JP2006300805A - Flow sensor and infrared gas analyzer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a flow sensor of enhanced sensitivity easy to be manufactured, and an infrared gas analyzer using the same. <P>SOLUTION: This flow sensor with two heating resistors arranged in a gas flow passage under a condition of keeping a fixed interval is provided with: a substrate arranged to make a plane in parallel to a gas flowing direction in the flow passage; a hole provided in the plane of the substrate; and the two heating resistors provided with the prescribed interval on the substrate astride the hole to be crossed with the flowing direction. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to the structure of a flow sensor used in a non-dispersive infrared gas analyzer (NDIR) or the like.
More specifically, the present invention relates to a flow sensor with improved sensitivity characteristics and easy manufacture, and an infrared gas analyzer using the same.

  Prior art documents related to a flow sensor and an infrared gas analyzer using the same include the following.

JP 2002-131230 A JP 2002-081982 A

  9 is a diagram for explaining the configuration of a conventional example that is generally used conventionally. FIG. 10 is a diagram for explaining the concept of FIG. 9. FIG. 11 is a diagram for explaining the bridge circuit of FIG. It is.

In FIG. 9, the infrared light emitted from the infrared light source 1 is divided into two by the distribution cell 2 and enters the reference cell 3 and the sample cell 4, respectively.
The reference cell 3 is filled with a gas that does not contain a component to be measured, such as an inert gas. A sample gas flows through the sample cell 4.
For this reason, the infrared light divided into two in the distribution cell 2 is absorbed by the measurement target component only on the sample cell 4 side and reaches the detector 5.

  The detector 5 comprises two chambers, a reference side chamber 501 that receives light from the reference cell 3 and a sample side chamber 502 that receives light from the sample cell 4, and a gas flow passage that connects the two chambers detects the passage of gas. A thermal flow sensor 51 is attached. The detector 5 contains a gas (detection gas) containing the same component as the measurement target. When infrared light from the reference cell 3 and the sample cell 4 is incident, the measurement target component in the detection gas is detected. Absorbs infrared light, so that the detection gas thermally expands in the reference side chamber 501 and the sample side chamber 502, respectively.

  Since the reference gas in the reference cell 3 does not include the measurement target component, the infrared light passing through the reference cell 3 is not absorbed by the measurement target component, and the measurement target component is included in the sample gas in the sample cell 4. In this case, a part of the infrared light is absorbed there, so that the infrared light incident on the sample side chamber 502 is reduced in the detector 5, and the thermal expansion of the detection gas in the reference side chamber 501 is caused in the sample side chamber 502. It becomes larger than the thermal expansion of the detection gas.

  The infrared light is intermittently interrupted by the rotating sector 6 and is repeatedly blocked and irradiated. When the infrared light is blocked, the infrared light does not enter both the reference side chamber 501 and the sample side chamber 502, so that the detection gas does not expand.

  Therefore, in the reference side chamber 501 and the sample side chamber 502, a differential pressure is periodically generated between the two chambers according to the concentration of the component to be measured in the sample gas, and the gas flow path provided between the two chambers is detected. Gas will come and go. The behavior of the detected gas is detected by the thermal flow sensor 51, is subjected to AC voltage amplification by the signal processing circuit 7, and is output as a signal corresponding to the concentration of the measurement target component. Reference numeral 8 denotes a synchronous motor that drives the rotating sector 6, and 9 denotes a trimmer that adjusts the balance of infrared light incident on the reference cell 3 and the sample cell 4.

  In this way, when the concentration of the measurement target component in the sample gas changes, the amount of infrared light incident on the detector 5 (sample side chamber 502) changes, so that the measurement target component concentration is accommodated via the signal processing circuit 7. Output signal can be obtained.

FIG. 10 is a conceptual explanatory diagram of FIG.
In the figure, when the arrow Usig is a moving direction of the detection gas generated in the gas flow path by absorption of infrared light, the first heater wire 511 and the second heater wire 512 constituting the thermal flow sensor 51 are predetermined. Are arranged along this movement (circulation) direction Usig, and a temperature (resistance value) change corresponding to the movement of the detection gas is generated.

That is, when the detection gas in the gas flow passage moves, the upstream heater wire is cooled by the detection gas, and the downstream heater wire is heated by the heat of the upstream heater wire. There will be a temperature difference.
The temperature change (resistance value change) in the two heater wires 511 and 512 is detected using a bridge circuit as shown in FIG.

FIG. 12 is a configuration explanatory diagram of the main part of FIG. 9 and is a configuration explanatory diagram showing an example of a conventional flow sensor.
In the example shown in the figure, two metal thin film heater substrates 210 and 220 formed by the same process of the semiconductor technology are stacked in two stages to form a flow sensor, and a metal is formed on the surface of each silicon substrate portion 211 and 221. The heater wires 212 and 222 made of a thin film are formed, and the silicon substrate located under the heater wires 212 and 222 is removed by anisotropic etching to form through holes 213 and 223 for gas flow.

  Therefore, by stacking such metal thin film heater substrates 210 and 220 in two stages, the two heater wires 212 and 222 are held at intervals according to the thickness of the silicon substrate portion 211 and arranged in the gas flow passage 230. be able to.

However, such an apparatus has the following problems.
Problems of such an apparatus include a problem of improving characteristics and a problem of manufacturing.
1. Challenges in improving characteristics (1) Loss due to flow path resistance Since the gas differential pressure generated in the gas cell is extremely small, it is essential that the flow resistance of the gas flow path be small. Since the gas flow path by the conventional apparatus is a combination of many parts, there is a problem that the gas flow is complicated and the gas stays easily and the fluid resistance increases.
For this reason, the differential pressure generated by the gas cell detector is lost due to the presence of fluid resistance, and there is an adverse effect of reducing the gas flow rate of the flow sensor unit.

(2) Difficult to achieve high sensitivity As a means to obtain higher sensitivity by improving the flow sensor structure of FIGS. 9 and 12 using a conventional apparatus, it is assumed that gas flow detection elements are arrayed in multiple stages to amplify the signal. However, it is very difficult to realize an array by extending the method of the conventional apparatus.

2. Manufacturing problems (3) Many parts and many processes As can be seen from the configuration example of FIG. 12, the following elements are necessary in the two-chip configuration.
a. A board component that hermetically fixes two silicon chips.
b. A spacer that adjusts the distance between the two board components to maintain airtightness.
c. Parts that hold the above combination parts in a gas-tight state while holding them in the gas detector body.

  As described above, in assembling the flow sensor, the number of parts is large, and accordingly, there is a trouble that an airtight seal of each part must be ensured. Along with this, there was a problem that the assembly work process increased and the cost was increased.

(4) The accuracy of the distance between the hot wires does not increase, and the component cost is high.
As can be seen from the conventional example of FIG. 12, the distance between the hot wires is determined by the thickness of each part involved, and the thickness of the flow sensor chip, the thickness of the adhesive layer, and the thickness of the spacer layer are all related to the distance between the hot wires. did. For this reason, there is a detrimental effect that the assembly accuracy is not improved because the processing accuracy of each part is limited. Also, increasing the processing accuracy increases the cost.

(5) Non-uniformity of substrate characteristics.
Since the two silicon substrates are independently manufactured and combined, as a variation in the resistance value of the silicon heat wire of the thermal flow meter,
・ Variation between production process lots ・ Variation between wafers ・ Variation within wafer (distribution)
As a result, the resistance value accuracy is difficult to improve.

  In the hot-wire flow meter, since these resistors are configured in a bridge detection circuit, the variation in the hot-wire resistance value is directly connected to the output offset. Since the amount of detection signal is usually small, if a large offset is superimposed, it causes problems such as degradation of output characteristics and a saturation phenomenon of the subsequent amplifier circuit. For this reason, an offset adjustment circuit is required.

(6) Problem of inter-element connection When configuring a bridge circuit using silicon resistance wires, the number of portions for electrically connecting silicon elements increases. Actually, wire bonding work between the chip and the chip or between the chip and the relay terminal has increased, resulting in an increase in cost. In addition, there is a problem that the work such as the necessity of preparing a jig part for bonding increases.

The object of the present invention is to solve the above problems.
(1) A sensor structure capable of improving sensitivity is proposed.
(2) The number of parts of the flow sensor to be used is reduced, the assembly is facilitated and the cost is reduced.
(3) Minimize the effect of characteristic variations associated with the silicon chip.
(4) Minimize the number of connection points between silicon heat wires. (5) Stabilize characteristics and reduce assembly and adjustment man-hours.

In order to achieve such a problem, in the present invention, in the flow sensor of claim 1,
In the flow sensor arranged in the gas flow path in a state where the two heating resistors are kept at a certain interval,
A substrate having a plane arranged in parallel to the gas flow direction of the flow path, a hole provided in the plane of the substrate, and a predetermined interval across the hole so as to cross the flow direction. And two heating resistors provided.

In the flow sensor according to claim 2 of the present invention, in the flow sensor according to claim 1,
A plurality of heating resistors are provided as a set of the two heating resistors.

In the flow sensor according to claim 3 of the present invention, in the flow sensor according to claim 1 or 2,
The heating resistor is formed by a semiconductor process.

In the flow sensor according to claim 4 of the present invention, in the flow sensor according to claim 3,
A wiring pattern provided on the substrate and connecting the heating resistor is provided.

In the flow sensor according to claim 5 of the present invention, in the flow sensor according to any one of claims 1 to 4,
A bridge circuit is configured by the heating resistor.

In the flow sensor of claim 6 of the present invention,
The flow sensor according to any one of claims 1 to 5,
The holes of the substrate are formed in the silicon substrate by anisotropic etching.

In the flow sensor according to claim 7 of the present invention,
The difference between infrared light that has passed through the sample cell and infrared light that has not passed through the sample cell is detected, and two heater wires are arranged in the gas flow path with a certain distance between them. In an infrared gas analyzer equipped with a sensor,
A substrate in which a plane is arranged in parallel to the gas flow direction of the flow passage, a hole provided in the substrate, and a predetermined interval on the substrate across the hole perpendicular to the flow direction. And a flow sensor including the two heating resistors.

In the flow sensor according to claim 8 of the present invention, in the flow sensor according to claim 7,
The flow sensor includes a plurality of heating resistors as a set of the two heating resistors.

According to claim 1 of the present invention, there are the following effects.
Conventionally, it has been manufactured by assembling heating resistors formed on each of two substrates. Adjustment work such as assembly and positioning between boards has increased, making production difficult and difficult.
In the present invention, since two heating resistors are arranged on a single substrate, a flow sensor can be obtained that can be easily manufactured and adjusted and can be reduced in cost.

According to claim 2 of the present invention, there are the following effects.
Since a plurality of heating resistors are provided with the two heating resistors as one set, a flow sensor with an increased signal amount and improved sensitivity can be obtained.
According to the present invention, a flow sensor that can be realized without increasing the manufacturing process and without being complicated is obtained.
On the other hand, in the conventional example, in order to realize the same configuration, a technique for stacking a large number of substrates while maintaining airtightness is required, which is difficult or impossible to realize.

According to claim 3 of the present invention, there are the following effects.
Since the heating resistor is formed by a semiconductor process, a flow sensor composed of a set of a plurality of heating resistors can be obtained more easily and accurately by applying the semiconductor process.

According to claim 4 of the present invention, there are the following effects.
A wiring pattern provided on the substrate and connecting the heating resistor was formed by a semiconductor process.
Conventionally, wiring work for each heating wire resistor has been performed by manual soldering work, but the work has become enormous as the number of sets increases.
According to the present invention, a flow sensor that can be stably formed at a time can be obtained by a wiring pattern forming technique frequently used in a semiconductor process.

According to claim 5 of the present invention, there are the following effects.
Since the bridge circuit is composed of heating resistors, the flow that can be realized easily and with few steps by designing and forming the necessary bridge detection circuit on the pattern in the wiring pattern. A sensor is obtained.

According to claim 6 of the present invention, there are the following effects.
Since the holes of the substrate are formed in the silicon substrate by anisotropic etching, a hole sensor of a large number of flow sensors can be collectively formed by applying a semiconductor process, and a flow sensor that can contribute to low cost is obtained.

According to claim 7 of the present invention, there are the following effects.
By using a flow sensor that is easy to manufacture, an infrared gas analyzer that can be simplified and reduced in cost can be obtained.

According to claim 8 of the present invention, there are the following effects.
In addition to simplification and cost reduction of the analyzer, an infrared gas analyzer that can achieve high sensitivity and high accuracy can be obtained.

Hereinafter, the present invention will be described in detail with reference to the drawings.
FIG. 1 is a diagram illustrating the configuration of the main part of one embodiment of the present invention, FIG. 2 is a side view of FIG. 1, FIG. 3 is a diagram illustrating an example of use of FIG. is there.
In the figure, configurations with the same symbols as in FIGS. 9 and 12 represent the same functions.
Hereinafter, only differences from FIGS. 9 and 12 will be described.

As shown in FIG. 3, the substrate 71 is provided with a plane parallel to the gas flow direction of the flow passage 72. In this case, a silicon substrate is used.
The hole 73 is provided in the substrate 71.
The two heating resistors 74 and 75 are provided at a predetermined interval on the substrate 71 across the hole 73 so as to be orthogonal to the gas flow direction.

In this case, a plurality of heating resistors 74a, 75a, 74b, and 75b are provided with two heating resistors 74 and 75 as a set.
In addition, the heating resistors 74a, 75a, 74b, and 75b are formed by a semiconductor process in this case.

Also, a wiring pattern (not shown) for electrically connecting the heating resistors 74a, 75a, 74b, and 75b is formed on the substrate 71 by a semiconductor process.
A heating circuit 74a, 75a, 74b, 75b constitutes a bridge circuit as shown in FIG.

In this case, the hole 73 of the substrate 71 is formed in the silicon substrate by anisotropic etching.
In this case, an insulating layer 76 is provided between the silicon substrate 71 and the heating resistors 74 and 75.

A represents the distance between the upstream heating resistor 74 and the downstream heating resistor 75, and B represents the distance between the heating resistors.
In FIG. 4, 81 and 82 are comparison resistors, 83 is a bridge output terminal, 84 is a bridge voltage Vb, and 85 is a COM potential.

In short, in the present invention, the entire flow sensor is formed on a single silicon substrate 71. An etching hole 73 processed by a technique such as anisotropic etching is formed in a part of the silicon substrate 71.
An insulating layer 711 is formed on the surface of the silicon substrate 71 as an electrical isolation layer, and resistors 74a, 75a, 74b, and 75b are patterned on the insulating layer 711. Part of the resistors 74a, 75a, 74b, and 75b is bridged to the space portion of the etching hole 73, and is floating without having a support portion. The floating portion 77 is heated and functions as a flow rate detection unit.

The following features are characteristic of the present invention.
(1) As a method of forming the resistors 74a, 75a, 74b, and 75b, a semiconductor patterning process is applied, and in the conventional example, the heat rays of the upstream 611 and the downstream 612 that are separately formed and combined are formed on the same substrate 71 surface. To do.

(2) The heat ray resistors 74a and 74b located on the upstream side of the gas and the heat ray resistors 75a and 75b arranged immediately downstream thereof are formed on the substrate 71 in pairs. Hereinafter, this pair of hot wire resistors is referred to as a “differential resistor”.
The distance between the upstream and downstream hot wires 74a, 75a, 74b, 75b greatly affects the flow velocity sensitivity and the measurement range. This parameter is determined from the characteristics of the gas cell.

(3) At least two pairs of “differential resistors” are formed on the same substrate 71 and connected to each other.
As a typical example of the plurality of “differential resistor” arrangements, there is a method of forming a row in the gas flow direction.

(4) FIG. 4 shows an example of a bridge detection circuit for measuring the gas linear velocity. In this example, among the operating resistors in pairs 74 and 75, those 74a and 74b located on the upstream side of the gas and those 75a and 75b located on the downstream side are connected to each other, and the entire half-bridge circuit is formed. Form.

In the above configuration, FIG. 5 shows differential resistors NO1, NO2, and NO3 (three pairs in the figure) arranged in an array, and the temperature distribution is also shown.
FIG. 6 shows the gas flow detection principle of one differential resistor NO1.
That is, in the two heat wire resistors 74a and 75a arranged in close proximity, if a gas flow exists, the heat generated by the upstream heat wire 74a reaches the downstream heat wire 75a, and the temperature of the two heat wires 74a and 75a. A non-equilibrium state appears.

Therefore, the gas flow can be detected by using a bridge circuit that detects the difference between the two resistance values.
In the present invention, a configuration is proposed in which a plurality of differential resistors are arranged in series and a flow rate signal detected by each differential resistor can be synthesized.

FIG. 7 shows a comparison of the characteristics of the conventional flow sensor of FIG. 9 and the flow sensor of the present invention.
In the figure, the A curve is the characteristic curve of the flow sensor of the present invention, and the B curve is the characteristic curve of the conventional flow sensor of FIG.
The horizontal axis indicates the flow velocity, and the vertical axis indicates the flow velocity signal.

The characteristics of the flow sensor of the present invention are improved by arraying the sensitivity near the flow velocity of zero. On the other hand, for large flow velocities, sensitivity decreases due to thermal interference between adjacent differential resistors.
The characteristics of the conventional example of FIG. 9 are characteristics due to a pair of resistors, and it is difficult to obtain a saturation characteristic of sensitivity, but the sensitivity is insufficient in a minute flow region.

As a result, in the present invention, a flow sensor with improved sensitivity characteristics and improved ease of manufacture can be obtained.
In detail,

By combining a plurality of differential resistors in multiple stages, an effect of improving sensitivity can be obtained particularly in a micro flow region.
When a plurality of differential resistors are formed in the gas flow direction, a thermal interference effect between adjacent differential resistors occurs in a relatively large flow region. At this time, saturation or reduction of sensitivity becomes a problem.
However, in applications such as gas analyzers, sensitivity characteristics in the minute flow rate range are used, and can be avoided by designing the flow sensor parameters.

The distance between a pair of differential resistors can be determined with high accuracy by semiconductor patterning technology and does not depend on the dimensional accuracy and combination accuracy of a large number of components.
Parts that determine the distance between the differential resistors are not required, and the number of parts can be reduced.

Resistor accuracy is improved. The bridge detection circuit captures the difference in resistance value between the upstream and downstream resistors, but in the present invention, the upstream and downstream resistors are formed from the adjacent portions of the resistance layer formed in the same process. Variation factors such as lot-to-lot variation, wafer-to-wafer variation, and wafer-to-wafer variation can be eliminated.
Since a part of the connection for forming the bridge circuit can be simultaneously formed in the semiconductor patterning process, an inexpensive flow sensor can be obtained.

FIG. 8 is a diagram illustrating the configuration of main parts of another embodiment of the present invention.
In the present embodiment, a full bridge circuit is configured by the heat ray resistors 74a, 75a, 74b, and 75b.
4 and 8, the connection between the heat ray resistors 74a, 75a, 74b, and 75b has been described from the outside for the sake of convenience of explanation. Of course, it may be labor-saving and simplified.

Further, the heat ray resistors 74a, 75a, 76c, 74b, 75b, and 76c of FIGS. 1 to 8 used for explaining the present invention are simplified for easy understanding of the explanation. Needless to say, the resistors 74a, 75a, 76c, 74b, 75b, and 76c include multiple folded shapes (meander shapes).

The above description merely shows a specific preferred embodiment for the purpose of explanation and illustration of the present invention.
Therefore, the present invention is not limited to the above-described embodiments, and includes many changes and modifications without departing from the essence thereof.

It is principal part structure explanatory drawing of one Example of this invention. It is a side view of FIG. It is a figure which shows the usage example of FIG. It is operation | movement explanatory drawing of FIG. It is operation | movement explanatory drawing of FIG. It is operation | movement explanatory drawing of FIG. It is operation | movement explanatory drawing of FIG. It is principal part structure description of the other Example of this invention. It is structure explanatory drawing of the prior art example generally used conventionally. 10 is a conceptual explanation of FIG. 9. FIG. 10 is an explanatory diagram of the bridge circuit of FIG. 9. It is principal part structure explanatory drawing of FIG.

Explanation of symbols

71 Substrate 72 Flow path 73 Hole 74 Heating resistor 75 Heating resistor 76 Insulating layer 77 Floating portion 81 Comparison resistor 82 Comparison resistor 83 Bridge output terminal 84 Bridge voltage Vb
85 COM potential

Claims (8)

In the flow sensor arranged in the gas flow path in a state where the two heating resistors are kept at a certain interval,
A substrate in which a plane is arranged in parallel with the flow direction of the gas in the flow path;
Holes provided in the plane of the substrate;
A flow sensor comprising: two heating resistors provided across the hole across the hole so as to cross the flow direction and at a predetermined interval on the substrate. The flow sensor according to claim 1, further comprising a plurality of heating resistors as a set of the two heating resistors. The flow sensor according to claim 1, wherein the heating resistor is formed by a semiconductor process. The flow sensor according to claim 3, further comprising a wiring pattern provided on the substrate and connecting the heating resistor. The flow sensor according to any one of claims 1 to 4, wherein a bridge circuit is configured by the heating resistor. The flow sensor according to any one of claims 1 to 5, wherein the hole of the substrate is formed in the silicon substrate by anisotropic etching. The difference between the infrared light that has passed through the sample cell and the infrared light that has not passed through the sample cell is detected, and the two heater wires are arranged in the gas flow path in a state of maintaining a certain interval. In an infrared gas analyzer equipped with a sensor,
A substrate in which a plane is arranged in parallel with the flow direction of the gas in the flow path;
Holes provided in the substrate;
An infrared gas analysis comprising: a flow sensor comprising: two heating resistors provided at predetermined intervals on the substrate across the hole in a direction orthogonal or oblique to the flow direction. Total. The infrared gas analyzer according to claim 7, wherein the flow sensor includes a plurality of heating resistors with the two heating resistors as a set.