21 July 2000 Comprehensive analysis of electron optical design of SCALPEL-HT/Alpha
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Abstract
This paper describes the design of the projection optics for the SCALPEL-HT/Alpha electron beam lithography tool. We first briefly review the main design requirements. We then describe the two main simulation software packages that have been used in the electron optical design -- (1) 'PROJECTION,' for optimizing the aberrations to fifth-order, including the electron lenses, deflectors, stigmators and dynamic focus coils, and (2) 'BOERSCHA,' for computing the combined effects of the aberrations and discrete and global Coulomb interactions. Recently developed key features include accurate simulation of blurring caused by plasmon losses in the SCALPEL mask, and quantitative assessment of the aberrations and distortions, by plotting through-focal series of point spread functions. A symmetric magnetic doublet lens is used, with a polepiece geometry that provides both low aberrations and telocentric (normal incidence) imaging at mask and wafer. Magnetic field clamps are used to improve the symmetry in the magnetic field of the doublet lens near the SCALPEl aperture. An image adjustment device ('Waskotron') is used to permit small adjustments of the magnification and image rotation. Deflectors, stigmators and dynamic focus coils are used to dynamically minimize the aberrations and landing angles along the stripe scan. The deflector and stigmator coils are wound on stacks of ferrite rings inside each lens, to enhance their sensitivities. The coil winding distributions are described, and we discuss how many deflectors, stigmators and focus coils are needed, and their inductances and drive currents are computed. An electrostatic deflector provides for high- bandwidth correction of small electron optical and mechanical position errors between the mask and wafer stages. The overall performance of the projection optics is predicted. For 100 keV beam energy and 15 (mu) A beam current, with a 400 mm mask-to- wafer distance, and a 0.25 mm square stub-field scanned over a 3 mm stripe at the wafer, using a silicon nitride membrane mask, the predicted 40 - 60% rise distance is d4060 equals 23 nm, the predicted 30% - 70% rise distance is d3070 equals 47 nm and the predicted 20% - 80% rise distance is d2080 equals 78 nm. These computed values are obtained at the optimum aperture angle of 6 mrad at the plane of best focus, which lies 11 micrometer beyond the Gaussian image plane.
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Xieqing Zhu, Xieqing Zhu, Eric Munro, Eric Munro, John A. Rouse, John A. Rouse, Warren K. Waskiewicz, Warren K. Waskiewicz, "Comprehensive analysis of electron optical design of SCALPEL-HT/Alpha", Proc. SPIE 3997, Emerging Lithographic Technologies IV, (21 July 2000); doi: 10.1117/12.390053; https://doi.org/10.1117/12.390053
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