Method for synthesizing diamond by distributed microwave plasmas

Abstract

The invention belongs to the field of diamond synthesis, and its application in particular in the field of protective coatings, biocompatible coatings and electronic devices comprising nanocrystalline, microcrystalline, polycrystalline and single-crystal diamond films. The invention relates to a method for synthesizing diamond by distributed microwave plasma, to a reaction chamber for implementing the method, to the substrate comprising a diamond layer obtained according to the method according to the invention, to a device chosen from hard and protective coatings for cutting and perforating tools, glasses, optics, windows, scalpels, biocompatible coatings for implantable medical devices, intracerebral electrodes and electronic devices, sensors, components for power electronics, heat sinks for electronic and optoelectronic components, encapsulated electronic devices, quantum sensors comprising a substrate comprising a layer of diamond obtained according to the method according to the invention.

Description

[0001] DIAMOND SYNTHESIS PROCESS USING DISTRIBUTED MICROWAVE PLASMAS

[0002] The invention belongs to the field of diamond synthesis, and to its application, in particular in the field of protective coatings, biocompatible coatings and electronic devices including nanocrystalline, microcrystalline, polycrystalline and monocrystalline diamond films.

[0003] The present invention relates to a method for synthesizing diamond using distributed microwave plasmas, to a reaction chamber for implementing the method and to a solid substrate comprising a layer of diamond obtained according to the method according to the invention.The invention also relates to a device chosen from hard and protective coatings for cutting and perforating tools, glasses, optics, windows, scalpels, biocompatible coatings for implantable medical devices such as prostheses, dental implants, intracerebral electrodes and electronic devices such as surface acoustic wave or guided acoustic wave devices, sensors, components for power electronics (diodes, transistors, metal-oxide gate field effect transistors (MOSFETs)), heat sinks for electronic and optoelectronic components, encapsulation of electronic devices, quantum sensors (magnetometers) comprising a solid substrate comprising a layer (or film) of diamond obtained according to the method according to the invention.

[0004] State of the art.

[0005] Diamond offers a combination of exceptional mechanical, electronic, thermal, quantum, and optical properties. These properties offer multiple potential for industrial applications in various fields. However, the properties of diamond depend crucially on the quality of the material and its origin.

[0006] Thus, natural crystals are of little interest for technological applications, due to their rarity, their prohibitive cost, their small size and the variability of their characteristics depending on their origins. The development of techniques allowing the reproducible synthesis of diamond and the control of its characteristics is therefore necessary. The first method developed in the 1950s for the synthesis of diamond is the so-called High Pressure / High Temperature (HPHT) method which is based on the conversion of carbon, mixed with a metallic catalyst, into diamond, by recreating extreme conditions of pressure and temperature identical to those produced by nature to form diamond.This technique allows the synthesis of diamond crystals but, even if major progress has recently been made, has a drawback linked to the presence in the crystal lattice of chemical impurities from the metallic solvents used, in addition to the incorporation of nitrogen which is difficult to control, which limits its use essentially to mechanical applications.

[0007] Other synthesis methods were developed in the 1980s to better control the composition and purity of diamonds while ensuring process reproducibility and offering the possibility of obtaining the material in the form of coatings. These include chemical vapor deposition (CVD) methods. The principle of these methods consists of activating a gas phase, consisting mainly of the elements hydrogen and carbon, by an external energy source, to generate radical species, such as methyl and atomic hydrogen, necessary for the construction of the crystalline structure of high-quality diamonds.

[0008] Depending on the type of energy source used to activate the gas phase, two types of CVD methods mainly exist for diamond synthesis.

[0009] The HFCVD (HF for "Hot Filament") process synthesizes diamond using a set of metal filaments, usually made of tungsten, heated by the Joule effect to very high temperatures. This method is inexpensive and allows the deposition of diamond on large surfaces and with complex geometry at speeds of up to 20 pm.h' 1. However, the filament placed very close to the substrate can degrade by heavily contaminating the deposited films, and by causing inhomogeneity in the temperature distribution along the filament. Another disadvantage of this method lies in the nature of the material constituting the filament. The latter, sensitive to oxidation and subject to corrosion, limits the choice of usable gas mixtures. Finally, the activation temperature of the gas phase remains limited by the melting temperature of the element constituting the filament, thus limiting the dissociation rate of the gaseous species. None of the aforementioned methods therefore allows for the production of high-purity diamond films (i.e. without graphite and / or other chemical impurities).

[0010] The second type of CVD process is based on the chemical activation of the gas phase using a plasma source, particularly microwave (MPA-CVD for "Microwave Plasma Assisted-CVD"). This is the most widely used technique for diamond synthesis for applications other than mechanical and tribological ones. There are several configurations of microwave reactors, most of which use resonant cavities, i.e., those whose dimensions allow a particular electromagnetic mode to be excited. These reactors can produce diamond films of good crystalline quality, ultra-pure, doped or including colored centers, but have several limitations. The CVD processes of the prior art use a resonant cavity with an antenna and a plasma that forms far from the antenna (e.g. 50 cm).The dimensions of the treated surface are limited by the size of the plasma which is itself limited, in resonant cavity reactors, by the frequency of the microwave wave used. Thus, the use of a microwave frequency of 2.45 GHz allows diamond to be deposited homogeneously on a surface of 2 to 3 inches in diameter maximum, while the use of a microwave frequency of 915 MHz allows diamond to be deposited on a surface of 4 to 6 inches in diameter maximum. In the latter case, the microwave power that must be injected is very high (30 to 60 kW), which poses efficiency problems at the industrial level (excessive power consumption). The substrate temperatures required for diamond growth are generally above 600 °C which prevents the use of heat-sensitive materials. The high reactivity of the plasma leads to high growth rates (> 1 pm.h'. 1), which does not allow the perfectly controlled synthesis of diamond films of submicrometer thickness. Finally, the use of a resonant cavity does not allow the processing of three-dimensional parts.

[0011] More recently, other microwave processes with linear antennas or surface waves have been developed to allow the synthesis of diamond on large surfaces (up to 30x30 cm 2 ) and at low temperature (up to 90 °C). However, these processes have various disadvantages, including: a) the impossibility of controlling the substrate temperature independently of other operating conditions; b) the need to inject high microwave powers, up to 20 kW; c) the very low growth rates of only a few nm.h' 1; d) the limitation to the treatment of flat substrates only; e) the difficulty of adapting experimental devices into industrial devices, to enlarge the deposition surface; f) obtaining nanocrystalline diamond films only.

[0012] Thus, regardless of the process chosen among the known processes, they all have different limitations which may be linked to the deposited surface, the nature of the diamond layer or its homogeneity. In addition, the geometry of the parts to be treated is a handicap in most of the known processes. Finally, the nature of the substrates which can be used is also sometimes restricted.

[0013] There is therefore a need to develop a process to overcome these limitations, particularly with regard to the deposited surface, the geometry of the parts to be treated, the nature of the substrates that can be used or the microstructure of the diamond layer obtained. There is also a need to obtain monocrystalline diamond films, preferably on a large scale.

[0014] It is to the applicant's credit to have developed a method for covering at least one solid substrate with a synthetic diamond coating. Thanks to the method according to the invention, the synthetic diamond coating is advantageously in monocrystalline, polycrystalline, microcrystalline or nanocrystalline form, and preferably monocrystalline. The coating corresponds to one or more layers of diamond.

[0015] A first subject of the invention is a method for covering at least one solid substrate with a synthetic diamond coating, comprising the steps: a) Placing the at least one solid substrate on a substrate holder, inside a reaction chamber comprising a cavity; b) Injecting a gas mixture into the chamber; c) Ionizing (or forming a plasma) the gas mixture by subjecting the gas mixture to a microwave electromagnetic field, the microwave frequency being in a range from 300 MHz to 6 GHz; and forming a diamond coating layer on the at least one solid substrate, characterized in that,

[0016] - the gas mixture consists of methane, dihydrogen, a third gas comprising at least one oxygen atom, and optionally a gaseous precursor of a doping element or modifier of the reactivity of the gas phase,

[0017] - the CH4 / H2 volume ratio is within a range of 0.1 to 10% (i.e. volume ratio of 0.001 to 0.1),

[0018] - the third gas / CH4 volume ratio is within a range from 0.1 to 120% (i.e. volume ratio from 0.001 to 1.2),

[0019] - the temperature of the solid substrate is regulated to a value in the range from 100 to 1000°C during the step of forming the diamond coating layer,

[0020] - the pressure inside the reaction chamber is maintained within a range of 10' 2 at 100 mbar,

[0021] - the microwave electromagnetic field being generated by means of a set of microwave sources.

[0022] At present, to the knowledge of the applicant, there is no method for forming a synthetic diamond coating, on a surface of solid substrate(s) made of diamond or not, with a dimension greater than or equal to 6 inches (dimension greater than 0.1016 μm or 0.1524 μm), in monocrystalline, polycrystalline, microcrystalline or nanocrystalline form.

[0023] The invention thus relates to a method for CVD synthesis of diamond by distributed microwave plasmas. The method according to the invention is a growth method which has a great versatility which is lacking in known synthesis process systems. The method according to the invention thus makes it possible to synthesize all forms of diamond, whether nano-, micro-, poly- or monocrystalline, preferably nano- and monocrystalline and also to produce an intrinsic material, doped or including colored centers, over a wide temperature range, that is to say, either at low temperature from 100 to 400 °C, which makes synthesis on thermosensitive solid substrates possible, or at high temperature from 400 to 1000 °C. The presence of the third gas comprising at least one oxygen atom makes it possible to provide additional etching species in the plasma which makes it possible to advantageously eliminate non-diamond phases (e.g. graphite, amorphous carbon, etc.).

[0024] Furthermore, the presence of the third gas is essential when one wishes to obtain monocrystalline diamond.

[0025] Furthermore, while most known experimental devices only allow diamond coating on flat surfaces (or "flat substrates"), the method according to the invention is suitable for both deposition on flat solid substrates and on so-called "three-dimensional" solid substrates.

[0026] The method according to the invention also makes it possible to homogeneously synthesize diamond films over large surfaces, with a diameter which may be greater than or equal to 8 inches (i.e. greater than or equal to 0.2032 m), when most known methods are limited to diameters of 4 inches (i.e. 0.1016 m).

[0027] The production conditions used in the process according to the invention lead to low growth rates, of a few nm.h' 1 , for example 5 nm.h' 1, several hundred nm.h' 1 , for example 200 nm.h' 1 , depending on the conditions, which allows precise control of thicknesses when these must be submicrometric. The growth rate can be in a range from 5 to 200 nm.h' 1 .

[0028] The device for implementing the method according to the invention is designed in such a way that the deposited thicknesses can be controlled in situ in real time using different optical techniques. Generally, the deposited thicknesses, per layer, can range from a few tens of nm, for example 30 nm, to several pm, for example 4 pm. The deposited thickness can be in a range from 30 nm to 4 pm. Thus, the solid substrate can be covered with one or more layers of diamond representing a thickness of the layers in a range from 30 nm to 4 pm.

[0029] Since the device for implementing the method according to the invention is not a resonant cavity, the reaction chamber has no physical limitations in terms of dimensions. Thus, the number of microwave sources can be increased as much as necessary in order to increase the volume of the plasma, in particular its diameter and, consequently, the deposition surface on the solid substrate, without modifying the original design of the reactor. The geometry, the dimensions of the chamber (height, diameter), the openings (windows, doors), the shape, the position and the dimensions of the substrate holder and the solid substrate do not impact the coupling of the electromagnetic wave to the plasma which takes place directly at the end of each microwave source, which constitutes fewer constraints compared to resonant cavities. In other words, the cavity of the reaction chamber in which the covering method according to the invention is implemented is a non-resonant cavity.The cavity is therefore not configured to excite a particular electromagnetic mode. Thanks to this non-resonant cavity, low growth rates are obtained which allow for homogeneous deposition.

[0030] The coupling of the electromagnetic wave to the plasma takes place directly at the end of each microwave source. The microwave sources are therefore preferably positioned so that one end is placed at the cavity. This allows the use of a non-resonant cavity.

[0031] The device (enclosure or reaction chamber) according to the invention can also be placed within a resonant cavity. This embodiment allows for two excitation sources (two types of plasma):

[0032] 1) the point microwave sources of the method and device according to the invention, thus making it possible to benefit from low growth rates, and

[0033] 2) a coupling which uses the resonant cavity allowing the implementation of a very energetic plasma and thus allowing high growth speeds, as in conventional reactors.

[0034] It is thus possible, in a device according to the invention (i.e. an enclosure or a reaction chamber comprising a non-resonant cavity) placed within a resonant cavity, to grow an intrinsic or doped film several tens of pm thick in resonant mode, then to make a layer according to the method according to the invention (for example delta-doped with nitrogen to create NV centers, or an encapsulation layer), over a small thickness (from 10 to 100 nm) in distributed plasma mode.

[0035] The method according to the invention allows a homogeneous deposition of the diamond layer. By "homogeneous deposition" is meant a deposit whose characteristics such as thickness, purity, grain size, texture do not vary by more than 10% over the entire surface of the sample and therefore the nano-, micro-, poly- or mono-crystalline nature remains unchanged over the entire sample. When the three-dimensional solid substrate has geometric patterns on its surface such as pads, pillars, grooves, etch pits or electrodes, with lateral dimensions and / or depth ranging from 100 nm to 5 cm, the synthesis is carried out in a conformal manner, that is to say that the deposit takes the form of the geometric patterns.

[0036] In the present invention, nanocrystalline diamond is understood to mean a diamond in the form of particles of average size ranging from approximately 5 to 50 nm, and preferably ranging from approximately 15 to 20 nm.

[0037] In the present invention, microcrystalline diamond is understood to mean a diamond in the form of particles with an average size ranging from approximately 51 nm to 1 pm, and preferably ranging from approximately 500 nm to 1 pm.

[0038] The size of the diamond particles can be determined by techniques well known to those skilled in the art and preferably by transmission electron microscopy, or X-ray diffraction.

[0039] In the present invention, polycrystalline diamond means a diamond in the form of several crystals and, in the usual terms, a diamond in the form of particles with an average size greater than 1 μm.

[0040] In the present invention, monocrystalline diamond means a diamond in the form of a single crystal.

[0041] The monocrystalline or polycrystalline nature of diamond can be determined by transmission electron microscopy.

[0042] According to step a) of the method according to the invention, at least one solid substrate is placed on a substrate holder, inside a reaction chamber. The solid substrate can be placed manually or by an automated process using a rod or a motorized arm.

[0043] Advantageously, the at least one solid substrate may be made of diamond or not, and preferably of diamond. A solid diamond substrate makes it possible in particular to promote the production of monocrystalline diamond. The solid substrate may be flat or three-dimensional. A flat solid substrate is defined as an object in which one of its dimensions (width, length or height) is significantly smaller, at least by a factor of 10, than the other two dimensions (width, length or height). When there are several solid substrates, they may be identical or different. These may be flat solid substrates of 8 inches in diameter or more (i.e. 0.2032 m in diameter or more), solid substrates of complex shape (three-dimensional) up to several centimeters in height or even several solid substrates, flat or three-dimensional, arranged simultaneously on the substrate holder as long as their lateral dimension allows it.

[0044] Advantageously, the at least one solid substrate may be composite or non-composite. Preferably, the solid substrate is composed of silicon, diamond, refractory metal, such as iridium, or its derivatives such as alloys, oxides or nitrides of said refractory metal, transition metal, such as titanium, or its derivatives such as alloys (e.g. titanium-based alloy such as TA6V), oxides or nitrides of said transition metal, stainless steel, a superalloy, a cemented carbide, a polymer, a ceramic, glass, an oxide (e.g. fused silica or alumina), a semiconductor chosen from semiconductors of columns III-V and II-VI of the periodic table of elements, a piezoelectric material, a nitride, a carbide, a sulfide, a silicide or a mixture thereof.Particularly preferably, the solid substrate is composed of diamond, refractory metal, such as iridium, and its derivatives such as alloys, oxides or nitrides of said refractory metal, transition metal, such as titanium, and its derivatives such as alloys (e.g. titanium-based alloy such as TA6V), oxides or nitrides, stainless steel, a superalloy, a cemented carbide, a polymer, a ceramic, glass, oxide (e.g. fused silica or alumina), a semiconductor chosen from semiconductors of columns III-V or II-VI of the periodic table of elements, a piezoelectric material, a nitride, a carbide, a sulfide, a silicide or a mixture thereof; and more particularly preferably the solid substrate is composed of diamond.

[0045] Advantageously, the thermal contact between the at least one solid substrate and the substrate holder is preferably homogeneous over the entire contact surface between the at least one solid substrate and the substrate holder. The surface of the substrate holder can thus be adapted to the surface of the solid substrate with which it is intended to be in contact. The term "contact surface" means the percentage of the face of the solid substrate that is in direct contact with the substrate holder on which the solid substrate rests. This percentage is ideally equal to 100%. If the solid substrate is corrugated or has a particular shape, the shape of the substrate holder must be adapted to the solid substrate. Alternatively, it is possible to use a substance to ensure contact between the solid substrate and the substrate holder if the dimensional differences between the solid substrate and the substrate holder (at the contact surface) are not too great, for example thermal paste.

[0046] Advantageously, the substrate holder may be made of molybdenum or inconel or coated with diamond. Preferably, the substrate holder is electrically insulated from the cavity of the reaction chamber. The substrate holder is preferably polarized. In particular, the substrate holder may be provided with a polarization system by which a voltage is applied to it via a metal conductor (rod, electric wire, coaxial cable, etc.), the reaction chamber or one or more metal parts arranged above the solid substrate, acting as counter-electrodes. The polarization may be DC, pulsed DC, AC (50 Hz), RF (13.56 MHz), low frequency or high frequency in the range 1 Hz to 1 MHz, according to various periodic patterns (pulses or saw teeth for example).

[0047] Advantageously, the substrate holder polarization system provides an additional degree of freedom compared to known microwave processes by assisting the growth steps, including germination, cleaning, etching, ion implantation, doping, or generation of vacancies in the deposits, by ion bombardment. This aspect is advantageous either to ensure good adhesion of the diamond film to the solid substrate, or to promote doping or the formation of colored centers involving vacancies (NV, SiV, GeV, or SnV centers).

[0048] The polarization system thus makes it possible to advantageously assist by ion bombardment, by superposition on the microwave plasma generated from elementary microwave sources:

[0049] - diamond germination, that is, the agglomeration of carbon atoms to form nuclei (germs) which will develop to then form isolated crystals, on the surface of non-diamond solid substrates and generally in the presence of excess carbon precursors in the gas phase;

[0050] - cleaning the solid substrate prior to diamond synthesis, advantageously using plasmas of gas mixtures containing hydrogen, argon or oxygen atoms;

[0051] - etching the solid substrate to, for example, remove the native oxide layer of solid silicon substrates or to reveal etching patterns (well known as "etch pits") on the surface of solid diamond substrates, advantageously using plasmas of gas mixtures containing atoms of hydrogen, argon, oxygen or halogenated precursors containing atoms such as chlorine or fluorine;

[0052] - the etching of diamond deposits, in particular to reduce their thickness, using plasmas of gas mixtures containing hydrogen atoms, noble gases such as argon or krypton, oxygen or halogenated precursors containing atoms such as chlorine or fluorine;

[0053] - the implantation of atoms on the surface of certain solid substrates, in particular carbon in the general presence of excess carbon precursors in the gas phase, in order to create a carbide-type conversion layer intended to promote the adhesion of the diamond film to the solid substrate;

[0054] - doping during growth with other atoms (boron, phosphorus, silicon, nitrogen, tin, etc.) likely to modify in particular the electronic, optical and quantum characteristics of the deposited diamond;

[0055] - the generation of carbon vacancies which, associated with the presence of substituting atoms such as nitrogen, silicon, germanium or tin, make it possible to create colored centers involving vacancies (NV, SiV, GeV, or SnV centers for example).

[0056] Advantageously, the substrate holder may be provided with a vertical translation system for adjusting the distance to the plane of the sources, said plane being defined by the source matrix and is parallel to the surface of the substrate holder. The substrate holder may be provided with a rotation system, for example in order to optimize the homogeneity of the deposition of the diamond layer over the entire surface of the solid substrate exposed to the plasma. This characteristic is useful, in particular for three-dimensional parts, some of whose faces may be more difficult to access for the plasma. The substrate holder may be provided with a heating / cooling system for regulating the temperature of the solid substrate during the implementation of the method. This temperature control can be done independently of the operating conditions (gas pressure, gas mixture, gas flow rate, microwave power applied to the sources, polarization power / voltage).

[0057] Advantageously, the temperature of the solid substrate is regulated to a value in the range from 100 to 1000°C during the step of forming the diamond coating layer, preferably from 600 to 1000°C and more preferably 800°C. The temperature of the solid substrate is regulated, i.e. the at least one solid substrate is preferably heated, cooled or maintained at temperature, by convection and / or thermal conduction through the substrate holder.

[0058] The different microstructures of diamond which can be obtained by the process according to the invention can be obtained according to the following temperature ranges:

[0059] - from 100 to 400°C: nanocrystalline,

[0060] - from 400 to 600°C: microcrystalline,

[0061] - from 600 to 800 °C: polycrystalline on non-diamond solid substrate, monocrystalline on diamond solid substrate, for a gas mixture composed of 96.4% by volume of H2, 2.6% by volume of CH4 and 1% by volume of CO2, a power of 187 W per source and at a distance of 10 cm from the sources.

[0062] According to a particularly preferred embodiment of the method of the invention, the method leads to a coating of monocrystalline synthetic diamond according to one or other of the following characteristics, and more preferably according to all of the following characteristics:

[0063] - the solid substrate is a solid diamond substrate,

[0064] - the temperature of the solid substrate ranges from 650 to 850°C,

[0065] - the pressure inside the reaction chamber ranges from 0.25 to 0.45 mbar, - the CH4 / H2 volume ratio ranges from 0.1 to 4%,

[0066] - the third gas / CH4 volume ratio ranges from 40 to 120%,

[0067] - a power ranging from 75 to 200 W per source,

[0068] - the solid substrate being at a distance of 4 to 12 cm from the sources.

[0069] Advantageously, the method according to the invention may further comprise one or more steps of preparing the surface of the solid substrate. Two cases may 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 such as those described in the invention). Thus, when the solid substrate is made of diamond, the method according to the invention may further comprise a step of preparing the surface of the solid substrate and optimizing the thermal contact with the substrate holder, chosen from a chemical treatment with acid (aqua regia), cleaning using organic solvents (such as ethanol, acetone), drying, etching by H2 / O2 plasma and bonding to the substrate holder.When the solid substrate is not made of diamond, the method according to the invention may further comprise a cleaning step using organic solvents (such as ethanol, acetone), abrasion or spin coating or immersion using a solution containing diamond grains of a size ranging from a few nm to a few tens of pm in suspension, drying, bonding to the substrate holder.

[0070] According to step b) of the method according to the invention, a gas mixture is injected into the reaction chamber. Step b) of injecting a gas mixture into the chamber may comprise three phases:

[0071] 1) before the introduction of microwaves, for several minutes, or even several tens of minutes, the time for the other growth parameters such as the temperature of the solid substrate or the pressure to stabilize,

[0072] 2) throughout the entire duration of microwave injection (or submission), which corresponds to the diamond growth phase. This injection (or submission) is generally done continuously, but the proportions of the gas mixture can be modified during this stage in order to modify the characteristics of the deposited diamond and consequently to create a stack of diamond layers with different properties, 3) once the microwaves are cut off in order to quickly cool the sample.

[0073] The composition of the gas mixture can be controlled using mass flow meters. The gas mixture can consist of methane, dihydrogen, and a third gas comprising at least one oxygen atom. The term "third gas comprising an oxygen atom" means a gas preferably chosen from CO2, CO, O2, and one of their mixtures. The gas mixture can consist of methane, dihydrogen, a third gas comprising at least one oxygen atom and optionally a gaseous precursor of a doping element or modifier of the reactivity of the gas phase (in particular by modifying the thermal conductivity of the gas mixture). It is for example possible to modify the thermal conductivity of the plasma by adding argon to the gas mixture as a substitute for hydrogen and by varying the [Ar] / [H2] ratio. This modification is likely to change the temperature of the gas and consequently its reactivity.For example, an increase in the [Ar] / [H2] ratio leads to a decrease in the thermal conductivity of the plasma and consequently to an increase in the temperature of the gas likely to promote radical chemical reactions within the gas phase. The precursor or modifier may preferably be a gas chosen from N2, N2O, NH3, B2H6, BBrs, B(CH3)3, PH3, PBrs, SiH4, GeH4, SnH4, Ar and He. The different gases composing the gas mixture are pre-mixed before introduction into the reaction chamber or enclosure. The introduction of the gas mixture into the reaction chamber or enclosure is done through one or more injectors (orifices) whose diameters may be identical or different and may be adjusted in order to modify and control the flow rate of the gas mixture through each injector (orifice) independently of the other injectors.

[0074] Gases containing nitrogen, such as N2, N2O and NH3, as a precursor or modifier, allow the incorporation into the diamond layer of nitrogen atoms which, associated with carbon vacancies (NV centers), have a spin state that can be read and manipulated optically at room temperature; which gives them the right properties for the design of quantum sensors. Gases containing a group XIV element, as a precursor or modifier, such as SiH4, GeH4 and SnH4, allow the incorporation of group XIV atoms, such as Si, Ge and Sn, respectively, which, associated with carbon vacancies, allow the creation of SiV, GeV and SnV colored centers, respectively; which gives them properties comparable to NV centers.Gases such as B2H6, BBrs, B(CH3)3 or PH3, PBrs, as precursor or modifier, allow to incorporate respectively boron B and phosphorus P atoms and to obtain P-type (acceptor) and N-type (donor) doping, respectively. For example, a gas mixture composed of 96.4% by volume of H2, 2.6% by volume of CH4, 1% by volume of CO2 and 0.04% by volume of N2 allows to obtain on a solid substrate of High Pressure / High Temperature (HPHT) diamond a layer of monocrystalline diamond containing NV centers.If we consider the basic gas mixture composed of 96.4% by volume of H2, 2.6% by volume of CH4 and 1% by volume of CO2 and we replace part of the H2 flow with an argon (Ar) flow so as to obtain, for example, a gas composition of 46.4% by volume of H2, 2.6% by volume of CH4, 1% by volume of CO2 and 50% by volume of Ar, the thermal conductivity of the gas mixture decreases by half, which causes an increase in the gas temperature leading to a modification of the reaction processes within the plasma.

[0075] Advantageously, in the gas mixture, the CH4 / H2 volume ratio can be within a range from 0.1 to 10% (i.e. volume ratio from 0.001 to 0.1), preferably from 2 to 3% (i.e. volume ratio from 0.02 to 0.03).

[0076] Advantageously, in the gas mixture, the third gas / CH4 volume ratio can be within a range from 0.1 to 120% (i.e. volume ratio from 0.001 to 1.2), preferably 40% to 120% (i.e. volume ratio from 0.4 to 1.2).

[0077] The ratio can be adapted according to the desired diamond microstructure. For example, a gas mixture composed of 96.4 vol% H2, 2.6 vol% CH4 and 1 vol% CO2 allows the synthesis of nanocrystalline diamond for a solid substrate temperature of 100 to 400 °C. At 750 °C a gas mixture composed of 94.4 vol% H2, 2.6 vol% CH4 and 3 vol% O2 allows the synthesis of monocrystalline diamond on solid HPHT diamond substrates. At 650 °C the use of a gas mixture composed of 96.4 vol% H2, 2.6 vol% CH4 and 1 vol% CO2 leads to the formation of microcrystalline diamond.

[0078] Advantageously, in the gas mixture, the volume ratio of gaseous precursor of doping element or reactivity modifier / CH4 can be in a range from 0.001 to 10000% (i.e. volume ratio from 0.00001 to 100). Illustrations have been given above.

[0079] According to step c) of the method according to the invention, the gas mixture is ionized (or transformed into plasma) by being subjected to a microwave electromagnetic field. The frequency of the microwaves can be in a range from 300 MHz to 6 GHz. The frequency is preferably chosen from 466 MHz, 915 MHz, 2.45 GHz and 5.8 GHz, 915 MHz and 2.45 GHz being the two standardized frequencies authorized for industrial applications. The microwave power can range from several watts to several hundred watts per source and can be injected continuously (continuous mode) or discontinuously (pulsed mode) following a cyclic pattern (pulses, sinusoids, sawtooth or others) by varying the repetition frequency of the pattern, the average power, the peak power and the duty cycle in the case of pulses.

[0080] Advantageously, microwaves are generated by microwave sources.

[0081] The term "microwave source" means an electromagnetic coupling device (antenna), generally axisymmetric, consisting of a central metallic core advantageously protected by an insulating refractory material, such as alumina, and having an impedance matching system. Microwave sources are for example described in patent application W003 / 103003A1. The power of a microwave source can be in a range from 1 W to 500 W, preferably the power is 200 W. In the invention, a set of microwave sources is implemented. In other words, the number of microwave sources is greater than or equal to two sources. The plasma generated by a source is axisymmetric and the surface defined by the intersection between the plasma and a plane parallel to the plane of the sources (hatched area, Fig. 5) varies from 1 to 20 cm 2depending on the plasma gas pressure, the microwave power and the distance from the source. For a set of / V coplanar sources d cm apart arranged in a square matrix, N being an integer greater than or equal to 4, the dimension of the plasma in a plane parallel to the source matrix (hatched area part 52 Fig. 5) is at least Ç(4N - 1) xd) cm 2 .

[0082] Advantageously, the sources can be magnetized using permanent magnets. Magnetizing the sources allows electrons to be trapped in an axisymmetric dipolar static magnetic field and to communicate a maximum of energy to them by electron cyclotron resonance, which generates dense and energetic plasmas that increase chemical reactivity and therefore the production of active species.

[0083] Advantageously, step c) can have a duration ranging from several minutes to several hours, or even tens of hours. Preferably, step c) is carried out for as long as necessary to obtain the desired diamond thickness, typically from 1 minute to 100 hours.

[0084] Advantageously, steps b) and c) are implemented simultaneously.

[0085] Advantageously, the microwave sources form a planar matrix of sources or have a three-dimensional configuration adapted to the geometry of the solid substrate(s) (Figs. 1, 2 and 3). Optimization of the growth conditions (pressure, microwave power, surface temperature and / or gas composition) makes it possible to obtain plasma homogeneity (electron density and temperature, gas temperature, density of active species) over several centimeters perpendicular to the solid substrate and over several tens, or even hundreds of centimeters depending on the number of microwave sources, parallel to the solid substrate, which is favorable to the homogeneous treatment of large planar parts and three-dimensional parts, without resorting to a three-dimensional configuration of the sources.

[0086] Advantageously, the distance between the at least one solid substrate and a microwave source may be in a range from 0.1 cm to 15 cm, preferably from 5 to 15 cm, more preferably 10 cm. Those skilled in the art may apply a distance determined according to the desired microstructure for the diamond film. Thus, the further the substrate holder is from the sources, the more the microstructure obtained on non-diamond solid substrates evolves from polycrystalline to nanocrystalline, passing through microcrystalline. The distance mentioned 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, therefore the total surface of the solid substrate less the surface of the latter which is in contact with the substrate holder which holds it.Advantageously, when the process 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 to 12 cm a few centimeters from the plane of the sources, this plasma is perfectly homogeneous in the plane parallel to the plane of the sources and homogeneous over a few centimeters perpendicular to this plane. Depending on the number of sources used, this configuration makes it possible to obtain homogeneous diamond deposits over large areas, typically 8 inches in diameter or more (1 inch = 0.254 m), which makes it possible to treat different configurations of solid substrates when these are placed on a substrate holder suitably located relative to the sources.

[0087] According to the method according to the invention, the ionization of the gas mixture leads to the formation of a diamond coating layer on the at least one solid substrate.

[0088] According to the method according to the invention, during the implementation of steps b) and c), the pressure inside the reaction chamber is maintained within a range of 10' 2 at 100 mbar, preferably 5x10 -2 at 50 mbar, and preferably still at 10 -1 at 1 mbar.

[0089] Advantageously, steps b) and c) can be repeated a number of iterations n, n being an integer greater than or equal to 2, preferably 2 < n < 10, so as to form a succession of diamond coating layers on the solid substrate, the gas mixture being identical or different at each iteration.

[0090] Advantageously, the method according to the invention may further comprise a step d) of measuring the temperature of the solid substrate. The temperature measurement may be carried out by thermocouple or by pyrometry. This step makes it possible in particular, for the treatment of three-dimensional solid substrates, for which temperature homogeneity is important, to make deposits that are homogeneous in thickness, microstructure and quality.

[0091] Advantageously, the method according to the invention may further comprise a step e) of in situ optical monitoring. The monitoring step e) may be implemented by means of an in situ optical monitoring system, preferably chosen from among monitoring systems by interference pyrometry, laser reflectance interferometry and by spectroscopic reflectometry. This step makes it possible to measure in real time the thickness and the optical index of the diamond layer during growth, or to detect all the modifications occurring on the surface of the solid substrate (for example germination, etching, a change in topography and a change in composition).

[0092] Advantageously, steps d) and e) can be implemented simultaneously with steps a), b) and c), throughout the implementation of the method according to the invention.

[0093] Another object of the invention is a reaction chamber comprising:

[0094] - a substrate holder, capable of receiving a solid substrate or set of flat or three-dimensional solid substrates,

[0095] - a set of microwave sources and

[0096] - optionally optical ports.

[0097] The optical ports, microwave sources, substrate holder and solid substrate (planar or three-dimensional) are as defined in the invention.

[0098] Advantageously, the reaction chamber consists of a vacuum chamber topped by a set of microwave probes. The dimensions of the reaction chamber can range from 25 liters for 16 sources to 100 liters for 36 sources. The microwave sources are preferably elementary plasma sources powered by one or more microwave generators. The different sources can be arranged in a two-dimensional matrix (all coplanar sources) or in a three-dimensional configuration. The arrangement of the set of sources can be adapted by a person skilled in the art depending on the solid substrate(s) on which the method will be implemented. These microwave sources can be magnetized or not.

[0099] Advantageously, the reaction chamber according to the invention can be arranged within a resonant cavity making it possible to excite a particular electromagnetic mode. By means of suitable electromagnetic coupling, in addition to point microwave sources, it then becomes possible to initiate two types of plasma: a distributed microwave plasma having the advantages described above, offering in particular low growth rates, and a very energetic microwave plasma allowing high growth rates equivalent to conventional reactors in a resonant cavity. The invention also relates to a solid substrate comprising one or more layers of diamond, obtained according to the method according to the invention.

[0100] The solid substrate is preferably as defined in the invention.

[0101] Advantageously, the solid substrate comprising one or more layers of diamond, obtained according to the method according to the invention, can be in various forms. The layer(s) of diamond can be in nanocrystalline, microcrystalline, polycrystalline and monocrystalline form, and preferably monocrystalline.

[0102] Depending on the nature of the gas mixture, the intrinsic properties of the diamond can be advantageously modified, for example doped (examples: with boron or phosphorus atoms) or including colored centers (examples: NV, GeV, SiV, SnV centers).

[0103] According to a particularly preferred embodiment of the invention, the layer(s) of diamond deposited on the solid substrate are layers including NV centers.

[0104] The synthesis of nanocrystalline diamond films at low solid substrate temperatures (i.e., 100 to 400 °C) makes it possible to process heat-sensitive materials. The synthesis of monocrystalline diamond films is effective at high solid substrate temperatures (i.e., 600 to 1000 °C) on solid diamond substrates. The synthesis of nanocrystalline, microcrystalline, and polycrystalline diamond films on several planar or three-dimensional solid substrates arranged simultaneously on the substrate holder is made possible by the method according to the invention. The method further allows the synthesis of monocrystalline diamond films on several planar diamond solid substrates arranged simultaneously on the substrate holder.The method according to the invention also allows very low growth rates, from a few nanometers (at low pressure <0.25 mbar, low power <75 W / source, low proportion of methane <1%) to a few tens of nanometers per hour (at higher pressure >0.35 mbar, high power >180 W / source, high proportion of methane >2%), which allows precise control of the thicknesses deposited in a range of a few nanometers to a few micrometers.

[0105] According to a more particularly preferred embodiment of the invention, the layer(s) of diamond deposited on the solid substrate are layers of undoped monocrystalline diamond or layers of monocrystalline diamond including NV centers.

[0106] The invention also relates to a device comprising a solid substrate obtained by the method according to the invention. The solid substrates coated with diamond films produced using the method according to the invention are used in particular in protective coatings for mechanics and optics, biomedical devices, electronic and quantum devices.

[0107] Advantageously, the device comprising a solid substrate obtained by the method according to the invention can be chosen from hard and protective coatings for cutting and perforating tools, glasses, optics, windows, scalpels, biocompatible coatings for implantable medical devices such as prostheses, dental implants, intracerebral electrodes and electronic devices such as surface acoustic wave or guided acoustic wave devices, sensors, components for power electronics (chosen from diodes, transistors, and MOSFET metal-oxide gate field effect transistors), heat sinks for electronic and optoelectronic components, encapsulated electronic devices and quantum sensors such as ultrasensitive magnetometers.

[0108] Advantageously, when the diamond layer(s) deposited on the solid substrate are monocrystalline, and in particular thanks to their controlled thicknesses, the device comprising a solid substrate obtained by the method according to the invention can be chosen from ultrasensitive quantum magnetometers. Indeed, this type of application requires colored centers located near the surface at known depths with nanometric precision, which requires the creation of diamond layers incorporating nitrogen atoms and carbon vacancies from a few nanometers to a few tens of nanometers thick (delta-doping).

[0109] Brief description of the figures:

[0110] Figure 1 represents an example of a reaction chamber 1 according to the invention comprising a substrate holder 11, on which is arranged a flat solid substrate 12, a set of microwave sources 13 and optical ports 14.

[0111] Figure 2 represents an example of a reaction chamber 2 according to the invention comprising a substrate holder 21, on which is arranged a three-dimensional solid substrate 22 (i.e. the entire surface of which is to be coated except for the parts in contact with the substrate holder) of cubic shape with a side of 2 cm, a set of microwave sources 23 and optical ports 24.

[0112] Figure 3 represents an example of a reaction chamber 3 according to the invention comprising a substrate holder 31, on which is arranged a set of solid substrates 32, a set of microwave sources 33 and optical ports 34.

[0113] Figure 4 shows (1) a substrate holder A of 10 cm diameter [dimension of this example], (2) the substrate holder A on which is arranged a large-sized (10 cm diameter) planar solid substrate B, (3) the substrate holder A on which is arranged a plurality of planar solid substrates B of 1 cm diameter and (4) the substrate holder A on which is arranged a plurality of three-dimensional substrates C of tungsten carbide drills and 6 mm diameter, 6 cm long including 3 cm of covered helix.

[0114] Figure 5 represents the plasma generated by a single microwave source (1) according to the prior art and by a set of sources according to the invention arranged in a flat matrix (2). The hatched area represents the intersection between the volume of the plasma and a plane arranged parallel to the plane of the sources.

[0115] Figure 6 represents: 1. Surface of a nanocrystalline diamond film observed by scanning electron microscopy (SEM). 2. Surface of a microcrystalline diamond film observed by SEM. 3. Surface of a polycrystalline diamond film observed by SEM. 4. Surface of a monocrystalline diamond film observed by SEM and transmission electron diffraction pattern showing the monocrystalline character of the deposit. 5. Photoluminescence spectra obtained with laser excitation at 473 nm of a nitrogen-doped monocrystalline film before and after electron irradiation, the peaks at 575 nm (NV°) and 673 nm (NV-) prove the presence of NV centers after irradiation.

[0116] The invention is further illustrated by the following examples, in a non-limiting manner.

[0117] Example 1: Implementation of the method according to the invention.

[0118] Consider a reactor 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. Consider a circular substrate holder 4 inches in diameter, temperature-regulated, arranged along the axis of this chamber.

[0119] 1. Synthesis of nanocrystalline diamond films (Fig. 6-1)

[0120] Circular non-diamond solid silicon substrate 4 inches in diameter and 0.5 mm thick.

[0121] Solid substrate preparation: seeding (spin coating or dip coating) with diamond powder with an average diameter of 25 nm; abrasion with diamond powder with an average diameter of 30 pm.

[0122] CH4 / H2 / CO2 gas mixture. [CP] = 2.6% and [CO2] = 1%. The gas mixture is injected at a flow rate of 50 SCCM (standard cubic centimeter per minute), throughout the synthesis.

[0123] Microwave power per source: 190 W.

[0124] Pressure: 0.35 mbar.

[0125] Substrate holder position (relative to microwave sources): 10 cm.

[0126] Solid substrate temperature: 250°C.

[0127] Synthesis time: approximately 5 hours. Steps b) and c) are carried out simultaneously.

[0128] In this example 1, the polycrystalline diamond obtained is nanocrystalline.

[0129] 2. Synthesis of microcrystalline diamond films (Fig. 6-2)

[0130] Circular non-diamond solid silicon substrate 4 inches in diameter and 0.5 mm thick

[0131] Solid substrate preparation: seeding (spin coating or dip coating) with diamond powder with an average diameter of 25 nm; abrasion with diamond powder with an average diameter of 30 pm

[0132] CH4 / H2 / CO2 gas mixture. [CH4] = 2.6% and [CO2] = 1%. The gas mixture is injected at a flow rate of 50 SCCM, throughout the synthesis.

[0133] Microwave power per source: 190 W.

[0134] Pressure: 0.25 mbar.

[0135] Substrate holder position (relative to microwave sources): 6 cm. Solid substrate temperature: 400°C.

[0136] Synthesis time: approximately 5 hours. Steps b) and c) are carried out simultaneously.

[0137] In this example 2, the polycrystalline diamond obtained is microcrystalline.

[0138] 3. Synthesis of polycrystalline diamond films (Fia. 6-3)

[0139] Circular non-diamond solid silicon substrate 4 inches in diameter and 0.5 mm thick.

[0140] Solid substrate preparation: seeding (spin coating or dip coating) with diamond powder with an average diameter of 25 nm; abrasion with diamond powder with an average diameter of 30 pm.

[0141] CH4 / H2 / CO2 gas mixture. [CP] = 2.6% and [CO2] = 1%. The gas mixture is injected at a flow rate of 50 SCCM, throughout the synthesis.

[0142] Microwave power per source: 190 W.

[0143] Pressure: 0.35 mbar.

[0144] Substrate holder position (relative to microwave sources): 10 cm.

[0145] Solid substrate temperature: 650°C.

[0146] Synthesis time: approximately 5 hours. Steps b) and c) are carried out simultaneously.

[0147] In this example 3, the polycrystalline diamond has a grain size greater than 1 pm.

[0148] 4. Synthesis of intrinsic single-crystal diamond films (Fig. 6-4)

[0149] Solid diamond substrate, High Pressure High Temperature (HPHT) type Ib

[0150] Solid substrate preparation: H2 / O2 microwave plasma etching (98% / 2%), 2.6 kW, 170 mbar, 780°C surface temperature, in a cavity reactor for 1 hour.

[0151] CH4 / H2 / O2 gas mixture. [CH4] = 2.6% and [O2] = 3%. The gas mixture is injected at a flow rate of 50 SCCM, throughout the synthesis.

[0152] Microwave power per source: 190 W.

[0153] Pressure: 0.35 mbar.

[0154] Substrate holder position (relative to microwave sources): 10 cm.

[0155] Solid substrate temperature: 750°C. Synthesis time: approximately 10 hours. Steps b) and c) are carried out simultaneously.

[0156] 5. Synthesis of Nitrogen-Doped Single Crystal Diamond Films for the Creation of Nitrogen-Vacancy (NV) Color Centers (Fia. 6-5)

[0157] Solid diamond substrate, intrinsic CVD type, 50 pm thick, deposited on a High Pressure High Temperature (HPHT) Ib substrate.

[0158] Solid substrate preparation: H2 / O2 microwave plasma etching (98% / 2%), 2.6 kW, 170 mbar, 780°C surface temperature, in a cavity reactor for 1 hour.

[0159] CH4 / H2 / O2 / N2 gas mixture. [CH4] = 2.6%, [O2] = 3% and [N2] = 0.04%. The gas mixture is injected at a flow rate of 50 SCCM, throughout the synthesis.

[0160] Microwave power per source: 190 W.

[0161] Pressure: 0.35 mbar.

[0162] Substrate holder position (relative to microwave sources): 10 cm.

[0163] Solid substrate temperature: 750°C.

[0164] Synthesis time: approximately 10 hours. Steps b) and c) are carried out simultaneously.

[0165] The concentration of nitrogen atoms measured by S IMS (secondary ion mass spectrometry) is 1.5x10 19 atoms / cm 3 in a 3 pm thick layer of monocrystalline diamond. The creation of the NV centers is completed by irradiating the sample with a 200 keV electron beam for a dose of 1018 electrons / cm 2

[0166] Nanocrystalline and microcrystalline diamond films can be used as hard and protective coatings, polycrystalline diamond films can be post-processed such as polishing and cutting to design optical windows and heat sinks, intrinsic single-crystal diamond films can be used as encapsulation layers, and single-crystal diamond films containing NV centers can be used for the design of ultrasensitive magnetometers.

Claims

Claims 1. Method for covering at least one solid substrate (12, 22, 32) with a synthetic diamond coating, comprising the steps: a) Placing the 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 the gas mixture to a microwave electromagnetic field, the microwave frequency being in the range from 300 MHz to 6 GHz; and forming a diamond coating layer on the at least one solid substrate, characterized in that, - the gas mixture consists of methane, dihydrogen, a third gas comprising at least one oxygen atom, and optionally a gaseous precursor of a doping element or modifier of the reactivity of the gas phase, - the CH4 / H2 volume ratio is within a range of 0.1 to 10%, - the third gas / CH4 volume ratio is within a range from 0.1 to 120%, - the temperature of the solid substrate (12, 22, 32) is regulated to a value in the range from 100 to 1000°C during the step of forming the diamond coating layer, - the pressure inside the reaction chamber (1, 2, 3) is maintained within a range of 10' 2 at 100 mbar, - the microwave electromagnetic field being generated by means of a set of microwave sources (13, 23, 33).

2. Method according to the preceding claim, further comprising one or more steps of preparing the surface of the solid substrate (12, 22, 32).

3. Method according to claim 1 or 2, in which steps b) and c) are repeated a number of iterations n, n being an integer greater than or equal to 2, preferably 2 < n < 10, so as to form a succession of diamond coating layers on the solid substrate (12, 22, 32), the gas mixture being identical or different at each iteration. Tl 4. Method according to any one of the preceding claims, in which the solid substrate (12, 22, 32) is composite or non-composite, and preferably composed of silicon, diamond, refractory metal or its derivatives, transition metal or its derivatives, stainless steel, a superalloy, a cemented carbide, a polymer, a ceramic, glass, a selected oxide, a semiconductor selected from the semiconductors of columns III-V and II-VI of the periodic table of elements, a piezoelectric material, a nitride, a carbide, a sulfide, a silicide or a mixture thereof.

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

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

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

8. Method according to the preceding claim, in which the substrate holder (11, 21, 31) is polarized.

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

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

11. Device comprising a solid substrate (12, 22, 32) according to the preceding claim, said device being chosen from hard and protective coatings of cutting and perforating tools, glasses, optics, windows, scalpels, biocompatible coatings of implantable medical devices such as prostheses, dental implants, intracerebral electrodes and electronic devices such as surface acoustic wave or guided acoustic wave devices, sensors, components for power electronics, heat sinks for electronic and optoelectronic components, encapsulated electronic devices and quantum sensors such as ultra-sensitive magnetometers.

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