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Effect of plasma immersion ion implantation on the thermal stability of diffusion barrier layers
Surface & Coatings Technology 186 (2004) 77 – 81 www.elsevier.com/locate/surfcoat
Effect of plasma immersion ion implantation on the thermal stability of diffusion barrier layers Mukesh Kumar a,b,*, Rajkumar c, P.M. Raole d, S.K. Gupta d, Dinesh Kumar b, P.J. George b a
Advanced Materials and Devices Group, TASC-INFM, Trieste, Italy Electronic Science Department, Kurukshetra University, Kurukshetra, Haryana, India c Semiconductor Complex Ltd., Industrial Area, Sector-72, S.A.S Nagar, Chandigarh, India d FCIPT, Institute of Plasma Research, Gandhinagar, Gujarat, India b
Available online 1 June 2004
Abstract Plasma immersion ion implantation technique has been used to synthesize titanium nitride and tantalum nitride layer to acts as diffusion barrier against copper diffusion. Two different doses of nitrogen, 1015 ions/cm2 (low dose) and 1017 ions/cm2 (high dose), were implanted in refractory metal-coated silicon wafers. Diffusion barrier properties of these nitride layers were evaluated after annealing the samples up to 700 jC for 30 min. Sheet resistance, SEM and X-ray diffraction were carried out to investigate the effect of annealing. The highly implanted layer reduces the diffusion of Cu metal through it. The enhancement in its diffusion barrier properties is supposed to be due to nitridation of metal films, which increases the activation energy involved for its chemical reaction with copper metal film. D 2004 Elsevier B.V. All rights reserved. Keywords: Diffusion barrier; Copper; Plasma immersion ion implantation; Scanning electron microscope; X-ray diffraction
1. Introduction The present-day high-speed, low-power and reliable silicon (Si) based integrated circuits (IC) make use of copper (Cu) metal as interconnection material [1– 5]. To solve the high diffusivity problem of this material into Si, SiO2 and other polymer layers, a thin, conformal and efficient diffusion barrier between Cu and these materials is mandatory [6– 8]. The properties required for an efficient diffusion barrier have been reviewed by Nicolet [9] and other workers [10]. The diffusion through a thin film can be reduced by carefully controlling their deposition conditions and postdeposition treatment. Although a lot of amorphous material layers, refractory materials and their nitrides have been investigated for diffusion barrier applications [11 –15], but titanium (Ti) material has gained more popularity as it behaves not only as diffusion barrier but also acts as adhesion promoter. The reactive sputtering and use of a * Corresponding author. c/o Rajkumar, Semiconductor Complex Ltd., Industrial Area, Sector 72, S.A.S. Nagar (Near Chandigarh), Distt. Ropar, Punjab 150059, India. Tel.: +91-9815501585 (mobile). E-mail address: [email protected] (M. Kumar). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.04.016
composite sputtering target has been the main tool for fabrication of nitride and carbide layers for diffusion barrier applications for so many years. However, all these techniques are limited by degree of nitridation achieved. Refractory metal nitride layers formed by ion implantation can also be a possible technique for diffusion barrier application as implantation causes both nitridation and some degree of amorphism of metal layers [16]. Due to the small thickness of diffusion barrier films, ion implantation is carried out at low energy. Beam line ion implantation (BLII) suffers from low dose rate at small energies, and thus cannot be used to achieve a high degree of nitridation of thin metal films. In addition to this increased complexity, its high operational cost has limited the use of BLII to synthesis barrier layers. Recently, plasma immersion ion implantation (PIII), a new technique that offers a high dose rate even at low energy, has been successfully utilized to carry out nitridation of a thin Ti metal layer [17]. In the present study, we have used PIII technique to synthesize a TiN and TaN layer and studied their performance as diffusion barrier layer. The advent of plasma immersion ion implantation (PIII) has overcome the limitations of conventional ion implanta-
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tion technique and so offers fresh possibilities for the implementation of ion implantation technique in the field of nitridation of thin metal films [18 –24]. The authors have successfully synthesized the titanium nitride and tantalum nitride films as diffusion barrier for copper metal using PIII.
The accelerating voltage was kept at 35 kV. The tube current was kept at 10 mA. The goniometer range used was at 25– 60j values. The X-ray diffraction pattern (2h vs. intensity) results were obtained and plotted using an X – Y recorder. JCDPS (Joint Committee Powder Diffraction Standards) files were used to identify the various peaks obtained in XRD spectrum.
2. Experimental details In the present study, n-type silicon with < 100> orientation having resistivity of 1– 10 V cm was used as substrate. The layers of Ti and Ta of approximately 100-nm thickness were deposited on separate pieces of Si samples by RF sputtering in argon plasma. The base pressure was 10 – 6 Torr while pressure during deposition was 20 mTorr. The separation between target and substrate was 7 cm and applied RF power was 200 W. These samples were implanted with nitrogen ions by plasma immersion ion implantation, to form Ti(N)/Si and Ta(N)/Si structure. Some metal-coated samples were kept unimplanted to compare the performance of elemental and implanted diffusion barrier layers. Nitrogen implantation in Ti/Si and Ta/Si samples was carried out by a PIII system. Two doses of nitrogen, 1015 ions/cm2 (low dose) and 1017ions/cm2 (high dose), were implanted at an energy of 20 keV. The chamber was evacuated to a base pressure of 10 6 Torr. Nitrogen was then introduced in the chamber and a working pressure of 10 3 Torr was maintained by controlling the gas flow. RF power of 100 W was used to generate plasma needed for carrying out implantation. Samples placed on a metallic chuck were connected with a 20 KV pulse voltage. Pulse voltage and ion current were measured using an oscilloscope. Different implant doses were obtained by varying the frequency and width of applied pulse voltage. For low dose, frequency and pulse width were maintained at 50 Hz and 10 As, respectively. For high dose, frequency and pulse width were selected at 1 kHz and 50 As, respectively. Total implantation time was maintained at 10 min for both, i.e., low- and high-dose, samples. After implantation, Copper layer of 200-nm thickness was deposited on Ti/Si, Ta/Si, Ti(N)/Si, Ta(N)/Si and Si samples by DC sputtering system. Measurement of thickness was performed by Dektek 3030 ST Profilometer. Annealing of samples was carried out in N2 ambient. For this purpose, first of all, the annealing chamber was evacuated using rotary pump and then flushed with N2 gas for few minutes. Annealing at different temperatures was carried out for 30 min. To avoid the oxidation of the top Cu layer, samples were allowed to cool down inside the chamber itself before unloading. X-ray diffraction and four probe sheet resistance measurements at room temperature were used for the characterization of the samples. For X-ray diffraction, Philips Model PW 1729 X-ray Diffractometer was used. The target consists of copper metal whereas nickel metal is used as h-filter.
3. Results Fig. 1 shows the sheet resistance variation of Cu/Si, Cu/ Ti/Si, Cu/Ti(1015 ions/cm2 N)/Si and Cu/Ti(1017 ions/cm2 N)/Si structures vs. annealing temperature. Sheet resistance of Cu/Si structure increases after annealing at 200 jC for 30 min. This increase in sheet resistance may be due to some reactions at the Cu – Si interface. Annealing up to 300 jC of Cu/Ti/Si structure does not lead the sheet resistance to increase. However, for samples annealed at 400 jC, a slight increase in sheet resistance is observed. But after annealing at 500 jC, an abrupt change in sheet resistance is observed, which indicates some type of interaction between constituents, so it shows that Ti layer no longer acts as the diffusion barrier at 400 jC. It is also observed that a dose of 1015 ions/ cm2 in Ti layer is not sufficient to cause any change in the sheet resistance curve to that of unimplanted sample. However, implantation with a high dose (1017 ions/cm2) of nitrogen in Ti layer has resulted to significant change in the sheet resistance curve. In this case, sheet resistance is found to remain constant even after annealing the sample at a temperature of 500 jC, but increase in sheet resistance is observed at 700 jC, which indicates that Ti layer with 1017 ions/cm2 dose of nitrogen does not act as stable diffusion barrier for copper metal. Fig. 2 shows the sheet resistance variation of Cu/Si, Cu/ Ta/Si, Cu/Ta(1015 ions/cm2 N2)/Si and Cu/Ta (1017 ions/cm2 N2)/Si structures vs. annealing temperature. Sheet resistance of Cu/Si structure increases after annealing at 200 jC for 30
Fig. 1. Sheet resistance variation of Cu/Si, Cu/Ti/Si, Cu/Ti(1015 ions/cm2 N2)/Si and Cu/Ti(1017 ions/cm2 N2)/Si structures vs. annealing temperature.
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Fig. 2. Sheet resistance variation of Cu/Si, Cu/Ta/Si, Cu/Ta(1015 ions/cm2 N2)/Si and Cu/Ta(1017 ions/cm2 N2)/Si structures vs. annealing temperature.
min. Annealing of Cu/Ta/Si structure up to 500 jC does not lead to increased sheet resistance. After annealing at 700 jC, abrupt change in sheet resistance is observed, which indicate some type of interaction between the materials. It is also observed that a dose of 1015 ions/cm2 in Ta is not sufficient to cause any change in the sheet resistance curve to that of unimplanted sample. However, implantation with a dose (1017 ions/cm2) of nitrogen in Ta layer has resulted to significant change in the sheet resistance curve. In this case, sheet resistance is found to remain constant even after annealing the sample at a temperature of 700 jC. It seems that interaction in the form of any chemical reaction or interdiffusion has not taken place in the structure even after annealing at 700 jC. Fig. 3 shows XRD patterns of Cu/Ti(1015N)/Si samples. Unannealed sample shows the peaks of Cu (111), Cu (200), along with a peak of Ti. It is evident from this figure that an implantation of 1015 ions/cm2 nitrogen does not cause any nitridation of the Ti film as no titanium nitride phase is observed. Formation of titanium silicide and copper silicide is detected in this sample after annealing at 400 jC. So, we can say that the Ti layer, after its implantation with 1015 ion/ cm2 dose of nitrogen, can work as diffusion barrier for Cu only up to 400 jC. The XRD of as-deposited and annealed Cu/Ti(1017N+)/Si structure is shown in Fig. 4. The presence of TiN peaks indicates that a nitrided phase has formed in the film due to implantation. Sample annealed at 500 jC shows that peaks of TiSi2 are obtained. No peak of copper silicide was detected. After annealing at 700 jC, copper silicide peak was also observed, which indicates that copper reacted with underlying silicon material. The intensities of copper peaks were also found to be reduced, which indicates that loss of copper material from the top surface has occurred. The results of present experiment show that diffusion barrier
Fig. 3. X-ray diffraction pattern of as-deposited and annealed Cu/Ti(1015 ions/cm2 N+)/Si samples.
properties of elemental titanium layer is modified and it becomes able to stop the copper diffusion up to at least 500 jC after implantation of 1017 ions/cm2 dose of nitrogen in it by plasma immersion ion implantation. Fig. 5 shows the XRD patterns of Cu/Ta(1015 N+)/Si samples. The XRD data of sample annealed at 700 jC indicates that some kind of chemical interaction between
Fig. 4. X-ray diffraction pattern of as-deposited and annealed Cu/Ti(1017 ions/cm2 N+)/Si samples.
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Fig. 5. X-ray diffraction pattern of as-deposited and annealed Cu/Ta(1015 ions/cm2 N+)/Si samples.
constituents has occurred. Formation of tantalum silicide and copper silicide is detected in this sample after annealing at 700 jC. The XRD of as-deposited and annealed Cu/Ta(1017N+)/ Si structure is shown in Fig. 6. Unannealed sample shows the peaks of Cu (111), Cu (200) and Ta (410) along with two peaks of Ta2N. Sample annealed at 500 jC shows that no interaction among the metal layers has taken place and all the peaks are intact. After annealing at 700 jC, peaks of TaSi2 are obtained, whereas no peak of copper silicide was observed in this sample. Intensity of Cu peaks was also found unchanged, which indicates that there is no loss of Cu from the top layer of the structure. It shows that even at 700 jC, the structure is stable and hence diffusion barrier properties of Ta has been improved by implantation of 1017 ions/cm2 dose of nitrogen in it by plasma immersion ion implantation.
reacting with and completely consuming the TiN diffusion barrier. This process does not seem to take place in the present experimental results due to the fact that height of TiN peaks are still intact after annealing at temperature of 700 jC, which indicates that there is no loss of TiN layer. No conclusive evidence could be found for the presence of reaction compound between titanium-nitride film and Cu or Si. The second path is migration of Cu atoms through the TiN diffusion barrier either by lattice diffusion or by diffusion along the grain boundaries and defects without necessarily reacting with the TiN [9]. This possibility depends on the diffusivity of Cu inside the grain and along the grain boundaries or defects of the TiN film, which in turn depends on temperatures and physical quality of the TiN film. Lattice diffusion in grains of TiN cannot be so significant to cause the failure of this barrier at 700 jC as high activation energy involved in this process. It may be possible that diffusion occurs through grain boundaries. But grain boundary diffusion alone cannot be held responsible for failure of diffusion barrier, because presence of Cu –Ti phase indicates that Cu has undergone a chemical reaction with Ti. Depending on the present experimental results, three important observations can be made. (1) Cu atoms react with Ti to form CuTi3; (2) Cu does not react with TiN as evidenced by unchanged heights of TiN even at 700 jC; (3) grain boundary diffusion may be the dominant mechanism for penetration of copper through TiN layer. The following mechanism for failure of TiN barriers is suggested. As the penetration depth of nitrogen ions in Ti is expected to be about 30 nm for the energy range used in our study, so TiN phase is expected only in upper side of about 30-nm thickness, and in lower portion, elemental Ti may be is present. Within the upper 30-nm-thick layer, the motion of Cu proceeds by penetration through the defects and grain
4. Discussion After high-dose nitrogen implantation, performance of Ti barrier was found to improve and it became stable up to 500 jC. Occurrence of a chemical reaction between Cu and Ti at 700 jC as indicated by formation of CuTi3 phase may be held responsible for the failure of the barrier. It is an interesting point to note that in case of nitrogen-implanted Ti, this reaction occurs at 700 jC, while in case of elemental titanium, this reaction was observed near about 400 jC [25 –27]. At 700 jC, formation of copper silicide is also observed, an indication that a high-dose-implanted Ti layer is no longer a stable barrier at this temperature. Two possible reaction paths exist between the surface Cu film and Si substrate. First, Cu and Si come in direct contact by
Fig. 6. X-ray diffraction pattern of as-deposited and annealed Cu/Ta(1017 ions/cm2 N+)/Si samples.
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boundaries without forming any reaction with it. The defects and grain boundaries in TiN film acts as failure sites at a higher temperature. After crossing the thin nitride phase at higher temperature, Cu reaches to elemental Ti layer and then it reacts with Ti to form CuTi3. Afterwards, barrier failure occurs in the same manner as in case of elemental titanium barrier by consuming this Ti layer. High thermal energy required for penetration through TiN may be held responsible for increase in temperature for Cu – Ti phase formation as compared to unimplanted titanium layer. In the case of the Ta barrier, no phases of tantalum nitride were found in the samples implanted at 1015 ions/cm2 dose of nitrogen, whereas at 1017 ions/cm2 dose of nitrogen, formation of nitride (Ta2N) phase is observed. This is consistent with the work carried out by other workers, which shows that low dose of nitrogen does not result in formation of any new nitride phase [28,29]. In this case, nitrogen is 10 at.% to that of tantalum. The addition of 1015 atoms/cm2 of nitrogen has no effect in stopping copper movement. However, if enough nitrogen is incorporated (1017 ions/cm2) to form the nitride phase, Ta2N, Cu – Si interaction is prevented at higher temperatures. The Ta– N equilibrium phase diagram shows that at room temperature, the solubility of nitrogen is just over 2 at.% in bcc Ta [30]. Solubility rises with temperature, but 5 at.% N would not be dissolved into the Ta until 800 jC is reached. So the excess nitrogen in case of 1017 ions/cm2 seems to get segregated at grain boundaries and may be responsible for stopping the penetration of Cu through the Ta metal film.
5. Conclusion It has been shown in this paper that the implantation of Ti and Ta layers with 1017 ions/cm2 dose of nitrogen by the PIII technique improves their diffusion barrier properties. This improvement in case of Ti is up to 500 jC, while in case of Ta layer is up to 700 jC. So we conclude that the plasma immersion ion implantation technique can be used as a tool to enhance diffusion barrier properties of Ti layer. Further studies in terms of depth profiling using RBS, XPS and SIMS are required to have a clear understanding of diffusion mechanism of copper through nitrogen-implanted barrier layers.
Acknowledgements The authors would like to acknowledge FCIPT, IPR Ahemdabad for providing PIII facility and RSIC, Panjab
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University Chandigarh for X-ray diffraction studies. MK and DK would like to thank University Grant Commission, New Delhi, for providing financial assistance.
References [1] K. Yamashita, S. Odanaka, IEEE Trans. Electron Devices 47 (2000) 90. [2] X.W. Lin, D. Parmanik, Solid State Technol. (1998) 63. [3] J.R. Lloyd, J.J. Clement, Thin Solid Films 262 (1995) 135. [4] D.S. Gardner, J. Onuki, K. Kudoo, Y. Misawa, Q.T. Vu, Thin Solid Films 262 (1995) 104. [5] S.P. Murarka, I.V. Verner, R.J. Gutmann, Copper-Fundamental Mechanism for Microelectronic Applications, Wiley, New York, 2000. [6] W.J. Ward, K.M. Carroll, J. Electrochem. Soc.: Solid State Sci. Technol. 129 (1982) 227. [7] M.M. Akhmedova, Sov. Phys., Semicond. 10 (1976) 1400. [8] S.K. Gandhi, VLSI Fabrication Principles, Wiley, New York, 1983, pp. 420 – 445. [9] M.A. Nicolet, Thin Solid Films 52 (1978) 415. [10] M. Wittmer, J. Vac. Sci. Technol. A2 (2) (1984). [11] H. Ono, T. Nakano, T. Ohta, Appl. Phys. Lett. 64 (1994) 151. [12] S. Saito, K. Matsuda, K. Nishizawa, K. Sakiyama, Mater. Res. Soc., (1987) 319. [13] S.-Q. Wang, I. Raaijmakers, B.J. Burrow, S. Suthar, S. Redkar, K.B. Kim, J. Appl. Phys. 68 (1990) 5176. [14] K. Holloway, P.M. Fryer, C. Cabral Jr., J.M.E. Harper, P.J. Bailey, K.H. Kelleher, J. Appl. Phys. 71 (1992) 5433. [15] J. Imahori, T. Oku, M. Murakami, Thin Solid Films 301 (1997) 142. [16] S. Mandl, R. Gunzel, E. Richter, W. Moller, J. Vac. Sci. Technol., B 17 (1999) 832. [17] K. Yukimura, K. Masanori, et al., J. Vac. Sci. Technol., B 17 (1999) 840. [18] A. Anders, Handbook of Plasma Immersion Ion Implantation, Wiley, New York, 2000. [19] J.R. Conard, L.J. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran, J. Appl. Phys. 62 (1987) 4591. [20] P.K. Chu, C. Chan, N.W. Cheung, Semicond. Int. 19 (1996) 165. [21] P.K. Chu, S.B. Fletch, P. Kellerman, F. Sinclair, L.A. Larson, B. Mizune, Solid State Technol. 42 (1999) 55. [22] J.D. Bernstein, S. Qin, C. Chung, T.J. King, IEEE Trans. Electron Devices 3 (1996) 1876. [23] T.J. Rajkumar, M. Kumar, P.J. George, S. Mukherjee, K.S. Chari, Surf. Coat. Technol. 156 (2002) 253. [24] M. Kumar, Rajkumar, D. Kumar, P.J. George, A.K. Paul, communicated to Thin Solid Films. [25] C. Apblett, D. Muira, M. Sullivan, P.J. Ficalora, J. Appl. Phys. 71 (1992) 4925. [26] W. Ono, J.M. Akhmedova, Sov. Phys. Semicond. 8 (1978) 1200. [27] J. Li, J.W. Strane, S.W. Russell, S.Q. Hong, J.W. Mayer, T.K. Marais, C.C. Theron, R. Pretorious, J. Appl. Phys. 72 (1992) 2810. [28] P.M. Raole, A.M. Narsale, D.C. Kothari, P.S. Pawar, S.V. Gogawale, L. Guzman, M. Dapor, Mater. Sci. Eng., A 115 (1989) 73. [29] H. Wilson, Thin Solid Films 33 (1976) 205. [30] K. Holloway, P.M. Fryer, C. Cabrai Jr., J.M.E. Harper, P.J. Bailey, K.H. Kelleher, J. Appl. Phys. 71 (1992) 5433.
Effect of plasma immersion ion implantation on the thermal stability of diffusion barrier layers Mukesh Kumar a,b,*, Rajkumar c, P.M. Raole d, S.K. Gupta d, Dinesh Kumar b, P.J. George b a
Advanced Materials and Devices Group, TASC-INFM, Trieste, Italy Electronic Science Department, Kurukshetra University, Kurukshetra, Haryana, India c Semiconductor Complex Ltd., Industrial Area, Sector-72, S.A.S Nagar, Chandigarh, India d FCIPT, Institute of Plasma Research, Gandhinagar, Gujarat, India b
Available online 1 June 2004
Abstract Plasma immersion ion implantation technique has been used to synthesize titanium nitride and tantalum nitride layer to acts as diffusion barrier against copper diffusion. Two different doses of nitrogen, 1015 ions/cm2 (low dose) and 1017 ions/cm2 (high dose), were implanted in refractory metal-coated silicon wafers. Diffusion barrier properties of these nitride layers were evaluated after annealing the samples up to 700 jC for 30 min. Sheet resistance, SEM and X-ray diffraction were carried out to investigate the effect of annealing. The highly implanted layer reduces the diffusion of Cu metal through it. The enhancement in its diffusion barrier properties is supposed to be due to nitridation of metal films, which increases the activation energy involved for its chemical reaction with copper metal film. D 2004 Elsevier B.V. All rights reserved. Keywords: Diffusion barrier; Copper; Plasma immersion ion implantation; Scanning electron microscope; X-ray diffraction
1. Introduction The present-day high-speed, low-power and reliable silicon (Si) based integrated circuits (IC) make use of copper (Cu) metal as interconnection material [1– 5]. To solve the high diffusivity problem of this material into Si, SiO2 and other polymer layers, a thin, conformal and efficient diffusion barrier between Cu and these materials is mandatory [6– 8]. The properties required for an efficient diffusion barrier have been reviewed by Nicolet [9] and other workers [10]. The diffusion through a thin film can be reduced by carefully controlling their deposition conditions and postdeposition treatment. Although a lot of amorphous material layers, refractory materials and their nitrides have been investigated for diffusion barrier applications [11 –15], but titanium (Ti) material has gained more popularity as it behaves not only as diffusion barrier but also acts as adhesion promoter. The reactive sputtering and use of a * Corresponding author. c/o Rajkumar, Semiconductor Complex Ltd., Industrial Area, Sector 72, S.A.S. Nagar (Near Chandigarh), Distt. Ropar, Punjab 150059, India. Tel.: +91-9815501585 (mobile). E-mail address: [email protected] (M. Kumar). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.04.016
composite sputtering target has been the main tool for fabrication of nitride and carbide layers for diffusion barrier applications for so many years. However, all these techniques are limited by degree of nitridation achieved. Refractory metal nitride layers formed by ion implantation can also be a possible technique for diffusion barrier application as implantation causes both nitridation and some degree of amorphism of metal layers [16]. Due to the small thickness of diffusion barrier films, ion implantation is carried out at low energy. Beam line ion implantation (BLII) suffers from low dose rate at small energies, and thus cannot be used to achieve a high degree of nitridation of thin metal films. In addition to this increased complexity, its high operational cost has limited the use of BLII to synthesis barrier layers. Recently, plasma immersion ion implantation (PIII), a new technique that offers a high dose rate even at low energy, has been successfully utilized to carry out nitridation of a thin Ti metal layer [17]. In the present study, we have used PIII technique to synthesize a TiN and TaN layer and studied their performance as diffusion barrier layer. The advent of plasma immersion ion implantation (PIII) has overcome the limitations of conventional ion implanta-
78
M. Kumar et al. / Surface & Coatings Technology 186 (2004) 77–81
tion technique and so offers fresh possibilities for the implementation of ion implantation technique in the field of nitridation of thin metal films [18 –24]. The authors have successfully synthesized the titanium nitride and tantalum nitride films as diffusion barrier for copper metal using PIII.
The accelerating voltage was kept at 35 kV. The tube current was kept at 10 mA. The goniometer range used was at 25– 60j values. The X-ray diffraction pattern (2h vs. intensity) results were obtained and plotted using an X – Y recorder. JCDPS (Joint Committee Powder Diffraction Standards) files were used to identify the various peaks obtained in XRD spectrum.
2. Experimental details In the present study, n-type silicon with < 100> orientation having resistivity of 1– 10 V cm was used as substrate. The layers of Ti and Ta of approximately 100-nm thickness were deposited on separate pieces of Si samples by RF sputtering in argon plasma. The base pressure was 10 – 6 Torr while pressure during deposition was 20 mTorr. The separation between target and substrate was 7 cm and applied RF power was 200 W. These samples were implanted with nitrogen ions by plasma immersion ion implantation, to form Ti(N)/Si and Ta(N)/Si structure. Some metal-coated samples were kept unimplanted to compare the performance of elemental and implanted diffusion barrier layers. Nitrogen implantation in Ti/Si and Ta/Si samples was carried out by a PIII system. Two doses of nitrogen, 1015 ions/cm2 (low dose) and 1017ions/cm2 (high dose), were implanted at an energy of 20 keV. The chamber was evacuated to a base pressure of 10 6 Torr. Nitrogen was then introduced in the chamber and a working pressure of 10 3 Torr was maintained by controlling the gas flow. RF power of 100 W was used to generate plasma needed for carrying out implantation. Samples placed on a metallic chuck were connected with a 20 KV pulse voltage. Pulse voltage and ion current were measured using an oscilloscope. Different implant doses were obtained by varying the frequency and width of applied pulse voltage. For low dose, frequency and pulse width were maintained at 50 Hz and 10 As, respectively. For high dose, frequency and pulse width were selected at 1 kHz and 50 As, respectively. Total implantation time was maintained at 10 min for both, i.e., low- and high-dose, samples. After implantation, Copper layer of 200-nm thickness was deposited on Ti/Si, Ta/Si, Ti(N)/Si, Ta(N)/Si and Si samples by DC sputtering system. Measurement of thickness was performed by Dektek 3030 ST Profilometer. Annealing of samples was carried out in N2 ambient. For this purpose, first of all, the annealing chamber was evacuated using rotary pump and then flushed with N2 gas for few minutes. Annealing at different temperatures was carried out for 30 min. To avoid the oxidation of the top Cu layer, samples were allowed to cool down inside the chamber itself before unloading. X-ray diffraction and four probe sheet resistance measurements at room temperature were used for the characterization of the samples. For X-ray diffraction, Philips Model PW 1729 X-ray Diffractometer was used. The target consists of copper metal whereas nickel metal is used as h-filter.
3. Results Fig. 1 shows the sheet resistance variation of Cu/Si, Cu/ Ti/Si, Cu/Ti(1015 ions/cm2 N)/Si and Cu/Ti(1017 ions/cm2 N)/Si structures vs. annealing temperature. Sheet resistance of Cu/Si structure increases after annealing at 200 jC for 30 min. This increase in sheet resistance may be due to some reactions at the Cu – Si interface. Annealing up to 300 jC of Cu/Ti/Si structure does not lead the sheet resistance to increase. However, for samples annealed at 400 jC, a slight increase in sheet resistance is observed. But after annealing at 500 jC, an abrupt change in sheet resistance is observed, which indicates some type of interaction between constituents, so it shows that Ti layer no longer acts as the diffusion barrier at 400 jC. It is also observed that a dose of 1015 ions/ cm2 in Ti layer is not sufficient to cause any change in the sheet resistance curve to that of unimplanted sample. However, implantation with a high dose (1017 ions/cm2) of nitrogen in Ti layer has resulted to significant change in the sheet resistance curve. In this case, sheet resistance is found to remain constant even after annealing the sample at a temperature of 500 jC, but increase in sheet resistance is observed at 700 jC, which indicates that Ti layer with 1017 ions/cm2 dose of nitrogen does not act as stable diffusion barrier for copper metal. Fig. 2 shows the sheet resistance variation of Cu/Si, Cu/ Ta/Si, Cu/Ta(1015 ions/cm2 N2)/Si and Cu/Ta (1017 ions/cm2 N2)/Si structures vs. annealing temperature. Sheet resistance of Cu/Si structure increases after annealing at 200 jC for 30
Fig. 1. Sheet resistance variation of Cu/Si, Cu/Ti/Si, Cu/Ti(1015 ions/cm2 N2)/Si and Cu/Ti(1017 ions/cm2 N2)/Si structures vs. annealing temperature.
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Fig. 2. Sheet resistance variation of Cu/Si, Cu/Ta/Si, Cu/Ta(1015 ions/cm2 N2)/Si and Cu/Ta(1017 ions/cm2 N2)/Si structures vs. annealing temperature.
min. Annealing of Cu/Ta/Si structure up to 500 jC does not lead to increased sheet resistance. After annealing at 700 jC, abrupt change in sheet resistance is observed, which indicate some type of interaction between the materials. It is also observed that a dose of 1015 ions/cm2 in Ta is not sufficient to cause any change in the sheet resistance curve to that of unimplanted sample. However, implantation with a dose (1017 ions/cm2) of nitrogen in Ta layer has resulted to significant change in the sheet resistance curve. In this case, sheet resistance is found to remain constant even after annealing the sample at a temperature of 700 jC. It seems that interaction in the form of any chemical reaction or interdiffusion has not taken place in the structure even after annealing at 700 jC. Fig. 3 shows XRD patterns of Cu/Ti(1015N)/Si samples. Unannealed sample shows the peaks of Cu (111), Cu (200), along with a peak of Ti. It is evident from this figure that an implantation of 1015 ions/cm2 nitrogen does not cause any nitridation of the Ti film as no titanium nitride phase is observed. Formation of titanium silicide and copper silicide is detected in this sample after annealing at 400 jC. So, we can say that the Ti layer, after its implantation with 1015 ion/ cm2 dose of nitrogen, can work as diffusion barrier for Cu only up to 400 jC. The XRD of as-deposited and annealed Cu/Ti(1017N+)/Si structure is shown in Fig. 4. The presence of TiN peaks indicates that a nitrided phase has formed in the film due to implantation. Sample annealed at 500 jC shows that peaks of TiSi2 are obtained. No peak of copper silicide was detected. After annealing at 700 jC, copper silicide peak was also observed, which indicates that copper reacted with underlying silicon material. The intensities of copper peaks were also found to be reduced, which indicates that loss of copper material from the top surface has occurred. The results of present experiment show that diffusion barrier
Fig. 3. X-ray diffraction pattern of as-deposited and annealed Cu/Ti(1015 ions/cm2 N+)/Si samples.
properties of elemental titanium layer is modified and it becomes able to stop the copper diffusion up to at least 500 jC after implantation of 1017 ions/cm2 dose of nitrogen in it by plasma immersion ion implantation. Fig. 5 shows the XRD patterns of Cu/Ta(1015 N+)/Si samples. The XRD data of sample annealed at 700 jC indicates that some kind of chemical interaction between
Fig. 4. X-ray diffraction pattern of as-deposited and annealed Cu/Ti(1017 ions/cm2 N+)/Si samples.
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Fig. 5. X-ray diffraction pattern of as-deposited and annealed Cu/Ta(1015 ions/cm2 N+)/Si samples.
constituents has occurred. Formation of tantalum silicide and copper silicide is detected in this sample after annealing at 700 jC. The XRD of as-deposited and annealed Cu/Ta(1017N+)/ Si structure is shown in Fig. 6. Unannealed sample shows the peaks of Cu (111), Cu (200) and Ta (410) along with two peaks of Ta2N. Sample annealed at 500 jC shows that no interaction among the metal layers has taken place and all the peaks are intact. After annealing at 700 jC, peaks of TaSi2 are obtained, whereas no peak of copper silicide was observed in this sample. Intensity of Cu peaks was also found unchanged, which indicates that there is no loss of Cu from the top layer of the structure. It shows that even at 700 jC, the structure is stable and hence diffusion barrier properties of Ta has been improved by implantation of 1017 ions/cm2 dose of nitrogen in it by plasma immersion ion implantation.
reacting with and completely consuming the TiN diffusion barrier. This process does not seem to take place in the present experimental results due to the fact that height of TiN peaks are still intact after annealing at temperature of 700 jC, which indicates that there is no loss of TiN layer. No conclusive evidence could be found for the presence of reaction compound between titanium-nitride film and Cu or Si. The second path is migration of Cu atoms through the TiN diffusion barrier either by lattice diffusion or by diffusion along the grain boundaries and defects without necessarily reacting with the TiN [9]. This possibility depends on the diffusivity of Cu inside the grain and along the grain boundaries or defects of the TiN film, which in turn depends on temperatures and physical quality of the TiN film. Lattice diffusion in grains of TiN cannot be so significant to cause the failure of this barrier at 700 jC as high activation energy involved in this process. It may be possible that diffusion occurs through grain boundaries. But grain boundary diffusion alone cannot be held responsible for failure of diffusion barrier, because presence of Cu –Ti phase indicates that Cu has undergone a chemical reaction with Ti. Depending on the present experimental results, three important observations can be made. (1) Cu atoms react with Ti to form CuTi3; (2) Cu does not react with TiN as evidenced by unchanged heights of TiN even at 700 jC; (3) grain boundary diffusion may be the dominant mechanism for penetration of copper through TiN layer. The following mechanism for failure of TiN barriers is suggested. As the penetration depth of nitrogen ions in Ti is expected to be about 30 nm for the energy range used in our study, so TiN phase is expected only in upper side of about 30-nm thickness, and in lower portion, elemental Ti may be is present. Within the upper 30-nm-thick layer, the motion of Cu proceeds by penetration through the defects and grain
4. Discussion After high-dose nitrogen implantation, performance of Ti barrier was found to improve and it became stable up to 500 jC. Occurrence of a chemical reaction between Cu and Ti at 700 jC as indicated by formation of CuTi3 phase may be held responsible for the failure of the barrier. It is an interesting point to note that in case of nitrogen-implanted Ti, this reaction occurs at 700 jC, while in case of elemental titanium, this reaction was observed near about 400 jC [25 –27]. At 700 jC, formation of copper silicide is also observed, an indication that a high-dose-implanted Ti layer is no longer a stable barrier at this temperature. Two possible reaction paths exist between the surface Cu film and Si substrate. First, Cu and Si come in direct contact by
Fig. 6. X-ray diffraction pattern of as-deposited and annealed Cu/Ta(1017 ions/cm2 N+)/Si samples.
M. Kumar et al. / Surface & Coatings Technology 186 (2004) 77–81
boundaries without forming any reaction with it. The defects and grain boundaries in TiN film acts as failure sites at a higher temperature. After crossing the thin nitride phase at higher temperature, Cu reaches to elemental Ti layer and then it reacts with Ti to form CuTi3. Afterwards, barrier failure occurs in the same manner as in case of elemental titanium barrier by consuming this Ti layer. High thermal energy required for penetration through TiN may be held responsible for increase in temperature for Cu – Ti phase formation as compared to unimplanted titanium layer. In the case of the Ta barrier, no phases of tantalum nitride were found in the samples implanted at 1015 ions/cm2 dose of nitrogen, whereas at 1017 ions/cm2 dose of nitrogen, formation of nitride (Ta2N) phase is observed. This is consistent with the work carried out by other workers, which shows that low dose of nitrogen does not result in formation of any new nitride phase [28,29]. In this case, nitrogen is 10 at.% to that of tantalum. The addition of 1015 atoms/cm2 of nitrogen has no effect in stopping copper movement. However, if enough nitrogen is incorporated (1017 ions/cm2) to form the nitride phase, Ta2N, Cu – Si interaction is prevented at higher temperatures. The Ta– N equilibrium phase diagram shows that at room temperature, the solubility of nitrogen is just over 2 at.% in bcc Ta [30]. Solubility rises with temperature, but 5 at.% N would not be dissolved into the Ta until 800 jC is reached. So the excess nitrogen in case of 1017 ions/cm2 seems to get segregated at grain boundaries and may be responsible for stopping the penetration of Cu through the Ta metal film.
5. Conclusion It has been shown in this paper that the implantation of Ti and Ta layers with 1017 ions/cm2 dose of nitrogen by the PIII technique improves their diffusion barrier properties. This improvement in case of Ti is up to 500 jC, while in case of Ta layer is up to 700 jC. So we conclude that the plasma immersion ion implantation technique can be used as a tool to enhance diffusion barrier properties of Ti layer. Further studies in terms of depth profiling using RBS, XPS and SIMS are required to have a clear understanding of diffusion mechanism of copper through nitrogen-implanted barrier layers.
Acknowledgements The authors would like to acknowledge FCIPT, IPR Ahemdabad for providing PIII facility and RSIC, Panjab
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University Chandigarh for X-ray diffraction studies. MK and DK would like to thank University Grant Commission, New Delhi, for providing financial assistance.
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