WO2018057490A1

WO2018057490A1 - Copper plating method and composition for semiconductor substrates

Info

Publication number: WO2018057490A1
Application number: PCT/US2017/052185
Authority: WO
Inventors: Elie NAJJAR; Wenbo Shao; Vincent Paneccasio; Richard Hurtubise; John Commander; Ivan Li; Han Verbunt; Frank R. KRAMER; Pingping Ye; Thomas Richardson; Tao-Chi Liu
Original assignee: Macdermid Enthone Inc.
Priority date: 2016-09-22
Filing date: 2017-09-19
Publication date: 2018-03-29
Also published as: TW201823519A

Abstract

A process for electrodepositing a copper layer on a metalizing substrate is described. The metalizing substrate comprising a seminal conductive layer positioned on and in electrically conductive communication with a photovoltaic cell panel comprising a semiconductor material. The process comprises the steps of (i) contacting the metalizing substrate with an aqueous electrodeposition composition, and (ii) supplying electrolytic current to the aqueous electrodeposition composition to cause deposit of copper on the metalizing substrate. The aqueous electrodeposition composition comprises (a) a source of copper ions, (b) an acid, (c) chloride ions, and (d) a depolarizer comprising an organic sulfonate anion selected from the group consisting of an O-alkyl-S-sulfohydrocarbylxanthate, mercaptopropane sulfonate, bis(sulfopropyl)disulfide, N,N-dimethylamino-dithiocarbamoyl-l -propane sulfonate, acid hydrolysis products of said organic sulfonates, and mixtures of said organic sulfonates and hydrolysis products.

Description

COPPER PLATING METHOD AND COMPOSITION FOR SEMICONDUCTOR

SUBSTRATES CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of U.S. Provisional Application Serial No. 62/398,307, filed on September 22, 2016, the subject matter of which is herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

[0002] This invention relates generally to copper plating of semiconductor substrates, and more particularly to a process for applying conductive copper terminals on semiconductor materials.

BACKGROUND OF THE INVENTION

[0003] Electrochemical deposition processes are commonly used in integrated circuit fabrication. Metal line interconnections drive a need for sophisticated electrodeposition processes and plating tools that have evolved in response to a need for ever smaller current carrying lines in device metallization layers that are formed by electroplating metal into very thin, high-aspect ratio trenches and vias.

[0004] It is important that with the need for smaller feature sizes and finer pitches, an amount of electrical conductivity provided by the features is not compromised.

[0005] High density fan-out (HDFO) wafer-level packaging (WLP) is a plating technology that is aimed at improving package performance, shrinking a form factor, and driving down associated costs. Fan-out (FO) WLP is viewed as an alternative to through silicon via (TSV) technology. FOWLP involves a semi-additive process in which fine redistribution layer (RDL) lines are formed, copper is plated in a patterned area, and the photoresist is stripped and a barrier and seed layer etched from the substrate.

[0006] FO technology comprises electro depositing single layer copper RDL lines which vary in thickness from 10 microns to 100 microns and in spacing between adjacent lines of about 10 to about 100 microns. HDFO technology is directed to much finer pitch RDL lines which may have a thickness of about 2 to about 10 microns and in spacing between adjacent lines of between about 2 to about 20 microns. [0007] FO and HDFO WLP are described for example in U.S. Pat. Pub. No. 2017/0243839 to Buckalew et al. and in U.S. Pat. No. 9,012,269 to Jin et al., the subject matter of each of which is herein incorporated by reference in its entirety.

[0008] One issue with FO WLP is warpage which may result from the stress of the expansion and contraction of the components of the WLP and can cause curvature of the integrated circuit. This curvature, which is focused at the edges and corners of the integrated circuit, can result in poor solder joint formation in certain types of packages and can also result in lack of

functionality of the integrated circuit. Thus, it would be desirable to improve the

electrodeposition process to reduce warpage and cracking that result in reduced or no current flowing through the integrated circuit.

[0009] In addition, to maximize efficiency of solar energy collection, it is desirable to preserve a maximum fraction of the front face of a solar panel free of obstructions that can block exposure to the sun and other light sources. Solar panels have been developed in which both P- and N- terminals of the solar cell are located on the backside of the panel.

[0010] Essential features of a solar panel are schematically illustrated in Fig. 1, which depicts both P- and N-terminals located on the backside of the panel, leaving the front side unobstructed. Current is transmitted through these terminals for delivery to distribution networks from which power is drawn for ultimate commercial, industrial and domestic consumption.

[0011] As part of the effort to develop solar energy systems of competitive efficiency, the thickness of semiconductor solar panels has been progressively reduced. While the use of ultra- thin solar panels has advanced the state of solar technology, panels now being manufactured and used have thicknesses significantly less than a millimeter, rendering them fragile and susceptible to bending, warpage or fracture in handling and usage. Thin creases or cracks in a handful of panels of a solar array do not necessarily detract from the capability of the panels to receive and convert solar energy. Nor do they compromise the efficiency of the solar array, provided that integrity of transmission to an external circuit is preserved.

[0012] However, recovery of energy absorbed by a panel is compromised if damage to a semiconductor panel extends to terminals that convey power to the external circuit. To maintain the integrity of the terminals, the constituent copper must exhibit high tensile strength, high flexural strength, high ductility (elongation), and be characterized by relatively low internal stresses. [0013] U.S. Pat. No. 3,732,151 to Abbott describes an electrolytic plating process that is said to provide copper plating having excellent brightness and ductility. The plating bath comprises a mixture of sulfurized sulfonated aromatic or hydroaromatic hydrocarbons, amino derivatives of tiarylmethane, and a thionyl or thione that may have the structure =N-C(=S)-S-, e.g., 1,3,4- thiadiazole-2,5-dithiol or A²-l,3,4-thiadiazoIine-5-thion_} K salt. The sulfurized sulfonated aromatics used by Abbott are described in U.S. Pat. No. 2,424,887 to Henricks, i.e., sulfonated thioaromatics such as-tbianthrene, diphenyl sulfide, thiophenol and the like.

[0014] U.S. Pat. No. 2,828,252 to Fischer et al. describes a process for electrodeposition of copper from a plating bath that comprises a urea or thiourea brightener. According to the reference, it was also found advantageous to add a small proportion of an alkali metal xanthate to the bath.

[0015] U.S. Pat. No. 6,251,249 to Chevalier describes a process for electrodeposition of precious metals from a composition comprising an organosulfur compound and/or a carboxylic acid and a source of precious metal ion. Among numerous examples of organosulfur compounds are thiourea (substituted and unsubstituted), 3-S-thiuronium propane sulfonate, diethanol disulfide and ethyl xanthate. Semiconductor materials are listed among a variety of substrates on which the precious metal may be deposited.

[0016] U.S. Pat. Pub. No. 2014/0174936 to Hamm et al. describes processes for plating copper on semiconductors. Monovalent copper plating baths are used to metallize current tracks on the front side or emitter side of semiconductor wafers, which may be used in the manufacture of photovoltaic devices. A group of three wafers was electrolytically plated from a composition formulated from cuprous oxide (10 g/L), sodium metabisulfite (35 g L), and dimethylhydantoin (100 g/L), buffered to pH 7.5-7.8 with KOH. In an alternative example, copper was plated from a solution formulated from 5,5'dimethylhydantoin, copper sulfate pentahydrate, sodium sulfite, and triethylene tetramine, adjusted to pH 8 with NaOH. The specification states that the

electrochemical composition may contain one or more additional components, including brighteners, grain refiners and ductility enhancers.

[0017] U.S. Pat. Pub. No. 2011/0089044 to Isono describes a process for high speed plating of substrates that may include through holes and blind vias. The reference explains the need for a leveler that is effective under high agitation and at high temperature for high throughput plating of substrates that may include through holes and blind vias. The plating bath includes a nitrogen-containing compound and a sulfur-containing compound. High-speed copper electrolytic plating is carried out at temperatures above 35°C in the presence of a polymeric leveler obtained from reacting one mole of morpholine with two moles of epichlorohydrin in an acidic aqueous solution to obtain a reaction intermediate, and further reacting the intermediate with imidazole. High-speed copper electrolytic plating on substrates having through-holes, blind via holes, posts or the like is said to be achieved while preserving throwing power and ensuring the physical properties of the deposit.

[0018] Sulfur containing compounds used in the Isono Ό44 electrodeposition formulation can include an O-alkyl-S-sulfoalkylxanthate. Working examples that describe plating copper on a laminated substrate operate at 10 or 15 A/dm², but use only SPS as the sulfur compound. Other levelers include Janus Green or a copolymer of diallyldialkylammonium and sulfur dioxide. Deposited copper foil specimens are tested for tensile strength and elongation, but not for internal stress. Elongations measured in the 28-30% range for deposits formed from plating baths that contained SPS as the sulfur compound.

[0019] U.S. Pat. No. 7,220,347 to Isono describes plating baths for simultaneously filling blind vias and through holes in silicon wafers using an electrodeposition composition that contains a water soluble-copper salt, sulfuric acid, and a leveler selected from the group consisting of a homopolymer of a quaternary salt of vinylimidazolium, or a copolymer of quaternized vinylimidazolium and vinylpyrrolidone. As a brightener, the composition may further contain any of variety of divalent sulfur compounds, including O-alkyl-S-sulfoalkylxanthanates.

Example 3 describes filling blind via holes in a silicon wafer by electrodeposition of Cu from a bath comprising Cu sulfate pentahydrate (250 g L), sulfuric acid (40 g L), chloride ions (150 mg L), 0-ethyl-S-(3-propylsulfonic acid-l)dithiocarbonate potassium salt (0.1 mg/L), and ethylene glycol-propylene glycol copolymer (MW 1500; 0.1 mg L).

[0020] WO 01/83854 is directed to a composition and process for filling interconnect structures in a semiconductor device. The compositions contain copper sulfate, sulfuric acid, chloride ions, and a sulfur compound that serves as an accelerator at low concentrations and a suppressor at high concentrations, the overall range of which is 1 to 500 μπιοΐε/ΐ^ preferably 8 to 250 μπιοΙεβ/ Among the sulfidic accelerators is 1-propanesulfonic acid, 3- [(ethoxy)thiomethylthio], K salt. None of the working examples include any xanthic acid derivative. [0021] U.S. Pat. No. 6,776,993 to Too et al. describes electroplating chemistry for filling submicron features of VLSI and ULSI interconnects from a bath comprising a copper salt, sulfuric acid, chloride ions, a bath soluble organic divalent sulfur compound, and a bath soluble polyether. Among the variety of organic divalent sulfur compounds used in the formulation are MPS, SPS, and 1-propanesulfonic acid, 3-[(ethoxythioxomethyl)-thio-]-potassium salt:

[0022] U.S. Pat. No. 3,770,598 to Creutz describes compositions for electrodepositing ductile, lustrous, low stress copper on substrates such as rotogravure rolls and printed circuit boards that contain through holes. Example 1 describes an electrodeposition composition formulated from copper sulfate pentahydrate (2 oz/gal Cu ion), sulfuric acid (30 oz/gal), HC1 (30 ppm), thioxanthate-S -propane sulfonic acid (10 ppm), and the reaction product of polyethyleneimine (MW 600) and benzyl chloride. Examples 2 and 3 also contain thioxanthate S -propane sulfonic acid in a concentration of 10 ppm. Each of these compositions was used in plating a substrate that is not described. Full, bright, leveled deposits were said to be formed.

[0023] U.S. Pat. No. 2,849,351 to Gundel describes plating solutions that may contain any of a variety of sulfidic brighteners, including sulfonic acids derived from xanthic acid, having the structure:

0-C(=S)-S-RS0₃H

[0024] Example V provides a lustrous copper coating on an object by electrodeposition from a solution containing copper sulfate (170 g L), sulfuric acid (60g/L), and n-butylxanthic acid-n- propyl ester-oo-sodium sulfonate at a current density of 4 to 6 A/dm². The bath apparently did not include chloride ion. The xanthic acid derivative comprises O-butyl-S-sulfopropylxanthate [0025] DE Auslegeschrift 1 668 600 describes copper plating baths that contain the potassium salt of 0-ethyl-S-(2-sulfopropyl)xanthate (preferably 5 to 10 mg/L), copper sulfate (220 g L), sulfuric acid (60 g L), and alkylaryl polglycol ether (2 g/L). The compositions possess good throwing power and high gloss in the copper deposit.

[0026] JP 2012/021202 discloses a plating bath comprising Cu sulfate pentahydrate (100-250 g/L), sulfuric acid (20-150 g/L), chloride ion (20-200 mg/L), and a sulfur compound.

[0027] CN 103834972 discloses additives for a bath from which an ultrathin 4 μ copper foil may be electrolytically deposited. The English abstract refers to what may be a single additive comprising raw material components: ethylene thiourea (10-15 mg/L, hydroxyethylcellulose (70- 80 mg L) polyethylene glycol (45-55 mg/L, propylethyldithiocarbonate K sulfonate (90-110 mg//L), Ν,Ν-dimethylthio argonmethylacyl propanesulfonate Na (sic) (160-1 0 mg L). The additives are said to reduce surface roughness, increase crystallization of crystal grains, and increase the tensile strength and peel strength. The copper foil has a surface density of 34-38 g/mm2, room temperature tensile strength of at least about 30 MPa, RT elongation of at least about 3%, high temperature tensile strength of at least about 20 MPa, and a high temperature elongation of at least about 4%.

[0028] The foil is useful as a 4μπι thick negative electrode (anode) current collector for a Li battery. Such application is said to require high tensile strength, low elongation, and low roughness. The working examples describe preparation of an unsupported foil, apparently unsupported in use. The nature of any support surface on which the copper film might be electrolytically formed is not described. Focus of the disclosure is on weight ratios of the various components and the method of preparing the composition. The xanthate derivative of formula (3) may function as a leveler.

SUMMARY OF THE INVENTION

[0029] It is an object of the present invention to provide a method of plating copper on semiconductor substrates.

[0030] It is another object of the present invention to provide a method of plating copper on semiconductor substrates that exhibits little or no cracking.

[0031] It is another object of the present invention to provide a method of plating semiconductor substrates that exhibits less warpage.

[0032] It is another object of the present invention to provide a method of improving the integrity of copper lines of RDLs on FO wafer level packaging substrates. [0033] It is another object of the present invention to provide a method of applying conductive copper terminals on the back face of photovoltaic cell panels or panel components constructed of a semiconductor material.

[0034] It is still another object of the present invention to maximize efficiency of solar energy collection.

[0035] It is still another object of the present invention to provide a copper deposit that is substantially free of columnar grains.

[0036] It is yet another object of the present invention to provide a copper deposit having low levels of impurities.

[0037] Disclosed herein are an aqueous composition and process for electrodeposition of a copper layer on a metalizing substrate. The metalizing substrate may be positioned on and in electrical communication with a semiconductor material. Preferred embodiments of the process as described herein produce a copper deposit that exhibits high ductility, high tensile strength and low internal tensile stress.

[0038] In some preferred embodiments the electrodeposition process described herein is used to electrodeposit copper lines on RDLs of a FO WLP. In other preferred embodiments, the copper layer is electrodeposited on a metallizing substrate in electrical communication with a photovoltaic cell comprising a semiconductor material.

[0039] In the disclosed process, the metalizing substrate is contacted with an aqueous electrodeposition composition comprising a source of copper ions, an acid, a divalent sulfur compound or acid hydrolysis product thereof, and chloride ion. In particularly preferred embodiments, the divalent sulfur compound comprises an O-alkyl-S-sulfohydrocarbylxanthate anion. Electrolytic current is supplied to the aqueous electrodeposition composition to cause deposit of copper on the metalizing substrate.

[0040] Electrodeposition according to the disclosed process produces a copper deposit that continues to self-anneal after termination of the electrolytic current. The self-annealed deposit reaches a stable or metastable condition at which it exhibits high tensile strength, high elongation, and low internal stress.

[0041] Also described herein is a FO WLP comprising copper lines on an RDL having improved properties, less warpage and less cracking. [0042] Also described herein is a photovoltaic cell comprising a semiconductor panel having a front side adapted to receive light energy and a back side, alternating p- and n-doped regions on said back side, and copper tracks that are on said p- and n-doped regions and may be electrically connected to an external circuit for transmission of electrical energy from said cell to such circuit, said copper tracks having a thickness between about 20 and about 60 μηι and comprising copper deposits structured of polygonal grains having a number average grain size between 250 and 400 angstroms and free of internal tensile stresses greater than about 10 MPa.

[0043] Other objects and features will be in part apparent and in part pointed out

hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Fig. 1 is a schematic illustration of a solar panel assembly of the type wherein copper terminals for transmission of power collected by the panel are located on the back face of the panel.

[0045] Figs. 2 to 4 display the X-ray diffraction patterns for the copper deposits formed in Examples 1 to 3, respectively.

[0046] Figs. 5 and 6 display X-ray diffraction patterns for the deposits formed in Example 7 from plating baths containing MPS and SPS, respectively.

[0047] Fig. 7 displays the X-ray diffraction pattern for the copper deposit obtained in Example 9.

[0048] Fig. 8 displays the X-ray diffraction patterns for the deposits formed in Example 11.

[0049] Fig. 9 depicts a view of a WLP with a fine redistribution layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] The present invention relates generally to a process for electrodepositing a copper layer on a metalizing substrate, the metalizing substrate comprising a seminal conductive layer. The metalizing substrate may be positioned on and in electrically conductive communication with a semiconductor material.

[0051] The process generally comprises the step of:

contacting the metalizing substrate with an aqueous electrodeposition composition comprising:

a) a source of copper ions, b) an acid,

c) chloride ions, and

d) a depolarizer comprising an organic sulfonate anion selected from the group consisting of an O-alkyl-S-sulfohydrocarbylxanthate, mercaptopropane sulfonate, bis(sulfopropyl)disulfide, N,N-dimethylamino-dithiocarbamoyl-l -propane sulfonate, acid hydrolysis products of said organic sulfonates, and mixtures of said organic sulfonates and hydrolysis products, and

supplying electrolytic current to the aqueous electrodeposition composition to cause deposit of copper on the metalizing substrate.

[0052] In one embodiment, the depolarizer comprises an O-alkyl-S-sulfocarbylxanthate anion and/or an acid hydrolysis product thereof that corresponds to the formula:

R'-0-C(=S)-S-R²S0₃M wherein R¹ comprises an alkyi group, R² is a hydrocarbylene moiety, M is hydrogen or an alkali metal, and wherein R¹ and R² are selected such that the O-alkyl-S-sulfohydrocarbylxanthate and its acid hydrolysis products are compatible with the aqueous electrodeposition composition.

[0053] In one embodiment, the O-alkyl-S-sulfohydrocarbylxanthate anion R^]-0-C(=S)-S-R²S0₃ and/or its hydrolysis products are protonated in the electrodeposition composition.

[0054] In the electrodeposition process described herein, electrodeposition baths containing a depolarizer comprising an organic sulfonate anion selected from the group consisting of an O- alkyl-S-sulfohydrocarbylxanthate, mercaptopropane sulfonate, bis(sulfopropyl)disulfide, N,N- dimethylamino-dithiocarbamoyl-1 -propane sulfonate, acid hydrolysis products of said organic sulfonates, and mixtures of said organic sulfonates and hydrolysis products, have been found to produce a bright copper deposit of high ductility, satisfactory tensile strength, high flexural strength, and relatively low internal stress.

[0055] The combination of such properties renders the deposit highly suitable for use in fabricating solar cell assemblies for generation of electrical current. The copper deposit formed by electrodeposition on a photovoltaic semiconductor panel functions as a conductive track or connector for low resistivity electrical communication from the semiconductor panel to a bus in which current is collected from an array of solar cells and supplied to a circuit that carries an electrical load. [0056] To implement the electrodeposition process, an electrolytic circuit is formed comprising a conductive metalizing substrate on a semiconductor substrate (which may be a solar cell or other photovoltaic cell panel), an anode, the aqueous electrodeposition composition, and a power source having a positive terminal in electrically conductive communication with the anode and a negative terminal in electrically conductive communication with the metalizing substrate.

Preferably, the metalizing substrate is immersed in the electrodeposition composition. An electrolytic current is delivered from the power source to the electrolytic composition in the circuit, thereby depositing copper on the metalizing substrate.

[0057] Features of a solar panel having copper terminals that can be formed according to the disclosed process are schematically illustrated in Fig. 1. The panel comprises an n-type silicon or other semiconductor wafer 1 , the frontside face 3 of which is adapted to receive photon energy. The panel is doped on its backside to provide alternating p- and n-regions 5 and 7.

Conductive metalizing substrates 11, 13 are provided along the back face of each of the p- and n- regions. The metalizing substrates are typically copper seed layers formed by vapor deposition, preferably physical vapor deposition. Copper is electrodeposited onto each of the metalizing substrates as described further herein to provide copper tracks 15 and 17 which serve as terminals for conducting electricity from the cell to an exterior circuit.

[0058] In order to insure the integrity of the power circuit, the integrity of the copper terminals must be preserved which in turn requires a proper balance of tensile strength, tensile modulus, elongation, flexural strength, flexural modulus, elongation, and internal stress. For example, it is important that the copper terminals maintain their integrity even when cracks develop in the underlying semiconductor sheet. The cells can continue to function satisfactorily if there are cracks in the silicon wafer, but not if there are ruptures in the copper terminals. Satisfactory properties of the copper deposits for this purpose may be indicated by a bending test in which a silicon panel bearing copper terminals is bent along a line that intersects the copper terminal tracks. Desirably, the copper terminals provided by the disclosed process suffer no rupture even when the underlying silicon sheet is deformed beyond recovery, cracked, or broken. This can be confirmed by a standard bending test in which a panel is bent along a line that is transverse to copper metal deposits on the panel, and the deposits are observed to still be intact, and to remain intact, at a bending angle at which the semiconductor panel cracks or breaks. Integrity of the copper deposits can be corroborated by passage of current through the copper deposit over the region of the underlying crack or break.

[0059] Ultimate properties of a copper deposit vary with the grain structure. However, the association between grain structure and properties is not definitively known. Generally, finer grains yield a brighter deposit that has a potential for favorable tensile and elongation properties, but grain configuration can also have an influence, as may the presence, absence and nature of impurities in the copper deposit. While finer grain structure may enhance ductility, it may have an unfavorable effect on conductivity.

[0060] The grain structure and properties yielded by electrodeposition from a copper plating bath of any given composition are generally unpredictable before the process has been attempted. Copper deposition solutions typically contain organic additives, the mechanistic effect of which is imperfectly understood, as is the fate of the additives and their contribution, if any, to impurities in the deposit.

[0061] It has now been discovered that copper tracks or layers suitable as conductive terminals for solar cells and other semiconductor devices can formed by electrodeposition from an aqueous acidic plating composition that contains a depolarizer comprising an organic sulfonate anion selected from the group consisting of an O-alkyl-S-sulfohydrocarbylxanthate, mercaptopropane sulfonate, bis(sulfopropyl)disulfide, N,N-dimethylamino-dithiocarbamoyl-l -propane sulfonate, acid hydrolysis products of said organic sulfonates, and mixtures of said organic sulfonates and hydrolysis products. In one preferred embodiment, the depolarizer comprises a xanthate derivative corresponding to the structure:

R'-0-C(=S)-S-R²S0₃M

[0062] wherein R comprises an alkyl group, R is a hydrocarbylene moiety, M is hydrogen or an alkali metal, and R¹ and R² are selected such that the O-alkyl-S-sulfohydrocarbylxanthate is compatible with the aqueous electrodeposition composition, i.e., it is soluble at the desired concentration in the aqueous plating composition, it does not cause precipitation or otherwise react adversely with any other component of the bath, and performs its desired function without materially compromising the function of other components of the bath. [0063] In one embodiment, R¹ is selected from the group consisting of methyl, ethyl, and propyl, and butyl and R is selected from the group consisting of sulfopropyl, sulfobutyl, sulfoethyl, p- sulfobenzyl and o-sulfobenzyl. One particularly preferred O-alkyl-S-sulfoalkylxanthate anion is O-ethyl-S-sulfopropylxanthate.

[0064] The depolarizer is believed to function as a grain refiner that promotes formation of polygonal grains of moderate size having properties that conduce to a combination of brightness, tensile strength and ductility, which adapt the deposit for use as durable and reliable connector for a solar cell panel.

[0065] The plating bath contains a source of copper ions, an acid, a suppressor, chloride ions, and a source of the O-alkyl-S-sulfohydrocarbylxanthate and/or its acid hydrolysis product.

[0066] Optionally, the composition may further comprise a leveler. Where a leveler is present, it preferably comprises a poly(allylamine), and more preferably a polyallylamine having a molecular weight in excess of 1,500, more preferably between 5,000 and 20,000, optimally between 5,000 and 15,000. Other levelers that can be used include polyvinylamine, and dipyridyl polymers, e.g., the reaction product of a dipyridyl and an alkylating agent comprising bis(2-chloroethyl)ether. Other useful dipyridyl polymers are described in U.S. Pat. No.

8,388,824 to Paneccasio, Jr. et al., the subject matter of which is herein incorporated by reference in its entirety. Preferably, the suppressor is present in a concentration between about 200 and about 10,000 mg L, more preferably between about 500 and about 5,000 mg/1, most preferably between about 1,000 and about 3,000 mg/L.

[0067] In another embodiment, the electrodeposition composition is devoid of any functional concentration of a leveler, or at least devoid of a functional concentration any additive having a leveling effect other than a component that may be present primarily as a suppressor, e.g., polyethylene glycol.

[0068] The copper ion source is preferably a salt of a mineral acid or an alkanesulfonic acid. Particularly preferred sources of copper ion include copper sulfate and copper methanesulfonate. The plating bath is formulated from portions of source material sufficient to yield a composition that preferably comprises between about 30 and about 80 g/L, more preferably between about 40 and about 50 g/L, cupric ions.

[0069] The acid component is preferably sulfuric acid or an alkane sulfonic acid such as methanesulfonic or ethanesulfonic acid. Preferably, the acid is present in a concentration between about 40 and 120 g/L, more preferably between about 50 and about 100 g L, still more preferably between about 65 and about 85 g/L. The pH of the electrodeposition composition is preferably less than 4, e.g., between 1 and 3, more preferably less than 2.

[0070] In one embodiment, the electrodeposition composition further comprises a suppressor. As a suppressor, the electrodeposition composition may comprise polyethylene glycol, polypropylene glycol, a block copolymer of ethylene oxide and propylene oxide and

combinations thereof. In one preferred embodiment, the suppressor comprises polyethylene glycol, typically having a weight average molecular weight in the range of 5,000 to 50,000, more preferably 10,000 to 30,000. Another suitable suppressor is a low molecular weight

polyglycidol, e.g., having a molecular weight between 200 and 600. Other conventional suppressors may also be used. Concentration of the suppressor is preferably between about 500 and about 5,000 mg L, preferably between about 1,000 and about 3,000 mg/L.

[0071] Chloride ion is present in a concentration between 30 and 110 ppm, preferably between 50 and 90 ppm.

[0072] In formulating the electrodeposition composition, the source of the O-alkyl-S- sulfohydrocarbylxanthate may be provided in the form of the free sulfonic acid or a salt of the sulfonic acid, e.g., an alkali metal salt, preferably a sodium or potassium salt. In whatever form the anion is provided, it is substantially protonated in the acid electrodeposition composition. It is also subject to acid hydrolysis in the low pH electrodeposition composition. However, neither protonation nor acid hydrolysis of the O-alkyl-S-sulfohydrocarbylxanthate compromises the effectiveness of incorporating O-alkyl-S-sulfohydrocarbylxanthate as a depolarizer in promoting the formation of copper deposits having high tensile strength, high elongation and low internal stresses. The O-alkyl-S-sulfohydrocarbylxanthate is preferably incorporated at a concentration between about 1 and about 100 mg/L, more preferably between about 1.5 and about 75 mg/L, still more preferably between about 1.5 and about 60 mg/L, advantageously between about 2.5 and about 25 mg/L. In other embodiments, the depolarizer may be present in the composition at a concentration between about 5 and about 100 mg/L, or between about 10 and about 100 mg/L, or between about 15 and about 70 mg/L, or between about 15 and about 60 mg/L, or between about 20 and about 45 mg/L.

[0073] In various embodiments, other organic divalent sulfur compounds may be used, including, for example, organic sulfonate anions such as mercaptopropane sulfonate, bis(sulfopropyl)disulfide, Ν,Ν-dimethylamino-dithiocarbamoyl-l -propane sulfonate, acid hydrolysis products of any of these, combination of these divalent sulfur compounds and acid hydrolysis products, and combinations of one or more of the foregoing. As described herein, current supplied to an electrodeposition bath containing any one or more of these depolarizers deposits conductive copper tracks that have high ductility and low internal stress at values conducive to the function of the copper deposit as a conductive track on the back face of a photovoltaic cell. All of these organic divalent sulfur compounds are also effective to provide favorable distributions of crystal orientation in the copper deposit. Electrodeposition baths comprising the O-alkyl-S-sulfohydrocarbylxanthate depolarizers are preferred because they most consistently yield low stress copper deposits at high current densities, best preserve throwing power of the composition and process, and generally allow a greater latitude in process conditions that are effective for producing bright, highly ductile and low stress deposits.

However, all the divalent sulfur compounds described herein are effective to produce copper deposits of high ductility, optimal grain size, and low resistivity.

[0074] In one embodiment, the metallizing composition may contain, for example, copper ions in a concentration between about 30 and about 80 g/L, sulfuric acid in a concentration between about 50 and about 100 g L, the depolarizer in a concentration between about 5 and about 50 mg/L, polyethylene glycol having a molecular weight between 5000 and about 50,000 in a concentration between about 100 and about 4,000 g/L, and chloride ions in a concentration between about 30 and about 100 mg/L.

[0075] The metalizing substrate comprises a seminal conductive layer that may, for example, comprise a copper seed layer or a conductive polymer layer. A seed layer may be provided by chemical vapor deposition onto the semiconductor substrate.

[0076] In the case of metallization of a photovoltaic cell, the p+ and n+ regions are coated with silicon dioxide, an etch resist is applied to the silicon dioxide layer except for the locations of contact holes, silicon dioxide is removed by etching to provide contact holes, and a metal stack is deposited on the exposed p+ and n+ regions to provide a conductive structure to which the subsequently deposited copper can securely adhere. Electrodeposition of copper then proceeds.

[0077] As described, for example, in U.S. Pat. No. 7,388,147 to Mulligan et al., the subject matter of which is herein incorporated by reference in its entirety, a thin (approximately 400 nm) 3-layer seed metal stack may be sputtered or evaporated onto the solar cell for contacts to the p+ and n+ regions. The first layer of the stack, aluminum in the preferred embodiment, makes ohmic contact to the semiconductor material and acts as a back surface reflector. In thin silicon solar cells, weakly absorbed infrared radiation passes through the thickness of silicon and is often lost by absorption in backside metallization. In one embodiment, the seed layer covers mostly silicon oxide, except in small contact openings where it contacts the silicon. The metallized silicon oxide stack is designed to be an excellent infrared reflector, reflecting light back into the cell and effectively multiplying the absorption path length. The front surface texture in combination with the back surface reflector can increase the optical path length to more than twenty times the wafer thickness. This design feature leads to higher photo-generated current in the solar cell.

[0078] In one embodiment, a second layer can be deposited on and in contact with the semiconductor material of the photovoltaic panel. The copper seed layer is then deposited on and in contact with the barrier layer. In one embodiment, titanium- 10 /tungsten-90% (TiW) can be used as the barrier layer and acts as a diffusion barrier to metals and other impurities. A third layer, copper (Cu) in the preferred embodiment, is used to provide a base or strike layer for initiating electroplating of metal. Alternatively, chromium, nickel, ruthenium or tantalum nitride can be used as the barrier layer instead of TiW. Because the seed layer, a Al(Si) TiW/Cu stack in the preferred embodiment, is not required to have significant current-carrying capacity, it can be made very thin. The metal layer comprises a Al(Si)/TiW/Cu stack, where the aluminum provides ohmic contact and back surface reflectance, TiW acts as the barrier layer, and Cu acts as the plating base. Alternatively, chromium (Cr) can be used as the barrier layer instead of TiW. The metal semiconductor contact can be annealed in a forming gas atmosphere, preferably at 400 °C. Alternatively, the contact anneal step can be eliminated.

[0079] For the electrodeposition step, either an inert anode or a copper anode may be used.

Anode to cathode area ratio is preferably at least 1 : 1. In preferred embodiments, the anode consists essentially of copper.

[0080] In the plating of photovoltaic cells, the plating process is preferably conducted at a relatively high current density, for example, at least about 10 A/dm² or at least about 15 A/dm². Generally, the current density is between about 10 and about 30 A/dm², more typically between about 15 and about 25 A/dm², and advantageously between about 20 and about 25 A/dm² based on the cathodic surface area of the metalizing substrate. The temperature of electrodeposition composition temperature is preferably elevated, for example, about 25° to about 50°C, more preferably between about 30° and about 40°C. Productivity is further enhanced by electrodeposition at about 40° to about 50°C, though with some combinations of depolarizer, suppressor and leveler, internal stress levels can become higher as the temperature approaches 50°C.

[0081] High electrodeposition bath temperature enhances the conductivity of the bath, reduces the resistance to mass transfer at the cathodic surface, and thus conduces to establishing and maintaining a high current density without undue anodic polarization. Within a plating cycle of 50 to 90 minutes, a copper deposit having a thickness between about 25 to 40 μπι can be formed at a cathodic current density in the range of about 20 to about 25 A/dm² and a temperature in the range of 25° to about 45°C. Copper deposition rates of at least 15 μηι hr, preferably between about 18 and about 30 μΐΉ/hour, can be achieved without compromising the integrity, uniformity and functional properties of the deposit. Final thickness of the uniform deposit may range from about 20 to about 60 μηι, or between about 30 and about 50 μηι. Without unduly extending the plating cycle, or the residence time in a continuous plating line, thicknesses of about 50 μπι can be consistently obtained at industrial production rates.

[0082] In the plating of copper lines on RDLs in FO WLP, the plating process is preferably conducted at a relatively high current density, for example, at least about 1 A/dm² or at least about 10 A/dm². Generally, the current density is between about 1 and about 8 A/dm², more typically between about 1 and about 6 A dm , and advantageously between about 1 and about 4 A/dm² based on the cathodic surface area of the metalizing substrate. The temperature of electrodeposition composition temperature is preferably at room temperature, for example about 25° to about 30°C. A view of a WLP with fine redistribution lines is shown in Figure 9 in which a silicon die is disposed on a wafer carrier and fine RDL are disposed thereon.

[0083] In the plating of copper lines of photovoltaic silicon cells, high electrodeposition bath temperature enhances the conductivity of the bath, reduces the resistance to mass transfer at the cathodic surface, and thus conduces to establishing and maintaining a high current density without undue anodic polarization. Within a plating cycle of 50 to 90 minutes, a copper deposit having a thickness between about 25 to 40 μπι can be formed at a cathodic current density in the range of about 20 to about 25 A/dm and a temperature in the range of 25° to about 45°C.

Copper deposition rates of at least 15 μπι/ΙΐΓ, preferably between about 18 and about 30 μΓη/hour, can be achieved without compromising the integrity, uniformity and functional properties of the deposit. Final thickness of the uniform deposit may range from about 20 to about 60 μιη, or between about 30 and about 50 μιη. Without unduly extending the plating cycle, or the residence time in a continuous plating line, thicknesses of about 50 μπι can be consistently obtained at industrial production rates.

[0084] Uniformity may be measured by the deviation from perfect planarity of the deposit. For example, the process as described herein has the capability of forming a copper deposit on the substrate that does not deviate at any location by more than 15 μηι, more typically not more than 10 μπι from perfect planarity, deviation being measured by the difference in thickness between the thickest and thinnest 1 cm² regions of the deposit. The process is further capable of providing a substantially uniform deposit that is essentially free of surface nodules.

[0085] It has been observed that re-nucleation and re-crystallization of copper occurs substantially throughout the course of electrodeposition. After the desired thickness of the copper deposit has been obtained, the electrodeposition current is terminated and the copper plated metalizing substrate is removed from contact with the electrodeposition composition, i.e., the solar panel bearing the plated metalizing substrate is withdrawn from the plating bath. It has been observed that the copper deposit continues to self-anneal for at least about 24 hours after the electrodeposition of copper is terminated by either removal of the panel bearing the metalizing substrate from the plating bath or by termination of the electrolytic current. Self- annealing comprises continuing re-crystallization of the copper deposit. After about 24 to about 48 hours of self-annealing proceeds, the condition of the copper deposit asymptotically approaches a stable or metastable state. It has also been observed that after 48 hours of self- annealing, the substantially stable or metastable copper deposit consists essentially of moderately fine polygonal grains of compact configuration.

[0086] Use of a depolarizer comprising an O-alkyl-S-sulfohydrocarbylxanthate or acid hydrolysis product thereof has been found to form a copper deposit of superior brightness, especially when the depolarizer is present in the plating bath at a concentration in the range of about 10 to about 100 mg/L, more preferably about 15 to about 60 mg L, optimally about 20 to about 45 mg L, and a bath temperature between room temperature and about 35°C.

[0087] In the stable or metastable state achieved after 48 hours of self-annealing, a copper deposit that is formed on the semiconductor substrate may typically exhibit an elongation of at least about 10%, more typically between about 15% and about 25%, or most typically between about 18% and about 21% when subjected to external tensile stress. Use of a depolarizer comprising an O-alkyl-S-sulfohydrocarbylxanthate or its acid hydrolysis product under the conditions described herein is particularly conducive to increase the ductility of the copper deposit, producing a deposit of high ductility so that it can be deformed to a considerable degree in manufacturing, shipping and handling of solar cells with minimal risk of rupture. Control of conditions to achieve the desired ductility can result in some sacrifice in tensile strength. But in copper tracks for connection of solar cells to an external circuit, inadequate ductility is more conducive to failure than low tensile strength. By "high ductility" what is meant is that the plated metalizing substrate can be bent along a line that is transverse to the copper deposit, and the deposit remains intact at the bending angle at which the underlying semiconductor panel cracks or breaks.

[0088] When conditions are controlled as described herein to achieve the elongation values summarized above, tensile strength remains high enough to provide a copper deposit of excellent toughness that is entirely adequate for the service contemplated. The stable or metastable deposit is substantially free of internal stresses, for example, when applied to thin solar panels having a thickness between about 300 μηι and about 400 μηι, internal stresses in the copper deposit do not cause warpage of the semiconductor panel. Thus, the substantially stable or metastable copper deposit exhibits low internal stress and high ductility. Thus, it has been observed that self-annealing increases the ductility of the copper deposit. More specifically, the process is capable of forming an electrodeposit wherein internal tensile stress is not greater than about 10 MPa, and in most instances not greater than about 6 MPa after self-annealing.

[0089] As described herein, the present invention also relates generally to a photovoltaic cell comprising a semiconductor panel having a front side adapted to receive light energy and a back side, alternating p- and n-doped regions on said back side, and copper tracks that are on said p- and n-doped regions and may be electrically connected to an external circuit for transmission of electrical energy from said cell to such circuit, said copper tracks having a thickness between about 20 and about 60 μηι and comprising copper deposits structured of polygonal grains having a number average grain size of at least about 250 angstroms, or between about 250 and about 500 angstroms, or between 250 and 400 angstroms, or between 300 and 400 angstroms, and free of internal tensile stresses greater than about 10 MPa. [0090] The photovoltaic cell has a front side adapted to receive light energy, a back side, and alternating p- and n-doped regions on said back side, and wherein a seminal conductive layer comprising a metalizing substrate is applied to a doped region on the back side, the

electrodeposition composition is brought into contact with the metalizing substrate, and current is supplied to the electrodeposition composition to deposit copper on said metalizing substrate. In a preferred embodiment, seminal conductive layers comprising metalizing substrates are applied to a plurality of n- and p-doped regions on the back side of the photovoltaic cell, the electrodeposition composition is brought into contact with each of the metalizing substrates, and current is supplied to the electrodeposition composition to deposit copper on each of the metalizing substrates.

[0091 ] The present invention also relates generally to a FO WLP comprising a fine redistribution layer and copper tracks or lines electrodepo sited on the RDL, said copper tracks having a thickness between about 5 and about 15 μηι and comprising copper deposits structured of polygonal grains having a number average grain size of at least about 250 angstroms, or between about 250 and about 500 angstroms, or between 250 and 400 angstroms, or between 300 and 400 angstroms, and free of internal tensile stresses greater than about 10 MPa.

[0092] The copper deposits are also substantially free of columnar grains, meaning that the copper deposits are free of weak physical properties that can lead to RDL cracking under harsh process conditions.

[0093] In one embodiment, the substantially stable or metastable copper deposit exhibits an elongation of at least about 10%, or between about 15 and about 25%, or between about 18 and about 21% when subjected to external stress.

[0094] The substantially stable or metastable copper deposit also preferably exhibits an internal ^" deposit stress of between about 5 and about 35 and a tensile strength of between about 300 and about 410 MPa.

[0095] Especially where the temperature is above 40°C and the concentration of the O-alkyl-S- sulfohydrocarbyxanthate is at the high end of the 1.5 to 50 mg/L range, crystal orientations <111> and <200> have been found to predominate in the stable or metastable deposit. Subjecting the copper deposit to X-ray diffraction after it has been allowed to self-anneal for 48 hours produces an X-ray diffraction pattern in which the ratio of the X-ray diffraction intensity for Miller Index orientation <111> to the sum of all X-ray diffraction intensities is between about 0.4 and about 0.7, the ratio of the X-ray diffraction intensity for Miller Index orientation <200> to the sum of all X-ray diffraction intensities is between about 0.2 and about 0.6, and the ratio of the X-ray diffraction intensity for Miller Index orientation <220> to the sum of all X-ray diffraction intensities in the deposit is between about 0 and about 0.2.

[0096] The use of an O-alkyl-S-sulfohydrocarbylxanthate additive also promotes a relative predominance of crystal orientation having a Miller index of <111>. For example, the ratio of the X-ray diffraction intensity for orientation <111> to the X-ray diffraction intensity for orientation <200> is at least about 1.0, at least about 1.2, at least about 1.3 , or at least about 1.5, or between 1.1 and 5, or between 1.5 and 4.0. Higher depolarizer concentrations, e.g., above 15 mg/L, especially 20 to 50 mg/L, are particularly conducive to a high ratio of the X-ray diffraction intensity for crystal orientation <111> relative to the intensity for crystal orientation <200>. This ratio can be as high as high as 3, but it can vary widely based on selection of additives and temperature.

[0097] A high ratio of <111> intensity plus <200> intensity to <220> indicates high ductility. The <111> and <200> form grain boundaries much less than other crystal orientations such as columnar. Such boundaries within the crystal structure determine whether the copper deposit bends, stretches, or breaks in response to stresses imposed in testing or service.

[0098] In another embodiment, X-ray diffraction analysis of the copper deposits produces an X- ray diffraction pattern in which the ratio of the X-ray diffraction intensity for Miller index orientation <220> to the sum of all X-ray diffraction intensities is not greater than 0.4, more preferably not greater than 0.33 and the ratio of the sum of the of X-ray diffraction intensities for Miller index orientations <111> and <200> to the sum of all X-ray diffusion intensities is at least 0.60, more preferably at least 0.67.

[0099] Without being bound to a particular theory, it is believed that the O-alkyl-S- sulfoalkylxanthate component functions to depolarize the substrate, enabling relatively high current density and high productivity. However, additives in the electrodeposition composition do not function to establish any significant polarization gradient such as would conduce to preferentially depositing copper in one region of the metalizing substrate in preference to another (as is the case, for example, in preferential bottom-filling of a concavity such as a submicron via feature or a through silicon via). [0100] It has been observed that the grains of the self-annealed copper deposit are relatively fine, but not exceptionally so. The configuration of the grains is not primarily columnar. Instead, the grains assume a relatively compact polygonal shape. This appears to reflect re-nucleation and re-crystallization throughout at least a significant fraction of the electrodeposition cycle, or substantially throughout the cycle. Examined microscopically, 95% by weight of the copper grains in the stable or metastable deposit are typically found to have a principal dimension of at least about 250 angstroms, e.g., between about 250 angstroms and about 500 angstroms, more typically between about 300 and about 400 angstroms. The number average dimension of the polygonal grains may also fall within these ranges. The substantially stable or metastable copper deposit also preferably has a resistivity of between about 1.85 and about 3.0 μθΐιπι. It has also been observed that when panels are plated with the electrolytic composition described herein, the plated panel can be bent along a line transverse to the copper deposit, and the deposit remains intact at the bending angle at which the underlying semiconductor panel cracks or breaks.

[0101] Impurity levels in the copper deposit are relatively low. For example, after three to four days of self-annealing, a substantially stable or metastable copper deposit from a two additive system comprising a depolarizer and a suppressor typically has a total impurity content of less than about 35 ppm, more typically between about 20 and about 30 ppm. After the same period, a substantially stable or metastable copper deposit from a three additive system that also contains a leveler typically has a total impurity content less than about 60 ppm, more typically between about 40 and about 50 ppm. The substantially stable or metastable copper deposit from the two additive system contains not greater than about 10 ppm, typically between about 3 and about 7 ppm carbon impurities, not greater than about 15 ppm, typically between about 7 and about 11 ppm oxygen impurities, not greater than about 8 ppm, typically between about 3 and about 7 ppm chlorine impurities, not greater than about 9 ppm, typically between about 4 and about 8 ppm sulfur impurities, and not greater than about 1 ppm, typically between and about 0.01 and about 0.2 ppm nitrogen impurities. The substantially stable or metastable copper deposit from the three additive system contains not greater than about 17 ppm, typically between about 10 and about 14 ppm carbon impurities, not greater than about 12 ppm, typically between about 7 and about 10 ppm oxygen impurities, not greater than about tl5 ppm, typically between about 10 and about 13 ppm chlorine impurities, not greater than about 15 ppm, typically between about 10 and about 13 ppm sulfur impurities, and not greater than about 1 ppm, typically between about 0.01 and about 0.2 ppm nitrogen impurities.

[0102] Resistivity of the deposit is typically no greater than about 2.1 microohm-cm.

[0103] Although the use of an O-alkyl-S-sulfohydrocarbylxanthate generally provides a superior balance of properties, i.e., ductility, internal stress, brightness, and crystal orientation, it has further been found that copper tracks for connection to an external circuit can be deposited from an electrodeposition bath that contains other divalent sulfur compound depolarizers, most particularly, mercaptopropane sulfonate (MSP), bis(sulfopropyl)disulfide (SPS), N,N- dimethylamino-dithiocarbamoyl-l -propane sulfonate (DPS), acid hydrolysis products of these organic sulfonates, and mixtures of such organic sulfonates and hydrolysis products. An electrodeposition bath comprising an O-alkyl-sulfohydrocarbylxanthate depolarizer provides excellent throwing power, and throwing power is preserved as the electrodeposition bath ages in commercial manufacturing. MPS can compromise throwing power, and MPS is readily formed upon hydrolysis of SPS. However, a bath comprising MPS or SPS can maintain good throwing power for operations extending, for example, to about 50 to about 100 amp-hours per liter. MPS and SPS function most effectively in a cell that uses consumable copper anodes. Where copper tracks are deposited on a semiconductor panel that is substantially planar, the adverse effect of lower throwing power is attenuated, as compared, for example, to a plating process for printed circuit board through holes, or for filling through silicon vias or submicron vias in an integrated circuit system.

[0104] Use of a consumable copper anode is advantageous in maintaining a constant and optimum concentration of copper ions in the electrolytic bath, and in inhibiting or preventing release of oxygen at the anode. Where oxygen release is not adequately suppressed, it not only detracts from anode current efficiency but can also result in anode passivation from formation of a copper oxide film over a copper anode surface. Oxygen generation at a copper anode can be inhibited by establishing and maintaining an anode to cathode electrodic surface area ratio of at least about 1.5, preferably higher, for example, at least about 2.0, 2.5 or 3.0. Various process and equipment options are known to the art for maintaining a desired minimum ratio of the area of consumable anode(s) to the relatively fixed area of the cathode.

[0105] Formation of other by-products at the anode can be inhibited by conditioning the anode by ramping the DC current in a plating bath that contains copper sulfate and chloride ions without any additives. For example, the anodic current can be initiated at 5 A/ft² for 90 minutes, thereafter ramped to 10 A/ft for 2 hours, and 20 A/ft for another 4 hours. Alternatively, the conditioning bath can contain a suppressor. Such conditioning forms a black, low impedance cupric oxide film that does not peel. Conditioning in this manner helps not only to inhibit byproduct formation, but also to reduce breakdown of brightener and suppressor. The

concentration of chloride ion in the conditioning solution is preferably in the range between about 35 and about 75 ppm.

[0106] Reverting to Fig. 1, dimensions of a semiconductor photovoltaic panel may typically be in the range of 5" x 6" to 6" x 5" or 5" x 5" to 6" by 6" with a thickness in the range of about 250 to about 400 μιη, preferably no greater than about 350 μπι. More generally, the ratio of the surface area on one side of the panel to the thickness of such ultra-thin panel is at least about 40,000 mm. Where copper connector tracks having a thickness of about 20 to 60 μιη are applied to such thin panels, the panel can be subject to warpage if there are excessive internal stresses in the copper deposit. But it has been found that when copper connector tracks are deposited according to the process as described herein, the stresses are generally less than 10 MPa, and low enough not to cause material warpage.

[0107] Warpage potential of the copper deposit can be evaluated using a standard 5 in. x 5 in. x 300 μπι thick semiconductor panel on which a 30 μηι copper layer fully covering one side of the panel has been deposited by the process described herein. It is desirable that the internal stress in the copper layer be insufficient to cause substantial warpage of the photovoltaic panel. The degree of warpage may be equated to the vertical displacement of a free edge of the panel when the panel is placed on a planar support surface and constrained by forcibly maintaining an edge segment of the panel diametrically opposite the free edge segment in contact with the support surface, provided that the panel is of sufficient planarity prior to deposit of copper so that the subjection of the panel to the same constraint would have limited vertical displacement of the free edge to no greater than about 300 to 400 angstroms.

[0108] Alternatively, warpage is measured on a standard semiconductor panel of the same thickness and lateral dimensions as described above, and having the same thickness of copper fully covering one side as further described above, by resting the panel convex side up on a planar support and measuring the peak height of the deformed panel above the planar support. Where the panel is deformed symmetrically, the value obtained by this alternative measurement will be essentially one half the value obtained from the method first described above.

[0109] In an industrial process for forming copper conductive connectors on the back side of solar cells, a series of cells is passed upwardly through the plating bath in parallel with a vertical anode that is immersed in the bath. At the high current densities which prevail in the process, a deposit of the desired thickness can be achieved in a residence time of about 30 to about 90 minutes. For a series of panels drawn through the bath on a carrier web, and an anode having a height of, e.g., 10 feet immersed in the bath, 20 to 30 panels can be plated per hour in a single longitudinal array, a rate that is multiplied in a process where there are several longitudinal arrays abreast on the carrier web.

[0110] The following examples illustrate the disclosed process.

Example 1

[0111] A series of four electrolytic plating baths was prepared, each containing copper sulfate (55 g/1 Cu²⁺ ion), sulfuric acid (75 g/I) and chloride ion (70 mg/i). Each of the baths contained a polyglycidol suppressor in a concentration of 400 mg/1. Three of the baths contained 3- mercaptopropane sulfonic acid (MPS) at concentrations of 5 mg/1, 20 mg/1 and 50 mg/1, respectively. Copper was deposited from each of the compositions onto a brass plate in a Hull cell at a current density of 5 A/dm² for 3 minutes at room temperature.

[0112] Focused ion beam and scanning electron microscope examination of the deposits showed that copper deposited from the bath containing only polyglycidol additive formed columnar crystals while the copper deposited from the baths containing the polyglycidol additive plus either 5 mg/1 or 20 mg/1 MPS formed a bright deposit composed of relatively compact polygonal crystals. X-ray diffraction analysis showed that crystal orientation <220> predominated in copper deposited from the bath containing only the polyglycidol additive, but crystal orientation was predominantly <111> and <200> in copper deposited from baths containing MPS. Also, the grain size was significantly larger in copper deposited from the bath that contained only the 320 additive. Twin bands decreased in the diffraction pattern of copper deposited from the baths containing MPS. X-ray diffraction patterns for the copper deposits from the respective plating baths of this example are presented in Fig. 2. Example 2

[0113] The plating process of Example 1 was repeated except that additive

bis(sulfopropyl)disulfide (SPS) was substituted for additive MPS. XRD analysis again showed that crystal orientation <220> predominated in copper deposited from the bath containing only the polyglycidol additive, but crystal orientation was predominantly <111> and <200> in copper deposited from baths containing MPS. X-ray diffraction patterns for the copper deposits from the respective plating baths of this example are presented in Fig. 3.

Example 3

[0114] The plating process of Example 1 was repeated except that O-ethyl-S- sulfopropylxanthate was substituted for MPS. XRD analysis again showed that crystal orientation <220> predominated in copper deposited from the bath containing only the polyglycidol additive, but crystal orientation was predominantly <111> and <200> in copper deposited from baths containing O-ethyl-S-sulfopropylxanthate. X-ray diffraction patterns for the copper deposits from the respective plating baths of this example are presented in Fig. 4.

Example 4

[0115] From the plating bath of Examples 1 to 3 that contained only LP1 additive, copper was deposited to a thickness of 10.0 μπι in a Hull cell at room temperature (23°C) and current densities varying from 25 to 200 A/ft² (2.69 to 21.5 A/dm²). Internal tensile stress was measured in these deposits promptly after termination of the electrolytic deposition, and again 1 day, 2 days, 3 days and 4 days after termination. Significant self-annealing was observed over the first 24 hours, and modest further self- annealing in the following 48 hours. Table 1 summarizes the internal stresses in the deposits as a function of the current density and the self-annealing period following termination of the electrodeposition. Table 1

[0116] Internal stress generally increased with current density. At each current density, internal stress asymptotically approached a stable or metastable value after self-annealing for 48 hours.

Example 5

[0117] Four plating baths, one containing no MPS and the other three containing additive MPS at concentrations of 5 mg/1, 20 mg/I and 50 mg/1, respectively, were prepared as described in Example 1. From each of these baths, copper was deposited on a brass cathode in a Hull cell at a current density of 175 A/ft (18.8 A/dm ) and room temperature. Internal tensile stress was measured in each of these deposits promptly after termination of the electrolytic deposition current, and again 1 day and 2 days after termination. The results are summarized in Table 2.

Table 2

[0118] It may be observed that the presence of additive MPS at a concentration of 5 mg/L materially reduced the internal stress in copper deposited at a current density of 175 A/ft , that increasing the additive concentration to 20 mg/L further reduced the internal stress, that increasing the additive concentration to 50 mg/L had essentially no further effect, and that all the deposits approached a stable or metastable condition with respect to internal stress after 24 hours.

[0119] Copper deposits from plating baths that contained additive MPS were bright. As measured by focused ion beam analysis, these deposits were found to consist predominantly of relatively compact polygonal crystals, the prevalence of which modestly increased as the deposits self-annealed. ·

Example 6

[0120] Example 5 was repeated except that additive O-ethyl-S-sulfopropyl-xanthate was used instead of MPS in the plating baths that contained an additive of this nature. Results are summarized in Table 3.

Table 3

The copper deposits of this Example that contained O-ethyl-S-sulfopropylxanthate showed generally the same characteristics and trends in properties as those deposited from the baths containing additive MPS as described in Example 5.

Example 7

[0121] A series of electrodeposition bath was prepared containing Cu sulfate (55 g/1 Cu²⁺), sulfuric acid (75 g/L), chloride ion (70 mg/L), additive Cu320 LPl (4 ml/L) and MPS additive concentrations of 0 mg/L, 5 mg/L, 20 mg/L and 50 mg/L, respectively. [0122] Copper was deposited from each of these plating baths onto a brass substrate in a Hull cell at a current density of 5 A/dm². Electrodepositions at this current density from the bath that contained no MPS additive were conducted at bath temperatures of 23°C (room temperature) and 30°C. From the baths that contained MPS additive, electrodepositions were conducted at 23°C, 30°C and 50°C.

[0123] In the deposits formed from the bath that contained no additive MPS, crystal orientation <220> predominated at both 23° and 30°C to the extent of substantial exclusion of orientations <11 1> and <200>, while in deposits formed from the baths that contained any of the several concentrations of MPS, crystal orientations <111> and <200> generally predominated. In deposits formed at room temperature from baths that contained MPS, <111> and <200> predominated to the near exclusion of <220>. The relative proportion of <220> increased slightly as the plating temperature rose in deposits formed from compositions containing MPS in concentrations of 5 mg L or 20 mg L. However, at 50 mg L MPS and 50°C, the prevalence of orientation <220> spiked to levels substantially greater than either <111> or <200>. Thus, concentrations of significantly greater than 20 mg/1 MPS are neither necessary nor desirable at the relatively high plating temperatures that are otherwise preferred because of their favorable effect on bath conductivity, mass transfer and plating rate. The X-ray diffraction patterns for the various MPS-based compositions of this example at 23°, 30° and 50°C are displayed in Fig. 5.

[0124] Similar experiments conducted using SPS rather than MPS additive revealed that <220> is substantially suppressed and both <111> and! <200> strongly promoted over a plating bath temperature range of 23° to 50°C and additive SPS concentrations of 5 mg/L, 20 mg/L and 50 mg/L, as shown in Fig. 6.

Example 8

[0125] Plating compositions were prepared containing Cu sulfate (55 g L Cu²⁺), sulfuric acid (75 g L), chloride ion (70 mg/L) and either no further additive or additive MPS at a concentration of 5 mg/L, 20 mg L or 50 mg/L. A control was prepared that had the same concentrations of copper sulfate, sulfuric acid and chloride ion, but did not contain an O-alkyl-S- sulfohydrocarbylxanthate additive. Copper deposits were formed from each of these

compositions at room temperature, 30°C and 50°C at a current density of 175 A/ft². The deposits from each run were allowed to self-anneal for two days after termination of electro deposition current, after which internal stress was determined for each copper deposit. Results of this example are set forth in Table 4.

Table 4

[0126] Focused ion beam analysis and scanning electron microscopy revealed that the copper crystals deposited in the absence of MPS were mainly columnar shaped independently of temperature. In general, copper consisting of columnar shaped grains possesses high strength, but the presence of many twins inside these grains could lead to decreased ductility. FIB and SEM analyses further indicate that, in the presence of additive MPS, a more compact polygonal shaped crystal structure is formed, which favors a higher ductility than the columnar deposit.

Example 9

[0127] Electrodepositions were conducted in the manner generally described in Example 1 from baths that contained copper sulfate (55 g/L Cu), sulfuric acid (75 g/L) and chloride ion (70 mg/L) and either additive MPS, additive SPS or additive O-ethyl-S-sulfopropylxanthate at

concentrations of either 5, 20 or 50 mg L and a current density of 175 A/ft².

[0128] Electrodepositions were conducted from the bath containing MPS at temperatures of 23° and 30°C, from the bath containing SPS at 30°C, and from the bath containing O-ethyl-S- sulfopropylxanthate at 23°, 30° and 50°C. The deposits from each run were allowed to self- anneal for three days, after which a grain size analysis was done on each of the deposits. Grain size was also determined on Day 0 for deposits formed at 23 °C from baths containing either no additive or additive MPS and at 30°C from baths containing either MPS or SPS. The results are summarized in Table 5. Table 5

[0129] It may be observed that grain size decreased with increase in working bath temperature in the presence of additives. However, after three days of self-annealing, the grain size increased to reach a stable phase.

[0130] Deposits from the bath that contained only a carrier at 175 ASD had small grain size uncharacteristic of a matte deposit. In contrast, the copper deposit from a bath which contains both carrier and additives such as MPS had a larger grain structure that is unusual for conventional bright copper deposits. Typically, a conventional plating bath with matte surface deposit at normal current density has a very large grain size of at least 1.0 μπι.

[0131] In plating baths containing 1, 4 or 10 ml/L O-ethyl-S-sulfopropylxanthate, the presence of the additive strongly promoted formation of a deposit in which crystal orientations <111> and <200> were predominant and <220> was suppressed. However, at 50°C and 5 mg L MPS, <220> again became competitive with <111> and <200>, at 50°C and 20 mg/1 O-ethyl-S- sulfopropylxanthate, Cu <220> again became the most prominent crystal orientation, and at 50°C and 50 mg L O-ethyl-S-sulfopropylxanthate, Cu <220> became overwhelmingly predominant while <111> and <200> were almost entirely suppressed. X-ray diffraction patterns for the compositions of this example at 23°, 30° and 50°C are displayed in Fig. 7.

[0132] Also at 50°C, it was observed that internal stress was lower without any additive than it was in deposits plated from formulations containing either 5 mg L, 20 mg L or 50 mg/L O-ethyl- S-sulfopropylxanthate.

Example 10

[0133] Electrodepositions were carried out as described in Example 8 except that additive SPS was used in place of additive MPS. Stress analysis yielded the data in Table 6.

Table 6

Example 11

[0134] In electrodeposition experiments and X-ray diffraction analyses similar to those described in above Examples, it was found that additive DPS at concentrations of 5 mg/L, 20 mg/L and 50 mg/L promoted Cu <111> and <200> at the expense of <220> at room temperature. At 30°C₅ and 5 mg/L DPS, Cu <220> becomes competitive with <111> and both exceed <200>, but <220> is substantially suppressed in favor of <111> and <200> at DPS concentrations of 5 mg/L and 50 mg/L. The pattern reverses at 50°C where 5 mg/L and 20 mg/L DPS are entirely ineffective for altering the predominance of <220>, but 10 ml/L DPS shows partial success in promoting <111> and <200>, though <220> remains the most prominent. The X-ray diffraction patterns for the various MPS-based compositions of this example at 23°, 30° and 50°C are displayed in Fig. 8.

Example 12

[0135] Electrodepositions were carried out as described in Example 8 except that additive D- ethyl-S-sulfopropylxanthate (SC A3) was used in place of additive MPS. Stress analysis yielded the data in Table 7:

Table 7

Example 13

[0136] Tensile tests were conducted to determine the elongation of the copper deposits formed in Example 8 at 30°C from baths containing LP1 (400 mg/L) and MPS (20 mg/L and 50 mg L). Elongation was 19.39% for the deposit formed from the bath containing 20 mg/1 MPS and 19.35% for the deposit formed from the bath containing 50 mg/L MPS.

Example 14

[0137] Tensile tests were conducted to determine the elongation of the copper deposits formed in Example 13 at 30°C and 50°C from baths containing LP1 (4 ml/L) and O-ethyl-S- sulfopropylxanthate (20 mg/L and 50 mg L). Elongation was 19.50% for the deposit formed from the bath containing 50 mg/L MPS at 30°C. Example 15

[0138] Three copper plating bath were as prepared, each containing Cu sulfate (40 g L Cu), sulfuric acid (145 g L), chloride ion (65 mg L), an O-alkyl-S-sulfohydrocarbylxanthate, and a leveler. A first bath contained leveler LP1 (20 mg L) and MPS (50 mg L). A second bath contained LP1 (400 mg L) and O-ethyl-S-sulfopropylxanthate (50 mg L), while the third contained C20 (2000 mg/L) and (50 mg/L).

[0139] When introducing elements of the present invention or the preferred embodiments(s) thereof, the articles "a", "an", "the" and "said" are intended to mean that there are one or more of the elements. The terms "comprising", "including" and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0140] 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.

[0141] As used herein, "room temperature" refers to a temperature of from about 20°C to about 30°C, more preferably between about 22 to about 28°C and may be about 22-24°C.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

[0142] As various changes could be made in the above compositions and processes without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

WHAT IS CLAIMED IS:

1. A process for electrodepositing a copper layer on a metalizing substrate, the metalizing substrate comprising a seminal conductive layer positioned on and in electrically conductive communication with a semiconductor material, the process comprising:

a) a source of copper ions,

b) an acid,

c) chloride ions, and

d) a depolarizer comprising an organic sulfonate anion selected from the group

consisting of an O-alkyl-S-sulfohydrocarbylxanthate, mercaptopropane sulfonate, bis(sulfopropyl)disulfide, Ν,Ν-dimethylamino-dithiocarbamoyl- 1 -propane sulfonate, acid hydrolysis products of said organic sulfonates, and mixtures of said organic sulfonates and hydrolysis products, and

2. The process as set forth in claim 1, wherein the depolarizer comprises an O-alkyl-S- sulfocarbylxanthate anion and/or an acid hydrolysis product thereof that corresponds to the formula:

R¹-0-C(=S)-S-R²S0₃M wherein R¹ comprises an alkyl group, R² is a hydrocarbylene moiety, M is hydrogen or an alkali metal, and wherein R¹ and R² are selected such that the O-alkyl-S-sulfohydrocarbylxanthate and its acid hydrolysis products are compatible with the aqueous electrodeposition composition.

3. The process as set forth in claim 2, wherein the O-alkyl-S-sulfohydrocarbylxanthate anion R¹-0-C(=S)-S-R²S0₃ and/or its hydrolysis products are protonated in said electrodeposition composition.

4. The process as set forth in claim 1, wherein the pH of said electrodeposition composition is less than 4.

5. The process as set forth in claim 3, wherein R¹ is selected from the group consisting of methyl, ethyl, and propyl, and butyl and R is selected from the group consisting of sulfomethyl, sulfopropyl, sulfobutyl, sulfoethyl, p-sulfobenzyl and o-sulfobenzyl.

6. The process as set forth in claim 1, wherein the aqueous electrodeposition composition further comprises a suppressor.

7. The process as set forth in claim 1, wherein the aqueous electrodeposition composition further comprises polyethylene glycol, the polyethylene glycol having a weight average molecular weight between about 5,000 and about 50,000.

8. The process as set forth in claim 1, wherein the acid is selected from the group consisting of sulfuric acid and alkane sulfonic acids.

9. The process as set forth in claim 1, wherein the aqueous electrodeposition composition further comprises a leveler,

wherein the leveler is selected from the group consisting of polyallylamine,

polyvinylamine, and dipyridyl polymers.

10. The process as set forth in claim 1, wherein the aqueous electrodeposition composition is free of any component having a leveling function other than polyethylene glycol.

11. The process as set forth in claim 1, wherein the concentration of the depolarizer in the aqueous electrodeposition composition is between 1 and 100 mg/L.

12. The process as set forth in claim 1, wherein the aqueous electrodeposition composition comprises copper ions in a concentration between about 30 and about 80 g/L, sulfuric acid in a concentration between about 50 and about 100 g/L, the depolarizer in a concentration between about 5 and about 50 mg L, polyethylene glycol having a molecular weight between 5000 and about 50,000 in a concentration between about 100 and about 4,000 g/L, and chloride ions in a concentration between about 30 and about 100 mg/L.

13. The process as set forth in claim 1, wherein the temperature of the aqueous

electrodeposition composition during electrolytic deposition of copper on the metalizing substrate is between about 25°C and about 50°C.

14. The process as set forth in claim 1, wherein electrolytic current is supplied at a rate of at least 15 A/dm² based on cathodic surface area of the metalizing substrate.

15. The process as set forth in claim 14, wherein copper is deposited on said metalizing substrate at a rate of at least 15 μηΊ hr.

16. The process as set forth in claim 1, wherein electrodeposition current is supplied until the thickness of the copper deposit is at least about 20 μπι.

17. The process as set forth in claim 16, wherein said copper deposit is of substantially uniform thickness.

18. The process as set forth in claim 1, wherein the copper deposit is allowed to self-anneal for at least 24 hours after electrodeposition of copper is terminated by removal of the metalizing substrate bearing the deposit from contact with the electrodeposition composition and/or termination of the electrolytic current, and

wherein the copper deposit approaches a stable or metastable state after self-annealing for 24 to 48 hours, the copper deposit exhibiting low internal stress and high ductility.

19. The process as set forth in claim 18, wherein internal tensile stress in said substantially stable or metastable copper deposit is not greater than 10 MPa.

20. The process as set forth in claim 18, wherein said substantially stable or metastable copper deposit exhibits an elongation of at least 10%, or between about 15% and about 25%, or between about 18% and about 21% when subjected to external tensile stress.

21. The process as set forth in claim 1, wherein the metalizing substrate is positioned on the backside of a photovoltaic cell panel, the photovoltaic cell comprising the semiconductor material.

22. The process as set forth in claim 18, wherein the substantially stable or metastable copper deposit has a resistivity no greater than about 2.1. microohm-cm.

23. The process as set forth in claim 1, wherein the electrodeposition composition comprises the depolarizer and a suppressor but not a leveler, and the total impurity content of said substantially stable or metastable copper deposit is less than 35 ppm or between 20 and 30 ppm.

24. The process as set forth in claim 23, wherein the stable or metastable copper deposit contains not greater than 10 ppm carbon impurities, not greater than 15 ppm oxygen impurities, not greater than 8 ppm chlorine impurities, not greater than 9 ppm sulfur impurities, and not greater than 1 ppm nitrogen impurities.

25. The process as set forth in claim 1, wherein the seminal conductive layer comprises a copper seed layer.

26. The process as set forth in claim 25, further comprising depositing a barrier layer on and in contact with the semiconductor material of said photovoltaic panel, and depositing said copper seed layer on and in contact with the barrier layer.

27. The process as set forth in claim 26, wherein the barrier layer comprises tantalum nitride or ruthenium.

28. The process as set forth in claim 1, wherein the semiconductor comprises a photovoltaic cell, wherein the photovoltaic cell has a front side adapted to receive light energy, a back side, and alternating p- and n-doped regions on said back side, and wherein a seminal conductive layer comprising a metalizing substrate is applied to a doped region on the back side, the

electrodeposition composition is brought into contact with the metalizing substrate, and current is supplied to the electrodeposition composition to deposit copper on said metalizing substrate.

29. The process as set forth in claim 28, wherein seminal conductive layers comprising metalizing substrates are applied to a plurality of n- and p-doped regions on the back side of the photovoltaic cell, the electrodeposition composition is brought into contact with each of the metalizing substrates, and current is supplied to the electrodeposition composition to deposit copper on each of the metalizing substrates.

30. The process as set forth in claim 1, wherein the metalizing substrate comprising a fine redistribution layer electrically connected to a semiconductor substrate.

31. A photovoltaic cell comprising a semiconductor panel having a front side adapted to receive light energy and a back side, alternating p- and n-doped regions on said back side, and copper tracks that are on said p- and n-doped regions and may be electrically connected to an external circuit for transmission of electrical energy from said cell to such circuit, said copper tracks having a thickness between about 20 and about 60 μπι and comprising copper deposits structured of polygonal grains having a number average grain size between 250 and 400 angstroms and free of internal tensile stresses greater than about 10 MPa.

32. A photovoltaic cell as set forth in claim 31 , where the copper deposits are substantially free of columnar grains.

33. A photovoltaic cell as set forth in claim 31 , wherein X-ray diffraction analysis of the copper deposits produces an X-ray diffraction pattern in which the ratio of the X-ray diffraction intensity for Miller index orientation <220> to the sum of all X-ray diffraction intensities is not greater than 0.4 and the ratio of the sum of the of X-ray diffraction intensities for Miller index orientations <111> and <200> to the sum of all X-ray diffusion intensities is at least 0.60.

34. A photovoltaic cell as set forth in claim 33, wherein the ratio of the X-ray diffraction intensity for Miller index orientation <220> to the sum of all X-ray diffraction intensities is not greater than 0.33 and the ratio of the sum of the of X-ray diffraction intensities for Miller index orientations <111> and <200> to the sum of all X-ray diffusion intensities is at least 0.67.

35. A photovoltaic cell as set forth in claim 31, wherein X-ray diffraction analysis of the copper tracks produces an X-ray diffraction pattern in which the ratio of the X-ray diffraction intensity for Miller index orientation <111> to the sum of all X-ray diffraction intensities is between about 0.4 and about 0.7, the ratio of the X-ray diffraction intensity for Miller index orientation <200> to the sum of all X-ray diffraction intensities is between about 0.2 and about 0.6 and the ratio of the X-ray diffraction intensity for Miller index orientation <220> to the sum of all X-ray diffraction intensities in the deposit is between about 0 and about 0.2.

36. A photovoltaic cell as set forth in claim 35, wherein the ratio of the X-ray diffraction intensity for Miller index orientation <111> to the X-ray diffraction intensity for Miller index orientation <200> is at least about 1.0, or between 1.1 and 5, or between 1.5 and 4.0.

37. A photovoltaic cell as set forth in claim 36, wherein the ratio of the X-ray diffusion intensity for Miller index <111> to the X-ray diffusion intensity for Miller index <200> is at least about 1.2, at least about 1.3 or at least about 1.5.

38. A semiconductor device comprising a redistribution layer, wherein the redistribution layer is metalized using the method of claim 1.