EP0963457A1

EP0963457A1 - Method for preparing thin films of fluorinated compounds

Info

Publication number: EP0963457A1
Application number: EP98964557A
Authority: EP
Inventors: Etienne Quesnel; Jean-Yves Robic; Bernard Rolland
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1997-12-31
Filing date: 1998-12-30
Publication date: 1999-12-15
Also published as: FR2773175A1; JP2001515542A; WO1999035299A1

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 FILMS OF FLUORINATED COMPOUNDS

DESCRIPTION

Technical area.

The invention relates to a process for the preparation of layers, in particular thin layers, of fluorinated compounds which 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 an optical function of the mirror, spectral filter or anti-reflective coating type, in the spectral range going from ultraviolet to infrared, or play the role of protective coating of optical components for example to protect them against intense laser fluxes or against corrosive atmospheres.

State of the prior art.

In the field of thin-film processing of optical components, the achievement of a given optical function, for example of a mirror, of a spectral filter or of an anti-reflective coating can be fulfilled by means of a layer or a stack of

30 layers of materials with different optical indices.

The total thickness of such a stack is generally from a few tenths of a micrometer to a few micrometers. Among the many materials commonly used, there may be mentioned: 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 infrared applications, such as chalcogenides (ZnS, ZnSe) or germanium.

- fluoride compounds which group together a wide range of compounds, for example of the general formula MF _X or Mi ... M _n F _x or M is an alkali or alkaline-earth metal or a lanthanide.

These compounds constitute a class of materials which are particularly advantageous for applications in a very wide spectral range including both the ultraviolet, the visible and the infrared. Indeed, most of these compounds have the distinction of having a very wide spectral range of transparency, typically ranging from 0.15 to 10 μm.

Among the most well-known compounds, mention may be made, for example, of YF ₃ and ThF _4, which are commonly used for applications in the infrared; MgF ₂ and CaF ₂ which are used as visible anti-reflective material, or also LaF ₃ ,

The thin fluoride layers 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 character, a low mechanical resistance and a significant sensitivity to the environment linked to a strong recovery of 'water.

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

This densification technique can however lead to cracking of the deposits, because the fluorides have relatively large 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 tension of several hundred MPa.

To avoid the drawbacks brought about by densification by heating, use has been made of other deposition methods known as energy deposition methods which generally operate cold and use ion beams, or applied directly to the growing layers in the case of evaporation assisted by ion beam (IAD or Ion

Assisted Déposition in English), either used to spray the source or target of the material to be deposited in the case of spraying with ion beams

(IBS or Ion Beam Sputtering in English). These deposition and densification methods constitute an effective means for limiting the porosity of the deposits, but on the other hand they generate a clear deficit in fluorine in the layers, which, optically, results in a large increase in absorption. in the spectral domain of the ultraviolet, even of the visible.

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

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

This phenomenon is frequently observed during deposition by spraying or by ionic assistance of fluoride layers, or even also during evaporation by electron beams as described in the document by N.N. Skvortsov,

Yu.K.Ustinov and S .V. Yalyshko, sov. J. Opt. echnol. ,

54 (9) (1987) 541.

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

In order for the energetic deposition processes of thin layers of fluoride to be able to be carried out efficiently and in a viable manner in the field of optics, it is therefore imperative that these processes are associated with methods of refluxing the layers, either during deposit, ie after deposit. For obvious reasons of simplification of the processes, it is preferable to simultaneously deposit and (re) fluorinate the layers.

So in thin film deposition processes in general, the compensation of stoichiometry deposits are made as far as possible by injection during the deposition of a reactive gas containing the deficient element, the process then taking the name of reactive process. In the particular case of the fluorides of the reactive gases generally used are CF ₄ , C ₂ F ₆ , SF ₆ , or else fluorine F ₂ , the implementation of which is however not without risk for the material.

Thus the document by 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, SVC (Society of Vacuum Coaters) 3βth annual Technical Conference proceedings (1993) 44-50 describe the deposition of thin layers of fluorides LaF ₃ , A1F ₃ and MgF ₂ by reactive methods of ion beam assisted evaporation (IAD) or ion beam atomization (IBS) for applications in the deep ultraviolet, in particular for mirrors.

Fluorinated gases such as F ₂ and CF ₄ are used as reactive gases to compensate for the substoichiometry in fluorine responsible for strong optical absorption at short wavelengths. However, if such gases allow a certain improvement compared to the same processes using oxygen as reactive gas, they however only allow a slight decrease in this absorption.

The same observation is made in the document by JG Cook, GH Yousefi, SRDas and DF Mitchell, Thin Solid Films, 217 (1992) 87-90 where in a sputtering process of CaF ₂ , the introduction of CF ₄ in the plasma, if it induces a clear compensation in fluorine of the layers generates, on the other hand, a strong absorption linked to the pollution of the layers by carbon. Furthermore, in a process of evaporation assisted by ion beams (IAD), the use of C ₂ F ₆ in the assistance gun leads to a low fluorine compensation. Finally, document JP-A-7 166 344 relates to the deposition of fluorides by spraying metal targets into 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 aim sought by this document is rather to allow a high deposition speed than to seek a particular transparency.

It emerges from the above that the refluxing of fluoride layers deposited by energy deposition techniques proves to be extremely difficult using fluorinated reactive gases and has so far been unable to be carried out successfully.

This is the reason why it is often preferred to resort to oxidation and to produce, instead of fluorides, oxyfluoride compounds of the MF _x _ _ε O _{ε type} which have good transparency in the ultraviolet but whose optical "gap" remains however superior to that of pure fluorides, which poses problems for applications in the deep ultraviolet.

Thus, the document by JDTargove, MJ Messerly, JPLehan, RH Potoff, HAMacleod, LCMc Intyre Jr and JALeavitt, SPIE vol. 678, (1986) 115, describes the assistance of the deposition of MgF ₂ by oxygen, while the document JP-A-8 134 637 relates to the use of a magnesium oxyfluoride target to ensure low optical absorption in the layers.

Similarly, the sputtering of a target consisting of a mixture of fluorides of the CaF ₂ type, Na3AlF ₆ , Na ₅ Al ₃ Fι ₄ or SrF ₂ by operating in a reactive atmosphere containing oxygen was the subject of document JP-A-8 171 002 which essentially aims at obtaining transparent layers, essentially in the visible.

The enumeration of the techniques cited above clearly shows that the preparation and (re) fluorination of thin layers of fluorinated compounds such as fluorides is extremely difficult and has so far not 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 in below k = 10 ^"3 in the visible.

Added to the fluorine desorption phenomena is the fact that the fluoride materials used as a vapor source for the condensation of a thin film are generally very sensitive to hydrolysis and oxidation, in particular when these materials are present. in powder form. Consequently, since the spray sources are generally in the form of sintered powders, they contain not only the fluoride compound (MF _X ) but also the hydrolysed derivatives

(MF _x _ _y (OH) _y ) or oxidized (MF _x - _z O _{z / 2} ). The proportion of these by-products can reach a few percent and they are responsible for a large part of the fluorine layer deficit.

With regard to evaporation, the high sensitivity of fluorides to humidity, or even to oxidation, means that the layer or possibly the Evaporation load deficient in fluorine will tend to compensate for the fluorine vacancies with oxygen or hydroxide groups -OH, within the limit of the concentration of these species in the residual vacuum of the deposit enclosure.

The object of the invention is to provide a process for preparing thin layers of fluorinated material which does not have the drawbacks, limitations and disadvantages of the prior art and which solves the problems of the prior art cited above.

This object and others are achieved in accordance with 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, one generates or at least one reducing chemical species and elementary, molecular or bound fluorine are also introduced into the gas phase, whereby the fluorination of the layer of fluorinated compound is carried out during its deposition.

The process according to the invention therefore differs fundamentally from the processes 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 fluorinated such as fluoride partially oxidized due to contamination by oxygen, and on the other hand, then, in its fluorination. These two reactions are made possible by the generation or introduction into the vacuum enclosure and / or at another point situated outside of it, not only of fluorine but also of reducing chemical species such as hydrogen. active. This approach is completely 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 enclosure, 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 elementary, molecular or bound fluorine, the following species may be mentioned, for example: F, F ₂ , CF, CF ₂ , CF ₃ , CHF, CF _4, etc.

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

The inventors have demonstrated that in the prior art the (re) fluorination reaction is carried out essentially with hydrolyzed or oxidized forms of the fluorinated compound such as a fluoride, and little with stoechio-metric forms of the fluorinated compound, such as stoichiometric forms of fluoride MF _x - _t .

In the (re) fluorination processes used up to now, where 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 substoichiometric fluorides, for example, according to the following reaction: MF _x - _y + tF - → MF _X (1)

To promote this reaction, this supposes reducing the oxidized or hydrolyzed forms of fluorinated compounds such as fluorides, to result in the compound MF _x - _t capable of reacting with fluorine, such reduction, fundamental in the process according to the invention, and made possible by the presence of reducing chemical species, preferably strongly reducing, is never carried out in the prior art.

According to the invention and thanks to the generation of reducing chemical species such as active hydrogen, and preferably fluorine monoatomic fluorine F, the production of sub stoichiometric species is increased, unlike the prior art. type MF _x _ _t and the balance of the (re) fluorination reaction (1) described above is shifted to the right, favoring the reduction reactions (2) and (3) which, in addition to the reaction (1 ) of fluoridation, come into play in the process.

MF _x - _t (OH) t + tH <-> MF _x _ _t + t (H ₂ 0) (2)

MFχ-tO _{t /} 2 + tH -> MF _x - _t + t / 2 (H ₂ 0) (3)

MF _x - _t + tF <→ MF _X (1)

In the process according to the invention, both a reduction, in particular of hydroxides or oxides of fluorinated compounds such as fluorides, is ensured, as well as effective fluorination.

The method according to the invention makes it possible to provide fluorine compensation for the layers accompanied, among other things, not only by a reduction in the oxygen contamination of the fluorides deposited, but also by a drastic reduction in optical absorption, in particular in the visible and the ultraviolet.

The layers prepared by the method according to the invention therefore exhibit a combination of properties never achieved by the layers of the prior art. The fluorinated compounds whose layers, in particular the thin layers, are prepared by the process according to the invention are not limited and can be both organic compounds and inorganic compounds. Among the inorganic compounds, mention may be made of the fluorides and in particular the metal fluorides already mentioned above, for example the alkali and alkaline earth metal fluorides, the lanthanide fluorides, and the fluorides of other metals such as YF, A1F ₃ , etc ..., and mixed fluorides thereof.

As an example, we can mention MgF ₂ , ThF ₄ , YF ₃ , LaF ₃ , NdF ₃ , CeF ₃ , CaF ₂ , LiF, ErF ₃ , BaF ₂ , KF or YLiF ₄ , Y ₃ KF ₀ , LiErF ₄ , KCeF ₄ , K ₃ CeF ₆ , Ba3AlF ₉ , Ba ₃ Al ₂ F ₁₂ , Ba ₂ Al ₃ Fι ₃ , Ba ₂ Y ₂ F ₈ and YBaF ₅ etc.

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

The term “reducing” and preferably highly reducing chemical species is generally understood to mean, according to the invention, energetic species, for example of the ionic or monoatomic type, which are reactive, preferably highly reactive, and which moreover have a sufficient lifetime to participate in the reactions described above. Said reducing species, preferably highly reducing, can be any suitable reducing species.

This species preferably comes from hydrogen or from a hydrogenated gas, for example from an alkane or alkene gas, such as methane, ethane, propane, butane, etc., but this species is of preferably still derived from hydrogen, that is to say that it is generally said 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 gas mixture is used, a gas rich in fluorine or fluorine, optionally diluted with a gas, is introduced simultaneously into the enclosure where the fluorination and / or the deposition is carried out. neutral or rare gas such as Ar, Ne, Kr or Xe, and on the other hand a gas capable during its decomposition of generating said reducing species, preferably strongly reducing, this latter gas is preferably hydrogen, said species reducing therefore being active hydrogen.

Preferably, said fluorine-rich gas is chosen from gaseous perfluorinated alkanes and alkenes of 1 to 4 carbon atoms, SF ₆ , NF ₃ and NH ₄ F.

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 CHF ₃ , CH ₂ F ₂ , CH ₃ F, and fluoride hydrogen HF. Preferably, this gas is diluted with a neutral or rare gas such as Ar, Ne, Kr and Xe.

To promote the production of highly reactive monoatomic species and for example the decomposition of the gas (s) introduced, recourse is preferably made, in whole or in part, to a plasma.

This plasma could be, for example, that generated by the ion source of spraying or bombardment assistance (IAD), or that generated by an ion source independent of the deposit.

Advantageously, said highly 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 location of the generation of the species and / or of 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 spraying or evaporation, and / or also near deposits, in particular for assisted ion beam evaporation (IAD). Indeed, the vacuum deposition process concerned by the invention is generally an energy deposition method chosen from ion beam spraying (IBS), ion beam assisted ion spraying (DIBS, D meaning "Dual That is, it is the IBS process assisted by a second ion beam), and the ion beam assisted evaporation (IAD).

The choice of the latter technique is advantageous because it produces a material flow ten to a hundred times more energetic than conventional evaporation and it allows compression stresses to be induced in the fluoride layers, which also allows the deposition of layers. thin, depositing layers with a thickness greater than 1 or even 2 μm and thick structures up to 20 μm. Such deposition 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 moderate temperature, generally less than or equal to 100 ° C., preferably, this temperature is room or ordinary temperature, that is to say, for example, a temperature close to 20 to 25 ° C. According to the invention, the target or source has no heating, unlike the prior art, because it is preferably maintained at said temperature, preferably, ambient or ordinary.

It will thus be possible "cold" to deposit anti-reflective and dense optical treatments on any type of substrate, for example plastic (organic glasses) which cannot withstand 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 notably thin layers, as already mentioned above, exhibit in particular properties of optical transparency over a wide spectral range, excellent compactness and a specific chemical composition. In other words, the process according to the invention makes it possible in particular in thin fluoride layers to promote an improvement in their physical and mechanical properties (density, hardness, internal compression stresses, etc.) while retaining low optical absorption, in particular in the spectral range not only visible but also UV.

More precisely, 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 5 10 ^{~ 4} .

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

The chemical composition of the thin fluoride layers according to the invention is further 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 solid material MF _X to within 10% or less.

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

Furthermore, and insofar as these layers come from energy processes using ion beams, they are also characterized by the fact that they contain a contamination in rare gas (s), that is to say that 'they contain in the state of impurities, from one to a few tenths of% 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 solid material.

It therefore follows that for non-hygroscopic fluoride materials such as for example YF ₃ , MgF ₂ and CaF ₂ the layers take up very little water. In other words the infrared optical losses measured at a wavelength λ of 2.9 μm remains less than 1

% for a 500 nm layer.

The layers according to the invention generally have a thickness of 0.01 to 5 μm. However, it will preferably be so-called thin layers.

It should be noted that, according to the invention, the term “thin layers” designates 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 to 2 μm and preferably 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 fluorides was not possible with the methods of the prior art in which the deposited fluoride layers have internal stresses of tension which favor the crazing 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 suitable for the production of high index layers, mention may be made of the fluorides ErF ₃ , YF ₃ , CeF ₃ , LaF ₃ , NdF ₃ and SrF ₂ . By way of example of fluoride materials suitable for producing the low index layers, mention may be made of the fluorides LiF, NaF, KF, BaF ₂ , MgF ₂ ,

Nas lFe, CaF ₂ and A1F ₃ .

The invention also relates to a multilayer stack comprising several layers in particularly thin layers as defined above. The number, nature, thickness, properties and 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 μm.

Such stacks are described in detail in application FR 96 07759 to which reference is expressly made here.

Finally, the invention relates to 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 for transporting the beam in the case, for example, of excimer or free electron lasers. The invention makes it possible in particular to produce thick “all fluoride” mirrors (with a thickness of 1 to 6 μm) and little absorbent usable in the ultraviolet field.

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 of 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 anti-reflective coatings in IR, visible or UV, transparent with little absorption, optically and mechanically stable 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, dense anti-reflective coatings formed by a layer or stack according to the invention on substrates sensitive to the heat, not supporting high temperatures such as plastics, such coatings will find their application in particular 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 content in the laser flux, excimer laser cavity portholes, against attack by corrosive atmospheres.

Other characteristics and advantages of the invention will appear better on reading the description which follows, given by way of illustration and not limitation, with reference to the accompanying drawings in which:

Brief description of the drawings

FIG. 1 is a graph illustrating the influence of the fluorination process on the transparency of layers of YF ₃ .

The extinction coefficient k was increased to 514 (nm (hatched area) and 351 nm (screened area) respectively for samples A, B, C, representing deposits produced by methods different, namely according to the prior art (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 YF ₃ / LiF layers prepared by the

1 invention.

The invention will now be described with reference to the following examples given by way of illustration and not limitation.

EXAMPLE 1 Deposition of monolayers of yttrium fluoride.

In this example, layers of yttrium fluoride (YF ₃ ) are deposited by ion beam sputtering of a target of YF ₃ .

EXAMPLE 1A.

In this example, a layer of yttrium fluoride is deposited by a process according to the prior art under non-reactive conditions and therefore using no reactive gas. The precise process conditions are as follows:

- Sintered YF ₃ target

- Spray gas: Xe

- Spray energy: 1 keV. The deposit thus obtained is called sample A.

EXAMPLE 1B. In this example, a layer of yttrium fluorides is deposited by a process according to the prior art under reactive conditions using only a fluorination gas (CF).

The precise process conditions are the same as before (Example 1A) except that an additional reactive gas, CF _{4, is used} .

The deposit thus obtained is called sample B.

EXAMPLE 1C.

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

The precise conditions of the process are the same as those of sample A, except that the mixture of reactive gas H ₂ and CF _{4 is also 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

* For comparison and for the calculation of the percentage of the density of massive YF _3, the latter has a density of 5.07.

It appears from this table I that the layers (sample C) deposited using the method of the invention, are distinguished from the layers deposited by the methods of the prior art (samples A and B) in that they exhibit times an almost stoichiometric composition (to the nearest 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 very much less than 5.10 ^{" 4} since it is equal to 1.5.10 ^-5 , instead of 3.10 ^"2 for sample A, and 1.5.10 ^{~ 2} for sample B.

The layers according to the invention (sample C) have, like the other sprayed layers of the prior art (sample B) a high density, in all cases equal to or greater than 85% of the density of solid yttrium fluoride, as well as 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 again makes it possible to demonstrate the effectiveness of the method 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 1B, sample B) only leads to a slight decrease in the absorption of fluoride, since a length wavelength 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 goes from l, 2.10 ^-2 to 10 ^{" 4} . On the other hand, the method according to the invention

(eg 1C, sample C) leads to a drastic reduction in the extinction coefficient of yttrium fluoride, which, at a wavelength of 351 nm, drops by three orders of 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 Deposit 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 spraying conditions which therefore use no 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 have, at a wavelength of 351 nm, an extinction coefficient k, measured by the photothermal deflection technique of 1.10 ⁻² which results from a high deficiency in fluorine.

EXAMPLE 2B

In this example, a layer of lithium fluoride (LiF) is deposited by the method according to the invention in which a mixture of gases consisting of H ₂ and CF 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 way as in Example 2A and at the same wavelength of 351 nm. It can be seen that, thanks to the method according to the invention, the extinction coefficient ka could be reduced by almost three orders of magnitude since it is no more than 4.5 × 10 ⁻⁵ .

Like Example 1, concerning the layers of YF ₃ , Example 2 demonstrates that the method according to the invention is very effective in improving the transparency of the layers of LiF. These examples prove that the method according to the invention is of general application and can therefore be extended to any type of fluoride material.

EXAMPLE 3: Production of an all-fluoride mirror formed by a stack of 30 pairs of layers.

In this example, a YF ₃ / LiF multilayer stack is produced, such a stack is the subject of French patent application No. 9β 07759, to which reference may be made.

Conventional stacks use a succession of quarter wave layers alternating the high index (H: YF ₃ ) and the low index (B: LiF). The refractive index of these two materials deposited by the method according to the invention is 1.53 and 1.40 respectively at the wavelength of 355 nm.

Under these conditions, the calculation shows that a stack of formula 30 (HB) H2B has a maximum reflectivity of 97.5% at 355 nm. 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 at all interfaces. The calculation shows that this assumes that the absorption coefficient of each of the layers does not exceed k = 10 ^{~ 4} .

Such a mirror of 30 pairs of layers was produced 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 presented in FIG. 2. There is a maximum reflection of 97.44%, very close to the theoretical value.

Such a result can only be obtained 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 offset from 355 nm because the mirror has been calculated for an incidence of 30 ° while the spectrophotometric measurement is made under an incidence of 6 °.

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

Claims

1. Process for the preparation of at least one layer of fluorinated compound by vacuum deposition, in which, simultaneously with the vacuum deposition operation, at least one species is further generated and / or introduced into the gas phase reducing chemical and elementary, molecular or bound fluorine, by means of which the fluorination of the layer of fluorinated compound is carried out during its deposition.

2. Method according to claim 1 wherein said fluorinated compounds are chosen from metal fluorides and fluorinated polymers.

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 elementary or monoatomic fluorine F.

5. The method of claim 1 wherein said reducing chemical species is a species of monoatomic or ionic type.

6. Method according to any one of claims 1 to 5 wherein said reducing species is derived from hydrogen or a hydrogenated gas.

7. Method according to 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 gas rich in fluorine, or fluorine, optionally diluted with a neutral or rare gas, and a gas susceptible during its decomposition to generate said reducing chemical species.

9. The method of claim 8 wherein said fluorine-rich gas is selected from gaseous perfluorinated alkanes and alkenes of 1 to 4 carbon atoms, SF ₆ , NF ₃ and NHF.

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 or rare gas.

11. The method of claim 8 wherein said gas capable during its decomposition of generating 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.

13. A method 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. Method according to any one of claims 1 to 13 wherein the vacuum deposition operation is carried out by an energy deposition technique chosen from atomization by ion beam (IBS), atomization by ion beam ionically assisted (DIBS) and ion beam assisted evaporation (IAD).

15. Method according to any one of claims 1 to 14, in which the target or source is maintained at ambient or ordinary temperature.

16. The method of claim 2 wherein said fluorides are chosen from alkali and alkaline earth metal fluorides, lanthanide fluorides, and fluorides of other metals such as YF ₃ and A1F ₃ , and mixed fluorides thereof.

17. The method of claim 2 characterized in that said fluoropolymer is poly (etrafluoroethylene).

18. Layer prepared by the method according to any one of claims 1 to 17.

19. Transparent layer of metal fluoride MF _X having an almost 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 ^"4 .

20. The layer of claim 19 further having a density greater than or equal to 85% of the density of the solid material.

21. Layer according to any one of claims 19 and 20 further having an extinction coefficient measured at 514 nm less than or equal to 10 ^"4 .

22. A layer according to any one of claims 19 to 21 further containing contamination in rare gas (ies).

23. Layer according to any one of claims 18 to 22 having a thickness of 0.01 to 5 μm.

24. The layer of claim 21 which is a thin layer with a thickness of 0.01 to 1 μm.

25. A multilayer stack comprising, several layers according to any one of claims 18 to 24.

26. Optical component comprising, possibly on a substrate, a layer according to one of claims 18 to 24 and / or a multilayer stack according to claim 25.

27. Optical component according to claim 26 playing the role of mirror, trichroic mirror, spectral filter, anti-reflective coating, protective coating.