Photoacoustic spectroscopy device

Abstract

Photoacoustic Spectroscopy Device This description relates to a photoacoustic spectroscopy device (100) comprising at least one mechanical resonator (101) having a first part (102) for detecting acoustic waves and a second part (104) for transducing the acoustic waves detected by the first part (102) of the mechanical resonator (101) into at least one transduction signal, the first part (102) of the mechanical resonator (101) being disposed in a first environment in which the acoustic waves are intended to propagate, and the second part (104) of the mechanical resonator (101) being disposed in a second vacuum environment separate from the first environment. Figure for the abstract: Fig. 1

Description

Title of the invention: Photoacoustic spectroscopy device technical field

[0001] The present description relates generally to the field of photoacoustic spectroscopy, or spectrometry, and in particular to that of gas detection by photoacoustic spectroscopy. Previous technique

[0002] Photoacoustic spectroscopy is an analytical method used, in particular, to determine the concentration of a specific gas in a medium. It is based on the following photoacoustic effect: modulated light is shone through a gas. If the gas molecules are sensitive to the wavelength of the light used, photons are absorbed by the gas molecules, causing them to enter an excited state. Thermal relaxation of the molecules occurs, resulting in the production of heat that is proportional to the concentration of the gas molecules and the amount of energy absorbed by them. Such a modulated temperature change generates a modulated pressure change, producing an acoustic wave whose amplitude is proportional to the concentration of the molecules and the amount of light absorbed by them.

[0003] This acoustic wave can be detected in various ways, and in particular by a mechanical microresonator coupled to capacitive sensing means, such a system being commonly called a resonant microelectromechanical system. However, due to the principles involved in capacitive transduction, particularly due to the small distance between the electrodes performing this transduction, a viscous damping phenomenon occurs which is extremely disadvantageous for detection. This viscous damping will result in a reduced quality factor and reduced detection sensitivity. Summary of the invention

[0004] There is a need to propose a photoacoustic spectroscopy device, or sensor, with improved performance compared to prior art devices.

[0005] An embodiment overcomes all or part of the drawbacks of known solutions and proposes a photoacoustic spectroscopy device comprising at least one mechanical resonator having a first part for detecting acoustic waves and a second part for transducing the acoustic waves detected by the first part of the mechanical resonator into at least one transduction signal, the first part of the mechanical resonator being disposed in a first environment in into which the acoustic waves are intended to propagate, and the second part of the mechanical resonator being disposed in a second vacuum environment separate from the first environment.

[0006] According to a particular embodiment:

[0007] - the first part of the mechanical resonator comprises a first portion of material configured to be set in motion by acoustic waves and a first portion of at least a second portion of material, at least one end of which is mechanically coupled to the first portion of material, and

[0008] - the second part of the mechanical resonator comprises a second part of the second portion of material of which at least one second end is capable of moving under the effect of energy transmitted from the first portion of material.

[0009] According to a particular embodiment, the mechanical resonator comprises several second portions of material extending around the first portion of material.

[0010] According to a particular embodiment, the mechanical resonator comprises several groups of second portions of material in each of which said second portions of material are mechanically coupled to each other.

[0011] According to a particular embodiment, in each group, the second portions of material are mechanically coupled together at least by a third portion of material disposed at the junctions of the first and second parts of each of the second portions of material.

[0012] According to a particular embodiment, in each group, the second parts of the second portions of material are mechanically coupled together by at least a fourth portion of material disposed in the second vacuum environment.

[0013] According to a particular embodiment:

[0014] - in each group, the second portions of material form beams straight lines arranged parallel to each other;

[0015] - the fourth portion of material forms a longitudinal portion extending perpendicular to the second portions of material.

[0016] According to a particular embodiment:

[0017] - the first part of the mechanical resonator comprises a first portion of material configured to be set in motion by acoustic waves, and

[0018] - the second part of the mechanical resonator comprises at least a second portion of material in the shape of a crown mechanically coupled to the first portion of material and capable of moving under the effect of energy transmitted from the first portion of material.

[0019] According to a particular embodiment, the first and second parts of the mechanical resonator correspond to parts of the same layer of material.

[0020] According to a particular embodiment, the second environment is delimited by at least one hood and walls arranged at at least one junction between the first and second parts of the mechanical resonator and resting on a first face of a support.

[0021] According to a particular embodiment, the support comprises at least one layer of material and at least one opening through said layer of material and opening opposite the first part of the mechanical resonator.

[0022] According to a particular embodiment, the photoacoustic spectroscopy device may further comprise at least one light source attached to a second face, opposite to the first face, of the support, and configured such that a modulated light beam intended to be emitted by the light source passes in front of the opening.

[0023] According to a particular embodiment, the photoacoustic spectroscopy device further comprises an acoustic resonator attached to the second face of the support and comprising at least one inlet configured to receive the light beam and at least one outlet opening opposite the opening of the support.

[0024] According to a particular embodiment, the second part of the mechanical resonator is configured to perform a capacitive transduction of the detected acoustic waves.

[0025] A method for implementing a photoacoustic spectroscopy device is also proposed, comprising at least the following steps:

[0026] - realization of at least one mechanical resonator having a first part of detection of acoustic waves and a second part of transduction of the detected acoustic waves into at least one transduction signal, the first part of the mechanical resonator being disposed in a first environment in which the acoustic waves are intended to propagate;

[0027] - creation of a second vacuum environment, distinct from the first environment and in which the second part of the mechanical resonator is arranged. Brief description of the drawings

[0028] These features and advantages, as well as others, will be described in detail in the following description of particular embodiments, given by way of non-limiting example, in relation to the accompanying figures, among which:

[0029] - Fig. 1 schematically represents a cross-sectional view of a device photoacoustic spectroscopy according to a particular example;

[0030] - [Fig.2] schematically represents part of a spectroscopy device photoacoustics according to a first embodiment;

[0031] - Figure 3 schematically represents part of a spectroscopy device photoacoustic according to a variant of the first embodiment;

[0032] - Figure 4 schematically represents a mechanical resonator of a device photoacoustic spectroscopy according to a second embodiment;

[0033] - Figure 5 schematically represents a cross-sectional view of an example of acoustic resonator of a photoacoustic spectroscopy device;

[0034] - Figure 6 schematically represents a cross-sectional view of a device photoacoustic spectroscopy including an acoustic resonator;

[0035] - Fig. 7, Fig. 8, Fig. 9 and Fig. 10 schematically represent steps in a process for manufacturing a photoacoustic spectroscopy device. Description of the implementation methods

[0036] The same elements have been designated by the same reference numerals in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same reference numerals and may have identical structural, dimensional and material properties.

[0037] For the sake of clarity, only the steps and elements necessary for understanding the described embodiments have been shown and are detailed. In particular, various elements of the photoacoustic spectroscopy device, such as control circuits, biasing circuits, transduction signal readout circuits, etc., are not detailed. A person skilled in the art will be able to implement these elements in detail from the functional description given here.

[0038] Unless otherwise specified, when referring to two elements connected together, this means directly connected without intermediate elements other than conductors, and when referring to two elements coupled together, this means that these two elements can be connected or linked through one or more other elements.

[0039] In the following description, when reference is made to absolute positional qualifiers, such as "front," "back," "top," "bottom," "left," "right," etc., or relative positional qualifiers, such as "above," "below," "superior," "inferior," etc., or to orientational qualifiers, such as "horizontal," "vertical," etc., unless otherwise specified, reference is made to the orientation of the figures. However, these terms do not imply the actual position and orientation of the device during its use.

[0040] Unless otherwise specified, the expressions "approximately", "roughly", and "on the order of" mean to within 10% or 10°, preferably to within 5% or 5°.

[0041] An example of the embodiment of a 100 photoacoustic spectroscopy device is described below in relation to [Fig. 1].

[0042] Throughout this description, the device 100 is intended for use in gas detection by photoacoustic spectroscopy. However, other uses for the device 100 are conceivable. For example, the device 100 can serve as a mass detector (e.g., gravimetric detection) or as a biosensor detector (e.g., biomarkers). The operation of the device 100 will then differ from that described below. For mass detection, it is possible, for example, to drive the device 100 at the resonance frequency and observe the frequency change. For biomarkers, it is possible to use the device 100 as for mass detection for particles and / or it is also possible to functionalize the surface of the device 100 (the central part used, in the example below, for the detection of photoacoustic waves).Device 100 can also be used as an acoustic sensor to detect acoustic waves of certain wavelengths (for example, to detect bats that emit ultrasound).

[0043] The device 100 includes at least one mechanical resonator 101 having a first part 102 for detecting acoustic waves and a second part 104 for transducing the detected acoustic waves into at least one transduction signal.

[0044] In a first embodiment of the device 100 shown in [Fig. 2], the first part 102 of the mechanical resonator 101 comprises a first portion of material 106 configured to be set in motion by the acoustic waves intended to be detected, for example forming a movable membrane, and a first part 108 of at least one second portion of material 109, at least one end of which is mechanically coupled to the first portion of material 106. In the example described, the mechanical resonator comprises several second portions of material 109 extending around the first portion of material 106. More particularly, in the example described, the mechanical resonator 101 comprises several groups of second portions of material 109, in each of which the second portions of material 109 are mechanically coupled to each other. In [Fig.2], four groups, each comprising four second portions of material 109, extend from the first portion of material 106 along two axes perpendicular to each other (parallel to the X and Y axes in Figures 1 and 2).

[0045] The first portion of material 106 is intended to be set in motion by the detected acoustic waves. The surface area presented by the first portion of material 106 to the environment in which the acoustic waves are intended to propagate can be maximized in order to maximize the collection of energy transmitted by the acoustic waves.

[0046] Furthermore, in the first embodiment, the second part 104 of the mechanical resonator 101 comprises a second part 110 of the second portion of material 109 (of each of the second portions of material 109 in the example described) capable of moving under the effect of the movement of the first portion of material 106.

[0047] In the described embodiment, in each group of second portions of material 109, these second portions of material 109 are mechanically coupled together by at least one third portion of material 112 disposed at the junctions of the first and second parts 108, 110 of each of the second portions of material 109. In addition, in this example, in each group of second portions of material 109, the second parts 110 of the second portions of material 109 are also mechanically coupled together by at least one fourth portion of material 114.

[0048] In the described embodiment, in each group of second portions of material 109, these second portions of material 109 form straight beams arranged parallel to one another, and the fourth portion of material 114 forms a longitudinal portion extending perpendicularly to the second portions of material 109. The cross-sections of these beams, in a plane perpendicular to their longest dimension, can be rectangular. In [Fig. 2], for each group of second portions of material 109, the fourth portion of material 114 mechanically connects the second ends, opposite to the first ends, of the second portions of material 109.

[0049] In the described embodiment, the elements of the mechanical resonator 101 form a resonant structure having at least one resonance frequency.

[0050] In the described embodiment, the first and second parts 102, 104 of the mechanical resonator 101, that is to say the first, second, third and fourth portions of material 106, 109, 112, 114, correspond to parts of the same layer of material. For example, this layer of material may comprise silicon.

[0051] In the examples of Figures 1 and 2, the mechanical resonator 101 is attached to a support 115. In the described embodiment, the support 115 includes a mechanical retaining layer 116, comprising, for example, a semiconductor such as silicon. Alternatively, the mechanical retaining layer 116 may include a conductive material or a material capable of creating an electrical capacitance with the mechanical resonator 101. In the described example, the support also includes fifth portions of material 118, comprising, for example, a dielectric material such as SiO2, interposed between the mechanical retaining layer 116 and the mechanical resonator 101, at the interface between the first and second parts 102, 104 of the mechanical resonator 101.

[0052] In the described example, the device 100 also includes anchoring portions 119 contributing to the mechanical retention of the mechanical resonator 101 to the support 115. Fifth portions of material 118 are also arranged between the anchoring portions 119 and the mechanical retention layer 116. The fifth portions of material 118 may have a different geometry from that of the anchoring portions 119. The fifth portions of material 118 provide electrical insulation between the mechanical resonator 101 and the support 115 and also contribute to sealing between the environments in which the first and second parts 102, 104 of the mechanical resonator 101 are located.

[0053] For example, the support 115, the mechanical resonator 101, the fifth portions of material 118, and the anchoring portions 119 may be derived from a SOI (Silicon-On-Insulator) substrate, or more generally from a semiconductor-on-insulator type substrate, with the mechanical resonator 101 and the anchoring portions 119 formed by portions of the semiconductor surface layer of the SOI substrate, the fifth portions of material 118 corresponding to portions of the buried dielectric layer, or BOX (Buried-Oxide), of the SOI substrate, and the mechanical support layer 116 corresponding to the support layer of the SOL substrate

[0054] In the described embodiment, the second part 104 of the mechanical resonator 101 is configured to perform capacitive transduction of the detected acoustic waves into an electrical transduction signal. This capacitive transduction is achieved here thanks to the electrical capacitance formed between the mechanical retaining layer 116 (forming a first electrode of this electrical capacitance) and the second parts 110 of the second portions of material 109 and the fourth portions of material 114 (forming second electrodes of this electrical capacitance). In the configuration described here, the mechanical retaining layer 116 and these elements of the mechanical resonator 101 form two electrically chargeable elements between which an air-filled or vacuum space is present. These two elements form an electrical capacitance of value Co, which changes when the mechanical resonator 101 is in motion.When the second portions of material 109 move at a frequency f0, this creates changes in capacitance at frequency f0. This can be seen as an alternating current of frequency f0 at the output of the sensor. The amplitude of the alternating component at frequency f0 is a function, in particular, of the amplitude of the displacement of the second portions of material 109.

[0055] Following the example of [Fig. 1], the device 100 further comprises at least one light source 120, for example a laser source, attached to a rear face 121 of the support 115 (which corresponds to the rear face of the mechanical support layer 116) is positioned opposite a front face of the support 115 to which the mechanical resonator 101 is attached. Furthermore, the light source 120 is configured such that a power-modulated light beam intended to be emitted by the light source 120 is collimated and passes through an opening 122 that passes through the support 115 and opens opposite the first part 102 of the mechanical resonator 101. The modulation of the beam emitted by the light source 120 can be frequency-modulated or amplitude-modulated. This modulation can be internal to the light source 120 or performed by an external device, for example, a chopper. The modulation can take the form of a square, sinusoidal, or triangular signal. The effectiveness of the resulting photoacoustic effect depends, in particular, on the modulation depth applied.The light source 120 can be configured to emit one or more modulated light beams of different wavelengths, depending on the nature of the gas molecules to be detected by the device 100. The light source 120 is chosen and configured such that the wavelength(s) of the emitted light beam(s) correspond to those absorbed by the gas molecules to be detected. The wavelength(s) emitted by the light source 120 may be in the infrared range. To detect several types of gas, the device 100 may include several light sources 120 emitting light of different wavelengths.

[0056] In the device 100, the first part 102 of the mechanical resonator 101 is arranged in a first environment in which the acoustic waves, resulting from the interaction of the gas molecules to be analyzed with the light beam intended to be emitted by the light source 120, are intended to propagate.

[0057] Furthermore, the second part 104 of the mechanical resonator 101 is disposed in a second vacuum environment separate from the first environment. In the example shown in [Fig. 1], this second environment is delimited by a hood 124 and walls 126 arranged at the junctions between the first and second parts 102, 104 of the mechanical resonator. The walls 126 are in contact with the mechanical retaining layer 116 and are designed so that they do not form a short circuit between the mechanical resonator 101 and the mechanical retaining layer 116. The walls 126 can be electrically insulated from the mechanical resonator 101 and the mechanical retaining layer 116. The walls 126 can also include parts comprising metal and / or semiconductor material so that they form a Faraday cage.When the device 100 is made from an SOI substrate, the walls 126 can be formed by parts of the semiconductor surface layer and the BOX of the SOI substrate. In . Furthermore, as in the example of [Fig.1], the mechanical support layer 116 on which the cover 124 rests also helps to hermetically separate this second environment from the first environment.

[0058] The vacuum level of the second environment can be such that it avoids a viscous damping effect on the mechanical resonator 101. This vacuum level can depend in particular on the dimensions of the second part 104 of the mechanical resonator 101. It can be determined by the Knudsen number Kn defined by the following equation: with A corresponding to the free path of the gas molecules considered, and

[0059] Lc corresponding to the characteristic dimension of the mechanical resonator 101.

[0060] For a Knudsen number Kn > 10, the quality factor of the mechanical resonator 101 is given by the intrinsic-internal and intrinsic-external damping. In this case, viscous damping can be considered negligible. Thus, the smaller the dimensions of the second part 104 of the mechanical resonator 101, the less stringent the vacuum conditions to be met. For example, the vacuum level in this second environment can be less than 100 mbar when the widths of the second and fourth portions of material 109, 114 are less than 1 pm, or less than 6 mbar when the widths of the second and fourth portions of material 109, 114 are less than 10 pm.

[0061] Furthermore, it can be advantageous to have a large number of second portions of material 109, which makes it possible to increase the nominal capacitance formed by the second part 104 of the mechanical resonator 101 compared to the parasitic capacitance, while maintaining a Knudsen number Kn > 10. In addition, the lower the mass of the second part 104 of the mechanical resonator 101, the greater its mechanical susceptibility and the greater the sensitivity of the device 100.

[0062] The mechanical susceptibility of device 100 is defined by the following equation: Q with Q n the quality factor of the nth mode of vibration, Q n the resonance frequency of the nth mode, and m „ the effective mass which corresponds to the resonance frequency 42 „, and n greater than or equal to 1.

[0063] In the described embodiment, the anchoring portions 119 can also serve as electrical connections allowing the collection of electrical transduction signals obtained and / or the transmission of an electrical bias signal to the mechanical resonator 101.

[0064] The walls 126, the fifth portions of material 118 and the third portions of material 112 are such as the first and second parts 102, 104 of the resonator mechanical 101 are mobile relative to each other in order to allow energy transfer from the first portion of material 106 to the second parts 110 of the second portions of material 109. The parameters of the walls 126, the fifth portions of material 118 and the third portions of material 112 to be considered in order to allow this mobility are in particular the material(s) used for their manufacture as well as their geometric characteristics in order to have sufficient flexibility at the junctions of the first and second parts 102, 104 of the mechanical resonator 101, with low angular rigidity in order to have low energy losses during the movement of second portions of material 109.

[0065] In the embodiment shown in [Fig. 2], this mobility corresponds to a rotational movement due to a torsional deformation of the second parts 110 of the second portions of material 109 relative to the first parts 108 of the second portions of material 109 along axes parallel to the principal plane in which the mechanical resonator 101 is at rest (plane (X,Y) in Figures 1 and 2), at the junctions between the first and second parts 102, 104, i.e., in the elements 112, 118, and 126. In this embodiment, the junctions between the first and second parts 102, 104 of the mechanical resonator 101 form pivot joints between them. Furthermore, these junctions are such that they significantly limit or prevent a translational movement in a direction outside the intended plane. In the embodiment shown in [Fig.[2], the ends of the second parts 110 of the second portions of material 109 are configured to move in phase when set in motion by acoustic wave detection, which creates constructive interference.

[0066] Alternatively, other types of movements and / or links are possible between the first and second parts 102, 104 of the mechanical resonator 101.

[0067] In addition, the walls 126, the material portions 118, the anchoring portions 116 and the third material portions 112 are such that they ensure gas-tight insulation between the first and second parts 102, 104 of the mechanical resonator 101.

[0068] During a gas analysis performed by the device 100 described above, a modulated light beam is emitted by the light source 120 in the presence of the gas to be analyzed in the first environment. The gas molecules absorb the light, causing localized and modulated heating. An acoustic wave with the same modulation as the emitted light is then generated and propagates due to the periodic pressure changes caused by the temperature modulation. The portion of this acoustic wave propagating through the aperture 122 causes the first portion of material 106 to move. The energy associated with the movement of the first portion of material 106 is transmitted to the first and second parts 108, 110 of the second portions of material 109. The movement of the second parts 110 of the second portions of material 109 relative to the mechanical support layer 116 generates a variation, with the same modulation as that of the acoustic wave, in the electrical capacitance formed between these elements. This variation in electrical capacitance is detected by a detection circuit, not shown in [Fig. 1], and then analyzed by a processing circuit, also not shown in [Fig. 1]. From the detected frequency or frequencies and the amplitudes of the variation in electrical capacitance at this or these frequencies, the nature of the gas or gases that interacted with the emitted light can be identified and their concentration can be determined. This operation is similar to that of a condenser microphone.

[0069] In the example of [Fig.2], several dimensions are designated and examples of values ​​are given below for these dimensions: .

[0070] L1: Width of each group of second portions of material 109, for example equal to 400 pm;

[0071] L2: Length of a second part 110 of each second portion of material 109, for example equal to 375 pm;

[0072] L3: Dimension of one side of the mechanical resonator 101, for example equal to 1500 pm;

[0073] wl: width of each second portion of material 109, for example equal to 7 pm;

[0074] w2: width of each fourth portion of material 114, for example equal to 7 pm;

[0075] w3: width of each third portion of material 112 or width of each wall 126, for example equal to 10 pm, and which is greater than wl and w2;

[0076] hl: thickness of the different portions of material of the mechanical resonator 101, for example less than 40 pm, or preferably less than 20 pm and for example equal to 15 pm;

[0077] h2: height of the walls 126, for example equal to 200 pm;

[0078] h3: thickness of portions 118 greater than 0.5 pm and preferably greater than 2 pm.

[0079] Furthermore, the dimension of the sides of portions 118 and 119 can be equal to 80 pm.

[0080] The resonant frequency of a mechanical resonator 101 whose different material portions are made according to the dimensions indicated above can, for example, be equal to 22 kHz.

[0081] In order to maximize the displacement amplitude of the mechanical resonator 101 during gas detection, it can be designed so that its resonant frequency corresponds to the frequency corresponding to the thermal relaxation time of the target gas intended to be detected by the device 100. For example, the relaxation time The thermal conductivity of methane (CH4) diluted in nitrogen is 11 ps, which corresponds to obtaining maximum acoustic pressure at a frequency of approximately 20 kHz. In this case, the previously mentioned dimensional examples can be considered well-suited for the construction of a mechanical resonator 101 intended for the detection of such a gas.

[0082] Furthermore, the photoacoustic force, denoted F Pa, obtained is proportional to the surface area of ​​the first part 102 of the mechanical resonator 101 which is exposed to acoustic waves and can be approximated by the following equation: Pa ~ with Sio2 corresponding to the surface area of ​​the first part 102 of the mechanical resonator 101 which is exposed to acoustic waves, Ap corresponding to the pressure difference between the two faces of the membrane formed by the first portion of material 106, and <e>corresponding to the shape function which describes the deformation of the mechanical resonator 101.

[0083] The signal-to-noise ratio (SNR) of device 100, calculated taking into account only the thermal fluctuation, can be approximated by the following equation: with kb, T, Af corresponding respectively to the Boltzmann constant, the temperature, and the bandwidth of the frequencies detected by the device 100. To increase the SNR of the device 100, it is therefore advantageous to increase the surface area S102 of the first part 102 of the mechanical resonator 101, to decrease the resonance frequency Qn, and to reduce the effective mass m, B of the mechanical resonator 101. The reduction of the effective mass m of the mechanical resonator 101 can be achieved by thinning the different portions of material of the mechanical resonator 101 (with, for example, a thickness hl < 20 qm), and by placing the junctions between the first and second parts 102, 104 of the mechanical resonator 101 such that the largest displacement amplitudes are obtained at the ends of the second parts 110 of the second portions of material 109.

[0084] In the embodiment described above, the second portions of material 109 of the mechanical resonator 101 are mechanically coupled together, in each group of second portions of material 109, by a single fourth portion of material 114 connecting together the ends of the second parts 110 of the second portions of material.

[0085] In an alternative embodiment shown in [Fig. 3], the mechanical resonator 101 may comprise, for each group of second portions of material 109, several fourth portions 114 mechanically linking the second portions together portions of material 109. In the embodiment shown in [Fig.3], for each group of second portions of material 109, the fourth portions 114 are distributed over at least part of the length of the second portions of material 109. This variant of the embodiment has the advantage of increasing the rigidity of the second part 104 of the mechanical resonator 101 and also the nominal capacitance formed between the second part 104 of the mechanical resonator 101 and the mechanical retaining layer 116.

[0086] In a second embodiment of the device 100, the mechanical resonator 101 may be such that its second part 104 comprises a second ring-shaped portion of material 109 mechanically coupled to the first portion of material 106 (which forms, for example, a movable membrane, as in the first embodiment) and which is capable of moving under the effect of energy transmitted from the first portion of material 106 to the second part 104. In the example of [Fig. 4], the second portion of material 109 forms two concentric rings mechanically connected to each other by retaining arms. Furthermore, in this second embodiment, the walls 126 arranged at the junctions between the first and second parts 102, 104 of the mechanical resonator 101 also have a ring shape which, in this example, is concentric with those formed by the second portion of material 109.

[0087] The different elements of the mechanical resonator 101 can be such that the displacement amplitude of the first portion of material 106, VFI06, is very small compared to that of the second portions of material 109 VF w, i.e. such that VF106 « VF 109. For example, these displacement amplitudes can be such that W 106< 10. VF im, or preferably VF 106 < 100. VF 109, or preferably VF 106 < 100.VF / W.

[0088] In the various examples and embodiments, the dimensions of the second portion or portions of material 109 are such that its dimensions in the (X,Y) plane are much greater than its thickness (dimension along the Z axis) in order to obtain mobility during the movement of the first portion of material 106.

[0089] Mechanical resonators 101 of different configuration and / or shape than those previously described are possible.

[0090] In an alternative that can be applied to the various embodiments of the device 100, the device 100 may further comprise an acoustic resonator 130 attached to the rear face 121 of the support 115. The acoustic resonator 130 has an inlet 132 configured to receive the light beam(s) intended to be emitted by the light source 120 and at least one outlet 134 opening opposite the aperture 122. Figure 5 schematically shows a cross-sectional view of a portion of an example of an embodiment of the acoustic resonator 130, and [Fig.6] schematically represents an example of an embodiment of the device 100 comprising such an acoustic resonator 130.

[0091] In the example of [Fig. 5], the acoustic resonator 130 has a "T" shape. In such an acoustic resonator, a standing wave is created in the larger hollow portion of this resonator 130, which forms the inlet 132. The shorter portion, forming the outlet 134, is positioned opposite the opening 122. In the example of [Fig. 5], the larger hollow portion has an end opposite the inlet 132, which is also open.

[0092] A length L mR of the longer hollow part of the acoustic resonator 130 can be chosen so as to be in accord with the resonance frequency of the mechanical resonator 101. The radius R mR of the longer hollow part can be chosen according to the dimensions of the light beam(s) intended to be emitted by the light source 120. The length 10 of the shorter hollow part forming the outlet 134 is for example equal to 0.5 mm and its radius r 0 is for example equal to 0.25 mm.

[0093] The acoustic resonator 130 can be configured such that the effective length Leff of its longest hollow part is equal to: Tcc - -1 i rnR ] L^f = T~R-~Rtan with c corresponding to the speed of sound, fmR corresponding to the resonance frequency of the acoustic resonator 130, S and S s corresponding to the cross-sectional areas of the longer and shorter hollow parts respectively, and t corresponding to the effective length of the shorter hollow part.

[0094] The acoustic resonator 130 increases the photoacoustic force obtained by confining the acoustic waves within the resonator 130. Using such an acoustic resonator 130 allows for a reduction in the dimensions of the first part 102 of the mechanical resonator 101, thereby reducing the effective mass of the mechanical resonator 101 and, consequently, increasing the mechanical susceptibility of the mechanical resonator 101 as well as the sensitivity of the device 100. Furthermore, the acoustic resonator 130 allows the use of highly divergent laser beams and prevents thermal excitation of the mechanical resonator 101. To optimize the photoacoustic force, the acoustic resonator 130 can be designed so that its resonant frequency corresponds to that of the mechanical resonator 101.

[0095] An example of a method for making device 100 is described below in relation to figures 7 to 10.

[0096] In this example, the device 100 is made from an SOI substrate intended to be used for the realization of the mechanical resonator 101 and the support 115 of this mechanical resonator 101. As an example, the SOI substrate may comprise a surface layer 136 of silicon with a thickness of 250 pm, a BOX 138 of SiO2 with a thickness of 0.5 pm, and a silicon support layer with a thickness of several hundred microns.

[0097] Lithography and etching are carried out through the support layer of the SOI substrate intended to form the mechanical holding layer 116 of the device 100, thus forming the aperture 122 (see [Fig. 7]). For example, this etching may correspond to a wet etching that can be carried out with a KOH solution, or may correspond to a deep reactive ion etching, or DRIE (Deep Reactive Ion Etching).

[0098] Lithography and etching steps of the surface layer 136 are then implemented in order to preserve at least part of the surface layer 136 intended to be used for the realization of the mechanical resonator 101, the walls 126 and the anchoring portions 119 (see [Fig.8]).

[0099] A localized etching of the remaining portion of the surface layer is then carried out to form the various parts 102, 104 of the mechanical resonator 101, the walls 126 and the anchoring portions 119 (see [Fig. 9]). For example, this etching may correspond to DRIE etching.

[0100] An etching of a portion of the BOX 138 of the substrate is then carried out so that the remaining portions of the BOX 138 correspond in particular to the fifth portions of material 118 described previously, forming the bonds between the mechanical support layer 116 and the mechanical resonator 101 and the anchoring portions 119 (see [Fig. 10]). For example, this etching may correspond to a wet or vapor-phase etching that can be carried out with an HF solution.

[0101] The hood 124 is then made in a vacuum environment so that the second part 104 of the mechanical resonator 101 is disposed in a vacuum environment.

[0102] Vacuum-tight electrical connections can then be made between the rear face 121 of the support 115 and the anchoring portions 119 with the walls 126. These electrical connections of the device 100 can be made by localized etching and metallization. For example, it is possible to deposit a layer of SiO2 on the side of the support 115 where the mechanical resonator 101 is located, and then to lithograph and etch this SiO2 before manufacturing the cover 124. After the cover 124 is manufactured, localized etching can be performed at the desired contact locations, and then the contacts are completed by metallization in the desired locations. The support and the resonator (SOI handle) are both electrically connected. The system also functions if one of them is not electrically connected. If both connections are present, one electrode will be connected to the polarity and the other to ground. Furthermore, the 120 light source is electrically isolated from the sensor.

[0103] The light source 120 and optionally the acoustic resonator 130 can be placed at the rear face 121 of the support 115. The resulting device 100 can correspond to that shown in [Fig. 1].

[0104] More generally than the example embodiment described above, the device 100 can be made by implementing the following steps:

[0105] - realization of at least one mechanical resonator 101 having a first part 102 of acoustic wave detection and a second part 104 of transduction of the detected acoustic waves into at least one transduction signal, the first part 102 of the mechanical resonator 101 being disposed in a first environment in which the acoustic waves are intended to propagate;

[0106] - creation of a second vacuum environment, distinct from the first environment and in which the second part 104 of the mechanical resonator 101 is disposed.

[0107] The device 100 as described above forms a resonant sensor of the MEMS (Micro-Electro-Mechanical System) or NEMS (Nano-Electro-Mechanical System) type used in photoacoustic spectroscopy, particularly for gas spectrometry, with motion transduction under vacuum. The device 100 can, in particular, form a compact, integrated photoacoustic gas sensor, for example, using VLSI (Very Large Scale Integration) technology, in which the signal transduction, for example capacitive, is implemented under vacuum.

[0108] Because the second part 104 of the mechanical resonator 101 performs transduction in a vacuum environment, viscous damping is greatly reduced, or even eliminated, compared to transduction that is not performed under vacuum, and also thanks to the presence of the walls 126. This results in a high quality factor of the mechanical resonator 101, and therefore better detection sensitivity of the photoacoustic spectroscopy device 100. Indeed, the sensitivity of a photoacoustic sensor increases when its mechanical susceptibility and the photoacoustic force to which it is subjected increase. The vacuum transduction implemented in the device 100 is advantageous because it avoids viscous damping, which is a source of reduction in the quality factor and therefore in the mechanical susceptibility of the device 100.

[0109] Finally, an advantage provided by the disposition of the first part 102 of the mechanical resonator 101 with respect to the opening 122 through which the acoustic waves pass before reaching the first part 102 is a further reduction of the viscous damping of the mechanical resonator 101.

[0110] The device 100 can be capable of detecting low concentrations of gas, and can be insensitive to noise (thanks to the high quality factor obtained), compact and integrated.

[0111] In the examples and embodiments described above, the second part 104 of the mechanical resonator 101 is configured to perform capacitive transduction of the acoustic waves detected by the first part 102 of the mechanical resonator 101. Alternatively, the second part 104 of the mechanical resonator 101 could be configured to implement another type of transduction, for example, piezoresistive, piezoelectric, optomechanical, etc. The transduction signal delivered by the second part 104 of the mechanical resonator 101 could, for example, be an electrical or optical transduction signal. In this case, the mechanical resonator 101 can be provided with transduction elements adapted to implement such transduction.

[0112] Several examples of applications of device 100 are given below.

[0113] For example, in the industrial sector, device 100 can be used for monitoring toxic gas emissions, protecting workers from toxic gases, and detecting leaks in pipelines or in the petrochemical industry. Device 100 can, for example, be configured to detect at least one of the following gases: H2S, SO2, NO, HF.

[0114] According to another application example, the device 100 can be used for air quality monitoring, for example for monitoring greenhouse gas emissions and / or in the automotive sector. The device 100 can, for example, be configured to detect at least one of the following gases: CO2, CH4.

[0115] According to another application example, the device 100 can be used in the field of security and defense, for example for the monitoring and / or detection of explosive and / or toxic substances. The device 100 can, for example, be configured to detect at least one of the following gases: CH4, NO, Sarin, CO.

[0116] According to another application example, the device 100 can be used in the medical field, for example for the non-invasive detection of pathologies based on breath analysis. In this case, the device 100 can, for example, be configured to detect at least one of the following gases: C2H4 (in ppb), CH4 (in ppm), CO (in ppm), NO (in ppb), acetone (in ppb). According to another example, the device 100 can be used to detect markers of certain diseases such as COVID-19. In this In this case, device 100 can for example be configured to detect at least one of the following gases: O3, NH3.

[0117] According to another application example, the device 100 can be used in the field of life sciences, for example for the detection of microorganism incubators (detection of CO2), the analysis of solids (detection of CH4 and / or CO2), botany (detection of C2H4), the emission of gas in the natural environment (detection of CH4).

[0118] Other examples of application of device 100 are conceivable, such as the preservation of food and beverages and quality control (detection of C2H4), integration into mobile phones and other objects carried on the person for multiple purposes (for example for personal security purposes), etc.

[0119] Device 100 can also be configured to detect gases other than those previously mentioned.

[0120] Various embodiments and variations have been described. A person skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will become apparent to a person skilled in the art.

[0121] Finally, the practical implementation of the embodiments and variants described is within the reach of a person skilled in the art, based on the functional indications given above.< / e>

Claims

Demands

1. Photoacoustic spectroscopy device (100) comprising at least one mechanical resonator (101) having a first part (102) for detecting acoustic waves and a second part (104) for transducing the acoustic waves detected by the first part (102) of the mechanical resonator (101) into at least one transduction signal, the first part (102) of the mechanical resonator (101) being disposed in a first environment in which the acoustic waves are intended to propagate, and the second part (104) of the mechanical resonator (101) being disposed in a second vacuum environment separate from the first environment.

2. Photoacoustic spectroscopy device (100) according to claim 1, wherein: - the first part (102) of the mechanical resonator (101) comprises a first portion of material (106) configured to be set in motion by acoustic waves and a first part (108) of at least a second portion of material (109) of which at least a first end is mechanically coupled to the first portion of material (106), and - the second part (104) of the mechanical resonator (101) comprises a second part (110) of the second portion of material (109) of which at least a second end is capable of moving under the effect of energy transmitted from the first portion of material (106).

3. Photoacoustic spectroscopy device (100) according to claim 2, wherein the mechanical resonator (101) comprises several second portions of material (109) extending around the first portion of material (106).

4. Photoacoustic spectroscopy device (100) according to claim 3, wherein the mechanical resonator (101) comprises several groups of second portions of material (109) in each of which said second portions of material (109) are mechanically coupled to each other.

5. A photoacoustic spectroscopy device (100) according to claim 4, wherein, in each group, the second portions of material (109) are mechanically coupled to each other at least by a third portion of material (112) disposed at junctions of the first and second parts (108, 110) of each of the second portions of material (109).

6. Photoacoustic spectroscopy device (100) according to claim 5, wherein, in each group, the second parts (110) of the second portions of material (109) are mechanically coupled together by at least a fourth portion of material (114) disposed in the second vacuum environment.

7. Photoacoustic spectroscopy device (100) according to claim 6, wherein: - in each group, the second portions of material (109) form straight beams arranged parallel to each other; - the fourth portion of material (114) forms a longitudinal portion extending perpendicularly to the second portions of material (109).

8. Photoacoustic spectroscopy device (100) according to claim 1, wherein: - the first part (102) of the mechanical resonator (101) comprises a first portion of material (106) configured to be set in motion by acoustic waves, and - the second part (104) of the mechanical resonator (101) comprises at least a second portion of material (109) in the shape of a crown mechanically coupled to the first portion of material (106) and capable of moving under the effect of energy transmitted from the first portion of material (106).

9. Photoacoustic spectroscopy device (100) according to any one of the preceding claims, wherein the first and second parts (102, 104) of the mechanical resonator (101) correspond to parts of the same layer of material.

10. Photoacoustic spectroscopy device (100) according to any one of the preceding claims, wherein the second environment is delimited by at least one hood (124) and walls (126) arranged at at least one junction between the first and second parts (102, 104) of the mechanical resonator (101) and resting on a first face of a support (115).

11. A photoacoustic spectroscopy device (100) according to claim 10, wherein the support (115) comprises at least a layer of material (116, 118) and at least one opening (122) through said layer of material (116, 118) and opening opposite the first part (102) of the mechanical resonator (101).

12. Photoacoustic spectroscopy device (100) according to claim 11, further comprising at least one light source (120) attached to a second face (121), opposite the first face, of the support (115), and configured such that a modulated light beam intended to be emitted by the light source (120) passes in front of the opening (122).

13. Photoacoustic spectroscopy device (100) according to claim 12, further comprising an acoustic resonator (130) attached to the second face (121) of the support (115) and comprising at least one inlet (132) configured to receive the light beam and at least one outlet (134) opening opposite the opening (122) of the support (115).

14. Photoacoustic spectroscopy device (100) according to any one of the preceding claims, wherein the second part (104) of the mechanical resonator (101) is configured to perform capacitive transduction of the detected acoustic waves.

15. Method of making a photoacoustic spectroscopy device (100), comprising at least the following steps: - making at least one mechanical resonator (101) having a first part (102) for detecting acoustic waves and a second part (104) for transducing the detected acoustic waves into at least one transduction signal, the first part (102) of the mechanical resonator (101) being disposed in a first environment in which the acoustic waves are intended to propagate; - making a second vacuum environment, distinct from the first environment and in which the second part (104) of the mechanical resonator (101) is disposed.

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