Abstract:
A 3-dimensional MEMS accelerometer fabricated on a single planar substrate deploys three co-planar sensor elements. Each sensor element is a capacitive device deploying a static electrode plate and a parallel dynamic electrode plate supported by a torsion beam. The dynamic electrode plate also includes a proof mass portion that displaces the center of gravity to below the plane of the plate. Two of the sensor elements are identical and rotated by 90 degrees on the planar substrate. The third capacitive sensor has two pairs of adjacent capacitive plates, each one having a dynamic electrode plate is suspended by a torsion beam. The proof mass on each dynamic electrode plates however is offset laterally from the torsion axis in opposite directions from the other plates to cancel the their respective capacitance charges induced by in-plane acceleration. However, this arrangement also adds the capacitive change induced by acceleration orthogonal to the planar substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to the US provisional application for an “Accelerometer” having application Ser. No. 60/988,114, which was filed on Nov. 15, 2007, which is incorporated herein by reference 
     
    
     BACKGROUND OF INVENTION 
       [0002]    The present invention relates to micro-mechanical electrical systems (MEMS) type device for measuring vibration and movement, and more particularly to a MEMS accelerometer. 
         [0003]    MEMS type devices for use as sensors and accelerometers are well known. Such devices are generally fabricated on a silicon or related planar substrate by semi-conductor manufacturing type methods, such as the use of photo-resists, masks and various etching processes to fabricate a proximity sensor that includes a suspended proof mass member and means to measure the deflection of the proof mass suspending means. Such devices have inherent limitations on the minimum size, detection limit, sensitivity and the like, largely due to the means used for detecting the deflection of the proof mass. 
         [0004]    It is therefore a first object of the present invention to provide 3-dimensional capacitive accelerometer that could be fabricated using a single process. 
         [0005]    Yet a further objective is to provide maximum capacitive sensitivity with minimum packaged size of the accelerometer. Obtaining this objective enable a highly efficient accelerometer that provides maximum response with minimum power demands. 
         [0006]    It is still a further object of the invention to provide a means to combine multiple accelerometers in a configuration for the simultaneous measurement acceleration in three dimensions. 
         [0007]    It is a further objective to provide such a 3-dimensional accelerometer that can be used in cardiovascular applications for example, in a linear structure that is easy for fabrication and packaging in a lead or catheter. 
       SUMMARY OF INVENTION 
       [0008]    In the present invention, the first object is achieved by providing an accelerometer device comprising a substantially planar substrate having an aperture frame therein, one or more static electrodes plates extend into an over the aperture frame from the edge thereof, at least one dynamic electrode plate disposed below said one or more first electrode and supported by at least one torsion beam that spans the aperture, a proof mass coupled to and disposed below said dynamic electrode plate such that the COG (center of gravity) is below the plane of the dynamic electrode, wherein at least one capacitive sensing circuit is defined by the electrical communication between said static electrode plate and said dynamic electrode plate. 
         [0009]    A second aspect of the invention is characterized in by the accelerometer for sensing acceleration perpendicular to a substantially planar substrate having at least two aperture frames disposed therein, one or more static electrodes plates extend into and over each aperture frame from the edge thereof, At least one dynamic electrode plate disposed below said one or more static electrode plates associated with each aperture frame, wherein at least one capacitive sensing circuit is defined by the electrical communication between said one or more static electrode plate and said dynamic electrode plates, each dynamic electrode plate comprising, at least one torsion beam portion that spans the aperture frame to suspend each dynamic electrode plate below said one or more static electrode plates associated with the aperture frame, each beam portion being parallel and disposed in the common plane parallel with the plane of said substrate, a proof mass coupled having at least a portion below the upper plan of the substrate, each proof mass is offset from the axis of the associated torsion beam portion; below each dynamic electrode plate such that the COG (center of gravity), and laterally in the opposite directions from another dynamic plate to cancel the their respective capacitive charges induced by acceleration in the plane of the substrate and add the capacitive charges induced by acceleration orthogonal to the plane of the substrate. 
         [0010]    Another object of the invention of providing a 3-dimension accelerometer is achieved by combining on a common planar substrate two orthogonal disposed accelerometer devices for measuring acceleration in the plane of the substrate in line adjacent a third accelerometer for sensing acceleration perpendicular to a substantially planar substrate. 
         [0011]    The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a cross-section view of the substrate. 
           [0013]      FIG. 2  is a cross-section view of the of  FIG. 1  substrate as etched to form a first embodiment. 
           [0014]      FIG. 3  is a cross-section view of the substrate of  FIG. 2  as etched to form a second embodiment. 
           [0015]      FIG. 4A  is a plan view of the static electrode layer of an embodiment of a 3-Dimensional accelerometer, which includes the embodiment shown in  FIG. 2  as well as that shown in  FIG. 3 . 
           [0016]      FIG. 4B  is a plan view of the dynamic electrode layer of the embodiment of  FIG. 4A . 
           [0017]      FIG. 5A  is a plan view of the Z-axis accelerometer of  FIG. 4A . 
           [0018]      FIG. 5B  is a plan view of a portion of the Z 2  component of the accelerometer in  FIG. 5A . 
           [0019]      FIG. 6A-C  is a schematic diagram illustrating the movement of each of the dynamic electrodes and proof masses in  FIG. 4  for X, Y and Z acceleration respectively. 
           [0020]      FIG. 7  is an electrical schematic of the capacitive circuit and sensing electronics. 
           [0021]      FIG. 8  is a graph of the capacitance changes of the X accelerometer vs. acceleration. 
           [0022]      FIG. 9  is a graph of the Capacitive sensitivity of the X accelerometer vs. acceleration. 
           [0023]      FIG. 10  is a graph of the capacitance of the Z accelerometer vs. acceleration. 
           [0024]      FIG. 11  is a graph of the capacitive sensitivity of the Z accelerometer vs. acceleration. 
           [0025]      FIG. 12A  is a plan view of an alternative embodiment of the accelerometer of  FIG. 3 . whereas  FIG. 12B  is a cross-section elevation of the same as reference line B-B. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Referring to  FIGS. 1 through 12 , wherein like reference numerals refer to like components in the various views, there is illustrated therein a new and improved accelerometer, generally denominated  100  herein. 
         [0027]    Accelerometers fabricated on semiconductor substrates, such as silicon wafers are well known. They frequently deploy one or more static electrodes spaced apart from a dynamic electric that moves in response to acceleration or de-acceleration. The silicon substrates is also etched in some manner to form a spring or hinge that allows the dynamic electrode to move. The change in capacitance between the static and dynamic electrodes, upon movement of the dynamic electrode is used to quantify the magnitudes and direction of the movement. The dynamic electrode acts as the proof mass that increases its movement in response to the acceleration. 
         [0028]    In accordance with the present invention, the accelerometer is preferably a MEMS device fabricated from a double silicon oxide layer substrate shown in  FIG. 1 . In this embodiment, double silicon oxide layer substrate  10  is preferably comprised of at least 5 layers. The first or device layer  101  is preferably made out of doped crystalline silicon (Si(c)) that is preferably 10 μm thick and is separated from the second device layer  103  by a buried oxide layer  102  which is about 2.5 μm thick. The second device layer  103 , also is preferably comprised of doped Si(c) and has a thickness of about 15 μm; it is, in turn, separated from the bottom or handle layer  105  by another buried oxide layer  104  (also about 2.5 μm thick). The handle layer is preferably about 680 μm thick and is also preferably comprised of Si(c). 
         [0029]    As shown in  FIGS. 2 and 3 , the double silicon oxide layer enables a preferred means for device fabrication wherein the static electrode  110  is formed in the first or device silicon layers  101  and a dynamic electrode  120  is formed in the second device layer  103 . As the dashed or broken line indicates an etch boundary, wherein portions of each silicon or silicon oxide layers are partially etched away to define and release the static  110  and dynamic electrodes  120 . Thus, as is illustrated in  FIG. 2  and  FIG. 3 , the etch boundaries also define the extent of the static and dynamic portions of the device. The upper silicon oxide layer  102  is etched away to release the static upper electrode portion  110  from the lower dynamic electrode  120 . However, only a portion of the handle layer  105  is etched away to provide for a large proof mass portion  122  that is attached to the bottom of the otherwise planar dynamic electrode  120 . 
         [0030]    As will be shown in additional embodiments and examples, the first silicon oxide layer  102  is preferably etched away through holes that are formed in the static electrode layer  110 , thus releasing the dynamic electrode  120 , which is connected to the substrate at a spring or beam element  121  formed in the first device layer  101 . Thus, a portion  102   a  of the first silicon oxide layers  102  and remain to connect this spring or beam element  121  to the dynamic electrode  120 . 
         [0031]    Another portion  104   a  of the second silicon oxide layer  104  remains to connect the bottom of the dynamic electrode  120  to the proof mass  122  formed in the handle layer  105 . As the proof mass  122  is attached to the back or lower side of the dynamic electrode  120  it is preferably defined by etching the back or handle side of the wafer  122 . 
         [0032]    The electrodes of the device  100  are formed within an aperture type frame  109  in a planar substrate, as shown in  FIG. 4 . The term aperture frame is intended to indicate the general region that is a least partially etched to define the static and dynamic electrode elements. Thus, the upper or front side of substrate  100  is masked to define the aperture  109  and the full extent of the static and dynamic electrode, while the lower or back side of substrate  10  is masked to define the proof mass  122  dimensions. The etching of a complete open aperture is not necessary, as a portion of the upper silicon layer  101  remains to connect the static electrode  110  mechanically, as well as provide one or more electrical contacts. However, to the extent that the static electrode contacts the edge of the frame, a trench  115  that extends to the first silicon oxide layer  102  is provided to electrically isolate the electrode from the surrounding silicon layer  101  of substrate  10  on the other side of the aperture frame. Further, in preferred embodiments, such trenches  115  are also used to subdivide the static electrode into two or more regions, labeled with A and B as a suffix to the reference numerals in  FIG. 5 , to provide differential capacitive sensing. Thus, the aperture frame  109  represents the linear extension of a plurality of isolation trenches and completely etched regions that collectively electrically isolate the static electrode. It should be appreciated that in the 3-dimensional sensing device of  FIG. 4 , all three accelerometer elements used to sense X, Y and Z axis acceleration can be formed within a single frame, or three separate adjacent frames on the same substrate  10 . 
         [0033]    In the embodiment in  FIG. 2  the proof mass is symmetrically disposed on opposite sides of the torsion beam portion  121 , with the center of gravity of the effective proof mass assembly disposed below the torsion beam axis. 
         [0034]    In contrast, in the device  100  of  FIG. 3 , the proof mass  122  is offset to the right of the torsion beam axis  121 , and comprises both an upper proof mass  122   a  and a lower proof mass  122   b . The upper proof mass  122   a  extends through an additional portion of the aperture  109 , or an additional aperture etched in the device layer  101  of substrate  10 . The extension of the proof mass above and below the dynamic electrode  120  increase the magnitude of the mass and further extends the center of gravity away from directly under the torsion beam axis to increase device sensitivity. 
         [0035]    As the center of gravity of the proof mass  122  and dynamic electrode  120  combination is below the plane of the dynamic electrode  120 , any acceleration in the plane of the substrate having a component perpendicular to the torsion beam axis  121   a  will cause the dynamic electrode to tilt about this axis. Hence the gap between the static and dynamic electrodes will vary from the constant value in the resting state, defined by the thickness of oxide layer  102 . That is, the gap will become smaller at one end of the dynamic electrode extended away from the torsion axis in the direction of the acceleration vector. As the gap at the other end of the dynamic electrode increase, it is desirable to electrically isolate opposing halves of at least one of the dynamic and static electrodes plates to form either a half or full bridge capacitive circuit. This permits differential measurements using the circuit shown in  FIG. 7 . Such isolation is provided by trench  115 ′. It should be understood that in the embodiments shown in  FIGS. 2 and 3  that the gap between static  110  and dynamic electrodes plates  120  varies with distance from the torsion beam axis  121   a.    
         [0036]    It should be appreciated that the holes in the static electrode plate  110  not only permit etching away the first silicon oxide layer  102 , and release of the release dynamic electrode  120 , but also reduce air damping effect by releasing (or admitting) air as the gap between the static and dynamic electrodes decreases (or increases). 
         [0037]    It is also preferred to limit the effective capacitive size of the static electrode  110  by using a trench to electrically isolate sub-region closest to the torsion spring member  121 , as this minimizes the response non-linearly as the gap in this sub-region changes more rapidly being closer to the torsion beam  121 . 
         [0038]    Alternatively, the static and dynamic electrodes need not be disposed one above the other as shown in  FIGS. 2 and 3 , but can be configures as shown in  FIGS. 12A and 12B  wherein a substrate  10  with a single buried oxide  102  layer is etch to provide the lower proof mass  122 , but with the static  110  and dynamic  120  electrodes both formed as a plurality of alternating interdigitated fingers in the upper silicon layer such that as the dynamic electrode rotates, as shown in  FIG. 12B , the gap between electrodes remains constant, but the projected overlapping area between electrodes decreases, as indicated by the widely hatched regions  1201   a  and  1201   b.    
         [0039]      FIG. 4A  is a plan view showing the substrate  10  with at least one aperture frame  109  supporting an array of static electrodes  110  in three accelerometer devices denoted X, Y and Z for measuring acceleration in the X, Y and Z axis respectively. Conductor traces  130  lead from each accelerometer to a series of terminal pads at the right edge of the device. Broken lines  121   a  illustrate the orientation of the torsion beam axis for each accelerometer element. The electrically isolated halves of each static electrode are donated  110 A and  110 B, each leading to a separate terminal pad at the right side of device  100 . The dynamic electrode  120  of each separate X, Y and X device on substrate is labeled C and is connected to a separate terminal pad thus labeled at the right side of device  100 . Thus for each of the X and Y accelerometer elements there is a set of three terminal pads, grouped by brackets labeled X and Y.  FIG. 4B  is a section parallel to the view of  FIG. 4A  to shown the dynamic electrode layout. 
         [0040]      FIG. 5A  is a more detailed plane view of the Z-axis accelerative sensor having two device Z 1  and Z 2 . Each of Z 1  and Z 2  has the static electrode split into two portions  110 A and  110  B. However, the A portions of static electrodes for Z 1  and Z 2  connect at a common terminal pad A (via metal or conductive traces  130 ) whereas the B portions of static electrode for Z 1  and Z 2  connect at a different common terminal pad each being electrically isolated from the other conductive layers or portion of substrate  10 . The dynamic electrodes of each of Z 1  and Z 2 ; labeled C 1  and C 2 ; are connected to different isolated pads with the same labels. 
         [0041]      FIG. 5B  illustrates an enlarged portion of the Z accelerometer showing portion of the two static electrodes  110 A and  110 B, two spring elements  121  and the structure  135  around the spring  121  for the purpose of providing electrical isolation between the electrodes of the structure, and it is grounded by a line which is connected to ground pad. The trenches  115  provide electrical insulations between regions with different potential. The conductive lines or traces  130  provide electrical contact between them. An electrical contact or via  127  traverses the buried oxide layer  102  to provide electrical continuity between the square pad  126   a  and spring element  121   a  that connects the dynamic electrode plate  120  to terminal pad at the edge of the device. The white areas in the figure denote etched areas; therefore there is electrical insulation between each of the regions of the structure. As can be seen from the figure, the static electrode contains many holes (grid pattern) the size of each hole is 3×3 μm. The depth of each hole is 10 μm (thickness of the device layer). The distance between two holes is also 3 μm. 
         [0042]      FIG. 5B  also illustrates a more detailed view of the torsion structure  121  connecting the dynamic electrode to the substrate at the frame boundary  109  in  FIG. 5A . Spring element  121  has two branched portions  124   a  and  124   b  that span the gap between the aperture  109  and the dynamic electrode  120 . The branch portions  124  a and b each connect via a narrower segment  125   a  and  b  respectively to square pads  126   a  and  b  that holds the proof mass  122 . The beam spring dimensions are 40×3×10 μm 3 . 
         [0043]      FIG. 6A-C  illustrates the general principle of operation of the 3-D accelerometer of  FIG. 4  for the simple case where the acceleration is restricted to a single coordinate axis. Thus each of  FIG. 6A-C  is an the x-axis elevation of the different dynamic electrode plate and proof mass for each of the X, Y and Z one dimensional accelerometers. It should be appreciated from these diagrams that acceleration in the two orthogonal directions X and Y that are in the plane of the substrate is primarily sensed by the accelerometers  100  of the type shown in  FIG. 2 . However, acceleration in the Z direction, orthogonal to the plane of the substrate is sensed by the accelerometer, denoted by bracket Z, that comprises two of the accelerometers of  FIG. 2 , denoted Z 1  and Z 2  in the figures. 
         [0044]    In  FIG. 6A , the relative movement of each dynamic electrode is shown for acceleration in the X-direction, as shown by the bold arrow below the figure title. However, for the Y-axis sensor the orthogonal elevation of the dynamic electrode and proof mass is also shown just below the x-axis elevation. The torsion axis of reach dynamic electrode, when viewed in section, is denoted by an upright triangle. The dashed lines show the equilibrium position of each dynamic electrode. Thus in  FIG. 6A , the X-dynamic electrode to the right tilts, but the Y-dynamic electrode is stable. However, as the Z 1  and Z 2  dynamic electrodes have their center of masses on opposite sides they tilt in the same direction, the right side tilting up and the right side tilting down. It should be appreciated that and since the A and B electrode pairs are constituted from opposite sides of the Z 1  and Z 2  device, this movement in the same direction will creates an equal and opposite change in capacitance for the combined electrodes so that the net change will be null. 
         [0045]    In  FIG. 6B , the relative movement of each dynamic electrode is shown for acceleration in the Y-direction, as shown by the letter “X” below the figure title to indicate the acceleration is into the plane of the paper. However, only the Y-dynamic electrode tilts. 
         [0046]    In  FIG. 6C , the relative movement of each dynamic electrode is shown for acceleration in the Z-direction, as shown by the bold arrow below the figure title. While the X and Y dynamic electrodes do not tilt, as proof mass has a center of gravity directly below the torsion axis. However, as the proof mass in each of the Z 1  and Z 2  dynamic electrodes is offset in a different direction laterally from the torsion axis, each electrode plates now tilts in opposite directions, forming an “x” shape profile. Now, rather the A and B electrodes pairs reinforce each other to increase the capacitance reading, rather than cancelling. 
         [0047]    While it is preferred for some application that each of accelerometer be placed adjacent to each other in a row to form a device with a 3:1 aspect ratio, such as for placement in narrow catheter leads, other arrangements and combinations may be desired in different applications. 
         [0048]    Preferably, the two Z-axis one dimensional accelerometer devices are co-planar with at least one of the X- and Y-one dimensional accelerometer devices, that is with the static electrode plates and torsion beam axis of all devices in a common plane. While the torsion beam component be a single rods that extends entirely across the dynamic electrode, preferably the torsion beams have two co-linear segments that extend from the frame edge on to the second electrode. 
         [0049]    The capacitive sensitivity was calculated by finite element methods (FEM) taking into account the grid structure of the top electrodes to account for the reduction in air damping due to hole in the upper or static electrode plate. 
         [0050]    The size of each hole in the electrode 3×3 μm and the distance between two adjacent holes is also 3 μm. The gap between the static electrode and dynamic electrodes is 2.5 μm (the thickness of the buried oxide layer) 
         [0051]    The static capacitance between moving and static electrodes was calculated at the equilibrium state. This required the calculation and accounting for the distribution of electric potential within the unit cell element of the electrode structure. The unit cell element for modeling purposes consisted of a segment of the moving electrode with a size of 6×6 μm (shown at the bottom of the figure) and a segment of the static electrode with a size of 6×6×10 μm 3 . The following boundary conditions were used in the calculations: 1) Bottom face of the structure corresponding to the moving electrode is grounded, 2) All facets corresponding to the static electrodes have potential V= and All other facets have symmetry boundary conditions. 
         [0052]    The capacitance was calculated from the formula: 
         [0000]    
       
         
           
             
               W 
               ς 
             
             = 
             
               
                 1 
                 2 
               
                
               ς 
                
               
                   
               
                
               
                 V 
                 2 
               
             
           
         
       
     
         [0053]    Where Wc the electric energy of the capacitor and C is the capacitance 
         [0054]    The simulations surprising showed that the resulting capacitance is only on a factor K=0.9738 which is smaller than the capacitance of the equivalent capacitor without the hole i.e. due to the grid pattern (and its hole structure) we lose only 2.62% from the capacitance. The air velocity within the unit cell due to the movement of the dynamic electrode was also considered in the model to calculate the thermo-mechanical noise of the structure that arises from the air damping arising that results from the movement of the proof mass. 
         [0055]    From the distribution of the air velocity resulting from the movement of the bottom of the moving electrodes in the Z direction the damping in the unit cell was calculated as the integral on the force that is applied against the direction of the motion. The resulting damping coefficient is: 
         [0000]    
       
         
           
             D 
             = 
             
               
                 
                   D 
                   ς 
                 
                 · 
                 A 
               
               36 
             
           
         
       
     
         [0056]    Where D 0 =9·10 −8  kg/sec and A is the area of the electrode in the μ 2 . 
         [0057]    In each of the one dimensional accelerometers the sense capacitance between two electrodes (A and C for example) increases when the sense capacitance between the other electrodes (B and C for example) decreases by the same amount. These two sense capacitors are connected to create a half-bridge capacitor circuit of  FIG. 7 . The signal from a crystal oscillator with amplitude Vo is applied to the static electrodes A and B. The sense signal is read from the electrode in the proof mass (electrode C) this signal is then amplified by the pre-amplifier of an ASIC. 
         [0058]    Following this, the signal is mixed with the original signal and following a low pass filter to obtain the output signal (V out ). 
         [0059]    The FEM model was extended for the X and Y accelerometers of the type shown in  FIG. 2 , to calculate the change of the capacitance vs. the acceleration.  FIG. 8  shows the calculated change in capacitance of the X or Y type accelerometer vs. acceleration for each pair of electrodes as well as the differential signal accelerometer. At the equilibrium the sense capacitance is about 0.5 pF. While the capacitance of the first electrode increases, the capacitance of the other sensing electrode decreases. This total change of the capacitance is also shown. 
         [0060]    Further, as shown in  FIG. 9  the capacitive sensitivity of the X, Y accelerometer was calculated vs. acceleration. The graph above shows the change of the capacitance sensitivity of the X (Y) accelerometer versus the acceleration. The total capacitance sensitivity is the difference of the capacitive sensitivities of the two sensing electrodes and is represented by the red line. As we can see, the capacitance sensitivity at the equilibrium is about 31 fF/g. 
         [0061]      FIGS. 10 and 11  shows the results of the corresponding calculations for the Z axis accelerometer, we calculated the change of the capacitance vs. the acceleration. At the equilibrium the sense capacitance is about 0.77 pF. Likewise, while the capacitance of the first electrode increases, the capacitance of the other sensing electrode decreases. As shown in  FIG. 11 , the capacitance sensitivity of the Z-axis accelerometer at the equilibrium is about 45 fF/g. 
         [0062]    The Table below summarizes the parameters for the specific embodiments of the X, Y and Z accelerometers described above 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                 Parameter 
                 Values 
                 Units 
               
               
                   
               
             
             
               
                 Sensitivity (X, Y, Z) 
                 30.6, 30.6, 44.7 
                 fF/g 
               
               
                 Sensing Electrode Capacitance (X, Y, Z) 
                 0.502, 0.502, 0.774 
                 pF 
               
               
                 Parasitic Capacitance From MEMS 
                 &lt;1 
                 pF 
               
               
                 Resonance Frequency (X, Y, Z) 
                 1.55, 1.55, 1.35 
                 kHz 
               
               
                 Nonlinearity 
                 2 
                 % 
               
               
                 Thermo-Mechanical Noise Floor (X, Y, Z) 
                 4.15, 4.15, 9.1 
                 μg/√Hz 
               
               
                 Q-Factor (X, Y, Z) 
                 1.52, 1.52, 0.5 
               
               
                 Capacitance Offset 
                 ±5 
                 % 
               
               
                   
               
             
          
         
       
     
         [0063]    While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be within the spirit and scope of the invention as defined by the appended claims.