Interface engineered ultrashort period Cr-Ti multilayers as high reflectance mirrors and polarizers for soft x rays of lambda = 2.74 nm wavelength

Cr-Ti multilayers with ultrashort periods of 1.39-2.04nm have been grown for the first time as highly reflective, soft-x-ray multilayer, near-normal incidence mirrors for transition radiation and Cherenkov radiation x-ray sources based on the Ti-2p absorption edge at E=452eV (lambda=2.74nm). Hard, as well as soft, x-ay reflectivity and transmission electron microscopy were used to characterize the nanostructure of the mirrors. To achieve minimal accumulated roughness, improved interface flatness, and to avoid intermixing at the interfaces, each individual layer was engineered by use of a two-stage ion assistance process during magnetron sputter deposition: The first 0.3nm of each Ti and Cr layer was grown without ion assistance, and the remaining 0.39-0.72nm of the layers were grown with high ion-neutral flux ratios Phi(PhiTi=3.3,PhiCr=2.2) and a low energy Eion (ETi=23.7andECr=21.2), ion assistance. A maximum soft-x-ray reflectivity of R=2.1% at near-normal incidence (∼78.8 deg.) was achieved for a multilayer mirror containing 100 bilayers with a modulation period of 1.379nm and a layer thickness ratio of Gamma=0.5. For a polarizing multilayer mirror with 150 bilayers designed for operation at the Brewster angle, 45 deg., an extinction ratio, Rs/Rp, of 266 was achieved with an absolute reflectivity of R=4.3%