Abstract
Flip-Chip technology is becoming a mainstream process due to the advantages of packaging density, functionality and cost. The reliability of solder ball and Under Ball Metallurgy(UBM) remains the critical issue for flip-chip structures. A systematic study of interfacial reaction between Sn, a major constituent of lead free solder, and Cu metallization was conducted. We investigated the interfacial reaction of Sn/Cu bimetallic thin film and the kinetics of Cu-Sn intermetallic compounds (IMC) formation by measuring the resistance change of the Cu/Sn bilayer thin films. Sn/Cu bilayer thin films were prepared by consecutive deposition of Cu film and Sn film onto oxidized Si substrates by sputtering and evaporation methods. Resistances of the Cu-Sn bimetallic thin film specimens were measured using an in-situ 4-point probe method when the specimens were heated from room temperature to 220□C at a fixed ramp rate in nitrogen ambient. The various Cu-Sn compounds were identified by X-ray diffraction (XRD), and the morphology of the Cu-Sn compounds was examined by scanning electron microscopy (SEM). The concertration depth profile of the Sn/Cu bimetallic samples were measured by Auger electron spectroscopy (AES). The activation energy of formation of Cu6Sn5 compounds was found to be 0.97±0.07 eV according to the in-situ resistivity measurement. The thickness of the Sn/Cu bilayer thin films was found to affect the sequence of IMC formation. Besides, the thinner reaction couples showed a better resolution for detecting interfacial reaction due to large change in thin film resistance.