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In all films the degree of crystallinity, crystal structure and impurities influence optical conductivity. It is
common knowledge, however, that thin films produced by a similar process in different laboratories, or indeed, in
different deposition systems of the same laboratory, have different optical constants. This is because different
geometry and conditions give rise to different thin film structures and composition. Thus, before a multilayer
device, heat mirror, is fabricated, it follows from above that, knowledge of the optical properties of individual
layers is essential. Moreover, by varying the material and thicknesses of the three layers, the optical properties of
the D/M/D or D/A/D films can be tailored to suit different applications.
The main objective of this work was the feasibility study to use Ag-alloy thin film as an IR-reflected
layer of heat mirror by simulation technique. The optical constant of Ag-alloy and TiO
2
thin films and the
performance of Ag-alloy based heat mirrors in this work was evaluated by the transmission spectrum which
simulated by the optical thin films design software.
Materials and Methods
Ag-alloy and TiO
2
thin films were deposited on well-cleaned glass slide and Si (100) by a home made
DC unbalanced magnetron sputtering system. Fig. 1 shows a diagram of home made coating system. The
cylindrical chamber of the system has 31 cm in diameter and 37 cm in height was connected to the diffusion pump
backed by rotary pump. The system has two unbalance magnetron cathodes, for deposited of Ag-alloy or TiO
2
,
however, during the deposition of Ag-alloy or TiO
2
films only one cathode was used. The targets are Ag-alloy
with 92.5%Ag-7.5%Cu composition and titanium with purity of 99.97%. Prior to deposit each film, the vacuum of
the chamber was evacuated to lower than 10
-5
mbar. Ag-alloy and TiO
2
films were deposited in DC sputtering
mode and DC reactive sputtering mode respectively with 30 nm thickness on glass slides and Si (100) substrate.
The thin films deposited conditions are listed in Table 1. The thickness of both films was control by deposition
rate (thickness per deposited time). The thickness of those films on Si (100) substrate was confirmed by AFM
technique (Nanoscope IV, Veeco Instrument Inc.). The transmission spectrum of those films which deposited of
glass slide substrate was measured by spectrophotometer (Shimadzu UV-VIS-NIR 3100 spectrophotometer)
in the wavelength range of 200 nm to 2500 nm. TFCalc, the optical thin films design software, was employed to
evaluate the optical constants such as the refractive index
n
and extinction coefficient
k
of those films from
transmittance spectra.
In this work, the feasibility study to use Ag-alloy thin film as an IR-reflected layer of heat mirror was
evaluated by the transmission spectrum which simulated by TFCalc base on the heat mirror with
TiO
2
/Ag-alloy/TiO
2
structure (Fig. 2). In order to simulation the transmission spectrum from TFCalc for evaluated
the performance of these heat mirrors. We used the optical constant,
n
and
k
, of Ag-alloy and TiO
2
film from
the first part and fixed the thickness of TiO
2
layer to 30 nm and then vary the thickness of Ag-alloy layer from
5 nm to 40 nm.
