Abstract:
A microelectromechanical sensor includes: a supporting structure, having at least one first electrode and one second electrode, which form a capacitor; and a sensing mass made of non-conductive material, which is arranged so as to interact with an electric field associated to the capacitor and is movable with respect to the supporting structure according to a degree of freedom so that a relative position of the sensing mass with respect to the first electrode and to the second electrode is variable in response to external stresses. The sensing mass is made of a material selected in the group consisting of: intrinsic semiconductor materials, oxides of semiconductor materials, and nitrides of semiconductor materials.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to a microelectromechanical sensor with a non-conductive sensing mass and to a method of sensing through a microelectromechanical sensor. 
         [0003]    2. Description of the Related Art 
         [0004]    Known to the art are microelectromechanical sensors of various types, which exploit the relative displacements of a movable mass with respect to a supporting structure. Sensors of this type are spreading to an ever-increasing extent in numerous apparatuses and may comprise, for example, accelerometers, gyroscopes, and electro-acoustic transducers (microphones/loudspeakers). 
         [0005]    The displacements of the movable mass are determined by variations of the quantity that is to be measured. In the case of an accelerometer, for example, forces applied to the supporting structure modify its state of motion and cause relative displacements of the movable mass. In gyroscopes, the movable mass, kept in controlled oscillation, displaces as a result of the Coriolis acceleration due to rotations of the supporting structure. 
         [0006]    In electro-acoustic transducers, the movable mass is in the form of a membrane that undergoes deformation in response to incident acoustic waves. 
         [0007]    Consequently, from the amount of the displacement of the movable mass, it is possible to derive the value of the quantity that has caused it. 
         [0008]    In many sensors, the movable mass is capacitively coupled to the supporting structure, and the capacitive coupling is variable in proportion to the position of the movable mass itself. From the information on the capacitive coupling, which can be easily obtained at electrical terminals, the quantity to be measured is derived. 
         [0009]    According to widely adopted solutions, the supporting structure and the movable mass have respective mutually facing conductive electrodes so as to form capacitors. The capacitances of the capacitors are determined by the distance between the electrodes of the supporting structure and the electrodes of the movable mass and hence depend upon the position of the latter. Between the electrodes generally air is present. 
         [0010]    Notwithstanding the wide range of applications, there are, however, some aspects that limit the performance and, sometimes, the possibility of use of capacitive sensors of this type. 
         [0011]    The most critical aspects, which are frequently in conflict, are in general the sensitivity and the linearity of the sensors. The sensitivity, defined as derivative of the capacitance with respect to the position in the sensing direction, basically depends upon the geometry of the sensor (surfaces of the electrodes and distance at rest) and upon the stiffness of the suspension elements that connect the movable mass to the supporting structure to enable elastic oscillations with respect to pre-determined degrees of freedom or else upon the stiffness of the membrane in the case of electro-acoustic transducers. In particular, stiffer springs or membranes enable displacements of modest proportions and, consequently, small capacitive variations. If, on the one hand, the linearity benefits from small displacements from a resting position, on the other hand, however, the sensitivity is limited and this results in lower accuracy and robustness to noise. Less stiff elastic connections and membranes are to the advantage of sensitivity, but reduce the linearity. In addition, the risk of impact between the movable parts and the fixed parts, which may even cause irreversible damage to the devices, increases. 
         [0012]    A further limit derives from the need to provide electrical connections both for the fixed electrodes and for the movable electrodes. The architecture of the microelectromechanical sensors is frequently complex, and providing numerous electrical connections may prove problematic. 
         [0013]    There has been proposed the use of sensors that exploit capacitors, which are provided on the surface of a substrate and are biased, and a movable mass, which is made of polymeric material (for example parylene) and is set at a variable distance from the substrate. The movable mass, according to the position with respect to the surface of the substrate, modifies differently the lines of field at the edge of the capacitors and, consequently, their capacitance. 
         [0014]    This solution presents, however, limitations because polymers, and in particular parylene, are far from suitable to create complex microstructures, as in many cases would be necessary, instead. The flexibility is consequently poor and the possibilities of use are rather limited. 
       BRIEF SUMMARY 
       [0015]    The present disclosure is directed to a microelectromechanical sensor and a sensing method that will enable the limitations described above to be mitigated. 
         [0016]    One embodiment of the present disclosure is directed to a microelectromechanical sensor that includes a supporting structure having a first electrode and a second electrode that is configured to form a capacitor with the first electrode. The sensor also includes a sensing mass of a non-conductive material, arranged to interact with an electric field associated with the capacitor and movable with respect to the supporting structure according to a degree of freedom, a relative position of the sensing mass with respect to the first electrode and to the second electrode being variable in response to an external stress. In one embodiment, the sensing mass is made of a material selected from the group consisting of intrinsic semiconductor materials, oxides of semiconductor materials, and nitrides of semiconductor materials. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0017]    For a better understanding of the disclosure, some embodiments thereof will now be described, purely by way of non-limiting example and with reference to the attached drawings, wherein: 
           [0018]      FIG. 1  is a simplified top plan view of a microelectromechanical sensor according to a first embodiment of the present disclosure; 
           [0019]      FIG. 2  is an enlarged perspective view of a detail of the microelectromechanical sensor of  FIG. 1 ; 
           [0020]      FIG. 3  is a simplified block diagram of a part of the microelectromechanical sensor of  FIG. 1 ; 
           [0021]      FIG. 4  is a cross section through a microelectromechanical sensor according to a second embodiment of the present disclosure; 
           [0022]      FIG. 5  is a simplified top plan view of a microelectromechanical sensor according to a third embodiment of the present disclosure; 
           [0023]      FIG. 6  is a cross-sectional view through a microelectromechanical sensor according to a fourth embodiment of the present disclosure; and 
           [0024]      FIG. 7  is a simplified block diagram of an electronic system incorporating a microelectromechanical sensor according to one embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIGS. 1 and 2  show in a simplified way a microelectromechanical sensor, in particular a monoaxial accelerometer, designated herein by the reference number  1 . 
         [0026]    The microelectromechanical sensor  1  comprises a supporting structure  2  made of semiconductor material, for example silicon, and a sensing mass  3 , which is housed in a cavity  4  of the supporting structure  2  and is movable with respect to the supporting structure  2  itself according to a degree of freedom, which in the embodiment described, is of a translatory type. In practice, the sensing mass  3  is constrained to the supporting structure through elastic connection elements  5 , which are configured to enable relative movements of the sensing mass  3  along a sensing axis X, with respect to a resting position. In the embodiment of  FIGS. 1 and 2 , the sensing axis X is moreover parallel to a bottom surface  4   a  of the cavity  4  in which the sensing mass  3  is housed. The sensing mass  3  is thus movable in a direction parallel to the surface  4   a , which it faces. 
         [0027]    The supporting structure  2  has a plurality of first electrodes  7   a  and second electrodes  7   b  made of doped semiconductor material (and hence conductive), which project towards the inside of the cavity  4 . The first and second electrodes  7   a ,  7   b  are parallel to one another and to the sensing axis X, are arranged facing one another and alternated and are evenly spaced apart. 
         [0028]    Moreover, adjacent first electrodes  7   a  and second electrodes  7   b  are electrically isolated from one another and define respective capacitors having plane and parallel plates. A first sensing terminal  8   a  and a second sensing terminal  8   b  are connected, respectively, to the first electrodes  7   a  and to the second electrodes  7   b.    
         [0029]    The movable mass  3  is made of non-conductive material, which, in the embodiment described herein, is intrinsic silicon. Alternatively, other intrinsic semiconductors may be used, such as germanium or gallium arsenide, or else oxides or nitrides of a semiconductor, such as, for example, silicon oxide (SiO 2 ) and silicon nitride (Si 3 N 4 ). In general, however, intrinsic semiconductors are preferred for the higher dielectric constant, which, as will be seen hereinafter, favors the sensitivity of the devices. 
         [0030]    As already mentioned, the sensing mass  3  is movable in the cavity  4  along the sensing axis X. The sensing mass  3  has moreover plates  3   a  that project laterally towards the first electrodes  7   a  and the second electrodes  7   b , parallel to the sensing axis X and to the surface  4   a.  The plates  3   a  are arranged at a uniform distance apart and are each inserted between a respective pair of adjacent first electrodes  7   a  and second electrodes  7   b.    
         [0031]    The elastic connection elements  5  are configured so that, in the absence of external stresses, the sensing mass  3  is in a resting position X R  with respect to the sensing axis X, as shown in  FIG. 1 . When a force is applied to the supporting structure  2 , the sensing mass  3  moves along the sensing axis X with respect to the resting position X R  in a direction determined by the sign of the stresses. The amplitude of the displacement is moreover determined by the amount of the stresses. 
         [0032]    Consequently, the space comprised between pairs of adjacent first electrodes  7   a  and second electrodes  7   b  is occupied by a respective plate  3   a  of the sensing mass  3  to an extent determined by the position of the sensing mass  3  itself with respect to the sensing axis X. 
         [0033]    As previously recalled, adjacent first electrodes  7   a  and second electrodes  7   b  define capacitors  10 , the capacitance of which is determined, amongst other things, by the extension of the portion of the plates  3   a  inserted between adjacent first electrodes  7   a  and second electrodes  7   b.  The plates  3   a  interact with the field associated to the capacitors  10  and affect the capacitance thereof. 
         [0034]    The overall capacitance of each capacitor  10  is determined by two contributions, one due to the portion of air between the plates and one due to the presence of the non-conductive material forming the plates  3   a  (see  FIG. 3 ). If we denote by L the dimension of the electrodes  7   a ,  7   b  in the direction of the sensing axis X, by X′ the length of the portion of the plates  3   a  inserted between the electrodes  7   a ,  7   b , by Z ( FIG. 2 ) the dimension of the electrodes  7   a ,  7   b  in the direction perpendicular to the surface  4   a  of the cavity  4  and to the sensing axis X, and by G the distance between adjacent first electrodes  7   a  and second electrodes  7   b , the overall capacitance of each capacitor  10  is given by 
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         [0035]    The simplified relation (1) applies when the width of the plates  3   a  is substantially the same, but for a minor play, as the distance G between adjacent first electrodes  7   a  and second electrodes  7   b  and, moreover, if the effect of the plates  3   a  on the adjacent capacitors  10  is assumed to be negligible. 
         [0036]    The overall capacitance of each capacitor  10  thus depends upon the position of the sensing mass along the axis X, which is, in turn, determined by the intensity of the forces applied to the supporting structure  2 . If the structure of the sensor does not satisfy the conditions indicated, the capacitance of the capacitors  10  is given by a relation that is more complex than the relation (1), but depends in any case in a substantially linear way upon the position of the sensing mass along the axis X. 
         [0037]    The total capacitance of the capacitors  10  (which are connected together in parallel,  FIG. 1 ) can be easily detected with a read circuit  15 , as shown in  FIG. 3 . In particular, the read circuit  15 , for example, applies a voltage V S  across the sensing terminals  8   a ,  8   b  and determines the integral of the current absorbed. 
         [0038]      FIG. 4  shows a different embodiment of the disclosure. A microelectromechanical sensor  100 , in particular a monoaxial accelerometer, comprises a supporting structure  102  and a sensing mass  103 , housed in a cavity  104  of the supporting structure  102  and movable with respect to the supporting structure  102  according to a degree of freedom, which in the embodiment described, is of a translatory type. 
         [0039]    In greater detail, the sensing mass  103  is constrained to the supporting structure  102  through elastic connection elements  105 , which are configured so as to enable relative movements of the sensing mass  103  along a sensing axis Z, with respect to a resting position. In the embodiment described, the sensing axis Z is perpendicular to a bottom surface  104   a  of the cavity  104  in which the sensing mass  103  is housed. Consequently, the degree of freedom enables the sensing mass  103  to move in a direction perpendicular to the surface  104   a  and hence to approach and recede with respect to the supporting structure  102 . 
         [0040]    The supporting structure  102  has a plurality of first electrodes  107   a  and second electrodes  107   b  made of doped semiconductor material (and hence conductive). The first electrodes  107   a  and the second electrodes  107   b  are perpendicular to the surface  104   a  of the cavity  104 , are arranged facing one another and alternated, and project towards the sensing mass  103  in a direction parallel to the sensing axis Z. 
         [0041]    The first electrodes  107   a  and the second electrodes  107   b  are arranged facing one another and are evenly spaced apart. Moreover, adjacent first electrodes  107   a  and second electrodes  107   b  are electrically isolated from one another and define respective capacitors with plane and parallel faces. A first sensing terminal  108   a  and a second sensing terminal  108   b  are connected, respectively, to the first electrodes  107   a  and to the second electrodes  107   b.    
         [0042]    The movable mass  103 , which is made of non-conductive material, in particular intrinsic silicon, is movable along the sensing axis Z in a direction perpendicular to the surface  104   a  of the cavity  104 . The sensing mass  103  has moreover plates  103   a  that project towards the first electrodes  107   a  and the second electrodes  107   b  parallel to the sensing axis Z. The plates  103   a  are arranged at a uniform distance apart and are inserted each between a respective pair of adjacent first electrodes  107   a  and second electrodes  107   b.    
         [0043]    The elastic connection elements  105  are configured so that, in the absence of external stresses, the sensing mass  103  is in a resting position Z R  with respect to the sensing axis Z, as shown in  FIG. 4 . When a force is applied to the supporting structure  102 , the sensing mass  103  moves along the sensing axis Z with respect to the resting position Z R  perpendicular to the surface  104   a , in a direction determined by the sign of the stresses. The amplitude of the displacement is moreover determined by the amount of the stresses. 
         [0044]    Consequently, the space comprised between pairs of adjacent first electrodes  107   a  and second electrodes  107   b  is occupied by a respective plate  103   a  of the sensing mass  103  to an extent that depends upon the position of the sensing mass  103  itself 
         [0045]    Adjacent first electrodes  107   a  and second electrodes  107   b  define capacitors  10 , the capacitance of which is determined, amongst other things, by the extent of the portion of the plates  103   a  inserted between adjacent first electrodes  107   a  and second electrodes  107   b  and, consequently, upon the position of the sensing mass  103  with respect to the sensing axis Z. 
         [0046]    In the embodiment illustrated schematically in  FIG. 5 , a gyroscope  200  comprises a supporting structure  202 , a sensing mass  203 , and a driving mass  204 . The sensing mass  203  and the driving mass  204  are movable with respect to the supporting structure  202  according to respective degrees of freedom, which in this case are translatory. More precisely, the driving mass  204  is constrained to the supporting structure  202  through first elastic connection elements  205 , which enable displacements of the driving mass  204  along a driving axis Y. The sensing mass  203  is constrained to the driving mass  204  through second elastic connection elements  206 , which enable displacements of the sensing mass  203  with respect to the driving mass  204  along a sensing axis X perpendicular to the driving axis Y. 
         [0047]    A driving device  210  keeps the driving mass  204  in oscillation with controlled amplitude and frequency, using for the purpose driving assemblies  209 , which comprise fixed electrodes (anchored to the supporting structure  202 ) and movable electrodes (anchored to the driving mass  204 ) in a combfingered configuration. 
         [0048]    The sensing mass  203  is made of non-conductive material, preferably intrinsic silicon, and comprises plates  203   a  arranged facing one another, which extend parallel to the sensing axis X. 
         [0049]    First fixed electrodes  207   a  and second fixed electrodes  207   b  extend from the supporting structure parallel to the sensing axis X. The first fixed electrodes  207   a  and the second fixed electrodes  207   b  are arranged facing one another and alternated. Moreover, the adjacent first electrodes  207   a  and second electrodes  207   b  are electrically isolated and form capacitors in pairs. 
         [0050]    The sensing mass  203  is arranged so that the plates  203   a  are located between respective pairs of first electrodes  207   a  and second electrodes  207   b , substantially as already described with reference to  FIGS. 1-3 . 
         [0051]    Rotations of the supporting structure about an axis perpendicular to the driving axis Y cause displacements of the sensing mass  203  along the sensing axis X as a result of the Coriolis force. The displacements are proportional to the angular velocity of the supporting structure and in turn determine capacitive variations of the capacitors defined between pairs of adjacent first electrodes  207   a  and second electrodes  207   b . Reading of said capacitive variations hence supplies signals indicating the angular velocity of the supporting structure  202 . 
         [0052]    The disclosure affords several advantages, both from the constructional standpoint and as regards performance. 
         [0053]    The use of a non-conductive material based upon semiconductor (such as an intrinsic semiconductor, silicon oxide or silicon nitride) instead of a polymer to provide the sensing mass enables exploitation of the usual micromachining techniques to obtain articulated structures, for example as the ones described with combfingered movable plates with fixed electrodes. The integration of polymeric structures, in fact, is frequently problematic, and the micromachining techniques do not enable comparable levels of precision to be achieved. 
         [0054]    The possibility of providing complex and articulated structures significantly reflects also on the performance. The introduction of non-conductive material to a variable extent within the plates of capacitors enables substantial modification of the capacitances of the capacitors themselves. The resulting modifications go well beyond the influence on the edge effect due to setting, alongside the capacitors, dielectric structures, which intercept the lines of field only outside of the plates. The sensitivity of the sensors is hence considerably increased. 
         [0055]    The sensitivity also benefits from the dependence of the material used upon the dielectric constant, which can be very high, in particular with respect to that of polymers that can be used. The increase in sensitivity due to the choice of the material is consequently independent of the configuration chosen for the sensing mass. An advantage, albeit less marked, would anyway be obtainable also in the case of a movable sensing mass in the vicinity of a capacitor, but arranged on the outside without being introduced between the plates, as in the embodiment of  FIG. 6 . In this case, in a microelectromechanical sensor  300  a sensing mass  303  is elastically connected to a supporting structure  302  through elastic connection elements  305 . In particular, the elastic connection elements  305  are configured to enable the sensing mass  303  to approach and recede alternately with respect to a surface  302   a  of the supporting structure  302  on which first electrodes  307   a  and second electrodes  307   b  are provided. The first electrodes  307   a  and second electrodes  307   b  are arranged aligned and alternate with one another. Moreover, adjacent first electrodes  307   a  and second electrodes  307   b  are electrically isolated and form capacitors. A first sensing terminal  308   a  and a second sensing terminal  308   b  are connected, respectively, to the first electrodes  307   a  and to the second electrodes  307   b.  In the presence of a sensing voltage V S  between the first sensing terminal  308   a  and the second sensing terminal  308   b , between the first electrodes  307   a  and the second electrodes  307   b  an electric field is set up, which is affected by the sensing mass  303  to an extent that is determined by the position of the sensing mass  303  itself with respect to the surface  302   a  of the supporting structure  302 . 
         [0056]    A further advantage of the solutions described, in which the sensing mass has plates inserted between pairs of fixed electrodes to a variable extent according to the position of the sensing mass itself, is the high linearity on a wide range of measurements. The relation that links the overall capacitance of the capacitors to the amplitude of the portion of the plates of the sensing mass that is located within the plates of the capacitors in fact applies with good approximation until the plates of the sensing mass are almost altogether extracted. 
         [0057]    In addition, the devices provided according to the disclosure do not suffer of the so-called “pull-in” phenomenon to which sensors that use conductive movable electrodes, in particular in a combfingered configuration, are, instead, subject. The movable electrodes are in fact subjected to electrostatic forces that tend to recall the movable electrodes themselves into the space within the fixed electrodes. Normally, the electrostatic forces are countered by the elastic force of the connection elements, which tend to bring the sensing mass and the movable electrodes back into the resting position. If, however, the displacements exceed a threshold, for example on account of an intense stress, the electrostatic forces, which depend in a non-linear way upon the position of the sensing mass, prevail. The sensing mass is blocked and does not return to the resting position, preventing operation of the device. The sensing mass according to the disclosure is, instead, made of non-conductive material and is hence immune from the pull-in phenomenon because the plates are not subject to voltage. 
         [0058]      FIG. 7  illustrates a portion of an electronic system  400  according to one embodiment of the present disclosure. The system  400  incorporates the microelectromechanical device  1  and can be used in electronic systems, such as, for example, a palmtop computer (personal digital assistant, PDA), a portable computer, possibly with wireless capacity, a cellphone, a messaging device, a digital audio player, a digital photographic camera or video camera, an inertial navigation system, an automotive system, or other devices designed to process, store, transmit, or receive information. For example, the microelectromechanical device  1  can be used in a digital camera for detecting movements and carry out an image stabilization. In a further embodiment, the microelectromechanical device  1  is included in a user interface activated by motion for computers or consoles for videogames. In a further embodiment, the microelectromechanical device  1  is incorporated in a satellite-navigation device and is used for temporary tracking of position in the case of loss of the satellite-positioning signal. 
         [0059]    The electronic system  400  can comprise a controller  410 , an input/output (I/O) device  420  (for example, a keyboard or a screen), the microelectromechanical device  1 , a wireless interface  440 , and a memory  460 , of a volatile or nonvolatile type, coupled to one another through a bus  450 . In one embodiment, a battery  480  can be used for supplying the system  400 . The scope of the present disclosure is not limited to embodiments having necessarily one or all of the devices listed. 
         [0060]    The controller  410  can comprise, for example, one or more microprocessors, microcontrollers, and the like. 
         [0061]    The I/O device  420  can be used for generating a message. The system  400  can use the wireless interface  440  for transmitting and receiving messages to and from a wireless communication network with a radio-frequency (RF) signal. Examples of wireless interface can comprise an antenna, a wireless transceiver, such as a dipole antenna, even though the scope of the present disclosure is not limited from this standpoint. In addition, the I/O device  420  can supply a voltage representing what is stored either in the form of digital output (if digital information has been stored) or in the form of analog output (if analog information has been stored). 
         [0062]    Finally, it is clear that modifications and variations may be made to the microelectromechanical sensor described and illustrated herein, without thereby departing from the sphere of protection of the present disclosure. 
         [0063]    In particular, the disclosure can be used to obtain also other types of devices, such as, for example, multiaxial accelerometers and gyroscopes and electro-acoustic transducers, and magnetometers. In addition, rotational accelerometers and gyroscopes with angularly oscillating driving mass may be obtained. 
         [0064]    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.