WO2007028060A2

WO2007028060A2 - Durable near-infrared blocking and emi shielding film for display filter

Info

Publication number: WO2007028060A2
Application number: PCT/US2006/034297
Authority: WO
Inventors: Chris R. Schmidt; Stanley Louie; Yukinori Asakawa
Original assignee: Southwall Technologies, Inc.
Priority date: 2005-09-02
Filing date: 2006-09-01
Publication date: 2007-03-08
Also published as: WO2007028060A3

Abstract

A plasma display filter is formed to include an alternating pattern (12) of infrared blocking layers (22, 24 and 26) and ZTO (zinc oxide/tin oxide) dielectric layers (14, 16, 18 and 20). In one embodiment, the alternating pattern comprises four ZTO dielectric layers and three infrared blocking layers. Benefits of the plasma display filter are enhanced if the ZTO dielectric layers are sputter deposited in an environment which is intentionally hydrogenated. As another possible enhancement, a sacrificial layer or glue layer may be deposited over the infrared blocking layers prior to formation of the subsequent ZTO dielectric layer, so as to protect the underlying layer during formation of the ZTO dielectric layer. Suitable materials include titanium and nichrome. The chemical durability of the ZTO dielectric layers enables the omission of the conventional protective overcoat layer (28), if desired.

Description

DURABLE NEAR-INFRARED BLOCKING AND EMI SHIELDING FILM FOR DISPLAY FILTER

TECHNICAL FIELD

[0001] The invention relates to a near-infrared blocking film and laminate structure with improved durability compared with the prior art. An electromagnetic interference (EMI) shielding film for a display is described.

BACKGROUND ART

[0002] A number of different factors are considered in the design of an optical filter for a plasma display panel (PDP). The factors include the degree of neutrality of transmitted color, the level of reflected light and the color shift with changes in the incidence angle of a viewer, structural stability, resistance to environment-induced changes, and the transmission levels of infrared and electromagnetic radiation. Unfortunately, modifying a filter to improve conditions with respect to one factor sometimes conflicts with maintaining a target level for another factor.

[0003] As one possible filter for a PDP, an Etalon filter is first formed on a polyethylene terephthalate (PET) substrate that is then affixed to the glass sheet by a layer of adhesive. Because a plasma display generates infrared radiation and electromagnetic interference (EMI) that must be controlled in accordance with legislated regulations, the filter layers are designed to reduce infrared and EMI from the display. Etalon filters based on multiple silver layers have been used to screen infrared wavelengths and electromagnetic waves. Interference between adjacent silver layers can be tuned to cause resonant transmission in the visible region, while providing desirable screening. U.S. Patent No. 5,071 ,206 to Hood et al. describes a suitable sequence of layers. [0004] A conventional filter for a PDP may also include an antireflection (AR) layer stack that was originally formed on a second PET substrate. Antireflection layer stacks are well known in the art. A second adhesive layer secures the PET substrate to the other elements of the structure.

[0005] While the PDP filter reduces infrared transmission and EMI from the display, the filter must also be cosmetically acceptable and must enable good fidelity in the viewing of displayed images. Thus, the transmissivity of the filter should be high in the visual region of the light spectrum and should be relatively colorless, so as not to change the color rendering of the plasma display. Further, a general expectation exists that displays should be low in reflectance and that the reflected color be bluish to slightly reddish.

[0006] Color can be expressed in a variety of fashions. In the above-cited Hood et al. patent, color is expressed in the CIE La*b*1976 color coordinate system and in particular the ASTM 308-85 method. Using this method, a property is shown by values for a* and b* near 0. Generally, consumers expect that computer displays will appear either neutral or slightly bluish in color.

[0007] As previously noted, different factors regarding the design of PDP filters may conflict. Generally, controlling reflected color competes with EM screening capability. Typical silver etalon filters work to screen infrared rays primarily by reflecting the rays. Infrared radiation is relatively close in wavelength to red and is therefore difficult to effectively control while simultaneously obtaining low reflection in the red region of the spectrum (i.e., 620-700 nm). The problem is particularly acute for plasma display, where it is desirable to shield from Xe emissions at 820 nm and 880 nm while maintaining high transmissivity in the red region of the spectrum.

[0008] Controlling reflection within the red region of the light spectrum is rendered even more difficult by the need for a low sheet resistance in the PDP filter. Attempts have been made to balance the goals of maximizing red transmission and minimizing sheet resistance. U.S. Patent No. 6,102,530 to Okamura et al. describes an optical filter for plasma displays, where the filter has a sheet resistance of less than 3 ohms/square. Generally, a sheet resistance of less than 1.5 ohms/square is required to meet Federal Communication Commission (FCC) Class B standard, even for PDP sets having the highest luminance efficiencies. Copper wire mesh PDP EMI filters having a sheet resistance of 0.1 to 0.2 omhs/square are often used to provide Class B compatibility.

[0009] A further complication is that the filter must be resistant to corrosion. One option is to provide a corrosion-inhibiting overcoat layer. However, such a layer can significantly affect the optical properties of the system.

SUMMARY OF THE INVENTION

[0010] One object of the invention is to provide a Fabry-Perot interference coating and laminate structure, preferably for application to a plasma display panel (PDP). Another object of the invention is to enable the use of multi- component tin oxide/zinc oxide layers, referred to herein as ZTO, in the coating to enhance the chemical durability of the structure by increasing adhesion within the structure. A further object of the invention is to provide a multi-layer electromagnetic interference (EMI) blocking film and/or infrared blocking film, comprising ZTO as the anti-reflective layer, with improved chemical durability when used in a display filter.

[0011] A plasma display filter in accordance with the invention includes a substrate and an alternating pattern of ZTO dielectric layers and infrared (IR) blocking layers. As defined herein, a "ZTO dielectric layer" is defined as a layer that is formed as a combination of zinc oxide and tin oxide. The ZTO dielectric layers and IR blocking layers are selected to achieve target optical properties for use as a filter for a plasma display. [0012] The ZTO dielectric layers and IR blocking layers may be sputtered onto the substrate. Thus, the ZTO dielectric layers may be formed using a metal alloy target having a composition of between five and fifty percent tin and between fifty and ninety-five percent zinc. However, it has been deter- mined that it is beneficial to use a metal alloy target having a composition of between five and thirty percent tin and between seventy and ninety-five percent zinc, with further improvements being provided when the metal alloy target has a composition of between five and fifteen percent tin and between eighty-five and ninety-five percent zinc. The alternating pattern may comprise three ZTO dielectric layers and three IR blocking layers. Each IR blocking layer may be silver or a silver alloy containing a second metal that is selected to inhibit corrosion, thereby enhancing chemical durability. The silver alloy may contain fifteen percent or less of the second metal, but preferably contains less than ten percent and most preferably contains less than five percent. The second metal may be selected from the group of copper, palladium and gold, but other materials may be selected. A protective overcoat layer may be formed atop the alternating pattern. It is known to use such a layer to improve the chemical durability of filter stacks. However, it has been determined that the improved performance of the ZTO dielectric layers as compared to the prior art plasma display filter coatings permits the omission of the protective overcoat layer.

[0013] A sacrificial layer or a glue layer may be deposited over the IR blocking layers on which a ZTO dielectric layer is to be formed. The sacrificial or glue layer protects the IR blocking layer during formation of the ZTO dielectric layer. A suitable material may be titanium or nichrome.

[0014] In addition to oxygen in order to provide oxide in the formation of the ZTO dielectric layer, the sputtering environment may be intentionally hydrogenated. A gaseous environment of at least thirty-five percent hydrogen is beneficial, but at least fifty percent hydrogen is preferred and at least seventy-five percent hydrogen is further beneficial. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Fig. 1 is a side view of one embodiment of a plasma display filter formed in accordance with the invention.

[0016] Fig. 2 is a side view of an application of the invention as a plasma display filter.

DETAILED DESCRIPTION

[0017] In a preferred embodiment, it has been determined that the stack design of Table 1 and Fig. 1 is useful as an infrared (IR) blocking structure. In Fig. 1 , a substrate 10 is coated 12 with an alternating pattern of ZTO dielectric layers and IR reflection layers. The coating 12 includes four ZTO dielectric layers 14, 16, 18 and 20 and three IR blocking layers 22, 24 and 26. The IR blocking layers will also be referred to as IR reflection layers. In addition to the alternating pattern, the coating 12 includes a protective overcoat layer 28. As will be described below, the overcoat layer may be omitted as a result of using the ZTO dielectric layers.

Table 1 - Coating Design of Fig. 1

[0018] The coating 12 on the substrate 10 may be used to make the display filter shown in Fig. 2. This display filter is constructed of an anti- reflection layer 30, a substrate 32, an adhesive or glue layer 34, the EMI shielding and near-infrared blocking coating 12, an adhesive or glue layer 36, a glass substrate 38, another adhesive or glue layer 40, and a conductive mesh layer 42.

[0019] In Fig. 1 , the dielectric layers 14, 16, 18 and 20 are comprised of a mixture of tin oxide and zinc oxide (ZTO) reactively sputtered from metal alloy targets comprising zinc and tin, wherein the composition of the alloy target is between five percent and fifty percent tin, and between fifty percent and ninety-five percent zinc. More preferably, the metal alloy target contains between five percent and thirty percent tin, and between seventy percent and ninety-five percent zinc. Even more preferably, the metal alloy targets contain between five percent tin and fifteen percent tin, and between eighty-five percent and ninety-five percent zinc.

[0020] The overcoat layer 28 may be comprised of titanium, titanium oxide, titanium nitride, nichrome, tantalum, or similar overcoat layers commonly known to those skilled in the art and known to improve the chemical durability of like structures. However, the improved performance of the ZTO layer over the prior art of display filter coatings has been determined to reduce the need for these protective overcoat layers, but their use is within the scope of the invention.

[0021] In another embodiment , a sacrificial or glue layer such as titanium or nichrome is deposited over each IR reflection layer 22, 24 and 26 to protect against degradation during ZTO or the related dielectric layer deposition. The invention is not limited to these sacrificial or glue layers and is expected to encompass the related layers known by those skilled in the art for protection of the IR reflection layer. Examples of such layers are disclosed in U.S. Patent Nos. 4,462,883 to Hart and 4,497,700 to Groth et al. While not being bound by any theory, the inventors believe the sacrificial layers tend to oxidize during ZTO deposition and become optically transparent. Using the modification of Fig. 2, the IR reflection layer is protected from degradation and the infrared reflectivity and electrical conductivity are maintained at the high levels.

Deposition Process

[0022] In order to achieve the highest performance in the IR reflection layers 22, 24 and 26 of Fig. 1 , i.e. the lowest infrared transmission, hydrogen gas may be added to the ZTO reactive deposition chambers to limit unwanted degradation of the IR reflection layer. The quantity of hydrogen employed may be at least thirty-five percent of the amount of oxygen gas used during reactive ZTO deposition, more preferably at least fifty percent of the amount of oxygen, and even more preferably at least seventy-five percent of the amount of oxygen gas used.

[0023] Magnetron sputtering of the ZTO dielectric layers 14, 16, 18 and 20 is typically controlled by adjusting the oxygen flow to maintain the metal target in an oxidized or "poisoned" mode as known to those skilled in the art. Reactive deposition may also be performed by using plasma emission control, or other common methods known in the art of reactive sputter deposition.

[0024] Each IR reflection layer 22, 24 and 26 may be comprised of silver, which has the highest IR reflectivity and electrical conductivity, or a silver alloy containing up to fifteen percent of another metal such as copper, palladium, gold, or other corrosion inhibiting metal to improve the chemical durability of the coating. More preferably, the IR reflection layer contains less than ten percent of another metal, and more preferably contains less than five percent of another metal to improve the chemical durability while maintaining infrared reflectivity and low cost. It has been determined that the unexpected improvement in chemical durability under temperature and humidity exposure that can be obtained with ZTO layers will generally limit the need for these silver alloy materials, but their use and application remains within the scope of the invention. [0025] The coating 12 may be deposited on web materials (substrate 10) such as polyethylene terephthalate (PET), cellulose triacetate (TAC), or similar transparent flexible substrates. Alternatively, the coating may be deposited on glass to form a display filter construction.

Chemical Durability

[0026] The coating 12 has been found to exhibit improved chemical durability after exposure to elevated temperature and humidity in relation to the display filter coatings of the prior art. In one preferred embodiment, the invention survived twenty-four hours of exposure to sixty degrees C and ninety percent relative humidity, while still forming silver spots greater than 0.1 mm in diameter with a frequency of two defects/m² or less. In the coatings of the related prior art of display filters, the formation of "white spots" or silver corrosion under elevated temperature and humidity are known to occur after twenty-four hours at sixty degrees C and ninety percent relative humidity with greater frequency and size.

Applications

[0027] The disclosed durable coating 12 may be applied to a rigid structure comprising a display filter. More specifically, the coating may be applied to the front surface of a Plasma Display Panel (PDP) in order to filter unwanted electromagnetic and near-infrared radiation.

Claims

WHAT IS CLAIMED IS:

1. A plasma display filter comprising: a substrate (10); and an alternating pattern (12) of ZTO dielectric layers (14, 16, 18 and 20) and infrared (IR) blocking layers (22, 24 and 26) on said substrate, each said ZTO dielectric layer being a combination of zinc oxide and tin oxide so as to enhance chemical durability, said ZTO dielectric layers and IR blocking layers being selected to achieve target optical properties for use as a filter for a plasma display.

2. The plasma display filter of claim 1 wherein each said ZTO dielectric layer (14, 16, 18 and 20) has properties defined by sputtering using a metal alloy target having a composition of between five and fifty percent tin and between fifty and ninety-five percent zinc.

3. The plasma display filter of claim 1 wherein each said ZTO dielectric layer (14, 16, 18 and 20) has properties defined by sputtering using a metal alloy target having a composition of between five and thirty percent tin and between seventy and ninety-five percent zinc.

4. The plasma display filter of claim 1 wherein each said ZTO dielectric layer (14, 16, 18 and 20) has properties defined by sputtering using a metal alloy target having a composition of between five and fifteen percent tin and between eight-five and ninety-five percent zinc.

5. The plasma display filter of claim 1 wherein said IR blocking layers (22, 24 and 26) are metallic layers and wherein there are at least three said metallic layers and at least four ZTO dielectric layers (14, 16, 18 and 20).

6. The plasma display filter of claim 5 wherein each said IR blocking layer (22, 24 and 26) is a silver alloy containing fifteen percent or less of a. second metal selected to inhibit corrosion, thereby enhancing chemical durability.

7. The plasma display filter of claim 5 wherein each said IR blocking layer (22, 24 and 26) is a silver alloy containing a second metal selected to inhibit corrosion, said second metal having a content of less than ten percent.

8. The plasma display filter of claim 5 wherein each said IR blocking layer (22, 24 and 26) is a silver alloy containing a second metal selected to inhibit corrosion, said second metal having a content of less than five percent.

9. The plasma display filter of claim 5 wherein each said IR blocking layer (22, 24 and 26) is a silver alloy having a second metal selected from the group of copper, palladium and gold.

10. The plasma display filter of claim 1 wherein said alternating pattern (12) is without a protective overcoat layer.

11. The plasma display filter of claim 1 further comprising one of a sacrificial layer and a glue layer deposited atop said IR blocking layers (22, 24 and 26) that precedes one of said ZTO dielectric layers (14, 16, 18 and 20) to inhibit degradation during deposition of said ZTO dielectric layer.

12. The plasma display filter of claim 1 wherein each ZTO dielectric layer (14, 16, 18 and 20) has properties determined by sputtering said ZTO dielectric layer in a gaseous environment of at least thirty-five percent hydrogen.

13. The plasma display filter of claim 1 wherein each ZTO dielectric layer (14, 16, 18 and 20) has properties determined by sputtering said ZTO dielectric layer in a gaseous environment of at least fifty percent hydrogen.

14. The plasma display filter of claim 1 wherein each ZTO dielectric layer (14, 16, 18 and 20) has properties determined by sputtering said ZTO dielectric layer in a gaseous environment of at least seventy-five percent hydrogen.

15. The plasma display filter of claim 1 wherein said substrate (10) is a flexible transparent substrate.

16. A method of forming a filter for a plasma display comprising: providing a transparent substrate (10) having a flexibility which enables efficient lamination onto a plasma display; and sputtering a layer stack (12) on said transparent stack to include an alternating pattern of ZTO dielectric layers (14, 16, 18 and 20) and IR reflective metallic layers (22, 24 and 26), each said ZTO dielectric layer being a combination of zinc oxide and tin oxide, said ZTO dielectric layers being selected to enhance chemical durability while maintaining target optical properties for a filter of a plasma display.

17. The method of claim 16 wherein sputtering said layer stack (12) includes depositing at least four said ZTO dielectric layers (14, 16, 18 and 20) and at least three said IR reflective metallic layers (22, 24 and 26).

18. The method of claim 16 wherein said sputtering includes sputtering said ZTO dielectric layers (14, 16, 18 and 20) in an environment that includes oxygen and hydrogen.

19. The method of claim 18 wherein said environment is at least thirty-five percent hydrogen.

20. The method of claim 16 further comprising forming a sacrificial layer or glue layer atop each said IR reflective metallic layer (22, 24 and 26) on which one of said ZTO dielectric layers (14, 16, 18 and 20) is to be sputtered.