Method and apparatus for measuring reflectance in two wavelength bands

Abstract

A method and system for performing reflectance measurements of a sample using radiation having UV frequency components (preferably in a broad UV band) and visible frequency components (preferably in a broad band). Preferably, two detectors simultaneously receive a sample beam reflected from the sample surface. One detector generates a signal indicative of the sample beam components in the UV band and the other detector generates a signal indicative of the sample beam components in the visible band. By processing these two signals, the invention enables accurate measurement of the thickness of a very thin film on the sample. Preferably, the system determines a single effective wavelength for the UV radiation incident on the first detector and a single effective wavelength for the visible radiation incident on the second detector. Embodiments of the system can also measure reflectance spectra and refractive indices, and can determine lithographic exposure times. Preferred embodiments include an objective lens assembly having a pupil stop with an entrance portion with one or more relatively large apertures therethrough and an exit portion with one or more relatively small apertures therethrough. Illuminating radiation passes through the relatively large apertures before reflecting from the sample, and then passes through the relatively small apertures after reflecting from the sample. This design and pupil stop orientation dramatically increases the insensitivity of the system to ripple on the sample surface.