Diffusion barrier properties of sputtered TaNx between Cu and Si using TaN as the target

Materials Chemistry and Physics 80 (2003) 690–695

Diffusion barrier properties of sputtered TaNx between Cu and Si using TaN as the target Yu-Lin Kuo a , Jui-Jen Huang a , Shun-Tang Lin a , Chiapyng Lee a,∗ , Wen-Horng Lee b a

Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10672, Taiwan, ROC b Department of Chemical Engineering, Lee-Ming Institute of Technology, Taishan, Taipei 243, Taiwan, ROC Received 18 October 2002; received in revised form 19 December 2002; accepted 30 December 2002

Abstract TaNx films sputtered from a TaN target were used as diffusion barriers between Cu thin films and Si substrates. Material characteristics of TaNx films and metallurgical reactions of Cu/TaNx /Si systems annealed in the temperature range 400–900 ◦ C for 60 min were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, cross-sectional transmission electron microscopy, and sheet resistance measurements. We found that the deposition rate decreased with increasing bias. TaN, ␤-Ta, and Ta2 N phases appeared and/or coexisted in the films at specific biases. A step change in N/Ta ratio was observed whenever a bias was applied to the substrate. After depositing a copper overlayer, we observed that the variation percentage of sheet resistance for Cu (70 nm)/TaNx (25 nm, x = 0.37 and 0.81)/Si systems stayed at a constant value after annealing up to 700 ◦ C for 60 min; however, the sheet resistance increased dramatically after annealing above 700 and 800 ◦ C for Cu/TaN0.37 /Si and Cu/TaN0.81 /Si systems, respectively. At that point, the interface was seriously deteriorated and formation of Cu3 Si was also observed. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Tantalum nitride; Sputtering; Bias; Diffusion barrier

1. Introduction Owing to their high stability and excellent conductivity, refractory metal nitrides are widely recognized as an attractive class of materials which can be used as diffusion barriers in metal–semiconductor contacts [1]. Among those refractory metal nitrides, tantalum nitride (TaN) has received extensive interest as a thin film diffusion barrier between silicon and metal overlayers of Ni [2], Al [3], and most recently, Cu [4,5]. Because tantalum nitride has a defective structure [6], and deviations from stoichiometry are common, the properties of TaN thin films are extremely sensitive to the film’s microstructure and growth morphology as well as deviations from stoichiometry. TaN has been prepared as thin films by sputtering tantalum in a mixture of nitrogen and argon [5,7–9]. According to previous literatures [5,7–9], the chemical composition, microstructure, and hence, properties of TaN films depend heavily on the deposition parameter. However, no study has ever been done by using TaN as the target to deposit TaNx films. ∗ Corresponding author. Tel.: +886-2-2737-6623; fax: +886-2-2737-6644. E-mail address: [email protected] (C. Lee).

In this work, TaNx films are deposited by rf sputtering from a TaN target. The variation of deposition rate, film chemical composition, and crystalline microstructure as a function of bias is studied. In addition, the thermal stability of TaNx barrier layer is investigated by using Cu/TaNx (25 nm)/Si structures and annealing at 400–900 ◦ C for 60 min.

2. Experimental The n-type Si(1 0 0) wafers of resistivity 1–10  cm were used as substrates in this study. The substrates were cleaned with acetone in an ultrasonic bath for 30 min just prior to loading into the deposition chamber. Thin films of TaNx were sputtered from a tantalum nitride (TaN) target with an rf power supply in an Ar ambient of 99.999% purity. The rf power and Ar flow rate were maintained at 250 W and 8 sccm, respectively. The TaN target was 3 in. in diameter and the target-to-substrate distance was about 7.5 cm. The substrate holder was neither heated nor cooled, but various dc substrate biases (0 to −200 V) were applied to the holder. The base pressure of the deposition chamber was 1×10−7 Torr and the operation pressure was 100 mTorr. The

0254-0584/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(03)00106-8

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thickness of the sputtered TaNx films for material characterization was 100 nm. TaNx films were characterized using X-ray diffraction (XRD) for phase identification, and X-ray photoelectron spectroscopy (XPS) for chemical compositions. The X-ray source for XPS was a monochromatized Al K␣ line (1486.6 eV). The film resistivity was calculated from the sheet resistance measured by a four-point probe and the film thickness measured by scanning electron microscopy (SEM). To test the thermal stability of the TaNx barriers, copper films of 70 nm in thickness were sputtered over the 25 nm thick TaNx layers. The Cu/TaNx /Si samples were then annealed in a tube furnace at 400–900 ◦ C in vacuum (10−6 Torr) for 60 min. Interfacial behavior and phase formation of the samples after annealing were characterized by cross-sectional transmission electron microscopy (XTEM) and XRD, respectively. Constituent elements at local regions of the samples were analyzed by energy-dispersive analysis of X-rays (EDAX). Variation in sheet resistance of samples, before and after annealing, was measured with a four-point probe. 3. Results and discussions 3.1. Material characteristics of sputtered TaNx thin films Fig. 1 presents the deposition rate of films for various biases, in which the deposition rate was deduced from the thickness measurements. This figure indicates that, as the bias increases, the deposition rate drops sharply from about 36.0 nm/min (≤−75 V) to 14.2 nm/min (−200 V). It is evident that the deposition rate has the highest value at low biases. Additionally, as the bias is raised from −75 to −200 V, the ion bombardment on the growing film is increased and causes resputtering leading to the decrease in deposition rate. By using XRD, diffraction peaks associated with the ␤-Ta [10], hexagonal-Ta2 N [11], and face-centered cubic TaN [12] phases are observed in the spectra of TaNx films deposited with substrate biases of 0 to −200 V (Fig. 2). A broad diffraction peak corresponded to the FCC-TaN (111, 2θ = 35.9◦ ) orientation was observed at a substrate bias of 0 V. At a substrate bias of −25 V, hexagonal-Ta2 N (101, 2θ = 38.5◦ ) is present in addition to FCC-TaN phase. For a bias between −50 and −125 V, diffraction peaks obviously show the coexistence of ␤-Ta and hexagonal-Ta2 N phases. As the bias exceeds −125 V, hexagonal-Ta2 N (101, 2θ = 38.5◦ ) is the preferred orientation. The chemical bonding states in the TaNx films were determined by XPS. Fig. 3 are the XPS spectra of Ta 4f (Fig. 3a) and N 1s (Fig. 3b) core levels in the TaNx films sputtered at various substrate biases. The binding energies of the Ta 4f and N 1s photoelectrons for TaNx films are consistent with the data found in Refs. [7,13]. Increasing the bias from 0 to −175 V causes the Ta 4f7/2 peak to shift from 22.64

Fig. 1. Deposition rate of sputtered TaNx films vs. substrate bias.

to 21.75 eV, while the N 1s peak to shift from 398.16 to 398.68 eV. The chemical composition x of the deposited film (TaNx ) can be evaluated from the ratio of the N 1s to Ta 4f peak areas corrected by suitable sensitivity factors of SN = 0.477 and STa = 2.589 [13]. Fig. 4 displays the film composition analyzed by XPS as a function of bias. This figure indicates that, as the bias increases, the N/Ta ratio drops sharply from 0.81 (0 V) to 0.39 (−25 V) and then 0.37 at a bias of −175 V. As generally assumed, the amount of ion bombardment is correlated with the substrate bias. For the multicomponent sputtering of a Cd–Co system, Cuomo and Gambino [14] proposed a kinetic model which considers the difference in sputtering yields of these two constituents. They showed a nice correlation between the Co/Cd ratio of the film and the applied bias. According to their model, the decrease in N/Ta ratio of the film in this study may result from the effect of resputtering when a dc bias is applied to the substrate. Tantalum nitride has a defect structure [6] and deviations from stoichiometry are frequent. This finding corresponds to the XPS observation in Fig. 3a that the Ta 4f peaks shift upon the variation in bias and in Fig. 4 that the amount of N in the deposited films decreases with increasing bias, since

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Fig. 4. N/Ta ratio and resistivity of TaNx films as a function of substrate bias.

Fig. 2. XRD spectra of TaNx films deposited at various substrate biases.

nitrogen doping should affect the chemical environment on the outermost electron orbitals of Ta. Fig. 4 also shows the dependence of the film resistivity on the negative bias applied on the substrate. The film

Fig. 3. XPS spectra of (a) Ta 4f and (b) N 1s core levels for TaNx films deposited at various substrate biases.

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resistivity increases with increasing negative bias. When the films were sputtered without a substrate bias, the film resistivity was 279 ␮ cm, while at −175 V bias, the film resistivity was 350 ␮ cm. For the TaNx film prepared by reactive sputtering, the resistivity usually increases with increasing nitrogen content in the film. In our study, the resistivity of TaNx film increases with decreasing N/Ta ratio. This is because the grain size decreases with increasing negative bias as observed by TEM micrographs (not shown here). In addition, our results are consistent with those of Mehrotra and Stimmel [15]. These resistivity values are higher than the reported resistivity for reactively sputtered TaN films (ρ ≈ 250 ␮ cm) [16], and slightly lower than the resistivity of plasma enhanced atomic layer deposited (PEALD) TaN films (ρ ≈ 400 ␮ cm) deposited at 260 ◦ C [17]. 3.2. Thermal stability of Cu/TaNx /Si system Twenty five nanometer TaN0.81 and TaN0.37 films sputtered at 0 and −175 V substrate biases were then employed as diffusion barriers for Cu/Si metallization. After deposition of a 70 nm thick copper film, the Cu/TaNx /Si samples were subjected to heat-treatment at 400–900 ◦ C for 60 min in vacuum (10−6 Torr). The variation of Cu sheet resistance as a function of the annealing temperature is commonly used to examine the capability of diffusion barrier against Cu diffusion. The difference of sheet resistance between the annealed and as-deposited samples, divided by the sheet resistance of as-deposited samples, is called the variation percentage of sheet resistance ( Rs /Rs %) and is defined as follows [8]: Rs,after anneal − Rs,as-deposited Rs %= × 100% (1) Rs Rs,as-deposited It is well known that Cu diffuses fast in Si and forms Cu–Si compounds at a temperature as low as 200 ◦ C, and the formation of Cu–Si compounds results in the increase of sheet resistance of Cu films. Fig. 5 presents the variation percentage of sheet resistance versus annealing temperature for the Cu/TaNx /Si samples. It is shown that the sheet resistance of the Cu/TaNx /Si samples remains after annealing at temperature up to 700 ◦ C for 60 min. However, a drastic increase in sheet resistance is found after annealing above 700 ◦ C for the Cu/TaN0.37 /Si sample and above 800 ◦ C for the Cu/TaN0.81 /Si sample. Therefore, TaN0.81 has a higher resistance to Cu diffusion as compared to TaN0.37 . In addition, the dramatic increase in sheet resistance is attributed to the formation of Cu3 Si precipitates according to the XRD and XTEM analyses. (as shown later in Figs. 6 and 7). To understand the reason for the variation in sheet resistance of Cu/TaNx (x = 0.37, 0.81)/Si samples after annealing, we performed XRD and XTEM analyses to investigate the reactions between the layers. Fig. 6 shows the XRD spectra of Cu/TaN0.37 /Si samples before and after annealing in the temperature range of 400–900 ◦ C for 60 min. The XRD results reveal only diffraction peaks of Cu for the as-deposited Cu/TaN0.37 /Si sample. Peaks for

Fig. 5. Variation percentage of sheet resistance vs. annealing temperature for the Cu (100 nm)/TaNx (25 nm, x = 0.37, 0.81)/Si samples.

the TaN0.37 layer which has a crystal structure of Ta2 N as observed in Fig. 2 are not clearly observed since it is very thin and the grains are small. As seen in Fig. 6, the spectra of the sample annealed at 400, 500, 600, and 700 ◦ C are similar to that of the as-deposited one except the intensity of the Cu (1 1 1) peak increases with annealing temperature. At 800 ◦ C, diffraction peaks of Cu3 Si [18] appear and the Cu peaks diminish. This indicates that most of the Cu is transformed to Cu3 Si at 800 ◦ C. The formation of high resistivity Cu3 Si phase results in the drastic increase of sheet resistance as shown in Fig. 5. The inset in Fig. 6 is an enlarged XRD spectrum for the Cu/TaN0.37 /Si sample annealed at 800 ◦ C. Both TaSi2 and Cu3 Si phases appear. Similar results of XRD and sheet resistance measurements were obtained for the Cu/TaN0.81 /Si sample, except that the appearance of Cu3 Si and the drastic increase of sheet resistance happened after 900 ◦ C annealing. Fig. 7 represents the XTEM micrographs of the as-deposited and the 800 and 900 ◦ C annealed Cu/TaN0.37 /Si samples. Interfaces between multilayers can be seen clearly for the as-deposited sample as shown in Fig. 7a. The 800 ◦ C annealed sample, as shown in Fig. 7b, indicates that the Cu film starts to agglomerate and the migration of Cu into Si substrate also starts at some specific defects. According

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Fig. 6. XRD spectra for the Cu/TaN0.37 /Si samples annealed at various temperatures for 60 min.

to SEM micrographs (not shown here), we found that the specific defects are randomly distributed microholes as reported by Chen and Wang [19] and they also proposed that these microholes let the Cu to diffuse through easily. An XTEM micrograph of a 900 ◦ C-annealed sample shows a triangular-like crystallite with sharp edges penetrating into the Si substrate (Fig. 7c). By using EDAX, we found that the triangular crystallites contained mostly Cu and a little Si. From the inclined angle of the edges, it is deduced that the crystallites are bounded by the Si {1 1 1} planes. These pyramids are analogous to the “pyramidal” spikes observed in the Al/Si system after annealing [20]. A similar pyramidal structure was also observed in the annealed Cu/Ta/Si system, and the pyramid was identified as Cu3 Si [21]. In the present case, the pyramids should also be Cu3 Si since EDAX reveals both Cu and Si signals within the pyramid. At failure temperature, except that Cu3 Si pyramids could be seen over the samples, we also observed the TaSi2 phase in the XRD spectrum (Fig. 6). Our results are similar to the previous studies of Yin et al. [22] and Holloway et al. [23]. This is a general case for failure behavior of Cu/Barrier/Si systems. The reason for the whole grain of Cu3 Si to get

Fig. 7. XTEM micrographs of the Cu/TaN0.37 /Si samples: (a) the as-deposited sample and samples annealed at (b) 800 and (c) 900 ◦ C for 60 min.

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pushed out was due to the stress force generated by the decrease in the density when Cu was converted into Cu3 Si, because copper has a larger density than copper silicide. In the current work, we did not detect any apparent reaction between Cu and TaNx . The breakdown of the Cu/TaNx /Si system after heat-treatment shall mainly originate from the diffusion of Cu and/or Si via the grain boundaries of the TaNx barrier. 4. Conclusion In this work, tantalum nitride (TaNx ) films were deposited on silicon substrates by rf sputtering from a TaN target. Our results indicate that the deposition rate decreases with increasing bias. TaN, ␤-Ta, and Ta2 N phases appeared and/or coexisted in the films at specific biases. A step change in N/Ta ratio is observed whenever a bias is applied to the substrate. When TaNx films of 25 nm in thickness was applied as a diffusion barrier between copper and silicon substrate, no change in sheet resistance and no evidence of reaction were observed for the Cu (70 nm)/TaNx (25 nm, x = 0.37 and 0.81)/Si samples annealed up to 700 ◦ C for 60 min. However, the sheet resistance increased dramatically after annealing above 700 and 800 ◦ C for Cu/TaN0.37 /Si and Cu/TaN0.81 /Si systems, respectively. The drastic increase of the sheet resistance is associated with the deterioration of the interface of samples as well as the formation of Cu3 Si. The failure mechanism may be associated with the diffusion of Cu and/or Si via the grain boundaries of the crystalline TaNx barriers. Acknowledgements The authors would like to thank the National Science Council of the Republic of China for financially sup-

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porting this research under contract no. NSC 91-2214-E-234001. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

[22] [23]

M.-A. Nicolet, Thin Solid Films 52 (1978) 415. S. Kanamori, Thin Solid Films 136 (1986) 195. M. Wittmer, J. Appl. Phys. 53 (1982) 1007. T. Oku, E. Kawakami, M. Uekubo, K. Takahiro, S. Yamaguchi, M. Murakami, Appl. Surf. Sci. 99 (1996) 265. A. Noya, K. Sasaki, M. Takeyama, Jpn. J. Appl. Phys. 32 (1993) 911. L.E. Thod, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971. J.C. Lin, C. Lee, J. Electrochem. Soc. 147 (2000) 713. W.L. Yang, W.-F. Wu, D.-G. Liu, C.-C. Wu, K.L. Ou, Solid-State Electron. 45 (2001) 149. J.C. Lin, C. Lee, J. Electrochemical Soc. 146 (1999) 1835. JCPDS Files card no. 25-1280. JCPDS Files card no. 26-0985. JCPDS Files card no. 32-1283. J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, 1995. J.J. Cuomo, R.J. Gambino, R. Rosenberg, J. Vac. Sci. Technol. 11 (1974) 1. B. Mehrotra, J. Stimmel, J. Vac. Sci. Technol. B 5 (6) (1987) 1736. S.C. Sun, M.H. Tsai, C.E. Tsai, H.T. Chiu, in: Proc. Symp. on VLSI Tech. Digest of Tech. Papers, vol. 29, 1995. J.S. Park, M.J. Lee, C.S. Lee, S.W. Kang, Electrochem. Solid State Lett. 4 (4) (1999) C17. JCPDS Files card no. 23-0224. J.S. Chenz, J.L. Wang, J. Electrochem. Soc. 147 (5) (2000) 1940. C.S. Pai, E. Cabreros, S.S. Lau, T.E. Seidel, I. Suni, Appl. Phys. Lett. 47 (1985) 652. J. Baumann, M. Stavrev, M. Rennau, T. Raschke, S.E. Schulz, C. Wenzel, C. Kauf-mann, T. Gessner, Mater. Res. Soc. Proc. ULSI XIV (1999) 321. K.-M. Yin, L. Chang, F.-R. Chen, J.-J. Kai, Mater. Chem. Phys. 71 (2001) 1. K. Holloway, P.M. Fryer, C. Cabral Jr., J.M.E. Harper, P.J. Bailey, K.H. Kelleher, J. Appl. Phys. 71 (11) (1992) 5433.