WO2025093454A1

WO2025093454A1 - Formulation for preparing an optical layer containing a metal oxide

Info

Publication number: WO2025093454A1
Application number: PCT/EP2024/080357
Authority: WO
Inventors: Simon SIEMIANOWSKI; Andreas BERKEFELD; Manuel HAMBURGER; Oliver Doll
Original assignee: Merck Patent GmbH
Current assignee: Merck Patent GmbH
Priority date: 2023-10-30
Filing date: 2024-10-28
Publication date: 2025-05-08
Anticipated expiration: 2026-04-30
Also published as: TW202536115A

Abstract

The present invention relates to a formulation for preparing an optical layer containing a metal oxide. The formulation may exhibit at least one of properties as an advanced material or as a high-performance material. The formulation may be used in the nanotechnology process to make semiconductor device/display device application, for example semiconductor chip, or a liquid crystal, quantum dot, OLED display fabricated on a substrate controlled by semiconductors.

Description

FORMULATION FOR PREPARING AN OPTICAL LAYER CONTAINING A METAL OXIDE

Field of the invention

The present invention relates to a formulation for preparing an optical layer containing a metal oxide, method for preparing a formulation, use of a formulation, method for preparing a composite, a composite, an optical device and a display device.

Background Art

Leading edge optical devices typically include optical gratings made from composite materials having a substrate as a support and complex and interlaced patterns thereon, the patterns being made up of different layers or stacks of layers. Usually, the creation of such complex and interlaced patterns demands for structuring processes, which become increasingly challenging with decreasing size of structural dimensions to be prepared.

In addition to a wide range of possible uses in various fields of application, such as in spectrometers or in optical storage systems (CD, DVD, etc.), diffractive gratings are the core components of so-called XR devices, usually in the form of glasses. In this context, R stands for the term reality and X denotes different attributes such as, for example, virtual, augmented, mixed and so forth. Hence, diffractive gratings form part of the core of the so-called optical engine in XR devices, specifically in augmented reality and mixed reality glasses. Virtual reality glasses, when built as a head mounted display, are often composed of a conventional liquid crystal (LC) organic light emitting diode (OLED) display being embedded in the device directly in front of the eyes of the user, and thus do not necessarily require diffractive gratings. In contrast, augmented and mixed reality glasses are designed to enable consumers to obtain visual impressions of their environment, at its best as if they would not wear any glasses at all. However, they also make it possible to provide and serve digital information and to also project it into the field of vision of individuals. Additional digital information is gathered from recognizing and analyzing the environment, the individual inspects or looks at. To convey and project supporting digital information into the eyes of an individual, the augmented or mixed reality glasses are equipped with an information supply unit, which is coupled to an optical waveguide system that transports the optically coded supporting information through it directly to the lens of the glasses. Here, the information passes a diffractive grating which couples the incident light into the lens and splits it according to its angular information and its spectral bands by diffraction. After incoupling of the light, the lens serves as waveguide enabling transport of the light to and into the pupil of an individual. The location of light incoupling is independent of any preferred position and thus of the implication of technical needs. The direction of traversal of light within the lenses is determined by the diffractive grating diffracting or splitting the light. At certain positions in the lens, a second and a third diffractive grating serves for changing the direction of light traversal and thereby enforcing the light to be projected into pupil of the user. The light traversal in the glasses is accomplished by total internal reflection (TIR) of the light, thus bouncing several times between the glass interfaces until reaching another diffractive grating, which changes the internal TIR direction of the light (see Figure 2). The second and third grating are geometrically aligned in different directions with respect to the first and incoupling grating, e. g. by a certain angular distortion of the longitudinal axis, thus allowing to change the direction of propagation of totally internally reflected light. Needless to say, the lens itself or the material of which lenses are made of shall not be absorbing.

Otherwise, the supportive information never reaches the pupil of the user or only with strongly depleted light intensity. The process works regardless of the use of reflection or transmission gratings. Usually, the lenses are equipped with both types of gratings to properly guide the light. It should also be mentioned that there are differences in the optical performance of reflection and transmission gratings, which, however, are of no further interest in the context of the current invention. The basic structure of the gratings is very similar, which is more important at this point. Nevertheless, there are different designs and structures such as surface relief (SR) or volume phase holographic (VPH) gratings to achieve waveguide. Both types are very similar in appearance. In the simplest case, the gratings are mounted onto the surface of a waveguiding material, here the lens. The grating itself is composed of an array of fine structures, mostly trenches of a first material type Material 01 with a refractive index Rl 01 , however, not limited thereto. The geometrical shape of the trenches may be manifold, from rectangular, over V-shaped trenches, U-shaped and there like. The width, including structures with different widths, the geometrical form of the trenches, their pitch as well as their depth, including different depths, are specially designed to influence the diffraction pattern of the incident light to be diffracted.

In case of VPH gratings, the trenches or structures of a first material type (Material 01 ) having a refractive index (Rl 01 ) are filled by a second material type (Material 02) having a refractive index (Rl 02), wherein Rl 02 is incrementally different from Rl 01 (see Figures 1 and 3). For the sake of completeness, it should be mentioned that Material 01 or Material 02 may be composed of a stack of structured layers, each containing a different material composition with different refractive index, stacked on top of each other, thereby forming Material 01 or Material 02 having an effective or graded refractive index Rl 01 or Rl 02, respectively. Incidentally, the (effective or graded) refractive indices Rl 01 and Rl 02 depend on the refractive index of the waveguide or the lens from which the glasses are made of. If a glass lens with high refractive index (n03 > 1 .46) is used, the (effective or graded) refractive indices of Material 01 and Material 02 are considered to be higher than that of the lens itself, whereby a Rl value of 2.0 can be reached and exceeded. Surface relief (SR) gratings may look similar and may also include a second type of material as a filler for the trenches, but the trenches can also be just air. High performance gratings, especially those of VPH-type, may be manufactured using standard lithography and deposition techniques known from micro-fabrication such as, for example, the manufacturing of integrated circuits.

Such standard techniques typically include physical vapor deposition (PVD) or chemical vapor deposition (CVD) processes and often suffer from incomplete gap filling due to unfavourable deposition and/or layer growth deposition properties including increased deposition and/or growth rates at corners and edges. Such incomplete gap filling results in the formation of voids within the structures to be filled by the PVD- and CVD-materials. In addition to the formation of voids, the surface of the substrate is covered by a PVD and/or CVD layer that is almost as thick as the maximum depth of the deepest structure to be filled by the deposited gap filling material (see Figures 4 and 5). In some applications, however, it may be necessary to expose the surface of the substrate so that it is available for further processing. Consequently, undesired overburden layers from PVD or CVD need to be removed, for example by chemical mechanical planarization (CMP) without harming the original substrate surface underneath. Although CMP is very well established in the process of manufacturing integrated circuits, CMP is a time consuming and costly process and can be seen as a potential economic drawback for mass production of leading-edge optical devices, particularly the mass production of diffractive gratings. It would therefore be desirable to have a solution for an advanced and cost-efficient manufacturing of optical gratings where gap filling does not require CMP (see Figure 6).

For that reason, more cost-effective production technology allowing for lower cost of ownership is required. Summary of the invention

The inventors newly have found that there are still one or more of considerable problems for which improvement is desired, as listed below: providing a printable formulation for preparing an optical layer/composite containing a material which provides sufficiently high refractive indices after curing; providing a formulation for fabricating an optical layer which enables to prepare a dense, crack-less or crack-free optical layer and enables to fill up of cavities, trenches or gaps after curing; providing a formulation for preparing an optical layer containing a metal oxide precursor material of a high refractive index material, which is well dispersed in the formulation; simpler and/or cost efficient method for preparing an optical layer/composite with using the formulation; realizing more stable formulation, zero or reduced viscosity change of the formulation, providing suitable formulation for wet printing, namely for spincoating or ink jetting, realizing continuous inkjet printing.

The inventors aimed to solve one or more of the above-mentioned problems.

Then, the present inventors have surprisingly found that one or more of the above-described technical problems can be solved by the features as defined in the claims.

Namely, it is found a novel formulation for preparing an optical layer containing a metal oxide, preferably for preparing a composite, more preferably for preparing a layered composite, comprising at least;

- a metal oxide precursor containing a group 4 and/or group 5 element of the periodic table, preferably said metal oxide precursor is a metal alkoxide, metal halide or a metal carboxylate containing a group 4 and/or group 5 element of the periodic table, more preferably said metal oxide precursor is a metal alkoxide containing a group 4 and/or group 5 element of the periodic table; - optionally an acid selected from one or more members of the group consisting of sulfonic acids, amine hydrochlorides and carboxylic acids; and

- a solvent, wherein the solvent is a secondary alcohol having one or two alkoxy groups or one or two alkyl groups where one or more non- adjacent groups of said alkyl group is replaced by oxygen atom; or secondary linear or branched >C3 alcohols, preferably it is selected from one or more members of 2-propanol, 2-butanol, 2-pentanol and 3- pentanol, preferably secondary linear or branched >C4 alcohols; preferably said solvent is selected from propylene glycol monoalkyl ethers, glycerol 1 ,3-dialkyl ethers, glycerol 1 ,3-dialkyl ethers or combination of propylene glycol monoalkyl ethers, glycerol 1 ,3-dialkyl ethers and glycerol 1 ,3-dialkyl ethers;

- wherein the total amount of the metal oxide precursor in the formulation is in the range from 0.1 to 40wt.% based on the total amount of the formulation, preferably it is in the range from 1 to 35wt%, more preferably from 2 to 30wt%, even more preferably from 3 to 28wt%.

In another aspect, the present invention further relates to a method for preparing a formulation of the present invention, containing at least the following step;

(X1 ) dissolving a metal oxide precursor in solvent 1 , preferably said solvent 1 is a dry or water-free solvent to form a metal oxide precursor solution; (X2) optionally dissolving an acid in solvent 2, preferably said solvent 2 is dry or water-free, wherein said acid is selected from one or more members of the group consisting of sulfonic acids, amine hydrochlorides and carboxylic acids to form an acid solution;

(X3) optionally adding said acid solution obtained in step (X2) to the metal oxide precursor solution obtained in step (X1 );

(X4) mixing water and solvent 3 to form an aqueous solvent; and

(X5) adding said aqueous solvent to the alkoxide solution obtained in step (X1) or the metal oxide precursor solution obtained in step (X3); (X6) optionally dissolving an acid and water in solvent 4, preferably said solvent 4 is dry or water-free solvent, wherein said acid is selected from one or more members of the group consisting of sulfonic acids, amine hydrochlorides and carboxylic acids to form an acid solution to form an aqueous acid solution;

(X7) optionally adding said aqueous acid solution (X6) to the metal oxide precursor solution (X1 ), wherein said solvents 1 to 4 are independent from each other, selected from one or more members of the group consisting of propylene glycol monoalkyl ethers, preferably it is propylene glycol monomethyl ether (PGME), propylene glycol monoethyl ether and/or propylene glycol monopropyl ether; glycerol 1 ,3-dialkyl ethers, 1 ,3-dimethoxy-2-propanol; a secondary linear or branched >C3 alcohol, preferably it is selected from 2- propanol, 2-butanol, 2-pentanol, 3-pentanol, more preferably it is linear or branched secondary >C4 alcohol; preferably said solvent is selected from propylene glycol monoalkyl ethers, glycerol 1 ,3-dialkyl ethers, glycerol 1 ,3- dialkyl ethers.

In another aspect, the present invention also relates to use of the formulation of the present invention for preparing an optical layer containing a metal oxide, preferably for preparing a composite, more preferably for preparing a layered composite.

In another aspect, the present invention further relates to a method for preparing a composite containing a metal oxide, preferably said metal oxide is selected from metal monoxide, metal dioxide and metal pentoxide, or a combination of these; comprising the following steps (a) and (b):

(a) providing the formulation of the present invention onto a surface of a substrate, preferably by wet deposition process, more preferably by spincoating or an area selective printing, preferably said area selective printing is an ink-jetting, even more preferably by ink-jetting; and (b) applying a thermal treatment to the formulation provided on the surface of the substrate to convert at least a part of the metal oxide precursor of the formulation to a metal oxide.

Preferably said composite being a layered composite, more preferably said layered composite is an optical layer.

In another aspect, the present invention further relates to a composite, preferably being a layered composite, preferably said layered composite is an optical layer, obtained or obtainable by the method of the present invention.

In another aspect, the present invention further relates to a composite, preferably being a layered composite, preferably said layered composite is an optical layer, derived from the formulation of the present invention.

In another aspect, the present invention further relates to an optical device comprising the composite of the present invention, and a substrate comprising a patterned surface or an uneven surface. Preferably a gap or trench of said patterned surface or an uneven surface of the substrate is at least partly filled with said composite.

Preferably said substrate is a patterned substrate comprising topographical features on the surface thereof. Preferably said composite fills at least a part of a gap of said topographical features, more preferably said composite fills a trench of the patterned substrate.

In another aspect, the present invention further relates to a display device comprising at least one functional medium configured to direct and modulate a light or configured to emit light; and the composite of the present invention.

Technical effects of the invention

The present invention provides one or more of following effects; providing a printable formulation for preparing an optical layer/composite containing a material which provides sufficiently high refractive indices after curing; providing a formulation which enables preparation of a dense, crack-less or crack-free optical layer, enables defect free filling of cavities, trenches or gaps after curing; providing a formulation for preparing an optical layer containing a metal oxide precursor material of a high refractive index material, which is well dispersed in the formulation; simpler and/or more cost efficient method for preparing an optical layer/composite via use of the formulation; realizing a more stable formulation, zero or reduced viscosity change of the formulation, providing suitable formulation for wet printing, namely for spincoating or ink jetting, realizing continuous inkjet printing.

Preferred embodiments of the present invention are described hereinafter and in the dependent claims.

Brief description of the figures

Fig. 1 : Schematic cross-sectional view of a VPH grating with a Material 01 and a Material 02, wherein the refractive index IR 01 of Material 01 is incrementally different to the refractive index IR 02 of Material 02.

Fig. 2 : Schematic cross-sectional view of a VPH grating enabling light diffraction (transmissive case) including propagation of diffracted light within waveguide (e.g. lens) by total internal reflection.

Fig. 3 : Schematic cross-sectional view of a VPH grating providing gaps (trenches) to be filled with a high refractive index material (Material 02), wherein the refractive index of Material 02 is incrementally different form the refractive index of Material 01 flanking the gaps (trenches).

Fig. 4 : Schematic representation of PVD- or CVD-mediated gap filling process and removal of undesired overburden.

Fig. 5 : Schematic representation of PVD- or CVD-mediated gap filling process creating and leaving voids within gaps and deposited layers. Fig. 6 : Schematic representation of gap filling process using formulations containing inventive metal complex or formulations thereof being converted to metal oxides.

Fig. 7a: shows a TEM image of Sample Zr-2.

Fig. 7b: shows a TEM image of Sample Zr-3.

List of reference signs

1. Material 02 with Rl 02

2. Material 01 with Rl 01

3. Substrate (e.g. glass)

4. Diffraction of incident light represented by broad arrow

5. Total internal reflection of light (TIR)

6. Waveguide

7. Structured layer stack with gaps (trenches)

8. Substrate (e.g. glass or silicon)

9. Overburden of material (e.g. high refractive index material or high etch resistant material)

10. Material (e.g. high refractive index material or high etch resistant material) providing gap fill

11 . Voids

12. Formulation (e.g. ink) of high refractive index material (e.g. metal oxide precursor)

13. High refractive index material (e.g. metal oxide) providing gap fill with optional concave geometry

14. Overburden layer (optional)

15. Energy

Definition of the terms

In the context of the present invention, the term “formulation medium” or the plural term “formulation media” as used herein, denote one or more compounds serving as a solvent, suspending agent, carrier and/or matrix for the metal oxide precursor compound and any other component included in the formulation. Formulation media are generally inert compounds that do not react with said metal oxide precursor compounds and said other components. Formulation media may be liquid compounds, solid compounds or mixtures thereof. Typically, formulation media are organic compounds.

The term “surfactant” as used herein, refers to an additive that reduces the surface tension of a given formulation.

The term “wetting and dispersion agent” as used herein, refers to an additive that increases the spreading and filling properties of a given formulation. In this way, the tendency of the molecules to adhere to each other is reduced.

The term “adhesion promoter” as used herein, refers to an additive that increases the adhesion of a given formulation.

The term “polymer matrix” as used herein, refers to an additive that acts as a macromolecular matrix for one or more components of a given formulation.

The term “optical device” as used herein, relates to a device containing one or more optical components for forming a light beam including, but not limited to, gratings, lenses, prisms, mirrors, optical windows, filters, polarizing optics, UV and IR optics, waveguides and optical coatings. Preferred optical devices in the context of the present invention are waveguides for augmented reality (AR) device, for virtual reality (VR) device and/or for mixed reality (MR) device, or preferred optical devices are augmented reality (AR) glasses, virtual reality (VR) glasses and/or mixed reality (MR) glasses. The term “display device” as used herein, is a kind of an optical device configured to output/present information in visual or tactile form. Examples are Liquid crystal display (LCD), Light emitting diode display (LED display), organic light emitting display (OLED), micro-LED display, quantum dot display (QLED), AR display, VR display, MR display, plasma (PDP) display, electroluminescent (ELD) display. Preferred optical devices in the context of the present invention is AR display, VR display or MR display.

Detailed description of the invention

The present invention relates to a formulation for preparing an optical layer containing a metal oxide, preferably for preparing a composite, more preferably for preparing a layered composite, comprising at least, essentially consisting of, or consisting of;

- a metal oxide precursor containing a group 4 and/or group 5 element of the periodic table, preferably said metal oxide precursor is a metal alkoxide, metal halide or a metal carboxylate containing a group 4 and/or group 5 element of the periodic table, more preferably said metal oxide precursor is a metal alkoxide containing a group 4 and/or group 5 element of the periodic table;

- optionally an acid selected from one or more members of the group consisting of sulfonic acids, amine hydrochlorides and carboxylic acids; and

- a solvent, wherein the solvent is a secondary alcohol having one or two alkoxy groups or one or two alkyl groups where one or more non- adjacent groups of said alkyl group is replaced by oxygen atom; or secondary linear or branched >C3 alcohols, preferably it is selected from one or more members of 2-propanol, 2-butanol, 2-pentanol and 3- pentanol, preferably secondary linear or branched >C4 alcohols; preferably said solvent is selected from propylene glycol monoalkyl ethers, glycerol 1 ,3-dialkyl ethers, glycerol 1 ,3-dialkyl ethers or combination of propylene glycol monoalkyl ethers, glycerol 1 ,3-dialkyl ethers and glycerol 1 ,3-dialkyl ethers; - wherein the total amount of the metal oxide precursor in the formulation is in the range from 0.1 to 40wt.% based on the total amount of the formulation, preferably it is in the range from 1 to 35wt%, more preferably from 2 to 30wt%, even more preferably from 3 to 28wt%.

- Metal oxide precursor

According to the present invention, said metal oxide precursor contains a group 4 and/or group 5 element of the periodic table, preferably said metal oxide precursor is a metal alkoxide, a metal halide or a metal carboxylate containing a group 4 and/or group 5 element of the periodic table, more preferably said metal oxide precursor is a metal alkoxide containing a group 4 and/or group 5 element of the periodic table.

As the metal oxide precursor, any publicly known metal oxide precursors can be used.

- Metal alkoxides

According to the present invention, a metal alkoxide containing a group 4 and/or group 5 element of the periodic table is used. As said metal alkoxide, commercially available metal alkoxides containing a group 4 and/or group 5 element of the periodic table, can be used.

Preferably, titanium tetra-n-butoxide, zirconium tetra-n-butoxide, niobium pentaethoxide can be used.

In a preferred embodiment of the present invention, said metal oxide precursor (preferably metal alkoxide) is represented by following chemical formula (IV) or formula (V):

M^b1O (R^b1 R^b2R^b3R^b4)₄ -(IV) M^b2O (R^b1 R^b2R^b3R^b4 R^b5)₅ -(V) wherein M^b1 is a tetravalent metal of a group 4 element of the periodic table, preferably it is Ti or Zr; M^b2 is a pentavalent metal of a group 5 element of the periodic table, preferably it is Nb or Ta;

R^b1, R^b2, R^b3, R^b4 and R^b5 are each independently selected from H, D, a straight-chain alkyl group having 1 to 25 carbon atoms, preferably 1 to 15 carbon atoms, more preferably 1 to 10 carbon atoms; a branched or cyclic alkyl group having 3-25 carbon atoms, preferably 3 to 15 carbon atoms, more preferably 3 to 10 carbon atoms; an aryl group having 3 to 25 carbon atoms, preferably from 3 to 15 carbon atoms, more preferably 3 to 10 carbon atoms; a straight chain alkyl-cycloalkyl group having 4 to 25 carbon atoms, preferably 4 to 15 carbon atoms; branched chain alkyl-cycloalkyl group having carbon atoms 6 to 25 carbon atoms, preferably 6 to 15 carbon atoms; a straight chain alkyl-aryl group having 4 to 25 carbon atoms, preferably 4 to 15 carbon atoms; branched chain alkyl-aryl group having 6 to 25 carbon atoms, preferably 6 to 15 carbon atoms; where one or more non-adjacent CH2 groups and/or one or more adjacent CH2 groups of the above-mentioned groups may be replaced by oxygen atom, and where one or more H atoms may be replaced by D, or NO2; where each of groups may be substituted by one or more groups R^a;

R^a is at each occurrence, identically or differently, H, D, a straight chain alkyl or alkoxy group having 1 to 15 carbon atoms, preferably 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms; a branched or cyclic alkyl or alkoxy group having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms, more preferably 3 to 5 carbon atoms; a straight-chain alkenyl or alkynyl group having 2 to 15 carbon atoms, preferably 2 to 10 carbon atoms, more preferably 2 to 5 carbon atoms; a branched alkenyl group or alkynyl group having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms, more preferably 3 to 5 carbon atoms; an aromatic or heteroaromatic ring system having 5 to 15 aromatic ring atoms, preferably 5 to 10 aromatic ring atoms; where in each of the above-mentioned groups, one or more H atoms may be replaced by D and where two or more adjacent substituents R^a here may optionally form a mono- or polycyclic, aliphatic ring system with one another.

According to the present invention, the total amount of the metal oxide precursor in the formulation is in the range from 0.1 to 40wt.% based on the total amount of the formulation, preferably it is in the range from 1 to 35wt%, more preferably from 2 to 30wt%, even more preferably from 3 to 28wt%.

It is believed that the above-mentioned total amount of the metal oxide precursor based on the total amount of formulation is very important to realize an improved gap filling or fabricating a thin layer placed directly onto a patterned surface or directly onto an uneven surface of a substrate I under layer. In case a thin layer is fabricated directly onto a patterned surface or an uneven surface of a substrate or a patterned surface or an uneven surface of an under layer, it is desirable that the bumps, trenches, grooves of the patterned surface and/or an uneven structure is filled with air and said thin layer is placed to cover said air filled patterned bumps, trenches, grooves. It is believed that by adjusting the total amount of the metal oxide precursor(s) of the formulation, an improved gap filling or good thin layer can be made preferably. Namely for an improved gap filling (filling gaps of trenches, bumps, grooves) of the patterned surface, uneven structure, lower amount of the metal oxide precursor is desirable to realize a lower condensation speed. Preferable lower amount of the metal oxide precursor to realize an improved gap filing is, for examples, in the range from 0.1 to 15wt% based on the total amount of the formulation, more preferably it is in the range from 1 to 10wt%, even more preferably from 2 to 8wt% based on the total amount of the formulation.

According to the present invention, the nominal relative weight content of metal oxide in the formulation of the present invention is theoretically in the range from 0.1 to 15wt.% based on the total amount of the composite, preferably it is in the range from 0.2 to 10wt%, more preferably from 0.5 to 5wt%, even more preferably from 0.7 to 4wt%.

According to the present invention, said nominal relative weight content of metal oxide in the formulation is calculated by using the following formula:

M(0R)x and MOx (w%) in the total amount of the formulation = generic formula for metal alkoxides and metal oxides (MO2 and M2O5) and their nominal relative weight contents in the formulation (sol-gel mixture).

-Solvent

According to the present invention, the formulation of the present invention contains a solvent, wherein the solvent is a secondary alcohol having one or two alkoxy groups or one or two alkyl groups where one or more non- adjacent groups of said alkyl group is replaced by oxygen atom.

It is believed that said secondary alcohol solvent chemically interacts with the metal oxide precursor in the formulation and allows controlling the rate of formation of a continuous metal oxide material during formulation deposition and final thermal curing process.

Preferably, said alkoxy group of the secondary alcohol is an alkoxy group having 1-10 carbon atoms, more preferably it is an alkoxy group having 1-5 carbon atoms, even more preferably an alkoxy group having 1-3 carbon atoms.

Preferably, said alkyl groups where one or more non-adjacent groups of said alkyl group is replaced by oxygen atom, is an alkyl group having 1-10 carbon atoms where one or more non-adjacent carbon groups of said alkyl group is replaced by oxygen atom, more preferably 2-5 carbon atoms, even more preferably 2 or 3 carbon atoms. Furthermore preferably, said solvent is selected from one or more members of the group consisting of propylene glycol monoalkyl ethers, preferably it is propylene glycol monomethyl ether (PGME), propylene glycol monoethyl ether and/or propylene glycol monopropyl ether; glycerol 1 ,3-dialkyl ethers, 1 ,3-dimethoxy-2-propanol, more preferably said solvent is selected from propylene glycol monoalkyl ethers, glycerol 1 ,3-dialkyl ethers, a secondary linear or branched >C3 alcohol, preferably it is selected from 2-propanol, 2- butanol, 2-pentanol, 3-pentanol, more preferably it is a linear or branched secondary >C4 alcohol; preferably said solvent is selected from propylene glycol monoalkyl ethers, glycerol 1 ,3-dialkyl ethers, glycerol 1 ,3-dialkyl ethers.

It is believed that the printing, especially ink jetting of structures is considered as a highly cost-efficient production step. Spin-coating is a convenient method and is preferable to form a uniform thin layer. Thus, suitable solvents of the formulation for spin-coating/inkjet printing the structures or filling up of cavities and structures, is described here.

It is believed that selection between chemically closely related secondary alcohols as solvents for a reactive sol-gel mixture containing a group 4 and/or group 5 element of the periodic table formed from a metal alkoxide allows controlling the rate of formation of a continuous metal oxide material during formulation deposition and final thermal curing.

After printing, deposition and fill up of structures, at least a part of the material as the metal oxide precursor need to become converted into the respective metal oxides by any known means know to the persons skilled in the art (thermally, photochemically, etc.).

-Acid

According to the present invention, the formulation may optionally contain an acid selected from one or more members of the group consisting of sulfonic acids, amine hydrochlorides and carboxylic acids. It is believed that the acid can function as a reaction mediator.

In a preferred embodiment of the present invention, the formulation of the present invention contains an acid selected from one or more members of the group consisting of sulfonic acids, amine hydrochlorides and carboxylic acids.

It is believed that these acids are suitable as the reaction mediator according to the present invention.

In a more preferred embodiment of the invention, said sulfonic acid is represented by following chemical formula (I).

R^a1SO₃H - (I) wherein

R^a1 is selected from the group consisting of a straight-chain alkyl group having 1 to 25 carbon atoms, preferably 1 to 15 carbon atoms, more preferably 1 to 10 carbon atoms; a branched or cyclic alkyl group having 3- 25 carbon atoms, preferably 3 to 15 carbon atoms, more preferably 3 to 10 carbon atoms; an aryl group having 3 to 25 carbon atoms, preferably from 3 to 15 carbon atoms, more preferably 3 to 10 carbon atoms; a straight chain alkyl-cycloalkyl group having 4 to 25 carbon atoms, preferably 4 to 15 carbon atoms; branched chain alkyl-cycloalkyl group having carbon atoms 6 to 25 carbon atoms, preferably 6 to 15 carbon atoms; a straight chain alkyl-aryl group having 4 to 25 carbon atoms, preferably 4 to 15 carbon atoms; branched chain alkyl-aryl group having 6 to 25 carbon atoms, preferably 6 to 15 carbon atoms; where one or more non-adjacent CH2 groups of the above-mentioned groups may be replaced by oxygen atom, C=O, C=S, C=Se, C=NH, SiH2, SO, SO2, OS, or CONH and where one or more H atoms may be replaced by D, F, Cl, Br, I, CN or NO2; where each of groups may be substituted by one or more groups R^ax;

R^ax is at each occurrence, identically or differently, H, D, a straight chain alkyl or alkoxy group having 1 to 15 carbon atoms, preferably 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms; a branched or cyclic alkyl or alkoxy group having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms, more preferably 3 to 5 carbon atoms; a straight-chain alkenyl or alkynyl group having 2 to 15 carbon atoms, preferably 2 to 10 carbon atoms, more preferably 2 to 5 carbon atoms; a branched alkenyl group or alkynyl group having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms, more preferably 3 to 5 carbon atoms; an aromatic or heteroaromatic ring system having 5 to 15 aromatic ring atoms, preferably 5 to 10 aromatic ring atoms; where in each of the above-mentioned groups, one or more H atoms may be replaced by D, F, Cl, Br, I, and where two or more adjacent substituents R^ax here may optionally form a mono- or polycyclic, aliphatic ring system with one another; said amine hydrochloride is represented by following chemical formula (II) HCI - R^a2 - (II) wherein

R^a2 is selected from the group consisting of N2H4, ammonia, hydroxylamine, imidazole and 1 ,4-diazabicyclo[2.2.2]octane; and/or said carboxylic acid is represented by following chemical formula (III) R^a3 - COOH wherein R^a3 is selected from the group consisting of a straight-chain alkyl group having 1 to 25 carbon atoms, preferably 1 to 15 carbon atoms, more preferably 1 to 10 carbon atoms; a branched or cyclic alkyl group having 3- 25 carbon atoms, preferably 3 to 15 carbon atoms, more preferably 3 to 10 carbon atoms; an aryl group having 3 to 25 carbon atoms, preferably from 3 to 15 carbon atoms, more preferably 3 to 10 carbon atoms; a straight chain alkyl-cycloalkyl group having 4 to 25 carbon atoms, preferably 4 to 15 carbon atoms; branched chain alkyl-cycloalkyl group having carbon atoms 6 to 25 carbon atoms, preferably 6 to 15 carbon atoms; a straight chain alkyl-aryl group having 4 to 25 carbon atoms, preferably 4 to 15 carbon atoms; branched chain alkyl-aryl group having 6 to 25 carbon atoms, preferably 6 to 15 carbon atoms; where one or more non-adjacent CH2 groups of the above-mentioned groups may be replaced by oxygen atom, C=O, C=S, C=Se, C=NH, SiH2, SO, SO2, OS, or CONH and where one or more H atoms may be replaced by D, F, Cl, Br, I, CN or NO2; where each of groups may be substituted by one or more groups R^ax;

R^ax is at each occurrence, identically or differently, H, D, a straight chain alkyl or alkoxy group having 1 to 15 carbon atoms, preferably 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms; a branched or cyclic alkyl or alkoxy group having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms, more preferably 3 to 5 carbon atoms; a straight-chain alkenyl or alkynyl group having 2 to 15 carbon atoms, preferably 2 to 10 carbon atoms, more preferably 2 to 5 carbon atoms; a branched alkenyl group or alkynyl group having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms, more preferably 3 to 5 carbon atoms; an aromatic or heteroaromatic ring system having 5 to 15 aromatic ring atoms, preferably 5 to 10 aromatic ring atoms; where in each of the above-mentioned groups, one or more H atoms may be replaced by D, F, Cl, Br, I, and where two or more adjacent substituents R^ax here may optionally form a mono- or polycyclic, aliphatic ring system with one another. In some preferred embodiments, the acid is an amine hydrochloride represented by following chemical formula (II) HCI - R^a2 - (II) wherein

R^a2 is selected from the group consisting of N2H4, ammonia, hydroxylamine, imidazole and 1 ,4-diazabicyclo[2.2.2]octane.

In a preferred embodiment, said acid is a sulfonic acid represented by following chemical formula (I).

R^a1SO₃H - (I) wherein

R^a1 is selected from the group consisting of a straight-chain alkyl group having 1 to 25 carbon atoms, preferably 1 to 15 carbon atoms, more preferably 1 to 10 carbon atoms; a branched or cyclic alkyl group having 3- 25 carbon atoms, preferably 3 to 15 carbon atoms, more preferably 3 to 10 carbon atoms; an aryl group having 3 to 25 carbon atoms, preferably from 3 to 15 carbon atoms, more preferably 3 to 10 carbon atoms; a straight chain alkyl-cycloalkyl group having 4 to 25 carbon atoms, preferably 4 to 15 carbon atoms; branched chain alkyl-cycloalkyl group having carbon atoms 6 to 25 carbon atoms, preferably 6 to 15 carbon atoms; a straight chain alkyl-aryl group having 4 to 25 carbon atoms, preferably 4 to 15 carbon atoms; branched chain alkyl-aryl group having 6 to 25 carbon atoms, preferably 6 to 15 carbon atoms; where one or more non-adjacent CH2 groups of the above-mentioned groups may be replaced by oxygen atom, C=O, C=S, C=Se, C=NH, SiH2, SO, SO2, OS, or CONH and where one or more H atoms may be replaced by D, F, Cl, Br, I, CN or NO2; where each of groups may be substituted by one or more groups R^ax; R^ax is at each occurrence, identically or differently, H, D, a straight chain alkyl or alkoxy group having 1 to 15 carbon atoms, preferably 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms; a branched or cyclic alkyl or alkoxy group having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms, more preferably 3 to 5 carbon atoms; a straight-chain alkenyl or alkynyl group having 2 to 15 carbon atoms, preferably 2 to 10 carbon atoms, more preferably 2 to 5 carbon atoms; a branched alkenyl group or alkynyl group having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms, more preferably 3 to 5 carbon atoms; an aromatic or heteroaromatic ring system having 5 to 15 aromatic ring atoms, preferably 5 to 10 aromatic ring atoms; where in each of the above-mentioned groups, one or more H atoms may be replaced by D, F, Cl, Br, I, and where two or more adjacent substituents R^ax here may optionally form a mono- or polycyclic, aliphatic ring system with one another.

In a preferred embodiment of the present invention, the stoichiometric mole ratio of the acid and the metal oxide precursor, preferably the metal alkoxide, is in the range from 0.01 :100 to 120:100, preferably the relative mole amount of the acid based on the total amount of the metal oxide precursor is in the range from 0.1 to 50, more preferably 1 to 10.

It is believed that when the stoichiometric mole ratio of the acid and the metal oxide precursor is in the above-mentioned range, it improves film stability, improves gap filing property and/or provides improved refractive index value.

- Water

According to the present invention, the formulation contains water, and the stoichiometric amount of water is in the range from 100 to 400 mol% based on the total amount of metal oxide precursor, preferably the stoichiometric amount of water based on the total amount of the metal oxide precursor is in the range from 150 to 300 mol%, even more preferably from 180 to 270 mol%.

-Additives

In some embodiments of the present invention, the formulation may optionally comprise one or more additives selected from surfactants, wetting and dispersion agents, adhesion promoters, and polymer matrices. Or, in some embodiments, the formulation of the present invention does not comprise any additives.

In some embodiments of the present invention, the formulation may further comprise additional one or more metal complexes, which may act as further metal oxide precursors.

It is preferred that the total content of the metal oxide precursors contained in the formulation is in the range from 0.1 % to 50 % (w/w), preferably 0.5 % to 40 % (w/w), more preferably 1 % to 30 % (w/w), based on the total mass of the formulation.

In a preferred embodiment of the present invention, the formulation is an ink formulation being suitable for inkjet printing. Typical requirements for ink formulations are surface tensions in the range from 20 mN/m to 30 mN/m and viscosities in the range from 5 mPa s to 30 mPa s.

-Method for preparing a formulation

In another aspect, the present invention also relates to a method for preparing a formulation of the present invention, containing at least, essentially consisting of or consisting of, the following steps;

(X1 ) dissolving a metal oxide precursor in solvent 1 , preferably said solvent 1 is dry or water-free to form a metal oxide precursor solution;

(X2) optionally dissolving an acid in solvent 2, preferably said solvent 2 is dry or water-free, wherein said acid is selected from one or more members of the group consisting of sulfonic acids, amine hydrochlorides and carboxylic acids to form an acid solution;

(X4) mixing water and solvent 3 to form an aqueous solvent; and (X5) adding said aqueous solvent obtained in step (X4) to the alkoxide solution obtained in step (X1 ) or the metal oxide precursor solution obtained in step (X3);

(X6) optionally dissolving an acid and water in solvent 4, preferably said solvent 4 is dry or water-free, wherein said acid is selected from one or more members of the group consisting of sulfonic acids, amine hydrochlorides and carboxylic acids to form an acid solution to form an aqueous acid solution;

(X7) optionally adding said aqueous acid solution (X6) to the metal oxide precursor solution (X1 ), wherein said solvents 1 to 4 are independent from each other, selected from one or more members of the group consisting of propylene glycol monoalkyl ethers, preferably it is propylene glycol monomethyl ether (PGME), propylene glycol monoethyl ether and/or propylene glycol monopropyl ether; glycerol 1 ,3-dialkyl ethers, 1 ,3-dimethoxy-2-propanol, more preferably said solvent is selected from propylene glycol monoalkyl ethers, glycerol 1 ,3-dialkyl ethers, glycerol 1 ,3-dialkyl ethers; a secondary linear or branched >C3 alcohol, preferably it is selected from 2-propanol, 2- butanol, 2-pentanol, 3-pentanol, more preferably it is linear or branched secondary >C4 alcohol; preferably said solvent is selected from propylene glycol monoalkyl ethers, glycerol 1 ,3-dialkyl ethers, glycerol 1 ,3-dialkyl ethers.

Preferably, the total amount of the metal oxide precursor added in step (X1 ) is set to be in the range from 0.1 to 40wt.% based on the total amount of the obtained formulation, preferably it is in the range from 1 to 35wt%, more preferably from 2 to 30wt%, even more preferably from 3 to 28wt%. Preferably said metal oxide precursor is a metal alkoxide or a metal halide or a metal carboxylate containing a group 4 and/or group 5 element of the periodic table, more preferably said metal oxide precursor is a metal alkoxide containing a group 4 and/or group 5 element of the periodic table.

-Use

In another aspect, the present invention also relates to use of the formulation of the present invention for preparing an optical layer containing a metal oxide, preferably for preparing a composite, more preferably for preparing a layered composite. The present invention may also relate to use of the formulation of the present invention for preparing an encapsulation layer of an electronic device.

-Method for preparing a composite containing a metal oxide

In another aspect, the present invention also relates to a method for preparing a composite containing a metal oxide, preferably said metal oxide is selected from metal monoxide, metal dioxide or metal pentoxide, or a combination of these; comprising at least the following steps (a) and (b):

(a) providing the formulation of the present invention onto a surface of a substrate or a surface of an under layer placed on a substrate, preferably by wet deposition process, more preferably by spin-coating or ink-jetting, even more preferably by ink-jetting; and

(b) applying a thermal treatment to the formulation provided on the surface of the substrate or on the surface of the underlayer placed on the substrate to convert at least a part of the metal oxide precursor of the formulation to a metal oxide.

Preferably said composite being a layered composite, more preferably said layered composite is an optical layer or an encapsulation layer.

-Step (a) According to the present invention, said formulation may preferably be provided onto a surface of a substrate or a surface of an underlayer by wet deposition process. Said wet deposition process is drop casting, coating, or printing. A more preferred coating method is spin-coating, spray coating, slit coating, or slot-die coating. A more preferred printing method is flexo printing, gravure printing, inkjet printing, EHD printing, offset printing, or screen printing. Furthermore, preferred printing method is spray coating and inkjet printing and the most preferred one is inkjet printing.

Thus, in a preferred embodiment, the formulation is applied onto a surface of a substrate or a surface of an underlayer by spin-coating or ink-jetting in step (a). From a viewpoint of cost effective, ink-jetting can preferably be used.

In a preferred embodiment of the present invention, the formulation provided in step (a) of the method is an ink formulation being suitable for inkjet printing. Typical requirements for ink formulations are surface tensions in the range from 20 mN/m to 30 mN/m and viscosities in the range from 5 mPa s to 30 mPa s.

Depending on the specific problem to be solved, the formulation needs to be deposited either as a homogeneous, dense and thin layer covering the entire surface of the substrate or the entire surface of an underlayer by a coating method or the formulation needs to be deposited locally in a structured manner, thus requiring for a printing method. Both, coating and printing methods require formulations to be formulated in an adequate manner to comply with the physico-chemical needs of the respective coating and printing method as well as to comply with certain needs regarding the surface of the substrate to be coated or printed.

In a preferred embodiment of the method of the present invention, the surface of the substrate is pre-treated by a surface cleaning process. Preferred surface cleaning processes are silicon wafer cleaning processes such as described in W. Kern, The Evolution of Silicon Wafer Cleaning Technology, J. Electrochem. Soc., Vol. 137, 6, 1990, 1887-1892 and in New Process Technologies for Microelectronics, RCA Review 1970, 31 , 2, 185-454. Such silicon wafer cleaning processes include wet cleaning process involving cleaning solvents (e.g., isopropanol (IPA)); wet etching processes involving hydrogen peroxide solutions (e.g., piranha solution, SC1 , and SC2), choline solutions, or HF solutions; dry etching processes involving chemical vapor etching, UV/ozone treatments or glow discharge techniques (e.g., O2 plasma etching); and mechanical processes involving brush scrubbing, fluid jet or ultrasonic techniques (sonification). The surface of the substrate can also be pre-treated by salinization or an atomic layer deposition (ALD) process. The pre-treatment of the surface of the substrate serves to modify the hydrophobicity/hydrophi licity of the surface. This can improve the adhesion and filling characteristics of the optical metal oxide layer on the surface of the substrate.

In a more preferred embodiment, a wet cleaning process involving cleaning solvents (e.g., isopropanol (IPA)) is combined with one or more of a wet etching process involving hydrogen peroxide solutions (e.g., piranha solution, SC1 , and SC2), choline solutions, or HF solutions; dry etching process involving chemical vapor etching, UV/ozone treatments or glow discharge techniques (e.g., O2 plasma etching); and mechanical process involving brush scrubbing, fluid jet or ultrasonic techniques (sonification).

In a most preferred embodiment, a wet cleaning process involving cleaning solvents (e.g., isopropanol (IPA)) is combined with a mechanical process involving brush scrubbing, fluid jet or ultrasonic techniques (sonification) and with a wet etching process involving hydrogen peroxide solutions (e.g., piranha solution, SC1 , and SC2), choline solutions, or HF solutions; Thus, in a preferable embodiment, in step (a), the formulation is applied to a surface of a substrate or to a surface of an underlayer by spin-coating or ink-jetting.

In a preferable embodiment, the formulation is at least partly converted on the surface of the substrate or the surface of an underlayer to a composite, wherein said composite contains a metal oxide, preferably selected from metal monoxide, metal dioxide and/or metal pentoxide; and a metal oxide precursor.

In a preferable embodiment, the substrate is a patterned substrate comprising topographical features on the surface thereof. Said patterned structure of the substrate can be an underlayer placed over the substrate (e.g., an underlayer of a semiconducting device).

-Step (b)

It is believed that the formulation is at least partly converted in step (b) on the surface of the substrate or on the surface of the underlayer to a metal oxide to form a composite by exposure to thermal treatment. Said composite is preferably a layered composite. And said solvent is usually removed in step (b).

Preferred thermal treatment includes exposure to elevated temperature from 50 to 600 °C, preferably it is from 80 to 500°C, more preferably from 100 to 300°C. It is believed that applying a higher temperature such as in the range from 100 - 600°C, preferably 125-450°C, more preferably from 150-250 °C can realize an improved gap fill.

It is preferred to use the formulation containing a lower amount of the metal oxide precursor based on the total amount of the formulation in step (a) to realize an improved gap filing, for examples, in the range from 0.1 to 15wt% based on the total amount of the formulation, more preferably in the range from 1 to 10wt%, even more preferably from 2 to 8wt% based on the total amount of the formulation; and applying a higher temperature in step (b) such as in the range from 100 - 600°C, preferably 125-450°C, more preferably from 150-250°C to realize an improved gap fill.

Thermal treatment method in Step (b) is not limited to any specific thermal treatment methods or times. Depending on the type of substrate and formulation, a person skilled in the art is able to determine suitable thermal treatment methods.

In some embodiments of the method for preparing an optical metal oxide layer according to the present invention, a pre-baking step can be applied before step (b) after step (a) to remove the solvent of the formulation. The formulation can also be partly converted on the surface of the substrate to an optical metal oxide layer by pre-baking (soft baking) at a temperature from 40 to 150 °C, preferably from 50 to 120 °C, more preferably from 60 to 100 °C; then, baking of step (b) (hard baking, sintering or annealing) at a temperature from 100 to 600 °C, preferably from 125 to 450 °C, more preferably from 150 to 255 °C is applied.

Pre-baking (soft baking) serves the purpose to remove volatile and low boiling components such as, e.g., volatile and low boiling formulation media or additives from the drop casted, coated or printed films. Pre-baking is preferably carried out for a period of 1 to 60 minutes. After pre-baking, layers of substrate adhering films of metal oxide precursor or metal oxide precursor mixtures are obtained. The films may still comprise residual formulation media or additives.

In an alternative preferred embodiment of the method for preparing an optical metal oxide layer according to the present invention, pre-baking is omitted so that the formulation is converted in step (b) on the surface of the substrate or on the surface of the under layer to an optical metal oxide layer directly.

Baking (hard baking, sintering or annealing) serves the purpose to convert the metal oxide precursor or metal oxide precursor mixture layers on the substrate into a metal oxide layer. Moreover, the final properties of the metal oxide layer may be adjusted by the baking treatment. Baking is preferably carried out at the time in the range from 1 to 60m in, preferably 2 to 20 min, more preferably 3 to 10m in.

Said Pre-baking and baking (step (b)) may be carried out under ambient atmosphere or atmospheres with increased oxygen content in order to decompose unwanted organic components, which can lead to a lower activation energy when the composite is formed and is believed to improve the physical-chemical properties of the resulting layered composite material.

In a preferred embodiment of the method of present invention, the substrate or the underlayer is patterned comprising topographical features on the surface thereof, and the layered composite, preferably it is an optical layer, forms a coating layer covering the surface of the substrate and filling said topographical features. As a result, the topographical features are filled and levelled by said composition.

Preferred topographical features include, for example, gaps, grooves, trenches and vias. Topographical features may be distributed uniformly or non-uniform ly over the surface of the substrate. Preferably, they are arranged as an array or grating on the surface of the substrate. It is preferred that the topographical features have different lengths, widths, diameters as well as different aspect ratios. It is preferred that said topographical features have an aspect ratio of 1 :20 to 20:1 , more preferably 1 :10 to 10:1 . The aspect ratio is defined as width of structure to its height (or depth). From the viewpoint of dimension, the depth of the topographical features is preferably in the range from 10 nm to 10 pm, more preferably 50 nm to 5 pm, and most preferably 100 nm to 1 pm.

It is also preferred that the topographical features are inclined at a certain angle, such as an angle from 10 to 80°, preferably from 20 to 60°, more preferably from 30 to 50°, most preferably about 40°. Such inclined topographical features are also referred to as slanted or blazed topographical features.

It may be also necessary to fill topographical features locally with optical metal oxide layer, either completely or to a certain level, but not to cover adjacent surfaces of the substrate, where no topographical features to be filled are available.

The substrate is preferably a substrate of an optical device. Preferred substrates are made of inorganic or organic base materials, preferably inorganic base materials. Preferred inorganic base materials contain materials selected from the list consisting of ceramics, glass, fused silica, sapphire, silicon, silicon nitride, quartz, and transparent polymers or resins. The geometry of the substrate is not specifically limited, however, preferred are sheets or wafers.

In step (a) of the method, the formulation is applied onto a surface of a substrate or a surface of an underlayer, wherein said surface may be either a surface of a base material of the substrate or a surface of a layer of a material being different from the base material of the substrate, wherein such layer has been formed prior to applying said formulation.

In this way, sequences of different layers (layer stacks) can be formed on top of one another. Such layer stacks may be also structured, wherein such structures typically have dimensions on the nanometer scale, at least with respect to diameter, width and/or aspect ratio.

Thus, in a preferable embodiment, in step (b), the formulation is at least partly converted on the surface of the substrate to a composite, preferably it is being of a layered composite, by baking it at a temperature from 100 to 600 °C, preferably it is from 125 to 450°C, more preferably from 150 to 250°C.

In a preferred embodiment of the present invention, said thermal treatment of step (b) is applied at the time in the range from 1 to 60m in, preferably 2 to 20 min, more preferably 3 to 10m in.

In some embodiments, the formulation is at least partly converted on the surface of the substrate to a composite during the thermal treatment process of step (b), wherein said composite contains a metal oxide, preferably selected from metal monoxide, metal dioxide and/or metal pentoxide; and a metal alkoxide or a metal halide, metal oxo halide or metal carboxylate.

- Composite

In another aspect, the present invention relates to a composite, preferably being a layered composite, preferably said layered composite is an optical layer, obtained or obtainable by the method of the present invention.

In another aspect, the present invention relates to a composite, preferably being a layered composite, preferably said layered composite is an optical layer, derived from the formulation of the present invention.

In a preferred embodiment of the present invention, the composite comprises at least a metal oxide derived from the metal oxide precursor of the formulation and metal oxide precursor as a non-converted part of the formulation used in step (a) of the method.

The detail of the metal oxide precursor is indicated in the section of metal oxide precursor above.

- Optical device

The present invention further relates to an optical device comprising the composite of the present invention, which is prepared by using the formulation according to the present invention as described above.

It is preferred that the optical device is a device containing one or more optical components for forming a light beam including, but not limited to, gratings, lenses, prisms, mirrors, optical windows, filters, polarizing optics, UV and IR optics, waveguides and optical coatings. Preferred optical devices in the context of the present invention are waveguides for augmented reality (AR) device, for virtual reality (VR) device and/or for mixed reality (MR) device, or preferred optical devices are augmented reality (AR) glasses, virtual reality (VR) glasses and/or mixed reality (MR) glasses.

- Display device

Finally, the present invention relates to a display device comprising at least one functional medium configured to modulate a light or configured to emit light; and the composite, or an optical device of the present invention.

Examples of said display device is selected from a Liquid crystal display (LCD), Light emitting diode display (LED display), organic light emitting display (OLED), micro-LED display, quantum dot display (QLED), Augmented Reality (AR) hardware, Virtual Reality (VR) hardware, Mixed Reality (MR) hardware, plasma (PDP) display and an electroluminescent (ELD) display. Said AR, VR and MR hardware are also called as AR, VR and MR display. Preferably said display device is AR hardware, VR hardware or MR hardware.

The present invention is further illustrated by the examples following hereinafter which shall in no way be construed as limiting. The skilled person will acknowledge that various modifications, additions and alternations may be made to the invention without departing from the spirit and scope of the invention as defined in the appended claims.

Examples

- Analytics and measurement methods

Ellipsometry is used to determine layer thickness, refractive index (n) and absorption index (k) of a metal oxide layer. Measurements are performed using an ellipsometer M2000 from J. A. Woolam and three different angles of incidence (65°, 70 ° and 75°). The measurement data is analyzed with software CompleteEase from J. A. Woolam, assuming either full or almost nearly complete transparent behavior above a wavelength of 600 nm (at 560nm) and applying B-spline fitting for obtaining refractive indices (n) as well as absorption indices (k). The optical constants are averaged from three to four measured samples each of them providing a different layer thickness either after soft bake or after hard bake or after combined soft and subsequent hard bake.

All chemicals for synthesis described are purchased from Sigma Aldrich and used without further purification, unless differently mentioned elsewhere.

of formulations

Used

Zirconium tetra-n-butoxide for working examples 1 to 5;

Titanium tetra-n-butoxide for working examples 6-11 ;

Niobium pentaethoxide for working examples 12-14 The stock solution and the organic cosolvents are weighed into a glass bottle and the formulations are stirred for 30 minutes prior to use.

Working example 1 : Preparation of formulation Zr-1 Formulation Zr-1 : Zirconium tetra-n-butoxide with 17 weight-% nominal solid content.

Zirconium tetra-n-butoxide is added to PGME (1-methoxy-2-propanol: dried over molecular sieves) in a two neck Schlenk flask connected to inert gas/vacuum line under argon atmosphere. The solution is stirred at room temperature and a solution of H2O and methanesulfonic acid (MSA) in PGME is added dropwise at room temperature, affording a clear solution that is stirred for another 60 mins. While the solvent and amounts of starting materials are varied, mass concentration of metal alkoxide starting material is not changed.

The wt% and mol% of each component used for fabricating formulation Zr-1 are indicated in table 1 of Zr-1 .

Working examples 2-5: Preparation of formulations Zr-2 to Zr-5a,b Formulations Zr-2 to Zr-5a,b are prepared in the same manner as described in Working example 1 except for that the materials described in Table 1 (Zr-2 to Zr-5) are used with the described amounts instead of the materials in Working example 1.

Working examples 6-11 : Preparation of formulations Ti-1 to Ti-6 Formulations Ti-1 to Ti-6 are prepared in the same manner as described in Working example 1 except for that the materials described in Table 1 (Ti-1 to Ti-6) are used with the described amount instead of the materials in Working example 1 .

Working examples 12-14: Preparation of formulation Nb-1 to Nb-3 Formulations Nb-1 to Nb-3 are prepared in the same manner as described in Working example 1 except for that the materials described in Table 1 (Nb-1 to Nb-3) are used with the described amount instead of the materials used in Working example 1 .

Table 1 :

*Top layer: A continuous layer of metal oxide material formed across 50- 150 nm wide trenches of the substate

Gap fill: 50-150 nm wide trenches of the substrate are filled with metal oxide

Tit (°C/min): baking temperature/baking time

SC/IJP: Spin Coating/lnk Jet Printing

Working example 15: forming a layer

Sample Zr-1 is prepared by the following process.

Formulation Zr-1 from working example 1 (W.E.1 ) is spin coated onto an O2 plasma pretreated SislS /Si substrate having 150nm width trenches on the surface. Then the coated layer is baked at 200°C for 5m in. The obtained sample Zr-1 is observed by TEM analysis.

Samples Zr-2, Zr-3, Zr-4, Zr-5a, Zr-5b are prepared in the same manner as described in working example 15 above except for that the different conditions as described in Table 1 is applied. Namely, about Sample Zr-3, two samples (Sample Zr-3 (baking temperature 200°C/5min) and Sample Zr-3 (baking temperature 300°C/5min) are prepared. About sample Zr-4, three samples (Sample Zr-4 (baking temperature 100°C/5min), Sample Zr-4 (baking temperature 200°C/5min) and Sample Zr-4 (baking temperature 300°C/5min)

For sol-gel derived ZrO2, selectivity for top layer formation vs. gap filling can be controlled through three different parameters. a) Curing temperature b) Solvent: PGME vs. DM2P c) Nominal ZrO2 solid content of formulation In general, reactive building blocks contained in ZrO2 sol-gel formulations show a strong preference toward top layer deposition on a O2 plasma pretreated SisN4/Si substrate. This behavior is attributed to the high rate of condensation of reactive Zr-oxo clusters formed from the controlled hydrolysis of zirconium(IV) n-butoxide during formulation preparation. When kept in diluted form, gelling of acid stabilized sols containing nominal ZrO2 solid contents <8w% is a slow process (days to weeks) at ambient conditions. Spin-coating deposition on O2 plasma pretreated SisN4/Si substrate causes rapid evaporation of solvent and condensation of reactive Zr-oxo clusters with surface Si-OH groups, which is believed to result in a high local concentration and therefore rapid condensation into a macroscopic network on the substrate surface.

In line with the mechanistic picture of a rapidly forming macroscopic ZrO2 domain, the impact of curing temperature on the selectivity for gap fill vs. top layer formation was observed only for ZrO2 sols with nominal solid contents <3.7w%. The viscoelasticity of a top layer formed initially from spin-coating a 1 ,3w% ZrO2 sol appears sufficient to induce material flow into gaps for curing at T> 200°C whereas a top layer persists at T = 100°C. Increasing the solid content to >~3w% ZrO2, however, results in initial top layers from spin-coating that are not prone to a temperature induced flow. This description is in line with the finding that no gap filling has been found for <100 nm gap widths.

Fig. 7a, b shows Temperature dependence of top layer formation vs. gap filling taken from examples Zr-2 and Zr-3, respectively, for 1 ,3w% ZrO2 solid content. Namely, Fig. 7a shows a TEM image of Sample Zr-2 and Fig. 7b shows a TEM image of Sample Zr-3.

The overall rate at which reactive Zr-oxo species condense forming a macroscopic ZrO2 network can be controlled through the choice of solvent. Indication from experimental data is that a lower rate of condensation correlates with improved gap filling, and it can be affected by solvents that i) act as terminal ligands to metal sites at the surface of reactive Zr-oxo clusters, and ii) are pH-responsive. In general, a secondary alcohol is preferred. More specifically for ZrO2, substituting 1-methoxy-2-propanol (PGME) for 1 ,3-dimethoxy-2-propanol (DM2P) allows filling >100 nm gaps selectively at curing temperatures T = 100-300°C. However, this positive solvent effect on gap filling only holds for nominal ZrO2 solid contents <~3w%, otherwise top layer formation predominates.

Table 2 shows the result of TEM analysis. It also shows solvent dependence of top layer formation vs. gap filling taken from examples Zr-2 and Zr-4, respectively.

Table 2:

Table 3. shows Nominal ZrO2 solid content dependence of top layer formation vs. gap filling taken from examples Zr-5 and Zr-4, respectively.

Table 3:

Table 4 provides pertinent optical properties of <100 nm thick ZrO2 films.

Table 4. Optical properties of sol-gel derived ZrO2 thin films; ft = film thickness deposited on plane Si wafer; Net abs. = net absorption determined at 460 nm, normalized to 100 nm film thickness.

Working example 16: forming a layer

In working example 16, O2 plasma pretreated SisN4/Si substrate having 50nm width trenches, O2 plasma pretreated SisN4/Si substrate having 100nm width trenches and O2 plasma pretreated SisN4/Si substrate having 150nm width trenches, are provided for each case.

Then, samples Ti-1 (50nm width trenches), Ti-1 (100 nm width trenches), Ti- 1 (150 nm width trenches), Ti-2 (50nm width trenches), Ti-2 (100 nm width trenches), Ti-2 (150 nm width trenches), Ti-3 (50nm width trenches), Ti-3 (100 nm width trenches), Ti-3 (150 nm width trenches), Ti-4a (50nm width trenches), Ti-4a (100 nm width trenches), Ti-4a (150 nm width trenches), Ti-4b (50nm width trenches), Ti-4b (100 nm width trenches), Ti-4b (150 nm width trenches), Ti-5 (50nm width trenches), Ti-5 (100 nm width trenches), Ti-5 (150 nm width trenches), Ti-6 (50nm width trenches), Ti-6 (100 nm width trenches), Ti-6 (150 nm width trenches), are prepared in the same manner as described in working example 15 above except for that different conditions as described in Table 1 (Ti-1 , 2, 3, 4a, 4b, 5 and 6) is applied.

For sol-gel derived TiO2, selectivity for top layer formation vs. gap filling can be controlled mainly by two parameters. a) Solvent: PGME vs. DM2P b) Nominal TiO2 solid content of formulation

In general, reactive building blocks in TiO2 sol-gel formulations show variable reactivity allowing TiO2 deposition either in the form of a top layer forming or as a gap filling material. Addressing these complementary modes of deposition is possible through solvent choice, which controls the rate of condensation of reactive Ti-oxo clusters formed from initial controlled hydrolysis of titanium(IV) n-butoxide. In addition to the solvent, the nominal TiO2 solid content is the second parameter that impacts the rate of gelation of acid stabilized sols. While 1 -methoxy-2-propanol (PGME) allows stabilizing reactive TiO2 sols with <8w% nominal solid content for days to weeks at ambient conditions, substitution for 1 ,3-dimethoxy-2- propanol reduces sol shelf-live and requires a reduction of the nominal solid content to < 2.3w%.

Table 5. shows solvent dependence of top layer formation vs. gap filling taken from examples Ti-5 and Ti-1 , respectively.

Table 5:

Spin-coating deposition on O2 plasma pretreated SisN4/Si substrate causes rapid evaporation of solvent and condensation of reactive Ti-oxo clusters with surface Si-OH groups, presumably leading to a high local concentration favoring facile condensation into a macroscopic network on the substrate surface. The impact of solvent on condensation rate, which reflects in sol shelf life and selectivity of TiO2 deposition as a top layer or gap filler, presumably relates to the acid-base properties of Ti-oxo species. In contrast to PGME, DM2P binding to surface Ti(IV) sites of Ti-oxo clusters results in a two-fold increase of methoxy groups acting as potential yet spatially more remote proton acceptors. The net solvent impact is a reduced stabilizing effect of the acid component in reactive TiO2 sols, which further reflects in the lack of DM2P derived sols to gap fill of 50 nm wide trenches.

Table 6 highlights the impact of nominal solid content on top layer vs. gap fill selectivity for 8 and ~1w% TiO2 sol formulations. While selective gap filling was achieved from the 1w% TiO2 sol for all gap widths, selective top layer formation from the 8w% sol was achieved only for the narrowest gap, whereas partial filling in addition to top layer deposition was found for >100 nm wide trenches.

Table 6. shows nominal TiO2 solid content dependence of top layer formation vs. gap filling taken from examples Ti-3 and Ti-1 , respectively.

Table 6:

The intrinsically high refractive index of crystalline TiO2 in rutile and other morphologies also reflects in the optical properties of thin films deposited from reactive TiO2 sol formulations, pertinent data are collected in table 7 along with processing conditions. Table 7. Representative optical properties of sol-gel derived TiO2 thin films; ft = film thickness deposited on plane Si wafer; Net abs. = net absorption determined at 460 nm, normalized to 100 nm film thickness.

Working example 17: forming a layer

In working example 17, O2 plasma pretreated SisN4/Si substrate having 50nm width trenches, O2 plasma pretreated SisN4/Si substrate having 100nm width trenches and O2 plasma pretreated SisN4/Si substrate having 150nm width trenches, are provided for all cases.

Then, samples Nb-1 (50nm width trenches), Nb-1 (100 nm width trenches), Nb-1 (150 nm width trenches), Nb-2 (50nm width trenches), Nb-2 (100 nm width trenches), Nb-2 (150 nm width trenches), Nb-3 (50nm width trenches), Nb-3 (100 nm width trenches), Nb-3 (150 nm width trenches), are prepared in the same manner as described in working example 15 above except for that different conditions as described in Table 1 (Nb-1 , 2, and 3) is applied for each cases.

For sol-gel derived Nb2Os, selectivity for top layer formation vs. gap filling can be controlled by two parameters. a) Solvent: PGME vs. DM2P b) Nominal Nb2Os solid content of formulation

The chemical and deposition properties of Nb2Os derived sol formulations are largely congruent to those of TiO2, except for a more pronounced solvent impact on sol shelf life and top layer vs. gap fill selectivity, in line with the similar ionic radii of 6-coordinate Ti(IV) and Nb(V). In keeping the amount of acid at 5 mole-%, DM2P derived sols allow for only half the nominal Nb20s solid content that can be handled in PGME (~3w%).

Table 8. shows solvent dependence of top layer formation vs. gap filling taken from examples Nb-3 and Nb-1 , respectively.

Table 8:

Table 9. shows nominal Nb2Os solid content dependence of gap fill performance taken from examples Nb-2 and Nb-1.

Table 10 provides pertinent optical data of Nb20s sol derived thin films. Table 10. Optical properties of sol-gel derived Nb2Os thin films on quartz wafer; ft = film thickness; Net abs. = net absorption determined at 460 nm and normalized to 100 nm film thickness

For Nb-4 of the table, a formulation containing nominal solid content of

Nb2Os at 12.6wt% (28.8wt% Niobium pentaethoxide as metal alkoxide) is prepared based on the total amount of the formulation and used.

Claims

1 . Formulation for preparing an optical layer containing a metal oxide, preferably for preparing a composite, more preferably for preparing a layered composite, comprising at least;

- a metal oxide precursor containing a group 4 and/or group 5 element of the periodic table, preferably said metal oxide precursor is a metal alkoxide, a metal halide or a metal carboxylate containing a group 4 and/or group 5 element of the periodic table, more preferably said metal oxide precursor is a metal alkoxide containing a group 4 and/or group 5 element of the periodic table;

- a solvent, wherein the solvent is a secondary alcohol having no, one or two alkoxy groups or one or two alkyl groups where one or more non- adjacent groups of said alkyl group is replaced by oxygen atom, or secondary linear or branched >C3 alcohols, preferably it is selected from one or more members of 2-propanol, 2-butanol, 2-pentanol and 3- pentanol, preferably secondary linear or branched >C4 alcohols; preferably said solvent is selected from propylene glycol monoalkyl ethers, glycerol 1 ,3-dialkyl ethers, glycerol 1 ,3-dialkyl ethers or combination of propylene glycol monoalkyl ethers, glycerol 1 ,3-dialkyl ethers and glycerol 1 ,3-dialkyl ethers;

2. Formulation of claim 1 , wherein said metal oxide precursor is represented by following chemical formula (IV) or formula (V): M^b1 O₄(R^b1 R^b2R^b3R^b4) - (IV)

M^b2Os(R^b1 R^b2R^b3R^b4 R^b5) - (V) wherein M^b1 is a tetravalent metal of a group 4 element of the periodic table, preferably it is Ti or Zr;

M^b2 is a pentavalent metal of a group 5 element of the periodic table, preferably it is Nb or Ta;

R^b1 , R^b2, R^b3, R^b4 and R^b5 are each independently selected from H, D, a straight-chain alkyl group having 1 to 25 carbon atoms, preferably 1 to 15 carbon atoms, more preferably 1 to 10 carbon atoms; a branched or cyclic alkyl group having 3-25 carbon atoms, preferably 3 to 15 carbon atoms, more preferably 3 to 10 carbon atoms; an aryl group having 3 to 25 carbon atoms, preferably from 3 to 15 carbon atoms, more preferably 3 to 10 carbon atoms; a straight chain alkyl-cycloalkyl group having 4 to 25 carbon atoms, preferably 4 to 15 carbon atoms; branched chain alkyl-cycloalkyl group having carbon atoms 6 to 25 carbon atoms, preferably 6 to 15 carbon atoms; a straight chain alkyl-aryl group having 4 to 25 carbon atoms, preferably 4 to 15 carbon atoms; branched chain alkyl-aryl group having 6 to 25 carbon atoms, preferably 6 to 15 carbon atoms; where one or more non-adjacent CH2 groups and/or one or more adjacent CH2 groups of the above-mentioned groups may be replaced by oxygen atom, and where one or more H atoms may be replaced by D, CN or NO2; where each of groups may be substituted by one or more groups R^a;

R^a is at each occurrence, identically or differently, H, D, a straight chain alkyl or alkoxy group having 1 to 15 carbon atoms, preferably 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms; a branched or cyclic alkyl or alkoxy group having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms, more preferably 3 to 5 carbon atoms; a straight-chain alkenyl or alkynyl group having 2 to 15 carbon atoms, preferably 2 to 10 carbon atoms, more preferably 2 to 5 carbon atoms; a branched alkenyl group or alkynyl group having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms, more preferably 3 to 5 carbon atoms; an aromatic or heteroaromatic ring system having 5 to 15 aromatic ring atoms, preferably 5 to 10 aromatic ring atoms; where in each of the above-mentioned groups, one or more H atoms may be replaced by Dand where two or more adjacent substituents R^a here may optionally form a mono- or polycyclic, aliphatic ring system with one another.

3. Formulation of claim 1 or 2, contains the acid selected from one or more members of the group consisting of sulfonic acids, amine hydrochlorides and carboxylic acids.

4. Formulation of any one of preceding claims, said sulfonic acid is represented by following chemical formula (I).

R^a1SO₃H - (I) wherein

R^a1 is selected from the group consisting of a straight-chain alkyl group having 1 to 25 carbon atoms, preferably 1 to 15 carbon atoms, more preferably 1 to 10 carbon atoms; a branched or cyclic alkyl group having 3- 25 carbon atoms, preferably 3 to 15 carbon atoms, more preferably 3 to 10 carbon atoms; an aryl group having 3 to 25 carbon atoms, preferably from 3 to 15 carbon atoms, more preferably 3 to 10 carbon atoms; a straight chain alkyl-cycloalkyl group having 4 to 25 carbon atoms, preferably 4 to 15 carbon atoms; branched chain alkyl-cycloalkyl group having carbon atoms 6 to 25 carbon atoms, preferably 6 to 15 carbon atoms; a straight chain alkyl-aryl group having 4 to 25 carbon atoms, preferably 4 to 15 carbon atoms; branched chain alkyl-aryl group having 6 to 25 carbon atoms, preferably 6 to 15 carbon atoms; where one or more non-adjacent CH2 groups of the above-mentioned groups may be replaced by oxygen atom, C=O, C=S, C=Se, C=NH, SiH2, SO, SO2, OS, or CONH; and where one or more H atoms may be replaced by D, F, Cl, Br, I, CN or NO2; where each of groups may be substituted by one or more groups R^ax;

R^a2 is selected from the group consisting of N2H4, ammonia, hydroxylamine, imidazole and 1 ,4-diazabicyclo[2.2.2]octane; and/or said carboxylic acid is represented by following chemical formula (III) R^a3 - COOH wherein

R^a3 is selected from the group consisting of a straight-chain alkyl group having 1 to 25 carbon atoms, preferably 1 to 15 carbon atoms, more preferably 1 to 10 carbon atoms; a branched or cyclic alkyl group having 3- 25 carbon atoms, preferably 3 to 15 carbon atoms, more preferably 3 to 10 carbon atoms; an aryl group having 3 to 25 carbon atoms, preferably from 3 to 15 carbon atoms, more preferably 3 to 10 carbon atoms; a straight chain alkyl-cycloalkyl group having 4 to 25 carbon atoms, preferably 4 to 15 carbon atoms; branched chain alkyl-cycloalkyl group having carbon atoms 6 to 25 carbon atoms, preferably 6 to 15 carbon atoms; a straight chain alkyl-aryl group having 4 to 25 carbon atoms, preferably 4 to 15 carbon atoms; branched chain alkyl-aryl group having 6 to 25 carbon atoms, preferably 6 to 15 carbon atoms; where one or more non-adjacent CH2 groups of the above-mentioned groups may be replaced by oxygen atom, C=O, C=S, C=Se, C=NH, SiH2, SO, SO2, OS, or CONH and where one or more H atoms may be replaced by D, F, Cl, Br, I, CN or NO2; where each of groups may be substituted by one or more groups R^ax;

R^ax is at each occurrence, identically or differently, H, D, a straight chain alkyl or alkoxy group having 1 to 15 carbon atoms, preferably 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms; a branched or cyclic alkyl or alkoxy group having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms, more preferably 3 to 5 carbon atoms; a straight-chain alkenyl or alkynyl group having 2 to 15 carbon atoms, preferably 2 to 10 carbon atoms, more preferably 2 to 5 carbon atoms; a branched alkenyl group or alkynyl group having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms, more preferably 3 to 5 carbon atoms; an aromatic or heteroaromatic ring system having 5 to 15 aromatic ring atoms, preferably 5 to 10 aromatic ring atoms; where in each of the above-mentioned groups, one or more H atoms may be replaced by D, F, Cl, Br, I, and where two or more adjacent substituents R^ax here may optionally form a mono- or polycyclic, aliphatic ring system with one another.

5. Formulation of any one of preceding claims, wherein the acid is a amine hydrochloride represented by following chemical formula (II) HCI - R^a2 - (II) wherein

6. Formulation of any one of claims 1 to 4, wherein said acid is a sulfonic acid represented by following chemical formula (I):

R^a1SO₃H - (I) wherein

7. Formulation of any one of preceding claims, wherein the stoichiometric mole ratio of the acid and the metal oxide precursor is in the range from 0.01 : 100 to 120: 100, preferably the relative mole amount of the acid based on the total amount of the metal oxide precursor is in the range from 0.1 to 50, more preferably 1 to 10.

8. Formulation of any one of preceding claims, said solvent is selected from one or more members of the group consisting of propylene glycol monoalkyl ethers, preferably it is propylene glycol monomethyl ether (PGME), propylene glycol monoethyl ether and/or propylene glycol monopropyl ether; glycerol 1 ,3-dialkyl ethers, 1 ,3-dimethoxy-2-propanol, more preferably said solvent is selected from propylene glycol monoalkyl ethers, glycerol 1 ,3-dialkyl ethers or a mixture of any of them.

9. Formulation of any one of preceding claims, wherein the formulation contains water, and the stoichiometric amount of water is in the range from 100 to 400 mol% based on the total amount of metal oxide precursor, preferably the stoichiometric amount of water based on the total amount of the metal oxide precursor is in the range from 150 to 300 mol%, even more preferably from 180 to 270 mol%.

10. Method for preparing a formulation of any one of the preceding claims, containing at least the following steps;

(X1 ) dissolving a metal oxide precursor in solvent 1 , preferably said solvent 1 is dry or water-free solvent to form a metal oxide precursor solution;

(X2) optionally dissolving an acid in solvent 2, preferably said solvent 2 is dry or water-free solvent, wherein said acid is selected from one or more members of the group consisting of sulfonic acids, hydrochlorides and carboxylic acids to form an acid solution;

(X4) mixing water and solvent 3 to form an aqueous solvent; and

(X5) adding said aqueous solvent to the alkoxide solution obtained in step (X1) or the metal oxide precursor solution obtained in step (X3);

(X6) optionally dissolving an acid and water in solvent 4, preferably said solvent 4 is dry or water-free solvent, wherein said acid is selected from one or more members of the group consisting of sulfonic acids, hydrochlorides and carboxylic acids to form an acid solution to form an aqueous acid solution;

(X7) optionally adding said aqueous acid solution (X6) to the metal oxide precursor solution (X1 ), wherein said solvents 1 to 4 are independent from each other, selected from one or more members of the group consisting of propylene glycol monoalkyl ethers, preferably it is propylene glycol monomethyl ether (PGME), propylene glycol monoethyl ether and/or propylene glycol monopropyl ether; glycerol 1 ,3-dialkyl ethers, 1 ,3-dimethoxy-2-propanol, more preferably said solvent is selected from propylene glycol monoalkyl ethers, glycerol 1 ,3-dialkyl ethers, glycerol 1 ,3-dialkyl ethers; a secondary linear or branched >C3 alcohols, preferably it is selected from 2-propanol, 2-butanol, 2-pentanol, 3-pentanol, more preferably it is a linear or branched secondary >C4 alcohol; preferably said solvent is selected from propylene glycol monoalkyl ethers, glycerol 1 ,3-dialkyl ethers, glycerol 1 ,3-dialkyl ethers.

11 . Method for preparing a composite containing a metal oxide, preferably said metal oxide is selected from metal monoxide, metal dioxide or metal pentoxide, or a combination of these; comprising the following steps (a) and (b):

(a) providing the formulation of any one of claims 1 to 9 onto a surface of a substrate or a surface of an underlayer, preferably by wet deposition process, more preferably by spin-coating or an area selective printing, preferably said area selective printing is an ink-jetting, even more preferably by ink-jetting; and

(b) applying a thermal treatment to the formulation provided on the surface of the substrate or on the surface of an underlayer to convert at least a part of the metal oxide precursor of the formulation to a metal oxide.

12. Method according to any one of claim 11 , wherein the formulation is at least partly converted on the surface of the substrate to a composite, wherein said composite contains a metal oxide, preferably selected from metal monoxide, metal dioxide and/or metal pentoxide; and a metal alkoxide, metal halide, metal oxo halide or metal carboxylate.

13. A composite, preferably being a layered composite, preferably said layered composite is an optical layer, derived from the formulation of any one of claims 1 to 9.

14. An optical device comprising the composite of claim 13, and a substrate comprising a patterned surface or an uneven surface.

15. A display device comprising at least one functional medium configured to direct and modulate a light or configured to emit light; and the composite of claim 13, or an optical device of claim 14.