A low emissivity film

Abstract: A low emissivity film which comprises: a substrate; and a coating of oxide and metallic films alternately formed on the substrate in a total of (2n+1) layers where n is an integer being equal to or more than 1, with the innermost layer being an oxide film, wherein the oxide film (B) formed on the outer side of the metallic film (A) being most apart from the substrate, has an internal stress which is equal to, or less than 1.1 x 10¹⁰ dyne/cm².

Description: This invention relates to a low emissivity film which is excellent in durability, especially in moisture resistance or in acid resistance. A film composed of (2n+1) layers (n≧1) such as a film composed of three layers in which an oxide film, an Ag film, and an oxide film are successively coated on a surface of a substrate, or a film composed of five layers in which an oxide film, an Ag film, an oxide film, an Ag film, and an oxide film are successively coated on a surface of a substrate, is a heat reflective film called Low-E (Low-Emissivity) film. A glass in which such a Low-E film is formed, is called a Low-E glass. This glass can prevent lowering of room temperature by reflecting the thermal infrared radiation emitted from within the heated room, which is used mainly in cold district on the purpose of decreasing heating load. Furthermore, since this glass has a heat insulating effect of the solar radiation energy, it is adopted in a windshield of an automobile. Since this glass is transparent and is electrically conductive, it has a usage as an electromagnetic shielding glass. When this glass is equipped with an electric heating such as a bus bar composed of an electrically conductive printing or the like, this glass can be used as an electrically heated window. As a major Low-E glass, a glass is exemplified having a film composition of ZnO/Ag/ZnO/glass. However, since this film is devoid of durability such as anti-scratching property or chemical stability, it can not be used on a single plate, and it is necessary to use the film in a laminated glass or in double glazing. This film has a problem especially in moisture resistance, in which white dot or white turbidity is caused by moisture in the air or by moisture contained in an intermediate film in case of the laminated glass. Furthermore, since ZnO is insufficient in acid resistance, the film may be deteriorated by acid substance in the air. Due to these shortcomings, caution is required in the storage or in the handling of the single plate. FR-A-2 586 245 and the corresponding DE-A-3 628 057 disclose a sheet of heat reflecting glass, wherein a laminate structure consisting of glass/ZnO/Ag/ZnO + Al 2 O 3 is formed. For forming the outer layer a target of Al 2 O 3 and ZnO is employed in a DC sputtering in an argon atmosphere having an oxygen concentration of 0-20 vol.-%. It is an object of the present invention to provide a Low-E film capable of overcoming the above shortcomings, and excellent in the durability, especially in the moisture resistance or in the acid resistance. According to the present invention, there is provided a low emissivity film which comprises: a substrate; and a coating of oxide and metallic films alternately formed on the substrate in a total of (2n+1) layers where n is an integer being equal to or more than 1, with the innermost layer being an oxide film, wherein the oxide film (B) formed on the outer side of the metallic film (A) being most apart from the substrate, is a multi-layer film having at least a layer whose major component is zinc oxide and a layer whose major component is tin oxide. In the drawings: Figures 1a and 1b are sectional diagrams showing embodiments of Low-E glasses on which low emissivity films are formed, according to the present invention. As mentioned above, in case of the conventional Low-E glass (film composition: ZnO/Ag/ZnO/glass), when it is left in a room, white turbidity or white dot appears on the film by the moisture in the air. When the film with white turbidity or white dot is observed by a scanning electron microscope (SEM), the existence of crack or wrinkle, and exfoliation of the film are recognized on the surface of the film. When an elementary analysis is performed on the exfoliated part of this film, with respect to Ag and Zn, Ag is found to exist at certain amount irrespective of the existence of the exfoliation. On the contrary, the detected amount of Zn is halved at the exfoliated part. Accordingly, the exfoliation is found to take place on the interface between the uppermost layer of ZnO and the Ag layer. Next, an investigation is performed on samples before and after a moisture resistance test (the samples are left for 6 days, at 50°C, in an atmosphere of relative humidity of 95%) by X-ray diffraction method. The diffraction angle 2 (center of gravity position of peak), interplanar spacing d, and the peak width (integral width) I.W. with respect to (002) diffraction line of hexagonal zinc oxide, and (111) diffraction line of cubic Ag, are respectively shown in Table 1. It is possible to detect degree of strain of the lattice due to an internal stress, by a deviation of peaks in X-ray diffraction method. In case of the sample of ZnO(b)/Ag/ZnO(a)/glass, a peak of ZnO(b) of the uppermost layer, is detected with an intensity 5 to 15 times as much as a peak of ZnO(a). Therefore, a peak of ZnO of X-ray diffraction method with respect to the total of the sample, is almost considered to be the peak of hexagonal ZnO(b) of the uppermost layer, although there may be more or less an influence of ZnO(a). ZnO (002) Ag (111) Before moisture resistance test After moisture resistance test Before moisture resistance test After moisture resistance test 2 (deg.) 33.78 33.91 38.09 38.09 d (nm) 0.265 0.2641 0.2361 0.2361 d (Å) (2.650) (2.641) (2.361) (2.361) l·w (deg.) 0.510 0.623 0.977 0.675 According to Table 1, (002) diffraction line of ZnO in the Low-E film before the moisture resistance test, is deviated in its position, compared with 2 = 34.44° of ZnO powder. This suggests the existence of a lattice strain. This lattice strain is due to an internal stress of the film. In the samples before the moisture resistance test, the interplanar space d 002 = 0.265 nm (2.650 Å), which is larger than d 002 = 0.2607 nm (2.602 Å) of ZnO powder by 1.8%. From this result, the film is found to receive a considerably large compressive stress. In case of samples after the moisture resistance test, the lattice strain is more or less decreased, as d 002 = 0.2641 nm (2.641 Å). This test result corresponds with the fact in which the internal stress of ZnO at the uppermost layer is partly relieved by the crack, the wrinkle, and the exfoliation. Concerning the (111) diffraction line of Ag, the peak width after the moisture resistance test is decreased. Therefore grain growth of Ag is considered to take place by performing the moisture resistance test. Accordingly, the mechanism of generation of the white turbidity or white dot, is considered as follows. The hexagonal ZnO film at the uppermost layer can not resist to the large internal stress. The film is exfoliated from the interface with Ag film, and is destructed. Next grain size of Ag is increased. The film displays the white turbidity or white dot since light is scattered by the destroyed surface and by the large silver grain. In the examples of Table 1, the internal stress is a compressive stress. However there are two kinds of internal stress, that is, a compressive stress and a tensile stress, both of which cause destruction of a film. From the above observation, in this invention, it is found that the decrease of the internal stress of ZnO film at the upper most layer, is effective to prevent the white turbidity or white dot due to moisture. Figures 1a and 1b are sectional diagrams showing embodiments of low emissivity films according to the present invention. Figure la is a sectional diagram of the low emissivity film composed of three layers, and Figure 1b is a sectional diagram of the low emissivity film composed of (2n+1) layers. A reference numeral 1 designates a substrate, 2, an oxide film, 3, a metallic film, and 4, an oxide film (B) having low internal stress. As a substrate 1 in this invention, a film or a plate substrate made of plastic or the like can be used as well as a glass plate. The oxide film (B) can be used, so long as the internal stress is equal to or less than 1.1 x 10 10 dyne/cm 2 . The internal stress of the film depends considerably on the deposition condition of the film. The deposition condition is necessary to be controlled precisely, when a film of low internal stress, is formed. As a method in which a tendency of decreasing internal stress of the film is shown, a method of changing the deposition condition (especially depositing by a sputtering method), such as, increasing the pressure of the atmosphere in deposition of the film (sputtering pressure), or heating the substrate in depositing the film, and a method in which a heat treatment is performed after depositing the film, are suggested. By these methods, the internal stress of the film can be decreased. The respective concrete conditions can be chosen depending upon each apparatus for depositing the film, and are not particularly restricted. When the oxide film (B) is deposited by reactive sputtering in the oxygen-containing atmosphere, it is preferable to first deposit a thin metal layer in non-oxidation atmosphere on the metallic film (A), in order to prevent the oxidation of the metallic film (A). The thin metal layer is oxidized to be an oxide layer during the deposition of the oxide film (B). Therefore the above preferable thickness of the oxide film (B) includes the thickness of the oxide layer formed by the oxidation of said thin metal layer. As an oxide film (B), a multi-layer film can be used, having a composition of at least two layers by combining films of high internal stress and those of low internal stress. As a film of low internal stress, although depending on the condition of the film deposition, SnO 2 film is suggested because a SnO 2 film of comparatively low internal stress of 7.0 x 10 9 dyne/cm 2 at most, is relatively easy to deposit. As concrete examples, three layer series such as ZnO/SnO 2 /ZnO, or SnO 2 /ZnO/SnO 2 , or five layer series such as ZnO/SnO 2 /ZnO/SnO 2 /ZnO, or SnO 2 /ZnO/SnO 2 /ZnO/SnO 2 , are suggested in which ZnO films and SnO 2 films are alternatively coated. The internal stress of the total of the oxide film (B) having these multi-layer films, may be 1.1 x 10 10 dyne/cm 2 at most, and the diffraction angle of ZnO (002) of the X-ray diffraction is not necessary to be the above value. Of course it is desirable that the ZnO film has low internal stress, and diffraction angle of ZnO (002) in X-ray diffraction method is within the above range. In case of a multi-layer film which is composed of at least two layers by combining films of high internal stress and those of low internal stress, as an oxide film (B), as shown in the above, the number of layers and the film thickness of a layer, may be chosen depending on an apparatus, and is not especially restricted, so long as the total thickness is within the range of 20 to 70 nm (200 to 700 Å). Moreover, the film thickness of each layer may be different. Table 2 shows the relationship among the internal stress of the oxide film (B), the diffraction angle 2 (center of gravity position) of (002) diffraction line of zinc oxide of a Low-E film in which the oxide film (B) is formed on Ag/ZnO/glass by a sputtering method, and the moisture resistance of the Low-E film. The internal stress in Table 2 is in compressive stress. The moisture resistance is evaluated by performing a test in which samples are left in an atmosphere of the relative humidity of 95% at 50°C, for 6 days. In this evaluation standard, O is for a sample having no white turbidity at adjacent to the edge of the film, and no white spot with a diameter of at least 1 mm, Δ for a sample having no turbidity at adjacent to the edge of the film, and white spot with a diameter of 1 to 2 mm, and x for a sample having white turbidity at adjacent to the edge of the film, and white spot with a diameter at least 2 mm. The doping quantities of Aℓ, Si, B, Ti, Sn, Mg, and Cr for all the samples are 4% in atomic ratio, of the total amount including Zn. In sample 2, the pressure of the atmosphere in film deposition, is increased compared with sample 1. In sample 3, the temperature of the substrate in film forming is elevated compared with sample 1. Sample 4 is heated after film deposition. It is found from Table 2, that the moisture resistance of the Low-E film, does not depend on the material of the film, or number of layers; a single layer or multi-layer, and depends on the internal stress and the diffraction angle 2 (center of gravity position) of (002) diffraction line of ZnO. Next, explanation will be given to an oxide film (B) in the invention of this application. By using a multi-layer film as the oxide film (B) composed of, at least one layer of film whose major component is zinc oxide, and one layer of film whose major component is tin oxide, a Low-E film excellent in acid resistance is realized. The tin oxide is excellent in acid resistance, and the optical property such as refractive index is almost the same with that of zinc oxide. Therefore, by replacing a portion of the zinc oxide film, with tin oxide, an oxide film (B) excellent in acid resistance can be composed, while maintaining the optical property. On the other hand, when these films are deposited by a sputtering method, especially by a direct current sputtering method, a zinc oxide film can be deposited by a higher rate than in tin oxide. Therefore, the film composition and film thickness of the oxide film (B) had better be determined considering the acid resistance and the film forming rate. The film thickness of the oxide film (B), although not especially restricted, is desirable to be in the range of 20 to 70 nm (200 to 700 Å), considering the color tone of the total of the Low-E film, and the visible light transmittance thereof. The number of layers and the film thickness of a single layer may be chosen according to an apparatus, and are not especially restricted. Furthermore, the thickness of each layer may be different. Zinc oxide can resist to the influence of acid from the edge of the film, when the zinc oxide layer is divided to a plurality zinc oxide films, and the film thickness of the single layer of zinc oxide is made thinner. Accordingly, the concrete film composition of oxide film (B), had better be composed as in three layer series such as ZnO/SnO 2 /ZnO, or SnO 2 /ZnO/SnO 2 , or five layer series such as ZnO/SnO 2 /ZnO/SnO 2 /ZnO, or SnO 2 /ZnO/SnO 2 /ZnO/SnO 2 , and the film thickness of a single layer of zinc oxide had better be 20 nm (200 Å) at most, favorably 18 nm (180 Å) at most. More favorably, the film thickness is particularly desirable to be 10 nm (100 Å) at most, and the film is composed by the five layer series. Considering the productivity of film depositing, a multi-layer composed of the five layer series is favorable, in which the film thickness of the each layer is unified to about 9 nm (90 Å), and total thickness of the film is about 45 nm (450 Å). Such oxide film layer (B) is further favorable, when the internal stress is 1.1 x 10 10 dyne/cm 2 at most. When the internal stress of zinc oxide is low, the film is difficult to be peeled off by the influence of acid from the edge of the film. Therefore, the low internal stress of zinc oxide is favorable also in view of the acid resistance and the moisture resistance. It is more preferable when the diffraction angle 2 (center of gravity position) of (002) diffraction line of zinc oxide by X-ray diffraction method, is within the range of 33.88° to 35.00°, particularly, from 34.00° to 34.88°. The material of the oxide film 2 other than oxide film (B) is not especially restricted. As the oxide film 2, a film of ZnO, SnO 2 , TiO 2 , and a multi-layer film containing at least 2 kinds of these, and a film further added with the other elements, can be utilized. Furthermore, considering the productivity, a film in which at least two layers of ZnO, SnO 2 , and ZnO-SnO 2 are alternatively laminated, and a film in which at least one of Aℓ, Si, B, Ti, Sn, Mg, and Cr, are added by 10 atomic % at most of the total quantity including Zn, are favorable. Considering the color tone and the visible light transmittance thereof, the thickness of the oxide film 2 is desirable to be in the range of 20 nm (200 Å) to 70 nm (700 Å). In case of a multi-layer film, the total thickness may be in the range of 20 nm (200 Å) to 70 nm (700 Å), and the film thickness of each layer is not restricted. As the metallic film 3 in the present invention, a metal layer with high heat reflective function, whose major component is Ag, or Ag added with at least one of Au, Cu, and Pd, can be utilized. The metallic film 3 may also be comprising a metal layer with various other function, other than the heat reflective metal layer (i.e. Ag layer), for example, a metal layer for controlling the adhesive strength between the heat reflective metal layer and the oxide film 2 and/or the oxide film (B), or a metal layer with a barrier function for preventing the diffusion of the metal from the heat reflective metal layer. Considering the balance between high heat reflective function and high visible light transmittance, the film thickness of the metallic film 3 is desirable to be in the range of 5 nm (50 Å) to 15 nm (150 Å), especially about 10 nm (100 Å). Especially in case of a Low-E film of five layers such as an oxide film, a metallic film, an oxide film, a metallic film, an oxide film alternately formed, or a Low-E film of more than five layers, it is desirable to use an oxide film having an internal stress of 1.1 x 10 10 dyne/cm 2 at most, as an oxide film 2 (an oxide film other than the oxide film (B).) Compared with a conventional Low-E film, the moisture resistance of the Low-E film of the present invention is considerably improved in the moisture resistance, by using a film of low internal stress of 1.1 x 10 10 dyne/cm 2 at most, as an oxide film (B). This is due to the fact that the oxide film is difficult to be destroyed by the low internal stress of the oxide film, and the deterioration by moisture is prevented. Furthermore, the acid resistance is improved, by introducing a film whose major component is tin oxide in the oxide layer (B). EXAMPLES EXAMPLE 1 A Low-E film is produced, which has a film composition of ZnO/SnO 2 /ZnO/SnO 2 /ZnO/Ag/ZnO/glass, by the same method as in REFERENCE EXAMPLE 2. The film thickness of Ag is 10 nm (100 Å), that of ZnO between Ag and glass, 45 nm (450 Å), and those of ZnO layer and SnO 2 layer on top of Ag layer, 9 nm (90 Å). ZnO layer and Ag layer are obtained by sputtering ZnO and Ag targets in Ar gas, and SnO 2 layer is obtained by sputtering SnO 2 target in an atmosphere of a mixed gas of Ar and O 2 . The sputtering pressure, the substrate temperature, and the RF power in film deposition of ZnO and Ag are the same in the above REFERENCE EXAMPLE 2. The power density is 1 W/cm 2 in film deposition of SnO 2 , gas flow rate ratio of Aℓ:O 2 is 8:2. The internal stress of film of ZnO/SnO 2 /ZnO/SnO 2 /ZnO, formed under the same condition as above, is 9.2 x 10 9 dyne/cm 2 . The moisture resistance of the Low-E film obtained, is as favorable as in the above Example. EXAMPLE 2 A Low-E film is produced, of which film composition is ZnO/SnO 2 /ZnO/SnO 2 /ZnO/Ag/ZnO/SnO 2 /ZnO/SnO 2 /ZnO/glass, by the same method as in Example 1. The thickness of Ag layer is 10 nm (100 Å), those of ZnO layer and SnO 2 layer are 9 nm (90 Å) for each layer. The target, and the sputtering gas, the sputtering pressure, the substrate temperature, and RF power density, are the same as in Example 1. The internal stress of the film of ZnO/SnO 2 /ZnO/SnO 2 /ZnO produced under this condition, is 9.2 x 10 9 dyne/cm 2 . The moisture resistance of the obtained Low-E film, is as favorable as in the above Example. EXAMPLE 3 A Low-E film having the composition of ZnO/SnO 2 /ZnO/SnO 2 /ZnO/Ag/ZnO/SnO 2 /ZnO/SnO 2 /ZnO/glass, is made by using metal targets made of zinc, tin, and silver, respectively, in which the Ag film is made by a direct current sputtering method, in argon atmosphere, and the SnO 2 film and the ZnO film are made by a reactive direct current sputtering method in an atmosphere containing oxygen. The thickness of Ag film is 10 nm (100 Å) and those of ZnO film and SnO 2 film are 9 nm (90 Å) for each layer, respectively. The visible light transmittance of the glass having such Low-E film is 86%, and its emissivity is 0.06. An acid resistance test was performed on the glass with Low-E film, in which the glass is immersed in 1 N hydrochloric acid. No change is observed until 2 minutes after the immersion. However, after 3 minutes the color of the sample begins to change into brownish color from edge of the film. After 5 minutes a part of the film is observed to be exfoliated. COMPARATIVE EXAMPLE 1 A Low-E film having the film composition of ZnO/Ag/ZnO/glass is made, by using metal targets made of zinc and silver, respectively, in which the Ag film is made by a direct current sputtering method, in argon atmosphere, and the ZnO film is made by a reactive direct current sputtering method, in a atmosphere containing oxygen. The thickness of the Ag film is 10 nm (100 Å), and the thickness of the ZnO film is 45 nm (450 Å). The visible light transmittance of the glass having the Low-E film, is 86%, and its emissivity is 0.06. An acid resistance test was performed on the glass with Low-E film, in which the glass is immersed in 1 N hydrochloric acid. The film begins to be exfoliated just after the immersion, and after 5 minutes, the Low-E film is totally exfoliated from the glass, and vanished. In the Low-E film of this invention, the moisture resistance and the acid resistance are significantly improved. Accordingly, the handling of the glass with such Low-E film on single plate, is likely to become easy. Furthermore, the possibility of the preservation of such glass in single plate for a long time in a room, is realized. Furthermore the reliability of the Low-E glass for an automobile or for a building, is promoted. When the film is used in a laminated glass, the glass is not deteriorated by the moisture contained in an intermediate film, which improves the durability of the laminated glass for an automobile or for a building. The Low-E film of this invention is electrically conductive, for it is comprising the metal layer. Therefore the Low-E film of the present invention can be used in various technical fields, for example, as an electrode in electronics field, or as a heating element for electrically heated window, or as an electromagnetic shielding film for a window or for electronics use. In some cases, the Low-E film of the present invention can be applied on a substrate with various functioning layers therebetween, in which case the optical property of this Low-E film can be adjusted depending on each purpose by choosing suitable film thickness for each layer comprised in the Low-E film of the present invention.

Claims: A low emissivity film which comprises: a substrate; and a coating of oxide films (B) and metallic films (A) alternately formed on the substrate in a total of (2n+1) layers where n is an integer being equal to or more than 1, with the innermost layer being an oxide film, wherein the oxide film (B) formed on the outer side of the metallic film (A) being most apart from the substrate, is a multi-layer film having at least a layer whose major component is zinc oxide and a layer whose major component is tin oxide. The low emissivity film according to Claim 1, wherein the oxide film (B) has a multi-layer film comprising at least three layers in which a single layer or a plurality of layers, the major component thereof being zinc oxide and a single layer or a plurality of layers, the major component thereof being tin oxide, are alternatively formed. The low emissivity film according to Claim 2, wherein the oxide film (B) has a multi-layer film comprising three layers or five layers in which a plurality of layers, the major component thereof being zinc oxide (C), and a single layer or a plurality of layers, the major component thereof tin oxide (D), are alternatively laminated in an order of (C) layer, (D) layer, and (C) layer. The low emissivity film according to Claim 2, wherein the oxide film (B) has a multi-layer film comprising three layers or five layers in which a single layer or a plurality of layers whose major component is zinc oxide (E), and a plurality of layers whose major component is tin oxide (F), are alternatively laminated in an order of (F) layer, (E) layer, and (F) layer. The low emissivity film according to Claim 1, wherein the crystal structure of said zinc oxide is hexagonal, and a value of a diffraction angle 2 (center of gravity position) of (002) diffraction line of the hexagonal zinc oxide in X-ray diffraction method using CuKα radiation, is not smaller than 33.88° and not larger than 35.00°. The low emissivity film according to Claim 1, wherein the metallic film (A) is a metallic film whose major component is Ag. The low emissivity film according to Claim 1, wherein at least one layer of the oxide films except the oxide film (B) is a multi-layer film having at least a layer whose major component is zinc oxide and a layer whose major component is tin oxide. The low emissivity film according to Claim 1, wherein the outermost layer of said oxide film (B) is a layer for controlling the adhesive strength thereof with a plastic intermediate film for lamination with another substrate.

Inventors: ANDO EIICHI, MIYAZAKI MASAMI, ANDO, EIICHI, MIYAZAKI, MASAMI
Applicants: ASAHI GLASS CO LTD, ASAHI GLASS COMPANY LTD.
Application year: 1991
Main classification: C03C17
IPC codes: C03C17, C23C14