GB2357281A - Fabrication of ceramic films - Google Patents

Abstract

In a method for fabricating thick ceramic films for both electrical and structural applications using a composite suspension of ceramic powder in an organo-metallic solution the suspension is deposited onto a substrate in one or more layers and one, or more layers of organo-metallic solution is optionally deposited on top of some or all of the layers of composite suspension to form a film. After each layer, the film is dried and fired. When the required thickness is achieved the film is sintered. A sintering aid is employed. The 'green' density of the film may also be increased prior to sintering by the use of uniaxial or isostatic pressure to reduce porosity. The production of lead zirconium titanate (PZT) films is described. Zirconium propoxide and titanium propoxide are used as the organo-metallic compounds and manganese acetate, antimony ethoxide and niobium ethoxide as dopants, with PbO and Cu<SB>2</SB>O as sintering aids.

Description

2357281 FABRICATION OF CERAMIC FILMS This invention relates to the

fabrication of ceramic thick films for both electrical and structural applications. More specifically, the present invention concerns the fabrication of high density ferroelectric thick films. By the term 'thick film' used herein is meant a film having a thickness of at least 2 microns.

This invention has particular, but not exclusive application, to the deposition of thick films of lead zirconate titanate (PbZr,-,,Ti.,0,), commonly known as PZT, on selected substrates.

During recent years the fabrication of ceramic films for both electrical and structural purposes has attracted an increasing amount of interest. In terms of electrical applications, encompassing dielectric, piezoelectric and pyroelectric devices, these presently rely on bulk ceramic technology.

This often requires high temperatures (> 120WC) for processing and ultimately the ceramic must be cut, lapped and polished down to the required thickness. This is both time consuming and costly for largescale manufacture. The high temperatures needed may also result in an incompatibility with the end device itself. For this reason the active layer must be fully processed and only then attached to the device.

Film technology offers many advantages over utilising ceramics. It is well recognised that much lower temperatures are required in the fabrication process, allowing the active layer to be deposited directly onto device components. Layers of the correct thickness and surface quality can also be fabricated such that further processing is not required.

2 The lead zirconate titanate (PbZr,-,Ti,,O,) family of ferroelectrics is already widely used in a number of electronic applications due to their exceptionally high piezoelectric and pyroelectric responses. The end applications are varied and depend in part on the region of the phase diagram used. However, the most common commercial devices include stress sensors, actuators and 'uncooled' infra-red imaging equipment.

In recent Years there has been increasing research activity in the fabrication of films, rather than ceramics, due to the growing demand for 'micro -electromechanical systems' (MEMS), and the need to reduce processing temperatures. Incorporating film, rather than ceramic technology can offer fully 'integrated' device manufacture with the potential to increase manufacturing speed and decrease costs.

Currently film synthesis methods for exploitation in the ferroelectric commercial market have limited application specifically in terms of the film thickness achievable. Other limitations are tendency for poor homogeneity, low deposition rates, high porosity, high sintering temperatures and the necessary inclusion of secondary inactive phases e.g.

glasses in the use of screen-printing which deteriorate electrical properties.

One of the favoured techniques is the sol-gel process. Organo-metallic precursors of the ceramic required are dissolved in solvents and the resulting solution spun down onto flat substrates. Usually, several layers are applied on top of each other to build up a required film thickness.

After each layer, an intermediate firing is carried out to remove organic material and form a stable crystalline layer on which subsequent layers can be deposited. In this way, a final film thickness of up to 2 microns can be achieved without cracking.

3 This procedure produces highly dense films with no second phase, high homogeneity for low-level dopants and requires only low-temperatures.

However, this procedure suffers from two drawbacks, namely low deposition rates and film thickness limitations.

The present invention is intended to remedy the drawbacks aforementioned.

According to one aspect of the present invention, this is broadly achieved by the formation of a stable composite suspension of ceramic powder within an organo-metallic solution for applying a film to a substrate in which the film is sintered in the presence of a sintering aid.

The addition of the ceramic powder to the organo-metallic solution is essentially an adaptation of the sol-gel process. The suspended ceramic powder acts as nucleation centres for promoting formation of the required film structure during the firing procedure. This maintains relatively low activation energies and thus keeps processing temperatures low assisted by the presence of the sintering aid while film surface quality is maintained as the organo-metallic solution fills voids between grains.

Varying the amount of powder in the suspension alters the electrical properties of the film and, by reducing the viscosity, the powder loading can be increased producing increased electrical properties in the film.

Manufacturing costs may also be reduced with increased powder loading as organo-metallic precursors are highly expensive in comparison to ceramic powder.

4 The viscosity of the composite suspension can be controlled in different ways but is preferably achieved by varying the pH. As a result, a different zeta-potential is developed on the surface of the suspended ceramic particles causing a change in the electrostatic forces between the particles, hence controlling the viscosity.

Organo-metallic solutions are often sensitive to moisture. The ceramic powder is therefore preferably dried to remove moisture, for example in a vacuum oven, prior to mixing with the organo-metallic solution under an inert atmosphere, for example by ball milling under nitrogen.

Advantageously, a dispersant is added to the composite suspension to assist in maintaining a stable suspension, especially for high ceramic powder loadings. A preferred dispersant is KR55 (Ken-React Lica 38, isopropanol isooctyl alcohol) which has been found highly successful in producing dispersed powder suspensions with high powder loading that remain stable for several weeks. It will be appreciated, however, that other suitable dispersants may be employed.

Preferably, the composition of the organo-metallic solution is closely matched to that of the ceramic powder. In this way, homogeneity both of the composite suspension and the film obtained therefrorn is promoted.

As a result, the electrical properties of the film are enhanced.

This is especially advantageous where the ceramic powder includes dopants such as manganese, antimony, and niobium for providing the film with enhanced electrical properties and may be achieved by the addition of precursors in the form of acetates and ethoxides.

The adhesion of the film to the substrate depends on various factors including thermal expansion mismatch between substrate and film, substrate surface roughness and organo-metallic solution-substrate chemical interaction. The adhesion will also depend on the degree of shrinkage which takes place in the sintering process.

In the case of PZT, the metal KOVARO (an alloy of iron with nickel, cobalt and manganese) has been found to have acceptable adhesion and can be fired without significant deterioration of electrical properties due to the formation of interfacial oxide layers. However, the method can be extended to substrates of other metals or materials such as mu-metal, platinum/titanium coated silicon or platinised alumina provided the interaction with the respective ceramic is considered.

The preferred technique for applying the composite suspension to the substrate is spin-coating but it will be understood that other deposition techniques may be used such as dip-coating and spray deposition.

When using the preferred technique of spin-coating, one side of the substrate is coated with a layer of the composite suspension and spun at a frequency typically in the range 1,000 to 4,000 rpm, depending on the viscosity of the composite suspension. The layer is then heated to remove organic vapour and form a stable polycrystalline structure.

The heating may be carried out on a hot-plate, preferably in two stages, a first drying stage in the temperature range 100-300C, preferably around 200C, and then a firing stage in the temperature range 300-900'C, with a preferred temperature of around 450C. It should be understood that the preliminary drying stage is optional but preferred.

6 By this procedure a film at least 2 microns thick can be obtained with a stable polycrystalline micro -structure. It will be understood that this process can be repeated. Thus, one or more subsequent layers can be deposited of the composite suspension to build up a film having a desired overall thickness, for example thick films can be generated having a thickness up to 200 microns.

In a preferred modification, at least one layer of an organo-metallic solution (i.e. without suspended powder) may be deposited on top of some or all of the layers of the composite suspension forming the film. For example, three layers of the organo-metallic solution may be deposited on some or all of the layers of the composite suspension. The or each layer of the organo-metallic solution is preferably deposited using the same organo-metallic solution used to form the composite suspension.

Preferably, the resulting film is dried on a hot-plate in the temperature range 100-300'C (preferably around 2000 and fired on a hot-plate in the temperature range 300-900'C (preferably around 450'C) before the next layer of the composite suspension is deposited. Again the preliminary drying stage is optional but preferred.

The or each layer of the organo-metallic solution increases the density of the layers and improves the surface finish which is found to result in enhanced electrical properties of the resultant film. By a procedure in which 3 pure organo-metallic solution layers are deposited on one composite layer, a film of at least 2.5 microns thick may be obtained with enhanced properties.

The sintering aid may be provided in the composite suspension and/or the organo-metallic solution where used for incorporation in the film to assist 7 densification during sintering. Alternatively, the sintering aid may be provided separately and incorporated into the film by depositing on top of the film and/or between one or more layers of the film.

Suitable sintering aids are metal oxides which form a liquid phase during subsequent sintering that reduces porosity in the film. A preferred sintering aid is a mixture of PbO and Cu,O but other materials that could be used include PbO on its own or mixtures of Li,OCO, with either Bi,O, or V'O'.

The final properties of the films depend primarily on the film density.

Although densification may be aided by the incorporation of a liquid phase sintering aid, a further preferred feature of this invention is the application of pressure, for example < 20OMPa, to form a higher green density prior to sintering which helps to reduce porosity in the film. This can be carried out in different ways but preferably by using either uniaxial pressure or cold, isostatic pressure.

In the fabrication of PZT films, uniaxial pressures (< 180MPa) have been demonstrated to enhance green density. Although this is a beneficial procedure, uniform uniaxial pressures are difficult to generate. Coating the film in latex prior to the application of pressure can improve uniformity, however, local maxima can create variations in pre-sintered density and hence variations in final sintered density. This in turn can produce unwanted stresses and subsequent film cracks with some ceramic compositions.

In the use of cold, isostatic pressure, the film and substrate are vacuum sealed in latex prior to pressing. Trials using PZT films have demonstrated that the uniformity of density, indicated by the uniformity of 8 electrical response, can actually increase in comparison to those films that undergo no pressure treatment. (Capacitance measurements measured randomly over a 2.5cm diameter film with 2mm respective electrodes, gave a variation of less than 5%.) Following deposition of the film and optional application of pressure, the film is fired either in air, oxygen, nitrogen or argon at temperatures up to 11000C and more preferably at temperatures between 4000C and 9000C depending on the particular substrate and coating.

The sintering time also varies depending on the amount and type of sintering aid used. The optimum value lies within the region 3 to 60 mins. For films without a liquid-phase sintering aid, optimum electrical properties have been exhibited after sintering hold times as low as 3 mins.

Densification through the use of sintering aids relies on a slower sintering process, i.e. through the use of capillary forces and grain reorientation and hence requires longer hold times.

According to another aspect of the present invention, there is provided a method for producing high density films on a substrate by:- (a) forming a stable composite suspension of ceramic powder in a carrier; (b) depositing a layer of composite suspension from (a) onto a substrate; and (c) sintering the film; characterised by (d) adjusting the viscosity of the composite suspension for adjusting the ceramic powder loading in step (a) by varying the pH, and (e) optionally enhancing the composite layer through deposition of pure carrier.

9 Preferably, the carrier is an organo-metallic solution and more preferably the same organo-metallic solution is used in steps (a) and (e).

Advantageously, one or more layers of carrier is/are dep osited in step (e) with drying and firing the or each layer to form a stable structure. The preliminary drying stage is optional but preferred.

The invention will now be described in more detail, by way of example only, with reference to the preparation of a thick film of PZT.

General Description

In the following examples, thick films of PZT are fabricated by depositing a plurality of layers of a composite suspension on a substrate and optionally depositing at least one layer of an organo-metallic solution on some or all of the layers of the composite suspension.

The composite suspension is produced by preparing and mixing an organo-metallic PZT solution close to the morphotropic phase boundary with PZT ceramic powder and a sintering aid. The composition of the organo-metallic solution is precisely matched to the composition of the PZT ceramic powder by incorporating common dopants through the use of commercially available acetates and ethoxides.

The organo-metallic solution is preferably provided by the same organo metallic solution used to produce the composite suspension.

The layers of the composite suspension and, where provided, layers of the organo-metallic solution are deposited with intermediate drying and firing to remove organic material and to form a stable surface between each layer followed by sintering to produce a thick film of PZT having the desired thickness and correct crystalline structure.

Detailed Description

Step 1 - Preparation of Pb solution The quantities below were refluxed together for 3 hours with a magnetic stirrer to achieve a homogeneous distribution.

52.5grams Lead acetate 30mI Acetic Acid The mixture was then allowed to cool before distillation was carried out to remove any contained water. As the solution is water sensitive, it was then allowed to cool and transferred to a nitrogen chamber.

Step 2 - Preparation of Ti, Zr solution and dopants.

The following quantities were refluxed with a magnetic stirrer under an atmosphere of nitrogen for 3 hours.

26.31grams Zirconium propoxide 17.99grams Titanium isopropoxide 0.43grams Manganese acetate 0.64grams Antimony ethoxide 0.79grams Niobium ethoxide 5OrnI 2ME In this example the dopants, Mn, Sb and Nb, were incorporated through the use of acetates and ethoxides. However, it will be appreciated that the type and levels of dopants can be altered depending on the electrical properties required for the intended application.

Step 3 - Preparation of PZT solution After cooling to room temperature, the solution from step 2 was then mixed with the lead solution from step 1 by a further refluxing procedure of 2 hours and the resulting 2ME/acetic acid solution was then allowed to cool. The solution obtained has a pH close to the isoelectric point resulting in maximum viscosity of the composite suspension that is formed from the organo-metallic solution. In this example, the solution has a pH of approximately 5 but it will be understood the pH may vary for different organo-metallic preparations.

Step 4 - Solution modifications for composite viscosity control For this specific composition, further treatment was carried out to stabilise the PZT solution from step 3 against the precipitation of Pb and Mn. This procedure can also be utilised to control both the pH and the viscosity of the organo-metallic solution. Both these factors can then be used to control the viscosity of the final composite suspension.

After cooling, the organo-metallic PZT solution from step 3 was distilled by increasing the temperature at first slowly to remove alcohol and then finally increasing to a temperature of 104'C, to remove any further traces of water. The combined volume of removed fluid was then replaced by adding 2ME to maintain molar concentration.

As a result, the pH of the solution increased resulting in a reduction in viscosity of the composite suspension that is formed from the organo metallic solution.

In this example the organo-metallic solution was distilled for approximately 2 hours, requiring the addition of 50 mI 2ME to be added 12 producing a pH of approximately 7.5. It will be understood however that by varying the length of the distillation process, the amount of fluid replaced and the resulting changes in pH and viscosity will change.

Hence by adjusting the distillation time, the degree of stability, the final pH and also viscosity can be altered.

After final cool down the solution was filtered with a 1 micron filter and then 5 grams of ethanediol added. Through all stages the solution was handled under a nitrogen atmosphere. Before further use all organo metallic solutions are allowed 24 hours to stabilise.

Step 5 - Suspension of PZT ceramic powder and sintering aid PZT ceramic powder and sintering aid (PbO and Cu20) were dried in a vacuum oven for 12 hours to remove any possible moisture obtained during storage. The below quantities were then mixed with the PZT solution from step 4 and dispersant under a nitrogen atmosphere in a glass Duran cylindrical vessel with milling pellets:

30mI PZT solution 45grn PZT ceramic powder 0Agm KR55 (Ken-React Lica 38, isopropanol isooctyl alcohol) dispersant 1.93gm PbO powder 0.311gm Cu,0 powder In this instance the cylindrical pellets were made from yttriastabilised zirconia. The powders, PZT solution and dispersant were mixed by ball milling for 24 hours. The PbO and Cu,0 were added in the approximate ratio 4A. With this ratio a liquid phase is formed around 680'C.

13 The powder loading may be increased by reducing the viscosity of the PZT solution as previously described to adjust the viscosity of the composite suspension. In this example, the viscosity of the PZT solution is reduced by increasing the pH above the isoelectric point. The same effect may also be obtained by reducing the pH below the isoelectric point, for example by the use of acetic acid.

Step 6 - Deposition onto Substrate (a) layers of composite suspension only Kovar and Pt/Ti coated silicon substrates were degreased and cleaned using high purity IPA. The composite suspension from step 5 was then dropped onto the substrates using a plastic pipette such that a large proportion of the substrate surface was covered. The substrates were then spun at 2,000rpm for 25 seconds.

The resulting layer was initially dried in air using a hot-plate at 200'C for 1 minute to remove organic vapour and then fired in air using a hot-plate at 450'C for 15 seconds to form a stable polycrystalline structure which will accept a subsequent layer.

Further layers of the composite suspension were then deposited using the same procedure. After each successive layer, the film is dried and fired until the desired film thickness is achieved. In this example, each layer after firing increases the film thickness by 2/3microns.

(b) layers of composite suspension and organo-metallic solution Pt/Ti coated silicon substrate was degreased and cleaned using high purity IPA. The composite suspension from step 5 was then dropped onto the substrate using a plastic pipette such that a large proportion of the 14 substrate surface was covered. The substrate was then spun at 2,000rpm for 25 seconds.

The resulting layer was then dried in air using a hot-plate at 200'C to remove organic vapour for I minute and fired in air usi ng a hot-plate at 450'C for 15 seconds to form a stable polycrystalline structure which will accept a subsequent layer.

Next a layer of the organo-metallic solution from step 3 was applied to the composite layer by spinning the substrate at 2,000rpm for 25 seconds.

The resulting layer was initially dried using a hot-plate at 200C for 1 minute to remove organic vapour and then fired using another hot-plate at 450'C to complete stabilisation. Two further layers of organo-metallic solution were then applied in the same manner.

The combination of one layer of the composite suspension and three layers of the pure organo-metallic solution (Le without suspended powder) forms a.set of layers to which further sets of layers were deposited using the same procedure until the desired film thickness is achieved.

After each successive layer is deposited, the film is dried and fired. In this example, each set of layers (one layer of composite suspension and three layers of the organo-metallic solution) after firing increases the film thickness by 2.5 to 3 microns.

Step 7 - Final sintering procedure When the required thickness is achieved, the film is sintered at a higher temperature to complete the phase transformation into the desired phase and reduce final porosity. A sintering aid is employed which produces a liquid phase at relatively low temperatures during sintering to aid densification and which, after film deposition, is homogeneously distributed around the grains of the film reducing porosity in the film.

Optimum sintering conditions for PZT vary depending on the choice of substrate and amount of sintering aid. This difference in firing, combined with the difference in interfacial oxide layers produces significant differences in electrical characteristics. In this example, films deposited on both KOVAR and Pt/Ti coated silicon were fired under an atmosphere of argon at 710'C for 30 mins. In each case a rapid thermal annealer was used with ramp rates of 30'C/min.

Results Physical and Electrical Properties The substrates were coated with films comprising either 7 individually spun layers of the composite suspension or 7 individually spun sets of layers each comprising one layer of the composite suspension and three layers of the organo-metallic solution.

Film cross-sections were mounted in resin such that film thickness could be established through careful optical microscopy. The films deposited on KOVAR and those that included additional organo-metallic solution layers, were observed to be around 18 microns in thickness. Films deposited on Pt/Ti coated silicon without additional organo-metallic solution layers, tended to be slightly thinner. The thicker deposition on KOVAR results from the relative increased roughness of the KOVAR substrate.

In this technique it has been found that the thickness of layers are dependent on composite viscosity and also the roughness of the underlying layer/substrate. Higher roughness increases adhesion of the composite 16 suspension during the spin process. This increase in adhesion in turn increases final film thickness.

For the purpose of electrical characterisation 4 gold-chrome, circular, electrodes (2mm diameter) were deposited by vacuum evaporation on the film surface. In respect of films consisting of layers of the composite suspension only deposited on KOVAR, dielectric constants up to 500 were measured using a GenRad digibridge with a respective dielectric loss of around 2%. Dielectric constants in respect of similar films deposited on Pt/Ti coated silicon were larger, up to 700, with a decreased dielectric loss of around 1%. Dielectric constants in respect of films consisting of the combination of layers of composite suspension and organo-metallic solution deposited on Pt/Ti coated silicon were still larger, up to 900, with only slightly higher dielectric loss than comparable films made up of layers of the composite suspens ion only on Pt/Ti coated silicon substrates.

The reduction in dielectric loss for films deposited on substrates of Pt/Ti coated silicon is expected as this choice of substrate eliminates lossy interfacial oxide layers that form on substrates of KOVAR. However, the flexibility of using a metallic substrate such as KOVAR may have mechanical advantages for certain applications.

The electrical properties of films deposited on Pt/Ti substrates with combined layers of composite suspension and organo-metallic solution are improved compared to layers of composite suspension only as demonstrated by the increase in dielectric constant. This is believed to be due to an increase in density of the layers and improved surface finish obtained with the addition of layers of organo-metallic solution.

17 P-E hysteresis loop measurements were also carried out using an RT66A Radiant Technology Unit. Ferroelectric behaviour of the PZT films could be detected prior to final sintering. This indicated that intermediate firing at temperatures as low as 45CC were sufficient for crystallisation of the correct tetragonal PZT perovskite structure. This was confirmed through X-ray diffraction. However, it should be noted that further firing may be required to reduce porosity and generate films with optimum electrical properties.

Although the invention has been described with particular reference to the fabrication of a thick film of PZT, it will be understood that the invention has application to thick films of other materials including lead titanate, lead zirconate, lead scandium tantalate, lead magnesium niobate, magnetic ferrite, alumina or alumina-silicate either separately or in any combination.

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Claims (50)

1 A method for producing high density films on a substrate by:- (a) mixing an organo-metallic solution with ceramic powder to form a stable composite suspension; (b) depositing a layer of composite suspension from (a) onto a substrate to form a film, and (c) sintering the film in the presence of a sintering aid.

2. A method according to claim 1 wherein step (b) is repeated to deposit additional layers to build up a desired film thickness.

3. A method according to claim 1 or claim 2 further including the step of depositing one or more layers of organo-metallic solution on top of a layer of composite suspension.

4. A method according to claim 3 wherein the organo-metallic solution and the organo-metallic solution of the composite suspension are the same.

5. A method according to claim 3 or claim 4 wherein one or more layers of organo-metallic solution are deposited between successive layers of composite suspension.

6. A method according to any one of claims 3 to 5 wherein three layers of organo-metallic solution are deposited on top of each layer of composite suspension.

7. A method according to any one of the preceding claims wherein the or each layer is fired at a temperature in the range 300-900'C.

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8. A method according to claim 7 wherein the firing temperature is around 450'C

9. A method according to claim 7 or claim 8 wherein the or each layer is dried prior to firing.

10. A method according to claim 9 wherein the or each layer is dried at a temperature in the range 100-300'C.

11. A method according to claim 10 wherein the drying temperature is around 200C.

12. A method according to any one of the preceding claims wherein the sintering aid is combined with the ceramic powder and organo-metallic solution in step (a).

13 A method according to any one of claims I to 11 wherein the sintering aid is combined with organo-metallic solution and deposited on top of the film prior to sintering in step (c).

14. A method according to any one of claims 1 to 11 wherein the sintering aid is combined with organo-metallic solution and deposited between composite layers of the film prior to sintering in step (c).

15. A method for producing high density thick films on a substrate by:(a) mixing an organo-metallic solution with ceramic powder to form a stable composite suspension, (b) depositing a layer of composite suspension from (a) onto a substrate; (c) depositing at least one layer of organo-metallic solution onto the layer of composite suspension from (b); and (d) sintering the layers from (b) and (c) in the presence of a sintering aid to form a high density thick film.

16. A method according to claim 15 including the step of drying and firing each layer to remove organic vapour and form a stable crystalline structure.

17. A method according to claim 15 or claim 16 including repeating steps (b) and (c) to build up a film of desired thickness with one or more layers of organo-metallic solution between successive layers of composite suspension.

18. A method according to any one of claims 15 to 17 wherein the organo-metallic solution in step (c) is the same as that used in step (a).

19. A method according to any one of claims 15 to 18 wherein three layers of organo-metallic solution are deposited in step (c).

20. A method according to any one of claims 15 to 19 wherein the sintering aid is present in the composite suspension used in step (b).

21. A method according to any one of claims 15 to 19 wherein the sintering aid is present in the organo-metallic solution used in step (c).

22. A method according to any one of claims 15 to 21 wherein each layer is fired at a temperature in the range 300-900'C prior to the next layer being deposited.

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23. A method according to claim 22 wherein each layer is dried at a temperature in the range 100-300'C prior to firing.

24. A method according to any one of the preceding claims wherein the sintering aid has a compositionXCU20. (1-x)PbO where 0. 10 < x < 0.30.

25. A method according to claim 24 wherein the sintering aid forms a eutectic having a liquid phase at a temperature below the sintering temperature.

26. A method according to any one of the preceding claims wherein the sintering temperature is up to 11OWC.

27. A method according to claim 26 wherein the sintering temperature is up to 90WC.

28. A method according to any one of the preceding claims including the step of applying pressure prior to sintering.

29. A method according to claim 28 wherein the applied pressure is less than 200 MPa.

30. A method according to claim 28 or claim 29 wherein the applied pressure is uniaxial.

31. A method according to claim 28 or claim 29 wherein the applied pressure is isostatic.

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32. A method according to any one of the preceding claims wherein dopants in the organo-metallic solution are matched with those of the ceramic powder.

33. A method according to claim 32 wherein one or more dopants is selected from the group comprising manganese, antimony and niobium.

34. A method according to any one of the preceding claims wherein the ceramic powder is selected from the group consisting of lead zirconate titanate, lead titanate, lead zirconate, lead scandium tantalate, lead magnesium niobate, magnetic ferrite, alumina or alumina-silicate.

35. A method according to any one of claims 1 to 33 wherein the ceramic powder comprises two or more materials selected from the group consisting of lead zirconate titanate, lead titanate, lead zirconate, lead scandium tantalate, lead magnesium niobate, magnetic ferrite, alumina and alumina-silicate.

36. A method according to any one of the preceding claims wherein the ceramic powder loading is increased by reducing viscosity of the composite suspension.

37. A method according to claim 36 wherein viscosity of the composite suspension is adjustable by altering the pH of the organo-metallic solution.

38. A method according to claim 37 wherein viscosity of the composite suspension is reduced by an increase or decrease in the pH of the organo metallic solution at the isoelectric point.

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39. A method according to any one of the preceding claims wherein the or each layer of composite suspension has a thickness of at least 2 microns.

40. A method according to any one of the preceding claims wherein each layer is deposited by any of the methods of spin-coating, spraying or dip-coating.

41. A method for producing high density thick films on a substrate by:(a) mixing an organo-metallic solution with ceramic powder to form a stable composite suspension; (b) depositing at least one layer of composite suspension from (a) and at least one layer of an organo-metallic solution onto a substrate; and (c) sintering the layers in the presence of a sintering aid to form a high density thick film.

42. A method according to claim 41 wherein said at least one layer of organo-metallic solution is deposited on top of said at least one layer of composite suspension.

43. A method according to claim 42 wherein a plurality of layers of organo-metallic solution are deposited on top of said at least one layer of composite suspension.

44. A method according to claim 43 wherein sets of layers are deposited on top of each other to build up a film of desired thickness, each set of layers comprising a layer of composite suspension and a plurality of layers of organo-metallic solution.

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45. A method according to any one of claims 41 to 44 wherein each layer is stabilised prior to application of the next layer.

46. A method according to claim 45 wherein each layer is stabilised by firing at a temperature in the range 300-900'C.

47. A method according to claim 46 wherein each layer is dried at a temperature in the range 100-300'C prior to firing.

48. A method according to any one of claims 41 to 47 wherein the layers are sintered at a temperature up to HOTC.

49. A method for producing high density films on a substrate substantially as hereinbefore described.

50. A substrate provided with a film according to any one of the preceding claims.