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<p>Replacing the AlN piezoelectric material in thin-film bulk acoustic resonators (FBAR) with scandium-doped AlN can enhance the electromechanical coupling, allowing for a wider bandwidth filter. Based on our theory, we estimate that the frequency bandwidth (the difference between the antiresonant and resonant frequencies) and the electromechanical coupling coefficient increase with higher scandium concentrations. Our model also evaluates optimized designs by considering the ratio of the average thickness of the metallic layers to the thickness of the piezoelectric layer. This optimized ratio is subsequently applied to bulk acoustic wave (BAW) filter components with varying bandwidths. Ultimately, the measured results of our fabricated filters validate the model’s optimal predictions. To further verify the design, three band filters—B40, Wi-Fi, and B41—were fabricated using optimized values of the metal-to-piezoelectric thickness ratio (<i>R</i><sub><i>d</i></sub>) and Sc concentrations. Experimental results show that the adjustment of <i>R</i><sub><i>d</i></sub> based on theoretical predictions effectively reduced band-edge insertion losses and improved filter performance across all bands. For the B40 filter, with a bandwidth (BW) ratio of 4.26%, increasing <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(R_{d}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>R</mi> <mi>d</mi> </msub> </math></EquationSource> </InlineEquation> from 0.09 to 0.1 improved insertion loss at 2400&#xa0;MHz from −2.9 to −1.7&#xa0;dB. The Wi-Fi filter, with a BW ratio of 3.28%, saw its <i>R</i><sub><i>d</i></sub> increase from 0.1 to 0.13, resulting in insertion losses of −1.6 and −1.4&#xa0;dB at 2402&#xa0;MHz and 2482&#xa0;MHz, respectively. For the B41 filter, a higher Sc concentration of 22% and an increased <i>R</i><sub><i>d</i></sub> from 0.075 to 0.1 led to a significantly improved insertion loss of −1.8&#xa0;dB at 2496&#xa0;MHz, compared to −2.9&#xa0;dB previously. These results demonstrate strong alignment between theoretical modeling and practical performance, confirming the feasibility of the proposed optimization method for bandwidth enhancement in sub-3&#xa0;GHz FBAR applications.</p>

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Optimizing Sc-Doped AlN FBARs for Wideband Performance in Sub-3 GHz RF Filters

  • 期刊论文

  • 发表日期：

    2025年11月14日

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  • Re-Ching Lin,
  • Chien-Chuan Cheng,
  • Shih-Jye Sun

摘要

Replacing the AlN piezoelectric material in thin-film bulk acoustic resonators (FBAR) with scandium-doped AlN can enhance the electromechanical coupling, allowing for a wider bandwidth filter. Based on our theory, we estimate that the frequency bandwidth (the difference between the antiresonant and resonant frequencies) and the electromechanical coupling coefficient increase with higher scandium concentrations. Our model also evaluates optimized designs by considering the ratio of the average thickness of the metallic layers to the thickness of the piezoelectric layer. This optimized ratio is subsequently applied to bulk acoustic wave (BAW) filter components with varying bandwidths. Ultimately, the measured results of our fabricated filters validate the model’s optimal predictions. To further verify the design, three band filters—B40, Wi-Fi, and B41—were fabricated using optimized values of the metal-to-piezoelectric thickness ratio (Rd) and Sc concentrations. Experimental results show that the adjustment of Rd based on theoretical predictions effectively reduced band-edge insertion losses and improved filter performance across all bands. For the B40 filter, with a bandwidth (BW) ratio of 4.26%, increasing \(R_{d}\) R d from 0.09 to 0.1 improved insertion loss at 2400 MHz from −2.9 to −1.7 dB. The Wi-Fi filter, with a BW ratio of 3.28%, saw its Rd increase from 0.1 to 0.13, resulting in insertion losses of −1.6 and −1.4 dB at 2402 MHz and 2482 MHz, respectively. For the B41 filter, a higher Sc concentration of 22% and an increased Rd from 0.075 to 0.1 led to a significantly improved insertion loss of −1.8 dB at 2496 MHz, compared to −2.9 dB previously. These results demonstrate strong alignment between theoretical modeling and practical performance, confirming the feasibility of the proposed optimization method for bandwidth enhancement in sub-3 GHz FBAR applications.