Patent Publication Number: US-2021188620-A1

Title: Process for manufacturing an optical microelectromechanical device having a tiltable structure with an antireflective surface

Description:
PRIORITY CLAIM 
     This application claims the priority benefit of Italian Application for Patent No. 102019000025042, 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 process for manufacturing an optical micro-electro-mechanical device having a tiltable structure with an antireflective surface. In particular, reference will be made hereinafter to the manufacture of a micromirror, using MEMS (Micro Electro-Mechanical System) technology. 
     BACKGROUND 
     As is known, a micromirror may be produced starting from semiconductor material. The small dimensions of MEMS semiconductor devices enable, in fact, integration of said devices in portable apparatuses, such as augmented-reality and virtual-reality viewers, portable computers, laptops, notebooks, PDAs, tablets, mobile phones, and smartphones, for optical applications. 
     Typically, such microelectromechanical devices are inserted in miniaturized projector modules (so-called picoprojectors), which are able to project images at a distance or generate desired patterns of light and have, for example, the structure illustrated in  FIG. 1 . 
       FIG. 1  is a schematic illustration of a picoprojector  1  comprising a light source  2 , typically a laser source, a light beam  3 , a micromirror  4 , and a screen  5 . Here, the micromirror  4  directs the light beams  3  coming from the light source  2  onto the screen  5 . 
     In the example illustrated in  FIG. 1 , the micromirror  4  is a mirror of a biaxial type, i.e., capable of rotating about two axes R′ and R″ that are mutually transverse, for example perpendicular to one another, to orient the light beams  3  onto a surface of the screen  5 . Alternatively, it is possible to use two uniaxial mirrors, each capable of rotating only about one axis. 
     A micromirror obtained with MEMS technology generally comprises a reflective structure suspended over a cavity and elastically supported by arms that enable inclination or rotation thereof with respect to the resting plane. Movement of the micromirror is guaranteed by an actuation system that may, for example, be of an electrostatic, piezoelectric, or electromagnetic type. 
     Electrostatic actuation systems in general require high operating voltages, whereas electromagnetic actuation systems in general involve a high-power consumption. Consequently, piezoelectric actuation systems are increasingly used. 
     As example of a microelectromechanical mirror device with actuation of a piezoelectric type, reference may be made to United States Patent Application Publication No. 20200192199 (corresponding to Italian Patent Application No. 102018000011112, filed on Dec. 14, 2018, and European Patent Application No. 19165958.0, filed on Mar. 28, 2019), all of which are incorporated by reference, which describes a microelectromechanical mirror device comprising a micromirror, which can be either of a biaxial type or of a uniaxial type. A top plan view of one of the possible embodiments of a uniaxial type of the aforesaid microelectromechanical device is represented schematically in  FIG. 2 . In detail, the microelectromechanical device, designated by  20 , is formed in a die of semiconductor material, in particular silicon, and is provided with a tiltable structure  22 , which has a main extension in a horizontal plane XY of a cartesian reference system XYZ with origin at the center of the microelectromechanical device  20  and is arranged to rotate about an axis of rotation A, coinciding with a first horizontal axis X of the aforesaid horizontal plane XY (for example, said axis of rotation A corresponds to the axis of rotation R′ represented in  FIG. 1 ). 
     The tiltable structure  22  is suspended over a cavity  23 , obtained in the die, and has, in the embodiment illustrated, a generically elliptical shape in the horizontal plane XY. The tiltable structure  22  carries, at the top, a reflective surface  22 ′ to define a mirror structure  36 . 
     The tiltable structure  22  is elastically coupled to a fixed structure  24 , defined by the die itself. In particular, the fixed structure  24  forms, in the horizontal plane XY, a frame  24 ′, having, for example, a substantially rectangular shape in the plane XY, which delimits and surrounds the aforesaid cavity  23 . The frame  24 ′ carries a first supporting element  25 A and a second supporting element  25 B, which extend along the axis X starting from the frame  24 ′ itself and are suspended over the cavity  23  on opposite sides of the tiltable structure  22 . 
     The tiltable structure  22  is supported by the first and second supporting elements  25 A,  25 B, to which it is elastically coupled by a first elastic suspension element  26 A and a second elastic suspension element  26 B, respectively. 
     The microelectromechanical device  20  further comprises an actuation structure  30 , which is coupled to the tiltable structure  22  and is configured in such a way as to cause rotation thereof about the axis of rotation A. The actuation structure  30  is set between the tiltable structure  22  and the frame  24 ′, and moreover contributes to supporting the tiltable structure  22  over the cavity  23 . 
     The actuation structure  30  comprises four driving arms  32 A- 32 D grouped together in two pairs. The first pair comprises a first driving arm  32 A and a second driving arm  32 B. The second pair comprises a third driving arm  32 A and a fourth driving arm  32 D. The two pairs are identical to one another and arranged symmetrically with respect to a second axis Y of the reference system XYZ. 
     Each driving arm  32 A- 32 D is suspended over the cavity  23  and has a first end fixedly coupled to the frame  24 ′ and a second end elastically coupled to the tiltable structure  22  by a respective elastic decoupling element  34 A- 34 D. In addition, each driving arm  32 A- 32 D carries, on a top surface thereof, opposite to the cavity  23 , a respective actuation region  33  of a piezoelectric type. 
     As illustrated schematically in  FIG. 2 , the microelectromechanical device  20  further comprises a plurality of electrical contact pads  38 , carried by the fixed structure  24  at the frame  24 ′ and are electrically connected (in a way not illustrated in detail in  FIG. 2 ) to the actuation regions  33  of the driving arms  32 A- 32 D to enable electrical biasing thereof. 
     During operation of the microelectromechanical device  20 , application of a biasing voltage V to the actuation regions  33  of the first and third driving arms  32 A and  32 C (having a positive value with respect to the biasing of the actuation regions  33  of the second and fourth driving arms  32 B and  32 D, which may, for example, be connected to a ground reference potential) causes a rotation of the mirror structure  36  in a first direction about the axis of rotation A. 
     Accordingly, application of a biasing voltage V to the actuation regions  33  of the second and fourth driving arms  32 B and  32 D (having a positive value with respect to the biasing of the actuation regions  33  of the first and third driving arms  32 A and  32 C, which may, for example, be connected to a ground reference potential) causes a rotation of the mirror structure  36  in an opposite direction about the axis of rotation A itself. 
     With reference to  FIG. 3 , the microelectromechanical device  20  is made up of two bodies: a sensor body  50  and a supporting body  49 . In a possible implementation, a protective cap  51  is fixed to the microelectromechanical device  20 . 
     The sensor body  50  has a generally parallelepipedal shape, which has a first surface  50 A and a second surface  50 B and houses the mirror structure  36  and the corresponding supporting elements  25 A,  25 B, the actuation structure  30 , the elastic decoupling elements  34 A- 34 D, and the elastic suspension elements  26 A,  26 B, described above with reference to  FIG. 2 . 
     The supporting body  49  is made, for example, of semiconductor material and is coupled to the second surface  50 B of the sensor body  50  by a bonding layer  60  (for example, of silicon oxide) at the frame  24 ′. The supporting body  49  has a recess  61  facing the tiltable structure  22  in order to enable rotation thereof during use. The recess  61  has a bottom surface  61 ′, which is generally blackened, for the reasons discussed below. 
     In the possible implementation considered, the protective cap  51  is made with molded plastic, for example having liquid-crystal polymers (LCPs), and is bonded to the first surface  50 A of the sensor body  50 . The protective cap  51  is substantially shaped like a cup turned upside down and has a chamber  54  closed at the top by a bottom wall  52  set at a distance from the first surface  50 A of the sensor body  50 . Moreover, the protective cap  51  has an opening  53  in the bottom wall  52  facing the reflective surface  22 ′. The bottom wall  52  and the opening  53  limit the light beam  62  that can reach the sensor body  50 . 
     The blackened bottom surface  61 ′ and the protective cap  51  that limits the light beam  62  allow the reflective properties of the microelectromechanical device  20  to be determined by the reflective surface  22 ′ and by the movement of the tiltable structure  22  in use. 
     This is particularly useful in the case where the microelectromechanical device  20  is used within augmented-reality or virtual-reality viewers, or more in general in all those head-mounted systems (HMSs) or head-mounted displays (HMDs), where it is desired to obtain excellent optical properties, in particular reflective properties, in order to help guarantee optimal performance of the devices themselves and protect the safety of users. 
     In fact, especially regarding the safety of users, the exposed metal surfaces of the sensor structure  50  represent a concern. In particular, spurious reflections of the light rays incident on the microelectromechanical device  20  may focus accidentally on the retinas of users, causing irritation thereof. 
     However, meeting the above safety desires is not simple with the microelectromechanical device  20  described above, where the two bodies  49 - 50  are manufactured separately and bonded to the protective cap  51  in a final stage (the so-called back-end stage). 
     In detail, according to what has been described in the Italian patent application cited above, the sensor body  50  is obtained starting from a SOI (Silicon-On-Insulator) wafer  40 , formed by two layers of semiconductor material (hereinafter referred to as first and second structural layers  40 A,  40 B), for example silicon, and by an intermediate insulating layer  40 C, for example silicon oxide. 
     Defined by chemical etching in the first structural layer  40 A (for example, having a thickness of 20 μm) are the tiltable structure  22 , the fixed structure  24 , the elastic decoupling elements  34 A- 34 D, the elastic suspension elements  26 A,  26 B (not illustrated in  FIG. 3 ), the supporting elements  25 A,  25 B (not illustrated in  FIG. 3 ), and the driving arms  32 A- 32 D. The cavity  23  is formed by chemical etching of selective portions of the second structural layer  40 B (for example, having a thickness of 140 μm) and of the intermediate insulating layer  40 C. 
     Underneath the tiltable structure  22 , following upon etching for formation of the cavity  23 , there remain reinforcement elements  41 , which have an extension along an orthogonal axis Z of the reference system XYZ and have the function of mechanical reinforcement. 
     Formed on a top surface  40 ′ of the first structural layer  40 A of the SOI wafer  40  are: the reflective surface  22 ′, at the tiltable structure  22 , made of an appropriate material (for example, aluminum, or else gold, according to whether the projection is in the visible or in the infrared); and moreover bottom-electrode regions  42 , made of an appropriate conductive material, at the driving arms  32 A- 32 D. 
     Regions of piezoelectric material  43  (constituted by a thin film of PZT-Lead Zirconate Titanate) are then obtained on top of the bottom-electrode regions  42 , and top-electrode regions  44  on top of the regions of piezoelectric material  43 , thus forming the actuation regions  33 . 
     A passivation layer  45 , made of an appropriate dielectric material, is formed, as covering, on top of the actuation regions  33 , and contact openings  46  are open through the passivation layer  45  to access the bottom-electrode regions  42  and the top-electrode regions  44 . 
     Metal routing regions  47  are then formed on the passivation layer  45  to contact, through the contact openings  46 , the bottom-electrode regions  42  and the top-electrode regions  44 , moreover extending up to respective electrical-contact pads  38  (here not illustrated). 
     A further wafer of semiconductor material, for example silicon, configured to form the supporting body  49 , is selectively etched to obtain the recess  61  and obtain blackening of the bottom surface  61 ′ of the latter. For instance, in a way known to the person skilled in the art, the bottom surface  61 ′ may be machined to increase the roughness thereof. 
     The further wafer of semiconductor material is then bonded to the SOI wafer  40  via the bonding layer  60 , and the composite wafer is then diced to form the microelectromechanical device  20 . 
     In the considered possible implementation, the protective cap  51  is molded by or on behalf of the manufacturer of the apparatus in which the microelectromechanical device  20  is to be mounted, who hence also fixes the protective body  51  to the microelectromechanical device  20 . 
     As is evident from the foregoing, with said implementation, the process of fixing of the protective cap  51  (capping step), carried out in the back-end stage, is complex and is not standardized in so far as it is carried out in a factory different from that where the microelectromechanical device  20  is produced and depends upon the specific application and the technology used. 
     In addition, use of the protective cap  51  made of plastic in the back-end may easily result in problems of alignment with the microelectromechanical device  20 , in particular between the opening  53  of the protective cap  51  and the tiltable structure  22  of the sensor body  50 , causing defects of operation of the microelectromechanical devices. 
     Therefore, further development is needed to provide a microelectromechanical device that allows the drawbacks of the prior art to be overcome. 
     SUMMARY 
     Embodiments herein concern a process for manufacturing an optical microelectromechanical device and an optical microelectromechanical device. 
     Indeed, described herein is a process for manufacturing an optical microelectromechanical device, including: forming, in a first wafer of semiconductor material having a first surface and a second surface, a suspended mirror structure, a fixed structure, surrounding the suspended mirror structure, elastic supporting elements extending between the fixed structure and the suspended mirror structure, and an actuation structure coupled to the suspended mirror structure; forming, in a second wafer, a chamber delimited by a bottom wall having a through opening; bonding the second wafer to the first surface of the first wafer and a third wafer to the second surface of the first wafer so that the chamber overlies the actuation structure, and so that the through opening is aligned to the suspended mirror structure, thus forming a device composite wafer; and dicing the device composite wafer to form an optical microelectromechanical device. 
     Prior to bonding the third wafer to the second surface of the first wafer, the process may include forming, on the third wafer, an inner antireflective surface facing the suspended mirror structure. 
     Bonding the second wafer to the first surface of the first wafer may be carried out prior to the step of bonding the third wafer to the second surface of the first wafer. 
     Forming the chamber may include: in a work wafer having a first surface and a second surface, selectively removing portions of the work wafer starting from the first surface to form a first recess and a second recess, wherein the first recess is surrounded by the second recess and extends within the work wafer from the second recess; and thinning the work wafer from the second surface until reaching the first recess, to form the second wafer having the through opening and an outer surface opposite to the first surface. 
     The outer surface of the second wafer may undergo blackening to cause the outer surface to be absorbant or diffusive with respect to light. 
     Blackening may include increasing the roughness of the outer surface. 
     Blackening may include depositing absorbant or dielectric layers onto the outer surface. 
     The second wafer may be made of semiconductor material. 
     Forming a suspended mirror structure may include selective chemical etching to release the suspended mirror structure, the selective chemical etching being carried out after bonding the second wafer to the first surface of the first wafer. 
     Forming a suspended mirror structure may include selective chemical etching to release the suspended mirror structure, the selective chemical etching being carried out prior to bonding the third wafer to the second surface of the first wafer. 
     The actuation structure may be of a piezoelectric type. 
     A bottom surface of the second recess of the second wafer and a side wall of the first recess of the second wafer may form an angle α comprised between 10° and 90°. 
     Also disclosed herein is an optical microelectromechanical device, which may include: a sensor body of semiconductor material, having a first surface and a second surface and having a suspended mirror structure, a fixed structure surrounding the suspended mirror structure, elastic supporting elements extending between the fixed structure and the suspended mirror structure, and an actuation structure, coupled to the suspended mirror structure; a protective cap, of semiconductor material or glass, bonded to the first surface of the sensor body and having a chamber overlying the actuation structure, the chamber being delimited by a wall having a through opening and having an outer surface; the outer surface being absorbant or diffusive to light, and the through opening being aligned to the suspended mirror structure; and a supporting body bonded to the second surface of the sensor body. 
     The supporting body may have an inner antireflective surface facing the suspended mirror structure. 
     Also disclosed herein is a picoprojector apparatus for use in a portable electronic apparatus, including: a light source, which may be operated for generating a light beam as a function of an image to be generated; the optical microelectromechanical device described above; and a driving circuit configured to supply electrical driving signals for rotating the suspended mirror structure. This portable electronic apparatus may be a viewer for augmented-reality or virtual-reality. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For better understanding, an embodiment is now described, purely by way of non-limiting example, with reference to the attached drawings, wherein: 
         FIG. 1  is a perspective schematic representation of a known picoprojector; 
         FIG. 2  is a top plan view of a microelectromechanical mirror device; 
         FIG. 3  is a cross-sectional view of the device of  FIG. 2 , taken along the line of section III-III. 
         FIGS. 4, 5A-5H, and 6-9  are cross-sectional views of wafers of semiconductor material in successive manufacturing steps of the microelectromechanical mirror device disclosed and described herein; 
         FIG. 10  is a perspective view of the microelectromechanical mirror device disclosed and described herein; 
         FIG. 11  is a block diagram of a picoprojector that uses the microelectromechanical mirror device disclosed and described herein; 
         FIG. 12  is a perspective view regarding a possibility of coupling between the picoprojector of  FIG. 10  and a portable electronic apparatus; and 
         FIG. 13  is a perspective view regarding a possibility of coupling between the picoprojector of  FIG. 10  and an augmented-reality viewing system. 
     
    
    
     DETAILED DESCRIPTION 
     Described hereinafter are steps for manufacturing a microelectromechanical device, in particular a mirror device obtained with MEMS technology, which can be used in picoprojectors and can be inserted in mobile apparatuses, in particular mobile phones and augmented-reality and virtual-reality viewers. 
     In particular, the manufacturing steps described below allow a microelectromechanical mirror device to be obtained having a general structure similar to the one illustrated in  FIGS. 2 and 3  (but with substantial improvements made) and described briefly hereinafter. In particular,  FIGS. 4, 5A-5H, and 6-9  refer to intermediate structures sectioned along lines of section corresponding to the line of  FIG. 2 . 
       FIG. 4  shows a SOI wafer  90  having a first surface  100 A and a second surface  100 A,  100 B and comprising a first structural layer  90 A and a second structural layer  90 B, which are, for example, made of silicon, and an intermediate insulating layer  90 C, which is, for example, made of silicon oxide. The SOI wafer  90  has already been subjected to first manufacturing steps that lead to formation, within the first structural layer  90 A, of actuation regions  83  (which comprise bottom-electrode regions, top-electrode regions, a passivation layer, contact openings, and metal routing regions, similar to the homologous structures of  FIG. 3  and here not illustrated). Moreover, by chemical etching of the first structural layer  90 A of the SOI wafer  90 , there have already been defined a tiltable structure  72 ; supporting elements  75 A,  75 B similar to the supporting elements  25 A,  25 B of  FIG. 2 , here visible only in part and delimited by a dashed line; elastic suspension elements similar to the elastic suspension elements  26 A,  26 B illustrated in  FIG. 2  (here not illustrated); four driving arms (here just two driving arms  82 A,  82 C are visible); and four elastic decoupling elements, of which only two elastic decoupling elements  84 A and  84 C are visible. In addition, a reflective surface  72 ′ has already been formed on top of the tiltable structure  72 , forming a mirror structure  86 . 
     Separately, a cap wafer  10 , made of semiconductor material, for example silicon, comprising a work substrate  10 ′ and an insulating layer  10 ″, for example of silicon oxide is machined, as shown in  FIG. 5A . The work substrate  10 ′ has a first work surface  10 A, set between the work substrate  10 ′ and the insulating layer  10 ″, and a second work surface  10 B, opposite to the first work surface  10 A. 
     In  FIG. 5B , a first opening  11 ′ is obtained in the insulating layer  10 ″ via known lithographic steps and chemical-etching steps. 
     A masking region  12  is then formed on top of the first work surface  10 A and on top of the insulating layer  10 ″ of the cap wafer  10 , via deposition and lithographic definition of a masking layer (for example, a resist layer); the masking region  12  forms an inner second opening  11 ″, for example concentric to the first opening  11 ′ ( FIG. 5C ). 
     Via a first chemical etching and using the masking region  12 , part of the work substrate  10 ′ is removed, for example for a depth of 50 μm, in a region corresponding to the second opening  11 ″, to create a first recess  13 ′. The masking region  12  is then removed ( FIG. 5D ). 
     Using the remaining portions of the insulating layer  10 ″ as a mask, a second chemical etching is carried out that further removes, for example for a depth of 100 μm, part of the work substrate  10 ′, making the first recess  13 ′ deeper and creating a second recess  13 ″, wider than the first recess  13 ′. In practice, the first and second recesses  13 ′,  13 ″ are arranged underneath the first opening  11 ′, illustrated in  FIG. 5B . The remaining portions of the insulating layer  10 ″ are then removed to obtain the intermediate structure of  FIG. 5E . The remaining portions of the first surface  10 A of the work substrate  10 ′ ( FIG. 5A ) form a temporary-contact surface  14  of the work substrate  10 ′, which has a smaller area than the first surface  10 A and surrounds the second recess  13 ″. The angle α, defined between a bottom surface  13 A of the second recess  13 ″ and a side wall  13 B of the first recess  13 ′, which in  FIG. 5E  is 90°, can have values comprised between 10° and 90° according to the specific chemical etching used and the desired profile of the opening for passage of the light beam (onto the reflective surface  72 ′ illustrated in  FIG. 4 ) in the final device, according to the application. 
     The work substrate  10 ′ of  FIG. 5E  undergoes a thinning step, for example through a grinding process, as illustrated in  FIG. 5F . To carry out this thinning process, the work substrate  10 ′ is conveniently supported according to techniques known to the person skilled in the art, for example by temporary bonding between the work substrate  10 ′, and a supporting wafer (here not illustrated), at the temporary-contact surface  14 . In particular, the bonding process may envisage possible intermediate layers of material that facilitate the above process, in a way that is also known. In particular, the thinning process is carried out from the back, starting from the second work surface  10 B of  FIG. 5E  and proceeds until the first recess  13 ′ is reached, to obtain a thinned substrate  15 , having an outer surface  15 A. Following upon the thinning step, the second recess  13 ″ forms a chamber  104  delimited underneath by a bottom wall  102 , and the first recess  13 ′ becomes a through recess and forms an opening  103  for the passage of light beams in the finished microelectromechanical device ( FIG. 9 ). 
     Next, with reference to  FIG. 5G , the outer surface  15 A of the thinned substrate  15  undergoes a blackening process to make the outer surface  15 A absorbant or diffusive in regard to light. For instance, the outer surface  15 A may undergo an oxygen plasma etching (O 2 ) or to laser etching, or some other work process such as to increase the roughness thereof, in a way known to the person skilled in the art. Alternatively, in a way equally known to the person skilled in the art, the outer surface  15 A may be coated with thin layers of light absorbing materials or with dielectric multilayers designed for suppressing the reflected component of light. 
     The thinned substrate  15  is then temporarily bonded to a temporary supporting wafer  16  via a layer of adhesive material  17 , with the layer of adhesive material  17  facing the outer surface  15 A of the thinned substrate  15  and the opening  103  ( FIG. 5H ). 
     In  FIG. 6 , the thinned substrate  15  is turned over and bonded on the first surface  100 A of the SOI wafer  90  of  FIG. 4  in such a way that the chamber  104  surrounds and houses the actuation regions  83 , and the opening  103  is aligned with, and faces, the mirror structure  86 . A first composite wafer  109  is thus formed. 
     In  FIG. 7 , the first composite wafer  109  undergoes chemical etches, starting from the second surface  100 B of the SOI wafer  90  (which now delimits the first composite wafer  109  and hence will also be referred to as bottom surface  100 B of the first composite wafer  109 ). In particular, the second structural layer  90 B of the SOI wafer  90  and the intermediate insulating layer  90 C are selectively removed to create a cavity  73  that delimits underneath, and releases, the tiltable structure  72 , the driving arms  82 A,  82 C (as well as the other two arms corresponding to the driving arms  32 B,  32 D of  FIG. 2 , here not visible), the elastic decoupling elements  84 A and  84 C, the supporting elements  75 A,  75 B (visible only in part and delimited by a dashed line in  FIG. 7 ), and elastic suspension elements similar to the elastic suspension elements  26 A,  26 B illustrated in  FIG. 2  and here visible. Each driving arm (here,  82 A,  82 C) forms, together with the respective actuation region  83 , an actuation structure  80 . In this step, reinforcement elements  91  are also formed, which extend from the tiltable structure  72  within the cavity  73 . Finally, in this step, a fixed structure  74  is defined, which surrounds the cavity  73  and is delimited in  FIG. 7  by a dashed line. 
     In  FIG. 8 , a supporting wafer  98  is bonded to the remaining portions of the bottom surface  100 B of the first composite wafer  109  by a bonding layer  110  to form a second composite wafer  112 . The supporting wafer  98  has been previously machined, in a way similar to what has been described with reference to  FIG. 3 , to form a rear recess  111  having a bottom surface  111 ′ which is blackened. In particular, the supporting wafer  98  is bonded in such a way that the rear recess  111  is located at the cavity  73 , facing and underneath the tiltable structure  72 . 
     Finally, as shown in  FIG. 9 , the temporary supporting wafer  16  and the layer of adhesive material  17  are removed, and the second composite wafer  112  is diced to obtain a microelectromechanical mirror device  70 . The microelectromechanical mirror device  70  thus comprises a sensor body  100 , a protective cap  101 , and a supporting body  99 , obtained, respectively, from the SOI wafer  90 , the cap wafer  10 , and the supporting wafer  98 . 
     The resulting microelectromechanical mirror device  70  is illustrated also in  FIG. 10 . In detail,  FIG. 10  shows the protective cap  101  bonded to the sensor body  100 , with the opening  103  aligned to the mirror structure  86  and to the bottom surface  111 ′, which is blackened, of the supporting body  99 . The protective cap  101  hence covers the actuation structures  80  (here not visible), leaving the control pads  88  free. 
     In this way, the sensor body  100 , the protective cap  101 , and the supporting body  99  may be obtained using the technologies and machines for semiconductor work, in one and the same factory. 
     Manufacturing the protective cap  101 , now integrated in the process cycle as described above, moreover makes it possible to carry out the blackening of the outer surface  15 A using efficient techniques, in particular in the case where the protective cap  101  is made of silicon. In this way, the outer surface  15 A can absorb or diffuse the light beams  105  incident thereon, preventing undesired spurious reflections. In this way, the radiation reflected by the microelectromechanical device  70  is determined only by the light beams  105  reflected by the reflective surface  72 ′. 
     Furthermore, the described manufacturing process allows for high freedom in the choice of the value of the angle α; in this way, it is possible to select in a controlled way the angle of incidence of the light beams  105  onto the tiltable structure  72 , helping guarantee better control of operation of the microelectromechanical mirror device  70 . 
     The techniques described herein also help guarantee a high versatility in manufacturing the protective cap  101 , preventing problems of alignment of the opening  103  with the tiltable structure  72  during post-manufacture back-end bonding, and hence possible malfunctioning of the microelectromechanical mirror device  70  during operation. 
     The microelectromechanical device  70  can be used in a picoprojector  120  designed to be functionally coupled to portable electronic apparatuses, as illustrated schematically in  FIGS. 11-13 . 
     In detail, the picoprojector  120  of  FIG. 11  comprises a light source  122 , for example a laser light source, designed to generate a light beam  123 ; the microelectromechanical mirror device  70 , designed to receive the light beam  123  and to direct it toward a screen or display surface  125  (external to and set at a distance from the picoprojector  120  itself); a first driving circuit  126 , designed to supply appropriate control signals to the light source  122 , for generation of the light beam  123  as a function of an image to be projected; a second driving circuit  128 , designed to supply driving signals to the microelectronic device  70 ; and a communication interface  129 , designed to receive, from an external control unit  130 , for example included in the portable apparatus  121 , information on the image to be generated, for example in the form of a pixel array. This information is sent at input for driving the light source  122 . 
     The picoprojector  120  may be obtained as a separate and stand-alone accessory with respect to an associated portable apparatus or may be integrated therein. 
     Consider, for example,  FIG. 12 , where the picoprojector  120  is arranged within a casing  133  of a portable electronic apparatus  121 . In this case, the portable electronic apparatus  121  has a respective portion  132 ′ transparent to the light beam  123  coming from the microelectronic device  70 . The picoprojector  120  is in this case for example coupled to a printed-circuit board present within the casing  133  of the portable electronic apparatus  121 . 
     In another configuration, thanks to the excellent optical properties of the present microelectromechanical device  70 , this can be integrated safely for the user also in a viewer  142  configured for being worn at a close distance from the eyes and for providing augmented-reality or virtual-reality images, as illustrated in  FIG. 13 . In detail, the viewer  142  comprises, for example, sensors  143 ,  144  for recording both the reality external to the user and movements of the user himself, such as movements of his hands or gaze. The information gathered by the sensors  143 ,  144  can be processed by a processing unit  145  and supplied to the control unit  130  for projecting images specific to the desired application by means of the picoprojector  120  on a lens  146  operating as screen. 
     Finally, it is clear that modifications and variations may be made to the microelectromechanical mirror device  70  and to the manufacturing process described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the annexed claims. 
     For instance, the mirror structure, the elastic suspension elements and the actuation system may have different shapes. Moreover, the protective cap may be manufactured starting from different materials, for example glass, using different blackening processes and may have a different shape. In addition, the opening of the protective cap may be obtained using different processes either of a mechanical type or of a chemical type, for example via deep chemical etching. 
     The actuation system may be different, and may even not be a piezoelectric system.