Scanned-spot-array extreme ultraviolet imaging for high-volume maskless lithography

Abstract

High-volume maskless lithography systems typically operate by raster-scanning a large array of focused radiation spots across an exposure surface while the spot intensities are modulated. This paper outlines a design concept and optical simulation results for a spot-scanning extreme ultraviolet (EUV) system using a 10×-reduction, flat-image Schwarzschild projection system consisting of only two mirrors. The spots are generated in the system's object space by means of blazed, multilevel zone-plate microlenses configured as Schupmann achromatic doublets, which are highly efficient and are designed to nullify geometric aberration in the projection system. Coherent proximity effects are eliminated by partitioning the exposure radiation into discrete, diffraction-limited image spots, which have convergence cones of numerical aperture 0.3 at the 13.5-nm operating wavelength. The image spot separation is 2.5 μm, and the spot array covers a 10-mm square image field, sufficient to achieve printing throughput of order 30 (300-mm) wafers per hour. For simplicity, the spot intensities are all controlled by a single source modulator, allowing printing of 2.5 -μm periodic patterns without a spatial light modulator. The microlens manufacturing technology would be similar to that used for EUV mirrors and phase-shift masks, but with dramatically less stringent tolerance requirements.