JOURNAL

OF M A T E R I A L S

S C I E N C E L E T T E R S 6 (1987) 115

117

Optical properties of reactively evaporated aluminium films M. J. VERKERK, W. A. M. C. B R A N K A E R T Philips Research Laboratories, PO Box 80 000, 5600 JA Eindhoven, The Netherlands

Diffusely reflecting layers are widely used in passive displays. For example, in liquid-crystal displays an external diffuse reflector is applied in the twistednematic configuration and an internal diffuse reflector in the guest-host configuration. In micromechanical displays, movable diffusely reflecting elements are located inside the cell. For the latter application a metallic reflector is preferred and requirements are made on the surface morphology. From the reflecting metals aluminium is a very suitable choice. Two methods can be used to obtain a diffusely reflecting aluminium layer. First, a smooth aluminium layer can be deposited on a substrate with the desired roughness. In this case the problem is how to make such a substrate. Secondly, an aluminium layer with the desired roughness can be deposited on a smooth substrate by choosing a suitable deposition method and the right deposition circumstances. In this study, we follow the latter approach. It has been reported [1-4] that residual gases, especially oxygen and water, have a strong influence on the morphology of evaporated aluminium films. As a consequence, a strong influence on the reflectance of these films can be expected. The aim of this letter is to investigate the influence of residual gases on the optical properties of evaporated aluminium films. Aluminium 99.999% was deposited with an e-gun in a Balzers BAK 550 evaporation unit. The mean film thickness was 0.5 pm, the substrate temperature 250°C and the evaporation rate 0.5 nm sec -1 . Reactive gases were introduced into the vacuum chamber through a needle valve. The composition of the residual gas was measured with a mass spectrometer. Experimental details can be found in [4]. The oxygen partial pressure, Po2, was varied between 1 x 10 8and 1 x 10-3pa. Fig. 1 shows thescanning electron micrographs (SEM) of several samples. At low values of Po2 a facetted surface morphology is present (Fig. l a). With increasing oxygen partial pressure the faceted surface morphology gradually disappears. AtPo2 = 1 x 10-5to5 x 10 5 P a a v e r y rough surface is obtained (Fig. lb, c). At higher oxygen partial pressures a smooth, fine-grained surface is present (Fig. ld). Fig. 2 shows the roughness, Ra (mean deviation from the centre-line) as measured with the stylus technique, tiP radius 2.5#m. As can be seen, the roughness increases strongly with Po2 until a maximum is reached at 5.5 x 10-SPa. After that the roughness decreases sharply. Fig. 2 also shows the diffuse and total reflectance as measured at 550 nm in an Ulbricht sphere with BaSO4 as a reference. At low values of Po, the diffuse reflec0261-8028/87 $03.00 + .12 © 1987 Chapman and Hall Ltd.

tance is low, i.e. specularly reflecting layers are obtained. With increasing Po2 the diffuse reflectance increases, which has to be ascribed to roughening of the surface [5]. A maximum in the diffuse reflectance is obtained atpo2 -- 1.1 x 10-SPa. These layers have a milky appearance; the diffuse reflectance is 78% and the total reflectance is 84%. At higher values ofpo2 the total and diffuse reflectance both decrease, resulting in grey/black layers. In the region ofPo2 = 2.2 x 10 5 to 1.0 x 10-4pa no specular reflectance is present, which has to be ascribed to the considerable roughness of these samples (Figs l c, 2). The decrease in total reflectance is attributable to absorption of light due to incorporation of oxide in the layer [6, 7] and to absorption due to multiple reflections. Fig. 3 shows the total and diffuse reflectance for the sample with the highest diffuse reflectance. With increasing wavelength the diffuse reflectance decreases, which has to be mainly ascribed to a smaller value of the roughness in comparison with the wavelength of the light [5]. Fig. 4 shows the distribution of the diffusely scattered light, as measured with the method described in [8, 9]. At oxygen partial pressures of 1 x 10-Spa to 2.5 x 10-6pa the intensity of the diffusely scattered light is strongly dependent on the scattering angle: at about ~ -- 15° a "bump" is present, which is correlated with the scattering of large grains in the surface [9]. Atpo2 = 1.1 x 10-SPa the intensity of the scattered light is almost constant in the region - 1 5 to + 15°. The water partial pressure, Paso, was varied between 1.8 x 10-6pa and 1.8 x 10 4pa. The phenomena observed with SEM in this region were analogous to those found by varying the oxygen partial pressure [4]. At low water partial pressures a facetted surface is obtained. AtpH2o = 3 x 10 S t o 5 x 1 0 - S P a a v e r y rough layer is present. At higher values of PH2o a fine-grained layer is obtained. The changes in morphology are accompanied by changes in the roughness, R1, and the total and diffuse reflectance, see Fig. 5. At low water partial pressures the layer is rather smooth and reflects the incoming light specularly. With increasing PH2o the layer becomes rougher, which is accompanied by an increase in the diffuse reflectance. AtpH~o = 3.5 x 10 -SPa both the roughness and the diffuse reflectance reach a maximum value. At 550 nm the total and diffuse reflectance are 75% and 71%, respectively (see Fig. 3). This layer has a milky appearance. At higher values ofpH2o the total and diffuse reflectance decrease with increasing PH2o. The optical properties of reactively evaporated aluminium films are influenced by the type of admitted 115

Figure 1 SEMs of e-gun-deposited aluminium layers as a function of the oxygen partial pressure. Evaporation rate: 0.5 n m sec-t ; substrate temperature: 250°C. (a) 1.5 x 10 7pa; (b) 1.I x 10-SPa; (c) 5.5 x 10-SPa; (d) 1.1 x 10-~Pa.

gas. Comparison of Figs 2 and 5 shows that for the system O2/A1 the diffuse reflectance increases at much lower partial pressures of the admitted gas than for the system HzO/A1. At the same time, for the system O2/A1 the maximum in the roughness is obtained at a higher value of Po2 than the maximum in the diffuse reflectance, whereas for the system HzO/A1 these maxima are found at the same value ofpH20. Finally, for the samples with the best optical properties, both the total and the diffuse reflectance are higher for the sample evaporated in 02 than in H 2 0 , a s can be seen in Fig. 3. In discussing the effects of residual gases on the growth and properties of evaporated aluminium films it is necessary to know more about the interaction of these gases with an aluminium surface. Surface studies [10-12] have shown that water and oxygen adsorb dissociatively accompanied by the formation o f - O and - O H groups. If the concentration of these impurities exceeds a critical value, nucleation and growth of a stable impurity phase (oxide/hydroxide)

take place. A transmission electron microscopy study of the growth of aluminium films in a water atmosphere has shown that the growth of the oxide/ hydroxide layer starts at the aluminium-substratevacuum boundary and extends up to the aluminium surface [13]. This layer stops the growth in the direction perpendicular to the covered surface, resulting in rough, hillock-like surfaces. In the case of deposition of aluminium in an oxygen atmosphere, oxide formation also plays an important part in the growth of the rough surface morphology. It is found that the growth of certain grains is hindered by oxidation whereas other grains can grow out more or less undisturbed [14]. To explain the observed differences in optical properties of aluminium films evaporated in 02 and H20, a detailed study of the growth mechanism and the oxide (hydroxide) layers is necessary. In conclusion, it has been shown in this study that the morphology and the optical properties of deposited aluminium films are strongly affected by the oxygen and water partial pressures during evaporation. The best optical properties are found for layers deposited in an oxygen atmosphere. At an oxygen partial press-

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ure of 1.1 x lO-SPa the deposited layer has a total reflectance of 84% and a diffuse reflectance of 78%.

Acknowledgements We thank D. M. N. Bemelmans for the scanning electron micrographs and W. Mesman for the laser scattering experiments.

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References 1. R. W. S P R I N G E R and D. S. C A T L E T T , J. Vac. Sci. Technol. 15 (1978) 210. 2. F. d ' H E U R L E , L. B E R E N B A U M and R. ROSENBERG, Trans. Met. Soc. A I M E 242 (1968) 502. 3. P. B. B A R N A , F. M. R E I C H A , G. B A R C Z A , L. GOSZTOLA and F. K O L T A I , Vacuum 33 (1983) 25. 4. M. J. V E R K E R K and W.A.M.C. BRANKAERT, Thin Solid Films 139 (1986) 77. 5. H. E. B E N N E T T and J. O. P O R T E U S , J. Opt. Soc. Am. 51 (1961) 123. 6. C. M. E A M P E R T , Solar Energy Mater. 1 (1979) 319. 7. H. G. C R A I G H E A D , R. B A R T Y N S K I , R. A. B U H R MAN, L. W O J C I K and A. J. SIEVERS, ibid. 1 (1979) 105. 8. E. L. C H U R C H , H. A. J E N K I N S O N and J. M. ZAVADA, Opt. Engng 16 (1977) 360. 9. M. J. V E R K E R K and I.J.J.M. RAAYMAKERS, Thin Solid Films 124 (1985) 271. 10. w . E B E R H A R T and C. K U N Z , Surf Sei. 75 (1978) 709. 11. C. W. B. M A R T I N S O N and S. A. F L O D S T R O M , ibid. 80 (1979) 306. 12. F. P. N E T Z E R and Y. E. MADEY, ibid. 127 (1983) L102. 13. G. J. van der K O L K and M. J. V E R K E R K , J. Appl. Phys. 59 (1986) 4062. 14. M. J. V E R K E R K and G. J. van der K O L K , International Conference on Metallic Coatings, San Diego, 7-11 April (1986).

log [PHzO (Pall Figure 5 (O) Total and (o) diffuse reflectance, and (

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Received 26 June and accepted 30 June 1986

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