EP4718870A1

EP4718870A1 - Electroacoustic actuator having improved sound pressure level performances

Info

Publication number: EP4718870A1
Application number: EP25202273.6A
Authority: EP
Inventors: Fabrizio Cerini; Federico CUNEO; Chiara Gazzola; Valentina ZEGA; Alberto Corigliano
Original assignee: STMicroelectronics International NV Switzerland; STMicroelectronics International NV
Current assignee: STMicroelectronics International NV Switzerland; STMicroelectronics International NV
Priority date: 2024-09-30
Filing date: 2025-09-15
Publication date: 2026-04-01

Abstract

An electroacoustic actuator (10) that is microelectromechanical includes: a supporting frame (12), containing semiconductor material; a membrane (13) of semiconductor material connected to the supporting frame (12) along a perimeter; a peripheral piezoelectric actuator (14) on a peripheral portion (13a) of the membrane (13); and a central piezoelectric actuator (15) on a central portion (13b) of the membrane (13). The electroacoustic actuator (10) further comprises: elastic elements (17), defined by respective portions of the membrane (13), wherein the central portion (13b) is connected to the peripheral portion (13a) of the membrane (13) by the elastic elements (17); and metal lines (30, 35, 36) extending on the peripheral portion (13a) of the membrane (13), on respective elastic elements (17) and on the central portion (13b) of the membrane (13), from the peripheral piezoelectric actuator (14) to the central piezoelectric actuator (15).

Description

Technical Field

The present invention relates to an electroacoustic actuator having improved sound pressure level performances.

Background

As is known, the users of the vast majority of mobile and land processing and communication devices, such as smartphones, tablets, portable and desktop computers, benefit from the use of headphones and earphones, which are by now extremely widespread. This spread, together with the fact that in many cases headphones and earphones are worn continuously for long periods of time, brings with it the need to provide comfortable and practical devices, without sacrificing the quality of audio reproduction. There is therefore an important push towards the manufacture of miniaturized electroacoustic transducers, such as speakers and microphones. Other miniaturized electroacoustic transducers towards which there is growing interest are used for example in probes for ultrasound inspection and, in general, in ultrasound imaging (Piezoelectric Micromachined Ultrasonic Transducer, PMUT).
However, the solutions currently available are not entirely satisfactory and do not represent a valid compromise between dimensions, performances in terms of high Sound Pressure Level (SPL - for transmitters) or sensitivity (for receivers) and costs.
A first type of electroacoustic transducer, in particular a speaker, utilizes traditional electromagnetic actuation and is capable of ensuring high reproduction quality. However, electromagnetic actuation speakers are not suitable for being miniaturized beyond a certain limit.
Other solutions based on MEMS (Micro-ElectroMechanical-Systems) technology allow better miniaturization levels to be obtained, but costs and/or performances are not yet suitable and such as to replace the electrodynamic speakers.
For example, hybrid devices are known wherein a microelectromechanical actuator, often of the piezoelectric type, is coupled to a polymeric membrane, which is caused to vibrate. The polymeric membrane has the advantage of high yielding (low Young's modulus), which allows a good response, but has critical issues from the point of view of process and costs. In fact, the membrane is applied to the portion of the device that houses the actuator only in the back-end step, i.e. in dedicated processing steps, successive to the manufacture of the same actuator. Furthermore, hybrid micro-speakers are not suitable for being assembled on boards by using SMT (Surface Mount Technology), because the membrane is not capable of withstanding soldering temperatures.
Other devices made entirely by using MEMS technology meet the miniaturization and cost requirements, but do not achieve sufficient performances as to response dynamics and bandwidth.
In particular, some micro-speakers comprise a semiconductor membrane connected to a supporting frame along its perimeter. The criticality of these devices lies mainly in the poor yielding of the membrane. In fact, to obtain a suitable sound pressure level, the area reduction of the membrane due to miniaturization should be compensated by a greater displacement (the sound pressure is in fact, as a first approximation, proportional to the product A*d, where A is the area of the membrane and d is the displacement). However, the stiffness of the semiconductor material does not allow a sufficient displacement, especially at low frequencies.
In other MEMS micro-speakers, the membrane is discontinuous in the inner portion precisely to have greater yielding and allow a wider displacement. More than a real membrane, the transducer comprises a plurality of cantilever structures, each of which defines a segment of a polygon or a sector of a circle and extends from a supporting frame. The vertices of the segments or sectors are adjacent to each other at the center of the transducer, without however being joined. A wider response dynamics may thus be obtained, which, however, is not constant on the audio bandwidth. A misalignment in the out-of-plane direction of the cantilever structures, again especially at low frequencies and resonance, may in fact lead to have uncontrolled widths between adjacent sectors, introducing vents in the membrane that may compromise the performances (acoustic short circuit), especially at low frequencies. Furthermore, the quality of the response of MEMS micro-speakers of this type is very influenced by process variations, since even small differences in the cantilever structures may cause nonuniform movements and dissymmetry in the sound emission, impacting on the Total Harmonic Distortion (THD). For example, the devices may simply be affected by the position in which they are formed on the semiconductor wafer. In particular, some characteristics such as zero-time deflection due to residual stresses of the materials may vary between devices that are located closer to the center or the periphery of the wafer.

Summary

It is therefore an aim of the present invention to provide an electroacoustic actuator that allows the limitations described to be overcome or at least mitigated.
According to the present invention, an electroacoustic actuator is provided as defined in the attached claims.

Brief Description of the Figures

For a better understanding of the present invention, preferred embodiments are provided, by way of non-limiting example, with reference to the attached drawings, wherein:

Figure 1 is a simplified block diagram of a processing and communication device;
Figure 2 is a top-plan view of an electroacoustic actuator in accordance with an embodiment of the present invention incorporated into the device of Figure 1;
Figure 3 shows the electroacoustic actuator of Figure 2 with parts removed for clarity;
Figure 4 shows an enlarged detail of the electroacoustic actuator of Figure 2;
Figure 5 is a cross-section through the electroacoustic actuator of Figure 2, sectioned along line V-V of Figure 2;
Figure 6 is a cross-section through the electroacoustic actuator of Figure 2, sectioned along line VI-VI of Figure 2;
Figure 7 shows another enlarged detail of the electroacoustic actuator of Figure 2;
Figure 8 shows a further enlarged detail of the electroacoustic actuator of Figure 2;
Figure 9 is a top-plan view of an electroacoustic actuator in accordance with a different embodiment of the present invention usable in the device of Figure 1;
Figure 10 is a top-plan view of an electroacoustic actuator in accordance with a further embodiment of the present invention usable in the device of Figure 1;
Figure 11 shows a time diagram of driving signals of the electroacoustic actuator of the present invention and two corresponding operating conditions;
Figure 12 is a top-plan view of an electroacoustic actuator in accordance with a further embodiment of the present invention usable in the device of Figure 1; and
Figure 13 shows an enlarged detail of the electroacoustic actuator of Figure 12 with parts removed for clarity.

Description of Embodiments

The following description refers to the arrangement shown in the drawings; consequently, expressions such as "above", "below", "upper", "lower", "top", "bottom", "right", "left" and the like relate to the accompanying Figures and are not to be interpreted in a limiting manner.
For convenience, hereinafter reference will be made to electroacoustic actuators used for example in micro-speakers, however without this being considered limiting.
With reference to Figure 1, an electronic system denoted as a whole with the number 1 comprises a processing and communication device 2 coupled in communication with a micro-speaker 3.
The processing and communication device 2 may be any portable or land device that supports audio communication with a reproduction peripheral, such as the micro-speaker 3. The processing and communication device 2 may be, but it is not limited to, a portable computer, a personal computer, a tablet, a smartphone or a wearable device, for example a smartwatch, and comprises, in particular, a processing unit 5 and a communication module 6, coupled with a corresponding communication module 8 of the micro-speaker 3. The processing and communication device 2 may generally comprise further components not illustrated, such as a display unit, memory units, insertion and pointing devices, peripherals, a battery, I/O interfaces.
The micro-speaker 3 comprises, in addition to the communication module 8, an electroacoustic actuator 10 and a driver 11. The driver 11 receives audio signals through the communication module 8 and actuates the electroacoustic actuator 10.
The communication modules 6, 8 of the processing and communication device 2 and of the micro-speaker 3 may be mutually coupled by a wireless or cable connection.
With reference to Figures 2-6, the electroacoustic actuator 10 is a piezoelectric-type membrane microelectromechanical actuator and comprises a supporting frame 12, a membrane 13, a peripheral piezoelectric actuator 14 and a central piezoelectric actuator 15.
The supporting frame 12 is of semiconductor material and has a cavity 16 (Figures 5 and 6) open on one side and closed on the opposite side by the membrane 13. More precisely, the supporting frame 12 may comprise a substrate, for example of monocrystalline silicon 12a, a dielectric layer 12b and one or more structural layers 12c which may include epitaxial layers, again of monocrystalline silicon, or layers of polycrystalline silicon grown from seed in an epitaxial reactor or deposited layers.
The membrane 13, also of semiconductor material, for example polycrystalline silicon in continuity with the outermost of the structural layers 12c of the supporting frame 12, is connected to the same supporting frame 12 along its perimeter. The membrane 13 may have a thickness comprised for example between 3µm and 25µm. In one embodiment, the membrane 13 is polygonal and has an N-fold rotational symmetry with respect to an axis perpendicular to the membrane and passing through the center, with N being an integer. It is understood that a body is provided with an N-fold rotational symmetry with respect to an axis when the body is invariant for rotations of 360°/N around the axis. For example, the membrane 13 may have the shape of a regular octagon. Furthermore, an N-fold rotational symmetry with N even may be advantageous in terms of balancing the stresses (e.g., for the arrangement of dummy connections, as explained in detail below).
With reference in particular to Figure 3, which for clarity shows only the supporting frame 12 and the membrane 13, the membrane 13 has a peripheral portion 13a, that accommodates the peripheral piezoelectric actuator 14 (Figure 2), and a central portion 13b, that accommodates the central piezoelectric actuator 15. The peripheral portion 13a and the central portion 13b of the membrane 13 are coupled to each other by connection portions of the same membrane 13 which define elastic elements 17. In a radial direction, the peripheral portion 13a and the central portion 13b may each extend for a distance comprised, for example, between 30% and 65% of an inscribed radius RI of the membrane 13, which is a regular polygon. In the embodiment of Figures 2-6, for example, the peripheral portion 13a and the central portion 13b occupy, respectively, about 40% and about 55% of the inscribed radius RI.
Furthermore, the membrane 13 is divided into a plurality of sectors 13c, delimited by radial slits 18 that extend in a radial direction from respective vertices of the membrane 13 towards the central portion 13b. Towards the outside, the radial slits 18 may reach the margin of the membrane 13 or even the supporting frame 12, in accordance with design preferences; towards the inside, however, the radial slits 18 extend for a section comprised between a quarter and two thirds of the central portion 13b of the membrane 13, here in particular for about one third. In one embodiment, the radial slits 18 all have the same width. Furthermore, the width of the radial slits 18 is lower than twice a thickness of a low-frequency viscous boundary layer of air (e.g. 100 Hz), in particular in an operating temperature range comprised for example between -20 °C and +40 °C. In one embodiment, the width is lower than the thickness of the low-frequency viscous boundary layer of air and is in any case not greater than 10 µm, e.g. 5 µm. Furthermore, a ratio between the width and a thickness of the membrane 13 is not greater than 1.
In the peripheral portion 13a of the membrane 13, the radial slits 18 define cantilever elements 13d of substantially trapezoidal shape, one in each sector 13c. In the central portion 13b of the membrane 13, the radial slits 18 define tabs 13e, one in each sector 13c. In particular, in each sector 13c the cantilever element 13d has a major base connected to the supporting frame 12 and a minor base connected to the corresponding tab 13e of the central portion 13b of the membrane 13 by a respective elastic element 17.
With reference in particular to the enlargement of Figure 4, each elastic element 17 is formed directly by a portion of the membrane 13 and is symmetrical with respect to an axis A that extends along a radial direction of the membrane 13 corresponding to the bisector of the respective sector 13c. The elastic elements 17 are identical to each other and for convenience reference will be made hereinafter to only one of them, being understood that what is described also applies to all the others. The elastic element 17 comprises an outer anchor 17a, an inner anchor 17b, outer arms 17c and inner arms 17d. The outer anchor 17a and the inner anchor 17b are fixed respectively to the cantilever element 13d and to the tab 13e of the respective sector 13c of the membrane 13 along the axis A. The outer arms 17c and the inner arms 17d are parallel to each other and are connected to each other, to the outer anchor 17a and to the inner anchor 17b so as to form a slot. In more detail, the outer arms 17c extend perpendicular to the axis A in opposite directions from the outer anchor 17a up to the radial slits 18 that delimit the respective sector 13c. Similarly, the inner arms 17d extend perpendicular to the axis A in opposite directions from the inner anchor 17b up to the radial slits 18 that delimit the respective sector 13c. The outer arms 17c and the inner arms 17d are joined to each other at the respective distal ends, relative to the outer anchor 17a and the inner anchor 17b.
Transversal slits 20, also perpendicular to the axis A, delimit the outer arms 17c and the inner arms 17d and separate them from the cantilever element 13d and the tab 13e of the respective sector 13c of the membrane 13. As shown in the enlargements of Figures 7 and 8, the ends of the transversal slits 20 are widened and rounded to avoid the concentration of force line and prevent the initiation of cracks.
In a direction perpendicular to the transversal slits 20, the outer arms 17c and the inner arms 17d have a width W1 comprised between 30µm and 70µm, for example 50µm, and a length comprised for example between 500µm and 1.5mm. The outer anchor 17a and the inner anchor 17b have a width W2 comprised between 70µm and 150µm, for example 100µm.
The peripheral piezoelectric actuator 14 (Figure 2) is arranged at the periphery of the membrane 13 and partly overlaps the supporting frame 12. More precisely, the peripheral piezoelectric actuator 14 comprises a plurality of peripheral actuator portions 14a of substantially trapezoidal shape, each arranged on the cantilever element 13d of a respective sector 13c of the membrane 13 and extending beyond the perimeter of the same membrane 13, on the supporting frame 12. Adjacent peripheral actuator portions 14a are connected to each other by bridges 14b, which have the same structure as the peripheral actuator portions 14a and extend on the supporting frame 12 around distal ends of respective radial slits 18.
The central piezoelectric actuator 15 (Figure 2) is arranged on the central portion 13b of the membrane 13 and comprises central actuator portions 15a that extend in a radial direction from an annular actuator region 15b and form lobes, each on the tab 13e of a respective sector 13c of the membrane 13.
The peripheral piezoelectric actuator 14 and the central piezoelectric actuator 15 use electrical connections that run partly on the membrane 13, including at least some of the elastic elements 17. The structure of the peripheral piezoelectric actuator 14, the central piezoelectric actuator 15, and the electrical connections, as well as the supporting frame 12 and the membrane 13, is shown in detail in the sections of Figures 5 and 6. A dielectric layer 21, for example of silicon oxide, is formed on the outermost of the structural layers 12c and covers the supporting frame 12 and portions of the membrane 13 corresponding to the peripheral piezoelectric actuator 14 and the central piezoelectric actuator 15. The peripheral piezoelectric actuator 14 and the central piezoelectric actuator 15 are formed from the same piezoelectric stack comprising: a bottom metallization structure, for example containing a layer of platinum; a layer of piezoelectric material, for example PZT, on the bottom metallization structure; and a top metallization structure, for example also containing a layer of platinum, on the layer of piezoelectric material. In particular, the peripheral piezoelectric actuator 14 and the central piezoelectric actuator 15 comprise respectively: a bottom peripheral electrode 14c and a bottom central electrode 15c, formed from the bottom metallization structure and arranged on respective portions of the dielectric layer 21; a peripheral piezoelectric body 14d and a central piezoelectric body 15d, formed from the piezoelectric layer and arranged on the bottom peripheral electrode 14c and the bottom central electrode 15c, respectively; and a top peripheral electrode 14e and a top central electrode 15e, formed from the top metallization structure and arranged on the peripheral piezoelectric body 14d and the central piezoelectric body 15d, respectively. The peripheral actuator portions 14a and the bridges 14b connecting them are parts of the peripheral piezoelectric actuator 14 and have the same general structure. A passivation structure 23, for example comprising a layer of silicon nitride and possibly covered by one or more electrically insulating layers, protects the supporting frame 12, the peripheral piezoelectric actuator 14 and the central piezoelectric actuator 15. Outside the peripheral piezoelectric actuator 14 and the central piezoelectric actuator 15, the surface of the membrane 13 opposite to the cavity 16 is substantially free.
Pads 25, 26, 27 on the supporting frame 12 (Figure 2) are accessible for biasing, respectively, the top peripheral electrode 14e, the top central electrode 15e and the bottom peripheral electrode 14c, which is maintained at the same potential as the bottom central electrode 15c as explained below.
The pad 25 is coupled to the top peripheral electrode 14e through a first buried metal line 28, for example of copper, aluminum or an alloy thereof, which extends on the dielectric layer 21 and is incorporated into the passivation structure 23.
The pad 26 is coupled to the top central electrode 15e (Figure 5) through a first exposed metal line 30, made of a conductive material that is immune to oxidation by exposure to the atmosphere and does not require passivation, for example gold or platinum. The first exposed metal line 30 extends along an arbitrary path from the pad 26 to the periphery of the membrane 13 and from there in a radial direction along the bisector of one of the sectors 13c. More precisely (Figures 2 and 5), the first exposed metal line 30 runs on the passivation structure 23 above the supporting frame 12 and the peripheral piezoelectric actuator 14, then on the free part of the cantilever element 13d, on the elastic element 17 and again along the bisector of the affected sector 13c on the tab 13e, to the corresponding actuator central portion 15a. A radially inner end of the first exposed metal line 30 overlaps an edge of the actuator central portion 15a and electrically coupled thereto by an interconnect 31, for example of copper, aluminum or an alloy thereof, through the passivation structure 23. In particular, the first exposed metal line 30 extends symmetrically on the outer arms 17c and on the inner arms 17d of the spring element 17. In one embodiment, the first exposed metal line 30 is formed directly on the membrane 13, where the piezoelectric actuators 14, 15 are not present, and on the elastic element 17. The first exposed metal line 30 (Figure 4) has a width W3 smaller than the width W1 of the outer arms 17c and the inner arms 17d, in one embodiment not greater than half the width W1 and for example equal to 20µm.
The pad 27 (Figure 6) is coupled to the bottom peripheral electrode 14c through a second buried metal line 33, that extends on the dielectric layer 21 and is incorporated into the passivation structure 23.
The bottom peripheral electrode 14c is coupled to the bottom central electrode 15c and maintained at the same potential through a second exposed metal line 35 that extends along the bisector of another of the sectors 13c of the membrane 13, different from the sector 13c accommodating the first exposed metal line 30. In a non-limiting embodiment, the sector 13c accommodating the second exposed metal line 35 is rotated by 90° with respect to the sector 13c accommodating the first exposed metal line 30. The second exposed metal line 35 has ends overlapping respective extensions of the bottom peripheral electrode 14c and the bottom central electrode 15c and electrically coupled thereto by interconnects 34 through the passivation structure 23. Furthermore, between the bottom peripheral electrode 14c and the bottom central electrode 15c, the second exposed metal line 35 extends on the cantilever element 13d, the elastic element 17 and the tab 13e of the respective sector 13c of the membrane 13. The second exposed metal line 35 is made of the same material as the first exposed metal line 30 and has the same shape, except for a rotation by 90° and the portion of the first exposed metal line 30 that extends on the peripheral piezoelectric actuator 14 and the supporting frame 12.
In one embodiment, dummy metal lines 36 are formed on sectors 13c of the membrane 13 opposite with respect to those accommodating the first exposed metal line 30 and the second exposed metal line 35. The dummy metal lines 36 extend between the peripheral actuator portions 14a and the central actuator portions 15a and on the elastic elements 17 of the respective sectors 13c of the membrane 13, are made of the same material and have the same shape as the first exposed metal line 30 and the second exposed metal line 35. The dummy metal lines 36 are decoupled from the piezoelectric actuators 14, 15, are floating and have the sole function of mechanically balancing the stresses applied to the membrane 13 by the first exposed metal line 30 and the second exposed metal line 35.
It is understood that the arrangement of the first exposed metal line 30, the second exposed metal line 35 and any dummy metal lines 36 may however be different from what has been described so far.
For example, in the embodiment of Figure 9, the second exposed metal line 35 is opposite to the first exposed metal line 30 and dummy metal lines 36 are not present.
In the embodiment of Figure 10, dummy metal lines 36 are present in all sectors 13c of the membrane 13 not occupied by the first exposed metal line 30 and the second exposed metal line 35.
In the example described above, the metal lines that connect the central piezoelectric actuator 15 to the pads 26, 27, as well as the dummy metal lines 36 if any, are exposed and free of passivating coating and, in general, of any coating. This is possible because such metal lines are made of a metal immune to oxidation by exposure to the atmosphere and the absence of coating is particularly advantageous because the effects on the deformability of the membrane 13 and the elastic elements 17 are minimal and, in fact, completely negligible. However, a passivating coating and/or another coating might still be present in accordance with design preferences, for example if the deformability of the membrane and the elastic elements is equally considered satisfactory. In this case, the metal lines would not be directly exposed to the atmosphere.
The electroacoustic actuator 10 of the present invention may be driven as described below. In particular, in the micro-speaker 3 the driving stage 11 provides the electroacoustic actuator 10 with electrical signals that are converted into deformations of the membrane 13 in order to generate acoustic waves.
With reference to Figure 11, the peripheral piezoelectric actuator 14 and the central piezoelectric actuator 15 are driven by respective electrical signals, for example unipolar sinusoidal signals (either only positive or only negative signals), having a phase shift of 180° therebetween. In detail, a first signal S1 is applied as a potential difference between the top peripheral electrode 14e and the bottom peripheral electrode 14c of the peripheral piezoelectric actuator 14 and, in one embodiment, has the following mathematical expression: $S_{1} = V_{bias} + V_{ac} \sin (2 πf)$ where,

V_bias is a half-dynamic voltage value, equal for example to 15V;
V_ac is a peak maximum amplitude voltage value, coinciding at maximum with V_bias (or more generally: V_ac ≤ V_bias) ; and
f is the driving frequency.

A second signal S2 is applied as a potential difference between the top central electrode 15e and the bottom central electrode 15c of the central piezoelectric actuator 15 and, in one embodiment, has the following mathematical expression: $S_{2} = V_{bias} + V_{ac} \sin (2 πf + π)$
The application of the first and the second signals S1, S2 allows to provide a so-called "push-pull" driving configuration of the electroacoustic actuator 10. In detail, as may be observed in Figure 11, when the first signal S1 assumes the peak maximum amplitude voltage value V_ac, the peripheral piezoelectric actuator 14 pushes the membrane 13 in the opposite direction with respect to the cavity 16 (i.e., it pushes it in the positive direction of a Z axis perpendicular to the membrane 13 - in Figure 11, upwards); concurrently, the second signal S2 is in phase-opposition and assumes a value equal to V_bias-V_ac, i.e. equal to 0V (in case V_bias=V_ac), with the central piezoelectric actuator 15 in a rest condition; in this condition the membrane 13 reaches a first deflection value z1. When instead the second signal S2 assumes the peak maximum amplitude voltage value V_ac, the central piezoelectric actuator 15 pushes the membrane 13 towards the cavity 16 (i.e., it pushes it in the negative direction of the Z axis - in Figure 11, downwards); concurrently, the first signal S1 is in phase-opposition and assumes a value equal to V_bias-V_ac, i.e. equal to 0V (in case V_bias=V_ac), with the peripheral piezoelectric actuator 14 in a rest condition; in this condition the membrane 13 reaches a second deflection value z2, having opposite sign with respect to the first deflection value z1. In a balanced embodiment, the second deflection value z2 coincides, in absolute value, with the first deflection value z1.
The electroacoustic actuator 10 of the present invention therefore allows a bidirectional movement of the membrane 13 along the Z axis with a single piezoelectric material layer (14d, 15d). In particular, the total deflection of the membrane 13, given by the sum of the absolute values of the first and the second deflection values z1, z2, is greater than the total deflection of a unidirectional electroacoustic actuator of the prior art that bends a membrane exclusively in the positive direction ("pull" only) or negative direction ("push" only) of the Z-axis. For example, the Applicant has verified that the total deflection obtainable by the electroacoustic actuator 10 is greater by 30%-50% than a unidirectional electroacoustic actuator of the prior art. Consequently, a significant improvement in the performances of sound pressure level and total harmonic distortion may be obtained, while maintaining a contained overall piezoelectric capacitance, for the same area of the electroacoustic actuator 10.
With reference to Figure 12, an electroacoustic actuator 110 comprises a supporting frame 112, a membrane 113, a peripheral piezoelectric actuator 114 and a central piezoelectric actuator 115. The membrane 113, for example of polycrystalline silicon, has the shape of a regular polygon with an N-fold rotational symmetry, for example a square, and is connected to the supporting frame 112 along its perimeter by elastic elements 117. The membrane 113 has a peripheral portion 113a, which accommodates the peripheral piezoelectric actuator 114, and a central portion 113b, which accommodates the central piezoelectric actuator 115. The peripheral portion 113a and the central portion 113b of the membrane 113 are coupled to each other by connection portions of the same membrane 113 that define elastic elements 117.
Furthermore, the membrane 113 is divided into a plurality of sectors 113c, delimited by radial slits 118 that extend in a radial direction from respective vertices of the membrane 113 towards the central portion 113b. In the peripheral portion 113a of the membrane 113, the radial slits 118 define cantilever elements 113d of substantially trapezoidal shape, one in each sector 113c. In the central portion 113b of the membrane 13, the radial slits 118 define tabs 113e, one in each sector 113c. In particular, in each sector 113c the cantilever element 113d has a major base connected to the supporting frame 112 and a minor base connected to the corresponding tab 113e of the central portion 113b of the membrane 113 by a respective elastic element 117.
With reference in particular to the enlargement of Figure 13, each elastic element 117 is formed directly by a portion of the membrane 113 and is symmetrical with respect to a bisector (generally indicated by the axis A) of the respective sector 113c. The elastic elements 117 are identical to each other and for convenience hereinafter reference will be made to only one of them, meaning that what has been described also applies to all the others. The elastic element 117 comprises a pair of outer anchors 117a, a pair of inner anchors 117b, an outer arm 117c and an inner arm 117d. The outer anchors 117a and the inner anchors 117b are fixed respectively to the cantilever element 113d and to the tab 113e of the respective sector 113c of the membrane 113; furthermore, as anticipated, the outer anchors 117a and the inner anchors 117b are arranged symmetrically with respect to the bisector of the respective sector 113c. The outer arm 117c and the inner arm 117d are parallel to each other and are connected to each other at respective distal ends relative to the bisector of the respective sector 113c; furthermore, the outer arm 117c is connected to the outer anchors 117a and the inner arm 117d is connected to the inner anchors 117b so as to form a slot. In more detail, transversal slits 120, also perpendicular to the bisector of the respective sector 113c, delimit the outer arm 117c and the inner arm 117d and separate them from the cantilever element 113d and from the tab 113e of the respective sector 113c of the membrane 113. The outer arm 117c and the inner arm 117d extend perpendicular to the bisector of the respective sector 113c up to lateral slits 121, connected, on one side, to the outermost transversal slits 120 and, on the other side, to the innermost transversal slits 120 and to the radial slits 118 that delimit the respective sector 113c.
The peripheral piezoelectric actuator 114 is arranged at the periphery of the membrane 113 and partly overlaps the supporting frame 112. More precisely, the peripheral piezoelectric actuator 114 comprises a plurality of peripheral actuator portions 114a of substantially trapezoidal shape, each arranged on the cantilever element 113d of a respective sector 113c of the membrane 113 and extending beyond the perimeter of the same membrane 113, on the supporting frame 112. Adjacent peripheral actuator portions 114a are connected to each other by bridges 114b, that have the same structure as the peripheral actuator portions 114a and extend on the supporting frame 112 around distal ends of respective radial slits 118. The central piezoelectric actuator 115 is arranged on the central portion 113b of the membrane 13 and comprises central actuator portions 115a that extend in a radial direction from an annular actuator region 115b and form lobes, each on the tab 113e of a respective sector 113c of the membrane 113. The piezoelectric actuators 114, 115 have the structure of the piezoelectric actuators 14, 15 already described, with a bottom electrode, a piezoelectric body and a top electrode not further illustrated in detail.
Pads 125, 126, 127 on the supporting frame 112 are accessible for biasing respectively the top electrode of the peripheral piezoelectric actuator 114, the top electrode of the central piezoelectric actuator 115 and the bottom electrode of the peripheral piezoelectric actuator 114, which is maintained at the same potential as the bottom electrode of the central piezoelectric actuator 115.
The pad 126 is coupled to the central piezoelectric actuator 115 through a first exposed metal line 130, made of a conductive material that is immune to oxidation by exposure to the atmosphere and does not require passivation, for example gold or platinum. The exposed metal line 130 extends along an arbitrary path from the pad 126 to the periphery of the membrane 113, on the respective peripheral actuator portion 114a, then on the cantilever element 113d of the respective sector 113c, for example parallel to the bisector of the respective sector 113c, on the respective elastic element 117 and from there, for example along the bisector of the respective sector 113c, on the tab 113e of the respective sector 113c until it couples with an edge of the respective central actuator portion 115a.
A second exposed metal line 135, provided in a similar manner to the first exposed metal line 130, is formed in the sector 113c of the membrane 113 opposite to that wherein the first exposed metal line 130 is formed, for biasing the bottom electrode of the central piezoelectric actuator 115. The second exposed metal line 135 extends between the peripheral piezoelectric actuator 114 and the central piezoelectric actuator 115, and in particular on the cantilever element 113d of the respective sector 113c, for example parallel to the bisector of the respective sector 113c, on the respective elastic element 117 and from there, for example along the bisector of the respective sector 113c, on the tab 113e of the respective sector 113c until it couples with an edge of the respective central actuator portion 115a.
It is understood that the electroacoustic actuator 110 may also be driven according to what has been previously described for the electroacoustic actuator 10 and according to what has been shown in Figure 11, obtaining the same results and advantages.
The metal lines according to the invention allow biasing piezoelectric actuators placed in central portions of membranes connected to the respective supporting body by elastic elements without appreciably modifying the elastic behavior of the same membranes. More precisely, the use of metals immune to oxidation by exposure to air allows forming exposed metal lines that do not require passivation structures or, if desired in accordance with design preferences, providing the metal lines with very thin passivating coatings at least on the membrane and on the elastic elements. In other words, the addiction of material on the membrane may be strictly limited to the metal of the same lines, avoiding superfluous structures that would stiffen the membrane and might reduce the dynamics. Alternatively, when the deformability of the membrane and the elastic elements is still considered satisfactory in accordance with design preferences, the metal lines may be provided with thin coatings, in particular passivating coatings, which do not substantially alter the performances of the membrane and the elastic elements.
Furthermore, very high conductivity materials may be used and the dimensions of the metal lines may be correspondingly reduced. In general, this avoids stiffening the membrane, to the advantage of the sound pressure level. Furthermore, the metal lines may be narrow enough to run on the elastic elements, without significantly altering their mechanical properties and without the need for dedicated membrane portions.
Finally, it is clear that modifications and variations may be made to the electroacoustic actuator described, without departing from the scope of the present invention, as defined in the attached claims.

Claims

A microelectromechanical electroacoustic actuator (10; 110) comprising:
a supporting frame (12; 112), containing semiconductor material;

a membrane (13; 113) of semiconductor material connected to the supporting frame (12; 112) along a perimeter;

a peripheral piezoelectric actuator (14; 114) on a peripheral portion (13a; 113a) of the membrane (13; 113);

a central piezoelectric actuator (15; 115) on a central portion (13b; 113b) of the membrane (13; 113);

elastic elements (17; 117), defined by respective portions of the membrane (13; 113), the central portion (13b; 113b) being connected to the peripheral portion (13a; 113a) of the membrane (13; 113) by the elastic elements (17; 117); and

metal lines (30, 35, 36; 130, 135) extending on the peripheral portion (13a; 113a) of the membrane (13; 113), on respective elastic elements (17; 117) and on the central portion (13b; 113b) of the membrane (13; 113), from the peripheral piezoelectric actuator (14; 114) to the central piezoelectric actuator (15; 115).
The electroacoustic actuator (10; 110) according to claim 1, wherein at least one of the metal lines (30, 35, 36; 130, 135) also extends on the peripheral piezoelectric actuator (14; 114).
The electroacoustic actuator (10; 110) according to claim 1 or 2, wherein the membrane (13; 113) is divided into sectors (13c; 113c) by radial slits (18; 118) extending from the peripheral portion (13a; 113c) into the central portion (13b; 113b).
The electroacoustic actuator according to the preceding claim, wherein each sector (13c; 113c) comprises a respective one of the elastic elements (17; 117).
The electroacoustic actuator (10; 110) according to claim 3 or 4, wherein in the peripheral portion (13a; 113a) of the membrane (13; 113) the radial slits (18; 118) define a cantilever element (13d; 113d) of substantially trapezoidal shape in each sector (13c; 113c),
wherein in the central portion (13b; 113b) of the membrane (13; 113) the radial slits (18; 118) define a tab (13e; 113e) in each sector (13c; 113c), and

wherein the cantilever element (13d; 113d) and the tab (13e; 113e) of each sector (13c; 113c) of the membrane (13; 113) are coupled to each other by the respective elastic element (17; 117).
The electroacoustic actuator (10; 110) according to the preceding claim, wherein the peripheral piezoelectric actuator (14; 114) comprises a plurality of peripheral actuator portions (14a; 114a), each arranged on the cantilever element (13d; 113d) of a respective sector of the membrane (13; 113) and extending beyond the perimeter of the membrane (13; 113), on the supporting frame (12; 112), adjacent peripheral actuator portions (14a; 114a) being connected to each other by bridges (14b; 114b) extending on the supporting frame (12; 112) around distal ends of respective radial slits (18; 118), and
wherein the central piezoelectric actuator (15; 115) comprises a plurality of central actuator portions (15a; 115a) extending in a radial direction from an annular actuator region (15b; 115b), each on the tab (13e; 113e) of a respective sector (13c; 113c) of the membrane (13; 113).
The electroacoustic actuator (10; 110) according to any of claims 3 to 6, wherein each elastic element (17; 117) comprises at least one outer anchor (17a; 117a), connected to the peripheral portion (13a; 113a) of the membrane (13; 113), at least one inner anchor (17b; 117b), connected to the central portion (13b; 113b) of the membrane (13; 113), at least one outer arm (17c; 117c) and at least one inner arm (17d; 117d) extending transversely to the bisector (A) of the respective sector (13c; 113c).
The electroacoustic actuator (10; 110) according to the preceding claim, wherein in each elastic element (17; 117), the at least one outer arm (17c; 117c) and the at least one inner arm (17d; 117d) are parallel to each other and are connected to each other, to the outer anchor (17a; 117a) and to the inner anchor (17b; 117b) so as to form a slot.
The electroacoustic actuator (10; 110) according to any of claims 3 to 8, wherein each elastic element (17; 117) is symmetrical with respect to the bisector (A) of the respective sector (13c; 113c).
The electroacoustic actuator (10; 110) according to any of claims 3 to 9, wherein the central piezoelectric actuator (15; 115) comprises a bottom electrode (15c), a piezoelectric body (15d) on the bottom electrode (15c) and a top electrode (15e) on the piezoelectric body (15d) and wherein the metal lines (30, 35, 36; 130, 135) comprise a first metal line (30; 130) connecting the top electrode (15e) to a first pad (26; 126) on the supporting frame (12; 112) through the respective sector (13c; 113c) of the membrane (13; 113).
The electroacoustic actuator (10; 110) according to the preceding claim, wherein the metal lines (30, 35, 36; 130, 135) comprise a second metal line (35; 135) connecting the bottom electrode (15c) to a second pad (27; 127) on the supporting frame (12; 112) through the respective sector (13c; 113c) of the membrane (13; 113).
The electroacoustic actuator (10; 110) according to the preceding claim, wherein the sectors (13c; 113c) of the membrane (13; 113) accommodating the first metal line (30; 130) and the second metal line (35; 135) are rotated by 90° or 180° with respect to each other.
The electroacoustic actuator (10) according to the preceding claim, wherein the sectors (13c) of the membrane (13) accommodating the first metal line (30) and the second metal line (35) are rotated by 90° with respect to each other, wherein the metal lines (30, 35, 36) comprise dummy metal lines (36) in sectors (13c) of the membrane (13) opposite to the sectors (13c) accommodating the first metal line (30) and the second metal line (35), wherein the dummy metal lines (36) extend at least on the elastic element (17) of the respective sector (13c) of the membrane (13) and up to the central piezoelectric actuator (15) and wherein the dummy metal lines (36) are electrically insulated from the central piezoelectric actuator (15).
The electroacoustic actuator (10; 110) according to any of the preceding claims, wherein the metal lines (30, 35, 36; 130, 135) are of a metal immune to oxidation by exposure to the atmosphere, for example gold or platinum, and wherein the metal lines (30, 35, 36; 130, 135) are free of coating and are exposed on the membrane (13; 113) and on the elastic elements (17; 117).
The electroacoustic actuator (10; 110) according to any of the preceding claims, wherein the membrane (13; 113) has an N-fold rotational symmetry, N being an integer.
The electroacoustic actuator (10; 110) according to any of the preceding claims, wherein the supporting frame (12; 112) has a cavity (16) open on one side and closed on the opposite side by the membrane (13; 113) and wherein the metal lines (30, 35, 36; 130, 135) extend on a face of the membrane (13; 113) opposite to the cavity (16).
A method of driving an electroacoustic actuator (10; 110) according to any of claims 1 to 16, comprising applying a first electrical signal (S1) to the peripheral piezoelectric actuator (14; 114) and a second electrical signal (S2) to the central piezoelectric actuator (15; 115), the first signal (S1) and the second signal (S2) being unipolar and having a phase shift of 180° therebetween.