Advancements in Super-High Frequency Al(Sc)N BAW Resonators for 5G and Beyond

Abstract

With the booming development of the 5G market in recent years, super-high frequency (SHF) resonators will play an increasingly critical role in 5G and future communication systems. Facing the growing market demand for miniaturized, high-bandwidth, and low insertion loss filters, the design of SHF resonators and filters with a high effective electromechanical coupling coefficient (K²_eff) and quality factor, low insertion loss, high passband flatness, strong out-of-band rejection, and high power handling capacity has placed high demands on piezoelectric material preparation, process optimization, and resonator design. The polarity-inverted Al(Sc)N multilayer substrate has become one of the key solutions for SHF resonators. This review provides a comprehensive overview of the recent advances in SHF Al(Sc)N bulk acoustic wave (BAW) resonators. It systematically discusses the device design methodologies, structural configurations, and material synthesis techniques for high-quality Al(Sc)N thin films. Particular emphasis is placed on the underlying mechanisms and engineering strategies for polarity control in Al(Sc)N-based periodically poled multilayer structures. The progress in periodically poled piezoelectric film (P3F) BAW resonators is also examined, with special attention to their ability to significantly boost the operating frequency of BAW devices without reducing the thickness of the piezoelectric layer, while maintaining a high K²_eff. Finally, the review outlines current challenges and future directions for achieving a higher quality factor (Q), improved frequency scalability, and greater integration compatibility in SHF acoustic devices, paving the way for next-generation radio frequency (RF) front-end technologies in 5G/6G and beyond.

1. Introduction

The development of bulk acoustic wave (BAW) resonators is critical for the advancement of 5G communication systems, which demand devices capable of operating at higher frequencies while maintaining excellent performance metrics. To meet these requirements, BAW resonators must exhibit high-frequency operation, a high effective electromechanical coupling coefficient (K²_eff), a high quality factor (Q), and robust power handling capabilities. Traditional single-layer BAW resonators often struggle to achieve these specifications, particularly at frequencies exceeding 10 GHz, where device performance would degrade due to increased acoustic losses and reduced electromechanical coupling. In contrast, multilayer periodically poled films, such as periodically poled AlN (P3F AlN) and AlScN, enable high overtone mode excitation, allowing these resonators to operate at significantly higher frequencies while preserving device size and power durability [1,2,3]. As the demand for higher data rates and improved spectral efficiency continues to grow, the integration of multilayer P3F technology into BAW resonators is poised to play a pivotal role in the evolution of 5G and future 6G wireless communication systems [4,5].

In recent years, increasing attention has been paid to polarity reversal engineering, which enables the formation of both Al- and N-polar regions within the same structure. Polarity inversion allows for innovative heterostructures such as periodically poled superlattices, inversion domain-based modulation structures, and tunable piezoelectric or ferroelectric stacks. However, fundamental understanding of polarity formation and inversion in AlN and AlScN remains incomplete, and the realization of high-quality, scalable, and thermally stable polarity-reversed structures remains challenging.

While most existing reviews on BAW devices have mainly focused on the progress of conventional single-layer Al(Sc)N resonators, discussions on fabrication approaches for super-high frequency (SHF) band BAW resonators remain limited, and comprehensive reviews in this area are still lacking. Moreover, limited attention has been devoted to the underlying principles enabling high-frequency operation with P3F multilayer structures, as well as polarity control methods and fabrication strategies for P3F devices. To address these gaps, this review provides a systematic summary of recent advances in the simulation and structural design of Al(Sc)N BAW resonators, growth techniques of Al(Sc)N thin films, especially for the polarity engineering methods for P3F multilayers, and the development status and bottlenecks of P3F devices in the SHF band.

In detail, this review begins with the circuit models and simulation methodologies for AlN-based BAW resonators, discussing the operating principles of several types of bulk acoustic wave resonators along with their respective development bottlenecks and technical characteristics in high-frequency bands. Starting from various growth techniques for AlN and AlScN piezoelectric thin films, it examines current growth methods and the performance parameters of BAW resonators fabricated using different approaches. As an emerging hotspot in the field of radio frequency (RF) resonators, super-high-frequency AlN BAW resonators leverage periodically polarized film stacks of AlN or AlScN materials to achieve high-frequency operation without thinning the piezoelectric film. This approach not only prevents spurious mode generation and energy leakage caused by film thickness reduction but also maintains high effective electromechanical coupling coefficients, which has been proven as a viable strategy for preparing ultra-high-frequency AlN BAW resonators. By exploring current research on the mechanisms of polarity growth and regulation for AlN and AlScN across various domains and analyzing case studies of different polarity control techniques, this review deepens theoretical and practical perspectives on AlN polarity research while identifying future development directions.

2. AlN-Based Bulk Acoustic Wave Resonators: Models, Technologies, and Frequency Advancement

2.1. Mason and mBVD Modeling of AlN BAW Resonators

Two theoretical modeling approaches are widely applied to BAW resonators: the one-dimensional Mason model and the modified Butterworth–Van Dyke (mBVD) circuit model [6,7], which are shown in Figure 1a and Figure 1b, respectively.

As illustrated in Figure 1a, the Mason model represents a layered piezoelectric resonator using an equivalent transmission-line network, translating its mechanical or acoustic properties into an electrical circuit with elements like inductance, capacitance, and resistance [8]. This provides a physics-based understanding of resonator behavior across a wide frequency range, including higher-order modes and spurious responses, making Mason’s model well-suited to designing multi-layer structures like solidly mounted resonators (SMRs) [9,10]. In contrast, the mBVD model is a lumped-element circuit that accurately fits a resonator’s electrical response in the vicinity of its resonance and anti-resonance frequencies [11,12,13]. Figure 1b presents the mBVD model, which consists of a motional branch (series R_m, L_m, C_m) representing the piezoelectric resonant mode and shunt elements to account for static capacitance and losses. Additional series resistors R_s (electrode/interface loss) and R₀ (dielectric loss) in the mBVD model enable fitting of the measured impedance more closely around resonance [14]. These models are complementary: the mBVD equivalent circuit provides an intuitive and computationally quick way to characterize the resonator near its operating point, while the Mason model offers a more comprehensive description of the broadband behavior of the resonators and can include arbitrary layer stacks and mode shapes. Overall, both the Mason and mBVD models are indispensable tools for interpreting BAW resonator behavior: they connect the physical design to its electrical performance of the devices, guiding optimizations in geometry and material to meet target frequencies and bandwidths.

Figure 1. Equivalent circuits of BAW resonators [15]: (a) Equivalent Mason circuit model [7]; (b) The modified Butterworth–Van Dyke circuit model (mBVD) [7,16].

Figure 1. Equivalent circuits of BAW resonators [15]: (a) Equivalent Mason circuit model [7]; (b) The modified Butterworth–Van Dyke circuit model (mBVD) [7,16].

2.2. Finite-Element Analysis for Frequency Extraction and Thermal Effects

While circuit models provide analytic insight, finite-element analysis (FEA) has become essential for detailed BAW resonator studies. FEA simulation has become indispensable in the design and optimization of BAW resonators and filters, especially in addressing key challenges such as increasing power handling capacity, enhancing the quality factor, suppressing spurious modes, and enabling hybrid filter architectures. For example, FEA allows solving the full piezoelectric solid mechanics of an AlN resonator in 2D or 3D, yielding accurate resonant frequencies, mode shapes, and performance metrics for complex geometries that may not be easily captured by analytical models [17].

Figure 2 presents case studies where different FEA tools are used to model the device structure, evaluate power handling capability, and optimize the design of hybrid filter architectures. Specifically, power handling simulations are illustrated in the “Power handling” section of Figure 2. Thermal management and power durability play critical roles for BAW resonators and filters, particularly in RF front-end modules for 5G and beyond. FEA software enables accurate simulations of temperature distribution across the piezoelectric layers, electrodes, and underlying structures under high-power conditions. For instance, FEA tools allow researchers to analyze how various geometric parameters (e.g., electrode thickness, cavity dimension, and material selections) influence thermal dissipation and stress concentration [18,19,20]. Tag et al. [17] developed an advanced multi-physics modeling approach for BAW filters operating under high-power conditions, integrating acoustic, electromagnetic (EM), and thermal simulations. The method accounts for spatial distributions of dissipated power and temperature within individual resonators, enabling accurate thermal modeling through 3D FEM and infrared thermography validation. By modifying the resonator layer stack based on thermal simulation outputs, the model captures thermally induced geometry and material property changes, facilitating realistic prediction of performance degradation such as resonance frequency shifts. This comprehensive framework allows for reliable assessment of power-handling capabilities and thermal robustness of BAW filters in high-RF-power environments. Wu et al. [21] conducted finite element simulations to evaluate the thermal behavior and power handling of AlN-based film bulk acoustic resonators (FBARs). By coupling electrostatic and mechanical fields in FEA simulation software, they analyzed resonance displacement, parasitic modes, and temperature-induced frequency shifts. A SiO₂-based compensation layer was introduced to reduce the temperature coefficient of frequency (TCF), and the optimized structure achieved stable performance with a TCF of 7.09 ppm/°C. The extracted loss parameters were integrated into a refined 1D model for improved power capability prediction. By iteratively implementing resonator designs in simulation, device structures can be optimized to improve power handling without compromising Q-factor, thereby ensuring stable operation at power levels of 30 dBm and beyond [22,23].

By incorporating temperature-dependent material properties such as thermal expansion coefficients, temperature variation of elastic, and piezoelectric constants and applying a uniform temperature change, one can simulate the shift in resonant frequency with temperature, i.e., extract the TCF. For example, a range of temperatures can be applied in an BAW resonator FEA model and the frequency drift can be directly computed, capturing both material-intrinsic effects and thermo-mechanical stress effects in the device stack [24]. This approach has been used to design temperature-compensated BAW structures by adding compensating layers or selecting substrates that counteract the native −25 ppm/°C TCF of AlN material [25]. Moreover, FEA can incorporate dissipative mechanisms (e.g., material damping, anchor loss) to predict quality factors and identify loss mechanisms. Larson et al. provide a thorough review of FEA for BAW devices, highlighting techniques like applying perfectly matched layer (PML) boundaries to absorb radiating acoustic energy and thus simulate an infinite substrate [26].

In terms of Q enhancement, the spurious mode suppression structure is illustrated in the “Spurious mode suppression” section of Figure 2. The frame structure of BAW resonators plays a crucial role in reducing mechanical loss and enhancing energy confinement. FEA simulations are instrumental in understanding how the geometry of the frame impact device performance. By adjusting frame dimensions and incorporating advanced boundary designs such as phononic crystal boundaries, FEA models enable the reduction in energy leakage and anchoring losses [27,28,29]. This leads to significant improvements in Q-factor, particularly for devices operating at super-high frequencies where energy confinement is more challenging due to smaller device dimensions. Yang and Tam [30] employed finite-difference time-domain (FDTD) simulations to investigate a novel frame-like airgap structure for suppressing spurious modes in BAW resonators. By introducing a patterned airgap beneath the bottom electrode, they demonstrated effective confinement of acoustic energy and significant reduction in lateral Rayleigh–Lamb waves.

In frequency extraction, an eigenfrequency or harmonic FEA simulation computes the resonant frequencies and impedance spectrum by accounting the geometry including patterned electrodes, finite lateral dimensions, and support structures. This approach can reveal spurious modes (e.g., laterally propagating or shear modes) and their impact on the device response, complementing the Mason model which is inherently 1D [31]. FEA is also used to evaluate how material and structural variations affect resonance. For instance, the influence of electrode shapes, boundary conditions, or the additional layers can be analyzed by adjusting the mesh model rather than deriving a new analytic solution. Crucially, FEA methods enable multi-physics simulations, where the coupling of thermal and mechanical effects can be included to study the frequency behavior of BAW devices [32,33].

In terms of hybrid filter design, the “Hybrid filter design” section in Figure 2 provides a corresponding illustration. FEA is widely used in the design of hybrid filters that combine BAW resonators with complementary technologies. Hybrid filter architectures aim to achieve broader bandwidths, reduced insertion loss, and improved linearity by leveraging the strengths of both acoustic devices and electromagnetic circuit components. FEA allows simulation of individual resonator components and their interactions within the hybrid system. Moreover, advanced modules can model nonlinear behavior at higher power levels, ensuring robust operation across various frequency bands [34]. Hybrid filters offer several compelling advantages, including high selectivity at high frequencies, compact form factor with enhanced power handling, wide bandwidth or dual-band operation, and superior electromagnetic interference (EMI) suppression [34,35]. To fully realize these benefits, FEA tools are increasingly employed in the design process. These tools enable multi-physics optimization by accounting for critical factors like thermo-mechanical stress, frequency drift, and acoustic or electromagnetic energy leakage. Through such comprehensive modeling, the performance of hybrid filters can be significantly enhanced, making them particularly well-suited for demanding applications such as 5G RF modules, ultra-wideband (UWB) communication systems, EMI mitigation, and compact high-performance RF front-end architectures [30].

In summary, finite-element simulation has become a powerful method to complement analytical models. By enabling high-fidelity frequency extraction and facilitating detailed investigation of geometric and multi-physics coupling effects, this method plays a critical role in advancing wireless communication applications.

Figure 2. The application of finite element simulation in BAW device structural optimization, thermal and power simulation, and circuit modeling for BAW filters and hybrid filters: “Spurious mode suppression” [35,36,37,38,39]; “Power handling” [17,21,40]; “Hybrid filter design” [41,42,43,44].

Figure 2. The application of finite element simulation in BAW device structural optimization, thermal and power simulation, and circuit modeling for BAW filters and hybrid filters: “Spurious mode suppression” [35,36,37,38,39]; “Power handling” [17,21,40]; “Hybrid filter design” [41,42,43,44].

2.3. BAW Resonator Structures

Figure 3 depicts the schematic structures of three BAW resonators, where FBAR and solidly mounted resonators (SMRs) are the two foundational designs of BAW devices. Both employ a piezoelectric thin film sandwiched between metal electrodes, vibrating in a thickness-extensional bulk mode. The key distinction lies in how the acoustic energy is confined: FBARs use a free-standing membrane structure that includes an air cavity or void beneath the active region [45,46,47,48,49], whereas SMRs are mounted directly on the substrate but use an acoustic Bragg reflector beneath the resonator to achieve acoustic isolation [50]. In FBARs, as presented in Figure 3a,b, the free boundaries formed by the air or vacuum interface reflect the acoustic waves highly effectively, resulting in a high quality factor. In contrast, it can be observed from Figure 3c that SMRs rely on quarter-wavelength multilayer reflectors made of alternating layers with high and low acoustic impedance to trap sound waves—a concept that was first introduced in 1965 by W.E. Newell [51,52,53,54].

A more recent development in BAW technology is the laterally excited bulk acoustic resonators (XBARs), as illustrated in Figure 4. Unlike FBAR and SMR, which are excited via a vertical electric field between top and bottom electrodes [55], the XBAR uses interdigitated electrodes on the surface of the piezoelectric film to launch acoustic waves via a lateral fringing field. In essence, XBAR is a form of bulk acoustic resonator where the electric field is primarily horizontal, but it excites a bulk wave that travels vertically in the film. The device is typically implemented on a thin piezoelectric plate, which is usually suspended or supported on a substrate frame with periodic electrode fingers—somewhat analogous to a SAW interdigital transducer, but the energy is coupled into a bulk longitudinal mode rather than a surface Rayleigh wave [56,57]. Wang et al. reported that the K²_eff of the XBAR resonator based on AlScN films can reach over 12.1% [58].

The XBAR structure demonstrates strong potential for high frequency operation, as its resonant frequency is governed by both the piezoelectric stack thickness and electrode spacing. The lateral dimensions of the electrodes can be precisely adjusted using photolithography techniques, offering an additional method for frequency tailoring [59,60,61]. By carefully optimization of the XBAR structure, clean frequency response can be achieved without spurious modes across a wide bandwidth (e.g., 1.2 GHz), showing significant advantages over traditional Lamb wave resonators [62]. Moreover, the resonant frequency can be tuned through photolithographic patterning, facilitating the monolithic integration of multiple resonators and filters with different operating bands on a single chip, making it particularly suitable for the multi-band integration solutions required in 5G communication [63]. Due to its characteristics of high frequency, high K²_eff, absence of spurious modes, and ease of integration, the XBAR resonator has become a key candidate technology for 5G and 6G RF front-end filters [64]. With the continuous optimization of processing technology and device structure architectures, the performance of XBAR devices is expected to be further improved, positioning an increasingly competitive solutions for next-generation high-frequency applications.

2.4. Research Status and Development Bottlenecks of AlN BAW in the Super-High Frequency Band

Nowadays, commercial BAW filters (mostly FBAR or SMR designs using AlN) comfortably cover up to around 7 GHz (e.g., for 5G mid-band and Wi-Fi applications). However, pushing BAW resonators to higher frequency above 8 GHz or even millimeter-wave bands presents several challenges. Firstly, the resonance frequency f of a thickness-mode BAW device is inversely proportional to the film thickness t: f ≈ v/(2t) for the fundamental extensional mode, where v is the acoustic velocity. The acoustic velocity of AlN in the c-axis longitudinal mode is around 10,000–11,000 m/s [65]; as a result, the fundamental resonance at 10 GHz would require an active piezoelectric film thinner than 0.5 µm. Unfortunately, the crystalline quality of AlN films deposited by the conventional sputtering method deteriorates significantly with reduced thickness, leading to higher acoustic loss, lower K²_eff and compromised structural stability of BAW resonators. Moreover, high-frequency BAW resonator designs face additional constraints due to geometric scaling effects. The ratio of resonator area to its perimeter (A/P) drops for higher-frequency designs, meaning a larger fraction of the acoustic energy can escape through the side boundaries into the substrate via anchors. This leads to degradation in both coupling efficiency and Q at small device sizes. In essence, scaling a traditional FBAR to frequencies above 8 GHz by simply thinning the film and shrinking the area yields a resonator with much poorer performance (lower K²_eff and Q) than its lower-frequency counterpart. Another challenge at high frequency is the increased influence of the electrodes. For a 2 GHz FBAR, the thickness of metal electrodes (typically Mo or W) is only a small fraction of the half-wavelength of acoustic wave. However, at frequencies above 10 GHz, even a 100 nm thick electrode constitutes a substantial fraction of the resonant thickness, creating a fundamental trade-off between acoustic performance and electrical loss. Research studies have explored composite electrodes composed of light and heavy metals to address this issue. For instance, a bilayer Al/W top electrode design, as shown in Figure 5a, was deposited to improve coupling efficiency while maintaining low resistance [66]. Last but not least, high-frequency resonators also face increased challenges associated with spurious modes [67]. The piezoelectric stacks can support higher-order thickness modes, known as overtones. Second or third harmonic thickness modes are sometimes intentionally used to achieve higher frequency without the need for extreme thinning of the films. However, these modes generally exhibit inferior coupling coefficients and quality factors. To address this, several techniques have been developed, including frame structures [68], apodization [69], and phononic crystal boundaries [70], as shown in Figure 5b, offering distinct advantages for optimization of super high frequency BAW device.

A major breakthrough in pushing BAW resonators operating at high frequencies has been achieved through the concept of composite or multi-layer resonators, specifically the advent of P3F [71] and harmonic mode excitation. Since 2021, several groups have demonstrated that by stacking multiple piezoelectric layers with alternating polarization (for example, a multilayer AlScN stacks where each layer is sequentially polarized in opposite directions), high-order extensional modes can be excited, resulting in higher frequencies with relatively thick piezoelectric stacks.

Figure 6a shows two examples of FBAR resonator fabrication using periodically polarized piezoelectric thin films. Recently, researchers reported AlScN P3F resonators with up to four-layer AlScN films, operating in the 4th-order thickness-extensional mode, achieving superior performance with resonant frequency of 18.4 GHz and Q of 260 [72]. Vetury et al. [73] reported the first commercial-compatible P3F AlScN filters with insert loss of 1.86 dB and bandwidth of 680 MHz at 17.9 GHz. Mingyo Park et al. demonstrated a cascaded FBAR using a fully epitaxial 20% AlScN stack. The device consists of two AlScN layers with opposite polarization and achieves resonance at 19.11 GHz with a K²_eff of 10.14% [74]. This multi-layer approach effectively addresses the “thin-film problem” by using a thicker piezoelectric stack that enhances mechanical robustness and improves energy confinement, while still attaining high resonant frequencies through internal mode shaping. However, the method requires precise control of layer thickness and sharp transition of polarization between adjacent layers. Moreover, driving higher-order modes can introduce additional spurious responses if not properly managed. Nonetheless, P3F technology is a highly promising route for the development of 6G RF filters.

Figure 6b shows the fabrication of super-high-frequency resonators using the XBAR structure. Meanwhile, the XBAR architecture mentioned earlier is another critical piece in the high-frequency puzzle. By utilizing lateral field excitation and lithographically defined electrode patterns, XBARs avoid the necessity of employing extremely thin piezoelectric films. For example, XBAR resonators with operation frequencies around 5 GHz can be achieved using relatively thicker piezoelectric film (e.g., 1 µm AlN), wherein the electrode is appropriately designed to excite a half-wavelength lateral standing wave that couples with the thickness resonance. Meruyert Assylbekova et al. demonstrated excellent SHF performance using advanced piezoelectric materials, including a laterally excited AlN-on-Si resonator operating at 11 GHz with a remarkable figure-of-merit (K²_eff·Q_max ≈ 8) [75]. Xiang Chen et al. fabricated XBARs using AlN films, achieving K²_eff as high as 1.54% and resonance frequencies at 3.75 GHz [62].

These aforementioned reports illustrate that lateral excitation enables high frequency operation while preserving the quality factor, especially when combined with optimized material and designs. However, as operation frequencies extend to significantly higher ranges, XBAR also faces fundamental structural limitations and fabrication challenges. Specifically, the necessity to scale both the resonator footprint and interdigital transducer (IDT) dimensions places increasingly stringent demands on lithography precision.

In summary, both P3F multilayer stacks and the laterally excited bulk acoustic resonator (XBAR) architectures represent promising approaches to overcome the limitations of conventional single-layer BAW devices at super-high frequencies, offering enhanced frequency scalability, mechanical robustness, and integration potential. Nonetheless, further improvements in fabrication precision and spurious mode suppression remain essential.

3. Growth of AlN and AlScN Piezoelectric Films for Bulk Acoustic Wave Resonators

AlN has emerged as a cornerstone material for BAW resonators due to its exceptional piezoelectric properties, high acoustic velocity, thermal stability, and compatibility with semiconductor fabrication processes. These attributes enable AlN-based BAW devices to achieve high operating frequencies, low insertion loss, and robust performance in harsh environments, making them indispensable in wireless communication systems and sensors. The crystalline quality of AlN films fundamentally determines the performance of BAW devices as it directly affects the piezoelectric coefficients, acoustic losses, and electromechanical coupling efficiency. Therefore, the growth method of AlN films plays a key role in their structural perfection, orientation, and defect density.

Current growth strategies for AlN and AlScN piezoelectric films aim to balance scalability, cost, and crystalline quality. Physical vapor deposition (PVD), particularly the sputtering method, is widely adopted due to its high throughput and ability to achieve strong c-axis orientation at relatively low temperatures. However, challenges such as residual stress and columnar grain boundaries often limit the piezoelectric response. Metal-organic chemical vapor deposition (MOCVD) offers superior crystalline quality and stoichiometric control, ideal for epitaxial growth on substrates like sapphire or silicon carbide, yet requires high temperatures and costly precursors. To mitigate these limitations, hybrid approaches such as the MOCVD-PVD two-step method have been explored, where a thin epitaxial seed layer promotes subsequent high-quality PVD growth, enhancing crystallinity while reducing thermal budget [76,77,78].

Advanced techniques, such as molecular beam epitaxy (MBE) and pulsed laser deposition (PLD), enable atomic-level precision with Sc incorporation, yielding ultra-low-defect AlScN films with enhanced piezoelectricity. However, their high cost and low throughput hinder industrial adoption. Innovatively, the PLD-PVD two-step process combines interfacial control of PLD with scalability of PVD, demonstrating improved film adhesion and reduced interfacial defects [78,79,80,81,82,83,84]. Meanwhile, doping strategies during growth are critical for tailoring piezoelectric coefficients without compromising structural integrity.

In summary, the evolution of AlN and AlScN growth techniques, including direct PVD or MOCVD, hybrid multistep methods, and advanced epitaxy, reflects a concerted effort to address the trade-offs between crystalline quality, process complexity, and scalability. Future advancements hinge on refining these methodologies to further enhance BAW resonator performance for next-generation high-frequency and low loss applications.

3.1. PVD Deposition of AlN and AlScN Films

PVD, particularly magnetron sputtering, has become the predominant technique for the deposition of AlN and AlScN films in BAW resonator fabrication, owing to its process simplicity, industrial scalability, and compatibility with low-temperature integration. As illustrated in Figure 7, critical sputtering parameters, including growth temperatures and gas flow rates, directly govern the crystal quality, surface roughness, and stress of AlN and AlScN films. Balasubramanian Sundarapandian et al. [85] systematically explored temperature effects (300–700 °C) on the sputtering of AlN films, demonstrating that high temperatures can improve crystallinity, reduced oxygen content, and enhanced piezoelectricity (d₃₃ = 5.2 pm/V at 700 °C). Rossiny Beaucejour et al. [86] addressed these issues using reactive co-sputtering system for AlScN growth. By adjusting Sc concentration (20.3–36.6%), gas flow ratios, and introducing a graded seed layer, stress-controlled, defect-improved AlScN films were obtained. BAW resonators with 380–485 nm-thick AlScN piezoelectric films exhibited high electromechanical coupling above 20% and quality factors Q above 1500, significantly improved compared with pure AlN devices.

The growth of AlN using the PVD method still faces several limitations, such as residual stress sensitivity, mixed polarity, and moderate crystal quality, which hinders device performance and uniformity [87]. Advanced approaches and engineering techniques have been proposed to mitigate these issues but complicate process control. Overall, the PVD method offers a viable BAW fabrication route, yet balancing material quality, stress management, and scalability remains critical for high-performance devices.

3.2. MOCVD Deposition of AlN and AlScN Films

MOCVD exhibits significant advantages over PVD in growing high-quality AlN and AlScN materials. MOCVD enables precise control over crystal orientation and stoichiometry through optimized gas-phase reactions and thermal dynamics, resulting in AlN films with superior crystallinity.

Figure 8 shows a comparative analysis of characterization parameters for MOCVD-grown AlScN films grown on different substrates, as well as an innovative approach employing periodic AlScN/AlGaN stacking to improve crystalline quality. As illustrated in Figure 8a, Jingxiang Su et al. [88] found that direct growth of AlScN films on Si, SiO₂, and poly-Si substrates results in high density of misoriented grains. However, by introduction of a 20 nm AlN seed layer, the c-axis preferred orientation of the AlScN films was significantly improved, with only a minimal 4.5% reduction in overall piezoelectric coefficient d₃₃. On the other hand, Figure 8b depicts that J.B. Shealy et al. [89] developed single-crystal AlScN films via MOCVD for wide-band BAW resonators and filters, showing that AlGaN/AlScN superlattice structures with optimized stress and crystal alignment can dramatically improve film quality.

In terms of device fabrication, many research groups have also utilized high-quality MOCVD-grown AlN films to realize high-performance BAW resonators, for instance, Ya Shen et al. [90] employed MOCVD to prepare AlN films for XBAW device fabrication. Compared to polycrystalline AlN grown by PVD, single-crystal AlN films deposited by MOCVD exhibited superior crystal alignment, achieving a full-width-at-half-maximum (FWHM) of 0.057° in (0002) X-ray diffraction (XRD) measurements. In contrast, the FWHM of polycrystalline AlN films is typically around 1.6°. The significantly lower FWHM value indicates the high quality and enhanced thermal conductivity of single-crystal AlN films. In terms of device performance, the peak power handling capability of single-crystal BAW filters reached 44.8 W (46.6 dBm), and the insert loss ranged from 1.2 to 1.4 dB, which is notable improvement over the 1.6–2.0 dB range of polycrystalline counterparts.

Recent advances in epitaxial growth of AlN on SiC substrates highlight significant progress in piezoelectric material engineering. Michael D. Hodge et al. [91] demonstrated MOCVD-grown single-crystal AlN on c-axis semi-insulating SiC, achieving a (0002) XRD rocking curve FWHM of 0.027°, making it ideal for high-frequency Wi-Fi/UNII applications. J.B. Shealy et al. [92] further optimized the MOCVD approach to enhance crystal quality, thereby increasing the acoustic velocity and piezoelectric response, enabling thicker active layers at fixed frequencies. They also fabricated Al_0.8Sc_0.2N films on Si(111) substrates via the MOCVD method using graded AlGaN interlayers with progressively increasing Ga concentration [89]. This method overcomes conventional PVD limitations by enabling precise Sc incorporation control and reduced defect density, significantly improving filter performance and power handling capacity.

These MOCVD deposition advancements provide crucial technological foundations for next-generation SAW/BAW devices for 5G and beyond, offering an optimal combination of enhanced power handling capability and exceptional frequency stability.

3.3. MBE Deposition of AlN and AlScN Films

MBE offers superior growth parameter control such as temperature, pressure, and flux compared to MOCVD, enabling highly uniform and reproducible film thickness with atomic-level precision. Figure 9 shows BAW resonators fabricated with AlN films grown by MBE, demonstrating significant performance improvements that highlight the ability of the MBE method to enhance the crystalline quality of piezoelectric films. Wenwen Zhao et al. [93] demonstrated MBE-grown AlN films on SiC substrates with a (0002) XRD rocking curve FWHM of 0.1° and sub-atomic roughness of 0.26 nm. This advancement facilitated the first thickness-extensional mode AlN FBARs operating at 15–17 GHz. Mingyo Park et al. [94] pioneered MBE-grown 400 nm thick single-crystalline Al_0.88Sc_0.12N films on 50 nm Mo substrates, demonstrating the first multi-GHz FBARs on a fully epitaxial AlScN/Mo heterostructure. These MBE-based breakthroughs provide critical solutions for 6G and WiFi-7 frontends by combining atomic-scale interface control with high-frequency operational capabilities.

3.4. Two-Step Method for AlN Piezoelectric Films Growth

The two-step growth method typically involves MOCVD or PLD for the seed layer formation, followed by PVD deposition to complete the final layer. Figure 10 shows three typical cases of two-step growth of AlN or AlScN thin films. Qin et al. [84] demonstrated this technique by first growing a 200 nm single-crystal AlN template on Si(111) via MOCVD, followed by 300 nm PVD-sputtered AlN, achieving exceptional crystallinity with the FWHM of the (0002) XRD of 0.47°, closely matching the 0.45° grown solely by MOCVD, and significantly lower compared to the conventional PVD method of 2.13°. The optimized films enabled high-performance resonators operating at 3.3 GHz with Q_max of 865 and a K²_eff of 5%. Dou et al. [35] reported BAW resonators fabricated using sputtered 30% scandium-doped AlScN films on MOCVD-grown AlN single crystal template, achieving an exceptionally high K²_eff of 17.8% under 4.75 GHz resonant frequency. Yang’s team developed a MOCVD-sputtering hybrid technique, growing 100 nm AlN seed layer on Si(111) by MOCVD with the FWHM of the (0002) XRD of 0.5°, followed by 400 nm AlScN film by PVD with the FWHM of 0.8°, demonstrating effective strain accommodation for 5G front-end modules [83]. Ouyang et al. [84] developed a MOCVD-PLD method to obtain low-stress, high-quality piezoelectric AlN films. Compared to AlN films grown directly by PVD, the two-step method exhibits significantly improved material quality with the FWHM of the AlN (0002) XRD of 0.21°, boosting the Q-factor of the fabricated BAW resonators to 4097.

Table 1. Al(Sc)N material parameters grown by different methods and corresponding resonator performance.

Method	Material	FWHM/°	RMS/nm	fs/GHz	K2eff/%	Qmax	Ref.
PVD	Al0.68Sc0.32N	2.24	0.74	4.8	15.8	1180	[86]
MOCVD	Al0.75Sc0.25N	0.38	-	7.04	11.45	1087	[89]
MOCVD	AlN	0.027	-	5.2	6.32	1523	[91]
MBE	AlN	0.1	0.26	14.73	2.3	443	[93]
MBE	Al0.88Sc0.12N	0.8	-	4.6	1	1162	[94]
MOCVD + PLD	AlN	0.21	0.187	2.246	6.25	4097	[82]
PLD + PVD	Al0.8Sc0.2N	1	0.628	3.41	13.7	1362	[79]
MOCVD + PVD	Al0.8Sc0.2N	0.25	0.316	11.51	9.07	1244	[80]
MOCVD + PVD	Al0.87Sc0.13N	1.55	0.6	5.38	10.09	1451	[81]
MOCVD + PVD	AlN	0.46	-	14.57	5.13	551	[95]

summarizes the material properties of Al(Sc)N films deposited by different methods, including PVD, MOCVD, MBE, and two-step approaches, along with the corresponding performance of fabricated BAW resonators. As shown, epitaxial and hybrid deposition methods significantly enhance the crystalline quality of Al(Sc)N piezoelectric films, as evidenced by narrow full-width-at-half-maximum (FWHM) values and low root mean square roughness (RMS). These enhancements directly enable outstanding resonator performance with higher frequency and lower loss.

4. Polarity Control and Inversion Mechanisms in AlN and AlScN Films

Group III-nitride semiconductors, particularly AlN and its alloys such as AlScN and AlYbN, exhibit broad application prospects in RF devices, optoelectronic devices, sensors, and nonlinear optics due to their unique physical and chemical properties. The polarity of the material (typically referring to metal-polarity or N-polarity along the c-axis) has a crucial influence on the growth mode, defect introduction, doping behavior, and ultimately device performance. By precisely controlling and reversing the polarity of the material, novel heteropolar structures can be constructed to improve device performance. The following discussion will focus on the mechanism of polarity inversion in AlN and AlScN materials, the growth methods and case studies to achieve polarity inversion, the discussion of challenges and controversies in current research, and the application prospects of polarity-inverted AlN and AlScN stacked materials.

4.1. Mechanisms of Polarity Inversion in AlN and AlScN Materials

AlN exhibits intrinsic polarity due to its wurtzite crystal structure, which lacks inversion symmetry along the (0001) c-axis. The alternating -Al-N-Al-N- stacking sequence produces two chemically distinct terminations: Al-polar and N-polar surfaces. The broken symmetry generates a spontaneous polarization vector pointing from the N-polar surface toward the Al-polar surface, making polarity an intrinsic property of AlN [96].

First-principles studies have revealed two representative microscopic mechanisms for polarity inversion in Al(Sc)N: homogeneous switching and domain-wall-mediated switching. The homogeneous switching pathway involves a transition through a non-polar hexagonal-like configuration before reaching the inverted wurtzite state. For pure AlN, this requires overcoming a relatively high intrinsic barrier of ~0.5 eV per formula unit [97]. In contrast, the domain-wall-mediated mechanism proceeds via the nucleation of reversed domains at energetically favorable sites, such as Sc-rich regions or local stress concentrations, followed by the migration of inversion domain boundaries (IDBs). First-principles calculations indicate that certain IDB configurations can exhibit relatively low formation energies [98], while electric-field-driven simulations show that the activation barrier decreases with increasing Sc content, accompanied by transition states involving altered atomic coordination [99].

Building on the two polarity inversion mechanisms identified by the aforementioned first-principles calculations, the polarity of AlN and its alloys is primarily determined by the initial nucleation mode on the substrate, interfacial atomic arrangement, and subsequent growth conditions. Multiple factors can induce or influence polarity selection and inversion:

4.1.1. Interface Nucleation and Substrate Effect

As shown in Figure 11a, several studies have shown that Al_xO_yN_z acts as an inversion domain boundaries in the transition from N-polar to Al-polar [100,101,102]. Tamano et al. [103] observed that the IDB from N-polar to Al-polar in a multilayer polarity-inverted AlN structure consisted of approximately 8–10 monolayers of Al_xO_yN_z, while the IDB interface from Al-polar AlN to N-polar consists of a 3-monolayer O-Al-O structure. They also found that the position of the inversion IDB from N-polar to Al-polar shifts approximately 20–30 nm from the sputtering interface towards the sample surface, while the position of the IDB from Al-polar to N-polar coincides with the sputtering interface. Liu et al. previously reported an IDB structure on the AlN(10-10) plane with eight-fold and four-fold coordinated bonds [104]. This provides mechanistic insights into how the behavior of IDBs affects the polarity evolution during AlN growth.

4.1.2. Substrate Nitridation and Buffer Layers

For AlN growth on sapphire substrates, both nitridation treatment and the introduction of a low-temperature (LT) AlN buffer layer are key factors for polarity control. Typically, a properly annealed LT-AlN buffer layer favors Al-polar AlN growth, while direct growth on nitrided sapphire (without a buffer) tends to yield N-polar AlN [105,106].

Figure 11b shows a schematic diagram of the atomic structure arrangement of the Al_xO_yN_z layer used to regulate the polarity of AlN growth. Mohn et al. suggested that the nitridation process converts the sapphire surface into an Al_xO_yN_z layer, which acts to transform the initial N-polar surface into Al-polarity [100]. Stolyarchuk et al. observed that during high-temperature nitridation process (>1000 °C), in addition to the formation of a two-dimensional Al_xO_yN_z layer, three-dimensional Al-polar AlN islands also emerge. These islands subsequently induce the formation of Al-polar columnar inversion domains within the N-polar AlN film [101]. These findings elucidate the causal link between IDB evolution and polarity establishment in AlN epitaxy.

4.1.3. Al Interlayer or Seed Layers

As shown in Figure 11c, recent studies have demonstrated effective strategies for controlling AlN film polarity through Al interlayer or seed layers. Lee et al. reported that when growing AlN on Si(111) substrates, the deposition of a thin Al interlayer atop an initial AlN nucleation layer, which is formed on a nitrided and Al-soaked Si surface, can facilitate the transition of AlN film polarity from N-polar to Al-polar [107]. Milyutin et al. demonstrated that MOCVD-grown AlN seed layer can serve as templates to dictate Al-polarity in subsequently sputtered AlN films [108]. These reports explored the effective method to manipulate AlN polarity while improving the crystal quality and piezoelectric properties of the films.

4.1.4. Interfacial Al-to-N Stoichiometry and Surface Reconstruction

As illustrated in Figure 11d, Brubaker et al. found that the polarity of AlN growth on Si(111) by the MBE method is closely related to the Al/N flux ratio, where N-rich conditions (Al/N < 1) favor Al-polar growth, while Al-rich conditions (Al/N > 1) promote N-polar growth [109]. This finding was corroborated by Fan et al. [110], who observed that during plasma-assisted MBE growth of AlN on Si(111), sustained Al over-supply leads to an Al-Si eutectic layer stabilized N-polar growth. Yoshikawa and Xu further extended these observations to RF-MBE growth, revealing distinct polarity control mechanisms between GaN and AlN: while GaN maintains polarity across a wide Ga/N stoichiometry range, AlN readily transitions from N-polar to Al-polar under Al-rich conditions [111].

4.1.5. Spontaneous Polarity Inversion

Liu et al. reported a spontaneous polarity inversion phenomenon during MOCVD growth of AlN on sapphire substrates. While initial nucleation predominantly exhibits N-polarity, a gradual transition to Al-polarity occurs during subsequent epitaxial growth. This polarity reversal is believed to be attributed to the anisotropy of surface energy and substrate atomic steps [104]. The observed spontaneous polarity switching provides important insights into the fundamental growth mechanisms of III-nitride materials and offers potential pathways for controlling crystalline polarity without requiring additional interlayers or complex processing steps.

Figure 11. Polarity inversion mechanism in AlN or AlScN materials: (a) The formation of IDBs evidenced through scanning transmission electron microscopy (STEM) measurement [102]; (b) The distribution of Al₉O₃N₇ IDBs revealed by the ball-and-stick model [100]; (c) Inserting Al as a metallic interlayer to control the growth of AlN with different polarities, along with the characterization results before and after KOH solution etching [107]; (d) Al-polar AlN grown under N-rich conditions (left) and N-polar AlN grown under Al-rich conditions (right) [109].

Figure 11. Polarity inversion mechanism in AlN or AlScN materials: (a) The formation of IDBs evidenced through scanning transmission electron microscopy (STEM) measurement [102]; (b) The distribution of Al₉O₃N₇ IDBs revealed by the ball-and-stick model [100]; (c) Inserting Al as a metallic interlayer to control the growth of AlN with different polarities, along with the characterization results before and after KOH solution etching [107]; (d) Al-polar AlN grown under N-rich conditions (left) and N-polar AlN grown under Al-rich conditions (right) [109].

4.2. Polarity Control Approaches in AlN and AlScN Materials

Recent advances in understanding AlN polarity inversion mechanisms, combined with progress in first-principles computational modeling and precision growth technologies, have enabled the development of multiple reliable techniques for manipulating AlN growth polarity, including dopant incorporation, ferroelectric field application, interface engineering, and precise adjustment of growth parameters, as demonstrated in Figure 12.

Dopant incorporation has emerged as an alternative approach for polarity engineering in AlN. Sekimoto et al. demonstrated that Si doping can induce polarity inversion in AlN from Al-polar to N-polar. The experiments revealed that polarity inversion occurs when the Si concentration x is in the range of 0.024–0.13 in Al_xSi_1−xN [115,116]. This is attributed to the formation of defect complexes to maintain charge neutrality after Si substitutes for Al. Jonathan Wright et al. [117] attempted to achieve periodically poled AlN using Mg doping combined with electron beam lithography and regrowth in MBE. It can be observed from Figure 12a that Anggraini’s group further advanced the field by exploring Mg/Si co-doping. By carefully controlling the dopant ratios, Al-polar domains dominated at Mg/Si > 1, while N-polar orientation prevailed at Mg/Si < 1 [112].

For doped AlN with ferroelectric properties, post-growth polarization control through external electric fields enables localized polarity inversion. The method has been experimentally verified in Al_xSc_1−xN material [118], enabling non-volatile switching of pre-fabricated device structures without requiring additional growth steps. As shown in Figure 12b, Calderon et al. [113] demonstrated B-induced polarity switching in AlN via a transient nonpolar state revealed by in situ STEM and density functional theory (DFT) analysis. Furthermore, Eliseev et al. proposed the “proximity ferroelectricity” theory, which suggesting that a non-ferroelectric material in intimate contact with a thin ferroelectric layer can experience collective polarization switching. This is attributed to the built-in electric field at the interface, which renormalizes the ferroelectric potential barrier. This method provides the possibility to switch the polarity of already-grown non-ferroelectric material, complementing growth-based polarity control techniques [119].

Beyond dopant-based polarity control, lithographic approaches have also been explored to engineer well-defined laterally inverted polarity regions. By applying this method, Kirste et al. [114] demonstrated the fabrication of periodic lateral polarity structures using a patterned low-temperature AlN buffer on sapphire, as shown in Figure 12c.

Interlayer engineering has emerged as an effective strategy for controlling the polarity of AlN, as illustrated in Figure 12d. Lee et al. [107] reported that inserting an ultrathin Al interlayer on an Al-soaked and nitrided Si(111) surface during MBE growth effectively converted the AlN film from N-polar to Al-polar. Similarly, Milyutin et al. [108] demonstrated that introducing a thin SiO₂ layer on an AlN seed layer enables selective N-polar growth on SiO₂ while maintaining Al-polarity on the exposed AlN template during subsequent sputtering.

Furthermore, Lu et al. [95] developed a two-step growth method combining MOCVD and PVD to fabricate high-quality polarity-inverted AlN bilayers, achieving a narrow XRD FWHM of 0.46°, and subsequently demonstrated a Ku-band BAW resonator operating at 14.57 GHz.

Recent studies have explored oxygen incorporation as an effective approach for AlN polarity engineering. Kinoshita et al. reported that incorporating a relatively high concentration of oxygen (>10²¹ cm⁻³) during the MOCVD nucleation promotes Al-polar AlN growth [120]. Stolyarchuk et al. achieved the conversion of N-polar domains to Al-polarity by subjecting mixed-polarity AlN films grown by MOCVD to oxygen plasma annealing in an MBE chamber, followed by AlN regrowth in MOCVD. This inversion is believed to be induced by the Al_xO_yN_z layer formed under the oxygen plasma treatment on the surface of the initial N-polar domains [102]. Studies by Uesugi and Miyake [121] as well as Shojiki et al. [122] have further elucidated the critical role of oxygen in polarity control for face-to-face annealed sputtered AlN, where oxygen incorporation and Al_xO_yN_z layer formation were closely linked to polarity inversion.

Polarity control in AlN can also be achieved through manipulation of the growth process. Fan et al. demonstrated a novel metal-flux-modulated-epitaxy (MME) strategy. This approach employs periodic switching of the Al source during AlN growth on Si(111), enabling complete polarity inversion from initial N-polarity to final Al-polarity. The underlying mechanism involves the formation and merger of antiphase domain boundaries (APBs) on {2201} planes, ultimately leading to a fully Al-polar surface [110].

In summary, polarity control in AlN and AlScN films can be achieved through substrate engineering, interfacial design, dopant incorporation, external field application and advanced growth strategies such as two-step deposition. These approaches have deepened the understanding of polarity inversion mechanisms and provided practical pathways for fabricating high-quality multilayers, thereby enabling P3F BAW resonators operating in the super-high frequency (SHF) band. Nevertheless, critical challenges remain, including achieving uniform multilayer stacks, suppressing interfacial defects and ensuring reproducibility at large wafer scale. Overcoming these obstacles will be essential to fully unlock the potential of polarity-engineered Al(Sc)N films for next-generation high-frequency and wideband BAW devices.

5. Periodically Poled Al(Sc)N BAW Resonators for High-Frequency Applications

AlN and AlScN are pivotal materials for RF front-end modules, particularly for BAW resonators and filters, due to their excellent piezoelectric properties, high acoustic velocity, and CMOS compatibility. The advent of 5G and emerging 6G technologies necessitates BAW devices operating at increasingly higher frequencies (e.g., X-band and beyond) with wider bandwidths and high quality factors. Conventional BAW resonators face significant challenges in scaling to these frequencies, primarily due to the requirement of ultra-thin piezoelectric films which significantly degrades device performance.

Recent progress has been made in understanding polarity inversion mechanisms and growth techniques for Al(Sc)N piezoelectric films, including substrate engineering, dopant-mediated control and interfacial stoichiometry manipulation, these fundamental advances have enabled innovative BAW resonator designs. Particularly, the emergence of periodically poled Al(Sc)N heterostructures has unlocked new paradigms in BAW resonator design. Under electrical excitation, adjacent layers of P3F stacked with opposite polarities undergo alternating contraction and expansion, inducing constructive interference of strain fields at the interfaces. This periodic inversion enforces the excitation of acoustic waves with shorter effective wavelengths than those of the fundamental mode, thereby enabling the resonator to operate in higher-order thickness-extensional (TE_n) modes. Importantly, because the multilayer structure maintains a relatively large overall piezoelectric thickness, it enables frequency scaling without excessive film thinning, thus mitigating energy leakage and spurious mode generation. As a result, P3F architectures simultaneously achieve higher operation frequencies and preserve strong electromechanical coupling, offering a practical pathway to extend BAW filter performance into the super-high-frequency regime, which are essential for next-generation RF filters. This section synthesizes recent advancements in the fabrication processes of P3F AlN and AlScN stacked films, analyzes the performance and innovations of BAW devices derived from these multilayer structures, and discusses the future outlook and existing bottlenecks in this rapidly evolving field.

5.1. Multilayer P3F Stack Structure

Building upon the fundamental understanding of polarity control and inversion mechanisms in AlN and AlScN films discussed in Section 4, these advances have paved the way for innovative device-level implementations. The capability to deliberately engineer and periodically invert the polarity of piezoelectric layers has enabled the realization of P3F structures. Depending on the employed approach, P3F multilayers can be achieved by three representative methods: (i) external voltage poling of selected layers after growth, (ii) direct growth control through interface engineering and epitaxial techniques, and (iii) dopant-assisted polarity control during Al(Sc)N film growth to induce inversion. These multilayer stacks leverage polarity inversion not merely as a materials science curiosity but as a powerful design tool to enhance the electromechanical coupling and extend the operational frequency of BAW resonators.

Figure 13 illustrates the high-frequency operation mechanism and structural characteristics of the P3F. The P3F structure has been extensively investigated for BAW resonator applications, with pioneering work conducted by the research teams from the University of Pennsylvania and Akoustis Inc. Their seminal studies first demonstrated the successful implementation of P3F AlScN films in K-band BAW resonators, establishing the foundation for high-frequency acoustic devices using the innovative approach [2,3]. Figure 13a demonstrates how this breakthrough addresses fundamental limitations of conventional single-layer BAW resonators, where electrode-induced field cancellation causes the electromechanical coupling strength of higher-order modes to decay quadratically with mode order n [72], making it practically possible to operate at high-frequency operation. In Figure 13b, the P3F architecture overcomes this constraint through the polarization reversal at the heterointerfaces, which compensates the phase cancellation that suppresses these modes in homogeneous films, thereby enabling the excitation of high-frequency, high-order resonant modes with negligible attenuation losses [123]. Figure 13c shows the simulated frequencies and performance of different resonant modes obtained by Natalya F. Naumenko using a single-layer piezoelectric material and a three-layer periodically polarity-inverted piezoelectric structure, respectively. The results theoretically demonstrate that the P3F structure can effectively optimize the performance of high-frequency resonators [124].

P3F stacks can support super high frequency BAW resonators primarily due to their ability to effectively excite higher-order overtone modes. These multilayer structures can maintain a relatively large overall thickness while operating at higher frequencies, which mitigates the typical challenges associated with single-layer BAW resonators that require extremely thin piezoelectric films. Furthermore, the P3F structure can maintain or even enhance the effective coupling coefficient while substantially reducing acoustic propagation losses and increasing Q-factors, thereby facilitating resonator design and improving the device performance. With thicker piezoelectric films, the power density at a specific frequency can be effectively reduced, as a result, enhancing power handling capability and reducing the risk of non-linear effects or degradation of performance. The ability to engineer the polarity of each individual layer offers a powerful means of achieving customized properties, such as anisotropic electromechanical coupling and application-specific resonant frequency design.

5.2. BAW Resonators Fabricated Using P3F Stack

The P3F structure is typically fabricated by sequential deposition of Al(Sc)N layers, where the polarity of each layer is controlled using one or a combination of the methods described in Section 3. Izhar et al. [2,3] developed a P3F fabrication process combining deposition of AlScN layers with specific as-grown polarities via a proprietary PVD method, along with electrical poling of selected layers to create alternating polarity sequences. This hybrid approach enables precise control of the polarization configuration while maintaining film quality. Figure 14 summarizes representative cases of fabricating high-frequency P3F resonators using two approaches: external voltage poling and direct growth control.

From the perspective of device fabrication processes, current P3F devices can generally be classified into four main approaches. The first method incorporates periodic electrode layers during the growth of the piezoelectric film. By applying voltages to these electrodes, originally unipolar Al(Sc)N stacks can be periodically inverted to form a P3F substrate, on which the subsequent resonator structures are fabricated (Figure 14a–e). In contrast, the second method applies external voltage after device fabrication, directly inducing polarity inversion in the completed structure (Figure 14g). A third route employs elemental doping during AlN growth to modify the intrinsic polarity of the film, enabling bottom-up formation of periodically inverted stacks (Figure 14f). Finally, the most widely adopted approach leverages different film growth techniques to exploit spontaneous polarization, allowing direct epitaxial growth of P3F Al(Sc)N films without additional post-processing treatment (Figure 14h–j). However, this method usually requires deposition of piezoelectric films directly on Si substrate, layer transfer techniques including wafer bonding and substrate removal processes are needed for the integration of bottom electrode and acoustic reflectors.

It can be seen from Figure 14a that Mo et al. successfully fabricated a two-layer AlScN P3F BAW resonator, which can be switched between the first thickness extension resonant (TE₁) mode at 7.04 GHz and the TE₂ mode at 13.4 GHz [1]. The TE₁ mode exhibits a series quality factor (Q_s) of 115 and effective electromechanical coupling coefficient (K²_eff) of 10.1%, while the TE₂ mode shows a Q_s of 151 with a K²_eff of 10.7%. In Figure 14b, Izhar et al. [2] reported a BAW resonator with three-layer AlScN P3F stacks, showing the operation frequency of ~20 GHz, approximately four times higher compared to the fundamental resonant mode of unpoled device. In their subsequent work, they developed a four-layer AlScN P3F BAW resonator, which shows a dominant TE₄ resonant mode of around 18.8 GHz with a Q_s of 348, a maximum quality factor (Q_max) of 531, and a K²_eff of 4.37%, as shown in Figure 14c [3]. According to Figure 14d, Izhar et al. demonstrated a four-layer AlScN P3F BAW resonator that predominantly operates in the TE₄ mode at around 17.9 GHz, achieving a Q_s of 58.4 and a K²_eff of 11.8% [73]. As shown in Figure 14e, Nam et al. [126] demonstrated a trilayer AlN/AlScN/AlN FBAR with polarization-switchable operation, enabling excitation of a higher-order resonance at 31 GHz while suppressing the fundamental mode. The device exhibited a K²_eff of 5.5% and a Q_max of 61 in the third resonant mode. Peng et al. [127] demonstrated a trilayer AlN/AlScN/AlN FBAR operating in the fifth mode at 56 GHz, achieving a K²_eff of 4.2% and a Q_max of 60, as illustrated in Figure 14f. In Figure 14g, Lu et al. [128] reported a switchable AlN/Al_0.7Sc_0.3N BAW resonator that operates in TE₁ mode at 6.80 GHz or TE₂ mode at 16.73 GHz through ferroelectric polarization control, achieving K²_eff values of 9.37% and 6.29% and Q_max of 255 and 106, respectively. As shown in Figure 14h, Lu et al. [95] enhanced the K²_eff of P3F resonators by fabricating AlN/AlN multilayer structures via MOCVD and PVD. Simulation-guided optimization of the layer thickness enabled complete suppression of low-order modes, yielding a K²_eff of 5.13% in SHF BAW resonators. Building on this approach, Zhu et al. [129] further improved performance by introducing polarity-inverted AlN/Al_0.7Sc_0.3N multilayers, as shown in Figure 14i. The optimized design effectively suppressed spurious modes and enhanced high-order coupling, achieving a resonant frequency of 14.18 GHz, a K²_eff of 9.7%, and a Q_max of 439.

Table 2. Performance metrics of P3F Al(Sc)N BAW resonators, traditional BAW and XBARs.

Material	Number of Layers	fs/GHz	K2eff (%)	Qs	Qp	Qmax	FOM (K2eff Q)	Ref.
AlScN	Single layer	19.11	10.14	-	-	145	14.71	[74]
AlScN (XBAR)	Single layer	3.75	0.53	-	1137	-	6	[62]
AlScN	2-layer	7.04	10.1	115	-	-	11.6	[1]
		13.4	10.7	151	-	-	16.2
AlScN	3-layer	20.7	8.23	92	160	217	18	[2]
AlScN	4-layer	18.8	4.37	348	264	531	23.2	[3]
AlScN	2-layer	18.4	7.55	180	260	-	19.6	[72]
AlScN	4-layer	17.4	11.8	58.4	236.6	-	27.9	[73]
AlN + AlScN + AlN	3-layer	9	5.1	183	169	395	20	[126]
		31	5.5	61	61	61	3.4
AlN + AlScN + AlN	3-layer	55.7	4.2	55	60	60	2.5	[127]
AlN + AlScN	2-layer	14.18	9.7	-	-	439	42.6	[129]
AlN + AlScN	2-layer	16.73	6.29	101	106	-	6.7	[128]
AlN	2-layer	14.57	5.13	440	394	551	28.3	[95]
AlN	2-layer	4.5	5~6	-	-	-	-	[130]

presents a comparative summary of the performance parameters for traditional single layer and P3F BAW resonators reported in recent years, including piezoelectric layer configurations, series resonant frequency (f_s), effective electromechanical coupling coefficient (K²_eff), and quality factors. Compared with conventional single-layer Al(Sc)N resonators, P3F BAW resonators are capable of achieving significantly higher operating frequencies while maintaining relatively thick piezoelectric films, thereby overcoming the limitations of frequency scaling in single-layer devices that rely solely on film thickness reduction, making periodically poled piezoelectric film (P3F) architectures particularly suitable for further extension into the super-high-frequency (SHF) and millimeter-wave bands. Moreover, P3F multilayers can effectively suppress spurious modes and mitigate energy leakage. The fabricated devices demonstrated strong electromechanical coupling coefficients even above 10 GHz, showing great potential for high frequency and wideband applications in next generation 6G and satellite communications.

In addition to the opportunities offered by periodically poled piezoelectric film (P3F) architectures, substantial challenges remain for their practical implementation. From the film growth perspective, achieving high-quality periodically poled Al(Sc)N multilayers requires precise atomic-scale control of polarity inversion, uniform Sc distribution and minimization of interfacial defects, all of which are critical for maintaining high electromechanical coupling and quality factor. Stress accumulation and wafer bowing during the growth of multilayer stacks also pose major hurdles to large-scale manufacturability. From the device fabrication standpoint, integrating multilayer films into high-frequency AlN-based BAW resonators demands careful manufacturing processes to preserve periodicity, robust electrode patterning to ensure uniform poling fields, and advanced cavity and release techniques to suppress spurious modes. These challenges highlight the critical need for co-optimization of multilayer deposition, device architecture, and process integration to fully translate the potential of P3F resonators into manufacturable high-frequency BAW devices.

6. Discussion and Conclusions

This review has systematically examined the critical advancements and persistent challenges in AlN and AlScN-based BAW resonators for next-generation high-frequency and wideband wireless communication systems. Advanced modeling frameworks, particularly the Mason model for broadband physics-based analysis and the mBVD model for resonant frequency characterization, complemented by finite element analysis, provide indispensable tools for resonators design, spurious modes prediction, and thermal and geometric effect evaluations. The limitations of conventional FBAR and SMR designs at frequencies beyond 6 GHz are being addressed by innovative architectures of XBAR and P3F structures. Laterally XBARs, utilizing interdigitated electrodes to generate bulk waves via lateral fields, offer a complementary pathway to high frequencies with high figures of merit, wide bandwidths, and inherent feasibility for frequency tuning and multi-band integration on a single chip. P3F structure, leveraging multilayer stacks of AlN and AlScN with alternating polarity, enable excitation of high-order overtone modes for high frequency operation while maintaining a thick and robust piezoelectric stack compared to conventional single-layer devices.

The performance of BAW resonators and filters fundamentally depends on the crystalline quality of AlN and AlScN piezoelectric films. While PVD offers industrial scalability, challenges still exist including residual stress, mixed polarity, and moderate crystal quality. In contrast, MOCVD and MBE offer superior crystallinity, enabling enhanced thermal conductivity and power handling capability, which is essential for practical RF applications. Hybrid approaches, such as the combination of MOCVD and PVD methods, strike a balance between high crystal quality and manufacturability. A key advancement in BAW technology is the polarity engineering in AlN and AlScN. Periodically poled piezoelectric film (P3F) Al(Sc)N multilayers have emerged as a promising strategy to overcome the intrinsic limitations of conventional single-layer BAW resonators in the super-high-frequency regime. By exploiting periodic polarity inversion, P3F stacks enable efficient excitation of higher-order thickness-extensional modes while maintaining relatively thick piezoelectric films, thereby achieving higher operating frequencies without excessive thinning. The mechanisms and techniques of polarity inversion in AlN and AlScN films have been extensively explored to facilitate P3F BAW device fabrication for high frequency and wideband applications. Recent demonstrations of P3F-based BAW devices, fabricated through external voltage poling, doping, or direct growth, have validated their capability to operate in the super-high-frequency (SHF) bands with strong electromechanical coupling, enhanced Q factors, and effective suppression of spurious modes. These advances highlight P3F architectures as a practical pathway toward high-frequency, wideband, and power-robust BAW resonators for 6G and satellite communication applications.

Despite these significant advances, critical challenges remain in the development of next-generation BAW resonators. A primary obstacle is to understand and control the atomic interface and inversion domain boundaries for the fabrication of high-quality polarity-controllable piezoelectric film stacks. The intricate process of integrating multiple precisely engineered piezoelectric layers with intermediate poling electrodes in P3F architectures demands simplification to improve yield and enable commercial adoption. Achieving reliable BAW resonator operation in the SHF and millimeter-wave bands for 6G applications requires a multi-pronged strategy combining materials, design, and integration advances. Higher scandium content AlScN alloys can boost piezoelectric response and wide bandwidth, though challenges remain in abnormal oriented grains (AOGs) suppression, stress control and doping uniformity. Novel 3D electrode topologies for XBAR and Lamb wave devices must simultaneously enhance coupling efficiency and suppress spurious modes. Effective thermal management using high thermal conductivity substrates and efficient heat conduction structure becomes critical for power handling. Ultimately, monolithic integration of high-performance BAW resonators and filters with active components on substrates like Si or SiC will be key for compact, efficient 6G front-end modules supporting multi-band operation with low latency and high efficiency.

Future research should focus on the following areas to advance BAW technology:

I.

Polarity Characterization

Fundamental understanding of polarity mechanisms is essential for reliable device performance. Advanced characterization methods, such as high-resolution TEM, synchrotron X-ray diffraction, and scanning probe techniques, should be leveraged to elucidate polarity formation dynamics, inversion pathways, and inversion domain boundary evolution under electrical, thermal, and mechanical stresses.

II.

Material Innovation

Material innovation remains a cornerstone for performance enhancement. Research into higher Sc incorporation, alternative nitride alloy compositions and heterostructure designs may enable precise polarity control while enhancing piezoelectric response and thermal stability.

III.

Device Design and Fabrication Optimization

AI-assisted multi-objective optimization offers a powerful tool for guiding resonator and filter design, balancing trade-offs among frequency scalability, coupling efficiency and quality factor. Additionally, the development of robust in situ poling methods for P3F stacks, alongside refinements in lithography, etching, and cavity-formation processes, will be essential for advancing P3F resonators and XBAR devices toward manufacturable solutions.

IV.

Monolithic Integration and System-Level Demonstration

The integration of RF filters with CMOS technology is critical for compact, low-power and reconfigurable RF front-end modules that enable seamless operation across multiple bands. Key challenges lie in the co-integrating Al(Sc)N-based BAW filters with active circuits while managing thermal budgets, material compatibility, and electromagnetic interference. Successfully addressing these issues will establish a robust foundation for high-frequency, wideband filters essential for 5G-Advanced and 6G systems, ensuring compatibility with mainstream semiconductor manufacturing processes and supporting seamless multi-band operation.