Use of Harsh Wafer Probing to Evaluate Various Bond Pad Structures

(This is part 2, continued from “Use of harsh Wafer Probing to Evaluate Traditional Bond Pad Structures”). Goals for circuit under pad (CUP) in aluminum – silicon dioxide (Al – SiO2) based IC pad structures include the design of metal interconnects in all layers beneath the pad Al, and the ability to withstand harsh probing and wirebonding stress without SiO2 cracking, without deforming or bending the interconnects. CUP pad designs having thin pad Al and Cu wirebond represent significant challenges. Previous work showed that the traditional pad designs are not at all robust to cracking, so major improvement in pad structure design is sought. This study uses harsh wafer probing to compare cracking tendencies for various pad structures having slots or holes in the metal sub-layers directly beneath the pad window. The presence of a full sheet of metal in a pad sub-layer will dominate the top SiO2 cracking performance in the pad. A dramatic improvement in robustness to cracking is seen as the metal-top-minus-one (MT(-1)) layer reduces in pattern density. Interaction between cracks and metal pattern is analyzed, and further details regarding the mechanisms of cracking are discussed. Deformation in metal sub-layer features can be prevented or minimized, thus preventing bending of the top SiO2, preventing cracks, increasing the capability for high reliability CUP pads. Robustness to cracking also increases the probing process margin. CUP pad objectives can be achieved even with harsh probing by following pad sub-layer interconnect layout guidelines within the pad window based on these experimental results.