WO2010041278A1

WO2010041278A1 - A method for obtaining a coating of a metal compound onto a substrate, an apparatus and a substrate thereof

Info

Publication number: WO2010041278A1
Application number: PCT/IN2009/000555
Authority: WO
Inventors: Srinivasrao Ajjampur Shivashankar; Sanjaya Brahma
Original assignee: Indian Institute of Science IISC
Current assignee: Indian Institute of Science IISC
Priority date: 2008-10-06
Filing date: 2009-10-06
Publication date: 2010-04-15
Anticipated expiration: 2011-04-06

Abstract

The present invention relates to the method for obtaining adherent coating of metal compounds, onto a substrate. The present invention further relates to an apparatus for obtaining adherent coatings of metal compounds, onto a substrate. More particularly, the invention relates to the use of microwave irradiation of suitable chemical reactants in a solution to obtain coatings on surfaces of substrate of various shapes and sizes. The invention further also relates to the coated substrate comprising an adherent coating of a metal compound.

Description

A METHOD FOR OBTAINING A COATING OF A METAL COMPOUND ONTO A SUBSTRATE, AN APPARATUS AND A SUBSTRATE THEREOF FIELD OF THE EVVENTION

BACKGROUND AND PRIOR ART OF THE INVENTION

The formation of thin and thick coatings (usually called films) of a variety of materials is a major part of technology. For example, it forms the very foundation of computer hardware technology. That is, the fabrication of integrated circuits (VLSI), which are at the heart of personal computers, mobile phones, and solar cells, is based on the formation of thin films of various materials in a complex sequence. More mundane devices, such as cutting tools used in machining, use coating technology extensively. (The terms "thin film" and "coating" are used to mean the same article.)

As such, a great variety of techniques have been developed for thin film formation-a process that is usually called thin film deposition. These include "physical vapour deposition (PVD)" techniques, such as thermal and e-beam evaporation, sputtering, ion-beam deposition, plasma-spray coating, pulsed laser deposition (PLD), and molecular beam epitaxy (MBE). They also include chemical vapour deposition (CVD), atomic layer deposition (ALD), spin coating, spray pyrolysis, dip-coating, sol-gel, and electrochemical deposition. Many of these are employed regularly in the production of various devices for various applications. A large number of patents have been granted over the years to cover these techniques and devices made using them.

Many of the techniques listed above and used today involve vacuum systems that produce the low pressure needed for them to work. These tend to make the processes expensive and often relatively slow, and inefficient both in the use of the materials involved and in the use of energy. Some of the other techniques in the list above, such as dip and spin coating, do not require vacuum systems, but usually call for treatment at high temperature to obtain adherent coatings. Thus, such processes cannot be used to obtain coatings on the surfaces of plastics (polymers) because of their low melting point.

In many practical applications, it is necessary to apply a coating over large areas. These include modern semiconductor processing (microprocessor and cell phone devices), solar cells, and decorative coatings. While e-beam evaporation and sputtering are capable of providing coverage of large-area substrates, some other PVD processes such as PLD are not capable of the same. The PVD processes, in general, cannot provide "conformal coverage" of objects (substrates) of complex shapes. In other words, PVD processes are "line-of-sight" processes. As such, PVD processes can only provide acceptable coverage of one "flat" side of a substrate, and not of all of its contours and sides. For example, PVD cannot provide a coating of a spherical object. PVD is also a relatively slow process. That is, the coating rate can be rather low.

Chemical processes, such as CVD and dip coating, can provide coating of large-area substrates, as well as covering substrates of complex shape (conformal coverage). They can usually provide coatings at relatively high rates as well, which can translate into a lower cost of manufacturing. However, chemical processes generally require substrates to be maintained at elevated temperatures so that chemical reactions proceed at adequate rates. Thus, they cannot be used where the substrate is made of a low-melting material, such as a plastic (polymer).

What is therefore desirable is to have a process that is capable of coating large-area substrates of complex shapes at a high rate. In addition to coating high-melting substrates, the process must enable the coating of low-melting substrates, of the type that would be involved in the technology of "flexible electronics". Furthermore, such a process must be capable of providing coatings of "functional materials", such as complex oxides. Such a versatile process for coating is presently not available. The present invention overcomes the limitation associated in the prior art. Separate from the various processes so far developed in the prior art for thin film (coatings) deposition, there exist in the prior art various chemical methods, which work in the liquid (solution) medium, for the preparation of nanoparticles of different materials. Such methods usually depend on chemical reactions conducted under specific conditions in solution. One of them has employed microwave irradiation of metalorganic chemical reactants to produce nanoparticles of certain oxides. However, the potential of this method of formation of oxide nanoparticles to produce thin films of these oxides (or of other materials, such as sulphides) has not been recognized. This is probably because it is not at all obvious that, in a liquid chemical medium, films can be deposited, which adhere sufficiently well to the substrate. Sufficient adherence of the film to the substrate, which is a required in any practical application, generally depends on the substrate being at an elevated temperature, such as in the CVD process. Alternatively, it is achieved by using the "clean" environment of high vacuum, as in most practical methods of thin film preparation.

OBJECTIVES OF THE INVENTION

An objective of the present invention is to develop a method for obtaining an adherent coating of metal compound onto a substrate.

Another objective of the present invention is to develop an apparatus for adherent coating of a metal compound onto a substrate and to obtain the substrate coated thereof.

STATEMENT OF THE INVENTION

Accordingly, the present invention relates to a method for obtaining an adherent coating of metal compound onto a substrate, said method comprising steps of: a) dissolving a metalorganic compound in a solvent followed by stirring to obtain a precursor solution, and b) suspending the substrate to be coated in the solution and subjecting the solution to microwave irradiation to obtain a coating of metal compound onto the substrate; and an apparatus for an adherent coating of a metal compound onto a substrate, said apparatus comprising: a) a reaction chamber comprising microwave system to guide microwave irradiation on to a reaction vessel placed within the chamber, and b) a reaction vessel comprising the substrate in a solution of the metalorganic material for obtaining the coating of a metal compound onto the substrate. BRIEF DESCRIPTION OF ACCOMPANYING FIGURES

Figure 1: A schematic drawing of the apparatus for the microwave irradiation-assisted coating process, including the reaction vessel in which coating takes place.

Figure 2: A flowchart showing schematically the steps involved in the said coating process.

Figure 3: Scanning electron micrographs (SEM) of the coating of zinc oxide (ZnO) obtained on a Si (100) substrate using the process of the present invention, (a) Large- area SEM view and (b) Magnified SEM view

Figure 4: SEM of the cross-section of a coating of ZnO on Si(IOO), illustrating the uniform thickness and continuity of the coating.

Figure 5: X-ray diffraction pattern of a coating of ZnO on Si(IOO), confirming that it is made of crystalline ZnO, even though the coating process is conducted at a relatively low temperature.

Figure 6: A coating comprising ZnO nanorods grown on Ge(IOO) substrate (inset) and a low magnification image showing the large-area coating.

Figure 7: A coating comprising ZnO nanorods formed on ITO-coated glass substrate (inset) and a low-magnification image to show the large-area coating Figure 8: A coating comprising ZnO nanorods formed on acrylic (PMMA) substrate (inset) and a low-magnification image to show the large-area coating Figure 9: A coating comprising ZnO nanorods grown on Si (100) using cetyltrimethyl ammonium bromide (CTAB) as cationic surfactant. Inset provides a magnified view. Figure 10: A coating comprising ZnO thin film deposited on Si (100) (inset), low magnification image shows large area coating. No surfactant used.

Figure 11: A coating comprising ZnO nanorods grown on Si (100) using Zn(acac)₂.bipy as the starting precursor material. Inset provides a magnified view. Figure 12: A coating comprising Fe₂O₃ nanoparticles grown on Si (100) using Fe(acac)₃ as the starting precursor material. Inset provides a magnified view. The figure also shows the X-ray diffraction pattern, confirming that the coating comprises crystalline Fe₂O₃.

Figure 13: A coating comprising Ga₂O₃ nanoparticles grown on Si (100) using Ga(acac)₃ as the starting precursor material. Inset provides a magnified view. The figure also shows the X-ray diffraction pattern, confirming that the coating comprises crystalline Ga₂O₃.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for obtaining an adherent coating of a metal compound onto a substrate, said method comprising steps of: a. dissolving a suitable metalorganic compound in a solvent followed by stirring to obtain a precursor solution; and b. suspending the substrate to be coated in the solution, and subjecting the solution to microwave irradiation, to obtain an adherent coating of the desired metal compound onto the substrate.

In an embodiment of the present invention, the metal compound (to be coated) is selected from a group comprising metal oxides, non-oxides, metal sulphides, metal oxysulphides, oxy-chalcogenides, nitrides, oxynitrides, metals or metal alloys, or any combination thereof , and the metalorganic compound, acting as the "precursor" is selected from a group comprising metal acetates, metal beta-diketonates, metal alkoxides, metal amines, and thio-derivatives of metal complexes, and other suitable compounds of one or more metals, which are soluble in a liquid dielectric solvent. The dielectric solvent may be chosen from a group comprising, but not limited to, water, alkanes, alcohols, aromatics, or a combination thereof.

In another embodiment of the present invention, the precursor solution contains a surfactant.

In still another embodiment of the present invention, the surfactant is helpful in preventing agglomeration of solvent particles during irradiation.

In still another embodiment of the present invention, the surfactant is dissolved in distilled water to obtain a dilute solution having concentration ranging from about O. lmM to about 5mM. In still another embodiment of the present invention, the substrate is selected from a group comprising silicon, glass, alumina, fused quartz or polymeric plastic.

In still another embodiment of the present invention, the substrate optionally comprise a thin coating of an electrically conducting material, the thickness of said coating not exceeding 100 micrometers.

In still another embodiment of the present invention, the substrates are of arbitrary shape and size, with or without lithographed features.

In still another embodiment of the present invention, the substrates are either nonconducting or semiconducting substrates.

In yet another embodiment of the present invention, the substrates, such as silicon, glass, alumina, fused quartz, or polymeric plastic, each substrate optionally having been coated first with a thin adherent layer of an electrically conducting substance such as a metal or a metal alloy, or an electrically conducting metal compound.

In still another embodiment of the present invention, the source of the microwave irradiation is provided by reaction chamber comprising of microwave system.

In still another embodiment of the present invention, the microwave system comprises of domestic or industrial-type microwave ovens, or apparatus.

In still another embodiment of the present invention, the microwave system provides irradiation having frequency of about 900 MHz to about 10 GHz.

In still another embodiment of the present invention, the method comprises heat treatment of the coated substrate at a temperature of about 500⁰C for a period of about less than 5 minutes to remove residual surfactant from the coated substrate.

In still another embodiment of the present invention, the thickness of the coating ranges from about 100 nanometers to about 25 micrometers. The present invention relates to an apparatus for an adherent coating of a metal compound onto a substrate, said apparatus comprising: a. a reaction chamber comprising microwave system which guides microwave irradiation on to a reaction vessel placed within the chamber; and b. a reaction vessel comprising the substrate in a solution of the metalorganic material for obtaining an adherent coating of the desired metal compound onto the substrate.

In an embodiment of the present invention, a reflux system is placed outside the reaction chamber.

In an embodiment of the present invention, the reflux system is a water-cooled condenser.

In an embodiment of the present invention, the reaction vessel is transparent to microwaves provided by the microwave system.

The present invention relates to a substrate comprising an adherent coating of a metal compound.

In another embodiment of the present invention, the substrate is a semi-conductor or a dielectric material selected from a group comprising silicon, glass, alumina, fused quartz or polymeric plastic. In another embodiment of the present invention, the substrate optionally comprise a thin coating of an electrically conducting material, the thickness of said coating not exceeding 100 micrometers.

The present invention provides a method for coating a variety of different materials onto a variety of substrates at a low temperature, using chemical reactions assisted by microwave irradiation.

In another embodiment, the method utilizes a suitable solution in a liquid form containing chosen chemical reactions, so that chemical reactions occur upon said irradiation.

In another embodiment of the present invention, the method provides for coating of a thin or thick film of the desired material by immersing the substrate in the said solution during the said microwave irradiation.

In another embodiment, the method provides for coating a substrate of large and arbitrary physical size, so that the entire surface is covered uniformly with a coating of the desired material.

In another embodiment, the method provides for coating at a rapid rate, and for alteration in the chemical composition of the desired coating material by altering the composition of the liquid solution suitably.

In another embodiment, the method provides to control the microstructure of the material in the coating by choosing processing conditions, such as the molecular structure of the chemicals, the concentration of the chemicals in the solution, the solvent used to form the solution and its amount, and the duration of said irradiation.

In another embodiment, the method employs chemicals known as "surfactants" in the solution to control the microstructure of the material in the coating. In another embodiment, the method chooses appropriate surfactant to control the shape and size of the microscopic particles which comprise any such coating.

In another embodiment, the method provides coating in such a manner that fine lithographed features present in a suitably prepared substrate are covered uniformly by the coating material in a "conformal" manner, .i.e., conforming to the contours of the lithographed feature.

The present invention provides the apparatus and method for fabricating thin or thick coatings of a desired material, such as metal oxides, metals, composites of a metal and its oxide, metal sulphides, metal oxysulphides, etc. on a substrate, by subjecting an appropriately prepared liquid solution to microwave irradiation. The apparatus comprises a domestic-type microwave oven or, more generally, a reaction chamber in which the reaction vessel containing the reactants in a solution may be placed, together with the substrate in which the coating is to be applied. The method consists of taking a liquid solution in which the reactants are dissolved in a suitable solvent, together with a surfactant (if any) in a suitable vessel, and subjecting it to microwave radiation of chosen power and duration. The coating formed on the substrate due to the irradiation may have to be heated briefly to an elevated temperature (outside the microwave apparatus) to remove residue (if any) of the surfactant.

The present invention describes a microwave irradiation-assisted chemical process, which is conducted in a liquid solution of chemical reactants in a dielectric solvent, whereby a coating of desired thickness of the desired material is obtained rapidly on an electrically non-conducting or semiconducting substrates of arbitrary shape and size. Coatings may also be obtained on a substrate, which has a thin coating of an electrically conducting substance, such as a metal, metal alloy, a metal oxide, a metal nitride, a metal sulphide, etc.

The present invention provides a method for coating substrates of large sizes (and small sizes) of arbitrary shape with a desired material, at a rapid rate, through the microwave irradiation of a solution containing appropriate chemical reactants in a suitable liquid solvent. More specifically, the present invention provides a method and related apparatus for the coating of a substrate that is kept in the solution. The apparatus is illustrated schematically in Figure 1. It consists of a chamber into which microwave radiation can be "guided", using such devices as waveguides. Microwave radiation spans the frequency range from about 900 MHz to greater than 10 GHz, and is divided into various bands, such as the Q-band, etc. Many of these frequencies are reserved for communication channels, and certain frequencies, e.g., 2.45 GHz is reserved for other uses, such as domestic and industrial applications. Fig. 1 represents a chamber into which microwave radiation of such an allowed frequency is guided. The reaction vessel is made of a material that is transparent to microwaves, such as glass or plastic. The vessel may not be made of a metal or an alloy or any electrical conductor, as they are not transparent to microwaves.

For example, the domestic-type microwave oven can be used as the reaction chamber, as illustrated in the present invention. However, this is not a limitation. The process can be scaled up both to coat substrates of a large size and to coat many substrates in s a single coating process. To scale up the process, industrial-type microwave ovens (reactors) are used. These are available and have been in use for other applications, such as processing polymers and ceramics. Specialised microwave systems can be built and used, and the technology for this is available. The present invention involving the method for coating does not depend on the specific reactor configuration. It may be noted that the domestic-type and industrial-type microwave ovens generally available are of the so-called "multi-mode" type, referring to the fact that many electromagnetic "modes" of microwaves are present in such apparatus. It is also possible to use specialised "mono-mode" microwave systems for the application of coatings described in this invention. Such apparatus uses a single electromagnetic mode of microwaves. The present invention involving the method for coating does not depend on the specific reactor configuration, or on whether the microwaves are multi-mode or mono-mode. The reaction vessel contains a solution, of which the solvent is a microwaves- absorbing dielectric liquid such as (but not limited to) ethanol, methanol, decanol, water, n-hexane, etc. The choice of this solvent depends on the solute used to make the solution and carry out the coating reaction. The solute is a chemical compound (or compounds) containing the elements which comprise the material to be coated. The solution may also contain a chemical called the surfactant, which is employed to control the microstructure and other aspects of the material in the coating. Finally, the apparatus includes the substrate on which the desired coating is to be made. This is usually made of a semiconducting or insulating material, such as silicon, glass, alumina, fused quartz, or a polymeric plastic (such as acrylic or nylon). A number of different substrates of different shapes and sizes can be coated. The reason the process is capable of coating substrates of any shape is that the substrates are immersed (suspended) in a liquid, which therefore surrounds the substrate. As a result, chemical reaction occurs on the entire surface of the substrate, leading to a coating. In a typical thin film process, the substrates are PLACED ON A FLAT SURFACE in the apparatus. So, the underside of the substrate does not get coated. These are usually "line-of-sight" processes. The process of the present invention is similar to electrochemical plating in the limited sense that the substrate is suspended in a liquid, and all contours are coated (for example, electrochemical gold coating of jewelry by electroplating). Thus, in the process of the present invention, the solution penetrates crevices and other openings in the substrate, thus coating inside these crevices as well. This results in "conformal coverage" of any lithographed features in the substrate, including those of very fine dimensions, of the order of a micrometer or less. Further, in many applications, it is desirable to coat both sides of a flat substrate in "one shot". The present invention does this effectively, as both sides of such a substrate are exposed to the chemical solution. Therefore, the present process is more efficient, and thus less expensive in coating both sides of a substrate than a "line-of-sight" process. The size of the substrate to be coated is limited only by the size of the reactor. Large industrial reactors are available, and these can be used to coat large substrates.

In another embodiment of the present invention, some of the chemical reactions involved in the present invention are described below: Schematic of the chemical reactions: Formation of ZnO coating from the zinc metalorganic complex, Zn(acac)₂, where 'acac' is the abbreviation for the ligand acetylacetone

Schematic of the chemical reactions: Formation of ZnO coating from Zn(acac)₂ bipyridyl, wherein the complex Zn(acac)₂ is further adducted with the "bipyridiF moeity

Schematic of the chemical reactions: Formation of ZnO coating from Zn(acac)₂ phenanthroline, wherein the metalorganic complex Zn(acac)₂ is adducted with the "phenanthroline" moeity

Schematic of the chemical reactions: Formation of ZnO coating from the metalorganic complex Zinc acetate dihydrate

Schematic of the chemical reactions: Formation of ZnO coating from Zn (dpm)₂, where 'dpm' stands for dipivaloylmethanate, of the ligand 2,2,6,6-tetramethyl-3,5- heptanedione

^• •

Schematic of the chemical reactions: Formation of ZnS coating from Zn thio(dpm)₂, wherein the oxygen atom in the 'dpm' ligand is replaced by sulphur

In another embodiment of the present invention, figure 2 represents the method or the process of the present invention, which leads to a coating of the desired material on the substrate chosen and placed in the solution, as shown schematically in Fig. l. A solution of the chemical compound(s) that would react (together) under microwave irradiation to give a coating of the desired material, which solution is prepared to the desired concentration using a suitable solvent, is taken in a reaction vessel of suitable shape and size. The compounds may be metalorganic complexes, metal acetates, metal alkoxides, thio-derivatives of metal complexes, or any other metal compounds which dissolve in a solvent of the kind described above. The quantity of solution taken depends on the size o of the vessel and the thickness of the desired coating. The solution may not contain a surfactant. The solution may also comprise a surfactant, such as poly(vinylpyrrolidone), in an appropriate concentration. The solution is then subjected to microwave irradiation of a power and duration that results in the desired coating on the substrate. After irradiation is complete, the substrate is removed from the reaction chamber, to examine the coating and to use it for the intended purpose. If a surfactant has been used in the coating process, it may become necessary to heat the substrate briefly to about 500⁰C to remove any leftover surfactant material from the coating.

In another embodiment of the present invention, figures 3 and 4 show scanning electron micrographs (SEM) of ZnO coatings obtained on Si(IOO) by the process described in this invention, indicating the continuity and uniformity of the coating. Further, Fig.4 provides a measurement of the thickness of the ZnO coating. Figure 5 shows the X-ray diffraction pattern of the coating, which is in complete agreement with the pattern known for ZnO. Specifically, Fig.5 shows that the ZnO coating is crystalline in character, even though it has been prepared by a low-temperature process. This may be asserted because the temperature of the solution during microwave irradiation does not exceed about 100⁰C, and the treatment at an elevated temperature of 500⁰C (after the deposition) is too brief to lead to crystallinity. It is thus inferred that the coating is crystalline as prepared. Such crystallinity is important in many practical applications of ZnO and other materials.

In another embodiment of the present invention, treatment at an elevated temperature is not necessary in all cases, and is not required at all where no surfactant is used. This is especially important for coatings on polymer (plastic) substrates, which cannot withstand elevated temps. Treatments at elevated temperatures help crystallization and are carried out for two specific reasons: 1) to improve crystallization and 2) to remove the residual surfactant so that the coated material is as pure as possible. If no surfactant is used, no heat treatment is usually required, and is therefore more advantageous.

DETAILED STEP-BY-STEP PROCESS FOR COATING A SUBSTRATE

In another embodiment of the present invention, the generalized detailed procedure of coating involved in the present invention is illustrated as given below. These steps are further elaborated with the help of examples given below.

As the present invention is further elaborated by the following steps, these steps should not be construed to limit the scope of the invention.

Step 1:

One gram of high-purity (at least 99%) metal precursor (metalorganic complex) is taken in a round-bottomed flask and dissolved in a suitable dielectric solvent (HPLC- grade desirable, but lower grade is sufficient) and stirred for about fifteen minutes.

Step 2 (optional step)

A dilute solution of a surfactant in double-distilled water is prepared separately, and added to the precursor solution, followed by about fifteen minutes of stirring. The surfactant is helpful in preventing agglomeration of particles that might result from the microwave irradiation.

Step 3: A suitable substrate is suspended in the solution containing metalorganic complex (optionally along with surfactant) taken in a round-bottomed flask, or other suitable vessel.

Step 4: The round-bottomed flask is then placed in a domestic-type microwave oven (operating at 2.45 GHz, with variable power) or an industrial-type microwave reactor (with variable power), equipped with a water-cooled condenser (reflux system) placed outside the microwave oven (as shown in Figure 1). Microwave radiation is switched on, and the solution subjected to microwave radiation at a suitable power, and for a suitable duration of time.

Step 5: After irradiation, the substrate is removed carefully from the solution and washed with distilled water and acetone. A visible, adherent coating will be found on the substrate. Optionally, the substrate with the coating is heated for a few minutes in air at 500 ⁰C to remove any residual surfactant that might be present. The coated substrate may or may not need post-synthesis annealing, depending on the solvent and surfactant used.

The present invention is further elaborated by the following examples and figures. However, these examples should not be construed to limit the scope of the invention.

EXAMPLE 1 ZnO COATING ON Si(IOO) SUBSTRATE

To prepare a coating of zinc oxide (ZnO) on the single-crystalline substrate of silicon, Si(IOO), the compound of zinc used is a metalorganic complex of zinc, e.g., zinc acetylacetonate. A solution is prepared with one gram of zinc acetylacetonate dissolved in 40 ml of ethanol and stirred well. To this is added a solution of about 0.3 gram of the surfactant poly(vinylpyrrolidone) in about 40 ml of water, and stirred well. This solution is taken in a glass vessel in which is placed a Si(IOO) substrate measuring about 20 mm x 20 mm. The vessel is placed in a microwave oven (reaction chamber) and microwave power at 2.45 GHz turned on at about 800 W. The power is maintained for about 5 minutes and then turned off. The substrate is then taken out of the vessel and heated to about 500⁰C in air for about 10 minutes. A coating of ZnO measuring about 2 micrometers in thickness will then be found on the substrate.

EXAMPLE 2

ZnO COATING ON AN ACRYLIC SUBSTRATE Conditions as described in Example 1 above are used, except that the substrate is made of acrylic [poly(methyl methacrylate), usually abbreviated as PMMA] measuring about 20 mm x 20 mm in size, and a thickness of about 1 mm. The deposition process as in Example 1 is repeated to get a coating on the substrate. To remove the residual surfactant from the coating, the substrate is heated rapidly and briefly (less than one minute) in air to about 500⁰C (using, for example, halogen lamps), so that the polymer plastic substrate (acrylic) does not melt or deform. The result is a uniform coating crystalline ZnO on the acrylic substrate measuring about two micrometers in thickness.

EXAMPLE 3

A COATING OF ZnO NANORODS GROWN ON Ge(IOO) SUBSTRATE Conditions as described above in Example 1 are used, except that the substrate is single-crystalline germanium, Ge (100). The deposition process as in Example 1 is repeated to get a coating on the substrate. To remove the residual surfactant from the coating, the substrate is heated rapidly and briefly (less than 5 minutes) in air to about 500⁰C in air. The result is a uniform coating crystalline ZnO on the Ge substrate measuring about one micrometer in diameter (Figure 6).

EXAMPLE 4

A COATING OF ZnO NANORODS GROWN ON ITO-COATED GLASS

Conditions as described above in Example 1 are used, except that the substrate is indium tin oxide-coated glass (ITO-coated glass). The deposition process as in Example 1 is repeated to get a coating on the substrate. To remove the residual surfactant from the coating, the substrate is heated rapidly and briefly (less than 5 minutes) in air to about 500⁰C. The result is a uniform coating crystalline ZnO on the ITO-coated glass substrate measuring about two micrometers in thickness (Figure 7).

It must be noted that ITO is an electrically conducting, transparent coating, useful in devices such as photovoltaics. It must also be noted that, in general, electrical conductors reflect microwaves (and other electromagnetic) radiation. Hence, it is not at all obvious that the process invented and described here would be successful in coating conducting substrates. Coatings of ZnO may be similarly obtained on substrates such as soda glass and Si(IOO), which are coated with a thin layer of a metal such as aluminium or chromium. EXAMPLE 5

A COATING OF ZnO NANORODS GROWN ON ACRYLIC (PMMA)

SUBSTRATE

Conditions as described above in Example 1 are used, except that the substrate is a thin (flexible) plate of acrylic, as in EXAMPLE 2, and the surfactant is cetyltrimethyl ammonium bromide (CTAB) which is a cationic surfactant. The synthesis conditions described in Example 1 are used, and involves Zn(acac)₂ (5 mmole = 1.31 g) dissolved in 40 ml of absolute ethanol and stirred for -15 minutes. A previously prepared aqueous solution of 2 mmole (-0.728 g) of cetyltrimethyl ammonium bromide (CTAB) was added to the precursor solution and stirred for 15 minutes. The deposition process as in Example 1 is repeated to get a coating on the substrate. No post-synthesis annealing is required when CTAB is used as surfactant. Highly oriented ZnO nanorods are grown (Figure 8). Coating of ZnO on a flexible polymer substrate, such as that of PMMA, is technologically important "Flexible Electronics". It is not possible to obtain a large- area, crystalline coating of a metal oxide such as ZnO on PMMA at a temperature low enough not to melt PMMA. Thus, the process of Example 5 is suitable for polymer and other substrates, which cannot be exposed to high temperatures.

EXAMPLE 6

A COATING OF ZnO NANORODS GROWN IN Si(IOO) USING A CATIONIC SURFACTANT (CTAB)

The synthesis conditions described in Example 1 are used, and involves Zn(acac)₂ (5 mmole = 1.31 g) dissolved in 40 ml of absolute ethanol and stirred for -15 minutes. A previously prepared aqueous solution of 2 mmole (-0.728 g) of cetyltrimethyl ammonium bromide (CTAB) was added to the precursor solution and stirred for 15 minutes. The deposition process as in Example 1 is repeated to get a coating on the substrate. The substrate was cleaned repeatedly by distilled water and acetone and finally dried in air. No post-synthesis annealing is required when CTAB is used as surfactant (Figure 9). EXAMPLE 7

ZnO THIN FILM OF Si(IOO) (WITHOUT THE USE OF ANY SURFACTANT)

The synthesis conditions described in Example 1 are used but no surfactant is used here. The deposition process is very simple where the precursor is dissolved in the solvent decanol followed by irradiation of microwaves. About one gram of Zn(acac)₂ and -80 ml of decanol are taken in a round bottom flask, stirred for about 30 minutes and subjected to microwave irradiation at 800 W for about 5 minutes. The substrate was washed with distilled water and acetone and dried in air. No post-synthesis annealing is required (Figure 10).

EXAMPLE 8

ZnO NANORODS GROWN IN Si(IOO) USING AN ADDUCTED PRECURSOR OF Zn(acac)₂ bipy

Conditions as described above in Example 1 are used, except that the starting precursor material is the adducted precursor of Zn(acac)₂. The starting precursor material is adducted with a 2, 2' bipyridyl ligand and the molecular formula is (C₁₀H₁₄O₄. Zn. CioH₈N₂), denoted by Zn(acac)₂(bipy). The deposition process as in Example 1 is repeated to get a coating on the substrate. To remove the residual surfactant from the coating, the substrate is heated rapidly and briefly (less than 5 minutes) in air to about 500⁰C in air. The result is a uniform coating crystalline ZnO on the Si(IOO) substrate measuring about one micrometer in diameter (Figure 11). As this Figure shows, the process and the precursor of Example 8 illustrates the capability of the present invention to yield coatings of a material with controlled microstructure (tapered ZnO nanorods, in this case).

EXAMPLE 9

Fe₂O₃ NANOPARTICLE THIN FILM ON Si(IOO)

The thin film deposition process of the present invention has the capability to deposit thin films or coating of various metal oxides. To prepare a coating of iron oxide (Fe₂O₃) on a substrate of Si(IOO), the compound of iron used is a metalorganic complex of iron, e.g., iron acetylacetonate, (Fe (CsH₇O₂)₃), designated by Fe(acac)₃. A solution is prepared with one gram of iron acetylacetonate dissolved in 40 ml of methanol and stirred well. To this is added a solution of about 0.3 gram of the surfactant poly(vinylpyrrolidone) (raw - 360000) in about 40 ml of water, and stirred well. This solution is taken in a glass vessel in which is placed a Si(IOO) substrate measuring about 20 mm x 20 mm. The vessel is placed in a microwave oven (reaction chamber) and microwave power at 2.45 GHz turned on at about 800 W. The power is maintained for about 5 minutes and then turned off. The substrate is then taken out of the vessel and heated to about 500⁰C in air for about 10 minutes. A coating of Fe₂O₃ measuring about -150 nm in thickness will then be found on the substrate (Figure 12).

EXAMPLE 10

Ga₂O₃ NANOPARTICLE THIN FILM ON Si(IOO). The condition reported in Example 9 may be used, except that the starting precursor material is Ga(acac)3. The method is repeated and nanoparticle thin film of Ga₂O₃ is deposited on Si(IOO). To remove the residual surfactant from the coating, the substrate is heated rapidly and briefly (less than 5 minutes) in air to about 500⁰C in air. The result is a uniform coating crystalline Ga₂O₃ on the Si(IOO) substrate measuring about one micrometer in thickness (Figure 13).

EXAMPLE 11

THIN FILM OF THE FERRITE ZnFe₂O₄

The conditions cited in Example 9 may be employed, except that the precursor solution is made of both Zn(acac)₂ and Fe(acac)₃, taken in 1:2 molar proportion, and dissolved in methanol and stirred well. To this is added a solution of about 0.3 gram of the surfactant poly(vinylpyrrolidone) (mw - 360000) in about 40 ml of water, and stirred well. This solution is taken in a glass vessel in which is placed a Si(IOO) substrate measuring about 20 mm x 20 mm. The vessel is placed in a microwave oven (reaction chamber) and microwave power at 2.45 GHz turned on at about 800 W. The power is maintained for about 5 minutes and then turned off. The substrate is then taken out of the vessel and heated to about 500⁰C in air for about 10 minutes. A coating of crystalline zinc ferrite, ZnFe₂O₄ will be found on the substrate.

Similarly, a thin film of a perovskite, such as BaTiO₃, may be formed by subjecting a solution of a metalorganic complex of strontium (Sr) and a metalorganic complex of titanium (Ti), taken in 1:1 molar proportion, in a solvent such as methanol, and subjecting the solution to microwave irradiation, in the presence of a substrate.

Claims

We Claim

1) A method for obtaining an adherent coating of a metal compound onto a substrate, said method comprising steps of: a. dissolving metalorganic compound in a solvent followed by stirring to obtain a precursor solution; and b. suspending the substrate to be coated in the solution and subjecting the solution to microwave irradiation to obtain a coating of metal compound onto the substrate.

2) The method as claimed in claim 1, wherein the metal compound is selected from a group comprising metal oxides, metal sulphides, metal nitrides, metal oxy-chalcogenides and oxynitrides or any combination thereof.

3) The method as claimed in claim 1, wherein the metalorganic compound is selected from a group comprising metal acetates, metal alkoxides, metal beta- diketonates and thio-derivatives of metal complexes or any combination thereof. 4) The method as claimed in claim 1, wherein the solvent is a dielectric solvent.

5) The method as claimed in claim 4, wherein the dielectric solvent is selected from a group comprising, but not limiting to methanol, ethanol, decanol, n- hexane and water or any combination thereof.

6) The method as claimed in claim 1, wherein the precursor solution contains a surfactant.

7) The method as claimed in claim 6, wherein the surfactant is helpful in preventing agglomeration of solvent particles during irradiation.

8) The method as claimed in claim 1, wherein the surfactant is dissolved in distilled water to obtain a dilute solution having concentration ranging from about 0.1 mM to about 5mM.

9) The method as claimed in claim 1, wherein the substrate is a semiconductor or a dielectric material selected from a group comprising, but not limiting to silicon, glass, alumina, fused quartz and polymeric plastic

10) The method as claimed in claim 9, wherein the substrate optionally has placed on it a thin coating of an electrically conducting material, with thickness of the coating not exceeding 100 micrometers.

1 1) The method as claimed in claim 9, wherein the substrates are of arbitrary shape and size, with or without lithographed features. 12) The method as claimed in claim 1, wherein source of the microwave irradiation is provided by reaction chamber comprising of microwave system.

13) The method as claimed in claim 12, wherein the microwave system comprises of domestic or industrial-type microwave ovens. 14) The method as claimed in claim 12, wherein the microwave system provides irradiation having frequency of about 900 MHz to about 10 GHz.

15) The method as claimed in claim 1, wherein said method comprises heat treatment of the coated substrate at a temperature of about 500⁰C for a period of about less than 5 minutes to remove residual surfactant from the coated substrate.

16) The method as claimed in claim 1, wherein thickness of the coating ranges from about 100 nanometers to about 25 micrometers.

17) An apparatus for an adherent coating of a metal compound onto a substrate, said apparatus comprising: a. a reaction chamber comprising microwave system to guide microwave irradiation on to a reaction vessel placed within the chamber; and b. a reaction vessel comprising the substrate in a solution of the metalorganic material for obtaining the coating of a metal compound onto the substrate. 18) The apparatus as claimed in claim 17, wherein a reflux system is placed outside the reaction chamber.

19) The apparatus as claimed in claim 18, wherein the reflux system is a water- cooled condenser.

20) The apparatus as claimed in claim 17, wherein the microwave system comprises of domestic or industrial-type microwave ovens.

21) The method as claimed in claim 17, wherein the microwave system provides irradiation having frequency of about 900 MHz to about 10 GHz.

22) The apparatus as claimed in claim 17, wherein the reaction vessel is transparent to microwaves provided by the microwave system. 23) A substrate comprising an adherent coating of a metal compound.

24) The substrate as claimed in claim 23, wherein the substrate is a semiconductor or a dielectric material selected from a group comprising silicon, glass, alumina, fused quartz, or polymeric plastic 25) The method as claimed in claim 23, wherein the substrate optionally comprise a thin coating of an electrically conducting material, with thickness of the coating not exceeding 100 micrometers.