FR2773175A1 - METHOD FOR PREPARING THIN FILMS OF FLUORINE COMPOUNDS USED IN OPTICS AND THIN FILMS THUS PREPARED - Google Patents

Abstract

The invention concerns a method for preparing at least one fluorinated compound film by vacuum deposit formation, which consists in, simultaneously with the vacuum deposition, also generating and/or introducing in the gas phase, at least a reducing species and elemental, molecular or bound fluorine, thereby producing during its deposit formation the fluorination of the fluorinated compound film. The invention also concerns the resulting thin films and thin film layers. Said films or film layers deposited on a substrate can ensure an optical function, such as mirror, spectral filter or antiglare coating, in the spectral field ranging from ultraviolet to infrared, or act as protective coating on optical components for example for protecting them against intense laser flux or against corrosive atmosphere.

Description

PROCESS FOR THE PREPARATION OF THIN LAYERS OF COMPOUNDS
OPTICALLY OPTICAL FLUOROS AND THIN LAYERS THUS
PREPARED
DESCRIPTION
Technical area.

 The invention relates to a method for preparing layers, in particular thin layers, of fluorinated compounds that can be used in optics.

 The invention also relates to the thin layers and stacks of thin layers thus prepared.

 These layers or stacks deposited on a substrate can provide a mirror-like optical function, spectral filter or antireflection coating, in the spectral range from ultraviolet to infrared, or play the role of protective coating of optical components for example to protect them against intense laser flux or against corrosive atmospheres.

 State of the prior art.

 In the field of thin-film processing of optical components, the realization of a given optical function, for example mirror, spectral filter or antireflection coating can be filled by means of a layer or a stack of layers of different optical index materials.

 The total thickness of such a stack is generally a few tenths of microns to a few micrometers.

Among the many materials commonly used, mention may be made of
oxide compounds comprising, in particular, silica (low index) but also the various oxides of titanium, tantalum, zirconium, hafnium, aluminum, etc. (high indices). These materials are widely used for applications in the visible and, for some, in the ultraviolet.

 - materials specifically intended for applications in the infrared, such as chalcogenide (ZnS, ZnSe) or germanium.

 the fluoride compounds which comprise a wide range of compounds, for example of the general formula MF, or M1... MnFx or M is an alkaline or alkaline-earth metal or a lanthanide.

 These compounds constitute a class of materials of particular interest for applications in a very wide spectral range including ultraviolet, visible and infrared. Indeed, most of these compounds have the particularity of having a very broad spectral range of transparency, typically ranging from 0.15 to 10 Wm.

 Among the best known compounds include for example YF3, and ThF which are commonly used for applications in the infrared; MgF2 and CaF2 which are used as visible antireflection material, or LaF3, Na3A1FG.

 Thin layers of fluorides are generally obtained by vacuum deposition techniques, in particular vacuum evaporation techniques. The simplest of these techniques operates by resistive heating and vaporization of the material to be deposited.

 The materials thus deposited, by their chemical purity and their stoichiometry, are optically very transparent, but they often have a high porosity, even an inhomogeneous nature, a low mechanical strength and a significant sensitivity to the environment related to a strong recovery of 'water.

 To remedy this problem and densify these layers, the first solution consists in depositing the layers while hot for example by heating the substrates on which they are deposited.

 This densification technique can, however, lead to a cracking of the deposits, since the fluorides have relatively high thermal expansion coefficients, which are on average ten times those of the oxides, and which result after deposition by the presence in the stress layers of voltage of several hundred MPa.

To avoid the disadvantages brought about by densification by heating, other deposition methods, called energy deposition methods, which generally operate at cold temperature and use ion beams, have been applied directly to the growing layers in the process. case of ion beam assisted evaporation (IAD or Ion
Assisted Deposition), or used to spray the source or target of the material to be deposited in the case of ion beam sputtering (IBS or Ion Beam Sputtering).

 These deposition and densification processes are an effective way to limit the porosity of the deposits but they generate a significant deficit in fluoride in the layers, which optically results in a sharp increase in the absorption in the spectral domain of the ultraviolet, or even the visible.

 More precisely, the fluoride deficit of the fluoride materials is essentially due to two phenomena, one of these phenomena - as already indicated above - is related to the physics of the vacuum deposition processes. other of these phenomena being related to the chemical behavior of these materials.

 It is known that when fluorides are subjected to particle bombardment, they have a strong tendency to release fluoride.

This phenomenon is frequently observed during sputter deposition or by ionic assistance of fluoride layers, or even during electron beam evaporation as described in the document by NN Skvortsov.
Yu.K.Ustinov and SVYalyshko, sov.J.Opt.Technol., 54 (9) (1987) 541.

 This lack of fluorine may be attributed either to a preferential ionic sputtering of fluorine or to a desorption of fluorine under the effect for example of an electron beam.

 In order that the energetic deposition processes of thin films of fluoride can be implemented efficiently and in a viable manner in the field of optics, it is therefore imperative that these methods be associated with methods for refluishing the layers, ie during deposit, either after the deposit.

 For obvious reasons of simplification of the processes, it is preferable to simultaneously deposit and (re) fluorinate the layers.

 Thus, in the thin film deposition processes in general, the compensation of the stoichiometry of the deposits is done as far as possible by injection during the deposition of a reactive gas containing the deficit element, the process then taking the name of the process reagent.

In the particular case of fluorides reactive gases generally used are CF4, C2F6, SF6, or fluorine F2 whose implementation is however not without risk for the equipment
Thus the document of H. Schink, J. Kolbe,
F. Zimmermann, D. Ristau and H. Welling, SPIE vol.1441 (1990) 327. and the document by J.Kolbe, H.Schink and
D.Ristau, Society of Vacuum Coaters (SVC) 36th annual
Technical Conference proceedings (1993) 44-50 describe the deposition of thin layers of fluoride LaF3, AlF3 and MgF2 by reactive methods of ion beam assisted evaporation (IAD) or ion beam scattering (IBS) for applications in the deep ultraviolet especially for mirrors.

 Fluorinated gases such as F2 and CF are used as reactive gases to compensate for the fluorine sub-stoichiometry responsible for high optical absorption at short wavelengths. But, if such gases allow some improvement over the same processes using oxygen as a reactive gas, they can only achieve a slight decrease in absorption.

 The same observation is made in the document of J. G. Cook, G. H. Yousefi, S.R.Das and D.F.

Mitchell, Thin Solid Films, 217 (1992) 87-90, where in a cathodic sputtering process of CaF2, the introduction of CF into the plasma, while it induces a clear fluorination of the layers, generates, on the other hand, strong absorption due to the pollution of the layers by carbon.

Moreover, in an ion beam assisted evaporation (IAD) process, the use of
C2F6 in the assistance barrel leads to low fluorine compensation.

 Finally, JP-A-7 166 344 relates to the deposition of fluoride by sputtering metal targets in a plasma composed of an inert gas and fluorine.

 The optical quality of the layers deposited in the ultraviolet or visible, however, is not mentioned, because the purpose of this document is rather to allow a high deposition rate to seek a particular transparency.

 It follows from the above that the refluoridation of fluoride layers deposited by energy deposition techniques is extremely difficult by using fluorinated reactive gases and has not been successfully achieved so far.

 This is why it is often preferred to resort to oxidation and achieve, instead of fluorides, MFX-EOE type oxyfluoride compounds which have a good transparency in the ultraviolet but whose optical gap remains higher than that pure fluorides, which poses problems for applications in the deep ultraviolet.

 Thus, J.D.Targove, M.J.

Messerly, JPLehan, RH Potoff, HAMacleod, LCMC
Intyre Jr and JALeavitt, SPIE vol.678, (1986) 115, describes the assistance of the deposition of MgF2 with oxygen, while JP-A-8 134 637 relates to the use of a magnesium oxyfluoride target to provide low optical absorption to the layers.

 Similarly, the sputtering of a target consisting of a mixture of fluorides of the CaF2, NalF6, Na5A13F14 or SrF2 type operating in a reactive atmosphere containing oxygen was the subject of JP-A-8 171 002 which essentially aims to obtain transparent layers, essentially in the visible.

 The enumeration of the abovementioned techniques makes it clear that the preparation and (re) fluorination of thin layers of fluorinated compounds such as fluorides is extremely difficult and has not hitherto been demonstrated and demonstrated.

 Thus, currently the fluorination techniques used, the principle of which is based on the decomposition of a fluorinated gas and on the reaction of the fluorine thus produced with the deposited substoichiometric material, make it difficult to reduce the extinction coefficient of fluoride monolayers below k = 10-3 in the visible.

 In addition to the fluorine desorption phenomena, fluoride materials used as a source of vapor for the condensation of a thin film are generally very sensitive to hydrolysis and oxidation, particularly when these materials are present. in powder form.

 As a result, since the spray sources are generally in the form of sintered powders, they contain not only the fluoride compound (NF) but also the hydrolysed (MFX-y (OH) y) or oxidized (MF, ~, O) derivatives. / Z). The proportion of these byproducts can reach a few percent and they are responsible for much of the deficit in fluoride layers.

 As far as evaporation is concerned, the high sensitivity of fluorides to moisture, or even oxidation, means that the fluorine-deficient layer or possibly evaporation charge will tend to compensate for fluorine deficiencies. oxygen or hydroxide groups -OH, within the limit of the concentration of these species in the residual vacuum of the deposition chamber.

 The object of the invention is to provide a process for the preparation of thin films of fluorinated material which do not have the disadvantages, limitations and disadvantages of the prior art and which solves the problems of the prior art mentioned above.

 This and other objects are achieved according to the invention by a process for the preparation of at least one layer of fluorinated compound by vacuum deposition, in which, simultaneously with the vacuum deposition operation, there is generated or at least one reducing chemical species and elemental fluorine, molecular or bonded, are introduced into the gaseous phase, whereby the fluorination of the fluorinated compound layer is carried out during its deposition.

 The method according to the invention is thus fundamentally different from the methods of the prior art described above, in that its principle is based on the combination of a double chemical reaction, which consists on the one hand in a reduction of the compound fluoro such as fluoride partially oxidized because of contamination by oxygen, and secondly, in its fluorination.

 These two reactions are made possible by the generation or introduction into the vacuum chamber and / or at another point outside thereof, not only fluorine but also reducing chemical species such as active hydrogen.

 This approach is totally surprising and goes against that hitherto followed in the prior art involving only fluorination.

 Advantageously, this reducing chemical species and / or this fluorine are generated and / or introduced in situ, that is to say directly into the chamber, or reactor, or the vacuum deposition operation takes place.

 It is obvious that this reducing chemical species and / or this fluorine can also be generated and / or introduced ex-situ, for example in a separate reactor and / or enclosure.

By way of example, for elemental, molecular or linked fluorine, mention may be made, for example, of the following species: F, F 2, CF, CF 2, CF 3,
CHF, CF4 etc ...

 To activate fluorination, an advantageous embodiment of the invention will consist in promoting in the gaseous phase the production of elemental or monoatomic fluorine F. This generation of elemental or monoatomic fluorine will preferably be in situ.

 The inventors have demonstrated that in the prior art the (re) fluorination reaction is essentially carried out with hydrolysed or oxidized forms of the fluorinated compound such as fluoride, and little with sub stoichiometric forms of the fluorinated compound, such as sub stoichiometric forms of MF fluoride.

 In the (re) fluorination processes used hitherto, in which only fluorinated gases are introduced and decomposed, the sole purpose of this is to simply generate fluorine.

This fluorine must chemically interact with fluorinated compounds such as stoichiometric fluorides, for example, according to the following reaction
MFx-y + tF <-> MFx (1)
To promote this reaction, this involves reducing the oxidized or hydrolysed forms of the fluorinated compounds such as fluorides, to lead to the MFXe compound capable of reacting with fluorine, such a reduction, fundamental in the process according to the invention, and made possible. by the presence of reducing chemical species, preferably strongly reducing, is never achieved in the prior art.

 According to the invention, and thanks to the generation of reducing chemical species such as active hydrogen, and fluorine, preferably of monoatomic fluorine F, the production of stoichiometric species is increased, contrary to the prior art. NK type. and the equilibrium of the (re) fluorination reaction (1) described above is displaced to the right by promoting the reduction reactions (2) and (3) which, in addition to the fluorination reaction (1), game in the process.

MFX (OH) t + tH ++ MFxt + t (H2O) (2)
MFx-tOt / 2 + tH MF,.: T + t / 2 (H 2 O) (3)
MFXt + tF ++ MF, (1)
In the process according to the invention, a reduction is ensured, in particular hydroxides or oxides of fluorinated compounds such as fluorides, as well as effective fluorination.

 The process according to the invention makes it possible to ensure a fluorine compensation of the layers accompanied inter alia not only by a reduction in the oxygen contamination of the deposited fluorides, but also by a drastic reduction in the optical absorption, in particular particularly in the visible and the ultraviolet.

 The layers prepared by the method according to the invention therefore have a combination of properties never reached by the layers of the prior art.

 Fluorinated compounds whose layers, in particular thin layers, are prepared by the process according to the invention are not limited and may be both organic compounds and inorganic compounds.

 Among the inorganic compounds, mention may be made of the fluorides and in particular the fluorides of metals already mentioned above, for example the fluorides of alkali and alkaline-earth metals, the fluorides of lanthanides, and the fluorides of other metals such as YF3, AlF3, etc., and mixed fluorides thereof.

By way of example, mention may be made of MgF2,
ThF4, YF3, LaF3, NdF3, CeF3, CaF2, LiF, ErF3, BaF2, KF or YLiF4, Y3KF10, LiErF4, KCeF4, K3CeF5, Ba3AlFg, Ba3Al2F12, Ba2A13F13, Ba2Y2Fe and YBaFs, etc.

 Among the organic compounds, mention may be made, for example, of fluorinated polymers, preferably poly (tetrafluoroethylene).

 By reducing chemical species, and preferably strongly reducing, generally means according to the invention energetic species for example of ionic or monoatomic type which are reactive, preferably highly reactive, and which also have a sufficient lifetime to participate in reactions described previously.

 Said reducing species, preferably strongly reducing, may be any suitable reductive species.

 This species is preferably derived from hydrogen or a hydrogenated gas, for example an alkane or gaseous alkene, such as methane, ethane, propane, butane, etc., but this species is preferably preferably again from hydrogen, that is to say that it is usually called active hydrogen.

 The reducing species and fluorine such as monoatomic fluorine are generally produced by decomposition of a single gas or a mixture of gases.

In the case where a mixture of gases is used, the fluorination and / or deposition, on the one hand, a gas rich in fluorine or fluorine, which is optionally diluted by neutral or rare gas such as
Ar, Ne, Kr or Xe, and on the other hand a gas susceptible during its decomposition to generate said reducing species, preferably strongly reducing, the latter gas is preferably hydrogen, said reducing species being then active hydrogen.

 Preferably, said fluorine-rich gas is chosen from perfluorinated alkanes and alkenes gas of 1 to 4 carbon atoms, SF6, NF3 and NH4F.

 In the case where a single gas is used, it is preferably chosen from partially fluorinated alkanes and alkenes of 1 to 4 carbon atoms such as CHF3, CH2F2, CH3F, and HF hydrogen fluoride. Preferably, this gas is diluted with a neutral or rare gas such as Ar, Ne, Kr and Xe.

 In order to promote the production of highly reactive monoatomic species and, for example, the decomposition of the introduced gas (s), it is preferable to use, in whole or in part, a plasma.

 This plasma may be for example that generated by the ionic source of atomization or assistance bombardment (lAD), or that generated by an ion source independent of the deposit.

 Advantageously, said strongly reducing species and / or said fluorine such as said monoatomic fluorine are generated near the surface to be fluorinated and / or targets or sources.

 In other words, the localization of the generation of species and / or fluorine, such as said monoatomic fluorine, is preferably near or just at the surface of the targets of fluoride materials or sources of evaporation, in the case respectively of the methods of spraying or evaporation, and / or also near deposits, especially for ion beam assisted evaporation (IAD).

 Indeed, the vacuum deposition method of the invention is generally an energetic deposition process chosen from ion beam sputtering (IBS), ionically assisted ion beam sputtering (DIBS, D meaning Dual c that is, IBS assisted by a second ion beam), sputtering, and ion beam assisted evaporation (IAD).

 The choice of the latter technique is advantageous because it produces a flow of material ten to one hundred times more energetic than conventional evaporation and it makes it possible to induce compression stresses in the fluoride layers, which in addition allows the deposition of layers thin layers of thickness greater than 1 or 2 μm and thick structures up to 20 Wm.

 Such a deposit is not possible in the prior art because the internal stresses of tension favor the crazing of the layers.

 It is noted that these deposition processes operate at a moderate temperature, generally below 100 ° C, it will thus be possible to cold deposit antireflet and dense optical treatments on any type of substrate for example plastic (organic glasses) not supporting high temperatures.

 The invention also relates to the layer (s), in particular the thin layer (s) prepared by the process according to the invention.

 These particularly thin layers, as already mentioned above, have both properties of optical transparency over a broad spectral range, excellent compactness and a specific chemical composition. In other words, the process according to the invention makes it possible in particular in the thin fluoride layers to promote an improvement in their physical and mechanical properties (density, hardness, internal compressive stresses, etc.) while retaining a low optical absorption, especially in the spectral domain not only visible but also UV.

 More specifically, the layers according to the invention and in particular the fluoride layers are characterized by an optical transparency such that the extinction coefficient k measured at 351 nm is less than or equal to 10-4.

 This transparency generally remains valid in the visible spectral range, ie the extinction coefficient k measured at 514 nm is less than or equal to 10 -4.

 The chemical composition of the fluoride thin films according to the invention is furthermore characterized by the fact that they are almost stoichiometric, generally to within 10% or less, that is to say that the composition ratio x = [F ] / [M] is close to the composition ratio for the MFx solid material at 10% or less.

 The composition ratio is accompanied by a contamination (pollution) very limited oxygen generally less than 3 at% and preferably less than 1 at%.

 Moreover, and insofar as these layers are derived from energy processes using ion beams, they are also characterized by the fact that they contain a contamination of rare gas (s), that is to say that they contain, in the form of impurities, from one to a few tenths of a% to one to a few percent, for example from 0.1 to 10% of rare gas atoms such as Ne, Ar, Kr, Xe.

 The fluoride layers according to the invention are finally defined by an excellent compactness characterized by a high density generally greater than or equal to 85%, preferably greater than or equal to 90%, of the density of the bulk material.

 It follows therefore that for non-hygroscopic fluoride materials such as for example YF3, MgF2 and CaF2 the layers take up very little water.

In other words, the infrared optical losses measured at a wavelength λ of 2.9 ssm remain less than 1 for a layer of 500 nm.

 The layers according to the invention generally have a thickness of 0.01 to 5 clam.

However, it will preferably be thin layers.

It should be noted that, according to the invention, the term "thin layers" refers to layers whose thickness is preferably from 0.01 to 1 μm.
As already indicated above, when the deposition technique used in the process of the invention is an IBS technique, it is possible to deposit relatively thick layers, that is to say of a thickness greater than 1. at 2 μm and preferably from 0.01 to 5 μm, and therefore also thick structures or stacks with a thickness (see below) of 0.1 to 20 μm.

 Such a deposition of thick layers of fluoride was not possible with the methods of the prior art in which the deposited fluoride layers have internal stresses of tension which favor the faiençage of the layers.

 The layers according to the invention can be for example so-called high index or low index layers.

 By way of example of fluoride materials that are suitable for producing high-index layers, mention may be made of ErF3, YF3, CeF3, LaF3, NdF3 and SrF2 fluorides.

By way of example of fluoride materials that are suitable for producing low-index layers, mention may be made of LiF, NaF, KF, BaF2 and NgF2 fluorides.
Na3A1F6, CaF2 and AlF3.

 The invention also relates to a multilayer stack comprising several layers, in particular thin layers as defined above. The number, the nature, the thickness, the properties and the arrangement of the layers of the stack can be arbitrary.

 Generally, the multilayer stack will comprise from 2 to 100 layers and the total thickness of such a stack is for example from 0.1 to 20 pin.

 Such stacks are described in detail in the application FR 96 07759 which is expressly referred to herein.

 The invention finally relates to the optical components comprising, optionally on a substrate, a layer or a multilayer stack as described above.

 The multilayer stacks according to the invention can be used for producing mirrors in the ultraviolet range, in particular centered at typical wavelengths, for example 193, 248 or 350 nm.

 These mirrors can be used in laser cavities or beam transport in the case, for example, of excimer or free electron lasers. The invention makes it possible in particular to make mirrors any thick fluoride (of a thickness of 1 to 6 pm) and little absorbent usable in the field of ultraviolet.

 Stacks can also be used to perform more complex functions of trichroic mirrors or spectral filters capable of withstanding high laser fluences, this is particularly the case for harmonic separator mirrors centered at 351 nm for application in very high energy lasers where precisely resistance to high laser fluences is required.

 The stacks are also suitable for producing antireflection coatings in the IR, the visible or the urethane, transparent low absorbency, stable optically and mechanically for any application requiring such a coating.

 The invention will also relate to the fact that the deposition is carried out cold as already mentioned above, antireflection and dense coatings consisting of a layer or a stack according to the invention on heat-sensitive substrates, not supporting high temperatures such as plastics, such coatings will find their application particularly in the field of eyewear for so-called organic glasses.

 The stacks can also be used as protection for already existing optical components, for example oxide mirrors to improve their laser flux content, excimer laser cavity portholes, against attack by corrosive atmospheres.

Other features and advantages of the invention will appear better on reading the description which follows given by way of nonlimiting illustration with reference to the accompanying drawings in which:
Brief description of the drawings
Figure 1 is a graph illustrating the influence of the fluorination process on the transparency of YF3 layers.

 The extinction coefficient k was respectively raised to 514 (nm (shaded area) and 351 nm (halftone area) for samples A, B, C, representing deposits made by different methods, namely according to the art. previous (samples A and B) and according to the present invention (sample C).

 FIG. 2 is a diagram illustrating the reflection R as a function of the wavelength λ (in nm) of an all-fluoride mirror comprising a stack of 30 pairs of YF3 / LiF layers prepared by the method of the invention.

 The invention will now be described with reference to the following examples given for illustrative and non-limiting.

 EXAMPLE 1 Deposition of Monolayers of Yttrium Fluoride

 In this example, layers of yttrium fluoride (YF3) are deposited by ion beam sputtering of a YF3 target.

 EXAMPLE 1A

In this example, a yttrium fluoride layer is deposited by a method according to the prior art under non-reactive conditions and therefore using no reactive gas. The precise conditions of the process are as follows
- Target in sintered YF3
- Spray gases: Xe
- Spray energy: 1 keV.

 The deposit thus obtained is called sample A.

 EXAMPLE 1B

 In this example, a yttrium fluoride layer is deposited by a method according to the prior art under reactive conditions using only a fluorination gas (CF4).

 The precise conditions of the process are the same as above (Example 1A) except that an additional reactive gas, CF4, is used.

 The deposit thus obtained is called sample B.

 EXAMPLE 1C

 In this example, a yttrium fluoride layer is deposited by the process according to the invention, in which a mixture of gases consisting of hydrogen and CF.sub.4 is used, allowing both a reduction in hydrogen and fluorination.

 The precise conditions of the process are the same as those of sample A, except that in addition the reactive gas mixture H2 and CF4 are used.

 The deposit thus obtained is called sample C.

 The characteristics of the layers deposited in Examples 1A, 1B and 1C (samples A, B, and C) are reported in Table I.

TABLE I

<tb><SEP> coefficient <SEP> Density * <SEP> and <SEP> F / Y <SEP> Oxygen <SEP> Gas <SEP> of
<tb><SEP> extinction <SEP> (% <SEP> of <SEP> the <SEP> (%) <SEP> sprays
<tb><SEP> k <SEP> to <SEP> 351 <SEP> nm <SEP> density <SEP> of <SEP> -tion
<tb><SEP> YF3 <SEP> massive) <SEP> Xe <SEP> (8) <SEP>
<tb> Ech <SEP> 3.10-2 <SEP> 4.5 <SEP> 2.2-2.3 <SEP> 7 <SEP> 0.3
<tb> A <SEP> (88,88) <SEP>
<tb> Ech <SEP> 1.5.10-2 <SEP> 4.8 <SEP> 2.3 <SEP> 2 <SEP> 0.3
<tb><SEP> B <SEP> (94,78) <SEP>
<tb> Ech <SEP> 1.5.10-5 <SEP> 4.7 <SEP> 2.75-3 <SEP> S <SEP><SEP> i <SEP> 0.3
<tb><SEP> C <SEP> 1 <SEP> (92.7%) <SEP>
<tb> AA comparison title and for the calculation of the percentage of the density of massive YF3 the latter has a density of 5.07.

It emerges from Table I that the layers (sample C) deposited using the process of the invention are distinguished from the layers deposited by the processes of the prior art (samples A and B) in that they present to the a nearly stoichiometric composition (to within ten percent), a very low oxygen contamination rate, less than 3%, and even less than or equal to 1%, and an extinction coefficient at 351 nm much lower than 5.10- 4 since it is equal to 1.5.10-5, instead of 3.10-2 for the sample
A, and 1.5.10-2 for sample B.

The layers according to the invention (sample
C) have, like the other pulverized layers of the prior art (sample B) a high density, in all cases equal to or greater than 85% of the density of massive yttrium fluoride, and a rare gas contamination (in the Xenon table) which is characteristic of the spraying process used.

 FIG. 1 also shows the extinction coefficients of the various deposits produced under the conditions of Examples 1A, 1B and 1C.

 This graph once again demonstrates the effectiveness of the process according to the invention.

 The extinction coefficients of the different deposits (samples A, B, C) were measured at wavelengths of 351 nm and 514 nm by spectrophotometry or by the photothermal deflection method.

 It is found that a conventional process of the prior art, but using only a fluorination gas (eg IB, sample B) leads only to a slight decrease in fluoride absorption, since At a wave of 351 nm, k evolves from 3.10-2 to 1.5 × 10 -2 while in the visible spectral range, at a wavelength of 514 nm, k passes from 1.2 × 10 -2 Ω.

 On the other hand, the process according to the invention (eg IC, sample C) leads to a drastic reduction in the extinction coefficient of yttrium fluoride, which, at a wavelength of 351 nm, falls by three orders of magnitude. magnitude to reach a value of 1.5.10-5. The same trend is observed at the wavelength of 514 nm in Figure 1.

 EXAMPLE 2 Deposition of Monolayers of Lithium Fluoride

 In this example, layers of lithium fluoride (LiF) are deposited by ion beam sputtering of a LiF target.

 EXAMPLE 2A

 In this example, a layer of lithium fluoride (LiF) is deposited by a method according to the prior art under non-reactive sputtering conditions which therefore do not use any reactive gas, and in particular no fluorination gas. .

 The precise conditions of the process are identical to those of Example 1A.

 The layers thus prepared according to the prior art exhibit, at a wavelength of 351 nm, an extinction coefficient k, measured by the photothermal deflection technique of 1.10-2 which results from a strong deficiency of fluorine.

EXAMPLE 2B
In this example, a layer of lithium fluoride (LiF) is deposited by the method according to the invention in which a gas mixture consisting of H 2 and CF 4 is used, allowing both a reduction by hydrogen, and fluorination.

 The precise conditions of the process are identical to those of Example 1C.

 The extinction coefficient k is measured in the same manner as in Example 2A and at the same wavelength of 351 nm. It can be seen that, thanks to the process according to the invention, the extinction coefficient k could be reduced by almost three orders of magnitude since it is only 4.5 × 10 -5.

 Like Example 1, concerning the YF3 layers, Example 2 demonstrates that the process according to the invention is very effective in improving the transparency of the LiF layers.

 These examples prove that the process according to the invention is of general application and can therefore be extended to any type of fluoride material.

 EXAMPLE 3 Production of a fluoride-free mirror formed of a stack of 30 pairs of layers.

 In this example, a multilayer YF3 / LiF stack is produced, such a stack is the subject of French Patent Application No. 96 07759, to which reference may be made.

 Conventional stacks use a succession of quarter-wave layers alternating the high index <H: YF3) and the low index (B: LiF). The refractive index of these two materials deposited by the process according to the invention is respectively 1.53 and 1.40 at the wavelength of 355 nm.

Under these conditions, the calculation shows that a stack of formula (HB) H2B exhibits at 355 nm a maximum reflectivity of 97.52. In practice, such a reflection can only be achieved if the absorption in the layers of the stack does not exceed a certain level to allow the incident light to penetrate to the bottom of the mirror and thus benefit from the reflection to all interfaces. The calculation shows that this assumes that the absorption coefficient of each layer does not exceed k =
Such a mirror of 30 pairs of layers was made using the method of the invention: that is to say that the deposition conditions are those of Examples 1C and 2B above.

 The optical response in reflection of such a mirror is shown in FIG.

 A maximum reflection of 97.44 i is observed, very close to the theoretical value.

 Such a result can be obtained only because the reactive method according to the invention used makes it possible to guarantee, in the stack, the deposition of sufficiently transparent monolayers. It should be noted that the centering of the mirror is shifted with respect to 355 nm because the mirror has been calculated for an incidence of 30 while the spectrophotometric measurement is made at an incidence of 6 ".

 Finally, let us add that the treatment presented in this example, despite its thickness of 4 pin, shows a perfect mechanical stability.

Claims (26)

 A process for preparing at least one fluorinated compound layer by vacuum deposition, in which, simultaneously with the vacuum deposition operation, at least one species is generated and / or introduced into the gaseous phase. chemical reducing agent and elemental fluorine, molecular, or bonded, whereby it is carried out during its deposition fluorination of the fluorinated compound layer.  2. Method according to claim 1 wherein said fluorinated compounds are selected from metal fluorides and fluoropolymers.  3. Method according to any one of claims 1 and 2 wherein said reducing chemical species and / or said fluorine are generated and / or introduced in-situ.  4. Method according to any one of claims 1 to 3 wherein said fluorine is elemental fluorine or monoatomic F.  The method of claim 1 wherein said reducing chemical species is a monoatomic or ionic type species.  6. Process according to any one of claims 1 to 5 wherein said reducing species is derived from hydrogen or a hydrogenated gas.  The process of any one of claims 1 to 6 wherein said reducing species and said fluorine are produced by decomposition of a single gas or a mixture of gases.  8. The method of claim 7 wherein said gas mixture comprises a fluorine-rich gas, or fluorine, optionally diluted with a neutral or rare gas, and a gas susceptible in its decomposition to generate said reducing chemical species.  9. The method of claim 8 wherein said fluorine-rich gas is selected from perfluorinated alkanes and alkenes gas of 1 to 4 carbon atoms, SF6, NF3 and NH4F.  10. The method of claim 7 wherein said single gas is selected from partially fluorinated alkanes and alkenes of 1 to 4 carbon atoms, and hydrogen fluoride HF, optionally diluted with a neutral gas or rare.  11. The method of claim 8 wherein said gas susceptible in its decomposition to generate said reducing chemical species is hydrogen and wherein said species is active hydrogen.  12. The method of claim 7 wherein said decomposition is carried out totally or partially under the influence of a plasma.  Process according to any one of claims 1 to 12 wherein said reducing species and / or said fluorine are generated near the surface to be fluorinated and / or targets or sources.  14. A method according to any one of claims 1 to 13 wherein the vacuum deposition operation is performed by an energetic deposition technique selected from ion beam sputtering (IBS), sputtering, sputtering ionically assisted ion beam (DIBS) and ion beam assisted evaporation (IAD).  15. The method of claim 2 wherein said fluorides are selected from alkali and alkaline earth metal fluorides, lanthanide fluorides, and fluorides of other metals such as YF3 and AlF3, and mixed fluorides thereof.  16. The method of claim 2 characterized in that said fluoropolymer is poly (tetrafluoroethylene).  17. Layer prepared by the process according to any one of claims 1 to 16.  18. Transparent MFX metal fluoride layer having a quasi-stoichiometric composition, an oxygen contamination rate of less than 3% by weight, and an extinction coefficient k measured at 351 nm less than or equal to 5. 10-.  19. The layer of claim 18 further having a density greater than or equal to 85% of the density of the bulk material.  20. Layer according to any one of claims 18 and 19 further having an extinction coefficient measured at 514 nm less than or equal to 10-4.  21. Layer according to any one of claims 18 to 20 further containing a rare gas contamination (s).  Layer according to any one of claims 17 to 21 having a thickness of 0.01 to 5 Am.  23. A layer according to claim 20 which is a thin layer with a thickness of 0.01 to 1 pin.  24. Multilayer stack comprising a plurality of layers according to any of claims 17 to 23.  Optical component comprising, optionally on a substrate, a layer according to one of claims 17 to 23 and / or a multilayer stack according to claim 24.  26. Optical component according to claim 25 acting as mirror, trichroic mirror, spectral filter, antireflection coating, protective coating.