process for producing a thin layer of amorphous boron by thermal CVD deposition

Abstract

The invention relates to a method for producing a thin layer of pure amorphous boron by thermal CVD deposition. For this, a carrier gas is used that is inert to the B2H6 dissociation reaction and the deposition temperature is less than or equal to 550°C. Figure for abstract: Fig. 2A

Description

Title of the invention: method for producing a thin layer of amorphous boron by thermal CVD deposition Technical field

[0001] The field of the invention is that of methods for producing a thin layer by thermal gas phase chemical deposition. STATE OF THE PRIOR ART

[0002] In certain micro or optoelectronic applications, it is necessary to produce a thin layer of a material that is both a good electrical insulator and a good thermal insulator. Such a thin layer must also be able to be deposited conformally on a silicon or silicon oxide support, among others. In addition, such a material must be able to be deposited at a temperature less than or equal to 550°C, or even 400°C, so as to be compatible with numerous technologies (for example, so as not to affect the thermal budget of an underlying CMOS circuit).

[0003] This is the case in particular in the field of thermal detectors, or microbolometers, which comprise an absorbent membrane suspended above a reading substrate by anchoring pillars and holding arms. The latter ensure the thermal insulation of the absorbent membrane and comprise conductive polarization tracks intended to electrically polarize a thermometer transducer located in the absorbent membrane. The conductive tracks can then be located between two thin layers made of an electrically and thermally insulating material, which can be amorphous silicon deposited by chemical vapor phase.

[0004] However, it appears that a thin layer of unintentionally doped amorphous silicon, deposited by plasma-enhanced chemical vapor deposition (PECVD), may see its electrical and thermal insulation properties degraded during a subsequent heat treatment carried out for example at 400°C, in particular when the temperature of said subsequent heat treatment is higher than the temperature at which the PECVD deposition was carried out.

[0005] In order to improve the thermal stability of amorphous silicon, it may be sought to deposit it by thermal CVD (not plasma-assisted). However, to avoid having to deposit it at too high a temperature, B2H6 may be used as a catalyst for the Si2H6 dissociation reaction. However, it appears that boron is incorporated as a dopant in the thin layer of amorphous silicon deposited, which leads to a degradation of the electrical insulation properties.

[0006] There is therefore a need to have a method for producing by CVD deposition a thin layer of a material which has good electrical insulation properties (electrical resistivity pEL greater than 50 Q.cm, or even 2000 Q.cm) and thermal insulation properties (thermal conductivity oTH less than 1.8 Wm *.K '), at a temperature less than or equal to 550°C and preferably 400°C, the insulation properties of which are preserved following a subsequent heat treatment at 400°C for example.

[0007] We also know the scientific article by Sarubbi et al. entitled Chemical Vapor Deposition of a-Boron Loyers on Silicon for Controlled Nanometer-Deep p+n Junction Formation, Journal of Electronic Materials, Vol.39, No.2, 2010 which describes the production by CVD deposition of a thin layer of amorphous boron deposited on a silicon substrate. However, the deposited material does not have the desired electrical insulation properties. In addition, the amorphous boron could not be deposited on a silicon oxide. Finally, the deposition speed is very low, and the thin layer is not deposited in a conformal manner. Statement of the invention

[0008] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose a method for producing by CVD deposition a thin layer of a material which has good electrical and thermal insulation properties, and whose insulation properties remain significant following a subsequent heat treatment.

[0009] For this, the subject of the invention is a method for producing by CVD deposition a thin layer of pure amorphous boron on a support substrate, comprising a step of injecting, into a deposition chamber of a CVD reactor, a precursor gas of B2 H6 diluted in a carrier gas, at a predefined deposition temperature.

[0010] According to the invention, the carrier gas is inert to a B2H6 dissociation reaction, and the deposition temperature is less than or equal to 550°C.

[0011] Some preferred but non-limiting aspects of this method are as follows.

[0012] The deposition temperature may be less than or equal to 400°C.

[0013] The carrier gas may be chosen from argon (Ar) and / or nitrogen (N2).

[0014] The gas mixture present in the deposition enclosure, formed of the precursor gas and the carrier gas, may have a total pressure less than or equal to 10 Torr.

[0015] The total pressure can be between 1 Torr and 10 Torr.

[0016] The precursor gas may have a partial pressure less than or equal to 100 mTorr.

[0017] The partial pressure of the precursor gas may be between 1 mTorr and 100 mTorr.

[0018] The support substrate may have an upper face, on which rests the thin layer of amorphous boron, made of a silicon-based material.

[0019] The upper face of the support substrate may be made of silicon, a silicon oxide, a silicon nitride, alumina A12O3, titanium Ti, titanium nitride TiN, or a titanium oxynitride TiOxNy.

[0020] The invention also relates to a structure comprising a support substrate and a thin layer of amorphous boron resting on and in contact with the support substrate, in which the thin layer has: an electrical resistivity pEL at least equal to 50 Q.cm, and preferably at least equal to 2000 Q.cm; a thermal conductivity oTH at most equal to 1.8 W / m / K. Brief description of the drawings

[0021] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which:

[0022] [Fig.l] is a schematic and partial view of an example of equipment for CVD deposition of a thin layer of amorphous boron according to one embodiment;

[0023] [Fig.2A] and [Fig.2B] are images of a thin layer of amorphous boron deposited on the support substrate whose upper face is flat. These are images obtained by scanning transmission electron microscopy (STEM), respectively in bright field and dark field;

[0024] [Fig.3A] and [Fig.3B] are images of a thin layer of amorphous boron deposited on the structured support substrate. [Fig.3A] is a wide-angle annular dark-field STEM image, and [Fig.3B] is a STEM image with chemical analysis by energy dispersive X-ray spectrometry;

[0025] [Fig.4A], [Fig.4B] and [Fig.4C] are schematic and partial cross-sectional views of a stack of a thin layer of amorphous boron deposited from a support substrate, according to different embodiment variants.

[0026] DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0027] In the figures and in the remainder of the description, the same references represent identical or similar elements. In addition, the different elements are not shown to scale so as to favor the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and can be combined with each other. Unless otherwise indicated, the terms “substantially”, “approximately”, “of the order of” mean to within 10%, and preferably to within 5%. elsewhere, the terms "between ... and ..." and equivalents mean that the limits are included, unless otherwise stated.

[0028] The invention relates to the production by chemical vapor deposition (CVD), and more precisely by thermal CVD, of a thin layer of amorphous boron which has an electrical resistivity at least equal to 50 Q.cm, and preferably at least equal to 2000 Q.cm, and a thermal conductivity at most equal to 1.8 Wm *.K '.

[0029] Thermal CVD deposition means CVD deposition using thermal energy as the energy source to carry out the chemical reactions (dissociation of B2H6). Thus, the substrate is brought to a temperature which depends on the desired deposition temperature. On the other hand, the gas mixture can be injected at room temperature. CVD deposition is therefore not plasma-assisted.

[0030] By thin layer of boron, it is meant that the thin layer comprises substantially only boron atoms. A residual atomic concentration of hydrogen is however possible but remains low (all the lower the higher the deposition temperature), for example of the order of 5%at for a temperature of 388°C. In any case, it is not a question of boron nitride or carbide, nor of a binary compound of type BxSi i_x, where x is the atomic proportion of boron.

[0031] By amorphous boron is meant that the thin layer does not contain a crystalline phase (more precisely, when the diffraction pattern of the transmission electron microscope (TEM) does not reveal a crystallite).

[0032] The inventors have demonstrated that, under certain operating conditions, i.e. when the carrier gas is inert to the B2H6 dissociation reaction, and the deposition temperature, to which the substrate holder on which the support substrate rests is brought, is at most equal to 550°C (for example between 250°C and 550°C), and preferably at most equal to 400°C, a thin layer of amorphous boron is deposited on the support substrate, which then has good electrical insulation and thermal insulation properties. These insulation properties remain high following a subsequent heat treatment, for example at 400°C for at least 1 h.

[0033] [Fig.l] illustrates an example of a CVD deposition equipment 1 that can be used to implement the method according to the invention.

[0034] The equipment 1 comprises a CVD reactor 2, a deposition chamber of which is connected to a source 3 of precursor gas, here diluted in a carrier gas. A source 4 of a diluent gas is also connected to the deposition chamber. The fluid lines may contain valves 5 and flow meters 6. A pressure gauge 7 is connected to the deposition chamber to measure the total pressure of the gas mixture present there. A discharge fluid line is connected to an outlet orifice of the deposition chamber, and here comprises a regulating valve 9 and a primary vacuum pump 10. The regulating valve 9 makes it possible to precisely control the total pressure in the deposition chamber.

[0035] The deposition chamber comprises a substrate holder 8 which is brought to the desired deposition temperature. The substrate holder 8 can receive large substrates such as substrates with a diameter of 200 mm. The support substrate on which the thin layer of amorphous boron will be deposited can thus be placed there. Furthermore, the gas mixture (precursor gas and carrier gas) can be injected into the deposition chamber so as to ensure a homogeneous distribution of the gases in the chamber, in particular in the vicinity of the support substrate.

[0036] The precursor gas is B2H6. It can be stored in the source 3, for example diluted in the carrier gas with a predefined dilution, for example here 1%. The mass flow meter 6 makes it possible to precisely control the feed rate of this gas mixture. For example, the feed rate can be between 0 and 1300 sccm (standard cubic centimeter per minute). In a known manner, the sccm unit corresponds to a unit of gas flow in cm3 / min, at a density defined by standard conditions of temperature (0°C) and pressure (1 atm, or 760 Torr).

[0037] The carrier gas is an inert gas in the B2H6 dissociation reaction, called neutral gas. It is chosen from argon Ar and / or nitrogen N2. In this example, the carrier gas is present in the first source 3 and ensures a first dilution of the precursor gas, here before introducing it into the reactor enclosure 2. The carrier gas is also present in the second source 4 and ensures an additional dilution of the precursor gas, here in the deposition enclosure, so as to simply modify the partial pressure of the precursor gas B2H6. The carrier gas of the two sources may be identical or different. In other words, in the deposition enclosure, the precursor gas B2H 6 is diluted in a carrier gas which may be formed of a single chemical species (for example Ar or N2) or of several chemical species (for example Ar and N2). In any case, the carrier gas is inert in the dissociation of B2H6.It is also possible to supply the enclosure with only the carrier gas, for example during a preliminary thermalization step of the support substrate before the deposition step.

[0038] The equipment 1 is adapted to maintain the total pressure constant in the deposition enclosure, both during the preliminary thermalization step and during the deposition step. This contributes to improving the thickness homogeneity of the thin layer deposited.

[0039] The support substrate is placed on the substrate holder 8. It has an upper face on which the thin layer of amorphous boron is deposited. This upper face may be flat or not. It may be formed from the same material or from different materials. For example, it may be made of silicon (amorphous or crystalline), in a silicon oxide (stoichiometric SiO2 as obtained for example by thermal oxidation of silicon, or non-stoichiometric SiOx as obtained for example by PECVD deposition of a tetraethyl orthosilicate precursor called TEOS), or even in a silicon nitride SiNx, in alumina A12O3, in titanium Ti, in titanium nitride TiN, in a titanium oxynitride TiOxNy, among others.

[0040] The method for producing the thin layer of amorphous boron may thus comprise a preliminary step of thermalizing the substrate, followed by the deposition step. Thus, during the thermalization step, the carrier gas alone is introduced into the deposition chamber. The total pressure in the chamber (thermalization pressure) may be equal to the total deposition pressure. The substrate holder, on which the support substrate rests, is brought to a temperature which depends on the deposition temperature, which is at most equal to 550°C, for example between 250°C and 550°C, and preferably at most equal to 400°C.

[0041] Then, the gas mixture formed from the precursor B2H6 diluted in the carrier gas (preferably argon Ar and / or nitrogen N2) is introduced into the deposition chamber with a predefined deposition mass flow rate. The total deposition pressure and the deposition temperature have predefined constant values.

[0042] The substrate holder, on which the support substrate rests, is brought to a deposition temperature at most equal to 550°C, and preferably between 250°C and 550°C.

[0043] It appears that, under these conditions, a thin layer of pure amorphous boron is deposited on the support substrate. This then has good electrical and thermal insulation properties. In addition, these electrical and thermal insulation properties remain present following a subsequent heat treatment, for example annealing at 400°C for 3 hours, in particular when this heat treatment is carried out at a temperature higher than the deposition temperature. Note that the deposited thin layer is dense and homogeneous, i.e. without the columnar microstructures which are generally present in thin layers obtained by PVD, and extends conformally over the support substrate and its possible topological variations.

[0044] The total pressure is preferably much lower than atmospheric pressure (low pressure CVD deposition), for example between 1 Torr and 10 Torr, for example between 1.4 Torr and 9 Torr. For example, the total pressure may be equal to 7 Torr. It is therefore a low pressure CVD deposition, in the sense that the total pressure of the gas mixture in the enclosure is of the order of a few torrs, preferably at most 10 Torr, and not several tens to hundreds of torrs.

[0045] Preferably, the precursor gas B2H6 is diluted in the carrier gas to 1% or less (for example to 0.1%), so that it has a partial pressure of between 10 mTorr and 100 mTorr, for example of between 14 mTorr and 90 mTorr. A supply Additional addition of a pure carrier gas (without B2H6) is possible to reduce the partial pressure of B2H6, which can then be between 1 mTorr and 10 mTorr, for example 1.4 mTorr and 9 mTorr.

[0046] The process makes it possible to produce thin layers of amorphous boron with a high deposition rate, which increases from 0.5 nm / min to 122 nm / min when the deposition temperature varies from 250°C to 550°C. It can thus be of the order of 50 nm / min, which, curiously, is almost 100 times higher than the expected deposition rate. This high deposition rate therefore makes it possible to reduce the operating time of the reactor, to reduce the consumption of the gases necessary for deposition, and consequently to reduce the cost of the operation.

[0047] The thin layer of amorphous boron can have a thickness of between 10 nm and 300 nm, or even up to 1 pm, with sufficient thickness uniformity.

[0048] The thin layer of amorphous boron has a high electrical resistivity, that is to say the electrical resistivity pEL is at least equal to 50 Q.cm, and preferably at least equal to 2000 Q.cm.

[0049] Furthermore, it appears that such a thin layer can be formed on a support substrate without there being a selective character with respect to the material of the upper face. As indicated previously, an upper face made of silicon, silicon oxide or nitride, among others, can be used for the deposition of the thin layer of amorphous boron. This effect is surprising and goes against the teaching of the article by Sarubbi et al. 2009 where deposition on a silicon oxide was not possible. This non-selective character in the deposition increases tenfold the possibilities of using the process, in the sense that it makes it possible to manufacture components requiring several other materials, without constraints on the choice of these other materials.

[0050] Furthermore, it is a conformal deposit, so that the thin layer can cover an upper face of the support substrate which would have a three-dimensional topology (reliefs, steps, mesas, etc.), while maintaining a sufficient thickness so that the thin layer remains continuous (see further on with reference to [Fig. 3A] and 3B).

[0051] Finally, the deposited material is resistant to hydrofluoric acid (HF) in vapor or liquid phase which can be subsequently used, in particular in the context of the removal of sacrificial silicon oxide layers.

[0052] After a predefined deposition time and obtaining the thin layer of amorphous boron, the enclosure is purged, and the support substrate can be removed from the deposition enclosure and returned to room temperature.

[0053] Figures 2A and 2B illustrate two images of a thin layer 21 of amorphous boron deposited on the support substrate 20 whose upper face is flat. These are images obtained by scanning transmission electron microscopy (STEM for Scanning Transmission Electron Microscopy, in English), respectively in bright field and dark field.

[0054] The thin layer 21 of amorphous boron has a thickness of 50 nm, and is deposited on a thin layer of SiO2 of 500 nm obtained by thermal oxidation of a crystalline silicon substrate. This thin layer 21 was formed with a total pressure of 7 Torr, a partial pressure of B2H6 of 70 mTorr, at a deposition temperature of 388°C, and using argon alone as the carrier gas.

[0055] These images show a thin layer 21 of homogeneous amorphous boron, without columnar microstructure. They also show a clear and abrupt interface between the SiO2 of the support substrate 20 and the thin layer 21 of amorphous boron, without the presence of aggregates at the interface which would result from a non-uniform growth start at the surface of the upper SiO2 face.

[0056] Furthermore, these STEM images do not reveal the presence of any crystallites in the thin layer 21 of amorphous boron, which indicates that the deposited thin layer is indeed totally amorphous boron.

[0057] The thin layer 21 of amorphous boron with a thickness of 50 nm here has an electrical resistivity pEL measured at room temperature (300°K) equal to 1.7xl04 Q.cm, therefore clearly beyond 50 Q.cm, or even 2000 Q.cm. This makes it possible to manufacture a wide range of electrical devices which can use this material for its electrical insulation properties.

[0058] Furthermore, it appears that the electrical resistivity remains high after a subsequent heat treatment. Thus, the thin layer of amorphous boron with a thickness of 50 nm obtained at a deposition temperature of approximately 380°C, having an electrical resistivity of 1.7xl04 Q.cm, retains a high resistivity, equal to LlxlO4 Q.cm, or equal to 7.5xl03 Q.cm, after a heat treatment under N2 carried out respectively at 380°C for 3 hours, or at 400°C for 3 hours. The stability of the electrical properties of the thin layer of amorphous boron thus exhibits greater stability than that of a thin layer of amorphous silicon which would have a similar initial resistivity.

[0059] In addition, the thin layer of amorphous boron has a thermal conductivity oTH < 1.8Wm *.K '. Thus, for a thin layer of amorphous boron with a thickness of between 30 nm and 90 nm produced according to the method at a deposition temperature chosen, for example, at approximately 380°C, the thermal conductivity oTH measured at room temperature (300°K) is typically equal to 1.5 Wm *.K ', therefore significantly lower than the maximum value desired for manufacturing a wide range of devices whose operation requires a thermally insulating element. This thermal insulation property oTH < 1.8Wm *.K1 is furthermore retained after a heat treatment under vacuum at a temperature of 400°C for 3 hours.

[0060] As examples, tests were carried out by changing the composition of the carrier gas. Thus, a thin layer of amorphous boron is produced by injecting B2H6 with a mass flow rate of 1100 sccm diluted to 1% in argon as the initial carrier gas. A second gas (diluting gas which helps to form the carrier gas), here also argon, is injected with a mass flow rate of 1000 sccm. The deposition temperature is 388°C and the total pressure is 7 Torr. The partial pressure of B2H6 is therefore approximately 1100x0.01 / 2100x7 = 36.7 mTorr. The thin layer of amorphous boron then has an electrical resistivity pEL equal to approximately 10.6 kQ.cm, and the deposition rate is equal to 44 nm / min. In the case where the second diluting gas is nitrogen also injected at 1000 sccm, the carrier gas is therefore a mixture of argon and nitrogen. Here we obtain a thin layer of amorphous boron with an electrical resistivity pEL equal to 10.2 kΩ.cm approximately, deposited at a deposition rate of 45 nm / min. Thus, it appears that the nature of the carrier gas does not modify the properties of the deposited thin layer.

[0061] Other tests were also carried out where the partial pressure of B2H6 was varied. They were carried out at a deposition temperature of 388°C, and at a total pressure of 7 Torr. The precursor gas was B2H6 diluted to 1% in argon as the carrier gas, and a diluent gas, here also argon, was injected at 1000 sccm. The results are summarized in the table below. It is noted that the electrical resistivity pEL is particularly high, and increases as the partial pressure of B2H6 increases. Mass flow rate B2H6 diluted to 1% in Ar Partial pressure B2H 6 Deposition rate Resistivity 20 sccm 1.37 mTorr 5.25 nm / min 1.07 kQ.cm 100 sccm 6.36 mTorr 11.2 nm / min 1.92 kQ.cm 250 sccm 14 mTorr 18.4 nm / min 2.85 kQ.cm 500 sccm 23.3 mTorr 29.0 nm / min 3.29 kQ.cm 1000 sccm 36.7 mTorr 44.0 nm / min 10.6 kQ.cm

[0062] Finally, other tests were carried out where the partial pressure of B2H6 and the deposition temperature were varied. The total pressure remained equal to 7 Torr. The results are summarized in the table below. It is noted that the electrical resistivity pEL remains high, and is particularly so (more than 100 kQ.cm) when the temperature is lower (300°C). Mass flow rate B2H6 diluted to 1% in Ar Diluent gas (Ar) Partial pressure B2H6 Deposition temperature Deposition rate Resistivity pEL 1100 sccm 0 70 mTorr 550°C 122 nm / min 51.3 Q.cm 1100 sccm 0 70 mTorr 300°C 6.3 nm / min 712 kQ.cm 20 sccm 1000 sccm 1.37 mTorr 550°C 6.2 nm / min 644 Q.cm 20 sccm 1000 sccm 1.37 mTorr 300°C 1.5 nm / min 134 kQ.cm

[0063] It therefore appears that the method according to the invention has advantages that are very different from the teaching presented in the article by Sarubbi et al. 2009, in particular with regard to the pEL electrical resistivity. Thus, a thin layer of amorphous boron, deposited at a deposition temperature of 550°C and with a partial pressure of B2H6 of 1.37 mTorr, has a pEL electrical resistivity of 644 Q.cm, i.e. more than 20,000 times higher than the electrical resistivity of 0.03 Q.cm which can be deduced from the tests of Sarubbi et al. 2009. In addition, the deposition speed is then 6.2 nm / min, which is curiously very high. And the thin layer of amorphous boron has a homogeneous density and uniform thickness, which is not obtained by Sarubbi et al. 2009. Finally, the process allows the thin layer of amorphous boron to be deposited on any type of material, including silicon oxide, whereas this was not achieved by Sarubbi et al. 2009.

[0064] Figures 3A and 3B illustrate two images of a stack of thin layers deposited on the structured support substrate 20 (non-planar upper face). [Fig. 3A] is a high angle annular dark field (HAADF) STEM image, and [Fig. 3B] is a STEM image with chemical analysis by energy dispersive X-ray spectrometry (EDX).

[0065] The support substrate 20 comprises a thin layer of SiO2 structured to form a first step 25, and a first intermediate layer 22 of approximately 20 nm of titanium nitride TiN, deposited above the thin layer 20 of SiO2, then structured by etching (with partial etching of the upper face of the SiO2 to a depth of the order of 15 nm) to form a second step 26 substantially steeper than the first step 25 and with a total height equal to 35 nm (20 +15 nm).

[0066] A first thin layer 21 of amorphous boron approximately 45 nm thick is then deposited. It can be seen that the thickness is continuous and uniform at the level of the passage of the second step 26 (conformal deposition).

[0067] The first thin layer 21 of boron is then structured by localized etching to form a third step 27 of a height equal to the thickness of the first thin layer 21 of amorphous boron (approximately 45 nm).

[0068] Then, a second intermediate layer 23 of the same nature as the first intermediate layer 22, but of lower thickness (approximately 8 nm) is then deposited. This second intermediate layer 23, used here to better reveal the interfaces between the different thin layers of the stack, is deposited by physical vapor deposition (PVD), and shows as expected a poorly conformal deposit with a thickness deposited on the side of the third step 27 substantially less than the thickness deposited on the upper face of the first thin layer 21 of amorphous boron.

[0069] A second thin layer 24 of amorphous boron is then deposited with a thickness of approximately 45 nm, and shows a uniform thickness (conformal deposition) on the side of the third step 27, unlike the intermediate layer 23 produced by PVD deposition.

[0070] As a result, the method makes it possible to obtain a first thin layer 21 of amorphous boron with a uniform thickness over the second steep step 26 of 35 nm, and a second thin layer 24 of amorphous boron with a uniform thickness over all three steps: a first non-steep step 25, a second steep step 26 of height 35 nm, and a third steep step 27 of approximately 53 nm.

[0071] Furthermore, this EDX image also confirms the substantially pure nature of the deposited amorphous boron (atomic concentration of boron at least equal to 95% for a CVD deposition at 380°C, and residual atomic concentration of hydrogen of 5% or less).

[0072] Figures 4A to 4C are schematic and partial cross-sectional views of a stack of a thin layer 21 of amorphous boron deposited from a support substrate 20, according to different embodiment variants.

[0073] It may be advantageous to produce a stack comprising the thin layer 21 of amorphous boron (aB) with a thin layer comprising an amorphous binary compound of boron and silicon a-BxSii_x, where x and 1-x respectively represent the atomic proportion of boron and silicon in the binary compound. The two layers may be deposited in the same reactor during the same process chaining two deposition steps to respectively form the two layers in question.

[0074] With reference to [Fig.4A], the stack comprises, from bottom to top, the support substrate 20, the thin layer 21 of amorphous boron, and a thin layer 28 of a-BxSi ix, deposited on and in contact with the thin layer 21 of amorphous boron. In this case, the thin layer 28 of a-BxSii_x completely covers the thin layer 21 of amorphous boron. This arrangement can be used to protect the thin layer 21 of amorphous boron, for example against a chemical agent which would attack the amorphous boron. The thin layer 28 in a-BsSi|X can also be provided to improve adhesion with third layers that will be deposited above the layers resulting from the process. In general, a thin layer of a-BxSii_x with a thickness of the order of 3 nm to 10 nm can be sufficient to achieve these functions, but any greater thicknesses can also be suitable.

[0075] With reference to [Fig.4B], the stack comprises, from bottom to top, the support substrate 20, a thin layer 29 of a-BxSii_x, then the thin layer 21 of amorphous boron. It can also be considered that the thin layer 29 of a-BxSii_x is an upper sub-layer of the support substrate 20. The thin layer 29 of a-BxSii_x can be used to improve the adhesion of the layers resulting from the process with third layers already present on the substrate to be covered. In this case too, a thin layer of a-BxSii_x can be sufficient.

[0076] With reference to [Fig.4C], the stack is here a combination of the cases of [Fig.4A] and 4B. Thus, the stack comprises, from bottom to top, the support substrate 20, a first thin layer 29 in a-BxSii_x, the thin layer 21 of amorphous boron, and a second thin layer 28 in a-BxSii x. The thin layer 21 of amorphous boron is therefore located between and in contact with the two thin layers 29, 28 in a-BxSii_x. Note that the atomic proportion x of boron may be identical or different in the two thin layers.

[0077] Such a three-thin-layer arrangement may be useful for compensating for the differential mechanical stress between the amorphous boron thin layer and the a-BxSii_x thin layer, and thus avoiding mechanical deformation of this stack of thin layers, in particular if this stack of thin layers is formed in a suspended form (such a suspended form may be obtained by interposing a sacrificial layer between the substrate and the stack of the 3 thin layers) which could lead to a bimetallic-type deformation if this compensation of mechanical stresses is not provided.

[0078] Note that the atomic proportion of boron and silicon in the compound a-BxSii_ x is obtained as a function of the flow rates of the two precursor gases B2H6 and Si2H6 introduced into the reactor enclosure. The latter then comprises an additional fluidic line equipped with a mass flow meter to supply the enclosure with Si2H6 at the desired flow rate.

[0079] Particular embodiments have just been described. Different variants and modifications will appear to those skilled in the art.

Claims

Claims

1. Method for producing by chemical vapor deposition CVD a thin layer (21) of pure amorphous boron on a support substrate (20), comprising a step of injecting, into a deposition chamber of a CVD reactor (2), a precursor gas of B2H6 diluted in a carrier gas, at a predefined deposition temperature, characterized in that: • the carrier gas is inert to a B2H6 dissociation reaction; • the deposition temperature is less than or equal to 550°C.

2. The method of claim 1, wherein the deposition temperature is less than or equal to 400°C.

3. A method according to claim 1 or 2, wherein the carrier gas is selected from argon (Ar) and / or nitrogen (N2).

4. Method according to any one of claims 1 to 3, in which the gas mixture present in the deposition enclosure, formed of the precursor gas and the carrier gas, has a total pressure less than or equal to 10 Torr.

5. The method of claim 4, wherein the total pressure is between 1 Torr and 10 Torr.

6. A method according to any one of claims 1 to 5, wherein the precursor gas has a partial pressure less than or equal to 100 mTorr.

7. The method of claim 6, wherein the partial pressure of the precursor gas is between 1 mTorr and 100 mTorr.

8. Method according to any one of claims 1 to 7, in which the support substrate (20) has an upper face, on which rests the thin layer (21) of amorphous boron, made of a silicon-based material.

9. A method according to any one of claims 1 to 8, wherein the upper face of the support substrate is made of silicon, a silicon oxide, a silicon nitride, alumina Al2O3, titanium Ti, titanium nitride TiN, or a titanium oxynitride TiOxNy.

10. Structure comprising a support substrate (20) and a thin layer (21) of amorphous boron resting on and in contact with the support substrate (20), in which the thin layer (21) has: an electrical resistivity Pel at least equal to 50 Q.cm, and preferably at least equal to 2000 Q.cm; and a thermal conductivity oTH at most equal to 1.8 W / m / K.

Images