Achieving ultrahigh-strength Cu sintering joints prepared by ultrasonic spraying of thin Cu nanoparticle films

The application of nanoparticle sintering technology for third-generation semiconductor packaging is usually hindered by the difficulty in volatilization of organic vehicles, the nanoparticle (NP) agglomeration of its solder paste zone, and poor mechanical properties due to the inherent pores/cracks. In this work, these obstacles can be overcome using ultrasonic-spraying Cu NP film and vacuum thermal-pressure sintering. This work first controlled the ultrasonic spraying process parameters (power, flow, pressure) to achieve uniform and stable atomization effects yielding a homogeneous Cu NP film. The Cu NP film was further employed to realize Cu die-Cu substrate interconnection, and the microstructure and mechanical properties of the sintering joints was emphatically analyzed. Results demonstrate that near-monolayer fine Cu sintering structures are successfully produced. The sintering joints have good interfacial bonding ratio (∼ 94 %) and low porosity (∼ 1.6 %), resulting in shear strengths of up to 120.1 MPa. Even when sintering at 250 °C, the shear strength can reach 96.1 MPa. Because of the "contact activation effect" and the removal of interfacial cracks, these ultrasonic-spraying samples are stronger than printing sintering joints (with cracks) under the same sintering processes. We expect this approach to improve the performance of the sintering joints and broaden the application of Cu NP films, the microstructure of which less dependent on paste composition and sintering procedure (temperature, pressure, and time, etc.).