical (pMEMS) resonators to a broad audience by reviewing design techniques including use of finite element modeling, testing and qualification of resonators, and fabrication and large scale manufacturing techniques to help inspire future research and entrepreneurial activities in pMEMS. The authors discuss the most exciting developments in the area of materials and devices for the making of piezoelectric MEMS resonators, and offer direct examples of the technical challenges that need to be overcome in order to commercialize these types of devices. Some of the topics covered include: Widely-used piezoelectric materials, as well as materials in which there is emerging interest Principle of operation and design approaches for the making of flexural, contour-mode, thickness-mode, and shear-mode piezoelectric resonators, and examples of practical implementation of these devices Large scale manufacturing approaches, with a focus on the practical aspects associated with testing and qualification Examples of commercialization paths for piezoelectric MEMS resonators in the timing and the filter markets ...and more! The authors present industry and academic perspectives, making this book ideal for engineers, graduate students, and researchers.