Patent ID: 7639375

Claim:
A method of determining a transmittance of a photomask for an exposure wavelength using an optical metrology system, the optical metrology system using an incident beam with a range of wavelengths, the method comprising: measuring a diffraction signal off a portion of a photomask, the diffraction signal generated by directing an incident beam with a range of wavelengths onto the portion of the photomask; determining one or more geometric features of the photomask using the measured diffraction signal off the portion of the photomask; determining one or more optical properties of the portion of the photomask using the determined one or more geometric features and an exposure wavelength; determining a wave coupling of the of the portion of the photomask using the determined one or more optical properties of the portion of the photomask at the exposure wavelength; and determining the transmittance of the portion of the photomask at the exposure wavelength.