- Email: [email protected]
Improvement of the properties and electrical performance on TiCl4-based TiN film using sequential flow chemical vapor deposition process
Thin Solid Films 518 (2010) 2285–2289
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Improvement of the properties and electrical performance on TiCl4-based TiN film using sequential flow chemical vapor deposition process Ming-Da Cheng, Tuung Luoh ⁎, Chin-Ta Su, Ta-Hung Yang, Kuang-Chao Chen, Chih-Yuan Lu Macronix International Co., LTD., No. 16, Li-Hsin Road, Science Park, Hsin-chu, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 28 October 2008 Received in revised form 25 September 2009 Accepted 7 October 2009 Available online 26 October 2009 Keywords: Titanium nitride Chemical vapor deposition Metal organic chemical vapor deposition Titanium chloride Secondary ion mass spectroscopy Transmission electron microscopy
a b s t r a c t Sequential flow chemical vapor deposition (SFCVD), utilizing TiCl4/NH3 as reactants and immediate NH3 treatment after film deposition, is applied to produce TiN barrier films in the contact process. Secondary ion mass spectroscopy results indicate that the SFCVD TiN film can effectively block the diffusion of WF6 into the underlying Ti layer during W deposition. NH3 treatment immediately after film deposition causes SFCVD TiN films to be less contaminated with carbon than TiN films that are formed by metallic organic compounds chemical vapor deposition (MOCVD) and to contain less chlorine residue than conventional TiCl4/NH3 CVD TiN layers even at a low reaction temperature. According to the resistance measurement of Kelvin contacts, the SFCVD process yields a lower resistance and a more uniform distribution than the MOCVD or CVD process. Transmission electron microscopic observations demonstrate that WF6 can diffuse through the MOCVD TiN to react with the underlying Ti layer, causing a rupture at the Ti/TiN interface and poor W adhesion. The SFCVD TiN can serve as a sufficient diffusion barrier against WF6 penetration during W CVD deposition. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Titanium nitride (TiN) is an attractive diffusion barrier material that is employed in the semiconductor industry, because of its thermal and chemical stability [1]. TiN films are traditionally deposited by physical vapor deposition (PVD) [2], which forms films with little impurity and low resistivity [3]. However, this technique offers poor step coverage and fails to meet requirements of semiconductor technology as devices continue to be scaled-down. The limitations of PVD have led to the use of chemical vapor deposition (CVD) to deposit TiN. The precursors that are most commonly used for the CVD of TiN films are titanium tetrachloride (TiCl4) and metallic organic compounds such as tetrakis diethylamido titanium and tetrakis dimethylamido titanium (TDMAT) [4]. TiN films formed by metallic organic compounds chemical vapor deposition (MOCVD) typically are contaminated with carbon (C), oxygen and hydrogen, which degrade their electrical properties. Plasma treatment is generally applied to reduce the amount of impurities [5]. However, as devices continue to shrink, the contacts or vias in the interconnection metallization become formed on the nanometer scale. Plasma cannot easily be used to treat TiN films in small contacts or vias. If the plasma treatment is not sufficient, it will degrade the resistance of contacts or vias. Thus, the efficiency of plasma treatment of MOCVD TiN films is an important issue. K. W. Chen et al. [6] suggested that a
⁎ Corresponding author. Tel.: +886 3 5786688x78173; fax: +886 3 5789087. E-mail address: [email protected] (T. Luoh). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.10.013
high treatment energy could eliminate C or impurities in MOCVD TiN films but cause poor tungsten contact holes to be formed. The TiCl4-based CVD process is another commonly used method for depositing TiN. The TiCl4 precursor can react with nitrogen (N2) and hydrogen (H2) to form TiN films according to the equation, 2TiCl4 ðgÞ + N2 ðgÞ + 4H2 ðgÞ→2TiNðsÞ + 8HClðgÞ
ð1Þ
J. P. Dekker et al. [7] proposed that the TiN growth rate depends on the reactant concentration, but noted that the deposition temperature, in the range 727 – 1000 °C, is relatively high. The usage of ammonia (NH3) can reduce the deposition temperature by the following equation [8], 2TiCl4 ðgÞ + 2NH3 ðgÞ + H2 ðgÞ→2TiNðsÞ + 8HClðgÞ
ð2Þ
The effect of NH3 on the growth characteristics of TiN films at a deposition temperature as low as to 600 °C has been studied [9]. The TiN growth rate and the intensity of (200) orientation reportedly increase with the flow rate of NH3. Decreasing the deposition temperature by using a NH3 reactant favors the CVD of TiN. However, the contamination with chlorine (Cl) is another important issue in the TiCl4/NH3-based CVD TiN process. Cl contamination in the film can reduce the reliability of a device. The Cl content is typically inversely proportional to the deposition temperature [8]. Reducing the deposition temperature increases the amount of Cl residue in the TiN film.
2286
M.-Da Cheng et al. / Thin Solid Films 518 (2010) 2285–2289
As mentioned above, achieving a low Cl content and a low deposition temperature simultaneously in CVD TiN processes is difficult. This work describes the deposition of a TiCl4/NH3-based TiN film by sequential flow chemical vapor deposition (SFCVD). It investigates barrier properties of various TiN films that are prepared by SFCVD, conventional TiCl4/NH3-based CVD and MOCVD. Kelvin contact resistance is measured to evaluate the electrical performance of the films. The diffusion behavior on stack films, Si/Ti (TiSix)/TiN/W, is also studied using secondary ion mass spectroscopy (SIMS). 2. Experimental Single crystalline, p-type (100)-oriented silicon wafers were covered with a Ti adhesion layer. Then, three TiN films, MOCVD TiN, CVD TiN and SFCVD TiN, were deposited on Ti. The MOCVD TiN film was carried out with a TDMAT precursor. A total MOCVD TiN film thickness of 7 nm was obtained by repeating a single process of 3.5 nm with in-situ plasma treatment using mixed N2/H2 gas. The deposition of TiN by CVD was achieved in a conventional TiCl4/NH3based CVD chamber at 600 °C, and post NH3 treatment was performed after TiN deposition to reduce the Cl content. SFCVD TiN films with thicknesses of 7 and 4 nm were deposited in the same CVD TiN chamber at 490 °C. As shown in Fig. 1, a single SFCVD cycle comprised four steps: deposition with TiCl4/NH3, purging with N2, treatment with NH3 and purging with N2. The TiN thickness obtained in a single cycle in SFCVD depended on the processing time in the deposition step, and the final TiN thickness was achieved by repeating the cycle. A rapid thermal process was performed at 600 °C in N2 ambient after the deposition of TiN to form TiSix. W was deposited on TiN films by the chemical vapor reaction of tungsten fluoride (WF6), silane and H2. To investigate the barrier properties of various TiN films, the worst deposition of W with a thin nucleation layer (with a thickness of about 15 nm) was intentionally employed. A thin W nucleation layer may fascinate the penetration of WF6 during W deposition. SIMS was used to analyze the concentration depth profiles in the Si/Ti (TiSix)/TiN/W stack that was deposited on blanket wafers. A contact short loop was prepared to evaluate the contact resistance. The contact process was employed in the deposition of Ti, TiN, and W films, as described above. The contact resistance was measured by obtaining Kelvin contact patterns with a contact hole with an aspect ratio of 6.5 and a critical bottom dimension of 84 nm on N+ and P+ silicon. Contact resistance was measured using an Agilent 4070 series semiconductor parametric tester (HP 4072). Transmission electron microscopy (TEM) was employed to obtain contact profiles and analyze the failed samples. Energy dispersive X-ray spectrometry (EDX) was used to determine the film composition.
Fig. 1. Representation of a SFCVD cycle for TiN deposition.
3. Results and discussion 3.1. Contact resistance Fig. 2(a) plots the N+ contact resistance of various TiN films that are covered with a thin W nucleation layer, which allows WF6 to penetrate easily into the diffusion barrier layer. The MOCVD TiN film has the highest resistance and the widest resistance distribution. In contrast, the CVD TiN and SFCVD TiN films have a low contact resistance and a narrow resistance distribution. The thin SFCVD TiN film with a thickness of 4 nm has the lowest resistance. Fig. 2(b) plots the P+ contact resistance. The MOCVD TiN film has the highest P+ contact resistance and a broad distribution. The CVD TiN and SFCVD TiN films still have sharp resistance profiles and lower resistances. The 4 nm-thick SFCVD TiN layer has a low and almost equal resistance in both N+ and P+ contacts (Fig. 2). 3.2. Physical failure analysis TEM is used to analyze failed samples with a high P+ contact resistance. Fig. 3 displays TEM cross sections of a normal sample (with a resistance of 584 Ω, as in Fig. 3a) and failed sample (with a resistance of 3640 Ω, as in Fig. 3b) in P+ contacts for samples that were prepared with a 7 nm-thick SFCVD TiN film and a 7 nm-thick MOCVD TiN film, respectively. A poor W structure is detected in the contact when MOCVD is employed to produce TiN films (Fig. 3b). In contrast, the SFCVD TiN has a complete contact profile, as shown in Fig. 3(a). The titanium silicide (TiSix) at the Ti/Si interface reduces the contact resistance for local interconnections. The formation of TiSix is
Fig. 2. Contact resistance for (a) N+ and (b) P+ type silicon. Various TiN films are deposited by SFCVD, CVD and MOCVD with thicknesses of 7 and 4 nm.
M.-D. Cheng et al. / Thin Solid Films 518 (2010) 2285–2289
2287
Fig. 4. (a) The TEM image of failed P+ contact profile for 7 nm-thick MOCVD TiN film, and (b) the results of EDX linear scan from point A to B labeled in (a). Fig. 3. TEM images of P+ contact profiles for 7 nm-thick TiN films prepared with (a) SFCVD and (b) MOCVD.
critical to ensure the establishment of the contact resistance. A comparison of the contact structures in Fig. 3 indicates the formation of a conformal TiSix layer with a thickness of around 13.5 nm at the Si/ TiN interface at the bottom of the contacts in both MOCVD and SFCVD samples. Hence, the large difference between the resistance of the failed and normal contacts is not caused by the TiSix layers. Some researches have reported that WF6 can be reduced by Ti during W deposition, and F atoms can penetrate the TiN film to form TiFx compounds according to the reaction [10,11], 2WF6 ðgÞ + 4TiðsÞ→2WðsÞ + 4TiF3 ðsÞ
ð3Þ
The formation of TiFx can rupture the TiN layer, resulting in the delamination of TiN/Ti interface and then the weak adhesion of W films. Fig. 4(a) presents the failed contact profile of the top region of the MOCVD TiN barrier. Fig. 4(b) displays an EDX analysis with a linear scan from point A to B (labeled in Fig. 4a). The signals that correspond to F, Ti, and W atoms all appear at the same position (about 30 nm) and start to decrease at 50 nm. W and the TiFx compound are thus formed at the original SiO2/(Ti + TiN) interface (the sidewall of the contact) according to Eq. (3). In contrast to the failed MOCVD TiN sample, the SFCVD TiN sample has an intact contact profile, which is presented in Fig. 5(a). The results of the EDX linear scan (Fig. 5b) from point C to D (marked in Fig. 5a) show that the normal structure is SiO2/Ti/TiN/W at the margin of the contact. As
shown in Fig. 5(b), the Ti signal appears at the SiO2/Ti interface (at 25 nm) and the N signal arises when encountering the Ti/TiN interface (near 35 nm). An abruptly rising W peak is found as the scan approaches the TiN/W interface. No F signal is generated during the scan, indicating that SFCVD TiN acts as a sufficient barrier against WF6 penetration. As discussed above, WF6 can diffuse into MOCVD TiN and to attack the underlying Ti layer, causing poor W adhesion inside the contact and inducing a high contact resistance, whereas SFCVD TiN prevents the penetration of WF6, and provides a stable contact resistance.
3.3. SIMS results The stack structure of Si/Ti (TiSix)/TiN/W is investigated by SIMS to evaluate the degree of contamination during film deposition. Fig. 6 presents the SIMS depth profiles of various elements, which are N, F, C and Cl. The N profile is viewed in the light of the depth profile of each element to determine the relative position of the TiN film in the Si/Ti (TiSix)/TiN/W stack. Fig. 6(a) shows the F concentration profile in the Si/Ti (TiSix)/TiN/W stack, indicating that much F diffuses deeply into the Ti (TiSix) region through the MOCVD TiN film, while less F is found in the Ti (TiSix) region in the SFCVD TiN sample. This result suggests that the SFCVD TiN is a more effective barrier against WF6 penetration during W CVD deposition, and is consistent with the data on contact resistances (Fig. 2) and TEM images (Fig. 3). F can penetrate through the MOCVD TiN to react with Ti (Fig. 3), causing a high contact
2288
M.-Da Cheng et al. / Thin Solid Films 518 (2010) 2285–2289
Fig. 5. (a) The TEM image of P+ contact profile in the SFCVD TiN 7 nm sample, and (b) the results of EDX linear scan data from point C to D labeled in (a).
resistance (Fig. 2). Fig. 6(b) displays the C depth profile. The C contamination levels are similar among the three TiN films. Some studies have reported that plasma treatment removes impurities from MOCVD TiN films [5,6]. In this work, the MOCVD TiN film has the same amount of C impurity as the SFCVD and CVD TiN films, revealing that in-situ plasma treatment in the MOCVD process can effectively reduce C contamination. The plot in Fig. 6(c) compares Cl depth profiles of the CVD TiN and SFCVD TiN films. The SFCVD TiN sample has less Cl residue. Notably, the SFCVD TiN film is deposited at 490 °C while the CVD TiN film is deposited at 600 °C. The Cl content typically increases as the deposition temperature declines [8]. Therefore, as presented in Fig. 6(c), a lower Cl content in the TiN film can be obtained using a lower deposition temperature in SFCVD. The disadvantage of Cl is its corrosive property. In Al metallization, Al can be corroded by the adsorption of chloride ions, reducing the lifetime of the device. Increasing the amount of chlorine residual also increases the contact resistivity [12]. The Cl content in the SFCVD process is low because of the immediate NH3 treatment in every cycle: Cl that is adsorbed to the surface is removed effectively by NH3 treatment immediately following the deposition of a film in a single cycle in the SFCVD process. Unlike in the SFCVD process, NH3 treatment in the conventional TiCl4/NH3-based CVD is generally conducted after the final deposition. A TiN film with low Cl contamination can be obtained at a low deposition temperature by the SFCVD process. The SFCVD TiN film acts as a robust barrier against WF6 penetration in the thin W nucleation layer. When the SFCVD TiN film is thinned to 4 nm, it maintains stable contact resistance. The SFCVD TiN film is a promising
Fig. 6. SIMS depth profiles as a function of various TiN films: (a) N and F profiles, (b) N and C profiles, and (c) N and Cl profiles.
candidate barrier film in the contact process in the generation of nanometer-scale devices. 4. Conclusion In this work, a SFCVD TiN film is used as a barrier layer in the contact process in this study. The TiN film with little Cl residue and low contact resistance is formed by the SFCVD process at a low deposition temperature. The SFCVD TiN film is a robust barrier, inhibiting penetration by WF6 during W deposition. The N+ and P+ Kelvin test patterns exhibit uniform and low contact resistance. Accordingly, the SFCVD process can replace conventional TiCl4/NH3 CVD and MOCVD TiN processes in the formation of barrier films in integrated semiconductor devices. References [1] J.P. Lu, W.Y. Hsu, Q.Z. Hong, G.A. Dixit, J.D. Luttmer, R.H. Havemann, P.J. Chen, H.L. Tsai, L.K. Magel, Thin Solid Films 320 (1998) 20.
M.-D. Cheng et al. / Thin Solid Films 518 (2010) 2285–2289 [2] Y.M. Sung, H.J. Kim, Surf. Coat. Technol. 171 (2003) 75. [3] J. Zhao, E.G. Garza, K. Lam, C.M. Jones, Appl. Surf. Sci. 158 (2000) 246. [4] A. Berry, R. Mowery, N.H. Turner, L. Seitzman, D. Dunn, H. Ladouceur, Thin Solid Films 323 (1998) 10. [5] A. Sabbadini, F. Cazzaniga, S. Alberici, C. Bresolin, G. Casati, V. Cusi, G. Pavia, G. Queirolo, Microelectron. Eng. 55 (2001) 205. [6] K.W. Chen, Y.L. Wang, L. Chang, F.Y. Li, G.J. Hwang, Thin Solid Films 498 (2006) 64. [7] J.P. Dekker, P.J. van der Put, H.J. Veringa, J. Schoonman, J. Electrochem. Soc. 141 (1994) 787.
2289
[8] M.J. Buiting, A.F. Otterloo, A.H. Montree, J. Electrochem. Soc. 138 (1991) 500. [9] H.H. Huang, M.H. Hon, M.C. Wang, J. Cryst. Growth 240 (2002) 513. [10] S. Parikh, L. Akselrod, J. Gardner, K. Armstrong, N. Parekh, Thin Solid Films 320 (1998) 26. [11] G. Ramanath, J.E. Greene, J.R.A. Carlsson, L.H. Allen, J. Appl. Phys. 85 (1999) 1961. [12] Y. Mitsui, F. Yano, Y. Nakamura, K. Kimoto, T. Hasegawa, S. Kimura, K. Asayama, Electron Devices Meeting, 1998. IEDM '98 Technical Digest., International, 1998, p. 329.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Improvement of the properties and electrical performance on TiCl4-based TiN film using sequential flow chemical vapor deposition process Ming-Da Cheng, Tuung Luoh ⁎, Chin-Ta Su, Ta-Hung Yang, Kuang-Chao Chen, Chih-Yuan Lu Macronix International Co., LTD., No. 16, Li-Hsin Road, Science Park, Hsin-chu, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 28 October 2008 Received in revised form 25 September 2009 Accepted 7 October 2009 Available online 26 October 2009 Keywords: Titanium nitride Chemical vapor deposition Metal organic chemical vapor deposition Titanium chloride Secondary ion mass spectroscopy Transmission electron microscopy
a b s t r a c t Sequential flow chemical vapor deposition (SFCVD), utilizing TiCl4/NH3 as reactants and immediate NH3 treatment after film deposition, is applied to produce TiN barrier films in the contact process. Secondary ion mass spectroscopy results indicate that the SFCVD TiN film can effectively block the diffusion of WF6 into the underlying Ti layer during W deposition. NH3 treatment immediately after film deposition causes SFCVD TiN films to be less contaminated with carbon than TiN films that are formed by metallic organic compounds chemical vapor deposition (MOCVD) and to contain less chlorine residue than conventional TiCl4/NH3 CVD TiN layers even at a low reaction temperature. According to the resistance measurement of Kelvin contacts, the SFCVD process yields a lower resistance and a more uniform distribution than the MOCVD or CVD process. Transmission electron microscopic observations demonstrate that WF6 can diffuse through the MOCVD TiN to react with the underlying Ti layer, causing a rupture at the Ti/TiN interface and poor W adhesion. The SFCVD TiN can serve as a sufficient diffusion barrier against WF6 penetration during W CVD deposition. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Titanium nitride (TiN) is an attractive diffusion barrier material that is employed in the semiconductor industry, because of its thermal and chemical stability [1]. TiN films are traditionally deposited by physical vapor deposition (PVD) [2], which forms films with little impurity and low resistivity [3]. However, this technique offers poor step coverage and fails to meet requirements of semiconductor technology as devices continue to be scaled-down. The limitations of PVD have led to the use of chemical vapor deposition (CVD) to deposit TiN. The precursors that are most commonly used for the CVD of TiN films are titanium tetrachloride (TiCl4) and metallic organic compounds such as tetrakis diethylamido titanium and tetrakis dimethylamido titanium (TDMAT) [4]. TiN films formed by metallic organic compounds chemical vapor deposition (MOCVD) typically are contaminated with carbon (C), oxygen and hydrogen, which degrade their electrical properties. Plasma treatment is generally applied to reduce the amount of impurities [5]. However, as devices continue to shrink, the contacts or vias in the interconnection metallization become formed on the nanometer scale. Plasma cannot easily be used to treat TiN films in small contacts or vias. If the plasma treatment is not sufficient, it will degrade the resistance of contacts or vias. Thus, the efficiency of plasma treatment of MOCVD TiN films is an important issue. K. W. Chen et al. [6] suggested that a
⁎ Corresponding author. Tel.: +886 3 5786688x78173; fax: +886 3 5789087. E-mail address: [email protected] (T. Luoh). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.10.013
high treatment energy could eliminate C or impurities in MOCVD TiN films but cause poor tungsten contact holes to be formed. The TiCl4-based CVD process is another commonly used method for depositing TiN. The TiCl4 precursor can react with nitrogen (N2) and hydrogen (H2) to form TiN films according to the equation, 2TiCl4 ðgÞ + N2 ðgÞ + 4H2 ðgÞ→2TiNðsÞ + 8HClðgÞ
ð1Þ
J. P. Dekker et al. [7] proposed that the TiN growth rate depends on the reactant concentration, but noted that the deposition temperature, in the range 727 – 1000 °C, is relatively high. The usage of ammonia (NH3) can reduce the deposition temperature by the following equation [8], 2TiCl4 ðgÞ + 2NH3 ðgÞ + H2 ðgÞ→2TiNðsÞ + 8HClðgÞ
ð2Þ
The effect of NH3 on the growth characteristics of TiN films at a deposition temperature as low as to 600 °C has been studied [9]. The TiN growth rate and the intensity of (200) orientation reportedly increase with the flow rate of NH3. Decreasing the deposition temperature by using a NH3 reactant favors the CVD of TiN. However, the contamination with chlorine (Cl) is another important issue in the TiCl4/NH3-based CVD TiN process. Cl contamination in the film can reduce the reliability of a device. The Cl content is typically inversely proportional to the deposition temperature [8]. Reducing the deposition temperature increases the amount of Cl residue in the TiN film.
2286
M.-Da Cheng et al. / Thin Solid Films 518 (2010) 2285–2289
As mentioned above, achieving a low Cl content and a low deposition temperature simultaneously in CVD TiN processes is difficult. This work describes the deposition of a TiCl4/NH3-based TiN film by sequential flow chemical vapor deposition (SFCVD). It investigates barrier properties of various TiN films that are prepared by SFCVD, conventional TiCl4/NH3-based CVD and MOCVD. Kelvin contact resistance is measured to evaluate the electrical performance of the films. The diffusion behavior on stack films, Si/Ti (TiSix)/TiN/W, is also studied using secondary ion mass spectroscopy (SIMS). 2. Experimental Single crystalline, p-type (100)-oriented silicon wafers were covered with a Ti adhesion layer. Then, three TiN films, MOCVD TiN, CVD TiN and SFCVD TiN, were deposited on Ti. The MOCVD TiN film was carried out with a TDMAT precursor. A total MOCVD TiN film thickness of 7 nm was obtained by repeating a single process of 3.5 nm with in-situ plasma treatment using mixed N2/H2 gas. The deposition of TiN by CVD was achieved in a conventional TiCl4/NH3based CVD chamber at 600 °C, and post NH3 treatment was performed after TiN deposition to reduce the Cl content. SFCVD TiN films with thicknesses of 7 and 4 nm were deposited in the same CVD TiN chamber at 490 °C. As shown in Fig. 1, a single SFCVD cycle comprised four steps: deposition with TiCl4/NH3, purging with N2, treatment with NH3 and purging with N2. The TiN thickness obtained in a single cycle in SFCVD depended on the processing time in the deposition step, and the final TiN thickness was achieved by repeating the cycle. A rapid thermal process was performed at 600 °C in N2 ambient after the deposition of TiN to form TiSix. W was deposited on TiN films by the chemical vapor reaction of tungsten fluoride (WF6), silane and H2. To investigate the barrier properties of various TiN films, the worst deposition of W with a thin nucleation layer (with a thickness of about 15 nm) was intentionally employed. A thin W nucleation layer may fascinate the penetration of WF6 during W deposition. SIMS was used to analyze the concentration depth profiles in the Si/Ti (TiSix)/TiN/W stack that was deposited on blanket wafers. A contact short loop was prepared to evaluate the contact resistance. The contact process was employed in the deposition of Ti, TiN, and W films, as described above. The contact resistance was measured by obtaining Kelvin contact patterns with a contact hole with an aspect ratio of 6.5 and a critical bottom dimension of 84 nm on N+ and P+ silicon. Contact resistance was measured using an Agilent 4070 series semiconductor parametric tester (HP 4072). Transmission electron microscopy (TEM) was employed to obtain contact profiles and analyze the failed samples. Energy dispersive X-ray spectrometry (EDX) was used to determine the film composition.
Fig. 1. Representation of a SFCVD cycle for TiN deposition.
3. Results and discussion 3.1. Contact resistance Fig. 2(a) plots the N+ contact resistance of various TiN films that are covered with a thin W nucleation layer, which allows WF6 to penetrate easily into the diffusion barrier layer. The MOCVD TiN film has the highest resistance and the widest resistance distribution. In contrast, the CVD TiN and SFCVD TiN films have a low contact resistance and a narrow resistance distribution. The thin SFCVD TiN film with a thickness of 4 nm has the lowest resistance. Fig. 2(b) plots the P+ contact resistance. The MOCVD TiN film has the highest P+ contact resistance and a broad distribution. The CVD TiN and SFCVD TiN films still have sharp resistance profiles and lower resistances. The 4 nm-thick SFCVD TiN layer has a low and almost equal resistance in both N+ and P+ contacts (Fig. 2). 3.2. Physical failure analysis TEM is used to analyze failed samples with a high P+ contact resistance. Fig. 3 displays TEM cross sections of a normal sample (with a resistance of 584 Ω, as in Fig. 3a) and failed sample (with a resistance of 3640 Ω, as in Fig. 3b) in P+ contacts for samples that were prepared with a 7 nm-thick SFCVD TiN film and a 7 nm-thick MOCVD TiN film, respectively. A poor W structure is detected in the contact when MOCVD is employed to produce TiN films (Fig. 3b). In contrast, the SFCVD TiN has a complete contact profile, as shown in Fig. 3(a). The titanium silicide (TiSix) at the Ti/Si interface reduces the contact resistance for local interconnections. The formation of TiSix is
Fig. 2. Contact resistance for (a) N+ and (b) P+ type silicon. Various TiN films are deposited by SFCVD, CVD and MOCVD with thicknesses of 7 and 4 nm.
M.-D. Cheng et al. / Thin Solid Films 518 (2010) 2285–2289
2287
Fig. 4. (a) The TEM image of failed P+ contact profile for 7 nm-thick MOCVD TiN film, and (b) the results of EDX linear scan from point A to B labeled in (a). Fig. 3. TEM images of P+ contact profiles for 7 nm-thick TiN films prepared with (a) SFCVD and (b) MOCVD.
critical to ensure the establishment of the contact resistance. A comparison of the contact structures in Fig. 3 indicates the formation of a conformal TiSix layer with a thickness of around 13.5 nm at the Si/ TiN interface at the bottom of the contacts in both MOCVD and SFCVD samples. Hence, the large difference between the resistance of the failed and normal contacts is not caused by the TiSix layers. Some researches have reported that WF6 can be reduced by Ti during W deposition, and F atoms can penetrate the TiN film to form TiFx compounds according to the reaction [10,11], 2WF6 ðgÞ + 4TiðsÞ→2WðsÞ + 4TiF3 ðsÞ
ð3Þ
The formation of TiFx can rupture the TiN layer, resulting in the delamination of TiN/Ti interface and then the weak adhesion of W films. Fig. 4(a) presents the failed contact profile of the top region of the MOCVD TiN barrier. Fig. 4(b) displays an EDX analysis with a linear scan from point A to B (labeled in Fig. 4a). The signals that correspond to F, Ti, and W atoms all appear at the same position (about 30 nm) and start to decrease at 50 nm. W and the TiFx compound are thus formed at the original SiO2/(Ti + TiN) interface (the sidewall of the contact) according to Eq. (3). In contrast to the failed MOCVD TiN sample, the SFCVD TiN sample has an intact contact profile, which is presented in Fig. 5(a). The results of the EDX linear scan (Fig. 5b) from point C to D (marked in Fig. 5a) show that the normal structure is SiO2/Ti/TiN/W at the margin of the contact. As
shown in Fig. 5(b), the Ti signal appears at the SiO2/Ti interface (at 25 nm) and the N signal arises when encountering the Ti/TiN interface (near 35 nm). An abruptly rising W peak is found as the scan approaches the TiN/W interface. No F signal is generated during the scan, indicating that SFCVD TiN acts as a sufficient barrier against WF6 penetration. As discussed above, WF6 can diffuse into MOCVD TiN and to attack the underlying Ti layer, causing poor W adhesion inside the contact and inducing a high contact resistance, whereas SFCVD TiN prevents the penetration of WF6, and provides a stable contact resistance.
3.3. SIMS results The stack structure of Si/Ti (TiSix)/TiN/W is investigated by SIMS to evaluate the degree of contamination during film deposition. Fig. 6 presents the SIMS depth profiles of various elements, which are N, F, C and Cl. The N profile is viewed in the light of the depth profile of each element to determine the relative position of the TiN film in the Si/Ti (TiSix)/TiN/W stack. Fig. 6(a) shows the F concentration profile in the Si/Ti (TiSix)/TiN/W stack, indicating that much F diffuses deeply into the Ti (TiSix) region through the MOCVD TiN film, while less F is found in the Ti (TiSix) region in the SFCVD TiN sample. This result suggests that the SFCVD TiN is a more effective barrier against WF6 penetration during W CVD deposition, and is consistent with the data on contact resistances (Fig. 2) and TEM images (Fig. 3). F can penetrate through the MOCVD TiN to react with Ti (Fig. 3), causing a high contact
2288
M.-Da Cheng et al. / Thin Solid Films 518 (2010) 2285–2289
Fig. 5. (a) The TEM image of P+ contact profile in the SFCVD TiN 7 nm sample, and (b) the results of EDX linear scan data from point C to D labeled in (a).
resistance (Fig. 2). Fig. 6(b) displays the C depth profile. The C contamination levels are similar among the three TiN films. Some studies have reported that plasma treatment removes impurities from MOCVD TiN films [5,6]. In this work, the MOCVD TiN film has the same amount of C impurity as the SFCVD and CVD TiN films, revealing that in-situ plasma treatment in the MOCVD process can effectively reduce C contamination. The plot in Fig. 6(c) compares Cl depth profiles of the CVD TiN and SFCVD TiN films. The SFCVD TiN sample has less Cl residue. Notably, the SFCVD TiN film is deposited at 490 °C while the CVD TiN film is deposited at 600 °C. The Cl content typically increases as the deposition temperature declines [8]. Therefore, as presented in Fig. 6(c), a lower Cl content in the TiN film can be obtained using a lower deposition temperature in SFCVD. The disadvantage of Cl is its corrosive property. In Al metallization, Al can be corroded by the adsorption of chloride ions, reducing the lifetime of the device. Increasing the amount of chlorine residual also increases the contact resistivity [12]. The Cl content in the SFCVD process is low because of the immediate NH3 treatment in every cycle: Cl that is adsorbed to the surface is removed effectively by NH3 treatment immediately following the deposition of a film in a single cycle in the SFCVD process. Unlike in the SFCVD process, NH3 treatment in the conventional TiCl4/NH3-based CVD is generally conducted after the final deposition. A TiN film with low Cl contamination can be obtained at a low deposition temperature by the SFCVD process. The SFCVD TiN film acts as a robust barrier against WF6 penetration in the thin W nucleation layer. When the SFCVD TiN film is thinned to 4 nm, it maintains stable contact resistance. The SFCVD TiN film is a promising
Fig. 6. SIMS depth profiles as a function of various TiN films: (a) N and F profiles, (b) N and C profiles, and (c) N and Cl profiles.
candidate barrier film in the contact process in the generation of nanometer-scale devices. 4. Conclusion In this work, a SFCVD TiN film is used as a barrier layer in the contact process in this study. The TiN film with little Cl residue and low contact resistance is formed by the SFCVD process at a low deposition temperature. The SFCVD TiN film is a robust barrier, inhibiting penetration by WF6 during W deposition. The N+ and P+ Kelvin test patterns exhibit uniform and low contact resistance. Accordingly, the SFCVD process can replace conventional TiCl4/NH3 CVD and MOCVD TiN processes in the formation of barrier films in integrated semiconductor devices. References [1] J.P. Lu, W.Y. Hsu, Q.Z. Hong, G.A. Dixit, J.D. Luttmer, R.H. Havemann, P.J. Chen, H.L. Tsai, L.K. Magel, Thin Solid Films 320 (1998) 20.
M.-D. Cheng et al. / Thin Solid Films 518 (2010) 2285–2289 [2] Y.M. Sung, H.J. Kim, Surf. Coat. Technol. 171 (2003) 75. [3] J. Zhao, E.G. Garza, K. Lam, C.M. Jones, Appl. Surf. Sci. 158 (2000) 246. [4] A. Berry, R. Mowery, N.H. Turner, L. Seitzman, D. Dunn, H. Ladouceur, Thin Solid Films 323 (1998) 10. [5] A. Sabbadini, F. Cazzaniga, S. Alberici, C. Bresolin, G. Casati, V. Cusi, G. Pavia, G. Queirolo, Microelectron. Eng. 55 (2001) 205. [6] K.W. Chen, Y.L. Wang, L. Chang, F.Y. Li, G.J. Hwang, Thin Solid Films 498 (2006) 64. [7] J.P. Dekker, P.J. van der Put, H.J. Veringa, J. Schoonman, J. Electrochem. Soc. 141 (1994) 787.
2289
[8] M.J. Buiting, A.F. Otterloo, A.H. Montree, J. Electrochem. Soc. 138 (1991) 500. [9] H.H. Huang, M.H. Hon, M.C. Wang, J. Cryst. Growth 240 (2002) 513. [10] S. Parikh, L. Akselrod, J. Gardner, K. Armstrong, N. Parekh, Thin Solid Films 320 (1998) 26. [11] G. Ramanath, J.E. Greene, J.R.A. Carlsson, L.H. Allen, J. Appl. Phys. 85 (1999) 1961. [12] Y. Mitsui, F. Yano, Y. Nakamura, K. Kimoto, T. Hasegawa, S. Kimura, K. Asayama, Electron Devices Meeting, 1998. IEDM '98 Technical Digest., International, 1998, p. 329.