Resonant Granted US Patent ‘Transversely-excited film bulk acoustic resonator using pre-formed cavities’
https://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.htm&r=1573&f=G&l=50&d=PTXT&p=32&S1=20211109&OS=20211109&RS=20211109