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The effect of silicon ion implantation on the structure of tantalum–silicon contacts
Thin Solid Films 449 (2004) 187–191
The effect of silicon ion implantation on the structure of tantalum–silicon contacts a ¨ M. Peikerta,*, R. Bhandaria,b, E. Wiesera, C. Wenzelb, D. Lippb, H. Reuthera, A. Mucklich a
Forschungszentrum Rossendorf, Institute of Ion Beam Physics and Materials Research, P.O. Box 510119, 01314 Dresden, Germany b ¨ Dresden, 01062 Dresden, Germany Institute of Semiconductor and Microsystems Technology, Technische Universitat Received 14 August 2003; received in revised form 6 October 2003; accepted 19 October 2003
Abstract The effect of ion beam mixing on the formation of tantalum–silicon contacts was studied. Silicon implantation into 50 nm Ta layers on nq-Si (1 0 0) was carried out at temperatures from 150 to 500 8C and fluences between 1=1015 and 1=1017 Siycm2. The microstructural changes were characterized by Auger electron spectroscopy, X-ray diffraction and cross-sectional transmission electron microscopy. The contact resistance was measured before and after implantation using the four-point Kelvin method. Implantation at low temperature (;150 8C) leads to the formation of an amorphous Ta(Si) interface-layer. The width of the mixed layer increases linearly with implanted dose and temperature. Substrate amorphization results in an increase of the contact resistance. An improved contact resistance was achieved by implantation with 5=1016 Siycm2 at 400 8C. TaSi2 formation at the interface was observed in this case. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Tantalum; Silicon; Contacts; Silicides; Ion implantation
1. Introduction Increasing attention has been devoted recently to the problem of low resistance ohmic contacts to silicon owing to down scaling in ultra large-scale integration (ULSI). The choice of a suitable metallization system and the method of contact formation are significant aspects for the production of metal–semiconductor contacts. Metal silicides have been widely used as ohmic contacts, Schottky barriers and gate electrodes in microelectronic devices because of their low resistance and high thermal stability. The formation of silicides from metal films on Si by thermal diffusion leads to problems because of difficulties experienced in obtaining reproducible results w1x. An alternative method for silicide formation is the ion beam mixing process, i.e. the intermixing of two layers as result of the collision cascades caused by bombardment with energetic ions w2–4x. Ion beam mixing offers some advantages in *Corresponding author. Forschungszentrum Rossendorf, Bautzner Landstraße 128, 01328 Dresden, Germany. Tel.: q49-351-260-3422; fax: q49-351-260-2703. E-mail address: [email protected] (M. Peikert).
comparison to a thermally excited solid-state reaction. Silicide formation by thermal annealing of refractory metal layers on silicon needs relatively high temperatures of approximately 700 8C. Using ion beam mixing, silicides may be generated at lower thermal load. Thermally formed silicides show non-uniform morphology of the interfaces, which results in non-uniformity of the sheet resistance of the silicide across the silicon wafer w5,6x. The non-uniform silicidation is also connected with rough interface structures leading to very poor junction integrity and large variation in leakage under reverse bias conditions. In contrast, well-defined silicide interfaces can be obtained using ion beam mixing. An improved interface morphology can be expected from ion-induced decomposition and redistribution of possible contaminants such as the natural oxide layer at the metal–silicon boundary. In the past, tantalum has gained attention as a promising contact material because of its good electrical properties together with a high chemical and thermal stability. Ta is also attractive as a reliable diffusion barrier in ULSI technologies with copper metallization w7x. The aim of this work was to study structural changes
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090(03)01409-3
M. Peikert et al. / Thin Solid Films 449 (2004) 187–191
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Fig. 1. Comparison of AES depth profiles of a 50 nm Ta layer on Si in the initial state and after implantation of 5=1015 and 1=1016 Siycm2, respectively.
and possible silicide formation at the interface of tantalum layers deposited on silicon substrates using Si implantation at different substrate temperatures and their effect on the contact resistance. 2. Experimental details Ta films of 50 nm thickness were deposited by HF magnetron sputtering on Si(1 0 0) wafers. To remove a possible native oxide layer on the silicon substrate, the wafer surface was cleaned by Arq-ion sputtering before the tantalum deposition. Ion beam mixing was carried out by implantation of Siq ions at 120 and 190 keV. Because of lower interface mixing and higher radiation damage in the substrate after implantation with 190 keV, an ion energy of 120 keV was chosen for the experiments reported here. The Si fluence was in the range from 1=1015 to 1=1017 ionsycm2. The sample temperature during implantation was varied between approximately 150 and 500 8C. The lowest temperature is due to beam heating alone. The intermixing of Ta and Si was studied using AES in combination with sputter etching by 3 keV Arq ions.
X-ray diffraction (XRD) of Cu Ka radiation at grazing incidence was used to detect amorphization and formation of silicides. The microstructure of the modified layer system was investigated by cross-sectional transmission electron microscopy (XTEM) and electron diffraction using a Philips 300 kV CM300 microscope. The specimens for XTEM were prepared by the standard ion milling technique using Precision ion polishing system GETAN-691 with Arq beam of 4.5 keV for prethinning of the first and the final process. In order to examine the contact resistance, Kelvin test structures were prepared w8,9x. For this structure, a 300nm n-poly Si layer with a sheet resistance of Rs31 Vy h was deposited on 100 nm thermal SiO2 on top of the Si substrate. Subsequently, a 50 nm Ta was deposited by magnetron sputtering. The contact resistance was measured through a contact area of 130 mm2, before and after implantation. 3. Results and discussion The interface mixing of 50 nm Ta on a (1 0 0) Si substrate in dependence on the implanted Si dose at 150 8C is demonstrated by the AES depth profiles shown in Fig. 1. After implantation with 1=1015 Siycm2 no change at the interface was observed in comparison to the initial state. With increasing dose the width of the mixed interface layer rises and approaches approximately 35 nm after implantation with 1=1016 Siycm2. XTEM was used to get more detailed information of the structural changes due to the implantation. Fig. 2a shows the initial tantalum layer before implantation. The asdeposited layer is polycrystalline with a columnar structure. The grain length achieves the film thickness. The lateral dimension of the grains is approximately 10 nm. By XRD b-Ta was detected as the dominating phase. A nearly 2 nm wide amorphous layer at the Ta–Si interface is visible as shown in the inset. It may be due to the sputter cleaning of the Si-substrate before deposition or to Ta–Si mixing at the beginning of the sputter deposi-
Fig. 2. XTEM images of a 50 nm Ta layer on silicon: (a) before implantation; (b) implanted with 1=1016 Siycm2 at 150 8C.
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Fig. 3. XTEM images of a 50 nm Ta layer on silicon implanted with 1=1016 Siycm2 at different substrate temperatures: (a) 300 8C; (b) 400 8C and (c) 500 8C.
tion. The formation of an amorphous metal–Si interface layer due to deposition was also reported in the systems HfySi, CrySi, MoySi, PtySi, NbySi, and TaySi w10– 13x. After implantation with 1=1016 Si ycm2 at 150 8C a layered structure is formed (Fig. 2b) which consists of a polycrystalline tantalum film extending 30 nm from the surface and an 18 nm amorphous Ta(Si) layer at the interface. The interfaces of the crystalline Ta, the mixed Ta–Si layer and the Si substrate are sharp. The silicon substrate is amorphized by the implantation up to a depth of approximately 200 nm. Substrate amorphization has to be avoided because of the worsening of the contact, as shown later. The critical dose values for the amorphization will vary substantially with the implantation temperature w14x. Therefore, in a second implantation series the influence of elevated substrate temperature was studied (300, 400 and 500 8C) for implantation with 1=1016 Siycm2. The effect is shown in the XTEM images of Fig. 3 for different temperatures. After implantation at 300 8C a stacked layer system is observed consisting of polycrystalline Ta on top of an amorphous Ta(Si) layer at the interface with a width of approximately 20–25 nm, similar to the result for implantation at 150 8C. As remarkable difference to the implantation at the lower temperature the silicon substrate remains crystalline. Increasing the implantation temperature to 400 and 500 8C leads to further increase of the interfacial mixing, i.e. to a broader amorphous region (Fig. 3b and c). The increased thickness of the mixed layer can be explained by radiation-enhanced diffusion additional to the ballistic mixing dominating at 150 8C. Radiation enhanced diffusion has to be assumed because no intermixing was observed at these temperatures by annealing only as discussed below. In this amorphous region the AES depth profiles of Ta and Si exhibit a plateau with a concentration ratio of 1:1 as shown in Fig. 4a for the temperature of 400 8C. Such a plateau is typical for
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compound formation and indicates growth of the monosilicide TaSi. However, neither by XRD nor by XTEM and electron diffraction crystalline silicides could be detected. We take this as possible evidence that the monosilicide exists only in an X-ray amorphous state in agreement with the fact that in the equilibrium phase diagram no TaSi phase is reported w15x. In order to examine the influence of higher doses on the interfacial mixture and silicide formation, implantations with doses from 2=1016 to 1=1017 were performed at 400 8C. As expected, with rising implantation dose the extension of the mixed region grows and the Si concentration near the interface increases. After implantation with 3=1016 Siycm2 the concentration ratio of Ta to Si near the interface approaches values of approximately 1:2. This indicates the beginning formation of TaSi2 (Fig. 4b). The implantation with 1=1017 Siycm2 leads to a complete transformation of all Ta into TaSi2 according to the concentration ratio (Fig. 4c). In the XTEM image of Fig. 5 stack of three layers is found on the silicon substrate for the implantation with 5=1016 Siycm2. At the interface a crystalline band with a thickness of 25–30 nm is formed, followed by an amorphous Ta(Si) mixed layer with a thin layer of residual polycrystalline Ta on top. In the high resolution image of the crystalline interface layer lattice planes with a distance characteristic for TaSi2 have been identified. The Si substrate keeps its crystalline structure also after this high dose implantation. The XRD investigation of the layer implanted at 400 8C approved that formation of TaSi2 starts after irradiation with 3=1016 Siycm2. The effect of annealing after implantation with 1=1016 Siycm2 at low temperature (150 8C) was studied and compared to implantations performed with the same dose at enhanced temperature. Post-implantation annealing at 500 8C for 1 h leads to partial recrystallisation of
Fig. 4. AES depth profiles of 50 nm Ta on Si measured after implantation at 400 8C with: (a) 1=1016 Siycm2; (b) 3=1016 Siycm2 and (c) 1=1017 Siycm2.
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Fig. 7. AES depth profile of a non-implanted 50 nm Ta layer on Si, annealed at 500 8C for 1 h and 650 8C for 1 h, respectively.
Fig. 5. XTEM image of a 50 nm Ta layer on Si implanted with 5=1016 Siycm2 at 400 8C.
the amorphous Ta(Si) layer. Fig. 6a shows the XTEM image of the Ta–Si interface after annealing at 500 8C for 1 h. A crystalline band of approximately 7 nm thickness is formed between the Si substrate and the amorphous layer. The thickness of the amorphous layer is reduced to approximately 13 nm. The remaining surface layer consists of polycrystalline tantalum. Annealing at 600 8C for 1 h leads, beside the recrystallisation at SiyTa(Si) interface, also to a complete recrystallisation of the amorphized substrate layer as shown in Fig. 6b. Fourier analysis from HREM pictures, resulting from the interface near crystalline band do not give clear information on the crystal structure. Due to the very similar lattice distances the phases TaSi2 and Ta3Si5 come into consideration. Reference samples were prepared by vacuum annealing without ion implantation. The intermixing of Ta and Si, and the growth of amorphous layer near the interface after annealing was reported in Refs. w13,16x. The AES
depth profile in Fig. 7 show that at 500 8C for 1 h annealing temperature no changes in comparison to the initial state are caused. Tantalum and silicon intermixing starts after annealing at 600 8C for 1 h. Formation of the TaSi2 phase is proven by XRD after annealing at 650 8C for 1 h. The AES depth profile for the 650 8C for 1 h annealing shows increased interface intermixing with the concentration ratio typical for TaSi2. Fig. 8 shows the behavior of the contact resistivity of Ta on n-Si contacts in dependence on implantation and annealing treatment. Because the implantation was carried out at enhanced temperature, the effect of a corresponding annealing without implantation was also studied. The contact resistivity of the as-deposited layers is already relatively low with approximately 1.4=10y5 V cm2 as shown in Fig. 8a. It can be slightly improved by vacuum annealing at temperatures of 400–500 8C for 1 h (Fig. 8b). Relaxation effects at the interface may explain this improvement. At 600 8C for 1 h annealing the contact becomes worsened. After ion bombardment with 1=1016 Siycm2 at temperatures from 300 to 500 8C the contact resistivity exhibits no significant changes in comparison to the initial state (Fig.
Fig. 6. XTEM image of 50 nm Ta layer on Si implanted with 1=1016 Siycm2 at 150 8C, post-annealed (a) 500 8C for 1 h; (b) 600 8C for 1 h.
M. Peikert et al. / Thin Solid Films 449 (2004) 187–191
Fig. 8. Contact resistivity of the 50 nm Ta layers on nq poly-Si: (a) as-deposited; (b) non-implanted, annealed at 500 8C for 1 h; (c) implanted with 1=1016 Siycm2 at 300 8C; (d) implanted with 1=1016 Siycm2 at 400 8C; (e) implanted with 1=1016 Siycm2 at 500 8C; (f) implanted with 5=1016 Siycm2 at 400 8C; (g) implantation with 1=1016 Siycm2 at 150 8C and post-implantation annealing at 600 8C.
8c–e). This can be understood as competition between the positive effect of interface mixing and the negative influence of the amorphous Ta(Si) region. Implantation at 400 8C with 5=1016 Siycm2 leads to an improvement of the contact resistivity by approximately 40% (Fig. 8f). The low temperature implantation at 150 8C results in enhanced values of the contact resistivity. This increase can be explained by the amorphization of the Si substrate near the interface. Post-implantation annealing at 600 8C for 1 h leads here to a contact improvement in contrast to the result of the non-implanted sample as shown in Fig. 8g. This can be attributed to the substrate recrystallisation. Silicide formation at the interface, occurring at this high temperature, may also contribute. 4. Summary and conclusions The effect of ion beam mixing using 120 keV Siq ions on the microstructure of 50 nm Ta layers deposited on Si and the phase formation at the Ta–Si interface as well as its effect on the contact resistivity have been investigated. Mixing of Ta with Si by medium doses (;1=1016 Siycm2) results in the formation of an amorphous Ta(Si) layer near the interface at all investigated temperatures. Implantation with enhanced temperature leads to a Ta:Si ratio of 1:1 over an extended depth range indicating the formation of an X-ray amorphous monosilicide. For higher doses (03=1016 Siy cm2) at high temperature beginning from the interface a crystalline TaSi2 layer is formed. Complete transfor-
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mation of the 50 nm tantalum film into TaSi2 is possible by irradiation with 1=1017 Siycm2 assisted by substrate heating to 400 8C. Without implantation no silicides are formed at the interface below 650 8C for 1 h. This leads to the conclusion that ion beam mixing lowers the necessary temperature for the silicide reaction from 0650 to 400 8C by ballistic mixing and radiationenhanced diffusion. Implantation at temperatures )300 8C is necessary to avoid amorphization of the silicon substrate, which is disadvantageous for the contact resistivity. A significant reduction of contact resistivity can be obtained by formation of a TaSi2 layer at the interface between Si and the contact material. In conclusion, we have demonstrated that an improvement of the Ta–Si contact resistivity by approximately 40% has been obtained by ion beam mixing. However, because of the high substrate doping, the contact resistivity of the non-implanted layer is already very low. Further studies on Ta–Si contacts with lower substrate doping are in progress to investigate the effect of silicide formation at systems where a more pronounced contact improvement is expected. References w1x R.K. Shuka, P.W. Davies, B.M. Tracy, J. Vac. Sci. Technol. B 4 (1986) 1344. w2x N. Bibic, S. Dhar, M. Milosavljevic, K. Removic, L. Rissanen, K.P. Lieb, Nucl. Instrum. Methods Phys. Res. B, 161–163 (2000) 1011. w3x M.Y. Tsai, C.S. Peterson, F.M. d’Heurle, V. Maniscalco, Appl. Phys. Lett. 37 (1980) 295. w4x A.A. Galuska, J. Vac. Sci. Technol. B 5 (1987) 1. w5x Z.G. Tao Jiang, Du. Anyan, Wu. Guoying, J. Vac. Sci. Technol. B 6 (1988) 1746. w6x K.N. Tu, J.W. Mayer, Thin Films—Interdiffusion and Reactions, Wiley, New York, 1978, p. 359. w7x D. Fischer, M. Stavrev, N. Urbansky, C. Wenzel, E. Neubauer, J.G. Bauer, T. Scherg, H.J. Schullze, Conference Proceedings ULSI XIV, Mater. Res. Soc. (1999) 337. w8x S.S. Cohen, Thin Solid Films 104 (1983) 361. w9x S.J. Proctor, L.W. Linholm, J.A. Mazer, IEEE Trans. Electron. Dev. ED-30 (1983) 1535. w10x W.Y. Hsieh, J.H. Lin, L.J. Chen, Appl. Phys. Lett. 62 (1993) 1088. w11x J.M. Liang, L.J. Chen, Appl. Phys. Lett. 64 (1994) 1224. w12x J.R. Abelson, K.B. Kim, D.E. Mercer, C.R. Helms, R. Sinclair, T.W. Sigmon, J. Appl. Phys. 63 (1988) 689. w13x J.Y. Cheng, L.J. Chen, J. Appl. Phys. 69 (1991) 2161. w14x J.R. Dennis, E.B. Hale, J. Appl. Phys. 49 (1978) 1119. w15x T.B. Massalski, H. Hiroaki, P.R. Subramanian, L. Kacpzak, Binary Alloy Phase Diagrams, American Society for Metals, William W. Scott-Publisher, second ed., vol. 3, (1990) p. 3364, ISBN: 0-87170-406-4. w16x A. Noya, M. Takeyama, J. Vac. Sci. Technol. A 15 (1997) 253.
The effect of silicon ion implantation on the structure of tantalum–silicon contacts a ¨ M. Peikerta,*, R. Bhandaria,b, E. Wiesera, C. Wenzelb, D. Lippb, H. Reuthera, A. Mucklich a
Forschungszentrum Rossendorf, Institute of Ion Beam Physics and Materials Research, P.O. Box 510119, 01314 Dresden, Germany b ¨ Dresden, 01062 Dresden, Germany Institute of Semiconductor and Microsystems Technology, Technische Universitat Received 14 August 2003; received in revised form 6 October 2003; accepted 19 October 2003
Abstract The effect of ion beam mixing on the formation of tantalum–silicon contacts was studied. Silicon implantation into 50 nm Ta layers on nq-Si (1 0 0) was carried out at temperatures from 150 to 500 8C and fluences between 1=1015 and 1=1017 Siycm2. The microstructural changes were characterized by Auger electron spectroscopy, X-ray diffraction and cross-sectional transmission electron microscopy. The contact resistance was measured before and after implantation using the four-point Kelvin method. Implantation at low temperature (;150 8C) leads to the formation of an amorphous Ta(Si) interface-layer. The width of the mixed layer increases linearly with implanted dose and temperature. Substrate amorphization results in an increase of the contact resistance. An improved contact resistance was achieved by implantation with 5=1016 Siycm2 at 400 8C. TaSi2 formation at the interface was observed in this case. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Tantalum; Silicon; Contacts; Silicides; Ion implantation
1. Introduction Increasing attention has been devoted recently to the problem of low resistance ohmic contacts to silicon owing to down scaling in ultra large-scale integration (ULSI). The choice of a suitable metallization system and the method of contact formation are significant aspects for the production of metal–semiconductor contacts. Metal silicides have been widely used as ohmic contacts, Schottky barriers and gate electrodes in microelectronic devices because of their low resistance and high thermal stability. The formation of silicides from metal films on Si by thermal diffusion leads to problems because of difficulties experienced in obtaining reproducible results w1x. An alternative method for silicide formation is the ion beam mixing process, i.e. the intermixing of two layers as result of the collision cascades caused by bombardment with energetic ions w2–4x. Ion beam mixing offers some advantages in *Corresponding author. Forschungszentrum Rossendorf, Bautzner Landstraße 128, 01328 Dresden, Germany. Tel.: q49-351-260-3422; fax: q49-351-260-2703. E-mail address: [email protected] (M. Peikert).
comparison to a thermally excited solid-state reaction. Silicide formation by thermal annealing of refractory metal layers on silicon needs relatively high temperatures of approximately 700 8C. Using ion beam mixing, silicides may be generated at lower thermal load. Thermally formed silicides show non-uniform morphology of the interfaces, which results in non-uniformity of the sheet resistance of the silicide across the silicon wafer w5,6x. The non-uniform silicidation is also connected with rough interface structures leading to very poor junction integrity and large variation in leakage under reverse bias conditions. In contrast, well-defined silicide interfaces can be obtained using ion beam mixing. An improved interface morphology can be expected from ion-induced decomposition and redistribution of possible contaminants such as the natural oxide layer at the metal–silicon boundary. In the past, tantalum has gained attention as a promising contact material because of its good electrical properties together with a high chemical and thermal stability. Ta is also attractive as a reliable diffusion barrier in ULSI technologies with copper metallization w7x. The aim of this work was to study structural changes
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090(03)01409-3
M. Peikert et al. / Thin Solid Films 449 (2004) 187–191
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Fig. 1. Comparison of AES depth profiles of a 50 nm Ta layer on Si in the initial state and after implantation of 5=1015 and 1=1016 Siycm2, respectively.
and possible silicide formation at the interface of tantalum layers deposited on silicon substrates using Si implantation at different substrate temperatures and their effect on the contact resistance. 2. Experimental details Ta films of 50 nm thickness were deposited by HF magnetron sputtering on Si(1 0 0) wafers. To remove a possible native oxide layer on the silicon substrate, the wafer surface was cleaned by Arq-ion sputtering before the tantalum deposition. Ion beam mixing was carried out by implantation of Siq ions at 120 and 190 keV. Because of lower interface mixing and higher radiation damage in the substrate after implantation with 190 keV, an ion energy of 120 keV was chosen for the experiments reported here. The Si fluence was in the range from 1=1015 to 1=1017 ionsycm2. The sample temperature during implantation was varied between approximately 150 and 500 8C. The lowest temperature is due to beam heating alone. The intermixing of Ta and Si was studied using AES in combination with sputter etching by 3 keV Arq ions.
X-ray diffraction (XRD) of Cu Ka radiation at grazing incidence was used to detect amorphization and formation of silicides. The microstructure of the modified layer system was investigated by cross-sectional transmission electron microscopy (XTEM) and electron diffraction using a Philips 300 kV CM300 microscope. The specimens for XTEM were prepared by the standard ion milling technique using Precision ion polishing system GETAN-691 with Arq beam of 4.5 keV for prethinning of the first and the final process. In order to examine the contact resistance, Kelvin test structures were prepared w8,9x. For this structure, a 300nm n-poly Si layer with a sheet resistance of Rs31 Vy h was deposited on 100 nm thermal SiO2 on top of the Si substrate. Subsequently, a 50 nm Ta was deposited by magnetron sputtering. The contact resistance was measured through a contact area of 130 mm2, before and after implantation. 3. Results and discussion The interface mixing of 50 nm Ta on a (1 0 0) Si substrate in dependence on the implanted Si dose at 150 8C is demonstrated by the AES depth profiles shown in Fig. 1. After implantation with 1=1015 Siycm2 no change at the interface was observed in comparison to the initial state. With increasing dose the width of the mixed interface layer rises and approaches approximately 35 nm after implantation with 1=1016 Siycm2. XTEM was used to get more detailed information of the structural changes due to the implantation. Fig. 2a shows the initial tantalum layer before implantation. The asdeposited layer is polycrystalline with a columnar structure. The grain length achieves the film thickness. The lateral dimension of the grains is approximately 10 nm. By XRD b-Ta was detected as the dominating phase. A nearly 2 nm wide amorphous layer at the Ta–Si interface is visible as shown in the inset. It may be due to the sputter cleaning of the Si-substrate before deposition or to Ta–Si mixing at the beginning of the sputter deposi-
Fig. 2. XTEM images of a 50 nm Ta layer on silicon: (a) before implantation; (b) implanted with 1=1016 Siycm2 at 150 8C.
M. Peikert et al. / Thin Solid Films 449 (2004) 187–191
Fig. 3. XTEM images of a 50 nm Ta layer on silicon implanted with 1=1016 Siycm2 at different substrate temperatures: (a) 300 8C; (b) 400 8C and (c) 500 8C.
tion. The formation of an amorphous metal–Si interface layer due to deposition was also reported in the systems HfySi, CrySi, MoySi, PtySi, NbySi, and TaySi w10– 13x. After implantation with 1=1016 Si ycm2 at 150 8C a layered structure is formed (Fig. 2b) which consists of a polycrystalline tantalum film extending 30 nm from the surface and an 18 nm amorphous Ta(Si) layer at the interface. The interfaces of the crystalline Ta, the mixed Ta–Si layer and the Si substrate are sharp. The silicon substrate is amorphized by the implantation up to a depth of approximately 200 nm. Substrate amorphization has to be avoided because of the worsening of the contact, as shown later. The critical dose values for the amorphization will vary substantially with the implantation temperature w14x. Therefore, in a second implantation series the influence of elevated substrate temperature was studied (300, 400 and 500 8C) for implantation with 1=1016 Siycm2. The effect is shown in the XTEM images of Fig. 3 for different temperatures. After implantation at 300 8C a stacked layer system is observed consisting of polycrystalline Ta on top of an amorphous Ta(Si) layer at the interface with a width of approximately 20–25 nm, similar to the result for implantation at 150 8C. As remarkable difference to the implantation at the lower temperature the silicon substrate remains crystalline. Increasing the implantation temperature to 400 and 500 8C leads to further increase of the interfacial mixing, i.e. to a broader amorphous region (Fig. 3b and c). The increased thickness of the mixed layer can be explained by radiation-enhanced diffusion additional to the ballistic mixing dominating at 150 8C. Radiation enhanced diffusion has to be assumed because no intermixing was observed at these temperatures by annealing only as discussed below. In this amorphous region the AES depth profiles of Ta and Si exhibit a plateau with a concentration ratio of 1:1 as shown in Fig. 4a for the temperature of 400 8C. Such a plateau is typical for
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compound formation and indicates growth of the monosilicide TaSi. However, neither by XRD nor by XTEM and electron diffraction crystalline silicides could be detected. We take this as possible evidence that the monosilicide exists only in an X-ray amorphous state in agreement with the fact that in the equilibrium phase diagram no TaSi phase is reported w15x. In order to examine the influence of higher doses on the interfacial mixture and silicide formation, implantations with doses from 2=1016 to 1=1017 were performed at 400 8C. As expected, with rising implantation dose the extension of the mixed region grows and the Si concentration near the interface increases. After implantation with 3=1016 Siycm2 the concentration ratio of Ta to Si near the interface approaches values of approximately 1:2. This indicates the beginning formation of TaSi2 (Fig. 4b). The implantation with 1=1017 Siycm2 leads to a complete transformation of all Ta into TaSi2 according to the concentration ratio (Fig. 4c). In the XTEM image of Fig. 5 stack of three layers is found on the silicon substrate for the implantation with 5=1016 Siycm2. At the interface a crystalline band with a thickness of 25–30 nm is formed, followed by an amorphous Ta(Si) mixed layer with a thin layer of residual polycrystalline Ta on top. In the high resolution image of the crystalline interface layer lattice planes with a distance characteristic for TaSi2 have been identified. The Si substrate keeps its crystalline structure also after this high dose implantation. The XRD investigation of the layer implanted at 400 8C approved that formation of TaSi2 starts after irradiation with 3=1016 Siycm2. The effect of annealing after implantation with 1=1016 Siycm2 at low temperature (150 8C) was studied and compared to implantations performed with the same dose at enhanced temperature. Post-implantation annealing at 500 8C for 1 h leads to partial recrystallisation of
Fig. 4. AES depth profiles of 50 nm Ta on Si measured after implantation at 400 8C with: (a) 1=1016 Siycm2; (b) 3=1016 Siycm2 and (c) 1=1017 Siycm2.
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Fig. 7. AES depth profile of a non-implanted 50 nm Ta layer on Si, annealed at 500 8C for 1 h and 650 8C for 1 h, respectively.
Fig. 5. XTEM image of a 50 nm Ta layer on Si implanted with 5=1016 Siycm2 at 400 8C.
the amorphous Ta(Si) layer. Fig. 6a shows the XTEM image of the Ta–Si interface after annealing at 500 8C for 1 h. A crystalline band of approximately 7 nm thickness is formed between the Si substrate and the amorphous layer. The thickness of the amorphous layer is reduced to approximately 13 nm. The remaining surface layer consists of polycrystalline tantalum. Annealing at 600 8C for 1 h leads, beside the recrystallisation at SiyTa(Si) interface, also to a complete recrystallisation of the amorphized substrate layer as shown in Fig. 6b. Fourier analysis from HREM pictures, resulting from the interface near crystalline band do not give clear information on the crystal structure. Due to the very similar lattice distances the phases TaSi2 and Ta3Si5 come into consideration. Reference samples were prepared by vacuum annealing without ion implantation. The intermixing of Ta and Si, and the growth of amorphous layer near the interface after annealing was reported in Refs. w13,16x. The AES
depth profile in Fig. 7 show that at 500 8C for 1 h annealing temperature no changes in comparison to the initial state are caused. Tantalum and silicon intermixing starts after annealing at 600 8C for 1 h. Formation of the TaSi2 phase is proven by XRD after annealing at 650 8C for 1 h. The AES depth profile for the 650 8C for 1 h annealing shows increased interface intermixing with the concentration ratio typical for TaSi2. Fig. 8 shows the behavior of the contact resistivity of Ta on n-Si contacts in dependence on implantation and annealing treatment. Because the implantation was carried out at enhanced temperature, the effect of a corresponding annealing without implantation was also studied. The contact resistivity of the as-deposited layers is already relatively low with approximately 1.4=10y5 V cm2 as shown in Fig. 8a. It can be slightly improved by vacuum annealing at temperatures of 400–500 8C for 1 h (Fig. 8b). Relaxation effects at the interface may explain this improvement. At 600 8C for 1 h annealing the contact becomes worsened. After ion bombardment with 1=1016 Siycm2 at temperatures from 300 to 500 8C the contact resistivity exhibits no significant changes in comparison to the initial state (Fig.
Fig. 6. XTEM image of 50 nm Ta layer on Si implanted with 1=1016 Siycm2 at 150 8C, post-annealed (a) 500 8C for 1 h; (b) 600 8C for 1 h.
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Fig. 8. Contact resistivity of the 50 nm Ta layers on nq poly-Si: (a) as-deposited; (b) non-implanted, annealed at 500 8C for 1 h; (c) implanted with 1=1016 Siycm2 at 300 8C; (d) implanted with 1=1016 Siycm2 at 400 8C; (e) implanted with 1=1016 Siycm2 at 500 8C; (f) implanted with 5=1016 Siycm2 at 400 8C; (g) implantation with 1=1016 Siycm2 at 150 8C and post-implantation annealing at 600 8C.
8c–e). This can be understood as competition between the positive effect of interface mixing and the negative influence of the amorphous Ta(Si) region. Implantation at 400 8C with 5=1016 Siycm2 leads to an improvement of the contact resistivity by approximately 40% (Fig. 8f). The low temperature implantation at 150 8C results in enhanced values of the contact resistivity. This increase can be explained by the amorphization of the Si substrate near the interface. Post-implantation annealing at 600 8C for 1 h leads here to a contact improvement in contrast to the result of the non-implanted sample as shown in Fig. 8g. This can be attributed to the substrate recrystallisation. Silicide formation at the interface, occurring at this high temperature, may also contribute. 4. Summary and conclusions The effect of ion beam mixing using 120 keV Siq ions on the microstructure of 50 nm Ta layers deposited on Si and the phase formation at the Ta–Si interface as well as its effect on the contact resistivity have been investigated. Mixing of Ta with Si by medium doses (;1=1016 Siycm2) results in the formation of an amorphous Ta(Si) layer near the interface at all investigated temperatures. Implantation with enhanced temperature leads to a Ta:Si ratio of 1:1 over an extended depth range indicating the formation of an X-ray amorphous monosilicide. For higher doses (03=1016 Siy cm2) at high temperature beginning from the interface a crystalline TaSi2 layer is formed. Complete transfor-
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mation of the 50 nm tantalum film into TaSi2 is possible by irradiation with 1=1017 Siycm2 assisted by substrate heating to 400 8C. Without implantation no silicides are formed at the interface below 650 8C for 1 h. This leads to the conclusion that ion beam mixing lowers the necessary temperature for the silicide reaction from 0650 to 400 8C by ballistic mixing and radiationenhanced diffusion. Implantation at temperatures )300 8C is necessary to avoid amorphization of the silicon substrate, which is disadvantageous for the contact resistivity. A significant reduction of contact resistivity can be obtained by formation of a TaSi2 layer at the interface between Si and the contact material. In conclusion, we have demonstrated that an improvement of the Ta–Si contact resistivity by approximately 40% has been obtained by ion beam mixing. However, because of the high substrate doping, the contact resistivity of the non-implanted layer is already very low. Further studies on Ta–Si contacts with lower substrate doping are in progress to investigate the effect of silicide formation at systems where a more pronounced contact improvement is expected. References w1x R.K. Shuka, P.W. Davies, B.M. Tracy, J. Vac. Sci. Technol. B 4 (1986) 1344. w2x N. Bibic, S. Dhar, M. Milosavljevic, K. Removic, L. Rissanen, K.P. Lieb, Nucl. Instrum. Methods Phys. Res. B, 161–163 (2000) 1011. w3x M.Y. Tsai, C.S. Peterson, F.M. d’Heurle, V. Maniscalco, Appl. Phys. Lett. 37 (1980) 295. w4x A.A. Galuska, J. Vac. Sci. Technol. B 5 (1987) 1. w5x Z.G. Tao Jiang, Du. Anyan, Wu. Guoying, J. Vac. Sci. Technol. B 6 (1988) 1746. w6x K.N. Tu, J.W. Mayer, Thin Films—Interdiffusion and Reactions, Wiley, New York, 1978, p. 359. w7x D. Fischer, M. Stavrev, N. Urbansky, C. Wenzel, E. Neubauer, J.G. Bauer, T. Scherg, H.J. Schullze, Conference Proceedings ULSI XIV, Mater. Res. Soc. (1999) 337. w8x S.S. Cohen, Thin Solid Films 104 (1983) 361. w9x S.J. Proctor, L.W. Linholm, J.A. Mazer, IEEE Trans. Electron. Dev. ED-30 (1983) 1535. w10x W.Y. Hsieh, J.H. Lin, L.J. Chen, Appl. Phys. Lett. 62 (1993) 1088. w11x J.M. Liang, L.J. Chen, Appl. Phys. Lett. 64 (1994) 1224. w12x J.R. Abelson, K.B. Kim, D.E. Mercer, C.R. Helms, R. Sinclair, T.W. Sigmon, J. Appl. Phys. 63 (1988) 689. w13x J.Y. Cheng, L.J. Chen, J. Appl. Phys. 69 (1991) 2161. w14x J.R. Dennis, E.B. Hale, J. Appl. Phys. 49 (1978) 1119. w15x T.B. Massalski, H. Hiroaki, P.R. Subramanian, L. Kacpzak, Binary Alloy Phase Diagrams, American Society for Metals, William W. Scott-Publisher, second ed., vol. 3, (1990) p. 3364, ISBN: 0-87170-406-4. w16x A. Noya, M. Takeyama, J. Vac. Sci. Technol. A 15 (1997) 253.