Optical system and method for angle-dependent reflection or transmission

Abstract

An angle-dependent reflectometer or transmissometer includes an optical imaging array in the incident and reflected or transmitted light path that breaks up an incident light beam into mutually spatially incoherent light bundles. The individual light bundles are then focused to a common spot by a high numerical aperture objective lens so as to provide a range of incidence angles on a sample surface. In a reflectometer, reflected light returns through the objective lens and imaging array and is imaged onto a detector array where different incidence and reflection angles are received by different groups of detection elements. In the angle-dependent transmissometer, the imaging array and high numerical aperture focusing objective lens are used for illuminating a spot on the sample, with a second high numerical aperture collection objective lens and detector array used for receiving transmitted light over a wide range of collection angles. The angle-dependent reflectance or transmittance measurement provided by the detector array can be analyzed to determine a desired characteristic parameter of the illuminated area of the sample surface. For example, a periodic text pattern on a wafer or mask surface can be illuminated to obtain a linewidth measurement. The break up of the light by the imaging array into light bundles allows the spot size to be controlled independently of the range of illumination angles so that areas much larger than the diffraction limit can be illuminated.