Ultra High-Precision Wavefront Metrology Using EUV Low Brightness Source

Abstract

Extreme ultraviolet (EUV, λ=13.5 nm) lithography is expected as the next generation lithography to the ArF immersion lithograph (λ =193 nm ). Projection optics is composed of several aspheric mirrors and its aberration should be about 1/30λ rms (0.45 nm rms) or smaller. In order to fabricate such accurate optics, very high precision metrology tool with the sensitivity of 0.1 nm rms or higher is required. In addition, after completion, aberration may change because of the environmental change or the aged deterioration. So the aberration must be monitored at intervals on the exposure apparatus. As an accurate metrology tool using EUV source, point diffraction interferometer (PDI) or other several tools have been proposed [1]-[4]. In these interferometers, an approximately spherical wavefront emerging from the pinhole is used as a reference surface. Because the pinhole size is usually about 100 nm or less, the radiant intensity drastically decreased through the pinhole, so a very high brightness source like a synchrotron radiation source (SR) is required. However, such a large-scale equipment is not suitable for on-machine sensing. Plasma-induced EUV sources are widely used for various purposes as the compact source. This source has low brightness that is 10− 6 to 10− 7 lower than that of SR, and the real-time monitoring is impossible. We developed a new wavefront metrology tool using such a low brightness compact EUV source.