Seima Kato

EUV wavefront measurement of six-mirror optics using EWMS

The wavefront measurements have been performed with the EUV Wavefront Metrology System (EWMS) for the first time using a prototype projection optic as a test optic. The wavefronts of the test optic was measured at the five positions in the exposure field with the Digital Talbot Interferometer (DTI). The RMS magnitude of the wavefront errors ranged from 0.71 λ (9.58 nm) to 1.67 λ (22.75 nm). The results obtained with the DTI were compared to those with the Cross Grating Lateral Shearing Interferometer (CGLSI). As a result of a repeatability assessment, it was found that the EWMS can stably measure the wavefronts of the test optic. Additionally, unwrapping of the phase map was found to be related to the precision of the measurement.

Wave-front errors of reference spherical waves in high-numerical aperture point diffraction interferometers

In phase-shifting point diffraction interferometry (PS/PDI), a pinhole with a diameter of 34 nm is necessary to measure a wave-front aberration of extreme ultraviolet projection optics of numerical aperture (NA) 0.20. However, it is extremely difficult to process such a small pinhole, and light transmission through the pinhole becomes too low. Here, the diameter of a pinhole is optimized, together with the thickness of a Ta membrane, for a converging wave of NA 0.20 with no aberration so that the difference between a wave front produced by the pinhole and that of a spherical wave is minimized. On that condition, the optimum values are a diameter of 50 nm and a thickness of 200 nm. For these values, behaviors are examined in real cases, including focal point shifts and aberrations with incident light. Astigmatism in the aberrations has the most impact on a wave-front error, and a 0°–90° astigmatism (Z5) coefficient in the FRINGE Zernike polynomials of test optics is required to be less than 50 mλ to use th...

Comparisons between EUV at-wavelength metrological methods

Comparisons between several at-wavelength metrological methods are reported. The comparisons are performed by measuring one test optic with several kinds of measurement methods from the viewpoints of accuracy, precision and practicality. According to our investigation, we found that the PDI, the LDI, and the CGLSI are the most suitable methods for evaluating optics for EUV lithography.

Ultra High-Precision Wavefront Metrology Using EUV Low Brightness Source

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.