Analysis and Solution of a Yield-Limiting Patterned-Fail Mechanism in a 1 Mbit DRAM

During development phases of a 1 Mbit DRAM, two distinctive types of bit fail maps were frequently observed. 'Top/Bottom Fails' (TBF) were groups of bit-line fails located at the top and/or bottom of arrays. 'Curtain Fails' (CF) were U-shaped streaks occurring in the interior of the array (see Fig 1). The problem was highly variable in its occurrence but the overall yield limit from this mechanism was significant. The fails could not be correlated to any regularly monitored device or process parameter.