EP4075422B1 - Micrometric loudspeaker - Google Patents

Description

The present invention relates to the field of micrometric loudspeakers. Its particularly advantageous application is the integration of at least one loudspeaker into computers, mobile phones and other headphones, particularly wireless ones.

STATE OF THE ART

The loudspeaker is used to transform an electrical signal into sound pressure.

For many years, speakers have been miniaturized to be integrated into computers, mobile phones, connected speakers and other headphones, for example wireless.

The loudspeaker is an electro-mechano-acoustic transducer. In its linear principle, the operation of the loudspeaker involves the actuation of a membrane or a rigid plate, coupled to the ambient air.

The electrical signal passes through an electromechanical transducer which converts the speaker supply voltage into displacement. A mechanical-acoustic transducer, very often a membrane, converts this displacement into acoustic pressure.

A good loudspeaker is one that reproduces all perceptible sound frequencies (typically 20 Hz to 20 kHz) at the same amplitude, with a low distortion rate. In practice, the lowest frequency at which a loudspeaker effectively produces sound is determined by the resonant frequency of the mechanical-acoustic transducer.

In the context of miniaturization, the membrane guidance system is more rigid and the mass of the mechano-acoustic transducer is lower, which increases the resonance frequency of the system and therefore reduces the bandwidth.

Furthermore, the sound pressure level radiated by a loudspeaker depends on the volume of air accelerated by the mechanical-acoustic transducer. The volume of air accelerated by a loudspeaker depends on the product of the area of the mechanical-acoustic transducer and the maximum displacement of the mechanical-acoustic transducer.

In the context of miniaturization, the surface area of the mechanical-acoustic transducer is greatly reduced, and a significant displacement is necessary to obtain a satisfactory sound pressure level. Micrometric loudspeakers (also known as “MEMS loudspeakers” or micro-loudspeakers) are mainly based on exploiting the compliance of flexible membranes. However, these are becoming more rigid under the effect of their deformations, which explains why micrometric drivers with flexible membranes suffer from high geometric nonlinearities.

The micrometric speakers with flexible membrane fitted to mobile phones have dimensions, typically 11 × 15 × 3 mm ³ , advantageous for their integration, and make it possible to generate a satisfactory radiated pressure, typically 85 dB, over a wide range of frequencies relative to the extent of the range of perceptible sound frequencies. However, the bulk of this type of speaker is less and less compatible with the thickness of portable devices which continues to be reduced.

Furthermore, to achieve large displacements and obtain a satisfactory sound pressure level, electromagnetic transduction, to convert the power supply voltage of the loudspeaker into displacement of its membrane or its rigid plate, remains a solution of choice, and this is what equips the vast majority of current speakers. However, the dimensions of this type of loudspeaker do not allow integration into portable systems and the use of a magnet makes it incompatible with microfabrication processes.

Another means of converting the loudspeaker's supply voltage into displacement of its membrane (or its rigid plate), which shows notable performance, is piezoelectric transduction. Although not necessarily conferring large displacements on the membrane or the rigid plate, piezoelectric transduction has the advantage of being compatible with microfabrication processes. More particularly, by using the bimetallic effect of a piezoelectric transducer positioned on the membrane to be moved, as for example in the patent document US 2012/057730 A1 , performances comparable to those of electromagnetic transducers are achievable. In other cases, such as for example in the MEMS loudspeaker described by the patent applications referenced US 20170094418 A1 And CN 111 918 179 A , the piezoelectric transducers are offset from the membrane, and this solution allows a pistonic movement of the latter. This last solution, with remote actuators, makes it possible to overcome the problems of the previous solution. This advantageous solution also requires a smaller silicon surface area and the footprint of the loudspeaker can thus be advantageously reduced. Despite these advantages, this solution does not allow large movements, and the bandwidth that such MEMS speakers can achieve remains reduced. Additionally, by enlarging the driver to obtain a lower resonant frequency, and therefore wider bandwidth, piezoelectric actuators tend to adopt non-linear behaviors which have a direct negative impact on the performance of the loudspeaker. Also, when the loudspeaker is made up of a “MEMS motor” and a membrane, for example made of polymer, assembled in a heterogeneous manner, non-linearities linked to the deformation of the polymer membrane appear which again affect, negatively affects the performance of the speaker.

An object of the present invention is therefore to propose a micrometric loudspeaker which makes it possible to overcome at least one of the disadvantages of the state of the art.

An object of the present invention is more particularly to propose a micrometric loudspeaker which has satisfactory performance, in particular in terms of bandwidth and/or pressure level produced and/or which has improved performance, in particular by avoiding that piezoelectric actuators adopt non-linear behaviors. The other objects, characteristics and advantages of the present invention will appear on examination of the following description and the accompanying drawings. It is understood that other benefits may be incorporated.

SUMMARY

• Un cadre,
• Un transducteur électromécanique, et
• Un transducteur mécano-acoustique comprenant une plaque rigide, montée mobile dans le cadre,

le transducteur électromécanique et le transducteur mécano-acoustique étant couplés entre eux de sorte qu'une sollicitation du transducteur électromécanique déplace le transducteur mécano-acoustique relativement au cadre et qu'un déplacement correspondant du transducteur mécano-acoustique soit converti en pression acoustique.To achieve this objective, according to one embodiment, a micrometric loudspeaker is provided comprising:

• A frame,
• An electromechanical transducer, and
• A mechano-acoustic transducer comprising a rigid plate, movably mounted in the frame,

the electromechanical transducer and the mechano-acoustic transducer being coupled together such that a bias of the electromechanical transducer moves the mechano-acoustic transducer relative to the frame and a corresponding movement of the mechano-acoustic transducer is converted into acoustic pressure.

• le transducteur électromécanique comprend deux actionneurs piézoélectriques et deux lames élastiques, chaque actionneur piézoélectrique étant associé à une lame élastique pour induire, lorsqu'il est alimenté électriquement, une déformation de la lame élastique par effet bilame,
• le cadre comprend une traverse centrale depuis laquelle s'étendent, solidairement et à l'opposé l'une de l'autre, les deux lames élastiques,
• les deux lames élastiques s'étendent depuis la traverse centrale du cadre jusqu'à mettre en prise deux rebords latéraux dits de couplage du transducteur mécano-acoustique.

The micrometric loudspeaker according to the invention is such that:

• the electromechanical transducer comprises two piezoelectric actuators and two elastic blades, each piezoelectric actuator being associated with an elastic blade to induce, when electrically powered, a deformation of the elastic blade by bimetallic effect,
• the frame comprises a central crosspiece from which the two elastic blades extend, integrally and opposite each other,
• the two elastic blades extend from the central crosspiece of the frame until two so-called coupling lateral edges of the mechanical-acoustic transducer engage.

In this way, each elastic blade is in a so-called “recessed-guided” bending configuration according to which, when the piezoelectric actuators are electrically powered, the elastic blades deform and cause with them a movement of the rigid plate of the mechanical-acoustic transducer in a direction substantially perpendicular to a main extension plane of the frame.

The mechanical-acoustic transducer further comprises at least two linearization springs each extending from one of the lateral coupling edges to a lateral edge of the rigid plate which is located opposite, the linearization springs being configured so as to allow, during deformation of the elastic blades, a movement of at least part of the two lateral coupling edges towards the central crosspiece of the frame.

This reduces the longitudinal stresses experienced by the elastic blades during their deformation due to their bending configuration.

Thus, to counter the stiffening of the elastic blades due to their deformation in their recessed-guided configuration, a degree of freedom is added by means of the linearization springs. The latter are thus named because, by authorizing a movement of at least part of the two lateral coupling edges towards the central crosspiece of the frame during deformation of the elastic blades, they make it possible to reduce the stresses suffered by the elastic blades and by consequently to reduce, or even avoid, their stiffening during their deformation. Such stiffening would have the consequence of inducing non-linear behavior of the rigid plate during its movements. The linearization springs advantageously affect the pressure level produced, allowing optimal flexibility over the entire stroke of the rigid plate, and thus reduce, or even cancel, the geometric nonlinearities which would be linked in particular to the aforementioned stiffening phenomenon if it was observed.

Another aspect concerns a method of manufacturing a micrometric loudspeaker as introduced above, comprising or even being limited to, deposition and etching steps relating to microelectronics. The micrometric loudspeaker according to the first aspect of the invention can therefore advantageously be microfabricated.

BRIEF DESCRIPTION OF THE FIGURES

• La figure 1 représente une vue en perspective d'un mode de réalisation du haut-parleur micrométrique selon l'invention.
• La figure 2 représente une vue éclatée du mode de réalisation illustré sur la figure 1.
• La figure 3 représente une vue en perspective du transducteur mécano-acoustique selon un mode de réalisation du haut-parleur micrométrique selon l'invention.
• La figure 4 représente une vue en perspective d'une partie du cadre d'un mode de réalisation du haut-parleur micrométrique selon l'invention.
• Les figures 5 et 6 représentent chacune une vue en perspective d'une coupe transversale d'un mode de réalisation du haut-parleur micrométrique selon l'invention, selon des angles de vue différent entre eux.
• La figure 7 est une représentation schématique en coupe transversale de l'assemblage formé par le transducteur électromécanique et le transducteur mécano-acoustique, dans deux positions différentes entre elles, relativement à la traverse centrale du cadre selon un mode de réalisation du haut-parleur micrométrique selon l'invention.
• La figure 8 est un schéma de fonctionnement en demi-coupe d'un mode de réalisation du haut-parleur micrométrique selon l'invention, lorsque le transducteur électromécanique est alimenté électriquement.
• La figure 9 est un schéma d'un système équivalent à celui représenté en figure 8.
• La figure 10 est une vue schématique en demi-coupe d'un mode de réalisation du haut-parleur micrométrique selon l'invention, générant des ondes acoustiques.
• La figure 11 est une vue schématique en demi-coupe de l'interstice entre les bords extérieurs du transducteur mécano-acoustique et le pourtour intérieur du cadre selon un mode de réalisation du haut-parleur micrométrique selon l'invention.
• La figure 12 est un graphique montrant la réponse en niveau de pression acoustique d'un haut-parleur micrométrique selon un mode de réalisation de l'invention sur une gamme de fréquences d'excitation dudit haut-parleur.
• Les figures 13 à 16 illustrent chacune une vue en coupe d'une étape du procédé de fabrication d'un haut-parleur micrométrique selon un mode de réalisation de l'invention.

The aims, objects, as well as the characteristics and advantages of the invention will emerge better from the detailed description of an embodiment of the latter which is illustrated by the following accompanying drawings in which:

• There figure 1 represents a perspective view of an embodiment of the micrometric loudspeaker according to the invention.
• There figure 2 represents an exploded view of the embodiment illustrated on the figure 1 .
• There Figure 3 represents a perspective view of the mechanical-acoustic transducer according to one embodiment of the micrometric loudspeaker according to the invention.
• There Figure 4 represents a perspective view of part of the frame of an embodiment of the micrometric loudspeaker according to the invention.
• THE figures 5 and 6 each represent a perspective view of a cross section of an embodiment of the micrometric loudspeaker according to the invention, from different viewing angles between them.
• There figure 7 is a schematic representation in cross section of the assembly formed by the electromechanical transducer and the mechanical-acoustic transducer, in two different positions between them, relative to the central crosspiece of the frame according to one embodiment of the micrometric loudspeaker according to invention.
• There figure 8 is a half-section operating diagram of one embodiment of the micrometric loudspeaker according to the invention, when the electromechanical transducer is electrically powered.
• There Figure 9 is a diagram of a system equivalent to that represented in figure 8 .
• There Figure 10 is a schematic half-section view of one embodiment of the micrometric loudspeaker according to the invention, generating acoustic waves.
• There Figure 11 is a schematic half-section view of the gap between the outer edges of the mechanical-acoustic transducer and the inner periphery of the frame according to one embodiment of the micrometric loudspeaker according to the invention.
• There Figure 12 is a graph showing the sound pressure level response of a micrometric loudspeaker according to one embodiment of the invention over a range of excitation frequencies of said loudspeaker.
• THE figures 13 to 16 each illustrate a sectional view of a step of the process of manufacturing a micrometric loudspeaker according to one embodiment of the invention.

The drawings are given as examples and do not limit the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily on the scale of practical applications. In particular, the thicknesses of the different layers or other elements extending mainly in two directions of main extension are not necessarily representative of reality, in particular when these thicknesses are compared to the dimensions, according to their directions of main extension, of the said layers or said other elements, respectively.

DETAILED DESCRIPTION

Before beginning a detailed review of embodiments of the invention, optional characteristics of the micrometric loudspeaker according to the first aspect of the invention are set out below which may possibly be used in combination or alternatively:
According to one example, each of the two piezoelectric actuators extends at most over half of the elastic blade associated with it from the lateral coupling edge of the mechanical-acoustic transducer which is engaged by said elastic blade. As an optional complement to this example, each of the two piezoelectric actuators extends at least over a quarter of the elastic blade associated with it from the lateral coupling rim of the mechanical-acoustic transducer which is engaged by said elastic blade.

According to another example, the micrometric loudspeaker is preferably substantially symmetrical with respect to a longitudinal section plane of the central crosspiece of the frame which is perpendicular to the main extension plane of the frame.

According to another example, the micrometric speaker is free of an actuator, in particular a piezoelectric actuator, directly covering all or part of the rigid plate. According to another example, the piezoelectric actuators of the electromechanical transducer are offset relative to the rigid plate; that is, the piezoelectric actuators of the electromechanical transducer are at a distance from the rigid plate. According to another example, the mechano-acoustic transducer is free of an electromechanical transducer and/or the electromechanical transducer is free of a mechano-acoustic transducer. More particularly, the rigid plate is free from, or is not directly covered, preferably even partially, with an electromechanical transducer. Preferably, the rigid plate is free of a flexible membrane. According to another example, the electromechanical transducer and the mechano-acoustic transducer are mechanically coupled to each other, preferably only via the two lateral coupling edges of the mechano-acoustic transducer. According to one example, the mechanical-acoustic transducer only includes two lateral coupling edges. Preferably, the two lateral coupling edges extend from lateral edges of the rigid plate which are opposite each other and/or extend from lateral edges of the rigid plate substantially perpendicular to a plane in which the rigid plate. According to one example, the other lateral edges of the rigid plate than those through which the rigid plate extends to form the two lateral coupling edges do not extend beyond the rigid plate. Preferably, each of the two lateral coupling edges is linked only to one edge of one of the two linearization springs and to one edge of one of the two elastic blades. According to one example, the mechanical-acoustic transducer is free of a lateral rim other than said two lateral coupling rims. According to one example, the mechanical-acoustic transducer is free of a lateral rim linking together the two lateral coupling rims of the mechanical-acoustic transducer. Preferably, the rigid plate only extends outside the plane in which it fits via the two lateral coupling edges of the mechanical-acoustic transducer. According to one example, the elastic blades are each uniform over their extent. According to one example, the mechanical-acoustic transducer does not extend beyond an area delimited by the interior periphery of the exterior edges of the frame. According to another example, the mechanical-acoustic transducer does not cover, nor intersect, the outer edges of the frame.

According to another example, each linearization spring has a stiffness at least ten times, preferably at least one hundred times, greater than a stiffness of the elastic blades. In this way, we ensure not to alter the linear behavior of the micrometric loudspeaker, over the entire range of perceptible sound frequencies.

According to another example, the central crosspiece of the frame extends at most over a first half of a thickness of the frame and the two elastic blades comprise the same layer secured to one face of the central crosspiece which is oriented towards a center of the frame. It is thus structurally easy to provide that the assembly formed of the electromechanical transducer and the mechano-acoustic transducer moves within the frame, so as to be protected by it. Said layer is for example made from silicon. According to the previous example, no elastic blade extends from a face of the central crosspiece which is different from the face of the central crosspiece oriented towards the center of the frame.

According to another example, the rigid plate and the linearization springs comprise the same layer, a greater stiffness of the rigid plate relative to a stiffness of the linearization springs being due to structuring patterns that the rigid plate includes and which are extend, from said layer, over a surface of the latter defining an extent of the rigid plate, the linearization springs being made up of portions of said layer which extend on either side of said surface. Said layer is for example made from silicon. Preferably, said portions which extend on either side of the surface from which the structuring patterns extend are themselves free of structuring patterns. structuring.

According to another example, the frame is configured so that the mechanical-acoustic transducer is located, on all sides, at a distance from the interior periphery of the frame of between 1 and 100 µm, preferably between 2 and 80 µm, for example substantially equal to 9 µm. The gap between the frame and the mechanical-acoustic transducer is thus such that, at this gap, the propagation of acoustic waves is mainly dominated by thermo-viscous behavior. This avoids any acoustic short-circuit phenomenon.

According to another example, the frame has, in its main extension plane, dimensions each between 1 and 10 mm, preferably between 3 and 8 mm. An advantageous compromise is thus proposed between the maximum sound pressure level achievable by the micrometric loudspeaker and the size of the latter.

According to another example, the lateral coupling edges of the mechanical-acoustic transducer extend from one of the two linearization springs over a distance greater than 750 µm, preferably greater than 500 µm. The thermo-viscous losses due to the compression of the air below the rigid plate are thus advantageously minimized.

According to another example, the elastic blades have a thickness of between 1 and 100 µm, preferably between 5 and 20 µm. An advantageous compromise is thus proposed between the choice of a low resonance frequency and the choice of a high radiated pressure level.

According to another example, the two piezoelectric actuators are based on, or even made of, PZT and each extend on one face of one of the two elastic blades which is opposite the rigid plate of the mechanical-acoustic transducer.

According to another example, the elastic blades of the electromechanical transducer have a first resonance frequency and the linearization springs of the mechanical-acoustic transducer have a second resonance frequency, the second resonance frequency being at least a hundred times, preferably at least a thousand times , greater than the first resonance frequency. This gives the micrometric speaker a wide bandwidth.

According to another example, the frame comprises first and second parts superimposed and concentric with each other, a second part of the frame supports the central crosspiece and comprises two electrical connection pads to the piezoelectric actuators, the electrical connection pads being preferably located in the extension of the central crosspiece, and the second part of the frame comprising two notches configured to each be located opposite one of the two electrical connection pads. The resumption of contact of the piezoelectric actuators is thus such that it does not increase the bulk of the micrometric loudspeaker.

By “micrometric” is meant the quality of a device or element having a volume, or included in an envelope, of less than 1 cm ³ , preferably less than 0.5 cm ³ .

It is specified that, in the context of the present invention, the term "rigid" describes a part or an element of the loudspeaker which does not deform or only slightly under the effect of the constraints generally applied to it in operation. normal. More particularly, it can be considered that the rigidity of the plate of the mechano-acoustic transducer is ten times, or even a hundred times, greater than the rigidity of the actuators.

It is specified that, in the context of the present invention, the term "elastic" describes a part or an element of the loudspeaker which deforms under the effect of the stresses generally applied to it in normal operation. More particularly, it can be considered that the rigidity of the elastic blades is ten times, or even a hundred times, lower than the rigidity of the so-called rigid plate of the mechanical-acoustic transducer. For example, the terms “elastic blades” could be reformulated more specifically by the terms “bending deformable blades”.

By a film based on a material A is meant a film comprising this material A and possibly other materials.

By a parameter “substantially equal/greater/less than” a given value we mean that this parameter is equal/greater/less than the given value, to plus or minus 20%, or even 10%, close to this value. We understand by a parameter "substantially between" two given values that this parameter is at least equal to the smallest given value, plus or minus 20%, or even 10%, close to this value, and at most equal to the largest large given value, plus or minus 20%, or even 10%, close to this value.

• Un cadre 11,
• Un transducteur électromécanique 12, et
• Un transducteur mécano-acoustique 13.

According to its first aspect, a structural description of which is given below with reference to the figures 1 to 9 , the invention relates to a micrometric loudspeaker comprising:

• A frame 11,
• An electromechanical transducer 12, and
• A mechano-acoustic transducer 13.

The mechanical-acoustic transducer 13 comprises a rigid plate 131 movably mounted in the frame 11. In this, the micrometric loudspeaker according to the first aspect of the invention is distinguished from micrometric speakers with flexible membrane.

The electromechanical transducer 12 and the mechanical-acoustic transducer 13 are coupled together so that a bias of the electromechanical transducer 12 moves the mechanical-acoustic transducer 13 relative to the frame 11 and a corresponding movement of the mechanical-acoustic transducer 13 is converted in sound pressure.

More particularly, and in particular with reference to the figure 2 , the electromechanical transducer 12 comprises two piezoelectric actuators 121a, 121b and two elastic blades 122a, 122b. Each piezoelectric actuator is associated with an elastic blade to induce, when electrically powered, a deformation of the elastic blade by bimetallic effect. In other words, each piezoelectric actuator is associated with an elastic blade so that, when an electrical voltage is applied to the piezoelectric actuator, the blade deforms in flexion.

With reference to figures 5 and 6 , the frame 11 comprises a central crosspiece 111 from which the two elastic blades 122a, 122b extend, integrally and opposite each other. The two elastic blades 122a, 122b extend from the central crosspiece 111 of the frame 11 to engage two so-called coupling lateral edges 132a, 132b of the mechanical-acoustic transducer 13.

In this way, each elastic blade 122a, 122b is in a so-called “recessed-guided” bending configuration. In this configuration, when the piezoelectric actuators 121a, 121b are electrically powered, the elastic blades 122a, 122b deform in flexion and cause with them a movement of the rigid plate 131 of the mechanical-acoustic transducer 13 in a direction substantially perpendicular to a plane main extension of the frame 11. It thus appears that the mechano-acoustic transducer 13 is more particularly movably mounted in the frame 11 via the electromechanical transducer 12.

The mechanical-acoustic transducer 13 further comprises at least two linearization springs 133a, 133b. The two linearization springs 133a, 133b each extend from one of the lateral coupling edges 132a, 132b of the mechanical-acoustic transducer 13 to a lateral edge of its rigid plate 131 which is located opposite. The linearization springs 133a, 133b are thus configured so as to allow, during deformation of the elastic blades 122a, 122b, a movement of at least part of the two lateral coupling edges 132a, 132b towards the central crosspiece 111 of the frame 11.

When the piezoelectric actuators 121a, 121b are electrically powered, the elastic blades each adopt a deformation with a substantially central inflection point and undergo longitudinal stresses, due to their recessed-guided bending configuration. The linearization springs 133a, 133b then make it possible to absorb at least part of these longitudinal stresses. To this end, in particular when the piezoelectric actuators 121a, 121b are made from PZT, which can only contract in the direction x as illustrated in the figure 7 , the piezoelectric actuators 121a, 121b are preferably arranged only on half of the surface of the elastic blades 122a, 122b. More particularly, the piezoelectric actuators 121a, 121b each extend continuously from the edge of the elastic blade 122a, 122b with which it is associated, as shown in the figure. Figure 10 , preferably on at least a quarter of the surface of said elastic blade, and preferably on at most half of this surface.

The linearization springs 133a, 133b add, to the micrometric loudspeaker 1, a degree of freedom by authorizing a movement of at least part of the two lateral coupling edges 132a, 132b of the mechanical-acoustic transducer 13 towards the central crosspiece 111 of the frame 11, during deformations of the elastic blades 122a, 122b. They thus make it possible to reduce the particularly longitudinal stresses experienced by the elastic blades 122a, 122b; however, such constraints could be the cause of a stiffening of the elastic blades 122a, 122b, which would have the consequence of inducing non-linear behavior of the rigid plate 131 during its movements, or at least for certain large amplitudes of its movements. Since the longitudinal stresses experienced by the elastic blades 122a, 122b are reduced, or even made negligible, we understand that the performance of the micrometric loudspeaker 1 is increased.

As illustrated on the figures 5 and 6 in particular, the micrometric loudspeaker 1 is preferably substantially symmetrical with respect to a longitudinal section plane of the central crosspiece 111 of the frame 11 which is perpendicular to the main extension plane of the frame 11.

With reference to figures 1 to 4 , the micrometric loudspeaker 1 according to the illustrated embodiment can also be considered as comprising two parts superimposed on each other concentrically. In particular, the frame 11 can be seen as consisting of two parts 11a and 11b, of which a first part 11a supports, preferably by itself, the central crosspiece 111 of the frame 11 and a second part 11b configured to closely accommodate the transducer there. mechano-acoustic 13. We will see, when we describe below an example of a manufacturing process, by means of microelectronics, of the micrometric speaker 1 as illustrated on the figures 1 to 9 , that this two-part vision of said micrometric loudspeaker 1 is linked to the fact that two silicon wafers or wafers are, according to said method, treated individually before being assembled to form the compact micrometric loudspeaker 1 as illustrated on the figure 1 . In particular, it will appear that each of the parts 11a and 11b of the frame 11 comes from one of the two silicon wafers.

There figure 7 shows a cutaway view of the loudspeaker in operation. It more particularly shows two superimposed views of the electromechanical transducer 12 and the mechanical-acoustic transducer 13, on the one hand in a non-deformation configuration of the elastic blades 122a, 122b (where the piezoelectric actuators are not electrically powered), d 'on the other hand in a configuration of deformation of the elastic blades 122a, 122b (where the piezoelectric actuators are electrically powered), relative to the central crosspiece 111 of the frame 11, the latter remaining fixed due to the fixing of the frame 11 itself, for example on a support (not shown). Thus, when an electric voltage is applied between the top and bottom of the layers of piezoelectric material constituting the piezoelectric actuators 121a, 121b, they contract in the x direction. Under the effect of contraction, and the fact that each of the two bimetallic strips which constitute the association of an elastic blade with a piezoelectric actuator is integral with the rigid plate 131, the elastic blades 122a, 122b adopt a deformation with a point inflection and their ends move in the z direction and more particularly in the -z direction, driving the whole formed of the electromechanical transducer 12 and the mechanical-acoustic transducer 13 in the same direction and in the same direction. When the piezoelectric actuators 121a, 121b are then no longer electrically supplied, the elasticity of the elastic blades 122a, 122b makes it possible to return the whole formed of the electromechanical transducer 12 and the mechano-acoustic transducer 13 to its starting position. In this so-called starting position, or equivalently of non-power supply to the piezoelectric actuators 121a, 121b, the rigid plate 131 can come flush with the periphery of the face of the frame 11 which is oriented upwards in the figures.

When the micrometric speaker 1 only allows movements of the rigid plate 131 in the -z direction by electrical supply of the piezoelectric actuators 121a, 121b, in particular because these are made from PZT, it is necessary to 'add a direct voltage across each piezoelectric actuator 121a, 121b to obtain a rest point in the middle of the dynamics of the speaker 1, to obtain an alternating movement around this operating point. For example, the piezoelectric actuators operate with an electrical supply voltage range substantially between 0 and 30 V, and the direct voltage added to the terminals of each piezoelectric actuator 121a, 121b is substantially equal to 15 V.

There figure 8 schematically shows the operating principle of the micrometric loudspeaker 1 according to the first aspect of the invention comprising an additional degree of freedom which is given to it by the linearization springs 133a, 133b. As illustrated in this figure, when an electrical voltage is applied to the terminals of the piezoelectric actuators 121a, 121b, the elastic blades 122a, 122b deform and move the rigid plate 131 by a distance δ ₀ along -z. For ideal linear operation, the length of the curve of each deformed elastic blade 122a, 122b must be identical to the length of the undeformed elastic blade 122a, 122b. On the figure 8 , the difference between the position of the distal end of the elastic blade 122a, before and after deformation, and in the y direction, is denoted Δ ₀ . This difference is authorized by the linearization spring 133a secured to the rigid plate 131 by its end opposite to that by which the linearization spring 133a is secured to the distal end of the elastic blade 122a. The idea is that the displacement Δ ₀ deforms the linearization spring 133a using the height h ₀ of the lateral coupling rim 132a of the mechanical-acoustic transducer 13 as a lever arm.

To significantly reduce geometric nonlinearities, it is preferable that the stiffness of each linearization spring, actuated via the lateral coupling rim, of height ho, which is associated with it and serving as a lever, is 10 times, preferably 100 times , lower than the apparent stiffness of the actuators along the axis outside the main extension plane of the frame.

In particular if the stiffness of the linearization spring 133a is much greater than the stiffness of the elastic blade 122a, the micrometric speaker 1 as described above allows guidance of the mechanical-acoustic transducer 13 similar to that which the system would allow equivalent shown on the Figure 9 . The diagram in this figure shows a piezoelectric actuator 121a and the elastic blade 122a associated with it in a deformed state, the elastic blade 122a being linked to the rigid plate 131 by a spring representing the stiffness of the linearization spring 133a along the axis z. Considering the piezoelectric actuator 121a and the elastic blade 122a as a mechanical actuator, and knowing that, according to the principle diagram of the Figure 9 , the characteristic of the mechanical actuator thus defined is the straight line connecting its blocked force (force generated by the actuator when the translation of its end is blocked according to z) and its free movement (maximum movement of the end of the actuator without load at its end), the stiffness of the spring illustrated on the Figure 9 cuts the mechanical actuator characteristic at its operating point. For a spring as shown in the Figure 9 which is quite stiff, the force corresponding to the operating point differs little from the blocked force of the mechanical actuator, which advantageously makes it possible to give the mechanical actuator a linear behavior over its operating range.

The principle diagram illustrated on the Figure 9 therefore allows us to illustrate why it is preferred that each linearization spring 133a, 133b has a stiffness at least ten times, preferably at least a hundred times, greater than a stiffness of the elastic blades 122a, 122b. The additional degree of freedom conferred by the linearization springs helps reduce nonlinearities. The fact that the linearization springs are more rigid than the actuators means that the frequency response of the micrometric speaker is not altered.

Another characteristic reflecting this same preference in a different way consists of specifying that the elastic blades of the electromechanical transducer 12 have a first resonance frequency and the linearization springs 133a, 133b of the mechanical-acoustic transducer 13 have a second resonance frequency, the second resonance frequency being at least one hundred times, preferably at least one thousand times, greater than the first resonance frequency. This ensures that the second resonance frequency is outside the desired bandwidth reached by the micrometric loudspeaker 1, and we thus give, to the micrometric loudspeaker 1, a wide bandwidth for a optimized range of perceptible sound frequencies.

By powering the piezoelectric actuators 121a, 121b with an alternating voltage, around a direct positive voltage, the rigid plate 131 moves up and down and generates acoustic waves, as illustrated in the Figure 10 .

In conventional loudspeakers, the acoustic short circuit, resulting from the interference between the positive (or negative) waves created by the front of the rigid plate in vibration, and the negative (or positive) waves created by the rear of this same plate, can be prevented by a deformable suspension. For the micrometric loudspeaker 1 according to the first aspect of the invention, the acoustic short circuit is prevented by using a dimension d of gap 2 between the frame 11 and the rigid plate 131, and more particularly between the inner periphery of the frame 11 and the side edges coupling 132a, 132b of the mechanical-acoustic transducer 13, such that the thermo-viscous behavior dominates in this gap 2. The Figure 11 shows a schematic representation of a part of the mechanical-acoustic transducer 13, the frame 11 and the gap 2 in question, on which the dimension d of the gap 2 is represented.

More particularly, the frame 11 is configured so that the mechanical-acoustic transducer 13 is located, on all sides, at an interstitial distance from the interior periphery of the frame 11 of between 1 and 100 µm, preferably between 2 and 80 µm. A finite element simulation can make it possible to determine, for each dimensioning of the micrometric loudspeaker 1 according to the first aspect of the invention, the interstitial distance making it possible to optimize the thermo-viscous behavior of the air in the gap 2. For the specific dimensions given below for purely illustrative purposes, this finite element simulation shows that the optimal dimension of gap 2 is approximately equal to 9 µm. The gap 2 between the frame 11 and the mechanical-acoustic transducer 13 is thus such that, at this gap 2, the propagation of the acoustic waves is mainly dominated by thermo-viscous behavior. This avoids any acoustic short-circuit phenomenon.

The dimensions of the micrometric loudspeaker 1 are obviously important because they influence the dimensions of the rigid plate 131 and the dimensions of the elastic blades 122a, 122b, and consequently on those of the piezoelectric actuators 121a, 121b. A larger loudspeaker will have a larger, heavier rigid plate 131, softer spring blades 122a, 122b and will generate more force. It will therefore have a lower resonant frequency, and therefore a wider bandwidth in low frequencies. There Figure 12 shows the frequency response of a micrometric loudspeaker 1 according to the first aspect of the invention, the rigid plate 131 of which has dimensions of 8x8 mm ² . We only observe that the resonance frequency of such a micrometric speaker 1 is substantially equal to 1 kHz. Smaller dimensions will give a higher resonant frequency, and therefore a narrower bandwidth. However, dimensions ranging from 1×1 mm ² to 10×10 mm ² of the micrometric loudspeaker 1 according to the first aspect of the invention are envisaged. Dimensions ranging from 3x3 mm ² to 8x8 mm ² will for example be favored for reasons of compromise between performance and size.

The height h ₀ of the lateral coupling edges 132a, 132b shown on the figure 8 is optimized so that the thermo-viscous losses due to the compressed air below the rigid plate 131 are minimized. For a height h ₀ greater than 500 µm, the thermo-viscous losses do not significantly modify the frequency response of the micrometric loudspeaker 1. This height, however, depends on the other dimensions of the micrometric loudspeaker 1. This is why, more generally, the lateral coupling edges 132a, 132b of the mechanical-acoustic transducer 13 extend from one of the two linearization springs 133a, 133b over a distance greater than 750 µm, preferably greater than 500 µm.

The frequency response of the micrometer loudspeaker 1 can also be greatly affected by the thickness of the elastic blades 122a, 122b supporting the piezoelectric actuators 121a, 121b. Thinner elastic blades 122a, 122b will give a lower resonance frequency and thicker elastic blades 122a, 122b will give more force to the micrometric loudspeaker 1 and therefore a higher level of radiated pressure. A compromise is therefore preferably to be determined in order to have a low resonance frequency and a satisfactory pressure level. This dimension again depends on the other dimensions of the micrometric loudspeaker 1. Typically, the elastic blades 122a, 122b can have a thickness of between 1 and 100 µm, preferably between 5 and 20 µm, and for example substantially equal to 12 µm.

THE figures 13 to 16 give an example of a method of manufacturing a micrometric loudspeaker 1 according to one embodiment of the first aspect of the invention. This process advantageously implements technological steps, in particular deposition and etching, which are common in microelectronics. These technological stages are for example carried out from two silicon wafers (or “wafer” according to Anglo-Saxon terminology). More particularly, as already introduced above and according to the example illustrated, two silicon wafers can be processed individually, assembled together, then the assembly can be processed in turn to obtain the micrometric speaker 1 according to one embodiment of the first aspect of the invention. However, any other conventional mechanical assembly method, than that illustrated in the figures, can be used.

According to the example illustrated, manufacturing begins with a BESOI wafer, composed of two layers of silicon separated by a layer of silicon oxide 201. On the thinnest layer, located on the front face FAV1 of the BESOI wafer and intended to constitute the elastic blades 122a, 122b, a stack comprising a first electrode layer, a layer of a piezoelectric material, then a second electrode layer, is deposited. As illustrated on the Figure 13 , a hard mask 202 is etched on the rear face FAR1 in order to subsequently carry out a deep etching step by this back side. The piezoelectric transducers 121a, 121b are then etched and protected by a passivation 203. Electrical contacts 204 allowing the electrical supply of the upper electrodes 205a, 206a and lower 205b, 206b of the piezoelectric actuators 121a, 121b and a material 207 intended to allow the bonding of the treated BESOI wafer to the second treated wafer are then deposited by the front face FAV1 of the BESOI wafer.

In reference to the Figure 14 , the second wafer, composed of two layers of silicon separated from an oxide layer 208, is intended to constitute a second part of the speaker 1, and in particular the rigid plate 131 and the second part 11b of the frame 11. A mask hard 209 is engraved on the front face FAR2 to allow the deep engraving of a rear cavity 210. A hard mask 211 is engraved on the rear face FAR2 to allow the subsequent engraving of structuring patterns 130b, taking for example the form of bars of reinforcement, of the rigid plate 131.

Once the two wafers are thus treated, they are assembled together by their respective front faces FAV1 and FAV 2, in the manner illustrated on the Figure 15 . The structuring patterns 130b of the rigid plate 131, as well as the gap d between the rigid plate 131 and the frame 11 are then etched by the rear face FAR2 of the second silicon wafer. The rear face FAR1 of the BESOI wafer is then etched to reach speaker 1 as illustrated on the Figure 16 .

We note that thus the two elastic blades 122a, 122b comprise the same layer 120a secured to one face of the central crosspiece 111 which is oriented towards a center of the frame 11. Said layer 120a is made from silicon.

Likewise, we note that the rigid plate 131 and the linearization springs 133a, 133b comprise the same layer 130a. A greater stiffness of the rigid plate 131 relative to a stiffness of the linearization springs 133a, 133b is due to the structuring patterns 130b that the rigid plate (131) comprises. More particularly, these structuring patterns 130b extend, from said layer 130a, over a surface of the latter defining the extent of the rigid plate 131. The linearization springs 133a, 133b are for their part made up of portions 130c, 130d of said layer 130a which extend on either side of said surface. Furthermore, it appears that said layer 130a is made from silicon.

The invention is not limited to the embodiments previously described and extends to all the embodiments covered by the claims.

In particular, the frame 11 includes a periphery, preferably closed. Preferably, but in a non-limiting manner, the crosspiece 111 of the frame 11 is integral with the inner periphery of the frame 11 by its two ends.

Although the frame 11 is represented as having a parallelepiped geometry, other shapes of the frame 11 are possible, whether for its interior periphery or its exterior periphery. Thus, a frame 11 of annular or oblong shape can be envisaged. Where applicable, the micrometric loudspeaker 1 will include more than two piezoelectric actuators each associated with each of a corresponding plurality of elastic blades.

Claims (15)

1. Micrometric speaker (1) comprising:

   • A frame (11),

   • An electromechanical transducer (12), and

   • A mechanical-acoustic transducer (13) comprising a rigid plate (131), movably mounted in the frame (11),

   the electromechanical transducer (12) and the mechanical-acoustic transducer (13) being coupled together such that an urging of the electromechanical transducer (12) moves the mechanical-acoustic transducer (13) relative to the frame (11) and that a corresponding movement of the mechanical-acoustic transducer (13) is converted into acoustic pressure,

   • the electromechanical transducer (12) comprises at least two piezoelectric actuators (121a, 121b) and at least two elastic strips (122a, 122b), each piezoelectric actuator being associated with an elastic strip to induce, when it is electrically powered, a deformation of the elastic strip by bimetal effect,

   • the frame (11) comprises a central crossmember (111) from which extend, securely and opposite one another, the two elastic strips (122a, 122b),

   characterised in that:

   • the two elastic strips (122a, 122b) extend from the central crossmember (111) of the frame (11) until engaging two so-called coupling lateral edges (132a, 132b) of the mechanical-acoustic transducer (13),

   such that each elastic strip (122a, 122b) is in a so-called "recessed-guided" bending configuration, according to which, when the piezoelectric actuators (121a, 121b) are electrically powered, the elastic strips (122a, 122b) are deformed and drive with them, a movement of the rigid plate (131) of the mechanical-acoustic transducer (13) according to a direction substantially perpendicular to a main extension plane of the frame (11),
   and in that:

   • the mechanical-acoustic transducer (13) further comprises at least two linearising springs (133a, 133b) each extending from one of the lateral coupling edges (132a, 132b) to a lateral edge of the rigid plate (131), which is located opposite, the linearising springs (133a, 133b) being configured so as to enable, during a deformation of the elastic strips (122a, 122b), a movement of at least some of the two lateral coupling edges (132a, 132b) to the central crossmember (111) of the frame (11).
2. Micrometric speaker (1) according to the preceding claim, wherein each of the two piezoelectric actuators (121a, 121b) extends at most over half of the elastic strip (122a, 122b) which is associated to it from the lateral coupling edge (132a, 132b) of the mechanical-acoustic transducer (13) which is engaged by said elastic strip (122a, 122b).
3. Micrometric speaker (1) according to any one of the preceding claims, wherein each linearising spring (133a, 133b) has a stiffness at least ten times, preferably at least one hundred times, greater than a stiffness of the elastic strips (122a, 122b).
4. Micrometric speaker (1) according to any one of the preceding claims, wherein the central crossmember (111) of the frame (11) extends at most over a first half of a thickness of the frame (11) and the two elastic strips (122a, 122b) comprise one same layer (120a) secured to a face of the central crossmember (111) which is oriented towards a centre of the frame (11).
5. Micrometric speaker (1) according to the preceding claim, wherein said layer (120a) is constituted of a silicon base.
6. Micrometric speaker (1) according to any one of the preceding claims, wherein the rigid plate (131) and the linearising springs (133a, 133b) comprise one same layer (130a), a greater stiffness of the rigid plate (131) relative to a stiffness of the linearising springs (133a, 133b) being due to structuring patterns (130b) that includes the rigid plate (131) and which extend, from said layer (130a), over a surface of the latter defining an extent of the rigid plate (131), the linearising springs (133a, 133b) themselves being constituted of portions (130c, 130d) of said layer (130a) which extend on either side of said surface.
7. Micrometric speaker (1) according to the preceding claim, wherein said layer (130a) is constituted of a silicon base.
8. Micrometric speaker (1) according to any one of the preceding claims, wherein the frame (11) is configured such that the mechanical-acoustic transducer (13) is located, from all sides, at a distance from the inner perimeter of the frame (11) of between 1 and 100µm, preferably between 2 and 80µm, for example substantially equal to 9µm.
9. Micrometric speaker (1) according to any one of the preceding claims, wherein the frame (11) has, in its main extension plane, dimensions each of between 1 and 10mm, preferably between 3 and 8mm.
10. Micrometric speaker (1) according to any one of the preceding claims, wherein the lateral coupling edges (132a, 132b) of the mechanical-acoustic transducer (13) extend from one of the two linearising springs (133a, 133b) over a distance greater than 750µm, preferably greater than 500µm.
11. Micrometric speaker (1) according to any one of the preceding claims, wherein the elastic strips (122a, 122b) have a thickness of between 1 and 100µm, preferably of between 5 and 20µm.
12. Micrometric speaker (1) according to any one of the preceding claims, wherein the two piezoelectric actuators (121a, 121b) are PZT-based, even constituted of PZT, and each extend over a face of one of the two elastic strips (122a, 122b) which is opposite the rigid plate (131) of the mechanical-acoustic transducer (13).
13. Micrometric speaker (1) according to any one of the preceding claims, wherein the elastic strips (122a, 122b) of the electromechanical transducer (12) have a first resonating frequency and the linearising springs (133a, 133b) of the mechanical-acoustic transducer (13) have a second resonating frequency, the second resonating frequency being at least one hundred times, preferably at least one thousand times, greater than the first resonating frequency.
14. Micrometric speaker (1) according to any one of the preceding claims, wherein the frame (11) comprises first and second parts (11a, 11b), superposed and concentric to one another, a second part (11a) of the frame (11) supports the central crossmember (111) and comprises two terminals (112a, 112b) for electrically connecting to the piezoelectric actuators (121a, 121b), the electrical connecting terminals (112a, 112b) preferably being located in the extension of the central crossmember (111), and the second part (11b) of the frame (11) comprises two notches (113a, 113b) configured to each be located opposite one of the two electrical connecting terminals (112a, 112b).
15. Method for manufacturing a micrometric speaker (1) according to any one of the preceding claims, comprising, even being limited to, deposition and etching steps falling under microelectronics.

Images