Ultraviolet and direct ultraviolet inspection of next generation lithography reticles

Abstract

The optical inspection of next generation lithography (NGL) patterned reticles, multilayer-coated blanks, and uncoated substrates is particularly challenging. The difficulties arise not only because of the higher sensitivity necessary at the smaller design rules, but also due to the specifics of the NGL mask materials and structures. Our research program is investigating the theoretical and practical operational limitations facing optical inspections of patterned and unpatterned NGL masks. We are constrained by the necessity to inspect only in reflected light, limitations in mask contrast, and interference effects caused by partially coherent illumination. We present inspection results and images of several types of NGL masks, blanks and substrates obtained on high resolution ultraviolet (UV) and direct ultraviolet (DUV) optical mask inspection systems. While electron projection lithography (EPL) stencil masks can be inspected in reflection, limited transmission through the mask suggests that subsurface defects will be detected with reduced sensitivity, compared to defects on the surface. For unpatterned inspections, we have demonstrated UV system sensitivity to 88 nm PSL spheres on quartz substrates and 117 nm PSLs on silicon substrates. On extreme ultraviolet masks we have achieved DUV system sensitivity down to defects as small as 80 nm in size, both in die-to-die inspections and in simulated die-to-database inspections. In addition to advancing the development of optical inspection systems for NGL reticles, these inspection results provide feedback to NGL mask developers.