Production implementation of a practical WLR program

Wafer Level Reliability (WLR) programs have been active in many semiconductor companies since the early 1980s. Test structures are designed to accelerate known reliability failure mechanisms. Large samples of these test structures are packaged and tested to determine reliability failure rates. These WLR programs typically focus on electromigration (EM), hot carrier, and TDDB testing of packaged parts. National Semiconductor has developed and implemented a WLR program which differs from the traditional approach. Wafer manufacturing personnel need to know which people, equipment, or process variables need to be better controlled to prevent reliability problems in the field. They also need faster feedback, Our WLR program looked at these issues and a more practical approach to eliminating fab-related reliability problems. Wafers, not packaged parts, are tested in-line and at the end of line (before sort test) to provide near-real-time feedback. We identified our top ten reliability failure mechanisms of concern based on past field failure rates and reliability monitors. WLR test structures and test methods were then designed to measure the effect of wafer fab process variability on reliability risk. Designed experiments were used extensively to correlate fab process monitors, WLR test results, and reliability test results.