Shape Control of Electrodeposited Copper Films and Nanostructures

Mar 3, 2014 - ... (XRD) methods are employed to investigate the effects of alcohol additives and the organic additive malachite green (MG), on copper ...
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Shape Control of Electrodeposited Copper Films and Nanostructures through Additive Effects Yunyu Joseph Han,† Xin Zhang, and Gary W. Leach* Department of Chemistry and 4D LABS, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia Canada V5A 1S6 S Supporting Information *

ABSTRACT: The use of electrolyte additives to affect nanocrystallite shape and film morphology in electrodeposited copper films is presented. Linear sweep and cyclic voltammetry, atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) methods are employed to investigate the effects of alcohol additives and the organic additive malachite green (MG), on copper electrodeposited onto polycrystalline gold electrodes. The use of additives affects the deposition process by increasing cathodic peak potentials and decreasing corresponding peak currents. Copper films deposited from electrolyte solutions with additives show additive-specific nanostructure and crystallite morphology. Film analysis reveals a greater than five times reduction in both film roughness and grain size in the presence of even small concentrations of the additive MG. Use of MG results in the preferential electrodeposition of oriented, square pyramidal crystallites, while alcohol additives result in tetrahedral crystallite textures. These shape-controlled additive effects are supported by additive adsorption energy calculations, which indicate preferential interactions, and differential growth kinetics on different facets of the film’s growing nanostructures during electrodeposition. This approach offers a new and costeffective route to achieve shape-controlled surface nanostructure.

1. INTRODUCTION The replacement of aluminum by copper as the material of choice for the fabrication of electrical interconnects in integrated circuitry by the electronics industry resulted in the adoption of electrochemical metal plating methods by the industry in 1997.1,2 The improved conductivity and electromigration resistance characteristics of copper thus led to the implementation of the damascene copper electroplating process for on-chip metallization, and a renewed interest in metal electrodeposition methods and the factors that control it. A key aspect of this control is the use of additives and combinations of additives in the electrochemical baths during electrodeposition, which aid in the superfilling and superconformal metal coating of high aspect ratio chip features for high quality interconnects. Typical copper-sulfate-based plating baths2 employ three or four component additive mixtures including suppressor additives such as chloride ions and polyethylene glycol (PEG), or polypropylene glycol (PPG), accelerating or brightening agents such as bis(sodiumsulfopropyl) disulfide (SPS), and leveling agent(s) such as thiourea, benzotriazole (BTA), or Janus Green B. The prevailing chemistry can be complex and dependent on the relative nature, abundance, and synergies of the particular additive combinations. While significant progress in the development of appropriate additive combinations has provided for high quality copper interconnects, understanding the complex mechanisms of action of © 2014 American Chemical Society

additives, additive combinations, and their effects on the electrodeposition of metals more generally, remains poorly understood. The controlled deposition of metal and metal nanostructures is also important in the development of plasmonic-based devices,3,4 X-ray optics,5 high density electronics,6 catalysts,7,8 chemical sensors9 and magnetic recording applications,10 where metal crystallinity and orientation can often determine performance characteristics. The ability to deposit shapecontrolled metal nanostructures and nanostructured surfaces through electrodeposition represents an attractive alternative to physical vapor-based metal deposition and lithographic methods, without their associated costs, complexities, and challenges. While shape-controlled electrodeposition requires fine control over the kinetic and thermodynamic parameters of complex heterogeneous interfacial processes, the use of electrochemical additives offers the potential to aid in this control. A molecular level understanding of additive effects can in principle, be exploited to direct metal nucleation and growth in an analogous manner to that achieved through the chemical control of growth and morphology in the solution phase synthesis of shape-controlled nanostructures.11 Received: January 12, 2014 Revised: February 26, 2014 Published: March 3, 2014 3589

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382.9 g/mol) was obtained from Sigma Chemical Co. Reagent grade 1-propanol and other alcohols were supplied by Caledon. All chemicals were used without further purification. The pH values for all 0.050 M CuSO4 solutions with or without additives, were found to be within 4.0 ± 0.5 and showed little to no change during deposition, although not buffered. Unless otherwise specified, the potential scan rate was chosen to be 20 mV/s to avoid convection effects, and the concentration of CuSO4 was selected to be 0.050 M to minimize the effects of uncompensated solution resistance. Nucleation, growth and morphology of Cu films deposited under potentiostatic control (Topometrix bipotentiostat-galvanostat) were investigated by AFM in contact mode in air using a ThermoMicroscopes Explorer AFM equipped with a silicon nitride tip of force constant 0.2 N/m. AFM images (5 × 5 μm2) were recorded with a resolution of 200 × 200 data points. Electrodeposition potentials corresponded to the half Cu reduction peak current of the corresponding CV curve. SEM micrographs were taken with an FEI Strata DB235 workstation or an FEI Nova NanoSEM430 system. XRD was performed with a Rigaku R-AXIS RAPID-S X-ray diffractometer (Model No. 2163A101) with a copper target (λκα = 1.542 Ǻ ) and image plate detector. Accelrys Materials Studio (MS) 4.3 software package was employed to perform the interaction and adsorption energy calculations. The adsorption mechanism of MG on Au and Cu single crystal (111) and (100) surfaces has been investigated with consistent valence forcefield (CVFF), polymer consistent forcefield (PCFF), and condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) using its Adsorption Locator module.21,22

Copper nucleation and growth processes have been extensively studied on various single crystal substrates using in situ electrochemical scanning tunneling microscopy (STM) methods in order to identify nucleation behavior, growth modes and the factors that affect them.12−18 Previous efforts to control copper film nanostructure have included the electrochemical deposition of nanoparticles onto ultrathin polypyrrole films deposited on gold film electrodes, where crystal size and number density were found to be dependent on the thickness of the gold and polypyrrole films, the applied potential, and electrolyte concentration.19 High-order hierarchical copper structures have also been fabricated electrochemically with the aid of surfactants.20 Here, we describe the effects of alcohol additives and the additive malachite green (MG) on the electrochemical process of Cu electrodeposition onto polycrystalline gold substrates studied with linear sweep and cyclic voltammetry techniques. Interest in these additives derives from their potential use as shape control agents, and in the case of MG, its large surface nonlinear optical response, which could offer the potential of direct additive measurement in the electrochemical environment via surface nonlinear spectroscopy. The ability of the additives 1-propanol and MG to affect the morphology, surface roughness, grain size and crystallinity of electrodeposited Cu films is reported. On the basis of the electrochemical measurements, atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) studies, we demonstrate that both 1propanol and MG have pronounced effects on the resulting properties of the Cu deposits, yielding films with smaller grains, improved smoothness and more uniform texture, compared with those from additive-free electrodeposition. Furthermore, we demonstrate that even small concentrations of the additive MG have the ability to modify the nucleation and growth characteristics of electrodeposited Cu significantly, resulting in the preferential growth and orientation of square pyramidal crystallites. This behavior is distinct from that observed with other additives and reflects the particular importance of the additive in controlling the nucleation and growth kinetics that drive this preferential growth. We propose a simple model to explain the observations and validate this model through calculation of the additive adsorption energies.

3. RESULTS AND DISCUSSION 3.1. LSV and CV of Additive Solutions and Electrolytes. LSV and CV of 0.050 M CuSO4-based electrolytes with and without additives have been investigated to elucidate their effects on the electrodeposition of Cu onto polycrystalline Au substrates. For the sake of brevity, only the LSV and CV data of 1-propanol are presented, since its behavior reflects that of the alcohol additive series more generally. LSV studies of solutions containing the additives alone have also been performed to determine their electroactivity in the absence of Cu2+. The results of these control experiments are described in detail in the Supporting Information. They indicate that, while the aliphatic alcohols are not electroactive, they result in a significant additional barrier to hydrogen evolution, consistent with adsorption of additive molecules at the electrochemical interface, “blocking” electrode active sites and hindering electrode reactions. These “blocking effects”23 result in a large overpotential in the presence of 1-propanol, but require significant concentrations of 1-propanol (∼0.1 M and higher) in order to be observed. In contrast, addition of even very small concentrations of the organic additive MG gives rise to comparable shifts in the hydrogen evolution potential. As MG is a chloride salt, it produces MG cations and chloride anions, both potentially capable of affecting the electrochemical process. While chloride is known to have a bridging effect that increases the current (decreases overpotential) of cathodic reactions,23−25 at MG concentrations in excess of ∼5 × 10−4 M, the presence of the organic cations dominates, resulting in an increase in overpotential, consistent with a MG blocking mechanism. In order to avoid complications in the interpretation of the additive effects observed with MG, we have employed MG additive concentrations of 5 × 10−3 M, where the effects of adsorbed chloride ions are expected to be overwhelmed by those of the adsorbed organic cations. CV studies of 5 × 10−3 M MG solutions show a reduction potential of −530 mV and oxidation peak potentials at −50 mV

2. EXPERIMENTAL DETAILS Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were used to investigate Cu electrodeposition from aqueous CuSO4-based electrolyte solutions containing various concentrations of MG or alcohols of different carbon chain length. A typical three-electrode cell containing a Ag|AgCl (3.5 M NaCl) reference electrode (RE) and a platinum counter electrode (CE) was used for electrochemical measurements and Cu electrodeposition (EGG potentiostat (VersaStat II), Power Suite software). The working electrode (WE) was gold, deposited by vacuum evaporation onto freshly cleaved mica. The Au film thickness was nominally 100 nm, deposited at 10−6 Torr, 370 °C and a 1.2−1.4 nm/s rate (evaporation). Under these deposition conditions and without flame annealing, the Au substrates are polycrystalline, as confirmed by XRD. Electrochemical measurements were carried out both with and without supporting electrolyte to minimize, where possible, the number of electrolyte components and simplify the interpretation of additive effects, and to verify that the behavior observed in the absence of supporting electrolyte was not due to its absence or other nonadditive effects (see Supporting Information for more information). Aqueous electrolyte solutions were made with Millipore water (resistivity 18.2 MΩ·cm) consisting of 0.050 M CuSO4 (99.995% metal basis, Aldrich) with or without additives. The additive malachite green hydrochloride (basic green 4, C23H26N2O·HCl, MW = 3590

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and 1016 mV, indicating that MG is electroactive. The 1016 mV anodic feature is attributed to the oxidation of Au and the formation of surface Au oxides.26,27 In accord with previous electrochemical studies of MG, the cathodic process is assigned to the transformation of malachite green to leuco-malachite green (LMG).28,29 Its CV characteristics illustrate that this process is not reversible. 3.2. LSV and CV of CuSO4 Solutions. 3.2.1. Effects of 1Propanol on the LSV and CV of CuSO4. The effects of 1propanol on the linear potential sweep voltammograms and cyclic voltammograms of CuSO4 solutions are presented in Figure 1a. Similar to the case of hydrogen evolution, 1-propanol is expected to demonstrate blocking effects on the copper deposition process. The LSV and CV curves demonstrate that copper deposition and dissolution peak currents decrease with increase of the 1-propanol concentration. The reduction peak potential shifts negatively while oxidation peak potential shifts positively, increasing the separation of peak potentials as the concentration of 1-propanol increases. Figure 1b shows the narrowed difference between cathodic and anodic peak currents, ΔJp = Jap − Jcp and expanded separations between peak potentials, ΔEp = Eap − Ecp with increasing concentration of 1-propanol. These results are consistent with a model in which molecules of 1-propanol are adsorbed at the working electrode surface, increasing the resistance to charge transfer reactions. The larger the number of molecules of 1-propanol in solution, the larger the fraction of electroactive sites of the electrode surface that are occupied, increasing the barrier to charge transfer and reducing the peak currents. As the resistance to redox reactions increases, larger driving forces (higher potentials) are required to reach peak currents, raising the overpotential for copper deposition and dissolution processes, and increasing the separation between peak potentials. The effects of other alcohols, methanol, ethanol, 1-butanol, and 1-pentanol, on the cyclic voltammograms of CuSO4 have also been investigated, and similar effects were observed. The results show that small concentrations of alcohols have minor effects on the electrochemical process of CuSO4, but at concentrations greater than ∼0.10 M, longer chain alcohols have slightly larger effects on the redox process of copper ions. Figure 1c depicts the decrease of peak currents and increase of peak potentials caused by the increasing carbon numbers in the alcohols. Alcohol molecules are not electroactive under these experimental conditions, so their blocking effects to copper redox processes are expected to increase with their size, as longer molecules are expected to be more surface active and to occupy more surface area of the electrode per molecule, increasing the overpotentials of electron transfer reactions. Consistent with these comments, the observed decrease of peak currents and increase of peak potentials are roughly proportional to the number of carbons in their molecules. In addition to the observed blocking effects, 1-propanol also appears to change the mechanism of copper redox processes at the Au surface. Both the negatively and positively polarized slopes of the LSV and CV curves are decreased by addition of 1-propanol to the electrolyte solution, which indicates a slower charge transfer constant. A plateau appears after the cathodic peak, which shows a limiting current due to the limiting diffusion of electro-active species. These limiting currents are decreased with increasing 1-propanol concentration. This result may suggest the formation of copper-propanol complexes such as [Cu(C3H8O)n(H2O)6−n]2+ in which some of the water

Figure 1. (a) CV and LSV (inset) data of 0.050 M CuSO4 with different concentrations of 1-propanol. Scan rate: 20 mV/s. (b) Blocking effects of 1-propanol on the redox reactions of Cu on Au electrode: Peak currents decrease and peak potentials increase with increasing concentration of 1-propanol. (c) Effect of alcohol carbon numbers on peak currents and potentials of CV curves of 0.050 M CuSO4 on Au electrode. Alcohol concentrations are all 0.20 M. Scan rate is 20 mV/s. Data series color matches the colors of the axes’ labels. Dashed trend lines are shown to guide the eye. 3591

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that in additive-free solution, there is no obvious cathodic current until the Cu nucleation potential of −114 mV versus Ag|AgCl (3.5 M) is reached. As the potential is swept more negatively, the reduction current increases quickly due to the reduction of Cu2+, followed by a reduction current peak and plateau. Similar shaped CVs from copper sulfate solution on a glassy carbon substrate have previously been observed.32 In CuSO4 solutions with the additive MG, the CVs are modified significantly. The defined potentials and copper nucleation peak parameters from the CV curves and how they change with MG concentration (Table ESI 1, Supporting Information) indicate a strong blocking effect of MG on Cu deposition and can be observed through the significant shift of cathodic peak potential. As expected for the blocking mechanism, the nucleation potential En and cathodic peak potential Epc are shifted to more negative values while the nucleation overpotential Enop (Figure ESI 7), increases with increasing concentration of MG. These shifts continue with increasing MG concentration until the critical concentration of 0.0030 M, where the nucleation potential, nucleation overpotential, and peak cathodic potential reach asymptotic values. This behavior suggests that at this concentration the MG coverage on the electrode surface has reached a critical value. There is no apparent indication of complex formation between MG and the Cu2+ species in solution. The limiting current at the plateau is −5.2 mA·cm−2 and does not change with increasing MG concentration. The effect of MG on the dissolution of Cu is similar to that observed in the case of 1-propanol. Addition of MG shifts the anodic peak potential positively and tends to decrease the peak currents. Evidently, these additives not only block copper ions from depositing, but also prevent the deposited copper film from dissolving. 3.3. Morphologies of Copper Films on Au Electrodes. In order to examine the morphology of electrodeposited films deposited under different conditions on a normalized basis, films were deposited at their half cathodic peak current potentials (Ec1/2), as determined by the corresponding CV: 0.050 M CuSO4, Ec1/2 = −200 mV; 0.050 M CuSO4 + 1.0 M 1propanol, Ec1/2 = −200 mV; and 0.050 M CuSO4 + 0.0050 M MG, Ec1/2 = −500 mV. These potentials maintain Cu film growth at a moderate rate, yielding films of acceptable quality in a relatively short time. The electrodeposition current densities for additive-free and 1.0 M 1-propanol electrolyte solutions were 2.2 mA·cm−2, and 1.8 mA·cm−2 for films deposited from 0.005 M MG solution. Cu films are deposited on 1 × 1 cm2 polycrystalline Au-coated mica substrates, and are obtained under potentiostatic conditions. Transferred charges are proportional to film deposition time. In all cases, the Au substrates are polycrystalline. 3.3.1. SEM Study of Film Morphologies. Investigation of the morphology of the resulting electrodeposited films was carried out by SEM. Figure 3 shows SEM images of the electrodeposited Cu films from electrolyte with and without additives after 30 min of deposition and with nominal thicknesses of 1.3 μm. The SEM images reveal Cu film morphologies that are highly dependent on the presence and nature of the additive. The SEM images show that Cu films deposited from additivefree, 0.050 M CuSO4 electrolyte do not have a regular texture and are comprised of irregular crystallites of ∼1 μm dimension. In contrast, Cu films formed from electrolyte solutions containing 1-propanol exhibit regular tetrahedral and truncated tetrahedral crystallites of somewhat smaller sizes, while Cu films

molecules of hydrated copper ions could be displaced by 1propanol molecules to form a larger hydrated complex, with a smaller diffusion coefficient and correspondingly smaller limiting current. However, the absorbance spectra of 0.050 M aqueous CuSO4 showed no dependence on 1-propanol concentration, indicating that 1-propanol is incapable of displacing the solvating water molecules bound directly to the Cu2+ ions. This is consistent with the observed condensed phase dipole moments of water (2.9 D)30 and 1-propanol (2.36 D).31 However, a molecular dynamics study of aggregation phenomena in aqueous 1-propanol has shown that, through hydroxyl groups, 1-propanol molecules can form intense hydrogen bonds with water molecules.31 The coordination number of water and 1-propanol may vary from 0 to 3. Such a 1-propanol modified hexaaquacopper(II) with increased size would be expected to move more slowly and may account for the decreased slope and limiting currents of the voltammograms. 3.2.2. Effect of MG on the LSV and CV of CuSO4. Figure 2 shows the CV and LSV response (inset) obtained from 0.050

Figure 2. CV and LSV (inset) data of 0.050 M CuSO4 with different concentrations of malachite green. Scan rate: 50 mV/s.

M CuSO4 in the presence of the organic additive MG with concentrations ranging from 0 to 0.0050 M. In comparison with the effects of 1-propanol, these data show that MG has pronounced effects at concentrations ∼104 times lower than the additive 1-propanol. The LSV data show that an increase in the MG concentration significantly shifts the cathodic peak potentials negatively, reaching a limiting value of −660 mV at a MG concentration of approximately 0.0030 M (for clarity, the LSV and CV data for 0.0030 M are not shown in Figure 2). This limiting potential represents a substantial increase in overpotential for Cu electrodeposition compared to the additive-free peak potential of −300 mV. The slopes of the LSV cathodic currents of CuSO4 (Figure 2, inset) do not change at concentrations of MG below 0.0010 M, but become slightly smaller as the concentration of MG is increased, indicating the kinetics become somewhat slower with further addition of MG. In accord with the LSV data, the CV data show 3592

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Figure 3. SEM images of electrodeposited Cu films from different electrolyte solutions. (a) 0.050 M CuSO4, (b) 0.050 M CuSO4 + 1.0 M 1-propanol, (c) 0.050 M CuSO4 + 0.0050 M MG. Tilt angles for images a−c are 0°. (d) a magnified view for image c at a tilt angle of 45°.

electrodeposited from electrolytes containing MG, yield square pyramidal crystallites with dimensions ranging from tens to hundreds of nanometers. Since SEM top view images do not always provide detailed information regarding the shape and orientation of surface features, Figure 3d shows a tilt view of the crystallites deposited from electrolyte containing MG at higher magnification. It is apparent that there is a strong preference for the formation of square pyramidal crystallites and that these crystallites grow with an orientation normal to the surface of the electrode, with a few crystallites oriented at small angles from the surface normal. The grain size and film roughness also appear to be reduced with the addition of additives and even at a nominal film thickness of 1.3 μm, as indicated by AFM analysis, Cu films electrodeposited from solutions containing additives become smoother and brighter to the naked eye. These observations suggest that the observed change in microstructure is due to the nature of the copper nucleation and growth dynamics resulting from the presence of these additives. 3.3.2. AFM Study of Film Morphologies. Further morphological characterization of the electrodeposited films was carried out using AFM. The topographic, internal sensor images shown in Figure 4 describe the morphologies of the electrodeposited Cu films and are effectively raw data, processed only with simple sample leveling. Figure 4a−c represent AFM topographic images of Cu films from additive-free 0.050 M CuSO4, with 1.0 M 1-propanol, and with 0.0050 M MG electrolyte solutions, respectively. Consistent with the SEM micrographs, the AFM images indicate that both the shape and size of the deposited Cu grains have been altered significantly by the presence of the additives. The Cu grains from the 0.050 M CuSO4 electrolyte without additives are coarse, with many facets and random shape orientation. Grains from 0.050 M CuSO4 with 1.0 M 1propanol are somewhat finer, and many have the shape of

Figure 4. 2D and 3D AFM topographic images (5 × 5 μm) of Cu films on Au-coated mica, electrodeposited for 5 min from electrolyte solutions containing (a) 0.050 M CuSO4, (b) 0.050 M CuSO4 + 1.0 M 1-propanol, and (c) 0.50 M CuSO4 + 0.0050 M MG. Z scale is (a) 1.3 μm, (b) 830 nm, and (c) 240 nm.

tetrahedra, while the grains resulting from the electrodeposition from 0.050 M CuSO4 with 0.0050 M MG are the finest, and have the shape of square pyramids. Figure 4c shows grains that are truncated and have not yet attained their full square pyramidal structure and thus appear with square tops under the deposition conditions employed (vide infra). Quantitative analysis of film roughness and grain properties can be extracted from the AFM images and illustrate the magnitude of film structural differences that result from additive use. Together with Zmax, the maximum peak-to-valley feature range in the image area, the average feature height, Z̅ , the average roughness, Ra, and the root-mean-square roughness, Rrms, all demonstrate that the electrodeposited Cu films become smoother in the presence of electrolyte additives, and significantly so in the presence of MG. Table 1 illustrates that the use of additives also increases the density of grains, while decreasing the average grain volume (Vavg), grain height (Havg), maximum height (Hmax), and maximum grain diagonal (Dmax) dimensions. All trends extracted from the AFM analysis indicate that grain size and roughness of Cu films decrease with the presence of electrolyte additives. The parameters from both the area roughness and 3593

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Table 1. Area Standard Roughness and Grain Parameters of Electrodeposited Cu Filmsa

a

electrolyte

Ra (nm)

RRMS (nm)

Z̅ (nm)

Zmax (nm)

grain density (#/100 μm2)

Vavg (106 cm3)

Havg (nm)

Hmax (nm)

Dmax (nm)

no electrolyte (Au-mica) 0.050 M CuSO4 0.050 M CuSO4 + 1.0 M 1-propanol 0.050 M CuSO4 + 0.0050 M MG

3 120 96 26

8 160 121 33

59 504 319 106

179 1104 664 220

36 40 76

10.4 10.3 1.5

86.8 56.2 19.1

549.9 332.1 110.1

3300 2100 867

Extracted from AFM images in Figure 4. Scan range: 5 × 5 μm, electrodeposition time 5 min.

Z̅ =

1 N Σ Z; N 1 i

Ra =

1 N Σ |Z N 1 i

− Z̅ | ;

R rms =

1 N Σ ⟨Zi N 1

− Z̅ ⟩2

grain size analysis show that the roughness of Cu films from electrolyte solutions containing MG is greater than 5 times smaller than that of Cu films from additive-free solutions. Similar leveling and brightening effects of additives on Cu electrodeposits have been observed for polyethylene glycol (PEG) + Cl−, and 4,5-dithiaoctane-1,8-disulfonic acid (SPS) + PEG systems,33,34 but the smoothing effect of MG in this work appears to be more significant than that for PEG + Cl−.34 3.3.3. Crystallite Dependence on MG Concentration. The morphologies of electrodeposited grains have been investigated by AFM following electrodeposition with different concentrations of MG in the 0.050 M CuSO4 electrolyte. For each electrolyte solution concentration, the CV was obtained to determine the corresponding half-wave potential for electrodeposition. The corresponding AFM topographic images are displayed in Figure ESI 9. At a MG concentration of 10−4 M, where the bridging effects of MG’s chloride ions are expected to dominate the blocking effects of the organic cation, the resulting Cu nanostructure is not well developed, and very few of the crystallites show hints of square pyramidal structure. At a MG concentration of 5.0 × 10−4 M, where the MG’s organic cation effects begin to predominantly influence the electrochemistry, the film morphology is dominated by square pyramidal crystallites. Deposition under larger additive concentrations, results in larger, more well-developed crystallites that appear to have formed by the merging of smaller crystallites. These results indicate that MG additive concentration is an important parameter determining Cu nanocrystallite shape and size. 3.3.4. Crystallite Dependence on Electrodeposition Potential. The effects of electrodeposition potential on the morphologies of Cu films deposited from 0.050 M CuSO4 + 0.0050 M MG solution have also been investigated (AFM topographic images are presented in Figure ESI 10). When the cathodic potential is more positive than −400 mV, electrodeposition occurs quite slowly, resulting in many small Cu crystallites that are not well developed. When the potential is more negative than −400 mV, square pyramidal structures begin to form (6b). At an electrodeposition potential of −500 mV, the square pyramidal structures appear fully developed. Deposition at higher overpotentials leads to film structures characterized by much larger crystallites and much larger variation in crystallite size, resulting in significant increases in film roughness. Based on these observations, it appears that −500 mV is a suitable potential for the deposition of the square pyramidal copper deposits from 0.050 M CuSO4 + 0.0050 M MG solution. 3.3.5. Crystallite Dependence on Copper Film Thickness. The crystallite size and structure have been investigated as a function of electrodeposition time and film thickness. AFM images of Cu films deposited up to 320 min have been analyzed over length scales ranging from 1 × 1 μm up to 80 × 80 μm in

order to examine the evolution of crystallite growth. The roughness and grain size appear to change and are dependent on electrodeposition time and thickness of the Cu films. Figure 5 shows 5 × 5 μm 3-D topographic images of copper films from 0.050 M CuSO4 + 0.0050 M MG solution at different electrodeposition times.

Figure 5. AFM topographic images (5 × 5 μm) of Cu films of different thickness from 0.050 M CuSO4 + 0.0050 M MG solution. Nominal thicknesses (deposition times) of Cu films are (a) 200 nm (5 min), (b) 1600 nm (40 min), (c) 6400 nm (160 min), and (d) 12800 nm (320 min).

Not only does the shape and morphology of the square pyramidal features become clearer with increasing thickness, but also the grain sizes get larger as the film thickness grows, resulting in increasing film roughness with deposition time. The increase in crystallite size and surface roughness with deposition time appears to be accompanied by a decrease in crystallite number density, consistent with crystallite growth that results from the merging of smaller crystallites to form larger structures, while maintaining the underlying mechanisms defining the square pyramidal shape preference. 3.4. X-ray Diffraction Studies. The SEM and AFM images described above indicate that use of electrolyte additives yields electrodeposited Cu films containing crystallites of preferential shape and orientation. X-ray diffraction has been employed to help quantify the degree of shape and orientation preference. The XRD patterns of electrodeposited Cu films onto polycrystalline Au electrodes from CuSO4 electrolytes with and without additives are presented in Figure 6. The observed diffraction peaks can be indexed to cubic Cu (JCPDS 4-836) or cubic Au (JCPDS 4-784) as indicated in Figure 6. The relative peak intensities of the electrodeposited Cu from all electrolytes are somewhat different to those of cubic Cu powder. Film 3594

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under similar conditions. With this definition, the ASTCs of all 7 of the Cu reflections displayed in Figure 6 have been calculated and are displayed in Table 2. Note that the ASTCs of the electrodeposited films obtained in the absence of electrolyte additives have, by definition, all ASTC(hkl) = 1. Thus, the ASTC values from films obtained in the presence of electrolyte additives are a measure of the degree to which the texture of the resulting film differs from that in the absence of additives. ASTC(hkl) values larger than 1 indicate a larger number of (hkl) planes parallel to the substrate surface, while values less than 1 indicate fewer (hkl) planes parallel to the substrate surface. The additive-specific texture coefficients displayed in Table 2 describe the shape and orientation preference of electrodeposited Cu crystallites resulting from additive effects. The magnitude of the ASTCs for CuSO4 + 1-propanol indicate modest texturing effects with ASTCs of magnitudes 80−90% of those for additive-free deposition for (111), (200), and (220) reflections and ASTCs close to those for additive-free deposition for the remaining reflections. The relative decrease in diffraction intensity from the (111), (200), and (220) reflection planes is consistent with the observation of tetrahedral Cu crystallites with a broad distribution of orientations, consistent with the SEM and AFM observations. In contrast, the ASTCs for MG-assisted deposition show more significant departures from 1, consistent with the high degree of shape and orientation preference observed in the SEM and AFM images. In particular, the magnitudes of the ASTCs for the (111) and (200) reflection planes indicate that there is considerably smaller (∼50%) diffraction intensity from (200) reflections relative to that from (111) reflection planes, compared to additive-free deposition. ASTC’s of this magnitude indicate that there are significantly fewer (200) reflection planes contributing to the diffraction intensity.

Figure 6. XRD patterns of Cu films deposited on Au/mica substrates from electrolytes containing (upper) 0.050 M CuSO4; (middle) 0.050 M CuSO4 + 0.0050 M MG; (lower) 0.050 M CuSO4 + 1.0 M 1propanol.

texture is typically assessed on the basis of the relative XRD peak intensities and compared to those expected from the corresponding powder sample. Texture coefficients (TCs) for each (hkl) reflection plane can be defined as TC(hkl) =

I(hkl) I0(hkl)

1 n

∑

I(hkl) I0(hkl)

(1)

where TC(hkl) is defined as the texture coefficient of a specific (hkl) plane, I(hkl) is the measured relative intensity, I0(hkl) is the relative intensity from a standard powder diffraction given by the JCPDS data, and n is the total number of reflections considered.35,36 In order to assess the effects of texturing that result from a particular additive, here we define an additive-specific texturing coefficient (ASTC), where, rather than comparing the relative XRD peak intensities to those of the corresponding powder, the ASTCs are calculated by comparison to the relative peak intensities I0(hkl) of the electrodeposited film in the absence of electrolyte additives. This definition is a more direct measure of the additive’s texturing abilities, since there is no a priori expectation that an electrodeposited film should manifest the XRD pattern of a random powder sample. Further, it has the added benefit of removing any instrumental artifacts from the calculated ASTCs, since all XRD data involved in their calculation has been obtained with the same instrument

4. PROPOSED GROWTH MECHANISM Insight into the nature of the preferred square pyramidal shape of copper crystallites deposited from MG-containing electrolytes can be garnered from the observed ASTCs and from direct measure of angles between the observed crystal facets from the AFM and SEM images. Measurement over multiple crystallites provides an average angle between square pyramidal top and sidewall crystal faces of 53.7 ± 5.4°. This is in agreement with the 54.7°angle between (111) and (200) reflection planes of a cubic crystal. This observation is consistent with the calculated ASTCs if one assumes that the preferred shapes of Cu grains observed in the presence of MG reflect square pyramids with angled sidewalls corresponding to (111) facets and the flat tops parallel to the substrate,

Table 2. Relative XRD Intensities (%) and Additive-Specific Texture Coefficients (ASTCs) of Electrodeposited Cu Films from Different Electrolyte Solutions Cu(hkl)

2θ (deg)

rel. Int. JCPDS powder

rel. int. CuSO4

rel. int. CuSO4 + propanol

rel. int. CuSO4 + MG

ASTC CuSO4

ASTC CuSO4 + propanol

ASTC CuSO4 + MG

(111) (200) (220) (311) (222) (400) (331)

43.3 50.4 74.1 89.9 95.1 116.9 136.5

100 46 20 17 5 3 9

100 42 7 10 4 2 8

100 44 8 13 5 3 12

100 21 5 14 4 3 10

1 1 1 1 1 1 1

0.81 0.85 0.89 1.06 1.08 1.04 1.24

0.99 0.48 0.61 1.36 1.19 1.11 1.25

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state, these simulations indicate that adsorption energies that differ by as little as 10% are sufficient to greatly modify the resulting nucleation and growth rates for Cu electrodeposition onto Au. The adsorption energies of MG on Cu(111) and Cu(200) surfaces are similar to those of Au. The lower adsorption energy on the Au(200) and Cu(200) surfaces indicates weaker interaction between MG and (200) facets, providing more opportunity for copper ions to be reduced and deposited on (200) facets, resulting in Cu crystals that grow along the Cu(200) direction to form square pyramidal structures. This growth is consistent with the observations of Randler et al., who have employed in situ STM and in situ Xray diffraction to show that Cu deposition on Au(100) proceeds in a layer-by-layer fashion.17 In an analogous manner to the shape controlled solution phase growth of nanostructures, the use of additives in electrodeposition offers the potential to fine-tune the nucleation and growth characteristics at the electrochemical interface and to generate shape and orientation controlled nanostructured surfaces. In situ electrochemical STM studies of Cu nucleation and growth on single crystal surfaces in the presence of additives should lend additional support for this mechanism. These experiments are currently underway and will appear in a forthcoming publication.

associated with (200) planes. Preferential shape and orientation of such crystallites would be expected to yield relatively weaker (200) diffraction intensity as well-developed square pyramidal (i.e., tapered) crystallites possess fewer Cu atoms in (200) reflection planes. Figure 7 shows an AFM image of a square pyramidal structure and the corresponding crystallite geometry consistent with its shape.

5. CONCLUSIONS The effects of electrolyte additives on Cu electrodeposition onto polycrystalline Au substrates have been examined using linear sweep and cyclic voltammetry. Both alcohols (1propanol) and malachite green (MG) act as blocking agents on electrochemical reactions and result in significant overpotentials to Cu nucleation, characterized by increasing cathodic peak potentials and decreasing peak currents, with increase in additive concentration. SEM and AFM studies of the resulting film morphology and crystallinity show evidence for the shape controlled electrodeposition of copper nanostructures with preferential orientation. Use of the additive 1propanol leads to tetrahedral Cu crystallites, while Cu deposition in the presence of the additive MG results in oriented square pyramidal crystallites with the Cu(200) plane preferentially oriented parallel to the substrate electrode. Film morphology can be tuned by control over the crystallite size and shape, and is achieved by manipulation of the deposition potential, additive concentration, and electrodeposition time. The use of additives is seen to reduce the area roughness and critical dimensions of the Cu grains significantly. Additiveguided electrodeposition is attributed to preferential interaction of the additives with particular crystal facets of growing nanostructures and is supported by calculated adsorption interaction energies. MG interactions with the Au(111) and Cu (111) surfaces are roughly 10% stronger than those with the corresponding (100) surfaces, leading to differential nucleation and growth kinetics and ultimately resulting in preferential crystallite shape and orientations. The observations in this work offer the potential to exploit additive interactions to produce shape-controlled and tailored nanostructured surfaces in a costeffective way. Further investigations into the factors responsible for these effects using in situ electrochemical scanning tunneling microscopy and in situ sum frequency generation (SFG) to probe the additives directly, are currently underway in our laboratory.

Figure 7. A model describing the growth of square pyramidal Cu crystallites in the presence of electrolyte additive MG. (a) Left: AFM image showing a square pyramidal Cu crystallite. Right: representation of the square pyramidal crystallite with (200) top facet and (111) angled side walls. (b) A molecular level model for crystallite growth based on adsorption energy calculations. Preferential interaction of the MG additive molecules with the (111) facets leads to additive blocking effects and differential growth kinetics on the two facets, resulting in faster growth in the (200) direction (see text).

Formation of specific shapes and orientations in the presence of additives may result from the preferential adsorption of additive molecules onto particular facets of the growing electrodeposited crystallites. One possible mechanism for this film growth is that MG molecules preferentially occupy surface sites on the (111) surface over those of the (200) surface. Stronger interaction of additive molecules with the (111) facet could effectively “block” the deposition of Cu onto this facet, leaving Cu to grow faster in other directions (e.g., the 200 direction). Figure 7 describes a plausible model illustrating the growth of square pyramids while MG is present in electrolyte solution. In order to assess the merits of this model, the Accelrys Materials Studio (MS) 4.3 software package was employed to perform interaction and adsorption energy calculations. The adsorption mechanism of MG on Au and Cu single crystal (111) and (100) surfaces has been investigated with consistent valence forcefield (CVFF), polymer consistent forcefield (PCFF), and condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) of the Adsorption Locator module of the program.21,22 The results of these calculations indicate preferential adsorption of MG on the Au(111) surface over the Au(100) surface and that interaction of the MG additive with the Au(111) surface is energetically more favorable than its interaction with the Au(100) facet by approximately 10%. Since adsorption reflects a dynamic equilibrium between MG in the electrolyte and in the adsorbed 3596

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ASSOCIATED CONTENT

S Supporting Information *

Additive effects on the hydrogen evolution reaction, cyclic voltammetry of MG, effect of MG on the CV of CuSO4, effects of supporting electrolyte, crystallite dependence on MG concentration, and crystallite dependence on electrodeposition potential. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 778 782 8065. E-mail: [email protected]. Present Address †

Biogate Laboratories, #110-4238 Lozells Ave., Burnaby, BC, V5A 0C4 Canada. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out with financial support from the Natural Sciences and Engineering Research Council of Canada and Simon Fraser University. This work also made use of 4D LABS shared facilities supported by the Canada Foundation for Innovation (CFI), British Columbia Knowledge Development Fund (BCKDF) and Simon Fraser University.

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ABBREVIATIONS LSV, linear sweep voltammetry; CV, cyclic voltammetry; AFM, atomic force microscopy; SEM, scanning electron microscopy; XRD, X-ray diffraction; MG, malachite green; STM, scanning tunneling microscopy; RE, reference electrode; CE, counter electrode; WE, working electrode; rms, root-mean-square; 2D, two-dimensional; 3D, three-dimensional

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