Thermally stable thin-film filters for high-power extreme-ultraviolet applications

We investigated several types of thin-film filters for high intensity work in the extreme-ultraviolet (EUV) spectral range. In our application, with a peak EUV intensity of 2.7 W cm −2 , Ni-mesh-backed Zr filters have a typical lifetime of 20 hours, at which point they suffer from pinholes and a 50 % loss of transmission. Initial trials with Si filters on Ni meshes resulted in rupture of the filters in less than an hour. A simple thermal calculation showed that the temperature rise in those filters to be about 634 K. A similar calculation indicated that using a finer mesh with thicker wires and made of Cu reduces the temperature increase to about 60 K. We have exposed a Si filter backed by such a mesh for more than 60 hours with little loss of transmission and no leaks.