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Autofocus methods and devices for lithography

Improved autofocusing (“AF”) methods and devices for lithography are provided. Some embodiments of the invention provide an AF system that includes one or more interferometers for measuring a distance to a wafer surface or a reticle surface. The invention includes methods and devices for calibrating the interferometer(s) according to known distances to a target. According to some embodiments of the invention, a spatial filter reduces the amount of undesired signal coming from the wafer or reticle surface. In some such embodiments, higher orders of diffracted light from the wafer or reticle multilayer surfaces are eliminated with a pinhole filter oriented to reject light that is not vertically directed. Other embodiments include a spatial filtering system that passes a selected diffraction order, e.g., the first order of diffracted light, from the target. These spatial filters reduce variations in the signal and make further signal processing easier. By including a beam expander, the interferometer beam can sample a larger or smaller area on the wafer. In another embodiment a polarizer will help attenuate scattered or diffracted light received by the interferometer. In some embodiments, the wavelength of the light used by the AF system is selected to maximize the absorption or reflection of this light by part of the target, e.g., by the reticle multilayer or the absorber layer.
Owner:NIKON CORP
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