In the last few decades, bulk-acoustic-wave filters have been essential components of 3G-to-4G radios. These devices rely on the high electromechanical coupling coefficient (k_t² ∼ 7%), attained by aluminum nitride (AlN) film-bulk-acoustic-resonators (FBARs), to achieve a wideband and low-loss frequency response. As the resonance frequency of FBARs is set by their thickness, the integration of multiple FBARs, to form filters, can only be attained through the adoption of frequency tuning fabrication steps, such as mass loading or trimming. However, as the ability to reliably control these steps significantly decays for thinner (or higher frequency) FBARs, manufacturing FBAR-based filters, addressing the needs of emerging IoT and 5G applications, is becoming more and more challenging. Consequently, there is a quest for new acoustic resonant components, simultaneously exhibiting high-k_t² and a lithographic frequency tunability. In this work, a novel class of AlN resonators is presented. These radio frequency devices, labeled as two-dimensional-resonant-rods (2DRRs), exploit, for the first time, the unconventional acoustic behavior exhibited by a forest of locally resonant rods, built in the body of a profiled AlN layer that is sandwiched between a bottom un-patterned metal plate and a top metallic grating. 2DRRs exhibit unexplored modal features that make them able to achieve high-k_t², a significant lithographic frequency tunability, and a relaxed lithographic resolution, while relying on an optimal AlN crystalline orientation. The operation of 2DRRs is discussed, in this work, by means of analytical and finite-element-methods. The measured performance of the first fabricated 2DRR, operating around 2.4 GHz and showing a k_t² in excess of 7.4%, is also reported.

Sc-doped aluminum nitride is emerging as a new piezoelectric material which can substitute undoped aluminum nitride (AlN) in radio-frequency MEMS applications, thanks to its demonstrated enhancement of the piezoelectric coefficients. Furthermore, the recent demonstration of the ferroelectric-switching capability of the material gives AlScN the possibility to integrate memory functionalities in RF components. However, its high-coercive field and high-leakage currents are limiting its applicability. Residual stress, growth on different substrates, and testing-temperature have already been demonstrated as possible knobs to flatten the energy barrier needed for switching, but no investigation has been reported yet on the whole impact on the dielectric and ferroelectric dynamic behavior of a single process parameter. In this context, we analyze the complete spectrum of variations induced by the applied substrate-RF, from deposition characteristics to dielectric and ferroelectric properties, proving its effect on all of the material attributes. In particular, we demonstrate the possibility of engineering the AlScN lattice cell to properly modify leakage, breakdown, and coercive fields, as well as polarization charge, without altering the crystallinity level, making substrate-RF an effective and efficient fabrication knob to ease the limitations the material is facing.

This work reports on the measured performance of an Aluminum Scandium Nitride (AlScN) Two-Dimensional Resonant Rods resonator (2DRR), fabricated by using a Sc-doping concentration of 24% , characterized by a low off-resonance impedance ( $\sim $ 25 $\Omega $ ) and exhibiting a record electromechanical coupling coefficient ( ${k}_{t}^{\textit {2}}$ ) of 23.9% for AlScN resonators. In order to achieve such performance, we identified and relied on optimized deposition and etching processes for highly-doped AlScN films, aiming at achieving high crystalline quality, low density of abnormally oriented grains in the 2DRR’s active region and sharp lateral sidewalls. Also, the 2DRR’s unit-cell has been acoustically engineered to maximize the piezo-generated mechanical energy within each rod and to ensure a low transduction of spurious modes around resonance. Due to its unprecedented ${k}_{t}^{\textit {2}}$ , the reported 2DRR opens exciting scenarios towards the development of next generation monolithic integrated radio-frequency (RF) filtering components. In fact, we show that $5^{th}$ -order 2DRR-based ladder filters with fractional bandwidths (BW) of $\sim $ 11% , insertion-loss (I.L) values of $\sim $ 2.5 dB and with $>$ 30 dB out-of-band rejections can now be envisioned, paving an unprecedented path towards the development of ultra-wide band (UWB) filters for next-generation Super-High-Frequency (SHF) radio front-ends. [2022-0024]

The increasing use of mobile broadband requires new acoustic filtering technologies that can operate efficiently at frequencies above 6 GHz. Previous research has shown that AlN Super High Frequency (SHF) Cross-Sectional Lamé Mode resonators (CLMRs) can address this challenge, but their performance is limited by the piezoelectric strength of AlN. In this work, we explore the use of substitutional doping of Al in AlN with Sc to enhance the $k{}_t{}^2$ values of SHF CLMRs. Our results showed that the measured $k{}_t{}^2$·$Q{}_m$ product of Al${}_{72}$Sc${}_{28}$N CLMRs was four times greater than that of AlN CLMRs operating at the same frequency. Additionally, the measured fractional bandwidth (FWB) of Al${}_{72}$Sc${}_{28}$N 2nd order ladder filters was 4.13%, a fourfold improvement over AlN filters with the same design. We also discuss other aspects of the technology, such as power handling, losses, and spurious mode suppression, and identify potential areas for future research.

This work describes the implementation of acoustic metamaterials (AMs) made of a forest of rods at the sides of a suspended Aluminum Scandium Nitride (AlScN) contour-mode-resonator (CMR) to increase its power handling without causing degradations of its electromechanical performance. The increase in usable anchoring perimeter with respect to conventional CMR-designs, enabled by the adoption of two AMs-based lateral anchors, permits to achieve an improved heat conduction from the resonator’s active region to the substrate. Furthermore, thanks to such AMs-based lateral anchors’ unique acoustic dispersion features, the attained increase of anchored perimeter does not cause any degradations of the CMR’s electromechanical performance, even leading to a ~ 15% improvement in the measured quality factor. Finally, we experimentally show that using our AMs-based lateral anchors leads to a more linear CMR’s electrical response, which is enabled by a ~ 32% reduction of its Duffing nonlinear coefficient with respect to the corresponding value attained by a conventional CMR-design that uses fully-etched lateral sides.