JPH09133563A - Thermal flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high response thermal flow sensor in which the flow rate can be measured accurately while retarding the saturation of current flowing through a heater even in the region of high flow rate. SOLUTION: A heater, i.e., a first diode 31, and a temperature measuring element, i.e., a second diode 32, are formed on a same substrate 30. Since the first diode 31 is formed on the substrate 30 through a diaphragm (or a cross- linked structure part or a cantilever), it is insulated thermally from the substrate 30 and has low thermal capacity. On the other hand, the second diode 32 is coupled thermally with the substrate 30 and it is not heated easily. Voltage difference is detected across the heater, i.e., the first diode 31, and across the temperature measuring element, i.e., the second diode 32, and a current is fed forward through the first and second diodes 31, 32 at same current density. Since the current flowing through the first diode 31 is substantially proportional to the root of fluid, the saturation of current is retarded as compared with a conventional thermal flow sensor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal type flow sensor having a fast response for measuring a flow velocity or a flow rate of a fluid and a small detecting element used in the thermal type flow sensor.

[0002]

2. Description of the Related Art When a heating element heated by applying an electric current is placed in a fluid, the amount of heat lost from the heating element in a unit time, that is, the amount of cooling depends on the velocity of the fluid. The thermal type flow sensor utilizes this principle to detect the velocity of the fluid.

In FIGS. 14 and 15, Japanese Patent Laid-Open No. 62-44.
The thermal type flow sensor described in Japanese Patent No. 627 is shown.

First, the detection element of the thermal type flow sensor will be described with reference to FIG.

The heating resistor 1 and the terminals 2A and 2B constituting the flow velocity detecting resistor Rh, and the temperature measuring resistor 3 and the terminals 4A and 8B constituting the temperature compensating resistor Rt are integrally formed on the surface of the silicon substrate 5. It is formed on the surface of the insulating layer 6 provided. The heating resistor 1 and the temperature measuring resistor 3 are formed of the same metal thin film having a large temperature coefficient of resistance such as platinum or nickel. Further, the silicon substrate 5 under the heating resistor 1 is removed by means such as anisotropic etching to form the space 7. As a result, the back surface of the insulating layer 6 is exposed, and the heating resistor 1 and the silicon substrate 5 are thermally insulated. Therefore, the heating resistor 1 generates heat with a minute current, and the thermal time constant until the temperature of the heating resistor 1 reaches a steady state becomes short. On the other hand, the thermal resistance between the resistance temperature detector 3 and the silicon substrate 5 is small, and both are thermally coupled. Therefore, even if a current flows and the resistance temperature detector 3 generates heat, the heat diffuses into the silicon substrate 5 and escapes. Therefore, the temperature of the resistance temperature detector 3 does not rise and is kept almost the same as the temperature of the fluid.

Next, the circuit configuration of a thermal type flow sensor using the above-mentioned detection element will be described with reference to FIG.

The power source Vcc is connected to the first input terminal T1 of the four-sided bridge circuit through the transistor 8 and the second input terminal T2 is grounded. A temperature compensation resistor Rt is connected between the first input terminal T1 and the first output terminal T3, and a resistor 9 is connected between the first output terminal T3 and the second input terminal T2. A flow velocity detecting resistor Rh is connected between the first input end T1 and the second output end T4, and a resistor 10 is connected between the second output end T4 and the second input end T2. The first output terminal T3 and the second output terminal T4 are connected to the differential amplifier 11. An OP amplifier is generally used as the differential amplifier 11. Output terminal T5 of the differential amplifier 11
The flow velocity detection signal Vo is taken out from. The flow velocity detection signal Vo is fed back to the base of the transistor 8 and applied.

The combined resistance value of the temperature compensating resistor Rt and the resistor 9 is made sufficiently larger than the combined resistance value of the flow velocity detecting resistor Rh and the resistor 10. As a result, the flow velocity detecting resistor Rh functions as a heating element that self-heats by the current from the power source Vcc. On the other hand, the temperature compensating resistor Rt does not generate heat by itself and detects the temperature of the fluid. Resistance 9 and resistance 1
With respect to the respective resistance values of 0, the voltages at the first output terminal T3 and the second output terminal T4 are balanced when the temperature difference between the fluid detection resistance Rh and the temperature compensation resistance Rt reaches a predetermined value. Is set as follows.

Next, the operation of the thermal type flow sensor will be described.

When the fluid passes through the flow velocity detecting resistor Rh which has generated heat, the flow velocity detecting resistor Rh is cooled. The cooling amount of the flow velocity detecting resistor Rh changes according to the flow velocity of the fluid. As a result, the resistance value of the flow velocity detecting resistor Rh changes according to the flow velocity of the fluid, and the voltage of the second output terminal T4 changes. A voltage corresponding to the temperature of the fluid is generated at the first output end T3 by the temperature compensating resistor Rt. These two voltages are input to the differential amplifier 11, the temperature of which is compensated from the output terminal T5 of the differential amplifier 11 and a voltage corresponding to the flow velocity of the fluid is taken out as the flow velocity detection signal Vo. Flow velocity detection signal V
o is applied to the base of the transistor 8. Power supply Vcc
Therefore, the current amplified by the transistor 8 flows through the flow velocity detection resistor Rh, and the flow velocity detection resistor Rh generates heat. As a result, the flow velocity detecting resistance Rh corresponding to the flow velocity of the fluid
Is compensated for, and the temperature difference between the flow velocity detecting resistor Rh and the temperature compensating resistor Rt is kept substantially constant.

Next, a method of determining the flow velocity vf of the fluid by using the thermal type flow sensor will be described.

When the flow velocity vf of the fluid is in a steady state, the amount P of heat lost from the flow velocity detecting resistance Rh in a unit time is obtained by the following equation according to King's equation. P = (A + B√vf)  (Th-Tt) (A and B are constants) where Th is the temperature in a steady state where the flow velocity detecting resistor Rh has generated heat, and Tt is the temperature of the fluid.

Further, the resistance value of the flow velocity detecting resistor Rh is r
Assuming that h is I, the current flowing through the flow velocity detection resistor Rh is V, and the voltage difference between both ends of the flow velocity detection resistor Rh is Vh, the heat generation amount Q of the flow velocity detection resistor Rh is calculated by the following equation. Q = I · I · rh = Vh · Vh / rh Since the calorific value P and the calorific value Q are equal, the following formula is established. (A + B√vf) · (Th−Tt) = I · I · rh = Vh · Vh / rh (1) In the equation (1), since the temperature difference (Th−Tt) is almost constant, the current I or The flow velocity vf of the fluid is obtained using the voltage difference Vh as a parameter.

The method of determining the flow velocity vf of the fluid is to control the current I or the voltage difference Vh so that the temperature difference (Th-Tt) between the flow velocity detecting resistor Rh and the fluid is substantially constant. is called. A method called constant current method in which a constant current I is passed through the flow velocity detecting resistor Rh and the temperature Th of the flow velocity detecting resistor Rh and the temperature Tt of the temperature compensating resistor Rt are measured to obtain the flow velocity vf of the fluid. There is also. However, since the response speed is slow, the constant temperature difference method is generally used.

Next, FIG. 16 shows an air flow sensing circuit of a thermal type flow sensor disclosed in Japanese Patent Publication No. 3-49374.

The air flow sensing circuit includes an ambient temperature branch 12
, An air flow branch 13, a temperature compensation branch 14, and an operational amplifier 15.

The ambient temperature branch 12 comprises a first semiconductor chip 16 and a current source 17. Inside the first semiconductor chip 16, a silicon diode 18 and a heating resistor 19 are formed in an electrically insulated state. A current source 17 is connected to the cathode end of the silicon diode 18 to form a series circuit. The silicon diode 18 side of the series circuit is connected to the power supply Vcc, and the other end side is grounded. The silicon diode 18 and the heating resistor 19 are thermally coupled.

The air flow branch 13 comprises a second semiconductor chip 20, a variable resistor 21 and a current source 22. Inside the second semiconductor chip 20, a silicon diode 23 and a heating resistor 24 are formed in an electrically insulated state. One end of the variable resistor 21 is connected to the cathode end of the silicon diode 23, and the current source 22 is connected to the other end of the variable resistor 21 to form a series circuit. The silicon diode 23 side of the series circuit is connected to the power supply Vcc, and the other end side is grounded. One end of the heating resistor 24 is connected to the power supply Vcc.

A connection point T6 between the silicon diode 18 and the current source 17, and a connection point T7 between the variable resistor 21 and the current source 22.
Is connected to the operational amplifier 15. The output end of the operational amplifier 15 is connected to the other end of the heating resistor 24 via the variable resistor 25.

The temperature compensation branch 14 includes a variable resistor 26,
27, a current source 28 and a jumper 29. A current source 28 is connected to one end of the variable resistor 26 to form a series circuit. The variable resistor 26 side of the series circuit is the power source V
cc, and the other end is grounded. Further, a series circuit including a variable resistor 27 and a jumper 29 is connected between the connection point of the variable resistor 26 and the current source 28 and the connection point T6 of the ambient temperature branch 12.

The electrical characteristics of the first semiconductor chip 16 and the second semiconductor chip 20 are almost the same,
The current ratings of the current sources 17 and 22 are the same.

Next, the operation of the air volume sensing circuit will be described.

When the temperatures of the first semiconductor chip 16 and the second semiconductor chip 20 are the same, the silicon diode 18 is used.
The voltage between both ends of and becomes equal. Therefore, the connection point T7
Is lower than the voltage at the connection point T6 by the voltage drop of the variable resistor 21. A flow velocity detection signal Vo proportional to the voltage difference between the connection points T6 and T7 is output to the output terminal of the operational amplifier 15. Since the current I flows through the heating resistor 24 by the flow velocity detection signal Vo, the heating resistor 24 generates heat and the silicon diode 23 is warmed.

In general, the forward voltage Vd of the silicon diode in the forward direction decreases as the temperature rises, and is expressed as a function of temperature as follows. Vd = Vd0−γ · (T−T0) where T is the ambient temperature, Vd0 is the forward voltage of the silicon diode in the forward direction at the temperature T0, and γ is a constant of about 2.
It is 3 mV / K.

Therefore, the voltage across the silicon diode 23 decreases, the voltage difference between the connection point T6 and the connection point T7 becomes equal, and the air flow sensing circuit is in a balanced state. At this time, the temperature of the second semiconductor chip 20 is the same as that of the first semiconductor chip 16
The temperature is higher than the temperature of 1 by a predetermined temperature. In this state, when the fluid passes through the second semiconductor chip 20, the second semiconductor chip 20 is cooled according to the flow velocity vf of the fluid, so that the voltage across the silicon diode 23 increases. Along with this, the flow velocity detection signal Vo changes so as to increase the current I flowing through the heating resistor 24, so that the temperature difference between the first semiconductor chip 16 and the second semiconductor chip 20 is kept substantially constant.

The current flowing through the silicon diode 18 is calibrated by adjusting the resistance values of the variable resistors 26 and 27 constituting the temperature compensating branch 14, and the temperature fluctuation of the air flow sensing circuit is compensated.

The flow velocity vf of the fluid is obtained from the above equation (1) using the current I flowing through the heating resistor 24 which changes according to the flow velocity detection signal Vo as a parameter.

[0028]

However, (1)
As is clear from the equation, there is a problem that the current flowing through the heating element or the voltage difference across the heating element is approximately proportional to the fourth root of the flow velocity of the fluid. That is, when the flow velocity of the fluid increases, the current flowing through the heating element or the voltage difference across the heating element is saturated. Therefore, the rate of change of the sensor output with respect to the change of the flow velocity becomes small, and the accuracy of the thermal type flow sensor is remarkably deteriorated.

Further, if the current flowing through the heating element is set to a large value in order to improve the accuracy, the temperature of the heating element becomes high, and it cannot be used for flammable gas or the like which has a risk of explosion. It was

Further, when the silicon diode and the heating resistor are formed inside one semiconductor chip, the manufacturing process is complicated because the silicon diode and the heating resistor are formed in separate steps. For this reason, the manufacturing cost of the semiconductor chip has increased, and consequently the price of the thermal type flow sensor using the semiconductor chip has increased.

Therefore, an object of the present invention is to provide a thermal type flow sensor for solving the above object.

[0032]

The present invention is configured as described below to achieve the above object. That is, firstly, it comprises a first diode, a second diode, and a substrate thermally coupled to the second diode, and a space is formed between the first diode and the substrate. The first diode and the substrate are thermally insulated, and a forward current having substantially the same current density is applied to the pn junction of the first diode and the pn junction of the second diode. It has a means.

By the current supplied by the circuit means:
The first diode thermally insulated from the substrate generates heat.
On the other hand, the second diode thermally coupled to the substrate is kept at the fluid temperature. As a result, the temperature difference between the first diode and the second diode is kept substantially constant.

Secondly, in the first invention, at least one of the first diode and the second diode is a diode array in which a plurality of basic diodes are connected in series.

Since the voltage difference between both ends of each basic diode is equal, the voltage difference between the first diode and the second diode is proportional to the ratio of the number of connecting stages of the diode array.

Thirdly, in the first or second invention,
The area of the pn junction of the first diode is smaller than the area of the pn junction of the second diode.

A larger amount of current flows through the first diode as compared with the second diode. Therefore, the temperature difference between the first diode and the second diode is controlled more accurately.

Fourthly, in the first, second or third invention, the first diode and the second diode are arranged so that the temperature difference between the first diode and the second diode becomes substantially constant. The circuit means for controlling the forward current of (1) is provided, and an output substantially proportional to the forward current of the first diode or the second diode is taken out as a flow velocity signal.

The flow velocity of the fluid is taken out as a signal which is approximately proportional to the forward current of the first diode or the second diode.

Fifth, in the first, second or third invention, a circuit means for controlling the forward currents of the first diode and the second diode so that the temperature of the first diode is substantially constant. And a means for calculating the flow velocity from the forward current of the first diode or the second diode and the differential voltage between the first diode and the second diode.

The flow velocity of the fluid is calculated and output using the forward current of the first diode or the second diode and the voltage difference between the first diode and the second diode as parameters.

[0042]

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Embodiment 1) The first detection element used in the thermal type flow sensor according to the present application will be described with reference to FIG.

The first detecting element is composed of a substrate 30, a first diode 31, and a second diode 32.

The substrate 30 is a rectangular plate made of silicon single crystal. On the surface near one long side edge of the substrate 30, thin rectangular lower insulating layers 33A, 33B having a constant thickness are formed.
Is provided. The lower insulating layers 33A and 33B are arranged with a predetermined gap. The lower insulating layers 33A, 33A
B is silicon dioxide (SiO2), silicon nitride (SiN
x) or another insulating material. In addition, the lower insulating layer 3
A space portion 34 is provided in a part of the substrate 30 in the lower portion of 3A, and the back surface of the lower insulating layer 33A is exposed. As a result, the lower insulating layer 33A and the substrate 30 form a diaphragm structure.

The first diode 31 is composed of an n-type silicon single crystal layer 35A, a p-type silicon conductive layer 36, and an n-type silicon conductive layer 37. The square-shaped n-type silicon single crystal layer 35A is provided at the center of the surface of the lower insulating layer 33A. The p-type silicon conductive layer 36 is a region in which boron ions are introduced as impurity ions into the surface layer of the n-type silicon single crystal layer 35A. The n-type silicon conductive layer 37 is a region where phosphorus ions are introduced as impurity ions into the surface layer of the n-type silicon single crystal layer 35A. The p-type silicon conductive layer 36 serves as an anode end, and the n-type silicon conductive layer 37 serves as a cathode end.

The second diode 32 is composed of an n-type silicon single crystal layer 35B, a p-type silicon conductive layer 38, and an n-type silicon conductive layer 39. The square-shaped n-type silicon single crystal layer 35B is provided at the center of the surface of the lower insulating layer 33B. The p-type silicon conductive layer 38 is a region in which boron ions are introduced as impurity ions into the surface layer of the n-type silicon single crystal layer 35B. The n-type silicon conductive layer 39 is a region in which phosphorus ions are introduced as impurity ions into the surface layer of the n-type silicon single crystal layer 35B. The p-type silicon conductive layer 38 serves as an anode end, and the n-type silicon conductive layer 39 serves as a cathode end.

The area of the pn junction between the n-type silicon single crystal layer 35B constituting the diode and the p-type silicon conductive layer 38 is determined by the area of the n-type silicon single crystal layer 35A constituting the diode.
And the area of the pn junction of the p-type silicon conductive layer 36. Also, the first diode 31 and the second diode 31
The cord 32 is formed in the same process. As a result, the electrical characteristics of the second diode 32 and the first diode 31 become almost the same.

Substrate 30, lower insulating layers 33A and 33B
And a first diode 31 and a second diode 32
A thin upper insulating layer 40 having a constant thickness is provided on the exposed portion. As a result, the surfaces of the first diode 31 and the second diode 32 are protected. The upper insulating layer 40 is
It is formed of an insulating material such as silicon dioxide or silicon nitride. The p-type silicon conductive layers 36 and 38 and the n-type silicon conductive layers 37 and 39 are provided at predetermined positions on the upper insulating layer 40.
Via holes 41 are formed to expose the respective surfaces.

Further, on the surface of the upper insulating layer 40 near the other long side edge of the substrate 30, square electrode pads 42A, 42 are formed.
Four B, 42C, and 42D are formed. Electrode pad 42
Are arranged at equal intervals along the long side of the substrate 30. The electrode pads 42 are connected to the p-type silicon conductive layers 36 and 38, n via the wiring electrodes 43 integrally provided inside the via holes 41 and on the surface of the upper insulating layer 40.
The surface of each of the silicon conductive layers 37 and 39 is electrically connected. The electrode pad 42 and the wiring electrode 43
Is formed of a thin film having a low resistivity, such as aluminum.

Since the first detection element has the above-described structure, the first diode 31 is thermally insulated from the substrate 30 and has a small heat capacity. On the other hand, the second diode 32 is thermally coupled to the substrate 30 and has a large heat capacity.

The voltage difference between both ends of the first diode 31 and the second diode 32 is taken out through the electrode pad 42.

Next, an example of a method for manufacturing the first detecting element will be described with reference to FIGS. 2 (a) to 2 (j).

An SOI (Silicon on Insulator) in which a lower insulating layer 33 and an n-type silicon single crystal layer 35 are sequentially laminated on the surface of a silicon single crystal substrate 30.
r) Form the substrate 44. In recent years, the SOI substrate 4
4 is commercially available, so it may be used.

Next, high-concentration boron ions are introduced as impurity ions into predetermined positions on the surface layer of the n-type silicon single crystal layer 35 by means of ion implantation or thermal diffusion. As a result, p-type silicon conductive layers 36 and 38 are formed.

Next, phosphorus ions are introduced as impurity ions into a predetermined position of the surface layer of the n-type silicon single crystal layer 35 by using a means such as ion implantation or thermal diffusion. As a result, n-type silicon conductive layers 37 and 39 which are regions containing phosphorus ions are formed. Next, RIE (React
iv Ion Etching)
A predetermined portion of the type silicon single crystal layer 35 is removed, and the square-shaped n-type silicon single crystal layers 35A and 35B are separated.
As a result, the first diode 31 and the second diode 3
2 are formed.

Next, a predetermined portion of the lower insulating layer 33 is removed by using a method such as wet etching or RIE. As a result, the rectangular lower insulating layers 33A and 33B are formed separately.

Next, the substrate 30 and the first diode 31
And a second diode 32 and lower insulating layers 33A and 33A.
The upper insulating layer 40 is formed so as to cover the exposed portion of B. The upper insulating layer 40 is formed by plasma CVD (Chemic).
Al Vapor Deposition) or the like.

Next, the p-type silicon conductive layers 36 and 38,
A via hole 41 is formed by means of RIE or the like to expose the surfaces of the n-type silicon conductive layers 37 and 39, respectively.

Next, the electrode pad 42 and the wiring electrode 43.
Are formed using a method such as vapor deposition or sputtering.

Next, the upper insulating layer 40 and the electrode pad 42.
A thin protective layer 45A of silicon nitride or the like is formed so as to cover the wiring electrodes 43. A protective layer 45B made of thin silicon nitride or the like is formed on the back surface of the substrate 30. Protective layer 45A
And 45B are formed by means of plasma CVD or sputtering. An opening 46 is provided in the protective layer 45B facing the lower insulating layer 33A, and the back surface of the substrate 30 is exposed.

Next, the substrate 30 is chemically etched using an alkaline aqueous solution such as potassium hydroxide (KOH). The substrate 30 is anisotropically etched from the back surface to the surface direction through the opening 46, and the space portion 34 is formed below the lower insulating layer 33A.

Thereafter, the protective layers 45A and 45B are
It is removed by means such as IE.

Since the first detecting element is formed by the above-mentioned method, the silicon diode and the heating resistor are formed in separate steps inside the semiconductor chip like a conventional thermal type flow sensor using a diode. There is no need, and the manufacturing process of the detection element is simplified. Moreover, when the commercially available SOI substrate 44 is used, the manufacturing process is further simplified.

FIG. 3 shows a first circuit configuration of a thermal type flow sensor using the first detecting element. This circuit configuration is
This is a constant temperature difference type drive circuit for keeping the temperature difference between the first diode 31 and the second diode 32 substantially constant.

The cathode end of the first diode 31 is connected to the anode end of the second diode 32. The anode end of the first diode 31 is connected to the power supply Vcc via the resistor 47. The cathode end of the second diode 32 is grounded via the resistor 48. The flow velocity detection signal Vo is taken out from the cathode end of the second diode 32. The first diode 31 and the second diode 32 have the same electrical characteristics.

The first buffer circuit 49, the second buffer circuit 50 and the third buffer circuit 51 are O
It is formed by a P amplifier. First buffer circuit 49
The non-inverting input terminal of the OP amplifier is a first diode 31.
Connected to the anodic end of the output terminal and the output terminal is fed back to the inverting input terminal. The non-inverting input terminal of the OP amplifier of the second buffer circuit 50 is connected to the cathode terminal of the first diode 31, and the output terminal is fed back to the inverting input terminal. The non-inverting input terminal of the OP amplifier of the third buffer circuit 51 is connected to the cathode terminal of the second diode 32, and the output terminal is fed back to the inverting input terminal.

The first subtraction circuit 52 is an OP amplifier 53.
And resistors 54, 55, 56 and 57. The resistors 54 and 55 are connected in series, and the resistor 5 of the series connection circuit is connected.
The 5 side is grounded and the resistor 54 side is the first buffer circuit 4
9 is connected to the output terminal. The non-inverting input terminal of the OP amplifier 53 is connected to the voltage dividing point of the resistors 54 and 55. The output terminal of the OP amplifier 53 is fed back to the inverting input terminal via the resistor 56. The inverting input terminal of the OP amplifier 53 is
It is connected to the output terminal of the second buffer circuit 50 via the resistor 57.

The second subtraction circuit 58 has an OP amplifier 59.
And resistors 60, 61, 62 and 63. The resistors 60 and 61 are connected in series, and the resistor 6 of the series connection circuit is connected.
One side is grounded, and the resistor 60 side is the second buffer circuit 5
It is connected to the output terminal of 0. The non-inverting input terminal of the OP amplifier 59 is connected to the voltage dividing point of the resistors 60 and 61. The output terminal of the OP amplifier 59 is fed back to the inverting input terminal via the resistor 62. The inverting input terminal of the OP amplifier 59 is
It is connected to the output terminal of the third buffer circuit 51 via the resistor 63.

The output terminal of the second subtraction circuit 58 is a resistor 64.
And 65 connected to the resistor 64 side of the voltage dividing circuit 66. The resistor 65 side of the voltage dividing circuit 66 is grounded.

The output terminal of the first subtraction circuit 52 is connected to the non-inverting input terminal of the OP amplifier 67 which is a differential amplifier circuit. The inverting input terminal of the OP amplifier 67 is connected to the voltage dividing point of the voltage dividing circuit 66. The output terminal of the OP amplifier 67 is connected to the anode terminal of the first diode 31.

Next, the circuit operation of the first circuit configuration will be described.

The resistor 47 supplies a minute current in the forward direction of the first diode 31 and the second diode 32 to the extent that the first diode 31 and the second diode 32 do not generate heat. It is a resistor provided to give a voltage for flowing.

When a voltage is applied to the anodic end of the first diode 31, the barrier potential difference between the pn junctions forming the first diode 31 and the second diode 32 is lowered, A current flows through the first diode 31 and the second diode 32. As a result, voltage differences Vd1 and Vd2 occur across the first diode 31 and the second diode 32. At this time, since only a minute current flows through the first diode 31 and the second diode 32, they hardly generate heat and are kept at the same temperature. Therefore, at this time point, the voltage differences Vd1 and Vd2 across the first diode 31 and the second diode 32 are equal.

Since the input impedances of the first buffer circuit 49 and the second buffer circuit 50 and the third buffer circuit 51 are very large, almost no current flows. Therefore, the voltage at the anode end of the first diode 31 is directly supplied to the non-inverting input terminal of the OP amplifier 53 via the resistor 54. The voltage at the cathode terminal of the first diode 31 is supplied to the OP amplifier 53 via the resistor 57.
Is supplied to the inverting input terminal of the
It is directly supplied to the non-inverting input terminal of the amplifier 59. The voltage at the cathode terminal of the second diode 32 is directly supplied to the inverting input terminal of the OP amplifier 59 via the resistor 63.

Resistances 54, 55, 5 of the first subtraction circuit 52
The resistance values of 6, 57 are r54, r55, r56,
r57. The resistance values of the resistors 54, 55, 56 and 57 are set so that rr54 = r55 = r56 = r57. The voltage at the output end of the first buffer circuit 49 is V1, the voltage at the output end of the second buffer circuit 50 is V2, and the voltage at the output end of the first subtraction circuit 52 is V3.
At this time, V3 is calculated by the following equation. Since V3 = V1-V2 (V1-V2) is the voltage difference Vd1 across the first diode 31, the voltage V3 at the output end of the first subtraction circuit 52 is Vd1.

The resistors 60, 61, 6 of the second subtraction circuit 58
The resistance values of 2, 63 are r60, r61, r62,
r63. When the resistance values of the resistors 60, 61, 62, 63 are set so that r60 = r61 = r62 = r63, the voltage at the output end of the second subtraction circuit 58 becomes Vd2.

The voltage Vd2 at the output end of the second subtraction circuit 58
Is divided by the voltage dividing circuit 66. Resistors 64, 65
Let r64 and r65 be the resistance values of, the voltage at the voltage dividing point is V
It becomes d2 · {r65 / (r64 + r65)}.

The OP amplifier 67 has a voltage Vd1 at the output end of the first subtraction circuit 52 and a voltage Vv at the voltage dividing point of the voltage dividing circuit 66.
Differentially output d2 · {r65 / (r64 + r65)}.

The voltage Vd1 is equal to the voltage Vd2 {r65 /
If it is larger than (r64 + r65)}, the OP amplifier 6
A current I is supplied from the output terminal of 7 to the anode terminal of the first diode 31. The first diode 31 having a small heat capacity generates heat when the current I flows. As a result, the voltage difference Vd1 across the first diode 31 decreases. On the other hand, a second diode with a large heat capacity is formed.
The current 32 does not generate heat even when the current I flows, and is maintained at a temperature substantially the same as the temperature of the fluid.

The voltage Vd1 and the voltage Vd2 · {r65 /
When (r64 + r65)} are the same, the current I supplied from the output terminal of the OP amplifier 67 to the anodic terminal of the first diode 31 is in a steady state. As a result, the first dio-
The temperature difference (Th-Tt) between the temperature Th of the gate 31 and the temperature Tt of the second diode 32 is substantially constant. That is, Vd1 = Vd2 * {r65 / (r64 + r65)} Vd1 = Vd0- [gamma] * (Th-T0) Vd2 = Vd0- [gamma] * (Tt-T0) The temperature difference (Th-Tt) is calculated by the following equation. Desired. Th-Tt = Vd2 (1 / γ)  {r64 / (r64 + r66)} = Vd1 (1 / γ)  (r64 / r65) Therefore, the first diode 31 and the second diode 32
The temperature difference (Th-Tt) is determined by the resistance values r64 and r65.

Since the same current I flows through the first diode 31, the second diode 32 and the resistor 48, the temperature difference (Th-Tt) is accurately controlled.

Heat value Q in the first diode 31
Is represented by the following equation. Q = Vd1 · I = (A + B · √vf) · (Th−Tt) If the resistance value of the resistor 48 is r48, the flow velocity detection signal Vo
Is represented by the following equation. Vo = r48 · I From these relationships, the flow velocity detection signal Vo is expressed by the following equation. Vo = r48 · (A + B · √vf) · (Th−Tt) / Vd1 = r48 · (A + B · √vf) · (1 / γ) · (r64 / r65) From this equation, the flow velocity vf is The detection signal Vo is obtained as a parameter. The flow velocity detection signal Vo
Is approximately proportional to the square root of the fluid flow velocity vf. Therefore, saturation of the flow velocity detection signal Vo is unlikely to occur, and the accuracy of the thermal flow velocity sensor does not deteriorate even when the flow velocity vf of the fluid is large.

FIG. 4 shows a second circuit configuration of the thermal type flow sensor using the first detecting element. This circuit configuration is
This is a constant temperature drive circuit that keeps the temperature of the first diode 31 constant.

The anode end of the first diode 31 is connected to the cathode end of the second diode 32, and the cathode end is grounded. The anode end of the second diode 32 is
It is connected to the power supply Vcc via the resistor 68.

The cathode end of the second diode 32 is O
It is connected to the non-inverting input terminal of the P amplifier 69. The inverting input terminal of the OP amplifier 69 is connected to the voltage dividing point of the voltage dividing circuit 72 including the resistors 70 and 71. Voltage dividing circuit 72
The resistor 70 side is connected to the voltage source Va, and the resistor 71 side is grounded. The output terminal of the OP amplifier 69 is connected to the anode terminal of the second diode 32 via the resistor 73.

The first buffer circuit 74, the second buffer circuit 75, the third buffer circuit 76, and the fourth buffer circuit 77 are OP amplifiers whose output ends are feedback-connected to the non-inverting input terminal. Is formed by. The non-inverting input terminal of the OP amplifier of the first buffer circuit 74 is connected to the anodic end of the second diode 32. The non-inverting input terminal of the OP amplifier of the second buffer circuit 75 is O.
It is connected to the output terminal of the P amplifier 69. Further, the non-inverting input terminal of the OP amplifier of the third buffer circuit 76 is connected to the anodic end of the second diode 32. Furthermore,
The non-inverting input terminal of the OP amplifier of the fourth buffer circuit 77 is connected to the cathode terminal of the second diode 32.

The first subtraction circuit 78 is an OP amplifier 79.
And resistors 80, 81, 82, and 83. The resistors 80 and 81 are connected in series, and the resistor 8 of the series connection circuit is connected.
One side is grounded and the resistor 80 side is the second buffer circuit 7
5 is connected to the output terminal. The output terminal of the OP amplifier 79 is
While being fed back to the inverting input terminal through the resistor 82,
It is connected to the arithmetic circuit 84. The inverting input terminal of the OP amplifier 79 is connected to the output terminal of the first buffer circuit 74 via the resistor 83.

The second subtraction circuit 85 is an OP amplifier 86.
And resistors 87, 88, 89, 90. The resistors 87 and 88 are connected in series, and the resistor 8 of the series connection circuit is connected.
The 8 side is grounded and the resistor 87 side is the third buffer circuit 7
6 is connected to the output end. The output terminal of the OP amplifier 86 is
It is fed back to the inverting input terminal via the resistor 89,
It is connected to the arithmetic circuit 84. The inverting input terminal of the OP amplifier 86 is connected to the output terminal of the fourth buffer circuit 77 via the resistor 90.

From the output end of the arithmetic circuit 84, the fluid velocity v
A signal corresponding to f is output.

Next, the circuit operation of the second circuit configuration will be described.

The resistor 68 supplies a minute current in the forward direction of the first diode 31 and the second diode 32 to the extent that the first diode 31 and the second diode 32 do not generate heat. It is a resistor provided to give a voltage for flowing.

When a voltage is applied to the anodic end of the second diode 32, voltage differences Vd1 and Vd2 occur between the first diode 31 and the second diode 32. The resistance values of the resistors 70 and 71 are r70 and r71. The voltage at the voltage dividing point of the voltage dividing circuit 72 is Va · {r71 / (r70 +
r71)}.

The voltage V is applied to the inverting input terminal of the OP amplifier 69.
a · {r71 / (r70 + r71)}, and the voltage Vd1 is supplied to the non-inverting input terminal. The OP amplifier 69 is supplied with a current I from the output terminal of the OP amplifier 69 to the anodic terminal of the second diode 32 so that the voltages at the non-inverting input terminal and the inverting input terminal become equal. . The first diode 31 having a small heat capacity generates heat, and the voltage difference Vd1 across the first diode 31 decreases.
As a result, the voltages at the inverting input terminal and the non-inverting input terminal of the OP amplifier 69 become equal, and the current I supplied from the output terminal of the OP amplifier 69 becomes a steady state. Therefore, the voltage difference Vd1 across the first diode 31 becomes constant, and the temperature of the first diode 31 becomes constant.

The voltage at the anodic end of the second diode 32 is applied via the first buffer circuit 74 to the inverting input terminal of the OP amplifier 79 of the first subtraction circuit 78 via the resistor 83. Will be supplied as is. Further, the voltage at the output end of the OP amplifier 69 is directly supplied to the non-inverting input terminal of the OP amplifier 79 of the first subtraction circuit 78 via the resistor 80 via the second buffer circuit 75.

The voltage at the anode end of the second diode 32 is supplied via the third buffer circuit 76 to the non-inverting input of the OP amplifier 86 of the second subtraction circuit 85 via the resistor 87. It is directly supplied to the terminal. Further, the voltage at the cathode terminal of the second diode 32 is directly supplied to the inverting input terminal of the OP amplifier 86 via the resistor 90 via the fourth buffer circuit 77.

Resistances 80, 81, 8 of the first subtraction circuit 78
The resistance values of 2, 83 are r80, r81, r82,
r83. The resistance values of the resistors 80, 81, 82 and 83 are set so that r80 = r81 = r82 = r83. The resistance value of the resistor 73 is r73. At this time, the voltage at the output end of the first subtraction circuit 78 is I · r73
Becomes

Resistances 87, 88, 8 of the second subtraction circuit 85
The resistance values of 9, 90 are r87, r88, r89,
r90. The resistance values of the resistors 87, 88, 89 and 90 are set so that r87 = 2 · r88 and r89 = 2 · r90. At this time, the voltage at the output end of the second subtraction circuit 85 becomes (Vd1-Vd2). Note that (Vd1-
Vd2) is proportional to the temperature difference (Th-Tt) between the temperature Th of the first diode 31 and the temperature Tt of the second diode 32.

The arithmetic circuit 84 has a first diode 31.
Current proportional to the current I flowing through r73 and the temperature difference between the first diode 31 and the second diode 32 (Th-T
(Vd1-Vd2) proportional to t) is input.

Next, a method of obtaining the fluid flow velocity vf will be described.

Vd1 · I = (A + B · √vf) · (Th−Tt) From the relationship of Vd1 = Va · {r71 / (r70 + r71)}, the current I flowing in the first diode 31 is
It is obtained by the following equation. I = (A + B ・ √vf) ・ (Th-Tt) ・ (1 / V
a) · {(r70 + r71) / r70} Therefore, the arithmetic circuit 84 is proportional to the current I and the temperature difference (Th-Tt) between the first diode 31 and the second diode 32 in this equation (( Vd1-Vd2) is used as a parameter to calculate the flow velocity vf of the fluid, and the flow velocity vf of the fluid is calculated.
The signal corresponding to is output.

The detection element used in the thermal type flow sensor according to the present application can take various forms other than FIG. The second detection element will be described with reference to FIG. Note that the portions corresponding to those of the first detection element have the same numbers, and the description thereof will be omitted.

The substrate 30 is a rectangular plate made of silicon single crystal. A thin rectangular lower insulating layer 33 having a constant thickness is formed on a surface of the substrate 30 near one long side edge.
B and 33C are provided. The substrate 30 below the lower insulating layer 33C is provided with a quadrangular pyramid-shaped recessed portion 91 that extends from the front surface to the back surface. A space is formed in the substrate 30 by the recess 91.

The lower insulating layers 33B and 33C are arranged with a predetermined interval. The lower insulating layers 33B and 33C
Is formed of an insulating material such as silicon dioxide or silicon nitride.

In the vicinity of each peripheral edge of the lower insulating layer 33C, four openings 92 having an isosceles trapezoidal shape are provided to penetrate the front and back surfaces of the lower insulating layer 33C. The opening 92 is arranged such that the long sides of the parallel sides of the isosceles trapezoid are aligned with the center of the opening edge of the recess 91 and the short side is on the center side of the lower insulating layer 33C. Therefore, the lower insulating layer 33C has the locking portion 9
3, the diode forming portion 94, and the beam portion 95 are integrally formed. That is, the locking portion 93 is formed in the lower insulating layer 33.
It is a rectangular frame-shaped portion having a constant width formed on the outer peripheral edge of C. Further, the diode forming portion 94 is a rectangular portion that is slightly smaller than the lower insulating layer 33C disposed in the recess 91. Further, the beam portion 95 is formed from the four corners of the diode forming portion 94 formed between the opening portions 92 to the concave portion 9.
It is a thin plate-shaped portion extending toward the locking portion 93 at the opening edge of the first corner portion. Therefore, the lower insulating layer 33C forms a bridge structure by the locking portion 93, the diode forming portion 94, and the beam portion 95.

The first diode 31 is formed on the surface of the diode forming portion 94. Therefore, the first dio-
The board 31 is thermally insulated from the substrate 30.

The substrate 30, the first diode 31, the second diode 32, the locking portion 93, the diode forming portion 94, and the exposed portion of the beam portion 95 are uniformly covered. A thick upper insulating layer 40 is provided.

The surfaces of the p-type silicon conductive layer 36 and the n-type silicon conductive layer 37 are covered with electrode pads 42C and 42D.
The wiring electrode 43 for connecting with is formed so as to pass through the upper insulating layer 40 above the beam portion 95.

An outline of the method for manufacturing the second detection element will be described with reference to FIGS. 6 (a) to 6 (d). It should be noted that description of the same steps as those in the method of manufacturing the first detection element is omitted, and the same numbers are used.

Using the same steps as the method for manufacturing the first detecting element described in FIGS. 2A to 2H, the first diode is used.
Mode 31, second diode 32, and lower insulating layer 33A
And 33B, the upper insulating layer 40, the via hole 41,
The electrode pad 42 and the wiring electrode 43 are formed.

After that, the upper insulating layer 40 and the electrode pad 4
45 A of protective layers are formed so that 2 and the wiring electrode 43 may be covered. A protective layer 45C is formed on the back surface of the substrate 30. The protective layers 45A and 45C are thin layers formed of silicon dioxide, silicon nitride, or the like. The protective layer 4
5A and 45C are formed by means of plasma CVD or the like.

The substrate 30 in contact with the back surface of the lower insulating layer 33A
In order to expose the surface of the protective layer 45A and the upper insulating layer 40 and the lower insulating layer 33A, an opening 92 having an isosceles trapezoidal opening is formed. The opening 92 is formed using a means such as RIE. Next, the substrate 30 is chemically etched using an alkaline aqueous solution such as potassium hydroxide. Since the substrate 30 is anisotropically etched from the front surface toward the rear surface through the opening 92, the recess 91 is formed in the lower portion of the lower insulating layer 33C. After that, the protective films 45A and 45C are removed by using a method such as RIE.

The second detecting element is formed by the manufacturing process as described above.

The structure of the third detection element will be described with reference to FIG. The same parts as those of the second detecting element are designated by the same reference numerals, and the description thereof will be omitted.

The lower insulating layer 33B is a square-shaped first insulating layer 96A formed so as to overlap the surface of the substrate 30.
And the second insulating layer 97A. The first insulating layer 96A is made of silicon dioxide, and the second insulating layer 97A is made of silicon nitride.

The lower insulating layer 33D is the second insulating layer 97B.
Consists of The second insulating layer 97B is the upper insulating layer 40.
Is held away from the substrate 30. For this reason,
Second insulating layer 97B, substrate 30 and upper insulating layer 40
A space 98 is formed by being surrounded by. Similar to FIG. 5, four openings 92 are formed near each peripheral edge of the second insulating layer 97B. The opening 92 may have a rectangular shape or an isosceles trapezoidal shape.

Next, an outline of the method of manufacturing the third detecting element will be described. The method other than the lower insulating layer 33 and the space 98 is the same as the method for manufacturing the second detection element, and therefore the description is simplified.

The lower insulating layer 33 is composed of a first insulating layer 96 and a second insulating layer 97. The first insulating layer 96 and the second insulating layer 97 are laminated and formed on the surface of the substrate 30 by means of plasma CVD or the like.

After that, the first insulating layer 96A is formed on the surface of the substrate 30 by using the same steps as in the method of manufacturing the second detecting element.
And a second insulating layer 97A, a pair of lower insulating layers 33B, a first diode 31, a second diode 32, and an upper insulating layer on the surface of each lower insulating layer 33B. 40, via holes 41, electrode pads 42 and wiring electrodes 43 are formed. Further, the upper insulating layer 40
Then, the protective layer 45A is formed so as to cover the electrode pad 42 and the wiring electrode 43. A protective layer 45C is formed on the back surface of the substrate 30.

Next, in order to expose the surface of the first insulating layer 96A which constitutes the lower insulating layer 33B on which the first diode 31 is formed, the protective layer 45A, the upper insulating layer 40 and the second insulating layer 96A are exposed. An opening 92 is formed to penetrate the insulating layer 97A. The opening 92 is formed using a means such as RIE.

Then, hydrofluoric acid (HF) is passed through the opening 92.
Alternatively, buffer hydrofluoric acid (mixture of HF and NH4F) is reacted with the first insulating layer 96A. The first insulating layer 96A is removed by chemical etching, and a space 98 is formed in the portion where the first insulating layer 96A was present. After that, the protective films 45A and 45C are removed by using a method such as RIE.

The third detecting element is formed by the manufacturing process as described above.

The fourth detection element will be described with reference to FIG. The high concentration p formed on the surface layer of the substrate 30
A first diode 31 and a second diode 32 are formed on the surfaces of the mold regions 99A and 99B, respectively, and a part of the substrate 30 under the high concentration p-type region 99A is removed,
It is the same as the first detection element except that the back surface of the high-concentration p-type region 99A is exposed.
Descriptions other than 9B are omitted.

The substrate 30 is a rectangular plate made of silicon single crystal. A pair of high-concentration p-type regions 99A and 99B having a high carrier concentration of 1 × 10 ¹⁹ / cm ³ or more are provided on the surface of the substrate 30 near one long side edge. In addition,
The high-concentration p-type regions 99A and 99B are formed with a predetermined interval.

A recess 100 is provided in the substrate 30 below the high-concentration p-type region 99A, and the back surface of the high-concentration p-type region 99A is exposed. As a result, the high-concentration p-type region 99A and the substrate 30 form a diaphragm structure.

The first diode 31 is formed on the surface of the high concentration p-type region 99A. Therefore, the first dio-
The board 31 is thermally insulated from the substrate 30.

Next, with reference to FIGS. 9A to 9D, an outline of the method for manufacturing the fourth detection element will be described. It should be noted that description of the same steps as those in the method of manufacturing the first detection element is omitted, and the same numbers are used.

Boron ions are introduced into the surface layer of the substrate 30 as impurity ions, and the high concentration p-type regions 99A and 99 are formed.
B is formed. The boron ions are introduced into the substrate 30 by using a means such as ion implantation or thermal diffusion.

Next, n-type silicon single crystal layers 35A and 35B are formed on the surfaces of the high-concentration p-type regions 99A and 99B by epitaxial growth.

After that, the first diode 31, the second diode 32, the upper insulating layer 40, and the via hole 41 are formed by using the same steps as in the method of manufacturing the first detecting element. ,
The electrode pad 42 and the wiring electrode 43 are formed.

Next, the upper insulating layer 40 and the electrode pad 42.
Then, a thin protective layer 45A is formed so as to cover the wiring electrode 43. Further, a thin protective layer 45B is formed on the back surface of the substrate 30. An opening 46 is provided in the protective layer 45B facing the high concentration p-type region 99A, and the back surface of the high concentration p-type region 99A is exposed. Protective layers 45A and 45B
Are formed by means of plasma CVD or sputtering.

Next, the substrate 30 is chemically etched using an alkaline aqueous solution such as potassium hydroxide. Opening 4
The substrate 30 is anisotropically etched from the back surface to the surface direction through the groove 6, and the recess 100 is formed in the lower portion of the high-concentration p-type region 99A. After this, protective films 45A, 45B
Are removed using means such as RIE.

The fourth detection element is formed by the manufacturing process as described above.

Further, in the detection element used for the thermal type flow sensor, the structure for thermally insulating the first diode 31 and the substrate 30 is not limited to the above-mentioned structure, and for example, the n-type silicon forming the first diode 31 is used. The single crystal layer 35A may have a cantilever (cantilever) structure and a gap may be provided between the single crystal layer 35A and the substrate 30.

(Embodiment 2) A fifth detection element used in the thermal type flow sensor according to the present application will be described with reference to FIG.

The first diode 31 is composed of m basic diodes 31A, and the second diode 32 is composed of n basic diodes 32A. The basic diodes 31A and 32A have the same electrical characteristics.
Since the first detection element is the same as the first detection element except that the first diode 31 and the second diode 32 form a diode array, the same components as those of the first detection element use the same numbers. One diode 31 and second diode 32
Will be described.

On the surface of the lower insulating layer 33A, m n-type silicon single crystals 35A are formed. The p-type silicon conductive layer 3 is formed on the surface layer of each n-type silicon single crystal 35A.
6 and the n-type silicon conductive layer 37 are formed to form the basic diode 31A. m basic diodes 31
A is connected in series via the connection conductor layer 101 so that the forward directions are the same.

On the surface of the lower insulating layer 33B, n n-type silicon single crystals 35B are formed. The p-type silicon conductive layer 3 is formed on the surface layer of each n-type silicon single crystal 35B.
8 and the n-type silicon conductive layer 39 are formed to form the basic diode 32A. n basic diodes 32
A is connected in series via the connection conductor layer 101 so that the forward directions are the same.

Substrate 30, lower insulating layers 33A and 33B
On the exposed portions of the first diode 31, the second diode 32 and the connection conductor layer 101, a thin upper insulating layer 40 having a constant thickness is provided. A via hole 41 is formed to expose the surface of the p-type silicon conductive layer 36 in the basic diode 31A at one end T1 of the first diode 31. Further, a via hole 41 is formed to expose the surface of the n-type silicon conductive layer 37 in the basic diode 31A at the other end T2. The p-type silicon conductive layer 36 and the n-type silicon conductive layer 37 exposed by the via holes 41 are interposed through the wiring electrodes 43 integrally provided inside the via holes 41 and on the surface of the upper insulating layer 40. The surfaces of are connected to the electrode pads 42, respectively. Further, the p-type silicon conductive layer 38 in the basic diode 32A at the end T3 of the second diode 32 is
A via hole 41 is formed to expose the surface of the. N in the basic diode 32A of the other end T4
A via hole 41 is formed to expose the surface of the type silicon conductive layer 39. Each via hole 41
Of the p-type silicon conductive layer 38 and the surface of the n-type silicon conductive layer 39 exposed by the via hole 41 through the respective wiring electrodes 43 integrally provided on the inside and the surface of the upper insulating layer 40. It is connected to the pad 42.

The voltage across the first diode 31 and the second diode 32 is taken out via the electrode pad 42.

FIG. 11 shows a third circuit configuration of the thermal type flow sensor using the fifth detecting element. Since the circuit configuration is the same as that of FIG. 1 except that the voltage dividing circuit 66 is omitted, the description of the other configurations will be omitted. Note that the same numbers are used for the same components as the circuit configuration of FIG.

Voltage difference Vd across the first diode 31
1 is proportional to m times the barrier potential difference of the pn junction of the basic diode 31A. The voltage difference Vd2 across the second diode 32 is proportional to n times the barrier potential difference of the pn junction of the basic diode 32A. Also, the first diode 3
When 1 does not generate heat, basic diodes 31A and 32
Since the barrier potential difference of A is the same, the first diode 3
The ratio (Vd1 / Vd2) of the voltage difference Vd1 between both ends of 1 and the voltage difference Vd2 between both ends of the second diode 32 becomes equal to the ratio (n / m) of the number of connection stages of the diode array. Therefore,
The voltage at the output end of the first subtraction circuit 52 is Vd1, and the voltage at the output end of the second subtraction circuit 58 is Vd2, that is, {Vd.
1 · (n / m)}. Therefore, if the stage number ratio (n / m) of the diode is set to a predetermined value, the voltage dividing circuit 66 can be omitted. As a result, the first circuit configuration is simplified.

When the number of basic diodes 31A forming the first diode array 31 and the number of basic diodes 32A forming the second diode array 32 are the same, the circuit configuration of FIG. To use.

(Embodiment 3) A sixth detecting element used in the thermal type flow sensor according to the present application will be described with reference to FIG.

The area of the pn junction between the n-type silicon single crystal layer 35A of the first diode 31 and the p-type silicon conductive layer 36 is the same as that of the n-type silicon single crystal layer 35B of the second diode 32. It is formed to be k times the area of the pn junction with the p-type silicon conductive layer 38. Since the elements other than the area of the pn junction are the same as those of the first detection element, the same components as those of the first detection element are denoted by the same reference numerals and description thereof will be omitted.

FIG. 13 shows a fourth circuit configuration of the thermal type flow sensor using the sixth detecting element.

The anode end of the first diode 31 is connected to the power supply Vcc via the resistor 102, and the cathode end is grounded via the resistor 103. The flow velocity detection signal Vo is taken out from the cathode end of the first diode 31.

The anodic end of the second diode 32 is connected to the anodic end of the first diode 31, and the cathode end of the second diode 32 is a JFET (junction field effect).
It is connected to the source terminal S of the ctTransistor) 104.

The drain terminal D of the JFET 104 has a resistance of 1
It is grounded via 05 and is also connected to the inverting input terminal of the OP amplifier 106. The gate terminal G of the JFET 104 is connected to the output terminal of the OP amplifier 106 via the resistor 107. The non-inverting input terminal of the OP amplifier 106 is
It is connected to the cathode end of the first diode 31.

The first buffer circuit 108 and the second buffer circuit 109 are formed by OP amplifiers. The non-inverting input terminal of the OP amplifier of the first buffer circuit 108 is connected to the anode terminal of the second diode 32, and the output terminal is fed back to the inverting input terminal. The non-inverting input terminal of the OP amplifier of the second buffer circuit 109 is connected to the cathode terminal of the second diode 32, and the output terminal is fed back to the inverting input terminal.

The output terminal of the first buffer circuit 108 is
Voltage dividing circuit 1 in which a resistor 110 and a resistor 111 are connected in series
The resistor 12 is connected to the resistor 110 side. The output terminal of the second buffer circuit 109 is connected to the resistor 111 side of the voltage dividing circuit 112.

The voltage dividing point of the voltage dividing circuit 112 is connected to the non-inverting input terminal of the OP amplifier 113 which is a differential amplifier circuit. The inverting input terminal of the OP amplifier 113 is the first diode.
It is connected to the cathode end of the terminal 31. The output terminal of the OP amplifier 113 is connected to the anode terminal of the first diode 31.

Next, the circuit operation of the fourth circuit configuration will be described.

The resistor 102 is a resistor provided in the forward direction of the first diode 31 for applying a voltage for passing a minute current to the extent that the first diode 31 does not generate heat. When a voltage is applied to the anode end of the first diode 31, a voltage difference Vd is applied across the first diode 31.
1 results.

A voltage for equalizing the voltages of the non-inverting input terminal and the inverting input terminal of the OP amplifier 106 is generated at the output terminal of the OP amplifier 106, and the voltage is passed through the resistor 107 to the gate terminal of the JFET 104. Applied to G. As a result, the JFET 104 becomes conductive and the second diode 32 is turned on.
A current flows through the second diode 31, and a voltage difference Vd2 is generated across the second diode 31. The resistor 107 is the JFET 104.
It is provided in order to prevent an overvoltage from being applied to the gate terminal G of.

Through the first buffer circuit 108, the voltage at the anodic end of the second diode 32 is divided by the voltage dividing circuit 1
It is directly supplied to the 12 resistor 110 side. In addition, the second diode 3 is connected via the second buffer circuit 109.
The voltage at the cathode terminal of 2 is the resistance 111 of the voltage dividing circuit 112.
It is supplied to the side as it is. Assuming that the resistance values of the resistors 110 and 111 are r110 and r111, respectively, and the voltage at the anode end of the second diode 32 is V4, the OP amplifier 1
The voltage V5 of the non-inverting input terminal of 13 is calculated by the following equation.

V5 = V4-Vd2 + Vd2 · {r111 / (r110 + r111)} = V4-Vd2 · {r110 / (r110 + r111)} Further, the voltage V6 of the inverting input terminal of the OP amplifier 106 is
It is obtained by the following equation. V6 = V4-Vd1 As a result, the voltage V7 at the output end of the OP amplifier 113 is obtained by the following equation. V7 = V5-V6 = Vd1-Vd2. {R110 / (r110 + r111)} At this point, the first diode 31 hardly generates heat, so the voltage difference V across the first diode 31
The voltage difference Vd2 across the second diode 32 is substantially equal to d1. Therefore, the first diode 31 and the second diode 32 are output from the output terminal of the OP amplifier 113.
A current is supplied to the anode end of the. First diode 3
1 heats up and the voltage difference Vd1 across both ends decreases. As a result, the voltages at the non-inverting input terminal and the inverting input terminal of the OP amplifier 113 become equal, and the current supplied from the output terminal of the OP amplifier 113 becomes a steady state.

The resistance value of the resistor 105 is set to be k times the resistance value of the resistor 103. Therefore, the first diode 3
The current flowing through 1 is k times the current flowing through the second diode 32. That is, the pn of the first diode 31
The current densities of the forward currents flowing through the junction and the pn junction of the second diode 32 are equal. Therefore, the first diode 31 having a small heat capacity acts as a heating element, while the second diode 32 having a large heat capacity hardly generates heat. Further, the area of the pn junction of the first diode 31 is equal to the area of the pn junction of the second diode 32.
Since it is k times the area of the junction, the temperature difference (Th-Tt) between the temperature Th of the first diode 31 and the temperature Tt of the second diode 32 is accurately controlled.

Next, a method for obtaining the fluid flow velocity vf will be described. The current flowing through the first diode 31 is I,
The resistance value of the resistor 103 is r103.

Vo = r103 · I Vd1 · I = (A + B√vf) · (Th−Tt) Vd1 = Vd2 · {r110 / (r110 + r11)
1)} Vd1 = Vd0−γ · (Th−T0) Vd2 = Vd0−γ · (Tt−T0) The flow velocity detection signal Vo is obtained by the following equation. Vo = r103 (A + B√vf)  (Th-Tt) / Vd1 = r103 (A + B√vf)  (1 / γ)  (r111 / r110) From this equation, the flow velocity detection signal Vo is a parameter. As
The flow velocity vf of the fluid is obtained.

[0160]

The present invention having the above-mentioned structure has the following effects.

(1) Since the first diode of the heating element is thermally insulated from the substrate, it has a small heat capacity. Therefore, the responsiveness to changes in the flow velocity of the fluid becomes high.

(2) The voltage difference between the first diode of the heating element and the second diode of the temperature sensing element is detected, and the first diode and the second diode are sequentially detected. A current having a current density equal to the direction is applied. Since the current flowing through the first diode is almost proportional to the square root of the fluid, the current is less likely to be saturated than the conventional thermal type flow sensor. Therefore, even when the flow velocity of the fluid is high, the flow velocity detection accuracy is high, and the flow velocity of the fluid in a wide range can be accurately detected.

(3) Since the first diode of the heating element and the second diode of the temperature measuring element can be formed in the same step, the conventional silicon diode and the heating resistor are the same. The manufacturing cost is lower than that in the case where the semiconductor chip is formed inside the semiconductor chip.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first detection element of a thermal type flow sensor according to the present invention.

FIG. 2 is a schematic view of the manufacturing process of the first detection element of the thermal type flow sensor according to the present invention.

FIG. 3 is a diagram showing a first circuit configuration of a thermal type flow sensor according to the present invention.

FIG. 4 is a diagram showing a second circuit configuration of the thermal type flow sensor according to the present invention.

FIG. 5 is a diagram showing a second detection element of the thermal type flow sensor according to the present invention.

FIG. 6 is a schematic manufacturing process diagram of a second detection element of the thermal type flow sensor according to the present invention.

FIG. 7 is a diagram showing a third detection element of the thermal type flow sensor according to the present invention.

FIG. 8 is a diagram showing a fourth detection element of the thermal type flow sensor according to the present invention.

FIG. 9 is a schematic manufacturing process diagram of a fourth detection element of the thermal type flow sensor according to the present invention.

FIG. 10 is a diagram showing a fifth detection element of the thermal type flow sensor according to the present invention.

FIG. 11 is a diagram showing a third circuit configuration of the thermal type flow sensor according to the present invention.

FIG. 12 is a diagram showing a sixth detection element of the thermal type flow sensor according to the present invention.

FIG. 13 is a diagram showing a fourth circuit configuration of the thermal type flow sensor according to the present invention.

FIG. 14 is a diagram showing a detection element of a thermal type flow sensor according to a conventional invention.

FIG. 15 is a diagram showing a circuit configuration of a thermal type flow sensor according to a conventional invention.

FIG. 16 is a diagram showing an air volume sensing circuit according to a conventional invention.

[Explanation of symbols]

 31 First diode 32 Second diode 30 Substrate 33A, 33B Lower insulating layer 34 Space 35A, 35B n-type silicon single crystal layer 36, 38 p-type silicon conductive layer 37, 39 n-type silicon conductive layer 40 Upper Insulating Layer 41 Via Hole 42 Electrode Pad 43 Wiring Electrode 49 First Buffer Circuit 50 Second Buffer Circuit 51 Third Buffer Circuit 52 First Subtraction Circuit 58 Second Subtraction Circuit 66 Voltage Dividing Circuit 67 OP amplifier (differential amplifier circuit)

Claims (5)

[Claims] 1. A first diode, a second diode, and a substrate thermally coupled to the second diode, wherein a space is formed between the first diode and the substrate. Circuit means for electrically insulating the first diode and the substrate from each other, and for supplying a forward current having substantially the same current density to the pn junction of the first diode and the pn junction of the second diode. A thermal type flow sensor having: 2. The thermal type flow sensor according to claim 1, wherein at least one of the first diode and the second diode is a diode array in which a plurality of basic diodes are connected in series. 3. The thermal type flow sensor according to claim 1, wherein the area of the pn junction of the first diode is smaller than the area of the pn junction of the second diode. 4. A circuit means for controlling the forward currents of the first diode and the second diode so that the temperature difference between the first diode and the second diode is substantially constant, The thermal type flow sensor according to claim 1, 2 or 3, wherein an output substantially proportional to the forward current of the first diode or the second diode is taken out as a flow velocity signal. 5. A circuit means for controlling the forward current of the first diode and the second diode so that the temperature of the first diode is substantially constant, and the order of the first diode or the second diode. 4. The thermal type flow sensor according to claim 1, wherein the flow rate is calculated from a directional current and a voltage difference between the first diode and the second diode.