JP2008170383A - Single-axis semiconductor acceleration sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a single-axis semiconductor acceleration sensor which enables reduction in damages to its beam due to resonance, and overall size reduction of the sensor. <P>SOLUTION: The sensor is equipped with a frame section made of semiconductor material; a displacement section, which is made of the semiconductor material, is arranged inside the frame section, receives acceleration in a first direction and is displaced with respect to the frame section; a plurality of connections which are connections made of the semiconductor material connect the frame section and the displacement section, respectively, and are arranged side by side in the first direction, having a cross-sectional shape, with a larger thickness in the second direction perpendicular to the first direction than a width in the first direction; and a plurality of strain-detecting elements arranged in the plurality of connections. The displacement section has one end and the other end arranged in a third direction perpendicular to the first and second directions, and at this one end, has a first protrusion and a first recession, arranged side by side that corresponds to the first protrusion; the frame section has a second recess corresponding to the first protrusion, and a second protrusion corresponding to the first recess. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a semiconductor acceleration sensor that detects acceleration, and more particularly to a uniaxial semiconductor acceleration sensor that detects a uniaxial acceleration component.

Accelerometers are widely used for measuring the impact strength of structures or analyzing acceleration waveforms in vibrations generated in structures. As a conventional acceleration sensor used in this way, for example, a technique of a uniaxial acceleration sensor made of metal is disclosed (for example, see Patent Document 1).
JP-A-7-167886

However, since metal is difficult to finely process and there is a limit to downsizing, it is difficult to reduce the size of a uniaxial acceleration sensor with the above-described technology using metal. In order to solve such problems, the present inventor has found a uniaxial semiconductor acceleration sensor that is miniaturized by using a semiconductor and using a micromachining technique.
Here, the beam (connection part) and the weight (displacement part) of the uniaxial semiconductor acceleration sensor have a resonance frequency. When an excitation force of the frequency is applied from the outside due to an impact or the like, the resonance resonates and a large displacement is generated in the weight. It was found that in extreme cases the beam could be damaged.
In view of the above, an object of the present invention is to provide a uniaxial semiconductor acceleration sensor that makes it possible to achieve both reduction in beam damage due to resonance and overall size reduction.

  A uniaxial semiconductor acceleration sensor according to the present invention includes a frame portion made of a semiconductor material, and a displacement portion made of the semiconductor material, disposed in the frame portion, and displaced with respect to the frame portion in response to acceleration in a first direction. And a plurality of connecting portions that are made of the semiconductor material, connect the frame portion and the displacement portion, and are arranged side by side in the first direction, and have a width in the first direction. A plurality of connection portions having a cross-sectional shape having a large thickness in a second direction perpendicular to the first direction, and a plurality of strain detection elements disposed in the plurality of connection portions, and the displacement The portion has one end and the other end arranged in a third direction orthogonal to the first and second directions, and arranged at the one end alongside the first convex portion and the first convex portion. A first concave portion, and the frame portion has a second concave portion corresponding to the first convex portion, and a front portion. And having a second convex portions corresponding to the first recess.

According to the above configuration, since a micromachining technique is used using a semiconductor, the uniaxial semiconductor acceleration sensor can be reduced in size.
In addition, a first convex portion and a first concave portion arranged side by side with the first convex portion are formed at one end in the third direction of the displacement portion, and the first convex portion of the displacement portion is formed in the frame portion. Since the second concave portion corresponding to the portion and the second convex portion corresponding to the first concave portion of the displacement portion are formed, air damping is applied to the movement of the displacement portion to prevent extreme resonance. Damage to the beam (connection portion) can be reduced.

Here, the connection portion has a cross-sectional shape in which the thickness in the second direction perpendicular to the first direction is larger than the width in the first direction. Therefore, the connection portion bends when it receives an acceleration of a component in one direction in the first direction.
In addition, since a plurality of connecting portions are provided side by side in the first direction, the displacement of the connecting portion is small with respect to acceleration in a direction orthogonal to the first direction (that is, the sensitive axis). Therefore, there is one sensitive axis, and the displacement of the connecting portion is small with respect to the acceleration orthogonal to the sensitive axis direction. Therefore, the semiconductor acceleration sensor according to the present invention can detect acceleration substantially only in one axis direction.

  ADVANTAGE OF THE INVENTION According to this invention, the uniaxial semiconductor acceleration sensor which enables coexistence of reduction of the damage of the beam by resonance and the miniaturization of the whole can be provided.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(First embodiment)
FIG. 1 is a perspective view showing a uniaxial semiconductor acceleration sensor 1 according to an embodiment of the present invention. The uniaxial semiconductor acceleration sensor 1 includes an acceleration detection unit 2 and an upper lid 3 and a lower lid 4 that are disposed so as to sandwich the acceleration detection unit 2. FIG. 2 is a perspective view showing the acceleration detection unit 2. FIG. 3 is a top view showing the uniaxial semiconductor acceleration sensor 1 of FIG. FIG. 4 is a partial cross-sectional view illustrating a state in which the uniaxial semiconductor acceleration sensor 1 is cut along AA in FIG. 3. FIG. 5 is a partial cross-sectional view illustrating a state in which the uniaxial semiconductor acceleration sensor 1 is cut along BB in FIG. 3.

The outer shape of the acceleration detection unit 2 is, for example, a rectangular parallelepiped shape with sides of 2500 μm in the X direction, 2000 μm in the Y direction, and 300 μm in the Z direction. The constituent material of the acceleration detector 2 can be a semiconductor, for example, polycrystalline or single crystal silicon. In the present embodiment, the single crystal whose surface orientation on the top surface of the acceleration detector 2 is (100). Silicon was used.
The outer shape of the upper lid portion 3 is, for example, a rectangular parallelepiped shape with sides of 2500 μm in the X direction, 1750 μm in the Y direction, and 300 μm in the Z direction. The outer shape of the lower lid portion 4 is, for example, a rectangular parallelepiped shape with sides of 2500 μm in the X direction, 2000 μm in the Y direction, and 300 μm in the Z direction. For example, a glass plate can be used as a constituent material of the upper lid portion 3 and the lower lid portion 4, and Pyrex (registered trademark) is used in the present embodiment.

When the constituent materials of the upper lid portion 3 and the lower lid portion 4 and the acceleration detection portion 2 are glass and Si, the upper lid portion 3 and the acceleration detection portion 2 are joined, and the lower lid portion 4 and the acceleration detection portion 2 are joined. For this, anodic bonding (also referred to as electrostatic bonding) can be used.
The upper lid 3 is shorter than the acceleration detector 2 and the lower lid 4 in the Y direction by 250 μm, for example, and a step W is formed on the front surface of the uniaxial semiconductor acceleration sensor 1 by the acceleration detector 2 and the upper lid 3. ing.
Moreover, between the acceleration detection part 2 and the upper cover part 3 in the uniaxial semiconductor acceleration sensor 1 and an acceleration detection part so that the displacement part 6 (after-mentioned) of the acceleration detection part 2 can displace by receiving the acceleration of a X direction. A space 10 is provided between 2 and the lower lid portion 4.
In the present embodiment, the distance between the acceleration detection unit 2 and the upper lid unit 3 and the acceleration detection unit 2 and the lower lid unit 4 in the space 10 is reduced (for example, 3 μm, respectively), and the slide film damping is applied to the movement of the displacement unit 6. By applying (described later), the amplitude of the displacement portion 6 at the time of resonance is reduced, and damage to the connection portions 8 and 9 due to resonance is reduced.

  The acceleration detection unit 2 is disposed on the displacement unit 6, the frame unit 7, the first connection unit 8, the second connection unit 9, the first connection unit 8, and the second connection unit 9. It comprises a plurality of piezoresistive elements R (R1 to R4). The acceleration detector 2 can be created by anisotropically etching the semiconductor substrate to form trench grooves 5a and 5b (elongated grooves).

The trench groove 5a extends in the X direction in a repeatedly folded shape in a region near the positive direction of the Y axis of the substrate, and the folded portions at both ends are side surfaces near the outer edge of the substrate in the connection portions 8 and 9, respectively. It extends long in the negative Y-axis direction so as to come into contact.
The trench 5b extends in the X direction in a repeatedly folded shape in a region near the negative Y-axis direction of the substrate, and is in contact so that the folded portions at both ends are sandwiched between the connection portions 8 and 9, respectively. .
The trench grooves 5a and 5b penetrate the front and back surfaces of the substrate, and the displacement portion 6 and the frame portion 7 are separated by the trench grooves 5a and 5b and are relatively movable.
By setting the width of the gap (gap) between the trench grooves 5a and 5b to 3 μm, for example, the movement of the displacement portion 6 is subjected to squeezed film damping, which will be described later, to reduce the amplitude at the time of resonance of the displacement portion 6 and It is possible to reduce damage to the connection portions 8 and 9.

The displacement part 6 is displaced with respect to the frame part 7 in response to the acceleration in the X direction. The displacement part 6 is separated from the frame part 7 and is provided in the opening of the frame part 7, and can be divided into a base part 6a and comb teeth parts 6b and 6c.
The base portion 6 a is a substantially rectangular substrate in plan view connected to one end of the first connection portion 8 and one end of the second connection portion 9.
The comb tooth portion 6b is a comb-shaped portion that extends in the Y positive direction from the end portion in the Y positive direction of the base body portion 6a.
The comb tooth portion 6c is a comb-shaped portion that extends in the Y negative direction from the end portion of the base portion 6a in the Y negative direction.

  In this specification, the comb tooth shape refers to a shape having comb teeth (that is, convex portions). In the present embodiment, from the viewpoint of obtaining a large squeezed film damping effect, the comb teeth of the comb teeth portions 6b and 6c, the comb teeth of the comb teeth portions 7b and 7c, and the comb teeth portions 6b and 6c, Although the external shape of the recessed part arrange | positioned between each comb teeth of 7b and 7c is made into the rectangular shape by planar view, these are not limited to a rectangular shape. Since the squeezed film damping effect can be obtained, the displacement part 6 has a convex part and a concave part arranged in line with the convex part at one or both ends in the Y direction, and the frame part 7 has the displacement part. What is necessary is just to have the recessed part corresponding to the convex part of 6 and the convex part corresponding to the recessed part of the displacement part 6. FIG.

  The connecting portions 8 and 9 and the displacement portion 6 of the uniaxial semiconductor acceleration sensor 1 have a resonance frequency. When an excitation force of the frequency is applied from the outside, the displacement portion 6 resonates and a large displacement is generated in the displacement portion 6. The connection parts 8 and 9 may be damaged. Therefore, the comb-tooth portions 6b and 6c of the displacement portion 6 are formed in a comb-tooth shape having a large attenuation coefficient due to air damping, the amplitude at the time of resonance of the displacement portion 6 is reduced, and damage to the connection portions 8 and 9 due to resonance is reduced. I am trying. The reason why the damping coefficient due to air damping increases when the displacement portion 6 has a comb-tooth shape is that a large squeezed film damping effect is obtained as will be described later.

  Here, the damping effect will be described. In the present specification, air damping refers to attenuating the movement of the displacement portion 6 using gas as a medium. When a plate-like vibrating body is disposed opposite to a fixed wall surface via a gap, the effect of air viscosity increases as the distance between the vibrating body and the wall surface decreases, so the air damping effect increases. For air damping, slide film damping that occurs when the vibrating body is vibrated in a direction substantially parallel to the wall surface, and a direction in which the vibrating body presses the wall surface (for example, the vibrating body is substantially perpendicular to the wall surface). An example is squeezed film damping that occurs when the vibration is applied.

  FIG. 6 is a diagram illustrating the side surfaces 6 b-1 to 6 b-3 of the comb tooth portion 6 b of the displacement portion 6 and the side surfaces 6 c-1 to 6 c-3 of the comb tooth portion 6 c of the displacement portion 6. FIG. 7 is a diagram illustrating the side surfaces 7 b-1 to 7 b-3 of the comb-tooth portion 7 b of the frame portion 7 and the side surfaces 7 c-1 to 7 c-3 of the comb-tooth portion 7 c of the frame portion 7. In FIGS. 6 and 7, the connecting portions 8 and 9 are also illustrated in order to clarify the positional relationship. The comb teeth 7b and 7c of the frame 7 will be described later.

  Since the displacement portion 6 is substantially displaced only in the X direction in response to the acceleration in the X direction, the side surfaces 6b-1 and 6b-2 parallel to the X axis of the comb tooth portion 6b are comb tooth portions facing the side surfaces. 7b vibrates in substantially parallel directions with respect to the side surfaces 7b-1 and 7b-2. Further, the side surfaces 6c-1 and 6c-2 parallel to the X axis of the comb tooth portion 6c vibrate in a substantially parallel direction with respect to the side surfaces 7c-1 and 7c-2 of the comb tooth portion 7c facing the side surface. . Therefore, the side surfaces 6b-1, 6b-2, 6c-1, 6c-2 of the comb teeth portions 6b, 6c and the side surfaces 7b-1, 7b-2, 7c- of the comb teeth portions 7b, 7c facing the side surfaces thereof. Between 1 and 7c-2, a slide film damping effect is obtained.

  Since the displacement portion 6 is substantially displaced only in the X direction in response to the acceleration in the X direction, the side surface 6b-3 parallel to the Y axis of the comb tooth portion 6b is the side surface 7b of the comb tooth portion 7b facing the side surface. Vibrates in a direction substantially perpendicular to -3. Further, the side surface 6c-3 parallel to the Y axis of the comb tooth portion 6c vibrates in a substantially vertical direction with respect to the side surface 7c-3 of the comb tooth portion 7c facing the side surface. Therefore, the squeezed film damping effect is provided between the side surfaces 6b-3 and 6c-3 of the comb teeth portions 6b and 6c and the side surfaces 7b-3 and 7c-3 of the comb teeth portions 7b and 7c facing the side surfaces. Is obtained.

  Since the sum of the areas of the side surfaces 6b-3, 6c-3 is larger than the sum of the areas of the side surfaces 6b-1, 6b-2, 6c-1, 6c-2, it is squeezed rather than slide film damping in terms of area. The contribution of film damping is large. Even when compared with the same area, squeezed film damping has a damping effect several orders of magnitude higher than slide film damping. For this reason, since the uniaxial semiconductor acceleration sensor 1 employs a comb-teeth shape for the comb-tooth portions 6b, 6c, 7b, and 7c, a large air-damping effect can be obtained by squeezed film damping. Even if the air is under atmospheric pressure, the amplitude of the displacement portion 6 at the time of resonance can be reduced, and damage to the connection portions 8 and 9 due to resonance can be reduced.

  Here, the air damping medium is not limited to the air under atmospheric pressure used in the present embodiment, and for example, a medium having a larger viscosity coefficient than air such as rare gas such as He or Ne or air such as nitrogen gas is used. In this case, a larger damping effect can be obtained. Even if an air damping medium with increased pressure is used, a larger damping effect can be obtained.

  The uniaxial semiconductor acceleration sensor 1 can use air under atmospheric pressure as an air damping medium. When air under atmospheric pressure is used as an air damping medium, the air-tightness is more important than using other media. It can be said that the nature is low. Therefore, the uniaxial semiconductor acceleration sensor 1 can be easily manufactured by using air under atmospheric pressure as an air damping medium, and has high durability.

Although the comb-tooth part formed in the displacement part 6 may be formed in one end of a Y direction, it is preferable to form the two comb-tooth parts 6b and 6c in the both ends of a Y direction like this Embodiment. The reason why the two comb-tooth portions 6b and 6c are formed in the displacement portion 6 is that the squeezed film damping effect can be further increased and damage to the connection portions 8 and 9 due to resonance can be further reduced.
The reason why the two comb-tooth portions 6b and 6c are formed in the displacement portion 6 is to achieve both miniaturization and high sensitivity of the uniaxial semiconductor acceleration sensor 1. If the uniaxial semiconductor acceleration sensor 1 is made smaller (smaller capacity) without forming the comb teeth 6c but only the comb teeth 6b, the displacement portion 6 also has a smaller capacity and its mass becomes smaller. There is also a possibility that the sensitivity to the may decrease. The mass of the displacement part 6 is ensured by dividing and arranging the comb-tooth parts 6b and 6c in two. As a result, the uniaxial semiconductor acceleration sensor 1 can be both reduced in size and increased in sensitivity.

  Further, if the number of comb teeth of the comb teeth portions 6b and 6c is increased, the damping effect can be increased. When the length of the comb teeth is the same, and the number of comb teeth of the comb teeth portions 6b and 6c is increased, the side surfaces 6b-1 and 6b-2 of the comb teeth portion 6b and the side surfaces 6c-1 of the comb teeth portion 6c, This is because the slide film damping effect does not change because the total area of 6c-2 is the same, but the total area of the side surfaces 6b-3 and 6c-3 increases, so that the squeezed film damping effect can be increased. .

The number of teeth of the comb teeth 6b and 6c is not limited to five as shown in FIG. 2, and the length and width of the comb teeth 6b and 6c are not limited. It can be determined based on external dimensions, desired damping ratio, and the like.
In the uniaxial semiconductor acceleration sensor 1, for example, the number of teeth of the comb teeth portion 6b is 66, the length of the comb teeth is 390 μm, the width of the comb teeth is 12 μm, the interval between the comb teeth is 22 μm, and the number of teeth of the comb teeth portion 6c is 63, the length of the comb teeth can be 390 μm, the width of the comb teeth can be 12 μm, and the interval between the comb teeth can be 22 μm.

The frame portion 7 can be divided into a frame body portion 7a and comb teeth portions 7b and 7c. The frame body portion 7a is a frame-shaped substrate whose outer periphery and inner periphery are both substantially rectangular in plan view, and the upper lid 3 is joined to the upper surface and the lower lid 4 is joined to the lower surface.
The comb tooth portion 7b is a comb-shaped portion that faces the displacement portion 6 so as to mesh with the comb tooth gap of the comb tooth portion 6b and extends in the Y negative direction. A trench groove 5a is formed between the comb tooth portion 7b and the comb tooth portion 6b and is spaced apart from each other.
The comb-tooth portion 7c is a comb-tooth-shaped portion that faces the displacement portion 6 so as to mesh with the comb-tooth gap of the comb-tooth portion 6c and extends in the positive Y direction. A trench groove 5b is formed between the comb tooth portion 7b and the comb tooth portion 6b, and is spaced apart from each other.

  In the present embodiment, the number of teeth, the length, the width, and the comb teeth of the comb teeth 6b and 6c so that the comb teeth of the comb teeth 7b and 7c respectively mesh with the gaps of the comb teeth of the comb teeth 6b and 6c. For example, the number of teeth of the comb teeth portion 7b is 65, the length of the comb teeth is 390 μm, the width of the comb teeth is 12 μm, and the interval of the comb teeth is 22 μm. 62, the length is 390 μm, the width is 12 μm, and the interval between comb teeth can be 22 μm.

  The first connecting portion 8 and the second connecting portion 9 have both ends integrally coupled to the displacement portion 6 and the frame portion 7, respectively, and are parallel to each other and have the same length. Moreover, the 1st connection part 8 and the 2nd connection part 9 are arrange | positioned along with the X direction, are provided along the Y direction, respectively, and the width | variety of a X direction is thin with respect to the thickness of a Z direction. belongs to. In this Embodiment, the longitudinal direction of the 1st connection part 8 and the 2nd connection part 9 is each arrange | positioned in the <110> direction.

  The width in the X direction of the connecting portions 8 and 9 is thinner than the thickness in the Z direction. Therefore, the connection portion bends when it receives an acceleration in the X direction. Further, since a plurality of connection portions 8 and 9 are provided side by side in the X direction, the displacement of the connection portions 8 and 9 is small with respect to acceleration in the Y and Z directions orthogonal to the X direction. Therefore, since the perception axis is the X direction and the displacement of the connecting portions 8 and 9 is small with respect to the acceleration in the Y and Z directions orthogonal to the perception axis direction, the acceleration sensor 1 is substantially accelerated in the uniaxial direction. Only detect.

  The 1st connection part 8 and the 2nd connection part 9 function as a beam which can be bent. The displacement part 6 can be displaced with respect to the frame part 7 by receiving the acceleration and bending the first connection part 8 and the second connection part 9. Specifically, when an acceleration component in the X-axis direction is applied, a force acts on the displacement portion 6 and the displacement portion 6 is linearly displaced in the X positive direction and the X negative direction with respect to the frame portion 7. That is, “displacement” here refers to movement of the X axis in the positive and negative directions.

By detecting the displacement of the displacement unit 6 in the X-axis direction, the acceleration in the X-axis direction can be measured. FIG. 8 is an enlarged view of a region surrounded by a dotted-line ellipse in FIG. 2, and is a top view in the vicinity of the piezoresistive elements R 1 and R 2 formed in the first connection portion 8. FIG. 9 is an enlarged view of a region surrounded by a dotted-line ellipse in FIG. 2, and is a top view in the vicinity of the piezoresistive elements R3 and R4 formed in the second connection portion 9. As shown in FIGS. 8 and 9, the longitudinal direction of the piezoresistive element is <110 at the outer edge of the (100) plane near the both ends of the connection portions 8 and 9 and the upper surface of the connection portions 8 and 9. The piezoresistive elements R (R1 to R4) are formed so as to be arranged in the> direction.
The longitudinal direction of the piezoresistive element R is arranged along the current direction of the wiring 11. The piezoresistive element R functions as a strain detection element. The piezoresistive element R is for detecting the bending (or distortion) of the connecting portions 8 and 9 as a change in resistance and the displacement of the displacement portion 6. Details of this will be described later.

  In the present embodiment, the acceleration detector 2 is made of single crystal silicon whose principal surface has a plane orientation of (100). The longitudinal directions of the first connecting portion 8 and the second connecting portion 9 are arranged in the <110> direction, respectively, and the longitudinal direction of the piezoresistive element R is also directed in the <110> direction on the upper surface of the connecting portions 8 and 9. Arranged. According to the above configuration, by using the anisotropy of the silicon single crystal, it is possible to obtain a uniaxial semiconductor acceleration sensor 1 that is small in size, relatively high in sensitivity, and high in accuracy. Details of this will be described later.

(Second Embodiment)
The uniaxial semiconductor acceleration sensor of this embodiment is different from the first embodiment in the following points. First, the acceleration detector 2 is composed of a single crystal silicon substrate having a principal surface with a surface orientation of (110). Secondly, the longitudinal directions of the first connection portion 8 and the second connection portion 9 are arranged in the <111> direction, respectively. Thirdly, the longitudinal direction of the piezoresistive element R is arranged toward the <111> direction on the outer edge of the (110) plane which is the upper surface of the connecting portions 8 and 9.
In the other respects, the present embodiment is not particularly different from the configuration of the first embodiment, and thus the description thereof is omitted. According to the above-described configuration, similarly to the uniaxial semiconductor acceleration sensor 1 of the first embodiment, it is possible to achieve both reduction of beam damage due to resonance and overall size reduction. Further, by utilizing the anisotropy of the silicon single crystal, a small and highly sensitive uniaxial semiconductor acceleration sensor can be obtained. Details of the high sensitivity of the uniaxial semiconductor acceleration sensor will be described later.

(Operation of single-axis semiconductor acceleration sensor)
The principle of acceleration detection by the uniaxial semiconductor acceleration sensor according to the present invention will be described. As shown in FIG. 8 and FIG. 9, a total of four piezoresistive elements R1 are provided in the vicinity of both end portions of the first connection portion 8 and the second connection portion 9 and on the outer edges of the upper surfaces of the connection portions 8 and 9. -R4 is arranged. These piezoresistive elements R can be constituted by P-type or N-type impurity doped regions formed on the upper surfaces of the first connection portion 8 and the second connection portion 9 of the single crystal silicon substrate.

  The piezoresistive elements R1 to R4 each function as an X-axis direction component displacement detection unit (strain detection element) that detects the displacement of the X-axis direction component of the displacement unit 6.

  From the amount of expansion / contraction of the piezoresistive element R, the acceleration in the X direction can be detected. The expansion and contraction of the piezoresistive element R can be detected as a change in the resistance of the piezoresistive element R.

  Assume that each piezoresistive element R is configured by doping P-type impurities into silicon. In this case, the resistance value in the longitudinal direction of the piezoresistive element R increases when a stress in the expansion direction is applied, and decreases when a stress in the contraction direction is applied. When the piezoresistive element R is constituted by doping N-type impurities into silicon, the change in resistance value is reversed.

  FIG. 10 is a circuit diagram showing a configuration example of a detection circuit for detecting the acceleration in the X-axis direction from the resistance of the piezoresistive element R. In this detection circuit, in order to detect an acceleration component in the X-axis direction, a bridge circuit including four piezoresistive elements R1 to R4 is configured and the bridge voltage is detected.

In these bridge circuits, the relationship between the output voltage Vout and the input voltage Vin is expressed by the following equation.
Vout / Vin =
[R3 / (R1 + R3) -R4 / (R2 + R4)] (1)

  Since the amount of expansion and contraction of the piezoresistive element R is proportional to the change in the resistance value R, the ratio of the output voltage to the input voltage (Vout / Vin) is proportional to the acceleration, and the X-axis acceleration can be measured. It becomes.

(High-performance single-axis acceleration sensor using anisotropy of silicon single crystal)
Theoretical explanation will be given for making the uniaxial semiconductor acceleration sensor according to the present invention highly sensitive and / or highly accurate using the anisotropy of the silicon single crystal.

  First, the influence of the anisotropy of the silicon single crystal on the electrical characteristics will be explained, and a theoretical explanation for improving the detection sensitivity will be given.

Figure 11 is a diagram showing the direction of stress sigma _t perpendicular to the longitudinal direction of the stress sigma _l and the longitudinal direction of the piezoresistive element R acting on the piezoresistive element R. As shown in FIG. 11, the longitudinal direction of the piezoresistive element R is arranged in the direction in which current flows. To act and the direction of the stress sigma _t perpendicular to the longitudinal direction of the stress sigma _l and the longitudinal direction of the piezoresistive element R as shown in FIG. 11, the resistance change rate [Delta] R / R of the piezoresistive element is of approximately the following equation It is expressed as follows.
ΔR / R = π _l σ _l + π _t σ _t (2)

Here, R is the resistance of the piezoresistive element before the force due to acceleration is applied, ΔR is the resistance (R + ΔR) of the piezoresistive element after the force due to acceleration and the force due to acceleration are applied This is the difference from the resistance R of the piezoresistive element. π _l and π _t are piezoresistance coefficients for the stresses σ _l and σ _t , respectively.

The characteristics in the crystal direction of the piezoresistance coefficients π _l and π _t will be described by taking as an example the case where p-type silicon is used for the piezoresistive element. Using a piezoresistive element made of p-type silicon has a higher piezoresistance coefficient related to shear stress than n-type, and therefore the sensitivity of the uniaxial semiconductor acceleration sensor according to the present invention can be increased.
FIG. 12 shows piezoresistance coefficients π _l and π _t in the (100) plane of p-type silicon. FIG. 13 shows the piezoresistance coefficients π _l and π _t in the (110) plane of p-type silicon. The length from the origin represents the magnitude of the piezoresistive coefficient, and the direction from the origin represents the arrangement direction of the piezoresistive elements in the longitudinal direction on the respective substrate surfaces. 12 and 13 are shown on the same scale.

As shown in FIGS. 12 and 13, π _l has a value larger than π _t in any crystal plane single crystal silicon substrate. Therefore, if the value of σ _l is increased, the value of ΔR / R in equation (2) can be increased, and as a result, the output voltage from the Heanstone bridge is also increased, and the detection sensitivity can be improved. it can.
In order to increase the value of σ _l , the longitudinal direction of the piezoresistive element R and the expansion / contraction direction of the piezoresistive element R may be matched. For this reason, in the embodiment of the present invention, as shown in FIGS. 8 and 9, the piezoresistive element R is provided in order to match the longitudinal direction (current direction) of the piezoresistive element R with the expansion / contraction direction of the piezoresistive element R. It arrange | positions at the outer edge of the upper surface of the one connection part 8 and the 2nd connection part 9, and has improved the detection sensitivity.

As shown in FIG. 13, since the <111> direction of the (110) plane has the maximum piezoresistance coefficient π ₁ , the detection sensitivity can be improved most. Further, as shown in FIG. 12, in the (100) plane, the <110> direction has a relatively large piezoresistance coefficient π _l , so that the detection sensitivity can be improved.

  Next, the influence of the anisotropy of the silicon single crystal on the mechanical characteristics will be explained, and a theoretical explanation for improving the detection accuracy will be given.

For sufficiently small deformation, strain is proportional to stress (Hooke's law). Since the stress component is a linear function of the strain component and the silicon single crystal is cubic, it can be expressed as follows.

Here, σ _xx , σ _yy , and σ _zz are normal stresses, and σ _xy , σ _xz , and σ _yz are shear stresses. FIG. 14 is a diagram illustrating a stress component acting on a unit area of a cubic crystal. As shown in FIG. 14, the first letter of the suffix (x of σ _xy ) indicates the direction of the force, and the second letter ( _y of y of σ _xy ) indicates the normal direction of the surface to which the force is applied. ε _xx , ε _yy , and ε _zz represent strain components in the x-axis direction, y-axis direction, and z-axis direction, respectively, and γ _xy , γ _xz , and γ _yz represent shear strains. No angular acceleration unit cube shown in FIG. 14, and in the condition that there is no rotational force as a _{whole, γ xy = γ yx, γ} xz = γ zx, a γ _yz = γ _zy. D ₁₁₁₁ , D ₁₁₂₂ ... Are elastic stiffness constants.

When the x, y, and z axes of a single crystal silicon substrate with a principal plane orientation of (100) are taken in the <100>, <010>, and <001> directions, respectively, Hooke's law is as follows: It becomes a shape.

As can be seen from the matrix of Equation (4), the values of the normal stresses σ _xx , σ _yy , and σ _zz are not affected at all by the shear strains γ _xy , γ _xz , and γ _yz . Therefore, the uniaxial semiconductor acceleration sensor does not twist due to shear strain when subjected to normal stress. Since no torsion occurs, the amount of expansion / contraction of the piezoresistive elements R1 and R3 is the same when subjected to acceleration in the Y-axis direction or the Z-axis direction, and the amount of expansion / contraction of the piezoresistive elements R2 and R4 is also the same. The same. Therefore, even if the acceleration in the Y-axis direction or the Z-axis direction is received from the expression (1), the output is canceled and can be theoretically made zero.
Here, in general, the acceleration sensor has a slight sensitivity to acceleration in a direction orthogonal to the sensitive axis, which is called lateral sensitivity, and contributes to a measurement error. For this reason, it is necessary to reduce the lateral sensitivity of the acceleration sensor to the limit.

  As described above, when a single crystal silicon substrate having a main surface with a plane orientation of (100) is used, the longitudinal directions of the first connection portion 8 and the second connection portion 9 of the uniaxial semiconductor acceleration sensor are set to <100>, respectively. If arranged in the direction, the lateral sensitivity can theoretically be zero, and the accuracy of the uniaxial semiconductor acceleration sensor can be improved.

Next, a case will be described in which the x, y, and z axes of a single crystal silicon substrate whose plane orientation of the main surface of the substrate is (100) are taken in the <110>, <110>, and <001> directions, respectively. In this case, Hooke's law can be expressed as follows.

As can be seen from the matrix of equation (5), the values of the normal stresses σ _xx , σ _yy , and σ _zz are not affected at all by the shear strains γ _xy , γ _xz , and γ _yz . Therefore, the uniaxial semiconductor acceleration sensor 1 does not twist due to shear strain when subjected to normal stress. Since no torsion occurs, the amount of expansion / contraction of the piezoresistive elements R1 and R3 is the same when subjected to acceleration in the Y-axis direction or the Z-axis direction, and the amount of expansion / contraction of the piezoresistive elements R2 and R4 is also the same. The same. Therefore, even if an acceleration in the Y-axis direction or the Z-axis direction is received from Equation (1), the output is canceled and the lateral sensitivity can theoretically be zero.
Therefore, when a single crystal silicon substrate having a main surface orientation of (100) is used, the longitudinal directions of the first connection portion 8 and the second connection portion 9 of the uniaxial semiconductor acceleration sensor 1 are each <110. When arranged in the> direction, the lateral sensitivity can theoretically be zero, and the uniaxial semiconductor acceleration sensor 1 can be highly accurate.

Next, the case where the x, y, and z axes of a single crystal silicon substrate whose main surface is (110) is taken in the <111>, <112>, and <110> directions will be described. In this case, Hooke's law can be expressed as follows.

D ₂₂₂₃ , D ₃₃₂₃ ... _Are non-zero elastic stiffness constants. As can be seen from the matrix of Equation (6), the values of the normal stresses σ _xx , σ _yy , and σ _zz are affected by the shear strains γ _xy , γ _xz , and γ _yz . Therefore, the uniaxial semiconductor acceleration sensor is twisted due to shear strain when subjected to normal stress. Since twisting occurs, the amount of expansion / contraction of the piezoresistive elements R1 and R3 is different and the amount of expansion / contraction of the piezoresistive elements R2 and R4 is different when subjected to acceleration in the Y-axis direction or the Z-axis direction. Therefore, when the acceleration in the Y-axis direction or the Z-axis direction is received from Expression (1), the output is not canceled and it is difficult to make the lateral sensitivity zero.
For this reason, when a single crystal silicon substrate having a principal surface orientation of (110) is used, if the longitudinal directions of the connecting portions 8 and 9 of the uniaxial semiconductor acceleration sensor according to the present invention are arranged in the <111> direction, the Although there is a merit that it is sensitivity, it is difficult to make the lateral sensitivity zero.

From the above, in the case of using a single crystal silicon substrate whose principal surface has a (110) plane orientation, the maximum piezoresistance coefficient π ₁ is taken in the <111> direction. Therefore, if the longitudinal directions of the connecting portions 8 and 9 are respectively arranged in the <111> direction and the longitudinal direction of the piezoresistive element R is the <111> direction, there is an advantage that the detection sensitivity can be improved most. . On the other hand, it is difficult to make the lateral sensitivity zero.

A case where a single crystal silicon substrate having a main surface with a plane orientation of (100) is used will be described. If the longitudinal directions of the first connection portion 8 and the second connection portion 9 of the uniaxial semiconductor acceleration sensor are arranged in the <100> or <110> direction, respectively, the lateral sensitivity can theoretically be zero. However, since the piezoresistance coefficient π _l in the <100> direction is very small and the sensitivity is low, it is difficult to apply to the uniaxial semiconductor acceleration sensor according to the present invention. On the other hand, the <110> direction of the (100) plane takes a relatively large piezoresistance coefficient [pi _l. Therefore, if the longitudinal directions of the connecting portions 8 and 9 are arranged in the <110> direction and the longitudinal direction of the piezoresistive element R is set to the <110> direction, a highly accurate and relatively sensitive uniaxial semiconductor acceleration sensor. 1 can be obtained.

(Other embodiments)
Embodiments of the present invention are not limited to the above-described embodiments, and can be expanded and modified. The expanded and modified embodiments are also included in the technical scope of the present invention.
For example, in this embodiment, the case where p-type silicon is used for the piezoresistive element has been described. However, the present invention is not limited to this, and an n-type piezoresistive element may be used. Of course.

It is a perspective view showing the uniaxial semiconductor acceleration sensor concerning one embodiment of the present invention. It is a perspective view showing the acceleration detection part of the uniaxial semiconductor acceleration sensor which concerns on one Embodiment of this invention. It is a top view of the uniaxial semiconductor acceleration sensor of FIG. FIG. 4 is a partial cross-sectional view illustrating a state where the uniaxial semiconductor acceleration sensor is cut along AA in FIG. 3. FIG. 4 is a partial cross-sectional view illustrating a state where the uniaxial semiconductor acceleration sensor is cut along BB in FIG. 3. It is the figure which divided and represented the side surface of the comb-tooth part of a displacement part. It is the figure which divided and represented the side surface of the comb-tooth part of a frame part. FIG. 3 is an enlarged view of a region surrounded by a dotted-line ellipse in FIG. 2, and is a top view in the vicinity of a piezoresistive element formed in a first connection portion. FIG. 3 is an enlarged view of a region surrounded by a dotted-line ellipse in FIG. 2, and is a top view in the vicinity of a piezoresistive element formed in a second connection portion. It is a circuit diagram which shows the structural example of the detection circuit for detecting the acceleration of a X-axis direction from the resistance of a piezoresistive element. It is a figure showing stress (sigma) _l of the longitudinal direction of the piezoresistive element which acts on a piezoresistive element, and stress (sigma) _t of the direction orthogonal to a longitudinal direction. It is a figure showing the piezoresistance coefficients (pi) _l and (pi) _t about (100) surface which is a main surface of a single crystal silicon substrate. It is a figure showing the piezoresistance coefficients (pi) _l and (pi) _t about (110) surface which is a main surface of a single crystal silicon substrate. It is a figure showing the stress component which acts on the unit area of a cubic crystal.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Uniaxial semiconductor acceleration sensor, 2 ... Acceleration detection part, 3 ... Upper cover part, 4 ... Lower cover part, 5a, 5b ... Trench groove, 6 ... Displacement part, 6a ... Base | substrate part, 6b, 6c ... Comb tooth part, 7 ... Frame part, 7a ... Frame body part, 7b, 7c ... Comb tooth part, 8 ... First connection part, 9 ... Second connection part, 10 ... Space, 11 ... Wiring, R (R1-R4) ... Piezo Resistance element.

Claims (8)

A frame made of a semiconductor material;
A displacement portion made of the semiconductor material, disposed in the frame portion, and displaced with respect to the frame portion in response to acceleration in a first direction;
A plurality of connecting portions made of the semiconductor material, each connecting the frame portion and the displacement portion and arranged side by side in the first direction, wherein the first portion is wider than the width in the first direction; A plurality of connecting portions having a cross-sectional shape having a large thickness in a second direction perpendicular to the direction of 1;
A plurality of strain sensing elements arranged in the plurality of connecting portions;
Comprising
The displacement part has one end and the other end arranged in a third direction orthogonal to the first and second directions, and the first protrusion and the first protrusion are arranged at the one end. A first recess disposed,
The uniaxial semiconductor acceleration sensor, wherein the frame portion includes a second concave portion corresponding to the first convex portion and a second convex portion corresponding to the first concave portion. The displacement part further includes a third convex part and a third concave part arranged alongside the third convex part at the other end in the third direction,
2. The uniaxial semiconductor according to claim 1, wherein the frame portion further includes a fourth concave portion corresponding to the third convex portion and a fourth convex portion corresponding to the third concave portion. Acceleration sensor.   2. The outer shape of each of the first and third convex portions and the first and third concave portions disposed at one end and / or the other end of the displacement portion is rectangular, respectively. Uniaxial semiconductor acceleration sensor.   4. The uniaxial semiconductor acceleration sensor according to claim 3, wherein one side of the rectangle of the first and third protrusions is arranged in parallel to the third direction. 5.   The frame portion, the displacement portion, and the plurality of connection portions are integrally formed from a single crystal silicon substrate having a main surface having a surface orientation of (100), and the plurality of connection portions are formed of the frame portion and the frame portion. The uniaxial semiconductor acceleration sensor according to claim 1, wherein the azimuth in the connecting direction of the displacement portion is <110>.   6. The uniaxial semiconductor acceleration sensor according to claim 5, wherein a longitudinal direction of the strain detection element is substantially parallel to the <110> direction.   The frame portion, the displacement portion, and the plurality of connection portions are integrally formed from a single crystal silicon substrate having a main surface having a surface orientation of (110), and the plurality of connection portions include the frame portion and the frame portion. The uniaxial semiconductor acceleration sensor according to claim 1, wherein the azimuth in the connecting direction of the displacement portion is <111>.   8. The uniaxial semiconductor acceleration sensor according to claim 7, wherein a longitudinal direction of the strain detection element is substantially parallel to the <111> direction.

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