| This dissertation project includes the experimental research, analysis and modeling that explains certain reliability failure modes arising from the mechanical stress of probing and wirebonding of those integrated circuit (IC) bond pads consisting substantially of aluminum (Al) and silicon dioxide (SiO2) thin film structures. Business needs coupled with the analytical results have led to the development of new bond pad designs and design rules, facilitating the design and manufacture of high reliability bond pads in circuit under pad (CUP) structures capable of copper (Cu) wirebond, even when the pad Al is very thin. Bond pads are a key component in integrated circuit technologies, enabling electrical connections from the chip to the outside world. CUP pads have potential issues with both electrical and mechanical failures if they are not well designed. Thin pad Al presents additional engineering challenges including increased coupling of stress to the fragile bond pad sub-layers. Cu wirebond imparts significantly more stress to the pads than the standard gold (Au) wirebond. No published method existed to improve bond pads to meet these requirements simultaneously without adding cost or new materials. For each CMOS technology of interest, test chips were designed, wafers fabricated, probe cards designed and built, and bonding diagrams created and IC packages obtained. The experimental bond pads were subjected to probing and wirebonding, and then destructively analyzed to compare mechanical robustness to the various stresses among a number of pad designs. New bond pad design rules were developed based on the large pool of experimental data. Certain CUP bond pad designs which performed well were then fabricated again, probed, wire bonded (some with Au, others with Cu) and packaged, then subjected to electrostatic discharge (ESD) testing or extended reliability stress testing, all with passing results. Finite element modeling (FEM) adds to our knowledge of stress locations and important design factors. A number of products having the improved bond pads are successfully implemented in manufacturing. |
