Ta barrier substrates by alternation of Pb-UPD and Cu-SLRR

Electrochimica Acta 206 (2016) 45–51

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Direct, sequential growth of copper film on TaN/Ta barrier substrates by alternation of Pb-UPD and Cu-SLRR J.S. Fanga,* , J.H. Chena , G.S. Chenb , Y.L. Chengc , T.S. Chinb a b c

Department of Materials Science and Engineering, National Formosa University, Huwei,Yunlin 63201, Taiwan Department of Materials Science and Engineering, Feng Chia University, Taichung 40724, Taiwan Department of Electrical Engineering, National Chi-Nan University, Nan-Tou 64561, Taiwan

A R T I C L E I N F O

Article history: Received 24 February 2016 Received in revised form 22 April 2016 Accepted 23 April 2016 Available online 25 April 2016 Keywords: underpotential deposition surface limited redox replacement electrochemical atomic layer deposition Cu film barrier layer

A B S T R A C T

This study uses an electrochemical form of layer-by-layer deposition to grow Cu films on TaN/Ta barriers without the need for a copper seed layer. The process involves sequential underpotential deposition (UPD) of Pb and surface-limited redox replacement (SLRR) of Cu. Additionally, this work elucidates the influence of pH and Pb deposition potential on the growth mechanism and electrical properties of the resulting Cu films on TaN/Ta barriers. The results demonstrate that during Cu-SLRR, the onset of Cu2+ substitution of Pb2+ is delayed when Pb coverage is increased via deposition at a higher Pb-UPD. This is because perchlorate anions are adsorbed on the underlying UPD-deposited Pb adatoms. Cu films formed at a Pb-UPD of 1150 mV exhibit the lowest resistivity of 7.6 mVcm in the as-deposited state because these films are formed with the Cu(111) texture. Additionally, the Cu layer remains thermally stable after annealing at 700  C. The results of this study may be of interest for the fabrication of microelectronics, particularly for forming Cu-interconnections. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Copper (Cu) has been adopted as an interconnect material in ultra-large-scale integrated circuits (ULSI) because of its high electrical conductivity and high resistance to electromigration. Conventionally, Cu interconnects are fabricated by the damascene process during the manufacturing of ULSI. The process includes electrochemical deposition of a Cu film onto TaN/Ta barriers that are previously sputter-deposited with a copper seed layer. As the feature sizes used in the ULSI shrink, it becomes increasingly difficult to deposit a conformal TaN/Ta barrier and Cu seed layer by sputter-deposition. Chemical vapor-phase atomic layer deposition (ALD) is typically employed in order to achieve conformal deposition because it is capable of atomic-level layer control and conformal growth through sequential, self-limiting surface reactions [1]. Apart from chemical vapor-phase ALD, liquid-based electrochemical atomic layer deposition (ECALD) has also been suggested as a way to deposit conformal films for applications in ULSI metallization [2–4]. In this study, the use of liquid-based electrochemical layer-by-layer deposition is reported for the

* Corresponding author. E-mail address: [email protected] (J.S. Fang). http://dx.doi.org/10.1016/j.electacta.2016.04.129 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.

formation of Cu films on TaN/Ta barriers without using a copper seed layer. Electrochemical deposition of Cu from a copper sulfate solution onto a copper seed layer has been studied in detail [5–11]. For direct Cu-electrodeposition on TaN/Ta barriers, the magnitude of the deposition potential is a dominating factor that determines Cu nucleation densities [12–15]. Further, TaN/Ta barriers usually cause a negative shift in the nucleation overpotential relative to the formal deposition potential because of the native Ta oxide that is inevitable formed on the surface. To avoid this, a two-step method has been reported for electrodeposition of copper on a TaN barrier in a copper-citrate complex solution. First the native Ta oxide is removed using a cathodic reduction reaction. In the second step, copper is deposited using a Cu2+ electrolyte [16,17]. This two-step method was used to control nucleation and crystal-growth in copper films [18]. However, direct Cu-electrodeposition on TaN (or Ta) has rarely been studied because of the difficulty of direct nucleation and growth of Cu deposits on TaN/Ta barriers. In this study, Cu films were deposited on TaN/Ta barriers by ECALD using sequential underpotential deposition (UPD) of Pb and surface-limited redox replacement (SLRR) of Cu. The UPD of Pb refers to the deposition of an atomic layer of Pb at a potential smaller than that required for the deposition of the element. A Cu2+ electrolyte, provided by a copper perchlorate solution, was used to replace sacrificial UPD-Pb adatoms deposited on the surface at an

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open-circuit potential (OCP). UPD results in the deposition of less than one monolayer of Pb, so subsequent repetitive steps result in the layer-by-layer growth of Cu. This work also reports the effects of pH and Pb deposition potential on the growth mechanism and the electrical properties of the resulting Cu films deposited on TaN/ Ta barriers, and the results are of interest for the fabrication of microelectronics, particularly for the formation of Cu interconnects between nanometer-sized features. 2. Experimental procedures Cu films were deposited on Si (100) substrates previously coated with TaN (10 nm)/Ta (5 nm). The TaN/Ta layers were deposited by direct current sputtering at a power of 100 W. The base pressure of the vacuum chamber was 4.0  10 4 Pa, and the sputtering was performed at a working pressure of 5.3  10 1 Pa under a constant argon flow rate of 40 sccm for Ta deposition and under a mixture of argon and nitrogen at flow rates of 40 sccm and of 20 sccm, respectively, for TaN deposition. No intentional heating or cooling of the substrate was performed during sputtering. Prior to the electrochemical deposition process, the TaN/Ta/Si substrate (with a deposition area of 1.5  1.5 cm2 in the deposition cell) was cleaned in a 10 mM HClO4 solution by cyclic voltammetry (CV) at a scan rate of 10 mV/s, scanning from +2000 mV to 1750 mV for 1 cycle. Following this, the first Cu monolayer was deposited on the TaN/Ta/Si substrate by UPD at 1100 mV (also determined by CV scanning). This was followed by a sacrificial Pb layer, which was also deposited by UPD (denoted as UPD-Pb) at a potential in the range of 1100 mV to 1250 mV according to the reductive potential obtained from CV scans of the Pb solution. Both UPDs were performed for 60 s. The next Cu monolayer was then prepared by replacing the UPD-Pb using SLRR in a Cu electrolyte at an open circuit potential for 60 s (denoted as SLRR-Cu). Subsequent alternations of UPD-Pb followed by SLRR-Cu were repeated 50 times. The Cu electrolyte was composed of 1.0 mM Cu (ClO4)2
5H2O, 2.5 mM HCl, and 0.5 mM C10H16N2O8 (EDTA), all purchased from Sigma-Aldrich and used as received. The electrolyte used for Pb-deposition was a mixture of 1.0 mM Pb(ClO4)2
H2O and 2.5 mM HClO4, also products of Sigma-Aldrich. A 1.0 mM HClO4 solution (Sigma-Aldrich) was used as a blank solution to remove the previous solution from the deposition cell. All of the solutions were prepared using deionized water that had been previously deaerated by bubbling with high purity nitrogen for 1 h prior to deposition. The auxiliary electrode was a gold wire embedded on the cell wall opposite to the substrate. A 1.0 mM sulfuric acid solution (Samakyu’s Pure Chemical) was used to strip the Cu film in

pH=3.0 pH=3.5 pH=4.0

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Current Density ( Acm )

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E (mV) vs Ag/AgCl Fig. 1. Cyclic voltammetry scans of Cu electrolyte at pH 3.0, 3.5, and 4.0 on TaN/Ta/Si substrates and at a scan rate of 20 mV/s.

order to estimate the monolayer (ML) deposition. For this study, a ML was defined as coverage of one adsorbate per substrate surface atom. Each cycle yielded deposition of a ML or less of the Cu, and the number of cycles determined the thickness. The reference electrode was Ag/AgCl (3 M NaCl, Bioanalytical Systems Inc.). Cu films were annealed in a rapid thermal annealer (RTA) at a fixed temperature in the range of 300  C to 800  C for 5 minutes in a mixture of argon and hydrogen (5%). X-ray diffractometry (XRD, Bruker AXE D8A25) with CuKa1 radiation was performed to investigate the crystal structure of the deposited films. The electrical resistivity of the film was derived from the sheet resistance (determined by a four-point probe facility) and film thickness. The morphology of the Cu films was examined by atomic force microscopy (AFM, Digital Instruments, Dimension 3100). 3. Results and discussion 3.1. pH-dependence of reductive potential changes during cyclic voltammetry scans UPD of Cu and UPD of Pb were first examined at various pH values prior to layer-by-layer deposition of SLRR-Cu. Fig. 1 presents the CV scans of the Cu2+ electrolyte on the TaN/Ta/Si substrate at a scan rate of 20 mV/s and at pH values of 3.0, 3.5, and 4.0. No current is generated initially. At a pH of 3.0, the current density of the Cu2+ electrolyte begins to decrease at 1000 mV during the downward scan from +2000 mV to 1750 mV. A current peak occurs at 1320 mV, followed by evolution of hydrogen at potentials below 1500 mV. During the upward scan from 1750 mV to +2000 mV, Cu oxidation begins at 1250 mV. Fig. 1 also shows that the peak current densities are slightly reduced when the pH is increased, and the reduction potential shifts toward 1180 mV for pH 3.5 and 1200 mV for pH 4.0; i.e., the reductive potential differs for different pH values. As the pH is increased, the reductive peak occurs at relatively less negative potentials with respect to the electrolyte. In this study, HCl was used to adjust the pH of the Cu electrolyte. The CV characteristics are closely correlated with the electrolyte concentration and pH [19], and a suitable concentration of Cl ions may promote the deposition of Cu atoms on Au(111) [20,21]. The concentration of Cl ions increases with increase concentration of HCl in the Cu electrolyte, and a large amount of adsorbed Cl ions induces desorption of some of the formerly absorbed Cu [9], thus lowering the reductive potential. The CV characteristics for copper deposition on TaN, as observed in Fig. 1, are significantly different from those on Pt and Ru. The reductive potential of copper shifts negatively by 1 V for the case of deposition on TaN/Ta barriers compared to that on Pt (or Ru). This is because the deposition potential of metallic ions on a foreign substrate is usually higher than that on an electrode made of the same metal due to the crystalline misfit between the substrate and metal [2–4,15,22,23]. Therefore, deposition of copper on the TaN/Ta/Si samples requires more negative potentials compared with the redox potential of Cu/Cu2+. During upward scans in CV, the current becomes zero at approximately 250 mV, implying that no stripping peaks are observed. This finding indicates that the Cu film becomes electrically isolated from the TaN/Ta/Si substrate due to the formation of Ta oxide on the surface during the previous downward scan [15]. It is known that Ta oxide can easily cause isolation from the anodic reaction during upward scans [24]. To avoid this, a K4P2O7 solution was used to remove the Ta oxide during direct Cu electrodeposition [16,17]. The thin Ta oxide layer formed here also facilitates electrodeposition of Cu, although anodic stripping of Cu may be completely blocked in this stage.

J.S. Fang et al. / Electrochimica Acta 206 (2016) 45–51

5

-2

Current Density ( Acm )

pH=3.0 pH=3.5 pH=4.0 0

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-10 -1500

-1000

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0

500

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1500

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E (mV) vs Ag/AgCl Fig. 2. Cyclic voltammetry scans of Pb electrolyte at pH 3.0, 3.5, and 4.0 and at a scan rate of 20 mV/s.

CV scans of the Pb electrolyte at pH values of 3.0, 3.5, and 4.0 are shown in Fig. 2. The CV scans at pH 3.0 reveal that a sharp increase in the reductive current density starts at 1000 mV, after which a distinct reductive peak is observed at 1370 mV. No obvious Pb oxidation is observed during the upward scan from 1750 mV to +2000 mV, which is also attributed to the formation of Ta oxide [15]. In the CV scans of the Pb2+ electrolyte on the TaN/Ta/Si substrates at pH values of 3.5 and 4.0, the peak current density decreases slightly lowers with increasing pH values, and the reduction potentials are 1250 mV and 1450 mV for pH 3.5 and 4.0, respectively. 3.2. Influence of Pb deposition potential on the onset of Cu-SLRR A pH of 3.5 was chosen for both the Cu2+ electrolyte and Pb2+ electrolyte solutions in the following deposition processes, and the UPD of Pb was varied from 1100 mV to 1250 mV (lower than the Pb reductive potential of 1250 mV). Fig. 3 shows the potentialtime diagram for Cu-SLRR deposition in the first three cycles. It has been found that when Pb-UPD and Cu-SLRR are performed after an initial UPD-Cu, the resultant SLRR-Cu films have slightly lower oxygen content compared to the deposition of only Pb-UPD followed by the SLRR-Cu [25,26]. Thus, an initial UPD-Cu film was grown on the TaN/Ta/Si substrates for 60 s at 1100 mV (smaller

E (mV) vs Ag/AgCl

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-400 100

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Time (s) Fig. 3. The time-potential-current diagram for the first three cycles Cu-SLRR deposition at a Pb-UPD in the range from 1100 mV to 1250 mV. The figure inset compares the OCP for HCl and HClO4 solutions.

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than the Cu reductive potential of 1180 mV). This was followed by the blank solution, also at 1100 mV, to flush out the Cu2+ electrolyte. After this, the Pb2+ electrolyte was flushed into the cell and sustained at 1100, 1150, 1200, and 1250 mV, respectively, for 60 s to form a UPD-Pb adlayer. The blank solution was again used to flush out the Pb2+ electrolyte for 10 s at the reductive potential of Pb, and the auxiliary electrode was then disconnected to enable the cell to go to its OCP. Simultaneously, the Cu electrolyte was introduced for the Cu-SLRR. As shown in Fig. 3, over the next 60 s during the Cu-SLRR, the OCP in the stage shows a positive shift due to the difference in the redox potential from approximately 430 mV to 13 mV (Cu2+/Cu). In this stage, the Cu2 + ions migrate to the substrate surface and exchange with the UPDPb atoms forming a stable adlayer. The replaced Pb atoms are converted into Pb2+ ions and are thus dissolved into the solution. The potential stabilizes when the replacement is completed. As seen in Fig. 3, for increasing Pb-UPD, progressively longer times are required to achieve the onset of rapid increase in potential during Cu-SLRR. The Cu deposition is controlled by the kinetics of SLRR if the transport of the Cu2+ ions towards the electrode surface is sufficiently fast. However, this work used a dilute Cu2+ solution, thus leading to a prolonged SLRR because SLRR is also controlled by the surface reaction. The onset of SLRR is delayed at higher Pb potentials, especially for 1200 mV and 1250 mV, because these potentials are near the reductive potential of Pb. UPD implies that surface-limited deposition occurs at potentials that are lower than the formal bulk deposition potential (i.e., reductive potential). Thus, UPD-Pb at a less negative Pb-deposition results in less Pb coverage on the substrate. This reduced Pb coverage, in turn, leads to a lower Cu coverage during the subsequent SLRR because fewer Pb atoms on the substrate are available for replacement by Cu2+ ions. Moreover, for the four cases of Pb-UPDs studied, the OCPs progressively shift toward 13 mV, the equilibrium potential of Cu2+/Cu, implying that the exchange between Cu2+ and Pb2+ is completed over 60 s. The results shown here indicate that Pb-UPD alters the onset of SLRR. The slope of the rapid increase in the potential-time curve (Fig. 3) gives the exchange rate of Cu-SLRR. The Pb-UPDs studied have a similar exchange rate but different delays before the onset of Cu-SLRR. It is well known that additives affect the exchange rates of Cu-SLRR. For example, Cl and EDTA in a Cu electrolyte react with the Cu2+ ions [3,25,27]. In particular, the reaction rate decreases once the Cu complex is formed in the Cu2+ electrolyte. However, the delayed onset of the Cu-SLRR found in this study is attributed to the perchlorate anions (from Pb2+ electrolyte and/or blank solution) adsorbed on the underlying UPD-Pb because the exchange rates during the Cu-SLRR are similar. Further, the figure inset in Fig. 3 compares the OCP for the Cu-SLRR, using HCl as an additive in the Pb2+ solution and as the blank solution. The OCP changes rapidly when using HCl, suggesting that the delayed onset of Cu-SLRR can be attributed to the adsorption of perchlorate. The adsorption of perchlorate anions has also been observed on a Au substrate and was found to affect the formation of Ag atomic layers during deposition [28]. Co-adsorption of perchlorate anions is considered to be unlikely for Cu-UPD on a Pt substrate using HClO4 as a supporting electrolyte [28–30]. Conversely, adsorption of perchlorate anions can be observed for UPD-Pb on Ag/Si substrates [31], and the weakly adsorbing perchlorate anion on UPD-Pb can be desorbed before the Cu-SLRR process. The perchlorate anions adsorbed onto UPD-Pb adatoms could cause a depletion of the incoming Cu2+ at the interface before it exchanges with the UPD-Pb adatoms. The Cu2+-depleted layer may lead to delays in the CuSLRR reaction by desorption of the perchlorate anions on the UPDPb adatoms. Thus, the Cu-SLRR reaction starts after the time required for the perchlorate anions to be desorbed from the UPDPb. A surface with a higher UPD-Pb coverage, deposited at a more

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negative Pb-UPD, adsorbs more perchlorate anions. Therefore, the delay in onset of Cu-SLRR becomes longer for higher values of PbUPD.

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Indensity (a. u.)

Fig. 4 shows the XRD results of the as-deposited Cu films prepared at various Pb-UPDs. The XRD patterns exhibit peaks associated with Cu(111), Cu(200), and Cu(220) (JCPDS 70-3039) for Pb-UPD at 1100 mV, and the intensities of the Cu peaks gradually increase with increasing Pb-UPD at 1150 and 1200 mV. However, Cu2O (JCPDS 05-0667) is formed at a Pb-UPD of 1200 and 1250 mV because the accumulation of residual oxygen dissolved in the electrolyte during the Cu-SLRR is inevitable, even though the de-ionized water used in this study was deaerated by bubbling with high-purity nitrogen for 1 hr before deposition [25,26,32]. Moreover, at higher Pb-UPD, oxygen reduction can be dominated relative to the Cu coverage because Pb is a known catalyst for the oxygen reduction reaction [33]. Thus, Cu2O formation occurs at a higher Pb-UPD, as seen from XRD results shown in Fig. 4. When Cu-SLRR occurs on a film with high Pb-coverage, i.e., Pb deposited at a potential of 1200 or 1250 mV, there can be further accumulation of residual oxygen. This oxygen can react with the incoming Cu to form Cu2O. The films prepared at Pb-UPDs of 1100 mV and 1150 mV have a (111) texture, which is preferable for Cu films used in interconnections. No trace of the Pb-phase was observed, indicating that the process can be used for layer-by-layer growth of Cu for interconnections via SLRR with the risk of contamination. Fig. 5 shows sheet resistances of the as-deposited Cu films prepared at Pb-UPDs ranging from 1100 mV to 1250 mV, followed by replacement of the UPD-Pb using the Cu-SLRRs. Sheet resistance of the TaN/Ta/Si substrate is 150 V/&. The Cu film prepared at a Pb-UPD of 1100 mV has a sheet resistance of 12.6 V/&, confirming the occurrence of the Cu-SLRR. The Cu film deposited at a Pb-UPD of 1150 mV has the lowest sheet resistance of 7.5 V/&. For Pb-UPD at 1200 mV and 1250 mV, the sheet resistance of the Cu films increases slightly to 12.5 V/& and 15.2 V/&, respectively. The high sheet resistance is due to Cu2Oformation although a higher Pb-UPD would induce a higher Cu-coverage during SLRR. Higher apparent deviations are seen in the sheet resistance for Pb-UPD at 1200 mV and 1250 mV. Deviations in thickness can be observed for ECALD deposits formed using SLRR with the flow cell [26]. In this study, slightly higher

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Fig. 4. XRD results of the as-deposited Cu film prepared at various Pb-UPDs.

Sheet Resistance (ohm / sq.)

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3.3. Properties of SLRR-Cu films

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UPD Potential (mV) Fig. 5. Electrical properties of SLRR-Cu films prepared at various UPDs of Pb in the range from 1100 mV to 1250 mV.

deposition of UPD-Pb has been observed at the inlet due to convection effects, which become worse when Pb is deposited at a potential near its reductive potential, i.e., 1250 mV. Thus the coverage of UPD-Pb at the inlet is higher when the film is deposited at a higher potential, which in turn results in a higher Cu coverage at the inlet after SLRR. This process may induce a thickness variation, which is reflected in greater sheet resistance deviation for Pb-UPD at 1200 mV and 1250 mV, as shown in Fig. 5. This issue can be eliminated by a lower flow rate or by slowing down the SLRR by decreasing the concentration of the electrolyte or adding a complexing agent [25]. Additionally, Cu-SLRRs are delayed for Pb-UPD at 1200 mV and 1250 mV (shown in Fig. 3), resulting in faster reduction of a small amount of oxygen, both during Pb-UPD and during Cu-SLRR [27,34]. A longer time is required to exchange Pb for Cu when UPD-Pb is carried out at these potentials. More oxygen reduction occurs when a longer amount of time is required for the Cu2+ ions to obtain electrons from UPD-Pb [3]. Thus, the combination of the reduction of a small amount of oxygen and high Pb-coverage at the inlet leads to higher sheet resistances and deviations for Pb-UPD at 1200 mV and 1250 mV. AFM images of Cu films deposited for 50 cycles (Cu UPD, followed by Pb UPDs replaced by Cu) deposited at a Pb-UPD of 1100 mV, 1150 mV, 1200 mV, and 1250 mV are shown in Fig. 6. The Cu film deposited using Pb-UPD at 1100 mV has a rough morphology as shown in Fig. 6a. The low Pb coverage obtained at potentials less than the reductive potential may be the reason for the rougher surface of the SLRR-Cu films. The SLRR-Cu films formed using Pb-UPD of 1150 mV and 1200 mV have a uniform morphology, as shown in Fig. 6b and c. The uniform growth of layers-clusters is frequently observed on the films made using UPD-Pb followed by SLRR growth [3,35]. However, a rough morphology is observed when the Cu film is formed using a Pb-UPD of 1250 mV, as shown in Fig. 6d. Many 3D clusters are formed on the surface, indicating that Cu films deposited at the Pb-UPD produce the grains that are usually observed on films deposited at a more negative potential [36]. The morphology of the Cu films is consistent with the results showing that Cu films have a slightly higher sheet resistance when the films are formed at Pb-UPD of 1100 mV and 1250 mV. Fig. 7 shows that anodic stripping of the Cu films prepared at various Pb-UPDs, occurred in an electrolyte consisting of 1 mM sulfuric acid and results in Cu coverage, For example, integration of the stripping curve for the film prepared at a Pb-UPD of 1150 mV

J.S. Fang et al. / Electrochimica Acta 206 (2016) 45–51

Fig. 6. AFM images of SLRR-Cu films deposited at different Pb-UPDs of (a)

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Current Density ( Acm )

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E (mV) vs Ag/AgCl Fig. 7. Stripping curves of Cu films prepared at various UPDs of Pb in the range from 1100 mV to 1250 mV.

yields 149.5 mAV/cm2 where the current density has been previously obtained by dividing the electrode area of 1.5  1.5 cm2. By dividing this value by a scan rate of 5 mV/s, the electric charge for the removal of the monolayer can be determined

1100 mV, (b)

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1150 mV, (c)

1200 mV, and (d)

1250 mV.

(29901 mC/cm2). This value refers to one monolayer on the TaN/Ta/ Si substrate and indicates that two electrons are involved in the reduction of Cu(II) to Cu(0). The electric charge is divided by 2 (because two electrons are transferred when one Pb atom is replaced by one Cu atom) and is then normalized for the number of cycles performed. In addition, the energy required to form one Cu ML is 440 mC/cm2 [37,38]. Thus, the stripping charge gives 0.67 ML coverage of Cu for each cycle at a Pb-UPD of 1150 mV. Table 1 summarizes the electric charge and Cu-coverage at the various PbUPDs studied. The Cu-coverage increases from 0.61 ML to 0.80 ML with an increase in Pb-UPD, suggesting that a higher Pb coverage causes more deposition of Cu atoms. The obtained Cu coverage on the TaN/Ta/Si substrates used in this study is slightly higher than that of the film deposited on an Au(111) single crystal substrate because of the crystalline mismatch between the TaN/Ta/Si substrate and Cu [3,37,38]. The values of electrical resistivity and thickness for the as-deposited Cu films are also shown in Table 1, which shows that Cu-SLRR on Pb-UPD at 1150 mV has the lowest resistivity of 7.6 mVcm in the as-deposited state. Although the Pb-UPDs at 1200 mV and 1250 mV have a higher deposition rate, the formation of Cu2O increases their electrical resistivity. Additionally, the electrical resistivity of the film significantly increases when the film thickness decreases below a certain value. A survey of the literature on nanometer-thick Cu films shows that a 50-nm-thick Cu film prepared by electrodeposition has an electrical resistivity of 8.2–12.0 mVcm [39]. A 30-nm-thick Cu

Table 1 Electrical charge, Cu coverage, thickness, and electrical resistivity of the Cu film prepared at various Pb-UPDs. Pb potential (mV) 1100 1150 1200 1250

Charge (mC cm 27583 29901 33656 34786

2

Atomic layer (ML)

Thickness (nm)

Resistivity (m Vcm)

0.61 0.67 0.75 0.80

9.3 10.0 11.3 11.8

11.7 7.6 14.1 17.8

)

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the diffusion of Cu across TaN/Ta and its reaction with underlying Si, indeed causes the abrupt increase in DR/R at 800  C, as shown in Fig. 8. This demonstrates that the SLRR-Cu film is thermally stable up to 700  C.

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4. Conclusions

R/R (%)

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Annealing Temperature (°C) Fig. 8. Variations of DR/R with varying annealing temperature for Cu films prepared via UPD-Pb at 1150 mV and SLRR-Cu for 60 s.

Intensity (a. u.)

film has a low resistivity of 2.28 mVcm when fabricated via a controlled a two-step electrodeposition process [18]. The Cu film prepared in this study shows an electrical resistivity comparable to the expected electrical resistivity for an electrodeposited Cu film of thickness 10 nm. The film is suitable to be used as a seed layers for electrochemical deposition [40,41]. To study the thermal stability of the the Cu films prepared on the TaN/Ta/Si substrates via UPD-Pb at 1150 mV and SLRR-Cu for 60 s. Fig. 8 shows the variations with annealing temperature of DR/R (DR/R = ((Rf R)/R)  100%, where Rf is the sheet resistance of the film after annealing and R is the sheet resistance of the film before annealing). The DR/R of the film remains low at an annealing temperature between 300  C and 650  C. However, DR/R significantly increases when the films are annealed at 700  C and 800  C, indicating that the film has started to agglomerate and/or intermix with the underlying TaN/Ta/Si. XRD results for various samples are shown in Fig. 9. The diffraction patterns exhibit only Cu and Ta peaks for the as-deposited samples and for the samples annealed at temperatures below 700  C. There is a drop in the Cu peak intensity, as well as evidence of trace amounts of Cu3Si upon annealing at 800  C. The formation of Cu3Si phase, which occurs by

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2 theta (degree) Fig. 9. XRD results of the post-annealed Cu films prepared at Pb-UPD of

1150 mV.

This study confirms that layer-by-layer growth of a Cu film can be obtained on a TaN/Ta/Si substrate via repetitive Pb-UPD and Cu-SLRR using lead perchlorate and copper perchlorate, respectively. Reductive potentials of Pb and Cu are dependent on the pH of the electrolytes. The adsorption of perchlorate anions on UPD-Pb adatoms delays the onset of Cu-SLRR. Structural analysis indicates that the Cu film prepared via SLRR of the sacrificial Pb layer with Cu has a (111) texture. The electrical resistivity of the as-deposited Cu film is 7.6 mVcm when the film is prepared at a Pb-UPD of 1150 mV. Varying Pb deposition potentials yield a Cu coverage between 0.61 ML to 0.80 ML per cycle. The Cu layer is thermally stable to annealing up to 700  C annealing. The results reported in the present process should promote future applications of this method for fabrication of Cu interconnects in microelectronics. Acknowledgments The authors acknowledge the financial provided by the Ministry of Science and Technology, Taiwan under the grant 104-2221-E150-005-MY2 and the experimental support provided by the Common Laboratory for Micro/Nano Science and Technology of National Formosa University for this work. References [1] S.M. George, Atomic layer deposition: an overview, Chem. Rev. 110 (2010) 111– 131. [2] B.W. Gregory, J.L. Stickney, Electrochemical atomic layer epitaxy (ECALE), J. Electroanal. Chem. 300 (1991) 543–561. [3] C. Thambidurai, Y.G. Kim, N. Jayaraju, V. Venkatasamy, J.L. Stickney, Copper nanofilm formation by electrochemical ALD, J. Elecrochem. Soc. 156 (2009) D261–D268. [4] J.Y. Kim, Y.G. Kim, J.L. Stickney, Cu nanofilm formation by electrochemical layer deposition (ALD) in the presences of chloride ions, J. Electroanal. Chem. 621 (2008) 205–213. [5] J.J. Kelly, A.C. West, Leveling of 200 nm features by organic additives, Electrochem. Solid-State Lett. 2 (1999) 561–563. [6] J.J. Kelly, C. Tian, A.C. West, Leveling and microstructural effects of additives for copper electrodeposition, J. Electrochem. Soc. 146 (1999) 2540–2545. [7] A.C. West, Theory of filling of high-aspect ratio trenches and vias in presence of additives, J. Electrochem. Soc. 147 (2000) 227–232. [8] Y. Cao, P. Taephaisitphongse, R. Chalupa, A.C. West, Three-additive model of superfilling of copper, J. Electrochem. Soc. 148 (2001) C466–C472. [9] T.P. Moffat, J.E. Bonevich, W.H. Huber, A. Stanishevsky, D.R. Kelly, G.R. Stafford, D. Josell, Superconformal electrodeposition of copper in 500–90 nm features, J. Electrochem. Soc. 147 (2000) 4524–4535. [10] T.P. Moffat, D. Wheeler, W.H. Huber, D. Josell, Superconformal electrodeposition of copper, Electrochem. Solid-State Lett. 4 (2001) C26–C29. [11] C. Wheeler, T.P. Witt, Seedless superfill: copper electrodeposition in trenches with ruthenium barriers, Electrochem. Solid-State Lett. 6 (2003) C143–C145. [12] O. Chyan, T.N. Arunagiri, T. Ponnuswamy, Electrodeposition of copper thin film on ruthenium: A potential diffusion barrier for Cu interconnects, J. Electrochem. Soc. 150 (2003) C347–C350. [13] S. Kim, D.J. Duquette, Nucleation characteristics of directly electrodeposited copper on TiN, J. Electrochem. Soc. 153 (2006) C673–C676. [14] S.K. Kim, S.K. Cho, J.J. Kim, Y.S. Lee, Super conformal Cu electrodeposition on various substrates, Electrochemical and Solid-state Lett. 8 (2005) C19–C21. [15] A. Radisic, Y. Cao, P. Taephaisitphongse, A.C. West, P.C. Searson, Direct copper electrodeposition on TaN barrier layers, J. Electrochem. Soc. 150 (2003) C362– C367. [16] D. Starosvetsky, N. Sezin, Y. Ein-Eli, Seedless copper electroplating on Ta from a Single electrolytic bath, Electrochim. Acta 55 (2010) 1656–1663. [17] D. Starosvetsky, N. Sezin, Y. Ein-Eli, Seedless copper electroplating on Ta from an alkaline activated bath, Electrochim. Acta 82 (2012) 367–371. [18] S. Kim, D.J. Duquette, Morphology control of copper growth on TiN an TaN diffusion barriers in seedless copper electrodeposition, J. Electrochem. Soc. 154 (2007) D195–D200.

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