Aspects of current reference generation and distribution for IDDx pass/fail current threshold determination

Pass/fail Iddx current thresholds have been set arbitrarily by various authors to determine the current limits beyond which a fault is said to be present. These current limits are of vital concern in producing Iddx threshold values which are able to detect a majority of faults while not excessively rejecting good product. A number of factors determine the absolute magnitude of the Iddq current and the scatter of current values about this nominal value. This paper reports some theoretical analyses of the causes of Iddq current variation both across a single wafer and from wafer to wafer for both biasing and biased cells. Comparison is made between these theoretical analyses and results measured from silicon fabricated for the purpose. Current biasing schemes are suggested based on this analysis and the predicted absolute current accuracy of these schemes is presented. Accurate determination of Iddq current variation then allows a more accurate current threshold to be determined for Iddx pass/fail analysis of circuits