EP2195472A1 - Device for very high frequency plasma assisted cvd under atmospheric pressure, and applications thereof - Google Patents

Abstract

The invention relates to a method for CVD on a substrate under atmospheric pressure, characterised in that it is assisted by a very-high-frequency plasma generated by a field applicator with an elongated conductor of the micro-ribbon or hollow conducting line type. The invention also relates to the use thereof for applying an electrically conductive inorganic layer on elements of vehicle bodywork, particularly the bumpers.

Description

 HIGH VOLTAGE PLASMA-ASSISTED CVD ASSISTED DEVICE AND METHOD AT ATMOSPHERIC PRESSURE, and

ITS APPLICATIONS.

The invention relates to a CVD deposition process using very high frequency plasma (including microwaves) at atmospheric pressure, and also relates to a device for its implementation as well as to the applications of said method. There are many potential applications for thin-film functional coatings on glass, metal, and polymer substrates. These thin layers are advantageously produced by means of plasma-assisted chemical vapor deposition technology, commonly known as plasma-CVD or PECVD. The principle of the latter is to excite in a plasma produced by an electric discharge a chemical vapor in contact with a substrate. The effect of plasma is to create in the gas phase very reactive unstable precursors which have the property of condensing and reacting on the surface of the substrate to bring in new atoms which progressively constitute a thin surface film of material.

By choosing the nature and the proportions of the gaseous chemical precursors, it is possible to develop materials of different compositions that can be adjusted with great flexibility (for example amorphous carbo-oxy-nitride silicon alloys SiO _x N _y C _z ). It is even possible to make gradients of properties according to the thickness by continuously controlling the characteristics of the plasma phase which proved impossible with the older methods of physical vapor deposition (PVD) such as sputtering where the material first movies are brought from solid sources. In addition, the PECVD is potentially better suited to the uniform deposition of material on objects of three-dimensional shape because the transport of chemical species is less directive than that of physical species (evaporated or atomized atoms) and can be controlled by acting on hydrodynamics and diffusion in the phase gas.

Originally, plasma-CVD technology was developed for the elaboration of thin layers of materials constituting microelectronic circuits, LCD flat screens and solar cells. These applications require the use of ultra-clean reactors with very high purity gases, and a substrate temperature of at least about 200 ^° C.

For new applications of coatings carrying out one or more functions of the anti-abrasion type, chemical barrier, thermal resistance, anti-corrosion, optical filtering, adhesion primer, anti-UV, etc., the requirements on materials, processes and equipment are very different. A material considered to be of good quality for these applications must above all have a dense structure with good average connectivity of the atomic network, a minimum porosity at the nanoscale and the absence of heterogeneous columnar or granular structures at the same time. micron scale. On the other hand, localized electrically active defects are generally not of significant importance. Moreover, these new thin-layer functional coatings will address products of added value per unit area much lower than, for example, a microcircuit slab or a display screen. It is therefore absolutely necessary to minimize the depreciation and operating costs of the depositing machines compared to the treated m ² . The deposit rate must therefore be the higher possible. Most of the industrial products covered by these new functional coating applications, including polymers, thin-foil steels and aluminum alloys, do not withstand a temperature of more than a few tens of degrees above ambient. Flat glass can, on the other hand, withstand heating, but since the treatment takes place at a later time after hot manufacture, reheating would be an energy waste not desired by the manufacturers. The objects to be coated are generally larger than a silicon wafer, a solar cell or an LCD screen and may be of three-dimensional shape. It may also be necessary to treat continuous thin substrates at the parade.

Based on these different application requirements, low pressure PECVD solutions have been progressively developed and are available today at the laboratory or industrial pilot level. They generally associate:

High-density plasma sources, microwave, inductive or relaxed arc which are capable of delivering a high density of excited free electrons, making it possible to generate by inelastic collisions a large quantity of deposit precursors and thereby to obtain the highest growth rates minimizing treatment times;

A high and controllable supply of non-thermal energy to the substrates, in the form of internal physical excitation of the species or ion bombardment;

PECVD reactors of large size and complex engineering to allow to create, transport and deliver a high and uniform flow of chemical and physical non - thermal active species at any point on the surface of the substrate. This results in distributed plasma sources, highly studied chemical gas injection modes, and distributed pumping. It is often advantageous to work at minimal pressures, of the order of 0.1 Pa, in order to obtain a large mean free path and to minimize the influence of hydrodynamics.

Clever and complex arrangements are needed to consider the possibility of achieving uniformity over large areas. For example, the changes that have been necessary over the years for the distributed electron cyclotron resonance concept of Metal Process SARL. The delocalized gas injection devices remain of a delicate mechanical execution with a large number of holes of very small diameter. Distributed turbomolecular pumping is also expensive (several small pumps are more expensive than a large one with equivalent cumulative capacity).

This type of technology remains reserved for the realization of coatings with rather complex functionalities and sufficient added value: optical filters, multiple protections (wear, aging outside, chemical barrier) innovative nanomaterials, etc.

For plasma-based design of simpler surface features, addressing common products with low added value, which can be large in size and left-handed, manufactured in very large quantities, there is a real need for a technology. PECVD deposition simple, inexpensive and easy to implement at atmospheric pressure. Indeed, the constraints related to vacuum maintenance infrastructure are very important. In addition to the operating costs (energy, maintenance, spare parts and consumables, qualified personnel), a vacuum installation Large requires a specific know-how and infrastructure to operate reliably and reliably over the next 24 hours, by managing complex sequential or continuous airlock systems, bottleneck loading and unloading operations, etc ..

Moreover, in case of failure, the breakage of the vacuum required for an intervention is time consuming which is difficult to compatible with lines of continuous production in tight critical flow.

However, at atmospheric pressure, compared to the case in which the gas in which we seek to maintain a non-thermal plasma is in a rarefied state, the elementary physical and chemical processes are modified, placing greater constraints on the development of technologies. plasma-CVD and restricting the application possibilities.

First of all, the very existence of non-thermal discharge regimes at atmospheric pressure represents a singular physical situation, the obtaining of which requires very particular arrangements of devices and procedures. Indeed, when the density of particles in the gas increases, the collisions also become much more frequent and tend to establish the local thermodynamic equilibrium, that is to say the transition to the arc regime which without particular precautions is a cause of degradation or destruction of the device.

Then, due to the much more frequent interactions between particles, all the gradients are strongly charged in a plasma device or a PECVD reactor at atmospheric pressure. The electrons and ions, as well as the active chemical and physical species involved in the plasma process, disappear in much smaller characteristic lengths than in the case of a vacuum plasma. It becomes all the more difficult to generate a plasma and distribute active species homogeneously on other than very elementary geometries. In particular, it is not possible to make an atmospheric pressure plasma deposition reactor capable of treating a substrate of three-dimensional shape that would be held stationary relative to one or more plasma generating devices. It is even often impossible to arrange such a substrate inside the plasma zone because of geometric constraints. The strong interactions between particles in the gaseous phase have another consequence in terms of the quality of the deposited materials: the radical chemical species that constitute the raw material of the deposit will have an important tendency to react prematurely between them even before reaching the surface of the film. This can result in homogeneous phase nucleation and irreversible generation of totally undesirable solid particles. To a lesser degree, the radicals will aggregate into a cluster of larger bound atoms which, just after their arrival on the surface, will be more difficult to rearrange by non-thermal energy than condensing atoms. isolation. However, still due to the more frequent interactions between particles in the gaseous phase, the species carrying this non-thermal energy lose their internal excitation more easily than in a rarefied gas before reaching the surface. This deficit can not be compensated by the application of ionic bombardment to the substrate because it is impossible, at atmospheric pressure, to significantly bias the substrate relative to the plasma. It is therefore particularly problematic to obtain atmospheric PECVD layers of a quality comparable to those produced by PECVD at reduced pressure. Another condition for a non-thermal atmospheric discharge to be used to make PECVD deposits is that the energetic electrons, which will then be responsible for the generation of depositing species, are created by the ionization processes of a homogeneous way in volume and continuous in time, as is the case in a vacuum plasma. Otherwise, the deposited material would have an irregular and heterogeneous structure and an inappropriate quality. Amongst the non-thermal atmospheric discharges, the best known type is the dielectric barrier discharge (DBD), maintained between two electrodes supplied with low frequency AC voltage, and whose surfaces are coated with a dielectric material. This dielectric prevents the passage to the arc regime by limiting the discharge current. However, this arrangement does not generally make it possible to obtain a homogeneous discharge. As soon as a sufficient power is applied to obtain the initiation or "breakdown" of the discharge (ie a regime where the ionization compensates for the losses of charged particles), we observe that the ionization intensifies and propagates very rapidly. rapidly along paths perpendicular to the electrodes, giving a large number of plasma filaments ("streamers") separated by dark spaces where there are no charges and therefore no depositing active species can be created. The presence of the dielectric "aborts" each streamer before it increases indefinitely to switch to the arc regime, but the discharge is not homogeneous and it is not usable for the PECVD. In recent years, however, it has been possible to obtain homogeneous dielectric barrier discharges: atmospheric discharge in luminescent mode in noble gases or in the Townsend regime in nitrogen. To initiate a discharge maintained while remaining in a "soft" ionization regime distributed throughout the entire volume between the electrodes, it is necessary to favor the ionization mechanisms which involve, not the direct inelastic electronic collisions (which would lead to the filament regime) but transfers of energies between species other than the electrons carrying internal excitation (ie being in one of their quantized energetic levels higher than the fundamental one), in particular atoms and metastable molecules. The amplitude and the frequency of the voltage signal which adjusts the energy deposition regime in the discharge, and thus the creation of the species that will control the desired ionization regime, are correlatively adapted. These conditions of existence of homogeneous landfills are however tenuous and constraining. For example, it may be necessary to add to the plasma gas a gas whose excitation will give a particular metastable species necessary for good control of the ionization regime, but this gas may be otherwise undesirable for the process. Conversely, the addition of a chemical precursor vapor can react with an excited species involved in the homogeneous ionization process and make it disappear prematurely, and thus return the discharge to the filament regime. The conditions for maintaining the homogeneous regime can also be sensitive to additional constraints imposed by the PECVD process such as the dynamics of the gas flow and the heating of the substrate.

In addition, the geometries in which these homogeneous regimes can be maintained are also restrictive: the parallel flat electrodes may have a relatively large surface area, but on the other hand the spacing may not exceed a few millimeters in the case of the discharge. Townsend homogeneous in nitrogen and a little more in the case of homogeneous glow discharge in rare gases. This excludes processing substrates other than planar and thin. In addition, inherently of the physical mechanisms of homogeneous discharge maintenance, these substrates must be of relatively insulating material. The introduction of any conductive substrate within the discharge immediately induces the transition to inhomogeneous filamentary mode.

There are other homogeneous cold atmospheric discharges which are essentially discharges maintained in flow in a tubular geometry of cylindrical or parallelepipedic section, the latter possibly extended in width (concept called "APPJ": for Atmospheric Pressure Plasma Jet sold by the company SurfX Technologies). Plasma is initiated between the tube wall and an internal counter electrode, without the need for a dielectric barrier. The stabilization of the discharge and the maintenance out of the arc regime result from the use of a high flux of almost pure helium as a plasma gas, ensuring a very efficient dethermalization. All attempts to add other plasma gases, such as nitrogen or argon, made the discharge unusable for applications. In addition to the cost, the use of helium, a non-renewable resource that is often in short supply on the market, which can be reserved for more strategic applications, is not desirable.

There is also a concept developed by the University of Wisconsin (see US 6,764,658) which consists of a plurality of coaxial dielectric barrier discharges in juxtaposed streams in parallel. These tubular sources are formed in a parallelepiped block facing which one can have a substrate of extended shape. The gas flow has the effect of partially ejecting the plasma (luminescent zone where there are charged particles) outwards towards the surface of the substrate to be treated, but the decay of these species is rapid and the treatment is carried out at the limit of the post-discharge where the active species are at the same time less many and less energetic. High deposition rate with good layer quality can not be achieved with this system.

Finally, there are a number of concepts of "cold torches" supplied with high or low frequency alternating voltage, or pulsed DC, which by suitable arrangements make it possible to transform an arc-shaped filament into more diffuse and colder plasma. These torches do not need stabilization by rare gases and can for example operate in the air. However, they do not make it possible to create high density plasmas or to have very good control of the production of the active species. These torches are very useful tools to perform simple operations of cleaning, stripping, deoxidation, surface activation. On the other hand, even if nothing prevents to associate an injection of precursors of deposit, one will be able to carry out only very simple polymerizations and not to realize thin layers with the precise specifications, controllable and reproducible and especially not at a speed required for most industrial applications. Homogeneous cold atmospheric discharges have electronic densities which are of the same order as capacitance radiofrequency capacitance discharges under vacuum (10 ⁸ to 10 ⁹ cm ^-3 ) .The rate of creation of active species under these conditions does not lead to very high deposition rates.

On the other hand, microwave atmospheric discharges have notoriously high electronic densities, from 10 ¹² to 10 ¹⁵ cm ^-3 as close as possible to the coupling of microwaves with plasma, and electronic inelastic collisions produce a large number of active chemical and physical species that promote a high deposition rate with good layer quality. It has therefore also been envisaged to implement microwave atmospheric discharges for surface treatment.

There are different families of devices for generating a microwave plasma and some of them can a priori operate at atmospheric pressure. The main types of sources are for example described in Microwave-Excited Plasmas, eds. M. Moisan and J. Pelletier, Chap. 4-5, Elsevier (1992): Sources located inside a microwave circuit in a guide, resonant cavities, surface wave launchers and torches. With the exception of the resonant cavities, these devices maintain plasmas in low volumes (generally inside small diameter dielectric tubes) which makes them poorly adapted to CVD deposition on extended forms. There are also microsewave field planar geometry applicators for maintaining plasmas over large areas, for example radiating slot waveguides, planar propagators or flat surface wave launchers.

However, this is true only in the case of vacuum plasmas. At atmospheric pressure indeed appears the phenomenon of contraction and filamentation of microwave discharges

(Y. Kabouzi et al., Journal of Applied Physics 91 (3), 1008

(2002)). This inhomogeneity is of a physical origin very different from that prevailing in cold atmospheric discharges. It results from the inhomogeneous heating of the gas by the elastic electronic collisions. This mechanism naturally tends to establish steep temperature gradients, which are associated with gradients of electronic density of the plasma in the same direction. In a large volume, the plasma is concentrated in very intense discrete filaments, separated by spaces containing little or no charge, and therefore a negligible number of active species. Homogeneous deposition is impossible and the substrate to be treated would suffer localized thermal damage.

By way of exception to this rule, two cases are known in which an atmospheric microwave plasma can be obtained homogeneously in a significant volume. The first is the Cyrannus source from iPlas GmbH which uses a resonant cavity fed from a slot annular waveguide. It is the high flow of argon that helps prevent inhomogeneous heating of the gas. This regime is, however, inherently unstable and the transition to inhomogeneous mode can occur on normal fluctuations of the process. Even in homogeneous operation, tests of PECVD (SiN silicon nitride) resulted in unacceptable inhomogeneities. The deposition rate did not appear considerable, not more than a few hundred nanometers per minute, which can be explained by the fact that the high flow of carrier gas "dilutes" the injected power, thus decreasing the rate the creation of depositing species. The very high consumption of argon is also not a favorable economic factor.

The second example is the AtmoPlas ™ technology from Dana Corp. (now owned by BTU International). In this concept, the plasma is homogenized on average by dispersing conductive particles in the gas which act as relocated ignition centers and thus permanently induce the absorption of microwaves to ionize the gas in the whole volume. . The presence of these particles, however, does not seem compatible with performing a CVD deposit of well controlled composition and microstructure.

The definition of the low limit of the frequency range corresponding to what is usually referred to as microwaves is not absolute. One of the legally permitted frequencies for industrial, scientific and medical (ISM) applications is the 434 MHz spectrum that some authors do not use as microwave (although the name is used from the next higher frequency) 915 MHz). We will therefore speak rather in the wake of very high frequencies, to designate those located well beyond 100 MHz.

The present inventors have described in the patent application filed that same day by the Applicant a very high frequency plasma source elongated conductor (microstrip type or hollow conductive line). The principle of this plasma source is based on a very high frequency wave propagator structure, consisting of the conductive (hollow or micro-ribbon) line applied to a dielectric substrate that separates it from the plasma. The latter is generated by the very high frequency power absorbed during its propagation along the conductor.

More specifically, the patent application filed by the Applicant on the same day as the present application relates to a plasma generating device which comprises at least one very high frequency power source (frequency greater than 100 MHz), connected via an adaptation system of the present invention. impedance to an elongated conductor of small cross section relative to its length (for example of microstrip or hollow conductor type) fixed in intimate contact over its entire lower surface on a dielectric support, at least one cooling means of said conductor , and at least one supply of gas close to the dielectric support on the side opposite the side supporting the conductor. The principle of this plasma generation mode is therefore to propagate the electromagnetic power along the power transmission line based on the micro-ribbon, to distribute this power and excite the plasma delocalized along the line. In fact, the actual existence of said line requires the presence of a mass reference which, in the prior art, is in the form of a continuous conducting metal plane.

According to advantageous modes of implementation of this plasma generator device, the Applicant has had the merit of having thought to consider that the plasma sheet is a driver with a potential that can therefore perfectly serve as a potential reference for the power transmission line. However, for the device to actually operate, it is necessary to add an absolute local reference of potential making it possible to format the propagation mode: the device comprises a partial electrical ground plane extending opposite the face of the dielectric opposite to the side supporting the driver, the partial character of the ground plane being expressed in that only a minority surface of the line of the driver is facing a ground plane.

- The partial ground plane is located at the origin of the line of the driver, where the microwaves arrive in the device.

the launch zone of the wave at the entrance of the conductive line has a conventional structure assembling the elongate conductor, the dielectric and the partial ground plane, the ground plane interrupting at a short distance from the input of the conductive line and then being replaced as a potential reference of the plasma transmission line extending with the conductor over the rest of the length of the conductive line.

the launch zone of the wave, at the entrance of the conductive line, has a conventional structure assembling the elongate conductor, the dielectric and the partial ground plane, the ground plane interrupting at a short distance from the input of the conductive line and then being replaced by the plasma, the conductor not extending substantially beyond the limit of the ground plane. Thus plasma plays both the reference role of potential and guide support of the propagation of the wave (mode similar to a surface wave but here in planar geometry).

The present invention is based on the use of this type of ultra-high frequency field plasma micro-ribbon applicator to produce a plasma-CVD module delivering a "curtain" active gas flow previously excited in the plasma dense and homogeneous, said curtain of active gas impacting the surface of a substrate. At the latter, the active gas may still have the characteristics of a plasma, that is to say contain a significant proportion of charged particles, or be essentially a post-discharge medium, that is to say containing only neutral active and / or excited species. These are the fastest flux rates that favor the survival of charged species (which are the ones whose population decreases most rapidly) at a distance from their place of creation by coupling the energy of the electromagnetic wave to the gas. This plasma device has the best efficiency in terms of the use of electrical energy to create depositing active species. Electrical energy is not massively converted into heat as would be the case in an arc plasma for example and the temperature of the gas remains low enough that the treatment of heat sensitive substrates is possible, by adjusting the rate of passage of the substrate in the jet of active gas. The plasma module can be used to deposit thin layers of material on moving planar substrates, or be embedded on a robot arm to perform these same treatments by a controlled scanning movement on three-dimensional substrates. In particular, the invention is well suited to the application of an electrically conductive inorganic layer on automotive body elements, particularly bumpers, before the application of the electrostatic spray paint. This layer is intended to replace liquid-applied conductive adhesion primer solutions requiring time-consuming drying.

In a very high frequency microstrip plasma device, according to the concept underlying the present invention, when the wall of the dielectric in contact with the plasma is flat and extended and the gas is almost static, it is found that the plasma at atmospheric pressure takes a filamentary structure irrespective of the plasma gas used, in particular argon, or else helium or their mixtures. In order to stabilize and homogenize the plasma, a provision has been adopted allowing tests where a lateral gas flow is established at the limit of the intense plasma zone corresponding to the coupling with the electromagnetic power at a very different frequency, said plasma zone being confined in a narrow channel in the dielectric substrate. The results of these tests show characteristics very well adapted to the development of a PECVD device.

Thus, the present invention relates to a CVD deposition process on a substrate which is led to pressure characterized by the fact that it is assisted by a very high frequency plasma produced by means of a field applicator using an elongated conductor of small section in front of its length (whether of the microstrip or line type). hollow, for example cylindrical). The plasma source is powered by electromagnetic power

(eg 434 MHz) by specially developed solid state generators. These generators benefit from the power electronics technologies of the telecommunications industry, and in particular from the large-scale production of power transistors that ensure both security of supply and a rapid cost-cutting effect with the customers. quantities ordered. In addition, they require no periodic maintenance, unlike generators based on vacuum tubes (magnetrons ...) which all have a limited life.

As will be understood from reading the above, the expression "very high frequencies" according to the invention means frequencies greater than 100 MHz, and in particular the "discrete" frequencies at 434 MHz, 915 MHz, 2450 MHz and 5850 MHz which are authorized by the international regulations for the Industrial, Scientific and Medical band.

In the process according to the invention, the plasmagenic gas is preferably argon, optionally supplemented with from 0.1 to 5%, preferably from 0.2 to 4% and even more preferably from 0.5 to 2% by weight. nitrogen volume. In argon, the plasma maintained in the geometry of the device according to the invention remains visually homogeneous without any apparent manifestation of contraction or filamentation. On the other hand, the operation at atmospheric pressure in pure nitrogen is impossible: on the one hand we do not have sufficiently powerful microwave sources, but also the structure is not designed to accommodate the minimum power densities corresponding to the maintenance of an atmospheric nitrogen plasma. The use of argon is perfectly acceptable economically for most of the industrial processes covered by the invention. The possible addition of a few percent of nitrogen can help modify the energy transfers in the landfill to help obtain certain depositing radicals.

The chemical nature of the precursor will obviously be chosen in the first place depending on the chemical elements to constitute the solid material to be deposited. However, other criteria specific to the implementation of the precursor in the atmospheric PECVD process will be taken into account.

Some of these precursors will be "normal" gases stored in compressed form, or liquefied under a high vapor pressure at room temperature, such as, for example, silane, methane, acetylene, etc. However, if one wants to extend the range of the possible materials (metals and their oxides, nitrides, carbides ...) one must in general consider to use in addition low-vapor liquid organometallic sources which will be conveyed in a carrier gas at atmospheric pressure. This carrier gas may be chosen from the group comprising argon, nitrogen, helium, krypton, xenon and neon. It is not present at the level of the plasma generation zone and its plasmagenic properties are therefore of no importance. On the other hand, its nature can have an influence on the transport of the active species in the vicinity of the substrate (hydrodynamic and diffusion) or even on their deexcitation / recombination. These precursors are incorporated in said carrier gas with a partial pressure sufficient to provide, after dissociation into active radicals in the plasma or in the immediate vicinity of the latter (so-called post-discharge zone), a sufficient flow of atoms in the jet of gas active impacting the substrate to form the layer material with the required growth rate. This involves vaporizing the precursor at a sufficient temperature, at which temperature the carrier gas will be maintained until injection into the curtain of active gas extracted from the plasma creation zone by coupling with the very high frequency waves. This temperature has a practical upper limit imposed by the resistance of the PECVD module materials (it is obviously assumed that the precursor does not decompose prematurely by a simple thermal effect at this maximum temperature).

According to one particular embodiment, the precursors are chosen from the group comprising gases stored in compressed form, or liquefied under a high vapor pressure at ambient temperature, low vapor pressure liquid organometallics, and mixtures thereof. The gaseous precursors are chosen from the group comprising in particular silane, methane, acetylene, ethylene and their mixtures. The organometallics are chosen from the group comprising the precursors of solid materials oxides, nitrides, metal carbides and mixtures thereof, more particularly the organic compounds of titanium, tin and tetramethylsilane.

Like any plasma-CVD process at atmospheric pressure, the process which is the subject of the invention is subject to limitations resulting from the much more frequent interactions between particles in the gas phase.

According to the invention, several original elements are associated to minimize these effects on the processing speed and the quality of the layers. First, the main plasmagenic carrier gas, typically argon, is highly excited at the channel underlying the microstrip line. The plasma thus created has the characteristics of a microwave plasma atmospheric, homogenized by the dynamic flow of the gas. Its electron density at this point is of the order of 10 ¹¹ - 10 ¹² cm ^-3 and the temperature of the gas can be from 1000 to 2000 K. The general principle of this mode of deposition by active gas jet extracted from a high-density plasma consists in using this high concentration of energy to generate, after injection of a chemical precursor, a large flow of active physical and chemical species, and at the same time to transport these species in the flow of gas in the shortest time to the surface of the substrate, so that 1) the decrease of the number of precursor radicals is limited in order to maintain a high deposition rate, 2) it also limits the losses of excited physical species that will assist the rearrangement incident atoms and densify the deposited material, 3) reduces the probability of oligomerization of precursors into clusters of larger and more difficult atoms to accommodate optimally in the film, which another factor of non-quality.

Thus, the chemical compound precursor deposition must be introduced into the main stream at a not too great distance downstream of the plasma excitation zone, so that the dissociation of the precursor to form active radicals is sufficiently complete. On the other hand, there is no point in prolonging the transit path of these radicals towards the surface of the substrate, since they will have a higher probability of reacting in the gas phase, or of becoming inactive and being lost to the deposition process, either to undergo an oligomerization prejudicial to the quality. However, it is not always advisable to minimize the distance between the plasma generation zone (channel under the microstrip) and the surface of the substrate to be treated. This allows as we have just said a maximum supply of non-thermal active species of the growing film surface (for the highest deposition rate and best quality). However, the further a substrate is disposed at a small distance from the plasma excitation source, the higher the temperature, which can exceed the endurance limit of the material, especially when it is a polymer. In dynamic deposition mode (relative tangential displacement of the source PECVD and the treated substrate) the maximum temperature also depends on the speed of scrolling or scanning). Thus, the skilled person is able to adapt this distance to the nature of the support to be treated and the speed of movement of the support relative to the plasma source.

As mentioned above, the method of the present invention is implemented using a device as described in the patent application filed today by the Applicant (described again above in the present description. ) associated with a precursor power supply. Thus, the invention relates to a plasma-enhanced vapor phase thin film deposition device which comprises at least one very high frequency source (> 100 MHz) connected via an impedance matching device to an elongated section conductor. low in front of its length (whether it is of the micro-ribbon type or hollow line type, for example cylindrical) conductive fixed on a dielectric support, at least one cooling means of said conductor, at least one supply of plasma gas close to the support dielectric on the side opposite to the conductor-supporting side, the plasma being generated under the dielectric along the conductive line, and at least one precursor feed into the active gas stream extracted from the plasma creation zone by coupling with the microphones -ondes. The characteristics of the device described and claimed in the application filed today by the Applicant are included by reference in the description of the device of the present invention. By a supply of gas "near" or "near" the dielectric support means an inlet typically emerging less than 15 mm from the support, and preferably less than 10 mm from the support.

In the present invention, the term "microstrip" means an electrically conductive element of elongated shape and thin, typically of the order of one millimeter or less than one millimeter. The length and the width of the microstrip are not arbitrary and will be dimensioned so as to optimize the propagation properties of the power along the transmission line constituted by the microstrip. As a variant, as already mentioned above, the microstrip may be replaced by a hollow elongated element, in particular of round, rectangular or square section, the thickness of the wall of the hollow tube being sufficient for good mechanical strength. and without effect on electrical behavior. The micro-ribbon is not constrained to a flat, straight geometry, but may also adopt a curved shape in the plane or a left shape in the direction of its length with concave or convex curvatures.

As will have been understood, it will be understood indifferently in what precedes and what follows of conductor, hollow conductive line, or microstrip, without at any time that the present invention can not be restricted to only one of these types of line.

Given the fact that high frequency currents circulate by obeying the effect of skin or "skin effect" and that it depends on the frequency and conductivity of the skin. material constituting the conductor, the practical thickness in which the current will flow will be much less than 0.1 mm. However, given the fact that the powers transported are high, of the order of a few hundred watts, and the conductivity of the metal decreases with the increase in temperature, the thickness of the micro-ribbon will be much greater than the Theoretical thickness defined by the skin effect and it will be necessary to cool the micro-ribbon so that it retains its physical integrity. Thus, the microstrip will have a thickness of the order of a millimeter and be made of a good electrical and thermal conductor material selected from those having good mechanical strength, which may be copper alloys such as brass or preferably beryllium copper. Advantageously, the device according to the invention comprises, below the channel formed in the dielectric substrate and confining the region of creation of the plasma by coupling with the microwave power, a slot through which the curtain of active flux gas extracted from said plasma creation zone and the precursor feed means are placed in such a way that the precursors arrive in the slot perpendicularly to the active gas flow. In a particularly advantageous manner, the flow of plasmagenic gas is symmetrically fed by two opposite lateral inputs at the level of the active coupling zone of the microwave power to the plasma. These inputs can lead to a variable distance from the surface of the dielectric substrate to give a dynamics of the adapted gas flow in the confinement channel of the plasma. For example, the inputs may open near the lower limit of the microwave coupling zone, or even slightly beyond. In this case, there is creation of a vortex effect in the plasma channel that effectively extracts the active species, but it avoids a "blow" effect of the plasma by the flow that could be detrimental to the stability of the latter. The flow is then forced in the direction perpendicular to the injection slot of the jet or "curtain" of active gas towards the surface of the substrate. The carrier gas of the chemical precursors, bringing the constituent atoms of the material to be deposited, is injected symmetrically in bypass in the flow of active gas. According to a particular embodiment, the feed means of the precursors are arranged in a power supply unit placed under the device. Said power supply unit can be removably placed. It is then possible to have a set of power supplies of different heights. Thus, the choice of the power supply makes it possible to adapt both the distance of the plasma excitation zone by coupling the very high frequency power under the microstrip at the outlet of the jet into the free space. as well as the distance between the precursor injection and the substrate to be treated, under the conditions of the treatment.

Since the device according to the invention is implemented at atmospheric pressure, because of the dynamics of the gaseous flow at impact on the surface, all the incident radicals do not directly reach the latter for s to permanently incorporate into the film and there is established recirculations in the vicinity of the surface which will increase the residence time of the radicals in the gas phase and promote the interactions within said gas phase, in a way detrimental to the quality of the material deposited on both sides of the impact of the plasma curtain. It is therefore interesting to adapt the shape of the plasma injection slot by adding, for example, devices deflectors on the treatment head to reduce recirculation.

By way of illustration, an exemplary implementation is described below: Thus, the optimized shape of the microstrip line makes it possible to generate the plasma in the underlying slot over a length close to 150 mm and a section close to 8 mm with an incident power of 300 W used with a yield of 97%, which represents a linear density of energy, and therefore of very substantial active species. The device used in a plasma gas where argon is very much in the majority, however, can withstand very significantly higher powers, for example from 500 to 600 W, thereby improving the speed and quality of the deposit. The range of total gas flows (plasmagene, carrier and precursors) allowing this operation, approximately 10 to 100 standard liters per minute (slm), offers extensive possibilities for controlling the transfer dynamics of jet active species from plasma on the substrate to be treated in order to optimize the process. The device is finally remarkable for the quality of its plasma energy transmission efficiency (impedance matching). Even more than a very low average value of the reflected power (3%), this value is maintained over a very wide range of variation of the operational parameters. The operation of the PECVD module will therefore be particularly robust and insensitive to variations and fluctuations in the operating conditions imposed by the application (multi-step processing, idling between passes, etc.).

Different devices according to the invention may be juxtaposed in particular to increase the speed of travel of the substrate under each of said devices and thus increase the productivity of the process. The device implemented according to the invention will be better understood from the description of the drawings below in which: FIG. 1 represents a cross-section of a device according to the invention; Figure 2 shows a cross section of an alternative with a cylindrical section transmission line incorporating an internal water circulation.

FIG. 1 shows a device 1 according to the invention consisting of the following different elements stacked on each other: a base 2 traversed by two symmetrical longitudinal channels 3a and 3b in which the precursor elements for deposition of solid materials circulate, these channels being each symmetrically connected by a slot 4a and 4b of distribution of the precursor to a central outlet slot 5 for extracting the active gas stream from the plasma 6; a dielectric 7 in the form of a parallelepiped plate; a microstrip 8 disposed centrally on the face 7a of the dielectric 7, consisting of a conductive metal strip, connected to the connector (not shown); the width of the microstrip is greater than that of the slot 5 so that the upper face of the base 2 acts as a partial ground plane; a dielectric radiator 9 made of ceramic, having a longitudinal channel 10 in which water circulates, is plated over the entire surface of the microstrip 8; a main distribution block 11 having two symmetrical halves 11a and 11b of generally parallelepipedal shape with in the lower part a releasement lie, Hd stretching towards the center of the device, on the free surface of which bears the dielectric, the two free ends of these recesses facing each other and leaving free the central slot 5; each half Ha and Hb of the block 11 being traversed at its upper part by a longitudinal cylindrical channel 12a and 12b in which cooling water circulates and in its lower part by a longitudinal cylindrical channel 13a and 13b, through which the gas arrives; plasmagene, each of the channels 13a and 13b opening through a slot 14a and 14b in the central slot 5; an inverted U-shaped dielectric support block 14 placed above the dielectric radiator 10 maintains between the dielectric substrate 7, the lower part of the distribution block 11 and the base 2; a metal closure plate 15 is fixed on the block 11 and allows the integration of a clamping system

16 for maintaining the block 11 and the dielectric radiator 9 on the base 2, on the one hand, and the maintenance of the dielectric block 14 plating the dielectric substrate

7 on the block 11. An O-ring 17 located in the lower part of the block 11 and an O-ring 19 located under the dielectric 7 ensure the tightness of the volume in which develops the discharge.

The metal plate 15 closes the block 11 in the upper part, the assembly thus constituting a Faraday cage in order to confine the very high frequency electromagnetic radiation delivered by the microstrip so as not to lose energy and not to disturb the environment (problems of electromagnetic compatibility and operator safety). Below the base 2 is disposed a priming lock 18 low pressure plasma. This airlock makes it possible to lower if necessary, by means of external pumping means (not shown), the pressure at the coupling zone of the electromagnetic power under the microstrip to facilitate priming (the latter being notoriously less easy to atmospheric pressure). This airlock is shown in dotted lines because it is mobile and is removed as soon as the plasma is primed. FIG. 2 shows another embodiment of the plasma generating device of the invention which differs from that of FIG. 1 in that the dielectric assembly 7 / ribbon 8 / insulating radiator 10 has been replaced by a system comprising a dielectric 19 of parallelepipedal general shape on the surface 19a of which is formed a longitudinal recess conforming to the profile of a propagation line element in the form of hollow conductive tube 21 in which circulates cooling water 22, said hollow tube being overcome a dielectric holding block 23.

A device according to the invention may advantageously be arranged on a robotic arm so that it is possible to process a substrate that can have a large size and a left shape without displacement of the substrate but by scanning the surface of the substrate using of the robotic arm.

The method of the invention and / or the device of the invention can be implemented in various applications, in particular for coatings producing one or more features of the anti-abrasion type, chemical barrier, thermal resistance, anti-corrosion , optical filtering, adhesion primer, anti-UV, etc.

In particular, the invention is well suited to the application of an electrically conductive inorganic layer on Automotive polymer body parts, particularly bumpers, prior to the application of electrostatic spray paint. This layer is intended to replace liquid - applied conductive adhesion primer solutions which require expensive drying time.

Thus, another object of the invention is the use of the method as described above for applying an electrically conductive inorganic layer to automotive body elements, particularly bumpers, prior to the application of the electrostatic spray paint. In this particular use, the material is chosen from the group comprising, in particular, tin oxides, indium tin oxide (ITO) and indium tin oxide, TiN titanium nitride, nitrogen doped titanium oxide. , and optionally doped silicon and / or carbon alloys. The corresponding precursors will be in particular tetra-n-butyltin, titanium isopropoxide, tetramethylsilane, ethylene. Indeed, the materials deposited from such precursors make it possible to satisfy the functional criterion of the ability of the primary coating to evacuate the electrostatic charges, which is expressed in terms of given surface resistivity in ohm per square (Ω / D) (any square portion of the coating having the same resistance regardless of its side). Values on the order of 1000 Ω / D seem well suited for the application. If we stick to thin layers of reasonable thickness (compared to the expected processing time), typically of the order of 1000 nm thick, this gives the material a resistivity of less than 10 ^{~ 3} Ω.m.

Claims

A method of CVD deposition on a substrate which is conducted at atmospheric pressure, characterized in that it is assisted by a very high frequency plasma produced by an elongate conductor field applicator of microstrip type or hollow conductive line .

2. Method according to claim 1, characterized in that it implements a plasma gas which is argon, optionally supplemented with 0.1 to 5%, preferably from 0.2 to 4% and more preferably still 0.5 to 2% by volume of nitrogen.

3. Method according to claim 1 or 2, characterized in that it implements one or more precursors which are selected from the group comprising gases stored in compressed or liquefied form under a high vapor pressure at room temperature, low vapor pressure liquid organometallic compounds, and mixtures thereof.

4. Method according to claim 3, characterized in that the gaseous precursors are selected from the group comprising in particular silane, methane, acetylene, ethylene and mixtures thereof.

5. Method according to claim 3 or 4, characterized in that the organometallics are selected from the group comprising in particular the precursors of titanium, tin, zinc and silicon.

6. Method according to any one of claims 1 to 5, characterized in that the excitation frequency of the plasma is preferably 434 MHz.

7. Method according to any one of claims 1 to 6, characterized in that the range of total gas flow used is between 10 and 100 standard liters per minute.

8. A CVD deposition device on a substrate comprising at least one very high frequency power source connected via an impedance matching device to an elongate conductor of microstrip type or conductive conductive line, conductive fixed in contact intimate on its entire lower surface on a dielectric support, at least one cooling means of said conductor, at least one plasma gas supply close to the dielectric support on the side opposite the side supporting the conductor, the plasma being generated by coupling the power very high frequency under the dielectric along the conductive line, and at least one precursor feed into an active gas stream extracted from the generated plasma.

9. Device according to claim 8, comprising, below the dielectric, a slot through which the active gas from the generated plasma escapes, the precursor feed means being placed in such a way that the precursors arrive in the slot perpendicularly. to the flow of active gas.

10. Device according to one of claims 8 or 9, characterized in that it comprises a partial electrical ground plane extending opposite the face of the dielectric opposite to the side supporting the conductor, the partial character of the ground plane being expressed in that only a minor surface of the conductor line is facing a ground plane.

11. Device according to claim 10, characterized in that the partial ground plane is located at the origin of the conductor line, where the microwaves arrive in the device.

12. Device according to claim 11, characterized in that the launching zone of the wave, at the entrance of the conductive line has a conventional structure assembling the elongated conductor, the dielectric and the partial ground plane, the plane of mass interrupting at a short distance from the input of the conductive line and then being replaced as a potential reference for the guided propagation of the wave by the plasma extending with the conductor over the rest of the length of the conductive line.

13. Device according to claim 11, characterized in that the launching zone of the wave, at the entrance of the conductive line, has a conventional structure assembling the elongate conductor, the dielectric and the partial ground plane, the plane of mass interrupting at a short distance from the input of the conductive line and then being replaced by the plasma, the conductor not extending substantially beyond the limit of the ground plane.

14. Use of the method according to any one of claims 1 to 7, or the device according to one of claims 8 to 13, for applying an electrically conductive inorganic layer on bodywork elements. polymer automobiles, particularly bumpers, prior to the application of electrostatic spray paint.

15. Use according to claim 14, characterized in that the material of the layer is selected from the group consisting of tin oxides, tin oxide and indium, titanium nitride TiN, oxide nitrogen-doped titanium, and optionally doped silicon and / or carbon alloys, and gaseous precursors among tetra-n-butyltin, titanium isopropoxide, tetramethylsilane and ethylene.