Fabrication and Experimental Characterization of a MEMS Microphone Using Electrostatic Levitation

Microelectromechanical systems (MEMS) microphone sensors have significantly improved in the past years, while the readout electronic is mainly implemented using switched-capacitor technology. The development of new battery powered “always-on” applications increasingly requires a low power consumption. In this paper, we show a new readout circuit approach which is based on a mostly digital Sigma Delta () analog-to-digital converter (ADC). The operating principle of the readout circuit consists of coupling the MEMS sensor to an impedance converter that modulates the frequency of a stacked-ring oscillator—a new voltage-controlled oscillator (VCO) circuit featuring a good trade-off between phase noise and power consumption. The frequency coded signal is then sampled and converted into a noise-shaped digital sequence by a time-to-digital converter (TDC). A time-efficient design methodology has been used to optimize the sensitivity of the oscillator combined with the phase noise induced by and thermal noise. The circuit has been prototyped in a 130 nm CMOS process and directly bonded to a standard MEMS microphone. The proposed VCO-based analog-to-digital converter (VCO-ADC) has been characterized electrically and acoustically. The peak signal-to-noise and distortion ratio (SNDR) obtained from measurements is 77.9 dB-A and the dynamic range (DR) is 100 dB-A. The current consumption is 750 A at 1.8 V and the effective area is 0.12 mm. This new readout circuit may represent an enabling advance for low-cost digital MEMS microphones.

Dual-frequency piezoelectric microelectromechanical systems (MEMS) microphones based on highly doped AlSc30%N thin films are developed for aeroacoustic measurements with high sensitivity, wide bandwidth, and large dynamic range. Comparing with the previous version based on lead zirconate titanate (PZT) thin films, the sensitivity of AlSc30%N based dual frequency MEMS microphones is significantly improved and no direct current bias is required for the new AlSc30%N material. The microfabrication, packaging, and testing of MEMS microphones with two pairs of triangular cantilevers were conducted using the cleanroom facilities and characterization tools. The MEMS die structure was characterized by a focused ion beam scanning electron microscopy (FIB-SEM). The resonant frequencies and mode shapes of large and small cantilevers were measured by laser Doppler vibrometer (LDV) to evaluate their mechanical performance. The aeroacoustic characterizations were conducted at the BACCHUS test bench in Airbus. The sensitivity of 3.2mV/[email protected], signal-to-noise ratio (SNR) of 60.2 dB (ref. 94 dB SPL, 30kHz), frequency range up to 55 kHz, and dynamic range up to 147 dB SPL were obtained in the first preliminary test. The MEMS microphone samples were also acoustically characterized within high-speed air flow. The overall performance of the dual-frequency piezoelectric MEMS microphones based on AlSc30%N thin films is better than the state-of-the-art microphones taking into account key parameters such as form factor, acoustic performance, and product price.

A unidirectional microelectromechanical system (MEMS) microphone single module for handsfree and voice recognition systems in automobiles is presented. Because in-cabin noise and temperature variation affect the reduction in the handsfree and voice recognition system performance, noise suppression and increasing the signal-to-noise ratio (SNR) of the microphone are required. In this study, a capacitive MEMS microphone module with a high SNR and a unidirectional characteristic is achieved by designing the structure, package, and module of the MEMS microphone. To improve the SNR, the microphone is developed using a slit-edged membrane. The slit structure is designed to release the residual stress of the membrane to achieve improved sensitivity and SNR. The unidirectional characteristic of the microphone enables suppression of noise signals from undesired directions. The directional characteristic of the microphone is realized by attaching a porous SU-8 filter to delay the time to one of the two acoustic ports on the package. Tests on the proposed unidirectional MEMS microphone package and module show that an SNR of 62.4 dB and a front-back ratio of 27.1 dB are achieved.

Impact-Echo (IE) is a nondestructive testing technique for plate like concrete structures. We propose a new sensor concept for air-coupled IE measurements. By using an array of MEMS (micro-electro-mechanical system) microphones, instead of a single receiver, several operational advantages compared to conventional sensing strategies in IE are achieved. The MEMS microphone array sensor is cost effective, less sensitive to undesired effects like acoustic noise and has an optimized sensitivity for signals that need to be extracted for IE data interpretation. The proposed sensing strategy is justified with findings from numerical simulations, showing that the IE resonance in plate like structures causes coherent surface displacements on the specimen under test in an area around the impact location. Therefore, by placing several MEMS microphones on a sensor array board, the IE resonance is easier to be identified in the recorded spectra than with single point microphones or contact type transducers. A comparative measurement between the array sensor, a conventional accelerometer and a measurement microphone clearly shows the suitability of MEMS type microphones and the advantages of using these microphones in an array arrangement for IE. The MEMS microphone array will make air-coupled IE measurements faster and more reliable.

A method for the direct measurement of the pressure sensitivity of a microelectromechanical system (MEMS) microphone is developed for frequencies from 10 to 100 kHz. The use of a simultaneous pressure field measurement reduces the errors in calculating the pressure sensitivity from free field measurements, and it allows calibration of non-reciprocal and non-capacitive transduction schemes. An ionophone is used as a calibration source in an anechoic chamber. An aluminum plate is mounted in the anechoic chamber and contains a linear array of pressure field condenser microphones. Treatment of the plate edge is used to minimize diffraction within the pressure field. The microphone data are used to characterize the directivity of the ionophone and localize sources of acoustic diffraction that could impact future MEMS microphone magnitude and phase calibrations.

Preparing and pre-testing experimental setups for flight tests is a lengthy but necessary task. One part of this preparation is comparing newly available measurement technology with proven setups. In our case, we wanted to compare acoustic Micro-Electro-Mechanical Systems (MEMS) to large and proven surface-mounted condenser microphones. The task started with the comparison of spectra in low-speed wind tunnel environments. After successful completion, the challenge was increased to similar comparisons in a transonic wind tunnel. The final goal of performing in-flight measurements on the outside fuselage of a twin-engine turboprop aircraft was eventually achieved using a slim array of 45 MEMS microphones with additional large microphones installed on the same carrier to drawn on for comparison. Finally, the array arrangement of MEMS microphones allowed for a complex study of fuselage surface pressure fluctuations in the wavenumber domain. The study indicates that MEMS microphones are an inexpensive alternative to conventional microphones with increased potential for spatially high-resolved measurements even at challenging experimental conditions during flight tests.

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.

We fabricate a microelectromechanical systems (MEMS) microphone that incorporates a graphene-based membrane that vibrates in response to acoustic forcing. We employ a novel fabrication process, where a graphene/poly(methyl methacrylate) (PMMA) bilayer membrane is transferred over a cavity on a separate chip before being affixed to the surface of another chip containing an electrode, resulting in the fabrication of a moveable capacitor with a membrane-to-electrode gap of 8 μm. The gap, which is less than half the size of other reported graphene membrane-based audio transducers, allows for the device to operate with the low-DC bias voltages of about 1 V and, when integrated with a custom-designed readout circuit, demonstrates a sensitivity to sound pressure between 0.1 and 10 mV/Pa across the range 100 Hz-20 kHz. As well as a sensitivity that is comparable with the previous work, the flat frequency response is stable when the sound pressure is varied between 70 and 80 dBSPL, with the sensitivity value not varying by more than 0.2 mV/Pa.

Surface-mount microphones based on MEMS (micro-electromechanical system) technologies have recently become viable as component-level engineering solutions for acoustic measurements. In addition, advances in microelectronics, flexible circuitry, and array processing have motivated the design of a high-speed, low-cost acoustic array system for aeroacoustic measurements. A variety of microphones are available in the market today. Each type of microphone has its benefits and drawbacks. For example, standard condenser microphones have excellent sensitivity, stability, and high frequency response, but tend to be unwieldy, expensive, and require relatively high operational voltages. Electret microphones are small and fairly inexpensive but their performance deteriorates if exposed to moderately elevated temperatures. MEMS microphones combine the best features of the electret and condenser microphones while occupying a volume of less than 20 cubic millimeters. This paper details the custom-made 128-channel MEMS-based acoustic array for wind tunnel applications as well as the electrical, mechanical, and acoustic properties of the MEMS microphones utilized here.

Detecting and finding people are complex tasks when visibility is reduced. This happens, for example, if a fire occurs. In these situations, heat sources and large amounts of smoke are generated. Under these circumstances, locating survivors using thermal or conventional cameras is not possible and it is necessary to use alternative techniques. The challenge of this work was to analyze if it is feasible the integration of an acoustic camera, developed at the University of Valladolid, on an unmanned aerial vehicle (UAV) to locate, by sound, people who are calling for help, in enclosed environments with reduced visibility. The acoustic array, based on MEMS (micro-electro-mechanical system) microphones, locates acoustic sources in space, and the UAV navigates autonomously by closed enclosures. This paper presents the first experimental results locating the angles of arrival of multiple sound sources, including the cries for help of a person, in an enclosed environment. The results are promising, as the system proves able to discriminate the noise generated by the propellers of the UAV, at the same time it identifies the angles of arrival of the direct sound signal and its first echoes reflected on the reflective surfaces.

Development of a process modeling for residual stress assessment of multilayer thin film structure

Towards a high bias voltage MEMS filter using electrostatic levitation

The research and design of sensor for aircraft engine gas path debris electrostatic monitoring are given in this paper. The mechanism of charged debris and principle of electrostatic induction of sensor are researched firstly, and when the charged material near or keep away from the electrostatic sensor, the induced charge of opposite polarity will be induced on the sensor’s probe. And then, the structure of electrostatic monitoring sensor is designed based on the principle of electrostatic induction. Electrostatic sensor is constituted by sensitive element, dielectric, internal circuit and grounding shell. Due to the particularity of working environment of electrostatic sensor, various factors such as performance requirements and using occasions should be considered in designed process. Finally, the effectiveness of electrostatic monitoring sensor is verified using experimental system. From the experimental results we can know when the charged particles through the electrostatic sensor, the sensing signal amplitude and the activity level are change obviously. These works provide theoretical method for the early fault warning of aircraft engine gas path components, and the monitoring and management of aircraft engine working condition.

Clivia biosensor: Soil moisture identification based on electrophysiology signals with deep learning

The design, fabrication, and characterization of a surface micro-machined, front-vented, 64 channel (8×8), capacitively sensed microphone array-on-a-chip are described. The element pitch is 1.3-mm center-to-center and is targeted at resolving the high-wavenumber components of the pressure spectra underneath the turbulent boundary layer. Care is taken to minimize package topology to reduce flow generated self-noise. The array was fabricated using the MEMSCAP PolyMUMPs® process, a three layer polysilicon surface micromachining process, with the addition of a Parylene-C coating in post-processing. An acoustic lumped element model, including mechanical components, environmental loading, fluid damping, and other acoustic elements, is detailed. Laboratory calibration indicates a sensitivity of 1 mV/Pa for each microphone over a 200–40 000 Hz band. A strong resonance occurs at 280 kHz in close accordance with modeled results. Spatial mapping of the array reveals minimal electrical and physical cross-talk between elements. Preliminary element-to-element sensitivity comparisons exhibit a standard deviation of 13%, 12%, and 25% at 250 Hz, 500 Hz, and 1 kHz. Phase matching showed a 9, 10, and 15 deg standard deviation difference at the same frequencies. A more in depth analysis of acoustic sensitivity is ongoing as well as element-to-element variability.