JPH08228015A - Acceleration sensor - Google Patents

Abstract

PURPOSE: To obtain an acceleration sensor, which can meet an acceleration parallel to a substrate, by a method wherein the sensor is constituted into such a structure that a voltage is fed to a beam for supporting a weight from one direction over the beam to a piezoelectric resistor formed on a diffused layer and an output voltage is led out from the direction intersecting orthogonally the one direction. CONSTITUTION: An acceleration sensor is provided with a vibrator 20, which has a weight 22 formed by a through etching through a substrate 11 and a beam 21 for supporting the weight 22 and is vibrated in the directions vertical and parallel to the substrate surface. Moreover, the sensor is constituted of a P-type or N-type diffused region arranged on the beam 21, is provided with a piezoelectric resistor 23, whose resistance value is changed by the deflection of the beam 21 due to the vibration of the vibrator 20, and an acceleration is detected from a change in the resistance value. Such the acceleration sensor is constituted into a structure, wherein a voltage Vi is fed to a U-axis in one direction over the beam 21 to the resistor 23, an output voltage Vo is led out from a V-axis direction, which intersects orthogonally the U-axis direction, over the beam 21 and a shearing stress in the beam 21 is measured from the resistance value, which is found by the supply voltage Vi and the output voltage Vo, of the resistor 23 to detect the acceleration.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an acceleration sensor which is mounted on an automobile, a robot or the like and detects an acceleration by a change in resistance value of a piezoresistor provided on a beam.

[0002]

2. Description of the Related Art Conventionally, as a technique in such a field,
For example, some documents were described in the following documents. Reference 1: Esashi, Fujita et al., “Micromachining and Ma
Ikuro Mechatronics ”(1992) Baifukan, P.198-201 Reference 2; IEEE TRANSACTION ON ELECTRON DEVICES,12
[ED-26] (1979) Lynn Michelle Roylance et al.
“A Batch-Fablicated Silicon Accelerometer” P.1911
-1917 Reference 3; Micro System Technologies 90-Proceedings o
f 1st International Conference on Micro Electro, Opt
o, Mechanic System and Components, Ed, H, Reichi Ber
lin, Sept. 10-13 (1990), J. Mohr et.al. “Movable M
icrostructures Manufactured by the LIGA Process as
 Basic Elements for Microsystems "Springer-Verlag
 , P.529-537 Automotive airbag, navigation, ABS control, and
Suspension control, robot control, etc.
The acceleration sensor has various uses. Their acceleration
The sensor has a weight supported by a beam, and the weight is added.
It changes according to the speed. Detecting displacement of weight
Then, the acceleration is obtained. For detecting displacement
Therefore, the acceleration sensor is divided into the gauge type and the capacitance type.
Can be In the gauge type, a strain gauge is attached to the beam to
Capacitance generated by displacement, for detecting and capacitance type
To detect changes in

2 (I) and (II) are views showing the conventional acceleration sensor shown in the above-mentioned Document 1 and Document 2. FIG. 2 (I) is a plan view and FIG. 2 (II) is its plan view. FIG. This acceleration sensor includes a cantilever beam 2 formed on a silicon substrate 1, and a weight 3 is formed at the tip of the beam 2. The beam 2 is provided with a piezoresistor 4 of a diffusion layer, and the lead wire 6 is connected to the piezoresistor 4 via a diffusion layer 5. The acceleration sensor is provided with a cover 7 in which the moving range of the weight 3 and the beam 2 is removed by etching. When acceleration is applied to the sensor, the weight 3 is displaced and the beam is bent. Due to the bending of the beam, the diffusion resistance changes due to the piezoresistance effect of the piezoresistance 4. A detector (not shown) connected to the lead wire 6 detects the change in the diffusion resistance through the diffusion layer 5 and the lead wire 6 and calculates the acceleration. In order to reduce the influence of the acceleration component parallel to the substrate 1, the weight 3 is designed so that the center of gravity of the weight 3 coincides with the extension line of the beam 2 in which the piezoresistor 4 exists.

[0004]

However, the conventional acceleration sensor has the following problems. Most of the conventional acceleration sensors measure acceleration perpendicular to the plane of the substrate 1, and almost none measure acceleration parallel to the substrate 1. This is to manufacture beams and weights that vibrate only in the direction parallel to the substrate.
This is because micromachining is difficult. FIG.
[FIG. 3] is a perspective view for explaining the problems of the conventional acceleration sensor, and portions common to FIG. 2 are denoted by common reference numerals. In FIG. 3, a substrate 1, a beam 2 and a weight 3 are shown. The weight 3 has a mass of m, and the beams have widths, thicknesses, and lengths of a, b, and l, respectively. The thickness b corresponds to the vertical direction Z of the substrate 1. Here, the length direction of the beam is X, and the direction parallel to the substrate 1 and perpendicular to X is Y.

When an acceleration α _{y in} a direction parallel to the substrate 1 and perpendicular to the beam 2 is applied, the beam 2 receives a force in the -Y direction and bends. At this time, the radius of curvature R _xy at the distance x from the fulcrum of the beam 2 is given by the following expression (1). R _xy = Ea ³ b / {12 × m (l−x) α _y } (1) However, E; Young's modulus Further, when the acceleration α _z perpendicular to the substrate 1 is received, the weight 3
Is displaced in the -Z direction, and the beam 2 is distorted. The radius of curvature R _zx of the beam 2 at this time is given by the following expression (2). R _zx = Eab ³ / {12 × m (l−x) α _z } (2) Thus, the radius of curvature of the beam is affected by both the accelerations α _y and α _z . When measuring the acceleration alpha _z, in order to reduce the influence of the acceleration alpha _z is from (1) and (2),
It is understood that the ratio of b and a, that is, the aspect ratio of the beam 2 may be increased. However, obtaining a structure with a large aspect ratio by micromachining is
It's extremely difficult. As a method for obtaining a structure with a large aspect ratio by micromachining, there is a LIGA (Lithografie, Galvanoformung, Abformung) process described in Reference 3. However, since this LIGA process requires synchrotron radiation, it is generally difficult to use, and mass productivity is also poor.

[0006]

Means for Solving the Problems The first to fourth inventions are as follows.
In order to solve the above problems, a vibrator having a weight formed by through-etching of a substrate and a beam supporting the weight and vibrating in a direction parallel to a direction perpendicular to the substrate surface, and a p-type disposed on the beam A diffusion region or an n-type diffusion region,
A piezoresistor having a resistance value that changes due to the bending of the beam due to the vibration, and an acceleration sensor that detects acceleration from a change in the resistance value of the piezoresistor has the following structure. That is, a voltage is supplied to the piezoresistor on the u-axis in one direction on the beam and an output voltage is taken out from the v-axis on the beam orthogonal to the u-axis direction. The structure is such that the shear stress in the beam is measured from the resistance value of the piezoresistance obtained and the acceleration is detected.

[0007]

In the first to fourth aspects of the invention, since the acceleration sensor is configured as described above, the vibrator vibrates when receiving acceleration. Since the beam supports the weight, acceleration causes the beam to bend and generate shear stress. The resistance value of the piezoresistor arranged on the beam changes due to bending. Here, the supply voltage is applied to the piezoresistor from, for example, the u-axis in the longitudinal direction of the beam, and the output voltage is taken out from the v-axis direction in the width direction orthogonal to the u-axis. From these supply voltage and output voltage, the change in resistance value of the piezoresistor can be obtained. As a result, the shear stress of the beam is determined and the acceleration is detected. Therefore, the above problem can be solved.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment FIGS. 1 (I), (II), and (III) are views showing the structure of an acceleration sensor according to a first embodiment of the present invention.
Is a top view, (II) is a side view thereof, and (III) is a partially enlarged view. This acceleration sensor has a vibrator 20 formed on a substrate 11. The oscillator is manufactured by penetrating an n-type silicon wafer which is the substrate 11 and etching, and is composed of a beam 21 and a weight 22.
Glass covers 31 and 32 are provided above and below the vibrator 20. The shape of the glass covers 31 and 32 is the vibrator 2
It does not interfere with 0 vibration. Figure 1 (III)
Shows the portion A in FIG. 1 (I). A p-type piezoresistor 23 is formed on the beam 21.
The aluminum electrodes 24a, 24b, 24c, 24d of the book are connected.

The beam 21 has a width a of 20 μm and a thickness b of 50 μm.
m and length l are as long as 2000 μm, and this beam 21 is ± Z
The structure vibrates in both directions (directions perpendicular to the substrate 11) and ± Y directions (directions parallel to the substrate 11 and perpendicular to the beams 20). In order to increase the sensitivity of the acceleration sensor, the weight 22 may be added with an additional mass of several mg. However, the center of gravity of the entire weight 22 needs to be set so as to be on the extension line of the beam 21. If no additional mass is applied,
For example, the weight 22 has a mass of 0.1 mg, and in the design of FIG. 1, the natural frequency of the vibrator 20 in the ± Y direction is 76.
The natural frequency in the 0 Hz and ± Z directions is 1900 Hz.
The substrate 11 is n-type silicon with a (001) plane orientation, and each side of the piezoresistor is <1, 0, 0>, <0, 1, with respect to the plane orientation, as shown in (I) of FIG. 0> direction. The piezoresistor 23 is a p-type region having an area of 12 × 12 μm ² which is formed by a diffusion method on an n-type silicon wafer. A voltage is supplied to the piezoresistor 23 by the electrodes 24a and 24b, and an output voltage is taken out via the electrodes 24c and 24d. That is, the input direction of the supply voltage and the output direction of the output voltage are at right angles.

FIG. 4 is a flowchart showing a manufacturing process of the acceleration sensor of FIG. The acceleration sensor of this embodiment is manufactured through processes P1 to P7 shown in FIG. (Process P1) n of (001) plane orientation as the substrate 11
Prepare mold silicon. As the silicon substrate, a thin substrate of 50 μm or a thick substrate is prepared. (Process P2) When the prepared n-type silicon is thick,
A portion corresponding to the vibrator 20 is selectively etched from the back side of the substrate in a KOH aqueous solution. With this etching,
The thickness of the peripheral portion of the vibrator 20 is reduced to 50 μm. (Process P3) After forming an oxide film on the substrate, a portion corresponding to the piezoresistor 23 is etched by photolithography to form a mask of the oxide film. Then, boron (B) or the like is diffused using the oxide film as a mask to form the piezoresistor 23. (Process P4) After depositing aluminum on the entire surface of the piezoresistive film 23 and the oxide film, patterning is performed to form the electrode 24a
~ 24d are formed.

(Process P5) A silicon oxide film having a thickness of 5 μm and a tungsten silicide film having a thickness of 0.5 μm are formed on the substrate which has been subjected to the process P4 by the CVD method.
m vapor deposition. By etching the silicon oxide film using tungsten silicide as a mask, a mask for penetrating etching of the silicon wafer is formed. (Process P6) By using the mask formed in the process P5 and performing selective vertical etching by the reactive ion etching method, the silicon wafer around the beam 21 and the weight 22 penetrates. A mixed gas of SF ₃ and CCl ₂ F ₂ is used as a reaction gas in the vertical etching. (Process P7) The remaining mask is removed, and the manufacturing of the vibrator 20 of the acceleration sensor in FIG. 1 is completed.

In the acceleration sensor of this embodiment, since the input direction of the supply voltage V _i and the output direction of the output voltage V _{o to} the piezoresistor 23 are perpendicular to each other, the acceleration in the Y direction parallel to the surface of the substrate 11 is obtained. It is possible to detect.
Acceleration α (α _x , α _y , α) is applied to the oscillator 20 of the acceleration sensor.
_{When z} ) is applied, shear stress is generated in the beam 21 due to the bending. The shear stresses τ _zx and τ _xy are uniform on the beam 21, and are expressed by the following equation (3). τ
By measuring _xy , α _y is detected. [tau] _xy = -M [alpha] _y / ab, [tau] _xy = -M [alpha] _z / ab (3) Generally, the piezoresistive effect is expressed by the following formula (4).

[Equation 1] As a result, under the condition that the electric field direction 1 of the supply voltage (the u-axis) and the current direction 2 of the output current (the v-axis) are orthogonal to each other, the electric field E1 in the one direction is given by the equation (5).

[0013]

[Equation 2] Here, π ₆₁ = π ₆₂ = π ₆₃ = π ₆₄ = π ₆₅ = 0
If the crystal orientation of the piezoresistor is selected so that (6)
It becomes an expression. E ₁ = ρJ 2 π ₆₆ τ ₆ (that is, E _x = ρJ _y π ₆₆ τ _xy ) ... (6) Among the orientations that satisfy the formula (6), the one in which π ₆₆ is large is <1, A combination of 0,0> and <0,1, -1> directions or a combination of <0,1, -1> and <1,1,0> directions. In the case of the former combination, (00
If a silicon wafer having a 1) plane orientation, and in the latter case a (011) plane orientation, is used, the u axis and v axis can be taken in the plane. The supply voltage V _i and the output voltage V _o are represented by the following equation (7), and the output voltage V ₀ is (8) according to the equations (3) and (7).
It becomes an expression. V _o = π ₆₆ τ _xy V _i (7) V _o = −π ₆₆ Mα _y V _i / (ab) (8) FIG. 5 shows output voltage characteristics with respect to acceleration in the acceleration sensor of FIG. 1. FIG. 4 is a diagram showing the output voltage characteristics when the specific resistance of the piezoresistor is 10 Ωcm. As shown in FIG. 5, the acceleration sensor of this embodiment can obtain the output voltage V _o proportional to the Y-direction component α _y of the acceleration α.

[0014] As described above, in the present embodiment, the piezoresistive 23 formed in the cantilever 21 vibrates in the Z direction and the Y direction, and a right angle taking out the output voltage V _o with respect to the supply voltage V _i . Therefore, the acceleration component α parallel to the substrate 11
It is possible to detect _y without mixing a component perpendicular to the substrate 11. Further, since the detection is by piezo resistance, it is possible to perform detection with high linearity. Furthermore, it is not necessary to form the beam 21 having an extremely high aspect ratio,
Since a simple structure may be formed by a normal semiconductor process without using the A process, an acceleration sensor that is inexpensive and excellent in mass productivity can be realized.

Second Embodiment The structure of the acceleration sensor of this embodiment is similar to that of the first embodiment. However, the first embodiment is different from the first embodiment in that (001) plane oriented p-type silicon is used for the substrate and the piezoresistor is used as the n-type diffusion region formed on the substrate. And
The supply voltage V _i is given from the <1,1,0> direction, and the output voltage V _o is taken out from the <1, -1,0> direction. This acceleration sensor also operates similarly to the first embodiment except that the piezoresistive coefficient is different, and can detect the acceleration component parallel to the substrate. Of the orientations in n-type silicon, π ₆₁ = π ₆₂ = π ₆₃ = π ₆₄ = π ₆₅ = 0
And the value of π ₆₆ becomes large is a combination of <1,1,0><1,-1,0> in the directions of the u and v axes. By taking the u and v axes in this way, it is possible to selectively measure only the shear stress τ ₆ with high sensitivity. The present invention is not limited to the above embodiment, and various modifications can be made. FIG. 6 is a diagram of an acceleration sensor showing an application example of FIG. As shown in FIG. 6, for example, when the acceleration sensor of the first embodiment is formed on the substrate 50 at two places, each acceleration sensor detects acceleration in two directions parallel to the substrate. That is, a two-dimensional acceleration sensor is realized.

[0016]

As described in detail above, according to the first to third aspects of the present invention, a voltage is supplied from one direction of the beam to the piezoresistor formed in the diffusion region in the beam supporting the weight. The output voltage is taken out from the direction orthogonal to the direction. Therefore, if the direction of the beam is appropriately set with respect to the crystal structure of the substrate, the acceleration component parallel to the substrate can be detected without mixing the component perpendicular to the substrate, and the acceleration sensor corresponding to the acceleration parallel to the substrate can be detected. Will be realized. Also,
There is no need to make the aspect of the beam extremely large,
For example, it is not necessary to use the LIGA process. Therefore, the acceleration sensor can be formed by a normal semiconductor process, and an inexpensive and mass-producible acceleration sensor can be realized.

[Brief description of drawings]

FIG. 1 is a structural diagram of an acceleration sensor showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a conventional acceleration sensor.

FIG. 3 is a perspective view illustrating a problem of a conventional acceleration sensor.

FIG. 4 is a flowchart showing a manufacturing process of the acceleration sensor of FIG.

5 is a diagram showing an output voltage characteristic with respect to the acceleration of FIG.

FIG. 6 is a diagram of an acceleration sensor showing an application example of FIG.

[Explanation of symbols]

11 substrate 20 vibrator 21 beams 22 weight 23 piezoresistive 24a~24d electrodes 31 glass cover V _i supply voltage V _o output voltage

Claims (4)

[Claims] 1. A vibrator having a weight formed by through-etching a substrate and a beam supporting the weight and vibrating in a direction parallel to a direction perpendicular to the substrate surface, and a p-type diffusion region arranged on the beam. A piezoresistor configured of an n-type diffusion region, the piezoresistor having a resistance value that changes due to the bending of the beam due to the vibration, wherein the acceleration sensor detects acceleration from a change in the resistance value of the piezoresistor. A voltage is supplied to the u-axis in one direction on the beam, an output voltage is taken out from a v-axis direction on the beam orthogonal to the u-axis direction, and the resistance value of the piezoresistor is obtained by the supply voltage and the output voltage. An acceleration sensor having a structure for measuring shear stress in the beam to detect acceleration. 2. The substrate is an n-type silicon substrate having a (001) plane orientation, the piezoresistor is formed by a p-type diffusion region, and each of the u and v axis directions is <1, 0, 0>, < 0, 1,
0> direction combination, or <0, 1, 0>, <1,
The acceleration sensor according to claim 1, wherein a combination of 0,0> directions is set. 3. The substrate is a p-type silicon substrate having a (001) plane orientation, the piezoresistor is formed by an n-type diffusion region, and each of the u and v axis directions is <1, 1, 0>, < 1,-
1,0> direction combinations or <1, -1,0>,
The acceleration sensor according to claim 1, wherein a combination of <1,1,0> directions is set. 4. The substrate is an n-type silicon substrate having a (011) plane orientation, the piezoresistor is formed by a p-type diffusion region, and each of the u and v axis directions is <1, 0, 0>, < 0, 1,
-1> direction combination, or <0, 1, -1>, <
The acceleration sensor according to claim 1, wherein the acceleration sensor is set to a combination of 1,1,0> directions.