Assessing the mask clamping ability of a low thermal expansion material chuck

Abstract

The successful implementation of extreme ultraviolet lithography (EUVL) for patterning in the sub-32 nm regime will require significant improvements in image placement (IP) accuracy over the next few years. The IP error budget for the mask system is extremely stringent; consequently, the reduction or elimination of all sources of error is essential. One potential source of IP error is the effect of the residual nonflatness of the patterned surface of the mask during exposure scanning. The focus of this article is to characterize the clamping ability of a Coulombic electrostatic pin-type chuck and to assess its ability to adequately flatten an EUVL mask. The chuck was fabricated from low thermal expansion material, with a nonflatness over the pin area of approximately 74 nm. Experimental results illustrate that a highly bowed substrate (1149 and 1047 nm for the frontside and backside nonflatness, respectively) could be chucked flat to less than 100 nm. Numerical models were also used to simulate electrostatic chucking and to predict the final nonflatness of the pattern surface of the mask. Excellent agreement was found between the experimental and numerical results. In addition, the modeling tools developed here can be used to optimize the chuck design parameters and to establish the requirements on nonflatness of both the EUVL substrate and the surface of the chuck