FR2719120A1 - Measuring and controlling reactive gas during physico-chemical treatment of substrate - Google Patents

Abstract

Determination and control of the composition of a reactive gas mixture acting on a substrate during a physico-chemical treatment under a rarefied atmosphere, involves the evaporation of a target submitted to an arc discharge. The process consists of effecting a spectroscopic analysis on the plasma emissions of the vapour generated at the target, deducing from the results of that analysis the concentration of the emitted species and if necessary acting on the different parameters of the installation, such as the electrical and magnetic fields applied to the target and to the gas flow, in order to maintain that concentration at the desired level. The device used to put this process into service is also claimed.

Description

METHOD AND DEVICE FOR DETERMINING AND

  CONTROL OF THE COMPOSITION OF THE REACTIVE GAS MIXTURE

  ACTING ON A SUBSTRATE DURING A TREATMENT

  PHYSICO-CHEMISTRY UNDER RAREFIED ATMOSPHERE.

  The subject of the present invention is a method and a device for determining and controlling the composition of the reactive gas mixture acting on a substrate during a physico-chemical treatment under

rarefied atmosphere.

  It relates more particularly, but not exclusively, to treatments such as deposits of single or multi-element layers such as metallic carbonitrides, for example titanium carbonitrides which are carried out in a sealed enclosure under a low pressure treatment atmosphere and at a temperature of

specific treatment.

  According to these treatments, the carbon and nitrogen elements contained in the treatment atmosphere are provided by the introduction then the excitation, the ionization and the dissociation in plasma medium in the reactor.

  methane and molecular nitrogen respectively.

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  The metallic element (titanium) in the vapor phase is produced by the evaporation of a titanium target subjected to a

arc discharge.

  At present, in treatments of this kind, the material flows which determine the quality of the multi-element layer are not controlled: the only controls relate, on the one hand, to the flow rates and pressures of the gases introduced. and, on the other hand, on the intensity of the evaporation arc current as well as on the bias potential of the substrate which will determine its temperature as a function of the energy of the

ions that bombard it.

  These checks alone do not make it possible to know the composition of the medium in the reactor, which also depends on the interaction with the surfaces.

  the nature of which changes during the processing operation.

  The invention therefore more particularly aims to fill this gap. To this end, it proposes to determine the composition of the above-mentioned medium by optical emission spectroscopy, a non-disturbing technique which makes it possible to carry out in situ diagnostics and to monitor the concentrations of radiative species in real time, calculating the concentration being based on the fact that the intensity of a line is directly proportional to the density of the upper level of the radiative transition which is at the origin

of the show.

  It exploits the fact that the plasma is a medium made up of electrons, ions, molecular, radical, atomic species in fundamental states or permanently excited, in mutual interaction and with the walls, and whose stationary state is conditioned by the electric and magnetic fields applied as well

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  that gas flows. This state and therefore its reproducibility

  are related to the emission of excited species.

  Thus, the method according to the invention is more particularly characterized in that it consists in performing a spectroscopic analysis of the emissions of the vapor plasma generated at the target, in deducing from the result of this analysis the concentration of the radiative species, and possibly to act on the various parameters of the installation such as the electric and magnetic fields applied to the target as well as the gas flows to maintain this

concentration at the desired level.

  For the reasons explained above, this control also applies to the quality of the layers obtained.

on the substrate.

  An embodiment of the method according to the invention will be described below, by way of nonlimiting example, with reference to the appended drawings in which: FIG. 1 is a schematic representation of a treatment installation able to perform a deposition of titanium carbonitride, by low pressure evaporation (PVD arc); Figure 2 is a partial schematic view to illustrate the principle of the reactor used in the installation of Figure 1; Figures 3 to 10 are emission spectra around 360 nm for different values of the proportion of nitrogen; Figures 11 to 13 are emission spectra around 430 nm for different values of the N2 / CH4 ratio;

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  FIG. 14 is a curve representative of the variation of the ratio I (N2) / I (Ti) as a function of the percentage of nitrogen; FIG. 15 is a curve representative of the variation of the C / N ratio as a function of the CH4 flow rate / N2 flow rate; FIG. 16 is a curve representative of the ratio I (CH) / I (N2 +) as a function of the ratio C / N; FIG. 17 is a curve representative of the variation of the intensity ratio I (CH) / I (Ti) with the ratio of the quantities C / Ti deposited; FIG. 18 is a curve representative of the variation of the intensity ratio I (N2) / I (Ti)

  with the ratio of the N / Ti quantities deposited.

  In the example shown in FIG. 1, the installation comprises an oven comprising a sealed enclosure 1, for example metallic, in which is disposed a metallic support structure 2 intended to receive the

parts to be treated 3 (substrate).

  This structure 2 is intended to be brought to a

cathodic potential.

  It is therefore connected to one of the two terminals of a high voltage (250 V) direct or pulsed electric generator 4. The other terminal of the generator is connected to the interior wall of the enclosure 1 which then constitutes an anode. (This anode can be

  possibly made up of heating elements).

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  The interior volume of the enclosure 1 is moreover connected, on the one hand, to a pumping device 6 capable of ensuring a relative vacuum and, on the other hand, to a treatment gas injection circuit 7 connected to gas sources 8 via conduits fitted with valves

  electrically controllable flow control valves (block 9).

  The reactive mixture can consist of a hydrocarbon such as methane, nitrogen and, optionally, argon, the flow rates of which, in variable proportion, are controlled by mass flow meters. Secondary pumping

  allows then to maintain the pressure at approximately 810-3 mb.

  Electrical resistances can also be provided inside the enclosure 1 to carry and maintain

  parts at processing temperature.

  The production of titanium vapor is ensured by means of a generator 10 comprising a titanium target opposite which is an electrode 12, these two elements being electrically connected to an external current source 13 capable of delivering an electric current to relatively high current (100 A) at a relatively low voltage (20 V). This generator 13 is more particularly designed so as to generate an arc discharge between the electrode 12 and the target 11, with the aim of causing vaporization, preferably by sublimation of the surface of the target 11. A field generator magnetic (for example about ten Gauss) placed in the vicinity of the target as well as a guard ring 14 placed around the cathode, brought to a floating potential, make it possible to confine the arc to the surface of the target 11 thereby promoting evaporation. To allow the implementation of the method according to the invention, the enclosure 1 of the oven is equipped with an optically transparent window 15, for example in

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  optical quartz, tightly sealed, and focused on an area located in the vicinity of the target 11. In the axis of this window is arranged, outside the furnace, a sighting optic 16 connected by optical fibers 17 to a focusing optic 18 which focuses the light radiation emitted by the radiant species generated by the titanium vapor generator 10 on an apparatus

optical spectroscopy 19.

  This apparatus can, for example, consist of the "JOBIN-YVON HR 320 Sn spectroscope equipped with an intensified multichannel detector having an interface module 20 allowing control by computer 21, so as to

  ability to analyze plasma generated by generator 10.

  This device is more particularly designed so as to be able to follow in real time the emission of the plasma in an interval of approximately 10 nm with a resolution of approximately 0.1 nm. The optics used (optical fibers + focusing lens) allows a spatial analysis with a

  resolution of approximately 5 mm at a distance of 50 cm.

  Advantageously, the parts to be treated can be placed on an electrically rotating metal plate

  connected to generator 4, as illustrated in figure 2.

  Thanks to the movement of the plate, these parts (block 3) pass opposite the steam generator 10, for example

  at a distance of about twenty centimeters.

  The treatment carried out by the installation described above is based on the following process: The arc produced at the level of the target 11 vaporizes the titanium, which results locally in an elevation

significant pressure.

  Almost all of the arc voltage (20 V) is transferred to this region which constitutes the cathodic task of the discharge in which the

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  ionization reactions for maintaining the discharge, as well as molecular excitation and dissociation reactions. The neutral species created then diffuse into the enclosure, producing very few inelastic collisions. Among these species, the electronically excited states will de-energize in a time which is linked to their lifespan (some 10-8 s) and over a distance which is linked to the kinetic energy acquired (a maximum of 20th V) for the fast atoms or molecules backscattered by the surface and which can be about 1 cm. After de-excitation, the neutral species diffuse in the enclosure and deposit on the walls and the substrate. Because of the polarization, the ions which have sufficient kinetic energy to escape the electric field which reigns near the target will go towards the parts to be treated (substrate) which are polarized at -250 V. On this substrate, these ions have two effects, a heating up to 300 C thereof and a hammering of neutral species deposited on the surface. These two combined effects contribute to the diffusion of the atoms of titanium, nitrogen, carbon and hydrogen in the solid medium to a depth depending on the deposition conditions. These conclusions are attested to by the fact that in the absence of polarization of the substrate, on the one hand, the temperature measured by optical pyrometry remains that of the environment and that, on the other hand, the analysis of the deposit carried out under these conditions shows that it is the same

  nature only with polarization, but is not adherent.

  Table I appearing below and referring to FIG. 2, gives a simple synthetic presentation of the complex operation of the process for treating titanium carbonitride deposition by arc as can be deduced from optical spectroscopy studies, taking into account account of the analyzes of the nature of the deposit and of the

substrate temperature.

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TABLE I

  At the surface of the titanium target (C) with macroscopic reactions: potential of -20 Volts. heating, melting, evaporation of titanium, formation of a crater, local pressure rise, expansion Zone Z1 (Fig. 2) Microscopic reactions: ion + C - e ion + C - Ti (by sputtering) In the immediate vicinity of the target, Presence of a magnetic field, arc region. strong pressure gradient, inelastic collisions: e + mol (CH4, N2) e + at (ri, C, N, H) Zone Z2 (Fig. 2) fast + neutral ion leading to: dissociation, e + at reaction, excitation , de-excitation, quasi-free diffusion, ionization, acceleration

  to the substrate, charge exchange.

  At the surface (S) of the atom substrate + substrate - molecule deposition + substrate - deposition

Zone Z3 (Fig. 2) ion + substrate -

  heating, diffusion in the substrate. The spectrographic analysis of the radiation emitted in the zone Z2 located in the immediate vicinity of the target made it possible to record around 360 nm the emission spectra represented in FIGS. 3 to 10 during treatments in which the proportion was varied nitrogen and methane CH4 as follows: 0% N2% CH4 (Figure 3); 30% N2 70% CH4 (Figure 4); % N2 50% CH4 (Figure 5); 60% N2 40% CH4 (Figure 6); 70% N2 30% CH4 (Figure 7); 80% N2 20% CH4 (Figure 8); 90% N2 10% CH4 (Figure 9); 95% N2

5% CH4 (Figure 10).

  These spectra show a relationship between the intensity of the N2 lines of nitrogen and the intensity of the titanium lines. This relation is defined by the curve represented on figure 14 which gives the variations of the

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  I (N2) / I (Ti) ratio as a function of the percentage of nitrogen N2. Similarly, Figures 11, 12 and 13 show the emission spectra around 430 nm for three different values of N2 / CH4 ratio, namely: CH4 40% N2 60% (Figure 11); CH4 20% N2 80% (figure

12); CH4 10% N2 90% (Figure 13).

  These different experiments made it possible to show an almost linear dependence between, on the one hand, the methane / nitrogen ratios and the carbon / nitrogen ratios of the deposit obtained analyzed by ESCA (figure) and, on the other hand, the ratios of intensity I (CH) / I (N2 +) obtained by emission spectroscopy and the

C / N ratio of the deposit (Figure 16).

  The curves represented in FIGS. 17 and 18 give the variation of the intensity ratio I (CH) / I (Ti) as a function of the quantities C / Ti deposited (FIG. 17) and the variation of the intensity ratio I (N2 +) / I (Ti) with the

  report of the quantities deposited N / Ti (Figure 18).

  It clearly appears that thanks to these different relationships, it becomes possible to control the composition of the medium in the reactor and therefore the quality

  layers deposited on the substrate.

  In particular, the computer can be programmed so as to control, via an interface, the flow rate of the treatment gases as a function of the proportions of carbon C, titanium Ti and nitrogen N that are desired. deposit on the substrate. This control can be carried out using correspondence tables established according to the curves mentioned above and / or by calculation, in particular when dealing with linear relationships such as those existing between the report.

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  CH4 flow / N2 flow and the C / N ratio or the ratio

I (CH) / I (N2 +) and the C / N ratio.

  It should be noted that reliability tests have been carried out during the operation of the titanium arc plasma reactor alone at the limit pressure of 10-5 mbar, then by adding nitrogen, and finally, in the mixture used. in treatment, that is to say with nitrogen and methane. In all cases, the reproducibility of the emission spectra is observed when the conditions

  of operation of the reactor are identical.

  Of course, the invention is not limited to the type of treatment described above: in fact, it remains valid for the control of deposits of most transition metals such as nickel Ni, tungsten

W, or vanadium Va.

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Claims (6)

Claims   1. Method for determining and controlling the composition of the reactive gas mixture acting on a substrate during a physicochemical treatment under a rarefied atmosphere according to which one proceeds to the evaporation of a target subjected to a discharge of arc, characterized in that it consists in carrying out a spectroscopic analysis of the emissions of the vapor plasma generated at the level of the target, in deducing from the result of this analysis the concentration of the radiative species, and possibly in acting on the various parameters of the installation such as the electric and magnetic fields applied to the target as well as the gas flows to maintain this concentration at the desired level. 2. Method according to claim 1, characterized in that the concentration calculation comprises the measurement of the intensities of the spectral lines corresponding to the elements entering into the composition, and the determination of the concentration from said intensities based on the fact that l line intensity is directly proportional to the density of the upper level of the radiative transition which is the origin of the broadcast.   3. Method according to one of claims 1 and 2,   characterized in that the above physical-   chemical is a deposit of single or multi-element layers such as metallic carbonitrides in which the reactive gas mixture consists of a hydrocarbon   such as methane, nitrogen and possibly argon.   4. Method according to claim 3, characterized in that it comprises the calculation of the ratio of the intensity of the spectral lines of nitrogen and - 12 - 2719120   the intensity of the spectral lines corresponding to the vaporized metal, the ratio of the intensity of the spectral lines corresponding to the radical CH and the intensity of the spectral lines of nitrogen, and the ratio of the intensity of the spectral lines corresponding to the radical CH and of the intensity of the spectral line of the vaporized metal, and the determination from these ratios of the carbon / nitrogen ratio, of the carbon / metal deposited ratio and of the nitrogen / metal deposited ratio, using diagrams of   variation and / or preset correspondence tables.   5. Method according to one of claims 3 and   4, characterized in that the above metal is a transition metal such as titanium, nickel, tungsten or vanadium.   6. Device for implementing the method   according to one of the preceding claims,   characterized in that it comprises a sealed enclosure (1) in which is disposed a metal support structure (2) intended to receive the parts to be treated (3) and brought to cathodic potential by means of a high voltage electric generator (4 ), the interior volume of the enclosure (1) being also connected, on the one hand, to a pumping device (6) capable of generating a relative vacuum and, on the other hand, to an injection circuit of process gas (7) connected to gas sources (8) via conduits equipped with electrically controllable flow control valves (9) and mass flow meters and a metallic steam generator (10) comprising a metal target opposite which is an electrode (12), said target and said electrode being connected to an external current source (13) capable of delivering an electric current capable of generating an arc discharge between the electrode ( 12) and the target, and in that the enclosure (1) is equipped with a window - 13 - 2719120   optically transparent (15) focused on an area located in the vicinity of the target and an external aiming optic (16) arranged in the axis of this window (15) and connected by optical fibers (17) to a focusing optic (18) which focuses the light radiation emitted by the radiating species generated by the generator (10) on an optical spectroscopy device (19) coupled, via an interface (20), to a computer (21) programmed so as to control the gas flow according to the information supplied by   the optical spectroscopy apparatus (19).