Abstract:
A sensor for detecting a first component in a gas mixture is disclosed having a gas-sensitive electrode and a catalyst which is arranged on and/or spaced apart from the electrode in a porous carrier ceramic. The catalyst has the effect that a second component in the gas mixture is chemically altered such that the component contributes to no substantial change in the potential of the electrode.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2010/063763, filed on Sep. 20, 2010, which claims the benefit of priority to Serial No. DE 10 2009 046 317.8, filed on Nov. 3, 2009 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     The present disclosure relates to a sensor for detecting at least a first medium in a media mixture comprising at least the first medium and a second medium, to a method for producing the sensor and to a chip having the sensor. 
     For example, sensors in the form of field effect transistors are used, inter alia, to determine gas components in gas mixtures. In this case, a gate electrode of the field effect transistor, for example, is sensitive to the gas components to be determined, thus resulting in a change in the potential of the gate electrode. A resultant change in the current flow between a source electrode and a drain electrode of the field effect transistor is associated with the concentration of a gas component. Such sensors are referred to as ChemFETs. Such ChemFETs are used, in particular in exhaust gas lines of internal combustion engines, to measure, for example, the proportion of nitrogen oxide (referred to as NO x  below) in the exhaust gas, as described in DE 10 2007 040 726 A1. 
     SUMMARY 
     The sensor according to the disclosure, the method according to the disclosure, and the chip according to the disclosure provide the advantage over conventional solutions that the catalyst whose coefficient of thermal expansion generally differs from that of the electrode is arranged at a distance from the electrode on and/or in the fully porous carrier ceramic, and no thermal strain therefore results between the catalyst and the electrode when the latter are exposed to highly varying temperatures, as is typically the case in the exhaust gas line of an internal combustion engine. In addition, the catalyst adheres comparatively well to the porous carrier ceramic on account of the high surface roughness of the latter. Furthermore, a catalytic activity of the electrode is not influenced by the catalyst on account of the distance, thus improving the measurement accuracy of the sensor. 
     Advantageous developments and improvements are also disclosed below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the disclosure are illustrated in the drawing and are explained in more detail in the following description. 
       In the drawing: 
         FIG. 1  shows a sectional view of a sensor according to one exemplary embodiment of the disclosure; 
         FIG. 2  shows a sectional view of a sensor according to a further exemplary embodiment of the disclosure; and 
         FIG. 3  shows a plan view of a chip according to one exemplary embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the figures, identical reference symbols denote identical or functionally identical elements, unless stated otherwise. 
       FIG. 1  shows a sectional view of a sensor  2  according to one exemplary embodiment of the disclosure. 
     According to the exemplary embodiment, the sensor  2  is arranged in or adjacent to an exhaust gas stream  3  in an exhaust gas line (not illustrated any further) of an internal combustion engine. 
     The exhaust gas stream  3  has a media mixture comprising a first medium  4  and a second medium  6 . According to the exemplary embodiment, the first medium is NO x  gas and the second medium  6  is hydrocarbon gas. It goes without saying that the exhaust gas stream  3  may also contain yet further media. 
     The sensor  2  is intended to be used to detect the NO x  gas  4  in the exhaust gas stream  3 . In the present case, the term “detect” is used to mean both the pure detection of the presence of NO x  gas  4  in the exhaust gas stream and preferably the measurement of the quantity of the NO x  gas  4 . For this purpose, the sensor  2  according to this exemplary embodiment is designed as follows: 
     The sensor  2  is, for example, in the form of a ChemFET and has a gate electrode  8  which is in contact with a semiconductor substrate  12  via an insulation layer  10 . 
     The semiconductor substrate  12  may be formed from gallium nitride, aluminum nitride, gallium-aluminum nitride or silicon carbide. According to this exemplary embodiment, it is in the form of silicon carbide. 
     The gate electrode  8  is preferably formed from an oxidation-stable noble metal such as platinum, palladium, gold, iridium, rhenium, rhodium or mixtures thereof and/or from admixture of element compounds of base metals such as hafnium, tantalum and/or aluminum. 
     The gate electrode  8  may also have a porous coating  14  made of a noble metal or from a noble metal/metal oxide mixed material. The material for the porous coating  14  is preferably selected from the transition elements such as hafnium, tantalum, niobium, tantalum, molybdenum, rhodium, platinum, palladium, silver, gold or mixtures thereof. Electrically conductive compounds such as nitrides, carbides or silicides of the transition elements, for example tungsten or tantalum silicide, are also possible. Cermets which, in addition to said transition elements or their carbides or silicides, have a ceramic component, for example aluminum oxide, silicon dioxide, zirconium dioxide, rare earth metals, such as, in particular, magnesium oxide, are particularly suitable as the noble metal/metal oxide mixed material. 
     The electrode  8  preferably comprising the porous coating  14  is set up to change its electrical potential if it comes into contact with the NO x  gas  4  or the hydrocarbon gas  6  in the exhaust gas stream  3  or both gases. In addition to the gate electrode  8 , the sensor  2  has source and drain electrodes (not illustrated), a change in the electrical potential of the gate electrode  8  changing a current in a channel region  15  in the semiconductor substrate  12  between the source and drain electrodes. The change in the current is evaluated in an evaluation unit (not illustrated) in order to determine which medium is present and the quantity in which this medium is present at the electrode  8 . 
     However, since only the NO x  gas is intended to be detected according to the exemplary embodiment, a catalyst  16  is arranged between the electrode  8  and the exhaust gas stream  3 . The catalyst  16  is used to eliminate individual gas components in the exhaust gas stream  3  to which the electrode  8  has an undesirable cross-sensitivity. In this case, the catalyst  16  converts these undesirable gas components into gas components which do not impede a measurement of the gas component to be determined. Therefore, according to the present exemplary embodiment, the catalyst  16  is in the form of an oxidation catalyst which oxidizes the hydrocarbon gas  6  to form carbon dioxide and water. The catalytic oxidation process preferably predominantly takes place at a temperature of the exhaust gas stream  3  in the range between 100 and 650°, preferably between 250 and 550°. The reaction products of carbon dioxide and water do not have a substantial influence on the electrical potential of the electrode  8 . In contrast, the NO x  gas  4  can pass unimpeded through the oxidation catalyst  16  to the electrode  8  where it contributes to a change in the electrical potential of the latter. 
     According to the exemplary embodiment, the catalyst  16  is in the form of a multiplicity of particles each having a ceramic core, for example made of aluminum oxide or zirconium oxide, which has a catalytic sheath, in particular made of noble metal such as platinum, rhodium, palladium, iridium or mixtures thereof. Alternatively, the catalyst  16  itself may be in the form of a porous layer, in particular made of one of the abovementioned noble metals. 
     According to the exemplary embodiment, the catalyst  16  is partially arranged on, that is to say on the exhaust gas stream side, and partially inside a fully porous carrier ceramic  18  which is in the form of a layer and is itself arranged between the electrode  8  and the exhaust gas stream  3 . The carrier ceramic  18  preferably consists of silicon or silicon carbide. In a region  20  on the exhaust gas stream side, the carrier ceramic  18  has pores which have a first diameter of preferably between 2 and 20 μm and in which the catalyst  16  in the form of particles is accommodated. The carrier ceramic also has an electrode-side region  22  which is designed with pores having a second diameter of approximately 0.2-2 μm or less. The catalyst  16  cannot pass into these pores or through these pores with the second diameter, with the result that the catalyst particles  16  cannot come into contact with the electrode  8  in an unwanted manner and cannot fall through the carrier ceramic  18 . 
     Alternatively, the catalyst  16  may also be in the form of a porous layer which is arranged on the carrier ceramic  18 . 
     According to the exemplary embodiment, the carrier ceramic  18  is arranged at a distance from the electrode  8  by means of a gap  24 . This avoids the carrier ceramic  18  and the electrode  8  directly touching, thus avoiding strain between them, for example on account of different coefficients of thermal expansion. 
     The sensor  2  also has a passivation  26  which, together with the catalyst  16 , the carrier ceramic  18  and the semiconductor substrate  12 , surrounds the electrode  8 . The passivation  26  is gastight and is preferably formed from silicon dioxide and/or silicon nitride. 
     The semiconductor substrate  12  and the carrier ceramic  18  are advantageously formed from the same material or from different materials with substantially the same coefficients of thermal expansion. For example, the semiconductor substrate  12  and the carrier ceramic  18  are preferably both formed from silicon carbide. This makes it possible to avoid strains between the carrier ceramic  18  and the semiconductor substrate  12 ; a large force flux is thus avoided by the component passivation  26 . The component passivation  26  and the insulation layer  10  are so thin ( FIG. 1  does not illustrate the size proportions realistically) that they are not relevant to strain between the components of the sensor  2 . 
     A method for producing the sensor  2  according to the exemplary embodiment shown in  FIG. 1  is explained in more detail below. 
     The semiconductor substrate  12  is first of all provided and the insulation layer  10  is applied to the latter. 
     The electrode  8  is then applied to the insulation layer  10 . A passivation  26  is applied to the semiconductor substrate  12  before or after the application of the insulation layer  10  or the electrode  8 . 
     In a further step, a protective layer, for example a protective photoresist, is applied to the electrode  8 . The carrier ceramic  18  is then produced, in particular deposited, on the protective layer. The carrier ceramic  18  is deposited, for example, in such a manner that it has the continuous pores already described above. On the other hand, it is also possible to first of all produce the carrier ceramic  18  on the protective layer and then to produce the pores in the carrier ceramic  18 . 
     In a further step, the catalyst  16  is applied to the carrier ceramic  18  and is at least partially introduced into the latter. 
     According to an exemplary embodiment which is not illustrated, the catalyst  16  may be applied to the carrier ceramic  18  on the exhaust gas stream side as a separate layer. However, according to the present exemplary embodiment, the catalyst  16  is applied to the carrier ceramic  18  in the form of particles already described above and is introduced into the exhaust gas stream side region  20  of the carrier ceramic  18 . This may be effected by dipping the carrier ceramic  18  into a suspension of the catalyst  16  or by spraying such a suspension onto the carrier ceramic  18  or else by printing a catalyst paste onto the carrier ceramic  18  in a screen printing method. All of these methods have the feature in common that a certain proportion of the catalyst particles  16  is embedded in the pores of the exhaust gas stream side region  20  of the carrier ceramic  18 . As a result of the fact that the pores in the region  22  have a smaller diameter than the catalyst particles  16 , the catalyst particles  16  are prevented from being able to pass to the protective layer or subsequently to the electrode  8 . 
     In a further step, the arrangement consisting at least of the catalyst  16  and the carrier ceramic  18  is heat-treated or sintered in order to increase the adhesion between the catalyst  16  and the carrier ceramic  18 . Such a heat treatment may be effected, for example, at a temperature of between approximately 200 and 600°. 
     It is also conceivable to first of all produce the carrier ceramic  18  and to coat it with the catalyst  16  and to heat-treat or sinter the arrangement produced in this manner, if necessary. In a further step, this arrangement could be applied to a further arrangement consisting at least of the semiconductor substrate  12 , the insulation layer  10  and the electrode  8 . 
     After the application of the catalyst  16  or preferably after the heat treatment, sintering or porification of the carrier ceramic  18 , the protective layer is removed, with the result that the free gap  24  is produced between the carrier ceramic  18  and the electrode  8 . 
     There are various possibilities for removing the protective layer. The common feature of these possibilities is intended to be the fact that the protective layer is removed from the gap  24  through the pores in the carrier ceramic  18 . 
     If the protective layer is in the form of a protective resist, the latter can be removed by means of an organic solvent in which the protective resist is readily soluble. For this purpose, the organic solvent is supplied, through the pores in the carrier ceramic  18 , to the protective layer which thus dissolves and is washed out through the pores. According to another variant, the protective layer can be removed by thermal heating, the protective layer first of all being thermally decomposed and then evaporating through the pores in the carrier ceramic  18 . For this method, it is favorable if the protective layer is formed from a thermally decomposable photopolymer. According to an advantageous exemplary embodiment, the protective layer is removed from the gap  24  using the same heat treatment and the adhesion between the catalyst  16  and the carrier ceramic  18  is simultaneously increased. This dispenses with the separate heat treatment of the catalyst  16  together with the carrier ceramic  18 , as described above, thus simplifying the method for producing the sensor  2  overall. 
     The sensor  2  according to the exemplary embodiment shown in  FIG. 2  differs from that shown in  FIG. 1  only by virtue of the fact that, in the exemplary embodiment shown in  FIG. 2 , the carrier ceramic  18  is directly applied to the electrode  8  which comprises the coating  14  if necessary. Therefore, the method step, which was described above in connection with  FIG. 1  and according to which the protective layer is applied to the electrode  8  and is subsequently removed again, is dispensed with in the method for producing the sensor  2  according to the exemplary embodiment shown in  FIG. 2 . For the rest, the described methods for producing the sensor according to  FIG. 1  and the sensor  2  according to  FIG. 2  are identical. 
       FIG. 3  shows a plan view of a chip  28  having the sensor  2  according to the exemplary embodiment shown in  FIG. 1  or  2 . The chip  28  also has a further sensor  30 . The further sensor  30  is a conventional ChemFET and has an electrode (not illustrated) which is set up to change its potential if it comes into contact with the first medium  4 , for example nitrogen oxide gas, and/or the second medium  6 , for example hydrocarbon gas. The further sensor  30  according to the exemplary embodiment shown in  FIG. 3  does not have a catalyst or the like which would be suitable for changing the second medium  6  in such a manner that the latter substantially does not contribute to changing the potential of the electrode of the further sensor  30 . The further sensor  30  does not have, in particular, a catalyst which would be suitable for converting hydrocarbon gas into carbon dioxide and water. The further sensor  30  is thus set up to detect the first and second media  4 ;  6 . 
     The sensor  2  will therefore produce a measured value for the first medium  4 , while the further sensor  30  will produce a measured value for the first and second media  4  and  6 . 
     The chip  28  can now have an evaluation unit  32  which is coupled, for signaling purposes, to the sensors  2  and  30 , indicated by the lines with the arrow tips in  FIG. 3 , the evaluation unit  32  being set up to also determine the proportion of the second medium, that is to say the hydrocarbon gas for example, in the exhaust gas stream  3  from the abovementioned measured values from the sensor  2  and from the further sensor  30 . 
     Although the disclosure was specifically described using exemplary embodiments, it is not restricted thereto but rather can be modified in various ways.