Development of projection optics set-3 for high-numerical-aperture extreme ultraviolet exposure tool (HiNA)

Abstract

We have developed a high-numerical-aperture extreme ultraviolet exposure tool (HiNA). HiNA is equipped with an illumination system, projection optics, a mask stage, and a wafer stage in the vacuum chamber. The projection optics consist of two aspherical mirrors (M1 and M2). The numerical aperture of the optics is 0.3. Thus far, we fabricated two sets of projection optics (set-1 and set-2). The wave-front errors of set-1 and set-2 were 7.5 and 1.9nm rms, respectively. We developed a third set of projection optics (set-3), the target wave-front error of which was less than 1nm rms. In set-3, we also attempted to reduce flare. We completed the mirror polishing, coating, and mirror adjustment of set-3. By using a recently developed polishing method, we reduced low-spatial-frequency roughness (LSFR), mid-spatial-frequency roughness (MSFR), and high-spatial-frequency roughness, simultaneously. The predicted wave-front error calculated from the LSFR number was 0.69nm rms. MSFR, which strongly affects the flare of the optics, was significantly reduced to less than 0.2nm rms. The estimated flare was 7%, which is significantly reduced to one-forth that of set-2. The wave-front error of set-3 was measured with the visible-light point diffraction interferometer after coating and assembly. The wave-front error measured after adjustment was 0.85nm rms, which is less than one-half the wave-front error of set-2. Set-3 will be completed after the cramping of the adjustment system, which is performed to prevent mirror displacement during transportation.