HIGH-TEMPERATURE STABLE AU-SN AND CU-SN INTERCONNECTS FOR 3D STACKED APPLICATIONS

The desire to directly integrate MEMS with ASICs in a 3D stack is the main motivation behind the development of a bonding technology suitable for both interconnects and seal rings. SLID (Solid Liquid Inter-Diffusion) bonding processes based upon Au-Sn and Cu-Sn (high melting point metal/low melting point metal) are therefore investigated. SLID bonding allows for repeated high temperature processing cycles as in the case for chip stacking, or for interconnections and seal rings bonded at different process steps. This work describes results obtained for fluxless bonding of SLID Au-Sn and Cu-Sn interconnects and seal rings, where a thin layer of intermetallic compound (IMC) on the Cu or Sn surface protects the metal surfaces from oxidizing at elevated temperatures. To evaluate the bond strength, test dies bonded at various temperatures were subjected to SEM/EDX bond line analysis, and shear testing at both room and elevated temperatures. Au Sn samples bonded at 280 degrees C re-melt at elevated temperatures; whereas samples bonded at 350 degrees C remain intact past the initial bonding temperature. For the Cu Sn samples, the measured shear strength is comparable to conventionally bonded interconnects. In order to remain within the uniformity requirements for SLID bonding, the pattern density of electroplated interconnects and seal rings require an optimized layout which can be calculated based upon the effective area.