Abstract:
The mirror group is formed by a monolithic frame bent along a bending line and including a first and a second supporting portions carrying, respectively, a first and a second chips forming two micromirrors made using MEMS technology. The first and second supporting portions are arranged on opposite sides of the bending line of the frame, angularly inclined with respect to each other. The mirror group is obtained by separating a shaped metal tape carrying a plurality of frames, having flexible electric connection elements. After attaching the chips, the frames are precut, bent along the bending line, and separated.

Description:
BACKGROUND 
       [0001]    Technical Field 
         [0002]    The present disclosure relates to a mirror package, in particular for a picoprojector, comprising micromirrors obtained using the MEMS technology. 
         [0003]    Description of the Related Art 
         [0004]    As is known, picoprojectors are devices of small dimensions which are able to project, on a wall or other surface, images, video clips, and other visual documents stored in small portable electronic equipment, such as cellphones, tablets, laptops, and the like. Furthermore, it has already been proposed to integrate such picoprojectors on the dashboard of motor vehicles, for example for projecting onto the windscreen information on satellite-navigation functions and other route or communication information. 
         [0005]    In order to reduce the size, picoprojectors use MEMS micromirror systems, which are able to turn about two axes in order to perform a movement of scanning of a two-dimensional area. In particular, in some solutions, micromirror systems comprise a pair of MEMS mirrors driven for turning about two mutually perpendicular rotation axes. 
         [0006]    For instance,  FIG. 1  is a schematic illustration of a picoprojector wherein a light source  1 , typically a laser source, generates a light beam  2  (generally formed by three monochromatic beams, one for each base color), which, through an optical system  3  shown only schematically, is deflected by a pair of MEMS mirrors  5 ,  6 . The first MEMS mirror  5  may, for example, be a horizontal micromirror, rotating about a first axis A and generating a fast horizontal scan, and the second MEMS mirror  6  may, for example, be a vertical micromirror, rotating about a second axis B, transverse, in particular perpendicular, to the first axis A, and generating a slow vertical scan, typically of a sawtooth type. The combination of the movements of the two MEMS mirrors  5 ,  6  causes the light beam  2  to perform a complete two-dimensional scanning movement and, once projected on a projection display  7 , to generate a two-dimensional image thereon. Such a system is described, for example, in WO 2010/067354, which also published as U.S. Pat. Pub. No. 2011/234898. 
         [0007]    MEMS mirrors  5 ,  6  may be formed as shown in  FIG. 2 . Here, a chip  10  comprises a mirror element  11  having a reflecting surface  9  and suspended over a semiconductor substrate  8 . The mirror element  11  is supported by a pair of arms  12 , which extend from opposite sides of the mirror element  11  and define the rotation axis of the mirror element  11  (for example, rotation axis B of the vertical micromirror  6 ). The arms  12  are connected to a fixed peripheral portion  13  of the chip  10 , which is fixed with respect to the substrate  8 , via torsion springs  14  enabling rotation of the arms  12  about the axis B. The arms  12  are further coupled to a driving structure formed by actuation units  18  of an electrostatic type. Each actuation unit  18  comprises mobile electrodes  19  facing fixed electrodes  20 . 
         [0008]    In detail, the mobile electrodes  19  are fixed with respect to the arms  12  and are comb-fingered with respect to the fixed electrodes  20  for generating a capacitive coupling. The fixed electrodes  20  are carried by supporting regions  21 , which are generally fixed with respect to the substrate  8  of the chip  10 . By virtue of the arrangement of the electrodes  19 ,  20 , the driving structure is also defined as “comb-drive structure”. 
         [0009]    By applying appropriate voltages between the mobile electrodes  19  and the fixed electrodes  20 , it is possible to generate attraction/repulsion forces between them and cause rotation of the mobile electrodes  19  with respect to the fixed electrodes  20 , rotation of the arms  12  about axis B, and thus the corresponding controlled rotation of the mirror element  11 . 
         [0010]    Currently, MEMS mirrors  5 ,  6  are manually mounted in a picoprojector, each already fixed to a respective support. For instance,  FIG. 3  shows a mirror package  24  wherein a chip, designated again by  10 , is fixedly coupled to a support  25 , generally of metal. An electric connection structure  26 , for example a flexible printed circuit (FPC) carrying electric connection lines  27 , is also fixedly coupled to the support  25 . The electric connection lines  27  are connected to the chip  10  via electric wires  28 , in a per se known manner. 
         [0011]    During assembly in a portable electronic apparatus, an operator picks up two mirror chips, one for a horizontal MEMS mirror and the other for a vertical MEMS mirror, and using a jig and microactuators in a specific apparatus, positions the two chips to mate them, until they are brought into the desired alignment. Next, the operator applies a curable glue using ultraviolet light, and the two mirror chips are fixed in the operative position. 
         [0012]    This type of assembly process is slow, difficult, and sensibly error-prone. It follows that, with this method, the throughput and yield of correctly mounted pieces are not optimal. 
       BRIEF SUMMARY 
       [0013]    According to one or more embodiments of the present disclosure, a mirror package, a tape, and a method for assembly of the mirror package are provided. 
         [0014]    In one embodiment, two micromirrors, horizontal and vertical, are assembled on a same metal frame and electrically couple by wire to a respective electric connection element (flexible printed circuit) already fixed to the frame. Then, the frame is bent so as to position the two micromirrors with the desired mutual angular arrangement. Bending may be carried out automatically, without human intervention, using a forming press similar to those used in the semiconductor industry for shaping connection conductors in packages of standard integrated circuits. The frame may be carried by a tape, together with a plurality of similar frames, and the individual frames may be separated after a single bending step. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0015]    For a better understanding of the present disclosure, preferred embodiments thereof are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein: 
           [0016]      FIG. 1  is a schematic perspective view of a picoprojector; 
           [0017]      FIG. 2  is a schematic illustration of an embodiment of a micromirror of  FIG. 1 ; 
           [0018]      FIG. 3  is a top plan view of a micromirror mounted on a support prior to assembly in an electronic apparatus; 
           [0019]      FIG. 4  is a perspective view of the present mirror package; 
           [0020]      FIG. 5  is a side view of the mirror package of  FIG. 4 ; and 
           [0021]      FIGS. 6-10  show, in perspective view, a tape comprising a plurality of frames for manufacturing micromirror packages in successive assembly steps. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIGS. 4 and 5  show a mirror package  30  designed to be mounted in a portable electronic apparatus (not shown). 
         [0023]    The mirror package  30  comprises a frame  31  of rigid material bent into a V-shape for forming a first supporting portion  32 , carrying a first chip  35 , and a second supporting portion  33 , carrying a second chip  36 . The frame  31  is in general of sheet metal, typically aluminum, for example with a thickness of about 0.5 mm so as to enable bending thereof in a purposely designed machine (as explained in detail hereinafter) but for have, at the same time, a sufficient stiffness degree to prevent any deformation during handling and a sufficient planarity of the supporting portions  32 ,  33  for precise positioning the chips  35 ,  36 . 
         [0024]    The first chip  35  is here, for example, a vertical MEMS mirror, and the second chip  36  is here, for example, a horizontal MEMS mirror. The chips  35 ,  36  are fixed to the respective supporting portions  32 ,  33  via a glue, for example a bi-adhesive layer (so called die attach film), standard in the semiconductor industry. 
         [0025]    Furthermore, the frame  31  carries two electric connection structures  37 ,  38 , one for each chip  35 ,  36 , formed, for example, as sticky printed circuits, so called flexible printed circuits (FPCs) of a known type, embedding electric connection lines (not visible here) similar to the electric connection lines  27  of  FIG. 3 . The electric connection structures  37 ,  38  are coupled to the chips  35 ,  36  via wires  60 . 
         [0026]    The first supporting portion  32  is connected to the second supporting portion  33  via connection arms  49  having S-shaped bent portions  52 , as described in detail below with reference to  FIG. 8 . 
         [0027]    The frame  31  is obtained via shearing and bending of a plane metal sheet, as described hereinafter with reference to  FIGS. 6-10 . In particular, the frame  31  is bent about a median virtual line designated by C in  FIG. 5  so as to obtain a preset angle α, for example 60°, between the two supporting portions  32 ,  33 , and a preset height H, for example 2.6 mm, between the edge of the second chip  36  which is far with respect to the bending line C and the upper surface of the first chip  35 . In fact, bending determines a deformed area wider than the bending line. For simplicity, in the following, the median virtual line referred to above will be defined as a bending line C, and the deformed area will be referred to as a bending zone. 
         [0028]    In this way, reflecting surfaces  55 ,  56  of the chips  35 ,  36  correctly face one another and are in the correct position to be able to deflect a light ray, as in the picoprojector of  FIG. 1 . 
         [0029]    With reference to  FIG. 6 , a tape  40  of sheet metal, for example aluminum, is stamped so as to form two strips  41 , forming two longitudinal edges of tape  40 , and a plurality of frames  31 , arranged alongside one another in the longitudinal direction of the tape  40 , between the strips  41 . Each frame  31  has a roughly rectangular shape and is supported by the strips  41  at a plurality of peripheral links  42 , here four for each frame  31 , arranged at the corners of the roughly rectangular shape, and by supporting arms  43 . Each supporting arm  43  extends transversely to the strips  41 , has ends connected to the two strips  41  and is arranged between two adjacent frames  31 . In addition, each supporting arm  43  is connected to the two adjacent frames  31 , arranged on the two sides thereof, via median links  44  that extend between a median portion of the supporting arms  43  and a median portion of the frames  31 . 
         [0030]    In detail, the median links  44  are formed by S-shaped stretches of metal sheet, arranged in couple symmetrically with respect to the bending axis C so as to define a slit  47  between them, the slit having a longitudinal stretch aligned with the bending axis C. 
         [0031]    In each frame  31 , the supporting portions  32 ,  33  are arranged on opposite sides with respect to the bending axis C and are separated from each other by an opening  48  extending in a longitudinal direction of the tape  40 , along bending axis C. 
         [0032]    The supporting portions  32 ,  33  of each frame  31  have a generally rectangular shape and are connected to one another via connection portions, extending transversely with respect to the tape  40  and across the bending line C. In the embodiment shown, each connection portion comprises a connection arm  49  having a first end connected to the first supporting portion  32  in proximity of a respective outer corner of this first supporting portion  32 , close to the strip  41  that is adjacent thereto, and a second end connected to the second supporting portion  33  in proximity of a respective inner corner of the second supporting portion  33 , close to bending axis C. The median links  44  are connected to the frames  31  at the connection arms  49 . 
         [0033]    In this way, each connection arm  49  forms a bending zone  49 A, as clarified hereinafter, and has an arm portion  49 B extending along a transverse side of the respective first supporting portion  32 , between the bending zone  49 A and the adjacent outer corner of the first supporting portion  32 . 
         [0034]    The tape  40  further carries the electric connection elements  37 ,  38 , fixed to the respective supporting portions  32 ,  33  in a known way. The electric connection elements are here U-shaped. 
         [0035]    First alignment holes  50  are formed along the strips  41 , and each frame  31  has second alignment holes  51 , for example near the ends of the connection arms  49  for coupling to the first supporting portions  32 . 
         [0036]    In  FIG. 7 , the first and second chips  35 ,  36  are attached to the respective supporting portions  32 ,  33  in a known way, for example via a die attach film so as to expose the respective reflecting surfaces  55 ,  56 . Then, in a known way, electric connection wires  60  are soldered for electrically connecting the chips  35 ,  36  to the respective electric connection elements  37 ,  38 . 
         [0037]    With reference to  FIG. 8 , the tape  40  is precut, for removing the peripheral links  42 , using a trimming tool (not shown) of a standard type used in packaging integrated circuit, for example for trimming pins of lead-frames. In this way, each frame  31  is supported by the strips  41  through the supporting arms  43  and the median links  44 . 
         [0038]    With reference to  FIG. 9 , the stretches  49 B of all the connection arms  49  are subjected to a shaping step, using a tool (not shown), of a standard type used in packaging integrated circuits, for example for bending pins of lead-frames, for forming, in each connection arm  49 , a respective S-shaped bent portion  52  and arranged along the arm portion  49 B, that is thus no longer planar. In particular, the each connection arm  49  bends so that a portion of each connection arm is in a different plane from the remaining portion of the connection arm. In one embodiment, an end of each connection arm  49  is in a different plane from the remaining portion of the connection arm. 
         [0039]    In particular, in this way, a first arm stretch  49 B 1  connected to the bending zone  49 A and a second arm stretch  49 B 2  connected to the first supporting portion  32  are obtained (see also  FIG. 4 ). Furthermore, the second arm stretches  49 B 2  and the first supporting portions  32  connected thereto are no longer co-planar with the rest of the tape  40 , in particular with the second supporting portions  33 , and the first chips  35  are raised and close to the second chips of the frame  31 . 
         [0040]    With reference to  FIG. 10 , the frames  31  are bent along the bending line C. To this end, a shaping press is used for bending the individual frames  31 , still fixed to the strips  41  through the supporting arms  43  and the median links  44 , so as to bring the two chips  35 ,  36  into the desired mutual angular and spatial position. In this step, bending is simplified by the fact that the bending zone of the frames  31  is formed by the connection arms  49 , having a width (in the longitudinal direction of the tape  40 ) smaller than the respective frames  31 , by virtue of the presence of the openings  48  and the slits  47 . In this step, the median links  44  undergo deformation, by twisting, by virtue of their small section. In this way, bending along the bending line C is obtained easily, without any risk of incorrect bending in other parts of the frames  31 . 
         [0041]    The individual frames  31  are separated, by cutting the median links  44  in proximity of the connection arms  49 , to obtain a plurality of mirror packages  30 , shown in  FIGS. 4 and 5 . These may then be dispatched for their final assembly and easily and automatically mounted, also due to the presence of the second alignment holes  51 , without requiring complex and delicate operations of manual assembly. 
         [0042]    The mirror packages described herein have numerous advantages. 
         [0043]    In particular, in one or more embodiments the mirror packages may be supplied already complete with the horizontal micromirror and the vertical micromirror arranged in the desired mutual angular and spatial position for assembly in electronic apparatuses. In this way, assembly may be carried out automatically by pick-and-place machines for placing the chips on the tape, reducing assembly costs and risks of erroneous positioning, and thus increasing the yield. 
         [0044]    In some embodiments, the process of assembling the mirror packages in the electronic apparatuses may be carried out using automatic machines, without any or minimal human intervention, thus reducing the costs and increasing the productivity. 
         [0045]    Furthermore to the S-shaped bent portion  52  of the connection arms  49 , the first chip  35  may be brought spatially very close to the second chip  36 , for a same bending angle α, thus reducing the dimensions of the corresponding first supporting portion  32 . In this way, the tape  40  has a smaller width (in the transverse direction), enabling a reduction of material and of storage space, and thus a reduction of the associated costs. 
         [0046]    Finally, it is clear that modifications and variations may be made to the mirror package, to the tape, and to the assembling method described and illustrated herein, without thereby departing from the scope of the present disclosure. 
         [0047]    For instance, the bending lines C could be transverse instead of longitudinal, and the first and second supporting portions  32 ,  33  of each frame  31  could be longitudinally aligned with respect to the tape  40 . 
         [0048]    The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.