Through-focus scanning and scatterfield optical methods for advanced overlay target analysis

Abstract

In this paper we present overlay measurement techniques that use small overlay targets for advanced semiconductor applications. We employ two different optical methods to measure overlay using modified conventional optical microscope platforms. They are scatterfield and through-focus scanning optical microscope (TSOM) imaging methods. In the TSOM method a target is scanned through the focus of an optical microscope, simultaneously acquiring optical images at different focal positions. The TSOM images are constructed using the through-focus optical images. Overlay analysis is then performed using the TSOM images. In the scatterfield method, a small aperture is scanned at the conjugate back focal plane of an optical microscope. This enables angle-resolved scatterometry on a high-magnification optical platform. We also present evaluation of optical constants using the scatterfield method.