Compact and High Steep Skirts Hybrid Heterogeneous Integrated N77 Full Band BAW Filter Based on Band-Stop Theory

BAW (Bulk Acoustic Wave) filters, commonly implemented in ladder or lattice topologies, have been widely adopted as RF filters for high frequencies. However, the bandwidth of these filter topologies is limited by the effective electromechanical coupling coefficient of the resonators, which is decided by the piezoelectric material and is difficult to increase. In this study, a new topology is proposed to construct a N77 full band filter with high steep skirts which can coexist with N79 band by hybrid integrating BAW resonators with IPD (Integrated Passive Device) devices. The passband of the filter utilizes the left part of the band-stop filter to form large bandwidth and steep skirt. BAW resonators based on Al
$_{{0}.{8}}$
Sc
$_{{0}.{2}}\text{N}$
are connected to IPD circuit through flip-chip bonding. An N77-band 3.3-4.2 GHz filter has been designed and fabricated, demonstrating an average insertion loss of -2 dB, an out-of-band rejection of -28 dB, an average roll-off of 250 MHz and a compact effective area of only 1 mm
$\times0.85$
mm.