WO2007124725A1

WO2007124725A1 - Microsensor

Info

Publication number: WO2007124725A1
Application number: PCT/DE2007/000754
Authority: WO
Inventors: Barbara March; Geert Brokmann; Arndt Steinke; Andreas Albrecht
Original assignee: CiS Institut für Mikrosensorik gGmbH
Priority date: 2006-04-27
Filing date: 2007-04-26
Publication date: 2007-11-08
Also published as: DE102006019534A1; EP2010897A1

Abstract

The invention relates to a capacitive-effect or conductive-effect microsensor having a first and a second electrode, at least one sensitive layer or at least one functionalized surface between the electrodes and an insulating substrate on which the electrodes and at least one sensitive layer or at least one functionalized surface are arranged. The invention is based on the object of specifying a capacitive sensor in which the influence of interference variables on the reproducibility of the measured values ascertained by means of the sensor is reduced. The invention achieves the object by means of a sensor in which the electrodes are at approximately the same distance from the substrate and the electrode faces which are adjacent to the sensitive layer are arranged on the substrate surface at an inclination relative to the perpendicular, the inclination of the electrodes being oriented such that the distance between the respective opposite electrode faces decreases towards the substrate.

Description

microsensor

The invention relates to a sensor arrangement having a first and a second electrode arranged opposite one another on an insulating substrate, between which there is at least one sensitive layer or at least one functionalized surface, the electrode surfaces adjacent to the at least one sensitive layer or the at least one functionalized surface Tilt zui 'perpendicular to the substrate surface are arranged.

The capacitive or conductive microsensor can be used for a variety of applications. Preferred fields of application are moisture, fluidic and gas sensors, in particular sensors for determining the relative humidity and water activity.

For measuring the humidity, conventional methods of gas moisture measurement can be used. Preference is given to methods in which the water vapor pressure measurement in the air moisture measurement via the measurement of relative humidity takes place or via a dew point measurement.

Water activity, also referred to as aw, is a measure of freely available water in a material and is an important criterion for assessing the shelf life of food. The aw value is defined as the quotient p / pθ of the water vapor pressure over a material (p) to the water vapor pressure above pure water (pθ) at a certain temperature. The aw-value measurement is based on the measurement of the relative humidity of a sensor element after adjustment of the equilibrium moisture content between the measuring medium and the sensor.

The presently preferred capacitive methods utilize planar electrode structures fabricated by thin film and thick film techniques, overlying sensitive layers, partially supplemented by vertically above them in terms of area arranged transparent cover electrodes. Disadvantages of these structures are the influence of the capacitive influence, in particular in the form of changes in thickness, for example by moisture absorption or -abgäbe, or in the form of impurities of the sensitive layer or their interfaces. This means that all applications in which the sensors are subject to such environmental influences due to their use are characterized by a high degree of cross-sensitivity. In order to achieve a sufficient measurement accuracy, complex calibrations and adjustments are required. Therefore, only short operating times with the original accuracy are achievable, so that use for tasks with high reliability requirements and long-term requirements is only possible to a limited extent. The cross-sensitivity is caused by a number of operational influences, such as temperature, pressure, negative pressure, chemical influences, high and very low humidity and condensation. On the other hand, there are also operational influences, such as temperature influence by heating or cooling. These influences lead to changes in the sensitive layer of the sensors. Analogous effects are also found in fluidic sensors and in gas sensors. Environmental influences can lead to changes in the sensitive layer, which cause a change in capacitance without the measuring state having changed.

In the prior art capacitive sensors are known in various embodiments.

Such a sensor is a capacitor with at least two electrodes, between which there is a moisture-sensitive dielectric. At least one of the two electrodes is mounted in an electrically insulated manner on a carrier, which consists for example of glass or ceramic and is referred to as a substrate. The second electrode, which is located on the outside and likewise in the form of a metallic layer, is permeable to moisture: the water molecules in the air can diffuse through this electrode. Between the two electrodes there is the moisture-sensitive dielectric which is crucial for the moisture measurement. In general, the dielectric layer is formed by a polymer film.

The change in the capacity of such a humidity sensor in the presence of air of different moisture content is due to the fact that in the air located water molecules diffuse into the polymer film forming the dielectric and thus change the dielectric constant and consequently the capacitance value of the capacitor thus formed. While the dielectric constant of polymers is between 2 and 3, the dielectric constant of water is 80. This means that the capacitance of such a capacitor increases when water molecules penetrate into the dielectric layer, which can be exploited for moisture measurements.

According to EP 0 403 994 a capacitive humidity sensor is known, in which a capacitor is used consisting of a planar arrangement with superimposed layers, which contains two metallic layers forming the electrodes, of which at least one is water vapor permeable, and a moisture sensitive polyimide film as dielectric ,

Furthermore, DE 197 29 697 describes an arrangement for determining the relative humidity with a capacitive air humidity sensor, in which the sensor contains a polymer layer as a dielectric and two electrically conductive, moisture-permeable electrodes which are arranged on both sides of the polymer layer.

Various materials are used for the dielectric layer. The dielectrics used hitherto, in particular the polymers from the group of polyimides, frequently exhibit drift properties and low reproducibility with respect to the electrical characteristics when used in humidity sensors.

From EP 0 403 994 A1 it is known to use a special group of imides, namely polyetherimide, as a dielectric for moisture sensors. With this material, the drift properties and reproducibility compared to the previously used substances to be improved.

EP 1 387 164 A1 describes a capacitive sensor having a first and a second electrode, which has a gas-sensitive layer between opposing electrodes, the electrodes and the gas-sensitive layer being arranged on an insulating substrate. In this case, the linear expansion coefficient of the electrodes is lower than that of the gas-sensitive layer and that of the substrate. For this sensor the gas-sensitive layer is located between electrodes arranged vertically on the substrate and above the electrodes.

Further, according to DE 43 37 418 C2 discloses a method for producing a sensor element is known in which a sensitive material is introduced into the opening of a thin silicon substrate which extends from the front to the back and tapers towards the back. The openings are closed with a thin plate. The disadvantage here is that the arrangement requires a diffusion limitation and that no CMOS-capable arrangements can be realized.

A major disadvantage of the known arrangements is that the reproducibility of the values determined by the sensor is impaired by water absorption and contamination.

The invention has for its object to provide a capacitive sensor of the type mentioned, in which the influence of disturbances on the reproducibility of the measured values determined by the sensor is reduced.

According to the invention, the object is achieved with a sensor which contains the features specified in claim 1.

Advantageous embodiments are specified in the subclaims.

In the sensor according to the invention, the electrodes are arranged at approximately the same distance from the substrate. At approximately the same distance, it should be understood that the distances of the electrodes from the substrate have only slight, in particular tolerance-related, differences. The electrode pads adjacent to the sensitive layer are arranged with an inclination to the vertical on the substrate surface. The inclination of the electrodes is aligned in such a way that the distance between the respectively opposite electrode surfaces and the substrate decreases. The electrodes are arranged at the top of the assembly. With the top of this case, an area is referred to, which is located on the surface of the arrangement. Compared to the usual arrangements in the prior art, in which the electrodes and the sensitive layer are arranged parallel to each other or side by side, is located in the inventive arrangement, the sensitive layer between electrodes which are arranged inclined to the substrate surface. As a result, changes in the thickness of the sensitive layers, which are caused by shrinkage and swelling, do not or only negligibly affect the distance between the electrodes. Furthermore, impurities of the sensitive layer occur only on the upper side, which have a smaller proportion of the effective dielectric. The influence of contamination is so significantly reduced as they occur at the top of the arrangement. However, the surface layers have a larger electrode spacing than the underlying layers and thus have a lower partial capacitance, so that they influence the measured value only to a smaller extent.

An advantageous embodiment provides that passivation layers are arranged between the electrodes and the substrate and / or between the electrodes and the sensitive layer. This makes it possible to extend the life and affect the capacitive parameters of the arrangement. Depending on whether the electrodes are provided with an insulating passivation layer or not, it is a capacitive or a conductive sensor arrangement.

It is possible for the electrodes arranged in an interdigital structure to be designed such that their surfaces perpendicular to the substrate have an inclination, the distances between the opposing electrode surfaces, between which the sensitive layer is located, tapering towards the substrate.

In an advantageous embodiment, the electrodes are arranged in a V-shaped or in a rounded recess in the substrate. In this case, the electrodes can consist of a metallic layer applied to the substrate or can be formed by a highly doped region which is arranged in the substrate and which adjoins the sensitive layer. This can be achieved in particular with highly doped silicon on an insulating substrate. Furthermore, it is possible for the electrode arrangement to be arranged multiply in the depression. For example, in each case two electrodes can be attached adjacent to one side of a distributor, which thus form different capacities at different depths. This makes it possible to detect effects of different cross influences, so that statements on the measurement accuracy and its time course can be obtained.

Different polymers, which have different dielectric properties, can also be arranged at different depths. Optionally, these polymers are subdivided by a release layer acting as a diffusion barrier.

A further advantageous embodiment results from the fact that two recesses are arranged side by side in the substrate, in each of which at least one capacitive sensor arrangement is located. Here, for example, one of the arrangements can be used as a reference sensor.

In order to determine transverse influences, the recesses in the substrate can be made with different depths, so that different information is obtained which allows statements about the useful signal and error signals.

Further, it is possible to arrange a reference sensor in which the electrode gap increases toward the substrate. In this case, changes in the layer thicknesses have a particularly strong effect, so that comparison signals are obtained which allow statements about the error or can be used for error compensation.

To reduce disturbing cross influences filter layers may be mounted on the top of the sensor.

An extension of the scope can be achieved by the arrangement is additionally provided with a heater and or a temperature sensor, so that, for example, the detection of gases is possible. To reduce the heat capacity, the heater may be provided on its back with a membrane. In addition to the advantages described above, the arrangement according to the invention is also distinguished by the fact that it allows a good mechanical coupling, which can be carried out in both thin-film and thick-film technology. Another advantage results from the fact that the electrodes can be made of thin material because they do not have to perform mechanical strength tasks.

The invention is explained in more detail below with reference to an embodiment.

In the accompanying drawing show:

Figure 1 shows an arrangement in which the effective electrode surfaces are conductively connected to the sensitive layer, Figure 2 shows an arrangement with filter, Figure 3 shows an arrangement in which the effective electrode surfaces are capacitively connected to the sensitive layer, Figures 4 to 6 arrangements with several Partial Capacities FIG. 7 shows arrangements with a V-shaped depression and triangular cross-section of the sensitive layer, FIG. 8 shows arrangements with a V-shaped depression and trapezoidal cross-section of the sensitive layer, FIG. 9 shows an arrangement with a rounded cross-section of the sensitive elements

Layer,

FIGS. 10 to 12 arrangements in which the sensitive layer has different areas,

13 shows an arrangement with a measuring sensor arrangement and a

Reference sensor arrangement and

FIG. 14 shows an arrangement with several sensors and a heating device.

FIG. 1 shows an embodiment in which an insulator layer 3 is mounted on a substrate 4. On the insulator layer 3 electrodes 2 are arranged. Between the electrodes 2 there is a sensitive layer 1. In the case of the illustrated In embodiments, the electrodes 2 are conductively connected to the sensitive layer 1. The sensitive layer 1 is in this case freely accessible to the surrounding medium at the top of the arrangement. The inclined electrode arrangement ensures that the sensitive layer 1 does not cause any changes in the distance between the electrodes 2 even in the event of changes in the form of swellings, which are caused in particular by moisture absorption, so that the changes in the dielectric properties remain small. In Figure Ia, a detail is shown enlarged. In this case, the electrodes 2 arranged in an interdigital structure are designed so that the electrode surfaces adjacent to the sensitive layer 1 are arranged with an inclination relative to the vertical on the substrate surface. The inclination of the electrodes 2 is aligned so that the distance between the respective opposite electrode surfaces to the substrate 4 decreases.

Figure 2 shows an embodiment in which the arrangement shown in Figure 1 is provided at the top with an additional filter layer 6. This layer can be made of glass, ceramic or a polymer, for example, and reduces contamination of the sensor arrangement.

In the arrangement shown in FIG. 3, the electrodes 2 are provided with an insulating passivation layer 5. It is therefore a capacitive sensor arrangement.

In the figures 4 to 6 arrangements are shown, each having two different areas of the capacitive arrangement. This is achieved by various embodiments of the electrodes 2 and / or the sensitive layer 1. This allows the determination of the influence of certain disturbances.

FIG. 4 shows a sensor embodiment in which the electrodes 2 are formed in two regions with different heights. In the arrangement shown in Figure 5, the sensitive layer 1 is arranged in different ways. In the case of a number of electrodes 2, the sensitive layer 1 is designed according to the manner explained in FIG. 3, while a different number of the electrodes 2 is completely covered by the sensitive layer 1 and with a further number of electrodes 2 the sensitive layer 1 has a smaller height as the electrodes 2 on. FIG. 6 shows an embodiment in which, in addition to a number of electrodes 2 arranged in the manner described in FIG. 3, a planar arrangement is provided, in which the sensitive layer 1 covers a few electrodes 2. 1 and covers the entire planar partial area Electrode 2.2 is arranged.

Figures 7 and 8 illustrate examples of arrangements in which the sensor assembly is mounted in a V-shaped recess. The embodiment shown in FIG. 7 has a triangular cross section of the sensitive layer 1, while in the embodiment of a capacitive sensor arrangement shown in FIG. 8, the sensitive layer 1 has a trapezoidal cross section.

FIG. 9 shows an embodiment in which the sensitive layer 1 has a rounded cross-section.

Figures 10 to 12 show examples of arrangements with several partial capacities. In the embodiment shown in Figure 10, two partial capacities are arranged one above the other. In this case, the sensitive layer 1 consists of the two partial layers 1.1 and 1.2, which may consist of the same material or of different materials. In the example shown, the partial capacitances are separated from one another by a diffusion barrier 7.

The embodiments shown in FIGS. 11 and 12 contain, in addition to the arrangement shown in FIG. 10, further sensor arrangements which are provided with only one capacitance and can be used as reference sensors.

FIG. 13 shows an embodiment in which a reference sensor arrangement is arranged on the substrate surface next to a measuring sensor arrangement. The measuring sensor arrangement contains the electrodes 2.7, 2.8 and 2.9, between which the sensitive layer is arranged as described above. The reference sensor arrangement contains the electrodes 2.10, 2.11 and 2.12, in which the distance of the electrodes 2 from one another increases towards the substrate 4. In this case, changes in the layer thicknesses have a particularly strong effect, so that comparison signals are obtained that allow statements about the error or can be used for Felllerkompensation. By this arrangement of the electrode surfaces, information about the influence of source effects can be obtained. Due to the higher field line density in the upper In the region of the electrodes, changes in the electrode surface occupation by the sensitive layer lead to a significant signal change.

FIG. 14 shows an arrangement in which a plurality of individual sensors S 1, S 2, S 3 are arranged on the upper side of the substrate 4. On the underside of the substrate 4 is a recess containing an electric heater 8. As the heating device 8, an electrical resistance device can be used, which can serve as a temperature sensor at the same time. Furthermore, it is possible to arrange a micropelt element as a cooling element in this way.

LIST OF REFERENCE NUMBERS

1 sensitive layer

1.1 upper sensitive layer

1.2 lower sensitive layer 2 electrode

2.1 ... 2.12 individual electrodes

3 insulator layer

4 substrate

5 passivation layer

6 filters

7 diffusion barrier

8 heating device

Sl ... S3 single sensors

_ 1

Claims

PAT EN TANSPR

1. sensor arrangement having a first and a second electrode (2) arranged opposite one another on an insulating substrate (4), between which at least one sensitive layer (1) or at least one functionalized surface is located, whereby the at least one sensitive layer ( 1) or the at least one functionalized surface adjacent electrode surfaces are arranged with a slope to the vertical on the substrate surface, characterized in that the electrodes (2) are located at the top of the array and arranged at approximately the same distance from the substrate (4) are, wherein the inclination of the electrodes (2) is aligned so that reduces the distance of the respective opposite electrode surfaces to the substrate (4) out.

2. Sensor arrangement according to claim 1, characterized in that passivation layers (5) are arranged between electrodes (2) and substrate (4) and / or between electrodes (2) and sensitive layer (1).

3. Sensor arrangement according to claim 1 or 2, characterized in that the electrodes (2) are arranged in a V-shaped recess in the substrate (4).

4. Sensor arrangement according to claim 1 or 2, characterized in that the electrodes (2) are arranged in an interdigital structure.

5. Sensor arrangement according to claim 4, characterized in that an additional passivation layer (5) is arranged between the electrodes (2) and the sensitive layer (1).

6. Sensor arrangement according to claim 1 or 2, characterized in that the electrodes (2) are arranged in a rounded depression in the substrate (4).

7. Sensor arrangement according to one of claims 3 to 6, characterized in that the electrode arrangement (2) is arranged several times in the recess, wherein in each case at least two electrodes (2) are mounted adjacent.

8. Sensor arrangement according to one of claims 3 to 7, characterized in that at least two recesses are arranged adjacent in the substrate (4).

9. Sensor arrangement according to one of the preceding claims, characterized in that on the upper side of the sensor at least one filter layer (6) is applied.

10. Sensor arrangement according to one of the preceding claims, characterized in that the arrangement includes a reference sensor, in which the electrode spacing increases towards the substrate or remains constant.

11. Sensor arrangement according to one of claims 1 to 9, characterized in that the arrangement includes a reference sensor, in which the electrodes (2.1, 2.2) are in a planar arrangement.

12. Sensor arrangement according to one of the preceding claims, characterized in that the arrangement includes a reference sensor, wherein the sensitive material (1) is layered differently.

13. Sensor arrangement according to one of the preceding claims, characterized in that the arrangement additionally contains a heating and / or cooling and / or a temperature sensor.