Electrodeposition

1. Electrodeposition • Electrodeposition is the process of coating a thin layer of one metal on top of a different metal to modify its surface properties. • Objectives: • To achieve the desired electrical & corrosion • resistance. • To reduce wear & friction. • To improve heat tolerance. • Applications: • Automotive parts. • Printed circuitry and electrical contacts. • General engineering components. • Gold-silver wares and jewelry. • Decorative wares. • Production of micro parts for MEMS.

2. Electroplating Setup

3. Equilibrium Potential For Cu2+ + 2e  Cu ECu2+/Cu= EoCu2+/Cu + (RT/zF) ln [Cu2+] = 0.340 + 0.059/2 ln [Cu2+] at 25 ℃ Noble Corrosion Cu2+ 0.34 V ECu2+/Cu Cu Electroplating Active

4. Atomistic Aspects of Electrodeposition For charge transfer reaction in metal/solution interface: M(in lattice)  Mz+ (hydrated in sol.) + ze- M,lattice M,ad Actually, Mz+, sol.  M, adsorbed  M, lattice Mz+,sol

5. Ion Transfer Mechanism Step-edge ion transfer mechanism: 1) A direct transfer to kink site 2) A direct transfer to the step-edge site other than a kink, the transferred metal ion diffuses along the step edge until it finds a kink site. Step edge Terrace ion transfer mechanism: A metal ion is transferred from the solution to the flat face of the terrace region. At this position the metal ion is in the adion state having most of the water of hydration. It is weakly bound to the crystal lattice. From this position it diffuses on the surface, seeking a position of lower energy. The final position is a kink site.

6. ia ic Electrode Kinetics • At equilibrium, M Mz+ + ze, at E = Eeq , ia = ic = io I.H. O.H. Free energy G‡ (io : exchange current density) M Mz+ Metal Solution Reaction coordinate - + - + - + - + - + - + - + Capacitor

7. Electrode Kinetics • At anodic polarization, at E = Ep > Eeq , M Mz+ + ze-, a = Ep - Eeq =overvoltage  ie.a = ia - ic  f(a) I.H. O.H. Free energy ia ic M Metal Solution Mz+ Reaction co-ordinate

8. Anodic current ηa Ba=2.3RT / zF =0.059 / z Er E ηc Cathodic current log io log |io| Polarization Diagram • Polarization : An electrode is no longer at equil. when a net current flows from or to the surface. The extent of potential change caused by net current to or from an electrode, measured in volts, is overpotential (η). η= f ( ie ), in which ie is current flowing through external wire. where, io = exchange current density Er = equil. potential, or rest potential Ba, Bc : Tafel constant, -0.05 V < B < 0.15 V

9. Conc. M M+Z CB CS  : Nernst Layer IHP OHP Influence of Mass Transport on Electrode Kinetics (Conc. Polarization) For M+z + ze- M iL = DzFCB/ c = RT/zF ln (1 - i/iL) = 0.059/z log (1 - i/iL)at 298 K iL is increased by • higher solution concentration, CB; • higher temperature which increases diffusivity, D;• higher solution agitation, which decreases . Concentration polarization only becomes important when the current density approaches iL

10. E Er ct io Ba Linear region E Butler-Volmer exponential relationship (the Purely activation controlled current) 1 Er C Exponential region 2 3 log ic Mixed control (activation and mass transport) iL 4 iL Combined Cathodic Polarization T = ct + c = Bc log i/io + 2.3RT/zF log(1 - i/iL) E – log i plot E – i plot The limiting current density is; iL = nFD/ cb (D : diffusion coefficient of Mz+  : diffusion layer thickness n : number of electrons involved in the reaction F : Faraday constant)

11. Influencing Factors in Electrodeposition The morphology and composition of electrodeposits vary significantly, and depend on: • Current density • The nature of the anions or cations in the solution • Bath composition and temperature • Solution concentration • Power supply current waveform • The presence of impurities • Physical & chemical nature of substrate surface

12. Faraday’s Law • Faraday’s Law: The amount of electrochemical reaction that occurs at an electrode is proportional to the quantity of electric charge (Q) through an electrochemical cell. • The weight of a product of electrolysis is w, then w = ZQ (Z : the electrochemical equivalent) Q = It, it follows that w = ZIt • Production of one gram equivalent of a product at the electrode (weq) in a cell needs 96487 Coulombs. • The electrochemical equivalent of a metal Z(M) is the weight in grams produced or consumed by one coulombs of charge. weq = Awt/n Awt: the atomic weight of metal deposited on the cathode n : number of electrons involved in the deposition reaction • When Q = 1,w(Q=1) = Z weq = 96487Z Z = weq/96487 = weq/F = Awt/nF w = ZQ = AwtQ/nF

13. Current Efficiency • Current efficiency is the ratio between the actual amount of metal depositing (or dissolved) Ma to that calculated theoretically from Faraday’s law Mtotal in %. • C.E. indicates the faction of total current that generates desired products. • C.E in plating bath depends on: • Electrolyte or bath • pH and agitation • Current density • Concentration of chemical component C.E. = (Qa / Qtotal) x 100 = (Ma / Mtotal) x 100

14. Nucleation and Growth of Electrodeposit Layer growth : a crystal enlarges by a spreading of discrete layers. 3D crystallites growth mechanism : a coherent deposit is built up as a result of coalescence of these crystallites.

15. 2r adsorbed atom h substrate Effects of Overpotential on the Nucleation Rate When a nuclei with radius r and height h is formed from adsorbed atoms, G = -r2h/V + 2rh ( V : atomic volume,  : interface energy, k : overpotential  : chemical potential difference between adsorbed atom and atom in neclei. (=Zek)) Critical radius (Rc) and critical free energy (G*) for stable nucleation; Rc = h3/6Zek G* = h42/6Zek Nucleation rate J J = K1 exp(-G*/RT) = K1exp(-K2/k)  Higher overpotential  smaller or finer nuclei

16. Effects of Overpotential on Surface Morphology of Sn Smooth Sn compact dendrite Potential (V) Powdery Sn powdery Powdery structure is obtained at high cathodic overpotential. Cathodic current density (A/cm2)

17. 0.2 A/cm2 0.3 A/cm2 0.4 A/cm2 Influence of Overpotential on Deposits of Cu 30 seconds at 0.2 A/cm2 30 seconds at 0.4 A/cm2 30 seconds at 0.3 A/cm2

18. Pulse and Pulse-Reverse Plating • In dc-plating, constant current is used, and the rate of arrival of metal ions depends on their diffusion coefficient (electrode-to-part spacing and agitation). • In PC and PRC, a modulated current waveforms are used to get a better leveling of the deposit, and to minimize the porosity, contamination, etc. • The morphology of some metal and alloy deposits were found to be superior to the dc-plated deposits. • Complex current wave forms can be generated by using pulse rectifier: unipolar and bipolar pulses.

19. Pulse deposition technique • The main effects of pulse deposition ; • Under pulse deposition conditions the Nernst diffusion-layer is split into two diffusion-layers. Because of the thin Nernst layer, high current density can be applied. • High current density & high overpotential increase the nucleation rate, then fine deposit can be formed.

20. Three major wave form in pulse deposition Rectangular-pulse deposition The waveform consists of pulses of a current or potential of a rectangular shape separated by intervals of zero current or potential. Periodic reverse deposition The applied current or potential is periodically switched from cathodic to anodic polarization. Superimposed sinusoidal deposition The waveform is the sum of a sinusoidal alternating (ac) wave current and a direct cathodic current (dc). If the amplitude of the sine wave is greater than the dc offset, then the wave form consists of both a cathodic and an anodic portion.

21. pulse period applied current ip toff ton ip : peak current density ton : current on-time toff : current off-time Duty cycle = ton/(ton+toff) Characteristics of Unipolar Currents • To characterize a train of current pulses, three parameters need to be known: peak pulse current density (ip) pulse length (ton) interval between pulses (toff) • The average current density iav is measured, and it is expressed for PC as: iav = (ip x ton)/(ton + toff) • The duty cycle (γ) which represents the portion of the time in each cycle when the current is ON. • It is defined by γ = ton/(ton + toff) ; iav = ip x γ

22. deposit substrate deposit substrate Leveling and Throwing Power • Throwing power: - The ability of electroplating solution to deposit almost equal thickness on both recessed and prominent areas. - Ni plating (also other acidic Cu and Zn baths) show poor throwing power. If the C.E. values are close to 100 % at low and high current density values, then the macroscopic irregularities on the cathode will lead to non-uniform deposits. - Alkaline baths have better throwing power, since the metal ions are present as complex ions. • Leveling: - The ability of electroplating solution to fill in defects and scratches on the surface preferentially is called leveling.

23. additive deposit substrate substrate deposit substrate H2 substrate Effects of Additives on Electrodposits • Levelers : Levelers are adsorbed on the peaks, and more metal is deposited on the recessed areas than peaks. • Grain refiner : Grain refiner is adsorbed on the surface, and suppress the lateral growth, resulting grain refinement. • Stress reducer : Stress reducer is adsorbed in deposited layer, and reduce residual stress that occurs from cracks formed during deposition. • Wetting agent : Wetting agent reduces the surface tension of H2 bubble adsorbed on the surface of cathode, and the metal is deposited more easily.

24. Influences of Additive and Pulse Current on Electrodeposits additive Pulse deposition 20Pb-80Sn deposited by DC current No additives ↑overpotential with additives Additives

25. Effects of bath type on depostion of Cu • Acid bath : Low polarization; difficult to deposit Cu initially. Once Cu is present on cathode surface, subsequent high speed deposition is undertaken. Without adding agents in electrolyte, the deposit is dull and uneven. High current efficiency (90 %) • Complex bath : High polarization ;yield deposits of finer grain size, brighter, and more lustrous appearance. Low current efficiency (70 %)

26. Uniform Electrodeposit through the Use of Complex Baths • In most complex baths, the deposition potentials are amenable to hydrogen evolution which competes with metal deposition such that C.E. falls as current density is increased. This results in a more uniform deposit on cathodic macro irregularities. • When the ions are complexed, they encounter high concentration polarization (CP). If the CP is high, the micro throwing power is rather poor. • The ability to produce a deposit over a surface including recesses is called covering power.

27. Cu(CN)43- Cu Potential E ZnO22- Zn Current density, i Alloy Electrodeposition Generally, metals whose E° values differ by more than 0.2 V can be codeposited from simple salt solutions. In the case of large difference in E° values between two metals, complex ion bath for noble metal should be used. Ex) Cu-Zn alloy deposition in 25-50 g/L Cu(CN)2, 30 g/L Na2CO3, 10-30 g/L Zn(CN)2, 10-30 g/L NaOH, 50-75 g/L NaCN bath. Cu and Zn form cyanide complexes 2NaCn + CuCN  Na2Cu(CN)3 Cu(CN)32- + CN-  Cn(CN)43- 2NaCN + Zn(CN)2  Na2Zn(CN)4 Zn(CN)42- + 4OH-ZnO22- + 4CN- + 2H2O  Cn & Zn can be codeposited from Cu(CN)43- & ZnO22-.

28. e- mold Electroforming Electroforming: a process used for making metallic articles with tight dimensional tolerance. • By depositing a metal into or on to a mold or mandrel, a free-standing metal object is made. • Useful for the production of originals and making exact copies of originals.

29. Advantages & Applications of Electroforming Advantages Applications • Micro components and prostheses • Complex wave guides • Metal bellows • Reflectors, Nose cones • Heat exchangers, micro filters • Decorative ware • Accuracy of reproduction • Production of foils and mesh-products • Manufacture of complex shaped objects When combined with lithography, electroforming is extremely useful for making micro parts, and overcomes the difficulty of traditional machining.

30. Lithography Process X-rays Mask Mold Resist Substrate Develop Resist Embossing Plating μ-structure

31. Introduction Copper electroplating is a wide-spread and important process in the electronics industry. ■ Electroforming process Copper foil (12~70 ㎛) used in the printed circuit boards industries is mostly manufactured by electrodeposition process (electroforming) of copper. ■ Roller Copper foil Cathode drum Electrolytic cell Anode (Pb) Electrolytyte Cu2+ SO4- Rotating cathode drum

32. Manufacturing Process of Copper Foil Manufacturing Process of Cu foil Base foil Cathode drum sheet roll slitter sheeter Surface treatment (Nodule formation, Stain proof)

33. catalytic surface Mz+sol + RedsolMlattice + Oxsol catalytic surface Mz+sol + ze Mlattice catalytic surface Redsol Oxsol + me Current-potential curves for reduction of Cu2+ ions and for oxidation of reducing agent Red, formaldehyde, combined into one graph. Electroless Plating No power supply is necessary to drive the deposition reaction. The overall reaction is ; Applications; Cu electroless deposition for via hole Ni-P electroless deposition for corrosion resistance

34. Advantages of Electroless Plating • No electrical contact is needed. • It is possible to plate both conductive and insulating surfaces, provided the surfaces are first sensitized. • It is readily adaptable for three-dimensional coverage. • No field lines are present, and this enhances deposit uniformity. • Metallizing Nonconductors • Certain parts or components whose functions are fully utilized only when the properties of both a metal and non-metal are combined. • Generally a part is made of plastic or ceramic, and the metal added to its significant surfaces to impart specific metallic properties. • 1. For electrical conductivity, as in PCB. • 2. For metallic appearance, as in the buttons, door knobs, wheels in toys, etc.

35. Developing Good Adhesion Metal Deposition on Polymers • For metallizing nonconductors, their surfaces need to be mechanically roughened, or etched, or made hydrophilic. Nonconductors need pre-conditioning of their surfaces. • Polymers: Polyimides, Polysulfones, etc. surface etching : Cr2O3/H2SO4 (0.5:1.0) • Fluorocarbons: Na metal in anhydrous NH3 or THF • Need to be treated further in sensitizing, nucleating (catalytic), accelerator solutions prior to plating in electroless solution.

36. Electroless Plating Process Surface etching (e.g. sulfuric + chromic acids) Rinse water Sensitize (e.g. Sn(II)) “Catalyze” mixed colloid Activate (e.g. KF, HF) Catalyze Pd(II)) Electroless bath Sensitization and catalysis mean the absorbing of agents from a solution of Sn2+and/or Sn4+. A simplified model of sensitization and catalysis process is that the sensitizing ion reduce the active metal from catalyst solution, which most often is PdCl2. Sn2+ + Pd2+→ Sn4+ + Pd

37. Anodizing • Anodizing is an electrochemical process in which the part is made the anodic electrode in a suitable electrolyte. Sufficiently high voltage is deliberately applied to establish the desired polarization to deposit oxygen at the surface (O2 overvoltage). The metal surfaces or ions react with the oxygen to produce adherent , oxide coatings, distinguishing the process from electrobrightening or electropolishing processes. • Industrial anodizing processes are confirmed mainly to Al, Mg and Ti alloys. Anodic coating applications include: 1. Protection : corrosion, wear and abrasion resistance. 2. Decorative : clear coatings on polished or brightened surfaces, dyed (color) coatings. 3. Base for subsequent paint or organic coating.

38. Anodic oxide film of Al a : thin barrier layer b : rough surface localized dissolution c : formation of pores d : growth of anodic oxide film 4Al + 6H2SO4 = 2Al2O3 + 6SO3 + 3H2 + 6H+ + 6e- + 1250 kJ Heat generation ↑ Al oxide film breakdown Formation of porous oxide layer Formation of anodic oxide film of Al

39. Structure of Anodic oxide film of Al Cell wall Porous oxide Barrier layer Aluminum Barrier layer

40. Template-Assited Nanowire Fabrication

41. Alumina Templates on Silicon Wafers (I)

42. Alumina Templates on Silicon Wafers (II) SEM image of porous alumina anodized in 4 wt.% H2C2O4 at 45 V. The pore diameter is ~ 44 nm. 40 nm Bi nanowires deposited in the porous alumina template on a silicon wafer with a conducting adhesion layer. Aluminum