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Title:
PIEZOELECTRIC MEMS TRANSDUCER
Document Type and Number:
WIPO Patent Application WO/2016/107975
Kind Code:
A1
Abstract:
A piezoelectric MEMS transducer comprises a substrate, at least one membrane, at least one electrically isolating layer between the membrane and the substrate, a plurality of piezoelectric actuators arranged on the membrane, at least one electrically isolating layer between the membrane and piezoelectric actuators, electrodes for electrical control of each of the piezoelectric actuators. The structure further comprises at least one recess formed into the substrate, electrically isolating layer between the membrane and the sub-strate and/or to the membrane in order to release the membrane for movement. The transducer is further arranged to receive digital signals for controlling the piezoelectric actuators via the electrodes to convert the signals into mechanical actuation of the actuators and consequently to the actuation of the membrane, said actuators being further arranged to be controlled as groups pertaining to binary values. Corresponding systems and methods are also presented.

Inventors:
GUO BIN (FI)
DEKKER JAMES R (FI)
GORELICK SERGEY (FI)
Application Number:
PCT/FI2015/050914
Publication Date:
July 07, 2016
Filing Date:
December 21, 2015
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
TEKNOLOGIAN TUTKIMUSKESKUS VTT OY (FI)
International Classes:
H04R17/00
Foreign References:
US20140177881A12014-06-26
US20130294636A12013-11-07
US20120087522A12012-04-12
US20110051985A12011-03-03
JP2011182298A2011-09-15
Other References:
None
Attorney, Agent or Firm:
IPR PARTNERS OY (Helsinki, FI)
Download PDF:
Claims:
Claims

1. A piezoelectric MEMS transducer comprising: -a substrate,

-at least one membrane,

-at least one electrically isolating layer between the membrane and the substrate,

-a plurality of piezoelectric actuators arranged on the membrane,

-at least one electrically isolating layer between the membrane and piezoelectric actuators,

-electrodes for electrical control of each of the piezoelectric actuators, the structure further comprising at least one recess formed into the sub- strate, electrically isolating layer between the membrane and the substrate and/or to the membrane in order to release the membrane for movement, and the transducer being further arranged to receive digital signals for control- ling the piezoelectric actuators via the electrodes to convert the signals into mechanical actuation of the actuators and consequently to the actuation of the membrane, said actuators being further arranged to be controlled as groups pertaining to binary values. 2. The piezoelectric MEMS transducer of any preceding claim, constituting an electroacoustic transducer, such as a driver or a speaker.

3. The piezoelectric MEMS transducer of any preceding claim, comprising groups of piezoelectric actuators arranged to produce at least 8-bit digital sound reconstruction.

4. The piezoelectric MEMS transducer of any preceding claim, wherein a number of the piezoelectric actuators are arranged to comprise groups pertaining to a bit value such that a group comprises at least three piezoelectric actuators.

5. The piezoelectric MEMS transducer of any preceding claim, wherein a number of the piezoelectric actuators constituting a group per- taining to a bit value are essentially symmetrically positioned relative to the transducer structure geometry and to each other.

6. The piezoelectric MEMS transducer of any preceding claim, wherein a number of the piezoelectric actuators constitute different sized, functionally weighted arcs that constitute groups of at least one such piezoelectric actuator pertaining to corresponding unit bit values.

7. The piezoelectric MEMS transducer of any preceding claim, wherein a number of the groups comprise piezoelectric actuators with different function in how they produce mechanical stress or strain as a function to an applied electrical field.

8. The piezoelectric MEMS transducer of any preceding claim, wherein the piezoelectric actuators are arranged substantially close to the edge of the membrane to enable edge actuation.

9. The piezoelectric MEMS transducer of any preceding claim, wherein the piezoelectric actuators are arranged substantially in the prox- imity of the center of the membrane to enable center actuation.

10. The piezoelectric MEMS transducer of any preceding claim, wherein the piezoelectric actuators are arranged on either sides of the membrane to constitute a unimorph structure.

1 1. The piezoelectric MEMS transducer of any preceding claim, wherein the piezoelectric actuators are arranged on both sides of the membrane to constitute a bimorph structure. 12. The piezoelectric MEMS transducer of any preceding claim, wherein the membrane comprises at least one recess constituting a piston or reverse piston structure.

13. The piezoelectric MEMS transducer of any preceding claim, wherein the transducer is used to change the light reflection properties of the structure or to convert the digital signal actuated piezoelectric actuation and consequent membrane actuation into mechanical energy to actuate an object in connection with the transducer.

14. The piezoelectric MEMS transducer of any preceding claim, wherein the transducer constitutes micro pump, synthetic jet actuator, ultrasonic actuator or a deformable mirror.

15. A system comprising the piezoelectric MEMS transducer of claim 1, comprising also one or more of the devices selected from the group consisting of driver, microphone, such as a directional microphone, signal processing entity, communication entity and remote control entity.

16. The system of claim 15, wherein the system is wireless.

17. The system of claim 15-16, constituting a hearing aid, monitor or earphone device.

18. A method for manufacturing a piezoelectric MEMS transducer comprising:

-producing a substrate,

-producing at least one membrane,

-producing at least one electrically isolating layer in between said substrate and said membrane,

-producing a plurality of piezoelectric actuators on the membrane or on the at least one conductive plate,

-producing at least one electrically isolating layer between the membrane and piezoelectric actuators,

-producing electrodes for electrical control of each of the piezoelectric actuators,

-forming recesses.

Description:
PIEZOELECTRIC MEMS TRANSDUCER

FIELD OF THE INVENTION Generally the present invention concerns microelectromechanical systems (MEMS). Particularly, however not exclusively, the invention pertains to electromechanical transducers.

BACKGROUND

Currently, MEMS devices and manufacturing techniques are widely used in various different applications. MEMS structures allow for even smaller devices with mechanical and electric properties, which is convenient since there is a growing trend in electronics towards smaller and more efficient solutions.

One demanding but appealing application for these manufacturing means and structures comprise electroacoustic solutions such as loudspeakers and hearing aids. However, even with the current solutions there is a clear need for even better performance and lower costs. Especially small speakers with low power consumption and high acoustic quality are greatly desired due to the various applications that exist for them. Indeed, very small speaker structures are still predominantly poor in their sound quality and power consumption properties.

For example, hearing aid and small speaker solutions rely predominantly on miniaturized electromechanical coils which makes it difficult to manufacture these on a micro scale. Further on, it is very hard to achieve high acoustic performance with such miniaturized devices. Consequently they require amplification and often digital to analog conversion (DAC), which further leads to greater power consumption, less compact and more complicated and expensive systems..

Some of existing complementary metal-oxide-semiconductor (CMOS)- MEMS speakers solutions used in digital sound reconstruction need high driving voltage (e.g. 40V) and show bipolar response (i.e. there are both positive and negative peaks in a single output pressure pulse). In addition, the response time of a CMOS-MEMS speaker is long (i.e. 250μ8). Addi- tionally, some digital sound reconstructing speaker solutions that are electrostatic suffer from high driving voltage and the pull-in problem. Further on, array-based digital MEMS speakers suffer from high distortion due to speaker non-uniformity and phase cancellation. All the above mentioned limitations significantly lower the performance of the previous reported digital sound reconstructing MEMS speakers.

There is hence a clear need for compact and low driving voltage micro- structure transducer solution that may feasibly utilized in electroacoustic applications, such as hearing aids, and which has more predictable and controllable performance and high quality acoustic properties.

SUMMARY OF THE INVENTION The objective of the embodiments of the present invention is to at least alleviate one or more of the aforementioned drawbacks evident in the prior art arrangements particularly in the context of MEMS transducers and electroacoustic solutions. The objective is generally achieved with a device and a method according to the claims in accordance with the present invention.

One of the advantageous features of the present invention is that it omits the use of digital to analog conversion and consequently analog amplification by actuating the membrane directly with digital signals. A linear and analog-type membrane movement can thus be achieved without a digital- to-analog converter (DAC) and analog amplifier. This is especially beneficial when the membrane is used to produce sound and allows for digital sound reconstruction. In one sense, the solution may be seen to act as an electromechanical DAC.

Also, compared to some existing piezoelectric-based actuation solutions the present solution has the advantage of overcoming the intrinsic nonlinear and bipolar response of such existing solutions by utilizing the digital actuation.

Consequently, because the digital nature of the present solution allows for omitting some structures a simpler and more compact solution in terms of structure design and size is attained. This allows for utilizing the present solution in a myriad of transducer applications.

One of the further advantageous features of the present invention is that by using piezoelectric actuation the driving voltage may be kept low. Other benefits of the present invention include improved frequency response.

Overall, the proposed digital MEMS speaker features more predictable and controllable performance than the prior arts, while keeping low power consumption.

Further on, from the manufacturing perspective the present solution gains advantageously from that it may be produced in the wafer and integrated circuit (IC) manufacturing foundries. MEMS technology also benefits from high accuracy patterning and structuring by semiconductor mass production facilities, which is thus ideal for fabricating micro speakers with small form factors and ultra-low power consumptions. Compared to the conventional miniaturized magnet based approach, speakers based on piezoelectric or electrostatic actuation offers the IC-compatible process and thus the possibility for integration with electronics (driving, signal processing and wireless communication). Moreover, this integration with electronics can be achieved in a wafer scale thanks to the full compatibility of the process and the mature enabling technologies such as through- silicon via (TSV) and wafer bonding.

In accordance with one aspect of the present invention a piezoelectric MEMS transducer comprising:

-a substrate,

-at least one membrane,

-at least one electrically isolating layer between the membrane and the substrate,

-a plurality of piezoelectric actuators arranged on the membrane,

-at least one electrically isolating layer between the membrane and piezoe- lectric actuators,

-electrodes for electrical control of each of the piezoelectric actuators, the structure further comprising at least one recess formed into the substrate, electrically isolating layer, conductive layer and/or to the membrane in order to release the membrane for movement, and the transducer being further arranged to receive digital signals for controlling the piezoelectric actuators via the electrodes to convert the signals into mechanical actuation of the actuators and consequently to the actuation of the membrane, said actuators being further arranged to comprise groups pertaining to binary values.

According to an exemplary embodiment of the present invention the piezoelectric MEMS transducer is an electroacoustic transducer, such as a driver or a speaker. According to an exemplary embodiment of the present invention the piezoelectric actuators are arranged to comprise, or pertain to, groups to produce at least 8-bit digital sound reconstruction.

According to an exemplary embodiment of the present invention the groups may be arranged such that a group pertaining to a binary or a bit value comprises at least three piezoelectric actuators.

According to an exemplary embodiment of the present invention a number of the piezoelectric actuators constituting a group pertaining to bit values may be essentially symmetrically positioned relative to the transducer structure, or more specifically to the membrane structure, geometry and/or to each other.

According to an exemplary embodiment of the present invention at least some of the piezoelectric actuators may constitute different sized, functionally weighted arcs that constitute groups of at least one such piezoelectric actuator the groups further pertaining to corresponding unit bit values.

According to an exemplary embodiment of the present invention a number of the groups may comprise piezoelectric actuators with different function in how they produce mechanical stress or strain as a function to an applied electrical field. According to an exemplary embodiment of the present invention the piezoelectric MEMS transducer actuation may be used to change the light reflection properties of the structure or to convert the digital signal actuated piezoelectric actuation and consequent membrane actuation into mechani- cal energy to actuate for example fluid or an element, structure or system in connection with the transducer. Some feasible transducer applications in addition to the electroacoustic applications comprise inter alia micro pump, synthetic jet actuator, ultrasonic actuator or a deformable mirror devices.

In accordance with one aspect of the present invention a system comprising a piezoelectric MEMS transducer comprising additionally one or more of the devices selected from the group consisting of driver, microphone, such as a directional microphone, signal processing entity, communication entity and remote control entity.

According to an exemplary embodiment of the present invention the system may constitute e.g. a hearing aid, monitor or earphone device that may be also wireless.

In accordance with one aspect of the present invention a method for manufacturing a piezoelectric MEMS transducer comprising:

-producing a substrate,

-producing at least one membrane,

-producing at least one electrically isolating layer in between said substrate and said membrane,

-producing a plurality of piezoelectric actuators on the membrane or on the at least one conductive plate,

-producing at least one electrically isolating layer between the membrane and piezoelectric actuators,

-producing electrodes for each of the piezoelectric actuators,

-forming recesses. The previously presented considerations concerning the various embodiments of the piezoelectric MEMS transducer may be flexibly applied to the embodiments of the method mutatis mutandis and vice versa, as being appreciated by a skilled person. Similarly, the structure obtained by the method and corresponding arrangement is scalable in the limitations of the entities according to the arrangement.

As briefly reviewed hereinbefore, the utility of the different aspects of the present invention arises from a plurality of issues depending on each particular embodiment.

The term "unit bit value" is used to refer to the value (i.e. 0 or 1) of a binary digit of a binary number, with any bit-length.

The expression "a number of may herein refer to any positive integer starting from one (1). The expression "a plurality of may refer to any positive integer starting from two (2), respectively. The term "exemplary" refers herein to an example or example-like feature, not the sole or only preferable option.

Different embodiments of the present invention are also disclosed in the attached dependent claims.

BRIEF DESCRIPTION OF THE RELATED DRAWINGS

Next, some exemplary embodiments of the present invention are reviewed more closely with reference to the attached drawings, wherein

Fig. 1 illustrates a cross-sectional view of an embodiment of a piezoelectric MEMS transducer in accordance with the present invention.

Figs. 2a, 2b, 2c illustrate cross-sectional views of various embodiments of a piezoelectric MEMS transducer in accordance with the present inven- tion.

Figs. 3a, 3b illustrate cross-sectional views of various embodiments of a piezoelectric MEMS transducer in accordance with the present invention. Figs. 4a, 4b, 4c, 4d illustrate top views of embodiments of a piezoelectric MEMS transducer in accordance with the present invention.

Fig. 5 illustrates an embodiment of a system comprising a piezoelectric MEMS transducer in accordance with the present invention. Fig. 6 is a flow diagram illustrating an embodiment of a method for manufacturing a piezoelectric MEMS transducer in accordance with the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 illustrates a cross-sectional view of an embodiment of a piezoelectric MEMS transducer 100 in accordance with the present invention. The piezoelectric MEMS transducer 100 essentially comprises a substrate 102, at least two electrically isolating layers 104a, 104b at least one membrane 106, a plurality of piezoelectric actuators 1 10 and electrodes 108, 1 12, connected to the piezoelectric actuators 1 10 and at least one recess 1 14. Additional elements and structures known to a person skilled in the art, such as insulators, may be incorporated appropriately according to various embodiments.

The substrate 102 comprises e.g. typical semiconducting material such as silicon produced by growing or other wafer manufacturing means. The substrate 102 may comprise a number of recesses 1 14. Further on it may be microfabricated to form recesses 1 14 therein, etc. CMOS circuits may also be preformed on the substrate 102.

The electrically isolating layer 104a is of insulating material and is prefer- ably such on the substrate 102 that it separates the substrate 102 and the membrane 106 electrically isolating them. This layer is essentially used for electrical isolation and diffusion prevention etc. The electrically isolating layer 104b is also of insulating material and is preferably such on the membrane 106 that it separates the membrane 106 and the electrodes 108 on the bottom of the piezoelectric actuators 1 10 electrically isolating them. However, more isolating layers may be used in either isolating layer structures 104a, 104b, i.e. between the substrate 102 and the membrane 106 or between the membrane 106 and the (bottom) electrodes 108/piezoelectric actuators 1 10 for providing layers with different functions. Some feasible insulating materials comprise silicon dioxide (SiO2), aluminum oxide (A12O3), titanium dioxide (TiO2), silicon carbide (SiC), nitrides, oxyni- trides and polymers. The at least one membrane 106 comprises preferably single crystal silicon but other materials such as silicon-germanium (SiGe), diamond and gra- phene may also be used. The thickness of the membrane may be e.g. of some 5-10μηι. The membrane 106 may also comprise recesses. It is pref- erably shaped such that preferred form factor, actuation and bending properties are attained. The membrane 106 may be produced by growing, deposition or by other wafer manufacturing means such as wafer bonding e.g. as in the cases of silicon on insulator (SOI) and cavity SOI and further on be microfabricated. The membrane 1 10 may be attached on the isolating layer 104 and optionally on the preformed recess in the substrate and/or the insulating layer by wafer bonding, or sacrificial layer (e.g. oxide) etch through some etching-access holes or channels.

The electrodes 108, 1 12 are connected to each of the piezoelectric actua- tors 1 10. Each piezoelectric actuator has a first electrode 108 and a second electrode 1 12. This allows for controlling the piezoelectric actuators 1 10 individually or consequently as groups of actuators 1 10 to actuate the membrane 106. Both of the electrodes 108, 1 12 are used essentially to conduct digital signals to, or analog signals from, the piezoelectric actua- tors 1 10.

Some feasible materials for the electrodes 108,1 12 comprise platinum (Pt), molybdenum (Mo), gold (Au), titanium nitride (TiN), tin (solder) and aluminum (Al).

The piezoelectric actuators 1 10 comprise preferably thin film elements of piezoelectric material with a thickness e.g. of some Ιμηι. They may be mutually different or same sized. They may also be mutually essentially similar or different in their function, such as how they produce mechanical stress or strain as a function to an electrical field.

Some feasible piezoelectric actuator 1 10 materials in this context comprise lead zirconate titanate (PZT), aluminum nitride (A1N), scandium aluminum nitride (ScAIN), zinc oxide (ZnO) and lithium niobate (LiNbO3).

The overall structure of the piezoelectric MEMS transducer 100 may comprise recesses 1 14 in the substrate 102, either of the electrically isolating layers 104a, 104b, and/or membrane 106. However, at least such re- cess(es) 1 14 exist that the membrane 106 may move in accordance to the preferred properties of the transducer. This may include a number of venting holes from the recess(es) and out of the structure to enable i.a. pressure stabilization in the recess(es).

Further on, the cavity may include porous Si (or pores in the substrate) to increase effective air volume in the cavity, which will in turn increase the acoustic performance of the transducer. Such porous Si can be formed from either side of the substrate. When it is formed from backside, another wafer or plate etc. may be used to close the cavity from the backside.

Futher on, the recess 1 14 may be formed as a cavity by packaging the piezoelectric MEMS transducer 100. Hence the cavity may constitute an essentially closed space, with e.g. venting hole(s). This may be done by e.g. attaching the membrane 1 10 over the substrate-isolating layer structure 102,104a over the preformed recess in the substrate and/or the isolating layer 104a by wafer bonding, or sacrificial layer (e.g. oxide) etch through some etching-access holes or channels. Optionally the whole structure 100 may be attached (from the substrate 102) on a CMOS (formed on a anoth- er wafer) or other structure (e.g. to create a packaging) optionally to substantially enclose the recess(es) and for creating a cavity e.g. in between the underlying CMOS and the substrate of the MEMS transducer 100.

Figures 2a, 2b & 2c illustrate cross-sectional views of various embodi- ments of a piezoelectric MEMS 200 transducer in accordance with the present invention.

The overall structure of the piezoelectric MEMS transducer 200 essentially comprises a substrate 202, at least two electrically isolating layers 204a, 204b, a membrane 206, a plurality of piezoelectric actuators 210 and electrodes 108, 1 12, connected to the piezoelectric actuators 210. Additionally the structure also comprises at least one recess 214 substantially such that the membrane 206 may move inter alia as an effect to actuation by the piezoelectric actuators 210.

The illustration depicts some of the possible design solutions particularly from the aspect of recess 214 construction in the overall structure. It is in no manner exclusive but depicts some preferable embodiments. In the illustrated embodiments the piezoelectric actuators 210 are arranged to produce edge actuation. In other words, the piezoelectric actuators 210 reside essentially closer to the edge than the center of the membrane 206 (and the isolating layer 204b) (horizontally when viewed from this perspective).

As discussed the structure comprises at least one recess 214 in the substrate 202, either of the electrically isolating layers 204a, 204b and/or the membrane 206. The at least one recess 214 may constitute any feasible shape and may be determined in accordance to i.a. desired form factor and physical properties of the membrane 206 (and consequentially to the surrounding structures, such as the electrodes 208, 212, piezoelectric actuators 210, etc.). The recess may be formed in accordance to the transducer application. For example, the volume, other dimensions and shape are chosen differently for electroacoustic and mechanical (such as pump) transducers. Optionally additionally the recess(es) 214 may be formed in accordance to the actuation principle, i.e. center or edge actuation/or and unimorph or bimorph actuation. For example, the recesses may be essen- tially determined by the location of the piezoelectric actuators 210.

A person skilled in the art may come up with different embodiments but e.g. the recess 214 may be all the way through the substrate 202 and isolating layer 204a to the membrane 206, essentially continuing inside the membrane 206 or ending on its surface, such that there exists free space under the membrane 206 in which the membrane 206 may move. The recess 214 could be also essentially only in the isolating layer 204a creating free space between the substrate 202 and membrane 206, however leaving a separating portion of the isolating layer 204a between the substrate 202 and the membrane 206 to separate the from each other. Similarly, the recess 214 in between the membrane 206 and substrate 202 could continue to either of the structures. The cavity formed by the recess 214 may be essentially symmetric or it may be for example such that there exists one or more smaller venting holes for the larger cavity wherein the membrane is moved.

The at least one recess 214 may be formed such that the overall structure constitutes a piston or reverse piston structure in the membrane 206. Op- tionally in some embodiments the structure may be made to constitute a resonance cavity for reinforcing the output of the transducer 200. Optionally the recess or cavity structure may be such as that the actuation of the membrane doesn't actuate or physically affect the object on which the MEMS transducer is residing. Hence, an enclosed recess essentially forming a cavity with venting holes protects the object(s) underneath the MEMS transducer structure that could otherwise be damaged from changes in fluid pressure etc. Obviously, the optimal recess 214 shape and size depends on the application of the transducer 200 and consequently i.a. how much space for movement the membrane 206 and related structures require, including the piezoelectric actuators 210 and electrodes 208, 212.

The electrodes 208, 212 are further on always produced such on the piezoelectric actuators 210 that they are not in direct electrical connection with each other, such that they are for example isolated by a layer of insulating material, trench isolation, perforations or the piezoelectric actuator 210 residing in between them.

The depicted embodiments of the piezoelectric MEMS transducer 200 constitute unimorph actuation arrangements. In the unimorph structure the piezoelectric actuators 210 are on either one side of the membrane 206. The unimorph structure utilizing single crystal Si as the membrane 206 reduces the influence of the residue stress which otherwise might often happen for piezoelectric thin film membrane actuation.

However, the piezoelectric actuators 210 and electrodes 212 on them could be produced on both sides of the membrane 206 allowing for bi- morph actuation. According to this embodiment the same requirement of having the electrodes 208, 212 on the piezoelectric actuators 210 such that they are not in direct electrical connection with each other applies, i.e. they are e.g. isolated by the piezoelectric actuators 210 or other feasible isolation mean or material.

Figures 3a & 3b illustrate a cross-sectional view of various embodiments of a piezoelectric MEMS transducer 300 in accordance with the present invention. Similarly to the embodiments illustrated in figs. 2a, 2b & 2c the overall structure of the piezoelectric MEMS transducer 300 essentially comprises a substrate 302, at least two electrically isolating layers 304a, 304b, a membrane 306, a plurality of piezoelectric actuators 310 and electrodes 308, 312. Additionally essentially the structure also comprises at least one recess 314 such that the membrane 306 may move inter alia as an effect of actuation by the piezoelectric actuators 310.

In the illustrated embodiments the piezoelectric actuators 310 are arranged to produce center actuation. In other words, the piezoelectric actuators 310 reside essentially closer to the center than the edge of the membrane 306 (and the isolating layer 304b) (horizontally when viewed from this perspective). However, even though not explicitly illustrated, the piezoelectric actuators 310 may be arranged to produce actuation essentially in any location on the membrane 306. In other words, the piezoelectric actuators 310 may reside in preferred locations closer to the center, closer to the edge or somewhere in between these locations on the membrane 306 horizontally when viewed from this perspective. The piezoelectric actuators 310 may be also in different actuation positions, such as in an asymmetric manner, in relation to each other.

The illustration depicts some of the possible design solutions particularly from the aspect of recess construction in the overall structure. It is no manner exclusive but depicts some preferable embodiments. Essentially same considerations as discussed with the figs. 2a, 2b & 2c apply.

Figs. 4a, 4b, 4c, 4d illustrate top views of embodiments of a piezoelectric MEMS transducer 400 in accordance with the present invention.

In these embodiments the piezoelectric actuators 402a, 402b, 402c, 402d reside essentially around and closer to the edge of the membrane and isolating layer structure (surface) 404, which is actuated by the piezoelectric actuators 402a, 402b, 402c, 402d. The membrane and isolating layer structure (surface) may comprise a number of recesses 406a, 406b of any shape. Some preferable shapes comprise plate recess 406a structure and ribbed recess structure constituting of a plurality of recesses 406b. There are many ways to produce the piezoelectric actuators 402a, 402b, 402c, 402d on the structure. Therefore individual piezoelectric actuators 402a, 402b, 402c, 402d (and consequentially their electrodes) may be sep- arated from each other by trench isolation, perforation or other isolation between the piezoelectric actuators 402a, 402b, 402c, 402d. This allows for example the use of trench isolation manufactured advantageously e.g. by etching, printing, lithography printing, etc. The piezoelectric actuators 402a essentially depict piezoelectric actuators 402a with similar mechanical functioning and/or similar shape and/or structure. The piezoelectric actuators 402a are essentially symmetrically positioned and individual actuators 402a may be chosen to constitute groups pertaining to binary values. For example, at least three or four symmetrically positioned actuators 402a relative to the transducer 400 structure geometry and to each other could be chosen to constitute such group.

The piezoelectric actuators 402b, 402c, 402d depict different sized, weighted arcs and/or piezoelectric actuators 402b, 402c, 402d with different mechanical functioning. Such piezoelectric actuators 402b, 402c, 402d may be used to constitute groups of at least one or two piezoelectric actuators 402b, 402c, 402d pertaining to a unit bit value. That is they may be arranged such that e.g. 402b always controls the value of the first digit of a binary number and the 402c always controls the value of the second digit of a binary number and so on.

The mentioned group of at least one may comprise for example rings or circular piezoelectric actuators on the membrane area. The mentioned group of at least two may be preferably comprise such functionally weighted piezoelectric actuator 402b, 402c, 402d areas or arcs. The functionally weighted herein is used to refer to being arranged in accordance to e.g. weight determined by the (mechanical) functioning or effect of the piezoelectric actuator 402b, 402c, 402d.

The mechanical functioning herein refers to e.g. the mechanical stress or strain, such as the amount and/or direction of the stress or strain, attained by actuating a piezoelectric actuator 402a, 402b, 402c, 402d with an applied electrical field or a (digital) signal.

The piezoelectric MEMS transducer 400 may be used to produce essen- tially any binary value actuation and digital sound reconstruction.

However for the sake of simplicity, an example of 2-bit control in accordance to mechanically similarly functioning piezoelectric actuators 402a:

-00: no actuated actuators 402a,

-01 : one group of three actuators 402a,

-10: two groups of three actuators 402a each, constituting a total of six actuated actuators 402a,

-1 1 : three groups of three actuators 402a each, constituting a total of nine actuated actuators 402a.

However, it should be noted herein that for example actuating total of six actuators 402a in accordance to binary value (10) needn't be necessarily as two groups. Equivalently, it may be said to be accomplished by simultaneous actuation of six actuators 402a and others binary values following the same logic. Even further, the groups may be dynamical such that e.g. any multiple of groups such as six may be produced by the actuation of different groups of three. Also, the groups needn't comprise always the same piezoelectric actuators such that individual piezoelectric actuators may be comprised into many different groups.

Further, 3 -bit control could be also attained with mechanically differently functioning piezoelectric actuators 402b, 402c, 402d e.g. as such:

-000: no actuated actuators 402b, 402c, 402d,

-100: one group of two actuators 402b,

-1 10: two groups of two actuators 402b, 402c each constituting a total of four actuated actuators 402b, 402c,

-1 1 1 : three groups of two actuators 402b, 402c, 402d each constituting a total of 6 actuated actuators 402b, 402c, 402d,

-010: one group of two actuators 402c,

-01 1 : two groups of two actuators 402c, 402d each constituting a total of four actuated actuators 402c, 402d,

-001 : one group of two actuators 402d. -101 : two groups of two actuators 402b, 402d each constituting a total of four actuated actuators 402b, 402d.

Obviously, the total number of piezoelectric actuators 402a, 402b, 402c, 402d as well as the number of piezoelectric actuators 402a, 402b, 402c, 402d used to pertain to unit bit values and bit values depends on many aspects, such as the piezoelectric actuator mechanical functioning, number bits needed and the overall size of the piezoelectric MEMS transducer 400 structure.

The term "group" and its plural form "groups" which are used to refer to the number of actuators 402a, 402b, 402c, 402d assimilated or pertaining to unit bit values, bit values and binary values are not limiting in a physical sense, i.e. there doesn't necessarily exist any (physical) relation be- tween actuators 402a, 402b, 402c, 402d of a group. The group is used as a more of a figurative manner to express that actually a number of actuators 402a, 402b, 402c, 402d are used to produce another bit value. Determining the groups is as such more of a configuration and controlling matter than actual manufacturing result or actual device structure property.

Essentially however, electroacoustic applications gain from at least 8-bit digital sound reconstruction. However different transducer 400 applications may require different bit control, which again is a matter of design in relation to the specific application. In digital sound reconstruction the transducer is working in a digital "on-of ' mode and the actuators 402a, 402b, 402c, 402d are either actuated or not with a digital signal.

Further on, the piezoelectric actuators may be used for calibration of the transducer, i.e. the bending action, flatness, stress and actuation properties of the membrane. At least one or more of said plural piezoelectric actuators can be used for sensing the bending (and or the stress and flatness) of the membrane. At least one or more of said plural piezoelectric actuators can be used to adjust the bending (and or the stress and flatness) of the membrane. Further on, at least one or more of said plural piezoelectric ac- tuators can be used for sensing the deflection of the membrane and functioning as a pressure or acoustic sensor/receiver. The acoustic sensing may be used to give directionality information of the source. The MEMS transducer 400 structure may be also such that it comprises separated membranes: the one membrane being separated or having at least two individual membranes in the structure. This is advantageous from the perspective of for example having two drivers (driven with dif- ferent clock frequencies, such as tweeters: 44.1kHz and woofer: 1 1kHz) in the one overall piezoelectric MEMS transducer 400. Obviously recesses may be made such in these embodiments e.g. that separate drivers have separate resonant cavities, etc. According to an example, the membrane preferably works under DC driving condition, i.e. the working frequency f is much lower than the resonance frequency fO:

- to avoid unwanted resonances,

- to have fast response time, i.e. the rise/fall time < l/2f;

- on the contrary, the post-resonance response is much smaller than the DC response,

- the response is in phase vs the driving signal in DC mode

- the working frequency f can be set to be 44.1kHz (i.e. CD audio);

- the resonance frequency fO is designed to be > 80-100kHz.

Figure 5 illustrates an embodiment of a system 500 comprising a piezoelectric MEMS transducer 502 in accordance with the present invention. In addition to the piezoelectric MEMS transducer 502 the system 500 may comprise a driver 504, signal processing, communication or remote control unit 506 and/or a directional microphone 508.

This kind of a system could e.g. constitute a very small scale and compact integrated hearing aid or earphone. Because of the compact, low power consumption and optionally wireless properties of the system one it allows for example a completely in-the-canal type of electroacoustic solution. In this application the solution is not beneficial only from the perspective of comfort to use the device but also from the perspective of unnoticeability of the system. However other such integrated systems for various applications are possible. The system 500 structure may be essentially manufactured by stacking Si chips allowing for high level of compactness and integration. The system constituting e.g. a hearing aid device or a loudspeaker may be functionally connected and controlled by digital audio sources, such as a desktop or laptop terminal, tablet, smart phone, smart TV, digital broadcasting device, etc. Similarly, the controlling of such system 500 comprising a transducer used for other purposes, such as micro pump, synthetic jet actuator, ultrasonic actuator or a deformable mirror, may be controlled optionally wirelessly by an external terminal device or a processing entity.

Figure 6 illustrates a flow diagram of one feasible embodiment of a method for manufacturing a piezoelectric MEMS transducer in accordance with the present invention.

At 602, referring to the initial state of the method materials for the various elements, materials and structures in a piezoelectric MEMS transducer to be manufactured are chosen. Additionally, the configurations are made in accordance to used manufactured means. Optionally, these decisions may be made in accordance to the application solution or the system that the transducer is meant to be used in.

At 604, a substrate is produced. Optionally, the substrate may be pre- formed to a desired shape, such as to comprise the designed shape and/or recesses of the final structure. Additionally, the substrate may be preconditioned to allow for the manufacturing or attachment of the other elements and structures to the substrate. Recess and cavities may be preformed in the substrate. Further on, these cavities may include porous Si.

CMOS circuit may also be formed on the substrate.

The substrate may be manufactured and/or constitute a part of a larger wafer constituting essentially an area such as to allow for microfabrication, MEMS fabrication and wafer fabrication thereon, from which it may be diced. However, other means of manufacturing on said substrate are possible as well as having the substrate as an individual die. At 606, at least one (first) electrically isolating layer is produced to a preferred location on the substrate. Preferably, the isolating layer is produced such that it at least separates the substrate from a membrane. Obviously this may be accomplished by many means and depends on the design of the overall structure of the MEMS transducer. For example, the electrically isolating layer may completely cover a one face of the substrate's structure or a part of it e.g. essentially the area whereupon a membrane is produced. At 608, a membrane is produced. The membrane is preferably produced essentially directly to the substrate by deposition and preferably such that the electrically isolating layer separates the membrane and substrate.

The membrane can also be the Si layer of a SOI wafer. In this case the substrate, the membrane and the electrically isolating layer are obtained together as a starting wafer. Depending on the fabrication method, such SOI wafer can be formed by deposition or by bonding and thinning.

The membrane is chosen in accordance to the shape and form factor re- quired from the overall MEMS transducer structure. This may be determined in accordance to the properties required from the transducer. For example, an electroacoustic structure requires different properties from a mirror or mechanical actuation solution, such as a jet pump. The membrane may undergo microfabrication to be formed or produce structures. The microfabrication may comprise inter alia doping of the membrane, forming the membrane to a desired shape, optionally with recesses therein, such as by etching. At 610, at least one other (second) electrically isolating layer is produced to a preferred location on the substrate. Preferably, the isolating layer is produced such that it at least separates the membrane from the electrodes. Obviously this may be accomplished by many means and depends on the design of the overall structure of the MEMS transducer. For example, the electrically isolating layer may completely cover a one face of the membrane's structure or a part of it e.g. essentially the area whereupon electrodes/or and piezoelectric actuators are produced. At 612, a number of (bottom) electrodes are produced on the (second) isolating layer. The (bottom) electrodes for each piezoelectric actuator may be produced by forming a conductive plate as a one layer that is later separated into (e.g. trench) isolated sections of (bottom) electrodes. Optionally the electrodes may be produced individually. The conductive plate and/or electrodes may be produced by deposition or printing but other means known in the state of art may also be used.

The conductive plate may be further any desired size or shape preferably in accordance to the shape and locations of the piezoelectric actuators. The conductive plate may further on act as a growth seed layer and a bottom electrode for the piezoelectric actuators.

At 614, piezoelectric actuators are produced on the membrane and option- ally (directly) on the electrodes (or the conductive plate) on the isolating layer. The piezoelectric actuators may be produced on essentially on the same side of the membrane or to essentially opposite sides of the membrane. The piezoelectric actuators may be laid on the membrane (or on the conductive plate) as premanufactured units or they may be manufactured directly on the membrane by deposition or printing. They may be produced by first making as an essentially single layer on the conductive plate(s) that is then divided into individual actuators of preferred size and shape. The piezoelectric actuators may be formed as to constitute different sized structures, such as to have different geometrical dimensions in terms of height, width, cross-sectional area, etc. The piezoelectric actuators may constitute same sized units as well. The actuators are isolated from each other by trench isolation, perforation isolation or by additional insulation material structures.

The piezoelectric actuators may be situated on the membrane such as to reside in geometrically symmetrical locations in relation to a number of other piezoelectric actuators. The distance from each other and their rela- tion to the membrane surface area is totally a matter of design and depends on the preferred properties of the overall structure of the transducer. The number of piezoelectric actuators depends on the factors as explained hereinbefore.

At 616, (top) electrodes are produced on the piezoelectric actuators. Each piezoelectric actuator needs to have an electrode such that it may be actuated. They may be produced by deposition or printing but other means known in the state of art may also be used.

The electrodes are preferably produced on the top side of each piezoelec- trie actuator although other locations are also feasible. However, the face with largest area is preferred since the size of the electrode also affects the functioning of a piezoelectric actuators on which it is attached.

At 618, recesses are formed to the structure such as that the preferred overall design of the MEMS transducer is attained. In practice many different options for the recesses exist of which many depend on the desired properties of the structure. However, at least one recess must be formed such that the membrane may move about at least in either direction perpendicular to the initial state of the membrane.

The recess may hence be formed to the substrate, electrically isolating layer, conductive layer, membrane or a combination of these such that there exists a recess at some point in between the substrate and the membrane. As is obvious, this recess may exist in any of the aforementioned or a combination of them as any desired shape. Preferably, the shape of the recess is such that the membrane may move about as much as desired, i.e. as much as is wanted to produce by the actuation of the piezoelectric actuation. Optionally additionally, the recess or any additional recesses are made to improve other properties of the overall structure, such as the me- chanical properties and form factor of the membrane enabling for example different actuation as a function of the membrane's shape. Optionally the recess and cavities may be preformed in the substrate and these cavities may include porous Si. Optionally additionally one or more vent holes for the recess(es) are formed constituting e.g. a cavity from the bigger recess for the membrane out of the structure such as to allow for a fluid to flow through it e.g. as an effect of the membrane actuation and movement. The vent holes and the cavity may be designed to optimize the acoustic performance of the membrane. Recesses may be attained by for example etching but other means known in the state of art may also be used, for example, SOI wafers with pre- etched cavities beneath the device layer, e.g., Cavity-SOI or CSOI, which essentially includes forming recesses into the insulating layer and or the substrate and attach a single crystal Si membrane above the cavity

Recess may also be formed by release etch of at least one sacrificial material (e.t. oxide) below the membrane. Such release etch may be performed through some holes formed into the membrane, or from the substrate.

At 620, referring to the final state of the method the overall structure of the MEMS transducer may be added the conductors, such as by TSV or wafer bonding means. Additionally insulating layers and structures may be added as well as other desired finishes such as wafers or chips containing CMOS circuits, overlays or structures. Lithography and further etching may be done to form structures and isolators. Additionally, the MEMS transducer may be tested, packaged and/or integrated to another device such as a (micro)processor or digital signal processor (DSP) and config- ured.

Obviously, as also mentioned hereinbefore at least implicitly in many occasions and as is also clear from the reading to a person skilled in the art, the mentioned method items needn't be consecutive but instead many may be carried out essentially simultaneously or in a preferred order in accordance inter alia to the used manufacturing methods.

The scope of the invention is determined by the attached claims together with the equivalents thereof. The skilled persons will again appreciate the fact that the disclosed embodiments were constructed for illustrative purposes only, and the innovative fulcrum reviewed herein will cover further embodiments, embodiment combinations, variations and equivalents that better suit each particular use case of the invention.