High performance low power electroacoustic transducer

Abstract

The invention relates to a high performance low power electro-acoustic transducer. A microelectromechanical electroacoustic transducer comprising: a support frame comprising a semiconductor material; a film of semiconductor material peripherally connected to the support frame; and a piezoelectric transducer on a central portion of the membrane. The piezoelectric transducer is configured to cause a stationary deflection of the membrane from the planar configuration toward the first side of the membrane without electrical stimulation to the piezoelectric actuator, and to cause an inductive deflection of the membrane toward the second side of the membrane opposite the stationary deflection in response to an electrical drive signal.

Description

Cross-reference to related applications

[0001] This application claims priority to Italian Patent Application No. 102024000021603, filed on September 30, 2024, the contents of which are incorporated herein by reference in their entirety to the fullest extent permitted by law. Technical Field

[0002] This disclosure relates to high-performance, low-power electroacoustic transducers. Background Technology

[0003] As is well known, the vast majority of users of mobile and fixed processing and communication devices (such as smartphones, tablets, portable and desktop computers) benefit from the use of headphones and earphones, which are currently very common. This widespread use, and the fact that headphones and earphones are worn continuously for extended periods in many cases, creates a need for devices that provide comfort and practicality without sacrificing audio reproduction quality. Therefore, there has been a significant impetus for the manufacture of miniaturized electroacoustic transducers such as speakers and microphones. Other miniaturized electroacoustic transducers of increasing interest include those used in ultrasound probes and are commonly used in ultrasound imaging (piezoelectric micromachined ultrasonic transducers, PMUTs).

[0004] However, the currently available solutions are not entirely satisfactory and do not represent an effective trade-off between performance, consumption, and cost in terms of size, sound pressure level (SPL - for transmitters) or sensitivity (for receivers).

[0005] The first type of electroacoustic transducer, especially loudspeakers, utilizes conventional electromagnetic excitation and is able to ensure high reproduction quality. However, electromagnetically driven loudspeakers are not suitable for miniaturization beyond certain limits.

[0006] Other solutions based on MEMS (Micro-Electro-Mechanical Systems) technology allow for better miniaturization, but their cost and / or performance are not yet suitable for replacing motorized speakers.

[0007] For example, hybrid devices are known in which a microelectromechanical actuator (typically piezoelectric) is coupled to a polymer membrane that induces vibration. The polymer membrane offers the advantage of high flexibility (low Young's modulus), allowing for good response, but presents critical issues from a process and cost perspective. In practice, the membrane is only applied to the actuator-receiving portion of the device in a downstream step, a dedicated processing step after the actuator itself has been manufactured. Furthermore, hybrid microspeakers are unsuitable for on-board assembly using SMT (Surface Mount Technology) because the membrane cannot withstand the soldering temperatures.

[0008] Other devices manufactured entirely using MEMS technology meet the requirements for miniaturization and cost, but do not achieve sufficient performance in terms of dynamic response and bandwidth.

[0009] In particular, some miniature loudspeakers include a semiconductor film attached to a support frame along its periphery. A key issue with these devices lies primarily in the poor flexibility of the film. In fact, to achieve a suitable sound pressure level, the reduction in film area due to miniaturization should be compensated for by a larger displacement (sound pressure is actually related to the product A). d f is proportional to d, where A is the area of ​​the film, d is the displacement, and f is the frequency. However, the stiffness of semiconductor materials does not allow for sufficient displacement, especially at low frequencies.

[0010] In other MEMS microspeakers, the diaphragm is precisely discontinuous within its internal portion to allow for greater flexibility and wider displacement. Instead of a true diaphragm, the transducer comprises multiple cantilever structures, each defining a polygonal segment or circular sector extending from a support frame. The vertices of the segments or sectors are adjacent to each other at the center of the transducer but not connected. This allows for a wider response dynamic range; however, it is not constant over the audio bandwidth. Misalignment in the out-of-plane direction of the cantilever structures, especially at low frequencies and resonances, can actually lead to uncontrolled widths of fluid channels between adjacent segments, introducing vents into the diaphragm that can impair performance. Furthermore, the response quality of this type of MEMS microspeaker is severely affected by process variations, as even small differences in the cantilever structures can cause uneven motion and asymmetry in sound emission, affecting total harmonic distortion (THD).

[0011] Similarly, to increase the maximum displacement, MEMS microspeakers with different piezoelectric actuators have been proposed to move a diaphragm in opposite directions relative to a stationary structure without applied stress. Specifically, peripheral piezoelectric actuators are arranged along the periphery of the diaphragm and apply a force tending to deform the diaphragm in a first direction, while a central piezoelectric actuator is centrally arranged on the diaphragm and applies a force tending to deform the diaphragm in a second direction opposite to the first direction. However, in known devices of this type, the increase in displacement is limited in part by the fact that the diaphragm is continuous and therefore more rigid to accommodate the central actuator and its electrical connections. Furthermore, the power absorbed by the piezoelectric actuator depends not only on the voltage and actuation frequency but also on the capacitance of the actuator itself. The latter should be minimized to reduce the absorbed power and increase the battery life of devices typically powered by batteries. On the other hand, the area of ​​the actuators cannot be reduced beyond a certain limit because the force applied by each actuator will decrease accordingly, thus limiting the maximum displacement of the diaphragm. Summary of the Invention

[0012] Therefore, the purpose of this disclosure is to provide an electroacoustic transducer that allows for overcoming or at least mitigating the limitations described.

[0013] A microelectromechanical electroacoustic transducer includes a support frame comprising semiconductor material and a membrane of semiconductor material connected peripherally to the support frame. A piezoelectric transducer is located in the central portion of the membrane. The piezoelectric transducer is configured to cause a stationary deflection of the membrane from a planar configuration toward a first side of the membrane without electrical stimulation of the piezoelectric transducer, and to cause an induced deflection of the membrane toward a second side of the membrane in response to an electrical drive signal, opposite to the stationary deflection.

[0014] Piezoelectric transducers and membranes can form composite membranes and have a corresponding compressive residual stress state.

[0015] The piezoelectric transducer can be located on the first side of the membrane.

[0016] The membrane can be divided into sections by radial slits extending from the periphery of the membrane to a certain distance from the center of the membrane.

[0017] A piezoelectric transducer may include an annular actuator region and lobes extending radially from the annular actuator region, each lobe being on a corresponding segment of the membrane.

[0018] The transducer may include an elastic element defined by a corresponding portion of the membrane, the membrane being connected to a support frame via the elastic element. Metal wires may extend from the corresponding elastic element on the membrane to the piezoelectric transducer.

[0019] Metal wires can be metals that do not oxidize when exposed to the atmosphere, such as gold or platinum.

[0020] Metal wires can be uncoated and exposed on the membrane and elastic elements.

[0021] The piezoelectric transducer may include a bottom electrode, a piezoelectric element on the bottom electrode, and a top electrode on the piezoelectric element. The metal wire may include a first metal wire connecting the top electrode to a first pad on a support frame and a second metal wire connecting the bottom electrode to a second pad on the support frame.

[0022] Each segment may include a pair of corresponding elastic elements arranged symmetrically to each other with respect to an axis extending along the bisector of the respective segment. Each elastic element may include an outer anchor fixed to a support frame, an inner anchor connected to a central portion of the membrane, an outer arm extending in the opposite direction from the outer anchor, and an inner arm extending in the opposite direction from the inner anchor.

[0023] In each elastic element, the outer arm and the inner arm can be parallel to each other and connected to each other, connecting to the outer anchor and the inner anchor, thus forming a slot.

[0024] The first and second metal wires can extend on corresponding different sections of the membrane, and each extends on two elastic elements in the corresponding section.

[0025] Alternatively, the first and second metal wires may extend over the same section of the membrane and each extend over a corresponding elastic element of the section.

[0026] The metal wire may include a dummy metal wire in each segment of the membrane opposite to one of the segments containing the first metal wire and / or the second metal wire. The metal wire may extend at least on the elastic element of the corresponding segment of the membrane and extend to the piezoelectric transducer, and may be electrically insulated from the piezoelectric transducer.

[0027] The membrane can have N-fold rotational symmetry, where N is an integer. Attached Figure Description

[0028] To better understand this disclosure, preferred embodiments are provided by way of non-limiting example with reference to the accompanying drawings, in which:

[0029] Figure 1 It is a simplified block diagram of processing and communication equipment;

[0030] Figure 2 It is combined with Figure 1 A top view of an electroacoustic transducer in a device according to an embodiment of the present disclosure;

[0031] Figure 3 It shows Figure 2 The electroacoustic transducer, in which components have been removed for clarity;

[0032] Figure 4It shows Figure 2 Magnified details of the electroacoustic transducer;

[0033] Figure 5a It is in the first operation configuration. Figure 2 The cross-section of the electroacoustic transducer;

[0034] Figure 5b It is in the first operation configuration. Figure 2 A perspective view of a portion of the device;

[0035] Figure 6a It is in the second operation configuration. Figure 2 The cross-section of the electroacoustic transducer;

[0036] Figure 6b It is in the second operation configuration. Figure 2 A perspective view of a portion of the device;

[0037] Figure 7 It is along Figure 2 The line VII-VII cut Figure 2 A simplified cross-sectional view of an electroacoustic transducer;

[0038] Figure 8 It is along Figure 2 The line VIII-VIII cut Figure 2 A simplified cross-sectional view of an electroacoustic transducer;

[0039] Figure 9 It shows Figure 2 Another magnified detail of the electroacoustic transducer;

[0040] Figure 10 It shows Figure 2 Further magnified details of the electroacoustic transducer;

[0041] Figure 11 It can be used Figure 1 A top view of an electroacoustic transducer according to different embodiments of the present disclosure in a device;

[0042] Figure 12a It passes through the first operating configuration. Figure 11 A cross-sectional view of an electroacoustic transducer;

[0043] Figure 12b It is through the second operating configuration Figure 11 The cross-section of the electroacoustic transducer. Detailed Implementation

[0044] The following description refers to the arrangement shown in the accompanying drawings; therefore, expressions such as “above,” “below,” “upper,” “lower,” “top,” “bottom,” “right,” “left,” etc., are related to the drawings and should not be interpreted in a restrictive manner.

[0045] For convenience, reference will be made below to electroacoustic transducers used in miniature loudspeakers. However, this should not be construed as limiting. Electroacoustic transducers according to this disclosure can be used in various devices, including receivers and transmitters, including microphones and ultrasonic probes, and are commonly used in the field of ultrasonic imaging (PMUT, piezoelectric micromechanical ultrasonic transducers).

[0046] Furthermore, throughout this text and below, the term "transducer" is intended to generally refer to a device that converts a first physical quantity (or form of energy) into a corresponding (different) second physical quantity (or form of energy) or vice versa. In some cases, depending on operating conditions, a transducer can be used bidirectionally to convert a first physical quantity into a second physical quantity or vice versa. In particular, it should be understood that an electroacoustic transducer is a device that converts sound waves into corresponding electrical signals, or vice versa, converts electrical signals into corresponding sound waves. Electroacoustic transducers can be used bidirectionally to convert sound waves into corresponding electrical signals and electrical signals into corresponding sound waves (e.g., in ultrasonic probes or in some headphones with active noise cancellation). Furthermore, it should be understood that a piezoelectric transducer converts force or pressure applied to the transducer surface into a corresponding electrical signal and vice versa. Piezoelectric transducers are generally bidirectional.

[0047] refer to Figure 1 The electronic system, represented by the digit 1 as a whole, includes a processing and communication device 2 that is communicatively coupled to the miniature speaker 3.

[0048] The processing and communication device 2 can be any portable or stationary device that supports audio communication with a playback peripheral device such as the miniature speaker 3. The processing and communication device 2 can be, but is not limited to, a portable computer, personal computer, tablet computer, smartphone, or wearable device such as a smartwatch, and specifically includes a processing unit 5 and a communication module 6 coupled to a corresponding communication module 8 of the miniature speaker 3. The processing and communication device 2 may typically include other components not shown, such as a display unit, memory unit, input and pointing devices, peripheral devices, a battery, and I / O interfaces.

[0049] In addition to the communication module 8, the miniature speaker 3 also includes an electroacoustic transducer 10 and a driver 11. The driver 11 receives audio signals through the communication module 8 and actuates the electroacoustic transducer 10.

[0050] The communication modules 6 and 8 of the processing and communication device 2 and the miniature speaker 3 can be coupled to each other wirelessly or by cable.

[0051] refer to Figures 2-8The electroacoustic transducer 10 is a piezoelectric membrane microelectromechanical transducer and includes a support frame 12, a membrane 13 and a piezoelectric transducer, particularly a piezoelectric actuator 15.

[0052] The support frame 12 is made of semiconductor material and has a cavity 16 ( Figure 5a , Figure 5b , Figure 6a , Figure 6b , Figure 7 , Figure 8 The cavity 16 is open on one side and closed on the opposite side by a membrane 13. More precisely, the support frame 12 may include a substrate, such as a monocrystalline silicon 12a, a dielectric layer 12b, and one or more structural layers 12c, which may include an epitaxial layer, or monocrystalline silicon, or a polycrystalline silicon layer or deposited layer grown from a seed in an epitaxial reactor.

[0053] The membrane 13 is also made of a semiconductor material, such as polycrystalline silicon continuous with the outermost structural layer 12c of the support frame 12, and is connected to the support frame 12 along its periphery. The membrane 13 may have a thickness, for example, between 3 μm and 25 μm. In one embodiment, the membrane 13 is polygonal and has N-fold rotational symmetry with respect to an axis perpendicular to the membrane and passing through its center, where N is an integer. It should be understood that the body has N-fold rotational symmetry with respect to the axis while its shape remains unchanged after a rotation of 360° / N about the axis. For example, the membrane 13 may have a regular octagonal shape. Furthermore, having N-fold rotational symmetry can even be advantageous in terms of balancing stress (e.g., for dummy connection arrangements, as explained in detail below).

[0054] Specific reference Figure 3 For clarity, Figure 3 Only the support frame 12 and the membrane 13 are shown. The membrane 13 is connected to the support frame 12 along its periphery by an elastic element 17. The membrane 13 is divided into a plurality of segments 13a, which are defined by radial slits 18 extending radially inward from their respective apexes to a distance from the center of the membrane 13. In one embodiment, all radial slits 18 have the same width. Furthermore, the width of the radial slits 18 is less than twice the thickness of the viscous boundary layer of air, particularly in the operating temperature range, for example, between -20°C and +40°C. In one embodiment, the width is less than the thickness of the viscous boundary layer of air, and in any case, is no greater than 10 μm, for example, 5 μm. Furthermore, the width-to-thickness ratio of the membrane 13 is no greater than 1.

[0055] In membrane 13, radial slits 18 define protrusions 13b, with each segment 13a corresponding to one protrusion 13b. The protrusions 13b are connected to the support frame 12 by corresponding elastic elements 17 and are connected to each other by continuous central portions 13c of membrane 13, which are radially inward relative to the radial slits 18.

[0056] exist Figures 2-8 In the example, each protrusion 13b is connected to the support frame 12 by a pair of corresponding elastic elements 17, which are arranged symmetrically with respect to the axis A extending along the bisector of the respective segment 13a. The arrangement of the elastic elements 17 is the same in each segment 13a of the membrane 13, and for convenience, reference will be made below only to the elastic element of one of the segments 13a; it should be understood that what has been described also applies to all other segments. It should also be understood that the arrangement and shape of the elastic elements may differ from those described.

[0057] Specific reference Figure 4 The enlarged view shows that each elastic element 17 is formed directly from a portion of the membrane 13 and includes an outer anchor 17a, an inner anchor 17b, an outer arm 17c, and an inner arm 17d. The outer anchor 17a and inner anchor 17b are respectively fixed along axis A to a corresponding side of the support frame 12 defining the cavity 16 and to a protrusion 13b of a corresponding segment 13a of the membrane 13. The outer arm 17c and inner arm 17d are parallel to each other and connected to the outer anchor 17a and inner anchor 17b to form a slot. More specifically, the outer arm 17c extends perpendicularly to axis A from the outer anchor 17a in the opposite direction to a radial slit 18 defining the corresponding segment 13a. Similarly, the inner arm 17d extends perpendicularly to axis A from the inner anchor 17b in the opposite direction to the radial slit 18 defining the corresponding segment 13a. The outer arm 17c and the inner arm 17d are connected to each other at their respective distal ends relative to the outer anchor 17a and the inner anchor 17b.

[0058] Along axis A, the elastic element 17 is divided by a separating slit 19, which extends radially from the protrusion 13b to the support frame 12. A transverse slit 20 perpendicular to axis A (see also...) Figure 9 and Figure 10 Define the outer arm 17c and the inner arm 17d, and separate them from the corresponding side of the support frame 12 and the protrusion 13b of the corresponding segment 13a of the membrane 13. Figure 9 and Figure 10 As shown in the enlarged view, the ends of the transverse slit 20 are widened and rounded to avoid the concentration of force lines and prevent the formation of cracks.

[0059] In the direction perpendicular to the transverse slit 20, the width W1 of the outer arm 17c and the inner arm 17d is between 30 μm and 70 μm, for example, 50 μm, and the length is between 500 μm and 1.5 mm. The width W2 of the outer anchor 17a and the inner anchor 17b is between 70 μm and 150 μm, for example, 100 μm.

[0060] piezoelectric actuator 15 ( Figure 2 The piezoelectric actuator 15 is arranged on the central portion 13c of the membrane 13, and in one embodiment includes lobes 15a extending radially from the annular actuator region 15b, each lobe 15a being on a protrusion 13b of a corresponding segment 13a of the membrane 13. The piezoelectric actuator 15 has the same N-fold rotational symmetry as the membrane 13.

[0061] The piezoelectric actuator 15 and the membrane 13 form a composite membrane, wherein the deformation of the membrane at rest, i.e., the deformation without electrical stimulation of the piezoelectric actuator 15, is obtained by utilizing the residual stress state of the material. As described in Seung-Mock Lee, Tunehisa Tanaka, Kji Inoue, “Residual Stress and Membrane Deflection Influences on the Ultrasonic Sensor Device,” IEEE Sensors 2006, EXCO, Daegu, Korea, October 22-25, 2006 (incorporated herein by reference), in composite membranes, differences in material and process factors generate residual stress that tends to induce mechanical strain. For example, the composite membrane may comprise a semiconductor film and a stack forming the piezoelectric actuator, as in the case of the electroacoustic transducer 10. The semiconductor film is typically subjected to residual compressive stress, while the residual stress state of the piezoelectric actuator (particularly defined by a Pt / PZT / Pt stack) can be controlled to be tensile or compressive, determining the deflection of the composite membrane. If the residual stress state of the piezoelectric actuator is compressive, the composite film deflects toward the piezoelectric actuator in the direction opposite to the cavity beneath the film; if the residual stress state of the piezoelectric actuator is tensile, the composite film deflects toward the semiconductor film in the direction of the cavity beneath the film.

[0062] In the embodiments described herein, the residual stress state is particularly compressive and causes the membrane 13 to deflect relative to the planar structure toward the piezoelectric actuator 15 side 13d, such that the membrane 13, in a static state, i.e., without electrical stimulation of the piezoelectric actuator 15 (static deflection or deflection at rest), has a dome shape toward the opening of the cavity 16 (e.g., Figure 5a and Figure 5b(As shown). Furthermore, the piezoelectric actuator 15 is configured to deform the membrane 13 in response to, for example, an electrical drive signal VD applied by the drive stage 11, so as to induce a induced deflection of the membrane 13 opposite to the static deflection (due to residual stress in the material), i.e., the induced deflection is directed toward a side 13e of the membrane 13 opposite to the piezoelectric actuator 15 and facing the cavity 16. Figure 6a and Figure 6b In fact, in response to the drive signal VD, the membrane 13 takes on the shape of a cup that opens in the opposite direction to the cavity 16.

[0063] To provide an electrically driven signal VD, the electroacoustic transducer 10 includes an electrical connection extending partially on a diaphragm 13, comprising at least some elastic elements 17. Figure 7 and Figure 8 The structure of the piezoelectric actuator 15, electrical connections, support frame 12, and membrane 13 is shown in detail in the section below. For simplicity, the membrane 13 is shown in a planar configuration that does not correspond to a static configuration without electrical stimulation of the piezoelectric actuator 15. For example, a dielectric layer 21, such as silicon oxide, is formed on the outermost side of the structural layer 12c and covers the portions of the support frame 12 and membrane 13 corresponding to the piezoelectric actuator 15. The piezoelectric actuator 15 is formed of a piezoelectric stack including a bottom metallization structure, such as a platinum layer; a piezoelectric material layer, such as PZT, on the bottom metallization structure; and an upper metallization structure, such as also including a platinum layer, on the piezoelectric material layer. Specifically, the piezoelectric actuator 15 includes a bottom electrode 15c formed by the bottom metallization structure and disposed on the dielectric layer 21; a piezoelectric body 15d formed by the piezoelectric layer and disposed on the bottom electrode 15c; and an upper electrode 15e formed by the upper metallization structure and disposed on the piezoelectric body 15d. For example, a passivation structure 23, including a silicon nitride layer and possibly covered by one or more electrically insulating layers, protects the support frame 12 and the piezoelectric actuator 15. Outside the piezoelectric actuator 15, the surface of the diaphragm 13 opposite to the cavity 16 is substantially free.

[0064] Support frame 12 ( Figure 2 The pads 25 and 27 on the membrane 13 are accessible and are used to bias the upper electrode 15e and the lower electrode 15c respectively by metal lines extending on one side 13d of the membrane 13, thereby causing deformation of the membrane in the opposite direction to the cavity 16.

[0065] Pad 25 is coupled to upper electrode 15e via a first exposed metal line 30, which is made of a conductive material exposed to the atmosphere to avoid oxidation and requiring no passivation, such as gold or platinum. The first exposed metal line 30 ( Figure 7The first exposed metal wire 30 extends along any path on the passivation structure 23 above the support frame 12 from the pad 25 to the periphery of the membrane 13, then to the elastic element 17 of a corresponding segment of the membrane 13a, and from there to the corresponding convex corner 15a of the piezoelectric actuator 15 in the radial direction along the bisector of the same segment 13a. The radially inner end of the first exposed metal wire 30 overlaps with the edge of the convex corner 15a and is electrically coupled to the edge of the convex corner 15a by an interconnect 31, for example, copper, aluminum, or an alloy thereof, through the passivation structure 23. In particular, the first exposed metal wire 30 extends symmetrically on the outer arm 17c and inner arm 17d of the two elastic elements 17 in the corresponding segments of the membrane 13. In one embodiment, the first exposed metal wire 30 is formed directly on the membrane 13 and the elastic element 17 without the piezoelectric actuator 15. The width W3 of the first exposed metal line 30 is less than the width W1 of the outer arm 17c and the inner arm 17d, and in one embodiment is no more than half the width W1, for example, equal to 20 μm.

[0066] Pad 27 ( Figure 8 The first exposed metal line 30 is coupled to the bottom electrode 15c via a metal line 33 and a second exposed metal line 35. The metal line 33 extends on the dielectric layer 21 and is incorporated into the passivation structure 23. The second exposed metal line 35 extends along any path on the passivation structure 23 above the support frame 12 from the pad 27 to the periphery of the membrane 13, then onto the elastic element 17 of a corresponding segment of the membrane 13, which is different from the segment 13a that houses the first exposed metal line 30, and from there radially along the bisector of the same segment 13a to the corresponding lobe 15a of the piezoelectric actuator 15. In a non-limiting embodiment, the segment 13a housing the second exposed metal line 35 is rotated 90° relative to the segment 13a housing the first exposed metal line 30. The second exposed metal line 35 has an end that overlaps with an extension of the bottom electrode 15c and is electrically coupled to the bottom electrode 15c via the passivation structure 23 through an interconnect 34. Except for the 90° rotation, the second exposed metal wire 35 is made of the same material as the first exposed metal wire 30 and has the same shape.

[0067] In one embodiment, a dummy metal line 36 is formed on a segment 13a of the membrane 13 opposite to the segment containing the first exposed metal line 30 and the second exposed metal line 35. The dummy metal line 36 extends on the elastic element 17 and along the bisector of the corresponding segment 13a of the membrane 13 until it is near the corresponding lobe 15a of the piezoelectric actuator. The dummy metal line 36 is made of the same material and has the same shape as the first exposed metal line 30 and the second exposed metal line 35. The dummy metal line 36 is decoupled from the piezoelectric actuator 15, is floating, and has 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.

[0068] However, it should be understood that the arrangement and geometry of the first exposed metal line 30, the second exposed metal line 35, and any dummy metal line 36 may differ from those described above.

[0069] In one embodiment not shown, for example, the second exposed metal line is opposite to the first exposed metal line, and there is no dummy metal line.

[0070] In another embodiment not shown, dummy metal lines exist in all sections 13a of the membrane 13 that are not occupied by the first exposed metal line and the second exposed metal line. Figure 3 )middle.

[0071] In the example above, the metal wires connecting the piezoelectric actuator to pads 25 and 27, as well as the dummy metal wire 36 (if present), are exposed without any passivation coating, and typically have no coating at all. This is possible because such metal wires are made of a metal that will not oxidize due to exposure to the atmosphere, and the absence of a coating is particularly advantageous because the effect on the deformability of the membrane 13 and the elastic element 17 is minimal and practically negligible. However, depending on design preferences, for example if the deformability of the membrane and the elastic element is still considered satisfactory, a passivation coating and / or another coating may still be present. In this case, the metal wires are not directly exposed to the atmosphere.

[0072] refer to Figure 11 The electroacoustic transducer 110 includes a support frame 112, a membrane 113, and a piezoelectric transducer, particularly a piezoelectric actuator 115. The membrane 113, for example, is a polycrystalline silicon regular polygon (e.g., a hexagon) with N-fold rotational symmetry, and is connected to the support frame 112 along its periphery by elastic elements 117. The membrane 113 is divided into multiple segments 113a, each segment 113a being defined by a radial slit 118 extending radially inward from a corresponding vertex of the membrane 113 to a distance from the center of the membrane 113. In each segment 113a of the membrane 113, the radial slit 118 defines a protrusion 113b coupled to the support frame 112 by a corresponding elastic element 117. More specifically, each protrusion 113b is coupled to the support frame 112 by multiple corresponding elastic elements 117 (two in this case), which are arranged symmetrically with respect to the bisector of the corresponding segment 113a. Each elastic element 117 includes an outer anchor 117a fixed to the support frame 112, an inner anchor 117b fixed to the protrusion 113b, an outer arm 117c, and an inner arm 117d. The use of multiple elastic elements 117 in each segment 113a allows for proper mobility of the membrane 113, thereby preventing the elastic elements 117 from being weakened due to the size of the periphery of the protrusion 113a.

[0073] A piezoelectric actuator 115 is disposed on the central portion of the membrane 113 and includes lobes 115a extending radially from the annular actuator region 115b, each lobe being a protrusion 113b on a corresponding segment 113a of the membrane 113. The piezoelectric actuator 115 has the structure of the piezoelectric actuator 15 already described, having a bottom electrode, a piezoelectric body, and an upper electrode, which are not shown in detail. Furthermore, also in this case, the residual stress state of the piezoelectric actuator 115 causes the membrane 113 to deflect toward one side 113d of the piezoelectric actuator 115, such that the membrane 113, under static conditions, i.e., without electrical stimulation of the piezoelectric actuator 115, has a dome shape (e.g., ...) toward the opening of the cavity 116. Figure 12a (As shown). Furthermore, the piezoelectric actuator 115 is configured to deform the membrane 113 in response to an electrical drive signal VD applied, for example, by the drive stage 11, so as to cause a deflection of the membrane 113 toward a side 113e of the membrane 113 opposite to the piezoelectric actuator 115 and facing the cavity 116, opposite to the stationary deflection. Figure 12b ).

[0074] Pads 125 and 127 on the support frame 112 are accessible for biasing the upper and lower electrodes (not shown in detail here) of the piezoelectric actuator 115. Pads 125 and 127 are coupled to the piezoelectric actuator 115 via a first exposed metal line 130 and a second exposed metal line 135, respectively. Both the first and second exposed metal lines 130 are made of a conductive material that will not oxidize upon exposure to the atmosphere and does not require passivation, such as gold or platinum. The first exposed metal line 130 extends along an arbitrary path from the pad 125 to the periphery of the membrane 113, on a first elastic element 117 of one of the segments 113a, and from there on a protrusion 113b of the same segment 113a. The first exposed metal line 130 has a radially inner end that is coupled to the upper electrode of the piezoelectric actuator 115 at an edge of a corresponding lobe 115a. The second exposed metal line 135 extends along an arbitrary path from the pad 127 to the periphery of the membrane 113 on the second elastic element 117 of the same section 113a as the first exposed metal line 130, and from there extends on the protrusion 113b of the same section 113a. The second exposed metal line 135 has a radially inner end coupled to the bottom electrode of the piezoelectric actuator 115 at one edge of the lobe 115a.

[0075] A dummy metal wire 136 is formed on a section 113a of the membrane 113 that differs from the section containing the first exposed metal wire 130 and the second exposed metal wire 135. The dummy metal wire 136 extends on the corresponding elastic element 117 and on the protrusion 113b of the section 113a of the membrane 113 that does not contain the first exposed metal wire 130 and the second exposed metal wire 135, until it reaches the vicinity of the corresponding flap 115a of the piezoelectric actuator 115. The dummy metal wire 136 is made of the same material as the first exposed metal wire 130 and the second exposed metal wire 135 and has the same shape as the first exposed metal wire 130 and the second exposed metal wire 135. The dummy metal wire 136 is decoupled from the piezoelectric actuator 115, is floating, and has the sole function of mechanically balancing the stresses applied to the membrane 113 by the first exposed metal wire 130 and the second exposed metal wire 135.

[0076] As described above, piezoelectric transducers are capable of producing static deflection and induced deflection, respectively, without electrical stimulation and in response to a drive signal. In other words, the electroacoustic transducer according to this disclosure allows the use of a single piezoelectric actuator to utilize the wide dynamic range of the membrane. This offers a dual advantage: on the one hand, a satisfactory sound pressure level can be obtained, comparable to that of an electroacoustic transducer with different piezoelectric actuators arranged to move the membrane in opposite directions. On the other hand, the use of a single piezoelectric actuator with a reduced surface area significantly reduces the associated capacitance and thus power consumption. This reduction in power consumption, in turn, translates into greater battery life, which is highly desirable for users of mobile devices as it simplifies their use.

[0077] Furthermore, the metal wire allows for the biasing of piezoelectric actuators placed on a membrane connected to a support frame via an elastic element without significantly altering the membrane's elastic behavior. More precisely, using a metal unaffected by oxidation from air exposure allows for the formation of exposed metal wires that do not require passivation structures, or, if required by design preferences, allows for a very thin passivation coating, at least on the membrane and elastic element, on the metal wire. In other words, the addition of material to the membrane can be strictly limited to the metal of the wire itself, avoiding redundant structures that would stiffen the membrane and potentially reduce kinetics. Alternatively, when the deformability of the membrane is still considered satisfactory according to design preferences, the metal wire can be provided with a thin coating, particularly a passivation coating, which substantially does not alter the properties of the membrane and elastic element.

[0078] Furthermore, highly conductive materials can be used, and the size of the metal wires can be reduced accordingly. This typically avoids stiffening the membrane, offering advantages in sound pressure level (for transmitters or actuators) and sensitivity (for receivers or sensors). Additionally, the metal wires can be narrow enough to extend over elastic elements without significantly altering their mechanical properties and without requiring dedicated membrane sections.

[0079] Finally, it is obvious that modifications and variations can be made to the described electroacoustic transducer without departing from the scope of this disclosure as defined by the appended claims.

[0080] In particular, it should be understood that the electroacoustic transducer according to this disclosure can be effectively used in devices other than miniature loudspeakers, such as, but not limited to, microphones and probes for ultrasound examination and imaging. While maintaining the same overall structure, the electroacoustic transducer can operate as a transmitter (e.g., a miniature loudspeaker) or a receiver (e.g., a microphone), and in some applications, it can operate reversibly as both a transmitter and a receiver (e.g., in an ultrasound imaging probe PMUT). This is possible because a piezoelectric transducer present on the membrane can operate as an actuator in a transmitter, converting electrical signals into membrane deformation to generate sound waves, and as a sensor in a receiver, converting membrane deformation caused by impacting sound waves into electrical signals.

Claims

1. A microelectromechanical electro-acoustic transducer, comprising: a support frame comprising a semiconductor material; a membrane of semiconductor material connected to the support frame along a perimeter on a central portion of the membrane; wherein the piezoelectric transducer is configured to induce a resting deflection of the membrane from a planar configuration toward a first side of the membrane in the absence of electrical stimulation of the piezoelectric transducer, and to induce an induced deflection of the membrane toward a second side of the membrane opposite the resting deflection in response to an electrical drive signal.

2. The microelectromechanical electro-acoustic transducer of claim 1, wherein the piezoelectric transducer and the membrane form a composite membrane and have respective compressive residual stress states.

3. The microelectromechanical electro-acoustic transducer of claim 1, wherein the piezoelectric transducer is on the first side of the membrane.

4. The microelectromechanical electro-acoustic transducer of claim 1, wherein the membrane is divided into segments by radial slits extending from a perimeter of the membrane to a distance from a center of the membrane.

5. The microelectromechanical electro-acoustic transducer of claim 4, wherein the piezoelectric transducer comprises a ring-shaped actuator region and lobes extending from the ring-shaped actuator region in a radial direction, each lobe on a respective segment of the membrane.

6. The microelectromechanical electro-acoustic transducer of claim 1, wherein the piezoelectric transducer and the membrane form a composite membrane and have respective compressive residual stress states.

7. The microelectromechanical electro-acoustic transducer of claim 1, wherein the piezoelectric transducer is on the first side of the membrane.

8. The microelectromechanical electro-acoustic transducer of claim 1, comprising elastic elements defined by respective portions of the membrane and through which the membrane is connected to the support frame; and metal wires on respective elastic elements of the membrane extending from the respective elastic elements to the piezoelectric transducer.

9. The microelectromechanical electro-acoustic transducer of claim 8, wherein the metal wires are a metal exposed to the atmosphere that is immune to oxidation, such as gold or platinum.

10. The microelectromechanical electro-acoustic transducer of claim 8, wherein the metal wires are uncoated and exposed on the membrane and the elastic elements.

11. The microelectromechanical electro-acoustic transducer of claim 8, wherein the piezoelectric transducer comprises a bottom electrode, a piezoelectric body on the bottom electrode, and a top electrode on the piezoelectric body, and wherein the metal wires comprise a first metal wire connecting the top electrode to a first pad on the support frame and a second metal wire connecting the bottom electrode to a second pad on the support frame.

12. The microelectromechanical electroacoustic transducer of claim 11, wherein each segment comprises a pair of respective elastic elements arranged symmetrically to each other with respect to an axis extending along a bisector of the respective segment; and wherein each elastic element comprises an outer anchor fixed to the support frame, an inner anchor connected to a central portion of the membrane, an outer arm extending from the outer anchor in opposite directions, and an inner arm extending from the inner anchor in opposite directions.

13. The microelectromechanical electroacoustic transducer of claim 12, wherein in each elastic element the outer arm and the inner arm are parallel to each other and connected to each other, to the outer anchor and to the inner anchor to form a slot.

14. The microelectromechanical electroacoustic transducer of claim 12, wherein the first metal line and the second metal line extend over respective different segments of the membrane, and each over two elastic elements of the respective segment.

15. The microelectromechanical electroacoustic transducer of claim 12, wherein the first metal line and the second metal line extend over the same segment of the membrane, and each over a respective one of the elastic elements of the segment.

16. The microelectromechanical electroacoustic transducer of claim 11, wherein the metal line comprises a dummy metal line in each segment of the membrane opposite one of the segments housing the first metal line and / or the second metal line, wherein the metal line extends at least over the elastic elements of the respective segment of the membrane and up to the piezoelectric transducer, and wherein the metal line is electrically insulated from the piezoelectric transducer.

17. The microelectromechanical electroacoustic transducer of claim 1, wherein the membrane has N-fold rotational symmetry, N being an integer.

18. A method comprising: forming a support frame of semiconductor material; opening a cavity on one side of the support frame; connecting a membrane of semiconductor material to the support frame along a perimeter of the membrane by elastic elements, the membrane enclosing the cavity; dividing the membrane into segments by forming radial slits extending from respective vertices of the membrane towards a center of the membrane, the radial slits defining protrusions in the membrane coupled to the support frame by the elastic elements and connected to each other by a continuous central portion of the membrane; arranging a piezoelectric actuator on the central portion of the membrane, the piezoelectric actuator comprising a lower electrode, a piezoelectric body and an upper electrode; forming the radial slits with a width smaller than twice a thickness of a viscous boundary layer of air in an operating temperature range; forming exposed metal lines extending over the elastic elements, the exposed metal lines made of an electrically conductive material exempt from being oxidized by being exposed to atmosphere; wherein the piezoelectric actuator and the membrane form a composite membrane having a residual stress state causing a resting deflection of the membrane in a first direction on a side facing the piezoelectric actuator; and applying an electrical drive signal to the piezoelectric actuator through the exposed metal lines to cause an induced deflection of the membrane in a second direction opposite the first direction toward a side of the membrane opposite the piezoelectric actuator and facing the cavity.

19. The method of claim 18, wherein the membrane has N-fold rotational symmetry with respect to an axis perpendicular to the membrane and passing through the center, where N is an integer, and wherein the piezoelectric actuator has the same N-fold rotational symmetry as the membrane.

20. The method of claim 18, wherein the membrane has a shape of a regular polygon, and wherein N is even.

21. The method of claim 18, wherein each protrusion is coupled to the support frame by a pair of elastic elements arranged symmetrically with respect to each other about an axis extending along a bisector of the respective segment.

22. The method of claim 18, wherein the piezoelectric actuator comprises lobes extending radially from an annular actuator region, each lobe disposed on the protrusion of a respective segment of the membrane, and wherein applying the electrical drive signal comprises: biasing the upper and lower electrodes through the exposed metal lines extending along bisectors of respective segments of the membrane.

23. The method of claim 22, further comprising: forming dummy metal lines on segments of the membrane different from the segments housing the exposed metal lines, the dummy metal lines being made of the same material and having the same shape as the exposed metal lines, but electrically decoupled from the piezoelectric actuator, wherein the dummy metal lines mechanically balance the stress applied to the membrane by the exposed metal lines.

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