Bonding of SiC Chips to Copper Substrates using Ag-In System

Silicon carbide (SiC) is one of wide bandgap semiconductors which has attracted much attention in manufacturing high power devices because they have better physical properties over silicon (Si). Compared with Si, SiC has wider bandgap, lower intrinsic carrier concentration, higher saturated electron drift velocity, higher saturated electron drift velocity, higher thermal conductivity, and higher electric breakdown field. These superior properties make it suitable for high power or high voltage applications. Most power devices operate at high temperatures in excess of 200 degrees C, which limits the choices of proper solder because most lead-free solders have melting points below 220 degrees C. In this research, we have developed a bonding process using Ag-In system, in which the resulting solder joints have melting temperature higher than 900 C. To demonstrate this process, 4H SiC chips were bonded to copper (Cu) substrates. The process is entirely fluxless. The joint was evaluated by observing the microstructure by scanning electron microscopy (SEM) and the chemical composition was determined using energy dispersive X-ray spectroscopy (EDX). The joint consists of Ag, solid solution phase (Ag), and Ag2In in stack of Ag/(Ag)/Ag2In/(Ag)/Ag. In reacts with Ag thoroughly and transforms into (Ag) and Ag2In. The bonded sample was also annealed at 350 degrees C for 100 hours. After annealing, it is observed that Ag2In has been converted into (Ag) solid solution phase.