Calibration method for optical metrology

A method for wavelength to pixel location calibration of a spectrometer of an optical metrology system, comprising:

• providing an optical metrology system, comprising:
• a spectrometer having an array detector with pixel locations defined thereupon, for measuring diffraction spectra;
• an order sorting filter with at least two zones created thereon, each pair of adjacent zones of the order sorting filter defining zone joints therebetween,
• providing at least one metrology sample with known reflectance versus wavelength data,
• measuring the reflectance of the at least one metrology sample using the optical metrology system,
• defining a wavelength to pixel location calibration curve as a set of curves comprising:
• at least one first function of array detector pixel location that defines a portion of the wavelength to pixel location calibration curve affected by dispersion at an order sorting filter zone joint, the at least one first function containing adjustable fit coefficients;
• and
• at least two second functions of array detector pixel location that define portions of the wavelength to pixel location calibration curve not affected by dispersion at an order sorting filter zone joint, the at least two second functions containing adjustable fit coefficients,
• iteratively fitting adjustable fit coefficients of the at least one first function and the at least two second functions defining the wavelength to pixel location calibration curve to the measured reflectance data of the at least one metrology sample, and
• utilizing the fitted wavelength to pixel location calibration curve in subsequent measurement of diffraction spectra.