The goal of creating lithographic-quality EUV imaging systems has pushed researchers to develop the most accurate wavefront aberration measurement techniques ever created. During the development of EUV lithography (EUVL) technology, at-wavelength optical testing has proven valuable as a tool for understanding chromatic aberrations and multilayer (ML) influences on performance, and also as a highly deterministic independent measurement for comparison with visible-light interferometry. Because of the perceived trade-offs among complexity, cost, and necessity, no universal conclusion has yet been reached on the role at-wavelength testing will play as EUVL transitions to production. EUV interferometric measurements have achieved accuracy levels of 0.4â1.0 Ã
rms for primary aberration terms (verified by lithographic imaging), with state-of-the-art visible-light testing methods only slightly behind. Measurement precision levels have been higher still, with researchers claiming uncertainty levels on the order of 0.05 Ã
rms or below. But precision and accuracy are separate concerns: precision relies only on the measurement stability, while accuracy requires that the measurements be correct. Despite the substantial effort needed to achieve these levels, these techniques are not widely practiced, and it is generally acknowledged that some progress is still needed to meet the demands of commercial lithography tools.
Over the past several decades, interferometry has become the cornerstone measurement method in the development of high-accuracy, diffraction-limited optical systems. With countless varieties and variations in its implementation, interferometry refers to the class of wavefront measurement techniques that rely on the ability of coherent light to interfere and produce measurable intensity fluctuations. When well-controlled coherent light is passed through a test lens, it can be made to interfere with delayed, displaced, or filtered versions of the same wave packet in such a manner that an optical system's wavefront aberrations or wavefront slope errors can be revealed through an interference pattern, or interferogram. In this way, the light wavelength becomes the fundamental unit of measurement. In modern interferometers, the interferograms are recorded digitally on a light-sensitive charge-coupled device (CCD) camera, and many individual interferograms can be combined to form a single measurement.
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