Indian Patents. 269188:"A METHOD FOR DETERMINING A CONCENTRATION OF A TARGET GAS USING A FIELD EFFECT (FET) -BASED GAS SENSOR HAVING A GAS -SENSITIVE LAYER"

Title of Invention	"A METHOD FOR DETERMINING A CONCENTRATION OF A TARGET GAS USING A FIELD EFFECT (FET) -BASED GAS SENSOR HAVING A GAS -SENSITIVE LAYER"
Abstract	Disclosed is u method for measuring gases in FBT-based gas sensors whose sensor signal is generated by modifying the work function on a sensitive layer, the change in capacity of the sensitive layer being evaluated in addition to reading out the modification of the work function.

Full Text	Specification Method for Measuring Gases and/or Minimizing Cross-Sensitivities in FET-Based Gas Sensors This invention relates to a method for improving the selectivity of FET-based gas sensors, whereby interfering effects of cross-sensitivities are minimized. Gas sensors that utilize and evaluate the change in work function of sensitive materials as a physical parameter have been experiencing increased interest recently. The reasons for this are the ability to operate them with low operating energy (low operating power), the economical production and construction technology of such gas sensors (low production costs), and a broad palette of gases that can be detected with this platform technology (high versatility). Numerous different detection substances can be integrated into such structures. Their construction and mode of operation are disclosed by the patent applications [I - IV]. for example. A number of materials can be used for sensitive layers in such gas sensors. The basic structure of these work function gas sensors is shown in Figure 2. It shows schematically the structure of work functions with FET readout, particularly an SGFET (suspended gate field effect transistor, an FET with suspended gate electrode). In the presence of the gas to be detected, an electrical potential that corresponds to the change in the work function of the sensitive layer (typically 50-100 mV) is developed on the sensitive layer with which the bottom of the raised gate electrode is coated. This acts on the channel of the FET structure and changes the source-drain current. The changed source-drain current is measured directly. Altemataively, the change in source-drain current is restored by applying an additional voltage to the suspended gate or to the transistor trough. The additionally applied voltage represents the readout signal that directly correlates with the change in work function of the sensitive layer. A basic problem with all gas sensors including the type described is their limited selectivity. In other words, the sensors under some circumstances react not only to the target gas but also to other gases, which is called cross-sensitivity. The superimposed gas signals then lead in many applications to a situation in which the target gas concentration cannot be determined with adequate reliability from the sensor signal, since this is distorted unacceptably by the cross-sensitivity. Up to now, it has been necessary to accept the distortion of the sensor signal. - The effect can be partially eliminated by intelligent signal evaluation adapted to the application, but this possibility is very limited for many applications. - Alternatively, an additional sensor can be used that is sensitive specifically to the interfering gas and whose additional signal is used to compensate for the interference effect in an appropriate signal processor, which naturally involves substantially higher system costs. The goal of the invention is to minimize distortion of the sensor signal from cross-sensitivity in FET-based gas sensors. This goal is achieved by the combination of features in Claim 1. Advantageous embodiments can be found in the subclaims. The invention is based on the recognition that the effect of cross-sensitivities can be sharply reduced by using an FET-based gas sensor in which not only the change in work function (change in interface potential), but also the change in the capacitance of the sensitive layer is evaluated by appropriate control. In this way, two physically independent signals are read from the layer, which can represent different gas sensitivities. The mechanisms, the first causing a change in the work function in a reaction with gases, and the second causing a change in the capacitance of the sensitive layer, are widely different. Because of this, the two parameters show different gas sensitivities. In other words, the reactions to the target gas and to the interfering gas are different. Then if the reactions to both gases are known, the effect of the interfering gas on the signal can be compensated for, and with this the concentration of the target gas can be determined. Alternatively, both gas concentrations can also be calculated. In accordance with the invention, there are "two sensors in one," in other words two independent signals have been generated by the manner of operation in one sensor structure. This saves the costs for a second sensor structure. In addition, gas sensors are subject to drift effects in long-term operation. Two separate sensor structures have a stronger tendency under some circumstances toward different drift phenomena than one sensor structure, which makes it difficult to compensate for the errors in the signal processor. Other advantages consist of additional data that can be read out of the system, but with the need for only one sensor structure. The method permits a "two-sensors-in-one" procedure. Exemplary embodiments will be described below with reference to schematic figures that do not limit the invention. Figure 1 shows the structural principle of an FET gas sensor (CCFET type), Figure 2 shows schematically the structure of work function gas sensors with an FET readout, Figures 3 and 4 show gas reactions with mixed readout principle according to the invention on an FET. Sensor construction relevant to the invention: Both classical suspended gate FET gas sensors (SGFETs) with a structure corresponding to Figure 2, and gas FETs in which the capacitance is developed by the gas-sensitive layer and the air gap and the electrical potential is transmitted to a separately attached readout FET through an electrically conductive connection (CCFETs), corresponding to Figure 1, are suitable for this procedural method. The invention is also suitable for all other constructions with similar functionality. Implementation of separate readouts of capacitance and interface potential: In the SGFET structure according to Figure 2, the action of interface potentials on channel conductivity, source-drain current, is read in the described operation. The capacitance of the sensitive layer is read out by applying alternating voltage (typically 10-10,000 Hz) to the gate electrode. The coupling of the gate voltage to the channel conductivity changes as a function of the capacitance of the sensitive layer, in other words the alternating portion of the source-drain current brought about by the alternating voltage on the gate depends on the capacitance of the sensitive layer and is thus a direct measure of this capacitance. If the gate voltage is kept constant for specific application-related reasons, an alternating voltage can alternatively be applied to the transistor itself, i.e. to the bulk connection of the Si or to the transistor trough in case of a corresponding structure. The basic function with this procedural method is the same as described above. With a CCFET structure in which still another electrode, called a capacitance well, is attached below the floating gate, the alternating voltage can also be applied as described above through the top contact face of the sensitive layer, and also through the transistor. With this variant of gas sensor, however, the alternating voltage can beneficially be introduced through the electrode called the capacitance well. This variant avoids both an excessively severe change in potential conditions in the air gap and impairments caused by applying potentials to the transistor. This also applies similarly to the variant of the SGFET known as the FGFET, as shown in Figure 2. The following apply to all the variants: - Alternating voltage can be used either for capacitance readout simultaneously with the readout of the interfacial potential, with both the alternating and constant fractions of the source-drain current then being read, or with alternation between the two operating modes. - It is not absolutely necessary to use an alternating voltage on the gate. - Alternatively, a rapid change in potential can occur. The time curve of the action of this potential change on the source-drain current in this case likewise depends on the capacitance of the sensitive layer and can be utilized in precisely the same way to determine it. - Alternatively, the transistor characteristic, the change of source-drain current with gate voltage, can also be evaluated. Since the resulting transistor slope is also determined by the air gap capacitances, this is directly dependent on the capacitance of the sensitive layer. Use with gas-sensitive materials with different morphologies: A distinction has to be made here between porous, or open-pored materials, and solid, continuous or closed-pored materials. Wijh_p_ojro11 sjnateri ais, there is often a strong cross-effect of varying atmospheric humidity. This results from the deposition of moisture on the grains and causes a sharp change in capacitance of the porous layer. An example of this is BaCOj, which is prepared as an open-pored layer. This is characterized by - major sensitivity of the sensor material to COi, a potential occurring from a change in work function on the outer boundary of the layer, which is independent of layer thickness, - cross-sensitivity to moisture that occurs from changes of capacitance in the pores of the layer, and therefore depends linearly on the layer thickness. In a mixed readout, humidity changes can modify the useful signal to CO? in an unacceptable way. If the capacitance is then read separately according to the invention, it is possible to make a correction of the measurement from the separately obtained moisture signal. Other COa-sensitive materials such as BaTiOs or material variants doped with CuO, or all other porous sensor materials, can beneficially be utilized in a comparable way. With non-porous materials, this mechanism producing essentially moisture effects does not prevail. Here also, however, depending on the type of gas and the detection material, there are different effects of various gases on work function and capacitance. The former ordinarily occur from interracial reactions of the gases, and the latter from reactions of the gas in the interior of the sensor layer. This change in capacitance described above can be caused, for example, by a change in thickness and or modification of the dielectric constant of the sensitive layer. Figures 3 and 4 show gas reactions with the mixed readout principle on an FET according to the invention. References [I] Hybrid flip-chip structure for economical construction of work function gas sensors, German Patent Application No. 19814857, [II] Structure of the field effect transistor made by hybrid flip-chip technology, German Patent Application No. 19956744, [III] Gas detection by the principle of work function measurement, German Patent Application No. 19849932, [IV] Gas sensor and method for producing it, especially master-slave structure, German Patent Application No. 19956806. Claims 1. Method for measuring gases in FET-based gas sensors with suspended gate electrode, characterized in that the change in capacitance of the sensitive layer is evaluated. 2. Method for minimizing cross-sensitivities in FET-based gas sensors whose sensor signals are generated by the change in work function on a sensitive layer, characterized in that the change in capacitance of the sensitive layer is read in addition to reading the changes in work function. 3. Method according to Claim 1 or 2, in which an alternating voltage is applied to the gate electrode of an SGFET or of a CCFET to read the capacitance of the sensitive layer, wherein the source/drain current has an alternating voltage component that depends on the capacitance of the sensitive layer. 4. Method according to Claim 3, in which the alternating voltage is applied through a top contact face of the sensitive layer, an additional electrode, or through the transistor. 5. Method according to Claim 4, in which the time curve of the potential change on the source/drain current is used to determine the capacitance of the sensitive layer. 6. Method according to Claim 5, in which the change of the source/drain current with gate voltage is used to determine the capacitance of the sensitive layer. 7. Method according to one of the foregoing claims, in which cross-sensitivity to moisture is eliminated.

Full Text

Specification
Method for Measuring Gases and/or Minimizing Cross-Sensitivities in FET-Based Gas Sensors
This invention relates to a method for improving the selectivity of FET-based gas sensors, whereby interfering effects of cross-sensitivities are minimized.
Gas sensors that utilize and evaluate the change in work function of sensitive materials as a physical parameter have been experiencing increased interest recently. The reasons for this are the ability to operate them with low operating energy (low operating power), the economical production and construction technology of such gas sensors (low production costs), and a broad palette of gases that can be detected with this platform technology (high versatility). Numerous different detection substances can be integrated into such structures. Their construction and mode of operation are disclosed by the patent applications [I - IV]. for example. A number of materials can be used for sensitive layers in such gas sensors.
The basic structure of these work function gas sensors is shown in Figure 2. It shows schematically the structure of work functions with FET readout, particularly an SGFET (suspended gate field effect transistor, an FET with suspended gate electrode).
In the presence of the gas to be detected, an electrical potential that corresponds to the change in the work function of the sensitive layer (typically 50-100 mV) is developed on the sensitive layer with which the bottom of the raised gate electrode is coated. This acts on the channel of the FET structure and changes the source-drain current. The changed source-drain current is measured directly. Altemataively, the change in source-drain current is restored by applying an additional voltage to the suspended gate or to the transistor trough. The additionally applied voltage represents the readout signal that directly correlates with the change in work function of the sensitive layer.

A basic problem with all gas sensors including the type described is their limited selectivity. In other words, the sensors under some circumstances react not only to the target gas but also to other gases, which is called cross-sensitivity. The superimposed gas signals then lead in many applications to a situation in which the target gas concentration cannot be determined with adequate reliability from the sensor signal, since this is distorted unacceptably by the cross-sensitivity.
Up to now, it has been necessary to accept the distortion of the sensor signal.
- The effect can be partially eliminated by intelligent signal evaluation
adapted to the application, but this possibility is very limited for many applications.
- Alternatively, an additional sensor can be used that is sensitive specifically
to the interfering gas and whose additional signal is used to compensate for the
interference effect in an appropriate signal processor, which naturally involves
substantially higher system costs.
The goal of the invention is to minimize distortion of the sensor signal from cross-sensitivity in FET-based gas sensors.
This goal is achieved by the combination of features in Claim 1. Advantageous embodiments can be found in the subclaims.

The invention is based on the recognition that the effect of cross-sensitivities can be sharply reduced by using an FET-based gas sensor in which not only the change in work function (change in interface potential), but also the change in the capacitance of the sensitive layer is evaluated by appropriate control.
In this way, two physically independent signals are read from the layer, which can represent different gas sensitivities.
The mechanisms, the first causing a change in the work function in a reaction with gases, and the second causing a change in the capacitance of the sensitive layer, are widely different. Because of this, the two parameters show different gas sensitivities. In other words, the reactions to the target gas and to the interfering gas are different. Then if the reactions to both gases are known, the effect of the interfering gas on the signal can be compensated for, and with this the concentration of the target gas can be determined. Alternatively, both gas concentrations can also be calculated.
In accordance with the invention, there are "two sensors in one," in other words two independent signals have been generated by the manner of operation in one sensor structure. This saves the costs for a second sensor structure. In addition, gas sensors are subject to drift effects in long-term operation. Two separate sensor structures have a stronger tendency under some circumstances toward different drift phenomena than one sensor structure, which makes it difficult to compensate for the errors in the signal processor.
Other advantages consist of additional data that can be read out of the system, but with the need for only one sensor structure. The method permits a "two-sensors-in-one" procedure.

Exemplary embodiments will be described below with reference to schematic figures that do not limit the invention.
Figure 1 shows the structural principle of an FET gas sensor (CCFET type),
Figure 2 shows schematically the structure of work function gas sensors with an FET readout,
Figures 3 and 4 show gas reactions with mixed readout principle according to the invention on an FET.
Sensor construction relevant to the invention:
Both classical suspended gate FET gas sensors (SGFETs) with a structure corresponding to Figure 2, and gas FETs in which the capacitance is developed by the gas-sensitive layer and the air gap and the electrical potential is transmitted to a separately attached readout FET through an electrically conductive connection (CCFETs), corresponding to Figure 1, are suitable for this procedural method. The invention is also suitable for all other constructions with similar functionality.
Implementation of separate readouts of capacitance and interface potential:
In the SGFET structure according to Figure 2, the action of interface potentials on channel conductivity, source-drain current, is read in the described operation. The capacitance of the sensitive layer is read out by applying alternating voltage (typically 10-10,000 Hz) to the gate electrode. The coupling of the gate voltage to the channel conductivity changes as a function of the capacitance of the sensitive layer, in other words the alternating portion of the source-drain current brought about by the alternating voltage on the gate depends on the capacitance of the sensitive layer and is thus a direct measure of this capacitance.

If the gate voltage is kept constant for specific application-related reasons, an alternating voltage can alternatively be applied to the transistor itself, i.e. to the bulk connection of the Si or to the transistor trough in case of a corresponding structure. The basic function with this procedural method is the same as described above.
With a CCFET structure in which still another electrode, called a capacitance well, is attached below the floating gate, the alternating voltage can also be applied as described above through the top contact face of the sensitive layer, and also through the transistor. With this variant of gas sensor, however, the alternating voltage can beneficially be introduced through the electrode called the capacitance well. This variant avoids both an excessively severe change in potential conditions in the air gap and impairments caused by applying potentials to the transistor.
This also applies similarly to the variant of the SGFET known as the FGFET, as shown in Figure 2.
The following apply to all the variants:
- Alternating voltage can be used either for capacitance readout
simultaneously with the readout of the interfacial potential, with both the
alternating and constant fractions of the source-drain current then being read, or
with alternation between the two operating modes.
- It is not absolutely necessary to use an alternating voltage on the gate.
- Alternatively, a rapid change in potential can occur. The time curve of the
action of this potential change on the source-drain current in this case likewise
depends on the capacitance of the sensitive layer and can be utilized in precisely the
same way to determine it.

- Alternatively, the transistor characteristic, the change of source-drain
current with gate voltage, can also be evaluated. Since the resulting transistor slope
is also determined by the air gap capacitances, this is directly dependent on the
capacitance of the sensitive layer.
Use with gas-sensitive materials with different morphologies:
A distinction has to be made here between porous, or open-pored materials, and solid, continuous or closed-pored materials.
Wijh_p_ojro11 sjnateri ais,
there is often a strong cross-effect of varying atmospheric humidity. This results from the deposition of moisture on the grains and causes a sharp change in capacitance of the porous layer.
An example of this is BaCOj, which is prepared as an open-pored layer. This is characterized by
- major sensitivity of the sensor material to COi, a potential occurring from a
change in work function on the outer boundary of the layer, which is independent of
layer thickness,
- cross-sensitivity to moisture that occurs from changes of capacitance in the
pores of the layer, and therefore depends linearly on the layer thickness.
In a mixed readout, humidity changes can modify the useful signal to CO? in an unacceptable way. If the capacitance is then read separately according to the invention, it is possible to make a correction of the measurement from the separately obtained moisture signal.

Other COa-sensitive materials such as BaTiOs or material variants doped with CuO, or all other porous sensor materials, can beneficially be utilized in a comparable way.
With non-porous materials,
this mechanism producing essentially moisture effects does not prevail. Here also, however, depending on the type of gas and the detection material, there are different effects of various gases on work function and capacitance. The former ordinarily occur from interracial reactions of the gases, and the latter from reactions of the gas in the interior of the sensor layer.
This change in capacitance described above can be caused, for example, by a change in thickness and or modification of the dielectric constant of the sensitive layer.
Figures 3 and 4 show gas reactions with the mixed readout principle on an FET according to the invention.

References
[I] Hybrid flip-chip structure for economical construction of work function
gas sensors, German Patent Application No. 19814857,
[II] Structure of the field effect transistor made by hybrid flip-chip
technology, German Patent Application No. 19956744,
[III] Gas detection by the principle of work function measurement, German
Patent Application No. 19849932,
[IV] Gas sensor and method for producing it, especially master-slave
structure, German Patent Application No. 19956806.

Claims
1. Method for measuring gases in FET-based gas sensors with suspended
gate electrode,
characterized in that the change in capacitance of the sensitive layer is evaluated.
2. Method for minimizing cross-sensitivities in FET-based gas sensors whose
sensor signals are generated by the change in work function on a sensitive layer,
characterized in that the change in capacitance of the sensitive layer is read in
addition to reading the changes in work function.
3. Method according to Claim 1 or 2, in which an alternating voltage is
applied to the gate electrode of an SGFET or of a CCFET to read the capacitance of
the sensitive layer, wherein the source/drain current has an alternating voltage
component that depends on the capacitance of the sensitive layer.
4. Method according to Claim 3, in which the alternating voltage is applied
through a top contact face of the sensitive layer, an additional electrode, or through
the transistor.
5. Method according to Claim 4, in which the time curve of the potential
change on the source/drain current is used to determine the capacitance of the
sensitive layer.

6. Method according to Claim 5, in which the change of the source/drain current with gate voltage is used to determine the capacitance of the sensitive layer.
7. Method according to one of the foregoing claims, in which cross-sensitivity to moisture is eliminated.

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Patent Number

269188

Indian Patent Application Number

6537/DELNP/2006

PG Journal Number

41/2015

Publication Date

09-Oct-2015

Grant Date

07-Oct-2015

Date of Filing

03-Nov-2006

Name of Patentee

MICRONAS GMBH

Applicant Address

HANS -BUNTE-STRASSE 19,78108 FREIBURG GERMANY

Inventors:

#	Inventor's Name	Inventor's Address
1	PHOLE ROLAND	FICHTENSTRASSE 22,85570 HERDWEG GERMANY
2	FLEISCHER MAXIMILIAN	SCHLOSSANGERWEG 12,D-85635 HOHENKIRCHEN GERMANY
3	LAMPE UWE	BEIM KLOSTER DOHREN 13,D-21614 BUXTEHUDE GERMANY
4	MEIXNER HANS	MAX-PLANCK -STRASSE 5,D-85540 HARR GERMANY
5	SIMON ELFRIEDE	WINTHIR STRASSE 18,D-80639 MIINCHEN GERMANY

PCT International Classification Number

G01N 27/22

PCT International Application Number

PCT/EP2005/004318

PCT International Filing date

2005-04-22

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	10 2004 019604.4	2004-04-22	Germany