Overcoming BEOL Thermal Constraints in Heterogeneous Integration of α -Ga₂O₃ 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 (< 400° C) of silicon CMOS back-end-of-line (BEOL) processing. Although α-Ga₂O₃ exhibits an excellent solar-blind UV-C photoresponse, its high epitaxial growth temperature (> 450° C) has hindered its 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. The 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 thermomechanical 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 (PDCRs) comparable to those of devices fabricated under high-temperature growth conditions, even under ultralow-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 the BEOL thermal constraints. This work highlights a packaging-level heterogeneous integration strategy rather than a device-level performance optimization.