Patent US 6,133,576

Broad spectrum ultraviolet inspection methods employ an achromatic catadioptric system to image the surface of an object, such as a semiconductor wafer or photomask, at multiple ultraviolet (UV) wavelengths over a large flat field (with a size on the order of 0.5 mm) in order to detect and identify defects. The imaging system provides broad band correction of primary and residual, longitudinal and lateral, chromatic aberrations for wavelengths extending into the deep UV. UV imaging applications include a method that illuminates an object with fluorescence-excitation radiation to stimulate fluorescent emission at a plurality of UV wavelengths, then images the fluorescent emissions and detects the images so formed in UV wavelength bands distributed over at least 50 nm (preferably 100-200 nm) wavelength. Photoresist patterns can be analyzed in this way. Another method uses multi-wavelength UV illumination and imaging to inspect photoresists, patterned wafers, phase-shift photomasks and the like based on the varying response to different UV wavelengths (such as wavelength-dependent reflectivities) of different materials. Yet another method takes advantage of the small depth of focus of imaging systems at UV wavelengths to generate image slices at various depths, such as on patterned wafers with nonplanar surface profiles, and at different wavelengths. The slices can be integrated to produce a composite 3-D UV-color image.