Improved MEMS sensor

Abstract

It is proposed a MEMS sensor (100) comprising a MEMS (10) housed in a housing (15), a support structure (14) and a set of layers stacked on the support structure (14) between the MEMS (1) and the support structure (14), the set of layers comprising:-a transfer element (12) on which the MEMS (1) is attached, the transfer element being arranged inside the housing (15) and forming a layer interposed between the MEMS (1) and a layer corresponding to a lower wall of the housing; -an electronic board substrate (13) arranged below the layer corresponding to the lower wall of the housing (15) and mounted on the support structure (14), the set of layers further comprising an impedance mismatch layer (16) made of foam forming a filter adapted to decouple the propagating waves propagating towards the MEMS (1).

Description

Technical Field

[0001] This invention relates generally to measurement systems, and more particularly to MEMS (microelectromechanical systems) accelerometer sensors. Background Technology

[0002] Navigation systems require increasingly high-performance sensors to perform their tasks. This is especially true for MEMS accelerometers or gyroscopes (motion sensors). Accelerometers provide signals representing the acceleration of the object to which they are attached, while gyroscopes provide information about the angular velocity of the MEMS inertial reference frame.

[0003] These MEMS sensors are particularly sensitive to external environmental factors, such as vibration or thermal changes. Therefore, it is necessary to limit the level of vibration transmitted to the sensitive element (MEMS component) as much as possible while maintaining the sensor's behavior under thermal loads.

[0004] In particular, the behavior of MEMS is susceptible to high-frequency perturbations far above 1 kHz. These high-frequency perturbations can disrupt the normal patterns of MEMS and alter the behavior of the sensor.

[0005] On the other hand, it is necessary to ensure that the assembled parts of the sensor have the same thermomechanical behavior and therefore their coefficients of thermal expansion need to be matched to ensure the optimal operation of the sensor.

[0006] The sensitivity of MEMS to high-frequency vibrations, especially those above 5 kHz, can lead to MEMS failure, as illustrated in the following literature: - "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, DuaneYounkin, Shin Nagpal, Ijaz Jafri and Farrokh Ayazi, Microsystems&Nanoengineering (2016) 2, 16015; doi:10.1038 / micronano.2016.15; - “Design and Simulation of a New Decoupled Micromachined Gyroscope”,Abdelhameed Sharaf, Sherif Sedky, S E -D Habib, Faculty of Engineering, CairoUniversity, - 12613, Giza, Egypt, Journal of Physics: Conference Series 34(2006) 464--469 International MEMS Conference 2006; - “Reducing Anchor Loss in Micromechanical Extensional ModeResonators”,Vahdettin Taş, Selim Olcum, 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); - “Vibroacoustic Effects in MEMS”,Roman Vinokur, Wieland AssociatesInc., Laguna Hills, California Sound and Vibration - September 2003。

[0007] - “Design strategies for controlling damping in micromechanical andnanomechanical resonators”,Surabi Joshi, Sherman Hung and Srikar Vengallatore-- EPJ Techniques and Instrumentation 2014。

[0008] MEMS sensors are associated with the transmission / reflection coefficient. Based on the principles of mechanical wave propagation, the transmission / reflection coefficient expresses the ability of a MEMS sensor to transmit mechanical waves. This coefficient primarily depends on the properties of the material and the geometry of the MEMS sensor, i.e., on the so-called mechanical impedance of the medium.

[0009] One known approach to addressing the sensitivity issues of MEMS is to minimize the transmission / reflection coefficients of the MEMS sensor to isolate the sensor from the external mechanical environment. This isolation is also known as "impedance disconnection".

[0010] To achieve this isolation, known solutions employ mechanical decoupling, which is accomplished by using transfer and attachment materials with very high mechanical resistance, close to that of silicon, the underlying material of MEMS. In such solutions, the MEMS is mounted on the transfer structure.

[0011] For example, application EP 1 340 220 B1 discloses a device for interrupting the acoustic impedance of a rod formed of an elastic solid, on which ultrasonic deformation waves can propagate. The device includes a mass block connected to the rod. The mass block includes a nut for screwing onto a first threaded region of the rod and a locking nut for screwing onto a second threaded region of the rod. The acoustic impedance of the nut and locking nut is much higher than that of the rod, and the nut and locking nut are tightened until the deformation of the threads in both threaded regions of the rod exceeds the maximum deformation of the rod due to wave propagation.

[0012] However, this solution does not allow for the decoupling of the MEMS from the structural waves that propagate through the structure and are transferred to it.

[0013] Solutions based on suspensions, such as elastomeric columns, have also been proposed. These suspensions effectively cut off frequency bands above the inherent resonant frequency of the suspension. However, they do not mitigate structural propagation. While these suspensions protect devices and MEMS from mechanical vibrations, they do not protect MEMS from vibrations propagating through the structure.

[0014] Therefore, an improved MEMS accelerometer sensor is needed. Summary of the Invention

[0015] Therefore, embodiments of the present invention enable the generation of mechanical impedance disconnection from foam materials, particularly metal foams. These materials have the advantages of being particularly porous and thus effectively limiting the transmission of mechanical waves, and on the other hand, achieving thermal expansion coefficients close to those of silicon and quartz, thereby ensuring optimal thermomechanical operation of the sensor.

[0016] In the field of inertial sensors, these materials have a significant impact on sensor performance in harsh environments.

[0017] By integrating suitable transfer materials via MEMS, embodiments of the present invention can prevent these high-frequency waves from propagating into the MEMS. Therefore, these materials enable decoupling of propagating waves involving the mechanical impedance of various materials, particularly using foam-based decoupling materials.

[0018] Embodiments of the present invention also ensure thermomechanical mating between the assembled parts to maintain the alignment of the sensor and its attached axes, while ensuring the accuracy of the sensor under thermal loads.

[0019] This invention improves upon this situation by proposing a MEMS sensor comprising a MEMS housed within a housing, a support structure, and a set of layers stacked on the support structure along a stacking direction (Y) between the MEMS and the support structure, wherein the lower wall of the housing is substantially flat and forms one of the layers. Advantageously, the set of layers comprises: - A transfer element, to which the MEMS is attached and which is disposed inside the housing, the transfer element forming a layer interposed between the MEMS and a corresponding layer on the lower wall of the housing; - Electronic board substrate, which is disposed below the layer corresponding to the lower wall of the housing and mounted on the support structure.

[0020] The set of layers also includes an impedance mismatch layer made of foam, which forms a filter suitable for decoupling propagating waves toward the MEMS.

[0021] The thickness d of the impedance mismatch layer can be selected based on the operating frequency of the MEMS.

[0022] The thickness d of the foam forming the impedance mismatch layer can be chosen such that the cutoff frequency of the filter formed by the impedance mismatch layer is equal to the cutoff frequency of the MEMS.

[0023] In one embodiment, the impedance mismatch layer can be inserted between the housing and the electronic board substrate.

[0024] Impedance mismatch layer can be inserted between the electronic board substrate and the support structure.

[0025] The impedance mismatch layer can correspond to a transfer element inserted between the MEMS and the lower wall of the housing.

[0026] In one embodiment, the impedance mismatch layer may be made of at least one metal foam material.

[0027] In one embodiment, the impedance mismatch performance achieved using the impedance mismatch layer can be a function of the thickness and surface area of ​​the impedance mismatch layer.

[0028] In one embodiment, the impedance mismatch layer can be formed as a single piece.

[0029] The impedance mismatch layer may include at least two separate foam sections.

[0030] According to one aspect, the impedance mismatch layer may comprise two separate foam portions spaced apart from each other.

[0031] Alternatively, the impedance mismatch layer may include at least three separate foam portions, the portions of the impedance mismatch layer being regularly spaced apart at selected intervals.

[0032] In one embodiment, the MEMS sensor may be an accelerometer or a gyroscope.

[0033] In one embodiment, the MEMS can be made of silicon or quartz.

[0034] In one embodiment, the housing may be metal or ceramic-based.

[0035] In one embodiment, the electronic board substrate may be made of ceramic or may be an electronic board made of FR4 material.

[0036] In one embodiment, the support structure may be made at least partially of aluminum.

[0037] In one embodiment, the transfer element can be implemented by welding or by structural or epoxy bonding.

[0038] In one embodiment, the MEMS can be attached using screws or by bonding with a high Young's modulus epoxy structure.

[0039] Therefore, embodiments of the present invention enable the generation of mechanical impedance disconnection from foam materials, particularly metal foams. These materials have the advantages of being particularly porous and thus effectively limiting the transmission of mechanical waves, and on the other hand, achieving thermal expansion coefficients close to those of silicon and quartz, thereby ensuring optimal thermomechanical operation of the sensor.

[0040] In the field of inertial sensors, these materials have a significant impact on sensor performance in harsh environments.

[0041] By integrating suitable transfer materials via MEMS, embodiments of the present invention can prevent these high-frequency waves from propagating into the MEMS. Therefore, these materials enable decoupling of propagating waves involving the mechanical impedance of various materials, particularly using foam-based decoupling materials.

[0042] Embodiments of the present invention also ensure thermomechanical mating between the assembled parts to maintain the alignment of the sensor and its attached axes, while ensuring the accuracy of the sensor under thermal loads. Attached Figure Description

[0043] Other features, details, and advantages of the invention will become apparent from the following description with reference to the accompanying drawings, which are given by way of example and are respectively: [ Figure 1 ] Figure 1 A cross-sectional view of the MEMS sensor according to the present invention is shown.

[0044] [ Figure 2 ] Figure 2 A cross-sectional view of a MEMS sensor according to another embodiment is shown.

[0045] [ Figure 3 ] Figure 3 A cross-sectional view of a MEMS sensor according to another embodiment is shown.

[0046] [ Figure 4 ] Figure 4 The propagation of a wave in a MEMS sensor according to an embodiment of the present invention is illustrated.

[0047] [ Figure 5 ] Figure 5 This is a comparison table of the performance of foam materials and materials used in traditional sensors.

[0048] [ Figure 6 ] Figure 6 This shows that for those from Figure 5 The graph shows the changes in transmission coefficient and reflection coefficient with impedance ratio for different types of materials.

[0049] [ Figure 7 ] Figure 7 This illustrates the propagation of a wave in two media with the same cross-section.

[0050] [ Figure 8 ] Figure 8 The propagation of a wave in two media with different cross-sections is shown.

[0051] [ Figure 9 ] Figure 9 This is a graph showing the variation of the reflection coefficient R and the transmission coefficient with the impedance ratio.

[0052] [ Figure 10 ] Figure 10 A theoretical model representing the impedance disconnection of an acoustic medium.

[0053] [ Figure 11 ] Figure 11 The characteristics of different metal foams that can be considered when defining the composition of the impedance mismatch layer are shown.

[0054] [ Figure 12 ] Figure 12 This is a table showing examples of foams that can be used to manufacture impedance mismatch layers.

[0055] [ Figure 13 ] Figure 13 This is a graph showing the changes in transmission coefficient and reflection coefficient relative to impedance ratio when using a metal foam to implement an impedance mismatch material. Detailed Implementation

[0056] Figure 1 A cross-sectional view of a MEMS sensor according to an embodiment of the present invention is shown.

[0057] MEMS sensor 100 can be an accelerometer or a gyroscope.

[0058] MEMS sensor 100 includes a MEMS 11 housed in a housing 15, a rigid fixing structure 14 (hereinafter also referred to as "support structure") serving as a support for the MEMS sensor 100, and a MEMS located between the MEMS and the support structure 14 along the stacking direction. A set of stacked layers. The positioning of the elements of MEMS sensor 11 will be defined below in an orthogonal coordinate system (X, Y, Z), where the plane (XZ) corresponds to the plane of the layers and the MEMS. Therefore, Figure 1 It is shown as a cross-sectional view along the plane (XY).

[0059] The housing 15 is composed of walls, including a flat lower wall located below the MEMS 11 and extending in the XZ plane of the layer that forms the MEMS sensor and is interposed between other layers of the MEMS sensor. The housing 15 may, for example, have a rectangular parallelepiped shape.

[0060] The set of layers includes a transfer element 12, with a MEMS 11 attached to the transfer element and the transfer element disposed inside the housing 15. The set of layers also includes an electronic board substrate 13, with the housing 15 disposed on the electronic board substrate, comprising the group consisting of the transfer element 12 and the MEMS 11. The electronic board substrate 13 itself is mounted on the support structure 14.

[0061] MEMS 11 can be made of, for example, silicon or quartz.

[0062] The housing 15 may be, for example, made of metal or ceramic.

[0063] In this embodiment, the electronic board substrate 13 may be a ceramic substrate or an electronic board made of FR4 material (FR4 material refers to the National Electrical Manufacturers Association (NEMA) definition of glass fiber reinforced epoxy resin composite material). This material meets the UL94V-0 standard regarding the flammability of plastic materials and therefore ensures that the flame will not spread and can be extinguished quickly if the material is ignited.

[0064] A rigid support structure 14 forms the structure to which the sensor is attached. The support structure 14 may be made at least partially of aluminum. In particular, it may be made primarily of aluminum.

[0065] In one embodiment, the transfer element 12 may be made of a rigid material, such as, but not limited to, structural bonding using welding or rigid epoxy adhesive.

[0066] MEMS 11 can be attached using screws or by bonding with a high Young's modulus epoxy structure.

[0067] According to an embodiment of the invention, a set of layers of the MEMS sensor 100 located between the MEMS 11 and the rigid support structure 14 includes an impedance mismatch layer 16 made of a foam impedance mismatch material. The foam impedance mismatch layer 16 enables the creation of a filter suitable for decoupling propagating waves toward the MEMS 11 and thus isolating it from the external mechanical environment.

[0068] Impedance cutoff is a function of the frequency of vibration. Impedance cutoff corresponds to a filter because it defines a cutoff frequency below which vibrations are transmitted while vibrations above this frequency are attenuated.

[0069] In addition to ensuring the decoupling of propagating waves, the MEMS sensor 100 according to embodiments of the present invention also ensures that the assembled parts of the sensor 100 exhibit the same thermomechanical behavior and thus match their coefficients of thermal expansion to guarantee optimal operation of the sensor 100. When the coefficients of thermal expansion are equal, it is called matching. This means that the assembled materials have the same expansion. Therefore, there is no relative deformation between the assembled parts and thus no stress in the material.

[0070] exist Figure 1 In the embodiment shown, the impedance mismatch layer 16 is inserted between the housing 15 and the electronic board substrate 13.

[0071] The foam impedance mismatch layer inserted between the housing 15 and the electronic board substrate 13 creates a transfer portion made of a material with a high impedance ratio, and thus produces decoupling.

[0072] exist Figure 2 In the alternative embodiment shown, the impedance mismatch layer 16 can be inserted between the electronic board substrate 13 and the rigid support structure 14.

[0073] Alternatively, such as Figure 3 As shown, the impedance mismatch layer 16 can form the transfer element 12 on which the MEMS 11 is disposed and thus can be inserted between the MEMS 11 and the housing 13.

[0074] In one embodiment, the foam constituting the impedance mismatch layer 16 is advantageously metallic. Using a metallic foam allows for impedance disconnection using low-density materials, resulting in a coefficient of thermal expansion (CTE) compatible with MEMS materials (silicon or quartz), which provides thermomechanical coupling. Consequently, the wave propagation speed is very low compared to MEMS materials, causing waves to propagate more slowly within the foam.

[0075] The impedance mismatch performance achieved by the impedance mismatch layer 16 according to embodiments of the present invention is advantageously a function of the thickness of the impedance mismatch layer 16 and the surface area of ​​the impedance mismatch layer 16.

[0076] The performance of impedance disconnection is defined as the level of attenuation between the input and output vibrations of a damper formed by foam. Therefore, this is the impedance mismatch modulus (usually expressed in dB). The larger this modulus, the more effective the impedance disconnection.

[0077] In one embodiment, the impedance mismatch layer 16 can be formed as a single piece without interruption, such as Figure 1 and Figure 3 The example shown.

[0078] Alternatively, the impedance mismatch layer 16 may include at least two separate foam portions P1 and P2, such as Figure 2 The embodiment shown. Therefore, the impedance mismatch layer 16 does not contain any material between the different separate foam sections.

[0079] In embodiments where the impedance mismatch layer 16 includes two separate portions P1 and P2, as... Figure 2 As shown, the two parts P1 and P2 are arranged to be as far apart as possible, and are therefore located at opposite ends of the impedance mismatch layer. Thus, assuming L represents the total width of the impedance mismatch layer 16, l1 represents the width of the first part P1 of the impedance mismatch layer 16, and l2 represents the width of the second part P2 of the impedance mismatch layer 16, the spacing e between the two parts P1 and P2 is defined as: e = L - (l1 + l2).

[0080] The widths L, l1, l2, and spacing e are defined along the X-axis, which is perpendicular to the stacking direction Y and the plane of the layer (XZ).

[0081] The smaller the dimensions of widths l1 and l2, the better the impedance disconnection. Furthermore, the smaller the exchange surface between the sensor's sensing element and its base, the more difficult it is for acoustic waves to disturb the sensor / sensing element.

[0082] In embodiments where the impedance mismatch layer 16 comprises more than two separate portions, the portions of the impedance mismatch layer 16 may be regularly spaced at selected intervals, wherein the intervals may be fixed or variable.

[0083] The portions of the impedance mismatch layer 16 may have similar or different dimensions.

[0084] Using an impedance mismatch layer 16 composed of multiple separate and spaced sections allows for a more complex wave path and a greater reverberation.

[0085] The impedance mismatch layer 16 made of foam is characterized by having an area of ​​S in the plane (XZ). r The total cross-section (or cross section) and the thickness d defined along the Y-axis.

[0086] Impedance mismatch layer 16 can be attached to other layers by bonding or welding.

[0087] The MEMS sensor 100 has motion maintained at a specific frequency called the "operating frequency," which is characteristic of MEMS detectors. The filter cuts off at a frequency called the "cutoff frequency," which is characteristic of dampers (formed from foam). These frequencies are distinct in absolute terms. MEMS sensors can be advantageously configured such that these frequencies are numerically equal, thus enabling maximum attenuation of sound waves at that frequency.

[0088] The thickness d of the damper (foam) forming the impedance mismatch layer 16 can be advantageously selected based on the frequency of interest (or operating frequency) of the MEMS 11. Specifically, the cutoff frequency of the filter formed by the foam impedance mismatch layer 16 can be selected to be equal to the operating frequency of the MEMS 11. In other words, the filter attenuation is at its maximum at the frequency of interest of the MEMS sensor 100 to achieve maximum isolation.

[0089] Furthermore, the surface area of ​​the impedance mismatch layer 16 can be selected to obtain the highest possible impedance, which can be achieved by maximizing the area S of the impedance mismatch layer 16. r This is achieved by the difference in area (or "section") between the areas of each surface of the two layers adjacent to the impedance mismatch layer 16 (located on either side of the impedance mismatch layer 16 in the Y direction).

[0090] Therefore, in Figure 2 In the illustrated embodiment, the surface area of ​​the impedance mismatch layer 16 can be determined to maximize the area difference between the following: - On the one hand, the area S of the impedance mismatch layer 16 r Between the area of ​​the electronic board substrate 13 directly disposed above the impedance mismatch layer 16 and the area of ​​the substrate 13, and - On the other hand, the area S of the impedance mismatch layer 16 r Between the area of ​​the rigid support structure 14 directly disposed below the impedance mismatch layer 16 and the area of ​​the rigid support structure 14.

[0091] As used herein, the terms “above” and “below” refer to the positioning of one element of the MEMS sensor 100 relative to another element along the stacking axis Y.

[0092] Figure 4 It shows Figure 3 The wave propagation path in the illustrated embodiment is shown, wherein the impedance mismatch layer 16 is disposed within the housing 15, between the MEMS 11 and the lower portion of the housing 15, and thus decoupled within the MEMS 11. Curve 17 represents an external vibration that will transmit the perturbation vibration through mechanical impedance coupling.

[0093] In existing technical solutions, the fundamental vibration of MEMS 11 can be disturbed by such external vibrations. According to an embodiment of the invention, the foam impedance mismatch layer 16 of the MEMS sensor 100 advantageously forms a filter that allows the MEMS 11 to decouple from structural waves propagating through the structure in which it is transferred.

[0094] Therefore, the impedance mismatch layer 16 forms a barrier that makes it difficult for propagating waves to pass through, or even if they do propagate, they will be significantly attenuated.

[0095] Figure 5 This is a table that compares the properties of foam material (row 3) used to manufacture impedance mismatch material in sensor 100 according to an embodiment of the present invention with those of other conventional transfer and attachment materials used in prior art MEMS sensors (rows labeled with numbers 1, 2, and 4). The table specifically shows that conventional materials have very high mechanical impedance, approaching that of silicon, which is typically the base material of MEMS 11, while foam material has lower mechanical impedance.

[0096] Figure 6 This is illustrated in an embodiment using a foam impedance mismatch material, with respect to materials from... Figure 5 The graph shown illustrates the variation of reflection and transmission coefficients with impedance ratio α = Z2 / Z1 for acoustic waves passing through two media 1 and 2 separated by an interface (the first medium 1 has impedance Z1 and the second medium has impedance Z2), compared to conventional transfer and attachment materials in the prior art, with consideration given to a MEMS made of silicon.

[0097] Therefore, embodiments of the present invention provide a MEMS sensor 11 in which a layer 16 made of a suitable foam material is inserted, thereby having strong mechanical impedance disconnection and acoustic decoupling that impedes wave propagation toward the MEMS 11, while adhering to thermomechanical coupling, i.e., simultaneously limiting differential expansion.

[0098] In order to enable the impedance mismatch layer 16 to form a barrier that is difficult for propagating waves to pass through, or to attenuate the waves when they successfully propagate, the impedance mismatch layer 16 according to embodiments of the present invention can be advantageously configured to have strong impedance disconnection.

[0099] The inventors discovered that principles related to wave propagation in the field of acoustics can be applied to structural vibration waves propagating in a MEMS sensor 100 to determine the optimal parameters of the impedance mismatch layer 16.

[0100] In the field of acoustics, the acoustic impedance Z of a medium to a sound wave is given by the following equation (E1): Z = ρc (E1) In equation (E1), Z is defined by Rayleigh, c represents the longitudinal velocity of the sound wave in the medium (in m / s), and ρ represents the density of the medium (in kg / m³). 3 express).

[0101] When the medium under consideration is an acoustic component, the acoustic impedance is measured at the input of the component.

[0102] Two consecutive layers separated by a common interface can be modeled as a conductor consisting of two acoustic layers with corresponding cross-sections S1 and S2, sharing the same axis Y, and separated by an interface I in the plane XZ. The conductor impedance Zc of the layer is defined based on the dielectric impedance Z and the cross-sectional surface area S (or area) of the layer: Zc i = Z i / S i (E2) Figure 7 This illustrates the concept of acoustic impedance disconnection for sound waves passing through two media 1 and 2 separated by an interface I with cross-section S. Figure 7 In the example shown, consider sound waves propagating from medium 1 to medium 2, where the two media 1 and 2, for simplicity, correspond to acoustic components with the same cross-sections S1 and S2 (S1 = S2 = S) in the plane XZ (in the plane of interface I).

[0103] The characteristic conductor impedance Zc1 of dielectric 1 is defined as: (E3) In equation (E3), c1 represents the longitudinal velocity of the sound wave in the medium (in m / s), and ρ1 represents the density of the medium (in kg / m³). 3 express).

[0104] Similarly, for a sound wave propagating in medium 2, the conductor impedance Zc2 of medium 2 is defined as: (E4) In equation (E4), c2 represents the longitudinal velocity of the sound wave in the medium (in m / s) and ρ2 represents the density of the medium (in kg / m³). 3 express).

[0105] The conductor impedance Zc advantageously enables the integration of the concept of exchange surfaces and thus the geometry of the design, which would not be possible by simply considering the impedance of the medium while ignoring the influence of the exchange surfaces between waves.

[0106] like Figure 7 As shown, when a sound wave encounters an interface I between two media 1 and 2, which have different acoustic impedances Zc1 and Zc2, part of the sound wave is transmitted, another part is reflected, and the last part is absorbed. These three phenomena are characterized by the energy reflection coefficient R (corresponding to the reflected part of the wave), the energy transmission coefficient T (corresponding to the transmitted part of the wave), and the absorption coefficient A (corresponding to the absorbed part of the wave), respectively. The presence of the absorption coefficient A creates an acoustic impedance break at the interface between the two media 1 and 2.

[0107] According to the principle of energy conservation, there is a relationship between the energy reflection coefficient R, the energy transmission coefficient T, and the absorption coefficient A, defined by the following formula: R + T + A = 1 (E5) exist Figure 7 In the example shown, p i p represents the incident power. t p represents the transmitted power. r This represents the reflected power.

[0108] Since the acoustic impedance of a medium depends on the material properties of the medium, it also depends on the cross-section of the medium involved (in... Figure 7 In the example shown, the two media 1 and 2 have equal cross sections S, so acoustic impedance disconnection can also be achieved by changing the cross sections S1 and S2 of the two media 1 and 2 through which the wave propagates and / or by changing the properties of the corresponding materials constituting the two media 1 and 2.

[0109] exist Figure 8 In the example configuration shown, acoustic impedance disconnection is achieved not only by changing the cross sections S1 and S2 of media 1 and 2, but also by changing the material properties of the two media 1 and 2.

[0110] The MEMS sensor 100 according to an embodiment of the present invention undergoes wave propagation in a solid layer separated by an interface.

[0111] In the field of structural acoustics, absorption is negligible compared to reflection and transmission. Therefore, the energy conservation equation (E4) can be reformulated as follows: R + T = 1 (E6) The energy reflection coefficient R between two media 1 and 2 with corresponding impedances Z1 and Z2 is defined by the following formula: (E7) The reflection coefficient can also be expressed by the following equation based on the amplitude: (E8) In this case, the energy transmission coefficient T can be defined by equation (E9): (E9) Figure 9 This shows the ratio of the impedance Z1 to Z2 of the reflection coefficient R and transmission coefficient T as the wave passes through two different media 1 and 2. A graph showing the changes.

[0112] like Figure 9 As shown in the figure, when Z1≫ Z2 (or α ≪ 1) and when Z2≫ Z1 (or α ≫ 1), that is, when the impedances Z1 and Z2 of the two media differ greatly from each other, the transmission is very small.

[0113] Figure 10 This paper presents a theoretical model for the impedance disconnection of acoustic media, developed by LM Brekhovskikh in the field of acoustics (Springer Series on Wave Phenomenology, LM Brekhovskikh, OAGodin Acoustics of Layered Media I Plane and Quasi-Plane Waves). This model can be used to calculate the reflection and transmission coefficients of a stack of multiple layers i (where i is between 1 and n) with different thicknesses and / or mechanical properties in the case of a plane acoustic wave incident normally. Therefore, layer i has a thickness d in the Y-direction of the stacking axis. i and input acoustic impedance Z in (i) .

[0114] The wave number of the medium is defined by equation (E10): (E10) In equation (E10), λ i Denotes the wavelength in medium i and c i This represents the longitudinal velocity of medium i.

[0115] According to this acoustic model, for each layer i, the input impedance Z in (i) Defined as the impedance Z of the medium i The input impedance Z of the layer in The wavenumber k of the medium formed by this layeri and the thickness d of the layer i The function is shown in equation E11: (E11) By applying this model to structural vibration waves (vibration waves in a solid structure) propagating in the MEMS sensor 100, each layer i is also associated with a medium-based impedance Z according to the model. i and the cross-sectional area S of layer i i (Or area) Conductor impedance Zc defined by equation (E2) i Conductor impedance Zc i Defined by the following formula: Zc i = Z i / S i (E12) Equation (E12) can use the impedance Z of the medium. i Rephrased according to the definition of equation (E13): Zc i = ρ i c i / S i (E13) By adjusting equation (E11), for each layer i (in consecutive layers 11 to 16), the input impedance Z in (i) Defined as the impedance Z of the medium i The input impedance Z of the layer in The wavenumber k of the medium formed by this layer i and the thickness d of the layer i The function is shown in equation (E14): (E14) Therefore, by configuring the impedance mismatch layer 16 and the two adjacent layers on both sides of layer 16 in the MEMS sensor to have different cross-sections (or areas) S i Different impedance ratios can be obtained (thus achieving a very large reflection coefficient R or a very small coefficient T, such as...). Figure 9 (As shown).

[0116] The reflection coefficient can then be calculated iteratively according to the following equation (E15): (E15) In the field of acoustics, the transmission loss TL, expressed in dB, is related to the reflection coefficient R by the following equation (E16): (E16) Therefore, in the embodiments, the theoretical model can be simulated using suitable simulation tools (such as Matlab) to define the thickness d of each layer i of the MEMS sensor 100 using equations E10 to E16. i Speed ​​c i Density ρ i And the cross section S from equation E13 i To calculate the acoustic transmission T between the first layer 14 and the last layer 11 of the MEMS sensor 100.

[0117] The input data are the material properties, including velocity c. i Density ρ i and geometric design parameters, including thickness d i and cross section S i .

[0118] The model outputs the simulated TL as a function of frequency. The simulation is performed in this manner to find the optimal trade-off for the input dataset, maximizing TL (the objective in terms of impedance mismatch performance and frequency placement) at the sensor's frequencies of interest, while ensuring the damper dimensions (d...). i and S i Minimize (as much as possible) and limit the size of the sensor. Simulations can be performed on materials with thermomechanical properties close to those of silicon and quartz. Multiple simulations can be performed to optimize the results. Advantageously, the best trade-off is achieved with metal foam. In addition to its impedance mismatch function, metal foam is particularly good at tuning the coefficient of thermal expansion between the silicon MEMS and the substrate.

[0119] In one embodiment, the impedance mismatch layer 16 may be made of low-density foam, which enables acoustic decoupling of the MEMS 11.

[0120] In one embodiment, the impedance mismatch layer 16 may be made of a foam based on a polymer material, such as epoxy, polyurethane, plastic, or other rubber. However, while these types of foams are workable, they may not have very high temperature stability and may exhibit a deviation in the coefficient of thermal expansion compared to silicon (which may be the material of MEMS 11), potentially subjecting MEMS 11 to mechanical stress.

[0121] Alternatively, the impedance mismatch layer 16 can be made of a foam with a metal or mineral matrix. Such foams offer advantages such as higher temperature stability and no deviation in the coefficient of thermal expansion compared to silicon, thus not imposing mechanical stress on the MEMS 11. Examples of metal-based foams include aluminum or steel foams. For nickel foams, the density can be as low as 20 kg / m³. 3 .

[0122] The foam constituting the impedance mismatch layer 16 can also be implemented with a defined shape by using an additive manufacturing process.

[0123] The foam used to implement the impedance mismatch layer 16 can be selected based on the inherent properties of MEMS 11, taking into account the characteristics of the foam. Figure 11 For example, the characteristics of different metal foams that can be considered to limit the composition of the impedance mismatch layer 16 are given.

[0124] Metal foam not only offers the advantage of achieving acoustic decoupling, but can also be used to absorb significant shocks that the MEMS sensor 100 may experience.

[0125] Mechanical resistance can be calculated using the following equations (E17) to (E21): (E17) (E18) (E19) (E20) (E21) In equations E17, E18, E19, E20, and E21: - E represents Young's modulus; - K represents the flexural modulus; - M represents the plate modulus; - c represents the speed of sound; and -ρ represents the density of the medium.

[0126] Consider an example of using aluminum foam to form an impedance mismatch layer with a density of ρ = 50 kg / m³. 3 , where ρ s =2500 kg / m 3 E s =70 MPa, the obtained longitudinal velocity is equal to c l =1180 m / s, Young's modulus is equal to E=43.4 MPa.

[0127] Consider another example where the foam used to form the impedance mismatch layer 16 is INCOFOAM nickel foam with a density equal to ρ = 180 kg / m³. 3 The following values ​​were obtained: E = 120 MPa, K = 150 MPa, c l =1080 m / s, Poisson's ratio: 0.32, and CTE=13 ppm / ℃.

[0128] In another alternative embodiment, the foam used to form the impedance mismatch layer 16 may be foam glass.

[0129] In their article "Effects of porosity onseismic velocities, elastic moduli and Poisson's ratios of solid materials and rocks," published in the Journal of Rock Mechanics and Geotechnical Engineering in September 2015, Chengbo Yu, Shaocheng Ji, and Qi Li presented the mechanical properties, Young's modulus, longitudinal velocity, and transverse velocity of different rocks with varying porosities, which can be used to define the characteristics of this porous glass.

[0130] Figure 12 The table provides non-limiting examples of foams with advantageous properties for forming the impedance mismatch layer 16. Those skilled in the art will readily understand that other foams with similar properties or that meet the specific constraints of MEMS 11 can also be used.

[0131] Figure 13 The diagram in the image shows the use of Figure 12 The table shows the changes in the transmission coefficient and reflection coefficient relative to the impedance ratio when a metal foam (Z2 represents the impedance of the impedance mismatch layer, and Z1 represents the impedance of the previous layer in the Y direction) is implemented as the impedance mismatch material 16 of the MEMS sensor according to an embodiment of the present invention.

[0132] Therefore, embodiments of the present invention provide a compact MEMS sensor 100 without interfering with the proper operation of the MEMS 11. In particular, they allow for maintaining alignment and positioning, have very low thermal drift, and thus require very little stress.

[0133] Those skilled in the art will readily understand that the present invention is not limited to the embodiments described above as non-limiting examples. The present invention encompasses all alternative embodiments that may be conceived by those skilled in the art.

Claims

1. A MEMS sensor (100) comprising a MEMS (11) housed in a housing (15), a support structure (14), and a set of layers stacked on the support structure (14) along a stacking direction (Y) between the MEMS (11) and the support structure (14), wherein the lower wall of the housing is substantially flat and forms one of the layers, characterized in that, The set of layers includes: - Transfer element (12), the MEMS (1) is attached to the transfer element and the transfer element is disposed inside the housing (15), the transfer element forming a layer inserted between the MEMS (1) and a layer corresponding to the lower wall of the housing; - An electronic board substrate (13), which is disposed below the layer corresponding to the lower wall of the housing (15) and mounted on the support structure (14), Furthermore, the set of layers also includes an impedance mismatch layer (16) made of foam, which forms a filter suitable for decoupling propagating waves toward the MEMS (11). The cross-sectional area of ​​the impedance mismatch layer (16) is selected to maximize the area difference between the area of ​​the impedance mismatch layer (16) and the area of ​​each of the surfaces of the two layers adjacent to the impedance mismatch layer (16) in the stacking direction and disposed on either side of the impedance mismatch layer (16).

2. The MEMS sensor (100) according to claim 1, wherein, The thickness d of the impedance mismatch layer (16) is selected according to the operating frequency of the MEMS (11).

3. The MEMS sensor (100) according to claim 2, wherein, The thickness d of the foam forming the impedance mismatch layer (16) is selected such that the cutoff frequency of the filter formed by the impedance mismatch layer (16) is equal to the cutoff frequency of the MEMS (11).

4. The MEMS sensor (100) according to any one of the preceding claims, wherein, The impedance mismatch layer (16) is inserted between the housing (15) and the electronic board substrate (13).

5. The MEMS sensor (100) according to any one of claims 1 to 3, wherein, The impedance mismatch layer (16) is inserted between the electronic board substrate (13) and the support structure (14).

6. The MEMS sensor (100) according to any one of claims 1 to 3, wherein, The impedance mismatch layer (16) corresponds to the transfer element (12) inserted between the MEMS (11) and the lower wall of the housing (15).

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

8. The MEMS sensor (100) according to any one of the preceding claims, wherein, The impedance mismatch performance achieved by the impedance mismatch layer (16) is a function of the thickness of the impedance mismatch layer (16) and the cross-sectional area of ​​the impedance mismatch layer (16).

9. The MEMS sensor (100) according to any one of the preceding claims, wherein, The impedance mismatch layer (16) is formed as a single piece.

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

11. The MEMS sensor (100) according to claim 10, wherein, The impedance mismatch layer (16) comprises two separate foam sections spaced apart from each other.

12. The MEMS sensor (100) according to claim 10, wherein, The impedance mismatch layer (16) includes at least three separate foam portions, which are uniformly spaced apart at selected intervals.

13. The MEMS sensor (100) according to any one of the preceding claims, wherein, The MEMS sensor is an accelerometer or a gyroscope.

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

15. The MEMS sensor (100) according to any one of the preceding claims, wherein, The housing (15) is made of metal or ceramic.

16. The MEMS sensor (100) according to any one of the preceding claims, wherein, The electronic board substrate (13) is made of ceramic or FR4 material.

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

18. The MEMS sensor (100) according to any one of the preceding claims, wherein, The transfer element (12) is implemented by welding or by structural bonding or epoxy bonding.

19. The MEMS sensor (100) according to any one of the preceding claims, wherein, The MEMS (11) is attached by screws or by bonding with a high Young's modulus epoxy structure.

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