Controlled electropolishing of copper foils at elevated temperature

Applied Surface Science 307 (2014) 731–735

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Controlled electropolishing of copper foils at elevated temperature Gi Duk Kwon a , Young Woo Kim a , Eric Moyen a , Dong Hoon Keum a , Young Hee Lee a,c,d , Seunghyun Baik a,b,c , Didier Pribat a,∗ a

Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea School of Mechanical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea IBS Center for Integrated Nanostructure Physics (CINAP), Institute of Basic Science (IBS), Daejeon, Republic of Korea d Department of Physics, Sungkyunkwan Advanced Institute of Nano-technology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea b c

a r t i c l e

i n f o

Article history: Received 2 March 2014 Received in revised form 17 April 2014 Accepted 18 April 2014 Available online 9 May 2014 Keywords: Electropolishing Elevated temperature Rolled copper foils rms roughness Surface reflectance

a b s t r a c t We have studied the electrochemical polishing of copper foils at elevated temperature, in H3 PO4 electrolytes of various concentrations. Atomic force microscopy, surface reflectance measurements as well as optical microscopy and scanning electron microscopy (including electron backscattering diffraction) have been used throughout this study to characterize the surface of the electropolished foils. We have found that copper foils electropolished at 65 ◦ C in 2.17 M H3 PO4 , exhibited a lower surface roughness and a higher percent specular reflection, comparing with values obtained after classical electropolishing in concentrated H3 PO4 at room temperature or comparing with values obtained after chemical-mechanical polishing. This work could open up new prospects for the preparation of copper foils before the growth of high quality graphene layers. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Electrochemical polishing, or electropolishing, is a widely used process in industry, applied to a large number of metals and alloys for deburring as well as surface finishing and removal of tarnishing. Electropolishing, which is performed by controlled anodic dissolution of the metal surface in an appropriate electrolyte, can yield a significant decrease in surface roughness, usually characterized by an increase in the gloss of the metal [1]. The electropolishing (EP) of copper, which has been studied for more than 80 years [2,3], has been recently considered for the replacement/improvement of the chemical-mechanical planarization (CMP) of Cu interconnects in ultra-large-scale-integrated (ULSI) circuits [4–6]. More recently, EP of Cu has been used for the surface preparation of rolled foils on which graphene layers of good crystalline quality can be grown by chemical vapor deposition [7–11]. As with any epitaxial or thin film deposition process, some kind of surface preparation must be performed before graphene growth on Cu foils, in order to remove the rolling marks or other machining defects which are usually preferred and uncontrollable nucleation sites [12]. Although mechanical or chemical-mechanical polishing can be employed [12], electro-polishing is a much

∗ Corresponding author. Tel.: +82 31 299 6271. E-mail address: [email protected] (D. Pribat). http://dx.doi.org/10.1016/j.apsusc.2014.04.144 0169-4332/© 2014 Elsevier B.V. All rights reserved.

simpler process which can be performed on large surface areas, with elementary foil handling and manipulation operations. In particular, when using EP, there is no need for a complex polishing machine, highly incompatible with a roll-to-roll graphene synthesis process [13]. Also, there is no need for several abrasives, clothes and colloidal suspensions as with chemical-mechanical polishing and there is no risk of surface contamination by residues from the polishing slurry [14]. In most studies so far [2–21], even if various additives have been employed, Cu EP has been performed in electrolytes based on highly concentrated H3 PO4 and at room temperature. Moreover, in order to avoid preferential crystallographic etching of the Cu surface (leading to an increase in roughness), all authors have been working in the kinetic regime of mass transfer limitation, corresponding to a current plateau in the anodic polarization curves [22]. In this kinetic regime, two mechanisms have essentially been proposed to explain the current limitation, namely (i) the formation of a salt film on the Cu surface or (ii) the formation of a viscous boundary layer in which metal ions must be complexed by water molecules or other electrolyte species before they can diffuse away from the Cu surface (the so-called acceptor mechanism) [16,22]. The formation of a surface oxide has also been invoked in the literature [21,23]. When the anode voltage is increased, a concurrent oxygen evolution (corresponding to water decomposition) drives the current above the plateau value, even though mass transfer still limits the Cu etching reaction. In this regime of higher anode

G.D. Kwon et al. / Applied Surface Science 307 (2014) 731–735

2.1. Cu foil preparation and electropolishing conditions

2.2. Surface analysis Surface roughness measurements (rms) were performed with an atomic force microscope AFM SPA400, from Seiko Instruments. The surface reflectance was measured with a UV-VIS-NIR spectrophotometer (UV3600, Shimadzu). An ESCA 2000 instrument from VG MICROTECH was used for X-ray photoelectron spectroscopy analysis. Various electron microscopy observations were made inside a JEOL JSM-7600F field emission gun scanning electron microscope (SEM). Electron backscattering diffraction (EBSD) images and spectra were recorded in a JEOL JSM 7000-F SEM equipped with an EDAX-TSL EBSD detector. 2.3. Chemical-mechanical polishing (CMP) The CMP control experiments were performed using a MetPrep 3TM/PH-3 polisher (ALLIED HIGH TECH, USA). Starting with raw Cu foils, the typical polishing and CMP conditions are summarized in Table 1 below. 3. Results and discussion In the present work, we have explored the EP of Cu foils in H3 PO4 electrolytes of different concentrations and at various temperatures. The EP experiments were performed on high purity (99.99%) NILACO [24] and WACOPA [25] Cu foils which were originally 100 ␮m thick; the results are essentially the same for both types of foils. We have used a constant current mode (see Section 2), and when the H3 PO4 concentration is low, most of the

(b)

Roughness value (nm) Average specular reflection (%)

100 90

210

80

180

70

150

60

25°C

50

120

40

90

30

60

20

30

10 0

0

100

240

90

210

80

180

70

150

60

55°C

50

120

40

90

30

60

20

30

10 0

240

0

2

4

6

8

10

12

14

RMS value (nm)

Before any operation, the 100 ␮m-thick copper foils used in this work were thoroughly degreased by dipping in acetone, followed by rising with isopropylic alcohol and ethanol. They were then annealed at 1050 ◦ C in forming gas (10% H2 in Ar) for 30 min. After this annealing step, we have observed a large increase in grain size (up to several millimeters). A large grain size is usually preferred for graphene growth. We note however that this thermal treatment has no incidence on the EP process, since we work in the masscontrolled kinetic regime, which is not dependent on the surface structure/orientation of the Cu foils. The electrochemical cell was a simple 2 electrodes arrangement, the working electrode being the electropolished foil and a large Cu plate being used as counter electrode. The electrolytes were: 0.63; 2.17; 3.72; 4.85; 7.59 and 14 Molar (M) H3 PO4 (Sigma Aldrich). Ethylene glycol (Sigma Aldrich) was added in all electrolytes [20], typically to a concentration of 0.6 g/ml. Anodic polarization curves were recorded using a BioLogic potentiostat/galvanostat. All EP experiments were performed in the constant current mode, using an Agilent E3645A DC power supply. After EP, the copper samples were thoroughly rinsed with DI water and ethanol. Unless otherwise stated, those samples were then immediately transferred in a vacuum desiccator to protect them against further oxidation in air.

Average specular reflection (%)

2. Experimental

(a)

RMS value (nm)

voltage, pitting of the Cu surface also starts to occur and this has been explained by the local breakdown of the salt film or viscous layer [22]. Fig. S1 of the Supplementary Data, which shows polarization curves of Cu foil anodes in 7.59 M and 14 M H3 PO4 recorded in our laboratory at room temperature, summarizes the 3 regions of interest briefly described above.

Average specular reflection (%)

732

0

H3PO4 Concentration (M) Fig. 1. Average specular reflection (600–800 nm wavelength range) and surface roughness (rms values over a 15 ␮m × 15 ␮m surface area) of electropolished NILACO Cu foils as a function of H3 PO4 concentration. (a) Electropolishing experiments performed at 25 ◦ C. (b) Electropolishing experiments performed at 55 ◦ C. Note the low roughness and corresponding high specular reflection values for the ∼2 M H3 P04 concentration at 55 ◦ C.

potential drop between the working electrode (Cu foil) and counter electrode occurs due to the high resistance of the electrolyte. So, even when the voltage is high, the local electric field at the anode surface is sufficiently low to prevent pitting. The surface quality of the Cu foils after electro-polishing has been qualitatively assessed by optical microscopy first. Quantitative surface properties were then obtained, using atomic force microscopy (AFM) and reflectance measurements. By varying the electrolyte temperature, we find that, for low concentration H3 PO4 , there is a small temperature window around 65 ◦ C where the Cu surface roughness can be significantly decreased. This occurs concomitantly with a sharp increase in surface specular reflection. Fig. 1 shows the surface roughness variations (rms values measured on 15 ␮m × 15 ␮m surface areas) as well as the percent average surface specular reflection as a function of the H3 PO4 concentration for EP experiments performed on NILACO Cu foils at 2 distinct temperatures, namely 25 ◦ C and 55 ◦ C (see Fig. S2 for some qualitative optical characterizations). We first measured the diffuse reflection values of the samples using an integrating sphere (see Fig. S3) and then deduced the specular reflection having in mind that the total reflection at each wavelength is 100%. Also, since the reflectance of Cu decreases for wavelengths below 600 nm (∼2 eV), due to the onset of interband transitions [26], the surface specular reflection values were averaged over the 600–800 nm range, where there is no absorption. For all the above experiments, the EP time was kept constant at 5 min and the EP current density was held at 380 mA/cm2 (i.e., 190 mA/cm2 per face of the Cu foil, assuming a uniform distribution of the current), so that the total amount of charge passed through the electropolished samples was 114 C/cm2 . A quick calculation indicates that this charge corresponds to an etched thickness of ∼42 ␮m/cm2 ; cross section measurements made under an optical microscope show that the

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Table 1 CMP procedure for control copper substrates. Step

1

2

3

4

5

Abrasive Type Coolant Platen Speed (RPM) Sample Speed (RPM) Force(N) Time (min)

600Grit SiC Water 300 150 4 5

1200Grit SiC Water 300 150 4 5

3 ␮m Da Purple Lube 150c 150 5 5

1 ␮m Da Purple Lube 150c 150 5 10

0.04 ␮m Sb Water 150 150 4 60

a b c

Diamond. SiO2 colloidal suspension. Counter rotation.

Roughness value (nm) Average specular reflection (%)

100 90

Also, controlled chemical-mechanical polishing (CMP) experiments performed on WACOPA Cu foils (see Section 2 for the CMP conditions) yielded a rms surface roughness of ∼14 nm (still on a 15 ␮m × 15 ␮m area) and an average specular reflection value of ∼90% over the 600–800 nm wavelength range (see Fig. S5 of for optical and AFM characterizations of a CMP sample). Taken together, those values indicate a slightly lower Cu surface quality than after EP in 2.17 M H3 PO4 at 65 ◦ C. Fig. 3 shows the variation of the average specular reflection and rms surface roughness of samples electropolished in 2.17 M H3 PO4 at 65 ◦ C, during various times. Again here, the EP current was 380 mA/cm2 . Clearly, a minimum surface roughness value of ∼8 nm, corresponding to a surface specular reflection value of ∼92% is obtained after 5 min EP. To further check the quality of our elevated temperature EP process, we have performed scanning electron microscope (SEM) observations of the Cu surface around grain boundaries, after EP at 65 ◦ C in 2.17 M H3 PO4 during 5 min. Fig. 4 shows a SEM picture taken across a grain boundary and the corresponding electron backscattered diffraction (EBSD) inverse pole figure map of the same region. The EBSD map clearly shows the 2 different grain orientations, namely close to (1 1 3) for the pink region and close to (1 0 1) for the green one. On the SEM image, the Cu surface exhibits fine striations (probably surface steps) which are continuous across the grain boundary. Within the resolution of the SEM, there is no height difference between the 2 grains, as anticipated when the kinetics of the etching reaction is mass-transfer-controlled. The contrast of the SEM picture originates from the variation in secondary electron emission between the (1 1 3) and (1 0 1) crystallographic facets of the Cu foil [27]. Note that very different results are obtained after a simple chemical etch of the Cu surface in

80 70

80 60

70

50

60 50

40

40 30

30 20

20

10 0

RMS Value (nm)

Average specular reflection (%)

etched thickness is around 40 ␮m for samples with a ∼1 cm2 surface area. Inspection of Fig. 1 indicates that at 25 ◦ C, the rms roughness values of the electropolished Cu surface are smaller at high H3 PO4 concentrations (in agreement with previous work, see refs 2–21), whereas the opposite is observed for EP experiments performed at 55 ◦ C. Moreover, the minimum roughness value obtained at 55 ◦ C in low concentration H3 PO4 is close to the minimum roughness value obtained at 25 ◦ C for high concentration H3 PO4 . The percent average specular reflection data, also displayed in Fig. 1, are totally consistent with the roughness measurements, since high values (≥90%) are observed at 25 ◦ C for high concentration H3 PO4 , whereas at 55 ◦ C, the highest specular reflections are obtained for low concentration H3 PO4 . In order to get more insights into the high temperature EP process, we have performed more systematic roughness and reflectance measurements of the surface of Cu foils electropolished in low concentration H3 PO4 , for temperatures ranging between 25 and 75 ◦ C. Fig. 2 shows such measurements performed on WACOPA Cu foils electropolished in 2.17 M H3 PO4 . Here again, the average specular reflection was averaged between 600 and 800 nm. The diffuse reflectance curves recorded over the whole 300–800 nm spectrum are shown in Fig. S4. As before, the EP experiments have been performed during 5 min and using a current density of 380 mA/cm2 . The surface roughness and percent specular reflection values show considerable variations over the investigated temperature range. A minimum roughness value of ∼8 nm is obtained at 65 ◦ C, which corresponds to a 92% surface specular reflection. In comparison, the best rms roughness value that we have obtained for samples electropolished in 14 M H3 PO4 at room temperature was ∼12 nm and the corresponding average surface specular reflection was 91%.

10 20

30

40

50

60

Temperature (oC)

70

80

Fig. 2. Average specular reflection (600–800 nm) and rms surface roughness values of samples electropolished in 2.17 M H3 PO4 for temperatures between 25 and 77 ◦ C.

Fig. 3. Average specular reflection (600–800 nm wavelength range) and surface roughness values (rms) of samples electropolished in 2.17 M H3 PO4 at 65 ◦ C during various times.

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Fig. 4. Characterization of the surface of a Cu foil electropolished at 65 ◦ C in 2.17 M H3 PO4 electrolyte. (a) Scanning electron microscope picture of the region across a grain boundary. (b) The corresponding EBSD map showing a (1 1 3)-oriented grain on the left-hand side (pink) and a (1 0 1)-oriented one on the right-hand side (green). The contrast on the SEM picture is only due to the difference in secondary electron emission of the Cu (1 1 3) and (1 0 1) crystal faces. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Fe(NO3 )3 , where the roughness is increased and etch pits start to appear [28]. Table 2 below summarizes the best results which we have obtained in terms of surface roughness and average specular reflection values for samples polished by CMP and electropolished in various conditions. Finally, we have tried to identify some surface oxides layer which would be formed during EP, other than the native oxides/hydroxyde that naturally form upon exposure of a fresh Cu surface to air [29,30]. Fig. 5 compares X-ray photo-electron spectroscopy (XPS) data recorded on a bare Cu foil and on an electropolished sample (WACOPA foil). The electropolished sample was loaded in the XPS chamber less than 2 min after rinsing, while the bare Cu foils were analyzed “as received”, without any treatment. The Cu2p3/2 peak corresponds to binding energies of 932.6 eV, 932.5 eV, 933.7 eV and 935.1 eV respectively in metallic Cu, Cu2 O, CuO and Cu(OH)2 [29]. The O 1s peak corresponds to binding energies of 529.6 eV, 530.3 eV and 531.2 eV respectively in CuO, Cu2 O and Cu(OH)2 [29]. Clearly, the surface of the bare Cu foil is oxidized and exhibits the characteristic peak of Cu2 O at 932.68 eV (with probably a contribution from Cu, but we know from literature that the surface of Cu exposed to air is oxidized), as well as a peak at 934.88 eV which we assign to Cu(OH)2 , see Fig. 5a and b. We do not detect a clear peak at 933.7 eV, which would correspond to CuO, but there is a large hump between 940 and 947 eV which is generally ascribed to the presence of CuO (Fig. 5a) [30]. The small peak at 937.08 eV is unidentified. For the O 1s signal (Fig. 5c), the peak at 531.23 eV probably corresponds to Cu2 O (with a contribution from CuO), whereas the one at 532.78 eV would correspond to Cu(OH)2 although it is shifted by 1.5 eV. Those observations are consistent with reports from the literature for native oxides on Cu [29,30]. Now, concerning the electropolished sample, the Cu 2p3/2 peak at 932.48 eV can be ascribed to Cu and/or Cu2 O. However, the presence of a small hump between 936 and 944 eV (related to the presence of CuO [30]) makes us think that the surface is slightly oxidized, in agreement with recent results obtained on electropolished samples and based only on electrochemical measurements [21]. Moreover, it is well known that the formation of CuO at room temperature is very slow [30], so we can assign any CuO detection on the electropolished sample to the EP

process itself and not to oxidation in air during the small amount of time between rising and loading in the XPS machine. With this in mind, the Cu 2p peak at 934.08 eV would then represent the Cu binding energy in CuO and the small one at 935.98 eV would be

Fig. 5. X-ray photoelectron spectroscopy data recorded on a bare Cu foil and on an electropolished sample. (a) Comparison of the 2 spectra over the ∼925–970 eV range, evidencing the CuO satellite hump. (b) Deconvolution of the Cu2p3/2 peak for the bare Cu foil (black) and the electropolished sample (red dots). (c) Deconvolution of the O1s peak for the bare Cu and electropolished samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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Table 2 The best rms surface roughness and reflectance values of sample (a) without any treatment (raw), (b) CMP, (c) EP in 14 M H3 PO4 at room temperature, (d) EP in 7.59 M H3 PO4 at room temperature, (e) EP in 2.17 M H3 PO4 at 65 ◦ C.

RMS value (nm) Specular reflection (%)

a

b

c

d

e

72.27 25.98

14.36 90.03

14.46 93.20

29.21 88.03

8.444 93.11

Table 3 Summary of the copper and oxygen XPS data. Cu 2p3/2

Cu

Cu2 O

CuO

Cu(OH)2

Reference peaks (eV) (Ref. [29]) Raw Cu (eV) EP Cu (eV)

932.6 932.68 932.48

932.5 932.68 932.48

933.7 + Hump between 940 and 947 (Ref. 30) Large hump between 940 and 947 934.08 + Small hump between 936 and 944

935.1 934.88 935.98

O 1s

Cu2 O

CuO

Cu(OH)2

Reference peaks (eV) (Ref. [29]) Raw Cu (eV) EP Cu (eV)

530.3 531.23 530.88

529.6 531.23 530.88

531.2 532.78 532.58

related to the presence of Cu(OH)2 . If we examine the O 1s signal, and again assuming an energy shift of ∼1.5 eV, the peak at 530.88 eV would represent CuO while the one at 532.58 eV would represent Cu(OH)2 , the latter probably originating from the rising operation at the end of the EP process. Note that the peak at 530.88 eV could also be assigned to Cu2 O. To summarize at this point, our XPS data indicate that the Cu surface is slightly oxidized after EP, but nonetheless much less oxidized than a raw Cu foil. The various XPS data are grouped in Table 3. 4. Conclusions In conclusion, we have presented a novel elevated temperature electropolishing process for Cu foils, which yields better surface roughness and specular reflection values than classical electropolishing performed at room temperature in high concentration H3 PO4 electrolytes. Moreover, this elevated temperature electropolishing process compares favorably with a more complex chemical-mechanical polishing one. XPS data recorded on the Cu foils after electropolishing indicate a slight surface oxidation. We are currently using this electropolishing process for the preparation of Cu foils before graphene growth. Acknowledgements The authors would like to thank Yeo Jin Lee and Je Mee Joe for their help in several EP experiments. This work was partially supported by the Human Resources Development program (No. 20124010203270) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc. 2014.04.144.

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