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Atomic layer deposition of transition metals for silicide contact formation: Growth characteristics and silicidation
Microelectronic Engineering 106 (2013) 69–75
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Atomic layer deposition of transition metals for silicide contact formation: Growth characteristics and silicidation q Hyungjun Kim School of Electrical and Electronic Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 23 January 2013 Keywords: Atomic layer deposition Contact Cobalt Nickel Silicidation
a b s t r a c t The atomic layer deposition (ALD) is a promising thin film deposition technique in the fabrication of nanoscale semiconductors devices. In this paper, the results on the ALD of transition metals are reviewed for their applications as silicide contact of nanoscale semiconductor devices, especially focusing on the growth characteristics of ALD Co (and Ni) and comparison between plasma-enhanced ALD (PE-ALD) and thermal ALD (TH-ALD). For most of metal organic precursors, NH3 plasma is a good choice as a reactant to produce highly pure Co or Ni films, while H2 or N2 plasma does not produce high quality film. At optimal conditions, highly pure Co films were deposited with low resistivity down to 10 lX cm. Relatively good quality metal film formation by thermal ALD was possible by limited range of precursors including Co(iPr-AMD)2. Even for these precursors, the resistivity and other film properties were inferior to those of PE-ALD films. However, for PE-ALD using NH3 plasma, the conformality was not good enough for high aspect ratio nanoscale via structures, which necessitates the development of thermal ALD process. The formation of silicide by rapid thermal annealing of ALD Co thin films was also investigated showing different behavior for PE- and TH-ALD Co thin films. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Contact with low contact resistance between semiconductor and metal line is crucial in high performance nanoscale semiconductor devices. For this, selection of proper material is very important. Silicide has been standard contact material due to its good properties including low contact resistance, compatibility with Si device fabrication processes, and easy integration through SALICIDE process. However, with device scaling, narrow line effect became a big concern regarding to the increase in contact resistance. To relieve this problem, introduction of new materials is necessary more than ever. Recently, CoSi2 and NiSi have been studied and began to be implemented as an alternative contact material to TiSi2 [1]. Beside the introduction of new materials, however, device scaling into nanoscale regime sparked a great interest in novel thin film deposition process. The main concern regarding the metal deposition for silicide contact is mainly caused by the limited step coverage of physical vapor deposition (PVD) processes such as sputtering and evaporation. The poor conformality is especially problematic for the deposition of metal for contact in deep contact holes [2].
q Contains Material Reprinted with permission of the IEEE International Interconnect Technology Conference 2011. E-mail address: [email protected]
0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.01.016
As an alternative deposition technique, ALD is promising in nanoscale regime due to its excellent conformality and atomic thickness controllability. Thus, ALD has been studied in the formation of several essential components of modern semiconductor devices such as gate insulators, metal gates, dielectrics for capacitors, etc. [3]. Thus, we can expect that deposition of transition metals for the contact application would be expected to be implemented in nanoscale device. Even with the practical importance, however, the ALD of most transition metals have not been widely studied mainly due to the difficulty in the process development [3]. One of the biggest difficulties comes from the absence of proper precursor and reduction agents. Especially, ALD for Co and Ni thin films have been rarely reported [4,5]. We have carried out studies on the ALD of Co and Ni and the silicide formation of these thin films, some of which were reported previously [6–15]. In this paper, we summarize our efforts in producing high quality Co and Ni thin films by ALD for contact applications. Especially, we describe the comparative results between plasma-enhanced ALD (PE-ALD) and thermal ALD (TH-ALD) The film properties studied using various thin film analysis techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray reflectivity (XRR), Rutherford backscattering spectroscopy (RBS), and X-ray photoemission spectroscopy (XPS) are described. Also, the formation of silicide by rapid thermal annealing of ALD metal films is also discussed.
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Table 1 Process parameters for ALD of Co and Ni.
Co Co Co Co Ni Ni
Process (Equipment)
Precursor
Reactant
Typical growth temperature (°C)
Related figures
Refs.
PE-ALD (remote) PE-ALD (Quros Plus 150™) PE-ALD (Quros Plus 150™) TH-ALD (Quros Plus 150™) PE-ALD (remote) PE-ALD/TH-ALD (Quros Plus 150™)
CoCp(CO)2 or CoCp2 CoCp2 Co(iPr-AMD)2 Co(iPr-AMD)2 NiCp2 Ni(dmamb)2
NH3 plasma (200 sccm, 300 W) NH3 plasma (200 sccm, 300 W) NH3 plasma(400 sccm, 300 W) NH3, H2 (400 sccm) NH3 plasma (200 sccm, 300 W) NH3 plasma (200 sccm, 400 W), NH3
300 300 350 350 300 300
Figs. 2–5 Fig. 6 Fig. 7 Figs. 8 and 9 Fig. 10 Fig. 11
[6] [11] [7] [14,15]
(HR-TEM). The film resistivity was measured by 4-point probe. The microstructures of some films were analyzed by synchrotron radiation X-ray diffraction (SR-XRD, Pohang acceleration laboratory, 3C2 beam line). 3. Results and discussion 3.1. Plasma-enhanced ALD of Co thin films The initial experiments on the TH-ALD of Co thin films using commercially available Co metal organic (MO) precursor, CoCp(CO)2 was carried out [6]. To remove ligands to deposit pure Co thin films, molecular hydrogen and ammonia were tried as reducing agents. Fig. 2a shows the growth rate vs precursor exposure time and the change in resistivity for the H2 based process. As we can see, although films could be deposited using H2, the growth behavior is significantly deviated from the ideal ALD mode growth based on self-saturation. Moreover, the resistivity was too high for pure Co thin films. Chemical analysis has shown that the high resistivity is due to the large incorporation of carbon atoms, lead-
Fig. 1. The molecular structures of Co and Ni MO precursors used in this study.
2. Experimental procedures In the present study, a commercial 6 inch ALD chamber (Quros Plus 150™) with a loadlock chamber or home-made 8 inch remote plasma ALD system was used. The key process parameters are described in Table 1 and further more detailed processes can be found in our previous reports [6–15]. The Co and Ni precursors that we used in these experiments are shown in Fig. 1. Overall, each precursor was contained in a stainless steel or glass bubbler and the temperature was maintained at proper level to obtain suitable vapor pressure to ALD process. Ar was used as a carrier and purging gas. NH3 or H2 with or without plasma activation was used as a reactant. One ALD cycle was composed of four steps consisting of precursor exposure time (ts), purging (tp), reactant exposure time (tr), and purging. The Si(0 0 1) substrates were cleaned by dipping in BOE followed by DI water rinsing and N2 blowing, and the SiO2 substrates were cleaned by dipping in acetone, isopropyl alcohol, and DI water sequentially, followed by N2 blowing. For the post deposition annealing to form silicide film, Ti capping layer was sputtered on ALD Co or Ni. Post deposition annealing was carried out in N2 environment using a rapid thermal annealing (RTA) system at different annealing temperatures, typically, Ta = 600–900 °C. Field emission scanning electron microscopy (FE-SEM) was used for analyses of morphology and film thickness, and the chemical composition analysis was performed by XPS. Microstructure and interface, silicide formation were analyzed by high resolution transmission electron microscopy
Fig. 2. The growth rate and resistivity of TH-ALD Co using CoCp(CO)2 and H2 as a function of (a) precursor exposure time and (b) hydrogen flow rate.
H. Kim / Microelectronic Engineering 106 (2013) 69–75
Fig. 3. The thickness and resistivity of PE-ALD Co using CoCp(CO)2 and NH3 plasma on SiO2 as a function of ALD cycles.
Fig. 4. (a) XPS depth profile of PE-ALD Co using CoCp(CO)2 and NH3 plasma on Si(0 0 1) grown at 300 °C. (b) XPS N 1s core spectra for PE-ALD using CoCp(CO)2 and NH3 plasma on Si(0 0 1) grown at temperature from 150 to 300 °C.
ing to Co–C formation. To reduce the resistivity, the hydrogen flow rate was increased, but leading to only marginal decrease in resistivity, as shown in Fig. 2b. The poor results for thermal ALD of Co were attributed to the low reactivity of hydrogen molecules. Thus, we employed plasma of hydrogen to increase the reactivity, resulting in only small
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Fig. 5. RBS spectrum of PE-ALD Co using CoCp2 and NH3 plasma measured with nitrogen resonance mode.
reduction of reactivity;. Next, NH3 plasma, which is expected to have higher reducing power than H2 plasma, was employed. In contrast to TH-ALD using H2 with resistivity over 2000 lX cm, the resistivity of PE-ALD Co using ammonia plasma was as low as 20 lX cm on both of SiO2 and Si substrates without nucleation delay as Fig. 3 shows. The chemical composition of PE-ALD Co thin films deposited using NH3 plasma with low resistivity were analyzed by XPS. Fig. 4a shows the depth profile of the film deposited at Ts = 300 °C, showing that there is essentially no impurity in the films. It is rather surprising that there is almost no nitrogen incorporation in the PE-ALD Co thin films. Thus, the nitrogen incorporation was further investigated by XPS. Fig. 4b shows the N 1 s core peak spectra for the PE-ALD Co thin films grown at different growth temperatures from 150 to 300 °C. As the inset of Fig. 4b shows, while the nitrogen concentration in the film is as high as almost 30 at.% for Ts = 200 °C, almost no nitrogen was detected for the film grown at Ts = 300 °C. Thus, high enough growth temperature is essential to minimize the incorporation of nitrogen atoms with reduced resistivity. The use of NH3 plasma for pure metal ALD has been reported for other metal system; for example, pure Ru was deposited by PE-ALD using NH3 plasma [16]. While pure Co deposition is possible by PE-ALD using NH3 plasma, Fig. 2 shows that CoCp(CO)2 is not an appropriate precursor for ALD since self-saturated adsorption is not achieved. This is attributed to the molecular configuration of the precursor. Since carbonyl is removed with low thermal energy, CoCp(CO)2 is actually monovalence precursor, which may have difficulty in achieving self-saturated adsorption. Actually, initial experiments using Co2(CO)8 precursor showed that the deposition through chemical vapor deposition reaction due to the very easy ligand removal of carbonyl precursor, agreeing with previously reported low temperature CVD of Co using the same precursor [17]. Thus, we studied the PE-ALD of Co using divalent CoCp2 precursor and NH3 plasma. In contrast to the CoCp(CO)2 case, well saturation in the growth rates with increasing precursor exposure time was observed [6]. Also, lower value of resistivity down to 10 lX cm, which is close to the bulk value (6 lX cm), was achieved. The chemical composition analysis showed that the PE-ALD Co using CoCp2 is very pure with low impurity level. Fig. 5 is the nitrogen enhanced RBS spectrum showing that the nitrogen incorporation could be minimized at suitable growth conditions similar to PE-ALD Co using CoCp(CO)2 precursor. 3.2. Conformality of PE-ALD and thermal ALD of Co thin films Although PE-ALD based on NH3 plasma produce high quality Co films, the conformality of the film inside nanoscale vias was found
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contact. However, as described above, TH-ALD of Co using most of the commercially available precursors do not produce good quality Co films due to the difficulty in clean removal of ligands. Meanwhile, the deposition of relatively good quality metal thin films using lab-synthesized acetamidinate precursors was reported [5]. Thus, we investigate the PE-ALD and TH-ALD of Co using Co(iPrAMD)2. As for PE-ALD using Co(iPr-AMD)2, low resistivity Co thin films were deposited by NH3 plasma, similar to other MO precursors [10]. The SEM images of deposited films on Si oxide and Si(0 0 1) substrates are shown in Fig. 7. In contrast to previously mentioned precursors, however, Co thin films with relatively good property were also deposited by TH-ALD without plasma. For both H2 and NH3 as reactants, good saturation in growth rate was observed at exposure time longer than 1 s at 350 °C of substrate temperature [7]. The saturated growth rate is higher for H2, but with higher resistivity. The resistivity of TH-ALD Co using NH3 as reactant was 50 lX cm. Then, the comparative study on the film properties of TH-ALD Co were carried out using XPS and X-ray reflectivity. Chemical compositional analysis using XPS has shown that the carbon as well as nitrogen concentration in the films are low for both cases, while the oxygen concentration is significantly higher in H2 based TH-ALD Co. (Fig. 8a). Since the depositions were carried out under reducing environment and there is no oxygen atom in either precursor or reactant, the oxygen contamination in the film should be due to the post deposition oxidation. Thus, we can infer that the H2 based TH-ALD Co thin film has lower density than NH3 based TH-ALD Co thin film. Indeed, the density measurement from the critical angle of XRR as shown in Fig. 8b has shown that the density of NH3 based TH-ALD Co has significantly higher film density than H2 based TH-ALD Co. NH3 molecules effectively cleave Co–N bonds of the Co(iPr-AMD)2, forming pure Co film. The conformality of TH-ALD Co was estimated by depositing
Fig. 6. SEM images of PE-ALD Co using CoCp2 and NH3 plasma on (a) 1:1 trench and (b) 5:1 via.
to be limited. Fig. 6a shows the SEM image of PE-ALD Co on trench structure with about 300 nm with and aspect ratio (AR) = 1:1. The conformality of the PE-ALD Co using NH3 is about 80%, which is not very poor. However, for via structure with smaller size and higher AR, the conformality is significantly degraded. For example, the first figure in Fig. 6b is the PE-ALD Co deposited at the same condition with Fig. 6a on 100 nm diameter via with AR = 5:1. The film was deposited only at the top portion of the vias. According to a previous report by Gordon et al., the conformality is largely dependent on the precursor exposure time [18]. Based on their model, high amount of exposure is required for the deposition on the sidewall and bottom of the nanoscale vias. Calculation based on their model suggests that the required precursor exposure time for top, sidewall, and bottom is ttop:tbottom:twall = 1:4.8:57.5 for the current via structure and CoCp2 precursor. Thus, we have increased the precursor exposure time up to 20 s from the initial 2 s condition. Fig 6b shows the SEM images of PE-ALD Co with increasing precursor exposure time. Although the conformality was enhanced by the increase in precursor exposure, the conformality of the PEALD Co was turned out to be unacceptably poor. This is due to the inherent nature of plasma as a reactant, which is due to the recombination of radicals [19–21]. The limited step coverage of PE-ALD Co indicates that we need to develop TH-ALD Co process for the applications of nanoscale via
Fig. 7. SEM images of PE-ALD Co using Co(iPr-AMD)2 as a precursor and NH3 plasma.
H. Kim / Microelectronic Engineering 106 (2013) 69–75
Fig. 8. (a) Chemical compositions and (b) XRR spectra of TH-ALD Co deposited using Co(iPr-AMD)2 as a precursor and NH3 or H2 as a reactant.
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Fig. 10. (a) XPS Ni 2p core spectra and (b) SEM image of Ni PE-ALD using NiCp2 and NH3 plasma on Si(0 0 1).
3.3. Atomic layer deposition of Ni Compared to Co, the ALD of Ni thin film is more difficult due to the lack of appropriate precursors. Initially, we began to explore the possibility of using NH3 plasma as a reactant with commercially available MO precursor. The first precursor that we investigated was NiCp2, which is analogous to CoCp2. Similar to Co, low resistivity Ni film was deposited using NiCp2 with NH3 plasma. The XPS and SEM results are shown in Fig. 10. The XPS spectrum shows clean Ni 2p core peak. Also, SEM image shows similar morphology to PE-ALD Co thin films. Thus, we can infer that NH3 plasma is a good reactant for Ni as well as Co PE-ALD. However,
Fig. 9. SEM images of TH-ALD Co Co(iPr-AMD)2 as a precursor and NH3 on 5:1 via.
the films inside nanosize contact holes. Fig. 9 shows the SEM image of hole patterns with 5:1 aspect ratio showing almost 100% step coverage. Good conformality was observed for both NH3 and H2 based TH-ALD of Co.
Fig. 11. The growth rate of TH-ALD and PE-ALD Ni using Ni(dmamb)2 and NH3 or NH3 plasma as a function of precursor exposure time.
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Fig. 12. XRD spectra of PVD Ti (20 nm)/Co (20 nm)/Si(0 0 1) annealed at 900 °C. Co was grown by PVD, TH-ALD and PE-ALD as noted in the figure.
it should be noted that TH-ALD of Ni was not possible by using NiCp2. For thermal ALD of Ni thin films, laboratory synthesized Ni MO precursor, Ni(dmamb)2, was used for PE- and T-ALD [14,15]. Both PE- and TH-ALD produced Ni films on Si and SiO2 substrates. Fig. 11 shows the growth rates vs exposure time of PE- and THALD of Ni films showing almost four times higher growth rate for PE-ALD. PE-ALD Ni film has columnar structure while TH-ALD Ni film has granular structure. Also, the roughness was larger for TH-ALD, probably due to the poorer nucleation behavior compared to PE-ALD. However, the resistivity of TH-ALD Ni was as low as 25 lX cm at Ts = 300 °C with very good conformality close to 100% for sub 100 nm via.
was capped with 20 nm thick sputtered Ti and RTA was carried out at various RTA temperatures (Ta). For PVD Co, CoSi2 peaks began to appear at above Ta = 700 °C agreeing with previous reports [22,23]. For TH-ALD Co, similar to PVD Co, silicide diffraction peaks were observed at above Ta = 700 °C. However, the silicide peaks were observed only at above Ta = 800 °C for PE-ALD Co. The formation of other silicide phases such as CoSi or Co2Si was not observed at lower annealing temperature. Also, the observed diffraction peaks and their intensity are different each other. Fig. 12 shows the XRD diffraction spectra of PVD, TH-ALD, and PE-ALD of Co thin films with Ti capping layer annealed at Ta = 900 °C . While strong (2 2 0) and weak (1 1 1) peaks are observed for PVD Co, strong (2 0 0) peak is observed for TH-ALD Co. Meanwhile, only one diffraction peak is observed at h2 = 33.5° for annealed PE-ALD Co. This difference of PE-ALD Co during silicide formation indicates the formation of epitaxial silicide layer. XRD U scan analysis on the thermally annealed samples have shown the formation of epitaxial CoSi2 layer on Si(0 0 1) with a cube-on-cube relationship, CoSi2[1 0 0]kSi[1 0 0]. During the PE-ALD Co on Si, due to the exposure of Si substrate to NH3 plasma, thin SilNx interlayer is formed between PE-ALD Co and Si substrate [9]. The SiNx interlayer contributes to the epitaxial silicide by controlling the diffusion of Co to Si substrate. This process is commonly known as intermediated layer epitaxy (IME) [24]. HR-TEM images for PE-ALD Co annealed at
3.4. Co silicide formation by RTA of ALD Co The silicide formation by annealing of TH- and PE-ALD Co was comparatively studied with sputtered Co. For the silicidation, Co
Fig. 13. HR-XTEM images of PVD Ti (20 nm)/Co (20 nm)/Si(0 0 1) annealed at 800 °C XRD and schematic drawing showing the interlayer mediated epitaxy of silicide.
Fig. 14. HR-XTEM images of PE-ALD Co films using CoCp2 grown on Si(0 0 1) with different NH3 plasma exposure time (tr = 2 and 6 s).
H. Kim / Microelectronic Engineering 106 (2013) 69–75
800 °C are shown in Fig. 13 with a schematic drawing showing the IME process. The thickness of interlayer was found to be dependent on NH3 plasma exposure time. Fig. 14 shows the HR-TEM images of PE-ALD Co prepared at different plasma exposure time; about 1 nm thick interlayer for plasma exposure time (tr) of 2 s and 3 nm thick interlayer for plasma exposure time of 6 s. Thus, we infer that silicide formation can be controlled by changing the ALD process.
4. Summary Various PE- and T-ALD processes for Co and Ni thin films were developed based on NH3 and its plasma as reactants for nanoscale contact applications. NH3 plasma based PE-ALD was shown to be versatile process for deposition of Co and Ni with low resistivity. However, PE-ALD process suffered from limited conformality in nanoscale via holes. By using proper selection of MO precursor, good quality Co and Ni films with excellent conformality was achieved. Depending on the growth conditions, silicidation process has shown different aspect compared to conventional PVD metal layers. ALD of transition metal is expected to be a viable process for the formation of nanoscale contact in near future device fabrication.
Acknowledgments
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
[15]
[16] [17] [18] [19] [20] [21]
This work was supported by the Technology Innovation Program Industrial Strategic Technology Development Program, 10035430, Development of reliable fine-pitch metallization technologies funded by the Ministry of Knowledge Economy MKE, Korea. Ni(dmamb)2 precursor was provided by KRICT, Korea.
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[22] [23] [24]
S.L. Zhang, M. Ostling, Crit. Rev. Solid State 28 (2003) 1. D. Ha et al., IEEE Trans. Electron Devices 47 (2000) 1499. H. Kim, J. Vac, Sci. Technol. B 21 (2003) 2231. M. Knez, K. Nielsch, L. Niinistö, Adv. Mater. 19 (2007) 3425. B.S. Lim, A. Rahtu, R.G. Gordon, Nat. Mater. 2 (2003) 749. Han-Bo-Ram Lee, H. Kim, Electrochem. Solid State Lett. 9 (2006) G323. Han-Bo-Ram Lee, Woo-Hee Kim, J. Electrochem. Soc. 157 (2010) D10. Jae-Min Kim, Han-Bo-Ram Lee, Clement Lansalot, Christian Dussarrat, Julien Gatineau, Hyungjun Kim, Jap. J. Appl. Phys. 49 (2010) 05FA10. Han-Bo-Ram Lee, J.Y. Son, H. Kim, Appl. Phys. Lett. 90 (2007) 123509. Han-Bo-Ram Lee, Jaemin Kim, Hyungjun Kim, Woo-Hee Kim, J. Kor. Phys. Soc. 56 (2010) 104. Han-Bo-Ram Lee, Gil Ho Gu, J.Y. Son, C.G. Park, H. Kim, J. Appl. Phys. 102 (2007) 094509. Han-Bo-Ram Lee, Hyungjun Kim, J. Cryst. Growth 312 (2010) 2215. Han-Bo-Ram Lee, Gil Ho Gu, J.Y. Son, C.G. Park, Hyungjun Kim, Small 4 (2008) 2247. Han-Bo-Ram Lee, Sung-Hwan Bang, Woo-Hee Kim, Gil Ho Gu, Young Kuk Lee, Taek-Mo Chung, Chang Gyoun Kim, C.G. Park, Hyungjun Kim, Jap. J. Appl. Phys. 49 (2010) 05FA11. Woo-Hee Kim, Han-Bo-Ram Lee, Kwang Heo, Young Kuk Lee, Taek-Mo Chung, Chang Gyoun Kim, Seunghun Hong, Jong Heo, Hyungjun Kim, J. Electrochem. Soc. 158 (2011) D1. Sang-Joon Park, Woo-Hee Kim, Han-Bo Ram Lee, W.J. Maeng, H. Kim, Microelectron. Eng. 85 (2008) 39. D.-X. Ye, S. Pimanpang, C. Jezewski, F. Tang, J.J. Senkevich, G.-C. Wang, T.-M. Lu, Thin Solid Films 485 (2005) 95. R.G. Gordon, D. Hausmann, E. Kim, J. Shepard, Chem. Vap. Deposition 9 (2003) 79. Hyungjun. Kim, Thin Solid Films 519 (2011) 6639. J. Dendooven, D. Deduytsche, J. Musschoot, R.L. Vanmeirhaeghe, C. Detavernier, J. Electrochem. Soc. 157 (2010) G111. E. Langereis, H.C.M. Knoops, A.J.M. Mackus, F. Roozeboom, M.C.M. van de Sanden, W.M.M. Kessels, J. Appl. Phys. 102 (2007) 083517. C. Detavernier, R.L. Van Meirhaeghe, W. Vandervorst, Microelectron. Eng. 71 (2004) 252. K. Maex, Mater. Sci. Eng. Rep. 11 (1993) 7. K.K. Chong, M. Yeadon, W.K. Choi, E.A. Stach, C.B. Boothroyd, Appl. Phys. Lett. 82 (2003) 1833.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Atomic layer deposition of transition metals for silicide contact formation: Growth characteristics and silicidation q Hyungjun Kim School of Electrical and Electronic Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 23 January 2013 Keywords: Atomic layer deposition Contact Cobalt Nickel Silicidation
a b s t r a c t The atomic layer deposition (ALD) is a promising thin film deposition technique in the fabrication of nanoscale semiconductors devices. In this paper, the results on the ALD of transition metals are reviewed for their applications as silicide contact of nanoscale semiconductor devices, especially focusing on the growth characteristics of ALD Co (and Ni) and comparison between plasma-enhanced ALD (PE-ALD) and thermal ALD (TH-ALD). For most of metal organic precursors, NH3 plasma is a good choice as a reactant to produce highly pure Co or Ni films, while H2 or N2 plasma does not produce high quality film. At optimal conditions, highly pure Co films were deposited with low resistivity down to 10 lX cm. Relatively good quality metal film formation by thermal ALD was possible by limited range of precursors including Co(iPr-AMD)2. Even for these precursors, the resistivity and other film properties were inferior to those of PE-ALD films. However, for PE-ALD using NH3 plasma, the conformality was not good enough for high aspect ratio nanoscale via structures, which necessitates the development of thermal ALD process. The formation of silicide by rapid thermal annealing of ALD Co thin films was also investigated showing different behavior for PE- and TH-ALD Co thin films. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Contact with low contact resistance between semiconductor and metal line is crucial in high performance nanoscale semiconductor devices. For this, selection of proper material is very important. Silicide has been standard contact material due to its good properties including low contact resistance, compatibility with Si device fabrication processes, and easy integration through SALICIDE process. However, with device scaling, narrow line effect became a big concern regarding to the increase in contact resistance. To relieve this problem, introduction of new materials is necessary more than ever. Recently, CoSi2 and NiSi have been studied and began to be implemented as an alternative contact material to TiSi2 [1]. Beside the introduction of new materials, however, device scaling into nanoscale regime sparked a great interest in novel thin film deposition process. The main concern regarding the metal deposition for silicide contact is mainly caused by the limited step coverage of physical vapor deposition (PVD) processes such as sputtering and evaporation. The poor conformality is especially problematic for the deposition of metal for contact in deep contact holes [2].
q Contains Material Reprinted with permission of the IEEE International Interconnect Technology Conference 2011. E-mail address: [email protected]
0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.01.016
As an alternative deposition technique, ALD is promising in nanoscale regime due to its excellent conformality and atomic thickness controllability. Thus, ALD has been studied in the formation of several essential components of modern semiconductor devices such as gate insulators, metal gates, dielectrics for capacitors, etc. [3]. Thus, we can expect that deposition of transition metals for the contact application would be expected to be implemented in nanoscale device. Even with the practical importance, however, the ALD of most transition metals have not been widely studied mainly due to the difficulty in the process development [3]. One of the biggest difficulties comes from the absence of proper precursor and reduction agents. Especially, ALD for Co and Ni thin films have been rarely reported [4,5]. We have carried out studies on the ALD of Co and Ni and the silicide formation of these thin films, some of which were reported previously [6–15]. In this paper, we summarize our efforts in producing high quality Co and Ni thin films by ALD for contact applications. Especially, we describe the comparative results between plasma-enhanced ALD (PE-ALD) and thermal ALD (TH-ALD) The film properties studied using various thin film analysis techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray reflectivity (XRR), Rutherford backscattering spectroscopy (RBS), and X-ray photoemission spectroscopy (XPS) are described. Also, the formation of silicide by rapid thermal annealing of ALD metal films is also discussed.
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H. Kim / Microelectronic Engineering 106 (2013) 69–75
Table 1 Process parameters for ALD of Co and Ni.
Co Co Co Co Ni Ni
Process (Equipment)
Precursor
Reactant
Typical growth temperature (°C)
Related figures
Refs.
PE-ALD (remote) PE-ALD (Quros Plus 150™) PE-ALD (Quros Plus 150™) TH-ALD (Quros Plus 150™) PE-ALD (remote) PE-ALD/TH-ALD (Quros Plus 150™)
CoCp(CO)2 or CoCp2 CoCp2 Co(iPr-AMD)2 Co(iPr-AMD)2 NiCp2 Ni(dmamb)2
NH3 plasma (200 sccm, 300 W) NH3 plasma (200 sccm, 300 W) NH3 plasma(400 sccm, 300 W) NH3, H2 (400 sccm) NH3 plasma (200 sccm, 300 W) NH3 plasma (200 sccm, 400 W), NH3
300 300 350 350 300 300
Figs. 2–5 Fig. 6 Fig. 7 Figs. 8 and 9 Fig. 10 Fig. 11
[6] [11] [7] [14,15]
(HR-TEM). The film resistivity was measured by 4-point probe. The microstructures of some films were analyzed by synchrotron radiation X-ray diffraction (SR-XRD, Pohang acceleration laboratory, 3C2 beam line). 3. Results and discussion 3.1. Plasma-enhanced ALD of Co thin films The initial experiments on the TH-ALD of Co thin films using commercially available Co metal organic (MO) precursor, CoCp(CO)2 was carried out [6]. To remove ligands to deposit pure Co thin films, molecular hydrogen and ammonia were tried as reducing agents. Fig. 2a shows the growth rate vs precursor exposure time and the change in resistivity for the H2 based process. As we can see, although films could be deposited using H2, the growth behavior is significantly deviated from the ideal ALD mode growth based on self-saturation. Moreover, the resistivity was too high for pure Co thin films. Chemical analysis has shown that the high resistivity is due to the large incorporation of carbon atoms, lead-
Fig. 1. The molecular structures of Co and Ni MO precursors used in this study.
2. Experimental procedures In the present study, a commercial 6 inch ALD chamber (Quros Plus 150™) with a loadlock chamber or home-made 8 inch remote plasma ALD system was used. The key process parameters are described in Table 1 and further more detailed processes can be found in our previous reports [6–15]. The Co and Ni precursors that we used in these experiments are shown in Fig. 1. Overall, each precursor was contained in a stainless steel or glass bubbler and the temperature was maintained at proper level to obtain suitable vapor pressure to ALD process. Ar was used as a carrier and purging gas. NH3 or H2 with or without plasma activation was used as a reactant. One ALD cycle was composed of four steps consisting of precursor exposure time (ts), purging (tp), reactant exposure time (tr), and purging. The Si(0 0 1) substrates were cleaned by dipping in BOE followed by DI water rinsing and N2 blowing, and the SiO2 substrates were cleaned by dipping in acetone, isopropyl alcohol, and DI water sequentially, followed by N2 blowing. For the post deposition annealing to form silicide film, Ti capping layer was sputtered on ALD Co or Ni. Post deposition annealing was carried out in N2 environment using a rapid thermal annealing (RTA) system at different annealing temperatures, typically, Ta = 600–900 °C. Field emission scanning electron microscopy (FE-SEM) was used for analyses of morphology and film thickness, and the chemical composition analysis was performed by XPS. Microstructure and interface, silicide formation were analyzed by high resolution transmission electron microscopy
Fig. 2. The growth rate and resistivity of TH-ALD Co using CoCp(CO)2 and H2 as a function of (a) precursor exposure time and (b) hydrogen flow rate.
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Fig. 3. The thickness and resistivity of PE-ALD Co using CoCp(CO)2 and NH3 plasma on SiO2 as a function of ALD cycles.
Fig. 4. (a) XPS depth profile of PE-ALD Co using CoCp(CO)2 and NH3 plasma on Si(0 0 1) grown at 300 °C. (b) XPS N 1s core spectra for PE-ALD using CoCp(CO)2 and NH3 plasma on Si(0 0 1) grown at temperature from 150 to 300 °C.
ing to Co–C formation. To reduce the resistivity, the hydrogen flow rate was increased, but leading to only marginal decrease in resistivity, as shown in Fig. 2b. The poor results for thermal ALD of Co were attributed to the low reactivity of hydrogen molecules. Thus, we employed plasma of hydrogen to increase the reactivity, resulting in only small
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Fig. 5. RBS spectrum of PE-ALD Co using CoCp2 and NH3 plasma measured with nitrogen resonance mode.
reduction of reactivity;. Next, NH3 plasma, which is expected to have higher reducing power than H2 plasma, was employed. In contrast to TH-ALD using H2 with resistivity over 2000 lX cm, the resistivity of PE-ALD Co using ammonia plasma was as low as 20 lX cm on both of SiO2 and Si substrates without nucleation delay as Fig. 3 shows. The chemical composition of PE-ALD Co thin films deposited using NH3 plasma with low resistivity were analyzed by XPS. Fig. 4a shows the depth profile of the film deposited at Ts = 300 °C, showing that there is essentially no impurity in the films. It is rather surprising that there is almost no nitrogen incorporation in the PE-ALD Co thin films. Thus, the nitrogen incorporation was further investigated by XPS. Fig. 4b shows the N 1 s core peak spectra for the PE-ALD Co thin films grown at different growth temperatures from 150 to 300 °C. As the inset of Fig. 4b shows, while the nitrogen concentration in the film is as high as almost 30 at.% for Ts = 200 °C, almost no nitrogen was detected for the film grown at Ts = 300 °C. Thus, high enough growth temperature is essential to minimize the incorporation of nitrogen atoms with reduced resistivity. The use of NH3 plasma for pure metal ALD has been reported for other metal system; for example, pure Ru was deposited by PE-ALD using NH3 plasma [16]. While pure Co deposition is possible by PE-ALD using NH3 plasma, Fig. 2 shows that CoCp(CO)2 is not an appropriate precursor for ALD since self-saturated adsorption is not achieved. This is attributed to the molecular configuration of the precursor. Since carbonyl is removed with low thermal energy, CoCp(CO)2 is actually monovalence precursor, which may have difficulty in achieving self-saturated adsorption. Actually, initial experiments using Co2(CO)8 precursor showed that the deposition through chemical vapor deposition reaction due to the very easy ligand removal of carbonyl precursor, agreeing with previously reported low temperature CVD of Co using the same precursor [17]. Thus, we studied the PE-ALD of Co using divalent CoCp2 precursor and NH3 plasma. In contrast to the CoCp(CO)2 case, well saturation in the growth rates with increasing precursor exposure time was observed [6]. Also, lower value of resistivity down to 10 lX cm, which is close to the bulk value (6 lX cm), was achieved. The chemical composition analysis showed that the PE-ALD Co using CoCp2 is very pure with low impurity level. Fig. 5 is the nitrogen enhanced RBS spectrum showing that the nitrogen incorporation could be minimized at suitable growth conditions similar to PE-ALD Co using CoCp(CO)2 precursor. 3.2. Conformality of PE-ALD and thermal ALD of Co thin films Although PE-ALD based on NH3 plasma produce high quality Co films, the conformality of the film inside nanoscale vias was found
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contact. However, as described above, TH-ALD of Co using most of the commercially available precursors do not produce good quality Co films due to the difficulty in clean removal of ligands. Meanwhile, the deposition of relatively good quality metal thin films using lab-synthesized acetamidinate precursors was reported [5]. Thus, we investigate the PE-ALD and TH-ALD of Co using Co(iPrAMD)2. As for PE-ALD using Co(iPr-AMD)2, low resistivity Co thin films were deposited by NH3 plasma, similar to other MO precursors [10]. The SEM images of deposited films on Si oxide and Si(0 0 1) substrates are shown in Fig. 7. In contrast to previously mentioned precursors, however, Co thin films with relatively good property were also deposited by TH-ALD without plasma. For both H2 and NH3 as reactants, good saturation in growth rate was observed at exposure time longer than 1 s at 350 °C of substrate temperature [7]. The saturated growth rate is higher for H2, but with higher resistivity. The resistivity of TH-ALD Co using NH3 as reactant was 50 lX cm. Then, the comparative study on the film properties of TH-ALD Co were carried out using XPS and X-ray reflectivity. Chemical compositional analysis using XPS has shown that the carbon as well as nitrogen concentration in the films are low for both cases, while the oxygen concentration is significantly higher in H2 based TH-ALD Co. (Fig. 8a). Since the depositions were carried out under reducing environment and there is no oxygen atom in either precursor or reactant, the oxygen contamination in the film should be due to the post deposition oxidation. Thus, we can infer that the H2 based TH-ALD Co thin film has lower density than NH3 based TH-ALD Co thin film. Indeed, the density measurement from the critical angle of XRR as shown in Fig. 8b has shown that the density of NH3 based TH-ALD Co has significantly higher film density than H2 based TH-ALD Co. NH3 molecules effectively cleave Co–N bonds of the Co(iPr-AMD)2, forming pure Co film. The conformality of TH-ALD Co was estimated by depositing
Fig. 6. SEM images of PE-ALD Co using CoCp2 and NH3 plasma on (a) 1:1 trench and (b) 5:1 via.
to be limited. Fig. 6a shows the SEM image of PE-ALD Co on trench structure with about 300 nm with and aspect ratio (AR) = 1:1. The conformality of the PE-ALD Co using NH3 is about 80%, which is not very poor. However, for via structure with smaller size and higher AR, the conformality is significantly degraded. For example, the first figure in Fig. 6b is the PE-ALD Co deposited at the same condition with Fig. 6a on 100 nm diameter via with AR = 5:1. The film was deposited only at the top portion of the vias. According to a previous report by Gordon et al., the conformality is largely dependent on the precursor exposure time [18]. Based on their model, high amount of exposure is required for the deposition on the sidewall and bottom of the nanoscale vias. Calculation based on their model suggests that the required precursor exposure time for top, sidewall, and bottom is ttop:tbottom:twall = 1:4.8:57.5 for the current via structure and CoCp2 precursor. Thus, we have increased the precursor exposure time up to 20 s from the initial 2 s condition. Fig 6b shows the SEM images of PE-ALD Co with increasing precursor exposure time. Although the conformality was enhanced by the increase in precursor exposure, the conformality of the PEALD Co was turned out to be unacceptably poor. This is due to the inherent nature of plasma as a reactant, which is due to the recombination of radicals [19–21]. The limited step coverage of PE-ALD Co indicates that we need to develop TH-ALD Co process for the applications of nanoscale via
Fig. 7. SEM images of PE-ALD Co using Co(iPr-AMD)2 as a precursor and NH3 plasma.
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Fig. 8. (a) Chemical compositions and (b) XRR spectra of TH-ALD Co deposited using Co(iPr-AMD)2 as a precursor and NH3 or H2 as a reactant.
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Fig. 10. (a) XPS Ni 2p core spectra and (b) SEM image of Ni PE-ALD using NiCp2 and NH3 plasma on Si(0 0 1).
3.3. Atomic layer deposition of Ni Compared to Co, the ALD of Ni thin film is more difficult due to the lack of appropriate precursors. Initially, we began to explore the possibility of using NH3 plasma as a reactant with commercially available MO precursor. The first precursor that we investigated was NiCp2, which is analogous to CoCp2. Similar to Co, low resistivity Ni film was deposited using NiCp2 with NH3 plasma. The XPS and SEM results are shown in Fig. 10. The XPS spectrum shows clean Ni 2p core peak. Also, SEM image shows similar morphology to PE-ALD Co thin films. Thus, we can infer that NH3 plasma is a good reactant for Ni as well as Co PE-ALD. However,
Fig. 9. SEM images of TH-ALD Co Co(iPr-AMD)2 as a precursor and NH3 on 5:1 via.
the films inside nanosize contact holes. Fig. 9 shows the SEM image of hole patterns with 5:1 aspect ratio showing almost 100% step coverage. Good conformality was observed for both NH3 and H2 based TH-ALD of Co.
Fig. 11. The growth rate of TH-ALD and PE-ALD Ni using Ni(dmamb)2 and NH3 or NH3 plasma as a function of precursor exposure time.
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Fig. 12. XRD spectra of PVD Ti (20 nm)/Co (20 nm)/Si(0 0 1) annealed at 900 °C. Co was grown by PVD, TH-ALD and PE-ALD as noted in the figure.
it should be noted that TH-ALD of Ni was not possible by using NiCp2. For thermal ALD of Ni thin films, laboratory synthesized Ni MO precursor, Ni(dmamb)2, was used for PE- and T-ALD [14,15]. Both PE- and TH-ALD produced Ni films on Si and SiO2 substrates. Fig. 11 shows the growth rates vs exposure time of PE- and THALD of Ni films showing almost four times higher growth rate for PE-ALD. PE-ALD Ni film has columnar structure while TH-ALD Ni film has granular structure. Also, the roughness was larger for TH-ALD, probably due to the poorer nucleation behavior compared to PE-ALD. However, the resistivity of TH-ALD Ni was as low as 25 lX cm at Ts = 300 °C with very good conformality close to 100% for sub 100 nm via.
was capped with 20 nm thick sputtered Ti and RTA was carried out at various RTA temperatures (Ta). For PVD Co, CoSi2 peaks began to appear at above Ta = 700 °C agreeing with previous reports [22,23]. For TH-ALD Co, similar to PVD Co, silicide diffraction peaks were observed at above Ta = 700 °C. However, the silicide peaks were observed only at above Ta = 800 °C for PE-ALD Co. The formation of other silicide phases such as CoSi or Co2Si was not observed at lower annealing temperature. Also, the observed diffraction peaks and their intensity are different each other. Fig. 12 shows the XRD diffraction spectra of PVD, TH-ALD, and PE-ALD of Co thin films with Ti capping layer annealed at Ta = 900 °C . While strong (2 2 0) and weak (1 1 1) peaks are observed for PVD Co, strong (2 0 0) peak is observed for TH-ALD Co. Meanwhile, only one diffraction peak is observed at h2 = 33.5° for annealed PE-ALD Co. This difference of PE-ALD Co during silicide formation indicates the formation of epitaxial silicide layer. XRD U scan analysis on the thermally annealed samples have shown the formation of epitaxial CoSi2 layer on Si(0 0 1) with a cube-on-cube relationship, CoSi2[1 0 0]kSi[1 0 0]. During the PE-ALD Co on Si, due to the exposure of Si substrate to NH3 plasma, thin SilNx interlayer is formed between PE-ALD Co and Si substrate [9]. The SiNx interlayer contributes to the epitaxial silicide by controlling the diffusion of Co to Si substrate. This process is commonly known as intermediated layer epitaxy (IME) [24]. HR-TEM images for PE-ALD Co annealed at
3.4. Co silicide formation by RTA of ALD Co The silicide formation by annealing of TH- and PE-ALD Co was comparatively studied with sputtered Co. For the silicidation, Co
Fig. 13. HR-XTEM images of PVD Ti (20 nm)/Co (20 nm)/Si(0 0 1) annealed at 800 °C XRD and schematic drawing showing the interlayer mediated epitaxy of silicide.
Fig. 14. HR-XTEM images of PE-ALD Co films using CoCp2 grown on Si(0 0 1) with different NH3 plasma exposure time (tr = 2 and 6 s).
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800 °C are shown in Fig. 13 with a schematic drawing showing the IME process. The thickness of interlayer was found to be dependent on NH3 plasma exposure time. Fig. 14 shows the HR-TEM images of PE-ALD Co prepared at different plasma exposure time; about 1 nm thick interlayer for plasma exposure time (tr) of 2 s and 3 nm thick interlayer for plasma exposure time of 6 s. Thus, we infer that silicide formation can be controlled by changing the ALD process.
4. Summary Various PE- and T-ALD processes for Co and Ni thin films were developed based on NH3 and its plasma as reactants for nanoscale contact applications. NH3 plasma based PE-ALD was shown to be versatile process for deposition of Co and Ni with low resistivity. However, PE-ALD process suffered from limited conformality in nanoscale via holes. By using proper selection of MO precursor, good quality Co and Ni films with excellent conformality was achieved. Depending on the growth conditions, silicidation process has shown different aspect compared to conventional PVD metal layers. ALD of transition metal is expected to be a viable process for the formation of nanoscale contact in near future device fabrication.
Acknowledgments
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
[15]
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This work was supported by the Technology Innovation Program Industrial Strategic Technology Development Program, 10035430, Development of reliable fine-pitch metallization technologies funded by the Ministry of Knowledge Economy MKE, Korea. Ni(dmamb)2 precursor was provided by KRICT, Korea.
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