JP3379736B2 - Heat propagation time measurement type flow sensor and its manufacturing method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat propagation time measuring type flow sensor.

[0002]

2. Description of the Related Art Flow sensors for measuring heat transit time are widely used for biological functions such as respiratory function tests in physiology to automobiles, those utilizing the principle of heat pulse wire method, those utilizing silicon technology, etc. Various types of flow sensors have been developed and put into practical use.

FIG. 18 is a sectional view for explaining the principle of a flow sensor using a conventional heat pulse wire method.

In the heat pulse wire method, a turbulent flow plate 72 made of wire mesh, a heating wire 73 made of thin metal wire, and a detection wire 74 are placed in the flow tube 71 in the order of the flow direction of the fluid, and the heating circuit 7 is provided.
A pulse current is applied to the heating wire 73 from 5 to instantly heat the heating wire 73 to heat the fluid passing therethrough, until the heated detection fluid 74 and the control circuit 76 detect the heated fluid. This is a method of obtaining the flow velocity from the time. Now, the flow rate of the fluid is Q (l / s), and the cross-sectional area of the flow tube 71 is S
(M ² ), the average flow velocity v is v = Q / S (m /
s). The distance between the heating wire plate 73 and the detection wire 74 is x
(M), where T is the heat propagation time, T = S · X / Q (1) In this measuring method, when the range of the flow rate Q is very wide, it becomes difficult to set the heating time period (pulse current period). That is, if the heating time period is lengthened, the resolution of the flow rate detection in the high flow rate region becomes poor, and if the heating time period is shortened, it becomes difficult to determine the correspondence between the signal detected at the downstream side and the heating pulse, and accurate time detection is possible. There is a problem that you cannot do it.

In order to solve this problem, a sing-around method has been proposed in which the next heating is performed with a delay time that is an integral multiple of the heat propagation time. Moreover, since the thermal conditions at the time of heating and at the time of detection differ depending on the gas composition and flow rate, an error occurs due to changes in the amplitude and waveform of the detected signal. Temperature compensation is performed to reduce this error. As shown in FIG. 19, the heat transfer type flow sensor provided with temperature compensation has a turbulent flow plate 72 having a double structure of wire mesh and gauze on the upstream side of a flow tube 71 made of acrylic resin, and is heated on the downstream side thereof. Line 7
3 is placed, and the detection line 74 and the temperature compensation line 75 are placed adjacent to each other on the downstream side.

FIG. 20 is a partially cutaway perspective view of an example of a conventional flow sensor using silicon.

In this sensor, the portions corresponding to the heating wire 73 and the detection wire 74 are made of silicon. The silicon substrate 81 is anodized to make the surface porous to a depth of 20 μm. Next, this is oxidized to form a porous silicon oxide film 82. A heating element 83 and a detection element 84 are formed on this with thin film resistors, and lead wires 83a, 83 of the metal thin film are formed.
Install b, 84a, 84b. Next, etching is performed from the back surface to remove the silicon below the heating element 83 (below the region shown by the broken line 85 in FIG. 20), and the porous silicon oxide film 8 is formed.
The thermal insulation structure is composed of 2 and the thin film resistor on it. This is installed in a stainless pipe to make a flow sensor. The heating element 83 and the detection element 84 are manufactured by silicon micromachine technology.

[0008]

The flow sensor in which the heating wire and the detection wire are made of thin metal wires must be dealt with by adding a complicated circuit in order to measure a wide range of flow velocity. Since it has a structure in which a thin metal wire is stretched in the tube, there are problems that mass production, cost reduction and miniaturization are difficult. On the other hand, a flow sensor using silicon is capable of integrating a heating element and a detection element, has a fast response, has a flat surface and does not disturb the flow of fluid, and can be used for liquids as well. Although there are many advantages such as cost reduction, there is a problem that the heat capacity of the detection element changes due to the contamination by the fluid, and the sensitivity changes.

An object of the present invention is to provide a heat propagation time measuring type flow sensor which has high sensitivity, can measure a wide range of flow velocity, has high resistance to dirt, is low in cost, and is highly producible, and a manufacturing method thereof. .

[0010]

According to the present invention, a silicon substrate of one conductivity type provided in a flow tube through which a fluid flows, an insulating film provided on the upper surface of the silicon substrate, and a space provided on the insulating film. In parallel, the heating element and the temperature detecting element formed of a thin film resistance film in order from the upstream side to the downstream side of the fluid, and the silicon substrate under the micro-heater portion of the heating element are selectively etched from the back surface. In a heat propagation time measuring type flow sensor having a recess for exposing the insulating film, a reference temperature detecting element is provided on the upstream side of the heating element, and the reference temperature detecting element, the heating element and the temperature detecting element The surface is covered with a silicon oxide film,
Under the resistance temperature detector of the reference temperature detection element, the micro-heater portion of the heating element, and the resistance temperature detector of the temperature detection element, opposite conductivity type regions are provided as soaking plates, respectively, and the opposite side is provided on the silicon substrate. It is characterized in that a recess is formed to expose the conductive type region and the insulating film in the periphery thereof.

The present invention is characterized in that the insulating film is composed of a double film of a silicon oxide film and a silicon nitride film.

The present invention is characterized in that at least two temperature detecting elements are provided at intervals.

The present invention is characterized in that the thin film resistance film is composed of a double film of a titanium film and a platinum film.

In the present invention, a cavity is formed between the reference temperature detecting element, the heating element, and the temperature detecting element, and the reference temperature detecting element, the heating element, and the temperature detecting element are supported on the silicon substrate in a double-supported beam form. Characterized by being supported.

According to the present invention, a cavity is formed between the reference temperature detecting element, the heating element and the temperature detecting element, and the reference temperature detecting element, the heating element and the temperature detecting element are cantilevered on the silicon substrate. Characterized by being supported.

In the present invention, a DC power source is connected between one terminal of the reference temperature detecting element and one terminal of the temperature detecting element, and the resistance value of the temperature detecting element is substantially equal to the DC power source in parallel with the power source. A first resistor having a resistance value, a variable resistor, and a second resistor having a resistance value slightly smaller than the resistance value of the reference temperature detecting element are connected in series to form a bridge circuit,
A first output terminal is taken out from a connection point between the first resistor and the variable resistor, and a second output terminal commonly connected to the other terminal of the reference temperature detecting element and the other terminal of the temperature detecting element is taken out, An output circuit is provided, which uses a voltage output between the first output terminal and the second output terminal as a measurement signal source.

According to the present invention, a DC power source is connected between two terminals of the temperature detecting element, and a first resistor having a resistance value substantially equal to the resistance value of the temperature detecting element is connected in parallel to the power source. A bridge circuit is configured by connecting a second resistor and a third resistor in series, and a first output terminal is taken out from a connection point between the second resistor and the third resistor, and the first resistor and the temperature detecting element. A second output terminal is taken out from a connection point with one terminal, and an output circuit is provided which uses a voltage output between the first output terminal and the second output terminal as a measurement signal source. To do.

According to the present invention, the reference temperature detecting element, the heating element and the temperature detecting element are supported on the silicon substrate in a double-supported beam form, and the two supporting portions are supported on the wall of the flow tube.
The reference temperature detecting element, the heating element, and the temperature detecting element are located in the flow tube.

According to the present invention, the reference temperature detecting element, the heating element and the temperature detecting element are supported on the silicon substrate in a cantilever manner, and the supporting portion is supported on the tube wall of the flow tube. And a heating element and a temperature detecting element are located in the flow tube.

The invention is characterized in that the flow tube is made of plastic or glass.

The present invention comprises a step of selectively diffusing an impurity of opposite conductivity type into a silicon substrate of one conductivity type at a high concentration to selectively form a region of opposite conductivity type as a soaking plate, and a first surface on the silicon substrate. the SiO ₂ film is provided in a step of providing a the Si ₃ N ₄ film thereon, a mask of photoresist and photoresist is patterned and deposited on the the Si ₃ N ₄ film formed, deposited Ti and Pt And forming a thin film resistance film composed of a double film of Ti and Pt, and dissolving and removing the mask of the photoresist to form a reference temperature detecting element, a heating element and a temperature detecting element of the double film of Ti and Pt. And a second SiO ₂ film is formed on both surfaces of the silicon substrate including the reference temperature detecting element, the heating element, and the temperature detecting element, and a photoresist mask is provided on the second SiO ₂ film to form the reference temperature detecting element and the heating element. And temperature detection Open windows and the connection pad exposed region for the child to the opposite conductivity type region exposed region where the steps of the second SiO ₂ film is etched off of the window portion, said opposite the silicon substrate is anisotropically etched And a step of exposing the conductive type region and the first SiO ₂ film in the periphery thereof.

The present invention comprises: (A) a step of selectively diffusing an impurity of opposite conductivity type in a silicon substrate of one conductivity type in a high concentration to form a counter conductivity type region as a soaking plate . A first SiO ₂ film is provided on the surface of the silicon substrate, and Si ₃ is formed on the first SiO ₂ film.
A step of providing the N ₄ film, the Si ₃ N ₄ and the photoresist is deposited patterned on the film a mask of the photoresist provided, a thin film resistor formed by depositing Ti and Pt from bilayer of Ti and Pt A step of forming a film, a step of dissolving and removing the mask of the photoresist to form a reference temperature detecting element, a heating element and a temperature detecting element of the double film of Ti and Pt, the reference temperature detecting element and the heating element And a second SiO ₂ film is formed on both sides of the silicon substrate including the temperature detecting element,
The mask of photoresist provided thereon, open the window connection pad exposed regions for the heating element and the temperature sensing element and the reference temperature sensing element and to said opposite conductivity type region exposed scheduled region, a second SiO ₂ of the window portion A step of etching away the film and a step of anisotropically etching the silicon substrate to expose the opposite conductivity type region and the first SiO ₂ film in the periphery thereof, and a set of reference temperature detection A process for manufacturing a wafer equipped with a flow sensor element, in which a plurality of flow sensor elements including one element, a heating element, and a temperature detection element as a flow sensor element are arranged and formed on one silicon substrate, (B) a glass or plastic plate A position corresponding to the reference temperature detecting element, the heating element and the temperature detecting element so that the gas flows on the one surface of the reference temperature detecting element, the heating element and the temperature detecting element. A step of forming a concave portion, forming an opening at a position corresponding to each connection pad of the reference temperature detecting element, the heating element and the temperature detecting element, and a step of forming a flat plate for glass or plastic on the one surface of the flat plate. A step of forming a flat plate for a lower flow pipe, in which a concave portion is formed at a position corresponding to the reference temperature detecting element, the heating element and the temperature detecting element so that gas flows under the reference temperature detecting element, the heating element and the temperature detecting element, and And (C) aligning the upper surface of the flow sensor element and the recess of the upper flow tube flat plate so that the upper surface of the flow sensor flat plate faces the upper surface of the flow sensor element mounting wafer. On the lower surface of the flow sensor element mounting wafer by aligning the lower surface of the flow sensor element and the recess of the flat plate for the lower flow tube so as to face each other. Laminating a tube flat plate and laminating the wafer with the flow sensor element, the flat plate for the upper flow tube, and the flat plate for the lower flow tube, (D) The wafer with the flow sensor element mounted, the flat plate for the upper flow tube, and the lower flow tube And a cutting step of cutting the flat plate into individual flow sensors with flow tubes.

The present invention is characterized in that the recess has a semicircular or rectangular cross section.

[0024]

In the present invention, since the reference temperature detecting element is provided on the upstream side of the heating element, a zero point calibration when the flow rate is zero can be performed by forming a bridge circuit between the reference temperature detecting element and the temperature detecting element on the downstream side of the heating element. It becomes possible and the reliability of measurement is improved. By coating the surfaces of the reference temperature detecting element, the heating element, and the temperature detecting element with a silicon oxide film, it is possible to prevent the reference temperature detecting element, the heating element, and the temperature detecting element from being contaminated by a fluid, and the measurement reliability is improved. Further improve. Areas of opposite conductivity type as soaking plates provided under the micro-heater part of the heating element respectively prevent local heat generation due to current concentration in the micro-heater part, prevent heater breakage, improve reliability and have a long service life. To do. By thermally etching the silicon substrate from below to expose the opposite conductivity type region and the insulating film around it, a thermal insulating structure is realized and sensitivity is improved.

When the Si ₃ N ₄ film is directly formed on the silicon substrate, cracks may occur in the Si ₃ N ₄ film.
_The occurrence of this crack can be suppressed by sandwiching the _two films between them. In general, SiO ₂ films against Si has a compressive stress, because with the Si ₃ N ₄ film is tensile stress, SiO ₂
The occurrence of cracks can be suppressed by controlling the film thickness with a double film structure of —Si ₃ N ₄ . Furthermore, Si
The O ₂ film alone tends to cause cracks in the corners of the diaphragm, but the SiO ₂ —Si ₃ N ₄ double film structure can prevent the cracks from occurring.
For these reasons, the SiO ₂ —Si ₃ N ₄ double film structure is adopted.

The measurable range of the flow velocity depends on the distance x between the heating element and the temperature detecting element. Therefore, if temperature detecting elements having various values of x are prepared, it will be different depending on the magnitude of the flow velocity. The measurable range can be widened by selecting whether to use the temperature detecting element. Therefore, at least two temperature detecting elements are provided at intervals.

When the thin film resistance film is composed of a double film of a titanium film and a platinum film coated thereon, platinum is an inert metal, so that it is prevented from being contaminated by a fluid and becomes a bonding pad for connecting fine metal wires. It is convenient.

When a cavity is formed between the reference temperature detecting element, the heating element and the temperature detecting element, the reference temperature detecting element, the heating element and the temperature detecting element are thermally insulated and separated,
The heat capacity is reduced and the sensitivity is improved. Since the elements are separated from each other, a structure in which the reference temperature detecting element, the heating element, and the temperature detecting element are supported on the silicon substrate in a double-supported beam form is used.

If the structure is supported by the cantilever type instead of the double-supported beam type, the thermal insulation separation is further promoted, the heat capacity is further reduced, and the sensitivity is further improved.

If a DC power source and a bridge circuit consisting of a first resistor, a variable resistor and a second resistor are connected to the reference temperature detecting element and the temperature detecting element, zero point calibration becomes possible when the flow rate is zero. , The reliability of measurement is improved. That is, when the variable resistance is adjusted to zero the voltage output between the first output terminal and the second output terminal when the flow rate is zero,
When the fluid flows, a voltage is output between the first output terminal and the second output terminal, and this voltage serves as a signal source for measuring the flow velocity or flow rate of the fluid.

In the measurement signal output circuit, a DC power supply is connected between the two terminals of the temperature detection element, and a first resistance having a resistance value substantially equal to the resistance value of the temperature detection element is connected in parallel with the power supply. A bridge circuit is formed by connecting the second resistor and the third resistor in series, and the first output terminal is taken out from the connection point of the second resistor and the third resistor, and the first resistor and one terminal of the temperature detecting element are connected. It can also be calibrated by taking out the second output terminal from the connection point. As described above, the voltage output between the first output terminal and the second output terminal serves as the measurement signal source.

The heat transfer time measuring type flow sensor is used by being installed in a flow tube through which a gas flows. In this case, the flow sensor element is formed in a double-supported beam type, and two supporting portions of the double-supported beam are connected to the flow tube. Stable measurement can be performed by supporting it on the tube wall.

Instead of the double-supported beam type as described above, the cantilever type may be used so that the support portion of the cantilever beam is supported on the tube wall of the flow tube.

The flow tube is made of plastic or glass.

According to the method of the present invention, a SiO ₂ film and a Si ₃ film are formed.
A heat propagation time measuring flow in which a reference temperature detecting element, a heating element and a temperature detecting element of a double film of Ti and Pt are formed on a double film of N ₄ film and are thermally insulated from a silicon substrate to improve sensitivity. The sensor can be manufactured.

If a plurality of flow sensor elements are arranged and formed on a silicon substrate and the upper flow tube flat plate and the lower flow tube flat plate are attached to each other and then separated into individual flow sensor chips, the flow sensor The heat propagation time measuring type flow sensor can be manufactured with high yield and a small number of steps without damaging the element.

The cross section of the recess of the flow tube may be either semicircular or rectangular. The point is that the gas should flow smoothly.

[0038]

1 is a plan view and a sectional view of a first embodiment of the present invention.

SiO is formed on the upper surface of the N-type silicon substrate 1.₂film
4, Si₃N_FourA membrane 5 is provided. Double layer of Ti film and Pt film
Reference temperature detecting element 6 and heating element
7. The temperature detecting elements 8 are formed in parallel with a space. each
SiO on the surfaces other than the connection pads 17 of the elements 6, 7 and 8₂
The membrane 9 is coated. Resistance temperature detector 6 of the reference temperature detection element 6
a, micro-heater portion 7a of heating element 7, temperature detection element
P as a soaking plate under the resistance temperature detector 8a of 8⁺⁺Mold area 3
Set up. Selectively etch silicon substrate 1 from the back ⁺⁺
SiO in the mold region 3 and its surroundings₂The membrane 4 is exposed. this
Temperature measuring resistor 6a, my 4 roh heater 7a, temperature measuring resistance
The antibody 8a has a heat insulating structure, and the sensitivity is improved. P⁺⁺Type
Area 3 prevents local heat generation due to current concentration,
It is provided to prevent disconnection of the minutes. Also, Si₃N_Fourfilm
When 5 is directly formed on the silicon substrate 1, Si is formed.₃N_FourOn membrane 5
Since cracks may occur, it is necessary to prevent the cracks from occurring.
For SiO₂The membrane 4 is sandwiched between them. In general, S with respect to Si
iO₂The film has compressive stress ₃N_FourThe membrane exerts tensile stress
I have, so SiO₂-Si₃N_FourThe double-layer structure of the membrane
By controlling the thickness, cracking can be suppressed.
It Furthermore, SiO₂The diaphragm alone
Although cracks tend to occur at the corners of the
O₂-Si₃N_FourIf a double film structure is used, this crack will occur
Can also be prevented. For these reasons SiO₂-Si
₃N_FourIt has a double film structure.

Next, the manufacturing method of the first embodiment will be described.

2A to 2D are sectional views showing the manufacturing method of the embodiment of FIG. 1 in the order of steps for explaining the manufacturing method.

As shown in FIG. 2A, the crystal orientation [10
0] on the lower surface of the N-type silicon substrate 1 with a SiO ₂ film 2 of about 7
The thickness is 50 nm, and the SiO ₂ film 11 is provided on the upper surface. A window is opened in the SiO ₂ film 11 by the photolithography technique, and P type impurities are introduced at a high concentration to form a P ⁺⁺ type region 3.

As shown in FIG. 2B, the SiO ₂ film 11 is formed.
Is removed, and a new SiO ₂ film 4 is formed to a thickness of about 500 nm, and a Si ₃ N ₄ film 5 is formed thereon to a thickness of about 80 nm. A photoresist mask 12 on the Si ₃ N ₄ film 5
Is provided, and a window is opened in a region where the reference temperature detecting element, the heating element, and the temperature detecting element are formed. Ti is deposited from above to form a Ti film 13 having a thickness of about 30 nm, and Pt is deposited thereon to provide a Pt film 14 having a thickness of about 200 nm. When the thin film resistance film is composed of a double film of the Ti film 13 and the Pt film 14 coated on the Ti film 13, the Pt film 14 is an inert metal film, so that it is prevented from being contaminated by a fluid and also serves as a bonding pad for connecting fine metal wires. , Convenient.

As shown in FIG. 2C, the mask 12 is removed together with the Ti film 13 and the Pt film 14 on the mask 12. The remaining portion becomes the reference temperature detecting element 6, the heating element 7, and the temperature detecting element 8 in FIG.

As shown in FIG. 2D, SiO _{2 is} formed on the surface.
A film 9 is formed, a photoresist mask 15 is provided on the film 9, and a photoresist mask 1 is formed on the SiO ₂ film 2 on the back surface.
6 is provided. A window is opened in the area of the mask 15 where the connection pad is to be formed, and a window is opened in the area of the mask 16 where the P ^{+ +} type area is exposed.

As shown in FIG. 2E, the SiO ₂ film 9 in the window 16 is selectively removed by etching to remove the Pt film 14.
Exposed to form the connection pad 17 and at the same time SiO
_{2 The} film 2 is selectively removed. Then, the photoresist masks 15 and 16 are removed.

As shown in FIG. 2E, the silicon substrate 1 is anisotropically etched with hydrazine or TMAH (tetramethylammonium hydroxide) to expose the P ⁺⁺ -type region 3 and the SiO ₂ film 4 around it. Let As a result, the flow sensor shown in FIG. 1 is manufactured.

Next, a method of using the first embodiment will be described.

FIG. 3 is a perspective view for explaining a fluid measuring method using the embodiment shown in FIG.

The pipe 21 is a pipe through which a fluid such as gas flows, and the fluid flows in the direction of the arrow 22 therein. The flow sensor of the first embodiment is installed in this tube 21. A heating power source 23 is provided on the connection pads 17 at both ends of the heating element 7 of the flow sensor.
And pulse current is applied. The pulse current is, for example, as shown in the figure, voltage Va = 2V, offset voltage V
_offset = 1V, duty = 20%, frequency f = 10k
A pulse current of Hz is applied. The power supply 24 is connected to the connection pad 17 on one side of each of the reference temperature detecting element 6 and the temperature detecting element 8.
Are connected and a voltage V is applied. A first resistor R ₁ , a variable resistor Ra, and a second resistor R ₂ are connected in series at both ends of the power source 24,
The terminal 25 is taken from the connection point between the first resistor R ₁ and the variable resistor Ra, the other connection pad 17 of each of the reference temperature detection element 6 and the temperature detection element 8 is connected, and the terminal 26 is taken. Terminal 2
The voltage appearing between 5 and the terminal 26 is Vout. The resistance value of the first resistor R ₁ is substantially equal to the resistance value of the temperature detecting element 8 (1 kΩ), and the resistance value of the second resistor R ₂ is slightly smaller than the resistance value of the reference temperature detecting element 6. (1
kΩ) to form a bridge circuit. First, the variable resistor Ra is adjusted without applying a pulse current to the heating element 7, and the bridge circuit is balanced (voltage Vout = 0). Next, when a pulse current is applied to heat the heating element 7 to heat the fluid, the balance of the bridge circuit is lost and the voltage V
out is output. This voltage is calculated by an arithmetic circuit (not shown)
Then, the average flow velocity v (m / s) is calculated.

Now, let the flow rate of the fluid flowing through the pipe 21 be Q (m /
s) and the cross-sectional area of the pipe 21 is S (m ² ), the average flow velocity v (m / s) is represented by v = Q / S (2) Assuming that the distance between the upstream heating element 7 and the downstream temperature detection element 8 is x (m), the heat propagation time T
(S) is expressed by T = S · x / Q (s) (3). That is, basically, by changing the distance between the heating element 7 and the temperature detecting element 8, it is possible to measure in a wide range from a low flow velocity to a high flow velocity.

FIG. 4 is a correlation diagram showing the relationship between the average flow velocity and the heat propagation time measured in the embodiment shown in FIG.

The horizontal axis represents the average flow velocity v and the reciprocal of the average flow velocity v, and the vertical axis represents the heat propagation time T, and the calculated value and the measured value are compared. From the equations (2) and (3), the calculated value is T = S · x / Q = S · x / v · S = x / v (4), and x = 2 × 10 ⁻³ (m) I asked. As shown in FIG. 4, the measured value is proportional to the heat propagation time T until the reciprocal of the average flow velocity v is 20, but is saturated when the reciprocal of the average flow velocity v is 20 or more and is not proportional to the heat propagation time T. That is, x = 2 × 10
The measurable range when set to ^-3 (m) is the average flow velocity v =
It shows that the measurement cannot be performed at a flow velocity lower than 0.05 (m / s). When it is desired to measure a fluid having a flow velocity smaller than 0.05 (m / s), it is necessary to change the distance x between the heating element 7 and the temperature detection element 8. Also,
The calculated value is different from the measured value, but this is due to the time constant (about 9 m) of the micro-heater portion 7a of the heating element 7, the reference temperature detecting element 6 and the temperature measuring resistors 6a and 8a of the temperature detecting element 8.
s). Since the calculated value and the actually measured value are shifted in parallel, it is possible to make a correction, and it is possible to obtain a highly reliable measured value by adding a correction term.

FIG. 5 is a front view and a sectional view of the second embodiment of the present invention.

Similar to the first embodiment, the N-type silicon substrate
SiO on the upper surface of 31₂Membrane, Si₃N _FourProvide a membrane and above it
The reference temperature is a thin resistance film consisting of a double film of Ti film and Pt film.
Degree detection element 32, heating element 33, a plurality of temperature detection elements 3
4, 35, 36 and 37 are formed in parallel with a space.
Providing a plurality of temperature detecting elements will be described with reference to FIGS. 3 and 4.
As described above, the measurable range of the flow velocity is the heating element and the temperature sensor.
The temperature of various values of x depends on the distance x to the child.
Create a detection element and select which temperature detection element to use according to the magnitude of the flow velocity.
The measurable range by selecting whether to use the child
This is to widen it.

The surfaces of the elements 32 to 37 are coated with a SiO ₂ film, the resistance temperature detector 32a of the reference temperature detecting element 32,
Providing a P ⁺⁺ type region (not shown) as a soaking plate below the micro-heater portion 33a of the heating element 33 and the resistance temperature detectors 34a to 37a of the temperature detection elements 34 to 37 is the same as the first embodiment. It is the same. Next, the silicon substrate 31 is selectively etched to form the cavity 38 between the elements 32 to 37, and at the same time, the silicon portion 31b below the resistance temperature detectors 32a to 37a.
Is removed to expose a soaking plate (not shown) in the P ⁺⁺ type region to thermally insulate the elements 32 to 37. When the cavity 38 is formed in this manner, the reference temperature detecting element, the heating element, and the temperature detecting element are thermally insulated from each other, the heat capacity is reduced, and the sensitivity is improved. Since the elements 32 to 37 are separated from each other, a double-supported beam support structure in which both ends are supported by the frame 31a of the silicon substrate 31 to support the elements 32 to 37 is formed. The difference from the first embodiment is that a cavity portion 38 is provided between the elements 32 to 37 and a double-supported beam support structure is provided. Next, the flow tube 40 is formed by plastic molding. Plastic molding can be performed after cutting it into chips, but it takes man-hours and may damage the element. It is better to mold it and then cut it into individual chips. The flow tube 40 is formed such that the resistance temperature detectors 32a to 37a are located at the center thereof and the connection pads 32b to 37b are located outside the flow tube 40. The fluid flows in the flow tube 40 in the direction of the arrow, and the flow velocity is measured.

Next, the manufacturing method of the second embodiment will be described.

FIG. 6 is a sectional view taken along the line CC ′ in the order of steps for explaining the manufacturing method of the embodiment shown in FIG. 5, and FIG. 7 is an order of steps for explaining the manufacturing method of the embodiment of FIG. It is the DD 'line sectional view shown.

[0059] FIG. 6 (a), the as shown in FIG. 7 (a), the SiO ₂ films 41 and 42 provided on the upper surface and the lower surface of the N-type silicon substrate 31 of the crystal orientation [100], SiO ₂ by photolithography A window is opened in the film 41, and a P-type impurity is introduced at a high concentration to form a P ^++- type region 43.

As shown in FIGS. 6 (b) and 7 (b), S
The SiO ₂ films 41 and 42 are removed and a SiO ₂ film 44 is newly formed on the lower surface of the silicon substrate 31 by the CVD method to a thickness of about 750 nm.
, A SiO ₂ film 45 having a thickness of about 500 nm on the upper surface, and a Si ₃ N ₄ film 46 having a thickness of about 80 nm thereon,
An SiO ₂ film 47 is formed on the SiO ₂ film to a thickness of about 500 nm. A photoresist mask 48 is provided on the SiO ₂ film 44, and a photoresist mask 49 is provided on the SiO ₂ film 47.
To provide. Then, a window is opened in the mask 49 in the region where the cavity 38 (see FIG. 5) is formed.

[0061] FIG. 6 (c), the as shown in FIG. 7 (c), d
After the SiO ₂ film 47 is selectively removed by etching, the mask 49 is removed. Next, the Si ₃ N ₄ film 46 is selectively etched using the SiO ₂ film 47 as a mask. Next, Si
_The SiO ₂ films 45 and 47 are selectively etched using the ₃ N ₄ film 46 as a mask. The mask 48 is removed, and a new photoresist mask 50 is provided on the lower surface of the silicon substrate 31,
A window is opened in the mask 50 in the region where the cavity 38 is formed, and S
The iO ₂ film 44 is selectively removed. After that, the mask 50 is removed.

As shown in FIGS. 6D and 7D, a photoresist mask 51 is provided on the upper surface of the silicon substrate 31, and a reference temperature detecting element 32, a heating element 33, and a plurality of temperature detecting elements 34, 35 are provided. , 36, 37 are formed in the mask 51 in the area. Ti and Pt are sequentially deposited from above to form a Ti film having a thickness of about 30 nm and Pt having a thickness of about 200 nm.
A thin film resistance film 52 made of a double film is provided.

As shown in FIGS. 6 (e) and 7 (e), the mask 51 is removed together with the Ti-Pt film 52 thereon.
The remaining portion becomes the reference temperature detecting element 32, the heating element 33, and the temperature detecting elements 34 to 37 in FIG. SiO _{2 on the} surface
A film 53 is formed, a photoresist mask 54 is provided on the film 53, and a photoresist mask 55 is provided on the SiO ₂ film 44 on the back surface. A window is opened in the area where the connection pad is to be formed in the mask 54 and the area where the cavity 38 is to be formed.
⁺⁺ Type area Open a window in the area to be exposed.

[0064] FIG. 6 (f), the as shown in FIG. 7 (f), d
The exposed SiO ₂ film 53 is selectively removed by etching to expose the Pt film surface of the Ti—Pt film 52, and the connection pad 56 is formed. Then, the photoresist masks 54 and 55 are removed.

As shown in FIGS. 6 (g) and 7 (g), the silicon substrate 1 is anisotropically etched with hydrazine or TMAH (tetramethylammonium hydroxide) to remove the P ⁺⁺ -type region 43 and its surroundings. The SiO ₂ film 45 is exposed. As a result, the flow sensor shown in FIG. 5 is manufactured.

FIG. 8 is a front view and a sectional view of a third embodiment of the present invention.

Similar to the second embodiment, an N-type silicon substrate
SiO on the upper surface of 61₂Membrane, Si₃N _FourProvide a membrane and above it
The reference temperature is a thin resistance film consisting of a double film of Ti film and Pt film.
Temperature detection element 62, heating element 63, a plurality of temperature detection elements 6
4, 65 are formed in parallel with each other. Multiple temperature detection
The output element is provided as described in FIGS. 3 and 4.
The fast measurable range is the distance between the heating element and the temperature sensing element.
Since it depends on the distance x, it is possible to make temperature detecting elements of various values of x.
Which temperature sensing element to use depending on the magnitude of the flow velocity
To expand the measurable range by selecting
is there. In the third embodiment, two temperature detecting elements 64 and 65 are used.
Although only provided, there are four as in the second embodiment.
Of course, more may be provided.

The surfaces of the elements 62 to 65 are coated with a SiO ₂ film, the resistance temperature detector 62a of the reference temperature detecting element 62,
Providing a P ⁺⁺ type region (not shown) as a soaking plate under the micro-heater portion 63a of the heating element 63 and the resistance temperature detectors 64a, 65a of the temperature detection elements 64, 65 is different from the second embodiment. It is the same. Next, the silicon substrate 61 is selectively etched to form the cavity 68 between the elements 62 to 65, and at the same time, the silicon portion 61b below the resistance temperature detectors 62a to 65a.
Is removed to expose a soaking plate (not shown) in the P ^{+ +} type region to thermally insulate the elements 62 to 65. The elements 62 to 65 have a cantilever structure in which one side is supported by the silicon substrate 61. This point is different from the second embodiment. If the structure is supported in a cantilever form, the thermal insulation separation further proceeds, the heat capacity further decreases, and the sensitivity further improves. Next, the flow tube 6 is formed by plastic molding.
9 is formed. Plastic molding can be performed after cutting it into chips, but it takes man-hours and may damage the element. It is better to mold it and then cut it into individual chips. The flow tube 69 has a resistance temperature detector 6 at its center.
2a to 65a are positioned, and the connection pad 62
It is formed so that b to 65b are located outside the flow tube 69. The manufacturing method of the third embodiment is almost the same as that of the second embodiment.

FIG. 9 is a perspective view for explaining a fluid measuring method using the embodiment shown in FIG. However, for simplification, the reference temperature detection element 62 not directly related to the description is used.
And the temperature detection element 65 are omitted.

Two connection pads 64 of the temperature detecting element 64
Between b, the first, second and third resistors R ₁ , R ₂ , having a resistance value substantially equal to the total resistance of the temperature detecting element 64,
Connect R ₃ in series. The temperature detecting element 64 and the first, second and third resistors R ₁ , R ₂ and R ₃ form a bridge circuit. A power supply 6 is provided across the first resistor R ₁ and the second resistor R _2.
7 is connected to the connection point of the second resistor R ₂ and the third resistor R ₃ ,
A terminal is attached to each connection point between the other end of the first resistor R ₁ and the connection pad 64b, and the output voltage appearing at this terminal is Vo.
ut. A heating power source 66 is connected to the connection pad 63b of the heating element 63, and a rectangular wave pulse current is applied at a predetermined frequency. When no fluid is flowing, the bridge circuit is balanced and the output voltage Vout is zero. When the fluid is made to flow in the direction of the arrow and the fluid heated by the heating element 63 flows to the temperature detection element 64, the resistance value of the temperature detection element 64 changes and the voltage Vout is output. This voltage Vou
An arithmetic circuit (not shown) calculates t to obtain the flow velocity.

FIG. 10 is a correlation diagram showing the relationship between the average flow velocity and the heat propagation time measured in the embodiment shown in FIG.

By modifying the equation (4), 1 / T = v / x (5) is obtained. When the average velocity v is plotted on the horizontal axis and the reciprocal 1 / T (ms) of the heat propagation time T is plotted on the vertical axis, the equation (5) is expressed by a straight line. When the values of x were set to 1 mm and 10.2 mm, the results shown in FIG. 10 were obtained by actual measurement. The measured value indicates that it can be measured with high accuracy by riding on a straight line.

FIG. 11 is a correlation diagram showing the relationship between the square root of the average flow velocity measured in the embodiment shown in FIG. 9 and the output voltage.

The square root of the average flow velocity v is plotted on the abscissa and the output voltage Vout is plotted on the ordinate, and the heat propagation time T was used as a parameter for actual measurement. Heat propagation time T is 2 ms, 5 ms, 10
There are four types, ms and 20 ms. As shown in the figure, in all cases, the measured values are on a straight line, and it can be measured with high accuracy. The relationship between heat transfer time and flow velocity is used to determine if the sensor voltage is contaminated by fluid contamination and the output voltage is not fluctuating. The data shown in FIG. 11 should be stored once at the beginning and then taken for a contamination check from time to time so that there is no change compared with the data taken at the beginning. Usually, the relationship between the output voltage and the flow velocity is measured.

FIG. 12 is a plan view and a sectional view of the fourth embodiment of the present invention.

N-type silicon substrate similar to the second embodiment
SiO on the upper surface of 91₂Membrane, Si₃N _FourProvide a membrane and above it
The reference temperature is a thin resistance film consisting of a double film of Ti film and Pt film.
Temperature detecting element 92, heating element 93, a plurality of temperature detecting elements 9
4 are formed in parallel at intervals. Table of elements 92 to 94
SiO on the surface₂Coating film, reference temperature sensing element 92
Temperature measuring resistor 92a, micro-heater part of heating element 93
93a, a temperature detecting element 94, and a temperature measuring element 94a.
P as a hot plate⁺⁺Providing mold region 95, silicon substrate
91 is selectively etched to form a cavity 98 between the elements 92 to 94.
Forming P,⁺⁺Exposing the heat equalizing plate 95 in the mold area
It is the same as the second embodiment that the elements 92 to 94 are thermally insulated.
It is like. Both sides of the elements 92 to 94 are on the silicon substrate 91.
It becomes a supported double-supported beam structure.

Next, the glass flow tube 96 is attached.
As shown in FIG. 13, the flow pipe 96 is composed of two prismatic upper flow pipe members 97 and a lower flow pipe member 98, and recesses 97 a and 98 a are provided inside each of them, and the upper flow pipe member 97 has a concave portion 97 a. A cutout portion 97b for exposing the connection pads 92b to 94b is formed on the outer wall. The silicon substrate 91 is sandwiched between the two members 97 and 98, and the resistance temperature detector 92
The a-94a are positioned inside the recesses 97a and 98a, and the connection pads 92b-94b are aligned so as to be exposed from the notch 97b, and are bonded by the anodic bonding method. Recesses 97a, 9
8a serves as a passage through which gas flows. Connection pad 92b-
The connecting conductor wire 99 is connected to 94b by a thermocompression bonding method or the like.

FIG. 14 is a plan view and a sectional view of a silicon wafer on which the flow sensor element of the fourth embodiment shown in FIG. 12 is formed.

On the silicon wafer 101, a plurality of flow sensor elements 102 are formed in line. The flow sensor element 102 comprises a set of reference temperature detecting element 92, heating element 93, and a plurality of temperature detecting elements 94 shown in FIG. The manufacturing method is the same as the method described in FIG.

FIG. 15 is a plan view and a sectional view of an upper glass plate on which the upper flow tube member of the fourth embodiment shown in FIG. 12 is formed.

A recess 97a is formed on the lower surface of the upper glass plate 111.
Are aligned and formed. A cutout portion 97b is formed in the form of a through hole corresponding to each flow sensor element 102. The recess 97a and the notch 97b are etched and machined using a photoresist (ultrasonic machining,
Laser processing, dicing, etc.).

FIG. 16 is a plan view and a sectional view of a lower glass plate on which the lower flow tube member of the fourth embodiment shown in FIG. 12 is formed.

A recess 98a is formed on the lower surface of the lower glass plate 121.
Are aligned and formed. The recess 98a is formed by etching using a photoresist and mechanical processing (ultrasonic processing, laser processing, dicing, etc.).

FIG. 17 is a plan view and a sectional view for explaining the manufacturing method of the fourth embodiment shown in FIG.

The upper glass plate 111 and the lower glass plate 121 are superposed on the silicon wafer 101, aligned with each other, and bonded by the anodic bonding method. Anodic bonding is performed by heating to a temperature of about 400 ° C. and applying a DC voltage of 600 to 1000V. In this way, the upper glass plate 11
1 and the lower glass plate 121 sandwiching the silicon wafer 101, the SiO ₂ film, the Si ₃ N ₄ film, and the reference temperature detecting element 92, the heating element 93, and the plurality of temperature detecting elements 94 of the thin film resistance film on the SiO ₂ film, the Si ₃ N ₄ film It is held between the recesses 97a, 98a of the plate and is protected from the outside.

Next, the upper glass plate 111 and the silicon
Cut the wafer 101 and the lower glass plate 121 with the cutting blade 131.
Disconnect and separate into individual flow sensor element 102 chips
You The cutting is indicated by broken lines 132 and 133 in FIG.
Then, it is performed in two directions, vertical and horizontal. SiO ₂Membrane, Si₃N_FourMembrane
Reference temperature detecting element 92 and heating element 9 of thin film resistive film on
3, the plurality of temperature detecting elements 94 and the upper glass plate 111 and the lower
Cut while being protected by being sandwiched between the glass plates 121
Therefore, it will not be damaged. Therefore, high yield
It is possible to manufacture heat flow time measuring type flow sensor with
It In the fourth embodiment, the recesses 97a, 9a of the glass plate are used.
Although the cross-sectional shape of 8a is rectangular, it may be semicircular.
Of course not.

[0087]

As described above, according to the present invention, the double film of the SiO ₂ film and the Si ₃ N ₄ film is provided on the silicon substrate,
A reference temperature detecting element, a heating element, and a temperature detecting element are provided at intervals with a thin film resistance film, the surface is covered with a SiO ₂ film, and heat is generated by removing the silicon under the resistance temperature detector of these elements. Since it is an insulating structure, it is possible to obtain a heat propagation time measuring type flow sensor having high sensitivity, capable of measuring a wide range of flow velocity, and having high resistance to dirt.

Further, a plurality of flow sensor elements are formed on a silicon substrate, and an upper glass plate and a lower glass plate which are upper and lower flow tube members are attached to each other and then cut into individual flow sensor chips. Therefore, the flow sensor element is not damaged, and the heat propagation time measuring type flow sensor can be manufactured with a high yield and a small number of steps.

[Brief description of drawings]

FIG. 1 is a plan view and a sectional view of a first embodiment of the present invention.

2A to 2D are cross-sectional views showing the manufacturing method of the embodiment in FIG. 1 in order of steps for explaining the manufacturing method.

FIG. 3 is a perspective view for explaining a fluid measuring method using the embodiment shown in FIG.

FIG. 4 is a correlation diagram showing the relationship between the average flow velocity measured in the example shown in FIG. 3 and the heat propagation time.

FIG. 5 is a front view and a sectional view of a second embodiment of the present invention.

6A to 6C are cross-sectional views taken along the line CC 'in the order of steps for explaining the manufacturing method of the embodiment of FIG.

7A to 7C are cross-sectional views taken along the line DD 'in the order of steps for explaining the manufacturing method of the embodiment in FIG.

FIG. 8 is a front view and a sectional view of a third embodiment of the present invention.

9 is a perspective view for explaining a fluid measuring method using the embodiment shown in FIG.

10 is a correlation diagram showing the relationship between the average flow velocity and the heat propagation time measured in the example shown in FIG.

11 is a correlation diagram showing the relationship between the square root of the average flow velocity measured in the embodiment shown in FIG. 9 and the output voltage.

FIG. 12 is a plan view and a sectional view of a fourth embodiment of the present invention.

13 is an exploded perspective view of the glass flow tube shown in FIG.

FIG. 14 is a plan view and a cross-sectional view of a silicon wafer on which the flow sensor element of the fourth embodiment shown in FIG. 12 is formed.

15A and 15B are a plan view and a sectional view of an upper glass plate on which an upper flow tube member of a fourth embodiment shown in FIG. 12 is formed.

16 is a plan view and a cross-sectional view of a lower glass plate on which a lower flow tube member of the fourth embodiment shown in FIG. 12 is formed.

17A and 17B are a plan view and a cross-sectional view for explaining the manufacturing method of the fourth example shown in FIG.

FIG. 18 is a sectional view for explaining the principle of a flow sensor using a conventional heat pulse wire method.

FIG. 19 is a partially cutaway perspective view of an example of a conventional heat transfer type flow sensor.

FIG. 20 is a partially cutaway perspective view of an example of a conventional flow sensor using silicon.

[Explanation of symbols]

1 N-type silicon substrate 2 SiO ₂ film 3 P ⁺⁺ type region 4 SiO ₂ film 5 Si ₃ N ₄ film 6 Reference temperature detecting element 6a Temperature measuring resistor 7 Heating element 7a Micro heater part 8 Temperature detecting element 8a Temperature measuring resistance Body 9 SiO ₂ film 17 Connection pad

Continuation of the front page (56) Reference JP-A-9-145442 (JP, A) JP-A-2-262013 (JP, A) JP-A-63-282662 (JP, A) JP-A-59-79118 (JP , A) Tokuheihei 5-508915 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01F 1/00-9/02

Claims (14)

(57) [Claims] 1. A one-conductivity-type silicon substrate provided in a flow tube through which a fluid flows, an insulating film provided on the upper surface of the silicon substrate, and an upstream side of the fluid parallel to and spaced from the insulating film. A heating element and a temperature detection element which are sequentially formed from a thin film resistance film toward the downstream side, and a recess for selectively etching the silicon substrate under the micro-heater portion of the heating element from the back surface to expose the insulating film. In a heat propagation time measurement type flow sensor comprising, a reference temperature detection element is provided on the upstream side of the heating element,
Surfaces of the reference temperature detecting element, the heating element, and the temperature detecting element are covered with a silicon oxide film, and a resistance temperature detector of the reference temperature detecting element, a micro-heater portion of the heating element, and a temperature measuring element of the temperature detecting element. Heat propagation, characterized in that regions of opposite conductivity type are provided below the resistor as heat equalizing plates, and a recess is formed in the silicon substrate to expose the region of opposite conductivity type and the insulating film around it. Time measurement type flow sensor. 2. The heat propagation time measuring type flow sensor according to claim 1, wherein the insulating film is composed of a double film of a silicon oxide film and a silicon nitride film. 3. The heat propagation time measuring type flow sensor according to claim 1, wherein the temperature detecting elements are provided at least at two intervals. 4. The heat propagation time measurement type flow sensor according to claim 1, wherein the thin film resistance film is composed of a double film of a titanium film and a platinum film. 5. A cavity is formed between the reference temperature detecting element, the heating element and the temperature detecting element, and the reference temperature detecting element, the heating element and the temperature detecting element are supported on the silicon substrate in a double-supported beam form. The heat transfer time measuring type flow sensor according to claim 1, wherein 6. A cavity is formed between the reference temperature detecting element, the heating element and the temperature detecting element, and the reference temperature detecting element, the heating element and the temperature detecting element are supported on the silicon substrate in a cantilever manner. The heat transfer time measuring type flow sensor according to claim 1, wherein 7. A DC power source is connected between one terminal of the reference temperature detecting element and one terminal of the temperature detecting element, and a resistance value substantially equal to the resistance value of the temperature detecting element is connected in parallel to the power source. And a variable resistor and a second resistor having a resistance value slightly smaller than the resistance value of the reference temperature detecting element are connected in series to form a bridge circuit,
A first output terminal is taken out from a connection point of the resistance and the variable resistance, and a second output terminal commonly connected to the other terminal of the reference temperature detecting element and the other terminal of the temperature detecting element is taken out, and the first output terminal is taken out. The heat propagation time measuring type flow sensor according to claim 1, further comprising an output circuit using a voltage output between the output terminal and the second output terminal as a measurement signal source. 8. A first direct current power supply is connected between two terminals of the temperature detecting element, and has a resistance value substantially equal to a resistance value of the temperature detecting element in parallel with the power supply.
A resistor, a second resistor, and a third resistor are connected in series to form a bridge circuit, and a first output terminal is taken out from a connection point between the second resistor and the third resistor, and the first resistor and the temperature. A second output terminal is taken out from a connection point with one terminal of the detection element, and an output circuit using a voltage output between the first output terminal and the second output terminal as a measurement signal source is provided. The heat transfer time measuring type flow sensor according to claim 1. 9. The reference temperature detecting element, the heating element, and the temperature detecting element are supported on the silicon substrate in a double-supported beam form, and the two supporting portions are supported on the wall of the flow tube, and the reference temperature detecting element is provided. The heating element and the temperature detecting element are located in the flow tube.
The heat transfer time measurement type flow sensor described. 10. The reference temperature detecting element, the heating element, and the temperature detecting element are supported by the silicon substrate in a cantilever manner, and the supporting portion is supported by the tube wall of the flow tube, and the reference temperature detecting element and the heating element are heated. The heat propagation time measuring type flow sensor according to claim 1 or 6, wherein an element and a temperature detecting element are located in the flow tube. 11. The heat transit time measuring type flow sensor according to claim 9, wherein the flow tube is made of plastic or glass. 12. A step of selectively diffusing an impurity of opposite conductivity type into a silicon substrate of one conductivity type at a high concentration to selectively form an opposite conductivity type region as a soaking plate , and forming a first SiO 2 film on the surface of the silicon substrate. ₂ film is provided, comprising: providing a the Si ₃ N ₄ film thereon, and a photoresist is deposited patterned on the the Si ₃ N ₄ film provided a mask of the photoresist, by depositing Ti and Pt Ti A step of forming a thin film resistance film composed of a double film of Pt and Pt, and dissolving and removing the mask of the photoresist to remove the Ti and P
a step of forming the reference temperature detecting element, the heating element and the temperature detecting element of the double film of t, and forming the second SiO ₂ film on both surfaces of the silicon substrate including the reference temperature detecting element, the heating element and the temperature detecting element. Then, a photoresist mask is provided thereon, and a window is opened in the connection pad exposed area of the reference temperature detecting element, the heating element and the temperature detecting element and the opposite conductivity type area exposed area, and the second portion of the window portion is exposed. A step of etching away the SiO ₂ film, and a step of anisotropically etching the silicon substrate to expose the opposite conductivity type region and the first SiO ₂ film in the periphery thereof. Method of manufacturing heat flow time measuring type flow sensor. 13. A step of: (A) diffusing an impurity of opposite conductivity type into a silicon substrate of one conductivity type at a high concentration to selectively form a region of opposite conductivity type as a soaking plate ; No. 1 SiO ₂ film is provided and a Si ₃ N ₄ film is provided thereon, and a photoresist is deposited on the Si ₃ N ₄ film and patterned to provide a photoresist mask, and Ti and Pt are deposited. Forming a thin resistance film composed of a double film of Ti and Pt, and removing the mask of the photoresist by dissolution to remove the Ti and P
a step of forming the reference temperature detecting element, the heating element and the temperature detecting element of the double film of t, and forming the second SiO ₂ film on both surfaces of the silicon substrate including the reference temperature detecting element, the heating element and the temperature detecting element. Then, a photoresist mask is provided thereon, and a window is opened in the connection pad exposed area of the reference temperature detecting element, the heating element and the temperature detecting element and the opposite conductivity type area exposed area, and the second portion of the window portion is exposed. A step of etching away the SiO ₂ film, and a step of anisotropically etching the silicon substrate to expose the opposite conductivity type region and the first SiO ₂ film in the periphery thereof. A process for manufacturing a wafer equipped with a flow sensor element, in which a plurality of flow sensor elements each having a temperature detection element, a heating element, and a temperature detection element as one flow sensor element are arranged and formed on a single silicon substrate. Forming a concave portion at a position corresponding to the reference temperature detecting element, the heating element and the temperature detecting element so that gas flows over the reference temperature detecting element, the heating element and the temperature detecting element on one surface of a flat plate of glass or plastic, A step of forming a flat plate for the upper flow tube that forms an opening at a position corresponding to each connection pad of the reference temperature detecting element, the heating element and the temperature detecting element; and the reference temperature detecting element on one surface of the glass or plastic flat plate. A flow tube flat plate having a step of forming a concave portion at a position corresponding to the reference temperature detecting element, the heating element and the temperature detecting element so that gas flows under the heating element and the temperature detecting element, and (C) The flow sensor element mounting wafer is aligned such that the upper surface of the flow sensor element and the recess of the flat plate for the upper flow tube face each other. The upper flow tube flat plate is placed on the upper surface of the flow sensor element, and the lower surface of the flow sensor element is aligned so that the lower surface of the flow sensor element and the recess of the lower flow tube plate face each other. And a step of laminating the wafer with the flow sensor element, the flat plate for the upper flow tube and the flat plate for the lower flow tube, and (D) cutting the bonded wafer with the flow sensor element, the flat plate for the upper flow tube and the flat plate for the lower flow tube And a cutting step for cutting the flow sensor with individual flow tubes into separate pieces. 14. The method of manufacturing a heat propagation time measuring type flow sensor according to claim 13, wherein the recess has a semicircular or rectangular cross section.