Reflectance of evaporated ruthenium films from 300 Å to 50 μm

G. Hass and W. R. Hunter Georg Hass is with U.S. Army Electronics R&D Com­mand, Night Vision Electro-Optics Laboratory, Fort Bel-voir, Virginia 22050; and W. R. Hunter is with U.S. Naval Research Laboratory, Washington D.C. 20375. Received 26 March 1981.

Ruthenium (Ru) vacuum deposited onto glass substrates forms hard corrosion-resistant coatings that show no loss in reflectance after long exposure to air. In this respect the Ru films are similar to rhodium (Rh) films1,2 which are frequently used as front surface mirrors. Ru coatings can be deposited on substrates fairly easily using a high-powered electron gun to vaporize the metal. At substrate distances of 35 cm, de­position rates of up to 80 Å/sec have been achieved. The re-

2334 APPLIED OPTICS / Vol. 20, No. 14 / 15 July 1981

Fig. 1. Normal incidence reflectance spectrum of evaporated Ru from 300 Å to 50 μm.

flectance and optical constants of evaporated Ru in the VUV (300-2200 Å) have been published,3 but no reflectance values for the UV, visible, and IR regions have been reported for vacuum deposited films. Of the measurements made using single crystal Ru,4-6 those of Weaver et al.6 agree closely with the present values in the wavelength range where overlapping occurs. This Letter combines data from the VUV studies3

with reflectance values at longer wavelengths to present the normal incidence reflectance spectrum of evaporated Ru coatings over an extended wavelength range from 300 Å to 50 μm.

The Ru films were prepared by evaporation with an electron gun capable of supplying a beam current of 1 Å at 10 kV, al­though the actual power required was 2-4 kW, depending on the deposition rate desired. Vacuum-melted and outgassed Ru of 99.93% purity was placed in a dimple in a heavy water-cooled copper block and bombarded with electrons for the evaporation. The vacuum evaporator was an ion-titanium-pumped system, and the pressures during evaporation ranged from 10 - 7 to 10 - 6 Torr. The substrates were of superpolished fused silica7 maintained at a temperature of ~300° C during the evaporation to improve the adherence of the Ru films to the substrate.

The technique used to measure the reflectance of the Ru films in the VUV has been described.3 For the longer wave­lengths, 2000 Å to 50 μm, the reflectance was measured at close to normal incidence with Perkin-Elmer models 350 and 621 double-beam spectrophotometers equipped with specular reflectance attachments. The reflectance data obtained in this manner were relative to freshly evaporated aluminum and rhodium standards of well-known reflectance8-10 and may be considered accurate to ±0.3%. Additional reflectance values in the visible and IR were determined with a high-precision Strong-type reflectometer.11

Figure 1 shows the normal incidence specular reflectance

Table I. Comparison of the Reflectances of Evaporated Films of Rh and Ru from 0.22 to 15 μm

spectrum of vacuum-deposited Ru films from 300 Å to 50 μm. There are two maxima in the spectrum: the first at about 600 Å with a reflectance value of 24% and the second at about 4000 Å with a reflectance value of 74%. For wavelengths longer than 1.1 μm the reflectance increases rapidly to a value of 93.5% at 3 μm and ever more slowly to a value of 97.8% at 50 μm.

Of all the platinum metals, evaporated Rh is the only one used as a front surface mirror in the visible and IR because it has a rather high reflectance in these regions and is very hard and corrosion-resistant.2 We had hoped that films prepared with Ru, which is closely related to Rh, would have the same or even higher reflectance than Rh films. This hope proved not to be true as shown in Table I which lists the reflectance values of Ru and Rh10 in the wavelength region from 0.2 to 15 μm.

References 1. J. K. Coulter, G. Hass, and J. B. Ramsey, Jr., J. Opt. Soc. Am. 63,

1149 (1973). 2. J. T. Cox, G. Hass, and W. R. Hunter, J. Opt. Soc. Am. 61, 360

(1971). 3. J. T. Cox, G. Hass, J. B. Ramsey, and W. R. Hunter, J. Opt. Soc.

Am. 64, 423 (1974). 4. J. J. Carroll and A. J. Melmed, J. Opt. Soc. Am. 66, 1050

(1976). 5. M. M. Kirillova, L. V. Nomerovannaya, M. M. Noskov, N. B.

Gorina, V. P. Polyakova, and E. M. Savitskii, Sov. Phys. Solid State 20, 994 (1978) [Fiz. Tverd. Tela. Leningrad 20, 1718 (1978)].

6. J. H. Weaver, C. G. Olson, and D. W. Lynch, unpublished data, Synchrotron Radiation Center, U. Wisconsin, Stoughton 53589. Note that this information and the results for other metals are to be published; details are given J. H. Weaver, C. Krafka, D. W. Lynch, and E. E. Koch, Appl. Opt. 20, 1124 (1981).

7. R. W. Dietz and J. M. Bennett, Appl. Opt. 5, 881 (1966). 8. H. E. Bennett, M. Silver, and E. J. Ashley, J. Opt. Soc. Am. 53,

1089 (1963). 9. E. T. Hutcheson, G. Hass, and J. K. Coulter, Opt. Commun. 3, 213

(1971). 10. G. Hass, in Applied Optics and Optical Engineering, Vol. 3, R.

Kingslake, Ed. (Academic, New York, 1965), pp. 309-327. 11. H. E. Bennett and W. F. Koehler, J. Opt. Soc. Am. 50, 1

(1960).

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