Low Temperature Bonding of High Density Large Area Array Interconnects for 3D Integration

The results of bonding and stress testing of Cu/Sn-Cu bonded dice and Cu-Cu thermocompression bonded dice at 10μm and 15μm pitch in large area arrays are shown. The interconnect bonding process pressure and temperature required for the formation of low resistance (<100 mΩ), high yielding (99.99 % individual bond yield), and reliable interconnects is described. In the case of Cu/Sn-Cu, use of a mechanical key was found to improve yield. A run of 22 consecutive bond pairs was made with the mechanical key, resulting in 98 % aggregate channel yield at 10μm pitch in area arrays containing 325,632 individual bonds per die to achieve an interconnect density of 10⁶ / cm². SEM cross sections of Cu/Sn-Cu and Cu-Cu bonded samples and EDS analysis of Cu/Sn intermetallic compounds both before and after stress testing are presented. The results of thermal cycling and humidity-temperature testing on electrical yield and resistance are presented for Cu/Sn-Cu with underfill. Comparison of the electrical and shear test performance of Cu/Sn-Cu and Cu-Cu is made. Low temperature bonding (at 210°C, below the melting point of tin) is demonstrated to produce high electrical yield, high shear strength and similar intermetallic compound formation to devices bonded at 300°C. The low temperature process may prove useful for integrating IC devices that have low thermal budgets.