DE10161214B4 - Gas sensor and method for the detection of hydrogen according to the principle of work function measurement, and a method for producing such a gas sensor - Google Patents

Abstract

Gas sensor, which works on the principle of work function measurement, for the detection of hydrogen and / or other gases, the molecules of which contain hydrogen, the gas sensor comprising a sensitive layer (8) and the adsorption of molecules of the gas to be detected on the sensitive layer Work function of the sensitive layer (8) can be changed, characterized in that the sensitive layer (8) has platinum and is separated from an insulator (2) by an air gap (4).

Description

The invention relates to a gas sensor and a method for the detection of hydrogen and / or other gases, the molecules of which contain hydrogen according to the preambles of claims 1 and 21, respectively. Such a gas sensor and such a method are known, for example, from US Pat DE 42 39 319 C2 as well as the post-published DE 100 32 062 A1 known. The invention also relates to a method for producing such a gas sensor.

The detection is the Measurement of the presence and / or concentration of the person concerned Gases understood. Since such a gas sensor generally for examination the composition of gases is used in the following Term "gas sensor" used; the sensor is also for the detection of substances in liquids suitable.

Hydrogen (H ₂ ) is a colorless, odorless gas which, at concentrations between 4% and 73% in air, becomes an explosive mixture with the oxygen contained in air. Since no greenhouse gases or other pollutants are released when hydrogen is burned with oxygen to water, hydrogen is a future energy source, for example for motor vehicles, airplanes, rockets and other mobile applications. For example, hydrogen can be obtained from water using solar energy and may replace fossil energy sources in the future.

However, the use of hydrogen as a mobile energy source is fraught with dangers, since even small amounts of hydrogen escaping from the energy store can lead to an explosion. In order to contain this danger, gas sensors could be used for control purposes, with which the hydrogen concentration in the vicinity of an energy store is measured and any leaks in the energy store can thus be detected. Due to the rapid dilution of hydrogen in air, these must respond well below 4% H ₂ concentration, for example at 1000 ppm (0.1%). Hydrogen is also a key gas for fire detection.

As gas sensors for hydrogen in the stand technology e.g. electrochemical cells used, but only a limited lifespan of typically one year, one low measurable Concentration range and high cross-sensitivity in relation to have other gases. Furthermore, conductivity sensors are known which consist of a gas-sensitive semiconductor, which when deposited of hydrogen changes its conductivity. such Need sensors however, due to their high working temperature of over 200 ° C, a high one Heating power and are therefore for Applications unsuitable, their high energy consumption with the relative low energy densities of batteries cannot be covered for adequate periods of time, e.g. in a cell phone, laptop or in a stationary car.

Another type of gas sensor who work according to the principle of work function measurement In contrast, only a low energy requirement. With such gas sensors the detection is based on the fact that molecules of the detecting substance on the surface of a sensitive material be adsorbed. This changes the work function of the sensitive material and thus that electrical potential, for example by a field effect transistor (FET) structure can be measured. In principle come for the sensitive material all materials from insulator to metal in question.

An example of such a so-called gas sensitive FET (GasFET), such as that from the above DE 42 39 319 C2 is known is schematically in 2 shown. In a field effect structure 1 there is a channel between a source region S and a drain region D. 3 through which a drain current I _DS can flow. Between a passivation layer applied to it 2 and the gate electrode G is an air gap 4 , into which the gas to be examined is diffused or flowed in by external influences (e.g. a pump). On their air gap 4 facing side is the gate electrode 6 with a sensitive material 8th coated, to which molecules of the substance to be detected attach. This creates on the surface of the sensitive layer 8th a dipole layer or a chemical compound and thus an electrical potential, which across the air gap 4 the conductivity of the channel 3 and thus influences the drain current I _DS . From a change in I _{DS, there} can be a change in the contact voltage and thus in the work function change Δφ on the surface of the layer 8th be concluded, which in turn is a measure of the concentration of the substance to be detected.

The drain current is simply calculated from: I DS = 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 _T the threshold voltage, Δφ the contact potential change, Δφ times the elementary charge e the work function change and U _DS the voltage between source and drain (drain voltage). If, for example, the change in the 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 necessary for the production of GasFET's Semiconductor components are 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 commonly referred to as a Suspended Gate FET (SGFET). From the DE 42 39 319 C2 is known a SGFET from two components, namely a substrate with a field effect structure without a gate. and to produce 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 in FIG DE 198 14 857 A1 described (hybrid flip chip FET, abbreviated HFC-FET). The contacting and fixing is done with a conductive adhesive. A slightly different structure is from the DE 43 33 875 C2 known. 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, for example, in the case of a SGFET with the same parameters.

Before the development of gas-sensitive FETs with an air gap (suspended gate FETs), the gate electrode arranged directly above the channel was made of a sensitive material that was permeable to the gas to be detected. The change in work function occurs due to the dissolving of the gas in the sensitive layer. Out I , Lundstrom, "A Hydrogen-Sensitive MOS Field Effecting Transistor" Applied Physics Letters 26, 55-57, 1975, for example, discloses such a sensor with a palladium layer for the detection of hydrogen. However, since the hydrogen in this sensor first has to be dissolved in the palladium film before a sensor signal is triggered, the response times are relatively long. Furthermore, iridium is also known as a sensitive material for hydrogen, which also has long response times.

Work function changes can be an alternative to a GasFET also using the Kelvin method ("Vibrating Capacitor Method") can be measured. Here is a Transducer (e.g. a metal plate) using of a piezo element excited to vibrate. At the end of the transducer is a capacitor plate attached with a gas sensitive material is coated and a second, differently coated, capacitor plate opposite. By the vibration of the capacitor plates generates an alternating current, which is the difference between the work functions on the two capacitor plates depends.

The aim of the present invention is it, an inexpensive and durable gas sensor and an inexpensive one To provide processes using hydrogen, hydrogen sulfide and other gases that are chemically similar are too hydrogen, with short response times and over one wide concentration range can be detected. Another goal is it, a simple and inexpensive To provide methods for producing such a gas sensor.

The invention provides a Gas sensor for the detection of hydrogen, hydrogen sulfide and / or other gases that are chemically similar are ready to hydrogen, which works on the principle of work function measurement works, the gas sensor comprises a sensitive layer and through the adsorption of molecules the work function of the gas to be detected on the sensitive layer the sensitive layer changeable is. The gas sensor includes a sensitive layer that has platinum.

Furthermore, the invention stops Method for the detection of hydrogen, hydrogen sulfide and / or other gases that are chemically similar to hydrogen, in which by the adsorption of the gas on a sensitive Shift caused change the work function is measured. The sensitive layer in turn points Platinum on.

Finally, the invention represents a method for producing a hybrid suspended gate FET (HSGFETs) ready with the following steps: (a) Create a field effect structure (MeßFET or measuring transistor) with a channel between a source and a drain region; (B) Applying a platinum-containing layer to a support; (C) Put on the carrier on the field effect structure, so that the platinum layer the channel is facing and there is an air gap between the two.

Platinum is a precious metal on whose surface e.g. unsaturated organic compounds are easily bound, making the excellent Platinum's catalyst properties explained. Speaks due to its surface activity Platinum quickly changes in hydrogen concentration even at room temperature indicates, but nevertheless has a relatively low sensitivity to moisture on. Furthermore, you can Platinum layers can be produced simply and therefore inexpensively. For the sensitive layer The gas sensor can be made of platinum, both pure and in alloys be used.

The sensitive layer preferably also has titanium. A single layer of a titanium-platinum alloy can be used for this however, a titanium-containing intermediate layer is preferably arranged between a platinum-containing layer and a carrier. The carrier is, for example, a capacitor plate of a Kelvin probe or the gate electrode of an SGFET. The titanium-containing intermediate layer facilitates the adhesion of the platinum-containing layer to the carrier. Since titanium is inert to hydrogen, titanium-containing materials are a good substrate for thin platinum layers, since hydrogen diffused through the platinum layer then does not react on the substrate. Therefore, titanium and titanium alloys are also suitable for the non-sensitive layer in a compensationFET for referencing the sensor signal (see below).

In the preferred embodiments the gas sensor is designed as a GasFET, which is described below Suppression functions of interferences e.g. due to temperature fluctuations and humidity.

On the one hand, it has been shown that the gate voltage U _G drifts strongly at high air humidities. The reason for this is that there is on the surface of the passivation layer 2 and / or the sensitive layer 8th forms a moisture film in which a leakage current can flow between areas 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. Like the one in 3 shows the measurement of the drain current in a SGFET according to the prior art at air humidity of alternately 0% and between 10% and 90%, the baseline of the sensor signal (here the gate voltage U _G ) drifts so much at humidity of over about 40% that a Concentration measurement is no longer possible.

Second, the electron mobility μ is in the channel 3 and the threshold voltage U _{T of} the field effect structure 1 strongly temperature dependent, so that the drain current I _DS decreases sharply with increasing temperature, like that in 4 shown measurement of the drain current in a SGFET according to the prior art at temperatures between -5 ° C and 65 ° C. 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 occurs that 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 I _DS 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 after the principle of work function measurement are so-called cross-sensitivities, i.e. the sensitive layer does not only respond to a single substance. Here falls in particular the change in work function caused by the adsorption of water Weight, as there is always high air humidity at room temperature, the moisture concentration is usually many times higher than that of the substances to be detected.

To suppress the interference effects due to temperature fluctuations, the gas sensor preferably has a first field effect structure with a channel between a source and a drain region (MessFET); and a first gate electrode with a sensitive layer, wherein through a change the work function of the sensitive layer, e.g. through adsorption of hydrogen molecules on the sensitive layer, the drain current in the channel of the measuring FET can be influenced. Moreover the gas sensor has a second field effect structure with a channel between a source and a drain area (compensationFET or compensation transistor) for compensation of the temperature response the drain current of the measuring FET on.

This structure is based on the knowledge that the temperature response of the sensor signal essentially results from the temperature response of the electron mobility μ and the threshold voltage U _T, 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 by comparing the two currents from the measured sensor signal. 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 technological, electrically and geometrically constructed in the same way as the MessFET and can be made on a common substrate with this so that for the Temperature compensation hardly result in additional production costs.

According to one embodiment the drain current in the channel of the compensation FET is not through a Influenceable gate electrode; i.e. in the case of a SGFET, the channel of the compensation FET is not one Covered gate electrode, or the air gap over the channel of the compensation FET is above the air gap MeßFET so widened that the the conductivity of the channel practically no longer due to a change in work function affects the gate electrode becomes. This construction has the advantage that the drain current in the channel of the compensation FET not from possible interfering effects on a gate electrode, but only depends on the temperature.

According to another preferred embodiment of the invention, however, the drain current in the channel of the compensation FET is through a Change in the work function of a second gate electrode can be influenced, which contains a material that is not sensitive to the substance to be detected. Non-sensitive means that the material responds to the substance to be detected at least significantly more valuable than the sensitive material platinum. 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 a material for the non-sensitive layer that responds approximately as strongly to atmospheric moisture as platinum.

For the non-sensitive material in the gate electrode of the compensation FET titanium is therefore preferably used, since titanium over many Gases are inert and platinum and titanium both have approximately the same sensitivity to moisture and have ammonia. Alternatively, silicon nitride can also be used.

These are particularly preferred and the non-sensitive material on a common gate structure applied. The gate structure is e.g. a substrate made of silicon or silicon carbide, which is used to manufacture a SGFET Field effect structure is put on. Advantage of the silicon substrate is its smooth surface. Before touchdown, the gate structure is preferably initially whole coated with a titanium-containing material, and in the area of the Channel of the measuring FET thereupon a platinum-containing layer is applied to the titanium-containing layer. Apart from the general simplicity of manufacture this has the particular advantage that the titanium-containing layer for the sensitive platinum layer serves as an adhesion promoter. Without one such an intermediate layer platinum adheres poorly to silicon.

In some embodiments, the measurement and the CompensationFET each, a common drain or a common Source area on. The drain and source regions are preferred of the two transistors, however, spatially separated from each other so that this each other if possible do not affect. For example, they are in two separate doping wells of a silicon substrate.

In a particularly easy to manufacture embodiment the gate structure consists entirely of a platinum-containing material, e.g. made of platinum.

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 readjustment of 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 reference 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 magnitude 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.

To compensate for the above parasitics due to air humidity, the channel of the measuring FET is preferably one so-called guard electrode to protect against electrical interference.

The guard electrode prevents, for example the radiation of leakage currents and capacitive interference, and it prevents processes charge balance on the surface. In particular, at high humidity due to the formation of a moisture film the passivation layer of the FET and on the sensitive layer leakage currents on the surfaces flow, which is the electrical conductivity of the channel. A falsification of the measurement result would be the result. The guard electrode, for example, kept at constant potential interruption of this charge exchange, and the influence of moisture at least significantly reduced.

The two are particularly preferred above-mentioned Suggested solutions to each other combined, i.e. the gas sensor with a compensationFET as well as with guard electrode (s) to both moisture as well as temperature influences to compensate. Such a sensor also delivers at room temperatures reproducible sensor signals and therefore does not need to be heated to keep the temperature constant and / or the humidity to reduce. The gas sensor therefore has only a small one in operation Energy requirements in the micro to milliwatt range are in the making Cheap and therefore for mobile and battery-powered applications ideally suited.

The guard electrode preferably forms a closed ring (guard ring) around the channel of the measuring FET. If is present, the compensation FET with its own is also preferred Guard electrode or a guard ring. Alternatively, you can a single guard electrode surround both channels.

The guard electrode consists of a conductive higen material, preferably a metal, such as aluminum, platinum or gold, and can be applied by any thin-film process on an insulator or passivation layer on the field effect structure, for. 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, as a result of which atoms are shot out of the target and are deposited as a homogeneous, compact layer on the surface to be coated. The thickness of the guard electrode thus produced is, for example, between 10 and 500 nm.

Is preferred in the isolator or Passivation layer recessed around the channel, on which the guard electrode is arranged. This arrangement offers especially when in the passivation layer covering the field effect structure above the A recess is arranged and the step in the side walls of the channel Deepening is integrated. With a SGFET, a recess can serve as a spacer for the gate electrode in the passivation layer.

The one enclosed by the guard electrode is advantageous Area if possible small so that there are no charges within this range or leakage currents flow can. The guard electrode should therefore be placed as close as possible to the channel become. However, it has been shown that the potential of the guard electrode - if this e.g. is kept at a constant potential - a disruptive influence on the Apply drain current in the sewer can. The guard electrode is therefore preferably so far from the respective one Channel spaced that the Drain current in the channel is not significant due to the potential of the guard electrode affected becomes. Preferably is the distance 1 to 15 μm, e.g. approx. 5 μm.

Another preferred way to exclude, that this Potential of the guard electrode has an interference effect on the drain current in the channel, is the potential of the guard electrode to the potential of the Equate gate of the measuring FET. In this way there are none between the gate and guard electrodes Differences in time and therefore none in the air gap The guard electrode generated electrical fields that have a leakage current trigger could. A different possibility is to put the guard electrode at constant potential, e.g. 0V (ground)

The gas sensor of the present invention can both as a Kelvin probe, as a Suspended Gate FET (SGFET), or as Capacitive controlled FET (CCFET) can be formed. In particular with SGFET are the channels of the measuring and possibly the compensation FET preferably meandering, i.e. the area between The source and drain areas are in the plane parallel to the passivation layer serpentine. This results in a space-saving use of the substrate surface favorable Width-length ratio W / L of the transistor of 10,000, for example, so that a high Signal-to-noise ratio is achievable. With the CCFET, such a broadening of the Channel is not absolutely necessary because of the change in work function evoked potential change not over here transmit an air gap is and therefore with a larger capacity C in the Coupled channel. An alternative construction of the CCFET is preferred used where the extended Gate electrode through which the potential change on the sensitive material electrically into the channel of the measuring FET is coupled in completely covered by a passivation layer and therefore less exposed to interference is. This causes voltage fluctuations ("floating") of the gate electrode reduced.

The sensitive layer is preferably and optionally the titanium-containing intermediate layer is formed as thin layers. The Layers are e.g. by electrochemical deposition, sputtering, reactive sputtering, vapor deposition, spin coating, sublimation, epitaxy or spraying on the carrier applied, whereby layers with approx. 10-500 nm layer thickness can be created are. Alternatively, however, thick-film technology can also be used, e.g. For this purpose, platinum or titanium atoms are placed in a polymer layer brought in.

The gas sensor is preferably used in an application in which low power consumption is important, for example a motor vehicle in the idle state or in systems in which the functionality must be ensured if the power supply fails. It is also intended to transmit the sensor signal to a monitoring station by radio. Compared to known sensors for hydrogen detection, the gas sensor is distinguished by an extremely low energy requirement and is therefore also suitable for battery operation. The explosion protection conditions are also much easier and cheaper to meet with a sensor that works like the gas sensor at room temperature or only slightly above it. For example, the gas sensor in a motor vehicle with hydrogen as the fuel can be used to detect leaks in the energy store. Typical concentrations to be detected are then 100 ppm to 4% hydrogen concentration. The gas sensor can also be used for leak detection in ultra-high vacuum systems. To do this, the system is filled with hydrogen and typical leaks result in a gas concentration of around 10 ppm. A portable device with low power consumption is also advantageous here. Another application is for example oil-cooled high-voltage transformers. With these, hydrogen can split off from methane in the oil, for example in the event of a sparkover. The hydrogen can become a correspondingly high concentration Explosion of the transformer. Here the sensor is suitable for monitoring the hydrogen content.

The invention is now based on embodiments and the accompanying drawing explained. The drawing shows:

1 a schematic sectional view through a gas sensor according to a first embodiment;

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

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

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

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

6 a schematic diagram of a sensor circuit;

7a . b schematic sectional views of a second embodiment of a gas sensor;

8th a schematic sectional view of a third embodiment of a gas sensor;

9 a diagram of the drain current in a gas sensor with guard electrode and in a gas sensor with guard electrode and compensation FET at various relative atmospheric humidity;

10 a diagram of the drain currents in the compensation and MessFET and their difference in a gas sensor with temperature compensation as a function of temperature;

11 a diagram of the sensor signal U as a function of the hydrogen concentration present;

12 a diagram of the sensor signal U against time t with different hydrogen partial pressures p present;

13 a bar graph of the change in work function of platinum when exposed to various gases at room temperature and at 130 ° C.

14a . b schematic sectional views of a fourth embodiment of a gas sensor during manufacture (a) and in the finished state (b);

15 Current-voltage characteristics of a field effect structure 8th at different voltages U _k .

Functionally identical or similar Parts are identified in the drawing with the same reference symbols.

1 shows a gasFET according to the invention in a suspended gate type, which has temperature compensation by means of a compensationFET (KompFET) as well as guard electrodes 10 Is provided. For the sensitive layer 8th of the measuring FET (measuring FET) became platinum in this example and for the non-sensitive layer 8th' of the compensation FET uses titanium so that the gas sensor responds to hydrogen, hydrogen sulfide and other substances that are chemically similar to hydrogen. The titanium layer 8th' covers the entire bottom of a gate structure 6 , and in the area above the measuring FET there is a sensitive platinum layer on the titanium layer 8th applied.

The measuring FET and the compensation FET are in two separate, eg p-doped wells 11 and 11 ' in a silicon substrate 12 arranged. The p-doped trough runs between the corresponding n ⁺ -doped source S and drain regions D of the measuring FET and the compFET 11 . 11 ' one channel each 3 . 3 ^` , The arrangement of the two field effect structures in separate tubs 11 . 11 ' has the advantage that the FETs cannot influence each other electrically. In particular, no current can flow between the transistors, since there are limits on the wells 11 . 11 ' to the substrate 12 Form barrier layers. In addition, the field effect structures can be applied, for example, by applying a voltage to the tubs 11 . 11 ' specifically influenced in their electrical properties, in particular their threshold voltage U _T.

On the substrate 12 is a passivation layer 2 applied, which on the one hand electrically isolates the field effect structures and on the other hand protects them from environmental influences such as oxidation. Silicon nitride is preferably used for the passivation, since it is inert to most substances, ie it does not show its own work function change when gas is applied. Over the canals 3 . 3 ' are in the passivation layer 2 Wells are arranged, each by part of the gate structure 6 (also called sensor cover) are covered and therefore the air gaps 4 . 4 ' form. The passivation layer is in the area of the depressions 2 just a few microns thick, and the distance between the passivation layer 2 and gate structure 6 (Air gap height) is approx. 1-3μm. The gate structure 6 here forms the gate electrode G as a whole and is made, for example, of highly doped silicon or a highly conductive metal. The gate structure is in the areas opposite the channels of the transistors 6 on the underside of the measuring FET with a platinum layer 8th and with the KompFET with a titanium layer 8th' coated.

The gas or liquid to be examined reaches the air gaps 4 . 4 ' about the gas inlets 14 in the gate structure. In other examples (not shown), the air gaps extend 4 . 4 ' to the edge of the gate structure 6 , so that the medium can be fed 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 lies door 6 directly on the substrate 12 on and the air gaps 3 . 3 ' are formed by depressions in the passivation layer 2 realized; In other exemplary embodiments, spacers with a corresponding height of 1 to 3 μm are used for this.

The substrate is used to manufacture the sensor shown 12 with the field effect structures and the gate structure 6 initially made separately from silicon wafers. The gate can also be made from another material, for example plastic. The tubs 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 upside down with the help of a swivel arm on the substrate 12 positioned, which is held on a hot plate. Using a beam splitter optics and a cross table, the components are positioned laterally before the swivel arm is folded and the gate structure 6 at the desired position above the substrate 12 comes to rest. 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, for example, to an in 6 shown sensor circuit connected ..

At the in 1  The gas sensor shown are the channels 3 . 3 '  in each of the field effect structures KompFET and MeßFET by a guard electrode 10  protected against electrical interference, the each in the area of the air gap 4 . 4 '  on the passivation 2  is applied and in this level the channel 3  respectively. 3 '  encloses. The smallest distance d between the channel areas 3 . 3 '  and the guard electrodes 10  is at least 5, preferably 10 μm, so the guard electrode has no control on the channel 3 . 3 '  exercises.

5 shows a plan 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 areas, but meandering. 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 10,000 results, for example.

The channel area 3 is at a distance of 10 μm as a whole from a guard electrode 10 surround. The top view of the guard electrode is shown here as a continuous rectangle, but other configurations such as an open ring or several individual electrodes are of course also possible.

The SGFET shown is operated with a sensor circuit, the principle of which is shown in 6 is shown. 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 works in the so-called feedback mode, ie the drain current I _DS is kept constant at a constant drain voltage U _DS 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 KompFET and the measuring FET are each converted into an equivalent voltage in an I / U converter and the two voltages are compared in an integrator. Possibly. the two voltages are scaled differently before the comparison. The integrator adjusts the gate voltage (usually the same gate voltage is present at both transistors) when one of the two input voltages from the I / U converters 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 change in temperature, the currents I _{DS of} the compFET and of the measuring FET change in the same way, which means 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 I _{DS 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 U _T. In order for the drain current to be kept constant, the gate voltage must be readjusted by a value ΔU _G , which means that I DS = μ CW / L ((U G + ΔU G ) - (U T + Δφ)) U DS = Const. results.

If the contact potential of the sensitive layer changes by Δφ when gas is applied, this can be done directly on the basis of the change in the gate voltage .DELTA.U G = Δφ be measured. A corresponding readjustment of the gate voltage is of course also possible with a gas sensor without a compensation FET, in which case half of the figures shown are omitted Circuit. The gate voltage U _G and thus the sensor signal is simply readjusted whenever the drain current changes in the measuring FET. Even without the readjustment, the sensor can be operated by reading out the changing drain current I _DS .

7 shows such a gas sensor according to the invention with only one field effect structure, and opposite 1 slightly modified designs of the guard electrode 10 , In this example, too, the gate structure lies on a passivation layer 2 of a substrate 12 on, the air gap 4 again through a recess in the passivation layer 2 is realized. In the side walls 18 the depression, which runs approximately along the boundaries between the channel and the source and drain regions, is a step 20 trained on the guard electrode 10 is arranged. The step serves as well as the lateral distance from the channel in the example of the 1 , in addition, the interference of the guard electrode on the channel 3 to minimize. In the embodiment of the 7b is the guard electrode 10 Processed as the highest layer on the field effect structure and therefore extends to the sensitive layer 8th up and thus also protects them from disruptive charge shifts. So that in the air gap 4 If possible, the guard electrode can form no electrical interference field and thus leakage currents on the surfaces 10 be electrically connected to the gate electrode G. This helps all the air gap 4 enclosing surfaces at the same potential so that the only change in potential is the change in work function on the sensitive layer 8th affects the drain current in the channel. Alternatively, the guard electrode potential can be kept at a constant potential, for example ground.

In 8th is schematically one against the DE 43 33 875 C2 modified form of a CCFET shown. In contrast to DE 43 33 875 C2 here is the extended gate electrode 22 into the hatched passivation layer 2 buried 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 mounted on a support above and separated by an air gap through which the gas flows. The sensor shown is with a sensitive layer 8th made of a platinum-containing material, a measuring and a compensation FET as well as guard electrodes 10 by type of 7b equipped, but the applicants reserve the right to claim the modified structure of the CCFET regardless of these features.

There is a sensor cover over the passivation layer 6 with the help of e.g. feet 26 arranged and fastened, for example, by adhesive (not shown). The support feet are as elevations on the sensor cover 6 formed, whose cross section in the plane of the passivation layer 2 is as small as possible, so when mounting between support feet 26 and passivation layer 2 no dust particles are caught that change the distance. The underside of the sensor cover is 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 compensation FET is coated 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 between the platinum layer 8th' and sensor cover 6 there is a titanium layer that facilitates the adhesion of the platinum layer to the sensor cover. The thickness of the titanium layer is 20 nm, for example, that of the platinum layer 100 nm.

The functioning of the sensor is as follows: Are on the sensitive or the non-sensitive layer 8th . 8th' Adsorbed gas molecules, their work function changes. The change in contact potential at the layer 8th . 8th' acts through the air gap 4 away on the buried gate electrode 22 and is through the electrode 22 to the one above the canal area 3 lying part 22a transferred to the electrode. Because between the part 22a and the channel 3 there is no air gap, but couples the potential change with large capacitance into the channel 3 and 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, for example, with the in 6 shown circuit.

The so-called capacitive coupling of the change in contact potential takes place here via one of the layers 8th and the gate electrode 22 formed capacitance with a capacitance between the gate electrode 22 and a doped tub 24 (called CC-Well) is connected in series in the substrate. By different positioning of the buried gate electrode 22 within the passivation layer 2 these two capacitances can be varied and the capacitive voltage divider formed by them can be set differently. Through the capacitive voltage divider layer 8th , Gate electrode 22 and CC-Well 24 you lose some signal.

To those in the substrate 12 under the air gap 4 doped tub 24 a voltage U _{k can also be} applied. By changing the voltage U _k , the threshold voltage U _{T of} the FET can be shifted, which is equivalent to a shift in the current-voltage characteristic of the FET. This is in 15 shown as an example, in the I _DS against the gate voltage U _G at different voltage U _k on the tub 24 is applied. While U _{K is} varied between 0 and 0.5 volts in 0.025 V steps, U _T shifts by a total of approx. 15 V. A change in U _k by approx. 25 mV results in a shift in the threshold voltage by 0.5 V. Effect can be used, for example, to differentiate the characteristics due to technology to compensate for the manufacturing tolerances between the measuring FET and the compensation FET or between different gas sensors on the wafers or batches. You can also by the voltage U _k on the tub 24 shift the threshold voltage U _T so far that the FET blocks and is thus effectively switched off. In this idle state, it does not consume any current even though the voltages and potentials are still present. In this operating mode, the gas sensor can be switched on and measured immediately.

Setting the properties of the field effect structure via U _k is also advantageous if the gas sensor is heated slightly above the ambient temperature by the drain current I _DS . In this case, the heating can be regulated without power via the gate. This light heating of the gas sensor - which of course also in the embodiments of the 1 and 7 can be carried out - is particularly advantageous since it prevents condensation of, for example, water in the air gap, but consumes only little energy, since the sensor temperature should only be slightly above the ambient temperature. In addition, the above-mentioned temperature compensation by a compensation FET works even more effectively, the lower the temperature fluctuations are. A relatively strong drain current I _{DS is} set to heat the gas sensor. In order to be able to detect gases at the same time and to regulate the heating power, in the embodiment the 8th the characteristic of the measuring FET by adjusting the voltage U _k on the tub 24 are shifted such that the measuring FET, in spite of different source-drain voltages and currents, also responds to changes in the contact potential at the sensitive layer with the aid of the shift in the threshold voltage. Alternatively, the gas sensor can also be operated in such a way that the regulation of the heating is carried out alternately in a timed manner or the gas sensor is read out.

In 14a and are shown a further construction variant of the gas sensor, in which the gate electrode is made entirely of a platinum-containing material 8th exists, i.e. in the finished sensor ( 14b ) no carrier or gate structure 6 is available. The platinum layer 8th rather lies directly on a passivation layer 2 on, to form an air gap 4 a depression above the canal 3 of the measuring FET is embedded. A support structure 6 is however used as an assembly aid for the platinum layer 8th used. For this purpose, a support structure 6 , for example a silicon substrate, coated with a platinum layer and in the same way as a gate structure 6 on the substrate 12 positioned ( 14a ). Because the platinum layer 8th does not adhere well to the silicon substrate, it falls off the carrier 6 and comes correctly positioned on the passivation layer 2 of the substrate 12 to lie ( 14b ). The carrier 6 is then removed and the platinum layer 8th contacted. In this case, the associated reference transistor works, for example, without a gate. The preferred readout method would then be to measure the change in current of I _DS .

11 and 12 demonstrate the suitability of a gas sensor with a platinum layer for the detection of hydrogen. 11 shows the sensor signal U of a GasFET of the type of 1 with a 100nm thick platinum layer depending on the hydrogen concentration at room temperature: as you can see, in the concentration range between 0 and 2% there is an almost logarithmic relationship between the sensor signal and the H ₂ concentration, whereby the sensor is particularly suitable at low hydrogen concentrations of less than 1% is sensitive. 12 shows the sensor signal at different hydrogen concentrations as a function of time, which shows the time response of the sensor. The response time of the sensor is in the range of a few seconds, the decay time is approx. 30 seconds.

13 shows the cross-sensitivities of the platinum layer with respect to the gases NO ₂ , NH ₃ , CO, SO ₂ , Cl ₂ and O ₃ , as well as the sensitivity to N ₂ , at room temperature (gray bars) and at 130 ° C (black). The diagram shows that a platinum-containing layer as a sensitive layer for a gas sensor can cover two gases according to the principle of work function measurement: at room temperature it is very sensitive and selective to hydrogen, at a temperature of 130 ° it shows good sensitivity and selectivity to ozone.

The in the 9 and 10 The diagrams shown demonstrate the good results of a sensor in the manner of the example of 1 or 7 with a sensitive layer 8th made of platinum and possibly a non-sensitive layer made of titanium with regard to the compensation of moisture and temperature influences. 9 shows a measurement curve 30 of the drain current I _DS in a gas sensor of the type of the example of 7 , i.e. without compensation FET, but with guard ring, with humidity between 0% and between 10 and 90%. Compared to the state of the art (see 3 ) shows a much more stable baseline; 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 in the manner of the example of 1 , which is equipped with guard electrodes and also has a compensation FET with a non-sensitive layer which has approximately the same sensitivity to moisture as the sensitive layer of the measuring FET. With this referencing, the moisture signal can be suppressed almost to the level of the noise.

10 shows the temperature profile of the drain currents (channel currents) in the channel of the measuring FET according to the invention, measuring I _D , and the Kompensati onsFETs, Kom I _D , as well as the difference ΔI _{D of} these currents, with a gate voltage of U _G = -1 V and a drain voltage of U _D = - 200 m V, measured with a gas sensor similar to that 1 , From the figure it can be seen that the drain currents in the measurement and compensation FET have approximately the same temperature dependency, so that the temperature dependency can be reduced by a factor of 20 by calculating the two variables.

Claims (24)

Gas sensor, which works on the principle of work function measurement, for the detection of hydrogen and / or other gases, the molecules of which contain hydrogen, the gas sensor comprising a sensitive layer ( 8th ) and by the adsorption of molecules of the gas to be detected on the sensitive layer, the work function of the sensitive layer ( 8th ) can be changed, characterized in that the sensitive layer ( 8th ) Has platinum and an air gap ( 4 ) from an isolator ( 2 ) is separated. Gas sensor according to claim 1, wherein the sensitive layer ( 8th ) also has titanium, in particular between a platinum-containing layer and a carrier ( 6 ) a titanium-containing intermediate layer is arranged. Gas sensor according to claim 1 or 2, comprising: a first field effect structure with a channel ( 3 ) between a source and a drain area (MessFET); and - a first gate electrode (G) comprising the sensitive layer ( 8th ), whereby by changing the work function of the sensitive layer ( 8th ) by adsorption of hydrogen molecules or molecules containing hydrogen on the sensitive layer, the drain current in the channel ( 3 ) of the measuring FET can be influenced. Gas sensor according to one of the preceding claims, comprising: a second field effect structure with a channel ( 3 ' ) between a source and a drain area (compensation FET) to compensate for the influence of temperature and / or humidity and / or cross gases on the drain current (I _DS ) of the measuring FET. Gas sensor according to claim 4 with - a second gate electrode which has a titanium-containing layer ( 8th' ), whereby by changing the work function of the titanium-containing layer, the drain current in the channel ( 3 ' ) of the compensation FET can be influenced. Gas sensor according to claim 2 and 5, wherein the titanium-containing layer ( 8th ) and the platinum-containing layer ( 8th' ) on a common gate structure ( 6 ) are applied. 7 gas sensor according to one of claims 3-5, wherein the gate electrode ( 6 ) consists of a platinum-containing material. Gas sensor according to one of the preceding claims, which has a sensor circuit with which a difference between the drain current (I _DS ) of the measuring FET and the drain current (I _DS ) of the compensation FET, or another linear combination of these currents, by a readjustment of the potential at the _Gate electrode (U _gate ) of the measuring FET can be kept constant. Gas sensor according to one of the preceding claims, wherein the channel ( 3 ) of the measuring FET from a guard electrode ( 10 ) is surrounded. Gas sensor according to claim 4 and 9, wherein the channel ( 3 ' ) of the compensation FET from a guard electrode ( 10 ' ) is surrounded. Gas sensor according to one of claims 9 or 10, wherein the gate electrode (G) through the air gap ( 4 ) is spaced from the field effect structure (MeßFET, RefFET) and the guard electrode ( 10 . 10 ' ) on a passivation layer applied to the field effect structure ( 2 ) is arranged. Gas sensor according to claim 11, wherein the guard electrode ( 10 . 10 ' ) on one in the passivation layer ( 2 ) recessed level ( 20 ) is arranged. Gas sensor according to one of Claims 9-12, in which the guard electrode ( 10 ; 10 ' ) is stable at a constant potential (U _K ), or at which the guard electrode ( 10 . 10 ' ) is at the same potential as the gate electrode (G). Gas sensor according to one of the preceding claims, which is suspended Gate FET (SGFET) or as a Hybrid Suspended Gate FET (HSGFET) is. Gas sensor according to one of claims 1-13, which as a capacitive Controlled FET (CCFET) is formed. Gas sensor according to claim 14 or 15, wherein the channels ( 3 . 3 ' ) of the measuring and compensation FETs are meandering. Gas sensor according to claim 1 or 2, which is designed as a Kelvin electrode is. Gas sensor according to one of the preceding claims, wherein the platinum-containing sensitive layer ( 8th ) and the titanium-containing intermediate layer ( 8th' ) are designed as thin layers. Gas sensor according to one of the preceding claims for use in a System or in a tunnel, for that low power consumption is required. Method for the detection of hydrogen and / or other gases, the molecules of which contain hydrogen, in which the change in the work function caused by the adsorption of the gas to be detected on a sensitive layer (Δφ⋅e) is measured, characterized in that the sensitive layer ( 8th ) Has platinum, and the gas to be detected through an air gap ( 4 ) to the sensitive layer ( 8th ) arrives. The method of claim 20, which with a gas sensor one of the claims 1-18 executed becomes. Method for producing a hybrid suspended gate FET (HSGFET) according to one of claims 1-16, comprising the following steps: (a) producing a field effect structure (measurementFET) with a channel ( 3 ) between a source and a drain region; (b) application of a platinum-containing layer ( 8th ) on a carrier ( 6 ); (c) putting on the support ( 6 ) on the field effect structure (MeßFET), so that the sensitive layer ( 8th ) the channel ( 3 ) facing and an air gap between the two ( 4 ) consists. The method of claim 22, wherein the carrier ( 6 ) is finally removed and the sensitive layer ( 8th ) remains on the field effect structure (MeßFET). The method according to claim 22, wherein prior to the application of the platinum-containing layer to the carrier ( 6 ) a titanium-containing layer is applied. Method according to one of claims 22-24, wherein the platinum and / or the titanium-containing layer ( 8th . 8th' ) by electrochemical deposition, sputtering, reactive sputtering, vapor deposition, spin coating, sublimation, epitaxy or spraying onto the carrier ( 6 ) is applied.