Aluminum Nitride Thin Films Research Articles

Ti1-xAlxN or TiN + AlN: an investigation about Al influence on titanium aluminum nitride thin films structural organization

Ti1-xAlxN or TiN + AlN: an investigation about Al influence on titanium aluminum nitride thin films structural organization

Recent Advances in Aluminum Nitride (AlN) Growth by Magnetron Sputtering Techniques and Its Applications

This review explores the processes involved in enhancing AlN film quality through various magnetron sputtering techniques, crucial for optimizing performance and expanding their application scope. It presents recent advancements in growing AlN thin films via magnetron sputtering, elucidating the mechanisms of AlN growth and navigating the complexities of thin-film fabrication. Emphasis is placed on different sputtering methods such as DC, RF, pulsed DC, and high-power impulse DC, highlighting how tailored sputtering conditions enhance film characteristics in each method. Additionally, the review discusses recent research findings showcasing the dynamic potential of these techniques. The practical applications of AlN thin films, including wave resonators, energy harvesting devices, and thermal management solutions, are outlined, demonstrating their relevance in addressing real-world engineering challenges.

Impact of Deposition Temperature on Structural and Electrical Properties of Sputtered AlN/ Si (111) for CMOS compatible MEMS

Aluminium nitride (AlN) thin films are grown by reactive sputtering on Silicon (Si) with (111) orientation at low temperature (< 450 °C) for piezoelectric-based micro-electromechanical systems (MEMS). The grown AlN thin films were polycrystalline wurtzite hexagonal structure with a high texture coefficient for the (002) plane of orientation. The leakage current density by the current-voltage (I-V) measurement was found to be as low as 1.6 ×10-6Acm−2 in the grown films. The trapped charges are crucial in controlling these electrical characteristics, and low interface trap density (6.72 ×1010cm−2 eV) was observed for the sputtered AlN grown at a substrate temperature of 300 °C. This study on the structural and electrical properties of AlN/Si (111) at low substrate temperature is compatible with the complementary metal oxide semiconductor (CMOS) process for constructing piezoelectric-based MEMS devices for various applications.

Systematic approach for high piezoelectric AlN deposition

Aluminum nitride (AlN) is a wide band gap semiconductor with interesting piezoelectric properties for many applications, including micromechanical systems (MEMS), acoustic wave sensors or energy harvesting devices. The influence of DC reactive magnetron sputtering (DCRMS) deposition parameters on the structural, crystalline, and piezoelectric properties of AlN thin films are presented. The systematic approach of Design of Experiments (DoE) has been used to evaluate the role of magnetron power, nitrogen and argon flows on the deposition process and the film properties. Magnetron power and argon flow have resulted to be the parameters causing the most significant effects on the deposition rate. AlN films have been deposited with a high crystal quality, showing low values of FWHM (0.28) and c-axis orientation parallel oriented respect to the growth direction, as evidenced by X-ray diffraction (XRD) analysis and high-resolution transmission electron microscopy (HRTEM). These samples also showed a high piezoelectric coefficient (d33) of 5 pC/N for AlN thin films. This work highlights the importance of deposition parameters on the properties of the film and the important role that DoE play for its optimization with a minimum number of depositions.

Enhancing cardiovascular health monitoring: Simultaneous multi-artery cardiac markers recording with flexible and bio-compatible AlN piezoelectric sensors

Continuous monitoring of cardiovascular parameters like pulse wave velocity (PWV), blood pressure wave (BPW), stiffness index (SI), reflection index (RI), mean arterial pressure (MAP), and cardio-ankle vascular index (CAVI) has significant clinical importance for the early diagnosis of cardiovascular diseases (CVDs). Standard approaches, including echocardiography, impedance cardiography, or hemodynamic monitoring, are hindered by expensive and bulky apparatus and accessibility only in specialized facilities. Moreover, noninvasive techniques like sphygmomanometry, electrocardiography, and arterial tonometry often lack accuracy due to external electrical interferences, artifacts produced by unreliable electrode contacts, misreading from placement errors, or failure in detecting transient issues and trends. Here, we report a bio-compatible, flexible, noninvasive, low-cost piezoelectric sensor for continuous and real-time cardiovascular monitoring. The sensor, utilizing a thin aluminum nitride film on a flexible Kapton substrate, is used to extract heart rate, blood pressure waves, pulse wave velocities, and cardio-ankle vascular index from four arterial pulse sites: carotid, brachial, radial, and posterior tibial arteries. This simultaneous recording, for the first time in the same experiment, allows to provide a comprehensive cardiovascular patient's health profile. In a test with a 28-year-old male subject, the sensor yielded the SI = 7.1 ± 0.2 m/s, RI = 54.4 ± 0.5 %, MAP = 86.2 ± 1.5 mmHg, CAVI = 7.8 ± 0.2, and seven PWVs from the combination of the four different arterial positions, in good agreement with the typical values reported in the literature. These findings make the proposed technology a powerful tool to facilitate personalized medical diagnosis in preventing CVDs.

Optimization of Growth Temperature and V/III Ratio toward High-Quality Si-Doped Aluminum Nitride Thin Films on Sapphire

Optimization of Growth Temperature and V/III Ratio toward High-Quality Si-Doped Aluminum Nitride Thin Films on Sapphire

Reduction mechanism of loss tangent of scandium-doped aluminum nitride thin film by post-deposition annealing

Scandium-doped aluminum nitride thin films are key materials for MEMS applications including bulk acoustic wave devices for communication. Although one drawback is the increase in the loss tangent with increasing Sc concentration, the loss tangent is reported to decrease after post-deposition annealing. However, the underlying mechanisms remain unclear. In this study, we propose the hypothesis that a low-resistivity thin layer near the surface of a substrate is one of the main reasons for the high loss tangent, and that annealing enhances the resistivity, eventually decreasing the loss tangent. The reasonability of the hypothesis was successfully confirmed by analyzing the frequency response of the loss tangent using an equivalent circuit with current–voltage characteristics, cathodoluminescence, etc. This achievement represents a significant step toward advanced methods for reducing the loss tangent and its application to other thin-film materials.

High Sensitivity Performance of Piezoelectric Micromachined Ultrasonic Transducer Employing Sc-Doped AlN Thin Film

Abstract Aluminum Nitride (AlN) based piezoelectric micromachined ultrasonic transducers (PMUTs) have been the prime choice in acoustic imaging, because it has advantages of high frequency and thus high resolution. However, low piezoelectric constants of AlN gives rises to limitation in transmitting sensitivity. Scandium (Sc) doping method for AlN can be used to improve its piezoelectricity. In this paper, single element PMUT based on ScAlN thin film was proposed and modeled by performing finite element method (FEM). Influence of different electrode configurations on PMUT performance were studied. The results show that, compared with other electrode configurations, PMUT has the best overall performance when the upper electrode is platinum (Pt) and the lower electrode is molybdenum (Mo). In addition, the acoustic field characteristics and its dynamic analysis of PMUT based on ScAlN in water were studied. The investigation results demonstrated that the relative pulse-echo sensitivity level M of PMUT is significantly improved with the increase of Sc doping concentration in AlN thin film. When the Sc concentration is 40%, the M reaches -44 dB, which is 20 dB higher than that without Sc (-64 dB). The PMUT based on the high Sc-doped AlN thin film has a larger effective electromechanical coupling coefficient and sensitivity, which is beneficial to the application in high sensitivity ultrasound imaging.

Design and Characterization of SAW-Based Wireless and Passive Temperature Sensing System

Abstract The Surface Acoustic Wave (SAW) temperature sensor has received significant attention due to its wirelessly-powered, battery-free, and chipless capabilities. This paper proposes a wireless sensing system comprising a one-port SAW resonator, antennas, and transceiver circuit. The SAW resonator utilized in this system was based on Aluminum Nitride (AlN) thin film, which exhibits high SAW velocity and excellent piezoelectric properties. To completely investigate the performance of the SAW resonator, simulation and experiments were conducted. Upon connecting the SAW sensor to the temperature sensing system via the antenna, the reflection peak of the SAW resonator was clearly observed in the spectrum of the return signal. Additionally, during the temperature tests, the response frequency displayed almost linear behavior as the temperature increased from 30°C to 150°C. The fitted temperature coefficient of frequency (TCF) was 30.68ppm/°C, indicating that the wireless temperature sensing system has high temperature sensitivity.

Effects of sputtering pressure on microstructure and secondary electron emission properties of AlN film prepared by magnetron sputtering

In this paper, aluminum nitride (AlN) thin films were prepared by reactive radio-frequency magnetron sputtering at different sputtering pressures and the crystalline structure, surface morphology and secondary electron emission (SEE) properties of the AlN films were studied in detail. The experimental results reveal that AlN films present the preferred orientation of a-axis (100), and the grain size and surface roughness of the film decrease with the increase of sputtering pressure. Additionally, the SEE coefficient of the film reaches a maximum of 4.3 at 200 eV when the sputtering pressure is 0.55 Pa and is relatively stable under the continuous bombardment of an incident electron with the energy of 200 eV. Thus, the AlN thin film prepared under the optimized sputtering pressure has a relatively high and stable coefficient at a low incident electron energy, which promotes its potential application in the high-gain and long-life electron multipliers.

A piezoelectric MEMS microphone with proof mass and its array

Piezoelectric MEMS microphones (PMMs) have attracted increased attention in home automation, wearable devices and autonomous driving, since the high linearity, dustproof and waterproof, and easy manufacturing. In this paper, a hexagonal PMM with proof mass and its array in parallel are proposed using a 20 % Scandium-doped Aluminum Nitride (ScAlN) thin film on a Silicon-on-insulator (SOI) wafer. The theoretical analysis, simulation, fabrication and verification test were discussed. The performance of the devices was simulated using the finite element simulation, and the resonant frequency is 102.85 kHz. The sensitivity of element and its array were measured up to −68.3 dB and −68.4 dB, respectively (re: 1 V/Pa). And the sensitivities of the element and array were −38.6 dB and −37.9 dB at 1 kHz (re: 1 V/Pa) after amplification and filtering. The SNR of the array was increased to 55.3 dB since the increased capacitance of array. Besides, the total harmonic distortion (THD) was measured 1.7 % and 1.14 %, respectively. The PMM and its array with high SNR and good THD is suitable to commercial acoustic fields, especially the high-noise scenarios.

Design and Fabrication of High-Performance Piezoelectric Micromachined Ultrasonic Transducers Based on Aluminum Nitride Thin Films.

Ultrasound is widely applied in diverse domains, such as medical imaging, non-destructive evaluation, and acoustic communication. Piezoelectric micromachined ultrasonic transducers (PMUTs) capable of generating and receiving ultrasonic signals at the micrometer level have become a prominent technology in the field of ultrasound. It is important to enrich the models of the PMUTs to meet the varied applications. In this study, a series of PMUT devices featured with various top electrode configurations, square, circular, and doughnut, were designed to assess the influence of shape on the emission efficacy. It was demonstrated that the PMUTs with a circular top electrode were outperformed, which was calculated from the external acoustic pressure produced by the PMUTs operating in the fundamental resonant mode at a specified distance. Furthermore, the superior performance of PMUT arrays were exhibited through computational simulations for the circular top electrode geometries. Conventional microelectromechanical systems (MEMS) techniques were used to fabricate an array of PMUTs based on aluminum nitride (AlN) films. These findings make great contributions for enhancing the signal transmission sensitivity and bandwidth of PMUTs, which have significant potential in non-destructive testing and medical imaging applications.

Influence of aluminum addition on structure, hardness, and oxidation resistance of Ta1−xAlxN thin films

Ta1−xAlxN thin films with 5, 15, and 40 at.% Al addition were co‐deposited by reactive magnetron sputtering and characterized by Rutherford backscattering spectroscopy (RBS), grazing angle X‐ray diffraction (GAXRD), X‐ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), nanohardness, and oxidation tests. GAXRD and XPS analyses showed, regardless of the % Al added, the non‐existence of ternary nitride Ta1−xAlxN, but always as individual binary nitrides, TaN and AlN. Sample TaAlN_15 obtained the highest hardness and H3/E2 values, possibly due to the AlN grains presence, which were efficient in distorting the TaN lattice. All samples failed oxidation tests at 873 K, showing that the Al addition was not efficient in improving this property for tantalum aluminum nitride thin films.

MoS2-assisted growth of highly-oriented AlN thin films by low-temperature van der Waals epitaxy

Aluminum nitride (AlN) is a wide bandgap material used in acoustic devices, piezo- micro-electromechanical system and is promising for other electronic applications. However, for most applications, the AlN crystalline quality obtained by PVD or MOCVD is insufficient, and suitable growth substrates providing an adapted lattice match and coefficient of thermal expansion are limited. Alternatively, monocrystalline AlN wafers are not yet available in 200/300 mm sizes and suffer from high costs and quality issues. Here, we propose a novel approach involving a two-dimensional transition metal dichalcogenide (TMD) material as a seed layer, which displays an excellent lattice matching with AlN (&amp;gt;98%) allowing a strong enhancement in the c axis texture of sputtered AlN layers on Si(100)/SiO2 thermal oxide (500 nm) substrates. We have successfully demonstrated an eightfold improvement of the AlN (002) rocking curve compared to reference samples grown on thermal SiO2, thus providing a relevant and cost-effective process for the large-scale deployment of high-quality III-N materials on silicon-based substrates.

Structural and optical properties of epitaxial ScxAl1−xN coherently grown on GaN bulk substrates by sputtering method

Three scandium aluminum nitride (ScAlN) thin films with different Sc compositions of 6%, 10%, and 14% were heteroepitaxially grown on n-type GaN bulk substrates by a low-temperature sputtering method. Atomically flat and smooth surfaces were observed by atomic force microscopy. The ScAlN films were coherently grown on GaN, and the c-axis lattice constants increased with increase in the Sc composition, confirmed by x-ray diffraction. The refractive index and the extinction coefficient of ScAlN were extracted by variable angle spectroscopic ellipsometry. The refractive index slightly increased and the extinction coefficient showed red shift with increase in the Sc composition. The optical bandgap of the ScAlN films was also extracted, which slightly shrunk with increase in the Sc composition.

Compressive stress reduction in sputter-deposited yttrium aluminum nitride (Y0.2Al0.8N) thin films for BAW resonators with high electromechanical coupling

In the past decade, rare earth elements have found increasing interest in enhancing the piezoelectric coefficient of aluminum nitride (AlN) thin films. Until now, scandium has been the most successful rare earth element to enhance the piezoelectric coefficient (d33) up to 600 %. In this research paper, we present a 250 % increase in d33 to 12.5 pC/N for sputter-deposited yttrium alloyed aluminum nitride (Y0.2Al0.8N) thin films compared to pure AlN. However, this increase in d33 comes with a substantial increase in compressive layer stress well above 1 GPa. Therefore, a chamber pressure sweep technique is introduced, resulting in a 40 % reduction in the stress level without significantly affecting the crystallographic film parameters to facilitate device integration. Even more, high compressive stress in thin films is a critical concern in piezoelectric MEMS devices such as bulk acoustic wave (BAW) resonators, as it can adversely affect the piezoelectric material properties and hence, the electromechanical performance. For this purpose, we demonstrate that the pressure sweep technique improves the electromechanical coupling factor of Y0.2Al0.8N to 7.02 %, being substantially above the reference value of 6.02 % set by pure AlN. In summary, the findings from this study may stimulate further effort to integrate YxAl1-xN thin films in BAW filter devices for future telecommunication applications.

Characteristics of aluminium nitride thin film prepared by pulse laser deposition with varying laser pulses

In this work, the studies of Aluminium Nitride (AlN) thin film deposited on silicon (Si) (111) substrate using the pulsed laser deposition (PLD) method, specifically under different number of laser pulses, were performed. From the electron micrographs, the samples exhibited a densely packed granular morphology. The film thickness, particle size, and surface roughness were found to be increasing along with number of laser pulses. X-ray diffraction analysis revealed all AlN samples to have hexagonal wurtzite structure. Additionally, the calculated lattice constants suggest AlN experiences inward biaxial stress during growth. This was in agreement with the Raman results, aside from substantiating the presence of AlN with (100) and (101) plane to be grown. Room temperature photoluminescence (PL) measurements showed band-edge emission was attributed to AlN along with some defect-related bands. Additionally, the band-edge emission shifted to higher photon energy, i.e. 5.49–5.89 eV with increasing laser pulses. Such observations were consistent with that obtained from UV–Vis spectroscopy.

An Investigation of the Structural and Surface Topography of Aluminum Nitride Thin Films Deposited at Low Sputtering Power for Piezoelectric Applications

An Investigation of the Structural and Surface Topography of Aluminum Nitride Thin Films Deposited at Low Sputtering Power for Piezoelectric Applications

Comparison of aluminum nitride thin films prepared by magnetron sputter epitaxy in nitrogen and ammonia atmosphere

Wurtzite-type aluminum nitride (AlN) thin films exhibiting high thermal conductivity, large grain size, and low surface roughness are desired for both bulk acoustic wave and surface acoustic wave resonators. In this work, we use ammonia (NH3) assisted reactive sputter deposition of AlN to significantly improve these properties. The study shows a systematic change in the structural, thermal, and morphological properties of AlN grown in nitrogen (N2) and N2 + NH3 atmosphere. The study demonstrates that NH3 assisted AlN sputtering facilitates 2D growth. In addition, the study presents a growth model relating the 2D growth to improve the mobility of aluminum (Al) and nitrogen (N) ad-atoms in NH3 atmosphere. Consequently, the thermal conductivity and roughness improve by ≈76%, and ≈35%, while the grain size increases by ≈78%.

Design and Characterization of Surface Acoustic Wave-Based Wireless and Passive Temperature Sensing System.

The surface acoustic wave (SAW) temperature sensor has received significant attention due to its wirelessly powered, battery-free, and chipless capabilities. This paper proposes a wireless sensing system comprising a one-port SAW resonator, helix antenna, and transceiver circuit. The SAW resonator used in this system is based on aluminum nitride (AlN) thin film, which exhibits high velocity and excellent piezoelectric properties. Simulations and experiments were conducted to investigate the performance of the designed SAW resonator. A helix antenna was also designed using finite element simulation to facilitate signal transmission between the SAW temperature sensor and the transceiver. An impedance-matching network was introduced between the helix antenna and the SAW resonator to optimize signal transmission. When the wireless SAW temperature sensor was placed within a certain distance of the mother antenna, the reflection peak of the SAW resonator was observed in the spectrum of the return signal. The frequency of the echo signal increased almost linearly as the temperature increased during the temperature tests. The fitted temperature coefficient of frequency (TCF) was -31.34 ppm/°C, indicating that the wireless temperature sensing system has high-temperature sensitivity.