Patent Publication Number: US-2022229287-A1

Title: Resonant mems device having a tiltable, piezoelectrically controlled micromirror

Description:
PRIORITY CLAIM 
     This is a continuation of U.S. patent application Ser. No. 16/830,920, filed Mar. 26, 2020, which claims the priority benefit of Italian Application for Patent No. 102019000004797, filed on Mar. 29, 2019, the contents of which are hereby incorporated by reference in their entireties to the maximum extent allowable by law. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to a resonant Micro-Electro-Mechanical System (MEMS) device having a tiltable, piezoelectrically controlled structure, in particular a micromirror. The micromirror may be uniaxial. 
     BACKGROUND 
     MEMS devices are known that have a mirror structure obtained using a semiconductor material technology. 
     Such MEMS devices are, for example, used in portable apparatuses, such as portable computers, laptops, notebooks (including ultra-thin notebooks), PDAs, tablets, mobile phones or smartphones, for optical applications, in particular for directing beams of light radiation generated by a light source in desired patterns. 
     By virtue of the small dimensions of MEMS devices, such devices may meet stringent requirements regarding occupation of space, in relation to both area and thickness. 
     For instance, microelectromechanical mirror devices are used in miniaturized projector modules (e.g., picoprojectors), which are able to project images at a distance or generate desired light patterns. 
     MEMS mirror devices generally include a mirror element suspended over a cavity and obtained from a semiconductor body so as to be mobile, typically by inclination or rotation, to direct an incident light beam in the desired way. 
     For instance,  FIG. 1  schematically shows a picoprojector  1  comprising a light source  2 , typically a laser source, generating a light beam  3  made up of three monochromatic beams, one for each base color, which, through an optical system  4  illustrated schematically, is deflected by a mirror element  5  towards a screen  6 , where it produces a scan  7 . In the example illustrated, the mirror element  5  includes two micromirrors  8 ,  9 , arranged in sequence along the path of the light beam  3  and rotatable about their own axes; namely, a first micromirror  8  is rotatable about a vertical axis A, and a second micromirror is rotatable about a horizontal axis B perpendicular to the vertical axis A. Rotation of the first micromirror  8  about the vertical axis A generates a fast horizontal scan, as illustrated in the detail of  FIG. 1 . Rotation of the second micromirror  9  about the horizontal axis B generates a slow vertical scan. 
     In a variant of the system of  FIG. 1 , illustrated in  FIG. 2 , the picoprojector, designated by  1 ′, includes a mirror element  5 ′ of a two-dimensional type, controlled to rotate about both the vertical axis A and the horizontal axis B to generate the same scanning pattern  7  as that in  FIG. 1 . 
     In both cases, the movement about the vertical axis A is generally of a resonant type and is fast, and the movement about the horizontal axis B is generally of a linear type and slower. 
     Another application of micromirror systems is represented by 3D gesture-recognition systems. These normally use a picoprojector and an image-capturing device, such as a photographic camera. The light ray may here be in the visible light range, the invisible range or at any useful frequency. The picoprojector for this application may be similar to the picoprojector  1  or  1 ′ of  FIGS. 1 and 2 , and the light beam  3  deflected by the micromirror  5 ,  5 ′ is used for scanning an object in two directions. For example, the picoprojector can project small stripes on the object; possible projecting or recessed areas of the object (due to the depth thereof) create deformations in the light rays, which can be detected by the photo camera and processed by adapted electronics to detect the third dimension (i.e., depth). 
     In both applications, with the considered technology, rotation of the mirror element  5 ,  5 ′ is controlled through an actuation system, generally of an electrostatic, magnetic, or piezoelectric type. 
     Electrostatic-actuation systems in general utilize high operating voltages, whereas electromagnetic-actuation systems in general entail high power consumption. 
     It has thus been proposed to control the scanning movement in a piezoelectric way so as to generate a strong force, thus using a lower actuation voltage and achieving lower power consumption. 
     For instance, in the mirror device described in United States Patent Publication 2011/0292479 (incorporated by reference), a suspended frame is connected to a fixed structure through spring elements having a serpentine or folded shape formed by a plurality of arms parallel to each other and set side by side. Each arm carries a piezoelectric band, and adjacent piezoelectric bands are biased by voltages having opposite polarity. Due to the characteristics of the piezoelectric materials, the biasing causes deformation in opposite directions (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, the frame rotates in a second direction, opposite to the first. The vertical scan can thus be obtained by applying bipolar AC voltages to the arms. 
     A similar actuation system may control rotation about the vertical axis A to also control the horizontal scan. 
     Another piezoelectrically actuated mirror device is described in U.S. Pat. No. 10,175,474 (EP 3,178,783), incorporated by reference to the maximum extent allowable under the law. This mirror device has a tiltable structure, rotatable 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 transverse to the horizontal axis B, each driving arm carrying a respective piezoelectric band of piezoelectric material. The driving arms are divided into two sets driven in phase opposition to obtain rotation in opposite directions of the tiltable structure about the horizontal axis B. 
     To reduce sensitivity to spurious movements and provide a structure that is more resistant to impact, U.S. patent application Ser. No. 16/704,484 (IT 102018000011112, filed on Dec. 14, 2018 and EP 19165958.0) incorporated by reference to the maximum extent allowable under the law, describes a mirror device having a piezoelectric-actuation structure of a linear type formed by pairs of driving arms coupled to the tiltable structure through decoupling spiral springs, that are rigid for movements out of the horizontal plane and compliant to torsion about the rotation axis. 
     United States Patent Application Publication No. 2007/268950, incorporated by reference, discloses a mirror scanner having piezoelectric actuation having actuator arms directly coupled between torsional support arms of the tiltable structure and the anchoring frame. 
     The current market calls for high-frequency resonant driving solutions for high-resolution projection systems (for example, having 1440 projection rows with increasing angles of aperture). 
     However, to obtain increased angles of aperture, actuation structures of large area are involved, entailing high consumption. 
     There is a need in the art to provide a MEMS device having a tiltable structure able to perform a wide oscillation movement at high frequency. 
     SUMMARY 
     Disclosed herein is a method of making a microelectromechanical (MEMS) device. The method includes in a single structural layer, affixing a tiltable structure to an anchorage portion with first and second supporting arms extending between the anchorage portion and opposite sides of the tiltable structure, and forming first and second resonant piezoelectric actuation structures extending between a constraint portion of the first supporting arm and the anchorage portion, on opposite sides of the first supporting arm. The method further includes coupling a handling wafer underneath the structural layer to define a cavity therebetween, and forming a passivation layer over the structural layer, the passivation layer having contact openings defined therein for routing metal regions for electrical coupling to respective electrical contact pads, the electrical contact pads being electrically connected to the first and second resonant piezoelectric actuation structures. 
     The single structural layer may be formed as part of a double structural layer using silicon-on-insulator technology, with the handling wafer being coupled underneath the structure layer by a supporting layer. 
     Portions of the supporting layer may be selectively removed to form reinforcement elements extending underneath the tiltable structure into the cavity to mechanically reinforce the tiltable structure. 
     Additional portions of the supporting layer may be selectively removed to further define the cavity. 
     Portions of the handling wafer may be removed to define the cavity. 
     The handling wafer may be formed from monocrystalline silicon or polycrystalline silicon. 
     The handling wafer may be coupled underneath the structure layer by an adhesive layer. 
     The first supporting arm may be formed to include an elongate portion coupled between the tiltable structure and the anchorage portion, and the constraint portion may be formed at an intermediate position along the elongate portion between the tiltable structure and anchorage portion. 
     The first support arm may be formed to include first and second torsion springs coupled at the constraint portion, with the first and second torsion springs being configured to be resistant with respect to movements out of a plane of the tiltable structure but complaint to torsion about a longitudinal axis of the elongate portion. 
     The first and second resonant piezoelectric actuation structures may be configured to cause deflection of the tiltable structure about the longitudinal axis of the elongate portion. 
     Also disclosed herein is a microelectromechanical (MEMS) device, including a die having a cavity defined therein, an anchorage portion formed by the die, a tiltable structure elastically suspended over the cavity, first and second supporting arms extending between the anchorage portion and opposite sides of the tiltable structure, and first and second resonant piezoelectric actuation structures configured to cause rotation of the tiltable structure. The first supporting arm includes first and second torsion springs, the first torsion spring being coupled to the tiltable structure, the second torsion spring being coupled to the anchorage portion, the first and the second torsion springs being coupled together at a first constraint region. 
     The first resonant piezoelectric actuation structure may include a first actuation arm carrying a first piezoelectric actuator element and a first displacement-transfer beam, and the second resonant piezoelectric actuation structure may include a second actuation arm carrying a second piezoelectric actuator element and a second displacement-transfer beam. 
     The first resonant piezoelectric actuation structure may include a first actuation arm carrying a first piezoelectric actuator element and a first displacement-transfer beam, and the second resonant piezoelectric actuation structure may include a second actuation arm carrying a second piezoelectric actuator element and a second displacement-transfer beam. 
     The first and the second actuation arms may be arranged symmetrically with respect to a rotation axis of the tiltable structure and have a first length in a direction parallel to a first horizontal axis and a first width in a direction parallel to a second horizontal axis. The first and the second displacement-transfer beams may be arranged symmetrically with respect to the first supporting arm and have a second length in a direction parallel to the first horizontal axis and a second width in a direction parallel to the second horizontal axis, where the first width is greater than the second width. 
     The first and second actuation arms may extend laterally to the second torsion spring, to the first constraint region, and in part to the first torsion spring. The first and the second displacement-transfer beams may extend laterally to the first and the second actuation arms, respectively, the first displacement-transfer beam being arranged between the first actuation arm and a part of the first torsion spring, and the second displacement-transfer beam being arranged between the second actuation arm and part of the first torsion spring. 
     Also disclosed herein is a method of making a picoprojector apparatus, including configuring a light source to be driven to generate a light beam according to an image to be generated, defining a cavity in a die, forming an anchorage portion in the die, forming a tiltable structure to have a main extension in a horizontal plane defined by first and second horizontal axes perpendicular to each other, elastically suspending the tiltable structure over the cavity using first and second supporting arms extending between the anchorage portion and opposite sides of the tiltable structure, configuring first and second resonant piezoelectric actuation structures to be biased to cause rotation of the tiltable structure about a rotation axis parallel to the first horizontal axis, resisting movement of the tiltable structure out of the horizontal plane and permitting torsion of the tiltable structure about the rotation axis utilizing first and second torsion springs coupled together at a first constraint region, and configuring a driving circuit to supply electrical driving signals the first and second resonant piezoelectric actuation structures to cause rotation of the tiltable structure. 
     The method may further include forming the first resonant piezoelectric actuation structure as a first actuation arm carrying a first piezoelectric actuator element and a first displacement-transfer beam, and forming the second resonant piezoelectric actuation structure as a second actuation arm carrying a second piezoelectric actuator element and a second displacement-transfer beam. 
     The method may also further include coupling the first and the second actuation arms to the anchorage portion and configuring the first and second actuation arms to be rigid to torsion and compliant to movements out of the horizontal plane, and coupling the first and the second displacement-transfer beams to the first constraint region and configuring the first and second displacement-transfer beams to be compliant to torsion about the rotation axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For better understanding, some embodiments are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein: 
         FIG. 1  is a schematic representation of an embodiment of a picoprojector with two one-dimensional mirror elements; 
         FIG. 2  is a schematic representation of another embodiment of a picoprojector with a two-dimensional mirror element; 
         FIG. 3  is a top plan view of an embodiment of a MEMS device having a tiltable structure; 
         FIG. 4  is a cross-section, taken along section line IV-IV of  FIG. 3 , of a possible implementation of the MEMS device of  FIG. 3 ; 
         FIG. 5  is a cross-section, taken along section line IV-IV of  FIG. 3 , of another implementation of the MEMS device of  FIG. 3 ; 
         FIG. 6  is a schematic illustration of the MEMS micromirror device of  FIG. 3  with the tiltable structure in a rotated position; 
         FIG. 7  is a top plan view of a different embodiment of a MEMS device having a tiltable structure; 
         FIG. 8  is a top plan view of another embodiment of a MEMS device having a tiltable structure; 
         FIG. 9  is a schematic illustration of the MEMS device of  FIG. 3, 7 or 8  with the tiltable structure in a rotated position; 
         FIG. 10  is a top plan view of still another embodiment of a MEMS device having a tiltable structure; 
         FIG. 11  is a block diagram of a picoprojector using the present MEMS device; and 
         FIGS. 12 and 13  show variants of coupling between the picoprojector of  FIG. 11  and a portable electronic apparatus. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  schematically shows a MEMS device  20 , in particular a mirror device. 
     The MEMS device  20  is formed in a die of semiconductor material, in particular silicon, designated by  21 , and comprises a tiltable structure  22 , having a main extension in a horizontal plane XY of a Cartesian reference system XYZ and is arranged so as to rotate about a rotation axis, parallel to a first horizontal axis X of the horizontal plane XY. In particular, the rotation axis corresponds to the fast vertical axis A of a picoprojector apparatus of the type illustrated in  FIGS. 1 and 2 , and therefore is designated once again by A. 
     The rotation axis A is also a first median symmetry axis for the MEMS device  20 , thus also designated by A; a second median symmetry axis C for the MEMS device  20  is parallel to a second horizontal axis Y, which is orthogonal to the first horizontal axis X, extends through a center O of the tiltable structure  22 , and defines, with the first horizontal axis X, the horizontal plane XY. 
     The tiltable structure  22  is suspended over a cavity  23  formed in the die  21 , is carried by a fixed structure  24  and has, in the illustrated embodiment, a generically circular shape. However other shapes are possible, from an elliptical to a polygonal shape, for example, square or rectangular. The tiltable structure  22  carries, at its top, a reflecting surface  22 ′ so as to define a mirror structure. The reflecting surface  22 ′ is of an appropriate material, for example, aluminum, or gold, according to whether the light projection is in the visible spectrum or in the infrared spectrum. 
     The tiltable structure  22  is elastically coupled to an anchorage structure, here formed by a frame portion  24 ′ of the fixed structure  24 , through a supporting structure comprising first and second supporting arms  25 A,  25 B. The supporting arms  25 A,  25 B extend longitudinally along the first median symmetry axis A, over the cavity  23 , between the frame portion  24 ′ of the fixed structure  24  and the tiltable structure  22 , on opposite sides of the latter. 
     In the instant embodiment, the frame portion  24 ′ has, in top view, a generally rectangular shape formed by a first and a second longer sides, indicated by  70 ,  71  and extending here parallel to the first horizontal axis X, and a first and a second shorter sides, indicated by  72 ,  73 , extending parallel to the second horizontal axis Y. 
     The supporting arms  25 A,  25 B have the same shape and are arranged symmetrically with respect to the second median symmetry axis C; the parts that make them up are thus designated by the same reference numbers and the letter A or B according to whether they belong to the first or to the second supporting arm  25 A,  25 B. 
     In detail, the first supporting arm  25 A has a first end  25 A′ rigidly coupled to the tiltable structure  22  and a second end  25 A″ rigidly coupled to the first shorter side  72  of the frame portion  24 ′ of the fixed structure  24 , and comprises first and second torsion springs  27 A,  28 A. The first and second torsion springs  27 A,  28 A extend as a prolongation of each other along the first median symmetry axis A between a first side of the tiltable structure  22  and a first side of the frame portion  24 ′ of the fixed structure  24 . 
     The second supporting arm  25 B has a first end  25 B′ rigidly coupled to the tiltable structure  22  and a second end  25 B″ rigidly coupled to the second shorter side  73  of the frame portion  24 ′ of the fixed structure  24 , and comprises third and fourth torsion springs  27 B,  28 B. The third and the fourth torsion springs  27 B,  28 B extend as a prolongation of each other along the first median symmetry axis A between a second side, opposite to the first side, of the tiltable structure  22  and a second side, opposite to the first side, of the frame portion  24 ′ of the fixed structure  24 . 
     The torsion springs  27 A,  27 B,  28 A,  28 B are shaped as a linear beam, along the first horizontal axis X (dimension referred to hereinafter as “length”), and are thin in the direction of the second horizontal axis Y (dimension referred to hereinafter as “width”); furthermore, in the illustrated embodiment, they have a thickness (in a direction parallel to a vertical axis Z of the Cartesian reference system XYZ) greater than the width; consequently, they have an increased flexural stiffness along the first and the second horizontal axes X, Y of the horizontal plane XY and are compliant to torsion about the first median symmetry axis A. 
     In the first supporting arm  25 A, the first and the second torsion springs  27 A,  28 A are connected together at a first constraint structure  29 A; in the second supporting arm  25 B, the third and the fourth torsion springs  27 B,  28 B are connected together at a second constraint structure  29 B. The first and the second constraint structures  29 A,  29 B are rigid and here have a generally rectangular shape with a width (in a direction parallel to the second horizontal axis Y) much greater than the torsion springs  27 A,  27 B,  28 A,  28 B and a length (in a direction parallel to the first horizontal axis X) comparable to the torsion springs  27 A,  27 B,  28 A,  28 B. 
     The MEMS device  20  further comprises at least first and second actuation structures  30 A 1 ,  30 A 2 , which extend between the frame portion  24 ′ of the fixed structure  24  and the first constraint structure  29 A, on first and second sides, respectively, of the first supporting arm  25 A, and have a symmetrical structure with respect to the first median symmetry axis A. 
     The first actuation structure  30 A 1  comprises a first actuation arm  31 A 1  and a first displacement-transfer beam  32 A 1 . 
     In the embodiment of  FIG. 3 , the first actuation arm  31 A 1  is rigidly coupled to the first shorter side  72  of the fixed structure  24  at its first end and extends parallel to and alongside the second torsion spring  28 A for a length (in a direction parallel to the first horizontal axis X) roughly equal to the second torsion spring  28 A. Furthermore, the first actuation arm  31 A 1  is coupled, at its second end, to the first displacement-transfer beam  32 A 1 . The first displacement-transfer beam  32 A 1  here extends as a prolongation of, and parallel to, the first actuation arm  31 A 1 , between this and the first constraint structure  29 A, and is coupled to the first constraint structure  29 A through a first coupling region  35 A 1 . In practice, the first displacement-transfer beam  32 A 1  extends laterally to the first constraint structure  29 A, throughout the entire length of the latter (in a direction parallel to the first horizontal axis X). 
     Likewise, in the second actuation structure  30 A 2 , the second actuation arm  31 A 2  is coupled between the second shorter side  73  of the fixed structure  24  and the second displacement-transfer beam  32 A 2 , parallel to and alongside the second torsion spring  28 A. The second displacement-transfer beam  32 A 2  extends as a prolongation of, and parallel to, the second actuation arm  31 A 2 , between the latter and the first constraint structure  29 A, and is coupled thereto through a second coupling region  35 A 2 . Also here, the second displacement-transfer beam  32 A 2  extends laterally to the first constraint structure  29 A, throughout the entire length thereof. 
     In the illustrated embodiment, the MEMS device  20  further comprises third and fourth actuation structures  30 B 1 ,  30 B 2 , which extend between the frame portion  24 ′ of the fixed structure  24  and the second constraint structure  29 B, on first and second sides, respectively, of the second supporting arm  25 B. The third and fourth actuation structures  30 B 1  and  30 B 2  are symmetrical to each other with respect to the first median symmetry axis A and are symmetrical to the first and the second actuation structures  30 A 1 ,  30 A 2 , respectively, with respect to the second median symmetry axis C. 
     Consequently, the third actuation structure  30 B 1  comprises a third actuation arm  31 B 1  and a third displacement-transfer beam  32 B 1 , and the fourth actuation structure  30 B 2  comprises a fourth actuation arm  31 B 2  and a fourth displacement-transfer beam  32 B 2 . 
     In the third actuation structure  30 B 1 , the third actuation arm  31 A 2  is coupled between the fixed structure  24  and the third displacement-transfer beam  32 B 1 , parallel to and alongside the fourth torsion spring  28 B. The third displacement-transfer beam  32 B 1  extends as a prolongation of, and parallel to, the third actuation arm  31 B 1 , between the latter and the second constraint structure  29 B, and is coupled thereto through a third coupling region  35 B 1 . The third displacement-transfer beam  32 B 1  extends laterally to the second constraint structure  29 B, throughout the entire length of the latter. 
     Moreover, the fourth actuation arm  31 B 2  is coupled between the fixed structure  24  and the fourth displacement-transfer beam  32 B 2 , parallel to and alongside the fourth torsion spring  28 B. The fourth displacement-transfer beam  32 B 2  extends as a prolongation of, and parallel to, the fourth actuation arm  31 B 2 , between it and the second constraint structure  29 B, and is coupled thereto through a fourth coupling region  35 B 2 . Also here, the fourth displacement-transfer beam  32 B 2  extends laterally with respect to the second constraint structure  29 B, throughout the entire length thereof. 
     In the embodiment illustrated in  FIG. 3 , the actuation arms  31 A 1 ,  31 A 2 ,  31 B 1 ,  31 B 2  have a substantially rectangular shape, with a width (in a direction parallel to the second horizontal axis Y) much greater than the displacement-transfer beams  32 A 1 ,  32 A 2 ,  32 B 1 ,  32 B 2 . In addition, they are suspended over the cavity  23  and carry, on a top surface thereof (not facing the cavity  23 ) respective piezoelectric actuator elements  38 A 1 ,  38 A 2 ,  38 B 1 ,  38 B 2 , substantially having the same area in the horizontal plane XY as the respective actuation arm  31 A 1 ,  31 A 2 ,  31 B 1 ,  31 B 2 . 
     As illustrated schematically in  FIG. 3 , the MEMS device  20  further comprises a plurality of electrical contact pads  40 , carried by the frame portion  24 ′ of the fixed structure  24  and electrically connected, in a way not illustrated in detail, to the piezoelectric actuator elements  38 A 1 ,  38 A 2 ,  38 B 1 ,  38 B 2 , to enable electrical biasing thereof by electrical signals fed from outside of the MEMS device  20  (for example, supplied by a biasing device of an electronic apparatus integrating the MEMS device  20 ). 
     Moreover, the MEMS device  20  comprises a displacement sensor  41 , here of a piezoresistive (PZR) type, formed, for example, by surface diffusion of dopant atoms, and arranged in the frame portion  24 ′ of the fixed structure  24 , in proximity of the second end  25 A″ of the first supporting arm  25 A and suspended over the cavity  23 , so as to supply a detection signal related to the rotation of the tiltable structure  22  about the rotation axis A. The displacement sensor  41  is electrically coupled to a respective electrical contact pad  40  so as to supply an electrical detection signal outside of the MEMS device  20 , for example, to an electronic processing circuit, such as an ASIC (Application Specific Integrated Circuit), not illustrated. 
       FIG. 4  shows a longitudinal section of the MEMS device  20 , taken along section axis IV-IV of  FIG. 3 , coinciding to a fair extent with the first median symmetry axis A, but extending through the first actuation arm  31 A 1 , to show the structure of the first piezoelectric actuation element  38 A 1 . In particular,  FIG. 4  shows the MEMS device  20  when the latter has suspended parts formed in a single structural layer  45 , typically of mono- or polycrystalline silicon, defined to have the geometry visible in  FIG. 3 , where the tiltable structure  22 , the supporting arms  25 A,  25 B, and the actuation structure  30 A 1 ,  30 A 2 ,  30 B 1 ,  30 B 2  are separated from the frame portion  24 ′ of the fixed structure  24  by a trench  46  (partly visible in  FIG. 4 ). 
     In detail, in  FIG. 4 , a handling wafer  46 , for example also of mono or polycrystalline silicon, is coupled underneath the structural layer  45  through an adhesive layer  44  and forms the cavity  23 . 
       FIG. 4  further shows the structure of the piezoelectric actuator element  38 A 1  (the other piezoelectric actuator elements  38 A 2 ,  38 B 1 ,  38 B 2  are formed in the same way). In detail, the piezoelectric actuator element  38 A 1  comprises a stack of layers including a bottom electrode  47 , of conductive material, a piezoelectric region  48  (for example, of PZT—Lead Zirconate Titanate), and a top electrode  49 . A passivation layer  50 , of dielectric material, is formed, as a cover, over the structural layer  45 , and has a contact opening for routing metal regions  51  for electrically coupling to respective electrical contact pads  40  (here not visible). 
     In the embodiment of  FIG. 5 , the MEMS device  20  (represented in the same section as  FIG. 4 ) has suspended parts formed in a double structural layer  55  formed, for example, by a SOI (Silicon-On-Insulator) wafer. In this case, an active layer  56  of the SOI wafer forms the tiltable structure  22 , the supporting arms  25 A,  25 B, and the actuation structures  30 A 1 ,  30 A 2 ,  30 B 1 ,  30 B 2 , separated by the trench  46 , and a supporting layer  57  has been selectively removed to form part of a cavity  23 ′ and forms reinforcement elements  60  extending underneath the tiltable structure  22 , parallel to the vertical axis Z. The reinforcement elements  60  have a function of mechanical reinforcement for the tiltable structure  22  and project in the cavity  23 ′. The supporting layer  57  moreover defines the shape of the frame portion  24 ′ of the fixed structure  24 . A dielectric layer  58  extends between the active layer  56  and the supporting layer  57 , selectively removed as the supporting layer  57 . The handling wafer  46  extends underneath the supporting layer and is also selectively removed to form a bottom part of the cavity  23 ′. 
     In the MEMS device  20  of  FIGS. 3-5 , the torsion springs  27 A,  27 B,  28 A and  28 B are designed so as to have a high stiffness to movements out of the horizontal plane XY (along the vertical axis Z) and be compliant to torsion (about the first median symmetry axis A). The actuation arms  31 A 1 ,  31 A 2 ,  31 B 1 ,  31 B 2  are designed so as to be flexible to deformations out of the plane (controlled by the respective piezoelectric actuator elements  38 A 1 ,  38 A 2 ,  38 B 1 ,  38 B 2 ) and stiff to torsion. The displacement-transfer beams  32 A 1 ,  32 A 2 ,  32 B 1 ,  32 B 2  are designed so as to enable a decoupling torsion along their longitudinal axis (parallel to the first median symmetry axis A). In particular, the pairs of beams formed by the first and the third displacement-transfer beams  32 A 1 ,  32 B 1  and by the second and the fourth displacement-transfer beams  32 A 2 ,  32 B 2 , respectively, rotate with a rotation direction concordant with each other and discordant with respect to the other pair. Furthermore, the displacement-transfer beams  32 A 1 ,  32 A 2 ,  32 B 1 ,  32 B 2  are designed so as to be flexurally rigid (so as to transfer the movement of the actuation arms  31 A 1 ,  31 A 2 ,  31 B 1 ,  31 B 2  to the supporting arms  25 A,  25 B, causing rotation thereof and decoupling the flexural movement from the torsional movement). The constraint structures  29 A,  29 B and the coupling regions  35 A,  35 A 2 ,  35 B 1 ,  35 B 2  are designed to be substantially rigid to deformations, both of a flexural and of a torsional type so as to co-operate with the displacement-transfer beams  32 A 1 ,  32 A 2 ,  32 B 1 ,  32 B 2  in flexural/torsional decoupling. 
     For instance, the entire suspended structure (including the tiltable structure  22 , the supporting arms  25 A,  25 B, and the actuation structures  30 A 1 ,  30 A 2 ,  30 B 1 ,  30 B 2 ) has a thickness greater than 80 μm, for example 200 μm; the first and the third torsion springs  27 A,  27 B have a width (in a direction parallel to the second horizontal axis Y) comprised between approximately 150 μm and 200 μm, and a length (in a direction parallel to the first horizontal axis X) comprised between approximately 2 mm and 4 mm; the second and the fourth torsion springs  28 A,  28 B have a width comprised between approximately 100 and 200 μm, and a length comprised between approximately 0.3 and 1 mm; the constraint structures  29 A,  29 B have a width comprised between approximately 0.8 and 2 mm and a length comprised between approximately 0.2 and 1 mm; the displacement-transfer beams  32 A 1 ,  32 A 2 ,  32 B 1 ,  32 B 2  have a width comprised between approximately 100 and 200 μm and a length comprised between approximately 1 and 2 mm; and the coupling regions  35 A,  35 A 2 ,  35 B 1 ,  35 B 2  have a width comprised between approximately 300 and 600 μm and a length comprised between approximately 500 and 800 μm. 
     Thereby, during operation of the MEMS device  20 , application of a biasing voltage V to the third piezoelectric actuator element  38 A 1  (and here also to the second piezoelectric actuator element  38 B 1 ) having a positive value with respect to the second piezoelectric actuator element  38 A 2 , (and possibly to the fourth piezoelectric actuator element  38 B 2 , which may, for example, be connected to a ground reference potential) causes rotation by a positive angle about the first median symmetry axis A, as illustrated in  FIG. 6 . 
     Instead, application of a biasing voltage V to the second piezoelectric actuator element  38 A 2  (and possibly to the fourth piezoelectric actuator element  38 B 2 ) having a positive value with respect to the first piezoelectric actuator element  38 A 1  (and possibly to the third piezoelectric actuator element  38 B 1 , which may for example be connected to the ground reference potential), causes rotation by a negative angle about the first median symmetry axis A, opposite to the illustration in  FIG. 6 . 
     By virtue of the presence of two torsion springs in each supporting arm  25 A,  25 B and coupling of the actuation structures  30 A 1  and  30 A 2  to the constraint structures  29 A,  29 B in an intermediate position in each supporting arm  25 A,  25 B, the tiltable structure  22  can rotate through a wider angle with respect to other resonant MEMS devices, reaching increased inclination angles (up to 14°) at a biasing voltage V lower than 30 V (for example, 20 V). Thereby, the MEMS device  20  can work at a high frequency (for example, 60 kHz), with a mirror of a large area (for example, with a diameter of 1.1 mm), and an area of the piezoelectric actuator elements  38 A 1 ,  38 A 2 ,  38 B 1 ,  38 B 2  equal, for example, to 6.5 mm 2 , albeit having a reduced dissipation (proportional to the square of the biasing voltage), and small dimensions of the die  21  (for example, 13×2.6 mm 2 ). 
     Furthermore, the displacement sensor  41  (arranged in proximity of the second end  25 A″ of the first supporting arm  25 A) is close to the frame portion  24 ′ of the fixed structure  24 , and therefore close to the electrical contact pads  40 . Therefore, the connection lines (not illustrated) coupling the displacement sensor  41  to the respective electrical contact pads  40  are short and extend in a non-stressed area of the die  21 , and are therefore more robust and ensure a good sensitivity. 
       FIG. 7  shows a MEMS device  120 , where the actuation arms, here designated by  131 A 1 ,  131 A 2 ,  131 B 1 ,  131 B 2 , extend alongside both the second torsion spring, here designated by  128 A, respectively the fourth torsion spring, here designated by  128 B, and along a fair extent of the first torsion spring, here designated by  127 A, respectively the third torsion spring, here designated by  127 B, and the displacement-transfer beams, here designated by  132 A 1 ,  132 A 2 ,  132 B 1 ,  132 B 2 , extend between the respective actuation arm  131 A 1 ,  131 A 2 ,  131 B 1 ,  131 B 2  and the respective first torsion spring  127 A or third torsion spring  127 B. 
     In detail, in the MEMS device  120 , the first and the second actuation arms  131 A 1 ,  131 A 2  extend along the second torsion spring  128 A and along approximately two-thirds of the length of the first torsion spring  127 A and are connected to the respective displacement-transfer beams  132 A 1 ,  132 A 2  through own intermediate-coupling regions  142 A 1 ,  142 A 2  reversed C-shaped. Vice versa, the third and the fourth actuation arms  131 B 1 ,  131 B 2  extend along the fourth torsion spring  128 B and along approximately two-thirds of the length of the third torsion spring  127 B and are connected to the respective displacement-transfer beams  132 B 1 ,  132 B 2  through own C-shaped intermediate-coupling regions  142 B 1  and  142 B 2 . 
     Here, the piezoelectric actuator elements, designated by  138 A 1 ,  138 A 2 ,  138 B 1 ,  138 B 2 , extend on part of the respective actuation arms  131 A 1 ,  131 A 2 ,  131 B 1 ,  131 B 2 . 
     The displacement-transfer beams  132 A 1 ,  132 A 2 ,  132 B 1 ,  132 B 2  are connected to the torsion springs  127 A,  127 B,  128 A,  128 B at the constraint structures, here designated by  129 A,  129 B, and are of smaller dimensions than in  FIG. 3 . 
     Moreover, in the embodiment of  FIG. 7 , in top plan view, the actuation arms  131 A 1 ,  131 A 2 ,  131 B 1 ,  131 B 2  no longer have a regular rectangular shape, but have a part, close to the intermediate-coupling regions  142 A 1 ,  142 A 2 ,  142 B 1  and  142 B 2 , having a trapezoidal shape with a decreasing width towards the intermediate-coupling regions  142 A 1 ,  142 A 2 ,  142 B 1 , and  142 B 2 . Also the outer perimeter of the cavity, here designated by  123 , is no longer rectangular, but follows the profile of the actuation arms  131 A 1 ,  131 A 2 ,  131 B 1 ,  131 B 2 . Here, the tiltable structure is designated by  122 . 
     For instance, in the embodiment of  FIG. 7 , the actuation arms  131 A 1 ,  131 A 2 ,  131 B 1 ,  131 B 2  have a length comprised between approximately 2 and 3 mm and a width comprised between 400 and 800 mm; the displacement-transfer beams  132 A 1 ,  132 A 2 ,  132 B 1 ,  132 B 2  have a length comprised between approximately 1 and 2 mm; the first and the third torsion springs  127 A,  127 B have a length comprised between approximately 2 and 4 mm; and the second and fourth torsion springs  128 A,  128 B have a length comprised between approximately 300 and 600 μm. 
     By virtue of the position of the displacement-transfer beams  132 A 1 ,  132 A 2 ,  132 B 1 ,  132 B 2 , the MEMS device  120  has a smaller length (in a direction parallel to the rotation axis A) than the MEMS device  20  of  FIGS. 3-5 . 
       FIG. 8  shows a MEMS device  220  similar to the MEMS device  120  of  FIG. 7 , where the actuation arms, here designated by  231 A 1 ,  231 A 2 ,  231 B 1 ,  231 B 2 , are connected together through a first and a second coupling arm  243 A,  243 B. 
     In detail, in  FIG. 8 , the actuation arms, here designated by  231 A 1 ,  231 A 2 ,  231 B 1 ,  231 B 2  have an elongated shape, substantially rectangular in top plan view, and extend for a length roughly equal to two-thirds of the length of the first torsion spring, here designated by  227 A, and of the third torsion spring, here designated by  227 B, respectively. 
     The actuation arms  231 A 1 ,  231 A 2 ,  231 B 1 ,  231 B 2  are connected also here to the respective displacement-transfer beams, here designated by  232 A 1 ,  232 A 2 ,  232 B 1 ,  232 B 2  through intermediate-coupling regions of their own, here designated by  242 A 1 ,  242 A 2 ,  242 B 1  and  242 B 2 , having the shape, for example, of an isosceles trapezium with the base side aligned with the side of the respective actuation arm  231 A 1 ,  231 A 2 ,  231 B 1 ,  231 B 2  that faces the frame portion, here designated by  224 ′. 
     Furthermore, the first and the third actuation arms  231 A 1 ,  231 B 1  are coupled together by the first coupling arm  243 A, which extends as a prolongation of the actuation arms and of the intermediate-coupling regions  242 A 1 ,  242 B 1  with the side facing the frame portion  224 ′ aligned to the corresponding sides of the associated actuation arms  231 A 1 ,  231 B 1  and of the associated intermediate-coupling regions  242 A 1 ,  242 B 1 . 
     Likewise, the second and the fourth actuation arms  231 A 2 ,  231 B 2  are coupled together by the second coupling arm  243 B, which is arranged and configured symmetrically to the first coupling arm  243 A with respect to the first median symmetry axis A. 
     The first and the second coupling arms  243 A,  243 B have a width (in a direction parallel to the second horizontal axis Y) smaller than the actuation arms  231 A 1 ,  231 A 2 ,  231 B 1 ,  231 B 2 , thereby they may deform during the rotation of the MEMS device  220 . For instance, the coupling arms  243 A,  243 B may have a width comprised between approximately 100 and 200 μm. 
     By virtue of the presence of the coupling arms  243 A,  243 B, the MEMS device  220  of  FIG. 8  is able to remove spurious mechanical modes that could be problematic. In fact, due to non-ideality of the real structure, in some cases the constraint structures  129 A,  129 B may rotate in opposite directions, and the actuation arms  231 A 1  and  231 B 1  (or  231 A 2 ,  231 B 2 ) arranged on the same side of the rotation axis X and the corresponding displacement-transfer beams  232 A 1  and  232 B 1  (or  232 B 2  and  232 B 2 ) may tend to move in opposite directions. The coupling arms  243 A,  243 B prevent such undesired opposite movement. 
     The MEMS devices  120  and  220  operate like the MEMS device  20 .  FIG. 9  shows, for example, rotation of the MEMS device  220  of  FIG. 8  when the second and the fourth piezoelectric actuator elements, designated by  238 A 2  and  238 B 2  in  FIG. 8 , are biased by a biasing voltage V that is positive with respect to the first and the second piezoelectric actuator elements, here designated by  238 A 1  and  238 B 1 , and grounded. In particular,  FIG. 9  shows the negative rotation of the tiltable structure, designated by  222  in  FIG. 8 , through an angle of 16° for a voltage V=30 V at a frequency of 25 kHz. Studies conducted by the present applicant have shown that, in this condition, the MEMS device  220  has an acceptable stress level and a lower power consumption, by a factor of 2, with a 30% smaller area of the die, designated by  221  in  FIG. 8 , than known solutions. 
       FIG. 10  shows a MEMS device  420  with a tiltable structure denoted here by  422 , where first and second actuation arms, here designated by  431 A 1 ,  431 A 2 , extend alongside the first torsion spring, here designated by  427 A, roughly parallel thereto, and the third and fourth actuation arms, here designated by  431 B 1 ,  431 B 2 , extend alongside the third torsion spring, here designated by  427 B, roughly parallel thereto. In addition, the first and second displacement-transfer beams, here designated by  432 A 1 ,  432 A 2 , extend transversely to the first supporting arm, here designated by  425 A, between the first constraint structure  429 A and the respective actuation arm  431 A 1 ,  431 A 2 ; and the third and fourth displacement-transfer beams, here designated by  432 B 1 ,  432 B 2 , extend transversely to the second supporting arm, here designated by  425 B, between the second constraint structure  429 B and the respective actuation arm  431 B 1 ,  431 B 2 . 
     In addition, in the MEMS device  420 , the first, second, third and fourth actuation arms  431 A 1 ,  431 A 2 ,  431 B 1 ,  431 B 2  are fixed to the frame portion, here designed by  424 ′, on sides thereof different from the embodiments of  FIGS. 3-9 . While, in the embodiments of  FIGS. 3-9 , both the supporting arms ( 25 A,  25 B in  FIG. 3 ) and actuation arms ( 30 A 1 ,  30 A 2 ,  30 B 1 ,  30 B 2  in  FIG. 3 ) are fixed to the shorter sides ( 72 ,  73  in  FIG. 3 ) of the frame portion  24 , in  FIG. 10 , the supporting arms  425 A,  425 B are fixed to the shorter sides, here designed by  472 ,  473 , of the frame portion  424 ′ and the actuation arms  431 A 1 ,  431 A 2  are fixed to the longer sides, here designed by  470 ,  471 , of the frame portion  424 ′. 
     Furthermore, in the embodiment of  FIG. 10 , the first, second, third and fourth actuation structures, here designated by  430 A 1 ,  430 A 2 ,  430 B 1 ,  430 B 2 , are generally S-shaped, with a smooth, curved connection between each actuation arms  431 A 1 ,  431 A 2 ,  431 B 1 ,  431 B 2  and the respective displacement-transfer beams  432 A 1 ,  432 A 2 ,  432 B 1 ,  432 B 2  and the actuation arms  431 A 1 ,  431 A 2 ,  431 B 1 ,  431 B 2  are sort of bent back from the respective displacement-transfer beam  432 A 1 ,  432 A 2 ,  432 B 1 ,  432 B 2  toward the tiltable structure  422 . 
     In addition, the first, second, third and fourth actuation structures  430 A 1 ,  430 A 2 ,  430 B 1 ,  430 B 2  have a variable width, increasing from the respective constraint structure  429 A,  429 B toward the frame portion  422 ′. 
     Analogously to the embodiments of  FIGS. 3-9 , the piezoelectric actuator elements, here designated by  438 A 1 ,  438 A 2 ,  438 B 1 ,  438 B 2 , extend on the respective actuation arms  431 A 1 ,  431 A 2 ,  431 B 1 ,  431 B 2 . 
     Moreover, also here a displacement sensor  441  of a piezoresistive (PZR) type is arranged in the frame portion  424 ′ of the fixed structure  424 , in proximity of the end of the first supporting arm  425 A fixed to the frame portion  424 ′ and suspended over the cavity (designated here by  423 ). 
     In practice, in the MEMS device  420 , the first and third torsion springs  427 A,  427 B form main torsional springs for allowing rotation of the tiltable structure  422  and the second and fourth torsion springs  428 A;  428 B form secondary torsional springs to allow sensing of the rotation angle of the tiltable structure  422 . Here, the first and third torsion springs  427 A,  427 B are longer than the second and fourth torsion springs  428 A,  428 B. 
     By virtue of the above arrangement of the actuation arms  431 A 1 ,  431 A 2 ,  431 B 1 ,  431 B 2  and the respective piezoelectric actuator elements  438 A 1 ,  438 A 2 ,  438 B 1 ,  438 B 2  laterally to the first and third torsion springs  427 A,  427 B, the MEMS device  420  of  FIG. 10  has a shorter length along the first horizontal axis X and thus a highly compact size in terms of die area. In addition, due to the variable width of the actuation structures  430 A 1 ,  430 A 2 ,  430 B 1 ,  430 B 2 , the stiffness thereof may be differentiated and optimized according to the specific task of the different portions. In particular, the lower width of the displacement-transfer beams  432 A 1 ,  432 A 2 ,  432 B 1 ,  432 B 2  allows them to better transfer the movement and minimize the stresses due to deformation and the larger width of the actuation arms  431 A 1 ,  431 A 2 ,  431 B 1 ,  431 B 2  allows arranging larger piezoelectric actuator elements  438 A 1 ,  438 A 2 ,  438 B 1 ,  438 B 2 , which thus operate with higher force. The structure is thus very efficient. 
     The MEMS device  20 ;  120 ;  220 ;  420  may be used in a picoprojector  301  adapted to be functionally coupled to a portable electronic apparatus  300 , as illustrated hereinafter with reference to  FIGS. 11-13 . 
     In detail, the picoprojector  301  of  FIG. 11  comprises a light source  302 , for example, of a laser type, intended to generate a light beam  303 ; the MEMS device  20 ;  120 ;  220 , intended to receive the light beam  303  and to direct it towards a screen or display surface  305  (external to and set at a distance from the picoprojector  301 ); a first driving circuit  306 , intended to supply appropriate control signals to the light source  302 , for generating the light beam  303  according to an image to be projected; a second driving circuit  308 , intended to supply driving signals for rotating the tiltable structure  22 ;  122 ;  222  ( FIGS. 4, 8, and 9 ) of the MEMS device  20 ;  120 ;  220 ; and a communication interface  309 , intended to receive, from an external control unit  310 , for example included in the portable apparatus  300 , light information on the image to be generated, for example in the form of an array of pixels. The light information is input for driving the light source  302 . 
     Furthermore, the control unit  310  may include a unit for controlling the angular position of the tiltable structure  22 ;  122 ;  222  of the MEMS device  20 ;  120 ;  220 . To this end, the control unit  310  may receive the signals generated by the displacement sensor  41 ;  141 ;  241  ( FIGS. 4, 8, and 9 ) through the interface  309  and accordingly control the second driving circuit  308 . 
     The picoprojector  301  may be a stand-alone accessory separate from an associated portable electronic apparatus  300 , for example, a mobile phone or smartphone, as shown in  FIG. 12 . In this case, the picoprojector  301  is coupled to the portable electronic apparatus  300  through appropriate electrical and mechanical connection elements (not illustrated in detail). Here, the picoprojector  301  has its own casing  341 , with at least a portion  341 ′ transparent to the light beam  303  coming from the MEMS device  20 ,  120 ,  220  and the casing  341  of the picoprojector  301  is coupled in a releasable way to a respective case  342  of the portable electronic apparatus  300 . 
     Alternatively, as illustrated in  FIG. 13 , the picoprojector  301  may be integrated within the portable electronic apparatus  300  and arranged inside the case  342  of the portable electronic apparatus  300 . In this case, the portable electronic apparatus  300  has a respective portion  342 ′ transparent to the light beam  303  exiting the MEMS device  20 ,  120 ,  220 . The picoprojector  301  is in this case, for example, coupled to a printed circuit within the case  342  of the portable electronic apparatus  300 . 
     The MEMS device described herein has numerous advantages. By virtue of two torsion springs in the supporting structures and of the connection of the actuation structure between the two torsion springs, it is possible to obtain a dual decoupling between the flexural movement of the PZT actuators and the tiltable structure. This enables a particularly wide rotation angle of to be obtained as compared to known solutions, also for the resonant movement (about the first median symmetry axis A). 
     Moreover, by virtue of the above geometry, the second and fourth torsion springs can transfer the stress (indirectly measuring the rotation angle) in the proximity of the frame portion of the fixed structure. It is therefore possible to position the displacement sensors (PZR sensors) in the suspended area near the frame portion, reducing the length of the conductive paths between the sensor and the contact pads, and above all without the latter extending over highly stressed areas because of the torsional and/or flexural movement. Consequently, the MEMS device is more reliable and is less subject to failure and electrical interruptions that might jeopardize proper operation thereof. 
     Finally, it is clear that modifications and variations may be made to the MEMS device described and illustrated herein, without departing from the scope of the present invention, as defined in the attached claims. For instance, the various embodiments described may be combined so as to provide further solutions. 
     Moreover, if so required, the MEMS device may include a driving structure along the second median symmetry axis, between the anchorage portion and the rest of the fixed structure. Also the structure of  FIGS. 3-5  may moreover be provided with coupling arms similar to the coupling arms  243 A,  243 B and connecting the first and the third displacement-transfer beams  32 A 1 ,  32 B 1  together and, respectively, the second and the fourth displacement-transfer beams  32 A 2 ,  32 B 2  together. 
     Moreover, the dimensions and shapes illustrated and described herein may vary; for example, the torsion springs  27 A,  27 B,  28 A,  28 B;  127 A,  127 B,  128 A,  128 B,  227 A,  227 B,  228 A, and  228 B may have a thickness equal to or greater than the width. In general, the dimensions, as well as the dimensional ratios, may be designed according to the desired mechanical and elastic characteristics, in a way known to the designer.