Proposal of improved ZERODUR as low-thermal-expansion material for EUV photomask blanks: a comparison of thermal and mechanical properties of different materials

Low-thermal-expansion material (LTEM) is the backbone of EUV photomask blanks. However, the consideration of thermal and physical properties of LTEMs is underrepresented in the discussion of EUV photomask substrates. Reconsideration of ZERODUR® as LTEM for EUV photomasks is discussed in this contribution. We compare curvatures of the coefficient of thermal expansion (CTE) of glass ceramics, Ti-doped fused silica and ceramics with the plateau-like CTE of ZERODUR®. In addition, we present our most recent material optimization of an adapted ZERODUR® composition which further improves thermal properties and relaxation behavior of ZERODUR®. The study is backed up by experimental data and material-property simulations. Besides material properties, we show proof of the processability of ZERODUR® with inhouse super-polishing results. The presented results impose the usage of optimized ZERODUR® for future EUV photomask substrates.