AlN FBARs via Thin-Film Transfer for Heterogeneous Integration

Heterogeneous integration has attracted significant research attention as a pathway toward high-performance, multifunctional systems, especially under rising on-chip power densities. Here, we introduce a sacrificial-layer-free layer-transfer integration process for aluminum nitride (AlN) film bulk acoustic resonators (FBARs), enabling their use in high-temperature heterogeneous systems. Pre-fabricated AlN FBAR chip is bonded onto a coefficient of thermal expansion (CTE)-matched host substrate using Au–Au flip-chip bonding. Unlike prior approaches, no sacrificial layer or pre-etched substrate is required: the bonding process itself defines an embedded air cavity beneath the resonant stack, simplifying fabrication and enhancing integration flexibility. The integrated resonators exhibit a high quality factor (Q) of 934 at 3.25 GHz and stable resonance characteristics following cumulative annealing at 800 ^◦C for 30 mins, with minimal performance degradation. This approach circumvents conventional fabrication constraints and advances monolithic integration of high-figure-of-merit radio frequency (RF) filters and resonators for harsh-environment applications.