JPS63282662A - Flow speed sensor - Google Patents

Abstract

PURPOSE:To perform measurement in a high speed flow region, by thermally connecting a heat sensor and a heater by a double-layer structure. CONSTITUTION:This flow speed sensor consists of a pair of membrane heat sensors 122, 124, a membrane heater 126 and a substrate 20, and the sensors 122, 124 and the heater 126 are brought to a non-contact state with respect to the substrate 20 by a bridge part 132. The sensors 122, 124 are arranged to the layer on the heater 126, and the layer of the heater 126 and the layers of the upstream and downstream sensors 122, 124 are formed to the bridge part 132 as respective separate layers to be formed into an almost double-layer structure. In this structure, a silicone wafer 20 is used as the substrate 20 and a silicon nitride layer 129 is formed to the surface thereof. The propagation of the air stream due too the heat generated from the heater 126 is measured at two positions close to each other by forming the double-layer structure and the effect of the heat loss caused by the air stream on an output signal is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a flow rate sensor that measures the flow rate of air.

[Conventional technology]

Commercially available airflow velocity sensors typically include a long probe inserted into the airflow.
A single heat source or a thermistor is installed. 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. Furthermore, recently, a microbridge flow rate sensor consisting of two sensors and a heater element has been known. Such semiconductor sensors are disclosed, for example, in US Pat.
07? ,4548078.4581928.46241
37 etc.

One conventional example of a microbridge flow rate sensor is shown in FIGS. 3 to 7. These are shown in US Pat. No. 4,501,144. This is two sensors 22.24
and a heater 26 on the substrate 20. Sensors 22 , 24 are placed on both sides of heater 26 . Substrate 20 is a semiconductor made of a material that is easy to etch and produce, such as Si. Two identical temperature-detecting resistance elements forming a grid shape formed on this substrate act as thin film heat sensing sensors 22 and 24, and forming a grid shape arranged in the center of these two sensors. The heating resistor element functions as a thin film heater 26. Heat detection sensor 22
．． 24 and the heater 26 are made of an alloy of iron and nickel,
For example, it is suitable to use something like permalloy, which is made of 80% nickel and 20% iron. These heat sensing sensors 22, 24 and heater 26 are surrounded by insulating layers 28, 29 of gJ film made of silicon nitride, for example.
Form a thin film member. As shown in the embodiments of FIGS. 3 and 4, the sensors include half of the heater 2G and the heat sensing sensor 22.
and a thin film member 32 consisting of half of the heater 26 and the heat sensing sensor 24, and has a width of 15 mm.
It has a size of 0μ and a length of 400μ.

Additionally, the sensor has an air space 30 that effectively surrounds the heat sensitive sensor 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 wires with a width of 5μ with a thickness of about 0°08 to 0.12μ and an interval of about 5μ between the lines, and furthermore, these have a total thickness of about 0. The structure is such that it is surrounded by a thin silicon nitride film having a thickness of .8 μm or less.

And by etching (air space 30 which is a recess)
is formed in the silicon substrate 20 below the thin film members 32, 34 to a depth of exactly 125μ. Thin film member 32.3
4 is 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.
4 is configured to bridge the air space 30.

Silicon nitride is a very good 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 through the silicon nitride film is extremely small. Most of the heat transferred from the heater 26 to the heat sensitive sensors 22, 24 will be transferred through the air surrounding the heater 26. That is, since the thermal conductivity of the silicon nitride film is low, the heat sensing sensors 22, 24 can be placed very close to the heater 26, and most of the heat from the heater 26 is transferred to the heater 26 without passing through the silicon nitride film. It will be transmitted through the air surrounding it. Heat sensing sensors 22 and 24 are effectively and firmly supported in the air near the heater 26.
will act as a probe to measure the temperature of the air surrounding the heater 26 and above the heater 26.

The principle of detecting the air flow velocity will be explained based on FIG. 4. The heater 26 is heated to a constant temperature 200°C higher than the temperature of the substrate 20. Even though the temperature of the heater 26 is kept 200°C higher than the temperature of the flowing air, the temperature is 0. O
Only less power is required than IW.

Most of the thermal conductivity from heater 26 is in air space 30
In the prior art, when there is no airflow, the thermal sensor 22.24 is heated to an average temperature of about 140°C (about 70% of 200°C). That is, as shown in the figure, the heat sensing sensor 22
．． 24 is placed exactly symmetrically with respect to the heater 26, so when the air flow velocity is O, the temperatures of these two sensors are the same, and there is no difference in the resistance values of these two sensors. Sometimes, a heat sensing sensor 2 located upstream
2 is cooled as the heat is carried away by the flow of air toward the heater 26, while the heat sensing sensor 2 located downstream
4 will be heated by the air flow from the heater 26. The resulting heat sensing sensors 22 and 24
The difference in resistance value between the heater and the heat sensing sensor causes a difference in voltage value and the flow velocity is measured. This voltage difference when not amplified is of the order of 0.1 V at an air flow velocity of 760 cm/s. Due to the extremely small heat capacity of the sensor, the thermal insulation provided by the silicon nitride film that is the connection means to the substrate, and the presence of air space, the response of this sensor is reduced to a time constant of 0.005 seconds according to measurement results. It becomes very fast.

This means that the thermal sensors 22 and 24 can respond very quickly to changes in air flow.

In the conventional example, 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 will be detected as a change in resistance value. Examples of circuits for realizing these functions are shown in FIGS. 6 and 7.

The temperature control circuit shown in FIG. 6 is comprised of a Whiston bridge circuit 46 for keeping the temperature of the heater 26 at a constant temperature higher than the ambient temperature detected by the comparison resistor 38. As mentioned above, this constant value is approximately 2
It is set to 00°C. whiston bridge circuit 4
6 has one side connected to the heater 26 and the resistor 40, and the comparison resistor 3
8 and resistors 42 and 44 constitute one side. The integrating 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. 7 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. This circuit includes a constant current power supply section 52 consisting of an amplifier 72, and an amplifier 6.
It is composed of a differential amplifier section 54 consisting of 8 and 70. The constant current power supply section 52 has a high impedance resistor 56.5 on one side.
8, and a zero adjustment variable resistor 60 and a heat sensing sensor 22 on the other side.
．． Drive a Whiston bridge circuit with 24. The gain of the differential amplifier section 54 is adjusted by a variable resistor 62.

Output terminal 64 outputs an output voltage proportional to the difference in resistance between heat sensitive sensors 22 and 24.

[Problem that the invention seeks to solve]

The conventional flow rate sensor shown in Fig. 3.4.5 has a heater 26
The heat capacity of the insulating nitride layer between the sensor and the thermal sensor 22, 24 is very small, and therefore the temperature of each sensor resistor varies greatly depending on the state of the surrounding air.

The upstream detector (which delivers the output signal) is cooled by the air flow until it approaches the temperature of this air flow. As a result, the output signal reaches saturation at a flow rate of about 1520 cm/sec. The sensor resistance on the downstream side is heated to some extent by heat transfer due to the airflow, so the output signal changes accordingly, but when the saturation state is reached, cooling becomes stronger than heating and no change can be detected.

The present invention is based on the above-described conventional flow velocity sensor technology, and further enables measurement in a high-speed flow region that is difficult to measure with a conventional microbridge flow velocity sensor. In the conventional micro-bridge flow rate sensor, when the flow rate on the sensor exceeds an average of 1520 cm/sec, heat transfer reaches a saturated state, and changes at a flow rate higher than this cannot be detected. The present invention is capable of measuring a flow rate approximately six times higher than that of conventional sensors, and has a configuration in which heat transfer does not reach a saturated state up to an average flow rate of 10,140 cm/sec.

[Means for solving problems]

In order to expand the measurable area, it is effective to thermally connect the sensor 122, 124 and the heater 126 with a two-layer structure as shown in FIG. Shown in Figure 1. The present invention includes a pair of thin film thermal sensors 122, 124 and a thin film heater 126.
, and a base 20, and the sensor and heater are connected to the bridge part 1.
32, it is in a non-contact state with the base body. sensor 1
22 and 124 are arranged in a layer above the heater 126.

The fundamental difference between this configuration and the conventional example shown in FIG. 3.4.5 is that the device shown in FIG. 1 is constructed from two parallel thin film resistive layers. A heater layer and upstream and downstream heat sensing sensor layers are formed in separate layers in the bridge portion 132. The reason for the two-layer structure is that it is necessary to arrange the upstream and downstream heat sensing sensors 122 and 124 above the heater 126.

A silicon wafer 20 is used to manufacture this sensor,
A silicon nitride layer 129 is formed on this surface. layer 12
9 is typically sputtered to a thickness of 4000 nm. Next, 8 layers of permalloy were added using the same spun technique.
00 people are formed.

Using a suitable photomask, photoresist and etching solution, permalloy elements 126 similar to those of Niremen 1-22, 24, 26, and 38 described above are drawn. This element 126 is formed over the entire width of the bridge section 132.

The second silicon nitride layer is made of this permalloin membrane.
It is formed by sputtering to a thickness of 4000 mm so as to completely cover 126. A second layer of permalloy is then formed over layer 128. The sensor elements 122 and 124 are then drawn in the same manner as the heater element 126 described above. Silicon nitride No. 37W12
7 is formed by sputtering to a thickness of 4000 mm so as to completely cover the permalloy elements 122 and 124.

This prevents oxidation of the element. In this manner, a pair of sensors 122, 124 are placed proximate a layer parallel to the heater 126 layer. There will be 4000 insulating layers between the pair of sensors and the heater. The manufacturing process for the space directly below the bridge section 132 is the same as that described in the conventional example shown in FIG. 1.2.3.

〔Effect of the invention〕

This configuration requires a higher velocity air flow to cool both sensors. For example, if the heater maintains a temperature of 160° C., each upstream and downstream sensor will more faithfully follow the temperature change of the heater. This means that by adopting the 2N structure shown in Figure 1, the propagation of heat generated by the heater through the airflow can be measured at two closer locations, thereby reducing the effect of heat loss due to the airflow on the output signal. It can be said that this is the case.

FIG. 2 shows the temperature distribution on the surface of the original bridge section 132 at a flow rate of 0. The present invention (a and C)
The sensors 122 and 124 in the conventional example (b and d)
It can be seen that the temperature distribution at the heater end is significantly different compared to the sensor.

[Brief explanation of the drawing]

Fig. 1 is a sectional view showing an embodiment of the invention, Fig. 2(a)
, (b) are cross-sectional views showing the configurations of the present invention and the conventional example, respectively, FIGS. 2(c) and (d) are temperature distribution diagrams corresponding to FIGS. 2(a) and (b), respectively, and FIG. , FIGS. 4 and 5 are cross-sectional views and plan views showing the configuration of a conventional sensor, and FIGS. 6 and 7 are diagrams showing an example of a circuit for achieving the function of the sensor. 122.124-Heat anger sensor element, 126-
Heater element, 127.128.129 - silicon nitride layer. Patent Applicant Yamatake Honeywell Co., Ltd. Agent Patent Attorney Yoshiharu Matsushita Figure 1 Figure 2 (0) Figure 2 (b) Figure 2 (C)
) Figure 2 (dl Procedures Principal Authorization (Method) Kunio Ogawa, Commissioner of the Patent Office, 1. Indication of the case, Patent Application No. 51319 of 1988, 2. Title of the invention, 3. Person making the amendment. Related Patent Applicant Address: 2-12-19 Shibuya, Shibuya-ku, Tokyo Name (666) Yamatake Honeywell Co., Ltd. Representative: Akira Ido 4, Agent Address: 4-28 Nishirokuro, Ota-ku, Tokyo 144 No. 1 Yamatake Honeywell Co., Ltd. 1 Ta Factory Shipped on May 31, 1988 7. Contents of amendment (1) "Figure 7" on page 2, line 14 of the horizontal tube of the detailed description of the specification and detailed explanation of the invention. (2) In the specification and detailed explanation of the invention, page 7, line 9 of the transverse flute is amended as “Fig. 8.”
"Figs. 6 and 7" are corrected to "Figs. 7 and 8 J. (3) "Figure 6" on page 7, line 10 of the detailed description of the invention is corrected to "Figure 7." (4) "In the second line of page 8 of the horizontal column of the specification and detailed explanation of the invention"
"Fig. 7" is corrected to "Fig. 8." (5) Insert “6” after “3.4.5” on page 8, line 16 in the description and detailed explanation of the invention. (6) In the description and brief explanation of the drawings. Insert "Fig. 6" after "Fig. 5" on the 3rd line on page 13 of the horizontal beam. (7) Insert "Fig. 6 J" on the 4th line on page 13 of the horizontal beam in the brief explanation of the specification and drawings. (8) Brief explanation of the specification and drawings, page 13, page 4.
Correct the line ``Figure 7'' to ``Figure 8''. that's all

Claims (3)

[Claims] (1) A thin film heater, a pair of thin film heat sensing sensors,
A flow velocity sensor comprising a semiconductor substrate, wherein a space is formed between the semiconductor substrate and the thin film heater and the pair of thin film heat sensing sensors, and the pair of thin film heat sensing sensors are connected to the thin film heater. A flow velocity sensor, characterized in that it is formed in an upper layer substantially parallel to the heater with an insulating section interposed therebetween. (2) The flow rate sensor according to claim 1, wherein the thin film heater and the pair of thin film heat sensing sensors are made of permalloy. (3) The flow rate sensor according to claim 1, wherein the insulating portion is made of silicon nitride.