Multi-tier die stacking through collective die-to-wafer hybrid bonding

A collective die-to-wafer bonding flow is extended beyond the N=2 tier to the N=3 and N=4 tier by collectively bonding multiple layers of dies on top of a target wafer. The N=2 die-level is shown to be consistently free of bonding voids (bond yields close to 100%) with +99% die transfer yields. For the N=3 and N=4 die-levels, both the transfer yields and bonding yields are significantly lower. Both N=2 and N=3 stacks show similar die-to-target wafer alignment. A cross sectional SEM of a 5-micron pitch shows excellent alignment and connectivity between Die 1 and Die 2 (N=3).