Electricity: measuring and testing – Determining nonelectric properties by measuring electric... – Semiconductors for nonelectrical property
Reexamination Certificate
2002-04-05
2004-08-10
Le, N. (Department: 2858)
Electricity: measuring and testing
Determining nonelectric properties by measuring electric...
Semiconductors for nonelectrical property
Reexamination Certificate
active
06774613
ABSTRACT:
The present invention relates to a semiconducting gas sensor in accordance with the preamble of Patent claim
1
, a gas sensor system, and a method of gas analysis using a semiconducting gas sensor.
The present invention relates to a semiconducting gas sensor, and to a method of gas analysis using a semiconducting gas sensor,
In some fields, gas analysis is of great importance. For example, in the combustion of fossil fuels, carbon monoxide and nitrous oxides or NO
x
, are produced, which may then be converted to O
3
. The damage to the environment caused by these substances is considerable. For this reason it is highly imperative that exhaust gases produced by internal combustion engines be analyzed, with an eye to reducing their emission of pollutants.
One possibility for gas analysis is presented by semiconducting gas sensors, in which a gas-sensitive metallic oxide layer, such as SnO
2
, is brought to a specific measuring temperature. By measuring the electrical resistance of the gas-sensitive layer at a specific temperature, the gas concentrations, for example of CO, NO
x
, or O
3
, can be determined.
The article by B. Ruhland, et al., “Gas-Kinetic Interactions of Nitrous Oxides with SnO
2
Surfaces”, Sensors and Actuators B 50 (1998) pp 85-94, discusses a semiconducting gas sensor of this type. In this known gas sensor, a thin layer of SnO
2
is placed on a heating structure. An SiO
2
layer separates a heating element from the gas-sensitive SnO
2
layer. The heating structure with the gas-sensitive layer is arranged on a Si
3
N
4
membrane, which is then placed over a silicon substrate. In the measurement process, the gas that is to be analyzed flows over the sensor element. The bombardment with the gas components to be analyzed can also be accomplished via diffusion.
In the measurement of gases comprising several components, the problem arises that the effects of the individual gas components may become superimposed in the measuring signal. For example, at a measuring temperature of 400° C., a bombardment of the gas-sensitive layer with CO or NO leads to a reduction in the electrical resistance of the gas-sensitive layer, while a bombardment with NO
2
at this temperature results in an increase in electrical resistance. Furthermore, the contact between the gas-sensitive layer and ozone results in increased resistance. For this reason, the individual concentrations in the gas mixture frequently cannot be precisely determined.
One possibility for solving this problem consists in providing an arrangement comprising several sensors having different measuring temperatures. While a considerable degree of NO
2
sensitivity is present even at relatively low temperatures of 150° C. to 250° C., a suitable measuring temperature for CO, for example, lies between 350° C. and 450° C. The arrangement with the whole sensor array, however, is expensive, and thus associated with relatively high costs.
Another approach to solving the problem involves obtaining comparative sets of data for defined individual gases and gas mixtures at various temperatures via experimentation. To this end, the above-mentioned publication provides for a bombardment of several sensor elements with individual gas components at defined concentrations, in order to determine the behavior of electrical resistance, as a function of temperature. With the resistance behavior determined in this manner, it is then possible to analyze a gas mixture comprised, for example, of CO and NO
2
using two sensors, wherein one sensor is operated at 200° C. and one sensor is operated at 400° C. One disadvantage of this process is that it allows only very simple gas mixtures to be analyzed. Furthermore, interactions between the gases are not taken into account.
In addition, the high O
3
sensitivity disrupts the measurement process significantly. In many cases, the ozone sensitivity outweighs all other effects. For example, with ozone concentrations that are higher than 100 ppb the measuring signal can be interpreted only as an ozone signal.
It is thus one object of the present invention to create a semiconducting gas sensor and a gas sensor arrangement that is suitable for analyzing a gas or gas mixture comprising a number of components, such as, for example, ozone and that can be produced simply and cost-effectively. Furthermore, a method of gas analysis is to be provided, which will enable the analysis of a gas or gas mixture comprising a number of components via semiconducting sensors.
This and other objects and advantages are achieved by the semiconducting gas sensor according to the invention, which comprises a gas-sensitive layer, whose electrical conductivity can be altered via contact with a gas, a heating apparatus for heating the layer to a defined measuring temperature, contact electrodes for measuring the electrical resistance or the electrical conductivity of the gas-sensitive layer, and a chamber in which the gas sensitive layer is positioned. The chamber can be sealed from the outside; and the volume of the chamber is small enough that at least one component of the gas or gas mixture is largely exhausted via conversion, within a predetermined measuring interval, for example on the gas-sensitive layer.
In this manner, the disruptive effects of ozone on the measurement process can be eliminated.
With the small chamber volume, individual components of the gas become converted during the measuring process, so that they do not contribute, or contribute only slightly, to the measuring signal. The remaining measuring signal is then no longer superimposed by the effects of the gas components that have already been converted, allowing the concentrations of the remaining components to be more easily determined. With the invention it is possible to determine the concentrations of different gas components in a gas mixture, without requiring a multitude of sensors operating at different temperatures, which require costly evaluation. In addition, the gas analysis can be accomplished within a relatively short period of time, with the chamber volume being dependent upon the type of gas to betanalyzed and the desired duration of the measuring interval.
Advantageously, the semiconducting gas sensor comprises a regulating device that enables the heating of the gas-sensitive layer in stages, thus allowing individual components of the gas mixture to be selectively converted at predetermined measuring temperatures. Preferably, the semiconducting gas sensor is produced using micromechanical technology, for example via Si technology. This enables a simple, cost-effective production, and a standard implementation of the sensor.
A platinum heating resistor, arranged in a meandering pattern, is preferably used as the heating device. The contact electrodes are preferably also made of platinum. This serves to produce increased temperature stability, while preventing mutual interference between the electrodes and the resistance material.
Advantageously, a passivating layer, comprised, for example, of SiO
2
, is positioned between the heater and the gas-sensitive layer and serves as an insulator. Specifically, a silicon substrate may be provided as the supporting material, along with a nitride membrane, which separates the heater from the substrate.
The gas-sensitive layer is preferably comprised of SnO
2
, however it can also be made of other metallic oxides such as WO
3
and titanium oxide, or of organic materials such as phthallocyanine.
The semiconducting gas sensor is preferably designed to be suitable for measuring concentrations of CO, NO
2
, NO, and/or O
3
. The chamber is preferably a microchamber made, for example, of silicon. The chamber volume advantageously measures approx. 10 to 500 &mgr;l, preferably 10 to 100 &mgr;l, and most preferably approx. 40 &mgr;l.
In accordance with a further aspect of the invention, a gas sensor system is provided, which comprises several semiconducting gas sensors as specified in the invention, along with an arrangement of regulated valves and lines for the inlet and outlet of gas. In this manner, it i
Becker Thomas
Muehlberger Stephan
Mueller Gerhard
Crowell & Moring LLP
EADS Deutschland GmbH
He Amy
Le N.
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