Method for depositing a coating by DLI-MOCVD with direct recycling of the precursor compound

Abstract

Process for the chemical vapor deposition by DLI-MOCVD on a substrate of a protective coating composed of at least one protective layer comprising a transition metal M: a) having available, in a feed tank, a mother solution containing a hydrocarbon solvent devoid of oxygen atom and a precursor of bis(arene) type containing the transition metal M to be deposited, and, if appropriate, a carbon-incorporation inhibitor; b) vaporizing said mother solution and introducing it into a CVD reactor in order to carry out the deposition of the protective layer on said substrate; c) collecting, at the outlet of the reactor, a fraction of the gaseous effluent comprising the unconsumed precursor, the aromatic byproducts of the precursor and the solvent, these entities together forming a daughter solution, and; d) pouring the daughter solution thus obtained into the feed tank in order to obtain a new mother solution capable of being used in step a).

Claims

1. Process for the deposition on a substrate of a protective coating composed of one or more layers, at least one being a protective layer comprising a transition metal M in the form of at least one protective material selected from the group consisting of a carbide, an alloy and a metal in native or virtually pure form, the deposition process being a process for the chemical vapor deposition of an organometallic compound by direct liquid injection (DLI-MOCVD) which comprises the following steps: a) having available, in a feed tank, a mother solution containing: a hydrocarbon solvent devoid of oxygen atom, said organometallic compound composed of a precursor of bis(arene) type having a decomposition temperature of between 300° C. and 600° C. and comprising the transition metal M, and if appropriate, a carbon-incorporation inhibitor; b) vaporizing said mother solution in an evaporator and then introducing said vaporized mother solution into a chemical vapor deposition reactor in which said substrate to be covered is found; c) depositing the protective layer on said substrate in the chamber of the reactor, the atmosphere of which is at a deposition temperature of between 300° C. and 600° C. under reduced deposition pressure, the deposition of the protective layer on said substrate comprising consumption of the precursor which comprises the production of a gaseous effluent comprising aromatic byproducts of the precursor; d) collecting, at an outlet of the reactor, a fraction of the gaseous effluent comprising the unconsumed precursor, the solvent and the aromatic byproducts of the precursor, the collecting step comprising an operation of selective condensation of said fraction of the gaseous effluent so as to condense the unconsumed precursor, the unconsumed solvent and the aromatic byproducts of the precursor, wherein the unconsumed precursor, the unconsumed solvent and the aromatic byproducts of the precursor together form, under standard conditions, a daughter solution; and e) pouring the daughter solution thus obtained into the feed tank in order to obtain a new mother solution capable of being used in step a).

2. Process according to claim 1, wherein step b) of vaporizing, step c) of reacting and depositing, and step d) of collecting said fraction of the effluent are carried out so that the atmosphere of the chamber of the reactor is at a reduced deposition pressure of between 133 Pa and 6666 Pa.

3. Process according to claim 1, wherein the transition metal M is chosen from Cr, Nb, V, W, Mo, Mn or Hf.

4. Process according to claim 3, wherein the transition metal M is at the zero oxidation state.

5. Process according to claim 3, wherein the protective material is a base alloy of the transition metal M.

6. Process according to claim 3, wherein the protective material is the transition metal M in native form, or is a virtually pure composition of a single transition metal M, each of any other chemical element in said protective material being present at a level of less than 0.5 atomic %.

7. Process according to claim 1, wherein the transition metal M is chromium.

8. Process according to claim 1, wherein the protective material is a carbide.

9. Process according to claim 8, wherein the carbide of the transition metal M composing the protective material is of CrC, WC, NbC, MoC, VC or HfC type, or has the stoichiometric Cr 7C 3, Cr 3C 2, Mo 2C, Mn 3C, V 2C or V 4C 3.

10. Process according to claim 1, wherein the precursor of bis(arene) type is devoid of oxygen atom and has the general formula (Ar)(Ar′)M, where M is the transition metal at the zero oxidation state (M 0) and Ar and Ar′, which are identical or different, each represent an aromatic group of the type of benzene or benzene substituted by at least one alkyl group.

11. Process according to claim 10, wherein the aromatic groups Ar and Ar′ each represent a benzene radical or a benzene radical substituted by from 1 to 3 identical or different groups chosen from a methyl, ethyl or isopropyl group.

12. Process according to claim 1, wherein the solvent is a monocyclic aromatic hydrocarbon of general formula C.sub.xH.sub.y which is liquid under the standard conditions and which has a boiling point of less than 150° C. and a decomposition temperature of greater than 600° C.

13. Process according to claim 1, wherein said mother solution further contains, as carbon-incorporation inhibitor, a chlorine-comprising or sulfur-comprising additive, devoid of oxygen atom and with a decomposition temperature of greater than 600° C., in order to obtain the protective material composed of the transition metal M or of the alloy of the transition metal M.

14. Process according to claim 13, wherein the carbon-incorporation inhibitor is sulfur-comprising.

15. Process according to claim 1, wherein, in step d), aliphatic byproducts of the precursor and aliphatic byproducts of the solvent which are present in the effluent at the outlet of reactor are not condensed.

16. Process according to claim 1, wherein step d) of collecting said fraction is followed by a step d1) of determining the concentration of the precursor in the daughter solution obtained, and wherein step e) comprises an operation e0) of adjusting the concentration of the precursor, as a function of the concentration of the precursor of the daughter solution poured into the feed tank.

17. Process according to claim 1, wherein the concentration of the precursor in the daughter solution is less than the initial concentration of the precursor in the mother solution.

18. Process according to claim 1, wherein steps a) to d) are repeated sequentially N times and the N daughter solutions are saved, and then step e) is carried out by pouring said N daughter solutions into the feed tank in order to obtain a new mother solution capable of being used in step a).

19. Process according to claim 1, wherein the daughter solution obtained in step d) is poured continuously into the feed tank, during the chemical vapor deposition process.

20. Process according to claim 1, wherein the protective coating has a mean thickness of between 1 μm and 50 μm.

21. Process according to claim 1, wherein, in step d), the operation of selective condensation is performed with a cryogenic trap at a temperature ranging between −200° C. and −50° C.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows the UV/visible spectra in transmittance of deposit-free quartz slides (Blank) and after treatment at 500° C., 600° C., 750° C. and 800° C. during the injection of toluene alone (without bis(ethylbenzene)chromium precursor).

(2) FIG. 2 exhibits the change in the intensity of the absorbance at the wavelength of 500 nm measured on the spectra in transmittance of FIG. 1 as a function of the pyrolysis temperature.

(3) FIG. 3 represents a calibration line for BEBC in UV/visible spectrophotometry.

(4) FIG. 4 exhibits a comparison of the microstructures of the coating obtained with fresh precursor and recycled precursor (views in section).

(5) FIG. 5 exhibits a comparison of the microstructures of the coating obtained with fresh precursor and recycled (top views).

(6) FIG. 6 exhibits a comparison of the Energy-Dispersive Spectra (EDS) of the coating obtained with fresh precursor (at the top) and recycled (at the bottom).

(7) FIG. 7 exhibits a comparison of the X-ray diffractograms of a coating made of amorphous chromium carbides obtained with fresh precursor (at the top) and recycled precursor (at the bottom).

(8) FIG. 8 is a diagrammatic view of a DLI-MOCVD device suitable for the implementation of the deposition process of the invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

(9) The specific embodiments of the process of the invention relate to the deposition of coatings based on chromium (chromium carbides or chromium metal) by decomposition of the two precursors BBC or BEBC, in toluene taken as solvent.

Example 1: Deposition of Chromium Carbide

(10) The deposition of a coating of chromium carbide CrC was carried out under the following conditions:

(11) Injection conditions: open time of the injector: 0.5 ms frequency: 10 Hz

(12) Reactant: BEBC (5 g)

(13) Solvent: Toluene (50 ml)

(14) Carrier gas: N.sub.2 (flow rate of 500 sccm, namely 500 cm.sup.3/min

(15) under standard conditions)

(16) Duration of the deposition: 20 minutes

(17) Temperature of deposition in the reactor: 450° C.

(18) Deposition pressure: 50 Torr

(19) Vaporization temperature in the evaporator: 200° C.

(20) Temperature of the cryogenic trap: −120° C.

(21) Amount of daughter solution recovered: 30 ml

(22) Two experiments N1 and N2 were carried out with a BEBC mother solution. On conclusion of these experiments, two daughter solutions are collected by cryogenic trapping of a fraction of the gaseous effluent recovered at the outlet of the CVD reactor.

(23) In a third experiment, the two daughter solutions recovered are combined in order to form a recycled mother solution which was used as source of precursor for carrying out a third deposition operation N3.

(24) For N1 and N2, the thickness of the deposit is typically 5 μm. A deposit with a thickness of approximately 1.5 μm is obtained on conclusion of N3. The concentration of BEBC was determined and the yield calculated for N1 and N2 (see Table 1).

(25) TABLE-US-00001 TABLE 1 Yield [BEBC] in [BEBC] in with the Daughter the respect Injected injected solution daughter to the solution solution recovered solution precursor No. (ml) (g/ml) (ml) (g/ml) (%) N1 55 0.078 30 0.031 60% N2 50 0.083 30 0.034 59% N3 30 + 30 0.040 35 Not N/A measured, as very low

Example 2: Deposition of Chromium Metal with Recycling of the Precursor

(26) The deposition of a coating of chromium metal Cr was carried out under the following conditions:

(27) Injection conditions: open time of the injector: 0.5 ms frequency: 10 Hz

(28) Reactant: BEBC (5 g)

(29) Solvent: Toluene (50 ml)

(30) Additive: Thiophenol C.sub.6H.sub.5SH (additive/precursor molar ratio=2%)

(31) Carrier gas: N.sub.2 (flow rate 500 sccm)

(32) Duration of the deposition: 1 h

(33) Temperature of deposition in the reactor: 450° C.

(34) Deposition pressure: 50 Torr

(35) Vaporization temperature in the evaporator: 200° C.

(36) Temperature of the cryogenic trap: −100° C.

(37) Amount of daughter solution recovered: 30 ml

(38) Two experiments were necessary in order to recover 60 ml of daughter solution. The mother solution thus recycled was reinjected into the CVD reactor in order to carry out a third deposition operation under the same conditions: a protective coating of approximately 1 μm is obtained.

Example 3: Choice of a Solvent: Toluene

(39) In order to study if toluene can be used as solvent in the process according to the invention, it was confirmed that it does not decompose within the range of temperatures which are swept over by the process and under hydrodynamic conditions comparable to the true conditions of depositions by DLI-MOCVD.

(40) Tests were carried out by injecting only toluene into the CVD reactor. Quartz slides are placed in the chamber of the CVD reactor on a sample holder and, after each deposition, a UV/visible transmittance spectrum was recorded. Several temperatures of the reactor were tested between 500° C. and 800° C. The spectra obtained are presented in FIG. 1. The spectrum of a control slide, which has not been subjected to deposition of carbon, is also represented (Blank).

(41) The mean transmittance at the wavelength of 500 nm was plotted for the different temperatures of the reactor. Above 600° C., it decreases because the quartz slide turns opaque following the formation of a thin film of carbon. It is justifiable to believe that toluene begins to decompose at this temperature, with an accentuation at 750° C., more marked still toward 800° C., as is shown by FIG. 2.

(42) Consequently, toluene is an appropriate solvent for depositions for which the temperature does not exceed 600° C.

(43) Furthermore, this result allows it to be believed that, when a bis(arene)chromium precursor decomposes with the release of the benzene ligands, the latter do not decompose either below 600° C. in the homogeneous phase.

Example 4: Assaying of the Precursors

(44) Numerous techniques exist for determining the concentration of precursor of the solutions used, all more or less reliable and problematic to carry out. The concentration of precursor of the mother solution injected initially into the CVD deposition reactor is known. The concentration of the recycled daughter solution is to be determined.

(45) For this, the concentration of BBC and of BEBC is determined by the change in their absorption band at 315 nm in the UV range, which is monitored by spectrophotometry (Douard, A., in Institut Carnot CIRIMAT. 2006, INP Toulouse). This absorption band corresponds to the M(4e2g).fwdarw.L(5e2g) charge-transfer transition, brought about by the chromium-ligand bond of the precursor molecule, which bond will be cleaved in the initial phase of the mechanism of growth of the coating.

(46) The principle thereof is as follows. The Beer-Lambert law, relating the concentration to the absorbance, is:
A=ε*C*l, with

(47) A: the absorbance of the solution at 315 nm;

(48) ε: the molar extinction coefficient of the precursor;

(49) C: the concentration of precursor;

(50) l: the length of the cell.

(51) In order to construct a calibration line, known concentrations of BEBC or BBC solutions are related to the measurement of their absorbance (see FIG. 3). The concentration of any solution can subsequently be determined by UV/visible spectrophotometry: its measured absorbance is directly related to its concentration using the calibration line.

(52) Access to the yields of the cryogenic trap of the CVD reactor is also possible. By withdrawing a small volume of daughter solution at the reactor outlet, its concentration can be determined in order to decide, if necessary, to enrich the daughter solution with precursor in order to reinject it subsequently into the system. The absorbance of the daughter solution can also be measured in line by incorporating an optic cell in the circuit for recovery of the daughter solution: this is a nondestructive analytical method.

Example 5: Coatings Obtained on Various Substrates

(53) No definite basic mechanism has been put forward for accounting for the growth of chromium carbides or chromium metal by decomposition of the BBC or BEBC precursor, any more than the influence of the presence of toluene on the reaction mechanism has been explained. Further, the data available for operating temperatures of less than 600° C. are very scarce.

(54) It has been demonstrated experimentally that the process of the invention makes it possible to deposit protective films and coatings exhibiting the desired characteristics.

(55) A) the Characteristics of the Films do not Depend on the Concentration of Precursor of the Injected Solution.

(56) Numerous variations in the parameters can cause the concentration of precursor of the injected solution to vary and, by extension, that of the reactive gas phase sent into the reactor. The films deposited by the process of the invention are nevertheless comparable. The following parameters were thus tested: Nature of the precursor used: BEBC; Injection parameters which modify the proportion of injected solution with respect to the flow rate of carrier gas: frequency between 1 Hz and 20 Hz; open time between 0.5 ms and 5 ms; Relative amounts of precursor and of solvent: concentrations of precursor of 1.0×10.sup.−2 mol.Math.l.sup.−1 to 5.0×10.sup.−1 mol.Math.l.sup.−1.

(57) The fact of injecting a solution based on fresh precursor and a solution based on recycled precursor does not change the characteristics of the films (see below). This is because the compositions of the protective coatings obtained, of the type of amorphous chromium carbides, with a composition close to Cr.sub.7C.sub.3, are always similar. The morphologies are also equivalent, with a typical microstructure of a homogeneous amorphous film, a completely dense and very smooth protective layer.

(58) Colorimetric assaying by spectrophotometry has made it possible to measure that the mother solution based on recycled precursor was approximately 60% less concentrated in precursor than the mother solution based on fresh precursor, without impacting the quality of the films deposited.

(59) Moreover, the fact that these characteristics are independent of the precursor/solvent ratio is consistent with previous results which have shown that MOCVD depositions (without solvent) are also comparable, just like DLI-MOCVD depositions (with solvent) with cyclohexane in place of toluene. This is consistent with the fact that the solvent is not involved in the mechanism of decomposition of the precursor and that it is not itself decomposed during the process.

(60) B) Morphology, Microstructure (SEM, Roughness)

(61) The microstructures of the protective coatings obtained from fresh or recycled mother solution are in every respect similar during the observations by Scanning Electron Microscopy (SEM). Each coating is dense, compact and homogeneous in thickness over the entire surface area of the sample, as shown in FIG. 4.

(62) The interface with the Si substrate is well-defined. Furthermore, in top view (see FIG. 5), they have the same very smooth appearance without major heterogeneities but with a few surface contamination elements. The maximum thicknesses achieved with the fresh precursor are significantly greater than those with the recycled precursor, because the concentration of the recycled solution was lower. As much precursor is consumed in the reactor, only a small part is recovered using the cryogenic trap.

(63) C) Composition (EDS, EPMA)

(64) The EDS spectra are also comparable, with slight contamination with oxygen visible in both cases, fresh precursor or recycled precursor. The peaks of the chromium and the carbon have identical intensities, as is shown by the spectra in FIG. 6

(65) The elemental compositions found with the Electron Probe MicroAnalysis (known under the English acronym “EPMA”) analyses do not reveal any glaring disparity between the samples prepared with the fresh or recycled precursor: BEBC-500° C. (amorphous): Cr.sub.0.65C.sub.0.32O.sub.0.03 standardized at Cr.sub.0.67C.sub.0.33 and C/Cr=0.49 BEBC-450° C. (amorphous): Cr.sub.0.64C.sub.0.33O.sub.0.03 standardized at Cr.sub.0.66C.sub.0.34 and C/Cr=0.52 recycled BEBC-450° C. (amorphous): Cr.sub.0.64C.sub.0.30O.sub.0.05 standardized at Cr.sub.0.68C.sub.0.32 and C/Cr=0.48

(66) As a reminder, the C/Cr ratio has the value of 0.43 for Cr.sub.7C.sub.3 and 0.66 for Cr.sub.3C.sub.2. The mean composition observed is thus very close to Cr.sub.7C.sub.3.

(67) D) Structure (XRD)

(68) The analysis by X-Ray Diffraction (XRD) shows that the coatings are always amorphous, as is testified by the broad hump which is centered at approximately 2θ=42°. Examples of diffractograms obtained for a deposition starting from fresh and recycled precursor are presented in FIG. 7. The broad hump centered around 2θ=69° is characteristic of the amorphous a-Si.sub.3N.sub.4 layer, which acts as barrier on the silicon substrate. It is present on the bare substrates and its contribution is greater when the deposit is thinner (case of the mother solution containing recycled precursor).

(69) E) Mechanical properties: Hardness (nanoindentation)

(70) The nanoindentation device is provided with an indenter of Berkovich type (triangular-based pyramid with an angle of 65.27° between the vertical and the height of one of the faces of the pyramid). The measures are carried out in accordance with the rule of the tenth: the indenter drives in by less than one tenth of the thickness of the coating. A measurement cycle is carried out in three steps: increasing load up to the maximum load, in 30 s; maintenance of the maximum load for 30 s; unload for 30 s.

(71) The nanoindentation measurements were carried out on samples coated starting from fresh precursor (thickness of 3.5 μm) and recycled precursor (thickness of 1 μm). The calculations made by the measurement and analysis software take into account a Poisson coefficient of the coating of 0.2. The measurements of hardnesses and of Young's modulus are presented in Table 2.

(72) The values found for the coating deposited starting from recycled precursor are higher for the hardness but lower as regards the Young's modulus. They remain in any case consistent with values expected for a very hard coating.

(73) TABLE-US-00002 TABLE 2 Driving in (nm) in Hardness Modulus Load proportion with the Sample (GPa) (GPa) (mN) thickness CrC 3.5 μm 25 294 4 ~100 (3%) 21 296 21 292 3 24 280 Mean 23 291 CrC 1 μm 32 260 3  ~90 (9%) “recycled” 33 279 26 240 31 257 Mean 31 259

Example 6: Device for Deposition by DLI-MOCVD

(74) A device for deposition by DLI-MOCVD which may be suitable for the implementation of the deposition steps a) and b) of the process of the invention is, for example, described in its main characteristics in the document WO 2008009714.

(75) The DLI-MOCVD device which can be used for the deposition of the protective coating with the deposition process of the invention according to steps a) to d) comprises mainly a feed tank, an evaporator, an injector, a CVD reactor and a unit for collecting the daughter solution for the purpose of the recycling thereof in the device. This DLI-MOCVD device is described more specifically with reference to FIG. 8.

(76) A pressurized feed tank 1 feeds the injector 2 with mother solution. The injector 2 is generally constituted of a commercial pulsed injection system, for example a diesel automobile injector.

(77) The opening and the closing of the injector 2 can be computer-controlled, which makes possible the injection of the mother solution into the evaporator 3.

(78) The evaporator 3 is positioned coaxially above the generally vertical CVD deposition chamber 10 into which it emerges.

(79) A carrier gas feed line 4 emerges in the evaporator 3 next to the outlet of the injector 2. The stream of carrier gas entrains the vaporized mother solution from the evaporator 3 toward the CVD deposition chamber 10. At the inlet of the latter, a baffle 8 stops the possible unvaporized droplets at the outlet of the evaporator 3 and a screen 9 pierced with holes uniformly dispenses the gas stream. This screen 9 makes possible good distribution of the gas stream in the CVD deposition chamber 10, which contributes to a good surface state of the coatings and a uniformity in thickness being obtained.

(80) A slide valve 5 can isolate the evaporator 3 from the remainder of the CVD deposition chamber 10: the volume thus delimited below the slide valve 5 comprises the CVD reactor proper in which is found the susceptor 13 on which the substrate to be covered is placed.

(81) The additional pipe 6 above the slide valve 5 makes possible the arrival of a reactive gas, such as, for example, a carbon-incorporation inhibitor. The additional pipe 7 above the slide valve 5 makes it possible for the evaporator 3 to be pumped out during the cycles of purging or cleaning the latter. The collar 14 on which the connections of the additional pipes 6 and 7 are made, and also the slide valve 5 at the inlet of the CVD reactor, are heated to a temperature close to that of the evaporator 3.

(82) A protective layer is deposited on the substrate starting from the vaporized mother solution in the CVD reactor.

(83) On conclusion of this reaction of deposition by DLI-MOCVD, an outlet pipe 12 at the outlet of the CVD deposition chamber 10 collects a fraction of the gaseous effluent produced during the reaction. This fraction comprises the unconsumed precursor of bis(arene) type, the aromatic byproducts of the precursor and the solvent, indeed even, if appropriate, the carbon-incorporation inhibitor.

(84) The outlet pipe 12 emerges on a selective condensation unit 14 (such as, for example, a cryogenic trap), in which the main undesirable compounds (in particular the light hydrocarbons) of the fraction of the gaseous effluent are removed, in order to produce a daughter solution.

(85) When the deposition process of the invention is carried out continuously, a pipe 15 continuously dispatches the daughter solution thus produced in order to recycle it in the feed tank 1. A new mother solution is then formed for the purpose of the use thereof in a new cycle of the deposition process of the invention.

(86) A backing pump 11 can be used to purge the whole of the DLI-MOCVD device, for example before a new deposition.