Actinic detection of multilayer defects on EUV mask blanks using LPP light source and dark-field imaging

Abstract

The development of defect-free mask blanks including inspection is one of the big challenges for the implementation of extreme ultraviolet lithography (EUVL), especially when the introduction of EUVL is rescheduled to a later technology node. Among others, inspection of multilayer coated mask blanks with no oversight of critical defects and with minimal detection of false defects is a challenging issue for providing mask blanks free of defects or with thorough characterization of any existing defects. MIRAI Project has been developing a novel actinic (at-wavelength) inspection tool for detecting critical multilayer defects using a dark-field imaging and a laser-produced plasma (LPP) light source, expecting better sensitivity and better correlation with printability. The first experimental set up is completed for proof-of-concept (POC) demonstration using 20x Schwarzschild imaging optics and a backsideilluminated CCD.An in-house LPP light source is integrated to optimally illuminate the area of interest by EUV with a wavelength of 13.5nm. For its illuminator, a multilayer-coated elliptical mirror is used to illuminate a mask blank with the EUV that is collected within a wide solid angle from the light source. The first EUV dark-field image is obtained from a mask blank with programmed multilayer defects which are manufactured by locating well-defined patterns before depositing Mo/Si multilayer on EUV mask substrate. All the fabricated multilayer defects down to 70nm in width and 3.5nm in height are detected as clear signals that are distinguishable from the background intensity arising from the scattering by the surface roughness of the multilayer-coated mask blank. We have also detected a phase defect as low as 2nm in height. False defect count was not only zero within the area of view but also statistically confirmed to be less than one within the whole area of a mask blank assuming the extrapolation of observed fluctuation of background intensity is applicable. EUV pulse energy measurements and a CCD speed scaling suggested that the inspection throughput of 2 hours per mask blank will be feasible. The actinic tool based on this scheme will, not only serve for benchmarking with non-actinic tools or support multilayer deposition process improvements, but also be a viable choice for qualification of premium EUV mask blanks.

EUV exposure experiment using programmed multilayer defects for refining printability simulation

“…(1) The same deposition mode as this experiment or the "decoration" mode, (2) The "shrinkage" mode as has been used for fabricating very small programmed multilayer defects for inspection experiments 8 . Figure 10 shows the left halves of the cross sections of the structures for the two typical cases to be compared.…”

Actinic detection and screening of multilayer defects on EUV mask blanks using dark-field imaging

“…The detail of the experimental set up is already described in our previous papers 1,7 . Figure 1 illustrates the top view of our proof-of-concept (POC) experimental setup.…”

Next-generation lithography for 22 and 16 nm technology nodes and beyond

“…However, the sensitivity of current non-actinic inspection may be not high enough, because actinic methods can diagnose multilayer defects [112][113][114] with better sensitivity [115,116] than the state-of-the-art non-actinic mask blank inspection system.…”