Effectiveness of Barrier Layer Metallisations in Long Term High Temperature Endurance Tests on Wire Bond Interconnections

Abstract

Silicon on Insulator (SOI) device technology has been shown to be capable of functioning satisfactorily at operating temperatures of >200°C, with device lifetimes of 5 years at 225°C being declared. One of the key areas governing the lifetime of the packaged electronic devices is the reliability of the wire bond interconnection between the device and the package or substrate connection. Extended temperature storage testing at 250°C of packaged SOI devices has highlighted end of life failure modes associated with wire bond connections. SOI devices are normally supplied with an aluminium based bond pad metallisation, which are not suitable for direct connection of Au wire at operating temperatures of >125°C, due to the formation of Au-Al intermetallics. It is possible to post-process silicon wafers to deposit barrier and connection materials to create a mono-metallic Au-Au joint at the surface. For long term endurance at temperatures >200°C, the effectiveness of the barrier layer in preventing diffusion of the aluminium bond pad metallisation to interact with the Au is a critical factor. This paper presents results of studies carried out on two post-process metallisation systems Au/TiW and Au/Pd/Ni deposited onto aluminium bond pads, which have been Au wire bonded and exposed to 250°C temperature storage for up to 13,000 hours. The results have shown that the barrier layers are not effective in preventing diffusion of the aluminium bond pad metallisation to create Au-Al based intermetallics. The results are compared with Al-1%Si wire bonding to the aluminium bond pad, where the 2nd wedge bond is attached to a Au/Ni plated metallisation, where the degradation appears to be less severe. Recommendations for designing stable wire bond interconnection systems for extended high temperature operation will be presented.