-0 wr7%

METALLIZATION OF POLYPYRROLE FILMS PART II: ELECTRODEPOSITION OF COPPER PbF

ABSTRACT

Maria Hepel, Yi-Ming Chen and Laura Adams Department of Chemistry

State University of New York at Potsdam Potsdam, New York 13676

The electrodeposition of copper on composi te conduct ive polymer polypyrrole/polystyrenesulfonate PPy(PSS) has been studied. The morphology of copper deposits was investigated in the presence of thiourea, benzotriazole, boric acid, 1 ,&naphthalene disulfonic acid, chloral hydrate, EDTA, and p-aminophenol. In the presence of thiourea in the solution, the rate of copper deposition on PPy(PSS) substrate was slightly inhibited but the rate of copper stripping was faster than in its absence. The addition of benzotriazole to the solution in mM concentration range results in large separation between the deposition and stripping peaks. The formation of Cu(l)-benzotriazole film in the intermediate potential range was . confirmed with the Electrochemical Quartz Crystal Microbalance and Voltammetric techniques.

The Electrochemical Quartz Crystal Microbalance (EQCM) technique in conjunction with Scanning Electron Microscopy (SEM) was used. The EQCM technique allowed us to simultanously monitor voltamperometric and resonance frequency vs. potential or time characteristics. The amount of electrodeposited copper was controlled by monitoring the EQCM resonant frequency.

INTRODUCTION

Polypyrrole, a widely known conductive polymer, has a potential to be a useful component of microelectronic devices. Polymers of the polypyrrole type can be switched in a controlled way between conducting and poorly conducting states. The possibilities of its electronic applications would be substantially increased if metal layers could be deposited onto the polypyrrole film. A few papers appeared in the literature dealing with metallization of conductive polymer films [ 1-91.

Among the variety of methods used in polymer metallization, electrodeposition from aqueous solutions is very promising. Electrodeposition of copper on a polypyrrole precoat formed by chemical polymerization on an insulating substrate, with potential applications for metallization of printed circuit boards has been reported[2]. The electrodeposition of various metals on polypyrrole carried out in typical industrial plating baths was examined by cyclic voltammetry by Tan et al. [3]. The electrodeposition of palladium, platinum, lead, and ruthenium on polypyrrole film was reported by Pletcher e t al. [4]. Eiectrodeposition of nickel on polypyrrole in the presence and absence of additives studied by the EQCM technique has been investigated [ 5 ] . The effect of the composition of polypyrrole substrate on the electrodeposition of copper and nickel has been found[6].

The electrodeposition of copper is a process of great practical importance,

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particularly in the electronics industry. However, the deposition process is mechanistically very complex and the final quality of copper electrodeposit depends on many interlinking factors, including bath composition, convection, temperature, current density and the presence of additives used as brightening and levelling agents [IO-161. The presence of organic compounds in an electrochemical system causes significant changes that affect the nucleation process and growth of metal nuclei. For this reason, many additives are used to optimize industrial electrochemical plating processes. The most desired properties of deposits are mirror-brightness, high-levelling, low internal stress of the coatings, as well as, stability during use. Even trace levels of additives present in the electroplating baths can import tailor-made properties to the electrodeposits. In most cases, these additives act as inhibitors in the electrodeposition process. The purpose of this work is the investigation of copper electrodeposition on conductive polymer using a new technique, the Electrochemical Quartz Crystal Microbalance (EQCM) in combination with scanning electron microscopy (SEM).

In this work, we report on electrodeposition of copper on composite polypyrrole (PPy(PSS)) coated AU piezoelectrodes in the presence and absence of additives. These data are compared with electrodeposition of copper on unmodified gold electrodes. Using the EQCM technique, we were able to follow changes in the electrode mass by measuring the oscillation frequency of these piezoelectrodes. in addition to the simultaneously recorded ,... r..a-t L.UlIGIIL.

EXPERIMENTAL

An Electrochemical Quartz Crystal Microbalance (EQCM) with 10 MHz AT-cut quartz oscillators was used in this study. The EQCM is a hybrid technique allowing for simultaneous monitoring of voltamperometric and resonance frequency vs. potential or time characterics. The EQCM technique is based

on the piezoelectric effect. The resonant frequency of the quartz crystal lattice vibrations in a thin quartz crystal wafer is measured as a function of the mass attached to the crystal interfaces. For thin rigid films, the interfacial mass changes are related to the changes in oscillation frequency of the EQCM through the Sauerbrey equation:

A f = - 2Amnfo2

In this formula, the change in the resonant oscillation frequency (Af) is equal to minus the change in the interfacial mass (Am) per unit area (A) times a constant. Thus, the frequency decreases as the mass increases. The constant is evaluated with knowledge of the oscillation frequency of the fundamental mode of the EQCM (fo), the overtone number (n), the density of quartz (dq = 2.648 g/cm3) and the shear modulus of quartz (pq = 2.947 x 1011 g cm-1s-2). The EQCM is well suited to the measurements of mass transport that can accompany redox processes occuring in thin rigid films on electrodes [26-301. The excellent sensitivity and large dynamic range of this device permit characterization of mass transport (both ions and solvent) in films.

A model EQCN-700 Electrochemical Quartz Crystal Nanobalance (ELCHEMA, Potsdam, NY) was used to monitor frequency variation (mass changes) of piezoelectrodes. The active surface area of the working Au electrode was 0.25 cm2. The oscillator crystals were sealed to the side opening in a glass vessel of 30 mL capacity. The working electrode was polarized using a Pt-wire coufiter electrcde and its potentia! measured vs. a saturated calomel electrode (SCE). A model PS-605 Precision I_ Potentiostat/Galvanostat (ELCHEMA) was used in the measurements. The program waveform was generated and measurements

acquisition system with 16-bit precision. The software trigger utility of the VOLTSCAN was used to control precisely the amount of deposited metal. This feature appeared to be

performed by VOLTSCAN real-time data - _^I

71 0 - 2 -

extremely useful for measurements oi nucleation density and nuclei size which could have been done for the given amount (mass) of deposit and irrespective of the deposition rate. The software trigger allowed us to stop electrolysis after a given present mass of the metal had been deposited. A Scanning Electron Microscope MModel IS1 SX-40A was employed to obtain surface images of metal depositions.

The electropolymerization of pyrrole was carried out on gold piezoelectrode from aqueous electrolyte solutions containing 20- 70 mM pyrrole, and supporting electrolyte at constant potential E = +600 mV vs. SCE and at E = +650 mV vs. SCE in the presence of organic dopant, eg. poly(styrenesu1fonate) (PSS). Films with thicknesses below 1 pm were typically grown. All chemicals were of analytical grade purity and were used without further purification except for pyrrole which was purified by distillation. Solutions were prepared using Millipore Milli-Q deionized water. They were deoxygenated by bubbling with purified nitrogen. All experiments were performed at room temperature, 2 2 T .

RESULTS AND DISCUSSION

Electrodeposition of Copper on Composite Polypyrrole Without Additives

The deposition of copper from 0. I M H2S04 solution without any additives usually begins in the potential range from +lo0 to - iOG mV vs. SCE. In this potential range, the composite PPy(PSS) film electrode remains highly conductive and, thus, copper can be readily deposited on such a film substrate.

In Figure 1, typical voltammetric and microelectrogravimetric characteristics obtained for a PPy(PSS) modified Au-

EQCM electrode in 10 mM Cu(I1) -t 0.1 M H2S04 solution, are presented. The increase in cathodic current observed during the negative potential scan, at potentials less than E = -100 mV, is accompanied by a simultaneous mass increase manifested on the microelectrogravimetric curve. The cathodic peak potential is E, , = -140 mV and coincides with highest rate of mass increase on the m-E curve. During the reverse potential scan, the copper stripping begins at the potential, E = 0 mV, as

0.F

0.4

e t E cr

-0.4 B -0.8

3

-1.2

-1.6 J -800.0 -400.0 -0.0 400.0

Figure 1. Linear potential scan current vs. potential and mass vs. potential characteristics for copper deposition on PPy(PSS) modified Au- EQCM electrode obtained in 10 mM Cu(I1) + 0.1 M H,SO, solution without additives, at scan rate v = 50 mV/s.

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demonstrated by the onset of the anodic current on the i-E curve and the beginning of t h e m a s s d e c r e a s e o n t h e m - E characteristic. The anodic peak potential E,,,, = +lo0 mV. The total mass increase, dm = 3270 ng, due to the copper deposition, corresponds to approximately 82.5 mono- layers of copper, assuming that smooth deposit is being formed and the mean

M'/3N-'/3d2/-' = 201.7 ng/cm2, where M is the atomic mass of copper, N is the Avogadro

- monolayer mass of copper is: mmon,, -

number, and d is the density of copper metal (d = 8.96 g/cm3). As seen in Figure 1, the mass change in one potential cycle is not well balanced, i.e. the final mass does not return to the initial level. The current stripping peak also shows a long tail indicating incompleteness of the stripping process. This is all caused by some degree of the system's irreversibility.

In Figure 2, the results of cyclic chronoamperometric experiments performed

0

-2.0

4 E c-; -4.0 i3 p: p: ' -6.0

-8.C

-1o.c

-800 -8OOmV - 8 0 0 m V

E

- 4000.0

0.0

I 400.0 800.0 1200.0

TIME, s

Figure 2 (a). Current and mass transients for sequential copper deposition and stripping obtained on a Au-EQCM electrode in 10 mM Cu(I1) + 0.1 M H,SO, solution without additives for potential steps from E = +400 mV to E = - 800 mV and pulse duration tdep = 30 s.

-2.0 4 E

p: 0: 2

-6.0

8 E

-8.0 -

' -0.0 -10.0 400.0 800.0 1200.0

0

TIME, s

__ Figure 2 (b). Current and mass transients for sequential copper deposition and stripping obtained on a Au-EQCM electrode in 10 mM Cu(I1) + 0.1 M H,SO, solution without additives for potential steps from E = +400 mV to E = - 500 mV and pulse duration tdep = 30 s.

-~ __

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on PPy(PSS) modified Au-EQCM electrode in 10 mM Cu(I1) + 0.1 M H,SO, solution, are presented. In experiments of Figure 2 (a), the electrode potential was stepped between E = -800 mV (deposition) and E = +400 mV (stripping). Thus, the potential for copper stripping is the same as the anodic potential limit E,, in experiments of Figure 1. The deposition pulse duration was tdep = 30 s, and the stripping period was tdi,, = 370 s. As seen in Figure 2 (a), this longer dissolution time is more than sufficient to allow all deposited copper to be removed from the electrode surface. As expected, the amount of copper deposited is strongly dependent on the electrode potential. This is confirmed by a gravimetric characteristic presented in Figure 2 (b), obtained for potential steps from E = +400 mV to E = - 500 mV. The mass of copper deposited at the latter potential, at tde,, = 30 s, is 4000 ng, versus 6700 ng deposited at E = -800 mV, Figure 2 ( a ) . The conductivity of polypyrrole is decreased at potentials more negative than E = -500 mV. It is interesting to note that it does not inhibit the copper electrodeposition process.

Effect of Thiourea

Thiourea is probably the most commonly used additive for copper plating. In spite of the long history of its utilization and numerous research studies performed [14-171, the detailed mechanism of the brightening action of this additive is still not well understood.

In Figure 3, a typical voltammetric and microelectrogravimetric characteristics obtained for a PPy(PSS) modified Au- EQCM electrode in 10 mM Cu(I1) + 0.1 M

H,SO, + 5.3 mM thiourea solution, are presented. The increase in cathodic current observed during the negative potential scan, at potentials less than E = -100 mV, is accompanied by a mass increase manifested on the microelectrogravimetric curve. During the reverse potential scan, the copper stripping begins at the same potential, E = - 100 mV, as demonstrated by the onset of the anodic current on the i-E curve and the beginning of the mass decrease on the m-E characteristic. The total mass increase, dm

/

\ 2

.---- *c

100.0

bo C

!4 E

000.0

.o

800.0 -400.0 -0.0

POTENTIAL m V vs. SCE

Figure 3. Linear potential scan current Vs. potential and mass vs. potential characteristics for copper deposition on PPy(PSS) modified Au- EQCM electrode obtained in 10 mM Cu(I1) + 0.1 M H,SO, + 5.3 mM thiourea solution, at scan rate v = 50 mV/s.

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= 2600 ng, due to the copper deposition, corresponds to approximately 65.6 mono- layers of copper, assuming that a smooth deposit is being formed. As seen in Figure 3, the mass change in one potential cycle is well balanced, Le. the final mass value returns to the initial level. This is indicative of t h e h i g h d e g r e e of t h e s y s t e m reversibility. The experiments performed on the same PPy(PSS) film electrode, under the same conditions, but in the absence of thiourea

4.c

2 . [

( 4 E +- -2.1 E E 5 -4 .1

-6..

-E.(

-10.1

(Figure l), show a faster copper

200.0 400.0 600.0 800.0

T!ME, E

Figure 4. Current and mass transients for sequential copper deposition and stripping obtained on a Au-EQCM electrode in 10 mM Cu(I1) + 0.1 M H,SO, + 5.3 mM thiourea solution for potential steps from E = +400 mV to E = -800 mV and pulse duration tdep = 30 s .

deposition process, larger amount of copper deposited (3300 ng), and a faster initial rate of dissolution. However, in the absence of thiourea, the copper stripping peak has a more sluggish falling branch (a longer tail) and, as a result, a rather large amount of copper (1300 ng) remains undissolved at the end of the potential cycle.

In Figure 4, the results of cyclic chronoamperometric experiments performed on PPy(PSS) modified Au-EQCM electrode in 10 mM Cu(I1) + 0.1 M H2S04 + 5.3 mM thiourea solution, are presented. The electrode potential was stepped between E = -800 mV (deposition) and E = M O O mV (stripping). The deposition pulse duration was tdeP = 30 s, and the stripping period was tdiss = 300 s. The amount of copper deposited is 4700 ng, substantially less than in the experiment carried out under the same conditions in the absence of thiourea (Fig- ure 2).

Effect of Benzotriazole

Benzotriazole has been shown to be one of the most effective brightening agents for copper plating from sulfuric acid solutions [11,17-201. The minimum concentration of benzotriazole in the plating bath is usually around 0.1 mM. Our experiments were performed using 10 mM Cu( I1 ) + 0.1 M H 2 S 0 4 + 1 . 3 m M benzntriazznle. In Figure 5, voltammetric and microelectrogravimetric characteristics obtained for a PPy(PSS) modified Au- __ EQCM electrode, are presented. These characteristics show a very complex behavior of this system. A general conclusion which can be drawn from Figure 5 is that the deposition and stripping processes are strongly irreversible in the

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presence of benzotriazole, in contrast to those in the presence of thiourea. In order to analyze the system behavior in more detail, we have to distinguish multiple potential regions in both the cathodic going potential scan and the anodic potential scan.

In the cathodic potential scan, four potential regions have to be considered:

(A) from E = +400 mV to E = -110 mV; (B) from E = -1 10 mV to E = -250 mV; (C) from E = -250 mV to E = -500 mV; (D) from E = -500 mV to E = -800 mV.

800.0 -400.0 -0.0

POTENTIAL, m-V m.

6000.0

4000.0

2

!! 2

2000.0

1.0

Figure 5. Linear potential scan current vs. potential and mass vs. potential characteristics for copper deposition on PPy(PSS) modified Au- EQCM electrode obtained in 10 mM Cu(I1) + 0.1 M H,SO, + 1.3 mM benzotriazole solution, at scan rate v = 50 mV/s.

The potential region A can be associated with the initial stage of the copper deposition. It is represented on the i-E characteristic by a rather sluggish prewave beginning at E = +150 mV. However, on the m-E characteristic, it is represented by a very well defined wave with the major mass increase taking place from E = +80 mV to E = -40 mV. The mass increase is cu. 800 ng. Further mass increase ceases beyond E = -40 mV. This behavior could be explained by assuming that benzotriazole is not adsorbed on the PPy(PSS) film and during the initial copper deposition process it gradually blocks the surface sites on the copper nuclei eventually poisoning the surface completely and hindering further mass increase. The potential region B can be ascribed to the onset of copper deposition. It is manifested by a current peak with sharp rising edge and a well defined mass wave, slightly higher than the mass wave in region A. The onset of copper deposition in region B i s undoubtedly associated with the breakdown of the tight blocking film of benzotriazole and the following increase in the nucleation density favored by the increasing cathodic overvoltage. The potential region C is ascribed to a steady deposition process. It is characterized on the m-E curve by a steady mass increase. The i-E curve shows more complex behavior: a secondary current feature burried on the falling branch of the main cathodic peak. In the potential region D, we still have a steady deposition as evidenced by a steady mass increase nn the 7n-E characteristic. The i -E curve is again more complex showing a prewave at E = -620 mV and a current rise near the cathodic potential limit, E,,,. Since this prewave has no corresponding feature on the m-E curve, it can be rationally ascribed to a Brdicka prewave. Similarly, the current rise near E,,, has no resemblance on the m-E characteristic

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and can be associated with the beginning of the hydrogen evolution process.

In the anodic potential scan, four potential regions have to be considered:

(E) from E = -800 mV to E = -200 mV; (F) from E = -200 mV to E = +lo0 mV; (G) from E = +lo0 mV to E = +170 mV; (H) from E = +170 mV to E = +400 mV.

The potential region E can be associated with the continuous growth of the copper film. The m-E characteristic in this region shows a steady mass increase, and the i-E characteristic shows a steady cathodic current flow. The potential region F is perhaps the most confusing. While the electrode is still gaining mass in a steady fashion as indicated by the electrogravi- metric characteristic, the current flowing is definitely anodic and gradually increasing. To rationalize this anomalous behavior, we have to recognize that the anodic current flow does not necessarily mean that copper is being dissolved, but rather, in general, a certain oxidation process is taking place. It is highly probable that this anodic current is due to copper oxidation with formation of sparringly soluble Cu(1) compound. Indeed, the formation of Cu(1)-benzotriazole surface compound has recently been postulated [ 171. The mass increase concomitant with the growth of such a surface film must then take place due to the uptake of benzotriazole as required by the compound stoichiometry. Thus, the apparently anomalous behavior, actually confirms the formation of Cu(1)- benzotriazole film formation. In the poten- tial region G, the sharp anodic copper stripping peak accompanied by a step mass decrease can be ascribed to the breakdown of the Cu(1)-benzotriazole protective film. Since the mass decrease is rather miniscule, it has to be concluded that either: (i) this breakdown is not complete and that

rather limited amount of copper is released through crevices and pinholes in the film still present on the surface, or: (ii) the surface film Cu(1)-benzotriazole is oxidized and released from the surface, i.e. both copper (as Cu2+) and benzotriazole, leaving the underlying copper film with a clean surface, or: (iii) the surface compound Cu(1)-benzo- triazole is oxidized but only copper is released from the surface as Cu2+, whereas the freed benzotriazole enters into another surface associate Cu"-benzotriazole hindering any avalanche dissolution of the underlying copper film but not blocking it entirely.

The microelectrogravimetric charac- teristic presented in Figure 5, confirms the mechanism (iii). Indeed, if the electrode surface were freed from the benzotriazole, a spontaneous dissolution of copper at potentials higher than E = 120 mV would result in large current peak and mass decrease to the initial level (i.e., m = 0). In the potential region H, copper dissolves slowly through the adsorbed benzotriazole film.

Effect of Other Additives

The effect of other additives on the rate of copper deposition on PPy(PSS) modified Au-EQCM electrode has also been studied. In Table I , presented are results obtained in cyclic chronoamperometric experiments with potential stepped between E = +400 mV and E = -800 mV. The pulse duration for copper deposition was tdeP = 30 s.

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TABLE 1. Mass of Copper Deposited from 10 mM Cu(I1) + 0.1 M H,SO, solution Containing Different Additives, at a Constant Potential E = -800 mV and tdep = 30 s

Additive Conc. Mass, ug

none 6.7 NDSA' 0.69 mM 8.7 EDTA 2.4 mM 6.0 C l 5.0 mM 5.0 %BO, 2.0 mM 6.0

' NDSA - 1,5-Naphthalene Disulfonic Acid.

The most inhibitive effect from this group of additives is exerted by chloride ions, and the NDSA actually increases the rate of copper deposition on PPy(PSS) film. Figure 6a: SEM micrograph of Cu film

electrodeposited on PPy(PSS) gold modified EQCM electrode E = -600 mV vs. SCE for 180 seconds from 10 mM CuSO4 +O.lM H2S04 + 0.63 mM thiourea solution. Effect Of Additives On Copper

Morphology

The SEM micrographs presented in Figure 6 (a)-(g) illustrate the effect of various additives on the morphology of copper deposited on PPy(PSS) modified Au- EQCM electrode from 10 mM Cu(I1) + 0.1 M H,SO, solution containg: (a) thiourea, (b) benzotriazole, (c) boric acid, (d) 1,5- naphthalene disulfonic acid, (e) chloral hydrate, (f) EDTA, and (8) p-aminophenol. Thz copper deposit obtained in soliiiion containing thiourea shows a microgranular structure with the mean size of nuclei cu. 1 um. The effect of benzotriazole is shown to reduce the intergranular distance. The deposit is very smooth with virtually no tendency to form pores. In the presence of boric acid in high concentration, also a

smooth deposit is being formed but some cracks and a tendency to form dendritic clusters is visible. The deposit obtained in 1,5-naphthalene disulfonic acid is micro- crystalline, somewhat similar to that obtained with added thiourea, but is more dense and shows less empty channels. It is also more crystalline. The addition of chloral hydrate to the copper plating solution also produces a microcrystalline deposit with somewhat uneven distribution of crystallites. In the presence of EDTA, copper deposit with very small grains is formed but is covered with copper flakes of varying size and shape. In solutions containing p-aminophenol, two copper layers are clearly seen: the underlayer with large fused spherical grains, and the

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Figure 6b: SEM micrograph of Cu film electrodeposited on PPy(PSS) gold modified E M M electrode at constant potential E =

Figure 6c: SEM micrograph of Cu film electrodeposited on PPy(PSS) gold modified EQCM electrode at constant potential E =

-\-- -

-600 mV vs. SCE for 180 seconds from lOmM CUSO,I to +0.1 M H7S04 + 0.03 s/L

-600 mV vs. SCE for 600 seconds from lOmM CuSO4 + +0.1 M H2SO4 + 2mM -

benzotriazole iolution.

electrodeposited on PPy(P%Sj gold modified

boric acid solution.

Figure 6e: SEM micrograph of Cu film electrodeposited on PPy(PSS) gold modified

EQCM dectrode at constant iotential E = -650 m M vs. SCE for 600 seconds from 1OmM CuSO4 + 0.1M H2SO4 + 0.2 g/L 1,5- naphthalene disulfonic acid solution.

EQCM dectrode at constant potential E = -650 mV vs. SCE for 600 seconds from lOmM CuS04 + 0.1M H2SO4 + 0.4gL chloral hydrate solution.

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_-

Figure 6f SEM micrograph of Cu film Figure 6g: SEM micrograph of Cu film electrodeposited on PPy(PSS) gold modified electrodeposited on PPy(PSS) gold modified EQCM electrode at constant potential E = EQCM electrode at constant potential E = -650 mV vs. SCE for 600 seconds from -650 mV vs. SCE for 600 seconds from lOmM CuSO4 + 0.1M H2SO4 + 2.4 mM 10mM CuSO4 to +O.lM HzSO4 + 0.4 g/L p- EDTA. aminophenol.

overlayer consisting of microcrystalline dendrites.

CONCLUSIONS

The experiments performed show that copper can be readily deposited on the composite PPy(PSS) polymer surface and its morphology can be controlled with a a variety of organic additives. The main difference between the copper deposition on gold (and other metal substrates) and on the PPy(PSS) surface lies in different interaction of the latter surface with additives and with copper. Thus the initial stage of the copper

nucleation process is mostly affected by the presence of the PPy(PSS) film. The results obtained indicate that it is possible to deposit on the surface of this polymer dense and smooth copper films without any cracks or voids.

The Electrochemical Quartz Crystal Microbalance has proven tn be an inva!~ah!e tool in studies of the electrodeposition process. In conjunction with Voltammetry and Cyclic Chronoamperometry, the EQCM technique allowed us to solve very complex behavior of systems with various additives, including the interaction between additives, the PPy(PSS) substrate and the copper deposit .

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ACKNOWLEDGEMENTS

This work was partially supported by the Research Corporation Grant and the NATO Grant No. CRG 930020.

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