Use of Harsh Wafer Probing to Evaluate Traditional and CUP Bond Pad Structures

IC pad damage from a wafer probe can be detrimental to wire-bond yield and product reliability. In this paper, bond pads are harshly probed on traditional pads and a variety of experimental circuit-under-pad (CUP) structures in technologies having aluminum (Al) metallization and silicon dioxide (SiO₂) dielectric films. Probe marks and cracking behavior are analyzed, seeking process margins for high-reliability products. Results follow the well-known dependencies on chuck overdrive, probe touch counts, and cantilever probe tip length. Additional detail is revealed regarding the probe mark area, the interaction of cracks with top vias, sublayer film deformation that leads to cracks, and decreased cracking with increased pad Al thickness. The presence of a full sheet of metal in a pad sublayer dominates the top SiO₂ cracking performance in the pad. A dramatic improvement in robustness to cracking is seen as the bond pad sublayer metal films reduce in pattern density. Deformation or ripple in metal sublayer features can be prevented or minimized, thus preventing bending and cracking of the top SiO₂ and increasing the capability for high-reliability CUP pads. Based on these experimental results, CUP pad objectives can be achieved even with harsh probing on thin pad Al.