Durability of Ru-based EUV masks and the improvement

In EUV Lithography, an absence of promising candidate of EUV pellicle demands new requirements of EUV mask cleaning which satisfy the cleaning durability and removal efficiency of the various contaminations from accumulated EUV exposure. It is known that the cleaning with UV radiation is effective method of variety of contaminants from surface, while it reduces durability of Ru capping layer. To meet the expectation of EUV mask lifetime, it is essential to understand the mechanism of Ru damage. In this paper, we investigate dominant source of Ru damage using cleaning method with UV radiation. Based on the mechanism, we investigate several candidates of capping to increase the tolerance from the cycled UV cleaning. In addition, we study durability difference depending on the deposition method of Ru capping. From these studies, it enables to suggest proper capping material, stack and cleaning process.