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Structural Reliability Analysis of
Interconnect Structures
for Principal Investigators Inclusive Dates: 04/01/98 - Current Background - The 1997 National Technology Roadmap for Semiconductors (the Roadmap), prepared with input from over 200 industry experts, is recognized as the most important document for guiding the industry’s $20 billion/year world-wide R&D effort. As indicated in the Roadmap, building the interconnecting metal lines on integrated circuit (IC) chips represents more than half of the total manufacturing costs. The Roadmap correctly predicted that the industry would migrate from aluminum- to copper-based interconnect lines. It also explained how new and better dielectric materials would be introduced to insulate these interconnect lines from each other. As these new materials are now being introduced and as the size of the interconnects continues to shrink, their structural reliability has become one of the industry’s top technical concerns. Approach - The objective of this project is to provide a comprehensive and technically robust methodology for evaluating the structural reliability of interconnect structures used in deep submicron ICs. Three of SwRI’s structural reliability analysis skills were adapted for this project, including 1) finite element stress analysis (FEA), 2) probabilistic analysis, and 3) computer-vision-assisted microscopy for deformation and failure analysis (DISMAP). During the first task of this project, a representative interconnect structure was identified for study. The stress in this representative structure was then simulated using an FEA model, which was then linked to SwRI’s probabilistic analysis software tool, NESSUS™. Using NESSUS™ and the FEA model, the effects of variations in the structure’s material properties and process conditions on the likelihood of failure were quantified. The computational approach was complemented by SwRI’s unique and powerful DISMAP capability, in which cross-sectioned integrated circuits are thermally loaded in a scanning electron microscope. The images from before- and after-loading were processed by SwRI’s existing computer vision system to produce deformation and strain plots. Accomplishments - During 1999, significant technical results were produced. Recent results are shown in the top illustration, which shows displacement vectors resulting from thermal cycling. The plot is produced by the Institute’s DISMAP technology and reveals, for the first time, the way submicron interconnect structures interact with the surrounding dielectric field. The results show previously unseen permanent structure deformation after the structure was heated to deposition temperatures. They also reveal how material interfaces tend to slip during heating. Similarly, as shown in the illustration on the bottom, this project has produced probabilistic reliability estimates of interconnect structures. For the first time, the relative importance of materials and process quality control are understood.
Displacement vectors produced on a Motorola chip sample using the DISMAP technology identify possible failure modes due to thermal cycling. This chart shows the relative importance of statistical uncertainty on the probability of interconnect failure. Materials Research and
Structural Mechanics Program |
