Patent Publication Number: US-11656539-B2

Title: Microelectromechanical device with a structure tiltable by piezoelectric actuation having improved mechanical and electrical characteristics

Description:
PRIORITY CLAIM 
     This application claims the priority benefit of Italian Application for Patent No. 102018000011112, filed on Dec. 14, 2018, and claims the priority benefit of European Application for Patent No. 19165958.0, filed on Mar. 28, 2019, the contents of which are hereby incorporated by reference in their entirety to the maximum extent allowable by law. 
     TECHNICAL FIELD 
     The present solution relates to a microelectromechanical device, made with MEMS (Micro-Electro-Mechanical System) technology, with a structure tiltable by piezoelectric actuation, having improved mechanical and electrical characteristics. 
     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. 
     BACKGROUND 
     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 miniaturized 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.  1 A  is a schematic illustration of a pico-projector  1  comprising a light source  2 , typically a laser source, which generates a light beam that is deflected by a mirror arrangement  3  towards a screen  4 . 
     In the example illustrated schematically in the aforesaid  FIG.  1 A , the mirror arrangement  3  comprises: a first mirror device  3   a , 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  3   b , 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  3   a ,  3   b  co-operate for generating a scanning pattern on the screen  4 , which is illustrated schematically and designated by  5  in  FIG.  1 A . In particular, the first mirror device  3   a , rotating about the vertical axis A, “draws” a horizontal line on the second mirror device  3   b ; and the second mirror device  3   b , rotating about the horizontal axis B, directs the projected image onto a desired rectangular surface on the screen  4 . 
     Alternatively, as illustrated schematically in  FIG.  1 B , the mirror arrangement  3  of the pico-projector  1  may comprise a single mirror device, designated by  3   c , 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 United States Patent Application Publication No. 2011/0292479, incorporated by reference, 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 European Patent No. 3178783 A1, incorporated by reference. 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. 
       FIGS.  2 A- 2 B  are schematic illustrations of a portion of a mirror device, designated by  10 , according to the teachings of the aforesaid document EP 3178783. By way of example, only a first driving arm  11   a  and a second driving arm  11   b  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  11   a ). 
     The aforesaid first driving arm  11   a  has a first end connected to the second driving arm  11   b  and a second end connected to the tiltable structure  12 , which carries a mirror surface  13 , at a corresponding end or edge portion thereof. 
     As illustrated in  FIG.  2 B , application of the biasing voltage V causes bending out of the horizontal plane (along an orthogonal axis Z) of the first driving arm  11   a  and in particular of the second end connected to the tiltable structure  12 . Consequently, also the same tiltable structure  12  undergoes a corresponding out-of-plane displacement. 
     Given that the extent of the out-of-plane displacement of the tiltable structure  12  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 maximization 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 Inventors have 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 100 Hz). 
     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. 
     There is a need in the art to provide a microelectromechanical device with actuation of a piezoelectric type that will enable the drawbacks of the prior art to be overcome. 
     SUMMARY 
     In an embodiment, a microelectromechanical device comprises: a fixed structure defining a cavity; a tiltable structure elastically suspended in the cavity and having a main extension in a horizontal plane; and a piezoelectrically driven actuation structure that is biased for causing rotation of the tiltable structure about at least a first rotation axis parallel to a first horizontal axis of said horizontal plane, said actuation structure being interposed between the tiltable structure and the fixed structure; wherein said piezoelectrically driven actuation structure comprises at least a first pair of driving arms including respective regions of piezoelectric material and which are elastically coupled to the tiltable structure on opposite sides of the first rotation axis by respective first decoupling elastic elements, said respective first decoupling elastic elements being stiff in regard to movements out of the horizontal plane and being compliant in regard to torsion about said first rotation axis 
     The first pair of driving arms have a first end elastically coupled to said tiltable structure through said respective first decoupling elastic elements and a second end, longitudinally opposite with respect to said first end, fixedly coupled to said fixed structure. A main extension of said first pair of driving arms is oriented parallel to a second horizontal axis of said horizontal plane, orthogonal to said first horizontal axis. 
     In one aspect, the driving arms have a general C shape, with a major side of the C shape that extends longitudinally parallel to said first rotation axis on the opposite side of said tiltable structure with respect to said first rotation axis, and with minor sides of the C shape that extend parallel to a second horizontal axis of said horizontal plane on opposite sides of said tiltable structure with respect to said second horizontal axis. Each driving arm is fixedly coupled to the fixed structure at said major side and is elastically connected to the tiltable structure through a respective pair of decoupling elastic elements at said minor sides. 
     In another aspect, a first lever mechanism is coupled between the first supporting element and a first driving arm of said first pair. The first lever mechanism comprises a lever arm having a longitudinal extension along said first horizontal axis and a first end coupled to said first driving arm and a second end coupled through a torsional elastic element to a coupling element that is integral with said first supporting element. A first pair of piezoresistors is formed at this coupling element to define a piezoresistive sensor configured to detect the displacement of said tiltable structure. 
     Similarly, a second lever mechanism is coupled between the first supporting element and a second driving arm of said first pair. The second lever mechanism is arranged symmetrically to said first lever mechanism with respect to said first horizontal axis. The second lever mechanism comprises a respective lever arm having longitudinal extension along said first horizontal axis and a respective first end coupled to said second driving arm and a respective second end coupled through a respective torsional elastic element to a respective coupling element that is integral with said first supporting element on the opposite side with respect to the coupling element of said first lever mechanism. A second pair of piezoresistors is formed at said respective coupling element to define, together with said first pair of piezoresistors, said piezoresistive sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIGS.  1 A- 1 B  are schematic illustrations of respective pico-projectors with a pair of uniaxial mirror devices or with a single biaxial mirror device; 
         FIGS.  2 A- 2 B  show a portion of a mirror device with piezoelectric actuation of a known type, respectively in a schematic top plan view and in a schematic side section view; 
         FIG.  3    is a schematic top plan view of a microelectromechanical device according to a first embodiment of the present solution; 
         FIGS.  4 A- 4 B  are schematic illustrations of the microelectromechanical device of  FIG.  3    with a first rotation movement of a corresponding tiltable structure; 
         FIGS.  5 A- 5 B  are schematic illustrations of the microelectromechanical device of  FIG.  3    with a second rotation movement of the corresponding tiltable structure; 
         FIG.  6    is a schematic top plan view of a portion of the microelectromechanical device of  FIG.  3   , according to a further aspect of the present solution; 
         FIGS.  7 A and  7 B  are schematic cross-section views of parts of the microelectromechanical device shown in  FIG.  6   , during a respective detection movement; 
         FIG.  8 A  is an equivalent electrical diagram of connections between diffused piezoresistors in the microelectromechanical device shown in  FIG.  6   ; 
         FIG.  8 B  is a schematic top plan view of a part of the microelectromechanical device of  FIG.  6   , showing electrical connections between the diffused piezoresistors; 
         FIG.  9    is a cross-section view of a possible implementation of the microelectromechanical device, starting from a SOI (Silicon-On-Insulator) wafer; 
         FIGS.  10 A and  10 B  show a biaxial embodiment of the microelectromechanical device, highlighting the rotation movements thereof about a first rotation axis and a second rotation axis, respectively; 
         FIG.  11    is a schematic top plan view of a microelectromechanical device according to a further embodiment of the present solution; 
         FIGS.  12 A- 12 B  are schematic illustrations of the microelectromechanical device of  FIG.  11    with a first rotation movement and a second rotation movement of a corresponding tiltable structure about a rotation axis; 
         FIG.  13    is a schematic top plan view of a microelectromechanical device according to yet a further embodiment of the present solution; 
         FIG.  14    is a schematic illustration of the microelectromechanical device of  FIG.  13   , with a rotation movement of a corresponding tiltable structure about a rotation axis; 
         FIG.  15    is a block diagram of a pico-projector that uses the present microelectronic device; and 
         FIGS.  16  and  17    show variants of coupling between the pico-projector of  FIG.  15    and a portable electronic apparatus. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  3    is a schematic illustration of a microelectromechanical device  20 , in particular a mirror device based on MEMS technology, according to a first embodiment of the present solution. 
     The microelectromechanical device  20  is formed in a die of semiconductor material, in particular silicon, and is provided with a tiltable structure  22 , 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.  1 A ). 
     The aforesaid first rotation axis represents a first median axis of symmetry X for the microelectromechanical device  20 . A second median axis of symmetry Y for the same microelectromechanical device  20  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  22  is suspended above a cavity  23 , 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  22  defines a supporting structure that carries, at the top, a reflecting surface  22 ′ so as to define a mirror structure. 
     The tiltable structure  22  is elastically coupled to a fixed structure  24 , defined in the same die. In particular, the fixed structure  24  forms, in the horizontal plane XY, a frame  24 ′ that delimits and surrounds the aforesaid cavity  23  and moreover has a first supporting element  25   a  and a second supporting element  25   b , which extend longitudinally along the first median axis of symmetry X within the cavity  23  starting from the same frame  24 ′, on opposite sides of the tiltable structure  22 . 
     The tiltable structure  22  is supported by the first and the second supporting elements  25   a ,  25   b , to which it is elastically coupled, respectively, by means of a first elastic suspension element  26   a  and a second elastic suspension element  26   b , 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  26   a ,  26   b  hence extend along the first median axis of symmetry X, between the first, respectively second, supporting element  25   a ,  25   b  and a facing side of the tiltable structure  22 , to which they are coupled at a corresponding central portion. 
     In the embodiment illustrated, the first and the second elastic suspension elements  26   a ,  26   b  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  26   a ,  26   b  couple the tiltable structure  22  to the fixed structure  24 , 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  20  further comprises an actuation structure  30 , coupled to the tiltable structure  22  and configured to cause rotation thereof about the first rotation axis. The actuation structure  30  is interposed between the tiltable structure  22  and the fixed structure  24  and moreover contributes to supporting the tiltable structure  22  above the cavity  23 . 
     The actuation structure  30  comprises at least one first pair of driving arms formed by a first driving arm  32   a  and a second driving arm  32   b , which are arranged on opposite sides of, and symmetrically with respect to, the first median axis of symmetry X and first supporting element  25   a , and here have a longitudinal extension parallel to the first horizontal axis X and to the aforesaid first supporting element  25   a.    
     In the embodiment illustrated in  FIG.  3   , the driving arms  32   a ,  32   b  have a generically trapezoidal (or fin-like) shape, with a major side directed parallel to the second horizontal axis Y fixedly coupled to the frame  24 ′ of the fixed structure  24  and a minor side directed parallel to the same second horizontal axis Y elastically coupled to the tiltable structure  22 . 
     Each driving arm  32   a ,  32   b  is suspended above the cavity  23  and carries, on a top surface thereof (opposite to the same cavity  23 ), a respective first region made of piezoelectric material  33  (e.g., PZT—lead zirconate titanate), having substantially the same extension in the horizontal plane XY as the driving arm  32   a ,  32   b . Each driving arm  32   a ,  32   b  moreover has a respective first end, fixedly coupled to the frame  24 ′ of the fixed structure  24 , and a respective second end, elastically coupled to the tiltable structure  22 , by means of a first and a second decoupling elastic element  34   a ,  34   b , respectively. 
     In the example illustrated, the frame  24 ′ has a substantially rectangular shape in the horizontal plane XY, and the first end of the driving arms  32   a ,  32   b  is fixedly coupled to the sides of the same frame  24 ′ having an extension parallel to the second horizontal axis Y (in a direction transverse to the first rotation axis of the tiltable structure  22 ). 
     The aforesaid first and second decoupling elastic elements  34   a ,  34   b  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  34   a ,  34   b  hence extend parallel to the first horizontal axis X, between the first and the second driving arms  32   a ,  32   b , respectively, and a same side facing the tiltable structure  22 . 
     In particular, as is illustrated in the aforesaid  FIG.  3   , the first and second decoupling elastic elements  34   a ,  34   b  are coupled to the tiltable structure  22  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 10 μm and 1500 μm in a typical embodiment and may moreover be in general between 1/10 and ½ of the main dimension (in the example, along the second median axis of symmetry Y) of the tiltable structure  22 . 
     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  22 . 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  22  and the vertical displacement of the driving arms  32   a ,  32   b , caused by the piezoelectric effect (as it is discussed in detail hereinafter). 
     Each elastic suspension element  26  has a first folded part elastically coupling the end of supporting element  25  to the tiltable structure on one side of the axis X and a second folded part elastically coupling the end of supporting element  25  to the tiltable structure on an opposite side of the axis X. The coupling locations for the first and second folded parts of the elastic suspension element  26  are positioned between the axis X and the respective coupling points Pa, Pb. 
     In the embodiment illustrated in  FIG.  3   , the first and the second decoupling elastic elements  34   a ,  34   b  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.  3   , the aforesaid actuation structure  30  further comprises a second pair of driving arms formed by a third driving arm  32   c  and a fourth driving arm  32   d , which are arranged on opposite sides of the first median axis of symmetry X and, this time, of the second supporting element  25   b , and have a longitudinal extension parallel to the first horizontal axis X and to the aforesaid second supporting element  25   b  (it should be noted that the second pair of driving arms  32   c ,  32   d  is hence arranged symmetrically to the first pair of driving arms  32   a ,  32   b  with respect to the second median axis of symmetry Y). 
     As it has been discussed for the first pair of driving arms  32   a ,  32   b , each driving arm  32   c ,  32   d  of the second pair carries, on a top surface thereof, a respective first region made of piezoelectric material  33  (e.g., PZT—lead zirconate titanate) and has a respective first end fixedly coupled to the frame  24 ′ of the fixed structure  24  and a respective second end elastically coupled to the tiltable structure  22 , by means of a third and a fourth decoupling elastic element  34   c ,  34   d , respectively (which are arranged on the opposite side of the first and the second decoupling elastic elements  32   a ,  32   b  with respect to the second median axis of symmetry Y). 
     The aforesaid third and fourth decoupling elastic elements  34   c ,  34   d  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.  3   , also the third and fourth decoupling elastic elements  34   c ,  34   d  are hence coupled to the tiltable structure  22  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  34   c ,  34   d  are of a folded type. 
     As illustrated schematically in the aforesaid  FIG.  3   , the microelectromechanical device  20  further comprises a plurality of electrical-contact pads  38 , which are carried by the fixed structure  24  at the frame  24 ′ and are electrically connected (in a way not illustrated in detail in  FIG.  3   ) to the first regions of piezoelectric material  33  of the driving arms  32   a - 32   d , to enable electrical biasing thereof by electrical signals coming from outside of the electromechanical device  20  (for example, supplied by a biasing device of an electronic apparatus in which the electromechanical device  20  is integrated). 
     Moreover, the microelectromechanical device  20  comprises a piezoresistive (PZR) sensor  39 , appropriately arranged so as to provide a detection signal associated with rotation of the tiltable structure  22  about the first rotation axis. This detection signal can be provided as a feedback to the outside of the microelectromechanical device  20 , for example to the aforesaid biasing device, through at least one of the electrical-contact pads  38 . 
     In the embodiment illustrated in  FIG.  3   , the piezoresistive sensor  39  is provided (for example, by surface diffusion of dopant atoms) on the first supporting element  25   a  (different arrangements may, however, be envisaged for the same piezoresistive sensor  39 ). 
     Advantageously, the elastic suspension elements  26   a ,  26   b  are able to transmit the stresses to the supporting elements  25   a ,  25   b  and hence towards the piezoresistive sensor  39 , enabling arrangement of the latter on the supporting elements  25   a ,  25   b  and a consequent simplification of routing of the electrical connections to the electrical-contact pads  38 . 
     During operation of the microelectromechanical device  20 , application of a biasing voltage V to the first region of piezoelectric material  33  of the first driving arm  32   a  (having a positive value with respect to the biasing of the first region of piezoelectric material  33  of the second driving arm  32   b , 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  FIGS.  4 A and  4 B . 
     In a corresponding manner, application of a biasing voltage V to the first region of piezoelectric material  33  of the second driving arm  32   b  (having a positive value with respect to the biasing of the first region of piezoelectric material  33  of the first driving arm  32   a , 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  FIGS.  5 A and  5 B . 
     It should be noted that the same biasing voltage V is applied to the first region of piezoelectric material  33  both of the first driving arm  32   a  and of the third driving arm  32   c , and, likewise, in order to cause rotation in the opposite direction, to the first region of piezoelectric material  33 , of both the second driving arm  32   b  and the fourth driving arm  32   d , so as to contribute in a corresponding manner to the rotation of the tiltable structure  22  about the first rotation axis (as on the other hand emerges clearly from the foregoing description). 
     Advantageously, the decoupling elastic elements  34   a - 34   d  elastically decouple displacement by the piezoelectric effect of the driving arms  32   a - 32   d  along the orthogonal axis Z from the consequent rotation of the tiltable structure  22  along the first rotation axis. 
     In particular, by virtue of the proximity of the coupling points Pa-Pd between the decoupling elastic elements  34   a - 34   d  and the tiltable structure  22  to the rotation axis, a wide angle of rotation of the tiltable structure  22  about the first rotation axis corresponds to a small displacement out of the horizontal plane XY of the aforesaid driving arms  32   a - 32   d  (as on the other hand is highlighted in the aforesaid  FIGS.  4 B and  5 B ), 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  22 . For example, the ratio between the extent of these displacements may be equal to five in a possible embodiment. 
     The tiltable structure  22  can reach wide tilting angles (for example, greater than 10°) with a low value of the biasing voltage V (for example, lower than 40 V). 
     Moreover, the maximum amount of stress occurs in the elastic suspension elements  26   a ,  26   b  that couple the tiltable structure  22  to the fixed structure  24 ; the Inventors have 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 Inventors have shown that the microelectromechanical device  20  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  22 , 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  32   a - 32   d  (this displacement being smaller even by a factor of ten as compared to traditional solutions), the microelectromechanical device  20  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  20  than in known solutions). 
     A possible embodiment of the piezoresistive sensor  39  is now described in greater detail, configured to detect the rotation of the tiltable structure  22  about the first rotation axis. 
     The Inventors have realized that the stress transferred by the elastic suspension elements  26   a ,  26   b  to the supporting elements  25   a ,  25   b  can be limited by the elastic characteristics of the same elastic suspension elements  26   a ,  26   b , which in fact are generally thin and compliant to ensure the desired range of movement of the tiltable structure  22 . 
     In a particular embodiment, shown in  FIG.  6   , an additional mechanical amplification structure, denoted with  100 , is therefore provided, configured to maximize the sensitivity of the piezoresistive sensor  39 . 
     This mechanical amplification structure  100  comprises a first lever mechanism  102 , coupled, in the illustrated example, between the first supporting element  25   a  and the first driving arm  32   a.    
     In detail, the first lever mechanism  102  comprises a lever arm  103  having a longitudinal extension (along the first horizontal axis X) and a first end coupled to the first driving arm  32   a  and a second end coupled, by means of a torsional elastic element  104  (in the example having extension along the second horizontal axis Y), to a coupling element  105 , fixed and integral with the first supporting element  25   a  (of which constitutes a protrusion towards the aforesaid first driving arm  32   a ). 
     A first pair of diffused piezoresistors  106  of the aforementioned piezoresistive sensor  39  are formed in this coupling element  105 . 
     During operation, and as shown schematically in  FIG.  7 A , the movement of the first driving arm  32   a  (due to biasing of the respective first region of piezoelectric material  33 ) determines a corresponding movement out of the horizontal plane XY (in the example an upwards movement) of the lever arm  103 , in particular of the corresponding first end (at which a force F is generated). A torsion of the torsional elastic element  104  is consequently generated, in the example clockwise around the same torsional elastic element  104 . 
     The diffused piezoresistors  106  of the first pair are arranged so as to detect the stress resulting from the aforementioned rotation of the torsional elastic element  104 , which is therefore indicative of the rotation of the tiltable structure  22 . In particular, the presence of the first lever mechanism  102  advantageously allows to amplify the stress detected by the diffused piezoresistors  106 . 
     As shown in the same  FIG.  6   , advantageously, the mechanical amplification structure  100  further comprises a second lever mechanism  112 , coupled between the same first supporting element  25   a  and, this time, the second driving arm  32   b , thus being arranged in a symmetrical manner to the first lever mechanism  102  with respect to the first horizontal axis X. 
     In a similar manner, the second lever mechanism  112  comprises a respective lever arm  113  having a longitudinal extension (along the first horizontal axis X) and a respective first end coupled to the second driving arm  32   b  and a respective second end coupled, by means of a respective torsional elastic element  114  (in the example having an extension along the second horizontal axis Y), to a respective coupling element  115 , fixed and integral with the first supporting element  25   a  (of which constitutes a protrusion towards the aforesaid second driving arm  32   b ); the coupling elements  105 ,  115  are therefore symmetrical with respect to the first horizontal axis X. 
     A second pair of diffused piezoresistors  116  of the piezoresistive sensor  39  are formed at the aforesaid respective coupling element  115 . 
     During operation, and as shown schematically in  FIG.  7 B , movement of the second driving arm  32   b  (due to biasing of the respective region of piezoelectric material  33 ) determines a corresponding movement outside the horizontal plane XY (in the example a downwards movement) of the lever arm  113 , in particular of the corresponding first end (at which a respective force F′ is generated). A torsion of the torsional elastic element  114  of the second lever mechanism  112  is consequently generated, in the example counterclockwise around the same torsional elastic element  114 . 
     The diffused piezoresistors  116  of the second pair are arranged so as to detect the stress resulting from the aforementioned rotation of the torsional elastic element  114 . 
     In particular, as shown in the equivalent electric diagram of  FIG.  8 A , the diffused piezoresistors  106  of the first pair may constitute a first half of a detection Wheatstone bridge, denoted as  120 , of the piezoresistive sensor  39 , while the diffused piezoresistors  116  of the second pair may in this case constitute the second half of the same detection Wheatstone bridge  120 . 
     As shown schematically also in  FIG.  8 B , suitable conductive paths  122  are formed in the first supporting element  25   a  (for example being constituted by surface metal traces), so as to connect the diffused piezoresistors  106 ,  116  of the first and second pairs together according to the Wheatstone bridge connection scheme, and also so as to connect the same diffused piezoresistors  106 ,  116  to associated electrical-contact pads  38  (here not shown). 
     In particular, a common end of the diffused piezoresistors  106  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.  8 A ), 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  116  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  120  allows to detect the opposite displacements of the first and second lever mechanisms  102 ,  112 , thereby maximizing and making symmetrical the detection and the corresponding detection signal provided at the output (outside of the microelectromechanical device  20 ). 
     Electrical-contact pad  38  (here not shown) are suitably coupled to the aforesaid conductive paths  122 , which provide the routing of the electrical connections to the same electrical-contact pads  38 . 
     This embodiment advantageously allows to maximize the detection sensitivity of the piezoresistive sensor  39 , thus ensuring a more effective control of the electromechanical device  20  (for example, by the electronic apparatus in which the same electromechanical device  20  is integrated). 
     With reference to  FIG.  9   , a cross-section view provided by way of example of a possible implementation of the microelectromechanical device  20  is now illustrated, in this case obtained starting from a SOI (Silicon-On-Insulator) wafer  40  made of semiconductor material, in particular silicon. 
     In a way that will be clear to a person skilled in the art, the tiltable structure  22 , the fixed structure  24 , the elastic elements  34 - 34   d , and the driving arms  32   a - 32   d  are defined by chemical etching in an active layer  40   a  of the SOI wafer  40  (e.g., having a thickness of 20 μm). The cavity  23  is formed, once again by chemical etching, in a rear layer  40   b  of the SOI wafer  40  (e.g., having a thickness of 140 μm) and in a dielectric layer  40   c  of the same SOI wafer  40 . 
     It should be noted that underneath the tiltable structure  22 , following upon etching for the formation of the cavity  23 , there remain reinforcement elements  41 , having an extension along the orthogonal axis Z and operating as mechanical reinforcement. 
     Formed on a top surface  40 ′ of the active layer  40   a  of the SOI wafer  40  are: the reflecting surface  22 ′, on the mobile structure  22 , made of an appropriate material (e.g., aluminum, or else gold, according to whether projection is in the visible or in the infrared); and moreover, bottom electrode regions  42 , made of an appropriate conductive material, on the driving arms  32   a - 32   d.    
     The first regions of piezoelectric material  33  (constituted by a thin film of PZT) are formed on the aforesaid bottom electrode regions  42 , and top electrode regions  44  are formed on the first regions of piezoelectric material  33 . 
     A passivation layer  45 , made of an appropriate dielectric material, is formed, as a cover, on the active layer  40   a  of the SOI wafer, and contact openings  46  are opened through the same passivation layer  45  so as to enable access to the aforesaid bottom electrode regions  42  and top electrode regions  44 . 
     Metal routing regions  47  are then formed on the passivation layer  45  so as to contact, through the contact openings  46 , the bottom and top electrode regions  42 ,  44 , moreover extending as far as respective electrical-contact pads  38  (here not illustrated). 
     Through the above passivation layer  45  a further contact opening  46 ′ is moreover formed, in order to reach a diffused region  48 , arranged at the front surface  40 ′ of the active layer  40   a  of the wafer  40 , which defines the PZR sensor  39 . A further metal routing region  47 ′ is formed on the passivation layer  45 , so as to contact, through the further contact opening  46 ′, the PZR sensor  39 , moreover extending as far as a respective electrical-contact pad  38 , which is also not illustrated herein. 
     As shown in  FIG.  9   , a supporting wafer (the so-called “handling wafer”)  49  is coupled underneath the SOI wafer  40  and has, underneath the cavity  23  and in the mobile structure  22 , a recess  49 ′, to enable rotation of the mobile structure  22 . 
     With reference to  FIGS.  10 A- 10 B  a biaxial embodiment of the microelectromechanical device  20  is now described, where the tiltable structure  22  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.  1 B ). 
     In this case, the tiltable structure  22  comprises an inner frame  50 , having, for example, a substantially rectangular shape in the horizontal plane XY and internally defining a window  51 . 
     The inner frame  50  is elastically coupled to the driving arms  32   a - 32   d  of the actuation structure  30  in a way altogether equivalent to what has been discussed previously with reference to  FIG.  3   . The inner frame  50  is hence driven in rotation about the first rotation axis by application of an appropriate biasing to the first regions of piezoelectric material  33 , as described previously in detail, which causes generation of a force F (as indicated in  FIG.  10 A ). 
     The tiltable structure  22  further comprises in this case a distinct supporting element  52 , which is housed in the window  51  and is elastically coupled to the inner frame  50  by means of elastic elements  54 , compliant to torsion about the second rotation axis, and moreover carries at the top the reflecting surface  22 ′. For instance, the supporting element  52  has a circular or elliptical shape in the horizontal plane XY. 
     During rotation about the first rotation axis, as shown in  FIG.  10 A , the supporting element  52  is fixedly coupled to the inner frame  50  so as to be driven in the same rotation, thus causing the desired movement of the reflecting surface  22 ′. 
     The actuation structure  30  in this case comprises second regions of piezoelectric material  53 , designed to be appropriately biased for causing rotation of the supporting element  52  about the second rotation axis. The inner frame  50 , as illustrated in  FIG.  10 B , decouples the rotations about the first and the second rotation axes. 
     In the embodiment illustrated, the second regions of piezoelectric material  53  are also carried by the driving arms  32   a - 32   d , at the respective ends coupled to the tiltable structure  22 , hence on the inside with respect to the first regions of piezoelectric material  33 . 
     In particular, as is highlighted in the aforesaid  FIG.  10 B , the second regions of piezoelectric material  53  carried by the driving arms  32   a - 32   b ,  32   c - 32   d  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  50  to generate resonant rotation of the supporting element  52  about the second rotation axis. 
     In a way not described in detail, further electrical-contact pads  38  are in this case present, which are, for example, once again arranged on the frame  24 ′ of the fixed structure  24 , for biasing the aforesaid second regions of piezoelectric material  53 . 
     With reference to  FIG.  11   , a further embodiment of the present solution is now described. 
     In this case, the actuation structure  30  of the microelectromechanical device  20  comprises just one pair of driving arms, designated once again by  32   a ,  32   b.    
     These driving arms  32   a ,  32   b  are in this case generically C-shaped, with the major side of the “C” shape 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” shape that extend parallel to the second horizontal axis Y, on opposite sides of the tiltable structure  22  with respect to the second median axis of symmetry Y. 
     The aforesaid driving arms  32   a ,  32   b  are arranged on opposite sides of the tiltable structure  22  with respect to the first median axis of symmetry X and jointly define an inner window  58 , arranged inside which is the tiltable structure  22 , 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  32   a ,  32   b  is suspended over the cavity  23  and carries, on a top surface thereof (opposite to the cavity  23 ) a respective first region of piezoelectric material  33  (e.g., PZT—lead zirconate titanate), having substantially the same extension in the horizontal plane XY as said driving arm  32   a ,  32   b.    
     In this case, each driving arm  32   a ,  32   b  is moreover fixedly coupled to the frame  24 ′ of the fixed structure  24  along all of the long side of the “C” shape and is elastically connected to the tiltable structure  22  by means of a respective pair of elastic decoupling elements, at the minor sides of the “C” shape: in particular, the first driving arm  32   a  is connected to the tiltable structure  22  by means of the first decoupling elastic element  34   a  and the third decoupling elastic element  34   c , whereas the second driving arm  32   b  is connected to the same tiltable structure  22  by means of the second decoupling elastic element  34   b  and the fourth decoupling elastic element  34   c.    
     In a way altogether similar to what has been discussed previously, also in this embodiment the aforesaid decoupling elastic elements  34   a - 34   d  are coupled to the tiltable structure  22  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  34   a - 34   d  are of a folded type (but could alternatively be of a linear type). 
     In the example illustrated in the aforesaid  FIG.  11   , the first and the second elastic suspension elements  26   a ,  26   b , which also in this case elastically connect the tiltable structure  22  to the fixed structure  24 , 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  20 . In particular, application of the biasing voltage V to the first region of piezoelectric material  33  carried by the first or second driving arms  32   a ,  32   b  causes rotation, in a positive direction (as illustrated in  FIG.  12 A ) or a negative direction (as illustrated in  FIG.  12 B ) of the tiltable structure  22  about the first rotation axis. 
     The embodiment described determines a different form factor of the microelectromechanical device  20 , which enables a smaller resulting size to be obtained in the case where the tiltable structure  22  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  38 , given the low number (two) of piezoelectric actuators. 
     With reference to  FIG.  13   , yet a further embodiment of the present solution is now described, where the actuation structure  30  of the microelectromechanical device  20  once again comprises a first and a second pair of driving arms, designated also in this case by  32   a - 32   b  and  32   c - 32   d.    
     Unlike what has been illustrated previously, the aforesaid driving arms  32   a - 32   d  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  22 , and a much smaller extension in a direction parallel to the first horizontal axis X. In the example, the driving arms  32   a - 32   d  have a substantially rectangular shape in the horizontal plane XY. 
     Also in this case, each driving arm  32   a - 32   d  carries, on a top surface thereof (opposite to the cavity  23 ) a respective first region of piezoelectric material  33 , and moreover has a respective first end fixedly coupled to the frame  24 ′ of the fixed structure  24  and a respective second end elastically coupled to the tiltable structure  22 , by means of a respective decoupling elastic element  34   a - 34   d.    
     In this embodiment, in which once again the frame  24 ′ has a substantially rectangular shape in the horizontal plane XY, the first end of the driving arms  32   a - 32   d  is fixedly coupled to the sides of said frame  24 ′, which have an extension parallel to the first horizontal axis X (parallel to the first rotation axis of the tiltable structure  22 ). 
     In the example illustrated in the aforesaid  FIG.  13   , the first and the second elastic suspension elements  26   a ,  26   b , which also in this case elastically couple the tiltable structure  22  to the fixed structure  24 , 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  20 . In particular, application of the biasing voltage V to the first regions of piezoelectric material  33  carried by the first and third driving arms  32   a ,  32   c  causes rotation in a positive direction (as illustrated in  FIG.  14   , by way of example), whereas application of the biasing voltage V to the first regions of piezoelectric material  33  carried by the second and the fourth driving arms  32   b ,  32   d  causes a corresponding rotation in the negative direction of the tiltable structure  22  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  22  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  20 . 
     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  20  does not have spurious modes (of any type) at frequencies close to the actuation frequency. Moreover, the microelectromechanical device  20  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  39 , discussed with reference to  FIGS.  6 ,  7 A- 7 B and  8 A- 8 B  allows, considering the same other characteristics of the microelectromechanical structure, to maximize the detection sensitivity and, therefore, the controllability of the microelectromechanical device  20  (for example by an external electronic apparatus). 
     Advantageously, the microelectromechanical device  20  can be used in a pico-projector  60  designed to be functionally coupled to a portable electronic apparatus  61 , as illustrated schematically with reference to  FIGS.  15 - 17   . 
     In detail, the pico-projector  60  of  FIG.  15    comprises: a light source  62 , for example of a laser type, adapted to generate a light beam  63 ; the microelectronic device  20 , acting as a mirror and designed to receive the light beam  63  and direct it towards a screen or display surface  65  (external to, and set at a distance, from the pico-projector  60 ); a first driving circuit  66 , designed to provide appropriate control signals to the light source  62 , for generation of the light beam  63  as a function of an image to be projected; a second driving circuit  68 , designed to provide driving signals to the actuation structure  30  of the microelectronic device  20 ; and a communication interface  69 , designed to receive, from an external control unit  70 , for example included in the portable apparatus  61 , 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  62 . 
     The pico-projector  60  can be provided as a separate and stand-alone accessory with respect to an associated portable apparatus  61 , for example a smartphone, as illustrated in  FIG.  16   . In this case, the pico-projector  60  is coupled to the portable electronic apparatus  61  by means of appropriate electrical and mechanical connection elements (not illustrated in detail). Here, the pico-projector  60  is provided with a casing  72  of its own, which has at least one portion  72 ′ transparent to the light beam  63  coming from the microelectronic device  20 . The casing  72  of the pico-projector  60  is coupled in a releasable manner to a respective case  73  of the portable electronic apparatus  61 . 
     Alternatively, as illustrated in  FIG.  17   , the pico-projector  60  can be integrated within the portable electronic apparatus  61  and be set within the case  73  of the same portable electronic apparatus  61 . In this case, the portable electronic apparatus  61  has a respective portion  72 ′ transparent to the light beam  63  coming from the microelectronic device  20 . The pico-projector  60  is, in this case, for example, coupled to a printed circuit board within the case  73  of the portable electronic apparatus  61 . 
     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  20  could be envisaged, based on the embodiments of  FIGS.  11  and  10   , in a way substantially similar to what has been described in detail with reference to  FIGS.  10 A- 10 B . 
     Moreover, it is evident that variants may in general be envisaged as regards the shape of the elements forming the microelectromechanical device  20 ; for example, the tiltable structure  22  (and the corresponding reflecting surface  22 ′) may have different shapes. 
     Moreover, it is highlighted that, as mentioned previously, the elastic suspension elements  26   a - 26   b  and the decoupling elastic elements  34   a - 34   d  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  39  and the associated mechanical amplification structure  100  could be formed at the second supporting element  25   b , in this case the first and the second lever mechanism  102 ,  112  being coupled between the same second supporting element  25   b  and the third, respectively the fourth, driving arms  32   c ,  32   d.    
     Moreover, the same amplification mechanical structure  100  could be applied, with suitable modifications, also to the second and third embodiments of the present solution (discussed previously with reference to  FIGS.  11  and  13   ).