WO2003050525A1

WO2003050525A1 - Gas sensor and method for detecting substances according to the work function principle

Info

Publication number: WO2003050525A1
Application number: PCT/EP2002/014231
Authority: WO
Inventors: Ignaz Eisele; Martin Zimmer; Markus Burgmair
Original assignee: Voigt, Wolfgang, M.
Priority date: 2001-12-13
Filing date: 2002-12-13
Publication date: 2003-06-19
Also published as: AU2002358703A1; DE10161213B4; DE10161213A1

Abstract

The invention relates to a gas sensor and a method for detecting one or more substances in a gas and/or a liquid, according to the work function principle. The invention is first characterised in that the gas sensor has two field effect structures - measurement FET and compensation FET (MessFET and KompFET), each comprising one channel (3,3') between a source and a drain region, in addition to a gate electrode (G) equipped with a sensitive material (8). The drain current (IDS) in the channel (3) of the MessFET can be influenced by a modification of the work function of the sensitive material (8), e.g. by the adsorption of molecules from the substance on the surface of the sensitive material (8). The temperature characteristic of the drain current (IDS) of the MessFET is referenced by a comparison with the drain current (IDS) of a compensation FET (KompFET). The invention is also characterised in that the channel (3) of the MessFET is protected by a guard electrode (10), which surrounds the channel (3), against electrical disturbances that occur more frequently in increased humidity.

Description

Gas sensor and method for the detection of substances according to the principle of

Work function measurement

The invention relates to a gas sensor for the detection of certain substances in a gas or a liquid, which works on the principle of work function measurement, according to the preamble of claims 1 and 9, and a detection method according to the preamble of claims 19 and 21. Such a gas sensor and such a method are, for example known from DE 42 39 319 C2.

Detection here means the measurement of the presence and / or the concentration of the substance. Since such a gas sensor is generally used to examine the composition of gases, the term “gas sensor” is used in the following; however, the sensor is also suitable for the detection of substances in liquids. In such gas sensors, which work on the principle of work function measurement, the detection is based on the fact that molecules of the substance to be detected are adsorbed on the surface of a sensitive material. This changes the work function of the sensitive material and thus the electrical potential, which can be measured, for example, by a field effect transistor (FET) structure. In principle, all materials from the insulator to the metal can be used for the sensitive material.

An example of such a so-called gas-sensitive FET (GasFET), as is known, for example, from the above-mentioned DE 42 39 319 C2, is shown schematically in FIG. 2. In a field effect structure 1 there is a channel 3 between a source region S and a drain region D, through which a drain current I _D s can flow. There is an air gap 4 between a passivation layer 2 applied thereon and the gate electrode G, into which the gas to be investigated is diffused or flowed in by external influences (for example a pump). On its side facing the air gap 4, the gate electrode 6 is covered with a sensitive material 8, to which molecules of the substance to be detected attach. This creates a dipole layer or a chemical compound on the surface of the sensitive layer 8 and thus an electrical potential which influences the conductivity of the channel 3 and thus the drain current I _D s across the air gap 4. A change in I _DS can be used to infer the change in contact voltage and thus the change in work function Δφ on the surface of layer 8, which in turn is a measure of the concentration of the substance to be detected.

The drain current is simply calculated from:

l _∞ = μ CW / L (U _G - (U _T + Δφ)) U _DS ,

where μ is the electron mobility in the channel, C the capacitance between gate G and channel 3, W / L the width-to-length ratio of the channel, U _G the gate voltage, U _τ the threshold voltage, Δφ the contact potential change, Δφ times the elementary charge e the work function change and U _D s is the voltage between source and drain (drain voltage). If, for example, the change in drain current I _{DS is} measured as a sensor signal when gas is applied, this gives a measure of the concentration of the substance to be detected. The semiconductor components required for the production of GasFETs can be mass-produced extremely cost-effectively.

Different types of GasFETs are known. The gas sensor shown with an air gap 4 between gate G and the passivation layer 2 over a channel is generally referred to as a suspended gate FET (SGFET). From DE 42 39 319 C2 it is known to produce a SGFET from two components, namely a substrate with a field effect structure without a gate and a gate structure placed thereon (hybrid suspended gate FET, HSGFET). Conversely, it is also possible to mount a prefabricated CMOS transistor using flip-chip technology on a substrate on which the gate electrode and the sensitive layer are applied, as described in DE 198 14 857 A1 (hybrid flip chip FET, abbreviated) HFC-FET). The contacting and fixing is done with a conductive adhesive. A somewhat different structure is known from DE 43 33 875. Here the sensitive layer is not directly above the channel of the FET, but is spatially separated from it, namely arranged on an extension of the gate electrode of the transistor. The potential changes caused by the adsorption of molecules on the sensitive layer are capacitively coupled into the channel area via the extended gate electrode (capacitive controlled FET, abbreviated CCFET). This structure has the advantage that the gate capacitance C is larger than in the SGFET, since there is no air gap between the channel and the gate, so that the measured change in the drain current and thus the sensor signal! according to the above formula is greater than e.g. with a SGFET with otherwise the same parameters.

Although gas sensors based on the principle of work function measurement offer a number of advantages over conventional sensors such as electrochemical cells, conductivity sensors or pellistors, in particular low energy consumption and thus suitability for battery-operated applications, as well as low manufacturing costs and a wide range of detectable substances, they have not yet established on the market. Among other things, this is because the measured signals have so far been influenced too much by external interference effects:

On the one hand, it has been shown that the gate voltage U _G drifts strongly at high atmospheric humidity. The reason for this is that a moisture film is formed on the surface of the passivation layer 2 and / or the sensitive layer 8, in which a leakage current can flow between regions of different potential. Such a current Flow between the gate and the channel, as well as a change in the potential of the moisture film, have a strong influence on the drain current in the channel directly below, since the effect couples into the channel without an air gap and therefore via a high capacitance. As the measurement of the drain current shown in FIG. 3 in a SGFET according to the state of the art shows for air humidity of alternately 0% and between 10% and 90%, the baseline of the sensor signal (here the gate voltage U _G ) drifts when the humidity is above about 40% so strong that a concentration measurement is no longer possible.

Second, the electron mobility μ in channel 3 and the threshold voltage U _{τ of} the field effect structure 1 are strongly temperature-dependent, so that the drain current I _D s decreases sharply with increasing temperature, like the measurement of the drain current shown in FIG. 4 in a SGFET according to the prior art Temperatures between -5 ° C and 65 ° C shows. The sensitivity of the gate voltage to the temperature can be up to one volt / K. This means that with a temperature change of one degree, a signal change arises which is equivalent to a strong gas application. This is insufficient for operation in the temperature range between 0 and 60 ° C, which is required for most applications. In order to be able to measure the influence of the work function change on l _D s at all, the sensor would have to be heated to a constant temperature, which would negate the advantage of the low energy requirement of such sensors.

Another interference effect with gas sensors based on the principle of work function measurement are so-called cross-sensitivities, i.e. the sensitive layer does not only respond to a single substance. The change in work function caused by the adsorption of water is particularly important here, since there is always high air humidity at room temperature, so the moisture concentration is generally many times higher than that of the substances to be detected.

The aim of the present invention is to provide a gas sensor and a detection method which work on the principle of work function measurement, with which a detection of one or more substances in a gas and / or a liquid is possible which is as independent of external interference as possible.

The invention achieves this aim by the subject matter of claims 1, 9, 19 and 21. Advantageous embodiments of the invention are specified, inter alia, in the respective dependent claims. Accordingly, according to a first aspect of the invention, a gas sensor for detecting one or more substance (s) in a gas and / or a liquid, which works on the principle of the work function measurement, is provided, which has a first field effect structure (measuring FET or measuring transistor) with a Channel between a source and a drain region; and comprises a first gate electrode with a sensitive material. The drain current in the channel of the measuring FET can be influenced by changing the work function of the sensitive material, for example by adsorbing molecules of the substance on the surface of the sensitive material. The gas sensor has a second field effect structure (compensation FET or compensation transistor) with a channel between a source and a drain region for compensation of the temperature response of the drain current of the measuring FET.

According to a first aspect of the method according to the invention, the method for the detection of one or more substance (s) in a gas and / or a liquid according to the principle of the work function measurement with the aid of a first field effect structure (MessFET) with a channel between a source and a drain region and a first gate electrode with a sensitive material. By changing the work function of the sensitive material, the drain current in the channel of the measuring FET is influenced. The temperature response of the drain current of the measuring FET is referenced by a comparison with the drain current in the channel of a second field effect structure (reference FET) or also compensated (compFET).

This aspect of the sensor and the method according to the invention is based on the finding that the temperature response of the sensor signal essentially results from the temperature response of the electron mobility μ and the threshold voltage U _τ, that is to say, it has its cause in the channel of the field effect structure. Since the drain current in the channel of a compensation FET is exposed to the same temperature influences as that in the measuring FET, the temperature effect can be eliminated from the measured sensor signal by comparing the two currents. The sensor can therefore be operated in any temperature range (in principle between approx. -60 and 200 ° C) and does not need to be heated to a constant temperature.

The compensation FET is preferably constructed technologically, electrically and geometrically the same as the measurement FET and can be on a common substrate with this sem are produced, so that there are hardly any additional costs for the production of temperature reference.

According to a first embodiment, the drain current in the channel of the compensation FET cannot be influenced by a gate electrode; i.e. in the case of a SGFET, the channel of the compensation FET is not covered by a gate electrode, or the air gap over the channel of the compensation FET is so widened compared to the air gap over the measuring FET that the conductivity of the channel is practically no longer influenced by a change in work function at the gate electrode. This structure has the advantage that the drain current in the channel of the compensation FET does not depend on possible interference effects on a gate electrode, but only on the temperature.

According to a second embodiment, however, the drain current in the channel of the compensation FET can be influenced by changing the work function of a second gate electrode which contains a material which is insensitive to the substance to be detected. Non-sensitive means that the material responds to the substance to be detected at least significantly less than the sensitive material. However, the sensitive material of the first and the non-sensitive material of the second gate electrode preferably have approximately the same cross-sensitivities, that is to say sensitivities to other substances. This has the great advantage that the compensation with the compensation FET not only eliminates the influence of temperature, but also the influence of cross-sensitivity, since both transistors are equally exposed to these disturbances. In particular, it is advantageous to choose materials for the sensitive and non-sensitive layer which respond approximately equally strongly to atmospheric moisture.

Titanium is preferably used for the non-sensitive material in the gate electrode of the compensation FET, since titanium is inert to many gases. Alternatively, silicon nitride can also be used. Platinum is preferably used as the sensitive material, in particular in combination with titanium as the non-sensitive material, since platinum and titanium both have approximately the same sensitivity to moisture and ammonia. Platinum can be used as a sensitive layer for hydrogen at room temperature and for ozone at higher temperatures. The layers can each contain pure platinum or titanium or alloys thereof.

The sensitive and the non-sensitive material are particularly preferred on a common gate structure applied. The gate structure is, for example, a substrate made of silicon or silicon carbide, which is placed on a field effect structure to produce a SGFET. The advantage of the silicon substrate is its smooth surface. Before being put on, the gate structure is preferably initially coated with a total of a titanium-containing material, and then a platinum-containing layer is applied to the titanium-containing layer in the region of the channel of the measuring FET. Apart from the general simplicity of manufacture, this has the particular advantage that the titanium-containing layer serves as an adhesion promoter for the sensitive platinum layer. Without such an intermediate layer, platinum adheres poorly to silicon.

In some embodiments, the measurement and compensation FETs each have a common drain or source area. However, the drain and source regions of the two transistors are preferably spatially separated from one another, so that they do not influence one another as far as possible. For example, they are housed in two separate doping wells of a silicon substrate.

The temperature compensation is particularly preferably carried out with the aid of a sensor circuit with which the difference between the drain currents of the measuring and reference FETs is kept constant by readjusting the voltage U _G at the gate electrode of the measuring FET. Alternatively, it is not the difference, but another linear combination of these currents that is kept constant, for example the drain current of the measuring FET is scaled by a constant factor of, for example, 1.5 before it is subtracted from the drain current of the compensation FET. Such scaling can compensate for structural differences between the two transistors, so that greater freedom is given in the design of the gas sensor and the permissible manufacturing tolerances are greater. The said readjustment of the gate voltage U _{G also} has the advantage over a direct measurement of the drain current I _DS that the size of the readjustment of U _G corresponds directly to the change in contact voltage Δφ. In addition, the gas space above the channel thereby always has the same potential (as I _DS = constant shows), and thus possible drifts due to changing potentials across the channel are excluded.

According to a further aspect, the present invention provides a gas sensor for the detection of one or more substance (s) in a gas and / or a liquid according to the principle of the work function measurement, which has a first field effect structure (MessFET) and a gate electrode with a sensitive one Material after The preamble of claim 9, wherein the channel of the measuring FET is surrounded by a so-called guard electrode for protection against electrical interference.

The invention also provides a method for the detection of one or more substance (s) in a gas and / or a liquid according to the principle of the work function measurement, which is carried out with the aid of a field effect structure (MessFET) and a gate electrode with a sensitive material, the drain current in the channel of the measuring FET is influenced by a change in the work function of the sensitive material. The channel of the measuring FET is protected from electrical interference by a guard electrode surrounding the channel.

The guard electrode, for example, prevents the penetration of leakage currents and capacitive disturbances, and it prevents processes of charge equalization on the surface. In particular, at high atmospheric humidity, the formation of a moisture film on the passivation layer of the FET and on the sensitive layer can cause leakage currents to flow on the surfaces, which influence the electrical conductivity of the channel. This would falsify the measurement result. The guarde electrode, which is kept at a constant potential, for example, interrupts this charge exchange and the influence of moisture is at least considerably reduced.

The two aspects of the present invention are particularly preferably combined with one another, that is to say the gas sensor is equipped both with a compensation FET and with a guard electrode (s) in order to compensate for both moisture and temperature influences. Such a sensor delivers reproducible sensor signals even at room temperatures and therefore does not need to be heated in order to keep the temperature constant and / or to reduce the air humidity. The gas sensor therefore only has a low energy requirement in the micro to milliwatt range, is inexpensive to manufacture and is therefore extremely suitable for mobile and battery-powered applications.

The guard electrode preferably forms a closed ring (guard ring) around the channel of the measuring FET. If available, the compensationFET is preferably equipped with its own guard electrode or a Guärd ring. Alternatively, a single guard electrode can surround both channels. The guard electrode consists of a conductive material, preferably a metal, for example aluminum, platinum or gold, and can be applied to an insulator or passivation layer on the field effect structure by any thin-film method, e.g. B. by electrochemical deposition, sputtering = cathode ray sputtering, reactive sputtering, vapor deposition, spin coating, sublimation, epitaxy or spraying. During sputtering, ions are accelerated in a vacuum and directed as a beam onto a target, whereby atoms are shot out of the target and are deposited on the surface to be coated as a homogeneous, compact layer. The thickness of the guard electrode thus produced is, for example, between 10 and 500 nm.

A step on which the guard electrode is arranged is preferably embedded in the insulator or passivation layer around the channel. This arrangement is particularly useful when a recess is arranged in the passivation layer covering the field effect structure above the channel and the step is integrated into the side walls of the recess. In a SGFET, a recess in the passivation layer can serve as a spacer for the gate electrode.

The area enclosed by the guard electrode is advantageously as small as possible. The guard electrode should therefore be placed as close as possible to the duct. However, it has been shown that the potential of the guard electrode - if this e.g. is kept at a constant potential - can have a disruptive influence on the drain current in the channel. The guard electrode is therefore preferably spaced so far from the respective channel that the drain current in the channel is not significantly influenced by the potential of the guard electrode. Preferably the distance is 1 to 15 µm, e.g. approx. 5 μm.

Another preferred way to rule out that the potential of the guard electrode has an interference effect on the drain current in the channel is to equate the potential of the guard electrode with the potential of the gate electrode of the measuring FET. In this way, there are no potential differences between the gate and guard electrodes at any time, and thus there are no electrical fields generated by the guard electrode in the air gap that could trigger a leakage current. Another possibility is to put the guard electrode at a constant potential, e.g. 0V (ground) The gas sensor of the present invention can be designed both as a suspended gate FET (SGFET) and as a capacitive controlled FET (CCFET). In the case of the SGFET in particular, the channels of the measuring and, if appropriate, the compensation FET are preferably meandering, ie the area between the source and drain areas is serpentine in the plane parallel to the passivation layer. In this way, a favorable width-length ratio W / L of the transistor of, for example, 10,000 is achieved with space-saving utilization of the substrate area, so that a high signal-to-noise ratio can be achieved. In the CCFET, such a broadening of the channel is not absolutely necessary, since the change in potential caused by the change in work function is not transmitted through an air gap here and therefore couples into the channel with a larger capacitance C. An alternative construction of the CCFET is preferably used, in which the elongated gate electrode, by means of which the potential change in the sensitive material is electrically coupled into the channel of the measuring FET, is completely covered by a passivation layer and is therefore exposed to fewer interferences. As a result, voltage fluctuations ("floating") of the gate electrode are reduced.

The invention will now be explained in more detail with reference to exemplary embodiments and the accompanying drawing. The drawing shows:

Fig. 1 is a schematic sectional view through a gas sensor according to a first

Embodiment;

Figure 2 is a schematic sectional view through a gas sensor in SGFET design according to the prior art.

3 shows a diagram of the drain current in a gas sensor according to the prior art at different relative atmospheric humidities;

4 shows a diagram of the drain current in a gas sensor according to the prior art as a function of the temperature;

5 shows a schematic plan view of an exemplary embodiment of a field effect structure;

6 shows a basic circuit diagram of a sensor circuit; 7a, b are schematic sectional images of a second exemplary embodiment of a gas sensor;

8 shows a schematic sectional view of a third exemplary embodiment of a gas sensor;

9 shows a diagram of the drain current in the case of a gas sensor with guard electrode and in the case of a gas sensor with guard electrode and compensation FET at different relative atmospheric humidities;

Fig. 10 is a diagram of the drain currents in the Komp- and MessFET and their difference in a gas sensor with temperature referencing as a function of temperature.

Parts with the same or similar functions are identified in the drawing with the same reference symbols. In the figures, the two aspects of the invention are usually shown in combination with each other, but each aspect can also be realized individually.

1 shows a suspended-gate type GasFET according to the invention, which is equipped both with temperature referencing by a compensationFET (KompFET) and with guard electrodes 10. The measuringFET and the compFET are arranged in two separate, for example p-doped wells 11 and 11 'in a silicon substrate 12. A channel 3, 3 'runs in the p-doped well 11, 11' between the corresponding n ⁺ -doped source S and drain regions D of the measuring FET and the compFET. The arrangement of the two field effect structures in separate troughs 11, 11 'has the advantage that the FETs cannot influence one another electrically. In particular, no current can flow between the transistors, since 12 barrier layers form at the boundaries of the wells 11, 11 'to the substrate. In addition, the field effect structures can be specifically influenced in their electrical properties, in particular their threshold voltage U _τ , for example by applying a voltage to the wells 11, 1.

A passivation layer 2 is applied to the substrate 12, which on the one hand electrically insulates the Feideffekt structures and on the other hand protects against environmental influences such as Oxidation protects. Silicon nitride is preferably used for the passivation, since it is inert to most substances, that is to say does not show any change in work function when gas is applied. Above the channels 3, 3 ', depressions are arranged in the passivation layer 2, each of which is covered by part of a gate structure 6 (also called a sensor cover) and thereby form the air gaps 4, 4'. In the area of the depressions, the passivation layer 2 is only a few micrometers thick, and the distance between the passivation layer 2 and the gate structure 6 (air gap height) is approximately 1-3 μm. Here, the gate structure 6 as a whole forms the gate electrode G and is made, for example, from highly doped silicon or a highly conductive metal. In the areas that lie opposite the channels of the transistors, the bottom of the gate structure 6 is coated with a sensitive material 8, 8 ′ for the measuringFET and for a non-sensitive material for the compFET. In the example shown, platinum is used for the sensitive layer 8 and titanium for the non-sensitive layer 8 ', so that the gas sensor responds to hydrogen, hydrogen sulfide and other substances which are chemically similar to hydrogen.

The gas or liquid to be examined reaches the air gaps 4, 4 'via the gas inlets 14 in the gate structure. In other (not shown) examples, the air gaps 4, 4 'extend to the edge of the gate structure 6, so that the medium can be supplied from the side. Despite the low air gap height of only 1 to 3 μm, gas exchange takes place in less than one second due to diffusion.

In the example shown, the gate structure 6 lies directly on the substrate 12 and the air gaps 3, 3 'are realized by depressions in the passivation layer 2; In other exemplary embodiments, spacers with a corresponding height of 1 to 3 μm are used for this.

To manufacture the sensor shown, the substrate 12 with the field effect structures and the gate structure 6 are first produced separately from silicon wafers. The gate can also be made from another material, for example plastic. The wells 11, 11 'and the S and D regions are doped by a standard microelectronic method such as diffusion, ion implantation or epitaxy. When assembling the two components, the gate structure 6 is positioned upside down with the aid of a swivel arm on the substrate 12, which is held on a heating plate. Using a beam splitter optics and a cross table, the components are positioned laterally Before the swivel arm is flipped over and the gate structure 6 comes to lie at the desired position above the substrate 12. For permanent fixation, the connection using a two-component adhesive - designated 16 in the drawing - has proven to be the most reliable, simplest, and most cost-effective option. The height h of the adhesive space, which is filled by the adhesive, is approximately 20 μm. After the gate structure has been fixed on the substrate, the contact surfaces (not shown) of the source and drain regions of the field effect structures and the gate electrode are contacted and connected, for example, to a sensor circuit shown in FIG. 6.

In the gas sensor shown in Fig. 1, the channels 3, 3 'in each of the field effect structures KompFET and MeßFET are protected by a guard electrode 10 against electrical interference, which is applied to the passivation layer 2 in the area of the air gap 4, 4' and in this Level encloses the channel 3 or 3 '.

5 shows a top view of one of the transistors KompFET or MeßFET. The field effect structure shown has the special feature that the channel 3 does not run in a straight line between drain and source regions, but in a meandering shape. This increases the width-to-length ratio W / L of the transistor with the same overall size! and thereby increases the sensor signal. With a channel length L of 0.2 μm and a channel width of 2 mm increased by the meandering, a ratio W / L of 10000 results, for example.

The channel region 3 as a whole is surrounded by a guard electrode 10. The top view of the guard electrode is shown here as a solid rectangle, but other configurations such as e.g. an open ring or several individual electrodes possible.

The SGFET shown is operated with a sensor circuit, the principle of which is shown in FIG. 6. The open-drawn gates of the KompFET and the MeßFETs symbolize the hybrid structure of the gas sensor with an air gap. With this circuit, the gas sensor operates in so-called feedback mode, ie the drain current I _D s is kept constant at a constant drain voltage U _D s of, for example, 100 mV by readjusting the gate voltage U _G , for example to approximately 100 μA. For this purpose, the drain currents I _{DS of} the KomFET and the measuring FET are each converted into an equivalent voltage in an I / U converter and the two voltages compared in an integrator. chen. Possibly. the two voltages are scaled differently before the comparison. The integrator always adjusts the gate voltage (there is usually the same gate voltage on both transistors) when one of the two input voltages from the I / U converter changes. It stops regulating when the input voltages are the same again. The size of the readjusted gate voltage ΔU _G is output as a sensor signal.

In the event of a temperature change, the currents bs of the compFET and of the measuring FET change in the same way, so that both input voltages of the integrator also change proportionally to one another. The integrator is therefore not active, the gate voltage is not readjusted and therefore no sensor signal is output. The sensor signal is therefore not temperature-dependent. When a substance to be detected is acted upon, only the drain current bs of the measuring FET and thus the associated input voltage at the integrator changes, and this now adjusts the gate voltage until the original current flows again in the channel.

Expressed in the physical quantities of the above formulas, a change in the contact potential by Δφ can be regarded as a shift in the threshold voltage UT. In order that the drain current can be kept constant, the gate voltage must be readjusted by a value ΔU _G , which means that the

bs = μ CW / L ((U _G + ΔU _G ) - (U _τ + Δφ)) U _D s = const.

results.

If the contact potential of the sensitive layer changes by Δφ when gas is applied, this can be done directly by changing the gate voltage

ΔU _G = Δφ

be measured.

Corresponding readjustment of the gate voltage is of course also possible in the case of a gas sensor without a KompFET, in which case half of the circuit shown is omitted. The gate voltage U _G and the sensor signal is easy then readjusted in ^¬ mer, when the drain current changes in MeßFET. Even without that The sensor can be readjusted by reading out the changing drain current bs.

FIG. 7 shows such a gas sensor according to the invention with only one field effect structure, and configurations of the guard electrode 10 that are slightly modified compared to FIG. 1. In this example too, the gate structure rests on a passivation layer 2 of a substrate 12, the air gap 4 again being formed by a Depression in the passivation layer 2 is realized. A step 20, on which the guard electrode 10 is arranged, is formed in the side walls 18 of the depression, which run approximately along the boundaries between the channel and the source and drain regions. The step, like the lateral spacing from the channel in the example of FIG. 1, serves to minimize the interference of the guard electrode on channel 3. In the embodiment of FIG. 7b, the guard electrode 10 is processed as the highest layer on the field effect structure and therefore extends up to the sensitive layer 8 and thus also protects it from disruptive charge shifts. The guard electrode 10 can be electrically connected to the gate electrode G so that as far as possible no electrical interference field and thus leakage currents can form on the surfaces. This helps to keep all the surfaces surrounding the air gap 4 at the same potential, so that the only change in potential that the work function change on the sensitive layer 8 has on the drain current in the channel. Alternatively, the guard electrode potential can be kept at a constant potential, e.g. Mass.

8 schematically shows a form of a CCFET which is modified compared to DE 43 33 875 C2. In contrast to DE 43 33 875 C2, the extended gate electrode 22 is buried in the hatched passivation layer 2 and thus encapsulated, so that it is less susceptible to interference from the presence of gases or from electrical currents and charges. The voltage of the gate electrode protected in this way therefore "floats" less. The gas-sensitive layer is attached to a carrier above it and separated by an air gap through which the gas flows. The sensor shown is equipped with a sensitive layer 8 made of a platinum-containing material, a measuring and a KompFET as well as guard electrodes 10 according to the type of FIG. 7b, but the applicants reserve the right to claim the modified structure of the CCFET independently of these features , A sensor cover 6 is arranged above the passivation layer using, for example, support feet 26 and is fastened, for example, by adhesive (not shown). The support feet are designed as elevations on the sensor cover 6, the cross-section of which is as small as possible in the plane of the passivation layer 2, so that no dust particles that change the distance are caught between the support feet 26 and the passivation layer 2 during assembly. The underside of the sensor cover is coated in the area of the air gap 4 of the measuring FET with a sensitive layer, for example platinum, and in the area of the air gap 4 'of the KompFET with a non-sensitive layer, for example titanium. In the example shown, the titanium layer covers the entire underside of the sensor cover, so that a titanium layer runs between the platinum layer 8 'and the sensor cover 6, which facilitates the adhesion of the platinum layer to the sensor cover. The thickness of the titanium layer is, for example, 20 nm, that of the platinum layer 100 nm.

The functioning of the sensor is as follows: If gas molecules are adsorbed on the sensitive or non-sensitive layer 8, 8 ', their work function changes. The change in contact potential at the layer 8, 8 ′ acts across the air gap 4 on the buried gate electrode 22 and is transmitted through the electrode 22 to the part 22a of the electrode lying above the channel region 3. Since there is no air gap between the part 22a and the channel 3, the potential change couples into the channel 3 with a large capacitance and thus causes relatively large changes in the drain current. The signal-to-noise ratio should therefore be at least as good as that of the HSGFET variant. The calculation and referencing of the sensor signal can e.g. with the circuit shown in Fig. 6.

The so-called capacitive coupling of the change in contact potential takes place via a capacitance formed by the layer 8 and the gate electrode 22, which is connected in series with a capacitance between the gate electrode 22 and a doped well 24 (called CC well) in the substrate. By differently positioning the buried gate electrode 22 within the passivation layer 2, these two capacitances can be varied and the capacitive voltage divider formed thereby can be set differently. Due to the capacitive voltage divider layer 8, gate electrode 22 and CC well 24, some signal is lost.

The diagrams shown in FIGS. 9 and 10 demonstrate the good results of a sensor of the type of the example of FIG. 1 or 7 with regard to the compensation of moisture and temperature influences. 9 shows a measurement curve 30 of the drain currents I _DS for a gas sensor according to the type of FIG. 7, that is to say without a compensation FET, but with a guard ring, in the case of humidity between alternately 0% and between 10 and 90%. In comparison to the prior art (see FIG. 3), a significantly more stable baseline is shown; only a rash can be seen, which is caused by the known effect that water molecules accumulate on the sensitive layer and / or the passivation layer and thereby cause a sensor signal. Curve 32, on the other hand, shows the estimated (not measured) sensor signal of a gas sensor according to the type of the example from FIG. 1, which is equipped with guard electrodes and also has a KompFET with a non-sensitive layer, which has approximately the same sensitivity to moisture as the sensitive one Has layer of the MeßFETs. With this compensation, the moisture signal can be suppressed almost to the level of the noise. The gas sensor used had a sensitive layer 8 made of platinum and a non-sensitive layer made of titanium.

Fig. 10 shows the temperature curve of the drain currents (channel currents) in the channel of MeßFETs, measuring l _D, and KompFETs, b Ref, as well as the difference .DELTA.I _D of these currents, at a gate voltage U _G = -1V and a drain voltage of U _D = - -200mV, measured in a gas sensor according to the invention according to the example of FIG. 1. It can be seen from the figure that the drain currents in the measurement and reference FET have approximately the same temperature dependency, so that the temperature dependency changes when the two quantities are offset can reduce the factor 20.

Claims

Expectations

1. Gas sensor for the detection of one or more substance (s) in a gas and / or a liquid, which works on the principle of the work function measurement, with: - a first field effect structure (MessFET) with a channel (3) between a source and a drain area; and a first gate electrode (G) with a sensitive material (8), whereby by changing the work function of the sensitive material, e.g. by the adsorption of molecules of the substance on the surface of the sensitive material (8), the drain current in the channel (3) of the measuring FET can be influenced; characterized in that the gas sensor has a second field effect structure

(Compensation FET) with a channel (3 ') between a source and a

Has drain area for compensation of the temperature response and / or the humidity and / or cross gases of the drain current (I _DS ) of the measuring FET.

2. Gas sensor according to claim 1, wherein the drain current (bs) in the channel (3 ') of the compensation FET can be influenced by a change in the work function of a second gate electrode which contains a material (8') which is not sensitive to the substance.

3. Gas sensor according to claim 2, wherein the sensitive material (8) and the non-sensitive material (8 ') have approximately the same cross-sensitivities, in particular approximately the same sensitivity to moisture.

4. Gas sensor according to claim 2 or 3, wherein the non-sensitive material (8 ') in the gate electrode of the compensation FET contains titanium.

5. Gas sensor according to one of the preceding claims, wherein the sensitive material (8) in the gate electrode (6) of the measuring FET contains platinum.

6. Gas sensor according to one of claims 2 to 5, wherein the sensitive material (8) and the non-sensitive material (8 ') are applied to a common gate structure (6) through an air gap (4) from the channel areas (3 , 3 ') of the measuring and compensation FETs is separated.

7. Gas sensor according to one of the preceding claims, wherein the measuring and the compensation FET each have separate drain and source regions (S, D).

8. Gas sensor according to one of the preceding claims, which has a sensor circuit with which a difference between the drain current (bs) of the measuring FET and the drain current (bs) of the compensation FET, or another linear combination of these currents, by a readjustment of the potential at the Gate electrode (U _G ) of the measuring FET can be kept constant.

9. Gas sensor for the detection of one or more substance (s) in a gas and / or a liquid, which works on the principle of work function measurement, in particular according to one of the preceding claims, with a field effect structure (MessFET) with a channel (3) between a source and a drain region; and a gate electrode (G) with a sensitive material (8), whereby by changing the work function of the sensitive material, e.g. by the adsorption of molecules of the substance on the surface of the sensitive material (8), the drain current in the channel (3) can be influenced; characterized in that the channel (3) of the measuring FET is surrounded by a guard electrode (10) for protection against electrical interference.

10. Gas sensor according to claim 9 and one of claims 1-8, wherein the channel (3 ') of the compensation FET is also surrounded by a guard electrode (10).

11. Gas sensor according to claim 9 or 10, wherein the guard electrode (10) forms a closed ring (guard ring) around the respective channel area (3,3 ').

12. Gas sensor according to one of claims 9-11, wherein the gate electrode (10) by an air gap (4) from the field effect structure (MeßFET, KompFET) spaced and the guard electrode (10) is arranged on a passivation layer (2) applied to the field effect structure, for example vapor-deposited or sputtered on.

13. Gas sensor according to claim 12, wherein the guard electrode (10) is arranged on a step (20) embedded in the passivation layer (2).

14. Gas sensor according to claim 12 or 13, wherein the guard electrode has direct contact with the sensitive layer (8) and / or the non-sensitive material (8 ').

15. Gas sensor according to one of claims 9-14, in which the guard electrode is stable at a constant potential (UK), or in which the guard electrode (10) is at the same potential as the gate electrode (G).

16. Gas sensor according to one of the preceding claims, which is designed as a Suspended Gate FET (SGFET), in particular as a Hybrid Suspended Gate FET (HSGFET).

17. Gas sensor according to claim 16, wherein the channels (3, 3 ') of the measuring and possibly the compensation FET are meandering.

18. Gas sensor according to one of claims 1-14, which is designed as a capacitive controlled FET (CCFET).

19. Method for the detection of one or more substance (s) in a gas and / or a liquid according to the principle of the work function measurement using a first field effect structure (MessFET) with a channel (3) between a source and a drain region; and a first gate electrode (G) with a sensitive material (8), the drain current in the channel being changed by changing the work function of the sensitive material (8), for example by the adsorption of molecules of the substance on the surface of the sensitive material (8) (3) the measuring FET is affected; characterized in that the temperature response and / or the humidity and / or cross gases are compensated for by a comparison with the drain current (bs) in the channel of a second field effect structure (KompFET).

20. The method according to claim 19, wherein a difference between the drain current (I _DS ) of the measuring FET and the drain current (bs) of the compensation FET, or another linear combination of these currents, is constant by readjusting the potential (Ugate) at the gate electrode of the measuring FET is held.

21. A method for the detection of one or more substance (s) in a gas and / or a liquid according to the principle of the work function measurement, in particular according to one of claims 19-21, with the aid of a field effect structure (MessFET) with a channel (3) between one Source and a drain region; and a gate electrode (G) with a sensitive material (8), whereby by changing the work function of the sensitive material (8), e.g. by the adsorption of molecules of the substance on the surface of the sensitive material, the drain current (bs) in the channel (3) of the measuring FET is influenced; characterized in that the channel of the measuring FET is protected from electrical interference by a guard electrode (10) surrounding the channel.

22. The method according to claim 21, wherein the potential of the guard electrode (10) is kept constant or is equated to the potential of the gate electrode (G) of the measuring FET.

23. The method according to any one of claims 19-22, which is carried out with a gas sensor according to one of claims 1-18.