Controlled Deposition of ${\rm SiO}_{2}$ Nanoparticles of NIST-Traceable Particle Sizes for Mask Surface Inspection System Characterization

Particulate contamination of masks is a serious challenge in extreme ultraviolet lithography (EUVL) technology due to the unavailability of conventional pellicles. EUVL mask surface inspection tools, operated at low pressure, are used not only for mask contamination control/monitoring but also for mask surface cleaning studies. In EUVL, contaminant particles can be generated during low-pressure stages of integrated circuit (IC) manufacturing and may contaminate the mask critical surface without protective pellicles. It is therefore needed to characterize the EUVL mask surface inspection tools with contaminants commonly seen in vacuum processes. We have developed a method to deposit particles of known material and NIST-traceable sizes on the mask surface for the purpose of calibrating the EUVL mask surface inspection tools. Our method can produce particles with 98% size-uniformity. SiO₂ particles with NIST-traceable sizes of 50 nm, 60 nm, and 70 nm were separately deposited on quartz mask blanks with a controlled deposition spot size and number density, and detected by a Lasertec M1350 mask surface scanner. The results demonstrate high capture efficiencies for 60 and 70 nm SiO₂ particles, and significantly lower capture efficiency for 50 nm SiO₂ particles. The sizing accuracy of Lasertec M1350 deteriorates with decreasing particle size.