Grid spectral purity filters for suppression of infrared radiation in laser-produced plasma EUV sources

摘要

We have developed a grid-type spectral purity filter (SPF) for suppression of infrared radiation in laser-produced plasma (LPP) EUV sources for high-volume EUV lithography. The SPF is a silicon grid with sub-wavelength periodicity that is metalized to make it reflective for infrared radiation. EUV radiation is transmitted geometrically through the open area of the grid. The first prototype samples show an in-band EUV transmittance of 74% at normal incidence. Infrared spectrometry exhibits a clear cut-off behavior as expected, with a transmittance of <0.1% at a wavelength of 10.6 μm. In a first power load test, a grid was exposed to a CO_2 laser at 100 W/cm~2 in vacuum for 8 hours. Another grid was kept at 800 °C in a vacuum oven for 24 hours. Both grids remained structurally intact and maintained an infrared transmittance of<0.1%.