Metastable fine grain copper for hybrid bonding applications

Abstract

A method of electrodepositing a copper film comprising a fine grain copper structure on a substrate. The method includes the steps of: (a) bringing the substrate into contact with an electrodeposition composition; and (b) depositing the copper film having the fine grain copper structure on the substrate. A largest dimension of the copper grains in the copper film is typically less than about 200 nm. In addition, the largest dimension of the copper grains in the copper film remains at less than about 200 nm when the copper film is annealed at a temperature of up to and including 170°C for a period of time. The copper grains grow by at least 20% or more when the copper film is annealed at a temperature of 200°C or higher.

Description

Attorney Docket No. 33225-37 (AES)-WO-AMETASTABLE FINE GRAIN COPPER FOR HYBRID BONDING APPLICATIONSFIELD OF THE INVENTION

[0001] The present invention relates generally to a method of electrodepositing metastable fine grain copper.BACKGROUND OF THE INVENTION

[0002] An integrated circuit (IC) requires conductive interconnects between semiconductor domains in order to communicate signals therebetween. Based thereon, in order to create ever faster microprocessors, smaller dimension interconnects of higher conductivity materials is an ongoing goal. Wafer Level Packaging (WLP) is a type of IC packaging technology that is performed at wafer level. This means that the packaging is applied on whole wafers and wafers are diced after the packaging is successfully completed. In WLP, the components used in assembly (i.e., bumps) are applied to the wafer pre-dicing, i.e., at wafer level. In contrast, in traditional semiconductor manufacturing, the wafers are first diced into separate dies and are then assembled into a semiconductor package.

[0003] As I / O requirements keep increasing and demand greater interconnect density, redistribution layers (RDL) have been developed to reroute connects of chips via conductive metal traces, which enables WLP to contain different chips with different functionalities. Another benefit to using WLP is the ability to stack chips vertically and / or tile them horizontally, resulting in the development of 2.5D and 3D ICs which make use of techniques such as through silicon vias (TSVs) to connect multiple layers of horizontal interconnects (which typically comprise copper) to ensure greater bandwidth with less power consumption.

[0004] WLP with heterogeneous integration boosts I / O counts, while reducing footprint, as both feature and pitch sizes shrink. Traditional methods such as C4 (i.e., controlled collapse chip connection) bump, C2 (i.e., chip connection) bump and micropillar with SnAg cap have advanced the field to include Cu-Cu direct bonding and hybrid bonding without solder cap, particularly for high density 2.5D and 3D packaging.

[0005] Both 2.5D and 3D solutions offer enhanced performance, size reduction, and improved power efficiency. 2.5D packaging is advantageous for combining various components and reducing footprints and is suitable for applications in high-performance computing and Al{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A accelerators. 3D packaging provides unparalleled integration, efficient heat dissipating, and reduced interconnected lengths, making it ideal for high-performance applications.

[0006] 2.5D is an intermediate step between traditional 2D packaging and fully 3D packaging. In 2.5D packaging, multiple semiconductor dies, typically from different process technologies, are placed side by side on a silicon interposer, which acts as a bridge, connecting the individual dies and providing a high-speed communication interface. These arrangements allow for greater flexibility in combining different functionalities on a single package. A common 2.5D integration technology combines a silicon interposer with TSVs. Advantages of 2.5D packaging include (1) enhanced performance, enabling the integration of diverse components on a single package; (2) overall reduced footprint of the package; and (3) improved power efficiency due to shorter interconnects and optimized chip placement.

[0007] 3D packaging takes integration to the next level by stacking multiple semiconductor dies on top of each other to create a three-dimensional structure that enhances the overall performance and functionality of the package, resulting in even shorter interconnects and smaller package footprints. Advantages of 3D packaging include (1) compact integration of a wide range of components and functionalities; (2) improved heat dissipation; and (3) reduced interconnect lengths which minimize signal delays and power consumption.

[0008] Conventional bonding technologies involve copper bumps and pillars in combination with solder materials and are widely employed in packaging applications. However, these processes can suffer from relatively large nonuniformities in the range of several micrometers, especially with regard to coplanarity and total thickness variation.

[0009] Increasing the interconnect density generally involves shrinking distance between the copper bumps and pillars. However, conventional solder technologies may not be suitable for fine pitch applications since the solder material could be squeezed out from the structures during bonding, creating electrical shorts. Direct formation of copper-to-copper interconnects may overcome these issues by omitting conventional solder caps for microbumps and, thereby scaling down the pitch between the bumps to values below 20 pm.

[0010] Several attempts have been suggested to improve bond formation, including, for example, surface activation by ion beams, chemical-mechanical polishing (CMP), surface passivation by alloying the copper with other elements, metal and organic finishes, and structural design.{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A

[0011] Current bonding techniques typically require high temperatures, which may exceed 300°C. To address this, innovative materials and techniques have been explored, focusing primarily on fine-grain and nanotwinned Cu (nt-Cu). Nanotwinned copper offers the potential for lower bonding temperatures due to its (111) plane-dominated texture, enhancing atomic diffusivity and reducing the thermal budget.

[0012] Alternatively, fine-grained copper provides another avenue for lowering the thermal budget. Metastable fine-grain copper maintains stability during the queue time before bonding, with grain growth occurring during bonding. The energy released during grain growth at the bonding process promotes copper interdiffusion, resulting in stronger bonding.

[0013] Hybrid bonding integrates dielectric and metal bonds, bypassing under-bump metallization and underfill, facilitating direct connections, and enabling pitches below 10 pm, where electrical and thermomechanical limitations constrain solder. This approach, reminiscent of a monolithic package, can also significantly reduce latency and heat generation. Cu-Cu hybrid bonding involves embedding metal contacts between dielectric materials and using heat treatment for solid-state diffusion of copper atoms. Advantages of hybrid bonding include, but are not limited to, enabling ultra-fine pitch and small contact sizes and facilitating high I / O counts, which is critical in packaging solutions in which devices require a growing number of connections to meet performance demands. In addition, unlike flip-chip soldering, which often relies on underfill materials, Cu-Cu hybrid bonding eliminates the need for underfill, thus reducing parasitic capacitance, resistance, and inductance, as well as thermal resistances.

[0014] U.S. Pat. Pub. No. 2024 / 0170428 to Schmidt, the subject matter of which is herein incorporated by reference in its entirety, describe a method for copper-to-copper direct bonding that includes an annealing step with an annealing temperature of 200°C or less. However, the copper grains were only found to be stable when stored at room temperature.SUMMARY OF THE INVENTION

[0015] It is an object of the present invention to provide a method of electrodepositing copper.

[0016] It is another object of the present invention to provide a method of electrodepositing fine grain copper.

[0017] It is still another object of the present invention to provide a method of electrodepositing fine grain copper for hybrid bonding.{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A

[0018] It is still another object of the present invention to strengthen copper bonding.

[0019] It is still another object of the present invention to provide a method of electrodepositing fine grain copper that remains stable over a period of time of self-annealing.

[0020] It is still another object of the present invention to provide a method of electrodepositing fine grain copper that exhibits a controlled grain growth upon annealing.

[0021] To that end, in one embodiment, the present invention relates generally to a method of electrodepositing a copper film comprising a fine grain copper structure on a substrate, the method comprising the steps of a) bringing the substrate into contact with an electrodeposition composition; and b) depositing the copper film having the fine grain copper structure on the substrate, wherein a largest dimension of the copper grains in the copper film is less than about 200 nm; and wherein the largest dimension of the copper grains in the copper film remains at less than about 200 nm when the copper film is annealed at a temperature of up to and including 170°C for a period of time.

[0022] In one embodiment, the copper grains grow by at least 20% or more when the copper film is annealed at a temperature of 200°C or higher.BRIEF DESCRIPTIONS OF THE FIGURES

[0023] The detailed description is set forth to the following figures which illustrate certain aspects of the present invention.

[0024] Fig. 1 depicts a top view of an electron back scattered diffraction (EBSD) grain map of a copper film plated in accordance with one aspect of the present invention.

[0025] Fig. 2 depicts a focus ion beam (FIB) image of a copper film to characterize grain size in accordance with one aspect of the present invention.

[0026] Fig. 3 depicts a 2D XRD diffraction frame showing a Debye diffraction pattern .

[0027] Fig. 4 depicts a comparison of sheet resistance of two different copper films as a function of room temperature self-annealing time.

[0028] Fig. 5 depicts grain stability of a copper deposit in accordance with Example 1.

[0029] Fig. 6 depicts grain stability of a copper deposit in accordance with Example 2.

[0030] Fig. 7 depicts grain stability of a copper deposit in accordance with Comparative Example 3.{N6091519}Attorney Docket No. 33225-37 (AES)-WO-ADETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] The present invention relates generally to methods of depositing fine grain copper on a workpiece and for characterizing the copper microstructure for copper grain evolution associated with both self-annealing and high-temperature annealing. The present invention is directed to methods of fabricating metastable fine-grain Cu on blanket and patterned wafers and provides a solution for a low thermal budget fine-grain process for a hybrid bonding sequence. In addition, the present invention also describes the joining of substrates having a fine grained film deposited thereon, including during a hybrid Cu-Cu bonding sequence.

[0032] The present invention also relates generally to different aspects of the fine grain copper deposit before and after bonding in view of grain size of the copper grains and purity of the copper deposits after annealing and self-annealing and describes improved conditions for subsequent copper-to-copper bonding in a hybrid bonding process sequence, including with respect to both time and annealing temperature.

[0033] As used herein, “a,” “an,” and “the” refer to both singular and plural referents unless the context clearly dictates otherwise.

[0034] As used herein, the term “about” refers to a measurable value such as a parameter, an amount, a temporal duration, and the like and is meant to include variations of + / -15% or less, preferably variations of + / -10% or less, more preferably variations of + / -5% or less, even more preferably variations of + / -1% or less, and still more preferably variations of + / -0.1% or less of and from the particularly recited value, in so far as such variations are appropriate to perform in the invention described herein. Furthermore, it is also to be understood that the value to which the modifier “about” refers is itself specifically disclosed herein.

[0035] As used herein, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, are used for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. It is further understood that the terms “front” and “back” are not intended to be limiting and are intended to be interchangeable where appropriate.

[0036] As used herein, the terms “comprises” and / or “comprising,” specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A

[0037] As used herein, the term “substantially free” or “essentially free” if not otherwise defined herein for a particular element or compound means that a given element or compound is not detectable by ordinary analytical means that are well known to those skilled in the art of metal plating for bath analysis. Such methods typically include atomic absorption spectrometry, titration, UV-Vis analysis, secondary ion mass spectrometry, and other commonly available analytical techniques.

[0038] As used herein, the term “features” refers to vias, through-silicon vias (TSV), trenches, pillars, pad, bumps, etc. that may be present on a microelectronic substrate.

[0039] All amounts are percent by weight unless otherwise noted. All numerical ranges are inclusive and combinable in any order except where it is logical that such numerical ranges are constrained to add up to 100%. The term “average” is equivalent to the mean value of a sample.

[0040] The terms “plating” and “deposit” or “deposition” are used interchangeably throughout this specification.

[0041] The terms “composition” and “bath” and “electrolyte” and “solution” are used interchangeably throughout this specification.

[0042] As used herein, the term “having a polycrystalline structure” defines a case where X-ray diffraction of an arbitrary number (one or more, preferably three or more, further preferably five or more) of parts of the deposited copper film are observed with a Debye (diffraction) pattern (also referred to as a Debye (diffraction) ring), as depicted, for example, in Fig. 3.

[0043] In one embodiment the present invention relates generally to a method of electrodepositing a copper film comprising a fine grain copper structure on a substrate, the method comprising the steps of: c) bringing the substrate into contact with an electrodeposition composition; and d) depositing the copper film having the fine grain copper structure on the substrate, wherein a largest dimension of the copper grains in the copper film is less than about 200 nm; and wherein the largest dimension of the copper grains in the copper film remains at less than about 200 nm when the copper film is annealed at a temperature of up to and including 170°C for a period of time.

[0044] The required time for grain growth stability will depend in part on the requirements of the customer. This time frame refers to the queue time prior to a step of performing a hybrid Cu-Cu{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A bond between two substrates, such as a Cu-pad bonding step in the integration process, with a minimum requirement of a self-annealing period of at least 1 week, or at least 2 weeks, or at least 3 weeks, or even at least 4 weeks. As described herein, it is highly desirable that the copper grains remain stable during this time period, meaning that sheet resistance does not decrease by more than about 5% in a 24-hour period, more preferably, sheet resistance does not decrease by more than about 3% in a 24-hour period. In addition, it is also highly desirable that the copper grains remain stable upon exposure to an annealing temperature of up to and including 170°C for between about 30 minutes and about 2 hours, more preferably between about 30 minutes and about 60 minutes, or that the copper grains remain stable upon exposure to an annealing temperature of up to and including 150°C for between about 30 minutes and about 4 hours, more preferably about 1 hour to about 2 hours. Thereafter, after exposure to an annealing temperature of at least about 200°C or between about 200°C and about 250°C for between about 30 minutes and about 4 hours, more preferably about 1 hour to about 2 hours, the copper grains grow by at least 20% or at least 30% or at least 40% as compared to the initial largest dimension of the copper grains before growth commences.

[0045] As used herein, the term “self-annealing” is used to describe a step of storing the substrate with the copper film deposited thereon at room temperature and under atmospheric conditions for a period of time.

[0046] As used herein, the term “annealing” is used to describe a step in which the substrate with the copper film deposited thereon is heated to an elevated temperature and then held at that elevated temperature in a nitrogen environment for a period of time.

[0047] In one embodiment, the copper electroplating composition comprises: a. a source of copper ions; b. a source of halide ions, preferably a source of chloride ions; c. an acid; and d. one or more bath additives, wherein the one or more bath additives are selected from the group consisting of suppressors, accelerators, and levelers.

[0048] In one embodiment, the one or more bath additives comprise one or more suppressors, one or more accelerators, and one or more levelers.

[0049] In one embodiment, as described herein, the basic make-up of the electroplating composition comprises an aqueous solution comprising copper ions, sulfuric acid, and chloride{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A ions. The electroplating composition is used in an electroplating process as described herein that includes make-up solutions and one or more organic bath additives.

[0050] In one embodiment, the copper ions are provided by a copper salt selected from the group consisting of copper sulfate, copper methane sulfonate, copper pyrophosphate, copper propanesulfonate and combinations of one or more of the foregoing. In one preferred embodiment, the copper salt comprises copper sulfate. The concentration of the copper ions is preferably in the range of about 1 to about 100 g / L, more preferably in the range of about 20 to about 80 g / L, most preferably in the range of about 40 to about 60 g / L.

[0051] Halide ions (i.e., most typically chloride ions) act as a bridge to assist in adsorption of certain organic additives onto a substrate surface. Halide ions include, but are not limited to, chloride ions, bromide ions, iodide ions, and combinations of one or more of the foregoing. In one embodiment, the halide ions comprise chloride ions. The concentration of chloride ions in the electroplating solution is preferably within the range of about 1 to about 150 mg / L, more preferably about 30 to about 120 mg / L, most preferably within the range of about 45 to about 75 mg / L.

[0052] In one embodiment, the electroplating composition contains an acid to control the conductivity of the plating bath, and suitable acids include sulfuric acid and methane sulfonic acid. In one embodiment, the acid is sulfuric acid. The concentration of acid in the electroplating solution is generally within the range of about 1 to about 240 g / L, more preferably within the range of about 2 to about 60 g / L. In one preferred embodiment, the concentration of the acid is in the range of about 5 to about 15 g / L, more preferably about 10 to about 12 g / L. In another preferred embodiment, the concentration of the acid is in the range of about 30 to about 60 g / L, more preferably about 40 to about 50 g / L.

[0053] As described herein, the bath additives typically comprise one or more accelerators, one or more suppressors, and one or more levelers.

[0054] In one embodiment, the accelerator comprises an organic sulfur compound, which may be an organic sulfur salt. Suitable organic sulfur compounds include, but are not limited to bis- (3-sulfopropyl)-disulfide (SPS), 3 -mercapto- 1 -propanesulfonic acid (MPS), 3-(benzothizolyl-2- mercapto)-propylsulfonic acid (ZPS), N,N-dimethyldithiocarbamylpropyl sulfonic acid (DPS), 3- S-isothiuronium propyl sulfonate (UPS), and (O-ethyldithiocarbonato)-S-(3-sulfopropyl)ester (OPX) and combinations of the foregoing.{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A

[0055] The concentration of the accelerator depends in part on the particular accelerator that is used in the copper electroplating solution but is preferably within the range of about 0.1 to about 100 mg / L, more preferably about 1 to about 60 mg / L, more preferably about 2 to about 40 mg / L.

[0056] In one embodiment, the suppressor comprises a reaction product of an amine or sulfur- containing compound with 2,3 -epoxy- 1 -propanol. The resulting linear or branched polyhydroxyl suppressor generally has a molecular weight of about 200 to about 10,000 g / mol, more preferably about 500 to about 5,000 g / mol, most preferably about 1,000 to about 3,000 g / mol.

[0057] Examples of suitable amines, include ethanolamine, diethanolamine, triethanolamine, propanolamine, isopropanolamine, diisopropanolamine, triisopropanolamine, N-methyl diethanolamine, N-ethyl diethanolamine, N-propyl diethanolamine, methyl monoethanolamine, N,N-dimethyl ethanolamine, N,N-diethyl ethanolamine, N-propyl monoethanolamine, N-propyl diethanolamine, N-butyl ethanolamine, N-butyl diethanolamine, N,N-dibutyl ethanolamine, hydroxy ethyl morpholine, 2-piperidino ethanol, diethanol isopropanolamine, N-(2- hydroxyethyl) pyrrolidine, 4-pyridinemethanol, 4-pyridineethanol, 4-pyridinepropanol, 2- hydroxy-4-methylpyridine, 2-hydroxymethyl-l -methylimidazole, 4-hydroxymethyl-5- methylimidazole, choline chloride, b-methylcholine chloride, bi s(2- hydroxyethyl)dimethylammonium chloride, tris(2-hydroxyethyl)methylammonium chloride, carnitine chloride, (2-hydroxyethyl)dimethyl(3-sulfopropyl)ammonium chloride, l-(2- hydroxyethyl)-3-methylimidazolium chloride, and combinations of the foregoing.

[0058] Other amines include tertiary amines, such as, 3 -hydroxypropyldimethylamine, n- butyldimethylamine, di(3-hydroxypropyl)methylamine, 2,3-dihydroxypropyldimethylamine, 3- hydroxypropyldiethylamine, 2-hydroxypropyldimethylamine, 4-hydroxybutyldimethylamine, 2- hydroxyethyldimethylamine, n-propyldimethylamine, 2-hydroxyethoxyethyldimethylamine, di(2-hydroxyethyl)methylamine, benzyldimethylamine, and 4-hydroxybenzyldimethyleamine, 4- methylpyridine, 3 -ethylpyridine, 4-propylpyridine, 4-tertbutylpyridine, 4-cyanopyridine, 4- isopropylpyridine, 4-methoxypyridine, 3,4-lutidine, 3-methoxypyridine, and 4-pyridinemethanol, 2-dimethylamino-l -ethanol, n-butyl dimethylamine, and N,N-dimethylbenzylamine, 4- ethylpyridine, and 1 -methylimidazole, 1 -benzylimidazole, N-methylmorpholine, 2-[2- (dimethylamino)ethoxy]ethanol.

[0059] Another suitable amine compound is bis(2-hydroxyethyl)dimethyl ammonium chloride.{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A

[0060] Other similar amine compounds that are capable of reacting with the 2,3-epoxy-l- propanol to produce a reaction compound are also usable as the suppressor of the instant invention.

[0061] Examples of suitable sulfur compounds include, but are not limited to, thiogly colic acid, thiomalic acid, sodium hydrogen sulfide, thiodiglycolic acid, thiodiethylene glycol, thiourea, N,N,N’N’ -tetramethylthiourea, 2-mercaptoethanol, 3 -mercaptopropanol, 2-mercaptoimidazole, 2-mercaptopyridine, 4-mercaptopyridine, 4-mercaptophenol, 3-mercapto-l -propanesulfonic acid, 3,6-dithia-l,8-octanediol, 2,2’ -thiodi ethanethiol, 2-hydroxy ethyl disulfide, 3,3 ’-thiodipropanol, and 2, 2'-(ethylenedi oxy )di ethanethiol.

[0062] In another embodiment, the suppressor comprises polyether groups covalently bonded to an amine species, which may include primary, secondary, tertiary and quaternary amines. Examples of suppressors of this type can be found, for example, in U.S. Pat. No. 7,303,992 to Paneccasio et al., the subject matter of which is herein incorporated by reference in its entirety.

[0063] In one embodiment, the leveler may be a reaction product of a pyridyl compound or a dipyridyl compound with an alkylating agent, preferably a difunctional alkylating agent.Examples of these levelers are described, for example, in U.S. Pat. No. 11,124,888 to Richardson et al., the subject matter of which is herein incorporated by reference in its entirety.

[0064] In one embodiment, alkylating agents that react with the pyridyl or dipyridyl compounds have the general structure (Illb):Y - ( CH2)p- B - ( CH2)q- ZStructure ( I l lb ) whereinB may be selected from among:{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A a single bond, an oxygen atom ( 0 ), a methenyl hydroxide (carbonylamino (p and q may be the same or different, are integers between 0 and 6, preferably from 0 to 2, wherein at least one of p and q is at least 1;X is an integer from one to about four, preferably one or two; andY and Z are leaving groups. The leaving group may be selected from among, for example, chloride, bromide, iodide, tosyl, triflate, sulfonate, mesylate, methosulfate, fluorosulfonate, methyl tosylate, brosylate, or nosylate.

[0001] In each B group above, the single line emanating from the functional moiety denotes a bond between an atom in the B moiety, e.g., oxygen, nitrogen, or carbon, and a carbon of theand akylene groups. Additionally, the Ri through Ri4 groups in denoted in the B moieties of Structure (Illb) are independently hydrogen; substituted or unsubstituted alkyl having from one to six carbon atoms, preferably one to three carbon atoms; substituted or unsubstituted alkylene having from one to six carbon atoms, preferably from one{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A to three carbon atoms; or substituted or unsubstituted aryl. The alkyl may be substituted with one or more of the following substituents: halogen, heterocyclo, alkoxy, alkenoxy, alkynoxy, aryloxy, hydroxy, protected hydroxy, hydroxycarbonyl, keto, acyl, acyloxy, nitro, amino, amido, nitro, phosphono, cyano, thiol, ketals, acetals, esters and ethers. In general, the various R groups are hydrogen or unsubstituted alkyl, and even more preferably, the R groups are hydrogen.

[0002] In one preferred embodiment, B is selected from the group consisting of: an oxygen atom (OH> - c I> - a methenyl hydroxide (H),an ethylene glycol groupa propylene glycol group

[0003] In another preferred embodiment, B is selected from the group consisting of: an oxygen atom (OH> - c I> - a methenyl hydroxide (H), a carbonyl{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A a phenylene group (an ethylene glycol group (

[0004] In one preferred embodiment, the structure -(CH2)P-B-(CH2)q- is aliphatic. In another preferred embodiment, in the alkylating agents of Structure (Illb), p and q are both one or are both two, and Y and Z are both chloride.

[0005] Another class of alkylating agents that may form a polymeric leveler when reacted with the di pyridyl compounds includes an oxirane ring and has the general structure (IIIc):Structure ( I I Ic ) whereinRii, R12, and R13 are hydrogen or substituted or unsubstituted alkyl having from one to six carbon atoms, preferably from one to three carbon atoms; o is an integer between one and six, preferably one or two; andY is a leaving group. The leaving group may be selected from among, for example, chloride, bromide, iodide, tosyl, triflate, sulfonate, mesylate, methosulfate, fluorosulfonate, methyl tosylate, brosylate, or nosylate.

[0006] In one embodimentEx, Rn, R12, and R13 are hydrogen and the alkylating agent has the following general structure: DY - ( CH2)o- C - CH2H wherein o and Y are as defined in connection with Structure (IIIc).{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A

[0065] Examples of suitable leveler compositions include, but are not limited to, one of the following structures:Structure (3) where the value of n is preferably between about 5 and about 20, such as between 6 and 9 in a low molecular weight embodiment or between 10 and 15 in a higher molecular weight embodiment, and / or is such that the polymer or oligomer has a number average molecular weight between about 1000 and about 5000;{N6091519}Attorney Docket No. 33225-37 (AES)-WO-AStructure (4) wherein X is an integer of at least 2, preferably from 2 to 100, such as from 2 to 50, and more preferred from 3 to about 20; andStructure (5) wherein X is an integer of at least 2, preferably from 2 to 100, such as from 2 to 50, and more preferred from 3 to about 20, still more preferably between about 5 and about 20, most preferably between about 10 and about 15;

[0066] The leveler may also be a pyridyl compound or a dipyridyl compound which may be substituted. For example, the leveler may be a pyridinium compound and, more particularly, a quatemized pyridinium salt. Examples of these substituted pyridyl and dipyridyl compounds include, but are not limited to, derivatives of a vinyl pyridine, such as derivatives of 2-vinyl pyridine and derivatives of 4-vinyl pyridine, homo-polymers of vinyl pyridine, co-polymers of vinyl pyridine, quaternized salts of vinyl pyridine, and quaternized salts of these homo-polymers and co-polymers. Specific examples of such compounds include, for example, poly(4-vinyl pyridine), the reaction product of poly(4-vinyl pyridine) with dimethyl sulfate, the reaction product of 4-vinyl pyridine with 2-chloroethanol, the reaction product of 4-vinyl pyridine with benzylchloride, the reaction product of 4-vinyl pyridine with allyl chloride, the reaction product of 4-vinyl pyridine with 4-chloromethylpyridine, the reaction product of 4-vinyl pyridine with 1,3-propane sultone, the reaction product of 4-vinyl pyridine with methyl tosylate, the reaction{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A product of 4-vinyl pyridine with chloroacetone, the reaction product of 4-vinyl pyridine with 2- methoxyethoxymethylchloride, the reaction product of 4-vinyl pyridine with 2-chloroethylether, the reaction product of 2-vinyl pyridine with methyl tosylate, the reaction product of 2-vinyl pyridine with dimethyl sulfate, the reaction product of vinyl pyridine and a water soluble initiator, poly(2-methyl-5-vinyl pyridine), and l-methyl-4-vinylpyridinium trifluoromethyl sulfonate, among others.

[0067] In one embodiment, it is desirable that the molecular weight of the leveler is less than about 12,000 g / mol, preferably less than about 10,000 g / mol, even more preferably less than about 8,000 g / mol, more preferably less than about 5,000 g / mol.

[0068] The concentration of the leveler depends in part on the particular leveler being used as well as the particular suppressor and accelerator and the process conditions. In one embodiment, the leveler is present in the copper electrolyte at a concentration within the range of about 0.1 to about 10 mg / L, more preferably about 1 to about 5 mg / L, more preferably about 2 to about 4 mg / L.

[0069] It is desirable that the copper deposit is pure copper. By pure copper, what is meant is that the copper content of the deposit is at least 99 wt.%, preferred of 99.9 wt.%, more preferred of 99.99 wt.%. It is desirable that the pure copper deposit does not contain any alloying metals or organic residues within the deposit. In other words, the copper deposit is at least substantially free of impurities. By substantially free of impurities, what is meant is that the copper deposit contains less than 300 ppm total impurities, preferably less than 100 ppm total impurities, most preferably less than 75 ppm total impurities.

[0070] In another embodiment, the present invention also relates generally to the use of a copper electroplating composition as described herein to create a metastable copper-copper hybrid bond between bonding substrates. The metastable copper-copper hybrid bond has a grain size that is larger than the grain size of the initial deposits before bonding. That is, in one embodiment, a copper deposit is provided on two substrates and the substrates are brought into contact with each other. A copper film comprising a fine grain copper structure is deposited ono two substrates and the two substrates with the copper film deposited thereon are brought into contact with each other. Once the copper films are brought into contact with each other, the resulting structure is annealed, and the grain size of the copper grains of the copper film increases and the copper film are joined together to form a copper-copper hybrid bond. As describe herein, the{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A grain size grows by at least 20% or at least 30% or at least 40% as compared to the initial largest dimension of the copper grains when the copper fdm is exposed to an annealing temperature of at least 200°C or between about 200°C to about 250°C for a period of time in a nitrogen environment. In one embodiment, the period of time is between about 30 minutes and about 4 hours or between about 1 hour and about 2 hours.

[0071] In one embodiment, the substrate is a blanket substrate. In another embodiment, the substrate is a patterned substrate, and the pattern may comprise one or more features such as one or more microvias. In one embodiment, the substrate comprises a copper seed layer deposited thereon.

[0072] The first substrate and the second substrate can be part of an assembly after applying the inventive method and / or can be part of a microelectronic device as a wafer-like substrate or a die-like substrate to be processed by the method described herein. The connecting of the bonding surfaces according to the inventive method may be one step in a wafer to wafer (W2W) process, a die-to-wafer (D2W) process, or a die-to-die (D2D) process, by way of example and not limitation.

[0073] The first substrate and the second substrate may further comprise, independently from each, other one or more additional metal deposits having a different metal composition, e.g., a copper alloy, forming a vertical stack. The additional metal deposit of the first substrate is connected with the copper film deposited on the first substrate and the additional metal deposit of the second substrate is connected with the second copper film deposited on the second substrate. Each additional metal deposit has a respective coefficient of thermal expansion (CTE). The first copper film and second copper film, in addition to the additional metal deposits, may be confined by dielectric material.

[0074] The first substrate may comprise further a first nonconductive material, e.g., a dielectric material, having a bonding surface and at least partially confining the first copper film and wherein the second substrate comprises further a second nonconductive material, e.g., a dielectric material, having a bonding surface and at least partially confining the second copper film.

[0075] The first copper film of the first substrate and the second copper film of the second substrate can be independently selected from the group consisting of a via, a bump, a pillar and / or a pad.{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A

[0076] As described herein, the inventors have found that the crystalline structure of the copper film remains unmodified after plating and remains stable for an extended self-annealing period under ambient temperature and pressure of at least 1 day or at least 1 week or at least 2 weeks or at least 3 weeks or at least 4 weeks. Thereafter, the copper deposit may be annealed at a temperature of up to 170°C for a period of time and the crystalline structure of the copper film remains unmodified and is conserved until the copper-to-copper bonding process is initiated at an annealing temperature of between about 200°C and about 250°C. In addition, the inventors of the present invention have found that the initiated grain growth of grains within the copper-to- copper bonding process leads to grains that grow by at least about 20% or at least 30% or at least 40% or more as compared with the initial grain structure when the copper film is exposed to the annealing temperature of about 200°C and about 250°C. The resulting copper-copper bond shows improved bonding strength and highly improved reliability of the connected (and converted) deposits.

[0077] Various methods can be used to characterize grain size and grain structure, including, for example, electron backscattered diffraction (EBSD), focus ion beam (FIB) images.

[0078] The most direct method is EBSD and an example of a typical EBSD grain map is shown in Fig. 1. Grains can be determined with a 5° tolerance angle and a 5-pixel minimum requirement. The grain size can be calculated through propriety software. However, EBSD requires a smooth surface and is not widely available.

[0079] Another direct method is FIB, as shown in Fig. 2. Different crystal orientations affect the number of secondary electrons escaping from the specimen, making the different grains appear with different contrast, offering a high spatial resolution. The FIB image is the primary method to characterize the grain size on the patterned wafer in the Examples below.

[0080] Indirect methods include X-ray diffraction (XRD) and sheet resistance. XRD (Bruker AXS, D8 discovered with VANTEC-500 area detector) and sheet resistance (Filmetrics, R50, 4- point probe) to characterize the grain size during the chemistry screening process. SIMS determines the impurities in the plated Cu.In the chemistry screening process described herein, XRD is utilized with a 2D detector to assess grain stability swiftly. Fig. 3 illustrates three XRD scans of distinct Cu deposits generated via electrodeposition with varying additives, following a room temperature self-annealing period of two weeks.{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A

[0081] Without extensive data integration and analysis, these area detector frames provide a quick overview of texture and grain size of the copper deposit. As shown in Fig. 3, the observed Debye rings exhibit smooth and continuous patterns for the first two deposits, indicating that both of these deposits are polycrystalline and fine-grained. In contrast, the ring for the third deposit appears continuous but sporadic, suggesting that this deposit is polycrystalline but is large grain. Based thereon, based on the XRD results, it can be seen that the first two deposits can maintain a fine grain structure after two weeks of self-annealing, while the third deposit does not exhibit this characteristic.

[0082] Without extensive data integration and analysis, these area detector frames provide a quick overview of texture and grain size of the copper deposit. As shown in Fig. 3, the observed Debye rings exhibit smooth and continuous patterns for the first two deposits, indicating that both deposits are polycrystalline and fine-grained. In contrast, the ring for the third deposit appears continuous but sporadic, suggesting that this deposit is polycrystalline but is large grain. Based thereon, based on the XRD results, it can be seen that XRD can be used to show deposits that can maintain a fine grain structure after two weeks of self-annealing versus those that cannot.

[0083] Sheet resistance measurements were utilized as another means to assess grain stability. As seen in Fig. 4, the sheet resistance change was observed over a 7-day room temperature selfannealing period for electrodeposited copper films. In the case of the first copper film, the sheet resistance gradually decreases through the seven days. In contrast, for the second copper film, there is a significant decrease in sheet resistance by the second day, followed by a stable level after that.

[0084] This change in sheet resistance is due to changes in grain size and the number of grain boundaries. The increase in grain size, accompanied by a reduction in grain boundary density, increases the electron-free path, thereby reducing electron scattering and consequently lowering sheet resistance. The gradual reduction in sheet resistance observed for the first copper film suggests a slow evolution in grain size and a gradual reciystallization / growth process, indicating that the first copper film consists of metastable fine grains. In contrast, the second copper film exhibits a rapid rate of recrystallization / growth, achieving thermodynamic stability after just one day of self-annealing. Therefore, sheet resistance is another rapid and straightforward method that can be used for screening the chemistry involved in fine-grain processes.{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A

[0085] The invention will now be described in reference to following non-limiting examples.Examples:

[0086] A VSP Biologic Potentiostat was used to perform coupon-level testing with either a minicell, water jacket, or beaker. A water bath was used to control the plating temperature at 25°C. The anode was either an insoluble titanium anode or a soluble copper anode.Example 1:

[0087] A copper electroplating composition was prepared comprising:

[0088] A coupon was plated with the electroplating solution of Example 1 at a current density of about 3 ASD for about 150 seconds to deposit a copper film. Fig. 5 depicts various annealing conditions of the copper plated coupon. As seen in Fig. 5, the as plated copper layer has grain sizes of about 100 nm in a largest dimensions and the grain size remains at about 100 nm when the coupon is self-annealed for 4 weeks under ambient conditions (ambient temperature and pressure) and upon exposure to an annealing temperature of 150°C for 2 hours. Thereafter, when the copper film is annealed at a temperature of about 250°C for 2 hours in a nitrogen environment, the size of the copper grains increases to about 200 nm or more.

[0089] Compared to the four weeks of self-annealing, where the Cu grain size remains around 100 nm, annealing at 250°C increases grain size to about 200 nm. This anticipated strengthening of Cu bonding is attributed to the enhanced grain size resulting from reduced grain boundaries at the bonding annealing temperature.Example 2:

[0090] A copper electroplating composition was prepared comprising:{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A

[0091] A coupon was plated with the electroplating solution of Example 2 at a current density of about 1 ASD for about 270 seconds and 3 ASD for about 100 seconds.

[0092] Fig. 6 depicts various annealing conditions of the copper plated coupon. As seen in Fig.6, the as plated copper layer has an average grain size of about 100 nm and the grain size remains at about 100 nm when the coupon is exposed to an annealing temperature of 170°C for about 40 minutes.Comparative Example 3:

[0093] A copper electroplating composition was prepared comprising:

[0094] A coupon was plated with the electroplating solution of Comparative Example 3 at a current density of about 1.5 ASD for about 150 seconds and 300 seconds respectively.

[0095] Fig. 7 depicts various annealing conditions of the copper plated coupon. As seen in Fig. 7, the plated copper layer did not remain stable when the coupon is exposed to a temperature of 170°C for about 40 minutes or a temperature of 150°C for 2 hours.{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A

[0096] Recrystallization describes the spontaneous self-annealing of Cu films during storage at room temperature. Microstructural Evolution typically involves the movement of grain boundaries, and the equation determines the velocity of a grain boundary. v = M * 4G (1)M represents grain boundary mobility, and AG is the driving force. Mobility M is thermally active and can be expressed by:where T is temperature and Ea is activation energy.

[0097] Assuming that mobility is only a thermally activated process and remains consistent between the blanket and patterned wafers, the discrepancy in grain growth rate is attributed to the additional driving force in the Cu coating with the patterned wafer. Not all the Cu structure evolution on the blanket wafer can transfer to the patterned wafer.

[0098] Hybrid bonding, occurring without external force, involves the thermal expansion of Cu during elevated temperature annealing to facilitate Cu interdiffusion. The concept behind employing fine-grain Cu for low thermal budget hybrid bonding lies in utilizing the energy released during grain growth in the bonding process to promote Cu interdiffusion, thereby enhancing bonding strength. This energy can arise from grain boundary energy and stress changes resulting from grain growth. The key criterion is to design metastable fine-grain Cu capable of maintaining its fine-grain structure at room temperature for at least 2 to 3 weeks while allowing grain growth at bonding temperatures, typically about 200 to about 250°C. It is imperative to reduce the grain boundary velocity to ensure the stability of the metastable fine grain at room temperature for at least 2 to 3 weeks prior to bonding, as indicated in equation (1). The driving force AG, promoting the growth, originates from numerous factors such as grain boundaries, stacking faults, dislocation, surface energy, and elastic strain, with the opposite force due to the Zener pinning.

[0099] Zener pinning refers to the dispersion of fine particles which influences grain size distribution via movement of grain boundaries in a polycrystalline material. Grain size distribution in polycrystals has a significant impact on their properties including physical,{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A chemical, mechanical, and optical properties, among others. Controlling grain size is a vital aspect of material tailoring for specific applications. Reduced grain size in metals can increase their strength while maintaining ductility and toughness, while larger grain sizes are preferred for high-temperature applications due to their improved creep resistance.

[0100] The approach described herein involves meticulous chemistry design to lower the driving force while relying less on Zener pinning to extend the self-annealing duration.

[0101] Additionally, the investigation into grain growth during annealing at 250°C for 2 hours, as depicted in Fig. 5(d), reveals higher mobility at this elevated temperature, promoting significant grain growth.

[0102] The various approaches for assessing fine-grain structures and detail the screening methodology to develop the metastable Cu fin-grain process are delineated. Additionally, how the geometry constraints from patterned wafers induce variances in copper grain evolution between uniform blanket wafers and patterned wafers under ambient conditions were investigated. Metastable fine-grain copper is introduced highlighting its stability over four weeks of self-annealing and its resilience during annealing at 150°C for two hours (or 170°C for 40 minutes), with observable grain growth commencing when the annealing temperature is increased to 250°C for a period of time.

[0103] Finally, it should be understood that the following clauses and claims are intended to cover all of the generic and specific features of the invention described herein and all statements of the scope of the invention that as a matter of language might fall therebetween.

[0104] Clause 1 : A method of electrodepositing a copper film comprising a fine grain copper structure on a substrate, the method comprising the steps of: i) bringing the substrate into contact with an electrodeposition composition; and ii) depositing the copper film having the fine grain copper structure on the substrate, wherein a largest dimension of the copper grains in the copper film is less than about 200 nm; and wherein the largest dimension of the copper grains in the copper film remains at less than about 200 nm when the copper film is annealed at a temperature of up to and including 170°C for a period of time.

[0105] Clause 2: The method according to Clause 1, wherein the largest dimension of the copper grains in the copper film is less than about 150 nm when the copper film is annealed at a{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A temperature of up to and including 170°C for the period of time, preferably wherein the largest dimension of the copper grains in the copper film is less than about 100 nm when the copper film is annealed at a temperature of up to and including 170°C for the period of time.

[0106] Clause 3: The method according to Clause 1 or Clause 2, wherein the period of time is between 30 minutes and about 4 hours, preferably wherein the period of time is between about 30 minutes and about 3 hours, more preferably wherein the period of time is about 40 minutes to about 2 hours.

[0107] Clause 4: The method according to any of Clauses 1 to 3, comprising the step of exposing the copper film to temperatures up to and including 170°C for the period of time, more preferably exposing the copper film to temperatures up to and including 150°C for the period of time.

[0108] Clause 5: The method according to any of Clause 1 to 4, further comprising the step of self-annealing the copper film at ambient temperature and ambient pressure for at least 1 day or at least 7 days or at least 14 days or at least 21 days or at least 28 days.

[0109] Clause 6: The method according to Clause 5, wherein the step of self-annealing the copper film is performed prior to the step of exposing the copper film to temperatures up to and including 170°C for the period of time.

[0110] Clause 7: The method according to any of Clauses 1 to 6, wherein the copper grains grow by at least about 20% or more when the copper film is annealed at a temperature of 200°C or higher, optionally wherein the copper grains grow by at least about 20% or more when the copper film is annealed at a temperature of 250°C or higher.

[0111] Clause 8: The method according to any of Clauses 1 to 7, wherein the copper film is at least substantially free of impurities, optionally wherein the copper film contains less than 300 ppm impurities or less than 100 ppm impurities or less than 75 ppm impurities.

[0112] Clause 9: The method according to any of Clauses 1 to 8, wherein the copper electrodeposition composition comprises: a. a source of copper ions; b. a source of chloride ions; c. an acid;{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A d. one or more bath additives, wherein the one or more bath additives are selected from the group consisting of accelerators, levelers, suppressors, and combinations of the foregoing, optionally wherein the bath additives comprise: i) one or more accelerators, wherein the one or more accelerators comprises an organic sulfur compound; ii) one or more levelers, wherein the one or more levelers comprise a pyridyl -based leveler; and iii) one or more suppressors.

[0113] Clause 10: The method according to Clause 9, wherein the leveler has a molecular weight of about 10,000 g / mol or less, preferably wherein the leveler has a molecular weight of about 5,000 g / mol or less.

[0114] Clause 11 : The method according to any of Clauses 1 to 10, wherein the copper film exhibits a decrease in sheet resistance of less than 5% over a period of 7 days or more, preferably wherein the copper film exhibits a decrease in sheet resistance of between about 2% and about 5% over a period of 7 days or more.

[0115] Clause 12: The method according to any of Clauses 1 to 11, wherein the substrate with the copper film deposited thereon is brought into contact with a second substrate having the copper film deposited thereon, wherein the copper film of the substrate and the copper film of the second substrate are annealed, the grain size of the copper grains of the copper film increases and the copper film are joined together to form a copper-copper hybrid bond.

[0116] Clause 13: The method according to any of Clauses 1 to 12, wherein the substrate is selected from the group consisting of blanket substrates and patterned substrates, optionally wherein the substrate is a patterned substrate comprising one or more features.

[0117] Clause 14: The method according to Clause 13, wherein the substrate comprises a copper seed layer deposited thereon.

[0118] Clause 15: The method according to any of Clauses 1 to 14, wherein the electrodeposition step is performed at a current density within the range of about 0.1 to about 3.0 ASD for 2.5 minutes to 45 minutes, preferably wherein the current density is less than about 2 ASD, more preferably wherein the current density is less than about 1.5 ASD.{N6091519}

Claims

Attorney Docket No. 33225-37 (AES)-WO-AWHAT IS CLAIMED IS;1. A method of electrodepositing a copper film comprising a fine grain copper structure on a substrate, the method comprising the steps of: a) bringing the substrate into contact with an electrodeposition composition; and b) depositing the copper film having the fine grain copper structure on the substrate, wherein a largest dimension of the copper grains in the copper film is less than about 200 nm; and wherein the largest dimension of the copper grains in the copper film remains at less than about 200 nm when the copper film is annealed at a temperature of up to and including 170°C for a period of time.

2. The method according to claim 1, wherein the largest dimension of the copper grains in the copper film is less than about 150 nm when the copper film is annealed at a temperature of up to and including 170°C for the period of time.

3. The method according to claim 2, wherein the largest dimension of the copper grains in the copper film is less than about 100 nm when the copper film is annealed at a temperature of up to and including 170°C for the period of time.

4. The method according to any of claims 1 to 3, wherein the period of time is between 30 minutes and about 4 hours.

5. The method according to claim 4, wherein the period of time is between about 30 minutes and about 3 hours.

6. The method according to claim 5, wherein the period of time is about 40 minutes to about 2 hours.

7. The method according to any of claims 1 to 3, comprising the step of exposing the copper film to temperatures up to and including 170°C for the period of time.

8. The method according to claim 7, comprising the step of exposing the copper film to temperatures up to and including 150°C for the period of time.

9. The method according to claim 7, further comprising the step of self-annealing the copper film at ambient temperature and ambient pressure for at least 1 day or at least 7 days or at least 14 days or at least 21 days or at least 28 days.{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A10. The method according to claim 9, wherein the step of self-annealing the copper fdm is performed prior to the step of exposing the copper film to temperatures up to and including 170°C for the period of time.

11. The method according to any of claim 1 to 3, wherein the copper grains grow by at least about 20% or more when the copper film is annealed at a temperature of 200°C or higher.

12. The method according to claim 11, wherein the copper grains grow by at least about 20% or more when the copper film is annealed at a temperature of 250°C or higher.

13. The method according to any of claims 1 to 3, wherein the copper film is at least substantially free of impurities.

14. The method according to claim 13, wherein the copper film contains less than 300 ppm impurities.

15. The method according to claim 14, wherein the copper film contains less than 100 ppm impurities.

16. The method according to claim 15, wherein the copper film contains less than 75 ppm impurities.

17. The method according to any of claims 1 to 3, wherein the copper electrodeposition composition comprises: a. a source of copper ions; b. a source of chloride ions; c. an acid; d. one or more bath additives, wherein the one or more bath additives are selected from the group consisting of accelerators, levelers, suppressors, and combinations of the foregoing.

18. The method according to claim 17, wherein the bath additives comprise: a. one or more accelerators, wherein the one or more accelerators comprises an organic sulfur compound; b. one or more levelers, wherein the one or more levelers comprise a pyridyl-based leveler; and c. one or more suppressors.

19. The method according to claim 18, wherein the leveler has a molecular weight of about 10,000 g / mol or less.{N6091519}Attorney Docket No. 33225-37 (AES)-WO-A20. The method according to claim 19, wherein the leveler has a molecular weight of about 5,000 g / mol or less.

21. The method according to any of claims 1 to 3, wherein the copper fdm exhibits a decrease in sheet resistance of less than 5% over a period of 7 days or more.

22. The method according to claim 21, wherein the copper film exhibits a decrease in sheet resistance of between about 2% and about 5% over a period of 7 days or more.

23. The method according to any of claims 1 to 3, wherein the substrate with the copper film deposited thereon is brought into contact with a second substrate having the copper film deposited thereon, wherein the copper film of the substrate and the copper film of the second substrate are annealed, the grain size of the copper grains of the copper film increases and the copper film are joined together to form a copper-copper hybrid bond.

24. The method according to any of claims 1 to 3, wherein the substrate is selected from the group consisting of blanket substrates and patterned substrates.

25. The method according to claim 23, wherein the substrate is a patterned substrate comprising one or more features.

26. The method according to claim 23, wherein the substrate comprises a copper seed layer deposited thereon.

27. The method according to any of claims 1 to 3, wherein the electrodeposition step is performed at a current density within the range of about 0.1 to about 3.0 ASD for 2.5 minutes to 45 minutes.

28. The method according to claim 27, wherein the current density is less than about 2 ASD.

29. The method according to claim 28, wherein the current density is less than about 1.5 ASD.{N6091519}

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