Quantitative Optimization of Optical Beam Alignment for Throughput Enhancement in Dark-Field Wafer Inspection Systems

Dark-field wafer inspection systems are central to early defect detection, but the standard time (ST) of the inspection step limits throughput. We present a quantitative optimization framework that removes most of the dominant intra-tool component of ST, optical beam alignment, without equipment investment. Unlike periodic alignment and conventional fault detection and classification (FDC) monitoring, it derives the action limit of each monitored equipment parameter from a physically established beam-position tolerance at the wafer plane and applies those limits, cycle by cycle, to decide whether alignment is executed. Throughput log analysis attributed 4.0 of the 5.5 percentage-point performance loss, or 72.7%, to beam alignment. A geometric relation, calibrated empirically at two incidence angles, showed that the beam displacement from stage height deviation scales with the cotangent of the incidence angle, making shallow-incidence recipes the worst case. Over 28 days, the measured beam shift stayed within ±0.1 μm against a ±0.2 μm tolerance. Because alignment only re-centres the beam and sets no detection parameter, it can be skipped while the beam stays inside that tolerance; repeated low-angle inspections and a 300-lot production trend showed no degradation in sensitivity, matching rate, or false detection rate within the resolution of the evaluation. For the 20° standard recipe family, suppressing 80% of alignment executions on production tools lowered the median processing time from 4.16 min to 4.02 min, a 3.5% throughput gain; the interquartile range did not narrow, so the demonstrated benefit is the removal of a mean processing overhead, not a reduction in dispersion. The method applies to oblique-incidence dark-field platforms whose alignment routine only re-centres the beam; its coefficients, tolerance, and thresholds are platform-specific and must be re-measured before transfer.