Patent Number: 
Section: claims

1. A method for manufacturing a transmissive spectral purity filter configured to transmit extreme ultraviolet radiation, the method comprising:providing a semiconductor substrate having a top Si layer, a bottom Si layer and an intermediate etch stop layer in between the top Si layer and the bottom Si layer;etching a plurality of apertures in the substrate using an anisotropic etching process, the anisotropic etching process comprisingapplying a hard mask of an aperture pattern on the substrate,deep reactive ion etching the aperture pattern vertically through the top Si layer to the intermediate etch layer,removing the bottom Si layer, andremoving at least part of the etch stop layer to open up the plurality of apertures. 2. The method according to claim 1, wherein the etching creates textured sidewalls that define the apertures. 3. The method according to claim 2, further comprising depositing a metal or reflective layer on top of the substrate and depositing the metal or reflective layer on at least a part of each sidewall. 4. The method according to claim 1, further comprising removing all of the etch stop layer after having manufactured the apertures in the substrate. 5. A transmissive spectral purity filter configured to transmit extreme ultraviolet radiation, the transmissive spectral purity filter manufactured by the method according to claim 1, wherein the etching creates textured sidewalls that define the apertures. 6. The method according to claim 1, wherein the etch stop layer comprises SiO2.