WO2012164545A1

WO2012164545A1 - Energy scavenging from a rotating gear using an impact type piezoelectric mems scavenger

Info

Publication number: WO2012164545A1
Application number: PCT/IB2012/052805
Authority: WO
Inventors: Don ISARAKORN; Pattanaphong JANPHUANG; Danick Briand; Nico De Rooij
Original assignee: Ecole Polytechnique Federale De Lausanne (Epfl)
Priority date: 2011-06-03
Filing date: 2012-06-04
Publication date: 2012-12-06

Abstract

A device for energy harvesting from an impact by using a piezoelectric MEMS harvester comprising at least a gear (1) driven by an inertial mass system and a piezoelectric transducer (2) located over the gear in order to keep the system as compact as possible wherein the piezoelectric transducer, such as a cantilever structure, is coupled to the inertial mass system through a tip (3) at the free end of the cantilever so that when the gear rotates, the piezoelectric cantilever is set into motion by the impact between the gear teeth (4) and the tip of cantilever. On the other hand, the piezoelectric cantilever can be set into motion using locked and released of the tip using the gear. In this matter, the tip and gear shape need to be optimized. Moreover, the frictionless coating of the tip using SiC or diamond would be taken into account in order to improve the efficiency of the systems.

Description

ENERGY SCAVENGING FROM A ROTATING GEAR USING AN IMPACT TYPE

PIEZOELECTRIC MEMS SCAVENGER CROSS RELATED APPLICATIONS

The present application claims priority to the European patent application ΝΈΡ 1 168703.4 filed on June 3, 201 1 in the name of the same applicant, the content of which is incorporated by reference in its entirety in the present application.

FIELD OF THE INVENTION

The present invention concerns energy harvesting from an impact by using a piezoelectric MEMS scavenger.

In the present invention, useful electrical power is generated by the impact between a rotating object, for example a gear, and a MEMS piezoelectric transducer. The design of the device structure is based on both analytical and FEM models for the estimation of induced voltage generated from a piezoelectric transducer.

The electrical characteristics of a prototype piezoelectric MEMS based impact harvester consisting of a piezoelectric unimorph transducer were evaluated using a gearbox with a speed of 25 rpm.

For the cantilever type harvester occupying approximately 4.2 mm³, an average output power of 14.30 pW is measured across a resistive load of 680 kQ. BACKGROUND OF THE INVENTION

Vibration energy harvesting is potentially interesting for powering low-power electronic devices and various types of sensors. It not only provides a regenerative energy source, but also offers an advantage for systems in which battery replacement is challenging (i.e. remote locations). The development of energy harvesters to produce energy from vibrations has rapidly advanced during the past few years since motions and vibrations are the most ubiquitous ambient energy source available. Typical examples are industrial machines, vehicles, and the human body. There are three types of transduction methods for turning ambient vibrations into useful energy: electromagnetic, electrostatic and piezoelectric. Among these methods, piezoelectric materials have received much attention due to its high conversion efficiency, its high power density, its miniaturization potential using simple structures, and its mass manufacturability [see reference 1].

Several piezoelectric MEMS harvesters have been proposed and investigated over the years and most of them are based on resonant mass spring systems. To optimize the power generation, the harvesters have to be excited very close to the resonant frequency of the oscillator, which is typically more than 100 Hz [see references 1-5]. Harvesting energy from lower frequencies, such as motions of the human body or other slow moving objects (typically less than 30 Hz) is also attractive, but challenging. Using a mass spring resonant system requires an extremely large mass and/or very soft springs such that the system resonates at these low frequencies. This results in a low power density and is difficult to scale down with MEMS technology. For this reason, an alternative principle of piezoelectric harvesters in which a mechanical impact is applied by direct contact of a piezoelectric transducer is a promising solution. As for the well known piezoelectric shoe using PVDF [see reference 6] and piezoelectric windmill [see reference 7] using PZT, significant electrical output power can be obtained with these types of harvesters.

The first example has not been used to harvest energy from a rotating object and the windmill concept does not scale to down to compact harvesting systems. Only very limited studies have been reported on the use of MEMS devices to harvest energy from mechanical impact [see reference 2].

SUMMARY OF THE INVENTION

In the present application, in one embodiment, we present the characteristics and performances of a MEMS harvester based on the impact between a rotating object and a piezoelectric compliant structure. The impact type piezoelectric harvester consisting of a piezoelectric layer (bulk and thin film), for example a 135 pm-thick PZT sheet, bonded on an AFM like cantilever beam (based on silicon and metallic substrates) was successfully fabricated. The electrical characteristics of the harvester (, i.e. voltage, power and load) were experimentally investigated and the results obtained are in good agreement with the analytical model and FEM (finite elements method) simulations.

In an embodiment, the invention concerns a device for energy harvesting from an impact by using a piezoelectric MEMS scavenger comprising at least a gear with teeth driven by an inertial mass system and a piezoelectric transducer located next to the gear, wherein the piezoelectric transducer is coupled to the inertial mass system through a tip so that when the gear starts moving, the piezoelectric transducer is set into motion by the impact between the gear teeth and the tip of the transducer.

The transducer may be a cantilever structure. The cantilever structure may be placed on a top side or a bottom side of the gear.

In an embodiment the cantilever may be a single element.

In an embodiment the cantilever may have several arms, each arm having a tip.

In an embodiment the cantilever has five arms.

In an embodiment the transducer is a piezoelectric membrane with at least one tip. In an embodiment the membrane may comprise a plurality of tips and the membrane may be circular.

In an embodiment the piezoelectric transducer may comprise a silicon substrate with a silicon nitride film and a PZT layer covered with a nickel top and bottom layer each forming an electrode layer.

The invention also concerns a method for fabricating the piezoelectric transducer as defined herein, the method comprising at least the following steps: -) provision of a 100 mm in diameter and 390 μιτι-thick silicon substrate;

-) patterning of the shape of rectangular cantilevers from the top side by DRIE, -) a 150 nm-thick LPCVD silicon nitride film is then deposited on both sides for electrical isolation and used as mask during a KOH etching of silicon;

-) backside openings in the nitride are defined using UV lithography followed by RIE etching;

-) the exposed silicon area is wet etched in a KOH solution to accomplish the silicon AFM like cantilever resulting in a silicon tip with a height of 240 pm;

-) the bonding of 135 pm-thick PZT layer 5 covered with nickel top and bottom electrodes is performed using spin coated UV curable epoxy.

An embodiment of the invention concerns an object comprising at least a device as defined herein. The object may be a watch. DETAILED DESCRIPTION OF THE INVENTION

The present invention will be better understood from the following detailed description and from the drawings which show Figure 1A and 1 B illustrates a schematic diagram of the concept of harvesting energy from a rotating object;

Figure 2A and 2B illustrates a schematic of cantilever based energy harvesters (2A unimorph, 2B bimorph);

Figure 3 illustrates the result of a FE; simulation for the induced voltage generated by PZT film;

Figures 4(a) to 4(e) illustrates the fabrication process of an impact type harvester; Figure 5 illustrates optical images of a PZT/Silicon cantilever and SEM image obtained from the silicon tip;

Figure 6 illustrates a schematic diagram of an experimental setup for testing the impact type PZT MEMS harvester;

Figure 7 illustrates an optical image of the harvester mounted on X, Y and Z stage over the gear box; Figure 8 illustrates a comparison between the analytically calculated, the simulated, and the experimentally measured open circuit voltage as a function of the tip's depth;

Figure 9 illustrates an output voltage signal waveform for different resistive loads at a gearbox speed of 25 rpm and tip's depth of 100 μηη;

Figure 10 illustrates a variation of the output power as a function of the resistive loads for different tip's depth;

Figures 1 1A, 1 1 B, 12A, 12B, 13A, 13B, 14A, and 14B illustrate possible configurations of the device according to the present invention;

Figure 15 illustrates operation modes of the piezoelectric transducers according to the invention.

Figure 16 illustrates the potential application to integrate an impact-type piezoelectric energy harvesting array with a commercially available mechanical watch mechanism.

HARVESTING CONCEPT AND MODELING

The concept of the proposed system according to the present invention is illustrated in Figures 1A and 1 B which shows a schematic drawing of the concept for harvesting energy from a rotating object.

The system comprises basically a piezoelectric harvester 2 positioned beside (Figure 1A) or above a gear 1 (Figure 1 B) to be coupled to a rotating inertial mass in order to keep the system as compact as possible. The piezoelectric harvester 2 is coupled to the gear 1 through a point 3 at the free end of the cantilever 2 (AFM like design). For demonstration purposes, the inertial mass system coupled to the gear was represented by an electric motor. As the gear 1 rotates, the piezoelectric harvester 2 is set into motion by the impact between the gear teeth 4 and the tip 3 of cantilever 2.

Consequently a portion of kinetic energy of the rotating gear 1 is converted to electrical energy through the impact with the piezoelectric structure. Here a silicon substrate served as a support layer 6 for the piezoelectric material 5 (see figure 2A) since silicon micromachining allows the easy fabrication of AFM like cantilever structure with a tip which allows a vertical integration. Among common MEMS compliant structures (i.e. cantilevers, bridges and diaphragms), the cantilever structure was chosen since it is the most compliant structure for a given input force and it can generate larger strain compared to bridge and diaphragm structures. As an exemplary embodiment, a unimorph (figure 2A) or bimorph (figure 2B) piezoelectric cantilever beam structure may be used with a piezo layer 5 on a support elastic layer 6, or two piezo layers 5, 5' on each side of the support layer 6 or even multiple piezo stacks.

The power generation from the strained piezoelectric layer(s) in cantilever structure can be analytically modeled using the bending beam theory of the piezoelectric bimorph reported in reference [10]. Since the configuration of the harvesting device considered here is the unimorph embodiment, its open circuit voltage is a half of the open circuit voltage obtained from the bimorph. Thus, the open circuit voltage of the piezoelectric unimor h can be expressed by:

where H: the total thickness of the beam, L: the length of the beam, δ_ζ displacement of the beam, h₃i: the piezoelectric constant expressed as:

where g₃₁ and g₃₃: piezoelectric voltage constant, c _x , c°₂ and c° : piezoelectric elastic stiffness coefficients.

The parameters for the unimorph used in the prototype was h₃i = -4.62 ^χ 10⁸ V/m. The magnitude of h₃i was calculated using the material specifications available for the piezoelectric ceramic PZT-5A4E from Piezo Systems, Inc.

DESIGN AND FABRICATION

The design of the impact type piezoelectric harvester was supported by FEM simulation in ANSYS 12.0 to determine the induced voltage generated by the deflection of the piezoelectric unimorph. The harvester was comprised of a PZT film stacked on a silicon beam. The geometry of the harvester is defined by three main parameters: the area of the beam, the thickness of the support layer and the thickness of the piezoelectric film. As a starting point, as illustrated in figure 2, a rectangular shape with a length of 5 mm and a width of 3 mm was chosen to maximize the size of the capacitor while maintaining a compact design since the potential power output is directly related to this parameter [see reference 1].

The piezoelectric layer 5 made of 135 pm-thick PZT-5A was first selected because bulk piezoelectric ceramics can provide greater electromechanical coupling, structural strength than deposited thin films [see reference 3].

The piezoelectric film 5 completely covers the silicon support layer 6. As demonstrated in reference 4, to maximize the electrical energy generated from the harvester the ratio of the thickness of the bender (t_p / t_c) has to be close to 0.64 (Figure 3A). Due to the thickness of the PZT film the silicon support layer thickness has been defined to be thinner than 150 pm to maintain a certain flexibility of the harvester.

In simulation, the tip at the free end of a cantilever was neglected for simplification and therefore the force or displacement was directly loaded to the free end of cantilever in the direction of approach (z-direction). The boundary conditions included displacement constraints in the clamped position of the cantilever to prevent the beam from moving away. Therefore the free end of cantilever was allowed to move only in z-direction. The bottom surface of the PZT layer 5 was set as ground and top surface as floating. Variations in the tip depth, from 80 pm to 200 pm, were simulated by applying the corresponding displacement to the tip of the cantilever, from 10 pm to 54 pm, in the static analysis.

Figure 3B shows the generated voltage distributed along the PZT when strained by 54 pm of tip deflection. The average voltage generated through the PZT film thickness is simulated by coupling the entire nodes at top surface together in ANSYS [see reference 8].

An example of a fabrication process of the impact type PZT harvester is illustrated in Figures 4(a) to 4(e).

The process starts from a 100 mm in diameter and 390 pm-thick silicon substrate 7. The shape of rectangular cantilevers 8 is first patterned from the top side by DRIE (see Fig. 4(a)). A 150 nm-thick LPCVD silicon nitride film 9 is then deposited on both sides for electrical isolation and used as mask during the KOH etching of silicon (see Fig. 4(b)).

Backside openings 10 in the nitride are defined using UV lithography followed by RIE etching (see Fig. 4(c)).

The exposed silicon area is wet etched in a KOH solution to accomplish the silicon AFM like cantilever resulting in a silicon tip with a height of 240 pm (see Fig. 4(d)).

The bonding of 135 pm-thick PZT layer 5 covered with nickel top 12 and bottom 13 electrodes is performed using spin coated UV curable epoxy 14 (see Fig. 4(e)).

Fig. 5 shows one of the fabricated impact type PZT harvesters (Fig.5a) and SEM image obtained from the silicon tip 1 1 (Fig.5b).

EXPERIMENTS

The energy harvesting performance of the impact type PZT harvester was first investigated by a complete set of experiments on power generation as illustrated in Fig. 6.

A gearbox 20 with speed of 100 rpm comprising a mini DC motor and a gear 1 with eighteen teeth 4 was used as a rotating object to impact the harvester 2 according to the present invention. The harvester was then mounted on a micrometric X, Y and Z stage 21 to precisely adjust the position of the tip 3 over and into the gearbox 20 and connected to an oscilloscope 22 to detect the output voltage from the impact type PZT harvester.

A laser Doppler vibrometer (LDV), Polytec OFV 502 fiber interferometer 23 driven by Polytec OFV 3000 vibrometer controller, is used to monitor the displacement of the harvesting device.

RESULTS AND DISCUSSION

In a first experiment, the orientation of the cantilever over the gear was adapted from the proposed concept, as shown in Fig. 7 which is an image of the harvester of figure 6 mounted over the gearbox 20 (gear 1 with teeth 4). The power generation from the impact type PZT harvester is observed by varying the depth of the tip 3 into the gearbox 4. The measured results on the output voltage generated from the impact type PZT harvester and the displacements of the beam obtained as function of the tip's depth are given in table 1 below. The results of the analytical model and FEM simulation were then compared to the measured electrical response of the impact type PZT harvester (see Figure 8). A good agreement between measurements, analytical and FEM simulation is achieved.

Table 1.

Results of the open circuit voltages generated from the impact type PZT harvester for different tip depth

The voltage level depends on the design parameters and electrical load used in the circuit.

Figure 9 gives an example of the output voltage peaks measured with different tip depth. To determine the output power, the harvester was connected with various resistive loads and the generated current through the resistive loads for different tip depth was recorded by a multimeter (Agilent 3441 1A). An average output power of

14.30 pW was obtained at 200 pm tip depth with an optimal resistive load of 680 kQ at a gearbox speed of 100 rpm (see Figure 10).

Figures 1 1 A, 1 1 B, 12A, 12B and 13A, 13B illustrate other possible configurations of the device according to the present invention all with the aim of minimizing the space or volume taken by the elements, in order to be usable in many technical fields. One typical application is the watch industry. Of course, other fields may be envisaged.

For example, figures 1 1A and 1 1 B illustrate configurations with cantilever above and/or below a horizontal gear.

The configuration of Figure 1 1A corresponds to the basic one illustrated in figure 1 with a gear 1 comprising teeth 4, a cantilever 2 with a tip 3. Figure 1 B illustrates a variant of the basic configuration which uses two cantilevers 2, 2' each with a tip 3, 3', each being placed on one side of the gear 1 . This allows a compact arrangement of multiple elements.

Figure 12A and 12B illustrate configurations with multi-cantilevers above and/or below a horizontal gear. For example figure 12A illustrates an embodiment with five cantilevers 2-1 , 2-2, 2-3, 2-4 and 2-5, each with a tip 3-1 to 3-5. These multi- cantilevers 2-1 to 2-5 are placed on one side of the gear 1 with teeth 4.

Figure 12B illustrates a variant of the embodiment of Figure 12A where multi- cantilevers 2-1 to 2-5 and 2-1 ' to 2-5' are placed on each side of the gear 1 each with tips 3-1 to 3-5 and 3-1 ' to 3-5'.

Figures 13A and 13B illustrate other embodiments with a membrane comprising tips on top and/or bottom of the gear.

In Figure 13A, there is a disk 25 with tips 25-1 , 25-2 and 25-3 which cooperate with the teeth 4 of the gear 1 according to the principles of the present invention. The disk may comprise up to several tips depending on the number of the gear teeth in order to improve the harvesting efficiency.

Figure 13B illustrates the modification of Figure 13A by separating the membrane into several parts. The electrical characteristics of this embodiment can be modified using the connection of the small membranes. In the applications where voltage is dominant, the membranes (or multi-cantilevers) will be connected in series in order to increase the voltage generated from the harvester. Of course other configurations (in parallel for example) are possible in the frame of the present invention.

Figure 14A illustrates the in plane movement configuration. The piezoelectric cantilever 2-6 with tip 3-6 will be bent in the same plane as the surface of the gear 1 instead of bending up and down.

Figure 14B illustrates a variant of the embodiment of figure 14A where multi- cantilevers 2-6, 2-7, 2-8 are used with tips 3-6 to 3-8 and placed on the gear 1 . This embodiment is implemented to avoid the torsional movement of the beam 2-6 to 2-8 which could reduce the output power generated from the piezoelectric harvester due to the charge cancellation effect.

Piezoelectric operation modes (dy, dw) see figure 15 At present, many designs for piezoelectric MEMS devices utilized the polycrystalline PZT (bulk or films), which allow the operations in either d₃i or d₃₃ modes. The d₃i mode transducers have a PZT layer 5 in between top 27 and bottom 28 electrodes, while the d₃₃ mode transducers dismiss the need of a bottom electrode 28 by utilizing interdigitated (IDT) electrodes 29 on the top of the PZT layer 5. In d₃i mode, the polarization direction is through the film's thickness and perpendicular to the plane of the film, but the generated displacement to be used or the applied stress to be detected is in plane of the film. On the other hand, the use of PZT films that are polarized in the plane of the film instead of through the thickness enables the d₃₃ to be employed. The d₃₃ piezoelectric coefficient is typically around twice the magnitude of the d₃i piezoelectric coefficient in PZT [see reference 1 1]. Therefore, the use of d₃₃ mode could enhance the performance of the devices.

Integration of piezoelectric materials onto silicon substrate

The piezoelectric materials can be integrated onto several substrates such as silicon, metal, and polymer or plastic. Using silicon micromachining, it allows us the easy fabrication of the MEMS structure. At present, there are several piezoelectric materials available. However, the materials of interest for application in MEMS device are mainly PZT (lead zirconate titanate), ZnO (zinc oxide), AIN (aluminium nitride), PMN-PT, and PVDF. Among these, PZT is commonly used in energy harvesting application because of its properties (high electro mechanical coupling and high piezoelectric coefficient) which is desirable in piezoelectric harvesters. A piezoelectric (PZT) layer can be integrated onto silicon substrate in three forms: thin films with a thickness of less than 3 pm; thick films with a thickness ranging from several microns to 100 pm and bonded bulk piezoelectric sheets [see reference 12].

1. Piezoelectric thin films on silicon

High-quality PZT thin films can be directly grown on silicon using several techniques such as sol-gel, sputtering, metalorganic chemical vapor deposition (MOCVD) and pulse laser deposition (PLD) [see reference 12-14]. To improve the properties of the PZT thin films, the crystallization, orientation, and the interface between the film and the electrode of these films need to be optimized. Recently, the growths of epitaxial PZT (epi-PZT) thin films on silicon substrates are receiving a lot of interest [see reference 15] since epitaxial films in general exhibit properties, including piezoelectric coefficient, polarization, and dielectric constant superior to polycrystalline films which are promising for the realization of high performance MEMS devices.

2. Piezoelectric thick films on silicon

PZT thick films with a thickness in the range of 10-100pm have shown a larger piezoelectricity compared to deposited thin film which are suitable for micro-sensors and actuators. However, thick PZT films are difficult to directly produce on silicon substrate using conventional thin film processing. Recently, the sol-gel composite method developed [see reference 6] can produce PZT films up to 100μιη thickness but film's density is low due to insufficient sintering leading to reduce dielectric constant and piezoelectric coefficient. Screen-printing is another approach to produce the PZT film thicker than Ι Ομππ [see reference 17]. However, the screen printed thick PZT films are not well crystallized because the sintering must be done at temperature less than 950°C to avoid a reaction between the PZT and Si [see reference 16]. Consequently, the film cannot shrink densely and their piezoelectricity is lower compared to bulk PZT ceramic.

3. Bonding of bulk PZT on silicon

Bulk PZT ceramics provide greater electromechanical coupling, structural strength, charge capacity, piezoelectricity and dielectric constant than deposited piezoelectric thin films (a few μιη), which are attractive in many MEMS applications including high force actuators, harsh environment sensors and micro power generators [see reference 18]. A key to enabling the fabrication of microdevices using bulk PZT ceramics is the bonding of the PZT to Si substrates. The thickness of the intermediate adhesion layer should be less than several microns in order to obtain a good mechanical property. Moreover, the bonding temperature needs to be as low as possible to reduce the residual stress due to the difference in thermal expansion between PZT and Si [see reference 14]. Previous studies for integration of bulk PZT in MEMS focused on bonding by adhesives, conducting glues, Epoxy resin, Cytop, Au or AuSn and InSn [see reference 9, 18]. Recently, a CMOS compatible piezoelectric power generator based on the integration of bulk PZT films on silicon has been demonstrated [see reference 3]. The technology offers advantages of integration of bulk PZT in silicon by using Auln solder-bonding and thinning of bulk PZT pieces on Si. By conserving the bulk piezoelectric properties of the PZT material, high generator output voltage and high output impedance are obtained.

Fabrication technique

Piezoelectric materials (bulk or films) could be directly integrated on several substrates before or after its structuring depending on fabrication process, processing temperature and complexity of the structure.

Applications

One of the potential applications is to integrate an impact-type piezoelectric energy harvesting array with a commercially available mechanical watch mechanism consisting of an oscillating mass and a mutli-toothed gear wheel (see Figure 16). Motion causes the heavy mass 30 to move which in turn rotates the gear wheel 1 which plucks the piezoelectric cantilevers 2 creating strain in the piezoelectric layer and generating an electrical signal. In this way, mechanical energy recovered from the oscillating mass will be converted into useable electrical energy by the piezoelectric harvester which will act as an autonomous, alternative and especially compact micro-power source in watches but also for the other systems. The embodiment illustrated in figure 16 as an example, corresponds to the one described in relation to figure 12A and reference is made to its description above. Of course other configurations are possible in this application of the present invention.

Other applications than watches are possible with the principle of the invention: for example the system could be a module with an inertial mass, a gear, a harvester that could be implemented in different systems and objects, wearable on the body of a user, on machine tools, in different moving objects etc, not moving in resonance, the movement of the system/object generating energy harvesting in accordance with the principle of the present invention. The examples and embodiments, sizes and materials disclosed in the present application are of course only for illustrative purposes and should not be construed in a limiting fashion. Other variants using equivalent means are of course possible as well without imparting from the spirit a scope of the present invention. In addition, different embodiments disclosed herein may be combined together if desired.

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Claims

1 . A device for energy harvesting from an impact by using a piezoelectric MEMS scavenger comprising at least a gear (1 ) with teeth (4) driven by an inertial mass system and a piezoelectric transducer (2) located next to the gear, wherein the piezoelectric transducer is coupled to the inertial mass system through a tip (3) so that when the gear starts moving, the piezoelectric transducer (2) is set into motion by the impact between the gear teeth (4) and the tip (3) of the transducer (2).

2. The device as defined in claim 1 , wherein the transducer is a cantilever structure.

3. The device as defined in the preceding claim wherein the cantilever structure (2) is placed on a top side or a bottom side of the gear (1 ).

4. The device as defined in one of the preceding claims 2 or 3, wherein the cantilever (2) is a single element.

5. The device as defined in one of claims 2 to 4, wherein the cantilever (2) has several arms (2-1 , 2-2, 2-3, 2-4, 2-5), each arm having a tip (3-1 to 3-5).

6. The device as defined in claim 5, wherein the cantilever has five arms.

7. The device as defined in claim 1 wherein the transducer is a piezoelectric membrane (25, 26) with at least one tip (25-1 , 26-1 ).

8. The device as defined in claim 7, wherein the membrane comprises a plurality of tips (25-1 to 25-3; 26-1 to 26-5).

9. The device as defined in claim 7 or 8, wherein the membrane is circular.

10. The device as defined in one of the preceding claims, wherein the

piezoelectric transducer comprises a silicon substrate with a silicon nitride film and a PZT layer covered with a nickel top (12) and bottom layer (13) each forming an electrode layer.

1 1 . A method for fabricating the piezoelectric transducer as defined in one of the preceding claims, comprising at least the following steps:

-) provision of a 100 mm in diameter and 390 μιη-thick silicon substrate;

-) patterning of the shape of rectangular cantilevers (8) from the top side by DRIE, -) a 150 nm-thick LPCVD silicon nitride film (9) is then deposited on both sides for electrical isolation and used as mask during a KOH etching of silicon;

-) backside openings (10) in the nitride are defined using UV lithography followed by RIE etching;

-) the exposed silicon area is wet etched in a KOH solution to accomplish the silicon AFM like cantilever resulting in a silicon tip (1 1 ) with a height of 240 pm;

-) the bonding of 135 pm-thick PZT layer 5 covered with nickel top (12) and bottom (13) electrodes is performed using spin coated UV curable epoxy (14).

12. An object comprising at least a device as defined in one of claims 1 to 10.

13. An object as defined in claim12, wherein said object is a watch.