Etched-multilayer black border formed on EUV mask: Does it cause image degradation during pattern inspection using EB optics?

Abstract

In Extreme Ultraviolet (EUV) lithography, to enhance the wafer printing performance, a black border surrounding the patterned EUV mask is suggested. In this study, influence of the black border formed on the EUV mask on the image quality captured by electron beam (EB) microscope was investigated. First, we prepared EUV masks with etched multilayer black border. The electrical conductivity between inside and outside of the black border was controlled by inserting a conductive layer before coating the multilayer. By forming the black border the EUV reflectivity was suppressed about 70 % compared with the EUV mask without the black border. Next, the influence of the electrical conductivity on the EB microscope images was investigated using two types of EB microscopes. One was an EB microscope that does not employ retarding method. The other was an EB microscope that employs retarding method to accelerate the secondary electrons from the surface of the EUV mask to the sensor plane. The retarding voltage was supplied to the outside of the black border. By applying the conductive layer, degradation of the image quality was not observed regardless of whether the EB microscopes employ the retarding method or not.