Solid-state bonding of silicon chips to silver layer plated on copper substrate

In this project, a novel solid-state bonding technique was developed to bond silicon (Si) chips to silver (Ag) layer plated on copper (Cu) substrates at temperature (260°C) far below Ag melting temperature of 961°C. This is possible because the gold (Au) layer coated on Si chip is made to contact the Ag atoms in atomic scale, thus allowing the Au atoms and Ag atoms to see each other and share electrons. This is our fundamental belief behind this technology. Ag is ductile and has low yield strength. It can deform to accommodate shear strain caused by large mismatch in coefficient of thermal expansion (CTE) between Si (2.7×10⁻⁶/°C) and Cu (17×10⁻⁶/°C). Ag has the highest electrical and thermal conductivities among all metals. It is the dream bonding medium. The question is how to bond Ag to other materials at temperature compatible to electronic packaging processes used in industries. In experiments, Si chips were coated with thin chromium (Cr) and Au layers and bonded to Ag layer plated on Cu substrates. The bonding was performed at 260°C with 1000psi static pressure in 90 mtorr vacuum environment to suppress oxidation. There is no molten phase involved during the bonding process. The resulting Ag joints are nearly perfect. The joints are strong and no voids are observed on Ag/Cu and Si/Ag bonding interfaces. This new solid-state bonding technology should be valuable in improving the thermal performance and reliability of high power devices.