Mask roughness challenges in extreme ultraviolet mask development - eScholarship

Mask roughness challenges in extreme ultraviolet mask development Patrick Naulleau, 1 Brittany McClinton, 2 Kenneth A. Goldberg, 1 Iacopo Mochi, 1 and Abbas Rastegar 3 Center for X-Ray Optics, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Applied Sci. & Technol. Graduate Group, University of California, Berkeley, CA 94720 SEMATECH, Albany, NY 12203 Abstract Despite significant progress in the commercialization of extreme ultraviolet (EUV) lithography, many challenges remain. Although availability of a reliable high power source is arguably the most daunting of these challenges, important mask issues are also of major concern. The issues of EUV phase roughness that can arise from either multilayer or capping layer roughness has recently become of increasing concern. The problem with mask phase roughness is that it couples to image plane speckle and thus line-edge roughness (LER). The coupling, however, depends on many factors including roughness magnitude, roughness correlation length, illumination partial coherence, aberrations and defocus, and numerical aperture. Analysis shows that only on the order of 50 pm multilayer roughness may be tolerable at the 22-nm half-pitch node. The analysis, however, also shows that the difficulty does not scale with future node reductions. Moreover, it is found that ruthenium is a particularly bad choice for capping layer from the perspective of phase roughness and that cleaning damage in such a multilayer could lead to unacceptable image-plane LER. 1. Introduction Despite significant progress in the commercialization of extreme ultraviolet (EUV) lithography [1], important challenges remain. Although availability of a reliable high power source [2-4] is arguably the most daunting of these challenges, several mask issues are also of major concern. The most pressing and well known of these issues is mask defectivity [5]. Significant progress has been made in this area, but another two orders of magnitude defect reduction is still required to meet current pilot lines goals [5]. As described in the literature [6], as a work around to the availability of zero defect mask blanks, a variety of methods have been proposed including defect repair, defect