Method for wavelength calibration of an optical measurement system

A method for wavelength calibration of an optical measurement system having a spectrophotometer and a ellipsometer, comprising the steps of:

• calibrating the spectrophotometer and the ellipsometer being independently of one another in an initial calibration,
• ascertaining upon calibration of the spectrophotometer, an allocation of the wavelength (λ) incident onto a CCD chip to the position of the pixels of the CCD chip,
• determining a layer thickness (delli) of a specimen using the ellipsometer, as a reference measured value;
• determining a spectrophotometer layer thickness (dvisphoto) in the visual spectral region and a spectrophotometer layer thickness (dUVphoto) in the UV region, at an initial angle of incidence (θinit);
• modifying coefficients of a wavelength-dependent function until the absolute value of the difference between a re-measured visual spectrophotometer layer thickness (dvisphoto) and a re-measured UV spectrophotometer layer thickness (dUVphoto) is less than a predefined absolute value;
• and
• varying the initial angle of incidence (θinit) until the absolute value of the difference between the visual spectrophotometer layer thickness (dvisphoto) and the ellipsometer layer thickness (delli), and the absolute value of the difference between the UV spectrophotometer layer thickness (dUVphoto) and the ellipsometer layer thickness (delli), are less than a predefined absolute value.