When measurements are made during the production of integrated circuit wafers on wafers that are out of specification, the wafers are considered misprocessed. Measurement accu-racy must be better than the specification to assure that the measurements are not part of the misprocessing problem. Optical tools have been used in the past for these critical dimension (CD) measurements. These tools have evolved from optical filars to TV pattern recognition to stepping a PMT aperture. This has improved the repeatability of measuring a single site multiple times by many operators to ±0.04 pm, 3.0 σ or better. However, with CDs of 1.0 μm or less, optical tools are reaching their limits of usability.