Patent Number: 
Section: claims

1. A spectral purity filter, comprising:a base having a thickness defined by two opposing major surfaces with a first array of apertures formed therein, the first array of apertures extending through the thickness of the base and defining a sidewall within each aperture; anda coating provided on the sidewalls of the first array of apertures, wherein the first array of apertures are configured to diffract a first wavelength of radiation and to allow at least a portion of a second wavelength of radiation to be transmitted, the second wavelength of radiation being shorter than the first wavelength of radiation,wherein the base further comprises a second array of apertures, each aperture of the second array of apertures having a diameter less than the first wavelength. 2. The spectral purity filter of claim 1, wherein the coating is arranged to absorb at least a portion of the first wavelength of radiation. 3. The spectral purity filter of claim 1, wherein the coating is arranged to inhibit reflection of at least a portion of the first wavelength of radiation. 4. The spectral purity filter of claim 1, wherein the coating is arranged to promote reflection of at least a portion of the second wavelength. 5. The spectral purity filter of claim 1, wherein the coating is arranged to inhibit degradation or environmental damage to the first array of apertures. 6. The spectral purity filter of claim 1, wherein the first array of apertures each have a diameter greater than 20 μm. 7. The spectral purity filter of claim 1, wherein the diameter is less than half of the first wavelength. 8. The spectral purity filter of claim 7, wherein the diameter is greater than the second wavelength. 9. The spectral purity filter of claim 1, wherein the first array of apertures have a periodicity such that the first diffraction order of the first wavelength of radiation is substantially separated from the zero diffraction order. 10. The spectral purity filter of claim 1, wherein the base comprises at least one of a plate, a mesh, and a wire grid. 11. The spectral purity filter of claim 1, wherein the first array of apertures or the second array of apertures are circular apertures arranged in a periodic array. 12. The spectral purity filter of claim 11, wherein the periodic array comprises a hexagonal pattern array. 13. The spectral purity filter of claim 1, wherein the first array of apertures are elongated slots arranged in a linear array.