DE102011120328A1 - Etching process for metal mixed oxides - Google Patents

Abstract

In the context of the invention, a method for etching a metal mixed oxide has been developed. A mixed metal oxide is understood as meaning a compound of at least two different metals and oxygen. According to the invention, an etchant is selected which brings at least one of the metals in the mixed oxide to a different oxidation state in which it is bound weaker and / or less directionally. The etchant has a redox potential of less than +2 V and includes either a reducing agent that lowers its redox potential by at least 0.15V, or an oxidizing agent that increases its redox potential by at least 0.05V. In this case, the change in the oxidation state and the redox potential in the claimed region act synergistically, so that the etchant as a whole is just as aggressive as is necessary in order to etch the mixed metal oxide at a rate which is of interest for industrial applications. Thus, the etching process becomes selective and spares other materials that are not to be etched, such as the photoresist used for lithographic patterning.

Description

The invention relates to etching processes for metal mixed oxides and to a process for producing a functional structure from a metal mixed oxide layer.

State of the art

Functional structures of metal mixed oxides are usually prepared by lithographic patterning with photoresist and subsequent etching. For this purpose, the most selective etching method is needed, chemical etching being preferred.

Disadvantages are the metal mixed oxides which are of interest for industrial applications, for example SrRuO ₃ and the room temperature both ferromagnetic and electrically conductive (La, Sr) MnO ₃ , only very poorly chemically etchable. From the US 6436723 B1 For example, an etching process for SrRuO _{3 is} known in which ozone dissolved in water is used as an etchant. Ozone is unstable and therefore neither storable nor safe to handle. It is also less selective and attacks the photoresist used for lithographic patterning. The hydrofluoric acid and concentrated hydrochloric acid hitherto used for the structuring of (La, Sr) MnO ₃ are likewise too aggressive and not very selective. Citric acid is a milder etchant, but only at temperatures at which the photoresist used for structuring already softens.

Task and solution

It is therefore the object of the invention to provide an etching process for metal mixed oxides which acts at a practicable etching rate and yet selectively only to the mixed oxide. It is a further object of the invention to provide a method for producing a functional structure from a metal mixed oxide layer, which damages the crystal structure less than in the prior art.

These objects are achieved by methods according to the main and secondary claims. Further advantageous embodiments will be apparent from the dependent claims.

Subject of the invention

In the context of the invention, a method for etching a metal mixed oxide has been developed. A mixed metal oxide is understood as meaning a compound of at least two different metals and oxygen.

According to the invention, an etchant is selected which brings at least one of the metals in the mixed oxide to a different oxidation state in which it is bound weaker and / or less directionally. This causes it to better dissolve at the other oxidation state. The etchant has a redox potential of less than +2 V and includes either a reducing agent that lowers its redox potential by at least 0.15V, or an oxidizing agent that increases its redox potential by at least 0.05V.

Critical to the chemical behavior of an oxide in etching solutions is the chemical balance between the crystalline oxide, poorly soluble reaction products, dissolved cations, soluble cation complexes and possibly other soluble or volatile reaction products. To etch an oxide, one wants to shift the chemical balance to the side of soluble reaction products and thus dissolve the crystalline oxide in the etching solution. It has been recognized that etching of the metal composite oxide in the core is a competition for which the metals are preferentially bound. Once the respective bonding of a metal to the etchant is preferred over the bond in the mixed oxide energetically, the metal is a compound with the etchant. In this case, the system of the mixed metal oxide and the etchant strives for the state in which the free enthalpy is minimal. Thus, the free enthalpy is decisive for whether the metal remains bound in the mixed oxide or newly binds to the etchant.

The strength and direction of the bonds that hold the metal in the mixed oxide is thus crucial to how well the metal is etched. The oxides of many metals readily dissolve in acids because they first react with water to form hydroxides, which in turn are generally well dissolved by acids. Examples of this behavior are alkali and alkaline earth metals and most rare earths. The cations of these metals form only undirected ionic bonds, which is why they easily form a hydration shell in water and dissolve well. Other oxides, especially of many transition metals, can be brought by acids or bases only poorly or not at all. The cations form directional and relatively strong bonds, especially to oxygen, rendering the oxides and hydroxides resistant to dissolution. The solubility of a metal compound in an etchant is usually determined by the less soluble component. For example, SrTiO ₃ is very stable, as is TiO ₂ , while SrO is very soluble in acids. Thus, in the case of SrTiO ₃ , the solubility of TiO ₂ in the etchant is the limiting factor for the solubility of the entire composite oxide.

It has now been recognized that the bonding conditions in different oxidation states, especially of transition metals, differ greatly. Manganese is known, for example, as MnO, Mn ₂ O ₃ , Mn ₃ O ₄ , MnO ₂ and Mn ₂ O ₇ . These compounds have completely different chemical behavior: MnO, Mn ₂ O ₃ and Mn ₃ O ₄ dissolve in acids, MnO ₂ is insoluble in acids and bases, and Mn ₂ O ₇ is a highly reactive liquid that dissolves well in water because it forms soluble MnO ₄ ^- anions.

Therefore, in the method according to the invention, such an etchant is selected, which by changing the oxidation state initially changes the bonding conditions of at least one metal in such a way that it is more easily soluble. If a redox potential subsequently presented by the etchant, which is lowered by at least 0.15 V either by a reducing agent or increased by at least 0.05 V by an oxidizing agent, then from the point of view of the metal the bond to the etchant becomes opposite to the bond preferred in mixed oxide. If in this way the less soluble component of the mixed oxide is broken, the more easily soluble component is attacked. Reduction or increase of the redox potential are understood in this context relative to an etchant in which the reducing agent or oxidizing agent is thought of.

In this case, the change in the oxidation state and the redox potential in the claimed region act synergistically, so that the etchant as a whole is just as aggressive as is necessary in order to etch the mixed metal oxide at a rate which is of interest for industrial applications. Thus, the etching process becomes selective and spares other materials that are not to be etched, such as the photoresist used for lithographic patterning. For this purpose, the redox potential is below +2 V. According to the prior art, many mixed metal oxides, such as mixed oxides with ruthenium and another metal (metal-ruthenium oxides), wet-chemically either not at all or only with highly reactive and hard to handle etchants such as ozone etch. Photoresist was also affected, so that functional structures of metal-ruthenium oxides could only be lithographically produced with limited quality.

The skilled person is usually asked to etch a given metal mixed oxide. He knows in which oxidation states the metals contained in the mixed oxide occur. For each of these oxidation states, in turn, it is known how well it is soluble in common etchants; In addition, the overall solution behavior of the metal mixed oxide is known. The oxidation state whose solution behavior most closely resembles that of all metal mixed oxide is the limiting factor for the etching of the metal composite oxide. Thus, the skilled person can specifically use an oxidizing or reducing agent, which converts exactly this oxidation state into a more easily soluble. Unless this oxidation state dissolves directly in the chosen oxidation or reduction agent, one skilled in the art may add another solvent known to dissolve this oxidation state. In this case, the redox potentials of the components used and of the entire etching solution are easily accessible to him, so that he has no difficulty in meeting the claimed secondary conditions for the redox potential of the etching solution. Thus, the skilled person is given a complete teaching at hand, with which he can solve the task asked him without undue difficulties.

For this purpose, the expert in particular Pourbaix diagrams are available, in which the stability of metals and metal complexes is shown vividly. The pH is plotted on the X axis and the electrochemical potential on the Y axis relative to the standard hydrogen electrode. The higher this potential is, the more oxidizing are the ratios in the solution, and the lower the more reducing. The diagram contains two parallel oblique reference lines: the potential in the case of oxygen saturation (oxidizing) and of hydrogen saturation (reducing). In each of the fields, the registered species is predominant, and at the boundary lines of fields, the two neighboring species are present at a ratio of 1: 1. The species may in particular be oxidation states.

These diagrams can be calculated for almost any system, as far as the thermodynamic data for the individual reactions and reaction products are presented. For many connections, such data is available in spreadsheets, and the most important are a program for calculating such diagrams as a database. The inventors have for the exemplary embodiments listed in the specific part Medusa ( http://www.kemi.kth.se/medusa/ ) with its standard database.

From the Pourbaix diagrams it can be seen whether oxidation or reduction can convert an insoluble oxide to a soluble form, how strong the oxidizing or reducing agent must be and at what pH to work to convert the oxide to a soluble form. By superimposing the Pourbaix diagram of the etchant and the oxide to be etched, it is possible to determine whether and in which pH range the etchant acts. If the stability range of the oxidized ( reduced) form of the metal with the stability region of the reduced (oxidized) form of the oxidant in the etching solution, the reaction equilibrium is on the side of the products, and the mixed metal oxide is attacked.

Pourbaix diagrams depict the case of thermodynamic equilibrium. In many cases, however, the reactions are so slow at room temperature that the equilibrium sets only after a very long time. For example, hydrogen and oxygen can coexist, although from a thermodynamic point of view, water would have to form. Therefore, the Pourbaix diagrams do not necessarily reflect what is actually in the reaction mixture, but only in which direction reactions would take place. Therefore, the reduction or oxidizing agent with low activation energy contained in the etching solution according to the invention is also necessary in such cases to achieve technically interesting etching rates in which the oxidation of the metal originally present in the metal mixed oxide from the thermodynamic point of view should be soluble. Oxidation or reduction in this case ensures that the stable bonds of the metal with the surrounding oxygen are broken down and rearranged so that the etching solution can dissolve the metal out of the metal mixed oxide.

Some metal mixed oxides are exemplified below. However, the person skilled in the art can easily identify further metal mixed oxides which behave similarly, and also exclude individual oxides from the classes of metal mixed oxides mentioned below, the bond ratios of which deviate greatly therefrom. There are crystallographic catalogs that record the structure and bonding relationships of most known crystalline solids.

In a particularly advantageous embodiment of the invention, an etchant is selected which does not form a compound with any metal of the mixed oxide which does not dissolve in the etchant. Then, the mixed metal oxide is completely converted into solution without remaining solid residues.

In a particularly advantageous embodiment of the invention, an etchant is selected which contains a solvent for the metal brought to a different oxidation state. Advantageously, this solvent also dissolves the other metal of the metal mixed oxide or the oxidation state in which this metal occurs in the metal mixed oxide. If the metal is dissolved in the solvent, it can be removed without residue after etching together with the solvent.

The oxidizing or reducing agent can also act as a solvent at the same time. Furthermore, the solvent can expressly also be a mixture of two solvents, one of which has the metal brought to a different oxidation state and the other the other metal of the metal mixed oxide or the oxidation state in which this metal occurs in the metal mixed oxide, converted into solution.

Advantageously, water, an organic solvent, an ionic liquid or a melt is chosen as the solvent. In particular, water and organic solvents dissolve RuO ₄ , WO ₄ , MoO ₄ and ReO ₄ well. Lower-quality metal oxides and in particular the oxides of strontium dissolve in water, an acidic medium, an ionic liquid or a melt.

Advantageously, an organic solvent selected from the group consisting of propylene carbonate, dimethyl sulfoxide, acetonitrile, dimethylformamide (DMF) or dimethylpropyleneurea (DMPU). These solvents are highly polar, d. H. they contain molecules with a large dipole moment. Thus, to a free charge (anion or cation), several of these molecules can each bind to the oppositely charged end and prevent the anion or cation from binding to a cation or anion. Thus, by interfering with the connection of anions and cations to ionic species, a polar solvent dissolves this species relatively well. In addition, the solvents from the listed group also have a high dielectric constant. This has the effect of shielding the local electric field from free charges (anion or cations) and weakening the attractive effect to the respective partner, which lacks the formation of an ionic species (cation or anion).

Again, this has the effect of disrupting the association of anions and cations with ionic species, thus favoring the dissolution of these species. The better solubility is effected in each case by the fact that the disruption of the bond between anion and cation reduces the activation energy required to separate the anion and cation from one another and thus to dissolve the ionic species into solution. The solvents mentioned also allow the dissociation of certain acids.

Advantageously, an organic solvent selected from the group of alkanols, in particular methanol, ethanol or isopropanol, because ionic species are at least slightly soluble in these solvents and at the same time PMMA, a common photoresist for electron beam lithography, is insoluble in these solvents.

In a particularly advantageous embodiment of the invention, an etchant is selected which includes a complexing agent. A complexing agent is a substance whose compound with a metal cation is soluble in the etchant on the one hand and on the other hand has a lower free enthalpy than a solid precipitate, such as an oxide or hydroxide, of the metal cation. He thus helps to keep metal cations in solution and shifts the related balances in the Pourbaix diagram. If the properties of all compounds and reactions of the complexing agent with the other species present are known, the corresponding Pourbaix diagram can also be calculated for the case of an added complexing agent. Effective complexing agents include, for example, chloride, fluoride, bromide, iodide, citrate, tartrate, EDTA, oxalate, cyanide, thiocyanate, thiosulfate, ammonium, crown ethers and cryptates. Chelating agents are generally Lewis bases and bind to metal centers that act as Lewis acids. A complexing agent is particularly advantageous when titanium is one of the metals in the metal mixed oxide.

Advantageously, a perovskite or a pyrochlore is chosen as metal mixed oxide. Many of these mixed metal oxides have interesting properties or combinations of properties for research and technological use. They are characterized in that in them the metal cations sit on very specific lattice sites and their bonds are particularly strong. Thus, their solubility with the method according to the invention can be increased to a particular extent.

In a particularly advantageous embodiment of the invention, a mixed metal oxide is selected which contains at least one transition metal or cerium as the metal. The cations of these metals form directional and relatively strong bonds, in particular to oxygen, so that these metals often occur in oxidation states that dissolve poorly or not at all in acids or bases. If they are contained in a metal mixed oxide, they are the limiting factor for its solubility. By the process according to the invention first of all starts at the oxidation stage of the metal, the etching effect is purposefully improved and at the same time no more aggressive etchant is used than necessary. This is especially true when the transition metal is selected from the group Os, Rh, Ti, Cr, Mn, Re, Ru, W.

In a particularly advantageous embodiment of the invention, an etchant is selected which contains a reductone, in particular ascorbic acid, or an aldehyde, in particular glucose or benzaldehyde, as a reducing agent. In general, all aldehydes are reducing agents because they can be easily oxidized to the corresponding carboxylic acid. All reductones are also reducing agents, since they can easily be oxidized to the corresponding diketone. Reductones and aldehydes are particularly useful for reducing Mn (IV) to Mn (II) insoluble in a mixed metal oxide with manganese as the metal. In particular, a perovskite of the form (La, Sr) MnO ₃ can be dissolved with such an etchant. Since lanthanum and strontium are generally readily soluble in acids, the etch rate is largely determined by the degradation of Mn (IV). Glucose and ascorbic acid are also characterized by the fact that they are not hazardous substances and that they are biodegradable. Glucose is an effective reducing agent with a standard potential of about -0.05 V. especially in alkaline medium.

A reducing agent is also advantageous for the etching of mixed oxides with trivalent manganese, since Mn (III) is not stable in an aqueous medium and disproportionates to soluble Mn (II) and insoluble Mn (IV).

By being generally capable of reducing trivalent and tetravalent manganese in oxidic compounds, the etchant may also etch manganese dioxide (MnO ₂ ) and other compounds of the form MMnO ₃ wherein M is an alkaline earth metal, a rare earth, or combinations thereof , Especially in conjunction with other ingredients, such as. As complexing agents, even more oxidic compounds of trivalent or tetravalent manganese can be attacked. For example, citric acid can be used as complexing agent in conjunction with glucose as a reducing agent.

In the prior art, oxidic compounds were etched with trivalent or tetravalent manganese with buffered hydrofluoric acid, hydrochloric acid or citric acid. Hydrofluoric acid is dangerous to handle and less selective. Concentrated hydrochloric acid etches too quickly and too seldom, while dilute hydrochloric acid etches more slowly, but leaves residues (probably MnO ₂ ). Concentrated hydrochloric acid is also a hazardous substance that must be disposed of properly after use. Citric acid is relatively harmless, biodegradable and can be removed without residue by ashing. However, it works only at elevated temperature and must be tempered exactly for a uniform result. At the required temperature, photoresist already begins to soften, so that lithographic patterning with citric acid as etchant does not give good edges.

Ascorbic acid is a stronger and faster reductant than citric acid, which works at room temperature. The inventors have successfully tested a solution of ascorbic acid in water. However, the ascorbic acid can also be partially or completely neutralized with a base (pH buffer) to affect the etch rate or selectivity. It can also be combined with other acids or complexing agents. Generally, ascorbic acid is a beneficial etchant when a material is to be attacked by a combination of mild reduction and acidity.

In general, the invention also relates to a method for etching manganese compounds with tetravalent manganese, wherein an etchant is selected which contains a reductone, in particular ascorbic acid, or an aldehyde, in particular glucose, as a reducing agent. The features disclosed in connection with the method described above are expressly also disclosed as disclosed for this method.

If a sparingly soluble mixed oxide such as (La, Sr) MnO _{3 is} present, it behaves in a first approximation chemically similar to the sparingly soluble MnO ₂ , because the manganese is present in both cases at least in part as Mn (IV) (tetravalent manganese) , Both La and Sr are readily soluble in acids. The inventors were able to show in their experiments that such mixed oxides can be chemically attacked by a combination of oxidation or reduction together with a suitable other solvent (eg an acid in water). In this example, the insoluble Mn (IV) may be reduced by ascorbic acid or another reducing agent to Mn (II), which readily dissolves in an acid.

In a particularly advantageous embodiment of the invention, an aqueous solution containing the anion and / or the free acid of a halo-oxygen acid is selected as an etchant. The anion and / or the free acid act as an oxidizing agent on the metal, in particular on metals from the group ruthenium, tungsten, molybdenum, rhenium. Water is a good solvent for both RuO ₄ , WO ₄ , Mo ₄ , ReO ₄ and many lower valence metal oxides such as SrO. Acid media also dissolve many metal oxides.

Many aqueous solutions of this type are the stationary state in a reaction equation in which the free acid is on one side with water and on the other side the anion with the cation. Both sides each have a tendency to transform into each other. Which stationary state occurs depends in particular on the pH value. By changing the pH, for example with other acids or bases, therefore, in particular the ratio between the free acid and the anion can be changed. Frequently, the free acid and the anion differ significantly in their aggressiveness. Thus, the etching rate can be adjusted in a wide range via the pH value.

It is within the skill of the art to adjust the concentration and pH of the solution to remain stable for at least the duration of the etching process. Extensive technical literature with phase diagrams showing the stable states as a function of concentration, pH and other conditions exists.

• – 0,4-molare Periodsäurelösung ätzt sehr schnell mit ca. 25 nm/s.
• – 0,4-molare Natriumperiodat-Lösung ätzt mit ca. 1 nm/s.
• – 0,4-molare Kaliumchlorat-Lösung ätzt sehr langsam und ungleichmäßig mit ca. 1 nm/min.

In a particularly advantageous embodiment of the invention, the oxidizing agent contains a perhalogenate, halogenate, halide or hypohalite. These may be, for example, iodates, periodates, bromites, bromates, perbromates, chlorates, chlorites, hypochlorites or perchloric acid in a high concentration ≥ 70%. The anions can be associated with virtually any cation, such. B. Na ⁺ or Ka ⁺ . The inventors have successfully tested the following etchants on SrRuO ₃ :

• - 0.4 molar periodic acid solution etches very quickly at about 25 nm / s.
• - 0.4 molar sodium periodate solution etches at about 1 nm / s.
• - 0.4 molar potassium chlorate solution etches very slowly and unevenly at about 1 nm / min.

• – zum Strukturieren mittels Lift-Off-Technik,
• – zur Herstellung freistehender epitaktischer Oxid-Strukturen durch Unterätzen oder
• – als „Hard mask” zum Strukturieren von schwer angreifbaren Schichten, wo Fotolack nicht geeignet ist.

All of these etchants have a standard redox potential below 2 V. Their etch rate can be adjusted by changing the pH with bases, such as NaOH, in a wide range. Also, the etch rate and the selectivity to metals and other oxides can be adjusted in this way. The metal ruthenium oxide, such as SrRuO ₃ , can then be used as a chemically highly stable, but etchable easily and rapidly etchable intermediate or overcoat with this etchant, which is completely removed after use, for example

• For structuring by means of lift-off technology,
• - For the production of freestanding epitaxial oxide structures by undercutting or
• As a "hard mask" for structuring hard-to-attack layers where photoresist is not suitable.

SrRuO ₃ is a stable oxide with a perovskite structure, which has an exceptionally high electrical conductivity of about 1.8 × 10 ⁵ S / m for an oxide (copper has 5.9 × 10 ⁷ S / m). It is advantageous for components made of epitaxial oxide layers to produce printed conductors or contacts from this material by lithography and chemical etching.

In a further particularly advantageous embodiment of the invention, an oxidizing agent is selected which contains an oxidizing metal complex with a transition metal and / or a rare earth. The metal acts as the actual oxidant. The inclusion of the metal in a complex, on the one hand, causes the metal to be dissolvable in solution and protects it at the same time avoiding other reactions before the oxidation of the metal to be attacked in the mixed metal oxide was effected. Which reactions prefer to proceed depends on the pH. By addition of other acids or bases, the pH can be adjusted so that the oxidation of the attacking over other reactions is preferred and unwanted reactions are prevented. As the metal, for example, cerium (IV) can be used. The metal is reduced, the metal to be attacked is oxidized. In particular, ruthenium and chromium can be attacked in this way. The inventors have successfully used a solution of perchloric acid as a pH stabilizer and ammonium cerium (IV) nitrate as the oxidizing agent as an etchant for SrRuO ₃ . This solution etches extremely fast at 500 nm / s or more. It also has a standard redox potential below 2 V. In an analogous manner, for example, LaCrO ₃ and (La, Sr) CrO ₃ attack.

Other oxidants based on permanganate, chlorine, Caro's acid (concentrated sulfuric acid with hydrogen peroxide) are also suitable for oxidizing Ru in compounds to RuO ₄ . Caro's acid is less suitable if the ruthenium compound contains an alkaline earth metal because insoluble alkaline earth sulfates form. Osmium is very similar to ruthenium in its chemical properties and is therefore oxidized to OsO ₄ by the same oxidants. Cr, Mo, W, Re are also attacked by oxidation to form the anions CrO ₄ ^2- , WO ₄ ^2- MoO ₄ ^2- and ReO ₄ ^- in which the metal resembles RuO ₄ and OsO ₄ in a very stable configuration tetrahedrally bound by oxygen. Particularly advantageous here is a combination of permanganate, perchloric acid for stabilizing the pH and optionally a small amount of added hydrochloric acid.

Advantageously, a mixed metal oxide of ruthenium and an alkaline earth metal and / or a rare earth (metal ruthenium oxide) is selected as metal mixed oxide. Oxides of alkaline earth metals and rare earths are particularly easily soluble. In addition, metal-ruthenium oxides with alkaline earth metals or rare earths are partly electrically conductive (in particular with strontium as alkaline earth metal), which gives additional functionality to a functional structure produced by the etching. In a particularly advantageous embodiment of the invention, SrRuO ₃ is therefore selected as the metal mixed oxide. In addition to compounds of the formula MRuO ₃ , for example, compounds of the formulas M ₂ RuO ₄ , M ₃ Ru ₂ O ₇ and M ₂ Ru ₂ O _{7 are} suitable for being etched by the process according to the invention, because the bonding ratios of the ruthenium are similar. M herein denotes the metal (s) besides ruthenium in the mixed metal oxide.

In a particularly advantageous embodiment of the invention, a mixed metal oxide of ruthenium and a further metal M is selected with a layer structure in which the metal M is contained both in layers of the formula MRuO ₃ and in other layers as pure oxide. Then, in the layers of the formula MRuO _3, similar bonding conditions prevail between the ruthenium and the respective next oxygen atoms, as in RuO ₂ . These layers are thus primarily attacked by the oxidant, while the pure oxide in the remaining layers is primarily attacked by the solvent. In particular, a Ruddlesden popper phase of the formula (MO) (MRuO ₃ ) _{n can} be chosen as metal mixed oxide. This crystal structure contains MO n layers (MRuO ₃ ) for each layer of the metal oxide.

In view of the foregoing, the invention generally also relates to a method of chemically etching metal ruthenium oxide. "Metal" refers to metals other than ruthenium. The features disclosed in connection with the previously described methods are expressly also disclosed as being disclosed for this method.

• – mindestens ein Oxidationsmittel, das RuO₂ zu RuO₄ zu oxidieren vermag, sowie
• – mindestens ein Lösungsmittel, das sowohl RuO₄ als auch mindestens eine Oxidationsstufe des Metalls in Lösung zu überführen vermag und für die Dauer des Ätzprozesses in Anwesenheit des Oxidationsmittels chemisch stabil ist,

umfasst.According to the invention, an etchant is selected which

• - At least one oxidizing agent, which is able to oxidize RuO ₂ to RuO ₄ , as well as
• At least one solvent which is capable of converting both RuO ₄ and at least one oxidation state of the metal into solution and is chemically stable for the duration of the etching process in the presence of the oxidizing agent,

includes.

It was recognized that the further oxidizability of ruthenium bound in the metal ruthenium oxide is mainly determined by the bonding ratios of the ruthenium to the nearest oxygen atoms. It has also been recognized that in many metal ruthenium oxides, these bonding ratios are very similar to those in RuO ₂ .

If the ruthenium bound similarly to RuO _{2 is} exposed to the oxidizing agent, it is oxidized to RuO ₄ . By dissolving this RuO ₄ again in the solvent, the ruthenium is removed from the solid phase of the metal ruthenium oxide. The result is a ruthenium defect, where there is only one oxidation state of the metal locally. This is also dissolved in the solvent, whereby further areas of the solid phase are exposed to the attack of the etchant. In this way, the etching attack continues to eat through the metal-ruthenium oxide.

If the oxidation state of the metal is not dissolved, it can form a protective layer on the metal ruthenium oxide, which no longer contains ruthenium and thus inhibits the attack of the etchant. The further etching then proceeds either strongly anisotropic or comes to a complete halt. However, the oxidation state may also remain as an impurity without forming a closed layer. Both are undesirable if a defined functional structure of the metal-ruthenium oxide is to be produced by the etching.

In the prior art, only ozone dissolved in water was known for SrRuO ₃ . Ozone had to be produced on site in an ozonizer because it was not storable for long. His instability also made it explosive. By contrast, the etchants used according to the invention consist of components which are available on the market in liquid form or else as a solid, to which only water has to be added before use. They are also much more selective than ozone, which also attacks the photoresist used in the lithographic production of functional layers, as many oxides are not further attacked by oxidation and other materials such as silicon are passivated by formation of a protective oxide layer. It is the merit of the inventors to have realized that the oxidizing agents known for oxidizing RuO _{2 are} the key to the chemical etching of compounds such as SrRuO ₃ , heretofore considered virtually non-etchable.

Under certain circumstances, one component of the mixed oxide is attacked much faster than another. This may cause the slower attacked component to linger, while the faster attacked will leach out of the structure. This leads to residues on the surface. By presenting the more soluble component in the etching solution in a suitable concentration, for example in the form of a soluble salt, the dissolution of this component can be slowed down so that a uniform etching progress is achieved. Advantageously, the component of the metal mixed oxide is added to the etchant, which dissolves better in it. This component can be present in particular in dissolved form, for example as a dissolved salt. By already having the product of the etching with respect to this one component, the tendency to continue to produce this product, that is, to continue to attack this component, is diminished.

In addition, under certain circumstances, a targeted termination of the crystal surface can be achieved after the etching process, when a component is presented in the etching solution: The resolution of the component may possibly be slowed down so far that after completion of the etching, the uppermost atomic layer formed by a majority of the submitted component becomes. The termination of the surface has a great influence on the properties of an interface in heteroepitaxy and also on the growth of the subsequent layer. Control over the termination is therefore technologically very advantageous.

In addition, an anhydrous solvent can be used to rinse after etching to prevent reaction of the surface with water.

The surface may also be etched a second time at a low etch rate, for example in an anhydrous solvent, after a first etch step of relatively high etch rate but poor surface area to improve the surface. In this second Ätzgang also mechanical friction can be used to remove impurities from the surface (chemical-mechanical polishing, CMP).

Oxidizing or reducing solutions can also be used only for surface treatment, such as for termination or in conjunction with CMP of mixed oxides. Again, a component of the oxide may be presented to achieve a particular termination or prevent leaching.

In the context of the invention, a method for producing a functional structure from a metal mixed oxide layer has been developed. In this case, a photoresist is first applied to the layer. This is then removed lithographically in the areas in which not belonging to the functional structure metal mixed oxide is to be removed. According to the invention, the exposed metal mixed oxide is etched using the etching method according to the invention.

Chemical etching is particularly advantageous for epitaxial layers over other etching methods, such as ion beam etching or plasma etching, because it produces less steep edges and leaves the crystal structure undamaged, allowing the growth of subsequent epitaxial layers. By using the etching method according to the invention, compounds such as SrRuO _{3 become} amenable to chemical etching for the first time on a practical scale, a new field of structurability by means of lithography opens up for these compounds. Advantageously, in this case the type and thickness of the photoresist and the etchant are coordinated so that the photoresist does not lose its protective effect until the completion of the Ätzprozeesses. The method is used by the inventors to produce conductor tracks and contacts from SrRuO ₃ and to undercut epitaxial structures deposited on SrRuO ₃ .

The etching process can also be used to leach valuable ruthenium from catalysts and other recycling equipment or even ores.

Special description part

The subject matter of the invention will be explained below with reference to figures and exemplary embodiments, without the subject matter of the invention being limited thereby. It is shown:

1 : Pourbaix diagrams of iodine in an aqueous environment (panel a), ruthenium (panel b) and superposition of the two diagrams (panel c).

2 : Pourbaix diagrams of chlorine (partial image a), manganese (partial image b) and superposition of the two diagrams (partial image c).

3 : Pourbaix diagram of titanium alone (panel a) and titanium with complexing agent (panel b).

1a shows the Pourbaix diagram of iodine in an aqueous environment. It is shown for different oxidation states of the iodine, such as periodate IO ₄ ^- for the oxidation state VII, respectively, in which combinations of pH and electrochemical potential, this oxidation state is predominant. 1b shows the Pourbaix diagram of ruthenium.

The comparison of 1a With 1b shows that there is a field in which, on the one hand, IO ₃ or HIO ₃ as reduced forms of the periodate and, on the other hand, RuO ₄ as the oxidized form of RuO _{2 are} predominant. 1c shows an overlay of 1a With 1b in which this area is hatched. It extends over the entire pH range shown. For higher electrochemical potentials, it is limited by the limit 1a to areas where H _x IO ₆ are predominant as oxidized forms of periodate. At lower electrochemical potentials it is limited by the limit 1b to areas where RuO ₂ is predominant as a reduced form of RuO ₄ . In the conditions that the hatched area in 1c define, the periodate IO ₄ ^- reduced to IO ₃ ^- and at the same time oxidizes RuO ₂ to more soluble RuO ₄ (redox reaction).

2a shows the Pourbaix diagram of chlorine. It is shown for different oxidation states of the chlorine, such as chloride Cl ^- or chlorate CIO ₃ ^- respectively, in which combinations of pH and electrochemical potential, this oxidation state is predominant. 1b shows the Pourbaix diagram of manganese.

The comparison of 2a With 2 B shows that there is a small area at low pH and high chloride concentration (corresponding to concentrated hydrochloric acid) in which Cl ₂ as the oxidized form of chloride and Mn ²⁺ as the reduced form of MnO _{2 are} predominant. 2c shows an overlay of 2a With 2 B in which this area is hatched. It is limited to pH values below 0. For higher electrochemical potentials, it is limited by the limit 2 B to the areas where MnO _{2 is} predominant as the oxidized form of manganese. At lower electrochemical potentials it is limited by the limit 2a to the areas where Cl ^- is prevalent as a reduced form of Cl ₂ . In the conditions that the hatched area in 2c manganese dioxide MnO _{2 is} reduced to more soluble Mn ²⁺ and at the same time Cl ^- oxidizes to Cl ₂ (redox reaction).

3a shows the Pourbaix diagram of titanium. It is shown for different oxidation states of titanium, such as TiO _{2 in} each case, in which combinations of pH and electrochemical potential, this oxidation state is predominant. 3b shows the Pourbaix diagram of titanium with the addition of the complexing agent EDTA.

The comparison of 3a With 3b shows that the stability ranges have shifted significantly and in particular the soluble EDTA complexes of the trivalent titanium Ti (HEDTA) and Ti (EDTA) are comparatively stable, ie predominate over a larger potential and pH range than trivalent Ti species without added complexing agent. In 3a and 3b each area is hatched in which hydrogen would be able to reduce tetravalent titanium as in TiO ₂ to trivalent titanium as in Ti ³⁺ . This range is limited to positive potentials by the range in which tetravalent titanium such as TiO ₂ , TiO (EDTA) ^2- or TiO ^{2- is} predominant. To negative potentials it is limited by the dashed line, which marks the stability limit of water, ie below which hydrogen is formed from water. The marked area is limited to 3a to a small area at very acidic pH, while the area in 3b extends to neutral pH in the presence of EDTA. Reducing agents such as glucose or ascorbic acid are comparable in their strength to hydrogen, ie they have a standard reduction potential of nearly 0 V. Within the hatched area Ti (IV) is reduced to Ti (III) and at the same time the reducing agent is oxidized (redox reaction) , The comparison of 3a and 3b shows that the etching of TiO ₂ is advantageous in the presence of EDTA as complexing agent.

Examples of metals to attack

manganese

For manganese mixed oxides, for example (La, Sr) MnO ₃ , the effect of the etchant is based on the reduction of insoluble Mn (IV) and unstable Mn (III) to Mn (II). The prior art used concentrated hydrochloric acid, citric acid at elevated temperature and buffered hydrofluoric acid. According to the invention, ascorbic acid, glucose with citric acid as complexing agent and other reducing agents in the neutral to acidic medium are used.

• – AMnO₂, A in Li, Na, K, Rb, Cs
• – AMnO₃, A in Ca, Sr, Ba, Sc, Y, La-Lu, Pb, Co, Fe, Ni, In, Bi
• – Li₂MnO₃
• – A₆MnO₈, A in Mg, Ni, Cu
• – AMn₂O₄, A in Li, Na, Mg, Ca, Sr, Co, Ni, Zn, Cd, Cu
• – AMn₂O₅, A in Y, La-Lu
• – A₂Mn₂O₇, A in Sc, Y, La-Lu
• – (AO)_n(MnO₂)_m, für n, m in den natürlichen Zahlen, n >= 2, n >= m, A in Ca, Sr, Ba, Pb, Co (sog. Ruddelsden-Popper-Phasen)
• – A₆Mn₂O₆, A in K, Rb

Examples of known simple ternary Mn (IV) and Mn (III) compounds (in each case with the same crystal motif also mixtures, doping and odd-numbered stoichiometry, in particular variation of the oxygen content):

• - AMnO ₂ , A in Li, Na, K, Rb, Cs
• AMnO ₃ , A in Ca, Sr, Ba, Sc, Y, La-Lu, Pb, Co, Fe, Ni, In, Bi
• - Li ₂ MnO ₃
• A ₆ MnO ₈ , A in Mg, Ni, Cu
• - AMn ₂ O ₄ , A in Li, Na, Mg, Ca, Sr, Co, Ni, Zn, Cd, Cu
• - AMn ₂ O ₅ , A in Y, La-Lu
• A ₂ Mn ₂ O ₇ , A in Sc, Y, La-Lu
• - (AO) _n (MnO ₂ ) _m , for n, m in the natural numbers, n> = 2, n> = m, A in Ca, Sr, Ba, Pb, Co (so-called Ruddelsden-Popper phases)
• A ₆ Mn ₂ O ₆ , A in K, Rb

The list is far from complete!

molybdenum

For molybdenum mixed oxides, such as SrMoO ₃ and CaMoO ₃ , the effect of the etchant is due to the oxidation of Mo (IV), as occurs in MoO ₂ , to Mo (VI), as occurs in MoO ₄ ^2- . Furthermore, the so-called molybdenum bronzes A _x MoO ₃ with A in alkali metals or thallium; x between 0 and 1, in which molybdenum is formally present as a mixture of Mo (V) and Mo (VI), ie partially reduced, completely oxidized by the etchant to Mo (VI) and thus soluble under alkaline conditions. There are no known wet chemical etchants. According to the invention salts of halogen-oxygen acids and hexacyanidoferrate (III) are proposed in alkaline solution for good solubility of the molybdate anion MoO ₄ ^2- as an etchant. The oxidizing agent need only be moderately strong to oxidize Mo (IV) to Mo (VI).

• – LiMoO₂
• – AMoO₃, A in Ca, Sr, Ba
• – Li₂MoO₃
• – A₂MoO₄, A in Fe,
• – CrMoO₃: siehe Cr-Mischoxide

Examples of known simple ternary Mo (IV) compounds (in each case with the same crystal motif also mixtures, doping and odd-numbered stoichiometry, in particular variation of the oxygen content):

• - LiMoO ₂
• - AMoO ₃ , A in Ca, Sr, Ba
• Li ₂ MoO ₃
• A ₂ MoO ₄ , A in Fe,
• CrMoO ₃ : see Cr mixed oxides

tungsten

In the case of tungsten mixed oxides, the effect of the etchant is based on the oxidation of tungsten bronzes A _x WO ₃ (A in alkali metals, Tl, In, Sn, Ge, Ba, Pb, Sb, rare earths or mixtures, x between 0 and 1) in which tungsten is formally present as a mixture of W (V) and W (VI), ie partially reduced, to W (VI), as occurs in WO ₄ ^2- . There are no known wet chemical etchants. According to the invention salts of halogen-oxygen acids and hexacyanidoferrate (III) are proposed in alkaline solution for good solubility of the tungstate anion WO ₄ ^2- as an etchant.

titanium

For titanium mixed oxides, for example SrTiO ₃ , the effect of the etchant is based on the reduction of Ti (IV) to Ti (III). As etchant buffered hydrofluoric acid and a mixture of hydrofluoric acid and nitric acid are known. According to the invention ascorbic acid or glucose are proposed with EDTA as a complexing agent in a pH neutral to acidic solution.

• – ATiO₃, A in Erdalkali-Metallen, Pb, Fe, Co, Hg, Mn, Ni, Cd, Zn,
• – A₂TiO₃, A in Tl, Rb, K, Ag
• – (AO)_n(TiO₂)_m, für n, m in den natürlichen Zahlen, n >= 2, n >= m, A in Erdalkali-Metallen, Fe, Zn, Sn, Mn, Zn,
• – A₂TiO_{5 :} A in Al, Seltene Erden, Sc, Y, In, Ga, Fe,
• – ATi₂O₅, A in Erdalkali-Metallen, Co, Fe
• – A₂Ti₂O₇, A in Sc, Y, Seltenen Erden, Bi,

Examples of known simple ternary Ti (IV) compounds (in each case with the same crystal motif also mixtures, doping and odd-numbered stoichiometry, in particular variation of the oxygen content):

• - ATiO ₃ , A in alkaline earth metals, Pb, Fe, Co, Hg, Mn, Ni, Cd, Zn,
• A ₂ TiO ₃ , A in Tl, Rb, K, Ag
• - (AO) _n (TiO ₂ ) _m , for n, m in the natural numbers, n> = 2, n> = m, A in alkaline earth metals, Fe, Zn, Sn, Mn, Zn,
• A ₂ TiO _{5 :} A in Al, rare earths, Sc, Y, In, Ga, Fe,
• ATi ₂ O ₅ , A in alkaline earth metals, Co, Fe
• A ₂ Ti ₂ O ₇ , A in Sc, Y, rare earths, Bi,

chrome

In the case of chromium mixed oxides, for example LaCrO ₃ or (La, Sr) CrO ₃ , the effect of the etchant is based on the oxidation of Cr (III), as occurs in Cr ₂ O ₃ , or Cr (IV), as in CrO ₂ occurs, to Cr (VI). There are no known wet chemical etchants. According to the invention, ammonium cerium nitrate in acidic solution is proposed since it is a known chromium metal etchant and its etching effect is due to the fact that the passivation layer of chromium (Cr ₂ O ₃ ) is attacked by the etchant by oxidation to Cr (VI).

• – ACrO₂, A in Alkali-Metallen, Cu, Ag
• – ACrO₃, A in Erdalkali-Metallen, Seltenen Erden, Sc, Y, Pb, Bi
• – ACrO₄, A in Seltenen Erden, Y, Sc
• – (AO)_n(CrO₂)_m, für n, m in den natürlichen Zahlen, n >= 2, n >= m, A in Erdalkali-Metallen, Fe, Zn, Sn, Mn, Zn
• – ACr₂O₄, A in Erdalkali-Metallen, Zn, Cd, Cu, Ni, Fe, Co, Mn

Examples of known simple ternary Cr (III) and Cr (IV) compounds (in each case with the same crystal motif also mixtures, doping and odd-numbered stoichiometry, in particular variation of the oxygen content):

• - ACrO ₂ , A in alkali metals, Cu, Ag
• - ACrO ₃ , A in alkaline earth metals, rare earths, Sc, Y, Pb, Bi
• - ACrO ₄ , A in rare earths, Y, Sc
• - (AO) _n (CrO ₂ ) _m , for n, m in the natural numbers, n> = 2, n> = m, A in alkaline earth metals, Fe, Zn, Sn, Mn, Zn
• - ACr ₂ O ₄ , A in alkaline earth metals, Zn, Cd, Cu, Ni, Fe, Co, Mn

rhenium

In the case of rhenium mixed oxides, for example CaReO ₃ or SrReO ₃ , the effect of the etchant is based on the oxidation of Re (IV), as occurs in ReO ₂ , or Re (VI), as occurs in ReO ₃ , to Re (VII ), as occurs in ReO ₄ ^- . There are no known wet chemical etchants. According to the invention, nitric acid, halogen-oxygen acids and hexacyanidoferrate (III) are proposed.

• – AReO₃, A in Erdalkali-Metallen
• – AReO₄, A in Bi, Co, Cr,
• – A₂ReO₅, A in Seltenen Erden, Y, Sc, Erdalkali-Metallen
• – A₃ReO₆, A in Ca
• – A₃ReO₂, A in Seltenen Erden, Y, Sc
• – A₆ReO₁₂, A in Seltenen Erden, Y, Sc
• – ARe₂O₆, A in Bi, Pb, Sb, Sc, Co, Ni
• – A₂Re₂O_7-x, A in Pb, Cd,
• – A₃Re₂O₉, A in Seltenen Erden, Y, Sc
• – A₅Re₂O₁₂, A in Seltenen Erden, Y, Sc

Examples of known simple ternary Re (IV) and Re (VI) compounds (in each case with the same crystal motif also mixtures, doping and odd-numbered stoichiometry, in particular variation of the oxygen content):

• - AReO ₃ , A in alkaline earth metals
• AReO ₄ , A in Bi, Co, Cr,
• - A ₂ ReO ₅ , A in rare earths, Y, Sc, alkaline earth metals
• - A ₃ ReO ₆ , A in Ca
• - A ₃ ReO ₂ , A in rare earths, Y, Sc
• - A ₆ ReO ₁₂ , A in rare earths, Y, Sc
• - ARe ₂ O ₆ , A in Bi, Pb, Sb, Sc, Co, Ni
• A ₂ Re ₂ O _7-x , A in Pb, Cd,
• - A ₃ Re ₂ O ₉ , A in rare earths, Y, Sc
• - A ₅ Re ₂ O ₁₂ , A in rare earths, Y, Sc

ruthenium

For ruthenium mixed oxides, for example SrRuO ₃ and CaRuO ₃ , the effect of the etchant is based on the oxidation of Ru (IV) to Ru (VIII). As a wet-chemical etchant is known only dissolved in water ozone. According to the invention, halogen-oxygen acids and ammonium cerium nitrate are used.

• – ARuO₃, A in Erdalkali-Metallen, Seltenen Erden, Sc, Y, Pb, Bi
• – A₂RuO₃, A in Alkali-Metallen
• – A₂RuO₅, A in Seltenen Erden, Y, Sc
• – (AO)_n(RuO₂)_m, für n, m in den natürlichen Zahlen, n >= 2, n >= m, A in Erdalkali-Metallen, Fe, Zn, Sn, Mn, Zn, Co, Pb,
• – A₃RuOa, A in Ag,
• – A₃RuO₇, A in Seltenen Erden, Y, Sc
• – A₂Ru₂O₇ A in Seltenen Erden, Y, Sc, Erdalkali-Metallen, Bi, Cd, Hg, Tl, Pb
• – A₅Ru₂O₁₂, A in Seltenen Erden, Y, Sc
• – A₃Ru₂O₇, A in Erdalkali-Metalle

Examples of known simple ternary Ru (IV) compounds (in each case with the same crystal motif also mixtures, doping and odd-numbered stoichiometry, in particular variation of the oxygen content):

• - ARuO ₃ , A in alkaline earth metals, rare earths, Sc, Y, Pb, Bi
• - A ₂ RuO ₃ , A in alkali metals
• - A ₂ RuO ₅ , A in rare earths, Y, Sc
• - (AO) _n (RuO ₂ ) _m , for n, m in the natural numbers, n> = 2, n> = m, A in alkaline earth metals, Fe, Zn, Sn, Mn, Zn, Co, Pb,
• A ₃ RuOa, A in Ag,
• - A ₃ RuO ₇ , A in rare earths, Y, Sc
• - A ₂ Ru ₂ O ₇ A in rare earths, Y, Sc, alkaline earth metals, Bi, Cd, Hg, Tl, Pb
• - A ₅ Ru ₂ O ₁₂ , A in rare earths, Y, Sc
• - A ₃ Ru ₂ O ₇ , A in alkaline earth metals

osmium

For osmium mixed oxides, for example SrOsO ₃ , the effect of the etchant is based on the oxidation of Os (IV) or Os (VI) to Os (VIII). There are no known wet chemical etchants. According to the invention, halogen-oxygen acids and ammonium cerium nitrate are proposed.

• – AOsO₃, A in Erdalkali-Metallen
• – A₃OsO₇, A in Seltenen Erden, Y, Sc
• – A₂Os₂O₇, A in Seltenen Erden, Y, Sc, Erdalkali-Metallen, Bi, Cd, Hg, Tl, Pb

Examples of known simple ternary Os (IV) or Os (VI) compounds (in each case with the same crystal motif also mixtures, doping and odd-numbered stoichiometry, in particular variation of the oxygen content):

• - AOsO ₃ , A in alkaline earth metals
• - A ₃ OsO ₇ , A in rare earths, Y, Sc
• - A ₂ Os ₂ O ₇ , A in rare earths, Y, Sc, alkaline earth metals, Bi, Cd, Hg, Tl, Pb

rhodium

For rhodium mixed oxides, for example LaRhO ₃ , the effect of the etchant is based on the reduction of Rh (IV) or Rh (III) to Rh (II) or Rh (I) and complex formation with cyanide in an alkaline medium. Wet chemical etchants are not yet known. According to the invention, a mixture of hexacyanidoferrate (II) and hexacyanidoferrate (III) with cyanide in an alkaline medium is proposed.

• – ARhO₃, A in Erdalkali-Metallen, Seltenen Erden, Y, Sc, Bi,
• – (AO)_n(RhO₂), für n, m in den natürlichen Zahlen, n >= 2, n >= m, A in Erdalkali-Metallen, Fe, Zn, Sn, Mn, Zn
• – ARh₂O₄, A in Erdalkali-Metallen, Zn, Cd, Cu, Ni, Fe, Co, Mn
• – A₂Rh₂O₇, A in Seltenen Erden, Y, Sc, Erdalkali-Metallen, Bi, Cd, Hg, Tl, Pb
• – A₃Rh₂O₇, A in Erdalkali-Metallen

Examples of known simple ternary Rh (III) or Rh (IV) compounds (in each case with the same crystal motif also mixtures, doping and odd-numbered stoichiometry, in particular variation of the oxygen content):

• - ARhO ₃ , A in alkaline earth metals, rare earths, Y, Sc, Bi,
• - (AO) _n (RhO ₂ ), for n, m in the natural numbers, n> = 2, n> = m, A in alkaline earth metals, Fe, Zn, Sn, Mn, Zn
• - ARh ₂ O ₄ , A in alkaline earth metals, Zn, Cd, Cu, Ni, Fe, Co, Mn
• - A ₂ Rh ₂ O ₇ , A in rare earths, Y, Sc, alkaline earth metals, Bi, Cd, Hg, Tl, Pb
• - A ₃ Rh ₂ O ₇ , A in alkaline earth metals

cerium

For cerium mixed oxides, for example SrCeO ₃ , and also for CeO ₂ , the effect of the etchant is based on a reduction of Ce (IV) to Ce (III). There are no known wet chemical etchants. According to the invention, an acidic reducing solution, such as ascorbic acid or an acidic glucose solution, is proposed.

• – ACeO3, A in Erdalkali-Metallen,
• – (AO)_n(CeO₂)_m, für n, m in den natürlichen Zahlen, n >= 2, n >= m, A in Erdalkali-Metallen
• – Mit anderen Seltenen Erden, Cu, Mn, Ca, Bi oder Sb dotiertes CeO₂

Examples of known simple ternary Ce (IV) compounds (in each case with the same crystal motif also mixtures, doping and odd-numbered stoichiometry, in particular variation of the oxygen content):

• - ACeO3, A in alkaline earth metals,
• - (AO) _n (CeO ₂ ) _m , for n, m in the natural numbers, n> = 2, n> = m, A in alkaline earth metals
• CeO ₂ doped with other rare earths, Cu, Mn, Ca, Bi or Sb

Examples of oxidizing and reducing agents

Hexacyanidoferrate (III) is a very fast oxidation and hexacyanidoferrate (II), a redox potential reducing agent stable only in the alkaline state. Nitrite is a reducing agent and nitrate is an oxidant with medium redox potential. Chlorine is a strong oxidant, chlorate and especially hypochlorite are even stronger, perchlorate is less strong. At high concentration and very acidic pH chloride can act as a reducing agent, such as. B. with manganese dioxide in concentrated hydrochloric acid. Iodide and iodine can be easily oxidized / reduced at medium redox potential. Iodate and periodate are strong oxidants. Cerium and nitrate form an acid-stable complex; Ammonium cerium nitrate is a very strong oxidizer. Bromate is also a strong oxidizer. Reductones, such as ascorbic acid, or aldehydes, such as glucose or benzaldehyde, are reducing agents. Aldehydes are readily oxidized to the corresponding carboxylic acid and reductones are readily oxidized to the corresponding diketone. Other reducing agents are oxalic acid, formic acid, thiosulfate, hydrazine, sulfite, dithionite, lithium aluminum hydride, sodium boron hydride, hydrogen, alkali and alkaline earth metals, zinc and divalent iron salts.

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

• US 6436723 B1 [0003]

Cited non-patent literature

• http://www.kemi.kth.se/medusa/ [0015]

Claims (23)

A process for etching a metal mixed oxide, characterized in that an etchant is selected which - brings at least one of the metals in the mixed oxide to another oxidation state in which it is weaker and / or less directionally bound, - a redox potential of below +2 V - and includes either a reducing agent that lowers its redox potential by at least 0.15V, or an oxidizing agent that increases its redox potential by at least 0.05V. A method according to claim 1, characterized in that an etchant is selected which does not form a compound with any metal of the mixed oxide which does not dissolve in the etchant. Method according to one of claims 1 to 2, characterized in that an etchant is selected which contains a solvent for the metal brought to a different oxidation state. A method according to claim 3, characterized in that water, an organic solvent, an ionic liquid or a melt is selected as the solvent. A method according to claim 4, characterized in that an organic solvent from the group propylene carbonate, dimethyl sulfoxide, acetonitrile, dimethylformamide (DMF) or dimethylpropyleneurea (DMPU) is selected. A method according to claim 4, characterized in that an organic solvent from the group of alkanols, in particular methanol, ethanol or isopropanol, is selected. Method according to one of claims 1 to 6, characterized in that an aqueous solution having a reducing action and a pH greater than 0 is selected as etchant. Method according to one of claims 1 to 7, characterized in that an etchant is selected which includes a complexing agent. A method according to claim 8, characterized in that a complexing agent selected from the group consisting of chloride, fluoride, bromide, iodide, citrate, tartrate, EDTA, oxalate, cyanide, thiocyanate, thiosulfate, ammonium, crown ethers or cryptate. Method according to one of claims 1 to 9, characterized in that a perovskite or a pyrochlore is chosen as metal mixed oxide. Method according to one of claims 1 to 10, characterized in that a mixed metal oxide is selected which contains at least one transition metal or cerium as the metal. A method according to claim 11, characterized in that the transition metal from the group Os, Rh, Ti, Cr, Mn, Re, Ru, W is selected. Method according to one of claims 1 to 12, characterized in that an etchant is selected which contains a reductone or an aldehyde as a reducing agent. Method according to one of claims 1 to 13, characterized in that an aqueous solution containing the anion and / or the free acid of a halo-oxygen acid is selected as an etchant. A method according to claim 14, characterized in that an oxidizing agent is selected which contains a perhalogenate, halogenate, halide or hypohalite. Method according to one of claims 1 to 15, characterized in that an oxidizing agent is selected, which contains an oxidizing metal complex with a transition metal and / or a rare earth. Method according to one of claims 1 to 16, characterized in that a metal mixed oxide of ruthenium and an alkaline earth metal and / or a rare earth is selected as metal mixed oxide. A method according to claim 17, characterized in that SrRuO _{3 is selected} as metal mixed oxide. Method according to one of claims 1 to 18, characterized in that a metal mixed oxide of ruthenium and another metal M is selected with a layer structure in which the metal M both in layers of the formula MRuO ₃ and in other layers as a pure oxide is included. A method according to claim 19, characterized in that a Ruddlesden-Popper phase of the formula (MO) (MRuO ₃ ) _{n is selected} as metal mixed oxide. Method according to one of claims 1 to 20, characterized in that the etchant, the component of the metal mixed oxide is added, which dissolves better in the etchant. Process for producing a functional structure from a metal mixed oxide layer, wherein a photoresist is applied to the layer and the photoresist is removed lithographically in the areas in which non-functional structure belonging metal mixed oxide is to be removed, characterized in that the exposed metal mixed oxide etched by a method according to one of claims 1 to 21 becomes. A method according to claim 22, characterized in that the type and thickness of the photoresist and the etchant are coordinated so that the photoresist does not lose its protective effect until the end of the etching process.

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