Method and apparatus for imaging dense linewidth features using an

Abstract

A method and apparatus for imaging a substrate (such as silicon or silicon dioxide) beneath and between a dense array of strips (such as photoresist strips composed of dielectric or other material) using a polarizing optical microscope. In preferred embodiments, the apparatus of the invention includes a polarizer for polarizing optical illuminating radiation, an analyzer for receiving polarized radiation reflected from the sample, and a variable retarder whose retardation characteristic can be controlled to enhance the light signal transmitted through the analyzer from the substrate in a region of interest of the sample. The variable retarder can be a fixedly mounted retarder whose birefringence is variable in response to a control signal, or a rotatably mounted retarder plate having fixed birefringence which is mechanically rotatable to control the orientation of its optical axis. In one embodiment, the retarder is a quarter wave plate, the projection on the sample of the polarization axis of linearly polarized radiation from the polarizer forms an angle of about 45 degrees to the sample's strips, and the optical axis of the quarter wave plate is oriented at an optimal angle to the strips, so that the quarter wave plate converts the linearly polarized illuminating radiation into elliptically polarized radiation, the elliptically polarized radiation undergoes a polarization change upon reflection from the sample, and the quarter wave plate converts the reflected elliptically polarized radiation from the sample into radiation whose polarization is oriented so that it can be transmitted by the analyzer.