Method for synthesizing diamond by distributed microwave plasma

Abstract

The invention belongs to the field of diamond synthesis and application thereof, and particularly relates to application in the fields of protective coatings containing nanocrystalline, microcrystalline, polycrystalline and monocrystal diamond films, biocompatible coatings and electronic devices. The invention relates to a method for synthesizing diamond by distributed microwave plasma, to a reaction chamber for carrying out the method, to a substrate comprising a diamond layer obtained according to the method of the invention, to a device comprising a substrate comprising a diamond layer obtained according to the method of the invention, and to a method for synthesizing diamond by distributed microwave plasma. The present invention relates to a coating selected from the group consisting of hard and protective coatings for tools for cutting and perforating, eyeglasses, optical elements, windows, scalpels, biocompatible coatings for implantable medical devices, intracerebral electrodes, and electronics, sensors, power electronics, heat sinks for electronic and optoelectronic elements, encapsulated electronics, quantum sensors.

Description

[0001] The present invention is in the field of diamond synthesis and in particular its application in the field of protective coatings, biocompatible coatings and electronic devices comprising nanocrystalline, microcrystalline, polycrystalline and single crystalline diamond thin films.

[0002] The present invention relates to a method for synthesizing diamond by distributed microwave plasma, to a reaction chamber for carrying out the method, to a solid substrate comprising a diamond layer obtained according to the method of the invention. The present invention also relates to a device comprising a substrate comprising a diamond layer (or thin film) obtained according to the method of the invention, selected from the group consisting of tools for cutting and perforating, eyeglasses, optical elements, windows, hard and protective coatings for surgical knives, biocompatible coatings for implantable medical devices such as prostheses, dental implants, intracerebral electrodes, and electronic devices such as surface acoustic or guided acoustic wave devices, sensors, power electronic components (diodes, transistors, metal oxide semiconductor field effect transistors (MOSFETs)), heat sinks for electronic and optoelectronic components, packaged electronic devices, quantum sensors (magnetometers). State of the art

[0003] Diamond offers an exceptional combination of mechanical, electronic, thermal, quantum and optical properties. These properties offer a large potential for industrial applications in various fields. However, the performance of diamond is critically dependent on the quality of the material and its origin.

[0004] Therefore, natural crystals are of little interest for technical applications because of their rarity, high cost, small size and the variability of their properties depending on their origin. It is therefore essential to develop a technology that enables reproducible synthesis of diamond and control of its properties.

[0005] The first method for diamond synthesis developed in the 1950s was the so-called high pressure high temperature method (HPHT), which is based on mixing carbon with a metallic catalyst, by reproducing the same extreme pressure and temperature conditions as those that form diamond in nature, to transform it into diamond. This technology can be used to synthesize diamond crystals, but despite recent significant advances, it still has the disadvantage of containing chemical impurities from the metallic solvent used in the lattice and of difficult-to-control nitrogen incorporation, which limits its use mainly to mechanical applications.

[0006] Other synthesis methods were developed in the 1980s to better control the composition and purity of diamond, while ensuring the reproducibility of the method and offering the possibility of obtaining the material in the form of a coating. These methods include the chemical vapor deposition (CVD) method. The principle behind these methods is to activate a gas phase consisting mainly of hydrogen and carbon elements by an external energy source to produce the radical species needed to build the structure of a high-quality diamond crystal, such as methyl and atomic hydrogen.

[0007] Depending on the type of energy source used to activate the gas phase, there are two main types of CVD methods for diamond synthesis.

[0008] The HFCVD (HF stands for "hot filament") method uses a set of metallic filaments, usually tungsten, heated to very high temperatures by the Joule effect to synthesize diamond. This method is low cost and allows the deposition of diamond on large surfaces with complex geometry at a rate of up to 20 pm.h -1 However, the filaments, placed very close to the substrate, can degrade, severely polluting the deposited film and causing inhomogeneities along the filament temperature profile. Another drawback of this method lies in the properties of the material constituting the filament. The filament, sensitive to oxidation and easily corroded, limits the choice of gas mixtures that can be used. Finally, the activation temperature of the gas phase is still limited by the melting point of the element constituting the filament, thus limiting the dissociation rate of the gaseous species.

[0009] Therefore, none of the above methods can produce diamond films of high purity, i.e. free of graphite and / or other chemical impurities.

[0010] The second type of CVD method is based on the chemical activation of the gas phase using a plasma source, in particular a microwave source (MPA-CVD, stands for "microwave plasma assisted CVD"). This is the most widely used technique for the synthesis of diamond for non-mechanical applications and tribological applications. There are multiple configurations of microwave reactors, most of which use a resonant cavity, i.e. a cavity whose dimensions allow the excitation of a specific electromagnetic mode. These reactors can produce diamond films of ultrapure, doped or containing color centers of good crystalline quality, but present some limitations. The CVD method of the prior art uses a resonant cavity with an antenna and the plasma is formed far from the antenna (e.g. 50 cm). The size of the treated surface is limited by the size of the plasma, which in turn is limited in a resonant cavity reactor by the microwave frequency used. For example, the use of a microwave frequency of 2.45 GHz allows the uniform deposition of diamond in an area of maximum diameter of 2 to 3 inches, while the use of a microwave frequency of 915 MHz allows the deposition of diamond in an area of maximum diameter of 4 to 6 inches. In the latter case, the microwave power injected is very high (30 to 60 kW), which poses an efficiency problem (excessive power consumption) at the industrial level. The substrate temperature required for diamond growth is usually above 600°C, which excludes the use of heat-sensitive materials. The high reactivity of the plasma leads to high growth rates (> 1 pm.h -1 ), which do not allow perfect control of the synthesis of sub-micron thickness diamond films. Finally, the use of a resonant cavity does not allow the treatment of three-dimensional parts.

[0011] More recently, other microwave methods using linear antennas or surface waves have been developed, allowing the synthesis of diamond at large surfaces (up to 30 x 30 cm2) and low temperature (down to 90°C). However, these methods have many drawbacks, including:

[0012] a) the substrate temperature cannot be controlled independently of other operating conditions;

[0013] b) high microwave power up to 20 kW needs to be injected;

[0014] c) the growth rate is very low, only a few nm.h -1 ;

[0015] d) limited to the treatment of planar substrates;

[0016] e) it is difficult to adapt the experimental setup to an industrial setup to enlarge the deposition surface;

[0017] f) only nanocrystalline diamond thin films are produced.

[0018] Thus, whatever the choice among the known methods, they have different limitations which can concern the deposition surface, the properties of the diamond layer or its homogeneity. Moreover, the geometry of the part to be treated is a limitation in most of the known methods. Finally, the properties of the substrate that can be used are sometimes limited too.

[0019] Therefore, there is a need to develop a method overcoming these limitations, in particular in terms of deposition surface, geometry of the part to be treated, properties of the substrate that can be used and microstructure of the resulting diamond layer. There is also a need for single crystalline diamond thin films, preferably produced on a large scale.

[0020] The Applicant's contribution is that they have developed a method for coating at least one solid substrate with a synthetic diamond coating. By virtue of the method according to the invention, the synthetic diamond coating is advantageously in the form of single crystalline, polycrystalline, microcrystalline or nanocrystalline, preferably single crystalline. The coating corresponds to one or more than one layer of diamond.

[0021] The first subject of the invention is a method for coating at least one solid substrate with a synthetic diamond coating, comprising the following steps:

[0022] a) placing at least one solid substrate on a substrate holder inside a reaction chamber comprising a non-resonant cavity;

[0023] b) injecting a gas mixture into the chamber;

[0024] c) ionizing the gas mixture (or forming a plasma from the gas mixture) by subjecting the gas mixture to a microwave electromagnetic field, the microwave frequency being between 300 MHz and 6 GHz; and

[0025] forming a diamond coating on said at least one solid substrate, characterized in that,

[0026] - the gas mixture consists of methane, hydrogen, a third gas comprising at least one oxygen atom, and optionally a gaseous precursor of a doping element or of a gas-phase reactivity modifier,

[0027] - the CH4 / H2 volume ratio is between 0.1% and 10% (i.e. between 0.001 and 0.1 in volume),

[0028] - the third gas / CH4 volume ratio is between 0.1% and 120% (i.e. between 0.001 and 1.2 in volume),

[0029] - the temperature of the solid substrate is regulated between 100°C and 1000°C during the step of forming the diamond coating,

[0030] - the internal pressure of the reaction enclosure (i.e. the reaction chamber) is maintained between 10 -2 millibars and 100 millibars,

[0031] - the microwave electromagnetic field is generated by a set of microwave sources.

[0032] To the best of the applicant's knowledge, no method is currently available to form a synthetic diamond coating in the form of single crystal, polycrystalline, microcrystalline or nanocrystalline on the surface of one or more diamond or non-diamond solid substrates of size greater than or equal to 6 inches (size greater than 0.1016 m or 0.1524 m).

[0033] The present invention thus relates to a method for synthesizing diamond using a distributed microwave plasma CVD. The method according to the invention is a growth method with greater versatility, which is lacking in known synthesis method systems. The method according to the invention thus makes it possible to synthesize all forms of diamond, whether nanocrystalline, microcrystalline, polycrystalline or single crystal, preferably nanocrystalline and single crystal diamond, and also to produce doped or intrinsic material containing color centers over a wide temperature range, i.e. at low temperatures of between 100°C and 400°C, which makes it possible to synthesize on heat-sensitive solid substrates or at high temperatures of between 400°C and 1000°C.

[0034] The presence of a third gas comprising at least one oxygen atom makes it possible to provide additional etching species in the plasma, which advantageously makes it possible to eliminate non-diamond phases (such as graphite, amorphous carbon, etc.).

[0035] Furthermore, the presence of a third gas is essential when single crystal diamond is to be obtained.

[0036] Furthermore, while most known experimental setups only allow diamond coating on planar surfaces (or "planar substrates"), the method according to the present application is suitable for deposition on planar and "three-dimensional" solid substrates.

[0037] The method according to the present application also enables uniform synthesis of diamond thin films on large surfaces with a diameter greater than or equal to 8 inches (i.e. greater than or equal to 0.2032 m), while most known methods are limited to 4 inches in diameter (i.e. 0.1016 m).

[0038] The preparation conditions used in the method according to the present application result in low growth rates, from a few nm.h -1 , for example 5 nm.h -1 , to a few hundred nm.h -1 , for example 200 nm.h -1 , depending on the conditions, which enables precise control of the thickness when sub-micron sizes are required. The growth rate can range from 5 nm.h -1 to 200 nm.h -1 .

[0039] The device used to implement the method of the present application is designed so that the deposition thickness can be monitored in situ and in real time by various optical techniques. Typically, the deposition thickness of each layer can range from a few tens of nm, for example 30 nm, to a few μm, for example 4 μm. The deposition thickness can be from 30 nm to 4 μm. In this way, the solid substrate can be coated with one or more than one diamond layer, resulting in a layer with a thickness of from 30 nm to 4 μm.

[0040] Since the device used to implement the method of the present application is not a resonant cavity, the reaction chamber is not physically limited in size. In this way, it is possible to increase the number of microwave sources as much as necessary to increase the volume of the plasma, in particular its diameter, and thus the deposition area on the solid substrate, without having to modify the original design of the reactor. The geometry, the size of the chamber (height, diameter), the openings (windows, doors), the shape, position and size of the substrate holder and of the solid substrate have no influence on the coupling of the electromagnetic waves with the plasma, which occurs directly at the end of each microwave source, thus reducing the limitations compared to a resonant cavity. In other words, the cavity of the reaction chamber in which the coating method according to the present application is implemented is a non-resonant cavity. Therefore, this cavity is not configured to excite specific electromagnetic modes. By virtue of this non-resonant cavity, it is possible to achieve low growth rates, thus enabling uniform deposition.

[0041] The coupling of the electromagnetic waves with the plasma occurs directly at the end of each microwave source. Therefore, the microwave sources are preferably positioned with one end of them located at the level of the cavity. This makes it possible to use a non-resonant cavity.

[0042] The device (reaction chamber or enclosure) according to the invention can also be placed inside a resonant cavity. This embodiment allows the use of two excitation sources (two types of plasma):

[0043] 1) The microwave point source of the method and device according to the invention, allowing low growth rates, and

[0044] 2) The use of a resonant cavity for coupling to create a high-energy plasma, allowing high growth rates as in conventional reactors.

[0045] Thus, when the device according to the invention (i.e. a reaction chamber or enclosure containing a non-resonant cavity) is placed inside a resonant cavity, it is possible to grow intrinsic thin films or doped thin films of several tens of μιη thick in resonant mode, then to make layers of small thickness (10 nm to 100 nm) in distributed plasma mode according to the method of the invention (e.g. delta doping with nitrogen to create NV centers, or encapsulation layers).

[0046] The method according to the invention allows the uniform deposition of diamond layers. By "uniform deposition" is meant a deposition whose characteristics such as thickness, purity, grain size and texture vary by no more than 10% over the entire sample area, so that the nanocrystalline, microcrystalline, polycrystalline or single crystalline nature is maintained over the entire sample. When the surface of a three-dimensional solid substrate has a geometric pattern, such as a stud, a pillar, a groove, an etch pit or an electrode, with a lateral dimension and / or a depth of 100 nm to 5 cm, the synthesis is carried out in a conformal manner, i.e. the deposit assumes the shape of the geometric pattern.

[0047] In the present invention, nanocrystalline diamond means diamond in the form of particles with an average size of about 5 nm to 50 nm, and preferably of about 15 nm to 20 nm.

[0048] In the present invention, microcrystalline diamond means diamond in the form of particles with an average size of about 51 nm to 1 μιη, and preferably of about 500 nm to 1 μιη.

[0049] The diamond particle size can be determined by techniques well known to those skilled in the art, preferably by transmission electron microscopy or X-ray diffraction.

[0050] In the present invention, polycrystalline diamond means diamond in the form of a plurality of grains, and, as generally understood, means diamond in the form of particles with an average size greater than 1 μιη.

[0051] In the present invention, single crystalline diamond means diamond in the form of a single crystal.

[0052] The single crystalline or polycrystalline nature of the diamond can be determined by transmission electron microscopy.

[0053] According to step a) of the method of the present application, at least one solid substrate is placed on a substrate holder inside the reaction chamber. The solid substrate can be placed manually or by automated methods using motorized arms or rods.

[0054] Advantageously, the at least one solid substrate can be a diamond or non-diamond and is preferably a diamond. Solid diamond substrates are particularly useful for obtaining single crystal diamond.

[0055] The solid substrate can be planar or three-dimensional. A planar solid substrate is defined as an object whose one dimension (width, length or height) is significantly smaller than the other two dimensions (width, length or height) by a factor of at least 10. When there are multiple solid substrates, they can be the same or different. These can be planar solid substrates with a diameter of 8 inches or greater (i.e. a diameter of 0.2032 m or greater), solid substrates of complex (three-dimensional) shape with a height of up to a few centimetres, or even multiple planar or three-dimensional solid substrates, provided their lateral dimensions allow them to be arranged on the substrate holder simultaneously.

[0056] Advantageously, the at least one solid substrate can be composite or non-composite. Preferably, the solid substrate is composed of silicon, diamond, refractory metals (such as iridium) or derivatives thereof (such as alloys, oxides or nitrides of refractory metals), transition metals (such as titanium) or derivatives thereof (such as alloys (for example titanium-based alloys such as TA6V), oxides or nitrides of said transition metals), stainless steel, superalloys, cemented carbides, polymers, ceramics, glass, oxides (for example fused silica or alumina), semiconductors selected from group III-V and group II-VI semiconductors of the periodic table, piezoelectric materials, nitrides, carbides, sulphides, silicides or mixtures thereof. Particularly preferably, the solid substrate is composed of diamond, refractory metals (such as iridium) and derivatives thereof (such as alloys, oxides or nitrides of refractory metals), transition metals (such as titanium) and derivatives thereof (such as alloys (for example titanium-based alloys such as TA6V), oxides or nitrides), stainless steel, superalloys, cemented carbides, polymers, ceramics, glass, oxides (for example fused silica or alumina), semiconductors selected from group III-V or group II-VI semiconductors of the periodic table, piezoelectric materials, nitrides, carbides, sulphides, silicides or mixtures thereof; and more particularly preferably, the solid substrate is composed of diamond.

[0057] Advantageously, the thermal contact between the at least one solid substrate and the substrate holder is preferably uniform over the entire contact surface between the at least one solid substrate and the substrate holder. Thus, the surface of the substrate holder can be adapted to the surface of the solid substrate intended to be in contact therewith. By "contact surface" is meant the percentage of the surface of the solid substrate that is in direct contact with the substrate holder on which it is placed. Ideally, this percentage is 100%. If the solid substrate is undulating or has a particular shape, the shape of the substrate holder must be adapted to the solid substrate. Alternatively, a substance, for example a thermally conductive paste, can be used to ensure contact between the solid substrate and the substrate holder, provided that the dimensional differences between the solid substrate and the substrate holder (in terms of contact surface) are not too great.

[0058] Advantageously, the substrate holder can be made of molybdenum or an Inconel alloy or diamond-coated. Preferably, the substrate holder is electrically isolated from the cavity of the reaction chamber. The substrate holder is preferably biased. In particular, the substrate holder can be equipped with a biasing system to which a voltage is applied via a metallic conductor (rod, wire, coaxial cable, etc.), the reaction chamber or one or more metallic parts arranged above the solid substrate acting as a counter electrode. The biasing can be direct current, pulsed direct current, alternating current (50 Hz), radiofrequency (13.56 MHz), low frequency or high frequency (1 Hz to 1 MHz) in various periodic modes (for example, pulsed or sawtooth).

[0059] Advantageously, the biasing system of the substrate holder provides additional degrees of freedom compared to known microwave methods, by ion bombardment during the growth phase, in particular nucleation, cleaning, etching, ion implantation, doping or creation of vacancies in the deposit. This aspect is advantageous, whether to ensure good adhesion of the diamond film to the solid substrate or to promote the formation of dopants or color centers (NV centers, SiV centers, GeV centers or SnV centers) involving vacancies.

[0060] In this way, the biasing system provides an advantageous assistance by ion bombardment on top of the microwave plasma generated by the basic microwave source:

[0061] - diamond nucleation, i.e. the aggregation of carbon atoms to form nuclei (seeds) which, in the presence of an excess of carbon precursor in the gas phase, grow on the surface of the non-diamond solid substrate to form isolated crystals;

[0062] - cleaning of the solid substrate before diamond synthesis, advantageously using a plasma of a gas mixture containing hydrogen, argon or oxygen atoms;

[0063] - etching of the solid substrate, for example to remove the natural oxide layer of a solid silicon substrate or to reveal etch pits on the surface of a solid diamond substrate, advantageously using a plasma of a gas mixture containing hydrogen, argon or oxygen atoms or halogenated precursors containing atoms such as chlorine or fluorine;

[0064] - etching of the diamond deposit, in particular to reduce its thickness, using a plasma of a gas mixture containing hydrogen atoms, an inert gas such as argon or krypton, oxygen or halogenated precursors containing atoms such as chlorine or fluorine;

[0065] - implantation of atoms, in particular carbon atoms, on the surface of certain solid substrates, generally in the presence of an excess of carbon precursor in the gas phase, to create a carbide-type conversion layer intended to promote the adhesion of the diamond thin film to the solid substrate;

[0066] - doping with other atoms (boron, phosphorus, silicon, nitrogen, tin, etc.) during growth, which can in particular modify the electronic, optical and quantum properties of the deposited diamond;

[0067] - creation of carbon vacancies, associated with the presence of substitution atoms such as nitrogen, silicon, germanium or tin, creating color centers involving vacancies (for example NV centers, SiV centers, GeV centers or SnV centers).

[0068] Advantageously, the substrate holder can be equipped with a vertical translation system for adjusting the distance to the source plane, said plane being defined by the source array and being parallel to the surface of the substrate holder. The substrate holder can be equipped with a rotation system, for example to optimize the uniformity of the deposition of the diamond layer over the entire solid substrate surface exposed to the plasma. This feature is particularly useful for three-dimensional parts, certain sides of which can be more difficult to access by the plasma. The substrate holder can be equipped with a heating / cooling system to regulate the temperature of the solid substrate during the process. This temperature control can be carried out independently of the operating conditions (gas pressure, gas mixture, gas flow, microwave power applied to the source, bias voltage / power).

[0069] Advantageously, the temperature of the solid substrate is regulated to 100°C to 1000°C, preferably 600°C to 1000°C, and more preferably 800°C during the step of forming the diamond coating. The temperature of the solid substrate is regulated, i.e. said at least one solid substrate is heated, cooled or temperature-maintained, preferably by convection and / or by thermal conduction of the substrate holder.

[0070] The various diamond microstructures that can be obtained according to the method of the application can be obtained in the following temperature ranges:

[0071] - 100°C to 400°C: nanocrystalline,

[0072] - 400°C to 600°C: microcrystalline,

[0073] - 600°C to 800°C: polycrystalline on a non-diamond solid substrate, single crystalline on a diamond solid substrate,

[0074] For a gas mixture consisting of 96.4 vol% H2, 2.6 vol% CH4 and 1 vol% CO2, a power of 187 W per source and a distance of 10 cm from the source.

[0075] According to a particularly preferred embodiment of the method of the application, the method leads to a single-crystalline synthetic diamond coating which complies with one or more than one of the following characteristics, and more preferably with all the following characteristics:

[0076] - the solid substrate is a solid diamond substrate,

[0077] - the temperature of the solid substrate is comprised between 650°C and 850°C,

[0078] - the pressure inside the reaction chamber is comprised between 0.25 mbar and 0.45 mbar,

[0079] - the CH4 / H2 volume ratio is comprised between 0.1% and 4%,

[0080] - the third gas / CH4 volume ratio is comprised between 40% and 120%,

[0081] - the power of each source ranges from 75 W to 200 W,

[0082] - the distance of the solid substrate from the source is comprised between 4 cm and 12 cm.

[0083] Advantageously, the method according to the application can also comprise one or more than one step of surface preparation of the solid substrate. Two cases can be distinguished: when the solid substrate is made of diamond, or when it is not (i.e. when the solid substrate is made of a material other than diamond, as mentioned in the present application). Thus, when the solid substrate is made of diamond, the method according to the application can also comprise the steps of preparing the surface of the solid substrate and optimizing the thermal contact with the substrate holder, chosen from chemical treatment with an acid (aqua regia), cleaning with an organic solvent (such as ethanol, acetone), drying, H2 / O2 plasma etching and bonding to the substrate holder. When the solid substrate is not made of diamond, the method according to the application can also comprise the steps of cleaning with an organic solvent (such as ethanol, acetone), lapping or spin or dip coating with a solution containing suspended diamond particles of size ranging from a few nanometers to a few tens of microns, drying, and bonding to the substrate holder.

[0084] According to step b) of the method of the application, the gas mixture is injected into the reaction chamber. Step b) of injecting the gas mixture into the chamber can comprise three phases:

[0085] 1) a few minutes or even tens of minutes before the introduction of the microwaves, in order to stabilize the other growth parameters such as the temperature of the solid substrate or the pressure,

[0086] 2) the whole duration of the microwave injection (or application), which corresponds to the diamond growth phase. This injection (or application) is generally continuous, but the proportions of the gas mixture can be modified at this stage to vary the properties of the deposited diamond, thus creating a stack of diamond layers with different properties,

[0087] 3) once the microwave is turned off, the sample is rapidly cooled.

[0088] The composition of the gas mixture can be monitored using mass flow meters. The gas mixture can consist of methane, hydrogen and a third gas comprising at least one oxygen atom. By "third gas comprising at least one oxygen atom" is meant a gas preferably selected from CO2, CO, O2 and mixtures thereof. The gas mixture can consist of methane, hydrogen, a third gas comprising at least one oxygen atom and optionally a gaseous precursor of a doping element or a gas-phase reactivity modifier, in particular by modifying the thermal conductivity of the gas mixture. For example, the thermal conductivity of the plasma can be modified by adding argon to the gas mixture instead of hydrogen and by modifying the [Ar] / [H2] ratio. This modification can change the temperature of the gas and thus its reactivity. For example, an increase in the [Ar] / [H2] ratio leads to a decrease in the thermal conductivity of the plasma, which leads to an increase in the temperature of the gas, which can promote radical chemistry within the gas phase. This precursor or modifier can preferably be a gas selected from N2, N2O, NH3, B2H6, BBr3, B(CH3)3, PH3, PBr3, SiH4, GeH4, SnH4, Ar and He. The various gases making up the gas mixture are pre-mixed before being introduced into the reaction chamber. The gas mixture is introduced into the reaction chamber through one or more injectors (ports), the diameter of which can be identical or different and can be adjusted to modify and control the flow of gas mixture through each injector (port) independently of the others.

[0089] N2, N2O and NH3, enable the incorporation of nitrogen atoms in the diamond layer, which bind to carbon vacancies (NV centres) with spin states that can be read and optically manipulated at room temperature, endowing them with properties suitable for designing quantum sensors. Gases containing Group XIV elements, such as SiH4, GeH4and SnH4, as precursors or modifiers, can incorporate Group XIV atoms such as Si, Ge and Sn, respectively, which bind to carbon vacancies, creating SiV colour centres, GeV colour centres and SnV colour centres, respectively, endowing them with properties comparable to NV centres. Gases such as B2H6, BBr3, B(CH3)3or PH3, PBr3, as precursors or modifiers, enable the incorporation of boron B and phosphorus P atoms, respectively, and obtain P-type (acceptor) and N-type (donor) doping, respectively. For example, a gas mixture consisting of 96.4 vol% H2, 2.6 vol% CH4, 1 vol% CO2and 0.04 vol% N2, produces a single-crystalline diamond layer containing NV centres on a high pressure high temperature (HPHT) solid diamond substrate. Considering the base gas mixture consisting of 96.4 vol% H2, 2.6 vol% CH4and 1 vol% CO2, and replacing part of the H2 stream with argon (Ar) to obtain a gas composition of, for example, 46.4 vol% H2, 2.6 vol% CH4, 1 vol% CO2and 50 vol% Ar, the thermal conductivity of the gas mixture is halved, resulting in an increase in the gas temperature and, consequently, in the reaction processes within the plasma.

[0090] Advantageously, in the gas mixture, the CH4 / H2 volume ratio can be comprised between 0.1% and 10% (i.e. a volume ratio comprised between 0.001 and 0.1), preferably between 2% and 3% (i.e. a volume ratio comprised between 0.02 and 0.03).

[0091] Advantageously, in the gas mixture, the third gas / CH4 volume ratio can be comprised between 0.1% and 120% (i.e. a volume ratio comprised between 0.001 and 1.2), preferably between 40% and 120% (i.e. a volume ratio comprised between 0.4 and 1.2).

[0092] This ratio can be adjusted depending on the desired diamond microstructure. For example, a gas mixture consisting of 96.4 vol% H2, 2.6 vol% CH4and 1 vol% CO2allows the synthesis of nanocrystalline diamond at a solid substrate temperature comprised between 100°C and 400°C. At 750°C, a gas mixture consisting of 94.4 vol% H2, 2.6 vol% CH4and 3 vol% O2allows the synthesis of single-crystalline diamond on a HPHT solid diamond substrate. At 650°C, the use of a gas mixture comprising 96.4 vol% H2, 2.6 vol% CH4and 1 vol% CO2results in the formation of microcrystalline diamond.

[0093] Advantageously, in the gas mixture, the volume ratio of gaseous doping element precursor or reactive modifier / CH4may range from 0.001% to 10000% (i.e. from 0.00001 to 100 in volume ratio). See above for illustration.

[0094] According to step c) of the method of the application, the gas mixture is ionized (or converted into plasma) by subjecting it to a microwave electromagnetic field. The microwave frequency can range from 300 MHz to 6 GHz. The frequency is preferably chosen from 466 MHz, 915 MHz, 2.45 GHz and 5.8 GHz, among which 915 MHz and 2.45 GHz are two standardized frequencies authorized for industrial applications. The microwave power of each source can range from a few watts to a few hundred watts and can be injected in a cyclic mode (pulsed, sinusoidal, sawtooth or other) either continuously (continuous mode) or discontinuously (pulsed mode), with, in the pulsed case, the mode repetition frequency, the average power, the peak power and the duty cycle being varied.

[0095] Advantageously, the microwaves are generated by a microwave source.

[0096] By "microwave source" is meant an electromagnetic coupling device (antenna), generally axisymmetric, composed of a metallic central core, advantageously protected by an insulating refractory material such as alumina, and having an impedance matching system. The microwave source is described, for example, in patent application WO 03 / 103003 A1. The power of the microwave source can range from 1 W to 500 W, preferably 200 W. The application uses a set of microwave sources. In other words, the number of microwave sources is greater than or equal to two. The plasma generated by the source is axisymmetric and the size of the plasma in the shadow area of the zone defined by the intersection of the plasma with a plane parallel to the plane of the source (shaded area in Figure 5 ) varies from 1 cm2to 20 cm2, as a function of the plasma gas pressure, the microwave power and the distance from the source. For a set of N coplanar sources arranged in a square matrix with a pitch of d cm, N being an integer greater than or equal to 4, the size of the plasma in the shadow area portion 52 of the zone parallel to the plane of the source array (shaded area in Figure 5 ) is at least .

[0097] Advantageously, the sources can be magnetized using permanent magnets. Magnetized sources trap the electrons in a static axisymmetric dipole magnetic field and deliver maximum energy to them by electron cyclotron resonance, generating a dense, high-energy plasma, thereby enhancing the chemical reactivity and, in turn, the yield of active species.

[0098] Advantageously, step c) can last from a few minutes to several hours, even tens of hours. Preferably, the time during which step c) is implemented is adapted to obtain the desired thickness of diamond, generally ranging from 1 minute to 100 hours.

[0099] Advantageously, steps b) and c) are carried out simultaneously.

[0100] Advantageously, the microwave sources form a planar array of sources or have a three-dimensional configuration adapted to the geometry of the solid substrate(s) Figure 1 , Figure 2 and Figure 3 ). By optimizing the growth conditions (pressure, microwave power, surface temperature and / or gas composition), plasma homogeneity (electron density and temperature, gas temperature, active species density) can be achieved in the range of a few centimeters perpendicular to the solid substrate and a few tens or even hundreds of centimeters parallel to the solid substrate, depending on the number of microwave sources, which is ideal for the uniform treatment of large-size planar parts and three-dimensional parts, without the need for a three-dimensional configuration of the sources.

[0101] Advantageously, the distance between the at least one solid substrate and the microwave sources can be between 0.1 cm and 15 cm, preferably between 5 cm and 15 cm, and more preferably 10 cm. The person skilled in the art is able to apply a distance determined by the desired microstructure of the diamond thin film. The further the substrate holder is from the sources, the greater the extent of the microstructure obtained on the non-diamond solid substrate, from polycrystalline via microcrystalline to nanocrystalline. The distance corresponds to the shortest distance between the plane of the microwave sources and the surface of the solid substrate closest to the microwave sources, whether the solid substrate is planar or 3D. The surface of the solid substrate to be considered is the surface in contact with the plasma, i.e. the total surface area of the solid substrate minus the surface area of the substrate in contact with the substrate holder supporting it.

[0102] Advantageously, when the method is implemented at low pressure, i.e. < 1 mbar, the plasma is generated and maintained by applying microwave power of a few tens to a few hundred watts per source. At a distance of 5 cm to 12 cm from the plane of the sources, which is a few centimeters, the plasma is perfectly homogeneous in the plane parallel to the plane of the sources and homogeneous over a few centimeters in the direction perpendicular to this plane. Depending on the number of sources used, this configuration makes it possible to obtain a uniform diamond deposit over a large area, generally 8 inches or more in diameter (1 inch = 0.0254 m), different configurations of solid substrates being able to be treated when the solid substrates are placed on the substrate holders in the appropriate position relative to the sources.

[0103] In this method, ionization of the gas mixture forms a diamond coating on the at least one solid substrate.

[0104] According to the method of the application, the internal pressure of the reaction chamber is maintained during the implementation of steps b) and c) between 10 -2 and 100 mbar, preferably between 5x10 -2 and 50 mbar, and more preferably between 10 -1 and 1 mbar.

[0105] Advantageously, steps b) and c) can be repeated n times, n being an integer greater than or equal to 2, preferably 2 < n < 10, so as to form a series of diamond coatings on the solid substrate, the gas mixture being the same or different at each iteration.

[0106] Advantageously, the method according to the application can also comprise a step d) of measuring the temperature of the solid substrate. The temperature can be measured by means of a thermocouple or a pyrometry. In particular when treating a three-dimensional solid substrate, the temperature homogeneity is important, this step allows to produce a deposit which is uniform in thickness, microstructure and quality.

[0107] Advantageously, the method according to the application can also comprise a step e) of in situ optical monitoring. The monitoring step e) can be performed using an in situ optical monitoring system, preferably selected from the group consisting of interferometric pyrometry, laser reflectometry interferometry and spectroscopic reflectometry monitoring systems. This step makes it possible to measure in real time the thickness and the optical refractive index of the diamond layer under growth, or to detect any change occurring on the surface of the solid substrate (for example, nucleation, etching, topography change and composition change).

[0108] Advantageously, steps d) and e) can be performed simultaneously with steps a), b) and c), throughout the method according to the application.

[0109] Another subject of the application is a reaction chamber comprising:

[0110] - a substrate holder adapted to fix a solid substrate or a set of planar or three-dimensional solid substrates,

[0111] - a set of microwave sources, and

[0112] - optionally, an optical port.

[0113] The optical port, the microwave sources, the substrate holder and the solid substrate (planar or three-dimensional) are as defined in the invention.

[0114] Advantageously, the reaction chamber consists of a vacuum chamber, above which is a set of microwave probes. The size of the reaction chamber ranges from 25 liters for 16 sources to 100 liters for 36 sources. The microwave sources are preferably basic plasma sources powered by one or more microwave generators. The various sources can be arranged in a two-dimensional array (all sources coplanar) or in a three-dimensional configuration. The arrangement of the set of sources can be adapted by the person skilled in the art depending on the solid substrate(s) for which the method is to be applied. These microwave sources can be magnetized or non-magnetized.

[0115] Advantageously, the reaction chamber according to the present application can be arranged within a resonant cavity for exciting specific electromagnetic modes. By means of a suitable electromagnetic coupling, in addition to the point microwave source, it is possible to trigger two types of plasma: a distributed microwave plasma, which has the above-mentioned advantages, in particular the advantage of providing a low growth rate; and a high-energy microwave plasma, allowing a high growth rate, comparable to that of a traditional resonant cavity reactor.

[0116] The present application also relates to a solid substrate comprising one or more diamond layers obtained according to the method of the present application.

[0117] The solid substrate is preferably as defined in the present application.

[0118] Advantageously, the solid substrate comprising one or more diamond layers obtained according to the method of the present application can take various forms. The one or more diamond layers can be nanocrystalline, microcrystalline, polycrystalline or single crystalline, preferably single crystalline.

[0119] Depending on the nature of the gas mixture, it is possible to modify the intrinsic properties of the diamond, for example by doping (for example with boron or phosphorus atoms) or by including color centers (for example, NV centers, GeV centers, SiV centers, SnV centers).

[0120] According to a particularly preferred embodiment of the present application, the one or more diamond layers deposited on the solid substrate are layers comprising NV centers.

[0121] The synthesis of nanocrystalline diamond films at low solid substrate temperatures (i.e. from 100°C to 400°C) allows the treatment of heat-sensitive materials. The synthesis of single crystalline diamond films is efficient at high solid substrate temperatures (i.e. from 600°C to 1000°C) on solid diamond substrates. By the method according to the present application it is possible to synthesize nanocrystalline, microcrystalline and polycrystalline diamond films on several planar or three-dimensional solid substrates arranged simultaneously on a substrate holder. The method also allows the synthesis of single crystalline diamond films on a plurality of planar solid diamond substrates arranged simultaneously on a substrate holder. The method according to the present application also allows very low growth rates, from a few nanometers per hour (at low pressure < 0.25 mbar, low power < 75 W / source, low methane content < 1%) to several tens of nanometers per hour (at higher pressure > 0.35 mbar, high power > 180 W / source, high methane content > 2%), thus allowing precise control of the deposition thickness in the range from a few nanometers to a few micrometers.

[0122] According to a more particularly preferred embodiment of the present application, the one or more diamond layers deposited on the solid substrate are undoped single crystalline diamond layers or single crystalline diamond layers comprising NV centers.

[0123] The present invention also relates to a device comprising a solid substrate obtained by the method according to the present invention. The diamond thin film coated solid substrate produced using the method according to the present invention is particularly useful for protective coatings in mechanical engineering and optical engineering, biomedical devices, electronic devices and quantum.

[0124] Advantageously, the device comprising a solid substrate obtained by the method according to the present invention can be chosen among tools for cutting and perforation, eyeglasses, optical elements, windows, hard and protective coatings for surgical knives, biocompatible coatings for implantable medical devices such as prostheses, dental implants, intracerebral electrodes, and electronic devices such as surface acoustic or guided acoustic wave devices, sensors, power electronic components chosen among diodes, transistors and MOSFETs, heat spreaders for electronic and optoelectronic components, packaged electronic devices and quantum sensors such as ultra-sensitive magnetometers.

[0125] Advantageously, when the one or more diamond layers deposited on the solid substrate are single crystals, in particular by virtue of their controlled thickness, the device comprising a solid substrate obtained by the method according to the present invention can be chosen among ultra-sensitive quantum magnetometers. Indeed, this application requires the color centers to be located at a known depth near the surface with nanometric precision, which requires the creation of a diamond layer doped with nitrogen atoms and carbon vacancies with a thickness of a few nanometers to tens of nanometers (delta-doping). BRIEF DESCRIPTION OF DRAWINGS

[0126] Figure 1 An example of a reaction chamber 1 according to the present invention is shown, comprising a substrate holder 11 on which a planar solid substrate 12 is arranged, a set of microwave sources 13 and an optical port 14.

[0127] Figure 2 An example of a reaction chamber 2 according to the present invention is shown, comprising a substrate holder 21 on which a three-dimensional solid substrate 22 in the shape of a cube with an edge of 2 cm is arranged (i.e. its entire surface is to be coated except the part in contact with the substrate holder), a set of microwave sources 23 and an optical port 24.

[0128] Figure 3 An example of a reaction chamber 3 according to the present invention is shown, comprising a substrate holder 31 on which a set of solid substrates 32 is arranged, a set of microwave sources 33 and an optical port 34.

[0129] Figure 4 An example of (1) a substrate holder A of 10 cm diameter [dimensions of this example], (2) a large (10 cm diameter) planar solid substrate B arranged on the substrate holder A, (3) a plurality of 1 cm diameter planar solid substrates B arranged on the substrate holder A, and (4) a plurality of three-dimensional tungsten carbide drill bit substrates C arranged on the substrate holder A, 6 mm in diameter and 6 cm in length, comprising a 3 cm long covering helix.

[0130] Figure 5 Plasmas generated from a single microwave source (1) according to the prior art and from a set of sources (2) arranged in a planar array according to the present application are shown. The hatched areas represent the portion of the plasma volume intersected by a plane parallel to the plane of the sources.

[0131] Figure 6 1. Nanocrystalline diamond film surface as observed by scanning electron microscope (SEM). 2. Microcrystalline diamond film surface as observed by SEM. 3. Polycrystalline diamond film surface as observed by SEM. 4. Single crystal diamond film surface as observed by SEM and transmission electron diffraction pattern showing the single crystalline nature of the deposit. 5. Photoluminescence spectra of a nitrogen-doped single crystal thin film obtained at 473 nm laser excitation before and after electron irradiation, which peaks at 575 nm (NV 0 and 673 nm (NV - ) prove the presence of NV centres after irradiation.

[0132] The present application is further illustrated by the following non-limiting examples.

[0133] Example 1: Implementation of the method of the present application.

[0134] A reactor was provided consisting of a cylindrical reaction chamber 30 cm high and 40 cm in diameter, topped by a set of 16 sources arranged in a 4x4 matrix. A temperature-controlled 4-inch diameter circular substrate holder was also provided, arranged along the axis of the enclosure.

[0135] 1. Synthesis of nanocrystalline diamond films Figure 6-1 )

[0136] A circular non-diamond solid silicon substrate, 4 inches in diameter and 0.5 mm in thickness.

[0137] Solid substrate preparation: spin or dip coating with diamond powder of average diameter 25 nm; milling with diamond powder of average diameter 30 μm.

[0138] CH4 / H2 / CO2 gas mixture. [CH4] = 2.6% and [CO2] = 1%. The gas mixture was injected at a flow rate of 50 SCCM (standard cubic centimetres per minute) for the entire synthesis process.

[0139] Microwave power per source: 190 W.

[0140] Pressure: 0.35 mbar.

[0141] Substrate holder position (relative to microwave sources): 10 cm.

[0142] Solid substrate temperature: 250 °C.

[0143] Synthesis time: about 5 hours. Step b) and step c) are performed simultaneously.

[0144] In Example 1, the polycrystalline diamond obtained is nanocrystalline.

[0145] 2. Synthesis of microcrystalline diamond thin films Figure 6-2 )

[0146] Cylindrical non-diamond solid silicon substrate, 4 inches in diameter, 0.5 mm in thickness.

[0147] Solid substrate preparation: spin coating or dip coating with diamond powder having an average diameter of 25 nm; milling with diamond powder having an average diameter of 30 pm.

[0148] CH4 / H2 / CO2 gas mixture. [CH4] = 2.6% and [CO2] = 1%. The gas mixture is injected at a flow rate of 50 SCCM for the entire synthesis.

[0149] Microwave power per source: 190 W.

[0150] Pressure: 0.25 mbar.

[0151] Substrate holder position (relative to the microwave source): 6 cm.

[0152] Solid substrate temperature: 400 °C.

[0153] Synthesis time: about 5 hours. Step b) and step c) are performed simultaneously.

[0154] In Example 2, the polycrystalline diamond obtained is microcrystalline.

[0155] 3. Synthesis of polycrystalline diamond films Figure 6-3 )

[0156] Cylindrical non-diamond solid silicon substrate, 4 inches in diameter, 0.5 mm in thickness.

[0157] Solid substrate preparation: spin coating or dip coating with diamond powder having an average diameter of 25 nm; milling with diamond powder having an average diameter of 30 pm.

[0158] CH4 / H2 / CO2 gas mixture. [CH4] = 2.6% and [CO2] = 1%. The gas mixture is injected at a flow rate of 50 SCCM for the entire synthesis.

[0159] Microwave power per source: 190 W.

[0160] Pressure: 0.35 mbar.

[0161] Substrate holder position (relative to microwave source): 10 cm.

[0162] Solid substrate temperature: 650 °C.

[0163] Synthesis time: ~ 5 hours. Step b) and step c) were performed simultaneously.

[0164] In this example 3, the grain size of the polycrystalline diamond was greater than 1 pm.

[0165] 4. Synthesis of intrinsic single-crystal diamond films Figure 6-4 )

[0166] High pressure high temperature (HPHT) Type Ib solid diamond substrate.

[0167] Solid substrate preparation: H2 / O2 (98% / 2%) microwave plasma etch, 2.6 kW, 170 mbar, surface temperature 780 °C, treated in a chamber reactor for 1 hour.

[0168] CH4 / H2 / O2 gas mixture. [CH4] = 2.6% and [O2] = 3%. The gas mixture was injected at a flow rate of 50 SCCM for the entire synthesis process.

[0169] Microwave power per source: 190 W.

[0170] Pressure: 0.35 mbar.

[0171] Substrate holder position (relative to microwave source): 10 cm.

[0172] Solid substrate temperature: 750 °C.

[0173] Synthesis time: ~ 10 hours. Step b) and step c) were performed simultaneously.

[0174] 5. Synthesis of nitrogen-doped single crystal diamond films for creation of nitrogen-vacancy (NV) color centers Figure 6-5 )

[0175] Intrinsic CVD solid diamond substrate, thickness 50 pm, deposited on a high pressure high temperature (HPHT) Type Ib substrate.

[0176] Solid substrate preparation: H2 / O2 (98% / 2%) microwave plasma etch, 2.6 kW, 170 mbar, surface temperature 780 °C, treated in a chamber reactor for 1 hour.

[0177] CH4 / H2 / O2 / N2 gas mixture. [CH4] = 2.6%, [O2] = 3% and [N2] = 0.04%. The gas mixture was injected at a flow rate of 50 SCCM for the entire synthesis process.

[0178] Microwave power per source: 190 W.

[0179] Pressure: 0.35 mbar.

[0180] Substrate holder position (relative to microwave source): 10 cm.

[0181] Solid substrate temperature: 750 °C.

[0182] Synthesis time: about 10 hours. Step b) and step c) are performed simultaneously.

[0183] The nitrogen atomic concentration, measured by SIMS (Secondary Ion Mass Spectrometry), is 1.5 x 1018 atoms / cm3 in a 3 μm thick single crystal diamond layer. 19 The creation of NV centers is accomplished by irradiating the sample with a 200 keV electron beam at a dose of 10 18 electrons / cm2.

[0184] Nanocrystalline and microcrystalline diamond films can be used as hard and protective coatings, polycrystalline diamond films can be post-processed, for example polished and cut, to design optical windows and heat spreaders, intrinsic single crystal diamond films can be used as encapsulation layers, while single crystal diamond films containing NV centers can be used to design ultra-sensitive magnetometers.

Claims

1. A method for coating at least one solid substrate (12, 22, 32) with a synthetic diamond coating, comprising the following steps: a) placing at least one solid substrate (12, 22, 32) on a substrate holder (11, 21, 31) inside a reaction chamber (1, 2, 3) comprising a non-resonant cavity; b) injecting a gas mixture into the chamber (1, 2, 3); c) ionizing the gas mixture by subjecting it to a microwave electromagnetic field, the microwave frequency being comprised between 300 MHz and 6 GHz; and forming a diamond coating on the at least one solid substrate, characterized in that: - the gas mixture consists of methane, hydrogen, a third gas comprising at least one oxygen atom, and optionally a gaseous precursor of a doping element or of a gas-phase reactivity modifier, - the CH4 / H2 volume ratio is comprised between 0.1% and 10%, - the third gas / CH4 volume ratio is comprised between 0.1% and 120%, - the temperature of the solid substrate (12, 22, 32) is regulated during the step of forming the diamond coating between 100°C and 1000°C, - the internal pressure of the reaction chamber (1, 2, 3) is maintained between 10 -2 and 100 millibars, - the microwave electromagnetic field is generated by a set of microwave sources (13, 23, 33).

2. The method according to the preceding claim, further comprising one or more than one step of preparing the surface of the solid substrate (12, 22, 32).

3. The method according to claim 1 or 2, wherein steps b) and c) are repeated n times, n being an integer greater than or equal to 2, preferably 2 < n < 10, so as to form a series of diamond coatings on the solid substrate (12, 22, 32), the gas mixture being the same or different at each iteration.

4. The method according to any one of the preceding claims, wherein the solid substrate (12, 22, 32) is composite or non-composite and preferably consists of silicon, diamond, refractory metals or derivatives thereof, transition metals or derivatives thereof, stainless steel, superalloy, cemented carbide, polymer, ceramic, glass, selected oxides, semiconductors selected from the group consisting of group III-V and group II-VI semiconductors of the periodic table, piezoelectric materials, nitrides, carbides, sulfides, silicides or mixtures thereof.

5. The method according to any one of the preceding claims, wherein the thermal contact between the at least one solid substrate (12, 22, 32) and the substrate holder (11, 21, 31) is uniform over the entire contact surface between the at least one substrate and the substrate holder.

6. The method according to any one of the preceding claims, wherein the temperature of the solid substrate (12, 22, 32) is regulated by convection and / or by thermal conduction through the substrate holder (11, 21, 31).

7. The method according to any one of the preceding claims, wherein the substrate holder (11, 21, 31) is electrically isolated from the cavity of the reaction chamber (1, 2, 3).

8. The method according to the preceding claim, wherein the substrate holder (11, 21, 31) is biased.

9. The method according to any one of the preceding claims, wherein the distance between the at least one solid substrate (12, 22, 32) and the microwave source (13, 23, 33) is from 0.1 cm to 15 cm, preferably 10 cm.

10. A solid substrate (12, 22, 32) comprising one or more diamond layers obtained according to the method of any one of the preceding claims.

11. A device comprising the solid substrate (12, 22, 32) according to the preceding claim, selected from the group consisting of tools for cutting and perforating, eyeglasses, optical elements, windows, hard and protective coatings for scalpels, biocompatible coatings for implantable medical devices such as prostheses, dental implants, intracerebral electrodes, and electronic devices such as surface acoustic wave or acoustic wave devices, sensors, power electronics components, heat sinks for electronic and optoelectronic components, encapsulated electronic devices, and quantum sensors such as ultra-sensitive magnetometers.

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