Patent Description:
In particular, hereinafter reference will be made, without this implying any loss of generality, to a microelectromechanical mirror device, in which the tiltable structure carries a suitable reflecting surface.

In a known manner, microelectromechanical mirror devices are used in portable apparatuses, such as smartphones, tablets, notebooks, and PDAs, for optical applications, in particular in order to direct, with desired patterns, beams of light radiation generated by a light source, for projecting images at a distance. Thanks to the reduced size, these devices allow to meet stringent requisites as regards occupation of space, in terms of area and thickness.

For instance, microelectromechanical mirror devices are used in miniaturised projector apparatuses (the so-called pico-projectors), which are able to project images at a distance and generate desired light patterns.

Microelectromechanical mirror devices generally include a mirror structure elastically supported above a cavity and obtained starting from a body of semiconductor material so as to be movable, for example with a tilting or rotation movement out of a corresponding plane of main extension, in order to direct the incident light beam in a desired manner.

Typically, a deviation of the light beam along two axes is required, which can be obtained by two microelectromechanical mirror devices of a uniaxial type, or else by a single microelectromechanical mirror device of a biaxial type.

<FIG> is a schematic illustration of a pico-projector <NUM> comprising a light source <NUM>, typically a laser source, which generates a light beam that is deflected by a mirror arrangement <NUM> towards a screen <NUM>.

In the example illustrated schematically in the aforesaid <FIG>, the mirror arrangement <NUM> comprises: a first mirror device 3a, of a uniaxial type, driven so as to rotate about a first axis (defined as "vertical axis A") with a resonant movement, to generate a fast horizontal scan; and a second mirror device 3b, which is also of a uniaxial type, driven so as to rotate about a second axis (defined as "horizontal axis B") with a linear or quasi-static movement (i.e., at a frequency much lower than the frequency of the resonant movement), for generating a slow vertical scan, for example with a sawtooth profile.

The first and the second mirror devices 3a, 3b cooperate for generating a scanning pattern on the screen <NUM>, which is illustrated schematically and designated by <NUM> in <FIG>. In particular, the first mirror device 3a, rotating about the vertical axis A, "draws" a horizontal line on the second mirror device 3b; and the second mirror device 3b, rotating about the horizontal axis B, directs the projected image onto a desired rectangular surface on the screen <NUM>.

Alternatively, as illustrated schematically in <FIG>, the mirror arrangement <NUM> of the pico-projector <NUM> may comprise a single mirror device, designated by 3c, of a two-dimensional type, i.e., controlled so as to rotate both about the vertical axis A with a resonant movement, and about the horizontal axis B with a linear movement.

In any case, rotation of the mirror device is driven via an actuation system that may be of an electrostatic, electromagnetic, or piezoelectric type.

Electrostatic actuation systems in general require high operating voltages, whereas electromagnetic actuation systems in general entail a high-power consumption.

It has hence been proposed to control the scanning movement, in particular at least the quasi-static linear movement about the horizontal axis B, in a piezoelectric way.

For instance, in the mirror device described in <CIT>, a suspended frame is connected to a fixed structure via spring elements having a serpentine shape constituted by a plurality of mutually parallel arms arranged alongside one another. Each arm carries a piezoelectric band, and adjacent piezoelectric bands are biased with voltages of opposite polarity. Given the characteristics of piezoelectric materials, this biasing causes deformation in the opposite direction (upwards and downwards) of adjacent arms and consequent rotation of the suspended frame in a first direction about the horizontal axis B. By applying an opposite biasing, rotation of the frame is obtained in a second direction, opposite to the first. The vertical scan can hence be obtained by applying alternating bipolar voltages to the arms.

An actuation system of a similar type may drive rotation about the vertical axis A, so as to also control the horizontal scan.

Another mirror device with piezoelectric actuation is described in <CIT>, in the name of the present Applicant. This mirror device has: a tiltable structure, which rotates about the horizontal axis B; a fixed structure; and an actuation structure of a piezoelectric type, coupled between the tiltable structure and the fixed structure. The actuation structure is formed by spring elements having a spiral shape. The spring elements are each formed by a plurality of driving arms, which extend in a direction transverse to the horizontal axis B, each driving arm bearing a respective piezoelectric band made of piezoelectric material. The driving arms are divided into two sets driven in phase opposition to obtain rotation of the tiltable structure in opposite directions about the horizontal axis B.

<FIG> are schematic illustrations of a portion of a mirror device, designated by <NUM>, according to the teachings of the aforesaid document <CIT>. By way of example, only a first driving arm 11a and a second driving arm 11b that belong to the aforesaid two sets driven in phase opposition are illustrated, and moreover highlighted is a biasing voltage V applied to just one of the aforesaid driving arms (in the example, the first driving arm 11a).

The aforesaid first driving arm 11a has a first end connected to the second driving arm 11b and a second end connected to the tiltable structure <NUM>, which carries a mirror surface <NUM>, at a corresponding end or edge portion thereof.

As illustrated in <FIG>, application of the biasing voltage V causes bending out of the horizontal plane (along an orthogonal axis z) of the first driving arm 11a and in particular of the second end connected to the tiltable structure <NUM>. Consequently, also the same tiltable structure <NUM> undergoes a corresponding out-of-plane displacement.

Given that the extent of the out-of-plane displacement of the tiltable structure <NUM> is substantially equal to the overall bending of the driving arms, it is evident why the spring elements have a shape folded to form a spiral, with a plurality of driving arms, thus enabling at the same time maximisation of the extent of the aforesaid displacement.

Mirror devices with piezoelectric actuation have the advantage of requiring actuation voltages and power-consumption levels reduced as compared to devices with electrostatic or electromagnetic actuation.

However, the present Applicant has realized that known solutions for mirror devices with piezoelectric actuation generally have a high sensitivity to spurious out-of-plane movements (along the orthogonal axis z). The driving arms, on which the piezoelectric bands are provided, are in fact thin and have a great length (as mentioned previously, in order to achieve high values of displacement), thus causing the presence of multiple spurious modes even at low frequency (i.e., at frequencies close to the frequency of the driving movement, for example around <NUM>).

Moreover, once again on account of the length of the driving arms (arranged in folded configuration), the structure proves particularly subject to impact and shock along the axis z.

<CIT> discloses an optical scanner provided with a reflecting mirror, movable beams, driving sections, a fixed section, and extending sections. The reflecting mirror is provided with a reflecting surface, which can swing about a swinging axis line, reflecting light and performing scanning. The movable beams are composed of first beam portions and second beam portions. The extending sections respectively extend in the direction perpendicular to the swinging axis line, i.e., in the X axis direction, in a plane parallel to the reflecting surface from both sides of a beam connecting portion where the first beam portions and the second beam portions are connected to each other.

<CIT> discloses a piezoelectric non-uniform folded beam-actuated MOEMS scanning raster micromirror. The MOEMS scanning raster micromirror is composed of a raster, a micromirror, a torsion beam, piezoelectric non-uniform folded beam micro-actuators, angle sensors and a fixing frame. The micromirror is suspended on the fixing frame through the torsion beam, so that the micromirror can rotate around the torsion beam, but is limited to perform translational motion in other directions.

The aim of the present solution is to provide a microelectromechanical device with actuation of a piezoelectric type that will enable the drawbacks of the prior art to be overcome.

According to the present solution, a microelectromechanical device is provided, as defined in the appended claims.

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

<FIG> is a schematic illustration of a microelectromechanical device <NUM>, in particular a mirror device based on MEMS technology, according to a first embodiment of the present solution.

The microelectromechanical device <NUM> is formed in a die of semiconductor material, in particular silicon, and is provided with a tiltable structure <NUM>, having a main extension in a horizontal plane xy and arranged so as to rotate about a first rotation axis, parallel to a first horizontal axis x of the aforesaid horizontal plane xy (for example, the first rotation axis corresponds to the horizontal rotation axis B of a pico-projector apparatus - see <FIG>).

The aforesaid first rotation axis represents a first median axis of symmetry X for the microelectromechanical device <NUM>. A second median axis of symmetry Y for the same microelectromechanical device <NUM> is parallel to a second horizontal axis y, orthogonal to the first horizontal axis x and defining, with the first horizontal axis x, the horizontal plane xy.

The tiltable structure <NUM> is suspended above a cavity <NUM>, provided in the die, and has, in the embodiment illustrated, a generically elliptical shape in the horizontal plane xy, with major axis arranged along the second median axis of symmetry Y. The tiltable structure <NUM> defines a supporting structure that carries, at the top, a reflecting surface <NUM>' so as to define a mirror structure.

The tiltable structure <NUM> is elastically coupled to a fixed structure <NUM>, defined in the same die. In particular, the fixed structure <NUM> forms, in the horizontal plane xy, a frame <NUM>' that delimits and surrounds the aforesaid cavity <NUM> and moreover has a first supporting element 25a and a second supporting element 25b, which extend longitudinally along the first median axis of symmetry X within the cavity <NUM> starting from the same frame <NUM>', on opposite sides of the tiltable structure <NUM>.

The tiltable structure <NUM> is supported by the first and the second supporting elements 25a, 25b, to which it is elastically coupled, respectively, by means of a first elastic suspension element 26a and a second elastic suspension element 26b, which have a high stiffness in regard to movements out of the horizontal plane xy (along an orthogonal axis z, transverse to the horizontal plane xy) and are compliant in regard to torsion about the first horizontal axis x. The first and the second elastic suspension elements 26a, 26b hence extend along the first median axis of symmetry X, between the first, respectively second, supporting element 25a, 25b and a facing side of the tiltable structure <NUM>, to which they are coupled at a corresponding central portion.

In the embodiment illustrated, the first and the second elastic suspension elements 26a, 26b are of a folded type, i.e., they are formed by a plurality of arms having a longitudinal extension parallel to the first horizontal axis x, connected two by two at the ends by connecting elements (having an extension parallel to the second horizontal axis y).

Advantageously, the first and the second elastic suspension elements 26a, 26b couple the tiltable structure <NUM> to the fixed structure <NUM>, enabling rotation thereof about the first rotation axis and providing a high stiffness in regard to movements out of the plane, hence guaranteeing a high ratio between the frequencies of spurious movements out of the horizontal plane xy and the frequency of rotation about the first rotation axis.

The microelectromechanical device <NUM> further comprises an actuation structure <NUM>, coupled to the tiltable structure <NUM> and configured to cause rotation thereof about the first rotation axis. The actuation structure <NUM> is interposed between the tiltable structure <NUM> and the fixed structure <NUM> and moreover contributes to supporting the tiltable structure <NUM> above the cavity <NUM>.

The actuation structure <NUM> comprises at least one first pair of driving arms formed by a first driving arm 32a and a second driving arm 32b, which are arranged on opposite sides of, and symmetrically with respect to, the first median axis of symmetry X and first supporting element 25a, and here have a longitudinal extension parallel to the first horizontal axis x and to the aforesaid first supporting element 25a.

In the embodiment illustrated in <FIG>, the driving arms 32a, 32b have a generically trapezoidal (or fin-like) shape, with major side directed parallel to the second horizontal axis y fixedly coupled to the frame <NUM>' of the fixed structure <NUM> and minor side directed parallel to the same second horizontal axis y elastically coupled to the tiltable structure <NUM>.

Each driving arm 32a, 32b is suspended above the cavity <NUM> and carries, on a top surface thereof (opposite to the same cavity <NUM>), a respective first region made of piezoelectric material <NUM> (e.g., PZT - lead zirconate titanate), having substantially the same extension in the horizontal plane xy as the driving arm 32a, 32b. Each driving arm 32a, 32b moreover has a respective first end, fixedly coupled to the frame <NUM>' of the fixed structure <NUM>, and a respective second end, elastically coupled to the tiltable structure <NUM>, by means of a first and a second decoupling elastic element 34a, 34b, respectively.

In the example illustrated, the frame <NUM>' has a substantially rectangular shape in the horizontal plane xy, and the first end of the driving arms 32a, 32b is fixedly coupled to the sides of the same frame <NUM>' having an extension parallel to the second horizontal axis y (in a direction transverse to the first rotation axis of the tiltable structure <NUM>).

The aforesaid first and second decoupling elastic elements 34a, 34b have a high stiffness in regard to movements out of the horizontal plane xy (along the orthogonal axis z) and are compliant in regard to torsion (about a rotation axis parallel to the first horizontal axis x). The first and the second decoupling elastic elements 34a, 34b hence extend parallel to the first horizontal axis x, between the first and the second driving arms 32a, 32b, respectively, and a same side facing the tiltable structure <NUM>.

In particular, as is illustrated in the aforesaid <FIG>, the first and second decoupling elastic elements 34a, 34b are coupled to the tiltable structure <NUM> at a respective coupling point Pa, Pb, which is located in the proximity of the first median axis of symmetry X, at a short distance d from the same first median axis of symmetry X. For instance, this distance d may range between <NUM> and <NUM> in a typical embodiment and may moreover be in general between <NUM>/<NUM> and <NUM>/<NUM> of the main dimension (in the example, along the second median axis of symmetry Y) of the tiltable structure <NUM>.

In any case, the distance between the respective coupling point Pa, Pb and the first median axis of symmetry X is preferably smaller, in particular much smaller, than the distance between the same coupling point Pa, Pb and end or edge portions (considered along the second median axis of symmetry Y) of the tiltable structure <NUM>. In fact, the closer these coupling points Pa, Pb are to the first rotation axis, the greater the ratio between the vertical displacement of the end of the tiltable structure <NUM> and the vertical displacement of the driving arms 32a, 32b, caused by the piezoelectric effect (as it is discussed in detail hereinafter).

In the embodiment illustrated in <FIG>, the first and the second decoupling elastic elements 34a, 34b are also of a folded type; i.e., they are formed by a plurality of arms having a longitudinal extension parallel to the first horizontal axis x, connected two by two by connecting elements (having an extension parallel to the second horizontal axis y).

In the embodiment illustrated in <FIG>, the aforesaid actuation structure <NUM> further comprises a second pair of driving arms formed by a third driving arm 32c and a fourth driving arm 32d, which are arranged on opposite sides of the first median axis of symmetry X and, this time, of the second supporting element 25b, and have a longitudinal extension parallel to the first horizontal axis x and to the aforesaid second supporting element 25b (it should be noted that the second pair of driving arms 32c, 32d is hence arranged symmetrically to the first pair of driving arms 32a, 32b with respect to the second median axis of symmetry Y).

As it has been discussed for the first pair of driving arms 32a, 32b, each driving arm 32c, 32d of the second pair carries, on a top surface thereof, a respective first region made of piezoelectric material <NUM> (e.g., PZT - lead zirconate titanate) and has a respective first end fixedly coupled to the frame <NUM>' of the fixed structure <NUM> and a respective second end elastically coupled to the tiltable structure <NUM>, by means of a third and a fourth decoupling elastic element 34c, 34d, respectively (which are arranged on the opposite side of the first and the second decoupling elastic elements 32a, 32b with respect to the second median axis of symmetry Y).

The aforesaid third and fourth decoupling elastic elements 34c, 34d also have a high stiffness in regard to movements out of the horizontal plane xy (along the orthogonal axis z) and are compliant in regard to torsion (about a rotation axis parallel to the first horizontal axis x).

In particular, as is illustrated in the aforesaid <FIG>, also the third and fourth decoupling elastic elements 34c, 34d are hence coupled to the tiltable structure <NUM> at a respective coupling point Pc, Pd, which is located in the proximity of the first rotation axis, at the short distance d from the same first rotation axis. Moreover, also the third and fourth decoupling elastic element 34c, 34d are of a folded type.

As illustrated schematically in the aforesaid <FIG>, the microelectromechanical device <NUM> further comprises a plurality of electrical-contact pads <NUM>, which are carried by the fixed structure <NUM> at the frame <NUM>' and are electrically connected (in a way not illustrated in detail in <FIG>) to the first regions of piezoelectric material <NUM> of the driving arms 32a-32d, to enable electrical biasing thereof by electrical signals coming from outside of the electromechanical device <NUM> (for example, supplied by a biasing device of an electronic apparatus in which the electromechanical device <NUM> is integrated).

Moreover, the microelectromechanical device <NUM> comprises a piezoresistive (PZR) sensor <NUM>, appropriately arranged so as to provide a detection signal associated with rotation of the tiltable structure <NUM> about the first rotation axis. This detection signal can be provided as a feedback to the outside of the microelectromechanical device <NUM>, for example to the aforesaid biasing device, through at least one of the electrical-contact pads <NUM>.

In the embodiment illustrated in <FIG>, the piezoresistive sensor <NUM> is provided (for example, by surface diffusion of dopant atoms) on the first supporting element 25a (different arrangements may, however, be envisaged for the same piezoresistive sensor <NUM>).

Advantageously, the elastic suspension elements 26a, 26b are able to transmit the stresses to the supporting elements 25a, 25b and hence towards the piezoresistive sensor <NUM>, enabling arrangement of the latter on the supporting elements 25a, 25b and a consequent simplification of routing of the electrical connections to the electrical-contact pads <NUM>.

During operation of the microelectromechanical device <NUM>, application of a biasing voltage V to the first region of piezoelectric material <NUM> of the first driving arm 32a (having a positive value with respect to the biasing of the first region of piezoelectric material <NUM> of the second driving arm 32b, which may, for example, be connected to a ground reference potential), causes a rotation of a positive angle about the first rotation axis (parallel to the first horizontal axis) x, as illustrated in <FIG>.

In a corresponding manner, application of a biasing voltage V to the first region of piezoelectric material <NUM> of the second driving arm 32b (having a positive value with respect to the biasing of the first region of piezoelectric material <NUM> of the first driving arm 32a, which may, for example, in this case be connected to a ground reference potential), causes a corresponding rotation of a negative angle about the same first rotation axis, as illustrated in <FIG>.

It should be noted that the same biasing voltage V is applied to the first region of piezoelectric material <NUM> both of the first driving arm 32a and of the third driving arm 32c, and, likewise, in order to cause rotation in the opposite direction, to the first region of piezoelectric material <NUM>, of both the second driving arm 32b and the fourth driving arm 32d, so as to contribute in a corresponding manner to the rotation of the tiltable structure <NUM> about the first rotation axis (as on the other hand emerges clearly from the foregoing description).

Advantageously, the decoupling elastic elements 34a-34d elastically decouple displacement by the piezoelectric effect of the driving arms 32a-32d along the orthogonal axis z from the consequent rotation of the tiltable structure <NUM> along the first rotation axis.

In particular, by virtue of the proximity of the coupling points Pa-Pd between the decoupling elastic elements 34a-34d and the tiltable structure <NUM> to the rotation axis, a wide angle of rotation of the tiltable structure <NUM> about the first rotation axis corresponds to a small displacement out of the horizontal plane xy of the aforesaid driving arms 32a-32d (as on the other hand is highlighted in the aforesaid <FIG> and <FIG>), or, likewise, a large displacement out of the horizontal plane xy of the end portions (considered along the second horizontal axis y) of the same tiltable structure <NUM>. For example, the ratio between the extent of these displacements may be equal to five in a possible embodiment.

The tiltable structure <NUM> can reach wide tilting angles (for example, greater than <NUM>°) with a low value of the biasing voltage V (for example, lower than <NUM> V).

Moreover, the maximum amount of stress occurs in the elastic suspension elements 26a, 26b that couple the tiltable structure <NUM> to the fixed structure <NUM>; the present Applicant has found that stresses of this amount can be withstood without any problem and fall within the design requirements.

Tests and simulations carried out by the present Applicant have shown that the microelectromechanical device <NUM> shows improved mechanical and electrical characteristics as compared to known devices.

In particular, the first spurious mode, due to the movement out of the plane of the tiltable structure <NUM>, in this case has a frequency that is much higher than the frequency of the main mode, represented by rotation about the first rotation axis (for example, the ratio between the two frequencies is higher than four), unlike known solutions in which these frequencies have comparable values.

Moreover, also by virtue of the small displacement in the direction of the orthogonal axis z of the driving arms 32a-32d (this displacement being smaller even by a factor of ten as compared to traditional solutions), the microelectromechanical device <NUM> is less subject to shock acting along the orthogonal axis z (in other words, a same shock causes a level of stress and a displacement out of the horizontal plane xy much smaller in the present microelectromechanical device <NUM> than in known solutions).

A possible embodiment of the piezoresistive sensor <NUM> is now described in greater detail, configured to detect the rotation of the tiltable structure <NUM> about the first rotation axis.

The present Applicant has realized that the stress transferred by the elastic suspension elements 26a, 26b to the supporting elements 25a, 25b can be limited by the elastic characteristics of the same elastic suspension elements 26a, 26b, which in fact are generally thin and compliant to ensure the desired range of movement of the tiltable structure <NUM>.

In a particular embodiment, shown in <FIG>, an additional mechanical amplification structure, denoted with <NUM>, is therefore provided, configured to maximize the sensitivity of the piezoresistive sensor <NUM>.

This mechanical amplification structure <NUM> comprises a first lever mechanism <NUM>, coupled, in the illustrated example, between the first supporting element 25a and the first driving arm 32a.

In detail, the first lever mechanism <NUM> comprises a lever arm <NUM> having a longitudinal extension (along the first horizontal axis x) and a first end coupled to the first driving arm 32a and a second end coupled, by means of a torsional elastic element <NUM> (in the example having extension along the second horizontal axis y), to a coupling element <NUM>, fixed and integral with the first supporting element 25a (of which constitutes a protrusion towards the aforesaid first driving arm 32a).

A first pair of diffused piezoresistors <NUM> of the aforementioned piezoresistive sensor <NUM> are formed in this coupling element <NUM>.

During operation, and as shown schematically in <FIG>, the movement of the first driving arm 32a (due to biasing of the respective first region of piezoelectric material <NUM>) determines a corresponding movement out of the horizontal plane xy (in the example an upwards movement) of the lever arm <NUM>, in particular of the corresponding first end (at which a force F is generated). A torsion of the torsional elastic element <NUM> is consequently generated, in the example clockwise around the same torsional elastic element <NUM>.

The diffused piezoresistors <NUM> of the first pair are arranged so as to detect the stress resulting from the aforementioned rotation of the torsional elastic element <NUM>, which is therefore indicative of the rotation of the tiltable structure <NUM>. In particular, the presence of the first lever mechanism <NUM> advantageously allows to amplify the stress detected by the diffused piezoresistors <NUM>.

As shown in the same <FIG>, advantageously, the mechanical amplification structure <NUM> further comprises a second lever mechanism <NUM>, coupled between the same first supporting element 25a and, this time, the second driving arm 32b, thus being arranged in a symmetrical manner to the first lever mechanism <NUM> with respect to the first horizontal axis x.

In a similar manner, the second lever mechanism <NUM> comprises a respective lever arm <NUM> having a longitudinal extension (along the first horizontal axis x) and a respective first end coupled to the second driving arm 32b and a respective second end coupled, by means of a respective torsional elastic element <NUM> (in the example having an extension along the second horizontal axis y), to a respective coupling element <NUM>, fixed and integral with the first supporting element 25a (of which constitutes a protrusion towards the aforesaid second driving arm 32b); the coupling elements <NUM>, <NUM> are therefore symmetrical with respect to the first horizontal axis x.

A second pair of diffused piezoresistors <NUM> of the piezoresistive sensor <NUM> are formed at the aforesaid respective coupling element <NUM>.

During operation, and as shown schematically in <FIG>, movement of the second driving arm 32b (due to biasing of the respective region of piezoelectric material <NUM>) determines a corresponding movement outside the horizontal plane xy (in the example a downwards movement) of the lever arm <NUM>, in particular of the corresponding first end (at which a respective force F' is generated). A torsion of the torsional elastic element <NUM> of the second lever mechanism <NUM> is consequently generated, in the example counterclockwise around the same torsional elastic element <NUM>.

The diffused piezoresistors <NUM> of the second pair are arranged so as to detect the stress resulting from the aforementioned rotation of the torsional elastic element <NUM>.

In particular, as shown in the equivalent electric diagram of <FIG>, the diffused piezoresistors <NUM> of the first pair may constitute a first half of a detection Wheatstone bridge, denoted as <NUM>, of the piezoresistive sensor <NUM>, while the diffused piezoresistors <NUM> of the second pair may in this case constitute the second half of the same detection Wheatstone bridge <NUM>.

As shown schematically also in <FIG>, suitable conductive paths <NUM> are formed in the first supporting element 25a (for example being constituted by surface metal traces), so as to connect the diffused piezoresistors <NUM>, <NUM> of the first and second pairs together according to the Wheatstone bridge connection scheme, and also so as to connect the same diffused piezoresistors <NUM>, <NUM> to associated electrical-contact pads <NUM> (here not shown).

In particular, a common end of the diffused piezoresistors <NUM> of the first pair is electrically connected to a first output (positive, in the example) of the Wheatstone bridge (denoted with 'Out+' in the aforementioned <FIG>), while the ends not in common are connected electrically to a first and a second biasing voltage (for example a positive voltage 'Bias+' and a negative voltage 'Bias-').

Similarly, a common end of the diffused piezoresistors <NUM> of the second pair is electrically connected to a second output (negative in the example) of the Wheatstone bridge (denoted with 'Out-'), while the ends not in common are electrically connected to the same first and second biasing voltages (the positive voltage 'Bias+' and the negative voltage 'Bias-').

Advantageously, the presence of the two fully symmetrical halves of the detection Wheatstone bridge <NUM> allows to detect the opposite displacements of the first and second lever mechanisms <NUM>, <NUM>, thereby maximizing and making symmetrical the detection and the corresponding detection signal provided at the output (outside of the microelectromechanical device <NUM>).

Electrical-contact pad <NUM> (here not shown) are suitably coupled to the aforesaid conductive paths <NUM>, which provide the routing of the electrical connections to the same electrical-contact pads <NUM>.

This embodiment advantageously allows to maximize the detection sensitivity of the piezoresistive sensor <NUM>, thus ensuring a more effective control of the electromechanical device <NUM> (for example by the electronic apparatus in which the same electromechanical device <NUM> is integrated).

With reference to <FIG>, a cross-section view provided by way of example of a possible implementation of the microelectromechanical device <NUM> is now illustrated, in this case obtained starting from a SOI (Silicon-On-Insulator) wafer <NUM> made of semiconductor material, in particular silicon.

In a way that will be clear to a person skilled in the art, the tiltable structure <NUM>, the fixed structure <NUM>, the elastic elements <NUM>-34d, and the driving arms 32a-32d are defined by chemical etching in an active layer 40a of the SOI wafer <NUM> (e.g., having a thickness of <NUM>). The cavity <NUM> is formed, once again by chemical etching, in a rear layer 40b of the SOI wafer <NUM> (e.g., having a thickness of <NUM>) and in a dielectric layer 40c of the same SOI wafer <NUM>.

It should be noted that underneath the tiltable structure <NUM>, following upon etching for the formation of the cavity <NUM>, there remain reinforcement elements <NUM>, having an extension along the orthogonal axis z and operating as mechanical reinforcement.

Formed on a top surface <NUM>' of the active layer 40a of the SOI wafer <NUM> are: the reflecting surface <NUM>', on the mobile structure <NUM>, made of an appropriate material (e.g., aluminium, or else gold, according to whether projection is in the visible or in the infrared); and moreover, bottom electrode regions <NUM>, made of an appropriate conductive material, on the driving arms 32a-32d.

The first regions of piezoelectric material <NUM> (constituted by a thin film of PZT) are formed on the aforesaid bottom electrode regions <NUM>, and top electrode regions <NUM> are formed on the first regions of piezoelectric material <NUM>.

A passivation layer <NUM>, made of an appropriate dielectric material, is formed, as a cover, on the active layer 40a of the SOI wafer, and contact openings <NUM> are opened through the same passivation layer <NUM> so as to enable access to the aforesaid bottom electrode regions <NUM> and top electrode regions <NUM>.

Metal routing regions <NUM> are then formed on the passivation layer <NUM> so as to contact, through the contact openings <NUM>, the bottom and top electrode regions <NUM>, <NUM>, moreover extending as far as respective electrical-contact pads <NUM> (here not illustrated).

Through the above passivation layer <NUM> a further contact opening <NUM>' is moreover formed, in order to reach a diffused region <NUM>, arranged at the front surface <NUM>' of the active layer 40a of the wafer <NUM>, which defines the PZR sensor <NUM>. A further metal routing region <NUM>' is formed on the passivation layer <NUM>, so as to contact, through the further contact opening <NUM>', the PZR sensor <NUM>, moreover extending as far as a respective electrical-contact pad <NUM>, which is also not illustrated herein.

As shown in <FIG>, a supporting wafer (the so-called "handling wafer") <NUM> is coupled underneath the SOI wafer <NUM> and has, underneath the cavity <NUM> and in the mobile structure <NUM>, a recess <NUM>', to enable rotation of the mobile structure <NUM>.

With reference to <FIG> a biaxial embodiment of the microelectromechanical device <NUM> is now described, where the tiltable structure <NUM> is able to perform rotation movements both about the first rotation axis (coinciding with the first median axis of symmetry X parallel to the first horizontal axis x) and about a second rotation axis (coinciding with the second median axis of symmetry Y parallel to the second horizontal axis y; the second rotation axis corresponds, for example, to the vertical rotation axis A of a pico-projector apparatus; see, for example, <FIG>).

In this case, the tiltable structure <NUM> comprises an inner frame <NUM>, having, for example, a substantially rectangular shape in the horizontal plane xy and internally defining a window <NUM>.

The inner frame <NUM> is elastically coupled to the driving arms 32a-32d of the actuation structure <NUM> in a way altogether equivalent to what has been discussed previously with reference to <FIG>. The inner frame <NUM> is hence driven in rotation about the first rotation axis by application of an appropriate biasing to the first regions of piezoelectric material <NUM>, as described previously in detail, which causes generation of a force F (as indicated in <FIG>).

The tiltable structure <NUM> further comprises in this case a distinct supporting element <NUM>, which is housed in the window <NUM> and is elastically coupled to the inner frame <NUM> by means of elastic elements <NUM>, compliant to torsion about the second rotation axis, and moreover carries at the top the reflecting surface <NUM>'. For instance, the supporting element <NUM> has a circular or elliptical shape in the horizontal plane xy.

During rotation about the first rotation axis, as shown in <FIG>, the supporting element <NUM> is fixedly coupled to the inner frame <NUM> so as to be driven in the same rotation, thus causing the desired movement of the reflecting surface <NUM>'.

The actuation structure <NUM> in this case comprises second regions of piezoelectric material <NUM>, designed to be appropriately biased for causing rotation of the supporting element <NUM> about the second rotation axis. The inner frame <NUM>, as illustrated in <FIG>, decouples the rotations about the first and the second rotation axes.

In the embodiment illustrated, the second regions of piezoelectric material <NUM> are also carried by the driving arms 32a-32d, at the respective ends coupled to the tiltable structure <NUM>, hence on the inside with respect to the first regions of piezoelectric material <NUM>.

In particular, as is highlighted in the aforesaid <FIG>, the second regions of piezoelectric material <NUM> carried by the driving arms 32a-32b, 32c-32d of the first or second pairs are in this case biased at a same biasing voltage V' (at an appropriate resonance frequency) so as to generate a force F' on the driving arms, which is transmitted through the inner frame <NUM> to generate resonant rotation of the supporting element <NUM> about the second rotation axis.

In a way not described in detail, further electrical-contact pads <NUM> are in this case present, which are, for example, once again arranged on the frame <NUM>' of the fixed structure <NUM>, for biasing the aforesaid second regions of piezoelectric material <NUM>.

With reference to <FIG> a further embodiment of the present solution is now described.

In this case, the actuation structure <NUM> of the microelectromechanical device <NUM> comprises just one pair of driving arms, designated once again by 32a, 32b.

These driving arms 32a, 32b are in this case generically C-shaped, with the major side of the "C" that extends longitudinally parallel to the first horizontal axis x and crosses the second median axis of symmetry Y, and the minor sides of the "C" that extend parallel to the second horizontal axis y, on opposite sides of the tiltable structure <NUM> with respect to the second median axis of symmetry Y.

The aforesaid driving arms 32a, 32b are arranged on opposite sides of the tiltable structure <NUM> with respect to the first median axis of symmetry X and jointly define an inner window <NUM>, arranged inside which is the tiltable structure <NUM>, which has for example in this case a generically rectangular shape in the horizontal plane xy, with a main extension parallel to the first horizontal axis x.

Again, each driving arm 32a, 32b is suspended over the cavity <NUM> and carries, on a top surface thereof (opposite to the cavity <NUM>) a respective first region of piezoelectric material <NUM> (e.g., PZT - lead zirconate titanate), having substantially the same extension in the horizontal plane xy as said driving arm 32a, 32b.

In this case, each driving arm 32a, 32b is moreover fixedly coupled to the frame <NUM>' of the fixed structure <NUM> along all of the long side of the "C" and is elastically connected to the tiltable structure <NUM> by means of a respective pair of elastic decoupling elements, at the minor sides of the "C": in particular, the first driving arm 32a is connected to the tiltable structure <NUM> by means of the first decoupling elastic element 34a and the third decoupling elastic element 34c, whereas the second driving arm 32b is connected to the same tiltable structure <NUM> by means of the second decoupling elastic element 34b and the fourth decoupling elastic element 34c.

In a way altogether similar to what has been discussed previously, also in this embodiment the aforesaid decoupling elastic elements 34a-34d are coupled to the tiltable structure <NUM> at respective coupling points Pa-Pd (here not illustrated), which are located in proximity of the first rotation axis (coinciding with the first median axis of symmetry X), at the short distance d from the first rotation axis.

For instance, also in this embodiment, the decoupling elastic elements 34a-34d are of a folded type (but could alternatively be of a linear type).

In the example illustrated in the aforesaid <FIG>, the first and the second elastic suspension elements 26a, 26b, which also in this case elastically connect the tiltable structure <NUM> to the fixed structure <NUM>, have a linear shape, instead of a folded shape, having in any case once again a high stiffness in regard to the movement out of the horizontal plane xy and being, instead, compliant in regard to torsion about the first horizontal axis x.

Altogether similar considerations apply as regards operation of the microelectromechanical device <NUM>. In particular, application of the biasing voltage V to the first region of piezoelectric material <NUM> carried by the first or second driving arms 32a, 32b causes rotation, in a positive direction (as illustrated in <FIG>) or a negative direction (as illustrated in <FIG>) of the tiltable structure <NUM> about the first rotation axis.

The embodiment described determines a different form factor of the microelectromechanical device <NUM>, which enables a smaller resulting size to be obtained in the case where the tiltable structure <NUM> has a main extension along the first rotation axis (as is common in the case of mirror devices that perform a raster scan).

Moreover, the above embodiment may enable a more convenient routing of the electrical signals towards the electrical-contact pads <NUM>, given the low number (two) of piezoelectric actuators.

With reference to <FIG>, yet a further embodiment of the present solution is now described, where the actuation structure <NUM> of the microelectromechanical device <NUM> once again comprises a first and a second pair of driving arms, designated also in this case by 32a-32b and 32c-32d.

Unlike what has been illustrated previously, the aforesaid driving arms 32a-32d have a main extension parallel to the second horizontal axis y, hence in a direction transverse to the first rotation axis of the tiltable structure <NUM>, and a much smaller extension in a direction parallel to the first horizontal axis x. In the example, the driving arms 32a-32d have a substantially rectangular shape in the horizontal plane xy.

Also in this case, each driving arm 32a-32d carries, on a top surface thereof (opposite to the cavity <NUM>) a respective first region of piezoelectric material <NUM>, and moreover has a respective first end fixedly coupled to the frame <NUM>' of the fixed structure <NUM> and a respective second end elastically coupled to the tiltable structure <NUM>, by means of a respective decoupling elastic element 34a-34d.

In this embodiment, in which once again the frame <NUM>' has a substantially rectangular shape in the horizontal plane xy, the first end of the driving arms 32a-32d is fixedly coupled to the sides of said frame <NUM>', which have an extension parallel to the first horizontal axis x (parallel to the first rotation axis of the tiltable structure <NUM>).

In the example illustrated in the aforesaid <FIG>, the first and the second elastic suspension elements 26a, 26b, which also in this case elastically couple the tiltable structure <NUM> to the fixed structure <NUM>, have a folded configuration, having, however, also in this case a high stiffness in regard to the movement out of the horizontal plane xy and being instead compliant in regard to torsion about the first horizontal axis x.

Altogether similar considerations apply as regards operation of the microelectromechanical device <NUM>. In particular, application of the biasing voltage V to the first regions of piezoelectric material <NUM> carried by the first and third driving arms 32a, 32c causes rotation in a positive direction (as illustrated in <FIG>, by way of example), whereas application of the biasing voltage V to the first regions of piezoelectric material <NUM> carried by the second and the fourth driving arms 32b, 32d causes a corresponding rotation in the negative direction of the tiltable structure <NUM> about the first rotation axis (in a way here not illustrated).

The above embodiment may advantageously be used to achieve driving of the tiltable structure <NUM> at low frequency and in resonance condition (instead of a linear or quasi-static driving). Moreover, this embodiment enables a reduction of the dimensions of the electromechanical device <NUM>.

The advantages of the present solution emerge clearly from the foregoing description.

In any case, it is underlined that the solution described, in its various embodiments, enables exploitation of the advantages of piezoelectric actuation (i.e., the use of low biasing voltages with a low energy consumption to obtain large displacements), at the same time presenting improved mechanical and electrical performance as compared to known solutions.

In fact, the microelectromechanical device <NUM> does not have spurious modes (of any type) at frequencies close to the actuation frequency. Moreover, the microelectromechanical device <NUM> is less sensitive to strains and stresses along the orthogonal axis z as compared to known solutions that adopt the same piezoelectric-actuation principle.

Moreover, as indicated above, the particular configuration of the piezoresistive sensor <NUM>, discussed with reference to <FIG> and <FIG> allows, considering the same other characteristics of the microelectromechanical structure, to maximize the detection sensitivity and, therefore, the controllability of the microelectromechanical device <NUM> (for example by an external electronic apparatus).

Advantageously, the microelectromechanical device <NUM> can be used in a pico-projector <NUM> designed to be functionally coupled to a portable electronic apparatus <NUM>, as illustrated schematically with reference to <FIG>.

In detail, the pico-projector <NUM> of <FIG> comprises: a light source <NUM>, for example of a laser type, adapted to generate a light beam <NUM>; the microelectronic device <NUM>, acting as a mirror and designed to receive the light beam <NUM> and direct it towards a screen or display surface <NUM> (external to, and set at a distance, from the pico-projector <NUM>); a first driving circuit <NUM>, designed to provide appropriate control signals to the light source <NUM>, for generation of the light beam <NUM> as a function of an image to be projected; a second driving circuit <NUM>, designed to provide driving signals to the actuation structure <NUM> of the microelectronic device <NUM>; and a communication interface <NUM>, designed to receive, from an external control unit <NUM>, for example included in the portable apparatus <NUM>, information on the image to be generated, for example in the form of a pixel array. This information is sent as an input for driving of the light source <NUM>.

The pico-projector <NUM> can be provided as a separate and stand-alone accessory with respect to an associated portable apparatus <NUM>, for example a smartphone, as illustrated in <FIG>. In this case, the pico-projector <NUM> is coupled to the portable electronic apparatus <NUM> by means of appropriate electrical and mechanical connection elements (not illustrated in detail). Here, the pico-projector <NUM> is provided with a casing <NUM> of its own, which has at least one portion <NUM>' transparent to the light beam <NUM> coming from the microelectronic device <NUM>. The casing <NUM> of the pico-projector <NUM> is coupled in a releasable manner to a respective case <NUM> of the portable electronic apparatus <NUM>.

Alternatively, as illustrated in <FIG>, the pico-projector <NUM> can be integrated within the portable electronic apparatus <NUM> and be set within the case <NUM> of the same portable electronic apparatus <NUM>. In this case, the portable electronic apparatus <NUM> has a respective portion <NUM>' transparent to the light beam <NUM> coming from the microelectronic device <NUM>. The pico-projector <NUM> is, in this case, for example, coupled to a printed circuit board within the case <NUM> of the portable electronic apparatus <NUM>.

Finally, it is clear that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the appended claims.

In particular, it is evident that further embodiments, of a biaxial type, of the microelectromechanical device <NUM> could be envisaged, based on the embodiments of <FIG> and <FIG>, in a way substantially similar to what has been described in detail with reference to <FIG>.

Moreover, it is evident that variants may in general be envisaged as regards the shape of the elements forming the microelectromechanical device <NUM>; for example, the tiltable structure <NUM> (and the corresponding reflecting surface <NUM>') may have different shapes.

Moreover, it is highlighted that, as mentioned previously, the elastic suspension elements 26a-26b and the decoupling elastic elements 34a-34d could be linear, instead of a folded type, having in any case a high stiffness in regard to movements out of the horizontal plane xy and being compliant in regard to the rotation.

Furthermore, in a completely corresponding manner to what discussed above, the piezoresistive sensor <NUM> and the associated mechanical amplification structure <NUM> could be formed at the second supporting element 25b, in this case the first and the second lever mechanism <NUM>, <NUM> being coupled between the same second supporting element 25b and the third, respectively the fourth, driving arms 32c, 32d.

Claim 1:
A microelectromechanical device (<NUM>), comprising:
a fixed structure (<NUM>) defining a cavity (<NUM>);
a tiltable structure (<NUM>) elastically suspended in the cavity (<NUM>) and having a main extension in a horizontal plane (xy) ;
a piezoelectrically driven actuation structure (<NUM>), biased for causing rotation of the tiltable structure (<NUM>) about at least a first rotation axis (X) parallel to a first horizontal axis (x) of said horizontal plane (xy), said actuation structure (<NUM>) being interposed between the tiltable structure (<NUM>) and the fixed structure (<NUM>),
wherein said actuation structure (<NUM>) comprising at least a first pair of driving arms (32a, 32b), which carry respective regions of piezoelectric material (<NUM>);
and said tiltable structure (<NUM>) is elastically coupled to said fixed structure (<NUM>) at said first rotation axis (X), by means of elastic suspension elements (26a, 26b), which have a high stiffness in regard to movements out of the horizontal plane (xy) and are compliant in regard to torsion about said first rotation axis (X), said elastic suspension elements (26a, 26b) extending along said first rotation axis (X), between a central portion of opposite sides of the tiltable structure (<NUM>) and said fixed structure (<NUM>),
characterized in that said first pair of driving arms (32a, 32b) are elastically coupled to the tiltable structure on opposite sides of the first rotation axis (X), by means of respective decoupling elastic elements (34a, 34b), having a high stiffness in regard to movements out of the horizontal plane (xy) and being compliant in regard to torsion about said first rotation axis (X).