DE102006004826B4 - Metal- and cyanide-free etching solution for wet-chemical structuring of metal layers in the semiconductor industry and their use in an etching process - Google Patents

Abstract

Etching solution for wet-chemical structuring of metal layers, in particular silver layers, in the production of semiconductor devices characterized by ammonium peroxodisulfate as at least one oxidant and ammonium thiosulfate as at least one complexing agent L, wherein the complexing agent L and the liberated by the oxidizing agent during the etching metal cations M of the metal layer Complex M (S2O3) x with x equal to 1 to 2; form and wherein the etching solution is free of metal and / or cyanide ions.

Description

The invention relates to an etching solution according to the preamble of claim 1 and a use of an etching solution according to claims 9 to 12.

Resistive storage materials have long been known, in particular under the heading "Programmable Metallization Cell (PMC)" as part of the "Nonvolatile Memory Technology". Based on inorganic materials, concepts for changing the conductivity by ion diffusion in glasses, in particular the system GeSe _x / Ag or GeS _x / Ag, are used.

An example of such a system is a Conductive bridging (CB) RAM cell. The CB-RAM cell, in its simplest form, consists of three layers. On a lower electrode made of silver, tungsten or similar materials, a germanium chalcogenide is deposited as an active material, which contains metals such as silver or copper to form a solid solution. Subsequently, a silver layer is deposited as the upper electrode and to provide the conductive path. Upon application of a voltage, the positively charged metal ions from the germanium chalcogenide layer migrate toward the cathode and deposit on the cathode to form a low resistance film. When a blocking voltage is applied, the deposition process is reversed and the deposited metal is dissolved, whereby a high resistance is restored.

In the context of a wider application of this technology, it is desirable to pattern the metal layers formed in the cell, in particular silver layers.

The structuring of silver layers makes special demands here, since silver has a relatively high reaction potential as a noble metal and is therefore relatively inert to chemical reactions. The structuring of silver layers therefore requires the use of specific etching solutions.

In particular, etching solutions used hitherto contain cyanide ions as strong complexing agents, since the metal-cyano complexes formed have a high degree of complex stability.

However, the known high toxicity of cyanide solutions and the high cost of the currently preferred tetraalkylammonium cyanides prove to be disadvantageous for the production. Thus, the cost of 200 l of a 0.1-1 molar cyanide solution in TMA as a developer solution is currently several tens of thousands of euros.

Cyanide-free etching solutions are also known. So will in the DE 23 27 878 C3 discloses a method for etching low blocking current, high reverse voltage electrodeposited semiconductor wafers wherein a silver layer is treated with a mixture of hydrogen peroxide and ammonium hydroxide.

The DE 20 55 251 A describes stabilized peroxo compounds in conjunction with complexing agents for metal ions and their use in etching processes, in particular of copper layers. In a preferred embodiment, the etching solution consists of a peroxodisulfate, z. As ammonium peroxodisulfate and ammonium hydroxide or ethylenediamine as a complexing agent.

DE 1 119 625 A refers to a method of etching the surface of a semiconductor, e.g. From germanium, using an etching solution of hydrogen peroxide, caustic alkali and a complexing agent, e.g. As potassium cyanide, sodium cyanide, ethylenediaminetetraacetic acid (EDTA) or polyphosphoric acids.

DD 114 283 A5 relates to a cyanide-free, thiosulfate-containing aqueous bath for the electrodeposition of silver coatings. As the bath, an aqueous thiosulfate solution of soluble silver or silver complex compounds, such as. As silver sulfate, silver chloride, Alkalsilbercyanide, Alkalisilbersulfite or silver complexes with nitrogen-containing compounds, such as. As ammonia, amines, polyamines used.

It is therefore an object of the present invention to provide etching solutions for the wet-chemical structuring of metal layers which are free of metal ions and cyanide ions. In particular, etching solutions are to be provided which enable the structuring of silver layers in the form of silver electrodes or what is left after a thermal, electrical or photoinduced diffusion thereof into CB-RAM cells. In particular, this method should find application for ADM.

This object is achieved by an etching solution with the features of claim 1.

The etching solution according to the invention for wet-chemical structuring of metal layers, in particular of silver layers, in the production of semiconductor components is characterized by ammonium peroxodisulfate as at least one oxidizing agent and ammonium thiosulfate as at least one complexing agent L, wherein the complexing agent L and the metal cations liberated by the oxidizing agent during the etching process the metal layer form a complex M (S ₂ O ₃ ) _x with x equal to 1 to 2, preferably 2, where x represents the number of bound ligands. The etching solution according to the invention is free from metal and / or cyanide ions.

Advantageously, the oxidizing agent used has a normal potential of greater than +1 V, in particular +2 V, on. Thus, the strong oxidizing agent peroxodisulfate in the reaction 2 SO ₄ ^2- ⇆ S ₂ O ₈ ^2- + 2e ^- a normal potential of +2 V on.

The formed complex ML _x is preferably characterized by a formation constant of greater than or equal to 10 KB ^{x 6} l / mol ^x, in particular greater than or equal to 10 ^{x 7} l / mol ^x. So z. For example, a [Ag (NH ₃ ) ₂ ] ⁺ complex formed during the etching of silver layers is characterized by a complexation constant of 10 ⁷ l ² / mol ² .

During the etching process, the metal is transformed by means of a suitable oxidizing agent into the corresponding metal cation according to the equation Ox + ne- → Red (1) transferred.

The redox potential of the reaction is determined according to the Nernst equation E = E ₀ + RT / nFnn Cox / Cred (2) where E _{0 is} the normal potential, R the general gas constant, T the temperature, n the number of transmitted electrons, F the Faraday constant, c _Ox the concentration of the free metal cations and C _Red the concentration of the metal

Thus, in the case of oxidation of a silver layer, the metallic silver represents the reduced component and the silver ions the oxidized portion, the normal potential of silver being + 0.7991 V and the metallic silver C _{Red being} equal to 1.

Since silver is difficult to etch, it is desirable to shift the equilibrium of equation (1) to the side of the oxidized step, ie, the side of the soluble silver cations. For the desired shift of the balance, the cations must be removed, which z. By the use of strong complexing agents according to the reaction Ag ⁺ + 2L ↔ AgL ₂ ⁺ (3) can be done.

beschreibbar.The constant of the complex formation reaction and thus the stability of the complex formed are by means of the equation

writable.

• a) einem starken Oxidationsmittel in Form von Ammoniumperoxodisulfat zur Überführung des Metalls in das korrespondierende lösliche Kation, sowie
• b) einem starken Komplexbildner in Form von Ammoniumthiosulfat, der das gebildete Metallkation dem Redoxgleichgewicht entzieht.

The process of etching, especially of hard-to-etch metal layers, is thus positively influenced by two components:

• a) a strong oxidizing agent in the form of ammonium peroxodisulfate for the conversion of the metal into the corresponding soluble cation, and
• b) a strong complexing agent in the form of ammonium thiosulfate, which extracts the metal cation formed the redox equilibrium.

The etching solution according to the invention thus makes it possible to move the electrode potential of the metal layers to be structured, in particular of silver layers, into a region which is favorable for the etching, preferably into a range of less than 0.4 V.

Strong oxidizing agents with a normal potential of greater than +1 V, such as. For example, peroxodisulfate with a +2.1 V normal potential should be oxidatively strong enough per se to etch metal layers such as +0.799 V of normal potential or + 0.521 V of normal copper. However, the singular use of peroxodisulfate for etching is prevented by the formation of oxide layers on the said metals and the associated build-up of an overvoltage. The simultaneous use of complexing agents reduces this overvoltage. It is even possible to lower the potential below the value of the normal potential of, for example, Cu, which is known to be very well etched by peroxodisulfate without the addition of complexing agents.

The lowering of the potential through the use of oxidants in conjunction with complexing agents can be z. B. for the Nernst equation effective silver ion concentration according to Table 1 as follows estimate.

Table 1 shows the influence of various complexing agents on the concentration of free silver ions using the example of silver halides and silver sulfide. salt Solubility product mol ² / l ² Max. Ag + concentration mol / l Soluble through complex images AgCl 10 ^-9.7 10 ^-4.85 NH ₃ ; S ₂ O ₃ ^2- ; CN ^- AgBr 10 ^-12.2 10 ^-6.1 S ₂ O ₃ ^2- ; CN ^- AgJ 10 ^-16 10 ^-8 CN ^- Ag ₂ S 10 ^-ca.50 10-25 (less than 1 atom / l) Completely insoluble

Considering the logarithmic scale and the solubility series of the silver halides, the following sequence of free silver ions is obtained: AgCl> [Ag (NH _{3) 2]} ^+> AgBr> [Ag (S ₂ O _{3) 2]} ^3-> Agl> [Ag (CN) _2] ^-> Ag ₂ S.

In a silver-containing thiosulfate solution thus max. 10 ^-7 mol / l Ag ⁺ ions. Using Nernst's equation (2), the potential of a silver-containing thiosulfate solution is given considering all the constants of equation (2) and the normal potential for silver of +0.799 V according to E = 0.799 V - 0.059 V / n · 7 (with n = 1) to +0.386 V. For a silver-containing thiosulfate solution, the potential is in the range of copper. Since the concentrations of free silver ions of a silver thiosulfate complex generally assume values greater than 2 × 10 ^-7 , the potential is even lower by about 100-150 mV.

When using the etching solutions according to the invention for structuring silver layers, care must also be taken that the etching solution is advantageously free of sulfide ions. In the presence of sulfide ions, formation and deposition of the poorly soluble silver sulfide would occur, adversely affecting the overall etch process.

In addition to its ability to reduce the potential of a metal layer, the etching solution according to the invention represents a cost-effective and health-safe alternative to the hitherto known cyanide-containing etching solutions.

The etching solution advantageously has from 0.1 to 10% by weight of ammonium peroxodisulfate as oxidizing agent and from 0.1 to 10% by weight of ammonium thiosulfate as complexing agent.

Generally suitable complexing agents L are compounds with nitrogen and / or sulfur donor atoms. Preference is given to using linear, cyclic, chelating amines, oximes and / or nitrates and thiols, and / or thiosulphates.

The number of ligands that can form complexes with transition metals is extraordinarily large. The classification of the ligands is usually based on the number of complex-forming atoms with which they are able to attach to a center. This property is described by the term "denticity." Monodentate ligands and multimetal chelating ligands are known.

The complex formation takes place by means of an addition of molecules provided with free electrons or electron pairs into the coordination sphere of the transition metal. Ligands with nitrogen and sulfur donor atoms have proven to be very advantageous for the solution of the problem, since these form on the one hand very stable complexes and on the other hand are inexpensive and non-toxic.

Chelating amines form particularly stable complexes and thereby very effectively reduce the metal cation concentration during the etching process.

Advantageously used as linear amines primary, secondary and / or tertiary amines according to the formula NR ₁ R ₂ R ₃ , wherein the radicals R ₁ , R ₂ and R _{3 may be} independently alkyl and / or aryl radicals. Preferred alkyl or aryl radicals are methyl, ethyl, propyl, benzyl and / or phenyl.

Preferred cyclic amines are non-substituted pyridine, alkyl-substituted pyridines and / or pyridinecarboxylic acids and, as chelating amines, bipyridine, phenanthroline, ethylenediamine and / or oligomeric analogs of ethylenediamine. The oxime used is advantageously diacetyldioxime (dimethylglyoxime).

As thiols, propanethiol, ethanethiols, butanethiol and / or thiophenol are advantageously used.

The complexing agents L used are preferably in the form of compounds of thiosulfates, thiocyanates and / or nitrates as anions and tetraalkylammonium as cations. The alkyl radicals are methyl, ethyl, propyl, benzyl or their permutation. It is also lithium, sodium, potassium, calcium or magnesium ions can be used as cations using metal ion-containing etching solutions.

The use of ammonia and / or ammonium thiosulphate as complexing agent L is particularly advantageous. Both compounds have very good complex-forming properties and are very cost-effective.

As the oxidizing agent, peroxo compounds can be generally used. To be particularly advantageous, the use of hydrogen peroxide H ₂ O ₂ and / or peroxodisulfates, especially ammonium peroxodisulfate (NH _{4) 2} S ₂ O _8, proved.

Further advantageous oxidizing agents are hypochlorites, hypobromides, peroxophosphates, peroxodiphosphates, permanganates, peroxochromates, perrhenates, ozonides and / or organic peroxides, in particular di-tert-butyl peroxide or dibenzoyl peroxide.

The salt-form oxidants are advantageously present in the form of lithium, sodium, potassium, calcium, magnesium in the case of metal ion-containing etching solutions and / or tetraalkylammonium salts in the case of metal ion-free etching solutions, the alkyl radicals being methyl, ethyl, propyl, Benzyl, and their permutations are used.

The etching solution also includes solvents and other additives.

The solvents used are preferably water, alcohols, in particular methanol, ethanol, isopropanol or n-propanol, cyclic ethers, in particular dioxane or tetrahydrofuran, dimethyl sulfoxide and / or dimethylformamide, which are present in pure form or in mixtures. The mixtures are adjusted in such a way that the oxidizing agent and complexing agent and the metal-ion complexes formed dissolve well therein.

As additives 0.01 to 10 vol .-% of surfactants, alcohols, solvents, inert salts and / or bases or acids are used to adjust the pH in an advantageous concentration.

In general, additives which serve to adjust the wetting behavior of the formulation serve as additives. These are the commercially available additives from various manufacturers and surfactants (cationic, anionic and neutral). The addition of alcohols such as methanol, ethanol, i, n-propanol, i, n, t-butanol and other higher alcohols, and phenols such as phenol or o, m, p-cresol, causes an improvement in the wetting behavior. Other water-miscible or conditionally miscible additives are: acetonitrile, acetone, tetrahydrofuran, dioxane, γ-butyrolactone, cyclohexanone, N-methylpyrrolidinone, triethylamine, pyridine, dimethyl sulfoxide, dimethylformamide, etc.

The term additives also includes admixtures of inert salts, such as halides, sulfates, tosylates, acetates, propionates, phosphates of ammonium or tetraalkylammonium ions in metal ion-free solutions and lithium, sodium, potassium, calcium, Magnesium ions in metal ion-containing solutions.

Additions of ammonia, tetraalkylammonium hydroxide, where the alkyl radical may be a methyl, ethyl, propyl, benzyl radical and their permutations, soda or potassium hydroxide or formic acid, acetic acid, hydrochloric acid, phosphoric acid or nitric acid are used to adjust the pH ,

The object is also achieved by the use of an etching solution according to the invention according to claims 10 to 12.

Thus, the etching solution according to the invention can be used for structuring metal layers, in particular silver layers in conducting bridging (CB) RAM cells.

However, the etching solution according to the invention can also be used for structuring GeS and / or GeSe. This feature is particularly advantageous for the CB-RAM application.

The etching process using the etching solution according to the invention makes it possible to pattern metal layers, in particular silver layers, in conducting bridging (CB) RAM cells.

The etching solution is also usable for silver conductive lines in electronic components, in particular in HF components or antenna structures. Since the solution according to the invention in particular etches the amine-containing solution, including copper, it can likewise be used for structuring copper-silver-containing alloys or multilayer systems.

The invention will be explained in more detail below with reference to the figures of the drawings of several embodiments. Show it:

1 Schematic representation of a CB-RAM cell

2 Top view of an etched with an etching solution of H ₂ O ₂ and NH ₃ silver layer

3 Top view of an etched with an etching solution of (NH ₄ ) ₂ S ₂ O ₈ and NH ₃ silver layer

4 Top view of a silver layer structured with an etching solution of H ₂ O ₂ and (NH ₄ ) ₂ S ₂ O ₃

5 Top view of a silver layer structured with an etching solution of (NH ₄ ) ₂ S ₂ O ₈ and (NH ₄ ) ₂ S ₂ O ₃

1 schematically illustrates the structure of a CB-RAM cell, wherein the upper electrode consists of a thin layer of silver to be structured.

Embodiment 1 (Prior Art)

2 shows the result of patterning a silver layer with a freshly prepared aqueous H ₂ O ₂ / NH ₃ solution at a magnification of 20,000. The solution used with a pH of 10 contains 1% H ₂ O ₂ as the oxidizing agent and 0.27 ml of a 28% aqueous NH ₃ solution in 20 ml of water as a complexing agent.

For structuring, all embodiments are prepared as follows. First, an approximately 35 nm thick silver layer was evaporated on a silicon wafer in a vacuum. Subsequently, a Spun photoresist at 2500 rpm and dried at 110 ° C for 1 min on a hotplate. The photoresist was exposed to i-line (365 nm) through a chromium-on-quartz mask for 7 seconds at an intensity of 7 mW / cm ² . The development is carried out in a standard Tetramethylammonioumhydroxid developer (2.38%) for 45 seconds, whereby the exposed areas are peeled off. The unexposed areas stop and serve as an etching mask. However, the unexposed areas are only stable for a certain time over strong bases. Therefore, it is advantageous if the pH of the etching solution is not too basic, ie is more in the neutral range.

The silver layer is patterned at an etch rate of 5.0 nm / s. After the etching, the resist is removed again by rinsing with acetone.

The etching rate plays an important role in the structuring process. Very high etching rates make it difficult to control the etching process. Undercutting / overetching is the consequence d. H. small structures are no longer resolved. Great areas etch i. a. a little slower d. H. The small structures come off before large areas are etched through.

Etch rate and etch rate are also heavily affected by ambient temperature. Thus, increasing the temperature causes an increase in the etching rate.

The top view of 2 shows a relatively good resolution of the structured silver surface. The structures, however, have a strong regression of the metal. In addition, a strong evolution of oxygen is observed.

Embodiment 2 (Prior Art)

To structure the in 3 The silver layer shown (magnification 2000 times) was an aqueous solution with a total volume of 20 ml of 1% (NH ₄ ) ₂ S ₂ O ₈ as an oxidant and 5 drops of 0.27 ml of an aqueous 28% NH ₃ solution in 20 ml of water used. The pH of the etching solution is 10. The silver layer was patterned at an etch rate of 5.0 nm / s.

However, the resolution of the structures is not very good here. In addition, the shelf life of the prepared solution is only a few hours, which is disadvantageous for use in an industrial process.

Embodiment 3 (Prior Art)

In 4 For example, the plan view is shown on a silver surface (5000x magnification) treated with an aqueous etch solution containing 1% H ₂ O ₂ and 5% (NH ₄ ) ₂ S ₂ O ₃ . The silver layer was patterned at an etch rate of 1.5 nm / s.

The structure resolution is very good, but it also comes here due to the reduction of H ₂ O ₂ for the development of oxygen.

Embodiment 4 (according to the invention)

5 shows the result of structuring a silver layer (10000 times magnification) with an aqueous etching solution with 1% (NH ₄ ) ₂ S ₂ O ₈ as oxidizing agent and 5% (NH ₄ ) ₂ S ₂ O ₃ as complexing agent. The pH of the solution is 5, which avoids attack on the resist.

The structuring took place with an etching rate of 0.58 nm / s. The etching rate can also be controlled by increasing the pH by adding NH ₃ . Thus, the etching rate at a pH of 7 is 0.88 nm / s.

(NH ₄ ) ₂ S ₂ O ₈ / (NH ₄ ) ₂ S ₂ O ₃ form metastable solutions which after about one day have low sulfur precipitates. It is therefore important that the components of the solution are mixed just before use. The stability of the solution is also controllable by the pH.

The top view of 5 shows a very good resolution and clear structures.

The improved resolution of Embodiments 3 and 4 compared with Embodiments 1 and 2 may be related to the high complexation constant of the thiosulfate complex, which is larger by several orders of magnitude than that of the ammonium complex. The high complex formation constant of thiosulfate is due to their action as a chelate ligand via a sulfur and an oxygen atom. Furthermore, peroxodisulfate complexes of silver are also known.

Table 2 summarizes the results of the experiments performed.

The results clearly show that an etching solution of an oxidizing agent and a complexing agent is well suited for structuring silver layers. The best result was achieved with a (NH ₄ ) ₂ S ₂ O ₈ / (NH ₄ ) ₂ S ₂ O ₃ solution, wherein the etching rate can be controlled by the pH. The solution is also non-toxic and the substances used are inexpensive.

LIST OF REFERENCE NUMBERS

1

upper electrode

2

active material

3

lower electrode

Claims (12)

Etching solution for wet-chemical structuring of metal layers, in particular silver layers, in the production of semiconductor devices characterized by ammonium peroxodisulfate as at least one oxidizing agent and ammonium thiosulfate as at least one complexing agent L, wherein the complexing agent L and the released by the oxidizing agent during the etching metal cations M of the metal layer Complex M (S ₂ O ₃ ) _x with x equal to 1 to 2; form and wherein the etching solution is free of metal and / or cyanide ions. Etching solution according to claim 1 characterized by the formation of a metal complex M (S ₂ O ₃ ) _x with x equal to 2. Etching solution according to at least one of the preceding claims, characterized in that the etching solution is free of sulfide ions. Etching solution according to at least one of the preceding claims, characterized by 0.1 to 10 wt .-% of ammonium peroxodisulfate as the oxidizing agent. Etching solution according to at least one of the preceding claims, characterized by 0.1 to 10 wt .-% of ammonium thiosulfate as a complexing agent. Etching solution according to at least one of the preceding claims, characterized by the use of water, alcohols, in particular methanol, ethanol, iso- or n-propanol, cyclic ethers, in particular dioxane or tetrahydrofuran, dimethyl sulfoxide and / or dimethylformamide as solvent. Etching solution according to at least one of the preceding claims, characterized by 0.01 to 10 vol .-% of other additives in the form of surfactants, alcohols, solvents, inert salts and / or bases or acids for adjusting the pH. Use of an etching solution according to at least one of the preceding claims for structuring metal layers, in particular silver layers, in conducting bridging (CB) RAM cells. Use of an etching solution according to at least one of Claims 1 to 8 for structuring silver conductor tracks in electronic components. Use of an etching solution according to at least one of claims 1 to 8 for structuring copper- and silver-containing alloys and / or multilayer systems. Use of an etching solution according to at least one of Claims 1 to 8 for structuring layers of germanium chalcogenides, in particular germanium sulphide and / or germanium selenide. Etching solution according to claim 1 or 2, characterized in that the complex M (S ₂ O ₃ ) _x formed during the etching process has a complex formation constant KB of greater than or equal to 10 ⁶ l ^x / mol ^x , in particular greater than or equal to 10 ⁷ l ^x / mol ^x , having.

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