High-Performance N79 Band AlScN BAW Resonator and Filter With the Consideration of Area Effect

Abstract

With the increase of operating frequency above 4 GHz, the required resonator size is reduced to hundreds of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}^{{2}}$ </tex-math></inline-formula> , which is proportional to 1/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}^{\,{2}}$ </tex-math></inline-formula> . In traditional, the size reduction of the resonator will induce its parallel resonant frequency decrease, thus resulting in the decrease of its effective coupling coefficient ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${k}_{\,{t}}^{\,{2}}$ </tex-math></inline-formula> ). Besides the parallel resonant frequency, we found that the serial resonant frequency of resonators working above 3 GHz also varies with its area size. However, this effect is not taken into account in conventional circuit models, leading to the failure of high-performance filter design. To address this issue, a modified Mason model of bulk acoustic wave (BAW) resonators was proposed in this work. Using the modified model, a high-performance N79 band (4.8–4.96 GHz) filter based on Al0.904Sc <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{0.096}}\text{N}$ </tex-math></inline-formula> piezoelectric film was designed and fabricated, which achieved a minimum insertion loss of −0.93 dB and a bandwidth of 240 MHz. The power capacity and wire-bond package of BAW filters were further studied. The whole process of the Mason model modification, filter design, and fabrication pave the road for high-frequency filter applications.

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