Influence of Surface Morphology and Micro-Defects on the Electrical Performance and Implied Mechanical Reliability of Fine-Pitch Copper Bonding Wires

Abstract

With the continuous miniaturization of semiconductor devices, copper (Cu) has largely replaced gold (Au) as the primary material for wire bonding due to its lower cost and superior electrical properties. However, the long-term reliability of copper wire interconnects is highly sensitive to material quality, particularly surface integrity. This study investigates the influence of surface morphology and manufacturing-induced micro-defects on the performance of fine-pitch palladium-coated copper (PCC) bonding wires. We employed Scanning Electron Microscopy (SEM) to perform a detailed morphological characterization, revealing the presence of longitudinal striations, pits, and critical transverse micro-cracks on the wire surface, which are artifacts of the wire drawing process. To quantitatively characterize the surface topography, Atomic Force Microscopy (AFM) was utilized, which provided high-resolution topographical data and quantified the significant nanoscale roughness of the defect-rich areas. Furthermore, the electrical properties were assessed using a four-point probe method. The results indicate that wires with pronounced surface defects exhibit a measurable increase in electrical resistivity. This is attributed to a combination of a reduced effective conductive cross-section and enhanced electron scattering at the roughened, defective surfaces. The observed micro-cracks are also discussed as potent stress concentration sites that could severely compromise the mechanical reliability and fatigue life of the interconnects. This work underscores the critical importance of controlling surface quality during the manufacturing of copper bonding wires to ensure the performance and long-term reliability of modern microelectronic packages.