EP2329256A1

EP2329256A1 - Protective layers suitable for exhaust gases for high-temperature chemfet exhaust gas sensors

Info

Publication number: EP2329256A1
Application number: EP09780679A
Authority: EP
Inventors: Thomas Wahl; Oliver Wolst; Alexander Martin
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2008-09-16
Filing date: 2009-07-16
Publication date: 2011-06-08
Also published as: DE102008042139A1; CN102159941B; WO2010031609A1; US20110260219A1; CN102159941A; JP5340390B2; JP2012503171A

Abstract

The invention relates to a method for producing a sensor element (1), comprising at least one sensitive component (3), wherein a masking layer (31) made of a residue-free thermally decomposable material is applied onto the sensitive component (3), wherein the sensitive component (3) is substantially completely covered by the masking layer (31). A protective layer (33) made of a temperature-stable material is applied onto the masking layer (31), and the masking layer (31) is removed by pyrolysis or a low temperature-guided oxygen plasma. The invention further relates to a sensor element, comprising at least one sensitive component (3) and a protective layer (33) made of a temperature-stable material, wherein the sensitive component (3) is covered by the protective layer (33) made of temperature-stable material, wherein the sensitive component (3) and the protective layer (33) are disposed at a distance from each other.

Description

description

title

Exhaust gas protective layers for high temperature ChemFET exhaust gas sensors

State of the art

The invention relates to a method for producing a sensor element, comprising at least one sensitive component. Furthermore, the invention is based on a sensor element according to the preamble of claim 11.

Sensor elements which comprise a sensitive component are used, for example, for measuring at least one property of a gas in a measuring gas space. This at least one property may be a physical and / or chemical property of the gas, in particular a composition of the gas. For example, such a sensor element can be used to measure a concentration and / or a partial pressure of a specific gas component in the gas, for example in the exhaust gas of an internal combustion engine, or to qualitatively and / or quantitatively detect these gas components. Instead of or in addition to a gas component, however, it is also possible, for example, to detect other types of analytes, for example analytes in states other than the gaseous state, for example liquid analytes and / or analyte particles.

In order to determine at least one property of a gas in a measurement gas space, the sensor elements generally comprise a component with a gas-sensitive layer, in particular a gas-sensitive layer semiconductor component. Such semiconductor devices with a gas sensitive layer are generally gas sensitive field effect transistors. In such gas-sensitive field effect transistors, the gate electrode is provided with a coating on which gas molecules can adsorb, which lead to a potential change at the gate, which in turn changes the charge carrier density in the transistor channel and thus the characteristic of the transistor. This is a signal for the presence of the respective gas. As a coating, in each case a material is selected which is selective for certain gases to be detected. For this purpose, the coating generally contains a catalytically active material. Through the use of different gas sensitive field effect transistors, each having specific gate coatings, different gases can be detected.

The gases to be detected can interact in various ways with the sensor element, in particular with the gas-sensitive layer, for example by adsorption and / or chemisorption, chemical reactions or else in other ways. The interaction of the gas to be detected with the gas-sensitive layer causes the potential change at the gate, which influences the charge carrier density of the field effect transistor in the channel region below. The potential change at the gate is caused by the changed work function of the gate metal with respect to the gate dielectric and / or the change in the interface state density between the dielectric (insulator) and the semiconductor material. As a result, the characteristic of the transistor is changed, which can be interpreted as a signal for the presence of the respective gas. Examples of such gas-sensitive field-effect transistors are shown for example in DE 26 10 530, so that reference may be made to this document for possible structures of such gas-sensitive field-effect transistors.

Sensor elements are used for the detection of gas components, for example also in exhaust gas lines of motor vehicles. With such sensor elements, for example, the presence of nitrogen oxides, ammonia or hydrocarbons in the exhaust gas can be determined. Due to the high temperatures of the exhaust gas of the internal combustion engine, however, high demands are placed on the sensor elements. In addition, particles may also be present in the exhaust gas, for example, which can lead to abrasion of the gate coatings. This requires protection of the gate coatings, but the function must not be impaired by such protection.

In order to protect the gate coatings, it is known, for example, to apply a gas-sensitive coating to the gas-sensitive field-effect transistor. Such a gas-sensitive field-effect transistor with an open-pore porous sensitive layer is described, for example, in DE-A 10 2005 008 051. However, the application of a porous layer on the gas-sensitive field effect transistor has the disadvantage that the very sensitive gate coating can be damaged. In addition, it is possible in particular when using the sensor element at high temperatures that due to different coefficients of thermal expansion of the semiconductor device and the protective layer tensions occur, which can lead to cracks in the protective layer or even flaking of the protective layer especially at layer thicknesses of a few micrometers to a few millimeters , Disclosure of the invention

Advantages of the invention

An inventive method for producing a sensor element, comprising at least one sensitive component, comprises the following steps:

(a) applying a masking layer of a residue-free thermally decomposable material to the sensitive component, wherein the sensitive component is completely covered by the masking layer,

(b) applying a protective layer of a temperature-stable material to the masking layer,

(c) removing the masking layer by pyrolysis or a low-temperature oxygen plasma.

By removing the masking layer after the application of the protective layer, a cavity is created between the sensitive device and the protective layer. The protective layer does not lie directly on the sensitive component. This has the particular advantage that at high temperature load and temperature changes with large differences in the thermal expansion coefficient of the protective layer and the sensitive component thermal stresses are avoided. At the same time, this leads to a stabilization of the protective layer, since no cracks are formed in the protective layer and no deposition of the protective layer occurs if the protective layer and the sensitive component expand differently due to high temperature load and temperature change.

In general, the sensitive component is applied to a carrier substrate. The carrier substrate usually comprises conductor tracks, with which the sensitive component is contacted. In particular, at high temperature use of the sensor element, the carrier substrate is made of a ceramic material. However, when used in low temperature applications, it is also possible for the carrier substrate to be, for example, a polymeric carrier substrate, as commonly used in printed circuit board manufacturing.

However, when the sensor element is used in high temperature applications, such as exhaust gas analysis of internal combustion engines, the material from which the substrate substrate is made is a ceramic. Suitable ceramic material Examples of materials for producing the carrier substrate are silicon nitride (Si ₃ N ₄ ), silicon oxide (SiO ₂ ), aluminum oxide (Al ₂ O 3) or zirconium oxide (ZrO) or mixtures of two or more of these materials. In order to achieve a protection of the sensitive component, it is necessary that the sensitive component is enclosed on all sides. In order to realize a corresponding protection, the protective layer is connected to the carrier substrate. In order to avoid that due to different coefficients of thermal expansion of the material of the protective layer and the material of the carrier substrate thermal stresses occur which can lead to damage of the protective layer, such as a breaking of the protective layer, it is preferred that the material for the protective layer is the same material as the material for the carrier substrate.

The temperature-stable material for the protective layer is thus preferably a ceramic material, particularly preferably silicon nitride, silicon oxide, aluminum oxide, zirconium oxide or mixtures thereof.

The residue-free thermally decomposable material from which the masking layer is made is preferably a thermally decomposable polymer. Materials or material classes of suitable thermally decomposable polymers which can be used as a masking layer are, for example, polyesters, polyethers such as polyethylene glycol, polypropylene glycol, polyethylene oxide or polypropylene oxide. Also suitable are polyacrylates, polyacetates, polyketals, polycarbonates, polyurethanes, polyether ketones, cycloaliphatic polymers, aliphatic polyamides, polyvinylphenols and epoxy compounds. Also suitable are co- or ter-polymers of the material classes mentioned here.

Preferably, the decomposable material is photosensitive or photostructurable, such as a resist.

In particular, a photoimageable resist may be one of the following combinations of a base polymer and a photoactive component. As a base polymer, for example, polyacrylates, polymethacrylates, polyacetates, polyacetals, co-polymers with maleic anhydride, aliphatic, aromatic or cycloaliphatic polymers with tert-butyl ester (COOC (C _n H _{2n +} I) ₃ with n = 1, 2, 3) such as tert Butyl methacrylate, or with tert-butoxycarbonyloxy groups (OCOO (C _n H _{2n +} I) ₃ ), such as tert-butoxycarbonyloxystyrene. Examples of suitable photoactive components are diazoketones, diazoquinones, triphenylsulfonium salts or diphenyliodonium salts. Thus, the resist can be patterned, for example by lithography and etching. Suitable solvents for obtaining coatable polymeric solutions or mixtures of a base polymer and a photoactive component are, for example, methoxypropyl acetate, ethoxypropyl cetate, ethoxyethyl propionate, N-methylpyrrolidone, γ-butyrolactone, cyclohexanone, cyclopentanone or ethyl acetate.

The layer thickness with which the residue-free thermally decomposable material for the masking layer is applied to the sensitive component is preferably in the range from 1 μm to 2 mm. The small layer thickness of the masking layer makes it possible in particular to realize a compact construction. Furthermore, the masking has the advantage that in the subsequent application method of the ceramic protective layer the sensitive semiconductor component is protected by its mechanically sensitive electrodes and a partially occurring abrasion of the electrode materials during the protective layer production is avoided. In addition, the thermally decomposable material may be removed during the application process, e.g. in thermal plasma spraying, already thermally decomposed and thus act as pore formers. If the coating process takes place at temperatures below the decomposition point of the marking layer, the protective layer is preferably set up in such a way that it is already permeable to decomposable material.

Since the sensitive component has a three-dimensional structure, it is necessary for applying the masking layer of the residue-free thermally decomposable material to use a method that is 3D-capable, that is, in which a coating over at least one stage is possible. Suitable methods for applying the residue-free thermally decomposable material are, for example, dispensing, ink-jet printing, pad printing, spin-coating or dipping. Dispensing, inkjet printing or pad printing also have the advantage that additive application is possible in order to produce the desired layer thickness. In order to apply the residue-free thermally decomposable material to the sensitive component by one of the methods mentioned above, the polymer used for the layer is dissolved, for example, in a solvent or dispersed in a solvent. The application of the residue-free thermally decomposable material is followed in this case by a drying step in order to remove the solvent from the thermally decomposable material, in particular the polymer.

In addition to the use of polymers which are dissolved or dispersed in a solvent, it is alternatively also possible, for example, to use monomers and / or polymers with radiation-curing properties, in particular UV-curing properties, in order to form the masking layer. In this case, after the application of the monomers and / or polymers for the masking layer, an exposure step is carried out by which the monomers and / or polymers crosslink and thus harden to a rigid, generally insoluble polymer layer. Suitable monomers and / or polymers with radiation For example, cure-hardening properties are those which contain epoxide groups, acrylate groups and / or methacylate groups as functional groups.

After drying, that is, removal of solvent, and / or curing, for example by an exposure step, the protective layer of the temperature-stable material is applied to the masking layer. For applying the protective layer, a spraying method is usually used as the application method. To produce a thick, abrasion-resistant protective layer, various spraying methods are conceivable and advantageously usable. Preference is given to plasma spraying processes with which the protective layer of the temperature-stable material is applied to the masking layer. The masking layer avoids uncontrolled action of the plasma on the gas-sensitive layer during the plasma spraying process, which leads to a more robust design of the protective layer production process and thus to a cost reduction. The action of the plasma during the plasma spraying process results, for example, from a mechanical loading of the sensitive component during application.

The advantage of using a plasma spraying method is that a defined porosity of the protective layer can be set. The porosity of the protective layer is necessary so that the gas to be detected or the gas mixture to be examined passes through the protective layer to the gas-sensitive component. However, particles contained in the gas stream are retained by the sensitive component through the protective layer, so that mechanical damage to the sensitive component is prevented.

After the protective layer has been applied, the masking layer is removed by pyrolysis or a low-temperature-controlled oxygen plasma. The gaseous product formed in the pyrolysis or the low-temperature-controlled oxygen plasma can be removed through the pores of the porous protective layer. Furthermore, it is also possible to adjust the porosity of the protective layer by the pyrolysis or the low-temperature-guided oxygen plasma. In particular, in the case of a protective layer of a ceramic material, it is furthermore preferred if the protective layer is sintered during the pyrolysis or through the low-temperature-guided oxygen plasma. In this case, a porous sintering is usually carried out to adjust the porosity of the protective layer.

The pyrolysis for removing the masking layer may be carried out, for example, in air or in an oxygen-rich atmosphere. It is also possible to change the composition of the atmosphere during pyrolysis. As an oxygen-rich atmosphere, for example, pure oxygen or oxygen-enriched air is introduced. puts. In the case of oxygen-enriched air, the oxygen content in the atmosphere is preferably in the range of 21 to 100% by volume, more preferably in the range of 22 to 50% by volume. In addition, a pyrolysis in a hydrogen-rich atmosphere is possible. The required decomposition temperature is mainly dependent on the choice of thermally decomposable masking materials. However, the temperature can be influenced via the pyrolysis parameters, for example the ambient pressure.

An inventively designed sensor element, which is produced for example by the method described above, comprises at least one sensitive component and a protective layer of a temperature-stable material, wherein the sensitive component is covered by the protective layer of the temperature-stable material. The sensitive device and the protective layer are spaced apart. As described above, with the sensitive component and the protective layer spaced from each other, thermal stresses due to high temperature stress or temperature changes are avoided.

The sensitive component is preferably a semiconductor sensor element, in particular a semiconductor sensor element with a semiconductor material comprising silicon carbide and / or gallium nitride. The sensitive component may in particular comprise a field effect transistor or a sensor component based on a field effect transistor. The sensitive component is particularly preferably a chemosensitive field effect transistor, in particular a gas sensitive field effect transistor.

A sensitive component has, for example, a sensor body with at least one sensor surface accessible to the gas to be measured. The sensor surface is usually designed so that at least one property of the gas can be measured with the sensor surface. In particular, with the sensor surface, a concentration of at least one gas component in the gas to be measured can be determined quantitatively and / or qualitatively selectively. For this purpose, it is possible, for example, that the sensor surface comprises a semiconductor surface of an inorganic semiconductor material, which may optionally be additionally provided with a sensitive coating. Thus, for example, a sensitive coating may be included which increases the selectivity of detecting a particular gas component. For example, the sensor surface may be a gate surface of a transistor element, in particular of a field-effect transistor. Preferably, the sensor surface is arranged on an outer surface of the sensor body, for example on an outer surface of an inorganic semiconductor layer structure, in particular of a semiconductor chip. The gas-sensitive layer generally contains a catalytically active material, so that upon contact with the gas to be measured, a chemical reaction is initiated, by which the electrical property of the gas-sensitive layer changes.

In order to allow access of the gas to be measured to the gas-sensitive surface, the protective layer of the temperature-stable material is porous. The protective layer preferably has a porosity in the range of 2 to 50%, in particular in the range of 10 to 30%.

Brief description of the drawings

Embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description.

Show it:

FIG. 1-4 Method steps for producing a sensor element according to the invention using the example of a gas-sensitive field-effect transistor.

In the figure 1, a sensor element is still shown without coating. A sensor element 1 comprises a sensitive component 3, which is connected to a carrier substrate 5.

The sensitive component 3 is a gas-sensitive field-effect transistor in the embodiment shown here. As an alternative to the embodiment illustrated here with a field effect transistor as a sensitive component 3, it is also possible to use a plurality of field effect transistors 3, for example in the form of an array of gas sensitive field effect transistors. An array of gas-sensitive field-effect transistors is used, for example, for the simultaneous detection of different gas components. The sensor element 1 can serve, for example, for the qualitative and / or quantitative detection of one or more gas components of a gas in a gas-containing environment. The gaseous environment may be, for example, an exhaust system of an internal combustion engine.

The sensitive component 3 designed as a gas-sensitive field-effect transistor comprises a sensor body 7 which is formed completely or partially from silicon carbide (SiC) and / or gallium nitride (GaN) as semiconductor material, optionally in different dopings. Usually, the sensor body 7 is constructed as a semiconductor chip. In general, the sensor body 7 comprises a source region 9 and a drain region 11, which are produced, for example, by corresponding dopants in the sensor body 7. can be. For example, the sensor body 7 has an n-doping in the source region 9 and drain region 11, whereas the remaining region of the sensor body 7 may be p-doped, for example. For electrical control, the source region 9 is connected to a source electrode 13 and the drain region 11 is connected to a drain electrode 15. The electrical contacting of the source electrode 13 and the drain electrode 15 takes place via contacting means 17 Contacting means 17 may be printed on the sensitive component 3, for example, conductor track structures which connect the source electrode 13 and the drain electrode 15 with conductor tracks 19 on the carrier substrate 5. Alternatively, however, it is also possible to use, as contacting means 17, for example, wirings in the form of wire bonds or any other contacting known to the person skilled in the art in order to connect the source electrode 13 and the drain electrode 15 with the electrodes Interconnects 19 to connect. In addition, in a specific embodiment, a flip-chip structure is conceivable. The sensor surface with the gas-sensitive coating 25 points in the direction of the ceramic carrier 5, wherein the gas inlet is secured in the carrier 5 via an additional channel.

In the electrical control of the sensor element 1, a current channel is formed between the source region 9 and the drain region 11 in the sensor body 7. The expansion and the electrical properties of this current channel and thus a current flow between the source region 9 and the drain region 11 are influenced by a gate electrode 21 in conventional field effect transistors. The role of the gate electrode 21 is taken over in a gas-sensitive field effect transistor on the one hand by a metallic electrode in conjunction with a semiconductor oxide material, or on the other hand, for example by a gate layer stack 23, which is provided with a gas-sensitive coating 25. The gate layer stack is typically made of a dielectric material, such as SiC> 2, SΪ3N4, SiO _x Ny, AI2O3, Hfθ2, ZrO 2 and mixtures made therefrom. Suitable gate layer stack 23 is any desired gate layer stack known to the person skilled in the art, as used for gas-sensitive field-effect transistors according to the prior art.

The gas-sensitive coating 25 is usually used to adsorb, absorb or chemisorb selectively gas molecules or other analytes to be detected or to trigger chemical reactions with these analytes. The presence of the analyte to be detected, for example the gas molecules of the gas component to be detected in the gas to be examined thus determines the electrical properties of the gate electrode 21 and thus the position, the expansion and the other electrical properties in the flow channel between the source region 9 and the drain. Region 11. The current flow between the source region 9 and the drain region 11 is thus influenced by the presence or absence of the analyte to be detected. In addition to the embodiment shown here with gate layer stack 23, on which the gas-sensitive coating 25 is applied, it is alternatively also possible, for example, for the gas-sensitive coating 25 to be applied directly to a surface 27 of the sensor body 7. Usually, however, a gate layer stack 23 is used.

The source electrode 13 and the drain electrode 15 are usually ohmic contacts made of a good conductive material. Commonly used as materials for the source electrode 13 and the drain electrode 15 are metals, for example tantalum, tantalum silicide, chromium, titanium, nickel, aluminum, titanium nitride, platinum, suicides, gold or all possible layer sequences.

In order to protect the source electrode 13, the drain electrode 15, the gate layer stack 23 and the sensor body 7 from aggressive media in the gas to be investigated, it is preferable to use the sensor body 7, the source electrode 13 and the drain body. Electrode 15, a passivation layer 29 applied. The passivation layer 29 can be dispensed with if the sensor element 1 is used in non-aggressive media. The material used for the passivation layer 29 is usually ceramic materials, for example silicon nitride (Si 3 N ₄ ), silicon oxide (SiC ^), aluminum oxide (Al 2 O 3), titanium dioxide (TiC> 2) and mixtures thereof. A preferred mixture is a mixture of silicon nitride and silicon oxide. In order not to impair the gas-sensitive property of the sensitive component 3, however, the passivation layer 29 does not cover the gas-sensitive coating 25.

However, the sensor element 1 shown in Figure 1 still has the disadvantages described above, since in particular the source electrode 13 and drain electrode 15 and the contacting means 17 and the gas-sensitive coating 25 can be damaged by aggressive media. In addition, all surfaces of the sensor element 1 can also be mechanically damaged by particles in a gas stream to be examined, for example an exhaust gas of an internal combustion engine which flows over the surface of the sensor element 1. To remedy this problem, the sensitive component 3 is covered with a protective layer. The erfmdungsgemäße production of the protective layer is shown in Figures 2 to 4.

A first step for applying the protective layer is shown in FIG.

In a first step, the sensor element 1 is covered with a masking layer 31. The masking layer is made of a residue-free thermally decomposable Material produced. The residue-free thermally decomposable material is preferably a polymer. Suitable polymers are, as described above, for example, polyesters, polyethers such as polyethylene glycol, polypropylene glycol, polyethylene oxide, polypropylene oxide, polyacrylates, polyacetates, polyketals, polycarbonates, polyurethanes, Polyetherke- tone, cycloaliphatic polymers, aliphatic polyamides, polyvinyl phenols and epoxy compounds and their Co - or ter-polymers. For example, to apply the masking layer 31, it is possible to dissolve or disperse the polymer in a solvent. In this case, after applying the residue-free thermally decomposable material, a drying step is performed to remove the solvent. Alternatively, however, it is also possible to use, for example, radiation-curing or thermosetting monomers and / or polymers which form the masking layer. In this case, after applying the material for the masking layer, the sensor element 1 is irradiated or heated to cure the polymers. Suitable radiation-curing or thermosetting monomers and / or polymers are those which, as already described above, contain, as functional groups, for example epoxide groups, acrylate groups and / or methacylate groups.

The application of the residue-free thermally decomposable material for the masking layer 31 is carried out by any method with which a coating of a three-dimensional element is possible. This is necessary because the sensitive component 3 is higher than the carrier substrate to which the sensitive component 3 is applied. The application method for the masking layer 31 must therefore be able to overcome at least one stage. Suitable methods for applying the masking layer 31 are, for example, dispensing, ink jet printing, pad printing, spin coating or dipping. Any other suitable methods which are known to the person skilled in the art can also be used for applying the masking layer.

After the masking layer has been applied, a protective layer 33 made of a temperature-stable material is applied to the masking layer 31. A sensor element 1 with a protective layer 33 applied to the masking layer 31 is shown in FIG.

The application of the protective layer 33 is preferably carried out by a spraying method, in particular by a plasma spraying method. The protective layer 33 applied by the plasma spraying process is preferably characterized by a high porosity. To produce the protective layer 33, it is possible, for example, to use ceramic powders or, in the case of a suspension plasma spraying process, suspensions containing ceramic constituents. The advantage of the plasma spraying method for producing the protective layer 33 is that it allows the porosity to be well adjusted by parameter variation of the plasma spraying method. Decisive is the residence time of the powder or the suspension in the plasma. A long residence time causes a completely melted starting substance and thus a rather closed protective layer 33, whereas a short residence time produces only a superficially melted starting substance and thus a porous layer on the masking layer 31.

Furthermore, in a plasma spraying process, the impact velocity of the particles can also be varied. Typical are impact speeds between 150 m / s up to 450 m / s. It is also possible to produce thick layers, typically between 80 μm and 2 mm, and in the case of suspension plasma spraying, thinner layers, for example in the range between 20 μm and 300 μm.

Due to the masking layer 31, damage to the sensitive component 3 due to the high impact velocity of the particles during the plasma spraying process can be avoided.

Also, a temperature load of the sensor element 1 during the production of the protective layer 33 can be kept low by the plasma spraying process. Despite very high temperatures in the plasma of up to 30,000 K, the temperature at the sensor element 1 or at the sensor body 7 can be kept smaller than, for example, 400 ^° C. The temperature at the sensor element 1 is dependent in particular on the distance of the masked sensor from the plasma source. In a separate temperature treatment step, in particular a high-temperature step, for crosslinking the starting material to the porous layer of the protective layer 33 may be omitted in a plasma spraying, since this is already included in the injection process. In addition, a plasma spraying is very reproducible perform and can be well integrated into a production line. Also, a precise coating for the production of the protective layer 33 is possible by a targeted movement of the sensor element 1 in the plasma.

With a plasma spraying process, for example, an entire sensor tip of a sensor element 1, which comprises the entire sensitive component 3, can be overmoulded completely and easily with a porous protective layer 33. Such a protective layer 33 has an advantageous effect, for example, even when used in an exhaust system of an internal combustion engine as a thermal shock protection and prevents thermal shock loading, for example, by impinging small water droplets on the heated sensor element. 1

As the material for the protective layer 33, ceramic materials, for example silicon nitride, silicon oxide, aluminum oxide, zirconium oxide, titanium dioxide or silicon are usually used. from it. Preferably, the same material is used, from which the carrier substrate 5 is made. The use of a ceramic material for the carrier substrate 5 is particularly preferred if the sensor element 1 is to be exposed to high temperatures, since the ceramic materials are resistant to high temperatures. In particular, damage to the carrier substrate 5 in a pyrolysis step that is carried out after the application of the protective layer 33 in order to remove the masking layer 31 can thus also be avoided. A sensor element 1, in which the masking layer 31 has been removed, is shown in FIG.

By the pyrolysis step, the masking layer 31 is pyrolyzed and the resulting gaseous product is removed through the porous layer 33. In order to achieve a complete and residue-free decomposition of the organic constituents of the masking layer 31, the pyrolysis is preferably carried out in air and / or an oxygen-rich or hydrogen-rich atmosphere. In order to obtain an oxygen-rich atmosphere, it is possible, for example, to increase the oxygen content in the air or alternatively to use pure oxygen. The pyrolysis step during which the masking layer 31 is removed may be used simultaneously for the porous sintering of the protective layer 33. In addition, it is possible that the porosity of the protective layer 33 is adjusted by the pyrolysis of the masking layer 31. Thus, for example, the porosity of the protective layer 33 can be increased thereby. As an alternative to pyrolysis of the masking layer 31, it is also possible to carry out a treatment in a low-temperature-guided oxygen plasma. In the low-temperature-guided oxygen plasma, the masking layer 31 is also decomposed and the decomposition product is removed by the protective layer 33.

A sensor element 1 manufactured in accordance with the method described above can be used particularly advantageously for measuring a concentration in at least one gas component in an exhaust gas line of an internal combustion engine. Particularly preferred is the use of the sensor element 1 for selective measurement, that is for the qualitative and / or quantitative detection of nitrogen oxides, ammonia or hydrocarbons in the exhaust gas. The inventive protective layer 33, which is formed spaced apart from the sensitive component 3, makes it possible to protect the complete sensitive component 3 from abrasion, for example from particles contained in the exhaust gas. A protection of the sensitive component 3 against chemical constituents of the exhaust gas and thus against corrosion takes place through the passivation layer 29.

Claims

claims

1. A method for producing a sensor element (1), comprising at least one sensitive component (3), comprising the following steps:

(a) applying a masking layer (31) of a residue-free thermally decomposable material to the sensitive component (3), the sensitive component (3) being substantially completely covered by the masking layer (31),

(b) applying a protective layer (33) of a temperature-stable material to the masking layer (31),

(c) removal of the masking layer (31) by pyrolysis or a low-temperature-guided oxygen plasma.

2. The method according to claim 1, characterized in that the residue-free thermally decomposable material is a thermally decomposable polymer.

3. The method according to claim 1 or 2, characterized in that the temperature-stable material is a ceramic material, in particular silicon nitride, silica, alumina, zirconia, titanium dioxide or mixtures thereof.

4. The method according to any one of claims 1 to 3, characterized in that the residue-free thermally decomposable material having a layer thickness in the range of 10 .mu.m to 2 mm is applied to the sensitive component (3).

5. The method according to any one of claims 1 to 4, characterized in that the residue-free thermally decomposable material by dispensing, ink jet printing, pad printing, spin-coating or dipping on the sensitive component (3) is applied.

6. The method according to any one of claims 1 to 5, characterized in that the residue-free thermally decomposable material for application to the sensitive component (3) is dissolved in a solvent or is present as a suspension in a solvent.

7. The method according to claim 6, characterized in that the sensitive component (3) after applying the masking layer (31) is dried from the residue-free thermally decomposable material to remove the solvent.

8. The method according to any one of claims 1 to 7, characterized in that the temperature-stable material for the protective layer (33) is applied by a plasma spraying process.

9. The method according to any one of claims 1 to 8, characterized in that the temperature-stable material of the protective layer (33) during the pyrolysis in step (c) is sintered.

10. The method according to any one of claims 1 to 9, characterized in that the pyrolysis in step (c) is carried out in the presence of an oxygen-rich atmosphere.

11. Sensor element, comprising at least one sensitive component (3) and a protective layer (33) made of a temperature-stable material, wherein the sensitive component (3) of the protective layer (33) is covered from the temperature-stable material, characterized in that the sensitive component (3) and the protective layer (33) are spaced from each other.

12. Sensor element according to claim 11, characterized in that the sensitive component (3) is a gas-sensitive field-effect transistor.

13. Sensor element according to claim 11 or 12, characterized in that the protective layer (33) made of the temperature-stable material is porous.