DE102010042017A1 - Method for coating e.g. silicon carbide substrate for manufacture process of gas sensor utilized to detect pollutants in air, involves partially removing carbon from substrate surface subsequent to coating substrate with metal and carbon - Google Patents

Abstract

The method involves coating a substrate (12) with metal and carbon by using a metal source e.g. fixed metal body (16), and a carbon source, where a solid carbon body (18) i.e. graphite body, and carbon-containing gas e.g. hydrocarbon-containing gas such as ethene or ethyne, are used as the carbon source. The carbon is partially removed from a surface of the substrate subsequent to coating the substrate by annealing in air or oxygen or by plasma treatment with reactive gas i.e. oxygen. The coating step is performed at temperature in a range of 250-350 degree Celsius.

Description

The present invention relates to a method of coating a substrate by sputtering. In particular, the invention relates to a method for coating a substrate for coating the substrate with a metal and with carbon for producing porous, chemically sensitive layers for sensors, in particular gas sensors.

State of the art

The use of chemosensitive field effect transistors (ChemFETs) as sensors for the detection of noxious gases in the air or in exhaust gases is widespread. In particular, such sensors are used for the detection of nitrogen oxides (NO _x ) in air and exhaust gas. In particular, chemosensitive field-effect transistors have a semiconductive substrate in which two semiconductive regions of a different doping are introduced, which are referred to as source or drain. Between source and drain is an electrically insulating layer, on which an electrode called a gate is arranged. In a chemosensitive field-effect transistor, a substance which is selective for the component to be detected is arranged on the gate electrode, or the gate electrode itself has such a selectivity. As a result, the component to be detected is deposited on the gate electrode or on the layer, as a result of which a change in the gate potential can be measured. This makes it possible to draw conclusions about the presence and the amount of noxious gases.

It is known to apply as a chemically sensitive layer, a spatially resolved coating with nanoscale noble metal particles, such as platinum with admixture of other precious metals or metal oxides. This is often realized in a wet chemical process by using carbonaceous solvents with the corresponding metals. After drying, a carbonaceous, gelatinous layer forms. Further annealing processes at higher temperatures and in an oxidizing or reducing atmosphere form a porous and electrically conductive layer, which consists predominantly of noble metal nanoparticles with a low residual carbon content.

Out CN 101299461 A For example, a method for producing a multi-phase catalyst electrode is known. In this process, metals are deposited from metallic powders comprising platinum, ruthenium, and rare earth lanthanides onto a carbon support by sputtering.

In KR 2008048846 A For example, a method of making a noble metal source for sputtering is described in which noble metals are deposited on a carbon support by the application of a plasma.

Out JP 07260728 A a carbon monoxide sensor is known which comprises an insulating substrate on which a sensitive layer is arranged, wherein the sensor can be produced by cathode sputtering. In this case, the sensor further comprises a plurality of catalytically active layers.

Disclosure of the invention

The present invention is a method for sputter-coating a substrate, wherein the substrate is coated with metal and carbon by using a metal source and a carbon source, and wherein the carbon is at least partially removed after coating from the substrate surface.

The method according to the invention is based on the principle of cathode sputtering, which is also referred to as sputtering or sputter deposition. Within the scope of the invention, cathode sputtering can be understood as meaning a process in which a substrate to be coated is arranged in a coating device together with a source, referred to as a target, of a substance, usually a metal, provided as a coating. The substrate is polarized as an anode, or arranged on an electrode polarized as an anode, wherein the substance source is polarized as a cathode or disposed on an electrode polarized as a cathode. Between the substrate and the substance source, a plasma is usually formed at negative pressure. In a bombardment of the surface of the substance source with ions of the plasma in particular atoms of the substance are dissolved from the substance source and promoted by a mostly electric field on the surface of the substrate to be coated, where they condense and form a coating.

According to the invention, all sputtering methods or sputtering methods are possible, that is, in particular DC sputtering, RF sputtering, diode sputtering, triode sputtering, magnetron sputtering or ion beam sputtering.

The process according to the invention is largely compatible with the processes generally used in semiconductor production, so that the process according to the invention is based on simple processes Can be incorporated into the manufacturing process of semiconductor-based sensors. The process control is thereby unproblematic according to the invention. By means of the method according to the invention, such sensors are thus considerably less laborious and can thus be produced in a time-saving and cost-saving manner.

In particular, according to the invention it is possible to produce layers which have a thickness in a range of ≥ 10 nm to ≦ 300 nm and are therefore particularly suitable for use in a chemosensitive gas sensor. In addition, after removal of the carbon, the coating applied according to the invention typically has a porosity in a range of ≥ 10% to ≦ 50%. As a result, a very large surface area is created, as a result of which, for example, a sensor produced by the method according to the invention can have a very high sensitivity.

Moreover, after removal of the carbon, the layer applied according to the invention typically has a resistivity in a range of ≦ 10 ^-3 ohm.m.

The metal source may be both a pure metal source, in particular a pure source of a noble metal, such as platinum, palladium or rhodium. Furthermore, sources of alloys of various metals are possible, for example, a platinum alloyed with rhodium and chromium, or even various individual sources.

The method according to the invention therefore makes it possible to obtain at least equivalent chemical-sensitive layers for wet-chemical coating processes when using a thin-layer process which can easily be incorporated into semiconductor manufacturing processes. The method according to the invention provides a cathode sputtering process, in particular for metals or even metal mixtures using carbon for the production of porous, chemically sensitive, gas-permeable and electrically conductive layers, in particular for gas sensors.

In the context of an advantageous embodiment of the method according to the invention, a solid carbon body, in particular graphite, is used as the carbon source. Such a carbon source is particularly inexpensive to produce and also offers a well-defined coating result.

In an alternative embodiment of the present invention, a carbonaceous gas, in particular a hydrocarbon-containing gas, such as ethene or ethyne, is used as the carbon source in addition to or as an alternative to a solid carbon source. Other carbonaceous gases, such as low hydrogen content hydrocarbons and preferably multiple bonds, especially alkenes, alkynes and cyclic hydrocarbons having less than 10 carbon atoms, are useful as carbon source in the present invention. The carbon-containing gas can be passed together with a sputtering gas into the coating apparatus, so that only a solid metal source must be arranged in the device. In this way, the inventive method can be easily applied in conventional sputtering devices, without disproportionately expensive conversion work is necessary. Rather, it is sufficient to provide a further gas connection for the carbon-containing gas.

In this case, the setting of a desired concentration of the carbon-containing gas is particularly simple, since gases are easy to dose in itself. It is particularly advantageous for the carbon-containing gas to have a concentration in a range from ≥ 30% by volume to ≦ 70% by volume, in particular the concentration of the carbon-containing gas in the sputtering gas according to the invention. In this area, the sputtering gas is not displaced too much, so that the discharge of the metal from the metal source is still easily possible. Furthermore, however, sufficient carbon is present to ensure sufficient coating of the substrate with carbon.

In the context of a further advantageous embodiment of the method according to the invention, a DC cathode sputtering is used. Such a method uses a DC gas discharge and allows working with a particularly low expenditure on equipment, which makes the inventive method particularly simple.

In a further advantageous embodiment of the method according to the invention, the coating is carried out at a temperature in a range of ≥ 0 ° C to ≤ 500 ° C, in particular in a range of ≥ 250 ° C to ≤ 350 ° C. At such temperatures good results can be achieved, in particular with respect to the desired nature of the Coating, such as composition, porosity or conductivity.

In the context of a further advantageous embodiment of the method according to the invention, the coating is carried out at a pressure in a range of ≦ 100 Pa, in particular in a range of ≥ 0.1 Pa to ≦ 1 Pa. This is a suitable pressure, wherein good results can be achieved, in particular with a negative pressure, in particular with regard to the desired nature of the coating, such as composition, porosity or conductivity.

It is also advantageous if structuring of the applied layer takes place, in particular by using a shadow mask. This makes it possible to introduce desired structures on the surface of the substrate. The use of a shadow mask is particularly easy and achieves good results. Furthermore, the inventive method in this embodiment has the advantage that a good spatial resolution in a. Range of for example 10 microns or less is possible. Such a spatial resolution is significantly higher than in comparable wet-chemical methods and, moreover, according to the invention, irrespective of the surface texture or the surface structure of the substrate to be coated. As a result, the performance of a sensor produced by a method according to the invention can be significantly improved.

In the context of a further advantageous embodiment of the method according to the invention, the removal of the carbon by annealing, in particular in air or oxygen, or by a plasma treatment with a reactive gas, in particular oxygen. Thereby, the carbon can be removed in a defined manner and to the desired extent by a relatively simple method, whereby a coating can be formed which largely comprises only metal clusters, such as noble metal clusters.

The method according to the invention is particularly preferably part of a production process of a chemosensitive sensor, in particular of a gas sensor. In particular, for the production of a chemosensitive field effect transistor for the detection of noxious gases, the inventive method is well suited.

Further advantages and advantageous embodiments of the objects according to the invention are illustrated by the drawing and explained in the following description. It should be noted that the drawing has only descriptive character and is not intended to limit the invention in any way. It shows

1 a schematic sectional view of a coating device of. the page for carrying out the method according to the invention.

In 1 is a schematic sectional view of a coating apparatus 10 shown. In such a coating device 10 is the inventive method for coating a substrate 12 by cathode sputtering feasible.

In the coating device 10 is the substrate to be coated 12 arranged. The substrate 12 is, for example, a pre-processed semiconductor wafer, such as silicon carbide (SiC). The coating of the substrate is used 12 in particular as a chemically sensitive gate electrode of a sensitive sensor, such as a gas sensor for detecting pollutants in the air or in exhaust gases. Consequently, the method according to the invention can be used in a particularly suitable manner for the production of silicon carbide or gallium nitride based high temperature suitable transducers or sensors based on chemosensitive field effect transistors.

To the substrate 12 To coat, can in the coating device 10 Target sources of substances may be arranged, with which the substrate 12 should be coated. Thus, in the coating apparatus 10 For example, be arranged a metal source and a carbon source.

The metal source is preferably a solid metal body 16 , and includes, for example, a noble metal such as palladium, platinum or rhodium. In this way, the substrate can 12 be coated with a single metal. The metal source may further comprise an alloy of different metals, in particular noble metals, to deposit a plurality of metals on the substrate 12 apply. In this way, in a possible coating of more than one metal, only one precious metal source is needed. Alternatively, instead of the one metal source, a plurality of different metal sources, in particular on solid metal bodies 16 in the coating apparatus 10 be arranged.

In addition to the at least one metal source, according to the invention, the use of a carbon source is provided around the substrate 12 not only with a metal, in particular a precious metal, but also with carbon to coat. The carbon source may be formed, for example, as a solid carbon source. Thus, preferably adjacent to the metal source, a solid carbon body 18 , such as graphite, in the coating apparatus 10 be arranged. As an alternative to the solid carbon source, a gaseous carbon source may be further provided, as will be explained later.

According to an exemplary and non-limiting embodiment of the method according to the invention, a direct current gas discharge may be used. To this also as DC Sputtering or sputtering is the substrate 12 in the coating apparatus 10 on an electrode 20 arranged. Here is this electrode 20 polarized as an anode, for example by a voltage source 22 , Alternatively it can be provided that the substrate 12 itself polarized as an anode to allow back etching, or sputtering, or glowing. In contrast, the at least one metal source and optionally the solid carbon source is on an electrode 24 arranged, which is polarized as a cathode. It can also be several voltage sources 22 and electrodes 24 be used for each source, or a voltage source 22 alternating to a metal source or carbon source on each one electrode 24 be switched on. Alternatively it can be provided that the solid metal body 16 as the metal source and / or the carbon body 18 or a conductive substrate 12 itself can be used as the anode or cathode. This can also be achieved by the voltage source through a suitable connection 22 or by the plurality of voltage sources 22 be achieved.

In an exemplary embodiment, the metal and carbon bodies 16 . 18 have a diameter of about 50 mm, wherein the spacings of the surfaces of the sources to the substrate to be coated 12 also about 50 mm, and wherein between the surface normals of the sources and the substrate 12 an angle, in particular of about 60 ° is set. This may cause the substrate 12 rotate during coating. The power of the voltage source 22 can range from ≥ 50 W to ≤ 300 W. By varying the distance or the orientation of the individual sources in each case to the substrate 12 or by inserting apertures in front of the sources, adjustments can be made to the composition of the substrate 12 deposited layers take place. An adaptation can also be made by metal or carbon sources from separate voltage sources 22 be supplied whose performance or duration of operation are adjusted individually.

Furthermore, the coating device 10 preferably a housing 26 in which an entrance 28 and an exit 30 are arranged. Through the entrance 28 For example, sputtering gas can enter the housing 24 be introduced. In particular, inert gases such as noble gases, for example argon, are used as the sputtering gas. In addition to the sputtering gas, another gas may be introduced as needed, which may include, for example, a reactive gas. According to the invention, this reactive gas may in particular comprise a carbon-containing gas and thus serve as a gaseous carbon source. Exemplary and usable according to the invention carbon-containing gases include in particular hydrocarbon gases with a low hydrogen content, which can be caused for example by double or triple bonds. Particularly suitable as a gaseous carbon source are ethene or ethyne. When using a gaseous carbon source, ie in particular a carbon-containing gas, this is preferably present in the sputtering gas in a concentration in a range of ≥ 30 vol .-% to ≤ 70 vol .-% before. With a combination of a solid and a gaseous carbon source, the concentration of the carbon-containing gas may be approximately in a range of ≦ 10% by volume.

Hence the entrance 28 preferably connected to a source of the sputtering gas and optionally to a gaseous carbon source.

The exit 30 For example, it can be connected to a vacuum pump. That way, inside the case 26 a duck suitable for the method can be set, in particular in a preferred range of ≥ 0.1 Pa to ≦ 1 Pa.

In addition, in the coating apparatus 10 a tempering device 32 be provided. The tempering device 32 serves to in the coating apparatus 10 to set a temperature which is particularly suitable for the coating process according to the invention. Advantageously, the inventive method is carried out at a temperature in a range of ≥ 0 ° C to ≤ 500 ° C, in particular in a range of 250 ° C to ≤ 350 ° C. To set such a temperature, the tempering means 32 expediently a heating device and optionally a cooling device.

In order to set and maintain the individual process parameters, such as pressure, temperature, gas flow and the like, and to maintain the desired value, a control device may be provided which is connected to corresponding sensors, such as a temperature sensor, gas pressure gauge or flow meter. Depending on the selected values of the process parameters and the measured values, the control device can then keep the process parameters within a desired range.

The method of the invention then proceeds in an exemplary and non-limiting manner as follows. After placing the substrate 12 , the metal source, a sputtering gas and a carbon source in the coating apparatus 10 the desired process parameters, such as pressure, temperature and the like are set. Further, preferably and in the Using a DC sputtering by the voltage source 22 a voltage applied. Between the electrodes 20 . 24 For example, an ionization process can produce a plasma from the sputtering gas whose high-energy ions strike the carbon source and the metal source. As a result, particles, usually atoms or atomic clusters, are discharged from these sources, which are due to the potential difference to the anode or to the substrate 12 move and condense, thereby forming a coating comprising at least one metal and carbon. The carbon for the metal preferably forms a matrix in which the metal is arranged.

Following the coating of the substrate 12 with carbon and metal, the carbon gets from the substrate 12 at least partially, preferably substantially completely, removed again. This process step serves to produce a pure metal layer on the substrate, which is advantageous in particular for use in a chemoselective gas sensor. This can be achieved, for example, by an annealing process, in particular in air or oxygen, which is typically used in a range of ≥ 500 ° C. to ≦ 600 ° C. In this case, an oxidizing atmosphere is advantageous for leaving the carbon residue-free from the substrate, for example as carbon dioxide 12 to remove. Alternatively, the removal of the carbon may be accomplished by a plasma treatment with a reactive gas, such as an oxygen plasma.

After removal of the carbon, a layer remains on the substrate 12 essentially comprising a metal or a plurality of metals. The layer is usually present as a result of the effect of temperature during the removal of the carbon as at least partially sintered layer. In a particularly advantageous manner, the finished coating has a carbon content after the end of the process, which is preferably in a range of ≦ 0.1%. The layer may, for example, have a thickness in a range of 10 nm to ≦ 300 nm and a porosity in a range of ≥ 10% to ≦ 50%. In this case, the layer preferably comprises noble metal clusters in the order of magnitude of typically ≥ 5 nm to ≦ 10 nm.

It may also be desirable, not the entire substrate 12 to coat but about a desired structure of the coating on the substrate 12 to obtain. For this purpose, the use of removable shadow masks, such as laser-cut metal foils, is preferably possible. In addition, structuring can be achieved by the use of a lift-off layer, in particular of photoresist or of a photoresist-structured sacrificial layer, in the context of a lift-off method.

The coating with carbon and metal can preferably take place simultaneously, as described above, or else offset in time. For this purpose, for example, first a sputtering with a carbon source can be carried out in order to obtain a porous carbon matrix. Subsequently, a metal source 16 be used to deposit on the or in the porous carbon matrix, the metal or metals. Thereafter, again, the carbon may be at least partially removed to obtain a porous metal coating.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• CN 101299461 A [0004]
• KR 2008048846 A [0005]
• JP 07260728 A [0006]

Claims (10)

Process for coating a substrate ( 12 ) by means of cathode sputtering, whereby the substrate ( 12 ) is coated with metal and carbon by using a metal source and a carbon source, and wherein the carbon is at least partially removed after coating from the substrate surface. A method according to claim 1, characterized in that as carbon source a solid carbon body ( 18 ), in particular graphite, is used. A method according to claim 1 or 2, characterized in that a carbon-containing gas, in particular a hydrocarbon-containing gas, such as ethene or ethyne, is used as the carbon source. A method according to claim 3, characterized in that the carbonaceous gas has a concentration in a range of ≥ 30 vol .-% to ≤ 70 vol .-%. Method according to one of claims 1 to 4, characterized in that a DC cathode sputtering is used. Method according to one of claims 1 to 5, characterized in that the coating is carried out at a temperature in a range of ≥ 0 ° C to ≤ 500 ° C, in particular in a range of ≥ 250 ° C to ≤ 350 ° C. Method according to one of claims 1 to 6, characterized in that the coating is carried out at a pressure in a range of ≤ 100 Pa, in particular in a range of ≥ 0.1 Pa to ≤ 1 Pa. Method according to one of claims 1 to 7, characterized in that a structuring of the applied layer, in particular by using a shadow mask, takes place. Method according to one of claims 1 to 8, characterized in that the removal of the carbon by annealing, in particular in air or oxygen, or by a plasma treatment with a reactive gas, in particular oxygen, takes place. Method according to one of claims 1 to 9, characterized in that the method is part of a manufacturing process of a chemosensitive sensor, in particular a gas sensor.

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