Electrodeposition of copper on plasma and thermally oxidized titanium substrates

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Surface and Coatings Technology89 (1997) 225-232

Electrodeposition of copper on plasma and thermally oxidized titanium substrates Jian Zhang, Kuo-shi Teng, Thomas J.

O'Keefe *

Department of Metallurgical Engineering and Graduate Centerfor Materials Research, Universityof Missouri-Rolla, Rolla, MO 65401, USA Received 30 June 1995; accepted 11 December 1995

Abstract

The effect of plasma and thermal oxidation of titanium cathode blanks on copper electrodeposition was studied. Polarization, potential and current step experiments were performed to evaluate electrochemical changes caused by the oxidation treatments. Scanning electron microscopy, Auger electron spectroscopy and X-ray diffraction were used to characterize the titanium substrates and the deposited copper films. Particular emphasis was given to the initial stages of copper nucleation and growth. The results showed that copper electrocrystallization was strongly influenced by the condition of the oxidized titanium surface. High density, uniform copper coverage was found on titanium substrates oxidized at 400 and 500 °C for 3 h under plasma or low oxygen pressure (80 mTorr) environments. The copper nucleation density and uniformity decreased considerably on titanium surfaces oxidized at 100 and 200 °C. Attempts were made to identify changes in the mechanism of copper nucleation on the oxidized titanium substrates using models developed by Thirsk and Harrison (A Guide to the Study of Kinetics, Academic, New York, 1972, p. 115). © 1997 Elsevier Science S.A.

Keywords: Electrocrystallization; PoIarization; Overpotentiat; Nucleation

1. In~oducfion

Titanium has been used successfully as an inert cathode blank in copper electrodeposition in recent years, not only for bulk extraction but also for producing copper foil for electronic applications. The protective oxide film provides corrosion resistance and allows for easy stripping and high quality, uniform deposits [ 1]. Various studies have been made concerning the application of titanium cathodes in copper electrodeposition. Ives et al. [1] discussed the factors affecting the nucleation, growth and retention of copper deposits on the titanium substrates. The strippability of the copper and the influence of organic jnhibitors on the electrocrystallization and structure of the copper were reported by Cooper and coworkers [2,3]. Recently, several studies focused on the electrodeposition of copper on titanium substrates with various surface pretreatments such as anodizing, mechanical and chemical polishing [4-6]. The nucleation and growth of copper electrodeposited on titanium substrates varied substantially for the * Correspondingauthor. 0257-8972/97/$17.00© 1997 ElsevierScienceS.A. All rights reserved PII S0257-8972 (96) 02892-7

different treatments and the mechanisms controlling the copper nucleation and growth were examined and correlated with the initial surface oxide. Plasma surface treatment techniques are now being utilized more extensively in both research and industrial applications. For instance, plasma oxidation of silicon has shown potential in the formation of high quality, electrically insulating oxide layers for use in integrated circuits. The plasma oxidation of titanium thin films deposited on silicon substrates to form layers with high dielectric constants and resistivities in metal-insulatorsemiconductor structures has also been reported [7]. In general, however, few studies of the plasma oxidation of metals and their subsequent behavior in bulk metal production operations have been reported. Schneider et al. [8] exposed thin aluminum samples to an oxygen plasma produced by an electron beam. A considerable increase in oxygen diffusion was observed, the magnitude depending on the duration of the plasma exposure. In a study of the oxidation of a Cu-AI alloy, selective oxidation and thicker layers were found when a plasma was involved [9]. Thus there is growing evidence that prior surface treatment can modify the nature or perfor-

226

aT. Zhang et aL / Szoface and Coatings Technology89 (1997) 225-232

mance of thin surface films, particularly where electrical conductivity is a critical factor. In this study the titanium substrates were treated by plasma and thermal oxidation at various temperatures (to 500 °C) and oxygen partial pressures. The nucleation of copper electrodeposited on the pretreated titanium electrodes was then investigated and, where possible, the mechanisms controlling the initial deposition were determined.

2. Experimental details

2.1. Deposition system A copper sulphate-sulfuric acid electrolyte was used for the deposition studies. The solutions were prepared from Fisher certified grade cupric sulfate pentahydrate and reagent grade sulfuric acid to give a stock electrolyte with a concentration of 40 g 1-1 copper and 180 g1-1 sulfuric acid. The temperature of the electrolyte was maintained at 40 °C for all the electrochemical tests. Titanium electrodes were cut from commercially pure titanium sheets 1 mm thick, and cleaned with acetone in an ultrasonic bath for 5 min before the oxidation treatments. Platinum was the counter electrode and a mercury/ mercurous sulphate electrode (656 mV vs. SHE (standard hydrogen electrode)) was used as the reference. All potentials are reported with respect to the mercury/ mercurous sulphate reference electrode.

2.3. Electrochemical tests 2.3.1. Potentiodynamic measurements An E G and G PARC Model 273 potentiostat-galvanostat was used to generate the cyclic voltammetry curves. The voltammograms were started at - 3 5 0 mV(Hg/Hg2SO4) and driven in the cathodic direction to - 700 mV before reversing to the initial potential, all at a scan rate of 0.5 mV s -1. 2.3.2. Potential step measurements A cathodic potential of - 6 0 0 mV(Hg/HgzSO4) was applied to the pretreated titanium cathodes for the potential step experiments. The values of the current densities were obtained as a function of time and stored for analysis of the nucleation mechanism. The current densities were plotted against (t-to)", where to is the induction time prior to a measurable faradaic current and n is related to the particular nucleation mechanism, as described by Thirsk and Harrison [11]. 2.4. Scanning electron microscopy, Auger electron spectroscopy and X-ray diffraction examination A JEOL T330A scanning electron microscope was used to examine the copper deposited on the various titanium substrates. Surface composition profile analyses of treated titanium substrates were carried out with a Physical Electronics Model 545 Auger electron spectrometer. The approximate sputter rate was 45 A min-1. A General Electric XRD-5 diffractometer with Cu K~ radiation was employed for the orientation determinations.

2.2. Plasma and thermal oxidation treatments

3. Results and discussion

The plasma treatments were conducted in an inductively coupled, cylindrical, glow discharge reactor consisting of four main sections, i.e. reaction chamber, flow control unit, power supply unit and vacuum pump, which was described in detail previously [10]. A controlled resistance heating plate was used to give the desired temperature for the 3 h oxidation treatments. A constant flow rate of 0.5 sccm (standard cubic centimeters per minute) of oxygen was used during the plasma oxidation. The chamber pressure was about 80 mTorr before the generation of the glow discharge. An input r.f. power of 120 W was used to generate and maintain the oxygen glow discharge while the frequency was fixed at 13.56 MHz. Oxidation treatments were also carried out in the same reactor chamber but without the plasma. The titanium substrates were oxidized at 200 and 400 °C in air and with an oxygen pressure of about 80 mTorr, maintained by introducing pure oxygen at a flow rate of 0.5 sccm.

3.1. Oxidation treatments The color of the titanium surface changed with oxidation temperature and time, in a manner similar to that reported in Ref. [12]. As the treatment temperature increased from 100 to 500 °C, the color of the titanium oxide films changed from pale yellow to yellow, gold and then blue. According to Fukuzuka et al. [12], there is a quantitative relationship between the thickness and color of titanium oxide film. The color of the titanium surfaces was lighter for samples treated in air or at low oxygen pressure than in an oxygen plasma environment at the same temperature. According to the AES analyses, the plasma treatment produced thicker oxide films and the depth of penetration of dissolved oxygen was also greater, as shown in Table 1. The data for untreated and plasma oxidized titanium samples are shown in Fig. 1 and provide a semiquantitative analysis of the oxide film. The oxygen

227

3". Zhang et al. / Surface and Coatings Technology 89 (1997) 225-232 Table 1 Thickness and color of oxide films on titanium substrates treated by plasma and thermal oxidations Temperature (°C)

Environment

Treatment time (h)

Thickness (A)

CoIor

....

Untreated 100 400 500 400 400 400 400

Oxygen plasma Oxygen plasma Oxygen plasma 80 mTorr oxygen Air Air Oxygen plasma

3 3 3 3 3 62 62

200 290 500 960 320 360 560 1860

Colorless Pale yellow Gold Blue Gold GoId Gold Grey

100

Ti 75

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50

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0 0

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3do

SPUTTER TIME (rain.)

Fig. 1. Auger surface composition profiles of various titanium substrates: (a) untreated; (b) 100 °C Q ( P ) ; (c) 400 °C O2(P) (sputtering rate 45

Amin-b.

228

J'. Zhang et al. / Smface and Coatings Technology 89 (1997) 225-232

composition showed various characteristic profiles as the treatment temperature changed. The relative oxygen concentration was low at the surface and went through a peak concentration before gradnally decreasing to a negligible amount at a thickness of about 200A for the untreated samples (Fig. 1(a)). For the titanium sample oxidized at 100 °C the oxygen composition was relatively high at the outer surface but decreased rapidly into the bulk (Fig. 1(b)). The titanium sample treated at 400 °C had a broader oxygen composition profile with a plateau at about 65 at.% oxygen, which was near the composition for TiO2 (Fig. l(c)), and the overall oxygen content remained relatively high to a depth of 450 A. Compared with the untreated sample, thermal oxidation below 500 °C in air and at low oxygen pressure gave only moderate increases in the thickness of the oxide layer. Plasma oxidation significantly increased the thickness of the titanium oxide films, particularly at 500 °C. The enhancement of oxide formation by the plasma treatments may be due to the impact of the high energy plasma oxygen flux on the substrate surface resulting in a higher localized effective surface temperature and activity which increased oxygen diffusion. Similar phenomena have been found in ion plating [13]. The Auger analyses were in agreement with the thickness-color relationship proposed for titanium oxide films. Two samples were also oxidized in air at 400 °C for 62 h and at 200 °C for 800 h to determine the influence of time on the fills formed. The color of the 400 °C air oxidized samples changed from colorless to yellow after 3 h and then golden yellow after 62 h, corresponding to a film thickness of about 560 *. The color of the 200 °C air-oxidized samples only changed to pale yellow. The results indicate that temperature is the major factor for oxide film formation and time at temperature is less critical. Also, the growth rate of the oxide film is high during the early stages of thermal oxidation but becomes lower with time. The XRD patterns of the oxidized titanium samples did not show any peaks for titanium oxides, indicating that the fills formed were probably too thin, free grained or amorphous to be detected. Additional studies to determine the phases present in the titanium oxide films using more sensitive characterization techniques are being conducted. 32. Polarization measurements

The polarization curves generated using the plasma oxidized titanium substrates in the acidic copper sulphate bath are shown in Fig. 2. Surfaces plasma oxidized at 200 °C caused the overpotential for copper nucleation to became more polarized (more negative potential) when compared with an untreated titanium electrode.

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Fig. 2. Polarization curves obtained for copper sulfate electrolyte ( 4 0 g l -x Cu 2+, 180 g i -~ t-I2SO4) using titanium substrates plasma oxidized at various temperatures.

As the treatment temperature increased above 300 °C, the overpotential shifted to a more positive value and nucleation and growth were enhanced. The depolarization tendency continued to increase as the treatment temperature was raised from 300 to 500 °C. In Fig. 3, the polarization curves for copper deposition on the titanium electrode thermally oxidized in various environments (80reTort oxygen pressure, air and oxygen plasma) at 200 and 400 °C are compared. The 400 °C, 80 mTorr oxygen pressure thermal treatment showed a strong depolarizing effect of about t20 mV compared with the untreated substrate. The polarization curve for copper deposition on the titanium electrode treated in air at 400 °C was intermediate between the curves of the untreated electrode and 400 °C plasma i

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3. Zhang et al. / Swface and Coatings Technology 89 (1997) 225-232

treated electrode. Both the plasma oxidation and 80 mTorr oxygen pressure oxidation at 200 °C gave an increase in polarization, indicating that a transition temperature of about 300 °C may also exist for the 80 mTorr oxygen pressure oxidation for the 3 h times used in these experiments. Oxidation below 300 °C caused an increase in polarization, while above this temperature, depolarization occurred. 3.3. S E M examination

The morphology and surface coverage of copper nucleated on the oxidized titanium substrates after short time deposition pulses at - 6 0 0 mV(Hg/Hg2SO4) were examined using SEM techniques. For untreated and 100 °C plasma treated titanium substrates, the size distribution of the copper crystals indicated that nucleation was progressive (Figs. 4(a) and (b)). For the titanium substrate treated at 400 °C a high density of small copper nuclei was found after 5-s deposition (Fig. 4(c)). The copper nuclei appeared to be relatively uniform in size, indicating that the mechanism for this substrate is

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probably instantaneous nucleation, however, some localized preferred areas are in evidence. For the titanium electrode oxidized at 80mTorr oxygen pressure at 400 °C, submicron copper nuclei formed almost instantaneously to provide uniform coverage of the electrode surface (Fig. 4(d)). The results of the SEM examination implied that the titanium oxide film formed at lower temperatures acted as a blocking film and limited the formation of favorable sites for copper nucleation, while the titanium oxide films formed during higher temperature oxidation enhanced the nucleation process. 3.4. Potential step measurements

In Fig. 5 the current-time curves for copper deposition on the plasma oxidized titanium substrates in a copper sulfate electrolyte (40 g l - 1 Cu2+, 180 g 1-1 H 2 8 0 4 ) a r e shown. The initial pulse of the current-time curves is not shown, because the various surface treatments had little effect on their intensity or duration. Compared with the untreated surface, the current

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230

J. Zhang et al. / Surface and Coatings Technology 89 (1997) 225-232 i

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response of the plasma oxidized titanium electrodes during copper deposition can also be separated into two categories. For the electrodes treated at a temperature of 400 °C or higher the apparent current density for copper nucleation and growth is much higher than for 300 °C or untreated electrodes. The induction time before a reasonable faradaic current was observed was also shorter. As the treatment temperature increased, the corresponding current density for deposition increased. For the electrodes treated at a temperature lower than 300 °C the current density was lower than for untreated electrodes and the induction time was longer. The current-time transients on titanium electrodes which were oxidized in various environments at 200 and 400 °C are shown in Fig. 6. The current density on an electrode oxidized at 200 °C was lower than that of the untreated electrode, and the current density for the electrodes treated at 400 °C was higher than that of the untreated electrode. These trends were in agreement with the results obtained for the polarization curves. The nucleation current density on the titanium electrode treated at 8 0 m T o r r oxygen pressure at 400 °C was considerably higher than the rest and almost no induction time was observed. The potential step data were used to analyze the mechanism of copper nucleation. According to Thirsk and Harrison [11], during the early stages of metal deposition a linear relationship between the current density i and the function of deposition time (t-to) l"s corresponds to progressive nucleation and a threedimensional growth of the nuclei limited by a slow diffusion process. A linear relationship between the current density and (t-to) °5 corresponds to instantaneous

•

~ 2.0

1.5

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4.0

Time ( s e c o n d s )

Time (seconds)

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;

,

Fig. 6. Current-time transients for deposition of copper on various oxidized titanium substrates from copper sulfate electrolyte (40 g1-1 Cua+, i80 g 1-t I_t2SO4)at -600 mV(Hg/HgzSO4) and T= 40 °C. nucleation followed by three-dimensional growth under diffusion control. As shown in Fig. 7, an approximate linear relationship between the current density and (t-to) 1'5 was found for the untreated and 100 °C plasmaoxidized titanium electrodes. An approximate linear relationship between the current density and (t-to) °'s was found for the electrode plasma oxidized at 400 °C (Fig. 8). The minor deviation from a straight line might be caused by (1) grain overlap, (2) irregular growth on a small area of a growing grain or (3) birth and death of nuclei at the early nucleation stage. The analyses of the current density and (t-to) n relationships suggested that the higher temperature plasma or i

• •

~'~

untreated 100% O2(P )

4

vE tO E3

2 J_.

o

r

I 20

~

I 40

I 60

i

I 80

i

I 100

I

I 120

~

I 140

(t_ta)l.S(second)l.5

Fig. 7. Relationship between current density and (t-to) 1'5 for copper deposition on untreated and I00 °C plasma-oxidized titaniura substrates from copper sulfate electrolyte at -600 mV(tlg/Hg2SO4).

.7.. Zhang et at / Surface and Coatings Technology 89 (1997) 225-232 I

I

I

l

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0i 8

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(t-to)°'5(second) °.s Fig. 8. Relationship between current density and (t-to) °'5 for copper deposition on 400 °C plasma-oxidizedtitanium substrate. low oxygen pressure oxidations not only enhanced copper electrodeposition by reducing the overpotential, but the mechanism also changed from progressive to nearly instantaneous nucleation. The nucleation and growth behavior of the copper electrodeposits on the various substrates is based essentially on the respective charge transfer rates and active sites initially available on the cathode surface. It is known that any films formed on the electrodes can influence either of these factors. Various mechanisms have been proposed to describe the electron transfer reactions at metal electrodes covered by semiconducting or insulating films [14]. For thin films less than a few nanometers thick the dominating electron transfer mechanism is elastic or inelastic tunnelling. For thicker films the electron transfer depends on the semiconducting properties of the films. According to the Auger surface composition analysis (Table 1), the titanium oxide films formed by plasma and thermal oxidations should be treated as thicker films and their conductivity should play an important role in copper deposition. Earle [15] found that the conductivity of the rutile form of titanium oxide depended on the equilibrium conditions of temperature and oxygen pressure at which it was measured. Considering conductivity changes as one possible reason for the overpotential and current density changes observed, these hypotheses also seem qualitatively valid for the oxidized titanium cathodes used in this research. As the oxidation temperature increased (above 300°C) or the oxygen pressure decreased (in the sequence air, oxygen plasma and 80 mTorr oxygen), the titanium oxide films formed could become more conductive and enhance copper nucleation. Another possibility concerning the conductivity was based on the observation of clusters containing a small

231

number of highly oriented microcrystals in anodized titanium oxide films [16]. These clusters may then act as preferential conducting channels in the oxide films. A similar explanation might be appropriate for titanium oxide films formed by the plasma and low oxygen pressure oxidation. At lower temperatures a thinner, less defective and less conductive oxide film could be formed on the titanium substrate. As the treatment temperature increased, the oxygen vacancy concentration and conductivity of the oxide film would increase and the nucleation and growth of the copper could be enhanced. In some early reports a loss of oxygen in the lattice of titanium dioxide caused by heating in a low oxygen pressure or in hydrogen was mentioned [17]. In addition, for the titanium oxide film formed by thermal oxidation, defects such as microcracks were observed in electron micrographs [ 12]. Therefore possible phase transformations, microcracks or internal stress changes of the titanium oxide films may also influence the electrochemical properties of the oxide films. Further studies are necessary to confirm these possible mechanisms and their effects on electrochemical activity. The thickness of the oxide film was found to be less critical in the early stages of copper nucleation and growth. Compared with 400 °C oxygen plasma (500 A) and 80mTorr oxygen (320A), titanium substrates treated at 400 °C in air (360 A) had the worst uniformity of copper nucleation and showed a greater degree of polarization. Therefore the results seem to indicate that the structure of the oxide film on titanium is a primary factor in the initial nucleation and growth of copper from acid sulfate electrolytes.

4. Conclusions Plasma oxidation of titanium at various temperatures produced thin, interference oxide films. Compared with air or low oxygen pressure (80 mTorr) treatments, the plasma process gave thicker oxide films (about 150 thicker) with more intense colors. The effects of the plasma and low oxygen pressure oxidation of titanium cathodes on the copper nucleation can be divided into two categories separated by a transition temperature of approximately 300 °C. Below the transition temperature the titanium oxide acts as a blocking film, providing a barrier to electron transfer that causes copper nucleation to be progressive and giving limited surface coverage. At higher temperatures (400 and 500 °C) the titanium oxide may become more conductive, causing considerable depolarization and enhanced copper growth. Nucleation appeared to be instantaneous, with a very fine crystallite size giving nearly complete coverage of the surface in about 5 s at an overpotential of - 600 mV. Such behavior may result from a higher density of defects in the film, such as

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or. Zhang et al. / Surface and Coatings Technology 89 (1997) 225-232

oxygen vacancies, a different oxide p h a s e or localized clusters c o n t a i n i n g highly oriented microcrystals. T h e results o b t a i n e d in this study suggest t h a t c o p p e r n u c l e a t i o n can be strongly m o d i f i e d b y t h e r m a l oxidation a n d p l a s m a t r e a t m e n t o f t i t a n i u m in m u c h the same w a y as previously r e p o r t e d for a n o d i z e d t i t a n i u m [4-6]. A d d i t i o n a l research is n o w being c o n d u c t e d to further elucidate the causes o f this b e h a v i o r a n d to identify o t h e r processing aspects which c o u l d influence the surface activity a n d n u c l e a t i o n o f e l e c t r o d e p o s i t e d copper.

Acknowledgement T h e fi_nancial s u p p o r t p r o v i d e b y N S F - I N T 8818746 a n d K i d d Creek M i n e s Ltd., Trimmins, Ont. is gratefully acknowledged.

References [I] A.G. Ives, J.R.B. Gilbert and J.P.A. Wortley, Nucleation and growth of copper electrodeposits on titanium, Proc. 103rd Ann. Meet. AIME, Dallas, TX,, 1974, AIME, 197X.

[2] K.S. Fraser and W.C. Cooper, Surf TechnoL, 8 (1979) 385. [3] G.M. Rao and W.C. Cooper, Hydrometallurgy, 4 (I979) 185. [4] J.L. Delplancke, M. Sun, T.J. O'Keefe and R. Winand, Hydrometalturgy, 23 (I989) 47. [5] J.L. Delplancke, M. Sun, T.J. O'Keefe and R. Winand, Hydrometallurgy, 24 (I990) 179. [6] H. Sun, J.L. Delplancke, R. Winand and T.J. O'Keefe, Copper 91-Cobre 91, VoI. 3, Pergamon, New York, 1991, p. 405. [7] G.P. Bums, I.S. Baldwin, M.P. Hastings and J.G. Wilkes, J. Appl. Phys., 66(6) (I989) 2320. [8] T. Schneider, M. Baron, D. Edwards, R. Gaudy and J. Fukai, £ Appl. Phys., 67(3) (I990) 1601. [9] J. Takada, H. Kuwahara, Y. Manabe, M. Kimura and K. Yanagihara, J. Mater. Sei., 26 (1991) 6288. [I0] J.L. Li, W.J. James and T.J. O'Keefe, Mater. Sei. Eng., B7 I-2 (I990) 5. [ 11] H.R. Thirsk and J.A. Harrison, A Guide to the Study of Electrode Kinetics, Academic, New York, 1972, p. 115. [I2] T. Fukuzuka, K. Shimogori, H. Satoh and F. Kamikubo, Titanium '80 Science and Technology, Proc. Fourth Int. Conf. on Titanium, Vol. 4, Warrendale, PA. : Metallurgical Society of AIME,

1980, p. 2782. [13] D.M. Mattox, J. Vac. Sci. Teehnol., 10(1) (1973) 47. [14] R.R. Dogonadze, A.M. Kuznetsov and J. Zak, Eleetroehim. Acta, 22 (1977) 967. [15] M. Earle, Phys. Rev., 61 (I942) 56. [16] J.L. Delplancke, A. Gamier, Y. Massiani and R. Winand, unpublished results, Igxx. [I7] R.G. Breckenridge and W.R. Hosler, Phys. Rev, 91(4) (1953) 793.