Current status of EUV mask blanks and LTEM substrates defectivity and cleaning of blanks exposed in EUV ADT

The defectivity of EUV mask blanks remains as one of the key challenges in EUV lithography. Mask blank defects are a combination of defects or particles added on the substrate, added during MoSi multilayer deposition, and during subsequent handling. A recent upgrade to the Lasertec M7360 at SEMATECH has enabled us to detect new defects (sub-30 nm SEVD (Sphere Equivalent Volume Diameter)) on the substrate that were not previously detectable. In this paper, we report our recent investigation of defects on low thermal expansion material (LTEM) substrates and their creation and removal. Data obtained with atomic force microscope (AFM) imaging of defect topography, scanning electron microscope/energy-dispersive spectroscopy (SEM/EDS), and Auger characterization of defect composition is also discussed. Cleaning of mask particles which may have been added by handling in a clean room environment with the ASML Alpha Demo Tool (ADT) with and without static EUV exposure is discussed. Particle contamination on the backside of EUV masks can potentially impact overlay or focus during exposure. We have developed cleaning processes capable of removing backside defects without contaminating the front side of the masks. Backside defects are characterized by AFM, SEM/EDS, and auger microscopy and their topography and composition are presented.