Low Temperature Solid State Gold Bonding of Si Chips to Alumina Substrates

Pure gold (Au) is used as a bonding medium to bond silicon (Si) chips to alumina substrates. The bonding process is performed at 260 °C with only 150 psi (1.0 MPa) static pressure applied. This is a solid-state bonding without any molten phase involved. The Au layer plated on alumina is ductile enough to deform for its surface to mate with the thin Au layer coated on Si. Au atoms on both sides of the bond line are brought within atomic distance and bonding is achieved. The ductile Au joint also accommodates the significant mismatch in coefficient of thermal expansion (CTE) between Si and alumina. Scanning electron microscope (SEM) evaluations show that nearly perfect joints are achieved and no voids are observed. Five samples are shear tested. They all pass the MIL-STD-883G standard. This bonding technique can be applied to bonding any two objects that can be coated with smooth Au layers. The 260 °C bonding temperature is compatible with typical reflow temperature of Sn3.5Ag solders used in electronic industries.