Abstract:
A microelectromechanical structure, usable in an optical switch for directing a light beam towards one of two light guide elements, including: a mirror element, rotatably movable; an actuator, which can translate; and a motion conversion assembly, arranged between the mirror element and the actuator. The motion conversion assembly includes a projection integral with the mirror element and elastic engagement elements integral with the actuator and elastically loaded towards the projection. The elastic engagement elements are formed by metal plates fixed to the actuator at one of their ends and engaging the projection with an abutting edge countershaped with respect to the projection.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention refers to a microelectromechanical structure comprising distinct parts mechanically connected through a translation-to-rotation motion converting assembly. 
     2. Description of the Related Art 
     As is known, optical devices formed by microelectromechanical structures (MEMs) are currently studied for guiding laser light beams. These optical devices in general comprise switches that have the function of deflecting the laser light beams and are controlled by electronic circuitry, preferably integrated circuits, associated to the devices. 
     FIG. 1 is a schematic representation of an optical device  1  of the indicated type, which comprises a first optical transmission element  2 , a second optical transmission element  3 , and a third optical transmission element  4 . The optical transmission elements may be of any type, for example optical fibers, waveguides, etc. The second optical transmission element  3  is arranged at 90° with respect to the first optical transmission element  2 , whereas the third optical transmission element  4  is arranged at preset angle, different from 90°, with respect to the first optical transmission element  2 . 
     An optical switch  7  is arranged between the optical transmission elements  2 - 4  to direct an incident light ray, which traverses the first optical transmission element  2 , selectively towards the second optical transmission element  3  or the third optical transmission element  4 . The optical switch  7  comprises a mirror element  8  and a control structure (not shown) which rotates the mirror element  8  between a first position (indicated by the solid line) and a second position (indicated by the dashed line). In the first position, the mirror element  8  is arranged at 45° with respect to the first optical transmission element  2  and the second optical transmission element  3 , so that an incident ray  9 , supplied by the first optical transmission element  2 , is reflected towards the second optical transmission element  3  (reflected ray  10  represented by a solid line), whilst in the second position, the mirror element  8  is arranged at an angle different from 45° with respect to the first optical transmission element  2  and the second optical transmission element  3 , and the incident ray  9  is reflected towards the third optical transmission element  4  (reflected ray  11  represented by a dashed-and-dotted line). 
     The third optical transmission element  4  may not be present. In this case, the optical switch  7  operates as an on/off switch, which enables or disables transmission of the light ray  9 . 
     Rotation of the mirror element  8  is obtained by applying a twisting moment lying in the plane of the mirror element  8 , which is suspended from a bearing structure through spring elements (two or four, according to the number of desired freedom degrees). At present, the twisting moment necessary for rotating the mirror element  8  is generated in two ways: via electrostatic forces acting directly on the mirror element  8 , or via a mechanical conversion assembly which converts a translation of a linear actuator into a rotation. 
     FIG. 2 is a schematic representation of an electrostatic actuation system. The mirror element  8  is formed by a platform  15  of semiconductor material suspended from a frame  18  through two spring elements  17   a  extending in the X direction starting from two opposite sides of the platform  15 . The frame  18  is in turn supported by a first wafer  16  of semiconductor material through two spring elements  17   b  extending in the Y direction starting from two opposite sides of the platform  15 . The spring elements  17   a,    17   b  of each pair are aligned to one another and are sized in order to be substantially rigid to tension/compression and to be compliant to torsion, so as to form pairs of axes of rotation of the platform  15 . Specifically, the spring elements  17   a  define an axis of rotation parallel to the X axis, and the spring elements  17   b  define an axis of rotation parallel to the Y axis. In the vicinity of its four comers, the platform  15  has, on the underside, electrodes  20  facing corresponding counterelectrodes  21  arranged on a second wafer  22 , arranged underneath. When appropriate differences of potential are applied between one pair of electrodes  20  and the respective counterelectrodes  21 , one side of the platform  15  is subjected to an attractive force (arrows F in FIG.  2 ), which generates a twisting moment M about two opposed spring elements (in this case the spring elements  17   a ), so causing rotation of the platform  15  in the desired direction and with the desired angle. 
     FIG. 3 is a schematic representation of a mechanical actuation system. Also in this case, the mirror element  8  is formed by a platform  15  made of semiconductor material supported by the first wafer  16  through spring elements  17   a,    17   b  and through the frame  18 . 
     On the underside of the platform  15  is arranged an element having the shape of a frustum of a pyramid integral with the platform  15  and defining a lever  25 . The lever  25  is engaged by four projecting elements, in this case four walls  26  extending vertically upwards starting from a plate  27  and each arranged perpendicular to the adjacent walls  26 . The plate  27  (illustrated in greater detail in FIG. 4) is suspended from a frame  30  through two spring elements  28  extending in the X direction starting from two opposite sides of the plate  27 . The frame  30  is in turn supported by the second wafer  22  through two spring elements  31  extending in the Y direction starting from two opposite sides of the frame  30 . The spring elements  28 ,  31  are sized in such a way as to be compliant, respectively, in the Y direction and in the X direction, and to be more rigid to rotation. 
     According to what is illustrated in FIG. 5, the plate  27  is suspended above a cavity  34  present in one protection layer  36  (for instance, a layer of silicon dioxide) which overlies a substrate  35  belonging to the second wafer  22  and in which there are formed integrated components belonging to the control circuitry. The plate  27  is conveniently made in a third wafer  37  bonded between the first wafer  16  and the second wafer  22 . 
     The plate  27  may translate as a result of the electrostatic attraction between actuating electrodes  38 ,  39 . For this purpose, on the underside of the plate  27  there are present mobile electrodes  38  facing fixed electrodes  39  formed on the bottom of the cavity  34 . In use, the mobile electrodes  38  and the fixed electrodes  39  are biased in such a way as to generate a translation of the plate  27  in the X direction or in the Y direction or in a vector combination of the two directions, exploiting the elastic compliance of the spring elements  28  and  31  in both directions. 
     The walls  26 -lever  25  ensemble form a conversion assembly  40  that converts the translation of the plate  27  into a rotation of the platform  15 , as illustrated in FIG. 5, which illustrates the effect of a displacement in the X direction of the plate  27 . This displacement determines, in fact, a corresponding displacement of the walls  26 , in particular, of the wall  26  on the left in FIG. 5; this wall  26 , by engaging the lever  25 , draws it towards the right, thus determining a rotation of the platform  15  by an angle θ about the spring elements  17   b  (one of which may be seen in FIG.  3 ), which are represented by the axis  17  in FIG.  5 . 
     The linear actuation of the plate  27  thus enables rotation of the platform  15  about the axes defined by the spring elements  17   a  or  17   b  or both, so enabling the platform  15  to assume a plurality of angular positions that may be controlled through the actuation electrodes  38 ,  39 . 
     The described conversion assembly  40  is affected by hysteresis, which limits the precision in the control of the platform  15  and causes part of the movement of the plate  27  to be ineffective. In fact, to ensure the engagement of the lever  25  with the walls  26  also in presence of misalignments between the first wafer  16  and the third wafer  37  and to take into account the fabrication tolerances as regards the height of the walls  26 , as well as the shape of the latter and of the lever  25 , the pairs of facing walls  26  are arranged at a greater distance than necessary for engaging the lever  25 , as indicated by the solid line and, in an exaggerated way, in detail in FIG.  6 . As a result, in the first part of the movement of the plate  27 , it may happen that the wall  26 , which should interact with the lever  25 , fails to engage the lever  25  immediately and does not cause rotation of the platform  15  at once. For example, in FIG. 6, for a displacement of the plate  27  in the direction of the arrows, the rotation of the platform  15  starts only when the wall  26  on the left arrives in contact with the lever  25  and the plate  27  has displaced by the amount ΔX. The same applies, in the illustrated example, for a displacement of the plate  27  in the direction opposite to that of the arrow, even though in general the amount of displacement in one direction or the other is different and not correlated. 
     The same problem of hysteresis described above afflicts in general all the microstructures formed by a translating part and a rotating part connected by an assembly for converting the translation into a rotation, the assembly having a play or hysteresis as a result of the tolerances and fabrication imprecisions. 
     SUMMARY OF THE INVENTION 
     An embodiment of the invention provides a microelectromechanical structure having an motion converting assembly that is free from the problem referred to above. 
     The microelectromechanical structure is usable in an optical switch for directing a light beam towards one of two light guide elements. The structure includes: a rotatably movable mirror element; an actuator that is movable with translatory motion; and a motion conversion assembly arranged between the mirror element and the actuator. The motion conversion assembly includes a projection integral with the mirror element and elastic engagement elements integral with the actuator and elastically loaded towards the projection. The elastic engagement elements are formed by metal plates fixed to the actuator at one of their ends and engaging the projection with an abutting edge countershaped with respect to the projection. 
     A process for manufacturing a microelectromechanical structure is further provided, including the steps of forming a first part which is rotatably movable, the first part including a projection, forming a second part that is movable with translatory motion, the second part including elastic engagement elements, and assembling the first and second parts, in that, during the assembling step, the elastic engagement elements automatically and elastically engage the projection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For understanding the present invention, a description of a preferred embodiment thereof is now provided, purely as a non-limiting example with reference to the attached drawings, wherein: 
     FIG. 1 shows a simplified diagram of a known optical device having a switch; 
     FIG. 2 shows a simplified perspective view of a first embodiment of a known switch; 
     FIG. 3 shows a simplified perspective view of a second embodiment of a known switch; 
     FIG. 4 shows a perspective view, at an enlarged scale, of a detail of FIG. 3; 
     FIG. 5 shows a cross-section view of the known switch of FIG. 3; 
     FIG. 6 shows an enlarged detail of FIG. 5; 
     FIG. 7 shows a cross-section view, similar to that of FIG. 5, of a microelectromechanical structure comprising a motion converting assembly, of a self-centering type, according to the invention; 
     FIG. 8 shows a view from below on the self-centering assembly according to the invention, taken along line VIII—VIII of FIG. 7; 
     FIG. 9 shows a cross-section view of the microelectromechanical structure of FIG. 7, taken along line IX—IX; and 
     FIG. 10 shows a cross-section view, similar to that of FIG. 7, in an intermediate manufacturing step. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 7, a microelectromechanical structure  50  comprises a first wafer or body  51 , a second wafer or body  52 , and a third wafer or body  58 , the latter wafer being arranged between the first wafer  51  and the second wafer  52 , which are bonded together. 
     The first wafer  51  has a structure similar to that of the first wafer  16  of FIG. 3, and thus comprises a platform  53 , which is carried by spring elements  17   a  and a frame  55  and is integral with a lever  54  having the shape of a frusto-pyramid. In FIG. 7, only spring elements  17   a  are visible, which correspond to the spring elements  17   a  of FIG. 2; additional spring elements corresponding to  17   b  of FIG. 2, are not shown, but extend, perpendicular to the drawing plane. The second wafer  52  comprises a substrate  56 , in which electronic components are integrated belonging to the control circuitry, and a protection layer  63  (for instance, a silicon dioxide layer) in which a cavity or depression  57  is present. 
     The third wafer  58  forms a plate  60  similar to the plate  27  of FIG.  3  and supported by a frame  61  similar to the frame  30 , and spring elements similar to the spring elements  28  and  31  (of which spring elements  62  may be seen only in part). 
     The plate  60  has an underside  60   a  facing the cavity  57  and a top side  60   b  facing the platform  53 . 
     Mobile electrodes  70  are arranged on the underside  60   a  of the plate  60 , face the fixed electrodes  71  arranged on the bottom of the cavity  57  and form, with the mobile electrodes  70 , actuation electrodes, in a known way. In practice, the plate  60 , together with the frame  61 , the spring elements  62 , and the electrodes  70 ,  71  forms a linear actuator  65 . 
     Two engagement springs  73  are formed on the top side  60   b  of the plate  60 . Each engagement spring  73  is formed by a metal plate having an elongated, arched shape (FIG.  8 ), with the concavity upwards. In detail, each engagement spring  73  has a first end  73   a  fixed to the plate  60  and a second end  73   b  free, defining a side forming a V-shaped notch which engages a respective edge of the lever  54 . Each engagement spring  73  is formed by at least two metal layers arranged on top of one another and having different thermal expansion coefficients, so as to be subject to different stresses at room temperature and to cause deformation upwards after release, as explained hereinafter. In the illustrated example, three metal layers  74 ,  75  and  76  are present, for instance of titanium, aluminum, titanium, wherein the two titanium layers  74 ,  76  have a different thickness to ensure curving of the spring with the concavity upwards. 
     The engagement springs  73  and the lever  54  form an assembly  77 , of a self-centring type, for converting a translation into a rotation, wherein the engagement springs  73  automatically adapt to the shape and position of the lever  54  upon bonding the first wafer  51  to the third wafer  58 , as explained below. 
     The microelectromechanical structure is manufactured as described hereinafter. The first wafer  51  is etched on the back through a masked isotropic etch which stops on an oxide layer (not shown) arranged between a substrate (in which the lever  54  is to be defined) and an epitaxial layer (in which the platform  53  is to be defined); in this way, the lever  54  is formed. 
     The second wafer  52  is processed in a known way to form the desired components (not shown), the electrical connections (not shown either), and the dielectric protection layers, including the protection layer  63  in which the cavity  57  is formed. Subsequently, the fixed electrodes  71  are made inside the cavity  56 . 
     The mobile electrodes  70  are initially formed in the third wafer  58 . The third wafer  58  is then turned upside down, bonded to the second wafer  52 , and thinned out, as shown in FIG.  10 . On the top face, corresponding to the side  60   b  of the plate  60 , a sacrificial layer  68  is deposited and is then opened where the first ends  73   a  of the engagement springs  73  are to be made. The metal layers are deposited and then defined and form the engagement springs  73 . At this stage, the engagement springs  73  still have a planar shape, since they are withheld by the sacrificial layer  68 . 
     Next, trenches are formed in the third wafer  58  such as to define the plate  60 , the frame  61  and the springs  62 ; the trenches extend down to the cavity  57 . 
     Next, the first wafer  51  and the second wafer  58  are bonded together so that the lever  54  positions itself in the space existing above the second ends  73   b  of the engagement springs  73 , without being engaged thereby, as illustrated in FIG.  10 . At this point, the platform  53  and the spring elements  17   a ,  17   b  are defined. Before defining the platform  53  and the spring elements  17   a ,  17   b , the first wafer  51  preferably undergoes chemical-mechanical polishing (CMP) and is coated with a metal layer which increases the reflecting power of the mirror element. 
     Finally, the sacrificial layer  68  is removed; consequently, the differential stress existing between the layers  74 - 76  (due to the different thermal expansion coefficients of the two metals, to the existing geometrical conditions, and to the working temperature which is different from the metal layer deposition temperature) causes the second end  73   b  of the engagement spring  73  to curve and engage with the lever  54 , as indicated by the arrows of FIG.  10 . By appropriately choosing the metals of the engagement springs  73  and the dimensions of the latter, it is possible to ensure that, in this phase, curving of the engagement springs  73  occurs with the ends  73   b  upwards and that the degree of curving will certainly be sufficient to engage the lever  54 , as will be obvious to a person skilled in the field. On the other hand, the deformation of the engagement springs  73  ceases automatically when these have engaged the lever  54 , and may be different for each spring. In practice, the engagement springs  73  adapt to the existing geometrical and spatial conditions, so causing the engagement springs  73  to self-center with respect to the lever  54 . 
     In this way, the movement of the linear actuator  65 , and in particular of the plate  60 , always causes a corresponding rotation of the platform  53 , so eliminating completely the hysteresis existing in known motion conversion assemblies. Furthermore, the continuous engagement, in all operating conditions, between the engagement springs  73  and the lever  54  ensures a control of the position of the platform  53  that is faster and more precise and enables convenient compensation of offset errors in the rest position of the platform  53 . 
     The illustrated solution requires, for its manufacture, processing steps that are usual for the microelectromechanical structures and may thus be implemented easily and at contained costs. 
     Finally, it is clear that numerous modifications and variations can be made to the microelectromechanical structure described and illustrated herein, all falling within the scope of the invention, as defined in the attached claims. In particular, the material and the shape of the engagement springs  73  may vary with respect to the above description, as likewise the shape of the lever  54 . In addition, the lever  54  and the engagement springs  73  may be exchanged with each other, and, if necessary, it is possible to provide a different number of engagement springs  73 , for example four, extending perpendicular to one another in pairs, for engaging substantially opposite and planar walls of the lever. Finally, during manufacture, it is possible to release the engagement springs  73  prior to bonding the wafers  51 ,  52 ,  58 , and to obtain self-centring engagement between the engagement springs  73  and the lever  54  during assembly.