JPH0545193A - Flow sensor - Google Patents

Abstract

PURPOSE:To develop a flow sensor which has good responsibility and detecting sensitivity, is inexpensive and very convertible, and can be mass produced, because the isolation of a sensing part and a substrate is enough kept and the fluid of a measuring object flows in the vertical direction against the sensing part. CONSTITUTION:An insulating film is performed on the top of a substrate 1, and the substrate 1 has openings at the top and the bottom and a hole for penetrating a fluid through the openings. A sensing part 2 is provided in the state isolated from the substrate through the insulating film and built over the opening of the substrate top.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flow rate sensor for measuring a flow rate of a fluid, which can be used in various industrial fields such as electronics, environment, chemistry, water supply, food and other fields.

[0002]

2. Description of the Related Art A measuring apparatus and a measuring method for a flow rate have been actively researched and developed for a long time. Particularly, a technology relating to a flow rate sensor has been remarkably advanced, and a field closely related to the present invention will be considered as follows. Various technologies are considered as conventional technologies.

(1) Metal wire type The measurement principle of the metal wire type flow sensor is that a constant current is passed through the metal wire, and as a result, the temperature rises to a constant temperature, and thereafter,
The flow rate is measured by detecting the amount of change in electrical resistance due to the change in temperature depending on the flow rate of the fluid that is the measurement target. Metal line type sensors are widely used because of their excellent detection sensitivity, linearity and responsiveness. However, as a drawback of the metal wire type flow sensor, in order to manufacture the flow sensor, it is necessary to manufacture individual components and combine them, which is difficult to miniaturize, and the manufacturing cost is also high. high. In addition, the characteristics of the flow rate sensor itself vary widely, and the interchangeability between the devices is not good. There is also a problem that the environment resistance and corrosion resistance of the metal wire are not good and the measurement target is limited.

(2) Thin-Film Type The thin-film type flow rate sensor grows a metal or semiconductor film on an insulated substrate and patterns it into a sensing portion. As the manufacturing process has been simplified and the cost has been reduced, research and development have been actively conducted. However, as a drawback of the thin-film type flow sensor, the sensing part having a thickness of about 1 μm has a thickness of hundreds of μm and is in close contact with a substrate having a much larger heat capacity, so that it is easily affected by the substrate. However, there is a limit in terms of detection sensitivity and response speed, and improvement in characteristics has been desired.

(3) Improved Thin-Film Type The improved thin-film type flow sensor is an improvement of the above-mentioned thin-film type. In order to overcome the drawbacks of the above-mentioned thin-film type in which the sensing part is in close contact with the substrate, the sensing part This is a flow sensor that ensures sufficient isolation between the substrate and the substrate. This improved thin film type technique will be described with reference to FIG.
The improved thin film type flow sensor is manufactured by the following steps. First, an n-type contact portion is formed on a p-type silicon wafer substrate. Next, a silicon dioxide film 3 is formed on the silicon substrate 1. After that, the contact hole 14 is opened, the n-type polysilicon film 4 is deposited over the entire surface, the polysilicon film 4 is patterned, and then the silicon dioxide film located below the sensing portion 2 of the polysilicon film 4. Etch 3. In this structure, the sensing portion 2 and the substrate 1 are secured in isolation by an amount corresponding to the thickness of the silicon dioxide film 3. As a result, the detection sensitivity is improved as compared with a normal thin film type flow sensor.

[0006]

However, the drawbacks of the so-called improved thin film type flow sensor of the above (3) are as follows. The first drawback is that the distance between the substrate 1 and the sensing unit 2 is limited to a certain level, and is about 2 μm.
Although it is only about m, if an attempt is made to increase the distance between the substrate 1 and the sensing portion 2, a crack may occur in the silicon dioxide film 3. The second drawback is that
Since the contact hole 14 of the silicon dioxide film 3 must be completely filled with the polysilicon film 4, if the contact hole 14 is too deep, the polysilicon film 4 may not reach the substrate 1 and may not be conductive. The third drawback is
Since the improved thin film type flow sensor can grow only the polysilicon film 4 on the silicon dioxide film 3 and cannot grow the crystalline silicon film, the sensing unit 2 is composed of the polysilicon film 4 and the carrier movement. The degree is significantly lower than that of the bulk silicon film, and the detection sensitivity is lowered.

Further, as another problem related to the flow rate sensor, the conventional flow rate sensor generally has a problem in detection sensitivity, and particularly when the measurement target is a small amount, the flow rate cannot be measured with sufficient accuracy. ..

That is, the problem to be solved by the present invention is to have a good responsiveness and detection sensitivity, to sufficiently maintain the isolation between the sensing part and the support substrate, and to be inexpensive, highly exchangeable, and in large quantities. It is to develop a flow sensor that can be produced. Then, the following is required as a condition therefor. (1) To maintain the advantages of low cost and mass producibility of the sensor by using the processing technology such as silicon without losing the advantage that semiconductor technology can be mass-produced at low cost. (2) The sensing unit is insulated from the substrate and is in a bridged state so as not to be affected by heat between the sensing unit and the substrate. (3) The material of the sensing part is preferably made of bulk silicon in order to improve the detection sensitivity.

[0009]

In order to solve the above problems and satisfy the above conditions, the present invention is to manufacture a flow rate sensor having the following structure using semiconductor technology.
The flow sensor of the present invention has an insulating film on the upper surface of the substrate,
The substrate has openings on the upper surface and the lower surface, and holes for allowing fluid to pass through the openings. And
A flow rate sensing unit 2 made of a thin film semiconductor is provided in an opening on the upper surface of the substrate so as to be separated from the substrate via an insulating film and provided in a bridged state.

In this flow rate sensor, the fluid flowing from the opening in the lower surface of the substrate 1 is discharged from the opening in the upper part of the substrate, and the flow rate and the like are measured by the sensing unit 2 when the fluid is discharged. However, it is also possible to remove the protective film provided on the upper part of the sensing unit 2 and perform the measurement on the upper surface.

[0011]

[First Embodiment] First, the principle of measurement will be described in the case where silicon is used as the material of the sensor. The resistance R of the sensing unit has a relationship with the temperature t according to the equation (1). Rt = R (t0) [1 + α (t−t0)] ... (1) Rt: Resistance value at temperature t R (t0): Resistance value at temperature t0 α: Temperature coefficient of resistance In this case , When the flow rate is 0, the constant current I is applied to the sensing unit 2.
When the temperature is in equilibrium, the temperature of the sensing unit 2 becomes t0 and the resistance becomes R (t0). When a constant current I is passed through the sensing unit 2 when the flow rate is greater than 0,
At the time of thermal equilibrium, the temperature of the sensing unit 2 falls below t0, and the resistance also becomes Rt according to the equation (1). Then, α (t−t0), which is the amount of change in resistance, is measured,
Flow rate can be detected. That is, when the flow rate of the measurement target changes, the temperature of the sensing unit 2 changes, and the sensing unit 2
The resistance of will change accordingly. The flow rate is detected by detecting the amount of change in this resistance.

The flow rate sensor of the present invention has greatly improved detection sensitivity and response speed as compared with conventional sensors. The reason will be described below. t2: temperature of the sensing unit 2 t1: temperature of the fluid V: flow velocity C: heat capacity of the sensing unit. From the sensing unit 2, the difference between the velocity V, the temperature of the sensing unit 2 and the temperature of the fluid (t2-t1). A heat quantity Q proportional to and diverges into the fluid to be measured, and the temperature of the sensor decreases by Q / C. As can be seen from this principle, if the flow velocity is the same, the smaller the heat capacity C of the sensing unit 2, the higher the amount of change in the temperature of the sensing unit 2, that is, the sensitivity. However, in the conventional technique, since the substrate 1 and the sensing unit 2 are in close contact with each other, if the temperature of the sensing unit 2 rises, the temperature of the substrate 1 also changes accordingly, and eventually the heat capacity of the sensor increases. ,
The sensitivity and response speed of the sensor are reduced. In the sensor of the present invention, since the sensing unit 2 has a bridge structure with respect to the substrate 1, even if the temperature of the sensing unit 2 changes, the temperature of the substrate 1 hardly changes. Since the heat capacity of the sensing unit 2 is very small, the detection sensitivity and response speed can be improved.

In addition to this, since the present invention has an opening in the upper and lower portions of the substrate, the fluid to be measured always flows in the direction perpendicular to the substrate, and the cross-sectional area of the opening is very small. Therefore, if the fluid flow is limited to the sensor section,
The flow velocity of the fluid is much faster than the conventional one. As a result, the flow rate sensor according to the present invention can improve the detection sensitivity as compared with the sensor according to the prior art, so that it is possible to measure even when the flow rate is minute.

An outline of a method of manufacturing this flow sensor will be described below with reference to FIG. In the following description, the case where silicon, which is a particularly effective material, is used as the substrate will be described as an example. (1) First, with a thickness of 300 μm made of silicon,
There are an n-characteristic upper silicon substrate 11 having crystallographic growth in the (100) direction and a silicon substrate 1 having a 1.5 μm silicon dioxide film 3 formed by thermal oxidation. (2) Next, the upper substrate 11 and the substrate 1 are joined by the direct joining technique. (3) Only the thickness required for the sensor is left, and the upper substrate 11
To polish. As a result, the n-characteristic thin film silicon film 12 remains on the substrate 1. (4) The thin silicon film 12 is thermally oxidized to form a silicon oxide film of 1 μm, and a substantially square opening is formed on the lower side of the substrate 1. One side of the square is (11
0) Match the direction. The length of one side of the square varies depending on the type of sensor, but is about 100 to 1000 μm.
Then, using the silicon dioxide film 3 as a mask, anisotropic etching is performed with an EPW etchant (a mixture of ethylenediamine, pyrocatechol and water). The EPW etchant has an etching speed for the silicon dioxide film 3 and an etching speed for silicon of 1: 100, and it is easy to stop the etching at the interface of the silicon dioxide film 3. As a result, the silicon cup 13 is formed on the substrate 1. In this case, the shape of the silicon cup 13 is preferably a tapered structure from the viewpoint of the concentration of fluid on the sensing portion. (5) The sensing portion 2 is formed on the side of the upper substrate 11 by the photolithography technique, and then the silicon dioxide film 3 having a thickness of 5000 angstrom is formed in order to protect the portion of the sensing portion 2. (6) A contact hole 14 is formed in the silicon dioxide film 3 in order to connect the electrode to the sensing section 2. (7) Aluminum is vapor-deposited as an electrode, and 2.0 μm
And the aluminum film 5 is formed. Aluminum film 5
Wiring (not shown) is formed by photolithography. (8) The silicon dioxide film 3 on the bottom surface of the silicon cup 13 (in other words, the surface in contact with the opening on the top surface of the substrate 1) is etched and removed to form the final structure. In this final structure, the sensing part 2 and the substrate 1 are
And is thermally insulated by the silicon dioxide film 3 attached by thermal oxidation. Further, the sensing unit 2 is provided in the opening on the upper surface of the substrate 1 so as to be separated in a bridged state, and the influence of heat from the substrate is minimized.

The above process has the following advantages. 1) Since the normal silicon growth process can be used, the type, carrier concentration, and size of the sensing unit 2 can be changed in a wide range. 2) Having the sensing unit 2 in a completely crosslinked state by the direct bonding technique of silicon / silicon dioxide / silicon, the responsiveness is improved. 3) The sensing unit 2 is made of a bulk crystal, has high carrier mobility, and can improve the detection sensitivity of the sensor. 4) Since a silicon planar process can be used, mass productivity is good, and in addition, sensor interchangeability and parameter uniformity are good.

(Second Embodiment) Next, the flow rate sensor 6
An example in which the above is actually used in a measuring device will be described below with reference to FIG. Since the flow rate sensor 6 of the present invention has a structure in which the sensing unit 2 and the substrate 1 are completely separated, and is installed on the upper portion of the substrate 1 in a bridged state, and has openings in the upper and lower portions of the substrate 1. , It becomes possible to install the sensing unit 2 in a direction perpendicular to the flow direction of the measurement target,
It can contribute to the improvement of detection sensitivity when measuring minute flow rates. Hereinafter, an example of minute flow rate measurement of the gas controller will be shown. The opening of the flow sensor 6 and the opening of the package 7 are aligned, the lead wire 8 is drawn out by bonding, the flow sensor 6 and the connector 9 are integrated, and the connector 9 and the gas pipe (not shown). To connect.
Here, the measurement target is as follows. Measurement gas N2, O2 Flow rate 0.1 ml to 1 ml / min Gas pipe inner diameter 6 mm Average flow velocity 0.006 to 0.06 cm / s Since gas flows in the gas pipe closed except for the sensor part,
The flow rate of the sensor portion above the silicon cup 13 is equal to the flow rate of the pipe. The flow velocity of the fluid is inversely proportional to the cross-sectional area of the gas pipe. When the opening of the flow sensor 6 is a square having a side length of 0.5 mm, the ratio of the piping area S1 to the opening area S2 is calculated by the formula (2).
It is represented by. 3 × 3 × π / 0.5 × 0.5 = 113 (2)

That is, the ratio of the area of the pipe to the area of the sensor opening is 113, and when the ratio is multiplied by the flow velocity, it is 0.006 × 113 = 0.67 and 0.06 × 113 = 6.7, respectively. Become. As a result, the flow velocity of the gas flowing through the flow sensor 6 is 0.6.
It is 7 cm / s to 6.7 cm / s, and the detection sensitivity is increased by about 100 times as compared with the conventional technique. By incorporating the flow rate sensor 6 of the present invention into the measurement apparatus of the second embodiment, it is possible to further enhance the effect that the detection sensitivity is good when the flow rate sensor itself has a small measurement target. .. In addition, since the low-cost chip is used, the cost of the entire device is reduced. Further, as an additional advantage, the size of the opening of the flow sensor 6 can be arbitrarily changed according to the measurement target and the measurement range. For example, when the length of one side of the opening portion provided in the substrate 1 of the flow rate sensor 6 can be changed from 0.1 mm to 5 mm, for example, the opening area can be changed within a range of 2500 times, It is possible to measure in a wide range of measurement targets.

Next, a third embodiment will be described with reference to FIG. The third embodiment is an application of the flow rate sensor 6 of the present invention to a device for measuring the flow direction of an object to be measured. As an application area of the present embodiment, it can be applied to an air flow rate of an air conditioning system, a flow velocity distribution monitor, and the like. In this embodiment, the method of incorporating the flow sensor 6 into the package 7 is the same as that of the second embodiment, but with the connector 9, the guide 10 has a cross-sectional area in which the area of the opening is wider than the cross-sectional area of the connector 9. Have. Then, two flow rate sensors 6 (6a, 6b) of the present invention are provided as a pair, and the direction of the guide 10 of one sensor and the direction of the other guide 10 are set in opposite directions. Moreover, the respective sensors 6a and 6b are provided parallel to the direction in which the fluid to be measured flows. When the measurement target is stationary, the measurement device of the third embodiment is adjusted so that the outputs of the two flow rate sensors 6a and 6b are the same. When the measurement target is flowing from A direction to B direction, 6
In a, the object to be measured is narrowed and the flow velocity through the small hole is higher than that in 6b. On the contrary, when the measurement target is flowing from the B direction to the A direction, the output signal of 6b is larger.
That is, the absolute values of the outputs of the two sensors are determined by the flow velocity, and the signs of the values output from the two flow rate sensors 6a and 6b reflect the flow direction of the measurement target. Further, in the measuring device, when the measurement target is stationary, the output signals of 6a and 6b are adjusted to be equal, and this flow sensor can simultaneously monitor the flow direction and flow rate of the measurement target. .. As an advantage of the third embodiment, since the flow sensor 6 of the present invention which is inexpensive and has a long life is used, the cost of the entire system is reduced. If the flow rate sensor 6 is installed not only in the plane direction but also in various directions, the three-dimensional flow direction and flow rate of the measurement target can be measured.

Although the above embodiments have described the sensing portion provided on the silicon substrate by the semiconductor technology, it goes without saying that the substrate and the sensing portion may be made of other materials. Absent.

[0020]

[Effect] The present invention is formed by using the direct bonding technique of (silicon / silicon dioxide / silicon), and the sensing unit is placed in a state where it is completely separated from the substrate, and the fluid to be measured is directed from the lower surface to the upper surface of the substrate. Since the flow rate is measured by flowing through the penetrating portion provided as above, the following effects can be obtained. First, since the distance between the substrate and the sensing portion can be increased to almost 1.5 times the maximum side length of the opening, the influence of the substrate can be eliminated and the response speed can be improved. Second, the carrier mobility is 10 × 10c
Instead of the polysilicon film 4 having the order of m / vs, the carrier mobility is 400 to 1000 × 400 to 1000.
Since it is possible to fabricate the sensing unit with a cm / vs bulk silicon film, the detection sensitivity can be significantly improved. Third, since the normal silicon process can be used, the type, carrier concentration, and size of the sensing unit 2 can be changed in a wide range. Fourthly, since the fluid passage (penetration portion) extending over the upper and lower openings can be provided by the silicon process, that is, the directional etching, the fluid can be taken into the device in a tapered structure. Fifth, the flow rate sensor of the present invention has high sensitivity and can perform effective and accurate measurement even when the measurement target is a small amount.

As the application of the present invention, it is used in the following fields because of its high sensitivity, high-speed response, low cost, and good uniformity of parameters. (1) If it is introduced into a gas flow controller, a high-performance and compact system can be made. (2) Used for real-time monitoring of gas flow rate. (3) It is a disposable type and is used for medical and health purposes. For example, it is used for respiratory volume measurement, blood flow rate measurement, and the like. (4) Since it has excellent exchangeability and good stability, it is used in fields where the sensor surface is easily soiled by the target object such as sewage, petroleum processing, and food.

[0022]

[Brief description of drawings]

FIG. 1 is a diagram illustrating a conventional technique.

FIG. 2 is a perspective view and a front view showing the configuration of the present invention.

FIG. 3 is a diagram showing a manufacturing process of the present invention.

FIG. 4 is a diagram showing a first embodiment of the present invention.

FIG. 5 is a diagram showing a second embodiment of the present invention.

[Explanation of symbols]

 1 substrate. 2 Sensing section. 3 Silicon dioxide film. 4 Polysilicon film. 5 Aluminum film. 6 Flow sensor. 7 packages. 8 lead wires. 9 connectors. 10 guides. 11 Upper substrate. 12 Thin film silicon film. 13 Silicon cup. 14 contact holes.

Claims (6)

[Claims] 1. A substrate (1) having an upper surface provided with an insulating film, having openings in the upper surface and the lower surface, and having holes for allowing a fluid to pass through the opening, and the substrate (1) having the opening interposed therebetween. A flow rate sensor provided with a flow rate sensing portion (2) provided in a state of being separated from the substrate and being bridged to an opening on the upper surface of the substrate. 2. A semiconductor substrate (1) having an insulating film on the upper surface thereof, having openings on the upper surface and the lower surface, and having holes for allowing a fluid to penetrate through the opening, and an insulating film. A flow rate sensor comprising: a flow rate sensing section (2) which is separated from the substrate and is provided in a state of being bridged to an opening on the upper surface of the substrate. 3. The flow sensor according to claim 2, wherein the semiconductor substrate is made of silicon. 4. The flow sensor according to claim 2, wherein the thin film semiconductor member is made of bulk silicon. 5. The semiconductor substrate is made of silicon,
The flow sensor according to claim 2, wherein the thin film semiconductor member is made of bulk silicon. 6. The semiconductor substrate is made of silicon,
The flow rate sensor according to claim 2, wherein the hole has a tapered structure provided by anisotropic etching.