Design and development of micro-sensors for measuring localised stresses during copper wirebonding

In wirebonding, high stresses applied onto the pad during the ultrasonic bonding can result in pad damage, silicon cratering and aluminium splash — all of which ultimately result in poor joint quality. Cracking in the Cu/low-k and Cu/ultra low-k layers beneath the pad (also a result of high applied stresses) is a common issue with wirebonding. As a result of these failures in the substrate and the poor quality of joints made, there is much interest to measure and map the stress distribution (σxx, σyy, σzz, σxy) beneath the bond pad. This information coupled with failure analysis would provide engineers with the information required to form more robust wirebonds. Current methods to measure this stress involve mechanical simulation for extrapolation of stress from the measured regions to the unmeasured regions. This paper presents the novel micro-sensor designs by using the various piezoresistive stress sensors to measure the four components of stress below the bond pad so that the stresses can be correlated with failure mechanism of the wirebond. This would provide a complete picture of what causes the failure of wirebonds. The advantage of this method is that the four components of stress at a very localized region can be determined.