JPH0542607B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Technical Field to Which the Invention Pertains> The present invention relates to a pressure sensor that utilizes the piezoresistive effect of a semiconductor.

<Prior art> Generally, a pressure sensor that utilizes the piezoresistance effect of a semiconductor has a pressure receiving diaphragm formed by etching on a single crystal semiconductor substrate made of silicon, for example.
A gauge resistor is provided on the pressure-receiving diaphragm using diffusion technology or the like, stress based on the pressure difference applied to both sides of the pressure-receiving diaphragm is applied to the gauge resistor, and the pressure difference is detected from the change in the resistance value of the gauge resistor. Usually, two or four gauge resistors are provided on the pressure receiving diaphragm to form a half bridge or full bridge to obtain a signal representing the pressure difference across the diaphragm. However, in this type of pressure sensor, the gauge resistance has a large temperature dependence, so it is affected by changes in ambient temperature and has the disadvantage that the output fluctuates.

Therefore, output fluctuations are generally compensated for by controlling the power supply voltage of the bridge according to temperature changes using a temperature sensing element such as a thermistor, posistor, or transistor. In order to perform accurate compensation using this method, it is necessary to match the temperature characteristics of the gauge resistance and the temperature sensing element for compensation.
However, it is not easy to match these, and highly accurate compensation is difficult. Moreover, the adjustment man-hours required for such compensation are reduced by using a constant temperature bath.
The temperature characteristics of the gauge resistance and the compensation temperature sensing element must be measured one by one, which accounts for nearly half of the total assembly man-hours of the pressure sensor.

Furthermore, temperature compensation for measured values is performed by providing a temperature-compensating gauge resistor on the fixed part of the substrate, but it is not possible to cancel the temperature coefficient β of the piezoresistance coefficient of the gauge resistor.

For this reason, it is necessary to take measures such as giving the power supply voltage a temperature coefficient of -β.

<Object of the Invention> The present invention is directed to realizing a pressure sensor having a structure that can effectively compensate for the influence of changes in ambient temperature.

<Means for Solving the Problem> In order to achieve this object, the present invention provides a pressure receiving diaphragm and a cantilever provided on a single crystal semiconductor substrate, a fixing portion of the pressure receiving diaphragm, the cantilever, and the single crystal semiconductor substrate. a base on which the single crystal semiconductor substrate is fixed by applying a predetermined displacement to the cantilever in advance so as to generate a predetermined stress on the cantilever; and a base on which the single crystal semiconductor substrate is fixed; The temperature coefficient of the gauge resistance and the temperature coefficient of the piezoresistance coefficient are calculated and eliminated from three relational expressions related to the measurement pressure obtained from the resistance value, the temperature coefficient of the gauge resistance, and the temperature coefficient of the piezoresistance coefficient, and the measurement pressure is calculated. The pressure sensor includes a calculation section.

<Example> FIG. 1 is a perspective view showing an example of the pressure sensor of the present invention, and FIG. 2 is a sectional view of FIG. 1. In both figures, 10 is a single-crystal semiconductor substrate such as silicon having a (100) plane orientation, 11 is a rectangular pressure-receiving diaphragm formed on the substrate 10 by anisotropic etching, and 12
1 is a cantilever formed on the substrate 10 by anisotropic etching, and 13 is a fixing portion of the substrate 10. As shown in FIG. 3, a slight initial step δ (for example, 4000 Å) is provided between the tip 12a of the cantilever 12 and the fixed portion 13. 21,2
2 and 23 are gauge resistances such as diffused resistance, respectively, and 21
are formed on the surface of the pressure receiving diaphragm 11, 22 on the surface of the cantilever 12, and 23 on the surface of the fixed part 13, respectively. Reference numeral 30 denotes a base made of silicon, glass, or the like, to which the back surface of the fixed portion 13 of the single crystal semiconductor substrate 10 is fixed by anodic bonding, low melting point glass bonding, or the like. By joining the substrate 10 and the base 30, a constant displacement δ based on the initial step difference is given to the cantilever 12 in advance. Further, the base 30 is provided with an opening 31 for applying a reference pressure P ₀ (for example, atmospheric pressure) to the back surface of the pressure receiving diaphragm 11. Thereby, the pressure receiving diaphragm 11 is sensitive to the measured pressure P _M (difference from the reference pressure P ₀ ) applied to its surface. Note that the cantilever 12 is balanced against the measured pressure P _M ,
Further, the tip portion 12a is not joined to the base 30.

In the pressure sensor of the present invention configured as described above, first, a stress σ _MX acts on the gauge resistor 21 provided on the pressure receiving diaphragm 11 in the longitudinal direction (current direction) in accordance with the measured pressure P _M , and stress is applied in the perpendicular direction. σ _MY
acts. The resistance value R _M of the gauge resistor 21 is defined as: R ₀ is the initial resistance at the reference temperature t ₀ , α is the resistance temperature coefficient of R ₀ , and π is the piezoresistance coefficient in the longitudinal direction and the right angle direction at the reference temperature t ₀ Letting _l0 , _πt0 , the temperature coefficient of the piezoresistance coefficient be β, and the temperature change from the reference temperature _t0 to be t, it is given by the following equation.

R _M = R ₀ (1+αt) {1+(π _l0 σ _MX +π _t0 σ _MY ) (1+βt)} (1) If the constant determined by the structure of the pressure receiving diaphragm 11, the placement position of the gauge resistor 21, etc. is k ₁ , then , π _l0 σ _MX + π _t0 σ _MY =k ₁ P _M (2) holds, and R _M can be expressed by the following equation.

R _M = R ₀ (1+αt) {1+k ₁ P _M (1+βt)} (3) Next, a gauge resistor 22 installed at a position l ₁ from the tip of the surface of the cantilever 12 has a constant displacement δ based on the initial step difference. Therefore, a stress σ _S as shown in the following equation acts.

σ _S =3/2・dE/l ³・δ・l ₁ (4) where, d: Cantilever thickness l: Cantilever effective length E: Young's modulus The temperature coefficient of this stress σ _S is Board 1
It is almost determined by the temperature coefficient of Young's modulus of 0, and the substrate 10
In the case of silicon, the value is sufficiently small, approximately 43×10 ^-6 /°C, and σ _S is hardly affected by changes in ambient temperature. In addition, the cantilever 12 has a pressure to be measured P _M
σ _S does not change even with P _M . That is, σ _S becomes a reference stress that is not affected by changes in ambient temperature or changes in measured pressure. Therefore, the resistance value R _S of the gauge resistor 22 is
The value is based on the standard stress σ _S and is given by the following formula.

R _S = R ₀ (1+αt) {1+π _l0 σ _S (1+βt)} (5) Furthermore, the gauge resistor 2 provided on the surface of the fixed part 13
Since the stress due to the measured pressure P _M and the reference stress do not act on 3, its resistance value R _Z is given by R _Z =R ₀ (1+αt) (6).

Therefore, the resistance values of gauge resistors 21, 22, 23
If R _M , R _S , and R _Z are detected and the following equation is calculated, R _M −R _Z /R _S −R _Z =kP _M (7) Here, k=k ₁ /π _l0 σ _S , the terms of temperature coefficients α and β can be removed. That is, a signal representing the pressure to be measured P _M can be obtained with high accuracy without being affected by changes in ambient temperature. Furthermore, it is no longer necessary to measure the temperature characteristics of the gauge resistance by using a constant temperature oven, and it is only necessary to check the standard stress σ _S , which can also reduce the number of steps required to assemble the pressure sensor.

Furthermore, with regard to disturbance forces such as residual stress caused by the bonding between the single crystal semiconductor substrate 10 and the base 30, the influence thereof is generally reduced by increasing the thickness of the substrate 10 and the bonding width. Furthermore, since the stress caused by the disturbance force acts on the gauge resistors 21, 22, and 23 in the same way, the resistance value changes due to this are the same, and the influence of the disturbance force can be canceled by calculating equation (7). .

FIG. 4 is a connection diagram showing an example of a signal processing circuit used in the pressure sensor of the present invention. In the signal processing circuit 40 shown in FIG. 4, 41a, 41b, and 41c are sensor amplifiers, and a gauge resistor 21 is connected to the feedback circuit of the sensor amplifier 41a, a gauge resistor 22 is connected to the feedback circuit of the sensor amplifier 41b, and the sensor amplifier 41 has a gauge resistor 21 in the feedback circuit of the sensor amplifier 41b.
A gauge resistor 23 is connected to each feedback circuit of c. 42 is an error amplifier whose output E _C is sent to sensor amplifiers 41a, 41b, 41 via resistors 43a, 43b, 43c having the same resistance value, respectively.
It is added to the input of c. 44 and 45 are subtraction circuits, respectively. The subtraction circuit 44 is an operational amplifier 44a.
Four calculated resistors 44b, 44 having the same resistance value as
The sensor amplifier 41 consists of c, 44d, and 44e.
The difference ( _ES - E _Z ) between the output E _S of the sensor amplifier 41c and the output E _Z of the sensor amplifier 41c is calculated and applied to the input terminal (-) of the error amplifier 42 via the resistor 42a. The subtraction circuit 45 has the same resistance value as the operational amplifier 45a.
It calculates the difference ( _EM - E _Z ) between the output E _M of the sensor amplifier 41a and the output E _Z of the sensor amplifier 41c, and outputs a voltage to the output terminal OUT. Give as E _O.
Reference numeral 46 denotes a reference voltage source that applies a constant voltage E _R to the input terminal (+) of the error amplifier 42 . In the signal processing circuit having such a configuration, the resistors 43a, 43
Select the resistance values of b and 43c equally and set the value as R _C
Then, each sensor amplifier 41a, 41b, 41c
The outputs E _M , E _S , and E _Z are given by the following equations.

E _M = -R _M /R _C E _C E _S = -R _S /R _C E _C E _Z = -R _Z /R _C E _C (8) Then, the output of the subtraction circuit 44 (E _S Since the input voltage E _C of the sensor amplifiers 41a, 41b, and 41c is controlled so that the voltage E _Z ) becomes equal to the reference voltage _ER , the following relationship holds true.

1/R _C (R _S −R _Z )E _C =E _R (9) Therefore, the output voltage E _O (=E _M −E _Z ) obtained at the output terminal of the subtraction circuit 45 is E _O =R _M −R _Z /R _S −R _Z E _R (10) Therefore, the calculation of equation (7) can be executed, and the signal voltage E _O representing the measured pressure P _M can be obtained without being affected by changes in ambient temperature. be able to. In addition, as a signal processing circuit, a constant current is passed through each gauge resistor 21, 22, 23, and the voltage drop of each gauge resistor is converted into a digital quantity by an A/D converter, and then a microcomputer is used to convert it into a digital quantity, which corresponds to equation (7). Various configurations can be used, such as those that perform digital calculations. Furthermore, by forming the signal processing circuit 40 on the single crystal semiconductor substrate 10, it is possible to improve the S/N and reduce the size.

5 and 6 are cross-sectional views of other embodiments of the pressure sensor of the present invention. 5 and 6 differ from the embodiment shown in FIG. 2 in that the length h of the tip end 12a of the cantilever 12 is shortened. This is because materials are selected and manufactured to prevent the contact between the tip 12a of the cantilever 12 and the base 30, so that the bulges do not normally act on the contact area, but if bulges are present, the difference in the coefficient of thermal expansion between the two A massing force F based on the temperature is generated, and a constant moment M (=Fh) is generated at the tip of the cantilever 12.
occurs. As a result, stress due to this constant moment M acts on the gauge resistor 22, causing the resistance value R _S of the gauge resistor 22 to vary. Therefore, the length h of the tip portion 12a of the cantilever 12 is shortened to keep the constant moment M small even if there is a lump in the contact portion, and the fluctuation of R _S is made sufficiently small. Note that the tip end 12a of the cantilever 12 and the base 30 may be joined to avoid being affected by the massing force.

FIG. 7 is a perspective view of another embodiment of the pressure sensor of the present invention. 7, the difference from the embodiment shown in FIG. 1 is that the distance l ₁ from the tip of the cantilever 12,
The difference is that gauge resistors 22a and 22b are provided at the positions _l2 , respectively, so that the influence of the constant moment M due to the stiffness of the contact portion between the tip 12a of the cantilever 12 and the base 30 can be removed by calculation. That is, the resistance values R _S1 and R _S2 of the gauge resistors 22a and 22b are the stress caused by the constant moment M acting on the tip of the cantilever 12 as σ _N (6M/bd ² ).
Then, each is given by the following formula.

R _S1 = R ₀ (1+αt) {1+π _l0 (σ _S1 +σ _N (1+βt)} (11) R _S2 = R ₀ (1+αt) {1+π _l0 (σ _S2 +σ _N ) (1+βt)} (12) Here, σ _S1 = 3/2・hE/l ³・δ・l ₁ σ _S2 = 3/2・hE/l ³・δ・l ₂ Therefore, the gauge resistances 21, 22a, 22b, 2
3 resistance values R _M , R _S1 . If the following formula is calculated based on R _S2 and R _Z , R _M −R _Z /R _S1 −R _S2 =kP _M (13) Here, k=k ₁ /π _l0 σ _S σ _S =σ _S1 −σ _S2 , and the influence of the constant moment M can also be removed. In addition, the influence of disturbance force caused by joining etc. can be removed by calculating equation (13), so when the tip 12a of the cantilever 12 and the base 30 are joined,
You may choose not to be affected by the force. Note that the gauge resistor 22a is usually provided on the fixed part 13 side, and the gauge resistor 22b is usually provided on the tip part 12 side so that the value of (σ _S1 -σ _S2 ) does not become too small due to the calculation.

Note that in the above description, the case where a single crystal semiconductor substrate 10 having a plane orientation of (100) is used as an example,
The surface orientation may be (110) or (111). Further, although the shape of the pressure receiving diaphragm 11 is rectangular, it goes without saying that it may be circular. Furthermore, cantilever 1
2 may have a small width b, as shown in the perspective view of FIG.

<Effects of the Invention> In the present invention, (1) The measured pressure P _M and the temperature coefficient of the gauge resistance are determined from the resistance values of three gauge resistors provided at the fixing part of the pressure receiving diaphragm, the cantilever, and the single crystal semiconductor substrate. Since three relational expressions related to α and the temperature coefficient β of the piezoresistance coefficient are obtained, the temperature coefficient α of the gauge resistance and the temperature coefficient β of the piezoresistance coefficient can be calculated and canceled by the calculation unit, and the temperature coefficient β of the piezoresistance coefficient can be calculated and eliminated. Measurement pressure can be measured with high precision without being affected by changes.

(2) Since it is not affected by changes in ambient temperature, there is no need to measure the temperature characteristics of the gauge resistance using a constant temperature bath, which can significantly reduce assembly man-hours.

[Brief explanation of the drawing]

Fig. 1 is a perspective view showing an embodiment of the pressure sensor of the present invention, Fig. 2 is a cross-sectional view thereof, Fig. 3 is a cross-sectional view of main parts of the pressure sensor of the present invention, and Fig. 4 is a signal of the pressure sensor of the present invention. A connection diagram showing one embodiment of the processing section, FIGS. 5 and 6 are sectional views showing other embodiments of the pressure sensor of the present invention, and FIGS. 7 and 8 show other embodiments of the pressure sensor of the present invention. FIG. DESCRIPTION OF SYMBOLS 10... Single crystal semiconductor substrate, 11... Pressure receiving diaphragm, 12... Cantilever, 13... Fixed part, 21, 22, 23, 22a, 22b... Gauge resistor, 30... Base, 40... Signal processing circuit.

Claims (1)

[Scope of Claims] 1. A pressure receiving diaphragm and a cantilever provided on a single crystal semiconductor substrate; a gauge resistor provided respectively on the pressure receiving diaphragm, the cantilever, and a fixing portion of the single crystal semiconductor substrate; and a predetermined stress on the cantilever. a base on which the single-crystal semiconductor substrate is fixed by applying a predetermined displacement to the cantilever in advance so as to produce a measured pressure and a temperature coefficient of the gauge resistance obtained from the resistance values of the three gauge resistors; A pressure sensor comprising a calculation section that calculates a measured pressure by calculating and eliminating a temperature coefficient of a gauge resistance and a temperature coefficient of a piezoresistance coefficient from three relational expressions related to a temperature coefficient of a piezoresistance coefficient.