Optical measurement system with systematic error correction

A semiconductor wafer processing tool comprising:

• a wafer handler which exchanges a substrate between plural wafer processing units;
• and
• an optical measurement system associated with the wafer handler and including,
• an optical measurement tool configured to expose the substrate to a spectrum of energy, and
• the optical measurement tool including a detection unit configured to measure,
• a first diffraction spectrum from a standard substrate including a layer having a known refractive index and extinction coefficient by exposing the standard substrate to the spectrum of electromagnetic energy, and
• a second diffraction spectrum from a workpiece by exposing the workpiece to the spectrum of electromagnetic energy, and
• a computational unit configured to calculate a tool-perfect diffraction spectrum for the standard substrate, compare the measured diffraction spectrum to the calculated tool-perfect diffraction spectrum to generate a calculated hardware systematic error, and correct the measured second diffraction spectrum based on the calculated hardware systematic error to obtain a corrected diffraction spectrum.