Lithography beyond EUV with transmissive diffractive optics

In 1993, two approaches to soft-x-ray projection lithography were proposed, based on transmissive photomasks and planar arrays of transmissive diffractive Fresnel zone plates. Given the high cost of current EUV lithography systems, the problems of non-axial reflective optics and current efforts to develop free-electron lasers (FEL) for lithography, the use of transmissive diffractive microlenses and transmissive photomasks was revisited, as well as the maskless version proposed in 1996. The high intensity, collimated output and adjustable wavelength of the FEL, together with modulator concepts based on grazing incidence reflection, make maskless, soft-x-ray-based lithography attractive. The key elements of such a system, called XZPAL, are reviewed and prospects for innovations in semiconductor manufacturing presented.