FR2551581A1 - Method and device for heat treatment of various materials by a homogeneous plasma of large volume. - Google Patents

Abstract

Heat treatment of materials. The device for implementing the method comprises: - a sealed enclosure 1; - a magnetic confinement envelope 3 electrically isolated from the enlosure; - at least one device 9 for exciting a gaseous medium occupying the volume of the envelope; - a support 12 of material to be treated, placed inside the envelope and connected to polarising means 16. Application to boron case-hardening of components.

Description

 The present invention relates to the general field of heat treatment of materials, on the surface more generally or, also, in depth.

The invention relates to the general technical field of heat treatments aimed at producing recrystallization, monocrystallization, an association of chemically complementary particles, such as boriding, nitriding, alloy formation, doping by diffusion, etc.
To carry out heat treatments of materials, many techniques have already been proposed
The technique of thermal annealing in a heating oven has long been used, although many drawbacks are attached to it. This is mainly a long treatment time and a high energy consumption, without neglecting the long adaptation times to reach the desired temperature.

Such a technique is therefore, moreover, expensive.

 A second technique consists in carrying out an annealing by scanning, by means of a laser beam or an electron beam.

 Such a technique does not make it possible to obtain a uniform annealing over a large surface, since the impact of the beam on the material is materialized by a spot of small surface which has to be displaced, so as to cover all the useful surface to be treat.

Furthermore, it was found that the spot did not itself present homogeneity over its diameter and therefore did not allow a given material to be exposed to a homogeneous heat treatment and distributed over the entire surface to be treated.

 Another drawback of the scanning technique also lies in the cost of installation, in the complexity of the scanning means and in the difficulty of adjusting such scanning.

 Yet another technique has been to anneal in an image furnace or by lamp. This technique also does not make it possible to obtain homogeneous annealing over large areas and does not offer the possibility of precise temperature control during the treatment phase. This uncertainty, combined with the uncontrollable aging of the lamps, does not allow reproducible treatments to be obtained on materials of the same nature.

 Another technique, again, consists in subjecting a material to an annealing treatment by means of a heating graphite ribbon. Such a technique reveals the same drawbacks as those which have to be charged to the annealing by image furnace or by lamp.

 Yet another technique consists in carrying out an annealing of material by applying microwave waves. By such a technique, it seems possible to control the temperature more precisely and to generate such a temperature much faster than with the prior techniques. However, the means currently available for implementing such a technique do not make it possible to obtain homogeneous treatments on large area materials.

Also known is the annealing treatment by continuous discharges with cold cathodes. The disadvantages of such a technique are, mainly - the existence of a radial density gradient, incompatible with a
homogeneous treatment, - the dependence of the discharge current on the voltage ap
plicated, - the impossibility of obscuring part of the surface of the material has
process, without significantly modifying the properties of the plasma
of discharge, since the sample or the material to be treated is
placed on one of the discharge electrodes or possibly the
constitutes in itself, - the impossibility of providing the sample and, directly by the
plasma, an additional thermal contribution, since, given the mounting or association of the material with the discharge electrode, only one side of the material is bathed by the plasma.

 The invention aims to solve the drawbacks of the prior techniques mentioned above, by proposing a heat treatment method for plasma of large homogeneous volume making it possible to dissociate the parameters of excitation of the plasma from those of interaction of this plasma on the material. .

 Another object of the invention is to provide means which make it possible to completely control a heat treatment in its duration, its bombardment voltage, as well as the current of particles or ions generated, with high precision and a response time. particularly weak.

 Another object of the invention is to propose means making it possible to obtain a better yield by the complete transfer of the electrical energy applied in energy absorbed by the material.

 An additional advantage to be taken into account of the object of the invention is the possibility of carrying out a heat treatment of a material without causing pollution of its constituent material.

 In addition, the geometry of the surface to be treated can be arbitrary, which represents a very significant advantage in the industrial field.

To achieve the above goals, the heat treatment process according to the invention is characterized in that
- creates in a sealed enclosure a large plasma
homogeneous volume, from a gaseous medium
suitable for treatment, by means of excitation
own parameters independent of the parameters used
sation of the enclosure,
- maintains this plasma in the enclosure by confining
magnetic and / or electrostatic confinement,
- provides means to maintain in the enclosure
plasma at a substantially constant pressure
determined,
- places the material inside the enclosure
process that we independently polarize,
for a fixed period of time
bend the electrons or ions in its direction
plasma.

The invention also relates to a device for implementing the above method, device characterized in that it comprises
- a sealed enclosure associated with means of ap
wearing a gaseous medium and maintaining this mid
place at a determined pressure substantially cons
aunt,
- an electrostatic confinement envelope
and / or magnetic of the internal multipolar type
the enclosure and electrically isolated from this
last,
- at least one medium excitation device
gas placed in relation to the internal volume
has the containment envelope,
- a material support placed inside the
internal volume envelops and connects to
own means of polarization.

 Various other characteristics will emerge from the description given below with reference to the appended drawings which show, by way of nonlimiting examples, embodiments of the subject of the invention.

 Fig. I is a schematic sectional elevation of a device for implementing the object of the invention.

 Fig. 2 is a transverse view taken, substantially on a larger scale, along the line II-II of FIG. 1.

 Figs. 3 and 4 are partial cross sections showing two alternative embodiments of the constituent element according to FIG. 2.

 The objective of the invention is to provide means making it possible to bring a conductive surface of any material to a high temperature, with a view to subjecting it to heat treatment by bombardment, using charged particles (ions or electrons) provided by a dense homogeneous plasma, of very large volume. By material is meant any part of any shape, sample or serial production marketable.

it must also be considered that the object of the invention relates to heat treatment, in general, and therefore includes both crystallization, recrystallization or the formation of alloys. The plasma used can therefore be produced, indifferently, by the excitation of a gaseous medium which is neutral vis-à-vis the constituent material of the material or, on the contrary, chemically reactive, so as to obtain the desired effect, for example , bo ruracion, nitriding, sulfurization, etc
For these purposes, the invention recommends generating a homogeneous plasma of large volume. The method of the invention consists in generating such a plasma in a sealed enclosure, internally equipped with a confinement envelope, for example magnetic, of the mul type dipolar permanent magnets. Such a process makes it possible to obtain very homogeneous plasmas and which can have very large volumes and, for example, of the order of several m. The internal envelope of the enclosure can also be produced to ensure electrostatic confinement alone or in combination with magnetic confinement.

 The method of the invention also consists in p-re-providing means for maintaining or maintaining the density of such a plasma, despite the importance of the flow of charged particles capable of being extracted from the plasma to bombard it. the surface or surfaces of the material to be treated To this end, the means according to the invention can consist simply of a plasma of very large volume, when it is a question of treating a material of small surface.

These means can also involve a source of input from the gaseous medium concerned, as well as pumping means capable of maintaining a determined pressure that is substantially constant.

 The method of the invention provides, more particularly, for generating the plasma by own excitation means which are independent of the parameters of use of the element for the purpose of carrying out a heat treatment. To this end, the excitation means can be constituted by at least one thermoemissive filament or, again, by a microwave generator.

 The material to be treated is placed inside the confinement envelope and subjected to a polarization, so as to create, from plasma, a flow of charged, accelerated particles, bombarding the surface or surfaces presented to a such flow by the material to be treated.

 It should be noted that this process can involve a prior dissociation of the molecules from the gaseous medium during the creation of the plasma and that the rise in temperature imposed on the material to be treated serves, in the case of a chemical treatment, to favor the diffusion of charged, dissociated particles. in the material of the material.

 If the material is conductive, it is possible to polarize it by applying a positive DC voltage with respect to the plasma potential. The surface of the material is then bombarded by electrons extracted from the plasma and / or negative ions, if the latter exist. If the material is polarized by applying a negative DC voltage to the plasma potential, the exposed surface (s) will be bombarded with positive ions.

If the material of the material to be treated is of an insulating nature, the treatment can be carried out by providing for prior treatment of the surface or surfaces to be exposed by applying a film, a layer or a conductive coating.
According to a preferred arrangement of the invention, the method involves a continuous positive polarization of the material with respect to the plasma potential, in order to be able to benefit from a higher electronic current and to eliminate the drawbacks which may arise from bombardment by capable ions. to induce defects in the material of which the material is made, or even to produce ionic erosion.

By way of example, for the treatment of a silicon wafer, with a diameter of 10 cm, to which it is necessary to undergo a heat treatment, with a view to recrystallization, the following parameters can be retained - production of a density argon plasma from I has a few
He -3
101l cm - electronic temperature 20,000,000 K, - acceleration voltage between a few tens of volts
and several hundred volts.

 For such a plate to be presented simultaneously neT.ent on its two faces to the plasma, an acceleration voltage of 75 volts makes it possible to apply a power of 2.25 kW for an electronic current of 30 amps.

 Such treatment is applied for a period of a few seconds.

 Parameters of the same order, except the duration, can be retained when it is a question of carrying out a heat treatment of a material, with a view to making it subject to boration, nitriding, sulphurization, etc. ., the variable introduced then being, naturally, the gaseous medium to be chosen as a function of the type of charged particles to be diffused in the material.

For example, in the case of nitriding, the process involves a plasma of nitrogen or of ammonia, whereas, for boriding, the plasma generated is a plasma of borane or boron halide.

Depending on the treatment and the depth of diffusion sought) the value of the product may then range between a few fractions of a second and a few hours.

 The method of the invention makes it possible to carry out a heat treatment, in a homogeneous manner, over the entire surface or surfaces of a material or of a part to be treated, since such a part can be placed within a plasma of very large homogeneous volume and that the polarization applied to it makes it possible to generate and accelerate towards it, in a multidirectional manner, a flow of charged particles, capable of softening all the parts of an exposed surface, even if these parts can be regarded as assimilating to imprints or negative forms. The independent polarization of the part to be treated makes it possible to provide variable flow settings to accelerate or slow down the treatment process, without this having any influence on the production of the plasma. In some cases, it may be desirable to treat only one side. In such a case, it may then be necessary to place the face of the untreated material in relation to a heat exchange means, by a heat pipe, in order to remove or provide the calories necessary for the proper conduct of the process.

 The process of the invention also makes it possible to carry out a heat treatment, in a homogeneous manner in consideration of the entire surface of the exposed material. This can be the case when it is appropriate, for example, to carry out a local recrystallization treatment or monocrystallization by zone fusion.

 In such a case and according to the invention, the local bombardment of the surface part to be treated is ensured by masking some of the non-interested parts of the surface of the material. This can be obtained by coating the parts of the surface, not affected by the treatment, with a layer of non-conductive material or, alternatively, by placing, over the surface to be treated, a mask or a non-conductive mask which can, if necessary, be made relatively movable relative to the surface to be treated.

 Figs. 1 and 2 show, schematically, by way of example, an embodiment of a device for implementing the above method.

 Such a device comprises, according to FIG. 1, a sealed enclosure I which is connected to ground 2. The sealed enclosure is preferably made of a non-magnetic material and comprises, internally, a magnetic confinement envelope 3. This envelope 3 is preferably of the type multipolar and is connected to a voltage source 4 allowing it to be grounded or polarized, continuously or alternatively, positively or negatively.

 The sealed enclosure I is associated with a controlled introduction circuit of a gaseous fluid by means of a connection tube 6. The enclosure I is associated, moreover, with a pumping circuit 7 connected to a tubing 8.

 The enclosure 1 is associated, in addition, with an excitation device 9 which is constituted by a voltage source 10 electrically connected to an excitation member It constituted, in the example illustrated, by a thermoemissive filament extending inside the volume delimited by the confinement envelope 3. It must be considered that any other excitation device could be implemented within the framework of the invention, from the moment when the necessary supply means to its operation can be considered independent of the parameters of use. By way of example, therefore, provision may be made to constitute the excitation device by a microwave generator.

 The enclosure I further comprises a support 12 intended to hold, in the internal volume of the confinement envelope 3, a material as illustrated, only by way of example, in the form of a wafer. The support 12 includes, as appears more precisely in FIG. 2, an adaptation part 14 whose constructive characteristic depends on the shape of the material 13 to be treated. In the present case, the part 14 is produced in the form of a cradle in which a plate 13 can be placed. It is certain that this part 14 could be made in any other different way, taking into account the shape, the mass or the volume of the material or of the part to be treated. The support 12 also comprises, following the part 14, a foot or an arm 15 for fixing to the enclosure 1. This foot or arm offers an insulating surface which can be obtained by making the part 14 and the foot 15 in two different materials. It is also possible to make the support 12 in one piece and to provide an insulating surface coating on the foot 15. The fact of making the foot 15 totally or superficially insulating is provided to avoid subjecting it to the electronic bombardment, in order to locate the thermal gradient in the grip or fixing part 14 and not in the material 13 to be treated.

 In all cases, the adaptation part 14 is connected, either directly or by the foot 15, to a polarization source 16 making it possible to apply, to the material 13, a desired polarization voltage, so as to bring it to a potential different from that of plasma to create, as said above, a flow of particles bombarding it.

 The enclosure I is also provided with a probe 17 making it possible to determine, in particular, the plasma potential and with an opening 18 allowing the temperature of the surface of the sample to be measured by optical pyrometry. For this purpose, the opening 18, provided with a window 19, is made so that the support 12 can, in most cases, ensure the maintenance of a material 13 offering at least part of its surface in the sight line of the window 19 and the opening 18.

 Fig. 3 shows that the support 12, illustrated here in another embodiment, can be associated, on one of its faces, with a mask or mask 20 comprising an exposure window 21 limiting the surface of the material exposed to the bombardment of charged particles. Such a mask or mask can be fixed, when it is advisable to carry out only a local treatment or, again, mobile in one or the other direction of the arrow f, when it is necessary to carry out a treatment. surface thermal by zone fusion, in the case of recrystallization or monocrystallization research.

 Fig. 4 also shows that the support 12 can be produced so that only part of the surface of a planar material 13 embedded in a complementary cavity offered by the support 12 is exposed to the flow of charged particles.

 The invention is not limited to the examples described and shown, since various modifications can be made thereto without departing from its scope. In particular, the casing 3 can be of a type capable of ensuring; electrostatic confinement alone or in combination with magnetic confinement.

Claims (14)

 plasma.  bend ionized particles in its direction9  for a fixed period, so as to have access  process that we polarize in an independent manner  places the material inside the enclosure  determined aunt. #  you the plasma has a pressure substantially consce  - provides means to maintain in the building -  is lying,  - maintains this plasma in the enclosure by confining  reading the enclosure,  their own independent of the utility parameters  suitable for treatment, by means of exci  homogeneous volume, from a gaseous medium  - creates in a sealed enclosure a large plasma  I - Process for heat treatment of various materials, characterized in that  2 - Process according to claim 1, characterized in that the gaseous medium is neutral vis-à-vis the material.  3 - Process according to claim 1, characterized in that the gaseous medium is chemically reactive vis-à-vis the surface material of the material.  4 - Method according to claim 1, characterized in that one creates a plasma of density close to 1 to a few isol # cl 3.  5 - Method according to claim 1, characterized in that accelerates the ions or particles by applying to the material a voltage between several tens and several hundred volts, relative to the plasma potential.  6 - Process according to claim 1, characterized in that the material is subjected to a temperature regulation from one of its faces by a heat exchange means.  7 - Process according to claim 1, characterized in that the surface to be treated with the material is partly at least masked.  8 - Process according to claim 7, characterized in that the material to be treated is associated with a relatively movable mask.  9 - Device for implementing the method according to one of claims 1 to 8, characterized in that it comprises  - a sealed enclosure (1) associated with means  supply (5) of a gaseous medium and maintenance  (7) of this medium has a determined pressure sen  so constant,  - an electrostatic confinement envelope  and / or magnetic (3) of the internal multipolar type  è 11 enclosure and electrically isolated from this  last,  - at least one device (9) for excitation of the mi  gaseous place placed in relation to the volume  internal to the containment system,  - a support (12) of material placed inside  of the internal volume of the envelope and connected to  own polarization means (16).  10 - Device according to claim 9, characterized in that the enclosure (3) is of the magnetic multipolar cage type, internal the enclosure and electrically isolated from the latter.  11 - Device according to claim 9, characterized in that the excitation device (9) comprises a thermoemissive filament (11).  12 - Device according to claim 9, characterized in that the support (12) of material is associated with a mask or mask (20) delimiting a window (21) movable exposure relative to the material.  13 - Device according to claim 9, characterized in that the enclosure comprises a window (19) for optical pyrometric measurement of the material being processed.  14 - Device according to claim 9, characterized in that it comprises a heat exchanger in relation to the material.