Polymer Direct Bonding Characterization in Wafer Level Packaging for 3D Integration

Wafer-level polymer direct bonding technology suitable for 3D integration was conducted. Major parameters such as surface planarization, bonding temperature, and bonding strength are reviewed. According to atomic force microscope (AFM) inspection, as-cured PI films after the chemical mechanical polishing process, a flat bonding surface with Ra < 0.92 nm is obtained by planarization. The bonding temperature dependence (200°C, 250°C, 300°C, and 350°C) of the shear strength for PI/PI wafer-to-wafer bonding are explored. Wafer-level bonding temperature above the glass-transition temperature (around 260°C) of PI, the contact-area ratio of the PI surfaces achieves 100% contact by PI deformation. SAM observations have demonstrated a seamless PI interface, realizing with optimized bonding temperature 350°C. For the highest bonding temperature wafers, interfaces of bonded PI-to-PI areas are imaged by focused ion beam scanning electron microscopy (FIB/SEM), displaying the feasibility of these bonds. As the bonding temperature increase, the contact-area ratio is increased by PI deformation. Our shear tests and scanning acoustic microscopy (SAM) investigations suggest that the strongest PI-PI bonding is ascribed to a higher contact-area ratio. The bonding wafer exhibited a shear strength of 21.520 MPa with the highest bonding temperature, revealing the contacts successfully passed the shear tests standard MIL-STD-883, method 2019.9 (i.e., 5.938 MPa).