CA2572485A1 - Sensor for detecting and/or measuring concentration of electric charges contained in an atmosphere, corresponding uses and method for making same - Google Patents
Sensor for detecting and/or measuring concentration of electric charges contained in an atmosphere, corresponding uses and method for making same Download PDFInfo
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- G01N27/414—Ion-sensitive or chemical field-effect transistors, i.e. ISFETS or CHEMFETS
- G01N27/4141—Ion-sensitive or chemical field-effect transistors, i.e. ISFETS or CHEMFETS specially adapted for gases
- G01N27/4143—Air gap between gate and channel, i.e. suspended gate [SG] FETs
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Abstract
L'invention concerne un capteur pour la détection et/ou la mesure d'une concentration de charges électriques contenues dans une ambiance. Le capteur comprend une structure de transistor à effet de champ comprenant un pont (4) qui forme une grille et est suspendu au-dessus d'une couche active (10) située entre des zones de drain (6) et de source (7). Une tension de grille possédant une valeur déterminée est appliquée au pont. Une zone, dite air-gap (9), est comprise entre le pont (4) et la couche active (10) ou une couche isolante (8) déposée sur ladite couche active, et possède une hauteur déterminée. Un champ électrique E, défini comme le rapport entre la tension de grille et la hauteur de l'air-gap, est crée dans l'air-gap. Selon l'invention, le champ électrique créé dans l'air-gap possède une valeur supérieure ou égale à une valeur seuil déterminée (50 000 V/cm, 100 000 V/cm, ou encore préférentiellement 200 000 V/cm), suffisamment importante pour que le champ électrique E influence la distribution de charges électriques contenues dans l'ambiance et présentes dans l'air-gap, et permette l'obtention d'une sensibilité élevée du capteur par une accumulation de charges électriques sur la couche active.
Description
4. , SENSOR FOR DETECTING AND/OR MEASURING A CONCENTRATION
OF ELECTRICAL CHARGES CONTAINED IN AN ENVIRONMENT, CORRESPONDING USES AND METHOD OF MANUFACTURE THEREOF
Field of the invention The field of the invention is that of chemical and biological sensors capable of being used in gaseous or liquid environments.
More precisely, the invention relates to a highly sensitive sensor for detecting and/or measuring a concentration of electrical charges present in a gaseous or liquid environment.
The sensor of the invention belongs to the category of sensors comprising a field-effect transistor structure including a bridge that forms the gate and is suspended above an active layer situated between drain and source regions.
The invention has numerous applications, e.g., such as sensors sensitive to NH3, NO2, humidity, or smoke, in gaseous environments, or else sensors sensitive to the pH of solutions, in liquid environments.
.. . . .
OF ELECTRICAL CHARGES CONTAINED IN AN ENVIRONMENT, CORRESPONDING USES AND METHOD OF MANUFACTURE THEREOF
Field of the invention The field of the invention is that of chemical and biological sensors capable of being used in gaseous or liquid environments.
More precisely, the invention relates to a highly sensitive sensor for detecting and/or measuring a concentration of electrical charges present in a gaseous or liquid environment.
The sensor of the invention belongs to the category of sensors comprising a field-effect transistor structure including a bridge that forms the gate and is suspended above an active layer situated between drain and source regions.
The invention has numerous applications, e.g., such as sensors sensitive to NH3, NO2, humidity, or smoke, in gaseous environments, or else sensors sensitive to the pH of solutions, in liquid environments.
.. . . .
2 More generally, it can be applied in any gas or liquid environment containing electrical charges.
However, it is important to note that this invention does not apply to electrically neutral environments.
Prior art The history of the chemically sensitive field-effect transistor (or FET) began 30 years ago. It includes gas-sensitive structures in gaseous environments, as well as ion-sensitive structures in liquid environments.
Conventionally, a gas-sensitive field-effect transistor (FET structure) is produced by using:
- either a permeable gate, made of palladium or polymers, which is placed against the active layer situated between the drain and source regions, whereby the gas reaches the active layer by passing through openings passing through the permeable gate;
- or a suspended gate (also called a "suspended bridge"), which allows the presence of gas in the so-called "air-gap" region contained between the gate and the active layer situated between the drain and source regions, or between the gate and an insulating layer deposited on the active layer.
The suspended gate FET structure was described by J. Janata in the U.S. Patent Nos. 4 411 741 (1983) and 4 514 263 (1985). This structure uses a conventional P-type single-crystal silicon FET transistor with a suspended and perforated gate forming a bridge. The sensitive parameter is the work function of the bridge, which varies in relation to the adsorption of dipoles
However, it is important to note that this invention does not apply to electrically neutral environments.
Prior art The history of the chemically sensitive field-effect transistor (or FET) began 30 years ago. It includes gas-sensitive structures in gaseous environments, as well as ion-sensitive structures in liquid environments.
Conventionally, a gas-sensitive field-effect transistor (FET structure) is produced by using:
- either a permeable gate, made of palladium or polymers, which is placed against the active layer situated between the drain and source regions, whereby the gas reaches the active layer by passing through openings passing through the permeable gate;
- or a suspended gate (also called a "suspended bridge"), which allows the presence of gas in the so-called "air-gap" region contained between the gate and the active layer situated between the drain and source regions, or between the gate and an insulating layer deposited on the active layer.
The suspended gate FET structure was described by J. Janata in the U.S. Patent Nos. 4 411 741 (1983) and 4 514 263 (1985). This structure uses a conventional P-type single-crystal silicon FET transistor with a suspended and perforated gate forming a bridge. The sensitive parameter is the work function of the bridge, which varies in relation to the adsorption of dipoles
3 contained in the fluid, likewise requiring a variation of the flat-band voltage of the structure.
Another patent [S.C. Pyke, U.S. Patent No.
Another patent [S.C. Pyke, U.S. Patent No.
4 671 852 (1987)] discloses a method for forming a suspended gate, chemically sensitive FET based on the removal of a sacrificial layer. As for the preceding patent, a metal gate is used for the suspended bridge.
B. Flietner, T. Doll, J. Lechner, M. Leu and I.
Eisele (Sensors and Actuators B, 18-19 (1994) pp. 632-636) proposed a hybrid suspended gate FET
(HSGFET)making it possible to easily deposit sensitive layers between the gate and the channel of the transistor (i.e., between the gate and the active layer of the transistor) . In this method, the gate is formed separately and is then fastened onto the previously formed gateless FET.
Subsequent to these patents, a significant number of publications and patents were produced for the purpose of optimising the gas-sensitive suspended gate FET structure. These works mainly dealt with optimisation of the materials used to produce the sensitive layer on which the adsorption phenomenon occurs.
The gas sensitivity of these known FET structures is explained by the variation of the work function of the sensitive layer under exposure to gases, which produces a shift of the threshold voltage. In other words, the sensitive parameter is the work function, which varies in relation to the adsorption, by the sensitive layer, of molecules (e.g., dipoles) contained in the (so-called air-gap) region contained between the bridge and the active layer (and more precisely, in this case where there is adsorption, between the bridge and the sensitive layer).
It is recalled that, conventionally, in order to obtain an indication of the quantity of desired molecules present in the air gap, the current between the drain and source regions is measured (the current Ins that passes into the active layer) and it is determined how the measured current varies. Using the current-day technique described above, which is based on the adsorption phenomenon, the variation of the measured current results from the adsorption of molecules by the sensitive layer. For example, as explained in the aforesaid U.S. Patent No. 4 514 263, in the case where a positive charge is present on the bridge, the larger the quantity of dipoles adsorbed by a sensitive layer deposited on the active layer, the stronger the current IDS. As a matter of fact, in this case, the adsorbed dipoles align themselves, the positive end of each of the adsorbed dipoles being oriented towards the active layer, which leads to an increase in the number of electrons attracted and thus to an increase in the current IDS that passes into the active layer.
The ion-sensitive structure in a liquid environment (or solution) is called Bergveld's Ion-Sensitive FET (ISFET). This is a gateless structure comprising, on the one hand, a sensitive layer, which covers the channel insulator, and, on the other hand, a reference electrode dipped into the solution and fixing the gate bias.
Although the first publication about this structure is by P. Bergveld ["Development of an ion-sensitive solid-state device for neuro-physiological measurements", IEEE Trans. Biomed. Eng. 17 (1970) pp.
B. Flietner, T. Doll, J. Lechner, M. Leu and I.
Eisele (Sensors and Actuators B, 18-19 (1994) pp. 632-636) proposed a hybrid suspended gate FET
(HSGFET)making it possible to easily deposit sensitive layers between the gate and the channel of the transistor (i.e., between the gate and the active layer of the transistor) . In this method, the gate is formed separately and is then fastened onto the previously formed gateless FET.
Subsequent to these patents, a significant number of publications and patents were produced for the purpose of optimising the gas-sensitive suspended gate FET structure. These works mainly dealt with optimisation of the materials used to produce the sensitive layer on which the adsorption phenomenon occurs.
The gas sensitivity of these known FET structures is explained by the variation of the work function of the sensitive layer under exposure to gases, which produces a shift of the threshold voltage. In other words, the sensitive parameter is the work function, which varies in relation to the adsorption, by the sensitive layer, of molecules (e.g., dipoles) contained in the (so-called air-gap) region contained between the bridge and the active layer (and more precisely, in this case where there is adsorption, between the bridge and the sensitive layer).
It is recalled that, conventionally, in order to obtain an indication of the quantity of desired molecules present in the air gap, the current between the drain and source regions is measured (the current Ins that passes into the active layer) and it is determined how the measured current varies. Using the current-day technique described above, which is based on the adsorption phenomenon, the variation of the measured current results from the adsorption of molecules by the sensitive layer. For example, as explained in the aforesaid U.S. Patent No. 4 514 263, in the case where a positive charge is present on the bridge, the larger the quantity of dipoles adsorbed by a sensitive layer deposited on the active layer, the stronger the current IDS. As a matter of fact, in this case, the adsorbed dipoles align themselves, the positive end of each of the adsorbed dipoles being oriented towards the active layer, which leads to an increase in the number of electrons attracted and thus to an increase in the current IDS that passes into the active layer.
The ion-sensitive structure in a liquid environment (or solution) is called Bergveld's Ion-Sensitive FET (ISFET). This is a gateless structure comprising, on the one hand, a sensitive layer, which covers the channel insulator, and, on the other hand, a reference electrode dipped into the solution and fixing the gate bias.
Although the first publication about this structure is by P. Bergveld ["Development of an ion-sensitive solid-state device for neuro-physiological measurements", IEEE Trans. Biomed. Eng. 17 (1970) pp.
5 70-71], the first patent belongs to C.C. Johnson, S.D.
Moss, J.A. Janata, "Selective chemical sensitive FET
transducers, U.S. Patent 4 020 830 (1977).
Since this patent, more than 500 publications and 150 patents have been devoted to the ISFETs. The primary subjects addressed relate to improving the sensitivity and selectivity of the sensitive layer on which the adsorption phenomenon occurs, (U.S. Patents 5 319 226 / 5 350 701 / 5 387 328) , the study of drift as well as the effect of temperature and the use of a reference FET structure (J.M. Chovelon, Sensors and Actuators B8 (1992) pp. 221-225).
As for the gas-sensitive FET structures, the sensitivity of the ISFETs is explained by the variation in the threshold voltage induced by the variation in the flat-band voltage VFB. In other words, only the effect of the adsorption phenomenon is used in known ISFETs.
VFB is expressed by: VFB = Vref-'Fo-xBOi- (Ds q where Vref is the contribution of the reference electrode, xs 1 the surface dipole potential of the solution, 'Po the surface potential at the interface between the insulator and the solution, (D$ the semiconductor work function.
Moss, J.A. Janata, "Selective chemical sensitive FET
transducers, U.S. Patent 4 020 830 (1977).
Since this patent, more than 500 publications and 150 patents have been devoted to the ISFETs. The primary subjects addressed relate to improving the sensitivity and selectivity of the sensitive layer on which the adsorption phenomenon occurs, (U.S. Patents 5 319 226 / 5 350 701 / 5 387 328) , the study of drift as well as the effect of temperature and the use of a reference FET structure (J.M. Chovelon, Sensors and Actuators B8 (1992) pp. 221-225).
As for the gas-sensitive FET structures, the sensitivity of the ISFETs is explained by the variation in the threshold voltage induced by the variation in the flat-band voltage VFB. In other words, only the effect of the adsorption phenomenon is used in known ISFETs.
VFB is expressed by: VFB = Vref-'Fo-xBOi- (Ds q where Vref is the contribution of the reference electrode, xs 1 the surface dipole potential of the solution, 'Po the surface potential at the interface between the insulator and the solution, (D$ the semiconductor work function.
6 Only 'Fo is sensitive to the pH value. The relationship ' o - pH is given by (R.E.G. van Hal, J.C.T.
Eijkel, P. Berveld, "A general model to describe the electrostatic potential at electrolyte/oxide interfaces", Adv. Colloid. Interface Sci. 69 (1966) pp.
31-62) :
a'Y _ -2.3 kT a aPHeuIk q where a is a dimensionless parameter, ranging between 0 and 1. When a is equal to 1, the maximum sensitivity of 59 mV/pH is reached, also called the Nernstian sensitivity.
No previous patent or published work has reported higher sensitivity without using an amplifying circuit.
One disadvantage of known sensors comprising a field-effect transistor structure is that they have limited sensitivity. Typically, this sensitivity is limited to 59 mV/pH, in the case of a liquid environment.
Objectives of the invention In particular, the objective of the invention is to mitigate these various disadvantages of the prior art.
More precisely, one of the objectives of this invention, in at least one embodiment, is to provide a sensor comprising a field-effect transistor and having a higher degree of sensitivity that that of known sensors.
Eijkel, P. Berveld, "A general model to describe the electrostatic potential at electrolyte/oxide interfaces", Adv. Colloid. Interface Sci. 69 (1966) pp.
31-62) :
a'Y _ -2.3 kT a aPHeuIk q where a is a dimensionless parameter, ranging between 0 and 1. When a is equal to 1, the maximum sensitivity of 59 mV/pH is reached, also called the Nernstian sensitivity.
No previous patent or published work has reported higher sensitivity without using an amplifying circuit.
One disadvantage of known sensors comprising a field-effect transistor structure is that they have limited sensitivity. Typically, this sensitivity is limited to 59 mV/pH, in the case of a liquid environment.
Objectives of the invention In particular, the objective of the invention is to mitigate these various disadvantages of the prior art.
More precisely, one of the objectives of this invention, in at least one embodiment, is to provide a sensor comprising a field-effect transistor and having a higher degree of sensitivity that that of known sensors.
7 The invention also as the objective, in at least one embodiment, of providing a sensor such as this, which is capable of being used in a gaseous environment.
Another objective of the invention, in at least one embodiment, is to provide a sensor such as this, which is capable of being used in a liquid environment.
An additional objective of the invention, in at least one embodiment, is to provide a sensor such as this, which is simple to manufacture and inexpensive.
Yet another objective of the invention, in at least one embodiment, is to provide a sensor such as this, which makes it possible to lift the constraint in the choice of material used for the sensitive layer (on which the adsorption phenomenon occurs).
Essential characteristics of the invention These various objectives, as well as others that will become apparent subsequently, are attained according to the invention with the aid of a sensor for detecting and/or measuring a concentration of electrical charges contained in an environment, said sensor comprising a field-effect transistor structure including a bridge which forms a gate and is suspended above an active layer situated between the drain and source regions. A gate voltage having a specific value is applied to the bridge. A so-called air-gap region is included between the bridge and the active layer or an insulating layer deposited on said active layer, and has a specific height. An electric field E, defined as the ratio between the gate voltage and the air gap height, is generated in the air gap. According to the
Another objective of the invention, in at least one embodiment, is to provide a sensor such as this, which is capable of being used in a liquid environment.
An additional objective of the invention, in at least one embodiment, is to provide a sensor such as this, which is simple to manufacture and inexpensive.
Yet another objective of the invention, in at least one embodiment, is to provide a sensor such as this, which makes it possible to lift the constraint in the choice of material used for the sensitive layer (on which the adsorption phenomenon occurs).
Essential characteristics of the invention These various objectives, as well as others that will become apparent subsequently, are attained according to the invention with the aid of a sensor for detecting and/or measuring a concentration of electrical charges contained in an environment, said sensor comprising a field-effect transistor structure including a bridge which forms a gate and is suspended above an active layer situated between the drain and source regions. A gate voltage having a specific value is applied to the bridge. A so-called air-gap region is included between the bridge and the active layer or an insulating layer deposited on said active layer, and has a specific height. An electric field E, defined as the ratio between the gate voltage and the air gap height, is generated in the air gap. According to the
8 invention, the electric field E generated in the air gap has a value greater than or equal to a specific threshold value, which is sufficiently large for the electric field E to influence the distribution of electrical charges contained in the environment and present in the air gap, and to enable high sensor sensitivity to be obtained by an accumulation of electrical charges on the active layer.
The basic principle of the invention, whether the latter is applied in a gaseous or liquid environment, consists in creating a strong electric field in the air gap, making it possible to push the electrical charges towards the active layer as well as to improve the sensitivity of the sensor. Therefore, this invention does not apply to electrically neutral environments in which there are no electrical charges on which the electric field created in the air gap is able to act.
It is important to note that this invention rests on the effect produced by a new distribution of the charges in the air gap owing to the application of a strong electric field, and not on the adsorption phenomenon. In known sensors based on the adsorption phenomenon, the effect on which this invention rests does not exist because the electric field applied in the air gap is much too weak. As a matter of fact, the inventors take the position that the effect on which this invention rests exists only if the electric field applied in the air gap is a strong field, greater than or equal to 50,000 V/cm. Such being the case, the electric field applied in the air gap in known sensors is a weak field, generally much lower than 1,000 V/cm.
The basic principle of the invention, whether the latter is applied in a gaseous or liquid environment, consists in creating a strong electric field in the air gap, making it possible to push the electrical charges towards the active layer as well as to improve the sensitivity of the sensor. Therefore, this invention does not apply to electrically neutral environments in which there are no electrical charges on which the electric field created in the air gap is able to act.
It is important to note that this invention rests on the effect produced by a new distribution of the charges in the air gap owing to the application of a strong electric field, and not on the adsorption phenomenon. In known sensors based on the adsorption phenomenon, the effect on which this invention rests does not exist because the electric field applied in the air gap is much too weak. As a matter of fact, the inventors take the position that the effect on which this invention rests exists only if the electric field applied in the air gap is a strong field, greater than or equal to 50,000 V/cm. Such being the case, the electric field applied in the air gap in known sensors is a weak field, generally much lower than 1,000 V/cm.
9 It shall also be noted that the presumptions of those skilled in the art have always led to the belief that it was not necessary to increase the value of the electric field created in the air gap too much, so as to not saturate the adsorption by the surfaces of the air gap.
Two implementations of the sensor of the invention are thus possible:
- in a first implementation, the sensor uses only the effect characteristic of the invention (new distribution of the charges in the air gap owing to the application of a strong electric field), and thus does not use the adsorption phenomenon. In this case, no sensitive layer is necessary and the invention thus makes it possible to lift the constraint in the choice of the material used for the sensitive layer (on which the adsorption phenomenon occurs); and - in a second implementation, the sensor combines the effect characteristic of the invention (new distribution of the charges in the air gap owing to the application of a strong electric field) and the adsorption effect. In this case, a sensitive layer is necessary for adsorption.
The invention relates to any geometry wherein the field effect, due the voltage applied on the suspended bridge, is high enough to influence the distribution of electrical charges present in the environment. It is recalled that the modulation of the current between the drain and source regions is primarily due to the variation in the distribution of the charges present in the air gap, between the bridge and the active layer (or between the bridge and an insulating layer deposited on the active layer).
Preferentially, the electric field created in the air gap has a value greater than or equal to 5 100,000 V/cm.
Even more preferentially, the electric field created in the air gap has a value greater than or equal to 200,000 V/cm.
Advantageously, the height of the air gap is less
Two implementations of the sensor of the invention are thus possible:
- in a first implementation, the sensor uses only the effect characteristic of the invention (new distribution of the charges in the air gap owing to the application of a strong electric field), and thus does not use the adsorption phenomenon. In this case, no sensitive layer is necessary and the invention thus makes it possible to lift the constraint in the choice of the material used for the sensitive layer (on which the adsorption phenomenon occurs); and - in a second implementation, the sensor combines the effect characteristic of the invention (new distribution of the charges in the air gap owing to the application of a strong electric field) and the adsorption effect. In this case, a sensitive layer is necessary for adsorption.
The invention relates to any geometry wherein the field effect, due the voltage applied on the suspended bridge, is high enough to influence the distribution of electrical charges present in the environment. It is recalled that the modulation of the current between the drain and source regions is primarily due to the variation in the distribution of the charges present in the air gap, between the bridge and the active layer (or between the bridge and an insulating layer deposited on the active layer).
Preferentially, the electric field created in the air gap has a value greater than or equal to 5 100,000 V/cm.
Even more preferentially, the electric field created in the air gap has a value greater than or equal to 200,000 V/cm.
Advantageously, the height of the air gap is less
10 than 1 pm.
Preferentially, the height of the air gap is less than 0.5 pm.
It is understood that, by reducing the height of the air gap, it is possible to apply a stronger electric field without increasing the gate voltage VGS
applied to the bridge, or else to apply the same electric field with a weaker gate voltage VGS.
In one particular embodiment of the invention, at least one portion of the structure, including the drain and source regions, the suspended bridge and the active layer, is covered with an insulating material, so that the sensor can be dipped into a liquid environment.
In this embodiment specific to a liquid environment, the sensor according to the invention differs from the known ISFET structure (see above discussion) in that the gate (suspended bridge) serves as the reference electrode and in that the height of the air gap and the gate voltage applied to the bridge are appropriately selected so that a strong electric filed exists in this air gap, thereby making it
Preferentially, the height of the air gap is less than 0.5 pm.
It is understood that, by reducing the height of the air gap, it is possible to apply a stronger electric field without increasing the gate voltage VGS
applied to the bridge, or else to apply the same electric field with a weaker gate voltage VGS.
In one particular embodiment of the invention, at least one portion of the structure, including the drain and source regions, the suspended bridge and the active layer, is covered with an insulating material, so that the sensor can be dipped into a liquid environment.
In this embodiment specific to a liquid environment, the sensor according to the invention differs from the known ISFET structure (see above discussion) in that the gate (suspended bridge) serves as the reference electrode and in that the height of the air gap and the gate voltage applied to the bridge are appropriately selected so that a strong electric filed exists in this air gap, thereby making it
11 possible to push the electrical charges towards the active layer.
The invention also relates to a use of the aforesaid sensor (according to the invention) for detecting and/or measuring a concentration of electrical charges contained in an environment.
The environment containing electrical charges advantageously belongs to the group including gaseous environments and liquid environments.
In a first advantageous use of the sensor according to the invention, the electrical charges are NH3 molecules contained in a gaseous environment.
In a second advantageous use of the sensor according to the invention, the electrical charges are NO2 molecules contained in a gaseous environment.
It shall be noted that the NH3 and NO2 molecules are dipolar molecules and, on these grounds, can be qualified as electrical charges, within the meaning of this invention. As a matter of fact, the electric field created in the air gap influences the movement of the dipolar molecules present in this air gap (even if these dipolar molecules are electrically neutral overall).
In a third advantageous use of the sensor according to the invention, the electrical charges are H+ ions contained in a liquid environment.
In a fourth advantageous use, the sensor according to the invention is used for detecting and/or measuring the humidity ratio in a gaseous environment, by detecting and/or measuring a concentration of OH- ions contained in said gaseous environment.
The invention also relates to a use of the aforesaid sensor (according to the invention) for detecting and/or measuring a concentration of electrical charges contained in an environment.
The environment containing electrical charges advantageously belongs to the group including gaseous environments and liquid environments.
In a first advantageous use of the sensor according to the invention, the electrical charges are NH3 molecules contained in a gaseous environment.
In a second advantageous use of the sensor according to the invention, the electrical charges are NO2 molecules contained in a gaseous environment.
It shall be noted that the NH3 and NO2 molecules are dipolar molecules and, on these grounds, can be qualified as electrical charges, within the meaning of this invention. As a matter of fact, the electric field created in the air gap influences the movement of the dipolar molecules present in this air gap (even if these dipolar molecules are electrically neutral overall).
In a third advantageous use of the sensor according to the invention, the electrical charges are H+ ions contained in a liquid environment.
In a fourth advantageous use, the sensor according to the invention is used for detecting and/or measuring the humidity ratio in a gaseous environment, by detecting and/or measuring a concentration of OH- ions contained in said gaseous environment.
12 In a fifth advantageous use, the sensor according to the invention is used for detecting and/or measuring a concentration of smoke in a gaseous environment, by detecting and/or measuring electrical charges contained in said smoke and contained in said gaseous environment.
In a sixth advantageous use, the sensor according to the invention is used for measuring air quality, by measuring the quantity of negative electrical charges contained in the air.
In a seventh advantageous use, the sensor according to the invention is used for detecting and/or measuring a void fraction in a gaseous environment, by detecting and/or measuring electrical charges that have not been eliminated from said gaseous environment.
As a matter of fact, when the void is established, the air, and thus the charges contained in the environment, are eliminated.
In an eighth advantageous use, the sensor according to the invention is used for measuring the pH
of a liquid environment, by measuring a concentration of H+ ions contained in said liquid environment.
The pH sensitivity depends on the field effect via the thickness of the air gap. It decreases when the thickness of the air gap increases.
In a ninth advantageous use, the sensor according to the invention is used for detecting electrically charged biological entities contained in said environment.
The term biological entities is understood to mean, in particular but not exclusively, DNA cells or branches.
In a sixth advantageous use, the sensor according to the invention is used for measuring air quality, by measuring the quantity of negative electrical charges contained in the air.
In a seventh advantageous use, the sensor according to the invention is used for detecting and/or measuring a void fraction in a gaseous environment, by detecting and/or measuring electrical charges that have not been eliminated from said gaseous environment.
As a matter of fact, when the void is established, the air, and thus the charges contained in the environment, are eliminated.
In an eighth advantageous use, the sensor according to the invention is used for measuring the pH
of a liquid environment, by measuring a concentration of H+ ions contained in said liquid environment.
The pH sensitivity depends on the field effect via the thickness of the air gap. It decreases when the thickness of the air gap increases.
In a ninth advantageous use, the sensor according to the invention is used for detecting electrically charged biological entities contained in said environment.
The term biological entities is understood to mean, in particular but not exclusively, DNA cells or branches.
13 It is clear that numerous other applications can be anticipated without exceeding the scope of the invention.
The invention also relates to a method for manufacturing a sensor such as the aforesaid one (according to the invention). In this method, the suspended bridge, field-effect transistor structure is produced using a surface micro-technology technique.
The advantage in using the surface micro-technology technique is that it makes it possible to easily obtain an air gap having a small height, as recommended by this invention (a height advantageously less than or equal to 0.5 pm, and preferentially less than or equal to 1}am) .
List of the figures Other characteristics and advantages of the invention will become apparent upon reading the following description of a preferred embodiment of the invention, given for non-limiting and illustrative purposes, and from the appended drawings in which:
- figures la and lb each show a schematic view, as a sectional view and perspective view, respectively, of a first particular embodiment of a sensor according to the invention, suitable for use in a gaseous environment;
- figure lc is an electron-microscopic view of a sensor according to the invention, of the type shown schematically in figures la and lb;
- figure id is a zoomed-in view of a portion of figure ic, showing the air gap in particular;
The invention also relates to a method for manufacturing a sensor such as the aforesaid one (according to the invention). In this method, the suspended bridge, field-effect transistor structure is produced using a surface micro-technology technique.
The advantage in using the surface micro-technology technique is that it makes it possible to easily obtain an air gap having a small height, as recommended by this invention (a height advantageously less than or equal to 0.5 pm, and preferentially less than or equal to 1}am) .
List of the figures Other characteristics and advantages of the invention will become apparent upon reading the following description of a preferred embodiment of the invention, given for non-limiting and illustrative purposes, and from the appended drawings in which:
- figures la and lb each show a schematic view, as a sectional view and perspective view, respectively, of a first particular embodiment of a sensor according to the invention, suitable for use in a gaseous environment;
- figure lc is an electron-microscopic view of a sensor according to the invention, of the type shown schematically in figures la and lb;
- figure id is a zoomed-in view of a portion of figure ic, showing the air gap in particular;
14 - figure 2a shows two transfer characteristics (drain-source current IDS - gate voltage VGS) of the same particular embodiment of a sensor according to the invention, one being obtained when the sensor is placed in dry air, the other after 100 ppm of NH3 have been introduced into the environment;
- figure 2b shows two transfer characteristics (drain-source current IDS - gate voltage VGs) of the same particular embodiment of a sensor according to the invention, placed in air having a relative humidity ratio of 10%, obtained before and after the introduction of 2 ppm of NO2, respectively;
- figure 2c shows a plurality of transfer characteristics (drain-source current IDS - gate voltage VGS) of the same particular embodiment of a sensor according to the invention, obtained at various successive moments after the introduction of smoke into the environment;
- figure 2d completes figure 2c by showing a variation curve for the threshold voltage in relation to the time elapsed since introduction of the smoke;
- figure 2e shows a linear plotting (and not in a logarithmic scale as in the other figures) of a plurality of transfer characteristics (drain-source current IDS - gate voltage VGs) of the same particular embodiment of a sensor according to the invention, obtained at various successive moments after the introduction of smoke into the environment;
- figure 2f shows a plurality of transfer characteristics (drain-source current IDS - gate voltage VGs) of the same particular embodiment of a sensor according to the invention, obtained for various relative degrees of humidity in the environment;
- figure 2g completes figure 2f by showing a variation curve for the threshold voltage in relation 5 to the humidity ratio;
- figure 2h shows a plurality of transfer characteristics (drain-source current IDS - gate voltage VGS) for the same particular embodiment of a sensor according to the invention, obtained at 10% and 10 20% relative humidity and before and after the introduction of smoke into the environment;
- figure 3 shows a cross-sectional schematic view of a second particular embodiment of a sensor according to the invention, suitable for use in a liquid
- figure 2b shows two transfer characteristics (drain-source current IDS - gate voltage VGs) of the same particular embodiment of a sensor according to the invention, placed in air having a relative humidity ratio of 10%, obtained before and after the introduction of 2 ppm of NO2, respectively;
- figure 2c shows a plurality of transfer characteristics (drain-source current IDS - gate voltage VGS) of the same particular embodiment of a sensor according to the invention, obtained at various successive moments after the introduction of smoke into the environment;
- figure 2d completes figure 2c by showing a variation curve for the threshold voltage in relation to the time elapsed since introduction of the smoke;
- figure 2e shows a linear plotting (and not in a logarithmic scale as in the other figures) of a plurality of transfer characteristics (drain-source current IDS - gate voltage VGs) of the same particular embodiment of a sensor according to the invention, obtained at various successive moments after the introduction of smoke into the environment;
- figure 2f shows a plurality of transfer characteristics (drain-source current IDS - gate voltage VGs) of the same particular embodiment of a sensor according to the invention, obtained for various relative degrees of humidity in the environment;
- figure 2g completes figure 2f by showing a variation curve for the threshold voltage in relation 5 to the humidity ratio;
- figure 2h shows a plurality of transfer characteristics (drain-source current IDS - gate voltage VGS) for the same particular embodiment of a sensor according to the invention, obtained at 10% and 10 20% relative humidity and before and after the introduction of smoke into the environment;
- figure 3 shows a cross-sectional schematic view of a second particular embodiment of a sensor according to the invention, suitable for use in a liquid
15 environment;
- figure 4a shows a variation curve for the gate voltage in relation to the pH, for a drain-source current of 100 pA and for an air gap thickness of 0.5 pm; and - figure 4b shows a variation curve for the gate voltage in relation to the pH, for a drain-source current of 400 pA and for an air gap thickness of 0.8 pm; and - figure 5 shows a plurality of transfer characteristics (drain-source current IDS - gate voltage VGS) for the same particular embodiment of a sensor according to the invention, obtained after the sensor was dipped into various liquid environments:
deionised water, KOH solution, KC1 solution and NAC1 solution.
- figure 4a shows a variation curve for the gate voltage in relation to the pH, for a drain-source current of 100 pA and for an air gap thickness of 0.5 pm; and - figure 4b shows a variation curve for the gate voltage in relation to the pH, for a drain-source current of 400 pA and for an air gap thickness of 0.8 pm; and - figure 5 shows a plurality of transfer characteristics (drain-source current IDS - gate voltage VGS) for the same particular embodiment of a sensor according to the invention, obtained after the sensor was dipped into various liquid environments:
deionised water, KOH solution, KC1 solution and NAC1 solution.
16 Description of an embodiment of the invention This invention thus relates to a highly sensitive sensor for detecting and measuring the concentration of electrical charges contained in an environment. The sensitivity amplification effect is due to a field effect introduced via a bridge suspended above (a small height) a resistive region (active layer) contained between drain and source regions. The modulation of the current measured between the drain and source regions ("drain-source current" IDS) is due in large part to the modification of the distribution of the charges present in the air gap, between the bridge and the active layer (or between the bridge and an insulating layer deposited on the active layer).
A first particular embodiment of a sensor according to the invention, suitable for use in a gaseous environment, will now be presented in relation to figures la, lb, lc and ld.
In this first embodiment, the sensor according to the invention includes a typical field-effect transistor structure 3, deposited on a glass substrate covered with a silicon nitride film 2.
The field-effect transistor structure 3 includes a suspended bridge 4 serving as a gate (G), made of highly doped polycrystalline silicon.
In this example, the field-effect transistor is actually a thin-film transistor (TFT). The polycrystalline silicon bridge is produced by using surface micro-technology techniques. The structure thus made using the surface micro-technology techniques is, , . , .
A first particular embodiment of a sensor according to the invention, suitable for use in a gaseous environment, will now be presented in relation to figures la, lb, lc and ld.
In this first embodiment, the sensor according to the invention includes a typical field-effect transistor structure 3, deposited on a glass substrate covered with a silicon nitride film 2.
The field-effect transistor structure 3 includes a suspended bridge 4 serving as a gate (G), made of highly doped polycrystalline silicon.
In this example, the field-effect transistor is actually a thin-film transistor (TFT). The polycrystalline silicon bridge is produced by using surface micro-technology techniques. The structure thus made using the surface micro-technology techniques is, , . , .
17 for example, called a "Suspended Gate Thin-Film Transistor" (SGTFT).
However, it is clear that the invention relates to all field-effect transistor structures for which the electric field is sufficiently strong to influence the distribution of the electrical charges present in the environment.
The field-effect transistor structure 3 includes an unintentionally doped polycrystalline silicon film (active layer) 10, deposited on the glass substrate 1 covered with the silicon nitride layer 2. Any other insulating substrate or substrate covered with any electrical insulation can also be used. The polycrystalline silicon layer, for example, is deposited amorphously and is then crystallised. It can also be deposited directly in the crystallised state.
Any other undoped or lightly doped semiconductor can also be used.
A second polycrystalline silicon layer 5, which is this time highly in-situ doped, is then deposited and etched to form the source (S)7 and drain (D)6 regions.
It can also be deposited amorphously and then crystallised or deposited directly in the crystallised state. It can also be post-doped by any doping method.
Any other highly conductive material can also be used.
Optionally, a silicon dioxide/silicon nitride bi-layer or a silicon nitride layer alone 8 is then deposited and etched so as to cover the surface between the source and drain regions. Any type of electrical insulating layer can also be used.
However, it is clear that the invention relates to all field-effect transistor structures for which the electric field is sufficiently strong to influence the distribution of the electrical charges present in the environment.
The field-effect transistor structure 3 includes an unintentionally doped polycrystalline silicon film (active layer) 10, deposited on the glass substrate 1 covered with the silicon nitride layer 2. Any other insulating substrate or substrate covered with any electrical insulation can also be used. The polycrystalline silicon layer, for example, is deposited amorphously and is then crystallised. It can also be deposited directly in the crystallised state.
Any other undoped or lightly doped semiconductor can also be used.
A second polycrystalline silicon layer 5, which is this time highly in-situ doped, is then deposited and etched to form the source (S)7 and drain (D)6 regions.
It can also be deposited amorphously and then crystallised or deposited directly in the crystallised state. It can also be post-doped by any doping method.
Any other highly conductive material can also be used.
Optionally, a silicon dioxide/silicon nitride bi-layer or a silicon nitride layer alone 8 is then deposited and etched so as to cover the surface between the source and drain regions. Any type of electrical insulating layer can also be used.
18 A germanium layer (not shown) is then deposited and used as a sacrificial layer. An Si02 layer or any other material compatible with the other layers present in the structure can also be used as a sacrificial layer. The thickness h of the sacrificial layer provides the final value for the height of the air gap 9 (the space under the bridge).
It is recalled that the electric field E created in the air gap is defined as the ratio between the gate voltage VGS and the height of the air gap. According to the invention, this electric field created in the air gap has a value greater than or equal to a specific threshold value (50,000 V/cm, and preferably 100,000 V/cm, or even 200,000 V/cm) . The height h of the air gap and the gate voltage VGs are selected so that this condition involving the electric field E is met.
This air gap height h is low, for a given gate voltage VGs, so that the electric field created in the air gap is strong and thus so that the field effect will be the predominant effect on sensitivity. In other words, this height h must be sufficiently low so that a gate voltage VGs applied to the bridge creates a sufficiently strong electric field E to influence the distribution of electrical charges contained in the environment and present in the air gap. According to the invention, this height is less than or equal to 1 pm, and preferably less than or equal to 0.5 pm.
Thus, for an air gap height h equal to 0.5 pm, the electric field E is equal to at least 50,000 V/cm, 100,000 V/cm or 200,000 V/cm, depending on whether the
It is recalled that the electric field E created in the air gap is defined as the ratio between the gate voltage VGS and the height of the air gap. According to the invention, this electric field created in the air gap has a value greater than or equal to a specific threshold value (50,000 V/cm, and preferably 100,000 V/cm, or even 200,000 V/cm) . The height h of the air gap and the gate voltage VGs are selected so that this condition involving the electric field E is met.
This air gap height h is low, for a given gate voltage VGs, so that the electric field created in the air gap is strong and thus so that the field effect will be the predominant effect on sensitivity. In other words, this height h must be sufficiently low so that a gate voltage VGs applied to the bridge creates a sufficiently strong electric field E to influence the distribution of electrical charges contained in the environment and present in the air gap. According to the invention, this height is less than or equal to 1 pm, and preferably less than or equal to 0.5 pm.
Thus, for an air gap height h equal to 0.5 pm, the electric field E is equal to at least 50,000 V/cm, 100,000 V/cm or 200,000 V/cm, depending on whether the
19 gate voltage VGS is equal to at least 2.5 V, 5 V or 10 V, respectively.
A highly in-situ doped polycrystalline silicon layer 4 is then deposited and etched in order to form the bridge that serves as a gate (G). Any other highly conductive material can also be used, which is compatible with the other layers present in the structure, and which has sufficient mechanical strength properties for maintaining the bridge.
A metallic layer (not shown) can then be deposited and etched to form the electrical source, drain and bridge (serving as a gate) contacts. The field-effect transistor structure 3 can also be produced without this metallic layer.
The sacrificial layer is etched (i.e., eliminated) so as to free the space (air gap) 9 situated beneath the bridge 4, either before or after depositing the metallic contacts, depending on the compatibility between the various materials used. In this way, the gaseous environment can occupy this space 9.
The first embodiment of the sensor according to the invention, which was described above, is sensitive to various gases. Sensitivity to various environments has been shown. The structure is not sensitive to electrically neutral environments. The transistor characteristic is similar under vacuum, in an 02 environment, or in an N2 environment, for example. In all of these environments, the threshold voltage is very high. This high threshold voltage value is normal considering the usual MOS theory equations wherein the dielectric constant is 1 and the gate insulator has a thickness greater than or equal to 0.5 Pm. The transistor characteristic varies in electrically charged environments.
A theoretical explanation will now be given for 5 the effect characteristic of the invention (new distribution of the charges in the air gap, owing to the application of a strong electric field), as well as for its possible combination with the adsorption effect.
The context here involves the case of a sensor of 10 the invention in which the shift in the threshold voltage of the transistor is due to:
- the field effect (effect characteristic of this invention) : a strong electric field is created in the air gap region, which causes a new distribution of the 15 charges in the air gap; and - the adsorption effect (well-known effect) at the surface of a sensitive layer deposited on the active layer of the transistor. However, as already indicated above, it is clear that the invention also applies in
A highly in-situ doped polycrystalline silicon layer 4 is then deposited and etched in order to form the bridge that serves as a gate (G). Any other highly conductive material can also be used, which is compatible with the other layers present in the structure, and which has sufficient mechanical strength properties for maintaining the bridge.
A metallic layer (not shown) can then be deposited and etched to form the electrical source, drain and bridge (serving as a gate) contacts. The field-effect transistor structure 3 can also be produced without this metallic layer.
The sacrificial layer is etched (i.e., eliminated) so as to free the space (air gap) 9 situated beneath the bridge 4, either before or after depositing the metallic contacts, depending on the compatibility between the various materials used. In this way, the gaseous environment can occupy this space 9.
The first embodiment of the sensor according to the invention, which was described above, is sensitive to various gases. Sensitivity to various environments has been shown. The structure is not sensitive to electrically neutral environments. The transistor characteristic is similar under vacuum, in an 02 environment, or in an N2 environment, for example. In all of these environments, the threshold voltage is very high. This high threshold voltage value is normal considering the usual MOS theory equations wherein the dielectric constant is 1 and the gate insulator has a thickness greater than or equal to 0.5 Pm. The transistor characteristic varies in electrically charged environments.
A theoretical explanation will now be given for 5 the effect characteristic of the invention (new distribution of the charges in the air gap, owing to the application of a strong electric field), as well as for its possible combination with the adsorption effect.
The context here involves the case of a sensor of 10 the invention in which the shift in the threshold voltage of the transistor is due to:
- the field effect (effect characteristic of this invention) : a strong electric field is created in the air gap region, which causes a new distribution of the 15 charges in the air gap; and - the adsorption effect (well-known effect) at the surface of a sensitive layer deposited on the active layer of the transistor. However, as already indicated above, it is clear that the invention also applies in
20 the case where only the field effect is used (without being combined with the adsorption effect).
In this case, the threshold voltage VTH of the sensor (i.e., the value of the gate voltage VGs for which the drain-source current IDS saturates), is written as:
VTx - (DMs+2cPF+ QSC _ 1 ejxp(x)dx (1) C Ceox o where (DMS is the difference between the work functions of the gate and the semiconductor, cpF is the position of the Fermi level in relation to the middle
In this case, the threshold voltage VTH of the sensor (i.e., the value of the gate voltage VGs for which the drain-source current IDS saturates), is written as:
VTx - (DMs+2cPF+ QSC _ 1 ejxp(x)dx (1) C Ceox o where (DMS is the difference between the work functions of the gate and the semiconductor, cpF is the position of the Fermi level in relation to the middle
21 of the forbidden band, Qsc is the space charge in the semiconductor, C is the total capacity per surface unit between the bridge and the semiconductor, eo,ç is the total thickness of the insulator (sum of the air gap height h and the thickness of the insulating layer 8, e.g., a silicon dioxide (Si02)/silicon nitride (Si3N4) bi-layer or a silicon nitride (Si3N4) alone), and p(x) is the charge in the insulator at a distance x from the bridge.
Any variation of the environment in the air gap causes a variation in the total charge in the insulator and a possible variation in its distribution.
Furthermore, chemical reactions on the internal surface of the air gap (adsorption phenomenon) may occur, thereby leading to a variation in the parameter (DMs.
In the case of prior techniques, only this latter variation associated with the adsorption phenomenon is considered.
However, as this invention proposes, when a strong electric field is present in the air gap, the distribution of the charge in the air gap varies, which causes a variation in p(x). Furthermore, this strong field can influence the adsorption by pushing the charges onto the surface of the sensitive layer.
A11 of these effects lead to a variation in (DMs but also to the last term of the above expression (1).
Consequently, the variation in the threshold voltage VTH can be very large if, according to the invention, the effects of a strong electric field are taken into account.
Any variation of the environment in the air gap causes a variation in the total charge in the insulator and a possible variation in its distribution.
Furthermore, chemical reactions on the internal surface of the air gap (adsorption phenomenon) may occur, thereby leading to a variation in the parameter (DMs.
In the case of prior techniques, only this latter variation associated with the adsorption phenomenon is considered.
However, as this invention proposes, when a strong electric field is present in the air gap, the distribution of the charge in the air gap varies, which causes a variation in p(x). Furthermore, this strong field can influence the adsorption by pushing the charges onto the surface of the sensitive layer.
A11 of these effects lead to a variation in (DMs but also to the last term of the above expression (1).
Consequently, the variation in the threshold voltage VTH can be very large if, according to the invention, the effects of a strong electric field are taken into account.
22 Several examples of use of this first embodiment of the sensor according to the invention will now be presented in relation to figures 2a to 2h. In these examples of use, the transistor is a thin-film transistor with an N-type polycrystalline silicon suspended gate. The air gap has a height of 0.5 pm. It is clear that numerous other uses can be anticipated without exceeding the scope of this invention.
Figures 2a and 2b show that in an NH3 environment (Fig. 2a) or in an NOz environment (Fig. 2b), the structure has a significant degree of sensitivity. NO2 and NH3 were selected as test gases for their opposite effects on the characteristics of the transistors.
Figure 2a shows that when NH3 is introduced, the curve IDS(VGS) shifts towards the weakest voltages (negative shift in the threshold voltage). Figure 2b shows that the introduction of NOz has the opposite effect. Thus, a shift in the threshold voltage of 6 V is obtained with 100 ppm of NH3 gas or 2 ppm of NO2.
It is also seen in figures 2a and 2b that, with this sensor example according to the invention, the gate voltage VGs must be greater than 10 V in order for detection to be possible, and thus the electric field must be greater than 200,000 V/cm (=10V/0.5pm).
Figures 2c and 2d shown that, when smoke is introduced, the threshold voltage and the slope below the threshold drop sharply, and the transfer characteristic saturates. This is particularly visible on the linear plot of figure 2e.
In the same way, figures 2f and 2g show that, when humidity is introduced, the threshold voltage and the
Figures 2a and 2b show that in an NH3 environment (Fig. 2a) or in an NOz environment (Fig. 2b), the structure has a significant degree of sensitivity. NO2 and NH3 were selected as test gases for their opposite effects on the characteristics of the transistors.
Figure 2a shows that when NH3 is introduced, the curve IDS(VGS) shifts towards the weakest voltages (negative shift in the threshold voltage). Figure 2b shows that the introduction of NOz has the opposite effect. Thus, a shift in the threshold voltage of 6 V is obtained with 100 ppm of NH3 gas or 2 ppm of NO2.
It is also seen in figures 2a and 2b that, with this sensor example according to the invention, the gate voltage VGs must be greater than 10 V in order for detection to be possible, and thus the electric field must be greater than 200,000 V/cm (=10V/0.5pm).
Figures 2c and 2d shown that, when smoke is introduced, the threshold voltage and the slope below the threshold drop sharply, and the transfer characteristic saturates. This is particularly visible on the linear plot of figure 2e.
In the same way, figures 2f and 2g show that, when humidity is introduced, the threshold voltage and the
23 slope below the threshold drop sharply, and the transfer characteristic saturates. Thus, the threshold voltage varies by more than 18 V when the humidity ratio shifts from 25 to 70%.
Figure 2h shows that the sensitivity of the structure is selective for smoke for low relative humidity ratios (e.g. when the humidity ratio is held constant and is lower than 25%).
A second particular embodiment of a sensor according to the invention, which is suitable for use in a liquid environment, will no be presented in relation to figure 3.
This structure differs from that of figure la (first embodiment suitable for use in a gaseous environment) in that a silicon nitride layer 30 is deposited at its surface (and thus in particular at the surface of the drain 6 and source 5 regions, the active layer 10 and the suspended bridge 4) . The structure thus modified can be dipped into a liquid and enable in-situ measurement in the liquid. Any other material making it possible to insulate the structure from the solution can also be used. Furthermore, the contact regions are covered with resin or any other electrical insulator.
This structure, for example, is used to measure the quantity of charges contained in a liquid. It is called, for example, an "Ion-Sensitive Thin-Film Transistor" (ISTFT).
Figure 4a shows that a pH sensitivity of 285 mV/pH
is obtained with an air gap having a height equal to 0.5 }im. With an air gap height such as this, the
Figure 2h shows that the sensitivity of the structure is selective for smoke for low relative humidity ratios (e.g. when the humidity ratio is held constant and is lower than 25%).
A second particular embodiment of a sensor according to the invention, which is suitable for use in a liquid environment, will no be presented in relation to figure 3.
This structure differs from that of figure la (first embodiment suitable for use in a gaseous environment) in that a silicon nitride layer 30 is deposited at its surface (and thus in particular at the surface of the drain 6 and source 5 regions, the active layer 10 and the suspended bridge 4) . The structure thus modified can be dipped into a liquid and enable in-situ measurement in the liquid. Any other material making it possible to insulate the structure from the solution can also be used. Furthermore, the contact regions are covered with resin or any other electrical insulator.
This structure, for example, is used to measure the quantity of charges contained in a liquid. It is called, for example, an "Ion-Sensitive Thin-Film Transistor" (ISTFT).
Figure 4a shows that a pH sensitivity of 285 mV/pH
is obtained with an air gap having a height equal to 0.5 }im. With an air gap height such as this, the
24 variation in the gate voltage, between approximately 6.5V and 9V, corresponds to a variation in the electric field (in the air gap), between approximately 130,000 V/cm and 180,000 V/cm. Figure 4b shows that this sensitivity drops to 90 mV/pH for an air gap having a height equal to 0.8 pm. With an air gap height such as this, the variation in the gate voltage, between approximately 6.25V and 7.25V, corresponds to a variation in the electric field (in the air gap), between approximately 62,500 V/cm and 72,500 V/cm. This reduction in sensitivity, in comparison with the case of figure 4a, shows that the field effect is predominant in obtaining high sensitivity. In other words, in a liquid, the modified structure of the invention provides high pH sensitivity, approximately 2 to 6 times stronger that that of the ordinary ISFET
structures, this sensitivity being dependent on the thickness of the air gap.
In general, and as explained above in relation to the formula (1), the high sensitivity to electrically charged environments of the sensor according to the invention is explained by the strong field effect that is created (i.e., the creation of a strong electric field in the air gap, greater than or equal to 50,000 V/cm, or even 200,000 V/cm) owing, in particular, to a an air gap having a small thickness h (e.g., h<lpm if VGS>10V, or h<0.5pm if VGS>5V, in order to obtain an electric field E greater than or equal to 100,000 V/cm).
When the thickness of the air gap is large and the electric field E in the air gap is less than 50,000 V/cm (the case of the prior techniques where E is much less than 1,000 V/cm), the field effect is not sufficient and the distribution of the electric charges is uniform inside the air gap. This distribution is no longer uniform when the electric field E becomes strong 5 (greater than or equal to 50,000 V/cm), due in particular to the fact that the thickness of the air gap decreases (the case of the technique according to the invention). The sensitivity of the sensor according to the invention is heightened because of the larger 10 accumulation of charges on one of the faces of the air gap (unlike the case of the prior technique where the distribution of charges is uniform). This accumulation becomes increasingly larger when the gate-source voltage and thus the field effect increase. The 15 saturation of the transfer characteristic is explained by the saturation of the air gap surface when the electrical charges accumulate as a result of the field effect. This saturation appears for lower gate-source voltages (weaker field effect) when the quantity of 20 charges contained in the environment increases. Finally, the strength of the field effect is clearly demonstrated because the pH sensitivity decreases when the thickness of the air gap increases (see above discussion of figures 4a and 4b).
structures, this sensitivity being dependent on the thickness of the air gap.
In general, and as explained above in relation to the formula (1), the high sensitivity to electrically charged environments of the sensor according to the invention is explained by the strong field effect that is created (i.e., the creation of a strong electric field in the air gap, greater than or equal to 50,000 V/cm, or even 200,000 V/cm) owing, in particular, to a an air gap having a small thickness h (e.g., h<lpm if VGS>10V, or h<0.5pm if VGS>5V, in order to obtain an electric field E greater than or equal to 100,000 V/cm).
When the thickness of the air gap is large and the electric field E in the air gap is less than 50,000 V/cm (the case of the prior techniques where E is much less than 1,000 V/cm), the field effect is not sufficient and the distribution of the electric charges is uniform inside the air gap. This distribution is no longer uniform when the electric field E becomes strong 5 (greater than or equal to 50,000 V/cm), due in particular to the fact that the thickness of the air gap decreases (the case of the technique according to the invention). The sensitivity of the sensor according to the invention is heightened because of the larger 10 accumulation of charges on one of the faces of the air gap (unlike the case of the prior technique where the distribution of charges is uniform). This accumulation becomes increasingly larger when the gate-source voltage and thus the field effect increase. The 15 saturation of the transfer characteristic is explained by the saturation of the air gap surface when the electrical charges accumulate as a result of the field effect. This saturation appears for lower gate-source voltages (weaker field effect) when the quantity of 20 charges contained in the environment increases. Finally, the strength of the field effect is clearly demonstrated because the pH sensitivity decreases when the thickness of the air gap increases (see above discussion of figures 4a and 4b).
25 The field effect characteristic of the invention (new distribution of the electric charges in the air gap owing to the application of a strong electric field), as well as its possible combination with the adsorption effect, will now be illustrated by way of an example and in relation to figure 5.
26 Saline solutions of KC1 and NaCl and a basic solution of KOH were prepared with exactly the same concentration.
The pH does not change when saline solutions such as KCl and NaCl are used. Consequently, when tracking the transfer characteristics of a sensor according to the invention, which is placed in these solutions, only the effect of the electric field on the distribution of the charges is observed.
On the other hand, in the presence of KOH, the pH
changes and, as a result, not only is the effect of the new distribution of charges (under the effect of the electric field) observed, but also the adsorption effect.
Figure 5 shows the transfer characteristics (drain-source current IDS - gate voltage VGS) of the same particular embodiment of a sensor according to the invention, obtained after the sensor was dipped into the following liquid environments: deionised water ("DI
Water") and solutions of KOH, KCl and NaCl with the same concentration.
In the presence of KC1 or NaCl with the same concentration, the same shift in the transfer characteristic is observed, in relation to the transfer characteristic obtained with the deionised water. This shift is due only to the new distribution of the electrical charges in the air gap, which results from the application of a strong electric field. The shift in the threshold voltage VTH is induced by the variation in the last term of the above equation (1).
The same distribution of the charges yields the same
The pH does not change when saline solutions such as KCl and NaCl are used. Consequently, when tracking the transfer characteristics of a sensor according to the invention, which is placed in these solutions, only the effect of the electric field on the distribution of the charges is observed.
On the other hand, in the presence of KOH, the pH
changes and, as a result, not only is the effect of the new distribution of charges (under the effect of the electric field) observed, but also the adsorption effect.
Figure 5 shows the transfer characteristics (drain-source current IDS - gate voltage VGS) of the same particular embodiment of a sensor according to the invention, obtained after the sensor was dipped into the following liquid environments: deionised water ("DI
Water") and solutions of KOH, KCl and NaCl with the same concentration.
In the presence of KC1 or NaCl with the same concentration, the same shift in the transfer characteristic is observed, in relation to the transfer characteristic obtained with the deionised water. This shift is due only to the new distribution of the electrical charges in the air gap, which results from the application of a strong electric field. The shift in the threshold voltage VTH is induced by the variation in the last term of the above equation (1).
The same distribution of the charges yields the same
27 shift. With the KOH solution having the same concentration, an additional shift is observed. It is due to the pH of KOH and thus to the charges that are adsorbed at the surface of the insulating layer (referenced as 30 in figure 3) consisting of silicon nitride Si3N4 (first term of the above equation (1) ).
It shall be noted that, in this example, the insulating layer also serves as a sensitive layer for the adsorption process. Consequently, in the presence of KOH, the shift in the transfer characteristic is due, on the one hand, to the new distribution of charges (under the effect of the electric field) and, on the other hand, to the adsorbed charge. Thus, the two effects combine and contribute to the good pH
sensitivity of this example of a sensor according to the invention.
Although the invention has been described above in relation to a limited number of embodiments, those skilled in the art, upon reading this description, will understand that other embodiments can be imagined without exceeding the scope of this invention.
Consequently, the scope of the invention is limited only by the appended claims.
It shall be noted that, in this example, the insulating layer also serves as a sensitive layer for the adsorption process. Consequently, in the presence of KOH, the shift in the transfer characteristic is due, on the one hand, to the new distribution of charges (under the effect of the electric field) and, on the other hand, to the adsorbed charge. Thus, the two effects combine and contribute to the good pH
sensitivity of this example of a sensor according to the invention.
Although the invention has been described above in relation to a limited number of embodiments, those skilled in the art, upon reading this description, will understand that other embodiments can be imagined without exceeding the scope of this invention.
Consequently, the scope of the invention is limited only by the appended claims.
Claims (19)
1. Sensor for detecting and/or measuring a concentration of electrical charges contained in an environment, said sensor comprising a field-effect transistor structure including a bridge (4), which forms a gate and is suspended above an active layer (10) situated between drain (6) and source (7) regions, a gate voltage having a specific value being applied to the bridge, a so-called air gap region (9), being included between the bridge (4) and the active layer (10) or an insulating layer (8) deposited on said active layer, and having a specific height, an electric field E, defined as the ratio between the gate voltage and the air gap height, being created in the air gap, characterised in that the electric field created in the air gap has a value greater than or equal to a specific threshold value, which is sufficiently large for the electric field E to influence the distribution of electrical charges contained in the environment and present in the air gap, and to enable high sensor sensitivity to be obtained by an accumulation of electrical charges on the active layer.
2. Sensor of claim 1, characterised in that the electric field created in the air gap has a value greater than or equal to 50,000 V/cm.
3. Sensor of claim 2, characterised in that the electric field created in the air gap has as value greater than or equal to 100,000 V/cm.
4. Sensor of claim 3, characterised in that the electric field created in the air gap has a value greater than or equal to 200,000 V/cm.
5. Sensor as claimed in any of claims 1 to 4, characterised in that the height of the air gap is less than 1 µm.
6. Sensor as claimed in claim 5, characterised in that the height of the air gap is less than 0.5 µm.
7. Sensor as claimed in any of claims 1 to 6, characterised in that at least a portion of the surface of the structure, including the drain and source regions, the suspended bridge and the active layer, is covered with an insulating material (30), so that the sensor can be dipped into a liquid environment.
8. Use of the sensor as claimed in any of claims 1 to 7 for detecting and/or measuring a concentration of electrical charges contained in an environment.
9. Use of claim 8, characterised in that the environment containing electrical charges belongs to the group including gaseous and liquid environments.
10. Use of claim 9, characterised in that the electrical charges are NH3 molecules contained in a gaseous environment.
11. Use of claim 9, characterised in that the electrical charges are NO2 molecules contained in a gaseous environment.
12. Use of claim 9, characterised in that the electrical charges are H+ ions contained in a liquid environment.
13. Use of the sensor as claimed in any of claims 1 to 7 for detecting and/or measuring a humidity ratio in a gaseous environment, by detecting and/or measuring a concentration of OH- ions contained in said gaseous environment.
14. Use of the sensor as claimed in any of claims 1 to 6 for detecting and/or measuring a concentration of smoke in a gaseous environment, by detecting and/or measuring electrical charges contained in said smoke and contained in said gaseous environment.
15. Use of the sensor as claimed in any of claims 1 to 6 for measuring air quality, by measuring a quantity of negative electrical charges contained in the air.
16. Use of the sensor as claimed in any of claims 1 to 6 for detecting and/or measuring a void fraction in a gaseous environment, by detecting and/or measuring electrical charges that have not been eliminated from said gaseous environment.
17. Use of the sensor as claimed in any of claims 1 to 7 for measuring the pH of a liquid environment, by measuring a concentration of H+ ions contained in said liquid environment.
18. Use of the sensor as claimed in any of claims 1 to 7 for detecting electrically charged biological entities contained in said environment.
19. Method for manufacturing a sensor as claimed in any of claims 1 to 7, characterised in that the suspended bridge field-effect transistor structure is produced using a surface micro-technology technique.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR0407583A FR2872914B1 (fr) | 2004-07-07 | 2004-07-07 | Capteur pour la detection et/ou la mesure d'une concentration de charges electriques contenues dans une ambiance, utilisations et procede de fabrication correspondants |
| FR0407583 | 2004-07-07 | ||
| PCT/FR2005/001761 WO2006013289A1 (fr) | 2004-07-07 | 2005-07-07 | Capteur pour la détection et/ou la mesure d'une concentration de charges électriques contenues dans une ambiance, utilisations et procédé de fabrication correspondants. |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2572485A1 true CA2572485A1 (fr) | 2006-02-09 |
Family
ID=34947923
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002572485A Abandoned CA2572485A1 (fr) | 2004-07-07 | 2005-07-07 | Sensor for detecting and/or measuring concentration of electric charges contained in an atmosphere, corresponding uses and method for making same |
Country Status (9)
| Country | Link |
|---|---|
| US (1) | US20080134759A1 (fr) |
| EP (1) | EP1774307A1 (fr) |
| JP (1) | JP2008506099A (fr) |
| CN (1) | CN101048656A (fr) |
| CA (1) | CA2572485A1 (fr) |
| FR (1) | FR2872914B1 (fr) |
| IL (1) | IL180496A0 (fr) |
| RU (1) | RU2398222C2 (fr) |
| WO (1) | WO2006013289A1 (fr) |
Families Citing this family (19)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2934683B1 (fr) * | 2008-07-31 | 2012-11-16 | Mhs Electronics | Capteur biologique a transistor a effet de champ. |
| US8373206B2 (en) * | 2010-07-20 | 2013-02-12 | Nth Tech Corporation | Biosensor apparatuses and methods thereof |
| DE102011083644A1 (de) * | 2011-09-28 | 2013-03-28 | Robert Bosch Gmbh | Mikromechanische Sensorvorrichtung mit beweglichem Gate und entsprechendes Herstellungsverfahren |
| DE102012211460A1 (de) * | 2012-07-03 | 2014-01-09 | Robert Bosch Gmbh | Gassensor und Verfahren zum Herstellen eines solchen |
| US9599586B2 (en) * | 2012-08-27 | 2017-03-21 | Infineon Technologies Ag | Ion sensor |
| WO2014059080A1 (fr) * | 2012-10-12 | 2014-04-17 | Texas State University-San Marcos | Transistor à effet de champ à grille isolée mobile verticalement (vmgfet) sur une tranche de silicium sur isolant (soi), et procédé de fabrication d'un vmgfet |
| US9170165B2 (en) * | 2013-03-25 | 2015-10-27 | Globalfoundries U.S. 2 Llc | Workfunction modulation-based sensor to measure pressure and temperature |
| KR101616959B1 (ko) * | 2013-07-02 | 2016-04-29 | 전자부품연구원 | Fet 이온센서 및 이를 이용한 시스템 |
| GB2523173A (en) | 2014-02-17 | 2015-08-19 | Nokia Technologies Oy | An apparatus and associated methods |
| DE102014115980B4 (de) | 2014-11-03 | 2022-06-23 | Infineon Technologies Ag | Gerät zum Analysieren der Ionenkinetik in Dielektrika |
| CN105301079B (zh) * | 2015-10-13 | 2019-10-15 | 上海小海龟科技有限公司 | 用于待测物离子活度检测的半导体器件及其检测方法 |
| CN105353000B (zh) * | 2015-10-14 | 2019-04-19 | 深圳市共进电子股份有限公司 | 半导体器件及其检测方法 |
| DE102016209360A1 (de) | 2016-05-31 | 2017-11-30 | Continental Automotive Gmbh | Verfahren, Vorrichtung zum Betreiben eines Stickoxidsensors, Computerprogramm und Computerprogrammprodukt |
| TWI648864B (zh) * | 2017-09-26 | 2019-01-21 | 國立清華大學 | 感測裝置及離子檢測方法 |
| RU2675667C1 (ru) * | 2017-12-18 | 2018-12-21 | Общество с ограниченной ответственностью "Технологии Печатной Электроники" (ООО "ПРИНТЭЛТЕХ") | Способ селективного определения концентрации газообразных меркаптосодержащих и/или аминосодержащих соединений при помощи газового сенсора на основе органического полевого транзистора и устройство для селективного определения концентрации газообразных меркаптосодержащих и/или аминосодержащих соединений |
| US20240230747A1 (en) * | 2021-06-01 | 2024-07-11 | Eltek S.P.A. | Sensor device for monitoring the dielectric strength of a dielectric fluid, in particular a fluid for the thermal conditioning of a battery |
| CN113203898B (zh) * | 2021-07-05 | 2021-09-14 | 北京科技大学 | 一种用于离子化空气中的非接触表面电位测试方法 |
| US12449395B2 (en) | 2022-03-14 | 2025-10-21 | Saudi Arabian Oil Company | System and method for real-time drilling fluids pH measuring utilizing electrolyte insulator semiconductor field-effect sensors |
| KR102811340B1 (ko) * | 2022-06-15 | 2025-05-21 | 스웨센시 에이비 | Fet 가스 센서 장치 |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4020830A (en) * | 1975-03-12 | 1977-05-03 | The University Of Utah | Selective chemical sensitive FET transducers |
| CA1148279A (fr) * | 1979-12-14 | 1983-06-14 | Andreas Scheidweiler | Detecteur de fumee par ionisation a coefficient de fiabilite ameliore |
| US4514263A (en) * | 1982-01-12 | 1985-04-30 | University Of Utah | Apparatus and method for measuring the concentration of components in fluids |
| US4411741A (en) * | 1982-01-12 | 1983-10-25 | University Of Utah | Apparatus and method for measuring the concentration of components in fluids |
| GB8416994D0 (en) * | 1984-07-04 | 1984-08-08 | Emi Ltd | Gas sensor |
| JPS6133645A (ja) * | 1984-07-25 | 1986-02-17 | 住友電気工業株式会社 | 生体用センサ− |
| US4671852A (en) * | 1986-05-07 | 1987-06-09 | The Standard Oil Company | Method of forming suspended gate, chemically sensitive field-effect transistor |
| DE3834189C1 (de) * | 1988-10-07 | 1990-02-15 | Ignaz Eisele | Nicht-elektrochemische Herstellung von chemisch selektiven Schichten in Feldeffekttransistoren mit frei hängendem Gate |
| JPH0368857A (ja) * | 1989-08-09 | 1991-03-25 | Terumo Corp | Ｉｓｆｅｔセンサ及びその製造方法 |
| US5693545A (en) * | 1996-02-28 | 1997-12-02 | Motorola, Inc. | Method for forming a semiconductor sensor FET device |
| US5683569A (en) * | 1996-02-28 | 1997-11-04 | Motorola, Inc. | Method of sensing a chemical and sensor therefor |
| DE19849932A1 (de) * | 1998-10-29 | 2000-05-11 | Siemens Ag | Gasdetektion nach dem Prinzip einer Messung von Austrittsarbeiten |
| DE19956303A1 (de) * | 1999-11-23 | 2001-06-07 | Siemens Ag | Brandmelder mit Gassensoren |
| JP4467022B2 (ja) * | 2000-03-31 | 2010-05-26 | フィガロ技研株式会社 | ガスセンサ |
| RU2188411C1 (ru) * | 2001-06-13 | 2002-08-27 | Тамбовский государственный технический университет | Способ измерения активности ионов в растворах и устройство для его осуществления |
-
2004
- 2004-07-07 FR FR0407583A patent/FR2872914B1/fr not_active Expired - Fee Related
-
2005
- 2005-07-07 JP JP2007519844A patent/JP2008506099A/ja active Pending
- 2005-07-07 RU RU2007101659/28A patent/RU2398222C2/ru not_active IP Right Cessation
- 2005-07-07 CN CNA2005800297375A patent/CN101048656A/zh active Pending
- 2005-07-07 EP EP05788666A patent/EP1774307A1/fr not_active Withdrawn
- 2005-07-07 WO PCT/FR2005/001761 patent/WO2006013289A1/fr not_active Ceased
- 2005-07-07 CA CA002572485A patent/CA2572485A1/fr not_active Abandoned
- 2005-07-07 US US11/631,839 patent/US20080134759A1/en not_active Abandoned
-
2007
- 2007-01-02 IL IL180496A patent/IL180496A0/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| RU2007101659A (ru) | 2008-08-20 |
| EP1774307A1 (fr) | 2007-04-18 |
| IL180496A0 (en) | 2007-06-03 |
| FR2872914A1 (fr) | 2006-01-13 |
| JP2008506099A (ja) | 2008-02-28 |
| US20080134759A1 (en) | 2008-06-12 |
| WO2006013289A1 (fr) | 2006-02-09 |
| FR2872914B1 (fr) | 2006-10-13 |
| CN101048656A (zh) | 2007-10-03 |
| RU2398222C2 (ru) | 2010-08-27 |
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| Date | Code | Title | Description |
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| EEER | Examination request | ||
| FZDE | Discontinued |
Effective date: 20140708 |