Optimization and design of radio frequency piezoelectric MEMS resonators

Radio frequency (RF) microelectromechanical system (MEMS) resonators employing Lamb waves propagating in piezoelectric thin films have recently attracted much attention since they combine the advantages of the bulk acoustic wave (BAW) and surface acoustic wave (SAW) technologies: high phase velocity and multiple frequencies on a single chip. In particular, aluminum nitride (AlN) resonators based on fundamental symmetric (S0) Lamb mode have shown great promise because they can offer high phase velocities (10,000 m/s), low dispersive phase velocity characteristic, small temperature-induced frequency drift, low motional resistance, and monolithic integration compatibility with complementary metal–oxide–semiconductor (CMOS). However, there are still a few outstanding technical challenges, including spurious modes suppression, quality factor (Q) enhancement, frequency scalability, and electromechanical coupling improvement. These issues obstruct the wide deployment and commercialization of AlN Lamb mode resonators. This dissertation presents comprehensive investigations and solutions to these issues. This thesis is organized as follows: Chapter 1 gives a brief introduction of the basics on piezoelectric MEMS resonators and their promising applications. Chapter 2 first investigates the various available Lamb wave modes in AlN and then identifies the S0 mode as the promising resonator solution to overcome several challenges associated with SOA. Chapter 2 also discusses several outstanding challenges with S0 devices, including spurious mode suppression, Q enhancement, scaling resonant frequency, and enlarging fractional bandwidth. In response, Chapters 3-7 address these outstanding challenges by developing new designs and models, resorting to new acoustic mode, and incorporating new piezoelectric material. More specifically, Chapter 3 proposes two techniques to suppress the spurious modes in the responses of S0 resonators, namely mode conversion and mode shifting. Chapter 4 address the challenge of a conventionally vague question of reflection at the interface between released and unreleased regions in S0 resonators, and then demonstrates Q enhanced resonators with defined released regions achieved by a sandbox process. Chapter 5 first characterizes the S1 Lamb mode and optimizes its resonator configuration. A high-frequency S1 resonator at 3.5 GHz with a coupling of 3.5% is fabricated and demonstrated. Chapter 6 presents a hybrid filtering topology with a mode conversion AlN S0 resonator and lumped elements for widening the bandwidths of resonator-based filters. Chapter 7 proposes lithium niobate (LiNbO3) multilayered resonators with large electromechanical coupling, structure robustness, and good temperature stability. The analysis of Bragg reflectors, resonator simulation, stress control, fabrication, and measurements are covered in this chapter

Lithium niobate RF-MEMS oscillators for IoT, 5G and beyond

This dissertation focuses on the design and implementation of lithium niobate (LiNbO3) radiofrequency microelectromechanical (RF-MEMS) oscillators for internet-of-things (IoT), 5G and beyond. The dissertation focuses on solving two main problems found nowadays in most of the published works: the narrow tuning range and the low operating frequency (sub 3 GHz) acoustic oscillators currently deliver. The work introduced here enables wideband voltage-controlled MEMS oscillators (VCMOs) needed for emerging applications in IoT. Moreover, it enables multi-GHz (above 8 GHz) RF-MEMS oscillators through harnessing over mode resonances for 5G and beyond. LiNbO3 resonators characterized by high-quality factor (Q), high electromechanical coupling (kt2), and high figure-of-merit (FoMRES= Q kt2) are crucial for building the envisioned high-performance oscillators. Those oscillators can be enabled with lower power consumption, wider tuning ranges, and a higher frequency of oscillation when compared to other state-of-the-art (SoA) RF-MEMS oscillators. Tackling the tuning range issue, the first VCMO based on the heterogeneous integration of a high Q LiNbO3 RF-MEMS resonator and complementary metal-oxide semiconductor (CMOS) is demonstrated in this dissertation. A LiNbO3 resonator array with a series resonance of 171.1 MHz, a Q of 410, and a kt2 of 12.7% is adopted, while the TSMC 65 nm RF LP CMOS technology is used to implement the active circuitry with an active area of 220×70 µm2. Frequency tuning of the VCMO is achieved by programming a binary-weighted digital capacitor bank and a varactor that are both connected in series to the resonator. The measured best phase noise performances of the VCMO are -72 and -153 dBc/Hz at 1 kHz and 10 MHz offsets from 178.23 and 175.83 MHz carriers, respectively. The VCMO consumes a direct current (DC) of 60 µA from a 1.2 V supply while realizing a tuning range of 2.4 MHz (~ 1.4% tuning range). Such VCMOs can be applied to enable ultralow-power, low phase noise, and wideband RF synthesis for emerging applications in IoT. Moreover, the first VCMO based on LiNbO3 lateral overtone bulk acoustic resonator (LOBAR) is demonstrated in this dissertation. The LOBAR excites over 30 resonant modes in the range of 100 to 800 MHz with a frequency spacing of 20 MHz. The VCMO consists of a LOBAR in a closed-loop with two amplification stages and a varactor-embedded tunable LC tank. By the bias voltage applied to the varactor, the tank can be tuned to change the closed-loop gain and phase responses of the oscillator so that Barkhausen’s conditions are satisfied for the targeted resonant mode. The tank is designed to allow the proposed VCMO to lock to any of the ten overtones ranging from 300 to 500 MHz. These ten tones are characterized by average Qs of 2100, kt2 of 1.5%, FoMRES of 31.5 enabling low phase noise, and low-power oscillators crucial for IoT. Owing to the high Qs of the LiNbO3 LOBAR, the measured VCMO shows a close-in phase noise of -100 dBc/Hz at 1 kHz offset from a 300 MHz carrier and a noise floor of -153 dBc/Hz while consuming 9 mW. With further optimization, this VCMO can lead to direct RF synthesis for ultra-low-power transceivers in multi-mode IoT nodes. Tackling the multi-GHz operation problem, the first Ku-band RF-MEMS oscillator utilizing a third antisymmetric overtone (A3) in a LiNbO3 resonator is presented in the dissertation. Quarter-wave resonators are used to satisfy Barkhausen’s oscillation conditions for the 3rd overtone while suppressing the fundamental and higher-order resonances. The oscillator achieves measured phase noise of -70 and -111 dBc/Hz at 1 kHz and 100 kHz offsets from a 12.9 GHz carrier while consuming 20 mW of dc power. The oscillator achieves a FoMOSC of 200 dB at 100 kHz offset. The achieved oscillation frequency is the highest reported to date for a MEMS oscillator. In addition, this dissertation introduces the first X-band RF-MEMS oscillator built using CMOS technology. The oscillator consists of an acoustic resonator in a closed loop with cascaded RF tuned amplifiers (TAs) built on TSMC RF GP 65 nm CMOS. The TAs bandpass response, set by on-chip inductors, satisfies Barkhausen's oscillation conditions for A3 only. Two circuit variations are implemented. The first is an 8.6 GHz standalone oscillator with a source-follower buffer for direct 50 Ω-based measurements. The second is an oscillator-divider chain using an on-chip 3-stage divide-by-2 frequency divider for a ~1.1 GHz output. The standalone oscillator achieves measured phase noise of -56, -113, and -135 dBc/Hz at 1 kHz, 100 kHz, and 1 MHz offsets from an 8.6 GHz output while consuming 10.2 mW of dc power. The oscillator also attains a FoMOSC of 201.6 dB at 100 kHz offset, surpassing the SoA electromagnetic (EM) and RF-MEMS based oscillators. The oscillator-divider chain produces a phase noise of -69.4 and -147 dBc/Hz at 1 kHz and 1 MHz offsets from a 1075 MHz output while consuming 12 mW of dc power. Its phase noise performance also surpasses the SoA L-band phase-locked loops (PLLs). The demonstrated performance shows the strong potential of microwave acoustic oscillators for 5G frequency synthesis and beyond. This work will enable low-power 5G transceivers featuring high speed, high sensitivity, and high selectivity in small form factors

Micro-Resonators: The Quest for Superior Performance

Microelectromechanical resonators are no longer solely a subject of research in university and government labs; they have found a variety of applications at industrial scale, where their market is predicted to grow steadily. Nevertheless, many barriers to enhance their performance and further spread their application remain to be overcome. In this Special Issue, we will focus our attention to some of the persistent challenges of micro-/nano-resonators such as nonlinearity, temperature stability, acceleration sensitivity, limits of quality factor, and failure modes that require a more in-depth understanding of the physics of vibration at small scale. The goal is to seek innovative solutions that take advantage of unique material properties and original designs to push the performance of micro-resonators beyond what is conventionally achievable. Contributions from academia discussing less-known characteristics of micro-resonators and from industry depicting the challenges of large-scale implementation of resonators are encouraged with the hopes of further stimulating the growth of this field, which is rich with fascinating physics and challenging problems

Piezoelectric Materials

The science and technology in the area of piezoelectric ceramics are extremely progressing, especially the materials research, measurement technique, theory and applications, and furthermore, demanded to fit social technical requests such as environmental problems. While they had been concentrated on piezoelectric ceramics composed of lead-containing compositions, such as lead zirconate titanate (PZT) and lead titanate, at the beginning because of the high piezoelectricity, recently lead water pollution by soluble PZT of our environment must be considered. Therefore, different new compositions of lead-free ceramics in order to replace PZT are needed. Until now, there have been many studies on lead-free ceramics looking for new morphotropic phase boundaries, ceramic microstructure control to realize high ceramic density, including composites and texture developments, and applications to new evaluation techniques to search for high piezoelectricity. The purpose of this book is focused on the latest reports in piezoelectric materials such as lead-free ceramics, single crystals, and thin films from viewpoints of piezoelectric materials, piezoelectric science, and piezoelectric applications

Design and Implementation of Silicon-Based MEMS Resonators for Application in Ultra Stable High Frequency Oscillators

The focus of this work is to design and implement resonators for ultra-stable high-frequency ( \u3e 100MHz) silicon-based MEMS oscillators. Specifically, two novel types of resonators are introduced that push the performance of silicon-based MEMS resonators to new limits. Thin film Piezoelectric-on-Silicon (TPoS) resonators have been shown to be suitable for oscillator applications due to their combined high quality factor, coupling efficiency, power handling and doping-dependent temperature-frequency behavior. This thesis is an attempt to utilize the TPoS platform and optimize it for extremely stable high-frequency oscillator applications. To achieve the said objective, two main research venues are explored. Firstly, quality factor is systematically studied and anisotropy of single crystalline silicon (SCS) is exploited to enable high-quality factor side-supported radial-mode (aka breathing mode) TPoS disc resonators through minimization of anchor-loss. It is then experimentally demonstrated that in TPoS disc resonators with tethers aligned to [100], unloaded quality factor improves from ~450 for the second harmonic mode at 43 MHz to ~11,500 for the eighth harmonic mode at 196 MHz. Secondly, thickness quasi-Lamé modes are studied and demonstrated in TPoS resonators for the first time. It is shown that thickness quasi-Lamé modes (TQLM) could be efficiently excited in silicon with very high quality factor (Q). A quality factor of 23.2 k is measured in vacuum at 185 MHz for a fundamental TQLM-TPoS resonators designed within a circular acoustic isolation frame. Quality factor of 12.6 k and 6 k are also measured for the second- and third- harmonic TQLM TPoS resonators at 366 MHz and 555 MHz respectively. Turn-over temperatures between 40 °C to 125 °C are also designed and measured for TQLM TPoS resonators fabricated on degenerately N-doped silicon substrates. The reported extremely high quality factor, very low motional resistance, and tunable turn-over temperatures \u3e 80 °C make these resonators a great candidate for ultra-stable oven-controlled high-frequency MEMS oscillators

High-Performance Reconfigurable Piezoelectric Resonators and Filters for RF Frontend Applications

A conventional RF frontend module consists of many filters where each filter is allocated for a specific frequency band. These filters are connected through multiplexing switch networks to support multi-band wireless standards. Using an individual filter for each frequency band increases the module size, power consumption and cost. Therefore, implementation of reconfigurable filters that can operate at different frequency bands while maintaining key RF performance requirements such as low insertion loss, good linearity and power handling is necessary for manufacturing of future RF frontends. Acoustic wave resonators based on piezoelectric devices such as Surface Acoustic Wave (SAW) and Bulk Acoustic Wave (BAW) are the most commonly used technologies to manufacture filters for RF applications. The objective of the research described in this thesis is to investigate the feasibility of tunable filter solutions using piezoelectric SAW resonators. A tunable SAW technology which can maintain required performance parameters and can be commercially manufactured will constitute a technological breakthrough in wireless communications. Thin-Film Piezoelectric on Substrate (TPoS) resonators, based on Aluminum Nitride (AlN) piezoelectric material which are fabricated using commercially available Silicon on Insulator (SOI) PiezoMUMPs process, have been demonstrated. By combining the superior acoustic properties of AlN and single crystalline silicon substrate, this class of resonators achieves ultra-high quality factor (Q) values in excess of 3600. A 3-pole bandpass filter using direct electrical coupling between the resonators has been presented and we have studied the performance of the fabricated filter over a temperature range from -196ºC up to +120ºC and under high power. For the first time, we have demonstrated the integration of switching elements, based on Vanadium Dioxide (VO2) phase change material, with Incredible-High-Performance SAW (IHP-SAW) technology which allows us to design and implement switchable and reconfigurable SAW resonators and filters for wireless applications. Switchable multi-band filters using VO2 switches strategically imbedded within the resonators of the filter have been demonstrated. A switchable dual-band filter with four switching states and two channels was presented using hybrid integration approach where discrete VO2 switches were fabricated separately and then integrated with the SAW resonators and filters using wire bonds. The fabricated 5-pole dual-band filter demonstrated good insertion loss in both transmission states but had inadequate performance in terms of isolation between the channels due to the limitations of the hybrid integration approach. Moreover, hybrid integration does not allow us to use more than a few switching elements and cannot be used for the implementation of higher order filters. To address these issues, we have demonstrated the monolithic integration of VO2 switches using an in-house fabrication process that allows us to fabricate VO2 switches and SAW resonators and filters on a single chip. A dual-band switchable higher order 7-pole filter with six monolithically integrated VO2 switches, three for each channel, was demonstrated. The monolithic integration allows the single-chip implementation of the proposed switchable dual-band filter with improved performance along with significant size reduction and ease of manufacturing, paving the path for commercialization of this technology. Novel reconfigurable SAW resonators and filters with tunable center frequency were also presented for the first time. Tuning of the center frequency between two different states was achieved by changing the configuration of interdigitated electrodes within the SAW resonator and by using a set of tuning electrodes and VO2 switches. In the first implementation, the VO2 switches were integrated over the electrodes and inside the active area of the SAW resonator. Each resonator consists of hundreds of tuning electrodes and for a reliable switching each resonator requires a number of heater elements which results in increased DC power consumption and total size. A second reconfigurable resonator with a modified structure and using a modified in-house fabrication process to include a second electrode layer was proposed to reduce the number of required VO2 switching elements for an even more compact implementation and ten times reduction in the required DC power consumption. Design, implementation, and measurement results for a 3-pole tunable SAW filter based on the proposed reconfigurable resonators have been presented. The filter’s center frequency is tuned from 733 MHz to 713 MHz while the insertion loss was maintained below 2.5 dB. The fabricated SAW resonators and filters also showed acceptable linear and high-power performance characteristics. This is the first time a single-chip implementation of a reconfigurable SAW filter with center frequency tuning and acceptable RF performance using monolithically integrated VO2 switches is ever reported. The single-chip implementation of the proposed SAW resonators and filters enables the development of future low-cost RF multi-band transceivers with improved performance and functionality

Numerical simulations of microacoustic resonators and filters

This dissertation discusses numerical simulations of microwave acoustic resonators and bandpass filters employed in wireless telecommunication systems. In the first part of the dissertation, tailored finite element method (FEM) software with efficient numerical techniques is implemented and applied in the modeling of thin-film bulk-acoustic wave resonators (FBARs). Simulations with 3D FEM models of FBARs are carried out to investigate the effect of the electrode shape on the spurious resonances that often are present in the electrical response. The modeling results are validated through comparison of simulated and measured mechanical vibration amplitudes. The usability of the FEM tool is further demonstrated in simulations of a resonator design that features a clean electrical frequency response, free of spurious resonance peaks caused by anharmonic modes. Additionally, a method is proposed for determination of the elastic constants of a piezoelectric thin-film material. The technique is based on fitting of the computed dispersion curves of Lamb-wave modes to those measured from an FBAR using a scanning laser interferometer. In the second part of this dissertation, numerical simulations are used to study propagation properties of longitudinal leaky surface acoustic wave (LLSAW) mode under periodic electrode array on YZ-cut lithium niobate (LN). A combined FEM/boundary element method is employed to compute the electric admittance of one-port synchronous LLSAW resonators. Simulations and experiments are used to derive the dependence of the resonator resonance frequencies and Q values on the electrode dimensions. Ladder-type bandpass filters exploiting the LLSAW mode are implemented on YZ-cut LN in the frequency range from 2.5 GHz to 5.2 GHz, with fundamental mode LLSAW resonators as building blocks. The results demonstrate that the high phase velocity of the LLSAW mode on YZ-cut LN allows inexpensive fabrication of wide-band, low-loss filters up to 5 GHz using conventional optical lithography. The third topic considered is comprehensive modeling of a SAW duplexer, including electromagnetic modeling of the ceramic package. The parasitic capacitive and inductive couplings in the package are obtained using rigorous computation, allowing one to estimate the package effects on the duplexer performance.reviewe