Actinic defect counting statistics over 1-cm2 area of EUVL mask blank
As a continuation of comparison experiments between EUV inspection and visible inspection of defects on EUVL mask blanks, we report on the result of an experiment where the EUV defect inspection tool is used to perform at-wavelength defect counting over 1 cm2 of EUVL mask blank. Initial EUV inspection found five defects over the scanned area and the subsequent optical scattering inspection was able to detect all of the five defects. Therefore, if there are any defects that are only detectable by EUV inspection, the density is lower than the order of unity per cm2. An upgrade path to substantially increase the overall throughput of the EUV inspection system is also identified in the manuscript.
Review of progress in extreme ultraviolet lithography masks
Extreme ultraviolet lithography (EUVL) is a leading next generation lithography technology. Significant progress has been made in developing mask fabrication processes for EUVL. The mask blank for EUVL consists of a low thermal expansion material substrate having a square photomask form factor that is coated with Mo/Si multilayers. SEMI standards are being developed for mask substrates and mounting. Several commercial suppliers are developing polishing processes for LTEM substrates, and they are progressing toward meeting the requirements for flatness, surface roughness, and defects defined in the a draft SEMI standard. One of the challenges in implementing EUVL is to economically fabricate multilayer-coated mask blanks with no printable defects. Significant progress has been made in developing mask blank multilayer coating processes with low added defect density. Besides lowering the added defect density, methods to reduce defect printability, such as defect compensation and buffer layer smoothing, are b...
EUV mask absorber defect repair with focused ion beam
In this paper, the Extreme Ultra Violet (EUV) mask absorber repair using focused ion beam (FIB) is investigated. It is well known that focused ion beam repair for the opaque defect removal can easily cause both damage and gallium staining or contamination to the substrate. The damage and gallium staining will induce a localized transmission loss in the case of conventional transmission optical mask. The situation become even worse in the case of reflective EUV mask since gallium is very absorbent at EUV wavelength of 13.4 nm. In addition, the incident beam travels double path through the damaged region due to the reflective nature of the mask. To minimize or even eliminate the repair induced damage, we designed a mask flow to incorporate a SiO2 sacrificial layer beneath the metal absorber layer. This sacrificial layer will become a buffer layer for the metal defect repair. The repair, therefore, will only result in the damage on the sacrificial SiO2 layer instead of EUV reflective multilayer (ML). After the repair, the damaged and contaminated SiO2 layer will be etched away by either wet or dry etch. In the study, we performed both high voltage of 30 kv repair and low voltage of 10 kv repair. We found that in addition to use a sacrifice layer, low voltage focused ion beam repair is also necessary to ensure the damage and contamination free ML substrate.