Patent Publication Number: US-11655140-B2

Title: Micro-electro-mechanical device with a shock-protected tiltable structure

Description:
PRIORITY CLAIM 
     This application claims the priority benefit of Italian Application for Patent No. 102019000025084, filed on Dec. 20, 2019, the contents of which are hereby incorporated by reference in their entirety to the maximum extent allowable by law. 
     TECHNICAL FIELD 
     This disclosure relates to a micro-electro-mechanical device with a shock-protected tiltable structure. 
     BACKGROUND 
     In particular, reference will be made hereinafter, without any loss of generality, to a micro-electro-mechanical mirror device (manufactured using MEMS, Micro-Electro-Mechanical System, technology), wherein the tiltable structure has a reflecting surface. 
     As is known, micro-electro-mechanical mirror devices are used in portable apparatuses, such as, for example, smartphones, tablets, notebooks, PDAs, and other apparatuses with optical functions, in particular for directing light beams generated by a light source with desired modalities, for projecting images at a distance, in miniaturized projectors (so-called picoprojectors), and in enhanced-reality apparatuses. By virtue of the small dimensions, in fact, micro-electro-mechanical devices are able to meet stringent requirements regarding costs and bulk, both in terms of area and thickness. 
     Micro-electro-mechanical mirror devices generally include a mirror structure, manufactured from a semiconductor material body and elastically supported over a cavity to be movable, for example with a tilting or rotating movement out of a corresponding main extension plane, for directing the incident light beam in a desired way. 
     Typically, in the considered applications, deflection of the light beam along two axes is provided, and may be obtained by two uniaxial micro-electro-mechanical mirror devices, or by a single biaxial micro-electro-mechanical mirror device. 
     Hereinafter, reference will be made to a first design, with two uniaxial micro-electro-mechanical mirror devices, as illustrated in  FIG.  1    for better understanding, and the following also applies to a biaxial mirror structure, as will be evident to the person skilled in the art. 
     In detail,  FIG.  1    schematically shows a picoprojector  1  comprising a light source  2 , typically a laser source, which generates a light beam that is deflected by a system of mirrors  3  toward a screen  4 . 
     In particular, the system of mirrors  3  comprises a first mirror device  3 A, of uniaxial type, driven so as to rotate about an axis A with resonant movement, for generating a fast horizontal scan; and a second mirror device  3 B, also of uniaxial type, driven so as to rotate about a second axis B with 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. 
     In practice, the first mirror device  3 A forms a horizontal mirror device, and the second mirror device  3 B forms a vertical mirror device; they cooperate for generating a scanning scheme, designated schematically by  5  in  FIG.  1   , on the screen  4 . 
     Rotation of the vertical mirror device is controlled by an actuation system that may be of electrostatic, electromagnetic, or piezoelectric type. 
     Electrostatic actuation systems in general require high operating voltages, whereas electromagnetic actuation systems in general involve a high-power consumption. 
     Actuation systems of a piezoelectric type are therefore spreading into wide usage. 
     For instance, in the mirror device described in United States Patent Application Publication No. 20110292479 (incorporated by reference), a suspended frame carrying a mirror surface is connected to a fixed structure via spring elements having a serpentine shape formed by a plurality of mutually parallel arms arranged side-by-side. Each arm carries a piezoelectric band, and adjacent piezoelectric bands are biased by voltages of opposite polarity. Due to the properties of piezoelectric materials, biasing causes the deformation in opposite directions (upwards and downwards) of adjacent arms and the consequent rotation of the suspended frame in a first direction about the horizontal axis B. By applying an opposite biasing, rotation of the frame in a second direction, opposite to the first, is obtained. The vertical scan may therefore be obtained by applying a.c. bipolar voltages to the arms. 
     Another mirror device with piezo-electric actuation is described in United States Patent Application Publication No. 20200192199 (incorporated by reference) corresponding to Italian patent application N. 102018000011112 (incorporated by reference), filed on 14 Dec. 2018, corresponding to European patent application N. 19165958.0 (incorporated by reference), filed on 28 Mar. 2019, and includes an embodiment illustrated in  FIG.  2   . Here, the mirror device, designated by  20 , has a tiltable structure  22  carrying a reflecting surface  22 ′ and suspended over a cavity  23 . The tiltable structure  22  is elastically coupled to a frame  24 ′ belonging to a fixed structure  24  via supporting elements  25 A,  25 B and elastic suspension elements  26 A,  26 B. The tiltable structure  22  is rotatable about a rotation axis corresponding, for example, to the horizontal axis B of the picoprojector  1  of  FIG.  1    and therefore designated once again by B. 
     The tiltable structure  22  is coupled to an actuation structure  30  comprising two pairs of driving arms  32 A- 32 D, each carrying a respective piezoelectric region  33 . The driving arms  32 A- 32 D of each pair are coupled on opposite sides of the rotation axis B (which is here parallel to a first axis X of a Cartesian reference system XYZ) by respective elastic decoupling elements  34 A- 34 D. The elastic decoupling elements  34 A- 34 D are rigid to movements of the tiltable structure out of the tiltable plane defined by the mirror surface (plane AB) and are compliant to torsion about the rotation axis B. 
     In the mirror devices of the type considered, due to the presence of suspended and mobile parts, robustness and resistance to shocks are desired, particularly in directions perpendicular to the rotation axis B (directions parallel to a second and a third axis Y and Z of the Cartesian reference system XYZ of  FIG.  2   ). In fact, uniaxial mirror devices are generally rather rigid in a direction parallel to the rotation axis B (i.e., to the first axis X), but shocks in a direction parallel to the second or third axes Y, Z may cause sharp movements of the tiltable structure  22  along these directions, with possible damage to, and even breakage of, the elastic decoupling elements  34 A- 34 D, which are rigid in these directions, thus jeopardising functionality of the mirror device. 
     To prevent excessive movements of the tiltable structure in directions perpendicular to the rotation axis, stop elements may be arranged between the tiltable structure and the fixed supporting structure. 
     However, this approach only operates properly for shocks and stresses applied to the mirror device when it is in a rest position and does not offer protection when the tiltable structure is rotated. 
     There is accordingly a need in the art to provide a micro-mechanical device that has high robustness to shocks. 
     SUMMARY 
     In an embodiment, a micro-electro-mechanical device comprises: a fixed structure having a cavity; a tiltable structure, elastically suspended over the cavity, having a main extension in a tiltable plane, and rotatable about at least one rotation axis parallel to the tiltable plane; a piezoelectric actuation structure including a first and a second driving arm, the first and second driving arms carrying respective piezoelectric material regions and extending on opposite sides of the rotation axis, the first and the second driving arms being rigidly coupled to the fixed structure and being elastically coupled to the tiltable structure; and a stop structure configured to limit movements of the tiltable structure with respect to the actuation structure along a planar direction parallel to the tiltable plane and perpendicular to the rotation axis during operation, the stop structure including at least a first planar stop element formed between the first driving arm and the tiltable structure and a second planar stop element formed between the second driving arm and the tiltable structure. 
     The first and the second driving arms may be coupled to the fixed structure at a first coupling portion thereof and to the tiltable structure at a second coupling portion thereof. The tiltable structure may have a first stop portion and each driving arm has a respective second stop portion at the second coupling portion. 
     The first and second planar stop elements may each include each a projection and an abutment surface, the projection of each planar stop element being rigid with a stop portion chosen between the first and the second stop portions, and the abutment surface of each planar stop element being rigid with another stop portion, the projection and the abutment surface facing and extending transverse to the planar direction. 
     The abutment surface may be formed by a side wall of a recess and the projection extends within the recess at a distance from the side wall of the recess in a rest condition of the micro-electro-mechanical device. 
     The first and the second driving arms may have a longitudinal extension and have a first end, rigidly coupled to the fixed structure, and a second end, longitudinally opposite with respect to the first end and elastically coupled to the tiltable structure by respective elastic decoupling elements, the projection of each planar stop element extending from the second end of a respective driving arm, laterally with respect to a respective elastic decoupling element. 
     A third and a fourth driving arm may be arranged on a side opposite to the rotation axis and symmetrically, respectively, to the first and the second driving arms with respect to the planar direction of the tiltable plane. The third and fourth driving arms may carry respective piezoelectric material regions and may be elastically coupled to the tiltable structure on opposite sides of the rotation axis by respective elastic decoupling elements. The stop structure further may include a third planar stop element formed between the third driving arm and the tiltable structure and a fourth planar stop element formed between the fourth driving arm and the tiltable structure. 
     The stop structure may include at least one first vertical stop element formed between the first driving arm and the fixed structure and a second vertical stop element formed between the second driving arm and the fixed structure, the first and the second vertical stop elements being configured to limit movements of the tiltable structure along a vertical direction perpendicular to the tiltable plane and directed towards the cavity. 
     The first and the second vertical stop elements may each include a pillar and a stop surface, each pillar extending in a direction transverse to the tiltable structure from a respective driving arm and having a free end, and each stop surface facing the free end of a respective pillar. 
     Each driving arm may have a first planar surface facing the cavity and a second planar surface, opposite to the first surface. The fixed structure may have a frame structure surrounding the cavity, and a cap element fixed to the frame structure, extending underneath the tiltable structure and delimiting, together with the frame structure, the cavity. The pillar of each vertical stop element may extend from the first planar surface of a respective driving arm, and the stop surface of each vertical stop element may be formed by the cap element. 
     The stop structure may also include at least one third vertical stop element formed between the third driving arm and the fixed structure and a fourth vertical stop element formed between the fourth driving arm and the fixed structure, the third and fourth vertical stop elements configured to limit movements of the tiltable structure along the vertical direction perpendicular to the tiltable plane and directed towards the cavity. 
     The stop structure further may include at least one fifth vertical stop element formed between the first driving arm and the fixed structure and a sixth vertical stop element formed between the second driving arm and the fixed structure, the fifth and sixth vertical stop elements configured to limit movements of the tiltable structure along a vertical direction perpendicular to the tiltable plane away from the cavity. 
     The fifth and sixth vertical stop elements may be formed by a die rigid with the fixed structure and extending over the tiltable structure on a side thereof opposite the cavity, the die having a light transparent portion vertically aligned to the tiltable structure and delimited by an edge portion forming an abutment structure for the first and the second driving arms. 
     Also disclosed herein is a picoprojector apparatus for use in a portable electronic apparatus, including: a light source operatable for generating a light beam as a function of an image to be generated; the micro-electro-mechanical device of an optical type as described above, upon which the light beam impinges; and a driving circuit configured to supply electrical driving signals for causing the rotation of the tiltable structure. 
     The portable electronic apparatus may be a viewer for enhanced or virtual reality. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, embodiments thereof are now described purely by way of non-limiting example, with reference to the attached drawings, wherein: 
         FIG.  1    is a schematic representation of a known picoprojector having a pair of uniaxial mirror devices; 
         FIG.  2    is a schematic top view of a uniaxial mirror device; 
         FIG.  3    is a top view of an enlarged detail of a uniaxial mirror device having the general structure of the device of  FIG.  2    and modified to form shock-protection structures, in the rest position; 
         FIG.  3 A  is an enlarged perspective view of the detail of  FIG.  3   , with the tiltable structure in a rotated position; 
         FIG.  4    is a schematic top view of an embodiment of the uniaxial mirror device described and disclosed herein; 
         FIG.  5    is a top view of an enlarged detail of the device of  FIG.  4   ; 
         FIG.  5 A  is an enlarged perspective view of the detail of  FIG.  5   , in a different operating position; 
         FIG.  6    is a cross-sectional view of the device of  FIG.  5   , taken along section line VI-VI of  FIG.  4   ; 
         FIG.  7    is a cross-sectional view similar to  FIG.  6    for a different embodiment of the device of  FIG.  4   ; 
         FIG.  8    is a perspective view of the device of  FIG.  7   ; 
         FIG.  9    is a block diagram of a picoprojector using the micro-electro-mechanical device described and disclosed herein; 
         FIGS.  10  and  11    are perspective views of different coupling possibilities between the picoprojector of  FIG.  9    and a portable electronic apparatus; and 
         FIG.  12    is a perspective view regarding a coupling possibility between the picoprojector of  FIG.  9    and an enhanced-reality viewing system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  3    shows a possible embodiment of the micro-electro-mechanical device  20  for manufacturing a shock-protection structure formed like structures provided in known micro-electro-mechanical acoustic sensors and transducers. 
     Specifically, the micro-electro-mechanical device of  FIG.  3   , designated once again by  20  and of which only a portion at the elastic connection between the first driving arm  32 A and the tiltable structure  22  is illustrated, has a stop structure  40  between the tiltable structure  22  and the fixed supporting structure  24 . 
     In detail, the stop structure  40  has a stop column  41 , which extends, for example, from a substrate underneath the tiltable structure  22  (not visible) through the cavity  23  in a vertical direction (parallel to the third axis Z) and projects, in the tiltable plane AB, in an intermediate position between the first driving arm  32  and the tiltable structure  22 , laterally with respect to the first elastic decoupling element  34 A. 
     The stop column  41  has a fixed projection  42 A, extending within a respective recess  43 A formed on the periphery of the tiltable structure  22 . The fixed projection  42 A, here having a rectangular shape in top view, like the recess  43 A, has dimensions (in particular, a width in a direction parallel to the second Cartesian axis Y) smaller than those of the recess  43 A. In order not to hinder rotation of the tiltable structure  22 , the fixed projection  42 A has the same thickness (in a direction parallel to the third axis Z) as the tiltable structure  22 . In the rest position of the micro-electro-mechanical device  20  (when the tiltable structure  22  is not rotated and the tiltable plane AB is parallel to the plane XY of the Cartesian reference system XYZ), the fixed projection  42 A and the tiltable structure  22  have coplanar top and bottom surfaces, and the fixed projection  42   a  extends within the walls of the recess  43 A at a distance therefrom. 
     Consequently, the stop structure  40  enables free rotation of the tiltable structure  22  and, in a rest condition and in presence of direct shocks parallel to the second axis Y, the displacement of the tiltable structure  22  in this direction is stopped and limited by contact between the recess  43 A and the fixed projection  42 A, thus also limiting the stress applied to the elastic decoupling element  34 A. 
     Similar stop structures may be provided in proximity of the elastic decoupling elements  34 B- 34 D of  FIG.  2   . 
     The stop structure  40  operates properly if the tiltable structure  22  is not rotated at all or is rotated only by a small angle. However, when rotation of the tiltable structure  22  exceeds a preset angle, which depends upon the geometry of the micro-electro-mechanical device  20 , the fixed projection  42 A may no longer face and remain inside the recess  43 A, as illustrated, for example, in the enlarged detail of  FIG.  3 A , where the tiltable structure  22  is rotated in a clockwise direction, as indicated by the arrow R, and the fixed projection  42 A is lower than the tiltable structure  22  and therefore outside the recess  43 A. 
     Therefore, in this situation, the stop structure is not effective and, in presence of shocks directed parallel to the second axis Y, the tiltable structure  22  may undergo a major displacement and jeopardize the integrity of the elastic decoupling elements  34 A- 34 D. 
     A similar situation may arise for shocks or forces acting parallel to the third axis Z. 
       FIG.  4    shows an embodiment of a micro-electro-mechanical device disclosed and described herein, designated by reference  60 , which solves the above problem. 
     The micro-electro-mechanical device  60  has a general structure similar to the micro-electro-mechanical device  20 , but contains numerous improvements; the parts in common with the micro-electro-mechanical device  20  are designated in  FIG.  4    by numbers increased by 40. 
     In detail, the micro-electro-mechanical device  60  is formed in a die of semiconductor material, in particular silicon, and has a tiltable structure  62 . The tiltable structure  62  has a main extension in a plane (hereinafter referred to as tiltable plane AB), which, in a rest position of the micro-electro-mechanical device  60 , is parallel to a plane XY of a system of Cartesian coordinates XYZ (the axes thereof are referred to hereinafter as first Cartesian axis X, second Cartesian axis Y, and third Cartesian axis Z). Thus, in the following description, the thickness of the tiltable structure  62  will be neglected, except where explicitly indicated. 
     Consequently, based on the above, the tiltable structure  62  is arranged so as to rotate about a rotation axis parallel to the first Cartesian axis X, belonging to the tiltable plane AB and corresponding, for example, to the horizontal axis B of the picoprojector apparatus of  FIG.  1    (and therefore designated once again by B). The tiltable structure  62  (substantially like other parts of the micro-electro-mechanical device  60 ) is symmetrical with respect to both the rotation axis B and to a further axis, referred to hereinafter as symmetry axis A, here parallel to the second Cartesian axis Y. 
     The tiltable structure  62  carries a reflecting surface  62 ′, is suspended over a cavity  63  of the die, and is elastically coupled to a frame  64 ′ belonging to a fixed structure  64  via supporting elements  65 A,  65 B and elastic suspension elements  66 A,  66 B. 
     In detail, the supporting elements  65 A,  65 B extend longitudinally along the rotation axis B, on opposite sides of the tiltable structure  62 . 
     The elastic suspension elements  66 A,  66 B, which have a high stiffness to movements out of the tiltable plane AB (along the third Cartesian axis Z, transverse to the tiltable plane AB) and are compliant to torsion about the rotation axis B, extend in proximity of the rotation axis B between a respective suspension element  65 A,  65 B and the tiltable structure  62 . In the illustrated embodiment, the elastic suspension elements  66 A,  66 B comprise straight springs, formed by thinned portions of the supporting elements  65 A,  65 B, but they could be of a folded type having the main extension parallel to the rotation axis B. 
     The micro-electro-mechanical device  60  further comprises an actuation structure  70  formed by four driving arms  72 A- 72 D (hereinafter also referred to as first driving arm  72 A, second driving arm  72 B, third driving arm  72 C, and fourth driving arm  72 ), coupled to the tiltable structure  62  by respective elastic decoupling elements  74 A,  74 B,  74 C and  74 D. The first and second driving arms  72 A,  72 B form a first pair of driving arms and are arranged on opposite sides of the rotation axis B; the third and fourth driving arms  72 C,  72 D form a second pair of driving arms and are arranged on opposite sides of the rotation axis B, symmetrically to the first pair of driving arms  72 A,  72 B with respect to the symmetry axis A. 
     Each driving arm  72 A- 72 D carries a respective piezoelectric region  73 , for example, of PZT (lead zirconate titanate) and, in the embodiment of  FIG.  4   , has a generically trapezial (or fin-like) shape, with the large base fixedly coupled to the frame  64 ′ of the fixed structure  64  and the minor base (designated by  80  in  FIG.  4   ) elastically coupled to the tiltable structure  62 . 
     The elastic decoupling elements  74 A- 74 D are rigid to movements out of the tiltable plane AB and are compliant to torsion about rotation axis B. 
     A plurality of electrical-contact pads  78  are carried by the fixed structure  64  along the frame  64 ′ and are electrically connected (in a way not illustrated in detail in  FIG.  4   ) to the piezoelectric regions  73 , to enable electrical biasing thereof by electrical signals from outside the electro-mechanical device  60  (for example, supplied by a biasing device of an electronic apparatus into which the electro-mechanical device  60  is integrated), as discussed below. 
     The micro-electro-mechanical device  60  further comprises a planar stop structure arranged between the driving arms  72 A- 72 D and the tiltable structure  62  for limiting movement of the latter in the tiltable plane AB in a direction perpendicular to the rotation axis B (i.e., in a direction parallel to the symmetry axis A). 
     Specifically, the planar stop structure comprises at least a first pair formed by a projection and an abutment surface, wherein a first element of the first pair (the projection or the abutment surface) projects from or is fixed to a driving arm  72 A- 72 D, and the other element of the first pair (the abutment surface or the projection, respectively) is fixed to, or projects from, the tiltable structure  62 , and the projection and the abutment surface extend in a direction transverse to the symmetry axis A, are arranged adjacent to and side-by-side, and, in a rest condition of the micro-electro-mechanical device  60 , are arranged at a distance from each other. 
     In the illustrated embodiment, see also the enlarged detail of  FIG.  5   , the first projection/abutment surface pair (designated as a whole by  83 A) comprises a first projection  81 A, extending from the first driving arm  72 A of the first pair, and a first abutment surface formed by a recess  82 A formed in the tiltable structure  62 . In particular, the first projection  81 A extends from the inner end of the first driving arm  72 A (at the minor base  80  of the trapezial shape) toward the tiltable structure  62 , and the first recess  82 A extends from the periphery of the tiltable structure  62  toward the inside thereof; moreover, the free end of the first projection  81 A extends within the first recess  82 A. The first recess  82 A comprises a first and a second transverse wall  84 A,  85 A; both transverse walls  84 A,  85 A are here parallel to the rotation axis B and are spaced from each other by a gap such as to allow a small relative movement between the first projection  81 A and the first recess  82 A along the symmetry axis A before the first projection  81 A abuts against one of the transverse walls  84 A,  85 A. 
     In particular, with reference to  FIG.  5   , if L is the distance between the transverse walls  84 A and  85 A, L 1  is the width of the projection  81 A along the second Cartesian axis Y (in the rest condition of the tiltable structure  62 ), d 1  is the distance between the first projection  81 A and the first transverse wall  84 A, and d 2  is the distance between the first projection  81 A and the second transverse wall  85 B, at rest, then:
 
 L=L 1+ d 1+ d 2.
 
     The micro-electro-mechanical device  60  of  FIG.  4    further comprises a second projection/abutment surface pair  83 B between the second driving arm  72 B and the tiltable structure  62 ; a third projection/abutment surface pair  83 C between the third driving arm  72 C and the tiltable structure  62 ; and a fourth projection/abutment surface pair  83 D between the fourth driving arm  72 D and the tiltable structure  62 . The second, third and fourth projection/abutment surface pairs  83 B- 83 C here have the same structure as the first projection/abutment surface pair  83 A. 
     In use, the tiltable structure  62  may be rotated about the rotation axis B by simultaneously biasing the piezoelectric regions  73  of the first and third driving arms  72 A,  72 C to obtain a rotation in a first direction (as indicated by the arrow R 1  in  FIG.  4   ) and by simultaneously biasing the piezoelectric regions  73  of the second and fourth driving arms  72 B,  72 D to obtain a rotation in a second direction (as indicated by the arrow R 2  in  FIG.  4   ). 
     As described in United States Patent Application Publication No. 20200192199 (incorporated by reference) corresponding to Italian Patent Application No. 102018000011112 cited above, by biasing alternately and in sequence (for example, at a frequency linked to the vertical scan of the picoprojector  1  of  FIG.  1   ) the piezoelectric regions  73  as indicated above, it is thus possible to obtain successive and alternating rotations of the tiltable structure  62  about the rotation axis B. 
     During the rotation movement of the tiltable structure  62 , due to the position of the projection/abutment surface pairs  83 A- 83 D, at least two of the projection/abutment surface pairs  83 A- 83 D (the ones associated to the biased side of the piezoelectric regions  73 ) are active and, in case of undesired shocks or movements of the tiltable structure  62  parallel to the symmetry axis A, limit the degree thereof. 
     For instance,  FIG.  5 A  shows engagement of the projection/abutment surface pair  83 A during the rotation of the tiltable structure  62  in the first direction (arrow R 1  of  FIG.  4   ) in presence of a wide rotation angle. This engagement ensures that, in the event of shock in a direction parallel to the symmetry axis A, the tiltable structure  62  can move for a distance d 1  or d 2  (depending on which way the impact acts) in this direction, preventing excessive stresses on the elastic suspension elements  66 A,  66 B and on the elastic decoupling elements  74 A- 74 D. 
     Consequently, the actuation structure  70  has a high stiffness in the tiltable plane AB, not only along the rotation axis B but also along the symmetry axis A and, as a whole, the micro-electro-mechanical device  60  is robust in the planar direction. 
     According to another aspect of the present description, the micro-electro-mechanical device  60  has a vertical stop structure  90  (perpendicular to the tiltable plane AB of the tiltable structure  62 , parallel to the third Cartesian axis Z in a rest condition of the tiltable structure  62 ). 
     In detail, with reference to  FIGS.  5  and  6   , the vertical stop structure  90  comprises stop pillars  91  cooperating with a rear stop surface  92 . 
     In detail, the stop pillars  91  project from the rear side of the driving arms  72 A- 72 D toward the inside of the cavity  63  and are rigid with the driving arms  72 A- 72 D. For instance, the vertical stop structure  90  may comprise four stop pillars  91 , one for each driving arm  72 A- 72 D, arranged (as represented with a dashed line in  FIG.  5    for the first driving arm  72 A) in proximity of the minor base  80  of the trapezoidal shape of the driving arm. 
     The rear stop surface  92  is formed by a cap structure  93  illustrated in  FIG.  6   . As may be noted from this figure, the cavity  63  is closed at the rear by a substantially parallelepipedal body, for example, of semiconductor material, forming the cap structure  93 . In particular, the cap structure  93  has a top surface facing the cavity  63  forming the rear stop surface  92  and is bonded to the frame  64 ′ of the fixed structure  64  through an adhesive layer  95 , for example, of silicon oxide or polymeric material. A recess  96 , facing the cavity  63 , is formed in the cap structure  93  and extends towards the inside thereof, from the rear stop surface  92 , underneath the tiltable structure  62 , for enabling free rotation of the latter in use. 
     As may be noted from  FIG.  6   , the stop pillars  91  have such a height that their bottom ends, which are free, are arranged at a certain distance from the rear stop surface  92 . The stop pillars  91  thus enable free rotation of the inner ends of the driving arms  72 A- 72 D, and therefore of the tiltable structure  62 , during the actuation movement, but limit the vertical movement of the inner ends of the driving arms  72 A- 72 D in the event of shocks directed out of the plane. 
     As shown once again in  FIG.  6    and as has described in United States Patent Application Publication No. 20200192199 (incorporated by reference) corresponding to Italian Patent Application No. 102018000011112 cited above, the tiltable structure  62  has reinforcement elements  94 , which extend parallel to the third Cartesian axis Z towards the recess  96 . The reinforcement elements  94  are rigid with the tiltable structure  62  and have the function of mechanical reinforcement for the latter. 
     In the illustrated embodiment, the reinforcement elements  94  and the stop pillars  91  have the same height (in a direction parallel to the third Cartesian axis Z) and may be formed simultaneously. For instance, in the illustrated embodiment, where the fixed structure  64 , the tiltable structure  62 , and the elastic elements  74 A- 74 D and  66 A,  66 B are manufactured monolithically in a structural layer or a wafer of semiconductor material, the reinforcement elements  94  and the stop pillars  91  may be formed simultaneously by deep etching the structural layer or wafer to form the cavity  63 . 
     In this case, the reinforcement elements  94  and the stop pillars  91  may have the same height as the structural layer or wafer forming the structures  62 ,  64  and the elastic elements  74 A- 74 D and  66 A,  66 B, and the distance between the stop pillars  91  and the rear stop surface  92  is equal to the thickness of the adhesive layer  95  and is, for example, comprised between 1 and 10 μm. 
     In this way, in the presence of shocks parallel to the third Cartesian axis Z (so-called “out-of-plane direction”) downward (i.e., toward the cap structure  93 , first vertical direction W 1  in  FIG.  6   ), due also to the stiffness of the elastic decoupling elements  74 A- 74 D regarding out-of-plane movements, the movement of the ends of the driving arms  72 A- 72 D, and therefore of the tiltable structure  62 , is limited to a value equal to the thickness of the adhesive layer  95 , notwithstanding the high compliance of the actuation structure  70  in the out-of-plane direction. 
       FIGS.  7  and  8    show a different embodiment wherein the vertical stop structure  90  also comprises stop elements active in case of shocks directed opposite to the first vertical direction W 1 . 
     In detail, in  FIGS.  7  and  8   , the micro-electro-mechanical device  60  has a front die  97  coupled to the fixed structure  64  and provided with a light passage opening  98  at the reflecting surface  62 ′. For instance, the front die  97  may be formed by a semiconductor material wafer or other material opaque to light, for limiting the radiation entering/leaving the micro-electro-mechanical device  60 , and the light passage opening  98  may be concentric to the tiltable structure  62  and have a greater area than the latter. 
     The front die  97  may be fixed to the fixed structure  64  by connection elements  99 , for example formed by a plurality of fixing pillars, having an elongated shape, that have a first end fixed to the frame  64 ′ and a second, opposite, end fixed to the front die  97 . Two fixing pillars  99  arranged in the drawing plane and two fixing pillars  99  (represented by dashed lines) arranged at the back with respect to the drawing plane are visible in  FIG.  7   . 
     As an alternative thereto, the connection structure may be formed by a peripheral wall belonging to a front cap structure extending along the entire periphery of the front die  97 . 
     In both cases, the bottom surface of the front die  97 , facing the driving arms  72 A- 72 D, forms, in proximity of the edge of the light passage opening  98 , an abutment area for the inner ends of the driving arms  72 A- 72 D; this abutment area, in presence of shocks upwards, parallel to the third Cartesian axis Z, vertical direction W 2  of  FIG.  7   , limits movement thereof in said direction. 
     The micro-electro-mechanical device  60  may therefore be used in a picoprojector  101  adapted to be functionally coupled to a portable electronic apparatus  100 , as described hereinafter with reference to  FIGS.  8 - 10   . 
     In detail, the pico-projector  101  of  FIG.  8    comprises a light source  102 , for example a laser source, for generating a light beam  103 ; the micro-electro-mechanical device  60 , for receiving the light beam  103  and directing it toward a screen or display surface  105  (external to and arranged at a distance from the pico-projector  101 ); a first driving circuit  106 , for supplying suitable control signals to the light source  102 , for generating the light beam  103  as a function of an image to be projected; a second driving circuit  108 , for supplying control signals to biasing-voltage generators  76 ,  77  for actuating the micro-electro-mechanical device  60 ; and a communication interface  109 , for receiving, from an external control unit  110 , for example included in the portable apparatus  100  ( FIGS.  9  and  10   ), information on the image to be generated, for example in the form of a pixel array. This information is input for driving the light source  102 . 
     The pico-projector  101  may be manufactured as separate and stand-alone accessory with respect to an associated portable electronic apparatus  100 , for example a mobile phone or smartphone, as illustrated in  FIG.  9   . In this case, the pico-projector  101  is coupled to the portable electronic apparatus  100  by suitable electrical and mechanical connection elements (not illustrated in detail). Here, the pico-projector  101  has its own casing  131 , which has at least one portion  131 ′ transparent to the light beam  103  from the micro-electro-mechanical device  60 ; the casing  131  of the pico-projector  1  is coupled in a releasable way to a respective casing  132  of the portable electronic apparatus  100 . 
     Alternatively, as illustrated in  FIG.  10   , the pico-projector  101  may be integrated within the portable electronic apparatus  100  and be arranged within the casing  132  of the portable electronic apparatus  100 . In this case, the portable electronic apparatus  100  has a respective portion  132 ′ transparent to the light beam  103  from the micro-electro-mechanical device  60 . In this case, the pico-projector  101  is coupled, for example, to a printed circuit board in the casing  132  of the portable electronic apparatus  100 . 
     In another embodiment, the micro-electro-mechanical device  60  may also be integrated in a viewer  150  configured to be worn by a user at a close distance from his eyes and for projecting images for enhanced or virtual reality, as illustrated in  FIG.  11   . In detail, the viewer  150  here comprises sensors  143 ,  144  able, for example, of recording both the reality external to the user and movements of the user, such as movements of his hands or gaze. The information gathered by the sensors  143 ,  144  may be processed by a processing unit  160  and by the control unit  110 , for projecting images specific for the desired application on a lens  165  of the viewer  150 . 
     The advantages obtainable with the described micro-electro-mechanical device  60  are evident from the foregoing. 
     In particular, it is emphasized that, thanks to coupling of the stop structures to the actuation structure  70 , the stop structures are operative also in the rotated condition of the tiltable structure  62  so that the micro-electro-mechanical device  60  has high robustness. 
     Finally, it is clear that modifications and variations may be made to the micro-electro-mechanical device described and illustrated herein, without thereby departing from the scope of this disclosure, as defined in the attached claims. 
     For instance, even though the illustrated embodiment refers to a micromirror that may be actuated for performing a slow vertical scan with linear or quasi-static movement, the micro-electro-mechanical device is not limited thereto, but may form a micromirror device with low-frequency resonant movement or a device of a different, non-optical, type having a piezoelectrically driven, and elastically suspended tiltable structure, the displacements of which are to be limited in a direction perpendicular to the rotation axis. 
     Moreover, as mentioned, the projections  81 A- 81 D and the recesses  82 A- 82 D may be switched around; likewise, the stop pillars  91  may be made to project from the cap structure  93 , instead of from the driving arms  72 A- 72 D. 
     The recesses  82 A- 82 D may be replaced by simple abutment walls; in this case, to limit the in-plane movement perpendicular to the rotation axis B in both directions, the transverse walls may be arranged symmetrically to the rotation axis B, and/or a first transverse wall may be provided for a given actuation arm (for example, the first wall  84 A for the first projection/abutment surface pair  83 A) and a second transverse wall may be provided for an actuation arm of the same pair (in the example considered above, the second transverse wall  85 A of the third projection/abutment surface pair  83 C). 
     In some applications, the light passage opening  98  may be formed by a region transparent to electromagnetic radiation in the frequency range of interest.