A statistical model based ASIC skew selection method

Cross wafer speed variations in the 0.13 μm process and beyond are significant enough to be considered in ASIC skew selection. Traditional Idsat and Vt measurements on a few sites of the wafer to gauge wafer speed simply does not work. Multiple skew lot runs to get proper skew wafers is financially prohibitive and part skew uncertainty makes it impossible to qualify product with a reasonable number of skew parts. Typical design practice for high-speed ASICs is to add a PVT monitor on the die. For area critical applications, scribe PVT monitors can be used. The PVT circuitry can be as simple as an inverter ring oscillator chain. We present a methodology based on on-chip or on-scribe ring oscillator data to bin the ASIC skew parts. Statistical modeling, PVT monitor sensitivity and actual experiment data are discussed. Based on the proposed methodology, proper skew parts can be selected for product testing so that products are validated cross all process corners.