Abstract
Copper has been electrodeposited on copper (FCC) and mild steel (BCC) substrates from acidic sulphate bath with and without cetyl trimethyl ammonium bromide at 0.25, 2, 6 and 9 V. It is found that the surface morphology varies with the change in overpotential, but not with the change in substrate. On the contrary, the crystal shape is found to be independent of the applied overpotential, but varies with the bath chemistry or choice of substrate.
Introduction
Electrodeposited thin films have been a subject of keen research because of their unique physical, mechanical and chemical properties that differ substantially from their bulk counterparts. The applications of such films are in flip chip technology, advanced materials for batteries, solar cells, contact materials, automotive applications, lead-free solders, micro-patterning, high strength along with corrosion resistant coatings, etc. [1-13]. Among all, electrodeposited copper coating is the simplest, inexpensive, easy to handle, non-toxic and non-hazardous for the environment. A lot of literature reports that copper coatings can be electrodeposited from a number of standard baths (cyanide and non-cyanide baths such as sulphate bath, phosphate bath and chloride bath) and on different substrates (steel, copper, aluminium, titanium etc.) [14]. Depending on the industrial and eco-friendly applications of electrodeposited pure copper, copper-based alloys and composites, the combinations of substrates and baths are chosen to influence the adhesion between coating and substrate such as deposition of Cu–Sn solder material [7,15]. Anisotropic properties, such as surface energy, resistivity, corrosion-resistant properties and residual stress of electrodeposited copper, can be altered if the substrate is changed. In order to achieve the identical crystal shape and morphology of electrodeposited copper coating on different substrate, bath chemistry has to be modified. Active sites of such electrodeposited films vary not only with the morphology of the coatings but also with the equilibrium crystal shapes having facets of well-defined planes such as (100), (110) or (111) [7,16,17]. It is these crystal shapes which predominantly control the electrochemical and tribological properties. Different crystal facets have different surface energies which substantially alter catalytic properties, micro-roughness, etc. The structure, shape and energy of electrodeposited copper coating enhance mechanical properties of copper, without affecting its electrical properties. Copper-plated steel strip is very cost-effective with excellent brazeability, formability combined with optimum heat resistance alternate to bulk copper [18]. Nucleation and growth mechanisms of as-deposited pure copper from acidic copper bath have already been studied [19] and in order to modify the microstructure and refinement of grains, the role of additives and surfactants in acidic copper sulphate bath is also reported [20]. Out of all, cetyl trimethyl ammonium bromide (CTAB) is of keen interest because it is known to inhibit the growth of (111) planes [21-23]. It has been extensively reported that the morphology and grain size of a deposit depend on current density and bath chemistry [24-35]. But the effect of the substrates in determining the crystal shapes is not much explored. The bath chemistry designed to obtain a certain crystal facet may not apply if the substrate is changed. Thus, the objective to achieve the identical crystal shape and morphology on different substrate/bath chemistry has to be modified. The novelty of this work is to find the independent contribution of the substrate/bath chemistry and overpotential in determining the structure of electrodeposited copper.
Experimental
The copper electrodeposits were prepared from a highly acidic, aqueous electrolytic sulphate bath containing 0.8 M copper (II) sulphate (LobaChemie,
, Mumbai, India), 1 M Sulphuric acid (Merck, prepared using 98% conc. sulphuric acid, Darmstadt, Germany). The pH of the electrolyte was adjusted to be less than 1 (0.12 + 0.02 at 23.7°C). Two baths were prepared with and without CTAB (SRL, 99%, Mumbai, India) in acidic copper sulphate bath. Pulsed potentiostatic electrodeposition at 0.25, 2, 6 and 9 V was carried out in a two-electrode setup in Autolab PGSTAT 302N (Metrohm, Herisau, Switzerland) equipped with a 10A current booster on copper and mild steel with an exposed area of 2 × 2 cm2 with platinum (Pt) rod as a counter electrode. The depositions were run for 30 min. A number of experiments were done with modifications in the bath chemistry to separate out the individual contribution of bath and substrate.
The microstructural aspects of the copper deposits were analysed using scanning electron microscope (FEI Quanta 250 FEG, FEI, Hillsboro, OR, USA) equipped with energy-dispersive spectroscopy system (BrukerXFlash 6|100). The artificial crystal morphological studies were performed using the SHAPE V7.3 software [36]. It takes the crystal structure file as input and produces equilibrium crystal shape under different conditions. This software not only shows the final equilibrium structure but also depicts the consecutive sequential phases to obtain an equilibrium crystal shape.
Results and discussion
Figure 1(a, c, e and g) shows the morphology of electrodeposited copper on a copper substrate as a function of voltage. It shows that the morphology changes in accordance with the increment of voltage. At 0.25 V (Figure 1(a)), planar morphology is observed followed by spherical clustering at 2 V (Figure 1(c)). This morphology is formed by screw dislocation-assisted layer-by-layer growth [37,38]. Deposit formed at 6 V, (Figure 1(e)), is dendritic in nature, whereas at 9 V (Figure 1(g)), it is cauliflower-like structure (equal growth rate in all directions). Figure 1(b, d, f and h) shows the magnified micrographs of the corresponding images shown in Figure 1(a, c, e and g), respectively. In all the above cases, octahedral crystal shape is found with some clear facets.
A SEM micrograph of electrodeposited copper on copper substrate at (a) 0.25, (c) 2, (e) 6 and (g) 9 V. The magnified micrographs of (a), (c), (e) and (g) are shown in (b), (d), (f) and (h), respectively.
Nucleation and growth: Change in morphology
The nucleation rate (N) is proportional to the volume free energy
Schematic of ad-atoms getting preferentially attached to a kink that provides three free-bonding sites.
is Gibbs free energy, n is the number of electrons, F is Faraday's constant,
is the change in overpotential, k is the Boltzmann constant and T is the temperature. As overpotential
is increased, there is an increase in nucleation rate of the deposition. But mass transfer of Cu2+ ions fails to cope up with the flow of electron and results in the formation of different structure/morphology. At low overpotential, the available volume free energy is less because of which the deposition takes place in pre-existing high-energy surfaces. This can provide the necessary energy for nucleation. A screw dislocation which is exposed at the surface is at a very high-energy state and acts as a nucleation site. Hence, at low overpotential, screw dislocation growth is observed (Figure 1(a)). When overpotential is increased, there is more volume free energy available for deposition. But it is not high enough to nucleate a new grain. Hence, the deposition mimics the parent (substrate structure) and deposition takes place on jog/kink sites (Figure 1(c)). Schematic of ad-atoms getting preferentially attached is shown in Figure 2 [39].
As the overpotential is further increased, there is sufficient volume free energy available for nucleation but not enough for the material to grow at all directions. Hence, the nucleation is random, but growth is directional. This leads to the formation of dendrites (Figure 1(e)). At the last stage when the overpotential is very high, both nucleation and growth directions are random. Hence, a cauliflower-like structure is formed (Figure 1(g)).
Surface energy and equilibrium crystal shapes
In order to minimise its total surface energy, each crystal tries to attain a particular shape [7]. This minimisation is attained at a constraint of constant free energy which is available from the transformation. Unstable surfaces try to become stable by the creation of low-energy surfaces, also known as faceting due to the difference in surface energy. Wulff [40] formulated the formation of shape of a solid crystal based on the above-stated criteria. It is considered that a solid crystal shape to be composed of several pyramids with an apex at the centre of the crystal. According to Wulff, for a solid crystal only those planes could exist as the surface of the solid crystal, where the ratios of surface energy of plane to its perpendicular distance from the centre were the same. Mathematically this can be written as follows:
It was shown in the previous section how
term can change the morphology of the deposit. But the term
was not considered. In order to minimise its total surface energy, each crystal tries to attain a particular shape. Hence, this term governs the crystal shape of the deposit.
Figure 3(b) illustrates the simulated structure (corresponding to Figure 3(a)) prepared using the SHAPE V7.3 software under the above deposition condition in which the deposition of copper is made on the copper substrate. It shows the consecutive sequential shapes to obtain a final equilibrium crystal shape. An octahedral shape is simulated and it is found that the facets are of {111} types of planes. Figure 3(c) shows the Wulff construction for forming an octahedral shape with {111} facets. The sets of {100} and {110} planes are fast growing planes and they ultimately disappear. The slowest growing {111} planes sustain till the end and form octahedral shape. Figure 3(a) shows the octahedral shape seen under SEM at a relatively high magnification. CTAB is known to inhibit the growth of {111} planes and hence it actuated the formation of octahedral shape.
(a) A SEM micrograph of twined octahedral shape of electrodeposited copper on copper substrate. (b) Simulated crystal structure shows the sequential growth of copper crystal deposited on copper resulting in an equilibrium octahedral shape when {111} sets of planes are the slowest growing planes. (c) Wulff construction of 111 facets corresponding {110} plane in FCC crystal (marked by ABCD plane).
To further validate the finding that CTAB perhaps is the sole reason behind the formation of {111} facets, copper was again deposited from the same bath but without CTAB under the same deposition condition. Surprisingly, the octahedral shapes with {111} facets are again observed (Figure 4). This shows that the copper substrate (FCC) itself is a {111} growth inhibitor. As {111} sets of planes are the closest pack planes in FCC, growth parallel to [111] involves accommodation of more atoms (more energetically favourable) than growth perpendicular to it. Therefore, the growth velocity in the direction perpendicular to {100} and {110} planes is more than the direction perpendicular to {111} sets of planes. The {111} planes, being the slowest to grow, sustain and form the octahedral shapes.
A SEM micrograph of electrodeposition of copper on copper substrate at 2 V from the bath without CTAB.
It is found that if the deposit is done on copper substrate, copper octahedral can be formed in the bath without CTAB. Hence, it is difficult to conclude the role of CTAB in promoting the formation of octahedral-shaped crystals with {111} facets when the deposition is done on copper. In order to find the independent effect of CTAB, a different substrate (other than copper or any FCC metal) must be chosen. Therefore, a mild steel (BCC) substrate has been used to understand the role of CTAB in the bath. The deposition was done from the bath without CTAB followed by the bath having CTAB.
Figure 5 shows the SEM micrographs of electrodeposited copper on mild steel substrate deposited from the bath without CTAB. It does not show any change in morphology while changing overpotential as it has been observed in electrodeposited copper on the copper substrate. The morphology is layer-by-layer growth followed by dendritic growth and eventually by cauliflower-type growth. Therefore, it is validated that the substrate has no effect on the overall deposited morphology. However, a major change in the crystal shapes is observed. It is found that the crystal shapes are no longer octahedral. Rather they are mostly cube-shaped.
A SEM micrograph of cubical shaped electrodeposited copper on mild steel at (a) 2, (c) 6 and (e) 9 V. The morphology is layer-by-layer growth followed by dendritic growth followed by cauliflower-like growth. In (b), (d) and (f), the corresponding magnified images of (a), (c) and (e) are shown.
Figure 6(a) shows the cubic shape seen under SEM at a higher magnification. The exact shape was simulated using the SHAPE V7.3 software and it is found that those cubic facets are of {100} types of planes (Figure 5(b)). Figure 6(b) shows that the Wulff construction (Figure 6(c)) for the formation of 100 planes. In the case of mild steel substrate, the growth of {100} planes is inhibited by the relatively high growth rate of {111} and {110} sets of planes resulting in the cubic shape. Hence, it can be inferred that the mild steel, which has BCC structure, is solely responsible for the formation of the cubical shapes.
(a) A SEM micrograph of twined octahedral shape of electrodeposited copper on mild steel. (b) Simulated crystal structure shows the sequential growth of copper crystal deposited on mild steel leading to an equilibrium cubical shape when {100} sets of planes are the slowest growing planes. (c) Wulff construction of 100 facets corresponding {110} plane in FCC crystal (marked by ABCD plane).
In order to find out the role of CTAB in determining the crystal shape, copper was electrodeposited on mild steel (BCC crystal structure) from a bath containing CTAB. The SEM micrograph of the deposit (Figure 7(a)) shows the mixed mode of growth. The crystal shape (Figure 7(b)) was simulated using the SHAPE V7.3 software. Figure 7(c) shows the corresponding Wulff construction. From the simulated shape, it is found that those shapes are made of a combination of {100} and {111} types of planes. Hence, it can be inferred that the introduction of CTAB into the bath promotes the formation of {111} sets of planes, whereas, {100} sets of planes are promoted by the mild steel substrate. As both the effects are strong, a mixed crystal shape having both sets of planes is observed. Therefore, in the absence of copper substrate, the independent effect of CTAB as a {111} plane promoter is clearly observed. Therefore, it can be concluded that both the substrate and CTAB contribute to determine the crystal shape.
A SEM micrograph shows mixed mode (both (100) and (111) facets of electrodeposited copper on mild steel from Bath 1 at (a) 2 V. (b) The corresponding simulated crystal structure made using the SHAPE V7.3 software. (c) Wulff construction of mixed (111) and (100) facets corresponding {110} plane in FCC crystal (marked by ABCD plane).
Conclusions
This work reported the effect of substrate and overpotential on the morphology and crystal shape of electrodeposited Cu. It is found that the applied overpotential exclusively alters the morphology of the deposit irrespective of the substrate chosen. As low overpotential the deposition takes place in prior existing steps and kinks promoting layer-by-layer growth. At higher potential dendritic growth (directional) followed by cauliflower-like structure (growth in all directions) is found. This change growth pattern with increasing overpotential is found in both the cases (deposition on Cu and mild steel substrate). It is, therefore, concluded that the substrate has no role to play in determining the morphology. Morphology can only be controlled by tuning the voltage.
However, it is also observed that substrate has a profound effect on deposited crystal shapes. Deposition on Cu substrate is octahedral in shape, whereas, deposition on mild steel is cubic in shape.
When CTAB is used in combination with mild steel, a mix mode of crystal shape having facets of both {111} and {100} planes is observed. Hence it is not only bath chemistry, but the substrate also has a profound effect on deposited crystal shapes.
Footnotes
Disclosure statement
No potential conflict of interest was reported by the authors.