Aerial image measurement methods for fast aberration set-up and illumination pupil verification

As feature size decreases, factors like lens aberrations and pupil illumination become increasingly important for the lithographer. Both factors can be quantified using the Transmission Image Sensor (TIS), a built-in measurement device on ASML Step & Scan systems. Advantages of the TIS include robustness and speed, because it is a straightforward measurement technique that does not involve exposure of resist. To allow for on-site minimization of lens aberrations, new lens types will incorporate additional manipulators. Consequently, fast and accurate in-situ measurement of aberrations such as coma and spherical is required. This is achieved by measuring the position of the aerial image at multiple illumination settings, using the flexibility of modern scanners to automatically set the numerical aperture and partial coherence. The accuracy of this technique, proven by strong correlation with interferometric and lithographic data, makes it suitable for system optimization. Regarding measurements of pupil illumination, the new TIS-based technique is compared with the conventional resist-based method. The latter images a defocused pinhole in resist, and is ideal for visual inspection. For the new method, the TIS is scanned in a defocused lateral plane to record the far field image of the pinhole. Quantitative results like pupil uniformity, ellipticity and sigma accuracy are easily obtained.