Ag-alloy multilayer on glass

Ag-alloy multilayer on glass

Applied Surface Science 246 (2005) 48–51 www.elsevier.com/locate/apsusc Low-emissivity coating of amorphous diamond-like carbon/Ag-alloy multilayer o...

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Applied Surface Science 246 (2005) 48–51 www.elsevier.com/locate/apsusc

Low-emissivity coating of amorphous diamond-like carbon/Ag-alloy multilayer on glass Kiyoshi Chiba*, Toshiyuki Takahashi, Takashi Kageyama, Hironori Oda Department of Nano Material and Bio Engineering, Tokushima Bunri University, Sanuki, Kagawa 769-2193, Japan Received 17 August 2004; accepted 18 October 2004 Available online 8 December 2004

Abstract Transparent low-emissivity (low-e) coatings comprising dielectrics of amorphous diamond-like carbon (DLC) and Ag-alloy films are investigated. All films have been prepared by dc magnetron sputtering. An index of refraction of the DLC film deposited in a gas mixture of Ar/H2 (4%) shows n = 1.80 + 0.047i at 500 nm wavelength. A multilayer stack of DLC (70 nm thick)/Ag87.5Cu12.5-alloy (10 nm)/DLC (140 nm)/Ag87.5Cu12.5-alloy (10 nm)/DLC (70 nm) has revealed clear interference spectra with spectra selectivity. This coating performs low emittance less than 0.1 for black body radiation at 297 K, exhibiting a transparent heat mirror property embedded in DLC films. # 2004 Elsevier B.V. All rights reserved. PACS: 78.66.-w Keywords: Low-emissivity coating; Diamond-like carbon; Heat mirror; Multilayer

1. Introduction Low-emissivity (low-e) coatings with visible transparency, namely transparent heat mirror coatings have attracted increased interest in reducing heat radiation loss through window panes from ecological and sustainable aspects [1–3]. Due to flexibility of the design producing spectral selectivity, dielectric– metal–dielectric type of multilayer structures is * Corresponding author. Tel.: +81 87 894 5111; fax: +81 87 894 4201. E-mail address: [email protected] (K. Chiba).

widely used [4,5]. They consist of thin silver layers sandwiched with dielectric layers, enabling dereflection of the silver surface attributing to optical interference of multilayer films. Transparent oxide and sulfide films of TiO2, SnO2, ZnO, indium–tin-oxide (ITO), ZnS, etc. have been applied to dielectric– metal–dielectric multilayer structures, producing heat mirrors of TiO2/Ag/TiO2, SnO2/Ag/SnO2, ZnO/Ag/ ZnO, ITO/Ag/ITO, ZnS/Ag/ZnS, respectively [6,7]. The spectral selectivity could improve with increase of numbers of layers, for instance at five layers of TiO2/ Ag/TiO2/Ag/TiO2 structure [8]. In addition of the optical property, environmental stabilities attributing

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.10.046

K. Chiba et al. / Applied Surface Science 246 (2005) 48–51

mainly to instable manner of silver atoms implies another key factor of design of the film structures [9,10]. To prevent deterioration of silver surface, the control of the interface between silver and dielectric becomes a critical issue for the coating. The blocking layer of very thin metal of sub-nanometer thick acts for this purpose in association with alloying of silver with copper and gold [11–14]. An oxygen-free dielectric such as silicon nitride of Si3N4 has been proposed as another choice to improve the interface [15]. Additionally, abrasion resistance of a coating also plays an important role to protect a silver layer against scratch and eliminate exposure of the silver surface to air. Consequently, an amorphous diamondlike carbon (DLC) film [16,17] has become a potential candidate for the dielectrics of heat mirror coatings. In this letter, we report on DLC/Ag-alloy multilayer films for novel transparent low-emissivity coatings.

2. Experimental Multilayer films were deposited by dc magnetron sputtering on glass substrates. The base pressure and gas pressure was 2  10 6 and 4  10 3 Torr, respectively. The gas flow rate was controlled at 40 sccm. DLC films were deposited from a 4-in. diameter target of carbon with purity of 99.999% in a gas mixture of Ar/H2 (H2: 4%) or Ar (99.999%) without heating of the substrate. The sputtering power was 100 W. Ag-alloy films were deposited from a 4-in. diameter target of Ag90Cu10 alloy in a gas of Ar. The sputtering power was 50 W. The copper concentration in the film was 12.5 at.%. Multilayer films were prepared in the form of glass/DLC/AgCu/DLC/AgCu/ DLC structures. Spectral ellipsometry measurements of films from 280 to 860 nm were carried out on JASCO 220 ellipsometer at an angle of incidence of 458. Tauc–Lorentz model was applied to calculate refractive index of films. A microscopic Raman spectrometry (Jovan-Yvon T 64000) was employed to observe a ratio of amorphous, namely disorder (D) to graphite (G) Raman shift’s peaks. An approximately 1-mm diameter Ar+ laser (514.5 nm) beam at 20 mW was irradiated to film surfaces. Both transmission and reflection spectra were obtained with a Shimadzu UV-3150 spectrometer in the wavelength range of

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220–2500 nm at angles of incidence of 08 and 58, respectively. Transmission spectra were measured from the film side including both substrate and substrate/air reflection losses. Auger in-depth analyses (Perkin-Elmer PHI 610) were used to characterize the profile of elements in multilayer films. The sputter etching was performed at an acceleration voltage of 2 keV at Ar gas. Finally, emissivity measurements of multilayer films for black body radiation at 297 K were carried out using an emissiometer of Devices & Services Co.

3. Results and discussion Optical properties of the sputtered carbon films are studied. Fig. 1 shows transmission and reflection spectra of 68 nm-thick films deposited in either a gas of Ar or Ar/H2. In a gas of Ar, the films appear opaque, indicating transmittance (T) of 29% and reflectance (R) of 28.6% at 500 nm wavelength. Absorptance (A) is more than 30%. In a gas of Ar/H2, the transparency increases, that is T of 54.2% and R of 18.2%. Then, absorptance in the film decreases to less than 25%. Hydrogen atoms would be attributed to decrease graphite structures in the film. It is noted that the absorptance at films prepared in a gas of Ar/H2 decreases to less than 6% beyond 700 nm wavelength. Raman spectrometry of the 280 nm-thick film deposited in a gas mixture of Ar/H2 was performed. The ratio of the area of the disorder of graphite peak (D) of carbon around 1350 cm 1 to graphite (G) peak around 1580 cm 1 by peak fitting shows 2.78. The spectral dependence of the complex index of refrac-

Fig. 1. Transmission and reflection spectra of sputter-deposited amorphous diamond-like carbon films. The solid and dotted curves correspond to the samples prepared in a gas of Ar/H2 (H2: 4%) and Ar, respectively.

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Fig. 2. Spectral dependence of n, the real part and k, the imaginary part of the complex refractive index for a sputter-deposited amorphous diamond-like carbon film.

Fig. 4. Auger depth profiles of glass/DLC (70 nm thick)/AgCualloy (10 nm)/DLC (140 nm)/AgCu-alloy (10 nm)/DLC (70 nm) multilayer film. Sputter etching: acceleration voltage of 2 kV at Ar gas.

tion of the film deposited in a gas of Ar/H2 is shown in Fig. 2. It is found 1.80 + 0.047i at 500 nm wavelength. Multilayer coatings of glass/DLC/Ag-alloy/DLC/ Ag-alloy/DLC have been studied. Fig. 3 shows spectra of glass/DLC (70 nm thick)/AgCu-alloy (10 nm)/DLC (140 nm)/AgCu-alloy (10 nm)/DLC (70 nm) structure. The induced transmission is clearly observed in this structure. At visible region, transparency is obtained, whereas highly reflectance (R) retains at infrared region. T max is 50% at 500 nm wavelength. R is 90% beyond 2000 nm wavelength. Transmittance sharply decreases with wavelength at near infrared region. Fig. 4 shows in depth profiles of the multilayer structure observed by Auger spectra. The distinct shape of the carbon layers is found to cover Ag-alloy films by proper distances without notable elemental mixtures at the film boundaries. This gives rise to

induce interference optically, and would protect Agalloy layer from both environmental and mechanical stresses. To investigate influence of a carbon layer to the property of thermal radiation of the coating, the emittance of the film for black body radiation at 297 K was measured. It is determined 0.1 for the glass/DLC (70 nm thick)/AgCu-alloy (10 nm)/DLC (140 nm)/ AgCu-alloy (10 nm)/DLC (70 nm) structure. This implies that dielectrics of DLC do not significantly affect the emittance of the multilayer coating corresponding mainly to reflection of infrared ray at silver layers. It indicates that a carbon-based, that is, oxygen-free transparent heat mirror coating could be produced at metal–dielectric multilayer systems. Optical properties could be further improved with decrease with extinction factor of the dielectric films, namely k of amorphous diamond-like carbon films.

4. Conclusion

Fig. 3. Transmission and reflection spectrum of glass/DLC (70 nm thick)/AgCu-alloy (10 nm)/DLC (140 nm)/AgCu-alloy (10 nm)/ DLC (70 nm) multilayer film.

In summary, a novel transparent heat mirror has been demonstrated at metal–dielectric multilayer coatings comprising DLC and Ag-alloy films. This coating shows clear interference spectra with low-e. This system also employs a hard surface and oxygenfree properties for heat mirror coatings. The optical performance of the coatings, for instance transmittance could be further improved by optimizing deposition parameters of amorphous diamond-like carbon films.

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