Influence of titanium surface contamination on the reliability of Al wire bonds

Organic and inorganic contaminations on the metallization surface of wire bonds are known to reduce the reliability of the devices. In this study the influence of Ti contamination on bondability and quality of Al thick wire bonds was investigated. Ti layers with thicknesses between 2 nm to 15 nm were deposited either directly on AlSiCu metallization pads or after an atmospheric exposure of the wafer. Characterization of the metallization by means of ToF-SIMS revealed two types of surface configurations: An intermixed surface layer composed of Ti- and Al- oxides was formed after a direct deposition of Ti on the pads. Exposure of the wafer to atmospheric environment before Ti deposition resulted in formation of a Ti passivation layer at the top of metallization. Evaluation of the bonding quality by shear tests and an accelerated mechanical fatigue testing method showed that deposition of a 2 nm thin layer of Ti leads to higher shear strength and fatigue resistance. A clear reduction of the bonding strength was observed by increasing the thickness of titanium to 10 nm and 15 nm. Microstructural investigations of these samples showed increased non-bonded areas in the interface due to presence of thick oxide particles which hinder a free metal-metal contact and formation of a defect free interface during the wire bonding process.