Sensor System, Sensor Array and Process of Using the Sensor System

Abstract

In an embodiment a sensor system includes a first sensor having a first thermistor configured to sense a change in heat flow and a first heater configured to heat the first thermistor and a second sensor having a second thermistor configured to sense a change in heat flow and a second heater configured to heat the second thermistor, wherein a heat conduction path between the first heater and the first thermistor has a higher thermal conductivity than a heat conduction path between the second heater and the second thermistor.

Claims

1. A sensor system comprising: a first sensor having a first thermistor configured to sense a change in heat flow and a first heater configured to heat the first thermistor; and a second sensor having a second thermistor configured to sense a change in heat flow and a second heater configured to heat the second thermistor, wherein a heat conduction path between the first heater and the first thermistor has a higher thermal conductivity than a heat conduction path between the second heater and the second thermistor.

2. The sensor system according to claim 1, wherein the heat conduction path between the first heater and the first thermistor includes the first thermistor itself and the heat conduction path between the second heater and the second thermistor includes the second thermistor itself, and wherein the first thermistor has a higher thermal conductivity than the second thermistor.

3. The sensor system according to claim 1, wherein the heat conduction path between the first heater and the first thermistor includes a first interlayer arranged between the first heater and the first thermistor, and wherein the heat conduction path between the second heater and the second thermistor includes a second interlayer arranged between the second heater and the second thermistor.

4. The sensor system according to claim 3, wherein the first interlayer has a higher thermal conductivity than the second interlayer.

5. The sensor system according to claim 3, wherein the first interlayer and the second interlayer comprise a chemically similar material, and wherein the first interlayer has a higher degree of crystallinity and/or comprises crystallites with a larger average size than the second interlayer.

6. The sensor system according to claim 3, wherein the first and the second interlayer comprise a silicon oxide material.

7. The sensor system according to claim 3, wherein the first interlayer has a thermal conductivity of 5 W/(m.Math.K) or above and the second interlayer has a thermal conductivity of 1.4 W/(m.Math.K) or below.

8. The sensor system according to claim 1, wherein each of the first and second heater is an arrangement of one or several conductors wound to a meander-like form within a spatial plain.

9. The sensor system according to claim 1, wherein, under constant conditions or steady state conditions, the first heater together with a power supply for the first heater and the second heater together with a power supply for the second heater are configured to provide different temperatures to the first thermistor and the second thermistor.

10. A sensor array comprising: at least two sensor systems according to claim 1, which are configured to detect different detectants.

11. A method for using the sensor system according to claim 1, the method comprising: heating the first sensor and the second sensor to different temperatures, wherein one of the sensors acts as a measurement sensor and the other sensor acts as a reference sensor; and exposing the sensor system to a gas atmosphere comprising a detectant; and measuring a difference in temperature change between the measurement sensor and the reference sensor when compared to a steady state response of both sensors in a reference gas atmosphere without the detectant.

12. A sensor system comprising: a first sensor having a first thermistor for sensing a change in heat flow, a first heater is configured to heat the first thermistor and a first interlayer between the first thermistor and the first heater; and a second sensor having a second thermistor configured to sense a change in heat flow, a second heater configured to heat the second thermistor and a second interlayer between the second thermistor and the second heater, wherein the first interlayer and the second interlayer have a thermal conductivity of above 1.4 W/(m.Math.K).

13. The sensor system according to claim 12, wherein the thermal conductivity of both interlayers is higher than 2 W/(m.Math.K).

14. The sensor system according to claim 12, wherein the thermal conductivity of the first interlayer and the second interlayer is the same.

15. A sensor array comprising: at least two sensor systems according to claim 12, which are configured to detect different detectants.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In the following the invention is described in more detail with reference to schematic figures and experimental data. For the schematic figures it is noted that the components are not shown true to scale, but are only represented schematically. Accordingly, the components may be shown distorted in their size, lengths or length ratios. Accordingly, length or length ratios may not be taken from the schematic drawings.

[0039] FIG. 1 shows a first embodiment of the present application in a schematic cross-section representation;

[0040] FIG. 2 shows computationally simulated data of the dependence of a temperature difference between different CO.sub.2 gas concentrations in a test atmosphere in dependence on the thermal conductivity at 300° C. heating temperature;

[0041] FIG. 3 shows computationally simulated data of the dependence of a temperature difference between different CO.sub.2 gas concentrations in a test atmosphere in dependence on the thermal conductivity at 150° C. heating temperature; and

[0042] FIG. 4 shows the computationally simulated data of the sensitivity of CO.sub.2 gas detection for thermistors heated to different temperatures in dependence on the thermal conductivity.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0043] FIG. 1 shows a first representation of a sensor system 1. The sensor system 1 comprises two sensors, a first sensor 1 and a second sensor 9. The first sensor 1 comprises a first thermistor 2 which is connected to contacts 3 of the first thermistor 2 for read-out of the first sensor. Furthermore, the first sensor 1 comprises a first heating meander layer 6 as a first means for heating. The first heating meander layer 6 is provided on a first substrate 8. The first substrate 8 may comprise a further insulation layer arranged between the bulk material of the substrate and the heating meander layer 6 (not depicted). Furthermore, the first heating meander layer 6 comprises two external contacts 7 which allow the first heating meander layer 6 to be electrically contacted and to be supplied with power. In between the first heating meander layer 6 and the first thermistor 2 the first interlayer 4 is arranged.

[0044] The second sensor 9 comprises a second substrate 16 which is shown as a joint substrate with the first substrate 8. Alternatively, the substrates may also be separated substrates. Besides this, the setup of the second sensor 9 is similar to the first sensor 1. A second thermistor 10, which is electrically contacted by the contacts ii of the second thermistor 10, is arranged on top of a second interlayer 12. Below the second interlayer 12 a second heating meander layer 14 is provided as a second means for heating. The second heating meander layer 14 is arranged on the second substrate 16. The second heating meander layer 14 also comprises means for contacting (external contacts 15) in order to supply power to the second heating meander layer 14.

[0045] The substrates 8 or 16 may be any suitable substrate, for example a silicon substrate such as a silicon wafer.

[0046] The heating meander layers 6 are 14 are capable of providing heat to each thermistor 2 or 10. The heat is conducted from each of the heating meander layers 6 or 14 through each interlayer 4 or 12 to each thermistor 2 or 10, respectively. This can be identified as the heat conduction path in each sensor.

[0047] The thermistors 2 or 10 are NTC thermistor but, in variations of this embodiment, can be any suitable thermistor material such as a PTC thermistor.

[0048] he first interlayer 4 and the second interlayer 12 comprise silicon oxide as the main material, but variations of this embodiment may comprise any technically suitable material for an interlayer.

[0049] The first interlayer 4 has a higher thermal conductivity than the second interlayer 12. Thereby the heat conduction path between the first heating meander layer 6 to the first thermistor has a higher thermal conductivity than the other heat conduction path between the second heating meander layer 14 to the second thermistor 10. In particular, the second interlayer 12 may comprise polycrystalline silicon oxide material with finer crystallites than the first. Also the second interlayer 12 can comprise more amorphous silicon oxide material than the first interlayer 4. Accordingly, the first interlayer 4 preferably has a higher degree of crystallinity or on average larger crystallite sizes than the second interlayer 12. By these means the thermal conductivity of the first interlayer is higher than that of the second interlayer. For example, the thermal conductivity of the second interlayer 12 may be 1.4 W/(m.Math.K) or smaller, while the first interlayer 4 has a thermal conductivity of above 1.4 W/(m.Math.K). Most preferably the first interlayer has a thermal conductivity of 5 W/(m.Math.K) or above.

[0050] In addition, optionally also the thermistor material of the first thermistor 2 may have a higher thermal conductivity than the thermistor material of the second thermistor 10 by which means the thermal conductivity may be further enhanced.

[0051] In an application it is preferred that the second sensor 9 is used as the measurement sensor, while the first sensor 1 is used as the reference sensor. If the thermal conductivity of the heat conduction path of the second sensor 9 is smaller than that of the first sensor 1, the second sensor 9 is more strongly affected by any change in the detection environment than the first sensor 1.

[0052] FIGS. 2 and 3 show computed graphs for the temperature difference measured on a thermistor at two different atmospheric concentrations of CO.sub.2, namely 9000 ppm and 1000 ppm CO.sub.2, in dependence of the thermal conductivity of an insulation layer. FIG. 2 addresses an elevated temperature of 300 K, which is advantageously applied as the temperature at which the measurement sensor is operated. In FIG. 3 the graph is shown for the temperature of 150° C., which is an advantageous temperature for a reference sensor to be operated.

[0053] In both cases the individual graphs show that at low thermal conductivity of below 5 W/(m.Math.K) the difference in temperature of the thermistor is highest, while at the maximum computed thermal conductivity of 20 W/(m.Math.K) the temperature difference is lowest.

[0054] In a state of the art sensor system, the same thermal conductivity, wold be chosen for both interlayers. If it is assumed that in such a case both interlayers have the same thermal conductivity of 5 W/(m.Math.K), a difference in response between the two sensors operated at 300 and 150° C. of roughly 0.1 K apparent, if FIGS. 2 and 3 are compared.

[0055] However, this difference can be further enhanced if the measurement sensor heated to 300° C. has a low thermal conductivity such as below 5 W/(m.Math.K) or even lower, for example as low as 1.5 W/(m.Math.K), while the reference sensor that is heated to 150° C. has a high thermal conductivity, for example above 5 W/(m.Math.K), 10 W/(m.Math.K), 15 W/(m.Math.K) or 20 W/(m.Math.K). The exact differences in thermal conductivity can be directly obtained by comparing the two curves of FIGS. 2 and 3. However, as can also be seen from the graphs, the slightest difference in thermal conductivity may already produce a gradual enhancement of the difference between measurement sensor and reference sensor. For example, if the measurement sensor (second sensor) is operated at 300° C. and the thermal conductivity of the second interlayer is in the range of 3 W/(m.Math.K), while the reference sensor (first sensor) is operated at 150° C. and the thermal conductivity of the first interlayer is 20 W/(m.Math.K), a difference in detected temperature difference of nearly 0.16 K between both sensors can be obtained. Accordingly, a measurement signal can be enhanced by about 40%, compared to the above comparison.

[0056] In FIG. 4 a similar depiction is shown for the simulation of the thermal conductivity of the thermistor material for a sensor at 150° C. (lower eight dots) against the thermal conductivity at 300° C. (higher eight dots). It is shown that the sensitivity measured in K/ppm of the detected gas (CO.sub.2), decreases for the measurement sensor (operated at 300° C.) with increasing thermal conductivity of the thermistor material. However, at 150° C. the sensitivity remains mainly the same up to around 12.5 W/(m.Math.K), but at 15 W/(m.Math.K) a jump in sensitivity for the reference sensor operated at 150° C. is observed. Accordingly, it is again proven that it is advantageous to have a low thermal conductivity for the second thermistor material but to have a high thermal conductivity for the first thermistor material.

[0057] It is advantageous to combine the results as described for FIGS. 2 and 3 and for FIG. 4.

[0058] In an alternative embodiment which is not depicted separately, but which may have the same layer stacking as described for FIG. 1, both the first interlayer 4 and the second interlayer 12 have comparatively high thermal conductivity of higher than 1.5 W/(m.Math.K), such as 2 W/(m.Math.K) or more preferably 5 W/(m.Math.K) or higher. The inventors found that if the thermal conductivity is high, the response time of the sensor is increased. In particular, a steady state is reached much faster in the case that thermal conductivity is high. Accordingly, the sensor can detect additional change in gas composition much faster.

[0059] If the thermistor is sensitive enough, it is possible that both interlayers have the same high thermal conductivity. However, as described above, the presence of different thermal conductivities is preferred, which allows to partly compensate for a sensitivity loss due to overall increased thermal conductivity for both sensors.

[0060] While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.