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Effect of density on the diffusion barrier property of TiNx films between Cu and Si
Materials Chemistry and Physics 85 (2004) 444–449
Effect of density on the diffusion barrier property of TiNx films between Cu and Si Wen-Horng Lee a,∗ , Yu-Lin Kuo b , Hong-Ji Huang b , Chiapyng Lee b b
a Department of Chemical Engineering, Lee-Ming Institute of Technology, Taishan, Taipei 243, Taiwan, ROC Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10672, Taiwan, ROC
Received 11 November 2003; received in revised form 8 January 2004; accepted 3 February 2004
Abstract TiNx films sputtered from a TiN target were used as diffusion barriers between Cu thin films and Si substrates. The influence of density on the diffusion barrier property of TiNx films between Cu and Si was investigated. Material characteristics of TiNx films and metallurgical reactions of Cu (100 nm)/TiNx (25 nm)/Si systems annealed in the temperature range 500–800 ◦ C for 60 min were investigated by sheet resistance measurements, X-ray diffraction, and cross-sectional transmission electron microscopy. It was found that the TiN0.81 film with a density of 4.99 g cm−3 can prevent Cu–Si interaction up to 600 ◦ C for 60 min. The TiN0.86 film with a higher density of 5.12 g cm−3 is a more effective barrier to Cu penetration; higher density TiN0.86 film prevents the Cu reaction with the Si substrate for temperature up to at least 800 ◦ C for 60 min. When the composition of the film is similar, the film density is the dominant factor which determines the failure temperature. The failure mechanism of TiNx films as diffusion barriers between Cu and Si was also discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Titanium nitride; Bias voltage; Film density; Diffusion barrier
1. Introduction Copper is a promising interconnection material for advanced ultra-large-scale integrated devices due to its lower electrical resistivity and higher resistance against electromigration than aluminum and its alloys [1]. However, the interaction between Cu and Si is very strong and detrimental to electrical performance of Si even at temperatures as low as 200 ◦ C [2,3]. In addition, copper has a high diffusion coefficient into Si or SiO2 which can deteriorate the device operation [4–6]. For this reason, there have been considerable efforts to identify a suitable diffusion barrier for Cu metallization. Since titanium nitride (TiN) is the most common diffusion barrier used in Al metallization, there is the possibility of extending its applicability to Cu interconnections. Properties of TiN films deposited by reactive sputtering in a mixture of nitrogen and argon in Cu/TiN/Si systems have been reported in many papers [7–12]. However, very different film properties were obtained because well-controlled films are difficult to deposit with good reproducibility. Diffusion barrier thin films must be conformal and have low ∗ Corresponding author. Tel.: +886-2-29097811x2350; fax: +886-2-29097811x2301. E-mail address: [email protected] (W.-H. Lee).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.02.001
porosity. The leakage current of the contact is increased due to penetration of Cu to Si through grain boundaries in the columnar structure of TiN. Therefore, improvement in the film quality, control of grain growth, composition, and density are needed. Also, low film densities have been reported to influence film properties such as the residual stress, the resistivity, the film adhesion, and the crystallization behavior [13]. However, there is no report about the influence of the film density on the diffusion barrier ability by using TiN as the target to deposit TiNx films. In this study, TiNx films are deposited by rf sputtering from a TiN target. The variation of deposition rate, film density, film resistivity, film chemical composition, and crystalline microstructure as a function of bias is studied. In addition, the thermal stability of TiNx barrier layer is investigated by using Cu (100 nm)/TiNx (25 nm)/Si structures and annealing at 500–800 ◦ 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 and then were
W.-H. Lee et al. / Materials Chemistry and Physics 85 (2004) 444–449
dipped in a dilute HF solution for a few seconds just prior to loading into the deposition chamber. In the rf magnetron sputtering system, the working distance was 7.5 cm and the TiN target was 3 in. in diameter. Thin films of TiNx were sputtered from a titanium nitride (TiN) target with an rf power supply in an Ar ambient of 99.999% purity. The Ar flow rate was maintained at 4 sccm. In order to obtain a uniform deposit, the substrate holder is equipped with a rotating system. 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 10 mTorr. The thickness of the sputtered TiNx films for material characterization was 100 nm. The film resistivity was calculated from the sheet resistance measured by a four-point probe (FPP Napson RT-7) and the film thickness measured by scanning electron microscopy (SEM; JEOL JSM-6340F). The crystal structure and preferred orientation of the TiNx films were identified using X-ray diffraction (XRD; Rigaku RTP300RC) and X-ray photoelectron spectroscopy (XPS; VG Microtech MT-500) for chemical compositions. The X-ray source for XPS was a monochromatized Al K␣ line (1486.6 eV). TiNx film densities were estimated by X-ray diffraction and X-ray photoelectron measurement, with an error of around ±0.01. To test the thermal stability of the TiNx barriers, copper films of 100 nm thickness were sputtered over the
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25 nm thick TiNx layers without breaking the vacuum. The Cu/TiNx /Si samples were then annealed in a tube furnace at 500–850 ◦ C in vacuum (<10−5 Torr) for 60 min. Interfacial behavior and phase formation of the samples after annealing were characterized by cross-sectional transmission electron microscopy (XTEM; JEOL FX2000) and XRD, respectively. Variation in sheet resistance of samples, before and after annealing, was measured with a four-point probe.
3. Results and discussions 3.1. Barrier properties of as-deposited TiNx thin films The film composition and structure are dependent on the sputtering parameters. As the substrate is negatively biased during deposition, dense and highly textured films are obtained. Fig. 1 displays the high-resolution XPS spectra of Ti 2p (Fig. 1a) and N 1s (Fig. 1b) core levels in the TiNx films sputtered from a TiN target at various conditions. The binding energies of the Ti 2p1/2 , Ti 2p3/2 , and N 1s photoelectrons for TiNx films are consistent with the data found in Refs. [14,15]. The chemical composition x of the deposited film (TiNx ) can be evaluated from the ratio of the N 1s to Ti 2p peak areas corrected by suitable sensitivity factors of SN = 0.477 and STi = 1.798 which were obtained from a
Fig. 1. XPS spectra of (a) Ti 2p and (b) N 1s core levels for various as-deposited TiNx films.
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Table 1 Summary of the properties of the deposited TiNx films Properties
(g cm−3 )
Density Resistivity ( cm) N/Ti ratio (x in TiNx ) Grain size (nm)
Techniques
XPS and XRD Four-point probe XPS SEM
Deposition conditions (rf power (W)/substrate bias (V)) 100/0
250/0
250/−50
250/−100
4.99 415 0.81 29
5.06 170 0.82 37.4
5.09 90 0.85 5.9
5.12 75 0.86 3.1
standard TiN sample. Table 1 displays the film properties determined by various techniques. The N/Ti ratio (x) increases with increasing substrate bias. However, the grain size decreases with an increasing in negative bias as observed by SEM micrographs (not shown here). Fig. 2 shows a set of X-ray diffraction patterns for the as-deposited TiNx films. All films deposited exhibit a single strong (1 1 1) textured TiN films (NaCl-type structure) [16]. However, the TiN (1 1 1) peak shifts to the left as the substrate bias applied, indicating enlargement of the lattice parameter for the TiN phase. The incorporation of nitrogen increase in the lattice may explain the shift in 2θ. It has been documented that the TiN (1 1 1) is a superior diffusion barrier material for Cu than TiN (1 0 0) [17]. The easy penetration of Cu through granules of the loosely packed TiN (1 0 0) diffusion barrier is the factor behind this barrier’s ear-
Fig. 2. XRD spectra of various deposited TiNx films.
lier degradation than that of the densely packed TiN (1 1 1) barrier. In addition, Abe et al. [18] and Yanagisawa et al. [19] reported that electromigration (EM) resistance of Cu can be improved by realizing a high orientation of a Cu (1 1 1) film by choosing an appropriate underlayer. It has been reported that the Cu (1 1 1) film easily grows on TiN (1 1 1) film [17]. This phenomenon follows from the grain growth and elimination of lattice residual defects in the Cu films. The full width at half maximum (FWHM) of this peak became broader with increasing substrate bias, which indicates that the grain size of the film is small at higher substrate bias. This result is in good agreement with the grain size observed by SEM (not shown here). Fig. 3 shows the variation of electrical resistivity of TiNx as a function of density. The density of a barrier film (TiNx )
Fig. 3. The resistivity of the as-deposited TiNx films as a function of the density of the film.
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was calculated from XPS and XRD spectra as described by Callister [20]. We can obtain the sum of the molecular weights from XPS and lattice constant from XRD. The lower electrical resistivity of the TiNx film was attributed to differences in film microstructure and density. As the absolute value of the biasing voltage increased, the TiNx film density increased. The deeply biased film had density approach to the bulk density of TiN (ρ = 5.22 g cm−3 ) [21]. This is consistent with the results reported by Moshfegh and Akhavan [22]. They pointed out that films with lower resistivity have higher density. A similar result for ZrN sputtered on Si substrate is also observed by Chou et al. [23]. Titanium nitride has a defect structure and deviations from stoichiometry are frequent [10]. For the film with a lower density, the amount of lattice defect in the film is large and consequently a higher resistivity is expected. It has been documented that increased energy of the impinging ions and atoms promotes surface diffusion to fill voids. Furthermore, with higher substrate bias voltage, nucleation was enhanced and resulted in smaller grain size and higher packing density [22]. Increasing ion bombardment during deposition causes an interruption of the growing columns and nucleation of new grains. This effect combined with increasing mobility of the condensing atoms that favors the formation of a dense microstructure has also been reported earlier [24]. However, Moriyama et al. [7] pointed out that the resistivities of TiN films decrease with increasing the grain sizes. Our result quite differs from the result obtained by them, which can be attributed to the film density. Therefore, to obtain layers with high density and low electrical resistivity, high power and high substrate bias are preferred. 3.2. Stability of Cu (100 nm)/TiNx (25 nm)/Si structure The thermal stability of TiNx thin films between Cu and Si was investigated. Cu (100 nm)/TiNx (25 nm)/Si samples annealed at various temperatures were characterized by sheet resistance measurement and XRD. A four-point probe system can be employed to measure the relative change in sheet resistance of the Cu (100 nm)/TiNx (25 nm)/Si samples following annealing at various temperatures for 60 min. Fig. 4 displays the relative changes in sheet resistance ( R/R0 (%)) for the Cu/TiNx /Si samples as a function of annealing temperature. It is clear that the relative changes in sheet resistance for the Cu/TiN0.86 (higher density, ρ = 5.12 g cm−3 )/Si system slightly reduced, indicating densification and structural improvement of the Cu layer. A similar reduction for Cu/Ta/SiO2 /Si structure is observed by Itow et al. [25]. A sudden sharp increase in sheet resistance of the films at 800 ◦ C can be related to interdiffusion and/or reaction between layers (Cu, TiNx , Cu) that occurred faster at higher temperatures. However, for the Cu/TiN0.81 (lower density, ρ = 4.99 g cm−3 )/Si a rapid increase in R/R0 (%) value was measured at lower temperature, i.e. 600 ◦ C. Similarly for Cu/TiN0.82 (ρ = 5.06 g cm−3 )/Si and Cu/TiN0.85 (ρ = 5.09 g cm−3 )/Si systems, a sharp increase in R/R0
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Fig. 4. Relative changes in sheet resistance ( R/R0 (%)) of various Cu/TiNx /Si samples induced by annealing at various temperatures for 60 min.
(%) value was measured at 700 and 750 ◦ C, respectively. Apparently, the higher density barrier film is superior to the lower density barrier film since the former exhibits an electrical failure temperature approximately 200 ◦ C above that of the latter. To understand the differences in the sheet resistance relative variation for these samples, the materials’ characteristics of these structures are examined by XRD. Figs. 5 and 6 depict the evolution of XRD patterns as a function of annealing temperature for the Cu/TiN0.86 (higher density)/Si and Cu/TiN0.81 (lower density)/Si samples, respectively. The thicknesses of the Cu and TiNx (x = 0.86, 081) again were 100 and 25 nm, respectively. The XRD results reveal only diffraction peaks of Cu (1 1 1) for the Cu/TiN0.86 (higher density)/Si sample (Fig. 5). It has been reported that Cu (1 1 1) provides higher electromigration resistance than that of Cu (2 0 0) [18]. Peak (1 1 1) for the TiNx layer is not clearly observed since it is very thin and the grains are small. As seen in Fig. 5, the spectra of the sample annealed at 600, 650, and 700 ◦ C are similar to that of the as-deposited one except that the intensity of the Cu (1 1 1) peak increases with annealing temperature. Precipitations of trace amounts of Cu3 Si [26] are observed after annealing at 800 ◦ C, and the Cu (1 1 1) peak remained. The formation of high-resistivity Cu3 Si phase results in the drastic increase of sheet resistance as shown in Fig. 4. A series of XRD patterns in Fig. 6
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Fig. 5. Evolution of XRD patterns for as-deposition Cu/TiN0.86 /Si samples annealed at various temperature for 60 min.
shows that phase-transition behavior of Cu/TiN0.81 (lower density)/Si samples following high-temperature annealing resembles that of Cu/TiN0.86 (higher density)/Si samples as discussed for Fig. 5. However, intensities of Cu (1 1 1) XRD peaks diminish and diffraction peaks of Cu3 Si appear after annealing at 600 ◦ C. The inset in Fig. 6 is an enlarged XRD spectrum for the Cu/TiN0.81 (lower density)/Si sample an-
Fig. 6. Evolution of XRD patterns for as-deposited Cu/TiN0.81 /Si samples annealed at various temperature for 60 min.
nealed at 600 ◦ C; the peak from Cu3 Si is observed, which indicates intermixing of Cu and Si through the TiNx barrier layer. Similar results of XRD were obtained for Cu/TiN0.82 (ρ = 5.06 g cm−3 )/Si and Cu/TiN0.85 (ρ = 5.09 g cm−3 )/Si
Fig. 7. XTEM micrograph of the Cu/TiN0.81 /Si structure annealed at 600 ◦ C.
W.-H. Lee et al. / Materials Chemistry and Physics 85 (2004) 444–449
systems, that the appearance of Cu3 Si at 700 and 750 ◦ C, respectively. The XRD results also revealed that the increases in sheet resistance discussed in the previous section were perhaps caused by the loss of copper due to a Cu3 Si formation. The XTEM image of Cu3 Si had an inverted pyramid shape and was buried in Si substrate of Cu/TiN0.81 (lower density)/Si structure annealed at 600 ◦ C for 30 min as shown in Fig. 7. This suggests that Cu diffuses across TiNx into the Si and reacts with Si to form Cu3 Si. The oblique planes of the Si at the Cu3 Si/Si interface are the (1 1 1) type. This is a general case for failure behavior of Cu/barrier/Si systems [27]. The appearance of Cu3 Si pyramids in the interface of Cu/TiNx /Si samples is attributed to the failure mechanism which may be associated with the diffusion of Cu and Si via the grain boundaries of the nanocrystalline TiNx barriers. According to SEM micrographs (not shown here), we found that Cu3 Si pyramids were randomly distributed. However, one can hardly observe the diffraction peak of Cu3 Si phase. This is because TEM is more sensitive than XRD for microanalysis. Similar phenomena were also reported by Chen and Chen [28]. It was found that the thermal stability was strongly correlated with the density of the film which was found to be the key factor in controlling the thermal stability of the TiN barrier layers. The highest temperature at which the TiN0.86 (higher density) layer prevents Cu diffusion is 800 ◦ C for 60 min. These results satisfy the requirements for the barrier layer used in the special devices, which must withstand the thermal stability of 700 ◦ C for 30 min [7].
4. Conclusion By rf sputtering from a TiN target and applying substrate biases of 0 to −100 V, we obtained nanocrystalline TiNx films of 3.1–37.4 nm grain size. The TiNx films deposited with the substrate bias voltage showed higher density, strong (1 1 1) preferred orientation, and small size in comparison without the substrate bias voltage. When TiNx film of 25 nm in thickness is applied as a diffusion barrier between copper and silicon substrate, no apparent change in sheet resistance and no evidence of reaction are observed for the Cu/TiN0.86 (higher density, ρ = 5.12 g cm−3 )/Si samples annealed above 800 ◦ C for 60 min. However, the Cu/TiN0.81 (lower density, ρ = 4.99 g cm−3 )/Si samples showed a drastic increase in variation percentage of sheet resistance after annealing up to 600 ◦ C for 60 min. The drastic increase of the sheet resis-
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tance is associated with the deterioration of the interface of samples, as well as the formation of Cu3 Si. It is concluded that high density of the TiNx film is of primary importance in achieving a good diffusion. References [1] W.-L. Yang, W.-F. Wu, D.-G. Liu, C.-C. Wu, K.L. Ou, Solid-State Electron. 45 (2001) 149. [2] C.-A. Chang, J. Appl. Phys. 67 (1990) 566. [3] A. Cros, M.O. Aboelfotoh, K.N. Tu, J. Appl. Phys. 67 (1990) 3328. [4] T. Oku, E. Kawakami, M. Uekubo, K. Takahiro, S. Yamaguchi, M. Murakami, Appl. Surf. Sci. 99 (1996) 265. [5] J.C. Chuang, M.C. Chen, J. Electrochem. Soc. 145 (1998) 4029. [6] A.E. Kaloyeros, X. Chen, T. Stark, K. Kuar, S.C. Seo, G.G. Peterson, H.L. Frisch, B. Arkles, J. Sullivan, J. Electrochem. Soc. 146 (1999) 170. [7] M. Moriyama, T. Kawazoe, M. Tanaka, M. Murakami, Thin Solid Films 416 (2002) 136. [8] S.-Q. Wang, I. Raaijmakers, B.J. Burrow, S. Suthar, S. Redkar, K.-B. Kim, J. Appl. Phys. 68 (1990) 5176. [9] K.T. Nam, A. Datta, S.-H. Kim, K.-B. Kim, Appl. Phys. Lett. 79 (2001) 2549. [10] K.-C. Park, K.-B. Kim, I. Raaijmarkers, K. Ngan, J. Appl. Phys. 80 (1996) 5674. [11] J.O. Olowolafe, C.J. Mogab, R.B. Gregory, M. Kottke, J. Appl. Phys. 72 (1992) 4099. [12] I. Suni, M. Maenpaa, M.-A. Nicolet, M. Luomajarvi, J. Electrochem. Soc. 130 (1983) 1215. [13] H.M. Choi, S.K. Choi, O. Anderson, K. Bange, Thin Solid Films 358 (2000) 202. [14] T. Brat, N. Parikh, N.S. Tsai, A.K. Sinha, J. Poole, C. Wickersham Jr., J. Vac. Sci. Technol. B 5 (6) (1987) 1736. [15] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Eden Prairie, 1995. [16] JCPDS Files card no. 38-1420. [17] G.S. Chen, J.J. Guo, C.K. Lin, C.S. Hsu, L.C. Yang, J.S. Fang, J. Vac. Sci. Technol. A 20 (2) (2002) 479. [18] K. Abe, Y. Harada, H. Onoda, J. Vac. Sci. Technol. B 17 (4) (1999) 1464. [19] H. Yanagisawa, K. Sasaki, H. Miyake, Y. Abe, Jpn. J. Appl. Phys. 39 (2000) 5987. [20] W.D. Callister Jr., Fundamentals of Materials Science and Engineering, Wiley, New York, 2001. [21] R.C. Weast, CRC Handbook of Chemistry and Physics, 53rd ed., Chemical Rubber, Cleveland, Ohio, 1972, p. B149. [22] A.Z. Moshfegh, O. Akhavan, Thin Solid Films 370 (2000) 10. [23] W.-J. Chou, G.-P. Yu, J.-H. Huang, Thin Solid Films 405 (2002) 162. [24] P.H. Mayrhofer, F. Kunc, J. Musil, C. Mitterer, Thin Solid Films 415 (2002) 151. [25] H. Itow, Y. Nakasaki, G. Minamihaba, K. Suguro, H. Okano, Appl. Phys. Lett. 63 (1993) 934. [26] K.-M. Yin, L. Chang, F.-R. Chen, J.-J. Kai, Mater. Chem. Phys. 71 (2001) 1. [27] Y.L. Kuo, J.J. Huang, S.T. Lin, C. Lee, W.H. Lee, Mater. Chem. Phys. 80 (2003) 690. [28] G.S. Chen, S.T. Chen, J. Appl. Phys. 87 (2000) 8473.
Effect of density on the diffusion barrier property of TiNx films between Cu and Si Wen-Horng Lee a,∗ , Yu-Lin Kuo b , Hong-Ji Huang b , Chiapyng Lee b b
a Department of Chemical Engineering, Lee-Ming Institute of Technology, Taishan, Taipei 243, Taiwan, ROC Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10672, Taiwan, ROC
Received 11 November 2003; received in revised form 8 January 2004; accepted 3 February 2004
Abstract TiNx films sputtered from a TiN target were used as diffusion barriers between Cu thin films and Si substrates. The influence of density on the diffusion barrier property of TiNx films between Cu and Si was investigated. Material characteristics of TiNx films and metallurgical reactions of Cu (100 nm)/TiNx (25 nm)/Si systems annealed in the temperature range 500–800 ◦ C for 60 min were investigated by sheet resistance measurements, X-ray diffraction, and cross-sectional transmission electron microscopy. It was found that the TiN0.81 film with a density of 4.99 g cm−3 can prevent Cu–Si interaction up to 600 ◦ C for 60 min. The TiN0.86 film with a higher density of 5.12 g cm−3 is a more effective barrier to Cu penetration; higher density TiN0.86 film prevents the Cu reaction with the Si substrate for temperature up to at least 800 ◦ C for 60 min. When the composition of the film is similar, the film density is the dominant factor which determines the failure temperature. The failure mechanism of TiNx films as diffusion barriers between Cu and Si was also discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Titanium nitride; Bias voltage; Film density; Diffusion barrier
1. Introduction Copper is a promising interconnection material for advanced ultra-large-scale integrated devices due to its lower electrical resistivity and higher resistance against electromigration than aluminum and its alloys [1]. However, the interaction between Cu and Si is very strong and detrimental to electrical performance of Si even at temperatures as low as 200 ◦ C [2,3]. In addition, copper has a high diffusion coefficient into Si or SiO2 which can deteriorate the device operation [4–6]. For this reason, there have been considerable efforts to identify a suitable diffusion barrier for Cu metallization. Since titanium nitride (TiN) is the most common diffusion barrier used in Al metallization, there is the possibility of extending its applicability to Cu interconnections. Properties of TiN films deposited by reactive sputtering in a mixture of nitrogen and argon in Cu/TiN/Si systems have been reported in many papers [7–12]. However, very different film properties were obtained because well-controlled films are difficult to deposit with good reproducibility. Diffusion barrier thin films must be conformal and have low ∗ Corresponding author. Tel.: +886-2-29097811x2350; fax: +886-2-29097811x2301. E-mail address: [email protected] (W.-H. Lee).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.02.001
porosity. The leakage current of the contact is increased due to penetration of Cu to Si through grain boundaries in the columnar structure of TiN. Therefore, improvement in the film quality, control of grain growth, composition, and density are needed. Also, low film densities have been reported to influence film properties such as the residual stress, the resistivity, the film adhesion, and the crystallization behavior [13]. However, there is no report about the influence of the film density on the diffusion barrier ability by using TiN as the target to deposit TiNx films. In this study, TiNx films are deposited by rf sputtering from a TiN target. The variation of deposition rate, film density, film resistivity, film chemical composition, and crystalline microstructure as a function of bias is studied. In addition, the thermal stability of TiNx barrier layer is investigated by using Cu (100 nm)/TiNx (25 nm)/Si structures and annealing at 500–800 ◦ 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 and then were
W.-H. Lee et al. / Materials Chemistry and Physics 85 (2004) 444–449
dipped in a dilute HF solution for a few seconds just prior to loading into the deposition chamber. In the rf magnetron sputtering system, the working distance was 7.5 cm and the TiN target was 3 in. in diameter. Thin films of TiNx were sputtered from a titanium nitride (TiN) target with an rf power supply in an Ar ambient of 99.999% purity. The Ar flow rate was maintained at 4 sccm. In order to obtain a uniform deposit, the substrate holder is equipped with a rotating system. 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 10 mTorr. The thickness of the sputtered TiNx films for material characterization was 100 nm. The film resistivity was calculated from the sheet resistance measured by a four-point probe (FPP Napson RT-7) and the film thickness measured by scanning electron microscopy (SEM; JEOL JSM-6340F). The crystal structure and preferred orientation of the TiNx films were identified using X-ray diffraction (XRD; Rigaku RTP300RC) and X-ray photoelectron spectroscopy (XPS; VG Microtech MT-500) for chemical compositions. The X-ray source for XPS was a monochromatized Al K␣ line (1486.6 eV). TiNx film densities were estimated by X-ray diffraction and X-ray photoelectron measurement, with an error of around ±0.01. To test the thermal stability of the TiNx barriers, copper films of 100 nm thickness were sputtered over the
445
25 nm thick TiNx layers without breaking the vacuum. The Cu/TiNx /Si samples were then annealed in a tube furnace at 500–850 ◦ C in vacuum (<10−5 Torr) for 60 min. Interfacial behavior and phase formation of the samples after annealing were characterized by cross-sectional transmission electron microscopy (XTEM; JEOL FX2000) and XRD, respectively. Variation in sheet resistance of samples, before and after annealing, was measured with a four-point probe.
3. Results and discussions 3.1. Barrier properties of as-deposited TiNx thin films The film composition and structure are dependent on the sputtering parameters. As the substrate is negatively biased during deposition, dense and highly textured films are obtained. Fig. 1 displays the high-resolution XPS spectra of Ti 2p (Fig. 1a) and N 1s (Fig. 1b) core levels in the TiNx films sputtered from a TiN target at various conditions. The binding energies of the Ti 2p1/2 , Ti 2p3/2 , and N 1s photoelectrons for TiNx films are consistent with the data found in Refs. [14,15]. The chemical composition x of the deposited film (TiNx ) can be evaluated from the ratio of the N 1s to Ti 2p peak areas corrected by suitable sensitivity factors of SN = 0.477 and STi = 1.798 which were obtained from a
Fig. 1. XPS spectra of (a) Ti 2p and (b) N 1s core levels for various as-deposited TiNx films.
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Table 1 Summary of the properties of the deposited TiNx films Properties
(g cm−3 )
Density Resistivity ( cm) N/Ti ratio (x in TiNx ) Grain size (nm)
Techniques
XPS and XRD Four-point probe XPS SEM
Deposition conditions (rf power (W)/substrate bias (V)) 100/0
250/0
250/−50
250/−100
4.99 415 0.81 29
5.06 170 0.82 37.4
5.09 90 0.85 5.9
5.12 75 0.86 3.1
standard TiN sample. Table 1 displays the film properties determined by various techniques. The N/Ti ratio (x) increases with increasing substrate bias. However, the grain size decreases with an increasing in negative bias as observed by SEM micrographs (not shown here). Fig. 2 shows a set of X-ray diffraction patterns for the as-deposited TiNx films. All films deposited exhibit a single strong (1 1 1) textured TiN films (NaCl-type structure) [16]. However, the TiN (1 1 1) peak shifts to the left as the substrate bias applied, indicating enlargement of the lattice parameter for the TiN phase. The incorporation of nitrogen increase in the lattice may explain the shift in 2θ. It has been documented that the TiN (1 1 1) is a superior diffusion barrier material for Cu than TiN (1 0 0) [17]. The easy penetration of Cu through granules of the loosely packed TiN (1 0 0) diffusion barrier is the factor behind this barrier’s ear-
Fig. 2. XRD spectra of various deposited TiNx films.
lier degradation than that of the densely packed TiN (1 1 1) barrier. In addition, Abe et al. [18] and Yanagisawa et al. [19] reported that electromigration (EM) resistance of Cu can be improved by realizing a high orientation of a Cu (1 1 1) film by choosing an appropriate underlayer. It has been reported that the Cu (1 1 1) film easily grows on TiN (1 1 1) film [17]. This phenomenon follows from the grain growth and elimination of lattice residual defects in the Cu films. The full width at half maximum (FWHM) of this peak became broader with increasing substrate bias, which indicates that the grain size of the film is small at higher substrate bias. This result is in good agreement with the grain size observed by SEM (not shown here). Fig. 3 shows the variation of electrical resistivity of TiNx as a function of density. The density of a barrier film (TiNx )
Fig. 3. The resistivity of the as-deposited TiNx films as a function of the density of the film.
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was calculated from XPS and XRD spectra as described by Callister [20]. We can obtain the sum of the molecular weights from XPS and lattice constant from XRD. The lower electrical resistivity of the TiNx film was attributed to differences in film microstructure and density. As the absolute value of the biasing voltage increased, the TiNx film density increased. The deeply biased film had density approach to the bulk density of TiN (ρ = 5.22 g cm−3 ) [21]. This is consistent with the results reported by Moshfegh and Akhavan [22]. They pointed out that films with lower resistivity have higher density. A similar result for ZrN sputtered on Si substrate is also observed by Chou et al. [23]. Titanium nitride has a defect structure and deviations from stoichiometry are frequent [10]. For the film with a lower density, the amount of lattice defect in the film is large and consequently a higher resistivity is expected. It has been documented that increased energy of the impinging ions and atoms promotes surface diffusion to fill voids. Furthermore, with higher substrate bias voltage, nucleation was enhanced and resulted in smaller grain size and higher packing density [22]. Increasing ion bombardment during deposition causes an interruption of the growing columns and nucleation of new grains. This effect combined with increasing mobility of the condensing atoms that favors the formation of a dense microstructure has also been reported earlier [24]. However, Moriyama et al. [7] pointed out that the resistivities of TiN films decrease with increasing the grain sizes. Our result quite differs from the result obtained by them, which can be attributed to the film density. Therefore, to obtain layers with high density and low electrical resistivity, high power and high substrate bias are preferred. 3.2. Stability of Cu (100 nm)/TiNx (25 nm)/Si structure The thermal stability of TiNx thin films between Cu and Si was investigated. Cu (100 nm)/TiNx (25 nm)/Si samples annealed at various temperatures were characterized by sheet resistance measurement and XRD. A four-point probe system can be employed to measure the relative change in sheet resistance of the Cu (100 nm)/TiNx (25 nm)/Si samples following annealing at various temperatures for 60 min. Fig. 4 displays the relative changes in sheet resistance ( R/R0 (%)) for the Cu/TiNx /Si samples as a function of annealing temperature. It is clear that the relative changes in sheet resistance for the Cu/TiN0.86 (higher density, ρ = 5.12 g cm−3 )/Si system slightly reduced, indicating densification and structural improvement of the Cu layer. A similar reduction for Cu/Ta/SiO2 /Si structure is observed by Itow et al. [25]. A sudden sharp increase in sheet resistance of the films at 800 ◦ C can be related to interdiffusion and/or reaction between layers (Cu, TiNx , Cu) that occurred faster at higher temperatures. However, for the Cu/TiN0.81 (lower density, ρ = 4.99 g cm−3 )/Si a rapid increase in R/R0 (%) value was measured at lower temperature, i.e. 600 ◦ C. Similarly for Cu/TiN0.82 (ρ = 5.06 g cm−3 )/Si and Cu/TiN0.85 (ρ = 5.09 g cm−3 )/Si systems, a sharp increase in R/R0
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Fig. 4. Relative changes in sheet resistance ( R/R0 (%)) of various Cu/TiNx /Si samples induced by annealing at various temperatures for 60 min.
(%) value was measured at 700 and 750 ◦ C, respectively. Apparently, the higher density barrier film is superior to the lower density barrier film since the former exhibits an electrical failure temperature approximately 200 ◦ C above that of the latter. To understand the differences in the sheet resistance relative variation for these samples, the materials’ characteristics of these structures are examined by XRD. Figs. 5 and 6 depict the evolution of XRD patterns as a function of annealing temperature for the Cu/TiN0.86 (higher density)/Si and Cu/TiN0.81 (lower density)/Si samples, respectively. The thicknesses of the Cu and TiNx (x = 0.86, 081) again were 100 and 25 nm, respectively. The XRD results reveal only diffraction peaks of Cu (1 1 1) for the Cu/TiN0.86 (higher density)/Si sample (Fig. 5). It has been reported that Cu (1 1 1) provides higher electromigration resistance than that of Cu (2 0 0) [18]. Peak (1 1 1) for the TiNx layer is not clearly observed since it is very thin and the grains are small. As seen in Fig. 5, the spectra of the sample annealed at 600, 650, and 700 ◦ C are similar to that of the as-deposited one except that the intensity of the Cu (1 1 1) peak increases with annealing temperature. Precipitations of trace amounts of Cu3 Si [26] are observed after annealing at 800 ◦ C, and the Cu (1 1 1) peak remained. The formation of high-resistivity Cu3 Si phase results in the drastic increase of sheet resistance as shown in Fig. 4. A series of XRD patterns in Fig. 6
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Fig. 5. Evolution of XRD patterns for as-deposition Cu/TiN0.86 /Si samples annealed at various temperature for 60 min.
shows that phase-transition behavior of Cu/TiN0.81 (lower density)/Si samples following high-temperature annealing resembles that of Cu/TiN0.86 (higher density)/Si samples as discussed for Fig. 5. However, intensities of Cu (1 1 1) XRD peaks diminish and diffraction peaks of Cu3 Si appear after annealing at 600 ◦ C. The inset in Fig. 6 is an enlarged XRD spectrum for the Cu/TiN0.81 (lower density)/Si sample an-
Fig. 6. Evolution of XRD patterns for as-deposited Cu/TiN0.81 /Si samples annealed at various temperature for 60 min.
nealed at 600 ◦ C; the peak from Cu3 Si is observed, which indicates intermixing of Cu and Si through the TiNx barrier layer. Similar results of XRD were obtained for Cu/TiN0.82 (ρ = 5.06 g cm−3 )/Si and Cu/TiN0.85 (ρ = 5.09 g cm−3 )/Si
Fig. 7. XTEM micrograph of the Cu/TiN0.81 /Si structure annealed at 600 ◦ C.
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systems, that the appearance of Cu3 Si at 700 and 750 ◦ C, respectively. The XRD results also revealed that the increases in sheet resistance discussed in the previous section were perhaps caused by the loss of copper due to a Cu3 Si formation. The XTEM image of Cu3 Si had an inverted pyramid shape and was buried in Si substrate of Cu/TiN0.81 (lower density)/Si structure annealed at 600 ◦ C for 30 min as shown in Fig. 7. This suggests that Cu diffuses across TiNx into the Si and reacts with Si to form Cu3 Si. The oblique planes of the Si at the Cu3 Si/Si interface are the (1 1 1) type. This is a general case for failure behavior of Cu/barrier/Si systems [27]. The appearance of Cu3 Si pyramids in the interface of Cu/TiNx /Si samples is attributed to the failure mechanism which may be associated with the diffusion of Cu and Si via the grain boundaries of the nanocrystalline TiNx barriers. According to SEM micrographs (not shown here), we found that Cu3 Si pyramids were randomly distributed. However, one can hardly observe the diffraction peak of Cu3 Si phase. This is because TEM is more sensitive than XRD for microanalysis. Similar phenomena were also reported by Chen and Chen [28]. It was found that the thermal stability was strongly correlated with the density of the film which was found to be the key factor in controlling the thermal stability of the TiN barrier layers. The highest temperature at which the TiN0.86 (higher density) layer prevents Cu diffusion is 800 ◦ C for 60 min. These results satisfy the requirements for the barrier layer used in the special devices, which must withstand the thermal stability of 700 ◦ C for 30 min [7].
4. Conclusion By rf sputtering from a TiN target and applying substrate biases of 0 to −100 V, we obtained nanocrystalline TiNx films of 3.1–37.4 nm grain size. The TiNx films deposited with the substrate bias voltage showed higher density, strong (1 1 1) preferred orientation, and small size in comparison without the substrate bias voltage. When TiNx film of 25 nm in thickness is applied as a diffusion barrier between copper and silicon substrate, no apparent change in sheet resistance and no evidence of reaction are observed for the Cu/TiN0.86 (higher density, ρ = 5.12 g cm−3 )/Si samples annealed above 800 ◦ C for 60 min. However, the Cu/TiN0.81 (lower density, ρ = 4.99 g cm−3 )/Si samples showed a drastic increase in variation percentage of sheet resistance after annealing up to 600 ◦ C for 60 min. The drastic increase of the sheet resis-
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