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Introduction
There were two technology innovations involved in flat glass industry and its related products in the 20th
century. One is the float glass production technology, and the other is the large-area coating technology. Recently,
coated glass products for special purposes such as spectrally selective glazing, solar control, low-E are very
popular. One of these special coating is the silver-based low-E coating, which has been extensively studied for
more than 10 years, which are widely used for an energy-efficient window such as insulating glass units or solar
control or heat mirror because of their high performance on thermal insulation (Allendorf, 2001 ; Ebisawa and
Ando, 1998 ; Granqvist, 1990 ; Pulker, 1981). The basic layer systems of these coatings are composed of the
optical multilayer thin films of suitable materials with specified design. Now, in view of the growing energy
crises, there is widespread interest world over for higher performance products, especially those that could
contribute to energy savings (Granqvist, 1990 ; Lampert, 1981).
An energy-efficient window is a device capable of providing lighting and thermal comfort at minimum
paid of energy for air-conditioning. The solar spectrum is roughly split between the visible and the near-infrared
regions. When overheating from excessive solar input is a problem, one can obtain energy efficiency by using
multilayer thin film-coated glass windows called heat mirrors that are transparent for visible light and reflecting
for infrared (IR) solar radiation. A three-layer system of Dielectric/Metal/Dielectric (D/M/D) or
Dielectric/Alloy/Dielectric (D/A/D) on a glass substrate could be use as a spectrally selective filter that reflects IR
radiation and transmits most of the visible spectrum. The highly reflective metal film or alloy film (that would
otherwise be opaque to the visible light) is sandwiched between the two dielectric layers that act as antireflection
coatings for the visible thus increasing the visible transmission. By varying the material and thickness of the three
layers, the optical properties of the D/M/D or D/A/D films can be tailored to suit different applications.
Spectrally selective glazing or heat mirrors have been constructed by forming of D/M/D layers on glass
substrates. For example, TiO2/Ag/TiO2(Fu et al., 1997 ; Zhang et al., 1996), ZnS/Ag (or Al)/ZnS (Leftheriotis et
al., 1997), ZnS/Ag/ZnS (Heavens, 1991) and tin doped indium oxide (Johnson and Christy, 1972), coatings on
glass have been used to fabricate heat mirrors that are energy efficient window.
Ag thin films possess many excellent optical properties; it is suitable for use to be an IR-reflected layer
of heat mirror with D/M/D structure. However, it easily degrades when exposed to O2 during the oxides layer
deposition sequence. This problem may be solved by using Ag-alloy in which Ag is doped with Pt, Pd or Cu
instead of pure Ag (Arbab, 1997).
Performance of the heat mirror of both D/M/D and D/A/D structure depends critically on the optical
constants and thicknesses of the Ag alloy and the metal oxide films used. Oxide coatings depend heavily upon
stoichiometry, impurities and defects in the films (Johnson and Christy, 1972). Metal or alloy films are very
dependent upon nucleation and coalescence phenomena.