JPH1183580A - Thermal type air flow sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve air temperature dependency and mechanical strength by constituting electrical heating and temperature-sensing resistors with a dope-treated polycrystalline silicon semiconductor thin film and setting the impurity concentration of the electrical heating resistor at least at a specific value and increasing it as compared with that of the temperature-sensing resistor. SOLUTION: Electrical heating resistors 4a and 4b have larger impurity concentration, at least 3×10<19> cm<-3> , for temperature-sensing resistors 5 and 6. The voltage of terminals F and G of a bridge circuit including the temperature-sensing resistors 5 and 6 and resistors 25b and 25c is inputted to a control circuit 26, and a temperature Th of the temperature- sensing resistor 5 heated by the electrical heating resistors 4a and 4b is controlled to be higher than a temperature Ta of an air temperature-sensing resistor 6 by a constant value by the control circuit 26. When the temperature of the temperature-sensing resistor 5 is lower than a set value, a transistor 24 is turned on and a heating current flows to the electrical heating resistors 4a and 4b. When the temperature becomes high, the transistor 24 is turned off. The heating current value of the electrical heating resistors 4a and 4b is an air flow rate. On the other hand, the direction of an air flow can be detected by comparing the temperatures of the electrical heating resistors 4a and 4b, thus achieving an accurate flow detection for detecting the direction of the air flow without being affected even if an air temperature changes.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal air flow sensor, and more particularly to a thermal air flow sensor suitable for measuring an intake air amount of an internal combustion engine.

[0002]

2. Description of the Related Art Conventionally, as an air flow sensor which is provided in an electronic control fuel injection device of an internal combustion engine of an automobile or the like and measures an intake air amount, a thermal air flow sensor has become mainstream since it can directly detect a mass air amount. I have. Among them, an air flow sensor manufactured by a semiconductor micromachining technology has attracted particular attention because it can reduce cost and can be driven with low power. Such a conventional thermal air flow sensor using a semiconductor substrate is disclosed in, for example, Japanese Patent Publication No. 3-502966 and Japanese Patent Application Laid-Open No. Hei 8-54269. In the technique described in Japanese Patent Application Laid-Open No. 8-54269, polycrystalline silicon (polysilicon) is used as a heating resistor because of its advantages in heat resistance and material cost. There has been a problem that the flow rate measurement accuracy is not sufficient because the temperature dependency is not taken into consideration, and that the mechanical strength of the electrical insulating film supporting the heating resistor is different. In addition, in the technology described in Japanese Patent Publication No. 3-502966, an air temperature measuring resistor is provided in addition to the heating resistor to take into consideration the temperature dependency of the air temperature. However, there is a problem that the mechanical strength is not sufficient.

[0003]

The prior art has the following problems. In the above JP-A-8-54269,
Polycrystalline silicon (polysilicon) is used as a heating resistor on a semiconductor substrate via an electrical insulating film, but since an air temperature measuring resistor for detecting air temperature is not formed, the air temperature is reduced. However, there is a problem that the output corresponding to the flow rate of the air to be measured has an error when the value changes.

The electrical insulating film (diaphragm) on which the heating resistor is formed has a small overall thickness of several microns in order to achieve thermal insulation with a semiconductor substrate and to reduce heat capacity in order to increase responsiveness. The cavity formed on the semiconductor substrate is cross-linked. Therefore, when the heating resistor is repeatedly heated and cooled, or when the air flow is increased, the electric insulating film may be greatly stressed and broken.

On the other hand, the prior art described in Japanese Patent Publication No. 3-502966 has the following problems. FIG.
The longitudinal section of the air flow sensor described in Fig. 2 of 3-502966 is shown. Reference numeral 2 in FIG. 12 denotes a semiconductor substrate, and 3 denotes a doped layer (rim) formed by subjecting the semiconductor substrate 2 surrounding the cavity 8 to an impurity doping process with a predetermined thickness.
2b is an electric insulating film, 4 is a heating resistor, 5c, 5d, 6
Reference numerals a and 6b denote temperature measuring resistors.

[0006] The conventional air flow sensor constructed as described above measures the air temperature with the resistance temperature detectors 6a and 6b.
The heating current is supplied to the heating resistor so that the temperature of the heating resistor 4 and the temperature measuring resistor 5d for measuring the temperature of the heating resistor become higher than the measured air temperature by a certain temperature. Resistance thermometer 5
c is located between the temperature measuring resistor 6a, which is the air temperature, and the heating resistor 4, which is set at a certain temperature higher than the air temperature, and is a temperature at one point of a temperature gradient between the air temperature and the heating temperature of the heating resistor 4. It is arranged to detect. The air flow is 1 in FIG.
As shown in FIG. 1, when the air flows from left to right and the air flow rate increases, the temperature gradient of the portion where the temperature measuring resistor 5c is located changes, and the temperature measuring resistor 5c is cooled by the air flow to lower the temperature. The temperature change of the resistance temperature detector 5c is compared with the air temperature (temperature difference) to detect the air flow rate. Air flow rate is measured with resistance temperature detector 5
Since the temperature is detected from the temperature difference between the temperature c and the temperature measuring resistor 6a indicating the air temperature, the influence of the change in the air temperature can be reduced.

However, the electrical insulating films 12a and 12b on which the resistors 4, 5c and 5d are formed have several thicknesses in order to achieve thermal insulation with the semiconductor substrate 2 and to reduce the heat capacity in order to increase the responsiveness. It is made as thin as a micron and the cavity 8 is bridged. Although the peripheral portion of the cavity 8 is surrounded by a doped layer (rim) formed by subjecting the semiconductor substrate 2 to impurity doping with a predetermined thickness, the electrical insulating film 1 on the cavity 8 is formed.
2a and 12b remain thin, and when the heating resistor is repeatedly heated and cooled, or when the air flow is increased, a great deal of stress is applied to the electric insulating film, and the electric insulating film may be broken.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a low-cost thermal air flow sensor which solves the problems of the prior art and has improved air temperature dependency and mechanical strength.

[0009]

The object of the present invention is to provide a thermal air flow sensor for measuring an air flow rate by forming at least a heating resistor and a temperature measuring resistor on a semiconductor substrate via an electric insulating film. The heating resistor and the resistance temperature detector are composed of a doped polycrystalline silicon (Si) semiconductor thin film,
The impurity concentration of the heat generating resistor is set to 3 × 10 ¹⁹ (cm ⁻³ ) or more, and is higher than the impurity concentration of the temperature measuring resistor. Further, the semiconductor substrate extends from the boundary surface of the electric insulating film to the lower surface. Having a cavity, the electric insulating film on the cavity is doped with impurities to a predetermined depth in the semiconductor substrate, and is supported and reinforced by a beam-like supporting portion protruding from the periphery of the cavity, so that the air temperature dependence and the mechanical A thermal air flow sensor with improved strength can be provided at low cost.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described.
This will be described with reference to the drawings.

FIG. 1 is a plan view showing a thermal air flow sensor element 1 according to a first embodiment of the present invention, and FIG. 2 is a sectional view of the measuring element 1 taken along line AA 'of FIG.

In FIG. 1 and FIG. 2, an element 1 comprises a semiconductor substrate 2 of silicon or the like and an electric insulating film 12 (12a, 12b).
And a dope layer 3 made of the same material as the dope layer 3 and the dope layer 3 in which the semiconductor substrate 2 on the lower surface of the semiconductor substrate 2 is doped with impurities.
a, 3b, an upstream-side heating resistor 4a and a downstream-side heating resistor 4b formed on the electric insulating film 12a, a temperature measuring resistor 5 for detecting the temperature of the heating resistor, and formed at the tip of the substrate 2. Air temperature measuring resistor for measuring the measured air temperature 6,
Terminal electrode 10 for connecting the signal of element 1 to an external circuit
(10a, 10b, 10c, 10d, 10e, 10f,
10g), wiring connection portions 9 (9a, 9b, 9c, 9d, 9e, 9f, 9) for connecting each resistor to the terminal electrode 10.
g), an electrical insulating film 12b for protecting each resistor. Here, each of the resistors 4a, 4b, 5, 6 is made of an impurity-doped polycrystalline silicon (Si) semiconductor thin film, and the heating resistors 4a, 4b have an impurity concentration that is lower than that of the temperature measuring resistors 5, 6. Large and impurity concentration is 3 × 10
It is formed to be ¹⁹ (cm ^-3 ) or more.

The thermal air flow sensor according to the embodiment of the present invention operates as follows.

The pair of upstream and downstream heating resistors 4a and 4b are
Electrically connected in series, connection point (middle tap) D
Is connected to the terminal electrode 10f by the wiring connection portion 9f. The pair of heating resistors 4a and 4b formed on the electric insulating film 12a thermally insulated by the cavity 8 has a temperature of the temperature measuring resistor 5 for detecting the temperature of the heating resistors 4a and 4b. The heating (indirect heat) electricity is supplied so that the temperature becomes higher by a certain temperature than the temperature of the air temperature measuring resistor 6 indicating the above temperature.

The direction of the air flow 11 is detected by comparing the temperatures (resistance values) of the heating resistors 4a and 4b formed symmetrically with respect to the temperature measuring resistor 5. That is,
When the airflow is zero, the heating resistors 4a and 4b exhibit substantially the same temperature as the temperature of the temperature measuring resistor 5, and no temperature difference occurs.
On the other hand, in the direction (forward flow) of the airflow 11 in FIG. 1, the heating effect of the airflow 11 is larger in the heating resistor 4 a arranged mainly on the upstream side than in the heating resistor 4 b arranged in the downstream side. Since the heating resistors 4a and 4b are connected in series and the same heating current flows, the amount of heat generation is substantially constant, so that the temperature of the heating resistor 4a on the upstream side is lower than the temperature of the heating resistor 4b. Becomes Further, when the airflow 11 is opposite to the direction of FIG. 1 (backflow), the temperature of the downstream heating resistor 4b is lower than the temperature of the upstream heating resistor 4a. Thus, the heating resistors 4a, 4b
The direction of the airflow 11 can be detected by comparing the temperatures (resistance values) of the airflow.

On the other hand, the air flow rate is measured from the heating (indirect heat) current value flowing through the heating resistors 4a and 4b in order to control the temperature of the temperature measuring resistor 5 higher than the air temperature measuring resistor 6 by a certain temperature. I do. As described above, in the present embodiment, the direction and the flow rate of the air flow can be detected. FIG. 3 is a sectional view showing an embodiment of a thermal air flow sensor on which the element 1 of FIG. 1 is mounted. FIG. 2 is a cross-sectional view illustrating an example of a thermal air flow sensor mounted in an intake passage of an internal combustion engine of an automobile or the like. As shown in the figure, the thermal air flow sensor includes an element 1, a support 19, and an external circuit 20. And the main intake passage 1
The element 1 is arranged in a sub-passage 18 inside the element 7. The external circuit 20 is electrically connected to the terminal electrode 10 of the device 1 via the support 19. Here, the intake air normally flows in the direction indicated by 11, and the intake air flows in the opposite direction (backflow) to 11 depending on the condition of a certain internal combustion engine.

FIG. 4 is an enlarged view of the element 1 and the support 19 of FIG. As shown in FIG. 4, the element 1 includes a support 1
An external circuit 20 having a terminal electrode 21 and a signal processing circuit formed on an electrically insulating substrate such as alumina is fixed on the support 19. This element 1
And the external circuit 20 connect the gold wire 2 between the terminal electrodes 10 and 21.
2 and the like, and then the above-described gold wire 22, electrode terminals 10, 21 and external circuit 2 are connected.
0 is protected from above by a support 19 (not shown).

Next, the circuit operation of the embodiment of the present invention will be described with reference to FIG. FIG. 5 shows the resistors 4a, 4b, 5, 6 of the element 1 of FIG. 1 and an external circuit 20 for signal processing.
It is shown. In the figure, 23 is a power supply, 24 is a transistor for passing a heating (indirectly heated) current to the heating resistors 4a and 4b, 25a, 25b and 25c are resistors, 26 is an input circuit including an A / D converter and the like, and D A control circuit including an output circuit including an / A converter and a CPU for performing arithmetic processing and the like, and 27 is a memory circuit.

Here, the voltage of the terminals F and G of the bridge circuit composed of the temperature measuring resistor 5, the air temperature measuring resistor 6, and the resistors 25b and 25c is input to the control circuit 26, and the heating resistor 4
Temperature of the resistance temperature detector 5 indirectly heated by a and 4b (Th)
Is the temperature of the air temperature measuring resistor 6 corresponding to the air temperature (T
Each resistance value is set so as to be higher than a) by a certain fixed value (for example, ΔTh = 150 ° C.), and is controlled by the control circuit 26. When the temperature of the resistance temperature detector 5 is lower than the set value, the transistor 24 is turned on by the output of the control circuit 26, and a heating current flows through the heating resistors 4a and 4b. When the temperature becomes higher than the set temperature, the transistor 24 is turned off. And is controlled to be constant at the set value. Heating resistor 4 at this time
The heating current value (corresponding to the potential E of the resistor 25a) flowing through the a and 4b is the air flow rate (Q).

On the other hand, the direction of the air flow depends on the heating resistors 4a,
It is detected from the temperature difference of 4b. As described above, the resistance temperature detector 5 is set to a certain reference temperature (Th = Ta + ΔTh). Since the heating resistors 4a and 4b are connected in series and have the same heating current flow, when the air flow is a forward flow, the heating resistor 4a on the upstream side loses heat by the air flow. The temperature decreases. On the other hand, when the air flow is the reverse flow, the temperature of the heating resistor 4b is lowered. That is, the direction of the air flow can be detected by comparing the temperatures (resistance values) of the heating resistors 4a and 4b.

In the circuit shown in FIG. 5, the temperatures (resistance values) of the heating resistors 4a and 4b are compared based on the potentials at both ends of each of the resistors connected in series. The potential difference between points C and D in FIG. 5 corresponds to the temperature of the upstream heating resistor 4a, and the potential difference between D and E corresponds to the temperature of the downstream heating resistor 4b. Therefore, by inputting the potentials at the points C, D and E to the control circuit 26, the direction of the air flow is detected from the potential difference corresponding to each heating resistor.

By adding the air temperature measuring resistor 6 and the temperature measuring resistor 5 as described above, the thermal air flow sensor of the conventional example is constituted by only the heating resistor, whereas the conventional thermal air flow sensor is constituted only by the heating resistor. Therefore, even if the air temperature changes, the flow rate can be detected with high accuracy by detecting the direction of the air flow without being affected.

Next, a specific example of the manufacturing process of the thermal air flow sensor element of the present embodiment will be described with reference to FIG.

Referring to FIG. 6A, silicon dioxide (SiO ₂ ) layers 13 and 14 are formed on the upper and lower surfaces of the silicon semiconductor substrate 2 by thermal oxidation to a thickness of about 0.3 μm.

Next, in (b), the silicon dioxide layer 1 on the upper surface
After forming a resist into a predetermined shape by a known photolithography technique and etching it by a method such as reactive ion etching, the silicon dioxide layer 13 is used as a mask to a depth of about 5 microns on the surface of the silicon semiconductor substrate 2. An impurity such as P (phosphorus) or B (boron) is doped by a method such as thermal diffusion or ion implantation to form the doped layers 3 and 3a.

In (c), after the silicon dioxide layers 13 and 14 are removed by etching, the electrical insulating film 1 made of the silicon dioxide layer is again formed on the upper and lower surfaces of the silicon semiconductor substrate 2.
2a, 15 is formed to a thickness of about 0.5 micron. Here, as the electric insulating film 12a formed on the upper surface of the silicon semiconductor substrate 2, a constituent material other than the above-described silicon dioxide can be used. For example, even if silicon nitride (Si ₃ N ₄ ) having a high mechanical strength and a thermal expansion coefficient slightly larger than that of the silicon semiconductor substrate 2 is used, or a thermal expansion coefficient of the silicon semiconductor substrate 2 is 1 /
By employing a multi-layer structure of silicon dioxide having a coefficient of thermal expansion of 10 and silicon nitride having a coefficient of thermal expansion slightly larger than that of the silicon semiconductor substrate 2 so as to match the coefficient of thermal expansion, a temperature change between the silicon semiconductor substrate 2 and the electrical insulating film 12a Can be reduced and the strength can be improved.

In (d), a polycrystalline silicon (Si) semiconductor thin film having a thickness of about 1 micron is formed on the electric insulating film 12a as the heating resistors 4a and 4b and the temperature measuring resistors 5 and 6 by a method such as CVD. After the formation, a resist is formed in a predetermined shape by a known photolithography technique, and then the semiconductor thin film is patterned by a method such as reactive ion etching. Here, the polycrystalline silicon (Si) semiconductor thin film is formed by a method such as LPCVD using plasma, ECR-PCVD using electron cyclotron resonance, CVD using microwave, or the like. The source gas is monosilane (SiH ₄ ),
Using phosphine (PH ₃ ) and hydrogen (H ₂ ), the amount of phosphorus (P) as an impurity doping material can be controlled by the flow rate of phosphine (PH ₃ ) gas, and the impurity concentration is 3 × 10 ¹⁹ (c
m ⁻³ ) It is controlled to be below. Here, the impurity doping process can be performed by a method other than the above method. Even after a non-bored polycrystalline silicon (Si) semiconductor thin film is formed, the impurity doping process is performed by a method such as thermal diffusion or ion implantation. good.

Next, in (e), impurity doping is further performed to increase the impurity concentration of the heating resistors 4a and 4b to 3 × 10 ¹⁹ (cm ⁻³ ) or more. The temperature measuring resistors 5 and 6 other than the heating resistors 4a and 4b are made of a mask material 1 such as silicon dioxide.
6, the heating resistors 4a and 4b are further diffused by heat diffusion or ion implantation.
(Phosphorus) and other impurity doping treatments are performed to reduce the impurity concentration to 3 ×
Heating resistors 4a and 4b doped at a high concentration of 10 ¹⁹ (cm ⁻³ ) or more are obtained. Then, although not shown, the terminal electrodes 10 (10a, 10b, 10c, 10d, 10
e, 10f, 10g), and wiring connection portions 9 (9a, 9b, 9c, 9d, 9) for connecting each resistor to the terminal electrode 10.
e, 9f, 9g) are formed of aluminum, gold or the like.

In (f), the electric insulating film 12b is formed to a thickness of about 0.5 μm in order to protect portions other than the terminal electrode 10, like the electric insulating film 12a. Next, in order to form the cavity 8 in the silicon semiconductor substrate 2, the etching mask material 15 is patterned into a predetermined shape to expose only the etched portion of the semiconductor substrate 2. As the mask material, silicon dioxide or silicon nitride having a higher etching selectivity is used.

In (g), finally, anisotropic etching is performed from the back surface of the silicon semiconductor substrate 2 using silicon dioxide or silicon nitride as a mask material 16 using an etching solution such as potassium hydroxide (KOH). Cavity 8
To form Here, the etching rate of the semiconductor substrate 2 made of silicon depends on the impurity concentration, and the etching rate decreases as the impurity concentration increases. 7 to support and reinforce the cross beam
Only the silicon semiconductor substrate 2 in the cavity 8 region is etched leaving a and 3b. The cross-shaped support portions 3a and 3b doped with impurities are selected to have a width of about 5 to 30 microns and a thickness of about 2 to 5 microns so that sufficient mechanical strength can be maintained.

Although the impurity in the above embodiment was P (phosphorus),
Similarly, N (nitrogen), Sb (antimony), As (arsenic) as an n-type impurity or A as a p-type impurity
l (aluminum), B (boron) or the like may be used.

With the above configuration, the electric insulating film 7 on the cavity 8 in which the heating resistors 4a and 4b and the temperature measuring resistor 5 are formed achieves thermal insulation with the semiconductor substrate 2 and has high responsiveness. Even if the overall thickness is made as thin as about 1 micron to reduce the heat capacity to increase the heat capacity, an impurity-doped cruciform beam-shaped support formed to support and reinforce the electrical insulating film 7 By being integrated with the semiconductor substrate 2 by the portions 3a and 3b, sufficient mechanical strength can be maintained even when the heating resistor is repeatedly heated and cooled or when a large amount of stress is applied when the air flow is increased.

The heating resistors 4a and 4b and the temperature measuring resistors 5 and 6 are formed of doped polycrystalline silicon (Si) semiconductor thin films, and the impurity concentration of the heating resistors 4a and 4b is 3 × 10 ^19. (Cm ⁻³ ) or more and higher than the impurity concentration of the temperature measuring resistors 5 and 6, the resistivity (ρ) of the heating resistors 4 a and 4 b can be made relatively small. And the resistance temperature coefficient (α) of the resistance temperature detectors 5 and 6 can be kept relatively large, thereby improving the temperature measurement sensitivity.
Further, the heating resistors 4a and 4b and the temperature measuring resistors 5 and 6 are used.
Are made of polycrystalline silicon (Si) semiconductor thin films with different impurity concentrations, so that there is no need to use a separate material such as expensive platinum or the like, and the polycrystalline silicon (Si) semiconductor Since a thin film can be formed, a low-cost thermal air flow sensor can be provided.

FIG. 7 shows the relationship between the resistivity (ρ) of the polycrystalline silicon (Si) semiconductor thin film and the impurity concentration. FIG. 8 shows the relationship between the temperature coefficient of resistance (α) and the resistivity (ρ) of a polycrystalline silicon (Si) semiconductor thin film. As can be seen from FIGS. 7 and 8, as the impurity concentration increases, both the resistivity (ρ) and the temperature coefficient of resistance (α) of the polycrystalline silicon (Si) semiconductor thin film decrease.

A polycrystalline silicon (Si) semiconductor film generally exhibits a thermistor-like resistance-temperature characteristic. However, when the temperature range is relatively narrow and is doped with impurities, a metallic resistance-temperature characteristic (1) is obtained. ) Shows the equation.

R = R0 (1 + α (T + T0)) (1) where R is a resistance value of the semiconductor film at a temperature (T), and R
0 is the resistance film of the semiconductor film at the temperature (T0), and α is the temperature coefficient of resistance. The larger the temperature coefficient of resistance (α), the greater the change in resistance value with respect to temperature. Therefore, the larger the temperature coefficient of resistance (α), the higher the detection sensitivity and the measurement of the air flow rate. It is desired because the accuracy is improved. As the temperature measuring resistors 5 and 6, the temperature coefficient of resistance (α) of the region 30 shown in FIG.
0 ⁻⁶ / ° C.) or more, and as shown in FIG.
Thirty regions below ¹⁹ (cm ^-3 ) are selected.

On the other hand, as the heating resistors 4a and 4b, the resistivity (ρ) becomes too large in the same impurity concentration region 30 as the temperature measuring resistors 5 and 6. The desired temperature (eg 2
When the heating resistors 4a and 4b are heated to (00 ° C.), the resistance values of the heating resistors 4a and 4b become large, and a high driving voltage is required, which causes a problem that the heating cannot be performed sufficiently. In order to reduce the resistance values of the heat generating resistors 4a and 4b, it is conceivable to increase the thickness of the polycrystalline silicon (Si) semiconductor film. However, if the thickness is increased, it is possible to perform accurate etching to a desired pattern. It becomes difficult and is not preferable in terms of material cost. The thickness of a polycrystalline silicon (Si) semiconductor film that can be accurately etched is about 1 micron, and the heating resistors 4a and 4b that can be driven with a driving voltage of 10 volts or less at this thickness have a resistance value of 1 kΩ. 7 and the impurity concentration shown in the region 29 of FIG.
^A resistivity (ρ) of 30 (× 10 ⁻⁴ Ω-c) at ¹⁹ (cm ⁻³ ) or more
m) The following areas are selected.

As described above, since the impurity concentration of the heating resistors 4a and 4b is set to 3 × 10 ¹⁹ (cm ⁻³ ) or more and higher than the impurity concentrations of the temperature measuring resistors 5 and 6, heat generation is achieved. Since the resistivity (ρ) of the resistors 4a and 4b can be made relatively small, the degree of freedom in designing the resistance value of the heating resistor is improved, and the temperature coefficient of resistance (α) of the temperature measuring resistors 5 and 6 is increased.
Can be kept relatively large, and the temperature measurement sensitivity can be improved.

The resistance values of the heat generating resistors 4a and 4b of this embodiment are 50 to 90 based on the relationship between the power supply voltage and the heat generation amount.
0 Ω and 1 to 5 kΩ were selected as resistance values of the resistance temperature detectors 5 and 6.

Next, second and third embodiments of the present invention will be described. FIG. 9 and FIG. 10 are plan views of the element 1 of the thermal air flow sensor formed up to the electric insulating film 7 on the cavity 8 in the second and third embodiments of the present invention. Semiconductor substrate 2 formed on the lower surface to support and reinforce electric insulating film 7
The supporting portions 3b, 3c, 3d, 3e and 3f in the form of beams which are doped with impurities to a predetermined depth and protrude from the periphery of the cavity are indicated by broken lines.

In FIG. 9, the support members 3a, 3b of the first embodiment shown in FIG.
Each of 3e and 3f is composed of a beam-shaped support portion for supporting and reinforcing the electrical insulating film 7 from the peripheral portion (square) to the center partway. With this configuration, it is possible to reinforce the boundary portion between the electrical insulating film 7 where the stress is concentrated and the semiconductor substrate 2, and to achieve thermal insulation with the semiconductor substrate 2. FIG. 10 shows an intermediate structure between the first embodiment shown in FIG. 1 and the second embodiment shown in FIG. 9, and a supporting portion 3b connecting between the cavities 8 and a supporting portion 3c extending from the peripheral portion toward the center. , 3e to support and reinforce the electrical insulating film 7. With this configuration, an intermediate effect between the first embodiment and the second embodiment can be obtained. In addition to this, even when a larger number of support portions are formed from the peripheral portion as the beam-shaped support portions for supporting and reinforcing the electric insulating film 7, the cross-sectional area of the support portion decreases from the peripheral portion toward the center. Even if formed
By making the connection between the support and the semiconductor substrate 2 rounded to prevent stress concentration, it is possible to further reinforce the support.

FIG. 11 is a plan view of the element 1 of the thermal air flow sensor according to the fourth embodiment of the present invention. The difference from the first embodiment of FIG. 1 is that the temperature measuring resistors 5a and 5b are formed upstream and downstream of the heating resistor 4. Thus, in the case of a temperature difference detection method in which the temperature measuring resistors 5a and 5b are arranged upstream and downstream of the heating resistor 4 and the air flow rate is measured from the temperature difference between the upstream and downstream temperature measuring resistors 5a and 5b. In addition, the impurity concentration of the heating resistor 4 is set to 3 × 10 ¹⁹ (cm ⁻³ ) or more and higher than the impurity concentrations of the temperature measuring resistors 5a and 5b, and the electric insulating film 7 is formed in a beam-like supporting portion 3a. , 3b can provide a low-cost thermal air flow sensor with improved air temperature dependence and mechanical strength. In any other method, it is obvious that the above-described present invention can be applied to a thermal air flow sensor formed of a heating resistor and a temperature measuring resistor on the semiconductor substrate 2.

[0043]

According to the present invention, there is provided a thermal air flow sensor for measuring an air flow rate by forming at least a heating resistor and a temperature measuring resistor on a semiconductor substrate via an electric insulating film.
The heating resistor and the temperature measuring resistor are composed of a doped polycrystalline silicon (Si) semiconductor thin film, the impurity concentration of the heating resistor is 3 × 10 ¹⁹ (cm ⁻³ ) or more, and The impurity concentration is set higher than the impurity concentration of the temperature resistor, and the semiconductor substrate has a cavity extending from the boundary surface of the electric insulating film to the lower surface. By supporting and reinforcing with a beam-shaped supporting portion that is doped and protrudes from the periphery of the cavity, a thermal air flow sensor with improved air temperature dependence and mechanical strength can be provided at low cost.

[Brief description of the drawings]

FIG. 1 is a diagram showing a plan view of a thermal air flow sensor element 1 according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an AA ′ cross section of the device of FIG. 1;

FIG. 3 is a diagram showing a cross section of a thermal air flow sensor on which the element of FIG. 1 is mounted.

FIG. 4 is an enlarged view of a measuring element unit in FIG. 3;

FIG. 5 is a diagram showing an electric circuit of resistors 4a, 4b, 5, 6 and an external circuit 20.

FIG. 6 is a diagram illustrating a manufacturing process of the element 1.

FIG. 7 is a diagram showing the relationship between the resistivity (ρ) of a polycrystalline silicon semiconductor thin film and the impurity concentration.

FIG. 8 is a diagram showing the relationship between the temperature coefficient of resistance (α) and the resistivity (ρ) of a polycrystalline silicon (Si) semiconductor thin film.

FIG. 9 is a view showing a plane of the thermal air flow sensor element 1 formed up to the electric insulating film 7 on the cavity 8 in the second embodiment of the present invention.

FIG. 10 is a view showing a plane of the thermal air flow sensor element 1 formed up to the electric insulating film 7 on the cavity 8 in the third embodiment of the present invention.

FIG. 11 is a view showing a plane of a thermal air flow sensor element 1 according to a fourth embodiment of the present invention.

FIG. 12 is a diagram showing a cross section of a conventional thermal air flow sensor element 1.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Element, 2 ... Semiconductor substrate, 3, 3a, 3b, 3c, 3
d, 3e, 3f: doped layer (supporting portion), 4, 4a, 4b
... heating resistor, 5, 5a, 5b ... temperature measuring resistor, 6, 6a,
6b: Air temperature measuring resistor, 7, 12a, 12b, 1
3, 14, 15 ... electric insulating film, 8 ... cavity, 9, 9a, 9
b, 9c, 9d, 9e, 9f, 9g ... wiring connection part, 1
0, 10a, 10b, 10c, 10d, 10e, 10
f, 10 g, 21 terminal electrode, 11 air flow, 16 mask material, 17 main intake passage, 18 sub passage, 19 support, 20 external circuit, 22 gold wire, 23 power supply, 24 …
Transistors, 25a, 25b, 25c ... resistors, 26 ...
Control circuit, 27 ... memory.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Keiichi Nakata 2520 Odaiba, Hitachinaka-city, Ibaraki Pref. Inside the Automotive Equipment Division of Hitachi, Ltd.

Claims (4)

[Claims] 1. A thermal air flow sensor for measuring an air flow rate by forming at least a heating resistor and a temperature measuring resistor on a semiconductor substrate via an electric insulating film, wherein the heating resistor and the temperature measuring resistor are provided. A thermal air flow sensor, comprising a polycrystalline silicon (Si) semiconductor thin film doped with Pb, and wherein the impurity concentration of the heating resistor is higher than the impurity concentration of the temperature measuring resistor. 2. The thermal method according to claim 1, wherein the doped polycrystalline silicon (Si) semiconductor thin film of the heating resistor according to claim 1 has an impurity concentration of 3 × 10 ¹⁹ (cm ⁻³ ) or more. Air flow sensor. 3. A semiconductor substrate having an electrical insulating film formed on an upper surface and having a cavity extending from the boundary surface of the electrical insulating film to a lower surface;
In a thermal air flow sensor for measuring an air flow rate by forming at least a heating resistor on the electric insulating film on the cavity,
A thermal air flow sensor, wherein the electrical insulating film on the cavity is supported and reinforced by a beam-like supporting portion protruding from the periphery of the cavity formed of the semiconductor substrate. 4. The method according to claim 3, wherein the beam-shaped support portion comprises:
A thermal air flow sensor comprising a layer on an upper surface of the semiconductor substrate, which is doped with impurities to a predetermined depth.