Anti-reflective composite

Details Anti-reflective composite Anti-reflective composite

1998-05-28

2001-07-24

Spyrou, Cassandra (Department: 2872)

Optical: systems and elements

Light interference

Produced by coating or lamina

C359S580000, C428S697000, C428S699000, C428S701000

Reexamination Certificate

active

06266193

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to improved composite structures exhibiting optical properties and more specifically to such structures which exhibit anti-reflective properties utilized in display applications.
BACKGROUND OF THE INVENTION
For the last several years, anti-reflective composites have been used on an ever expanding basis for a myriad of purposes. Anti-reflective composites are most commonly used on windows, mirrors, and an assortment of display applications which includes television screens and computer monitor screens to minimize reflective “glare.” The most common design for such composites is one having a quarter-wave optical thickness at a particular wavelength. This design is capable of reducing reflectance of a surface to less than 1% over the visible range.
A typical anti-reflective composite consists of a light transmissive substrate and one or more transparent anti-reflective top layers. A transparent hard coat layer is often deposited between the substrate and the anti-reflective layers to give the composite both mechanical durability and physical strength. The materials used in each layer and the thicknesses of each layer are chosen so that a maximum amount of light is transmitted through the composite while a minimum amount of light is reflected by the composite.
Numerous anti-reflective composite designs are known to date, most of which are comprised of high and low refractive index materials in pairs. One of the earliest patents in this field, i.e., U.S. Pat. No. 2,478,385, describes a three-layer structure of medium/high/low refractive index materials over a glass substrate.
Another earlier patent dealing with anti-reflective coatings is U.S. Pat. No. 3,432,225, wherein is disclosed a method of combining a four-layer anti-reflective coating system using two different materials, i.e. ZrO
2
and MgF
2
. A basic problem with this approach is the inherent softness of MgF
2
, which limits the use of this approach in many applications.
Another multi-layer anti-reflective coating is disclosed in U.S. Pat. No. 3,565,509 wherein a three layer system is reduced to two using two materials for production simplicity.
The most common anti-reflective coating is a four layer structure. Such a design can be made from two anti-reflective coating materials rather than from three or four different materials as indicated in some earlier patents. The thickness of each layer is optimized to obtain the best properties over a broad range of the spectrum.
Most anti-reflective composite designs utilize high index dielectric anti-reflective layers as part of the construction. In applications where electromagnetic shielding and static discharge control are of primary concern, conductive high index oxides such as indium oxide or tin oxide are incorporated in the design structure. U.S. Pat. No. 4,422,721 covers the use of conductive coatings as part of the anti-reflective design structure.
U.S. Pat. No. 5,170,291 discloses a four-layer anti-reflective composite where DC reactive sputtering has been suggested as a preferred method of deposition. U.S. Pat. No. 5,579,162 discloses a multi-layer anti-reflective composite utilizing DC reactive sputtering as a preferred method of deposition for temperature sensitive substrates.
There are several problems with the anti-reflective composites presently known. A first problem is that special processes are required to deposit anti-reflective coatings onto a polymeric substrate.
A second problem is that most production techniques proposed for deposition of various layers of anti-reflective composites are possible, but few are practical. This is particularly important since there is no documented method of depositing anti-reflective coatings in a continuous roll coating (web) system.
A third problem with presently known anti-reflective composites is the general lack of manufacturing efficiency and low deposition rates previously inherent in the manufacture of anti-reflective composites.
A fourth problem with presently known anti-reflective composites is the great difficulty in being able to adhere anti-reflective coatings onto a polymeric substrate, especially a polymeric substrate which is covered with a hard coat.
Accordingly, there is a need for an improved anti-reflective composite which overcomes these problems in the prior art.
SUMMARY
The present invention solves these problems. The present invention is an anti-reflective composite having very high visible light transmission and negligible visible reflectance over 400 nm-800 nm wavelengths. Such coatings may be suitable for any surfaces requiring low light reflectance and requiring electrical and magnetic shielding. The proposed anti-reflective composites have utilization in a variety of display applications.
In one embodiment, the invention is an anti-reflective composite comprising: (a) a light transmissive substrate; (b) a hard coat deposited onto the substrate; (c) a first transparent oxide layer deposited onto the hard coat; and (d) a second transparent oxide layer deposited onto the first transparent oxide layer. Preferably, the transparent oxide layers are deposited by pulsed magnetron sputtering comprising either medium frequency AC sputtering or symmetric/asymmetric bi-polar DC sputtering.
In another preferred embodiment of the invention, a thin carbon layer, having an average thickness between about 2 Å and about 100 Å is deposited between the hard coat and the substrate. In another preferred embodiment of the invention, a thin carbon layer, having an average thickness of between about 2 Å and about 100 Å is deposited between the outermost transparent oxide layer and the low surface energy layer.
In more sophisticated embodiments of the invention, the composite comprises at least one pair of oxide layers deposited on top of the hard coat. Each pair of oxide layers comprises (i) a first transparent oxide layer deposited onto the hard coat, the first transparent oxide layer having a refractive index between about 1.65 and about 2.65 and having an average thickness between about 100 and about 3200 Å; and (ii) a second transparent oxide layer deposited onto the first transparent oxide layer, the second transparent oxide layer having a refractive index between about 1.2 and about 1.85 and having an average thickness between about 100 and about 3200 Å.
In a preferred embodiment of the composite described immediately above having at least one pair of oxide layers, one of the transparent oxide layers is a tertiary oxide layer.
In another preferred embodiment of the composite having at least one pair of oxide layers, a layer of aluminum oxide or zirconium oxide having a thickness between about 400 Å and about 100 Å is deposited between the hard coat and the pair of oxide layers. Other oxides possessing refractive indices of about 1.50 to about 2.20 can replace the above mentioned articles.
In still another preferred embodiment of the composite having at least one pair of oxide layers, the second transparent oxide layer in the outermost pair has an index of refraction between about 1.2 and about 1.85 and has a low surface energy of 40 dynes/cm or less.
In another preferred embodiment, the low surface energy layer is a vacuum deposited organic/inorganic mixture.
The transparent layers may be deposited by vacuum or non-vacuum processes or by a combination of both.
The hard coat layer may be an “ordinary” organic hard coat layer having an index of refraction between about 1.4 and about 2 and having an average thickness between about 0.5 and about 10 microns, preferably deposited by a wet chemistry process. The hard coat layer can also be either an inorganic material or an organic/inorganic material having an index of refraction between about 1.5 and about 2 and having an average thickness between about 0.5 and 10 microns, preferably deposited by a vacuum process.


REFERENCES:
patent: 2478385 (1949-08-01), Gaiser
patent: 2854349 (1958-09-01), Dreyfus et al.
patent: 3185020 (1965-05-01), Thelen
patent: 3272986 (1966-09

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