Thin Solid Films, 167 (1988) 291-297 GENERAL FILM BEHAVIOUR
HIGH EFFICIENCY REFLECTORS* M. VISWANATHAN,
291
FRONT AND REAR SURFACE
SILVER
C. L. NAGJTNDRA AND G. K. M. THUTUPALLI
Thin Film Laboratories, Indian Space Research Organisation. ISRO Satellite Centre, Airport Road, Bangalore 560017 (India) (Received December 7,1987)
Experimental studies relating to the development of front and rear surface silver mirrors have been carried out. The reflectance of these mirrors is found to be greater than 95% in the visible and IR regions. The mechanical and environmental durability characteristics have been studied and test results indicate excellent stability of the coatings with adhesion and abrasion tests in accordance with military standards and also against prolonged exposures to humidity, thermal cycling and accelerated sulphide tarnishing tests.
1. INTRODUCTION
Front and rear surface mirrors are used in a number of optical-electro-optical systems operating right from the visible to far-IR spectral regions’. Some of these include space-borne electro-optical systems for uses such as very high resolution radiometers, reflective optics for astronomical observations, high efficiency gratings, laser systems and thermal control of satellites’. Among the various film materials, silver coating is found to be the best choice and offers several advantages such as high spectral reflectance, low polarization effects3 and low IR emissivity4. However, the common problems associated with silver coatings are their poor adhesion characteristics to the substrates, the increased tarnishing effects when exposed to normal atmospheric conditions and the restricted availability of good adherent protective coating materials5. The preparation of durable silver coatings has therefore proven to be a challenging task and only in recent years has there been some progress in the development of such coatings3-5. The present authors over the last few years of extensive studies have developed a simple and unique technique for the preparation of high efficiency durable silver mirrors useful in the visible and IR regions6. The detailed coating process technology together with the spectral and environmental durability properties are presented in this paper. * Paper presented at the 7th International 1987.
0040-6090/88/$3.50
Conference on Thin Films, New Delhi, India, December 7-l 1,
0 Elsevier Sequoia/Printed
in The Netherlands
292
2.
M. VISWANATHAN,
C. L. NAGENDRA,
G. K. M. THUTUPALLI
EXF’ERIMENTAL DETAILS
The preparation of durable silver films is largely dependent on the appropriate selection of binder layer and protective overlayer materials’. It is, however, seen that only a very few materials such as aluminium oxide3, tantalum oxide4, yttrium oxide’, silicon dioxide, magnesium fluoride* are suitable as protective overcoatings. Similarly oxide3 and several metallic4 binder films have been studied and their results reported. However, the present authors’ experience shows that such systems can be used only with partial success for long-term (about 7 years) space applications. Even though a single layer of nichrome-chromium as binder film has been reported4*5*7*g,in the present investigation this has been further improved by using a thin layer of chromium and the composite layer of chromium and silver as the binder coating for the front surface mirror and tantalum oxide for the rear surface mirror. In both cases, a thick single layer of tantalum oxide is used as a protective coating. The deposition of the films is carried out by a vacuum evaporation technique. Before deposition of the materials, the substrates are subjected to glow discharge cleaning at 6 x lo-’ Torr for about 10 min. The materials used for evaporation are of high purity (99.999%). A thin binder layer (about 300 A) of chromium is first deposited at a pressure of 2 x 10F6 Torr and then co-evaporation of chromium and silver is continued until the thickness reaches 1000 A at gradually increasing rates of silver deposition. The evaporation of chromium is then terminated while silver film deposition is continued until the coating just becomes opaque. A thick single protective layer of tantalum oxide (about 800 A) is finally deposited at a pressure of 1 x 10T5 Torr using an electron gun source. Alternatively, a reflectance-enhancing pair of aluminium oxide and tantalum oxide of quarter-wave thickness can be used to improve the reflectance in the lower wavelength of the visible region. In the case of the rear surface mirror, a tantalum oxide film about 500 A thick is deposited as the adherent underlayer coating. 3.
RESULTS AND DISCUSSION
3.1. Spectral characteristics The reflectance spectra of a single-layer tantalum-oxide-protected front surface silver mirror (solid curve) from the visible to the far-IR region and of the rear surface mirror (broken curve) in the visible region are shown in Fig. l(a). A slight dip in the reflectance curve is observed at around 0.5 pm in the case of the front surface reflector. An increase in the thickness of the protective layer shifts this reflectance dip towards the longer wavelength while a decrease in thickness provides inadequate protection of the mirror coating. The dotted curve in Fig. l(a) represents the reflectance of the mirror with a pair of quarter-wave thick dielectric boosting layers tuned to 0.4 fun. A considerable improvement in reflectance is seen in the lower wavelength region of the visible spectrum. The spectral results for the silver mirrors after exposure to a hydrogen sulphide atmosphere are shown in Fig. l(b), which will be discussed in detail later.
HIGH EFFICIENCY SILVER REFLECTORS
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(b) Fig. 1. (a) Spectral reflectance of TazO,-protected and enhanced silver mirrors: -, front surface; ---, rear surface; ....., enhanced mirror. (b) Measured spectral reflectance of protected front surfaoe retkctor (-) and unprotected mirror (---) after sulphide corrosion tests.
294
M. VBWANATHAN, C. L. NAGENDRA, G. K. M. THUTUPALLI
(b) Fig. 2. Dark field optical micrographs of front surface silver mirrors: (a) silicon-dioxide-pro tectedm irror abraded using eraser; (b) tantalum-oxide-protected mirror abraded using 1 pm alumina powder and rubbed with eraser.
HIGH EFFICIENCY
SILVER REFLECTORS
Fig. 3. Dark field optical micrographs of (a) unprotected silver mirror after 10 min of exposure !toal solution of ammonium sulphide and(b) tantalum-oxide-protected mirror after 1 h of exposure.
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3.2. Mechanicalstability The coatings have excellent adhesion in the Scotch tape peel test according to the MIL-C-675A standard. The abrasion resistance of the coating is determined using a standard eraser loaded with a 0.5 kgfweight and rubbed along the same path on the coated surface 50 times (MIL-C-675A). A single layer of tantalum-oxideprotected mirror exhibits no evidence of any scratches while the silicondioxideprotected mirror is scratched as depicted in Fig. 2(a). Figure 2(b) shows the density of scratches on the tantalum-oxide-protected mirror coating when 1 pm alumina powder is used as the abrasive and rubbed with the standard eraser. White scattering spots are also observed on protected silver mirrors’O when viewed at near-normal incidence at 100 x magnification against black background. Scratches are also seen on a reflectance-enhanced mirror in spite of the additional aluminium oxide protective layer. This is largely due to the reduced thickness of the tantalum oxide coating. 3.3. Environmental stability The protected silver mirrors also survive severe thermal cycling tests (5000 cycles between +8O”C and -65 “C with a dwell time of 10 min at extreme temperatures), continuous exposure for more than 300 h to a relative humidity of 95% at 50 “C and immersion in boiling water for more than 30 min without any sign of deterioration or change in optical properties. A similar configuration with the absence of the intermediate composite layer of chromium and silver exhibits degradation when subjected to the boiled water immersion test as a result of which the adhesion at the chromium-silver interface becomes poor. This is also observed after a few days of exposure to high humidity. The degradation of these mirror coatings caused by sulphide tarnishing has been studied by exposing them to a 10% solution of ammonium sulphide (10 min for the unprotected mirror and 1 h for the protected mirror) and the results are illustrated in Fig. 3. The unprotected mirror is found to have a hazy appearance as a result of the severe corrosion of the silver film while no degradation is observed on the protected mirror. The reflectance of the unprotected mirror is drastically reduced in the visible region while the reflectance of the protected mirror remains unaffected as can be seen from the spectral results presented in Fig. l(b). These illustrations clearly indicate the excellent protective nature of tantalum oxide overcoatings on silver reflectors. 4.
CONCLUSIONS
Front and rear surface mirrors yielding high reflectance in both the visible and the IR regions and having excellent mechanical and environmental stability have been successfully developed. It is found that the selection and thickness control of appropriate binder film and protective film materials play a vital role in improving the durability of these mirror coatings. REFERENCES 1
G. Ham, J. Opt. Sot. Am., 72 (1982) 27.
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G. Hass, J. B. Heaney and A. R. Toft, Appl. Opt., 18 (1979) 1488. G. Hass, J. B. Heaney, J. F. Osantowski and J. J. Triolo, Appl. Opt., 14 (1975) 2639. D. Y. Song, R. W. Sprague, H. A. Macleod and M. R. Jacobson, Appl. Opt., 24 (1985) 1164. I. Lubezky, E. Ceren and Z. Klein, Appl. Opt., 19 (1980) 1895. Indian Patents 786/iUAS/86,787/MAS/86, 1986. G. Hass, J. B. Heaney and W. R. Hunter, in G. Hass, M. H. Francombe and J. L. Vossen @is.), Physics of Thin Films, Vol. 12, Academic Press, New York, 1982. 8 D. K. Burge, H. E. Bennett and E. J. Ashley, Appl. Opt., 12 (1973) 42. 9 S. M. Wong, R. Goggin and P. J. Call, Thin Solid Films, 83 (1981) 415. 10 S. F. Pellicori, Appl. Opt., 19 (1980) 3096.