EUV light source with spectral purity filter and power recycling

摘要

A plasma-generated EUV light source uses an EUV-diffracting collection mirror to channel spectrally pure in-band radiation through an intermediate-focus aperture and through EUV illumination optics. Out-of-band radiation is either undiffracted by the collection mirror or is diffractively scattered away from the aperture. The undiffracted portion, plus plasma-emitted radiation that does not intercept the collection mirror, can be efficiently recycled back to the plasma via retroreflecting mirrors, cat's-eye reflectors, or corner-cube reflectors, to enhance generation of in-band EUV radiation by the plasma.

主权项

1. An imaging system comprising:
apparatus for generating an ionized plasma from a laser-irradiated target, a collection mirror comprising a reflective diffraction grating, an intermediate-focus aperture, illumination optics, projection optics, an object surface, and an image surface wherein: the plasma generates in-band radiation comprising a range of emission wavelengths and out-of-band radiation comprising wavelengths greater than the in-band wavelengths; the collection mirror collects and reflects a portion of the in-band and out-of-band radiation from the plasma; the collection mirror is configured to reflectively direct undiffracted, zero-order radiation outside of the intermediate-focus aperture; the diffraction grating is either (a) a conformal-multilayer grating comprising a substrate having a phase-Fresnel surface-relief structure, and a multilayer, reflective film stack deposited conformally on the phase-Fresnel structure, or (b) a patterned-multilayer grating comprising a smooth substrate and a multilayer, reflective film stack deposited on the substrate, with a phase-Fresnel surface-relief structure patterned in the film stack; the diffraction grating is blazed for optimal diffraction efficiency of the in-band wavelengths, and is configured to diffractively reflect the collected in-band radiation toward the intermediate-focus aperture, whereby all or most of the collected in-band radiation transmits through the intermediate-focus aperture; the grating has a sufficiently short period to achieve separation of the diffracted in-band radiation and undiffracted out-of-band radiation at the intermediate-focus aperture; all or most of the out-of-band radiation is either not collected by the collection mirror, or is undiffracted by the grating, or is diffractively scattered outside of the intermediate-focus aperture, and in each case is thereby excluded from the intermediate-focus aperture; the collection mirror and illumination optics are disposed on opposite sides of the intermediate-focus aperture; the in-band radiation that transmits through the intermediate-focus aperture is conveyed by the illumination optics to illuminate a reflective photomask at the object surface; and the projection optics images the object surface onto the image surface.