Phase-Flip Lattice Bulk Acoustic Wave Filter With Unbalanced Terminals Using Hybrid Heterogeneous Integration Technology

Bulk acoustic wave (BAW) filters are crucial components in communications systems. Ladder and lattice types are two basic filter topologies used in unbalanced and balanced systems, respectively. Compared to ladder filters, lattice filters have advantages of larger bandwidth and better trade-off between out-of-band rejection and roll-off. However, the current lattice filters are restricted by their incompatible balanced terminals with other radio frequency devices. In this work, a hybrid integration of integrated passive device (IPD) and lattice-type bulk acoustic wave (BAW) filter with unbalanced terminals was proposed. The IPD network achieved phase flip and transformed the balanced terminal into unbalanced terminal of the lattice-type BAW filter. To realize a more compact integration of the BAW device and phase-flip network, for the first time we achieved Si microcap capacitor structure, which can also be adopted for 3D integration of other acoustic and electromagnetic devices. The fabricated filter achieved a minimum insertion loss of 1.39 dB and a 3-dB bandwidth of 145 MHz, a relative bandwidth fraction of 7.2%. Compared with the ladder type filter with same resonator number, the phase-flip lattice filter achieved ~30% improvement in bandwidth. This work brings new methodology for the topology designs of lattice filters and compact integration of capacitors and inductors with BAW devices.