WO1994011546A1

WO1994011546A1 - Preparation of metal or oxide films

Info

Publication number: WO1994011546A1
Application number: PCT/GB1993/002388
Authority: WO
Inventors: Michael Stuart Waite
Original assignee: Rank Brimar Limited
Priority date: 1992-11-19
Filing date: 1993-11-19
Publication date: 1994-05-26
Also published as: GB9224242D0

Abstract

A method is disclosed for forming a film on a substrate by coating at ambient temperatures a substrate with a solution of a polymer containing a metal bonded to the polymer chain through acid groups and heating the substrate to decompose the polymer. The metal is left on the substrate as such or as an oxide or carbonate. Suitable problems include vinyl acids and esters such as acrylates or methacrylates which decompose on heating. The method may be used to make rare earth doped YAG phosphor films or borate phosphor films.

Description

PREPARATION OF METAL OR OXIDE FILMS

FIELD OF THE INVENTION This invention relates to the production of oxide films, carbonate films or metal films, particularly though not exclusives for use as luminescent films. In particular it is concerned with the room temperature or low elevated temperature application of such films to substrates by techniques such as spin- or dip-coating using metal polymer precursors, in which the polymer will be a copolymer in which the units and other properties of the polymer are selected to provide suitable rheaological and viscoelastic properties.

BACKGROUND OF THE INVENTION

In general, simple and compound oxide films can be deposited by the following techniques:

(a) vacuum deposition by evaporation or sputtering; (b) chemical vapour deposition;

(c) spin or dip coating of a precursor material in the form of a viscous resin which can be thermally decomposed to give the oxide film (see, for example, C. D. Veitch, "Synthesis of polycrystalline yttrium iron garnet and yttrium aluminium garnet from organic precursors", J. Mater. Sci, 26, [1991], 6527-6532).

The technique (c) has certain advantages over the other methods. Vacuum or low pressure deposition (a) can lead to differential evaporation rates in the case of mixed oxides and nonstoichiometry through oxygen deficiency.

CVD methods (b) require the use of volatile precursors and the relative concentrations of these must be adjusted for differential decomposition rates in order to give stoichiometric compound oxides. Furthermore the technique of CVD for complex oxides is still developing and problems such as premature homogeneous decomposition in the gas phase can produce defects and blemishes in the film.

The liquid phase organic precursor technique (c) can be used when the substrate is stable at the decomposition temperatures required. The effective reaction temperatures for producing complex oxides by this technique are generally lower than for the equivalent reaction from the oxide powders. The method is applicable to all metals for which suitable organo- metallic compounds exist which are not volatile at the decomposition temperature. The systems which can be used in this technique tend to fall into two groups:

i) The organometallic compound, which can be a metal salt, is dissolved in a suitable solvent for spin or dip coating, with additives for viscosity control if necessary.

ii) the metal is bound by M-0 bonds to a polymer which determines the rheological properties of the solution. The systems which are the subject of this invention fall into this category, the bonding of the metal to the polymer chain being through acid groups.

A process in which metal oxides or hydroxides or other compounds are reacted with a carboxylic acid monomer containing polymerizable vinyl groups, the monomer is polymerized optionally with acrylonitrile, styrene or an ester of methacrylic acid, and the resulting polymer in DMF or DMSO is sprayed onto a substrate which is heated to decompose the polymer is disclosed in DE-A-4118749. The process is used to make superconductor films, films for use in the electrical and electronics industries, refractory and/or corrosion resistant coatings and decorative coloured coatings. However the film forming material is sprayed onto a substrate which is

maintained at 400-600 C so that the droplets of liquid impinging on the substrate dry and decompose immediately and the resulting film is composed of small crystallites and is light scattering.

SUMMARY OF THE INVENTION It is an object of the invention to provide a method for making films that can be stochiometric, of high transparency and good substrate adhesion.

The invention provides a method for forming a film on a substrate which comprises coating a substrate with a solution of a polymer containing a metal bonded to the polymer chain through acid groups at a temperature below that at which the polymer is decomposed, allowing the coated substrate to dry, and heating the substrate to decompose the polymer and leave the metal on the substrate as such or as an oxide or carbonate. The invention also provides a method for forming a film on a substrate which comprises dip- or film-coating the substrate with a solution of a polymer containing a metal bonded to the polymer chain through acid groups, allowing the coated substrate to dry, and heating the substrate to decompose the polymer and leave the metal on the substrate as such or an oxide or carbonate. For this purpose, the polymer will normally be selected to have rheological properties suitable for spin- or dip-coating and to be sufficiently flexible to avoid a film cracking when the solvent evaporates and when the film is heated.

The invention further provides a method for forming a luminescent film on a substrate which comprises coating a substrate with a solution of a polymer containing metals bonded to the polymer chain through acid groups and which decompose to form a luminecsent inorganic film, the coating being carried out at a temperature below that at which the polymer is decomposed, allowing the coated substrate to dry, and heating the substrate to decompose the polymer and leave on the substrate the luminecent inorganic film. DESCRIPTION OF PREFERRED FEATURES The polymer systems considered most suitable for this application are the vinyl acids and esters such as acrylates and methacrylates which depolymerise on heating and thus reduce the tendency to form graphitic residues. They have the additional property of acting as electron beam resists so that the electron beam lithography can be employed in device fabrication.

Films of complex oxides of electropositive elements may be prepared by appropriate mixture of individual metal copolymers. The invention may be used to prepare luminescent rare earth doped YAG films. Complex oxides incorporating acidic oxides as discrete anion units such as rare earth or indium borates can be prepared if the copolymer has a component which aids the dissolution of the acidic oxide by, for example, ester linkage or hydrogen bond formation to hydrogen substituted acrylate based units.

General Features of the Deposition Method

There are four stages in the preparation of a single layer film: STAGE 1: Preparation of individual metal-polymer resins STAGE 2: Mixing of components in ratios required for final stoichiometry where a complex and not a. simple oxide film is required STAGE 3: Spin or dip coating onto chosen substrate STAGE 4: Thermal decomposition of metal-polymer film

STAGE 1 - Preparation of Individual Metal-Polymer Resins The polymers used are preferably co-polymers of acrylic acid (I), methacrylic acid (II) or itaconic acid (III) with methyl methacrylate (IV) or a derivative such as hydroxyethyl acrylate (V) . It was found that the metal acrylates alone, when polymerised, give films which do not adhere well to substrates and which crack on drying. The film-forming resin used must bind to the metal cation, but also incorporate an effective proportion of units imparting viscoelastic behaviour so that the dried film does not develop cracks or peel. The presence of the ester acting as a cross-linking agent or chain extender in the polymer backbone produces films with improved adhesion to typical ceramic or glass substrates. Other inert chain-extending units e.g styrene units may also be present. The films containing an appropriate proportion of copolymer units can be applied to greater thickness before cracking or peeling from the substrate on drying.

(I) H₂C = CH

C0₂H

(II) H,C C-CH.

C0₂H

(IV) CH- = C-CH.

C0₂CH₃

(V) CH₂ = CH

C0₂CH₂-CH₂OH

A preferred method for the preparation of metal-polymer solutions is a two-step process: Step 1

Copolymers are prepared by conventional polymerisation in dimethylformamide solution with azoisobutronitrile initiator. The proportion of acid to ester varies from 1:2 to 2:1 with 1:1 being the most common combination used, The proportion of ester in the reaction mixture is monitored by GLC analysis and the heating is continued until the concentration of the monomeric ester falls below 5% of the starting concentration. The result is a colourless, viscous liquid. It may be analysed for polymer solid content by drying in vacuum and for acid content by titration with alkali.

The copolymers can also be prepared by emulsion polymerisation of the acid and ester in sodium lauryl

sulphate solution at 70 to 90°C with sodium persulphate initiator, followed by separation of the product by filtration and washing in water/propan-2-ol. Copolymers with high content of hydroxyl substituted components are obtained as an aqueous gel. Step 2

A metal-polymer complex is prepared by reaction of the copolymer with the metal hydroxide, carbonate or basic salt in refluxing solvent, usually DMF. The metal compound is added in the calculated stoichiometric quantity depending on the acid content of the polymer and the known or assumed oxidation state of the metal. The solution containing the reaction product is centrifuged or filtered to remove unreacted material. The equivalent metal oxide content is adjusted to >4% by vacuum evaporation of the solvent if necessary. At this concentration the material is usually viscous enough for coating. The equivalent oxide content of the resin is determined by airbake at 500°C.

STAGE 2 - Mixing of Individual Metal-Polymer Resins The individual oxide content of the required complex oxide is determined. Knowing the equivalent oxide content of each individual resin they are blended to give the required proportion of individual oxides.

STAGE 3 - Spin or Dip Coating Onto Chosen Substrates

The resin solution is applied to the substrate at a temperature below that at which the resin decomposes, usually below 140 C, preferably below 100°C and most preferably at ambient temperatures. The resin solution may be dip coated onto substrates e.g using a dip method, by flowing down the internal surface of tubing or by spin coating. The viscosity for each method of application is adjusted by variation of solvent concentration so that the resultant film has a thickness of ca 4μm. The coatings are dried at 120°C for half an hour prior to baking. A wide variety of heat resistant inorganic substrates may be used, e.g. transparent substrates such as glass or silica, or alumina or sapphire substrates.

STAGE 4 - Decomposition Of The Film

The important parameters in the decomposition are the atmosphere during firing, the temperature programming rate and the ultimate temperature. In most cases natural air firing is suitable. The initial stage of decomposition of most of the metal-polymer resins is the decomposition of the acrylate based units, starting at about 150°C and complete by 400°C. Air firing or oxygen enrichment is desirable to avoid residual carbon. The temperature programming rate over the initial decomposition stage is important. The rate of temperature rise should preferably be in the region 2 to 5°C/min. If the rate is too high there is possibility of nonstoichiometry in the resultant oxide, residual carbon in the film and in the case of complex oxides the formation of metastable phases. If it is too low, the processing time is unnecessarily long, although rates of

temperature rise below 2 C/min may sometimes be used though not preferred.

The ultimate temperature is determined by the nature of the substrate and the oxide film. The minimum ultimate temperature for formation of an oxide phase is 400°C, for transition metal oxide films for example. For complex oxides such a yttrium aluminium garnet the crystalline oxide phase is established at 900°C but crystallinity is improved by further annealing up to 1400°C (on sapphire substrates) .

The resulting films typically have thickness about 0.3μm per coating or layer. The invention will be further illustrated by reference to the following Examples.

EXAMPLE 1 STAGE 1 - Preparation of Acrylic Acid-Methyl Methacrylate

Copolymer

Acrylic acid (72g), methyl, methacrylate (lOOg) and dimethylformamide (lOOg) are refluxed with 0.5g AIBN until GLC analysis of the MMA/DMF ratio indicates that the MMA content is <5% of starting value. The residual MMA and excess solvent are removed by rotary evaporation. The solvent content is adjusted to a known value in the range 15 to 30 weight percent as determined by drying samples at 100°C. The acid content is determined by titration in alcohol solution with sodium hydroxide.

STAGE 2 - Preparation of Nickel-Polymer Solution Nickel carbonate is heated with the copolymer in stoichiometric quantity based on the acid content of the polymer until the carbonate has dissolved. The solution is centrifuged or filtered to remove residual powder. The solvent content is adjusted to give a Ni content of 2.5-4%. The preferred solvents are DMF or acetylacetone. DMF solutions are brown ,solutions containing acetyl¬ acetone are green.

EXAMPLE 2 STAGE 1 - Preparation of Acrylic Acid-Methyl Methacrylate

Copolymer

Acrylic acid and methyl methacrylate are copolymerised as in Example 1.

STAGE 2 - Preparation of Reactive In/Sn Hydroxides

To a solution of indium nitrate (30g) in water (100ml) is added stannous octanoate (1.6g) in propan-2-ol (20ml). Ammonia is added dropwise until precipitation is complete and the precipitate collected and dried in vacuum at room temperature.

STAGE 3 - Preparation of Metal-Polymer Solution The dried mixed metal hydroxides are refluxed with the calculated quantity of copolymer, filtered and the solvent (DMF) proportion adjusted to give an effective metal oxide content of ca 4%. STAGE 4 - Preparation of Oxide Film

The resin solution is applied to the substrate by spin coating or flow methods as appropriate and decomposed by heating to > 400°C for a minimum of 1 hour in air or oxygen to give an indium tin oxide film.

EXAMPLE 3

STAGE 1 - Preparation of Individual Metal-Polymer Solutions. The individual hydroxides of yttrium, aluminium and terbium are prepared as dried amorphous powders by ammoniacal hydrolysis of the corresponding nitrates. The hydroxides are reacted with acid-methyl methacrylate copolymer prepared as in Example 1 above. For additional substrate adhesion a proportion of the methyl meth¬ acrylate may be replaced by 2-hydroxyethyl methacrylate. The equivalent oxide content of the individual metal- polymer solutions is determined by pyrolysis. The individual metal-polymer solutions are hence combined in the proportions appropriate to give a Tb:Y molar ratio of 1:30 and a Y:A1 molar ratio of 3:5. STAGE 2 - Preparation of Oxide Films

The resin solution is applied to the substrate by spin coating or flow methods as appropriate. The heating rate for successful decomposition to give a transparent YAG layer must be controlled at <4°C/min to temperatures of

>900°C. The resulting layer forms a YAG:Tb luminescent film.

EXAMPLE 4

Preparation of InBO., : Eu Film

Acrylic acid, methyl methacrylate and 2-hydroxy ethyl acetate, in molar ratio 0.5 : 0.25 : 0.25, are dissolved

in dimethylformamide and heated to 100°C with azoisobutyronitrile initiator. The gelatinous product is dissolved in further DMF and heated with indium hydroxide, terbium hydroxide and boric acid in molar proportions 1 : 0.03 : 1.1 (slight boric acid excess). The solution can be spin coated onto refractory

substrates and decomposed at 950 C to give a film with charcteristic narrow line orange emission at 590nm. APPLICATIONS

1. Nickel oxide high resistivity wall coating on a narrow bore tubing

The Ni containing polymer can be used to deposit a high resistivity (lOMΩ.cm) film on the internal surface of a

CRT neck in the region of high field strength in order to reduce flashover. The coating is applied by drawing the resin up the tube by suction and allowing to drain back.

After decomposition a near transparent nickel oxide layer is left with good adhesion to the substrate and an absence of particulate clumps which could act as field emitting spots.

2. Indium tin oxide containing films The internal surface of a miniature CRT was provided with a wall electrode by flow coating with the indium tin containing resin and decomposition of the polymer at 420°C.

3. Luminscent films

Green emitting cathodoluminescent YAG:Tb films, annealed on sapphire substrates at 1400°C, can be prepared by spin coating of the mixed metal polymer resin with subsequent decomposition, (see Fig 1). Multiple layers can be built up to increase thickness. With other activators e.g Eu, Nd other emission bands are available. The other activators are incorporated as the Tb by preparation of the corresponding polymer. Fig 2 illustrates the cathodoluminescent spectrum of a Y203:Eu film prepared by decomposition at 900°C of a layer spin-coated onto sapphire from a methyl methacrylate-itaconic acid copolymer containing Y and Eu.

In both cases the film spectra are identical to that of the corresponding powder spectra prepared by conventional synthesis from the oxides.

One possible field of use of a luminescent film is as the phosphor of a microtips fluorescent display (R Meyer et al, Japan Display, 88, 512) which is a flat panel display based on an array of field emission cathodes. Normal phosphor screens are based on powder layers and therefore could generate particles which would destroy the cathodes. Hence the interest in a simply produced thin film luminescent screen. Another possible application is for a high luminance light source achieved by electron beam or other high energy excitation. The layers could be deposited on substrates of complex geometry and would be well heat sinked to avoid excessive temperatures. The layers could be transparent or granular to assist with light extraction (light trapping due to total internal reflection is well known in thin transparent films resulting in only 10-20% of the generated light being emitted). Possible fields of use include both displays and illumination systems.

Simply produced films avoiding the need for expensive vacuum deposition equipment etc means that the system has a broad commercial range of applications.

Other potential applications are:

a) Preparation of high temperature superconducting ceramic films e.g YBaCO.

b) There is evidence that Ni/Co spinels form from the appropriate precursor mix at 400°C suggesting that ferromagnetic spinel thin films can be produced by this method.

c) The application of insulating layers to the surface of those advanced semiconductors e.g GaAs for which it is not easy to form a native oxide layer by oxidation.

d) The most electropositive elements such as Gpl (alkali metals) and GpIIA (alkaline earth metals) form carbonates on pyrolysis, at least initially, and raises the possibility that miniature thermionic emitters could be fabricated - with possible application to "vacuum fluorescent" devices.

e) Surface acoustic wave layers e.g based on ZnO.

f) Gold, platinum and palladium are believed to decompose to the zerovalent state. This raises the possibility of a method for the formation of "nanoclusters" of such atoms.

Claims

CLAIMS:

1. A method for forming a film on a substrate which comprises coating a substrate with a solution of a polymer containing a metal bonded to the polymer chain through acid groups, the coating being carried out at a temperature below that at which the polymer is decomposed, allowing the coated substrate to dry, and heating the substrate to decompose the polymer and leave the metal on the substrate as such or as an oxide or carbonate.

2. A method according to claim 1, wherein the coating is carried out at ambient temperatures.

3. A method according to claim 2, wherein the coating is carried out by dip- or spin-coating.

4. A method according to any preceding claim, wherein the polymer contains free hydroxyl groups which can bond by ester or hydrogen bond formation to acidic oxides or hydroxides.

5. A method according to any preceding claim, wherein the polymer to which the metal is bonded contains units in which acid groups are absent.

6. A method according to any preceding claim, wherein the polymer comprises copolymerised carboxylic acid group containing units and units which do not contain carboxylic acid groups.

7. A method according to claim 6, wherein the polymer comprises units derived from acrylic acid, methacrylic acid or itaconic acid.

8. A method according to claim 6 or 7 , wherein the polymer comprises units derived from an ester or hydroxyester of a vinyl acid.

9. A method according to claim 9, wherein the proportion of acid to ester groups is 1:2 to 2:1.

10. A method according to any preceding claim, wherein the coated substrate is heated at a rate of 2-5°C/min to a temperature above 400°C at which the polymer becomes decomposed.

11 A method according to claim 10, wherein the heating is carried out in air or in an oxygen rich environment.

12. A method according to any preceding claim, when used to make a luminiscent film.

13. A method according to claim 12, when used to make yttrium aluminium garnet film doped with a rare earth.

14. A method according to claim 12, when used to make a rare earth-indium borate phosphor or other borate phosphor.

15. A method according to any preceding claim when used to make a transparent film.

16. A method according to claim 15, wherein the film is on a transparent substrate.

17. A method according to claim 16, wherein the substrate is of a glass or silica.

18. A method according to any of claims 1 to 11, wherein the film produced is of any of the following types:

(a) a high resistivity nickel oxide coating;

(b) a conductive indium tin oxide film; (c) a superconducting ceramic film;

(d) a ferromagnetic spinel thin film;

(e) an oxide layer on a gallium arsenide substrate;

(f) a carbonate of a Group I or Group II metal for use as a thermionic emitter; (g) a surface acoustic wave layer; (h) a film of a noble metal.

19. A method according to claim 19, when used to make a wall coating of a CRT.

20. A method according to any of claims 12 to 17, when used to make a phosphor of a microtips fluorescent display.

21. A method for forming a film on a substrate substantially as described in any Example.

22. A method for forming a film on a substrate which comprises coating a substrate with a solution of a polymer containing a metal bonded to it at a temperature below the decomposition temperature of the polymer and heating the substrate to decompose the polymer and leave the metal on the substrate as such or as an oxide or carbonate or other inorganic compound thereof.

23. A method for forming a luminescent film on a substrate which comprises coating a substrate with a solution of a polymer containing metals bonded to the polymer chain through acid groups and which decompose to form a luminecsent inorganic film, the coating being carried out at a temperature below that at which the polymer is decomposed, allowing the coated substrate to dry, and heating the substrate to decompose the polymer and leave on the substrate the luminecent inorganic film.

24. The method as claimed in claim 1, when used to form a transparent film.