WO2017013399A1

WO2017013399A1 - Metal oxide deposition

Info

Publication number: WO2017013399A1
Application number: PCT/GB2016/052113
Authority: WO
Inventors: David Stanton; Michael Hill; Andrew Johnson; Kieran Molloy; Amanda CATHERALL; Anna COLLEY
Original assignee: Pilkington Group Limited; University Of Bath
Priority date: 2015-07-17
Filing date: 2016-07-13
Publication date: 2017-01-26
Also published as: GB201512625D0; GB201802231D0; GB2557761A

Abstract

A process for depositing a transparent coating comprising a metal oxide on a substrate, the process comprising, providing a soda lime silica glass substrate having a surface, providing a precursor mixture comprising a source of a metal and a source of an oxidising anion,at least partially atomizing the precursor mixture, and delivering the precursor mixture to the surface of the substrate, wherein the surface of the substrate is at a substrate temperature sufficient for deposition of the metal oxide coating on the surface. The preferred metal oxides are tin, copper, iron, zinc, indium or antimony and the preferred oxidising anion is nitrate or nitrite.

Description

Metal Oxide Deposition

The present invention relates to processes for depositing metal oxides on substrates and to substrates having such metal oxide coatings on at least one surface.

Coatings on substrate surfaces find uses in many fields. Some of the more useful coatings are metal oxides, for example, tin oxide. Some metal oxides (including doped tin oxide) can form transparent conductive oxide (TCO) coatings.

Methods used to deposit metal oxide coatings include physical vapour deposition methods such as sputtering or liquid based methods such as sol-gel using spin coating or dip coating techniques among others.

US-A-2013/0217180 discloses a method of forming a metal oxide film for use in a thin film transistor device involving depositing the film using a liquid precursor composition containing a fuel and one or more oxidising agents either of which is a metal salt, and subsequently annealing the thin film at 350 °C or lower.

US-A-4,571,350 relates to deposition of metal oxide films using an atomised mist of a metal chloride.

Marks et al in Nature Materials 10 (2011) pages 382 to 388 disclose combustion processing techniques as a route to solution growth of electronic metal oxide films (ln₂0₃, a-ZnSnO, a-InZnO, indium tin oxide) at temperatures as low as 200 °C to produce optically transparent transistors on flexible plastic substrates.

US-A-2010/0184253 discloses the manufacture of a thin film transistor in which a metal oxide semiconductor is formed from a liquid precursor.

US-A-2008/0286807 discloses a method of making a zinc oxide semiconducting layer for a thin film transistor using solution processing at low temperature. US-B-6 495 709 discloses a precursor for an aluminium oxide film comprising a liquid solution of an aluminium organic precursor in an organic solvent.

One particularly useful method for deposition of coatings is chemical vapour deposition (CVD) wherein a fluid precursor in the form of a vapour is delivered to the surface of the substrate where the precursors react and/or decompose thereby depositing a coating. Particular types of CVD include metal organic (MO) CVD, combustion (C) CVD, plasma enhanced (PE) CVD and aerosol-assisted (AA) CVD.

US-B-6,235,343 discloses CVD deposition of doped tin oxide using a alkyl tin chloride precursor.

WO-A-91/ 03323 discloses deposition of a coating of superconducting ceramic oxide using plasma enhanced CVD.

US-A-5, 034,372 discloses deposition of superconducting ceramics on stainless steel or quartz using plasma enhancement.

WO-A-2013/085557 discloses deposition of ionically conductive oxides using CVD.

Gladfelter et al in Chemical Vapor Deposition (1998) 4(1) pages 9 to 11 disclose low temperature CVD of crystalline titanium dioxide films using tetranitratotitanium (IV).

WO-A-2012/143707 discloses nucleating a surface with aerosol assisted CVD tin oxide before CVD deposition of a further layer. The preferred tin precursor is monobutyl tin chloride.

GB-A-1 357 274 discloses aerosol deposition of indium tin oxide using indium chloride and tin chloride.

An important aspect of successful CVD processes is the choice of precursor. In the case of metal oxide coatings, especially tin oxide coatings, a number of metal precursors have been investigated. Unfortunately, many metal precursors (for example, Sn(CH3)4) or other components necessarily used with particular metal precursors (for example, flammable solvents), are highly reactive, toxic or expensive. There is, therefore, a need for alternative systems for deposition of metal oxides.

It is an aim of the present invention to address the problems with existing processes.

In a first aspect, the present invention accordingly provides, a process for depositing a transparent coating comprising a metal oxide on a substrate, the process comprising, providing a soda lime silica substrate having a surface, providing a precursor mixture comprising a source of a metal and a source of an oxidising anion, at least partially atomizing the precursor mixture, and delivering the precursor mixture to the surface of the substrate, wherein the surface of the substrate is at a substrate temperature sufficient for deposition of the metal oxide coating on the surface.

This is advantageous because at least partially atomized precursor mixtures allow the use of solutions of precursors (comprising a source of a metal and a source of an oxidising anion) without the precursors needing to be volatile under the conditions of precursor transport and delivery. Thus, a wider range of sources of metals and sources of oxidising anions may be used, allowing improved tuning of the properties of the metal oxide coating. The precursor mixture may be at least partially atomized using, for example, ultrasonic atomizers, preferably piezo ultrasonic atomizers.

The transparent coating has a visible light transmission preferably of 10% or greater, more preferably 15% or greater, most preferably 20% or greater. Usually, the transparent coating will have a visible light transmission 50% or greater, 55% or greater, or 60% or greater.

Preferably, the metal oxide comprises a transparent conductive metal oxide (e.g. tin oxide, zinc oxide and/or indium tin oxide).

The process is preferably aerosol assisted CVD.

The use of aerosol assisted CVD in combination with the use of a source of metal and source of oxidising anion is also advantageous because the sources of metal and sources of oxidising anions may be soluble or very soluble in a number of solvents, including for example water and/or lower alcohols. Thus, the use of AA CVD in combination with a source of the metal and source of oxidising anion may enable consistent and efficient deposition over a wide range of temperatures without the need to apply relatively high temperatures to the precursor mixture in order to volatilise the source of metal and source of oxidising anion, or the need to maintain the precursor mixture at relatively high

temperatures in order to ensure sufficient and consistent carry-over of the source of metal and source of oxidising anion to the zone of deposition of the metal oxide. Avoiding the need for relatively high temperatures is advantageous because it reduces the chance of premature reaction of the source of metal and source of oxidising anion or other components of the precursor mixture.

Thus, surprisingly, the process according to the invention in combination with AA CVD may, in a number of embodiments, enable the use of benign solvents such as water (or lower alcohols) which has significant environmental and safety benefits. Industrial coating processes that may use water as a solvent are greatly advantageous. Water solvent also has the advantage that it does not contribute to carbon contamination of the metal oxide coating.

Preferably, the precursor mixture further comprises a solvent. The more preferred solvents comprise an oxygen-containing solvent. The most preferred solvent comprises water. Alternatively, or additionally, the solvent may comprise a Ci to C4 alcohol such as methanol, ethanol, propanol or butanol. The most preferred alcohol solvent comprises ethanol.

The process of the invention may be suitable for a number of metals. Thus, the metal may be selected from one or more of Sn, Cu, Fe, Zn, Sb and In. The most preferred metal is tin. Other more preferred metals are Zn and Fe.

It is advantageous if the source of the metal comprises an inorganic salt of the metal.

It is advantageous if the source of the oxidising anion is an inorganic salt of the anion or an inorganic acid of the anion. The source of metal and the source of the oxidising anion may comprise the same salt. Alternatively, the source of metal and source of oxidising anion may be different salts, or a salt and an acid, added to the precursor mixture.

The metal oxide may be a doped metal oxide. This is advantageous because it may provide advantageous properties (including, for example, improved conductivity). The doped metal oxide may be doped with one or more dopants selected from (for example) F, Sb Al, Ga, and In, preferably F. If the metal oxide is, for example tin oxide, then advantageously the doped tin oxide comprises fluorine doped tin oxide (which is more electrically conductive than the undoped oxide and may form low e coatings). If the doped metal oxide is zinc oxide, then advantageously the doped zinc oxide comprises aluminium (or gallium) doped zinc oxide.

To form a doped metal oxide the precursor mixture preferably further comprises a source of a dopant. The source of the dopant may be (in the case of fluorine) a fluorine compound, for example R4 F, wherein R is H or Ci to C4 alkyl (e.g. t-butyl, propyl, ethyl, methyl). The preferred fluorine compound is ammonium fluoride. In the case of metal dopants, the source of the dopant may be a source of a second metal (e.g. for Al, aluminium nitrate).

One of the advantages of the present invention is that, in addition to metal oxides having one metal, the invention also enables the deposition of mixed metal oxides, for example binary metal oxides or ternary metal oxides. Examples of binary metal oxides than may conveniently be deposited according to the invention include Sn/Zn oxide, indium tin oxide or ZnO: Al.

Binary metal oxides may be formed of similar components as doped metal oxides having a metal species as dopant. In general, doped metal oxides doped with a second metal species generally have relatively low amounts of the second metal whereas binary metal oxides generally have higher amounts of the second metal, the distinction between metal oxide considered doped and binary (or ternary, with a third metal species) is a matter of degree. Thus, preferably the precursor mixture further comprises a source of a second metal. An example of a binary metal oxide is Sn/Zn oxide in which the preferred source of a second metal is zinc nitrate. For ternary metal oxides, the precursor mixture preferably further comprises a source of a third metal.

The source of an oxidising anion may be a source of an oxoanion (which may have oxidising properties, or in an alternative aspect of the invention, may not). In a particularly preferred embodiment, the oxoanion may be a nitrogen oxoanion.

Preferably, the oxidising anion (or oxoanion) is selected from the group consisting of nitrates, nitrites, perchlorates, chlorates, hypochlorites, peroxides, superoxides, persulfates and acids thereof.

The most preferred anion is nitrate or nitrite because the decomposition products of sources of nitrate or nitrite are usually volatile and so do not contribute to contamination of the metal oxide coating.

The substrate temperature sufficient for deposition of the metal oxide coating on the surface will depend on the choice of source of metal and source of oxidising anion. However, generally, the substrate temperature will be in the range 300 °C to 800 °C, preferably 350 °C to 750 °C, more preferably 360 °C to 720 °C, and most preferably 410 °C to 700 °C.

Substrate temperatures below this range may result in unreacted or poorly reacted precursors. Substrate temperatures above this range may be too high for some substrates (for example glass, the surface of which may become unsuitable for deposition). A sufficiently high temperature is advantageous because it may provide for improved properties of the metal oxide coating. Such improved properties include improved crystallinity or structure, and/or improved optical properties (e.g. improved transmission, controlled haze) and/or electrical properties (especially sheet conductivity where e.g. the metal oxide is a transparent conductive oxide). Furthermore, a sufficiently high temperature is advantageous because it reduces the chance of incomplete reaction and also may provide a relatively high growth rate of coatings. Relatively high growth rate is advantageous because it enables coatings of metal oxide of sufficient thickness to be deposited in a fixed period. This may be particularly advantageous wherein the substrate and precursor delivery apparatus are moving relative to one another (e.g. for deposition, on-line, during the float glass production process).

The preferred lower temperature is 350 °C or 360 °C because above this temperature the properties of many metal oxides improve. For example, the inventors have discovered that tin oxide deposited by a process according to the invention becomes electrically conductive at or above these temperatures and the crystal structure become more defined.

Usually, the precursor mixture further comprises a carrier gas, preferably argon and/or nitrogen and/or air (if an oxidising carrier gas is advantageous or required).

Preferably, the metal oxide is deposited to a thickness in the range 60 nm to 300 nm, preferably 100 nm to 250 nm.

The substrate comprises glass. The surface of the glass substrate may comprise a layer (e.g. comprising silicon oxide, silicon oxycarbide or silicon oxynitride) and the metal oxide may be deposited on the layer. Such a layer is advantageous because the layer may form a sodium blocking layer from e.g. the glass substrate and/or to modify e.g. the optical properties of the coating.

Additional layers may be situated under the layer or the metal oxide coating or may be deposited on the metal oxide coating.

The substrate may comprise a continuous ribbon of glass. Conveniently, the metal oxide coating may be deposited during the float glass production process.

As discussed above, the present invention enables deposition of metal oxide coatings on a substrate surface.

Thus, the present invention provides, in a second aspect, a process for depositing a coating comprising a metal oxide on a glass substrate, the process comprising, providing a glass substrate having a surface, providing a precursor mixture comprising a source of nitrate and a source of a metal (preferably a metal salt) wherein the metal is selected from the group consisting of tin, copper, iron, zinc, indium and antimony, and delivering the precursor mixture to the surface of the substrate, wherein the surface of the substrate is at a substrate temperature sufficient for deposition of the metal oxide coating on the surface.

The preferred source of metal is a source of tin and the preferred source of oxidising anion is a source of nitrate, enabling deposition of coatings comprising tin oxide.

In a third aspect, the present invention provides a process for depositing a coating comprising tin oxide on a glass substrate, the process comprising, providing a glass substrate having a surface, providing a precursor mixture comprising a tin salt and a source of nitrate, and delivering the precursor mixture to the surface of the substrate, wherein the surface of the substrate is at a substrate temperature sufficient for deposition of the tin oxide coating on the surface.

Surprisingly, the inventors have discovered that the tin salt may comprise a tin (II) salt. The tin salt may preferably comprise a tin halide, more preferably tin chloride and most preferably tin (II) chloride.

It is preferred that the source of nitrate is an inorganic nitrate. Preferably, the inorganic nitrate comprises ammonium nitrate or, alternatively, an alkali metal nitrate (e.g. sodium nitrate, or potassium nitrate).

In the embodiment wherein the tin salt comprises a tin (II) salt it is preferred if the source of nitrate is ammonium nitrate because such a mixture of precursors provides excellent coatings of tin oxide.

As discussed in relation to the first aspect, a great advantage of the invention as defined in the second or third aspect is that the precursor mixture may be an aqueous precursor mixture. The use of water as a solvent is beneficial environmentally and for safety.

Generally, for the reasons discussed in relation to the first aspect, it is preferred if the process of the second or third aspect further comprises a step of at least partially atomizing the precursor mixture before delivering the precursor mixture to the surface of the substrate. Thus, it is preferred if the process according to the second or third aspect is aerosol assisted CVD generally for the reasons discussed above.

Substrates having a coating of metal oxide deposited according to the invention find uses in many areas, for example as low emissivity glazings (where the substrate is glass), and in electrode, photovoltaic cells or other uses of e.g. electrically conducting transparent coatings e.g. wherein the metal oxide is tin oxide, especially doped tin oxide.

The present invention will now be described by way of example only, and with reference to, the accompanying drawings, in which: Figure 1 shows X-ray diffraction patterns for Examples 10 to 16, indicating substrate temperature of deposition.

Figure 2 shows scanning electron micrographs of a Sn0₂ coating deposited at 550 °C from 0.3 M solution (Example 14) a) 90° tilt; b) 80° tilt.

The invention is further illustrated, but not limited, by the following Examples.

Examples and Comparative Example

The synthetic procedure for producing the uniform and highly crystalline fluorine-doped, and undoped tin oxide coatings of the Examples is described below, as well as for producing mixed Sn/Zn oxide coatings (Example 25) and iron oxide coatings (Example 26). The procedure for the Examples involves synthesis of tin (II) nitrate and its reaction on the heated substrate to produce films of tin (IV) oxide. The same reaction, with the addition of ammonium fluoride and subsequent reaction on the heated substrate yields films of F-doped tin (IV) oxide. Since tin (II) nitrate is thermodynamically unstable, and therefore not isolatable, the synthesis requires the in situ generation of Sn(N0₃)₂ species via the salt metathesis of SnCl₂ and ¾Ν0₃.

The substrates were float glass (with the exception of Example 9a which used a Si wafer) having a visible light transmission above 70% with a coating of silicon oxy carbide that had undergone cleaning with isopropanol and oxygen plasma treatment. The tin oxide coatings were deposited on the silicon oxycarbide coated surface.

To produce a 0.1M solution of the Sn(N0₃)2 precursor, SnCl₂ (2.26 g, 0.01 mol) and H4N0₃ (1.60 g, 0.02 mol) were added to a conical flask and dissolved in 100 ml distilled water. The solution was stirred for one hour to fully dissolve the salts. The clear solution was then transferred into a stainless steel bubbler which was sealed shut.

Other concentrations of precursor solution were produced by varying the amounts of the reactants. For a precursor solution containing fluorine, H4F was added at varying concentrations, and a few drops of FIN0₃ to fully dissolve the solution. The concentration of the precursor solutions used to deposit coatings according to the Examples and Comparative Example are as indicated in Table 5.

Example 25 used a precursor solution additionally containing equimolar Zn(N0₃)2.6H₂0 and resulted in deposition of a mixed Sn/Zn oxide coating having approximately equal amounts of Sn and Zn. To produce a 0.5M solution of the Zn(N0₃)₂.6H₂0 / Sn(N0₃)₂ precursor, SnCl₂ (3.79 g, 0.02 mol), H₄N0₃ (3.2 g, 0.04 mol) and Zn(N0₃)₂.6H₂0 (5.95g, 0.02 mol) were added to a conical flask and dissolved in 40 ml distilled water. A few drops of FIN0₃ were added and the solution was stirred for one hour to fully dissolve the salts. The clear solution was then transferred into a stainless steel bubbler which was sealed shut.

Example 26 was an iron oxide coating deposited from a solution of iron nitrate using a method generally as indicated for Examples 1 to 25 discussed herein. Comparative Example A and Examples 1 to 9 used ethanol (EtOH) as solvent instead of water (as indicated in Table 5).

The bubbler containing precursor was degassed for 5 minutes with flowing nitrogen to clear the head space of any oxygen and then attached to an aerosol generator. Deposition of the tin (IV) oxide coating layers by Aerosol Assisted Chemical Vapour Deposition (AACVD) was accomplished using a quartz, hot- walled tubular reactor with gas flow for the deposition of the precursor onto the substrate. The reactor fitted horizontal substrates up to 30 ^χ 220 mm.

A Colli son type atomizer was used to generate an aerosol for AACVD of small droplet diameter. The precursor solution was delivered as an aerosol which was created using a TSI 3076 constant output Colli son type atomizer, using compressed air as the carrier gas (air pressure 20 psi (138 kPa)). This type of aerosol generator produces a constant droplet concentration of around 10⁷ particles cm^"3 and a mean droplet diameter of 0.3 μπι. The precursor solution flowing through the aerosol generator to the reactor is at a flow rate of 8 ml / hour.

The aerosol precursor was passed into the reactor and directed on to the surface of the heated substrate for 30 minutes or 60 minutes. The heated substrates were at the temperature indicated in Table 5, and the time of deposition for each sample is also as indicated in Table 5.

After deposition, the samples were left to cool to room temperature under flowing nitrogen before being removed, examined and analysed by X-ray diffraction, EDS and scanning electron microscopy.

The appearance of the coated substrates was as indicated in Table 5.

X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Discover

X-ray diffractometer using monochromatic Cu Και and Cu Ka₂ radiation of wavelengths 0.154056 and 0.154439 nm respectively, emitted with a voltage of 40 kV and a current of 40mA in an intensity ratio of 2: 1. The results of XRD are shown in Figure 1 for the Examples 10 to 16 as a function of substrate temperature.

Scanning electron microscopy (SEM) was performed using a Philips XL30

FEG operating in plan and cross section mode at instrument magnification x 50,000 and tilt angle of 90° and 80°.

The Examples all exhibited good uniformity and coverage of the substrate. Comparative Example A used tin (II) chloride as precursor without ammonium nitrate. Without the addition of H4NO3, no uniform deposition occurs. Thus the sample is unsuitable for further analysis. Some powdery deposits appear at the end of the reactor which wipe off easily.

X-ray diffraction

X-ray diffraction (XRD) patterns of the films of Examples 10 to 16 were recorded and are shown in Figure 1 as a function of substrate temperature at which the Examples were deposited (for which see Table 5) together with assigned Miller indices for the reflections. The structure of the coatings was assigned to tetragonal SnC with space group

The Miller indices assigned to the coatings of Examples 10 to 16 are as indicated in Table 1, below. Miller indices (indexed to tin oxide or zinc oxide) for Example 25 are also indicated in Table 1. The iron oxide coating of Example 26 was very poorly crystalline or amorphous, but with broad small reflections that were indexed to maghemite (Fe₂03, y-FeiC ; cubic unit cell, a = 8.35 A).

Table 1. Scanning electron microscopy

Scanning electron microscopy (SEM) was used to determine surface morphology and height profiles of the deposited film of Example 14. Figure 2a and 2b are images at x 50,000 magnification, Figure 2a at 90° tilt, Figure 2b at 80°. The thickness of film as determined by SEM was about 200 nm.

Energy-dispersive X-ray spectroscopy (EDS)

To aid detection of fluorine, the EDS data for sample Example 9a was collected from a duplicate film grown on a Si substrate. The elemental composition as determined by EDS is shown in Table 2.

Table 2

X-Ray Photo-Electron Spectroscopy (XPS)

Example 17 to 25 were analysed using XPS depth profile analysis. Example 25 contained a mixed tin and zinc oxide coating on silica coated glass (Sn:Zn atomic ratio being approximately 1). This sample also contained a small amount of chlorine contamination close to the air interface. Examples 17 to 24 were confirmed as tin oxide on silica coated glass. Results for XPS depth profile analysis as given in Table 3, below. Average concentration (atomic %) throughout the coating depth

Example Sn 0 F Zn

17 33.6 61.9 0.3 -

18 35.3 62.7 0.4 -

19 35.8 61.7 1.7 -

20 33.4 62.5 0.4 -

21 34.2 62.7 0.2 -

22 35.5 62.3 1.3 -

23 35.2 62.6 - -

24 35.2 62.3 0.7 -

25 22.1 55.3 - 17.0

0.6 atomic % chlorine average for this coating

Table 3

Electrical properties

The sheet resistance of Examples 1 to 9 were determined using a surface resistivity meter with a 4-point probe (Guardian Model SRM 232). Measurements were taken at the same thickness for each sample, and the mean of three measurements was taken. The values are as indicated in Table 4, below.

Example Mean Sheet resistance

Ω / square

(rounded)

El Too high to measure

E2 518

E3 313

E4 410

E5 724

E6 106

E7 41

E8 47

E9 41

Table 4

Example (E) or

Comparative

Example (CE) Ammonium Substrate

Nitrate NH₄F Concentration Temperature Deposition

added added Solvent (mol dm^"3) (°C) Time (min) Comments

El Yes No EtOH 0.1 300 30 Transparent, insulating

E2 Yes No EtOH 0.1 350 30 Transparent, Conducting

E3 Yes No EtOH 0.1 400 30 Transparent, Conducting

E4 Yes No EtOH 0.1 450 30 Transparent, Conducting

E5 Yes No EtOH 0.1 500 30 Transparent, Conducting

E6 Yes Yes EtOH 0.1 350 30 Transparent, Conducting

E7 Transparent, v.

Yes Yes EtOH 0.1 400 30 conductive

E8 Transparent, v.

Yes Yes EtOH 0.1 450 30 conductive

E9 Transparent, v.

Yes Yes EtOH 0.1 500 30 conductive

E9a Transparent, v.

conductive, deposited on

Yes Yes EtOH 0.1 500 30 Si wafer

E10 Yes No water 0.3 350 60 Transparent, insulating

El l Yes No water 0.3 400 60 Transparent, insulating

E12 Yes No water 0.3 450 60 Transparent, insulating

E13 Yes No water 0.3 500 60 Transparent, insulating

E14 Yes No water 0.3 550 60 Transparent, insulating

E15 Yes No water 0.3 600 60 Transparent, insulating

E16

Yes No water 0.3 650 60 Transparent, insulating

E17 1 : 1 ratio Sn:NH4F

Yes Yes water 0.5 450 60 Transparent, Conducting

E18 1 : 1 ratio Sn:NH4F

Yes Yes water 0.5 400 60 Transparent, Conducting

E19 1 : 1 ratio Sn:NH4F

Yes Yes water 0.5 350 60 Transparent, Conducting

E20 2: 1 ratio Sn:NH4F

Yes Yes water 0.5 400 60 Transparent, Conducting

E21 2: 1 ratio Sn:NH4F

Yes Yes water 0.5 450 60 Transparent, Conducting

E22 2: 1 ratio Sn:NH4F

Yes Yes water 0.5 350 60 Transparent, Conducting

E23 4: 1 ratio Sn:NH4F

Yes Yes water 0.5 450 60 Transparent, Conducting

E24 4: 1 ratio Sn:NH4F

Yes Yes water 0.5 400 60 Transparent, Conducting

E25 Zn nitrate added.

Yes No water 0.5 400 60 Transparent, Insulator

CE A No deposition on

No No EtOH 0.1 500 30 substrate.

Table 5

Claims

1. A process for depositing a transparent coating comprising a metal oxide on a substrate, the process comprising,

providing a soda lime silica glass substrate having a surface,

providing a precursor mixture comprising a source of a metal and a source of an oxidising anion,

at least partially atomizing the precursor mixture, and

delivering the precursor mixture to the surface of the substrate, wherein the surface of the substrate is at a substrate temperature sufficient for deposition of the metal oxide coating on the surface.

2. A process as claimed in claim 1, wherein the process is aerosol assisted chemical vapour deposition.

3. A process as claimed in either claim 1 or claim 2, wherein the metal is selected from one or more of Sn, Cu, Fe, Zn, Sb and In.

4. A process as claimed in any one of the preceding claims wherein the metal oxide is a doped metal oxide.

5. A process as claimed in claim 4, wherein the metal oxide is doped with one or more dopants selected from F, Sb Al, Ga, and In.

6. A process as claimed in any one of the preceding claims, wherein the source of the metal comprises an inorganic salt of the metal.

7. A process as claimed in claim 6, wherein the source of the oxidising anion is an inorganic salt of the anion or an inorganic acid of the anion.

8 A process as claimed in any one of the preceding claims wherein the oxidising anion is an oxoanion.

9. A process as claimed in claim 8, wherein the oxoanion is a nitrogen oxoanion

10. A process as claimed in claim 9, wherein the anion is nitrate or nitrite.

11. A process as claimed in any one of the preceding claims, wherein the oxidising anion is selected from the group consisting of nitrates, nitrites, perchlorates, chlorates, hypochlorites, peroxides, superoxides, persulfates and acids thereof.

12. A process as claimed in any one of the preceding claims, wherein the substrate temperature is in the range 300 °C to 800 °C, preferably 350 °C to 750 °C, more preferably 360 °C to 720 °C, most preferably 410 °C to 700 °C.

13. A process as claimed in any one of the preceding claims, wherein the precursor mixture further comprises a carrier gas, preferably argon and/or nitrogen.

14. A process as claimed in any one of the preceding claims, wherein the precursor mixture further comprises a solvent.

15. A process as claimed in claim 14, wherein the solvent comprises an oxygen-containing solvent.

16. A process as claimed in claim 14 or claim 15, wherein the solvent comprises water.

17. A process as claimed in either claim 15 or claim 16, wherein the solvent comprises a Ci to C4 alcohol.

18. A process as claimed in claim 17, wherein the solvent comprises ethanol.

19. A process as claimed in any one of the preceding claims, wherein the metal oxide is deposited to a thickness in the range 60 nm to 300 nm, preferably 100 nm to 250 nm.

20. A process as claimed in any one of the preceding claims, wherein the surface of the substrate comprises a layer comprising silicon oxide and the metal oxide is deposited on the layer comprising silicon oxide.

21. A process as claimed in any one of the preceding claims, wherein the substrate comprises a continuous ribbon of glass.

22. A process as claimed in claim 21, wherein the substrate comprises a continuous ribbon of glass undergoing the float glass production process.

23. A process for depositing a coating comprising a metal oxide on a glass substrate, the process comprising,

providing a glass substrate having a surface, providing a precursor mixture comprising a source of nitrate and a source of a metal, optionally a metal salt, wherein the metal is selected from the group consisting of tin, copper, iron, zinc, indium and antimony, and

24. A process for depositing a coating comprising tin oxide on a glass substrate, the process comprising,

providing a glass substrate having a surface,

providing a precursor mixture comprising a tin salt and a source of nitrate, and

delivering the precursor mixture to the surface of the substrate, wherein the surface of the substrate is at a substrate temperature sufficient for deposition of the tin oxide coating on the surface.

25. A process as claimed in claim 24, wherein the tin salt comprises a tin (II) salt.

26. A process as claimed in either claim 24 or claim 25, wherein the tin salt comprises a tin halide, preferably tin chloride and more preferably tin (II) chloride.

27. A process as claimed in any one of claims 23 to 26, wherein the source of nitrate is an inorganic nitrate.

28. A process as claimed in claim 27, wherein the inorganic nitrate comprises ammonium nitrate or an alkali metal nitrate.

29. A process as claimed in any one of claims 23 to 28, wherein the precursor mixture is an aqueous precursor mixture.

30. A process as claimed in any one of claims 23 to 29, further comprising a step of at least partially atomizing the precursor mixture before delivering the precursor mixture to the surface of the substrate.

31. A process as claimed in any one of the preceding claims, further comprising a source of a second metal, thereby depositing a binary metal oxide coating.

32. A process as claimed in claim 31, wherein the source of a second metal comprises a source of zinc.

33. A substrate having a coating comprising a metal oxide, obtainable by the process of any one of claims 1 to 32.

34. A glass substrate having a coating comprising tin oxide, obtainable by the process of any one of claims 23 to 32.