Extreme ultraviolet lithography mask flatness and electrostatic chucking analysis

Abstract

The Semiconductor Industry Association Roadmap for Extreme Ultraviolet Lithography (EUVL) calls for significant improvements in the quality and flatness of the substrate and patterned mask. Due to the difficulty of meeting the mask flatness specifications, an alternative strategy has been proposed to ensure that suitable substrates will be available for EUVL. A new SEMI standard for EUVL electrostatic chucks is currently being developed. If such a standard were established, then the flatness requirements for the substrate might be relaxed to permit certain types of bow that can later be flattened by the chuck. The purpose of this study is to identify which shapes can indeed be flattened by an electrostatic chuck. To support the proposed strategy, it is essential that the clamping ability of the electrostatic chuck be characterized and well-understood. The ability of the chuck to flatten the mask will depend primarily on the mechanical stiffness of the chuck and the clamping pressure. Legendre polynomials have been identified as an effective and efficient means of representing EUVL mask surface shapes. Finite element (FE) models have been developed to utilize the Legendre polynomials as input data to define the surfaces of the reticle. The FE models were then used to determine the clamping response of the various mask shapes. In particular, the maximum reticle-to-chuck gap within the flatness quality area and over the entire reticle has been tracked as a function of clamping pressure and chuck thickness for the low-order Legendre modes.