A Study of Undetectable Non-Feedback Shorts for the Purpose of Physical-DFT

Undetectable shorts may decrease the long term reliability of a circuit, cause intermittent failures, add noise or delay, or increase test pattern generation costs. This paper describes the undetectable non-feedback shorts that are likely to occur in standard cell implementations of the ISCAS'85 combinational test circuits. For ten layouts of each circuit, all shorts between adjacent wires were extracted and the undetectable ones analyzed. We found that approximately 0.4% are undetectable and that nearly half of these can be easily predicted before the physical layout of the circuit is generated. Since only a small percentage of the shorts are undetectable, and many of the undetectables are easily identiiable, it appears that it is possible to reduce the likelihood, or completely eliminate, the occurrence of a large portion of these shorts by incorporating design-for-test strategies into routing software. 1 Deenitions A short occurs when two or more lines are unintentionally connected during fabrication. We use the term short, rather than bridge fault, to help make it clear that we are talking about the physical manifestation of a fault rather than a behavioral model. The shorts we consider in this paper are interconnect shorts|shorts between two lines that are gate outputs or circuit inputs. We do not consider shorts that are internal to a logic gate. If the graph representing the unfaulted circuit contains a directed path between the two shorted lines, the short is a feedback short. If no such path exists, the short is a non-feedback short. This paper considers only non-feedback shorts; techniques for properly evaluating feedback shorts are currently under investigation. A short is detectable if, for some combination of the circuit inputs, at least one of the circuit outputs has a diierent logic value than it would in the defect free circuit with the same input combination. Likewise, a short is undetectable if no combination of the circuit inputs will generate a discrepancy on a circuit output (static voltage testing). This means that some of the shorts we categorize as undetectable are detectable via I DDQ Ack83] or delay testing. However, these techniques are generally more costly and less accepted than voltage-based testing. 2 Motivation Although an undetectable short does not change the logic function of a circuit, it can change other functional aspects. An undetectable short can cause reliability problems by creating unexpected delay, noise, power consumption, and heat. These may introduce intermittent errors or eventually …