Patent Description:
As is known, among the innumerable applications of the accelerometers, in particular microelectromechanical ones, the monitoring of vibrations in machines of industrial plants, such as motors, turbines, pumps and so on is particularly important. The early and accurate identification of anomalous vibrations is decisive to avoid not only failures and potentially serious damage to the plants, but also for the correct execution of suitable maintenance interventions, so as to optimize costs and in general maintain efficiency at high levels.

A problem for this type of applications arises from the fact that monitoring requires high performance not only in terms of sensitivity and noise, but also in terms of bandwidth, as anomalous vibrations may be related to a wide range of phenomena and, consequently, appear on a very broad spectrum. Among the accelerometers that are best suited for their characteristics are bulk piezoelectric accelerometers, open-loop capacitive microelectromechanical accelerometers and closed-loop microelectromechanical accelerometers. However, all suffer from limitations that make the results not entirely satisfactory.

Bulk piezoelectric accelerometers, for example, have a very wide band and an optimum dynamic range, but are bulky and expensive and require to be recalibrated relatively frequently, without having the possibility of self-diagnosis procedures. Furthermore, the sensitivity depends significantly on the temperature and generally only sensors of uniaxial type may be produced.

Open-loop capacitive microelectromechanical accelerometers have the advantage of small size and low cost, without excessively scarifying the band. Beyond a certain limit, however, a tradeoff is to be sought between the bandwidth on the one hand and the noise levels and sensitivity on the other hand. In fact, while the noise tends to increase as the bandwidth increases, to the detriment of the quality of the measurements, the sensitivity decreases quadratically.

Closed-loop microelectromechanical accelerometers apply feedback forces to cancel the inertial forces acting on a movable mass and maintain the movable mass in proximity to an equilibrium position; the intensity of the control is a measure of the inertial forces that are opposed. This solution allows to combine stability, linearity and low noise levels of sensors operating on relatively narrow frequency ranges with a large bandwidth. Closed-loop microelectromechanical accelerometers may also be used to manufacture triaxial devices, combining sensing structures of the in-plane and out-of-plane type. Particularly in sensors of the out-of-plane type, however, feedback forces may trigger spurious vibration modes of the movable masses.

As shown schematically in <FIG>, a closed-loop microelectromechanical accelerometer <NUM> of the out-of-plane type normally comprises a substrate <NUM> of semiconductor material, a movable mass <NUM>, defined by a plate also of semiconductor material, sensing electrodes <NUM> and feedback electrodes <NUM>. The movable mass <NUM> has a barycenter G and is connected to the substrate <NUM> at a fulcrum <NUM> with an offset with respect to the barycenter G. In practice, the movable mass <NUM> is connected so as to be able to rotate, with respect to the substrate <NUM>, around a non-barycentric fulcrum axis F. In the absence of external forces applied, the movable mass <NUM> is maintained in an equilibrium position, for example parallel to the substrate <NUM>, by flexures not shown here.

When an external force causes a displacement of the accelerometer <NUM> along an axis Z perpendicular to the substrate <NUM>, the movable mass <NUM> tends to rotate around the fulcrum axis F and the displacements are sesed by the sensing electrodes <NUM>. A control device (not shown) applies electrostatic feedback forces FFB, FFB' through the feedback electrodes <NUM> to bring the movable mass <NUM> back to the equilibrium position and cancel the displacements. However, since the electrostatic feedback forces FFB, FFB' may only be of attractive type, the movable mass <NUM> is subject to a non-zero net force due to the control. The net force caused by the control may trigger spurious vibration modes, which appear as a torque applied to one end of the movable mass <NUM> and modify the movement of the movable mass <NUM> itself. The control device has no way of discriminating the cause of the movements of the movable masses and reacts by trying to compensate also the forces caused by the spurious vibration modes, but in doing so equally spurious signal components are introduced which degrade the quality of the measurements.

The aim of the present invention is to provide a microelectromechanical accelerometer and a process for manufacturing a microelectromechanical accelerometer which allow the described limitations to be overcome or at least mitigated.

<CIT> discloses a closed-loop accelerometer comprising a supporting body of semiconductor material, an out-of-plane sensing mass of semiconductor material and feedback electrodes. The out-of-plane sensing mass has a first side facing the supporting body and a second side opposite to the first side and is connected to the supporting body to oscillate around a non-barycentric fulcrum axis parallel to the first side and to the second side and perpendicular to an out-of-plane sensing axis. The feedback electrodes are capacitively coupled to the sensing mass and are configured to apply opposite electrostatic forces and a torque around the fulcrum axis to the sensing mass. The feedback electrodes comprise a first group of feedback electrodes facing the first side of the out-of-plane sensing mass and a second group of feedback electrodes facing the second side of the out-of-plane sensing mass. Moreover the feedback electrodes comprise a first feedback electrode and a second feedback electrode, arranged on the supporting body symmetrically with respect to the fulcrum axis and facing the first side of the out-of-plane sensing mass; and a third feedback electrode and a fourth feedback electrode, supported by respective feedback supports symmetrically with respect to the fulcrum axis and facing the second side of the out-of-plane sensing mass. Another example of a known accelerometer is disclosed in <CIT>.

According to the present invention a microelectromechanical device and a process for manufacturing a microelectromechanical device are provided, as defined in claims <NUM> and <NUM>, respectively.

For a better understanding of the invention, some embodiments thereof will now be described, purely by way of non-limiting example and with reference to the attached drawings, wherein:.

With reference to <FIG>, a microelectromechanical device according to an embodiment of the present invention is schematically illustrated and indicated with the number <NUM>. In the illustrated example, in particular, the microelectromechanical device <NUM> is a triaxial accelerometer comprising a supporting body <NUM>, an in-plane sensing mass <NUM>, which responds to accelerations in an XY-plane parallel to a face of the supporting body <NUM>, and an out-of-plane sensing mass <NUM>, which responds to accelerations along an out-of-plane sensing axis Z perpendicular to the XY-plane. However, the use in triaxial accelerometers is not to be considered as limiting and it is understood that the invention may equally be used to make uniaxial or biaxial accelerometers with out-of-plane sensing along an axis.

<FIG> illustrates, in a simplified manner, the operation of the accelerometer <NUM> relative to the sensing of accelerations along the Z-axis. The accelerometer <NUM>, in particular, is provided with sensing electrodes <NUM> and with feedback electrodes <NUM> capacitively coupled to the out-of-plane sensing mass <NUM> and comprises a sensing stage <NUM>, a control device <NUM> and a driving stage <NUM>. The sensing stage <NUM>, through the sensing electrodes <NUM>, generates reading signals SR indicative of the angular position of the out-of-plane sensing mass <NUM> around a fulcrum axis F. From the reading signals SR, the control device <NUM> generates control signals SC which are applied to the feedback electrodes <NUM> through the driving stage <NUM> and tend to bring the out-of-plane sensing mass <NUM> back to an equilibrium position.

The micromechanical part of the accelerometer <NUM> relating to the out-of-plane sensing is shown in more detail in <FIG> and <FIG>, where the supporting body <NUM>, the out-of-plane sensing mass <NUM>, the sensing electrodes and the feedback electrodes are illustrated in particular. The supporting body <NUM> comprises a substrate <NUM> of semiconductor material, covered by an insulating layer <NUM>, and a perimeter wall <NUM>, which defines, with the substrate <NUM>, a cavity <NUM> where the out-of-plane sensing mass <NUM> is accommodated. A cap <NUM> is bonded to the perimeter wall <NUM> through an adhesion layer <NUM>, for example glass-frit, and closes the cavity <NUM>.

The out-of-plane sensing mass <NUM> is made of semiconductor material, for example polycrystalline silicon. The out-of-plane sensing mass <NUM> is also connected to the supporting body <NUM> through an anchor <NUM> and flexures <NUM>, configured to allow rotations of the out-of-plane sensing mass <NUM> around the fulcrum axis F, which is a non-barycentric axis parallel to a face of the substrate <NUM> and perpendicular to the out-of-plane sensing axis Z. For ease of representation, the anchor <NUM> and the flexures <NUM> are illustrated only schematically in <FIG>. To increase the imbalance of the out-of-plane sensing mass <NUM> with respect to the fulcrum axis F and the sensitivity of the accelerometer <NUM>, one end of the out-of-plane sensing mass <NUM> further away from the anchor <NUM> is provided with an additional mass <NUM>.

The sensing electrodes comprise a first group of sensing electrodes facing the first side of the out-of-plane sensing mass and a second group of sensing electrodes facing the second side of the out-of-plane sensing mass. In detail, the sensing electrodes include a first sensing electrode 15a, a second sensing electrode 15b, a third sensing electrode 15c and a fourth sensing electrode 15d, arranged in pairs symmetrically with respect to the fulcrum axis F so as to obtain differential signals. More precisely, the first sensing electrode 15a and the second sensing electrode 15b are formed on the insulating layer <NUM> in symmetrical positions with respect to the fulcrum axis F and face a first side 13a of the out-of-plane sensing mass <NUM> facing the substrate <NUM>. The first sensing electrode 15a and the second sensing electrode 15b are capacitively coupled to the out-of-plane sensing mass <NUM> and electrically insulated from each other. The third sensing electrode 15c and the fourth sensing electrode 15d are formed on respective sensing supports <NUM> in symmetrical positions with respect to the fulcrum axis F and face a second side 13b of the out-of-plane sensing mass <NUM> opposite to the first side 13a. The third sensing electrode 15c and the fourth sensing electrode 15d are arranged in positions corresponding to the positions of the second sensing electrode 15b and of the first sensing electrode 15a, respectively. The third sensing electrode 15c and the fourth sensing electrode 15d are also capacitively coupled to the out-of-plane sensing mass <NUM> and electrically (<FIG>) are insulated from each other and directly connected to the first sensing electrode 15a and to the second sensing electrode 15b, respectively. Therefore, for the symmetrical arrangement of the sensing electrodes, upon a rotation of the out-of-plane sensing mass <NUM> around the fulcrum axis F, the capacitive coupling varies in a differential manner with respect to the equilibrium position between the first sensing electrode 15a and the third sensing electrode 15c on one side and the second sensing electrode 15b and the fourth sensing electrode 15d on the other side.

Similarly, the feedback electrodes comprise a first group of feedback electrodes facing the first side of the out-of-plane sensing mass and a second group of feedback electrodes facing the second side of the out-of-plane sensing mass. In detail, the feedback electrodes include a first feedback electrode 17a, a second feedback electrode 17b, a third feedback electrode 17c and a fourth feedback electrode 17d, arranged in pairs symmetrically with respect to the fulcrum axis F. More precisely, the first feedback electrode 17a and the second feedback electrode 17b are formed on the insulating layer <NUM> in symmetrical positions with respect to the fulcrum axis F and face the first side 13a of the out-of-plane feedback mass <NUM>. The first feedback electrode 17a and the second feedback electrode 17b are capacitively coupled to the out-of-plane feedback mass <NUM> and electrically insulated from each other. The third feedback electrode 17c and the fourth feedback electrode 17d are formed on respective feedback supports <NUM> in symmetrical positions with respect to the fulcrum axis F and face the second side 13b of the out-of-plane feedback mass <NUM>. The third feedback electrode 17c and the fourth feedback electrode 17d are arranged in positions corresponding to the positions of the second feedback electrode 17b and of the first feedback electrode 17a, respectively. The third feedback electrode 17c and the fourth feedback electrode 17d are capacitively coupled to the out-of-plane feedback mass <NUM> and electrically (<FIG>) are insulated from each other and directly connected to the first feedback electrode 17a and to the second feedback electrode 17b, respectively.

The feedback electrodes 17a-17d are also in positions which are closer to the anchor <NUM> with respect to the sensing electrodes 15a-15b.

The sensing supports <NUM> and the feedback supports <NUM> are made of semiconductor material and are anchored to the substrate <NUM>. In more detail, the sensing supports <NUM> and the feedback supports <NUM> have respective first structures 35a, 37a, extending from the substrate <NUM> in a direction parallel to the out-of-plane sensing axis Z through openings <NUM> in the out-of-plane sensing mass <NUM>; and respective second structures 35b, 37b extending from the respective first structures 35a, 37a in a direction perpendicular to the out-of-plane sensing axis Z. The first structures 35a, 37a are anchored to the substrate <NUM> and are connected to respective conductive lines <NUM>, <NUM> formed in the insulating layer <NUM> (<FIG>). More precisely, the first structures 35a of the sensing supports <NUM> are connected to respective conductive lines <NUM>, which are insulated from each other. The first structures 37a of the feedback supports <NUM> are connected to respective conductive lines <NUM>, which are insulated from each other. The second structures 35b, 37b of the sensing supports <NUM> and of the feedback supports <NUM> face the second side 13b of the out-of-plane sensing mass <NUM>, opposite with respect to the substrate <NUM>. The third sensing electrode 15c, the fourth sensing electrode 15d, the third feedback electrode 17c and the fourth feedback electrode 17d are arranged between the second structures 35b, 37b of the respective sensing supports <NUM> and feedback supports <NUM> and the out-of-plane sensing mass <NUM>. Each sensing support <NUM> is adjacent to and aligned with a respective feedback support <NUM>, with the respective first structures arranged through the same openings <NUM>. The second structures 37b of the feedback supports <NUM> extend from the respective first structures 37a towards the anchor <NUM>; the second structures of the sensing supports <NUM> extend from the respective first structures 35a in a direction opposite to the anchor <NUM>.

The feedback electrodes 17a-17d apply electrostatic feedback forces FFB1, FFB2 to the out-of-plane sensing mass <NUM> due to the control signals SC supplied by the control device <NUM> through the driving stage <NUM> to balance the external forces and bring the out-of-plane sensing mass <NUM> back to the equilibrium position (<FIG>). Furthermore, for the symmetrical arrangement described, the electrostatic feedback forces FFB1, FFB2 have zero resultant, even if a feedback torque TFB is applied to the out-of-plane sensing mass <NUM> around the fulcrum axis F. In fact, the first feedback electrode 17a and the third feedback electrode 17c are always at an equal distance from the out-of-plane sensing mass <NUM> (according to the out-of-plane sensing axis Z), regardless of its angular position around the fulcrum axis. Since they are directly connected to each other and are at the same potential, the applied electrostatic feedback forces FFB1 have equal modulus and opposite directions. Similarly, the second feedback electrode 17b and the fourth feedback electrode 17d apply electrostatic feedback forces FFB2 with equal modulus and opposite directions. The zero resultant of the electrostatic feedback forces FFB1, FFB2 is practically unable to trigger spurious vibration modes, which, as discussed, appear in the form of a torque tending to rotate the out-of-plane sensing mass <NUM> around an axis other than the fulcrum axis F. Spurious signal components are also avoided. These signal components are in fact generated by the control device <NUM> to balance the effects of the spurious vibration modes, which cannot be discriminated from the inertial forces to be measured. Ultimately, therefore, the zero resultant of the electrostatic feedback forces FFB1, FFB2 avoids the triggering of spurious vibration modes and, consequently, spurious signal components that would otherwise be indistinguishable from the useful signal. The accelerometer according to the invention is therefore advantageous in terms of sensitivity, stability, linearity and bandwidth, as well as having a low cost with respect to sensors of the piezoelectric-type and being rather easily integrable into biaxial or triaxial devices.

The microelectromechanical device <NUM> of <FIG> may be made through the process described hereinbelow with reference to <FIG>. In practice, the out-of-plane sensing mass <NUM>, the anchor <NUM>, the sensing supports <NUM>, the feedback supports <NUM> and the respective sensing and feedback electrodes are obtained from two structural layers epitaxially grown on top of each other, as described in detail hereinbelow.

With reference to <FIG>, a wafer <NUM> of semiconductor material, for example monocrystalline silicon, initially comprises the substrate <NUM>, having the insulating layer <NUM>, for example of silicon oxide, grown thereon. A conductive layer not shown in full, for example of polycrystalline silicon, is deposited on the insulating layer <NUM> and shaped to form the first sensing electrode 15a, the second sensing electrode 15b, the first feedback electrode 17a, the second feedback electrode 17b and the conductive lines <NUM>, <NUM>. A first sacrificial layer <NUM>, for example of thermally-grown or deposited silicon oxide, is formed on the insulating layer <NUM> and covers the sensing electrodes 15a, 15b, the feedback electrodes 17a, 17b and the conductive lines <NUM>, <NUM>. The first sacrificial layer <NUM> is selectively etched in positions corresponding to the anchor <NUM> of the out-of-plane sensing mass <NUM>, to the sensing electrodes 15a, 15b, to the feedback electrodes 17a, 17b and to the conductive lines <NUM>, <NUM>, where the first structures 35a, 37a of the sensing supports <NUM> and of the feedback supports <NUM> will be formed later on.

Then, <FIG>, a first structural layer <NUM> is grown by epitaxy above the first sacrificial layer <NUM> from a deposited seed layer <NUM>' and contacts the sensing electrodes 15a, 15b, the feedback electrodes 17a, 17b and the conductive lines <NUM>, <NUM>. The first structural layer <NUM> has a thickness which is determined on the basis of the characteristics of the desired micro-electro-mechanical structures and may be comprised for example between <NUM> and <NUM>. After the structural growth, the first structural layer <NUM> is planarized and brought to the desired final thickness, for example by CMP (Chemical Mechanical Polishing).

The first structural layer <NUM>, <FIG>, is etched to define bottom portions of the desired structures and of other regions provided according to the design preferences. In particular, in this step the anchor <NUM>, the out-of-plane sensing mass <NUM>, the flexures <NUM>, a bottom portion <NUM>' of the anchor <NUM>, the first structures 35a, 37a of the sensing support <NUM> and of the feedback support <NUM> and bottom portions <NUM>' of the perimeter wall <NUM> are formed from the first structural layer <NUM>. For this purpose, the wafer <NUM> is covered by a resist mask not shown (first trench mask) and subject to a dry etching, forming trenches <NUM>, which completely extend through the first structural layer <NUM>. In this step, trenches (not shown) are also formed through the portion of the first structural layer <NUM> intended to form the out-of-plane sensing mass <NUM>, which will have a lattice structure. The trenches will also be used in the subsequent steps for removing the sacrificial layer <NUM>. The etching automatically stops on the first sacrificial layer <NUM>.

Then, <FIG>, a second sacrificial layer <NUM>, for example of TEOS (TetraethylOrthoSilicate), is deposited for a thickness equal to the desired width W of a gap between the out-of-plane sensing mass <NUM> and the sensing supports <NUM> and the sensing supports <NUM>. The second sacrificial layer <NUM> partially fills the trenches <NUM>, for example to one third of their depth although this filling, as well as the extent and depth of filling are not important. The second sacrificial layer <NUM> is then planarized.

The second sacrificial layer <NUM> is selectively thinned in zones where the third sensing electrode 15c, the fourth sensing electrode 15d, the third feedback electrode 17c and the fourth feedback electrode 15d will have to be subsequently formed. For this purpose, by using a masking layer not shown (bump mask) a masked etching, for example a time-etching, is performed in a per se known manner, to form recesses <NUM> in positions corresponding to the first sensing electrode 15a, the second sensing electrode 15b, the first feedback electrode 17a, the second feedback electrode 17b. With the same bump mask, the areas intended for other structures of the accelerometer <NUM> are also delimited, such as the contact structures that limit out-of-plane displacements (bumps).

Subsequently, <FIG>, the second sacrificial layer <NUM> is further etched and selectively removed throughout its thickness, using a masking layer not shown (second anchoring mask), forming openings <NUM>. The etching of the second sacrificial layer <NUM> leads to the formation of hard masking regions <NUM>' and automatically terminates on the first epitaxial layer <NUM>. The openings <NUM> are defined between adjacent hard masking regions <NUM>' and, in the illustrated embodiment, are arranged at the end of the out-of-plane sensing mass <NUM> further away from the fulcrum axis F, on the first structures 35a, 37a of the sensing supports <NUM> and of the feedback supports <NUM> and, in general, in zones where it is desired to form connection regions with the first epitaxial layer <NUM>, e.g. the perimeter wall <NUM>. Due to the recesses <NUM>, the hard masking regions <NUM>' have two different thicknesses: a greater thickness equal to that of the second sacrificial layer <NUM>, and a smaller thickness where the recesses <NUM> are formed.

Subsequently, <FIG>, a second structural layer <NUM> is grown, also in this case by epitaxy, for a thickness for example between <NUM> and <NUM>. The thickness of the second structural layer <NUM> is related to the desired micro-electro-mechanical structures, including the sensing supports <NUM> and the feedback supports <NUM>. In general, the second structural layer <NUM> may be thinner than the first structural layer <NUM>, even though the opposite may occur and the invention is not limited to any particular relationship between the thicknesses of the structural layers <NUM>, <NUM>. The semiconductor material, which during the epitaxial growth fills the recesses <NUM>, forms the third sensing electrode 15c, the fourth sensing electrode 15d, the third feedback electrode 17c and the fourth feedback electrode 17d. After the epitaxial growth, the second structural layer <NUM> is planarized and brought to the desired final thickness, for example by CMP (Chemical Mechanical Polishing).

Subsequently, the second structural layer <NUM> is etched as shown in <FIG>. For this purpose, the wafer <NUM> is covered by a resist mask (not illustrated) and subject to a dry etching. In this step, the portions of the second structural layer <NUM> not covered by the resist mask are removed throughout the thickness and the etching stops on the hard masking regions <NUM>'.

In particular, in this step the out-of-plane sensing mass <NUM> (with the additional mass <NUM> formed from the second structural layer <NUM>), the second structures 35b, 37b of the sensing supports <NUM> and of the feedback supports <NUM> and the upper part of the perimeter wall <NUM> are defined.

Then, the residual portions of the second sacrificial layer <NUM> and the first sacrificial layer <NUM> are removed, releasing the out-of-plane sensing mass <NUM>.

Finally, a cap wafer is bonded to the wafer <NUM> by the adhesion layer <NUM> and the composite wafer thus obtained is diced to form the accelerometer <NUM> of <FIG>.

<FIG> shows an electronic system <NUM> which may be of any type, in particular, but not limited to, a wearable device, such as a watch, a smart bracelet or band; a computer, such as a mainframe, a personal computer, a laptop or a tablet; a smartphone; a digital music player, a digital camera or any other device for processing, storing, transmitting or receiving information. The electronic system <NUM> may be a general purpose or device-embedded processing system, an equipment or a further system. For example, the electronic system <NUM> may be a system for monitoring vibrations in machinery of an industrial plant.

The electronic system <NUM> comprises a processing unit <NUM>, memory devices <NUM>, a microelectromechanical gyroscope according to the invention, for example the microelectromechanical gyroscope <NUM> of <FIG>, and may also be provided with input/output (I/O) devices <NUM> (e.g. a keyboard, a pointer or a touch screen), a wireless interface <NUM>, peripherals <NUM>,. N and possibly further auxiliary devices, not shown here. The components of the electronic system <NUM> may be coupled in communication with each other directly and/or indirectly through a bus <NUM>. The electronic system <NUM> may also comprise a battery <NUM>. It should be noted that the scope of the present invention is not limited to embodiments necessarily having one or all the listed devices.

The processing unit <NUM> may comprise, for example, one or more microprocessors, microcontrollers and the like, according to the design preferences.

The memory devices <NUM> may comprise volatile memory devices and non-volatile memory devices of various kinds, for example SRAM and/or DRAM memories for the volatile-type and solid-state memories, magnetic disks and/or optical disks for the non-volatile type.

Finally, it is apparent that modifications and variations may be made to the microelectromechanical accelerometer and to the process described, without departing from the scope of the present invention, as defined in the attached claims.

Claim 1:
A closed-loop microelectromechanical accelerometer comprising:
a supporting body (<NUM>) of semiconductor material;
an out-of-plane sensing mass (<NUM>) of semiconductor material, having a first side (13a) facing the supporting body (<NUM>) and a second side (13b) opposite to the first side (13a), wherein the out-of-plane sensing mass (<NUM>) is connected to the supporting body (<NUM>) to oscillate around a non-barycentric fulcrum axis (F) parallel to the first side (13a) and to the second side (13b) and perpendicular to an out-of-plane sensing axis (Z); and
feedback electrodes (17a-17d), capacitively coupled to the sensing mass (<NUM>) and configured to apply opposite electrostatic forces (FFB1, FFB2) and a torque (TFB) around the fulcrum axis (F) to the sensing mass (<NUM>);
wherein the feedback electrodes (17a-17d) comprise a first group of feedback electrodes (17a, 17b) facing the first side (13a) of the out-of-plane sensing mass (<NUM>) and a second group of feedback electrodes (17c, 17d) facing the second side (13b) of the out-of-plane sensing mass (<NUM>);
and wherein the feedback electrodes (17a-17d) comprise:
a first feedback electrode (17a) and a second feedback electrode (17b), arranged on the supporting body (<NUM>) symmetrically with respect to the fulcrum axis (F) and facing the first side (13a) of the out-of-plane sensing mass (<NUM>); and
a third feedback electrode (17c) and a fourth feedback electrode (17d), supported by respective feedback supports (<NUM>) symmetrically with respect to the fulcrum axis (F) and facing the second side (13b) of the out-of-plane sensing mass (<NUM>);
characterized in that the feedback supports (<NUM>) comprise respective first structures (37a), anchored to the supporting body (<NUM>) and extending in a direction parallel to the out-of-plane sensing axis (Z) through openings (<NUM>) in the out-of-plane sensing mass (<NUM>), and respective second structures (35b, 37b) which extend from the respective first structures (35a, 37a) in a direction perpendicular to the out-of-plane sensing axis (Z) and face the second side (13b) of the out-of-plane sensing mass (<NUM>).