JPH0795076B2 - Flow velocity sensor - Google Patents

Description

The present invention relates to a flow velocity sensor for measuring the velocity of air flow. Commercially available air flow velocity sensors typically place a single hot wire or thermistor at the tip of a long probe that is inserted into the air flow.

This is to measure the air flow velocity by the change in electric resistance value accompanied by the temperature decrease caused by the cooling effect of the air flow.

With such a device configuration, the sensor element is exposed to the air flow, and is therefore easily damaged or soiled. Moreover, since the temperature change due to the air cooling has no linearity, it is necessary to linearize the obtained electric signal by an electronic circuit. Moreover, these are expensive and are not suitable for mass production.

Related to the present invention are the following commercially available mass flow rate sensors. This sensor consists of a metal tube through which air and other measuring gases pass, a transformer that resistively heats one area of this metal tube, two large heat sinks mounted in this area, and the center of this heat area and the heat sink. It consists of two thermocouples symmetrically attached to the metal in the middle of the thermal zone between. The flow of air through the metal tube cools the upstream thermocouple and heats the downstream thermocouple. When the transformer is driven with a constant electric power, the difference in the output voltage of the thermocouple is a measure for measuring the mass flow rate.
This is a large device that requires considerable power. It is not suitable for flow measurement in large ducts or outdoors, is expensive, and cannot be mass-produced. Therefore, there is a need for a flow velocity sensor or mass flow velocity sensor having the following characteristics and a signal processing circuit related thereto. That is, long life, maintenance-free, small size, low power consumption,
It is easily applicable to a wide range of fields, has a large output signal, and has output characteristics that are linear or easily linearized over a wide speed range. Furthermore, it must be mass-produced and inexpensive.

In the literature, in connection with these requirements, several attempts have been made to improve flow sensors. These attempts generally seek to utilize pyroelectric materials, or silicon and its semiconductor properties, as described below.

Although these attempts have provided technical improvements in some respects, they are still unsatisfactory with respect to many of the properties required of current flow sensors. The present invention significantly meets these requirements compared to any prior art,
It is technically superior. The well-known and most relevant prior art will now be described.

The flow velocity sensor invented by Huising (* 1) et al. Has two identical temperature sensing elements consisting of diffusion type transistors embedded in the vicinity of both ends of the silicon chip, and the silicon chip placed in the center of them is 45 ° C above the air temperature. The heater element is composed of a diffusion type transistor for heating.

When air flows, the temperature sensing element located upstream of the flow
The temperature is slightly lower than that of the temperature detecting element located downstream, and the temperature difference between these two temperature detecting elements becomes the difference in current, which is converted into voltage and the flow velocity of air is measured. This temperature sensing element must be placed at opposite ends of the chip in order to realize a temperature difference that can be sensed, but the temperature difference that is still generated is small and is 0 to 500 cm / sec in the speed range. It is only a temperature change of less than 0.2 ℃.

* 1 JH Huijing, et al: IEEE Transactions on Elect
ron Devices, Vol.ED−29, No.1, pp.133−136, January, 19
82 The flow velocity sensor invented by Bapten (* 2) et al. Consists of the same diffusion resistance element embedded on each of the four sides of the silicon chip facing each other.

All resistive elements are self-heating, which heats the silicon chip well above the temperature of the incoming air. The resistance element is driven by an electric double bridge circuit. In the absence of air flow, all resistive elements are at the same temperature, so the double bridge circuit is electrically balanced. The upstream and downstream resistance elements, which are perpendicular to the flow of air, are cooled more than the resistance elements on both sides parallel to the flow. This temperature difference destroys the balance of the double bridge and the air velocity is measured.

* 2 AFPVan Putten, et al: Electronics Letters, V
ol.10 No.21 pp.425-426, October, 1974 The mass flow sensor invented by Marin (* 3) is a center between the two sensors consisting of a diffusion type resistive element on a large silicon strip. It is composed of a diffusion type heater element which is arranged at.

This technique is similar to commercially available metal tube heating type mass flow sensors. The air flow heats the downstream sensor and cools the upstream sensor so that the temperature difference between these sensors results in a voltage difference across the sensor and the mass flow rate is measured.

* 3 K. Malin, et al: IBM Technical Disclosure Bulle
tin, Vol.21, No.8 January, 1979 Lahanamai (* 4) et al. attached a thin metal film to the entire back surface of a thin plate of polished single crystal lithium tantalate arranged crystallographically A sensor including a heater resistance element attached in a thin film shape at the center and two thin film electrodes arranged at the same distance from the heater resistance element is disclosed.

Here, the size of lithium tantalate is 8 mm in length and 4 in width.
mm, minimum plate thickness 0.06 mm. As described 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 on the upstream side and the downstream side form the same capacitor separated between the electrode surface on the back surface and serve as a capacitor for temperature detection. The operation is to heat the heater element periodically with respect to the temperature of the flowing air by driving the heater element with a voltage having a low frequency such as 2 to 10 Hz. The sensor element is also correspondingly heated periodically by heat conduction through the lithium tantalate. Since this lithium tantalate is a pyroelectric material, it causes polarization depending on the temperature, but when there is no air flow,
The periodic polarization voltages generated in the two sensors are the same. Therefore, the voltage difference between the two sensors when there is no air flow is zero. As mentioned in the literature, when there is a flow of air, the sensor element located upstream is cooled more than the sensor element located downstream,
The difference in temperature of these sensors results in a difference in voltage and the flow velocity is measured.

* 4 H. Rahnamai, et al: paper presented at the 1980
International Electron Devices Society of IEEE, Wa
shington DC, pp 680-684 December 8-10, 1980 As mentioned above, these attempts have provided technological improvements in some respects, but still remain uncertain with regard to many of the properties demanded of today's sensors. It is unsatisfactory. The present invention is an advance of the technology to sufficiently satisfy these requirements.

The present invention comprises a single resistive grid 26 and a substrate 20 which holds them in a floating manner. This resistance grid has two voltage outlets and is divided into two takeout portions (heat detection sensor portion) and non-takeout portions (center portion). The two heat detection sensor portions are on both sides of the nontakeout portion. Is placed opposite to.

As a specific example of the present invention, the substrate 20 is a semiconductor, and silicon is selected from the viewpoint that a particularly precise etching technique can be applied and the chip productivity is high. And a single resistive grid in the form of a grid formed on this substrate
26 functions not only as a thin film heater but also as a heat detecting sensor.

As the resistance grid 26, it is suitable to use an alloy of iron and nickel, for example, a permalloy made of 80% nickel and 20% iron. This resistance grid 26
For example, it is wrapped with thin film insulating layers 28 and 29 made of silicon nitride to form a thin film member. As shown in the embodiment of FIG. 2, the sensor is composed of a thin film member 32F including a resistance grid 26F and has a size of width 150μ and length 400μ.

Further, the sensor disclosed in the present invention has an air space 30 that effectively encloses the resistive grid 26. This air space 30
Are microstructured on the surface 36 of silicon. That is, the resistance grid 26 has a thickness of about 0.08 to 0.12μ and a line spacing of about 5
.mu. and a line width of 5 .mu., and these are configured to be wrapped by a thin film of silicon nitride so that the total thickness is about 0.8 .mu. or less. Then, an air space 30, which is a depression, is accurately formed in the silicon substrate 20 below the thin film member 32 by etching to a depth of 125 μ.

Membrane member 32 is connected to the top of surface 36 of silicon substrate 20 at one or more edges of air space 30. For example, as shown in FIG. 1, the thin film member 32 may be configured to bridge the air space 30 or to form a cantilever as shown in FIG. 1A.

Silicon nitride is a very good thermal insulator. Since the silicon nitride film that surrounds the thin film member 32 is extremely thin and has a good heat insulating property, most of the heat transferred from the central portion of the resistance grid 26 to the heat sensing sensor section is transferred through the air surrounding the resistance grid 26.

The principle of the present invention of detecting the flow velocity of air will be described with reference to FIG. The resistance grid 26 is 200 degrees above the substrate 20 temperature.
It is heated to a constant temperature that is ℃ higher. This silicon substrate 20
The temperature of 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 sardip package, the temperature of the silicon substrate 20 is 0.5 ° C higher than the temperature of the flowing air. It just gets higher.

Further, even if the temperature of the resistance grid 26 is kept 200 ° C. higher than the temperature of the flowing air, electric power smaller than 0.001 W is merely required.

As shown in the figure, the heat detection sensor part (ejection port 98-102
Between the two sensors and between 100 and 104) are arranged symmetrically with respect to the central part of the resistance grid 26, so that there is no difference in the resistance values of the two sensors when the air velocity is zero.

When there is a flow of air, the heat-sensing sensor portion located upstream in this embodiment is cooled as heat is carried away by the air-flow towards the resistance grid 26, and the heat-sensing sensor portion located downstream from the resistance grid 26. It is heated by the flow of air. The resulting difference in resistance between the heat sensitive sensor portions causes a difference in voltage value and the flow velocity is measured.

Due to the extremely small heat capacity of the resistance grid 26, the thermal insulation provided by the silicon nitride film that is the means for connecting to the substrate, and the presence of the air space 30, its response has a time constant of 0.005 according to the measurement results. Very fast in seconds, the heat sensitive sensor portion responds very quickly to changes in air flow.

In the present invention, the resistance grid 26 is driven so as to have a constant temperature with respect to the temperature of the air, and the temperature change of the heat detecting sensor portion changes the resistance value and is detected as the voltage change.

The circuit of FIG. 4 is for controlling the temperature of the resistance grid 26.

In the present invention, the temperature of the ambient air depends on the silicon substrate 20.
Is monitored by a comparison resistor 38 formed as a heat sink. The comparative resistor 38 is composed of a lattice-shaped permalloy like the resistor lattice 26, and has a silicon surface 36.
It is arranged on top of which it is wrapped in insulating layers 28 and 29.

Since the insulating layers 28 and 29 have a total thickness of 0.8 μm and are relatively thin, they have relatively good heat conduction, so that heat flows in and out through these insulating layers in the vertical direction. The comparative resistor 38 is wrapped in an insulating layer and attached directly to the surface 36 of the substrate 20 and is within 0.5 ° C. of the ambient air temperature even if the heater 26 is heated 200 ° C. above the ambient temperature. The temperature of the substrate 20 can be easily monitored.

In other words, the comparison resistor 38 detects the temperature of the flowing air which almost matches the temperature of the substrate 20.

The temperature control circuit of FIG. 4 compares the temperature of the resistance grid 26 with the comparison resistance 38
It is composed of a Wheatstone bridge circuit 46 for maintaining a constant temperature higher than the ambient temperature detected by. As described above, in the embodiment of the present invention, this constant value is set to about 200 ° C. Wheatstone bridge circuit 46
Is composed of one side composed of the heater 26 and the resistor 40, and another side composed of the comparative resistor 38 and the resistors 42 and 44. Amplifier
The integrating circuit composed of 48 and 50 operates so that the bridge circuit 46 is balanced by changing the potential of the output, and keeps the electric power consumed by the heater 26 constant.

One of the characteristics of the sensor disclosed in the present invention is that it is configured to have a large difference from the temperature sensed by the heat sensing sensor portion with respect to the flow velocity of air in a wide range. As a result, the accuracy of the flow velocity measurement is remarkably enhanced, and the measurement becomes easy. A large temperature difference effect can be obtained by combining the outputs of two sensors, that is, a heat-sensing sensor portion located upstream that is cooled by the flow of air and a heat-sensing sensor portion located downstream that is heated by heat from the central portion. Of. In order to combine the heating and the cooling to obtain a large temperature difference, (1) it is necessary to thermally couple the heat detecting sensor portion to the air relatively strongly.

That is, the heat sensitive sensor portion must be substantially thermally isolated from the silicon substrate 20. And this is that the thermal conductivity in the direction along the longitudinal direction of the silicon nitride film enclosing the thin film members 32F and 32G is relatively small,
This is made possible by an air space 30 precisely formed between the thin film members 32F, 32G and the substrate 20 at a depth of about 125μ.

(2) In this embodiment, it is necessary to greatly cool the heat detection sensor portion located upstream of the air flow. This requires setting the temperature of the resistance grid 26 high. This is because the heat sensing sensor is thermally insulated from the silicon substrate 20 through the air space 30, and the thin film member is used.
This is possible due to the small heat conduction in the direction along the longitudinal direction of 32.

(3) In this embodiment, the heat detection sensor portion located downstream of the air flow needs to be heated significantly by transmitting heat from the central portion. This is an air space 30
To thermally insulate the heat sensing sensor portion from the silicon substrate 20, the heat conduction in the direction along the lengths of the thin film members 32F and 32G is small, and the temperature of the resistance grid 26 is set high. It becomes possible by doing.

(4) It is necessary to select an optimum value for the distance between the heat detection sensor portion and the central portion.

The embodiment shown in FIG. 2 consists of an electrically resistive grid 26F with voltage outlets 98, 100, which allows the measurement of the voltage across the resistance section at each end of the total resistance. I am trying. Voltage outlet 98 shown,
Although 100 takes out three grid lines at each end of a grid of 14 lines, it is not limited to this and any desired number of grid lines can be taken out. Further, the distance between the taken-out portion and the non-take-out portion (central portion) can be changed from the shown distance to a desired distance.

As shown, the electrically resistive grid 26F is a thin film member 32F.
Occupies most of the central area of. The grid lines of the grid 26F are arranged parallel to the longitudinal direction of the thin film member 32.

The total resistance of the grid 26F is 1510Ω, and the voltage outlets 98, 100
Makes it possible to measure the 330 Ω resistance section delimited by both ends of the total resistance. The size of the thin film member 32F supporting the grid 26F above the air space 30 is approximately 150μ wide and 400μ long in the illustrated embodiment. Permalloy plates 106 and 108 are electrically isolated,
It is used to strengthen the thin film member 32F.

In the embodiment shown in FIG. 3, the resistance grids are arranged along the longitudinal direction of the thin film member and the air flow is also typically parallel to the longitudinal direction of the thin film member as shown. Is located in.

Whereas a small turbulence may occur in the shape of the surface when the air flow is laterally aligned with the thin film member,
The advantage of these embodiments is that they guarantee a continuous surface that does not cause this small turbulence. A possible disadvantage of the embodiment in which the air flow is aligned with the longitudinal direction of the thin film member is that there is almost no air flow under the thin film member as compared to the embodiment in which the air flow is aligned with the lateral direction of the thin film member.

By connecting the leads 102, 104 to a circuit such as that shown in FIG. 4, the grid 26F is self-heated to a temperature approximately 200 ° C. above ambient.

At such temperatures, the total resistance of grid 26F is approximately 2500Ω. Since the air flow is directed in the lateral direction of the thin film member 32F, the upstream side is cooled more than the downstream side. If the two voltage outlet sections are the same, the voltage difference is zero when there is no air flow. When there is a flow of air, a temperature difference occurs between the two voltage outlet sections, so that a difference in resistance value occurs and a voltage difference occurs. This voltage difference becomes the output of the sensor corresponding to the air flow velocity.

In the embodiment shown in FIG. 3, the grid lines of the resistive grid 26G are arranged so as to be orthogonal to the center line of the thin film member 32G, and the air flow is relative to the center line of the thin film member 32G. 2 is substantially the same as the embodiment of FIG. 2 except that it is oriented parallel. The grid in the embodiment of FIG.
The total resistance value of 26G is 1420Ω, and the resistance value of the resistance section between the voltage extraction ports 98G and 100G and each terminal of the total resistance is 420Ω.
The total resistance of the gratings of the embodiments of FIGS. 2 and 3 is typically in the range of about 500Ω to 2000Ω at about 25 ° C. The resistance value of the resistance section between each end of the total resistance value and the voltage outlet is typically in the range of approximately 20% to 40% of the total resistance value of the grid. The other values shown in this embodiment are merely examples, and the present invention is not limited thereto.

The single resistance element with two voltage outlets shown in FIG. 2 and FIG. 3 has four terminals as compared with one in which two temperature detection elements and one heater element are individually formed. Since the distance between the temperature detecting element portion and the central heater element portion can be minimized, the overall size can be reduced. Therefore, it is not necessary to provide, for example, a strengthening permalloy plate or an etching opening between the temperature detecting element and the heater element, which is often required in the former case of 6 terminals.

According to a preliminary experiment, if the same flow velocity is observed, the output is larger when the resistance grid is arranged at the end of the chip than when the resistance grid is arranged at the center position of the chip. Therefore, when the side wall of the upstream portion or the downstream portion of the air space 30 is cut off, the side wall does not block the side wall, so that the air flows more easily. The output characteristics will be greater than the example where the chip is placed in the center of the chip, whichever is lacking, but the lack of the side wall in the upstream part of the air space shows significantly higher output than the one without the side wall in the downstream part. It was

As an alternative flow velocity sensor configuration, air space
Provided on one or more thin film members across 30
A device having two self-heating resistance elements can be considered. Here, one resistance element is placed downstream with respect to the other resistance element, both resistance elements being configured to act alternately as a heater and a sensor. That is, when the upstream resistance element is heated to a constant temperature higher than the surroundings, the temperature of the downstream resistance element is detected.

Next, the two resistance elements switch their functions, and the resistance element that previously worked as a sensor is heated to a constant temperature higher than the surroundings this time, and the temperature of the resistance element that worked as a heater is detected. .

In this way, the two resistance elements alternately switch the self-heating mode and the temperature detection mode. That is, when the resistance element is in the temperature detection mode, it is not heated by its own heat generation but is heated by another resistance element in the self-heat generation mode. If the two resistance elements are alternately heated to the same temperature in the self-heating mode, each resistance element in the temperature detection mode should be heated to almost the same temperature in the absence of air flow. become. Thus, under no air flow, the difference between the two pulsed temperature signals will be almost zero.

When there is a flow of air, the resistance element on the downstream side is heated more than the resistance element on the upstream side, so that there is a significant difference between the temperature signals of the two resistance elements, and as a result, the flow velocity signal is obtained. .

Another alternative configuration of the speed sensor is one having three resistance elements provided on one or more thin film members that traverse the air space 30. The central resistive element here is configured to be heated by current pulses having a pulse width and spacing such that the increase and decrease of the self-heating temperature pulse approaches thermal equilibrium at the top and bottom of the current pulse. Typically, this pulse width and pulse interval are set significantly. The two resistive elements, which act as sensors, sense what corresponds to the heat pulse by heat transfer through the air.

That is, as mentioned above, being affected by the flow of air,
The exothermic pulse causes a corresponding exothermic pulse in the sensor element. The circuit to heat the resistive element with pulse is
Although it is much more complicated than the circuit required for DC mode to generate heat at a constant temperature, it has some advantages in application.

For example, the output of the flow velocity sensor in this configuration is actually 2
Obtained by subtracting the voltage pulse corresponding to the temperature pulse sensed by the two sensor elements. That is, the difference between the two AC pulses is output.
The output of such an AC component is convenient when voltage insulation (DC insulation) is required between the flow velocity sensor and the signal processing circuit. Another advantage is that it is possible to reduce the error in the flow velocity measurement caused by the slight difference in the resistance values of the two resistance elements serving as the sensor. The difference in this change in resistance is
For example, it can be caused by dirt on the sensor or a difference in oxidation. By taking the difference of the AC voltage of the two sensors,
Such an error cannot be eliminated, but it can be reduced. This is because, by taking the difference, the change in the resistance value of the sensor due to oxidation or dirt can be regarded as a slight change in the AC pulse, not as a change in the resistance value itself.

Therefore, the magnitude of the output voltage error can be reduced to a factor that is approximately equal to the ratio of the resistance value to the pulse resistance value of the sensor.

Although the use of the pulse mode is desirable for application, it is not always necessary or typically used in the embodiments of the present invention because the signal processing circuit becomes complicated.
In addition, a 3310 function generator manufactured by Hewlett-Packard Co. was used to heat in a pulsed manner in a pulse mode operation. Such circuits are silicon substrates
It is possible to integrate on 20, ie in the region 116.

In the process of manufacturing the present sensor, a silicon wafer having a (100) crystal face is used, and an insulating layer 29 of silicon nitride is formed on the surface thereof. This insulating layer 29 is typically 4000 angstroms thick and is deposited by conventional low pressure gas discharge sputtering techniques.

Next, a uniform layer of Permalloy, typically 80% nickel and 20% iron, is sputter deposited onto the silicon nitride film to a thickness of 800 Å.

By using an appropriate photomask and photoresist etchant, a permalloy element as depicted at 26F in FIG. 2 is drawn.

A second insulating layer 28 of silicon nitride is then deposited by sputtering. This layer is typically 4000 angstroms thick and is formed to protect the resistive element from oxidation.

Openings 82F, 82G to form thin film members 32F, 32G
Is etched through the silicon nitride screen to the silicon surface of the (100) crystal plane. The size of the openings 82F, 82G is almost a design choice. Dashed lines 114F, 114G
Represents the approximate shape of the air space 30.

Finally, the silicon under the thin film members 32F and 32G is etched by a controlled method using an anisotropic etching solution that does not damage the silicon nitride. As an etching solution, KOH
A mixture of and isopropyl alcohol is suitable.

The inclined surface of the air space is surrounded by (111) or other crystal planes that are resistant to the etchant.
The bottom surface of the air space 30 is a (100) crystal plane that has almost no resistance to the etching liquid, and the thin film members 32F, 32
It is located at a certain distance from G, that is, at a depth of 125μ. This depth is achieved by adjusting the etching time. A stop layer that stops the boron-doped etch to control the depth of the air space can also be used, but is not specifically required for the formation of the present invention. By adjusting the etching time, the air space
The depth of 30 can be controlled with an accuracy of about 3μ or about 2%.

This accuracy leads to the heat transfer characteristics of the air space surrounding the thin film member and the accurate reproducibility of the characteristics with respect to the air flow velocity.

Under a cantilevered thin film member as shown in Figure 1A,
In order to effectively scrape under a bridging thin film member as shown in FIG. 1, the straight edge of thin film member 32, shown as 110 in FIG. 3, is non-zero with respect to the [110] crystallographic axis of silicon. Are arranged at an angle 112. (In the present invention, the straight edge or axis of the thin film member also includes the content of arranging at a certain angle with respect to the [110] crystal axis of silicon. However, it is possible to shape the thin-film member in such a way that it cannot be determined easily.However, as will be described below, the arrangement of the thin-film member is arranged at an angle that realizes this undercut in a minimum time. By setting 112 to approximately 45 °, the time to scrape under the thin film member can be minimized. Further, by not setting the angle to 0 °, it becomes possible to manufacture a bridge whose both ends are connected as shown in FIG. That is, such a bridging thin film member cannot be formed by arranging the straight edges of the thin film member substantially in the [110] axis direction. This is because the straight edge of the thin film member is [11
This is because when it is arranged in the [0] axial direction, the anisotropic etching solution does not scrape off the (111) crystal plane exposed along this straight edge.

When the angle 112 is 45 °, the supporting interface between the semiconductor and the thin film member is quickly rounded and flattened. As a result, it is possible to eliminate stress concentration points at the intersections of two (111) crystal planes below the silicon nitride insulating layer 29, which occur when the angle is not 45 °.

Also shown in FIGS. 1 and 1A is a region 116 for integration of the circuit of FIG. In the illustrated embodiment, the thin film member typically has a width of 127 μ to 178 μ and a length of 254 μm.
μ-508μ, thickness 0.8μ-1.2μ. Resistor grid 26, typically made of permalloy, comparative resistor 38 is approximately 800
It has a thickness of angstroms (typically between 800 and 1600 angstroms) and its resistance is room temperature
It is approximately between 200Ω and 2000Ω at 20 to 25 ° C. The resistance value of permalloy ranges from room temperature to 400 ° C.
Then, the value increases to about 3 times. The line width of the permalloy grating can be about 5μ and the line spacing can be about 5μ.
The depth of the air space 30 is typically 125μ, but this depth can easily be varied between approximately 25μ and 250μ. The thickness of the silicon substrate 20 is typically 20
It is 0μ. These values are merely examples and are not limiting.

When the size of the thin film member is the above-mentioned typical size, the heat capacity becomes very small. The heat capacity of the thin film member, the heater and the heat detecting sensor is extremely small, they are thermally insulated by being supported on the substrate by a thin insulating means such as a silicon nitride layer, and the presence of an air space surrounding them. Makes the response time very short.

The actual constant was 0.005 seconds in the actual measurement.

Therefore, the heat sensitive sensor can respond very quickly to changes in the air flow. Also, if desired, heater at 50Hz
Alternatively, it is possible to drive in a pulsed manner at a higher frequency.

The operating temperature of the resistance grid 26 is typically set between 100 and 400 ° C, but the desired operating temperature is set approximately 200 ° C above ambient. With a permalloy element, this can be achieved with only a few mW of power. These power levels can be accommodated by integrated circuits, so they can be fabricated with the sensor on the same silicon substrate if desired, as described above.

If a resistive grid with a resistance between 600 and 1000 Ω at 25 ° C. is used, then a voltage of 2-3 V and a current of 2-3 mA will be used to provide the power dissipation to reach the proper operating temperature. In the present embodiment, the reason why the resistance value of the permalloy resistance element is selected to be 600 to 1000Ω is that there is also a factor of element damage due to electromigration. Electromigration is a damage mechanism inside a conductor caused by movement of a substance when a current density exceeds a certain critical value, and is dependent on temperature. This critical value for permalloy is 25 ° C
In an order of ^{10 × 10 6 A / cm 2} , as the preferred embodiment, the resistance value of the heater element is typically 600-1000
Ω, line width is 5μ and thickness is 0.08μ, so
The current density is substantially less than about 0.6 × 10 ⁶ A / cm ² . At this current density, electromigration is not a detrimental factor.

The impedance of a standard temperature sensor used in the industry is about 100Ω. However, for the purposes of the present invention, such a low resistance sensor has a resistance value of 600-1000 Ω at 25 ° C. used in the embodiments of the present invention and a thickness of approximately 0.08μ. Compared to the one
Not desirable. For example, in manufacturing, it is desirable to match the resistance values of the two heat detection sensors located upstream and downstream with an accuracy of the order of 0.1%. This match is
Higher resistance sensors can also reduce undesired effects such as different resistance values associated with the leads on the silicon chip. Furthermore, in order to accurately obtain a voltage change due to a slight change in the air flow with a small current, it is necessary to use a higher resistance. In addition, a small current can be used to avoid self-heating of the heat sensing sensor itself. in this case,
The self-heating of the heat-sensitive sensor changes the heat field of the heater and reduces the temperature sensitivity to the air flow, but not so badly. In addition, the higher currents flowing through the heat sensitive sensors exacerbate the undesirable effects of various mismatches between the two sensors in the absence of air flow.

For manufacturing purposes, choosing the same permalloy thickness for both the heater and the heat-sensitive sensor makes it easier.
Become more economical. From this point of view, as described above, the resistance values of the heater and the heat detection sensor are also typically in this embodiment.
The value is similar to that of 0.08μ thick permalloy and can be easily realized.

For many planned applications, the preferred element of the present invention is a permalloy resistive element, as described above. Since the thin film members 32F and 32G are wrapped in a thin silicon nitride layer, the permalloy element is protected from oxidation by air,
It can also be used as a heater element having a temperature exceeding 0 ° C. The temperature dependence of the resistance of this permalloy element is similar to that of platinum, and both have a resistance temperature coefficient of about 4000 ppm at 0 ° C.

However, permalloy is superior to platinum for the structure of the present invention. Platinum is also commonly used as a resistance element for temperature detection, and permalloy is about 2% that of platinum.
It has the advantage of having double the resistance value. Moreover, if viewed as a thin film, the temperature coefficient of resistance of permalloy is 800-1600
The maximum thickness is angstroms, but platinum is at least 3500 angstroms thick.

The temperature coefficient of resistance of permalloy is maximum at a thickness of approximately 1600 angstroms, but 800 angstroms was selected in the present invention because the resistance value doubles and the temperature coefficient of resistance is also slightly lower than the value of 1600 angstroms. Because it is only small. Therefore, by using a 800 Å thick permalloy element, the same resistance can be achieved with only 1/8 the surface area required for platinum. That is, by using permalloy, the thermal efficiency of the heater and the sensor can be increased, and the required surface area can be reduced, so that the price can be reduced. In the present invention, as disclosed, the permalloy element is used as both a temperature change detection sensor element having a microstructure and a heater element.

Furthermore, by enclosing the heater and sensor made of permalloy in an insulating layer of silicon nitride having a thickness of about 1 μm, a protective film is provided against the phenomenon of oxidation, which is a problem particularly at high temperatures. The insulating layer of silicon nitride also has a function of thermally insulating the permalloy element from the silicon substrate. Since silicon nitride has a high resistance to etching, the dimensions of the thin film members 32F and 32G can be controlled accurately. Furthermore, due to the high resistance of silicon nitride to etching, the depth of the air space 30 is set to 25 to 250 by etching.
It can be deep, such as μ. This air space determines the most important heat transfer factor.

As described above, in the preferred embodiment of the present invention, the heat detection sensor and the heater are formed of permalloy in consideration of the disclosed microstructure. The insulating layer of silicon nitride is used as a supporting material and as a protective material to achieve the etching time required to form the desired structure. As described above, by appropriately orienting the thin film member with respect to the silicon crystal plane, it is possible to form a desired structure without using an artificial etching stopper, and it is possible to remove the thin film member in a minimum time.

Furthermore, by using anisotropic etching, 25-250μ
By forming such a deep air space, greater thermal insulation will be achieved compared to the way resistors elements are commonly placed in integrated semiconductor devices.

The gist of the present invention is not limited to what has been described in the embodiments. For example, the heat sensing sensor element and the heater element are not limited to permalloy, and any appropriate element may be used. Other examples would be pyroelectric materials such as zinc oxide films, thin film thermocouples, thermistor films of semiconductor materials, and metal films with a preferred temperature coefficient of resistance other than permalloy. It should be noted that while the text has sometimes stated that air is the medium of the measured flow, the present invention is applicable to many other gaseous substances. Add. That is, for the purpose of applying the present invention, the meaning of the term "air" is defined to include a general gaseous substance.

[Brief description of drawings]

1, 1A, 2 and 3 show an embodiment of the present invention, and FIG. 4 shows an example of a circuit used in the present invention. 20 ... Substrate 26, 26F, 26G ... Resistor grid 28, 29 ... Insulating layer 30 ... Air space 32, 32F, 32G ... Thin film member 38 ... Comparative resistance element 116 ... Integrated circuit

Claims (2)

[Claims] 1. A flow velocity sensor composed of a thin film single resistance element having a total resistance value and a semiconductor substrate, wherein the single resistance element has two voltage outlets. Is capable of measuring the voltage of the sensor resistance section separated from each end of the total resistance value, and the sensor resistance section is arranged on opposite side surfaces of the resistance element excluding the sensor resistance section. At the same time, the semiconductor substrate holds most of the single resistance element in a non-contact state with the semiconductor substrate, and further, the single resistance element has each voltage output port and each termination of total resistance value. The flow velocity sensor is characterized in that the two sensor resistance sections divided between and act as a temperature detecting element, and the central resistance section excluding these temperature detecting element portions is used as a heater element. 2. A semiconductor substrate having an air space formed on the uppermost surface thereof, and two self-supports provided on at least one thin film member crossing the air space held by the semiconductor substrate. The heat-generating resistance element and these two resistance elements are alternately switched to the self-heating mode and the temperature detection mode so that when one functions as a heater, the other functions as a sensor. And a circuit means for extracting the difference between the temperature signals alternately pulsed from the element in the temperature detection mode, the two resistance elements being the fluid to be measured. A flow velocity sensor characterized in that the flow velocity sensor is arranged in parallel so as to be located upstream and downstream of the flow.