Design and Fabrication of Silver Solid Solution Layer on Silicon and Its Solid-State Bonding Applications

Abstract

Silver (Ag) solid solution with indium (In) is further studied since Ag-In binary system has been demonstrated with an extraordinary anti-tarnish property and superior mechanical properties, such as malleability, strength, and ductility. How to fabricate that kind of bonding materials at relative low temperatures is a challenge. In this paper, the unique Ag-In solid solution joints are produced successfully between Si chips and Cu substrates. The Ag(15 µm)/In(4 µm)/Ag(3 µm) multilayers are electroplated on Si chips. In order to control the reaction rate between Ag and In during the electroplating step, the 15 µm Ag layer is initially annealed at 350 °C to increase its grain size and electroplated the following In(4 µm)/Ag(3 µm) layers at room temperature. A two-step annealing process is followed in a vacuum environment at 180 °C and 350 °C, respectively. These two different annealing temperatures are chosen to decompose AgIn2 and Ag2In. After the annealing process, the Ag-In alloys made on Si chips are then bonded to Cu substrate at 300 °C with 800 psi (5.5 MPa) pressure in 0.1 torr vacuum for 30 min. Cross-sectional scanning electron microscopic (SEM) images show that the Ag-In solid solution joints on Si chips are well bonded to the Cu substrates without visible voids and cracks. The scanning electron microscope with energy-dispersive X-ray (SEM/EDX) analysis studies the composition of bonding joints, which displays the homogeneous bonding layer structure. The melting range for Ag-rich Ag-In solid solution is from 670 °C to 962 °C, depending on In composition. Thus, the bonding joints are expected to exhibit impressive performance at high temperatures. This novel method may bring a chance to utilize the Ag-In binary system for those who need die-attach materials for high operating temperatures under severe environmental conditions.