MEMS accelerometer with high mechanical robustness

Abstract

The invention relates to a MEMS accelerometer with high mechanical robustness. The MEMS accelerometer has a substrate and a sense mass suspended at a distance from the substrate in an out-of-plane direction. The sense mass is coupled to the substrate to experience out-of-plane movement relative to the substrate in response to acceleration along an out-of-plane direction. The MEMS accelerometer also has a damping structure configured to damp in-plane movement of the sense mass relative to the substrate. The damping structure has a plurality of movable fingers integral with the sense mass and a plurality of fixed fingers integral with the base plate and intersecting the movable fingers. The movable fingers and / or the fixed fingers have a variable length along a first in-plane direction transverse to the out-plane direction.

Description

Technical Field

[0001] The present disclosure relates to a microelectromechanical system (MEMS) accelerometer having high mechanical robustness. Specifically, the present disclosure relates to out-of-plane MEMS accelerometers, and even more specifically, to MEMS accelerometers for high accelerations (high-g MEMS accelerometers). Background Art

[0002] It is well known that MEMS accelerometers have become popular due to their small size and high detection sensitivity.

[0003] Common applications of MEMS accelerometers include shock monitoring, such as to detect car accidents or the likelihood of a person falling to the ground; detecting a user's gestures (such as the rotation of a smart phone screen or a particular type of user touch); and use as a bone conductivity detector, such as use as a microphone in true wireless stereo (TWS) headphones.

[0004] Currently, low-g accelerometers known to be suitable for detecting low accelerations, such as having a full scale range (FSR) equal to 16 g or 32 g, and high-g sensors suitable for detecting high accelerations, such as having a full scale range equal to 128 g, are known, where g = 9.81 m / s 2 .

[0005] Compared to low-g accelerometers, high-g MEMS accelerometers need to withstand greater stress and typically may have lower sensitivity.

[0006] The applicant has confirmed that known MEMS accelerometers (in particular, MEMS accelerometers for high accelerations) may suffer from insufficient mechanical robustness and detection accuracy or sensitivity for certain applications.

[0007] For example, high-g MEMS accelerometers are known that have a high resonant frequency in order to ensure the required full scale range.

[0008] At the same time, these MEMS accelerometers are configured to have an overdamped frequency response and thus have a low quality factor Q. This allows ensuring high immunity to vibrations both along the sensing direction and along the direction orthogonal to the sensing direction.

[0009] One way to reduce the quality factor Q is to use in-plane mechanical damping structures. However, to ensure their effectiveness, known mechanical damping structures require a large area; however, this may compromise the mechanical robustness of the MEMS accelerometer.

[0010] Out-of-plane MEMS accelerometers are also known, in which, in response to a high in-plane acceleration, the sensing mass undergoes a high in-plane rotation. The high in-plane rotation of the sensing mass may cause mechanical fracture of the sensing mass and thus cause failure of the MEMS accelerometer.

[0011] US2023 / 314466 A1 discloses an inertial sensor that includes a movable body and a damper that attenuates displacement of the movable body relative to a substrate. The damper includes a comb-shaped first structure and a comb-shaped second structure that cross each other. The comb-shaped first structure includes a plurality of movable comb teeth whose proximal ends are coupled to the movable body, and the comb-shaped second structure includes a plurality of fixed comb teeth whose proximal ends are coupled to the substrate. The movable comb teeth are spaced apart from each other along a first axis, the fixed comb teeth are spaced apart from each other along the first axis, and the width of the proximal end of each of the movable comb teeth and the fixed comb teeth (measured along the first axis) is greater than the width of its tip (measured along the first axis).

[0012] This disclosure relates to at least partially overcoming the disadvantages of the prior art. Summary of the Invention

[0013] This disclosure relates to a MEMS accelerometer. For example, at least one embodiment of a MEMS: a substrate; a sensing mass suspended at a distance from the substrate in an out-of-plane direction, the sensing mass being coupled to the substrate to undergo an out-of-plane movement relative to the substrate in response to an acceleration in the out-of-plane direction; and a damping structure configured to damp the in-plane movement of the sensing mass relative to the substrate, the damping structure including a plurality of movable fingers integral with the sensing mass and a plurality of fixed fingers integral with the substrate and crossing the movable fingers, wherein at least one of the movable fingers and the fixed fingers has a variable length along a first in-plane direction transverse to the out-of-plane direction. Brief Description of the Drawings

[0014] To better understand this disclosure, embodiments of this disclosure are now described by way of non-limiting examples only with reference to the accompanying drawings, in which:

[0015] Figure 1 A top plan view of an out-of-plane MEMS accelerometer according to one embodiment is shown;

[0016] Figure 2 Shows along Figure 1 Section line II-II of Figure 1 A schematic cross-section of the accelerometer;

[0017] Figure 3A Shows Figure 1 A top plan view of an enlarged portion of the MEMS accelerometer;

[0018] Figure 3B Shows in detail the Figure 3A amplified portion;

[0019] Figure 4 Shows Figure 1 a top plan view of another amplified portion of the MEMS accelerometer of

[0020] Figure 5 Shows the Figure 1 MEMS accelerometer in use in the presence of an external in-plane shock;

[0021] Figure 6A and Figure 6B Shows Figure 1 a cross-section of some portions of the MEMS accelerometer of

[0022] Figure 7 Shows the Figure 1 MEMS accelerometer including a cover in a schematic top plan view;

[0023] Figure 8 Shows the cross-section of the Figure 7 MEMS accelerometer along section line VIII-VIII;

[0024] Figure 9 Shows Figure 7 a schematic top rendering of the cover in

[0025] Figure 10 a schematic top plan view of the layout of an inertial device including a plurality of inertial sensors; and

[0026] Figure 11 Shows a perspective view of the Figure 10 inertial device mounted on a support. Detailed Description

[0027] Figure 1 and Figure 2 Show a MEMS accelerometer 10 in a Cartesian reference system XYZ having a first axis X, a second axis Y, and a third axis Z.

[0028] The MEMS accelerometer 10 is formed by micromachining techniques starting from a body of semiconductor material such as silicon.

[0029] The MEMS accelerometer 10 is an out-of-plane MEMS accelerometer, which is configured to detect acceleration along an out-of-plane direction (i.e., here along the third axis Z). In particular, the MEMS accelerometer 10 can be a high-g accelerometer, i.e., it is configured to have a full-scale range greater than 32 g, particularly greater than 100 g, more particularly greater than 128 g, and even more particularly between 260 g and 700 g, where g is the acceleration due to gravity equal to 9.81 m / s 2 of the earth's gravitational acceleration.

[0030] The full-scale range can be defined as the maximum acceleration value (absolute value) that the MEMS accelerometer can detect. For example, an accelerometer with a 100-g full-scale range can be capable of detecting accelerations between -100 g and +100 g, which are accelerations with a maximum amplitude equal to -100 g and +100 g.

[0031] The MEMS accelerometer 10 includes a substrate 11 and a sensing mass 12 suspended on the substrate 11. The sensing mass 12 extends a certain distance from the substrate 11 along the third axis Z.

[0032] The sensing mass 12 is coupled to the substrate 11 to undergo out-of-plane movement (i.e., having a component along the third axis Z) relative to the substrate 11 in response to an external acceleration of the MEMS accelerometer 10 along the third axis Z.

[0033] Specifically, the sensing mass 12 is substantially planar and has a main extension in the plane XY formed by the first axis X and the second axis Y.

[0034] The MEMS accelerometer 10 includes a central anchoring region 14 fixed to the substrate 11 by corresponding struts 15 (indicated by the dashed lines in Figure 1 ), and springs 16A, 16B extending between the central anchoring region 14 and the sensing mass 12. The springs 16A, 16B hold the sensing mass 12 suspended on the substrate 11 at rest.

[0035] The springs 16A, 16B allow the sensing mass 12 to perform out-of-plane movement in response to the acceleration along the third axis Z experienced by the MEMS accelerometer 10.

[0036] The MEMS accelerometer 10 includes sensing electrodes 18A, 18B, which are fixed to the substrate 11 and arranged at a certain distance from the sensing mass 12 along the third axis Z.

[0037] The sensing electrodes 18A, 18B are capacitively coupled to the sensing mass 12, thereby forming corresponding capacitors C1, C2 therewith.

[0038] In this embodiment, the MEMS accelerometer 10 has a teeter - totter structure, as shown in the Figure 2 schematic cross - section.

[0039] The sensing mass 12 has a center of gravity G.

[0040] The springs 16A, 16B couple the sensing mass 12 to the central anchor region 14 at a fulcrum F, which is offset with respect to the center of gravity G of the sensing mass 12.

[0041] The springs 16A, 16B are torsional elastic elements configured to allow the sensing mass 12 to rotate about a rotation axis R passing through the fulcrum F.

[0042] The rotation axis R is transverse to (in particular, perpendicular to) the third axis Z.

[0043] The rotation axis R is a non - centroidal axis, i.e., the rotation axis does not pass through the center of gravity G of the sensing mass 12.

[0044] In the Figure 1 arrangement, the rotation axis R is parallel to the first axis X.

[0045] Specifically, the sensing mass 12 has a main portion 12A and a secondary portion 12B. The main portion extends along the second axis Y on a first side of the rotation axis R, and the secondary portion extends along the second axis Y on a second side of the rotation axis R opposite the first side.

[0046] The main portion 12A and the secondary portion 12B respectively form the outer (peripheral) walls 13A and 13B of the sensing mass 12.

[0047] The main portion 12A has a greater mass than the secondary portion 12B.

[0048] In particular, the main portion 12A has greater dimensions than the secondary portion 12B.

[0049] Specifically, as shown in Figure 5 , in this embodiment, the outer wall 13A extends along the second axis Y at a distance D1 from the rotation axis R, and the outer wall 13B extends along the second axis Y at a distance D2 from the rotation axis R, where the distance D2 is shorter than the distance D1.

[0050] Thus, the center of gravity G of the sensing mass 12 is arranged on the first side of the rotation axis R, in particular here inside the main portion 12A of the sensing mass 12.

[0051] The main portion 12A faces the sensing electrode 18A. The secondary portion 12B faces the sensing electrode 18B.

[0052] InFigure 1 In an embodiment, the through cavity 20 extends through the sensing mass 12 along the third axis Z. Specifically, the sensing mass 12 has an inner wall 20A that faces the through cavity 20 and the central anchoring region 14.

[0053] The central anchoring region 14, the struts 15, and the springs 16A, 16B extend inside the through cavity 20. This can allow the area occupancy of the MEMS accelerometer 10 to be optimized.

[0054] The central anchoring region 14 may include a protrusion 21 that extends along the first axis X towards the inner wall 20A of the sensing mass 12. The protrusion 21 extends at a certain distance along the first axis X from the inner wall 20A during rest; for example, it extends at a distance between 1.4 μm and 1.6 μm. The protrusion 21 is a stop region of the sensing mass 12 along the first axis X.

[0055] In this embodiment, the springs 16A, 16B have a T shape. Specifically, the twisted springs 16A, 16B each have a respective longitudinal portion 22 that extends from the central anchoring region 14 parallel to the first axis X and a respective transverse portion 23 that extends transverse to the respective longitudinal portion 22.

[0056] Specifically, the longitudinal portion 22 extends along the rotation axis R.

[0057] Specifically, the transverse portion 23 extends parallel to the second axis Y.

[0058] Specifically, here, the transverse portions 23 of the springs 16A, 16B are coupled to one end of the respective longitudinal portions 22.

[0059] The sensing mass 12 is coupled to the transverse portions 23 of the springs 16A, 16B, specifically, to the ends of the respective transverse portions 23.

[0060] The MEMS accelerometer 10 further includes one or more damping structures that are configured to damp the in-plane movement (along the first axis X and / or the second axis Y) of the sensing mass 12.

[0061] Specifically, in Figure 1 the embodiment, the MEMS accelerometer 10 includes four modified damping structures 25A, 25B, 25C, 25D and four additional damping structures 26A, 26B, 26C, 26D.

[0062] The modified damping structures 25A, 25C and the additional damping structures 26A, 26C are the damping structures for the main part 12A of the sensing mass 12. The modified damping structures 25B, 25D and the additional damping structures 26B, 26D are the damping structures for the secondary part 12B of the sensing mass 12.

[0063] As shown in detail for the modified damping structures 25A, 25B in Figure 3A and Figure 3B each of the modified damping structures 25A to 25D includes respective multiple movable fingers (or protrusions) (here four movable fingers 30A to 30D) integral with the sensing mass 12 and respective multiple fixed fingers (or protrusions) 31 (here five fixed fingers 31A to 31E) integral with the substrate 11.

[0064] For each of the modified damping structures 25A to 25D, the movable fingers 30A to 30D cross the respective fixed fingers 31A to 31E.

[0065] In this embodiment, each of the modified damping structures 25A to 25D is disposed within respective through - cavities 27A to 27D of the sensing mass 12; this can allow the die area occupancy of the MEMS accelerometer 10 to be minimized.

[0066] In this embodiment, the through - cavities 27A to 27D extend in the plane XY at a certain distance from the through - cavity 20.

[0067] The through - cavity 27A extends along the second axis Y in the main part 12A of the sensing mass 12 to one side of the rotation axis R, particularly to the side of the longitudinal portion 22 of the spring 16A.

[0068] The through - cavity 27C extends along the second axis Y in the main part 12A of the sensing mass 12 to one side of the rotation axis R, particularly to the side of the longitudinal portion 22 of the spring 16B.

[0069] The through - cavity 27B extends along the second axis Y in the secondary part 12B of the sensing mass 12 to one side of the rotation axis R, particularly to the side of the longitudinal portion 22 of the spring 16A.

[0070] The through - cavity 27D extends along the second axis Y in the secondary part 12B of the sensing mass 12 to one side of the rotation axis R, particularly to the side of the longitudinal portion 22 of the spring 16B.

[0071] The through cavities 27A, 27C extend along a first axis X on two opposite sides of the central anchoring region 14 inside the main part 12A of the sensing mass 12. The through cavities 27B, 27D extend along the first axis X on two opposite sides of the central anchoring region 14 inside the secondary part 12B of the sensing mass 12.

[0072] Specifically, each of the modified damping structures 25A to 25D includes a respective anchoring region 35 fixed to the substrate 11. The fixed fingers 31A to 31E extend from the respective anchoring regions 35 at a distance from each other.

[0073] In this embodiment, the fixed fingers 31A to 31E extend towards the inner wall 20A; specifically, they extend parallel to the first axis X and at a distance from each other along a second axis Y.

[0074] The movable fingers 30A to 30D extend between the fixed fingers 31A to 31E at a distance from each other and face each other.

[0075] Specifically, the movable fingers 30A to 30D extend at a distance from each other along the second axis Y, parallel to the first axis X, towards the respective anchoring regions 35. In this embodiment, each of the movable fingers 30A to 30D extends along the second axis Y between two adjacent ones of the fixed fingers 31A to 31E.

[0076] For each of the modified damping structures 25A to 25D, the movable finger 30A is arranged at a shorter distance from the rotation axis R along the second axis Y relative to the movable finger 30D. Similarly, the fixed finger 31A is arranged at a shorter distance from the rotation axis R along the second axis Y relative to the fixed finger 31E.

[0077] In addition, as Figure 3A shown in detail, the movable finger 30D and the fixed finger 31E face the curved portion 33 of the inner wall 20A along the first axis X.

[0078] The movable fingers 30A to 30D and the fixed fingers 31A to 31E have variable lengths along the first axis X.

[0079] Specifically, the movable fingers 30A to 30D and the fixed fingers 31A to 31E have lengths measured along the first axis X that decrease as they move away from the rotation axis R along the second axis Y. This can allow optimization of the trade-off between the damping effectiveness and the area occupation of the damping structures 25A to 25D, and at the same time, ensure the high mechanical robustness of the MEMS accelerometer 10.

[0080] In other words, for simplicity, referring toFigure 3B The fixed fingers 31A to 31E of the modified damping structure 25A shown have a maximum length Lmax and the fixed finger 31E has a minimum length Lmin.

[0081] The maximum length Lmax can be between, for example, 40 μm and 50 μm.

[0082] The minimum length Lmin can be between, for example, 25 μm and 40 μm.

[0083] In particular, in this embodiment, the fixed fingers 31A to 31E follow a linear length reduction trend; this can allow maximizing the trade-off between damping effectiveness, area occupancy, and mechanical robustness. For example, referring to Figure 3B , the ends of the fixed fingers 31A to 31E form an angle α with an axis parallel to the second axis Y cs .

[0084] The angle α cs can be between, for example, 20° and 45°; this can allow optimizing the trade-off between damping effectiveness, area occupancy, and mechanical robustness.

[0085] It will be clear to those skilled in the art that the geometric considerations discussed for the fixed fingers 31A to 31E with reference to Figure 3B also apply to the movable fingers 30A to 30D that cross them.

[0086] Additional damping structures 26A to 26D also extend inside the through cavities 27A to 27D, one for each of the through cavities 27A to 27D. In fact, the modified damping structures 25A to 25D each share the corresponding through cavities 27A to 27D with the corresponding additional damping structures 26A to 26D.

[0087] The additional damping structures 26A to 26D each include corresponding movable fingers 37 and corresponding fixed fingers 36 that cross each other.

[0088] The fixed fingers 36 of the additional damping structures 26A to 26D extend from the anchoring regions 35 of the corresponding modified damping structures 25A to 25D in the opposite direction with respect to the fixed fingers 31A to 31E, that is, towards the transverse portions 23 of the corresponding springs 16A, 16B.

[0089] In the illustrated embodiment, the fixed fingers 36 and the movable fingers 37 have a constant length along the first axis X. This can allow maximizing the damping effectiveness.

[0090] The applicant has verified that the presence of an in-plane damping structure with variable-length interdigitated fingers allows the trade-off between damping effectiveness, area occupancy, and mechanical robustness to be optimized.

[0091] In the case where the MEMS accelerometer 10 is a high-g out-of-plane accelerometer, variable length can be particularly useful for optimizing the performance of the MEMS accelerometer 10.

[0092] In particular, the lengths of the interdigitated fingers 30A to 30D and 31A to 31E can be adjusted during the design step to ensure high mechanical robustness in the regions where the sensing mass 12 is subject to high mechanical stress during use.

[0093] For example, referring to the described and illustrated embodiments, the applicant has verified that, in the case where there is a high in-plane acceleration (shock) along, for example, the first axis X, the sensing mass 12 is subject to high mechanical stress at the bending portion 33 of the inner wall 20A.

[0094] The fact that the lengths of the fingers 30D, 31E arranged at the bending portion 33 are the minimum lengths of the respective finger groups allows the mechanical robustness of the sensing mass 12 at the bending portion 33 to be increased.

[0095] Therefore, in the presence of high external acceleration, the risk of rupture of the MEMS accelerometer 10 can be reduced.

[0096] In other words, the presence of a damping structure with variable-length fingers contributes to optimizing the performance and reliability of the MEMS accelerometer 10.

[0097] Referring again to Figure 1 , the MEMS accelerometer 10 includes a frame 50 that is fixed to the substrate 11 by respective struts 51 (indicated by dashed lines in Figure 1 ), and the frame extends around the sensing mass 12 at a certain distance from the sensing mass 12.

[0098] The struts 51 can have a minimum width along the first axis X or the second axis Y, and the minimum width is between, for example, 4 μm and 15 μm. In particular, when other sensors (such as other accelerometers and / or gyroscopes) are formed on the substrate 11, the width of the struts 51 can have a smaller value.

[0099] The frame 50 includes a buffer region 53 (shown in detail in the enlarged portion of Figure 4 ), and the buffer region extends along the second axis Y toward the sensing mass 12 within the recess 55 of the sensing mass 12.

[0100] The buffer region 53 can be optional.

[0101] InFigure 1 In the embodiment, the recess 55 is formed on the outer wall 13A of the main portion 12A of the sensing mass 12.

[0102] The buffer region 53 extends at a certain distance from the sensing mass 12 during rest (i.e., in the absence of acceleration).

[0103] The buffer region 53 includes a plurality of protruding portions 54 that extend from the buffer region 53 towards the sensing mass 12 along the first axis X. A plurality of buffer regions 54 extend outwardly from one or more side walls of the buffer region 53.

[0104] The protruding portion 54 extends along the first axis X at a distance w from the sensing mass 12 during rest; for example, the distance w can be between 1.4 μm and 2 μm.

[0105] In use, in the presence of in-plane acceleration, such as along the first axis X or the second axis Y, the sensing mass 12 can undergo in-plane rotation (i.e., in the plane XY) about the fulcrum F.

[0106] The buffer region 53 with the corresponding protruding portions 54 serves as the in-plane stop region of the sensing mass 12. In fact, as Figure 5 shown, the buffer region 53 defines the maximum in-plane rotation angle θ that the sensing mass 12 may undergo max . The angle θ max is defined as the angle between the first line A1 and the second line A2 (additionally shown in the enlarged detail of Figure 4 ), where the line A1 is the line passing through the end of the protruding portion 54 and the fulcrum F, and where the line A2 is the line passing through the fulcrum F and the portion of the sensing mass 12 facing the protruding portion 54 along the first axis X.

[0107] Furthermore, the outer wall 13A is at a long distance along the second axis Y relative to the fulcrum F, particularly longer than the distances of the outer wall 13B and the inner wall 20A from the fulcrum F.

[0108] As a result, the fact that the buffer region 53 is formed together with the outer wall 13A allows the maximum in-plane rotation angle θ that the sensing mass 12 may experience max to be minimized, thereby reducing the risk of mechanical fracture of the MEMS accelerometer 10.

[0109] In other words, the buffer region 53 can contribute to further optimizing the performance and reliability of the MEMS accelerometer 10.

[0110] Figure 6AShows a cross-section of a portion of the MEMS accelerometer 10 at the outer wall 13A of the main portion 12A of the sensing mass 12. Figure 6B Shows a cross-section of a portion of the MEMS accelerometer 10 at the outer wall 13B of the secondary portion 12B of the sensing mass 12.

[0111] As can be seen in the details of Figure 6A and Figure 6B The substrate 11 may include a bulk region 60 of semiconductor material (e.g., silicon) and an insulating region 61 (e.g., of silicon oxide) overlying the bulk region 60.

[0112] The MEMS accelerometer 10 also includes out-of-plane lower stop regions 63A, 63B of the sensing mass 12.

[0113] The out-of-plane lower stop region 63A includes a first protruding portion 64A integral with the main portion 12A of the sensing mass 12 and a second protruding portion 65A integral with the substrate 11 and facing the first protruding portion 64A along the third axis Z.

[0114] Specifically, the first protruding portion 64A extends along the third axis Z from the lower surface 66A of the main portion 12A of the sensing mass 12 towards the substrate 11.

[0115] The first protruding portion 64A is arranged near the outer wall 13A along the second axis Y, e.g., at a short distance (e.g., about 70 μm) from the outer wall 13A.

[0116] Generally, the first protruding portion 64A may be arranged at a certain distance from the outer wall 13A along the second axis Y to ensure contact with the second protruding portion 65A before the portion of the sensing mass 12 at the outer wall 13A contacts the underlying second protruding portion 65A.

[0117] The out-of-plane lower stop region 63B includes a first protruding portion 64B integral with the secondary portion 12B of the sensing mass 12 and a second protruding portion 65B integral with the substrate 11 and facing the first protruding portion 64B along the third axis Z.

[0118] Specifically, in the illustrated embodiment, the sensing mass 12 includes a peripheral portion 68 that extends along the second axis Y from the secondary portion 12B towards the outside of the secondary portion 12B. In fact, the peripheral portion 68 forms a protruding portion of the outer wall 13B.

[0119] The first protruding portion 64B extends along the third axis Z from the lower surface 66B of the peripheral portion 68 of the sensing mass 12 towards the substrate 11.

[0120] The first protruding portion 64B is arranged at the outer wall 13B.

[0121] As shown in Figure 7 and Figure 8 shown therein, the MEMS accelerometer 10 may include a lid 70, which is formed, for example, starting from a wafer of semiconductor material (specifically silicon), and is fixed to the substrate 11 in a manner not shown here. The lid 70 may be bonded to the substrate 11 through a bonding area of an insulating material (such as glass frit).

[0122] The lid 70 and the substrate 11 may form a hermetic chamber (not shown here) for accommodating the sensing mass 12.

[0123] The lid 70 includes an upper region 72 and two protruding regions 73A, 73B, which extend above the sensing mass 12 along the third axis Z on opposite sides with respect to the substrate 11 (indicated by the gray area in Figure 7 for clarity).

[0124] For simplicity, in Figure 7 only the protruding regions 73A, 73B of the lid 70 are shown as gray areas in a transparent manner.

[0125] The upper region 72 extends along the third axis Z at a distance g c from the sensing mass 12, and this distance g c is, for example, between 30 μm and 80 μm.

[0126] The protruding region 73A has a lower surface 74 facing the sensing mass 12. The lower surface 74 extends along the third axis Z at a distance g c shorter than the distance g s at rest. For example, the distance g s may be between 4 μm and 10 μm.

[0127] The protruding region 73B has a lower surface 75 facing the sensing mass 12. The lower surface 75 extends along the third axis Z to the distance g s at rest. However, relative to the lower surface 74, the lower surface 75 may extend at a different distance from the sensing mass 12.

[0128] In fact, the protruding regions 73A, 73B are out-of-plane upper stop member regions of the sensing mass 12.

[0129] The protruding regions 73A, 73B and the upper region 72 are sized such that, in use, when there is a potential difference between the lid 70 and the sensing mass 12, the total torque M tot applied to the sensing mass 12 by the lid 70 with respect to the rotation axis R is zero.

[0130] For example, if within the tolerance range, the total torque Mtot can be considered as zero, and this tolerance range can depend on, for example, the process variability during the manufacture of the MEMS accelerometer 10, which may cause variations relative to the design values.

[0131] Specifically, consider a general point P on the cover 70 i , the torque M exerted by the cover 70 on the sensing mass 12 i is given by:

[0132] M i = br i · F i

[0133] where br i is the arm of the point P i relative to the axis of rotation R, and F i is the electrostatic force exerted by the cover 70 on the sensing mass 12.

[0134] For each region of the cover 70, the electrostatic force F i is a function of the facing area on the sensing mass 12 and the distance from the sensing mass 12 along the third axis Z.

[0135] Figure 9 The region of the cover 70 facing the sensing mass 12 is schematically shown from above.

[0136] The protruding region 73A can be divided into two parts CP1, CP2, which are arranged on opposite sides of the axis of rotation R along the second axis Y. The protruding region 73B can be divided into two parts CP5, CP6, which are arranged on opposite sides of the axis of rotation R along the second axis Y. The upper region 72 can be divided into two parts CP3, CP4 arranged on opposite sides of the axis of rotation R.

[0137] It can be verified that the total torque M tot can be expressed as:

[0138] M tot = ∑ i Mi = ∑ i br i · F i

[0139] where for each part Cpi (where i = CP1,..., CP6), the arm br i can be approximated as the distance of the centroid of the part Cpi from the axis of rotation R along the second axis Y, and F i is the electrostatic force exerted by the part Cpi on the sensing mass 12.

[0140] Specifically, where A i is the facing area of a portion of CPi on the sensing mass 12, and g i is the distance of the portion of CPi from the sensing mass 12 along the third axis Z.

[0141] By appropriately sizing the area of the cover 70 facing the sensing mass 70, M tot = 0 can thus be obtained, whereby the expression can be used as the figure of merit (FOM) of the MEMS accelerometer 10.

[0142] The applicant has verified that if the absolute value of the figure of merit lies between 0 and 1 (including or excluding the boundaries), the total torque M tot can be considered zero.

[0143] For example, the MEMS accelerometer 10 can be designed in such a way that the figure of merit lies between 0 and 1. Alternatively, even if the designed value of the figure of merit is equal to 0, the actual figure of merit of the final MEMS accelerometer 10 may differ from zero due to variability during the manufacturing process.

[0144] In use, a potential difference of even up to 1.5 V can be established between the cover 70 and the sensing mass 12.

[0145] For example, the potential difference may be caused by external electrostatic / electromagnetic interference. The potential difference can be used during use to determine whether the cover 70 is floating electrically; for example, if the cover 70 is bonded to the substrate 11 via an insulating material.

[0146] However, the fact that the cover 70 is sized to counteract the total torque M tot applied to the sensing mass 12 means that the potential difference does not cause a spurious rotational component of the sensing mass 12 about the axis of rotation R. As a result, the cover 70 helps to improve the sensing accuracy of the MEMS accelerometer 10.

[0147] In other words, the cover 70 can help to further optimize the performance and reliability of the MEMS accelerometer 10.

[0148] The MEMS accelerometer 10 can be integrated into an inertial device 100 (shown in Figure 9 and Figure 10 in a Cartesian reference frame XYZ), comprising a plurality of MEMS inertial sensors integrated into the same die 101.

[0149] Specifically, the inertial device 100 includes a three-axis gyroscope (GYRO 102), a first three-axis accelerometer 103 indicated by LG-XL in the figure, and a second three-axis accelerometer 104 indicated by HG-XL in the figure.

[0150] The gyroscope 102 has a known structure of the type described, for example, in Italian patent application 102021000020504 corresponding to US patent application US 4124827, and the gyroscope is arranged in the first half of the die 101.

[0151] The first three-axis accelerometer 103 is a low-g accelerometer having a full-scale range of less than 100 g, for example, and includes two in-plane uniaxial accelerometers 120, 122 for sensing in-plane accelerations parallel to the first axis X and the second axis Y, and an out-of-plane uniaxial accelerometer 121 for sensing out-of-plane acceleration parallel to the third axis Z.

[0152] For example, depending on the specific application, the out-of-plane uniaxial accelerometer 121 may have the same structure as or a different structure from the MEMS accelerometer 10.

[0153] The second three-axis accelerometer 104 is a high-g accelerometer and includes two in-plane uniaxial accelerometers 124, 126 for sensing in-plane accelerations parallel to the first axis X and the second axis Y, and the previously described out-of-plane uniaxial accelerometer 10 for sensing out-of-plane acceleration parallel to the third axis Z.

[0154] The low-g in-plane accelerometers 120, 122 and the high-g in-plane accelerometers 124, 126 may have similar structures to each other and may be appropriately sized to have different full-scale ranges according to the specification application, or may have different structures.

[0155] The inertial sensors 102 to 104 are integrated into adjacent positions in the die 101 as described below and have a major dimension parallel to the plane XY.

[0156] The first three-axis accelerometer 103 and the second three-axis accelerometer 104 are arranged in the second half of the die 101, which has an area slightly larger than the first half.

[0157] Specifically, the gyroscope 102, the first three-axis accelerometer 103, and the second three-axis accelerometer 104 all have rectangular regions, are arranged side by side in the direction of the first axis X, and have substantially the same height measured along the direction parallel to the second axis Y and respective widths indicated by W1, W2, W3 measured in the direction parallel to the first axis X.

[0158] The die 101 is through Figure 10The bonding area 112 shown is fixed to the lid 111 visible in Figure 11 The lid 111 may form a reference

[0159] and the lid 70 described. In other words, at the out-of-plane accelerometer 10, the lid 111 may have the shape of the lid 70 described in Figure 7 and Figure 8 . Figure 7 and Figure 8 In addition, at the out-of-plane accelerometer 121, the size of the lid 111 may be determined in a manner similar to that described for the lid 70 to prevent possible sensing errors of the out-of-plane accelerometer 121.

[0160] The lid 111 may be shaped to define two separate chambers schematically indicated by 113A and 113B in

[0161] and is intended to accommodate the gyroscope 102 and the triaxial accelerometers 103, 104 respectively. Further, here, the lid 111 has a recess 114 ( Figure 10 ) to expose the contact pads 115 formed on the die 101 near the die side. Figure 11 As shown in

[0162] , the wire 116 connects the contact pad 115 to the die 117 in which the control circuit (ASIC) of the inertial device 100 may be formed. The die 117 is also coupled to a surface mount support 118 of, for example, the LGA (land grid array package) type. Figure 11 The dies 101, 117, the lid 111, and the support 118 may be packaged together in a known manner not shown to form a packaged device having a standard size (e.g., 3.0×2.5×0.86 mm

[0163] ) 3 .

[0164] Finally, it is clear that the MEMS accelerometer described and illustrated herein may be modified and varied without departing from the scope of the present disclosure as defined in the appended claims.

[0165] The sensing mass 12 may generally be provided with a series of through holes 140 that occupy its entire area, as exemplarily shown in Figure 7 and only shown by way of example in the upper right corner of Figure 1 and Figure 5 .

[0166] For example, the MEMS accelerometer 10 may have a structure different from the seesaw structure, in which the sensing mass 12 is coupled to the substrate 11 to undergo out-of-plane movement (e.g., translation) other than rotation in response to acceleration of the MEMS accelerometer along the out-of-plane direction (Z).

[0167] The MEMS accelerometer 10 may have a seesaw structure, but may have a shape other than the shapes shown and described.

[0168] For example, the MEMS accelerometer 10 may have a different number of springs and central anchoring regions. For example, the springs and central anchoring regions may have shapes and arrangements other than the shapes and arrangements shown and described.

[0169] For example, the frame 50 may only partially extend around the sensing mass 12.

[0170] For example, depending on the specific application, the MEMS accelerometer may have a different number of (additional and / or modified) damping structures.

[0171] For example, for each modified damping structure 25A to 25D, only the fixed fingers 31A to 31E or only the movable fingers 30A to 30D may have variable lengths.

[0172] For example, the arrangement, number, and shape of the in-plane and / or out-of-plane stop regions may be different from the arrangements, numbers, and shapes shown.

[0173] In view of the foregoing disclosure, particularly with reference to that described for Figure 4 and Figure 5 A MEMS accelerometer may be summarized as including: a substrate; a sensing mass suspended at a distance from the substrate along an out-of-plane direction (e.g., axis Z), the sensing mass being coupled to the substrate to undergo out-of-plane movement relative to the substrate in response to acceleration along the out-of-plane direction; and a frame fixed to the substrate and at least partially extending around the sensing mass, wherein the sensing mass has an outer wall facing the frame and a recess in the outer wall, and the frame includes a buffer region configured to form an in-plane stop region for the sensing mass that extends into the recess towards the sensing mass.

[0174] Furthermore, in view of the foregoing disclosure, particularly with reference to that described for Figures 7 to 9The description of a MEMS accelerometer can be summarized as including: a substrate; a sensing mass suspended at a certain distance from the substrate along an out-of-plane direction (e.g., axis Z), the sensing mass being coupled to the substrate to undergo out-of-plane movement relative to the substrate in response to acceleration along the out-of-plane direction; and a cover fixed to the substrate and having a first region that extends above the sensing mass at a certain distance from the sensing mass along the out-of-plane direction, the first region being sized such that a zero total torque is applied to the sensing mass by the first region of the cover in the case where there is a voltage difference between the first region of the cover and the sensing mass.

[0175] Finally, the above-described different embodiments can be combined to provide other solutions.

[0176] At least one embodiment of a MEMS accelerometer (10) of the present disclosure is summarized as including: a substrate (11); a sensing mass (12) suspended at a certain distance from the substrate along an out-of-plane direction (Z), the sensing mass being coupled to the substrate to undergo out-of-plane movement relative to the substrate in response to acceleration along the out-of-plane direction; and damping structures (25A to 25D) configured to damp in-plane movement of the sensing mass relative to the substrate, the damping structures including a plurality of movable fingers (30A to 30D) integral with the sensing mass (12) and a plurality of fixed fingers (31A to 31E) integral with the substrate (11) and intersecting the movable fingers, wherein the movable fingers and / or the fixed fingers have a variable length along a first in-plane direction (X) transverse to the out-of-plane direction (Z).

[0177] In at least one embodiment, the out-of-plane movement is a rotation of the sensing mass about a rotation axis (R) that is non-centroidal and transverse to the out-of-plane direction (Z).

[0178] In at least one embodiment, the fixed fingers (31A to 31E) and the movable fingers (30A to 30D) can face each other along a second in-plane direction (Y) transverse to the first in-plane direction (X) and the out-of-plane direction (Z), wherein the length of the fixed fingers and / or the movable fingers along the first in-plane direction (X) can decrease parallel to the second in-plane direction (Y).

[0179] In at least one embodiment, the length of the fixed fingers and / or the movable fingers along the first in-plane direction (X) can decrease parallel to the second in-plane direction towards the outer wall (13A) of the sensing mass.

[0180] In at least one embodiment, the length of the fixed fingers and / or the movable fingers along the first in-plane direction (X) can have a linear trend along a second in-plane direction (Y) transverse to the first in-plane direction (X) and the out-of-plane direction (Z).

[0181] In at least one embodiment, the MEMS accelerometer further includes: a first through cavity (20) that extends through the sensing mass (12), the sensing mass having an inner wall (20A) facing the first through cavity; an anchoring region (14) of the sensing mass (12) that is fixed to the substrate (11), disposed in the first through cavity and facing the inner wall (20A) of the sensing mass parallel to the first in-plane direction (X); and second through cavities (27A to 27D) that extend through the sensing mass (12), the fixed fingers (31A to 31E) and the movable fingers (30A to 30D) of the damping structure extending in the second through cavities (27A to 27D), the fixed fingers extending toward the first through cavity.

[0182] In at least one embodiment, the fixed fingers (31A to 31E) and / or the movable fingers (30A to 30D) have a minimum length along the first in-plane direction (X) at a curved portion (33) of the inner wall (20A) of the sensing mass (12).

[0183] In at least one embodiment, the MEMS accelerometer includes a frame (50) that is fixed to the substrate (11) and extends at least partially around the sensing mass (12), wherein the sensing mass may have an outer wall (13A) facing the frame and a recess (55) in the outer wall (13A), and the frame may include a buffer region (53) that is configured to form an in-plane stop region for the sensing mass and extends toward the sensing mass inside the recess.

[0184] In at least one embodiment, the outer wall (13A) may be a first peripheral portion of the sensing mass, the first peripheral portion may extend at a first distance (D1) from the axis of rotation (R) along a second in-plane direction (Y) that is transverse to the first in-plane direction (X) and the out-of-plane direction (Z), the sensing mass having a second peripheral portion (13B) that may extend at a second distance (D2) from the axis of rotation along the second in-plane direction, the first distance being longer than the second distance.

[0185] In at least one embodiment, the MEMS accelerometer further includes a lid (70) that is fixed to the substrate (11) and has a first region (70, 73A, 73B) that extends above the sensing mass (12) at a certain distance from the sensing mass along the out-of-plane direction (Z), the first region being sized such that when there is a voltage difference between the first lid region and the sensing mass, the first lid region applies a zero total torque on the sensing mass (12).

[0186] In at least one embodiment, the first lid region includes at least one first portion (72) at a first distance (g c ) from the sensing mass along an out-of-plane direction (Z) and at least one protruding portion (73A, 73B) at a second distance (g c ) from the sensing mass along the out-of-plane direction (Z), the second distance being shorter than the first distance.

[0187] In at least one embodiment, the MEMS accelerometer senses high out-of-plane accelerations.

[0188] In at least one embodiment, the MEMS accelerometer further includes at least one sensing electrode (18A, 18B) integral with the substrate (11) and extending along the out-of-plane direction at a certain distance from the sensing mass at rest, the at least one sensing electrode being capacitively coupled to the sensing mass.

[0189] At least one embodiment of a MEMS accelerometer (10) is summarized as including: a substrate (11); a sensing mass (12) suspended at a certain distance from the substrate along an out-of-plane direction (Z), the sensing mass being coupled to the substrate to undergo out-of-plane movement relative to the substrate in response to an acceleration along the out-of-plane direction; and a frame (50) fixed to the substrate (11) and at least partially surrounding the sensing mass (12), wherein the sensing mass has an outer wall (13A) facing the frame and a recess (55) in the outer wall (13A), and the frame includes a buffer region (53) configured to form an in-plane stop region of the sensing mass and extending towards the sensing mass inside the recess.

[0190] At least one embodiment of a MEMS accelerometer (10) is summarized as including: a substrate (11); a sensing mass (12) suspended at a certain distance from the substrate along an out-of-plane direction (Z), the sensing mass being coupled to the substrate to undergo out-of-plane movement relative to the substrate in response to an acceleration along the out-of-plane direction; and a lid (70) fixed to the substrate (11) and having a first region (70, 73A, 73B) extending above the sensing mass (12) at a certain distance from the sensing mass along the out-of-plane direction (Z), the first region being sized such that in the case where there is a voltage difference between the first region of the lid and the sensing mass, the first region of the lid exerts zero total torque on the sensing mass (12).

[0191] The various embodiments described above can be combined to provide other embodiments. If necessary, aspects of the embodiments can be modified to incorporate the concepts of various patents, applications, and publications to provide other embodiments.

[0192] These and other changes may be made to the embodiments in light of the foregoing detailed description. Generally, in the following claims, the terms used should not be construed as limiting the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments within the entire scope of equivalents of the rights accompanying such claims. Accordingly, the claims are not limited by the present disclosure.

Claims

1. A MEMS accelerometer, comprising: a substrate; a sensing mass that is suspended at a certain distance from the substrate in an out-of-plane direction, the sensing mass being coupled to the substrate to experience an out-of-plane movement relative to the substrate in response to an acceleration along the out-of-plane direction; and a damping structure configured to damp an in-plane movement of the sensing mass relative to the substrate, the damping structure including: a plurality of movable fingers integral with the sensing mass; and a plurality of fixed fingers integral with the substrate and intersecting the movable fingers, wherein at least one of the movable fingers and the fixed fingers has a variable length along a first in-plane direction transverse to the out-of-plane direction.

2. The MEMS accelerometer according to claim 1, wherein the out-of-plane movement is a rotation of the sensing mass about a rotation axis that is non-centroidal and transverse to the out-of-plane direction.

3. The MEMS accelerometer according to claim 1, wherein the movable fingers are spaced apart from each other along a second in-plane direction that is transverse to the first in-plane direction and the out-of-plane direction, and the fixed fingers are spaced apart from each other along the second in-plane direction.

4. The MEMS accelerometer according to claim 3, wherein the respective lengths of the movable fingers and the fixed fingers are measured along the first in-plane direction, and wherein the respective lengths of the movable fingers and the respective lengths of the fixed fingers are different between different fixed fingers.

5. The MEMS accelerometer according to claim 4, wherein the movable fingers include a first movable finger and a second movable finger, the first movable finger being disposed at a first distance from the rotation axis along the second in-plane direction, the second movable finger being disposed at a second distance from the rotation axis along the second in-plane direction, the second distance being greater than the first distance, and the respective length of the second movable finger along the first in-plane direction being less than the respective length of the first movable finger along the first in-plane direction, and wherein the fixed fingers include a first fixed finger and a second fixed finger, the first fixed finger being disposed at a third distance from the rotation axis along the second in-plane direction, the second fixed finger being disposed at a fourth distance from the rotation axis along the second in-plane direction, the fourth distance being greater than the third distance, and the respective length of the second fixed finger along the first in-plane direction being less than the respective length of the first fixed finger along the first in-plane direction.

6. The MEMS accelerometer according to claim 5, wherein the fixed fingers and the movable fingers face each other along a second in-plane direction that is transverse to the first in-plane direction and the out-of-plane direction, and wherein at least one of the respective lengths of the fixed fingers and the movable fingers along the first in-plane direction decreases parallel to the second in-plane direction.

7. The MEMS accelerometer according to claim 6, wherein the respective length of at least one of the fixed fingers and the movable fingers along the first in-plane direction decreases parallel to the second in-plane direction toward the outer wall of the sensing mass.

8. The MEMS accelerometer according to claim 1, wherein the respective length of at least one of the fixed fingers and the movable fingers along the first in-plane direction has a linear trend along a second in-plane direction that is transverse to the first in-plane direction and the out-of-plane direction.

9. The MEMS accelerometer according to claim 1, further comprising: a first through cavity that extends through the sensing mass, the sensing mass having an inner wall facing the first through cavity; an anchoring region of the sensing mass that is fixed to the substrate, disposed in the first through cavity and facing the inner wall of the sensing mass parallel to the first in-plane direction; and a second through cavity that extends through the sensing mass, wherein the fixed fingers and the movable fingers of the damping structure extend in the second through cavity, and the fixed fingers extend toward the first through cavity.

10. The MEMS accelerometer according to claim 9, wherein at least one of the fixed fingers and the movable fingers has a minimum length along the first in-plane direction at a curved portion of the inner wall of the sensing mass.

11. The MEMS accelerometer according to claim 1, comprising a frame that is fixed to the substrate and extends at least partially around the sensing mass, wherein the sensing mass has an outer wall facing the frame and a recess in the outer wall, and the frame includes a buffer region that is configured to form an in-plane stop region of the sensing mass and extends toward the sensing mass inside the recess.

12. The MEMS accelerometer according to claim 8, wherein the outer wall is a first peripheral portion of the sensing mass that extends along a second in-plane direction at a first distance from the axis of rotation, the second in-plane direction being transverse to the first in-plane direction and the out-of-plane direction, and the sensing mass has a second peripheral portion that extends along the second in-plane direction at a second distance from the axis of rotation, and the first distance is longer than the second distance.

13. The MEMS accelerometer according to claim 1 further includes a cover, the cover being fixed to the substrate and having a first region that extends above the sensing mass at a certain distance from the sensing mass along the out-of-plane direction, the first region being sized such that when there is a voltage difference between the first cover region and the sensing mass, the first cover region applies zero total torque on the sensing mass.

14. The MEMS accelerometer according to claim 13, wherein the first lid region comprises: At least one first portion, at a first distance from the sensing mass along the out-of-plane direction; And at least one protruding portion, at a second distance from the sensing mass along the out-of-plane direction, the second distance being shorter than the first distance.

15. The MEMS accelerometer according to claim 1, wherein the movable fingers, the fixed fingers, and the sensing mass are configured to sense high out-of-plane accelerations during operation.

16. The MEMS accelerometer according to claim 1 further includes at least one sensing electrode, the at least one sensing electrode being integral with the substrate and extending at a certain distance from the sensing mass along the out-of-plane direction during rest, the at least one sensing electrode being capacitively coupled to the sensing mass.

17. A MEMS accelerometer includes: A substrate; A sensing mass that is suspended at a certain distance from the substrate along the out-of-plane direction, the sensing mass being coupled to the substrate to undergo out-of-plane movement relative to the substrate in response to an acceleration along the out-of-plane direction; And A frame that is fixed to the substrate and at least partially surrounds the sensing mass, Wherein the sensing mass has an outer wall facing the frame and a recess in the outer wall, the frame includes a buffer region configured to form an in-plane stop region for the sensing mass and extending into the recess towards the sensing mass.

18. The MEMS accelerometer according to claim 17, wherein: The buffer region includes a plurality of protrusions that extend outward from one or more sidewalls of the buffer region; and The recess is in a main portion of the sensing mass.

19. A MEMS accelerometer includes: A substrate; A sensing mass that is suspended at a certain distance from the substrate along the out-of-plane direction, the sensing mass being coupled to the substrate to undergo out-of-plane movement relative to the substrate in response to an acceleration along the out-of-plane direction; And A cover that is fixed to the substrate and has a first region that extends above the sensing mass at a certain distance from the sensing mass along the out-of-plane direction, the first region being sized such that when there is a voltage difference between the first region of the cover and the sensing mass, the first region of the cover applies zero total torque on the sensing mass.

20. The MEMS accelerometer according to claim 19, further comprising a frame, the frame being fixed to the substrate and at least partially surrounding the sensing mass, and wherein: The sensing mass has an outer wall facing the frame and a recess in the outer wall; And The frame further includes a buffer region configured to form an in-plane stop region of the sensing mass, extending towards the sensing mass inside the recess; The buffer region includes a plurality of protrusions extending outward from one or more sidewalls of the buffer region; And The recess is in a main portion of the sensing mass.

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