EP3440012A1

EP3440012A1 - Process for fabricating a micromechanical structure made of silicon carbide including at least one cavity

Info

Publication number: EP3440012A1
Application number: EP17714827.7A
Authority: EP
Inventors: Daniel Alquier; Rami KHAZAKA; Jean François Michaud; Marc PORTAIL
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite de Tours
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite de Tours
Priority date: 2016-04-06
Filing date: 2017-04-06
Publication date: 2019-02-13
Also published as: US10472230B2; WO2017174709A1; US20190152772A1; JP2019513904A; FR3049946A1; FR3049946B1

Abstract

The invention relates to a process for fabricating a micromechanical structure made of silicon carbide including a cavity (5), from a stack comprising a first silicon-carbide layer (2) and a silicon layer (3) on the first silicon-carbide layer, said process including a step of shaping the silicon layer (3) so as to form a discrete silicon structure (3') on the first silicon-carbide layer (2). The process according to the invention furthermore comprises, after the step of shaping the silicon layer (3): a step of carbonising initiating the removal of the discrete silicon structure (3'); a step of depositing a second silicon-carbide layer; and an annealing step, the discrete silicon structure (3') being entirely removed at the end of the annealing step.

Description

PROCESS FOR PRODUCING A MICROMECHANICAL SILICON CARBIDE STRUCTURE COMPRISING AT LEAST ONE CAVITY

TECHNICAL FIELD OF THE INVENTION

The technical field of the invention is that of micro-fabrication. The present invention relates to a method of manufacturing a micromechanical structure of silicon carbide having at least one cavity.

BACKGROUND OF THE INVENTION

Micromechanical structures, and in particular electromechanical microsystems, are used in many technical fields and in particular in the manufacture of sensors. These sensors are sometimes used in hostile environments, that is, environments with high temperatures or pressures, or extreme chemical conditions. For such applications, it is known to use electromechanical microsystems made of a resistant material such as silicon carbide (SiC). This material has indeed many advantages, including its high mechanical strength and inertia vis-à-vis most chemical agents.

Such sensors are known from the literature and described by N. Marsi et al. (The Mechanical and Electrical Effects of MEMS Capacitive Pressure Sensor Based on 3C-SiC for Extreme Temperature, Journal of Engineering, Vol 2014, Article ID 7151 67); DJ Young et al. (High-Temperature Single-Crystal 3C-SiC Capacitive Pressure Sensor, IEEE Sensors Journal, Vol 4, No. 4, August 2004); GS Chung (Manufacturing and Characterization of Polycrystalline 3C-SiC Piezoresistive Micro-pressure Sensor, Journal of the Korean Physical Society, Vol 56, No. 6, June 2010, pp 1759-1762); or by BV Cunning et al. (Graphitized silicon carbide microbeams: wafer-level, self-aligned graphene on silicon wafers, Nanotechnology, Vol. 25 (32) p. 325301, 2014).

However, this chemical inertia has the consequence of making it very difficult to produce micromechanical silicon carbide structures, since the agents conventionally used in conventional microsystem fabrication processes are difficult to apply to silicon carbide. EP21 68910B1 proposes a method in which a sacrificial silicon germanium (SiGe) structure is deposited and structured on a surface of silicon carbide. This sacrificial structure is then covered by a layer of silicon carbide. An opening is then formed on the surface of the silicon carbide layer, this opening opening on the sacrificial structure of silicon germanium. Wet etching is then performed, the etching agent penetrating through the opening in the silicon carbide layer and attacking the sacrificial structure. Once the silicon germanium structure has been fully etched, the opening in the silicon carbide layer is closed to obtain a cavity.

This method is however not without inconvenience. Firstly, in order to release the membrane, it is necessary to expose the sacrificial structure to the chemical etching agent, which requires an opening in the silicon carbide layer which will then be closed. In addition, the etching step requires a liquid chemical agent which can lead to a "sticking effect" (sticking effect in English) of the silicon carbide structure induced by the surface tension of the etching agent or the rinse agent rendering the micromechanical structure inoperative.

There is therefore a need for a method of manufacturing a micromechanical silicon carbide structure having a cavity, for example an electromechanical microsystem, not requiring an opening in the silicon carbide layer to perform the step of etching and to prevent the phenomenon of adhesion of the cavity membrane during this etching step.

SUMMARY OF THE INVENTION The invention offers a solution to the problems mentioned above, by making it possible to eliminate the discrete silicon structure by means of a carburation step, a deposition step and an annealing step. It is thus possible to overcome the etching step. The disadvantages concerning the opening formed in the silicon carbide layer and the bonding phenomenon associated with it are thus avoided.

For this, a first aspect of the invention relates to a method of manufacturing a micromechanical structure of silicon carbide having a cavity, from a stack comprising a first layer of silicon carbide and a silicon layer on the first silicon carbide layer, said method comprising a step of shaping the silicon layer so as to form a discrete structure of silicon on the first layer of silicon carbide. In order to obtain the micromechanical structure, the method according to the invention also comprises:

a carburation step initiating the elimination of the discrete silicon structure;

a step of depositing a second layer of silicon carbide;

an annealing step;

the discrete structure of silicon being completely eliminated at the end of the annealing step.

The term carburizing step is understood to mean an annealing step in an atmosphere containing a precursor of carbon, for example propane, but devoid of silicon, leading to the growth of a layer of silicon carbide at the level of the surface of the discrete structure of silicon. . This step will create empty spaces in sacrificial silicon structures. These empty spaces will be called voids in the rest of the text. In the process in question, a single discrete structure of silicon is formed. However, it is also possible to form a plurality of discrete silicon structures to obtain a plurality of cavities. In the rest of the text, the expression "a discrete structure" should therefore be understood as "at least one discrete structure" and the expression "a cavity" should therefore be understood as "at least one cavity". In the process according to the invention, during the carburizing steps, deposition of a second layer of silicon carbide and annealing, growth of silicon carbide occurs and causes a migration of the silicon atoms of the discrete structure of silicon. silicon towards the growth zones, that is to say towards the surface of the discrete structure of silicon. This diffusion has two consequences: it feeds the growth of the silicon carbide layer into silicon atoms at the surface of the discrete structure and it causes the formation of a cavity in this same structure. A micromechanical structure made of silicon carbide having a cavity without the use of an etching agent is thus obtained. In addition, since no etching agent is used, it is no longer necessary to provide an opening in the second layer of silicon carbide and can therefore obtain a sealed cavity without problem of "bonding" ( sticking effect in English) and sealing.

The shaping step of the silicon layer makes it possible to determine the geometry of the cavity and to control the size of the latter in all three dimensions. Thus, unlike the state of the art which describes the phenomenon of formation of voids as an undesirable phenomenon whose geometric properties are dictated by the crystalline orientation of silicon, the method according to the invention makes it possible to control this phenomenon in order to to obtain a hermetically sealed cavity having dimensions perfectly controlled and independent of the crystalline orientation of the silicon layer. More particularly, once the discrete structure of silicon formed on the surface of the first layer of silicon carbide, the formation of the cavity is ensured by the last three steps of the process. First, the carburizing step initiates the formation of voids and a silicon carbide layer on the surface of the discrete silicon structure. At the end of this first step, the size of the voids is less than one hundred nanometers. In order to ensure continuity of the membrane, the deposition step of a second layer of silicon carbide is then performed. This step will fill part of the openings created during the carburation stage. This step is performed so that, at the end of the deposition, openings allowing diffusion of the silicon atoms on the surface of the layer of silicon carbide are always present in the silicon carbide layer. Finally, the annealing step makes it possible to consume the carbon precursor and the remaining silicon so as to form a micromechanical structure of silicon carbide having a sealed cavity.

In addition to the characteristics that have just been mentioned in the preceding paragraph, the method of manufacturing a micromechanical silicon carbide structure comprising a cavity according to one aspect of the invention may have one or more additional characteristics among the following, considered individually. or in any technically possible combination.

Advantageously, the shaping step of the silicon layer is directly followed by a second annealing step. This second annealing step makes it possible to improve the surface state of the discrete silicon structure before the carburizing step and thus to control the crystalline orientation of the silicon carbide membrane. In particular, it makes it possible to obtain a silicon carbide layer whose crystalline orientation is of (1 1 1) type. This crystalline orientation makes it possible to obtain a low roughness compared to a membrane whose crystalline orientation is of (1 10) type.

Advantageously, a thermal transition step is performed between the carburizing step and the step of depositing the second layer of silicon carbide. During this thermal transition step, the temperature changes from a first temperature equal to the temperature of the carburizing step to a second temperature equal to the temperature of the deposition step of the second layer of silicon carbide.

Thus, during this step, the voids formed during the carburizing step will grow to reach a size of the order of a few hundred nanometers or even of the order of a few microns.

Preferably, a waiting step is performed between the thermal transition step and the deposition step of the second silicon carbide layer. This waiting time allows to modulate the size of the voids according to the size of the desired cavity.

Advantageously, the temperature during the annealing step is between 1100 ° C. and 1400 ° C. This temperature window ensures a good compromise between the speed of growth of the voids and the silicon carbide layer and the quality of the silicon carbide obtained during this annealing step.

Advantageously, the duration of the annealing step is chosen as a function of the width, the length and / or the thickness of the discrete silicon structure. Thus, the silicon structure is completely consumed at the end of the annealing step to form the micromechanical structure of silicon carbide having a cavity.

Preferably, the carburizing step is carried out in an atmosphere comprising a hydrocarbon gas. Advantageously, propane is chosen as carbon precursor gas and hydrogen is chosen as the carrier gas.

Advantageously, the stack comprises a substrate and the method according to the invention comprises, before the step of shaping the silicon layer:

a step of depositing a first layer of silicon carbide on the substrate;

a step of depositing a layer of silicon on the first layer of silicon carbide. The step of depositing a first silicon carbide layer and the step of depositing a silicon layer make it possible to obtain the stack comprising a first layer of silicon carbide and a layer of silicon on the first layer. of silicon carbide. Advantageously, the substrate forming the first layer of the stack is chosen so as to allow crystal growth of 3C-SiC. This substrate may be a substrate of silicon (Si), sapphire (Al ₂ O ₃ ), aluminum nitride (AlN), silicon carbide (SiC) or gallium nitride (GaN). Advantageously, the first silicon carbide layer and / or the second silicon carbide layer have a 3C-SiC crystal structure. Preferably, the first silicon carbide layer has a crystalline orientation of (001) type and / or the second silicon carbide layer has a crystalline orientation of (1 1 1) type.

Advantageously, the shaping step is performed so as to form in the silicon layer a plurality of connecting elements, each connecting element of the plurality of connecting elements joining at least a first discrete structure of the plurality from discrete structures to a second discrete structure of the plurality of discrete structures.

Preferably, for each link member of the plurality of link members, the width of said link member is less than the width of the discrete structures among the plurality of discrete structures that said link member joins.

By width of a discrete structure is meant the edge of smaller dimension of a section of said discrete structure obtained in a plane parallel to the surface of the first layer of silicon carbide and passing through said discrete structure. In the same way, the width of a connecting element is understood to mean the edge of smaller dimension of a section of said connecting element obtained in a plane parallel to the surface of the first layer of silicon carbide and passing through said connecting element. Thus, a cavity is obtained in a silicon carbide structure having wide areas corresponding to discrete silicon structures removed and areas of reduced width corresponding to the removed link elements. Indeed, since the bonding elements are made in the same layer as the discrete structures, they are eliminated together with the discrete silicon structures.

A second aspect of the invention relates to a sensor of the electromechanical microsystem type comprising a micromechanical structure of silicon carbide having a cavity sealed by a silicon carbide membrane, obtained by a method according to a first aspect of the invention.

Advantageously, the sensor is a piezoresistive or capacitive type pressure sensor.

Advantageously, the sensor is a chemical sensor comprising at least one layer sensitive to a chemical compound to be detected, said sensitive layer being deposited on said membrane.

A third aspect of the invention relates to the use of a sensor according to a second aspect of the invention in or on an organic tissue.

A third aspect of the invention relates to the use of a sensor according to a second aspect of the invention in a radiative environment.

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics and advantages of the invention will emerge on reading the detailed description which follows, with reference to the appended figures, which illustrate:

- Figure 1, a flow chart of the method according to a first embodiment of the invention;

FIGS. 2A to 2F, a diagram of the different steps of the method according to a second embodiment;

FIG. 3 is a flow diagram of the method according to a third embodiment of the invention;

FIG. 4, a flowchart of the method according to a fourth embodiment of the invention;

- Figure 5, a flow chart of the method according to a fifth embodiment of the invention;

- Figure 6, a flow chart of the method according to a sixth embodiment of the invention; FIGS. 7A and 7B, an illustration of the structure obtained at the end of the shaping step of the silicon layer according to one embodiment of the invention;

- Figure 8, an embodiment of a sensor according to a second aspect of the invention.

DETAILED DESCRIPTION

An aspect of the invention illustrated in FIG. 1 and in FIGS. 2A to 2F concerns a method of manufacturing a micromechanical structure of silicon carbide having a cavity 5, starting from a stack comprising a first layer of silicon carbide 2 and a silicon layer 3 on the first silicon carbide layer 2, said method comprising a step 102 for shaping the silicon layer 3 (FIG. 2C) so as to form a discrete 3 'structure of silicon on the first layer of silicon carbide 2.

In order to obtain a micromechanical structure of silicon carbide having a cavity 5, the method furthermore comprises:

a carburation step 103 initiating the elimination of the discrete 3 'silicon structure;

a step 104 for depositing a second layer of silicon carbide 4;

a step 105 of annealing.

the discrete structure of silicon being completely eliminated at the end of the annealing step. As previously explained, carburetion is understood to mean an annealing step in an atmosphere containing a carbon precursor, for example propane, but devoid of silicon, leading to the growth of a layer of silicon carbide at the level of the surface of the discrete structure of silicon and the formation of voids in these same structures.

The growth of the silicon carbide during the carburizing step 103, the step 104 of deposition of a second layer of silicon carbide 4 and during the annealing step 105 causes the formation of a layer of carbide of silicon on the surface of the discrete structure (FIG. 2D) as well as a migration of the silicon atoms SI (illustrated by dashed arrows in FIG. 2E) of the discrete structure 3 'of silicon towards the growth zones, that is to say towards the surface of the discrete structure 3 'of silicon. This diffusion has two consequences: it feeds the growth of the silicon carbide layer 4 into silicon atoms at the surface of the discrete structure 3 'and it causes the formation of a cavity 5 (illustrated by continuous arrows at Figure 2E) in these same structures 3 '. A micromechanical structure of silicon carbide having a cavity 5 is thus obtained without resorting to an etching agent. In addition, since no etching agent is used, it is no longer necessary to provide an opening in the silicon carbide layer 4 and thus a hermetically sealed cavity can be obtained.

More particularly, step 103 of carburation makes it possible to initiate the formation of the voids. At the end of this first step, the size of the voids is generally less than one hundred nanometers. In order to ensure a continuity of the membrane, the step 104 of deposition of a second layer of silicon carbide 4 is then performed. This step will make it possible to fill part of the openings created during the carburizing step 103. The latter is made so that, at the end of the step 104 of deposition of the second layer of silicon carbide 4, openings for the diffusion of the silicon atoms on the surface of the silicon carbide layer 4 are always present during the first two stages, the 3C-SiC layer continues to form and thicken. Finally, the annealing step 104 makes it possible to consume the precursor of the remaining carbon and silicon so as to form a micromechanical structure of silicon carbide having a sealed cavity 5.

In other words, a micromechanical structure of silicon carbide having a cavity 5 is formed by the consumption of the silicon atoms of the discrete structure 3 ', this consumption resulting in the formation of the silicon carbide layer 4 forming a silicon carbide membrane . The cavity 5 is thus self-sealed and the pressure within this cavity 5 is identical to the pressure used during the annealing step 105. In one embodiment, the silicon carbide component of the second layer of silicon carbide 4 has a crystal structure of the 3C-SiC type.

In one embodiment illustrated in FIG. 3, the stack further comprises a substrate 1. The method then comprises, before step 102 of shaping the silicon layer 3:

a deposition step 100 (FIG. 2A) of a first layer of silicon carbide 2 on the substrate 1;

a deposition step 101 (FIG. 2B) of a silicon layer 3 on the first layer of silicon carbide 2.

Step 100 of deposition of a first layer of silicon carbide 2 and step 101 of deposition of a silicon layer 3 make it possible to obtain the stack comprising a first layer of silicon carbide 2 and a layer of silicon 3 on the first layer of silicon carbide 2.

The substrate 1 of the stack is chosen so as to allow the growth of silicon carbide. It may be selected from a substrate of silicon (Si), sapphire (Al ₂ O ₃ ), aluminum nitride (AlN), silicon carbide (SiC) or gallium nitride (GaN). Indeed, these substrates make it possible to grow, during step 100 of deposition of the first layer of silicon carbide 2, a monocrystalline silicon carbide layer and thus improves the mechanical and electrical characteristics of the micromechanical structure obtained. In one embodiment, the first silicon carbide layer 2 is of the 3C-SiC type. In one embodiment, the crystalline orientation of the first silicon carbide layer 2 is of the (001) type. However, certain growth conditions can lead to obtaining a first layer of polycrystalline silicon carbide 2 or even amorphous. In one embodiment, the deposition step 100 of the first silicon carbide layer 2 on the substrate 1 may be preceded by a step of removing the native oxide present on the surface of said substrate 1. When the substrate 1 is a silicon substrate, this step of removing the native oxide may take the form of a homoepitaxy of a thin silicon layer on the silicon substrate 1 or an annealing under hydrogen atmosphere. If the substrate 1 is a nitride, this step of removing the native oxide may take the form of annealing in a nitrogen atmosphere.

In one embodiment, the deposition step 100 of the first silicon carbide layer 2 may be carried out by an epitaxial process, for example by means of a chemical vapor deposition technique (CVD for "Chemical Vapor Deposition" in English). In one embodiment, the thickness of the first silicon carbide layer 2 is between 100 nm and 20 μm, preferably equal to 5 μm.

The shaping step 102 (FIG. 2C) of the silicon layer 3 may be carried out using a conventional lithography technique. For example, the etching of the silicon layer may be carried out using inductively coupled plasma (ICP) etching in an atmosphere containing sulfur hexafluoride (SF). ₆ ), ethylene (C ₂ H ₄ ) and argon (Ar). In another example, it will be possible to use a wet etching. The removal of the resin layer after etching can be performed by an oxygen plasma step. This oxygen plasma cleaning step may optionally be followed by a cleaning step using a piranha solution.

In an embodiment illustrated in FIG. 4, step 102 of shaping (FIG. 2C) of the silicon layer 3 is followed by a second annealing step 106. During this annealing step, the temperature may be between 800 ° C. and 1000 ° C. The pressure during this annealing step 106 may be chosen less than 1 bar. The duration of this annealing step 106 may be substantially equal to 10 minutes or even 20 minutes. It makes it possible to obtain a better surface state before the carburizing step 103 and thus to control the crystalline orientation of the second layer of silicon carbide 4. In one embodiment, the second layer of silicon carbide 4a a crystalline orientation of the type (1 1 1).

In one embodiment, the temperature during the carburizing step 103 is between 860 ° C and 1300 ° C, preferably between 860 ° C and 1150 ° C. In a first variant, the temperature is modified so as to go from a minimum temperature of 860 ° C. to a maximum temperature of 1150 ° C. In a second alternatively, the temperature is varied so as to go from a minimum temperature of 860 ° C to a maximum temperature of 1100 ° C. Preferably, the carburation step 103 is carried out under an atmosphere comprising a precursor of carbon, for example propane, and hydrogen.

In an embodiment illustrated in FIG. 5, when the temperature between the step

103 and the step 104 of deposition of the second layer of silicon carbide 4 is different, a thermal transition step 107 is performed between the carburizing step 103 and the step 104 of deposition of the second layer of carbide of silicon 4. During this thermal transition step 107, the temperature is changed from a first value equal to the temperature of the carburation step 103 to a second value equal to the temperature of the deposition step 104. second layer of silicon carbide 4. During this thermal transition step 107, the voids formed during the carburation step 103 will grow to a size of the order of a few hundred nanometers or even of the order of a micrometer . For example, for a thickness of the silicon discrete structure of 300 nm, a carburation step 103 carried out at 1150 ° C. and a step

104 deposition of the second layer of silicon carbide 4 carried out at 1320 ° C, at the end of the thermal transition step 107, the size of the voids is between 500nm and 1 μηι. This thermal transition step 107 can be carried out under an atmosphere comprising hydrogen.

In an embodiment illustrated in FIG. 6, a waiting step 108 is performed between the thermal transition step 107 and the deposition step 104 of the second silicon carbide layer 4. This step 108 of waiting allows the voids to be grown before step 104 for depositing the second layer of silicon carbide 4. The size of the voids at the end of step 108 of waiting depends on the duration of this step 108 waiting . For example, for a thickness of the discrete silicon structure 3 'of 300 nm, a carburation step 103 carried out at 1150.degree. C. and a step 104 for depositing the second layer of silicon carbide 4 at 1320.degree. size of the voids obtained at the end of a step 108 waiting 5 minutes is between 1 .5μηι and 2.5μηι. In a second example, for a thickness of the discrete silicon structure of 300 nm, a carburation step 103 carried out at 1150.degree. C. and a step 104 for depositing the second layer of carbide of silicon 4 performed at 1320 ° C, the size of the voids obtained at the end of a step 108 waiting 10 minutes is between 3.5μηι and 5μηι.

The size of the voids is also dependent on the thickness of the discrete structure 3 '. For example, for a thickness of the discrete silicon structure of 100 nm, a carburation step 103 carried out at 1150.degree. C. and a step 104 for depositing the second layer of silicon carbide 4 at 1320.degree. voids obtained at the end of a waiting step 108 of 5 minutes is at least equal to 5μηι. In a second example, for a thickness of the discrete silicon structure of 500 nm, a carburation step 103 carried out at 1150.degree. C. and a step 104 for depositing the second layer of silicon carbide 4 at 1320.degree. size of the voids obtained at the end of a step 108 waiting 5 minutes is equal to 900 nm. In a third example, for a thickness of the discrete silicon structure of 1 .2μηπ, a carburation step 103 carried out at 1150 ° C. and a step 104 for depositing the second silicon carbide layer 4 carried out at 1320 ° C. , the size of the voids obtained at the end of a waiting step 108 of 5 minutes is equal to 400 nm.

In one embodiment, the temperature during the step 104 of deposition of the second layer of silicon carbide 4 is between 1100 ° C and 1400 ° C. This deposition step 104 can be carried out by epitaxy using a precursor of silicon, for example silane (SiH ₄ ) and a precursor of carbon, for example propane (C ₃ H ₈ ). This deposition can be carried out by a technique of chemical vapor deposition (CVD for "Chemical Vapor Deposition" in English). The duration of this step can be between 30 seconds and 10 minutes, preferably between 1 minute and 3 minutes.

In one embodiment, the temperature during the annealing step 105 is equal to the temperature during the deposition step 104 of the second silicon carbide layer 4, that is to say between 1 100 C. and 1400 ° C.The duration of annealing step 105 is a function of the size of the discrete structure of silicon 3 '. For example, if the discrete structure 3 'has a thickness of 200nm, a width of 20μηι and a length of 20μηπ, then the annealing step 105 has a duration substantially equal (at plus or minus 20%) to 30 minutes. In an embodiment illustrated in FIG. 7A, the shaping step 102 is made so as to form in the silicon layer 3 a plurality of connecting elements, each connecting element 3bis of the plurality of elements connecting link joining at least a first discrete structure 3 'of the plurality of discrete structures to a second discrete structure 3' of the plurality of discrete structures.

Preferably, for each connecting element 3bis of the plurality of connecting elements, the width of said connecting element 3bis is smaller than the width of the discrete structures 3 'among the plurality of discrete structures that said connecting element 3bis joins. Each connecting element also has a length equal to the distance separating the discrete structures 3 'from among the plurality of discrete structures that said connecting element 3bis joins. In an embodiment illustrated in FIG. 7B, the discrete structures 3 'are distributed on the surface of the first layer of silicon carbide 2 in the form of a matrix of discrete structures 3', the said discrete structures forming a plurality of lines L and a plurality of columns C, each comprising a plurality of discrete structures 3 '. In this embodiment, each discrete structure 3 'of the same column C is joined by means of a connecting element 3bis to the discrete structure 3' which precedes it in column C so as to obtain a continuous structure formed by an alternation of discrete structures 3 'and connecting elements 3bis. In one embodiment, the discrete structures have a square shape with a width of 25 μηι and the connecting elements have a width of 5 μηι and a length of 10 μηι. In other words, in this embodiment, two discrete structures 3 'consecutive joined by a connecting element 3bis are spaced apart by a distance of 10 μηι. The discrete silicon structures 3 'and the connecting elements 3a being made in the same silicon layer 3, they therefore have an identical thickness equal to the thickness of said silicon layer 3. This thickness may for example be equal to 200 nm . The steps performed on the discrete silicon 3 'structures, in particular the carburation step 103, the silicon carbide deposition step 104 and the annealing step 105, are also implemented on the connecting elements 3bis. A cavity 5 is thus obtained in a silicon carbide structure comprising zones having a width of 25 μηι corresponding to the discrete silicon structures 3 'suppressed and zones having a width of 5μηι and a length of 10 μηι corresponding to the elements of FIG. link 3bis deleted by the method object of the invention. Indeed, the connecting elements 3a being made in the same silicon layer 3 as the discrete silicon structures 3 ', they are therefore eliminated concomitantly with the discrete silicon structures. A particularly advantageous application of these micromechanical structures relates to the production of sensors, in particular sensors used in a severe environment, involving extreme pressure and / or temperature conditions, as well as in demanding chemical environments. Such sensors, in particular MEMS sensors or electromechanical microsystems, advantageously benefit from the mechanical and electrical properties of SiC, in particular its properties of thermal conductivity, mechanical strength as well as its stability towards most chemical compounds, even at high temperatures. exceeding 300 ° C make the SiC particularly suitable for this type of application. MEMS sensors are, for example, capacitive type pressure sensors. In an embodiment illustrated in FIG. 8, the sensor according to a second aspect of the invention comprises a support 1 0, for example a support made of silicon, separated from the micromechanical structure by an electrical insulating layer 1, for example a layer 1 of aluminum nitride (AlN). Under the effect of a pressure P, the membrane 4 undergoes an elastic deformation modifying the distance between the membrane 4 and the support 10. This variation in distance results in a proportional variation of the capacitance Cp. This variation can be measured, for example by means of a first PC contact disposed on the support and a second DC contact disposed on the membrane 4, and the pressure P exerted on the SiC membrane can be deduced from this measured.

MEMS sensors can also be piezoresistive type pressure sensors. In one embodiment (not shown), a piezoresistive sensor according to a second aspect of the invention comprises at least two deposited contacts. on the SiC membrane 4 of the micromechanical structure. Under the effect of pressure, a variation of the resistivity of the SiC membrane proportional to the pressure applied is measured and allows to deduce the pressure exerted on the SiC membrane.

In another embodiment of a sensor according to a second aspect of the invention, the sensor is a chemical sensor. Such a chemical sensor may, for example, be obtained by deposition, on the silicon carbide membrane 4 of the cavity of the micromechanical structure obtained by a method according to a first aspect of the invention, of at least one sensitive layer. a chemical compound to be detected, in particular graphene or metal oxide, for example a layer of SiO ₂ , TiO ₂ , ZnO, SnO ₂ .

The use of the properties of a membrane such as that obtained using a method according to a first aspect of the invention for detection purposes is a practice well known to those skilled in the art. The configurations that have just been presented serve above all to illustrate, through a few examples, the advantages that result from the use of a micromechanical structure obtained by means of a method according to a first aspect of the invention and of setting up such a structure in a sensor according to a second aspect of the invention.

Claims

Method for manufacturing a micromechanical structure in silicon carbide comprising a cavity (5), from a stack comprising a first layer (2) of silicon carbide and a layer of silicon (3) on the first layer of carbide of silicon, said method comprising a step (102) of shaping the silicon layer (3) so as to form a discrete structure (3') of silicon on the first layer

(2) silicon carbide; said method being characterized in that it comprises after the step (102) of shaping the silicon layer (3):

- a carburizing step (103) initiating the elimination of the discrete silicon structure (3');

- a step (104) of depositing a second layer of silicon carbide;

- an annealing step (105);

the discrete silicon structure (3') being entirely eliminated at the end of the annealing step (105).

Method according to the preceding claim characterized in that the step (102) of shaping the layer

(3) of silicon is directly followed by a second annealing step (106).

Method according to one of the preceding claims, characterized in that it comprises a thermal transition step (107) between the carburizing step (103) and the deposition step (104) of the second layer of silicon carbide during which the temperature changes from a first temperature equal to the temperature of the carburizing step (103) towards a second temperature equal to the temperature of the deposition step (104) of the second layer of silicon carbide.

4. Method according to the preceding claim characterized in that it comprises a step (108) of waiting between the step (107) of transition thermal and the step (104) of depositing the second layer of silicon carbide.

Method according to one of the preceding claims, characterized in that the temperature during the step (104) of deposition of the second layer of silicon carbide and/or the annealing step (105) is between 1,100° C and 1400°C.

Method according to one of the preceding claims, characterized in that the duration of the annealing step (105) is chosen as a function of the width, length and/or thickness of the discrete structure (3') of silicon.

Method according to one of the preceding claims, characterized in that the carburizing step (103) is carried out under an atmosphere comprising a hydrocarbon gas.

Method according to one of the preceding claims characterized in that the stack further comprises a substrate (1); said method comprising, before the step (102) of shaping the silicon layer (3):

- a step (100) of depositing a first layer (2) of silicon carbide on the substrate (1);

- a step (101) of depositing a layer of silicon (3) on the first layer of silicon carbide (2);

the step (100) of depositing a first layer (2) of silicon carbide and the step (101) of depositing a layer of silicon (3) making it possible to obtain the stack comprising a first layer ( 2) of silicon carbide and a layer of silicon (3) on the first layer of silicon carbide.

9. Method according to the preceding claim characterized in that the substrate forming the first layer of the stack is chosen from a substrate of silicon (Si), sapphire (Al ₂ 0 ₃ ), aluminum nitride (AIN), silicon carbide (SiC) or gallium nitride (GaN).

Method according to one of the preceding claims characterized in that the shaping step (102) is carried out so as to form in the silicon layer (3) a plurality of connecting elements (3bis) and a plurality of structures discrete structures (3'), each connecting element (3bis) of the plurality of connecting elements (3bis) joining at least one first discrete structure (3') of the plurality of discrete structures (3') to a second discrete structure (3') of the plurality of discrete structures (3').

1 1 . Electromechanical microsystem type sensor comprising a micromechanical structure in silicon carbide comprising a cavity (5) sealed by a silicon carbide membrane obtained using a method according to one of claims 1 to 10.

12. Sensor according to claim 1 1, for which said sensor is a pressure sensor of the piezoresistive type or of the capacitive type.

13. Sensor according to claim 1 1, for which said sensor is a chemical sensor comprising at least one layer sensitive to a chemical compound to be detected, said sensitive layer being deposited on said membrane.

14. Use of a sensor, as defined in any one of claims 1 1 to 13 in or on an organic tissue.

15. Use of a sensor, as defined in any one of claims 1 1 to 13 in a radiative environment.