Piezoelectric AlN Films Research Articles

Design of a multi-physics coupling MEMS pressure sensor

The pressure measurement of explosion shock wave puts forward high requirements on the temperature resistance and response speed of the sensor. In this paper, a multi-physics coupling pressure sensor is designed to meet requirements. The sensor mainly consists of a Fabry-Perot cavity optical fiber sensing unit, a piezoelectric pressure sensing unit and a thermometric resistance temperature measurement module. Firstly, the structure of the sensor is designed. The top and bottom surfaces of the Fabry-Perot cavity are composed of a silicon diaphragm and a quartz glass. The piezoelectric pressure sensing unit is an AlN film including its electrodes. Pt thermometric resistance is developed to realize real-time temperature monitoring. Secondly, the fabrication process of the sensor is discussed, especially three key technologies. An Al film is sputtered to increase the reflectivity of the quartz glass. The SU-8 photoresist is applied to accurately control the thickness of Fabry-Perot cavity. Etching is adopted to work out the graphics of the AlN piezoelectric film. At last, a complete fabrication process of the sensor is described. The fabrication of multilayer films begins with a double-sided polishing silicon wafer, while Fabry-Perot cavity was from a double-sided polishing quartz glass. Then two parts of the sensor are combined to ensure the multi-physics coupling sensor to achieve complete function.

Research on piezoelectric simulation characteristics based on ALN piezoelectric thin film structure

AlN piezoelectric thin films possess exceptional performance and find extensive applications in the fields of underwater acoustic communication and hydroacoustic sensing. To enhance the output voltage values of AlN piezoelectric thin films and optimize their structure, comprehensive simulations and analyses were carried out by using COMSOL on both circular and square film configurations. These investigations specifically focused on examining the impact of dimensional variations on resonance frequency and output voltage, as well as exploring the relationship between vibration frequency and output voltage. The findings revealed that by employing an appropriate cavity radius, increasing the thickness of the thin film can effectively enhance the output voltage. However, if the thickness surpasses 1.5 μm, the output voltage will gradually decline. Notably, the square film exhibited a significantly higher output voltage measurement of 2158 pm compared to the circular film.

Functionality in frequency tuning of magnetoelectric heterostructure integrated highly flexible bulk acoustic wave resonator

Functional flexible piezo-resonators are of vital interest for designing micro-electrometrical system (MEMS) based high-frequency wearable devices. The magnetoelectric (ME) heterostructure comprising highly magnetostrictive Ni–Mn–In and piezoelectric AlN films was fabricated over flexible substrates to develop the bulk acoustic wave (BAW) resonator. The BAW resonators fabricated over Ni and Kapton substrates show the fundamental resonance at ∼5.535 and 5.400 GHz, respectively. The comparative study of frequency tuning for both resonators has been executed in the presence of a dc magnetic field. A larger frequency shift (ΔfR) of ∼540 MHz was detected at 1200 Oe for a device fabricated over Ni with a sensitivity of ∼5.4 Hz/nT. However, it is 360 MHz in the case of Kapton, with a sensitivity of ∼3.0 Hz/nT. Moreover, the BAW resonator over the magnetostrictive Ni substrate shows a higher tunability of ∼11.4% compared to ∼8.5% for the resonator fabricated over non-magnetostrictive Kapton. The equivalent modified Butterworth–Van Dyke circuit parameters have been extracted by fitting the experimental data with and without an external magnetic field using advanced design system. The effect of an external magnetic field has been thoroughly investigated on device parameters, such as electromechanical coupling coefficient (K2), acoustic velocity, quality factor (Q), and figure of merit. The anisotropic functionality of the fabricated resonator has been studied by measuring the tunability of the resonator in parallel and perpendicular magnetic fields. The present study motivates the incorporation of flexible magnetostrictive substrates for futuristic multifunctional MEMS magnetic field sensor applications.

Acoustic Waves in Piezoelectric Layered Structure for Selective Detection of Liquid Viscosity.

The acoustic waves of higher orders propagating in a layered structure consisting of a silicon plate coated with piezoelectric ZnO and/or AlN films were used for the development of a sensor with selective sensitivity to liquid viscosity η in the range of 1-1500 cP. In that range, this sensor possessed low sensitivity to liquid conductivity σ and temperature T in the ranges of 0-2 S/m and 0-55 °C, respectively. The amplitude responses insensitive to the temperature instead of the phase were used to provide the necessary selectivity. The sensor was based on a weak piezoactive acoustic wave of higher order. The volume of the probes sufficient for the measurements was about 100 μL. The characteristics of the sensors were optimized by varying the thicknesses of the structure layers, number of layers, wavelength, wave propagation direction, and the order of the acoustic waves. It was shown that in the case of the layered structure, it is possible to obtain practically the same selective sensitivity toward viscosity as for acoustic waves in pure ST, X quartz. The most appropriate waves for this purpose are quasi-longitudinal and Lamb waves of higher order with in-plane polarization. It was found that for various ranges of viscosity η = 1-20 cP, 20-100 cP, and 100-1500 cP, the maximum sensitivity of the appropriate wave is equal to 0.26 dB/cP, 0.087 dB/cP, and 0.013 dB/cP, respectively. The sensitivity of the waves under study toward the electric conductivity of the liquid is much less than the sensitivity to liquid viscosity. These two responses become comparable only for very small η < 2 cP. The waves investigated have shown no temperature responses in contact with air, but in the presence of liquid, they increase depending on liquid properties. The temperature dependence of liquid viscosity is measurable by the same sensors. The results obtained have shown the possibility of designing acoustic liquid viscosity sensors based on multilayered structures. The set of possible acoustic waves in layered structures possesses modified propagation characteristics (various polarization, phase velocities, electromechanical coupling coefficients, and attenuations). It allows choosing an optimal acoustic wave to detect liquid viscosity only.

Single-crystalline AlN/sapphire and composite electrode based ultra-high temperature surface acoustic wave devices

Single crystalline AlN material is very attractive for the development of high temperature electronic devices owing to its high temperature resistance. However, at ultrahigh temperatures, AlN film still will be oxidized and metal electrodes used for the devices will aggregate and vaporize. The deterioration of the material and metal electrodes eventually fails the AlN-based devices. Here, we report a single crystalline AlN film-based ultrahigh temperature surface acoustic wave (SAW) device. By using a thin alumina (Al2O3) protection layer on surface, oxidation of the AlN piezoelectric film was effectively inhibited at high temperatures. A composite electrode based on multilayer structure of Pt-Rh and Al2O3 was designed and fabricated, with the Al2O3 layer as the diffusion barrier layers to suppressed inter-diffusion, agglomeration and vaporization of the metals. The results showed that the developed AlN SAW devices can withstand high temperatures, and work well at temperatures up to 1400 °C.

A Film Bulk Acoustic Resonator-Based Sensor with AlN Piezoelectric Material for Detecting Ethanol and Acetone

Measurement of a volatile solution is essential for laboratory safety and hospital clinic safety. In this paper, we present an ethanol-sensing and acetone-sensing device using an AlN piezoelectric material-based film bulk acoustic resonator (FBAR). In order to realize volatile solution sensing, the AlN-based FBAR was designed, fabricated, and characterized. In our sensor structure, the upper electrode is a Ti/Au (30 nm/150 nm) composite electrode, the bottom electrode is Mo material with 150 nm thickness, and the piezoelectric sensing material is 0.8 μm thickness AlN. We conducted the experiment of ethanol measurement and acetone measurement by using this FBAR detector on the probe station within the vector network analyzer. The resonance frequency of the FBAR detector decreased as the concentration of ethanol increases, while under the circumstance of acetone concentration increasing, the detector’s response is the opposite. The sensing mechanisms of both ethanol measurement and acetone measurement are discussed in this paper, demonstrating that this FBAR detector could be able to distinguish acetone from ethanol due to different sensing mechanisms.

The 3.4 GHz BAW RF Filter Based on Single Crystal AlN Resonator for 5G Application.

To meet the stringent requirements of 5G communication, we proposed a high-performance bulk acoustic wave (BAW) filter based on single crystal AlN piezoelectric films on a SiC substrate. The fabrication of the BAW filter is compatible with the GaN high electron mobility transistor (HEMT) process, enabling the implementation of the integration of the BAW device and high-performance monolithic microwave integrated circuit (MMIC). The single crystal AlN piezoelectric film with 650-nm thickness was epitaxially grown on the SiC substrate by Metal Organic Chemical Vapor Deposition (MOCVD). After wafer bonding and substrate removal, the single crystal AlN film with electrode layers was transferred to another SiC wafer to form an air gap type BAW. Testing results showed that the fabricated resonators have a maximum Q-factor up to 837 at 3.3 GHz resonant frequency and electromechanical coupling coefficient up to 7.2%. Ladder-type filters were developed to verify the capabilities of the BAW and process, which has a center frequency of 3.38 GHz with 160 MHz 3 dB bandwidth. The filter achieved a minimum 1.5 dB insertion loss and more than 31 dB out-of-band rejection. The high performance of the filters is attributed to the high crystallinity and low defects of epitaxial single crystal AlN films.

Increase of electromechanical coupling coefficient k t 2 in (0001)-oriented AlN films by chromium doping

Piezoelectric AlN films possess high bulk acoustic wave velocity, low acoustic attenuation, and good temperature characteristics. However, AlN film bulk acoustic wave resonators (FBARs) have a relatively small electromechanical coupling coefficient k eff 2. It was recently reported that Cr doping in AlN films increased the piezoelectric constant d 33. The k t 2 of AlN FBARs may thus be enhanced by Cr doping. In this study, we investigated the effect of Cr doping in (0001)-oriented AlN films on k t 2 from the frequency characteristics of high overtone bulk acoustic wave resonators. k t 2 of the Cr-doped AlN films was increased for Cr contents of less than 3%. The maximum k t 2 observed for the Cr0.012Al0.088N film was 5.9%, which was approximately 1.4 times higher than that of pure AlN film (k t 2 = 4.4%).

High-G and High-Bandwidth Bulk Acoustic Wave (BAW) Accelerometers Using a Metal-Less AlN-HARPSS Process With 95 nm Gaps

We present a 4.4-MHz resonant accelerometer based on the electrostatic softening effect in a bulk acoustic wave (BAW) resonator, fabricated using a metal-less AlN-HARPSS process with combined piezoelectric and capacitive transduction. BAW accelerometers can be used in high-g and high-dynamic range applications with a very wide bandwidth. The fabrication process supports both piezoelectric and nanogap capacitive transduction in a micro/nanoelectromechanical device, primarily applicable to stiff high-frequency devices that use a nanoscale (<200 nm) capacitive gap for actuation or frequency tuning purposes. It allows for high-temperature processing of substrates containing a sputtered AlN piezoelectric film, which is not typically possible for piezoelectric films sandwiched between metal electrodes. As a demonstration of the process, a metal-less AlN-on-Si BAW accelerometer with a 95-nm gap-changing electrode has been fabricated and experimentally characterized.

Converse Magnetoelectric Composite Resonator for Sensing Small Magnetic Fields

Magnetoelectric (ME) thin film composites consisting of sputtered piezoelectric (PE) and magnetostrictive (MS) layers enable for measurements of magnetic fields passively, i.e. an AC magnetic field directly generates an ME voltage by mechanical coupling of the MS deformation to the PE phase. In order to achieve high field sensitivities a magnetic bias field is necessary to operate at the maximum piezomagnetic coefficient of the MS phase, harnessing mechanical resonances further enhances this direct ME effect size. Despite being able to detect very small AC field amplitudes, exploiting mechanical resonances directly, implies a limitation to available signal bandwidth along with the inherent inability to detect DC or very low frequency magnetic fields. The presented work demonstrates converse ME modulation of thin film Si cantilever composites of mesoscopic dimensions (25 mm × 2.45 mm × 0.35 mm), employing piezoelectric AlN and magnetostrictive FeCoSiB films of 2 µm thickness each. A high frequency mechanical resonance at about 515 kHz leads to strong induced voltages in a surrounding pickup coil with matched self-resonance, leading to field sensitivities up to 64 kV/T. A DC limit of detection of 210 pT/Hz1/2 as well as about 70 pT/Hz1/2 at 10 Hz, without the need for a magnetic bias field, pave the way towards biomagnetic applications.

Characterization of highly textured piezoelectric AlN films obtained from aluminum and ammonium chloride by a simple vapor deposition process

We report a simple and efficient deposition technique for the growth of highly-textured AlN thin films. The proposed chloride-based chemical vapor deposition approach is based on widely available precursors and an unsophisticated experimental set-up. The films were characterized by electron microscopy, X-ray diffraction, Raman spectroscopy and cathodoluminescence. We studied the correlations between the deposition conditions and the properties of the resulting films, such as morphology, microstructure, features of the Raman spectra, and piezoelectric characteristics. In particular, we show that the minimum of the Raman bands A1(TO)/ E2(h) intensity ratio, under excitation along c-axis, corresponds to the maximum of the piezoelectric coefficient. The films deposited under the optimized conditions are composed of well-aligned column-like crystallites. These films have a piezoelectric coefficient of 5.5–6.5 pC/N, which approaches the highest values reported for this material. The simplicity, efficiency and high quality of the resulting films make the proposed deposition approach highly interesting for a wide range of practical applications.

A Geometrical Study on the Roof Tile-Shaped Modes in AlN-Based Piezoelectric Microcantilevers as Viscosity⁻Density Sensors.

Cantilever resonators based on the roof tile-shaped modes have recently demonstrated their suitability for liquid media monitoring applications. The early studies have shown that certain combinations of dimensions and order of the mode can maximize the Q-factor, what might suggest a competition between two mechanisms of losses with different geometrical dependence. To provide more insight, a comprehensive study of the Q-factor and the resonant frequency of these modes in microcantilever resonators with lengths and widths between 250 and 3000 µm and thicknesses between 10 and 60 µm is presented. These modes can be efficiently excited by a thin piezoelectric AlN film and a properly designed top electrode layout. The electrical and optical characterization of the resonators are performed in liquid media and then their performance is evaluated in terms of quality factor and resonant frequency. A quality factor as high as 140 was measured in isopropanol for a 1000 × 900 × 10 µm3 cantilever oscillating in the 11th order roof tile-shaped mode at 4 MHz; density and viscosity resolutions of 10−6 g/mL and 10−4 mPa·s, respectively are estimated for a geometrically optimized cantilever resonating below 1 MHz.

Sensing characteristics of pure-shear film bulk acoustic resonator in viscous liquids

We presented a pure-shear film bulk acoustic resonator (FBAR) and investigated its sensing characteristics in viscous liquids. In the resonator, the electrodes were located on the surface of c-axis-oriented AlN film to generate the lateral electric field and excite the shear acoustic resonance. Compared with the typical quasi-shear film bulk acoustic resonator based on inclined c-axis-oriented AlN or ZnO piezoelectric film, the proposed device exhibits significantly higher Q-factors and a notably improved detection limit, particularly in water and viscous liquids. The frequency shifts show a linear dependency on the square root of the product of the liquid viscosity and density of the glycerol solution in the viscosity range of 1–5 mPa[Formula: see text]s. Furthermore, we measured the mass sensitivity through real-time monitoring of the frequency change during the volatilization process of the loaded saline solutions. The proposed device shows the mass sensitivity of 465 Hz[Formula: see text]cm2/ng and the mass resolutions of 0.17 ng/cm2 in air, 0.25 ng/cm2 in water and 2.08 ng/cm2 in 50% glycerol solution, respectively. The obtained results clearly indicate that the proposed device is capable of using in liquid phase detection with high sensitivity requirements.

Estimation of Wire Bonding State by the Ensemble Based MT Method and Thin AE Sensor

With non-destructive, simple, and low-priced testing methods (which can be applied to manufacturing processes), we can assure high quality production by applying inspection across all products. Quality assurance in wire bonding is possible by using state diagnosis which utilizes detection of elastic waves close to the bonded joint. We succeeded in detecting elastic waves under difficult conditions and in harsh environments by using a thin AE (Acoustic Emission) sensor developed using AlN piezoelectric thin film technology. We attempted to conduct a wire bonding state estimation using the MT (Mahalanobis-Taguchi) method which is a simple and powerful pattern recognition technique. Two issues are raised when applying this method to the manufacturing process: 1) a large probability bias caused by an insufficient number of data samples, and 2) securing homogeneity of the Unit Space. In this paper, we present a state diagnosis method utilizing a real-world application which uses piezoelectric thin film sensing and an improved MT method to which ensemble learning is applied. Performance of this proposed method is also examined through a common benchmark dataset to assure efficient operation in any manufacturing process.

Study of microwave acoustic attenuation in a multifrequency bulk acoustic wave resonator based on a synthetic diamond single crystal

Acoustic attenuation in a multifrequency bulk acoustic wave resonator based on the synthetic diamond single crystal is investigated. The acoustic energy loss in a layered piezoelectric Al/AlN/Mo/(001) diamond structure is analyzed. The depth of a damaged surface layer with a thickness of 20–30 nm in diamond after abrasive finishing is estimated using high-energy electron backscattered diffraction and observation of Kikuchi lines. The estimation shows that the acoustic energy loss at a diamond substrate roughness of up to 20 nm, as well as acoustic loss in thin films, is lower than the bulk acoustic attenuation by an order of magnitude and is of no fundamental importance. However, the surface roughness of the piezoelectric AlN film can contribute comparably with the bulk attenuation in the substrate. It is demonstrated that the transition from the Akhiezer to Landau–Rumer regime in diamond occurs at a frequency of ~1 GHz and the phonon–phonon relaxation time is ~1.6 × 10–10 s. Calculation of acoustic attenuation showed that, although at a frequency of ~1 GHz the acoustic loss in diamond is somewhat higher than in well-known materials with a low attenuation level, the loss in diamond becomes noticeably lower as frequency increases to 8–10 GHz. The obtained maximum experimental value Qf ≈ 10 × 1013 Hz (9.5 GHz) makes synthetic diamond promising for microwave acoustoelectronic devices.

Carbon nanotube forests as top electrode in electroacoustic resonators

We grow carbon nanotube forests on piezoelectric AlN films and fabricate and characterize nanotube-based solidly mounted bulk acoustic wave resonators employing the forests as the top electrode material. The devices show values for quality factor at anti-resonance of ∼430, and at resonance of ∼100. The effective coupling coefficient is of ∼6%, and the resonant frequencies are up to ∼800 MHz above those observed with metallic top electrodes. AlN promotes a strong catalyst-support interaction, which reduces Fe catalyst mobility, and thus enforces the growth of forests by the base growth mechanism.

The Process Optimization of Smart Nanostructured AlN Thin Films Sputtered by Pulsed DC

A ‘Smart’ coating design, based on the incorporation of an AlN piezoelectric thin film sensor into a tribological coating system, has been explained in this paper. As part of the overall coating system, piezoelectric AlN thin films with highly preferred (002) orientations have been deposited by a pulsed closed field unbalanced magnetron sputtering system using Cr, Mo and Pt as the seed/adhesion layer. The texture, orientation and piezoelectric properties of AlN films were characterized by means of x-ray diffraction, field emission scanning electron microscopy and laser interferometry. A Michelson laser interferometer was designed and constructed to obtain the converse piezoelectric response of the deposited AlN thin films. It was found that the incorporation of different seed layers at various working pressure in the sandwiched structure, significantly affected the (002) orientation and the piezoelectric response of AlN thin films.

Carbon nanotube growth on piezoelectric AlN films: influence of catalyst underlayers

Integration of carbon nanotube (CNT) forests in acoustic wave resonators has appeared interesting for sensing applications. High quality CNTs can be grown on piezoelectric AlN by using a stabilising layer for catalyst nanoparticles.

A Multiple Resonant Mode Film Bulk Acoustic Resonator Based on Silicon-on-Insulator Structures

We report a multiple resonant mode film bulk acoustic resonator (FBAR) with different AlN film thicknesses of 605 nm, 640 nm and 680 nm. With the tilted c-axis orientation of the AlN piezoelectric film providing polarization vertical to the c-axis, acoustic wave resonant peaks have been observed for both the thickness shear modes (TSM 0th, TSM 1st, TSM 2nd) and the thickness extension modes (TEM 0th, TEM 1st). The corresponding parallel resonant frequencies are around 1.60 GHz, 2.41 GHz, 3.45 GHz, 2.75 GHz and 4.10 GHz, respectively. The latter two TEM modes also have good quality factors, and high equivalent electromechanical coupling coefficients Keff2 of 647, 3.13% and 113, 6.23%, respectively. By etching the 1.8 μm silicon sacrificial layer, the air gap FBAR devices have been fabricated in an easier and cleaner way resulting in a low insertion loss of −2.2 dB. The overall device structure of the top electrode/AlN film/bottom electrode on SiO2/silicon-on-insulator (SOI) substrate potentially enables CMOS compatibility. These multiple resonant mode FBAR devices will promote the integration of multi-band filters on a single chip. Improvements of the fabrication process, the influence of different AlN film thicknesses and theoretical analyses of the coexistence of multiple resonant modes are presented.

Magnetism modulation in Cu-doped AlN via coupling between AlN thin film and ferroelectric substrate

Cu-doped AlN (Cu:AlN) thin films were deposited on non-ferroelectric substrate (Si (111)) and ferroelectric substrates (LiTaO3 and LiNbO3) by direct current reactive magnetron sputtering. It was discovered that all the deposited Cu:AlN thin films showed room temperature ferromagnetism and the average magnetic moment (AMM) per Cu atom of the film increased with the decrease of the angle (θ) between AlN film plane and the c-axis of ferroelectric substrate. A giant AMM as high as 2.23μB/Cu was obtained when Cu:AlN thin film was deposited on LiTaO3(110) with θ of 0 degree, while a small AMM of about 0.11μB/Cu was observed when Cu:AlN thin film was deposited on LiNbO3 (006) with θ of 90°. It was discovered that the AMM of the film increased with decreasing of θ. The ferromagnetism modulation was ascribed to the coupling between AlN piezoelectric film and ferroelectric substrate, resulting from the change of host lattice structure and spontaneous polarization of AlN thin film. The in-plane anisotropy of AMM and the annealing relaxation results confirmed the anomalous coupling.