Ultra-sensitive graphene membranes for microphone applications

Microphones exploit the motion of suspended membranes to detect sound waves. Since the microphone performance can be improved by reducing the thickness and mass of its sensing membrane, graphene-based microphones are expected to outperform state-of-the-art microelectromechanical (MEMS) microphones and allow further miniaturization of the device. Here, we present a laser vibrometry study of the acoustic response of suspended multilayer graphene membranes for microphone applications. We address performance parameters relevant for acoustic sensing, including mechanical sensitivity, limit of detection and nonlinear distortion, and discuss the trade-offs and limitations in the design of graphene microphones. We demonstrate superior mechanical sensitivities of the graphene membranes, reaching more than 2 orders of magnitude higher compliances than commercial MEMS devices, and report a limit of detection as low as 15 dBSPL, which is 10 - 15 dB lower than that featured by current MEMS microphones.Comment: 34 pages, 6 figures, 7 supplementary figure

Data acquisition techniques based on frequency-encoding applied to capacitive MEMS microphones

Mención Internacional en el título de doctorThis thesis focuses on the development of capacitive sensor readout circuits and data converters based on frequency-encoding. This research has been motivated by the needs of consumer electronics industry, which constantly demands more compact readout circuit for MEMS microphones and other sensors. Nowadays, data acquisition is mainly based on encoding signals in voltage or current domains, which is becoming more challenging in modern deep submicron CMOS technologies. Frequency-encoding is an emerging signal processing technique based on encoding signals in the frequency domain. The key advantage of this approach is that systems can be implemented using mostly-digital circuitry, which benefits from CMOS technology scaling. Frequencyencoding can be used to build phase referenced integrators, which can replace classical integrators (such as switched-capacitor based integrators) in the implementation of efficient analog-to-digital converters and sensor interfaces. The core of the phase referenced integrators studied in this thesis consists of the combination of different oscillator topologies with counters and highly-digital circuitry. This work addresses two related problems: the development of capacitive MEMS sensor readout circuits based on frequency-encoding, and the design and implementation of compact oscillator-based data converters for audio applications. In the first problem, the target is the integration of the MEMS sensor into an oscillator circuit, making the oscillation frequency dependent on the sensor capacitance. This way, the sound can be digitized by measuring the oscillation frequency, using digital circuitry. However, a MEMS microphone is a complex structure on which several parasitic effects can influence the operation of the oscillator. This work presents a feasibility analysis of the integration of a MEMS microphone into different oscillator topologies. The conclusion of this study is that the parasitics of the MEMS limit the performance of the microphone, making it inefficient. In contrast, replacing conventional ADCs with frequency-encoding based ADCs has proven a very efficient solution, which motivates the next problem. In the second problem, the focus is on the development of high-order oscillator-based Sigma-Delta modulators. Firstly, the equivalence between classical integrators and phase referenced integrators has been studied, followed by an overview of state-of-art oscillator-based converters. Then, a procedure to replace classical integrators by phase referenced integrators is presented, including a design example of a second-order oscillator based Sigma-Delta modulator. Subsequently, the main circuit impairments that limit the performance of this kind of implementations, such as phase noise, jitter or metastability, are described. This thesis also presents a methodology to evaluate the impact of phase noise and distortion in oscillator-based systems. The proposed method is based on periodic steady-state analysis, which allows the rapid estimation of the system dynamic range without resorting to transient simulations. In addition, a novel technique to analyze the impact of clock jitter in Sigma-Delta modulators is described. Two integrated circuits have been implemented in 0.13 μm CMOS technology to demonstrate the feasibility of high-order oscillator-based Sigma-Delta modulators. Both chips have been designed to feature secondorder noise shaping using only oscillators and digital circuitry. The first testchip shows a malfunction in the digital circuitry due to the complexity of the multi-bit counters. The second chip, implemented using single-bit counters for simplicity, shows second-order noise shaping and reaches 103 dB-A of dynamic range in the audio bandwidth, occupying only 0.04 mm2.Esta tesis se centra en el desarrollo de conversores de datos e interfaces para sensores capacitivos basados en codificación en frecuencia. Esta investigación está motivada por las necesidades de la industria, que constantemente demanda reducir el tamaño de este tipo de circuitos. Hoy en día, la adquisición de datos está basada principalmente en la codificación de señales en tensión o en corriente. Sin embargo, la implementación de este tipo de soluciones en tecnologías CMOS nanométricas presenta varias dificultades. La codificación de frecuencia es una técnica emergente en el procesado de señales basada en codificar señales en el dominio de la frecuencia. La principal ventaja de esta alternativa es que los sistemas pueden implementarse usando circuitos mayoritariamente digitales, los cuales se benefician de los avances de la tecnología CMOS. La codificación en frecuencia puede emplearse para construir integradores referidos a la fase, que pueden reemplazar a los integradores clásicos (como los basados en capacidades conmutadas) en la implementación de conversores analógico-digital e interfaces de sensores. Los integradores referidos a la fase estudiados en esta tesis consisten en la combinación de diferentes topologías de osciladores con contadores y circuitos principalmente digitales. Este trabajo aborda dos cuestiones relacionadas: el desarrollo de circuitos de lectura para sensores MEMS capacitivos basados en codificación temporal, y el diseño e implementación de conversores de datos compactos para aplicaciones de audio basados en osciladores. En el primer caso, el objetivo es la integración de un sensor MEMS en un oscilador, haciendo que la frecuencia de oscilación depe capacidad del sensor. De esta forma, el sonido puede ser digitalizado midiendo la frecuencia de oscilación, lo cual puede realizarse usando circuitos en su mayor parte digitales. Sin embargo, un micrófono MEMS es una estructura compleja en la que múltiples efectos parasíticos pueden alterar el correcto funcionamiento del oscilador. Este trabajo presenta un análisis de la viabilidad de integrar un micrófono MEMS en diferentes topologías de oscilador. La conclusión de este estudio es que los parasíticos del MEMS limitan el rendimiento del micrófono, causando que esta solución no sea eficiente. En cambio, la implementación de conversores analógico-digitales basados en codificación en frecuencia ha demostrado ser una alternativa muy eficiente, lo cual motiva el estudio del siguiente problema. La segunda cuestión está centrada en el desarrollo de moduladores Sigma-Delta de alto orden basados en osciladores. En primer lugar se ha estudiado la equivalencia entre los integradores clásicos y los integradores referidos a la fase, seguido de una descripción de los conversores basados en osciladores publicados en los últimos años. A continuación se presenta un procedimiento para reemplazar integradores clásicos por integradores referidos a la fase, incluyendo un ejemplo de diseño de un modulador Sigma-Delta de segundo orden basado en osciladores. Posteriormente se describen los principales problemas que limitan el rendimiento de este tipo de sistemas, como el ruido de fase, el jitter o la metaestabilidad. Esta tesis también presenta un nuevo método para evaluar el impacto del ruido de fase y de la distorsión en sistemas basados en osciladores. El método propuesto está basado en simulaciones PSS, las cuales permiten la rápida estimación del rango dinámico del sistema sin necesidad de recurrir a simulaciones temporales. Además, este trabajo describe una nueva técnica para analizar el impacto del jitter de reloj en moduladores Sigma-Delta. En esta tesis se han implementado dos circuitos integrados en tecnología CMOS de 0.13 μm, con el fin de demostrar la viabilidad de los moduladores Sigma-Delta de alto orden basados en osciladores. Ambos chips han sido diseñados para producir conformación espectral de ruido de segundo orden, usando únicamente osciladores y circuitos mayoritariamente digitales. El primer chip ha mostrado un error en el funcionamiento de los circuitos digitales debido a la complejidad de las estructuras multi-bit utilizadas. El segundo chip, implementado usando contadores de un solo bit con el fin de simplificar el sistema, consigue conformación espectral de ruido de segundo orden y alcanza 103 dB-A de rango dinámico en el ancho de banda del audio, ocupando solo 0.04 mm2.Programa Oficial de Doctorado en Ingeniería Eléctrica, Electrónica y AutomáticaPresidente: Georges G.E. Gielen.- Secretario: José Manuel de la Rosa.- Vocal: Ana Rus

Micromachined in-plane acoustic pressure gradient sensors

textThis work presents the fabrication, modeling, and characterization of two first-generation acoustic in-plane pressure gradient sensors. The first is a micromachined piezoelectric microphone. The microphone structure consists of a semi-rigid beam structure that rotates about torsional pivots in response to in-plane pressure gradients across the length of the beam. The rotation of the beam structure is transduced by piezoelectric cantilevers, which deflect when the beam structure rotates. Sensors with both 10 and 20-μm-thick beam structures are presented. An analytical model and multi-mode, multi-port network model utilizing finite-element analysis for parameter extraction are presented and compared to acoustic sensitivity measurements. Directivity measurements are interpreted in terms of the multi-mode model. A noise model for the sensor and readout electronics is presented and compared to measurements. The second sensor is a capacitive sensor which is comprised of two vacuum-sealed, pistons coupled to each other by a pivoting beam. The use of a pivoting beam can, in principle, enable high rotational compliance to in-plane small-signal acoustic pressure gradients, while resisting piston collapse against large background atmospheric pressure. A design path towards vacuum-sealed, surface micromachined broadband microphones is a motivation to explore the sensor concept. Fabrication of surface micromachined prototypes is presented, followed by finite element modeling and experimental confirmation of successful vacuum-sealing. Dynamic frequency response measurements are obtained using broadband electrostatic actuation and confirm a first fundamental rocking mode near 250 kHz. Successful reception of airborne ultrasound in air at 130 kHz is also demonstrated, and followed by a discussion of design paths toward improve signal-to-noise ratio beyond that of the initial prototypes presented. A method of localizing sound sources is demonstrated using the piezoelectric sensor. The localization method utilizes the multiple-port nature of the sensor to simultaneously extract the pressure gradient and pressure magnitude components of the incoming acoustic signal. An algorithm for calculating the sound source location from the pressure gradient and pressure magnitude measurement is developed. The method is verified by acoustic measurements performed at 2 kHz.Electrical and Computer Engineerin

Experimental characterization of the electrostatic levitation force in MEMS transducers

In this study, a two-step experimental procedure is described to determine the electrostatic levitation force in MEMS transducers. In these two steps, the microstructure is excited quasi-statically and dynamically and its response is used to derive the electrostatic force. The experimental results are obtained for a 1 by 1 plate that employs 112 levitation units. The experimentally obtained force is used in a lumped parameter model to find the microstructure response when it is subjected to different dynamical loads. The natural frequency and the damping ratios in the model are identified from the experimental results. The results show this procedure can be used as a method to extract the electrostatic force as a function of the microstructure’s degrees of freedom. The procedure can be easily used for any microstructure with a wide variety of electrode configurations to predict the response of the system to any input excitation

Nanoelectromechanical Sensors based on Suspended 2D Materials

The unique properties and atomic thickness of two-dimensional (2D) materials enable smaller and better nanoelectromechanical sensors with novel functionalities. During the last decade, many studies have successfully shown the feasibility of using suspended membranes of 2D materials in pressure sensors, microphones, accelerometers, and mass and gas sensors. In this review, we explain the different sensing concepts and give an overview of the relevant material properties, fabrication routes, and device operation principles. Finally, we discuss sensor readout and integration methods and provide comparisons against the state of the art to show both the challenges and promises of 2D material-based nanoelectromechanical sensing.Comment: Review pape

MEMS microphone design.

This thesis presents an overview of microelectromechanical (MEMS) capacitive type microphone design for use in hearing instruments. A cohesive methodology is achieved via a mechanical equation of motion. Resulting in displacement, change in capacitance, sensitivity and pull-in voltage. All derived from one equation. From this investigation it is apparent that sensitivity is the most important factor in MEMS microphone design. The topics covered in the overview are: MEMS microphone design considerations, comparison of microphone types, signal detection methods, sources of dampening, modeling methods, sensitivity estimation, pull-in voltage estimation, bias voltage, ultimate tensile strength, design space optimization and MEMS microphone design flow. A current state of the art design is used as an example throughout the overview. The current state of the art design utilises a square diaphragm with width 2600, thickness 3 and air gap 4 mum, with 361 vent holes of effective radius 33.9 mum in a 13 mum thick backplate. With the initial modeling conclusions in place, two new MEMS capacitive microphone designs are introduced, modeled and analysed. (Abstract shortened by UMI.)Dept. of Electrical and Computer Engineering. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis2003 .S65. Source: Masters Abstracts International, Volume: 42-05, page: 1829. Adviser: W. C. Miller. Thesis (M.A.Sc.)--University of Windsor (Canada), 2003

Large-stroke capacitive MEMS accelerometer without pull-in

In this study, the feasibility of obtaining electrical read-out data from a capacitive MEMS accelerometer that employs repulsive electrode configuration is demonstrated. This configuration allows for large-stroke vibrations of microstructures without suffering from pull-in failure that exists in conventional accelerometers based on the parallelplate configuration. With initial fabrication gap of 2:75um, the accelerometer can reach a 4:2um dynamical displacement amplitude. The accelerometer is tested up to 95(V) without exhibiting pull-in failure. For comparison, the pull-in voltage of an accelerometer with same dimensions but with conventional parallel-plate electrode configuration is 0:8(V). The MEMS device is fabricated using the POLYMUMPs fabrication standard. An electrical circuit is built to measure the capacitance change due to motion of the accelerometer proof-mass. The accelerometer has a mechanical sensitivity of 35nm g and electrical sensitivity of 5:3mV g . The ability to use large bias voltages without the typical adverse effects on the stability of the moving electrode will enable the design of capacitive MEMS accelerometers with enhanced resolution and tunable frequency range

Additive manufacturing (3D print) of air-coupled diaphragm ultrasonic transdrucers

Air-coupled ultrasound is a non-contact technology that has become increasingly common in Non Destructive Evaluation (NDE) and material evaluation. Normally, the bandwidth of a conventional transducer can be enhanced, but with a cost to its sensitivity. However, low sensitivity is very disadvantageous in air-coupled devices. This thesis proposes a methodology for improving the bandwidth of an air-coupled micro-machined ultrasonic transducer (MUT) without sensitivity loss by connecting a number of resonating pipes of various length to a cavity in the backplate. This design is inspired by the pipe organ musical instrument, where the resonant frequency (pitch) of each pipe is mainly determined by its length. The −6 dB bandwidth of the "pipe organ" inspired air-coupled transducer is 55.7% and 58.5% in transmitting and receiving modes, respectively, which is ∼5 times wider than a custom-built standard device. After validating the concept via a series of single element low-frequency prototypes, two improved designs: the multiple element and the high-frequency single element pipe organ transducers were simulated in order to tailor the pipe organ design to NDE applications.Although the simulated and experimental performance of the pipe organ inspired transducers are proved to be significantly better than the conventional designs, conventional micro-machined technologies are not able to satisfy their required 3D manufacturing resolution. In recent years, there has been increasing interest in using additive manufacturing (3D printing) technology to fabricate sensors and actuators due to rapid prototyping, low-cost manufacturing processes, customized features and the ability to create complex 3D geometries at micrometre scale. This work combines the ultrasonic diaphragm transducer design with a novel stereolithographic additive manufacturing technique. This includes developing a multi-material fabrication process using a commercial digital light processing printer and optimizing the formula of custom-built functional (conductive and piezoelectric) materials. A set of capacitive acoustic and ultrasonic transducers was fabricated using the additive manufacturing technology. The additive manufactured capacitive transducers have a receiving sensitivity of up to 0.4 mV/Pa at their resonant frequency.Air-coupled ultrasound is a non-contact technology that has become increasingly common in Non Destructive Evaluation (NDE) and material evaluation. Normally, the bandwidth of a conventional transducer can be enhanced, but with a cost to its sensitivity. However, low sensitivity is very disadvantageous in air-coupled devices. This thesis proposes a methodology for improving the bandwidth of an air-coupled micro-machined ultrasonic transducer (MUT) without sensitivity loss by connecting a number of resonating pipes of various length to a cavity in the backplate. This design is inspired by the pipe organ musical instrument, where the resonant frequency (pitch) of each pipe is mainly determined by its length. The −6 dB bandwidth of the "pipe organ" inspired air-coupled transducer is 55.7% and 58.5% in transmitting and receiving modes, respectively, which is ∼5 times wider than a custom-built standard device. After validating the concept via a series of single element low-frequency prototypes, two improved designs: the multiple element and the high-frequency single element pipe organ transducers were simulated in order to tailor the pipe organ design to NDE applications.Although the simulated and experimental performance of the pipe organ inspired transducers are proved to be significantly better than the conventional designs, conventional micro-machined technologies are not able to satisfy their required 3D manufacturing resolution. In recent years, there has been increasing interest in using additive manufacturing (3D printing) technology to fabricate sensors and actuators due to rapid prototyping, low-cost manufacturing processes, customized features and the ability to create complex 3D geometries at micrometre scale. This work combines the ultrasonic diaphragm transducer design with a novel stereolithographic additive manufacturing technique. This includes developing a multi-material fabrication process using a commercial digital light processing printer and optimizing the formula of custom-built functional (conductive and piezoelectric) materials. A set of capacitive acoustic and ultrasonic transducers was fabricated using the additive manufacturing technology. The additive manufactured capacitive transducers have a receiving sensitivity of up to 0.4 mV/Pa at their resonant frequency