Patent Publication Number: US-11650221-B2

Title: MEMS tri-axial accelerometer with one or more decoupling elements

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to a MEMS tri-axial accelerometer with improved configuration. 
     Description of the Related Art 
     As it is known, current micromachining techniques enable manufacturing of so-called MEMS (Micro Electro Mechanical System) devices starting from layers of semiconductor material, which have been deposited (for example, a layer of polycrystalline silicon) or grown (for example, an epitaxial layer) on sacrificial layers, which are removed via chemical etching. For example, inertial sensors, accelerometers, and gyroscopes obtained with the above technology are today widely used, for example in the automotive field, in inertial navigation, in the field of portable devices, and in the medical field. 
     In particular, integrated accelerometers of semiconductor material made with MEMS technology are known, which comprise sensing masses coupled to which are mobile (or rotor) electrodes, which are arranged facing fixed (or stator) electrodes. 
     The inertial movement of the sensing mass, in response to a linear acceleration in a sensing direction, causes a capacitive variation of the capacitor formed between the mobile electrodes and the fixed electrodes, thus generating an electrical quantity that is variable as a function of the acceleration to be detected. 
     The frequency of the movement is determined by the acceleration; the amplitude of displacement of the sensing mass is linked to the resonance frequency via the following relation X=a/ω 2 , where X is the displacement of the sensing mass, a is the external acceleration applied, and ω is the resonance frequency expressed in rad/s. 
     The sensing masses of MEMS accelerometers are arranged above a substrate, suspended by anchoring and suspension structures comprising anchorage elements, fixed with respect to the substrate, and elastic suspension elements, configured to ensure one or more degrees of freedom for the inertial movement of the sensing masses in one or more sensing directions. 
     In particular, in several applications, it is required to provide a detection of linear accelerations acting in a number of sensing directions, for example along three sensing directions corresponding to the axes of a Cartesian triad. For this purpose, MEMS tri-axial accelerometers may be used, which are able to detect three components of acceleration acting in the three sensing directions. 
     In various fields, such as the medical field or the field of portable electronic devices, the need is also known to reduce as far as possible the dimensions, with the consequent need to reduce the dimensions of the MEMS tri-axial accelerometers. The size reduction of the sensors, in addition to affecting the overall occupation of area, further enables reduction of the manufacturing costs. 
     Currently, the majority of MEMS tri-axial accelerometers include a number of sensing masses, typically one sensing mass for each sensing direction. This has the advantage of enabling a design of each sensing mass (and of the coupled sensing electrodes and anchoring and suspension structures) focused and specific for the detection requirements in the respective sensing direction. However, this solution does not enable reduction of the occupation of area and entails in general high manufacturing costs and complexity. 
     To obtain the aforesaid reduction in dimensions and manufacturing costs, a further known solution envisages the use of a single sensing mass, which is able to detect the components of acceleration in the three sensing directions. In this known solution, the sensing mass is suspended above a substrate via single elastic suspension elements, configured to allow inertial movements in the three sensing directions and determine the resonant vibrational modes thereof. 
     This solution, albeit enabling a reduction of the area occupation and manufacturing costs, has some disadvantages. 
     In particular, it is known that the reduction of the planar area of the MEMS accelerometer is directly linked to a mass reduction of the sensing mass, given by:
 
 m=ρ·A  
 
where m is the mass, ρ is the density of the material, and A is the in-plane area.
 
     It is further known that the mechanical detection sensitivity of the MEMS accelerometer is given by: 
               dx   dg     =       m   ·   9.8     k           
where k is the elastic constant.
 
     A reduction of mass thus entails a reduction in the mechanical sensitivity, which may be compensated by a reduction of the elastic constant k. 
     However, the elastic constant k may not be reduced beyond a minimum value due to the presence of adhesion forces (the so-called “stiction” phenomenon) between mobile parts and fixed parts of the tri-axial accelerometer structure (these fixed parts being, for example, constituted by stopper elements, designed to limit the movement of the mobile parts to prevent failure thereof). 
     In particular, to prevent the stiction phenomena, the (mechanical) elastic force has to be greater than the adhesion force, so that the following relation has to be satisfied:
 
 F   mech   &gt;F   adh  
 
 k·x   stop   &gt;F   adh  
 
where F mech  is the elastic force, F adh  is the adhesion force, and x stop  is the displacement of the sensing mass for reaching the corresponding stopper elements. From this expression a minimum value k min  for the elastic constant k is obtained.
 
     In the aforesaid solution, which envisages single elastic elements providing the sensing movements of the single inertial mass in the three sensing directions, it is in general not possible to optimize the sensing performance (in terms, for example, of sensitivity), simultaneously in the three sensing directions. Further, it is difficult to ensure resonance frequencies that are substantially similar for the sensing modes in the above three sensing directions, with consequent further differences of behavior of the accelerometer in regard to the different acceleration components. 
     BRIEF SUMMARY 
     The present disclosure is directed to solving, at least in part, the problems previously highlighted, in order to provide an optimized configuration for a MEMS tri-axial accelerometer. 
     The present disclosure is directed to a MEMS tri-axial accelerometer that includes a single anchorage element fixed to the substrate, a single inertial mass on the substrate, having a main extension in a horizontal plane defined by a first horizontal axis and a second horizontal axis. The mass includes a first window through the mass, a thickness of the mass extending along a vertical axis orthogonal to said horizontal plane, the anchorage element being within the first window and a suspension structure in the first window and configured to elastically couple said inertial mass to the single anchorage element, the suspension structure suspends the inertial mass with respect to the substrate. The mass is configured to perform a first sensing movement in a first sensing direction parallel to said first horizontal axis in response to a first acceleration component, a second sensing movement along a second sensing direction parallel to said second horizontal axis in response to a second acceleration component, and a third sensing movement along a third sensing direction parallel to said vertical axis in response to a third acceleration component. The suspension structure includes a first decoupling element configured to decouple at least one of said first, second, and third sensing movements from the remaining sensing movements of said inertial mass. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       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: 
         FIG.  1 A  shows a schematic top plan view of a sensing structure of a MEMS tri-axial accelerometer according to a first embodiment of the present solution; 
         FIG.  1 B  shows an enlarged schematic top plan view of a portion of the sensing structure of  FIG.  1 A ; 
         FIGS.  2 A- 2 C  are schematic depictions of the sensing movements of the sensing structure of  FIG.  1 A , in response to respective linear acceleration components; 
         FIG.  3    shows a schematic top plan view of a sensing structure, according to a further embodiment of the present solution; 
         FIGS.  4 A- 4 C  are schematic representations of the sensing movements of the sensing structure of  FIG.  3   , in response to respective linear acceleration components; and 
         FIG.  5    shows a general block diagram of an electronic device incorporating the MEMS tri-axial accelerometer, according to a further aspect of the present solution. 
     
    
    
     DETAILED DESCRIPTION 
     As will described in detail hereinafter, one aspect of the present solution envisages providing, in the sensing structure of a MEMS tri-axial accelerometer with single inertial mass, at least one first decoupling element, designed to decouple at least one first sensing movement (i.e., one first resonant vibrational mode) of the inertial mass in a respective first sensing direction from the resonant vibrational modes of the same inertial mass in one or more of the other sensing directions. Advantageously, in this way, the characteristics of the first vibrational mode may be designed in an independent way and optimized with respect to the other vibrational modes. 
     In detail, and with reference to  FIGS.  1 A and  1   , the sensing structure, designated as a whole by 1, of a MEMS tri-axial accelerometer comprises a single inertial mass  2 , which has, in the example, a substantially rectangular shape in a horizontal plane xy, defined by a first horizontal axis x and a second horizontal axis y and constituting a plane of main extension for the sensing structure  1  (which has a substantially negligible extension, or in any case a much smaller extension, along a vertical axis z, which defines, with the aforesaid horizontal axes x, y, a Cartesian triad). 
     It should be noted that, according to the present solution (as, on the other hand, is described in detail hereinafter), the MEMS tri-axial accelerometer has a single inertial mass, i.e., a single mass that is designed, due to inertial effect, to carry out respective sensing movements in the three directions of the tri-axial sensor. 
     The inertial mass  2  is arranged suspended above a substrate of semiconductor material (shown, for example, in  FIG.  2 C , where it is designated by  3 ). 
     A central window  4  is present within the inertial mass  2  (i.e., within the footprint defined by the inertial mass  2  in the horizontal plane xy); the central window  4  traverses the inertial mass  2  throughout its thickness. In the example shown, the central window  4  has a main extension along the second horizontal axis y, at a central position of the inertial mass  2  with respect to the first horizontal axis x, and has a symmetrical configuration with respect to an axis of symmetry parallel to the same first horizontal axis x. 
     According to a particular aspect of the present solution, a first decoupling element  6  is arranged within the central window  4 , in a central position with respect to the same window. 
     The first decoupling element  6  has a central portion  6   a  with a substantially frame-like conformation, defining internally a central empty space  7 , in which an anchorage element  8  is arranged, fixed with respect to the substrate  3  (for example, the anchorage element  8  is constituted by a column extending vertically, along the vertical axis z, starting from the aforesaid substrate  3 ). The anchorage element  8  is arranged at the center of the central empty space  7  and of the central window  4 . 
     The decoupling element  6  is elastically connected to the anchorage element  8  by first elastic elements  10   a ,  10   b , which have a linear extension parallel to the first horizontal axis x and extend on opposite sides with respect to the same anchorage element  8  until they reach a respective inner side of the central portion  6   a  of the decoupling element  6 . 
     It should be noted that the axis of extension of the first elastic elements  10   a ,  10   b  divides the inertial mass  2  into a first portion  2   a  and a second portion  2   b , the first portion  2   a  having an extension along the second horizontal axis y greater than the corresponding extension of the second portion  2   b ; the centroid of the inertial mass  2  thus is located within the aforesaid first portion  2   a.    
     The first decoupling element  6  further has prolongation portions  6   b ,  6   c , extending linearly within the central window  4  along the second horizontal axis y, starting from a respective outer side of the central portion  6   a  of the same decoupling element  6 . 
     The first decoupling element  6  is moreover elastically connected to the inertial mass  2  by second elastic elements  12   a ,  12   b , which extend within the central window  4 , between an end portion of a respective prolongation portion  6   b ,  6   c  of the first decoupling element  6  and a respective inner side of the inertial mass  2 , facing the central window  4 . 
     In particular, in the embodiment illustrated, the second elastic elements  12   a ,  12   b  are aligned along the second horizontal axis y, and each has a so-called folded conformation, having a plurality of first portions, parallel to one another, with a linear extension along the first horizontal axis x, connected in pairs by second portions having an extension (much smaller) along the second horizontal axis y. 
     Two lateral windows  14   a ,  14   b  are further defined within the mobile mass  2 , arranged on opposite sides of the central window  4  along the first horizontal axis x, at the prolongation of the first elastic elements  10   a ,  10   b . The lateral windows  14   a ,  14   b  are arranged at a peripheral portion of the mobile mass  2 , in the proximity of the external perimeter of the same mobile mass  2 . 
     The sensing structure  1  further comprises: first mobile electrodes  15 , arranged within the lateral windows  14   a ,  14   b , having an extension along the first horizontal axis x and fixedly coupled to the mobile mass  2 ; and first fixed electrodes  16 , which are also arranged within the lateral windows  14   a ,  14   b , are fixedly coupled to the substrate  3  by respective anchorage elements (here not illustrated), and are arranged facing respective first mobile electrodes  15  (in particular, the electrodes are in combfingered configuration). 
     The sensing structure  1  further comprises: second mobile electrodes  17 , arranged within the central window  4 , on opposite sides of each prolongation portion  6   b ,  6   c  of the first decoupling element  6 , also having an extension along the first horizontal axis x and fixedly coupled to the mobile mass  2 ; and second fixed electrodes  18 , which are also arranged within the central window  4 , are fixedly coupled to the substrate  3  by respective anchorage elements (not illustrated herein), and are arranged facing, in particular combfingered with, respective second mobile electrodes  17 . 
     The sensing structure  1  further comprises third fixed electrodes  19 , fixed with respect to the substrate  3  and arranged above the same substrate  3 , underneath the inertial mass  2 . The third fixed electrodes  19  are arranged in pairs, on opposite sides of the axis defined by the first elastic elements  10   a ,  10   b , the fixed electrodes  19  of each pair being separated by the central window  4 ; in the example, the aforesaid fixed electrodes  19  have a substantially rectangular shape in the horizontal plane xy. 
     The operation of the sensing structure  1  is now described, for inertial sensing of a first acceleration component a x  oriented parallel to the first horizontal axis x, of a second acceleration component a y  oriented parallel to the second horizontal axis y, and of a third acceleration component a z  oriented parallel to the vertical axis z. 
     In detail, the first acceleration component a x , as illustrated schematically in  FIG.  2 A , causes a first inertial sensing movement, a rotation of the inertial mass  2  in the horizontal plane xy, about an axis of rotation parallel to the vertical axis z and passing through the center of the anchorage element  8 . This rotation, of an angle Δθ, is caused by the mass arrangement of the inertial mass  2  with respect to the axis of rotation, having a centroid shifted inside the first portion  2   a  of the same inertial mass  2 . 
     It should be noted that the aforesaid first sensing movement is allowed by the first elastic elements  10   a ,  10   b , which are compliant to bending in the horizontal plane xy. Moreover, during the aforesaid first movement of the inertial mass  2 , the first decoupling element  6  is rigidly coupled to the inertial mass  2 , given that the second elastic elements  12   a ,  12   b  are rigid with respect to the aforesaid rotation in the horizontal plane xy. 
     Rotation of the inertial mass  2  thus causes a variation of the facing distance between the first mobile electrodes  15  and the first fixed electrodes  16  (a variation of opposite sign in the two lateral windows  14   a ,  14   b ), and a corresponding differential capacitive variation, which may be detected by an appropriate electronic circuitry coupled to the sensing structure  1 . 
     The second acceleration component a y , as illustrated schematically in  FIG.  2 B , causes a second inertial sensing movement, a translation of the inertial mass  2  along the second horizontal axis y (translation designated by Δy in  FIG.  2 B ). 
     This second sensing movement is allowed by the second elastic elements  12   a ,  12   b , which are compliant along the second horizontal axis y. In particular, during this second movement of the inertial mass  2 , the first decoupling element  6  is completely decoupled from the inertial mass  2 , given the deformation of the second elastic elements  12   a ,  12   b , and thus remains substantially immobile with respect to the same inertial mass  2 . 
     The aforesaid translation of the inertial mass  2  causes a variation of the facing distance between the second mobile electrodes  17  and the second fixed electrodes  18  (a variation of opposite sign in the electrodes arranged on the opposite side of the anchorage element  8  along the second horizontal axis y), and a corresponding differential capacitive variation, which may be detected by the electronic circuitry coupled to the sensing structure  1 . 
     The third acceleration component a z , as illustrated schematically in  FIG.  2 C , causes a third inertial sensing movement, a rotation of the inertial mass  2  out of the horizontal plane xy, about the axis defined by the first elastic elements  10   a ,  10   b  (rotation designated by Δφ in  FIG.  2 C ). 
     In particular, the third sensing movement is once again allowed by the first elastic elements  10   a ,  10   b , which are compliant to torsion. During this third movement of the inertial mass  2 , the first decoupling element  6  is coupled to the inertial mass  2 , in so far as the second elastic elements  12   a ,  12   b  are, instead, rigid with respect to torsion. 
     The aforesaid rotation of the inertial mass  2  causes a variation of the facing distance between the inertial mass  2  (which in this case acts as a mobile sensing electrode) and the third fixed electrodes  19 , and a corresponding differential capacitive variation, which may once again be detected by the electronic circuitry coupled to the sensing structure  1 . 
     Thus, advantageously, the presence of the first decoupling element  6  and of the associated second elastic elements  12   a ,  12   b  decouples the vibrational mode of the inertial mass  2  in the second sensing movement (translation along the second horizontal axis y) from the remaining sensing movements and associated vibrational modes. Consequently, said vibrational mode is defined exclusively by the characteristics of the inertial mass  2  and of the second elastic elements  12   a ,  12   b.    
     Moreover, the use of elastic elements of a folded type (the second elastic elements  12   a ,  12   b ) is advantageous, in so far as it allows it to obtain a greater robustness with respect to the manufacturing process spread. In particular, the greater the number of folds (i.e., of parallel portions) of the second elastic elements  12 , the greater their width in the plane, thus ensuring a smaller spread in the value of the elastic constant k. 
     Likewise, the vibrational modes associated to the first sensing movement (in response to the acceleration component a x ) and to the third sensing movement (in response to the acceleration component a z ) are independent of the aforesaid vibrational mode associated to the second sensing movement, being defined exclusively by the characteristics of the inertial mass  2  (and of the first decoupling element  6 ) and of the first elastic elements  10   a ,  10   b.    
     In other words, detection of acceleration along the second horizontal axis y is decoupled from detection of accelerations along the first horizontal axis x and the vertical axis z. 
     The sensing structure  1  thus has a suspension structure arranged within the central window  4 , and comprising the aforesaid first decoupling element  6 , the first and second elastic elements  10   a - 10   b ,  12   a - 12   b , and the anchorage element  8 . 
     A second embodiment of the present solution is now discussed, envisaging a further degree of decoupling in the detection of the acceleration components a x , a y , and a z , thanks to the introduction, in the suspension structure  29  of the sensing structure  1 , of a further decoupling element, co-operating with the first decoupling element  6 . 
     In detail, as shown in  FIG.  3   , the first decoupling element  6  of the sensing structure, once again designated as a whole by 1, is in this case connected by the first elastic elements  10   a ,  10   b , having a linear extension parallel to the first horizontal axis x, to a second decoupling element  20 . The first decoupling element  6  is further connected to the inertial mass  2  by the second elastic elements  12   a ,  12   b , once again of a folded type, but aligned in this case along the first horizontal axis x. 
     In detail, the first decoupling element  6  has, also in this case, a central portion  6   a  with frame-like conformation, from the inner sides of which the first elastic elements  10   a ,  10   b  depart, with extension aligned along the first axis x, and defined inside which is the central empty space  7 . 
     In this embodiment, the prolongation portions  6   b ,  6   c  have a linear extension within the central window  4  along the first horizontal axis x, starting from a respective outer side of the central portion  6   a  of the same decoupling element  6 , as a prolongation of the first elastic elements  10   a ,  10   b.    
     The second elastic elements  12   a ,  12   b  extend from an end portion of a respective prolongation portion  6   b ,  6   c  of the decoupling element  6  up to a respective inner side of the inertial mass  2 , which faces the central window  4 . 
     In this case, the aforesaid second elastic elements  12   a ,  12   b  once again have a folded conformation, but with the plurality of first portions, which are parallel to one another and have a linear extension along the second horizontal axis y, connected in pairs by second portions having an extension (much smaller), this time along the first horizontal axis x. 
     The first decoupling element  6  further has lateral portions  22   a ,  22   b , which are fixedly coupled with respect to the corresponding central portion  6   a , and extend within the central window  4 , on opposite sides with respect to the central empty space  7 . Each lateral portion  22   a ,  22   b  has a frame-like conformation and internally defines a respective lateral empty space  23   a ,  23   b , which is fluidically connected to the central empty space  7 . 
     According to one aspect of the present embodiment, the second portion  2   b  of the inertial mass  2  is separated into a first part  2   b ′ and a second part  2   b ″, separated from one another by a gap  25 . The first part  2   b ′ is rigidly and fixedly connected to the first portion  2   a  of the inertial mass  2 , whereas the second part  2   b ″ is rigidly connected to the first decoupling element  6 . 
     In detail, the second decoupling element  20  has a conformation substantially equivalent to that of the first decoupling element  6 , being housed within the central empty space  7  and the lateral empty spaces  23   a ,  23   b.    
     Also the second decoupling element  20  thus has a central portion  20   a , with frame-like conformation, and lateral portions  26   a ,  26   b , which also have a frame-like conformation, arranged within the respective portions of the first decoupling element  6 . 
     In particular, the second decoupling element  20  internally defines a respective empty space within which the anchorage element  8  is housed. 
     As previously highlighted, the second decoupling element  20  is elastically connected to the first decoupling element  6  by the first elastic elements  10   a ,  10   b.    
     In particular, the central portion  20   a  of the second decoupling element  20  is elastically connected to the central portion  6   a  of the first decoupling element  6  by the first elastic elements  10   a ,  10   b , which extend starting from outer sides of the central portion  20   a  of the second decoupling element  20  towards facing inner sides of the respective central portion  6   a  of the first decoupling element  6 . 
     Furthermore, the second decoupling element  20  is elastically connected to the anchorage element  8 , which also in this case is single, for anchoring the sensing structure  1  to the substrate  3 . 
     In particular, third elastic elements  28   a ,  28   b , aligned along the second horizontal axis y, extend from inner sides of the lateral portions  26   a ,  26   b  up to prolongations  8   a ,  8   b  of the aforesaid anchorage element  8 , which are constituted by rigid arms that extend aligned along the aforesaid second horizontal axis y and are rigidly connected to the same anchorage element  8 . 
     Also the aforesaid third elastic elements  28   a ,  28   b  have a folded conformation, having a plurality of first portions, parallel to one another, with a linear extension along the first horizontal axis x, connected in pairs by second portions having an extension (much smaller) along the second horizontal axis y. 
     In this embodiment, the second mobile electrodes  17  are rigidly connected to inner sides of the lateral portions  26   a ,  26   b  of the second decoupling element  20 , facing the respective second fixed electrodes  18 , which are arranged, like the aforesaid second mobile electrodes  17 , within the empty spaces defined internally by the same lateral portions  26   a ,  26   b.    
     The mode of operation of the sensing structure  1  of  FIG.  3    is now described, for inertial sensing of the first acceleration component a x  oriented parallel to the first horizontal axis x, of the second acceleration component a y  oriented parallel to the second horizontal axis y, and of the third acceleration component a z  oriented parallel to the vertical axis z. 
     In detail, the first acceleration component a x , as illustrated schematically in  FIG.  4 A , causes a first inertial sensing movement of the inertial mass  2  in the horizontal plane xy, in this case constituted by a translation Δx parallel to the first horizontal axis x of the corresponding first portion  2   a  and of the first part  2   b ′ of the second portion  2   b . This translation is allowed by the second elastic elements  12   a ,  12   b , which are compliant to tensile forces in the horizontal plane xy along the aforesaid first horizontal axis x. 
     The first decoupling element  6  decouples the first movement of the inertial mass  2  so that the same first decoupling element  6 , the second decoupling element  20 , and the second part  2   b ″ of the second portion  2   b  of the inertial mass  2  are substantially immobile. In particular, both the first elastic elements  10   a ,  10   b  and the third elastic elements  28   a ,  28   b  are rigid with respect to the movement of translation along the first horizontal axis x. 
     The aforesaid first sensing movement thus causes a variation of the facing distance between the first mobile electrodes  15  and the first fixed electrodes  16  (a variation of opposite sign in the two lateral windows  14   a ,  14   b ), and a corresponding differential capacitive variation, which may be detected by an appropriate electronic circuitry coupled to the sensing structure  1 . 
     The second acceleration component a y , as illustrated schematically in  FIG.  4 B , causes a second inertial sensing movement, of translation of the inertial mass  2  along the second horizontal axis y (translation designated once again by Δy). This second sensing movement is allowed by the third elastic elements  28   a ,  28   b , which are compliant to tensile forces along the second horizontal axis y. 
     Instead, both the first elastic elements  10   a ,  10   b  and the second elastic elements  12   a ,  12   b  are rigid with respect to this translation, so that the first portion  2   a  and the second portion  2   b  (comprising the first and second parts  2   b ′,  2   b ″) of the inertial mass  2  are rigidly and fixedly connected to one another. In other words, the entire inertial mass  2  and the first and second decoupling elements  6 ,  20  move fixedly with respect to one another in the translation along the second horizontal axis y. 
     This translation causes a variation of the facing distance between the second mobile electrodes  17  and the second fixed electrodes  18  (a variation of opposite sign in the electrodes arranged on the opposite side of the anchorage element  8  along the second horizontal axis y), and a corresponding differential capacitive variation, which may be detected by the electronic circuitry coupled to the sensing structure  1 . 
     The third acceleration component a z , as illustrated schematically in  FIG.  4 C , causes a third inertial sensing movement, of rotation of the entire inertial mass  2  (comprising the first and second portions  2   a ,  2   b ) out of the horizontal plane xy, about the axis defined by the first elastic elements  10   a ,  10   b  (rotation designated by Δφ in  FIG.  4 C ). In particular, this third sensing movement is allowed by the first elastic elements  10   a ,  10   b , which are compliant to torsion. 
     Given the stiffness both of the second elastic elements  12   a - 12   b  and of the third elastic elements  28   a - 28   b  with respect to torsion, during the aforesaid third sensing movement, the first decoupling element  6  is again coupled to the inertial mass  2 , whereas the second decoupling element  20  is decoupled from the inertial mass  2  and from its rotation movement, remaining substantially immobile. 
     The aforesaid rotation of the inertial mass  2  causes a variation of the facing distance between the inertial mass  2  (which, in this case, acts as the mobile sensing electrode) and the third fixed electrodes  19 , and a corresponding differential capacitive variation, which may once again be detected by the electronic circuitry coupled to the sensing structure  1 . 
     Thus, advantageously, the joint presence of the first and second decoupling elements  6 ,  20  and of the associated elastic elements decouples each vibrational mode of the inertial mass  2  with respect to the other vibrational modes. The vibrational modes may thus be designed independently, and consequently independently optimized to obtain the desired sensing performance. 
     In particular, the first sensing movement (translation along the first horizontal axis x) is defined exclusively by the characteristics of the first portion  2   a  and of the first part  2   b ′ of the second portion  2   b  of the inertial mass  2  and by the characteristics of the second elastic elements  12   a ,  12   b ; the second sensing movement (translation along the second horizontal axis y) is defined by the characteristics of the entire inertial mass  2  (and of the first and second decoupling elements  6 ,  20 ) and by the characteristics of the third elastic elements  28   a ,  28   b ; and the third sensing movement (rotation out of the horizontal plane xy) is defined by the characteristics of the entire inertial mass  2  (and of the first decoupling element  6 ) and by the characteristics of the first elastic elements  10   a ,  10   b.    
     In yet other words, detection of each acceleration component along the respective sensing axis is independently determined by the respective elastic elements; namely: detection of the acceleration component a x  along the first horizontal axis x is determined by the second elastic elements  12   a ,  12   b ; detection of the acceleration component a y  along the second horizontal axis y is determined by the third elastic elements  28   a ,  28   b ; and detection of the acceleration component a z  along the vertical axis z is determined by the first elastic elements  10   a ,  10   b.    
     In particular, also in this case it is advantageous to use folded elastic elements to define the vibrational modes for detection along the first and second horizontal axes x, y (respectively, the second elastic elements  12   a ,  12   b  and the third elastic elements  28   a ,  28   b ). 
     The advantages of the MEMS tri-axial accelerometer emerge clearly from the foregoing description. 
     In any case, it is once again emphasized that the solution described, with the introduction of at least one decoupling element in the sensing structure, enables to decouple from one another the vibrational modes corresponding to detection of the acceleration components, and in particular decoupling of at least one vibrational mode (and detection of the associated acceleration component) from the remaining vibrational modes (and detection of the remaining acceleration components). 
     The sensing structure  1 , which comprises in any case a single inertial mass  2  for detecting the three acceleration components, is particularly compact and leads to reduced manufacturing costs. 
     In particular, the aforesaid sensing mass may also be reduced in size, without on the other hand reducing the sensing performance (for example, in terms of sensitivity), thanks to the possibility of optimizing in an independent way the mechanical characteristics of detection along the three sensing axes (for example, optimizing the value of the elastic constant k of the corresponding elastic elements). 
     Basically, the aforesaid characteristics render the MEMS tri-axial accelerometer particularly indicated for integration in an electronic device  30 , as shown in  FIG.  5   , which may be used in a plurality of electronic systems, for example in inertial navigation systems, in automotive systems or in systems of a portable type, such as: a PDA (Personal Digital Assistant); a portable computer; a cellphone; a digital audio player; or a photographic camera or video camera. The electronic device  30  is generally able to process, store, transmit, and receive signals and information. 
     The electronic device  30  comprises: the MEMS tri-axial accelerometer, here designated by  32 ; an electronic circuit  33 , operatively coupled to the MEMS tri-axial accelerometer  32 , to supply biasing signals to the sensing structure  1  (in a per se known manner, not illustrated in detail herein) and to detect the displacements of the sensing mass and thus determine the accelerations acting on the same structure; and an electronic control unit  34 , for example a microprocessor unit, connected to the electronic circuit  33 , and designed to supervise general operation of the electronic device  30 , for example on the basis of the accelerations detected and determined. 
     Finally, it is clear that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the scope of the present disclosure. 
     For example, it is highlighted that the particular conformation and configuration of the sensing mass and of the elastic elements in the sensing structure  1  may vary with respect to what has been illustrated. 
     The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet 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.