Experimental characterization and physical modeling of resolution limits in proximity printing x-ray lithography

Abstract

Image resolution in x-ray proximity printing is influenced by diffraction, photo and Auger electron range, and in some instances, shot noise. In this thesis each of these issues is addressed. For the first time, 30 nm-wide lines were replicated using A1K radiation ( = 0.8 nrm). No degradation in linewidth was seen for these lines as compared with lines imaged from the same mask with either CK ( = 4.5 nm) or CUL ( = 1.3 rnm) x-rays, indicating that, contrary to previous assumptions, photoelectron range is not an obstacle to resolution in the 50 nm linewidth regime for wavelengths as short as 0.8 nm. For reasons of process latitude and ease of mask fabrication slightly longer wavelengths are preferred. A chemically amplified resist, Ray-PF, was used to pattern 100 nm-wide lines in a 1 Am-thick film with a dose of 5 mJ/cm2 ; this dose corresponds to 100 photons/pixel where the pixel height is 100 nm and the pixel area is the linewidth squared. Based on image quality, 100 photon/pixel is taken as the shot-noise-limited minimum acceptable x-ray dose; higher doses are recommended for industrial processes. The question of maximum acceptable mask-to-substrate gap was explored by shadow casting a mask pattern onto a sloped wall. The pattern consisted of isolated lines, spaces, and gratings of nominal 0.2, 0.3, 0.4 and 0.5 pm width. The smallest features, 0.17 Mm-wide spaces printed to gaps as large as 35 Am with significant image degradation becoming apparent at 18 Am. Exposures were compared to theoretical linewidth-gap plots, each of which contained a family of equi-irradiance contours of diffracted linewidth as a function of gap. Correlation was seen between image degradation and divergence of the equi-irradiance contours. A slow widening of linewidth with gap was seen; but, linewidth oscillations with changing gap, predicted by theory, were not detected in the exposures. Thesis Supervisor: Henry I. Smith Title: Professor of Electrical Fngineering and Computer Science

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