1. A method of estimating integrated circuit yield, comprising: providing an integrated circuit layout; providing a set of systematic defects based on a manufacturing process; representing a systematic defect by modifying structures in said integrated circuit layout to create modified structures; performing a critical area analysis on said integrated circuit layout using said modified structures; computing a fault density value; and comparing said fault density value to a predetermined value, wherein said modifying of said structures comprises pre-expanding said structures when said structures comprise a higher systematic defect sensitivity level for short-circuit-causing defects and a lower systematic defect sensitivity level for open-circuit-causing defects, and wherein said modifying of said structures comprises pre-shrinking said structures when said structures comprise a lower systematic defect sensitivity level for short-circuit-causing defects and a higher systematic defect sensitivity level for open-circuit-causing defects.