John F. Osantowski

Reflectance of Aluminum Overcoated with MgF 2 and LiF in the Wavelength Region from 1600 Å to 300 Å at Various Angles of Incidence

Since the discovery that Al overcoated with MgF(2) or LiF produces high reflectances to wavelengths as short as 1150 A and 1000 A, respectively, these coatings have been used extensively in vacuum ultraviolet instruments in the wavelength region where their reflectance is high. If the instrument is intended to cover wavelengths shorter than the two given above, usually either Pt or Ir is used, with a loss of speed at the longer wavelengths. This paper presents reflectance data showing that fluoride-overcoated Al can be useful to wavelengths as short as 500 A. Measurements were made from 1600 A to about 300 A at normal, 35 degrees , and 85 degrees angles of incidence, angles used in normal, Seya, and grazing incidence spectrometers, respectively. These measurements show that from the boundary of the high reflectance region to 500 A, the reflectance at normal and 35 degrees depends on the thickness of the fluoride coating and can be as high as 24% at 800 A for a MgF(2) thickness of 150 A. For shorter wavelengths, the reflectance shows a decreasing thickness dependence and at 304 A is very low-about 1%. At grazing incidence, the reflectance shows some thickness dependence from 1500 A to about 1000 A, but toward shorter wavelengths the dependence disappears and the reflectance increases slowly to about 80% at 500 A. In addition to the reflectance measurements, polarization effects are discussed.

Reflectance and durability of Ag mirrors coated with thin layers of Al 2 O 3 plus reactively deposited silicon oxide

A technique for producing adherent and well-protected front surface Ag mirrors with an evaporated Al(2)O(3) + SiO(x) overcoat and an evaporated Al(2)O(3) underlayer is described. A thin Al(2)O(3) layer promotes adhesion between the Ag film and its substrate. The optimum thickness of each outer layer necessary to give good adhesion and protection to the Ag surface with minimal loss in reflectance due to ir absorption was found to be about 300 A for the Al(2)O(3) layer and between 1000 A and 2000 A for the SiO(x) film. Ag surfaces coated in this fashion retained a normal incidence reflectance in excess of 95% over the wavelength region from 450 nm to the far ir, even when exposed to harsh sulfide and humidity environments. Calculations demonstrate the advantage of using protected front surface Ag in comparison to more durable metal reflectors, such as Al or Rh, in terms of high reflectance and low visible region polarization. However, a very pronouncedrestrahlen reflectance and absorption effect in the thicker SiO(x) film is responsible for a calculated drop in reflectance from 98.5% to about 65% at lambda = 8.1 mum and 45 degrees incidence for the Ag + Al(2)O(3) + SiO(x) coating.

Normal incidence reflectance of ion beam deposited SiC films in the EUV

Results are presented from an experimental investigation of the normal-incidence reflectance at 58.4, 92.0, and 121.6 nm wavelength of 30- and 80-nm-thick SiC films produced by ion-beam deposition on unheated 5 x 5-cm microscope slides. The films were deposited in the 2-m evaporator described by Bradford et al. (1969) with chamber base pressure 1 microtorr, operating pressure 40 microtorr, and a 50-62-mA 750-eV Ar ion beam; the reflectance measurements were obtained in the reflector-monochromator system described by Osantowski (1974). Reflectances of over 30 percent were found at 92 and 121.6 nm, almost equal to those of polished CVD films of SiC and degrading only slightly after aging for 4 months. It is suggested that ion-beam deposition may be the best low-temperature technique for coating EUV optics for space astronomy.

Reflectance and optical constants for Cer-Vit from 250 to 1050 A

The reflectance for a bowl-feed polished Cer-Vit sample was measured at nine wavelengths and five angles of incidence from 15° to 85°. Optical constants were derived by the reflectance-vs-angle-of-incidence method and compared to previously reported values for ultra-low-expansion fused silica (ULE) and several other glasses. Surface-roughness corrections of the reflectance data and optical constants are discussed.

Preparation of Mirror Coatings for the Vacuum Ultraviolet in a 2-m Evaporator

The design and features of a 2-m evaporator suitable for coating large mirrors uniformly with Al + MgF(2) and Al + LiF films of high reflectance in the vacuum uv are described. The techniques used for monitoring film thicknesses during the film deposition and for producing films of uniform thicknesses over large areas are discussed. It is shown that the Al films for MgF(2)_ and LiF-protected mirrors of highest reflectance in the vacuum uv down to 1000 A should be 700-800 A thick. Data on the vacuum uv reflectance of Al coated with MgF(2) films of various thicknesses are presented. It was found that mirror coatings prepared in a large evaporator have a higher reflectance in the vacuum uv than those deposited under the same vacuum and deposition conditions in a small vacuum unit. At lambda = 1216 A, the reflectance of Al overcoated with 250 A of MgF(2) was measured to be about 85%.

Optical coating technology for the EUV

Abstract Advances in optical coating and materials technology have been one of the key motivators for the development of missions such as the Far Ultraviolet Spectroscopic Explorer recently selected by NASA for an Explorer class mission in the mid 1990s. We review the performance of a range of candidate coatings for normal incidence and glancing incidence applications, and discuss strengths and problem areas for their use in space. The importance of recent developments in multilayer films, chemical vapor deposited SiC (CVD-SiC) mirrors, and SiC films are discussed in the context of EUV instrumentation design. For example, the choice of optical coatings is a design driver for the selection of the average glancing angle for the FUSE telescope, and impacts efficiency, short wavelength cut-off, and physical size.

Low earth orbit environmental effects on osmium and related optical thin-film coatings

A number of samples of optical thin film materials were flown on Shuttle flight STS-8 as part of an experiment to evaluate their interaction with residual atomic oxygen in low earth orbit. Osmium was selected because of its usefulness as a reflective optical coating for far-UV instruments and for confirmation of results from previous Shuttle flights in which such coatings disappeared. Reflectance data and photographic evidence are presented to support the hypothesis that the osmium disappearance is due to reaction with oxygen to form a volatile oxide. Platinum and iridium, which were included for comparison, fared much better.

Extreme ultraviolet optical properties of two SiO 2 based low-expansion materials

Reflectances of two low-expansion materials, a recrystallized glass ceramic and a high silica glass, have been measured at five angles of incidence from 15° to 85° in the wavelength region from 80 to 310 A and in some cases up to 1050 A. Optical constants are derived and silicon core-level transitions analyzed.

Grazing incidence reflectance of SiC films produced by plasma-assisted chemical vapor deposition

The grazing incidence reflectance of silicon carbide films produced by plasma-assisted chemical vapor deposition has been evaluated in the spectral region from 256 to 1216 Å. The results show that reflectivities higher than conventional coatings can be obtained on coatings deposited both on silicon wafers and quartz substrates. Potential application of silicon carbide films for EUV astronomical instruments will be discussed.

Auger spectroscopic examination of MgF 2 -coated Al mirrors before and after uv irradiation

Freshly prepared samples of evaporated Al + 250 A of MgF(2) on glass were subjected to analysis by Auger electron spectroscopy coupled with surface erosion by Ar+ ion bombardment before and after uv irradiation. The analysis identified C and 0 on the mirror surfaces prior to irradiation and the addition of Si after uv irradiation in vacuum. The relative amounts of photolyzed surface contaminants were proportional to the observed decrease in mirror reflectance at 1216 A. The Auger analysis confirmed that a very thin layer of surface contamination and not bulk photolysis in the MgF(2) film was responsible for the irradiation induced reflectance loss.