CN111771016B - Method for electrodeposition of cobalt - Google Patents

Method for electrodeposition of cobalt Download PDF

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CN111771016B
CN111771016B CN201980015252.2A CN201980015252A CN111771016B CN 111771016 B CN111771016 B CN 111771016B CN 201980015252 A CN201980015252 A CN 201980015252A CN 111771016 B CN111771016 B CN 111771016B
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cobalt
electrolyte
acid
cavity
substrate
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CN111771016A (en
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文森特·梅费里克
多米尼克·祖尔
米卡卢·蒂亚姆
路易斯·凯拉德
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Alimont Solutions
MacDermid Enthone Inc
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Alchimer SA
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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D7/00—Electroplating characterised by the article coated
    • C25D7/12—Semiconductors
    • C25D7/123—Semiconductors first coated with a seed layer or a conductive layer
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D3/00—Electroplating: Baths therefor
    • C25D3/02—Electroplating: Baths therefor from solutions
    • C25D3/12—Electroplating: Baths therefor from solutions of nickel or cobalt
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
    • C25D5/02—Electroplating of selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
    • C25D5/48—After-treatment of electroplated surfaces
    • C25D5/50—After-treatment of electroplated surfaces by heat-treatment
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/28—Manufacture of electrodes on semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/268
    • H01L21/283—Deposition of conductive or insulating materials for electrodes conducting electric current
    • H01L21/288—Deposition of conductive or insulating materials for electrodes conducting electric current from a liquid, e.g. electrolytic deposition
    • H01L21/2885—Deposition of conductive or insulating materials for electrodes conducting electric current from a liquid, e.g. electrolytic deposition using an external electrical current, i.e. electro-deposition
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/70—Manufacture or treatment of devices consisting of a plurality of solid state components formed in or on a common substrate or of parts thereof; Manufacture of integrated circuit devices or of parts thereof
    • H01L21/71—Manufacture of specific parts of devices defined in group H01L21/70
    • H01L21/768—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics
    • H01L21/76838—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics characterised by the formation and the after-treatment of the conductors
    • H01L21/76877—Filling of holes, grooves or trenches, e.g. vias, with conductive material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/70—Manufacture or treatment of devices consisting of a plurality of solid state components formed in or on a common substrate or of parts thereof; Manufacture of integrated circuit devices or of parts thereof
    • H01L21/71—Manufacture of specific parts of devices defined in group H01L21/70
    • H01L21/768—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics
    • H01L21/76838—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics characterised by the formation and the after-treatment of the conductors
    • H01L21/76877—Filling of holes, grooves or trenches, e.g. vias, with conductive material
    • H01L21/76879—Filling of holes, grooves or trenches, e.g. vias, with conductive material by selective deposition of conductive material in the vias, e.g. selective C.V.D. on semiconductor material, plating

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Abstract

The present invention relates to a method for manufacturing cobalt interconnects and an electrolyte capable of implementing the method. The electrolyte has a pH of less than 4.0 and comprises cobalt ions, chloride ions and up to two low molecular weight organic additives. One of these additives may be an alpha-hydroxycarboxylic acid or a compound having a pKa value in the range of 1.8-3.5.

Description

Method for electrodeposition of cobalt
Technical Field
The present invention relates to electrodepositing cobalt on conductive surfaces. More particularly, the present invention relates to a method for electrodeposition of cobalt that may be used to fabricate electrical interconnects (electrical interconnection) in integrated circuits.
Background
Semiconductor devices include conductive metal interconnects such as trenches extending along the surface and vias connecting various levels of integration. The fabrication of the interconnects involves etching the structures in a dielectric material and then depositing a metal seed layer over the entire surface of the structures to enhance their conductivity in a subsequent step of filling with a conductive metal, which is typically performed electrochemically.
Conventional methods of filling interconnects with cobalt use electrolytes containing cobalt salts and various organic additives, including inhibitors (suppresor) and promoters (accelerants) with complementary functions to obtain the filling, known as bottom-up filling. Combinations of these additives are often required to obtain good quality cobalt agglomerates (mass), especially cobalt agglomerates without material voids. The inhibitor controls the deposition of cobalt at the cavity opening and on the planar surface of the substrate surrounding the cavity, either by adsorption to the cobalt surface or by complexation with cobalt ions. Thus, the compound may be a high molecular weight molecule, such as a polymer or cobalt ion complexing agent that is unable to diffuse within the cavity. The accelerator itself will diffuse to the bottom of the cavity, the presence of which is more necessary in very deep cavities. It is able to increase the deposition rate of cobalt at the bottom of the cavity and on its walls. The filling method using a bottom-up mechanism is in contrast to what is known as "conformal" filling methods, in which cobalt deposits grow at the same rate on the bottom and walls of the hollowed-out pattern.
These electrodeposition baths and their use have several drawbacks that ultimately limit the proper operation of the fabricated electronic devices and make them too costly to manufacture. In fact, they produce cobalt interconnects contaminated with organic additives, which in cobalt are necessary to limit the formation of filled holes. In addition, the filling rates obtained with these chemical methods are too slow to be compatible with industrial scale production.
Thus, there remains a need to provide an electrolytic bath that produces cobalt deposits with improved properties, i.e., that have extremely low impurity levels, that are formed at a rate sufficient to make device fabrication cost-effective, and that are free of material voids to ensure good electrical conductivity.
The inventors have found that the family of α -hydroxycarboxylic acids is capable of achieving this objective.
These additives are known in the "bottom-up" or "super-shape" filling methods. In these methods, the bath must contain several additives to accelerate deposition at the bottom of the cavity, slow deposition on the planar areas of the substrate and on the cavity walls. Such a system can prevent void formation in cobalt deposits within the cavity by closing the cavity opening in advance during filling.
However, accelerators also act on the flat surface of the substrate outside the cavity, so a leveler (leveler) derived from an alpha-hydroxy carboxylic acid is typically used to counter/reduce its effect, reduce excessive deposition of surface cobalt, and avoid excessively long subsequent polishing steps. Thus, when the trench and/or via is almost completely filled, the α -hydroxycarboxylic acid plays its role at the end of the electrodeposition process.
In conformal cobalt electrodeposition processes, the possibility of depositing cobalt at pH values below 4 using alpha-hydroxycarboxylic acids has never been suggested, which makes the results of the present invention even more surprising.
Disclosure of Invention
The invention thus relates to an electrolyte for electrodepositing cobalt in the form of an aqueous solution having a pH between 2.0 and 4.0 and comprising cobalt II ions, chloride ions and at most two non-polymeric organic additives, such as only one alpha-hydroxycarboxylic acid or two alpha-hydroxycarboxylic acids.
The term "polymer" refers to a compound that contains at least two repeating units in its chemical formula.
The invention also relates to a method for filling cavities with cobalt by means of a conformal deposition mechanism, which uses the above-described electrolyte.
The electrolyte and method of the invention enable to obtain continuous cobalt deposits of high purity in production times compatible with industrial applications, which can be shortened compared to the prior art. This is because the method for manufacturing cobalt conductive lines implements two different cobalt electrodeposition steps: a first electrodeposition step of filling the cavity with a first electrolyte containing cobalt ions, and a second electrodeposition step of depositing an "overcoating" layer on the entire surface of the substrate with a second electrolyte containing cobalt ions. Furthermore, the substrate must be rinsed and dried at the end of the first electrodeposition step before the second step is performed. The method of the invention advantageously enables filling of the cavity and deposition of the cover layer in a single electrodeposition step.
Furthermore, the cobalt deposit produced in the context of the present invention has the advantage of a very high purity.
Prior art methods for creating cobalt interconnects use alkaline electrolytes (e.g. pH above 9) by applying very low current densities and cobalt-specific inhibiting compounds to maintain pH above 4 within the trenches throughout the filling step, which results in a significant formation of cobalt hydroxide in the resulting cobalt deposit, which reduces the conductivity of the cobalt interconnect and reduces the performance of the integrated circuit.
The electrodeposition process that can be performed using the electrolyte of the present invention follows a conformal filling pattern and therefore does not require organic additives that are used in large amounts in the prior art bottom-up filling processes and can produce contamination.
Subsequently, operation in the pH range below 4.0 has the advantage of limiting cobalt hydroxide formation and can eliminate the use of buffer compounds such as boric acid, which are necessary to stabilize the pH during the polarization phase of the process using alkaline electrolytes, as described in the prior art. However, boric acid, which is typically used to perform this function, can decompose into boron derivatives that contaminate cobalt deposits. The contamination is more pronounced because the concentration of buffer compound in the electrolyte must be high to stabilize the pH of the electrolyte during electrodeposition.
Thus, the electrolyte and method of the present invention can greatly limit contamination of cobalt deposits by limiting the concentration of organic molecules (e.g., buffer species) and formation of cobalt hydroxide during electrodeposition.
The electrolyte of the present invention also has the advantage of creating cobalt lines or vias that do not contain voids.
Furthermore, the electrolyte and method of the present invention enable cobalt interconnects to be obtained with very low impurity levels, preferably below 1000ppm atomic, while being formed at a faster deposition rate.
Definition of the definition
The term "electrolyte" refers to a liquid containing a precursor of a metal coating used in an electrodeposition process.
The term "continuously packed" refers to a void-free cobalt mass. In the prior art, pores or voids ("sidewall voids") of material can be observed in the cobalt deposit between the pattern walls and the cobalt deposit. Voids equidistant from the pattern wall can also be observed in the form of holes or lines ("seams"). By making a cross section of the deposit, these voids can be observed and quantified by transmission electron microscopy or scanning electron microscopy. The continuous deposit of the invention preferably has an average void fraction of less than 10% by volume, preferably less than or equal to 5% by volume. The measurement of the void fraction inside the structure to be filled can be performed by an electron microscope with a magnification of between 50000 and 350000.
The term "average diameter" or "average width" of a cavity refers to the size measured at the opening of the cavity to be filled. For example, the cavity is in the form of a tapered channel or cylinder.
The term "conformal fill" refers to a filling mode in which cobalt deposits grow at the same rate on the bottom and walls of the hollowed-out pattern. This filling is in contrast to a faster bottom-up (referred to as "bottom-up") filling of cobalt at the bottom of the cavity.
The term "buffer substance" or "buffer compound" refers to a compound that is part of an electrolyte comprising cobalt ions and chloride ions and that has a pH in the range of 2.0-4.0. In the electrical step of the electrodeposition process, the compound is used in an amount sufficient to stabilize the pH of the electrolyte within + -0.3, preferably within + -0.2, after contact with the conductive surface of the substrate to be covered with cobalt metal. Thus, a compound may be a buffer substance at a given concentration in a given electrolyte, and no longer be a buffer substance in the same electrolyte if its concentration is insufficient to avoid pH changes of the electrolyte during the electrical step. The expression "substances in an amount sufficient to produce a buffering action" may also be used.
Drawings
Fig. 1 and 2 are transmission electron microscope images of cavities filled according to the method of the invention (examples 1 and 2).
Fig. 3 is a scanning electron microscope image of a cavity filled according to the electrodeposition method of the prior art (comparative example 3).
Detailed Description
According to a first embodiment, the invention relates to an electrolyte for the electrodeposition of cobalt, comprising cobalt II ions, chloride ions, an acid in an amount sufficient to obtain a pH between 1.8 and 4.0 (for example between 2.0 and 4.0), and at most two organic additives (preferably only one or at most two organic additives), at least one of which is selected from the group of α -hydroxycarboxylic acids, preferably free of sulphur.
In particular, the invention relates to an electrolyte for the electrodeposition of cobalt, characterized in that it is an aqueous solution comprising 1g/L-5g/L of cobalt II ions, 1g/L-10g/L of chloride ions, an amount of acid sufficient to obtain a pH between 1.8 and 4.0 (for example between 2.0 and 4.0), and at most two organic additives which are not polymers and preferably are free of sulphur, at least one or even two of the organic additives preferably being an α -hydroxycarboxylic acid.
The electrolyte preferably contains at most one organic additive, which may be a sulfur-free alpha-hydroxycarboxylic acid.
The molecular weight of the organic additive is preferably below 250g/mol, preferably below 200g/mol and above 50g/mol, more preferably above 100g/mol.
The concentration of the additive or the sum of the concentrations of the two additives is preferably between 5mg/L and 200 mg/L. In this embodiment, each of the additives may be a sulfur-free alpha-hydroxycarboxylic acid.
The mass concentration of cobalt II ions may range from 1g/L to 5g/L (e.g., 2g/L to 3 g/L). The mass concentration of chloride ions can range from 1g/L to 10g/L.
The relatively high cobalt ion concentration at strongly acidic pH has several advantages over prior art electrolyte baths having alkaline or weakly acidic pH, which are lower in cobalt ion concentration.
In fact, contrary to what is taught in the prior art, the inventors have found that it is not necessary to operate at a pH above 4 in order to limit corrosion of cobalt deposits. By increasing the concentration of cobalt ions and lowering the pH, it is possible, without being bound by any theory, to stabilize the deposit of cobalt metal by greatly increasing the concentration of cobalt ions present in the aqueous solution. Thus, the inventors observed a faster deposition rate than the prior art and observed larger sized cobalt grains (grains), typically greater than 100nm, in the deposit.
The chloride ion may be provided by dissolving cobalt chloride or a hydrate thereof (e.g., cobalt chloride hexahydrate) in water.
Since cobalt salts such as cobalt sulfate or hydrates thereof produce sulfur-containing contaminants of cobalt deposits (which are desirably avoided), the composition is preferably not obtained by dissolving cobalt salts such as cobalt sulfate or hydrates thereof.
The organic additive is preferably free of sulfur and is preferably selected from the group consisting of alpha-hydroxy carboxylic acids such as citric acid, tartaric acid, glycolic acid, lactic acid, malic acid, mandelic acid, maleic acid, oxalic acid and 2-hydroxybutyric acid compounds.
The additional organic compound may be of any nature, provided that it does not cause a bottom-up filling effect. The compounds may have various functions, such as the function of an accelerator, an inhibitor, a growth promoter or a leveler, but the electrolyte of the present invention advantageously does not contain these compounds. For example, the electrolyte of the present invention is free of inhibitor polymers, in particular polymers such as polyethylene glycol, polyvinylpyrrolidone or polyethyleneimine.
The hydroxylated carboxylate is for example tartrate and the electrolyte preferably comprises at most one organic additive.
In the electrolyte of the invention, it is preferred that the cobalt II ions are advantageously in free form, i.e. not complexed with organic additives, such as for example alpha-hydroxycarboxylic acids, glycine or ethylenediamine, before and during the polarization.
The large number of complexes without cobalt and organic molecules has many advantages: organic contamination of cobalt metal deposits can be reduced because the concentration of organic molecules in the bath can be very low; any uncontrolled changes in pH that may destabilize the solution can also be avoided throughout the deposition of cobalt in the structure. In addition, cobalt ions are not stabilized by the complex and are more easily reduced, so that the deposition rate of cobalt is faster. Finally, the very high concentration of cobalt ions protects the cavity conductive surfaces from corrosion. This effect is decisive when the substrate is covered with a cobalt layer (seed layer) of very small thickness.
When the organic additive or both organic additives are alpha-hydroxycarboxylic acids and the pH of the electrolyte is below 4.0, the additive does not complex with cobalt ions.
In the cavity to be filled in the context of the method of the invention, several hollows on the flat part and the flat part can be distinguished. One of the aims sought in the prior art is to slow down the deposition of cobalt on flat parts using inhibitors that adsorb specifically on the flat surface of the substrate without penetrating into the hollows of the pattern. In this specification, they are referred to as surface inhibitors.
The electrolyte of the invention advantageously comprises one of the following features, alone or in combination:
-no promoter of cobalt growth is included at the bottom of the pattern;
the electrolyte is free of organic inhibitor molecules capable of being adsorbed onto the flat portion of the substrate at the cavity opening by specificity during electrodeposition
Onto the cobalt at that location to slow down the growth of the cobalt;
-no polymer;
-free of sulphur-containing compounds;
free of buffer compounds useful in alkaline media, such as boric acid;
-a combination free of additives that cause a bottom-up filling mechanism, in particular a combination of inhibitors and accelerators, or a combination of inhibitors, accelerators and levelers.
Among the surface inhibitors, the following compounds may be mentioned: carboxymethyl cellulose, polyoxyethylene glycol ether, polyethylene glycol dimethyl ether, octanediol bis (polyalkylene glycol ether), octanol polyalkylene glycol ether, polyglycolic acid ester of oleic acid, poly (ethylene glycol-propylene glycol), polyethylene glycol, polyethylene imine, polyethylene glycol dimethyl ether, polyoxypropylene glycol, polypropylene glycol, polyvinyl alcohol, stearic acid polyglycolic acid ester, stearyl alcohol polyglycolic acid ether, butanol/ethylene oxide/propylene oxide copolymer, 2-mercapto-5-benzimidazole sulfonic acid, 2-mercaptobenzimidazole.
The accelerator is typically a compound containing a sulfur atom, such as (3-sulfopropyl) ester of N, N-dimethyldithiocarbamic acid, (3-sulfopropyl) ester of 3-mercaptopropane sulfonic acid, (3-sulfoethyl) ester of 3-mercapto-1-propane sulfonate, (3-sulfanyl-1-propane sulfonate), o-ethyl dithiocarbonate (dithiocarbonic acid o-ethyl esters-ester) with potassium salt of 3-mercapto-1-propane sulfonic acid, disulfopropyl disulfide, sodium salt of 3- (benzothiazolyl-s-thio) propyl sulfonic acid, sodium salt of pyridylpropyl sulfobetaine, sodium 3-mercapto-1-propane sulfonate, sodium (3-sulfoethyl) ester of N, N-dimethyl-dithiocarbamic acid, (3-sulfoethyl) ester of 3-mercaptoethyl propane sulfonic acid, sodium salt of 3-mercaptoethyl sulfonic acid, pyridylethylsulfobetaine or thiourea.
In the first embodiment, the pH of the electrolyte is preferably between 2.0 and 4.0. In a particular embodiment, the pH is between 2.0 and 3.5, or between 2.0 and 2.4, or between 2.5 and 3.5, or between 2.8 and 3.2.
The pH of the composition may optionally be adjusted using a base or acid known to those skilled in the art. The acid used may be hydrochloric acid.
In a preferred embodiment of the invention, the electrolyte contains less than 500ppm of buffer compounds (having at least 1 pKa) capable of preventing pH changes of the electrolyte during the polarizing step of the electrodeposition process. The concentration of buffer compound in the electrolyte is preferably below 400ppm, 300ppm or even 250ppm. In the electrical step, for example, for the separate addition of an acid (e.g., hydrochloric acid), the pH of the electrolyte may be readjusted if necessary.
However, in a preferred embodiment of the invention, the electrolyte does not contain a significant amount of buffer substance.
Although there is no fundamental limitation on the nature of the solvent (provided that it sufficiently dissolves the active substance in solution and does not interfere with electrodeposition), the solvent is preferably water. According to one embodiment, the solvent comprises mainly a volumetric dose of water.
According to a second embodiment, the invention relates to an electrolyte for the electrodeposition of cobalt, the pH of which is between 1.8 and 4.0 (for example between 2.0 and 4.0), and which comprises cobalt II ions, chloride ions and between 5 and 200mg/L of one or more compounds having at least 1 pKa (in the range 1.8 to 3.5, preferably in the range 2.0 to 3.5, more preferably in the range 2.2 to 3.0) in an aqueous solution.
This second embodiment may include the following features.
The molecular weight of the compound is preferably below 250g/mol, preferably below 200g/mol and above 50g/mol, preferably above 100g/mol.
In some cases, the compound having at least 1 pKa value (in the range of 2.0 to 3.5) may be the same as at least one of the organic additives used in the first embodiment. It may be chosen in particular from citric acid, tartaric acid, malic acid, maleic acid and mandelic acid.
It may also be selected from the compounds fumaric acid (pka=3.03), glyceric acid (pka=3.52), orotic acid (pka=2.83), malonic acid (pka=2.85), L-alanine (pka=2.34), phosphoric acid (pka=2.15), acetylsalicylic acid (pka=3.5) and salicylic acid (pka=2.98).
The prior art method of filling with cobalt uses alkaline electrolytes (e.g. pH above 9) while applying very low current densities and cobalt-specific inhibiting compounds to maintain pH above 4 in the trenches throughout the filling step, which results in a significant formation of cobalt hydroxide in the resulting cobalt deposit, which reduces the conductivity of the cobalt interconnect and reduces the performance of the integrated circuit.
The purpose of the electrolyte of the invention and the method of the invention is to solve this problem by significantly limiting the formation of cobalt hydroxide in such a way that it is present only in trace amounts in the deposited cobalt. Solutions to this problem include using an electrolyte having a pH between 1.8 and 4.0 (e.g., between 2.0 and 4.0) and adding to the electrolyte an additive, preferably having at least one or even all of the following characteristics such as:
a buffer capacity that enables the pH of the electrolyte to be maintained at a value higher than 1.8 or 2.0 and lower than 3.5 (preferably lower than 2.5) during the entire polarization of the substrate;
-a low molecular weight enabling diffusion of the additive in the structure with small opening diameter; and
very low concentrations in the electrolyte, so that the amount of additive present in the electrolyte before the onset of polarization is almost completely diffused into the cavities of the structure and the additive has a local buffer capacity.
The electrolyte containing such additives is able to limit in a selective way (for example only in the cavities of the structure, not on the flat surface of the substrate) the increase of the pH, said pH having a value lower than 4.0, preferably lower than 3.0 and more preferably comprised between 2.0 and 2.5. Thus, the additive may advantageously act as a buffer by acting locally (i.e. only in the cavity). Organic additives or compounds having at least 1 pKa (in the range of 1.8-3.5 or 2.0-3.5) may act as local buffers, the effect of which is only observed in the cavity.
This second embodiment may include other features, which may correspond to some of the features of the first embodiment of the invention described above.
The invention also relates to an electrochemical method for filling a cavity, the method comprising:
-a step of contacting the conductive surface of the cavity with one of the above electrolytes, and
-a step of polarizing the conductive surface for a time sufficient to completely fill the cavity by cobalt deposition to be obtained, such filling being preferably conformal.
The method preferably comprises the step of annealing the cobalt deposit obtained at the end of the polarisation step.
The invention also relates to an electrochemical method for depositing cobalt on a substrate comprising a conductive surface within a cavity hollowed out in the substrate and a conductive surface outside the cavity, the method comprising:
a step of contacting the conductive surface with an electrolyte in the form of an aqueous solution having a pH between 2.0 and 4.0 and comprising cobalt II ions, chloride ions and only one or at most two organic additives, which are not polymers,
a step of polarizing said conductive surface for a time sufficient to fill said cavity with cobalt and depositing a cobalt layer at least 50nm thick on the conductive surface outside the cavity,
-a step of annealing the cobalt obtained at the end of the polarization step.
The cobalt layer, also referred to as the capping layer, may have a thickness between 20nm and 300 nm. It advantageously has a constant thickness over the entire surface of the substrate outside the cavity. The layer was also uniform, glossy and dense (compact). The purity thereof is preferably less than 1000ppm atomic.
The method of the invention may be carried out using one of the above electrolytes comprising one or two non-polymeric organic additives according to the first embodiment, or comprising between 5mg/L and 200mg/L of one or more compounds having at least 1 pKa (in the range 1.8-3.5, preferably 2.0-3.5, and more preferably 2.2-3.0).
The electrolyte used in the context of the method of the invention may correspond to the electrolyte of the first embodiment described above or of the second embodiment described above.
The pH in the cavity is advantageously kept below 3.5 or even below 3.0 throughout the implementation of the filling step of the method of the invention, depending on the nature of the electrolyte used.
The cavity may be designed in the context of the implementation of a Damascene (Damascene) method or a Dual-Damascene (Dual-Damascene) method. In particular, the cavity may be obtained by performing the following steps:
a step of etching the structure on the silicon substrate,
a step of forming a silicon oxide layer on the silicon surface of the structure to obtain a silicon oxide surface,
-a step of depositing a metal layer on said silicon oxide layer to obtain a conductive surface of the cavity.
For example, the metal layer has a thickness between 1nm and 10 nm. It is preferably deposited on a silicon oxide layer in contact with silicon.
The method of the present invention is a conformal method as opposed to the prior art "bottom-up" or "super-conformal" methods. In the conformal filling method of the present invention, cobalt deposits grow at the same rate on the bottom and walls of the hollowed-out pattern to be filled. This filling is in contrast to other prior art methods in which cobalt is deposited at a faster rate at the bottom of the cavity than at the walls of the cavity.
The electrodeposition method of the present invention may use the bath described above as the subject of the first aspect of the present invention. All features described in relation to the first aspect of the invention are applicable to electrodeposition methods.
The total impurity content of the cobalt deposit obtained by the electrodeposition method of the present invention is less than 1000ppm atomic. The impurities are mainly oxygen, and secondarily carbon and nitrogen. The total content of carbon and nitrogen is less than 300ppm. The cobalt precipitate is advantageously continuous. It preferably has an average void fraction of less than 10% by volume or surface area, preferably less than or equal to 5% by volume or surface area. The porosity in the cobalt deposit can be measured by electron microscopy observations known to those skilled in the art who will choose the method they consider most suitable. One of these methods may be a Scanning Electron Microscope (SEM) or a Transmission Electron Microscope (TEM), which employs a magnification of between 50000 and 350000. Void volume can be assessed by measuring the void surface area observed over one or more cross-sections of the substrate containing the filled cavities. In the case of measuring multiple surface areas over multiple cross sections, the average of these surface areas will be calculated to assess void volume.
Typically the electrodeposition step is stopped when the cobalt deposit covers the planar surface of the substrate: in this case, the cobalt deposit comprises a cobalt deposit within the cavity and a cobalt layer covering the surface of the substrate in which the cavity is hollowed out. The thickness of the cobalt layer covering the surface may be between 50nm and 250nm and may for example be between 125nm and 175 nm.
Low impurity levels and extremely low void fractions enable cobalt deposits with lower resistivity to be obtained.
The cobalt deposition rate is between 0.1nm/s and 3.0nm/s, preferably between 1.0nm/s and 3.0nm/s, and more preferably between 1nm/s and 2.5 nm/s.
The through-vias and interconnects may be fabricated according to a damascene or dual damascene method known to those skilled in the art, which includes a series of steps including: -etching a pattern in or through a silicon wafer on a spindle perpendicular to the wafer surface to obtain a pattern with a vertical profile forming a hollowed-out; -depositing an insulating dielectric layer, typically consisting of silicon oxide; -depositing a layer of material for preventing cobalt migration in silicon; -optionally depositing a thin metal layer (called seed layer); -filling the pattern by electrodepositing cobalt; and-removing excess cobalt by polishing.
Conductive cobalt lines may be formed on the front (FEOL lines) or back (BEOL lines) of the semiconductor device metallization structure.
A silicon substrate may be used which has been etched according to a desired pattern and then covered with a silicon oxide layer, followed by a metal layer (which may be a seed layer of metal), a barrier layer to cobalt diffusion, a liner or a combination of at least two of the foregoing. The metal layer may have a thickness between 1nm and 10nm (e.g., between 2nm and 5 nm) and may comprise, for example, a single layer or multiple superimposed layers of various materials.
The conductive surface of the pattern is a surface of a metal layer including at least one compound selected from the group consisting of cobalt, copper, tungsten, titanium, tantalum, ruthenium, nickel, titanium nitride, and tantalum nitride. In a particular embodiment, the conductive surface is a cobalt surface.
Thus, the metal layer may comprise a tantalum nitride TaN layer having a thickness between 1nm and 6nm, itself covered with and in contact with a cobalt metal layer having a thickness between 1nm and 10nm (preferably between 2nm and 5 nm), on which cobalt is deposited during the electrical step.
The resistivity of the component comprising the metal layer and the cobalt deposit may range from 7ohm/cm to 10ohm/cm. The resistivity is preferably between 7.5ohm/cm and 8.5 ohm/cm.
According to the method of the invention, the width or diameter of the cavity intended to be filled with cobalt at its opening is preferably between 10nm and 200nm or between 10nm and 100nm, preferably between 10nm and 40nm, and more preferably between 15nm and 30nm, said opening being at the level of the surface of the substrate where the cavity is created. The depth of the cavity may range from 50nm to 250nm. According to one embodiment, the cavity has a width of between 10nm and 30nm and a depth of between 125nm and 175 nm.
It has been demonstrated that the electrolyte of the present invention is capable of filling very thin trenches or small vias with widths or diameters below 40nm at the openings at speeds higher than the speeds of the prior art methods and consistent with conformal filling. The electrolyte is also capable of obtaining continuous cobalt agglomerates without material voids and with very low levels of impurities (e.g. cobalt hydroxide).
The polarization intensity used in the electrical step is preferably in the range of 2mA/cm 2 -20mA/cm 2 However, in the prior art method using an alkaline electrolyte, the range is usually 0.2mA/cm 2 -1mA/cm 2 。
The electrical steps of the method of the invention may comprise only one or more polarization steps, wherein the person skilled in the art will be able to select the variables according to his general knowledge.
The electrical step may be performed using at least one polarization mode selected from the group consisting of a ramp mode (ramp mode), a constant current mode (galvanostatic mode), and a current pulse mode (galvano-pulsed mode).
Thus, the electrical steps may comprise at least one electrodeposition step in a current pulse mode and at least one electrodeposition step in a constant current mode, the electrodeposition step in the constant current mode preferably being subsequent to the electrodeposition step in the current pulse mode.
For example, the electrical step comprises a first step of polarizing the cathode in a current pulse mode in a current range of 3mA/cm 2 -20mA/cm 2 (e.g. 12 mA/cm) 2 -16mA/cm 2 Time (T) on ) Preferably between 5ms and 50 ms) and zero polarization (time (T off ) Preferably between 50ms and 150 ms).
In the first step, the substrate may be contacted with the electrolyte before or after polarization. Preferably in contact with the cavity prior to application of the voltage to limit corrosion of the metal layer in contact with the electrolyte.
In the second step, the cathode may be polarized in a constant current mode having a current range of 3mA/cm 2 -20mA/cm 2 . The two steps preferably have substantially equal durations.
The second step in constant current mode may itself comprise the following two steps: a first step in which the intensity of the current is in the range of 3mA/cm 2 -8mA/cm 2 The method comprises the steps of carrying out a first treatment on the surface of the And a second step in which the applied intensity is in the range of 9mA/cm 2 -20mA/cm 2 Is set in the above-described range).
This electrical step may be particularly used when the pH of the electrolyte is between 2.5 and 3.5.
According to another example, the electrical step comprises a first step of polarizing the cathode in a ramp mode, the current of which is preferably 0mA/cm 2 -15mA/cm 2 (preferably 0 mA/cm) 2 -10mA/cm 2 ) Then a step in a constant current mode, the constant current mode being applied in a range of 10mA/cm 2 -20mA/cm 2 (preferred range is 8 mA/cm) 2 -12mA/cm 2 ) Is set in the above-described range). This electrical step may be particularly used when the pH of the electrolyte is between 2.0 and 2.5.
Preferably, the method of the present invention comprises the step of annealing the cobalt deposit obtained at the end of the filling described above.
The annealing heat treatment may be carried out at a temperature between 350 ℃ and 550 ℃ (e.g., about 450 ℃), preferably under a reducing gas (e.g., N 2 H of 4% 2 ) The following is performed.
The method may include a pretreatment step by reducing the plasma to reduce native metal oxide present on the substrate surface. The plasma also acts on the surface of the trench, thereby making it possible to improve the quality of the interface between the seed layer and the electrodeposited cobalt. The subsequent electrodeposition step is preferably performed immediately after the plasma treatment to minimize the reformation of native oxide.
The present application also describes an electrochemical method for conformal filling of a cavity, the method comprising:
-a step of contacting the conductive surface of the cavity with an aqueous solution having a pH between 2.0 and 4.0 and comprising cobalt II ions, chloride ions and at most two organic additives, including an alpha-hydroxycarboxylic acid, and
-a step of polarizing said conductive surface so as to fill the cavity with cobalt according to a conformal filling pattern.
The alpha-hydroxycarboxylic acid preferably diffuses in the cavity due to its small volume.
The method may satisfy one or more of the features previously described with respect to the electrochemical method of the present invention.
The method of the present invention may be particularly useful in the fabrication of semiconductor devices during the creation of conductive metal interconnects (e.g., trenches extending along a surface and vias connecting various levels of integration).
Examples
The invention is further illustrated by the following exemplary embodiments.
Example 1: electrodeposition of 26nm wide, 150nm deep structures with organic additives at ph=3.0
The 26nm wide, 150nm deep trenches were filled with cobalt by electrodeposition over a cobalt seed layer. The deposition was carried out at pH3.0 using a composition containing cobalt chloride and an alpha-hydroxycarboxylic acid. Finally, the substrate is heat treated to improve the quality of the deposited metal.
A. Materials and equipment:
a substrate:
the substrate used in this example consisted of a 4cm silicon coupon (silicon coupon). The silicon is covered with silicon oxide, which is in contact with a 4nm thick TaN layer, which itself is covered with and in contact with a 3nm thick cobalt metal layer. Thus, the trench to be filled has a width of 26nm and a depth of 150 nm. The resistivity of the substrate (the cavity of which is filled with cobalt) was measured to be about 300 ohm/square.
Electrodeposition solution:
in the solution, co 2+ The concentration is equal to 2.47g/L, obtained from CoCl 2 (H 2 O) 6 . The concentration of tartaric acid is between 5ppm and 200ppm, for example equal to 15ppm. The pH of the solution was adjusted to ph=3.0 by means of the addition of hydrochloric acid.
The device comprises:
in this example, an electrowinning apparatus consisting of two parts was used: a tank intended to contain an electrodeposition solution, equipped with a fluid recirculation system to control the fluid dynamics of the system; and a rotating electrode equipped with a sample holder adapted to the dimensions of the sample embryo used (4 cm. Times.4 cm). The electrowinning cell comprises two electrodes:
-a cobalt anode;
-a structured silicon sample blank coated with the above layer, which constitutes the cathode;
-a reference connected to the anode.
The connector enables electrical contact of the electrodes, which are connected by wires to a potentiostat supplying up to 20V or 2A.
B. Experimental protocol:
the preparation steps are as follows:
by H 2 Plasma (0.5 mbar,70W,5 min) vs. silicon sampleThe product is subjected to a short treatment to reduce native cobalt oxide on the substrate.
The electrical method comprises the following steps:
the method comprises three steps:
a) In the first step, the cathode was polarized in a current pulse mode with a current range of 30mA (or 3.8mA/cm 2 ) 150mA (or 19 mA/cm) 2 ) For example 110mA (or 14mA/cm 2 ) The pulse time for cathodic polarization is between 5ms and 50ms, and the pulse time for zero polarization between two cathodic pulses is between 50ms and 150 ms. This step was carried out with a 50rpm cycle for 50 seconds. The electrolyte is brought into contact with the substrate before the voltage is applied.
b) In the second step, the cathode was polarized in constant current mode with a current range of 30mA (or 3.8mA/cm 2 ) 60mA (or 7.6 mA/cm) 2 ) For example, the current is 55mA (or 7mA/cm 2 ). This step was carried out with a cycle of 100rpm for 20 seconds.
c) In the final step, the cathode was polarized in constant current mode with a current range of 80mA (or 10mA/cm 2 ) 150mA (or 19 mA/cm) 2 ) For example 110mA (or 13.75mA/cm 2 ). This step was carried out with a cycle of 100rpm for 30 seconds.
After these three successive electro-grafting steps, a complete filling of the structure with high aspect ratio is obtained within 100s, on which a thick layer of cobalt is deposited.
And (3) annealing:
annealing in a reducing gas (N) 2 H of 4% 2 Referred to as forming gas) was carried out at 500 c for 10 minutes.
C. The results obtained:
analysis by transmission electron microscopy (mag=320 k, eht=100 kV) after annealing showed that the holes on the trench walls ("sidewall voids") were filled without defects, reflecting good nucleation of cobalt, and that there were no holes in the structure ("seam voids"), reflecting that annealing eliminated this type of defect (see fig. 1). The resistivity of the thick cobalt layer of 150nm on the structure was 8.0ohm cm.
The total impurity content of the obtained cobalt deposit was equal to 790ppm atomic. The total content of carbon and nitrogen is equal to 200ppm.
The cobalt deposition rate was equal to 1.5nm/s.
Example 2: electrodeposition of 16nm wide, 150nm deep structures with organic additives at ph=3.0
By electrodeposition on a cobalt layer, trenches (16 nm wide, 150nm deep) that are more challenging than example 1 are filled with cobalt. The deposition was still carried out at ph3.0 using a composition containing cobalt chloride and tartaric acid. The substrate is still heat treated in order to improve the quality of the deposited metal.
A. Materials and equipment:
a substrate:
the substrate used in this example was obtained from a 4cm silicon coupon embryo. The silicon is covered with silicon oxide, which is in contact with a 4nm thick TaN layer, which itself is covered with and in contact with a 3nm thick cobalt metal layer. Thus, the trench to be filled has a width of 16nm and a depth of 150 nm. The resistivity of the substrate was measured to be about 170 ohm/square.
Electrodeposition solution:
the solution was the same as that of example 1.
The device comprises:
the apparatus is the same as that of example 1.
B. Experimental protocol:
the preparation steps are as follows:
the plasma treatment was the same as that of example 1.
The electrical method comprises the following steps:
the process is as described in example 1; in this case too, the process is carried out in three steps and is identical.
And (3) annealing:
this annealing step is exactly the same as the annealing step of example 1.
C. The results obtained:
analysis by transmission electron microscopy (mag=320 k, eht=100 kV) after annealing showed that the holes on the trench walls ("sidewall voids") were filled without defects, reflecting good nucleation of cobalt, and that there were no holes in the structure ("seam voids"), reflecting that annealing eliminated this type of defect (see fig. 2). The resistivity of the thick cobalt layer of 150nm on the structure was 8.5ohm cm.
When the same process is carried out at a pH equal to 3.0, the total impurity content of the cobalt deposit obtained is equal to 790ppm atomic. The total content of carbon and nitrogen is equal to 200ppm.
The cobalt deposition rate was equal to 1.5nm/s.
Comparative example 3: electrodeposition of 26nm wide, 150nm deep structures with organic additives at ph=5.3
The 26nm wide, 150nm deep trenches were filled with cobalt by electrodeposition over a cobalt seed layer. The deposition was performed at ph5.3 using a composition containing cobalt dichloride hydrate and tartaric acid. Finally, the substrate is heat treated to improve the quality of the deposited metal.
A. Materials and equipment:
a substrate:
the substrate used was exactly the same as that of example 1.
Electrodeposition solution:
in the solution, co 2+ The concentration is equal to 2.47g/L, obtained from CoCl 2 (H 2 O) 6 . The concentration of tartaric acid is between 5ppm and 200ppm, for example equal to 15ppm. The pH of the solution was adjusted to ph=5 by means of the addition of hydrochloric acid.
The device comprises:
the apparatus is the same as that of example 1.
B. Experimental protocol:
the electrical method comprises the following steps:
in this case too, the process is carried out in three steps, as described in example 1, and is identical.
And (3) annealing:
the annealing step was exactly the same as that of example 1.
C. Results and discussion:
analysis of the substrate by scanning microscopy (mag=100k, wd=2.2 nm, signal a=esb, eht=2.0 kV) showed poor filling quality, many "joint void" type hole defects were present, visible in black in fig. 3.
These results emphasize the need to use a pH below 4 in order to obtain good electrodeposition with the solution used.
The resistivity of the 150nm thick cobalt layer on the substrate surface was 9.0ohm cm.
Example 4: electrodeposition of 16nm wide, 150nm deep structures with organic additives at ph=2.2
The 16nm wide, 150nm deep trenches were filled with cobalt by electrodeposition over a cobalt seed layer. This example differs from example 2 due to the conditions of the electrical step and its pH.
A. Materials and equipment:
a substrate:
the substrate is the same as that of example 2.
Electrodeposition solution:
the solution was the same as that of example 1, but the pH was adjusted to 2.2.
The device comprises:
the apparatus is the same as that of example 1.
B. Experimental protocol:
the preparation steps are as follows:
the plasma treatment was the same as that of example 1.
The electrical method comprises the following steps:
the method comprises two steps:
a) In a first step, the cathode is polarized in a current dynamic ramp mode (galvanodynamic ramp mode) in which the current is varied proportionally over time from a value greater than or equal to 0mA to a maximum of 110mA (or 13.75 mA/cm) 2 ). In the present embodiment, the current was changed from 0mA to 80mA (or 10 mA/cm) at a rate of 1.33mA/sec 2 ). This step was carried out with a 50rpm cycle for 60 seconds.
b) In the second step, the cathode was polarized in constant current mode with a current range of 80mA (or 10mA/cm 2 ) 150mA (or 19 mA/cm) 2 ) For example 80mA (or 10mA/cm 2 ). This step was performed with a 50rpm cycle for 40 seconds.
After these two successive steps, a complete filling of the structure with high aspect ratio is obtained within 100s, and a cobalt layer is also formed on the planar surface of the substrate.
And (3) annealing:
annealing in a reducing gas (N by a rapid thermal annealing method 2 H of 4% 2 Referred to as forming gas) was carried out at 450 c for 5 minutes.
C. The results obtained:
TEM analysis performed after annealing showed that the holes on the trench walls ("sidewall voids") were filled without defects, reflecting good nucleation of cobalt, and that there were no holes in the structure ("seam voids"), reflecting that annealing eliminated such defects. The resistivity of the thick cobalt layer of 150nm on the structure was 7.5ohm cm.
The cobalt deposition rate was equal to 1.9nm/s.
Comparative example 5: electrodeposition of 22nm wide, 75nm deep trenches with organic additives at ph=8.0
22nm wide, 75nm deep trenches having a free surface of copper obtained by depositing a copper layer on a substrate are filled with cobalt. The trenches were filled with a composition based on triethylenetetramine, which was present in a 1:1 ratio to cobalt stoichiometry.
A. Materials and equipment:
a substrate:
the substrate used in this example consisted of a 4cm long, 4cm wide silicon coupon blank covered with a structured silicon oxide layer having 22nm wide, 75nm deep trenches. The silicon oxide is covered by and in contact with a 2nm thick cobalt layer, which itself is covered by and in contact with a copper layer having a thickness below 4 nm. The resistivity of the copper layer was 250 ohm/square.
Electrodeposition solution:
in this solution, the concentration of triethylene tetramine present in stoichiometric amounts with cobalt was 1.32g/L (from a 60% commercial solution). CoSO 4 (H 2 O) 6 Is equal to 1.5g/L (i.e. 0.31g/L Co) 2+ Concentration).
Thioglycolic acid was also added at a concentration equal to 10 ppm. The pH of the solution was equal to 8.0.
The device comprises:
in this example, an electrowinning apparatus consisting of two parts was used: a tank intended to contain an electrodeposition solution, equipped with a fluid recirculation system to control the fluid dynamics of the system; and a rotating electrode equipped with a sample holder adapted to the dimensions of the sample embryo used (4 cm. Times.4 cm). The electrowinning cell comprises two electrodes:
inert carbon graphite anode (anode),
a structured silicon sample blank coated with a copper layer, which constitutes the cathode,
-a reference connected to the anode.
The connector enables electrical contact of the electrodes, which are connected by wires to a potentiostat supplying up to 20V or 2A.
B. Experimental protocol:
the cathode was polarized in a current pulse mode with a current range of 3mA (or 0.38mA/cm 2 ) 35mA (or 4.38 mA/cm) 2 ) For example 9mA (or 1.14mA/cm 2 ) The pulse frequency in cathodic polarization is between 1kHz and 10kHz, and the pulse frequency in zero polarization between two cathodic pulses is between 0.5kHz and 5kHz. The circulation of the cathode was set to 6rpm.
The duration of the electrodeposition step was 14 minutes to obtain a complete filling of 25nm wide, 75nm deep trenches.
C. The results obtained:
TEM analysis shows that the filling obtained with metallic cobalt by bottom-up mechanism is of good quality.
However, the compound deposited on the surface is entirely composed of cobalt oxide, and cobalt is desired.
In addition, the filling time is longer when cobalt is complexed in alkaline medium. Within 12min, a 20nm thick cobalt oxide layer formed on the substrate surface was obtained.
These results emphasize the need to work at acidic pH in order to obtain metallic cobalt at sufficient speed.

Claims (16)

1. Electrolyte for the electrodeposition of cobalt, characterized in that it is an aqueous solution consisting of 1g/L to 5g/L of cobalt II ions, 1g/L to 10g/L of chloride ions, an acid in an amount sufficient to obtain a pH between 1.8 and 4.0, and an organic additive, which is not a polymer, and which is selected from organic compounds having at least 1 pKa in the range of 1.8 to 3.5.
2. The electrolyte of claim 1 wherein the molecular weight of the organic additive is less than 250g/mol and greater than 50g/mol.
3. The electrolyte of claim 1 or 2, wherein the organic additive having at least 1 pKa in the range of 1.8-3.5 is selected from the group consisting of citric acid, tartaric acid, malic acid, mandelic acid, maleic acid, fumaric acid, glyceric acid, orotic acid, malonic acid, L-alanine, acetylsalicylic acid, and salicylic acid.
4. The electrolyte of claim 1 or 2 wherein the cobalt II ions are present in free form, i.e. not complexed with the organic additive.
5. The electrolyte of claim 1 or 2, wherein the pH of the electrolyte is between 2.0 and 3.5.
6. The electrolyte of claim 1 or 2, wherein the acid is hydrochloric acid.
7. The electrolyte of claim 1 wherein the concentration of the organic additive is between 5mg/L and 200 mg/L.
8. The electrolyte of claim 1 or 2 wherein the organic additive is selected from the group consisting of alpha-hydroxycarboxylic acids.
9. An electrochemical method for filling a cavity, the method comprising:
the step of contacting the conductive surface of the cavity with an electrolyte as claimed in any one of claims 1 to 8,
-a step of polarizing said conductive surface for a time sufficient to achieve a conformal and complete filling of said cavity by deposition of cobalt, and
-a step of annealing the cobalt deposit obtained at the end of the polarization step.
10. The method according to claim 9, wherein the cavity is obtained by performing the steps of:
a step of etching the structure on the silicon substrate,
a step of forming a silicon oxide layer on the silicon surface of the structure to obtain a silicon oxide surface,
-a step of depositing a metal layer on the silicon oxide surface to obtain a conductive surface of the cavity.
11. The method of claim 10, wherein the metal layer comprises at least one compound selected from the group consisting of cobalt, copper, tungsten, titanium, tantalum, ruthenium, nickel, titanium nitride, and tantalum nitride.
12. The method of claim 11, wherein the metal layer further comprises titanium nitride or tantalum nitride.
13. The method of any one of claims 10-12, wherein the cavity has a width or diameter at the opening of between 10nm and 40nm and a depth of between 50nm and 250 nm.
14. The method of any one of claims 10-12, wherein the cobalt deposit has an impurity content of less than 1000ppm atomic and has an average void fraction of less than 10% by volume or surface area as measured by electron microscopy.
15. The method of any one of claims 10-12, wherein the cobalt deposition rate is between 0.1nm/s and 3.0 nm/s.
16. An electrochemical process for depositing cobalt on a substrate, the substrate comprising a conductive surface within a cavity hollowed out in the substrate and a conductive surface outside the cavity, the process comprising:
the step of contacting the electrically conductive surface of the cavity with an electrolyte as claimed in any one of claims 1 to 8,
a step of polarizing said conductive surface for a time sufficient to effect filling of said cavity with cobalt and depositing a cobalt layer of at least 50nm thickness on the conductive surface outside said cavity to effect conformal and complete filling of said cavity by deposition of cobalt,
-a step of annealing the cobalt deposit obtained at the end of the polarization step.
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