Design and Fabrication of a WLP-Packaged Ultrawideband BAW Filter Based on Al0.7Sc0.3N Film With Cap-Wafer IPD Matching Inductors

Abstract

This work reports an ultrawideband bulk acoustic wave (BAW) filter based on the codesign of a high-effective coupling coefficient ( ${k}_{\textit {eff}}^{{2}}\text {)}$ Al0.7Sc0.3N resonator and wafer-level-packaging (WLP) compatible cap-wafer integrated passive device (IPD) matching inductors. The fabricated BAW resonators exhibit a ${k}_{\textit {eff}}^{{2}}$ of 21.1% and a maximum Bode Q of 690. The 2- $\mu $ m-thick Au cap-wafer inductors provide an inductance of approximately 0.95 nH and a measured Q value of about 16 at 3 GHz. The cap-wafer inductors are introduced at the filter ports to reshape the port impedance and maintain a low voltage standing wave ratio (VSWR) across the enlarged passband without requiring PCB-level or postpackaging matching components. The resulting filter achieves a center frequency of 3.1 GHz, a 355-MHz 3-dB bandwidth (11.5% fractional bandwidth), an in-band VSWR $\le 1.5$ , out-of-band rejection > 36 dB, and a minimum insertion loss of −1.53 dB. The packaged filter also exhibits a temperature coefficient of frequency (TCF) of −27 ppm/°C, a measured ${\mathrm{P}}_{\text {1$\,$ {dB}}}$ of 35 dBm, and a compact WLP size of $1.1\times 0.9\times 0.3$ mm. Failure analysis further indicates that the power-induced degradation is mainly associated with the BAW resonators rather than the cap-wafer inductors. These results demonstrate that cap-wafer IPD inductors provide an effective device–package–matching codesign route for wideband, miniaturized, and manufacturable BAW filters.