Overcoming BEOL Thermal Constraints in Heterogeneous Integration of $α$-Ga$_{2}$O$_{3}$ UV-C Photodetectors Using Low-Temperature Thermal Compression Bonding

This work demonstrates a low-temperature heterogeneous integration strategy for α-Ga₂O₃-based functional devices that complies with the stringent thermal budget constraints (450 °C) has hindered direct monolithic integration with CMOS circuitry. To address this incompatibility, α-Ga₂O₃ was synthesized at 470 °C and subsequently integrated at 350 °C using a Cu–Cu thermal compression bonding (TCB) platform. A dual-functional Ti interlayer architecture, consisting of a 10 nm passivation layer and a 50 nm adhesion layer, facilitates low-temperature bonding while maintaining interfacial stability between metal and oxide layers. Cu–Cu TCB performed at 350 °C under an applied pressure of 10 MPa resulted in a continuous and void-free metallurgical junction. Atomic-scale interfacial analyses indicate that the 50 nm Ti adhesion layer plays a key role in limiting Cu diffusion and mitigating thermo-mechanical stress, contributing to the preservation of the α-Ga₂O₃ active layer during bonding. The heterogeneously integrated UV-C photodetector exhibits photo-to-dark current ratios comparable to those of devices fabricated under high-temperature growth conditions, even under ultra-low-intensity UV-C illumination. These results demonstrate that low-temperature Cu–Cu TCB can be extended beyond conventional BEOL-compatible packaging to enable the integration of high-temperature functional oxides within BEOL thermal constraints. This work highlights a packaging-level heterogeneous integration strategy rather than device-level performance optimization.