Electrostatic chucking and EUVL mask flatness analysis

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Successful implementation of Extreme Ultraviolet Lithography (EUVL) depends on advancements in many areas, including the quality of the mask and chuck system to control image placement (IP) errors. One source of IP error is the height variations of the patterned mask surface (i.e., its nonflatness). The SEMI EUVL mask and chucking standards (SEMI P37 and SEMI P40) describe stringent requirements for the nonflatness of the mask frontside and backside, and the chucking surfaces. Understanding and characterizing the clamping ability of the electrostatic chuck and the effect on the mask flatness is therefore critical in order to meet these requirements. 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 coefficients (obtained from measured mask and chuck data) as input data to define the surfaces of the mask and the chuck. The FE models are then used to determine the clamping response of the mask and the resulting flatness of the pattern surface. The sum of the mask thickness nonuniformity and the chuck surface shape has a dominant effect on the flatness of the patterned surface after chucking. The focus of the present research is a comprehensive analysis of the flatness and interaction between the nonflat chuck and the mask. Experiments will be conducted using several sample masks chucked by a slab type electrostatic chuck. Results from the study will support and facilitate the timely development of EUVL mask/chuck systems which meet required specifications.