Plasma-enhanced atomic layer deposition (PEALD) of cobalt thin films for copper direct electroplating

    Plasma-enhanced atomic layer deposition (PEALD) of cobalt thin films for copper direct electroplating Jae-Hyung Park, Dae-Yong Moon, Dong-Suk Han, Yu-Jin Kang, So-Ra Shin, Hyung-Tag Jeon, Jong-Wan Park PII: DOI: Reference:

S0257-8972(14)00407-1 doi: 10.1016/j.surfcoat.2014.05.005 SCT 19388

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

10 October 2013 11 April 2014 4 May 2014

Please cite this article as: Jae-Hyung Park, Dae-Yong Moon, Dong-Suk Han, Yu-Jin Kang, So-Ra Shin, Hyung-Tag Jeon, Jong-Wan Park, Plasma-enhanced atomic layer deposition (PEALD) of cobalt thin films for copper direct electroplating, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.05.005

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Plasma-enhanced atomic layer deposition (PEALD) of

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cobalt thin films for copper direct electroplating Jae-Hyung Park1, Dae-Yong Moon1, Dong-Suk Han1, Yu-Jin Kang1, So-Ra Shin2, and

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Hyung-Tag Jeon2, Jong-Wan Park2*

Division of Nanoscale Semiconductor Engineering, Hanyang University, Seoul, Korea

Division of Materials Science & Engineering, Hanyang University, Seoul, Korea 133-791

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133-791

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It is challenging to produce reliable Cu wiring on the nanometer scale for scaled-down devices. We studied the use of Co films deposited by plasma-enhanced atomic layer deposition (PEALD) using dicobalt hexavarbonyl tert-butylacetylene (CCTBA) as a

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precursor for Cu direct plating. Electrical properties of PEALD Co films of sub-20 nm thickness were determined by assessing continuities, morphologies, and impurities. To decrease the resistivity of Co films, the TaNx substrate was pre-treated with H2 plasma and the flow rate of H2 gas during CCTBA feeding and reactant feeding pulses was increased. Co films were deposited on 3 nm-thick TaNx-covered SiO2 substrate with 24 nm-deep trenches, and Cu direct plating was successfully performed under conventional conditions.

Keywords: Cobalt, Atomic layer deposition, Direct plating, CCTBA, Copper interconnect

ACCEPTED MANUSCRIPT 1. Introduction Scaling-down of ultra-large integrated circuits (ULSI) needs a high quality diffusion barrier

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and Cu seed layer with ultrathin, continuous, and good step coverage. Traditionally, diffusion

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barrier and seed layer have been fabricated using physical vapor deposition (PVD). However,

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as the feature sizes decrease to sub 30 nm and below, deposition of the high quality diffusion barrier and Cu seed layer by PVD is challenging [1-6]. Conformal deposition in narrow line

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trenches via holes is difficult to achieve using the PVD method. Atomic layer deposition (ALD) has good step coverage and offers accurate thickness control, and is therefore a

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candidate technique for the formation of ultrathin conformal films [7, 8]. Moreover, an unwanted increase in the resistivity of Cu wires occurs due to the size effect on the resistivity

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of a conventional seed/diffusion barrier film [9, 10]. Hence, direct electro plating of Cu on

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the barrier without a seed layer to ensure the space needed for plating of Cu are being

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investigated [11-13]. Recent work on direct plating processes using Co films have attracted great attention due to better adhesion of Co to Ta than Cu, the high electrical conductivity of Co, and the increase in effective space for plating filling [14]. In this study, we used dicobalt

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hexacarbonyl tert-butylacetylene (C12H10O6Co2, CCTBA) as a Co precursor because of its high vapor pressure, liquid state, and excellent thermal stability under normal conditions [18]. We investigated the effects of H2 plasma pre-treatment of the TaNx substrate and variations of the feeding flow rate of H2 reactant gas on the resistivity of Co film deposition using CCTBA as a precursor. Furthermore, to evaluate the Co seed layer for direct electroplating, we performed Cu filling of 24 nm-deep trenches.

ACCEPTED MANUSCRIPT 2. Experimental Co films were deposited on a 3 nm-thick TaNx-covered SiO2/Si plane and trench substrate by

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radiofrequency plasma-enhanced atomic layer deposition (RF-PEALD) using H2 reactant gas

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and CCTBA as the Co precursor. TaNx was deposited using PEALD on a thermal oxide

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covered Si, and Co films were deposited without breaking the vacuum. CCTBA was heated up to 50˚C and fed into the chamber without introducing a carrier gas. To check the ALD

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window of CCTBA, Co films were deposited at different temperatures ranging from 100 to 250˚C. One deposition cycle of Co PEALD included four consecutive pulses: a pulse of

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CCTBA vapor, a purge pulse with 100 sccm N2 gas, a pulse for H2 plasma exposure, and a second purge pulse with 100 sccm N2 gas. To decrease the resistivity of Co films, the TaNx

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substrate was treated with H2 plasma and Co films were deposited at H2 gas flow rates from

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50 to 100 sccm. Original and modified deposition conditions are listed in Table 1. To evaluate

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the applicability of the Co films as the Cu seed layer, 24 nm-deep trenches were filled with Cu at a current density of 1 mA/cm2 for 240 s at room temperature using traditional electrochemical plating (ECP) solution, which contains copper sulfate (CuSO4), sulphuric

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acid (H2SO4), chloride ions, and two additives, namely polyethylene glycol as a suppressor and a sulfopropyl sulfonate as an accelerator [15]. Film thicknesses and morphology of Co films and the characteristics of Cu filling were analyzed using scanning transmission electron microscopy (STEM; Hitachi HD-2300A). Film resistivity was measured by a four-point probe (JANDEL) and the film thickness. Impurities in the Co films were analyzed by Auger electron spectroscopy (AES, PHI 680). Wetting angle was measured with a contact angle measurement system and the sessile-drop method using iodine, and atomic force microscopy (AFM) was used to characterize the surface roughness of films.

ACCEPTED MANUSCRIPT 3. Results and discussion We first investigated the growth rate of Co thin films at different substrate temperatures

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ranging from 100 and 250˚C, as shown in Fig. 1(a). The number of ALD cycles for Co thin

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films was 600. Co thin films deposited at substrate temperatures between 120 and 200˚C exhibited ideal self-limiting and complementary reactions, while substrate temperatures above 200˚C led to chemical vapor deposition growth. To confirm linear growth rate with

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number of cycles, film thickness was measured after varying the number of PEALD cycles

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from 50 – 500 at a substrate temperature of 150˚C. Thickness and resistivity of the resultant Co films are shown in Fig. 1(b). A linear trend in film thickness was observed, with a growth rate of 0.08 nm/cycle. Resistivity of Co films thicker than 20 nm was about 90 μΩ-cm and

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high in comparison with that of bulk Co (6.2 μΩ-cm). For films thinner than 20 nm, resistivity increased rapidly with increasing film thickness.

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Lim et al. investigated the electrical resistivity of Cu films deposited by ion beam deposition with or without a negative substrate bias voltage for different states of dependence on film

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thickness. And they concluded that the resistivity of metal film are increased by grain size, impurities, and morphological defects as well as surface scattering and grain boundary scattering [16]. Furthermore, as film thickness decreases, scattering due to grain boundaries and surface roughness becomes the primary factor resulting in increased resistivity [17]. To investigate the reasons for the poor electric conductivity of PEALD Co thin films, Co films were analyzed by both AES and TEM. Figure 2(a) shows the AES depth profile of a ~ 40 nmthick Co film deposited on TaNx/SiO2/Si substrate. It is clear that the Co film contained a large amount (> 20 at.%) of carbon. We attributed the high resistivity of Co films thicker than 20 nm to the high impurity content in these films induced by incomplete decomposition of organic compounds. Figure 2(b) shows the results of cross-sectional TEM analysis of Co

ACCEPTED MANUSCRIPT islands deposited on TaNx/SiO2/Si substrates with 24 nm-deep trenches. The surface morphology of ~ 5 nm-thick Co films was very rough and discontinuous, making it

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impossible to fill the trench by electroplating. Island-like growth occurred during initial Co

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growth due to the low nucleation density of Co on TaNx and the relatively high surface

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energy of Co.

CCTBA is very reactive with H, and H terminated on the substrate surface functions as a

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nucleation site [18, 19]. We therefore pre-treated the TaNx substrate with hydrogen plasma at a plasma power of 100 W for 1 min to hydrogenate the TaNx surface. Figure 3 shows the

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resistivities of Co films deposited on TaNx substrate with and without H2 plasma pretreatment. Resistivity of Co films with a thickness less than 20 nm deposited on TaNx

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substrate pre-treated with H2 plasma was much lower than that of films deposited on

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untreated TaNx substrate. This drastic decrease in resistivity means that continuous and dense

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Co films formed at lower thicknesses. This is expected because of the increase in nucleation density. Furthermore, to evaluate the effect of H2 plasma treatment on the wettability of the TaNx surface, we measured the contact angle of TaNx before and after treatment using a

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contact angle measurement system and a sessile drop method. Wetting angle of TaNx substrate decreased from 55.0˚ to 36.9˚ after H2 plasma treatment, as shown in the inset of Fig. 3. Compared to the untreated substrate, the reduced wetting angle indicates that the H2 plasma-treated TaNx substrate had a higher substrate surface energy or lower interfacial energy than the untreated substrate, which enhanced Co film growth by allowing layer-bylayer growth [19]. Increase in the surface energy of the TaNx substrate and the nucleation density of Co resulted in the formation of denser nuclei with smaller wetting angles. Continuous Co films formed on H2 plasma-treated TaNx substrate at lower thicknesses and these films had a higher density and lower resistivity than Co films deposited on untreated TaNx substrate.

ACCEPTED MANUSCRIPT Although H2 plasma pre-treatment of the TaNx substrate decreased the resistivity of the Co films, this resistivity was still too high for Cu direct plating. As mentioned above, the

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presence of impurities in films, such as carbon and oxygen, results in poor electrical

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conductivity. ALD Co thin films deposited on TaNx substrate using CCTBA as a precursor

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have not previously been reported. However, Kwon et al. reported reactions of CCTBA with SiO2 and H/Si surfaces at decarbonylation temperature (140˚C) and characterized the

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resulting Co films [19]. These authors reported that carbonyl groups are quite stable on H/Si in the form of Co2(CO)6 and semi-bridging CO groups during the initial reaction, and that the

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presence of surface-bound CO is negligible on SiO2 because of weakening of the bond strength of CO with the Co center. However, without annealing at 300˚C, organic ligands

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such as t-butyl, carbonyl, methyl, and their products remain in the Co films. To decrease the

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content of impurities in the Co film deposited at a substrate temperature of 150˚C without an

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additional annealing process, Co films were deposited as a function of H2 gas flow rate during the CCTBA feeding and reactant feeding pulses. Modified deposition process conditions are listed in Table 1 and the resistivity, carbon content, and surface roughness of

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the Co films deposited by the modified process are shown in Fig. 4. Resistivity of ~ 5 nmthick Co films decreased with increasing H2 gas flow rate as expected because of the decrease in carbon content. Increase of the H2 gas flow rate led to a change in surface roughness as well as resistivity of the Co films. Root mean square (rms) roughness measured by AFM decreased with an increase in H2 gas flow rate, as shown in Fig. 4. At lower H2 gas flow rates, organic ligands remaining on the surface of TaNx (during the nucleation stage) or Co (during the film growth stage) decreased the number of available reaction sites during the next CCTBA feed [20]. In contrast, at a high H2 gas feeding rate, effective elimination of these ligands promoted nucleation and reaction with CCTBA, resulting in a smoother surface morphology.

ACCEPTED MANUSCRIPT To form a Cu interconnect with excellent characteristics using electroplating, the seed layer must be continuous, highly conductive, and minimally rough. To evaluate the applicability of

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ALD Co film as a seed layer, we performed Cu direct plating at a current density of 1

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mA/cm2 for 240 s at room temperature. A 3 nm-thick TaNx diffusion barrier was deposited on

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substrate with 24 nm-sized trenches using PEALD, and then the substrate was treated with H2 plasma. As shown in Fig. 5(a), the Co thin film formed was continuous and had low

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resistivity to achieve efficient filling using a conventional ECP process. Figure 5(b) shows the cross sectional STEM images of Cu deposited in trench using direct plating on the PEALD

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Co glue layer. From the results obtained in the STEM analysis, it is concluded that a Cu interconnect with a thickness of about 500 nm was deposited with void-free filling of trench.

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These results indicate that H2 plasma pretreatment of the TaNx substrate and a high H2

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reactant gas flow process enabled the generation of continuous, highly conductive, and

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smooth surface Co film via ALD using a CCTBA precursor at the nanometer-thickness scale.

ACCEPTED MANUSCRIPT 4. Conclusions PEALD Co films for direct Cu plating were deposited using CCTBA as a precursor. Co film

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deposited at a substrate temperature of 150˚C had a carbon content above 20 at.% and was

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discontinuous and rough at a thickness below 5 nm. H2 plasma treatment of the TaNx

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substrate increased the nucleation density and wettability of the substrate, thereby decreasing the resistivity of the resultant Co films. Furthermore, the resistivity of the Co films decreased

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with increasing H2 gas flow rate during both CCTBA feeding and reactant feeding pulses. To evaluate applicability of ALD Co film as a seed layer for Cu interconnect, we performed

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direct electro plating on the Co/H2 plasma treated TaNx/24 nm-sized trench. And filling was achieved successfully without any defects. These results can be attributed to not only the

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formation of ALD Co thin films with high conductivity and conformality for Cu direct electro

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plating but also extension of the current plating process to devices with much smaller features

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than those of existing devices.

ACCEPTED MANUSCRIPT Acknowledgments This work was supported by Hynix Semiconductor Incorporated. We would also like to

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acknowledge the National Nanofab Center (NANO) for assistance with the STEM analysis.

ACCEPTED MANUSCRIPT References [1] International Technology Roadmap for Semiconductors (Edition 2009, Interconnect,

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2009).

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[3] H. Kim, Surf. Coat. Technol. 200 (2006) 3104.

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[2] Z. Li, A. Rahtu, and R. G. Gorden, J. Electrochem. Society 153(11) (2006) C787.

[4] D. Volger and P. Doe, Solid State Technol. 46 (2003) 35.

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[5] S. C. Sun, C. E. Tsai, S. H. Chuang and H. T. Chiu, J. Appl. Phys. 79 (1996) 6932. [6] T. Suntola, Mater. Sci. Rep. 4 (1989) 261.

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[7] L. Wu and E. Eisenbraun, J. Vac. Sci. Technol. B 25(6) (2007) 2581. [8] T. M. Mayer, J. W. Elam, S. M. George, P. G. Kotula, R. S. Goeke, Appl. Phys. Lett.

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82 (2003) 2883.

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[9] W. Steinhogl, G. Schindler, G. Steinlesberger, M. Engelgardt, Phys. Rev. B. 66 (2002)

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[10] S. M. Rissnagel, T. S. Kuan, J. Vac. Sci. Technol. B. 22 (2004) 240. [11] D. Josell, C. Witt, T. P. Moffat, Electrochem. Solid-State. Lett. 9 (2006) C41.

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[12] O. Chyan, T. N. Arunagiri, T. Ponnuswamy, J. Electrochem. Soc. 150 (2003) C347. [13] D. Josell, D. Wheeler, C. Witt, T. P. Moffat, Electrochem. Solid-State. Lett. 6 (2003) C143. [14] T. Nogami et al. IEEE 2010 Int. Interconnect Technology Conference. (2010) 3. [15] Y. Liu, J. Wang, L. Yin, P. Kondos, C. Parks, P. Borgesen, D. W. Henderson, E. J. Cotts, N. Dimitrov, J. Appl. Electrochem. 38 (2008) 1695. [16] J. W. Lim, M. Isshiki, J. Appl. Phys. 99 (2006) 094909. [17] M. J. Attardo, R. Rosenberg, J. Appl. Phys. 41 (1970) 2381. [18] K. W. Lee, T. Y. Park, J. S. Lee, J. W. Kim, J. T. Kim, N. J. Kwak, S. J. Yeom, H. T. Jeon, Jpn. J. Appl. Phys. 47 (2008) 5396

ACCEPTED MANUSCRIPT [19] J. Kwon, M. Saly, R. K. Kanjolia, and Y. J. Chabal, Chem. Mater. 23 (2011) 2068.

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[20] K. S. Gadre and T. L. Alford, J. Appl. Phys. 93 (2003) 15.

ACCEPTED MANUSCRIPT Table Captions Table 1. Process conditions and the four steps of the ALD cycle with pulse and purge times,

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plasma power, and flow rates.

ACCEPTED MANUSCRIPT Figure captions Fig. 1(a) Thickness of Co films deposited on TaNx/SiO2/Si substrate using ALD as a function

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150˚C as a function of the number of deposition cycles.

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of substrate temperature and (b) thickness and electrical resistivity of Co films deposited at

Fig. 2. AES depth profile of Co films deposited on TaNx/SiO2/Si substrate and (b) cross-

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sectional STEM image of Co film deposited on ~ 3 nm-thick TaNx covered substrate with 24-

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nm deep trenches.

Fig. 3. Resistivity of Co films deposited on TaNx substrate as a function of the thickness and

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the contact angle of the TaNx substrate, both with and without H2 plasma pretreatment.

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Fig. 4. Resistivity and surface roughness of ~ 5 nm-thick Co films and the atomic concentration of carbon in Co films according to H2 flow rate.

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Fig. 5. Cross-sectional STEM images of (a) Co film deposited on substrate with 24 nm-deep trenches at a H2 flow rate of 100 sccm during CCTBA feeding and reactant sequences for Cu direct plating, and (b) Cu line-filled using plating at a current density of 1 mA/cm2 for 240 s at room temperature.

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Base pressure

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Precursor feeding

Precursor purge

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Plasma power

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unit

Deposition temperature

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plasma power, and flow rates.

5/x/o 200

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10/N2/100

5/H2/ 50, 75, 100

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Modified process

Reactant purge

sec./x/sccm

Original process 150

Reactant feeding

4/H2/50 4/H2/ 50, 75, 100

5/N2/50

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ACCEPTED MANUSCRIPT Highlights  PEALD Co films for direct Cu plating were deposited using CCTBA precursor.

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 The H2 plasma treatment increased the nucleation density and wettibility of Co films.

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 The resistivity of Co films decreased with increasing the H2 gas flow rate.