WO2001090638A9

WO2001090638A9 - Two-layer multipurpose window coating for a transparent substrate

Info

Publication number: WO2001090638A9
Application number: PCT/US2001/016208
Authority: WO
Inventors: Floyd E Woodard; Yisheng Dai; Richard P Shimshock
Original assignee: Southwall Technologies Inc; Globamatrix Holdings Pte Ltd
Priority date: 2000-05-24
Filing date: 2001-05-17
Publication date: 2002-10-03
Also published as: AU2001263293A1; WO2001090638A1; TWI282837B

Abstract

A solar screening, thermally insulating, glare reducing, anti-reflecting coating (24) is formed of a two-layer stack on a substrate (12), with the first layer (22) being formed of titanium nitride and the second layer (26) being formed of a dielectric material having a high refractive index. The first layer is an absorbing layer and the dielectric layer has a refractive index in the range of 1.73 to 2.6. In the preferred embodiment, the dielectric layer is silicon nitride. Also in the preferred embodiment, a thin adhesive primer layer (20) is located between the substrate and the multipurpose window coating, while a lubricating layer (28) is formed on the dielectric layer.

Description

TWO-LAYER MULTIPURPOSE WINDOW COATING FOR A TRANSPARENT SUBSTRATE

TECHNICAL FIELD

The invention relates generally to providing desired optical properties to a surface of a light transmissive member, and relates more particularly to providing a coating on a surface of a substrate, such as automotive or architectural glass, which provides solar screening, thermal insulation and glare reduction while maintaining a low internal reflection.

BACKGROUND ART

Coatings are applied to light transmissive members in order to impart desired optical properties to the members. For example, one or more coatings may be applied to automotive glass or to commercial or architectural glass (e.g., glass for a home or commercial building). The coatings may be applied directly to the glass or to a substrate which can then be adhered to the glass as an aftermarket product.

There are a number of optical and mechanical properties that must be considered in the selection of materials for forming a multipurpose window coating. Aspects of seven such properties will be described. A first desired property is "low internal reflection." It is desirable to have a glass/ coating laminate with an internal visible reflection that is less than 2.6 percent, and more preferably with an internal reflection less than 2 percent. Moreover, this low internal reflection is preferably obtained with a coating on only one side of the glass. A reasonable way to limit the total internal reflection to the desired level is to limit the reflection from the coating to 1.3 percent, and more preferably 1 percent, and to limit the reflection from the backside of the glass

(i.e., the uncoated side) to 1.3 percent and preferably 1 percent. To achieve this objective, the visible light transmission of the coating must be reduced to a level that reduces the reflection from the uncoated glass surface. A "conventional" clear glass plate has a reflection of approximately 4 percent. Therefore, in order to achieve the limit of approximately 1.3 percent backside reflection contribution, the transmission of the coating must be less than 57 percent (the contribution of the front glass surface to internal reflection can be approximated by the equation: 4 percent * transmission²). If the backside reflection is to be limited to 1 percent, the visible transmission of the coating must be less than 50 percent.

A second desirable optical property is "low light transmission." In an aftermarket product to be applied to automotive glass or architectural glass, it is desirable to suppress the visible light transmission (T_vis) to reduce glare during daylight hours. A visible transmission of 20 percent to 57 percent (more preferably 25 percent to 50 percent) is desirable.

A third optical property of concern is "solar rejection," which is achieved by providing a low "shading coefficient." A multipurpose window coating should provide solar rejection when viewed from the side of the glass opposite to the side that bears the coating (i.e., the non-coated side). The degree of solar rejection is most readily quantified by the term shading coefficient. This parameter represents the fraction of solar energy that a glazing element passes relative to that passed by a double strength annealed reference glass, which is commonly 0.125 inch (3.175 mm) thick window glass. Thus, a glass structure with a shading coefficient of 0.5 passes only half of the solar energy that the reference glass would pass. The reference glass allows approximately 87 percent of the incident solar energy to pass. The equation used to calculate the shading coefficient is: SC = (solar transmission + 0.27 * solar absorption)/.87, where solar absorption equals 1 - solar reflection - solar transmission. The standardized method for calculating solar transmission and solar absorption is given by ASTM E424B.

A fourth optical property is "solar selectivity." Solar selectivity refers to the ability of a structure to pass visible light, while rejecting invisible light (i.e., near infrared light). Solar selectivity is typically calculated as T_vis T_S0|. Here, T_vis is an integrated weighted transmission that accounts for the eye's wavelength selectivity, as well as the spectral distribution of solar energy. T_so, is an integrated weighted transmission that accounts only for the spectral distribution of solar energy. Dyed window films generally have solar selectivity values less than 1 , since they primarily block only the visible portion of the solar spectrum. Grey metal films (as commonly used in inexpensive applied window films), such as stainless steel, titanium and nichrome, generally have a selectivity of about 1 , since they attenuate solar radiation equally across the solar spectrum.

"Low emissivity" is another desired optical property of a multipurpose window coating. In use, an antireflection coating on automotive or architectural glass is directly exposed to light on the inboard side of the window structure. For a low emissivity surface to be effective at suppressing heat loss from a room, it must be directly in contact with air or some other infrared-transparent medium. Thus, in this application, it is most desirable for the coating to have a low emissivity. An emissivity of less than 0.5 is most preferred, since it provides an increase in the R value of a monolithic window by more than approximately 19 percent. However, an emissivity of less than 0.6 is acceptable, since it is associated with an increase in the R value by more than 12 percent.

A physical property of concern is "chemical durability." Since window coatings are often exposed directly to the environment, it is imperative that the coatings be chemically stable. Many low emissivity and anti- reflection coatings contain metals which are not chemically stable. Such metals include silver and nichrome. Providing a top antireflection layer that is robust and chemically inert is important. The last property to be identified is "antisoiling." Since the window coating described here is an interference coating that is directly exposed to the atmosphere, it is susceptible to being soiled with a fingerprint or other surface contaminant. In order to minimize the extent to which contaminants are bound to the coating surface, a topcoat may be applied. What is needed is a cost efficient and practical method of fabricating an optical member that exhibits desirable optical and mechanical properties.

SUMMARY OF THE INVENTION

The multipurpose window coating is a two-layer stack on a substrate, with the first layer being formed of titanium nitride and the second layer being formed of a dielectric material having a high refractive index, i.e., a refractive index of at least 1.73. As a result of practical considerations, the titanium nitride layer will likely include oxygen. In the preferred embodiment, the dielectric layer is silicon nitride. Also in the preferred embodiment, a thin adhesive primer layer is located between the substrate and the first layer, while a lubricating layer is formed on the second layer of the window coating. With regard to the titanium oxynitride, the first layer preferably has a thickness in the range of 5 nm to 75 nm, and more preferably in the range of 10 nm to 40 nm. The first layer may be sputter deposited in an environment which results in a coating in which the oxygen content is less than 30 atomic percent, and more preferably less than 25 percent. Regarding the dielectric layer (e.g., silicon nitride, Si₃N₄ with a refractive index of approximately 2.0), the dielectric material may be sputter deposited to a thickness in the range of 15 nm to 90 nm, and more preferably 25 nm to 65 nm. The refractive index is in the range of 1.73 to 2.6, and more preferably in the range of 1.73 to 2.37.

Regarding the properties of the combined layers, the emissivity is less than 0.6, and more preferably less than 0.5. The T_vιs is in the range of 20 percent to 57 percent, and more preferably in the range of 25 percent to 50 percent. For reasons stated above, the R_vιs value on the coated side and the R_V1S value of backside contribution are below 1.3 percent, and more preferably below 1.0 percent. Solar selectivity (T_VIS/T_S0,) is greater than 1.1 , and more preferably greater than 1.15. Finally, the shading coefficient should be less than 0.9, and more preferably less than 0.8.

The first surprising aspect of the two-layer multipurpose window coating is that it provides antireflection of visible light while having a top layer with a relatively high refractive index (i.e., greater than or equal to 1.73). Conventionally, such a layer is formed of a lower index material, such as silicon dioxide with a refractive index of 1.46 to 1.48 or magnesium fluoride with a refractive index of approximately 1.39. A second surprising aspect relates to the T_vιs values corresponding to optimum antireflection properties. Generally, in an absorbing antireflection stack, the optimum antireflection performance is found for only a narrow range of thicknesses in the absorbing layer. Because of the correspondence between this thickness and the visible reflection by resulting coatings, many prior art coatings provide suitable antireflection properties only over a small range of visible transmissions. For example, when the titanium oxynitride is combined with silicon dioxide (rather than silicon nitride) to make an anti- reflection coating, the optimum T_vis is approximately 70 percent. In comparison, the absorbing antireflection coating in accordance with the invention is not optimized at high light transmissions. Rather, the antireflection coating provides light transmissions of less than 57 percent. For example, in the case in which the two-layer coating is TiO_xN_y and Si₃N₄, the optimum T_vιs of the unlaminated film is about 46 percent. (The optimum transmission for a particular antireflection coating in accordance with the invention is determined by the refractive index of the top dielectric layer.)

Regarding the adhesive primer layer between the substrate and the two-layer coating, the effectiveness of this layer is determined in part by the composition of the substrate or a hardcoat layer on the substrate. Typically, a hardcoat layer will be present. A silicon primer has been found to be effective for both acrylate-based and epoxy-based hardcoats. The adhesion-promoting layer may be as thin as 1 monolayer, with a thickness in the range of 0.2 nm to 2 nm. Although silicon is the preferred primer layer, other materials which readily oxidize are believed to be effective. Examples of these alternate primer layers include chromium, titanium and tantalum. The primer layer is deposited as a metal or suboxide and becomes more completely oxidized during subsequent processing and during aging.

Regarding the lubricating topcoat, the layer may be an ether or acrylate fluorocarbon polymer, or a mixture of the two, with an inherently low surface energy. To encourage the polymer to adhere to silicon-containing sputtered layers, such as silicon nitride, the polymer can include alkoxy-silane groups. The thickness of the topcoat can range from 5 Angstroms to 100 Angstroms. The surface energy should be less than 40 dynes per centimeter (dynes/cm). This surface energy rendering corresponds to a contact angle greater than 50 degrees or more preferably greater than 90 degrees. The purpose of the topcoat is to minimize smudging and to act as a surface lubricant which resists scratching.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a side sectional view of an optical member for attachment to automotive or architectural glass, with the optical member having an absorbing multipurpose window coating in accordance with the invention. Fig. 2 is a plot of the relationship between optimum visible transmission and the refractive index of the dielectric layer of the multipurpose window coating of Fig. 1.

Fig. 3 is a plot of the relationship between the lowest attainable visible reflection and the refractive index of the dielectric layer of Fig. 1. Fig. 4 illustrates plots of the relationships of reflected colors and the refractive index of the dielectric layer of Fig. 1.

Fig. 5 is a plot of the reflection spectra for an multipurpose window coating of type shown in Fig. 1.

Fig. 6 shows plots of the relationship between layer thicknesses and the refractive index of the dielectric layer of Fig. 5.

Fig. 7 is a side sectional view of a single pane of glass having the multipurpose window coating of Fig. 1 on one side. Fig. 8 is a side sectional view of a double-pane assembly of glass having the multipurpose window coating of Fig. 1 on one side.

Fig. 9 includes plots of changes in the R value with changes in emissivity, for both the single pane embodiment of Fig. 7 and the double-pane embodiment of Fig. 8.

Fig. 10 is a plot of the relationship between emissivity and a nitrogen-to-titanium ratio for five samples.

Fig. 11 is a plot of the relationship between emissivity and the nitrogen atomic percentage in the five samples of Fig. 10. Fig. 12 is the plot of the relationship between emissivity and the oxygen-to-titanium ratio for the same five samples of Figs. 10 and 11.

Fig. 13 is a plot of the relationship between emissivity and the oxygen atomic percentage of the same five samples of Figs. 10, 11 and 12.

Fig. 14 is a plot of the change in T_vιs with a change in the thick- ness of the titanium nitride layer.

Fig. 15 represents the relationship (for the antireflection coated surface alone) between R_vιs and percentage T_vιs for nine samples of a multipurpose window coating formed in accordance with the invention.

Fig. 16 shows the relationship (for the total reflection after lamination to glass) between percentage R_vιs and percentage T_vιs for the same nine samples of Fig. 15.

Fig. 17 represents the relationship between emissivity and percentage T_vιs for the same nine samples as Figs. 15 and 16.

DETAILED DESCRIPTION

With reference to Fig. 1 , an optical member 10 is shown as including a substrate 12. An acceptable substrate is one that is formed of polyethylene terephthalate (PET). However, other materials can be substi- tuted. An acceptable thickness of the substrate 12 is 0.5 mil to 10 mils. The refractive index of a PET substrate is in the range of 1.4 to 1.7. While only a small portion of the substrate and attached layers is shown, the substrate is preferably a web of flexible material that can be segmented to fit a particular pane of glass. Alternatively, the multipurpose window coating to be described below can be formed directly on the glass.

On one side of the substrate 12 is a pressure sensitive adhesive 14 for attachment to a window or the like. The use of pressure sensitive adhesives is well known in the art. An acceptable thickness is 0.5 mil to 2 mils. A release liner 16 is located on a side of the adhesive opposite to the substrate 12. In use, the release liner is removed immediately before the optical member 10 is attached to the glass. An acceptable material for forming the release liner is PET having a thickness of 0.25 mil to 1.0 mil. Often, PET substrates 12 include a hardcoat layer 18 on at least one side. The hardcoat layer improves the durability of the flexible substrate during processing and particularly during use of the end product. The hardcoat layer can be any one of a variety of known hardcoat materials, such as silica-based hardcoats, siloxane hardcoats, melamine hardcoats and acrylic hardcoats. An acceptable thickness range is 1 μm to 20 μm. The use of the hardcoat layer is not critical to the invention.

In the preferred embodiment, the optical member 10 includes an adhesive primer layer 20. The primer layer is a sputtered layer between the hardcoat layer 18 and the absorbing layer 22 of a two-layer multipurpose window coating 24. The effectiveness of the primer layer is determined in part by the composition of the hardcoat layer 18. However, a silicon primer has been found to be effective for both acrylate-based hardcoats and epoxy- based hardcoats. The primer layer preferably has a thickness in the range of 0.2 nm to 2 nm. Although silicon is the preferred primer material, other metals which readily oxidize are also believed to be effective. Examples of these alternate primer materials include chromium, titanium and tantalum.

The primer layer is preferably a metal that undergoes oxidation or nitridation in situ during processing, so as to yield a substantially transparent, substantially colorless inorganic metal oxide. The primer layer should be sufficiently thin to minimize disruption of the desired optical properties of the optical member 10.

The benefits of the adhesive primer layer 20 are most evident when a sputtered sample that includes the layers 20, 22 and 26 is rub tested after being exposed to a high temperature and high humidity for 48 hours (i.e., 50°C at 95 percent relative humidity). During a rub test, the layer was rubbed with a cloth soaked in acetone. The cloth was backed with a soft rubber pad having a diameter of 12.5 mm (i.e., 1.23 cm²). The rubbing was done with a weight of 2.2 kilograms on the cloth (i.e., approximately 1.79 kg/cm²). Wthout the primer layer, the multipurpose window coating 24 was removed after less than 25 rub cycles. With the primer layer 20, no removal was seen at up to 200 rub cycles.

The individual layers 22 and 26 of the multipurpose window coating 24 will be described fully below. On a side of the coating opposite to the primer layer 20 is a lubricating topcoat 28. The topcoat may be either an ether or acrylate fluorocarbon polymer (or a mixture of the two) with an inherently low surface energy. To encourage the polymer to adhere to silicon-containing sputtered material (such as the dielectric layer 26 of silicon nitride), the lubricating topcoat 28 may include alkoxy-silane groups. The thickness of the topcoat can range from 5 Angstroms to 100 Angstroms. The topcoat should have a surface energy of less than 40 dynes/cm. This surface energy rendering corresponds to use of a contact angle greater than 50 degrees and more preferably greater than 90 degrees. The main purposes of the topcoat are to minimize smudging and to act as a lubricant which resists scratching. A particularly desirable material for forming the layer is sold by 3M Company under the federally registered trademark FLUORORAD. For example, FLUORORAD FC-722, which is sold diluted to a 2 percent solution in a fluorinated solution, may be used. The multipurpose window coating 24 consists of the absorbing layer 24 and the dielectric layer 26. In the preferred embodiment, the absorbing layer is titanium nitride, more typically titanium oxynitride, and the dielectric layer 26 is silicon nitride, typically Si₃N₄ with a refractive index of approximately 2.0. The range of suitable refractive indices for the transparent dielectric layer 26 extends from 1.73 to 2.6. Examples of suitable materials

(i.e., transparent dielectrics with indices within the specified range) other than silicon nitride include aluminum oxide, indium oxide, zinc oxide, tin oxide, tantalum oxide, titanium oxide, zirconium oxide and niobium oxide, as well as some metal nitrides and metal oxynitrides. The lower limit of the refractive index of the dielectric layer 26 is determined by the desired upper limit of T_vιs for the optical member 10. If the "backside" reflection is to be limited to 1.3 percent, then the transmission of the multipurpose window coating 24 cannot exceed 57 percent, corresponding to a dielectric refractive index of 1.73. Fig. 2 illustrates a plot 30 of the correspondence between optimal trans- mission in the visible light spectrum to the refractive index of the dielectric layer 26 in a coating in which the absorbing layer is titanium oxynitride. As noted, the refractive index that corresponds to the 57 percent level is approximately 1.73.

The upper limit on the refractive index of the transparent dielec- trie layer 26 is primarily determined by the acceptable R_vιs value for the coated side of the substrate 12 (i.e., excluding backside reflection). If the acceptable frontside reflection has been set at 1.3 percent, the corresponding limit on refractive index is 2.6. Fig. 3 is a plot 32 of the relationship between R_vιs and the refractive index of the dielectric layer in an antireflection multipurpose window coating in which the absorbing layer is titanium oxynitride. As can be seen, 1.3 percent reflection corresponds to the refractive index upper limit of 2.6. More preferably, the upper limit on the refractive index of the dielectric layer 26 is 2.37, which corresponds to the visible reflection of 1 percent.

A second factor affecting the acceptability of materials for forming the high refractive index dielectric layer 26 is the reflected color (i.e., Ra* and Rb*). The correspondence between reflected color and the refractive index of the dielectric layer is shown by plots 34 and 36 in Fig. 4. As the refractive index of the dielectric layer increases, the reflected light becomes more intense (i.e., redder (more positive Ra^*)) and bluer (more negative Rb*).

Regarding the thickness of the dielectric layer 26 in the antireflection multipurpose window coating 24, in order to minimize the visible reflection of the antireflection absorbing coating, the antireflection "well" must be centered in the visible region of the electromagnetic spectrum. Thus, the "well" is centered at approximately 550 nm. Fig. 5 is a typical reflection spectra 38 for the coating 24. The reflection "well" corresponds to changes in the thickness of the dielectric layer by shifting left to shorter wavelengths when the thickness is reduced and by shifting right to longer wavelengths when the thickness of the dielectric layer is increased. Based on optical modeling, the optimized thicknesses for various dielectric refractive indices are reflected in Table 1 and in Fig. 6, which show data for ten examples considered in the optical modeling.

Fig. 6 illustrates the effect of the dielectric refractive index on the design of the multipurpose window coating 24. Plot 40 shows the optimized range of thicknesses of the dielectric layer 26, while plot 42 illustrates the optimized range of thicknesses of the absorbing layer 22. For the dielectric layer, the optimum range of thickness is approximately 28 nm to 59 nm. This represents only the theoretical optimum tuning for each case. In practice, a wider range of thicknesses may be employed. In the preferred embodiment, the range is 15 nm to 90 nm. In the more preferred embodiment, the range is 25 nm to 65 nm.

In the formation of the dielectric layer 26, Si₃N₄ is a particularly desirable material since (1) it is transparent, (2) it has an acceptable refractive index, and (3) it is sputter deposited in a nitrogen environment. The nitrogen-based plasma is advantageous because it facilitates the formation of the titanium nitride absorbing layer 22 (which is also deposited from a nitrogen-based plasma) with minimal oxygen contamination. Since one of the objects of the design of the multipurpose window coating is to provide a low emissivity, as evidenced by a lower sheet resistance, it is important to minimize the oxidative damage to the TiN_x layer. The titanium nitride layers of prior art antireflection coatings can be oxidized in two manners. In one case, if the titanium nitride and an oxide layer are being sputtered simultaneously in a given vacuum vessel, some of the oxygen from the oxide process can migrate into the titanium nitride sputtering region. The migrated oxygen is incorporated into the deposited titanium nitride film. Of course, the degree of migration depends on the degree of isolation between the two sputtering regions. The second manner relates to operations that occur after the titanium nitride has been deposited. Once the titanium nitride has been deposited, as it is passed through a reactive oxide plasma to deposit oxide, the surface of the titanium nitride can become oxidized. This is well known in the sputter deposition of silver low-emissivity coatings. The use of a silicon nitride layer atop the titanium nitride layer reduces the likelihood of oxygen contamination into the titanium nitride. Nevertheless, while the intent is to deposit an essentially pure titanium nitride layer, oxygen will invade the vacuum system as a result of absorbed water on vessel walls, water in the substrate, and air leaks. Therefore, in many cases the titanium nitride layer is likely to contain some oxygen, i.e., will be a titanium oxynitride layer.

The most obvious effects of the presence of the oxygen in the titanium nitride layer 22 are to increase the emissivity and the sheet resis- tance of the multipurpose window coating 24. This is shown in Table 2, which provides the information regarding formation of five titanium oxynitride samples having a T_vis selected to be compatible with a Si₃N₄ top dielectric layer. More specifically, the T_vis of the sputter deposited titanium oxynitride layer without the silicon nitride layer was maintained at approximately 32 percent. In the fabrication of these sputter deposited samples, oxygen (O₂ flow) was intentionally added to the TiN_x plasma. The "in situ" measurements were determined while the samples remained within the sputtering chamber. The QC measurements were calculated immediately after removal, while the QC2 measurement of sheet resistance (Ohms SQ) was determined two weeks layer. The selectivity measurement of T_vis/T₁₅₅₀ is the ratio of the T_vis value to the transmission at the wavelength of 1550 nm. The flow rates identified in Table 2 are in units of seem.

Table 3 identifies the concentrations of the three components within each of the five samples and identifies the ratios of the components. The measurements of component concentrations were obtained using the Rutherford Back Scattering (RBS) approach. This approach is known in the art. Charles Evans and Associates in Redwood City, California performed the analytical procedures for determining the data in Table 3.

One important concern in designing the optical member 10 of Fig. 1 is "how low must the emissivity of a coating be in order to significantly affect a window's thermal performance." Two cases are considered. Fig. 7 illustrates the case in which monolithic glass 44 includes an multipurpose window coating 46 on an interior side. Between the monolithic glass and the coating is a strip 48 that represents the hardcoated PET and the pressure sensitive adhesive described above. The second case is represented in Fig. 8. A dual pane insulated glass unit includes an outside glass pane 50, an inside glass pane 52, a PET strip 54, and the multipurpose window coating 56. In both cases, the emissivity of each glass surface was assumed to be 0.84. The only low-emissivity surface was the inboard surface (i.e., the side directly facing the interior of a building or automobile). If it is assumed that at least a 12 percent (or more preferably at least 19 percent) improvement of the R value of a monolithic glazing is required for thermal isolation to be a relevant issue, then an emissivity of less than 0.6 (and more preferably less than 0.5) is required. Moreover, this means that the oxygen atomic ratio in titanium oxynitride must be less than 35 percent (and more preferably less than 25 percent). These calculations of emissivity and oxygen atomic ratio are taken from the data of Tables 1 and 2 and the plots of Figs. 9, 10, 11 , 12 and 13. In Fig. 9, the lower plot 58 represents the monolithic arrangement of Fig. 7, while the upper plot 60 (IGU) represents the case for the double pane arrangement of Fig. 8. In Figs. 10-12, the plots 62, 64, 66 and 68 represent the data of the five samples from Tables 2 and 3.

Regarding the thickness of the titanium nitride layer 22 of Fig. 1 , direct physical measurements of thicknesses were used to determine the desired range of thickness. To do this, very thick samples (300 Angstroms) were deposited on 7 mil hardcoated PET. A 2 mil tape was applied to the PET. The tape was removed and the step height was measured with a DekTac (model 2D) Profilometer. The linespeed of the web coater was increased to obtain coatings with visible transmissions in the range considered to be relevant. By assuming that the coating thickness was inversely proportional to the linespeed, titanium nitride physical thicknesses were calculated and correlated to visible transmissions. The results are shown in a plot 70 in Fig. 14. With regard to the plot 70, it should be noted that the transmission (T_vιs) was measured for titanium nitride on a hardcoated PET substrate without the top dielectric layer 26 of the coating 24.

Thickness values for nine samples of multipurpose window coatings are given in Table 4. The samples are for coatings having an absorbing layer of titanium nitride and a dielectric layer of silicon nitride. The T_V1S values were measured with only the titanium nitride layer present. The desirable thickness of the titanium nitride is partially dependent upon the thickness of the dielectric layer (i.e., the refractive index of the dielectric layer). Based purely upon the data, the optimal thickness for the titanium nitride layer is in the range of 10 nm to 40 nm. However, when considering the variations in oxygen contamination, the preferred thickness range expands to 5 nm to 75 nm.

Returning to Figs. 7 and 8, while the multipurpose window coatings 46 and 56 may be applied to either the monolithic glass 44 or the dual pane glass 50 and 52, it is believed that the largest improvements in reflection and thermal isolation are afforded with the monolithic glass structures. The relationship between emissivity and the percentage improvement in the R value is shown in Table 5. The results in this table were obtained from the Windows 4.1 software supplied by Lawrence Berkeley National Laboratories.

Table 6 shows results of optical measurements on nine samples of laminates of antireflection (AR/glass), where the multipurpose window coating is comprised of titanium nitride and silicon nitride. The reflection of the AR coating was measured by applying a flat black tape (e.g., electrical tape) to the backside of the AR coating. The black tape eliminates backside reflection. The residual film reflectivities reported were measured with a Gamma Scientific spectrophotometer. The method for calculating visible reflection (R_vis) and visible transmission (T_vιs) is given by the standard ASTM E308-90. Table 6 also shows the Rvis measured for the sputtered film after being applied to glass. These reflectivities were measured with a Perkin Elmer Lambda 9 Spectrophotometer equipped with an integrating sphere. Measurements were made from both the glass side and the sputter coated side. Note that for all samples the glass side reflection was an order of magnitude greater than that measured for the coated side.

The shading coefficients of the titanium nitride/silicon nitride coatings shown in Table 6 range from 0.39 to 0.51. In modeling work, it was shown that for optimized coatings, the shading coefficients can vary from 0.40 to 0.62 as the dielectric refractive index varies over the acceptable range. As previously noted, as the refractive index of the top dielectric layer varies, the thickness of the underlying absorbing layer must also vary in order to obtain the desired AR properties. The range of shading coefficients obtained by the modeling work does not account for oxygen contamination effects in the titanium nitride layer. Therefore, in practice, shading coefficients can be higher than suggested by modeling. Using this reasoning, shading coefficients of less than 0.9 are preferred, and shading coefficients of less than 0.8 are more preferred.

Regarding solar selectivity of the titanium nitride/silicon nitride coatings of Table 6, values ranging from 1.37 to 1.53 were obtained. In the modeling work, it was shown that the selectivity can vary from 1.26 to 1.51 as the dielectric refractive index varies over the acceptable range. However, this range of selectivities does not account for oxygen contamination effects in the titanium nitride layer. In practice, selectivities could be lower than suggested by modeling. Selectivities of at least 1.1 are preferred and selectivities of at least 1.15 are more preferred.

The requirement of low emissivity affects the boundaries of the titanium nitride/silicon nitride coatings in two ways. Firstly, the titanium nitride layer must be sufficiently thick (i.e., T_vis should be sufficiently low) to provide the desired emissivity. Secondly, the degree of oxygen contamination in the titanium nitride layer must be limited. It is believed that the optimally tuned titanium nitride/silicon nitride coating has an emissivity of approximately 0.3, giving an improvement of R value for a monolithic glass of approximately 35 percent. The benefits of using a low-emissivity surface as an applied film were described above.

EXPERIMENT DETAILS

Tables 7, 8 and 9 identify fabrication parameters and measurement results for nine samples of an optical member having a titanium nitride/ silicon nitride multipurpose window coating. The thickness of the titanium nitride layer ranged from 200 to 300 Angstroms. It should be noted that in all of these samples, the refractive index of the top dielectric layer was fixed at approximately 2.02. The titanium nitride thickness variations corresponded to in situ T_vιs values of 21 percent to 36 percent (with no Si₃N₄ layer) or 31 percent to 43 percent (with both layers present).

For all nine samples, the layers were formed on a hardcoated side of a PET substrate. Initially, the hardcoated PET was subjected to a preglow. A PET web was moved through the coating zone of a deposition chamber at a linespeed of approximately 25 mm/second. Oxygen was introduced into the preglow at a rate of approximately 10.8 seem. An aluminum glow rod was powered at 1500 volts with a current of 10 mA.

The parameters for forming the adhesive primer layer of silicon are shown in Table 7. The silicon primer was applied using an AC powered cathode pair. The argon (Ar) flow rate was 97.9 seem for the first four samples and 98.2 for the last five samples. The argon gas flow was introduced into the sputtering chamber to obtain a pressure of approximately 3.19 milliTorr for the first four samples and 3.33 milliTorr for the last five samples. The applied power was either 0.93 kW or 1.25 kW.

The parameters for forming the titanium nitride layer in the second pass are shown in Table 8. After the N₂ flow was adjusted for maximum selectivity, as defined by T_VIS/T_1S50 at a given T_vιs, titanium nitride was deposited to a thickness that would lower the visible transmission of the total stack to approximately 32 percent. This was accomplished by adjusting the linespeed. As an example of the fabrication process, the titanium nitride layer was formed on the sixth sample by using a linespeed of 3.59 mm/second. A DC source, operating under manual control was the applied method. The argon flow rate was 117.3 seem, while the nitrogen flow rate was 27.0 seem. The total pressure was 3.58 milliTorr. The applied power was 5.90 kW (12.8 amps at 462 volts).

The parameters for forming the silicon nitride layer in the third pass are identified in Table 9. By changing the linespeed, Si₃N₄ was deposited to a thickness that would best match a reflection spectra overlay that was generated from a HMFIT model. Again referring to the formation of Sample 6, the linespeed was 7.29 mm/second. The Si₃N₄ was sputter deposited using the method of AC source, with manual control. The argon flow rate was 98.1 seem and the nitrogen flow rate was 43.0 seem. The total pressure was 3.58 milliTorr and the power was 5.00 kW (12.9 amps at 425 volts). 0

5

0

Fig. 15 shows results of Gamma Scientific reflection measurements as a function of the visible transmission of the antireflection (AR) coatings of Samples 1 through 9. For the Gamma Scientific measurements, 5 a section of flat black "electrical" tape was adhered to the backside of the substrate in order to eliminate backside reflections. From the data shown in Fig. 15, it is evident that Sample 6 is the most preferred, since it has the lowest visible reflection. The reflection spectra for this sample is the one that is illustrated in Fig. 5. Sample 6 had a film reflectivity (eye weighted) Q of only 0.71 percent.

Fig. 16 illustrates the visible reflectivity at the coated side of the nine samples of Ar/glass laminates. These measurements were made on a Perkin Elmer Lambda 900 spectrophotometer and include contributions from both the AR coating and backside of the glass. Once again, Sample 6 ₅ is the preferred sample. When applied to glass, the laminate reflectivity for Sample 6 was only 1.55 percent.

From the plots given in Figs. 15 and 16, it is evident that for a particular top dielectric layer (here, silicon nitride), optimum reflectivity is obtained over a relatively narrow range of visible transmissions. The preferred transmission of the two-layer stack appears to be around 45.2 percent when laminated to glass and around 45.9 percent when measured as a raw film. The titanium nitride thickness for this preferred transmission is calcu- lated to be around 227 Angstroms. In the series of samples, reasonable AR coatings having a R,_is of less or equal to 1.3 percent were obtained for Samples 6, 7, 8 and 9. These samples had a T_vis range (after lamination to glass) of 41.0 percent to 46.8 percent, and a titanium nitride thickness in the range of 211 Angstroms to 258 Angstroms. As evident in Fig. 17, as the titanium nitride layer is made thicker, thereby reducing the transmission, the emissivity of the AR coating decreases. As previously noted, this decrease in emissivity improves the thermal insulation properties of the optical member. However, if good anti- reflection properties and reasonably high visible transmission are most important, the thickness of the titanium nitride layer must be restricted. The better performing samples were Samples 6, 7, 8 and 9. These samples had emissivities ranging from 0.268 to 0.312. An emissivity as low as 0.241 was obtained for Sample 3. However, this sample had a higher R_vis and a lower T_vis than the most preferred sample mentioned above. The shading coefficient of Sample 6 was determined to be 0.49.

This is achieved with a selectivity ratio (i.e., T_vis/T_S0|) of 1.44. Thus, the multipurpose window coating is a much more selective solar filter than either dyed films or typical grey metal films. The stability of the optical properties of the titanium nitride/silicon nitride coating was tested by exposing the coating to a 48 hour heat treatment at 77°C. No significant change in the optical properties was detected after the heat treatment.

Claims

WHAT IS CLAIMED IS:

1. A method of forming an optical member comprising the steps of: providing a substrate having a first surface; and providing a multipurpose window coating on said substrate including:

(a) forming a first layer of titanium nitride; and

(b) forming a high refractive index layer in contact with said first layer on a side of said first layer opposite to said first surface, said high refractive index layer being substantially transparent and having a refractive index of at least 1.73.

2. The method of claim 1 wherein said multipurpose window coating is a two-layer coating consisting of said first layer and said high refractive index layer.

3. The method of claim 2 wherein said step of forming said high refractive index layer includes depositing silicon nitride.

4. The method of claim 2 wherein said step of forming said first layer includes depositing titanium nitride.

5. The method of claim 3 wherein said step of forming said titanium nitride includes sputter depositing material to a thickness in the range of 5 nm to 75 nm and wherein said step of depositing said silicon nitride includes sputter depositing material to a thickness in the range of 15 nm to 90 nm.

6. The method of claim 3 wherein said step of forming said titanium nitride includes sputter depositing material to a thickness in the range of 10 nm to 40 nm and wherein said step of depositing said silicon nitride includes sputter depositing material to a thickness in the range of 25 nm to 65 nm.

7. The method of claim 4 wherein said step of depositing said titanium oxynitride includes forming a deposition environment such that said first layer has an oxygen content of less than 30 atomic percent.

8. The method of claim 3 wherein said step of depositing said titanium oxynitride includes forming a deposition environment such that said first layer has an oxygen content of less than 25 atomic percent.

9. The method of claim 1 further comprising a step of sputtering an adhesive primer layer between said substrate and said multipurpose window coating, such that said primer layer has a thickness in the range of 0.2 nm and 2 nm.

10. The method of claim 9 further comprising a step of applying a topcoat to said multipurpose window coating such that said topcoat has a low surface energy.

11. The method of claim 10 wherein said step of providing said substrate includes providing a flexible polymeric web for attachment to a glass substrate.

12. A method of forming an optical member comprising the steps of: providing a substrate; and forming a two-layer multipurpose window coating on said substrate, including: depositing a titanium nitride layer having a thickness in the range of 5 nm to 75 nm; and depositing a layer of silicon nitride having a refractive index in the range of 1.73 to 2.6, said silicon nitride layer being formed on said titanium nitride layer and having a thickness in the range of 15 nm to 90 nm.

13. The method of claim 12 further comprising the steps of: forming a silicon-containing primer layer between said substrate and said two-layer multipurpose window coating; and forming a lubricating topcoat layer on said two-layer multipurpose window coating, said topcoat layer having a surface energy of less than 40 dynes/cm.

14. The method of claim 13 wherein said step of forming said primer layer includes sputtering material to a thickness in the range of 0.2 nm to 2 nm and wherein said step of forming said topcoat layer includes depositing material to a thickness in the range of 5 Angstroms to 100 Angstroms.

15. The method of claim 12 wherein: said step of depositing said titanium nitride layer includes sputter depositing titanium oxynitride to a thickness in the range of 10 nm to 40 nm; and said step of depositing said silicon nitride layer includes sputter depositing Si₃N₄ to a thickness in the range of 25 nm to 65 nm.

16. The method of claim 15 wherein said step of sputter depositing titanium oxynitride includes forming a deposition environment which produces a coating with an oxygen content of less than 25 atomic percent.

17. An optical member comprising: a substrate having a first surface; an absorbing layer of titanium nitride; and a dielectric layer of a high refractive index material, said dielectric layer being in contact with said absorbing layer and having a refractive index in the range of 1.73 to 2.6, wherein said absorbing and dielectric layers form an multi- purpose window coating.

18. The optical member of claim 17 wherein said absorbing layer is titanium oxynitride having a thickness in the range of 5 nm to 75 nm and wherein said dielectric layer is silicon nitride having a thickness in the range of 15 nm to 90 nm.

19. the optical member of claim 18 further comprising: an adhesive primer layer between said substrate and said absorbing layer, said adhesive primer layer having a thickness in the range of 0.2 nm to 2 nm; and a lubricating layer in contact with said dielectric layer, said lubricating layer having a surface energy of less than 40 dynes/cm.

20. The optical member of claim 17 wherein said absorbing layer is titanium oxynitride having a thickness in the range of 10 nm to 40 nm and wherein said dielectric layer is silicon nitride having a thickness in the range of 25 nm to 65 nm.

21. The optical member of claim 17 wherein said substrate is a web of a flexible polymeric material.