Method for manufacturing a substoichiometric oxygen layer of a titanium, vanadium, tungsten or molybdenum oxide

Abstract

The present invention relates to a method for manufacturing a substoichiometric oxygen layer of a titanium, vanadium, tungsten, or molybdenum oxide onto a substrate by magnetron sputtering of a target in a chamber, said method being characterized in that said target is made of said titanium, vanadium, tungsten, or molybdenum oxide and in that it comprises the steps of: a) evacuating the chamber and introducing an inert gas, b) simultaneously applying a first radiofrequency potential to the target and a second radiofrequency potential to the substrate, so as to generate in the chamber a plasma suitable for simultaneously i) sputtering the target to deposit a layer of said titanium, vanadium, tungsten, or molybdenum oxide onto the substrate, and ii) sputtering said layer of titanium, vanadium, tungsten, or molybdenum oxide onto the substrate. molybdenum deposited on the substrate to extract oxygen atoms from said layer.Figure 1.

Description

Title of the invention: Method for manufacturing a sub-stoichiometric oxygen layer of a titanium, vanadium, tungsten or molybdenum oxide GENERAL TECHNICAL FIELD

[0001] The present invention relates to the field of sub-stoichiometric oxygen materials and magnetron cathode sputtering.

[0002] More specifically, the present invention relates to the manufacture of a sub-stoichiometric oxygen layer of a titanium, vanadium, tungsten or molybdenum oxide by magnetron cathode sputtering. STATE OF THE ART

[0003] Substoichiometric oxygen materials of different oxides such as titanium, vanadium, tungsten or molybdenum oxides have particularly advantageous characteristics. Indeed, some of these materials, which are called Magneli phases, are very resistant and electrically conductive and are very useful for manufacturing electric batteries. For example, titanium Magneli phases have been marketed under the name Ebonex® by the English company Atraverda Ltd for the manufacture of electrodes in batteries. The choice of using titanium Magneli phases is justified both by their robustness in aggressive basic or acidic environments and by their good electrical conductivity. It has been demonstrated that Ebonex® titanium Magneli phases resist for 50 years in 4 mol / dm3 of H2SO4 at room temperature.The strength and conductivity of these materials were explained by the scientist Arne Magneli who discovered the Magneli phases in 1954. The particular characteristics of the Magneli phases are due to a deformation called shear, the shear itself being due to one or more lacks of oxygen atoms in the crystal structure.

[0004] Magneli phases have a precise chemical formula, in which the ratio between the number of oxygen atoms and the number of atoms of the metal (i.e. titanium, vanadium, tungsten, molybdenum) from one Magneli phase to another remains very close. For example, titanium Magneli phases have the formula TinO2n i with n being between 4 and 9 inclusive. The chemical formulas of titanium Magneli phases are therefore Ti4O7, Ti5O 9, Ti6On, etc... Thus, the O / Ti ratios between the first phase, i.e. 1.75 for Ti4O7, and the last phase, i.e. 1.89 for Ti90i7 are very close. It is understood here that to obtain Magneli phases, it is necessary to be able to very precisely control the under-oxidation of titanium, vanadium, tungsten or molybdenum oxides.

[0005] Currently, there are only three ways to obtain materials of the same composition: position than the Magneli phases which are therefore particular sub-stoichiometric materials in oxygen.

[0006] The first way consists of heating titanium, vanadium, tungsten or molybdenum powders with, respectively, titanium, vanadium, tungsten or molybdenum oxide powders under an inert atmosphere.

[0007] The second way consists of reducing titanium, vanadium, tungsten or molybdenum oxide at high temperature, for example at more than 1273 Kelvins for titanium oxide, under a reducing atmosphere, for example under a dihydrogen atmosphere.

[0008] In these first two cases, it is necessary to heat materials to high temperatures, which requires a lot of energy.

[0009] The third way is to sputter a titanium target by magnetron sputtering under an atmosphere of argon and dioxygen so that the titanium becomes enriched in oxygen. In this case where the atmosphere includes dioxygen, the target oxidizes. As a result, oxidized molecules of the target are sputtered, in addition to the sputtering of the titanium atoms of the target. It is then very difficult to control the process and ensure that Magneli phases are actually obtained.

[0010] The present invention improves the situation. PRESENTATION OF THE INVENTION

[0011] The invention provides a method for manufacturing a sub-stoichiometric oxygen layer of a titanium, vanadium, tungsten or molybdenum oxide which is more energy-efficient and easier to control.

[0012] The present invention thus relates, according to a first aspect, to a method for manufacturing a sub-stoichiometric oxygen layer of a titanium, vanadium, tungsten or molybdenum oxide on a substrate by magnetron cathode sputtering of a target in an enclosure, said method being characterized in that said target is made up of said titanium, vanadium, tungsten or molybdenum oxide and in that it comprises the steps of:

[0013] a) placing the enclosure under vacuum and introducing an inert gas,

[0014] b) simultaneous application of a first radiofrequency potential to the target and a second radiofrequency potential to the substrate, so as to generate in the enclosure a plasma suitable for simultaneously

[0015] i) sputtering the target to deposit a layer of said titanium, vanadium, tungsten or molybdenum oxide on the substrate, and

[0016] ii) sputtering said layer of titanium, vanadium, tungsten or molybdenum oxide deposited on the substrate to extract oxygen atoms from said layer.

[0017] According to advantageous and non-limiting characteristics:

[0018] The inert gas introduced into the enclosure is argon.

[0019] The substrate is a ceramic substrate.

[0020] A ratio of a density of the second radio frequency potential to a density of the first radio frequency potential is greater than a potential density ratio threshold.

[0021] The potential density ratio threshold is greater than 0.2.

[0022] The substrate is supported by a substrate holder, the second radiofrequency potential is applied to the substrate holder and the density of the second radiofrequency potential corresponds to the second radiofrequency potential applied to the substrate holder per unit area of ​​the substrate holder.

[0023] According to a second aspect, the invention relates to a system for implementing a method for manufacturing a sub-stoichiometric oxygen layer of a titanium, vanadium, tungsten or molybdenum oxide on a substrate by magnetron cathode sputtering of a target consisting of said titanium, vanadium, tungsten or molybdenum oxide, comprising:

[0024] - an enclosure connected to a vacuum device, the enclosure being capable of being put under vacuum using the vacuum apparatus, the enclosure being connected to an inert gas source configured to introduce inert gas into the enclosure,

[0025] - at least one first radiofrequency source adapted to be connected to the target, the first radiofrequency source being configured to apply a first radiofrequency potential to the target,

[0026] - at least one second radiofrequency source adapted to be connected to the substrate, the second radio frequency source being configured to apply a second radio frequency potential to the substrate. PRESENTATION OF FIGURES

[0027] Other characteristics and advantages of the present invention will appear on reading the following description of a preferred embodiment. This description will be given with reference to the appended figures, including:

[0028] - [Fig.l] shows schematically a system for the manufacture of an under-layer stoichiometric oxygen content of a titanium, vanadium, tungsten or molybdenum oxide according to a possible embodiment of the invention;

[0029] - [Fig.2] shows the steps of a method for manufacturing an underlayment layer stoichiometric oxygen content of a titanium, vanadium, tungsten or molybdenum oxide according to a possible embodiment of the invention;

[0030] - [Fig.3] is a graph of the electrical resistivity of a sub-layer stoichiometric oxygen content of a titanium oxide as a function of the density ratio of potential applied to the substrate holder on the potential density applied to the target according to a possible embodiment of the invention;

[0031] - [Fig.4] is a graph of the ratio of the number of oxygen atoms to the number of titanium atoms in a sub-stoichiometric oxygen layer of a titanium oxide as a function of the ratio of the potential density applied to the substrate holder to the potential density applied to the target according to a possible embodiment of the invention,

[0032] - [Fig.5] is a graph showing the evolution of the electric current as a function of the potential difference between a working electrode composed of the material of a sub-stoichiometric oxygen layer of a titanium oxide obtained by the method according to one embodiment and a saturated calomel reference electrode during three voltammetry cycles,

[0033] - [Fig.6] shows the evolution of the average transmittance in the domain of visible of the material of a sub-stoichiometric oxygen layer of a titanium oxide obtained by the claimed method as a function of the ratio of the potential density applied to the substrate holder to the potential density applied to the target according to a possible embodiment of the invention. DETAILED DESCRIPTION System

[0034] With reference to [Fig.l], the invention relates to a system 100 for implementing a method for manufacturing a sub-stoichiometric oxygen layer of a titanium, vanadium, tungsten or molybdenum oxide on a substrate 2 by magnetron sputtering of a target 1 in an enclosure. Magnetron sputtering consists of using the energy of a plasma, i.e. a partially ionized gas, on the surface of a target 1, i.e. the cathode, to tear off the atoms of the material of the target 1 one by one and deposit them on a substrate 2, i.e. the anode.

[0035] To implement the method for manufacturing a substoichiometric oxygen layer of an oxide, the magnetron sputtering system 100 comprises an enclosure 4 connected to a vacuum apparatus 6. The enclosure 4 is for example a frame that can be opened and closed and, when closed, is airtight. The vacuum apparatus 6 connected to the enclosure 4 is configured to evacuate the enclosure 4. Preferably, the vacuum apparatus 6 makes it possible to place the enclosure 4 under an ultrahigh vacuum, that is to say the residual air pressure in the enclosure 4 is less than 106 mbar. More preferably, the enclosure 4 is connected to two vacuum apparatuses 6a, 6b comprising a first vacuum apparatus 6a and a second vacuum apparatus 6b. The first vacuum apparatus 6a is configured to apply a primary vacuum in the enclosure 4, that is to say that the residual air pressure in the enclosure 4 is between 1 mbar and 102 mbar. The first vacuum device 6a is for example a primary oil pump. The second vacuum device 6b is configured to apply an ultra-high vacuum in the enclosure 4, once the primary vacuum has been applied. The second vacuum device 6b is for example a turbomolecular pump.

[0036] The enclosure 4 is also connected to an inert gas source 8 configured to introduce inert gas into the enclosure 4. The inert gas delivered by the inert gas source 8 is for example argon, neon or krypton. Preferably, the inert gas delivered by the inert gas source 8 is argon. The inert gas source 8 is typically an inert gas cylinder. The inert gas flow rate at which the inert gas is introduced into the enclosure 4 can be measured using a flow meter so that the flow rate can be controlled.

[0037] Preferably, the enclosure 4 is connected to a second gas source 9 to increase the pressure in the enclosure 4 until it reaches atmospheric pressure so that the enclosure 4 is no longer under vacuum. Preferably, the gas delivered by the second gas source 9 is nitrogen or air.

[0038] In the enclosure 4 are arranged the substrate 2 and the target 1. The substrate 2 is preferably supported by a substrate holder 3. Preferably, the target 1 has a thickness of between 3 mm and 10 mm. Typically, the target 1 is larger than the substrate 2. For example, the target 1 can be a circular plate 10 cm in diameter and 3 mm thick while the substrate 2 is a square plate 25 mm on each side and 1 mm thick.

[0039] Target 1 is made of an oxide of titanium, vanadium, tungsten or molybdenum. For example, target 1 is made of titanium dioxide, TiO2.

[0040] The substrate 2 is made of a ceramic or metallic material. Ceramic materials have the advantage of being very resistant to wear. Alternatively, the substrate 2 may be made of silicon.

[0041] Preferably, the system 100 comprises cooling systems 10a, 10b for cooling the target 1 and the substrate 2 respectively. The cooling systems 10a, 10b are for example cold water circuits circulating near the target 1 and the substrate 2 to cool them. Indeed, during the magnetron sputtering process, the target 1 and the substrate 2 are caused to heat up and it is necessary to cool them. In particular, it is necessary to be able to cool the target 1 because, when the target 1 is bombarded, it can heat up to the point of cracking.

[0042] Advantageously, the system 100 comprises a heating system 12 configured to heat the substrate 2. Heating the substrate 2 allows better crystallization of the sub-stoichiometric oxygen layer of an oxide which will be formed on the substrate 2.

[0043] The system 100 comprises at least one first radiofrequency source 14 adapted to be connected to the target 1, the first radiofrequency source 14 being configured to apply a first radiofrequency potential to the target 1. The first radiofrequency source 14 is typically a radiofrequency generator.

[0044] The system 100 comprises at least one second radiofrequency source 16 adapted to be connected to the substrate 2, the second radiofrequency source 16 being configured to apply a second radiofrequency potential to the substrate 2. The second radiofrequency source 16 is typically a radiofrequency generator. In a certain embodiment, the substrate 2 is fixed to a substrate holder 3 and the second radiofrequency source 16 is connected to the substrate holder 3. The second radiofrequency source 16 is therefore connected to the substrate 2 via the substrate holder 3 and is configured to apply a second radiofrequency potential to the substrate 2 via the substrate holder 3.

[0045] The first radiofrequency source 14 and second radiofrequency source 16 are typically radiofrequency generators generating waves with a frequency of 13.56 KHz. The first radiofrequency source 14 and second radiofrequency source 16 may also be microwave systems generating waves with a frequency of a few MHz. Process

[0046] With reference to [Fig.2], the invention relates to a method for manufacturing a sub-stoichiometric oxygen layer of a titanium, vanadium, tungsten or molybdenum oxide on a substrate 2 by magnetron cathode sputtering of a target 1 in an enclosure 4.

[0047] In a preliminary step a01), which is carried out outside the enclosure 4, the substrate 2 is washed and then dried. For example, the substrate 2 can be washed with acetone and then with ethanol in ultrasonic baths and then dried by a jet of nitrogen.

[0048] Then, in a prior step a02), the substrate 2 is fixed to a substrate holder 3 and the substrate holder 3 is arranged in the enclosure 4 and the substrate 2 (or the substrate holder 3) is connected to the second radiofrequency source 16. Furthermore, the target 1 is arranged in the enclosure 4 and is connected to the first radiofrequency source 14.

[0049] In a step a), the enclosure 4 is placed under vacuum using the vacuum device 6 and an inert gas is introduced into the enclosure 4. The vacuum can be implemented in two phases: a first phase of primary vacuuming by the first vacuum device 6a and a second phase of ultra-high vacuuming by the second vacuum device 6b.

[0050] The inert gas is then introduced into the enclosure 4 using the inert gas source 8. Only an inert gas is introduced into the enclosure 4 and not a reactive gas such as would be a gaseous mixture of oxygen and argon. Indeed, if the introduced gas included oxygen, target 1 would oxidize and the process of manufacturing a substoichiometric layer of oxygen would be difficult to control. Preferably, the introduced gas is argon.

[0051] In a step b0), the target 1 is pre-sputtered to clean it of any residues, for example water residues. Thus, a radiofrequency potential (called in the remainder of the text third radiofrequency potential) is applied to the target 1 by the first radiofrequency source 14. Consequently, the inert gas atoms in the enclosure 4 are ionized and these ions bombard the target 1 which is thus sputtered and cleaned. During this pre-sputtering, a mask is arranged on the substrate 2 so that it is not impacted by atoms of the inert gas or atoms torn off the target 1.

[0052] Still with reference to [Fig.2], in a step b), a first radiofrequency potential is applied to the target 1 by the first radiofrequency source 14 and, simultaneously, a second radiofrequency potential is applied to the substrate 2 by the second radiofrequency source 16. Consequently, a plasma is generated in the enclosure 4 which makes it possible to simultaneously sputter the target 1 to deposit a layer of a titanium, vanadium, tungsten or molybdenum oxide on the substrate 2, and to sputter the layer of titanium, vanadium, tungsten or molybdenum oxide deposited on the substrate 2 to extract oxygen atoms from the layer.

[0053] More precisely, when the first radiofrequency potential is applied to the target 1, atoms of the inert gas are ionized and these ions bombard the target 1. Consequently, atoms of the target 1, i.e. the atoms constituting the titanium, vanadium, tungsten or molybdenum oxide, are torn from the target 1 and are deposited on the substrate 2, thus forming a layer of titanium, vanadium, tungsten or molybdenum oxide on the substrate 2. The first radiofrequency potential may be equal to the third radiofrequency potential.

[0054] Simultaneously, when the second radiofrequency potential is applied to the substrate 2, ions from the atoms of the inert gas are attracted to the substrate 2 and thus bombard the substrate 2. Thus, during the deposition of the oxide on the substrate 2, ions from the atoms of the inert gas bombard the oxide layer deposited and growing on the substrate 2. As a result, oxygen atoms from the oxide layer deposited on the substrate 2 are stripped from the substrate 2, which means that the oxide layer deposited on the substrate 2 undergoes a loss of oxygen atoms, thus forming a substoichiometric oxygen layer of titanium, vanadium, tungsten or molybdenum oxide on the substrate 2. As the metal atoms (i.e.titanium, vanadium, tungsten or molybdenum) are heavier than oxygen atoms, the bombardment of the oxide layer in the growth phase by ions from the atoms of the inert gas will promote the depletion of the oxide layer in oxygen. This will create local defects, which are notably sites of oxygen deficiency. This explains the formation of substoichiometric oxide layers. The torn oxygen atoms are then pumped out of the enclosure.

[0055] Advantageously, the ratio of a density of the second radiofrequency potential to a density of the first radiofrequency potential is greater than a potential density ratio threshold. The radiofrequency potential density corresponds to the radiofrequency potential applied per unit area. For example, the density of the first radiofrequency potential corresponds to the radiofrequency potential applied to the target 1 per unit area of ​​the target 1. The radiofrequency potential density can be expressed in W / cm2.

[0056] Preferably, the density of the second radiofrequency potential corresponds to the radiofrequency potential applied to the substrate holder 3 per unit area of ​​the substrate holder 3.

[0057] As illustrated in [Fig. 3], from a certain potential density ratio threshold, the sub-stoichiometric oxygen layer of a titanium, vanadium, tungsten or molybdenum oxide formed on the substrate 2 is electrically conductive. [Fig. 3] is a graph plotting the electrical resistivity of the sub-stoichiometric oxygen layer of a titanium oxide formed on the substrate 2 as a function of the ratio of the density of the second radiofrequency potential to the density of the first radiofrequency potential, measured at room temperature (21°C). The electrical resistivity is expressed in Ohm*cm. It can be noted that from a potential density ratio of 0.2, the electrical resistivity of the layer formed on the substrate 2 is low in comparison to the electrical resistivity of the layer for a potential density ratio between 0 and 0.2. This is due to oxygen substoichiometry.Indeed, from a certain threshold of potential density ratio, the ratio of the number of titanium, vanadium, tungsten or molybdenum atoms to the number of oxygen atoms present in the sub-stoichiometric layer formed on the substrate 2 is low so that the electrical resistivity of the layer is low. By low electrical resistivity is meant an electrical resistivity lower than 0.11 Ohm*cm.

[0058] [Fig. 4] is a graph of the ratio of the number of oxygen atoms to the number of titanium atoms in a substoichiometric oxygen layer of a titanium oxide formed on the substrate 2 in the present method, O / Ti ratio, as a function of the ratio of the density of the second radiofrequency potential to the density of the first radiofrequency potential. The measurements were carried out by microprobe.

[0059] It is noted that from a potential density ratio of 0.2, the O / Ti ratio is low in comparison with the O / Ti ratio for a potential density ratio between 0 and 0.2.

[0060] Advantageously, the potential density ratio threshold is greater than 0.2. Indeed, from this threshold of 0.2, the electrical resistivity of the sub-stoichiometric oxygen layer of a titanium, vanadium, tungsten or molybdenum oxide formed on the substrate 2 is less than 0.11 Ohm*cm.

[0061] During the simultaneous sputtering of target 1 and substrate 2, cooling circuits 10a and 10b allow the target 1 and substrate 2 to be cooled respectively.

[0062] The simultaneous sputtering of the target 1 and the substrate 2 is carried out for a certain sputtering time. Once this sputtering time has elapsed, the first radiofrequency source 14 and the second radiofrequency source 16 are switched off so that no radiofrequency potential is applied to the target 1 and the substrate 2. Then, a gas is introduced into the enclosure 4 by the second gas source 9 to increase the pressure in the enclosure 4. Once the pressure in the enclosure 4 reaches atmospheric pressure, the enclosure 4 is opened and the substrate 2 is extracted from the enclosure 4.

[0063] In the same system, that is to say with in particular the same enclosure and the same means of application of the first and second radiofrequency potentials, it is possible to vary the stoichiometry of the oxide layer formed on the substrate by varying in particular the ratio of the density of the second radiofrequency potential to the density of the first radiofrequency potential. Also, for example, it is possible to vary other parameters such as the pressure, which will also require varying the ratio of the density of the second radiofrequency potential to the density of the first radiofrequency potential.

[0064] The experimental results show that the material obtained is indeed a sub-stoichiometric oxygen oxide whose composition is identical to the composition of Magneli phases. Tests were carried out on a sub-stoichiometric oxygen layer of a titanium oxide obtained by the present process. First of all, X-ray diffraction tests do not show the presence of metallic titanium. In addition, sectional observations by scanning and transmission electron microscopy were carried out and no metallic titanium atom was observed, which proves that the layer is indeed composed of a sub-stoichiometric oxide.

[0065] The experimental results concerning the properties of the obtained layer also confirm that the obtained layer has the same properties as the Magneli phases. First of all, the obtained material is very electrically conductive as illustrated in [Fig.2]. In addition, the obtained material has very good resistance to oxidation under acidic pH conditions. [Fig.5] shows the result of three volt-ammeter cycles applied to the obtained material. A working electrode (composed of the obtained material) and a saturated calomel reference electrode (SCE) were immersed in sulfuric acid (H2SO4 solution with a concentration of 1.7 mol / L) and a potential was applied to the working electrode. [Fig.5] shows curves plotting the electric current in mA flowing between the two electrodes as a function of the potential difference between the two electrodes V / ECS in Volts. Three cycles are represented (first cycle plotted in solid line, second cycle plotted in dotted line and third cycle plotted in broken line). We note a reduction of the water to -0.6 V, which shows that the working electrode is electrically conductive. We also note that the corrosion current is almost zero up to a potential difference of 1.5 V in oxidation (positive voltage). The superposition of the three cycles shows the absence of oxidation of the substoichiometric oxide layer as well as the electrochemical stability of the substoichiometric oxide layer over time.

[0066] Finally, [Fig.6] shows the evolution of the average transmittance in the visible range (wavelength between 400 nm and 800 nm) of the material obtained as a function of the potential density ratio. From a certain potential density ratio, the transmittance is low (less than 0.25, i.e. less than 25%), compared to TiO2 which has a very high transmittance, of the order of 0.8. This property is similar to that of Magneli phase composition materials.

[0067] The invention is not limited to the embodiment described and shown in the attached figures. Modifications remain possible, in particular from the point of view of the constitution of the various technical characteristics or by substitution of technical equivalents, without departing from the scope of protection of the invention.

Claims

Claims

1. A method of manufacturing a sub-stoichiometric oxygen layer of a titanium, vanadium, tungsten or molybdenum oxide on a substrate (2) by magnetron cathode sputtering of a target (1) in an enclosure (4), said method being characterized in that said target (1) is made of said titanium, vanadium, tungsten or molybdenum oxide and in that it comprises the steps of: a) placing the enclosure (4) under vacuum and introducing an inert gas, b) simultaneously applying a first radiofrequency potential to the target (1) and a second radiofrequency potential to the substrate (2), so as to generate in the enclosure (4) a plasma suitable for simultaneously i. sputtering the target (1) to deposit a layer of said titanium, vanadium, tungsten or molybdenum oxide on the substrate (2), and ii.spraying said layer of titanium, vanadium, tungsten or molybdenum oxide deposited on the substrate (2) to extract oxygen atoms from said layer.

2. Manufacturing method according to claim 1, wherein the inert gas introduced into the enclosure (4) is argon.

3. A manufacturing method according to any preceding claim, wherein the substrate (2) is a ceramic substrate (2).

4. A manufacturing method according to any preceding claim, wherein a ratio of a density of the second radio frequency potential to a density of the first radio frequency potential is greater than a potential density ratio threshold.

5. The manufacturing method according to claim 4, wherein the potential density ratio threshold is greater than 0.

2.

6. A manufacturing method according to one of claims 4 and 5, wherein the substrate (2) is supported by a substrate holder (3), the second radiofrequency potential is applied to the substrate holder (3) and the density of the second radiofrequency potential corresponds to the second radiofrequency potential applied to the substrate holder (3) per unit area of ​​the substrate holder (3).

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