Accelerating EUV learning with synchrotron light: mask roughness challenges ahead

Abstract

Despite significant progress in the commercialization of extreme ultraviolet (EUV) lithography, many important challenges remain, including in the area of masks. The issue of EUV phase roughness that can arise from either multilayer or capping layer roughness has recently garnered increasing concern. The problem with mask phase roughness is that it couples line-edge roughness (LER) through the formation of image plane speckle. The coupling from phase roughness to LER depends on many factors including roughness magnitude, roughness correlation length, illumination partial coherence, aberrations, 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. Results also show that Atomic Force Microscopy (AFM) may not be a suitable method for measuring mask phase roughness due to its sensitivity to the surface only. Capping layer roughness is another significant concern especially given that it has been shown to increase with cleaning cycles. In this case, however, AFM does provide a reasonable metric.

Effect of amplitude roughness on EUV mask specifications

“…Actinic scatterometry presents an alternative measurement technique capable of measuring the roughness as seen by the EUV imaging system. 4,5 However, the conversion of the scattering power to an equivalent phase roughness requires the assumption that the scattering is exclusively resulting from phase roughness. It is possible that there is amplitude roughness in addition to phase roughness.…”