Abstract:
A microelectromechanical systems (MEMS) capacitive sensor ( 52 ) includes a movable element ( 56 ) pivotable about a rotational axis ( 68 ) offset between ends ( 80, 84 ) thereof. A static conductive layer ( 58 ) is spaced away from the movable element ( 56 ) and includes electrode elements ( 62, 64 ). The movable element ( 56 ) includes a section ( 74 ) between the rotational axis ( 68 ) and one end ( 80 ) that exhibits a length ( 78 ). The movable element ( 56 ) further includes a section ( 76 ) between the rotational axis ( 68 ) and the other end ( 84 ) that exhibits a length ( 82 ) that is less than the length ( 78 ) of the section ( 74 ). The section ( 74 ) includes slots ( 88 ) extending through movable element ( 56 ) from the end ( 80 ) toward the rotational axis ( 68 ). The slots ( 88 ) provide stress relief in section ( 74 ) that compensates for package stress to improve sensor performance.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to microelectromechanical systems (MEMS) sensors. More specifically, the present invention relates to a MEMS differential capacitive accelerometer. 
     BACKGROUND OF THE INVENTION 
     An accelerometer is a sensor typically utilized for measuring acceleration forces. These forces may be static, like the constant force of gravity, or they can be dynamic, caused by moving or vibrating the accelerometer. An accelerometer may sense acceleration or other phenomena along one, two, or three axes or directions. From this information, the movement or orientation of the device in which the accelerometer is installed can be ascertained. Accelerometers are used in inertial guidance systems, in airbag deployment systems in vehicles, in protection systems for a variety of devices, and many other scientific and engineering systems. 
     Capacitive-sensing MEMS accelerometer designs are highly desirable for operation in high gravity environments and in miniaturized devices, due to their relatively low cost. Capacitive accelerometers sense a change in electrical capacitance, with respect to acceleration, to vary the output of an energized circuit. One common form of accelerometer is a capacitive transducer having a “teeter-totter” or “see saw” configuration. This commonly utilized transducer type uses a movable element or plate that rotates under z-axis acceleration above a substrate. The accelerometer structure can measure at least two distinct capacitances to determine differential or relative capacitance. 
     Referring to  FIGS. 1 and 2 ,  FIG. 1  shows a top view of a prior art capacitive-sensing MEMS sensor  20  constructed as a conventional hinged or “teeter-totter” type accelerometer, and  FIG. 2  shows a side view of MEMS sensor  20 . MEMS sensor  20  includes a static substrate  22  and a movable element  24  spaced from substrate  22 , each of which have opposed planar faces. Substrate  22  has a number of conductive electrode elements  26  of a predetermined configuration deposited on a substrate surface  28  to form capacitor electrodes or “plates.” In an exemplary scenario, electrode elements  26  may operate as excitation or sensing electrodes to receive stimulating signals. Electrode elements  26  may additionally operate as a feedback electrodes when a feedback signal is superimposed on the sensing signal. 
     Movable element  24 , commonly referred to as a “proof mass,” is flexibly suspended above substrate  22  by one or more suspension anchors, or rotational flexures  30 , for enabling movable element  24  to pivot or rotate about a rotational axis  32  to form capacitors  34  and  36 , labeled C 1  and C 2 , with electrode elements  26 . Movable element  24  moves in response to acceleration, thus changing its position relative to the static sensing electrode elements  26 . This change in position results in a set of capacitors whose difference, i.e., a differential capacitance, is indicative of acceleration in a direction  37 . 
     When intended for operation as a teeter-totter type accelerometer, a section  38  of movable element  24  on one side of rotational axis  32  is formed with relatively greater mass than a section  40  of movable element  24  on the other side of rotational axis  32 . The greater mass of section  38  is typically created by offsetting rotational axis  32 . That is, a length  42  between rotational axis  32  and an end  44  of section  38  is greater than a length  46  between rotational axis  32  and an end  48  of section  40 . In addition, electrode elements  26  are sized and spaced symmetrically with respect to rotational axis  32  and a longitudinal axis  50  of movable element  24 . 
     Many MEMS sensor applications require smaller size and low cost packaging to meet aggressive cost targets. In addition, MEMS sensor applications are calling for lower temperature coefficient of offset (TCO) specifications. The term “offset” refers to the output deviation from its nominal value at the non-excited state of the MEMS sensor. Thus, TCO is a measure of how much thermal stresses effect the performance of a semiconductor device, such as a MEMS sensor. A high TCO indicates correspondingly high thermally induced stress, or a MEMS device that is very sensitive to such stress. The packaging of MEMS sensor applications often uses materials with dissimilar coefficients of thermal expansion. Thus, an undesirably high TCO often develops during manufacture or operation. These thermal stresses, as well as stresses due to moisture and assembly processes, can result in deformation of the underlying substrate  22 , referred to herein as package stress. 
     Referring to  FIGS. 3-4 ,  FIG. 3  shows a cross-sectional edge view of MEMS sensor  20  along section lines  3 - 3  in  FIG. 1 , and  FIG. 4  shows a cross-sectional edge view of MEMS sensor  20  along section lines  4 - 4  in  FIG. 1 . A problem particular to the teeter-totter configuration shown in  FIG. 1  is that when teeter totter configuration of MEMS sensor  20  is subject to a bending moment from substrate  22  caused by package stress, the stress causes section  40 , i.e., the lighter section, to deform more than section  38 , i.e., the heavier section, resulting in an offset change. As illustrated in  FIGS. 3 and 4 , package stress can result in deformation of section  40  of movable element  24  that is significantly greater than the deformation of section  38  of movable element  24 . This non-symmetric bending induced by package stress can result in an undesirably high offset difference between sense capacitances  34  and  36  (i.e., poor TCO performance), thus adversely affecting capacitive accelerometer  20  output. 
     Thus, what is needed is a low cost, compact, single die teeter-totter type MEMS sensor that can sense along one or more axes and is less susceptible to thermally induced package stress gradients. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the Figures, wherein like reference numbers refer to similar items throughout the Figures, and: 
         FIG. 1  shows a top view of a prior art capacitive-sensing microelectromechanical systems (MEMS) sensor; 
         FIG. 2  shows a side view of the MEMS sensor of  FIG. 1 ; 
         FIG. 3  shows a cross-sectional edge view of the MEMS sensor along section lines  3 - 3  in  FIG. 1 ; 
         FIG. 4  shows a cross-sectional edge view of the MEMS sensor along section lines  4 - 4  in  FIG. 1 ; 
         FIG. 5  shows a top view of a microelectromechanical systems (MEMS) sensor in accordance with an embodiment of the invention; 
         FIG. 6  shows a cross-sectional edge view of the MEMS sensor along section lines  6 - 6  in  FIG. 5 ; 
         FIG. 7  shows a cross-sectional edge view of the MEMS sensor along section lines  7 - 7  in  FIG. 5 ; and 
         FIG. 8  shows a device in which the MEMS sensor may be installed. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 5  shows a top view of a microelectromechanical systems (MEMS) sensor  52  in accordance with an embodiment of the invention. Sensor  52  may be, for example, a capacitive-sensing accelerometer or another MEMS sensing device. MEMS sensor  52  is constructed as a hinged or “teeter-totter” type accelerometer. MEMS sensor  52  includes a substrate  54  and a movable element  56  spaced from substrate  54 , each of which have opposed planar faces. A static conductive layer  58  is deposited on a surface  60  of substrate  54 . Static conductive layer  58  is in the form of at least two electrically isolated electrodes or plates, including, for example, an electrode element  62  and an electrode element  64  (both of which are shown in ghost form). Electrode elements  62  and  64  may operate as excitation or sensing electrodes to receive stimulating signals. Electrode elements  62  and  64  may additionally operate as a feedback electrodes when a feedback signal is superimposed on the sensing signal. 
     Movable element  56  is suspended above and pivotally coupled to substrate  54  by a pair of suspension anchors  66 , or rotational flexures, for enabling movable element  56  to pivot or rotate about a rotational axis  68  to form capacitors (see, for example,  FIG. 2 ) between movable element  56  with respective electrode elements  62  and  64 . Only two electrode elements  62  and  64  are shown in  FIG. 5  for simplicity of illustration. However, in alternative embodiments, MEMS sensor  52  may include a different quantity and/or different configuration of electrode elements. In addition, it should be understood that a number of flexures, hinges, and other rotational mechanisms may be utilized to enable pivotal movement of movable element  56  about rotational axis  68 . 
     Movable element  56  exhibits an axis of symmetry  70  that is orthogonal to rotational axis  68 . An axis of symmetry is a line in a geometric figure which divides the figure into two parts such that one part, when folded over along the axis of symmetry, coincides with the other part. Accordingly, MEMS sensor  52  exhibits an equivalent size and placement of its components on either side of axis of symmetry  70 . In one embodiment, each of suspension anchors  66  is offset an equivalent distance  72  on opposing sides of axis of symmetry  70 . 
     A section  74  of movable element  56  on one side of rotational axis  68  is formed with relatively greater mass than a section  76  of movable element  56  on the other side of rotational axis  68 . The greater mass of section  74  is created by offsetting rotational axis  68 . That is, a length  78  between rotational axis  68  and an end  80  of section  74  is greater than a length  82  between rotational axis  68  and an end  84  of section  76 . Electrode element  62  faces section  74  of movable element  56  and electrode element  64  faces section  76  of movable element  56 . In addition, electrode elements  62  and  64  are sized and spaced symmetrically with respect to rotational axis  68  and longitudinal axis of symmetry  70  of movable element  56 . That is, each of electrode elements  62  and  64  is offset an equivalent distance  86  on opposing sides of rotational axis  68 , and each of electrode elements  62  and  64  extends an equivalent distance on either side of axis of symmetry  70 . 
     Movable element  56  moves in response to acceleration in direction  37  ( FIG. 2 ), thus changing its position relative to the static electrode elements  62  and  64 . Thus, electrode elements  62  and  64  are adapted to detect movement of movable element along an axis that is perpendicular to a plane of electrode elements  62  and  64 . This change in position results in a set of capacitors whose difference, i.e., a differential capacitance, is indicative of acceleration in direction  37 . The term “static” utilized herein refers to conductive layer  58  and electrode elements  62  and  64  that are stationary relative to movable element  56 . That is, while movable element  56  may rotate or pivot on suspension anchors  66  about rotational axis  68 , conductive layer  58  (including electrode elements  62  and  64 ) does not pivot, rotate, or otherwise move relative to movable element  56 .  FIG. 1  shows one possible configuration of MEMS sensor  52 . However, it should be understood that MEMS sensor  52  can take on a number of two- and/or three-layer forms. 
     Section  74  includes slots  88  extending through movable element  56 . In an embodiment, each of slots  88  extends from end  80  of section  74  toward rotational axis  68 . Each of slots  88  exhibits a dimension, referred to as a length  90 , and another dimension, referred to as a width  92 . In addition, slots  88  are uniformly distributed on opposing sides of longitudinal axis of symmetry  70 . That is, there is an equivalent quantity of slots  88  arranged on either side of axis of symmetry  70  that are also offset from axis of symmetry  70  by equivalent distances. Although an embodiment of MEMS sensor  52  illustrated herein includes an even quantity of slots  88  formed on opposing sides of axis of symmetry  70 , in another embodiment, MEMS sensor  52  may include an odd number of slots  88 . In such a configuration, one of slots  88  would thus be centered on axis of symmetry  70 . In addition, although generally rectangular slots  88  are illustrated herein, other shapes such as a sawtooth or triangular shape, may alternatively be utilized. 
     A function of slots  88  is to reduce the bending moment of inertia of section  74  caused by package stress. As a result, the bending moment of inertia between section  74  and section  76  are more closely matched. Referring to  FIGS. 6 and 7 ,  FIG. 6  shows a cross-sectional edge view of MEMS sensor  52  along section lines  6 - 6  in  FIG. 5 , and  FIG. 7  shows a cross-sectional edge view of MEMS sensor  52  along section lines  7 - 7  in  FIG. 5 . As illustrated in  FIGS. 6 and 7 , due to the presence of slots  88  in the “heavy end” (i.e., section  74 ) package stress results in deformation of section  74  of movable element  56  that is approximately equivalent to the deformation of section  76  of movable element  56  on an opposing side of axis of rotation  68 . This generally symmetric bending of movable element  56  induced by package stress results in an offset difference that is significantly less than that seen in prior art MEMS sensors, such as MEMS sensor  20  ( FIG. 1 ). Accordingly, TCO performance is correspondingly improved leading to more accurate acceleration output of MEMS sensor  52 . 
     A method of fabricating MEMS sensor  82  may entail the provision of substrate  54 . In accordance with conventional and upcoming MEMS sensor manufacturing processes, substrate  54  may be a semiconductor wafer comprising silicon, although any mechanically supporting substrate may be utilized. An insulating layer (not shown) may be formed on surface  60  of substrate  54 . The insulating layer may be silicon dioxide, silicon nitride, and the like. The insulating layer may be formed conformally and then patterned and etched. It functions to insulate static conductive layer  58  from substrate  54 . It should be understood, however, that if substrate  54  is nonconductive, an insulating layer may not be utilized. 
     Static conductive layer  58  may comprise polysilicon, although other conductive materials may be employed. Static conductive layer  58  may be formed by known methods such as deposition and sputtering. Static conductive layer  58  may be deposited over surface  60  of substrate  54  as a blanket layer and can then be patterned and etched to form electrode elements  62  and  64 . A protective layer (not shown) may optionally be disposed over static conductive layer  58  and patterned and etched as desired to protect substrate  54  during future processing steps and to prevent shorting and/or welding between static conductive layer  58  and movable element  56 . 
     A sacrificial layer (not shown) may be formed on the patterned and etched static conductive layer  58 . Like previous layers, the sacrificial layer may also be formed conformally and then patterned and etched as desired. The sacrificial layer may be formed of phosphosilicate glass and can be deposited by chemical vapor deposition, as known to those skilled in the art. It should be understood that other sacrificial materials may be employed in lieu of phosphosilicate glass. 
     The next conductive layer, i.e., movable element  56 , may comprise polysilicon and is formed as a teeter-totter structure positioned over static conductive layer  58 . Movable element  56  is mechanically coupled to substrate  54  by suspension anchors  66 . Movable element  56  may be formed by known methods such as deposition and sputtering. As such, movable element  56  may be deposited over the sacrificial layer as a blanket layer and can then be patterned and etched to form slots  88  of length  90  and width  92  extending from end  80  of movable element  56  toward rotational axis  68 . 
     Following the formation of the above described structure, the sacrificial layer is removed in accordance with conventional procedures. For example, a selective etchant may be employed that can remove the phosphosilicate glass sacrificial layer without appreciably damaging the polysilicon of static conductive layer  58 , movable element  56 , and suspension anchors  66 . Following etching, movable element  56  and a rotational portion of suspension anchors  66  is released from the underlying substrate  54 . 
     Prior to formation of slots  88  in section  74 , section  74  exhibits a mass that is greater than the mass of section  74  following the formation of slots  88 . The mass of section  74  decreases following formation of slots  88  because of the loss of material at slots  88 . However, in an embodiment of the invention, the slots are small enough that material loss results in a mass reduction of section  74  of approximately two to five percent less than the mass of section  74  prior to formation of slots  88 . Since the formation of slots  88  only slightly decreases the mass of section  74 , there is negligible change to the sensitivity of MEMS sensor  52 . For example, in one embodiment, width  92  of each of slots  88  may be approximately one and a half microns with a fifty-two micron pitch, which only reduces sensitivity of MEMS sensor by approximately three percent. 
       FIG. 8  shows a device  94  in which MEMS sensor  52  may be incorporated. Device  94  can be any of a number of devices such as a vehicle dynamic control system, an inertial guidance system, an airbag deployment system in a vehicle, a protection system for a variety of devices, and many other scientific and engineering systems. MEMS sensor  52  may be a single axis accelerometer capable of sensing acceleration along an axis that is perpendicular to a plane of electrode elements  62  and  64  ( FIG. 5 ). 
     Device  94  may include an accelerometer package  96  into which MEMS sensor  52  is incorporated. In this exemplary situation, accelerometer package  96  is in communication with a circuit  98 , which may include, for example, a processor, hard disk drive, and other components that are interconnected via conventional bus structures known to those skilled in the art. Those skilled in the art will recognize that device  94  may include many other components that are not discussed herein for brevity. Furthermore, device  94  need not have the structures specified herein. In this example, circuit  98  monitors signals from accelerometer package  96 . These signals can include acceleration in direction  37  ( FIG. 2 ). An acceleration signal  100  is output from MEMS sensor  52  and is communicated to a sense circuit of an input/output circuit chip  102  for suitable processing, as known to those skilled in the art, prior to output to circuit  98 . The acceleration signal  100  has a parameter magnitude (e.g. voltage, current, frequency, etc.) that is dependent on the acceleration. However, the inclusion of slots  88  ( FIG. 5 ) largely reduces any non-symmetric bending of movable element  56  on opposing sides of axis of rotation ( FIG. 5 ) so that acceleration signal  100  more accurately reflects acceleration in direction  37  ( FIG. 2 ). 
     An embodiment described herein comprises a device that includes a differential capacitive MEMS sensor. Another embodiment comprises a method of fabricating the microelectromechanical systems sensor of the present invention. The sensor may be a differential accelerometer fabricated as a teeter-totter structure, i.e., a movable element. Slots are formed in the heavier end of the movable element distal from and extending toward the rotational axis of the movable element. Due to the presence of the slots in the “heavy end” of the movable element, package stress results in a more symmetric deformation of the movable element on either side of the rotational axis. This symmetric bending of the movable element results in an offset difference that is significantly less than that seen in prior art MEMS sensors. Accordingly, the effects of package stress is greatly decreased, leading to correspondingly improved TCO performance and more accurate acceleration output of the MEMS sensor. 
     Although the preferred embodiments of the invention have been illustrated and described in detail, it will be readily apparent to those skilled in the art that various modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.