A Comprehensive Approach for Total Suppression of In-Band Spurious Modes in UHF Al $_{0.72}$ Sc $_{0.28}$ N Lamb Wave Resonators and Filters

In this paper, spurious mode suppression methods for Al
$_{0.72}$
Sc
$_{0.28}$
N Lamb wave resonators and filters using the lowest order symmetric (S
$_{0})$
Lamb wave were meticulously studied and exercised. By adopting the method formally known as apodization, which inherently incorporates controlled finger lengths and bus-interdigitated transducer (IDTs) separation, in conjunction with a full-width anchor and an aperture-matched release pit geometry, the complete eradication of spurious mode responses in the resonator passband was demonstrated. Owing to the high piezoelectric coefficient brought by the scandium alloying, and the resonator design that makes the most use of this coupling, the resonator exhibited an electromechanical coupling coefficient
$k_{\mathrm{t}}^{2}$
as high as 5.36%, an unloaded
$Q_{\mathrm{u,p}}$
of 2916 at the parallel resonant peak of 524.85 MHz and the associated figure of merit (FOM) of 156.0. Ladder filters built upon this design achieved an insertion loss of
$-$
3.72 dB and fractional bandwidths of 2.78%/1.8% for the reception (Rx)/transmission (Tx) branch, respectively. The ability to build spurious-free, frequency lithographically defined, and CMOS-compatible resonators and filters will be of great assistance in the realization of next-generation telecommunication systems.2024-0034