JPH04235338A - Humidity sensor - Google Patents

Abstract

PURPOSE:To provide a compact humidity sensor which can be used in a broad environment and maintains sufficient measuring accuracy. CONSTITUTION:A substrate 1 comprising thermally poor conductor is thermally connected to a heat sink. A moisture sensitive film 2 and a means 3 for heating or cooling the specified position of the substrate are provided on the substrate 1. A means 4 which detects the intended temperature of the substrate that is heated or cooled with the heating or cooling means is also provided on the substrate 1. The thermal resistance of the moisture sensitive film 2 is changed in response to the amount of adsorbed water contained in the moisture sensitive film 2. The change in temperature at the specified position of the substrate 1 caused by the change in the thermal resistance is detected. Thus, the humidity in atmosphere is measured.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compact humidity sensor that is easy to construct and has high measurement accuracy.

0002

2. Description of the Related Art Humidity is a measure of the dryness and humidity of the air, and is a physical quantity that is as familiar to daily life as temperature. Humidity has traditionally been measured using psychrometric bulb hygrometers, hair hygrometers, etc., but in recent years, with advances in electronics technology, various manufacturing industries have come to recognize the importance of humidity control. Demand for more sophisticated measurement and humidity control has increased. For this reason, many humidity sensors that can convert humidity into electrical signals have been developed. As described above, among chemical sensors, humidity sensors are in high demand as sensors for detecting the familiar scientific quantity of humidity, and many types of humidity sensors have been put into practical use. In addition, new attempts are continually being made in terms of detection principles, materials, structures, etc.
Humidity can be said to be one of the sensing targets with active research and development. However, to date, there is no sensor on the market that satisfies all aspects, and future results are expected.

As an example, the characteristics of a thermistor humidity sensor that has been put into practical use as a humidity sensor will be explained below. The thermal conductivity of air containing water simply increases as the amount of water vapor increases in the humidity range of 0 to 150 g/m3 expressed in absolute temperature. Using this fact, the amount of moisture in the air can be determined from changes in thermal conductivity. In addition,
Since changes in thermal conductivity exhibit almost constant behavior regardless of temperature, the humidity determined from thermal conductivity is an absolute temperature.

A thermistor temperature sensor detects changes in thermal conductivity in the atmosphere by self-heating the thermistor. One of the two measurement bead-type thermistors with well-matched characteristics is exposed to humid air, and the other is used for temperature compensation in dry air. The self-heated thermistor is cooled by moist air and its resistance changes, and this change in resistance is measured using a bridge circuit.

[0005]

However, the thermistor temperature sensor has the following problems. first,
Since changes in thermal conductivity in the atmosphere are measured, the output may shift if gases other than air with different thermal conductivities coexist. Next, when foreign matter adheres to the sensor section and the heat transfer coefficient from the sensor changes, the characteristics shift. Furthermore, since the thermistor humidity sensor is affected by pressure, it cannot be used at anything other than normal pressure. And because it is necessary to perform thermal cleaning regularly,
A separate heater was required. Furthermore, since semiconductor thin film technology cannot be used, there is also the problem that the size inevitably becomes large. As described above, the environment in which the thermistor humidity sensor can be used is extremely restricted in today's harsh environment where humidity sensors can be used.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a compact humidity sensor that can be used in a wide range of environments and maintains sufficient measurement accuracy. That is, the humidity sensor of the present invention includes a substrate that is thermally connected to a heat sink and has the characteristics of a thermally poor conductor, and a moisture sensitive film provided on a certain portion of the substrate. The heating or cooling means heats or cools a predetermined position of the substrate, and the detection means detects the temperature of the heated or cooled position of the substrate. Then, atmospheric humidity is measured by detecting the temperature brought about by a change in thermal resistance that changes depending on the amount of moisture contained in the moisture-sensitive film on the substrate.

[0007]

[Operation] The present invention measures the humidity in the atmosphere by utilizing the heat conduction (thermal resistance) characteristics of a moisture-sensitive membrane and the mechanism of a thermocouple. When the surrounding temperature rises, a moisture-sensitive membrane adsorbs or absorbs moisture, increasing thermal conductivity (lowering thermal resistance). This makes it easier for the heat generated (or absorbed) by the heating (or cooling) means to be conducted to (or from) the heat sink, and as a result, a change in the voltage detected by the thermocouple becomes observable. The mechanism of a thermocouple will be briefly explained below. When one end of two types of conductors is electrically connected and a temperature difference is applied between this connection point and the other end, a thermoelectromotive voltage corresponding to the temperature difference is generated between the two contact points due to the Seebeck effect. . Thermocouples are detection elements that apply this principle to find temperature differences by measuring the value of thermoelectromotive voltage. By keeping the reference junction at 0°C, the temperature of the hot junction can be directly read from the value of thermoelectromotive voltage. be able to.

This method applies to the present invention. When moisture in the atmosphere adheres to the moisture sensitive film 2, the resistance value of the substrate of the thermally poor conductor changes. Then, the voltage difference between the vicinity of the heater section, which generates heat at a constant temperature, and the end of the thermocouple is reduced. As a result, the resistance disappears and the voltage of the current flowing through the thermocouple changes. This phenomenon is used to determine temperature differences from changes in voltage and detect humidity percentages.

[0009]

[Example] The humidity sensor according to the present invention has a heat sink 5
(not shown); a moisture-sensitive film 2 provided on at least a portion of the substrate; and means for heating or cooling a predetermined position of the substrate 1. 3, and means 4 for detecting the temperature at a desired position on the substrate heated or cooled by the heating or cooling means 3. FIG. 1 is a diagram showing an embodiment of the humidity sensor according to the present invention, in which a thin film heater 31 for heating and a thin film thermoelectric conductor for temperature detection are placed on a substrate 1 of a thermally poor conductor that is thermally connected to a heat sink 5. A pair 41a and 41b are configured. FIG. 1(A) is a view of the substrate 1 viewed from below, and FIG. 1(B) is a cross-sectional view taken along arrow E--RO.

As for the material of the substrate 1 which is a thermally poor conductor, it is preferable that the thermal resistance changes greatly due to a slight change in the amount of moisture contained in the moisture sensitive film 2. A small one is appropriate. Therefore, as a material for the substrate 1 which is a thermally poor conductor, glass, ceramic, crystal, quartz, acrylic, or an organic film such as epoxy or polymide may be used.

The moisture-sensitive film 2 may be made of any absorbent material, such as PIQ (trade name: Polymide Film), porous silicon, porous alumina, and the like.

The means 3 for heating or cooling a predetermined position of the substrate 1 includes resistance heating using electricity, absorption heating of a light absorption film using light, and the like. Figure 1 shows the thin film heater 3
This is an example in which heating was performed using No. 1. On the other hand, as means 3 for cooling a predetermined position of the substrate 1, there is cooling by the Peltier effect using a compound of bismuth, tellurium, or the like. In short, in the humidity sensor according to the present invention, the substrate 1
It suffices to be able to heat or cool any desired location.

The means 4 for detecting the temperature at a desired position on the substrate 1 includes a thermocouple, a radiation thermometer, and the like. In Figure 1,
Temperature is detected using thin film thermocouples 41a and 41b and an ohmic electrode 8 (hereinafter also simply referred to as electrode 8) that detects the potential difference therebetween.

In the embodiment shown in FIG. 1, a portion of the thermally poor conductor substrate 1 is thermally connected to a heat sink 5. In the embodiment shown in FIG. The heat sink 5 may be a block-shaped object with a large heat capacity, or may be connected with a thin conductive wire to conduct heat well to the outside. By connecting to the heat sink 5, a part of the board 1 that is a thermally poor conductor is kept at a constant temperature (hereinafter, this part is also referred to as the constant temperature part 6).
. This constant temperature section 6 has a thin film heater 31 at its center and both ends,
That is, they are located on both sides of the thin film heater 31 in FIG. In this case, since the constant temperature section 6 is installed in a copper block or the like, a constant temperature is always maintained regardless of whether the thin film heater 31 is on or off. This constant temperature section 6 includes a thin film thermocouple 4
It is not only necessary for the cold junctions 1a and 41b, but is essential for creating a temperature gradient within the substrate 1 when the means 3 for heating or cooling is provided at a predetermined position on the substrate 1. . In FIG. 1, the electrodes for the thin film heater 31 are wired in a thin pattern in order to reduce the escape of heat from the thin film heater 31 through the electrodes of the heater, that is, to improve heating efficiency.

Here, FIG. 1 shows a humidity sensor according to the present invention.
One example of the manufacturing method of the embodiment shown in will be briefly explained.

As the thermally poor conductor substrate 1, as described above, glass, ceramic, crystal, quartz, acrylic, or an organic film such as epoxy or polymide is used. After thoroughly cleaning this substrate 1 with an organic solvent or the like, it is dried in a clean environment.

Next, thin film thermocouples 41a and 41b are formed as means 4 for detecting temperature. Thin film thermocouple 41a
, 41b preferably has a large Seebeck,
An amorphous semiconductor thin film such as amorphous silicon or amorphous germanium is used. These amorphous semiconductors are deposited by plasma CVD using gases such as silane, germane, and hydrogen. At this time, the amount of doping gas supplied to the n-type semiconductor such as phosphine or arsine, and the amount of doping gas supplied to the p-type semiconductor such as diborane is varied.

Unnecessary portions of the deposited amorphous semiconductor are removed using photo-etching technology to form predetermined thin film thermocouples 41a and 41b. In addition, thin film thermocouple 41
a and 41b may be composed of an amorphous thin film or the like with a large Seebeck coefficient and a small metal thin film or the like.

Next, a thin film heater 31 is formed. As this thin film resistor, a metal thin film such as nichrome or tantalum is used. These thin films are deposited by sputtering or vacuum evaporation. Unnecessary portions of this thin film are similarly removed to form a thin film heater 31 of a desired shape.

[0020] Furthermore, a thin metal film for electrodes such as gold is deposited,
Similarly, unnecessary parts were removed, and thin film thermocouples 41a and 41b were
Then, the electrode 8 for the thin film heater 31 is formed.

Next, a moisture sensitive film 2 made of polyimide film, PIQ, porous silicon, porous alumina, etc. is attached using an adhesive or semiconductor process technology. Porous silicon films and porous alumina can be easily produced by using a corrosion method or a positive oxidation method from a thin film deposited by sputtering or vacuum evaporation. Note that a silicon dioxide thin film, a silicon nitride thin film, or the like may be provided as the surface protective film.

The measurement principle of the present invention and the role of the constant temperature section 6 will be explained using FIG. FIG. 2 is a diagram of the temperature gradient at the stage when a predetermined position of the substrate 1 is heated (in this embodiment, only heating is performed since the thin film heater is used) in the embodiment described in FIG. In (a), when the moisture-sensitive membrane 2 does not contain moisture, (
b) is a case where the moisture sensitive film 2 contains moisture. In the figure, the constant temperature section 6 (indicated by diagonal lines on both sides of FIG. 2) is installed directly on a copper block or the like, so it always maintains a constant temperature regardless of whether the thin film heater 31 is on or off.

On the other hand, the desired position of the heating section 7 (FIG.
When the thin film heater 31 is off, the temperature of the area (shown by the dotted line in the center) is the same as that of the constant temperature section 6, but when the thin film heater 31 is on, the temperature is higher than that of the constant temperature section 6. Therefore, when the thin film heater 31 is on, a temperature difference ΔT1 occurs between the heating section 7 and the constant temperature section 6.

This temperature difference ΔT1 is determined by the product of the heat generation amount Q of the thin film heater 31 and the thermal resistance Rs, and is expressed by the following equation 1.

[0025]

[Math 1]

In this case, the thermal resistance Rs depends almost only on the thermal resistance of the substrate 1. Therefore, if the thermal conductivity of the substrate 1 is λs, the length is Ls, the width is Ws, and the thickness is ts,
The thermal resistance Rs is expressed by the following equation 2.

[0027]

[Math 2]

As described above, when the moisture-sensitive film 2 does not contain moisture, that is, when a predetermined position of the substrate 1 is heated, the temperature difference ΔT1 is constant.

Next, a step begins in which a first temperature after heating a desired position of the substrate 1 is detected. In FIG. 2, thin film thermocouples 41a and 41b are used to detect this temperature difference ΔT1.
is used. At this time, the thermoelectromotive voltage V1 of the thin film thermocouple 41
is the Seebeck coefficient S of the thin film thermocouples 41a and 41b
and the temperature difference ΔT1 (Equation 3).

[0030]

[Math 3]

[0031] Even at this point, since moisture is not contained in the moisture sensitive membrane 2, the generated thermal electromotive voltage V1 is constant. From this stage, a stage begins in which moisture is contained in the moisture-sensitive film 2 provided on the surface of at least a portion of the substrate 1. In the embodiment shown in FIG. 1, moisture is contained in the moisture sensitive film 2 on the surface opposite to the surface on which the thin film heater 31 and thin film thermocouples 41a, 41b are formed. Before moisture is contained in the moisture-sensitive film 2, heat flows only within the substrate 1 and reaches the heating section 7.
It is transmitted from the temperature control section 6 to the constant temperature section 6. On the other hand, when the moisture-sensitive film 2 contains moisture, a part of the heat that was flowing only inside the substrate 1 flows through the moisture-sensitive film 2 due to the moisture contained in the moisture-sensitive film 2 and from the heating section 7. It is transmitted to the constant temperature section 6. Therefore, the thermal resistance between the heating section 7 and the constant temperature section 6 is reduced. If the thermal conductivity of the moisture sensitive membrane 2 is λf, the length is Lf, the width is Wf, and the amount of moisture contained in the moisture sensitive membrane 2 is tf, then the thermal resistance of only the moisture sensitive membrane 2 is Rf as shown in the following equation 4. expressed.

[0032]

[Math 4]

Further, the composite thermal resistance R of the thermal resistances Rs and Rf of the substrate 1 is expressed by the following equation 5.

[0034]

[Math 5]

Similarly to Equation 1, the temperature difference △ between the heating section 7 and the constant temperature section 6 after the moisture-sensitive membrane 2 contains moisture is calculated.
T2 is expressed by the following equation 6.

[0036]

[Math 6]

As can be seen from equation 6, the temperature difference ΔT2 is
Temperature difference △T1 before moisture-sensitive membrane 2 starts to contain moisture
is decreasing.

[0038] Subsequently, after the moisture-sensitive membrane 2 begins to contain moisture, a step begins in which the second temperature at a predetermined position is detected. That is, in this embodiment, the temperature difference ΔT2 is measured. The second temperature detection method is the same as the first temperature detection method, and in the case of the embodiment shown in FIG.
It is detected by thermoelectromotive voltage generated by 1a and 41b. At this time,
The thermoelectromotive voltage V2 is determined in the same manner as Equation 3 above, and is obtained using Equation 7 below.

[0039]

[Math 7]

In this way, the thermoelectromotive voltage V1, which is the first temperature detection signal, and the thermoelectromotive voltage V, which is the second temperature detection signal,
2 is required.

[0041] Next, the step of determining the amount of moisture contained in the moisture-sensitive membrane 2 is started based on the difference between the first temperature and the second temperature. From Equations 3 and 7, the difference between ΔT1 and ΔT2 is expressed by Equation 8 below.

[0042]

[Math. 8]

Further, from Equations 1 and 6, the difference between ΔT1 and ΔT2 is expressed by Equation 9 below.

[0044]

[Math. 9]

From Equations 8 and 9, the following Equation 10 can be obtained.

[0046]

[Math. 10]

By substituting Equation 2, Equation 3, Equation 4, and Equation 5 into Equation 10 and rearranging, Equation 1 representing the amount tf contained in the moisture-sensitive film 2 is obtained.
1 is obtained.

[0048]

[Math. 11]

In Equation 11, λs, Ls, Ws, ts
, and λf, Lf, and Wf are known. Therefore, the amount of moisture tf contained in the humidity sensitive film 2 is determined by detecting the thermoelectromotive voltages V1 and V2, which are temperature detection signals.

FIG. 3 shows that in the embodiment shown in FIG. 1, Ls=10 (mm) and Ws=2(
mm), ts = 150 (μm), and the Seebeck coefficient of the thin film thermocouples 41a and 41b is 0.
．． 25 (mV/K), the amount of heat generated by the thin film heater 31 is 4
The amount of water t contained in the moisture sensitive membrane 2 at 00 (mW)
It is a figure showing the relationship between f and detected thermoelectromotive voltage Vo. The drawings are exaggerated and the dimensions have not been taken into account. The vertical axis represents the change in voltage from the temperature difference ΔT1 to the temperature difference ΔT2 as an absolute value, and the horizontal axis represents the water content tf. In this case, the Au adhesion layer is attached to the thin film heater 31 and the thin film thermocouple 41a.
, 41b are formed on the entire opposite surface. A detected thermal electromotive voltage Vo with good linearity is obtained with respect to the amount of water contained tf.

FIG. 4 is a diagram showing another embodiment of the present invention. In the embodiment shown in FIG. 4, the constant temperature section 6 includes an electrode 8 that extends circularly around the periphery, and the heating means includes:
The thin film heater 31 in the center of the figure is used as a temperature detection means.
Two pairs of thin film thermocouples 41a and 41b extending in a fan shape are used. In this embodiment, the moisture sensitive film 2 is provided on the back side of the substrate 1. The thermal resistance in this case is from the thin film heater 31 in the center to the electrode 8 that extends circularly around the periphery. Furthermore, two pairs of thermoelectromotive voltages generated by the thin film thermocouples 41a and 41b are wired in series, and the thermoelectromotive voltage for detecting temperature is doubled.

FIG. 5 shows a combination of the embodiments shown in FIG. 4, in which the output directions of the thermoelectromotive voltages of the thin film thermocouples 41a and 41b have opposite characteristics. Each thin film heater 31 in the center has an equal amount of heat Q, and before moisture is contained in the moisture sensitive film 2, the thin film thermocouples 41 of each other
The thermoelectromotive voltages from a and 41b are balanced, and the thermoelectromotive voltage of the humidity sensor as a whole is not output. On the other hand, if moisture is contained only in the moisture sensitive film 2 (not shown) of either one of the circular heat resistance sections (for example, section I on the left side and section II on the right side), Only the thermal resistance of the circular portion on the left side decreases. Therefore, the thermoelectromotive voltage of only the I section decreases, the balance between the thermoelectromotive voltages of both parts is lost, and the thermoelectromotive voltage of the entire humidity sensor is outputted in accordance with the moisture content of the humidity sensitive film 2.

FIG. 6 shows an example in which a thermistor thin film resistor 42 is used for temperature detection, and a bridge circuit is integrated on the humidity sensor. Note that a thin film heater 31 is used for heating, and the constant temperature section 6 is located on the opposite side of the thin film heater 31. Figure 6
Inside, the thin film resistor 42 closest to the thin film heater 31
Assuming that is a thermistor, when a bias voltage is applied to this bridge circuit, a voltage corresponding to the temperature is output from the bridge circuit.

FIG. 7 shows an example in which light is used for heating. Light is introduced from outside the humidity sensor, absorbed by the light absorption film 9 in the center, and converted into heat. A plurality of pairs of thin film thermocouples 41a and 41b are used to detect the temperature, and the constant temperature section 6 is an electrode 8 on the circular periphery. In the embodiment shown in FIG. 7, since light introduced from the outside is used for heating, there is an advantage that the number of wiring lines can be reduced when measuring humidity in a vacuum apparatus or the like.

In FIG. 8, a radiant film 43 such as a black body formed in a circular shape in the center is used, and the radiant heat from the radiant film 43 is utilized for temperature detection. A thin film heater 31 is used for heating, and the constant temperature section 6 is an electrode 8 on the circular periphery. The embodiment shown in FIG. 8 also does not require wiring for temperature detection, so it has the advantage that the number of wiring can be reduced when measuring the amount of moisture contained in the moisture sensitive film 2 in a vacuum device, etc. There is.

Similar to the embodiment shown in FIG. 8, FIG. 9 uses radiant heat for temperature detection. Similarly, the constant temperature section 6 is located at the electrode 8 at the circular periphery. However, in the embodiment shown in FIG. 8, the thin film heater 31 is formed so as to surround the radiant film 43 in the center.
Compared to the embodiment shown in FIG. 8, this embodiment has the advantage that the temperature of the radiant film 43 becomes more uniform and more accurate temperature measurement is possible.

In the embodiments shown in FIGS. 1 to 9 so far, examples have been described in which a predetermined position of the substrate is heated, but it is also possible to perform cooling instead of heating. In this cooling embodiment, instead of the heater, a Peltier element made of an N-type semiconductor, a metal, and a P-type semiconductor is used to flow a current from the N-type semiconductor to the P-type semiconductor. At this time, cooling (heating if the current is in the opposite direction) occurs due to the Peltier effect at the junction surfaces between the N-type semiconductor and the metal, and between the metal and the P-type semiconductor. In addition, there is no problem in cooling the predetermined position by a method other than the Peltier effect, for example, by spraying cooled gas within a range that does not affect the degree of vacuum. When using a humidity measurement method that uses cooling, there are cases where measurements that generate heat in a cooling atmosphere are undesirable, and
Its advantages are demonstrated when measuring near the high temperature limit.

[0058]

[Effects of the Invention] First, the humidity sensor of the present invention heats or cools a substrate and detects the temperature that changes depending on the amount of moisture contained in the humidity sensitive film 2 provided on the substrate. Detecting humidity.

As a result, unlike a thermistor humidity sensor, the output will not shift due to the influence of gases contained in the air, and the characteristics will not shift due to foreign matter adhering to the sensor section. Accurate measurements can be taken without any problems. Further, since a thermistor humidity sensor is affected by pressure, it cannot be used at any pressure other than normal pressure, but the sensor of the present invention does not have such limitations. Next, since the present invention consists of a heating or cooling means and a temperature detection means, it can be easily miniaturized by using semiconductor thin film technology, etc., and the sensor can be installed in a wide range of locations, allowing it to be used in any environment. It is possible to measure humidity. Furthermore, by using semiconductor thin film technology, etc., it can be manufactured at a very low cost compared to its performance. Then, in order to perform thermal cleaning, a heater unique to the heater may be used. Furthermore, since a simple measurement principle is used, the equipment configuration is simple, and therefore various troubles are less likely to occur and the equipment can be configured at low cost.

[0060]

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of a humidity sensor according to the present invention.

FIG. 2 is a diagram showing a temperature gradient within the substrate of the humidity sensor shown in FIG. 1;

FIG. 3 is a diagram showing the relationship between the amount of moisture contained in the humidity sensitive film 2 of the humidity sensor shown in FIG. 1 and the detected thermoelectromotive voltage.

FIG. 4 is a diagram showing another embodiment of the humidity sensor according to the present invention.

FIG. 5 is a diagram showing another embodiment of the humidity sensor according to the present invention.

FIG. 6 is a diagram showing another embodiment of the humidity sensor according to the present invention.

FIG. 7 is a diagram showing another embodiment of the humidity sensor according to the present invention.

FIG. 8 is a diagram showing another embodiment of the humidity sensor according to the present invention.

FIG. 9 is a diagram showing another embodiment of the humidity sensor according to the present invention.

[Explanation of symbols]

1. Board. 2 Moisture sensitive membrane. 3. Means for heating or cooling. 4. Means for detecting temperature. 5 Heat sink. 6. Constant temperature section. 7 Heating section. 8. Electrode. 9. Light absorption film. 31 Thin film heater. 41a, 41b Thin film thermocouple. 42 Thin film resistor. 43 Radiant film.

Claims (1)

[Claims] 1. A substrate (1) of a thermally poor conductor thermally connected to a heat sink, a moisture sensitive film (2) provided on at least a portion of the substrate, and a moisture sensitive film (2) provided at a predetermined position of the substrate. a heating or cooling means (3); and a means (4) for detecting a desired temperature of the substrate heated or cooled by the heating or cooling means; A sensor that measures atmospheric humidity by detecting the temperature caused by a corresponding change in thermal resistance.