EP2104752A2

EP2104752A2 - Method for stabilizing and functionalizing porous metallic layers

Info

Publication number: EP2104752A2
Application number: EP07821232A
Authority: EP
Inventors: Oliver Wolst; Markus Widenmeyer; Alexander Martin
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2006-10-17
Filing date: 2007-10-12
Publication date: 2009-09-30
Also published as: DE102006048906A1; WO2008046785A3; JP2010507018A; WO2008046785A2; CN101535526A

Abstract

The invention relates to a method for stabilizing and functionalizing a porous metallic layer (1), wherein the porous metallic layer (1) comprises a metallic matrix (3) with pores (5) therein. In a first step, ceramic material (7) or precursors of ceramic material (7) is introduced into the pores (5) of the metallic matrix (3). In a second step, the precursors of the ceramic material are converted into the ceramic material (7), if applicable. Subsequently, a thermal or chemical post-treatment of the porous metallic layer takes place. The invention further relates to a porous metallic structure, particularly for gate electrodes of semiconductor transistors, comprising pores (5) in a metallic matrix (3). The pores (5) of the metallic matrix (3) comprise ceramic material.

Description

description

title

Process for the stabilization and functionalization of porous metallic layers

State of the art

The invention relates to a method for stabilizing and functionalizing porous metallic layers, wherein the porous metallic layer comprises a matrix with pores contained therein.

Such porous metallic layers are used, for example, as gate electrodes of semiconductor transistors. Transistors with such a porous gate electrode can be used for example as gas sensors. The porous metallic gate electrode is produced, for example, by wet-chemical deposition of nanomaterials. Gate electrodes made of nanomaterials may exhibit advantageous properties in terms of stability, gas sensitivity, and response time as compared to metallizations produced in conventional semiconductor processes, for example, by vapor deposition or sputtering. However, especially at higher temperatures, the structures of such electrodes can also degenerate, thereby impairing the function of the sensor. The degeneration of the electrodes results, for example, from sintering processes and a structural enlargement. The electrochemical properties of the electrodes are determined by the selection of the metal and the interface material, for example, the semiconductor device.

Disclosure of the invention

Advantages of the invention

The method according to the invention for the stabilization and functionalization of porous metallic layers, wherein the porous metallic layer comprises a matrix with pores contained therein, comprises the following steps: (a) introducing ceramic material or precursors of the ceramic material into the pores of the metallic matrix,

(b) optionally converting the precursors of the ceramic material into the ceramic material,

(c) optionally thermal or chemical post-treatment of the layer.

Porous metallic layers produced according to the invention can be used, for example, as gate electrodes of semiconductor transistors which are used as chemosensitive components, for example as gas sensors. The material for the metallic layer is preferably selected from platinum, palladium, iridium, nickel, gold, silver, rhodium, copper, osmium, rhenium and alloys thereof. In particular, when using the porous metallic layer for chemosensitive components, the type of metallic layer has a significant influence on the chemosensitive function. Generally, high porosity of the metallic layer promotes desired sensor functions such as high sensitivity and fast response time.

By introducing ceramic material or precursors of the ceramic material into the pores of the metallic matrix, possible sintering paths of the metal are closed, at least in part. The metallic sintering process is thereby limited and the porous layer stabilized. As a result, for example, gas-sensitive transistors with gate electrodes which have been produced by the process according to the invention are stabilized with respect to their electrochemical properties. This extends the life. In addition, the use in less favorable conditions, for example at high temperatures or in a corrosive gas environment, allows.

Furthermore, the selection of the ceramic material which is introduced into the pores, the electrochemical properties, that is, the sensitivity, selectivity and the operating range of the sensor can be adjusted. Examples of suitable ceramic materials are the oxides, nitrides, carbides or suicides of magnesium, aluminum, silicon, indium, tin, zinc, iron, titanium, zirconium, scanidum, yttrium, lanthanum, cerium, boron, tungsten, vanadium, tantalum. Niobium, hafnium or molybdenum and the mixtures of these compounds. The ceramic material is preferably selected from the group consisting of aluminum oxide, silicon oxide, indium oxide, tin oxide, zinc oxide, iron oxide, titanium oxide, zirconium oxide, scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, aluminum nitride, silicon nitride, indium nitride, titanium nitride, boron nitride, aluminum silicide, tungsten silicide, Vanadium silicide, tantalum silicide, niobium silicide, zirconium silicide, hafnium silicide, molybdenum silicide, titanium silicide, silicon carbide, aluminum carbide, tungsten carbide, vanadium carbide, tantalum carbide, niobium carbide, zirconium carbide, hafnium carbide, molybdenum carbide and titanium carbide, as well as mixtures of these compounds

In addition to the stabilization of the porous, metallic layer, for example, mass transport processes of different species can be limited differently. In this case, it is possible to discriminate parasitic reaction paths which reduce the concentration of signal-forming species. Furthermore, the ceramic material also promotes the adsorptive and catalytic properties of the metal. In addition, in many cases, the above-mentioned ceramic materials themselves have adsorptive or catalytic properties that can be utilized.

In a preferred embodiment, the ceramic material is introduced into the pores of the matrix by a wet-chemical method. The ceramic material may in this case be in the form of nanoparticles, for example, which are dispersed in a liquid and applied as a suspension to the metallic matrix.

In one embodiment, the suspension contains at least one stabilizer additive. This stabilizes the nanocolloids in the suspension. Sedimentation or agglomeration of the colloids is thus restricted or prevented.

Suitable solvents in which the ceramic nanocolloids are dispersed are, for example, water, alcohol or other polar organic solvents and mixtures thereof.

Suitable additives for stabilizing the suspension are, for example, acids, diethylene glycol monobutyl ether or surfactants. Suitable acids are, for example, hydrochloric acid, acetic acid, nitric acid, oxalic acid and hydroxycarboxylic acid. Suitable surfactants are, for example, AOT (bis (2-ethyl-1-hexyl) sulfosuccinate), polyethylene oxide-polypropylene oxide block copolymers and tetraalkylammonium salts.

The ceramic nanoparticles preferably have an average diameter in the range of 1 to 1000 nm. Preferably, the average diameter is in the range of 2 to 250 nm. When precursors of the ceramic material are introduced into the pores of the metallic matrix, they are preferably introduced as a suspension or as a solution.

A precursor of a ceramic which is incorporated as a suspension is, for example, boehmite, a hydroxyaluminum oxide AIO (OH). The boehmite is introduced into the cavities of the metallic matrix in the form of nanoparticles in an acidic suspension. A subsequent heat treatment at a temperature in the range between 300 and 500 ⁰ C converts the AIO (OH) nanoparticles in sintered Al ₂ O ₃ .

In a further embodiment, the precursors of the ceramic material are introduced as a solution into the pores of the metallic matrix. Suitable precursors of the ceramic material are preferably dissolved metal salts which are converted in step (b) into their corresponding metal oxides. For this purpose, for example, salt solutions of magnesium, aluminum, silicon, indium, tin, zinc, iron, titanium, zirconium, scandium, yttrium, lanthanum or cerium are introduced into the metallic matrix. These salts may be, for example, nitrates, oxinitrates, halides, carbonates, acetylacetonates, acetates, carboxylic acid derivatives, alcoholates or organometallic compounds. These are dissolved, for example, in water, alcohol, a polar organic solvent or mixtures thereof. After impregnation, the precursor is transferred to the actual ceramic. This is done for example by calcination at a temperature in the range between 250 and 650 ⁰ C or by other methods of a chemical or physical nature, such as acidic or basic treatment, treatment with reactive plasma or low-temperature treatment, especially drying.

The application of the solutions or the suspension is realized, for example, by dipping, spin-coating, dispensing or thick-film printing of a paste. In general, a multiple coating is possible. As a result, the amount of the ceramic material can be adjusted independently of the concentration of the fluid. In addition, Schich- tabfolen different ceramic materials can be produced.

Alternatively, it is also possible to introduce the ceramic material or precursors of the ceramic material into the pores of the metallic matrix, for example by sputtering processes or by vapor deposition processes.

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

Show it:

FIG. 1 shows a schematic representation of a metallic matrix with pores contained therein,

FIG. 2 shows a metallic matrix with pores contained therein, wherein the pores are filled with ceramic material,

Figure 3 is a schematic representation of a metallic matrix with pores contained therein, which are filled with ceramic material, wherein the metallic matrix is additionally covered with ceramic material.

FIG. 1 shows a metallic matrix with cavities contained therein.

A porous structure 1 contains a metallic matrix 3, in which pores 5 are formed. Through the pores 5 results in a sponge-like structure of the metallic matrix 3. Such a porous structure 1, as shown in Figure 1, is already known from the prior art. In one embodiment, it is possible that the metallic matrix 3 also contains at least one ceramic material in addition to the at least one metal. However, if the metallic matrix 3 contains metal and ceramic material, it is necessary that the proportion of metal be so large as to ensure the electrical conductivity of the porous structure 1. This is particularly necessary when the porous structure 1 is used as a gate electrode for semiconductor transistors.

As material for the metallic matrix 3, metals of the 8th to 11th group of the Periodic Table of the Elements are preferably used. Particular preference is given to platinum, palladium, iridium, nickel, gold, silver, rhodium, copper, osmium, rhenium and alloys thereof.

To produce the metallic matrix 3, for example, a suspension containing particles of the metallic material is applied to a carrier. The suspension also contains at least one organic component which can harden to a polymer matrix. About a possibly contained in the suspension solvent, the viscosity of the suspension can be adjusted. The application of the suspension takes place, for example, by dripping or printing. Any further, known to the expert Way to apply the suspension is also possible. The viscosity of the suspension is adjusted depending on the type of job.

After the suspension has been applied, it is optionally pre-dried to remove the solvent. Furthermore, at least one organic component is cured to the polymer matrix. This is preferably also at elevated temperature. The particles of the metallic material are uniformly distributed in the polymer matrix. In a next step, the polymer matrix is removed. This is preferably done by a thermolysis or pyrolysis. Due to the temperatures which occur during the thermolysis or pyrolysis, the metallic particles which form the porous layer are sintered together. This creates a porous layer with evenly distributed pores.

In addition to the manner described above, however, it is also possible to produce the metallic matrix 3 with the pores formed therein in any other manner known to those skilled in the art. When such a porous structure 1 is used as a gate electrode for a semiconductor transistor, the metal particles of the metallic matrix 3 may converge on an oxide surface. The oxidic surface is generally the surface of the semiconductor transistor on which the gate electrode is formed. By the convergence of the metal particles, the life of the semiconductor transistor, which can be used, for example, as a gas sensor is reduced.

In order to increase the life of the semiconductor transistor having the gate electrode formed as a porous structure, it is necessary to stabilize the metallic matrix 3. According to the invention, the stabilization takes place by introducing ceramic material into the pores 5 of the metallic matrix 3. This is shown in FIG.

The ceramic material 7 is, for example, introduced into the pores 5 by a wet-chemical method, as already described above. For this purpose, it is possible, for example, that the ceramic material 7 is dispersed in a solvent and the suspension is applied to the porous structure 1. The suspension also penetrates into the pores 5 of the metallic matrix 3. After applying the suspension containing the ceramic material, a heat treatment is performed in which the ceramic material is sintered to the metallic matrix 3. As a result, the metallic matrix 3 and thus the porous structure 1 is stabilized. By sintering the ceramic material 7 in the pores 5 of the metallic matrix 3, the sintering paths of the metallic matrix 3 are closed. As a result, the convergence of metal particles observed in particular at relatively high temperatures is prevented or restricted. Alternatively, the ceramic material 7 can also be applied initially in the form of its precursors as a suspension or in solution to the metallic matrix 3. The precursors may be present on the one hand, for example, as nanoparticles or, on the other hand, may be dissolved in the form of the corresponding metal salts in a solvent. After application of the precursors of the ceramic material 7, these are converted into the ceramic material 7. This is generally done by a heat treatment. The heat treatment is carried out, for example, at a temperature in the range of 250 to 650 ^° C. It is possible that the heat treatment lasts up to several hours.

In addition to introducing the ceramic material 7 into the cavities 5 of the metallic matrix 3, it is also possible for a coating 9 containing the ceramic material 7 to be applied to the metallic matrix 3. Also by the coating 9, which is applied to the metallic matrix 3, the metallic matrix 3 and thus the porous structure 1 is stabilized. The thickness of the coating 9 is generally in the range of 1 to 500 nm.

Since the ceramic material 7 also gives a porous structure, the metallic matrix 3 is also not sealed by the coating 9 or the filling of the pores 5 with the ceramic material 7 against surrounding gases. Thus, it is further possible to detect gases when the porous structure 1 is used as a gate electrode of semiconductor transistors used as gas sensors.

example 1

A 200 nm thick porous metallic matrix 23 of platinum with cavities 5 having a diameter in the range of approximately 5 to 500 nm is provided with a zirconium dioxide coating. For this purpose, a dilute alcoholic solution of zirconium tetraisopropoxide is added to the porous structure 1. This is followed by drying and heat treatment at 500 ⁰ C in air. The zirconium tetraisopropoxide is converted to zirconia by the heat treatment. The cavities of the metallic matrix 3 are filled with zirconia. In addition, a coating 9 is produced on the metallic matrix 3 of zirconium dioxide.

Example 2 Alternatively, to obtain a zirconia coating, it is also possible to use, for example, a dilute, acidic, aqueous-alcoholic solution of zirconium nitrate. In this case as well, the zirconium nitrate is converted into zirconia by the heat treatment.

Example 3

In order to produce the pores 5 of the metallic matrix 3 and the coating 9, a layer of a dilute zirconia sol, wherein the particle size of the zirconia particles is in the range between 2 and 50 nm, is applied to the metallic matrix 3 on platinum , then dried and burned out at 500 ⁰ C in air. The zirconia sinters to the metallic matrix 3 and thus stabilizes the metallic matrix 3.

Example 4

In order to fill the pores 5 of the metallic matrix 3 with silicon dioxide and to produce a silicon dioxide coating, tetraethyl orthosilicate is first dissolved in ethanol. The amount of tetraethyl orthosilicate is chosen to result in 1 wt% SiO ₂ in the solution. This solution is pipetted onto the metallic matrix 3. Subsequently, the metallic matrix with the solution contained on it is heated to 250 ⁰ C in the presence of air. The tetraethylorthosilicate is thereby converted into silicon dioxide.

Claims

claims

Method for stabilizing or functionalizing a porous, metallic layer, wherein the porous metallic layer (1) comprises a metallic matrix (3) with pores (5) contained therein, comprising the following steps:

(a) introducing ceramic material (7) or precursors of the ceramic material into the pores (5) of the metallic matrix (3),

(b) optionally converting the precursors of the ceramic material (7) into the ceramic material,

(c) optionally thermal or chemical aftertreatment of the porous metallic layer (1).

2. The method according to claim 1, characterized in that the ceramic material (7) is in the form of ceramic particles having a particle diameter in the range of 1 nm to 1000 nm.

3. The method according to claim 1 or 2, characterized in that the ceramic material (7) is applied to the metallic matrix as a suspension containing dispersed ceramic particles.

4. The method according to claim 3, characterized in that the suspension is spun on the metallic matrix or dropped or that the metallic matrix is immersed in the suspension.

5. The method according to claim 4, characterized in that the suspension contains as solvent water, at least one organic solvent or mixtures thereof.

6. The method according to claim 4 or 5, characterized in that the suspension contains at least one additive for stabilization, preferably an acid, a surfactant or Diethylenglykolmonobutylether.

7. The method according to any one of claims 1 to 3, characterized in that the precursors of the ceramic material in dissolved form, preferably as a salt solution.

8. The method according to claim 1, characterized in that the ceramic material or the precursors of the ceramic material are applied by sputtering or vapor deposition on the metallic matrix.

9. The method according to any one of claims 1 to 8, characterized in that the ceramic material contains at least one oxide, at least one nitride, at least one silicide, at least one carbide or mixtures thereof.

10. The method according to claim 9, characterized in that the oxide is an alumina, silica, indium oxide, tin oxide, zinc oxide, iron oxide, titanium oxide, zirconium oxide,

The nitride is an aluminum nitrite, silicon nitrite, indium nitrite, titanium nitrite or boron nitride, the silicide is an aluminum silicide, tungsten silicide, vanadium silicide, tantalum silicide, niobium silicide, zirconium silicide, hafnium silicide, molybdenum silicide or titanium silicide and the carbide is a silicon carbide, aluminum carbide, Tungsten carbide, vanadium carbide, tantalum carbide, neobarbide, zirconium carbide, hafnium carbide, molybdenum carbide or titanium carbide.

11. The method according to any one of claims 1 to 10, characterized in that the porous, metallic layer contains an element of the 8th to 11th group of the Periodic Table of the Elements.

12. The method according to claim 11, characterized in that the porous metallic layer contains platinum, palladium, iridium, nickel, gold, silver, rhodium, copper, osmium, rhenium or alloys thereof.

13. Porous metallic layer, in particular for gate electrodes for semiconductor transistors, comprising pores (5) in a metallic matrix (3), characterized in that ceramic material (7) is contained in the pores (5) of the metallic matrix (3).

14. A structure according to claim 13, characterized in that the metallic matrix (3) is made of an element of the 8th, 9th, 10th or 11th group of the Periodic Table of the Elements.

15. Structure according to claim 13 or 14, characterized in that the ceramic material is selected from the group consisting of alumina, silica, indium oxide, tin oxide, zinc oxide, iron oxide, titanium oxide, zirconium oxide, scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, Aluminum nitride, silicon nitride, indium nitride, titanium nitride,

Boron nitride, aluminum silicide, tungsten silicide, vanadium silicide, tantalum silicide, niobium silicide, zirconium silicide, hafnium silicide, molybdenum silicide, titanium silicide, silicon carbide, aluminum carbide, tungsten carbide, vanadium carbide, tantalum carbide, niobium carbide, zirconium carbide, hafnium carbide, molybdenum carbide and titanium carbide and mixtures of these compounds.