Po-Yu Kung; Wei-Lun Huang; Chin-Li Kao; Yung-Sheng Lin; Yun-Ching Hung; C. R. Kao
Cu-to-Cu direct bonding plays an important role in three-dimensional integrated circuits. However, the bonding process traditionally requires high temperature, high pressure, and a high degree of consistency in height.
In this study, tin is passivated over electroplated copper. Because tin is a soft material with a low melting point, a successful bond can be achieved under low temperature and low pressure of 1 MPa without a planarization process.
Tin thickness, bonding temperature, and bonding pressure are the variables. Thicknesses of 1 μm, 800 nm, and 600 nm were used to calculate the minimum tin thickness required to compensate for height differences. Bonding was conducted at 220 °C and 250 °C, and optimized parameters and required pressure were identified.
The optimized parameters after copper planarization were also investigated, showing that bonding can succeed under severe conditions. Transmission electron microscopy was used to observe adhesion between different metals and intermetallic compounds.
Published in: Materials 2022, 15(21), 7783
