Influence of carbon dioxide gas on internal stress and film density of tin oxide films grown by magnetron sputtering

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Abstract

The internal stress, film density and dynamic deposition rate of amorphous tin oxide (SnO2) films were investigated as a function of the carbon dioxide (CO2) gas flow ratio [CO2/(O2 + CO2)] during sputter deposition. The internal stress and film density decreased with higher CO2 gas flow ratios. The dynamic deposition rate increased 1.6-fold with increases in the CO2 gas flow ratio. These results suggest that the stress relaxation of the SnO2 films was induced to decrease the interatomic repulsive force with decreases in the film density. Therefore, using CO2 gas was quite an effective method to reduce the compressive stress of the SnO2 films.

Introduction

In comparison with single pane windows and solar control glass windows used in buildings, low-emissivity (low-e) glass windows improve energy savings by reducing the capital and running costs of heating systems because of their low-emissivity and heat-shielding properties [1]. Low-e glass windows consist of double-glazing units with on pane coated with a low-e coating deposited on one side by sputtering. The basic structure of the low-e coating is glass/bottom-oxide/Ag/top-oxide, where zinc oxide (ZnO) and tin oxide (SnO2) films are used as the oxide because they have the advantage of high transmittance in the visible range [1], [2].

However, one problem associated with low-e coatings is that moisture in the air can cause white dots and white turbidity to be formed on the surface. It has been reported that the moisture-induced degradation of low-e coatings is due to the high internal stress of the top-oxide film [3], [4]. It has also been reported that the degradation can be suppressed by use of a top ZnO film with low internal stress [4].

Stress relaxation method using reactive sputtering gas is critically industrially important due to without improvement of the equipment. On the other hand, carbon dioxide (CO2) gas is a non-flammable and harmless gas, however CO2 gas has not been used for reactive sputtering gas; therefore, the effect of CO2 gas on physical and mechanical properties is not clear. Hence, CO2 gas was used as a discharge gas. In this study, the internal stress of SnO2 films used as low-e coatings was investigated as a function of the CO2 gas flow ratio [CO2/(O2 + CO2)] during sputter deposition. The influence of CO2 gas on internal stress of the SnO2 films is discussed.

Section snippets

Preparation

SnO2 films were prepared on 0.1 mm-thick and 3.0 mm-thick soda-lime-silicate glass substrates by dc magnetron sputtering. A conventional in-line sputtering system was used for the deposition process. The deposition parameters are listed in Table 1. The SnO2 films were deposited under various CO2 gas flow ratios [CO2/(O2 + CO2)] from 0 to 45% using a pure Sn metal target. The film thickness was set at 40 nm by adjusting the conveyance speed of the substrate. The total gas pressure of the O2 gas or O2 +

Results and discussion

In the case of using the SnO2 film as the low-e coating, high transmittance in the visible range is required. Therefore, the transmittance of the SnO2 films deposited with various CO2 gas flow ratio was investigated. All the SnO2 films in this study exhibited a high transmittance of over 80% in the visible range.

Fig. 1 shows the compressive stress of the SnO2 films as a function of the CO2 gas flow ratio with discharge gas. Although the compressive and tensile stresses were reported for

Conclusions

The internal stress, film density, and dynamic deposition rate were investigated as a function of the CO2 gas flow ratio. The following results were obtained:

  • (1)

    The compressive stress of the SnO2 films decreased from 2.4 to 1.2 GPa with increases in the CO2 gas flow ratio.

  • (2)

    The film density decreased from 6.9 to 6.5 g/cm3 with increases in the CO2 gas flow ratio.

  • (3)

    The dynamic deposition rate of the SnO2 films deposited with a CO2 gas flow ratio over 25% increased linearly.

From these experimental

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