Issue Events Semiconductor International November 1999 Laura Peters, Senior Editor Yield Test Structures Accelerate Learning Part 10 of Series B y employing large-area yield test structures early in the process development cycle, engineers rapidly identify the most significant yield detractors. 2-metal 2 leakage caused by TiW stringers. Optimum yield was reached in six months. The structures include serpentine patterns to test for opens, comb patterns to check for shorts and leakage, contact and via chains for all combinations, and arrays of transistors for each major transfer type.
Structures indicating random yield problems have losses approximately proportional to the area.
With systematic problems, the measured parameter value is proportional to the area. The engineers designed poly combs and serpentine structures with poly probe pads so they can perform probe testing after poly doping, patterning and activation for earlier results. The structures also must be positioned to represent a significant portion of wafer area. The engineers generate the probability data using computer simulation of large numbers of defects randomly dispersed on each critical layer. Then they count the number of defects causing shorts, for instance, and calculate the probability of failure by simply dividing the number causing shorts by the total number distributed.