Patent Publication Number: US-2021181089-A1

Title: Device for photoacoustic characterisation of a gaseous substance and method for manufacturing such a device

Description:
TECHNICAL FIELD OF THE INVENTION 
     The technical field of the invention is that of devices for photoacoustic characterisation of a gaseous substance, for example for detecting the presence of one or more gaseous elements in this gaseous substance, or to measure the concentration of one or more of these elements. 
     The invention relates in particular to such a device, integrated on a semiconductor chip. The invention also relates to a method for manufacturing such a device. 
     TECHNOLOGICAL BACKGROUND OF THE INVENTION 
     To measure the concentration of a particular element present in low quantity in a gaseous mixture (or in a gas), it is known to fill a chamber with the gaseous mixture in question, then to inject a light radiation into this chamber, the light radiation being chosen so as to be absorbed by the element of which it is wished to measure the concentration, preferably selectively (that is to say only by this element). This light radiation thus has a spectrum which, in general, is situated in the mid-infrared, for example between 3 and 10 microns. On absorbing this radiation, the gaseous mixture heats up. The luminous power of the injected radiation is then modulated, to a modulation frequency situated in the sonic or ultrasonic domain. The heating up in question is thus also time modulated which, in the gaseous mixture, generates an acoustic wave having the modulation frequency in question. The amplitude of this acoustic wave, which provides information on the concentration of the element to detect, is measured using a microphone. To make such a detection device more sensitive, the modulation frequency may be chosen equal to or close to a resonance frequency of the acoustic cavity that constitutes the chamber. 
     These last few years have seen the development of compact devices for detecting or measuring using such a photoacoustic effect, these devices being made of semiconductors using manufacturing techniques derived from the microelectronics industry. 
     The document FR 3019653, for example, describes an integrated photoacoustic detection device, produced by etching two silicon substrates then by sealing them one on the other to delimit chambers intended to receive the gaseous substance to be characterised (this device, which is of the “double Helmholtz differential” type, comprises two chambers connected by capillaries). 
     The light source employed, in this case a QCL (Quantum Cascade Laser) which emits in the mid-infrared, is optically coupled to one of these chambers by an integrated waveguide. On the chamber&#39;s side of this guide, the guide has a widened end and is provided with a so-called decoupling diffraction grating, which enables to inject the light radiation into the chamber in the form of a light beam which, at least in one direction, is not very divergent. This light beam next propagates in the chamber, while being reflected on an upper face of the chamber, made reflective by a gold layer deposited on this face. 
     In the absence of a reflective coating, an important part of the power of the light beam in question would be lost, when reflecting on this upper face. Indeed, during reflection on a bare silicon face (that is to say without reflective coating), an important part of the light beam would be transmitted into the silicon, then would pass through it to finally come out of the device (it will be noted that, at the wavelengths employed, silicon is essentially transparent), thus reducing the average luminous power which, in the chamber heats up the gas, thus reducing the detection sensitivity of the device. 
     Making one or more faces of the chamber reflective thanks to a metal deposition thus enables to better confine the light radiation in the chamber. But, on the other hand, these metal layers deposited on these faces are sources of noise, of a photoacoustic type, which can impair the operation of the device and which reduce its sensitivity. 
     Indeed, even in the mid-infrared, the power reflection coefficient, on a gold layer, is around 98%. And as soon as the layer has a thickness of around one hundred or so nanometres, its coefficient of transmission is close to 0%. The fraction of the incident luminous power that is not reflected by the layer, which represents around 2% of this incident power, is thus absorbed in the layer, which causes the layer to heat up, modulated at the aforementioned modulation frequency. 
     This modulated heating of the gold layer is the source of a parasitic acoustic wave, which is added to the acoustic wave produced by the heating up of the gas to be detected. A parasitic absorption of 2% at the level of a reflective face of the cavity may appear small, but it may in fact notably impair the measurement because the absorption in the gaseous substance to characterise is generally quite low as well. 
     In this context, it would thus be desirable to have a device for the photoacoustic characterisation of a gaseous substance, which enables efficient confinement of the light radiation, in the chamber intended to receive the gaseous substance to characterise, while avoiding the problem of photoacoustic noise that a metal reflective coating would cause. 
     SUMMARY OF THE INVENTION 
     To resolve at least in part the aforementioned problem, a device is proposed for the photoacoustic characterisation of a gaseous substance, the device comprising:
         a light source, emitting a light radiation, and   a chamber intended to contain the gaseous substance to characterise, the chamber being delimited, inter alia, by a first wall, formed in a material at least partially transparent to said radiation, said wall having:
           an inner face, substantially flat, situated on the side of the chamber, and   an outer face, a major part of the outer face being flat, contained in a same plane, and substantially parallel to an average plane of the inner face.   
               

     The light source is optically associated with the chamber so as to inject said light radiation into the chamber, in the form of a light beam, a part at least of the light beam being reflected on the inner face of the first wall. 
     And, in a remarkable manner, the inner face of the first wall is etched so as to have recesses, each recess being delimited laterally by a lateral surface, a part of said lateral surface, which is illuminated by said light beam, being tilted, with respect to the average plane of said inner face, by a given tilt angle α. 
     Here, the tilt angle α satisfies the following inequality F1: 
     
       
         
           
             
               
                 
                   
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     i C  is the critical angle of total internal reflection in said material and n is the optical index of said material. 
     In a device which would comprise a first wall  110  of which the inner face  120  would be completely flat, without the recesses mentioned above, a part of the light beam which reaches this inner face would come out of the device after having passed through the first wall, as is represented in  FIG. 1 . 
     Indeed, without the recesses in question, a light ray RA that reaches the inner face  120  of the first wall  110  would be refracted by it. Then, after having passed through this wall, this ray would encounter an outer face  130  of the wall  110  under an angle necessary smaller than the critical angle of total internal reflection, whereas the inner  120  and outer  130  faces of this wall  110  are substantially parallel with each other. This light ray RA would then come out of the device through the outer face  130  in question, which would cause, in the chamber, a loss of luminous power. 
     Etching recesses, of which the lateral surface is tilted with respect to the average plane P 1  of the inner face  12  ( FIG. 2 ), enables, for a light ray RB which encounters this lateral surface, to increase the angle of incidence with which this ray encounters, next, the outer face  13  of the first wall  11  (compared to an inner face which would be exempt of recesses). 
     And it is possible to show by calculations of refracted angles that any light ray, which is refracted by the part of the lateral surface of the recess tilted by the tilt angle α, next reaches the outer face  13  of the first wall  11  with an angle of incidence i R  which satisfies the following condition F2: 
     
       
         
           
             
               
                 
                   
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     Choosing the tilt angle α sufficiently large, so that it satisfies the inequality F1, thus enables the angle of incidence i R  to be greater than the critical angle i C  of total internal reflection, i R &gt;i C , and this is so whatever the initial direction (in the chamber) of the light ray considered. 
     Since the angle of incidence i R  is then greater than the critical angle i C , each of these light rays is totally reflected by the outer face  13  of the first wall (by total internal reflection), and is thus sent back in the direction of the chamber. 
     With this device, the reflection of the light radiation, which makes it possible to send it back to the chamber, is achieved by total internal reflection. Unlike reflection on a metal layer, this reflection is thus realised without absorption, which is particularly interesting since it avoids the photoacoustic noise mentioned above. 
     It will be noted that the critical angle i C  of total internal reflection designates, as is customary, the limit angle of incidence beyond which a light ray is totally reflected, at the interface between the material in question and the space situated beyond the first wall, on the outside of the device. When this space is occupied by a substance having an optical index close to 1, such as air or another gaseous substance, the expression of the critical angle i C  is given by formula F3 below, where n is the optical index of the material forming the first wall, at the average wavelength of the light radiation emitted by the source: 
         i   C =sin −1 (1/ n )  (F3).
 
     If the outer face of the first wall was covered with another material, having an optical index n2 (optical index at the average wavelength of the light radiation in question), the expression of the critical angle i C  would be given by formula F4 below: 
         i   C =sin −1 ( n 2/ n )  (F4).
 
     The inner face of the first wall is substantially flat, in the sense that an entire part of this inner face is flat, situated in a same average plane P 1  ( FIG. 2 ). But this face also comprises hollow portions corresponding to the recesses in question. In other words, the inner face comprises several flat portions situated in the same plane which constitutes the average plane P 1 , as well as portions situated outside of this plane (at the level of the recesses). The portions of the inner face situated in the average plane P 1  may for example represent the major part of the area of this inner face (area measured in projection on the average plane P 1 ). 
     The tilt angle α is the angle formed between, on the one hand, the average plane P 1 , and, on the other hand, the part of the lateral surface of the aforementioned recess, which is illuminated by the light beam injected into the cavity (part which may correspond for example to a flat facet forming one of the portions of the lateral surface of the recess). As illustrated in  FIG. 2 , the tilt angle α designates more specifically the angle of opening of the hollow angular sector (without material) which extends from the average plane P 1  up to the part of the aforementioned lateral surface. Thus, as an example, if the recesses were extremely shallow, the tilt angle α would be close to 0 degrees. 
     Here, the tilt angle α is below 90 degrees. 
     Apart from the characteristics described above, the device that has been described may comprise one or more optional and non-limiting characteristics among the following, considered individually or according to all technically possible combinations thereof:
         the major part of the inner face of the first wall, or even the totality of this face, is exempt of metal;   the major part of the outer face of the first wall, or even the totality of this face, is exempt of metal;   the entire part of the outer face which is situated opposite said recess, or at least the essential (that is to say the major part) of this part of the outer face is flat;   the outer face is flat;   the lateral surface that delimits the recess considered is formed of several flat facets, for at least some of said recesses;   each of said facets is tilted, with respect to the average plane of said inner face, by said tilt angle α;   at least some of said recesses, of which the lateral surface is formed of several flat facets, are produced in the form of rectilinear grooves having a V-shaped section;   said grooves are parallel with each other, and in which each of said grooves extends along a transversal axis (y) which forms, with a mean direction of propagation of the light beam injected into the chamber, an angle comprised between 70 and 90 degrees;   the mean direction of propagation of the light beam is parallel to the inner face of the first wall, to some more or less 20 degrees;   at least some of said recesses, of which the lateral surface is formed of several flat facets, have a hollow pyramid shape with a square or rectangular base;   said pyramids are each arranged such that one of the sides of their base is parallel to a same transversal axis, and in which this transversal axis forms, with a mean direction of propagation of the light beam injected into the chamber, an angle comprised between 70 and 90 degrees;   the first wall is formed in an essentially monocrystalline substrate, wherein the average plane of said inner face, and said flat facets, are parallel to different crystalline planes of said substrate;   the substrate is formed of silicon, in which the average plane of said inner face is parallel to a crystalline plane of which the Miller indices are 1,0,0 or 1,1,0 and said facets are parallel to crystalline planes of which the Miller indices are 1,1,1, or 1,1,−1, or 1,−1,1 or −1,1,1;   the chamber is also delimited by a second wall formed in said material, the second wall having an inner face, substantially flat, situated on the side of the chamber, and an outer face, substantially flat and parallel to the inner face of the second wall, the inner face of the second wall being etched so as to also have recesses, each recess being delimited laterally by a lateral surface, a part of said lateral surface, which is illuminated by said light beam, being tilted, with respect to an average plane of the inner face of the second wall, by said tilt angle α.       

     It may also be provided that said grooves, or said pyramids, are arranged such that two grooves, or two pyramids, situated one following the other along a longitudinal axis, are spaced apart, along the longitudinal axis(x), by a distance d i =x i+1 −x i  which is equal, at least within 20%, to the following quantity d i,opt : 
     
       
         
           
             
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     In the formula above,
         the coordinates x i  and x i+1  denote the respective positions, along the longitudinal axis, of the two bottoms of said two V-shaped grooves, or denote the respective positions, along the longitudinal axis, of the two summits of said two pyramids,   the longitudinal axis, which is orthogonal to said transversal axis and which is contained in the average plane of said inner face, has an origin which is situated upright a point of injection of said light radiation,   H is the depth of the V-shaped grooves, or pyramids, measured perpendicularly to the average plane of said inner face, and   W is the distance between the point of injection of the light radiation and the average plane of said inner face, measured perpendicularly to said average plane.       

     Another aspect of the invention relates to a method for manufacturing a device for the photoacoustic characterisation of a gaseous substance, such as described above, the method comprising the following steps:
         producing a chamber intended to contain the gaseous substance to characterise, the chamber being delimited, inter alia, by a first wall, formed in a material, said wall having an inner face, substantially flat and situated on the side of the chamber, as well as an outer face, the major part of the outer face being flat, contained in a same plane, and substantially parallel to an average plane of said inner face, and   producing, or mounting on the device, a light source that emits a light radiation, the light source being optically associated with the chamber so as to inject said light radiation into the chamber, in the form of a light beam, a part at least of the light beam being reflected on the inner face of the first wall.       

     In a remarkable manner, the step of producing the chamber comprises a step of producing recesses, etched on the inner face of the first wall, each recess being delimited laterally by a lateral surface, a part of said lateral surface, which is illuminated by said light beam, being tilted, with respect to the average plane of said inner face, by said tilt angle α. 
     Apart from the characteristics described above, the method that has been described may comprise one or more optional and non-limiting characteristics among the following, considered individually or according to all technically possible combinations thereof:
         the first wall is formed in a first substrate, and the step of producing the chamber comprises a step of etching a half-cavity, in the first substrate, the half-cavity having a bottom forming said inner face, the step of producing said recesses being executed after the step of etching the half-cavity;   the first substrate is formed of essentially monocrystalline silicon, the cavity bottom is parallel to a crystalline plane of which the Miller indices are 1,0,0 or 1,1,0, and the step of producing said recesses comprises the following operations:
           producing a protective layer covering the lateral faces of said half-cavity,   depositing a protective mask on the bottom of said half-cavity, the mask being deposited through a stencil and comprising orifices coinciding with the openings of the recesses to be etched,   wet etching the first substrate, by action of potassium hydroxide or tetramethylammonium hydroxide;   removing the mask and the protective layer.   
           the chamber being further delimited by a second wall, which is formed in a second substrate made of said material, the second wall having an inner face, substantially flat, situated on the side of the chamber, and an outer face, substantially flat and parallel to the inner face of the second wall, the step of producing the chamber further comprises:
           a step of producing recesses, etched on the inner face of the second wall, each recess being delimited laterally by a lateral surface, a part of said lateral surface, which is illuminated by said light beam, being tilted, with respect to an average plane of the inner face of the second wall, by said tilt angle α, and   a sealing of the second substrate on the first substrate, so that the respective inner faces of the first wall and the second wall are situated opposite one another, on either side of the chamber.   
               

     The invention and the different applications thereof will be better understood on reading the description that follows and by examining the figures that accompany it. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The figures are presented for indicative purposes and in no way limit the scope of the invention. 
         FIG. 1  schematically represents the path of a light ray, in a conventional photoacoustic characterisation device. 
         FIG. 2  schematically represents the path of a light ray, in a photoacoustic characterisation device implementing the teachings of the invention. 
         FIG. 3  schematically represents a photoacoustic characterisation device according to a first embodiment, seen from the side. 
         FIG. 4  schematically represents the device of  FIG. 3 , seen from the side. 
         FIG. 5  schematically represents the path of a light ray, in the device of  FIG. 3 . 
         FIG. 6  schematically represents a photoacoustic characterisation device according to a second embodiment, seen from the side. 
         FIG. 7  is a partial schematic view of the device of  FIG. 6 , seen from the side. 
         FIG. 8  shows the results of digital simulations, for a device implementing the teachings of the invention. 
         FIG. 9  shows the results of digital simulations, for another device. 
         FIG. 10  shows the results of digital simulations, for yet another device, of the prior art. 
         FIG. 11  schematically represents steps of a method for manufacturing a photoacoustic characterisation device implementing the teachings of the invention. 
         FIG. 12  represents in greater detail one of the steps of the method of  FIG. 11 . 
         FIG. 13  schematically represents a sequence of operations carried out in the course of the step of  FIG. 12 . 
         FIG. 14  schematically represents other operations carried out in the course of the step of  FIG. 12 . 
         FIG. 15  is a schematical sectional view of a substrate, used as a basis for the manufacture of a characterisation device implementing the teachings of the invention, after a step of deep etching of a half-cavity, in the substrate. 
         FIG. 16  schematically represents the substrate of  FIG. 15 , at another stage of its structuring process. 
         FIG. 17  schematically represents the substrate of  FIG. 15 , after producing a protective layer protecting the lateral faces of the half-cavity. 
         FIG. 18  schematically represents the substrate of  FIG. 15 , after producing a protective mask protecting the bottom of the half-cavity. 
         FIG. 19  schematically represents the substrate of  FIG. 15 , after etching recesses on the bottom of the half-cavity. 
         FIG. 20  schematically represents the substrate of  FIG. 15 , after etching recesses and removing the layer and the protective mask. 
         FIG. 21  schematically represents the characterisation device, of which a part is formed by the substrate in question. 
     
    
    
     DETAILED DESCRIPTION 
     As already mentioned, the invention relates to, among other things, a device  1 ;  1 ′ for the photoacoustic characterisation of a gaseous substance. 
     The device  1 ;  1 ′ comprises, inter alia (see  FIGS. 3, 6 and 21 ):
         a light source  2 , configured to emit a light radiation,   a chamber  3 , intended to contain the gaseous substance to characterise,   at least one fluidic port  4  for the admission of the gaseous substance in the chamber and/or for the discharge of this gaseous substance (this or these ports are not represented in  FIGS. 3 to 7 ),   at least one microphone (not represented in the figures), to acquire an acoustic wave, generated in the gaseous substance to be characterised by photoacoustic effect.       

     Here, the device  1 ;  1 ′ is a compact, integrated device. It is produced by structuring (and functionalising) different substrates, in this case silicon substrates. Here, the chamber  3  is produced more particularly by sealing a first silicon substrate  10  and a second silicon substrate  20  one on the other, the second substrate  20  playing in some way the role of lid. 
     The device  1 ;  1 ′ may comprise an additional chamber in addition to the aforementioned chamber  3 , the two chambers then forming acoustic cavities coupled one to the other by capillaries to produce a detector of the “double differential Helmholtz” type, as described in the document FR 3019653. But the device  1 ;  1 ′ may also comprise a single chamber (in this case, the chamber  3 , mentioned above). 
     It will be noted that the invention relates not only to the device  1 ;  1 ′ described above but also a similar device at an intermediate stage of its manufacture, said latter device already comprising the chamber  3  but not yet being provided with a microphone or a fluidic port, for example. 
     In a remarkable manner, at least one inner face of the chamber  3  is etched so that this face has particular recesses  14 ;  14 ′, each of these recesses being delimited laterally by a lateral surface of which a part at least is tilted by a given tilt angle α with respect to an average plane P 1 , P 2  of the inner face in question. 
     As explained in the section entitled “Summary of the invention”, thanks to this tilt, the lateral surface of the recess  14 ;  14 ′ deviates the light rays which encounter it, when refracting them, in a manner such that these light rays next encounter an outer face of the substrate under an angle of incidence sufficiently large so that there is total internal reflection on this outer face. These light rays are thus reflected back to the chamber  3 . Etching such recesses on one at least of the inner faces of the chamber thus enables to reflect back to the chamber  3  a part of the light radiation which would otherwise exit therefrom. Since the light radiation is sent back to the chamber  3  by total internal reflection, problems of absorption and photoacoustic noise which would arise with the use of a reflective metal layer are thus avoided. 
     A first embodiment of the device,  1 , is schematically represented in  FIGS. 3 and 4 , and a second embodiment of the device,  1 ′, is schematically represented in  FIGS. 6 and 7 . These two embodiments mainly differ from each other by the shape of the recesses  14 ;  14 ′ mentioned above. 
     In the first embodiment, these recesses  14  are in the form of rectilinear grooves, in this case grooves with a V-shaped section. In the second embodiment, these recesses  14 ′ are produced in the form of hollow pyramids (pyramid shaped holes), with a square base. These two embodiments have nevertheless numerous common points. Also, identical or corresponding elements of these two embodiments will be as far as possible marked by the same reference signs and will not be described each time. 
     In these two embodiments, the light source  2  of the device  1 ;  1 ′ is a laser of the QCL (Quantum Cascade Laser) type. The light radiation that it emits has a spectrum situated mainly in the mid-infrared, for example between 3 microns and 10 microns. 
     The microphone(s) of the device may be MEMs (microelectromechanical system) type integrated microphones. 
     As for the chamber  3 , it is delimited ( FIGS. 4 and 7 ):
         on the one hand, by a first lower wall  11 ;  11 ′, formed in the first substrate  10 , and   on the other hand, by a second upper wall  21 ;  21 ′, formed in the second substrate  20 .       

     The first wall  11 ;  11 ′ and the second wall  21 ;  21 ′ each have:
         an inner face  12 ,  22 ;  12 ′,  22 ′ situated on the side of the chamber  3 , and   an outer face  13 ,  23  substantially parallel to the inner face  12 ,  21 ;  12 ′,  22 ′ of the wall considered and situated outside of the chamber  3 .       

     The first and second walls  11 ,  21 ;  11 ′,  21 ′ are substantially flat, in the sense that their outer and inner faces are themselves substantially flat and parallel to each other. 
     The outer and inner faces of each of the walls  11 ,  21 ;  11 ′,  21 ′ are substantially parallel with each other, in the sense that they form between them an angle less than 5 degrees. The outer face and the inner face of each of these walls  11 ,  21 ;  11 ′,  21 ′ may in particular, as here, be exactly parallel with each other. 
     The inner face  12 ;  12 ′ of the first wall  11 ;  11 ′ is substantially flat, in the sense that an entire part of this inner face is flat, situated in a same average plane P 1  (see  FIG. 4 ). But this face also comprises hollow, and potentially projecting, portions situated below or above the average plane P 1 . In other words, the inner face  12 ;  12 ′ comprises several flat portions situated in a same plane, which constitutes the average plane P 1 , as well as portions situated outside of this plane. The portions of the inner face  12 ;  12 ′ situated in this average plane P 1  may represent here the major part of the area of this face (area measured in projection on the average plane P 1 ). 
     The inner face  22 ;  22 ′ of the second wall  21 ;  21 ′ is also substantially flat, in the sense that an entire part of this inner face  22 ;  22 ′ is flat, situated in a same average plane P 2 . This face comprises however, here again, hollow, and optionally projecting, portions situated below or above the average plane P 2 . 
     As for the outer faces  13 ,  23  of the first and second walls  11 ,  21 ;  11 ′,  21 ′, they are here completely flat, and parallel respectively to the average plane P 1  and to the average plane P 2 . 
     The inner faces  12 ,  22 ;  12 ′,  22 ′ extend along a longitudinal axis x and along a transversal axis y (these two axes, x and y, are thus parallel, each, to the average plane P 1  and to the average plane P 2 ). Here, the inner faces  12 ,  22 ;  12 ′,  22 ′ each have an overall rectangular shape. The lengths of their sides, parallel to the longitudinal axis x, and, respectively, parallel to the transversal axis y, are noted a and b respectively ( FIGS. 3 and 6 ). These lengths may, as here, be comprised between 3 and 20 millimetres. 
     The direction perpendicular to the average plane P 1  of the inner face  12 ;  12 ′ is identified as the normal axis z. The axes x, y and z introduced above are perpendicular two by two. 
     The inner faces  12  and  22 , or  12 ′ and,  22 ′, which form the lower and upper limits of the chamber  3 , are separated from each other by a distance e which may be comprised between 0.3 and 1.5 millimetres (the distance e is the distance, along the z axis, between the average plane P 1  of the inner face  12 ;  12 ′ on the one hand and the average plane P 2  of the inner face  22 ;  22 ′ on the other hand). 
     Here, the chamber  3  is thus more extended parallel to the inner faces  12 ,  22 ;  12 ′,  22 ′ than perpendicularly thereto and thus has an overall flattened shape. 
     The first and second walls  11  and  21 , or  11 ′ and  21 ′, have respective thicknesses, noted L 1  and L 2 , which may for example be comprised between 0.2 and 0.6 millimetres. The thickness L 1  designates the distance between the average plane P 1  and the outer face  13  of the first wall, and the thickness L 2  designates the distance between the average plane P 2  and the outer face  23  of the first wall. 
     Here, the chamber  3  is formed ( FIGS. 4 and 21 ):
         of a lower half-cavity  31 , formed in the first substrate  10  and of which the bottom is constituted by the inner face  12 ;  12 ′ of the first wall  11 ;  11 ′, and   of an upper half-cavity  32 , formed in the second substrate  20  and of which the bottom is constituted by the inner face  22 ;  22 ′ of the second wall  21 ;  21 ′.       

     Each of these half-cavities is formed by depth etching of the corresponding substrate  10 ,  20 . It is thus delimited by its bottom and by a lateral surface, while remaining open on the side opposite its bottom. The first and second substrates  10  and  20  are sealed one on the other while positioning the upper half-cavity  32  opposite the lower half-cavity  31 , upright thereof. 
     The light source  2  is optically associated with the chamber  3 , that is to say coupled thereto, so as to inject into the chamber  3  the light radiation that it emits, in the form of a light beam F; F′. 
     To do so, the light source  2  may for example be coupled to an integrated waveguide,  6  ( FIG. 3 ). A first end  61  of the guide is then coupled to the light source  2 . A second end  62  of the guide, through which the light beam F exits, emerges in the chamber  3 . It is this solution which is implemented in the first embodiment, of  FIGS. 3 and 4 . The second end  62  of the guide may for example be situated at the junction between the two substrates  10  and  20 . 
     The centre of the second end  62  of the guide, through which the light beam F exits, is called point of injection P i  hereafter (this second end has a small section, with respect to that of the chamber  3 ; the light beam F is thus injected into the chamber from a localised zone, centred on the point of injection P i ). 
     The second end  62  of the guide may, as here, comprise a widened part and be provided with a decoupling diffraction grating, as is described for example in the document FR 3019653. This enables reducing the divergence of the light beam F. The diffraction grating may for example be orientated so that a mean direction of propagation of the light beam, D, at the outlet of the guide, is parallel to the longitudinal axis x, or at least does not depart by more than 20 degrees (or even 10 degrees) from the longitudinal axis x. The diffraction grating may be oriented furthermore so that the direction of lowest divergence of the light beam F is parallel to the transversal axis y. The light beam F then has a higher divergence parallel to the z axis (the beam then has a transverse section more elongated in the z direction than in the y direction). The light beam then propagates in some way in the form of a fan-shaped layer, which, parallel to the transversal axis y, is not very thick. 
     This arrangement enables reflections of the light beam on the inner faces of the chamber to take place essentially on the inner face  12  of the first wall  11  (lower inner face), and on the inner face  22  of the second wall  21  (upper inner face), rather than on the lateral inner faces  33  and  34  of the chamber  3  (lateral inner faces which are parallel to the plane (x,z)—see  FIG. 3 ). This is interesting because, here, it is precisely the inner faces  12  and  22  which are provided with particular recesses  14 , which enable efficient reflection of the light beam. 
     In the second embodiment, of  FIGS. 6 and 7 , the aforementioned waveguide is omitted. An outlet port of the light source  2 , from where the light beam F′ comes out, is then placed directly opposite an inlet port in the chamber,  6 ′, (that is to say opposite an inlet opening in the chamber), without intermediate optical component. The light beam F′ may then have quite important divergence, for example of several tens of degrees, not just parallel to the transversal axis y but also parallel to the normal axis z. Here, the inlet port of the light beam,  6 ′, is situated at the junction between the two substrates  10  and  20 . Here again, the light beam F′ is injected into the chamber from a zone not very extended with respect to the section of the chamber (this zone corresponds for example to the inlet opening in question), centred on a point called point of injection P i . As previously, the light beam F′ has a mean direction of propagation D parallel to the longitudinal axis x, or at least does not depart by more than 20 degrees (or even 10 degrees) from the longitudinal axis x. 
     In the different embodiments considered here, a part of the light beam F; F′ injected into the chamber  3  is reflected on the inner face  12 ;  12 ′ of the first wall, and another part of this beam is reflected on the inner face  22 ;  22 ′ of the second wall. 
     The inner face  12 ;  12 ′ of the first wall  11 ;  11 ′ is etched so as to have the aforementioned recesses  14 ;  14 ′. Each of these recesses is delimited laterally by a lateral surface  15 ;  15 ′, a part  16 ;  16 ′ of said lateral surface, which is illuminated by the light beam F; F′, being tilted, with respect to the average plane P 1  of said inner face  12 ;  12 ′, by the aforementioned tilt angle α. 
     The inner face  22 ;  22 ′ of the second wall  21 ;  21 ′ is etched so as to have such recesses  14 ;  14 ′ as well. 
     The tilt angle α satisfies the inequality F1 that has been described above in the section entitled “Summary of the invention”: 
     
       
         
           
             
               
                 
                   
                     α 
                     - 
                     
                       
                         sin 
                         
                           - 
                           1 
                         
                       
                       ( 
                       
                         
                           sin 
                            
                           
                               
                           
                            
                           α 
                         
                         n 
                       
                       ) 
                     
                   
                   &gt; 
                   
                     
                       i 
                       C 
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   
                     F 
                      
                     
                         
                     
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                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     As already explained, choosing the tilt angle α sufficiently large, so that it satisfies this inequality, enables that each ray refracted by the part  16 ;  16 ′ of the lateral surface  15 ;  15 ′ in question next undergoes total internal reflection on the outer face  13 ,  23  of the wall considered  11 ,  21 ;  11 ′,  21 ′. 
     To satisfy the condition F1, the tilt angle α is chosen so as to be greater than a limit tilt angle α L  which corresponds to the case of equality, in the formula F1: 
     
       
         
           
             
               
                 
                   
                     
                       α 
                       L 
                     
                     - 
                     
                       
                         sin 
                         
                           - 
                           1 
                         
                       
                       ( 
                       
                         
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                            
                           
                               
                           
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                             α 
                             L 
                           
                         
                         n 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       i 
                       C 
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   
                     F 
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                     5 
                   
                   ) 
                 
               
             
           
         
       
     
     The material which forms the first and second walls is here silicon. In the wavelength domain considered here, comprised between 3 and 10 microns, this material has an optical index equal to 3.4. The critical angle i C  is thus equal to around 17 degrees (c.f.: formula F3 indicated above), and the limit tilt angle α L  is then equal to around 24 degrees. 
     In these two embodiments, for each recess  14 ;  14 ′, the lateral surface  15 ;  15 ′ that laterally delimits the recess considered is formed of several flat facets  16 ,  17 ;  16 ′,  17 ′,  18 ′,  19 ′ ( FIGS. 4 and 7 ). And each of these facets is tilted, with respect to the average plane P 1 , P 2  of the inner face considered  12 ,  22 ;  12 ′,  22 ′, by said tilt angle α. 
     In the embodiment of  FIGS. 3 and 4 , for which the recesses  14  are produced in the form of V-shaped grooves, the facets  16  and  17  in question correspond to the two lateral faces of the groove considered (these faces join up with each other along the ridge which forms the bottom of the V-shaped groove). 
     And in the embodiment of  FIGS. 6 and 7 , for which the recesses  14 ′ are produced in the form of hollow pyramids, the aforementioned facets  16 ′,  17 ′,  18 ′,  19 ′ correspond to the four faces of the pyramid considered. 
     For these two embodiments, the silicon substrates  10  and  20  are essentially monocrystalline (“essentially monocrystalline” substrate is taken to mean a substrate of which the major part of the volume, or even more than 90% of the volume, is monocrystalline, this substrate nevertheless being able to comprise a certain number of defects such as dislocations or insertions). 
     The inner faces  12 ,  22 ;  12 ′,  22 ′ have average planes P 1  and P 2  that are parallel to the crystalline planes of silicon of which the Miller indices are 1,0,0 (plane sometimes designated as the plane (100) in the literature), or 1,1,0 (plane (110)). 
     Furthermore, for these two embodiments, the recesses are produced by a wet etching process in the course of which the substrate  10 ,  20  is etched in an anisotropic manner (this anisotropic wet etching process will be described below, during the description of a method for manufacturing such a device). With this etching technique, the facets  16 ,  17 ;  16 ′,  17 ′,  18 ′,  19 ′ that laterally delimit the recesses  14 ;  14 ′ obtained are each parallel to a crystalline plane of silicon, of which the Miller indices are 1,1,1 (sometimes also noted plane (111)) or 1,1,−1 or 1,−1,1 or instead −1,1,1. 
     Given the tilt of these crystalline planes, with respect to the crystalline plane of silicon of indices 1,0,0, or 1,1,0, the tilt angle α obtained here is equal to 54.7 degrees (it thus indeed satisfies the relationship F1, whereas it is greater than the limit tilt angle α L , which here is equal to around 24 degrees). 
     The arrangement of the recesses  14 ;  14 ′, on the inner faces  11 ;  11 ′ and  21 ;  21 ′, will now be described, firstly for the first embodiment then for the second embodiment. 
     In the first embodiment ( FIGS. 3 and 4 ), the grooves  14  are etched parallel to each other, and parallel to the transversal axis y. 
     They have a same depth H, comprised for example between 10 and 100 microns. The depth H designates the depth of each of these grooves, measured perpendicularly to the average plane P 1 , P 2  of the inner face  12 ,  22  in which is etched the groove  14  (the depth H is the distance between the average plane P 1 , P 2  and the bottom, that is to say the ridge, of the V-shaped groove). 
     The positions of the grooves  14  on the inner face  12  are marked, along the longitudinal axis x, by the coordinates x 1 , . . . , x i , x i+1 , . . . . The integer index i corresponds to the number of the groove considered. The first groove, of index i=1, is that which is the closest to the point of injection P i  of the light beam (see  FIG. 5 ). The coordinate x 1  marks more precisely the position of the bottom of groove number i, along the longitudinal axis x, of which the origin O is situated upright the point of injection P i . 
     The first groove, of index i=1, is positioned on the inner face  12 , as a function of the angular opening of the light beam F in the plane (x,z), so that the limit light ray, situated at the lower limit of the light beam F, encounters the facet  16  of this first groove. This limit light ray is, among the light rays of the light beam F, that which is the closest to the average plane P 1  (the luminous intensity at the level of this limit ray being for example equal to half of the maximum of the luminous intensity in the light beam F). 
     It is next desirable to position the grooves  14  near to each other, with a relatively reduced spacing, so as to reduce the dimensions of the portions of the inner face  12  which are parallel to the outer face  13 . Indeed, when a ray of the light beam F encounters the inner face  12  at the level of such a portion, parallel to the outer face  13 , this ray comes out of the chamber, then the device, and no longer participates in the heating up of the gaseous substance to characterise (as with a device without recesses). 
     But, on the other side, if the grooves are too close to each other, a light ray refracted by the facet  16  of one of the grooves may reach the following groove and be reflected therefrom, instead of being reflected on the outer face  13 . In this case, the light ray may be deviated rearwards by reflection on the following groove, in the direction of the source, this light ray then no longer participating in the heating up of the gaseous substance to characterise. 
     It is thus desirable that the grooves  14  are neither too spaced apart from one another, nor too close to one another. It is then possible to provide to arrange the grooves such that two grooves  14 , of which the positions x i+1  and x i  along the longitudinal axis x immediately succeed each other, are spaced apart, along this longitudinal axis x, by a distance d i =x i+1 −x i  which is equal, to some 20%, or even better, to the quantity d i,opt  defined by the following formula F6: 
     
       
         
           
             
               
                 
                   
                     d 
                     
                       i 
                       , 
                       opt 
                     
                   
                   = 
                   
                     
                       H 
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                           1 
                           
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                                 sin 
                                 
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     In this formula, the distance W 1  designates the distance between the point of injection P i  of the light radiation into the chamber on the one hand, and the average plane P 1  of the inner face  12  of the first wall  11  on the other hand. This distance is measured perpendicularly to the average plane P 1  ( FIG. 5 ). 
     The distance d i,opt  is the smallest distance, between the grooves i and i+1, for which the light rays, refracted by the facet  16  of the groove i, reach the outer face  13 , without encountering the facet  17  of the groove i+1 (without being retroreflected by this facet). This distance thus corresponds to a good compromise between a gap which would be too small (then leading to retro-reflection of an important part of the light radiation, in the wall  11 ) and a gap which would be too big (then increasing luminous power losses at the level of the outer face  13 ). 
     The case x i+1 −x i =d i,opt  is schematically represented in  FIG. 5 . The light ray R L  represented in this figure is the ray which, among the rays refracted by the facet  16  of groove no  1 , is the closest to the junction between the facet  16  and the average plane P 1  (in some way, it is the highest ray, among these rays). It is thus the ray which is the most likely to encounter the facet  17  of the following groove, no  2 . In this limit situation, this light ray R L  passes just under the groove no  2  (while being at the limit of touching the bottom of this groove). It is this geometric construction that makes it possible to determine the expression of the distance d i,opt , given by formula F6. 
     On the inner face  22  of the second wall  21 , the grooves  14  are arranged as explained above for the inner face  12  of the first wall  11 , but by replacing the distance W 1  by a distance W 2 , in the formula F6, W 2  being the distance between the point of injection P i  and the average plane P 2  of the inner face  22  of the second wall  21 . 
     In this embodiment, the lateral inner faces  33  and  34  of the chamber  3 , which are parallel to the plane (x,z), are exempt of grooves. But, in an alternative, it could also be provided to also etch V-shaped grooves on these lateral faces, each groove being for example parallel to the normal axis z. Furthermore, in an alternative, it could be provided that only one of the two inner faces  12  and  22  is etched with grooves, instead, as here, of etching these two inner faces. 
     In the second embodiment ( FIGS. 6 and 7 ), the pyramids  14 ′ are etched so that, for each pyramid, two of the sides of the base (square) of the pyramid are parallel to the longitudinal axis x, the other two sides of this base being parallel to the transversal axis y. The pyramids  14 ′ are thus arranged so that the sides of their respective bases are parallel with each other, from one pyramid to the other. 
     On each of the two inner faces  12 ′ and  22 ′, the pyramid-shaped recesses  14 ′ are furthermore arranged so as to be aligned with each other ( FIG. 6 ). These recesses  14 ′ are thus arranged in lines and in columns, on each of the two inner faces  12 ′ and  22 ′ (in other words, the recesses are arranged at the nodes of a rectangular network). In  FIGS. 6 and 7 , for reasons of clarity, not all of the pyramids  14 ′ are represented. 
     The depth, H, of the different recesses  14 ′ is the same as previously. In this case, the depth H designates the distance between the average plane P 1 , or P 2 , and the summit of the pyramid  14 ′ considered. 
     The positions of the pyramids  14 ′ on the inner face  12 ′ are marked, along the longitudinal axis x, by the coordinates x 1 , . . . x i , x i+1 , . . . , and, along the transversal axis y, by the coordinates y −j , . . . , y 0  . . . , y j , y j+1  . . . . The integer indices i and j correspond respectively to the column number and to the line number of the pyramid considered. The pyramid, of indices i=1 and j=0, is that which is the closest to the point of injection P i  of the light beam. The pair of coordinates (x i ,y i ) marks more precisely the position of the summit of the pyramid considered  14 ′, of indices i and j, along the axes x and y, of which the common origin O is situated upright the point of injection P i . 
     The line of pyramids  14 ′ for which j=0 (central line) is situated here in the alignment of the point of injection P i . The first column of pyramids  14 ′ (those for which i=1) is positioned on the inner face  12 ′ as a function of the angular opening of the light beam F′ in the plane (x,z), so that the limit light ray, situated at the lower limit of the light beam F′, encounters the facet  16  of the pyramid  14 ′ of indices i=1, j=0. 
     As for the first embodiment, the recesses  14 ′ are arranged, on the inner face  12 ′, so that two recesses  14 ′ (here two pyramids  14 ′) of which the positions x i+1  and x i  along the longitudinal axis x follow each other immediately (i.e.: recess of indices (i,j), and, respectively, of indices (i+1,j)), are spaced apart, along this longitudinal axis x, by a distance d i =x i+1 −x i  which is equal, to some 20%, or even better, to the quantity d i,opt  defined by the formula F6 given above. In this second embodiment, the distance W 1  that intervenes in formula F6 designates, here again, the distance between the point of injection P i  and the average plane P 1  of the inner face  12 ′ of the first wall  11 ′. 
     As explained for the first embodiment, spacing the recesses  14 ′ apart in this way makes it possible, in the chamber, to obtain a high average luminous power since this spacing limits potential retro-reflections inside the first wall  11 ′, while corresponding to a quite important density of recesses on the inner face  12 ′ (which limits lateral luminous losses, at the level of the outer face  13 ). 
     For the same reasons, the recesses  14 ′ are arranged, on the inner face  12 ′, so that two recesses  14 ′ of which the positions y j+1  and y j  along the transversal axis y follow each other immediately (i.e. recess of indices (i,j), and of indices (i,j+1) respectively), are spaced apart, along this transversal axis y, by a distance d j =y j+1 −y j  which is equal, to some 20%, or even better, to the quantity d j,opt  defined by the following formula F7: 
     
       
         
           
             
               
                 
                   
                     d 
                     
                       j 
                       , 
                       opt 
                     
                   
                   = 
                   
                     
                       H 
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     On the inner face  22 ′ of the second wall  21 ′, the pyramids  14 ′ are arranged as explained above for the inner face  12 ′ of the first wall  11 ′, but by replacing the distance W 1  by a distance W 2 , in formulas F6 and F7, W 2  being the distance between the point of injection P i  and the average plane P 2  of the inner face  22 ′ of the second wall  21 ′. 
     Producing the recesses  14 ′ in the form of hollow pyramids is well suited to the geometry that the light beam F′ has, in this second embodiment. Indeed, unlike the first embodiment, the light beam F′ is here quite highly divergent, not just in a direction parallel to the normal axis z but also in a direction parallel to the transversal axis y. Certain light rays of this beam may thus depart quite considerably from the plane (x,z). These light rays are then refracted by the left and right lateral facets  18 ′,  19 ′ of the pyramids, instead of being refracted by the front  16 ′ and rear  17 ′ facets of these pyramids, which makes it possible to obtain similar refraction conditions for the rays situated in the plane (x,z) (refracted by the front and read facets  16 ′ and  17 ′), and for the rays which depart from this plane (refracted by the left and right facets  18 ′,  19 ′). 
     In this second embodiment, the lateral inner faces  33  and  34  of the chamber  3 , which are parallel to the plane (x,z), are exempt of pyramids. But, in an alternative, it could however be provided to also etch pyramid-shaped recesses on these lateral faces, the sides of the bases of these pyramids each being for example parallel to the normal axis z, or to the longitudinal axis x. Furthermore, in an alternative, it could be provided that only one of the two inner upper and lower faces  12 ′ and  22 ′ is etched with pyramids, instead, as here, of etching these two inner faces. 
     Furthermore, in other embodiments, the inner lower face and/or the inner upper face of the chamber could be etched so as to have at one and the same time V-shaped grooves and pyramid-shaped recesses, such as described above. The recesses could be produced in another form than V-shaped grooves or hollow pyramids, from the moment that the recesses in question each remain delimited by a lateral surface which comprises a part, illuminated by said light beam and tilted by said the tilt angle α. 
       FIGS. 8, 9 and 10  show the results of digital simulations which illustrate the interest of the device  1 ;  1 ′ described above. 
       FIG. 8  is a sectional view, along a sectional plane parallel to the plane (x,z), of a device similar to the device  1  of the first embodiment but for which the thickness of the chamber is smaller than that which has been indicated above (thickness of 20 microns, in the case of this simulation). The V-shaped grooves for their part have a depth of 20 microns. In this figure, the amplitude of the electrical field associated with the light radiation injected into the chamber is represented in grey levels, which darken as the amplitude increases. 
       FIG. 9  is a sectional view, along a sectional plane parallel to the plane (x,z), of a device comparable to that of  FIG. 8 , but in which the inner faces of the first and second walls are exempt of grooves. 
     The results of these simulations have shown that, in the case of  FIG. 8  (with grooves), the walls of the chamber each have an overall power reflection coefficient equal to around 99%. Whereas in the case of  FIG. 9  (without grooves), each of these walls has an overall power reflection coefficient equal to around 60%. It is moreover possible to see, in  FIG. 9 , that a part of the luminous power injected into the chamber comes out of the device through the outer faces of the walls of the chamber. 
       FIG. 10  is a sectional view comparable to those of  FIGS. 8 and 9 , but for a device in which the inner faces of the chamber are completely flat (without grooves) and covered by a reflective gold layer. In this case, the power reflection coefficient is 98% for each wall, and the luminous power injected remains well confined in the chamber. But, as mentioned in the foreword, residual absorption in the reflective gold layer then generates bothersome photoacoustic noise 
     These digital simulations, in particular that of  FIG. 8 , show that the grooves etched on the inner faces of the chamber indeed have the expected effect: they effectively make it possible to better confine in the chamber  3  the injected luminous power (with respect to flat inner faces and without reflective coating), and to do so by total internal reflection, thus without residual absorption. 
     A method making it possible to produce a device for the photoacoustic characterisation of a gaseous substance, such as the device  1 , or  1 ′, described above, will now be described, with reference to  FIGS. 11 to 21 . 
     This method here comprises the following steps, schematically represented in  FIG. 11 :
         S 1 : structuring of the first substrate  10 ,   S 2 : structuring of the second substrate  20 ,   S 3 : once steps S 1  and S 2  have been carried out, sealing of the second substrate  20  on the first substrate  10 ,   S 4 : thinning of the device  1 ;  1 ′, to the desired thickness,   S 5 : producing fluidic ports  4  for bringing in and evacuating the gaseous substance to characterise, and producing ports  5  for the passage, to one or more microphones, of the acoustic wave generated in the chamber.       

       FIG. 21  schematically represents the device  1 , such as it is obtained at the end of step S 5 , seen from the side (section along the plane (x,z)). 
     Before structuring, each of the two substrates  10  and  20  is a monocrystalline silicon substrate, of which the faces are parallel to a crystalline plane of which the Miller indices are 1,0,0 or 1,1,0, and which may be polished either on a single face (SSP, single-side polished, type substrate), or on both faces (DSP, double-side polished, type substrate), making it possible to obtain a device of which the structure is even better defined. Each of these substrates has for example a (standard) thickness of 725 microns, initially. 
     The light source  2  is here produced by structuring of a part of the first substrate  10 , during a step S 19  of step S 1  ( FIG. 12 ). 
     Step S 1  furthermore comprises a step S 10  of producing the lower half-cavity  31  of the chamber  3 , by etching of the first substrate  10 . 
     It is this step S 10  which is now described in detail, with reference to  FIGS. 12 to 20 .  FIGS. 12 to 14  schematically represent sequences of operations carried out in the course of step S 10 , whereas  FIGS. 15 to 20  show the first substrate  10  at different stages of the production of the half-cavity  31 . 
     As may be seen in  FIG. 12 , step S 10  of producing the lower half-cavity  31  here comprises:
         a step S 11  of deep etching the first substrate  10 , to define the lower volume of the lower half-cavity  31 , then   a step S 12  of producing recesses  14 ;  14 ′, on the bottom  12 ;  12 ′ of this half-cavity.       

     As already indicated, it is the bottom of the lower half-cavity  31  that here forms the inner face  12 ;  12 ′ of the first wall  11 , mentioned above in the description of the device  1 ;  1 ′. 
     Step S 11  of deep etching the first substrate  10  may be executed by carrying out the following steps ( FIG. 13 ):
         S 13 : producing a hard mask  40 , which extends around a surface which, after etching, will form the upper opening of the half-cavity  31 ,   S 14 : deep etching the substrate, through this hard mask  40  ( FIG. 15 ), and   S 14 ′: removing a polymer layer, previously deposited on the faces of the half-cavity in the course of the deep etching step S 14 .       

     The etching carried out at step S 14  is a deep dry etching, directive (anisotropic), for example of DRIE (Deep Reactive Ion Etching) type. This etching is carried out over a depth comprised for example between 300 and 450 microns, the depth which will be that of the half-cavity  31  obtained finally. 
     The hard mask  40  produced at step S 13  may be a mask made of silica SiO 2 , having a thickness of several microns, for example comprised between 3 and 4.5 microns, for the etching depths indicated above. This thickness makes it possible to give the hard mask sufficient resistance to the DRIE to etch in depth the first substrate  10  (over a depth of around 300 to 450 microns), without making this mask (which protects the parts of the substrate which are not to be etched) totally disappear. 
     Step S 13  may for example comprise the following steps ( FIG. 13 ):
         S 131 : depositing a layer of silica SiO 2  of several microns thickness (for example 3 microns thickness; this deposition may for example be a PECVD (Plasma Enhanced Chemical Vapour Deposition), the source of silicon used for the deposition being for example tetraethoxysilane, or TEOS, then   S 132 : depositing a photosensitive resin, here a positive resin, on the silica layer, then   S 133 : photolithography of the resin (exposure to light, development, and removal of the light exposed resin), to define the zones of the silica layer to etch, then   S 134 : etching the silica layer, then   S 135 : removing the remaining photosensitive resin.       

       FIG. 15  schematically shows the first substrate  10 , seen from the side, after step S 11  of deep etching. After this step, the lower half-cavity  31  is delimited by its bottom  12  and by lateral faces, for example four in number (if the chamber has a rectangular bottom). Two of these lateral faces,  351 ,  361 , are visible in  FIG. 15 . 
     As for step S 12  of producing recesses  14 ;  14 ′, it here comprises the following steps ( FIG. 14 ):
         S 15 : producing a protective layer  41  that covers the different lateral faces (including  351  and  361 ) of the half-cavity  31  ( FIGS. 16 and 17 ), then   S 16 : depositing a protective mask  42 , on the bottom  12  of the half-cavity  31 , the protective mask  42  being deposited through a stencil  43  and comprising orifices  44  coinciding with the openings of the recesses  14 ;  14 ′ to etch ( FIG. 18 ), then   S 17 : wet etching the substrate, through the protective mask  42 , by action of potassium hydroxide KOH or tetramethylammonium hydroxide TMAH, to etch the recesses  14 ;  14 ′, then   S 18 : removing the protective mask  42  and the protective layer  41 .       

     The protective mask  42  and the protective layer  41  are here formed of silicon nitride Si 3 N 4 , which confers thereon good resistance to the wet etching carried out at step S 17 . 
     Step S 15  may for example comprise the following operations:
         S 151 : depositing silicon nitride Si 3 N 4 , on all of the lateral faces and the bottom of the half-cavity  31 , by LPCVD (Low Pressure Chemical Vapour Deposition), then   S 152 : eliminating the silicon nitride Si 3 N 4  deposited on the bottom  12  of the half-cavity  31 , by dry etching of RIE (Reactive Ion Etching) type.       

       FIG. 16  schematically shows the substrate  10 , just after the deposition operation S 151 .  FIG. 17  shows this substrate just after the removal of the silicon nitride which had been deposited on the bottom  12 . 
     At step S 16 , the stencil  43  employed is for example a metal stencil, or a silicon stencil. The dimensions of the patterns present on this stencil are chosen as a function of the desired dimensions for the orifices  44  of the protective mask  42 . 
     The dimensions of the orifices  44  present in the protective mask  42  are chosen as a function of the desired dimensions for the openings of the recesses  14 ;  14 ′ (they are equal to the desired dimensions for the openings of the recesses  14 ;  14 ′). For example, if it is wished to obtain V-shaped grooves having a width of 28 microns (in the average plane P 1 ), the protective mask will be produced so that it has orifices in the form of rectilinear strips, also having a width C of 28 microns. 
     As for the depth H of the recesses  14 ;  14 ′ etched in the substrate  10 , it is in some way self-limited, and depends directly on the dimensions of the orifices  44 , for example their width C. Indeed, the wet etching of silicon by potassium hydroxide or by TMAH is anisotropic, and the faces obtained after etching, which laterally delimit the recesses  14 ;  14 ′, correspond to crystalline planes of which the Miller indices are 1,1,1 or 1,1,−1, or 1,−1,1, or −1,1,1 (the density of atoms in these crystalline planes is greater than in the crystalline planes of index 1,1,0 and 1,0,0; the etching is thus slower in a direction perpendicular to one of the planes 1,1,1, or 1,1,−1, or 1,−1,1, or −1,1,1, which explains why the faces which subsist after etching have the orientation indicated above). 
     As already indicated, the lateral faces of the recesses are thus tilted by 54.7 degrees with respect to the average plane P 1  of the inner face  12  (given the respective orientations of the aforementioned crystalline planes). For orifices  44  in the form of rectilinear strips, of width C, the depth H of the V-shaped grooves will thus be self-limited to (C/2)×tan(α)=)(C/2)×tan(54.7°). Thus, if it is wished to obtain a depth H of 20 microns, for example, then a protective mask will be deposited of which the orifices, in the form of strips, have a width C of 28.3 microns. 
     As for the wet etching step S 17 , it is for example carried out in accordance with the indications given in section 2.2 “Gravure chimique” of the document “ Micro - usinage des matériaux monocristallins ”, by Jean-Sébastien DANEL,  Techniques de l&#39;ingénieur , article BM 7 290, July 1998. 
     Finally, at step S 18 , the removal of the protective mask  42  and the protective layer  41  may be carried out by chemical attack with hydrofluoric acid HF, for example. In the course of this step, the hard mask  40  is also removed (by the chemical attack in question). 
     The upper half-cavity  32 , produced in the second substrate  20  in the course of step S 2 , is produced in the same way as the lower half-cavity  31 , by the same sequence of steps. The upper half-cavity  32  may nevertheless have a depth different to that of the lower half-cavity  31 , and the recesses may be arranged differently on the bottom  22  of the upper half-cavity  32 , and on the bottom  12  of the lower half-cavity  31 . 
     Once the two half-cavities  31  and  32  have been produced, the first and second substrates  10  and  20  are sealed one on the other, at step S 3 . This sealing is caned out by direct bonding of the substrates  10  and  20  one on the other. Beforehand, the substrates  10  and  20  are cleaned to be exempt of impurities (which may be of particular origin, organic and/or ionic). This cleaning may be wet chemical cleaning, cleaning by ultraviolet radiation and ozone, or be carried out by plasma treatment. Before being sealed, the substrates  10  and  20  are positioned one with respect to the other such that the lower half-cavity  31  and the upper half-cavity  32  are situated facing each other, that is to say opposite each other (each lateral wall of the upper half-cavity then extending in the extension of one of the lateral walls of the lower cavity). This alignment is achieved by means of alignment marks, produced on each of the two substrates  10  and  20  at the start of their structuring process. 
     In  FIG. 21  may be seen the device  1 , after sealing of the two substrates  10  and  20  one on the other, and after step S 5  of producing the fluidic ports  4  and the ports for microphones  5 . It will be noted that this figure, intended to illustrate the general structure of the device after sealing, is very schematic: it is not necessarily to scale, and all the parts of the device are not necessarily represented in this figure (this is furthermore also the case for  FIGS. 3 to 7 ). 
     Different alternatives may be made to the manufacturing method that has been described. This method may in particular comprise additional steps or sub-steps, before or after the sealing of the two substrates. Certain steps could be carried out differently from that which has been described above. 
     For example, the protective mask which covers the bottom of the half-cavity considered, to protect it during wet etching, could be produced by photolithography (for example of Spray type) instead of being deposited through a stencil. This alternative is however quite awkward to implement and requires specific photolithography systems. Indeed, the orifices  44  present in the protective mask must then be defined by photolithography at the cavity bottom, at an important depth with respect to the upper face of the substrate (generally more than 300 microns), which makes the focusing of the lithography light beam awkward.