Low-Temperature Solid-State Silver Bonding of Silicon Chips to Alumina Substrates

Pure silver (Ag) is used as a bonding medium to bond silicon (Si) chips to alumina substrates. The bonding process is performed at 260°C, which is a typical peak reflow temperature of Sn3.5Ag solders used in electronic industries. A static pressure of 1000 psi (6.9 MPa) is applied. This is a solid-state bonding process without any molten phase involved. The Ag foil sandwiched between the Si chip and the alumina substrate is ductile enough to deform and mate with the gold (Au) layers coated, respectively, on the Si chip and on the alumina substrate. Ag and Au atoms on both sides of the bonding interfaces are brought within atomic distance, and bonding is thus achieved. The ductile Ag joints can accommodate significant mismatch in coefficient of thermal expansion between Si and alumina. Scanning electron microscopy evaluations show that nearly perfect joints are achieved and no voids are observed. A standard shear test is performed to assess the bonding strength. The shear strength measured far exceeds the requirement specified in MIL-STD-883G standard.