Wafer Sleuth in Action

Purposeful reordering of wafer process sequence is an essential ingredient to successful yield analysis using Wafer Sleuth. In some fabs, this is accomplished through the use of wafer reader / sorters. Newer fabs have the ability to control wafer process sequence within the process tools themselves, thus not requiring the use of sorters to impart a unique wafer order to each process step. In either case, Wafer Sleuth analysis finds correlations between observational and test results to wafer process order, allowing quick identification of the process step and associated equipment causing a variation.

An example of Wafer Sleuth problem resolution
At one particular customer installation, a lot contained many wafers with unusually high threshold voltage variation (delta Vt) across the wafer. The delta Vt value of each wafer was plotted against that wafer's sequence at all process steps.

A correlation with wafer location in the gate oxidation furnace was discovered. No additional experimental batches were needed to determine the cause, which was a furnace boat contaminated by phosphorus-doped dummy wafers. After examining the wafer tracking plots, this problem was resolved.

Since many such correlation scattergrams can be generated for every lot, Wafer Sleuth detects those of greatest significance by automatically applying to each plot a series of mathematical models and filters. The program looks for many pattern types, such as linear correlation, continuity with and without breaks, communities of points, variation within batches, cyclical patterns and outlier trends. This allows a user to quickly recognize which piece of process equipment is causing a wafer-to-wafer variation.

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