EBSD characterization of twinned copper using pulsed electrodeposition

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2009 J. Phys. D: Appl. Phys. 42 215410

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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 42 (2009) 215410 (8pp) doi:10.1088/0022-3727/42/21/215410

EBSD characterization of twinned copperusing pulsed electrodepositionGuan-Tai Lui1, Delphic Chen1 and Jui-Chao Kuo1,2,3

1 Department of Materials Science and Engineering, National Cheng Kung University, No. 1, UniversityRoad, Tainan 701, Taiwan, Republic of China2 Center for Micro/Nano Science and Technology, National Cheng Kung University, No. 1, UniversityRoad, Tainan 701, Taiwan, Republic of China

E-mail: jckuo@mail.ncku.edu.tw (J C Kuo)

Received 19 January 2009, in final form 19 May 2009Published 23 October 2009Online at stacks.iop.org/JPhysD/42/215410

AbstractWe investigated the influence of pulse current density on a crystallographic microstructure oftwin Cu films through x-ray diffraction and electron backscatter diffraction (EBSD). With anincrease in the pulse current density, the intensity of the preferred orientation (texture), grainsize and the fraction of the twins decreased. The high-density twin copper films, including thenanotwins observed from the orientation image maps, were achieved using a pulsedelectrodeposition technique at the current density of 0.5 A cm−2, with an average grain size of818 nm. The crystallographic microstructure of the high-density twin copper films shows a〈0 1 1〉 preferred orientation associated with the 60◦〈1 1 1〉 twin having a {1 1 1} twin plane.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

The electrodeposition of Cu finds its applications in thefabrication of micro-components in electronic devices.Recently, in the field of microelectronic technology, copperhas replaced aluminium as a metal for interconnects, owingto its lower resistivity, higher thermal conductivity andbetter electromigration resistance [1, 2]. In contrast toconventional direct current (dc) electrodeposition, a pulseelectrodeposition process has been used due to its abilityto significantly reduce void formation [3]. Recently, high-density copper nanotwins synthesized using the pulsedelectrodeposition (PED) technique have shown ultrahigh-yieldstrength, ductility and electrical conductivity [1, 2]. Excellentbehaviour of ultrahigh strength and high electrical conductivityfor the nanotwinned Cu has been attributed to the lowelectrical resistance nature of the coherent twin boundariesin comparison with the conventional coarse-grained Cu.Therefore, these excellent mechanical and electrical propertiesmake the nanotwinned Cu a promising candidate for potentialinterconnect materials.

3 Author to whom any correspondence should be addressed.

A number of earlier studies have been conducted onthe microstructural evolution or ‘self-annealing’ of the dcelectrodeposited Cu thin films at room temperature [4–8].From the results of these studies, we know that recrystallizationoccurring at room temperature in the electrodeposited Cufilms leads to grain coarsening after a few hours ordays. The preferred orientation can be controlled bydeposition conditions such as current density, overpotential andelectrolyte composition [9–11]. Meanwhile, microstructureand texture are dependent on many different parametersincluding substrate structure and stress state, film depositionparameters, plating bath chemistry, annealing temperature andtime, alloying and geometrical constraints. The copper filmsdeposited without any additives have two different texturetypes, 〈1 1 1〉 and 〈1 1 0〉 textures. Furthermore, the textureschange from 〈1 1 1〉 orientation to 〈1 1 0〉 as the film thicknessincreases. Two fibre textures of 〈1 1 1〉 and 〈1 1 0〉 have beenobserved in the copper films deposited with EDTA and sodiumcitrate [12]. While {3 1 1}, {1 0 0}, {1 1 0} and {1 1 1} out-of-plane textures of Cu synthesized by PED have been found witha high density of {1 1 1} twins [13].

Although Cu nanotwins have already been investigatedin a number of studies [1, 2, 14–17], the electrodeposition

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J. Phys. D: Appl. Phys. 42 (2009) 215410 G-T Lui et al

Table 1. On-time, off-time and current density of samples 1–8.

Ton Toff θ Ip Ia

Sample (s) (s) (Duty cycle) (A cm−2) (A cm−2)

1 0.02 2.0 0.01 0.25 0.00252 0.02 2.0 0.01 0.50 0.00503 0.02 2.0 0.01 1.00 0.0104 0.02 2.0 0.01 1.50 0.0155 0.002 0.2 0.01 0.25 0.00256 0.002 0.2 0.01 0.50 0.00507 0.002 0.2 0.01 1.00 0.0108 0.002 0.2 0.01 1.50 0.015

parameters of composition, temperature and current densityare integral to gain an understanding of the effects ofthe electrodeposition parameters on grain size, grain sizedistribution, twin density and texture in the synthesis of a high-density Cu nanotwin. Given that the introduction of additiveswill lead to complex reactions of the cation in the solution,a basic electrolyte without a mixture of organic electrolyteadditives was used in order to understand the effect of thecurrent density on the microstructure. In this study, we utilizedelectron backscatter diffraction (EBSD) to characterize thecrystallographic microstructure of pulse deposited Cu as afunction of pulse current density. We did this in order tounderstand the preferred orientation and twin boundary on themesoscale. The EBSD analysis was performed within the day,right after the deposition process, in order to avoid the self-annealing phenomenon which occurs after a couple of days orweeks.

2. Materials and experiments

The PED was carried out in 0.5M CuSO4 and a pH valueof 1.0 by adjusting the amount of H2SO4. The electrolytetemperature was controlled at 20 ◦C by water cooling, afterwhich the electrolyte was mechanically stirred at 200 rpm.The PED system consisted of an electroplating cell and anexternal pulse power supply of Keithley Source Meter Model2611. During the deposition, the pulse current was turnedoff periodically with an on-time Ton of 0.02 s and an off-timeToff of 2.00 s. The nominal peak current density (PCD) variedfrom 0.25 to 1.5 A cm−2, which was calculated by the ratio ofa given current over the area of the Cu cathode (table 1). Theaverage current density Ia in a pulse plating system is presentedin terms of the following pulsed current density Ip [18]:

Ia = Ton

Ton + ToffIp = f · Ip. (1)

A copper sheet with a purity of 99.99% was prepared intothe dimensions of 30 mm × 30 mm × 3 mm for the anodematerial. Afterwards, amorphous graphite with a diameter of11.3 mm and a thickness of 2 mm was selected as the cathodesubstrate. The anode and cathode were polished with 1500,2000 and 4000 grit SiC abrasive papers and then with 0.03 µm

Figure 1. XRD scan of electrodeposited copper with currentdensities of (a) 0.25 A cm−2, (b) 0.5 A cm−2, (c) 1.0 A cm−2 and (d)1.5 A cm−2.

Table 2. Texture coefficients of the (1 1 1), (2 0 0), (2 2 0) and (3 1 1)peaks for Ton = 0.02 and Toff = 2.0.

Current density (Ip)

0.25 A cm−2 0.50 A cm−2 1.00 A cm−2 1.50 A cm−2

M111 0.14 0.02 0.08 0.11M200 0.06 0.04 0.11 0.10M220 3.60 3.86 3.60 3.58M311 0.20 0.09 0.21 0.21

SiO2. Sequentially, the anode and cathode were cleanedultrasonically for 10 min in acetone and ethanol.

X-ray diffraction (XRD) analysis was conducted on as-deposited copper using a Rigaku diffractometer operated at30 kV and 20 mA, with temperatures ranging from 30◦ to100◦. For the EBSD sample preparation, the surfaces ofthe materials were mechanically polished though a standardmetallographic procedure in order to achieve a final levelof 0.03 µm. This was followed by electropolishing using1.5 V for 60 s in a phosphoric acid electrolyte, whichconsisted of 825 ml phosphoric acid and 175 ml de-ionizedwater.

It is necessary to indicate the interface investigatedon the EBSD measurement that came in contact with thesubstrate called the deposit–substrate interface. Here, EBSDmeasurement was performed on the deposited copper in afield emission SEM of JEOL 7001F with an EDAX/TSLTechnology EBSD system operated at 20 kV. The measuredarea for EBSD analysis was 40 µm × 40 µm with a stepsize of 50 nm. The OIMTM software was used to analysethe orientation maps, grain sizes, twin boundaries andcrystallographic textures.

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Figure 2. IPF maps of the deposit–substrate interface with current densities of (a) 0.25 A cm−2, (b) 0.5 A cm−2, (c) 1.0 A cm−2 and(d) 1.5 A cm−2. Colour coding displays the out-of plane direction in terms of the IPF (blue: // [1 1 1], yellow: //[1 0 1]).

2.1. Effect of current density on preferred orientation andgrain size

The XRD analyses show that the texture of the deposits variedwith the plating current densities as presented in figure 1. Itis observed that the electrodeposited Cu films have a strongpreferred orientation of {1 1 0} peaks and a very weak textureof {1 1 1} peaks. The texture coefficient Mhkl of a (hkl) planeis defined by Barrett and Massalski [19] as

Mhkl = Ihkl/I0hkl

(1/n)∑

Ihkl/I0hkl

, (2)

where I 0hkl is the intensity of the powder pattern in the JCPDS

cards, Ihkl is the measured intensity of XRD andn is the numberof calculated peaks.

Four peaks of {1 1 1}, {2 0 0}, {2 2 0} and {3 1 1} wereused to calculate the texture coefficient. The index value of

Mhkl is a measurement of the volume fraction of grains withthe respective planes parallel to the film [20, 21]. From table 2,we can see that the texture coefficients of {1 1 1}, {2 0 0} and{3 1 1} are <1 and that of {2 2 0} is >3. This reveals that{2 2 0} is the preferred orientation of the deposits.

Figure 2 shows the EBSD orientation image maps(OIMs) investigated on the deposit–substrate interface for fourcurrent densities within the range 0.25–1.50 A cm−2. Thecorresponding colour in the inverse pole figure (IPF) displayedin figure 2 represents the out-of-plane direction. It is foundthat the deposited grains on the deposit–substrate interfacehave an out-of-plane direction of 〈1 1 0〉 as represented inyellow . The fraction of the 〈1 1 0〉 out-of-plane directioncan be quantitatively expressed in terms of the intensity ofthe central pole along the out-of-plane direction shown infigure 3. The intensities of the 〈1 1 0〉 out-of-plane directionare 1.97, 6.02, 3.53 and 1.72 times the random texture forthe current densities 0.25 A cm−2, 0.50 A cm−2, 1.0 A cm−2

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Figure 3. {1 1 0} pole figures of the deposit–substrate interface withcurrent densities of (a) 0.25 A cm−2, (b) 0.5 A cm−2, (c) 1.0 A cm−2

and (d) 1.5 A cm−2, where the central pole means the out-of-planedirection, called the ND direction here.

Table 3. Grain size and twin fraction of samples 1–8.

Toff Ip Grain size TwinSample Ton (s) (s) (A cm−2) (nm) fraction (%)

1 0.02 2.0 0.25 513 28.92 0.02 2.0 0.50 574 383 0.02 2.0 1.00 435 19.64 0.02 2.0 1.50 341 14.95 0.002 0.2 0.25 597 276 0.002 0.2 0.50 818 46.57 0.002 0.2 1.00 612 27.58 0.002 0.2 1.50 483 34.5

and 1.5 A cm−2, respectively. A high intensity at the centralpole represents a high fraction of deposited grains having the〈1 1 0〉 out-of-plane direction, that is a strong 〈1 1 0〉 preferredorientation. Therefore, for the current density at 0.50 A cm−2,the Cu deposit with an intensity of 6.02 has the strongest〈1 1 0〉 texture as compared with the other current densitiesin figure 1(b). It has also been reported that the 〈1 1 0〉 out-of-plane texture has been observed in the as-deposited Cu filmsproduced by PED [1, 2, 22].

Using OIM analysis software, the average area grain sizeν̄ is determined by the following:

ν̄ =

N∑i=1

Aiνi

N∑i=1

Ai

, (3)

where Ai is the area of grain i and νi is the grain size of grain i.

Figure 4. The central intensity of {1 1 0} pole figures and grain sizeas a function of the current density for electrodeposited copper.

Figure 5. The grain size as a function of the current density forelectrodeposited copper with the on-time of 0.02 and 0.002 s.

A decrease in grain size has been observed exceptfor the current density of 0.25 A cm−2 when the currentdensity increased (table 3). According to the principles ofelectrocrystallization, the rate of formation of new nuclei onthe electrode surface increases exponentially as the cathodicpolarization η increases as follows [23]:

ω = B exp

(−K

η2

), (4)

where the constants B and K are for a given metal andtemperature. If all other factors were kept constant, theincrease in cathodic polarization would lead to a larger numberof newly formed nuclei. An increase in current density resultsin an increase in the polarization effect, enhancing the Cunucleation process, which in turn, promotes the formation offine grains.

Charge transfer and surface diffusion are the rate-determining steps for the reduction mechanism of copper,when the pulse period lies in the millisecond range. Cupric

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Figure 6. Pattern quality maps showing the microstructure on the deposit–substrate interface copper with current densities of(a) 0.25 A cm−2, (b) 0.5 A cm−2, (c) 1.0 A cm−2 and (d) 1.5 A cm−2.

ions to copper ad-atoms are reduced by the disproportionationof cuprous ions and the dissolution of copper ad-atoms intobulk solution. It has been found that the reduction of cupricions can mainly account for the decrease in current efficiencyin the millisecond range pulse current in a surface diffusioncontrolled system [24]. The decrease in current density resultsfrom the decrease in the overpotential. The effect of thedecreasing overpotential resulting from the reduction of cupricions is of central importance in the case of a small given pulsecurrent density, such as 0.25 A cm−2. This phenomenon canbe used as an explanation for the observation of grain sizereduction at the small current density of 0.25 A cm−2. Incomparison with samples 5–8 with the on-time 0.002 s andthe off-time 0.2 s, another series of on-time 0.02 s and off-time2.0 s (samples 1–4) was conducted to confirm the maximumgrain size at the current density of 0.5 A cm−2 (figure 4). Thisrelationship between grain size and current density that affectsthe attainment of a maximum grain size is also observed insamples 1–4 as seen in figure 5 and table 3.

In addition, it is interesting to note that the grain size andthe {1 1 0} pole intensity as a function of the current densityshow the same trend (figure 4). As the grain size decreases,

except for 0.25 A cm−2, the {1 1 0} pole intensity parallel tothe normal direction decreases, that is the {1 1 1} pole intensityincreases (figure 2).

2.2. Effect of current density on twin boundary

Twin boundaries are observed in the image quality for the Cudeposits with 0.25, 0.50, 1.00 and 1.50 A cm−2 (figure 6). Adark pixel means that it is impossible to acquire a Kikuchipattern of the backscattered electrons because a grain boundaryexists. To identify the character of the grain boundaries, weused figure 7 (a magnified version of figure 2) to show the grainboundaries including the low- and high-angle grain boundariesas well as the twin boundaries for the Cu deposits. Here,low-angle grain boundaries are defined as a misorientationangle <15◦, while high-angle grain boundaries are >15◦. Inaddition, the twin copper films observed in the Cu deposits arerepresented as 60◦〈1 1 1〉, that is a 60◦ rotation about 〈1 1 1〉axes with the {1 1 1} twinning plane. It is observed that the60◦〈1 1 1〉 twins exist in the grains with the 〈1 1 0〉 out-of-planedirection (figure 7(b)), as well as in the nanotwins (figure 8). Inthe following, we will try to explain the relationship between

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Figure 7. IPF maps and the grain boundary character of the deposit–substrate interface magnified from figure 2 with current densities of(a) 0.25 A cm−2, (b) 0.5 A cm−2, (c) 1.0 A cm−2 and (d) 1.5 A cm−2 (black line: small grain boundaries; blue line: high grain boundaries; redline: twin boundaries).

the 〈1 1 0〉 preferred orientation and the 60◦〈1 1 1〉 twin infigure 9. When the {1 1 1} plane is parallel to the out-of-planedirection, it is possible to find two {1 1 0} planes separated bythe {1 1 1} plane called a twin plane. This accounts for the factthat the 60◦〈1 1 1〉 twins in the Cu deposits are associated withthe grains having the 〈1 1 0〉 out-of-plane orientation.

If the grain size and the twin fraction are plotted together(figure 10), it is noteworthy that the twin fraction is directlyproportional to the grain size in this study. This suggests thata high twin fraction of 46.5% can be produced at the grain sizeof 818 nm.

3. Conclusions

In this study, copper films were electrodeposited with differentapplied pulse current densities. The microstructure in theCu deposits depended on the applied pulse current density.

Figure 8. Magnification OIM showing nanotwinned grains for thecurrent density of 1.0 A cm−2.

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Figure 9. Schematic illustration of the crystal arrangement for a60◦〈1 1 1〉 twin containing a twin plane of {1 1 1} as a mirror planein the out-of-plane direction and its neighbour grains with a {1 1 0}plane.

Figure 10. Twin fraction as a function of grain size forelectrodeposited copper on the substrate–deposition interface forsamples 1–8.

In general, the {1 1 0} pole intensity of the out-of-planedirection, the grain size and the twin fraction of the Cu depositsdecreased as the current density increased. A high fraction oftwins (46.5%) including nanotwins was achieved using a PEDtechnique, under the current density of 0.5 A cm−2 with anaverage grain size of 818 nm. A high density of twins was alsoachieved by the crystallographic microstructure in which thegrains consisted of a {0 1 1} preferred orientation associatedwith the 60◦〈1 1 1〉 twin having a {1 1 1} twin plane.

Acknowledgments

This work was supported by the National Science Council(NSC 97-2221-E-006-007) and has received grants from

the Center for Frontier Materials and Micro/Nano Scienceand Technology, National Cheng Kung University, Taiwan,ROC (D97-2700).

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