Electroplating Techniques

The information acquired from voltammetry not only provides the potential difference between the anode and cathode for most efficient deposition, it also allows for deposits to be conducted using either constant voltage or constant current. Historically, until the advent of modern electrical generation, electroplating was conducted under potentiostatic, constant voltage, conditions. Potentiostatic deposition is typically used in cases where the thickness of the deposition layer is not subject to a strict tolerance, or when selection is needed due to more than one candidate for reduction present in solution. In cases where more than one metallic ion exists in an electrolyte as a candidate for reduction, selection may be used to provide a pure or alloyed deposit depending on the construction of the electrolyte.

The benefits of an alloyed film compared to a pure film depend largely on the alloy deposited. Some alloys have superior qualities including: density, hardness, corrosion resistance, wear resistance, or different magnetic properties, which are not available for a single metal metallic film. By definition, the electrodeposition of an alloy requires the co-deposition of two or more metals, meaning that a rapprochement between differing deposition potentials of at least two metals is needed within the electrolyte solution. Ultimately successful alloy deposition relies on the deposition potentials of the metals becoming close, or even identical. The deposition potential, E, of metal ions within a single electrolyte, as provided by the Nernst equation, Equation 2.5, is dependent on the standard electrode potential, E°, of the metal as well as the activity of the ion in the electrolyte, which is controlled by the concentration of ions in solution; the temperature of the electrolyte is moot as both ions share a single electrolyte. For the co-deposition of metals of greatly different E°red the rapprochement of Ered is achieved by changing the concentration of the respective ions in solution. Altering the activities by changing the concentration is exemplified in the alloyed co-deposition of Zn and Cu from a bath containing cyanide complexes of both metals. Maintaining the concentration of Cu+ ions to the order of 10–18 mol∙L–1 [5], or equivalently 63.5∙10-18 g·L-1, results in a high concentration ratio of the Zn ions relative to the Cu ions and brings the two deposition potentials closer to one another overcoming the approximately 1.284 V vs. SHE

difference between Cu and Zn; E°_Zn2+ = –0.763 V and E°_Cu+ = 0.521 V. The degree of

alloying may be determined to some degree by the ratio of the current densities of individual metals at a given potential as seen in the superposition of voltammetry curves, Figure 2.3.

Figure 2.3: Typical voltammetric curves for two different metals, M1 & M2; at potential V1 the more noble metal, M1, is deposited, at potential V1 metal M1 and M2 are deposited in a ratio of approximately I3/I2 [5]. [Image modified from Figure 1.22 “Modern Electroplating, 5th Edition”, with kind permission from John

Wiley & Sons, Inc. (2010).]

In addition to the option of alloying, metals of greatly different E° may be deposited individually from the same solution as multiple sequential layers, or multi- layers, provided their deposition potential within the electrolyte is sufficiently different. The electrodeposition of modern multi-layers is achieved by periodically alternating the potential in pulses allowing for the deposition of alternating layers of metals. As shown in Figure 2.3, the metal with the least negative E°, M1, may be deposited as a pure metal at potential V1, while the metal ions of M2 will have some contamination of metal M1 when deposited at potential V2. Much as in the case of alloying, the concentrations may be modified to make the voltammetric curves more distinct. Increasing the concentration of the metal with more negative E° will increase the ratio of metal M2 deposited. In order to ensure uniform layer thickness between each compositionally unique layer, the deposition time, or ‘pulse’, at the less negative deposition potential is longer than the ‘pulse’ at the more negative potential. The difference in pulse lengths accounts for higher current density, and hence deposition rate, at the more negative deposition potential. The limitation of layered deposits of this type is that the maximum difference

between the deposition potentials of differing metals is E°red. Attempting to electroplate alternating layer of metals of similar E°red such as Co, -0.28 V vs. SHE, and Ni, -0.25 V vs. SHE will result, at best, in the formation of Co-Ni alloys of differing Co-Ni ratio. Further details regarding the deposition of multi-layers are discussed in Section 2.3.3 of this chapter.

In addition to the co-deposition of conductive materials and metals, inert materials may also be co-deposited via electroplating. The purpose of co-depositing inert materials is often to increase the wear resistance of surfaces. The inclusion of mixed carbon materials such as silicon carbide {SiC}, tungsten carbide {WC}, or diamond particles, can be achieved by using low current densities allowing for the natural inclusion, trapping, of particles and other impurities within the deposit. One documented application is the inclusion of 100 ppm of carbon in a sulfamate {H2NSO3–} nickel bath which has been shown to increase the tensile strength of the deposit from 500 MPa to approximately 900 MPa [7].

Galvanostatic deposition, deposition at constant current or more specifically using a constant current density, is used when the electrolyte is well defined and a voltammetric curve has been established. Given a voltammetric curve providing the peak current for a system and the size of the cathode, it is possible to determine the current per area, or current density. Application of the optimum density establishes the optimum deposition potential just as establishing the optimum potential results in the peak current density. This method is most convenient when the deposition thickness requires precise control as it is the electrons that reduce the metal ions. The method in which the amount of material deposited and deposition thickness may be calculated is Faraday’s law. Faraday’s law states that the amount of electrochemical reaction that occurs at an electrode is proportional to the quantity of electric charge, q, passed through an electrochemical cell [5]. The weight, or more correctly the mass, of the deposited materials, Equation 2.8, can be expressed as the product of the electrochemical equivalent, Z, and the amount of charge, q; more practically q can be replaced by the product of the current, I, and the duration of the deposit, t. The electrochemical equivalent, Z, denotes the atomic weight of the element to be deposited per the number of electrons for the deposition of a single

ion per number of particles per electron charge Awt/nNae; Nae is known as Faraday’s constant, F. nF A e nN A w wt a wtIt It ZIt= = = (2.8) where: w = Z = I = t = Awt = n = Na = e = F =

Mass, sometimes referred to as weight, of material deposited (g) Electrochemical equivalent

Applied current (A)

Duration of the deposition (s)

Atomic weight of the deposited species (g·mol−1) number of electrons involved in the deposition reaction Avogadro’s Number, 6.022×1023 mol−1

Electron charge 1.6021×10−19 C Faraday constant, 9.6485×104 C·mol−1

The thickness of the resulting deposit, h, is determined as the volume of material deposited, V, over the area, a. Given that the density, d, may be expressed as the mass, w, of the deposit over its volume, V, useful relationships may be drawn between the deposition time, current, and desired thickness of the deposit, Equation 2.9.

t ad I ad w a V h       = = = nF A / / wt (2.9) where: h = V = a = d =

Thickness of the deposit (mm) Volume of material deposited (mm3) Area of the deposit (mm2)

Density of the deposit (g/mm3)

Though the thickness of the deposit is dependent on the duration of the provided current, the presence of other ionic species in solution provides other candidates for reduction by the supplied electrons. Even at the most efficient current density as set out by voltammetry, the efficiency of the metal deposition is typically less than 100 %. For example, during deposition of Cu from a solution of cupric nitrate {Cu(NO3)2} in dilute nitric acid {HNO3}, three cathodic reactions occur: the deposition of Cu (the reduction of cupric, Cu2+, ions) and the reduction of both nitrate and hydrogen ions [5]. The efficiency of the deposit, known as the current efficiency, CE, is calculated by the amount of charge used to reduce the desired species, qi, per total charge available, qtotal; or alternatively the mass of the reduced species desired, wi, per total mass reduced, wtotal, Equation 2.10.

CE = qi/qtotal or CE = wi/wtotal (2.10) where: CE = qi = qtotal = wi = wtotal = Current Efficiency

Charge used to reduce the desired species Total charge available

Mass of desired reduced material Total mass of reduced materials

Though current efficiency is typically below 100 %, the deposition of Au from alkaline baths, with sufficient agitation, have produced cathodic current efficiencies as high as 90–100 %. The concentrations of ions within the electrolyte, in addition to conditions during deposition and bath composition, have a large influence on the efficiency of the deposit. Current efficiency of 100 % has been obtained for deposition of Au from a 12 g·L-1 KAu(CN)2 solution under mild agitation at a current density of 10 mA·cm-2, while a current efficiency only about 50 % is obtained from a 4 g·L-1 KAu(CN)2 solution in otherwise similar conditions [8]. The inclusion of some additives can result in deposition rates of over 100 %. In these cases a chemical reduction of ions, termed electroless deposition, occurs in consort with the electroplating resulting in efficiencies of over 100 %. The chemical reduction of ions without the use of any outside current, known as electroless plating, is discussed in Section 2.3 of this chapter. It should also be noted that when deposition occurs in a magnetic field, the structure, texture, and throwing power9 of both magnetic and nonmagnetic materials can be negatively affected [5].