Improved MEMS sensor

Abstract

The invention relates to a MEMS sensor (100) comprising: a MEMS (10) which is housed in a housing (15); a support structure (14); and, between the MEMS (1) and the support structure (14), a set of layers which are stacked on the support structure (14), the set of layers comprising: - a carrier element (12) to which the MEMS (1) is attached and which is arranged inside the housing (15) and forms a layer inserted between the MEMS (1) and the layer corresponding to the lower wall of the housing; - an electronic board substrate (13) which is arranged under the layer corresponding to the bottom wall of the housing (15) and is mounted on the support structure (14), the set of layers further comprising an impedance breaking layer (16) made of foam, the foam impedance breaking layer forming a filter which is suitable for decoupling the propagating waves that propagate towards the MEMS (1).

Description

DESCRIPTION Title of invention: Improved MEMS sensor Technical field

[0001] The invention relates generally to measurement systems, and in particular to MEMS type accelerometric sensors (microelectronic systems or “Micro Electro Mechanical Systems”).

[0002] Navigation systems require increasingly powerful sensors to fulfill their missions. This is particularly the case for MEMS-type accelerometric or gyrometric sensors (motion sensors). An accelerometric sensor provides a signal representing the acceleration of the object to which it is attached, while a gyrometric sensor provides angular velocity information relative to a MEMS-type inertial reference frame.

[0003] Such MEMS type sensors are particularly sensitive to external environments such as vibrations or thermal variations. It is therefore necessary to limit as much as possible the level of vibrations transmitted to the sensitive element (MEMS component) while preserving the behavior under thermal load of the sensor, on the other hand.

[0004] In particular, the behavior of MEMS tends to be disturbed by high frequencies well above one kilohertz. These high frequencies interfere with the MEMS's natural modes and modify the behavior of the sensor.

[0005] On the other hand, it is necessary to ensure identical thermomechanical behavior between the assembled parts of the sensor and therefore to match their thermal expansion coefficients in order to guarantee optimal operation of the sensor.

[0006] The sensitivity of MEMS to high frequency vibrations, particularly at frequencies above 5 KHz, can cause MEMS malfunctions, as shown for example in:

[0007] - “Substrate-decoupled, bulk-acoustic wave gyroscopes: Design and evaluation of next-generation environmentally robust devices”, Diego E. Serrano, Mohammad F. Zaman, Amir Rahafrooz, Peter Hrudey, Ron Lipka, Duane Younkin, Shin Nagpal, Ijaz Jafri and Farrokh Ayazi, Microsystems & Nanoengineering (2016) 2, 16015; doi:10.1038 / micronano.2016.15;

[0008] - “Design and Simulation of a New Decoupled Micromachined Gyroscope” Abdelhameed Sharaf, Shérif Sedky, SE -D Habib, Faculty of Engineering, Cairo University, - 12613, Giza, Egypt, Journal of Physics: Conference Series 34 (2006) 464-469 International MEMS Conference 2006;

[0009] - “Reducing Anchor Loss in Micromechanical Extensional Mode Resonators” Vahdettin Ta§, Selim Oleum, Student Member, IEEE, M. Deniz Aksoy, and Abdullah Atalar, Fellow, IEEE, MEMS Application of Porous Silicon - Wolfgang Benecke and Alexandra Splinter - Institute for MicroSensors, Actuators and Systems (IMSAS);

[0010] - Vibroacoustic Effects in MEMS - Roman Vinokur, Wieland Associates Inc., Laguna Hills, California Sound and Vibration - September 2003.

[0011] - “Design strategies for controlling damping in micromechanical and nanomechanical resonators” - Surabi Joshi, Sherman Hung and Srikar Vengallatore - EPJ Techniques and Instrumentation 2014.

[0012] A MEMS sensor is associated with a transmission / reflection coefficient. According to the principles of mechanical wave propagation, the transmission / reflection coefficient expresses the ability of the MEMS sensor to transmit mechanical waves. This coefficient mainly depends on the material properties and geometry of the MEMS sensor, i.e., what is called the mechanical impedance of the medium.

[0013] A known approach to solving the MEMS sensitivity problem is to minimize the transmission / reflection coefficient of the MEMS sensor so as to isolate the sensor from external mechanical environments. This isolation is also referred to as "impedance breakdown".

[0014] To achieve this isolation, known solutions use mechanical decoupling made from transfer materials and fasteners with very high mechanical impedances, close to silicon, which is the basic material of MEMS. In such solutions, the MEMS is arranged on a transfer structure.

[0015] For example, in application EP 1 340 220 B1, there is proposed a device for breaking the acoustic impedance of a rod formed in an elastic solid, on which an ultrasonic deformation wave can propagate, comprising a mass connected to the rod. The mass comprises a nut which is screwed onto a first threaded zone of the rod and a locknut which is screwed onto a second threaded zone of the rod. The nut and the locknut have an acoustic impedance much higher than the acoustic impedance of the rod, the tightening of the nut and the locknut being carried out until a deformation of the threads of the two threaded zones of the rod is obtained which is greater than the maximum deformation of the rod due to the propagation of the wave in the rod.

[0016] However, such solutions do not allow MEMS to be decoupled from solid-borne waves propagating through the structure on which they are attached.

[0017] Suspension-based solutions, such as elastomeric pads, have also been proposed. These suspensions effectively cut a frequency band above the suspensions' natural resonance frequency. However, they do not attenuate structure-borne propagation. While these suspensions protect the equipment and MEMS from mechanical vibrations, they do not protect the MEMS from vibrations propagating through the structure.

[0018] Thus, there is a need for an improved MEMS accelerometer sensor. General definition of invention

[0019] The embodiments of the invention thus make it possible to achieve a mechanical impedance break from foam materials, and in particular metal foams. These materials have the advantage of being particularly porous and therefore effectively limiting the transmission of mechanical waves on the one hand, and of obtaining a thermal expansion coefficient close to silicon and quartz, guaranteeing optimal thermomechanical operation of the sensor on the other hand.

[0020] In the field of inertial sensors, these materials have a significant impact on the performance of the sensor in harsh environments.

[0021] By integrating materials suitable for the transfer by bonding of the MEMS, the embodiments of the invention make it possible to prevent these high waves frequencies do not propagate to MEMS. They thus allow decoupling of propagation waves involving the mechanical impedances of the different materials, in particular with foam-based decoupling materials.

[0022] The embodiments of the invention further make it possible to guarantee thermomechanical matching between the assembled parts so as to preserve the alignment of the sensor and its fixing axis while guaranteeing the accuracy of the sensor under thermal loading. General definition of invention

[0023] The invention improves the situation by proposing a MEMS sensor comprising a MEMS housed in a housing, a support structure and, between the MEMS and the support structure, a set of layers stacked on the support structure, in a stacking direction (Y), the lower wall of the housing being generally planar and forming one of said layers. Advantageously, the set of layers comprises:

[0024] - a transfer element on which the MEMS is fixed and being arranged inside the housing, the transfer element forming a layer interposed between the MEMS and the layer corresponding to the lower wall of the housing;

[0025] - an electronic card substrate arranged under the layer corresponding to the lower wall of the housing and mounted on the support structure.

[0026] The layer assembly further comprises an impedance break layer made of foam, the foam impedance break layer forming a filter adapted to decouple the propagating waves propagating towards the MEMS.

[0027] The thickness d of the impedance breakdown layer can be chosen depending on the operating frequency of the MEMS.

[0028] The thickness d of the foam forming the impedance breakdown layer can be chosen so that the cutoff frequency of the filter formed by the impedance breakdown layer is equal to the cutoff frequency of the MEMS.

[0029] In one embodiment, the impedance break layer may be interposed between the package and the electronic board substrate.

[0030] The impedance break layer can be interposed between the electronic board substrate and the support structure.

[0031] The impedance break layer may correspond to the transfer element interposed between the MEMS and the bottom wall of the package.

[0032] In one embodiment, the impedance breakdown layer may be comprised of at least one metal foam material.

[0033] In one embodiment, the impedance breakdown performance achieved with the impedance breakdown layer may be a function of the thickness of the impedance breakdown layer and the surface area of ​​the impedance breakdown layer.

[0034] In one embodiment, the impedance break layer may be made of a single piece.

[0035] The impedance break layer may comprise at least two disjointed foam portions.

[0036] In one aspect, the impedance break layer may comprise two disjointed foam portions spaced apart from each other.

[0037] Alternatively, the impedance break layer may comprise at least three disjointed foam portions, the portions of the impedance break layer being regularly spaced at a selected pitch.

[0038] In one embodiment, the MEMS sensor may be of the accelerometer or gyrometer type.

[0039] In one embodiment, the MEMS may be made of silicon or quartz.

[0040] In one embodiment, the housing may be metallic or ceramic-based.

[0041] In one embodiment, the electronic board substrate may be ceramic or may be an electronic board made of FR4 material.

[0042] In one embodiment, the support structure may be at least partially made of aluminum.

[0043] In one embodiment, the transfer element may be made using a structural or epoxy type brazing or bonding.

[0044] In one embodiment, the MEMS attachment may be achieved by screws or by high Young's modulus epoxy-type structural bonding.

[0045] The embodiments of the invention thus make it possible to achieve a mechanical impedance break from foam materials, and in particular metal foams. These materials have the advantage of being particularly porous and therefore effectively limiting the transmission of mechanical waves on the one hand, and of obtaining a thermal expansion coefficient close to silicon and quartz, guaranteeing optimal thermomechanical operation of the sensor on the other hand.

[0046] In the field of inertial sensors, these materials have a significant impact on the performance of the sensor in harsh environments.

[0047] By integrating materials suitable for the transfer by bonding of the MEMS, the embodiments of the invention make it possible to prevent these high-frequency waves from propagating to the MEMS. They thus allow decoupling of the propagation waves involving the mechanical impedances of the different materials, in particular with foam-based decoupling materials.

[0048] The embodiments of the invention further make it possible to guarantee thermomechanical matching between the assembled parts so as to preserve the alignment of the sensor and its fixing axis while guaranteeing the accuracy of the sensor under thermal loading. Brief Description of Figures

[0049] Other characteristics, details and advantages of the invention will emerge from reading the description given with reference to the appended drawings given by way of example and which represent, respectively:

[0050] [Fig.1] Figure 1 represents a sectional view of a MEMS sensor, according to one embodiment of the invention.

[0051] [Fig.2] Figure 2 shows a sectional view of a MEMS sensor, according to another embodiment.

[0052] [Fig.3] Figure 3 represents a sectional view of a MEMS sensor, according to another embodiment.

[0053] [Fig. 4] Figure 4 illustrates the propagation of waves in the MEMS sensor, according to embodiments of the invention.

[0054] [Fig.5] Figure 5 is a comparative table of the properties of foam materials versus materials used in conventional sensors.

[0055] [Fig.6] Figure 6 is a diagram showing the evolution of the transmission coefficient and the reflection coefficient as a function of the impedance ratio, for the different types of materials in Figure 5.

[0056] [Fig.7] Figure 7 illustrates the propagation of a wave in two media of identical sections.

[0057] [Fig.8] Figure 8 illustrates the propagation of a wave in two media of different sections.

[0058] [Fig.9] Figure 9 is a diagram representing the evolution of the reflection coefficient R and the transmission coefficient as a function of the impedance ratio.

[0059] [Fig.10] Figure 10 represents the theoretical model of impedance breakdown of acoustic media.

[0060] [Fig.11] Figure 11 represents characteristics of different metal foams that can be taken into account to define the constitution of the impedance breakdown layer.

[0061] [Fig .12] Figure 12 is a table showing examples of foams that can be used to make the impedance break layer.

[0062] [Fig.13] Figure 13 is a diagram illustrating the evolution of the transmission coefficient and the reflection coefficient with respect to the impedance ratio, in the case where the impedance breaking material is made using one of the metal foams. Detailed description of the request

[0063] Figure 1 shows a sectional view of a MEMS sensor according to embodiments of the invention.

[0064] The MEMS 100 sensor can be of the accelerometer or gyrometer type.

[0065] The MEMs sensor 100 comprises a MEMS 11 housed in a housing 15, a fixed rigid structure 14 (hereinafter also called “support structure”) serving as a support for the MEMS sensor 100, and, between the MEMS and the support structure 14, a set of layers stacked in a stacking direction Y. The positioning of the elements of the MEMS sensor 11 will be defined hereinafter in an orthogonal reference frame (X, Y, Z), the plane (XZ) corresponding to the plane of the layers and of the MEMS. Figure 1 is thus represented according to a sectional view along the plane (XY).

[0066] The housing 15 is made up of walls comprising a flat lower wall, located below the MEMS 11 and extending in the XZ plane which forms a layer of the MEMS sensor interposed between the other layers of the MEMS sensor. The housing 15 may for example have the shape of a rectangular parallelepiped.

[0067] The set of layers comprises a transfer element 12 on which the MEMS 11 is fixed and which is arranged inside the housing 15. The set of layers further comprises an electronic card substrate 13 on which the housing 15 comprising the assembly consisting of the transfer element 12 and the MEMS 11 is arranged. The electronic card substrate 13 is itself mounted on the support structure 14.

[0068] The MEMS 11 can be made of silicon or quartz, for example.

[0069] The housing 15 may, for example, be metallic or ceramic-based.

[0070] In embodiments, the electronic board substrate 13 may be a ceramic substrate or an electronic board made of FR4 material (FR4 material refers to a standard defined by the National Electrical Manufacturers Association (NEMA) for a fiberglass-reinforced epoxy resin composite). Such a material complies with the UL94V-0 standard for the flammability of plastic materials and thus ensures that fire does not propagate and is quickly extinguished when the material is ignited.

[0071] The rigid support structure 14 constitutes the structure on which the sensor is fixed. The support structure 14 may be at least partly made of aluminum. In particular, it may be mainly made of aluminum.

[0072] In one embodiment, the transfer element 12 may be made of a rigid material such as, for example and without limitation, a solder, a structural type bonding with a rigid epoxy type glue.

[0073] The MEMS 11 can be fixed by screws or by structural bonding of the epoxy type with high Young's modulus.

[0074] According to the embodiments of the invention, all of the layers of the MEMS sensor 100 located between the MEMS 11 and the rigid support structure 14 comprise an impedance breaking layer 16 made of a foam impedance breaking material. The foam impedance breaking layer 16 makes it possible to produce a filter adapted to decouple the propagation waves which propagate towards the MEMS 11 and thus isolate it from external mechanical environments.

[0075] The impedance break is a function of the frequency of the vibration. The impedance break corresponds to a filter in that it defines a cutoff frequency below which the vibrations are transmitted and above which the vibrations are attenuated.

[0076] The MEMS sensor 100 according to the embodiments of the invention makes it possible, in addition to guaranteeing the decoupling of the propagation waves, to guarantee identical thermomechanical behavior between the assembled parts of the sensor 100 and therefore to match their thermal expansion coefficients in order to guarantee optimal operation of the sensor 100. The thermal expansion coefficients are said to be matched when they are equal. This means that the assembled materials have the same expansions. Thus, there are no relative deformations between the assembled parts and therefore an absence of stresses in the materials.

[0077] In the embodiment of FIG. 1, the impedance breaking layer 16 is interposed between the housing 15 and the electronic card substrate 13.

[0078] The foam impedance breaking layer interposed between the housing 15 and the electronic card substrate 13 makes it possible to obtain a transfer part made of materials having a high impedance ratio, and thus to obtain decoupling.

[0079] In an alternative embodiment shown in FIG. 2, the impedance breaking layer 16 can be interposed between the electronic card substrate 13 and the rigid support structure 14.

[0080] Alternatively, as illustrated in FIG. 3, the impedance breaking layer 16 may form the transfer element 12 on which the MEMS 11 is disposed, and thus be interposed between the MEMS 11 and the housing 13.

[0081] In one embodiment, the foam constituting the impedance break layer 16 is advantageously metallic. The use of a metallic foam makes it possible to use a low-density material to achieve the impedance break, having a Thermal Coefficient of Expansion (CTE) compatible with the MEMS material (silicon or quartz), which makes it possible to obtain thermomechanical coupling. As a result, the propagation speed of the wave is very low compared to the MEMS material so that the wave propagates more slowly in the foam.

[0082] Advantageously, the impedance breakdown performance obtained with the impedance breakdown layer 16 according to the embodiments of the invention is a function of the thickness of the impedance breakdown layer 16 and the surface area of ​​the impedance breakdown layer 16.

[0083] The performance of an impedance break is defined as the level of attenuation between the input and output vibrations of the dampers formed by the foam. This is therefore the modulus of the impedance break (usually expressed in dB). The higher this modulus, the more efficient the impedance break.

[0084] In one embodiment, the impedance break layer 16 may be made in one piece, without interruption, as in the embodiments of Figures 1 and 3.

[0085] Alternatively, the impedance break layer 16 may comprise at least two disjointed foam portions P1 and P2, as in the embodiment of FIG. 2. Thus, the impedance break layer 16 does not comprise any material between the different disjointed foam portions.

[0086] In the embodiment where the impedance break layer 16 comprises two disjointed portions P1 and P2, as illustrated in FIG. 2, the two portions P1 and P2 are arranged so that they are as far apart as possible from each other, and therefore arranged at the opposite ends of the impedance break layer. Thus, considering that L denotes the total width of the impedance break layer 16, l the width of the first portion P1 of the impedance break layer 16 and l2 the width of the second portion P2 of the impedance break layer 16, the spacing e between the two portions P1 and P2 is defined by: e = L -(l^ + Z2)).

[0087] Widths L, Z 1; l2 and spacing e are defined along the X axis which is perpendicular to the stacking direction Y and the plane (XZ) of the layers.

[0088] The smaller the widths l and l2 are in terms of dimensions, the better the impedance breakdown. Furthermore, the smaller the exchange surface between the sensor's sensitive element and its base, the more difficult it is for a sound wave to disturb the sensor / sensitive element.

[0089] In the embodiment where the impedance break layer 16 comprises more than two disjointed portions, the portions of the impedance break layer 16 may be regularly spaced at a chosen pitch, this pitch being able to be fixed or variable.

[0090] The portions of the impedance breakdown layer 16 may have similar or different dimensions.

[0091] The use of an impedance breaking layer 16 consisting of a plurality of disjointed and spaced parts makes it possible to complicate the wave path and obtain more reverberation.

[0092] The foam impedance break layer 16 is characterized by a total section having an area S r (or section), in the plane (XZ), and by a thickness d defined in the direction of the Y axis.

[0093] The impedance break layer 16 can be attached to the other layers by gluing or soldering.

[0094] The MEMS sensor 100 has a sustained movement at a certain frequency called the "operating frequency". This is specific to the MEMS detector. The filter cuts at a frequency called the "cutoff frequency". This is specific to the dampers (formed by the foam). In absolute terms, these frequencies are distinct. The MEMS sensor can be advantageously configured so that these frequencies are equal in value so as to attenuate the acoustic waves at this frequency as much as possible.

[0095] Advantageously, the thickness d of the dampers (foam) forming the impedance break layer 16 can be chosen as a function of the frequency of interest (or operating frequency) of the MEMS 11. In particular, the cut-off frequency of the filter formed by the foam impedance break layer 16 can be chosen equal to the operating frequency of the MEMS 11. In other words, the attenuation of the filter is at its maximum at the frequency of interest of the MEMS sensor 100 for maximum isolation.

[0096] Furthermore, the surface area of ​​the impedance break layer 16 can be chosen so as to obtain the greatest possible impedance, which can be achieved by maximizing the difference in areas (or 'sections') between the area S rof the impedance break layer 16 and the area of ​​each of the surfaces of the two layers adjacent to the impedance break layer 16 (arranged on either side of the impedance break layer 16, along the Y direction).

[0097] Thus in the embodiment of Figure 2, the surface area of ​​the impedance break layer 16 can be determined so as to maximize the difference in areas:

[0098] - between area S r of the impedance break layer 16 and the area of ​​the substrate of the electronic card 13, arranged directly above the impedance break layer 16, on the one hand, and

[0099] - between area S r of the impedance break layer 16 and the area of ​​the rigid support structure 14, arranged directly below the impedance break layer 16, on the other hand.

[0100] As used herein, the terms "upper" and "lower" refer to the positioning of one element of the MEMS sensor 100 relative to another along the stack Y-axis.

[0101] Figure 4 illustrates the wave propagation path in the embodiment of Figure 3 where the impedance breaking layer 16 is arranged inside the housing 15 between the MEMS 11 and the lower part of the housing 15, and therefore with decoupling at the level of the MEMS 11. Curve 17 represents an external vibration which, by mechanical impedance couplings, will transmit the disturbing vibrations.

[0102] In the solutions of the state of the art, the fundamental vibration of the MEMS 11 can be disturbed by such an external vibration. Advantageously, the foam impedance breaking layer 16 of the MEMS sensor 100 according to the embodiments of the invention forms a filter which makes it possible to decouple the MEMS 11 solid waves which propagate through the structure on which they are transferred.

[0103] The impedance break layer 16 thus constitutes a barrier that the propagation waves can hardly cross or, if they propagate, they will be very strongly attenuated.

[0104] Figure 5 is a table that compares the properties of foam materials used to make the impedance breaking material in the sensor 100 according to the embodiments of the invention (line bearing the reference 3) with other conventional transfer and fixing materials used in MEMS sensors of the state of the art (lines referenced by the numbers 1, 2, and 4). The table shows in particular that the conventional materials have very high mechanical impedances close to silicon which is generally the base material of MEMS 1 1 , while the foam materials have lower mechanical impedances.

[0105] Figure 6 is a diagram showing the evolution of the reflection and transmission coefficients as a function of the impedance ratio a = 2 / ZI, for an acoustic wave which passes through two media 1 and 2, separated by an interface, the first medium 1 having an impedance Z1 and the second medium having an impedance Z2, in an embodiment using foam impedance breaking materials, compared to that of the conventional transfer and fixing materials of the state of the art of figure 5, considering the example of a MEMS made of silicon.

[0106] Thus, the embodiments of the invention provide a MEMS sensor 11 in which is interposed a layer 16 made of a suitable foam material, having a high mechanical impedance break and acoustic decoupling which opposes the propagation of waves towards the MEMS 11, while respecting the thermomechanical coupling, that is to say while limiting differential expansions.

[0107] In order for the impedance break layer 16 to constitute a barrier which can only be crossed with difficulty by the propagation waves or to attenuate them if they manage to propagate, the impedance break layer 16 according to the embodiments of the invention can be advantageously configured to have a high impedance break.

[0108] The inventors have found that it is possible to adapt principles relating to wave propagation in the field of acoustics to the solid-borne vibration waves which propagate in the MEMS sensor 100, to determine optimization parameters of the impedance break layer 16.

[0109] In the acoustic domain, the acoustic impedance Z of a medium for an acoustic wave is given by the following relation (E1):

[0110] Z = pc (E1)

[0111] In equation (E1), Z is defined in Rayleigh, c denotes the longitudinal speed of the acoustic wave in the medium (expressed in m / s) and p denotes the density of the medium (expressed in Kg / m3).

[0112] When the medium considered is an acoustic component, the acoustic impedance is measured at the input of the component.

[0113] Two successive layers separated by a common interface can be modeled as a conduit consisting of two acoustic layers of respective sections S1 and S2, with the same Y axis and separated by the interface / in the XZ plane. The conduit impedance Zc of a layer is defined from the impedance of the medium Z and the surface of the cross-section S (or area) of the layer by:

[0114] Zci = Zi / S f (E2)

[0115] Figure 7 illustrates the concept of acoustic impedance break, for an acoustic wave which crosses two media 1 and 2, separated by an interface / of section S. In the example of figure 7, an acoustic wave is considered which propagates from medium 1 to medium 2 and that the two media 1 and 2 correspond to acoustic components having identical sections SI and S2 in the XZ plane (in the plane of the interface / ) (SI = SI = S) for simplification.

[0116] The driving impedance Z C1 characteristic of medium 1 is then defined by:

[0117] Z cl = ^ (E3)

[0118] In equation (E3), denotes the longitudinal speed of the acoustic wave in the medium (expressed in m / s) and p denotes the density of the medium (expressed in Kg / m3).

[0119] Similarly, the driving impedance Z c2 for the medium 2 for an acoustic wave which propagates there is defined by:

[0120] Z C2 = ^ (E4)

[0121] In equation (E4), c2 denotes the longitudinal speed of the acoustic wave in the medium (expressed in m / s) and p2 denotes the density of the medium (expressed in Kg / m3).

[0122] The conduction impedance Zc advantageously allows the integration of the notion of exchange surface and therefore the geometry of the design, which is not possible with the sole consideration of the impedance of the medium which omits the impact of the exchange surface between the waves.

[0123] As illustrated in Figure 7, when an acoustic wave encounters the interface / separating the two media 1 and 2 with acoustic impedances Z C1 and Z c2different, a part of the acoustic wave is transmitted, another part of the acoustic wave is reflected and the last part of the acoustic wave is absorbed. These three phenomena are characterized respectively by an energy reflection coefficient (R) corresponding to the reflected part of the wave, an energy transmission coefficient (T) corresponding to the transmitted part of the wave and an absorption coefficient (4) corresponding to the absorbed part of the wave. The presence of the absorption coefficient A creates a break in acoustic impedance at the interface between the two media 1 and 2.

[0124] According to the principle of conservation of energy, there is a relationship between the energy reflection coefficient R, the energy transmission coefficient T and the absorption coefficient A defined by:

[0125] / ? + T + 4 = 1 (E5)

[0126] In the example of Figure 7, p tdenotes the incident power, p t the transmitted power and p r reflected power.

[0127] The acoustic impedance of a medium depends on the properties of the materials of the medium but also on the section of the medium considered (in the example of the Figure 7, the two media 1 and 2 have an equal section S), it is also possible to achieve an acoustic impedance break by varying the sections S1 and S2 of the two media 1 and 2 respectively, through which the waves propagate and / or by varying the properties of the respective materials constituting the two media 1 and 2.

[0128] In the configuration example of Figure 8, the acoustic impedance break is achieved not only by varying the sections S1 and S2 of media 1 and 2, but also by varying the material properties of both media 1 and 2.

[0129] The MEMS sensor 100 according to the embodiments of the invention is subject to wave propagation phenomena in solid layers separated by interfaces.

[0130] However, in the field of structure-borne acoustics, absorption is very negligible compared to reflection and transmission. Equation (E4) for the conservation of energy can therefore be reformulated as follows:

[0131] / ? + T = 1 (E6)

[0132] The energy reflection coefficient (R) between two media 1 and 2 having respective impedances Z1 and Z2 is defined by: [° 133 ]« =ÏH5 ( E7 >

[0134] The reflection coefficient can also be expressed in terms of amplitudes according to the following equation: [° 135 i r = IHI < E8 >

[0136] In this case, the energy transmission coefficient (T) can be defined by equation (E9):

[0138] Figure 9 is a diagram representing the evolution of the reflection coefficient R and the transmission coefficient T as a function of the ratio a = — Zi between the impedances Z and Z2 of two different media 1 and 2 crossed by a wave.

[0139] As illustrated by the diagram in Figure 9, the transmission is very small when Z » Z2 (i.e. a « 1) and when Z2 » (i.e. a » 1), that is to say when the impedances Z and Z2 of the two media are very far from each other.

[0140] Figure 10 illustrates the theoretical model of impedance breakdown of acoustic media developed by LM Brekhovskikh in the acoustic field (Springer Series on Wave Phenomena LM Brekhovskikh OAGodin Acoustics Of Layered Media I Plane and Quasi-Plane Waves). This model allows to calculate, in the case of a plane sound wave at normal incidence, the reflection and transmission coefficients of a stack of several layers i, with i between 1 and n, of different thickness and / or mechanical characteristics. Thus, layer i has a thickness in the direction of the stacking axis Y and an input acoustic impedance Z^.

[0141] The wave number of a medium is defined by equation (E10): denotes the wavelength in the medium i and c t denotes the longitudinal velocity of the medium i.

[0144] According to such an acoustic model, for each layer i, the input impedance Z in(l) is defined as a function of the impedance of the medium Z ( -, of the input impedance Z in of the layer, of the wave number k t of the medium formed by the layer and the thickness of the layer according to equation E11:

[0146] By transposing this model to solid vibration waves (vibration waves in a solid) which propagate in the MEMS sensor 100, according to which each layer i is also associated with a conduction impedance Zci i; defined from the impedance of the medium Zi and the surface area of ​​the cross-section S t (or area) of layer i according to equation (E2), the conduction impedance Zci is defined by:

[0147] Zci = Zi / St (E12)

[0148] Equation (E12) can be reformulated using the definition of the medium impedance Zi according to equation (E13): [OUOl Zci ^ (E13)

[0150] By adapting equation (E11), for each layer i (among successive layers 11 to 16), the input impedance Z in w is defined as a function of the impedance of the medium Z t , of the input impedance Z in of the layer, of the wave number k t of the medium formed by the layer and the thickness of the layer d according to the following equation (E14):

[0152] Thus, by configuring the impedance break layer 16 and the two adjacent layers on either side of the layer 16 of the MEMS sensor so that they have sections (or areas) S t different, it is possible to obtain different impedance ratios (and therefore a very large reflection coefficient R or a very small coefficient T as illustrated in figure 9).

[0153] It is then possible to iteratively calculate the reflection coefficient according to the following equation (E15):

[0155] In the acoustic domain, the transmission losses TL in dB are linked to the reflection coefficient R by the following equation (E16):

[0156] TL = 10 log g) = 10 log (^) (E16)

[0157] Thus, in embodiments, a simulation of this theoretical model can be implemented using a suitable simulation tool (such as Matlab for example) to calculate the acoustic transmission T between the first layer 14 and the last layer 11 of the MEMS sensor 100 by means of the equations E10 to E16 by defining the thickness the celerity c f , the density p t of each layer i of the MEMS sensor and the section S t of equation E13.

[0158] The input data are the characteristics of the materials including the speed c t , the density p t , as well as the geometric parameters of the design including the thickness and the section S t .

[0159] The model simulation provides TL as a function of frequency. The simulation is implemented in such a way as to search for the best compromises of input data sets that allow TL to be maximized at the sensor frequency of interest (objective in terms of impedance break performance and frequency placement) while guaranteeing dimensions (d f and S tas small as possible for dampers) and limiting the sensor footprint. The simulation can be implemented for materials with thermomechanical properties close to those of silicon and quartz. Several simulations can be carried out to optimize the result. The optimal compromise is advantageously obtained for foams of the metal foam type. Metal foams allow in particular an adjustment of the thermal expansion coefficients between the silicon MEMS and the substrate, in addition to their impedance breaking function.

[0160] In embodiments, the impedance break layer 16 may be made of low density foam, such foams providing acoustic decoupling from the MEMS 11.

[0161] In one embodiment, the impedance break layer 16 may be made of foam(s) based on polymeric materials such as epoxy, polyurethanes, plastics or other rubbers. However, these types of foams, although operational, may not be very stable in temperature and may have deviations in expansion coefficients compared to Silicon (which may be the material of the MEMS 11) and bring mechanical stresses to the MEMS 11.

[0162] Alternatively, the impedance break layer 16 may be made of foam(s) with a metallic or mineral base. Such foams have the advantage of being more stable in temperature and of not exhibiting differences in expansion coefficients compared to Silicon and therefore of not generating mechanical stresses on the MEMS 11. Examples of metal-based foams include aluminum or steel foams. The density of such foams can be as low as 20 kg / m3 for nickel foams.

[0163] The foams constituting the impedance break layer 16 can also be made with a well-defined shape using an additive manufacturing process to make them.

[0164] The foams used to produce the impedance break layer 16 can be chosen according to the specific properties of the MEMS 11, taking into account the characteristics of the foams. Figure 11 gives for example the characteristics of different metal foams which can be taken into account to define the constitution of the impedance break layer 16.

[0165] Metal foams not only have the advantage of allowing acoustic decoupling, but they can also be used to absorb high shocks that the MEMS 100 sensor may experience.

[0166] Mechanical impedances can be calculated from the following relations (E17) to (E21):

[0171] Z = pc, (E21)

[0172] In equations E17, E18, E19, E20 and E21:

[0173] - E denotes Young's modulus;

[0174] - K denotes the flexural modulus;

[0175] - M denotes the plate module;

[0176] - C denotes the speed of the acoustic wave; and

[0177] - p denotes the density of the medium.

[0178] Considering an example of using an aluminum foam to constitute the impedance break layer having a density equal to p=50 kg / m 3 with p s =2500 Kg / m 3 summer s =70 MPa, the longitudinal speed obtained is equal to Ci= 1180 m / s and the Young's modulus is equal to E=43.4 MPa.

[0179] Considering another example in which the foam used to constitute the impedance breaking layer 16 is a Nickel foam of the INCOFOAM type having a density equal to p=180 kg / m 3 , the following values ​​are obtained: E=120 MPa, K=150 MPa, Ci= 1080 m / s, Poisson's ratio: 0.32 and CTE=13 ppm / °C.

[0180] In another embodiment, the foam used to constitute the impedance breaking layer 16 may be cellular glass.

[0181] In the article "Effects of porosity on seismic velocities, elastic moduli and Poisson's ratios of solid materials and rocks" by Chengbo Yu, Shaocheng Ji, Qi Li, September 2015, Journal of Rock Mechanics and Geotechnical Engineering, the mechanical properties, Young's modulus, longitudinal velocity and transverse velocity of different rocks with different porosities are given which can be used to define characteristics of such a porous glass.

[0182] The table in Figure 12 indicates examples of foams having advantageous properties for constituting the impedance breakdown layer 16, by way of non-limiting example. Those skilled in the art will easily understand that other foams having similar properties or meeting the specific constraints of the MEMS 11 can also be used.

[0183] The diagram in Figure 13 illustrates the evolution of the transmission coefficient and the reflection coefficient with respect to the impedance ratio, in the case where the impedance breaking material 16 of the MEMS sensor according to the embodiments of the invention is produced using one of the metal foams in the table in Figure 12 (Z2 designates the impedance of the impedance breaking layer and Z1 the impedance of the previous layer in the Y direction).

[0184] The embodiments of the invention thus make it possible to provide a MEMS sensor 100 of compact size, without disturbing the proper functioning of the MEMS 11. In particular, they make it possible to preserve its alignment and positioning, with very low thermal drift and therefore very few reported constraints.

[0185] Those skilled in the art will readily understand that the invention is not limited to the embodiments described above as a non-limiting example. It encompasses all variant embodiments that may be envisaged by those skilled in the art.

Claims

CLAIMS 1. MEMS sensor (100) comprising a MEMS (11) housed in a housing (15), a support structure (14) and, between the MEMS (11) and said support structure (14), a set of layers stacked on the support structure (14), in a stacking direction (T), the lower wall of the housing being generally planar and forming one of said layers, characterized in that the set of layers comprises: - a transfer element (12) on which the MEMS (1) is fixed and being arranged inside the housing (15), the transfer element forming a layer interposed between the MEMS (1) and the layer corresponding to the lower wall of the housing; - an electronic card substrate (13) arranged under the layer corresponding to the lower wall of the housing (15) and mounted on the support structure (14), and in that the set of layers further comprises an impedance break layer (16) made of foam, the foam impedance break layer forming a filter adapted to decouple the propagation waves which propagate towards the MEMS (11), the surface of the impedance break layer (16) being chosen so as to maximize the difference in areas between the area of ​​the impedance break layer (16) and the area of ​​each of the surfaces of the two layers adjacent to the impedance break layer (16), arranged on either side of the impedance break layer (16), according to the stacking direction.

2. MEMS sensor (100) according to claim 1, wherein the thickness d of the impedance breakdown layer (16) is chosen as a function of the operating frequency of the MEMS (11).

3. MEMS sensor (100) according to claim 2, wherein the thickness d of the foam forming the impedance breaking layer (16) is chosen so that the cut-off frequency of the filter formed by the impedance breaking layer (16) is equal to the cut-off frequency of the MEMS (11).

4. MEMS sensor (100) according to one of the preceding claims, wherein the impedance breaking layer (16) is interposed between the housing (15) and the electronic card substrate (13).

5. MEMS sensor (100) according to one of claims 1 to 3, wherein the impedance breaking layer (16) is interposed between the electronic card substrate (13) and the support structure (14).

6. MEMS sensor (100) according to one of claims 1 to 3, in which the impedance breaking layer (16) corresponds to the transfer element (12) interposed between the MEMS (11) and the lower wall of the housing (15).

7. MEMS sensor (100) according to one of the preceding claims, wherein the impedance breaking layer (16) is made of at least one metal foam material.

8. MEMS sensor (100) according to one of the preceding claims, wherein the impedance breakdown performance obtained with the impedance breakdown layer (16) is a function of the thickness of the impedance breakdown layer (16) and the surface area of ​​the impedance breakdown layer (16).

9. MEMS sensor (100) according to one of the preceding claims, wherein the impedance breaking layer (16) is made of a single piece.

10. MEMS sensor (100) according to one of claims 1 to 8, wherein the impedance breaking layer (16) comprises at least two separate foam portions.

11. The MEMS sensor (100) of claim 10, wherein the impedance breaking layer (16) comprises two disjointed foam portions spaced apart from each other.

12. The MEMS sensor (100) of claim 10, wherein the impedance break layer (16) comprises at least three disjointed foam portions, the portions of the impedance break layer (16) being regularly spaced at a selected pitch.

13. MEMS sensor (100) according to one of the preceding claims, in which the MEMS sensor is of the accelerometer or gyrometer type.

14. MEMS sensor (100) according to one of the preceding claims, wherein the MEMS (11) is made of silicon or quartz.

15. MEMS sensor (100) according to one of the preceding claims, wherein the housing (15) is metallic or ceramic-based.

16. MEMS sensor (100) according to one of the preceding claims, in which the electronic card substrate (13) is made of ceramic or is an electronic card made of FR4 material.

17. MEMS sensor (100) according to one of the preceding claims, wherein the support structure (14) is at least partly made of aluminum.

18. MEMS sensor (100) according to one of the preceding claims, in which the transfer element (12) is produced using a structural or epoxy type solder or bonding.

19. MEMS sensor (100) according to one of the preceding claims, in which the fixing of the MEMS (11) is carried out by screw or by structural bonding of the epoxy type with high Young's modulus.

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