WO2000029839A1

WO2000029839A1 - Humidity sensor and method for the production thereof

Info

Publication number: WO2000029839A1
Application number: PCT/DE1999/003608
Authority: WO
Inventors: Patrick Keller
Original assignee: Fraunhofer Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 1998-11-17
Filing date: 1999-11-10
Publication date: 2000-05-25
Also published as: DE19852969A1

Abstract

The invention relates to a humidity detector and to a method for the production thereof. The inventive humidity detector consists of a support (1) on which at least two electrodes (2, 3) are arranged. A layer (4) made of a specific hygroscopic material extends between the electrodes. The hygroscopic material is characterized in that electrolysis of water occurs in said layer (4) when humidity is absorbed and an electrical voltage is applied to the electrodes (2, 3). The electrolytic current is used to detect humidity. The humidity sensor can be directly integrated into electronic circuits. It has good long-time stability and requires only a short time to react.

Description

Moisture detector and manufacturing method

The invention relates to a moisture detector, which is particularly suitable for the detection of low moisture in an environment requiring special monitoring, and a method for its production.

The moisture detector presented can be integrated directly into electronic circuits, so that the moisture is detected directly at the point of action.

When using moisture detectors or sensors, the aim is to continuously, reliably, accurately and reproducibly monitor the water content in the surrounding atmosphere. The use of further technical auxiliary systems (actuators) should also make it possible to influence the composition of the atmosphere to be monitored or to protect moisture-sensitive electronic components.

A large number of different moisture measuring systems are known in the prior art and are briefly explained below.

When measuring moisture with color hygrometers, the color change of salts containing water of crystallization is observed. Color hygrometers consist of a number of different cellulose platelets with scale values, which are impregnated with salts from, for example, cobalt (111) compounds, copper sulfate or nickel chloride. At temperatures of 15-25 ° C, an exact 0-25% RH can be achieved. However, this only allows use for a rough display of the moisture.

Mechanical hygrometers exploit the property of certain organic substances to expand or contract under moisture (e.g. hair hygrometer). These moisture meters are simple and inexpensive to manufacture, but often have a very high hysteresis and long response times. In addition, the conversion of the measured values into electrical signals is very difficult and causes high costs.

Psychrometers work with two identical temperature sensors that are located in an air flow. One of the two sensors is over one

Cotton stocking kept constantly moist. Due to the evaporative cooling, the moist temperature sensor cools down. This evaporative cold is proportional to the amount of water evaporated from the cotton stocking. Since the air flowing past is saturated by the amount of water evaporated from the cotton stocking, the measured temperature difference between the two sensors can be regarded as a measure of the humidity of the incoming air. The disadvantages of the method lie in the very long response time and the relatively high measuring errors caused by icing of the moist temperature sensor.

If moist air is cooled down to the temperature at which the saturation state arises due to the elimination of water, the absolute dew point temperature of the air in question can be determined using the determined dew point temperature. On this Working principle is dew point hygrometer. Technically, for this purpose, a small area in the dew point measuring device is cooled by means of a cooling medium or a Peltier element until the moist air to be measured on the cooled area reaches the saturation moisture and the water vapor begins to condense, ie the surface is condensed. A photoelectric cell, for example, is directed onto this surface and changes its output signal when it is thawed. The temperature of the measuring surface is defined as the dew point temperature. Dew point meters are very stable, accurate and fast.

The psychrometer and the dew point sensor are on the one hand very accurate, but on the other hand they are very expensive and bulky, which means that the use of the two measuring devices is very limited. They are mainly used only for precision measurements and permanently installed in climatic chambers. Because of the disadvantages mentioned above, they are difficult to use for measuring small and dry volumes. The compact humidity sensors shown below contrast this complex process.

A representative of compact moisture sensors based on organic polymers is the "Humicape" sensor, which consists of cellulose acetate dissolved in ethylene dichloride or cross-linked copolymers of styrene sulfone, vinyl polymer, N, N-methylene-bis-acrylamide and polyvinyl chloride. Compared to ceramic moisture sensors, the short lifespan of organic moisture sensors is very disadvantageous. In addition, with these materials there is the problem of reproducible sensor production by means of spray coating, dip coating or similar processes. Porous ceramic materials are particularly well suited for the production of moisture sensors, as they have very good properties with regard to their mechanical adhesive strength, reproducibility, durability, easy handling and resistance to environmental influences. The structure of these moisture sensors generally consists of a sensitive layer vapor-coated with metal on both sides, which consists of sintered, ceramic, porous, moisture-sensitive materials. The sensitive layer serves as the dielectric of a plate capacitor, the two metal films representing the electrodes. The relative humidity changes the dielectric constant of the sensitive layer and thus also the capacitance of the plate capacitor or the conductivity of the sensitive layer.

Hysteresis and aging phenomena have a disadvantageous effect on ceramic moisture sensors, particularly at high moisture concentrations.

Electrolytic moisture sensors are known as a further possibility for moisture measurement. This type is essentially a moisture meter that works on the basis of lithium chloride sensors. Two properties of LiCl are used here. On the one hand, lithium chloride is hygroscopic, i.e. it adsorbs water molecules, and on the other hand the resulting salt solution is an electrolyte that conducts the electric current.

A major disadvantage of electrolyte moisture sensors based on LiCl is their relatively slow response time. But they should also be used in very humid environments not be used as this leads to negative influences in the precision and the service life of the sensor. The aging effects of the sensor due to the crystal formation of the LiCl are particularly disadvantageous here.

The invention has for its object to provide a moisture detector and a method for its production, which has a fast response time, high sensitivity and good long-term stability.

The object is achieved with the moisture detector according to claim 1 and the method according to claim 13. Advantageous embodiments of the moisture detector and the method are the subject of the subclaims.

The moisture detector according to the invention consists of a carrier on which at least two electrodes, preferably in an interdigital structure, are arranged. For electrical insulation from the carrier, an insulating intermediate layer can additionally be provided between the carrier and electrodes. A layer of a certain hygroscopic material extends between the electrodes. This hygroscopic

Material is characterized in that when moisture is absorbed and an electrical voltage is applied to the electrodes, electrolysis of water takes place in the layer. The resulting electrolysis current is used to detect the moisture. In this case, a threshold value of the current is preferably specified, the moisture in the environment being exceeded when it is exceeded of the detector has reached a certain value. If the threshold value is exceeded, it is detected and, if necessary, output in a signal for triggering a subsequent action, for example triggering an alarm or actuating an actuator.

In contrast to the previously mentioned electrolytic moisture sensors based on ion crystals such as LiCl, the hygroscopic material used according to the invention causes electrolysis of the water in the sensor surface, ie in the hygroscopic layer located between the electrodes. In the case of a sensor with a layer of LiCl according to the prior art, the LiCl is converted into a salt solution by the moisture, so that electrolysis of Li ⁺ and Cl ^~ ions takes place. However, this creates the disadvantages mentioned above.

However, the use of the sensor according to the invention has the surprising advantage that

Utilization of the electrolysis of water does not change the sensor material, so that good long-term stability of the detector results.

A hygroscopic material is preferably used, which enters into a chemical reaction with water, which can be reversed by applying an electrical voltage to the electrodes. Examples of such materials are substances that form an acid or base with water. Due to the back reaction when the electrical voltage is applied to the electrodes, the water in the sensor surface is thus available for electrolysis. In a particularly advantageous embodiment, P ₂ O _{5 is} used as the hygroscopic material.

The special advantages of this material compared to the LiCl used in conventional electrolyte sensors are the achievable short response times of the detector, which can be attributed to the highly hygroscopic property of the P ₂ 0 ₅ . This enables more sensitive measurements. Another advantage is the good adhesion of this

Material on the substrate so that no additional binder is required. With this embodiment of the moisture detector, as already explained above, the aging of the "sensitive" material can be neglected.

The sensitive material consisting of P ₂ Ü5 is preferably applied as a closed layer to the electrode structure by spray coating, electrolysis and heating, as described below.

In a further advantageous embodiment of the moisture detector according to the invention, the carrier consists of a substrate made of semiconductor-compatible material, for example Al ₂ O ₃ , Si, SiO ₂ or Si ₃ N ₄ . As a result, the detector can advantageously be integrated directly into the electronic circuits or onto a common substrate with the circuits whose ambient humidity is to be monitored. The electrodes can, for example, be applied to the substrate using planar technology. The layer can either only between the Extend electrodes or cover them completely.

In the method according to the invention for producing the moisture detector, a substrate is first provided to which the electrodes are applied, for example using thin-film or thick-film technology. Furthermore, a hygroscopic material is used, which enters into a chemical reaction with water, which can be reversed by applying an electrical voltage. The hygroscopic material is applied to the substrate in aqueous form so that it extends at least between the electrodes. The application can take place, for example, by means of the spray coating technique. Finally, the hygroscopic material is dried at an elevated temperature by applying an electrical voltage sufficient for the electrolysis of water to the electrodes. The drying takes place due to the elevated temperature and the electrolysis of the water.

In a preferred embodiment for producing the layer, the electrodes are covered, for example by means of spray coating, with aqueous P ₂ 0 ₅ , so that a closed layer of moisture-sensitive material is formed. As already stated, this layer of aqueous P ₂ Os is dried by electrolysis of water at a permanently elevated temperature, so that a moisture-sensitive layer of dried sensor material is formed.

The applied layer is characterized by extremely hygroscopic and electrolytic properties whose electrical conductivity changes depending on the water vapor concentration. During the measurement, the absorbed water molecules are electrolyzed in hydrogen and oxygen, the equilibrium electrolysis current being an indirect measure of the moisture.

Of course, the hygroscopic layer for the moisture detector can also be applied to the substrate or the electrodes by other methods, for example by dropping in a suitable, volatile solvent which then volatilizes by itself.

The invention is based on a

Embodiment explained in connection with the figure.

The figure shows an example of the possible structure of a moisture detector according to the invention. The detector consists of a substrate (1), on the front of which two electrodes (2, 3) spaced apart from one another are applied in an interdigital structure (not shown), so that the individual "fingers" of the electrodes interlock. The interdigital structure is covered by a closed layer (4) made of the hygroscopic material. Such a detector can be implemented with dimensions of, for example, 2mm x 3mm for the substrate (1).

In the present example, the hygroscopic layer (4) consists of P0, a highly hygroscopic copy material. Interdigital electrodes (2, 3) made of platinum are applied to the substrate (1).

The interdigital electrodes are produced by depositing platinum and subsequent structuring using conventional photolithography techniques, as described, for example, in D. Widmann et al. , "Technology of highly integrated circuits", 2nd ed., Springer-Verlag, Berlin 1996, applied to the substrate. The interdigital structure can be configured as desired using thick or thin film technology.

To apply the hygroscopic layer, a solution of aqueous P ₂ 0 ₅ is first applied to the substrate by means of spray coating. In the resulting surface film, the water is decomposed by electrolysis of water with permanent heating and the layer is thus dried. The electrolysis of the water is effected by applying a voltage of a few volts to the interdigital electrodes. The simultaneous heating to a temperature above 100 ° C can be carried out by different heating techniques, for example in an oven, by radiation, by additional heating elements in the substrate or the like.

With increasing drying out of the P ₂ Os solution, the electrolysis current returns to an equilibrium state which varies with changing ambient air humidity. This effect is used for the detection of moisture or moisture dips in an environment requiring special monitoring. The actual measuring or detection process therefore consists in detecting a change in the electrolysis current when a voltage is applied to the electrodes. The detector shown as an example has short response times due to the strong hydrophilicity of the sensor material. This enables precise moisture measurements, even of very low humidity, through exact current measurements. The aging of the "sensitive" material can be neglected.

In a further embodiment of the moisture detector, there is a heating device on the back of the substrate for heating the hygroscopic

Layer provided. By means of this heating device, the hygroscopic layer after "moisture accidents", i.e. in the event of sudden very high water absorption, regenerate faster. Furthermore, the heating device can advantageously be used for heating when the layer is applied, as explained above. The heating device here consists, for example, of a meandering structure of a conductive material with high resistance, which can be applied in planar technology on the rear surface of the substrate.

Of course, the heating device can also be provided elsewhere, for example in an intermediate layer between the substrate and the electrode structure that is insulated from the electrode structure.

Claims

claims

Moisture detector which has on a carrier (1) a layer (4) of hygroscopic material arranged between two electrodes (2, 3), characterized in that the material is selected such that the moisture absorbed by the layer (4) when applied an electrical voltage to the electrodes (2, 3) is electrolyzed.

2. Moisture detector according to claim 1, characterized in that the hygroscopic material undergoes a chemical reaction with water, which can be reversed by applying an electrical voltage to the electrodes (2, 3).

3. Moisture detector according to claim 1 or 2, characterized in that the hygroscopic material forms an acid or base with water.

4. Moisture detector according to one of claims 1 to 3, characterized in that the hygroscopic material consists of P ₂ 0 ₅ .

5. Moisture detector according to one of claims 1 to 4, characterized in that a heating device for heating the layer (4) is provided on the carrier (1).

6. Moisture detector according to claim 5, characterized in that the heating device enables the setting of different operating temperatures of the layer (4).

7. Moisture detector according to one of claims 1 to 6, characterized in that the layer (4) has a thickness of at most 2 μ.

8. Moisture detector according to one of claims 1 to 7, characterized in that the electrodes (2, 3) form an interdigital structure.

9. Moisture detector according to one of claims 1 to 8, characterized in that the carrier (1) is a substrate made of semiconductor-compatible material.

10. Moisture detector according to claim 9, characterized in that the electrodes (2, 3) are applied in planar technology.

11. Moisture detector according to one of claims 1 to 10, characterized in that the electrodes (2, 3) are covered by the layer (4).

12. Moisture detector according to one of claims 1 to 11, characterized in that a voltage source for applying a DC voltage to the electrodes (2, 3) and a measuring device for detecting the size of an electrolysis current flowing between the electrodes are further provided.

13. A method for producing a moisture detector according to one of the preceding claims, comprising the following steps:

- Providing a substrate (1);

- Application of electrodes (2, 3) on the substrate; - Providing a hygroscopic material that undergoes a chemical reaction with water that can be reversed by applying an electrical voltage;

- Applying the hygroscopic material in aqueous form to the substrate (1) so that it extends between the electrodes (2, 3);

- Drying the hygroscopic material by applying an electrical voltage sufficient for the electrolysis of water to the electrodes (2, 3) at an elevated temperature.

14. The method according to claim 13, wherein P ₂ 0 _{5 is} used as the hygroscopic material.

15. The method according to any one of claims 13 or 14, wherein the hygroscopic material is applied in aqueous form by spray coating.

16. The method according to any one of claims 13 to 15, wherein the electrodes (2, 3) are applied in thin film technology.

17. The method according to any one of claims 13 to 15, wherein the electrodes (2, 3) are applied in thick film technology.