Reducing pattern delay variations for screening frequency dependent defects

The delay variations of a pattern set can come from two sources: (1) Different patterns sensitize different parts of the circuit and result in different delays. (2) The same pattern, applied on different chips, results in different delays because of process variations. For structural delay testing, these pattern variations may result in difficulty for finding an optimal test clock setting, which may significantly impact the defect screening effectiveness. This paper investigates the possibility of applying statistical timing analysis techniques to reduce pattern variations for structural delay testing. We develop an efficient statistical pattern-based timing simulator and devise pattern selection algorithms for reducing such variations. By constructing pattern sets with smaller variations, we show that higher screening effectiveness can be achieved. We present experimental results to demonstrate the advantages of our techniques based on benchmark circuits.