electrochemical deposition of copper on n-si/tin

1436 Journal of The Electrochemical Society, 146 (4) 1436-1441 (1999)S0013-4651(98)07-069-4 CCC: $7.00 © The Electrochemical Society, Inc.

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Electrochemical Deposition of Copper on n-Si/TiNGerko Oskam,* Philippe M. Vereecken, and Peter C. Searson**

Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, USA

In this paper we report on the electrochemical deposition of copper onto n-type silicon with a 30 nm TiN barrier film. We show thatthe mechanism of nucleation and growth is dependent on the applied potential. At potentials more negative than 20.35 V, in-stantaneous nucleation of hemispherical clusters is followed by diffusion-limited growth. In this potential regime, the nucleus densi-ty is essentially constant at about 5 3 108 cm22. At potentials more positive than 20.35 V, the nucleation and growth kinetics are morecomplex, and clusters consisting of several nuclei are formed. The cluster density decreases to about 2 3 105 cm22 at 20.05 V.© 1999 The Electrochemical Society. S0013-4651(98)07-069-4. All rights reserved.

Manuscript submitted July 20, 1998; revised manuscript received October 6, 1998.

As feature sizes in integrated circuits continue to decrease, copperinterconnect technology has emerged as a leading candidate toreplace conventional aluminum technology. Copper has a lower resis-tivity and higher maximum current density for electromigration,properties that are critical to improved device performance and relia-bility.1,2 These properties are sufficient to provide the necessaryincrease in performance and reliability for the next generation ofultralarge scale integration (ULSI) devices. In addition, with dual-Damascene processing,2 vias and interconnects can be fabricated atthe same time, thereby decreasing the number of deposition steps.Since copper is soluble in silicon it is necessary to deposit a thin dif-fusion barrier onto the silicon before deposition of the copper metal-lization. Candidate materials for diffusion barriers include transitionmetals, transition metal alloys, transition metal silicides, polycrys-talline metal nitrides, and ternary amorphous alloys.3

Copper can be deposited by a number of techniques, includingchemical vapor deposition, physical vapor deposition, electrolessdeposition,4,5 and electrochemical deposition.6,7 Of these tech-niques, electrochemical deposition is the leading candidate due to itsinherent advantages in filling high aspect ratio structures with com-plex geometries combined with high deposition rates. Due to the dif-ficulties in depositing directly onto most diffusion barriers, a copperseed layer is usually vapor-deposited prior to electrochemical depo-sition.2

In this paper, we report on the nucleation and growth kinetics ofcopper onto unpatterned n-type silicon wafers with a TiN barrierlayer. The goal of this work is to explore the possibility of electro-chemical deposition of high quality copper films without the use ofa copper seed layer. We show that under appropriate experimentalconditions, a high density of copper nuclei can be obtained at thesurface. Furthermore, continuous copper films can be obtained bysubsequent growth of the clusters at low overpotentials under kinet-ic or mixed control.

ExperimentalAll experiments were performed on n-Si(100), ND 5 1 3

1015 cm23 (Wacker Siltronic, AG) with a 30 nm TiN barrier layer.The TiN layer was radio frequency (RF) sputter deposited at roomtemperature for 1 min (VRF 5 620 V). In order to avoid limitationsassociated with the sheet resistance of the TiN film, an ohmic contactwas made to the back side of the silicon wafer using In/Ga eutectic;since the n-Si/TiN contact is ohmic, this method provides a goodelectrical contact to the thin TiN layer. The aqueous 50 mM Cu21

solution was prepared from 25 mM CuCO3?Cu(OH)2 with 0.32 MH3BO3 and 0.18 M HBF4. The pH of the solution was about 1.4.

The experiments were performed under ambient conditions usinga conventional three-electrode cell with a Ag/AgCl (3 M NaCl) ref-erence electrode and a platinum gauze counter electrode. The refer-ence electrode was placed close to the Si/TiN working electrode

** Electrochemical Society Student Member.** Electrochemical Society Active Member.

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using a Luggin capillary. All potentials are given with respect to thereference electrode (0.22 V vs. NHE). The electrochemical experi-ments were performed using a computer-controlled EG&G PAR 273potentiostat. Scanning electron microscopy (SEM) was done on anAMRAY 1860 FE at a beam acceleration voltage of 5 kV.

Results and DiscussionFigure 1 shows current-potential curves for n-Si/TiN in 0.32 M

H3BO3 1 0.18 M HBF4 with and without 50 mM Cu21 at a scan rateof 10 mV s21. In the absence of Cu21 ions, the onset of hydrogenevolution is observed at about 20.7 V, illustrating the slow kineticson TiN in this solution. In the 50 mM Cu21 solution, the open-circuitpotential was 0.20 V, and the first cycle was initiated from this poten-tial. The onset of the reduction of Cu21 occurs at about 0 V with acharacteristic diffusion-limited growth peak at 20.15 V. After thedeposition peak, the current again increases at a potential of about20.75 V due to the reduction of protons at the TiN surface, which ispartially covered with copper. From the cathodic current peak, weestimate the deposition of about 650 equivalent monolayers of cop-per at 20.8V (assuming 100% faradaic efficiency).

Figure 1. Current-potential curves for n-Si/TiN in 0.32 M H3BO3 1 0.18 MHBF4 (pH 1.4) solution with (a) 0 and (b) 50 mM Cu21 at a scan rate of10 mV s21. The first scan (1) was recorded starting from the open-circuitpotential at 0.2 V. Subsequent cycles (2 and 3) were essentially the same.

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Journal of The Electrochemical Society, 146 (4) 1436-1441 (1999) 1437S0013-4651(98)07-069-4 CCC: $7.00 © The Electrochemical Society, Inc.

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The reverse scan in the 50 mM Cu21 solution shows a steady-state, diffusion-limited deposition current density of about 5 mAcm22 over a wide potential range. At potentials positive to 0.02 V, astripping peak is observed since the n-Si/TiN contact is ohmic. Incontrast, if copper is electrodeposited onto silicon, a stripping peakis not observed due to the 0.63 eV barrier height of the Si/Cu Schott-ky junction.8

On subsequent cycles, the copper deposition peak is shifted toabout 20.02 V since the copper is not completely removed duringthe stripping wave. This shift of the deposition peak indicates that anucleation overpotential is required for the deposition of copper ontoTiN. The second and third sweeps are essentially the same, and theshape of the voltammograms suggests that deposition and dissolu-tion of copper on TiN/Cu is a quasireversible process. The voltam-mograms show a second peak at about 20.32 V. This peak was notobserved in the absence of Cu21 on a copper foil substrate and,hence, can be ascribed to the copper deposition process.

The position and magnitude of the current peak in the first scanwas found to be dependent on the scan rate. Cyclic voltammogramswere recorded at scan rates from 25 to 2025 mV s21. Prior to eachmeasurement, the electrodeposited copper from the previous scanwas stripped by holding the potential at 0.20 V until the current wassmaller than 0.7 mA cm22. Figure 2 shows the maximum currentdensity at the peak, ipeak, of the first scan vs. the square root of thescan rate, n. Figure 3 shows the potential at the current maximum,Upeak, vs. the logarithm of the scan rate. From Fig. 2 and 3 it can beseen that ipeak varies linearly with n1/2 and that Upeak varies linearlywith log(n), characteristic of a diffusion controlled reaction.9 Thenonzero intercept of the peak current at zero scan rate implies thatthe reaction may also involve a catalytic or chemical step.9 Note thatat a scan rate of 2 V s21, Upeak has shifted considerably to about20.4 V. The observation that the peak potential shifts to more nega-tive potentials upon increasing the scan rate indicates that the kinet-ics of charge transfer are not sufficiently fast to maintain steady-stateconditions upon changing the potential.

Experimentally, the mechanism of nucleation and growth can bedetermined from analysis of the current response to a potential step.Figures 4A and B show a series of current transients for copper depo-sition on n-Si/TiN in the 50 mM Cu21 solution for potential stepsfrom the open-circuit potential to deposition potentials in the range

Figure 2. Peak deposition current (first cycle) vs. the square root of the scanrate. Prior to each measurement, the electrodeposited copper from the previ-ous experiment was stripped at 0.20 V.

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from 0 to 20.55 V. All the transients exhibit an initial current peakdue to charging of the double layer. The nucleation and growthprocess is characterized by a current peak where the deposition cur-rent first increases due to nucleation of copper clusters and three-dimensional diffusion-controlled growth, and then decreases as thediffusion zones overlap resulting in one-dimensional diffusion-con-trolled growth to a planar surface. The deposition transients are char-acterized by a maximum current, imax, that occurs at time tmax. Cur-rent transients recorded in a solution without Cu21 exhibited only adouble-layer charging current.

At long times (t > 3tmax), the transient current is proportional tot21/2 in the potential regime between 20.55 to 0 V in accordancewith the Cottrell equation.9,10 The diffusion coefficient of Cu21 wasdetermined from i22 vs. t curves to be about 6 3 1026 cm2 s21 at20.2 V which is in good agreement with values reported in the lit-erature.11 The apparent coefficient increased to about 2 3 1025 cm2

s21 at 20.55 V which may be due to parasitic reactions such as theevolution of hydrogen. These results confirm that at longer times,linear diffusion to a planar surface is the rate-limiting step in thedeposition process in the potential range between 20.2 and 20.55V. At potentials in the range from 20.2 to 0 V, the current at longertimes is proportional to t21/2 but is smaller than the Cottrell limit. Inaccordance with the results from voltammetry, it can be inferred thatthe current is not purely diffusion controlled but also involves akinetic component.9

In general, nucleation of a metal on a foreign substrate is assum-ed to take place at active sites on the surface, such as steps, kinks, orother surface defects. The density of active sites represents the totalnumber of possible sites for nucleation, which may be potential de-pendent if different sites have a different activation energy for nucle-ation. The density of nuclei as a function of time, N(t), is usuallydescribed in terms of a nucleation rate constant, A 12,13

N(t) 5 N` [1 2 exp (2At)] [1]

where N` is the final nucleus density may be dependent on appliedpotential and solution composition. For the case where At >> 1, thenucleation is instantaneous and all nuclei are formed at the sametime. For At << 1, the nucleation is progressive and new nuclei areformed while existing nuclei are already growing. Hence, the sizedistribution for instantaneous nucleation is expected to be much nar-rower than for progressive nucleation.

Figure 3. The potential at the current peak maximum vs. the logarithm of thescan rate for the same measurements as shown in Fig. 2.

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The time-dependent deposition current density (normalized to thegeometric surface area), i(t), for instantaneous nucleation followed bythree-dimensional diffusion-limited growth is given by12,13

[2]

where z is the number of electrons transferred in the deposition reac-tion, F is the Faraday constant, D is the diffusion coefficient, c is thebulk metal ion concentration, M is the molar weight of the deposit,and r is the density of the film. For progressive nucleation, the time-dependent deposition current density is given by12,13

i tzFD c

tN Dt

cM( ) exp

/

/ /

/

5p

2 2 pp

r`

1 2

1 2 1 2

1 2

18

Figure 4. Current transients for the deposition of copper. (A, top) The poten-tial was stepped from the open-circuit potential to (a) 20.55, (b) 20.50, (c)20.45, (d) 20.40, (e) 20.30, (f) 20.20, and (g) 20.10 V. (B, bottom) Thepotential was stepped from the open-circuit potential to (a) 20.25, (b) 20.20,(c) 20.15, (d) 20.10, (e) 20.05, and (f) 0 V.

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[3]

In order to determine whether nucleation is instantaneous or pro-gressive, the transients can be analyzed in reduced form in terms ofthe maximum current, imax, and the time at which the maximum cur-rent is observed, tmax. 12,13 For instantaneous nucleation

[4]

and for progressive nucleation

[5]

Figure 5 shows representative deposition transients from Fig. 4replotted in reduced form, along with the theoretical curves forinstantaneous and progressive nucleation given by Eq. 4 and 5,respectively. In all cases, the raw data are plotted; the induction timewas always negligible compared to the time at the maximum. Fromthis figure it can be seen that the mechanism of deposition is depen-dent on the applied potential, and that two potential regions can bedistinguished. At potentials from 20.35 to 20.55 V, the analysis ofthe experimental results shows good agreement with the theoreticalcurves for instantaneous nucleation and diffusion-controlled growth.The current transients at potentials in the range 20.30 to 0 V followthe theoretical curve for instantaneous nucleation up to tmax butexhibit a higher current at longer times. Although the transients didnot follow simple models for instantaneous or progressive nucle-ation with kinetically limited growth in this potential range, it is like-ly that the deviation is due to partial kinetic control.

After the deposition transients, the deposited copper was strippedfrom the surface by holding the potential at 0.2 V until the currentwas smaller than 0.7 mA cm22. The current efficiency for the depo-sition process was determined by integration of the current transientsfor deposition and stripping. Figure 6 shows the efficiency defined

i

i

t

t

t

t

2

2

22

1 2254 1 2 3367max

max

max25 2 2. exp .

i

i

t

t

t

t

2

2

2

1 9542 1 1 2564max

max

max5 2 2. exp .

i tzFD c

tAN Dt

cM( ) exp

/

/ /

/

5p

2 2 pp

r`

1 2

1 2 1 22

1 2

12

3

8

Figure 5. Reduced parameter plots for selected current transients for thedeposition of copper shown in Fig. 4: (h) 20.55, (n) 20.40, (,) 20.30,(e) 20.25, and (s) 20.05 V. The curve at 20.05 V overlays that of 20.25 V.Also shown are the theoretical curves for instantaneous (solid line) and pro-gressive (dotted line) nucleation.





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as the ratio of the stripping charge, Qa, and the total cathodic charge,Qc. It can be seen that the deposition efficiency is close to 1 in thepotential range from 20.2 to 0 V, but decreases to about 0.25 atpotentials more negative than about 20.3 V. The decrease in effi-ciency is due to parasitic hydrogen evolution. Note that the deposi-tion efficiency is likely to be time dependent as the kinetics forhydrogen evolution are expected to be dependent on the cluster den-sity and size.

SEM was used to determine the nucleus density as a function ofthe applied potential. Figure 7 shows SEM images of copper nucleideposited on TiN surfaces at 20.15, 20.25, and 20.45 V. In allcases, 10-15 mC cm22 of charge was passed, corresponding to athickness of about 20 equivalent monolayers of copper; the deposi-tion time corresponded to t/tmax $ 3. From the image obtained at20.45 V it can be seen that the nuclei are randomly distributed overthe surface and are of uniform size, consistent with the instantaneousnucleation model. At 20.25 V, the copper clusters appear relativelyuniform in size and have a nodular structure. At 20.15 V, the clus-ters appear needle-like and exhibit some faceting. From theseimages it can be seen that growth of the nuclei is anisotropic at lowoverpotentials resulting in the formation of clusters with more com-plex geometries than the hemispherical growth centers seen at morenegative potentials. This suggests a kinetic contribution mechanismat positive potentials (20.3 to 0 V). These more complex geometriesevolving as a function of time may also contribute to the deviationof the growth current at times after tmax. The nucleus densitiesobtained from SEM images are shown in Fig. 8. The nucleus densi-ty increases from 2 3 105 cm22 at 20.05 V to about 5 3 108 cm22

at 20.7 V. In the potential range from 20.35 to 20.7 V the nucleusdensity is essentially independent of potential.

Figure 9 shows the nucleus size distribution determined from theSEM images. For the samples prepared at 20.15 and 20.25 V, lowermagnification images were used to improve the statistics. The sizedistributions are approximately Gaussian in shape with averagediameters: d 5 0.47 mm (60.16 mm) at 20.15 V, d 5 0.23 mm(60.09 mm) at 20.25 V, and d 5 0.11 mm (60.04 mm) at 20.45 V.The relative width of the distribution is independent of the deposi-tion potential, indicating that the nucleation mechanism is the samefor the three potentials analyzed. A distribution of nucleus sizes fora system that exhibits instantaneous nucleation (At >> 1) impliesstrong correlations between the growth of neighboring nuclei.

Figure 6. The current efficiency for copper deposition determined from thecurrent transients. The amount of copper deposited was determined by apply-ing a constant potential of 0.2 V after the deposition transient, and recordingthe current as a function of time until the current was smaller than 0.7 mAcm22. The curve represents the average of three sets of transients.

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The total charge associated with the nuclei in the SEM imagescan be determined by integrating the size distributions. Assuminghemispherical nuclei, the charge obtained from the distributions at20.15 and 20.25 V were 23 and 20 mC cm22, respectively, whichis larger than the charge passed of 10.5 and 15 mC cm22. The SEMimages show that the nuclei at these potentials are not fully dense sothat the assumption that the nuclei are hemispherical is expected toresult in an overestimation of the copper volume. At 20.45 V, thecharge obtained from integration of the distribution is 6.9 mC cm22

corresponding to an efficiency of 65%, which is about a factor two

Figure 7. SEM images of copper clusters on TiN at (a, top) 20.15 V, (b, mid-dle) 20.25 V, and (c, bottom) 20.45 V. In each case, the deposition time waslarger than 3tmax; the charge passed was (a) 15, (b) 10.5, and (c) 10.5 mC cm22.





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higher than the value obtained in Fig. 6. Since the areas of the SEMimages are 1025 to 1026 of the electrode area, these results show thatthe images are representative of the entire surface area.

The nucleus density at each deposition potential can also be de-termined from the current transients. According to the model forinstantaneous nucleation followed by three-dimensional diffusionlimited growth, tmax and imax are given by

[6]

[7]

Equations 6 and 7 can be combined to give the following expressionfor the nucleus density

[8]

Figure 8 shows that the values of the nucleus density obtainedfrom analysis of the current transients are in good agreement withthe values obtained from the SEM images. At potentials more nega-tive than 20.35 V, the nucleus density obtained from analysis of thetransients is lower than observed with SEM. This can be attributedto the parasitic reactions such as reduction of protons as shown inFig. 6. From Eq. 8 it can be concluded that if only 25% of the cur-rent observed at the maximum corresponds to copper deposition thatthe nucleus density would be underestimated by a factor 16. Hence,by correcting for the the current efficiency, the nucleus densitiesdetermined from SEM and the current transients agree very well.

In the potential range between 0 and 20.3 V, the calculated val-ues of the nucleus density are in good agreement with the SEMresults, indicating that despite the more complicated depositionprocess the model for instantaneous nucleation and diffusion-con-trolled growth can be applied to determine the cluster density. Thepotential dependence of the nucleus density indicates that the densi-ty of active sites for nucleation is potential dependent. In the contextof classical nucleation models14-16 this suggests that there is a distri-bution of activation energies for nucleation, and that in this potential

NcM

zFc

i t` 5r

p0 065

8

1 2 2

./

max max

i zFcDcM

Nmax 5p

r`0 6382

81 4

1 2. ( )/

/

t

N DcM

max 5

pp

r`

1 2564

81 2

./

Figure 8. Nucleus density determined from deposition transients (s) and fromscanning electron microscope images (n) plotted vs. deposition potential.

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range the overpotential is of similar order of magnitude as the acti-vation energy for nucleation. As a consequence, the nucleus densityincreases sharply with increasing overpotential. At potentials morenegative than 20.35 V, the overpotential is sufficiently large so thatnucleation can occur at all sites and the nucleus density becomesconstant. The potential dependence of the nucleus density shows that

Figure 9. Nucleus size distribution for deposition at (top) 20.15 V, (middle)-0.25 V, and (bottom) 20.45 V. For the experiments at 20.25 and 20.15 V,lower magnification images were used to improve the statistics. The numberof nuclei used in the analysis is shown in the respective figures.

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voltammetry experiments represent a complicated situation wherenucleation is effectively progressive during the potential sweep.

Figure 10 shows a plot of log (tmax) and log (imax) vs. the deposi-tion potential. Both curves show two linear regions corresponding tothe same two potential regimes observed in the transient analysis. Inthe potential regime between 20.35 and about 20.7 V, tmax and imaxare weakly dependent on the deposition potential. From Eq. 6 and 7it follows that this observation corresponds to a potential indepen-dent nucleus density, N`. In the potential regime between 0 and20.35 V, the potential dependence of tmax and imax is much strongerwith inverse slopes of dU/d log (tmax) 5 85 mV/decade and dU/d log(imax) 5 2160 mV/decade, respectively. According to Eq. 6) and 7these results indicate that the nucleus density is potential dependentin this regime. Also note that Eq. 6 and 7 predict that dU/d log(imax) 5 22 dU/d log (tmax), which is observed experimentally. Theconclusions from the transient analysis are in good agreement withthe results from the SEM images, which confirms the applicabilityof the nucleation and growth models for the characterization ofmetal deposition processes.

Figure 10. Potential dependence of tmax (h) and imax (s) obtained from thecurrent transients plotted vs. the deposition potential.

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The results from these experiments were used to determine theconditions for the deposition of copper films on TiN surfaces. Theapproach was to deposit a high density of copper nuclei of uniformsize and hemispherical shape by applying a potential pulse from theopen-circuit potential to a potential in the range from 20.5 to21.0 V. The length of the nucleation pulse was optimized in order toprevent coalescence of the nuclei under diffusion-controlled growthwhich can lead to dendritic deposits.17 After the nucleation pulse, thepotential was stepped to a potential in the range from 20.05 to0.02 V in order to grow the clusters under either kinetic or mixedcontrol and to obtain continuous copper films. Preliminary experi-ments showed that this approach results in good quality copper filmson TiN surfaces.

Acknowledgments

This work was supported by the National Science Foundationunder grant CTS-9732782.

The Johns Hopkins University assisted in meeting the publication costs ofthis article.

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