JPH0755835A - Semiconductor acceleration sensor - Google Patents

Abstract

PURPOSE:To detect an acceleration with good accuracy by a method wherein the interconnection material which is connected to a piezoresistance is formed of a heavily- doped diffused layer whose coefficient of thermal expansion is equal to that of a semiconductor substrate. CONSTITUTION:Piezoresistance 16 to 19 are constituted near the support part of an Si contilever 11. In addition, fixed resistances 16' to 19' are formed on an Si substrate 25 which is common to the cantilever 11. The resistances 16' to 19' are formed in parts which are not affected even when the cantilever 11 is broken and damaged. Then, the resistances 16 to 19 and the resistances 16' to 19' are connected in parallel one by one by interconnections 31 to 34 which are composed of a heavily-doped diffused layer. In addition, the interconnections 31 to 34 which are connected to the resistances 16 to 19 are formed of a heavily-doped diffused layer formed by diffusing impurities to the substrate 25, their coefficient of thermal expansion is identical to that of the contilever 11, and a thermal stress is not generated by the interconnections 31 to 34. Consequently, a change due to a temperature in resistance values of the piezoresistances 16 to 19 is suppressed to a minimum value, a temperature drift in the output voltage of a bridge circuit is reduced, and the accuracy of the detection of an acceleration is enhanced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microminiature semiconductor acceleration sensor using a cantilever beam formed on a semiconductor substrate.

[0002]

2. Description of the Related Art Recently, a microminiaturized semiconductor acceleration sensor formed on a semiconductor substrate has been developed. This semiconductor acceleration sensor is formed on a semiconductor substrate using a thin film technique such as etching, and detects acceleration by detecting a resistance change due to the piezoresistive effect of the semiconductor and a minute capacitance change due to deviation. It is like this. Since these semiconductor acceleration sensors are formed using the thin film technology as described above, for example, the length of the vibrating portion is about 100 μm, the thickness is about 1 μm, and the size of the entire chip is about 1 mm square, which is extremely small. In addition, it has an excellent feature that it can be formed on the same substrate as other elements by an integrated circuit. As the semiconductor acceleration sensor as described above, for example, "IEEE Electron Devices, Vol.ED-26, No.1"
2, p.1911, Dec.1979 “A Batch-Fabricated Silicon Ac
celerometer ”)”.

2A and 2B are views showing the above-mentioned semiconductor acceleration sensor. FIG. 2A is a plan view of a cantilever beam and a piezoresistor, and FIG. 2B is a circuit diagram of a bridge circuit. In FIG. 2, the Si cantilever beam 11 is displaced when acceleration is applied. As a result, stress is applied to the piezoresistor 12 formed at the base of the Si cantilever 11, and the resistance value of the piezoresistor 12 changes. Acceleration can be detected by detecting the change in the resistance value.

As a circuit for detecting the change in the resistance value, for example, a bridge circuit as shown in FIG. 2B is used. In addition, in (b), 12 is the same piezoresistor as (a), 13-15 is a fixed resistance.
The resistors 13 to 15 described above can also be formed on the semiconductor acceleration sensor chip. In the bridge circuit shown in FIG. 2B, when acceleration is applied, the resistance value R of the piezoresistor 12 changes to R + ΔR, and the other resistors 13 to
Assuming that the resistance value R of 15 does not change and the applied voltage of the bridge is V, the output ΔV of the bridge is expressed by the following (Equation 1).

[0005]

[Equation 1]

As can be seen from the equation (1), the bridge circuit using one piezoresistor has a problem of low sensitivity.

Therefore, as a method for improving the sensitivity,
For example, a structure as shown in FIG.
9-158566). In FIG. 3, four piezoresistors 16 to 19 are formed near the supporting portion of the Si cantilever 11, and the bridge circuit 20 is formed by the four piezoresistors. Reference numerals 21 to 24 are wirings for connecting the four piezoresistors to the outside, and metal wirings such as Al are used. Of the above four piezoresistors, 16 and 18 are formed perpendicular to the length direction of the Si cantilever, and 17 and 19 are formed parallel to the length direction of the Si cantilever. Therefore, 16 and 18 and 17 and 19 have opposite resistance changes when stress is applied. Therefore, when stress is applied to the Si cantilever due to acceleration, when the resistance value of 16 and 18 of the four piezoresistors becomes R-ΔR, the resistance value of 17 and 19 becomes R + Δ
Become R. When the bridge circuit as shown in FIG. 3B is configured by the above four piezoresistors 16 to 19, and the applied voltage of the bridge circuit is V, the output ΔV of the bridge circuit is expressed by the following equation (2). Shown.

[0008]

[Equation 2]

As can be seen by comparing the above equation (2) with the above equation (1), the sensitivity in the configuration of FIG. 3 is four times higher than that in the configuration of FIG.

[0010]

As described above, the structure shown in FIG. 3 can improve the sensitivity. However, in the configuration of FIG. 3, since the metal wirings 21 to 24 such as Al are used as the wirings connected to the piezoresistors 16 to 19, the following problems occur. That is, S
Due to the difference in the thermal expansion coefficient between the i cantilever beam 11 and the metal wiring layers 21 to 24, the thermal stress caused by the difference is caused by the Si cantilever beam 1.
1, the thermal stress applied to the four piezoresistors 16 to 19 differs depending on the direction, length, thickness, area, etc. of the metal wirings 21 to 24. Therefore, when the temperature changes, in the semiconductor acceleration sensor, the offset temperature characteristic is generated in the resistance value of each piezoresistor due to the disposition of the metal wirings 21 to 24, and the output voltage Vout of the bridge circuit has a temperature drift. This causes a problem that a detection error occurs.

The present invention has been made to solve the problems of the prior art as described above, and has a high detection sensitivity,
It is an object of the present invention to provide a semiconductor acceleration sensor that is highly accurate and has excellent reliability and stability.

[0012]

To achieve the above object, in the present invention, a bridge circuit is formed on a cantilever formed on a semiconductor substrate by four piezoresistors whose ends are directly coupled. The wiring of the bridge circuit is formed from each of the coupling portions, and at least the portion of the wiring on the cantilever is formed by the high-concentration diffusion layer formed on the semiconductor substrate.

[0013]

In the present invention, since the wiring connected to the piezoresistor is formed of the high-concentration diffusion layer having the same thermal expansion coefficient as the semiconductor substrate, the thermal stress due to the wiring does not occur. Therefore, with respect to the resistance value of the piezoresistor, the change due to the temperature is suppressed to the minimum value, the temperature drift of the voltage output from the bridge circuit is reduced, and the acceleration can be accurately detected. Further, as is apparent from the embodiment shown in FIG.
In the present invention, the end portions of the four piezoresistors 16 to 19 formed near the supporting portion of the cantilever 11 are directly joined, and there is no wiring between the piezoresistors. Such a structure is made possible by effectively utilizing the characteristics of the cantilever type acceleration sensor. That is, in a cantilever, the stress due to acceleration concentrates at some points (near the support section), so piezoresistors can be gathered at the concentrated points, which allows direct connection between piezoresistors without wiring. Of. As described above, by directly connecting the piezoresistors, it is possible to solve the problem that the detection sensitivity is lowered due to the wiring resistance and the detection accuracy is lowered due to the offset caused by the variation in the resistance value of each wiring resistance. I can.

[0014]

1 is a plan view of a cantilever beam and a resistance forming portion, and FIG. 1 (b) is a circuit diagram of a bridge circuit. In FIG. 1, Si cantilever 11
Four piezoresistors 16 to 19 are formed in the vicinity of the support portion of the. The ends of these four piezoresistors are directly connected, and there is no wiring for connection between the piezoresistors. Further, four fixed resistors 16 'to 19' are formed on the Si substrate 25 which is common with the Si cantilever 11. These four resistors 16 'to 19' are formed in a portion which is not affected even if the Si cantilever 11 is broken. The piezoresistors 16 to 19 and the resistors 16 'to 19' are connected in parallel one by one by wirings 31 to 34 made of high-concentration diffusion layers, respectively. The above four piezo resistors 16-
All of 19 are formed on the cantilever 11. Also,
The wirings 31 to 34 wired to the piezoresistors 16 to 19 are
It is formed of a high-concentration diffusion layer formed by diffusing impurities into the semiconductor substrate, and its thermal expansion coefficient is the same as that of the Si cantilever 11. Therefore, thermal stress due to the wirings 31 to 34 does not occur. Therefore, the piezo resistance is
With respect to the resistance value of 9, the change due to temperature is suppressed to the minimum value, the temperature drift of the output voltage Vout of the bridge circuit is reduced, and the accuracy of acceleration detection is improved.

The above-mentioned bridge circuit has a circuit configuration as shown in FIG. In the apparatus of FIG. 1, if the Si cantilever beam 11 is broken from the root and the four piezoresistors 16 to 19 are disconnected, the potential of the output terminal is changed by the four fixed resistors 16 'to 19'. It will be decided. For example, if all 16'-19 'have the same resistance value, if four piezoresistors are damaged,
The output voltage ΔV will be fixed at 0. Also, if the values of the four fixed resistors 16 'to 19' are set appropriately,
If the piezoresistors 16-19 are damaged, output voltage ΔV
The value of can be set to a fixed value. Therefore, the Si cantilever is broken and the piezoresistor 1
Even if 6 to 19 are disconnected, a stable value can be taken out as the output of the semiconductor acceleration sensor, and the value can be set to an arbitrary value, so when the set value is detected, the semiconductor It can be determined that the acceleration sensor is damaged. Therefore, it becomes possible to accurately detect the damage status of the semiconductor acceleration sensor with a simple detection circuit.

Next, the change in the sensitivity of the bridge circuit due to the connection of the resistors 16 'to 19' will be described. Assuming that the resistance values of the resistors 16 'to 19' are r and the other conditions are the same as in the case of FIG. 3,
The output voltage ΔV is expressed by the following equation (3).

[0017]

[Equation 3]

Therefore, if the resistance value r of the resistors 16 'to 19' is sufficiently larger than the resistance value R of the piezoresistors 16 to 19, the equation (3) becomes equal to the equation (2). That is,
By making r sufficiently larger than R, the resistors 16 'to 1
The decrease in sensitivity caused by connecting 9'in parallel can be almost ignored. For example, when it is assumed that the resistance values r of the resistors 16 'to 19' are 10 times the resistance values R of the piezo resistors 16 to 19, respectively.
The sensitivity is reduced by 10% as compared with the case where 19 'is not connected. If the decrease in sensitivity is within 10%, it is considered that there is no problem in practical use. Therefore, the resistance values of the resistors 16 'to 19' are set to the piezoresistor 16 as described above.
It is desirable to be 10 times or more than -19. As a method of forming the resistors 16 'to 19', the accuracy is high,
Moreover, it is desirable to use an ion implantation method which can realize a high resistance value in a small area.

[0019]

As described above, according to the present invention,
Since the wiring connected to the piezoresistor is formed of the high-concentration diffusion layer having the same coefficient of thermal expansion as the semiconductor substrate, thermal stress due to the wiring does not occur. Therefore, with respect to the resistance value of the piezoresistor, the change due to temperature is suppressed to the minimum value, the temperature drift of the voltage output from the bridge circuit is reduced, and the acceleration can be accurately detected. Further, by directly connecting the four piezoresistors, it is possible to solve the problem that the detection sensitivity is lowered due to the wiring resistance and the detection accuracy is lowered due to the offset due to the variation of the resistance value of each wiring resistance. The effect that can be obtained is obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing an example of a conventional device.

FIG. 3 is a diagram showing another example of a conventional device.

[Explanation of symbols]

11 ... Si cantilever 16 to 19 ... Piezoresistor 16 'to 19' ... Resistor 21 to 24 ... Wiring 25 ... Si substrate 31 to 34 ... Wiring

Claims (1)

[Claims] 1. A piezoresistor is formed in the vicinity of a supporting portion of a cantilever which is formed on a semiconductor substrate and whose one end is supported, and a piezoresistive change caused by the deviation of the cantilever is generated. In a semiconductor acceleration sensor that detects acceleration by detecting, a bridge circuit is formed by four piezoresistors whose ends are directly coupled, and the wiring of the bridge circuit is drawn out from each coupling portion, and at least the above wiring. A semiconductor acceleration sensor, wherein a portion on the cantilever is formed of a high concentration diffusion layer formed on the semiconductor substrate.