Wirebond enhancement on copper palladium bonding in a Over Pad Metalization

Abstract

This paper presents the wire bonding enhancement for copper palladium (CuPd) wires on Nickel Palladium (NiPd) Over Pad Metallization (OPM) on bond pads of Power Management Device. The enhancement was done to minimize Non-Stick on Pad (NSOP) and further improvement on ball bond adhesion using 33um CuPd Wire. A Design of Experiment (DOE) was conducted for the Wire bond process Improvement to reduced NSOP occurrence. The DOE showed strong dependence on the Forming Gas Flow in the Chamber, E-torch and 3rd Nozzle to improved ball bond adhesion. The ball bond adhesion was measured through ball shear test (BST). The Forming Gas Flow optimization indicated the need to reduce chamber and E-torch flow by about 30% form current value used for Bare Copper wire. However the 3rd nozzle should be increased about 33%. This resulted to higher BST readings by 40% from the original readings. NSOP occurrence was still observed despite the wire bond process improvement. Process mapping was performed with the aid of an ISHIKAWA Diagram. The analysis indicated that poor bond pad property was the major contributor of NSOP. A detailed physical analysis of the bond pad showed thin Pd plating. The Pd thickness was verified through cross sectioning of the pads. The thickness was correlated and confirmed by manual cross sectioning, Focus Ion Beam (FIB) and depth profiling using Top of Flight Secondary Ion Mass Spectroscopy (TOFSIMS). NSOP occurrence was observed for Pd thickness of 200nm and below. Good bondability was obtained with Pd thickness above 300nm. Other bond pad manifestation was noted such as discoloration and rough/granulated bond pad surface. The discoloration and rough surface was due to the pronounced nickel structure underneath the thin Pd plating. Forming Gas Flow shows significant effect on ball bond adhesion but does not completely eliminate the NSOP occurrence. The bond pad morphology plays strong correlation on the NSOP occurrence.