effects of organic additives on preferred plane and residual stress of copper electroplated on...
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Materials Chemistry and Physics 120 (2010) 341–347
Contents lists available at ScienceDirect
Materials Chemistry and Physics
journa l homepage: www.e lsev ier .com/ locate /matchemphys
ffects of organic additives on preferred plane and residual stress of copperlectroplated on polyimide
ongsoo Kim, Heesan Kim ∗
chool of Materials Sci. & Eng., Hongik University, 300 Sinan-ri, Jochiwon-eup, Yeongi-gun, Chungnam 339-701, Republic of Korea
r t i c l e i n f o
rticle history:eceived 20 June 2009eceived in revised form 30 October 2009ccepted 7 November 2009
eywords:etals
lectrochemical properties
a b s t r a c t
Effects of the preferred plane and the residual stress of an electroplated copper on polyethylene glycol(PEG) and 3-N,N-dimethylaminodithiocarbamoyl-1-propanesulfonic acid (DPS) were studied. Polyimidefilm coated with sputtered copper was used as a substrate. Preferred plane, residual stress, and impuritylevel in the electroplated copper were measured by an X-ray diffractometry (XRD), calculated by Stoney’sequation, and analyzed with secondary ion mass spectroscopy (SMS), respectively. With increasing theconcentration of PEG, the preferred plane changed in the order (1 0 0) and (1 1 0) while with increasingthe concentration of DPS, the preferred plane changed in the order (1 1 0), (1 0 0), and (1 1 1). Based on the
econdary ion mass spectroscopy (SMS)hin film
modified preferred growth model, where the amount of additive adsorbed on a plane is newly assumedto be proportional to its surface energy in vacuum, the predicted preferred planes correspond to theexperimental results. The residual stress of the electroplated copper depended on the type of additive aswell as its concentration but was independent of the preferred plane. For example, PEG and DPS inducedtensile and compressive residual stresses in the electroplated copper, respectively, and their magnitudesincreased with their concentrations. The dependency of residual stress on the additives was explained
ives
by the incorporated addit. Introduction
Generally flexible printed circuit board (FPCB), composed of aolyimide film as an insulator with low dielectric constant and aopper film as an excellent conductor, has been applied to com-licated electronic devices and repetitive moving parts such asaterials of liquid crystal display (LCD), tape automated bonding
TAB), and chip on flexible printed circuit (COF) [1].Because of the economical advantage, the copper film has
een manufactured by electroplating method rather than physicaleposition method. Several additives have been added to copperulfate bath to reduce the residual stress and the surface roughnessf the electroplated copper film for higher reliability. For example,olyethylene glycol (PEG) with 200–20,000 molecular weightMW) is usually added by 100–2000 ppm in order to inhibit plating2–4]. It was reported to function to impede the transport of copperons from bulk electrolyte [3], to suppress the surface diffusion of
opper atoms by adsorbed PEG [4], or to hinder the charge transferate by adsorbed PEG [3,5,6]. Like 3-mercapto-1-propanesulfoniccid (MPSA) and bis-(3-sodiumsulfopropyl) disulfide (SPS),-N,N-dimethylaminodithiocarbamoyl-1-propanesulfonic acid∗ Corresponding author. Tel.: +82 41 860 2685; fax: +82 41 866 8493.E-mail addresses: [email protected] (J. Kim), [email protected] (H. Kim).
254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2009.11.011
into the electroplated copper.© 2009 Elsevier B.V. All rights reserved.
(DPS) functions as an accelerator [7]. Its role was explained byenhancement of the number of nucleation sites [8], improvementof the reduction rate of cupric ion [9], or displacement of PEG–Cl−
complex [10].As the other attempt to reduce surface roughness, the pre-
ferred plane is required to be controlled by plating conditions suchas over-potential, bath temperature, and concentration of cop-per ion. Finch [11] proposed that the preferred plane of depositsdepended on cathodic over-potential as well as surface energy asfollows. In low over-potential where deposits are grown laterally,the electrodeposits with the (1 1 1) plane are preferentially grownto minimize surface energy. Meanwhile the electrodeposits withthe (1 1 0) plane are preferentially grown in high over-potentialwhere deposits are grown outwardly. In addition, Pangarov [12]proposed that preferred plane was determined by the minimumwork required to form the two-dimensional nuclei as given by Eq.(1):
Wh k l = Bh k l
(zF�/N) − Ah k l(1)
where h k l means the Miller index of crystallographic plane, N
is Avogadro number, � is over-potential, and Ah k l and Bh k l areconstants depending on energy required to release atoms. Hisproposal was also proved by the experimental results that theelectrodeposits with the (1 1 1) plane having the lowest W1 1 1 inlow over-potential were preferentially grown in the low over-
3 stry and Physics 120 (2010) 341–347
ptitptfgtratolabscpiFcwiHrtoep
ib[iepr(cdr
fpat
2
doCfisw(battw1c1
Fig. 1. Two-dimensional model showing development of texture based in low ionicconcentration (a), high ionic concentration (b), and higher ionic concentration (c)on preferred growth model: plane CE and plane CD indicate the most closely packedplane and a crystal plane, respectively; plane M and L indicate a reduced ion on flatsurface and on ledge, respectively [13].
42 J. Kim, H. Kim / Materials Chemi
otential, whereas the electrodeposits with the (1 1 0) plane havinghe lowest W1 1 0 in high over-potential were preferentially grownn the high over-potential. However, Lee [13] reported the con-rary results where the preferred plane changed from the (1 1 0)lane to the (1 1 1) plane with increasing over-potential. Fromhe experimental results on preferred planes, Lee proposed a pre-erred growth model [13] with the following assumption: (i) crystalrowth front is flat, (ii) the direction of impinging ions is normalo a deposit, (iii) surface diffusion rate is high enough to adjusteduced ions to stable position, and (iv) the relative surface energiesre not affected by environment covering the deposit. Accordingo the model, the change of preferred plane depending on bothver-potential and ionic concentration can be predictable as fol-ows. Higher concentration of metal ion adjacent to the depositsllows planes with higher surface energy to grow preferentiallyecause average surface diffusion distance in a crystal with higherurface energy as shown in Fig. 1 is shorter. Meanwhile, lower ioniconcentration allows planes with lower surface energy to growreferentially because the surface area to be covered by reduced
ons is larger for a plane with higher surface energy as shown inig. 1. Higher over-potential induces lower ion concentration adja-ent the deposits and results in the formation of preferred planesith lower surface energy. Similarly, lower over-potential results
n the formation of preferred planes with higher surface energy.owever, the independence of relative surface energies from envi-
onments, the last assumption mentioned in the model, provideshat it is difficult for the model to predict the preferred planesf deposits formed in a solution containing additives though sev-ral additives are practically added in the solutions for copperlating.
The residual stress of the deposits, which is induced during plat-ng and affects surface roughness after self-annealing [14], is causedy preferred-plane-induced stress [15] or entrapped impurities15–17]. The preferred-plane-induced stress gives an understand-ng of how the preferred plane affects the residual stress. Forxample, the deposits with the (1 1 1) preferred plane and the (1 0 0)referred plane had high residual stress and low residual stress,espectively, based on that strain energy decreases in the (1 1 1),1 1 0), and (1 0 0) order in face centered cubic (FCC) structure likeopper [18]. The entrapped impurity model enables to explain theependence of a kind of impurity as well as its concentration onesidual stress independent of a preferred plane.
Therefore, the aims of this work are to propose a modified pre-erred growth model, to approve it by examining how the preferredlanes of copper deposits depend on organic additives such as PEGnd DPS, and to find out what causes residual stress induced duringhe electroplating.
. Experiments
Cathodic polarization tests and galvanostatic tests were con-ucted in order to examine how PEG and DPS affect the kineticsn the electroplating of copper in copper sulfate solution (30 g L−1
uSO4·5H2O, 75 g L−1 H2SO4, 30 ppm Cl−) at 25 ± 1 ◦C. A polyimidelm (Dupont, Kapton E) on which Ni–Cr alloy and copper wereputtered sequentially was used as a working electrode. The filmas masked with an acid-resisting tape excluding the exposed area
5 cm × 1 cm) and the upper part which was electrically connectedy a clipper as shown in Fig. 2. Two copper rods with 99.99% puritynd a saturated calomel electrode (SCE) were used as counter elec-rodes and a reference electrode, respectively. Prior to the tests,
he cathode was degreased in an acidic cleaner at 45 ◦C for 3 min,ashed with distilled water, immersed into the test solution for0 s, and was polarized from the potential more noble than an openircuit potential (OCP) by 10 mV to −0.7 VSCE with the scan rate ofmV s−1. Fig. 2. Schematic diagram of cathode.
J. Kim, H. Kim / Materials Chemistry and Physics 120 (2010) 341–347 343
F
tfiswsaatwgtp[o5Eb
r
�
woi(cFct
T
wrdpmsraomto
current density, where a reduction rate is totally controlled by thediffusion of metal ions adjacent to the deposits. The change of over-potential with PEG gradually decreased until 10 ppm over whichover-potential was not affected by PEG. The results imply that PEG
Fig. 5. Cathodic polarization curves of copper with the concentrations of DPS in thecopper sulfate bath.
ig. 3. Sketch diagrams for measurements of the radius of the copper plated film.
For evaluating preferred plane and residual stress of the elec-roplated copper depending on PEG and/or DPS, the polyimidelm with 4 cm × 5 cm exposure area was prepared for a cathode ashown in Fig. 2. As an anode, a copper plate containing phosphorousas enveloped in polyethylene bag with micropores to minimize
urface defects on a cathode caused by particles separated from annode during plating. Electroplating was carried out at 25 ± 1 ◦C instagnant solution under the constant current of 7 mA cm−2 until
he copper film approached to 8 �m thick. After plating, the cathodeas washed with distilled water several times and dried with nitro-
en gas. In order to minimize the relief of stress and the change ofextures by the self-annealing, the residual stress and the preferredlane were analyzed within 2 h after plating by Stoney’s equation19] and texture coefficient (TC) [16], respectively. For the analysisf residual stresses, the electroplated copper films were cut intocm × 0.5 cm, and the curvature radii were measured according toq. (2) [20] and Fig. 3, and the residual stresses were calculatedased on Eq. (3) [19].
= L2 + ı2
2ı(2)
= Est2s
6(1 − vs)tf
(1r2
− 1r1
)(3)
here � is the stress of the copper film, Es is the elastic modulusf the polyimide film (=5379 MPa), ts is the thickness of the poly-mide film (=38 �m), �s is the Poisson’s ratio of the polyimide film=0.32), tf is the thickness of the copper film, and r1 and r2 are theurvature radius of the film before and after plating, respectively.or the analysis of preferred plane, texture coefficients (TCs) werealculated with the peak intensities of (h k l) reflections accordingo the equation [13].
C(h k l) = nI(h k l)/Io(h k l)∑
I(h k l)/Io(h k l)(4)
here I(h k l) and Io(h k l) are the integrated intensities of (h k l)eflections measured for the test film and a standard copper pow-er sample, respectively, and n is the total number of reflectionlanes. X-ray diffractometry (MACSCIENCE, M03XHF) was used foreasurements of intensities of (h k l). Finally, secondary ion mass
pectroscopy (SMS, CAMECA, IMS-6F) was carried out to find theelation between residual stress and incorporated impurities suchs hydrogen, carbon, oxygen, and sulfur. The favorable application
+
f SMS for detection of these non-metals required the use of Cs pri-ary ions and negatively charged secondary ions for analysis. Afterhe analysis, the etching rate was calibrated by the measurementsf etched depth by profile meter (VEECO, DEKTAK 150).
Fig. 4. Cathodic polarization curves of copper with the concentrations of PEG in thecopper sulfate bath.
3. Results and discussion
3.1. Effects of PEG and DPS on cathodic polarization
Fig. 4 shows that the addition of PEG increased over-potentialat the potential between −0.4 and 0 VSCE but did not affect limiting
Fig. 6. Effects of 30 ppm chloride ion and DPS concentrations on over-potentialduring galvanostatic plating (i = 7 mA cm−2) in the copper sulfate solution (30 g L−1
CuSO4·5H2O + 75 g L−1 H2SO4).
344 J. Kim, H. Kim / Materials Chemistry and Physics 120 (2010) 341–347
Fc
dbikcfDwaftnopsottaPpidFiidpD
Fi
ig. 7. Cathodic polarization curves of copper with the concentrations of DPS in theopper sulfate bath containing 300 ppm PEG.
oes not function as a barrier for the transport of copper ions fromulk electrolyte [2]. It agrees with that the copper plating is inhib-
ted by a PEG–Cl− complex [4–6] on the surface though it is notnown how the PEG–Cl−–Cu complex affects the plating rate. Theathodic polarization curves in Fig. 5 seem to imply that DPS alsounctions as an inhibitor on the contrary to the previous report thatPS plays a role of an accelerator [7]. In addition, the over-potentialas not saturated with the addition of DPS and increased again
t 5 ppm DPS. These unexpected results are postulated to resultrom that 10 s immersion prior to cathodic polarization is enougho absorb thiolate of DPS on the metal surface for inhibition [9] butot enough to form the environments related to the accelerationf copper electroplating such as the adsorption of DPS–Cl− com-lex [21,22]. The constant-current chronopotentiometry in Fig. 6hows that over-potential decreased independently of the presencef chloride ion as the concentration of DPS increased, indicating thathe postulation is correct. Furthermore, with the addition of DPS,he addition of 30 ppm Cl− decreased over-potential less effectivelynd increased over-potential at the solution containing 5 ppm DPS.ossibly, the inhibiting effect of DPS is caused by the fact that theresence of chloride ion more effectively impedes the tautomer-
zation of DPS known to accelerate copper plating or promotes theimerization of DPS known to suppress copper plating [7]. Unlikeig. 5, Fig. 7 shows that DPS effectively decreased over-potentialn the plating solution containing PEG as the concentration of DPS
ncreased. The effective decrease of over-potential is owing to theisplacement of PEG–Cl− complex by DPS. In other words, DPS dis-laces PEG in PEG–Cl− complex functioning as an inhibitor to formPS–Cl− complex functioning as an accelerator [10].ig. 8. Texture coefficients of electroplated copper with the concentrations of PEGn the copper sulfate bath.
Fig. 9. Texture coefficients of electroplated copper with DPS concentrations in thecopper sulfate bath.
From the cathodic polarization test and the constant-currentchronopotentiometry, the over-potential was increased by theformation of PEG–Cl− complex rather than diffusion from bulk solu-tion while the over-potential was decreased by the adsorption ofDPS–Cl− complex. Especially, in the solution containing PEG, theeffective decrease of over-potential by DPS results from the dis-placement of PEG in a PEG–Cl− complex by DPS.
3.2. Effects of PEG and DPS on preferred orientation
Figs. 8–10 show the trends in preferred planes of thecopper plated in the plating solution under constant current(i = 7 mA cm−2) as the concentrations of PEG and/or DPS increases.In the solution without the organic additives, the preferred planewas the (1 1 0) plane which agrees with the results predicted basedon the preferred growth model, for according to the model, planeswith higher surface energies are predicted to preferentially growunder low over-potential and surface energy of copper with FCCstructure increases in the order (1 1 0), (1 0 0), and (1 1 1) [18]. Forovercoming a shortcoming of the preferred growth model, thelast assumption – surface energy is assumed to be independentof environment in the model – is modified to that the amountof adsorbed additive is proportional to surface energy in vacuum.The assumption is considered to be valid according to the previousexperimental result that PEG was preferentially adsorbed on the
plane in the descending order (2 2 0), (2 0 0), and (1 1 1) [23] andthe surface characteristic in Fig. 1 that the surface of the electrode-posits consisted of most closely packed planes and ledges whosedensity is proportional to surface energies.Fig. 10. Texture coefficients of electroplated copper with DPS concentrations and300 ppm PEG in the copper sulfate bath.
J. Kim, H. Kim / Materials Chemistry and Physics 120 (2010) 341–347 345
Fcc
ppaphsepotasiwiet
cDecpthficppi
was independent of the preferred plane of the plated copper. Inthe solution containing DPS only, the residual stress of the electro-plated copper also monotonously changed with adding DPS, similarto PEG, except the sign of the stress as shown in Fig. 14. Hence, theindependence of residual stress from preferred plane is thought
ig. 11. Proposed model showing relation between preferred planes and PEG con-entrations (©: absorbed PEG); (a) low concentration of PEG, (b) intermediateoncentration of PEG, and (c) high concentration of PEG.
In case that PEG functioning as an inhibitor is added in the cop-er plating solution, PEG is preferentially adsorbed on the (1 1 0)lane whose surface energy is the highest among (1 1 0), (1 0 0),nd (1 1 1) planes, inhibiting the preferential growth of the (1 1 0)lane as shown in Fig. 11(a). If the concentration of PEG is notigh enough to be adsorbed on the (1 0 0) plane with next highesturface energy, the electrodeposits with the (1 0 0) plane is a pref-rentially grown. Increasing the concentration of PEG changes thereferred plane from the (1 0 0) plane to the (1 1 1) plane becausef the saturation of PEG adsorbed on the (1 0 0) plane as well ashe (1 1 0) planes as shown in Fig. 11(b). Finally, the saturation onll three planes allows the (1 1 0) plane to be a preferred plane ashown in Fig. 11(c). According to the modified model, the trendsn the predicted preferred plane with the addition of PEG agree
ith the experimental results except no appearance of (1 1 1) planen Fig. 8. No detection of the (1 1 1) preferred plane is consid-red to result from the interval of PEG concentration used in theest.
In other case of DPS which acts as an accelerator below a criticaloncentration and an inhibitor beyond a critical concentration [7],PS is preferentially adsorbed on the (1 1 0) plane, accelerating thelectrodeposits with the (1 1 0) plane as shown in Fig. 12(a). If theoncentration of DPS exceeds a critical concentration on the (1 1 0)lane with the next highest surface energy, the electrodeposits withhe (1 1 0) plane is inhibited and the (1 0 0) plane having the nextighest surface energy is accelerated as shown in Fig. 12(b). With
urther increasing the concentration of DPS, the preferred planes changed to the (1 1 1) plane as shown in Fig. 12(c). Exceeding a
ritical concentration on all the planes, finally, allows the (2 2 0)lane to be a preferred plane. The predicted trends in the preferredlane with the concentration of DPS in Fig. 12 also agree with thosen Fig. 9 as well as in Fig. 10.
Fig. 12. Proposed model showing relation between preferred planes and DPSconcentrations (�: absorbed DPS accelerating Cu deposition, : absorbed DPSinhibiting Cu deposition); (a) low concentration of DPS, (b) intermediate concen-tration of DPS, and (c) high concentration of DPS.
The comparison between experimental results and predictionof preferred planes confirms that the modified preferred growthmodel is capable of predicting the preferred planes of depositsformed during plating.
3.3. Effects of PEG and DPS on residual stress
Fig. 13 shows that the residual stress of the electroplated cop-per monotonously increased with the concentration of PEG but
Fig. 13. Residual stresses on electrodeposits with concentrations of 3350 MW PEGin the copper sulfate bath.
346 J. Kim, H. Kim / Materials Chemistry a
Fig. 14. Residual stresses of electrodeposits film with the concentrations of DPS inthe copper sulfate bath.
Fst
tp
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ig. 15. SMS profiles of carbon (no symbol) and oxygen (�): (a) and hydrogen (noymbol) and sulfur (�): (b) in from the surface of copper deposits electroplated inhe copper sulfate bath containing no organic additive (—),PEG (- - -), or DPS (. . .).
o be caused by the entrapped impurities [15–17] rather than thereferred-plane-induced stress [15].
Fig. 15 shows that the intensity of hydrogen entrapped in thelectroplated copper was the highest in the solution containingEG, and the intensity of the entrapped sulfur was the highestn the solution containing DPS. Meanwhile, the intensities of thentrapped carbon and oxygen were independent of the type of thedditive, PEG or DPS though these additives increased the intensity
f oxygen. From these results, the following facts can be interred:ydrogen [24] in PEG is incorporated into the deposits duringlating causing a tensile residual stress while sulfur [25] in DPSs incorporated into the deposits during plating causing a com-ressive residual stress. Copper sulfide which is produced by the
[[[
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nd Physics 120 (2010) 341–347
reaction between copper and adsorbed thiolate in DPS [9] is thoughtto be incorporated into interstitial sites of the electroplated copperduring the plating and to cause a compressive residual stress. Mean-while, it is difficult to even guess how PEG is incorporated becauseof lack of the information of dissociation and adsorption of PEG.The occurrence of the tensile residual stress by the entrapped PEGcould be explained with the release of hydrogen incorporated intothe plated copper [17].
From these results, the residual stress of the electrodepositsis thought to be caused by the kinds of additives, and its sign isthought to depend on if the entrapped impurities are presentedor diffused out. In addition, with increasing the concentration ofDPS in the solution containing PEG, the residual stress of the elec-trodeposits can also be expected to monotonously decrease likeFig. 14 though the residual stress was relatively higher because ofthe tensile component induced by incorporation of PEG.
4. Conclusions
In this paper, the factors affecting the preferred plane and theresidual stress of the electroplated copper film were investigatedwith the copper sulfate bath containing PEG and/or DPS, and the fol-lowing conclusions have been drawn with respect to copper platingrate, preferred plane, and residual stress:
(1) PEG and DPS function as an inhibitor and an accelerator, respec-tively. Added to the electroplating solution with PEG, DPS moreeffectively increases copper plating rate, which is caused by thedisplacement of PEG in PEG–Cl− complex by DPS.
(2) According to the modified preferred growth model where theamount of adsorbed additive is newly assumed to be propor-tional to surface energy in vacuum, the predicted preferredplanes with increasing PEG and with increasing DPS are in theorder (1 0 0), (1 1 1), and (1 1 0) and in the order (1 1 0), (1 0 0),and (1 1 1), respectively.
(3) The tensile residual stress and the compressive residual stressof the electroplated film monotonously increase with PEGand with DPS, respectively. The residual stress is thought tooriginate from the entrapped impurities. In other words, thetensile stress and the compressive stress are caused by the out-diffusion of entrapped hydrogen in PEG and the incorporationof entrapped sulfur in DPS, respectively.
Acknowledgments
The work was supported by Korea Institute of IndustrialTechnology Evaluation and Planning (ITEP) (Grant No. 10021564-2005-01).
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