MOISTURE SENSOR AND METHOD FOR MEASURING MOISTURE OF A GAS-
PHASE MEDIUM
[0001] The invention relates to a moisture sensor, which has a receiving area on its
surface for a moisture film, the layer thickness of which is dependent on the
relative humidity in the surrounding of the receiving area, wherein the moisture
sensor has a signal source for providing a control voltage to the moisture film which is
connected to at least one control electrode at at least one infeed, the electrode
abutting the receiving area. The invention further relates to a method for measuring
moisture of a gas-phase medium, wherein a moisture film, the layer thickness of
which is dependent on the relative humidity in the medium, is generated on a solid,
and wherein an electric potential is provided to the moisture film at at least one infeed.
[0002] Such a moisture sensor is disclosed in P. Ruther et al.: "Surfare
Conductivity of CMOS Silicon Nitride Layers," Proceedings of IEEE Sensor 2003,
Toronto, October 22-24, 2003, pages 920-925. On the surface of a nitride layer, the
moisture sensor has a plurality of ring electrodes concentrically arranged relative to
each other, which in each rase are spaced apart from each other by gaps. The
surface conductivity of the nitride layer, which is dependent on atmospheric moisture, is
measured by means of the ring electrodes. To this end, a temporally variable
control voltage is provided to the ring electrodes by means of a signal source. On the
surface of the nitride layer, the control voltage induces a current flow, which is
measured relative to the ambient temperature and the relative humidity and plotted in
the form of a grid. The surface conductivity of the nitride layer is determined by
means of the current. Furthermore, by means of the grid it is possible to determine the
relative humidity from the electric current. However, the moisture sensor has the disadvan-
tage that fouling between the ring electrodes ran drastically alter the electrical
resistance on the surface of the nitride layer and thus considerably distort the moisture
measurement signal.
[0003] DE 39 11 812 C2 discloses a moisture sensor which has a moisture-
sensitive layer composed of polytetrafluoroethylene, which is located between two
electrodes, namely a moisture-permeable cover electrode and a ground electrode.
The moisture sensor is used for the capacitive measurement of moisture. This
process exploits the effect in which water molecules are adsorbed in microscopic
cavities in the amorphous polytetrafluoroethylene and change the capacity between
the electrodes relative to the moisture.
[0004] DE 39 11 812 C2 further discloses a moisture sensor operating on the resistive
measurement p(incipie, wherein the polytetraftuoroethylene layer located between the
electrodes is conductive. The electrodes are interdigital comb-shaped electrodes in which the
fingers of one electrode are located in the gaps formell between the fingers of the other
electrode when the moisture sensor is viewed from above. The electrodes are connected to a
voltage source, which induces a current flow in the moisture sensor, wherein the current
flows from one of the electrodes via the polytetrafluoroethylene to the other electrode and is
measured by means of a sensor.
[0005] The saure disadvantage of fouling on the electrodes potentially distorting the
moisture measurement signal also resides in the moisture sensors disclosed in DE 39 11812
C2.
[00061 The object is therefore to create a moisture sensor and a method of the aforemen-
tioned type which make it possible to measure moisture as independently as possible of
fouling occurring in the surrounding of the measurement site. [0007] This objective is
achieved by a moisture sensor which has a receiving area on its surface for a moisture film,
the layer thickness of which is dependent on the relative humidity in the surrounding of the
receiving area, wherein the moisture sensor has a signal source wich means for generating a
control voltage, wherein the signal source for providing the control voltage to the moisture film
is connected to at least one control electrode at at least one infeed, the electrode abutting the
receiving area, wherein the moisture sensor has at least one potential sensor, which has at
least one sensor area under the receiving area which is spaced apart from the at least one
infeed and which is electrically insulated from the receiving area by an insulation layer located
between said sensor area and said receiving area in such a way that an electric potential can
be capacitively detected by means of the potential sensor, the potential being dependent on the
layer thickness of the moisture film and on the control voltage, wherein an evaluator is con-
nected to the measurement signal Output of the potential sensor,
- wherein the signal source has means for generating a variable control voltage and the
evaluator has means for generating a signal that is dependent on the rate at which the
potential sensor measurement signal changes in response to the change of the control signal
and/or
- wherein the evaluator has means for generating a signal for a potential steift between
the measurement signal of the potential sensor and a reference signal or a reference potential.
[0008] The aforementioned object is further achieved by a method for measuring moisture of
a gas-phase medium, wherein a moisture film is generated on a solid, the thickness of the film
being dependent on the relative humidity in the medium, and wherein an electric potential is
provided to the moisture film at at least one infeed, wherein at a site spaced apart from the at
least one infeed a measurement signal for an electric potential is capacitively measured
through an electrical insulation layer, the potential being dependent on the layer thickness of
the moisture film and on the control voltage.
[0009] In an advantageous manner, an electric potential on the moisture film is measured to
determine moisture. The measurement signal is thus largely independent of any
fouling that may be present on the control electrode. The thickness of the moisture film is
a measurement for the relative humidity in the surrounding, compare Michelle L. Gee et al.,
"Hydrophobicity Effects in the Condensation of Water Films on Quartz," Journal of Coliold and
Interface Science, Vol. 140, No. 2, pages 450-464 (December 1990). The potential on the
moisture film can be measured statically and/or dynamically. Due to the charge transfers
occurring with dynamic measurement, the measurement signal is dependent an the ohmic
resistance of the moisture film and therefore on its thickness in spite of the insulation layer
located between the receiving area and the potential sensor. The internal resistance of the
signal source is preferably considerably lass than the electrical resistance of the fouling on the
control electrode that can be expected during the use of the moisture sensor, so that the
control voltage provided to the moisture film via the control electrode is minimally affected by
the fouling and by an electric current discharged from the fouling.
[0010] As moisture decreases, the rate of change of the measurement signal decreases due
to the consequently reduced layer thickness and due to the greater electrical resistance of the
moisture film rohen the temporal progression of the control signal remains the saure. Analo-
gously, the rate of change increases as moisture increases. The rate of change is therefore a
measurement for the relative humidity. The analysis of the rate of change can be carried out
using an analog circuit (e.g., a differentiator) and/or with digital signal processing methods.
The evaluator used for this purpose can haue a microcomputer.
[0011] A change in the humidity in the surrounding of the moisture sensor results in a
change of the response curve of the Ion-sensitive field effect transistor (ISFET), which in turn
results in the constant component of the measurement signal increasing in absolute value as
relative humidity increases, and decreasing in absolute value as relative humidity decreases.
This is brought about by changes in the charge density
at the contact surface between the moisture film and the insulation layer. This change
in charge density acts as an additional potential on the ISFET channel area or on the Input
of the measurement amplifier, thus shifting the measurement signal towards potential
values greater in absolute value. Therefore, it is also possible to determine the relative
humidity statically from the potential steift of the measurement signal. This can be accom-
plished with, for example, a comparator, an adder, or a subtractor.
[0012] It is advantageous if the control electrode has at least two electrode areas con-
nected to each other by at least one strip conductor, and W the receiving area is located
between these electrode areas. The measurement signal of the potential sensor is then
rendered even more independent of fouling on the control electrode. Not even a short
circuit between the electrode areas would cause a change of the measurement signal.
[0013] In a preferred embodiment of the invention, the control electrode forms an uninter-
rupted perimeter around the receiving area and is in particular configured as a ring elec-
trode. Preference is given to the entre inner circumference of the ring electrode being in
contact with moisture film in such a way that the lauer is charged from all sides with the
control voltage.
[0014] In an advantageous embodiment of the invention, the potential sensor is a field
effect transistor, particularly an ISFET, which has a semiconductor substrate of a firnt
charge carrier type on which are provided a drain and a source of a second charge carrier
type, wherein a channel area forming the sensor area is formed between the drain and the
source. The field effect transistor then forms an electric capacity with the moisture film
across the Isolation layer, which capacity is connected in series to the ohmic resistance of the
moisture film and to the signal source and on which capacity a voltage drop occurs that
changes the conductivity in the channel area of the field effect transistor when the tempo-
rally variable control voltage is infed into the control electrode. A high impedance meas-
urement of the potential of the moisture film is possible with the field effect transistor.
Preference is given to location of the field effect transistor directly under the moisture film
or the receiving area for the moisture film, thus making it possible to obtain a measure-
ment signal that is largely insensitive to electromagnetic interference and a moisture
sensor with Compact dimensions.
[0015] In another embodiment of the invention, preference is given to a laminar measure-
ment electrode for forming the sensor area, the electrode being connected by a strip
conductor to the channel area of a field effect transistor and/or to an Input of a high-
impedance measurement amplifier. In this mannet the channel area or the measurement
amplifier can also be laterally spaced apart from the sensor area.
[0016] It is advantageous if the clear space between two electrode areas located
an both sides of the receiving area or the clear width of the ring electrode is smaller
than 100 pm, particularly smaller than 10 pm, and preferably smaller than 1µm. For
most applications, this dimensioning enables the measurement signal to react to a
change in moisture with sufficient Speed. With a ring electrode clear width of ca. 1 pm,
the time required for the sensor to detect a stepwise change in relative humidity from
5% to 95% is between ca. 0.1 and 2 seconds.
[0017] In an advantageous embodiment of the Invention, the signal source is
configured in such a way that it can generste a control voltage with at least one jump
discontinuity, particulady a rectangular signal. The high rate of change of the signal at
the jump discontinuity results in a rapid change of the measurement signal in
response to the jump discontinuity. Furthermore, the rate of change and the potential
shift of the measurement signal in response to a change in humidity can be easily
determined by means of the jump discontinuity of the measurement signal.
[0018] Preference is given to the insulation layer consisting of a silicone dioxide
layer and/or a silicone nitride layer. These layers enable a high specific electrical
resistance and are easily manufactured with Standard semiconductor production
processes.
[0019] It is advantageous if the signal source, the control electrode, the potential
sensor, the insulation layer, and the evaluator are integrated in a semiconductor chip.
The moisture sensor can then be economically mass produced using semiconductor
manufacturing techniques, and furthermore it can be manufactured with Compact
dimensions. The semiconductor chip can be coated with an electrically insulating
sealing compound, which has an opening that forms an access to the receiving area.
[0020] Other advantageous embodiments of the Invention are described in the
dependent Claims.
[0021] An illustrative embodiment of the Invention is explained in detail in the
following, with reference to the drawing. Shown are:
[0022] Fig. 1 a partial Cross-section of a moisture sensor comprising a control
electrode for providing a control voltage to a moisture film,
[0023] Fig. 2 a view from above of a section of a semiconductor chip of the
moisture sensor, in which the control electrode and an ISFET located under the
electrode can be discerned, and
[0024] Fig. 3 a graphic Illustration of two measurement Signals of a potential
sensor of the moisture sensor plotted at different relative humidities and of a control voltage
provided to the moisture film, wherein the time t is plotted on the x-axis and the measure-
ment signal amplitude s is plotted on the y-axis.
[0025] A moisture sensor designated in its entirety by 1 in Fig. 1 has a semi-
conductor substrate 2, on the surface of which is located an electrical insulation layer 3,
preferably a silicone dioxide layer or a silicone nitride layer. A doping of a first charge carrier
type is inserted in the semiconductor substrate 2.
[0026] A receiving area 4 for a moisture film 5a, 5b is provided on the insulation layer 3.
The moisture sensor 1 is extrusion-coated with an electrically insulating sealing compound,
which is not shown in any greater detail in the drawing and which has an opening forming an
access to the receiving area. The layer thickness 6a, 6b of the moisture film 5a, 5b and thus
the ohmic resistance of the moisture film 5a, 5b are dependent on the relative humidity of
the atmosphere in contact with the receiving area 4. This is indicated by a dashed live in
Fig. 1.
[0027] In the semiconductor substrate 2 is integrated a potential sensor, which has a sensor
area 9 under the receiving area for detecting electric charges in the vicinity of the receiving
area. The potential sensor is configured as an ISFET and comprises a drain 7 and a source 8
of a second charge carrier type. Between the drain 7 and the source 8 is located a channel
area, which forms the sensor area 9. The channel area is spaced apart from the receiving
area 4 or the moisture film 5a, 5b located therein by the insulation layer 3.
[0028] On its surface facing away from the channel area, the moisture sensor 1 has a
control electrode 10, which is in contact with the outer edge of the moisture film serving as an
infeed 11 for a control voltage. The infeed 11 is located on top of the rim of the channel area
or laterally spaced apart from it. In Fig. 2 it can be discerned that the control electrode 10
is configured as a ring electrode and defines the receiving area 4 for the moisture film
5a, 5b. Sections of the control electrode 10 are located on top of the source 8 and the drain 7.
The control electrode 10 is electrically insulated from the source 8 and the drain 7 by the
insulation layer 3.
[0029] The control electrode 10 is electrically connected by a strip conductor to a first output
terminal of a signal source, which is not shown in any greater detail in the drawing. A second
output terminal of the signal source is joined to the semiconductor substrate 2. Preference
is given to Integration of the signal source in the semiconductor substrate 2.
[0030] By means of the signal source, a temporally variable control voltage, namely a
rectangular signal having a predetermined frequency and a predetermined amplitude, is
generated and provided via the control electrode 10 to the moisture film 5a, 5b. The temporal
progression 12 of the control voltage is graphically illustrated in Fig. 3. With the ISFET, the
moisture film 5a, 5b forms an electric capacity, which is connected in series to the ohmic
resistance and the control electrode.
[0031] While the control voltage is residing on the moisture film 3, the electric
potential in the receiving area 4 is capacitively detected by means of the ISFET. The
measurement signal 13a, 13b of the ISFET is dependent on the temporal
progression 12 of the control voltage and on the layer thickness 6a, 6b of the
moisture film 5a, 515. The layer thickness 6a, 6b can be smaller than 500 nm and is
preferably smaller than 100 nm.
[0032] In Fig. 3 it can be discerned that with an unchanging progression 12 of the control
voltage, the rate at which the measurement signal 13a, 13b changes in response to a
change in the control voltage decreases as the thickness of the moisture film decreases.
The value of this rate of change can be determined, for example, by measuring the time t,
or t2 that the measurement signal 13a, 13b needs to change its value by a predefined
magnitude As when a jump discontinuity occurs in the control voltage and by calculating the
rate of change from the quotients As /1, or As/12.
[0033) In Fig. 3 it can be further discerned that with an unchanging progression 12 of the
control voltage, the measurement signal 13a, 13b shifts by an offset 14 if the layer
thickness 6a, 615 of the moisture film 5a, 5b, and thus the relative humidity, changes. As
the layer thickness 6a, 615 increases the magnitude of the measurement signal 13a, 1315
increases, and as the layer thickness decreases the magnitude of the measurement signal
13a, 13b decreases.
[0034] Reference values relative to the relative humidity are provided for the rate of
change and the offset 14-, these values can be in the form of response curves or in the form
of a grill for which reference points are stored. Preference is given to determination of the
reference values by measurement; however, they can also be calculated, compare
Michelle L. Gee et al., "Hydrophobicity Effects in the Condensation of Water Films
on Quartz," Journal of Colloid and Interface Science, Vol. 140, No. 2, pages 450-464
(December 1990).
[0035] By means of the potential sensor measurement signal 13a, 13b and the reference
values, it is then possible to determine the relative humidity in Order to generate, for
example, an analog signal and/or a corresponding digital signal proportional to the relative
humidity. To this end, a measurement signal Output of the potential sensor is connected to a
suitable evaluator. This evaluator has an electric circuit for generating a signal for the rate
of change and a signal for the offset 14 of the measurement signal 13a, 11315.
WE CLAIM
1. Moisture sensor (1) which comprises a receiving area (4) on its surface
for a moisture film (5a, 5b), the layer thickness (6a, 6b) of which is dependent on the relative
humidity in the surrounding of the receiving area (4), wherein the moisture sensor (1) has
a signal source that has means for generating a control voltage, wherein the signal source
for providing the control voltage to the moisture film (5a, 5b) is connected to at least one
control electrode (10) at at least one infeed (11), the electrode abutting the receiving area
(4), wherein the moisture sensor (1) has at least one potential sensor, which has at least
one sensor area (9) under the receiving area (4), which is spaced apart from the at least
one infeed (11) and which is electrically insulated by an insulation layer (3) located be-
tween said sensor area (9) and said receiving area (4) in such a way that an electric
potential can be capacitively detected by means of the potential sensor, the potential being
dependent on the layer thickness of the moisture film (5a, 5b) and on the control
voltage, wherein an evaluator is connected to the measurement signal Output of the poten-
tial sensor,
i. wherein the signal source has means for generating a variable control
voltage and the evaluator has means for generating a signal dependent on the rate at
which the potential sensor measurement signal changes in response to the change of the
control signal and/or
ii. wherein the evaluator has means for generating a signal for a poten-
tial shift between the measurement signal of the potential sensor and a reference
signal or a reference potential.
2. Moisture sensor (1) as in claim 1, characterized in that the control
electrode (10) has at least two electrode areas connected to each other by at least one strip
conductor, and further characterized in that the receiving area is located between these
electrode areas.
3. Moisture sensor (1) as in claim 1 or 2, characterized in that the control
electrode (10) preferably forms an uninterrupted pehmeter around the receiving area
(4) and is in particular configured as a ring electrode.
4. Moisture sensor (1) as in any one of Claims 1 bis 3, charac-
terized in that the potential sensor is a field effect transistor, particularly an ion-sensitive
field effect transistor, which has a semiconductor Substrate (2) of a firstcharge carrier type
on which are arranged a drain (7) and a source (8) of a second charge carrier type, and
further characterized in that a channel area forming the sensor area (9) is formed between
the drain (7) and the source (8).
5. Moisture sensor (1) as in any one of claims 1 through 4, character-
ized in that preference is given to a laminar measurement electrode for forming the sensor
area (9), the electrode being connected by a strip conductor to the channel area of a field
effect transistor and/or to an Input of a high-impedance measurement amplifier.
6. Moisture sensor (1) as in any - one of claims 1 through 5, character-
ized in that the clear spare between'two electrode, areas located on both sides of the
receiving area or the clear width of the ring electrode is smaller than 100 pm, particularly
smaller than 10 pm, and preferably smaller than 1 pm.
7. Moisture sensor (1) as in any one of claims 1 through 6, character-
ized in that the signal source is configured in such a way that it ran generate a
control voltage wich at least one jump discontinuity, particularly a rectangular signal.
8. Moisture sensor (1) as in any one of claims 1 through 7, character-
ized in that the insulation layer (3) consists of a silicone dioxide layer and/or a silicone nitride
layer.
9. Moisture sensor (1) as in any one of claims 1 through 8, character-
ized in that the signal source, the control electrode (10), the potential sensor, the
insulation layer (3), and the evaluator are integrated in a semiconductor chip.
10. Method (1) for measuring moisture of a gas-phase medium, wherein
a moisture film, the layer thickness of which is dependent on the relative humidity in the
medium, is generated on a solid, and wherein an electric potential is provided to the
moisture film at at least one infeed (11), and wherein at a site spaced apart from the at least
one infeed (11), a measurement signal for an electric potential is capacitively measured
through an electrical insulation layer (3), the potential being dependent on the layer
thickness (6a, 6b) of the moisture film (5a, 5b) and on the control voltage.
11. Method as in Claim 10, characterized in that a temporally variable
electric potential is provided to the moisture film.
12. Method as in Claim 10 or 11, characterized in that the rate at which
the electric potential changes in response to a change of the control signal is determined.
13. Method as in any one of Claims 10 through 12, charactedzed in that a
potential shift between the measured electric potential and a reference potential is
determined.

The Invention relates to a moisture sensor (1) which comprises a
receiving area (4) on its surface for a moisture film (5a, 5b), the layer thickness (6a,
6b) of which is dependent on the relative humidity in the surrounding of the receiving
area (4). The moisture sensor (1) has a signal source which is connected to at
least one control electrode (10) at at least one infeed (11), the electrode abut-
ting the receiving area (4), for providing a control voltage to the moisture film (5a,
5b). The moisture sensor (1) comprises, at least one potential sensor which has at
least one sensor area (9), under the receiving area (4), which is spaced apart from
the at least one infeed (11). The sensor area (9) is electrically insulated from the
receiving area (4) by an insulation layer (3), located between the sensor area (9)
and the receiving area (4), in such a way that an electrical potential can be capacitively detected by means of the potential sensor, the potential being dependent
on the layer thickness of the moisture film (5a, 5b) and the control voltage.