Reactive deposition of cobalt using bis(2,2,6,6-tetramethyl-3,5-heptanedionato) cobalt(II) from supercritical carbon dioxide

J. of Supercritical Fluids 107 (2016) 189–195

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Reactive deposition of cobalt using bis(2,2,6,6-tetramethyl-3,5-heptanedionato) cobalt(II) from supercritical carbon dioxide Masashi Haruki a,b,∗ , Shengkai Li a , Gang Qian a , James J. Watkins a,∗ a

Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003, USA Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University Japan, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan b

a r t i c l e

i n f o

Article history: Received 9 May 2015 Received in revised form 7 September 2015 Accepted 7 September 2015 Available online 11 September 2015 Keywords: Supercritical fluid deposition Supercritical carbon dioxide Bis(2,2,6,6-tetramethyl-3,5heptanedionato) cobalt(II) Cobalt thin film Copper oxidation

a b s t r a c t Cobalt depositions from supercritical carbon dioxide (scCO2 ) were conducted on various surfaces including the native oxide surface of silicon wafers, tantalum nitride (TaN), carbon, and copper in a cold-wall reactor using bis(2,2,6,6-tetramethyl-3,5-heptanedionato) cobalt(II) as the precursor. Deposition onto TaN barrier layers at temperature above 300 ◦ C yielded high purity cobalt films as determined by X-ray photoelectron spectroscopy with grain sizes of 200 nm or less. The volume resistivities, of about 200 nm thick films estimated from the surface resistivities, were about 2.5 times higher than that of the literature value of pure cobalt. Cobalt films could also be deposited on both carbon and copper surfaces with morphologies that varied depending on the amount of precursor loaded. Moreover, the cobalt film protected copper surface from oxidation in solution and also improved its resistance to oxidation in air as demonstrated respectively by cyclic voltammetry and X-ray photoelectron spectroscopy depth profiles. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Cobalt is an important material for electronic devices and functional catalysts. It has been used in giant magnetoresistance devices, and memory and data storage elements [1–3]. Moreover, the potential of cobalt to improve the performance of copper interconnect structures in integrated circuits has received considerable attention. Cobalt capping layers protect copper against electromigration [4,5], and improve the adhesion of copper to dielectric materials [6]. Cobalt also acts as barrier layer against copper diffusion [7] and oxidation, which is especially effective when cobalt layers are doped with small amounts of tungsten, phosphorous, or silicon [8,9]. As a catalyst, cobalt and cobalt oxide are reported both

∗ Corresponding authors. E-mail addresses: [email protected] (M. Haruki), [email protected] (J.J. Watkins). http://dx.doi.org/10.1016/j.supflu.2015.09.004 0896-8446/© 2015 Elsevier B.V. All rights reserved.

as an electrode catalyst in fuel cells [10–12] and as a catalyst of the Fischer–Tropsch process [13,14]. There are several methods typically used for deposition of metal thin films: electrodeposition [15–17], physical vapor deposition such as sputtering [18–20], chemical vapor deposition [21–23] and atomic layer deposition [24–26]. Compared to these methods, supercritical fluid deposition (SCFD) using carbon dioxide (scCO2 ) is advantageous for deposition in confined geometries, due to high diffusivity, low viscosity, solubility based transport of precursors, and miscibility with gaseous of reduction and oxidation agents [27–30]. Thus, SCFD has attracted much attention since the early 2000s. In such processes, metal complexes of metallocene or ˇdiketonate types are often used as precursors, and hydrogen (H2 ) is often used as the reducing agent. Up to now, the only reports of studies on cobalt deposition using SCFD method were reported by our group [8,31]. Using cobaltocene as a precursor, we showed that pure cobalt films could be deposited at temperatures between 285 and 320 ◦ C onto native oxide surface of silicon wafers (nSi), tantalum nitride or titanium nitride barrier layers on silicon wafers (TaN/Si and TiN/Si). The cobalt thin films were examined by X-ray diffractometry,

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X-ray photoelectron spectrometry and measurement of electrical resistivity. Furthermore, deposition using tris(2,2,6,6-tetramethyl3,5-heptanedionato) cobalt(III) (Co(tmhd)3 ) was also studied, and it was possible to deposit high purity cobalt films onto copper coated TaN layer at between 200 and 300 ◦ C. However, no significant deposition was observed even at 320 ◦ C on nSi and the barrier layers. To date only a limited number of precursors that could be used for the SCFD processing have been studied, and further fundamental investigation is needed to make the SCFD of cobalt more accessible. The present work extends the scope of precursors used for SCFD to include bis(2,2,6,6-tetramethyl-3,5-heptanedionato) cobalt(II) (Co(tmhd)2 ). The solubility of Co(tmhd)2 in scCO2 is comparable with Co(tmhd)3 , and is slightly lower than that of cobaltocene, according to previous reports (Fig. S1 in Supplementary material) [32–34]. This makes it a promising candidate as precursor for cobalt SCFD. H2 was used as the reducing agent, and substrates used included nSi, TaN barrier layers on surface oxidized silicon wafers (TaN/SiO2 /Si), carbon layers supported on silicon wafers (C/Si), copper plates (Cu plate), and copper films coated on surface oxidized silicon wafers with titanium nitride barrier layers (Cu/TiN/SiO2 /Si). We also show that cobalt films deposited by SCFD were effective barrier layers against oxidation of copper in both water and air. 2. Experiment 2.1. Materials and preparations of substrates Bis(2,2,6,6-tetramethyl-3,5-heptanedionato) cobalt(II) (purity of 99.9% metals basis, formula weight of 425.46, melting temperature of 143 ◦ C) was purchased from Alfa Aesar of Johnson Matthey Inc., and used as received. Bone dry grade CO2 (CD BD 300) and ultra high purity grade H2 (HY UHP 300) were purchased from Airgas, Inc. 1 in. × 0.5 in. of nSi, TaN/SiO2 /Si or Cu plates were used for deposition. Cu/TiN/SiO2 /Si substrates were prepared from sputter coating of copper on TiN/SiO2 /Si substrates and cut into 1 in. × 1 in. pieces. The copper layer was pure with a thickness of about 70 nm, and TiN barrier layer was 5 nm thick. As for C/Si, a thin porous carbon layer was formed on a 0.7-in square of nSi using a method reported by Meng et al. [35], namely, calcination of the polymer mixture of resol and Pluronic® F127 (ethylene oxide/propylene oxide/ethylene oxide triblock copolymer, BASF) in nitrogen (N2 ) atmosphere. The C/Si samples used as substrate were not characterized, but analysis of samples prepared by the same method were characterized using transmission electron microscopy (TEM) (Fig. S2 of Supplementary material), shows an ordered porous structure with d-spacing in good agreement with the literature. Before deposition, the substrates of nSi and TaN/SiO2 /Si were cleaned by ultrasonication in a concentrated sulfuric acid bath for about 15 min. Cu plates and Cu/TiN/SiO2 /Si were cleaned using ultrasonication in acetic acid, acetone and 2-propanol baths successively for about 5 min each. C/Si substrate was used without pre-cleaning. 2.2. Apparatus and procedure An apparatus similar to that described previously [31] was used (Fig. 1). It was a cold-wall reactor with inlets for different gases and an outlet for depressurization and removal of by-products. The cold-wall reactor had an inner volume of 153 mL. The wall was heated by four cartridge heaters and the pedestal was resistively heated using an embedded heating coil. The heaters were controlled by separate temperature controllers (OMEGA Engineering, Inc., CN76000). As the pedestal temperature was much higher than that of the wall, the reaction proceeded selectively on the heated substrate.

Fig. 1. Schematic diagram of the cold-wall deposition apparatus. (1) H2 cylinder, (2) high pressure cell for H2 injection, (3) pressure gauge for H2 cell, (4) syringe pump, (5) cold-wall reactor, (6) pedestal heater, (7) ceramic heat insulator, (8) pressure gauge for reactor, (9) thermocouple, (10) cartridge heaters, (11) temperature controller for pedestal heater, (12) temperature controller for cartridge heaters, (13) micrometering valve, (14) activated carbon bed, (15) bubbler, (S1–S7) stop valves.

In a typical experiment, a substrate was first placed onto the pedestal. TaN/SiO2 /Si and nSi substrates were placed side by side in a same deposition experiment, Cu plates were used two pieces side by side, and other substrates were used one piece at a time. The rest of the surface of pedestal was covered by a glass insulator to reduce cobalt deposition on pedestal. A known amount of cobalt precursor, Co(tmhd)2 , was put in a small polyimide container and placed onto the glass insulator. The reactor was then sealed, and air and moisture were flushed with N2 at room temperature for 30 min and at 90 ◦ C for 60 min. CO2 was then introduced into the reactor up to about 13 MPa using a high-pressure syringe pump (Isco, Inc., model 500HP), and the pedestal heater was turned on and set to 90 ◦ C. The time for dissolution of Co(tmhd)2 in scCO2 was about 180 min in all depositions. Just prior to deposition, H2 was added to the reactor from a 70 mL high-pressure vessel. The amount of H2 added was estimated from the pressure drop in the vessel using ideal gas model. In most cases, the molar ratios of H2 to Co(tmhd)2 introduced to the reactor were larger than 100. The pedestal heater was then set to the deposition temperature. The temperature of the pedestal heater rapidly increased, initiating the deposition. The deposition periods were 60 min, and the pressure and the temperature of CO2 bulk phase in the reactor increased gradually during the deposition. The final pressure and bulk scCO2 temperature inside the reactor were dependent on the deposition temperature (pedestal temperature), and were approximately 21 MPa and 190 ◦ C. After deposition was finished, the pedestal heater was turned off, and the reactor was then purged with pure scCO2 flow using the high-pressure syringe pump to remove unreacted reagents and by-products. In all cases, a small amount of precursor residue was found in the polyimide container after the deposition. Reducing the initial precursor loading did not eliminate the small amount of residue, ruling out the possibility that the precursor amount exceeded the dissolution limit in the process. In fact, the solubility limit was estimated to be 1.0 g/L at 90 ◦ C and 13 MPa (about 0.15 g for the 153-mL reactor), based on the Chrastil equation and reported experimental data [33], as shown in Fig. S3 of Supplementary material. This solubility limit was higher than the initial precursor loading in most cases. A possible reason of the residue was that the Co(tmhd)2 reagent contained impurities insoluble in scCO2 . The morphology and composition of deposited films were characterized by scanning electron microscope (SEM, JEOL, JSM 6320F), X-ray diffractometer (XRD, PANalytical, X’Pert PW3040: Cu K␣ radiation), and X-ray photoelectron spectrometer (XPS, Physical Electronics, Quantum 2000 Scanning ESCA).

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Fig. 2. SEM images and the results of XPS analysis for Co films on TaN/SiO2 /Si. (a)–(d) SEM images of the film surface deposited at the temperatures of (a) 320 ◦ C, (b) 310 ◦ C, (c) 300 ◦ C and (d) 280 ◦ C, respectively. (e) XPS survey of the film deposited at 310 ◦ C after 10 min sputtering with Ar+ . (f) Depth profiles of relative atomic concentrations of the film deposited at 310 ◦ C.

3. Results and discussion 3.1. Deposition on native oxide surface of Si and on TaN barrier layer Cobalt was deposited on nSi surfaces from 300 to 320 ◦ C, and on TaN/SiO2 /Si surfaces from 280 to 320 ◦ C. About 0.1 g (2.4 × 10−4 mol) of Co(tmhd)2 was loaded in the reactor in all cases. The loading corresponds to approximately 0.65 g/L in CO2 , assuming precursor dissolution was complete. The deposited cobalt did not form homogeneous films on the nSi surface regardless of deposition temperature. Large discontinuous grains were formed on the surface, and the deposited cobalt showed poor adhesion to the substrate and could easily be removed by wiping (shown in Fig. S4 in Supplementary material). Likewise, deposition of granular films was also observed on TaN/SiO2 /Si at a deposition temperature

of 280 ◦ C. However, at deposition temperatures ranging from 300 to 320 ◦ C, continuous cobalt films were deposited on TaN/SiO2 /Si with grain size of 200 nm or less, as shown in Fig. 2. Deposition attempts using Co(tmhd)3 in previous work failed to give significant deposition [31], indicated that the reduction of Co(tmhd)2 by H2 is more facile than that of Co(tmhd)3 . A thorough explanation to this difference in reactivity would require further studies of reaction mechanisms. XPS analysis was performed on the film deposited on TaN/SiO2 /Si at 310 ◦ C, and Fig. 2 shows the depth profile of the relative atomic concentrations during 10 min Ar+ sputtering, as well as the survey spectrum after sputtering. The only significant peaks apparent after 10-min sputtering are those of cobalt. The depth profile shows the purity of the film was high except for the film surface, where high concentrations of carbon and oxygen were found due to surface contamination as expected.

Fig. 3. XRD patterns of Co films. Films deposited at (a) 320 ◦ C and (b) 300 ◦ C, on surfaces of TaN/SiO2 /Si (upper curves) and nSi (lower curves). The peaks attributed to the crystalline structures of cobalt are: (1) hcp Co(1 0 0), (2) fcc Co(1 1 1), (3) hcp Co(0 0 2), (4) hcp Co(1 0 1), (5) fcc Co(2 0 0), (6) fcc Co(2 2 0), (7) hcp Co(1 1 0).

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Fig. 4. SEM images and the results of XPS analysis for Co films deposited on C/Si at 300 ◦ C. (a)–(c) SEM images of the film surfaces deposited using Co(tmhd)2 loadings of (a) 0.1 g, (b) 0.05 g and (c) 0.025 g, respectively. (d)–(f) Depth profiles of the relative atomic concentrations obtained by the XPS for the films deposited using Co(tmhd)2 loadings of (d) 0.1 g, (e) 0.05 g and (f) 0.025 g, respectively.

The crystalline structures of films deposited onto nSi and TaN/SiO2 /Si were analyzed by XRD, and the results at deposition temperatures of 320 and 300 ◦ C are shown in Fig. 3. The XRD peak patterns of the cobalt films showed the coexistence of the hexagonal-closed packed (hcp) with the face-centered cubic (fcc) structures in all cases, which is similar to results reported previously, using cobaltocene as the precursor [31]. It has also been reported that although the hcp structure is stable up to 420 ◦ C, the fcc structure also appears at ambient temperature because of size effect of the cobalt particles [36,37].

Volume resistivities of cobalt films deposited on TaN/SiO2 /Si at 300 and 320 ◦ C were also investigated at room temperature. They were estimated from surface resistivities measured by four-point probe method (Resistivity test unit, JANDEL Engineering, Ltd.), and film thicknesses measured by profilometer (Dektak 150, Veeco Instruments Inc.). The film thicknesses measured were 200 and 250 nm for deposition at 300 and 320 ◦ C, respectively. Surface and volume resistivities were estimated using the formulas below: Rs =

4.532 × V , I

Rv = Rs × t

(1)

Fig. 5. SEM images and the results of XPS analysis for Co films deposited on Cu surfaces at 300 ◦ C. (a) and (b) SEM images of the film surfaces deposited on Cu plate using 0.2 and 0.1 g loadings of Co(tmhd)2 , respectively. (c) SEM image of the film surface deposited on Cu/TiN/SiO2 /Si using 0.2 g Co(tmhd)2 . (d) and (e) Depth profiles obtained by the XPS for the films deposited on Cu plates using 0.2 and 0.1 g loadings of Co(tmhd)2 , respectively. (f) Depth profile of the XPS for the film deposited on Cu/TiN/SiO2 /Si using 0.2 g Co(tmhd)2 .

M. Haruki et al. / J. of Supercritical Fluids 107 (2016) 189–195

-0.004 (a)

I [A]

-0.002 0 0.002 0.004 -0.004 (b)

-0.002 0

I [A]

where Rs and Rv represent the surface and volume resistivities, respectively. V represents the potential difference between detection probes at arbitrary current, I, and t is the film thickness. The measurements were carried out at several positions on each sample, with currents up to 7 mA. Linear fits were applied to the V–I curves of each point, and the slopes of which were taken average to be the V/I value for each sample. The volume resistivities of the films deposited at 300 and 320 ◦ C were (1.5 ± 0.1) × 10−7 and (1.7 ± 0.2) × 10−7  m, respectively, which were about 2.5 times higher than that of the literature value of pure cobalt, 6.4 × 10−8  m at 20 ◦ C [38]. This difference can be explained by the deliterious influence of surface contamination, and the values of cobalt films here are comparable to those reported using other deposition methods [21,22].

193

0.002 0.004

-0.004 (c)

Cobalt depositions on C/Si were carried out at 300 ◦ C. The amount of Co(tmhd)2 loaded in the cell was varied from 0.1 to 0.025 g (2.4 × 10−4 to 5.9 × 10−5 mol). The SEM images and the depth profiles of the relative atomic concentrations obtained by XPS are shown in Fig. 4. At 0.1 g-Co(tmhd)2 loading, a pure cobalt thick film on the carbon layer was observed, and the grain size of the film was 300 nm or less. At 0.05 and 0.025 g-Co(tmhd)2 loadings, the maximum cobalt concentrations in the depth profiles were clearly lower, and the signals of carbon concentration attributed to the carbon layer could be found at shallow depth near the surface. Therefore, cobalt layers were considered to have incomplete coverages on the carbon layer. Although sputtering time cannot be directly converted to the depth from the surface because sputtering rate varied depending on material species, it can still be inferred from the difference in the slopes of cobalt concentration decrease in the depth profiles that the cobalt was loaded at deeper position of the carbon layer for the 0.025 gCo(tmhd)2 loading compared to that of the 0.05 g-Co(tmhd)2 loading.

3.3. Deposition on copper surfaces Cobalt depositions on the surface of Cu plates were carried out using 0.2 or 0.1 g of Co(tmhd)2 at 300 ◦ C. The SEM images of the surface and the relative atomic concentration profiles by the XPS are shown in Fig. 5. The SEM images show that the size of grains was bigger in the case of 0.2 g-Co(tmhd)2 loading than that of 0.1 gCo(tmhd)2 loading. XPS depth profiles of the cobalt film deposited using 0.2 g-Co(tmhd)2 showed a layer of high purity cobalt at sputtering times of up to 55 min, where the signal of copper appeared. On the other hand, the cobalt film deposited using 0.1 g-Co(tmhd)2 had noticeable copper signal as early as after 4 min of sputtering. This indicates that a thicker film with much better coverage was deposited in the case of 0.2 g Co(tmhd)2 initial loading compared to 0.1 g Co(tmhd)2 initial loading. The XPS depth profiles also show very low levels of carbon and oxygen contamination in both cases. Furthermore, XRD patterns (Fig. S5 of Supplementary material) show the layers of cobalt and copper to be pure without any alloy or solid solution formation. Cobalt deposition with 0.2 g-Co(tmhd)2 loading was also performed onto Cu/TiN/SiO2 /Si substrate for the study of oxidation resistance in air. The SEM image of the cobalt layer surface and the XPS-depth profile are also shown in Fig. 5. The films are comprised of cobalt grains of 100 nm or larger, similar to those on Cu plate with the same precursor loading, and XPS data indicate that high purity cobalt layer was deposited on the copper layer.

-0.002

I [A]

3.2. Deposition on carbon layer

0

1

-1.5

2

1

0.002 0.004

2

1

-1

2

1 2

-0.5 0 E [V] vs. Ag/AgCl

0.5

Fig. 6. CV curves for Cu and Co electrodes. (a) Bare Cu plate, (b) cobalt on TaN/SiO2 /Si, and (c) cobalt on Cu plate, measured at 100 mV/s in 0.1 M KOH. Dash lines: 1 = redox peaks of Cu, 2 = redox peaks of Co.

3.4. Antioxidation tests of copper coated with cobalt Electrochemical properties of the cobalt film deposited on the Cu plate using 0.1 g-Co(tmhd)2 loadings was investigated by cyclic voltammetry (CV) analysis (CH Instruments Inc., CHI600D) at room temperature. The coated Cu plate was used as the working electrode, and a platinum wire was used as the counter electrode. All CV analyses were carried out in 0.1 M KOH solution at a sweep rate of 100 mV/s, and an Ag/AgCl-saturated KCl electrode was used as reference. The CV curve of the Cu plate with deposited cobalt is illustrated in Fig. 6, along with those of Cu plate and cobalt on TaN/SiO2 /Si. In all measurements, the potential sweeps were started from 0 V, raised to a positive maximum of 0.6 or 0.7 V, and then back to a negative maximum of −1.3 V. The measurements were carried out 5 cycles continuously. According to the literature [39,40], the peaks for the redox reactions of the Cu plates obtained in this potential window are assigned as follows: two reduction peaks, from CuO to Cu2 O at around −0.6 V and from Cu2 O to Cu at around −0.95 V, and two oxidation peaks, from Cu to Cu2 O at around −0.25 V and from Cu2 O to CuO at around 0 V. These peaks remained nearly unchanged after 5 cycles. After coating with cobalt, the peaks of copper redox reactions drastically reduced. Instead, the peaks assigned to cobalt redox reactions could be observed, and they were almost unchanged during 5 cycles. Small peaks of copper redox reactions remained probably due to the not-fullycovered edges of the copper plate. Thus the surface of the copper plate was effectively protected by the deposited cobalt film against electrochemical oxidation. Oxidation tests in air were performed on samples of cobalt deposited on Cu/TiN/SiO2 /Si substrates, along with a control sample of bare copper plate. Samples were heated in a tube furnace for 1 h with air flow, and XPS-depth profiles obtained are shown in Fig. 7. The bare copper plate was oxidized at 280 ◦ C, as indicated by the presence of oxygen peaks at around 530 eV, typical of copper oxides [41,42]. Cobalt coated samples, on the other hand, were effectively protected from Cu oxidation up to 405 ◦ C (two more samples tested at 355 and 380 ◦ C are included in Fig. S6 of Supplementary material). The oxygen peaks apparent at

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100

(a)

90 80

Relative atomic concentration [%]

Relative atomic concentration [%]

100

Cu_2p

70 60 50 40 30

O_1s

20 10 0

C_1s 0

20

40

60

80

100

Co_2p

70 60 50 40

O_1s

30 20

N_1s Si_2p

10 0

20

100

60

80

100

120

4500

(c)

90

(d) 4000

Co_2p

80 70 60

Cu_2p

50 40

O_1s

30 20

Si_2p

Intensity [CPS]

Relative atomic concentration [%]

40

Sputtering time [min]

Sputtering time [min]

3500 3000

Co/Cu/TiN/SiO2/Si Cu plate

2500 2000

N_1s

10 0

Cu_2p

80

0

120

(b)

90

0

20

40

60

80

100

120

Sputtering time [min]

1500 545

540

535

530

525

Binding Energy [eV]

Fig. 7. XPS-depth profiles and oxygen spectra for the control Cu plate and Co/Cu/TiN/SiO2 /Si samples after oxidation in air for 1 h. (a) Cu plate after oxidation at 280 ◦ C. (b) and (c) Co/Cu/TiN/SiO2 /Si samples after oxidation at 405 ◦ C and 430 ◦ C respectively. (d) O 1s spectra after 100.5 min of Ar+ sputtering, of Cu plates after oxidation at 280 ◦ C (sample in (a)), and Co/Cu/TiN/SiO2 /Si after oxidation at 405 ◦ C (sample in (b)).

approximately 532 eV after 100.5 min of Ar+ sputtering peaks are assigned to SiO2 rather than copper oxides [43,44]. At higher temperature of 430 ◦ C, copper diffused across the cobalt layer to the surface and was oxidized. This breakdown temperature was higher than what has previously been reported (around 350 ◦ C) [9]. This is, however, probably because of the thicker cobalt coating on our samples.

4. Conclusion In the present work, the depositions of cobalt films from Co(tmhd)2 in scCO2 were carried out on several surfaces. High purity, cobalt films could be deposited on the surfaces of TaN/SiO2 /Si, C/Si, Cu plates and Cu/TiN/SiO2 /Si at deposition temperatures 300 ◦ C or higher. On the other hand, continuous cobalt films were not formed on nSi at temperatures up to 320 ◦ C. The volume resistivities of the cobalt films estimated from the surface resistivities were about 2.5 times higher than that of pure cobalt, which is a reasonable value for thin films. Moreover, the cobalt layer could effectively protect the Cu surface from both electrochemical oxidation and oxidation in air at 405 ◦ C.

Acknowledgments We gratefully acknowledge sponsor from the National Science Foundation through the Center for Hierarchical Manufacturing, CMMI-1025020, and IIP-1346442.

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