Gas Sensor
The invention relates to a gas sensor comprising a substrate of a first charge
carrier type whereon a drain and a source of a second charge carrier type are
arranged, wherein a channel area is formed between the drain and the source,
and also comprising a gas sensitive layer comprising poles between which a gas
induced voltage is produced according to the concentration of a gas which is in
contact with the layer, wherein the gas sensitive layer for measuring the voltage
is capacitatively coupled to the channel area by one of its poles over an air gap
and to a counter-electrode having a reference potential by its other pole.
Such a gas sensor is disclosed in DE 43 33 875 C2. It has a gas sensitive layer
that reacts to the effects of gasses with a change of its work function. There is an
electrically insulating layer arranged in the channel area of the gas sensor, which
covers the substrate and the source and drain areas. An air gap is formed
between the channel insulation and the gas sensitive layer. This is in accordance
with the suspended gate field effect transistor (SGFET) principle. The voltage
induced in the gas sensitive layer by the presence of the gas capacitatively
couples to the channel surface over the air gap and induces charges in the
SGFET structure. The channel area is surrounded by a guard electrode that
screens the channel area from electric potentials that are arranged outside of the
gas sensor surface area defined by the guard electrode. However, the
disadvantage of the gas sensor ties therein that the SGFET measuring signal is
not only dependent on the concentration of the gas to be measured but also on
the electric resistance between the guard ring and the channel zone, which is
influenced by humidity. Disadvantageous above all is that for gradual changes in
gas concentration, the measuring accuracy of the gas sensor decreases
disproportionately to increasing moisture content.
An improved gas sensor has already been disclosed in DE 101 18 367 C2, in
which there is a surface profiling having elevations and depressions formed
between the guard ring and the channel area. By means of this relatively simply
and economically achieved method, the distance between the guard ring and the
channel area is increased, consequently reducing the Faraday current flowing
between the guard ring and the channel. Although said gas sensor has proven its
efficacy in practice in a wide range of applications, it nevertheless has
disadvantages. The measuring accuracy of said gas sensor for gradual changes
in gas concentration also decreases disproportionately to increasing moisture
content.
Another gas sensor of the type mentioned in the introduction is disclosed in EP 1
191 332 A1, which sensor comprises a moisture sensitive layer in the same field
effect transistor in addition to the gas sensitive layer, and wherein said moisture
sensitive layer can be activated according to the same measuring principle as the
gas sensitive layer. According to the disclosure statement, it is thereby possible,
at a known temperature, to define moisture influences in comparison with the gas
reaction to be measured, and to reduce the cross-sensitivity to moisture in the
gas sensor by drawing on the moisture measuring signal. A disadvantage,
however, lies therein that a complex and expensive compensation switch
mechanism is also required, in addition to the additional moisture sensor. An
additional disadvantage lies therein that such compensation is only valid for a
specific temperature at a given time; therefore, in the event of temperature
fluctuations, a temperature measuring signal must also be detected and taken
into consideration.
The objective of the invention is therefore to create a gas sensor of the type
mentioned in the introduction that enables a high degree of measurement
accuracy and has a simple and compact construction. In doing so, the
measurement accuracy should be largely independent of moisture influences.
This objective is solved in that there is a hydrophobic layer arranged on the
surface of the gas sensor between the gas sensitive layer and the channel area
and/or on a sensor electrode that is electrically connected to a gate electrode
arranged on the channel area.
The adsorption of moisture on the surface of the gas sensor is impeded or even
prevented completely in an advantageous manner by means of this surprisingly
simple solution. By this means, ion transport between the channel area or the
sensor electrode and areas of the gas sensor separated therefrom having a
different electric potential than said channel area or sensor electrode, especially
with a gas having high moisture content, is substantially limited. The measuring
accuracy of the gas sensor thus remains largely constant even if the moisture
content of the gas changes. Furthermore, high long term stability of the
measurement accuracy is realized. The hydrophobic layer is advantageously
arranged on an electrically non-conductive or semi-conductive layer. However, it
is also conceivable to arrange the hydrophobic layer on an electrically conductive
layer if the electrically conductive layer is electrically insulated from the channel
area, the sensor electrode and/or another area separated from the electrically
conductive layer, the potential of which differs from that of the electrically
conductive layer. The hydrophobic layer is preferably constructed as an ultra-
hydrophobic layer.
In a preferred embodiment of the invention, the gas sensor has an electrically
conductive guard ring on its surface, which delimits the channel area and/or the
sensor electrode from the channel area and/or the sensor electrode through a
space, wherein the hydrophobic layer is arranged in at least one area of the gas
sensor located between the guard ring and the channel area and/or the sensor
electrode. By means of the guard ring, the potential over the channel area or the
potential of the sensor electrode is prevented from being drawn after a specific
time by the conductivity still remaining on the gas sensor surface to the potential
of the pole of the gas sensitive layer, which is capacitatively coupled to the
channel area, or to the potential of the guard ring. A potential drift is avoided by
this means and an even greater accuracy of measurement is achieved.
In a functional embodiment of the invention, the hydrophobic layer extends
continuously over the channel area and/or the sensor electrode. The gas sensor
is then especially easy and economical to produce, as the hydrophobic layer can
be superimposed to cover the entire surface of the gas sensor and in doing so a
masking step can be eliminated.
It is advantageous if the hydrophobic layer is separated from the channel area
and/or the sensor electrode and if it delimits the channel area and/or the sensor
electrode, preferably in a ring- or frame-like manner. By this means, in a
hydrophobic layer in which an interference voltage is induced by contact with an
interfering gas different from that to which the gas sensitive layer is sensitive, the
influence of said interference voltage on the measuring signal, and consequently
the cross-sensitivity of the gas sensor to the interfering gas, can be reduced.
In a preferred embodiment of the invention, the static contact angle of the
hydrophobic layer measured by water obtained on a planar surface measures is
at least 70°, if necessary at least 90°, especially at least 105° and preferably at
least 120°. Above all, with a contact angle of at least 120°, it is possible to
achieve an especially high measuring accuracy of the gas sensor that is largely
independent of the moisture content of the gas. The contact angle can be defined
by using known standard measurement methods at room temperature.
In a functional embodiment of the invention, molecules of the hydrophobic layer
are covalently bound to the surface of an adjacent, preferably semi-conducting or
electrically insulating layer of the gas sensor. By this means it is possible to
attach the hydrophobic layer directly to the adjacent layer of the gas sensor when
manufacturing the gas sensor.
It is advantageous if the hydrophobic layer contains at least one polymer. The
hydrophobic layer may then be superimposed on the surface of the gas sensor
during the manufacturing of the gas sensor at room temperature, thereby
protecting the implantation area and structures already present on the substrate
from heat.
It is especially advantageous if the polymer is a fluoride and preferably a
perfluoride polymer. A high level of accuracy in measuring the gas concentration
can still be achieved by means of the strongly electronegative CF groups
contained in said polymers, even when the gas to be measured has a high
moisture percentage, e.g., a relative humidity of 90%.
In another advantageous embodiment of the invention, the polymer is connected
to an adjacent, preferably semi-conducting or electrically insulating layer of the
gas sensor by means of an intermediate layer preferably in the form of a
monolayer, wherein the intermediate layer has at least one reactive group
anchored on the adjacent layer, and wherein the polymer is preferably coupled to
the intermediate layer by means of a covalent bond. In doing so, it is even
possible when manufacturing the gas sensor to first superimpose the
hydrophobic polymer on the intermediate layer so that it covers it completely, and
then to photochemically bind it to the intermediate layer only in specific subzones
of the surface of the gas sensor under the influence of an optical ray projected on
the surface of the gas sensor by means of a shadow mask. The hydrophobic
polymer can then be removed from the remaining subzones, for example, by
washing the surface of the gas sensor. By means of this entire procedure, a gas
sensor is produced comprising a structured hydrophobic layer arranged only on
specific sites of its surface.
It is advantageous if the hydrophobic layer has a surface profiling with projections
and depressions. Even greater measurement accuracy can be achieved thereby.
The depressions are preferably constructed as grooves or slots forming a frame
or a ring around the channel area and/or the sensor electrode.
In the following, exemplary embodiments of the invention are explained in more
detail, with reference to the^drawings. dSome parts are shown in a very
diagrammatic context.
Fig. 1 shows a lengthwise section of a gas sensor, which has an ISFET located
under a gas-sensitive layer represented by a dashed line,
Fig. 2 shows a cross section of the gas sensor shown in fig. 1 along the line of
intersection designated by II in fig. 1,
Fig. 3 shows a lengthwise section of a gas sensor, which has a CCFET, located
under a gas-sensitive layer represented by a dashed line,
Fig. 4 shows a cross section of the gas sensor shown in fig. 3 along the line of
intersection designated by IV in fig. 3,
Fig. 5 shows a schematic illustration of the photochemical bonding of a
hydrophobic polymer to a layer with linker molecules immobilized on an electrical
insulation layer.
A gas sensor designated in its entirety by 1 has a substrate 2 of a first charge
carrier type that may be composed, e.g., of p-type silicon. A drain 3 and a source
4 of a second charge carrier type are arranged on the substrate 2. The drain 3
and the source 4 may be composed, for example, of n-type silicon. The drain 3 is
connected to a drain connector 5 by means of electric conductor paths that are
only partially illustrated in the drawing. The source 4 is connected to a source
connector 6 in like manner. The drain connector 5 and the source connector 6
are each arranged on a layer 7 deposited on the substrate 2.
In an exemplary embodiment of fig. 1 and fig. 2, there is a channel formed in the
substrate 2 between the drain 3 and the source 4 whereon a thin oxide electric
insulation layer 9 is arranged which serves as a gate dielectric. The thin oxide
layer 9 is ca. 3 -150 nm thick.
As can be discerned especially easily in Fig. 2, the gas sensor 1 also has a gas
sensitive layer 10, and on the flat sides turned away from each other thereof
there are poles 11 and 12 between which a gas-induced electrical voltage is
produced according to the concentration of a gas in contact with the layer 10. For
the detection of the voltage, the gas sensitive layer 10 is capacitatively coupled
to the channel area 8 by one of its poles 12 over an air gap 14. The other pole 11
is connected to a counter-electrode 13 whereon there lies an electric reference
potential. The air gap 14 has an access to the gas to be detected and is between
the deposited layers 7, whereon the gas sensitive layer 10 rests.
In the exemplary embodiment of fig. 1 and fig. 2, the channel area 8 is openly
constructed (ISFET) and capacitatively coupled directly to the gas sensitive layer
10 over the thin layer oxide and the air gap 14. It can clearly be discerned that
the channel area 8 is arranged on the side of the air gap 14 that lies opposite the
gas sensitive layer 10.
In an exemplary embodiment according to fig. 3 and fig. 4, the channel area 8 is
arranged alongside of the gas sensitive layer 10 in the substrate 2 and covered
with a gate electrode 22. For the capacitative coupling of the channel area 8 to
the gas sensitive layer 10, the gate electrode 22 is connected by means of a
conductor path 15 to a sensor electrode 16, which is arranged on an insulation
layer 17 located on the substrate 2 on the side of the air gap 14 lying opposite to
the pole 12 of the gas sensitive layer 10. The insulation layer 17 may be, for
example, a Si02 layer.
Furthermore, the gas sensor 1 has an electrically conductive guard ring 18 on its
surface, which delimits the channel area 8 in the exemplary embodiment
according to fig. 1 and fig. 2 and the sensor electrode 16 leading to the channel
area 8 in the exemplary embodiment according to fig. 3 and fig. 4. A space is
provided thereby between the guard ring 18 and the channel area 8 of the
exemplary embodiment according to fig. 1 and fig. 2 and between the guard ring
18 and the sensor electrode 16 of the exemplary embodiment according to fig. 3
and fig. 4. The guard ring 18 lies on a defined electric potential in order to screen
the channel area 8 from electric potentials located outside of the surface zone of
the gas sensor substrate 2 defined by the guard ring 18.
In the exemplary embodiment according to fig. 1 and fig. 2, a hydrophobic layer
19 is arranged between the guard ring 18 and the channel area 8 on the surface
of the gas sensor 1. Said layer is located on an electric insulation layer 17, which
is arranged on the drain 3, the source 4 and the areas of the substrate 2 located
outside of the channel area 8. It can be discerned in fig. 1 that the hydrophobic
layer 19 delimits the channel area 8 in a frame-like manner and ends at a
distance from the channel area 8 and the guard ring 18. By means of the
hydrophobic layer 19, the adsorption of the water contained in the gas is
substantially impeded in the part of the gas sensor surface located between the
guard ring 18 and the channel area 8. By this means it is possible to attain a high
level of electrical resistance on the surface and a high level of measurement
accuracy of the gas sensor.
In the exemplary embodiment according to fig. 3 and fig. 4, the hydrophobic layer
19 is arranged on the insulation layer 17 between the guard ring 18 and the
sensor electrode 16. In fig. 4 it can be discerned that the hydrophobic layer 19
delimits the sensor electrode 16 in a frame-like manner and ends at a distance
from the sensor electrode 16 and the guard ring 18. By means of the
hydrophobic layer 19, the adsorption of the water contained in the gas is
substantially impeded in the part of the surface of the gas sensor 1 located
between the guard ring 18 and the sensor electrode 16.
The hydrophobic layer consists of a polymer, preferably made of
poly(heptadecafluoroacrylate). In the manufacturing of the gas sensor 1, the
hydrophobic layer 19 is attached to the insulation layer 17 by means of an
intermediate layer 20. In order to do this, the intermediate layer 20 in the form of
a benzophenone-functionalized silicon monochloride monolayer is first
superimposed on the insulation layer 17. In fig. 5 it can be discerned that upon
exposure to UV light, free radicals are produced in the intermediate layer 20,
which bind on contact to the insulation layer 17 and in doing so attach the
intermediate layer 20 to the insulation layer 17.
Afterwards, a thin film of poly(heptadecafluoroacrylate) is deposited on the
intermediate layer 20 so that it covers it entirely. Then the zones whereon the
hydrophobic layer 19 is to be supported at a later time are irradiated with UV rays
with the aid of a shadow mask. It can be discerned in fig. 5 that the intermediate
layer 20 has a photoreactive benzophenone group 21 which binds to an adjacent
polymer of the future hydrophobic layer 19 when irradiated with UV light. In doing
so, the benzophenone group 21 accepts a hydrogen atom from the adjacent
polymer in such a way that a covalent bond is formed between the
benzophenone group 21 and the adjacent polymer (see Prucker, O., Ruhe, J. et
al, Photochemical Attachment of Polymer Films to Solid Surfaces via Monolayers
of Benzophenone Derivates, J. Am. Chem. Soc. (1999), 121, p. 8766 - 8770).
After the polymer of the hydrophobic layer 19 is bound in specific zones to the
surface of the insulation layer 17 in this manner, the non-bound polymers for
forming the structured hydrophobic layer 19 remaining on the non-irradiated
zones of the surface are removed, for example, by washing them away with a
solvent.
It should still be mentioned that there are also other possible exemplary
embodiments wherein the hydrophobic layer 19 may extend without interruptions
over the channel area 8, the sensor electrode 16 and/or the guard ring 18. In the
production of such a gas sensor 1, the hydrophobic layer 19 may also be
deposited directly on the insulation layer 17. This may be accomplished by
precipitating hydrophobic trichloro(1H,1H,2H,2H-perfluorooctyl)silicate (TPFS)
from the gas phase at a temperature of ca. 100 °C onto the insulation layer 17.
TPFS is preferably precipitated in the absence of moisture so that cross-
connections and inhomogeneities in the TPFS film precipitated on the surface
are avoided. Furthermore, care must be taken to prevent dust particles from
adhering to the surface during the precipitation process.
WE CLAIM:
1. A gas sensor (1) comprising a substrate (2) of a first charge carrier
type, whereon a drain (3) and a source (4) of a second charge carrier
type are arranged wherein a channel area (8) is formed between the
drain (3) and the source (4), and with a gas-sensitive layer (10)
comprising poles (11, 12), between which a gas-induced voltage is
produced according to the concentration of a gas which is in contact
with the layer (10), wherein in order to measure the voltage, the gas-
sensitive layer (10) is capacitatively coupled by one of its poles (12) to
the channel area (8) over an air gap (14) and by its other poles (11) to
a counter-electrode (13) having a reference potential, characterized in
that a hydrophobic layer (19) is arranged on the surface of the gas
sensor (1) between the gas sensitive layer (10) and the channel area
(8) and/or a sensor electrode (16), which is electrically connected to a
gate electrode (22) arranged on the channel area (8).
2. A gas sensor (1) as claimed in claim 1, wherein it has an electrically
conductive guard ring (18) on its surface, which delimits the channel
area (8) and/or the sensor electrode (16) leading to the channel area
(8) from the channel area (8) and/or the sensor electrode (16) by
means of a space, and wherein the hydrophobic layer (19) is arranged
in at least one area of the surface of the gas sensor (1) located
between the guard ring (18) and the channel area (8) and/or the
sensor electrode (16).
3. A gas sensor (1) as claimed in claim 1 or claim 2, wherein the
hydrophobic layer (19) extends continuously over the channel area (8)
and/or the sensor electrode (16).
4. A gas sensor (1) as claimed in any one of claims 1 through 3, wherein
the hydrophobic layer (19) is separated from the channel area (8)
and/or the sensor electrode (8) and delimits the channel area (8)
and/or the sensor electrode (16) preferably in a ring- or frame-like
manner.
5. A gas sensor (1) as defined in any one of claims 1 through 4, wherein
the static contact angle of the hydrophobic layer (19) measured with
water and obtained on a planar surface is at least 70°, if necessary at
least 90°, especially at least 105° and preferably at least 120°.
6. A gas sensor (1) as claimed in any one of claims 1 through 5, wherein
molecules of the hydrophobic layer (19) are covalently bound to the
surface of an adjacent, preferably semi-conductive or electrically
insulating layer of the gas sensor (1).
7. A gas sensor (1) as claimed in any one of claims 1 through 6, wherein
the hydrophobic layer (19) contains at least one polymer.
8. A gas sensor (1) as claimed in any one of claims 1 through 7, wherein
the polymer is a fluoride and preferably a perfluoride polymer.
9. A gas sensor (1) as claimed in any one of claims 1 through 8, wherein
the polymer is connected by an intermediate layer (20) that is
preferably in the form of a monolayer to an adjacent, preferably semi-
conductive or electrically insulating layer of the gas sensor (1), and
further characterized in that the intermediate layer (20) has at least
one reactive group anchored on the adjacent layer, and that the
polymer is coupled preferably by means of a covalent bond to the
intermediate layer (20).
10. A gas sensor (1) as claimed in any one of claims 1 through 9, wherein
the hydrophobic layer (19) has a surface profiling with projections and
depressions.
11. A gas sensor (1) as claimed in any one of claims 1 through 10,
wherein the depressions are in the form of slots or grooves and
preferably form a frame or a ring around the channel area (8) and/or
the sensor electrode (16).
The invention relates to a gas sensor comprising a substrate of a first charge
carrier type whereon a drain and a source of a second charge carrier type are arranged. A
channel area is formed between the drain and the source. The gas sensor also comprises a gas
sensitive layer comprising poles between which a gas induced voltage is produced according
to the concentration of a gas which is in contact with the layer. In order to measure said
voltage, a pole of the gas sensitive layer is capacitatively coupled to the channel area by
means of an air gap and the other pole is connected to a counter-electrode having a reference
potential. A hydrophobic layer is arranged on the surface of the gas sensor between the gas
sensitive layer and the channel area and/or on a sensor electrode which is connected to a gate
electrode arranged on the channel area.