Abstract:
An acceleration sensor includes a substrate and a first mass element, which is connected to the substrate in such a way that the first mass element is rotatable about an axis, the first mass element being connected to a second mass element in such a way that the second mass element is movable along a first direction parallel to the axis, and the first mass element being connected to a third mass element in such a way that the third mass element is movable along a second direction perpendicular to the axis.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates to an acceleration sensor having a mass situated over a plane of a substrate. 
       BACKGROUND INFORMATION  
       [0002]    Triaxial acceleration sensors, in particular triaxial micromechanical acceleration sensors, are needed for applications in entertainment and automotive electronics. A maximally compact design of the acceleration sensors is desired in those cases. 
         [0003]    The basic principle of micromechanical acceleration sensors is that a seismic mass is movably supported with respect to stationary electrodes on a substrate with the aid of a suspension. The seismic mass and the stationary electrodes form one or more capacitors. A deflection of the seismic mass caused by an acceleration acting on the micromechanical acceleration sensor results in a change in the capacitances of these capacitors, which may be detected and represents a measure of the magnitude of the effective acceleration. To avoid zero deviations, capacitance changes are preferably evaluated differentially. 
         [0004]    In the related art, triaxial acceleration sensors are implemented using three sensor cores which are independent of each other and have separate seismic masses, which are situated next to each other on a shared chip. This results in large space requirements and comparatively large acceleration sensors. 
       SUMMARY OF THE INVENTION  
       [0005]    According to the present invention, an acceleration sensor includes a substrate and a first mass element, which is connected to the substrate in such a way that the first mass element is rotatable about an axis, the first mass element being connected to a second mass element in such a way that the second mass element is movable along a first direction parallel to the axis, and the first mass element being connected to a third mass element in such a way that the third mass element is movable along a second direction perpendicular to the axis. This acceleration sensor may be advantageously designed to be extremely compact. 
         [0006]    The first mass element is preferably designed asymmetrically with respect to the axis. An acceleration acting perpendicularly to the substrate&#39;s plane thus causes the mass element to tilt about the axis, which improves its detectability. 
         [0007]    In a preferred specific embodiment of the acceleration sensor, the substrate is a silicon substrate. A method compatible with conventional silicon processing may thus be used for manufacturing the acceleration sensor. 
         [0008]    One embodiment of the acceleration sensor provides that at least one detection electrode, which is fixedly connected to the substrate and allows a rotation of the first mass element about the axis to be detected, is situated opposite to the first mass element. In a refinement of this specific embodiment, at least two detection electrodes are provided, the detection electrodes allowing a differential evaluation of a rotation of the first mass element about the axis. Zero deviations of the acceleration sensor may be suppressed due to the differential evaluation. 
         [0009]    A detection electrode is preferably provided on each side of the axis, both detection electrodes being designed to be symmetrical to each other with respect to the axis. This symmetry provides advantages regarding the linearity and offset stability of the acceleration sensor. 
         [0010]    In one embodiment of the acceleration sensor, the first mass element has a frame, the second mass element being connected to the frame via at least one bending spring which is extensible in the first direction. In another embodiment, the third mass element is connected to the frame via at least one bending spring which is extensible in the second direction. These embodiments make a very compact design of the acceleration sensor possible. 
         [0011]    In a preferred specific embodiment, the second mass element has first finger electrodes, opposite to which first substrate electrodes fixedly connected to the substrate are situated, the first finger electrodes and substrate electrodes allowing a deflection of the second mass element in the first direction to be detected. 
         [0012]    In another preferred specific embodiment, the third mass element has second finger electrodes, opposite to which second substrate electrodes fixedly connected to the substrate are situated, the second finger electrodes and substrate electrodes allowing a deflection of the third mass element in the second direction to be detected. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]      FIG. 1  shows a first specific embodiment of a triaxial acceleration sensor. 
           [0014]      FIG. 2  shows a second specific embodiment of a triaxial acceleration sensor. 
       
    
    
     DETAILED DESCRIPTION  
       [0015]      FIG. 1  shows a schematic representation of a first specific embodiment of an acceleration sensor  300 , which is situated in the z direction above a surface of a substrate  322 , lying in the x-y plane. Acceleration sensor  300  is suitable for detecting accelerations in all three spatial directions x, y, z. Acceleration sensor  300  is manufactured, for example, from a silicon substrate, as a micromechanical component. 
         [0016]    Acceleration sensor  300  includes an external frame  313 , which is situated in the x-y plane. External frame  313  has a rectangular basic shape. The outer edges of external frame  313  are formed by a first frame part  316 , a second frame part  317 , a third frame part  318 , and a fourth frame part  319 . First frame part  316  and third frame part  318  are oriented parallel to the y axis. Second frame part  317  and fourth frame part  319  are oriented parallel to the x axis. The area enclosed by first, second, third, and fourth frame parts  316 ,  317 ,  318 ,  319  is subdivided into three sections, adjacent in the x direction, by a fifth frame part  320  and a sixth frame part  321 , which are oriented parallel to the y axis. First frame part  316  is wider compared to third frame part  318  and forms an additional mass  303 . 
         [0017]    A fixing point  301 , connected to the substrate, is situated in the central area section of the three area sections enclosed by external frame  313 , which runs between fifth frame part  320 , second frame part  317 , sixth frame part  321 , and fourth frame part  319 . Fixing point  301  is connected to external frame  313  via two z springs  308  oriented in the y direction. z springs  308  are designed as torsion springs. A first z spring  308  connects fixing point  301  to second frame part  307 . A second z spring  308  connects fixing point  301  to fourth frame part  319 . z springs  308  form an axis of rotation oriented in the y direction, about which external frame  313  may be tilted. 
         [0018]    The second area section enclosed by external frame  313  has an essentially rectangular shape and is situated between first frame part  316 , second frame part  317 , fifth frame part  320 , and fourth frame part  319 . A first internal frame  304 , which represents a mass element and has an essentially rectangular basic shape, is situated within this area section. First internal frame  304  is connected to external frame  313  via two y springs  307 . The edge of first internal frame  304 , adjacent to second frame part  317  of external frame  313 , is connected to second frame part  317  via first y spring  307 . The edge of first internal frame  304  adjacent to fourth frame part  319  of external frame  313  is connected to fourth frame part  319  via second y spring  307 . Both y springs  307  have a meandering or S shape. y springs  307  are designed to be elastic in the y direction, but rigid in the x and z directions. 
         [0019]    The third area section enclosed by external frame  313  has an essentially rectangular shape and is delimited by sixth frame part  321 , second frame part  317 , third frame part  318 , and fourth frame part  319 . This area section is essentially filled by a second internal frame  305 , which represents a mass element. Second internal frame  305  is connected to external frame  313  via two x springs  306 . The external edge of second internal frame  305 , adjacent to sixth frame part  321  of external frame  313 , is connected to sixth frame part  321  via first x spring  306 . The external edge of second internal frame  305 , adjacent to third frame part  318  of external frame  313 , is connected to third frame part  318  via second x spring  306 . x springs  306  are designed as meandering or S-shaped bar springs and are elastic in the x direction, but rigid in the y and z directions. 
         [0020]    First internal frame  304  is subdivided into three rectangular area sections adjacent to each other in the x direction. The central area section of first internal frame  304  has a flat design and is situated in the z direction above a first z electrode  311 , fixedly connected to the substrate. First z electrode  311  has essentially the same dimension in the x and y directions as the central area section of first internal frame  304 . The central area section of first internal frame  304  and first z electrode  311  form a capacitor, whose capacitance is a function of the distance between the central area section of first internal frame  304  and first z electrode  311 . 
         [0021]    The area elements of first internal frame  304  situated on both sides of the central area section of first internal frame  304  are designed as a grid having grid bars running in the x direction and forming a plurality of y electrode fingers  315 . y electrode fingers  315  are situated in the z direction above y substrate electrodes  310 , fixedly connected to the substrate. y electrode fingers  315  and y substrate electrodes  310  form a capacitor, whose capacitance is a function of the distance between y electrode fingers  315  and y substrate electrodes  310 . 
         [0022]    The area covered by second internal frame  305  is subdivided into three rectangular area sections having approximately the same size, adjacent to each other in the y direction. The central area section of second internal frame  305  has a flat design and is situated in the z direction above a second z electrode  312 , fixedly connected to the substrate. Second z electrode  312  has essentially the same dimension in the x and y directions as the central area section of second internal frame  305 . The central area section of second internal frame  305  and second z electrode  312  form a capacitor, whose capacitance is a function of the distance between the central area section of second internal frame  305  and second z electrode  312 . 
         [0023]    The area sections of second internal frame  305 , situated on both sides of the central area section of second internal frame  305 , are designed as a grid having grid bars running in the y direction and forming a plurality of x electrode fingers  314 . x electrode fingers  314  are situated in the z direction above x substrate electrodes  309 , which are fixedly connected to the substrate. x electrode fingers  314  and x substrate electrodes  309  form capacitors, whose capacitance is a function of the distance between x electrode fingers  314  and x substrate electrodes  309 . 
         [0024]    External frame  313 , y springs  307 , first internal frame  304 , x springs  306 , and second internal frame  305  together form a rocker mass  302 , or a mass element. Due to the additional mass  303  formed by first frame part  316  of external frame  313 , rocker mass  302  has an asymmetric design with respect to the axis of rotation formed by z springs  308 . On one side of the axis of rotation formed by z springs  308 , rocker mass  302  has a mass which is greater than that on the other side of the axis of rotation by additional mass  303 . 
         [0025]    An acceleration acting on acceleration sensor  300  in the x direction exerts a force acting on second internal frame  305  in the x direction. It results in an elastic deformation of x springs  306  and in a deflection of second internal frame  305  relative to external frame  313 . The distance between x electrode fingers  314  and x substrate electrodes  309  changes due to the deflection of second internal frame  305 , which changes the capacitance of the capacitor formed thereby. This may be detected by an electronic evaluation system connected to acceleration sensor  300 . The capacitance change represents a measure of the magnitude of the acceleration acting on acceleration sensor  300 . 
         [0026]    An acceleration acting in the x direction also generates forces acting on external frame  313  and first internal frame  304  in the x direction. However, since y springs  307  and z springs  308  have a rigid design in the x direction, these forces do not cause external frame  313  or first internal frame  304  to deflect. 
         [0027]    An acceleration acting on acceleration sensor  300  in the y direction results in a force acting on first internal frame  304  in the y direction and deflects it by an elastic deformation of y springs  307  against external frame  313 . The distance thus changed between y electrode fingers  315  and y substrate electrodes  310  changes the capacitance of the capacitor formed thereby, which may be detected and quantified by an electronic evaluation system connected to acceleration sensor  300 . The capacitance change is a measure of the magnitude of the acceleration acting in the y direction. Since x springs  306  and z springs  308  are not deformable in the y direction, second internal frame  305  and external frame  313  are not deflected. 
         [0028]    An acceleration acting on acceleration sensor  300  in the z direction generates a force acting on rocker mass  302  in the z direction, which, due to additional mass  303  on one side of the axis of rotation formed by z springs  308 , results in a torque acting on rocker mass  302  and in a tilt of rocker mass  302  about the axis of rotation formed by z springs  308 . The greater the acceleration acting on rocker mass  302 , the greater the tilt angle. Due to the tilting of rocker mass  302 , the distances between first internal frame  304  and first z electrode  311 , and between second internal frame  305  and second z electrode  312 , are changed. Depending on the direction of tilt of rocker mass  302 , one of the distances increases, while the other one decreases. This changes the capacitances of the capacitors formed by first internal frame  304  and first z electrode  311 , or second internal frame  305  and second z electrode  312 . This is detected with the aid of an electronic evaluation system. The changes in opposite directions of the two capacitances allow a differential evaluation of the capacitance changes, which provides a linearized relationship between output signal and input acceleration. 
         [0029]    Since y springs  307  and x springs  306  are not deformable in the z direction, first internal frame  304  and second internal frame  305  are not deflected with respect to external frame  313 . 
         [0030]      FIG. 2  shows a second specific embodiment of the present invention based on an acceleration sensor  400 . Acceleration sensor  400  is situated in a z direction above a substrate  422 , situated in an x-y plane. Substrate  422  may be a silicon substrate, for example. Acceleration sensor  400  may be manufactured, for example, using semiconductor microstructuring methods. 
         [0031]    Acceleration sensor  400  has an external frame  413  having a first frame part  416 , a second frame part  417 , a third frame part  418 , and a fourth frame part  419 , which are situated as lateral edges of a rectangle. First frame part  416  and third frame part  418  are oriented parallel to the y axis. Second frame part  417  and fourth frame part  419  are oriented parallel to the x axis. First frame part  416  is designed to be wider than third frame part  418  and thus represents an additional mass  403 . 
         [0032]    The area enclosed by external frame  413  is subdivided into three area sections adjacent in the x direction by a fifth frame part  420  and a sixth frame part  421 , which run parallel to the y axis and are situated between second frame part  417  and fourth frame part  419 . A fixing point  401 , fixedly connected to the substrate, is situated in the central area section delimited by fifth frame part  420 , second frame part  417 , sixth frame part  421 , and fourth frame part  419 . External frame  413  is connected to fixing point  401  via two z springs  408  oriented in the y direction. First z spring  408  extends from fixing point  401  to second frame part  413 . Second z spring  408  extends from fixing point  401  to fourth frame part  419 . z springs  408  are designed as bar-shaped torsion springs and form an axis of rotation which is parallel to the y axis, and about which external frame  413  may be tilted against the substrate situated in the x-y plane. 
         [0033]    The area section enclosed by first frame part  416 , second frame part  417 , fifth frame part  420 , and fourth frame part  419  is essentially filled by a rectangular first internal frame  404 , which represents a mass element. A lateral edge of first internal frame  404 , parallel to second frame part  417 , is connected to second frame part  417  via a first y spring  407 . The lateral edge of first internal frame  404  adjacent to fourth frame part  419  is connected to fourth frame part  419  via a second y spring  407 . y springs  407  are elastically deformable in the y direction, but are rigid in the x and z directions. y springs  407  are designed as meandering or S-shaped bar springs. y springs  407  allow first internal frame  404  to deflect against external frame  413 . 
         [0034]    The area section delimited by sixth frame part  421 , second frame part  417 , third frame part  418 , and fourth frame part  419  is essentially filled by a second internal frame  405 , which represents a mass element and has a basic rectangular shape. The external edge of second internal frame  405 , adjacent to sixth frame part  421 , is connected to sixth frame part  421  via a first x spring  406 . The external edge of second internal frame  405 , adjacent to third frame part  418 , is connected to third frame part  418  via a second x spring  406 . x springs  406  are designed as meandering or S-shaped bar springs and are elastically deformable in the x direction, but are rigid in the y and z directions. x springs  406  allow second internal frame  405  to deflect against external frame  413 . 
         [0035]    First internal frame  404  has a central rectangular area which has a flat design and is situated in the z direction above a first z electrode  411 , fixedly connected to the substrate. The flat area of first internal frame  404  and first z electrode  411  together form a capacitor, whose capacitance is a function of the distance between first internal frame  404  and first z electrode  411 . The area of first internal frame  404  surrounding the central area of first internal frame  404  is formed by two grid sections which are adjacent in the x direction. The grid sections of first internal frame  404  have grid bars oriented in the x direction and forming y electrode fingers  415 , which are situated in the z direction above a plurality of y substrate electrodes  410 , fixedly connected to the substrate. y electrode fingers  415 , oriented in the x direction, extend from the edge of frame  404  to the central, flat area of first internal frame  404 , i.e., to a bar of frame  404 , separating the two grid sections. y electrode fingers  415 , adjacent to the central, flat area of first internal frame  404 , are thus shorter than the two y electrode fingers  415 , adjacent to the bar of frame  404  separating the two grid sections. y electrode fingers  415  and y substrate electrodes  410  form capacitors, whose capacitances are a function of the distance between y electrode fingers  415  and y substrate electrodes  410 . 
         [0036]    Second internal frame  405  has a central, flat, rectangular section which is situated in the z direction above a z electrode  412 , fixedly connected to the substrate. The flat section of second internal frame  405  and second z electrode  412  form a capacitor, whose capacitance is a function of the distance between the flat section of second internal frame  405  and second z electrode  412 . The flat section of second internal frame  405  is enclosed by two grid sections of second internal frame  405 , which are adjacent in the y direction and have a plurality of grid bars oriented in the y direction, forming a plurality of x electrode fingers  414 . x electrode fingers  414 , oriented in the y direction, extend from the edge of frame  405  to the central, flat area of second internal frame  405 , i.e., to a bar of frame  405 , separating the two grid sections. x electrode fingers  414 , which are adjacent to the central, flat area of second internal frame  405 , are thus shorter than x electrode fingers  414 , adjacent to the bar of frame  405 , separating the two grid sections. x electrode fingers  414  are situated in the z direction above a plurality of x substrate electrodes  409 , fixedly connected to the substrate and, together with these, form capacitors, whose capacitance is a function of the distance between x electrode fingers  414  and x substrate electrodes  409 . 
         [0037]    External frame  413 , y springs  407 , first internal frame  404 , x springs  406  and second internal frame  405  together form a rocker mass  402 , or a mass element. Rocker mass  402  is asymmetrical with respect to the axis formed by z springs  408 . The part of rocker mass  402  enclosing first frame part  416  has additional mass  403  with respect to the other part of rocker mass  402 . 
         [0038]    An acceleration acting on acceleration sensor  400  in the x direction results in a force acting on second internal frame  405  in the x direction and deflects it against external frame  413  while x springs  406  are elastically deformed. This changes the distance between x electrode fingers  414  and x substrate electrodes  409 , which changes the capacitance of the capacitors formed thereby. The greater the acceleration acting on acceleration sensor  400 , the greater the deflection and thus the capacitance changes. The capacitance changes may be detected with the aid of an electronic evaluation system. y springs  407  and z springs  408  are rigid in the x direction; therefore, an acceleration acting in the x direction does not cause external frame  413  or first internal frame  404  to deflect. 
         [0039]    An acceleration acting on acceleration sensor  400  in the y direction results in a force acting on first internal frame  404  in the y direction and deflects it against external frame  413  while y springs  407  are elastically deformed. This changes the distance between y electrode fingers  415  and y substrate electrodes  410 , which results in a capacitance change of the capacitors formed by y electrode fingers  415  and y substrate electrodes  410 , which may be detected by an electronic evaluation system. The greater the deflection and thus the capacitance changes are, the greater the acceleration acting on the acceleration sensor. x springs  406  and z springs  408  are rigid in the y direction; therefore, there is no deflection of second internal frame  405  or of external frame  413 . 
         [0040]    An acceleration acting on acceleration sensor  400  in the z direction generates a force acting on rocker mass  402  in the z direction and, due to additional mass  403 , in a torque causing rocker mass  402  to tilt about the axis of rotation formed by z springs  408 . The greater the force acting on acceleration sensor  400 , the greater the angle of tilt. Due to the tilting of rocker mass  402 , the distances between first internal frame  404  and first z electrode  411 , and between second internal frame  405  and second z electrode  412  are changed, which results in a capacitance change, detectable by an electronic evaluation system, of the capacitors formed by first internal frame  404  and first z electrode  411 , and second internal frame  405  and second z electrode  412 . Since the capacitance changes have opposite signs, a differential evaluation is possible, thereby suppressing the zero deviations. Because x springs  406  and y springs  407  are rigid in the z direction, no deflection of first internal frame  404  or of second internal frame  405  occurs against external frame  413 . 
         [0041]    Acceleration sensor  400  shown in  FIG. 2  has the advantage over acceleration sensor  300  shown in  FIG. 1  that the flat central sections of first internal frame  404  and of second internal frame  405 , and first z electrode  411  and second z electrode  412  are symmetrical with respect to each other, which offers advantages regarding linearity and offset stability. On the other hand, in acceleration sensor  300 , the surface area of first internal frame  304  and second internal frame  305  is made better use of, which increases the basic capacitance and thus the sensitivity of the capacitors provided for detecting z accelerations. 
         [0042]    x substrate electrodes  309  and  409  and y substrate electrodes  310  and  410  of acceleration sensors  300 ,  400  shown in  FIGS. 1 and 2  may also be optionally designed in such a way that deflections of first internal frames  304 ,  404  and second internal frames  305 ,  405  caused by accelerations result in capacitance changes, which may be evaluated differentially. The technical details are known to those skilled in the art from the related art.