Patent Publication Number: US-2017363530-A1

Title: Sensor for detecting electrically conductive and/or polarizable particles, sensor system, method for operating a sensor, method for producing a sensor of this type and use of a sensor of this type

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is directed to a sensor for detecting electrically conductive and/or polarizable particles, in particular for detecting soot particles. The invention is also directed to a sensor system, to a method for operating a sensor, to a method for producing a sensor for detecting electrically conductive and/or polarizable particles and to a use of a sensor of this type. 
     2. Discussion of the Related Art 
     The prior art discloses sensors comprising a sensor carrier, with electrodes and heating structures being arranged on this sensor carrier in a planar arrangement. In a detecting mode of operation, polarizable and/or electrically conductive particles are deposited on this planar arrangement. The deposited particles bring about a reduction in the resistance between the electrodes, this drop in the resistance being used as a measure of the mass of deposited particles. When a predefined threshold value with respect to the resistance is reached, the sensor arrangement is heated by the heating structures, so that the deposited particles are burned and, after the cleaning process, the sensor can be used for a further detection cycle. 
     DE 10 2005 029 219 A1 gives a description of a sensor for detecting particles in an exhaust-gas flow of internal combustion engines, the electrode, heater and temperature-sensor structures having been applied to a sensor carrier in a planar arrangement. One disadvantage of this sensor arrangement is that the electrodes to be bridged have a necessary minimum length in order to be able to arrive at an acceptable sensitivity range when measuring conductive or polarizable particles, such as for example soot. However, a certain size of the sensor component is necessary for this, in order to be able to arrange the minimum length for the electrodes to be bridged. This is accompanied by corresponding cost disadvantages in the production of these sensor components. 
     The invention is based on the object of providing a further-developed sensor for detecting electrically conductive and/or polarizable particles, in particular for detecting soot particles, the sensor being minimized with regard to its size, so that the aforementioned disadvantages can be overcome. 
     The object of the present invention is also to provide a sensor system, a method for operating a sensor and a method for producing a sensor of this type. 
     SUMMARY OF THE INVENTION 
     This object is achieved according to the invention by a sensor for detecting electrically conductive and/or polarizable particles, in particular for detecting soot particles. 
     The invention is based on the idea of providing a sensor for detecting electrically conductive and/or polarizable particles, in particular for detecting soot particles, comprising a substrate and at least two electrode layers, a first electrode layer and at least a second electrode layer, which is arranged between the substrate and the first electrode layer, being arranged, at least one insulation layer being formed between the first electrode layer and the at least a second electrode layer and at least one opening being respectively formed in the first electrode layer and in the at least one insulation layer, the opening in the first electrode layer and the opening in the insulation layer being arranged at least in certain portions one over the other in such a way that at least one passage to the second electrode layer is formed. 
     A sensor is preferably provided, comprising a substrate, a first electrode layer, a second electrode layer, which is arranged between the substrate and the first electrode layer, a first insulation layer being formed between the first electrode layer and the second electrode layer, at least a third electrode layer being formed between the first insulation layer and the first electrode layer, and at least a second insulation layer being formed between the at least third electrode layer and the first electrode layer, at least one opening being respectively formed in the first electrode layer, in the at least second insulation layer, in the at least third electrode layer and in the first insulation layer, the opening in the first electrode layer, the opening in the at least second insulation layer, the opening in the at least third electrode layer and the opening in the insulation layer being arranged at least in certain portions one over the other in such a way that at least one passage to the second electrode layer is formed. 
     In other words, a sensor is made available, a first and a second electrode layer being arranged horizontally one over the other and a first insulation layer, optionally at least a third electrode layer and optionally at least a second insulation layer being formed between these two electrode layers. In order to form a passage to the second electrode layer, so that particles to be detected, in particular soot particles, can reach the second electrode layer with the aid of the passage, both the first and third electrode layers and the first and second insulation layers respectively have at least one opening, the opening in the first and third electrode layers and the opening in the first and second insulation layers being arranged at least in certain portions one over the other, so that the passage is formed or can be formed. 
     Particles can accordingly reach the second electrode layer by way of at least one passage only from one side of the sensor, to be specific from the side of the sensor that is made to be the closest to the first electrode layer. The electrically conductive and/or polarizable particles accordingly lie on a portion of the second electrode layer. 
     The sensor according to the invention may for example comprise at least three electrode layers and at least two insulation layers, an insulation layer preferably always being formed between two electrode layers. 
     An insulation layer may also consist of two or more sublayers, which may be arranged next to one another and/or one over the other. Two or more sublayers of an insulation layer may consist of different materials and/or comprise different materials. 
     An electrode layer may also consist of two or more sublayers, which may be arranged next to one another and/or one over the other. Two or more sublayers of an electrode layer may consist of different materials and/or comprise different materials. 
     It is possible that the sensor comprises more than three electrode layers and more than two insulation layers, also in this situation an insulation layer preferably always being formed between two electrode layers. From now on, the expression “at least third electrode layer” should be understood as meaning that a fourth and/or fifth and/or sixth and/or seventh and/or eighth and/or ninth and/or tenth electrode layer may also be intended instead of the stated third electrode layer. 
     From now on, the expression “at least second insulation layer” should be understood as meaning that a third and/or fourth and/or fifth and/or sixth and/or seventh and/or eighth and/or ninth insulation layer may also be intended instead of the stated second insulation layer. 
     The sensor according to the invention may in other words comprise a laminate which comprises at least three electrode layers and at least two insulation layers. The electrode layer closest to the substrate is referred to as the second electrode layer, the electrode layer at the maximum distance from the substrate is referred to as the first electrode layer. Between the first electrode layer and the second electrode layer there is for example at least a third electrode layer, at least one insulation layer being respectively formed between two electrode layers. 
     The electrode layers are arranged one over the other, in particular in layers one over the other, the electrode layers being respectively kept at a distance from one another by means of the insulation layers. In other words, the electrode layers do not lie in one plane. 
     Preferably, the opening in the first electrode layer is formed at a distance from the peripheral region of the first electrode layer, the opening in the optionally at least second insulation layer is formed at a distance from the peripheral region of the second insulation layer, the opening in the optionally at least third electrode layer is formed at a distance from the peripheral region of the third electrode layer and the opening in the first insulation layer is formed at a distance from the peripheral region of the first insulation layer. The openings are accordingly preferably not formed in a peripheral position, or not formed at the side peripheries of the layers concerned. 
     The first electrode layer and the optionally third electrode layer are insulated from one another by the second insulation layer located in between. The optionally third electrode layer and the second electrode layer are insulated from one another by the first insulation layer located in between. Such a structure allows a very sensitive sensor of a smaller overall size in comparison with sensors of the prior art to be formed. 
     The second electrode layer, formed for example with a flat extent, is indirectly or directly connected to the substrate. An indirect connection of the second electrode layer to the substrate may take place for example by means of a bonding agent, in particular a bonding agent layer. The bonding agent may also be formed in an insular manner between the substrate and the second electrode layer. For example, a drop-like formation of the bonding agent/the bonding agent layer is possible. A bonding agent layer may be formed between the second electrode layer and the substrate. 
     The bonding agent, in particular the bonding agent layer, may for example consist of an aluminum oxide (Al 2 O 3 ) or a silicon dioxide (SiO 2 ) or a ceramic or a glass or any desired combinations thereof. The bonding agent layer is preferably formed very thin, and consequently only has a small thickness. 
     The first insulation layer and/or the at least second insulation layer may have a thickness of 0.1 to 50 μm, in particular of 1.0 μm to 40 μm, in particular of 5.0 μm to 30 μm, in particular of 7.5 μm to 20 μm, in particular of 8 μm to 12 μm. With the aid of the thickness of the insulation layer(s), the distance of one electrode layer from another electrode layer is set. The sensitivity of the sensor can be increased by reducing the distance between the, for example flat-extending, electrode layers, located one over the other. The smaller the thickness of the insulation layer is formed, the more sensitive the sensor is made. 
     It is also possible that the thickness(es) of the electrode layers and/or the thickness(es) of the insulation layer(s) of a substrate vary. 
     The insulation layer(s) may be formed from aluminum oxide (Al 2 O 3 ) or silicon dioxide (SiO 2 ) or magnesium oxide (MgO) or silicon nitride (Si 2 N 4 ) or glass or ceramic or any desired combinations thereof. 
     Preferably, the first insulation layer laterally encloses the second electrode layer. In other words, the first insulation layer can cover the side faces of the second electrode layer in such a way that the second electrode layer is laterally insulated. For example, the at least second insulation layer laterally encloses the at least third electrode layer. In other words, the second insulation layer can cover the side faces of the third electrode layer in such a way that the third electrode layer is laterally insulated. 
     The first electrode layer and/or the second electrode layer and/or the optionally at least third electrode layer is formed from a conductive material, in particular from metal or an alloy, in particular from a high-temperature-resistant metal or a high-temperature-resistant alloy, particularly preferably from a platinum metal or from an alloy of a platinum metal. The elements of the platinum metals are palladium (Pd), platinum (Pt), rhodium (Rh), osmium (Os), iridium (Ir) and ruthenium (Rh). Nonprecious metals such as nickel (Ni) or nonprecious metal alloys such as nickel/chromium or nickel/iron may also be used. 
     It is possible that at least one electrode layer is formed from a conductive ceramic or a mixture of metal and ceramic. For example, at least one electrode layer may be formed from a mixture of platinum grains (Pt) and aluminum oxide grains (Al 2 O 3 ). It is also possible that at least one electrode layer comprises silicon carbide (SiC) or is formed from silicon carbide (SiC). The stated materials and metals or alloys of these metals are particularly high-temperature-resistant and are accordingly suitable for the forming of a sensor element that can be used for detecting soot particles in an exhaust-gas flow of internal combustion engines. 
     In a further embodiment of the invention, the second electrode layer is formed from a conductive material, in particular from a metal or an alloy, that has a higher etching resistance than the conductive material, in particular the metal or the alloy, of the first electrode layer. This has the advantage that the second electrode layer can be formed in a production process as a layer stopping the etching process. In other words, a second electrode layer formed in this way can determine the depth to be etched of a passage that is for example to be introduced into the sensor structure. 
     On the side of the first electrode layer that is facing away from the first insulation layer there may be formed at least one covering layer, which is formed in particular from ceramic and/or glass and/or metal oxide. In other words, the covering layer is formed on a side of the first electrode layer that is opposite from the first insulation layer. The covering layer may serve as a diffusion barrier and additionally reduces an evaporation of the first electrode layer at high temperatures, which in an exhaust-gas flow for example may be up to 850° C. 
     The at least one covering layer may laterally enclose the first electrode layer. In a further embodiment of the invention, the covering layer may additionally laterally enclose the at least second insulation layer. In a further embodiment of the invention, the covering layer may additionally laterally enclose the at least second insulation layer and the at least third electrode layer. 
     It is possible that at least one covering layer does not completely cover the uppermost electrode layer, in particular the first electrode layer. In other words it is possible that at least one covering layer only covers certain portions of the uppermost electrode layer, in particular the first electrode layer. If the uppermost electrode layer is formed as a heating layer, it is possible that only the portions of the heating loop/heating coil are covered by the at least one covering layer. 
     In a further embodiment of the invention, the at least one covering layer may additionally laterally enclose the at least second insulation layer and the at least third electrode layer and the first insulation layer. In other words, both the side faces of the first electrode layer and the side faces of the insulation layers and electrode layers arranged thereunder may be covered by at least one covering layer. It is also conceivable that the covering layer additionally laterally encloses the second electrode layer. The lateral enclosing part or lateral enclosing region of the covering layer may accordingly reach from the first electrode layer to the second electrode layer. This brings about a lateral insulation of the first electrode layer and/or of the insulation layers and/or of the at least third electrode layer and/or of the second electrode layer. 
     On the side of the first electrode layer that is facing away from the first insulation layer or on the side of the covering layer that is facing away from the first electrode layer there may be additionally formed at least one porous filter layer. With the aid of a porous filter layer of this type, large particle parts can be kept away from the arrangement of at least two, in particular at least three, electrode layers arranged one over the other. The pore sizes of the filter layer may be for example &gt;1 μm. Particularly preferably, the pore size is formed in a range from 20 μm to 30 μm. The porous filter layer may for example be formed from a ceramic material. It is also conceivable that the porous filter layer is formed from an aluminum oxide foam. With the aid of the filter layer, which also covers the at least one passage to the second electrode layer, the large particles, in particular soot particles, that disturb the measurement can be kept away from the at least one passage, so that such particles cannot cause a short circuit. 
     The at least one passage to the second electrode layer may for example be formed as a blind hole, a portion of the second electrode layer being formed as the bottom of the blind hole and the blind hole extending at least over the first insulation layer, over the optionally at least third electrode layer, over the optionally at least second insulation layer and over the first electrode layer. If the sensor has a covering layer, the blind hole also extends over this covering layer. In other words, not only the first electrode layer but also the optionally at least second insulation layer, the optionally at least third electrode layer and the first insulation layer and the covering layer then have an opening, these openings being arranged one over the other in such a way that they form a blind hole, the bottom of which is formed by a portion of the second electrode layer. The bottom of the blind hole may for example be formed on the upper side of the second electrode layer that is facing the first insulation layer. It is also conceivable that the second electrode layer has a depression that forms the bottom of the blind hole. 
     The opening cross section of the blind hole is formed by the peripheral portions of the first electrode layer, of the at least second insulation layer, of the at least third electrode layer and of the first insulation layer and, if there is one, of the covering layer that bound the openings. The opening cross section of the at least one blind hole may be round or square or rectangular or lenticular or honeycomb-shaped or polygonal or triangular or hexagonal. Other types of design, in particular free forms, are also conceivable. 
     For example, it is possible that the blind hole has a square cross section with a surface area of 3×3 μm 2  to 150×150 μm 2 , in particular of 10×10 μm 2  to 100×100 μm 2 , in particular of 15×15 μm 2  to 50×50 μm 2 , in particular of 20×20 μm 2 . 
     In a development of the invention, the sensor may have a multiplicity of passages, in particular blind holes, these blind holes being formed as already described. It is also conceivable that at least two passages, in particular at least two blind holes, have different cross sections, in particular different sizes of cross section, so that a sensor array with a number of zones can be formed, in which a number of measuring cells with blind-hole cross sections of different sizes can be used. Parallel detection of electrically conductive and/or polarizable particles, in particular of soot particles, allows additional items of information concerning the size of the particles or the size distribution of the particles to be obtained. 
     In a further embodiment of the invention, the openings in the first insulation layer, in the optionally at least third electrode layer, in the optionally at least second insulation layer, and in the first electrode layer may be respectively formed in a linear form or respectively formed in a meandering manner or respectively formed in a grid form or respectively formed in a spiral form. In other words, an opening in the first insulation layer, an opening in the optionally at least third electrode layer, an opening in the optionally at least second insulation layer, and an opening in the first electrode layer are respectively formed in a linear form or respectively formed in a meandering manner or respectively formed in a spiral form or respectively formed in a grid form. The openings in the individual layers are preferably formed similarly, so that a passage can be formed. The openings do not necessarily have to have exactly coinciding cross sections or exactly coinciding sizes of cross section. It is possible that, beginning from the second electrode layer, the cross sections of the openings respectively become greater in the direction of the first electrode layer. The basic forms of the openings are preferably formed similarly, so that all of the openings are formed either in a linear form or in a meandering manner or in a spiral form or in a grid form. 
     In a further embodiment of the invention it is possible that the sensor has a number of passages that are formed in a linear form and/or a meandering manner and/or a spiral form and/or a grid form. 
     If the second electrode layer has the form of a meander or the form of a loop, the at least one passage of the sensor is formed in such a way that the passage does not end in a gap or an opening in the form of the meander or the form of the loop. The at least one passage of the sensor is formed in such a way that a portion of the second electrode layer forms the bottom of the passage. 
     It is also possible that the at least one passage is formed as an elongate depression, a portion of the second electrode layer being formed as the bottom of the elongate depression and the elongate depression extending at least over the first insulation layer, over the optionally at least third electrode layer, over the optionally at least second insulation layer, and over the first electrode layer and over a/the optionally formed covering layer. 
     The elongate depression may also be referred to as a trench and/or groove and/or channel. 
     In a further embodiment of the invention it is possible that the sensor comprises both at least one passage in the form of a blind hole, which is formed as round and/or square and/or rectangular and/or lenticular and/or honeycomb-shaped and/or polygonal and/or triangular and/or hexagonal, and at least one passage in the form of an elongate depression, which is formed in a linear form and/or a meandering manner and/or in a spiral form and/or in a grid form. 
     In a further embodiment of the invention, the first electrode layer, the optionally at least second insulation layer, the optionally at least third electrode layer and the first insulation layer are respectively formed as porous, the at least one opening in the first electrode layer, the at least one opening in the optionally at least second insulation layer, the at least one opening in the optionally at least third electrode layer, and the at least one opening in the first insulation layer respectively being formed by at least one pore, the pore in the first insulation layer, the pore in the at least third electrode layer, the pore in the at least second insulation layer and the pore in the first electrode layer being arranged at least in certain portions one over the other in such a way that the at least one passage to the second electrode layer is formed. In other words, it is possible to dispense with an active or subsequent structuring of the passages, the first and at least third electrode layer and the first and at least second insulation layer being formed as permeable to the medium to be measured. 
     This can be made possible for example by a porous or granular structure of the layers. Both the electrode layers and the insulation layers can be produced by sintering together individual particles, with pores or voids for the medium to be measured being formed while they are being sintered together. The second electrode layer is preferably formed as non-porous. Accordingly, at least one passage that allows access to the second electrode layer for a particle that is to be measured or detected must be formed, extending from the side of the first electrode layer that is facing away from the first insulation layer to the side of the second electrode layer that is facing the insulation layer as a result of the one-over-the-other arrangement of pores in the electrode layers, in particular the first and the optionally at least third electrode layer, and in the insulation layers. If the sensor has a covering layer, this covering layer is also preferably to be formed as porous in such a way that a pore in the covering layer, a pore in the first electrode layer, a pore in the second insulation layer, a pore in the third electrode layer and a pore in the first insulation layer form a passage to the second electrode layer. 
     The pore size distribution and their number in the first and optionally third electrode layer and/or the first and optionally second insulation layer and/or the covering layer(s) can be optimized with regard to the measuring or detecting tasks to be carried out. 
     The first and/or third electrode layer and/or the first and/or second insulation layer and, if there is one, the at least one covering layer may have portions with different pore sizes in such a way that a sensor array with a number of zones of different pore sizes is formed. Parallel detection with portions of layers of different pore sizes allows a “fingerprint” of the medium that is to be analyzed or detected to be measured. Accordingly, further items of information concerning the size of the particles to be measured or the size distribution of the particles to be measured can be obtained. 
     The first electrode layer, the second electrode layer and the optionally at least third electrode layer respectively have an electrical contacting area that are free from sensor layers arranged over the respective electrode layers and are or can in each case be connected to a terminal pad. The electrode layers are connected or can be connected to terminal pads in such a way that they are insulated from one another. For each electrode layer there is formed at least one electrical contacting area, which is exposed in the region of the terminal pads for the electrical contacting. The electrical contacting area of the first electrode layer is free from a possible covering layer and free from a passive porous filter layer. In other words, above the electrical contacting area of the first electrode layer there is neither a portion of the covering layer nor a portion of the filter layer. 
     The electrical contacting area of the second or at least third electrode layer is free from insulation layers, free from electrode layers, and also free from a possibly formed covering layer and free from a passive porous filter layer. 
     In other words, on the electrical contacting area of the second or at least third electrode layer there is neither a portion of an insulation layer nor a portion of an electrode layer, nor a portion of the passive porous filter layer. 
     In a further embodiment of the invention, the first electrode layer and/or the second electrode layer and/or the at least third electrode layer has strip conductor loops in such a way that the first electrode layer and/or the second electrode layer and/or the at least third electrode layer is formed as a heating coil and/or as a temperature-sensitive layer and/or as a shielding electrode. The first electrode layer and/or the second electrode layer and/or the at least third electrode layer has at least one additional electrical contacting area that is free from sensor layers arranged over the electrode layer, that is to say the first and/or the second and/or the at least third electrode layer, and is connected or can be connected to an additional terminal pad. In other words, the first electrode layer and/or the second electrode layer and/or the at least third electrode layer has two electrical contacting areas, both electrical contacting areas being free from sensor layers arranged over the electrode layer. 
     The formation of two electrical contacting areas on an electrode layer is necessary whenever this electrode layer is formed as a heating coil and/or temperature-sensitive layer and/or as a shielding electrode. Preferably, the second and/or the at least third electrode layer has at least two electrical contacting areas. The second and/or the at least third electrode layer is preferably formed not only as a heating coil but also as a temperature-sensitive layer and as a shielding electrode. By appropriate electrical contacting of the electrical contacting area, the electrode layer can either heat or act as a temperature-sensitive layer or shielding electrode. Such a formation of the electrode areas allows compact sensors to be provided, since one electrode layer can assume a number of functions. Accordingly, no separate heating coil layers and/or temperature-sensitive layers and/or shielding electrode layers are necessary. 
     During the heating of at least one electrode layer, measured particles or particles located in a passage of the sensor may for example be burned away or burned off. 
     To sum up, it can be stated that a very accurately measuring sensor can be made available as a result of the structure according to the invention. The forming of a/a number of thin insulation layers allows the sensitivity of the sensor to be increased significantly. 
     Furthermore, the sensor according to the invention can be made much smaller than known sensors. The formation of the sensor in a three-dimensional space allows a number of electrode layers and/or a number of insulation layers to be built as a smaller sensor. Furthermore, significantly more units can be formed on a substrate or a wafer during the production of the sensor. This structure is consequently accompanied by a considerable cost advantage in comparison with normally planar-constructed structures. 
     A further advantage of the sensor according to the invention is that the cross sections of the passages can be dimensioned in such a way that specific particles of specific sizes cannot enter the passages. It is also possible that the cross sections of a number of passages can be of different sizes, so that only specific particles of corresponding particle sizes are allowed access into individual passages. 
     The sensor according to the invention may be used for detecting particles in gases. The sensor according to the invention may be used for detecting particles in liquids. The sensor according to the invention may be used for detecting particles in gases and liquids or gas-liquid mixtures. When the sensor is used for detecting particles in liquids, it is not always possible however to burn off or burn away the particles. 
     In the case of known sensors, the sensors are arranged in one plane and engage in one another. In the case of the present sensor, it is not necessary for the electrode structures to engage in one another, since the individual electrode layers are formed at a distance from one another as a result of the formation of insulation layers between the electrode layers. The electrode layers of the sensor according to the invention are not connected to one another, but lie one over the other, separated by at least one insulation layer. There is a “non continuous loop” between at least a first electrode layer and at least a second electrode layer. The at least two electrode layers are not twisted together or entwined. At least two electrode layers can only be electrically connected to one another by a soot particle located in at least one passage. 
     With the aid of at least three formed electrode layers, it is possible during a measurement of particles for example to deduce the particle size or to detect the particle size. If a particle bridges only two electrode layers arranged one over the other, the size of the particle is smaller than a particle that bridges more than two electrode layers. Different formations of the thickness of the insulation layers also allow the size of the particles to be deduced. 
     According to an independent aspect, the invention relates to a sensor system, comprising at least one sensor according to the invention and at least one controller, in particular at least one control circuit, which is formed in such a way that the sensor can be operated in a measuring mode and/or in a cleaning mode and/or in a monitoring mode. 
     The sensor according to the invention and/or the sensor system according to the invention may have at least one auxiliary electrode. Between an auxiliary electrode and an electrode layer and/or between an auxiliary electrode and a component of the sensor system, in particular the sensor housing, there may be applied such an electrical potential that the particles to be measured are electrically attracted or sucked in by the sensor and/or the sensor system. Preferably, such a voltage is applied to the at least one auxiliary electrode and to at least one electrode layer that particles, in particular soot particles, are “sucked into” the at least one passage. 
     The sensor according to the invention is preferably arranged in a sensor housing. The sensor housing may for example have an elongate tube form. The sensor system according to the invention may accordingly also comprise a sensor housing. 
     Preferably, the sensor and/or the sensor in the sensor housing and/or the sensor housing is formed and/or arranged in such a way that the sensor, in particular the uppermost layer of the sensor, or the layer of the sensor that is arranged furthest away from the substrate, is arranged obliquely in relation to the direction of flow of the fluid. The flow in this case does not impinge perpendicularly on the plane of the electrode layers. Preferably, the angle α between the normal to the plane of the first electrode layer and the direction of flow of the particles is at least 1 degree, preferably at least 10 degrees, particularly preferably at least 30 degrees. Also preferred is an arrangement of the sensor in which the angle β between the direction of flow of the particles and the longitudinal axis of for example elongate depressions lies between 20 and 90 degrees. In this embodiment, the particles to be detected more easily enter the passages, in particular blind holes or elongate depressions, in the sensor, and thereby increase the sensitivity. 
     The controller, in particular the control circuit, is preferably formed in such a way that the electrode layers of the sensor are interconnected with one another. Such voltages may be applied to the electrode layers or individual electrode layers that the sensor can be operated in a measuring mode and/or in a cleaning mode and/or in a monitoring mode. 
     According to an independent aspect, the invention relates to a method for controlling a sensor according to the invention and/or a sensor system according to the invention. 
     The method according to the invention allows the sensor to be operated according to choice in a measuring mode and/or in a cleaning mode and/or in a monitoring mode. 
     In the measuring mode, a change in the electrical resistance between the electrode layers or between at least two electrode layers of the sensor and/or a change in the capacitances of the electrode layers can be measured. 
     With the aid of the method according to the invention, particles can be detected or measured on the basis of a measured change in resistance between the electrode layers and/or by a measurement of the change in impedance and/or by a measurement of the capacitance of the electrode layer(s). Preferably, a change in resistance between the electrode layers is measured. 
     In the measuring mode, an electrical resistance measurement, that is to say a measurement on the resistive principle, may be carried out. This involves measuring the electrical resistance between two electrode layers, the electrical resistance decreasing if a particle, in particular a soot particle, bridges at least two electrode layers, which act as electrical conductors. 
     It applies in principle in the measuring mode that, by applying different voltages to the electrode layers, different properties of the particles to be measured, in particular soot particles, can be detected. For example, the particle size and/or the particle diameter and/or the electrical charging and/or the polarizability of the particle can be determined. 
     If at least one electrode layer is also used or can be connected as a heating coil or heating layer, an electrical resistance measurement may additionally serve the purpose of determining the point in time of the activation of the heating coil or heating layer. The activation of the heating coil or heating layer corresponds to a cleaning mode to be carried out. 
     Preferably, a decrease in the electrical resistance between at least two electrode layers indicates that particles, in particular soot particles, have been deposited on or between the electrodes (electrode layers). As soon as the electrical resistance reaches a lower threshold value, the activation of the heating coil or heating layer takes place. The particles are in other words burned off. With an increasing number of burnt-off particles or burnt-off particle volume, the electrical resistance increases. The burning off is preferably carried out for such a time until an upper electrical resistance value is measured. Reaching an upper electrical resistance value is taken as an indication of a regenerated or cleaned sensor. A new measuring cycle can subsequently begin or be carried out. 
     Alternatively or in addition, it is possible to measure a change in the capacitances of the electrode layers. An increasing loading of the arrangement of electrode layers leads to an increase in the capacitance of the electrode layers. The arrangement of particles, in particular soot particles, in at least one passage of the sensor leads to a charge transfer or a change in the permittivity (s), which leads to an increase in the capacitance (C). In principle: C=(ε×A)/d, where A stands for the active electrode area of the electrode layer and d stands for the distance between two electrode layers. 
     The measuring of the capacitance may be carried out by way of example by:
     determining the rate of voltage increase with a constant current and/or   applying a voltage and determining the charging current and/or   applying an AC voltage and measuring the current profile and/or   determining the resonant frequency by means of an LC oscillating circuit.   

     The described measurement of the change in the capacitances of the electrode layers may also be carried out in connection with a monitoring mode to be carried out. 
     According to OBD (on-board diagnosis) regulations, all parts and components that are relevant to exhaust gas must be checked for their function. The functional check is to be carried out for example directly after starting a motor vehicle. 
     For example, at least one electrode layer may be destroyed, this being accompanied by a reduction in the active electrode area A. Since the active electrode area A is directly proportional to capacitance C, the measured capacitance C of a destroyed electrode layer decreases. 
     In the monitoring mode, it is alternatively or additionally possible to form the electrode layers as conductor circuits. The conductor circuits may be formed as closed or open conductor circuits, which can be closed on demand, for example by a switch. It is also possible to close the electrode layers by way of at least one switch to form at least one conductor circuit, it being checked in the monitoring mode whether a test current is flowing through the at least one conductor circuit. If an electrode layer has a crack or is damaged or destroyed, no test current would flow. 
     According to an independent aspect, the invention relates to a method for producing a sensor for detecting electrically conductive and/or polarizable particles, in particular a method for producing a described sensor according to the invention. 
     The method comprises that a laminate with a first electrode layer, a second electrode layer, a first insulation layer, which is arranged between the first electrode layer and the second electrode layer, optionally at least a third electrode layer, which is arranged between the first insulation layer and the first electrode layer, and optionally at least a second insulation layer, which is arranged between the third electrode layer and the first electrode layer, is produced, at least one passage that extends over the first electrode layer, the optionally at least second insulation layer, the optionally at least third electrode layer, and the first insulation layer being subsequently introduced into the laminate, the bottom of the passage being formed by a portion of the second electrode layer. 
     The method is also based on the idea of producing a laminate which comprises at least three electrode layers and two insulation layers, in order to introduce at least one passage into this laminate. The passage serves as access to the second electrode layer for the particles to be detected, in particular soot particles. 
     The production of the laminate and/or of the individual layers of the laminate may take place by a thin-film technique or a thick-film technique or a combination of these techniques. As part of a thin-film technique to be applied, a vapor depositing process or preferably a cathode sputtering process may be chosen. As part of a thick-film process, a screen-printing process is conceivable in particular. 
     At least one insulation layer and/or at least one covering layer, which is formed on the side of the first electrode layer that is facing away from the first insulation layer, may be formed by a chemical vapor deposition (CVD process) or a plasma-enhanced chemical vapor deposition (PECVD process). 
     The first insulation layer may be produced in such a way that it laterally encloses the second electrode layer. An optionally present covering layer may likewise be produced in such a way that it laterally encloses the first electrode layer and/or the at least second insulation layer and/or the at least third electrode layer and/or the first insulation layer and/or the second electrode layer. Accordingly, both at least one of the insulation layers and at least one/the covering layer may form an additional lateral enclosure. 
     The passage may for example be formed as a blind hole or as an elongate depression, the at least one blind hole or a subportion of the blind hole or the at least one elongate depression or a subportion of the elongate depression being introduced into the laminate by at least one removing or etching process, in particular by a plasma-ion etching process, or by a number of successively carried out removing or etching processes which is adapted to the layer of the laminate that is respectively to be etched or to be removed. 
     In other words, a blind hole or an elongate depression may be introduced into the laminate in such a way that, for example for each layer to be penetrated or to be etched or to be removed, a process that is optimum for this layer is used, and consequently a number of etching or removing steps that are to be successively carried out are carried out. 
     It is also conceivable that the blind hole or a subportion of the blind hole or the elongate depression or a subportion of the elongate depression may be made in a chemical etching process from the liquid or vapor phase. The first electrode layer preferably consists of a metal, in particular a platinum layer, which is relatively easy to etch through or to etch. 
     In one possible embodiment of the method according to the invention it is possible that the etching process stops at the second electrode layer if the second electrode layer is produced from a material that is more resistant to etching in comparison with the first and third electrode layers and with the insulation layers. If the laminate or the sensor comprises an additional covering layer, the second electrode layer also comprises a material that is more resistant to etching in comparison with this covering layer. For example, the second electrode layer is produced from a platinum-titanium alloy (Pt/Ti). It is also conceivable that the second electrode layer consists of a layer filled with metal oxides. 
     In a further embodiment of the method according to the invention it is possible that the first insulation layer and/or the at least second insulation layer is formed as a layer stopping the etching process and, in a further step, a subportion of the blind hole or a subportion of the elongate depression is introduced into the first insulation layer and/or the at least second insulation layer by a conditioning process or a conditioning step with phase conversion of the first insulation layer and/or the at least second insulation layer. 
     In a further embodiment of the method according to the invention it is possible that the at least one passage and/or a passage is formed as a blind hole or as an elongate depression and this blind hole or the at least one blind hole or a subportion of the blind hole or this elongate depression or the at least one elongate depression or a subportion of the elongate depression is introduced into the laminate by a process of irradiating with electromagnetic waves or charged particles (electrons), the radiation source and/or the wavelength and/or the pulse frequency of the radiation being adapted to the layer of the laminate that is respectively to be machined. 
     It is preferably possible that the at least one passage and/or a passage is formed as a blind hole or as an elongate depression and this blind hole or the at least one blind hole or a subportion of the blind hole or this elongate depression or the at least one elongate depression or a subportion of the elongate depression is introduced into the laminate by a laser machining process, in particular by means of an ultrashort pulse laser, the laser source and/or the wavelength and/or the pulse frequency of the laser and/or the energy of the charged particles and/or the species of the charged particles being adapted to the layer of the laminate that is respectively to be machined. Particularly preferably, an ultrashort pulse laser is a femto laser or a pico laser. 
     One possibility for producing the passage that is formed as a blind hole or as an elongate depression is consequently the partial removal of the laminate by means of a laser. Laser sources with different wavelengths and/or pulse frequencies that are respectively made to suit the material to be removed can be used. Such a procedure has the advantage that, by making them suit the material of the layer that is to be removed, the respectively individual laser machining steps can be carried out quickly, so that overall an improved introduction of passages and/or blind holes and/or elongate depressions into the laminate is obtained. The use of an ultrashort pulse laser proves to be particularly advantageous. 
     Apart from electromagnetic radiation, charged or uncharged particles can however also be used for removing the electrode layers and/or insulation layers. Thus, apart from electron beams, other charged or uncharged particles can also be used for the ablation. This may be carried out with or without masks that contain the structural information to be transferred. 
     In a further embodiment of the method according to the invention it is possible that, when producing the laminate, the first insulation layer and/or the at least second insulation layer is created over the full surface area, in particular by a screen-printing process or spraying-on process or immersion process or spin-coating process, between the second electrode area and the at least third electrode area or between the at least third electrode area and the first electrode area and, in a subsequent method step, at least a portion of the first insulation layer and/or of the at least second insulation layer is removed, in particular by structured dissolving or etching or burning out, in such a way that the passage is formed in the sensor. 
     Such a method corresponds to the lost mold principle. Accordingly, it is possible, especially in the case of thermally stable materials, to perform structuring by the lost mold principle. A lost mold serves for creating a passage from the first electrode layer to the second electrode layer. The at least one insulation layer or insulating layer is created between the electrode layers from a thermally stable material, a portion of this insulation layer preferably being removed by dissolving or etching or burning out after the application of the first electrode layer. As a result of this, the first electrode layer located thereover is also removed. If a covering layer is formed, the portion of the covering layer that is located over the removed portion of the insulation layer is also removed by the dissolving or etching or burning out of the portion of the insulation layer. 
     Preferably, after the introduction of a passage and/or a blind hole and/or an elongate depression into the laminate, at least one passive porous filter layer is applied on the covering layer. The passive porous filter layer is formed for example by an aluminum oxide foam. This is also formed over the at least one passage or over the at least one blind hole or over the at least one elongate depression. 
     In a further independent aspect, the invention relates to a method that serves for producing a sensor for detecting electrically conductive particles and/or polarizable particles. 
     A laminate with a first electrode layer, a second electrode layer, a first insulation layer, which is arranged between the first electrode layer and the second electrode layer, at least a third electrode layer, which is arranged between the first insulation layer and the first electrode layer, and at least a second insulation layer, which is arranged between the third electrode layer and the first electrode layer, is produced, the first insulation layer, the at least third electrode layer, the at least second insulation layer and the first electrode layer being formed as porous layers. The pores in the first and third electrode layer and the first and second insulation layer are set in such a way that at least one pore in the first electrode layer, at least one pore in the at least second insulation layer, at least one pore in the at least third electrode layer and at least one pore in the first insulation layer are arranged at least in certain portions one over the other, so that at least one passage to the second electrode layer is produced. 
     If the sensor has a covering layer, this covering layer is also applied to the first electrode layer with a pore size and porosity, at least one pore in the covering layer being arranged at least in certain portions over a pore in the first electrode layer, over a pore in the at least second insulation layer, over a pore in the at least third electrode layer and a pore in the first insulation layer in such a way that, starting from the covering layer, at least one passage to the second electrode layer is formed. A passive porous filter layer may finally be applied to the covering layer. 
     In a further independent aspect, a method for producing a sensor for detecting electrically conductive particles and/or polarizable particles is provided, a laminate with a first electrode layer, a second electrode layer, at least a first insulation layer, which is arranged between the first electrode layer and the second electrode layer, optionally at least a third electrode layer, which is arranged between the first insulation layer and the first electrode layer, and optionally at least a second insulation layer, which is arranged between the third electrode layer and the first electrode layer, being produced, the first insulation layer, the at least third electrode layer, the at least second insulation layer and the first electrode layer being structured, in particular created by a lift-off process and/or an ink-jet process and/or in a stamping process, in such a way that, as a result of the structured application of the individual layers one over the other, a passage to the second electrode layer is formed. 
     In other words, already during the production of the insulation layer(s) and/or the first and/or third electrode layer, such a structure that has openings or clearances is produced, a number of openings that are arranged at least in certain portions one over the other forming at least one passage to the second electrode layer. If the sensor has a covering layer, this covering layer may also be applied in an already structured form to the first electrode layer. 
     In the case of all of the described processes for producing a sensor for detecting electrically conductive and/or polarizable particles, it is necessary that an electrical contacting area is respectively formed in the first electrode layer and/or in the second electrode layer and/or in the optionally at least third electrode layer. This is achieved by portions of the first electrode layer and/or of the second electrode layer and/or of the optionally at least third electrode layer being kept free from sensor layers arranged over the respective electrode layers. This may take place on the one hand by the electrical contacting areas being produced by removing and/or etching away and/or lasering away sensor layers arranged thereover. It is also conceivable that the insulation layers and/or the electrode layers and/or the covering layer(s) are applied to one another in a structured form, so that the electrical contacting areas are already kept free during the production of the individual sensor layers. 
     As an alternative or in addition, it is possible that at least the insulation layers, preferably all of the layers, of the laminate of the sensor are produced by means of an HTCC (high temperature cofired ceramics) process. The insulation layers are produced by combining powder, for example ceramic powder, metal powder, aluminum oxide powder and glass powder, and also an amount of binder and solvent, which together form a homogeneous liquid mass. This mass is applied to a film strip, so that green sheets are formed. The drying of the green sheets subsequently takes place. The dried green sheets may be cut and/or punched and/or shaped, in particular provided with openings. Subsequently, the green sheets may for example be rolled up and transported for further processing. 
     The electrode layers may for example be produced on the green sheet by printing, in particular by screen printing or stencil printing, from metal pastes. Alternatively, thin metal films may be produced and correspondingly prestructured. 
     Once the various substrate, electrode and insulation layers have been created, the green sheets are arranged in the desired sequence and positioned in exact register one over the other, pressed and joined together by thermal treatment. The binder may be of an organic or inorganic nature and during the thermal treatment either turns into a stable material or combusts or evaporates. The particles thereby fuse firmly to one another by melting and/or sintering processes during the thermal treatment. In this way, the three-dimensional structure of the sensor is formed or produced. 
     In a further embodiment of the invention it is conceivable that, when producing the laminate, the electrical contacting areas are covered with the aid of stencils, so that the electrical contacting areas cannot be coated with other sensor layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail below on the basis of exemplary embodiments with reference to the accompanying schematic drawings, in which: 
         FIGS. 1 a - c    show sectional representations of various embodiments of sensors for detecting electrically conductive and/or polarizable particles; 
         FIG. 2  shows a perspective plan view of a sensor according to the invention; 
         FIG. 3  shows a possible formation of a second electrode layer; 
         FIG. 4  shows a sectional representation of a further embodiment of a sensor for detecting electrically conductive and/or polarizable particles; 
         FIG. 5  shows a sectional representation of a further embodiment of a sensor for detecting electrically conductive and/or polarizable particles which comprises at least three electrode layers; 
         FIGS. 6 a - f    show representations of various embodiments of openings; 
         FIGS. 7   a+b  show representation of a possible arrangement of a sensor in a fluid flow; 
         FIGS. 8   a+b  show representations of various cross sections or cross-sectional profiles of passages; 
         FIG. 9  shows a sectional representation of undercuts in insulation layers or set-back insulation layers; and 
         FIGS. 10 a - d    show exploded representations of various embodiments of a sensor according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The same reference numerals are used below for parts that are the same and parts that act in the same way. 
       FIG. 1  a shows in a sectional representation a sensor  10  for detecting electrically conductive and/or polarizable particles, in particular for detecting soot particles. The sensor  10  comprises a substrate  11 , a first electrode layer  12  and a second electrode layer  13 , which is arranged between the substrate  11  and the first electrode layer  12 . An insulation layer  14  is formed between the first electrode layer  12  and the second electrode layer  13 . At least one opening is respectively formed in the first electrode layer  12  and in the insulation layer  14 , the opening  15  in the first electrode layer  12  and the opening  16  in the insulation layer  14  being arranged one over the other, so that a passage  17  to the second electrode layer  13  is formed. 
     For the purposes of a high-temperature application, the substrate  11  is formed for example from aluminum oxide (Al 2 O 3 ) or magnesium oxide (MgO) or from a titanate or from steatite. 
     The second electrode layer  13  is connected to the substrate  11  indirectly by way of a bonding agent layer  18 . The bonding agent layer  18  may be for example very thinly formed aluminum oxide (Al 2 O 3 ) or silicon dioxide (SiO 2 ). 
     In the exemplary embodiment, the first electrode layer  12  is formed by a platinum layer. In the example shown, the second electrode layer  13  consists of a platinum-titanium alloy (Pt—Ti). The platinum-titanium alloy of the second electrode layer  13  is a layer that is more resistant to etching in comparison with the first electrode layer  12 . 
     The distance between the first electrode layer  12  and the second electrode layer  13  is formed by the thickness d of the insulation layer  14 . The thickness d of the insulation layer may be 0.5 μm to 50 μm. In the present case, the thickness d of the insulation layer is 10 μm. The sensitivity of the sensor  10  according to the invention can be increased by reducing the distance between the first electrode layer  12  and the second electrode layer  13 , and consequently by reducing the thickness d of the insulation layer  14 . 
     The insulation layer  14  covers the second electrode layer  13  on the side face  19  shown, so that the second electrode layer  13  is laterally enclosed and insulated. 
     The passage  17  is formed as a blind hole, a portion of the second electrode layer  13  being formed as the bottom  28  of the blind hole. The blind hole or the passage  17  extends over the insulation layer  14  and over the first electrode layer  13 . The passage  17  is in other words formed by the openings  15  and  16  arranged one over the other. In the embodiment shown, the openings  15  and  16  are not formed peripherally. 
     A soot particle  30  can enter the passage  17 . In  FIG. 1   a,  the particle  30  is lying on the bottom  28  of the blind hole, and consequently on a side  31  of the second electrode layer  13 . However, the particle  30  is not touching the first electrode layer  12  in the peripheral region  32 , which bounds the opening  15 . As a result of the particle  30  being deposited on the bottom  28  and touching the second electrode layer  13  on the side  31 , the electrical resistance is reduced. This drop in the resistance is used as a measure of the accumulated mass of particles. When a predefined threshold value with respect to the resistance is reached, the sensor  10  is heated, so that the deposited particle  30  is burned and, after being burned free, the sensor  10  can detect electrically conductive and/or polarizable particles in a next detection cycle. 
       FIG. 1 b    likewise shows in a sectional representation a sensor  10  for detecting electrically conductive and/or polarizable particles, in particular for detecting soot particles. Likewise shown are a first electrode layer  12  and a second electrode layer  13 , which is arranged between the substrate  11  and the first electrode layer  12 . An insulation layer  14  is formed between the first electrode layer  12  and the second electrode layer  13 . With respect to the properties and the design of the openings  15  and  16 , reference is made to the explanations in connection with the embodiment according to  FIG. 1   a.    
     A covering layer  21 , which is for example formed from ceramic and/or glass and/or metal oxide, is formed on the side  20  of the first electrode layer  12  that is facing away from the insulation layer  14 . The covering layer  21  encloses the side face  22  of the first electrode layer  12 , the side face  23  of the insulation layer  14  and the side face  19  of the second electrode layer  13 . The covering layer  21  consequently covers the side faces  19 ,  22  and  23 , so that the first electrode layer  12 , the second electrode layer  13  and the insulation layer  14  are laterally insulated. The covering layer  21  consequently comprises an upper portion  24 , which is formed on the side  20  of the first electrode layer  12 , and a side portion  25 , which serves for the lateral insulation of the sensor  10 . 
       FIG. 1 c    shows in a sectional representation a sensor  10  for detecting electrically conductive and/or polarizable particles, in particular for detecting soot particles. The sensor  10  comprises a substrate  11 , a first electrode layer  12  and a second electrode layer  13 , which is arranged between the substrate  11  and the first electrode layer  12 . An insulation layer  14  is formed between the first electrode layer  12  and the second electrode layer  13 . At least one opening is respectively formed in the first electrode layer  12  and in the insulation layer  14 , the opening  15  in the first electrode layer  12  and the opening  16  in the insulation layer  14  being arranged one over the other, so that a passage  17  to the second electrode layer  13  is formed. 
     For the purposes of a high-temperature application, the substrate  11  is formed for example from aluminum oxide (Al 2 O 3 ) or magnesium oxide (MgO) or from a titanate or from steatite. 
     The second electrode layer  13  is connected to the substrate  11  indirectly by way of a bonding agent layer  18 . The bonding agent layer  18  may be for example very thinly formed aluminum oxide (Al 2 O 3 ) or silicon dioxide (SiO 2 ). 
     In the exemplary embodiment, the first electrode layer  12  is formed by a platinum layer. In the example shown, the second electrode layer  13  consists of a platinum-titanium alloy (Pt—Ti). The platinum-titanium alloy of the second electrode layer  13  is a layer that is more resistant to etching in comparison with the first electrode layer  12 . 
     The insulation layer  14  consists of a thermally stable material with a high insulation resistance. For example, the insulation layer  14  may be formed from aluminum oxide (Al 2 O 3 ) or silicon dioxide (SiO 2 ) or magnesium oxide (MgO) or silicon nitride (Si 3 N 4 ) or glass. 
     The distance between the first electrode layer  12  and the second electrode layer  13  is formed by the thickness d of the insulation layer  14 . The thickness d of the insulation layer may be 0.5 μm to 50 μm. In the present case, the thickness d of the insulation layer is 10 μm. The sensitivity of the sensor  10  according to the invention can be increased by reducing the distance between the first electrode layer  12  and the second electrode layer  13 , and consequently by reducing the thickness d of the insulation layer  14 . 
     A covering layer  21 , which is for example formed from ceramic and/or glass and/or metal oxide, is formed on the side  20  of the first electrode layer  12  that is facing away from the insulation layer  14 . The covering layer  21  encloses the side face  22  of the first electrode layer  12 , the side face  23  of the insulation layer  14  and the side face  19  of the second electrode layer  13 . The covering layer  21  consequently covers the side faces  19 ,  22  and  23 , so that the first electrode layer  12 , the second electrode layer  13  and the insulation layer  14  are laterally insulated. The covering layer  21  consequently comprises an upper portion  24 , which is formed on the side  20  of the first electrode layer  12 , and a side portion  25 , which serves for the lateral insulation of the sensor  10 . 
     In a further embodiment of the invention it is conceivable that the covering layer  21  also laterally encloses the substrate  11 . 
     A porous filter layer  27  is formed on the side  26  of the covering layer  21  that is facing away from the first electrode layer  12 . The sensitivity of the sensor  10  is increased as a result of the formation of this passive porous filter or protective layer  27  which is facing the medium that is to be detected with regard to electrically conductive and/or polarizable particles, since larger particles or constituents that could disturb the measurement or detection are kept away from the first electrode layer  12  and the second electrode layer  13 . Since the passage  17  is covered by the porous filter layer  27 , particles can still penetrate through the pores in the porous filter layer  27 , but short-circuits caused by large penetrated particles can be avoided as a result of the porous filter layer  27 . 
     The passage  17  is formed as a blind hole, a portion of the second electrode layer  13  being formed as the bottom  28  of the blind hole. The blind hole or the passage  17  extends over the insulation layer  14 , the first electrode layer  13  and over the covering layer  21 . For this purpose, the covering layer  21  also has an opening  29 . In other words, the passage  17  is formed by the openings  29 ,  15  and  16  arranged one over the other. 
     As a result of the choice of materials for the individual layers and the insulation of the individual layers from one another, the sensor  10  shown is suitable for a high-temperature application of up to for example 850° C. The sensor  10  can accordingly be used as a soot particle sensor in the exhaust-gas flow of an internal combustion engine. 
     After penetrating through the porous filter layer  27 , a soot particle  30  can enter the passage  17 . In  FIG. 1   c,  the particle  30  lies on the bottom  28  of the blind hole, and consequently on a side  31  of the second electrode layer  13 . However, the particle is not touching the first electrode layer  12  in the peripheral region  32 , which bounds the opening  15 . As a result of the particle  30  being deposited on the bottom  28  and touching the second electrode layer  13  on the side  31 , the electrical resistance is reduced. This drop in the resistance is used as a measure of the accumulated mass of particles. When a predefined threshold value with respect to the resistance is reached, the sensor  10  is heated, so that the deposited particle  30  is burned and, after being burned free, the sensor  10  can detect electrically conductive and/or polarizable particles in a next detection cycle. 
       FIG. 2  shows a perspective view of a sensor  10 . The sensor has nine passages  17 . For better illustration, the porous filter layer  27  is not shown in  FIG. 2 . The upper portion  24  of the covering layer  21  and also the side portion  25  of the covering layer  21  can be seen. The bottoms  28  of the passages  17  are formed by portions of the second electrode layer  13 . The nine passages  17  have a square cross section, it being possible for the square cross section to have a surface area of 15×15 μm 2  to 50×50 μm 2 . 
     The first electrode layer  12  has an electrical contacting area  33 . The second electrode layer  13  likewise has an electrical contacting area  34 . The two electrical contacting areas  33  and  34  are free from sensor layers arranged over the respective electrode layers  12  and  13 . The electrical contacting areas  33  and  34  are or can in each case be connected to a terminal pad (not shown). 
     The second electrode layer  13  has an additional electrical contacting area  35 , which is likewise free from sensor layers arranged over the electrode layer  13 . This additional electrical contacting area  35  may be connected to an additional terminal pad. The additional electrical contacting area  35  is necessary to allow the second electrode layer  13  to be used as a heating coil or as a temperature-sensitive layer or as a shielding electrode. Depending on the contacting assignment (see  FIG. 3 ) of the electrical contacting areas  34  and  35 , the second electrode layer  13  may either heat and burn the particle  30  or detect the particle  30 . 
     To be able to use an electrode layer, here the second electrode layer  13 , as a heating coil and/or temperature-sensitive layer and/or shielding electrode, the second electrode layer  13  has a small number of strip conductor loops  36 . 
     In  FIG. 4 , a further embodiment of a possible sensor  10  is shown. The first electrode layer  12  and the insulation layer  14  are respectively formed as porous, the at least one opening  15  in the first electrode layer  12  and the at least one opening  16  in the insulation layer  14  respectively being formed by at least one pore, the pore  41  in the insulation layer  14  and the pore  40  in the first electrode layer  12  being arranged at least in certain portions one over the other in such a way that the at least one passage  17  to the second electrode layer  13  is formed. In other words, it is possible to dispense with an active or subsequent structuring of the passages, the first electrode layer  12  and the insulation layer  14  being formed as permeable to the medium to be measured. The passages  17  are represented in  FIG. 4  with the aid of the vertical arrows. 
     The passages  17  may be formed by a porous or granular structure of the two layers  12  and  14 . Both the first electrode layer  12  and the insulation layer  14  can be produced by sintering together individual particles, with pores  40  and  41  or voids for the medium to be measured being formed while they are being sintered together. Accordingly, a passage  17  that allows access to the second electrode layer  13  for a particle  30  that is to be measured or detected must be formed, extending from the side  20  of the first electrode layer  12  that is facing away from the insulation layer  14  to the side  31  of the second electrode layer  13  that is facing the insulation layer  14  as a result of the one-over-the-other arrangement of pores  40  and  41  in the first electrode layer  12  and in the insulation layer  14 . 
     In the example shown, the second electrode layer  13  is completely enclosed on the side face  19  by the porous insulation layer  14 . The second electrode layer  13  is accordingly covered on the side  31  and on the side faces  19  by the porous insulation layer  14 . The porous first electrode layer  12  on the other hand encloses the porous insulation layer  14  on the side face  23  and on the side  37  facing away from the second electrode layer  13 . The insulation layer  14  is accordingly covered on the side  37  and on the side faces  23  by the first electrode layer  12 . 
     If this sensor  10  has a covering layer, this covering layer is also to be formed as porous in such a way that a pore in the covering layer, a pore  40  in the first electrode layer  12  and a pore  41  in the insulation layer  14  form a passage  17  to the second electrode layer  13 . 
     In  FIG. 5 , a section through a sensor  10  for detecting electrically conductive and/or polarizable particles, in particular for detecting soot particles, is shown. The sensor  10  can in principle be used for detecting particles in gases and in liquids. The sensor  10  comprises a substrate  11 , a first electrode layer  12 , a second electrode layer  13 , which is arranged between the substrate  11  and the first electrode layer  12 , a first insulation layer  14  being formed between the first electrode layer  12  and the second electrode layer  13 . 
     At least a third electrode layer  50  is formed between the first insulation layer  14  and the first electrode layer  12 , at least a second insulation layer  60  being formed between the third electrode layer  50  and the first electrode layer  12 . 
     According to sensor  10  of  FIG. 5 , therefore at least three electrode layers  12 ,  13 ,  50  and at least two insulation layers  14 ,  60  are formed. The first electrode layer  12  is in this case the electrode layer that is arranged furthest away from the substrate  11 . The second electrode layer  13  on the other hand is connected directly to the substrate  11 . It is possible that the second electrode layer  13  is connected indirectly to the substrate  11 , preferably by means of a bonding agent layer. 
     In the embodiment according to  FIG. 5 , a fourth electrode layer  51  is also formed and also a third insulation layer  61 . The sensor  10  consequently comprises altogether four electrode layers, to be specific the first electrode layer  12 , the second electrode layer  13 , and also the third electrode layer  50  and the fourth electrode layer  51 . Insulation layers are respectively formed between the electrode layers ( 12 ,  13 ,  50 ,  51 ), to be specific the first insulation layer  14 , the second insulation layer  60  and also the third insulation layer  61 . The sensor  10  also comprises a covering layer  21 , which is formed on the side of the first electrode layer  12  that is facing away from the substrate  11 . 
     At least one opening  15 ,  16 ,  70 ,  71 ,  72 ,  73  is respectively formed in the first electrode layer  12 , in the third insulation layer  61 , in the fourth electrode layer  51 , in the second insulation layer  60 , in the third electrode layer  50  and in the first insulation layer  14 . The covering layer  21  also has an opening  29 . The opening  15  in the first electrode layer  12 , the opening  73  in the third insulation layer  61 , the opening  72  in the fourth electrode layer  51 , the opening  71  in the second insulation layer  60 , the opening  70  in the third electrode layer  50  and the opening  16  in the first insulation layer  14  are arranged at least in certain portions one over the other in such a way that at least one passage  17  to the second electrode layer  13  is formed. 
     The distance between the electrode layers  12 ,  13 ,  50  and  51  is formed by the thickness of the insulation layers  14 ,  60  and  61 . The thickness of the insulation layers  14 ,  60  and  61  may be 0.1 μm to 50 μm. The sensitivity of the sensor  10  according to the invention can be increased by reducing the distance between the electrode layers  12 ,  13 ,  50  and  51 , and consequently by reducing the thickness of the insulation layers  14 ,  60  and  61 . 
     The passage  17  is formed as a blind hole, a portion of the second electrode layer  13  being formed as the bottom  28  of the blind hole. The blind hole or the passage  17  extends over the first insulation layer  14 , the third electrode layer  50 , the second insulation layer  60 , the fourth electrode layer  51 , the third insulation layer  61 , the first electrode layer  12  and over the covering layer  21 . In other words, the passage  17  is formed by the openings  16 ,  70 ,  71 ,  72 ,  73 ,  15  and  29  arranged over one another. In the embodiment shown, the openings  16 ,  70 ,  71 ,  72 ,  73 ,  15  and  29  are not formed peripherally. A perspective section through a passage  17  is shown. 
     A small soot particle  30  for example can enter the passage  17 . In  FIG. 5 , the particle  30  is lying on the bottom  28  of the blind hole, and consequently on a side  31  of the second electrode layer  13 . The particle  30  is also touching the third electrode layer  50 . If the determination of particles is performed on the basis of the resistive principle, the resistance between the second electrode layer  13  and the third electrode layer  50  is measured, this resistance decreasing if the particle  30  bridges the two electrode layers  13  and  50 . The size of the particle  30  is consequently relatively small. 
     The soot particle  30 ′ has also entered the passage  17 . The particle  30 ′ is lying on the bottom  28  of the blind hole, and consequently on the side  31  of the second electrode layer. The particle  30 ′ is also touching the third electrode layer  50 , the fourth electrode layer  51  and also the first electrode layer  12 . The particle  30 ′ consequently bridges a number of electrode layers, in the example shown all of the electrode layers  12 ,  13 ,  50  and  51 , so that the particle  30 ′ is detected as a particle that is larger in comparison with the particle  30 . 
     By applying different voltages to the electrode layers  12 ,  13 ,  50  and  51 , different particle properties, in particular different soot properties, such as for example the diameter and/or the size of the (soot) particle and/or the charging of the (soot) particle and/or the polarizability of the (soot) particle, can be measured. 
     Various embodiments of openings  80  are shown in  FIGS. 6 a    to  6   f.  The openings  80  may be formed both in insulation layers  14 ,  60  and  61  and in electrode layers  12 ,  50  and  51 . Accordingly, the openings  80  that are shown may be an arrangement of openings  15  in a first electrode layer  12 , openings  16  in a first insulation layer  14 , openings  70  in a third electrode layer  50 , openings  71  in a second insulation layer  60 , openings  72  in a fourth electrode layer  51  and also openings  73  in a third insulation layer  61 . 
     Preferably, the openings  80  in a laminate of the sensor  10  are formed similarly. The individual layers  12 ,  14 ,  21 ,  50 ,  51 ,  60  and  61  are arranged one over the other in such a way that the openings  15 ,  16 ,  29 ,  70 ,  71 ,  72  and  73  form passages  17 . As a result of the openings shown in  FIGS. 6 a    to  6   d,  elongate depressions  17 ′ and  17 ″ are respectively formed. 
     In  FIG. 6   a,  linear openings  80  are formed, the openings  80  being formed parallel to one another and all pointing in the same predominant direction. 
     In  FIG. 6   b,  a layer of the sensor  10  is subdivided into a first portion  45  and a second portion  46 . All of the openings  80 ,  80 ′ shown are formed as linear clearances, with both the openings  80  in the first portion  45  being formed parallel to one another, and the openings  80 ′ in the second portion  46  being formed parallel to one another. The openings  80  in the first portion  45  run parallel in the horizontal direction or parallel to the width b of the sensor layer, whereas the openings  80 ′ in the second portion  46  run parallel in the vertical direction or parallel to the length l of the sensor layer. The openings  80 ′ in the second portion  46  run in a perpendicular direction in relation to the openings  80  in the first portion  45 . 
     In  FIG. 6   c,  likewise a number of openings  80 ,  80 ′,  80 ″ are shown in the form of elongate clearances. In a central portion  47 , a number of linear openings  80 ′ running in the vertical direction are shown, in the example shown eight openings, which are formed parallel to the length l of the sensor layer. These openings are surrounded by further openings  80 ,  80 ″, forming a frame-like portion  48 . First openings  80 ″ are in this case formed parallel to the openings  80 ′ of the central portion  47 . Further openings  80  are formed perpendicularly in relation to the openings  80 ,  80 ″. The openings  80 ″ are of different lengths, so that the layer of the sensor  10  can be formed with a largest possible number of openings  80 . 
     In  FIG. 6   d,  a sensor layer with an elongate through-opening  80  is shown, the opening  80  running in a meandering manner. 
     In  FIG. 6   e,  a further sensor layer with a number of vertically running openings  80 ′ and a number of horizontally running openings  80  is shown. The vertical openings  80 ′ and the horizontal openings  80  form a grid structure. 
     Apart from rectangular grid structures, other angular arrangements can also be produced, or geometries in which the grid or network structure has round, circular or oval shapes. Furthermore, corresponding combinations of the structures, which may be regular, periodic or irregular, can be created. 
     In  FIG. 6   f,  a sensor layer with an elongate through-opening  80  is shown, the opening  80  running spirally. Apart from rectangular geometries, circular, oval geometries or combinations thereof can also be produced. 
     In each case a number of layers, which respectively have openings  80 ,  80 ′,  80 ″ according to an embodiment of  FIG. 6   a,    6   b,    6   c,    6   d,    6   e  or  6   f,  are arranged in layers one over the other, so that passages in the form of elongate depressions  17 ′ and  17 ″ are respectively formed in a sensor. 
     As shown in  FIG. 7   a,  a sensor  10  is introduced into a fluid flow in such a way that the direction of flow a of the particles does not impinge perpendicularly on the plane (x, y) of the electrode layers. The angle α between the normal (z) to the plane (x, y) of the first electrode layer and the direction of flow of the particles is in this case at least 1 degree, preferably at least 10 degrees, particularly preferably at least 30 degrees. The particles can consequently be guided more easily into the elongate depressions  17 ′,  17 ″, and consequently more easily to the walls of the openings of the electrode layers  12 ,  50 ,  51  formed therein. 
     In  FIG. 7   b,  a sensor  10  has thus been introduced into a fluid flow in such a way that the angle β between the direction of flow a of the particles and the longitudinal axis x of the elongate depressions lies between 20 and 90 degrees. 
     In  FIGS. 8 a    and  8   b,  a cross section which is taken perpendicularly to the sensor  10 , that is to say beginning from the uppermost insulation or covering layer  21  to the substrate  11 , is respectively shown. The sensors  10  of  FIGS. 8 a  and 8 b    have four electrode layers, to be specific a first electrode layer  12 , a second electrode layer  13  and also a third electrode layer  50  and a fourth electrode layer  51 . Also formed are three insulation layers, to be specific a first insulation layer  14 , a second insulation layer  60  and also a third insulation layer  61 . 
     In the sensor  10  according to  FIG. 8   a,  the cross-sectional profiles of two passages in the form of elongate depressions  17 ′,  17 ″ are shown. The left passage  17 ′ has a V-shaped cross section or a V-shaped cross-sectional profile. The right passage  17 ″ on the other hand has a U-shaped cross section or a U-shaped cross-sectional profile. The sizes of the openings or cross sections of the openings decrease from the covering layer  21  in the direction of the second electrode layer  13 . The cross sections of the openings  29 ,  15 ,  73 ,  72 ,  71 ,  70  and  16  become increasingly smaller from the first cross section of an opening  29  in the direction of the lowermost cross-sectional opening  16 . 
     With the aid of the V-shaped and U-shaped cross-sectional profiles, the measurements of round particles are improved. 
     In  FIG. 8 b    it is also shown that the passages  17 ′,  17 ″ can have different widths. The left passage  17 ′ has a width B 1 . The right passage  17 ″ shown has a width B 2 . B 1  is greater than B 2 . As a result of passages  17 ′,  17 ″ formed with different widths, size-specific measurements of the particles  30  can be carried out. 
     In  FIG. 9 , undercuts in insulation layers  14 ,  21 ,  60 ,  61  or set-back insulation layers  14 ,  21 ,  60 ,  61  are shown in cross section. In the case of round particles, the formation of level or smooth passage surfaces is unfavorable. The measurement of round particles can be improved by the formation of undercuts or set-back insulation layers. 
     The left passage  17 ′ shown has a first insulation layer  14 , a second insulation layer  60  and also a third insulation layer  61  and a covering layer  21 , which also serves as an insulation layer. The insulation layers  14 ,  60 ,  61  and  21  have undercuts or clearances  90 . The size of the openings  16 ,  71 ,  73  and  29  in the insulation layers  14 ,  60 ,  61  and  21  are consequently greater than the openings  70 ,  72  and  15  in the electrode layers  12 ,  50  and  51  that are respectively formed over and under the insulation layers  14 ,  60 ,  61  and  21 . 
     This also applies in connection with the passage  17 ″ shown on the right. In this case, the insulation layers  14 ,  16 ,  61  and  21  are formed as set-back in comparison with the electrode layers  50 ,  51  and  12 . The openings  16 ,  71  or  73  in an insulation layer  14 ,  60  or  61  is formed larger in each case than an opening  70 ,  72  or  15  formed thereover in an electrode layer  50 ,  51  or  12  arranged over the respective insulation layer. Since the cross-sectional profile of the right passage  17 ″ is formed in a V-shaped manner and the openings in all the layers  21 ,  12 ,  61 ,  51 ,  60 ,  50  and  14  become smaller in the direction of the substrate  11 , the openings  16 ,  71 ,  73  and  29  in the insulation layers  14 ,  60 ,  61  and  21  are not of coinciding sizes. 
     It should be pointed out in connection with the sensors  10  shown in  FIGS. 5, 8   a,    8   b  and  9  that it is possible that only two uppermost electrode layers have to be made accessible within a passage. In other words, in a method, preferably according to the invention, a passage  17 ,  17 ′,  17 ″ that is merely formed with respect to the uppermost electrode layers  12  and  51  may be formed in a sensor  10 . 
     It is also possible that a sensor  10  comprises a number of passages  17 ,  17 ′,  17 ″, at least a first passage merely reaching as far as the fourth electrode layer  51 . The fourth electrode layer  51  or the second insulation layer  60  forms the bottom of this passage formed. 
     A second passage reaches as far as the third electrode layer  50 . The third electrode layer  50  or the first insulation layer  14  forms the bottom of the passage formed. A third passage reaches as far as the second electrode layer  13 . The second electrode layer  13  forms the bottom of the passage formed. 
     This embodiment can be carried out or can be formed independently of the features of the sensors  10  shown in  FIGS. 5, 8   a,    8   b  and  9 . 
     The exploded representations of  FIGS. 10 a  to 10 d    illustrate that a number of openings can be formed in a number of layers of the sensor  10 , the layers being arranged one over the other in such a way that the openings are also formed one over the other, so that passages  17 ,  17 ′ and  17 ″ can be formed. 
     The sensors  10  shown comprise a substrate  11 , a second electrode layer  13  arranged thereupon, a first electrode layer  12  and also a first insulation layer  14 , which is arranged between the first electrode layer  12  and the second electrode layer  13 . A first covering layer  21  and also a second covering layer  42  are formed on the first electrode layer  12 . The first electrode layer  13  does not have an arrangement of openings for the forming of passages (see  FIG. 10 a   ). 
     Gaps  95  are formed within the second electrode layer  13 . The first insulation layer  14  is arranged on the second electrode layer  13  in such a way that the openings  16  in the first insulation layer  14  are not arranged above the gaps  95 . 
     On the other hand, the first electrode layer  12  is arranged in such a way that the openings  15  in the first electrode layer  12  are arranged above the openings  16  in the first insulation layer  14 . With the aid of the openings  15  in the first electrode layer  12  and the openings  16  in the first insulation layer  14 , passages  17  are formed, the side  31  of the first electrode layer  13  serving as the bottom  28  of the passages, in particular of blind holes and/or elongate depressions  17 ′,  17 ″. 
     In  FIG. 10   b,  the arrangement of the openings  15  and  16  in relation to one another is shown in an enlarged representation. It can be seen that a first portion  45  and a second portion  46  with openings  15  and  16  are respectively formed both in the first insulation layer  14  and in the first electrode layer  12 . The openings  15  and  16  arranged one over the other form in each case blind-hole-like passages  17 . 
     Also in  FIG. 10   c,  a first portion  45  and a second portion  46  are respectively formed in the first insulation layer  14  and also in the first electrode layer  12 . Elongate openings  15 ,  16  are respectively formed in the portions  45  and  46 , the elongate openings  15  and  16  being oriented in the same directions. 
     According to the representation of  FIG. 10 d    it is possible that the elongate openings  15  and  16  can also be aligned perpendicularly in relation to the orientations shown in  FIG. 10   c.    
     It is pointed out that some of the sensors  10  shown ( FIGS. 1 a   - 1   c,    FIG. 4 ,  FIG. 5 ,  FIGS. 8 a - b    and  FIG. 9 ) are in each case only shown as a detail. The measurement of the particles preferably takes place only in the passages  17 ,  17 ′,  17 ″ and not on side edges/side faces of the sensor and not on side faces/side edges of the sensor layers. 
     It is also possible that, in a further embodiment of the invention, all of the sensors  10  shown do not have an upper insulation layer/covering layer  21  and/or do not have a filter layer  27 . If sensors  10  do not have an upper insulation layer/covering layer  21  and/or do not have a filter layer  27 , large particles have no influence on the signal or on the measurement result. 
     With regard to a possible production process in connection with the sensors  10  according to the invention of  FIGS. 1 a   - c,    2 ,  4 ,  5 ,  8   a - b,    9  and  FIGS. 10 a   - d,  reference is made to the production possibilities already described, in particular to etching processes and/or laser machining processes. 
     At this stage it should be pointed out that all of the elements and components described above in connection with the embodiments according to  FIGS. 1 a  to 10 d    are essential to the invention on their own or in any combination, in particular the details that are shown in the drawings. 
     LIST OF DESIGNATIONS 
     
         
           10  Sensor 
           11  Substrate 
           12  First electrode layer 
           13  Second electrode layer 
           14  First insulation layer 
           15  Opening in first electrode layer 
           16  Opening in first insulation layer 
           17  Passage 
           17 ′,  17 ″ Elongate depression 
           18  Bonding agent layer 
           19  Side face of second electrode layer 
           20  Side of the first electrode layer 
           21  Covering layer 
           22  Side face of first electrode layer 
           23  Side face of insulation layer 
           24  Upper portion of covering layer 
           25  Side portion of covering layer 
           26  Side of covering layer 
           27  Porous filter layer 
           28  Bottom 
           29  Opening in covering layer 
           30 ,  30 ′ Particle 
           31  Side of second electrode layer 
           32  Peripheral region of first electrode layer 
           33  Electrical contacting area of first electrode layer 
           34  Electrical contacting area of second electrode layer 
           35  Additional electrical contacting area of second electrode layer 
           36  Strip conductor loop 
           37  Side of insulation layer 
           40  Pore in first electrode layer 
           41  Pore in insulation layer 
           42  Second covering layer 
           45  First portion 
           46  Second portion 
           47  Central portion 
           48  Frame-like portion 
           50  Third electrode layer 
           51  Fourth electrode layer 
           60  Second insulation layer 
           61  Third insulation layer 
           70  Opening in third electrode layer 
           71  Opening in second insulation layer 
           72  Opening in fourth electrode layer 
           73  Opening in third insulation layer 
           80 ,  80 ′,  80 ″ Opening 
           90  Undercut 
           95  Gap 
         a Direction of flow 
         b Width of sensor layer 
         l Length of sensor layer 
         B 1  Width of passage 
         B 2  Width of passage 
         d Thickness of insulation layer 
         x Longitudinal axis of the elongate depressions 
         α Angle between the normal to the electrode plane and the direction of flow 
         β Angle between the longitudinal axis and the direction of flow