High temperature sensor

Details High temperature sensor High temperature sensor

2001-06-29

2003-10-21

Larkin, Daniel S. (Department: 2856)

Measuring and testing

Gas analysis

Detector detail

C073S023200, C073S023310

Reexamination Certificate

active

06634212

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns a high temperature sensor, particularly an exhaust gas sensor in the exhaust line of an automobile.
2. Description of the Related Art
In order to meet the ever more stringent governmental requirements with respect to air quality, very selective gas sensors are necessary. Such sensors can be employed for example for monitoring pollutant levels, or to activate an alarm when a threshold concentration of a dangerous or poisonous gas in the environmental atmosphere has been exceeded. It is also possible to employ such gas sensors directly in the exhaust gas of an internal combustion process. Examples thereof include selective hydrocarbon sensors such as known for example from EP 0 426 989 or selective ammonia sensors as known for example from DE 197 03 796.
The mentioned examples concern gas sensors produced using planar technology (and in particular thick layer technology or thin layer technology). In
FIG. 1
various views of the typical design of such a sensor are schematically illustrated. A substrate
4
has provided on the sensor lower side a structure
6
for heating and eventually temperature measurement, and has provided on the sensor upper side at the sensor tip a capacitor structure. This structure, which is again shown in
FIG. 2
in enlarged representation, is comprised of a plurality of staggered or offset electrodes
8
which are alternatingly connected to conductor line
10
or conductor line
12
. The conductors
10
and
12
have respective contact pads
14
and
16
on the sensor connection side, onto which connector wires are applied. If an alternating current is applied to the two conductors, then the capacitates C
L
of this structure (referred to in the following as empty capacity) can be measured. Since this capacitor structure looks similar to inter-digitating fingers, such a structure is referred to also as interdigitatec capacitor (IDC). If now upon this IDC structure a functional layer
18
—not shown for purposes of better understanding—is applied, of which the electrical characteristic changes upon exposure to a gas, then one can construct therewith a gas sensor. Such a construction is in principle not only suitable for sensors which detect components of a gas mixture, but rather also for all chemical or substance sensors.
The term “substance sensor” is intended herein to mean a sensor for determination of concentrations of a substance in a substance mixture, that is, for example, a sensor for determining the concentration of a component of a gas mixture or a sensor for determining a component of a fluid or a sensor which changes its output signal on the basis of an interaction with a gas or a fluid.
The above described arrangement comprised of substrate, heating and/or temperature measurement resistor device, and IDC structure will in the following be referred to as “U-carrier”. A sensor in this respect is also comprised of at least a transducer and a functional layer.
Estimation of Signal Size
The signal change to be measured depends upon the geometry of the IDC structure. This is shown again in
FIG. 2
in enlarged view. The entire IDC structure has as external dimensions the length L and the breadth B. Across the breadth B electrode fingers of the breadth b are provided in separation s. One can therewith imagine the entire capacitor as a parallel circuit (electrically switched in parallel) comprised of multiple component capacitors, wherein each partial capacitor is comprised of two adjacent fingers. The empty capacity of these partial capacitors, and therewith also the total empty capacity C
L
, increases with the finger length L. With a reduction in the finger separation s the empty capacity of the partial capacitors likewise increases, since the density of the field line or line of electric flux between two fingers increases (in comparison: in plate capacitors the capacity is inversely proportional to plate separation). Since the total capacity is based upon the parallel circuitry of the partial capacities, the total capacity is the larger the greater the number of partial capacitors which can be provided within the breadth B with decreasing finger breadth b, thus the capacity of the total capacitor increases, since the number of the parallel switched partial capacitors increases with decreasing finger breadth b at constant outer dimension B. With decreasing finger spacing s in accordance therewith, the capacity of the total condenser even increases over-proportionally (almost quadratically), since on the one hand the number of the partial condensers and on the other hand their capacity increases.
The height of the electrode (layer thickness) is only of minimal consequence.
In the following, a few theoretical calculations of the total capacity C
L
will be presented, which are carried out using a finite element method. Therein, the measurements of a typical IDC structure, that is, approximately 5 mm×6 mm (L×B), is used as basis. For the relative dielectric constant, ∈
r
was presumed to have a value of ∈
r
≈10 as disclosed in published literature as conventional for Al
2
O
3
substrates. The results of the calculations confirm that the layer thickness of the IDC structures can be disregarded.
It has further been determined, as best seen in
FIG. 3
, that an optimal relationship of line separation s and finger breadth b of s/b≈2 exists, at which the total empty capacity C
L
reaches a maximum. In
FIG. 3
, a finger separation of s=20 &mgr;m was presumed. At a finger breadth of b=9.88 &mgr;m, there is the maximum empty capacity. If one varies the finger separation s, then one can determine that the value of the optimal relationship is almost independent of the separation of the fingers. One achieves for example at s=20 &mgr;m an optimal value for the finger breadth of b=9.88 &mgr;m (s/b=2.024) and at s=10 &mgr;m an optimal finger breadth of b=0.54 &mgr;m (s/b=1.203).
The optimal empty capacity for finger separations ranging from 10 &mgr;m to 30 &mgr;m is shown in FIG.
4
. One can recognize that at a finger separation of approximately 20 &mgr;m, a total empty capacity C
L
of almost 40 pF can be achieved. Table 1 clearly shows the relationship between the geometric size b and s and the total empty capacity C
L
. At structure breadths for s and b of approximately 100 &mgr;m, one achieves only a total empty capacity of C
L
<10 pF.
TABLE 1
Finger
Optimal Finger
Separation
Separation
Total Empty
Maximal Capacity
s/&mgr;m
b/&mgr;m
Capacity C
L
/pF
Change &Dgr;C
max
/pF
10
4.54
82.33
4.12
15
7.22
53.22
2.66
20
9.88
39.28
1.96
25
12.53
31.10
1.56
30
15.15
25.73
1.29
If one next applies the functional layer
18
, then the measurable capacity increases, depending upon the dielectric constant ∈
r
of the functional layer and its thickness. It can however be shown that the influence of the layer thickness of the functional layer in particular at values of the dielectric constant ∈
r
<5 hardly plays any roll. If one presumes that the supplemental capacity, which is attributable to the functional layer, corresponds to the half value of the empty capacity, and if one further presumes that the supplemental capacity during gas sampling changes at a maximal of 10% of its value, then one obtains the maximal capacity change &Dgr;C
max
to be measured, which is entered in the fourth column of Table 1. It is immediately evident from Table 1 that one, in order to even be able to make reliable measurements, must have as small as possible finger breadth b and finger separation s. This is in particular then the case, when long conductors or lead lines, which conventionally exhibit capacities of a few pF/m, are required. This is for example the case, when the sensor is to be employed in the exhaust gas stream of an automobile, in order to be able to measure the ammonia or hydrocarbon content in the exhaust gas of an automobile. Therein, it is to be observed, that even this lead line or conductor cap

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