Degradation in Euv Reflectance of Ion-Sputtered SiC Films
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DAN SCHWARCZ AND RITVA A.M. KESKI-KUHA NASA Goddard Space Flight Center, Code 717. 1, Greenbelt, MD 20771
ABSTRACT Silicon Carbide (SiC) formed by chemical vapor deposition (CVD) has the highest reflectivity in the extreme ultraviolet (EUV) of any currently used optical material. The high temperature required for the CVD process, however, limits its suitability for coating optical components. To address this problem thin films have been sputtered onto optical surfaces from CVD P3-SiC targets. These films, while having reflectivity lower than that of CVD SiC, are nonetheless the best coatings available for reflectance in the spectral region below 1000A. While the initial properties are good, the EUV reflectivity degrades with time after deposition. A relative decrease of about 25% is evident in the reflectivity at 920A after 2.5 years, and about 85% of this change occurs in the first three months. In fact, a decrease is observed in the minutes following deposition. In this study the degradation is characterized and a mechanism is proposed. Efforts underway to reduce or eliminate the degradation are discussed.
INTRODUCTION Advances in astrophysics often rely on the development of new instrumental techniques which give astronomers access to unexplored parts of the electromagnetic spectrum. Of particular interest is the extreme ultraviolet (EUV), in which the peak radiances of hot interstellar plasmas and stellar atmospheres lie, as well as important spectral lines of molecular hydrogen, deuterium,
and oxygen VI. Aluminum coated with magnesium fluoride (MgF 2) or lithium fluoride (LiF) provides high reflectance coatings for optical systems down to wavelengths of 1150A and 1050A, respectively. Below 1050A, in the EUV, the normal-incidence reflectance of all conventional mirror coatings is low'. Severe constraints are imposed on the design of normal-incidence, EUV space-based instruments due to this low reflectance. Instruments that overcome this limitation by using glancing-incidence optics are much more difficult and expensive to fabricate than those that employ normal-incidence optics. Choyke et a12 found that polished silicon carbide (SiC) formed by chemical vapor deposition (CVD) demonstrates a high (>40%) normal-incidence reflectance above 600A. However, the high temperatures (-.900'C) needed for the CVD process prevents the use of CVD SiC for coating conventional mirror components and diffraction gratings3 . High reflectance thin film (200-300A) SiC coatings have been produced by a lowtemperature (
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