Robert Littrell

Modeling and Characterization of Cantilever-Based MEMS Piezoelectric Sensors and Actuators

Piezoelectric materials are used in a number of applications including those in microelectromechanical systems. These materials offer characteristics that provide unique advantages for both sensing and actuating. Common implementations of piezoelectric transduction involve the use of a cantilever with several layers, some of which are piezoelectric. Although most analyses of such a cantilever assume small piezoelectric coupling (SPC), the validity of this assumption has not been fully investigated. This paper presents closed-form expressions for the voltage developed across a piezoelectric layer in an N-layer cantilever used as a sensor (e.g., as a microphone) and for the displacement profile of an N-layer cantilever used as an actuator. This represents the first time these closed-form expressions have been presented without making the SPC assumption and are used to determine the validity of the this assumption. Furthermore, a new, more robust experimental technique for identifying the piezoelectric coefficient is demonstrated using an aluminum nitride (AlN) cantilever beam. The developed expressions are also used to predict the voltage across a piezoelectric layer in a beam containing AlN layers in response to a pressure excitation and are shown to be in close agreement with experimental results.

Piezoelectric Cantilevers for Low‐Noise Silicon Microphones

Microphones fabricated using microelectromechanical systems (MEMS) technology are one of the fastest growing applications of MEMS. Capacitive sensing has been the dominant detection principle used in MEMS microphones. Piezoelectric sensing, however, offers advantages including simpler accompanying circuitry and the possibility for simpler fabrication. Piezoelectric microphones have been limited primarily by a high noise floor, typically at least an order of magnitude higher than, otherwise similar, capacitive microphones. We present a low noise piezoelectric cantilever microphone to overcome the main limitation of previously constructed piezoelectric microphones. Aluminum Nitride (AlN) has been selected as the piezoelectric material because its piezoelectric coupling coefficient, in combination with its electric permittivity, and its piezoelectric loss coefficient enable low‐noise devices. Through both mechanical and electrical optimization, models indicate that by combining several short, thin cantilevers made exclusively of Molybdenum and AlN, microphones with a die size of 1mm x 1mm, 10 kHz bandwidth, 2mV/Pa sensitivity, and noise floor below 40 dBA can be constructed using a simple 4 mask process. Analytical and numerical models and experimental results will be presented.Microphones fabricated using microelectromechanical systems (MEMS) technology are one of the fastest growing applications of MEMS. Capacitive sensing has been the dominant detection principle used in MEMS microphones. Piezoelectric sensing, however, offers advantages including simpler accompanying circuitry and the possibility for simpler fabrication. Piezoelectric microphones have been limited primarily by a high noise floor, typically at least an order of magnitude higher than, otherwise similar, capacitive microphones. We present a low noise piezoelectric cantilever microphone to overcome the main limitation of previously constructed piezoelectric microphones. Aluminum Nitride (AlN) has been selected as the piezoelectric material because its piezoelectric coupling coefficient, in combination with its electric permittivity, and its piezoelectric loss coefficient enable low‐noise devices. Through both mechanical and electrical optimization, models indicate that by combining several short, thin cantilever...

Microphone and microphone array characterization utilizing the plane wave tube method

Microelectromechanical Systems (MEMS) Microphone arrays are becoming ubiquitous in consumer electronics. Large and expensive anechoic chambers are commonly used to characterize these arrays. Individual MEMS microphones, on the other hand, are typically tested using one of three methods: a free field calibration in an anechoic chamber, a pressure field calibration in a pressure chamber, or a pressure field calibration in a plane wave tube (PWT). In this work, we present a PWT system for testing a single microphone as well as a second PWT system for testing an array of four MEMS microphones. Both systems utilize a 3D printed portion of the tube that is designed to minimize reflections and standing waves while allowing the sound pressure to reach a calibrated instrumentation microphone and the MEMS microphone(s) under test. With these PWT systems, we characterize individual microphones up to 30 kHz and microphone arrays up to 3 kHz. Further, the array test system is used to measure the polar pattern of the microphone array at several frequencies and measure the impact of microphone mismatch on array performance. This PWT test methodology is a size and cost effective way to characterize MEMS microphone arrays.Microelectromechanical Systems (MEMS) Microphone arrays are becoming ubiquitous in consumer electronics. Large and expensive anechoic chambers are commonly used to characterize these arrays. Individual MEMS microphones, on the other hand, are typically tested using one of three methods: a free field calibration in an anechoic chamber, a pressure field calibration in a pressure chamber, or a pressure field calibration in a plane wave tube (PWT). In this work, we present a PWT system for testing a single microphone as well as a second PWT system for testing an array of four MEMS microphones. Both systems utilize a 3D printed portion of the tube that is designed to minimize reflections and standing waves while allowing the sound pressure to reach a calibrated instrumentation microphone and the MEMS microphone(s) under test. With these PWT systems, we characterize individual microphones up to 30 kHz and microphone arrays up to 3 kHz. Further, the array test system is used to measure the polar pattern of the m...

Computational methods for the interior structural acoustics of small spaces

In some biomechanical systems and micro-electro-mechanical systems (MEMS), the interaction of a viscous compressible fluid confined in a space bounded in part by a flexible structure is of central importance. Two specific examples are MEMS microphones (condenser or piezoelectric) and the cochlea. In both the manmade and biological acoustical sensor, the interior space is typically smaller than an acoustic wavelength, and a successful design involves trade-offs between sensitivity, bandwidth, and noise (including thermal, mechanical, electrical, or channel generated noise); the latter two criteria depend critically on the viscous and thermal forces in the system. A direct numeric approach to modeling viscous and thermal effects is often prohibitively expensive, as boundary layers must be resolved in the mesh. In this talk, we will present approximate methods that enable the inclusion of viscothermal effects in a computational framework. In particular, a variational approach amenable to inclusion in a finit...

Minimizing noise in micromachined piezoelectric microphones

Piezoelectric microelectromechanical systems (MEMS) microphones have been researched for over 30 yr because they are relatively easy to build, output a signal without any biasing circuitry, and are relatively linear. The primary impediment to mass utilization of piezoelectric MEMS microphones has been the noise levels of these devices, which have been unacceptably high. The input referred noise of most piezoelectric MEMS microphones is greater than or equal to 55 dB(A) while commercial capacitive MEMS microphones typically have noise floors between 32 and 38 dB(A), roughly ten times lower. In order to achieve competitive noise levels in a piezoelectric MEMS microphone, a systematic approach to mechanical and electrical optimizations must be used. A key microphone metric for this optimization is acoustically generated electrical energy, as opposed to sensitivity. This optimization can be used to determine the minimum achievable noise floor for piezoelectric sensors with relatively few assumptions. Further,...

Advantages of piezoelectric microelectromechanical systems (MEMS) microphones.

Microphones fabricated using microelectromechanical systems (MEMS) technology are one of the fastest growing applications of MEMS. Capacitive sensing has been the dominant form of transduction in both traditional and recently commercialized MEMS microphones. Models and experiments, however, indicate that the thin layers and fine spatial resolution made possible by MEMS technology lend themselves more appropriately to piezoelectric microphones. Although piezoelectric MEMS microphones have typically been shown to have a relatively high noise floor, this limitation can be overcome with appropriate design and high quality piezoelectric material. Models indicate that piezoelectric MEMS microphones can achieve a comparable noise floor to capacitive MEMS microphones of similar size and bandwidth while achieving 1000 times greater dynamic range. A piezoelectric MEMS microphone utilizing aluminum nitride (AlN) will be presented. Previously experienced film quality issues have been addressed. This microphone is des...

An Alternate Geometry for a Piezoelectric MEMS Microphone, Part I: Design and Analysis

Microphones fabricated using microelectromechanical systems (MEMS) technology are one of the fastest growing applications of MEMS. While most commercial MEMS microphones are sensed capacitively, piezoelectric MEMS microphones require less accompanying electronics and offer increased linearity. Currently, piezoelectric MEMS microphones suffer from high noise levels, limiting their applicability. This paper presents an alternative sensor geometry consisting of a cantilever beam electrostatically clamped to the center of a diaphragm that both favorably concentrates stress from the applied acoustic load and eliminates the deleterious effects of residual stress in the piezoelectric material. A complete analysis of the sensitivity and noise characteristics of the electromechanical design (including the amplifying electronics) is performed and compared to a design employing a piezoelectric layer on a diaphragm. The analysis has shown that the proposed geometry can be used to build microphones sensed via aluminum nitride with noise levels around 48 dBA while similar materials and sizes result in noise levels around 57 dBA using the standard geometry.Copyright