WO2003024712A1

WO2003024712A1 - Durable, low ohm, high transmission transparent conductor

Info

Publication number: WO2003024712A1
Application number: PCT/US2002/025980
Authority: WO
Inventors: Mohtashim Saif; Hassan Memarian
Original assignee: Cpfilms Inc.
Priority date: 2001-09-14
Filing date: 2002-09-13
Publication date: 2003-03-27
Also published as: EP1451009A1

Abstract

A transparent electrically conductive flexible composite comprises a transparent flexible polymeric sheet (2) with a transparent electrically conductive coating (1) of indium zinc oxide on the sheet. The zinc content in the indium zinc oxide coating is in the range of 0.01 to 9.99 atomic percent relative to the total amount of indium and zinc in the oxide. The indium zinc oxide coating has a sheet resistance in the range of 3-1000 ohms per square. The electrically conductive coating of indium zinc oxide is sputter coated onto the sheet.

Description

DURABLE, LOW OHM, HIGH TRANSMISSION TRANSPARENT CONDUCTOR

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to the field of transparent electrical conductive coatings made of indium zinc oxide which are applied to transparent substrates, especially flexible transparent polymeric film substrates. The invention also pertains to the method of making the coated substrates and the use of the coated substrates in electronic devices which require transparent conductive oxide (TCO) films having excellent electrical conductivity, mechanical durability and high transparency. Such electronic devices include transparent electrodes, electroluminescent lamps, EMI/RFI shielding, liquid crystal displays, touch screens, heat mirrors and transparent window heaters.

2. Background Information

Substrates such as flexible transparent polymeric films having a TCO coating thereon are widely used in the above-noted devices because these coatings possess high optical transparency, high electrical conductivity and good mechanical durability. Typically indium oxide-tin oxide (indium-tin oxide commonly referred to ITO) is used as the TCO coating in electronic devices which comprise a TCO coating on a flexible polymeric film substrate. In particular, ITO with a refractive index (RI) of about 2.0 has been widely used for transparent conductive films for many applications. ITO generally has desirable optical properties, e.g. , ultraviolet (UN) radiation absorption below 330 (nm), high visible light transmission, and infrared (IR) radiation reflection at 800-1500 nm. U.S. Patent No. 6,028,654 discloses a liquid crystal display with ITO transparent electrodes having a surface resistivity of 20-70 ohms/square. A drawback to ITO films is a tendency to absorb blue light which results in a yellow or green tint. This absorption of visible light increases with film thickness.

ITO has an inherent bulk resistivity of about 0.0004 ohm-centimeter.

Lower resistance can be obtained by providing films with greater thickness. Highly conductive ITO films can be readily applied to rigid transparent substrates such as glass and polycarbonate. There is a tendency for ITO films on flexible substrates to have diminished structural integrity with increased thickness. However, as noted above, increasing the thickness is required to provide a film having lower resistance. This increased thickness makes it difficult to achieve durable, highly conductive ITO films on flexible substrates. The diminished structural integrity is manifest by reduced adhesion between the ITO coating and the substrate and a high radius of curl due to ITO brittleness. In addition, ITO with low structural integrity has low tolerance for further handling and/or etching.

Although ITO is routinely used in many applications, the limitation of ITO coatings at higher thicknesses leads to more stress in the film and also produces a pronounced color which makes its use prohibitive in some applications.

The prior art describes various types of indium zinc oxide (IZO) coatings including transparent electrically conductive coatings made from IZO.

However, none of the prior art IZO coated substrates provide the desired degree of structural integrity and thus they suffer from the same problems noted above with respect to the prior art ITO coated substrates. U.S. Patent No. 5,972,527 and EPO 677,593 Al disclose transparent electrically conductive layers of indium zinc oxide on various substrates. The indium zinc oxide coating used in U.S. Patent No. 5,972,527 and EPO 677,593 Al contains indium and zinc having an atomic ratio (In/In +Zn) of 0.5-0.9. In other words the percentage of indium atoms in the indium zinc oxide is 50-90 % based upon the total number of indium and zinc atoms in the indium zinc oxide.

Carter et al. discloses in European patent application EP0707320 Al transparent conductors which comprise zinc-indium oxide (40-75 atomic % indium) deposited onto glass and quartz substrates. Such films are said to be useful as transparent electrodes, particularly in the blue region.

Minami et al. disclose in Japanese Kokai HEI 8 [1996]-264021 transparent conductive films which are obtained by sputtering indium zinc oxide (5-45 atomic % zinc) onto substrates at a temperature of 350 °C.

Moldonado et al. disclose (J. Vac. Sci. Technol. A 15(6) Nov/Dec 1997 p. 2905) that substrate temperature is the most important parameter which influences the physical properties of indium zinc oxide films. Moldonado et al. disclose the spray deposition of an indium zinc oxide (3 atomic % indium) film on glass substrates at 675 °K (412°C) and 800°K (527°C).

Minami et al. also disclose Thin Solid Films 290-291 (1996) 1-5) that the minimum resistivity is achieved when indium zinc oxide films are prepared by DC magnetron sputtering and that the composition at which minimum resistivity can be attained depends upon the substrate temperature. Figure 1 of Minami et al. illustrates resistivity as a function of the atomic ratio of Zn/(In+Zn) at deposition temperatures of room temperature and 350°C.

In an article entitled "Transparent Conductive Material Consisting of Indium Oxide and Zinc Oxide Deposited at Low Temperature", A. Knijo of Idemitu Kosan Co., Ltd. discloses transparent films of indium zinc oxide wherein the atomic ratio of In/Zn is 2/8 to 9/1.

U.S. Patent No. 5,206,089 discloses a glass substrate coated with a layer of transparent conductive material. The transparent conductive material is a mixed oxide of indium and zinc containing 10-34 atomic % indium. The coating is obtained by pyrolysis of powdered compounds of indium and zinc in contact with a substrate heated to a high temperature.

U.S. Patent No. 5,443,862 describes a process for treating thin semiconducting layers of metal oxide. The treatment comprises subjecting the thin semi-conducting layer to an ion beam. Various semi-conducting layers are described including indium oxide doped with tin, tin oxide doped with fluorine and zinc oxide doped with indium (ZnO:In).

U.S. Patent No. 5,470,618 discloses a zinc oxide based transparent conductive film containing a small amount (0.4 to 8 atomic %) of gallium or gallium oxide. It is said that the gallium or gallium oxide may be replaced by indium or indium oxide.

U.S. Patent No. 5,589,274 discloses a thermal controlled coating which is applied like a paint to a substrate. The coating comprises a silicone polymeric matrix having doped zinc oxide particles distributed therein. Dopants include aluminum, gallium, indium, boron, zinc, tin or hydrogen.

U.S. Patent No. 6,040,056 discloses a transparent substrate which is coated with a transparent electrically conductive film. The electrically conductive film is a laminate which contains reflection preventing layers and one or more metal layers. The metal layers are silver or metal which comprises silver as the main component. The reflection preventing layer comprises a composite oxide of zinc and indium. The atomic ratio of indium and zinc expressed as zinc/(indium+zinc) in the reflection preventing layers is 0.03 to 0.9. The transparent electrically conductive film having the silver and IZO layers is structured to provide a sheet resistance of about 2.5 ohms per square to 1.5 ohms per square. Preferably the laminated structure of the electrically conductive film has a structure to provide a sheet resistance of 1.5 ohms per square or lower.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a transparent flexible substrate coated with a transparent electrically conductive film of indium zinc oxide which has enhanced mechanical and optical properties and which can be deposited by sputtering onto the flexible transparent substrate at low temperature (e.g. , ambient or room temperature). In particular, it is an objective of this invention to provide a transparent flexible polymeric substrate coated with an electrically conductive film of IZO which has improved conductivity, mechanical durability and optical clarity which cannot be achieved with indium tin oxide coatings or other indium zinc oxide coatings on the flexible polymeric substrates. It is also an objective of this invention to provide a transparent flexible substrate coated with an electrically conductive transparent coating of indium zinc oxide having a low sheet resistance within the range of 3-1 ,000 ohms per square, preferably 3-30 ohms per square, more preferably 3-20 ohms per square and most preferably 3-10 ohms per square.

It is also an objective of this invention to provide a low temperature (25°C-30°C) sputtering deposition process for applying such IZO films to the flexible substrate.

These and other objectives are obtained by sputter coating a transparent electrically conductive IZO film at low temperature (for example at room or ambient temperature) onto a transparent flexible polymeric film or sheet; wherein the IZO film contains zinc and indium in an amount so that the ratio of the total number of Zn atoms relative to the total number of In and Zn atoms (total number of Zn atoms divided by the sum of the total number of Zn atoms and the total number of In atoms; i.e. , Zn Zn+In) is 0.0001 to 0.0999. This ratio may be expressed as an atomic percent by multiplying the ratio by 100. Thus the atomic percent zinc used in this invention is 0.01 % to 9.99 %.

Since the atomic percent of Zn is relative to the total amount of In and Zn, then the atomic percent zinc when added to the atomic percent indium will equal 100% . Thus the atomic percent indium used in this invention is 90.01 % to 99.99% (i.e., (In In+Zn) x 100=90.01 to 99.99).

Preferably the IZO coating has an atomic percent of In which is 97 % with a corresponding atomic percent of Zn which is 3 % (referred to herein as having the atomic percent ratio 97:3). In this preferred embodiment the transparent electrically conductive film or coating on the transparent flexible polymeric substrate has a sheet resistance which is at least 3 ohms per square up to about 1000 ohms per square.

As noted above, the IZO coating is transparent and electrically conductive. Thus the invention is directed to an electrically conductive composite device which comprises a flexible transparent polymeric sheet having a transparent electrically conductive film or coating of IZO thereon. In one embodiment the transparent electrically conductive film or coating consists of a layer of transparent electrically conductive IZO without any other layers therein. Thus in this embodiment, a metal layer or layers (i.e. , metal in the elemental state) are excluded from within the IZO coating. However, a primer layer, which in some cases may be a metal as further described herein, may be interposed between the IZO layer and the polymeric substrate. Since the primer layer is between the IZO film and the polymeric material, it is not within the IZO coating. An example of an IZO coating which includes a metal layer therein (which is excluded in this embodiment of the present invention) is the electrically conductive film described in U.S. Patent No. 6,040,056. In Patent No. 6,040,056, the transparent electrically conductive film is a laminate which includes one or more silver layers with IZO layers on both surfaces thereof. Thus the transparent electrically conductive film of Patent No. 6,040,056 includes a silver layer within the IZO material.

Also, the embodiments of the present invention wherein the transparent electrically conductive coating has a sheet resistance which is more than 3 ohms per square, is further distinguished from the electrically conductive coating of U.S. Patent No. 6,040,056 which has a maximum sheet resistance of 2.5 ohms per square. It is also to be noted that the IZO film coating of the present invention is sputter coated at a low temperature which does not result in degradation (e.g. , melting, discoloration, decomposition or other loss of properties) of the polymeric sheet which would occur at high temperature sputtering such as the 350 °C sputter coating temperature used by Minami et al. in Japanese Kokai Hei 8[ 1996] -264021. Such a high temperature would destroy the polymeric substrates used in the present invention.

The ratio 97:3 with respect to IZO referred to in the figures and elsewhere in this application means that the IZO has 3 atomic percent Zn based on the total number of In and Zn atoms and 97 atomic percent In based on the total number of In and Zn atoms and thus has an atomic percent ratio (In:Zn) which is 97 : 3 (ratio of atomic percents) . Likewise the ratio 90 : 10 with respect to IZO referred to in the figures and elsewhere in this application means that the IZO has 10 atomic percent Zn based on the total number of In and Zn atoms and 90 atomic percent In based on the total number of In and Zn atoms and thus has an atomic percent ratio (In:Zn) which is 90: 10 (ratio of atomic percents). Also the ratio 90:10 with respect to ITO referred to in the figures (figures 15-17) and elsewhere in this application means that the ITO has 10 weight percent Sn and 90 weight percent In based on the total weight of In and Sn and thus has a weight percent ratio (In:Sn) which is 90: 10 (ratio of weight percents).

As noted above, in one embodiment of the present invention the ratio of the atomic percent of indium to the atomic percent of zinc (wherein both atomic percents are based upon the total amount of indium and zinc) is 97:3. However, it is contemplated in this invention to employ IZO coatings having an atomic percent of Zn, calculated as described above, which is less than 3 atomic percent zinc based on the total number of zinc and indium atoms in the IZO.

The IZO conductive coatings of the present invention may include any of the conventional dopants which are used in this field of technology. Thus, dopants such as Sn, Al, Sb, B etc. , may also be added to further improve the optical and/or electronic properties of the deposited films.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates resistivity as a function of zinc content for IZO films which is known from the prior art.

Figure 2 depicts schematically a sputter roll coater which can be used to deposit the IZO coating used in the present invention.

Figure 3 is a graph which shows the percent transmission versus wavelength and percent reflection versus wavelength for an IZO film having a sheet resistance of 20 ohms per square, sputter coated onto a transparent substrate by reactive sputter coating from indium and zinc metal target in an oxygen containing atmosphere.

Figure 4 is a graph which shows the percent transmission versus wavelength and percent reflection versus wavelength for an IZO film having a sheet resistance of 30 ohms per square, sputter coated onto a transparent substrate by reactive sputter coating from indium and zinc metal target in an oxygen containing atmosphere. Figure 5 is a graph which shows the percent transmission versus wavelength and percent reflection versus wavelength for an IZO film having a sheet resistance of 100 ohms per square, sputter coated onto a transparent substrate by reactive sputter coating from indium and zinc metal target in an oxygen containing atmosphere.

Figure 6 is a graph which shows the percent transmission versus wavelength and percent reflection versus wavelength for an IZO film having a sheet resistance of 200 ohms per square, sputter coated onto a transparent substrate by reactive sputter coating from indium and zinc metal target in an oxygen containing atmosphere.

Figure 7 is a graph which shows the percent transmission versus wavelength and percent reflection versus wavelength for an IZO film having a sheet resistance of 300 ohms per square sputter coated onto a transparent substrate by reactive sputter coating from indium and zinc metal target in an oxygen containing atmosphere.

Figure 8 is a graph which shows the percent transmission versus wavelength and percent reflection versus wavelength for an IZO film having a sheet resistance of 20 ohms per square, sputter coated onto a transparent substrate by sputter coating from an IZO ceramic target.

Figure 9 is a graph which shows the percent transmission versus wavelength and percent reflection versus wavelength for an IZO film having a sheet resistance of 30 ohms per square, sputter coated onto a transparent substrate by sputter coating from an IZO ceramic target. Figure 10 is a graph which shows the percent transmission versus wavelength and percent reflection versus wavelength for an IZO film having a sheet resistance of 100 ohms per square, sputter coated onto a transparent substrate by sputter coating from an IZO ceramic target.

Figure 11 is a graph which shows the percent transmission versus wavelength and percent reflection versus wavelength for an IZO film having a sheet resistance of 200 ohms per square, sputter coated onto a transparent substrate by sputter coating from an IZO ceramic target.

Figure 12 is a graph which shows the percent transmission versus wavelength and percent reflection versus wavelength for an IZO film having a sheet resistance of 300 ohms per square, sputter coated onto a transparent substrate by sputter coating from an IZO ceramic target.

Figure 13 is a graph which shows the percent transmission versus wavelength and the percent reflection versus wavelength for a variety of films which have a sheet resistance of 20 ohms per square (IZO films having In:Zn atomic percent ratios of 97:3 and 90: 10 based on the total number of In and Zn atoms.

Figure 14 is a graph which shows the percent transmission versus wavelength and percent reflection versus wavelength for a variety of films which have a sheet resistance of 30 ohms per square (IZO films having In:Zn atomic percent ratios of 97:3 and 90: 10 and an ITO (indium tin oxide) film having an In:Sn weight percent ratio of 90: 10. The weight percent ratio of 90: 10 for ITO is the ratio of weight percent In to the weight percent Sn wherein each weight percent is based on the total weight of In and Sn. Figure 15 is a graph which shows the percent transmission versus wavelength and percent reflection versus wavelength for a variety of films having a sheet resistance of 100 ohms per square (IZO films having an In:Zn atomic percent ratios of 97:3 and 90: 10, an indium tin oxide (ITO) film having In:Sn weight percent ratio of 90: 10 and an indium oxide (InO_x) film.

Figure 16 is a graph which shows the percent transmission versus wavelength and percent reflection versus wavelength for a variety of films having a sheet resistance of 200 ohms per square (IZO films having In:Zn atomic percent ratios of 97:3 and 90:10, an ITO film having an In:Sn weight percent ratio of 90: 10 and an indium oxide film.

Figure 17 is a graph which shows the percent transmission versus wavelength and percent reflection versus wavelength for a variety of films having a sheet resistance of 300 ohms per square (IZO films containing In:Zn atomic percent ratios of 97:3 and 90: 10, an ITO film having an In:Sn weight percent ratio of 90: 10 and an indium oxide InO_x film.

Figure 18 is a cross-sectional representative of an embodiment of the invention which comprises an IZO film of the invention coated onto a transparent flexible polymeric substrate.

Figure 19 is a cross-sectional illustration of another embodiment of the invention which corresponds to the embodiment illustrated in Figure 18 with a primer layer interposed between the IZO film and the transparent flexible polymeric substrate. DET AILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

Reference is made herein with respect to the present invention, to atomic ratios of In to Zn such as the atomic ratio 97:3 discussed above. All such references to atomic percent ratios of In:Zn refer to the ratio of the atomic percent of In to the atomic percent of Zn wherein each of the atomic percents are with reference to the total number of In and Zn atoms. Thus when the atomic percent ratio is 97:3, the atomic percent of In is 97% based on the number of In atoms relative to the total number of In and Zn atoms and the atomic percent of Zn is 3 % based on the total number of Zn atoms relative to the total number of In and Zn atoms. Likewise referenced herein to the atomic percent ratio of In:Zn which is 90: 10 means that the atomic percent of In is 90% based on the total number of In atoms relative to the total number of In and Zn atoms and the atomic percent of Zn is 10% based on the number of Zn atoms relative to the total number of In and Zn atoms.

Reference is also made herein to ITO (indium tin oxide) material having a weight percent ratio of In:Sn which is 90: 10. Reference herein to the atomic percent ratio with respect to ITO material means that the material contains 90% by weight of In with respect to the total weight of In and Sn and 10% by weight of Sn with respect to the total weight of In and Sn. Thus the ratio 90: 10 with respect to ITO refers to the above described weight percent ratio and the ratios 97:3 and 90: 10 with respect to IZO refers to the above-described atomic percent ratios.

Reference is also made herein to electrically conductive indium oxide coatings. Such coatings are referred to herein as InO_x coatings. It will be appreciated by those skilled in the art that such coatings have a less than stoichiometric amount of oxygen so that the indium oxide is electrically conductive. The value of x in the formula InO_x therefore refers to the amount of oxygen which is less than the stoichiometric amount of oxygen and which allows the indium oxide to be electrically conductive.

The transparent flexible polymeric substrate used in the present invention may be any of the commonly employed polymeric substrates which are typically used in electrically conductive composites which employ a transparent electrically conductive oxide (TCO) coating. An example of such a polymeric material is polyethylene terephthalate (PET).

Sputtering is advantageously used to deposit the indium zinc oxide film so that high temperatures and other physical and chemical conditions which could harm the polymeric substrate can be avoided. Sputtering is particularly advantageous because it can be used to deposit the IZO layer onto the polymeric substrate at ambient or room temperature (e.g., about 70°F). Any conventional sputtering method and apparatus may be used; it being understood that the ratio of In to Zn requires the use of a target which has a corresponding In:Zn ratio. Sputtering coaters which advance a polymeric sheet or web from one roll to another roll with one or more sputtering stations positioned in the path of the sheet or web are particularly advantageous because the substrate can be sputter coated with the IZO layer in one pass of the sheet as it advances from one roll to the other roll. Such sputtering coaters are well known and are commercially available. An example of such a sputtering device is shown in U.S. Patent No. 4,977,013, the specification of which is incorporated herein by reference. The IZO coated polymeric sheets described herein were made by using the sputter coater illustrated in figure 2. The sputtering target may be an indium-zinc alloy. When sputtering with an indium-zinc alloy, the sputtering is performed in an atmosphere which contains oxygen according to well known techniques so that the deposited IZO material has the desired oxides of indium and zinc which are transparent and electrically conductive. The sputtering conditions required to produce a transparent electrically conductive IZO coating are well known to those skilled in the art. For convenience, such IZO coatings sputter coated by reactive sputtering using a metal target (e.g., indium-zinc alloy) may be referred to herein as "metal" or "metallic" IZO coatings.

Alternatively, instead of using an alloy target in an oxidizing atmosphere, the sputtering may use a zinc oxide-indium oxide ceramic (e.g. , a sintered mixture of zinc oxide and indium oxide powder). IZO Coatings obtained by sputter coating from a ceramic target may be referred to herein as "ceramic" IZO.

In a preferred embodiment the sputtering is done in a closed loop using plasma emission monitoring especially when sputtering from a metal target in an oxidizing atmosphere. Plasma emission monitors and the use thereof and regulating the amount of oxygen during reactive sputtering procedures are well known to those skilled in the art. A description of a suitable plasma emission monitor which can be used in the present invention is described in the publication by Patel et al. ; Methods of Monitoring and Control of Reactive ITO Deposition Process on Flexible Substrates with DC Sputtering; Society of Vacuum Coaters; 39th Annual Technical Conference Proceedings (1996) ISSN 0737-5921; pp.. 441-445. The text of the aforementioned publication is incorporated herein by reference. Figure 18 illustrates an embodiment of the invention which comprises transparent electrically conductive IZO film 1 adhered directly to transparent flexible polymeric sheet or film 2. Figure 19 illustrates an embodiment of the invention wherein primer layer 3 is interposed between IZO film 1 and polymeric sheet 2.

The primer layer which may be interposed between the IZO film and the polymeric substrate may also be deposited by a conventional sputtering procedure. The same sputtering apparatus and temperature used for applying the IZO film may also be used to apply the primer layer. The primer layer improves the adhesion between the IZO and the polymeric substrate. Preferably the primer layer is a metal or an oxide or nitride thereof. In a preferred embodiment the primer layer has a thickness of .5 to 100 nm, more preferably .5 to 50 nm. Preferably the primer layer is made from a material selected from the group consisting of Ti, Cr, Ni, C, Zr, Hf, Ta, W, Al, oxides of any of the aforementioned metals, nitrides of any of the aforementioned metals and mixtures of any of the aforementioned metals, oxides or nitrides. Examples of suitable oxides include Ti0₂, A1₂0₃, Ta₂0₅, etc. Suitable nitrides include TiN, ZrN, etc.

The IZO film of the present invention may be doped with small amounts of other conventional doping elements to further improve the optical-electronic properties thereof. Such elements may include Sn, Sb, Al and others which are will known to those skilled in the art as dopants in this field of technology. The atomic percentage of these elements can vary from 0.1 % to 10% depending on the application wherein the atomic percentage of the dopant is based upon the total number of zinc atoms , indium atoms and dopant atoms . It is expected that this type of tertiary alloy composition may provide additional benefits and improve the overall properties of the deposited films.

The dopants are advantageously included in the target (alloy or ceramic) so that the sputter coated layer contains the dopant metal therein.

In a preferred embodiment the layer of indium zinc oxide which is coated onto the transparent flexible polymeric substrate has a sheet resistance of from 3 to 30 ohms per square. In addition, in a preferred embodiment the composite film which comprises the layer of IZO coated on a flexible transparent polymeric substrate, has a light transmission at 550 nanometers of at least 75 % and is essentially crack free after rolling over a 3 mm diameter mandril.

The IZO coatings of the present invention preferably have bulk resistivities of less than 2.0 x 10^"4 ohm centimeter, visible light transmission which is greater than 85% and low yellow index.

The IZO film which is coated onto the substrate has 0.01 to 9.99 atomic % zinc therein based upon the total number of indium and zinc atoms in the IZO film. Preferably the IZO film has 1 to 8 atomic % zinc, more preferably 2 to 6 atomic % zinc, and even more preferably 2 to 4 atomic % zinc.

The IZO coated polyester films of the invention have an IZO coating which is applied at a thickness which produces low surface resistivity, e.g., less than 100 ohms per square, and a low yellow index, e.g. , less than 15 YID. In addition, the IZO coated polyester films of the present invention have enhanced mechanical properties including enhanced adhesion, low curl, extreme flexibility and machinability.

The preferred IZO coated polyester film of this invention has a surface resistance in the range of 3-50 ohms per square, more preferably 3-30 ohms per square or even more preferably 3-20 ohms per square and most preferably 3-10 ohms per square. In addition, the sputter coated IZO films or coatings of the present invention have enhanced transparency in the visible region of the electromagnetic spectrum.

The mechanical properties of the IZO film of the present invention (which includes adhesion, abrasion, curl and flexibility) are superior than other transparent conductive coatings. These excellent mechanical properties are extremely important where room temperature sputtering is needed in roll-to-roll flexible films, especially for applications that need less than 50 ohms per square sheet resistance.

To illustrate the improved mechanical properties of the IZO films or coatings of the present invention, the IZO coatings of the invention were sputter coated onto a flexible polymeric film at room temperature using metallic indium/zinc target. The transparent conductive film produced by this sputtering procedure met all the optical requirements compared to that of ITO film with low absorption and less yellowness than ITO. The mechanical properties, however, were much superior compared to ITO film. The IZO film of the invention showed good adhesion, good mechanical durability /flexibility, curl and excellent machinability with good etching properties especially for films which have less than 20 ohms per square sheet resistance. The spectral optical properties (i.e. , absorptance, reflectance and transmittance) of the IZO coated flexible substrate composites made in accordance with this invention are determined by conventional procedures using a spectro-photometer. The surface resistivity or sheet resistance as it is sometimes called, of the IZO films of the present invention are measured in the conventional manner. Thus, the surface resistivity or sheet resistance is set forth in units of ohms per square. The methodology for measuring the sheet resistance is well known to those skilled in the art. Typically for surface resistivity of 10 ohms per square and higher, a 2 point probe is satisfactory. Although a 4 point probe method is sometimes recommended for surface resistivities of less than 10 ohms per square, a 2 point probe method may be used for the IZO materials of the present invention. Surface resistivity is measured on a transverse strip (about 5 cm wide and 5 cm long) of IZO film coated flexible substrate. "Transverse strip" means that a sample is cut across the width of a continuous web with a short dimension (5 cm) in the direction of web motion during coating. A useful probe comprises 5 cm long cylindrical conductive elastomer electrodes for contacting the IZO surface where the elastomer electrodes are spaced 5 cm apart by mounting them in a metal channel supported by a rigid insulator. Such conductive elastomers are well known to those skilled in the art and are commercially available from Chomerics.

Surface resistivity is determined by cleaning electrodes and electrodes that contact surfaces, e.g., metal channels and IZO surface, with a suitable solvent such as isopropyl alcohol. The IZO coated flexible substrate is placed IZO-side up on a flat surfaced rubber mat. The probe is placed on the IZO coating, loaded to about 6.9 kiloNewtons per square meter (1 psi) with the 5 cm length of the electrodes extending across the 5 cm width of the strip. Resistance measured in ohms is "ohms per square". As used herein the term "surface resistivity" or "sheet resistance" of an IZO surface means an average of surface resistivities measured at 5 cm (3 inch) intervals on at least two sample transverse strips 2 meters apart.

The following examples describe preferred embodiments of the invention and illustrate the improved features of the present invention compared to other electrically conductive transparent materials. The following examples are intended for illustration only and are thus not intended to limit the scope of the present invention.

Example 1 This example illustrates the deposition of an IZO coating on a flexible polyester substrate. A roll of 0.18 millimeter (7 mil) thick polyethylene terephthalate (PET) film was threaded through a roll-to-roll laboratory sputtering coater illustrated in Figure 2. The IZO coating was applied to the moving web of PET film by sputtering from an indium: zinc alloy target of atomic percent ratio of 97:3. The drum width is 40 cm (16 in) and the cathode size is 38 cm x 11 cm (15 in x 4.5 in). The first pass was used to condition/clean the PET surface by glow discharge plasma. The PET film was run through the coater in a second pass at various line speeds to deposit IZO at various thickness (and surface resistivity). The PET with IZO coating of about 30 ohms per square was run through the coater in a third pass to provide an IZO coating of about 20 ohms per square. All three passes were run with a single pump down to operating pressure and the film was re-wound after each pass. Sputtering parameters for all three passes are set out below in Tables 1-3. Table 1

First Pass: Glow Discharge Plasma Cleaning Parameters

Table 2 Second Pass: IZO Deposition Parameters

Table 3

The IZO coated PET substrates prepared according to this Example 1 were evaluated for spectral optical properties according to ASTM Standard E424 (Test Method for Solar Energy Transmittance and Reflectance of Sheet Materials) over the 316nm to 2765nm spectral range using a spectrophotometer calibrated according to ASTM Standard E275 (Recommended Practice for Describing and Measuring Performance of Spectrophotometers). Spectral reflectance (%R) and transmittance (%T) were measure; absorptance (% A) was determined from the Kirchoff relationship where the sum of absorptance, reflectance and transmittance equals unity. A yellowness index (YID) was determined from the CIE 1976 (L*a*b*) color space. The spectral optical properties are reported in Table 4. Table 4

Figures 3-7 show the optical spectrum of the visible region for the metallic (atomic percent ratio In:Zn=97:3) IZO of 20, 30, 100, 200 and 300 ohms/square.

Figures 8-12 show the optical spectrum of the visible region for the ceramic (atomic percent ratio In:Zn=97:3) IZO of 20, 30, 100, 200 and 300 ohms/ square.

The IZO coated PET substrates prepared according to this Example 1 were evaluated for mechanical properties, i.e. adhesion of IZO coating to the PET substrate, abrasion resistance of the surface of the IZO coating, flatness of the IZO coated PET substrate (reported as "curl") and machinability. Adhesion is determined according to ASTM Standard D3330 (Test Method for Peel Adhesion of Pressure Sensitive Tape of 180 degree angle) using No. 250 masking tape (3M Co., St. Paul MN). Surface resistivity is measured before (R₀) and after (R) the peel adhesion treatment of the IZO surface. All of the IZO coated PET substrates of Example 1 showed IZO coating adhesion with R₀/R = 1.

Abrasion resistance determined according to ASTM Standard D618 (Method for Conditioning Plastics and Electrical Insulating Materials for Testing) and D1003 (Test Method for Haze and Luminous Transmittance of Transparent Plastics) using "Kaydry" general purpose cleaning paper (Kimberly-Clark, Neenah, WI) to abrade the surface. Surface resistivity is measured before (R₀) and after (R) the abrasion treatment of the IZO surface. All of the IZO coated PET substrates of Example 1 showed abrasion resistance with R₀/R = 1.

Flatness is measured by laying a flat square (25cm x 25 cm) sample of the IZO coated PET substrate on a flat substrate convex side down. The average displacement of all four corners from the flat surface is reported as mm of "curl" . All of the IZO coated PET substrates of Example 1 showed flatness with a curl value of less than 5 mm.

Machinability is measured by observing for cracks in the IZO coating by back lit microscope after drawing the coated substrate over a 3 mm (1/8 inch) diameter mandrel. All of the IZO coated PET substrates of Example 1 showed no cracks indicating excellent machinability.

The IZO coated PET Substrates of Example 1 were tested for environmental resistance by measuring surface resistivity after (a) exposure to ambient (room temperature/humidity) conditions for 24 hours, (b) exposure to high temperature, i.e. , 80°C in an oven for 24 hours and (c) exposure to humidity, i.e. 95 % relative humidity at 65 °C for 24 hours. The surface resistivity for the before and after environmental exposure is reported in Table 5 for each of the nominal resistivity rated IZO coated PET substrates.

Table 5

Surface Resistivity of IZO Coated PET Substrates Using Metallized IZO 97:3 (atomic percent ratio In:Zn=97:3)

Table 5 shows good stable IZO (atomic percent ratio In:Zn=97.3) for the 24 hour environmental test.

Example 2:

This example illustrates the deposition of an IZO coating on one side of a PET web by sputtering from a sintered ceramic indium oxide/zinc oxide target of an atomic percent ratio In:Zn = 97:3. As noted above this atomic percent ratio is a ratio of atomic percents; i.e. , the ratio of the atomic percent of indium to the atomic percent of Zn wherein the percentage of In and Zn atoms is based upon the total number of In and Zn atoms. The method and apparatus described in Example 1 was used except that the atmosphere was increased to 2.0m Torr with an argon/oxygen ratio increased to 70:6; and, the plasma power was doubled. Sputtering parameters for three passes are set out below in Tables 6-8. Table 6

First Pass: Glow Discharge Plasma Cleaning Parameters

Table 7

Second Pass: IZO Deposition Parameters

Table 8

Third Pass: IZO Deposition Parameters

The IZO coated PET substrates prepared according to the Example 2 were evaluated for spectral optical properties, mechanical properties and environmental resistance as described in Example 1. The Spectral optical properties are reported in Tables 9 and 10.

Table 9

Spectral Optical Properties of IZO Coated PET Substrates

All of the IZO coated PET substrates prepared in Example 2 showed IZO coating adhesion with R₀/R = 1, abrasion resistance with R₀/R = 1 , flatness with a curl value of less than 5 mm and excellent machinability flex.

The IZO coated PET substrates prepared in Example 2 were also tested for environmental resistance with results reported in Table 10.

Table 10

Surface Resistivity of IZO Coated PET Substrates

Table 10 shows good stable IZO (atomic percent ratio In:Zn=97:3) for the 24 hour environmental test.

Comparative Example 3

This example illustrates properties of (a) indium oxide, (b) indium tin oxide (weight percent ratio In:Sn=90: 10) (c) IZO (atomic percent ratio In:Zn=90: 10) and IZO (atomic percent ratio In:Zn=97:3) of the invention on the PET substrate. For comparison purposes, InO_x, ITO (90: 10), IZO (90: 10) and IZO (97:3) films of various sheet resistances were prepared using the sputter coater of figure 2 wherein the coating was deposited onto the substrate using DC magnetron sputtering. This comparison demonstrates the superior mechanical properties of IZO (97:3) compared to InO_x, ITO (90: 10) and IZO (90: 10).

Comparison of 20 ohms/sq. Transparent Conductors

20 ohms/sq. sheet resistance was achieved on PET only by using IZO (90: 10) ceramic and IZO (97:3) metallic. ITO (90: 10) and InO_x peeled at this low sheet resistance on polyester film (PET). To achieve the same ohms/sq. , IZO (90: 10) coating was thicker than IZO (97:3). See Figure 13.

Table 11

It can be seen from the above Table 11 that IZO (97:3) 20 ohms/sq. has low stress and more flexibility compared to IZO ceramic (90: 10) 20 ohms/sq. Ceramic which shows that IZO (97:3) is much more durable than IZO (90: 10). To achieve the same ohms/sq. IZO (90: 10) coating was thicker than IZO (97:3). See Figure 13. Comparison of 30 ohms/sq. Transparent Conductors

It can be seen from Table 12 and Figure 12 that IZO metallic shows best flexibility results and is thinnest to achieve same ohms/sq. compared with IZO (90: 10) and ITO (90: 10).

Table 12

30 ohms/sq. IZO (97:3) has lower stress and more flexibility compared to IZO (90: 10) and ITO (90: 10). Optical spectrum of 30 ohms/sq. shows that IZO (97:3) is thinner than all 30 ohms/sq. IZO (90: 10), and ITO (90: 10).

Comparison of 100 ohms/sq. Transparent Conductors

It can be seen from the Table 13 and Figure 15 that 100 ohms/sq. IZO (97:3) shows good durability results, slightly better than IZO (90: 10), but much superior than ITO (90: 10) and InO_x. Table 13

Optical spectrum of 100 ohms/sq. transparent conductors shows that 100 ohms/sq. IZO (97:3) and IZO (90: 10) thickness are close while ITO (90: 10) and InO_x are thicker. The flexibility and durability of 100 ohms/sq. IZO (97:3) is good due to thinner thickness achieved at the same resistance.

Comparison of 200 ohms/sq. Transparent Conductors

Table 14

200 ohms/sq. Transparent Conductors

It can be from Table 14 that mechamcal properties durability and flexibility of all 200 ohms/sq. sheet resistance are good except InO_x. Figure 16, shows that IZO (97:3) is the thinnest when compared with the rest of 200 ohms/sq. sheet resistance transparent conductors.

Comparison of 300 ohms/sq. Transparent Conductors

Table 15

300 ohms/sq. Transparent Conductors

Table 15, shows all 300 ohms/sq. sheet resistance transparent conductors with the exception of InO_x shows good durability and flexibility. Figure 17 shows IZO (97:3) is superior to all other 300 ohms/sq. transparent conductors as it is thinner than other Transparent Conductors to achieve the same 300 ohms/sq.

The reported spectral optical, mechanical and environmental properties for transparent, conductive metal oxide coatings show the advantages of IZO coatings of this invention. In general the IZO coatings having the In:Zn atomic percent ratio of 97:3 showed excellent optical quality with a low yellow index, e.g. compared to ITO coatings. Mechanical properties were much superior to those of ITO coated flexible substrates.

While the present invention has been described in terms of certain preferred embodiments, one skilled in the art will readily appreciate that various modifications, changes, omissions and substitutions may be made without departing from the spirit thereof. It is intended, therefore, that the present invention be limited solely by the scope of the following claims.

Claims

What is claimed is:

1. A transparent electrically conductive flexible composite which comprises a transparent flexible polymeric sheet with a transparent electrically conductive coating of indium zinc oxide thereon; wherein the zinc content in said indium zinc oxide coating is in the range of 0.01 to 9.99 atomic percent relative to the total amount of indium and zinc in said oxide; with the proviso that said indium zinc oxide coating has a sheet resistance in the range of 3-1000 ohms per square.

2. The composite of claim 1 wherein said sheet resistance is in the range of 3-50 ohms per square.

3. The composite of claim 2 wherein said sheet resistance is in the range of 10-50 ohms per square.

4. The composite of claim 1 which further includes a primer layer interposed between said indium zinc oxide coating and said polymeric sheet.

5. The composite of claim 4 wherein said primer layer is carbon, or a metal selected from the group consisting of Ti, Cr, Ni, Zr, Hf, Ta, W and Al, or said primer layer is an oxide or nitride of said metal or a nitride of said carbon, or said primer layer is a mixture of any of said metals, oxides or nitrides.

6. The composite of claim 4 wherein said primer layer is selected from the group consisting of metal, metal oxide, metal nitride and mixtures thereof and said primer layer has a thickness in the range of 0.5 to 100 nm.

7. The composite of claim 1 wherein said indium zinc oxide coating has a sheet resistance of 3-30 ohms per square; said composite has a light transmission at 550 nanometers of at least 75 % and said indium zinc oxide coating is crack free after rolling over a 3mm diameter mandrel.

8. The composite of claim 2 wherein said indium zinc oxide coating contains about 97 atomic percent indium and about 3 atomic percent Zn wherein the atomic percent of In and Zn is based on the total number of In and Zn atoms in said indium zinc oxide coating whereby the ratio of the atomic percent indium to the atomic percent zinc is about 97:3.

9. A transparent electrically conductive flexible composite which comprises a transparent flexible polymeric sheet having a transparent electrically conductive coating thereon; said electrically conductive coating consisting of transparent electrically conductive indium zinc oxide and said composite optionally includes a primer layer between said transparent electrically conductive coating and said transparent flexible polymeric sheet.

10. The composite of claim 9 wherein said indium zinc oxide of said coating has a sheet resistance which is less than 50 ohms per square.

11. The composite of claim 9 wherein said composite further includes a primer layer interposed between said indium zinc oxide of said coating and said polymeric sheet.

12. The composite of claim 11 wherein said primer layer is carbon, or a metal selected from the group consisting of Ti, Cr, Ni, Zr, Hf, Ta, W and Al, or said primer layer is an oxide or nitride of said metal or a nitride of said carbon, or said primer layer is a mixture of any of said metals, oxides or nitrides.

13. The composite of claim 11 wherein said primer layer is selected from the group consisting of metal, metal oxide, metal nitride and mixtures thereof and said primer layer has a thickness of 0.5 to 50 nm.

14. The composite of claim 9 wherein said indium zinc oxide of said coating has a sheet resistance which is less than 30 ohms per square, said composite has a light transmission at 550 nanometers of at least 75 % and said indium zinc oxide coating is crack free after rolling over a 3mm diameter mandril.

15. The composite of claim 9 wherein said indium zinc oxide of said coating contains about 97 atomic percent indium and about 3 atomic percent Zn wherein the atomic percent of In and Zn is based on the total number of In and Zn atoms in said indium zinc oxide whereby the ratio of the atomic percent indium to the atomic percent zinc is about 97:3.

16. The composite of claim 1 wherein said indium zinc oxide has an atomic ratio of Zn expressed as Zn/(In+Zn) which is equal to or less than 0.03.

17. The composite of claim 9 wherein said indium zinc oxide has an atomic ratio of Zn expressed as Zn/(In+Zn) which is equal to or less than 0.03.

18. A method for depositing a transparent conductive film of indium zinc oxide which comprises sputtering from an indium-zinc metal alloy wherein the atomic percent zinc in said alloy is 0.01-9.99% zinc based upon the total number of zinc and indium atoms in said alloy.

19. A method for depositing a transparent conductive film of indium zinc oxide which comprises sputtering from a target of ceramic indium oxide and zinc oxide wherein the content of zinc in said ceramic target is 0.01 to 9.99 atomic percent zinc based upon the total number of zinc and indium atoms in said ceramic target.

20. A transparent electrically conductive flexible composite which comprises a transparent flexible polymeric sheet with a transparent electrically conductive coating of indium zinc oxide thereon; wherein the zinc content in said indium zinc oxide coating is in the range of 0.01 to 9.99 atomic percent relative to the amount of indium and zinc in said oxide; with the proviso that said indium zinc oxide coating is doped with a metal dopant; said metal dopant being present in the range of 0.01 to 10 atomic percent based upon the total number of zinc atoms, indium atoms, and dopant atoms in said indium zinc oxide coating.

21. The composite of claim 20 wherein said dopant metal is selected from the group consisting of Al, B, Sn and Sb.

22. The composite of claim 20 which further includes a primer layer interposed between said doped indium zinc oxide coating.

23. The composite of claim 22 wherein said primer layer is carbon, a metal selected from the group consisting of Ti, Cr, Ni, Zr, Hf, Ta, W and Al or said primer layer is an oxide of said metal, or nitride of said metal or of said carbon, or said primer layer is a mixture of any of said metals, oxides or nitrides.

24. The composite of claim 23 wherein said primer layer has a thickness in the range of 0.5 to 100 nm.

25. A target for use in a sputter coating process; said target being either an indium zinc alloy target or a zinc oxide-indium oxide ceramic target; wherein said target optionally includes a metal dopant therein; and wherein said target has a zinc content which is in the range of 0.01 to 9.99 atomic percent relative to the total amount of indium and zinc in said target; with the proviso that when said dopant is present, the amount of dopant is in the range of 0.1 to 10 atomic percent based upon the total number of zinc atoms, indium atoms and dopant atoms in said target.

26. The target of claim 25 wherein said alloy target consists of said indium and zinc and said ceramic target consists of indium oxide and zinc oxide.

27. The target of claim 25 wherein said alloy target consists of said indium, zinc and said dopant and said ceramic target consists of indium oxide, zinc oxide and said dopant.

28. The target of claim 25 which includes said dopant as a component thereof.

29. The target of claim 25 which contains about 97 atomic percent indium and about 3 atomic percent zinc wherein the atomic percent of indium and zinc is based upon the total number of indium and zinc atoms in said target whereby the ratio of the atomic percent indium to the atomic percent zinc is about 97:3.

30. The target of claim 25 which has an atomic ratio of Zn expressed as Zn (In+Zn) which is equal to or less than 0.03.

31. The target of claim 27 wherein said dopant is selected from the group consisting of Al, B, Sn and Sb.

32. The target of claim 28 wherein said dopant is selected from the group consisting of Al, B, Sn and Sb.