Through-focus scanning and scatterfield optical methods for advanced overlay target analysis
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.
This paper was published in SPIE Proceedings Vol. 7272