Electrodeposition of copper films onto conductive substrates

Abstract

The copper electrolyte comprises (a) a water-soluble copper salt, (b) a complexing agent, (c) a leveler, (d) an optional polar solvent, (e) an optional pH adjuster, and (f) balance water. The copper electrolyte has a pH ranging from about 8 to about 11, preferably from about 8.2 to about 9.5. The copper electrolyte can be used to deposit ultra-thin copper layers on conductive substrates that are free of pinholes and other defects and inhibit metal dissolution.

Description

[Technical Field]

[0001] The present invention generally relates to copper electrolytes and methods for electrodepositing copper films onto conductive substrates. [Background technology]

[0002] One sequence commonly used to fabricate copper interconnects is called the "dual damascene" process. To form electrical interconnects, a dielectric layer is patterned to form features in which the metal interconnects will be formed. Generally, the features used to form the metal interconnects are recesses of any shape formed in a substrate or a layer deposited on the substrate.

[0003] Because copper is difficult to etch and has a high diffusivity in many materials, interconnects are generally fabricated by a series of steps including the following: a. depositing an insulating dielectric layer; b. etching interconnect features into the dielectric layer; c. depositing a barrier layer (generally made of, for example, tantalum, titanium nitride, tantalum nitride, tungsten nitride, or tungsten carbide) used to prevent copper migration; d. filling the lines and interconnect holes with copper; and e. Removing excess copper by chemical mechanical polishing.

[0004] This sequence of steps is known as the "Damascene process" and is described, for example, by Y. Chang and SMSe, "ULSI Technology", McGraw-Hill, New York, (1996), pp. 444-445.

[0005] Electronic connections between electronic devices within integrated circuit (IC) chips are currently typically made using copper metal or copper metal alloys. Devices within an IC chip can be distributed across the entire surface of the IC chip, or they can be stacked in multiple layers on the IC chip. Electrical interconnections between electronic devices are constructed using features that may include one or more vias and / or trenches filled with conductive material. Layer(s) of insulating material, often a low-k dielectric material, separate the various components and devices within the IC chip. The substrate on which devices of an IC circuit chip are constructed can be, for example, a silicon wafer or a silicon-on-insulator substrate. Other substrates include, but are not limited to, germanium, indium antimonide, lead telluride, indium arsenide, indium phosphide, gallium arsenide, gallium antimonide, and / or other III-V materials, alone or in combination with silicon, silicon dioxide, or other insulating materials.

[0006] Nanoscale features are etched into a dielectric layer (typically SiO2), after which diffusion barrier layer(s) are deposited. A thin copper seed layer is applied, such as by sputtering, followed by copper electroplating to form the intricate nanopattern. The multilayer interconnect network may include several levels (e.g., 7-10 interconnect levels). Interconnects in the back-end-of-line (BEOL) of a chip typically distribute clock and other signals, provide power and ground to various electronic system components, and interconnect transistors in the front-end-of-line (FEOL) of the chip.

[0007] Barrier liner layers are used between metal interconnects and dielectric materials to prevent diffusion and electromigration of the metal (i.e., copper) into surrounding materials. Device failure can occur, for example, in situations where copper metal is in contact with the dielectric material because the copper metal can ionize and penetrate the dielectric material. Barrier layers placed between the dielectric material, silicon, and / or other material(s) and the copper interconnect can also function to promote adhesion of the copper to the other materials.

[0008] As the trend toward miniaturization in integrated circuit manufacturing continues, alternative liner materials for use in conjunction with copper interconnects have been explored, typically including conductive materials such as cobalt, tungsten, ruthenium, molybdenum, combinations of the foregoing, and alloys of the foregoing. Cobalt has been widely studied and used due to its low resistivity and integration compatibility at the nanoscale, as well as its advantages in improving copper electromigration resistance. However, direct electrodeposition of copper onto cobalt is problematic, in part due to cobalt's stability issues in acid copper damascene processes. As the width and aspect ratio of electrofilled features become more challenging, the subsequent processes of copper seeding and copper electroplating face increasing technical difficulties.

[0009] In BEOL copper damascene, cobalt has been used as a liner material to provide good copper seed coverage on the sidewall and longer electromigration lifetime of the copper interconnects. Attempts have been made to fill copper directly onto the cobalt liner without using a copper seed layer, but this approach may result in high defectivity in the copper fill at advanced nodes.

[0010] U.S. Patent Nos. 10,648,097 and 11,168,407 (both to Velmurugan et al.), the subject matter of each of which is incorporated herein by reference in its entirety, describe an electroplating system for depositing copper on a cobalt layer.

[0011] WO 2021 / 207254 to LAM Research Corporation describes combinations of suppressors and accelerators selected from the group consisting of (i) polyallylamine (suppressor) and thiourea (accelerator), (ii) polyallylamine (suppressor) and ammonium thiocyanate (accelerator), and (iii) saccharin (suppressor) and thiourea (accelerator), which are directed to superconformal filling of vias and trenches (i.e., via fill or gap fill).

[0012] U.S. Patent No. 10,883,185 to Religieux et al. describes an electrolyte composition for depositing copper onto a metal seed layer, which may contain at least two materials selected from the group consisting of copper, cobalt, or ruthenium, by combining two separate copper-complexing electrolytes. The present invention also describes a method for filling cavities with copper, and the semiconductor device obtained by this method.

[0013] U.S. Pat. No. 10,472,726 to Mevellec et al., the subject matter of which is incorporated herein by reference in its entirety, describes alkaline electroplating of copper in vias and trenches, where the substrate is first lined with ruthenium or sputtered copper on TiN / Ti. Summary of the Invention

[0014] It is an object of the present invention to provide a copper electrolyte for use in copper damascene processes.

[0015] Another object of the present invention is to provide a copper electrolyte for producing thin copper films.

[0016] It is yet another object of the present invention to provide a copper electrolyte for producing ultra-thin copper films.

[0017] It is yet another object of the present invention to provide a copper electrolyte for producing ultra-thin copper films on conductive substrates.

[0018] It is yet another object of the present invention to provide a copper electrolyte configured to produce ultra-thin copper films by electrodeposition on conductive substrates such as cobalt, tungsten, ruthenium, molybdenum, combinations thereof, and alloys thereof.

[0019] It is yet another object of the present invention to provide a method for electrochemically depositing continuous, pinhole-free, ultra-thin copper, preferably films thinner than about 2 nm, onto a conductive liner.

[0020] It is yet another object of the present invention to provide improved liner materials for use in copper damascene processes, including BEOL copper damascene processes.

[0021] To that end, in one embodiment, the present invention generally relates to a copper electrolyte, the copper electrodes comprising: a. a water-soluble copper salt; b. a complexing agent; c. A leveller; d. optionally a polar solvent; e. optionally, a pH adjuster; f. residual water, The copper electrolyte has a pH in the range of about 8 to about 11, preferably about 8.2 to about 9.5.

[0022] In another embodiment, the present invention generally relates to a method for depositing a copper film on a conductive substrate, the method comprising: a) optionally pretreating the conductive substrate, the pretreatment removing surface oxides; b) immersing the conductive substrate in a copper electrolyte, the copper electrolyte comprising: a. a water-soluble copper salt; b. a complexing agent; c. A leveller; d. optionally a polar solvent; e. optionally, a pH adjuster; f. residual water, the copper electrolyte has a pH in the range of about 8 to about 11, preferably about 8.2 to about 9.5; c) electrodepositing a copper film onto a conductive substrate using the copper electrolyte described herein, wherein the copper film is an ultra-thin copper film having a thickness of less than about 5 nm, wherein the ultra-thin copper film is a continuous copper film free of pinholes or other defects. [Brief explanation of the drawings]

[0023] Features and aspects of the embodiments are described below with reference to the accompanying drawings, in which elements are not necessarily drawn to scale and in certain views parts may be exaggerated or removed for clarity.

[0024] Exemplary embodiments of the present disclosure are further described with reference to the accompanying drawings, in which it is noted that the various features, steps, and feature / step combinations described below and illustrated in the figures may be arranged and organized differently to result in embodiments that are still within the scope of the present disclosure.

[0025] The invention will now be described with reference to the following drawings: [Figure 1(a)-1(c)] 1 shows the surface morphology of different amounts of copper deposited on a cobalt substrate. [Figure 2] 1 shows the dependence of substrate sheet resistance on the nominal thickness of the copper film. [Figure 3(a)-3(c)] Auger surface mapping of a 1.4 nm thick copper film on a cobalt substrate is shown. [Figure 4(a)-4(c)] 1 shows the surface morphology of copper electrodeposition on tungsten substrate at different process stages or conditions. DETAILED DESCRIPTION OF THE INVENTION

[0026] It should be understood that the disclosed embodiments are merely illustrative of the present disclosure, which may be embodied in various forms. Accordingly, the details disclosed herein with reference to exemplary assembly / manufacturing methods and associated processes / techniques of assembly and use should not be construed as limiting, but merely as a basis for teaching those skilled in the art how to make and use the advantageous assemblies / systems described herein.

[0027] As used herein, "a," "an," and "the" refer to both the singular and the plural, unless the context clearly dictates otherwise.

[0028] As used herein, the term "about" refers to a measurable value of a parameter, amount, duration, etc., and is meant to include a variation of no more than + / -15%, preferably no more than + / -10%, more preferably no more than + / -5%, even more preferably no more than + / -1%, and still more preferably no more than + / -0.1% of the specifically recited value, to the extent that such variations are appropriate for practicing the invention described herein. It is also to be understood that the value to which the modifier "about" refers is itself specifically disclosed herein.

[0029] As used herein, spatially relative terms such as "beneath," "below," "lower," "above," "upper," "front," "back," etc. are used for ease of description to describe the relationship of one element(s) or feature(s) to another element or feature. It will be further understood that the terms "front" and "back" are not intended to be limiting and are intended to be interchangeable where appropriate.

[0030] As used herein, the terms "comprise(s)" and / or "comprising" specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0031] As used herein, "substantially-free" or "essentially-free," with respect to a particular element or compound, unless otherwise defined above, means that the given element or compound is not detectable by conventional analytical means well known to those skilled in the art of metal plating for bath analysis. Such methods typically include atomic absorption spectroscopy, titration, UV-Vis spectroscopy, secondary ion mass spectroscopy, and other commonly available analytical methods.

[0032] As used herein, the term "sheet resistance" refers to a quantity used to measure the electrical resistance of a thin film or layer. It is expressed in ohms / square and is equivalent to the resistivity of a two-dimensional system, i.e., a system in which the current flows in the plane of the layer and not in a plane perpendicular to this layer. Mathematically, the value of sheet resistance is obtained by dividing the resistivity of the constituent materials of the layer (expressed in ohm.m or microohm.cm) by the thickness of this layer (expressed in m or nm).

[0033] In one embodiment, the present invention generally relates to a copper electrolyte, the copper electrolyte comprising: (2) a water-soluble copper salt; (3) a complexing agent; (4) A leveler; (5) optionally a polar solvent; (6) optionally, a pH adjuster; (7) residual water, The copper electrolyte has a pH in the range of about 8 to about 11, preferably about 8.2 to about 9.5.

[0034] In another embodiment, the present invention generally relates to a method for producing a pharmaceutical composition comprising: (1) a water-soluble copper salt; (2) a complexing agent; (3) A leveler, (4) 0 to 300 g / L of a polar solvent; (5) optionally, a pH adjuster; (6) residual water, for a copper electrolyte consisting essentially of: The copper electrolyte has a pH in the range of about 8 to about 11, preferably about 8.2 to about 9.5.

[0035] By "consisting essentially of," it is meant that the composition does not contain components that would render it unable to form an ultra-thin copper layer on a cobalt substrate that is free of pinholes and other defects. That is, the composition is preferably at least substantially free of or free of oxidizers (i.e., nitrates, including cupric nitrate) and catalyst poisons (i.e., Hg, Pb, As, Bi, Cr, Se, and Sb).

[0036] In one embodiment, the water-soluble copper salt is a water-soluble copper(II) salt selected from the group consisting of copper(II) sulfate, copper(II) chloride, and copper(II) acetate. In one embodiment, the copper salt comprises copper(II) sulfate. The copper(II) salt is preferably contained in the electrolyte at a concentration ranging from about 2 to about 100 g / L. The inventors of the present invention have found that in compositions that do not contain a polar solvent, a higher concentration may be required to provide good results. That is, in compositions that do not contain a polar solvent, the copper(II) salt concentration may be in the range of about 40 to about 80 g / L, more preferably about 45 to about 65 g / L, and more preferably about 50 to about 60 g / L. On the other hand, in compositions that contain a polar solvent such as glycerol, the copper(II) salt concentration may be lower, ranging from about 0.1 to about 25 g / L, more preferably about 0.1 to about 10 g / L, and more preferably about 0.1 to about 1 g / L. This is believed to be, in part, to achieve better uniformity of the plated copper film.

[0037] In one embodiment, the complexing agent is selected from the group consisting of citric acid, tartaric acid, ethylenediaminetetraacetic acid (EDTA), N-(hydroxyethyl)ethylenediaminetriacetic acid (HEDTA), ethylenediamine, 1,6-diaminocyclohexane, diethylenetriamine, triethylenetetramine, N,N,N,N-tetramethylethylenediamine, N,N-bis(2-hydroxyethyl)ethylenediamine, N,N,N,N-tetrakis(2-hydroxyethyl)ethylenediamine, glycine, 2-aminoethylphosphonic acid, salicylic acid, salicylhydroxamic acid, catechol, 1,2-dihydroxybenzene-4-sulfonic acid, acetylacetone, acetylacetonate, dimethylglyoxime, and 1,3-diaminopropane. In one embodiment, the complexing agent comprises diethylenetriamine. In another embodiment, the complexing agent comprises one or more of N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine and ethylenediaminetetraacetic acid. In one embodiment, the complexing agent has a concentration in the range of about 0.1 to about 20 g / L. As with the copper(II) salt, the concentration of the complexing agent in the composition depends in part on whether the composition contains a polar solvent. That is, in compositions that do not contain a polar solvent, the concentration of the complexing agent may be in the range of about 4.0 to about 20 g / L, more preferably about 5.0 to about 9.0 g / L, and more preferably about 4.5 to about 7.5 g / L. In contrast, in compositions that contain a polar solvent, the concentration of the complexing agent may be in the range of about 0.1 to about 2.0 g / L, more preferably about 0.2 to about 1.2 g / L, and more preferably about 0.4 to about 0.8 g / L.

[0038] In one embodiment, the present invention also includes a leveler. Suitable leveling agents include, but are not limited to, one or more of polyethyleneimine and its derivatives, quaternized polyethyleneimine, polyglycine, poly(allylamine), polyaniline, polyurea, polyacrylamide, poly(melamine-co-formaldehyde), reaction products of amines and epichlorohydrin, reaction products of amines, epichlorohydrin, and polyalkylene oxides, reaction products of amines and polyepoxides, polyvinylpyridine, and polyvinylimidazole. One example of a suitable leveling agent is a hydroxyethylated (ethoxylated) water-soluble polyethyleneimine formed by the reaction of a relatively high molecular weight polyethyleneimine with ethylene oxide, commercially available from BASF under the trade name Lupasol® SC-61B. In one embodiment, the concentration of the leveling agent ranges from about 2 to about 25 g / L. As with other components in the composition, the concentration of the leveling agent in the composition will depend in part on whether the composition contains a polar solvent. That is, in compositions that do not contain a polar solvent, the concentration of the leveling agent may range from about 15 to about 25 g / L, more preferably from about 18 to about 20 g / L. In contrast, in compositions that contain a polar solvent, the concentration of the leveling agent may range from about 2 to about 8 g / L, more preferably from about 3 to about 6 g / L.

[0039] In one embodiment, the electrolyte comprises a polar solvent. Examples of suitable polar solvents include, but are not limited to, ethylene glycol, propylene glycol, glycerol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, dipropylene glycol monomethyl ether, and combinations thereof. In one embodiment, the polar solvent comprises glycerol. When used, the concentration of the polar solvent is typically in the range of about 150 to about 250 g / L, more preferably about 175 to about 225 g / L, and more preferably about 190 to about 210 g / L.

[0040] In one embodiment, the electrolyte of the present invention is at least substantially free of halide atoms, where "substantially free" means that the composition contains less than 50 ppm, less than 25 ppm, or less than 10 ppm of halide atoms.

[0041] The pH of the electrolyte may be optionally adjusted with a suitable pH adjuster to maintain the pH at a desired level and to complex the copper ions with the complexing agent. In one embodiment, the pH adjuster comprises tetraethylammonium hydroxide or tetrabutylammonium hydroxide, or a buffer such as borate, glycine, and / or phosphate. As noted above, the pH is preferably within the range of about 7 to about 11, more preferably about 8 to about 11, and more preferably about 8.2 to about 9.5.

[0042] The electrolytes described herein can be used to provide ultra-thin copper films on conductive substrates, which may include, for example, cobalt, tungsten, ruthenium, molybdenum, combinations thereof, and alloys thereof. In one embodiment, the conductive substrate includes a cobalt or cobalt alloy substrate, such as a cobalt liner deposited on a barrier layer of an insulating dielectric layer. By "ultrathin," we mean that the thickness of the electroplated copper layer is less than 10 nm, or less than 5 nm, or less than 2 nm, or less than 1 nm. At the same time, the ultra-thin copper layer is continuous and pinhole-free. In one embodiment, the conductive substrate is formed by CVD deposition.

[0043] In one embodiment, the present invention is a method for depositing a copper film, preferably an ultra-thin copper film, on a substrate, preferably a conductive substrate, the method comprising: a) optionally pretreating the substrate, the pretreatment removing surface oxides; b) electrodepositing a copper film onto a conductive substrate using the copper electrolyte described herein.

[0044] In one embodiment, the present invention includes a method for depositing an ultra-thin copper film on a conductive substrate, the method comprising: A) optionally pretreating the conductive substrate to remove surface oxides; B) immersing the conductive substrate in a copper electrolyte, the copper electrolyte comprising: (1) a water-soluble copper salt; (2) a complexing agent; (3) A leveler, (4) optionally a polar solvent; (5) optionally, a pH adjuster; (6) residual water, the copper electrolyte has a pH in the range of about 8 to about 11, preferably about 8.2 to about 9.5; C) electrodepositing a copper film onto a conductive substrate using the copper electrolyte described herein, wherein the copper film is an ultra-thin copper film having a thickness of less than about 5 nm, or less than about 4 nm, or less than about 3 nm, or less than about 2 nm, wherein the ultra-thin copper film is a continuous copper film free of pinholes or other defects.

[0045] In the case of fresh conductive substrates, such as virgin cobalt substrates, the pretreatment step may be omitted. Prior to deposition, the conductive substrate may be immersed in a pretreatment solution to remove surface oxides. The pretreatment solution may contain a dilute weak acid, such as a solution of about 0.1 to about 8 g / L, more preferably about 5 to about 6 g / L, of citric acid, or a ligand capable of complexing with cobalt cations and removing native cobalt oxide, such as tartaric acid, ethylenediamine, or diethylenetriamine. The pretreatment step preferably lasts for about 0 to 30 seconds, followed by a rinse with deionized water. The pretreatment step can be omitted in the case of virgin cobalt substrates. A "fresh" cobalt substrate means that it has been exposed to air for no more than a few hours after vacuum break. The pretreatment step can also be replaced by immersion in a copper plating solution under open-circuit conditions. The immersion time is 0 to 60 seconds.

[0046] In one embodiment, the conductive substrate is a conductive liner, which may be, for example, a cobalt liner deposited on a barrier layer, such as a diffusion barrier layer made of one or more of tantalum, titanium, titanium nitride, tantalum nitride, tungsten, tungsten nitride, tungsten carbide, or manganese. The diffusion barrier layer generally has a thickness of about 0.5 to about 20 nm and may be deposited using various techniques, such as chemical vapor deposition, physical vapor deposition, or atomic layer deposition (ALD). The barrier layer is formed on a dielectric substrate and covers the surface of the substrate, including the sidewalls and bottom of features formed therein. The aspect ratio of the features may vary from 2:1 to 20:1, for example, from 3:1 to 10:1.

[0047] In one embodiment, the features have opening dimensions between 5 and 45 nm and aspect ratios between about 3:1 and about 10:1.

[0048] In one embodiment, the thickness of the cobalt layer is in the range of about 0.1 to about 3 nm, more preferably about 0.1 to about 2 nm. The cobalt liner may be formed by various methods, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), and electroless deposition. In one embodiment, the cobalt liner is formed by CVD. The thickness of the cobalt liner layer is preferably in the range of about 0.5 to 20 nm, more preferably about 1 to 10 nm, e.g., about 2 to 5 nm.

[0049] In one embodiment, the electrodeposition process described herein is carried out at a temperature in the range of about 20 to about 35° C., more preferably at about room temperature.

[0050] The alkaline copper electrolyte described herein can be used in copper damascene processes, including back-end electrical wiring board (BEOL) copper damascene processes, to deposit ultra-thin copper layers on conductive substrates that may contain cobalt, tungsten, ruthenium, molybdenum, combinations thereof, or alloys thereof, and may also be a liner layer, such as a cobalt liner. The ultra-thin copper film may have a thickness of less than 5 nm, preferably less than 4 nm, and more preferably less than 2 nm. The electrodeposited ultra-thin copper layer prevents metal dissolution. That is, the ultra-thin copper layer prevents dissolution caused by one or more of hydrogen evolution, copper displacement, and oxygen oxidation. Because the solution is alkaline, corrosion is not an issue. The thin copper film can then be used as a seed layer for acid copper. That is, a substrate containing features can be electroplated with copper using a suitable electrolyte, including an acid copper electrolyte.

[0051] As described herein, the method of the present invention can be used to plate ultra-thin copper layers onto conductive substrates to produce copper electrodeposits with good adhesion and that are substantially pinhole-free. In one embodiment, the electrolyte described herein can be used to plate ultra-thin copper layers onto conductive substrates using an electroplating method. The electrolyte is contained in an electrolytic cell that includes an anode, more preferably an inert anode, more preferably an anode selected from the group consisting of titanium or titanium platinide coated with mixed iridium, ruthenium, platinum, rhodium, and tantalum metal oxides, and a conductive substrate as the cathode.

[0052] In one embodiment, the electroplating method comprises: A) a "cold entry" step in which the surface to be coated is contacted with an electrolyte as described herein while the conductive substrate is maintained at an open circuit potential condition for a period of from about 0 seconds to about 30 seconds, more preferably from greater than 0 seconds to about 20 seconds, more preferably from about 0.5 seconds to about 10 seconds, more preferably from about 1.0 seconds to about 2.0 seconds; B) a coating step in which the cathode is polarized in a constant current mode to form an ultra-thin copper coating on the conductive substrate; and C) a "hot exit" step in which the substrate is removed from the electroplating bath.

[0053] The invention will now be discussed with reference to the following non-limiting examples.

[0054] Working Example: The electrolysis cell comprises an anode made of either an inert metal (e.g., platinum-coated titanium) or the same metal as that constituting the seed layer (in this case, copper), a silicon wafer coated with TaN or a barrier layer, and a conductive substrate that constitutes the cathode of the cell.

[0055] The electrolytic cell also includes a device for supporting the wafer to be coated, which device includes means for rotating the wafer at a predetermined speed.

[0056] The electroplating method used in the examples included the following successive steps, carried out in order: 1) Cold entry: The substrate was introduced into the electrolytic cell so that the surface bearing the cobalt liner layer was in contact with the electrolyte under open circuit conditions, the contacting step generally lasting no more than about 5 seconds, or no more than about 2 seconds, or no more than about 1 second, and was performed while no power was applied to the electrolytic cell.

[0057] 2) Copper film formation: The cathode assembly was then polarized in galvanostatic mode over a range of currents while simultaneously rotating at speeds of 20 to 200 revolutions per minute. The duration of the electroplating step was approximately 7 seconds to obtain a film of approximately 2 nm on the conductive substrate.

[0058] 3) Hot Exit: After the electroplating step, the copper-coated cathode was removed from the electroplating solution at the previous rotation speed while still under voltage bias. The duration of this step was approximately 2 seconds.

[0059] A deionized water rinse was then performed on the copper coated substrate and the substrate was allowed to dry.

[0060] Example 1: This example describes a copper layer formed with an electrolyte that does not contain a polar solvent.

[0061] A thin layer of copper, 0.3-3 nm thick, was deposited on a blanket cobalt liner. The substrate consisted of a piece of silicon wafer covered with a blanket silicon dioxide dielectric layer, which itself was coated with a copper diffusion barrier coating such as tantalum nitride or titanium nitride and a 16 nm thick CVD cobalt liner. The sheet resistance of the substrate was in the range of 9-10 ohms / square.

[0062] Prior to deposition, the cobalt substrate was immersed in a pretreatment solution to remove cobalt surface oxide. The pretreatment solution contained a dilute weak acid, such as 5 g / L citric acid, or a ligand that complexes with cobalt cations. The pretreatment step typically lasts approximately 0-30 seconds and is followed by a rinse with deionized water. The pretreatment step can be omitted in the case of unoxidized cobalt substrates.

[0063] The substrate is then transferred into the plating solution described below and maintained at open circuit conditions for 0-30 seconds. After that, the cobalt cathode is charged at 2 mA / cm 2 ~10mA / cm 2 The plating solution was adjusted to a pH of about 8.9 and maintained at that pH level throughout the test.

[0064] [Table 1]

[0065] Figure 1 shows the surface morphology of different amounts of copper deposited on a cobalt substrate. The substrate coated with approximately one layer of copper in Figure 1(b) exhibits the same surface morphology as the substrate, indicating the initial conformal copper layer growth on the cobalt. As more copper is deposited on the surface, the root-mean-square surface roughness decreases from 2.6 nm in Figure 1(a) when no copper is present on the surface to 2.1 nm in Figure 1(c) when 1.6 nm of copper is deposited. The copper film deposited from the plating solution described in this example does not roughen the substrate. Instead, the thin copper film smoothes the substrate, and the surface exhibits reduced surface roughness compared to the bare cobalt layer. When a thicker copper layer is deposited (i.e., approximately 4-5 nm), the surface topography of the cobalt substrate disappears. Due to the lack of hydrogen evolution and copper displacement, no characteristic markers are observed on the surface.

[0066] Example 2: This example describes a copper layer formed using an electrolyte containing a polar solvent (i.e., glycerol).

[0067] A thin layer of copper, 0.3-3 nm thick, was deposited on a blanket cobalt liner. The substrate consisted of a piece of silicon substrate covered with a blanket silicon dioxide dielectric layer, which itself was coated with a diffusion barrier coating such as tantalum nitride or titanium nitride and a 3 nm thick CVD cobalt liner. The sheet resistance of the substrate was in the range of 200-300 ohms / square.

[0068] Prior to deposition, the cobalt substrate was immersed in a pretreatment solution to remove cobalt surface oxide. The pretreatment solution may contain a dilute weak acid, such as 0.5 g / L citric acid, or a ligand that complexes with cobalt cations. Pretreatment typically lasts approximately 0-30 seconds and is followed by a rinse with deionized water. The pretreatment step can be omitted in the case of unoxidized cobalt substrates.

[0069] The substrate was then transferred into the plating solution described below and maintained at open circuit conditions for 0-30 seconds. A cobalt cathode was then applied at 0.5 mA / cm 2~3mA / cm 2 The composition was polarized in constant current mode over a current range of 1000 VA to 1000 VA. The composition was adjusted to a pH of about 8.9 and maintained at that pH level throughout the test.

[0070] [Table 2]

[0071] Figure 2 shows the dependence of substrate sheet resistance on the nominal thickness of the copper film. The sheet resistance drops at a nominal thickness of 3 nm, which roughly corresponds to a true thickness of 1 nm, due to low Coulombic efficiency in the initial growth stages. This observation indicates that the coalescence of the copper film is achieved sooner than the 1 nm thickness, indicating an extremely high copper nucleation density on the cobalt.

[0072] Figure 3 shows Auger surface mapping of a 1.4 nm thick copper film on a cobalt substrate with low impurity levels. The escape depth of the cobalt Auger electrons is approximately 1.2 nm, slightly shorter than the copper film thickness. The dominant upper copper intensity shown in Figure 3(c) and the noise-level lower cobalt intensity shown in Figure 3(b) indicate a dense, pinhole-free copper film.

[0073] Example 3: This example demonstrates that thin layers of copper can be formed not only on cobalt liners, but also on other conductive substrates including, but not limited to, tungsten, molybdenum, and ruthenium.

[0074] The substrate used in this example consisted of a piece of silicon wafer covered with a blanket silicon dioxide dielectric layer and a CVD tungsten layer. The electrical sheet resistance of this substrate is in the range of 0.3-0.4 ohms / square.

[0075] Prior to deposition, the tungsten substrate can be immersed in a pretreatment solution to remove tungsten surface oxides. The pretreatment solution may contain a dilute weak acid, such as 5 g / l citric acid, or a ligand that complexes with tungsten cations. Pretreatment typically lasts for 0-30 seconds and is followed by a rinse with deionized water. The pretreatment step may be omitted for new tungsten substrates. The substrate is then transferred to a copper plating solution and maintained at an open circuit condition for 0-30 seconds. The tungsten cathode is then irradiated with 0.5 mA / cm. 2 ~5mA / cm 2 The electrodes were polarized in constant current mode in the current range of 0.1 to 1.0 V.

[0076] The composition was adjusted to a pH of about 9.2 and maintained at that pH level for the duration of the test. Copper films 6 nanometers thick were deposited on tungsten substrates using the following plating solution:

[0077] [Table 3]

[0078] Figure 4 shows the surface morphology of copper electrodeposition on tungsten substrates at different process stages or conditions. Figure 4(a) shows the coarse grains of the as-received CVD tungsten film, with a grain size of approximately 300 nm. Without surface pretreatment before copper deposition, agglomerates of copper particles with a diameter of approximately 40 μm are formed on the tungsten substrate, as shown in Figure 4(b). In contrast, with the aid of appropriate surface pretreatment, copper forms a conformal coating layer on the tungsten particles, as shown in Figure 4(c).

[0079] The examples demonstrate that it is possible to produce ultra-thin copper layers from alkaline electrolytes on various conductive substrates. These ultra-thin copper layers can then be subsequently processed, if desired, to electrolytically deposit and fill copper onto the ultra-thin copper layer. As described herein, the ultra-thin copper layers have been shown to provide pinhole-free copper deposits on conductive substrates that can resist dissolution.

Claims

1. A copper electrolyte comprising: a. a water-soluble copper salt; b. a complexing agent; c. a leveller; d. optionally a polar solvent; e. optionally a pH adjuster; f. residual water, A copper electrolyte, wherein the copper electrolyte has a pH in the range of about 8 to about 11, preferably about 8.2 to about 9.

5.

2. The copper electrolyte of claim 1 , wherein the water-soluble copper salt comprises a copper (II) salt.

3. 3. The copper electrolyte of claim 2, wherein the copper (II) salt is selected from the group consisting of copper (II) sulfate, copper (II) chloride, and copper (II) acetate.

4. 4. The copper electrolyte of claim 2 or claim 3, wherein the copper (II) salt comprises copper (II) sulfate.

5. 5. The copper electrolyte of claim 1, wherein the complexing agent is selected from the group consisting of citric acid, tartaric acid, ethylenediaminetetraacetic acid (EDTA), N-(hydroxyethyl)ethylenediaminetriacetate (HEDTA), ethylenediamine, 1,6-diaminocyclohexane, diethylenetriamine, triethylenetetramine, N,N,N,N-tetramethylethylenediamine, N,N-bis(2-hydroxyethyl)ethylenediamine, N,N,N,N-tetrakis(2-hydroxyethyl)ethylenediamine, glycine, 2-aminoethylphosphonic acid, salicylic acid, salicylhydroxamic acid, catechol, 1,2-dihydroxybenzene-4-sulfonic acid, acetylacetone, acetylacetonate, dimethylglyoxime, and 1,3-diaminopropane.

6. The copper electrolyte of any one of claims 1 to 5, wherein the complexing agent comprises diethylenetriamine.

7. 7. The copper electrolyte according to claim 1, wherein the leveler is selected from the group consisting of polyethyleneimine and its derivatives, quaternized polyethyleneimine, polyglycine, poly(allylamine), polyaniline, polyurea, polyacrylamide, poly(melamine-co-formaldehyde), reaction products of amines and epichlorohydrin, reaction products of amines, epichlorohydrin and polyalkylene oxides, reaction products of amines and polyepoxides, polyvinylpyridine, and polyvinylimidazole.

8. The copper electrolyte according to any one of claims 1 to 7, wherein the pH adjuster is selected from the group consisting of tetraethylammonium hydroxide and tetrabutylammonium hydroxide.

9. 9. The copper electrolyte of claim 1, wherein the polar solvent present in the electrolyte is selected from the group consisting of ethylene glycol, propylene glycol, glycerol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, dipropylene glycol monomethyl ether, and combinations thereof.

10. The copper electrolyte of claim 9 , wherein the polar solvent is glycerol.

11. a. about 40 to about 80 g / L, more preferably about 45 to about 65 g / L, more preferably about 50 to about 60 g / L of said water-soluble copper salt; b. about 4 to about 20 g / L, more preferably about 5 to about 9 g / L, more preferably about 4.5 to about 7.5 g / L of said complexing agent; c. about 15 to about 25 g / L, more preferably about 18 to about 20 g / L, of said leveler.

12. a. about 2 to about 25 g / L, more preferably about 3 to about 10 g / L, more preferably about 4 to about 8 g / L of said water-soluble copper salt; b. about 0.1 to about 2.0 g / L, more preferably about 0.2 to about 1.2 g / L, more preferably about 0.4 to about 0.8 g / L of said complexing agent; c. about 2 to about 8 g / L, more preferably about 3 to about 6 g / L, of said leveler; and d) about 150 to 250 g / L of said polar solvent, more preferably about 175 to about 225 g / L, more preferably about 190 to about 210 g / L.

13. A copper electrolyte comprising: a. a water-soluble copper salt; b. a complexing agent; c. a leveller; d. 0 to 300 g / L of a polar solvent; e. optionally a pH adjuster; f. residual water; A copper electrolyte, wherein the copper electrolyte has a pH in the range of about 8 to about 11, preferably about 8.2 to about 9.

5.

14. A copper electrolyte comprising: a. a water-soluble copper salt; b. a complexing agent; c. a leveller; d. 0 to 300 g / L of a polar solvent; e. optionally a pH adjuster; f. residual water; A copper electrolyte, wherein the copper electrolyte has a pH in the range of about 8 to about 11, preferably about 8.2 to about 9.

5.

15. 1. A method for electrodepositing a copper film on a conductive substrate, the method comprising: a) optionally pretreating the conductive substrate, wherein the pretreatment removes surface oxides; b) immersing the conductive substrate in a copper electrolyte, the copper electrolyte comprising: a. a water-soluble copper salt; b. a complexing agent; c. a leveller; d. optionally a polar solvent; e. optionally a pH adjuster; f. residual water, the copper electrolyte having a pH in the range of about 8 to about 11, preferably about 8.2 to about 9.5; c) electrodepositing the copper film onto the conductive substrate using the copper electrolyte described herein, wherein the copper film is an ultra-thin copper film having a thickness of less than about 5 nm, and wherein the ultra-thin copper film is a continuous copper film free of pinholes or other defects.

16. 16. The method of claim 15, wherein the pretreatment step comprises contacting the conductive substrate with a pretreatment solution containing a dilute weak acid.

17. 17. The method of claim 16, wherein the dilute weak acid comprises a solution of citric acid.

18. 18. The method according to any one of claims 15 to 17, wherein the ultra-thin copper film has a thickness of less than 5 nm, preferably less than 4 nm, more preferably less than 2 nm.

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