Reactivation of spares for off-chip memory repair after die stacking in a 3-D IC with TSVs

Memory, especially DRAM, is one of the candidates to be considered in three-dimensional integrated circuit (3-D IC), and in particular, to be heterogeneously stacked with a system on chip (SOC) for mobile applications. Even though the memory is tested and repaired beforehand, the known good die (KGD) can become bad during the integration process. Traditional schemes may not be able to redo the repair and obtain a known good stack (KGS), let alone unused spares be reused. We propose an off-chip repair scheme to deal with the inaccessibility from outside of the memory die. Using a through silicon via (TSV) to access the redundancy control circuit (RCC), we reactivate the unused spares by overwriting their states as if the corresponding fuses are blown. Even when the row or column, which has already been repaired, is damaged again, we are able to replace it with a new spare. Our simulation using a 65 nm process technology shows that the maximum timing penalty of the off-chip repair is only 93ps, compared to the on-chip method. The area overhead is estimated to be 490 μm² per fuse set by using a 5 μm diameter TSV process. Most importantly, the yield improvement of a two-die stacked memory can be over 50% with yield excursion reduced to 8%.