Improvement of inter layer dielectric crack for LQFP C90FG wafer technology devices in copper wire bonding process

This paper aims to investigate the root causes of and implement the improvements for the inter layer dielectric (ILD) crack for LQFP C90FG (CMOS90 Floating Gate) wafer technology devices in copper wire bonding process.

Failure analysis was conducted including cratering, scanning electron microscopy inspection and focus ion beam cross-section analysis, which showed ILD crack. Root cause investigation of ILD crack rate sudden jumping was carried out with cause-and-effect analysis, which revealed the root cause is shallower lead frame down-set. ILD crack mechanism deep-dive on ILD crack due to shallower lead frame down-set, which revealed the mechanism is lead frame flag floating on heat insert. Further investigation and energy dispersive X-ray analysis found the Cu particles on heat insert is another factor that can result in lead frame flag floating.

Lead frame flag floating on heat insert caused by shallower lead frame down-set or foreign matter on heat insert is a critical factor of ILD crack that has never been revealed before. Weak wafer structure strength caused by thinner wafer passivation1 thickness and sharp corner at Metal Trench (compared with the benchmarking fab) are other factors that can impact ILD crack.

For ILD crack improvement in copper wire bonding, besides the obvious factors such as wafer structure and wire bonding parameters, also should take other factors into consideration including lead frame flag floating on heat insert and heat insert maintenance.