The promise of diffractive optics in maskless lithography
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 k1 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.
diffractive optics maskless lithography blazed zone plate