MEMS-type inertial sensor with specific mechanical linkage

Abstract

MEMS-type inertial sensor with specific mechanical linkage. Microelectromechanical device of the inertial sensor type, comprising a support, a movable frame (3) guided in translation along a displacement axis (X) parallel to the support, and comprising a test mass (4) extending from a first end (41), connected to the support by a mechanical linkage, to a second end (42), the mass being connected, on the side of the second end, to a detection element (7) for pivoting the mass relative to the frame. Said linkage comprises a thin and flexible wall (50) extending parallel to the support, from the frame to the first end of the test mass, along a mean line parallel to the displacement axis of the frame. Figure to be published with the abbreviation: Figure 9

Description

Title of the invention: MEMS type inertial sensor with specific mechanical connection Technical field

[0001] The technical field of the invention is that of micro-electromechanical devices or MEMS (for "MicroElectroMechanical Systems" in English). The invention relates in particular to such a device used as an inertial sensor, and comprising a test mass which can pivot relative to a frame or a support under the effect of the forces to which it is subjected. TECHNOLOGICAL BACKGROUND

[0002] Different types of micro-electromechanical gyrometers have been developed in recent years, in particular gyrometers with “out-of-plane” movement, such as that shown schematically in [Fig.l], which make it possible to measure a rotation speed around a Y axis parallel to the mean plane of the substrate from which the gyrometer laa is made.

[0003] The major part of this substrate forms a thick layer, which serves as support 2aa. The gyrometer 1a comprises two movable frames 3aa and 3aa' each guided in displacement relative to this support 2aa, along an axis X which is parallel to the support 2aa (parallel to the substrate), and perpendicular to the axis of measurement of angular speed of rotation, Y. During operation of the gyrometer, the two frames are set in motion, for example by electrostatic actuation by means of interdigital combs (not shown), so as to oscillate parallel to the axis X, in phase opposition to each other (symmetrically). The two frames then have displacement speeds, relative to the support 2aa, which are opposite to each other.

[0004] Each frame 3aa, 3aa' carries with it a test mass 4aa, 4aa' (also called Coriolis mass), which is connected to the frame by a connection allowing the test mass to pivot around an axis of rotation (axis of the connection) parallel to the Y axis. This connection is thus similar, in part, to a pivot connection, or, in other words, to a hinge.

[0005] When the gyrometer laa rotates (i.e.: when the support 2aa rotates) relative to an inertial frame of reference (for example relative to the geocentric frame of reference), around the Y axis, with an angular velocity q _ Qy, each mass 4aa, 4aa' then undergoes a Coriolis force, which is expressed as — 2 m,or (vx) A °where mœr represents the mass of any one of these two test masses 4aa, 4aa' and where v is its speed of movement (along the X axis). This force ("out of plane") is therefore directed along the Z axis and it is of the same amplitude but in opposite directions for the two test masses 4aa and 4aa' (since the two masses are driven in opposite directions). For each mass, this force therefore causes an out-of-plane displacement of the mass, more precisely a displacement of a moving end of the mass, in a direction parallel to the Z axis. This force therefore causes a pivoting of the mass around its axis of rotation (axis of the mass-frame connection). In this case, this out-of-plane displacement (this pivoting) takes place in an opposite direction for the two masses. The out-of-plane displacement of these two masses then causes a detection lever 7aa to rotate around a detection axis (parallel to the Y axis). This rotation of the lever is then measured, for example using strain gauges (piezoresistive, piezoelectric or resonant detection) to deduce the angular velocity Q.These gauges may be connected to the pivot detection lever on the one hand, and to the support on the other hand, and be arranged to stretch or compress when the lever pivots.

[0006] In a gyrometer such as this, it is known to make the connection, which connects one of the frames to the corresponding test mass, by means of one or more torsion beams (figures 2 and 3): • each having a significant tMEMS thickness along the Z axis, to be rigid with respect to a movement along the Z axis, in order to fix the Z position of one end of the test mass (to effectively fulfill a “hinge” function), • and having a reduced extension (width) in a direction parallel to the X axis, to facilitate the torsion of the beam around the Y axis and thus allow the mass to pivot around its axis of rotation without too much resistance.

[0007] Figures 2 and 3 show an example of embodiment of such a connection based on this principle. In this example, the connection comprises four torsion beams 61aa, 62aa, 63aa, 64aa, each of which forms a sort of wall, parallel to the plane containing the Y and Z axes. The beams 61aa and 62aa are each connected (rigidly), on the one hand to the frame 3aa, and on the other hand to a common central island 65aa. Similarly, the beams 63aa and 64aa are each connected, rigidly, on the one hand to the mass 4aa (at its first “connected” end), and on the other hand to the central island 65aa.

[0008] But this type of connection is not completely satisfactory. Indeed, it is desirable that the connection between frame and mass: • a) is capable of transmitting a force parallel to the Z axis, with little deformation in this direction, to fix the position in Z of one of the ends (or in any case of a given point) of the test mass, • b) is capable of transmitting a force parallel to the X axis, deforming little in this direction, so that the frame can carry the mass with it, And • c) opposes as little as possible a rotation of the mass around its axis of rotation (pivoting), by presenting the smallest possible stiffness with respect to this rotation.

[0009] Concerning criterion c), it is indeed noted that the sources of elastic stiffness, which oppose the rotation of the test mass 4aa include: the rotational stiffness due to the connection, as well as the stiffness of the strain gauge(s) (used to measure the rotation of the test mass around its axis of rotation), and possibly other sources of stiffness opposing such rotation. However, it is desirable to minimize the stiffness which is not due to the gauges themselves, so that a maximum of energy is directed towards the gauges in order to maximize the rotation measurement signal (in terms of detection and from an energy point of view, it is desirable to avoid storing energy - in the form of elastic energy - in the mass-frame connection).

[0010] However, for a torsion beam such as beams 61aa to 64aa, criteria b) and c) are partly antagonistic. Indeed, to make the beam very flexible in torsion (around the Y axis), its width a could be reduced enormously, but this would make the beam very inflexible with respect to a displacement along the X axis.

[0011] More precisely, such a beam opposes a displacement of its end, in a direction parallel to the X axis, with a coefficient of stiffness in bending k'x which is proportional to ^tNIEMS a3) / b3, °where b is the length of the beam along the Y axis. And it opposes a torsion around the Y axis with a coefficient of stiffness in bending C'Y which is proportional to (tMFMS a3) / b (when a is very small compared to tMEMS). Reducing the width a therefore has the effect of reducing the stiffness in bending, but also the stiffness in bending whereas it would be desirable on the contrary to increase it (because it is useful, since it allows a drive of the mass by the frame).

[0012] It is possible, to a certain extent, to reduce both the width a of the beam and its length b, in an attempt to obtain a high stiffness coefficient k'x while reducing C'Y (indeed, the ratio k'x / C'Y is proportional to 1 / b2). But this then leads to very small values ​​of width a which can pose manufacturing problems, particularly in terms of reproducibility. Indeed, engraving a block about twenty microns high (extension along the Z direction), to delimit in this block a wall less than 1 micron wide (extension of the beam in the X direction) and twenty microns high (20 microns thick) generally leads to a wall whose exact width is difficult to control precisely, this width also varying over the height (i.e.: over the thickness) of the wall in question, so that the mechanical characteristics of the connection are then difficult to control.

[0013] For a gyrometer as described above, there is therefore a need for a more efficient connection than the connection of the prior art described above, allowing in particular a better compromise between • on the one hand, a high stiffness with respect to a displacement parallel to the direction of displacement X and Z of the mobile frame, and • on the other hand, flexibility with respect to rotation around an axis parallel to the detection axis of the gyrometer (parallel to the Y axis). SUMMARY

[0014] To remedy at least in part the limits of the prior art, the present technology then relates to a micro-electromechanical device comprising: • a support, • a mobile frame, guided in translation relative to the support along a displacement axis parallel to a mean plane of the support, • a proof mass, which extends from a first end to a second end of the proof mass, • its first end being connected to the mobile frame by a mechanical connection allowing the mass to pivot, relative to the frame, around an axis of rotation which is parallel to the mean plane of the support and perpendicular to the axis of movement of the frame, • the test mass being connected, on the side of its second, movable end, to a member for detecting a pivoting of the test mass around its axis of rotation, • device in which the connection comprises a thin wall which extends parallel to the mean plane of the support, from the frame to the first end of the proof mass, along a mean line which is parallel to the axis of movement of the frame, and which, along a Z axis perpendicular to the mean plane of the support, has a thickness less than the thickness of the proof mass, or even less than one tenth of the thickness of the proof mass.

[0015] The mean line in question connects the frame to the first end of the test mass. This is, for example, the neutral fiber of the wall which connects these two elements.

[0016] Due to its small thickness along the Z axis, this wall is very flexible: its stiffness, with respect to bending around the axis of rotation of the test mass (and therefore with respect to rotation of the mass around this axis), is very low.

[0017] On the other hand, as this connecting wall extends along a mean line which is parallel to the axis of movement of the frame, it works in compression or in extension, during the transmission of movements parallel to this axis, and it is therefore very rigid with respect to the transmission of such movements (or with respect to the transmission of forces parallel to this axis).

[0018] Connecting the mass to the frame by means of such a wall therefore makes it possible to obtain a very good compromise between criteria b) and c) mentioned above.

[0019] More precisely, for the transmission of movements parallel to the axis of movement of the frame, such a wall has a stiffness coefficient kx (stiffness in compression / extension) which is proportional to (1 .ep)f L, where: • 1 is the width of the wall, that is to say its extension parallel to the axis of rotation of the mass (axis of the connection), • ep, also noted tNEMS below, is the thickness of the wall along the Z axis, and • L is the length of the wall, that is to say its extension parallel to the axis of movement of the frame (L corresponds for example to the distance which separates the frame from the first end of the test mass).

[0020] On the other hand, the stiffness of this wall with respect to bending, around the axis of rotation of the mass, is associated with a stiffness coefficient CY which is proportional to (1 .ep3) / L. It is therefore possible to obtain both a high stiffness coefficient kx and a low stiffness coefficient CY by using such a wall, which is fairly short (low L), with a fairly large section 1 .ep, and a low thickness ep. Indeed, the ratio kx / CY is proportional to 1 / ep2 (instead of 1 / b2 in the prior art connection presented above), and the thickness ep can be very low while being very well controlled since it is the thickness of an element parallel to the mean plane of the support (parallel to the mean plane of the substrate), resulting for example from a layer obtained by a planar technology or from a Si-top layer of an SOI substrate (this connection wall is typically formed in a NEMS layer of the device).However, it is easier to control the thickness, regularity and reproducibility of such a layer than the reduced "width" of a thin and very "high" wall (significant thickness) obtained by etching a block, perpendicular to the average plane of this block. This configuration therefore makes it possible, among other things, to resolve the manufacturing difficulties mentioned above (and this thin layer may also be necessary for the production of measuring gauges).

[0021] In addition to the wall mentioned above, the connection in question may comprise a retaining element comprising a retaining wall: • which is perpendicular to the mean plane of the support, • which, in a direction parallel to the axis of movement of the frame, has an extension (called width in the following, for the retaining wall) less than a quarter of the thickness of the retaining wall along the Z axis, • and which connects on the one hand to the mobile frame, and on the other hand to the first end of the test mass.

[0022] As this retaining wall is parallel to the Z axis and has a height (ie: a significant thickness) along the Z axis, it is rigid with respect to a displacement of one of its ends parallel to the Z axis (like a flat ruler, whose plane would be vertical, perpendicular to the mean plane of the support); it therefore makes it possible to fix the position in Z of the first end of the test mass, and therefore makes it possible to fulfill the mechanical connection criterion a) mentioned above, in the background part. This retaining wall prevents in particular a deformation of the connecting wall described previously, under the effect of forces parallel to Z.

[0023] And since the retaining wall is not very thick, it remains flexible in torsion, with respect to a torsion around the axis of rotation of the mass. In this regard, since the rigidity function along the X axis is ensured by the first wall, mentioned above (wall parallel to the mean plane of the support), one can freely reduce the width, or increase the length of this second retaining wall (thus reducing its torsional stiffness). And the possible variability (difficulty of control) of the width of this retaining wall, when this width is very small, is not problematic here, precisely because this second wall is not intended to transmit forces in the X direction (in other words, this variability is then no longer a problem because the stiffness is so reduced that it becomes negligible compared to the others, in particular that due to the gauges. The variation of something negligible has no high-level consequences).

[0024] The use of these two thin walls, one parallel to the mean plane of the support, and the other perpendicular to it, therefore makes it possible to decouple the functions of the connection to be made and thus to obtain a better compromise between the different constraints a) to c) mentioned above.

[0025] The configuration presented above, in which the forces parallel to the X axis are transmitted by this first wall, thin and parallel to the support, brings many improvements, as explained above.

[0026] However, this configuration appears at first sight to be problematic in terms of detection for this inertial sensor type device.

[0027] Indeed, in practice, the first wall, parallel to the mean plane of the support, most often extends in the extension of either a lower face or an upper face of the inertial mass. In general, it extends in the extension of the lower face of this mass (face located on the side of the support), and it comes from a Si-top layer of an SOI substrate (while the mass and the frame itself are etched in a thick layer deposited on this Si-top layer).

[0028] The forces parallel to the X axis, transmitted by the connection, are therefore transmitted at the level of a lower (or upper) face of the test mass. These forces therefore do not pass through the center of mass of the test mass, and induce a torque which therefore tends to rotate this mass around its axis of rotation. In In practice, this undesirable effect can be very significant, because the forces transmitted parallel to the X axis are often significantly greater than the Coriolis forces to be detected (as an illustration, the amplitude of oscillation along the X axis can be around ten microns, while the out-of-plane displacement of the free end of the test mass is, for example, of the order of 0.1 micron, or even less).

[0029] This undesirable effect, due to the torque of the forces transmitted by the connection, can nevertheless be compensated for, and even cancelled out by the use of a second frame-mass assembly, symmetrical to the frame-mass assembly presented above, the two test masses then being connected to the same pivoting detection member, in this case a lever, respectively on one side and the other of this lever.

[0030] It should be noted that the fact that this double frame and common lever configuration makes it possible to compensate for this undesirable moment of force (due to the vertical offset of the axis of rotation of the connection relative to the center of gravity of the mass) is however not immediate, and requires careful analysis of the mechanical forces present, represented schematically in [Fig.9].

[0031] In any case, it can be provided that the device is of the gyrometer type, the frame, the test mass and the connection mentioned above being called respectively first frame, first test mass and first connection, the device then further comprising: • a second mobile frame, guided in translation relative to the support along said axis of movement, • a second proof mass, which extends from a first end to a second end of the second proof mass, said second end being located on the side of the first proof mass while the first end is located on the opposite side, • the first end of the second mass being connected to the second frame by a second mechanical connection identical to the first connection, • the second test mass being connected, on the side of its second movable end, to the same pivot detection member as the first test mass, • said detection member being a lever, movable in rotation around a detection rotation axis which is parallel to the rotation axis of the first test mass, the lever being connected on one side to the first test mass, and on the other to the second test mass.

[0032] In this document, the term "first end", as well as the term "second end", does not necessarily designate a terminal end of the proof mass (i.e. located at the very end of the proof mass and which terminates this proof mass). In other words, the first end can delimit the test mass in an area that is not located completely at the end of this mass (and the same for the second end). But in any case, the first and second ends are located opposite each other (i.e.: on one side and the other) relative to the center of mass (center of gravity) of the test mass. The first end, as well as the second end, is for example located closer to a terminal end of the test mass than to the center of mass of the test mass. It can also correspond directly to the terminal end of the test mass.

[0033] Furthermore, by connection we mean an element, or a set of elements guiding the movement of the test mass relative to the frame (i.e.: arranged to articulate the mass with the frame).

[0034] By detection member, we mean: a mechanical part (in practice, a micropart) driven by the second end of the test mass, or an electromechanical transducer (for example a strain gauge) linked to this end and whose electrical characteristics vary during the movement of the second end in a direction parallel to the Z axis, or even an assembly comprising such a part and such a transducer.

[0035] By wall is meant an element delimited by two free surfaces substantially parallel to each other (parallel at better than 15 degrees) and separated by a distance, smaller, and even significantly smaller than the dimensions of these free surfaces. In other words, it is an element (not necessarily planar) whose surface area is significantly greater than the distance between the two free surfaces of the wall (i.e.: significantly greater than the extent of the element in a transverse direction).

[0036] In this document, for the first wall as for the second wall, the term “thickness” designates the extension of the wall in the Z direction, perpendicular to the layers (perpendicular to the substrate).

[0037] On the other hand, in this document, the term "parallel" (and "perpendicular") means: parallel (or, respectively, perpendicular) to better than 15 degrees, or even to better than 5 or even 1 degree.

[0038] In addition to the characteristics mentioned above, the device which has just been presented may include one or more of the following optional characteristics, considered individually or in all technically conceivable combinations: • the thickness of said wall, parallel to the mean plane of the support, is less than one thirtieth of the thickness of the proof mass; said wall connects to the frame along a connection line which is parallel to the axis of rotation of the proof mass; said wall connects to the proof mass along a another connecting line which is also parallel to the axis of rotation of the proof mass; said wall is delimited laterally, on one side and the other, by two free edges which each extend from the frame to the proof mass; each of these two free edges is parallel to the axis of movement of the frame; neither of these two edges is parallel to the axis of rotation of the proof mass; the test mass is delimited by a lower face, located on the side of the support, and, on the other side, by an upper face; said wall, parallel to the mean plane of the support, extends in the extension of the lower face of the test mass; the device comprises a micro-electromechanical actuator configured to impose an oscillation on the mobile frame along said axis of movement; the test mass defines a mean plane, which is parallel to the mean plane of the support when the mass is at rest; the support of the device is formed by a layer, for example of semiconductor material, thicker than the frame or the proof mass; the mean plane of the support is the plane defined by this layer; the retaining wall is flat; the retaining wall is perpendicular to the axis of movement of the movable frame; the retaining wall is delimited, on the support side, by a lower, free edge, and, on the opposite side, by an upper, also free edge. the extension of the retaining wall in a direction parallel to the axis (X) of movement of the frame is less than one tenth, or even one twentieth of a length, presented by this wall in a direction parallel to said axis of rotation, between a point of connection to the mobile frame and a point of connection to the test mass; the extension of the retaining wall in a direction parallel to the axis (X) of movement of the frame is less than one tenth of its thickness (of its height); the retaining wall extends in a direction parallel to the axis of rotation of the test mass, from a first end to a second end of the retaining wall, and: • the first end and the second end of the retaining wall are both connected to the movable frame, while the test mass is connected to the retaining wall in a middle zone of said wall, between these two ends, • or, the first end and the second end of the retaining wall are both connected to the test mass, while the frame connects to the retaining wall in a middle zone of said wall, between these two ends, • the retaining wall extends in a direction parallel to the axis of rotation of the test mass, from a first end to a second end of the retaining wall, the first end of the retaining wall being connected to the movable frame while its second end is connected to the test mass, or vice versa; • the device comprises at least one additional retaining wall, which is perpendicular to the mean plane of the support, which has, in a direction parallel to the axis of movement of the frame, an extension less than a quarter of the thickness of the retaining wall along the Z axis, and which extends, in a direction parallel to the axis of rotation of the test mass, from a first end to a second end of the additional retaining wall, the first end of the additional retaining wall being connected to the movable frame while its second end is connected to the test mass, or vice versa; • the retaining wall and the additional retaining wall are symmetrical to each other, with respect to a plane of symmetry of the frame and the test mass, plane of symmetry parallel to the X,Z plane; • said wall, parallel to the mean plane of the substrate, is formed by a portion of a thin upper silicon layer of a silicon-on-insulator SOI substrate, and the support of the device is formed by a thick support layer of said SOI substrate; • over the majority of its thickness, the proof mass is formed of silicon (which can be polycrystalline, or monocrystalline, deposited on said thin upper silicon layer); • said thin upper layer of the SOI substrate is essentially monocrystalline; • the connection connecting the detection lever to the first test mass, and the connection connecting the detection lever to the second test mass are located opposite each other with respect to the detection rotation axis; • the device comprises an electromechanical actuation system configured to impose on each of the movable frames an oscillation along said axis of movement, the movement of the first frame relative to the support, and the movement of the second frame relative to the support having the same amplitude and opposite directions to each other.

[0039] The present technology and its various applications will be better understood by reading the following description and examining the accompanying figures. BRIEF DESCRIPTION OF THE FIGURES

[0040] The figures are presented for information purposes only and are not limiting.

[0041] [Fig.l] schematically represents a gyrometer with two moving frames of the prior art.

[0042] [Fig.2] schematically represents a mechanical connection of the gyrometer of the [Fig.l], this connection connecting one of the mobile frames to a test mass of the gyrometer, this connection being represented in perspective and in a situation where the frame pulls on the test mass to drive it in its movement.

[0043] [Fig.3] schematically represents the connection of [Fig.2], seen from above.

[0044] [Fig.4] is a schematic perspective representation of a two-axis gyrometer mobile frames implementing this technology.

[0045] [Fig.5] schematically represents a part of the gyrometer of [Fig.4], seen from the side.

[0046] [Fig.6] is a schematic perspective representation of a mechanical connection of the gyrometer of [Fig.4], this connection connecting one of the moving frames to one of the test mass of the gyrometer.

[0047] [Fig.7] is a partial schematic representation of the connection of [Fig.6], seen in section according to a section plane AA marked in [Fig.8].

[0048] [Fig.8] is a schematic representation of the connection of [Fig.6], seen from above.

[0049] [Fig.9] schematically represents the forces transmitted by the gyrometer connections of [Fig.4]. DETAILED DESCRIPTION

[0050] Figures 4 and 5 show a device 1, of the gyrometer type, implementing the present technology. This gyrometer 1 is a double-frame gyrometer with out-of-plane movement, which makes it possible to measure a rotation speed around a Y axis parallel to the mean plane of the substrate from which the gyrometer 1 is made.

[0051] The majority of this substrate forms a thick layer which serves as support 2.

[0052] The gyrometer 1 comprises two mobile frames 3 and 3' each guided in de placement, relative to this support 2, along an axis X which is parallel to the mean plane P of the support, mean plane which is also the mean plane of the gyrometer itself, generally flat. The axis of movement of the frames, X, is also perpendicular to the axis of measurement of angular rotation speed, Y. The axis of movement of the frames, X, and the axis of measurement of angular speed, Y, are reported on the different figures, as well as an axis Z, perpendicular to the mean plane of the support (perpendicular to X and Y).

[0053] For each frame, the guiding of the frame relative to the support is obtained for example by means of four springs 20, arranged at four points of the frame distant from each other, each spring 20 connecting the frame to the support while allowing a movement relative parallel to the X axis. The springs 20 here comprise blades working in flexion, which connect the support 2 to the frame considered 3, 3'. The frames 3 and 3' have a rectangular overall shape, here.

[0054] During operation of the gyrometer, the two frames are set in motion, for example by electrostatic actuation by means of interdigital combs (not shown), so as to oscillate parallel to the X axis, in phase opposition to each other (symmetrically). The two frames then have directions of movement, relative to the support 2, which are opposite to each other.

[0055] The two frames 3 and 3' are arranged opposite each other, on either side of a central portion 8, fixed (i.e.: without movement relative to the support 2) of the gyrometer. In [Fig.4], the detail of this central portion 8 is not shown, so as not to clutter the figure. In other words, [Fig.4] is a partial view. The boundary between the non-shown parts and the shown parts is marked in the figure by three thick wavy lines.

[0056] Each frame 3, 3' carries with it a test mass 4, 4' (also called Coriolis mass), which is connected to the frame by a link 5, 5' which allows the test mass to pivot around an axis of rotation Ai, A\ parallel to the Y axis. This link is partly similar to a pivot link, or, in other words, to a hinge. As will be seen below, the link 5, 5' is very rigid with respect to relative movements between the mass and the frame directed along the X axis. Due to this strong coupling, for the oscillation movement of each mass-frame assembly, relative to the support, parallel to the X axis, we obtain almost a single resonance frequency, typically between 1 and 100 kHz (or even between 5 and 50 kHz).

[0057] As can be seen in [Fig. 4], each test mass 4, 4' here has an overall shape which is that of a plate (approximately parallelepiped), parallel to the mean plane P of the support 2 when the mass is at rest. This plate is surrounded by the corresponding frame, over the majority of its perimeter. Each mass 4, 4' extends from a first end 41, 41' to a second end 42, 42'. The mean axis of the test mass 4, 4', which connects its first end to its second end, is parallel to the axis of movement, X. Each mass 4, 4' is delimited, on the side of the support 2, by a lower surface 44, 44' (parallel to the support when the mass is at rest) and, on the opposite side, by an upper surface 45, 45'.

[0058] The first end 41, 41' is connected to the frame 3, 3' by the connection 5, 5' in question, while the second end 42, 42' of the mass can move "out of plane", in a direction parallel to the Z axis, when the mass 4, 4' pivots around its axis of rotation Ab A'p

[0059] For each mass 4, 4', the first end 41, 41' is located, relative to the rest of the test mass 4, 4', opposite the other test mass 4', 4 (and therefore at the opposite of the other frame 3'). While the second end 42, 42', is located on the contrary on the side of the other test mass 4', 4 almost opposite the other mass, in a central zone of the gyrometer 1. In other words, each test mass 4, 4' is connected to its frame 3, 3' (by the connection 5, 5') on the sort of external side of the frame, on one side of this mass located opposite the other test mass 4', 4.

[0060] Each test mass 4, 4' is connected, on the side of its second end 42, 42', to a common rotation detection lever 7. This lever 7 pivots around a detection axis A3, which is parallel to the axis Y and which is fixed relative to the support 2. The lever 7 is located in the central zone of the gyrometer, between the two test masses.

[0061] When the gyrometer 1 rotates (i.e.: when the support 2 rotates) with respect to an inertial frame of reference, for example with respect to the geocentric frame of reference, around the Y axis, with an angular velocity q _ each mass 4, 4' then undergoes a Coriolis force, which is expressed as — 2 meor (y Y) A (Qy) °where HLor represents the mass of any one of the test masses 4, 4' and where v is its speed of movement (along the X axis). This force is therefore directed along the Z axis and it is of the same amplitude but in opposite directions for the two test masses 4 and 4' (since the two masses are driven in opposite directions). For each of these two masses, this force therefore causes a displacement of its second end 42, 42', along the Z axis (or, formulated differently, a pivoting of the mass around its axis of rotation Ai, A' i), in an opposite direction for the two masses 4, 4', which then causes the detection lever 7 to rotate around the detection axis A3 (see [Fig.5]).This rotation of the lever is measured, here using strain gauges (piezoresistive, piezoelectric, or resonant detection) to deduce the angular velocity Q.

[0062] In Figures 4 and 5, the displacement amplitude along the Z axis is exaggerated (i.e.: this displacement is not shown to scale), to make it clearly visible. In practice, the oscillation amplitude of the frames, along the X axis, is of the order of ten microns (which is very high, for a MEMS), while the displacement, along the Z axis, of the second ends of the masses is for example of the order of 0.1 micron. In these figures, the displacement of the frames and the masses along the X axis is represented schematically by the double arrows M1 and M1', while the displacement along the Z axis of the second ends of the masses is shown schematically by the double arrows M2 and M2'.

[0063] Each connection 5, 5' has a certain stiffness, opposing a rotation of the corresponding test mass 4, 4' around the axis Ab or A'p. To this rotational stiffness is added the stiffness of the strain gauges and the rotational stiffness of the connection 9, 9' between the mass 4, 4' and the lever 7. The rotational movement of the mass 4, 4' around its axis of rotation Ab A\ is associated with a resonance frequency, chosen for example so as to be close to (slightly higher, for example by 1 to 10%, than) the resonance frequency of the frame-mass assembly in its oscillation parallel to X. This makes it possible to obtain a larger angular velocity measurement signal Q.

[0064] In any case, the overall architecture of the gyrometer, with two test masses 4 and 4' which oscillate symmetrically and which actuate the same rotation detection lever 7, is particularly interesting because it allows differential detection of the angular rotation speed Q, which greatly improves the signal-to-noise ratio of this gyrometer 1. Indeed, any mechanical vibrations to which the gyrometer 1 may be subjected, for example in the X direction or in the Z direction, have the same influence on the mass 4 and on the mass 4'. In other words, they cause a parasitic displacement which is identical for these two masses (for example in the +Z direction for the two masses), and which therefore has no influence on the detection lever 7 (i.e.: does not cause this lever to pivot).This architecture therefore allows effective rejection of these parasitic vibrations, directly at the level of the mechanical detection organ 7, even before the electromechanical conversion by the gauges.

[0065] The fact, for each mass, of decoupling the connection 5, 5' on the one hand, and the mass rotation detection member, 7, on the other hand (the two being arranged respectively at two different ends of the mass), makes it possible to configure each of these two elements independently of one another, with a great deal of freedom.

[0066] The 5 and 5' bonds are now described in more detail. These two bonds are identical here, so that only the 5 bond is described in detail (with reference to Figures 6 to 8).

[0067] This link 5 comprises at least: • a first wall 50, also called a connecting wall, or transmission wall, parallel to the mean plane P of the support 2; this wall allows the frame 3 to carry the mass 4 with it, with high rigidity in the X direction, and • a second wall, 61, also called a retaining wall, perpendicular to the mean plane of the support, and which fixes the position in Z of the first end 41 of the test mass.

[0068] As can be seen in [Fig.8], the connection 5 also comprises a third wall 55, parallel to the mean plane of the substrate. The connection thus comprises two transmission walls (in a way as if this wall were divided into two parts, instead of being in one piece). Alternatively, the connection could also comprise a different number of transmission walls, or retaining walls. Thus, in the example of [Fig.8], the connection 5 comprises a single retaining wall (the wall 61). But, as a variant, it could comprise two separate retaining walls, or even more than two, the set of retaining walls being advantageously symmetrical with respect to a plane of symmetry which is parallel to the X and Z axes (and which is also a plane of symmetry for the mass and the frame). Each of these retaining walls is thin (in the sense given below for the wall 61), parallel to the Z axis, perpendicular to the X axis, and it connects on the one hand to the frame, and on the other hand to the test mass.

[0069] The first, second and third walls 50, 61 and 55 each connect the first end 41 of the mass 4 to the frame 3, more precisely to a part of this frame which corresponds to one of the sides of this rectangular frame, a side which is parallel to the Y axis. These three walls thus connect: • a face, presented by this part of the frame, and which is perpendicular to the X axis, • at the first end 41 of the mass 4, first end which forms an end face, also perpendicular to the X axis (these two faces being separated by a sort of slot).

[0070] The walls 50 and 55 each have a thickness denoted tNEMS. This is their extension parallel to the Z axis. This thickness is less, and even significantly less than the thickness tMEMS of the proof mass (extension of the proof mass along the Z axis). The thickness tNEMS of these walls may for example be less than one tenth, and even one thirtieth of the thickness tMEMS of the proof mass. tNEMS is for example less than 2 microns, or even 1 micron or even 0.5 microns. As for tMEMS> it is for example between 5 and 100 microns, or between 15 and 50 microns.

[0071] The walls 50 and 55 each extend in the extension of the lower face 44 of the test mass 4.

[0072] The walls 50 and 55 may each be formed by a portion of a thin upper silicon layer 12 of an SOI substrate, silicon on insulator, from which the gyrometer is manufactured.

[0073] Such an SOI substrate comprises a thick support layer (generally at least 100 microns thick, generally more), covered by an insulating layer, generally made of silicon oxide, itself covered by the thin upper layer, made of silicon, often called the Si-top layer. This Si-top layer, has, by manufacturing, a reduced thickness (for example 250 nm) and very well controlled. It is therefore suitable for the production of the transmission walls 50 and 55. It is also essentially monocrystalline, and therefore suitable for the production of piezoresistive gauges. Here, the gauges are also produced in this layer 12. During the manufacture of the gyrometer, an additional layer of silicon (more generally, of semiconductor material), quite thick, of thickness h = tMEMS- tNEMS, is deposited on the layer 12, to form the bulk of the proof masses and frames (total thickness tMEMS). This additional layer can be polycrystalline or monocrystalline, and its thickness h is typically a few microns or tens of microns. During the manufacture of the gyrometer, layer 12 and this additional layer are etched to define the different elements of the gyrometer. The silicon oxide layer mentioned above, located under the NEMS layer 12, is removed (by chemical etching) in particular under the parts of the gyrometer which are mobile relative to the support (frames and proof masses in particular), to free these mobile parts. After manufacture, the thick support layer of the SOI substrate forms the support 2 of the gyrometer.

[0074] It is noted that the walls 50 and 55 are obtained in some way by laterally cutting the layer 12. Their thickness is therefore fixed by the initial thickness of the layer 12; this reduced thickness therefore does not result from a refinement or local etching of a thicker layer (in other words, it is not obtained by partial etching. Its thickness is therefore very well controlled, and any possible “over-etching” of the edges has only a limited influence on the stiffness of the mechanical connection).

[0075] The third wall 55 is here identical to the first wall 50, so that only the geometry of the first wall is now described in more detail.

[0076] This first wall 50 extends, from the frame 3 to the first end 41 of the proof mass, along a mean line which is parallel to the axis X. The mean line in question connects the frame 3 to the first end 41 of the proof mass 4 (this mean line corresponds for example to the line (not necessarily rectilinear) which passes through the centers of the different straight sections of the wall 50). The junction between the first wall 50 and the frame 3 corresponds to a rectangular, very elongated connecting surface, which extends parallel to the axis of rotation Ai of the proof mass 4. The junction between the first wall 50 and the proof mass 4 also corresponds to a rectangular, very elongated connecting surface, which extends parallel to the axis of rotation of the proof mass.The first wall 50 is delimited laterally, on one side and the other, by two edges 53, 54, free (that is to say which are not linked, in any case not directly, to another element), which each extend from the frame 3 to the proof mass 4. Each of these two free edges, 53, 54 is parallel to the axis of movement of the frame, X (and neither of these two edges is parallel to the axis of rotation of the proof mass).

[0077] The first wall 50 has a length L, along the X axis (this is the distance between the side of the frame 3 connected to the mass 4, and the first end 41 of this mass). In practice, it is for example between 5 and 50 microns, and it is chosen to be rather short, if possible (to increase the stiffness coefficient kx, presented in the part "summary"). As already indicated, the width of the wall 50, that is to say its extension parallel to the axis of rotation of the mass (axis of the connection 5), is noted 1. It is for example between 5 and 200 microns (more generally, the sum of the widths of the different transmission walls (here 50 and 55) is between 10 and 500 microns), and is chosen to be large if possible, to increase the stiffness coefficient kx. It will be noted that, in the present document, by convention, for the first wall of the connection 5, 5' (i.e.: for the wall of small thickness t^Ms, and which is parallel to the plane of the layers), the term "length" (in this case the length L) designates the extension of the first wall in a direction parallel to the axis X of movement of the frames, while the term "width" (in this case the width 1) designates its extension in a direction parallel to the axis of rotation Y.It should be noted in this regard that the distance (L), called the length of the first wall, can be smaller than the distance (1) called the width of the first wall. Furthermore, this lexical convention concerns the first wall, but not necessarily the other walls of the device (in particular not the retaining wall).

[0078] The second retaining wall 61 and the retaining element 60 of which it is part are now described in more detail.

[0079] The second wall 61 is connected on the one hand to the frame 3, and on the other hand to the first end 41 of the proof mass 4. This wall is flat, here. It is perpendicular to the axis X. This wall is thin: its width a (extension along the axis X) is less than a quarter, and even a tenth of its thickness h. Its width a is moreover less than a tenth, or even a thirtieth of a length, b, presented by this wall in a direction parallel to the axis of rotation Ai, between a connection point (E) to the movable frame 3 and a connection point (F) to the proof mass 4. The width a is for example between 0.5 and 5 microns (and is generally less than 2 microns). The length b is for example between 10 and 100 microns, and the height h is typically between 5 and 100 microns.

[0080] The retaining wall 61 extends, parallel to the Y axis, from a first end 611 to a second end 612. These two ends are connected here to the frame 3 (respectively by a first short connecting wall 62 and by a second short connecting wall 63, also called connecting elements), both parallel to the Z axis. The test mass 4 is connected to the retaining wall 61 in a middle zone of this wall, located between the two ends 611 and 612 (midway between these two ends), here also by means of a short connecting wall 64, parallel to the Z axis.

[0081] Alternatively, the retaining wall could be connected differently to the movable frame and the test mass. For example, its two ends could be connected to the mass, the frame then being connected to a middle part of the wall (at instead of the reverse). And, as indicated above, the connection could include several thin retaining walls, perpendicular to the X axis.

[0082] The retaining wall 61 is delimited, on the support side, by a lower, free edge, and, on the opposite side, by an upper, also free edge.

[0083] The retaining element 60, which comprises the retaining wall 61 and the three connecting elements 62, 63, 64, is not in contact with the transmission walls 50 and 55. The retaining element 60 is produced by etching: • the additional silicon layer mentioned above, over the entire height of this layer, • as well as the NEMS 12 layer located below (unless this layer has been previously removed in this area).

[0084] This engraving thus produces openings 11 (in the form of fairly narrow slots) which extend over the entire height of the proof mass.

[0085] Here, on the side of the first end 41, the test mass 4 and the frame 3 are connected to each other only by the transmission walls (50 and 55) and by the retaining element 60.

[0086] As explained in the “summary” section, this connection 5 based on at least two thin walls, one, 50, parallel to the mean plane of the support, and the other, 61, perpendicular to it, makes it possible to obtain a very stiff connection in the direction of movement of the frame (axis X), and also very stiff in the direction Z, thus defining the axis of rotation of the test mass 4 relative to the frame 3 (axis AJ), while being flexible, of reduced stiffness, with respect to a rotation around this axis.

[0087] On this subject, it is recalled that the retaining wall 61, due to its significant height (along the Z axis), couples the mass to the frame with a very high stiffness in Z, and thus contributes to defining an axis of rotation (no movement other than rotation) between the two parts, this axis being in this case the axis Ai. This wall requires the points E, F and H identified in [Fig.8] to have the same, or almost the same position in Z (these three points being located respectively at the level of the connecting element 62, the connecting element 64 and the connecting element 63, on the free upper face of the retaining element 60).

[0088] As the walls 50, 55 and 61 are thin (in the sense defined above), very rigid in the X direction for the walls 50 and 55, and very rigid in the Z direction for the wall 61, the axis of rotation Ai is located here at the intersection between ([Fig.7]): • a plane PI which is the average plane of walls 50, 55 (and which is parallel to plane P), and • a plane P2 which is the average plane of wall 61.

[0089] This axis of rotation is therefore located here at the level of the NEMS layer 12 (very low torsional rigidity of the wall 61).

[0090] [Fig.9] schematically represents the gyrometer 1, in a sort of functional form, showing certain forces which are exerted in the connections of the gyrometer. As the frames 3 and 3' are actuated in phase opposition to each other: • the force F transmitted by the connection 5 because the frame 3 carries with it the mass 4 (force which is parallel to X), • is of the same amplitude but opposite to the force F', transmitted by the 5' bond because the 3' frame carries the 4' mass with it.

[0091] These two forces F and F' are exerted on the masses 4 and 4' almost at the level of their lower faces 44 and 44'. They are therefore exerted on these masses with a non-zero lever arm relative to the center of mass, G, G' of the mass considered, and therefore tend to pivot this mass around its axis of rotation, possibly much more than the Coriolis force to be detected. But this undesirable effect is counteracted by the use of the common lever 7.

[0092] Indeed, in terms of forces, this non-zero moment of forces F and F' translates, at the level of links 9 and 9', into forces Fp and F'p both directed along the +Z axis, and of the same amplitude. Links 9 and 9' are the links which connect, respectively, mass 4 and mass 4', to lever 7. Now these links, which are intended to transmit to the lever displacements parallel to the Z axis, transmit these forces Fp and Fp' without practically modifying them. The detection lever 7 is thus subjected, at its two opposite ends, to two equal forces Fp and F'p, so that the combination of these two forces does not tend to rotate lever 7 around its axis of rotation A3 (located in the middle of the lever, and fixed relative to the support).Finally, even if the forces transmitted in the links 5 and 5' have a non-zero lever arm relative to the centers of mass G and G' of the test masses, they do not disturb the measurement of the angular velocity Q, in this device.

[0093] A complete numerical example is now presented, for illustration purposes. The dimensions of the various elements of the connection 5 are shown in [Fig.8]. The values ​​of these dimensions are given in Table 1, in microns.

[0094] The values ​​of the stiffness coefficients kx and CY corresponding to this dimensioning, values ​​obtained by numerical simulation, are given in table 3.

[0095] A numerical example corresponding to the prior art connection, of [Fig. 3], is also presented for comparison. The dimensions of the different elements of this connection are shown in [Fig. 3]. The values ​​of these dimensions are given in Table 2, in microns. The values ​​of the stiffness coefficients k'x and C'y corresponding to this dimensioning, obtained by numerical simulation, are given in Table 3.

[0096] [Tab. 1] Table 1 a 1.7 b 100 c 10 d 3.6 e 3.6 f 10 g 120 1 100 m 2 n 2 o 5 P 1 Imems 20 Inems 0.25

[0097] [Tab. 2] Table 2 a 4.15 b 30 c 12 d 13 e 5 f 15 g 120 Imems 20

[0098] [Tab. 3] Table 3 stiffnesses Prior art (figure 3) Example of implementation of the present technology (figure 8) report Stiffness in x, in N / m 5.103 6.2.105 120 times greater Stiffness in rotation (rotation around Ab parallel to Y), in Nm / rd 1.4.106 5.9 .108 24 times smaller

[0099] As can be seen in this example, the new connection configuration presented here makes it possible to both increase the stiffness in x, while substantially reducing the stiffness in rotation around the axis of rotation of the mass considered (and while facilitating the manufacture / reproducibility of the device).

Claims

Claims

1. Micro-electromechanical device (1) comprising: - a support (2), - a movable frame (3), guided in translation relative to the support along an axis (X) of movement parallel to a mean plane (P) of the support, - a test mass (4), which extends from a first end (41) to a second end (42) of the test mass, • its first end (41) being connected to the mobile frame by a mechanical connection (5) allowing the mass to pivot, relative to the frame, around an axis of rotation (AJ which is parallel to the mean plane (P) of the support and perpendicular to the axis (X) of movement of the frame, • the test mass being connected, on the side of its second end (42), movable, to a member (7) for detecting a pivoting of the test mass around its axis of rotation, - characterized in that said connection (5) comprises: • a thin wall (50) which extends parallel to the mean plane (P) of the support, from the frame (3) to the first end (41) of the proof mass, along a mean line which is parallel to the axis of movement (X) of the frame, and which, along an axis Z perpendicular to the mean plane of the support, has a thickness (tNEMS) less than the thickness (tMEMS) of the proof mass (4), or even less than one tenth of the thickness (tMEMS) of the proof mass (4); • a retaining element (60) comprising a retaining wall (61) - which is perpendicular to the mean plane (P) of the support (2), - which, in a direction parallel to the axis (X) of movement of the frame, has an extension (a) less than a quarter of the thickness of the retaining wall along the Z axis, - and which is connected on the one hand to the movable frame (3), and on the other hand to the first end (41) of the test mass (4).

2. Device (1) according to the preceding claim, wherein said extension (a) of the retaining wall is less than one tenth of its thickness.

3. Device (1) according to one of the preceding claims, wherein the retaining wall (61) extends in a direction parallel to the axis of rotation (Y) of the test mass (4), from a first end (611) to a second end (612) of the retaining wall, and wherein: - the first end (611) and the second end (612) of the retaining wall are both connected to the movable frame (3), while the test mass (4) is connected to the retaining wall (61) in a middle zone of said wall, between these two ends, or wherein - the first end and the second end of the retaining wall are both connected to the test mass, while the frame is connected to the retaining wall in a middle zone of said wall, between these two ends.

4. Device according to one of claims 1 to 2, wherein the retaining wall extends in a direction parallel to the axis of rotation of the test mass, from a first end to a second end of the retaining wall, the first end of the retaining wall being connected to the movable frame while its second end is connected to the test mass, or vice versa.

5. Device according to claim 4, comprising an additional retaining wall, - which is perpendicular to the mean plane of the support, - which, in a direction parallel to the axis (X) of movement of the frame, has an extension less than a quarter of the thickness of the retaining wall along the Z axis, - which extends, in a direction parallel to the axis of rotation of

6.

7. the test mass, from a first end to a second end of the additional retaining wall, the first end of the additional retaining wall being connected to the movable frame while its second end is connected to the test mass, or vice versa. Device (1) according to one of the preceding claims, in which said wall (50) is formed by a portion of a thin upper silicon layer (12) of a silicon-on-insulator SOI substrate, and in which the support (2) of the device is formed by a thick support layer of said SOI substrate. Device (1) according to one of the preceding claims, of the gyrometer type, in which the frame, the test mass and the connection are respectively called first frame (3), first test mass (4) and first connection (5), the device (1) and further comprising: - a second movable frame (3'), guided in translation relative to the support (2) along said axis of movement (X), - a second test mass (4'), which extends from a first end (41') to a second end (42') of the second test mass (4'), its second end (42') being located on the side of the first test mass (4) while its first end (41') is located on the opposite side, • the first end (41') of the second mass (4') being connected to the second frame (3') by a second mechanical connection (5') identical to the first connection (5'), • the second test mass (4') being connected, on the side of its second movable end (42'), to the same pivoting detection member (7) as the first test mass (4), - said detection member being a lever, movable in rotation around a detection rotation axis (Delta3) which is parallel to the rotation axis (Deltal) of the first test mass (4), the lever being connected on one side to the first test mass (4), and on the other to the second test mass (4').

8. Device (1) according to the preceding claim, further comprising an electromechanical actuation system configured to impose on each of the movable frames (3, 3') an oscillation along said axis of movement (X), the movement of the first frame (3) relative to the support, and the movement of the second frame (3') relative to the support (2) having the same amplitude and opposite directions to each other.

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