Acoustic Bragg Reflectors for Q-Enhancement of Unreleased MEMS Resonators

This work presents the design of acoustic Bragg reflectors (ABRs) for unreleased MEMS resonators through analysis and simulation. Two of the greatest challenges to the successful implementation of MEMS are those of packaging and integration with integrated circuits. Development of unreleased RF MEMS resonators at the transistor level of the CMOS stack will enable direct integration into front-end-of-line (FEOL) processing, making these devices an attractive choice for on-chip signal generation and signal processing. The use of ABRs in unreleased resonators reduces spurious modes while maintaining high quality factors. Analysis on unreleased resonators using ABRs covers design principles, effects of fabrication variation, and comparison to released devices. Additionally, ABR-based unreleased resonators are compared with unreleased resonators enhanced using phononic crystals, showing order of magnitude higher quality factor (Q) for the ABR-based devices.United States. Defense Advanced Research Projects Agency (DARPA Young Faculty Award)Semiconductor Research Corporation (Center for Materials, Structures and Devices (MSD)

2DEG electrodes for piezoelectric transduction of AlGaN/GaN MEMS resonators

A 2D electron gas (2DEG) interdigitated transducer (IDT) in Gallium Nitride (GaN) resonators is introduced and demonstrated. This metal-free transduction does not suffer from the loss mechanisms associated with more commonly used metal electrodes. As a result, this transducer can be used for both the direct interrogation of GaN electromechanical properties and the realization of high Q resonators. A 1.2 GHz bulk acoustic resonator with mechanical Q of 1885 is demonstrated, with frequency quality factor product (f·Q) of 2.3×10[superscript 12], the highest measured in GaN to date

The Resonant Body Transistor

With quality factors (Q) often exceeding 10,000, vibrating micromechanical resonators have emerged as leading candidates for on-chip versions of high-Q resonators used in wireless communications systems, sensor networks, and clocking sources in microprocessors. However, extending the frequency of MEMS resonators generally entails scaling of resonator dimensions leading to increased motional impedance. In this dissertation, I introduce a new transduction mechanism using dielectric materials to improve performance and increase frequency of silicon-based RF acoustic resonators. Traditionally, electrostatically transduced mechanical resonators have used air-gap capacitors for driving and sensing vibrations in the structure. To increase transduction efficiency, facilitate fabrication, and enable GHz frequencies of operation, it is desirable to replace air-gap transducers with dielectric films. In my doctoral work, I designed, fabricated, and demonstrated dielectrically transduced silicon bulk-mode resonators up to 6.2 GHz, marking the highest acoustic frequency measured in silicon to date. The concept of internal dielectric transduction is introduced, in which dielectric transducers are incorporated directly into the resonator body. With dielectric films positioned at points of maximum strain in the resonator, this transduction improves in efficiency with increasing frequency, enabling resonator scaling to previously unattainable frequencies. Using internal dielectric transduction, longitudinal-mode resonators exhibited the highest frequency-quality factor (f.Q) product in silicon to date at 5.1 x 10 exp(13) s exp(-1) . These resonators were measured by capacitively driving and sensing acoustic vibrations in the device. However, capacitive detection often requires 3-port scalar mixer measurement, complicating monolithic integration of the resonators with CMOS circuits. The internal dielectric bulk-mode resonators can be utilized in a 2-port configuration with capacitive drive and piezoresistive detection, in which carrier mobility is dynamically modulated by elastic waves in the resonator. Piezoresistive sensing of silicon-based dielectrically transduced resonators was demonstrated with 1.6% frequency tuning and control of piezoresistive transconductance gm by varying the current flowing through the device. Resonant frequency, determined by lithographically defined dimensions, was demonstrated over a wide frequency range. These degrees of freedom enable acoustic resonators spanning a large range of frequencies on a single chip, despite design restrictions of the CMOS process. As we scale to deep sub-micron (DSM) technology, transistor cut-off frequencies increase, enabling the design of CMOS circuits for RF and mm-wave applications greater than 60 GHz. However, DSM transistors have limited gain and integrated passives demonstrate low Q, resulting in poor efficiency. A successful transition into DSM CMOS requires enhanced-gain and high-Q components operating at microwave frequencies. In this work, a merged NEMS-CMOS device is introduced that can function as a building block to enhance the performance of RF circuits. The device, termed the Resonant Body Transistor (RBT), consists of a field effect transistor embedded in the body of a high-frequency NEMS resonator implementing internal dielectric transduction. The results of this work indicate improved resonator performance with increased frequency, providing a means of scaling MEMS resonators to previously unattainable frequencies in silicon. With the transduction methods developed in this work, a hybrid NEMSCMOS RBT enables low-power, narrow-bandwidth low noise amplifier design for transceivers and low phase-noise oscillator arrays for clock generation and temperature sensing in microprocessors

SI-based unreleased hybrid MEMS-CMOS resonators in 32nm technology

This work presents the first unreleased Silicon resonators fabricated at the transistor level of a standard CMOS process, and realized without any release steps or packaging. These unreleased bulk acoustic resonators are driven capacitively using the thin gate dielectric of the CMOS process, and actively sensed with a Field Effect Transistor (FET) incorporated into the resonant body. FET sensing using the high f[subscript T], high performance transistors in CMOS amplifies the mechanical signal before the presence of parasitics. This enables RF-MEMS resonators at orders of magnitude higher frequencies than possible with passive devices. First generation CMOS-MEMS Si resonators with Acoustic Bragg Reflectors are demonstrated at 11.1 GHz with Q~17 and a total footprint of 5μm × 3μm using IBM's 32nm SOI technology.United States. Defense Advanced Research Projects Agency. Leading Edge Access ProgramUnited States. National Security Agency. Trusted Access Program OfficeInternational Business Machines Corporatio

Approximating the influence of monotone boolean functions in O(√n) query complexity

Author Manuscript received 27 Jan 2011. 14th International Workshop, APPROX 2011, and 15th International Workshop, RANDOM 2011, Princeton, NJ, USA, August 17-19, 2011. ProceedingsThe Total Influence (Average Sensitivity) of a discrete function is one of its fundamental measures. We study the problem of approximating the total influence of a monotone Boolean function f : {0, 1}[superscript n] → {0, 1}, which we denote by I[f]. We present a randomized algorithm that approximates the influence of such functions to within a multiplicative factor of (1 ± ) by performing O([√n log n[over]I[f]] poly(1/Є ))queries. We also prove a lower bound of Ω ([√ n [over] log n·I[f]])on the query complexity of any constant-factor approximation algorithm for this problem (which holds for I[f] = Ω(1)), hence showing that our algorithm is almost optimal in terms of its dependence on n. For general functions we give a lower bound of Ω ([n [over] I[f]]), which matches the complexity of a simple sampling algorithm

An unreleased mm-wave resonant body transistor

In this work, we present the first fully unreleased Micro-Electro-Mechanical (MEM) resonator. The 1st harmonic longitudinal resonance of a silicon FinFET fully clad in SiO[subscript 2] is demonstrated. The device exhibits two resonances at 39 and 41 GHz, corresponding well with simulation results. The quality factor (Q) of 129 at 39 GHz is ~4× lower than that of its released counterpart. Methods to improve Q and reduce spurious modes are introduced. This first demonstration of unreleased resonators in a hybrid MEMS-CMOS technology can provide RF and microwave CMOS circuit designers with active high-Q devices monolithically integrated in Front-End-of-Line (FEOL) processing without the need for post-processing or special packaging.Microelectronics Advanced Research Corporation (MARCO)United States. Defense Advanced Research Projects Agenc

Shape optimization of solid-air porous phononic crystal slabs with widest full 3D bandgap for in-plane acoustic waves

The use of Phononic Crystals (PnCs) as smart materials in structures and microstructures is growing due to their tunable dynamical properties and to the wide range of possible applications. PnCs are periodic structures that exhibit elastic wave scattering for a certain band of frequencies (called bandgap), depending on the geometric and material properties of the fundamental unit cell of the crystal. PnCs slabs can be represented by plane-extruded structures composed of a single material with periodic perforations. Such a configuration is very interesting, especially in Micro Electro-Mechanical Systems industry, due to the easy fabrication procedure. A lot of topologies can be found in the literature for PnCs with square-symmetric unit cell that exhibit complete 2D bandgaps; however, due to the application demand, it is desirable to find the best topologies in order to guarantee full bandgaps referred to in-plane wave propagation in the complete 3D structure. In this work, by means of a novel and fast implementation of the Bidirectional Evolutionary Structural Optimization technique, shape optimization is conducted on the hole shape obtaining several topologies, also with non-square-symmetric unit cell, endowed with complete 3D full bandgaps for in-plane waves. Model order reduction technique is adopted to reduce the computational time in the wave dispersion analysis. The 3D features of the PnC unit cell endowed with the widest full bandgap are then completely analyzed, paying attention to engineering design issues