NÚMERO SECUENCIAL

The presence of an electrical double layer and diffusion layer on the surface of an electrode complicate the process of electroplating beyond the straightforward reduction of metal ions alone. The cathodic deposition of metals, whether alloyed or pure, can be broken down into three main stages, 1) ionic migration, 2) electron transfer, and 3) incorporation [5]. During the ionic migration phase, hydrated ions in the electrolyte migrate toward the cathode under the influence of the applied potential. The beginning

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Throwing power is a measure of the ability of an electrolyte to plate to a uniform thickness over a cathode of irregular shape. Throwing power may be improved with use of an anode which conforms to the irregular

of the electron transfer stage is characterized by the hydrated metal ions entering the diffusion/double layer on the surface of the cathode. As the hydrated ions approach the cathode, water molecules of the hydrated ions are aligned by the field present in the double layer and ultimately the hydrated shell around the metal ion is lost due to the high electrical field present in the layer [5]. The electrons present on the cathode then neutralize the metal ion as it is adsorbed onto the surface. The final stage of incorporation is the migration of the adsorbed atom along the cathode to a region of lower energy and finally incorporation of the atom into the growing three-dimensional, regular periodic geometric order of atoms, or unit cells, which define the lattice. The three dimensional constant distance between the beginning of set of atoms and the same pattern as set out by a translation define the lattice constants for the unit cell10 of a crystal lattice.

The initial layers, up to a few microns, of a continuous deposit are typically referred to as the thin film deposits while further thickening of the deposit is known as bulk deposition. In practise the only difference between the stages is the thin film is deposited on the substrate, often different material than the deposit, while the bulk deposit occurs upon the thin film made of the same material as the deposit. During both these stages, deposition occurs wherever electrons are present to reduce the metal from ionic form. The path the ions take to the substrate is defined in part by electrical field lined established by the charge present on the anode and cathode. Due to the dependence of the deposit on field lines, deposits are most even where the electric field lines are perpendicular to the surface. Fringing fields from the edge of a substrate result in uneven deposition as the aggregation of field lines at the edge produce a thicker deposit at the edges of the substrate compared to the middle. The effect of the fringing fields is commonly known as the “dog-bone” effect as the resulting deposit is shaped like a stereotypical bone one would give to a dog. Additionally, the line of sight limitation brought about by the electric field lines often results in non-existent or extremely poor quality deposits within recessed areas. Both the dog-bone effect and difficulties plating recessed areas can be mitigated by using anodes customized for the substrate.

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A unit cell is the smallest repetition of atoms within a crystal lattice, the translational repetition of which produces the entire crystal lattice.

The atomistic perspective of lattice growth provides explanation as to the process of migration of the neutralized atom within the incorporation stage. This approach treats the metal as a fixed lattice of positively charged atoms with the electrons permeating between the atoms moving nearly unimpeded as a gas of free valance electrons [5]. Interactions between the free electrons and the metal ions are largely responsible for the metallic bond [5]. The lattice formed by the deposited metal ions, especially electroplating, is not a perfect, ideal, atomically smooth crystalline structure11 as it contains a variety of defects including vacancies, missing atoms; dislocations, atoms shifted from the periodic geometric ordering; mono-atomic steps in the lattice; clusters of adsorbed atoms, or adatoms; and non-periodic impurities. For example, the density of metal surface atoms is about 1015 cm–2, while the density of dislocations on a non-ideal surface is of the order of 108 cm–2 [5]. The presence of defects in an electroplated deposit are principally the result of coordination chemistry and diffusion layer effects on the migration of reduced atoms/ions along the surface to the position of lowest energy, typically a kink site, Figure 2.4.

Figure 2.4: Ion transfer to a terrace site, surface diffusion, and incorporation at kink site [5].

Note: The hydrated shell is lost in stages as the ion is transferred to the surface.

[Image reproduced from Figure 1.13 in “Modern Electroplating 5th Edition”, with kind permission from John Wiley & Sons, Inc. (2010).]

One unmentioned mechanism of note is Ostwald ripening, which is a thermodynamically-driven, spontaneous process that occurs due to greater stability and lower energy configuration of larger particles compared smaller particles [9]. Compared

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to the nucleation of the deposit by electroplating, the role of Ostwald Ripening is effectively non-existent. By definition, the phenomenon of Ostwald ripening can play a more significant role in incorporation of adsorbed molecules, especially the formation of immersion deposits where the rate for crystallite formation is significantly slower. In the case of immersion deposits, the mass transport of material away from smaller particles towards larger particles in a supersaturated environment, Ostwald ripening, is more likely to occur. The theoretical treatment of the deposition mechanisms has been conducted in work by others [6, 10-13] and information beyond the overview presented is not covered within this work.

Beyond the atomistic perspective, the growth of electroplated deposits resulting from the reduction of metal onto the surface of an electrode by the acquisition of electrons can be characterized as a combination of two processes or mechanisms; layer growth and nucleation-coalescence growth, or three-dimensional (3D) crystallite growth, Figure 2.5.

Figure 2.5: Schematic representation of (a, b) layer growth and (c) the nucleation–coalescence mechanism

[5].[Image reproduced from Figure 1.16 in “Modern Electroplating 5th Edition”, with kind permission from John Wiley & Sons, Inc. (2010).]

The ideal layered growth mechanism occurs when single, discrete layers are deposited across the surface with the next layer growing upon completion of the previous layer. The nucleation-coalescence growth of 3D crystallites is characterized by four stages, namely, 1) the formation of isolated nuclei and their growth to 3D crystallites, 2) the coalescence of the crystallites, 3) formation of a linked network, and 4) formation of a continuous deposit [5]. While in practice both growth mechanisms occur during electroplating, control over the deposition conditions may be used in order to favour one

mechanism over another. Such modifications include changing the deposition rate by changing the concentration of ions or bath temperature, as well as the incorporating additives to make the deposit more compact, smoother, or change other qualities associated with crystallite size. Additives adsorbed onto the surface are able to affect the kinetics of electroplating as well as the growth mechanism by changing the concentration of growth sites on a surface, the concentration of adions12, or the diffusion coefficient of the diffusion layer.

The influencing deposition conditions by means of the inclusion of additives, ion concentration, and deposition conditions ultimately influence the quality of the thin film coating by means of the crystal structure of the deposit. The arrangement of crystallites that form a deposit may be considered as either highly crystalline, meaning the charged atoms/ions are arranged in large, ongoing periodic structures, or amorphous, meaning that the size of the crystallites are of the order of the periodic pattern itself [5]. The higher the degree of crystallinity, the longer the scale upon which the periodic structure is retained up to the formation of a single crystal. Interruptions in the periodicity of the lattice are called grain boundaries with the individual crystallites known as grains [5]. Within a polycrystalline coating, grains typically share periodicity, though the size of individual grains varies. The degree of crystallinity of electroplated structures depends on the competing formation of new crystals and the growth of those existing. A large number of variables during electroplating including metal ion concentration, additives, current density, temperature, agitation, and polarization affect the structure and size of the crystallites as well as the formation of defects within [5]. There is some variation of the term ‘grain’ with some authors attributing the term to groupings or clumps of crystallites which some authors refer to as ‘islands’ [5]; within the context of this work the term grain will refer only to individual crystallites having crystal lattice planes of the same direction. The structures of lattices that make up the crystallites of commonly deposited metals, Table 1.2, fall under one of three different crystal structures, Figure 2.6.

Figure 2.6: Unit cells of the three crystal lattices of commonly deposited metals.

(Figure assembled from individual original images by Bob Mellish; reprinted under GNU Free Documentation License.)

Crystal Structure Commonly Deposited Elements

BCC Cr, Fe, W

FCC Al, Ni, Cu, Ag, Au

HCP Co, Zn

Table 2.2: Crystal structure of commonly deposited metals.

As illustrated in Figure 2.6, body-centered cubic (BCC) lattices have unit cells with an atom at each corner as well as the center of the body of the cube with each atom in contact with 8 other adjacent atoms within the lattice, also known as having a coordination number of 8. The more commonly deposited face-centered cubic (FCC) lattices have unit cells of an atom in each corner as well as the center of each face of the cube and have a coordination number of 12. Hexagonal close packed (HCP) lattices are made up from planes of hexagonal lattices with an atom at the center of each hexagon and each plane offset within the tetrahedral hole of the previous plane. Each atom has a coordination number of 12 and the unit cell of the HCP lattice is outlined with bold lines within the figure. The voids between atoms in the FCC and HCP structures account for 25.96 % of the total volume while the BCC structure has voids accounting for 31.98 % of its total volume. The reason for similarities between the voids and coordination number of the HCP and FCC lattices is that the FCC lattice can be constructed from an HCP-like lattice where the tetrahedral holes of a hexagonal plane are not filled symmetrically by atoms above and below the plane; this packing is known as cubic-close packed (CCP) and contains the FCC lattice.