Transparent glass or plastic substrates (e.g., optical substrates) can experience a substantial loss of optical performance due to unwanted reflections from an air-substrate interface. When multiple interfaces are present within a display, the loss of viewing efficiency can substantially increase. The losses from the air-substrate interface can be described by the Fresnel equation:((nd−1)/(nd+1))2*100=% reflection
In this equation nd represents the refractive index of the optical substrate and 1 represents the approximate refractive index of air.
This loss of optical performance becomes apparent when one tries to view an image or text through a transparent glass or plastic substrate, such as a cellular phone lens or a touch screen. In conditions of high ambient lighting, the surface reflection becomes so intense that one cannot readily view the text or the images through the transparent substrate.
Various solutions to this problem have been disclosed in the prior art. The most common of these is to coat the transparent substrate with a layer of material, which has a refractive index lower than that of the substrate, and an optical thickness of approximately one-quarter the wavelength of the light of interest. For instance, by coating a poly(ethylene terephthalate) (PET) film with a single layer (e.g., about 0.100 microns thickness) of gas phase deposited silicon dioxide (SiO2), the percent reflectance can be decreased from about 5.75 percent per side to about 1.50 percent per side, with concomitant improvements in viewing efficiency. As the number of functional layers increases, the efficiency of these coatings also improves dramatically. The coatings go from being quite narrow in their performance characteristics to quite broad, as one goes from 2-layers to greater than 3-layers in an optical stack.
Traditionally, high performance anti-reflection films are comprised of: polyethylene terephthalate film with an abrasion resistant coated surface, followed by alternating layers of metallic oxides and silicon dioxide. It is typical that the total thickness of the sputtered layers will exceed about 200 nm. Anti-reflection films produced via this method exhibit excellent anti-reflection performance. However, the anti-reflection film is too expensive for many applications due to the relatively low processing rate, which is related to the total physical thickness that must be sputtered.
The outer surface of an anti-reflection film produced by this method is typically composed of silicon dioxide. Silicon dioxide exhibits extremely high surface energy and therefore is susceptible to mark-off from fingerprints and other stains. In addition, this high surface energy makes the silicon dioxide surfaces difficult to clean; thus, a thin lubrication layer (i.e. anti-fingerprint layer) is usually provided to the anti-reflection film's surface in a separate processing step. This added processing step adds to the cost of the film.
U.S. Pat. No. 6,464,822 assigned to 3M Innovative Properties Company teaches a process for providing anti-reflective coated articles. The patent discloses anti-reflection coated articles prepared by a combination of vacuum sputtering of metallic inorganic oxides followed by application of curable polymer coatings. The patent indicates that substantial improvements in production efficiencies are gained by the reduction in the total sputtered thickness. For example, the patent teaches an anti-reflection coated film, wherein the anti-reflective layers are comprised of about 20 nm of metal oxide, about 20 nm of silica and approximately 85 nm of a polymer outer layer—for a total sputtered thickness of about 40 nm. The patent discloses that in the production of films that parallel the performance of sputtered multi-layer films, the number of layers increases, as does the sputtered thickness. In this example, the physical thickness of the sputtered layers is between about 80 nm and about 160 nm, which results in the above-mentioned production inefficiency.
Modern electronic input devices, such as touch a panel, are comprised of two transparent electrodes separated by a thin space mounted in front of a display terminal. The rear electrode is typically rigid while the front electrode is flexible. As a rule, the more transparent these conductive electrodes are, the better. The most widely used transparent conductive oxide, Indium Tin Oxide (ITO), is very reflective when applied to the necessary conductivity. This results in reduced display performance due to the reflective losses generated within the touch panel. While the prior art indicates that these losses can be minimized via thin multi-layer dielectric stacks under the ITO layer, the reduction in production efficiencies are quite substantial.
U.S. Pat. No. 6,583,935 assigned to CPFilms, Inc. teaches a process whereby a high transmission transparent conductive metal oxide is formed that exhibits high transmission and low reflectance. The coated articles of reference are formed by vacuum deposition of several layers of inorganic oxide materials followed by a thin layer of a conductive metal oxide. For example, the patent teaches a high transmission ITO coated film comprised of about 28 nm of titanium dioxide, about 64 nm of silicon dioxide, and about 16 nm of ITO. Again, as the total physical thickness of the sputtered layers increases, the rate of production decreases from more than 5 meters per minute to less than 0.50 meters per minute with concomitant increases in cost.