Fabrication of Multilayer Ceramic Substrates via Low-Temperature Cu–Cu Bonding Using Gold Passivation Layer

Multilayer ceramic substrates technology is essential for achieving high integration and improved performance of advanced radio-frequency (RF) devices. However, traditional interconnect processes typically adopt solder-based eutectic bonding, which suffers from complex processes, circuit failure, and reliability decline (with most joints failing by melting at temperatures above $300~^{\circ }$ C), thus limiting their use in high-density packaging with rising thermal demands. Herein, Cu-Cu direct bonding with surface sputtered Au passivation treatment was proposed to fabricate multilayer ceramic substrates. The process was formed by one-step bonding with atomic-level Cu diffusion at a low temperature of $200~^{\circ }$ C. Under harsh conditions with a thermal power density of 171.6 W/cm³ and junction temperatures exceeding $300~^{\circ }$ C, the directly bonded multilayer ceramic substrates remained thermally stable without structural or functional failure. Cu-Cu joints bonded with an Au passivation layer exhibited outstanding mechanical strength and hermeticity, achieving an average shear strength of 21.01 MPa and a maximum helium leak rate of ${9}.{1}\times {10} ^{\mathbf {-{8}}}$ Pa $\cdot $ m³/s. This approach offers a promising route for fabricating multilayer ceramic packaging structures capable of high temperature resistance and flexible integration.