Surface Density Gradient Engineering Precedes Enhanced Diffusion; Drives CMOS In-Line Process Flow Compatible Cu–Cu Thermocompression Bonding at 75 °C

Diffusion is one of the most critical and key factor for achieving low temperature and low pressure thermocompression bonding. In this work, we propose a novel concept of modifying density conditions within the Cu films. The density of film was varied by varying the DC bias applied for the sputter deposition. While two samples of different densities are brought in contact with each other, Cu atoms from the thin film having higher density of atoms tends to interdiffuse into the thin film having lower density. Optimal dc biasing conditions for pair of wafers were figured out based on the pre-bonding characterizations and it yielded an excellent bonding at Sub 100 °C. No observable bond interface X-TEM images and bond strength of 164 MPa confirmed the quality of bonding. Furthermore, bonded structure undergone various reliability tests include multiple current stressing test, temperature cycling test and relative humidity test confirms Cu-Cu bonding using surface density gradient engineering is stable and promising candidate for future 3D IC integration applications.