Dual Ion Beam Sputtering of Carbides for EUV Reflectance
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ABSTRACT There is a need for thin optical coatings that can be produced at low temperatures and have a high reflectance at the extreme ultraviolet (EUV), , < 1200 A. Currently, the best such materials are silicon carbide (SiC) and boron carbide sputtered onto optical surfaces. The EUV reflectance of sputtered SiC, however, is not as high as that of the sputtered target materials (sintered or CVD 13-SiC). More significantly, the reflectance of both sputtered films degrades with time. In previous work the reflectance degradation of the SiC films has been quantified, and efforts to ameliorate it via ion-beam assisted deposition (IBAD) were described. Further work on sputtered SiC films is reported, including chemical and structural analysis. In addition, the degradation behavior of sputtered boron carbide is discussed. INTRODUCTION At wavelengths below 1050A, in the extreme ultraviolet (EUV), the normal-incidence reflectance of all conventional mirror coatings is low'. Polished f3silicon carbide produced by chemical vapor deposition (CVD P-SiC) has a high (>40%) normal-incidence reflectance above 600A2, making it a very promising alternative. However, the high temperatures (-900'C) needed for the CVD process prevents the use of P-SiC for coating conventional mirror components and diffraction gratings3 . Several methods have been explored for coating optical components with a SiC film at low temperatures 4,5,6,'. The method that best combines high EUV reflectance and large area coverage is ion-beam sputtered (IBS) SiC, produced by sputtering from a CVD l3-SiC target'. Reflectances of up to 80% of those of CVD 1-SiC are achievable with these films. Unfortunately, these sputtered SiC films, unlike their CVD counterparts, undergo an irreversible loss of reflectance in 1000eV, 40mA, 9 sccm Ar, ~40A, Target Si enriched. atmosphere. In previous reports, the reflectance degradation of sputtered SiC films was quantified and unsuccessful efforts to halt or slow the 12161 degradation were described8 9 . It was determined 0 that the degradation rate is exponential with time. Figure 1 shows a typical reflectance vs. log[time] 920A curve. o0 It was recently found that IBS boron carbide has a higher reflectance than fresh IBS SiC at 1 wavelengths below 600A, but is also subject to , degradation on exposure to atmosphere. 30 Specimen SCC9533 In the present work we report a more detailed characterization of IBS SiC films, as well as 101 102 101 10' 101 further efforts to stabilize these films. In addition, Minutes In Atmosphere some preliminary work on the optical degradation and attempted stabilization of sputtered boron Figure 1 carbide is presented. 503
Mat. Res. Soc. Symp. Proc. Vol. 396 ©1996 Materials Research Society
PROCEDURE
The SiC and boron carbide films (400A - 800A thick) were prepared using a modified Commonwealth 1160 ion-beam deposition chamber, which is described elsewhere8'9 . Rutherford Backscattering (RBS) was performed using 2.27 MeV 'He"+. X-ray photoelectron spectroscopy (XPS) was done on a Perkin-Elmer 5000
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