JPH057659B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a flow rate sensor for measuring air flow rate. This is a commercially available long probe that is inserted into the air stream and has a hot wire or thermistor placed at the end. This measures the air flow velocity based on the temperature drop caused by the cooling effect of the air flow and the accompanying change in electrical resistance. Because of this device configuration, the sensor element is exposed to air currents and is therefore susceptible to damage and contamination. Moreover, since the temperature change caused by this air cooling is not linear at all, it is necessary to linearize the obtained electrical signal using an electronic circuit. Furthermore, these are expensive and not suitable for mass production.

Related to the present invention are the following commercially available mass flow sensors: The sensor consists of a metal tube through which air or other gas to be measured passes, a transformer that resistively heats a section of the tube, two large heat sinks attached to this section, and a center of the heated section. It consists of two thermocouples mounted symmetrically on the metal in the middle of the thermal zone between the heat sink and the heat sink. This flow of air through the metal tube cools the upstream thermocouple and heats the downstream thermocouple. When the transformer is driven with constant power, the difference in the output voltage of this thermocouple provides a measure of mass flow rate. This is a large device that requires considerable power. It is not suitable for measuring flow rates inside large ducts or outdoors, is expensive, and cannot be mass-produced.

Therefore, there is a need for a flow rate sensor or mass flow sensor and associated signal processing circuit that have the following characteristics. Long life, maintenance free, small size, low power consumption, easy application in a wide range of fields, large output signal, and linear or easily linearized over a wide speed range It is like having output characteristics. Furthermore, it must be able to be mass-produced and be inexpensive.

The literature presents several attempts to improve flow rate sensors in relation to these demands. These attempts, as described below,
Generally, pyroelectric materials seek to exploit silicon and its semiconducting properties. Although these attempts have brought about technical improvements in some respects, they are still unsatisfactory with respect to many of the characteristics desired in current flow rate sensors. The present invention is technologically advanced to meet these requirements to a greater extent than any prior art. Next, we will discuss the well-known and most closely related prior art.

The flow velocity sensor invented by Huising (*1) et al. consists of two identical temperature sensing elements consisting of diffusion transistors embedded near both ends of a silicon chip, and a sensor placed in the center of these elements that detects the air temperature. It consists of a heater element consisting of a diffusion type transistor that heats the device by 45 degrees Celsius. When air flows, the temperature sensing element located upstream of the flow is slightly cooler than the temperature sensing element located downstream, and the temperature difference between these two temperature sensing elements becomes a difference in current, which is converted into voltage. The air flow rate is measured. This temperature sensing element must be placed at opposite ends of the chip in order to achieve a perceivable temperature difference, but the temperature difference is still small and zero at speeds from 0 to 50 cm/sec.
It is only a temperature change of ~0.2℃ or less.

*1 JHHuijsing, et al: IEEE
Transactions on Electron Devices, Vol.ED
-29, No. 1, pp. 133-136, January, 1982 The flow velocity sensor invented by Buptuten (*2) et al. consists of identical diffused resistance elements embedded on each of the four opposing sides of a silicon chip. be done. All resistive elements self-heat, which causes the silicon chip to heat up considerably due to the temperature of the flowing air. The resistive element is driven by an electrical double bridge circuit. When there is no air flow, all resistive elements are at the same temperature, so the double bridge circuit is electrically balanced. When there is air flow, the upstream and downstream resistive elements perpendicular to the flow will be cooler than the resistive elements on either side parallel to the flow. This temperature difference unbalances the double bridge and the air flow velocity is measured.

*2 AFPVan Putten, et al: Electronics
Letters, Vol.10.No.21, pp.425−426,
October, 1974 The mass flow sensor invented by Mullin (*3) et al. consists of two sensors consisting of a diffused resistance element on a large piece of silicon and a diffused heater element placed centrally between the sensors. configured. This technology is similar to commercially available heated metal tube type mass flow sensors. As the air flow heats the sensors located downstream in the flow and cools the sensors located upstream, the difference in temperature of these sensors results in a voltage difference across the sensors and a mass flow rate is measured.

*3 K. Malin, et al: IBM Technical
Disclosure Bulletin, Vol.21, No.8,
January, 1979 Rahanamayi (*4) et al. deposited a thin metal film on the entire back surface of a thin plate of polished single-crystal lithium tantalate arranged crystallographically, and deposited a thin film on the center of the surface. A sensor is disclosed that is comprised of a heater resistor element and two thin film electrodes spaced the same distance therefrom. Here, the lithium tantalate has a length of 8 mm, a width of 4 mm, and a minimum thickness of 0.06 mm. As stated in the literature, both ends of the plate are supported on large screw heads to allow air to flow under the plate. The two electrodes located upstream and downstream constitute the same capacitor separated from the back electrode surface and serve as a temperature sensing capacitor. In operation, the heater element is driven with a voltage at a low frequency, such as 2 to 10 Hz, to periodically heat the heater element relative to the temperature of the flowing air. The sensor element will also be correspondingly heated periodically due to heat conduction through the lithium tantalate. Since this lithium tantalate is a pyroelectric material, it will polarize in response to temperature, but when there is no air flow, the periodic polarization voltages generated in the two sensors will be the same. Therefore, the difference in voltage between the two sensors when there is no air flow is zero. As stated in the literature, when there is air flow, the sensor element located upstream will be cooler than the sensor element located downstream, so the difference in temperature of these sensors will be the difference in voltage. resulting in the flow rate being measured.

*4 H. Rahnamai, et al: paper presented
at the 1980 International Electron Devices
Society of IEEE, Washington DC, pp.680
-684, December 8-10, 1980 As mentioned above, although these attempts brought about technical improvements in some respects, many of the characteristics required of current sensors are still unsatisfactory. It is. The present invention advances technology to fully satisfy these demands.

The first invention of the present application is to position two thin film members, each of which surrounds a heat sensing sensor and a heater with a thin insulating layer having a relatively low thermal conductivity, in an air space formed by etching on the surface of a semiconductor substrate. As such, at least one end thereof is held by the semiconductor substrate, so that it can be placed in a non-contact state with the substrate and in close proximity to the substrate.

Next, a second invention relates to an improvement of the first invention, and has a configuration in which a connecting means is provided to keep the first and second thin film members on the same plane.

Furthermore, the third and fourth inventions provide, in addition to the thin film heater and the pair of thin film heat sensing sensors, an ambient temperature detection means using a silicon substrate and a heat sink to monitor the ambient temperature, and a bridge circuit control means. The structure is such that the thin film heater can be driven at a constant temperature higher than the ambient temperature.

In a specific embodiment of the present invention, the substrate 20 is selected from semiconductors, in particular silicon, because it can be applied with precise etching technology and has high chip productivity. The identical temperature-measuring resistance elements formed on this substrate without the grid shape function as thin-film heat sensing sensors 22 and 24, and these two
A heating resistor element disposed in the center of each sensor that eliminates the grid shape functions as a thin film heater 26.
As the heat sensing sensors 22, 24 and the heater 26, it is suitable to use an alloy of iron and nickel, such as permalloy made of 80% nickel and 20% iron. This heat detection sensor 22, 2
4 and the heater 2 are surrounded by thin film insulating layers 28 and 39 made of silicon nitride, for example, to form a thin film member. As shown in the embodiments of FIGS. 1 and 2, the sensor includes a thin film member 32 comprising half of the heater 26 and the heat sensing sensor 22, and a thin film member 34 comprising half of the heater 26 and the heat sensing sensor 24. It has a width of 150μ and a length of 400μ.

Additionally, the sensor disclosed herein has an air space 30 that effectively surrounds the heat sensitive sensors 22, 24 and the heater 26. This air space 30 is formed with a microstructure on the surface 36 of the silicon. That is, the heat sensing sensors 22, 24 and the heater 26 are made of loose wires with a thickness of about 0.08 to 0.12μ and a width of 5μ with a space of about 5μ between the lines, and the total thickness of these is about 0.8μ or less. It is constructed so that it is surrounded by a thin film of silicon nitride.
Then, by etching, a recessed air space 30 is formed in the silicon substrate 20 under the membrane members 32, 34 to a depth of exactly 125 microns.
Thin film members 32 , 34 are connected to the top of the surface 36 of the silicon substrate 20 at one or more edges of the air space 30 . For example, as shown in FIG.
It can also be constructed with a cantilever beam as shown in Figure A.

Silicon nitride is an excellent thermal insulator. Since the silicon nitride film surrounding the thin film members 32 and 34 is extremely thin and has good thermal insulation, the heat loss of the heater 26 due to the silicon nitride film is extremely small, and the heat sensing sensors 22 and 24 are
Most of the heat transferred to the heater 26 will be transferred through the air surrounding the heater 26. In other words, since the thermal conductivity of the silicon nitride film is low, the heat sensing sensors 22 and 24 can be placed very close to the heater 266, and most of the heat from the heater 26 does not pass through the silicon nitride film. Heater 26
It will be transmitted through the air surrounding it.
The heat sensing sensors 22 and 24 effectively and firmly supported in the air near the heater 26 function as probes that measure the temperature of the surface of the heater 26 and the air near it.

The principle of the present invention of detecting the air flow velocity will be explained based on FIG. 2. The heater 26 is connected to the substrate 20
is heated to a constant temperature that is 200°C higher than the temperature of The temperature of this silicon substrate 20 is almost the same as the temperature of the flowing air. Specifically, when the silicon substrate 20 is mounted on a heat sink such as a TO-100 type metal head or a cerdip package, the temperature of the silicon substrate 20 is lower than the temperature of the flowing air.
It will only increase by about 0.5℃. Also, the heater 26
Whenever the temperature of the air is kept 200°C higher than the temperature of the flowing air, less than 0.01W of power is required.

Most of the heat transfer from heater 26 occurs through the surrounding air, which also includes air space 30;
When there is no air flow in the embodiment of the present invention,
The average temperature of the thermal sensors 22 and 24 is approximately 140°C.
(approximately 70% of 200℃). That is, as shown in the figure, the heat sensing sensors 22 and 24 are connected to the heater 2.
6, the temperature of these two sensors is the same when the air flow rate is 0, and there is no difference in the resistance values of the two sensors. Therefore, even if a minute current of 0.1 to 1.0 mA is applied, no voltage difference will occur between the two heat sensing sensors.

When there is air flow, the upstream heat sensing sensor 22 in this embodiment is connected to the heater 26.
The heat is carried away by the flow of air towards
will be heated by the air flow from heater 26. The resulting difference in resistance between the heat sensing sensors 22 and 24 results in a difference in voltage and the flow velocity is measured. This voltage difference without amplification is equal to
It is about 0.1V.

In the present invention, the heat sensing sensors 22 and 24 are driven by a constant current and are configured to detect changes in temperature balance under flowing air as described above. In addition, if the method causes a signal to be generated, such as driving two sensors in constant voltage mode, constant temperature mode, or constant power mode, other methods can be used instead. However, it is possible to realize it.

The heat capacity of the heater 26 and the heat sensing sensors 22 and 24 is extremely small, and the thermal insulation provided by the silicon nitride film, which is the connection means to the substrate,
Due to the presence of the air space 30, the responsiveness of the present invention is very fast with a time constant of 0.005 seconds according to measurements. That is, heat sensing sensors 22 and 24
will be able to respond very quickly to changes in air flow.

In the present invention, since the heater 26 is driven to maintain a constant temperature relative to the air temperature, and the heat sensing sensors 22 and 24 are driven with a constant current,
A change in temperature of the heat sensing sensors 22 and 24 is detected as a change in resistance value. Examples of circuits for realizing these functions are shown in FIGS. 4 and 5. The circuit shown in FIG. 4 is for controlling the temperature of the heater 26, and the circuit shown in FIG. It is meant to be obtained.

In the present invention, the temperature of the surrounding air is monitored by a comparison resistor 38 formed with the silicon substrate 20 as a heat sink. Comparison resistance 38
Like the heat sensing sensors 22, 24 and the heater 26, it is made of grid-like permalloy and is placed on the silicon surface 36 surrounded by insulating layers 28 and 29.

In the present invention, the temperature of the surrounding air is monitored by a comparison resistor 38 formed with the silicon substrate 20 as a heat sink. Comparison resistance 38
Like the heat sensing sensors 22, 24 and the heater 26, it is made of grid-like permalloy and is placed on the silicon surface 36 surrounded by insulating layers 28 and 29.

The total thickness of the insulating layers 28 and 29 is very thin, 0.8μ, so heat conduction is relatively good.
Heat flows into and out of the thermal sensors 22 and 24, the heater 26, and the comparison resistor 38 through the vertical direction of these insulating layers. The comparison resistor 38 is wrapped in an insulating layer and attached directly to the surface 36 of the substrate 20, and the heater 26 is 200°C lower than the surrounding temperature.
Even when heated to a high temperature, the temperature of the surrounding air is 0.5℃
This means that it is possible to easily monitor the temperature of the substrate 20 within the range shown in FIG. That is, by detecting the temperature of the substrate 20, the comparison resistor 38 detects the temperature of the flowing air which almost matches the temperature of the substrate 20.

The temperature control circuit shown in FIG.
A Wheatstone bridge circuit 46 maintains the temperature at a constant temperature higher than the ambient temperature detected by the comparison resistor 38. As mentioned above, in the embodiment of the present invention, this constant value is set at approximately 200°C. The Wheatstone bridge circuit 46 has one side made up of the heater 26 and the resistor 40, and one side made up of the comparison resistor 38 and the resistors 42 and 44. The integrator circuit consisting of amplifiers 48 and 50 operates to balance the bridge circuit 46 by changing the output potential, thereby keeping the power consumed by the heater 26 constant.

The circuit shown in FIG. 5 is for detecting the difference in resistance between the heat sensing sensor 22 located upstream of the air flow and the heat sensing sensor 24 located downstream in this embodiment. . This circuit includes a constant current power supply section 52 consisting of an amplifier 72, and an amplifier 6.
8 and 70. The constant current power supply section 52 has high impedance resistors 56 and 58 on one side and a zero adjustment variable resistor 60 on the other side.
and a Wheatstone bridge circuit having heat sensing sensors 22,24. The gain of the differential amplification section 54 is adjusted by a variable resistor 62. Output end 6
4 is an output voltage proportional to the difference in resistance between the heat sensing sensors 22 and 24.

In this embodiment, amplifiers 48, 50, 6
For amplifiers 6 and 72, each of the LM324 amplifiers having four amplifiers is used, and for amplifiers 68 and 70, each of the OP-10 amplifiers having two amplifiers is used.

One of the features of the sensor disclosed in the present invention is that it is configured so that the difference in temperature detected by the heat sensing sensors 22 and 24 is large over a wide range of air flow speeds. It will be done.
As a result, the output 6 is a function of the air flow velocity.
4, the accuracy of flow velocity measurement is significantly improved and measurement becomes easy. The fact that this temperature difference can be made large is shown in FIG. 6 obtained by a specific embodiment of the present invention. In other words, the heat sensing sensor 2 located upstream is cooled by the air flow.
By linking the outputs of two sensors: 2 and the heat sensing sensor 24 located downstream which is heated by the heat from the heater 26, a large temperature difference effect can be obtained. In order to combine heating and cooling to obtain a large temperature difference, (1) it is necessary to bring the heat sensing sensors 22, 24 into relatively strong thermal contact with the air. That is, the thermal sensors 22, 24 must be substantially thermally isolated from the silicon substrate 20.
This means that the thermal conductivity of the silicon nitride film surrounding the thin film members 32 and 34 in the direction along the flow direction is relatively small, and the thin film members 32 and 3
This is made possible by the air space 30 being precisely formed between 4 and the substrate 20 to a depth of approximately 125μ.

(2) In this embodiment, it is necessary to significantly cool the heat sensing sensor 22 located upstream of the air flow. This requires setting the temperature of the thermal sensor 22 high. And this is connected to the heat sensing sensor 22 through the air space 30.
thermally insulating from the silicon substrate 20;
This is possible because the thermal conductivity in the longitudinal direction of the thin film member 32 is low and by setting the temperature of the heater 26 high. In order to set the temperature of the heater 26 high, it is necessary to thermally insulate the heater 26 from the silicon substrate 20 via the air space 30 and to
It is necessary that the thermal conductivity in the direction along the length of 2, 34 is small.

(3) In this embodiment, it is necessary to greatly heat the heat sensing sensor 24 located downstream of the air flow by transmitting the heat from the heater 26. This is because the heat sensing sensor 24 is thermally insulated from the silicon substrate 20 via the air space 30, the thermal conductivity in the longitudinal direction of the thin film member 34 is small, and the temperature of the heater 26 This is possible by setting . In order to set the temperature of the heater 26 high, it is necessary to thermally insulate the heater 26 from the silicon substrate 20 via the air space 30, and to increase the thermal conductivity in the longitudinal direction of the thin film members 32, 34. is small.

(4) The center of the heat detection sensors 22 and 24 and the heater 2
It is necessary to select the distance between the edge portions of No. 6 to an optimum value.

FIG. 7 shows an idealized air temperature distribution curve 74 when the air flow velocity is 0, and an idealized air temperature distribution curve 76 when the air flow velocity is a certain arbitrary value. , which shows the most suitable placement distance of the ideally narrowed thermal sensing sensors 22 and 24. Since the upstream side of the flow with respect to the heater 26 is cooled by air, this temperature distribution curve will be lowered by a value of ΔT1, which is a function of distance, on this side. Here, this △T
1 is largest at a position D1 away from the edge on the upstream side of the heater 26. On the other hand, on the downstream side of the flow, the temperature increases by a value of ΔT2, which is a function of distance, due to the heat transferred by this flow. Here, ΔT2 is largest at a position D2 away from the downstream edge of the heater 26. D1 and D2 do not need to be equal values, but in order to ensure that the output from the flow rate sensor is 0 when the flow rate is 0,
At this time, the temperatures of the heat sensing sensors 22 and 24 must be equal. Therefore, in a preferred embodiment of the present invention, the heat sensing sensors 22, 2
4 actually has a width, it is necessary to make the distance D1 to the center of the heat sensing sensor 22 and the distance D2 to the center of the heat sensing sensor 24 equal. From now on, under the condition that D1 and D2 are equal, the output of the flow velocity sensor will be maximum unless the sign is considered, so the heat sensing sensor 22
Average value of △T1 over the above and heat sensing sensor 2
The sum of the average values of ΔT2 over 4 becomes the maximum. It has been found that the above occurs at a certain appropriate position, and in the flow velocity range of several hundred feet/minute, the value of D1 (=D2) is approximately 1/2 of the width of the heater.
is the length of

While the flow velocity sensor according to the prior art could only obtain a small temperature difference, in the present invention, to give specific numerical values, at a flow velocity of 860 cm/sec, the average value of △T1 and the average value of △T2 can be obtained. The total is approx.
It can be relatively large at 50℃. This is because the thin film members 32 and 34 are wrapped in a very thin silicon nitride film with low conductivity in the longitudinal direction, and an air space 30 is provided so that the heat sensing sensors 22 and 24 and the heater 26 are surrounded by air. Therefore, heat detection sensor 2
This became possible because it became possible to thermally insulate 2, 24 and the heater 26 from the silicon substrate 20. Accurately dimensioning the width of the air space 30 and the depth of the recess formed under the heat sensing sensors 22, 24 and the heater 26 is achieved by a precision etching technique described below. By forming this air space 30 with high precision, variations in thermal insulation between sensor chips can be made uniform. That is, variations in response to air flow between sensor chips can be made uniform.

In the prior art, these sensor elements are
The structure was such that it was buried in or closely attached to a substrate such as a silicon chip. Therefore, in order to extract a significant temperature difference between the sensor elements, it is necessary to separate the sensor element from the heater by a distance that is long compared to the width of the heater. However, the temperature difference obtained with such conventional technology is only about 1/100 of that of the present invention, which is constructed based on thermal insulation.

Taking the prior art of Lahanamai et al. as an example, the heater and sensor elements are deposited on a lithium tantalate plate. (The specific conductivity of lithium tantalate is lower than that of silicon, but 70% higher than that of silicon nitride.)
The heater and sensor emerents are closely mounted to a lithium tantalate plate having a thickness of at least 60 microns. This value of 60μ is 75 times thicker than the value of 0.8μ which is the thickness of the silicon nitride film surrounding the thin film members 32 and 34 of the present invention. The conduction of heat between the heater element, which is in close contact with the lithium substrate, and the sensor element, which is placed at a distance between 0 and 500μ from the edge of the heater element, is faster than that through the air between the sensor and the heater. , it is dominated by the 60μ lithium substrate. That is, since the heater is not effectively surrounded by air, most of the heat from the heater to the sensor will be transferred through the lithium substrate. As a result, this
A sensor with a width of 500μ will be able to change only a small temperature value with respect to the air flow compared to the present invention. Here, the sensor of the invention is disposed extending over a distance of between 0 and 100 microns from the edge of the heater.

Another advantage of the present invention over the prior art is the air space 30 provided to effectively thermally isolate the thermal sensors 22, 24 and heater 26 from the silicon substrate 20. As a result, as described above, the temperature increase in the silicon substrate 20 can be suppressed to approximately 0.5° C. or less, which is an almost negligible temperature increase. Therefore, in the present invention, the output characteristics with respect to the flow rate are completely independent of the thermal contact between the silicon substrate and the housing supporting it. In contrast, in the prior art, the output characteristics of the sensor were greatly influenced by thermal contact with the mounting body. In fact, even in the sensor of Lahanamai et al., the output characteristics are greatly influenced by the method of mounting structure. In the prior art by Huising et al., it is also stated that the fixed base of the silicon chip deteriorates the output characteristics with respect to the flow velocity. In the conventional technology of Banputsuten et al., the thickness of the silicon chip is reduced from the commonly used 200 μm to 50 μm in order to minimize the thermal shortening of the silicon chip. This makes it susceptible to a decrease in sensitivity. Marin et al.'s prior art uses long silicone strips to prevent thermal brittleness, but is similarly affected by the fixation stage.

A second advantage of the present invention is that the temperature difference between the thermal sensors 22 and 24 is linear over a wide flow rate range. In the prior art, this temperature difference is proportional to the square root of the flow rate.
Because of this dependence, known as a parabolic output characteristic, if the flow velocity changes by a certain value when the flow velocity is V, the difference in temperature between the two sensors will be 1/2√V times that value. It's going to change. Because of this incremental characteristic, the faster the flow rate, the smaller the change. As a result, they are more susceptible to errors due to noise and drift in the electronic circuitry. The present invention has good linearity of output characteristics with respect to changes in flow velocity, and has
Since the electrical output signal obtained in the flow velocity region of ~1016 cm/sec or higher can be obtained without amplification, errors due to drift etc. can be reduced to an almost negligible level even in fast flow velocity measurements. A typical output characteristic of this temperature difference versus flow rate for a sensor according to the invention is shown in FIG.
This figure also shows the characteristics of the sensor from the housing for comparison.

As described above as a specific example, in the sensor of the present invention, the total resistance value of the heater 26 is
The circuit shown in the figure allows it to remain constant at any ambient temperature. The above-mentioned characteristic of good linearity is due to the fact that the heat sensing sensor 22 is sufficiently wide and appropriately arranged so as to cover most of the region where the temperature distribution in the vicinity of the heater 26 has a slope (see FIG. 7).
It can be obtained by using and 24.

In such a configuration, the internal edge 76 of the thermal sensor 22 is connected to the proximal edge 78 of the heater 26.
(for example, with a line width of 5μ). When there is no air flow, close edge 78
The temperature of the air in the vicinity at this distance from almost corresponds to the temperature of the proximate edge 78. As a specific example, the heat sensing sensors 22 and 24 are approximately 100μ
, the outer edge 80 of the thermal sensor 22 is located approximately 100 microns from the proximal edge 78 of the heater 26. When there is no air flow, the air temperature at this position approximately 100 μm away is closer to the ambient air temperature, that is, the temperature of the silicon substrate, than the temperature of the heater 26 (see FIG. 7). As a result, the external edge 80 of the thermal sensor 22 is easily cooled to near the temperature of the silicon substrate 20 even at low air flow rates. On the other hand, the inner part of the heat sensing sensor 22 is closely thermally coupled to the heater 26, so that it will not be cooled to near the temperature of the silicon substrate even with a fast air flow. easily respond to When the effects of temperature changes from each grid section of the thermal sensor 22 are combined, the temperature versus flow rate characteristic of this sensor located upstream in the flow becomes more constant over a wide flow rate range. This characteristic contrasts with that of a single point or line element located anywhere away from the edge of the heater, which is generally the case in the prior art. This linear temperature characteristic over a wide flow velocity range of the heat sensing sensor 22, which was actually measured according to a preferred embodiment of the present invention, is expressed as
As shown in the figure.

For the heat sensing sensor 24 installed downstream of the flow, an increase in resistance value occurs due to heat transfer due to the air flow, but for the heat sensing sensor 24 located upstream, the resistance value increases.
This is smaller than the resistance change due to air cooling in No. 2. However, in the present invention, the spacing between the inner edge 84 of the heat sensitive sensor 24 and the proximal edge 86 of the heater 26 is 5 μm, and the spacing between the outer edge 88 of the heat sensitive sensor 24 and the proximal edge 86 of the heater 26 is approximately
100μ, which is closer than conventional technology,
Heat conduction by the air flow from the heater 26 to the heat sensing sensor 24 is effective over a wide flow velocity range. This effective heat transfer and thermal isolation of the thermal sensor 24, primarily by the air space 30, results in a significant increase in the temperature of the thermal sensor 24 with increasing flow rate over a wide flow rate range. A thermal sensor 24 located downstream, measured according to a preferred embodiment of the present invention.
Figure 6 shows the temperature change with respect to the flow rate.
When the characteristics with respect to the flow velocity of the heat sensing sensor 22 located upstream and the heat sensing sensor 24 located downstream are combined, the characteristics shown in FIG. 8 are obtained. From this figure, it can be seen that in comparison with the characteristic curve of the prior art of Huising et al., there is linearity over a wide flow velocity range and that a large signal level is obtained.

One of the advantages of the sensor of the present invention having a large power versus flow velocity characteristic is that it is desirable to reduce the flow velocity in order to extend sensor life and avoid the effects of turbulence. There is a point of promoting application in the field. In addition, in this sensor, the air flow is parallel to the silicon substrate surface, and since a slow-flowing surface layer is formed on the silicon substrate surface, regions with fast airflow are essentially separated from this surface layer. Therefore, the thin film heat sensing sensors 22, 24 and the thin film heater 26 are
Another advantage is that it is protected from damage caused by collisions with particles contained in the air.

The thermal sensors 22, 24 and heater 26 shown schematically in FIG. 2 are shown in detail in FIG. 9, which is one embodiment. An opening 82 is cut into the silicon nitride to facilitate etching as described below. Lead portion 92 and permalloy plate 90 are made symmetrically to ensure symmetrical heat transfer characteristics on membrane members 32 and 34. In this embodiment, the dimensions of membrane members 32 and 34 are approximately
150μ and length 300μ. Heat sensing sensor 22,2
The thickness of the heater 26 is 0.08μ, the resistance value of the heat sensing sensors 22 and 24 is 740Ω, and the thickness of the heater 26 is 0.08μ.
The resistance of 6 is 840Ω. Heat sensing sensor 22,2
The size of 4 is approximately 100μ in width and 175μ in length. As mentioned above, thermal sensors 22 and 24 are separated from heater 26 by one line width (5 μ). That is, the internal edge 7 of the heat sensing sensor 22
6 is 5μ from the proximal edge 78 of the heater 26, and the internal edge 84 of the thermal sensor 24 is 5μ from the proximal edge 86 of the heater 26. Regarding other embodiments of the present invention, unless otherwise stated, the thicknesses of the heat sensing sensors 22, 24 and the heater 26 are
0.08μ, the width of the lines forming the grid is 5μ, the distance between the lines is 5μ, and the grid is made of permalloy consisting of 80% nickel and 20% iron. As with other dimensions mentioned in this invention, these values are appropriate as used in actual devices, but are limited as they should vary depending on the application. It's not a thing.

Another embodiment of the invention is shown in FIG. Similar to the embodiment shown in FIG. 9, the embodiment shown in FIG.
It consists of two membrane members labeled A. A divided heater 26A is used, and this heater 26A is placed on the thin film member 32A.
The other half of the heater 26A is placed on the thin film member 34. The sensors 22A, 24A are
It is narrower than the heat sensing sensors 22 and 24 described above, and its size is approximately 90μ in width and 175μ in length. Additionally, sensors 22A and 24A are located 25 microns further from heater 26A than in the previously described embodiment. That is, the edge 76A of the sensor 22A is 25μ from the edge 78A of the heater 26A, and the sensor 24
Edge 84A of A is edge 86A of heater 26A.
It is located 25μ away from the In this embodiment, the dimensions of the thin film members 32A, 34A are approximately 150μ in width and 350μ in length, and the sensors 22A, 2
The resistance value of 4A is 670Ω, and the resistance value of heater 26A is 840Ω. Furthermore, in the embodiment of FIG.
They are connected by a connecting portion 94 reinforced by 6. This connecting portion 94 is connected to the thin film members 32A and 34A.
It plays a role in keeping the surfaces on the same plane. The lead portion 92A is arranged along the center line of the thin film members 32A, 34A in order to increase the strength of the center portions of the thin film members 32A, 34A.

According to preliminary experiments, the output of the embodiment of FIG. 10 was approximately 100% greater than the output of the embodiment of FIG. 9 for the same flow velocity.
However, in these experiments, the embodiment of FIG. 9 was placed at the center of the chip as shown in FIG. 1, while the embodiment of FIG. 10 was placed at the edge of the chip. Therefore, as shown in FIG. 11, the upstream portion of the air space 30A is opened, or
Alternatively, air space 30 as shown in FIG.
Since the downstream part of the AA is open and not obstructed by the side walls, air can flow more easily. 1st
In the layout example shown in Figure 1 and the layout example shown in Figure 12, the output characteristics are both larger than in the layout example shown in Figure 9, which places the chip in the center of the chip, but the output characteristics in Figure 11 are considerably higher than in Figure 12. It showed a large output. 10th
It is still unclear whether the fact that the embodiment shown in the figure has a larger output than the embodiment shown in FIG. 9 is due to the difference in placement, design, or both. I haven't. Note that the embodiment of FIG. 11 can be modified by removing at least a portion of the leading edge of substrate 20A. That is,
For example, by removing portion 118 of substrate 20A up to line 120, the tip edge of thin film member 32A is exposed to a greater extent. It should be noted that the fact that the embodiment of FIG. 11 exhibited considerably greater output characteristics than the embodiment of FIG. put. The first is that, as mentioned above, in the embodiment of FIG. 11, the upstream portion of the air space 30A is open and thus receives flow unobstructed by the side walls. Thus, the membrane members 32A and 34A, particularly the distal edges of 32A, will be more susceptible to flow. The second point is, as shown in FIG. 17, the silicon substrate 20A
Free flow velocity envelope 37A starting from the tip edge of
However, from the free flow velocity envelope 37 starting from the tip edge of the silicon substrate 20 shown in FIG. 16, it can be seen that the thin film members 32A and 34A are placed close to a high flow velocity. In the case of the envelope 37, the thin film members 32 and 34 shown in FIG. 16 are far away from the high flow velocity compared to the thin film members 32A and 34A shown in FIG. 17, so they are less likely to be exposed to the high flow velocity. Become.

In the embodiment shown in FIG.
4C and heater 26C are placed on separate membrane members bridging the air space. In this embodiment, heater 26C is placed 50 microns from the near edge of sensors 22C, 24C. That is, the edge 78C of the heater 26C is the sensor 22C.
The edge 86C of the heater 26C is spaced 50 microns from the edge 76C of the sensor 24C. In this embodiment, sensor 22
The resistance value of C, 24C and heater 26C is 1000Ω
The size is approximately 135μ in width and 150μ in length.
The size of the three bridges is approximately 150μ in width.
The length is 300μ. The permalloy plate 90C is added to increase the strength of the thin film member and to make the heat transfer characteristics at both ends of the bridge portion substantially the same when viewed from the lead portion 92C.

The output characteristics of the sensor shown in FIG. 13 were considerably smaller than those of the sensors shown in FIGS. 9 and 10. This is because the space between the heater 26C and the sensors 22C and 24C is widened to 50μ,
This is thought to be due to the fact that increasing the width of the sensor widens the distance between the center line of the sensor and the heater. Therefore, referring to FIG. 7 in conjunction with the embodiment of FIG. 13, sensors 22C and 24
The centerline of C will be placed much further apart than the appropriate distance D1, D2, so that at a given flow rate the average temperature difference obtained between sensors 22C and 24C will be much smaller. It is.

As mentioned above in connection with FIG. 7, the sum of the average value of ΔT1 over one sensor and the average value of ΔT2 over the other sensor is maximized. is desirable. Ideally, this would be best accomplished using a sensor element that would have very low resistance but is very narrow. However, from practical circuit considerations, the resistance value of the sensor should be at least
A resistance of 100Ω or more is required, and preferably between 700Ω and 1000Ω. Combining this requirement with the practical limitations on the length of the thin film structure, the airflow density limitations, and the associated thin film thickness of the resistive element, it is possible to This means that the grid width needs to be at least 75μ. For these reasons, the width of the sensor in the preferred embodiment cannot be made very narrow, but rather has some width. The average value of △T1 on the ideal temperature curve shown in Figure 7 and △
In order to maximize the sum of the average values of T2, the width should be
75μ or more, but because the edge of the sensor must be close to the edge of the heater, in the embodiment of FIG. can no longer be ignored. Therefore, the output characteristics are also smaller than the maximum. Due to the small space of 5μ, the actually measured output characteristics were 3 to 5 times that of the conventional technology, but were almost the same as, or even somewhat smaller than, the output characteristics of the embodiment shown in Figure 10. . The following conclusions can be drawn from the results of these considerations. In the example, if the thickness of the silicon nitride film is increased by 0.8 μm while the other dimensions remain the same, the output characteristics with respect to the flow rate will be significantly reduced. This is because increased thickness causes the sensor to be more closely thermally bonded to the heater due to increased heat conduction therethrough.

Possible drawbacks of the embodiment of FIG. 13 include:
Because the heater and the two sensors are on separate membrane members bridging the air space, various types of physical deformation throughout the manufacturing process can occur.
The problem is that a portion of the heater and sensor tend to be out of the same plane. This drawback is explained in Figs. 9 and 1.
Note that in the embodiment of FIG. 0, this is virtually negligible. That is, each sensor is directly adjacent to a portion of the heater on the same membrane member, thereby ensuring that the heater and sensor are substantially coplanar.

In the embodiment shown in FIG. 14, the connecting portion 94D
The thin film members 32D and 34D connected by
A heater 26D and sensors 22D, 24 are respectively placed thereon.
The two halves of D are placed in series and bridged over the air space. Permalloy grid 2 is used to ensure thermal balance between sensors 22D and 24D.
2D, 24D, 26D and permalloy plates 90D, 9
0DD and lead portion 92D have a rotational symmetry of 180°. The permalloy plate 90D is provided to increase the strength along the centerline of the thin film member, and the permalloy plate 90DD is provided to increase the strength along the centerline of the thin film member and provide contrasting heat transfer characteristics. be. In this embodiment, the air flow is typically aligned along the flow direction of the membrane member, unlike the previously described embodiments where the air flow is aligned along the lateral direction of the membrane member. Sensors 22D, 24
D is relatively narrow, and half the size of each sensor is the width
75μ and length 135μ. The edge of the heater 26D and the sensors 22D and 24D are separated by 25 μ. Sensors 22D, 24D and heater 2 are placed on each thin film member.
Having 6D halves helps ensure coplanarity. In the embodiment shown in FIG. 14, each thin film member has a width of 150 μm and a length of 480 μm, the resistance value of the heater 26D is 1300Ω, and the resistance value of the sensors 22D and 24D is 1050Ω. The output characteristics with respect to the flow of this embodiment are smaller than those of the previously described embodiments. This is due to the relatively small flow of air beneath the membrane member and the fact that the sensor is closer to the wall of the air space and therefore more closely coupled thermally to the silicon substrate.

In the embodiment shown in FIG. 15, the membrane element bridging the air space is comprised of a single membrane element, and the flow is typically aligned in the direction of flow of the membrane element. The size of this thin film member is approximately 150μ in width.
The length is 480μ, and the resistance value of heater 26E is 710Ω.
The resistance value of the sensors 22E and 24E is 440Ω.
This embodiment is substantially the same as the embodiment shown in FIG. 14, except that the resistance value is small and the elements are arranged on one thin film member, and includes a permalloy plate 90E and a lead portion 92EE. increases strength along the center axis of the membrane member, and leads 92E and 92EE provide 180° thermal symmetry.

In the embodiments shown in FIGS. 14 and 15, the heaters and sensors are arranged along the length of the membrane member, and the airflow is also typically shown in the membrane member. placed parallel to the longitudinal direction of the The advantage of these embodiments is that a continuous flow that does not cause this small turbulence can occur when the air flow is aligned laterally across a thin film member, whereas small turbulence can occur due to the shape of the surface. It is about guaranteeing the surface. 1st
As mentioned in connection with the embodiment of FIG. 4, possible disadvantages of the embodiment that aligns the air flow along the length of the membrane member include the following: This means that there is almost no air flow under the thin film member.

In the process of manufacturing this sensor, a silicon wafer having a (100) crystal plane is used, and an insulating layer 29 of silicon nitride is formed on the surface 36 of the silicon wafer. This insulating layer 29 is typically 4000 Å thick and is deposited and formed by conventional low pressure gas discharge sputtering techniques. Then, typically
A uniform layer of permalloy consisting of 80% nickel and 20% iron is sputtered onto the silicon nitride film to a thickness of 800 Å.

By using a suitable photomask and photoresist etching solution, 22, 24, 2 in Fig.
Permalloy elements as shown in 6, 38 are drawn.

Then, a second silicon nitride insulating layer 28 is formed.
It is attached by spatter. This layer is typically 4000 Å thick and is formed to protect the resistive element from oxidation.

Openings 82 (in other embodiments 82A, 82C, 82D,
etc) is etched through the silicon nitride to the silicon surface in the (100) crystal plane. The size of opening 82 is largely a matter of design choice. Dashed lines 114 (labeled 114A, 114C, 114D, etc. in other embodiments) represent the approximate shape of air space 30.

Finally, the silicon beneath the thin film members 32, 34 is etched in a controlled manner using an anisotropic etchant that does not damage the silicon nitride. A mixture of KOH and isopropanol alcohol is suitable as the etching solution.
The inclined planes of air space 30 are surrounded by (111) or other crystal planes that are resistant to etching fluids. The bottom of the air space 30 is
It is a (100) crystal plane with almost no resistance to the etching solution and is located at a certain distance from the thin film members 32, 34, that is, at a depth of 125 microns. This depth is achieved by adjusting the etching time. An etch stop layer, such as a boron-doped etch stop layer, may be used to control the depth of the air spaces, but is not specifically required for the formation of the present invention. By adjusting the etching time, the air space can be increased by 30
This means that the depth can be controlled with an accuracy of about 3μ, and the inkling can be controlled with an accuracy of about 2%. This precision leads to an accurate reproduction of the heat transfer characteristics of the air space surrounding the membrane member and its characteristics relative to the air flow velocity.

In order to effectively scrape off the bottom of a cantilevered thin film member as shown in Fig. 3A or the bottom of a bridging thin film member as shown in Fig. 3,
The straight edge of membrane member 34, designated as 110, is
It is arranged at a non-zero angle 112 with respect to the [110] crystal axis of silicon. (In the present invention,
Although it includes the content that the straight edge or axis of the thin film member is arranged at a certain angle to the silicon [110] crystal axis, the edge of the thin film member is not formed in a straight line or the axis cannot be easily determined. It is also possible to form the thin film member into a certain shape. However, as will be discussed, the placement of the membrane members is angled to achieve this undercut in a minimum amount of time. ) By making the angle 112 approximately 45 degrees, the time to scrape under the membrane member can be minimized. Furthermore, by not setting the angle to 0°, it is possible to manufacture a bridge that connects both ends as shown in Figure 3. That is, such a crosslinking thin film member cannot be formed by arranging the straight edges of the thin film member substantially in the [110] axial direction. This means that the straight edge of the thin film member is [110]
This is because, when placed in the axial direction, the anisotropic etching liquid cannot follow the etching of the (111) crystal plane exposed along this straight edge.

A 45 degree angle 112 will quickly round and flatten the support interface between the semiconductor and the thin film member. This eliminates the 2nd layer under the silicon nitride insulating layer 29 that would otherwise occur if the angle was not 45°.
The stress concentration point at the intersection of two (111) crystal planes can be eliminated.

As previously mentioned, in some applications it is desirable to connect two membrane members using a connecting means. (As this connecting means, for example, the connecting portion 94 in FIG. 10 or the connecting portion 94 in FIG.
(see D). In FIG. 10, the connecting portion 94 is
It serves to maintain a uniform spacing between each membrane member and the bottom of the air space 30. In other words, it maintains uniformity of heat transfer characteristics between them, and also helps uniformity of characteristics within the device.
For the same reason, as mentioned above, it is more convenient to arrange one or more resistive elements on one thin film member, or to arrange one element and part of another element simultaneously. It is. (See Figures 1, 2, 9, 10, 11, 12, 14, and 15). The small rectangular etched openings 82 seen at both ends of the bridges of membrane members 32 and 34 in FIG. 9 make it easier to undercut the silicon substrate 20 beneath membrane members 32 and 34. established for the purpose of However, sensor performance is satisfactory even without such a small etching rectangular opening 82.

FIGS. 3 and 3A show an area 116 for circuit integration as shown in FIGS. 4 and 5.
is also shown. In the examples shown therein, typical dimensions of the thin film members range from 127μ to 127μ in width.
178μ, flow rate 254μ~508μ, thickness 0.8μ~1.2μ.
Heat sensing sensor 2, typically made of permalloy
2, 24, heater 26, and comparison resistor 38 are approximately 800 Å thick (typically between 800 Å and 1600 Å) and have a resistance value at room temperature, i.e. 20-25
It is approximately between 200Ω and 2000Ω at °C.
The resistance value of permalloy varies from room temperature to 400°C.
When the temperature reaches ℃, the value increases approximately three times. The line width of the permalloy lattice can be approximately 5μ, and the line spacing can also be approximately 5μ. The depth of the air space 30 is typically 125μ, but this depth can vary from approximately 25μ.
Can be easily varied between 250μ. The thickness of the silicon substrate 20 is typically 200μ.
These values shown above are only examples and are not limited thereto.

If the thin film member has the typical size described above, the heat capacity will be very small. The heat capacity of the thin film member, heater, and thermal sensor is extremely small; the fact that they are thermally insulated by being supported by a thin insulating means called a silicon nitride layer on the substrate; and the existence of an air space surrounding them. As a result, the response time is extremely short. The time constant was actually measured to be 0.005 seconds.
The thermal sensor can therefore respond very quickly to changes in air flow. It is also possible to drive the heater in pulses at a frequency of 50 Hz or higher if desired.

The operating temperature of the heater 26 is typically 100°C ~
It is set between 400℃, but the desired operating temperature is approximately 200℃ higher than the ambient temperature.
If a permalloy element is used, this is only 2
This can be achieved with a power of ~3 mW. These power levels can be accommodated by integrated circuits, so they can be fabricated on the same silicon substrate with the sensor if desired, as mentioned above.

If we use a heater resistor with a resistance between 600Ω and 1000Ω at 25°C, a voltage of 2-3V and a current of 2-3mA will give the power dissipation to reach the appropriate operating temperature. .
In addition, in this example, the resistance value of the permalloy heater element was selected between 600Ω and 1000Ω because
This is because there is also a factor of damage to the element due to electromigration. Electromigration is a temperature-dependent damage mechanism inside a conductor caused by the movement of matter when the current density exceeds a certain critical value. This critical value for permalloy is of the order of 10×10 ⁶ A/cm ² at 25°C. In a preferred embodiment, the resistance of the heater element is typically
The current density is set to 600Ω to 1000Ω, the line width is 5μ, and the thickness is 0.08μ, so the current density is effectively about 0.6
It becomes smaller than ~10 ⁶ A/cm ² . At current densities of this order, electromigration is not a detrimental factor.

The impedance of a standard temperature sensor used industrially is about 100Ω. However, for purposes of the present invention, such a low resistance sensor should have a resistance of 600Ω to 1000Ω at 25°C and a thickness of approximately 0.08μ, as used in the embodiment of the present invention. Comparatively speaking, it is not desirable. For example, in manufacturing, it is desirable that the resistance values of two heat sensing sensors located upstream and downstream match with an accuracy of about 0.1%. This matching is made easier by using higher resistance values. Furthermore, if a sensor with a higher resistance value is used,
Undesirable effects such as resistance differences associated with leads on silicon chips can also be reduced. Furthermore, in order to accurately obtain changes in voltage due to small changes in air flow using a small current, it becomes necessary to use a higher resistance. In addition, if a small current is used,
Self-heating of the heat sensing sensor itself can be avoided. In this case, the self-heating of this thermal sensor is
It changes the heat field of the heater and reduces its temperature sensitivity to air flow, but the effect is not too severe. In addition, increasing the current flowing through the thermal sensor increases undesirable effects such as various mismatches between the two sensors in the absence of air flow.

For manufacturing purposes, it is simpler and more economical to choose the same permalloy thickness for both the heater and the thermal sensor. From this point of view, as mentioned above, in this example as well, the resistance values of the heater and the heat sensing sensor are typically similar to each other due to the permalloy having a thickness of 0.08μ, and are values that can be easily achieved. .

For many contemplated applications, the preferred element of the present invention is a permalloy resistive element, as described above. Because the thin film members 32, 34 are surrounded by a thin silicon nitride layer, the permalloy element is protected from oxidation by air, allowing it to be used as a heater element at temperatures above 400°C. The temperature dependence of the resistance of this permalloy element is similar to that of platinum, and both are
It has a temperature coefficient of resistance of 4000ppm. However, permalloy is superior to platinum for the structure of the present invention. Platinum is also commonly used as a resistance element for temperature sensing, but it has the advantage of having a resistance value roughly twice that of permalloy platinum. Moreover, when viewed in the form of a thin film, permalloy's temperature coefficient of resistance reaches its maximum at a thickness of 800 to 1,600 Å, while platinum's thickness reaches a thickness of at least 3,500 Å. The temperature coefficient of resistance of permalloy reaches its maximum at a thickness of approximately 1600 Å, but with the present invention it reaches a maximum at a thickness of 800 Å.
was chosen because the resistance value is doubled and the temperature coefficient of resistance is only slightly smaller than the value of 1600 Å. Therefore, by using a permalloy element with a thickness of 800 Å, the thickness required for platinum is only 1/
The same resistance value can be achieved with a surface area of 8.
This means that permalloy increases the thermal efficiency of heaters and sensors, and reduces cost by reducing the required surface area.

That is, in the present invention, as disclosed, the permalloy element is used for both the microstructured temperature change detection sensor element and the heater element.

Furthermore, the heater and sensor made of permalloy are 1μ
Encasing it in a moderately thick insulating layer of silicon nitride provides a protective layer against oxidation phenomena, which are particularly problematic at high temperatures. The silicon nitride insulating layer also has the function of thermally insulating the permalloy element from the silicon substrate. Since silicon nitride has high resistance to etching, the dimensions of the thin film members 32 and 34 can also be precisely controlled. Further, due to the high etching resistance characteristic of silicon nitride, the depth of the air space 30 can be made as deep as 25 to 250 microns by etching. This air space determines the heat transfer factor which is the most important.

As described above, in the preferred embodiment of the present invention, due to the balance with the disclosed microstructure,
Form a heat sensing sensor and heater using permalloy.
The insulating layer of silicon nitride is used as a supporting material and as a protective material to achieve the etch time required to form the desired structure.
Furthermore, as previously mentioned, by properly orienting the thin film member with respect to the silicon crystal plane, the desired structure can be formed without the use of artificial etching stoppers and can be removed in a minimum amount of time. Furthermore, by using anisotropic etching to create deep air spaces of 25 to 250 µm, greater thermal isolation is achieved compared to the conventional placement of resistive elements in integrated semiconductor devices. That will happen.

The gist of the present invention is not limited to what has been described in the examples. For example, the heat sensing element and the heater element are not limited to permalloy, but may be any suitable material. Other examples would include pyroelectric materials such as zinc oxide films, thin film thermocouples, thermistor films of semiconductor materials, and metal films with favorable temperature coefficients of resistance other than permalloy. It should be noted that although the text has sometimes referred to air as the flow medium being measured, the invention is applicable to many other gaseous substances. Let me add something. That is, for the purpose of application of the present invention, the term "air" is defined to include general gaseous substances.

[Brief explanation of drawings]

1, 2, 3, 3A and 9 to 15 show embodiments of the present invention. FIGS. 4 and 5 show examples of circuits used in the present invention. 6th, 7th, 8th, 1617th
The figure shows a characteristic diagram of the present invention. 20... Board, 22, 24... Heat sensing sensor,
26... Heater, 28, 29... Insulating layer, 30...
...Air space, 32, 34...Thin film member.

Claims (1)

[Claims] 1. A silicon semiconductor substrate having a crystal plane (100) and a crystal axis [110] and an air space formed on the uppermost surface, one of a pair of heat sensing sensors and a heater. a thin film member which includes a part of the film and is surrounded by a thin film insulating layer, the thin film member being arranged on a plane substantially parallel to the crystal plane (100) and located in the air space; a first thin film member having at least one end held by the semiconductor substrate and most of which is disposed in a non-contact state with the substrate; the other of the pair of heat sensing sensors and the remainder of the heater; a thin film member surrounded by an insulating layer, at least one end of which is disposed on a plane substantially parallel to the crystal plane (100) and positioned in the air space; a second thin film member held by the substrate, most of which is disposed in a non-contact state with the substrate, and the pair of thin film heat sensing sensors are positioned with the heater in between. A flow velocity sensor characterized by: 2. The flow velocity sensor according to claim 1, wherein the thin film insulating layer is made of silicon nitride. 3. A silicon semiconductor substrate having a crystal plane (100) and a crystal axis [110] and having an air space formed on the uppermost surface, one of a pair of heat sensing sensors and a part of a heater, A thin film member surrounded by a thin insulating layer, the thin film member being arranged on a plane substantially parallel to the crystal plane (100) and located in the air space, at least one end of which is connected to the semiconductor. The first thin film member is held on the substrate and most of the part is disposed in a non-contact state with the substrate, the other of the pair of heat sensing sensors and the remainder of the heater, and is surrounded by a thin film insulating layer. a thin film member, at least one end of which is held on the semiconductor substrate so as to be disposed on a plane substantially parallel to the crystal plane (100) and located in the air space; a second thin film member that is mostly disposed in a non-contact state with the substrate; and a connecting member that connects the first and second thin film members, and the pair of thin film heat sensing sensors. is a flow velocity sensor, characterized in that it is located across the heater. 4. The flow rate sensor according to claim 3, wherein the thin film insulating layer is made of silicon nitride. 5. A silicon semiconductor substrate having a crystal plane (100) and a crystal axis [110] and an air space formed on the uppermost surface, and a silicon semiconductor substrate on a plane substantially parallel to the crystal plane (100). a thin film heater whose at least one end is held by the semiconductor substrate so as to be located in the air space, and whose most part is arranged in a non-contact state with the substrate; and the crystal plane (100). at least one end of which is held on the semiconductor substrate, and most of which is disposed in a non-contact state with the substrate, so as to be located on a plane substantially parallel to the air space and in the air space. a thin film heat sensing sensor; a comparison resistance element provided on the semiconductor substrate for detecting ambient temperature; and an electronic circuit for controlling a bridge circuit including the comparison resistance element and the thin film heater. , each of the pair of thin film heat sensing sensors is arranged on opposite sides of the heater, and the heater is driven by the electronic circuit at a constant higher temperature than the ambient temperature. . 6. A silicon semiconductor substrate having a crystal plane (100) and a crystal axis [110] and an air space formed on the uppermost surface, and a thin film member is formed by being wrapped with a thin film insulating layer, and further, At least one end of the thin film member is held on the semiconductor substrate so that it is disposed on a plane substantially parallel to the crystal plane (100) and located in the air space, and almost the majority of the thin film member is held on the semiconductor substrate. a thin-film heater disposed in a non-contact state with the thin-film heater; and a thin-film heater surrounded by a thin-film insulating layer to form a thin-film member, further disposed on a plane substantially parallel to the crystal plane (100) and the above-mentioned a pair of thin film thermal sensing sensors, at least one end of the thin film member being held in the semiconductor substrate so as to be located in an air space, and a substantially large portion of the thin film member being disposed in a non-contact state with the substrate; Each of the pair of thin film heat sensing sensors is provided with a comparison resistance element for detecting ambient temperature, and an electronic circuit for controlling a bridge circuit including the comparison resistance element and the thin film heater. A flow velocity sensor disposed on opposite sides of the heater, wherein the heater is driven by the electronic circuit at a constant temperature higher than ambient temperature.