Actinic, spot-scanning microscope for EUV mask inspection and metrology
View Patent ↗An actinic, through-pellicle EUV mask inspection or metrology system acquires image information by scanning an array of focused illumination spots across a photomask and detecting the mask reflectance signal from each spot in synchronization with the scan motion. The radiation from each spot is detected by a detector comprising four quadrant sensors to provide information on the angular reflectance distribution, which is sensitive to the reflectance phase. The focal spots are generated from achromatic EUV microlenses (phase-Fresnel, Schupmann doublets), enabling the use of a high-bandwidth, high-power, laser-produced-plasma EUV source for high-throughput operation. The microlens foci are projected through illumination optics (EUV mirrors) onto the focal spots at the mask, and the microlenses nullify the illumination optics' geometric aberrations for substantially aberration-free point imaging. Aberration-correcting micro-optics may also be used for the collection optics between the mask and the detector array.
1. A spot-scanning microscope comprising illumination optics, collection optics, an array of optical detectors, and a scanning mechanism, wherein:
the illumination optics focus each of a plurality of discrete radiation beams onto a corresponding focal spot on or proximate an inspection surface,
the radiation beams transmit through, or reflect from, the inspection surface, and are conveyed by the collection optics to the detector array, whereby each detector senses optical radiation from a corresponding radiation beam,
each radiation beam has an angular radiance distribution after transmitting through or reflecting from the inspection sample,
each detector comprises a plurality of optical sensor elements, which sense different portions of the corresponding radiation beam's angular radiance distribution,
the scanning mechanism establishes relative motion between the focal spots and the inspection surface so that the paths traversed by the focal spots relative to the inspection surface comprise a set of scan lines on the surface, and
the detectors sense the radiation in synchronization with the scan motion to synthesize a high-resolution raster image of the inspection surface, or to optically measure characteristics of the inspection surface.
2. The spot-scanning microscope of claim 1 further comprising a radiation source and spot-generation optics, wherein the spot-generation optics partition radiation from the radiation source into the discrete radiation beams.
3. The spot-scanning microscope of claim 2 wherein the spot-generation optics comprise microlenses, and each radiation beam traverses and is formed by a corresponding microlens.
4. The spot-scanning microscope of claim 3 wherein the microlenses are configured to neutralize geometric aberrations of the collection optics at the focal spots.
5. The spot-scanning microscope of claim 3 wherein the microlenses are achromatic Schupmann doublets.
6. The spot-scanning microscope of claim 5 wherein the radiation source is a laser-produced plasma, and the Schupmann doublets comprise phase-Fresnel lens elements.
7. The spot-scanning microscope of claim 6 wherein the radiation is 13.5-nm EUV radiation.