JP2005049320A - Acceleration sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a structure of an acceleration sensor that eliminates an ineffective space in the acceleration sensor, increases the mass, lengthens the beam length, improves the detection sensitivity, or can be downsized.
SOLUTION: A plurality of bead rods arranged on a plane that does not include the center of gravity of a movable mass disposed inside a frame portion and that connects the mass and the frame portion, and a plurality of resistance elements provided on the surfaces thereof In the acceleration sensor structure configured to measure the electrical change in proportion to the displacement of the mass due to the inertial force generated in the mass by the acceleration by the resistance change of the resistance element, the inner periphery of the frame portion is substantially square, In addition, the central axis of each beam is substantially linear and is arranged in parallel with each side of the square on the inner periphery of the frame. Furthermore, the sides of the outer periphery of the mass should be parallel to each beam. The four beams should be arranged in a “卍” shape. Alternatively, the four beams have both ends fixed to the frame and the center is fixed to the mass.
[Selection] Figure 1

Description

  The present invention relates to an acceleration sensor. More specifically, the present invention relates to an acceleration sensor configured to output an electrical change substantially proportional to the displacement of a movable mass by a piezoresistive effect.

  In recent years, the acceleration sensor has been miniaturized, and a miniaturized sensor structure has been proposed that can detect not only a one-dimensional but also a two-dimensional or three-dimensional acceleration by utilizing a fine processing technique. FIG. 8 (plan view) and FIG. 9 (center CC cross-sectional view) show the structure of a conventional example of an acceleration sensor using the piezoresistive effect. Also, Cartesian coordinate axes are shown in each figure to define the direction.

  In both figures showing a conventional example of a sensor structure, the constituent material is thinly embedded at a depth of several μm to several tens of μm from the upper surface of an SOI (Silicon On Insulator) substrate, that is, a Si substrate having a thickness of several hundred μm. A material having the embedded SiO2 layer 1 is used, and a number of sensors are simultaneously formed by applying a microfabrication process including etching and thin film formation techniques to the material. The single sensor has an external shape that is a thin cube as shown.

  The outer side is a square frame 2 (the lower surface is used by being fixed to the surface of a sensor support not shown), the center has a mass 3, both of which are four linear and identical beams 5, It is connected at 6, 7, and 8. The thickness of each beam is the thickness of the Si layer above the buried SiO2 layer 1 of the SOI substrate, which is considerably smaller than the total thickness of the sensor (the thickness of the SOI substrate and also the thickness of the frame 2). Therefore, it can be easily bent.

  Inside the frame portion 2, there are four “<”-shaped through holes 10, which are formed by etching Si at the position of the through holes 10 from the upper surface of the SOI substrate and from the lower surface of the through holes 10. The position and the lower surface of the beams 5, 6, 7, 8 are removed by deep etching, and finally the buried SiO 2 layer 1 exposed is removed by etching. A slight Si layer is also removed from the lower surface of mass 3 to create a movable space of mass 3.

  Mass 3 and beams 5, 6, 7, 8 are thus formed. Prior to these steps, a plurality of resistance elements 9 having a piezoresistive effect in which a substantial part is formed of a thin layer of P-type Si doped with boron or the like and is substantially strip-shaped at a necessary position on the upper surface of each beam. Provided. In addition, an aluminum wiring pattern serving as an insulating layer and a lead wire to the outside is also provided along with each resistance element 9, but it is not shown.

  The mass 3 is elastically suspended by four thin and thin beams 5, 6, 7, and 8. The thickness of the mass 3 is slightly thinner than the thickness of the frame portion 2 so that there is room for movement inside the frame portion 2. I'm leaving. Each resistance element 9 on the upper surface of each beaker is incorporated in a Wheatstone bridge circuit, and the bridge circuit is balanced when stationary. When acceleration is applied to the sensor, that is, the frame portion 2 through a sensor support (not shown), an inertial force opposite to the acceleration as viewed from the sensor is generated at the center of gravity G of the mass 3, and the elastic support force of the beam It is displaced slightly in the frame against Due to this displacement, the beam is deformed, and the resistance value of the resistance element 9 is changed. As a result, a non-equilibrium output almost proportional to the applied acceleration can be obtained from the bridge circuit.

  When the center of gravity G of the mass 3 moves up and down (Z direction), the mass 3 moves in parallel, and each beam is bent and deformed in a similar waveform (or should be a gentle step type). Since the center of gravity G is located at a position considerably below the plane located at the center of the thickness of the beak bowls 5, 6, 7 and 8, when the center of gravity G is displaced in the X or Y direction, each beam is caused by the moment of inertia. Make different waveform deformations. Since the beam deformation mode varies depending on the direction of acceleration, for example, if three types of bridge circuits are prepared, outputs proportional to acceleration components in three orthogonal directions can be obtained separately.

The conventional acceleration sensor described above has the following problems.
(1) The four thick “く” -shaped through holes 10 are dead spaces that are not effectively utilized, and hinder the downsizing of the sensor.
(2) Since the beams 5, 6, 7, and 8 are directed in the diameter direction of the sensor, the length thereof cannot be sufficiently obtained and the detection sensitivity is low. If the beam is short, the bending rigidity becomes high and it becomes difficult to deform, and if it is made thin to avoid it, it becomes brittle and easily broken. Moreover, the resistive element that can be formed must be shortened, and the amount of resistance change with respect to the mass displacement, which is the detection sensitivity, cannot be sufficiently obtained.
(3) Further, an attempt to secure the necessary beam length prevents the mass 3 from being enlarged.

  The present invention has been made to solve these problems, and its purpose is to eliminate the ineffective space in the acceleration sensor to increase the mass and to lengthen the beam as much as possible to improve the acceleration detection sensitivity. An object of the present invention is to provide a structure of an acceleration sensor that can be made or can be miniaturized.

In order to achieve the above object, the acceleration sensor of the present invention has the following characteristics.
(1) A frame portion that surrounds the outer periphery, a movable mass disposed inside the frame portion, and a plurality of bees that are disposed on a plane not including the center of gravity of the mass and connect the mass and the frame portion. A plurality of resistance elements provided at predetermined positions on the surfaces of the plurality of beams, and an electrical change substantially proportional to the displacement of the mass due to an inertial force generated in the mass due to acceleration through the deformation of the beam In a sensor structure having an acceleration detection function configured to measure by a resistance change of the resistance element, an inner circumference of the frame portion is substantially square, and central axes of the plurality of beams are substantially linear. And arranged in parallel with each side of the square of the frame.

The acceleration sensor of the present invention may further have at least one of the following features (2) to (4).
(2) The outer peripheral portion of the mass includes each side of the square on the inner periphery of the frame portion and a plurality of sides substantially parallel to the central axes of the plurality of beams.

(3) There are four beams, one end of each beam is fixed to the frame, the other end is fixed to the mass, and each beam has a substantially “卍” shape around the mass. Be placed in.

(4) There are four bees, and both ends of each beam are fixed to the frame, and the center is fixed to the mass.

  In the present invention, since the beam is arranged along the inner periphery of the frame portion, the length of the beam can be increased, and the sensitivity of resistance change to mass displacement can be increased. At the same time, the intensity of the individual beams can be increased. Furthermore, since the mass plane area can be increased by bringing the outer circumference of the mass closer to the side surface of the beam, the mass value itself increases, increasing the mass displacement for the same acceleration, and also increasing the acceleration detection sensitivity. Can do. And since the whole area of a sensor plane can be used effectively, it can also contribute to size reduction of an acceleration sensor.

  In the acceleration sensor structure of the present invention, four beams provided with resistance elements having a piezoresistive effect are arranged along the inner circumference of the frame portion and the respective sides of the inner circumference of the mass. Two examples are described below. In the drawings showing these structures, parts corresponding to those of the conventional example and having the same functions are denoted by the same reference numerals, and will not be repeated without being described again. Each of the embodiments is basically manufactured by a microfabrication process according to the above-described conventional example. Therefore, the details and application of the improved processing method are free, and there is no particular limitation.

  FIG. 1 is a plan view of a first embodiment of the three-dimensional acceleration sensor of the present invention, and FIG. In this example, the sensor plane appearing at time 1 is taken as the (100) plane of the Si single crystal, the X axis is taken in the <110> direction, and the Y axis is taken in the <1-10> direction. This is a result of considering the anisotropy and symmetry of the piezoresistive effect. The beak bowls 5, 6, 7, and 8 connect the side surface of the convex portion 4 provided at the center of each side of the mass 3 having a square planar shape and the portion close to the end of each side inside the frame portion 2. Thus, it is formed on the upper surface side of the sensor. (The planar shape of the frame 2 and the mass 3 is preferably a square because of the simplicity of the design and the equalization of X and Y sensitivities, but the sensor can be realized even if these shapes are rectangular. The length of and does not necessarily have the same shape or shape.)

  Therefore, the planar arrangement of each beam is arranged around the center mass 3 so as to form a “卍” shape. Of course, the direction of these beams may be clockwise or counterclockwise. Such an arrangement improves the length of each beam considerably longer. Further, since the planar outer shape of the mass 3 can be close to the side surface of each beam (by reducing the width of the four through holes 10 as much as possible), the value of the mass can be set to be considerably large. The mass values of the four convex portions 4 are also added together with that of the mass 3.

  The advantages associated with the lengthening of the beams 5, 6, 7, and 8 will be described. First, the bending rigidity is reduced by increasing the length, and a large deformation is obtained for the same mass and the same acceleration. Moreover, since the surface which produces the same distortion on a beam increases, the length and magnitude | size of the resistive element 9 formed on it can be increased, and a characteristic, a sensitivity, and a processing precision can be improved. Also, the beam thickness that gives the same bending rigidity can be increased, the distance from the neutral surface to the surface (half the thickness) becomes large, and even if the neutral surface bends with the same curvature, the expansion and contraction of the surface is Increases and contributes to an increase in the piezoresistive effect. When the mass 3 moves violently due to impact or the like, its kinetic energy is absorbed by each beam, but the energy that can be absorbed until the beam is broken is proportional to the volume of the beam. This also improves the fracture strength of the steel.

  Rx1 to Rx4, Ry1 to Ry4, and Rz1 to Rz4, which are resistive elements 9 having a piezoresistive effect, are schematically illustrated on the surfaces of the beads 5 to 8 in order to individually measure acceleration components in the X, Y, and Z directions. 1 is formed. These are arranged so that the deformation of the beam due to the tilt displacement of the mass 3 in a predetermined direction when the acceleration in the X or Y direction is applied, and the deformation of the beam due to the vertical translation of the mass 3 can be detected separately in the case of the Z direction acceleration. .

  Each resistance element is incorporated in three Wheatstone bridge circuits, which are the main parts of the acceleration detection circuit, as shown in FIGS. 3 (a), 3 (b), and 3 (c). A rectangular frame indicates the resistance value of each resistance element 9. Each bridge circuit is supplied with a constant voltage input Vin between itself and GND in the vertical direction of the drawing, and the resistance of each side is adjusted so as to balance when stationary. The resistance balance is lost due to the distortion of the beam based on the applied acceleration. In the figure, the arrow attached to each resistance represents the direction of resistance change with respect to acceleration acting in the direction of the coordinate axis. VoutAx, VoutAy, and VoutAz, which are voltage outputs proportional to the acceleration components in the X, Y, and Z directions, are obtained from the detection terminals provided in the horizontal direction.

  A modification of the first embodiment will be described. In FIG. 1, there is still a wide portion in a part of each through hole 10. In order to increase the mass 3 by utilizing this space, the four additional masses 3a can be provided integrally with the mass 3 as indicated by the dotted line. As another space utilization method, the positions of the convex portions 4 that are fixed ends on the mass 3 side of the beams 5, 6, 7, and 8 are set on the sides of the mass 3 on the frame portion 2 side of each beak bowl. By shifting in a direction farther from the fixed end, the length of each beam can be further extended (to nearly the side length of mass 3). In addition, the position of each resistance element 9 is not necessarily the best in the illustrated position (the end of the beam). For example, some may be near the center of the beam. Of course, the configuration of the bridge circuit changes with it.

  There may be various other modifications. For example, in the structure of the present embodiment, a slight twist may occur in some beams depending on the state of displacement of the mass 3. If there is a piezoresistive effect due to this torsion and it is added to the piezoresistive effect based on the bending of the beam, it will become the sensitivity of other axes for the detection output in a certain direction, and the detection accuracy and sensitivity of the sensor may be affected. There is sex. Since the piezoresistance effect of p-type Si has anisotropy, there is a piezoresistance effect in bending deformation, but in the case of torsional deformation, a crystal plane or orientation that makes the piezoresistance effect zero is used to cut out the sensor orientation. It is also possible to obtain a sensor configuration that is insensitive to torsion by tilting.

  FIG. 4 is a plan view of a second embodiment of the three-dimensional acceleration sensor of the present invention, and FIG. In the present embodiment, the beams (which may be considered partial beams) 5, 5 a are arranged so as to be parallel to each side from both side surfaces of the convex portion 4 provided at the center of each side of the mass 3 having a square planar shape. 6, 6 a, 7, 7 a, 8, 8 a are fixedly provided, and the other end thereof is fixed to the frame portion 2. Therefore, if a pair of beams (for example, 5 and 5a) that are in a straight line is regarded as one beam, the mass 3 is fixed and supported at the center of each of the four beams that are both supported by the four ends. Can be said. The advantages of the second embodiment are almost inherited from the advantages of the first embodiment described above, but it is expected that the effect of twisting is negligible because the symmetry is further increased.

  Since the resistance elements 9 in this embodiment are provided on both side portions of the beam, the number thereof increases, R′z1 to R′z4 increase, and they are arranged as shown in FIG. These are incorporated in the Wheatstone bridge circuit shown in FIGS. 6 (a), 6 (b) and 6 (c). The detection operation and various effects of the bridge circuit are essentially the same as in the first embodiment. In this example, for example, the position of the resistance element, the shape of the mass, the beam length, and other various modifications may be possible.

  FIG. 7 is a plan view of a modification of the second embodiment of the present invention. This modified example is different from the basic shape of FIG. 4 in the arrangement of the resistance elements 9. The cross-sectional shape of the sensor may be the same as in FIG. 5, and the same bridge circuit for detection as in FIG. 6 can be used, so only a plan view of a modification is shown. In the basic form of FIG. 4, Rx1 to Rx4 that are X direction detection resistance elements and Ry1 to Ry4 that are Y direction detection resistance elements are arranged on the same straight line, but in the modified example of FIG. One pair of detection resistive elements 9 (Rx1 and Rx2, Rx3 and Rx4) and one pair of Y direction detection resistive elements 9 (Ry1 and Ry2, Ry3 and Ry4) are different (on the opposite side). Placed on top. According to the operation simulation, this arrangement can provide higher X and Y detection sensitivity.

  A third embodiment of the present invention will be described without using the drawings. This example has a partial structure that is a mixture of the first and second embodiments. That is, it has a planar shape obtained by removing, for example, the beak bowls (partial beams) 5a, 6, 7a, and 8 from FIG. In other words, it can be said that the extending directions of the beam 6 and the beam 8 of the first embodiment are reversed. Even in this configuration, the three-dimensional acceleration can be detected, and various features are close to those of the first embodiment. However, the position of each resistance element 9 on the bridge circuit may be different from that in FIG. There can be various modifications of this example as well.

  According to the present invention, the sensitivity of the acceleration sensor can be improved or downsized (for example, about 1 mm × 1 mm × 0.5 mm), so that the acceleration detection accuracy can be improved, the configuration of the detection circuit can be simplified, and the power can be saved. The package that encloses the sensor can be expected to be downsized, and the device itself in which the acceleration sensor is incorporated can be expected to rationalize the component arrangement and save the space.

  It is a top view of the 1st example of the acceleration sensor of the present invention.   It is AA sectional drawing of the 1st Example of the acceleration sensor of this invention.   Each of (a), (b), and (c) is a circuit diagram of the main part of the detection circuit in the first embodiment of the acceleration sensor of the present invention.   It is a top view of the 2nd example of the acceleration sensor of the present invention.   It is BB sectional drawing of the 2nd Example of the acceleration sensor of this invention.   Each of (a), (b), and (c) is a circuit diagram of the main part of the detection circuit in the second embodiment of the acceleration sensor of the present invention.   It is a top view of the modification of the 2nd Example of the acceleration sensor of this invention.   It is a top view of the prior art example of an acceleration sensor.   It is CC sectional drawing of the prior art example of an acceleration sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Embedded SiO2 layer 2 Frame part 3 Mass 3a Additional mass 4 Convex part 5, 5a, 6, 6a, 7, 7a, 8, 8a Beam 9 Resistance element 10 Through-hole Ax, Ay, Az Acceleration component G Gravity center GND Grounding terminal Vin Power input terminal Vout Acceleration output X, Y, Z Coordinate axis

Claims (4)

  A frame surrounding the outer periphery, a movable mass disposed inside the frame, a plurality of beams disposed on a plane not including the center of gravity of the mass, and connecting the mass and the frame; and the plurality of beams A plurality of resistance elements provided at predetermined positions on the surface of the beet bowl, and the resistance elements undergo an electrical change substantially proportional to the displacement of the mass due to an inertial force generated in the mass by acceleration through deformation of the beam. In the sensor structure having an acceleration detecting action, which is configured to measure by a resistance change, the inner circumference of the frame portion is substantially square, and the central axes of the plurality of beams are substantially linear, An acceleration sensor, wherein the acceleration sensor is arranged in parallel with each side of the square of the frame portion.   2. The acceleration sensor according to claim 1, wherein an outer peripheral portion of the mass includes each side of a square of an inner periphery of the frame portion and a plurality of sides substantially parallel to central axes of the plurality of beams.   There are four beams, one end of each beam is fixed to the frame, the other end is fixed to the mass, and the beams are arranged in a substantially “卍” shape around the mass. The acceleration sensor according to claim 1, wherein the acceleration sensor is provided.   The acceleration sensor according to claim 1 or 2, wherein the number of the beaks is four, and each beam is fixed to the frame at both ends and the center is fixed to the mass.

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