Aluminum Nitride MEMS Research Articles

Vibration Analysis at Castello Ursino Picture Gallery (Sicily, Italy) for the Implementation of Self-Generating AlN-MEMS Sensors.

This work explores the potential of self-powered MEMS devices for application in the preventive conservation of cultural heritage. The main objective is to evaluate the effectiveness of piezoelectric aluminum nitride MEMS (AlN-MEMS) for monitoring vibrations and to investigate its potential for harvesting energy from vibrations, including those induced by visitors. A preliminary laboratory comparison was conducted between AlN-MEMS and the commercial device Tromino®. The study was then extended to the Picture Gallery of Ursino Castle, where joint measurements with the two devices were carried out. The analysis focused on identifying natural frequencies and vibrational energy levels by key metrics, including spectral peaks and the Power Spectral Density (PSD). The results indicated that the response of the AlN-MEMS aligned well with the data collected by the commercial device, especially observing high vibrational energy around 100 Hz. Such results validate the potential of AlN-MEMS for effective vibration measurement and for converting kinetic energy into electrical power, thereby eliminating the need for external power sources. Additionally, the vibrational analysis highlighted specific locations, such as the measurement point Cu4, as exhibiting the highest vibrational energy levels. These points could be used for placing MEMS sensors to ensure efficient vibration monitoring and energy harvesting.

Aluminum Nitride MEMS Resonant Pressure Gauges Without Vacuum Packaging

Aluminum Nitride MEMS Resonant Pressure Gauges Without Vacuum Packaging

A fluid filled MEMS hydrophone

MEMS hydrophones are of interest to provide a small form factor acoustic sensing capability in water. The majority of previous work on MEMS hydrophones (e.g., Bernstein 1997, Moon, 2010, Gu, 2018) use air backed diaphragms in order to provide increased sensitivity. However, this limits the operating depth of the device due to a reduction in the diaphragm burst pressure. In this work we investigate an architecture that has the potential to increase the operating depth by filling the backing cavity with a heavy fluid. We have designed architectures with multiple acoustic ports (Moon, 2010) and coupled piezoelectric diaphragms, in a bid to maintain sufficient sensitivity while incorporating the filling fluid. The sensing elements are Parylene coated, repackaged Vesper VM1000 aluminum nitride MEMS microphones. It was reported (Travaglione, 2018) that such a device can sense underwater sound, but characterization was limited. In our prototypes, the microphone die are co-packaged onto modified printed circuit boards which include the acoustic coupling elements and required preamplifiers. We report on computational predictions of sensitivity and resolution, and provide initial test results as part of our ongoing investigation. Support from OUSD (R&E) Innovation and Modernization Office.

Investigation of high-quality-factor aluminum nitride MEMS cantilever resonators

This paper presents the design, fabrication, and characterization of cantilever-type resonators with a novel stacked structure. Aluminum nitride is adopted as the material for both the structural layer and the piezoelectric layer; this simplifies the fabrication process and improves the quality factor of the resonator. Both in-plane and out-of-plane flexural modes were investigated. The effect of the structural dimensions and electrode patterns on the resonator’s performance were also studied. Finite-element simulations and experiments examining anchor loss and thermoelastic damping, which are the main loss mechanisms affecting the quality factor of these resonators, were carried out. The optimal structural dimensions and electrode patterns of the cantilever-type resonators are presented. A quality factor of 7922 with a motional impedance of 88.52 kΩ and a quality factor of 8851 with a motional impedance of 67.03 kΩ were achieved for the in-plane and out-of-plane flexural-mode resonators, respectively. The proposed resonator design will contribute to the development of high-performance devices such as accelerometers, gyroscopes, and pressure sensors.

CMOS-Integrated Aluminum Nitride MEMS: A Review

Aluminum nitride (AlN) has gained wide interest owing to its high values of elastic modulus, band gap, dielectric strength, resistivity, thermal conductivity and acoustic velocities, especially because it retains most of its properties and versatility in the thin-film form. This review focuses on applications where the CMOS integration of AlN MEMS has been effectively demonstrated. First, the fundamental concepts of piezoelectricity on polycrystalline <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$c$ </tex-math></inline-formula> -axis oriented thin-films are introduced and AlN is compared to other common piezoelectric materials, namely LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , quartz, lead zirconate titanate (PZT), ZnO and GaN by thoroughly discussing the material properties, processing and technological implications. After presenting the possible MEMS-CMOS integration strategies, recent demonstrations of AlN-based devices are reviewed, namely energy harvesters, film bulk acoustic resonators (FBAR), contour mode resonators (CMR), gas sensors, imagers, microphones, transducers for chip-scale communication and calorimetric sensors. Finally, other recent applications/integration opportunities are outlined for AlN-based micro-mirrors, flexible sensors and transducers for liquid media and harsh environments. [2022-0006]

A piezoelectric AlN MEMS hydrophone with high sensitivity and low noise density

This paper presents a micromachined hydrophone with high sensitivity and low noise density. The hydrophone is composed of a 10by10 piezoelectric aluminum nitride (AlN) membrane array, a low noise amplification circuit, and packaged by an acoustic transparent material. Equivalent noise model is used to analyze the equivalent input noise of receiving system, which can be effective to reducing the noise floor by choosing appropriate components. Moreover, a new pouring method is applied to improve the acoustic performance of the matching layer. The experiment results show that the packaged MEMS hydrophone achieves an acoustic sensitivity of -178 dB (Ref. 1 V/μPa), a bandwidth from 100 Hz to 1600 Hz, and an equivalent noise density of 52.6 dB@100 Hz (Ref. μPa/√Hz). The enhanced hydrophone is beneficial to achieve weak signals detecting in pipeline leak detection, marine noise monitoring and many other low frequency engineering applications.

Local residual stress monitoring of aluminum nitride MEMS using UV micro-Raman spectroscopy

Localized stress variation in aluminum nitride (AlN) sputtered on patterned metallization has been monitored through the use of UV micro-Raman spectroscopy. This technique utilizing 325 nm laser excitation allows detection of the AlN E2(high) phonon mode in the presence of metal electrodes beneath the AlN layer with a high spatial resolution of less than 400 nm. The AlN film stress shifted 400 MPa from regions where AlN was deposited over a bottom metal electrode versus silicon dioxide. Across wafer stress variations were also investigated showing that wafer level stress metrology, for example using wafer curvature measurements, introduces large uncertainties for predicting the impact of AlN residual stress on the device performance.

Multi-sensitive temperature sensor platforms using aluminum nitride MEMS resonators

This paper presents three MEMS platforms that exhibit multiple thermal sensitivities and multi-frequency capabilities. Multiple sensors with a variety of operating frequencies and thermal sensitivities can co-exist in the same device wafer. Aluminum nitride is the active layer of the three platforms. The impact of stack and substrate modifications on the performance of test devices is discussed as well. To test the platforms’ performance, temperature sensors are realized using Lamb-acoustic-wave micro-electro-mechanical (MEMS) resonators, each one featuring a different thermal coefficient of frequency that scales with frequency. Resonators designed for frequencies between 200 MHz to 1.5 GHz operate at their first symmetric mode (S0) and feature first-order TCFs in the range from −12 up to −30 ppm °C−1 depending on the frequency and design. Furthermore, TCFs of devices can be tailored to get smaller values through process variations. The platforms exhibit high electromechanical performance, with quality factors in excess of 1500 and maximum effective coupling coefficient of 6.43% for radio frequency (RF) applications above 1 GHz.

Researching the Aluminum Nitride Etching Process for Application in MEMS Resonators

We investigated the aluminum nitride etching process for MEMS resonators. The process is based on Cl2/BCl3/Ar gas chemistry in inductively coupled plasma system. The hard mask of SiO2 is used. The etching rate, selectivity, sidewall angle, bottom surface roughness and microtrench are studied as a function of the gas flow rate, bias power and chamber pressure. The relations among those parameters are reported and theoretical analyses are given. By optimizing the etching parameters, the bottom surface roughness of 1.98 nm and the sidewall angle of 83° were achieved. This etching process can meet the manufacturing requirements of aluminum nitride MEMS resonator.

Aluminum nitride MEMS resonator-based sensors and transducers

Aluminum nitride MEMS resonator-based sensors and transducers

A novel checker-patterned AlN MEMS resonator as gravimetric sensor

This paper presents the design and experimental verification of a new class of piezoelectric aluminum nitride (AlN) resonator with checker-patterned electrode architecture for mass sensing applications. The mass sensitivity of the developed sensor is analytically derived and confirmed with FEM simulations and experimental results. The results show that a 773MHz resonator has mass sensitivity of 175.42μm2/ng and detection limit of 18.7ng/cm2. The developed sensor has an obtained mass sensitivity of 2–3 times higher than that of state-of-the-art mass sensors and provides high potential for developed piezoelectric transducers for mass sensing applications.

Piezoelectric aluminum nitride MEMS annular dual contour mode filter

The following work presents the analytical, numerical and experimental characterization of a novel piezoelectric aluminum nitride MEMS bandpass filter. In contrast to multi-pole filters employing distinct mechanically or electrically coupled resonator building blocks, the passband of the device in the present work is defined by the proximity of two natural contour modes of vibration in a single annular resonator. The proposed implementation, albeit currently limited to dual-pole filters, results in smaller form factors and reduces device sensitivity to across wafer fabrication tolerances. A filter with 4.8 dB of insertion loss, a 22.4 MHz center frequency, a TCF of approximately −20 ppm/°C, and 0.5% fractional bandwidth at −3 dB is demonstrated.