CA2161282C

CA2161282C - Multilayer antireflective coating with a graded base layer

Info

Publication number: CA2161282C
Application number: CA002161282A
Authority: CA
Inventors: George A. Neuman
Original assignee: PPG Industries Ohio Inc
Current assignee: PPG Industries Ohio Inc
Priority date: 1994-12-27
Filing date: 1995-10-24
Publication date: 2001-07-24
Anticipated expiration: 2015-10-24
Also published as: DK0721112T3; US5811191A; ES2182862T3; CN1134555A; KR960022314A; JPH08231248A; DE69528143T2; EP0721112B1; EP0721112A3; US5948131A; CA2161282A1; AU669319B1; ATE224064T1; KR100186922B1; DE69528143D1; EP0721112A2

Abstract

An antireflectance coating is disclosed comprising a first graded layer wherein the composition is varied throughout the thickness of the layer such that the refractive index of the graded layer varies from a low refractive index approximately matching the refractive index of the substrate at the interface of the graded layer and the substrate to a higher refractive index at the surface of the graded layer opposite the interface with the substrate, and a second substantially homogeneous layer of a composition selected to have a refractive index which is approximately the square root of the product of the higher refractive index of the graded layer and the refractive index of the incident medium at the surface of the second layer opposite the interface of the second layer with the graded layer, having an optical thickness of approximately at least one quarter of a selected design wavelength. The antireflectance properties of the coating can be expanded to a broader range of reflected wavelengths by incorporating, between the graded layer and the second substantially homogeneous layer, an intermediate layer having a relatively high refractive index and an optical thickness of about half the design wavelength.

Description

MULTILAYER ANTIREFLECTIVE COATING
WITH A GRADED BASE LAYER
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates generally to the art of antireflective coatings, and more particularly to the art of multilayer antireflective coatings on glass.
to RELEVANT ART
A method for attenuating the refractive index difference between a metal oxide and a glass substrate is disclosed in U.S. Patent No. 3,378,396 to Zaromb, wherein a glass substrate is coated by simultaneously directing separate Zs sprays of a tin chloride solution and a silicon chloride solution onto a stationary heated glass substrate in an oxidizing atmosphere, e.g. air. The heat of the glass substrate thermally converts the metal chlorides to their metal oxides. The ratio of the sprays to each other are gradually zo varied to vary the ratio of the weight percent of the metal oxides in the coating. The resultant coating has a continuously changing composition throughout its thickness, e.g. near the glass-coating interface, the coating is predominantly silicon oxide, the surface of the coating 2s furthest from the glass-coating interface is predominantly tin oxide, and between the surfaces the coating is made up of varying weight percent amounts of silicon oxide and tin oxide.
U.S. Patent Nos. 4,206,252 and 4,440,882 teach the depositing of a second coating composed of fluorine-doped tin 30 oxide on a gradient coating such as described above.

- 2 -SUMMARY OF THE DISCLOSURE
The present disclosure relates to a method of coating a substrate with a multilayer antireflective coating comprising a first layer having a continuously changing chemical s composition, and a correspondingly continuously changing refractive index, as the distance from the glass-coating interface increases, and a relatively homogeneous second layer with a refractive index and thickness selected to provide antireflectance. The first layer, referred to herein as the to graded layer, gradually changes in refractive index from an approximate match to the refractive index of the substrate at the substrate surface to a higher value at the surface of the graded layer away from the interface of the coating and substrate. The second layer, placed on top of the graded is layer, is referred to herein as the antireflective layer. The approximate refractive index at the high value of the graded layer is chosen such that a composition is selected for the antireflective layer that has a refractive index which is approximately the square root of the refractive index of the 2o incident medium times the refractive index of the graded layer at the higher value. The antireflective layer is applied at an optical thickness that is one quarter the design wavelength, i.e. the wavelength selected for optimum antireflectance.
Thus, for example, at normal incidence, if the refractive index 2s of the graded layer at the high value is 2.1 and the incident medium is air, with a refractive index of 1.0, then the refractive index of the antireflective layer would be approximately 1.45, with a physical thickness of approximately 900 Angstroms for a design wavelength of 5000 Angstroms. The 3o performance of the antireflective coating of the present invention can be extended to a broader range of reflected wavelengths by placing between the graded layer and the 2161282,

- 3 -antireflective layer an intermediate layer having a relatively high refractive index and an optical thickness of about half the design wavelength.
DESCRIPTION OF THE DRAWING
Figure la illustrates the refractive index profile for a coating having minimum luminous reflectance Y at a 65°
viewing angle, showing the index of refraction as a function of distance from the substrate surface, through the graded layer io and then the homogeneous layer, where the thickness of the graded layer is 1189 Angstroms and the Si02 layer is 1121 Angstroms. Figure lb shows the reflectance as a function of wavelength over the visible spectrum at 65° from normal observation angle where the luminous reflectance Y is 5.5 and the average reflectance is 8 percent.
Figure 2a illustrates the refractive index of a coating with the same graded layer as Figure 1, but with a silica layer thickness of 886 Angstroms thick for luminous reflectance Y optimization at normal incidence. Figure 2b is 2o the corresponding reflectance curve for the coating, with average reflectance of 1.3 percent and luminous reflectance Y
of 0.33 percent.
Figure 3a illustrates the refractive index profile for a coating with the graded layer optimized for average 2s reflectance minimization at normal incidence. The graded layer thickness is 1373 Angstroms and the silica layer thickness is 911 Angstroms. Figure 3b is the corresponding reflectance as a function of wavelength for the coating, with an average reflectance of 0.8 percent and luminous reflectance Y of 0.44 3a percent.
Figure 4a illustrates a different gradient for luminous reflectance Y minimization, having a graded layer 216 1282 ~ .

- 4 -thickness of 1338 Angstroms and silica layer thickness of 934 Angstroms. Figure 4b is the reflectance as a function of wavelength for the coating, having an average reflectance of 1.02 percent and luminous reflectance Y of 0.33 percent.
s Figure 5a illustrates the refractive index profile for the same graded layer as in Figure 4, with a graded layer thickness of 1338 Angstroms, but with a silica layer thickness chosen to minimize the reflectance at 7000 Angstrom wavelength.
The silica layer thickness is 1200 Angstroms. Figure 5b shows io the reflectance as a function of wavelength, with zero reflectance at the 7000 Angstrom design wavelength.
Figure 6a illustrates the refractive index profile for the same graded layer as shown in Figures 4 and 5, but with a half wave thick layer of Ti02 between the graded layer and the is silica layer. The result is a broader band reflectance well and lower luminous reflectance Y than for the example in Figure 4. Figure 6b shows the reflectance as a function of wavelength.
Figure 7a illustrates the refractive index profile of 2o a coating with a graded layer optimized for use with a half wave thick titanium dioxide layer for minimization of luminous reflectance Y. The graded layer is 960 Angstroms, the half wave layer of titanium dioxide is 1008 Angstroms, and the silica layer is 908 Angstroms. Figure 7b shows the very low 2s reflectance across the spectrum. Average reflectance is 1.4 percent and luminous reflectance Y is 0.03 percent.
Figure 8a illustrates the normal reflectance as a function of wavelength for three different thicknesses of silica over a graded layer of mixed oxides of silicon and tin.
3o Figure 8b illustrates the reflectance of the same three silica layer thicknesses over a graded layer of mixed oxides of silicon and tin at a 65° from normal angle of incidence.

- 5 -DESCRIPTION OF PREFERRED EMBODIMENTS
The method for depositing the graded layer of the present invention preferably includes the steps of directing a s vapor coating composition of metal-containing precursors, e.g.
a silicon containing precursor and a tin-containing precursor, onto the surface of the glass ribbon; moving a first portion of the vapor along a first region of the ribbon surface in a first direction and a second portion of the vapor along a second to region in a second opposite direction; and maintaining the first portion of the coating composition on the first region for a longer period of time than the second portion of the coating composition on the second region, to provide a coating on the glass ribbon having a varying composition of metal is oxides as the distance from the coating-glass interface increases.
The invention preferably involves a transparent substrate, e.g. a glass substrate, having a coating thereon composed of mixed metal oxides, e.g. silicon oxide and tin zo oxide. The coating composition has a continuously changing ratio of silicon oxide to tin oxide as the distance from the glass-coating interface increases, e.g. substantially all-silicon oxide at the glass coating interface and substantially all tin oxide at the opposite surface of the graded layer.
2s Between the glass-coating interface and the opposite coating surface there are minimal, if any, strata of a fixed ratio of silicon oxide to metal oxide and there may be dispersed in the coating small amounts of phosphorus, boron, and/or aluminum when compounds containing those elements are used as 3o accelerants to increase the coating deposition rate and control the coating morphology.

- .6 -An article embodying the present invention includes a substrate, e.g. clear or colored glass, having a coating that exhibits minimum reflection of visible light by having a continually varying refractive index first layer and a s second layer of specifically selected refractive index and thickness. An optional layer of half wave optical thickness may be deposited between the graded layer and the antireflective layer to provide improved antireflectance performance over a broader.wavelength spectrum. The refractive io index of this intermediate layer is relatively high, preferably in the range of 1.7 to 3.0, more preferably in the range of 1.8 to 2.8. A preferred composition for the intermediate layer is titanium dioxide, Ti02. In the following examples, the substrate is a glass substrate. The first layer of the coating is preferably comprises a mixture of silicon oxide and a metal oxide, such as tin oxide. The coating has a continuously changing composition as the distance from the glass-coating interface increases. Generally, near the glass-coating interface, the coating is predominantly silicon oxide, while at 2o the opposite surface of the coating, i.e. the coating surface farthest from the glass-coating interface, the composition of the coating is predominantly tin oxide. Although the coating is discussed using a first layer of tin oxide and silicon oxide, the invention is not limited thereto and, as will be 2s appreciated from the discussion below, any two or more compounds may be used in the practice of the invention to vary the refractive index of the graded layer.
The coating method, apparatus and reactants described in detail in Canadian Patent Application 2,114,971 filed 4 February 1994, corresponding to U.S. Patent 5,356,718 of Athey et al. are especially effective for the chemical vapor deposition (CVD) of coatings from mixtures of silicon and metal containing precursors to form the graded layer in accordance with the present invention.
The first graded layer of the antireflective coating of the present invention is preferably made from a mixture of s tin containing precursors and silicon containing precursors capable of being volatilized and converted to their corresponding oxides in the presence of oxygen at temperatures in the range of about 750° to about 1500°F (about 400°C
to about 815°C). As will be appreciated, the invention is not limited to thereto and other metal containing precursors may be used with the coating apparatus and in the coating processes discussed above.
When the substrate, e.g. a glass substrate, is subjected to chemical vapor deposition of mixed oxides, for is example, a mixture of silicon oxide and tin oxide, to obtain the graded layer thereon in accordance with the invention, the graded layer is characterized by having a continuously varying composition as the distance from the glass-coating interface increases, resulting in a substantial reduction in reflectance zo of the coated product. Assuming a coating composed of substantially silicon oxide and tin oxide, that portion of the coating adjacent to the glass-coating interface is composed largely of silicon oxide and as the distance from the glass-coating interface increases, each succeeding region of the 2s continuously varying composition contains a silicon oxide to tin oxide ratio that varies as the distance from the glass-coating interface increases. More particularly, the percent of silicon oxide decreases as the percent of tin oxide increases, so that as the opposite surface of the graded layer is reached, 3o the region is composed predominantly of tin oxide.
An advantage of the present invention is that the change in refractive index of the graded layer from an ~~ 8 ~ 282 _8_ approximate match at the substrate surface to the higher value can be accomplished in a flexible manner. The simplest, manner is a linear change with gradual transitions at the substrate and high index ends. However many additional profiles are s suitable. The primary requirement of the graded layer of the present invention is that a gradual change in refractive index occurs between the substrate refractive index and the higher index value. Gradual is defined herein as such that the change in refractive index between one area of the graded coating to io an adjacent area is not greater than about 0.1. The refractive index of the graded layer can decrease in value for portions of the coating as long as the change is gradual and the net change through the coating is an increase to the higher value. The refractive index change may be provided as a plurality of fixed is index layers. The thickness of the graded layer must be at least about a quarter wave optical thickness, but the performance of the antireflective coating improves as this layer thickness increases. The maximum thickness is limited primarily by cost. Subtle variations of the antireflective zo coating thickness and the graded layer composition and thickness may be necessary to optimize the coating performance for a given application or to compensate for observation angle effects.
An additional advantage of the use of a graded layer 2s in an antireflective coating is achieved when the coated article is intended for use at a high installation angle, such as an automotive windshield. When the installation angle is higher than the Brewster angle (defined as the inverse tangent of the refractive index) of the coating, the s (perpendicular) 3o and p (parallel) polarization states cannot be simultaneously minimized. Since the graded layer provides a new surface having a higher refractive index, and therefore, a higher Brewster angle, antireflection performance may be optimized at higher installation angles. For example, a glass substrate with a refractive index of 1.52 has a Brewster angle of 56°. If the higher refractive index value of the graded layer is e.g.
s 2.0, the Brewster angle is 63°. If the higher refractive index value of the graded layer is e.g. 2.4, the Brewster angle is 67°. With a higher refractive index surface on which to deposit the low refractive index antireflectance layer, e.g. silica at about 1.45, the multilayer coating can be designed to better to minimize both s and p polarization states to give better overall antireflectance performance at higher installation angles. Such improved antireflectance can also be achieved in accordance with the present invention with thinner coatings, and a simple and flexible multiple layer coating configuration.
is The present invention will be further understood from the description of specific examples which follow. In the first seven examples, data are obtained for a first graded layer comprising a varying mixture of silicon oxide having a refractive index of about 1.5 and an oxide of zinc and tin 2o having a refractive index of about 2.0, and a second homogeneous layer of silica, having a refractive index of about 1.5, on a substrate of green tinted glass having a refractive index of about 1.5, with the incident medium being air, with a refractive index of 1Ø The last example involves three 2s different silica layer thicknesses over a graded layer of oxides of silicon and tin produced by chemical vapor deposition in a float glass bath. Data are obtained for reflectance at both normal incidence and 65° from normal incidence.
3o EXAMPLE I
A ribbon of green tinted glass 3.9 millimeters thick is coated with a first graded layer comprising the oxides of ~1~1282 - 1~ -silicon, zinc and tin. The graded layer comprises primarily silicon oxide at the glass-coating interface and changes composition to primarily metal oxides at the opposite coating surface. The thickness of the graded layer is about 1190 s Angstroms. Over the graded layer is deposited a homogeneous layer of silica. The thickness of the Si02 layer is about 1120 Angstroms. The luminous reflectance Y of the coated surface is about 5.5 percent, and the average reflectance from the coated surface over the visible spectrum is about 8 percent. The io refractive index gradient is illustrated in Figure la, and the reflectance, optimized for a 65° from normal angle of incidence, is illustrated in Figure lb.
EXAMPLE II
is A ribbon of green tinted glass 3.9 millimeters thick is coated with a first graded layer comprising the oxides of silicon, zinc and tin, with primarily silicon oxides at the glass-coating interface and primarily metal oxides at the opposite coating surface, with a thickness of about 1190 2o Angstroms. The graded layer is overcoated with silica as in the previous example except that the silica thickness is about 886 Angstroms. The average reflectance from the coated surface is about 1.34 percent, and the luminous reflectance Y is less than 1 percent. The refractive index gradient is illustrated 2s in Figure 2a, and the reflectance, optimized for normal (90°) angle of incidence, is illustrated in Figure 2b.
EXAMPLE III
A ribbon of green tinted glass 3.9 millimeters thick 3o is coated with a graded layer as in the previous examples, except that the thickness of the graded layer is about 1373 Angstroms. The graded layer is overcoated with silica as in 2~ ~ 1 282 - l~ -the previous examples, except that the thickness of the silica layer is about 911 Angstroms. The luminous reflectance Y from the coated surface is about 0.44 percent, and the average reflectance from the coated surface is about 0.8 percent. The s refractive index gradient is shown in Figure 3a. This coating configuration optimizes, i.e. minimizes, the average reflectance as shown in Figure 3b.
EXAMPLE IV
io A ribbon of green tinted glass 3.9 millimeters thick is coated with a graded layer as in the previous examples, except that the thickness of the graded layer is about 1338 Angstroms. The graded layer is overcoated with silica as in the previous examples, except that the thickness of the silica is layer is about 934 Angstroms. The average reflectance from the coated surface is about 1 percent, and the luminous reflectance 4 is about 0.33 percent. The refractive index gradient is shown in Figure 4a. This coating configuration optimizes i.e.
minimizes, the luminous reflectance Y at normal incidence as zo shown in Figure 4b.
EXAMPLE V -A ribbon of green tinted glass 3.9 millimeters thick is coated as in the previous examples with a graded layer zs having a thickness of 1338 Angstroms. The graded layer is overcoated with silica as in the previous examples, except that the thickness of the silica layer is 1200 Angstroms. The refractive index gradient is shown in Figure 5a. The coating configuration of this example exhibits essentially zero 3o reflectance at the design wavelength of 7000 Angstroms as can be seen in Figure 5b.

1~~282 EXAMPLE VI
A ribbon of green tinted glass 3.9 millimeters thick is coated as in the previous examples with a graded layer having a thickness of 1338 Angstroms. The graded layer is overcoated with a layer of titanium dioxide 1115 Angstroms thick. A layer of silica 933 Angstroms thick is deposited over the titanium oxide layer. The refractive index gradient is illustrated in Figure 6a. The coated surface has an average reflectance of 3.2 percent and a luminous reflectance Y of 0.2 io percent as illustrated in Figure 6b.
EXAMPLE VII
A ribbon of green tinted glass 3.9 millimeters thick is coated as in the previous examples with a graded layer is having a thickness of 960 Angstroms. The graded layer is overcoated with titanium oxide as in the previous examples, but with a thickness of 1008 Angstroms. A final layer of silica is 908 Angstroms thick. The refractive index gradient is shown in Figure 7a. The coated surface has a very low reflectance 2o across the visible spectrum, with an average reflectance of 1.4 percent and a luminous reflectance of 0.03 percent as shown in Figure 7b.
EXAMPLE VIII
2s A soda-lime-silica float glass ribbon supported by and moving 280 inches (about 7.1 meters) per minute along a molten metal bath comprising primarily tin was coated on its top surface with a graded layer by contacting the surface at a temperature of about 1200°F (about 650°C) with a vaporized 3o mixture of varying concentrations of tetraethoxysilane (TEOS) and monobutyltin trichloride (MBTC). A first chemical vapor deposition (CVD) cell was supplied with a coating mixture at ~1 6 ~ 282 concentrations, in mole percent in a 90 SCFM carrier gas stream of 14 percent oxygen in nitrogen, ofØ4 water, 0.2 MBTC, 0.29 TEOS and 0.28 triethylphosphite (TEP, an accelerant). A second cell downstream from the first was supplied with a mixture of s 0.58 water, 0.62 MBTC, 0.62 TEOS and 0.1 TEP. The resulting graded layer was coated, outside the float bath, with silica layers at three thicknesses, 749 Angstroms, 815 Angstroms, and 1113 Angstroms. The combined reflectance from both surfaces as a function of wavelength in the visible spectrum at normal to incidence for these three configurations is shown in Figure 8a.
The reflectance from the coated surface only at a 65° from normal observation, or installation, angle is illustrated in Figure 8b.
Zs The above examples illustrate the antireflectance coating of the present invention. Various compositions may be used for the graded layer as well as the homogeneous layer, particularly if substrates or incident media of different refractive index are employed. Any convenient method of 2o coating deposition may be used, especially for the homogeneous layer, where sputtering may be preferred. The scope of the present invention is defined by the following claims.

Claims

CLAIMS:

1. A low reflectance article comprising:
a supporting surface having a refractive index, and a visible reflectance of a predetermined value;
a first film comprising a mixture of oxides, the first film having a first surface and a second surface opposite to the first surface with the second surface of the first film on the supporting surface, the first film at interface provided by the second surface of the first film and the supporting surface having a refractive index approximately equal to the refractive index of the supporting surface and between the first and second surfaces of the first film having a refractive index having a generally increasing value as the distance from the second surface of the film increases, and a second film having a first surface and a second surface opposite to the first surface with the second surface of the second film contacting the first surface of the first film, the second film having a substantially uniform refractive index throughout its thickness, with the refractive index substantially equal to the square root of the product of the refractive index of the first film at the first surface of the first film and refractive index of medium in contact with the first surface of the second film and an optical thickness approximately equal to one quarter of a selected design wavelength.

2. The article as set forth in claim 1 wherein the supporting surface is a major surface of a substrate.

3. The article as set forth in claim 1 wherein change in the refractive index between one area of the first film and adjacent area of the first film between the first and second surface of the first film is not greater than about 0.1.

4. The article as set forth in claim 1, wherein the substrate is glass.

5. The article as set forth in claim 1, wherein the thickness of the first film is the range of about 500 to 3000 Angstroms.

6. The article as set forth in claim 1, wherein the thickness of the second film is in the range of about 400 to 2000 Angstroms.

7. The article as set forth in claim 1 wherein the substrate has a visible reflectance from the support surface in the range of about 4 to 40 percent.

8. The article as set forth in claim 4, wherein the mixture of oxides includes silicon and tin.

9. The article as set forth in claim 4, wherein the second film has a refractive index in the range of about 1.4 to 1.5.

10. The article as set forth in claim 8, wherein the second film is silica.

11. The article as set forth in claim 9, wherein the first film has a refractive index in the range of about 1.4 to 1.65 at the second surface of the first film.

12. The article as set forth in claim 11, wherein the first film has a refractive index in the range of about 1.7 to 2.8 at the first surface of the first film.

13. The article as set forth in claim 12, wherein the average reflectance in the visible spectrum from the article is in the range of 0.01 to 1.5 percent.

14. The article as set forth in claim 12, wherein the luminous reflectance Y is in the range of about 0.01 to 1.5 percent.

15. The article as set forth in claim 12, wherein the average reflectance in the visible spectrum is in the range of 0.1 to 1.5 percent and the luminous reflectance is in the range of 0.01 to 1.5 percent.

16. A low reflectance article comprising:
a supporting surface having a refractive index, and a visible reflectance of a predetermined value;
a first film comprising a mixture of oxides, the first film having a first surface and a second surface opposite to the first surface with the second surface of the first film on the supporting surface, the first film at interface provided by the second surface of the first film and the supporting surface having a refractive index approximately equal to the refractive index of the supporting surface and between the first and second surfaces of the first film having a refractive index having a generally increasing value as the distance from the second surface of the film increases;
a second film having a first surface and a second surface opposite to the first surface with the second surface of the second film spaced from the first surface of the first film, the second film having a substantially uniform refractive index throughout its thickness, with the refractive index substantially equal to the square root of the product of the refractive index of the first film at the first surface of the first film and refractive index of medium in contact with the first surface of the second film and an optical thickness approximately equal to one quarter of a selected design wavelength, and a third film between the first and second films, the third film having an optical thickness of about one-half the selected design wavelength.

17. The article as set forth in claim 16 wherein the supporting surface is a major surface of a substrate.

18. The article as set forth in claim 17, wherein the substrate is glass.

19. The article as set forth in claim 17, wherein the thickness of the first film is the range of about 500 to 3000 Angstroms.

20. The article as set forth in claim 17, wherein the thickness of the second film is in the range of about 400 to 2000 Angstroms.

21. The article as set forth in claim 17, wherein the substrate has a visible reflectance from the support surface in the range of about 4 to 40 percent.

22. The article as set forth in claim 17 wherein change in the refractive index between one area of the first film and adjacent area of the first film between the first and second surfaces of the first film is not greater than about 0.1.

23. The article as set forth in claim 17, wherein the third film contacts the second surface of the second film and the first surface of the first film.

24. The article as set forth in claim 18, wherein the mixture of oxides includes silicon and tin.

25. The article as set forth in claim 18, wherein the second film has a refractive index in the range of about 1.4 to 1.5.

26. The article as set forth in claim 24, wherein the second film is silica.

27. The article as set forth in claim 25, wherein the first film has a refractive index in the range of about 1.4 to 1.65 at the second surface.

28. The article as set forth in claim 27, wherein the first film has a refractive index in the range of about 1.7 to 2.8 at the first surface.

29. The article as set forth in claim 28, further wherein the third film has a refractive index in the range of about 1.7 to about 3Ø

30. The article as set forth in claim 29, wherein the average reflectance in the visible spectrum from the article is in the range of 0.01 to 1.5 percent.

31. The article as set forth in claim 29, wherein the luminous reflectance Y is in the range of about 0.01 to 1.5 percent.

32. The article as set forth in claim 22, wherein the thickness of the graded layer is at least a quarter wave optical thickness.

33. The article as set forth in claim 23 wherein the third film is a titanium dioxide film.