Focus latitude enhancement of symmetrical phase mask design for deep submicron contact hole patterning

Abstract

The mechanism of focus latitude enhancement for contact/via hole printing is explained by approximating the axis intensity distribution of an image as a series of cosine functions to characterize the interference between each pair of diffraction beams. It is found that a phase-shifting mask (PSM) with symmetrical assist features improves the depth of focus (DOF) by introducing destructive interference to counterbalance the intensity fluctuation from constructive interference as defocus. A simple formula was derived to represent the capability of focus latitude enlargement. It shows that the extent of enhancement depends on the exposure wavelength and numerical aperture of a projection lens only. Increasing the degree of partial coherence degrades the focal range enlargement because a larger illumination angle elongates the destructive interference pattern in the optical-axis direction to weaken its ability for intensity compensation. On the other hand, the lack of constructive interference in dense hole imaging fails the mask pattern transfer, which limits the application of the phase-shifting method to pattern pitch greater than root2lambda/NA. A tiny amount of spherical aberration results in prominent asymmetrical defocus behavior because the wave deformation in the projection lens shifts the distribution of constructive and destructive interference patterns to opposite defocus directions. The printing characteristics of 0.17 mum contact using an 18% transmission, rim-type attenuated phase-shifting mask are investigated to corroborate our analysis of defocus behavior. The dependence of depth of focus on pattern duty is stressed to elucidate the difference in mechanisms of focus latitude improvements for a sparse hole and periodic dense hole. (C) 2001 American Vacuum Society.