Surface characterization of copper electroless deposition on atomic layer deposited palladium on iridium and tungsten

Surface & Coatings Technology 200 (2006) 5760 – 5766 www.elsevier.com/locate/surfcoat

Surface characterization of copper electroless deposition on atomic layer deposited palladium on iridium and tungsten Young-Soon Kim a , Jiho Shin b , Joong-Hee Cho a , Gregory A. Ten Eyck c , De-Li Liu c , Samuk Pimanpang c , Toh-Ming Lu c , Jay J. Senkevich d , Hyung-Shik Shin a,⁎ a

Thin Film Technology Lab, School of Chemical Engineering, Chonbuk National University, Jeonju, 561756, South Korea b Korean Minjok Leaders Academy, Hoengsung-gun, Kangwon-do 225-823, South Korea c Department of Physics, Rensselaer Polytechnic Institute, Troy, NY 12180 USA d Brewer Science Inc., Rolla, MO 65401 USA Received 4 November 2004; accepted in revised form 18 August 2005 Available online 18 October 2005

Abstract Iridium and tungsten refractory metals possess high melting points, hardness, and good electrical resistivity. Palladium has been recently deposited on W and Ir via atomic layer deposition and it is also an appropriate catalyst for the electroless deposition of copper. Palladium was deposited at 80 ± 5 °C with a PdII(hfac)2 sublimation temperature of 46.0 ± 0.5 °C using 13 sccm Ar as a carrier gas, and 105 sccm H2 as a reducing gas on Ir and W substrates for 150 cycles. The thickness of Pd was 20 and 30 Å on Ir and W substrates, respectively, with a low surface roughness. For the Cu electroless process, ethylenediamine-tetraacetic acid (EDTA) was used as a chelating agent, glyoxylic acid as a reducing agent, and additional chemicals such as polyethylene glycol and 2,2′dipyridine as surfactant and stabilizer respectively. Electroless Cu was undertaken at 60 °C with good adhesion to iridium and tungsten with Pd as a catalytic layer. © 2005 Elsevier B.V. All rights reserved. Keywords: Electroless deposition; Copper; XPS; Iridium; Tungsten

1. Introduction It is well-known that the electroless process is based on redox reactions, by a simultaneous reduction and oxidation of metal ions and chemical reducing agent in solution, respectively. Many examples of electroless exist in the literature including printed circuit boards [1,2], magnetic storage media [3,4] and metallization of polymers [5,6]. Scientists have favorably worked with electroless processes due to its promising use in nano-scale materials fabrication of the metal nanowire for biosensor, ultra large scale integration (ULSI) and micro electromechanical system (MEMS), for example, the progress of electrodeposited Cu interconnection of fine wiring for ULSI applications [7–10]. Some advantages of electroless process for advanced metallization are a low processing temperature, high selectivity, selfalignment, and conformal deposition ability for high aspect ratio features filling [11,12]. ⁎ Corresponding author. Tel.: +82 63 2702438; fax: +82 63 2702306. E-mail address: [email protected] (H.-S. Shin). 0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.08.142

Copper continues to attract attention for the preparation of on-chip metallization and has been substituting aluminum in wiring material since it has lower resistivity and better electromigration resistance. In contrast to Al, Cu diffuses rapidly into low k dielectrics and forms weak chemical bonds with these materials. Therefore, copper needs both a diffusion barrier layer and attention to adhesion. An elemental refractory metal may improve barrier electrical resistivity over metal nitrides, which have been shown effective for Cu/low κ structures [11–14]. Noble metal seeds, for instance ruthenium, have been proposed as a replacement for the Cu seed layer that is physical vapor deposited [15]. A catalyst is desired for the electroless plating process of copper to oxidize the organic reducing agent and to affect the texture of the resulting film. As a result, various deposition techniques such as acid solution, sputtering, and metallorganic chemical vapor deposition have been proposed for the preparation of this thin metal catalyst [16–19]. In a previous study, Senkevich et al. [20] and Ten Eyck et al. [21] examined a palladium layer by plasma-assisted atomic layer deposition (PA-

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2. Experimental 110 nm of Ir (99.95%) and 120 nm W deposited onto the Si (100) substrate by electron beam and DC sputtering, respectively, were used to deposit palladium Pd was deposited on Ir and W via ALD [21] followed by electroless Cu onto this stack. The deposition chamber walls were kept at 75–80 °C while plasma was struck and maintained at ∼120 mTorr. During each deposition for Pd, 55 sccm Ar (99.999%) was flowed as a purge gas, 13 sccm Ar as a carrier gas for PdII(hfac)2, and 105.0 sccm H2 as a reducing gas. The pulse sequence was 15 s of PdII(hfac)2 followed by 8 s of ‘dead time’ with 55 sccm Ar (purge gas), then 7 s of H2 to allow for a steady state in the plasma, and then 15 s of remote H2 inductively coupled plasma with 105 sccm H2 flow and 65 W of net forward power. The number of cycles for each deposition was kept constant at 150. The thickness of Pd film was measured by Rutherford backscattering spectrometry (RBS) on the 4.0 MeV Dynamitron accelerator at the Ion Beam Laboratory in the Department of Physics at the University at Albany, and revealed that the Pd thickness on Ir and W are 20 and 30 Å, respectively. Details of the experimental arrangement and data acquisition system for RBS are described elsewhere [23– 25]. In the second step, cupric sulfate (CuSO4·5H2O; 7.615 g/l) was used as the Cu source material, ethylenediaminetetraacetic acid (C10H16N2O8 (EDTA); 10.258 g/l) as the chelating agent, glyoxylic acid monohydrate (C2H2O3·H2O; 7 g/ l) as the reducing agent, and additional chemicals such as polyethylene glycol (5 μl) and 2,2′dipyridine (4 mg) as surfactant and stabilizer, respectively. The bath pH was adjusted at room temperature to ∼12.5 with tetramethyammonium hydroxide (TMAH). The solution temperature was then raised and maintained at 60 °C. The surface chemical structure on each sample was analyzed by using X-ray photoelectron spectroscopy (XPS). The samples were loaded into the XPS chamber with a chamber base pressure at ∼1 × 10− 9 Torr and during spectra sweeps the pressure raised to ∼4 × 10− 9 Torr. The X-ray MgKα source (PHI model 04-151) used in this experiment has a primary energy 1253.6 eV, and a double pass cylindrical mirror analyzer (PHI

Model 15-255G) with a pass energy 50 eV used for highresolution scans. The microstructures of samples were examined by a field emission scanning electron microscope (FESEM, JEOL JSM6330F). The surface morphologies of the films were examined by an atomic force microscope using an Auto Probe CP (Park Scientific Instruments, TM Microscope) operated in the tapping mode. A triangular Silicon cantilever with silicon conical tip (Veeco Metrology Group) was used in non-contact mode to measure the surface topography. Additionally this sensor offers typical tip radius of curvature of less than 10 nm. The AFM examinations were undertaken at room temperature in air. 3. Results and discussions Fig. 1 shows the XPS spectra of the Ir substrate deposited onto a hydrogen terminated Si(100) using electron beam deposition. Fig. 1(a) shows a typical XPS wide scan spectrum. All the standard spectral lines of Ir element are present: Ir 4f, Ir 4d, Ir 4p, C 1s, and O 1s. The most intense one, located at 61 eV for Ir 4f, is due to metallic iridium. Doublet separation of the Ir

(a) wide scan spectral line Ir 4d

O 1s Ir 4p

800

600

Ir 4f C 1s

400

(b) Ir 4f spectral line

Intensity (arb. unit)

ALD) for a broad range of applications on Ir, W and Si substrates. This PA-ALD Pd catalyst layer had good step coverage. In addition Kim et al. demonstrated copper electroless deposition on TaNX with an ultrathin Pd catalyst by PA-ALD [22]. These results were described the deposition a continuous catalyst in high aspect ratio trenches and vias and not voiding during the electroless plating. The investigation of basic mechanisms and processes involved for the electroless Cu deposition on barrier layers is of primary interest. In this paper, we report the analysis of the surface of iridium and tungsten as barrier materials deposited by electron beam and DC sputtering, respectively for Cu metallization. Further, examination is also undertaken for the Pd catalyst layer deposited on iridium and tungsten by ALD. The structure and properties of the electroless Cu films are also investigated.

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63.86

200 60.85 4f

4f

80

75

535

7/2

5/2

70

65

(c) O 1s spectral line

540

0

60

55

529.44

530

525

520

Binding energy (eV) Fig. 1. XPS spectra of Ir substrate deposited onto native silicon oxide using electron beam.

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4f spectral line is observed in the spectrum. Fig. 1(b) is characterized by a binding energy of 60.85 eV (4f7 / 2) and 63.86 eV (4f5 / 2) with a separation energy of 3.01 eV and a FWHM of 1.8 eV. Spectral lines of both the Ir 4f7 / 2 and 4f5 / 2 are close to the values reported in the literature. Ir 4f7 / 2 has been shown to have a binding energy of 60.90 [26] and 60.84 eV [27]. These discrepancies of the chemical shift are due to inaccuracies in the calibration of the binding energy [27]. Spectral line of the O 1s in Fig. 1(c) at 529.44 eV is due to chemisorbed oxygencontaining carbon on the Ir surface. Fig. 2 shows the XPS spectrum of Pd (thickness of 20 Å) deposited by ALD for 150 cycles on the Ir substrate (Fig. 1). Fig. 2(a) shows a typical XPS wide scan spectrum, all the standard photoelectron lines of Pd element are present by Pd 3d, Pd 3p, and C 1s without O 1s. The MVV identified by specifying the initial vacancy in the M shell and final vacancies in valence levels of the Auger transition [26]. The most intense one, located around 926 eV, is characterized by MVVAuger line of Pd. The Like Pt, Ag, Rh, Os, and Re, Pd is a noble metal and does not oxidize under ambient conditions. Fig. 2(b) shows the XPS spectrum with binding energies for Ir, Pd, Cu substrates and referenced to the adventitious carbon peak at 284.75 eV for C 1s peak. The doublet separation of the Pd 3d spectral line in Fig. 2(c) is characterized by a binding energy of 335.85 eV (3d5 / 2) with a FWHM of 1.5 and 341.13 eV (3d3 / 2) with separation energy of 5.28 eV. Spectral lines of both the Pd 3d5 / 2 and 3d3 / 2 are very close to the values reported in the literature till date. Pd 3d5 / 2 has been shown to have a binding energy of

335.10 eV with a separation energy of 5.26 [20–22], 335.19 [26], and 335.8 eV[27]. Spectral line of Pd 3p3 / 2 in Fig. 2(d) at 532.86 eV has a FWHM of 2.5 eV. Previously Senkevich et al. demonstrated that Pd was deposited on various substrate (Ir, W, Ta, TaN, TiN, SiO2, SAM, polymer dielectric materials) with a PdII(hfac)2 source using ALD [20,21,28]. Pd (111) textured films have been successfully deposited by ALD via metalorganics with a thickness of 20–100 Å at a deposition temperature of 80–130 °C [20,21,28]. Fig. 3 shows the FESEM surface images of the Cu films deposited onto the Pd / Ir stack by ELD as a function of different immersion times. The micrographs indicate that the crystal grains become larger as a function of immersion time. Fig. 3(a) shows the micrographs as a function of immersion time for 1 min, (b) for 2 min, and (c) for 5 min. Fig. 3(a), presents the initial phenomenon analogous to a case where the crystallization growth rate is faster than the nucleation rate. It can also be seen in Fig. 3(b) that very homogenous copper grains had coalesced to form a uniform continuous coverage without a protuberant formation. In Fig. 3(c) fine equated copper grains are observed. The deposition rates for electroless copper plating on Pd/Ir were measured by cross-sectional FESEM and AFM images (Fig. 4). Fig. 4(a) and (b) shows that the thicknesses of the films with respect to plating time. They were 70 nm for 2 min and 200 nm for 5 min. Therefore, a 200 nm Cu film was deposited at ∼40 nm/min. Cross-sectional FESEM images presents a nice agreement that copper and iridium have a good adhesion

Intensity (arb. unit)

(a) wide scan spectral line Pd Auger

(b) C 1s spectral line

284.75

Pd 3d Pd 3p C 1s

800

600

400

200

335.85

Intensity (arb. unit)

(c) Pd 3d spectral line 341.13

0

290

345

280

(d) Pd 3p spectral line 532.86

3d5/2

3d3/2

350

285

3p3/2

340

335

Binding energy (eV)

330

325 540

535

530

Binding energy (eV)

Fig. 2. XPS spectra of Pd (20 Å) deposited onto Ir substrate by atomic layer deposition.

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Fig. 3. Plane-view FESEM images of Cu films deposited onto Pd(20 Å)/Ir substrate by electroless deposition versus at different time, (a) 1 min, (b) 2 min, and (c) 5 min.

Cu 70 nm

Ir 110 nm

Fig. 4. Cross-section FESEM and AFM 3-D images of Cu films deposited onto Pd(20 Å)/Ir substrate by electroless deposition versus at different time, (a, c) 2 min and (b, d) 5 min.

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(a) wide scan spectral line O 1s

W 4d C 1s

W 4p

Intensity (arb. unit)

600

400

W 4f

200

0

(b) W 4f spectral line 35.8

4f 5/2

5p3/2

31.13 4f

37.76

7/2

33.39

45

40

35

30

25

and 33.88 eV [26]. Fig. 5(c) shows the spectral line of O 1s at 531.44 and 532.46 eV which are due to the oxidization of WO3, WO2, and WOx on the surface of W exposed to ambient conditions for extended periods. Fig. 6(a) shows the XPS spectra of Pd with a thickness of 30 Å, deposited by ALD for 150 cycles on W substrate. It shows a typical XPS wide spectrum scan and all the standard photoelectron lines of Pd element are present by Pd 3d, Pd 3p, C 1s without O 1s. Fig. 6(b) contains the AFM photograph that shows the surface morphology of Pd. The observed RMS roughness is ∼5 Å. Fig. 7 shows the XPS spectra of Cu with a thickness of 80 nm deposited by ELD for 2 min on the Pd (30 Å)/W stack. Fig. 7(a) shows a typical XPS wide scan spectrum, indicating that all the standard photoelectron lines of elemental Cu are present: Cu 3p, Cu 2p, O 1s, C 1s. The peak, located at around 335 eV, is characterized as the Cu LMM Auger line. Fig. 7(b) predicts the doublet separation of Cu 2p spectral line and characterized by a binding energy of 934.24 (2p3 / 2) and 954.59 eV (2p1 / 2) with

(c) O 1s spectral line (a)

531.25

Pd Auger

532.46

540

535

530

525

520

Binding energy (eV) Fig. 5. XPS spectra of W substrate deposited onto native silicon oxide using DC sputtering.

between them. This Cu film did not delaminate during a Scotch™ tape peeling test suggesting that the film had reasonable adhesion. Although a quantitative value was not measured, no delamination during the test suggests that there was no serious adhesion problem. If adhesion is poor, the film typically delaminates during Cu deposition since the electroless solution is highly alkaline. Fig. 4(c) and (d) shows the AFM 3-D images of the Cu layers on Pd/Ir clearly indicating time dependence. It has been noted that Cu films have demonstrated good coverage, smooth texture, and good uniformity. The root mean square roughness of the Cu films surface were 7 nm and 18 nm at 2 and 5 min, respectively. The surface morphology analysis shows that the increase of surface roughness might be related to the increase of film thickness via deposition time. Fig. 5 shows the XPS spectra of the W substrate deposited onto Si (100) by DC sputtering. Fig. 5(a) shows a typical XPS wide scan spectrum. All the standard spectral lines of elemental W are present: W 4f, W 4d, W 4p, C 1s, and O 1s same as the spectra reported in the literature [26]. W 4f and 5p peaks in Fig. 5(b) exist at binding energies of 31 and 38 eV. WOx compounds are also formed and the spectral lines of both the W 4f7 / 2 and 4f5 / 2 are very close to the values reported in the literature. For pure W, the W 4f7 / 2 and W 4f5 / 2 have a binding energy of 31.73

Intensity (arb. unit)

Pd 3d Pd 3p

C 1s

800

600

400

200

0

Binding Energy (eV)

(b)

Fig. 6. XPS wide scan spectrum and AFM image of Pd (30 Å) deposited onto W substrate by atomic layer deposition.

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(a) wide scan spectral line

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(d)

Cu 2p O 1s

Cu Auger C 1s

800

600

400

Cu 3p

200

0

Intensity(arb. unit)

(b) Cu 2p spectral line 2p1/2 2p3/2

(e)

960

950

940

930

920

(c) O 1s spectral line 531.55

538

536

534

532

530

528

526

Binding energy(eV) Fig. 7. XPS spectra (a–c), plane-view FESEM image (d) and AFM image (e) of Cu film deposited onto Pd(30 Å)/Ir substrate by electroless deposition for 2 min.

satellite shake-up peaks. The Cu 2p3 / 2 and 2p1 / 2 intensities of shake-up satellites approach that of the main line with satellite intensity at around 943.74 and 962.73 eV while the doublet separation of Cu 2p spectral line with the separation energy of 20.13 eV. The spectral lines of both the Cu 2p3 / 2 and 2p1 / 2 are close to the values reported in the literature previously. The Cu 2p3 / 2 has a binding energy of about 934.1 eV with the separation energy of 19.6 [26] and 932.67 eV [27]. Fig. 7(c) shows the spectral line of O 1s at 531.55 eV, which is due CuOX at the Cu surface. Fig. 7(d) and (e) shows SEM and AFM images of Cu, both of which indicate that the Cu deposits were uniformly deposited on Pd/W stacks. It can also be observed in Fig. 7(d) that grains of copper deposited for 2 min have an equiaxial shape with a distribution of size. The equiaxial shape is the same to geography deposited on bare tungsten for 5–12 min [25]. Kim et al. have reported a deposition rate of 16.6 nm/min on bare tungsten without Pd catalyst [25]. In addition, the bare tungsten substrate needs the incubation time for 3–5 min since the state of surface changes tungsten oxide to tungsten oxide ions [25]. The root mean square surface roughness in Fig. 7(e) for this Cu film is ∼5 nm. This Cu film did not delaminate by Scotch tape

peeling test that the adhesive strength is good for Cu metallization such as the Cu film on iridium of Fig. 4(a) and (b). 4. Conclusions Cu electroless deposition on Ir and W elemental barrier layers has been demonstrated in this paper. Films of Ir, W, Pd/Ir, Pd/W and Cu/Pd/W were subjected to detailed analysis by XPS, FESEM and AFM. The XPS analyses indicate that Ir has surface contamination and W oxidizes both under ambient conditions. Also, XPS spectra revealed that atomic layer deposited Pd on Ir and W passivates these surfaces and acts as an appropriate catalyst for electroless deposition. The Cu films spread homogenously and smoothly on these barrier surfaces from AFM and FESEM results. For Cu ELD, the pH of bath was kept constant at 12.5. The concentration of glyoxylic acid was kept as 7 g/l, and solution temperature was constantly maintained at 60 °C. The cross sectional FESEM images show that ELD rate is about 20 nm/min. The roughness of the Cu film deposited, for the time period of 2 min, was 5–7 nm on Ir and W substrates. The FESEM images show equiaxial shaped Cu grains for a deposition time of 2 min.

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Acknowledgment We acknowledge the financial support of the Korea Science and Engineering Foundation (KOSEF), Korea Ministry of Science and Technology (KMOST) and SRC (Semiconductor Research Corporation) Center for Advanced Interconnect Systems and Technologies (CAIST). References [1] F. Hanna, Z.A. Hamid, A. Abdel Aal, Mater. Lett. 58 (1–2) (2004) 104. [2] S. Miura, H. Honma, Surf. Coat. Technol. 169–170 (2003) 91. [3] S.-S. Kim, S.-T. Kim, J.-M. Ahn, K.-H. Kim, J. Magn. Magn. Mater. 271 (1) (2004) 39. [4] X.Y. Yuan, T. Xie, G.S. Wu, Y. Lin, G.W. Meng, L.D. Zhang, Physica, E, Low-Dimens. Syst. Nanostruct. 23 (1–2) (2004) 75. [5] W.C. Wang, E.T. Kang, K.G. Neoh, Appl. Surf. Sci. 199 (2002) 52. [6] Z.J. Yu, E.T. Kang, K.G. Neoh, Polymers 43 (2002) 4137. [7] H. Honma, T. Kobayashi, J. Electrochem. Soc. 141 (3) (1994) 730. [8] Y.Y. Shacham-Diamand, Electrochem. Solid-State Lett. 3 (6) (2000) 279. [9] Z. Wang, T. Ida, H. Sakaue, S. Shingubara, T. Takahagi, Electrochem. Solid-State Lett. 6 (3) (2003) C38. [10] J.J. Kim, S.K. Kim, Y.S. Kim, J. Electrochem. Soc. 151 (1) (2004) C97. [11] A. Kohn, M. Eizenberg, Y. Shacham-Diamand, Appl. Surf. Sci. 212-213 (2003) 367. [12] J.S. Park, H.S. Park, S.W. Kang, J. Electrochem. Soc. 149 (1) (2002) C28. [13] J. Kim, H. Hong, K. Oh, C. Lee, Appl. Surf. Sci. 210 (2003) 231.

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