arXiv subjects

Byeongjin Kim

Publications and source records attributed to Byeongjin Kim.

9 recordsLinked to original sources

arXiv2026

In-Plane Q Anisotropy of Higher-Order XBARs

128$^\circ$Y-cut lithium niobate (LN) laterally field-excited higher-order antisymmetric bulk acoustic resonators (XBARs) have attracted interest for high-frequency acoustic devices thanks to their high electromechanical coupling coefficient ($k^2$), high quality factor ($Q$) from low metal coverage ratio, and thickness-defined resonant frequency. So far, the in-plane orientation of these resonators is commonly chosen to maximize $k^2$, thereby maximizing bandwidth. More recently, in-plane-rotated XBARs in 128$^\circ$Y-cut LN have been built to provide greater design flexibility in filter synthesis. However, the in-plane anisotropy of $Q$ has been far less explored. This leaves an important gap in understanding whether the propagation direction that determines $k^2$ also affects $Q$. In this work, we investigate the anisotropic $Q$ of higher-order antisymmetric modes (namely, A$_3$, A$_5$, and A$_7$) in 500-nm-thick 128$^\circ$Y-cut LN on Si. By characterizing resonator performance in various in-plane orientations, we observe that both Bode $Q$ and $Q_{\mathrm{3dB}, f_p}$ show minimum values at 90$^\circ$ to the material x-axis and maximum values around 0$^\circ$, following a trend similar to $k^2$. The A$_3$, A$_5$, and A$_7$ modes around 10.4, 17, and 24 GHz exhibit averaged Bode $Q$/$Q_{\mathrm{3dB}, f_p}$ values of 735/556, 204/149, and 59/37, respectively. At 90$^\circ$, the average Bode $Q$ values are reduced to 66, 9, and 12. Finite element analysis (FEA) results suggest that the orientation-dependent degradation of $Q$ near 90$^\circ$ is associated with stronger transverse displacement near the inactive and anchor regions, resulting in enhanced energy leakage. These results reveal an orientation-dependent loss pathway in 128$^\circ$Y-cut LN XBARs and provide design guidance for jointly optimizing $k^2$ and $Q$.

physics.app-ph↗

arXiv2026

PPA-Plan: Proactive Pitfall Avoidance for Reliable Planning in Long-Context LLM Reasoning

Large language models (LLMs) struggle with reasoning over long contexts where relevant information is sparsely distributed. Although plan-and-execute frameworks mitigate this by decomposing tasks into planning and execution, their effectiveness is often limited by unreliable plan generation due to dependence on surface-level cues. Consequently, plans may be based on incorrect assumptions, and once a plan is formed, identifying what went wrong and revising it reliably becomes difficult, limiting the effectiveness of reactive refinement. To address this limitation, we propose PPA-Plan, a proactive planning strategy for long-context reasoning that focuses on preventing such failures before plan generation. PPA-Plan identifies potential logical pitfalls and false assumptions, formulates them as negative constraints, and conditions plan generation on explicitly avoiding these constraints. Experiments on long-context QA benchmarks show that executing plans generated by PPA-Plan consistently outperforms existing plan-and-execute methods and direct prompting.

cs.CL↗

arXiv2026

Gradient Residual Stress in Transferred Thin-Film Lithium Niobate and Its Compensation Using Periodically Poled Piezoelectric Bilayers

In this work, we experimentally investigate the gradient stress (sigma1) in 128 deg Y-cut transferred thin film lithium niobate (TFLN) films with thicknesses from 100 to 460 nm using cantilever curvature analysis. The results reveal a strong dependence of sigma1 on both crystallographic orientation and film thickness, with stress-free orientations at approximately 55 deg and 125 deg for 220-460 nm films, shifting to approximately 20 deg and 160 deg for 100 nm films. The extracted normalized sigma1 ranges from -0.1 to 3.4 MPa/nm (100 nm), -0.8 to 0.34 MPa/nm (220 nm), and -0.12 to 0.08 MPa/nm (460 nm), indicating a pronounced thickness-dependent through-thickness stress gradient. Finite element simulations show excellent agreement with the measurements, validating the curvature-based extraction method and confirming that sigma1 originates from an orientation-dependent residual stress gradient. To mitigate this effect, a bilayer TFLN structure with opposite crystallographic orientations, forming a periodically poled piezoelectric film (P3F), is investigated, enabling partial cancellation of sigma1. A 90/110 nm P3F bilayer reduces the equivalent normalized sigma1 to -0.4 to -0.04 MPa/nm, resulting in significantly reduced deformation. These results establish gradient stress engineering through orientation, thickness, and bilayer design as an effective strategy for achieving mechanically stable and scalable TFLN microelectromechanical systems (MEMS) devices.

physics.app-ph↗

arXiv2026

Lattice XBAR Filters in Thin-Film Lithium Niobate

This work presents the demonstration of lattice filters based on laterally excited bulk acoustic resonators (XBARs). Two filter implementations, namely direct lattice and layout-balanced lattice topologies, are designed and fabricated in periodically poled piezoelectric film (P3F) thin-film lithium niobate (TFLN). By leveraging the strong electromechanical coupling of XBARs in P3F TFLN together with the inherently wideband nature of the lattice topology, 3-dB fractional bandwidths (FBWs) of 27.42\% and 39.11\% and low insertion losses (ILs) of 0.88 dB and 0.96 dB are achieved at approximately 20 GHz for the direct and layout-balanced lattice filters, respectively, under conjugate matching. Notably, all prototypes feature compact footprints smaller than 1.3 mm\textsuperscript{2}. These results highlight the potential of XBAR-based lattice architectures to enable low-loss, wideband acoustic filters for compact, high-performance RF front ends in next-generation wireless communication and sensing systems, while also identifying key challenges and directions for further optimization.

eess.SP↗

arXiv2026

62.6 GHz ScAlN Solidly Mounted Acoustic Resonators

We demonstrate a record-high 62.6 GHz solidly mounted acoustic resonator (SMR) incorporating a 67.6 nm scandium aluminum nitride (Sc0.3Al0.7N) piezoelectric layer on a 40 nm buried platinum (Pt) bottom electrode, positioned above an acoustic Bragg reflector composed of alternating SiO2 (28.2 nm) and Ta2O5 (24.3 nm) layers in 8.5 pairs. The Bragg reflector and piezoelectric stack above are designed to confine a third-order thickness-extensional (TE) bulk acoustic wave (BAW) mode, while efficiently transducing with thickness-field excitation. The fabricated SMR exhibits an extracted piezoelectric coupling coefficient (k2) of 0.8% and a maximum Bode quality factor (Q) of 51 at 63 GHz, representing the highest operating frequency reported for an SMR to date. These results establish a pathway toward mmWave SMR devices for filters and resonators in next-generation RF front ends.

eess.SP↗

arXiv2026

MATANet: A Multi-context Attention and Taxonomy-Aware Network for Fine-Grained Underwater Recognition of Marine Species

Accurate fine-grained recognition of marine organisms is important for scalable biodiversity monitoring and ecological assessment using underwater imagery. However, existing methods mainly focus on target appearance and make limited use of surrounding environmental cues and biological taxonomy. We propose the Multi-Context Attention and Taxonomy-Aware Network (MATANet) for region-of-interest (ROI)-guided marine organism recognition. MATANet contains two complementary components. The Multi-Context Environmental Attention Module uses the ROI representation as a query to aggregate spatial patch features from ROI-centered contextual views at multiple scales, enabling target-conditioned modeling of the surrounding environment. Level-wise auxiliary classifiers further incorporate higher taxonomic ranks during training, encouraging hierarchically consistent representations without changing the finest-label prediction space or inference procedure. On the official FathomNet 2025 Private test split, MATANet achieves a hierarchical distance of 1.570 with the base backbone and 1.423 with the large backbone, substantially outperforming the strongest benchmark value of 2.603. On FishCLEF2015, MATANet achieves an accuracy of 0.793 and a hierarchical distance of 1.120, outperforming the strongest benchmark values of 0.766 and 1.327, respectively. Ablation studies show that surrounding scene information provides complementary evidence beyond repeated multi-scale observations of the target and that target-conditioned aggregation outperforms direct multi-view concatenation. Additional experiments on FAIR1M v2.0 examine the applicability of the proposed design beyond underwater imagery. In post-detection evaluation, MATANet improves fine-grained classification accuracy on matched detector-generated ROIs from 0.828 to 0.959 supporting its engineering applicability to automated marine monitoring.

cs.CV↗

arXiv2025

Residual Stress Anisotropy In Thin-Film Lithium Niobate For Stress-Managed MEMS

In this work, we present the first experimental study of residual stress and post-release beam deflection in 128-degree Y-cut thin-film lithium niobate (TFLN) on Si, revealing pronounced stress anisotropy with in-plane orientation. Using optical profilometry with curvature fitting, we extract the stress gradient (sigma1) and generate orientation-resolved stress maps across multiple film thicknesses (100 nm, 220 nm, and 460 nm). For films in the 220 to 460 nm range, we identify stress-free in-plane orientations near approximately 55 degrees and 125 degrees, enabling extremely flat suspended beams. In contrast, ultra-thin 100 nm films exhibit shifted stress-free orientations near approximately 20 degrees and 160 degrees. Leveraging these orientations, we demonstrate very long suspended beams up to 2 cm in length, 10 micrometers in width, and 460 nm in thickness without collapse. These results establish in-plane stress anisotropy and thickness selection in TFLN as practical design levers for mechanically stable, scalable, and stress-managed microelectromechanical systems (MEMS).

physics.app-ph↗

arXiv2025

An 11.7-GHz ScAlN FBAR Filter: Case Study on Scaling Limits and Challenges

This paper reports an 11.7 GHz compact 50 ohm ladder filter based on single layer Scandium Aluminum Nitride (ScAlN) film bulk acoustic resonators (FBARs) with platinum (Pt) electrodes, and uses it as a quantitative case study of the limits encountered when directly scaling to higher frequencies. The measured filter achieves a 3 dB fractional bandwidth (FBW) of 4.0% and an out of band rejection greater than 23.1 dB, with a minimum insertion loss (IL) of 6.8 dB. We analyze the origin of this performance through a quantitative framework: (1) a loss decomposition study, (2) frequency shift sensitivity that explains the discrepancy between simulated and measured center frequency, (3) FBW sensitivity to series shunt separation and port impedance, and (4) stress limited aperture that constrains device size. The results establish a realistic, fabricable baseline for directly scaled single layer ScAlN FBAR filters and outline materials, electrode, and stress management directions toward lower loss mmWave acoustic filters.

physics.app-ph↗

arXiv2025

Bimorph Lithium Niobate Piezoelectric Micromachined Ultrasonic Transducer

This work demonstrates a prototype bi-layer piezoelectric micromachined ultrasonic transducer (PMUT) based on transferred periodically poled piezoelectric film (P3F) X-cut lithium niobate (LN). Opposite in-plane polarizations in the piezoelectric film stack are employed to enable efficient lateral field excitation of the flexural mode. Thanks to its high piezoelectric coefficient and low dielectric loss, the X-cut LN exhibits high figure of merits (FoMs) as both sensors and transducers. The fabricated PMUT demonstrates an out-of-plane mode near 1 MHz with an electromechanical coupling of 3.6\%. Laser Doppler vibrometry further validates the finite element analysis, showing a peak center displacement of 340 pm/V. These results establish bi-layer P3F LN PMUTs as a promising platform for compact and high-performance ultrasonic transducers. Future work will focus on theoretical analysis, modeling of the measured data, improving the design of the transducer topology, and mitigating feedthrough effects.

physics.app-ph↗