Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/18—Electroplating using modulated, pulsed or reversing current
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/38—Electroplating: Baths therefor from solutions of copper

Definitions

• the present invention is directed to a reverse pulse plating composition and method. More specifically, the present invention is directed to a reverse pulse plating composition and method that reduces brightener decomposition and reduces defects of an electroplated metal layer.
• compositions and methods for electroplating articles with metal layers or coatings may involve passing a current between two electrodes in a plating composition or solution where one of the electrodes is an article to be metal plated.
• a plating solution may contain (1) dissolved copper (cupric ions), usually copper sulfate, (2) an acid electrolyte such as sulfuric acid in an amount sufficient to impart conductivity to the solution, and (3) additives to improve efficiency of the plating reaction and the quality of the metal deposit.
• additives include, for example, surfactants, brighteners, levelers, suppressants, and corrosion inhibitors.
• Metals that may be electroplated include, for example, copper, copper alloys, nickel, tin, lead, gold, silver, platinum, palladium, cobalt, chromium, and zinc. Electrolytic metal plating solutions are used for many industrial applications. For example, they may be used in the automotive industry as base layers for subsequently applied decorative and corrosion protective coatings. They also may be used in the electronics industry, such as in the fabrication of printed circuit or wiring boards, and for semiconductor devices. For circuit fabrication in a printed circuit board, a metal such as copper is electroplated over selected portions of the surface of a printed circuit board and onto the walls of through-holes passing between the surfaces of the circuit board base material. The walls of the through-holes are metallized to provide conductivity between circuit layers on each surface of the board.
• Dog boning is believed to be caused by a voltage drop between the top surface of the through-hole and the center of the through-hole.
• the potential drop is a function of current density, a ratio of the length of the through-hole to the through-hole diameter (aspect ratio) and board thickness. As the aspect ratio and the thickness of the board increase, dog boning becomes more severe due to a voltage drop between the surface of the board and the center of the through-hole.
• This voltage drop is believed to be caused by a combination of factors including solution resistance, a difference in surface to through-hole over potential due to mass transfer, i,e., a difference in the flow of solution through the through-hole compared to the movement of the solution over the surface of the board, and a charge transfer difference as a consequence of the concentration of solution additives in the through-hole compared to the surface.
• the printed circuit board industry continuously seeks greater circuit densification.
• the industry has resorted to multi-layer circuits with through-holes or interconnections passing through multiple layers, Multi-layer circuit fabrication results in an overall increase in the thickness of the board and a concomitant increase in the length of an interconnection passing through the board. This means that increased circuit densification results in increased aspect ratios and through-hole length and an increase in the severity of the dog boning problem.
• aspect ratios may exceed ten to one.
• Another problem encountered in metal electroplating are defects such as intermittent surface roughness and non-uniform surface appearance of the plated metal. Intermittent surface roughness and non-uniform surface appearance are believed to be caused by non-uniform current distribution across the surface of the printed wiring board that is being plated. The non-uniform current distribution results in an unequal or uneven deposit of metal on the board surface resulting in the surface roughness and non-uniformity of plated metal layers.
• Whiskers are believed to be crystals of the metal being plated and grow out of the plated surface. Whiskers may range in diameter of from less than 1 micron to as large as several millimeters. Although the cause of whisker growth has been the subject of some debate, there is no question that whiskers are undesirable for a variety of electrical, mechanical, and cosmetic reason. For example, whiskers are readily detached and carried by cooling air flows into electronic assemblies, both within and external to electronic article housings, where they may cause short-circuit failure.
• Plating metal is a complex process that involves multiple ingredients in a plating bath.
• many plating baths contain chemical compounds that improve various aspects of the plating process.
• Such chemical compounds or additives are auxiliary bath components that are used to improve the brightness of the metal plating, the physical properties of the plated metal especially with respect to ductility and throwing power of an electroplating solution or bath, Throwing power of the solution defined as the ratio of current density flowing at the center of the through-hole to the current density flowing at the surface of the through-hole.
• Throwing power of the solution defined as the ratio of current density flowing at the center of the through-hole to the current density flowing at the surface of the through-hole.
• Optimum throwing power is achieved when the current density at the center of the through-hole is the same as the current density flowing at the through-hole surface. However, such a current density is difficult to achieve.
• a main concern is additives that have an effect on the bright finish, leveling and uniformity of metal deposition on surfaces. Maintaining bath concentrations of such additives within close tolerances is important to obtain high quality metal deposits.
• the additives do breakdown during metal plating, The additives breakdown due to oxidation at the anode, by reduction at the cathode, and by chemical degradation.
• Reverse pulse plating is an electroplating process where the electrical current is alternated between a cathodic current (forward pulse) and an anodic current (reverse pulse) during the electroplating process.
• Typical pulses or waveforms are a reverse to forward voltage ratio of 3 to 1 and times of 10 to 20 milliseconds for the forward waveform and 0.5 to 1 millisecond for the reverse.
• waveforms often result in undesirable intermittent surface roughness and non-uniform surface appearance on plated metal layers, especially at current densities of 100 amps/cm 2 .
• optimum bath performance is continuous (from 6 months to at least a year).
• the short life of a reverse pulse plating bath is due to additive breakdown, especially due to the build-up of brightener by product.
• the rate at which byproducts form is primarily governed by the brightener concentration and secondarily by the idle time at which the by product is formed on an anode surface.
• Reverse pulse plating often uses high brightener concentrations, i.e., in excess of 1 ppm (part per million), to help prevent or reduce poor performance in leveling, throwing power and corner cracking. Poor throwing power results in rough metal surfaces and non-uniform metal layers. Corner cracking is a condition where the plated metal layer begins to separate from the plated substrate.
• high brightener concentrations may result in high concentrations of byproducts, which may shorten the electroplating bath life. Accordingly, there is a need for an improved reverse pulse plating composition or bath and an improved reverse pulse plating method to address the foregoing problems.
• the methods prevent or at least reduce dendrite or whisker formation on metal plated substrates, reduce dog boning as well as intermittent surface roughness, and provide a uniform metal layer on the substrates.
• Other advantages include improved leveling performance, improved throwing power and reduced corner cracking. Also additive decomposition is reduced to provide electroplating baths having a longer operating life.
• a primary objective of the present invention is to provide a method of metal plating a substrate that reduces metal plating defects.
• Another objective is to provide a method of plating a metal that has an improved throwing power.
• compositions include chloride ion and brighteners in concentration ratios of from 20:1 to 125: 1, and a brightener concentration preferably of 0.001 ppm to 1.0 ppm.
• the compositions also may include other additives depending on the particular function of the composition.
• the present invention is a reverse pulse plating method to electroplate a metal on a substrate
• An electromotive force (emf) is generated from a suitable electrical source to provide an electric field around an electroplating apparatus including an anode, cathode and a composition including chloride ions and brighteners in a concentration ratio of from 20:1 to 125:1 and metal ions.
• the anode, cathode and composition are in electrical communication with each other to provide a complete electrical circuit with the source of the electromotive force.
• the cathode is the substrate on which the metal is plated.
• the electric field around the electroplating apparatus is modified to provide (i) a cathodic current (forward pulse or waveform) followed by an anodic current (reverse pulse or waveform); (ii) a cathodic current followed by an anodic current (reverse pulse or waveform) followed by cathodic DC current (direct current), (iii) a cathodic current followed by an anodic current (reverse pulse or waveform) followed by equilibration (open circuit); (iv) a cathodic current followed by an anodic circuit (reverse pulse or waveform) followed by cathodic DC current (direct current) then followed by equilibration (open circuit); or combinations of pulse patterns (i), (ii), (iii), or (iv) provided that the net result of the pulse electroplating process results in a metal layer formed on the substrate to be metal plated.
• Net current for each pattern or combination of patterns is in the cathodic or plating direction.
• cathodic current AC or alternating current
• a metal is being plated on the cathode, while during anodic current metal is being removed or stripped from the cathode.
• cathodic DC current metal is again being plated on the cathode, and during equilibration there is no metal being deposited on the cathode or stripped from the cathode. There is no plating or stripping during equilibration because the electrical circuit is open and there is no emf to plate or strip.
• each pulse pattern and the time duration during an electroplating process of each pulse pattern and their respective waveforms, DC currents and equilibrations may vary depending on the dimensions of the substrate and the desired thickness of the metal layer(s), Reverse to forward voltage ratios range from 1.5 to 5.5, preferably from 2.5 to 3.5.
• the pulse patterns provide for reduced intermittent surface roughness and improved uniform metal layer(s) in contrast to many conventional pulse plating patterns.
• the pulse plating patterns also have improved throwing power in contrast to many conventional pulse plating patterns.
• pulse patterns examples include pulse pattern (i) by itself during the entire electroplating process; a combination of pulse patterns (i) and (ii); a combination of pulse patterns (i), (ii) and (iii); a combination of pulse patterns (i), (ii), (iii), and (iv); or a combination of pulse patterns (i), (iii) and (iv).
• the particular order of each pulse pattern and the time duration of each including their respective waveforms, DC currents and equilibrations may vary depending on the dimensions of the substrate and the desired thickness of the metal layer(s), Some minor experimentation may be employed to determine which combination of pulse patterns and duration of the pulse patterns optimize the electroplating process for a given substrate. Such minor experimentation is common in the electroplating art to optimize electroplating processes.
• a preferred pulse pattern is (i) a cathodic current (forward pulse or waveform) followed by an anodic current (reverse pulse or waveform).
• Current densities may range from 5 milliamps(mA)/cm 2 to 200 mA/cm 2 , preferably from 5 mA/cm 2 to 125 mA/cm 2 , more preferably from 5 mA/cm 2 to 50 mA/cm 2 .
• Forward pulses range in time from 40 milliseconds (ms) to 1 second, preferably from 40 ms to 800 ms, and reverse pulses may range from 0.25 ms to 15 ms, preferably from 1 ms to 3 ms for pulse pattern (i).
• Forward pulses range from 40 ms to 1 second, preferably from 40 ms to 800 ms and reverse pulses range from 0.25 ms to 15 ms, preferably from 1 minute to 10 ms, and the DC current ranges from 5 seconds to 90 seconds, preferably from 10 seconds to 60 seconds for pulse pattern (ii).
• Forward pulses range from 40 ms to 1 second, preferably from 40 ms to 800 ms and reverse pulses range from 0.25 ms to 15 ms, preferably from 1 minute to 10 ms, and the equilibration ranges from 5 seconds to 90 seconds, preferably from 10 seconds to 60 seconds in pulse pattern (iii)
• Forward pulses range from 40 ms to 1 second, preferably from 40 ms to 800 ms
• reverse pulses range from 0.25 ms to 15 ms, preferably from 1 minute to 10 ms
• DC current ranges from 5 seconds to 90 seconds, preferably from 10 seconds to 60 seconds
• equilibration ranges from 5 seconds to 90 seconds, preferably from 10 seconds to 60 seconds for pulse pattern (iv).
• Pulse times, pulse patterns and applied voltages of the cathodic and anodic waveforms may be adjusted to provide that the overall process is cathodic, i.e., there is a net deposition of metal on a substrate. Workers may adapt the pulse time waveforms and their frequencies to a particular application based on the teachings of the process of the invention.
• suitable copper compounds include copper halides, copper sulfates, copper alkane sulfonate, copper alkanol sulfonate, or mixtures thereof. Such copper compounds are water-soluble.
• a sufficient amount of a copper salt is included in the electroplating composition such that the concentration of the copper ion , preferably ranges from 0.01 to 100 grams/liter, more preferably from 0.10 grams/liter to 50 grams/liter.
• Solvents of the electroplating composition may be water or an organic solvent such as alcohol or other suitable organic solvent employed in electroplating. Mixtures of solvents also may be employed.
• Sources of chloride ion include any suitable chloride salt or other source of chloride that is soluble in the electroplating compositions solvent.
• chloride ion sources are sodium chloride, potassium chloride, hydrogen chloride (HCl), or mixtures thereof.
• a sufficient amount of chloride ion source is included in a composition such that the chloride ion concentration ranges from 0,02 ppm to 125 ppm, preferably from 0.25 ppm to 60 ppm, more preferably from 5 ppm to 35 ppm.
• Brighteners that may be employed in the methods of the present invention include any brightener that is suitable for copper to be electroplated, Brighteners may be specific for the metal that is plated, Workers in the art are familiar with the particular brightener that may be employed with copper. Brighteners are included in the electroplating compositions at a range of from 0,001 ppm to 1.0 ppm, preferably from 0.01 ppm to 0.5 ppm, more preferably from 0.1 ppm to 0.5 ppm. Thus, chloride to brightener concentrations of the compositions range from 20:1 to 125:1, preferably 25:1 to 120:1, more preferably from 50:1 to 70:1.
• chloride ion to brightener are suitable for reducing or preventing whisker formation, corner cracking and brightener byproduct formation during electroplating, copper. Such chloride to brightener ratios also improves leveling, and throwing power of an electroplating bath in copper electroplating.
• suitable brighteners include sulfur containing compounds that have a general formula S-R-SO 3 , where R is substituted or unsubstituted alkyl or substituted or unsubstituted aryl group. More specifically, examples of suitable brighteners include compounds having structural formulas HS-R-SO 3 X, XO 3 -S-R-S-S-R-SO 3 X or XO 3 -S-Ar-S-S-Ar-SO 3 X where R is a substituted or unsubstituted alkyl group, and preferably is an alkyl group having from 1 to 6 carbon atoms, more preferably is an alkyl group having from 1 to 4 carbon atoms; Ar is an aryl group such as phenyl or naphthyl; and X is a suitable counter ion such as sodium or potassium.
• Such compounds include n,n-dimethyl-dithiocarbamic acid-(3-sulfopropyl)ester, carbonic acid-dithio-o-ethylester-s-ester with 3-mercapto-1-propane sulfonic acid (potassium salt), bissulfopropyl disulfide (BSDS), 3-(benzthiazolyl-s-thio)propyl sulfonic acid (sodium salt), pyridinium propyl sulfonic sulfobetaine, or mixtures thereof.
• BSDS bissulfopropyl disulfide
• BSDS bissulfopropyl disulfide
• benzthiazolyl-s-thio)propyl sulfonic acid sodium salt
• pyridinium propyl sulfonic sulfobetaine or mixtures thereof.
• suitable brighteners are described in U.S. Pat. Nos. 3,770,
• Examples of other suitable brighteners include 3-(benzthiazoyl-2-thio)-propylsulfonic acid sodium salt, 3-mercaptopropane-1-sulfonic acid sodium salt, ethylenedithiodipropylsulfonic acid sodium salt, bis-(p-sulfophenyl)-disulfide disodium salt, bis( ⁇ -sulfobutyl)-disulfide disodium salt, bis-( ⁇ -sulfohydroxypropyl)-disulfide disodium salt, bis-( ⁇ -sulfopropyl)-disulfide disodium salt, bis-( ⁇ -sulfopropyl)-sulfide disodium salt, methyl-( ⁇ -sulfopropyl)-disulfide sodium salt, methyl-( ⁇ -sulfopropyl)-trisulfide disodium salt, o-ethyl-dithiocarbonic acid-S-(
• compositions of the present invention also may include levelers, suppressors (carriers), surfactants) buffering agents and other compounds used in conventional electroplating baths.
• levelers include lactam alkoxylates having a formula: where A represents a hydrocarbon radical such as -CH 2 -, R 1 is hydrogen or methyl, n is an integer from 2 to 10, preferably from 2 to 5, and n' is an integer from 1 to 50.
• A represents a hydrocarbon radical such as -CH 2 -
• R 1 is hydrogen or methyl
• n is an integer from 2 to 10, preferably from 2 to 5
• n' is an integer from 1 to 50.
• Such compounds include ⁇ -propiolactam ethoxylate, ⁇ -butyrolactam-hexa-ethoxylate, ⁇ -valerolactam-octa-ethoxylate, ⁇ -valerolactam-penta-propoxylate, ⁇ -caprolactam-hexa-ethoxylate, or ⁇ -caprolactam-dodeca-ethoxylate.
• Such leveling agents are included in electroplating compositions in amounts of from 0.002
• levelers include polyalkylene glycol ethers of formula: [R 2 O(CH 2 CH 2 O) m (CH(CH 3 )-CH 2 O) p -R 3 ] a where m is an integer of from 8 to 800, preferably from 14 to 90, p is an integer of from 0 to 50, preferably from 0 to 20, R 2 is a (C 1 -C 4 ) alky), R 3 is an aliphatic chain or an aromatic group and a is either 1 or 2.
• Amounts of polyalkylene glycol ether that may be included in the compositions ranges from 0.005 to 30 grams/liter, preferably from 0.02 to 8.0 grams/liter. Relative molecular mass may be from 500 to 3500 grams/mole, preferably from 800 to 4000 grams/mole.
• polyalkylene glycol ethers are known in the art or may be produced according to processes which are known in the art by converting polyalkylene glycols with an alkylating agent such as dimethyl sulfate or tertiary butene.
• polyalkylene glycol ethers examples include dimethyl polyethylene glycol ether, dimethyl polypropylene glycol ether, di-tertiary butyl polyethylene glycol ether, stearyl monomethyl polyethylene glycol ether, nonylphenol monomethyl polyethylene glycol ether, polyethylene polypropylene dimethyl ether (mixed or block polymer), octyl monomethyl polyalkylene ether (mixed or block polymer), dimethyl-bis(polyalkylene glycol)octylene ether (mixed or block polymer), and ⁇ -naphthol monomehtyl polyethylene glycol.
• Additional levelers that may be employed to practice the present invention include nitrogen and sulfur containing levelers with a formula N-R 4 -S, where R 4 is a substituted or unsubstituted alkyl group or a substituted or unsubstituted aryl group.
• the alkyl groups may have from 1 to 6 carbons, typically from 1 to 4 carbons.
• Suitable aryl groups may include substituted or unsubstituted phenyl or naphthyl.
• Substituents of the alkyl and aryl groups may be, for example, alkyl, halo, or alkoxy.
• levelers examples include 1-(2-hydroxyethyl)-2-imidazolidinethione; 4-mercaptopyridine, 2-mercaptothiazoline, ethylene thiourea, thiourea, and alkylated polyalkyleneimine. Such levelers are included in amounts of 500 ppb (parts per billion) or less, preferably from 100 to 500 ppb, Other suitable leveling agents are described in U.S. Pat, Nos. 3,770,598 , 4,374,709 , 4,376,685 , 455,315 and 4,673,459 .
• any suppressor (carrier) that is employed in metal plating may be employed in the practice of the present invention. While the concentrations of suppressors may vary from one electroplating bath to another, suppressors typically range from 100 ppm or greater, Examples of such suppressors are polyhydroxy compounds such as polyglycols, e.g., poly(ethylene glycol), poly(propylene glycol) and copolymers thereof, An example of a preferred suppressor is poly (ethylene glycol). A suitable concentration range for poly(ethylene glycol) is from 200 ppm to 2000 ppm. The poly(ethylene glycol) may range in molecular weight from 1000 to 12000, preferably from 2500 to 5000.
• pH adjusters may include, for example, inorganic acids such as sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, or mixtures thereof. Sufficient acid is added to the compositions such that the pH ranges from 0 to 14, preferably from 0 to 8.
• Copper electroplating baths may be maintained at a temperature range of from 20° C to 80° C with acid copper baths (pH from 0 to 4) at temperatures of from 20° C to 50° C. Copper plating is continued for a time sufficient to form a deposit of desired thickness. Plating time for a printed wiring board may range from 45 minutes to 8 hours. For circuit board manufacture, a desired thickness may range from 62 mils to 400 mils (0.001 mils/inch and 2,54 cm/inch).
• the method of the present invention is suitable for copper plating through-holes of multi-layer circuit boards with aspect ratios of at least 10:1 and through-hole interconnections of at least 0.16 cm, and blind vias of 0,063 cm.
• the method of the present invention in addition to the other advantages, reduces or eliminates dog-boning in contrast to many conventional electroplating methods.
• Both vertical and horizontal plating processes may be employed.
• the substrate such as a printed wiring board
• the substrate which functions as a cathode
• the substrate and the anode are connected to a current source and an electrical current or electric field is generated the substrate, anode and plating composition.
• Any suitable source for emf may be employed.
• Various apparatus for generating an emf are well known in the art.
• Plating composition is directed continuously through a container with the cathode, anode and plating composition by means of transporting equipment such as a pump. Any suitable pump employed in electroplating processes may be employed to practice the present invention. Such pumps are well known in the electroplating industry and are readily available.
• the substrate or cathode In the horizontal plating process, the substrate or cathode is transported through a conveyorized unit in a horizontal position with a horizontal direction of movement. Electroplating composition is injected continuously from below and/or above and onto the substrate by means of splash nozzles or flood pipes. Anodes arc arranged at a spacing relative to the substrate and are brought into contact with the electroplating composition by means of a suitable device. The substrate is transported by means of rollers or plates.
• Such horizontal apparatus are well known in the art.
• the method of the present invention eliminates or reduces dog-boning, increases throwing power, reduces or prevents corner cracking as well as whisker formation, and provides an improved metal layer surface and leveling performance. Additionally, the compositions used in the present invention are more stable than many conventional plating compositions. Accordingly, the present invention is an improvement in the metal plating art.
• the present invention is described with an emphasis on electroplating in the printed wiring board industry, the present invention may be employed in any suitable plating process.
• the method may be employed in copper plating in the manufacture of electrical devices such as printed circuit and wiring boards, integrated circuit, electrical contact surfaces and connectors, electrolytic foil, silicon wafers for microchip applications, semi-conductors and semi-conductor packaging, lead frames, optoelectronics, and optoelectronic packaging, and solder bumps, such as on wafers.
• Each electroplating composition or bath was an aqueous bath that contained 80 grams/liter of copper sulfate pentahydrate as the metal ion source, 225 grams/liter of sulfuric acid to maintain the pH of the baths at 4.0.
• Chloride ion concentration in each of the baths was 25 ppm.
• the chloride ion source was HCl.
• each bath also contained a carrier component at a concentration of either 0.25 ppm or 1 ppm, and a brightener (BSDS) in an amount of either 0.1 ppm or 0.2 ppm to provide a chloride to brightener ratio of either 125:1 1 or 250:1.
• BSDS brightener
• Carriers that were employed in each solution are disclosed in the table below. All of the carriers listed in the table below are block copolymers.
• Each bath was placed in a separate standard 1.5 liter Gornell cell and a 9.5 cm x 8.25 cm copper clad panel (cathode) was placed in each cell with air circulation and mechanical agitation during the electroplating process.
• a copper anode was employed as the auxiliary electrode.
• Current density during the electroplating process was maintained at 32 mAmps/cm 2 .
• Each panel was electroplated for 60 minutes using a forward to reverse waveform of 10 ms to 0.2 ms.
• the source of the emf was a Technu pulse rectifier.

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• 2003
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