Overview of Mask Metrology

Abstract

Extreme ultraviolet (EUV) lithography is the successor to optical lithography and will enable advanced patterning in semiconductor manufacturing processes down to the 8 nm half pitch technology node and beyond. However, before EUV can successfully be inserted into high volume manufacturing a few challenges must be overcome. Central among these remaining challenges is the requirement to produce “defect free” EUV masks. Mask blank defects have been one of the top challenges in the commercialization of extreme ultraviolet (EUV) lithography. To determine defect sources and devise mitigation solutions, detailed characterization of defects is critical. However, small defects pose challenges in metrology scale‐up. SEMATECH has a comprehensive metrology strategy to address any defect larger than a 20 nm core size to obtain solutions for defect‐free EUV mask blanks. SEMATECH's Mask Blank Development Center has been working since 2003 to develop the technology to support defect free EUV mask blanks. Since 2003, EUV mask blank defects have been reduced from 10000 of size greater than 100 nm to about a few tens at size 70 nm. Unfortunately, today's state of the art defect levels are still about 10 to 100 times higher than needed. Closing this gap requires progress in the various processes associated with glass substrate creation and multilayer deposition. That process development improvement in turn relies upon the availability of metrology equipment that can resolve and chemically characterize defects as small as 30 nm. The current defect reduction efforts at SEMATECH have intensively included a focus on inspection and characterization. The facility boasts nearly $100M of metrology hardware, including an FEI Titan TEM, Lasertec M1350 and M7360 tools, an actinic inspection tool, AFM, SPM, and scanning auger capabilities. The newly established Auger tool at SEMATECH can run a standard 6‐inch mask blank and is already providing important information on sub‐100 nm defects on EUV blanks. Complementary to Auger analysis, TEM provides ultimate resolution in the defect imaging of sub‐nanometer structures. Crystalline and phase information generated by this metrology technique also indicates the sources of defects. SEMATECH's TEM capability is further equipped with energy dispersive X‐ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS), which provide higher analytical power than similar techniques in traditional secondary electron microscopy (SEM). This paper will describe SEMATECH's comprehensive effort to develop robust inspection of EUV mask defects. We will discuss the development of hardware and procedures for inspecting particles 70 nm and smaller. This paper will also outline challenges in the metrology of current defects on EUV mask blanks and metrology issues that arise with increasingly smaller defects.