Possible extreme ultraviolet mask thermal deformation during exposure

Background: Repeated exposure of extreme ultraviolet (EUV) masks to UV radiation exacerbates the thermal deformation of the constituent materials. In particular, for systems with high numerical apertures (NAs) in next-generation EUV lithography machines, new structures and materials are needed to negate the shadow and three-dimensional effects of masks. However, because masks with such materials and structures have larger absorption and thermal expansion coefficients than conventional EUV masks, they may undergo new, unidentified thermal deformations. Aim: This study aimed to evaluate the temperature accumulation and thermal deformation of EUV masks owing to radiation exposure. Approach: The temperature accumulation and thermal deformation in EUV masks were compared and analyzed using various cooling conditions and patterns as well as various materials and structures. Results: The thermal deformation increased with increasing pattern density. The results indicate that more careful control could be required regarding the pattern density and shape. Compared with conventional EUV masks, the high-NA EUV masks exhibited considerable thermomechanical deformations owing to their different multilayers and absorbers. However, the deformation could be reduced to the level of conventional EUV mask deformation through appropriate cooling. Conclusions: The thermal deformation due to exposure is intensified depending on the mask structure, material, and pattern change; however, the thermal deformation can be alleviated with proper mask cooling.