Giant piezoelectric response and electromechanical coupling in AlScN thin films via multi-scale lattice engineering

Giant piezoelectric response and electromechanical coupling in AlScN thin films via multi-scale lattice engineering | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Giant piezoelectric response and electromechanical coupling in AlScN thin films via multi-scale lattice engineering Zhenxiang Cheng, Zhenyue Cheng, Tong Liu, Daojian Su, Changhong Yang, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9122069/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract High-performance AlScN thin films are critical for next-generation lead-free piezoelectric applications. Here, exceptional electromechanical coupling and piezoelectric response are demonstrated in Al0.9Sc0.1N thin films via a multi-scale lattice engineering strategy. Post-growth rapid thermal annealing significantly boosts the piezoelectric coefficient to 15.0 pC N-1 and electromechanical coupling to 34.9%. This performance enhancement stems from a synergistic mechanism: a reduced c/a lattice ratio and homogenized nanodomain alignment, fundamentally underpinned by a transition toward higher bond ionicity as evidenced by reduced minimum electron density at the bond saddle points. Leveraging an industrial-grade 6-inch wafer process, these films exhibit tailored electromechanical characteristics with a resonance frequency of 368.2 kHz. The optimized piezoelectric response enables record-breaking acoustic sensitivity of -162.4 dB at 10 Hz and a noise floor of 59 dB at 1 kHz, with robust stability across diverse media and temperatures up to 150 °C. This work provides a scalable, mechanism-driven pathway for tailoring the electromechanical properties of AlScN thin films, addressing the long-standing challenge of high-fidelity weak signal detection in extreme environments. Physical sciences/Materials science/Condensed-matter physics/Ferroelectrics and multiferroics Physical sciences/Physics/Condensed-matter physics/Ferroelectrics and multiferroics AlScN thin films lattice engineering MEMS receivers ultrasensitivity low-frequency applications Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction Piezoelectric micro-electromechanical systems (MEMS) exhibit remarkable advantages in passive operation, extreme miniaturization, and scalable manufacturability, effectively addressing the inherent constraints of conventional bulk materials, such as excessive dimensions, high power consumption, and integration complexities 1-3 . By leveraging efficient mechanical-to-electrical energy conversion, piezoelectric MEMS architectures have underpinned a new generation of precision acoustic transducers 4,5 , intelligent robotic skins 6,7 , environmental energy harvesters 8 , and high-resolution medical imaging arrays systems 9 . As micro-sensor functionalities continue to evolve, piezoelectric MEMS are rapidly advancing to the forefront of autonomous sensing and ubiquitous computing. In the realm of acoustic reception, piezoelectric MEMS have redefined the capabilities of transducers, particularly for low-frequency detection critical for long-range underwater surveillance and seismic monitoring 10,11 . These devices typically operate by converting low-frequency (≤1000 Hz) acoustic waves into electrical signals via thin-film piezoelectricity. While traditional materials like Pb(Zr,Ti)O 3 (PZT) and ZnO are widely utilized, they face significant limitations in low-frequency regimes. For instance, PZT-based cantilevers hydrophones often exhibit a dramatic sensitivity roll-off in the low frequencies, while ZnO sensors typically suffer from substantial signal attenuation due to their modest piezoelectric response and dielectric loss 12,13 . These challenges necessitate the exploration of novel material systems that can deliver high sensitivity and a flat response at low-frequencies without compromising structural integrity. Recently, aluminum nitride (AlN) has emerged as a compelling lead-free alternative due to its high CMOS compatibility, inherent polarity (eliminating the need for high-voltage poling), exceptional thermal conductivity (200 W m -1 K -1 ), and elevated operating temperature (up to 1150 °C) 14-18 . The alloying of scandium (Sc) into the AlN lattice has been shown to exponentially enhance the piezoelectric response by softening the crystal lattice 1,19 . For example, Al 0.71 Sc 0.29 N thin film can achieve a piezoelectric coefficient ( d 33 ) of ~12.6 pC N -1 , more than double the value for pure AlN film 20,21 . Additionally, Sc substitution markedly increases the dielectric constant, with ɛ r ≈ 15 observed for Al 0.85 Sc 0.15 N compared to ɛ r ≈ 10 for AlN 22,23 . However, a critical bottleneck exists: excessively high Sc concentration (typically >20%) often triggers phase instability, the formation of non-polar grains, and prohibitive fabrication cost 24-27 . Consequently, moderate Sc doping represents a more robust and feasible pathway for practical sensing applications, provided that the piezoelectric response can be further augmented through structural engineering. Nevertheless, the seamless integration of Sc-doped AlN into low-frequency MEMS is still impeded by the challenges in synthesizing high-performance thin films, achieving wafer-scale process compatibility, and optimizing device topologies. In this work, we selected an AlN thin film containing approximately 10 at.% Sc (abbreviated as AlScN), which as demonstrated in our density functional theory (DFT) calculations (Supplementary Fig. S1 and Table S1) and supported by the previous literatures (Supplementary Table S2 and Fig. S2), exhibited an optimal ratio of piezoelectric coefficient ( d 33 )/relative permittivity ( ɛ r ) for high receiving sensitivity. Crucially, we demonstrate that post-growth rapid thermal annealing (RTA) dramatically boosts the piezoelectric performance, elevating the d 33 from 8.2 pC N -1 in the as-grown (AG) state to 15.0 pC N -1 . This enhancement is shown to originate from a reduction in the lattice parameter ratio ( c / a ) and the promotion of highly ordered nanoscopic domain orientations. Leveraging a 6-inch wafer-scale fabrication process, we developed a MEMS chip integrating a 5 × 5 array of RTA-AlScN sensing elements. The packaged device achieves a record acoustic pressure sensitivity of -162.4 dB (re: 1 V μPa -1 ) at 10 Hz and a noise equivalent pressure of 59 dB (re: 1 μPa Hz -0.5 ) at 1kHz. Furthermore, the sensor demonstrates robust functionality across diverse media, including water, silicone oil, and solid ceramics, maintaining high fidelity at temperatures up to 150 °C. This study provides a scalable and high-performance framework for lead-free Sc-doped AlN based acoustic sensing in complex and extreme environments. 2. Results 2.1 Microstructure and electrical performance of AlScN thin films To investigate the effects of Sc doping and the RTA process on the crystalline structure of AlScN, X-ray diffraction (XRD) was performed. As shown in Fig. 1a, all samples exhibit a sharp (002) diffraction peak according to the standard wurtzite AlN pattern (JCPDS 76-0565), indicating a high degree of c -axis orientation. This is attributed to the high kinetic energy of sputtered particles during the deposition process 28,29 . Scanning electron microscopy (SEM) reveals both AG and RTA treated AlScN films possess uniform, spherical grains (Fig. 1bi and Supplementary Fig. S3a). Notably, the average grain size of RTA-AlScN (100 nm) is significantly larger than previously reported values for AlScN thin film with similar Sc content (Al 0.90 Sc 0.10 N ~ 50 nm, Al 0.85 Sc 0.15 N ~ 30 nm) 25,30 . Cross-sectional SEM images further confirm that both AG-AlScN and RTA-AlScN films feature well-defined columnar grains with thicknesses of approximately 1 μm (Fig. 1bii and Supplementary Fig. S3b). Guided by our simulation results, this thickness was optimized to achieve a low resonant frequency of 390 kHz and a relatively high surface charge output (5.74×10 -17 C), while ensuring mechanical robustness for practical applications (Supplementary Fig. S4) 31 . The chemical composition and element distribution were further verified by EDS and XPS (Supplementary Fig. S5 and S6). Atomic-scale structural insights were obtained via high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), providing high-precision atomic position measurements with uncertainties below 50 pm. As shown in Fig. 1c, d, N and Al(Sc) atoms form alternating hexagonal close-packed layers along the c -axis. Each Al(Sc) atom resides in a tetrahedral arrangement with four neighboring N atoms; this coordination confirms an “N-polar” growth direction, with the permanent dipole moment oriented toward the substrate. Critically, this N-polar characteristic in the AG-AlScN film remains stable and unchanged after RTA treatment. Statistical analysis of the lattice parameters across multiple regions (Supplementary Fig. S7, S8) reveals that following RTA, both a and c lattice parameters exhibit an increasing trend (Fig. 1e, f). The average a -axis parameter rose significantly from 3.163 Å to 3.279 Å, while the c -axis parameter increased marginally from 4.931 Å to 4.951 Å. The expansion of the a -axis in RTA-AlScN is attributed to stress relaxation, as post-growth RTA weakens the transverse interaction forces between adjacent columnar grains 32 . Consequently, the c / a ratio of RTA-AlScN decreased to 1.510 compared with 1.559 for AG-AlScN (Supplementary Table S3). RTA at high temperatures promotes the substitution of larger Sc 3+ ions into Al 3+ sites and the relaxation of epitaxial strain, driving the lattice towards a flatter, more frustrated hexagonal state with a reduced c/a ratio (Supplementary Fig. S9). The improved piezoelectricity is fundamentally rooted in the synergistic evolution of chemical bonding and lattice symmetry. As corroborated by our DFT calculations (Supplementary Fig. S1f), Sc substitution reduces the minimum electron density (MED) at the bond saddle point—from 0.195 in pure AlN to 0.178 in Al 0.875 Sc 0.125 N—indicating a transition toward higher bond ionicity (weakened covalency). This lattice softening, characterized by decreased MED and electron localization function (ELF) values, facilitates a more compliant structural response to external strain. While Sc substitution establishes the chemical foundation for this softening, the RTA process serves as a structural trigger that further destabilizes the cation position via c/a ratio reduction. This dual-effect—combining intrinsic bonding modification with extrinsic lattice strain engineering—collectively drives the material toward a metastable state with exceptionally high polarizability. Theoretically, the lower c / a ratio resulting from this modified bonding state enhances polarization-strain coupling coefficient ( e 33 ) and the polarization-electric field coupling coefficient ( χ 33 ), thereby significantly boosting the effective piezoelectric coefficient ( d 33 ≈ ε 0 e 33 χ 33 ) 33-35 . To characterize the effective electromechanical coupling coefficient ( k t 2 ), capacitance-electric field ( C - E ) loops were measured (Fig. 2a). The extracted k t 2 values for RTA-AlScN reached 34.9%, a substantial improvement over the 14.1% of AG-AlScN, outperforming reported values for AlScN with Sc concentrations exceeding 30% (Fig. 2b). The d 33 reached 15.0 pC N -1 for RTA-AlScN, representing a 2.5-fold enhancement over pure AlN (6 pC N -1 , Supplementary Fig. S10) and outperforming most reported Sc-doped AlN materials (Fig. 2b). This performance remains highly uniform across the 6-inch wafer, with a d 33 variation of less than 0.7% (Supplementary Fig. S11), highlighting its suitability for mass production. Additionally, the RTA-AlScN film exhibits a relative permittivity ( ε r ) of 16 and a remarkable low dielectric loss (tan δ ) of < 0.0006 at 1 kHz (Supplementary Fig. S12). Surface topography evaluated by AFM (Fig. 2c) shows that RTA maintains high surface quality (RMS roughness of 0.65 nm). Figure 2d compares the out-of-plane PFM amplitude and phase images. While the AG-AlScN film reveals disordered nanodomains—evidenced by a high density of randomly oriented spots, indicating non-uniform piezoelectric activity—the RTA-AlScN film exhibits a marked reduction in these features and a high overall amplitude. This correlates with the enhanced c -axis ordering of nanodomains, a process intrinsically supported by the lowered ferroelectric switching barrier resulting from the increased bond ionicity and lattice flattening. This facilitates easier dipole rotation and long-range ordering during the thermal process. The distinct phase contrast confirms effective domain-switching under bias, successfully transitioning the central region from the “N-polar” state to the “Al(Sc)-polar” state 36,37 . Furthermore, RTA-AlScN displays superior ferroelectric behavior with a high remanent polarization ( P r ~107 μC cm -2 ) and robust fatigue endurance (~10 5 cycles) (Supplementary Fig. S13). 2.2 Finite element simulation design To guide the structural design of the MEMS receiver, we performed multiphysics coupling finite element analysis (FEA) of a single sensing element. The cross-sectional schematic in Fig. 3a depicts the multilayer architecture of the sensing element, which is integrated on an SOI substrate and comprises an AlN seed layer, Cr/Mo electrodes, and an AlScN piezoelectric functional layer. In piezoelectric MEMS transducers, a back cavity must be engineered beneath the piezoelectric membrane to release mechanical constraints and enable structural deformation. We first optimized the cavity diameter, as shown in the simulation results in Fig. 3a. As the back cavity radius ( R C ) increases, the resonant frequency of the diaphragm consistently decreases. However, an excessively large R C can compromise mechanical reliability, potentially leading to fatigue and fracture under continuous acoustic excitation. Balancing the requirements for low-frequency sensitivity and structural durability, a R C of 250 μm was selected to ensure a sufficiently low resonant frequency while maintaining structural integrity. With R C fixed at 250 μm, we further investigated the dependencies of the induced charge and resonant frequency on the back cavity-to-(piezoelectric film and top electrode) ratio ( R C / R PF&TE ) and (back cavity and piezoelectric film)-to-top electrode ( R C&PF / R TE ) ratio. As illustrated in Fig. 3b, the total charge on the electrode surface initially increases with both R C / R PF&TE and R C&PF / R TE , peaking at a ratio 1.25, before exhibiting a slightly decline. Conversely, the resonant frequency follows an opposite trend. At an R C / R PF&TE ratio of 1.25, the device achieves an optimal combination of a low resonant frequency (349.0 kHz) and a maximum induced charge (3.4×10 -16 C). Notably, at R C / R PF&TE = 1.25, the surface charge distribution exhibits a uniformly positive character (Fig. 3c), which supports the maximization of the total harvested charge. This phenomenon is attributed to the optimized distribution of compressive stress across the AlScN film under this configuration (Fig. 3d). For comparison, the results for R C / R PF&TE ratios of 1.0 and 1.5 are provided in Supplementary Fig. S14, S15. Similarly, at R C&PF / R TE = 1.25, the film also exhibits positive surface charges (Fig. 3e) and compressive stress in the top electrode (Fig. 3f). Comparative cases for R C&PF / R TE at 1.0 and 1.5 are detailed in Supplementary Fig. S16, S17. Nevertheless, the R C&PF / R TE = 1.25 configuration remains superior, providing a lower resonant frequency and higher charge density. Using this optimized configuration, we simulated the device response under an applied acoustic field of 1 Pa at 1 kHz (source positioned 1 m above the sensor). These simulations reveal the relationship between external acoustic pressure and the resulting charge density distribution. Under the simulated acoustic field (Fig. 3g), the receiver undergoes predictable mechanical stress and deformation (Fig. 3h). As acoustic pressure of approximately 116 dB ( pref = 1 μPa) induces a surface charge density ranging from 0.5 × 10 -5 to 2.5 × 10 -5 C m -2 (Fig. 3i). This rigorous FEA framework validates the design’s effectiveness for high-quality low-frequency acoustic sensing. 2.3 Structure design and fabrication of MEMS device Based on the aforementioned FEA optimization and superior piezoelectric properties of the RTA-AlScN film, we proceeded with the micro-fabrication and characterization of the devices on a 6-inch wafer. To enhance the output signal-to-noise ratio and enable independent operation, a typical rectangular unit cell was designed, featuring a 5×5 arrays of piezoelectric sensing elements connected in parallel 38 . The micromachining process was streamlined to employ only three chromium masks (M1-M3), as detailed in Supplementary Fig. S18. Figure 4a presents an optical photograph of the completed wafer-level transducer surface. Each sensing element pattern consists of a stacked piezoelectric functional layer and a top electrode ( t = ~1.2 μm, r = 200 μm), as shown in the magnified views in Fig. 4b-d. To release the sensing diaphragm, the backside silicon was effectively etched to create a cavity with a diameter of 500 μm and a depth of 620 μm (Fig. 4e-h). The measured cavity-to-piezoelectric film radius ratio is approximately 1.25, in excellent agreement with the finite element simulation results and demonstrating the high precision of our MEMS fabrication process. A micrograph of the single chip containing 25 sensing elements is presented in Supplementary Fig. S19, showcasing a compact footprint with an area of only ~36 mm² (Supplementary Fig. S20). This “three-mask and four-etching” manufacturing protocol minimizes error accumulation and ensures high fabrication reliability and batch consistency. The electrical integrity of the bare chips was initially evaluated using a probe station by connecting the electrodes to two probes (Fig. 4i). The continuity between top electrodes (top-top) and bottom electrodes (bottom-bottom) is illustrated in Fig. 4j. We observed ideal Ohmic contact behavior with excellent conductivity, characterized by low resistance values of 8.05 Ω for the top electrodes and 5.67 Ω for the bottom electrodes. This high electrode continuity across the wafer provides a reliable foundation for low-loss piezoelectric signal transmission. To evaluate the leakage and resistive performance of the RTA-AlScN film itself, current-voltage ( I - V ) measurements were conducted between the top and bottom electrodes. Under a bias voltage of 5 V, the inter-electrode insulation consistently exceeded 4×10 9 Ω across ten consecutive measurements (Fig. 4k), confirming the high-quality insulation and structural integrity of the annealed film. Figure 4l shows the impedance and phase spectra of the RTA-AlScN unit. The measured resonant frequency of 368.2 kHz is only 5.2% higher than the simulated value (349.0 kHz), validating the accuracy of our design parameters and the effectiveness of the device in responding to low-frequency signals. The overall yield of functional bare dies exceeds 90%, with isolated failures (such as edge-effect-included shorts) occurring only at the wafer periphery. To optimize the overall sensing performance, a specialized matching circuit and protective packaging were developed. A preamplifier circuit with a gain of 40 dB was fabricated to amplify the weak acoustic-induced charges (Supplementary Fig. S21-S23). For system-level measurements, the MEMS chips were wire-bonded to a printed circuit board (PCB) using a Gold Wire ball bonder, ensuring stable electrical and mechanical interconnections (Supplementary Fig. S24, S25). Finally, the sensors encapsulated using silicone oil and polyurethane to achieve high sensitivity and environmental robustness, as displayed in Supplementary Fig. S26. 2.4 Acoustic receiving performance of MEMS sensor The packaged MEMS sensor was fully characterized using industry-standard calibration instrument. The operational principle of the sensor is illustrated in Fig. 5a. During underwater deployment, acoustic waves from the environment penetrate the protective shell and the liquid insulating medium to reach the sensitive MEMS diaphragm. The integrated AlScN chips convert the incident acoustic pressure into electrical signals, which are subsequently amplified and processed. The receiving sensitivity, a critical performance metric, was evaluated using the configuration shown in Supplementary Fig. S27. As demonstrated in Fig. 5b, the sensor exhibits a remarkably flat sensitivity response across the 10 Hz to 1000 Hz operational bandwidth, with a sensitivity of -172.2 dB at 1 kHz. Notably, the sensitivity reaches as high as -162.4 dB at 10 Hz, significantly outperforming conventional AlN-based devices (-178 dB re: V μPa -1 ) 39 , thereby highlighting the enhanced acoustic reception enable by the RTA-AlScN film. Time-domain measurements (Supplementary Fig. S28) reveals a stable and clean sinusoidal output voltage, confirming distortion-free signal acquisition and amplification. Furthermore, the self-noise equivalent pressure spectral level, which dictates the detection threshold for weak signals, was analyzed (Fig. 5c). The self-noise amplitude decreases rapidly from 20 Hz to 1 kHz before leveling off above 2.7 kHz. At 1kHz, the noise floor is approximately 59 dB (re:1 μPa Hz -0.5 ), which is comparable to the ambient ocean noise level under Sea State 2 40 . To assess the spatial response, the directivity was measured in both vertical and horizontal planes (Fig. 5d, e). The measured fluctuation was only -1.51 dB and -1.07 dB for the vertical and horizontal planes, respectively, confirming the excellent omnidirectional characteristics of the device. The receiver’s capability was further evaluated through pitch-catch measurements using a PZT ceramic transmitter. In liquid media (water and silicone oil), the peak-to-peak voltage output ( V PP ) follows a predictable attenuation pattern as the distance from the sound source increases (Fig. 5f, g), consistent with acoustic propagation loss and the decay of particle vibration amplitude 41 . To verify the potential for high-temperature applications, measurements were performed at various temperatures up to 150 °C. Although the V PP values exhibit a marginal decreasing trend with a 12% loss (from 890 m V PP to 800 m V PP at 150 °C), the performance remains highly stable within this range. This robustness indicates that the sensor is well-suited for harsh environments, such as deep-sea offshore wells, automotive underhood sensing, and oil shale exploration 42,43 . Additionally, the sensor’s versatility was demonstrated in solid media by coupling the device to a ceramic panel. As the medium layer thickness increased from 2 mm to 15 mm (Fig. 5h), the V pp signal exhibited a monotonic decrease, mirroring the behavior observed in liquid medium. The maximum output voltage of 900 mV observed in silicone oil is attributed to optimal impedance matching between the sensor’s internal insulating liquid and the external medium (Supplementary Table S4). For practical applications, long-term stability is paramount. As shown in Fig. 5i, the device demonstrates exceptional sensitivity stability in seawater; after six months of continuous immersion, the cumulative attenuation was only 2.8 dB. This validates the effectiveness of the polyurethane/silicone oil packaging in protecting the core MEMS chip. To contextualize the achievement in MEMS receivers, a comprehensive comparison of key performance metrics between this work and state-of-the-art reported devices is summarized in Table 1. Notably, our RTA-AlScN MEMS receiver represents a significant breakthrough, being the first to achieve the highest reported sensitivity at such an ultra-low frequency. The following salient merits underscore its potential as a paradigm-shifting solution for low-frequency acoustic sensing. (i) Industrial Scalability: While the majority of prior research has focused on small-scale laboratory prototypes, our work leverages an industrial-grade 6-inch wafer platform. This transition from chip-level to wafer-level fabrication demonstrates the immense potential for high-volume mass production. (ii) Manufacturing Efficiency: The device utilizes a streamlined "three-mask and four-etching" protocol. This minimalist process significantly reduces error accumulation and production costs while ensuring exceptional fabrication reliability and yield. (iii) Record-breaking Sensitivity: To the best of our knowledge, the RTA-AlScN MEMS sensor demonstrates a record-breaking sensitivity of -162.4 dB at the lowest operating frequency of 10 Hz reported to date. This capability addresses a critical gap in detecting infrasonic signals with high fidelity. (iv) Proven Long-term Stability: For the first time in lead-free piezoelectric MEMS research, we report a cumulative sensitivity attenuation as low as 2.8 dB over six months of seawater immersion. This exceptional wet-storage stability validates the effectiveness of our system-level packaging and highlights its readiness for sustained service in harsh marine environments. Table 1 | Comparisons of sensing performances of our RTA-AlScN receiver with other reported representative MEMS devices Thin films Wafer size (inch) Etching count Masks Usable frequency range (Hz) Highest sensitivity (dB re 1 V μPa -1 ) Noise resolution (dB re 1 μPa Hz -0.5 at 1000 Hz) Directivi-ty Ref. PZT _ 6 _ 100 -10000 -227.5 @ 2000 Hz _ _ 51 PZT _ 5 5 80-20000 -189.3 @ 920 Hz _ Omnidir-ectional 12 PZT _ _ _ 10 -5000 -195.5 @ 2500 Hz _ _ 11 AlN 4 3 4 10-10000 -166.5 @ 750 Hz 40 _ 52 AlN _ 4 4 10-8000 -178 @ 1000 Hz 58.7 Omnidir-ectional 38 AlN _ 6 6 10-1600 -178 @ 1000 Hz 52.6 @ 100 Hz Omnidir-ectional 39 AlN _ 4 _ 20-1000 -177.6 @ 1000 Hz 58.4 _ 10 Al 0.905 Sc 0.095 N _ _ _ 20000 -160000 -175 @160 kHz _ Omnidir-ectional 53 Al 0.8 Sc 0.2 N _ 4 3 10 -200000 -172 @ 1000 Hz _ Omnidir-ectional 54 Al 0.8 Sc 0.2 N _ _ _ 10 -1000 -162.9 @ 1000 Hz 46.1 Omnidir-ectional 55 RTA-AlScN 6 4 3 10-1000 -162.4 @10 Hz 59 Omnidir-ectional This work 3. Conclusion In summary, we have successfully demonstrated an ultra-sensitive piezoelectric MEMS receiver by a multi-scale lattice engineering strategy. By integrating high-performance AlScN thin film via an industrial-grade fabrication process on 6-inch SOI wafers, we provide a compelling pathway for the development of next-generation ultra-sensitive, low-frequency acoustic devices. The Al 0.9 Sc 0.1 N thin film, optimized via post-growth RTA, exhibits significantly enhanced piezoelectric properties, including a d 33 of 15.0 pC N -1 and a k t 2 of 34.9%. This record-breaking performance is fundamentally rooted in a synergistic mechanism: at the atomic scale, Sc-doping reduces the bond covalency and minimum electron density; at the mesoscopic scale, RTA minimizes the lattice parameter ratio ( c / a ) and homogenizes nanoscopic domain orientations. By leveraging optimized structural design and a robust “three-mask, four-etch” micromachining process, we have established a reliable scalable wafer-level manufacturing route. The resulting MEMS receiver demonstrates unprecedented acoustic responsiveness, highlighted by a record-high sensitivity of -162.4 dB (re: 1 V μPa -1 ) at 10 Hz, a noise equivalent pressure of 59 dB (re: 1 μPa Hz -0.5 ) at 1 kHz, and omnidirectional receiving characteristics. Furthermore, the device maintains outstanding long-term stability with minimal attenuation over six months of seawater immersion and reliable functionality up to 150 °C. This work underscores the critical synergy between fundamental materials physics, optimized device architecture, and robust system integration, paving a transformative framework for designing high-performance lead-free piezoelectric microsystems in demanding sensing applications. 4. Experiments and computational modelling The development of the AlScN-based MEMS receiver proceeded through three integrated stages: (i) deposition of high-quality AlScN films on 6-inch SOI wafers, (ii) micromachining of the MEMS sensing units, and (iii) system-level integration and packaging (Supplementary Fig. S29). 4.1 Procedure of AlScN film P-type (100)-oriented silicon-on-insulator (SOI) wafers (6-inch diameter, resistivity: 0.002-0.005 Ω cm) were employed as substates. The wafer architecture comprises a 5 µm device silicon layer, a 0.5 µm buried oxide (SiO₂) layer, and a 625 µm handle silicon layer. Initially, the SOI wafers underwent Standard Radio Corporation of America (RCA) Cleaning to ensure a pristine surface. Subsequently, a multilayer stack consisting of a seeding layer (20 nm AlN), an adhesion layer (20 nm Cr), a bottom electrode (250 nm Mo), the functional piezoelectric layer (1 μm AlScN), a second adhesion layer (20 nm Cr), and a top electrode (200 nm Mo) was sequentially deposited using a magnetron sputtering system (Explorer 14, DENTON Vacuum LLC, USA). The AlN and AlScN were synthesized under a base pressure of < 5 × 10 -4 Pa and a working pressure of 0.4 Pa, with an RF power of 350 W and a substrate rotation of 15 rpm. To optimize piezoelectricity, the as-deposited AlScN films were subjected to rapid thermal annealing at 700 °C. 4.2 Fabrication of the MEMS by micromachining technology The MEMS fabrication followed a “three-mask and four-etching” protocol on a commercial production line. First, the top electrode and piezoelectric layers were patterned using Mask M1. The Cr/Mo top electrode was defined via Ion Beam Etching (IBE), followed immediately by inductively coupled plasma (ICP) etching of the AlScN layer. The photoresist (AZ 5214, Merck KGaA, Germany) was stripped using AZ400T. Second, a 200 nm SiO 2 insulating layer was deposited via plasma-enhanced chemical vapor deposition (PECVD), and contact holes were patterned using Mask M2 via reactive ion etching (RIE). Third, the interconnection metal was patterned. Finally, the handle Si was etched from the backside using deep reactive ion etching (DRIE, SPTS Rapier, UK) to release the diaphragms. The wafer was diced (NANO 150G), and the resulting chips were mounted onto printed circuit boards (PCB) using epoxy and electrically interconnected via gold wire bonding (53XX BDA, F&S Bondtec, Germany). 4.3 Preparation of AlScN MEMS sensor To minimize system-level noise, the MEMS chip was integrated with a custom-designed 40 dB preamplifier circuit. Based on acoustic transmission theory, the device was encapsulated using polyurethane and silicone oil. Silicone oil was injected into the sensing cavity to ensure acoustic coupling and hydrostatic pressure balance. The acoustic impedance gradient among seawater (1.57 MRayl), polyurethane (1.62 MRayl), and silicone oil (1.26 MRayl) were optimized to maximize the acoustic-electric conversion efficiency (Supplementary Table S4 and Fig. S30). 4.4 Characterization Morphological and element distributions were analyzed using SEM (GeminiSEM 360, Carl Zeiss, Germany) and EDS. Crystalline phases were identified via XRD (D8 Advance, Bruker, Germany) using Cu Kα radiation. The quasi-static d 3 3 was measured using a d 33 meter (ZJ-6A, Institute of Acoustics, Chinese Academy of Sciences, China). Impedance spectra, including resonant and anti-resonant frequencies were acquired using an Agilent 4294A analyzer. Electrode conductivity was verified via four-point probe measurements (Crexbox, Deyi Tech). For pitch-catch characterization, a function generator (AFG31000, Tektronix, USA) and oscilloscope (MDO34, Tektronix, USA) were used to record the acoustic response at 20 Hz. Ferroelectric properties and C-V / P-E loops were characterized using a Radiant Precision Multiferroic system. To facilitate through-thickness poling of the 1-μm-thick films, samples were thinned to ~100 nm using focused ion beam (FIB, Helios G4, USA) milling. The surface morphology was characterized using an atomic force microscope (AFM; Dimension Icon, Bruker, Germany). Following this, the amplitude and phase images of both AG-AlScN and RTA-AlScN films were acquired using the AFM system (Dimension Icon, Bruker, Germany) in piezoelectric mode. Atomic-scale imaging was performed via HAADF-STEM (Spectra 300, Thermo Fisher Scientific, USA) at 300 kV with an 11 mrad convergence angle. 4.5 Finite element models Multiphysics simulations were conducted with finite element models, coupling the Pressure Acoustics , Solid Mechanics , and Electrostatics modules. The sensing area was defined by the top electrode geometry. A background pressure field of 1 Pa was applied to evaluate the acoustic-to-electric conversion efficiency via surface charge density analysis. 4.6 DFT calculations The atomic properties and electronic structure of the materials were calculated using first-principles simulations within density functional theory (DFT) 44 . The projected augmented wave pseudopotentials method was used as implemented in the Vienna Ab initio Simulation Package 45 . The exchange correlation energy was calculated using the generalized gradient approximation of the Perdew-Burke-Ernzerhof form 46 . The plane wave cutoff energy was set to 500 eV. For Brillouin zone sampling, a Monkhorst-Pack k -point mesh with a spacing finer than 0.03 Å -1 was used for all structures 47 . Utilizing the conjugate gradient method, the plane lattice constant and atomic coordinates were fully relaxed until the energy and force converge to 10 -7 eV and 10 –3 eV Å -1 , respectively. The valence electron configurations were set to Sc-2s 2 p 6 3d 1 4s 2 , Al-3s 2 3p 1 , and N-2s 2 2p 3 . The piezoelectric e ij tensors were predicted by the density-functional perturbation theory method 48-50 . Declarations Data availability The data that support the findings of this study are available from the corresponding author upon reasonable request. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 51972144, U24A2040), the Key Technology Research and Development Program of Shandong Province (Grant No. 2022CXPT045), the Taishan Scholar Foundation of Shandong Province (Grant No. tstp20221130), Taishan Industrial Experts Program (Grant No. tscx202306035), and the 111 Project of International Corporation on Advanced Cement-based Materials (Grant No. D17001). Author contributions Z.Y. Cheng, T. Liu, and D. Su contributed equally to this work, leading the majority of the experiments and data analysis. C. Wu contributed to theory and discussions. Y. Liu provided project funding support. W. Sun performed the density functional theory (DFT) calculations. S. Huang supervised this study. C. Yang and Z. X. Cheng oversaw the conceptualization and design of the study, and led the writing and revision of the manuscript. Competing interests The authors declare no competing interests. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9122069","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":606862592,"identity":"78128513-5481-4462-8e02-9cb2bba2c53f","order_by":0,"name":"Zhenxiang Cheng","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAuUlEQVRIiWNgGAWjYDACCRBRgcQmUssZkrUwtpGihX9287GHX+cdljM4wHzwNg+DXWIDQUvuHEs3lt122NjgAFuyNQ9DMmEtBhI5ZtKS2w4nbjvAYybNw8BMjJb8b9KSc0Ba+L8BtdQTZQub5McGsC1sQC2HifDLjTQzaQagf+wPsxlbzjE4bkxQC/+M5GeSP2qs5STbmx/eeFNRLUtQCwgw84BJsDuJUQ8EjD+IVDgKRsEoGAUjFAAAUaE3ol82WHoAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0003-4847-2907","institution":"University of Wollongong","correspondingAuthor":true,"prefix":"","firstName":"Zhenxiang","middleName":"","lastName":"Cheng","suffix":""},{"id":606862593,"identity":"df2db253-4aca-4c21-93a4-5b25c343744a","order_by":1,"name":"Zhenyue Cheng","email":"","orcid":"","institution":"University of Jinan","correspondingAuthor":false,"prefix":"","firstName":"Zhenyue","middleName":"","lastName":"Cheng","suffix":""},{"id":606862594,"identity":"1feeaf9c-1295-4e04-961a-24fc2a93d996","order_by":2,"name":"Tong Liu","email":"","orcid":"","institution":"Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, MEMS Institute of Zibo National High-tech Development Zone","correspondingAuthor":false,"prefix":"","firstName":"Tong","middleName":"","lastName":"Liu","suffix":""},{"id":606862595,"identity":"d7c49959-88c2-4577-941c-3ea540893f5f","order_by":3,"name":"Daojian Su","email":"","orcid":"","institution":"University of Jinan","correspondingAuthor":false,"prefix":"","firstName":"Daojian","middleName":"","lastName":"Su","suffix":""},{"id":606862596,"identity":"adebb54a-f332-4caa-9cba-4f48f27f2bb5","order_by":4,"name":"Changhong Yang","email":"","orcid":"","institution":"Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan","correspondingAuthor":false,"prefix":"","firstName":"Changhong","middleName":"","lastName":"Yang","suffix":""},{"id":606862597,"identity":"16d8bd57-2528-45e3-8eae-85d5e8c7e97f","order_by":5,"name":"Chuancheng Wu","email":"","orcid":"","institution":"University of Jinan","correspondingAuthor":false,"prefix":"","firstName":"Chuancheng","middleName":"","lastName":"Wu","suffix":""},{"id":606862598,"identity":"ca112cd0-0504-47cf-bc52-5e650cb753ef","order_by":6,"name":"Yao Liu","email":"","orcid":"","institution":"Shandong University","correspondingAuthor":false,"prefix":"","firstName":"Yao","middleName":"","lastName":"Liu","suffix":""},{"id":606862599,"identity":"42abc38d-e290-48a1-b8a3-8b024727a239","order_by":7,"name":"Wei Sun","email":"","orcid":"https://orcid.org/0009-0002-7023-5553","institution":"University of Jinan","correspondingAuthor":false,"prefix":"","firstName":"Wei","middleName":"","lastName":"Sun","suffix":""},{"id":606862600,"identity":"48a19db7-c959-42a9-9092-738fa54283a5","order_by":8,"name":"Shifeng Huang","email":"","orcid":"https://orcid.org/0000-0002-9757-4267","institution":"Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials","correspondingAuthor":false,"prefix":"","firstName":"Shifeng","middleName":"","lastName":"Huang","suffix":""}],"badges":[],"createdAt":"2026-03-14 11:15:17","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9122069/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9122069/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":104938662,"identity":"d5cb77c0-9c66-48c2-af57-1a64fd99714c","added_by":"auto","created_at":"2026-03-19 02:29:21","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1862296,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStructural and morphological characterization of AlScN thin films\u003c/strong\u003e. a) XRD patterns of the AlN, AG-AlScN and RTA-AlScN thin films. b) Surface morphology and cross-sectional SEM images of RTA-AlScN film. c,d) Cross-sectional HAADF-STEM images of revealing the atomic structures of (c) AG-AlScN and (d) RTA-AlScN. The direction of polarization is indicated by the arrow labeled “P”. e,f) Comparison of lattice parameters of (\u003cem\u003ea\u003c/em\u003e and \u003cem\u003ec\u003c/em\u003e) for the AG-AlScN and RTA-AlScN films.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-9122069/v1/d690b2eccb43b219be61cc26.png"},{"id":104938712,"identity":"4a76646d-84fe-4a35-86ea-60ca0dd0cb47","added_by":"auto","created_at":"2026-03-19 02:29:27","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":3031377,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePiezoelectric and ferroelectric characterization of AG-AlScN and RTA-AlScN thin films\u003c/strong\u003e. a) Capacitance-electric field (\u003cem\u003eC\u003c/em\u003e-\u003cem\u003eE\u003c/em\u003e) loops of the AG-AlScN and RTA-AlScN thin films. b) Comparison of \u003cem\u003ed\u003c/em\u003e\u003csub\u003e33\u003c/sub\u003e and \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e with previously reported AlN-based films\u003csup\u003e25,56-68\u003c/sup\u003e.\u003csup\u003e \u003c/sup\u003ec) AFM surface morphology of the AG-AlScN and RTA-AlScN thin films. d) Out-of-plane PFM amplitude and phase images of the AG-AlScN and RTA-AlScN thin films. The central 1.5μm × 1.5 μm region was written with a box polarization configuration using a DC tip voltage of -30 V, yielding the polarization reversal from the downward to the upward.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-9122069/v1/c47680fd17e8f8fe166e4fca.png"},{"id":104938741,"identity":"939382ae-a539-4244-a2bc-eb50ad8c2ad4","added_by":"auto","created_at":"2026-03-19 02:29:29","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":2619733,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFinite element simulation and structural optimization of the AlScN-based MEMS element\u003c/strong\u003e. a) Resonant frequency as a function of the back-cavity radius (\u003cem\u003eR\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e). The inset shows the chip array and the cross-sectional schematic of a single sensing element. b) Variations of induced charge and resonant frequency across different radius ratios (\u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e/\u003cem\u003eR\u003c/em\u003e\u003csub\u003ePF\u0026amp;TE\u003c/sub\u003e and \u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u0026amp;PF\u003c/sub\u003e/\u003cem\u003eR\u003c/em\u003e\u003csub\u003eTE\u003c/sub\u003e). c,d) Simulated surface charge density and stress distribution profiles at \u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e/\u003cem\u003eR\u003c/em\u003e\u003csub\u003ePF\u0026amp;TE\u003c/sub\u003e=1.25. e,f) Simulated surface charge density and stress distribution profiles at \u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u0026amp;PF\u003c/sub\u003e/\u003cem\u003eR\u003c/em\u003e\u003csub\u003eTE\u003c/sub\u003e =1.25. g) Multiphysics model of the acoustic pressure field for excitation. h,i) Resulting stress distribution and surface-induced charge density of the piezoelectric thin film under the acoustic excitation condition defined in g).\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-9122069/v1/0fd8a2173775bf6f95a152a3.png"},{"id":104938719,"identity":"ffacf89f-5efe-4884-9849-23814b6bed2c","added_by":"auto","created_at":"2026-03-19 02:29:28","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":3284274,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMorphological and electrical characterization of the AlScN-based MEMS chip\u003c/strong\u003e. a) Optical photograph of the micromachined top surface. b) Magnified view of the 5 × 5 element array. c,d) Three-dimensional topography and corresponding step-height profile of a single sensing element. e) Photograph of the silicon backside after Deep-RIE. f,g) Three-dimensional topography and enlarged view of the etched back-cavity array. h) Cross-sectional profile of the back cavity along the dashed line in g). i) Photograph of the probe-station setup for electrode characterization. j) \u003cem\u003eI\u003c/em\u003e-\u003cem\u003eV\u003c/em\u003e characteristic of the top and bottom electrodes, demonstrating Ohmic contact. k,l) Leakage resistance and impedance spectra of a representative sensing unit, showing a resonant frequency near 368.2 kHz.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-9122069/v1/74696705e4d82eeea821b35b.png"},{"id":104938685,"identity":"df305746-cc78-4404-a432-f35772d468f6","added_by":"auto","created_at":"2026-03-19 02:29:23","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":2005793,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eAcoustic receiving performance and environmental robustness of the MEMS receiver\u003c/strong\u003e. a) Schematic illustration of the operational principle. b) Sensitivity response curve from 10 Hz to 1 kHz. c) Self-noise pressure spectral density level. d,e) Vertical and horizontal directivity patterns. f) Relationship between output voltage amplitude and propagation distance in water. g) Variations of voltage amplitude with distance and temperature in silicone oil. h) Variation of voltage amplitude with ceramic solid medium thickness. i) Long-term sensitivity stability in a shallow-sea environment over six months.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-9122069/v1/5c54b67af4701382cf66daf9.png"},{"id":104938916,"identity":"8923b2a8-0178-4398-9e82-2183db8fd469","added_by":"auto","created_at":"2026-03-19 02:29:52","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":13385405,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9122069/v1/0c258d5a-3e05-4515-9a7e-ff1de8eb6e11.pdf"},{"id":104938705,"identity":"85856e0d-5d1f-447e-aba6-ba073569d741","added_by":"auto","created_at":"2026-03-19 02:29:26","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":13894639,"visible":true,"origin":"","legend":"Supplementary information","description":"","filename":"Supplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-9122069/v1/bf4405bfdac82e545eb13388.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Giant piezoelectric response and electromechanical coupling in AlScN thin films via multi-scale lattice engineering","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003ePiezoelectric micro-electromechanical systems (MEMS) exhibit remarkable advantages in passive operation, extreme miniaturization, and scalable manufacturability, effectively addressing the inherent constraints of conventional bulk materials, such as excessive dimensions, high power consumption, and integration complexities\u003csup\u003e1-3\u003c/sup\u003e. By leveraging efficient mechanical-to-electrical energy conversion, piezoelectric MEMS architectures have underpinned a new generation of precision acoustic transducers\u003csup\u003e4,5\u003c/sup\u003e, intelligent robotic skins\u003csup\u003e6,7\u003c/sup\u003e, environmental energy harvesters\u003csup\u003e8\u003c/sup\u003e, and high-resolution medical imaging arrays systems\u003csup\u003e9\u003c/sup\u003e. As micro-sensor functionalities continue to evolve, piezoelectric MEMS are rapidly advancing to the forefront of autonomous sensing and ubiquitous computing.\u003c/p\u003e\n\u003cp\u003eIn the realm of acoustic reception, piezoelectric MEMS have redefined the capabilities of transducers, particularly for low-frequency detection critical for\u0026nbsp;\u0026nbsp;long-range\u0026nbsp;underwater\u0026nbsp;surveillance and seismic monitoring\u003csup\u003e10,11\u003c/sup\u003e. These\u0026nbsp;devices typically operate by converting low-frequency (\u0026le;1000 Hz) acoustic waves into electrical signals via thin-film piezoelectricity. While traditional materials like Pb(Zr,Ti)O\u003csub\u003e3\u0026nbsp;\u003c/sub\u003e(PZT) and ZnO are widely utilized, they face significant limitations in low-frequency regimes. For instance, PZT-based\u0026nbsp;cantilevers\u0026nbsp;hydrophones often exhibit a dramatic sensitivity roll-off in the low frequencies, while ZnO sensors typically suffer from substantial signal attenuation due to their modest piezoelectric response and dielectric loss\u003csup\u003e12,13\u003c/sup\u003e.\u0026nbsp;These challenges necessitate the exploration of novel material systems that can deliver high\u0026nbsp;sensitivity and a flat response at\u0026nbsp;low-frequencies without compromising structural integrity.\u003c/p\u003e\n\u003cp\u003eRecently, aluminum nitride (AlN) has emerged as a compelling lead-free alternative due to its high CMOS compatibility,\u0026nbsp;inherent polarity\u0026nbsp;(eliminating the need for high-voltage poling), exceptional thermal\u0026nbsp;conductivity\u0026nbsp;(200 W\u0026nbsp;m\u003csup\u003e-1\u003c/sup\u003e\u0026nbsp;K\u003csup\u003e-1\u003c/sup\u003e), and elevated operating temperature (up to 1150\u0026nbsp;\u0026deg;C)\u003csup\u003e14-18\u003c/sup\u003e.\u0026nbsp;The alloying of scandium (Sc) into the\u0026nbsp;AlN\u0026nbsp;lattice has been shown to exponentially enhance the piezoelectric response by softening the crystal lattice\u003csup\u003e1,19\u003c/sup\u003e. For example,\u0026nbsp;Al\u003csub\u003e0.71\u003c/sub\u003eSc\u003csub\u003e0.29\u003c/sub\u003eN thin film\u0026nbsp;can achieve a\u0026nbsp;piezoelectric coefficient (\u003cem\u003ed\u003c/em\u003e\u003csub\u003e33\u003c/sub\u003e) of\u0026nbsp;~12.6 pC\u0026nbsp;N\u003csup\u003e-1\u003c/sup\u003e, more than double the value for pure AlN film\u003csup\u003e20,21\u003c/sup\u003e.\u0026nbsp;Additionally, Sc substitution markedly increases the dielectric constant, with \u003cem\u003eɛ\u003csub\u003er\u003c/sub\u003e\u003c/em\u003e\u003csub\u003e\u0026nbsp;\u003c/sub\u003e\u0026asymp; 15 observed for Al\u003csub\u003e0.85\u003c/sub\u003eSc\u003csub\u003e0.15\u003c/sub\u003eN compared to \u003cem\u003eɛ\u003csub\u003er\u003c/sub\u003e\u003c/em\u003e\u003csub\u003e\u0026nbsp;\u003c/sub\u003e\u0026asymp; 10 for AlN\u003csup\u003e22,23\u003c/sup\u003e. However, a critical bottleneck exists: excessively high Sc concentration (typically \u0026gt;20%) often triggers phase instability, the formation of non-polar grains, and prohibitive fabrication cost\u003csup\u003e24-27\u003c/sup\u003e. Consequently, moderate Sc doping represents a more robust and feasible pathway for practical sensing applications, provided that the piezoelectric response can be further augmented through structural engineering. Nevertheless, the seamless integration of Sc-doped AlN into low-frequency MEMS is still impeded by the challenges in synthesizing high-performance thin films, achieving wafer-scale process compatibility, and optimizing device topologies.\u003c/p\u003e\n\u003cp\u003eIn this work, we selected an AlN thin film containing approximately 10 at.% Sc (abbreviated as AlScN), which as demonstrated in our density functional theory (DFT) calculations (Supplementary Fig.\u0026nbsp;S1 and Table S1) and supported by the previous literatures (Supplementary Table S2 and Fig. S2), exhibited\u0026nbsp;an optimal ratio of\u0026nbsp;piezoelectric coefficient (\u003cem\u003ed\u003c/em\u003e\u003csub\u003e33\u003c/sub\u003e)/relative\u0026nbsp;permittivity (\u003cem\u003eɛ\u003csub\u003er\u003c/sub\u003e\u003c/em\u003e)\u003cem\u003e\u003csub\u003e\u0026nbsp;\u003c/sub\u003e\u003c/em\u003efor high receiving sensitivity.\u0026nbsp;Crucially, we demonstrate that post-growth rapid thermal annealing (RTA) dramatically boosts the\u0026nbsp;piezoelectric performance, elevating the \u003cem\u003ed\u003c/em\u003e\u003csub\u003e33\u0026nbsp;\u003c/sub\u003efrom 8.2 pC N\u003csup\u003e-1\u003c/sup\u003e in the as-grown (AG) state to 15.0 pC N\u003csup\u003e-1\u003c/sup\u003e. This enhancement is shown to originate\u0026nbsp;from a reduction in the lattice parameter ratio (\u003cem\u003ec\u003c/em\u003e/\u003cem\u003ea\u003c/em\u003e) and the promotion of highly ordered nanoscopic domain orientations. Leveraging a 6-inch wafer-scale fabrication process,\u0026nbsp;we developed\u0026nbsp;a MEMS\u0026nbsp;chip\u0026nbsp;integrating\u0026nbsp;a 5 \u0026times; 5 array of\u0026nbsp;RTA-AlScN\u0026nbsp;sensing elements. The packaged device\u0026nbsp;achieves\u0026nbsp;a\u0026nbsp;record\u0026nbsp;acoustic pressure sensitivity of\u0026nbsp;-162.4\u0026nbsp;dB (re: 1 V\u0026nbsp;\u0026mu;Pa\u003csup\u003e-1\u003c/sup\u003e)\u0026nbsp;at 10 Hz and a\u0026nbsp;noise\u0026nbsp;equivalent pressure of\u0026nbsp;59\u0026nbsp;dB (re: 1\u0026nbsp;\u0026mu;Pa\u0026nbsp;Hz\u003csup\u003e-0.5\u003c/sup\u003e) at 1kHz. Furthermore, the sensor demonstrates robust functionality across diverse media, including water, silicone oil, and solid ceramics, maintaining high fidelity at temperatures up to 150 \u0026deg;C. This study provides a scalable and high-performance framework for lead-free Sc-doped AlN based acoustic sensing in complex and extreme environments.\u003c/p\u003e"},{"header":"2. Results","content":"\u003cp\u003e\u003cstrong\u003e2.1 Microstructure and electrical performance of AlScN thin films\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo investigate the effects of Sc doping and the \u003cstrong\u003eRTA\u003c/strong\u003e process on the crystalline structure of AlScN, X-ray diffraction (XRD) was performed. As shown in Fig.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e1a, all samples exhibit a sharp (002) diffraction peak according to the standard wurtzite AlN pattern (JCPDS 76-0565), indicating a high degree of \u003cem\u003ec\u003c/em\u003e-axis orientation. This is attributed to the high kinetic energy of sputtered particles during the deposition process\u003csup\u003e28,29\u003c/sup\u003e. Scanning electron microscopy (SEM) reveals both AG and RTA treated AlScN films possess uniform, spherical grains (Fig. 1bi\u0026nbsp;and\u0026nbsp;Supplementary\u0026nbsp;Fig.\u0026nbsp;S3a). Notably, the average grain size of RTA-AlScN (100 nm) is significantly larger than previously reported values for AlScN thin film with similar Sc content (Al\u003csub\u003e0.90\u003c/sub\u003eSc\u003csub\u003e0.10\u003c/sub\u003eN ~ 50 nm, Al\u003csub\u003e0.85\u003c/sub\u003eSc\u003csub\u003e0.15\u003c/sub\u003eN ~ 30 nm)\u003csup\u003e25,30\u003c/sup\u003e.\u0026nbsp;Cross-sectional SEM images further confirm that both AG-AlScN and RTA-AlScN films feature well-defined\u0026nbsp;columnar grains\u0026nbsp;with thicknesses of approximately 1 \u0026mu;m (Fig. 1bii and Supplementary Fig. S3b). Guided by our simulation results, this thickness was optimized to achieve a low resonant frequency of 390 kHz and a relatively high surface charge output (5.74\u0026times;10\u003csup\u003e-17\u003c/sup\u003e C), while ensuring mechanical robustness for practical applications (Supplementary Fig. S4)\u003csup\u003e31\u003c/sup\u003e. The chemical composition and element distribution were further verified by EDS and XPS (Supplementary Fig. S5 and S6).\u003c/p\u003e\n\u003cp\u003eAtomic-scale structural insights were obtained via high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), providing high-precision atomic position measurements with uncertainties below 50 pm. As shown in Fig. 1c, d, N and Al(Sc) atoms form alternating hexagonal close-packed layers along the \u003cem\u003ec\u003c/em\u003e-axis. Each Al(Sc) atom resides in a tetrahedral arrangement with four neighboring N atoms; this coordination confirms an \u0026ldquo;N-polar\u0026rdquo; growth direction, with the permanent dipole moment oriented toward the substrate. Critically, this N-polar characteristic in the AG-AlScN film remains stable and unchanged after RTA treatment. Statistical analysis of the lattice parameters across multiple regions (Supplementary Fig. S7, S8) reveals that following RTA, both \u003cem\u003ea\u003c/em\u003e and \u003cem\u003ec\u003c/em\u003e lattice parameters exhibit an increasing trend (Fig. 1e, f). The average \u003cem\u003ea\u003c/em\u003e-axis parameter rose significantly from 3.163 \u0026Aring; to 3.279 \u0026Aring;, while the \u003cem\u003ec\u003c/em\u003e-axis parameter increased marginally from 4.931 \u0026Aring; to 4.951 \u0026Aring;. The expansion of the \u003cem\u003ea\u003c/em\u003e-axis in RTA-AlScN is attributed to stress relaxation, as \u003cstrong\u003epost-growth RTA\u0026nbsp;\u003c/strong\u003eweakens the transverse interaction forces between adjacent columnar grains\u003csup\u003e32\u003c/sup\u003e. Consequently, the \u003cem\u003ec\u003c/em\u003e/\u003cem\u003ea\u003c/em\u003e ratio of RTA-AlScN decreased to 1.510 compared with 1.559 for AG-AlScN (Supplementary Table S3). RTA at high temperatures promotes the substitution of larger Sc\u003csup\u003e3+\u003c/sup\u003e ions into Al\u003csup\u003e3+\u003c/sup\u003e sites and the relaxation of epitaxial strain, driving the lattice towards a flatter, more frustrated hexagonal state with a reduced\u003cem\u003e\u0026nbsp;c/a\u003c/em\u003e ratio (Supplementary Fig. S9).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe improved piezoelectricity is fundamentally rooted in the synergistic evolution of chemical bonding and lattice symmetry. As corroborated by our DFT calculations (Supplementary Fig.\u0026nbsp;S1f), Sc substitution reduces the minimum electron density (MED) at the bond saddle point\u0026mdash;from 0.195 in pure AlN to 0.178 in Al\u003csub\u003e0.875\u003c/sub\u003eSc\u003csub\u003e0.125\u003c/sub\u003eN\u0026mdash;indicating a transition toward higher bond ionicity (weakened covalency). This lattice softening, characterized by decreased MED and electron localization function (ELF) values, facilitates a more compliant structural response to external strain. While Sc substitution establishes the chemical foundation for this softening, the RTA process serves as a structural trigger that further destabilizes the cation position via \u003cem\u003ec/a\u003c/em\u003e ratio reduction. This dual-effect\u0026mdash;combining intrinsic bonding modification with extrinsic lattice strain engineering\u0026mdash;collectively drives the material toward a metastable state with exceptionally high polarizability. Theoretically, the lower \u003cem\u003ec\u003c/em\u003e/\u003cem\u003ea\u003c/em\u003e ratio resulting from this modified bonding state enhances polarization-strain coupling coefficient (\u003cem\u003ee\u003c/em\u003e\u003csub\u003e33\u003c/sub\u003e) and the polarization-electric field coupling coefficient (\u003cem\u003e\u0026chi;\u003c/em\u003e\u003csub\u003e33\u003c/sub\u003e), thereby significantly boosting\u0026nbsp;the effective piezoelectric coefficient (\u003cem\u003ed\u003c/em\u003e\u003csub\u003e33\u003c/sub\u003e \u0026asymp; \u003cem\u003e\u0026epsilon;\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e\u003cem\u003ee\u003c/em\u003e\u003csub\u003e33\u003c/sub\u003e\u003cem\u003e\u0026chi;\u003c/em\u003e\u003csub\u003e33\u003c/sub\u003e)\u003csup\u003e33-35\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eTo\u0026nbsp;characterize\u0026nbsp;the\u0026nbsp;effective electromechanical coupling coefficient\u0026nbsp;(\u003cem\u003ek\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e), capacitance-electric field\u0026nbsp;(\u003cem\u003eC\u003c/em\u003e-\u003cem\u003eE\u003c/em\u003e) loops were measured (Fig. 2a). The extracted \u003cem\u003ek\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e values for RTA-AlScN reached 34.9%, a substantial improvement over the 14.1% of AG-AlScN, outperforming reported values for AlScN with Sc concentrations exceeding 30% (Fig. 2b). The \u003cem\u003ed\u003c/em\u003e\u003csub\u003e33\u003c/sub\u003e reached 15.0 pC N\u003csup\u003e-1\u003c/sup\u003e for RTA-AlScN, representing a 2.5-fold enhancement over pure AlN (6 pC N\u003csup\u003e-1\u003c/sup\u003e, Supplementary Fig. S10) and outperforming most reported \u003cstrong\u003eSc-doped AlN\u003c/strong\u003e materials (Fig. 2b). This performance remains highly uniform across the 6-inch wafer, with a \u003cem\u003ed\u003c/em\u003e\u003csub\u003e33\u003c/sub\u003e variation of less than 0.7% (Supplementary Fig. S11), highlighting its suitability for mass production. Additionally, the RTA-AlScN film exhibits a relative permittivity (\u003cem\u003e\u0026epsilon;\u003csub\u003er\u003c/sub\u003e\u003c/em\u003e) of 16 and a remarkable low dielectric loss (tan \u003cem\u003e\u0026delta;\u003c/em\u003e) of \u0026lt; 0.0006 at 1 kHz (Supplementary Fig. S12).\u003c/p\u003e\n\u003cp\u003eSurface topography evaluated by AFM (Fig. 2c) shows that RTA maintains high surface quality (RMS roughness of 0.65 nm). Figure 2d compares the out-of-plane PFM amplitude and phase images. While the AG-AlScN film reveals disordered nanodomains\u0026mdash;evidenced by a high density of randomly oriented spots, indicating non-uniform piezoelectric activity\u0026mdash;the RTA-AlScN film exhibits a marked reduction in these features and a high overall amplitude. This correlates with the enhanced \u003cem\u003ec\u003c/em\u003e-axis ordering of nanodomains,\u0026nbsp;a process intrinsically supported by the lowered ferroelectric switching barrier resulting from the increased bond ionicity and lattice flattening.\u0026nbsp;This facilitates easier dipole rotation and long-range ordering during the thermal process.\u0026nbsp;The distinct phase contrast confirms effective domain-switching under bias, successfully transitioning the central region from the \u0026ldquo;N-polar\u0026rdquo; state to the \u0026ldquo;Al(Sc)-polar\u0026rdquo; state\u003csup\u003e36,37\u003c/sup\u003e.\u0026nbsp;Furthermore,\u0026nbsp;RTA-AlScN\u0026nbsp;displays\u0026nbsp;superior ferroelectric behavior\u0026nbsp;with a high remanent polarization (\u003cem\u003eP\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e ~107 \u0026mu;C cm\u003csup\u003e-2\u003c/sup\u003e) and\u0026nbsp;robust\u0026nbsp;fatigue endurance\u0026nbsp;(~10\u003csup\u003e5\u0026nbsp;\u003c/sup\u003ecycles) (Supplementary\u0026nbsp;Fig. S13).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2 Finite element simulation design\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo guide the structural design of the MEMS receiver, we performed multiphysics coupling finite element analysis (FEA) of a single sensing element. The cross-sectional schematic in Fig. 3a depicts the multilayer architecture of the sensing element, which is integrated on an SOI substrate and comprises an AlN seed layer, Cr/Mo electrodes, and an AlScN piezoelectric functional layer. In piezoelectric MEMS transducers, a back cavity must be engineered beneath the piezoelectric membrane to release mechanical constraints and enable structural deformation. We first optimized the cavity diameter, as shown in the simulation results in Fig. 3a. As the back cavity radius (\u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e) increases, the resonant frequency of the diaphragm consistently decreases. However, an excessively large \u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e can compromise mechanical reliability, potentially leading to fatigue and fracture under continuous acoustic excitation. Balancing the requirements for low-frequency sensitivity and structural durability, a \u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e of 250 \u0026mu;m was selected to ensure a sufficiently low resonant frequency while maintaining structural integrity.\u003c/p\u003e\n\u003cp\u003eWith \u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e fixed at 250 \u0026mu;m, we further investigated the dependencies of the induced charge and resonant frequency on the back cavity-to-(piezoelectric film and top electrode) ratio (\u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e/\u003cem\u003eR\u003c/em\u003e\u003csub\u003ePF\u0026amp;TE\u003c/sub\u003e) and (back cavity and piezoelectric film)-to-top electrode (\u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u0026amp;PF\u003c/sub\u003e/\u003cem\u003eR\u003c/em\u003e\u003csub\u003eTE\u003c/sub\u003e) ratio. As illustrated in Fig. 3b, the total charge on the electrode surface initially increases with both \u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e/\u003cem\u003eR\u003c/em\u003e\u003csub\u003ePF\u0026amp;TE\u0026nbsp;\u003c/sub\u003eand\u003csub\u003e\u0026nbsp;\u003c/sub\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u0026amp;PF\u003c/sub\u003e/\u003cem\u003eR\u003c/em\u003e\u003csub\u003eTE\u003c/sub\u003e, peaking at a ratio 1.25, before exhibiting a slightly decline. Conversely, the resonant frequency follows an opposite trend. At an \u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e/\u003cem\u003eR\u003c/em\u003e\u003csub\u003ePF\u0026amp;TE\u003c/sub\u003e ratio of 1.25, the device achieves an optimal combination of a low resonant frequency (349.0 kHz) and a maximum induced charge (3.4\u0026times;10\u003csup\u003e-16\u0026nbsp;\u003c/sup\u003eC).\u003c/p\u003e\n\u003cp\u003eNotably, at \u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e/\u003cem\u003eR\u003c/em\u003e\u003csub\u003ePF\u0026amp;TE\u0026nbsp;\u003c/sub\u003e= 1.25, the surface charge distribution exhibits a uniformly positive character (Fig. 3c), which supports the maximization of the total harvested charge. This phenomenon is attributed to the optimized distribution of compressive stress across the AlScN film under this configuration (Fig. 3d). For comparison, the results for \u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e/\u003cem\u003eR\u003c/em\u003e\u003csub\u003ePF\u0026amp;TE\u003c/sub\u003e ratios of 1.0 and 1.5 are provided in Supplementary Fig. S14, S15. Similarly, at \u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u0026amp;PF\u003c/sub\u003e/\u003cem\u003eR\u003c/em\u003e\u003csub\u003eTE\u003c/sub\u003e = 1.25, the film also exhibits positive surface charges (Fig. 3e) and compressive stress in the top electrode (Fig. 3f). Comparative cases for \u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u0026amp;PF\u003c/sub\u003e/\u003cem\u003eR\u003c/em\u003e\u003csub\u003eTE\u003c/sub\u003e at 1.0 and 1.5 are detailed in Supplementary Fig. S16, S17. Nevertheless, the \u003cem\u003eR\u003c/em\u003e\u003csub\u003eC\u0026amp;PF\u003c/sub\u003e/\u003cem\u003eR\u003c/em\u003e\u003csub\u003eTE\u003c/sub\u003e = 1.25 configuration remains superior, providing a lower resonant frequency and higher charge density.\u003c/p\u003e\n\u003cp\u003eUsing this optimized configuration, we simulated the device response under an applied acoustic field of 1 Pa at 1 kHz (source positioned 1 m above the sensor). These simulations reveal the relationship between external acoustic pressure and the resulting charge density distribution. Under the simulated acoustic field (Fig. 3g), the receiver undergoes predictable mechanical stress and deformation (Fig. 3h). As acoustic pressure of approximately 116 dB (\u003cem\u003epref\u0026nbsp;\u003c/em\u003e= 1 \u0026mu;Pa) induces a surface charge density ranging from 0.5 \u0026times; 10\u003csup\u003e-5\u003c/sup\u003e to 2.5 \u0026times; 10\u003csup\u003e-5\u003c/sup\u003e C m\u003csup\u003e-2\u003c/sup\u003e (Fig. 3i). This rigorous FEA framework validates the design\u0026rsquo;s effectiveness for high-quality low-frequency acoustic sensing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.3 Structure design and fabrication of MEMS device\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBased on the aforementioned FEA optimization and superior piezoelectric properties of the RTA-AlScN film, we proceeded with the micro-fabrication and characterization of the devices on a 6-inch wafer. To enhance the output signal-to-noise ratio and enable independent operation, a typical rectangular unit cell was designed, featuring a 5\u0026times;5 arrays of piezoelectric sensing elements connected in parallel\u003csup\u003e38\u003c/sup\u003e. The micromachining process was streamlined to employ only three chromium masks (M1-M3), as detailed in Supplementary Fig. S18. Figure 4a presents an optical photograph of the completed wafer-level transducer surface. Each sensing element pattern consists of a stacked piezoelectric functional layer and a top electrode (\u003cem\u003et\u003c/em\u003e = ~1.2 \u0026mu;m,\u003cem\u003e\u0026nbsp;r\u003c/em\u003e = 200 \u0026mu;m), as shown in the magnified views in Fig. 4b-d. To release the sensing diaphragm, the backside silicon was effectively etched to create a cavity with a diameter of 500 \u0026mu;m and a depth of 620 \u0026mu;m (Fig. 4e-h). The measured cavity-to-piezoelectric film radius ratio is approximately 1.25, in excellent agreement with the finite element simulation results and demonstrating the high precision of our MEMS fabrication process. A micrograph of the single chip containing 25 sensing elements is presented in Supplementary Fig. S19, showcasing a compact footprint with an area of only ~36 mm\u0026sup2; (Supplementary Fig. S20). This \u0026ldquo;three-mask and four-etching\u0026rdquo; manufacturing protocol minimizes error accumulation and ensures high fabrication reliability and batch consistency.\u003c/p\u003e\n\u003cp\u003eThe electrical integrity of the bare chips was initially evaluated using a probe station by connecting the electrodes to two probes (Fig. 4i). The continuity between top electrodes (top-top) and bottom electrodes (bottom-bottom) is illustrated in Fig. 4j. We observed ideal Ohmic contact behavior with excellent conductivity, characterized by low resistance values of 8.05 \u0026Omega; for the top electrodes and 5.67 \u0026Omega; for the bottom electrodes. This high electrode continuity across the wafer provides a reliable foundation for low-loss piezoelectric signal transmission. To evaluate the leakage and resistive performance of the RTA-AlScN film itself, current-voltage (\u003cem\u003eI\u003c/em\u003e-\u003cem\u003eV\u003c/em\u003e) measurements were conducted between the top and bottom electrodes. Under a bias voltage of 5 V, the inter-electrode insulation consistently exceeded 4\u0026times;10\u003csup\u003e9\u003c/sup\u003e \u0026Omega; across ten consecutive measurements (Fig. 4k), confirming the high-quality insulation and structural integrity of the annealed film. Figure 4l shows the impedance and phase spectra of the RTA-AlScN unit. The measured resonant frequency of 368.2 kHz is only 5.2% higher than the simulated value (349.0 kHz), validating the accuracy of our design parameters and the effectiveness of the device in responding to low-frequency signals. The overall yield of functional bare dies exceeds 90%, with isolated failures (such as edge-effect-included shorts) occurring only at the wafer periphery.\u003c/p\u003e\n\u003cp\u003eTo optimize the overall sensing performance, a specialized matching circuit and protective packaging were developed. A preamplifier circuit with a gain of 40 dB was fabricated to amplify the weak acoustic-induced charges (Supplementary Fig. S21-S23). For system-level measurements, the MEMS chips were wire-bonded to a printed circuit board (PCB) using a Gold Wire ball bonder, ensuring stable electrical and mechanical interconnections (Supplementary Fig. S24, S25). Finally, the sensors encapsulated using silicone oil and polyurethane to achieve high sensitivity and environmental robustness, as displayed in Supplementary Fig. S26.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.4 Acoustic receiving performance of MEMS sensor\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe packaged MEMS sensor was fully characterized using industry-standard calibration instrument. The operational principle of the sensor is illustrated in Fig. 5a. During underwater deployment, acoustic waves from the environment penetrate the protective shell and the liquid insulating medium to reach the sensitive MEMS diaphragm. The integrated AlScN chips convert the incident acoustic pressure into electrical signals, which are subsequently amplified and processed.\u003c/p\u003e\n\u003cp\u003eThe receiving sensitivity, a critical performance metric, was evaluated using the configuration shown in Supplementary Fig. S27. As demonstrated in Fig. 5b,\u0026nbsp;the sensor exhibits a remarkably flat sensitivity response across the 10 Hz to 1000 Hz operational bandwidth, with a sensitivity of -172.2 dB at 1 kHz. Notably,\u0026nbsp;the sensitivity reaches as high as -162.4\u0026nbsp;dB\u0026nbsp;at 10 Hz, significantly outperforming conventional AlN-based devices (-178 dB re:\u0026nbsp;V\u0026nbsp;\u0026mu;Pa\u003csup\u003e-1\u003c/sup\u003e)\u003csup\u003e39\u003c/sup\u003e, thereby highlighting the enhanced acoustic reception enable by the RTA-AlScN film.\u0026nbsp;Time-domain measurements (Supplementary Fig. S28) reveals a stable and clean sinusoidal output voltage, confirming distortion-free signal acquisition and amplification. Furthermore, the self-noise equivalent pressure spectral level, which dictates the detection threshold for weak signals, was analyzed (Fig. 5c). The\u0026nbsp;self-noise amplitude decreases\u0026nbsp;rapidly\u0026nbsp;from 20 Hz to 1 kHz before leveling off above 2.7 kHz.\u0026nbsp;At 1kHz, the noise floor is approximately 59 dB (re:1\u0026nbsp;\u0026mu;Pa\u0026nbsp;Hz\u003csup\u003e-0.5\u003c/sup\u003e), which is comparable to the ambient ocean noise level under Sea State 2\u003csup\u003e40\u003c/sup\u003e. To assess the spatial response, the directivity was measured in both vertical and horizontal planes (Fig. 5d, e). The measured fluctuation was only -1.51 dB and -1.07 dB for the vertical and horizontal planes, respectively, confirming the excellent omnidirectional characteristics of the device.\u003c/p\u003e\n\u003cp\u003eThe receiver\u0026rsquo;s capability was further evaluated through pitch-catch measurements using a PZT ceramic transmitter. In liquid media (water and silicone oil), the peak-to-peak voltage output (\u003cem\u003eV\u003c/em\u003e\u003csub\u003ePP\u003c/sub\u003e) follows a predictable attenuation pattern as the distance from the sound source increases (Fig.\u0026nbsp;5f, g), consistent with acoustic propagation loss and the decay of particle vibration amplitude\u003csup\u003e41\u003c/sup\u003e. To verify the potential for high-temperature applications, measurements were performed at various temperatures up to 150 \u0026deg;C. Although the \u003cem\u003eV\u003c/em\u003e\u003csub\u003ePP\u003c/sub\u003e values exhibit a marginal decreasing trend with a 12% loss (from 890 m\u003cem\u003eV\u003c/em\u003e\u003csub\u003ePP\u0026nbsp;\u003c/sub\u003eto 800 m\u003cem\u003eV\u003c/em\u003e\u003csub\u003ePP\u0026nbsp;\u003c/sub\u003eat 150 \u0026deg;C), the performance remains highly stable within this range. This robustness indicates that the sensor is well-suited for harsh environments, such as deep-sea offshore wells, automotive underhood sensing, and oil shale exploration\u003csup\u003e42,43\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eAdditionally, the sensor\u0026rsquo;s versatility was demonstrated in solid media by coupling the device to a ceramic panel. As the medium layer thickness increased from 2 mm to 15 mm (Fig. 5h), the \u003cem\u003eV\u003c/em\u003e\u003csub\u003epp\u003c/sub\u003e signal exhibited a monotonic decrease, mirroring the behavior observed in liquid medium. The maximum output voltage of 900 mV observed in silicone oil is attributed to optimal impedance matching between the sensor\u0026rsquo;s internal insulating liquid and the external medium (Supplementary Table S4). For practical applications, long-term stability is paramount. As shown in Fig. 5i, the device demonstrates exceptional sensitivity stability in seawater; after six months of continuous immersion, the cumulative attenuation was only 2.8 dB. This validates the effectiveness of the polyurethane/silicone oil packaging in protecting the core MEMS chip.\u003c/p\u003e\n\u003cp\u003eTo contextualize the achievement in MEMS receivers, a comprehensive comparison of key performance metrics between this work and state-of-the-art reported devices is summarized in Table 1. Notably, our RTA-AlScN MEMS receiver represents a significant breakthrough, being the first to achieve the highest reported sensitivity at such an ultra-low frequency. The following salient merits underscore its potential as a paradigm-shifting solution for low-frequency acoustic sensing. (i) Industrial Scalability: While the majority of prior research has focused on small-scale laboratory prototypes, our work leverages an industrial-grade 6-inch wafer platform. This transition from chip-level to wafer-level fabrication demonstrates the immense potential for high-volume mass production. (ii) Manufacturing Efficiency: The device utilizes a streamlined \u0026quot;three-mask and four-etching\u0026quot; protocol. This minimalist process significantly reduces error accumulation and production costs while ensuring exceptional fabrication reliability and yield. (iii) Record-breaking Sensitivity: To the best of our knowledge, the RTA-AlScN MEMS sensor demonstrates a record-breaking sensitivity of -162.4 dB at the lowest operating frequency of 10 Hz reported to date. This capability addresses a critical gap in detecting infrasonic signals with high fidelity. (iv) Proven Long-term Stability: For the first time in lead-free piezoelectric MEMS research, we report a cumulative sensitivity attenuation as low as 2.8 dB over six months of seawater immersion. This exceptional wet-storage stability validates the effectiveness of our system-level packaging and highlights its readiness for sustained service in harsh marine environments.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1 | Comparisons of sensing performances of our RTA-AlScN receiver with other reported representative MEMS devices\u003c/strong\u003e\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"586\" class=\"fr-table-selection-hover\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 14.4804%;\"\u003e\n \u003cp\u003eThin films\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 9.02896%;\"\u003e\n \u003cp\u003eWafer size\u003c/p\u003e\n \u003cp\u003e(inch)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 10.3918%;\"\u003e\n \u003cp\u003eEtching\u003c/p\u003e\n \u003cp\u003ecount\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.8586%;\"\u003e\n \u003cp\u003eMasks\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.4361%;\"\u003e\n \u003cp\u003eUsable frequency range (Hz)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.7768%;\"\u003e\n \u003cp\u003eHighest\u003c/p\u003e\n \u003cp\u003esensitivity\u003c/p\u003e\n \u003cp\u003e(dB re 1 V \u0026mu;Pa\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.0954%;\"\u003e\n \u003cp\u003eNoise resolution\u003c/p\u003e\n \u003cp\u003e(dB re\u0026nbsp;1 \u0026mu;Pa\u0026nbsp;Hz\u003csup\u003e-0.5\u003c/sup\u003e at 1000 Hz)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11.414%;\"\u003e\n \u003cp\u003eDirectivi-ty\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.51789%;\"\u003e\n \u003cp\u003eRef.\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 14.4804%;\"\u003e\n \u003cp\u003ePZT\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 9.02896%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 10.3918%;\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.8586%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.4361%;\"\u003e\n \u003cp\u003e100\u003c/p\u003e\n \u003cp\u003e-10000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.7768%;\"\u003e\n \u003cp\u003e-227.5\u003c/p\u003e\n \u003cp\u003e@ 2000 Hz\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.0954%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11.414%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.51789%;\"\u003e\n \u003cp\u003e51\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 14.4804%;\"\u003e\n \u003cp\u003ePZT\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 9.02896%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 10.3918%;\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.8586%;\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.4361%;\"\u003e\n \u003cp\u003e80-20000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.7768%;\"\u003e\n \u003cp\u003e-189.3\u003c/p\u003e\n \u003cp\u003e@ 920 Hz\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.0954%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11.414%;\"\u003e\n \u003cp\u003eOmnidir-ectional\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.51789%;\"\u003e\n \u003cp\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 14.4804%;\"\u003e\n \u003cp\u003ePZT\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 9.02896%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 10.3918%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.8586%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.4361%;\"\u003e\n \u003cp\u003e10\u0026nbsp;-5000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.7768%;\"\u003e\n \u003cp\u003e-195.5\u003c/p\u003e\n \u003cp\u003e@ 2500 Hz\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.0954%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11.414%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.51789%;\"\u003e\n \u003cp\u003e11\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 14.4804%;\"\u003e\n \u003cp\u003eAlN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 9.02896%;\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 10.3918%;\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.8586%;\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.4361%;\"\u003e\n \u003cp\u003e10-10000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.7768%;\"\u003e\n \u003cp\u003e-166.5\u003c/p\u003e\n \u003cp\u003e@ 750 Hz\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.0954%;\"\u003e\n \u003cp\u003e40\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11.414%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.51789%;\"\u003e\n \u003cp\u003e52\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 14.4804%;\"\u003e\n \u003cp\u003eAlN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 9.02896%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 10.3918%;\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.8586%;\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.4361%;\"\u003e\n \u003cp\u003e10-8000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.7768%;\"\u003e\n \u003cp\u003e-178\u003c/p\u003e\n \u003cp\u003e@ 1000 Hz\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.0954%;\"\u003e\n \u003cp\u003e58.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11.414%;\"\u003e\n \u003cp\u003eOmnidir-ectional\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.51789%;\"\u003e\n \u003cp\u003e38\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 14.4804%;\"\u003e\n \u003cp\u003eAlN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 9.02896%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 10.3918%;\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.8586%;\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.4361%;\"\u003e\n \u003cp\u003e10-1600\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.7768%;\"\u003e\n \u003cp\u003e-178\u003c/p\u003e\n \u003cp\u003e@ 1000 Hz\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.0954%;\"\u003e\n \u003cp\u003e52.6 @ 100 Hz\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11.414%;\"\u003e\n \u003cp\u003eOmnidir-ectional\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.51789%;\"\u003e\n \u003cp\u003e39\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 14.4804%;\"\u003e\n \u003cp\u003eAlN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 9.02896%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 10.3918%;\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.8586%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.4361%;\"\u003e\n \u003cp\u003e20-1000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.7768%;\"\u003e\n \u003cp\u003e-177.6\u003c/p\u003e\n \u003cp\u003e@ 1000 Hz\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.0954%;\"\u003e\n \u003cp\u003e58.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11.414%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.51789%;\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 14.4804%;\"\u003e\n \u003cp\u003eAl\u003csub\u003e0.905\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003eSc\u003csub\u003e0.095\u003c/sub\u003eN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 9.02896%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 10.3918%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.8586%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.4361%;\"\u003e\n \u003cp\u003e20000\u003c/p\u003e\n \u003cp\u003e-160000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.7768%;\"\u003e\n \u003cp\u003e-175\u003c/p\u003e\n \u003cp\u003e@160 kHz\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.0954%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11.414%;\"\u003e\n \u003cp\u003eOmnidir-ectional\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.51789%;\"\u003e\n \u003cp\u003e53\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 14.4804%;\"\u003e\n \u003cp\u003eAl\u003csub\u003e0.8\u003c/sub\u003eSc\u003csub\u003e0.2\u003c/sub\u003eN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 9.02896%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 10.3918%;\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.8586%;\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.4361%;\"\u003e\n \u003cp\u003e10 -200000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.7768%;\"\u003e\n \u003cp\u003e-172\u003c/p\u003e\n \u003cp\u003e@ 1000 Hz\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.0954%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11.414%;\"\u003e\n \u003cp\u003eOmnidir-ectional\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.51789%;\"\u003e\n \u003cp\u003e54\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 14.4804%;\"\u003e\n \u003cp\u003eAl\u003csub\u003e0.8\u003c/sub\u003eSc\u003csub\u003e0.2\u003c/sub\u003eN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 9.02896%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 10.3918%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.8586%;\"\u003e\n \u003cp\u003e_\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.4361%;\"\u003e\n \u003cp\u003e10 -1000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.7768%;\"\u003e\n \u003cp\u003e-162.9\u003c/p\u003e\n \u003cp\u003e@ 1000 Hz\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.0954%;\"\u003e\n \u003cp\u003e46.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11.414%;\"\u003e\n \u003cp\u003eOmnidir-ectional\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.51789%;\"\u003e\n \u003cp\u003e55\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 14.4804%;\"\u003e\n \u003cp\u003eRTA-AlScN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 9.02896%;\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 10.3918%;\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.8586%;\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.4361%;\"\u003e\n \u003cp\u003e10-1000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.7768%;\"\u003e\n \u003cp\u003e-162.4\u003c/p\u003e\n \u003cp\u003e@10 Hz\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 12.0954%;\"\u003e\n \u003cp\u003e59\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11.414%;\"\u003e\n \u003cp\u003eOmnidir-ectional\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8.51789%;\"\u003e\n \u003cp\u003eThis work\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e"},{"header":"3. Conclusion","content":"\u003cp\u003eIn summary, we have successfully demonstrated an ultra-sensitive piezoelectric MEMS receiver by a multi-scale lattice engineering strategy. By integrating high-performance AlScN thin film via an industrial-grade fabrication process on 6-inch SOI wafers, we provide a compelling pathway for the development of next-generation ultra-sensitive, low-frequency acoustic devices. The Al\u003csub\u003e0.9\u003c/sub\u003eSc\u003csub\u003e0.1\u003c/sub\u003eN thin film, optimized via post-growth RTA, exhibits significantly enhanced piezoelectric properties, including a \u003cem\u003ed\u003c/em\u003e\u003csub\u003e33\u003c/sub\u003e of 15.0 pC N\u003csup\u003e-1\u003c/sup\u003e and \u003cstrong\u003ea\u0026nbsp;\u003c/strong\u003e\u003cem\u003ek\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e of 34.9%. This record-breaking performance is fundamentally rooted in a synergistic mechanism: at the atomic scale, Sc-doping reduces the bond covalency and minimum electron density; at the mesoscopic scale, RTA minimizes the lattice parameter ratio (\u003cem\u003ec\u003c/em\u003e/\u003cem\u003ea\u003c/em\u003e) and homogenizes\u0026nbsp;nanoscopic domain\u0026nbsp;orientations.\u003cstrong\u003e\u0026nbsp;By leveraging\u003c/strong\u003e optimized structural design and a robust \u0026ldquo;three-mask, four-etch\u0026rdquo; micromachining process, we have established a reliable scalable wafer-level manufacturing route. The resulting MEMS receiver demonstrates unprecedented acoustic responsiveness, highlighted by a record-high sensitivity of -162.4 dB (re: 1 V \u0026mu;Pa\u003csup\u003e-1\u003c/sup\u003e)\u0026nbsp;at 10 Hz, a\u0026nbsp;noise\u0026nbsp;equivalent pressure of\u0026nbsp;59\u0026nbsp;dB (re: 1\u0026nbsp;\u0026mu;Pa\u0026nbsp;Hz\u003csup\u003e-0.5\u003c/sup\u003e) at 1 kHz, and omnidirectional receiving characteristics. Furthermore, the device maintains outstanding long-term stability with minimal attenuation over six months of seawater immersion and reliable functionality up to 150 \u0026deg;C. This work underscores the critical synergy between fundamental materials physics, optimized device architecture, and robust system integration, paving a transformative framework for designing high-performance lead-free piezoelectric microsystems in demanding sensing applications.\u003c/p\u003e"},{"header":"4. Experiments and computational modelling","content":"\u003cp\u003eThe development of the AlScN-based MEMS receiver proceeded through three integrated stages: (i) deposition of high-quality AlScN films on 6-inch SOI wafers, (ii) micromachining of the MEMS sensing units, and (iii) system-level integration and packaging (Supplementary Fig.\u0026nbsp;S29).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4.1 Procedure of AlScN film\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eP-type (100)-oriented silicon-on-insulator (SOI) wafers (6-inch diameter, resistivity: 0.002-0.005 \u0026Omega; cm) were employed as substates. The wafer architecture comprises a 5 \u0026micro;m device silicon layer, a 0.5 \u0026micro;m buried oxide (SiO₂) layer, and a 625 \u0026micro;m handle silicon layer.\u0026nbsp;Initially, the SOI wafers underwent\u0026nbsp;Standard Radio Corporation of America (RCA) Cleaning\u0026nbsp;to ensure a pristine surface. Subsequently, a multilayer stack consisting of a seeding layer (20 nm AlN), an adhesion layer (20 nm Cr), a bottom electrode (250 nm Mo), the functional piezoelectric layer (1 \u0026mu;m AlScN), a second adhesion layer (20 nm Cr), and a top electrode (200 nm Mo) was sequentially deposited using a magnetron sputtering system (Explorer 14, DENTON Vacuum LLC, USA).\u0026nbsp;The\u0026nbsp;AlN\u0026nbsp;and AlScN were\u0026nbsp;synthesized\u0026nbsp;under\u0026nbsp;a base pressure of \u0026lt;\u0026nbsp;5 \u0026times; 10\u003csup\u003e-4\u003c/sup\u003e Pa and a working pressure of 0.4 Pa, with an RF power of 350 W and a substrate rotation of 15 rpm. To optimize piezoelectricity, the as-deposited AlScN films were subjected to rapid thermal annealing at 700 \u0026deg;C.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4.2 Fabrication of the MEMS by micromachining technology\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe MEMS fabrication followed a \u0026ldquo;three-mask and four-etching\u0026rdquo; protocol on a commercial production line. First, the top electrode and piezoelectric layers were patterned using Mask M1. The Cr/Mo top electrode was defined via Ion Beam Etching (IBE), followed immediately by inductively coupled plasma (ICP) etching of the AlScN layer. The photoresist (AZ 5214, Merck KGaA, Germany) was stripped using\u0026nbsp;AZ400T. Second, a 200 nm SiO\u003csub\u003e2\u003c/sub\u003e insulating layer was deposited via plasma-enhanced chemical vapor deposition (PECVD), and contact holes were patterned using Mask M2 via reactive ion etching (RIE). Third, the interconnection metal was patterned. Finally, the handle Si was etched from the backside using deep reactive ion etching (DRIE, SPTS Rapier, UK) to release the diaphragms. The wafer was diced (NANO 150G), and the resulting chips were mounted onto printed circuit boards (PCB) using epoxy and electrically interconnected via gold wire bonding (53XX BDA, F\u0026amp;S Bondtec, Germany).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4.3 Preparation of AlScN MEMS sensor\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo minimize system-level noise, the MEMS chip was integrated with a custom-designed 40 dB preamplifier circuit. Based on acoustic transmission theory, the device was encapsulated using polyurethane and silicone oil. Silicone oil was injected into the sensing cavity to ensure acoustic coupling and hydrostatic pressure balance. The acoustic impedance gradient among seawater (1.57 MRayl), polyurethane (1.62 MRayl), and silicone oil (1.26 MRayl) were optimized to maximize the acoustic-electric conversion efficiency (Supplementary Table S4 and\u0026nbsp;Fig. S30).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4.4 Characterization\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMorphological and element distributions were analyzed using SEM (GeminiSEM 360, Carl Zeiss, Germany) and EDS. Crystalline phases were identified via XRD (D8 Advance, Bruker, Germany) using Cu K\u0026alpha; radiation. The quasi-static \u003cem\u003ed\u003c/em\u003e\u003csub\u003e3\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e was measured using a \u003cem\u003ed\u003c/em\u003e\u003csub\u003e33\u003c/sub\u003e meter (ZJ-6A, Institute of Acoustics, Chinese Academy of Sciences, China). Impedance spectra, including resonant and anti-resonant frequencies were acquired using an Agilent 4294A analyzer. Electrode conductivity was verified via four-point probe measurements (Crexbox, Deyi Tech). For pitch-catch characterization, a function generator (AFG31000, Tektronix, USA) and oscilloscope (MDO34, Tektronix, USA) were used to record the acoustic response at 20 Hz. Ferroelectric properties and \u003cem\u003eC-V\u003c/em\u003e/\u003cem\u003eP-E\u003c/em\u003e loops were characterized using a Radiant Precision Multiferroic system. To facilitate through-thickness poling of the 1-\u0026mu;m-thick films, samples were thinned to ~100 nm using focused ion beam (FIB, Helios G4, USA) milling. The surface morphology was characterized using an atomic force microscope (AFM; Dimension Icon, Bruker, Germany). Following this, the amplitude and phase images of both AG-AlScN and RTA-AlScN films were acquired using the AFM system (Dimension Icon, Bruker, Germany) in piezoelectric mode. Atomic-scale imaging was performed via HAADF-STEM (Spectra 300, Thermo Fisher Scientific, USA) at 300 kV with an 11 mrad convergence angle.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4.5 Finite element models\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMultiphysics simulations were conducted with finite element models, coupling the \u003cem\u003ePressure Acoustics\u003c/em\u003e, \u003cem\u003eSolid Mechanics\u003c/em\u003e, and \u003cem\u003eElectrostatics\u003c/em\u003e modules. The sensing area was defined by the top electrode geometry. A background pressure field of 1 Pa was applied to evaluate the acoustic-to-electric conversion efficiency via surface charge density analysis.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4.6 DFT calculations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe atomic properties and electronic structure of the materials were calculated using first-principles simulations within density functional theory (DFT)\u003csup\u003e44\u003c/sup\u003e. The projected augmented wave pseudopotentials method was used as implemented in the Vienna \u003cem\u003eAb\u003c/em\u003e initio Simulation Package\u003csup\u003e45\u003c/sup\u003e. The exchange correlation energy was calculated using the generalized gradient approximation of the Perdew-Burke-Ernzerhof form\u003csup\u003e46\u003c/sup\u003e. The plane wave cutoff energy was set to 500 eV. For Brillouin zone sampling, a Monkhorst-Pack \u003cem\u003ek\u003c/em\u003e-point mesh with a spacing finer than 0.03 \u0026Aring;\u003csup\u003e-1\u003c/sup\u003e was used for all structures\u003csup\u003e47\u003c/sup\u003e. Utilizing the conjugate gradient method, the plane lattice constant and atomic coordinates were fully relaxed until the energy and force converge to 10\u003csup\u003e-7\u003c/sup\u003e eV and 10\u003csup\u003e\u0026ndash;3\u003c/sup\u003e eV \u0026Aring;\u003csup\u003e-1\u003c/sup\u003e,\u0026nbsp;respectively.\u0026nbsp;The valence electron configurations were set to Sc-2s\u003csup\u003e2\u003c/sup\u003ep\u003csup\u003e6\u003c/sup\u003e3d\u003csup\u003e1\u003c/sup\u003e4s\u003csup\u003e2\u003c/sup\u003e, Al-3s\u003csup\u003e2\u003c/sup\u003e3p\u003csup\u003e1\u003c/sup\u003e, and N-2s\u003csup\u003e2\u003c/sup\u003e2p\u003csup\u003e3\u003c/sup\u003e. The piezoelectric \u003cem\u003ee\u003c/em\u003e\u003cu\u003e\u003csub\u003eij\u003c/sub\u003e\u003c/u\u003e tensors were predicted by the density-functional perturbation theory method\u003csup\u003e48-50\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Natural Science Foundation of China (Grant Nos. 51972144, U24A2040), the Key Technology Research and Development Program of Shandong Province (Grant No. 2022CXPT045), the Taishan Scholar Foundation of Shandong Province (Grant No. tstp20221130), Taishan Industrial Experts Program (Grant No. tscx202306035), and the 111 Project of International Corporation on Advanced Cement-based Materials (Grant No. D17001).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eZ.Y. Cheng, T. Liu, and D. Su contributed equally to this work, leading the majority of the experiments and data analysis. C. Wu contributed to theory and discussions. Y. Liu provided project funding support. W. Sun performed the density functional theory (DFT) calculations. S. Huang supervised this study. C. Yang and Z. X. Cheng oversaw the conceptualization and design of the study, and led the writing and revision of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary information\u003c/strong\u003e The online version contains supplementary material available.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eWang, Q. et al. 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IEEE, 301\u0026ndash;304 (2019).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"AlScN thin films, lattice engineering, MEMS receivers, ultrasensitivity, low-frequency applications","lastPublishedDoi":"10.21203/rs.3.rs-9122069/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9122069/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"High-performance AlScN thin films are critical for next-generation lead-free piezoelectric applications. Here, exceptional electromechanical coupling and piezoelectric response are demonstrated in Al0.9Sc0.1N thin films via a multi-scale lattice engineering strategy. Post-growth rapid thermal annealing significantly boosts the piezoelectric coefficient to 15.0 pC N-1 and electromechanical coupling to 34.9%. This performance enhancement stems from a synergistic mechanism: a reduced c/a lattice ratio and homogenized nanodomain alignment, fundamentally underpinned by a transition toward higher bond ionicity as evidenced by reduced minimum electron density at the bond saddle points. Leveraging an industrial-grade 6-inch wafer process, these films exhibit tailored electromechanical characteristics with a resonance frequency of 368.2 kHz. The optimized piezoelectric response enables record-breaking acoustic sensitivity of -162.4 dB at 10 Hz and a noise floor of 59 dB at 1 kHz, with robust stability across diverse media and temperatures up to 150 °C. This work provides a scalable, mechanism-driven pathway for tailoring the electromechanical properties of AlScN thin films, addressing the long-standing challenge of high-fidelity weak signal detection in extreme environments.","manuscriptTitle":"Giant piezoelectric response and electromechanical coupling in AlScN thin films via multi-scale lattice engineering","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-19 02:24:07","doi":"10.21203/rs.3.rs-9122069/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"0e2a1dec-5bdc-43a2-855a-3f0b4b17706e","owner":[],"postedDate":"March 19th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":64575514,"name":"Physical sciences/Materials science/Condensed-matter physics/Ferroelectrics and multiferroics"},{"id":64575515,"name":"Physical sciences/Physics/Condensed-matter physics/Ferroelectrics and multiferroics"}],"tags":[],"updatedAt":"2026-04-15T09:26:15+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-19 02:24:07","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9122069","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9122069","identity":"rs-9122069","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}