EP1969155A1 - Method for producing a ceramic oxide based coating conformable with the geometry of a substrate having raised designs - Google Patents

Abstract

The invention concerns a method for making conformable ceramic oxide layers on substrates having raised designs including: a step of depositing on said substrate a layer of sol-gel solution precursor of said ceramics; a step of heat treatment of said layer to transform same into ceramics; said steps being optionally repeated one or more times. The invention is characterized in that the sol-gel solution precursor of said ceramics is prepared by a method including the following successive steps: a) preparing a first solution by contacting one or more molecular precursors of metals designed to be constituents of the ceramics in a medium comprising a diol solvent and optionally an aliphatic monoalcohol; b) allowing the solution obtained in a) to rest during sufficient time required to obtain a solution having a substantially constant viscosity; c) diluting at a predetermined proportion the solution obtained in b) with a diol solvent optionally identical to the one in step a) or a solvent miscible with the diol solvent used in step a).

Description

 PROCESS FOR MAKING A COATING BASED ON A

OXIDE CERAMIC CONFORMING TO THE GEOMETRY OF A SUBSTRATE

PRESENTING REASONS FOR RELIEF

DESCRIPTION

TECHNICAL AREA

The subject of the present invention is a process for producing a coating based on an oxide ceramic conforming to the geometry of a substrate having raised patterns, in particular micron-sized patterns.

The general technical field of the invention can therefore be defined as that of ceramic coatings for subtrate.

The coatings have, for example, the function of modifying the properties of a substrate, such as the mechanical properties, the thermal properties, the electrical properties and the chemical properties, the optical properties.

Substrate coatings therefore find application in many fields such as microelectronics, optics or energy. Thus, in the field of microelectronics, the trend is to go to systems of increasingly smaller size, involving the implementation of relief structures to increase in particular the active surface of these structures. The realization of such structures requires knowing to coat them with thin ceramic layers, generally having dielectric properties. In the field of energy, in particular that of fuel cells, the trend is to go towards portable systems, whose size is a few millimeters and including components whose critical dimensions are of the order of 0 , 1 to 10 μm. Their manufacture requires the production of coatings on substrates of complex geometry, such as LiCoO ₂ coatings acting as cathodes. Finally, in the field of optics, particularly in photonic systems, the trend is also towards miniaturization, in particular of diffraction gratings. These diffraction gratings are generally in the photonic systems, in the form of tunnels having on their surface micron-sized patterns. The production of ceramic coatings on these systems generally contributes to providing better resistance to these optical systems. Whether in the field of microelectronics, energy and optics, ceramic coatings must have a uniform thickness on the substrates on which they are deposited, so as to ensure uniformity of properties. brought by these coatings.

The oxide coating deposition processes can be classified into two categories: namely, dry processes and wet processes. In the dry mode, there is mainly a difference in the literature for substrates with micron units, chemical vapor deposition (known by the abbreviation CVD for "Chemical Vapor Deposition"), physical vapor deposition (known by the abbreviation PVD for "Physical Vapor Deposition") of which a variant specific is ion implantation.

Chemical vapor deposition is a method in which the volatile compounds of the material to be deposited are transformed into reactive species, such as radicals generated by microwaves, plasma torches, etc., thus forming a vapor phase, which reacts with the heated substrate to give a coating. The volatile compounds of the material to be deposited are optionally diluted in a carrier gas, such as dihydrogen. This method has a certain number of advantages among which one can cite a good selectivity of the deposits, a good adaptability in the chains of production. However, this method has the following drawbacks: the coatings obtained are not very dense;

they are often contaminated by very reactive gases resulting from the chemical reaction (hydrogen, halogens);

they have poor adhesion to the substrate; the coatings have poor sharpness at the edges of the relief patterns.

A more advantageous method may be to achieve physical vapor deposition coatings, such as evaporation, spraying and ablation. For example, evaporation consists of simply to evaporate or sublimate the material to be deposited in a vacuum crucible by heating it at high temperature. The evaporated material is deposited by condensation on the substrate to be coated and a layer is formed on the substrate. Although this method makes it possible to obtain denser layers, this method proves to be difficult to implement, because of the apparatus to be used, and of high cost, and does not ensure uniform layer thickness on substrates. with raised patterns.

In summary, whether by CVD or PVD, the coatings on substrates having micron-size patterns obtained by these techniques do not have a uniform thickness over the entire deposition length and, in particular, have excess thicknesses at the edges. patterns in relief. This can cause, when the substrates thus coated are intended to be used as electronic components, capacity variations as well as risks of breakdown at the edges of the raised patterns.

Some authors have used the sol-gel process to form deposition solutions for substrates with raised patterns (Journal of the European Ceramic Society, 199, 18 (3), p.255-260

[1] Journal of Materials Research, 2003, 18 (5); p.1259-1265 [2]). However, it is in these documents to make moldings (that is to say the negative impression) of the substrate and in no case to achieve a coating of uniform thickness conforming to the shape of the substrate. There is thus a real need for a method of producing a coating conforming to the geometry of a substrate having patterns in relief, which does not have the disadvantages encountered in the prior art with dry techniques.

STATEMENT OF THE INVENTION

The object of the invention is achieved by a method of producing an oxide ceramic coating conforming to the geometry of a substrate having relief patterns comprising:

a step of depositing on said substrate a sol-gel solution layer precursor of said ceramic; a step of heat treatment of said layer in order to transform it into said ceramic; said steps being optionally repeated one or more times, characterized in that the precursor sol-gel solution of said ceramic is prepared by a process comprising successively the following steps: a) preparing a first solution by contacting the molecular precursor (s) with metals and / or metalloids intended to enter the constitution of the ceramic with a medium comprising a solvent comprising at least two -OH functions and optionally an aliphatic monoalcohol; (b) put at rest the solution obtained in (a) for a sufficient time necessary to obtain a solution having a substantially constant viscosity; c) diluting at a predetermined rate the solution obtained in b) with a solvent identical to that of step a) or a solvent miscible with the solvent used in step a) but different from this.

According to the invention, the term "miscible solvent" means a solvent which can be mixed with the solvent comprising at least two -OH functions and, where appropriate, with the aliphatic monolalcohol, forming a homogeneous mixture, and this in all proportions at room temperature, that is to say at a temperature of the surrounding atmosphere, generally between 20 and 25 ^° C.

The method of the invention, implementing sol-gel technology to form the deposition solution, has the following advantages:

it makes it possible to produce coatings on complex surfaces of various sizes and without the need for heavy equipment;

it makes it possible to obtain homogeneous deposits in composition;

since the mixture of species takes place on a molecular scale, complex oxides comprising, for example, three or more elements and controlling stoichiometry can be easily produced by this process.

In addition, the method of the invention advantageously makes it possible to obtain coatings which comply with the geometry of the substrate, that is to say coatings having a substantially uniform thickness over the entire deposition length, in particular thanks to the stability properties of the sol-gel solution obtained prior to deposition.

According to the invention, the oxide ceramics constituting the coating may be chosen from oxides with a perovskite structure such as lead zircono- titanate (known by the abbreviation PZT), barium titanate, barium titanate and strontium ( known as BST), lead titanate, niobium and zinc (known as PZNT), zinc and lead niobate (known as PZN), magnesium niobate and lead (known as PMN), lead titanate (known as PT), potassium potassium niobate, bismuth potassium titanate (known as BKT), bismuth titanate and strontium (known by the abbreviation SBT), potassium tantalate (known as KLT), solid solutions of PMN and PT.

The oxide ceramics constituting the coating may also be chosen from simple oxides, such as SiO ₂ , HfO ₂ , ZrO ₂ , Al ₂ O ₃ and Ta ₂ O ₅ .

Thus, the process of the invention comprises the preparation of a stable sol-gel solution. This preparation comprises, in a first step, bringing into contact one or more molecular precursors of metals and / or metalloids intended to enter the constitution of the ceramic with a medium comprising a solvent comprising at least two -OH functions and optionally an aliphatic monoalcohol.

The metal may be selected from the group consisting of alkali metals, such as K, alkaline earth metals, such as Mg, transition metals, lanthanide metals and post-transition metals of columns IIIA and IVA of periodic classification of the elements. The transition metals can be selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt. The lanthanide metals can be selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Yb. The post-transition metals can be chosen from the elements of group IIIA Al, Ga, In and T1 and the elements of group IVA Ge, Sn and Pb.

The metalloids can be selected from Si, Se, Te.

The molecular precursors of metal and / or metalloid may be in the form of inorganic metal or metalloid salts such as halides (fluorides, chlorides, bromides, iodides), nitrates or oxalates. The molecular precursors of metal and / or metalloid may also be in the form of organometallic compounds of metal or metalloid, such as alkoxides having the formula (RO) _n M, wherein M denotes the metal or metalloid, n represents the number of ligands bound to M, this number also corresponding to the valence of M and R represents a linear or branched alkyl group which may contain from 1 to 10 carbon atoms or an aromatic group comprising from 4 to 14 carbon atoms, such as a phenyl group. The molecular precursors of metal or metalloid may also be in the form of organometallic compounds of formula: in which: M represents a metal or a metalloid;

X represents a hydrolyzable group chosen from halogen, acrylate, acetoxy and acyl groups, OR 'with R' representing a linear or branched alkyl group, which may comprise from 1 to 10 carbon atoms or an aromatic group comprising from 4 to 14; carbon atoms, such as a phenyl group;

- R ¹ represents a non-hydrolyzable group selected from alkyl groups, linear or branched, which may comprise from 1 to 10 carbon atoms, optionally perfluorinated, the aromatic groups may comprise from 4 to 14 carbon atoms; y and z are integers selected so that (y + z) is equal to valence M. In addition to the aforementioned molecular precursors, the first solution of step a) may contain, in addition, one or more compounds polymerizable, such as ethylenic monomers, such as styrene. Thus, when the coating that one wants to achieve is a PZT coating, the precursors The molecular molecules to be used for the preparation of the sol-gel solutions are respectively lead molecular precursors, zirconium molecular precursors and titanium molecular precursors. By way of example, organic lead salts such as acetates, lead inorganic salts such as chlorides or organometallic compounds of lead such as alkoxides containing a number of carbon atoms may be used as lead precursors. ranging from 1 to 4. Preferably, the lead precursor used is a hydrated organic salt such as lead acetate trihydrate. This precursor has the advantage of being stable, very common and cheap. However, when using such a hydrated precursor, it is preferable to dehydrate the latter. In fact, the presence of water during the mixing of the sol-gel solutions with one another can lead to premature hydrolysis of the metal precursors followed by polymerization and thus to a sol-gel solution that is unstable over time.

For example, the dehydration of the lead acetate trihydrate can be carried out by distillation of the latter in the solvent comprising at least two -OH functions used to effect the mixing of the sol-gel solutions.

Preferably, the titanium precursors are alkoxides, such as titanium isopropoxide. Likewise, zirconium precursors are preferably alkoxides, such as zirconium n-propoxide. When the coating to be obtained is a BST coating, the molecular precursors to be used for the preparation of the sol-gel solution are respectively molecular precursors of barium, molecular precursors of strontium and molecular precursors of titanium.

When the coating that is to be obtained is a PZNT coating, the molecular precursors to be used for the preparation of the sol-gel solution are respectively molecular precursors of lead, zirconium molecular precursors, molecular precursors of niobium and molecular precursors of titanium.

When the coating that is to be obtained is a PMN coating, the molecular precursors to be used for the preparation of the sol-gel solution are respectively molecular precursors of lead, molecular precursors of magnesium, and molecular precursors of niobium. When the coating that is to be obtained is a PT coating, the molecular precursors to be used for the preparation of the sol-gel solution are respectively molecular precursors of lead, molecular precursors of titanium.

When the coating that is to be obtained is a BKT coating, the molecular precursors to be used for the preparation of the sol-gel solution are respectively molecular precursors of bismuth, molecular precursors of potassium, and titanium molecular precursors. When the coating that is to be obtained is a coating of SBT, the molecular precursors to be used for the preparation of the sol-gel solution are respectively molecular precursors of strontium, molecular precursors of bismuth, molecular precursors of titanium.

When the coating that is to be obtained is a coating of SiO ₂ , HfO ₂ , Ta ₂ O ₅ , ZrO ₂ or Al ₂ O ₃ , the molecular precursors to be used for the preparation of the sol-gel solution are precursors, respectively. silicon, hafnium, tantalum, zirconium or aluminum.

The precursors as mentioned above are brought into contact with a medium comprising a solvent comprising at least two -OH functions and optionally an aliphatic monoalcohol.

The solvent comprising at least two -OH functions used in step a) and optionally c) may be an alkylene glycol having a number of carbon atoms ranging from 2 to 5. This type of solvent contributes to facilitating the solubilization of precursors and, in addition, acts as a stabilizing agent of the sol-gel solution. A solvent comprising at least two -OH functions that can be used is ethylene glycol or even diethanolamine.

In addition to the solvent comprising at least two -OH functions, the medium of step a) may also comprise an aliphatic monoalcohol, which may, for example, comprise from 1 to 6 carbon atoms. An aliphatic monoalcohol comprising from 1 to 6 carbon atoms can also be used as dilution solvent in step c). As an example of an aliphatic monoalcohol, mention may be made of n-propanol. The contacting of the molecular precursors with the medium comprising a solvent comprising at least two -OH functions can be carried out in different ways and will depend on the nature of the precursors, the essential point being to obtain a sol-gel solution of homogeneous appearance. .

For example, when the sol-gel solution is a precursor of a PZT ceramic, the bringing into contact may consist in preparing a first sol-gel solution based on lead in a diol solvent, by dissolving a molecular precursor based on lead in this diol solvent, to which is added a second mixed sol-gel solution based on titanium and zirconium, said mixed sol-gel solution being prepared by dissolving a molecular precursor based on zirconium and a molecular precursor based on titanium in the same diol or in a solvent compatible with said diol, namely a solvent miscible with said diol, as is the case of aliphatic monoalcohols such as propanol. It is specified that the lead sol-gel solution is preferably initially in excess of 10% with respect to the stoichiometry. The mixture of said sol-gel solutions can then be refluxed, with stirring, at a temperature close to the boiling temperature of the reaction mixture. Reflux makes it possible, advantageously, to homogenize the sol-gel solutions mixed with each other. Once the sol-gel solution obtained at the end of step a), the sol-gel solution is quenched, in accordance with the invention, for a suitable time until a solution having a viscosity is obtained. substantially constant. Generally, step b) is preferably carried out at room temperature, for example, for a period ranging from one week to 4 months. During this so-called ripening phase, the precursors of solubilized metals and / or metalloids condense to a state of equilibrium. This condensation results in an increase in the viscosity of the sol-gel solution, until reaching a substantially constant value as a function of time, when the equilibrium state is reached. In practice, the solution prepared in a) is placed at rest, generally at room temperature and in the absence of any heating. In parallel, the viscosity of the solution is measured at regular intervals. Once it has a substantially constant viscosity, usually reached after a period of 1 week to 4 months, the solution is diluted at a predetermined rate of dilution (step c). This dilution rate will be chosen by those skilled in the art according to the envisaged use of the sol-gel solution, and in particular according to the desired coating thickness after deposition and treatment of such a solution on a substrate and also according to the technique. deposit .

This dilution may consist in diluting the sol-gel solution obtained after step b) by a dilution factor ranging from 1 to 20. According to the invention, the dilution solvent must be miscible with the solvent for preparing the solution of step a). It may be identical to the solvent comprising at least two -OH functions for preparing the sol-gel solution of step a) or be another solvent comprising at least two -OH functions. This alternative, consisting of using a solvent comprising at least two -OH functions identical to or different from that used in the context of step a), is especially preferably chosen, when the deposition technique is spin coating. Examples of solvents comprising at least two possible OH functions are ethylene glycol and propylene glycol. The solvent may be different from a solvent used in step a) and chosen, for example, from solvents having a lower viscosity than that of the solvent used in step a). Solvents corresponding to this specification are, for example, aliphatic monoalcohols comprising from 1 to 6 carbon atoms as defined above. In particular, it is advantageous to use, as a diluting solvent, a solvent having a lower viscosity than that of the solvent used in step a), in order to obtain conformal coatings, when the deposition technique used is the dip coating.

Once prepared, the sol-gel solution is deposited on a substrate in the form of a layer.

This deposit can be performed by any technique to obtain a deposit in the form of thin layers. The thicknesses of thin layers deposited, according to the invention, can range from 1 to 500 nm.

The deposition can be done according to one of the following techniques: - soaking-withdrawal (known under the English terminology "dip-coating");

centrifugal coating (known under the terminology "spin-coating"); laminar coating (known under the terminology "laminar-flow-coating or meniscus coating"); spraying (known under the terminology "spray-coating");

However, preferably, the deposition will be achieved by the technique of soaking-withdrawal (commonly called "dip-coating" in English) or by the technique of spin coating (commonly called "spin-coating" in English). These techniques facilitate, in particular, precise control of the thicknesses of deposited layers.

With regard to the soaking-shrinking technique, the substrate is immersed in the sol-gel solution prepared beforehand and then removed at a suitable speed to obtain a conformal deposit, as defined above. The advantage of this technique is that several substrates can be processed at the same time, which allows a gain in productivity. With regard to the technique of the centrifugal coating, the substrate for depositing is plated on a rotating support. Then, a volume of sol-gel solution is deposited to cover said substrate. The centrifugal force spreads said solution in the form of a thin layer. The thickness of the layer is in particular a function of the centrifugation speed and the concentration of the solution. Since the parameter of the concentration of the solution is fixed, the person skilled in the art can easily choose a desired centrifugation speed for a desired layer thickness. As mentioned above, in the case of the use of the centrifugal coating technique, the dilution solvent used in step c) will preferably be a solvent comprising at least two -OH functions identical to that used in step a) or optionally another solvent comprising at least two -OH functions.

According to the invention, the substrate for depositing is a substrate comprising raised patterns, for example of micrometric size. It is specified that, by micrometric size patterns, is generally meant relief patterns having dimensions (such as height, width) ranging from 1 to 100 microns, these patterns being spaced equally by a distance of 1 at 100 μm.

These relief patterns may be in particular in the form of trenches, for example of parallelepipedal shape, having, for example, a depth, a height and a size spacing micrometric. This substrate may be in the form of a silicon wafer, optionally covered with a metallization layer, when the application domain is microelectronics.

Once the deposition of the sol-gel solution is carried out on one side of the substrate, the method of the invention comprises a heat treatment of the deposited layer (s) so as to transform them into the desired ceramic. This heat treatment can take place in different ways, depending on whether the method of the invention comprises the deposition of one or more layers.

In general, this heat treatment comprises:

a step of drying each layer deposited, so as to gel the layer and optionally to remove a part of the solvent;

optionally, a step of pyrolysis of each layer deposited, so as to remove the organic compounds from the layer;

- Optionally, a relaxation step of each deposited layer, so as to eliminate the stresses generated during the shrinkage of the layer; possibly a step of densification of the layer or set of deposited layers.

The heat treatment can be limited to a simple drying step, if it is sufficient to obtain a ceramization of the layer. This is particularly the case of single oxide layers, such as SiO ₂ , HfO ₂ , Ta ₂ O ₅ , ZrO ₂ or Al ₂ O ₃ . For oxide layers of perovskite structure, the heat treatment generally requires a drying step, a pyrolysis step, a relaxation step and a densification step.

Thus, each deposited solution layer undergoes, according to the invention, a step consisting of a step of drying the deposited layer so as to ensure gelation of the layer. This step is intended to ensure the evaporation of part of the solvent of step a) and a part of the dilution solvent and optionally secondary products, such as esters, resulting from the reactions between the metal precursors. At the end of this step, the sol-gel solution deposited is transformed into a gel layer of constant thickness adhering to the surface of the substrate. The temperature and time effective for gelation can be readily determined by those skilled in the art using, for example, UV-visible spectrometry techniques.

For example, the drying step according to the invention can be carried out at ambient temperature for a duration ranging from 1 to 10 minutes. In other words, this deposition step will consist in resting for a suitable duration the layer, just after the deposition, so that it dries. This drying step may also be carried out at a temperature ranging from 40 to 8O ⁰ C, for example, by making use of a hot plate. In this case, this step will be qualified, in the experimental part, of prepyrolysis step.

After drying, each layer generally undergoes a pyrolysis step carried out at a temperature and time effective to completely remove organic compounds from the deposited layer and in particular the solvents for preparing and diluting the sol-gel solution and the compounds generated by the reaction of the molecular precursors with each other. The effective temperature and time can be easily determined by those skilled in the art through techniques such as IR spectroscopy (Infra ^¬ red).

The pyrolysis time for a given temperature corresponds to a duration that makes it possible to obtain a constant layer thickness. The layer thickness is controlled, for example, by profilometry techniques. The pyrolysis step is stopped to obtain a layer homogeneous in thickness and free of organic compounds.

For example, when the deposited layer (or all of the deposited layers) is a precursor of a PZT ceramic, this pyrolysis step can be carried out at a temperature ranging from about 300 to about 400 ^° C., preferably from 350 ^° C. and 37O ⁰ C, and for a time ranging from about 5 minutes to 10 minutes.

After the pyrolysis step, each deposited layer can be brought to undergo a relaxation step, in order to release the stresses generated during the shrinkage of the layer, in particular those accumulated at the level of the relief patterns. We need that by shrinkage is meant the reduction of the dimensions of the deposited layer, after drying and optionally pyrolysis thereof. This step can be carried out by maintaining the deposited layer at a temperature slightly higher than the pyrolysis temperature, for example greater than 10 to 30 ^° C., for a duration ranging from 10 to 30 minutes.

For example, when the layer is a precursor of a PZT ceramic, the relaxation temperature is greater than 10 to 30 ^° C. at the pyrolysis temperature, but should preferably not exceed 400 ^° C., so as to avoid formation. of a pyrochlore phase.

Finally, the deposited layer or all of the deposited layers may be subjected to a densification (or annealing) step at a time and a temperature effective to allow the crystallization of the deposited layer or of all the deposited layers. The crystallization of the layer corresponds to obtaining a layer of stabilized thickness and crystallized structure, of the perovskite type. The temperature and the annealing time are chosen so as to obtain this crystallization, easily verifiable by structural analysis, such as X-ray diffraction analysis.

Preferably, the densification is carried out at a temperature ranging from about 500 to about 800 ^° C. for a duration of between about 30 seconds and about 1 hour, in particular from 1 minute to 10 minutes. The annealing can be performed by different techniques. Preferably, the annealing is carried out by a rapid heating mode, obtained, for example, with the technique of "rapid thermal annealing" (commonly referred to by the abbreviation RTA for "Rapid Thermal Annealing" or RTP for "Rapid Thermal Process" ).

After this heat treatment, the thermal coatings are homogeneous, continuous, conform to the geometry of the substrate and strongly adhere to the substrate.

The compliance factor, defined by the ratio of the thicknesses at the bottom of the patterns and at the top or on the flanks of the patterns, is close to 1. This result, added to the simplicity of implementation of the sol-gel technique, its cost and its productivity gain augurs favorably for the use of such a process in the industrial environment.

The deposition steps of the sol-gel solution and heat treatment can be repeated one or more times, until a coating having the desired thickness, for example a thickness ranging from 30 to 200 nm, is obtained.

This coating method finds its application in particular for the production of electronic components, such as capacities ranging from 100 nF / mm ² to 1 μF / mm ² .

The invention will now be described with reference to the following examples given for illustrative and not limiting. BRIEF DESCRIPTION OF THE DRAWINGS

The single figure illustrates a cross-section of a portion of a substrate having lined trench patterns and illustrating the magnitudes necessary to determine compliance factors.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The examples which follow illustrate, first of all, the preparation of sol-gel solutions used for the production of conformal coatings, and secondly, the production of conformal coatings on substrates having relief patterns in the form of trenches.

The conformity of the coating with respect to the geometry of the substrate is determined by the compliance factors (b / a) and (b / c), for which:

- a corresponds to the thickness of the coating at the top of the trench;

- b corresponds to the thickness of the coating at the bottom of the trench;

- c corresponds to the thickness of the coating at half height on the side of the trench.

These magnitudes a, b and c are shown in FIG. 1. As the compliance factors (b / a) and (b / c) approach 1, the conformity of the coating is considered ideal.

In practice, to determine these compliance factors, the substrate is firstly cleaved after heat treatment along the desired line of observation and then the interface coating / substrate is observed by scanning electron microscopy.

EXAMPLE 1

In this example, the experimental protocol for the preparation of a precursor sol-gel solution of a lead zirconium titanate ceramic is described.

(PZT) as well as the method of deposition by soaking-removal of this solution on metallized silicon wafers whose surface has micrometric patterns in relief in order to obtain coatings perfectly in accordance with the geometry of the substrate. The patterns used in this example are trenches 1 μm deep, 2 μm wide and spaced 2 μm apart.

a) Preparation of the sol-gel solution

Initially, a solution comprising a lead precursor is prepared. To do this, 751 g (1.98 mol) of lead acetate trihydrate and 330.2 g of ethylene glycol are weighed into a flask surmounted by a distillation assembly. The mixture is homogenized for 30 minutes at

70 ^° C. so as to allow complete dissolution of the lead acetate. Then the temperature of the homogeneous solution is increased to dehydrate the lead precursor by distillation. During the distillation, the solution turns yellow. The recovered distillate has a lead concentration of the order of 2.05 mol / kg.

Under argon flushing and stirring, 225.1 g (0.792 mol) of titanium isopropoxide was stirred at 264 g of 1-propanol. Under the same conditions, 401.5 g (0.858 mol) of 70% zirconium propoxide in 1-propanol and 458.7 g of ethylene glycol are then added. The mixture is stirred for 20 minutes at room temperature. In a tricolor, 883 g (1.815 mol) of the previously prepared lead alkoxide solution is poured, ie a 10% excess to compensate for the loss of lead oxide during the heat treatment. Under an argon flow, the Ti / Zr solution is rapidly added to the tricol with vigorous stirring (600 rpm). At the end of the addition, a refrigerant surmounted by a desiccant is installed on the assembly. The mixture is refluxed for 2 hours (101 ^° C.). After refluxing, the solution obtained has a concentration in mass equivalent PZT of the order of 26%.

The solution is then stored at room temperature during its ripening phase. It is diluted after one week of curing by addition of methanol, so as to obtain a solution having a concentration of 15% by mass equivalent PZT. The viscosity then obtained is of the order of 3 mPa.s. Dilution stabilizes the viscosity of the solution for several months.

b) Deposition of a conformal coating on the substrate. The substrate is a silicon wafer 6 inches in diameter, covered with a layer of silica obtained by thermal oxidation. It is metallized by sputtering with a platinum layer with a thickness of the order of 100 nm. The surface of the wafer has relief patterns of the trench type, the depth of which is 1 μm and the width is of the order of one micrometer.

The diluted solution prepared beforehand is deposited by soaking-withdrawal on the wafer. More specifically, the wafer, whose back side has been protected by an adhesive film, is placed one minute in the sol-gel solution and then removed at a shrinkage speed adjusted between 2 and 10 cm. . min ^"1 Once the wafer is removed from the treatment bath, it is subjected to a heat treatment This heat treatment comprises the following steps:. a first step called" prepyrolysis "of heating the wafer on a hotplate for a time ranging from 2 to 10 min at a temperature of 50 ^° C., this step being intended to reduce the drying time compared with a conventional drying at ambient temperature, a pyrolysis step at a temperature of 36 ^° C. for 5 at 10 minutes, this step being intended to eliminate the residues of organic compounds and to initiate the crystallization phase without trapping residues - a relaxation step at a temperature of 39O ⁰ C for a period ranging from 10 to 20 minutes intended to allow relaxation of the stresses generated during removal of the PZT film; a densification step at a temperature of 600 ^° C. for a period ranging from 5 to 10 minutes intended to crystallize the film in a perovskite phase.

To characterize the conformity of the deposit on the surface of the wafer, the deposition / substrate interface is observed by scanning electron microscopy. For this, the sample is cleaved after heat treatment along the desired line of observation.

The thickness of the coating was evaluated at 90 nm with compliance factors (b / a) equal to 1.4 and (b / c) equal to 1.3.

EXAMPLE 2

In this example, the experimental protocol for the preparation of a precursor sol-gel solution of a lead zirconium titanate ceramic is described.

(PZT) as well as the centrifugal coating deposition method of this solution on metallized silicon wafers whose surface has micrometric 3-dimensional patterns in order to obtain coatings perfectly in accordance with the geometry of the substrate. The patterns used in this example are trenches 1 μm deep, 2 μm wide and spaced 2 μm apart.

a) Preparation of the sol-gel solution with nominal composition PbZr ₀ , 52Ti ₀ , 4 ₈ C> 3 Initially, a solution comprising a lead precursor is prepared. To do this, 751 g (1.98 mol) of lead acetate trihydrate and 330.2 g of ethylene glycol are weighed into a flask surmounted by a distillation assembly. The mixture is homogenized for 30 minutes at 70 ^° C. so as to allow complete dissolution of the lead acetate. Then the temperature of the homogeneous solution is increased to dehydrate the lead precursor by distillation. During the distillation, the solution turns yellow. The recovered distillate has a lead concentration of the order of 2.05 mol / kg.

Under argon flushing and stirring, 225.1 g (0.792 mol) of titanium isopropoxide are added to 264 g of 1-propanol. Under the same conditions, 401.5 g (0.858 mol) of 70% zirconium propoxide in 1-propanol and 458.7 g of ethylene glycol are then added. The mixture is stirred for 20 minutes at room temperature.

In a tricolor, 883 g (1.815 mol) of the previously prepared lead alkoxide solution is poured, ie a 10% excess to compensate for the loss of lead oxide during the heat treatment. Under an argon flow, the Ti / Zr solution is rapidly added to the tricol with vigorous stirring (600 rpm). At the end of the addition, a refrigerant surmounted by a desiccant is installed on the assembly. The mixture is refluxed for 2 hours (101 ^° C.). After refluxing, the solution obtained has a concentration in mass equivalent PZT of the order of 26%. The solution is then stored at room temperature during its ripening phase.

It is diluted after one week of curing by adding ethylene glycol, so as to obtain a solution having a concentration of 10% by mass equivalent PZT. The viscosity then obtained is of the order of 25 mPa.s. Dilution stabilizes the viscosity of the solution for several months.

b) Deposition of a conformal coating on the substrate

The substrate is a silicon wafer 6 inches in diameter, covered with a layer of silica obtained by thermal oxidation. It is metallized by sputtering with a platinum layer with a thickness of the order of 100 nm. The surface of the wafer has relief patterns of the trench type, the depth of which is 1 μm and the width is of the order of one micrometer.

The diluted solution prepared beforehand is filtered at 0.2 μm and is deposited by centrifugal coating on the wafer. The rotational speed is set to 4500 tr.min ^"1 After deposition, the layer subjected to the following heat treatment:. A first said stage

"Prepyrolysis" of heating the hot plate wafer for a period ranging from 2 to 10 min at a temperature of 50 ^° C., this step being intended to reduce the drying time compared with conventional drying at room temperature;

a pyrolysis step at a temperature of 36O ⁰ C for 5 to 10 minutes, this step being intended to eliminate the residues of organic compounds and to initiate the crystallization phase without trapping residues.

The deposition followed by a heat treatment as mentioned above is repeated 6 times. The wafer coated with 6 layers undergoes a final heat treatment comprising:

a relaxation step at a temperature of 39 ^° C. for a duration ranging from 10 to 20 minutes, this step being intended to allow a relaxation of the stresses generated during the shrinkage of the PZT film; a densification step at a temperature of 600 ⁰ C for 5 to 10 minutes to crystallize the film in a perovskite phase.

To characterize the conformity of the deposit on the surface of the wafer, the deposition / substrate interface is observed by scanning electron microscopy. For this, the sample is cleaved after heat treatment along the desired line of observation.

The thickness of the coating was evaluated at

90 nm with compliance factors (b / a) equal to 1.4 and (b / c) equal to 1.3.

Claims

1. A method of producing an oxide ceramic coating conforming to the geometry of a substrate having raised patterns comprising: - a step of depositing on said substrate a sol-gel solution precursor layer of said ceramic; a step of heat treating said layer to transform it into said ceramic; said steps being optionally repeated one or more times, characterized in that the precursor sol-gel solution of said ceramic is prepared by a process comprising successively the following steps: a) preparing a first solution by contacting the molecular precursor (s) with metals and / or metalloids intended to enter the constitution of the ceramic with a medium comprising a solvent comprising at least two -OH functions and optionally an aliphatic monoalcohol; b) quenching the solution obtained in a) for a sufficient time necessary to obtain a solution having a substantially constant viscosity; c) diluting at a predetermined rate the solution obtained in b) with a solvent identical to that of step a) or a solvent miscible with the solvent used in step a) but different from this. 2. The process according to claim 1, wherein the oxide ceramic is selected from lead zircono- titanate (known by the abbreviation PZT), barium titanate, barium and strontium titanate (known by the abbreviation BST). ), lead, niobium and zinc titanate (known by the abbreviation PZNT), zinc and lead niobate (known by the abbreviation PZN), magnesium and lead niobate (known by the abbreviation PMN), lead titanate (known as PT), potassium and calcium niobate, potassium and bismuth titanate (known as BKT), bismuth and strontium titanate (known under the abbreviation BKT), abbreviation SBT), potassium tantalate (known by the abbreviation KLT), solid solutions of PMN and PT. 3. The method of claim 1, wherein the oxide ceramic is selected from SiO ₂ , HfO ₂ , ZrO ₂ , Al ₂ O ₃ , Ta ₂ O ₅ . The process of any one of claims 1 to 3, wherein the molecular precursor of metal or metalloid is an inorganic metal or metalloid salt. The process of any one of claims 1 to 3, wherein the metal or metalloid molecular precursor is an organometallic metal or metalloid compound. The process according to claim 5, wherein the organometallic metal or metalloid compound is an alkoxide having the formula (RO) _n M, wherein M denotes the metal or metalloid, n represents the number of ligands bound to M, this number also corresponding to the valence of M and R represents a linear or branched alkyl group which may comprise from 1 to 10 carbon atoms or an aromatic group comprising from 4 to 14 carbon atoms. The process according to claim 5, wherein the organometallic metal or metalloid compound is an organometallic compound of the formula: in which : M represents a metal or a metalloid; X represents a hydrolyzable group chosen from halogen, acrylate, acetoxy and acyl groups, OR 'with R' representing a linear or branched alkyl group, which may contain from 1 to 10 carbon atoms, or an aromatic group comprising from 4 to 14 atoms of carbon; - R ¹ represents a non-hydrolyzable group selected from alkyl groups, linear or branched, which may comprise from 1 to 10 carbon atoms, optionally perfluorinated, the aromatic groups may comprise from 4 to 14 carbon atoms; y and z are integers selected so that (y + z) is equal to the valence M. The process according to any of claims 1 to 7, wherein the first solution of step a) further comprises one or more polymerizable compounds, such as ethylenic monomers. The process according to any of claims 1 to 8, wherein the solvent comprising at least two -OH functions used in step a) and optionally c) is an alkylene glycol, having a number of carbon atoms ranging from from 2 to 5. 10. Process according to any one of claims 1 to 9, wherein the optional aliphatic monoalcohol of step a) comprises from 1 to 6 carbon atoms. 11. A method according to any one of the preceding claims, wherein the sol-gel solution prepared in step a) is quiescent, as part of step b), for a period ranging from 1 week to 4 hours. month. 12. A process according to any one of the preceding claims, wherein the deposition is carried out by soaking-removal or by centrifugal coating. 13. The method of claim 12, wherein, when the deposition is by coating centrifugal, the dilution solvent used in step c) is a solvent comprising at least two functions - OH identical to or different from that used in step a). 14. The method of claim 12, wherein, when the deposition is by soaking-withdrawal, the dilution solvent used in step c) is a solvent having a lower viscosity than the solvent comprising at least two functions - OH used in step a). 15. The process according to claim 14, wherein the diluting solvent is an aliphatic monoalcohol comprising from 1 to 6 carbon atoms. The process of any of the preceding claims wherein the oxide ceramic is lead zirconium titanate (PZT). 17. A method according to any one of the preceding claims, wherein the heat treatment comprises: a step of drying the layer (s) deposited (s), so as to gel the layer (s); optionally, a step of pyrolysis of the deposited layer (s) so as to eliminate the organic compounds from the layer (s); - optionally, a relaxation step of the layer (s) deposited (s), so as to eliminate the stresses generated during the shrinkage of the layer (s); optionally, a step of densification of the layer (s) deposited. 18. The method of claim 17, wherein the drying step is carried out at room temperature for a period of 1 to 10 minutes. 19. The method of claim 17, wherein the pyrolysis step is carried out at a temperature ranging from about 300 to about 400 ⁰ C and for a time ranging from about 5 minutes to 10 minutes. 20. The method of claim 17, wherein the relaxation step is carried out at a temperature of 10 to 30 ^° C above the pyrolysis temperature for a period ranging from 10 to 30 minutes. 21. The method of claim 17, wherein the densification step is carried out at a temperature ranging from 500 to 800 ⁰ C for a period ranging from 1 minute to 10 minutes. 22. The method of any one of claims 1 to 21, wherein the coating has a thickness of from 30 to 200 nm. 23. A method according to any one of claims 1 to 22, wherein the substrate has micrometric size patterns.