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Effect of pulse frequency and current density on anomalous composition and nanomechanical property of electrodeposited Ni–Co films
Thin Solid Films 517 (2009) 4800–4804
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effect of pulse frequency and current density on anomalous composition and nanomechanical property of electrodeposited Ni–Co films C.K. Chung ⁎, W.T. Chang Department of Mechanical Engineering, and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan 701, ROC
a r t i c l e
i n f o
Available online 20 March 2009 Keywords: Anomalous codeposition Nickel–cobalt alloy Pulse electrodeposition Hardness
a b s t r a c t Effect of pulse frequency and current density on the anomalous cobalt content and nanomechanical property of the electrodeposited nickel–cobalt (Ni–Co) films has been investigated. The composition, morphology, phase and hardness of the Ni–Co alloy films were examined by scanning electron microscope with an attached energy dispersive X-ray spectroscope, X-ray diffraction and nanoindentation techniques, respectively. The different Co composition of the Ni–Co films codeposited from the fixed sulfamate–chloride bath is subject to the pulse frequencies and current densities. The frequencies varied from 0 to 100 Hz and current densities varied from 1 to 20 ASD (ampere per square decimeter). The Co composition has no significant variation in pulse electrodeposition but it is greatly influenced by current densities from 22.53% at 1 ASD decreased to 13.39% at 20 ASD under DC codeposition. The mean hardness of Ni–Co films has no eminent change at a pulse frequency of 10–100 Hz but it decreases with current densities from 8.72 GPa (1 ASD) to 7.13 GPa (20 ASD). The smoother morphology can be obtained at higher pulse frequency or lower current density. Good Ni–Co films with high hardness and smooth morphology can be obtained by reducing current density and increasing pulse frequency. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Electroplating is a powerful surface-finishing process for meeting the requirements of decorative and functional applications. It promotes the appearance, extends the life, and improves the performance of materials. Ni–Co alloy is one of the most important engineering materials for several decades because of their unique properties, such as high hardness, good wear resistance, electrocatalytic activity [1]. How to electroplate the Ni-Co film with high hardness and smooth morphology is a crucial issue for application. The electroplating of Ni–Co films has been recognized as an anomalous codeposition, that is, the composition of alloy films is different from that in electrolyte composition [2–4]. The anomalous composition was related to the current density, temperature, pH, applied potential and other variables in electrolyte and would affect the properties of alloy. Iron group alloys i.e. Fe, Co and Ni usually exhibit the anomalous phenomena during codeposition. Some researchers reported that the anomaly may be attributed to the Fe (OH)2 formation depresses Ni deposition for Ni–Fe alloys [5] or more active metal monohydroxide preferred adsorption on the deposits, and inhibiting the less active metal ions [6–9]. Also, the anomalous codeposition can be inhibited by applying cyclic voltammetry (CV) [10–12]. The addition of different anion [13] or cationic surfactant [14] even nano-particle [15] in the electrolyte could also modify the
⁎ Corresponding author. Tel.: +886 6 2757575x62111; fax: +886 6 2352973. E-mail address: [email protected] (C.K. Chung). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.087
composition and morphology of deposited films. The effect of the pulse electrodeposition on pure Ni was investigated for higher hardness, smooth and dense films [16]. The pulse frequency effect on the anomaly and property of the electrodeposited Ni–Co films will be worthy of studying. With regard to nanomechanical property of the electrodeposited Ni–Co films, it was measured by nanoindentation techniques. The nanohardness of films is calculated by the proportion of loading force to the indentation area at the unloading step in the loading-andunloading curve whose slope is used for stiffness [17]. Continuous stiffness measurement (CSM) in nanoindentation testing allows the measurement of contact stiffness along with load and displacement data at any point on the loading curve and not just at the unloading point in the conventional indentation measurement [17,18]. In this article, the influence of current density and pulse frequency on the anomalous Co content and property variation of the Ni–Co alloy films has been investigated. CSM technique was used to measure the nanohardness of Ni–Co films. The relationship between the anomalous Co content, morphology, phase and nanohardness was correlated and discussed. 2. Experimental procedures The p-type Si(100) wafers were diced into 5 mm × 5 mm chips as the substrates of electrodeposition. The Cr/Au seed layers about 40/80 nm thick were deposited on the Si substrates by electron beam evaporator (ULVAC, VT1-10CE, Japan) and cleaned by
C.K. Chung, W.T. Chang / Thin Solid Films 517 (2009) 4800–4804
conventional steps of acetone (ACE), isopropanol (IPA) and sulfuric acid (H2SO4) solution prior to electrodeposition in the electrolyte. The composition of the electrolyte bath solution consisted of nickel sulphamate of 500 ± 20 ml/l, cobalt sulfamate 20 ± 1 ml/l, nickel chloride of 4 ± 1 g/l, wetting agent 0.5 ±0.05 ml/l and boric acid 42 ± 2 g/l. The temperature was kept at 50 ± 1 °C. The initial pH of the electrolyte was controlled at 4.2. The power supply was controlled by potentiostat/golvanostat (Jiehan5000, Taiwan). Pulse frequency was ranged from 10 to 100 Hz with a duty cycle (Ton/Toff) of 1 at constant current density of 3 ASD (ampere per square decimeter) and DC current density varied from 1 to 20 ASD. The average thickness of alloy films was about 600–700 nm which was measured by Alpha-Step Profilometer (Surfcorder, ET3000, Kosaka Laboratory Ltd., Japan). The microstructure, phases, and crystallinity of Ni–Co films were identified by grazing incidence xray diffractometry (GIXRD, PANalytical X'Pert PRO, Philips, Netherlands) using CuKα radiation (0.15418 nm). The surface morphology and composition was determined by Field-Emission Scanning Electron Microscope (FE-SEM) with an attached Energy Dispersive X-ray (EDX) Spectroscope (FE-SEM, JEOL JSM-7000, Japan). The nanomechanical properties of the films were measured using a MTS Nano Indenter® with CSM in XP mode and a Berkovich indenter. Since the thickness of the soft Ni–Co films on hard Si substrates is about 600–700 nm, the hardness is obtained from a contact depth of 120–180 nm to avoid the surface and substrate effects. Six
Fig. 1. The effect of: (a) pulse frequency at constant 3 ASD and (b) DC current density on the anomalous Co content of the codeposited Ni–Co films.
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Table 1 The EDX measured cobalt (Co) content in Ni–Co alloys. Co content in Ni–Co alloys Varied frequency
Co %
Varied current density
Co %
DC (0 Hz) 10 Hz 50 Hz 100 Hz
21.20 ± 0.54 19.94 ± 0.47 20.31 ± 0.36 20.16 ± 0.78
1 ASD 3 ASD 5 ASD 10 ASD 20 ASD
21.18 ± 0.74 21.20 ± 0.54 20.81 ± 0.42 15.26 ± 1.23 13.19 ± 1.20
indentations were done in each sample for the average value with a deviation.
3. Results and discussion The electrodeposition of Ni–Co alloy films has been recognized as an anomalous codeposition, i.e., cobalt atomic content (at.%) in the codeposited films is more than that in the electrolyte composition. Fig. 1(a)–(b) shows the effect of the pulse frequency at constant 3 ASD and the DC current density on the anomalous Co content of the codeposited Ni–Co films, respectively. The frequency varies from 0 Hz (DC) to 100 Hz and current density varies from 1 ASD to 20 ASD. Table 1 lists the EDX measured Co content of each film. Compared to about 4% Co content in electrolyte, the anomalous Co composition in
Fig. 2. GIXRD patterns of Ni–Co alloy films codeposited: (a) at various pulse frequency of 0–100 Hz at constant 3 ASD. (b) at various DC current densities of 1–20 ASD. The diffraction peaks (⁎) come from the Au seed layer.
4802
C.K. Chung, W.T. Chang / Thin Solid Films 517 (2009) 4800–4804
Fig. 3. SEM micrographs of the electroplated Ni–Co films at frequencies of 0 Hz (DC), 10 Hz, 50 Hz and 100 Hz. The morphology becomes smoother at higher pulse frequency.
the Ni–Co films is around 13%–21% due to the high exchange current density of Co. In terms of pulse frequency, the Co content decreases from 21.20% (0 Hz) to 20.16% (100 Hz) at constant 3 ASD (Fig. 1a). It indicates that DC and pulse deposition modes have no significant effects on the composition. However, the Co content distinctly
decreases with increasing current density from 21.18% (1 ASD) to 13.19% (20 ASD) at DC codeposition (Fig. 1b). Although the mechanisms of anomalous codeposition are not well understood, two different mechanisms can be used to describe the anomalous codeposition behavior, i.e., two-step adsorption and hydroxide
Fig. 4. SEM micrographs of the electroplated Ni–Co films at DC current densities of 1–20 ASD. The morphology becomes rougher at higher current density.
C.K. Chung, W.T. Chang / Thin Solid Films 517 (2009) 4800–4804
mechanisms. The first two-step adsorption mechanism is assumed that metal deposition occurs in two step manner via an adsorbed intermediate as follows: M2+ + e − YM + ðadÞ M + ðadÞ + e − YM − ðsÞ
ð1Þ
One M2+ ion is assumed to be reduced to an M+ (ad) ion, which adsorbs on the electrode surface and is further reduced to the metal M(s). Metal ions do not directly arrive at cathode from anode but in the middle compartment of bulk electrolyte; other reactions would occur to influence the deposition of Ni ions. The standard equilibrium electrode potentials of Ni2+ + 2e±Ni and Co2+ + 2e±Co are −0.257 V and −0.277 V versus standard hydrogen electrode (NHE), respectively. The Co metal with low electrode potential is preferentially to be deposited on the cathode subject to anomalous behavior. In electrodeposition process, besides metal is adsorbed on electrode surface, hydrogen ion was also reduced simultaneously. The other hydroxide mechanism is that the pH value at the cathode surface during process is assumed to rise due to H+ ions consumption at the electrode and resulting in hydroxide ions. It is expressed in chemical reactions below: 2H + + 2e − YH2 2H2 O + 2e − YH2 + 2OH −
ð2Þ
Under these conditions, the surface pH could rise sufficiently high so that metal ions would undergo hydrolysis reaction with OH−. The local increase of pH results in the formation of hydroxide precipitate of the low electrode potential metal (Co in this case) at the cathode and its hydroxide (Co(OH)+) may suppress deposition of the high electrode potential metal (Ni). Therefore, increasing current density may consume metal ions rapidly to reduce the concentration gradient of metal ions with the deposition time and leads to the decrease of anomalous behavior near cathode surface with reduced diffusion. Fig. 2(a)–(b) show the GIXRD patterns of Ni–Co alloy films codeposited at frequencies of 0–100 Hz at constant 3 ASD and DC current density of 1–20 ASD, respectively. The strong intensity of peak at 2θ = 44.7° are from Ni(111) plane together with other two distinct peaks Ni(200) and Ni(220) at approximately 2θ = 52° and 77° for the polycrystalline phase characteristics. The peaks marked by ⁎ symbol are diffracted from the Au seed layer. The Ni(111) peak intensity increases with increasing current density due to high Ni or low Co content in the Ni–Co film. It implies that the highest packing density of fcc-Ni(111) plane dominates the growth orientation of film during codeposition. Fig. 3 shows the SEM micrographs of the electroplated Ni–Co films at pulse frequencies of 0 Hz (DC), 10 Hz, 50 Hz and 100 Hz. The morphology becomes smoother at higher pulse frequency. This is due to more new nuclei formed on the surface of the cathode to result in a refined grain size as well as relaxation time enhanced adsorption of inhibiting species for blocking of grains growth at high frequency pulse deposition [16]. Moreover, Fig. 4 shows the SEM micrographs of the electroplated Ni–Co films at DC current densities of 1, 3, 5, 10 and 20 ASD. The morphology becomes rougher at higher current density. This is similar to a common phenomenon of a rougher deposit obtained for higher current density deposition with higher deposition rate. Fig. 5(a)–(b) shows the hardness of Ni–Co films as a function of the pulse frequency and current density, respectively. The hardness of all Ni–Co films is higher than pure Ni (4 GPa) due to the addition of hard Co content. In terms of pulse frequency, the mean hardness at 10–100 Hz is close to being 6.67–6.82 GPa (10–100 Hz) while that at 0 Hz (DC deposited film) is high for 8.45 GPa. Since the Co content is not significantly affected by varying the deposition mode from DC to pulse plating, hardness of Ni–Co deposits should be related to grain size, roughness and/or inner stress. The smaller grain size is usefully beneficial for enhanced hardness. The calculated mean grain size of
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Ni–Co deposits slightly decreases with frequency from 12.3 nm to 10.5 nm. Also, the inner stress of the electrodeposited film is less than GPa which can have little contribution to the film hardness. Therefore, roughness effect is the most likely reason for the DC deposit with higher hardness than pulse deposits. Because the hardness by nanoindentation is calculated from the ratio of loading force to the indentation area, the rougher surface of the DC deposit may remain the underestimated area for the higher calculated hardness value. In terms of DC current density, the mean hardness decreases a little from 8.7 GPa (1 ASD) to 7.13 GPa (20 ASD) due to less amount of hard Co content. The effect of current density on the trend of hardness variation of Ni–Co films is similar to that of Co content (Fig. 1(b)). And the morphology becomes rougher at high current density (Fig. 3). Overall, reducing current density and increasing pulse frequency will be beneficial for the formation of good Ni–Co films with high hardness and smooth morphology.
4. Conclusions The composition, surface morphology and nanomechanical properties of the anomalous Ni–Co alloy films have been investigated at various pulse frequencies and current densities. The anomalous behavior of electrodeposited Ni–Co films is due to the high exchange current density of Co content with low electrode potential and preferred adsorption of more active Co(OH)+ on the deposit
Fig. 5. (a) The hardness of Ni–Co films as a function of pulse frequency at constant 3 ASD, and (b) The hardness of Ni–Co films as a function of DC current density.
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C.K. Chung, W.T. Chang / Thin Solid Films 517 (2009) 4800–4804
compared to Ni content. The Co content decreases from 22.53% (1 ASD) to 13.39% (20 ASD) at DC codeposition but has no pronounced variation for the DC and pulse deposition modes. The pulse Ni–Co deposits with close Co content also have near hardness. The surface morphology of films becomes smoother at high pulse frequency and low current density. The reduced Co content in Ni–Co films can result in lower hardness which decreases from 8.72 GPa (1 ASD) to 7.13 GPa (20 ASD) at DC current. Overall, the formation of good Ni–Co films with high hardness and smooth morphology can be obtained by reducing current density and increasing pulse frequency.
Acknowledgements This work is partially sponsored by National Science Council under contract No 96-2622-E-006-007-CC3. We would like to thank the Center for Micro/Nano Science and Technology in National Cheng Kung University for the access of analysis equipments.
References [1] R. Orinakova, A. Turonova, D. Kladekova, M. Galova, R.M. Smith, J. Appl. Electrochem. 36 (2006) 957. [2] G. Qiao, T. Jing, N. Wang, Y. Gao, X. Zhao, J. Zhou, W. Wang, J. Electrochem. Soc. 153 (2006) C305. [3] L. Wang, Y. Gao, Q. Xue, H. Liu, T. Xu, Appl. Surf. Sci. 242 (2005) 326. [4] Y. Zhuang, E.J. Podlaha, J. Electrochem. Soc. 150 (2003) C219. [5] H. Dahms, I.M. Croll, J. Electrochem. Soc. 112 (1965) 771. [6] J.O'M. Bockris, D. Drazic, A.R. Despic, Electrochem. Acta 4 (1961) 325. [7] S. Hessami, C.W. Tobiaas, J. Electrochem. Soc. 136 (1989) 3611. [8] K.M. Yin, J.H. Wei, J.R. Fu, B.N. Popov, S.N. Popova, R.E. White, J. Appl. Electrochem. 25 (1995) 543. [9] J. Vaes, J. Fransaer, J.P. Celis, J. Electrochem. Soc. 147 (2000) 3718. [10] A. Bai, C.C. Hu, Electrochem. Acta 47 (2002) 3447. [11] C.C. Hu, A. Bai, J. Electrochem. Soc. 149 (2002) C615. [12] A. Bai, C.C. Hu, T.C. Wen, Electrochem. Acta 48 (2003) 2425. [13] D. Golodnitsky, N.V. Gudin, G.A. Volyanuk, J. Electrochem. Soc. 147 (2000) 4156. [14] E. Gomez, S. Pane, X. Alcobe, E. Valles, Electrochem. Acta 51 (2006) 5703. [15] L. Shi, C.F. Sun, F. Zhou, W.M. Liu, Mater. Sci. Eng. A 397 (2005) 190. [16] C.K. Chung, W.T. Chang, Microsys. Technol. 13 (2007) 537. [17] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [18] http:// www.mtsnano.com.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effect of pulse frequency and current density on anomalous composition and nanomechanical property of electrodeposited Ni–Co films C.K. Chung ⁎, W.T. Chang Department of Mechanical Engineering, and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan 701, ROC
a r t i c l e
i n f o
Available online 20 March 2009 Keywords: Anomalous codeposition Nickel–cobalt alloy Pulse electrodeposition Hardness
a b s t r a c t Effect of pulse frequency and current density on the anomalous cobalt content and nanomechanical property of the electrodeposited nickel–cobalt (Ni–Co) films has been investigated. The composition, morphology, phase and hardness of the Ni–Co alloy films were examined by scanning electron microscope with an attached energy dispersive X-ray spectroscope, X-ray diffraction and nanoindentation techniques, respectively. The different Co composition of the Ni–Co films codeposited from the fixed sulfamate–chloride bath is subject to the pulse frequencies and current densities. The frequencies varied from 0 to 100 Hz and current densities varied from 1 to 20 ASD (ampere per square decimeter). The Co composition has no significant variation in pulse electrodeposition but it is greatly influenced by current densities from 22.53% at 1 ASD decreased to 13.39% at 20 ASD under DC codeposition. The mean hardness of Ni–Co films has no eminent change at a pulse frequency of 10–100 Hz but it decreases with current densities from 8.72 GPa (1 ASD) to 7.13 GPa (20 ASD). The smoother morphology can be obtained at higher pulse frequency or lower current density. Good Ni–Co films with high hardness and smooth morphology can be obtained by reducing current density and increasing pulse frequency. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Electroplating is a powerful surface-finishing process for meeting the requirements of decorative and functional applications. It promotes the appearance, extends the life, and improves the performance of materials. Ni–Co alloy is one of the most important engineering materials for several decades because of their unique properties, such as high hardness, good wear resistance, electrocatalytic activity [1]. How to electroplate the Ni-Co film with high hardness and smooth morphology is a crucial issue for application. The electroplating of Ni–Co films has been recognized as an anomalous codeposition, that is, the composition of alloy films is different from that in electrolyte composition [2–4]. The anomalous composition was related to the current density, temperature, pH, applied potential and other variables in electrolyte and would affect the properties of alloy. Iron group alloys i.e. Fe, Co and Ni usually exhibit the anomalous phenomena during codeposition. Some researchers reported that the anomaly may be attributed to the Fe (OH)2 formation depresses Ni deposition for Ni–Fe alloys [5] or more active metal monohydroxide preferred adsorption on the deposits, and inhibiting the less active metal ions [6–9]. Also, the anomalous codeposition can be inhibited by applying cyclic voltammetry (CV) [10–12]. The addition of different anion [13] or cationic surfactant [14] even nano-particle [15] in the electrolyte could also modify the
⁎ Corresponding author. Tel.: +886 6 2757575x62111; fax: +886 6 2352973. E-mail address: [email protected] (C.K. Chung). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.087
composition and morphology of deposited films. The effect of the pulse electrodeposition on pure Ni was investigated for higher hardness, smooth and dense films [16]. The pulse frequency effect on the anomaly and property of the electrodeposited Ni–Co films will be worthy of studying. With regard to nanomechanical property of the electrodeposited Ni–Co films, it was measured by nanoindentation techniques. The nanohardness of films is calculated by the proportion of loading force to the indentation area at the unloading step in the loading-andunloading curve whose slope is used for stiffness [17]. Continuous stiffness measurement (CSM) in nanoindentation testing allows the measurement of contact stiffness along with load and displacement data at any point on the loading curve and not just at the unloading point in the conventional indentation measurement [17,18]. In this article, the influence of current density and pulse frequency on the anomalous Co content and property variation of the Ni–Co alloy films has been investigated. CSM technique was used to measure the nanohardness of Ni–Co films. The relationship between the anomalous Co content, morphology, phase and nanohardness was correlated and discussed. 2. Experimental procedures The p-type Si(100) wafers were diced into 5 mm × 5 mm chips as the substrates of electrodeposition. The Cr/Au seed layers about 40/80 nm thick were deposited on the Si substrates by electron beam evaporator (ULVAC, VT1-10CE, Japan) and cleaned by
C.K. Chung, W.T. Chang / Thin Solid Films 517 (2009) 4800–4804
conventional steps of acetone (ACE), isopropanol (IPA) and sulfuric acid (H2SO4) solution prior to electrodeposition in the electrolyte. The composition of the electrolyte bath solution consisted of nickel sulphamate of 500 ± 20 ml/l, cobalt sulfamate 20 ± 1 ml/l, nickel chloride of 4 ± 1 g/l, wetting agent 0.5 ±0.05 ml/l and boric acid 42 ± 2 g/l. The temperature was kept at 50 ± 1 °C. The initial pH of the electrolyte was controlled at 4.2. The power supply was controlled by potentiostat/golvanostat (Jiehan5000, Taiwan). Pulse frequency was ranged from 10 to 100 Hz with a duty cycle (Ton/Toff) of 1 at constant current density of 3 ASD (ampere per square decimeter) and DC current density varied from 1 to 20 ASD. The average thickness of alloy films was about 600–700 nm which was measured by Alpha-Step Profilometer (Surfcorder, ET3000, Kosaka Laboratory Ltd., Japan). The microstructure, phases, and crystallinity of Ni–Co films were identified by grazing incidence xray diffractometry (GIXRD, PANalytical X'Pert PRO, Philips, Netherlands) using CuKα radiation (0.15418 nm). The surface morphology and composition was determined by Field-Emission Scanning Electron Microscope (FE-SEM) with an attached Energy Dispersive X-ray (EDX) Spectroscope (FE-SEM, JEOL JSM-7000, Japan). The nanomechanical properties of the films were measured using a MTS Nano Indenter® with CSM in XP mode and a Berkovich indenter. Since the thickness of the soft Ni–Co films on hard Si substrates is about 600–700 nm, the hardness is obtained from a contact depth of 120–180 nm to avoid the surface and substrate effects. Six
Fig. 1. The effect of: (a) pulse frequency at constant 3 ASD and (b) DC current density on the anomalous Co content of the codeposited Ni–Co films.
4801
Table 1 The EDX measured cobalt (Co) content in Ni–Co alloys. Co content in Ni–Co alloys Varied frequency
Co %
Varied current density
Co %
DC (0 Hz) 10 Hz 50 Hz 100 Hz
21.20 ± 0.54 19.94 ± 0.47 20.31 ± 0.36 20.16 ± 0.78
1 ASD 3 ASD 5 ASD 10 ASD 20 ASD
21.18 ± 0.74 21.20 ± 0.54 20.81 ± 0.42 15.26 ± 1.23 13.19 ± 1.20
indentations were done in each sample for the average value with a deviation.
3. Results and discussion The electrodeposition of Ni–Co alloy films has been recognized as an anomalous codeposition, i.e., cobalt atomic content (at.%) in the codeposited films is more than that in the electrolyte composition. Fig. 1(a)–(b) shows the effect of the pulse frequency at constant 3 ASD and the DC current density on the anomalous Co content of the codeposited Ni–Co films, respectively. The frequency varies from 0 Hz (DC) to 100 Hz and current density varies from 1 ASD to 20 ASD. Table 1 lists the EDX measured Co content of each film. Compared to about 4% Co content in electrolyte, the anomalous Co composition in
Fig. 2. GIXRD patterns of Ni–Co alloy films codeposited: (a) at various pulse frequency of 0–100 Hz at constant 3 ASD. (b) at various DC current densities of 1–20 ASD. The diffraction peaks (⁎) come from the Au seed layer.
4802
C.K. Chung, W.T. Chang / Thin Solid Films 517 (2009) 4800–4804
Fig. 3. SEM micrographs of the electroplated Ni–Co films at frequencies of 0 Hz (DC), 10 Hz, 50 Hz and 100 Hz. The morphology becomes smoother at higher pulse frequency.
the Ni–Co films is around 13%–21% due to the high exchange current density of Co. In terms of pulse frequency, the Co content decreases from 21.20% (0 Hz) to 20.16% (100 Hz) at constant 3 ASD (Fig. 1a). It indicates that DC and pulse deposition modes have no significant effects on the composition. However, the Co content distinctly
decreases with increasing current density from 21.18% (1 ASD) to 13.19% (20 ASD) at DC codeposition (Fig. 1b). Although the mechanisms of anomalous codeposition are not well understood, two different mechanisms can be used to describe the anomalous codeposition behavior, i.e., two-step adsorption and hydroxide
Fig. 4. SEM micrographs of the electroplated Ni–Co films at DC current densities of 1–20 ASD. The morphology becomes rougher at higher current density.
C.K. Chung, W.T. Chang / Thin Solid Films 517 (2009) 4800–4804
mechanisms. The first two-step adsorption mechanism is assumed that metal deposition occurs in two step manner via an adsorbed intermediate as follows: M2+ + e − YM + ðadÞ M + ðadÞ + e − YM − ðsÞ
ð1Þ
One M2+ ion is assumed to be reduced to an M+ (ad) ion, which adsorbs on the electrode surface and is further reduced to the metal M(s). Metal ions do not directly arrive at cathode from anode but in the middle compartment of bulk electrolyte; other reactions would occur to influence the deposition of Ni ions. The standard equilibrium electrode potentials of Ni2+ + 2e±Ni and Co2+ + 2e±Co are −0.257 V and −0.277 V versus standard hydrogen electrode (NHE), respectively. The Co metal with low electrode potential is preferentially to be deposited on the cathode subject to anomalous behavior. In electrodeposition process, besides metal is adsorbed on electrode surface, hydrogen ion was also reduced simultaneously. The other hydroxide mechanism is that the pH value at the cathode surface during process is assumed to rise due to H+ ions consumption at the electrode and resulting in hydroxide ions. It is expressed in chemical reactions below: 2H + + 2e − YH2 2H2 O + 2e − YH2 + 2OH −
ð2Þ
Under these conditions, the surface pH could rise sufficiently high so that metal ions would undergo hydrolysis reaction with OH−. The local increase of pH results in the formation of hydroxide precipitate of the low electrode potential metal (Co in this case) at the cathode and its hydroxide (Co(OH)+) may suppress deposition of the high electrode potential metal (Ni). Therefore, increasing current density may consume metal ions rapidly to reduce the concentration gradient of metal ions with the deposition time and leads to the decrease of anomalous behavior near cathode surface with reduced diffusion. Fig. 2(a)–(b) show the GIXRD patterns of Ni–Co alloy films codeposited at frequencies of 0–100 Hz at constant 3 ASD and DC current density of 1–20 ASD, respectively. The strong intensity of peak at 2θ = 44.7° are from Ni(111) plane together with other two distinct peaks Ni(200) and Ni(220) at approximately 2θ = 52° and 77° for the polycrystalline phase characteristics. The peaks marked by ⁎ symbol are diffracted from the Au seed layer. The Ni(111) peak intensity increases with increasing current density due to high Ni or low Co content in the Ni–Co film. It implies that the highest packing density of fcc-Ni(111) plane dominates the growth orientation of film during codeposition. Fig. 3 shows the SEM micrographs of the electroplated Ni–Co films at pulse frequencies of 0 Hz (DC), 10 Hz, 50 Hz and 100 Hz. The morphology becomes smoother at higher pulse frequency. This is due to more new nuclei formed on the surface of the cathode to result in a refined grain size as well as relaxation time enhanced adsorption of inhibiting species for blocking of grains growth at high frequency pulse deposition [16]. Moreover, Fig. 4 shows the SEM micrographs of the electroplated Ni–Co films at DC current densities of 1, 3, 5, 10 and 20 ASD. The morphology becomes rougher at higher current density. This is similar to a common phenomenon of a rougher deposit obtained for higher current density deposition with higher deposition rate. Fig. 5(a)–(b) shows the hardness of Ni–Co films as a function of the pulse frequency and current density, respectively. The hardness of all Ni–Co films is higher than pure Ni (4 GPa) due to the addition of hard Co content. In terms of pulse frequency, the mean hardness at 10–100 Hz is close to being 6.67–6.82 GPa (10–100 Hz) while that at 0 Hz (DC deposited film) is high for 8.45 GPa. Since the Co content is not significantly affected by varying the deposition mode from DC to pulse plating, hardness of Ni–Co deposits should be related to grain size, roughness and/or inner stress. The smaller grain size is usefully beneficial for enhanced hardness. The calculated mean grain size of
4803
Ni–Co deposits slightly decreases with frequency from 12.3 nm to 10.5 nm. Also, the inner stress of the electrodeposited film is less than GPa which can have little contribution to the film hardness. Therefore, roughness effect is the most likely reason for the DC deposit with higher hardness than pulse deposits. Because the hardness by nanoindentation is calculated from the ratio of loading force to the indentation area, the rougher surface of the DC deposit may remain the underestimated area for the higher calculated hardness value. In terms of DC current density, the mean hardness decreases a little from 8.7 GPa (1 ASD) to 7.13 GPa (20 ASD) due to less amount of hard Co content. The effect of current density on the trend of hardness variation of Ni–Co films is similar to that of Co content (Fig. 1(b)). And the morphology becomes rougher at high current density (Fig. 3). Overall, reducing current density and increasing pulse frequency will be beneficial for the formation of good Ni–Co films with high hardness and smooth morphology.
4. Conclusions The composition, surface morphology and nanomechanical properties of the anomalous Ni–Co alloy films have been investigated at various pulse frequencies and current densities. The anomalous behavior of electrodeposited Ni–Co films is due to the high exchange current density of Co content with low electrode potential and preferred adsorption of more active Co(OH)+ on the deposit
Fig. 5. (a) The hardness of Ni–Co films as a function of pulse frequency at constant 3 ASD, and (b) The hardness of Ni–Co films as a function of DC current density.
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compared to Ni content. The Co content decreases from 22.53% (1 ASD) to 13.39% (20 ASD) at DC codeposition but has no pronounced variation for the DC and pulse deposition modes. The pulse Ni–Co deposits with close Co content also have near hardness. The surface morphology of films becomes smoother at high pulse frequency and low current density. The reduced Co content in Ni–Co films can result in lower hardness which decreases from 8.72 GPa (1 ASD) to 7.13 GPa (20 ASD) at DC current. Overall, the formation of good Ni–Co films with high hardness and smooth morphology can be obtained by reducing current density and increasing pulse frequency.
Acknowledgements This work is partially sponsored by National Science Council under contract No 96-2622-E-006-007-CC3. We would like to thank the Center for Micro/Nano Science and Technology in National Cheng Kung University for the access of analysis equipments.
References [1] R. Orinakova, A. Turonova, D. Kladekova, M. Galova, R.M. Smith, J. Appl. Electrochem. 36 (2006) 957. [2] G. Qiao, T. Jing, N. Wang, Y. Gao, X. Zhao, J. Zhou, W. Wang, J. Electrochem. Soc. 153 (2006) C305. [3] L. Wang, Y. Gao, Q. Xue, H. Liu, T. Xu, Appl. Surf. Sci. 242 (2005) 326. [4] Y. Zhuang, E.J. Podlaha, J. Electrochem. Soc. 150 (2003) C219. [5] H. Dahms, I.M. Croll, J. Electrochem. Soc. 112 (1965) 771. [6] J.O'M. Bockris, D. Drazic, A.R. Despic, Electrochem. Acta 4 (1961) 325. [7] S. Hessami, C.W. Tobiaas, J. Electrochem. Soc. 136 (1989) 3611. [8] K.M. Yin, J.H. Wei, J.R. Fu, B.N. Popov, S.N. Popova, R.E. White, J. Appl. Electrochem. 25 (1995) 543. [9] J. Vaes, J. Fransaer, J.P. Celis, J. Electrochem. Soc. 147 (2000) 3718. [10] A. Bai, C.C. Hu, Electrochem. Acta 47 (2002) 3447. [11] C.C. Hu, A. Bai, J. Electrochem. Soc. 149 (2002) C615. [12] A. Bai, C.C. Hu, T.C. Wen, Electrochem. Acta 48 (2003) 2425. [13] D. Golodnitsky, N.V. Gudin, G.A. Volyanuk, J. Electrochem. Soc. 147 (2000) 4156. [14] E. Gomez, S. Pane, X. Alcobe, E. Valles, Electrochem. Acta 51 (2006) 5703. [15] L. Shi, C.F. Sun, F. Zhou, W.M. Liu, Mater. Sci. Eng. A 397 (2005) 190. [16] C.K. Chung, W.T. Chang, Microsys. Technol. 13 (2007) 537. [17] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [18] http:// www.mtsnano.com.