Assessing stability of metal tellurides as alternative photomask materials for extreme ultraviolet lithography

Tellurium (Te) is one of the elements with highest extinction coefficient κ at the 13.5 nm extreme-ultraviolet (EUV) wavelength. It is being considered as an alternative absorber material for binary photomasks in EUV lithography. The absorber material is required to remain chemically stable during EUV exposure, at elevated temperatures up to 150 °C, during mask cleaning, and in the low hydrogen pressure environment that is present in the EUV scanner. However, Te is known to react with oxygen and hydrogen, forming less EUV absorbing TeO₂ and more volatile H₂Te, respectively. Since the melting temperature of Te is only 449.5 °C at normal pressure, alloying Te with a more stable metal might result in a high κ material that will remain thermally and chemically stable over a wider range of operating conditions. In this paper, the authors report on the stability assessment of metal telluride (M-Te) alloys for the EUV absorber material. They combined Te with high κ metals, noble metals, and etchable metals. High κ and noble M-Te materials are both thermally more stable than etchable M-Te, but they cannot be patterned easily for use in an EUV photomask. High κ M-Te exhibits polycrystal morphology at room temperature compared to quasiamorphous noble M-Te though both can crystallize at a higher temperature. Hydrogen stability and cleaning solution stability of M-Te materials are improved considerably compared to Te, but their higher surface reactivity cannot be completely mitigated without the addition of an inert capping layer. Furthermore, etchable M-Te alloys are easily oxidized during deposition, resulting in lower electron density and hence lower κ. Nevertheless, M-Te alloys may be a way to stabilize Te for usage as the EUV absorber material.