The promise of diffractive optics in maskless lithography

Abstract

For photon-based maskless lithography, a system based on the incoherent addition of scanned, multiplexed, on-axis focal spots from an array of diffractive-optical elements, such as binary-phase zone plates, has advantages over systems based on partially coherent projection of the image of a micromechanical mirror array through a large refractive or reflective lens. We have fabricated binary-phase zone-plate arrays with numerical apertures up to 0.9 with near-ideal performance, and used them to achieve high quality lithography of arbitrary patterns. Using a wavelength of 400 nm we achieve k 1 factors as low as 0.32, without the use of any resolution enhancement techniques. Sufficient contrast is achieved in dense patterns despite the presence of diffracted orders other than +1. Advantages of our diffractive-optical approach include: the feasibility of wavefront engineering by using elements other than binary-phase zone plates; the simplicity of the planar processes needed to manufacture large arrays of high-NA diffractive-optical elements; and the ease of wavelength scaling, with the promise of ultimately approaching the limits of the lithographic process.