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The Influence of Organic Additives on Surface Microroughnessof Copper Deposits from Cuprous Solutionunder Potentiostatic Conditions
Aphichart Rodchanarowan1,2,+ and Michael L. Free3
1Department of Materials Engineering, Faculty of Engineering, Kasetsart University,50 Ngamwongwan Rd., Ladyao, Chatuchak, Bangkok 10900, Thailand2Center of Advanced Studies in Industrial Technology, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand3University of Utah, 135 S. 1460 E. Rm 412, Salt Lake City, UT 84112, USA
In this study, surface roughness values of electrodeposits from chloride solution (0.1mol/L CuCl, 0.05mol/L HCl and 4.0mol/L NaClelectrolytes) under potentiostatic conditions were characterized in terms of root mean square roughness (RMS roughness) using a Mirauinterferometer, an optical microscope, and computer software analysis. A variety of organic additives were used to study their influence on RMSroughness values. The surface roughness of copper electrodeposits in the presence of gelatin (RMS roughness = 0.38 µm) is significantly lowerthan in the absence of additives (RMS roughness = 15.19µm). The use of gelatin also resulted in lower roughness than other additives (RMSroughness ranging from 16.75 to 3.30 µm). The copper electrodeposition tests were performed under rotating disk electrodeposition conditions(100mV cathodic overpotential for 1 h under argon purging at 500 rpm). The effect of additive concentration on RMS roughness values was alsoevaluated. As cathodic overpotential and deposition time increased, the RMS roughness increased at low overpotential and deposition time.RMS roughness did not increase significantly at longer deposition times in the presence and absence of gelatin.[doi:10.2320/matertrans.M2012095]
(Received March 13, 2012; Accepted June 18, 2012; Published August 1, 2012)
Keywords: copper, electrodeposition, chloride, additives, microroughness, potentiostatic
1. Introduction
Electrodeposition is an important technique widely usedin many industries such as electronics, decoration, electro-winning, etc. Researchers have proposed and evaluated manyroutes to recover copper from sulfide minerals (chalcopyrites,CuFeS2) via hydrometallurgical means.1) It was found thatthe use of halide solution together with an oxidant such ascupric ions helped to leach the copper from chalcopyrite.1,2)
Halide dissolution of chalcopyrite results in cuprous ionformation. Consequently, the electron requirement forelectrowinning from halide solutions is half of the traditionalrequirement of two electrons for reduction from the cupricion state in sulfate systems. However, copper electrodepositsin halide media are morphologically rougher than those insulfate media.2)
A simple way of obtaining the surface roughness values ofelectrodeposits involves a Mirau interferometer in conjunc-tion with an optical microscope and computer software.3,4)
The surface roughness was characterized mathematically asthe root mean squared roughness (RMS roughness) shown ineq. (1).
RMS roughness ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1
L
Z1
0
y2ðxÞdx
vuuut ð1Þ
L is the length of the measured sequence and y is thetopographic height as a function of the x-position along thelength of the surface.
Typically, there are different ways to reduce the surfaceroughness of a copper electrodeposit. One way to reduceroughness is to appropriately control the mass transport
of ions to the depositing surface. Another way to reduceroughness is to use smoothing additives. Adding organiccompounds is one of the most effective and most frequentlyused methods to control the quality of the electrodeposits.57)
Researchers have shown that the addition of compoundssuch as thiourea, polyethylene glycol, gelatin, guar, glycine,etc. results in reduced roughness.514) Many researchershave tried to understand the mechanism by which organicadditives modify morphology during electrodeposition.
Rodchanarowan and coworkers showed that high molecu-lar weight molecules such as gelatin, polyethyleneoxide(PEO), polyacrylamide (PAA), polyvinylpyrrodilone (PVP)and polyvinylalcohol (PVA) reduce surface roughnesssignificantly.3,4,15,16) These high molecular weight additivestended to suppress anomalous growth. In addition, theseadditives increased the nucleation rate, resulting in smootherelectrodeposits than in the absence of such additives.3,4,15,16)
This study was undertaken to extend the work byRodchanarowan and coworkers performed under galvano-static control to include potentiostatic control. Thus, thispaper is focused on evaluating the influence of organicadditives on the surface roughness of copper electrodepositsin chloride based media under potentiostatic control.
2. Experimental Procedures
2.1 Electrodeposition instruments and experimentsAll electrochemical tests were performed using either an
EG&G 273 potentiostat operated through PowerSuite soft-ware made by Princeton Applied Research or a PCI4/750potentiostat operated through Virtual Front Panel softwaremade by Gamry Instruments. The EG&G potentiostat wasconnected to a computer which was connected to an AFASRrotator from Pine Instruments. The PCI4/750 potentiostat+Corresponding author, E-mail: [email protected]
Materials Transactions, Vol. 53, No. 9 (2012) pp. 1695 to 1698©2012 The Japan Institute of Metals EXPRESS REGULAR ARTICLE
was connected to a computer which was connected to a MSRrotator from Pine Instruments.
All of the electrodeposition experiments were performed ina three electrode cell with a counter electrode (Pt electrode),a reference electrode (saturated calomel electrode: SCE) anda working electrode (Cu disc electrode). The copper discworking electrode (99.999% purity) was mounted in a Teflonholder with a copper surface area of 0.203 cm2 exposed to theelectrolyte. All of the solutions were prepared using reagentgrade chemicals and ASTM Type I water. The base solutionused in the experiments contained 0.1M CuCl (97% purity,Alfa Aesar), 0.05M HCl (36.538% purity, EMP) and 4MNaCl (99.0% purity, Mallinckrodt Chemicals). All of thesechemicals were used without further purification.
The working electrode was polished with 600-grit polish-ing paper, then rinsed with pure water, and ultrasonicallycleaned. Each working electrode was further polished withMicrocut silicon carbide grinding paper (P4000), and Micro-polish alumina powder (1.0 µm) to obtain a smooth, cleansurface. Finally, the electrode was cleaned in an ultrasonicwater bath to remove polishing particles from the surface.
2.2 Topography measurementIn this study, surface morphology was characterized by
root mean squared roughness (RMS roughness). The workingprinciple for the measurement of roughness is based on Mirauinterferometry. As shown in Fig. 1, the light beam ‘A’ from alight source is split into beam ‘B’ and ‘C’ by a beam splitter.Beam ‘B’ is reflected back as beam ‘D’ from the surface. Theinterference between the beams ‘C’ and ‘D’ is then recordedby a CCD camera through the Lead Capture software. Theelectrodeposit specimen is moved up or down at a constantspeed so that the interference fringes are observed at everypixel in each of the sequential images recorded at eachvertical position. The image files are later converted to adesired format by Microimage software. The image files areconverted to a topographic image and RMS roughness valuesare calculated from the topographic image using MATLAB.The RMS roughness is defined mathematically as shown ineq. (1).
3. Results and Discussion
The chemistry of the copper-chloride-water system wasstudied using potentiodynamic scans. The effect of rotationalspeed on copper-chloride reaction is presented in Fig. 2. Asseen in Fig. 2, the mass transport of cuprous ion is greatly
improved when the rotational speed of the copper electrodeincreases. The limiting current densities (iL) at 250, 500 and1000 rpm were measured to be 23.9, 34.7 and 47.8mA/cm2,respectively using the data in Fig. 2. The values of limitingcurrent density (iL) were directly proportional to the squareroot of angular velocity (½) of the copper disc electrode.
Two different electrodeposition techniques can be used todeposit copper from dissolved cuprous ions from the chloridemedia: galvanostatic and potentiostatic. Potentiostatic con-ditions were used in this study since this information has notbeen reported previously.
The effect of different additives on RMS roughness underthe potentiostatic conditions was studied as shown in Fig. 3.It was found that among all of the additives studied, gelatinprovided the lowest RMS roughness value of approximately0.38 µm. Similar results have been reported using galvano-static conditions.14,1517) The presence of other additivesyielded RMS roughness values ranging from 3.30 to16.75 µm. Figure 4 presents the effect of the organicadditives concentration on RMS roughness. The RMS
computer control
light source
CCDcamera
sample(electrodeposit)
interferometer in optical microscope
sample moves up and downto collect all interferences
A
B
D
C
Fig. 1 Schematic diagram of topography equipment measurement system.
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23.9 34.7 47.8 mA·cm -2
Fig. 2 Potentiodynamic scans for electrolyte with 0.1M CuCl, 0.05M HCland 4M NaCl as a function of different rotational speeds of a copperworking electrode, using a scan rate of 5mV/s, and argon purging.
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Fig. 3 Effect of different additives (A = no additives, B = 0.01M sodiumcitrate, C = 0.1M sodium acetate, D = 0.1M sodium monolaurate,E = 0.001M EDTA, F = 0.001M C10TAB, G = 0.001M C16TAB,H = 0.01M DPC, I = 0.1M ethylene glycol and J = 0.001M gelatin) onsurface roughness (root mean squared roughness) of copper electro-deposits obtained from an electrolyte containing 0.1M CuCl, 0.05M HCland 4M NaCl using a cathodic overpotential of 100mV for 1 h underargon purging and a 500 rpm disc rotational speed.
A. Rodchanarowan and M. L. Free1696
roughness was not affected significantly by additive concen-tration over the range of concentrations evaluated (generally0.0001 to 0.1M).
The topographical images of copper electrodeposits in theabsence of additives and in the presence of 0.001M gelatinfrom an electrolyte are shown in Fig. 5. The surfacemorphology of copper electrodeposits in the presence ofgelatin is significantly smoother than that in the absence ofadditives.
Because the cathodic overpotential is the main drivingforce for the electrodeposition process, the effect of cathodicoverpotential on surface roughness of copper electrodepositswas evaluated, and the results of this evaluation are shown inFig. 6. The data in Fig. 6 show that the RMS roughness ofthe copper electrodeposits increases as the magnitude ofcathodic overpotential increases. This is related to the factthat as the magnitude of the cathodic overpotential increases,the electrodeposition moves from the electrochemicalreaction controlled kinetics to the mass transport controlledkinetics as seen in Fig. 2 as the deposition approaches thelimiting current density. This trend is observed in the absenceof organic additives and the presence of gelatin. It should be
noted that the RMS roughness value of copper electro-deposits in the presence of gelatin is much lower than that inthe absence of gelatin.
According to Fig. 7, the RMS roughness of the copperelectrodeposit surface increases rapidly with deposition timeduring the initial deposition phase. However, as the chargetransferred increases with time, the rate of increase inroughness decreases, eventually approaching a steady-statevalue. This trend is observed in the presence and absence ofgelatin.
Deposition onto an atomically smooth substrate alwaysresults in roughening that increases with time as nuclei grow.The initial roughness is due to the size and distribution ofgrowing nuclei. However, as the deposition reaches a stagewhere the surface is covered with new growth and newnuclei, the opportunity for roughening directly from a smoothsurface is removed. Consequently, at long deposition times,the surface roughness is a function of the combination ofnucleation and growth. Constant surface roughness can beobtained if the rate of nucleation and associated growth isbalanced by the removal of nuclei through overlappinggrowth from adjacent nuclei.
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C16TAB
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NaAcetate
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0
Fig. 4 Effect of concentration levels of different additives (no additives,sodium citrate, sodium acetate, sodium monolaurate, EDTA, C10TAB,C16TAB, DPC and gelatin) on surface roughness (root mean squaredroughness) of copper electrodeposits obtained from an electrolytecontaining 0.1M CuCl, 0.05M HCl and 4M NaCl using a cathodicoverpotential of 100mV for 1 h under argon purging with a rotating discspeed of 500 rpm.
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Fig. 5 Topographical color images of copper electrodeposits (a) in the absence of organic additives, and (b) in the presence of 0.001Mgelatin using an electrolyte containing 0.1M CuCl, 0.05M HCl and 4M NaCl, a cathodic overpotential of 100mV for 1 h with argonpurging and an electrode rotational speed of 500 rpm.
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Fig. 6 Effect of cathodic overpotential on surface roughness (RMSroughness) of copper electrodeposits with and without 0.001M gelatin.Tests were performed using an electrolyte containing 0.1M CuCl, 0.05MHCl and 4M NaCl with deposition occurring for 1 h under argon purgingwith a rotating disc speed of 500 rpm.
The Influence of Organic Additives on Surface Microroughness of Copper Deposits from Cuprous Solution 1697
4. Conclusions
The effect of organic additives on surface roughness ofcopper which was potentiostatically electrodeposited in CuClbased solution was investigated using a Mirau interferometer,optical microscope and computer software tools. Surfaceactive, complexation and macromolecular additives wereshown to decrease surface roughness relative to control testswith no additives. Among the additives evaluated, copperelectrodeposited in the presence of gelatin had the lowestRMS roughness.
Acknowledgements
The financial support for this project provided by theCenter for Advanced Separation Technologies is gratefullyacknowledged. Financial support by the funding agency does
not constitute endorsement of the contents of this paper. Theauthors also would like to thank the Center of AdvancedStudies in Industrial Technology, Faculty of Engineering,Kasetsart University.
REFERENCES
1) M. L. Free: Electrochemical Coupling of Metal Extraction andElectrowinning, ed. by J. A. Gonzales and J. Dutrizac, (Electro-metallurgy, 2001) pp. 235260.
2) M. L. Free, R. Bhide and A. Rodchanarowan: Fray Inter. Symposiumon Metals and Materials Processing in a Clean Environment, (2011).
3) M. L. Free, R. Bhide and A. Rodchanarowan: ECS Trans. 13 (2006)1323.
4) M. L. Free, R. Bhide, A. Rodchanarowan and N. Phadke: ECS Trans. 3(2006) 335343.
5) W. Plieth: Electrochim. Acta 37 (1992) 21152121.6) Y. M. Loshkarev and E. M. Govorova: Protect. Met. 34 (1998) 399
415.7) Y. K. Lee and T. J. O’Keefe: JOM 54 (2002) 3741.8) M. Stelter, H. Bombach and N. Nesterov: JOM 54 (2002) 3236.9) C. P. Fabian, M. J. Ridd and M. E. Sheehan: Hydrometallurgy 86
(2007) 4455.10) M. R. H. Hill and G. T. Rogers: J. Electroanal. Chem. 86 (1978) 179
188.11) D. R. Turner and G. G. Johnson: J. Electrochem. Soc. 109 (1962) 918
923.12) L. Mauresan, S. Varvara, G. Mauring and S. Dorneanu: Hydro-
metallurgy 54 (2000) 161169.13) H. M. A. Soliman: Appl. Surf. Sci. 195 (2002) 155156.14) O. Kardo and D. G. Foulke: Adv. Electrochem. Electrochem. Eng. 2
(1962) 145233.15) A. Rodchanarowan, R. Bhide and M. L. Free: Hydrometallurgy 2008:
Proc. 6th Int. Symp., ed. by C. A. Young, P. R. Tay and C. G.Anderson, (SME, 2008) pp. 638645.
16) M. L. Free, R. Bhide, A. Rodchanarowan and N. Phadke: SohnInternational Symposium, Advanced Processing of Metals andMaterials, Proceedings of the International Symposium, New Improvedand Existing Technologies, Aqueous and Electrochemical Processing,(2006) pp. 479585.
17) M. Schlesinger and M. Paunovic: Modern Electroplating, (John Wiley& Sons, Inc., New York, 2000) pp. 6667.
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Fig. 7 Effect of electrodeposition time on surface roughness (RMSroughness) of copper electrodeposits in the absence of organic additivesand in the presence of 0.001M gelatin obtained from an electrolytecontaining 0.1M CuCl, 0.05M HCl and 4M NaCl at cathodicoverpotential of 100mV for 1 h under argon purging and a rotationalspeed of 500 rpm.
A. Rodchanarowan and M. L. Free1698
