Phase defect detection with spatial heterodyne interferometry

Phase shift techniques introduced in photolithography to further improve resolution produce a new set of challenges for inspection. Unlike the high contrast provided by patterned and unpatterned areas on a binary mask, phase errors do not provide significant contrast changes, since the phase change is imparted by a difference in material thickness. Surface topology measurements can be used to identify phase defects, but methods for surface topology inspection are typically slow or can damage the surface to be measured. In this study, Spatial Heterodyne Interferometry (SHI) has been considered as a possible method for high-speed non-contact phase defect detection. SHI is an imaging technique developed at Oak Ridge National Laboratory that acquires both phase and amplitude information from an optical wavefront with a single high-speed image capture. Using a reflective SHI system, testing has been performed with a mask containing programmed phase defects of various sizes and depths. In this paper, we present an overview of the SHI measurement technique, discuss issues such as phase wrapping associated with using SHI for phase defect detection on photolithographic masks, and present phase defect detection results from die-to-die comparisons on a 248nm alternating aperture phase shift mask with intentional phase defects.