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Ti bilayers on SiO2
Materials Chemistry and Physics 62 (2000) 29±34
Thermal stability of Co/Hf and Co/Ti bilayers on SiO2 Youngjae Kwon, Chongmu Lee* Department of Metallurgical Engineering, Inha University, 253 Yonghyun-dong Nam-ku, Inchon 402-751, South Korea Received 12 February 1999; received in revised form 17 June 1999; accepted 23 June 1999
Abstract The Co/Ti bilayer silicidation technique is widely known as a method to obtain an epitaxial CoSi2. The Co/Hf bilayer also can be used for this purpose. During the silicidation the metal and the spacer SiO2 can react. Any residue of this reaction can degrade device performance by compromising the oxide integrity or by producing pattern bridging. In this paper the reaction of the Co/Hf bilayer with SiO2, as well as that of the Co/Ti bilayer with SiO2, during the silicidation annealing are reported. The collapse of the upper side of the SiO2 substrate occurred at 8008C in the Co/Ti/SiO2 sample and at 7008C in the Co/Hf/SiO2 sample. The sheet resistance of both the samples increased rapidly in the temperature range above 5008C. The rapid increase in the sheet resistance of Co/metal/SiO2 may be owing to the reaction between the metal layer and the SiO2 substrate, and the agglomeration of the Co layer on SiO2. Considering the temperature to get epitaxial CoSi2, along with the temperature from which the SiO2 substrate starts collapsing, we may conclude that optimum silicidation annealing temperatures for Co/Ti/Si and Co/Hf/Si are 700 and 6008C, respectively. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Salicide; Co silicide; Bilayer; Oxide collapsing
1. Introduction Self-aligned silicide (salicide) technology is widely used to reduce the contact resistances and the sheet resistances of the shallow junctions of source/drain regions as well as the interconnect resistances of polysilicon lines [1]. Titanium silicide and cobalt silicide are the two main silicides for application to salicide structures. Silicidation of a single Co or Ti layer on Si makes it dif®cult to form a shallow junction owing to the Si overconsumption. In addition, due to the non-uniform reaction [2,3] it is dif®cult to form a uniform Si/silicide interface necessary to get good electrical properties. Recently a method of silicidation using Co/Ti bilayer was tried to solve these problems related to the single metal layer silicidation [4,5]. In the Co/Ti bilayer silicidation technique known as TIME (Ti interlayer mediated epitaxy) the thin Ti layer is used to enhance the epitaxial growth of CoSi2 without overconsuming the Si atoms in the Si substrate. Other refractory metals like Ta and Zr have been used as epitaxy promoters [6,7]. The authors recently reported the silicidation of Co/Hf and Co/Nb bilayers as well as the Co/Ti bilayer [8]. In this work perfect and local epitaxial CoSi2 *
Corresponding author. Tel.: 82-32-860-7536; fax: 82-32-862-5546 E-mail address: [email protected] (C. Lee)
layers were obtained by the silicidation of Co/Ti and Co/Hf, respectively, but a non-epitaxial CoSi2 layer was obtained with Co/Nb under the same process conditions. During the silicidation the metal and the spacer SiO2 can react. Any residue of this reaction can degrade device performance by compromising the oxide integrity or by producing bridging. For example, during silicidation Ti can react with SiO2 to form electrically conducting TiSix and TiOx, which may result in electrical short between source/drain and gate or leakage current [9]. In general refractory metals like Ti, Hf, and Nb have much higher oxidation potentials than Si, while Co has a lower one than Si. The reaction between Ti and Co on SiO2 has been investigated in detail by previous workers [11,12], but little has been reported on the reactions between other refractory metals and SiO2. In this paper we report the reaction of SiO2 with bilayers of Co/Hf or Co/Ti during silicidation annealing. 2. Experimental P-type (100)Si wafers were thermally oxidized to grow SiO2 ®lms 300 nm thick. Co/Ti/SiO2 and Co/Hf/SiO2 samples were prepared by the sputter deposition on the SiO2 ®lm
0254-0584/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 1 5 6 - X
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Y. Kwon, C. Lee / Materials Chemistry and Physics 62 (2000) 29±34
of Ti and Hf, respectively, followed by Co. The thicknesses of Co, Ti and Hf ®lms were ®xed to about 28, 8, and 8 nm, respectively. The base pressure of the sputter chamber was 5 10ÿ7 Torr. The samples were then annealed under a vacuum of 2 10ÿ5 Torr from 330 to 8008C for 30 s in a halogen lamp rapid thermal annealing (RTA) system. A four point probe technique was used to measure the sheet resistances of the ®lms in the samples. Glancing angle Xray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to identify the phases and oxidation state in the samples formed during RTA, respectively. Also Auger electron emission spectrometry (AES) depth pro®ling and atomic force microscopy (AFM) were performed to characterize the chemical compositions and surface morphologies of the samples. 3. Results and discussion Fig. 1 shows the glancing angle XRD spectra of the Co(28 nm)/Ti(8 nm)/SiO2 samples annealed from 330 to 8008C for 30 s in an RTA system. In an as-deposited state the Co layer seems to be in between polycrystalline with the (100) texture and amorphous, but the Ti layer is completely amorphous since no Ti peak is found in Fig. 1. The Ti layer seems to be virtually in a state of titanium oxide as can be seen from AES depth pro®les later. The Co peak seems to increase as the annealing temperature increases from 330 to 7008C, which implies that crystallization and grain growth proceeds by annealing. On the other hand the XRD spectra of the Co(28 nm)/ Hf(8 nm)/SiO2 samples annealed from 500 to 6508C for 30 s in an RTA system are shown in Fig. 2. The Co/Hf/SiO2 system also shows the tendency that Co ®lms are crystallized with increasing the annealing temperature from 500 to
Fig. 1. The XRD spectra of Co(28 nm)/Ti(8 nm)/SiO2 after RTA annealing at various temperatures for 30 s.
Fig. 2. The XRD spectra of the Co(28 nm)/Hf(8 nm)/SiO2 after RTA annealing at various temperatures for 30 s.
6508C. In addition, Co-Hf compounds seem to form during annealing in the Co/Hf/SiO2 system. It is one of the major concerns in the silicidation of Co/Ti/ SiO2 and Co/Hf/SiO2 if electrically conducting reaction products form or not as a result of the reactions between Ti (or Hf) and the SiO2 substrate as pointed out earlier. Since XRD spectra did not give suf®cient information, XPS analyses were performed to investigate the reactions between Ti (or Hf) and SiO2. The XPS spectra of the Co/ Ti/SiO2 samples annealed at 6008C for 30 s followed by etching of the Co residues are shown in Fig. 3. The spectra were split into many component peaks using a curve ®tting technique. Ti2p curve shows the Ti ®lms were totally oxidized. TiO2 (2a, 2b) and TiO (Ti rich TiO: 1a, 1b, and O rich TiO: 3a, 3b) [12] are mixed in the oxidized Ti layers. TiO2 is the most stable phase, but it does not seem that the oxidation of Ti has been completed after annealing at 6008C
Fig. 3. The Ti2p XPS region of Co/Ti/SiO2 sample after RTA annealing for 30 s at 6008C and subsequent etching of the upper Co layer.
Y. Kwon, C. Lee / Materials Chemistry and Physics 62 (2000) 29±34
Fig. 4. The XRD spectra of Hf/SiO2 sample after RTA annealing for 30 s.
for 30 s. The formation of titanium oxide implies two possibilities: one is the reaction between Ti and oxygen impurities existing in the ambient gas of the RTA furnace. The other is the reaction between Ti and SiO2 at the interface of the Ti layer and the SiO2 substrate. Ting et al. [10] reported SiO2 loss during the furnace annealing of 100 nm Ti on SiO2 areas in Ar from 500 to 6008C for 30±120 min to be 5.5±15 m. The amount of SiO2 loss during silicidation failed to be measured in this study since the thickness of the Ti layer in the Co/Ti/SiO2 sample was as thin as 8 nm and the RTA annealing time was as short as 30 sec. However, the amount of the SiO2 loss seems to be negligible. No evidence for the formation of conducting TiSix was found from the sheet resistance measurement, and XRD and XPS analyses.
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Fig. 4 shows the XRD spectra for the RTA-annealed Hf(10 nm)/SiO2 samples. HfO2 peaks are found for the samples annealed at temperatures higher than 5008C. The same HfO2 peak as is observed here was also found for the Co/Hf/SiO2 samples (Fig. 2). The sheet resistance of the asdeposited Hf ®lm on SiO2 was measured to be as high as a few thousand /&, which suggests that a part of the HfO2 formed during the sputter-deposition of Hf, although no HfO2 peak is found in the XRD spectra for the as-deposited Hf/SiO2 sample. The sheet resistance of the Co/Hf/SiO2 sample annealed at temperatures higher than 5008C approaches that of the SiO2 substrate implying that electrically conducting phases like Hf silicides did not form during silicidation annealing. It should be pointed out that the lowest sheet resistance and the best epitaxial CoSi2 layer was obtained for the Co/Hf/(100)Si sample annealed at 6008C in our previous work [8]. The XRD spectra of the Co/Hf/SiO2 and Co/Ti/SiO2 samples annealed at 8008C are shown in Fig. 5. In the case of Co/Hf/SiO2 no SiO2 peak is found at all, unlike Co/Ti/SiO2. This may be because the top of the SiO2 substrate in contact with the Hf layer was completely decomposed. Since the tendency of oxidation for Hf (Gibbs's free energy for formation, Gf ÿ253.6 kcal moleÿ1) is much stronger than that for Ti (Gf ÿ212.5 kcal moleÿ1), the reaction between Hf and SiO2 occurs more actively than that between Ti and SiO2. The easier decomposition of the SiO2 in contact with Hf can also be explained using a structural concept as follows: SiO2 has a tetrahedral structure in which a Si ion is surrounded by four O ions. A transition metal atom like Ti and Hf is a network modi®er which occupies an interstitial site between matrix atoms [13]. Since the size of the Hf atom (atomic volume 13.6 cm3 moleÿ1) is larger than that of the Ti atom (atomic volume 10.6 cm3 moleÿ1), Hf atoms are more likely to collapse the SiO2 tetrahedral structure
Fig. 5. XRD spectra of (a) Co/Hf and (b) Co/Ti on SiO2 substrate after RTA annealing for 30 s at 8008C.
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Y. Kwon, C. Lee / Materials Chemistry and Physics 62 (2000) 29±34
Fig. 6. AES depth profiles of the Co(28nm)/Ti(8nm)/SiO2 samples (a) asdeposited, and RTA annealed for 30 s at (b) 7008C, (c) 8008C.
resulting in its decomposition. The oxygen atoms coming out of the SiO2 tetrahedra react with Hf atoms to form HfO2. Fig. 6 shows AES depth pro®les of the Co/Ti/SiO2 samples. In the case of as-deposited Co/Ti on SiO2 (Fig. 6a), Ti and O peaks are nearly overlapped although the O peak is positioned slightly on the righthand side of the Ti peak. This suggests that Ti reacted with SiO2 when Ti atoms with high kinetic energies bombarded the SiO2 substrate during their sputter deposition. The high oxygen concentration at the surface of the Co layer seems to be due to the incorporation of the oxygen atoms existing as a residual gas in the sputter chamber. In the case of annealing at 7008C (Fig. 6b) there is little change in the layer structure compared with the as-deposited samples. The Ti and O peak positions are perfectly coincident suggesting that substantial reactions occurred between Ti and SiO2. Since the Ti layer was very thin (8 nm), all the Ti atoms in the Ti layer participated in the reaction with SiO2, although only the SiO2 molecules in the upper side of the SiO2 substrate did participate in the reaction with Ti. The x value of the TiOx in the Co/Ti/SiO2 sample annealed at 6008C is higher than that
Fig. 7. AES depth profiles of the Co(28 nm)/Hf(8 nm)/SiO2 samples (a) as-deposited, and RTA annealed for 30 s at (b) 6008C and (c) 7008C.
in the as-deposited Co/Ti/SiO2 sample. Also some interdiffusion of Ti and Co atoms seems to have occurred at the Co/Ti interface. In the case of Co/Ti/SiO2 annealed at 8008C (Fig. 6c) the resulting layer structure of the sample is quite different from that annealed at 7008C. Co atoms diffused deep into the SiO2 substrate during the annealing at 8008C. As a result of Co diffusion the relative positions of Co and Ti layers are reversed. The tetrahedral structures of the upper side of the SiO2 substrate collapsed and many oxygen atoms released from the SiO2 tetrahedra bonded with Ti atoms, which resulted in the increase of the oxygen concentration in the Ti layer. Thus, the x value of the TiOx in the Ti layer increased from 1 to 2 during annealing at 8008C. The as-deposited Co/Hf/SiO2 sample and the one annealed above 6008C for 30 s are shown in Fig. 7. There is little change after annealing at 6008C (Fig. 7b) except that the Hf (virtually HfOx) layer became thicker and the x value of HfOx increased, but there is a signi®cant change after annealing at 7008C (Fig. 7c). At 7008C Co atoms diffused deep into the SiO2 substrate and the upper side of the SiO2 substrate was collapsed with the result that the composition
Y. Kwon, C. Lee / Materials Chemistry and Physics 62 (2000) 29±34
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Table 1 Sheet resistances of Co/Hf and Co/Ti bilayers on SiO2 ( /&)
Co/Ti/Si2 Co/Hf/Si2
As depo.
3308C
4208C
5008C
5508C
6008C
7008C
15.0 42
9.7 ±
8.7 ±
7.6 8.5
± >10,000
952 >10,000
5,341 >10,000
of the SiO2 changed to SiO1.7. Also it can be seen that the Co and Hf peak positions are reversed after annealing at 7008C. It should be noted that the collapse of the upper side of the SiO2 substrate occurred at 8008C in the Co/Ti/SiO2 sample, compared with 7008C in the Co/Hf/SiO2 sample. Table 1 compares the sheet resistances of Co/Ti/SiO2 with those of Co/Hf/SiO2. In this Table `>10,000' means the sheet resistance value out of the measurement range with the four point probe used in this experiment. In the temperature range below 5008C the sheet resistances for both samples tend to somewhat decrease with increasing the annealing temperature, due to the crystallization and grain growth of the metal ®lms. However, the sheet resistances of Co/Ti/ SiO2 and Co/Hf/SiO2 increase rapidly in the temperature range above 600 and 5008C, respectively. The sheet resistance of the Co/Hf/SiO2 sample at 5508C is in the range of dielectric materials. The rapid increase in the sheet resistance of Co/Ti/SiO2 at 6008C may be owing to the reaction between the Ti layer and the SiO2 substrate and the agglomeration of the Co layer on SiO2. The reaction between the Ti and SiO2 was con®rmed from the XPS results (Fig. 3) earlier. The agglomeration of the Co layer is shown in the AFM results (Fig. 8). Surface roughness of Co/Ti/ SiO2 starts increasing rapidly from 4208C, while that of Co/Hf/SiO2 does from 5008C. The temperature ranges for the rapid increase in the sheet resistances of Co/Ti/SiO2 and Co/Hf/SiO2 agree roughly with those for the rapid increase in the surface roughnesses of those two systems.
However, the high sheet resistances in these temperature ranges do not necessarily mean that the silicidation annealing should not be performed at these temperatures. Since the metals on SiO2 will be removed after silicidation annealing eventually, if only the SiO2 substrate is not collapsed or decomposed, the high resistances are not a big problem. An important thing is the temperature from which the SiO2 substrate starts collapsing, because silicidation annealing should be performed below this critical temperature. The authors reported previously that silicidation annealing should be performed at a temperature higher than 7008C to get epitaxial CoSi2 for Co/Ti/Si but that the optimum silicidation annealing temperature to get epitaxial CoSi2 for Co/Hf/Si is 6008C. Considering the temperature to get epitaxial CoSi2 along with the temperature from which the SiO2 substrate starts collapsing we may conclude that optimum silicidation annealing temperatures for Co/Ti/Si and Co/Hf/Si are 7008C and 6008C, respectively. 4. Conclusions The reaction of the Co/Hf bilayer with SiO2 as well as that of the Co/Ti bilayer with SiO2 during silicidation annealing were investigated. The collapse of the upper side of the SiO2 substrate occurred at 8008C in the Co/Ti/SiO2 sample, while that occurred at 7008C in the Co/Hf/SiO2 sample. The sheet resistance of both samples began to increase rapidly in the temperature range above 5508C. The rapid increase in the sheet resistance of Co/Ti/SiO2 may be owing to the reaction between the Ti layer and the SiO2 substrate and the agglomeration of the Co layer on SiO2. Considering the temperature to get epitaxial CoSi2 along with the temperature from which the SiO2 substrate starts collapsing we may conclude that optimum silicidation annealing temperatures for Co/Ti/ Si and Co/Hf/Si are 700 and 6008C, respectively.
Acknowledgements The authors wish to gratefully acknowledge ®nancial support for the present work by Samsung Electrics.
References Fig. 8. The AFM surface morphology and standard deviation of roughness of (a) Co/Ti/SiO2 and (b) Co/Hf/SiO2 samples as a function of various annealing temperatures.
[1] S. Wolf, Silicon Processing for VLSI Era, Process Integration, vol. 2, Lattice Press, CA, 1990, pp. 84±175. [2] S.P. Muraka, D.S. Williams, J. Vac. Sci. Technol. B5, 1674, 1987.
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[3] A.E. Morgan, E.K. Broadbent, M. Delfino, B. Coulman, D.K. Sadana, J. Elcectrochem. Soc. 134 (1987) 925. [4] M. Lawrence, A. Dass, D.B. Fraser, C.S. Wei, Appl. Phys. Lett. 58 (1991) 1308. [5] A. Vantomme, M.-A. Nicolet, N.D. Theodore, J. Appl. Phys. 75 (1994) 3882. [6] J.S. Byun, Ph.D. Thesis, Seoul Natl. University, 1992. [7] J.S. Byun, J.J. Kim, W.S. Kim, H.J. Kim, J. Elcectrochem. Soc. 1428 (1995) 2805. [8] Y. Kwon, C. Lee, H.K. Kang, D.L Bae, Mat. Chem. Phys., submitted.
[9] A.E. Morgan, E.K. Broadbent, K.N. Ritz, D.K. Sadana, B.J. Burrow, J. Appl. Phys. 64 (1988) 344. [10] C.Y. Ting, M. Wittmer, S.S. Iyer, S.B. Brodsky, J. Electrochem. Soc. 131 (1984) 2934. [11] G.J.P. Krooshof, F.H.P.M. Habraken, W.F. van der Weg, L. Van den hove, K. Maex, R.F. De Keersmaecker, J. Appl. Phys. 63 (1988) 5110. [12] Y.N. Kim, S.C. Kang, J.S. Park, N.I. Lee, I.K. Kim, Y.W. Kim, J. Kor. Vac. Soc. 21 (1993) 28. [13] K.H. Sun, J. Am. Ceram. Soc. 30 (1949) 277.
Thermal stability of Co/Hf and Co/Ti bilayers on SiO2 Youngjae Kwon, Chongmu Lee* Department of Metallurgical Engineering, Inha University, 253 Yonghyun-dong Nam-ku, Inchon 402-751, South Korea Received 12 February 1999; received in revised form 17 June 1999; accepted 23 June 1999
Abstract The Co/Ti bilayer silicidation technique is widely known as a method to obtain an epitaxial CoSi2. The Co/Hf bilayer also can be used for this purpose. During the silicidation the metal and the spacer SiO2 can react. Any residue of this reaction can degrade device performance by compromising the oxide integrity or by producing pattern bridging. In this paper the reaction of the Co/Hf bilayer with SiO2, as well as that of the Co/Ti bilayer with SiO2, during the silicidation annealing are reported. The collapse of the upper side of the SiO2 substrate occurred at 8008C in the Co/Ti/SiO2 sample and at 7008C in the Co/Hf/SiO2 sample. The sheet resistance of both the samples increased rapidly in the temperature range above 5008C. The rapid increase in the sheet resistance of Co/metal/SiO2 may be owing to the reaction between the metal layer and the SiO2 substrate, and the agglomeration of the Co layer on SiO2. Considering the temperature to get epitaxial CoSi2, along with the temperature from which the SiO2 substrate starts collapsing, we may conclude that optimum silicidation annealing temperatures for Co/Ti/Si and Co/Hf/Si are 700 and 6008C, respectively. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Salicide; Co silicide; Bilayer; Oxide collapsing
1. Introduction Self-aligned silicide (salicide) technology is widely used to reduce the contact resistances and the sheet resistances of the shallow junctions of source/drain regions as well as the interconnect resistances of polysilicon lines [1]. Titanium silicide and cobalt silicide are the two main silicides for application to salicide structures. Silicidation of a single Co or Ti layer on Si makes it dif®cult to form a shallow junction owing to the Si overconsumption. In addition, due to the non-uniform reaction [2,3] it is dif®cult to form a uniform Si/silicide interface necessary to get good electrical properties. Recently a method of silicidation using Co/Ti bilayer was tried to solve these problems related to the single metal layer silicidation [4,5]. In the Co/Ti bilayer silicidation technique known as TIME (Ti interlayer mediated epitaxy) the thin Ti layer is used to enhance the epitaxial growth of CoSi2 without overconsuming the Si atoms in the Si substrate. Other refractory metals like Ta and Zr have been used as epitaxy promoters [6,7]. The authors recently reported the silicidation of Co/Hf and Co/Nb bilayers as well as the Co/Ti bilayer [8]. In this work perfect and local epitaxial CoSi2 *
Corresponding author. Tel.: 82-32-860-7536; fax: 82-32-862-5546 E-mail address: [email protected] (C. Lee)
layers were obtained by the silicidation of Co/Ti and Co/Hf, respectively, but a non-epitaxial CoSi2 layer was obtained with Co/Nb under the same process conditions. During the silicidation the metal and the spacer SiO2 can react. Any residue of this reaction can degrade device performance by compromising the oxide integrity or by producing bridging. For example, during silicidation Ti can react with SiO2 to form electrically conducting TiSix and TiOx, which may result in electrical short between source/drain and gate or leakage current [9]. In general refractory metals like Ti, Hf, and Nb have much higher oxidation potentials than Si, while Co has a lower one than Si. The reaction between Ti and Co on SiO2 has been investigated in detail by previous workers [11,12], but little has been reported on the reactions between other refractory metals and SiO2. In this paper we report the reaction of SiO2 with bilayers of Co/Hf or Co/Ti during silicidation annealing. 2. Experimental P-type (100)Si wafers were thermally oxidized to grow SiO2 ®lms 300 nm thick. Co/Ti/SiO2 and Co/Hf/SiO2 samples were prepared by the sputter deposition on the SiO2 ®lm
0254-0584/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 1 5 6 - X
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Y. Kwon, C. Lee / Materials Chemistry and Physics 62 (2000) 29±34
of Ti and Hf, respectively, followed by Co. The thicknesses of Co, Ti and Hf ®lms were ®xed to about 28, 8, and 8 nm, respectively. The base pressure of the sputter chamber was 5 10ÿ7 Torr. The samples were then annealed under a vacuum of 2 10ÿ5 Torr from 330 to 8008C for 30 s in a halogen lamp rapid thermal annealing (RTA) system. A four point probe technique was used to measure the sheet resistances of the ®lms in the samples. Glancing angle Xray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to identify the phases and oxidation state in the samples formed during RTA, respectively. Also Auger electron emission spectrometry (AES) depth pro®ling and atomic force microscopy (AFM) were performed to characterize the chemical compositions and surface morphologies of the samples. 3. Results and discussion Fig. 1 shows the glancing angle XRD spectra of the Co(28 nm)/Ti(8 nm)/SiO2 samples annealed from 330 to 8008C for 30 s in an RTA system. In an as-deposited state the Co layer seems to be in between polycrystalline with the (100) texture and amorphous, but the Ti layer is completely amorphous since no Ti peak is found in Fig. 1. The Ti layer seems to be virtually in a state of titanium oxide as can be seen from AES depth pro®les later. The Co peak seems to increase as the annealing temperature increases from 330 to 7008C, which implies that crystallization and grain growth proceeds by annealing. On the other hand the XRD spectra of the Co(28 nm)/ Hf(8 nm)/SiO2 samples annealed from 500 to 6508C for 30 s in an RTA system are shown in Fig. 2. The Co/Hf/SiO2 system also shows the tendency that Co ®lms are crystallized with increasing the annealing temperature from 500 to
Fig. 1. The XRD spectra of Co(28 nm)/Ti(8 nm)/SiO2 after RTA annealing at various temperatures for 30 s.
Fig. 2. The XRD spectra of the Co(28 nm)/Hf(8 nm)/SiO2 after RTA annealing at various temperatures for 30 s.
6508C. In addition, Co-Hf compounds seem to form during annealing in the Co/Hf/SiO2 system. It is one of the major concerns in the silicidation of Co/Ti/ SiO2 and Co/Hf/SiO2 if electrically conducting reaction products form or not as a result of the reactions between Ti (or Hf) and the SiO2 substrate as pointed out earlier. Since XRD spectra did not give suf®cient information, XPS analyses were performed to investigate the reactions between Ti (or Hf) and SiO2. The XPS spectra of the Co/ Ti/SiO2 samples annealed at 6008C for 30 s followed by etching of the Co residues are shown in Fig. 3. The spectra were split into many component peaks using a curve ®tting technique. Ti2p curve shows the Ti ®lms were totally oxidized. TiO2 (2a, 2b) and TiO (Ti rich TiO: 1a, 1b, and O rich TiO: 3a, 3b) [12] are mixed in the oxidized Ti layers. TiO2 is the most stable phase, but it does not seem that the oxidation of Ti has been completed after annealing at 6008C
Fig. 3. The Ti2p XPS region of Co/Ti/SiO2 sample after RTA annealing for 30 s at 6008C and subsequent etching of the upper Co layer.
Y. Kwon, C. Lee / Materials Chemistry and Physics 62 (2000) 29±34
Fig. 4. The XRD spectra of Hf/SiO2 sample after RTA annealing for 30 s.
for 30 s. The formation of titanium oxide implies two possibilities: one is the reaction between Ti and oxygen impurities existing in the ambient gas of the RTA furnace. The other is the reaction between Ti and SiO2 at the interface of the Ti layer and the SiO2 substrate. Ting et al. [10] reported SiO2 loss during the furnace annealing of 100 nm Ti on SiO2 areas in Ar from 500 to 6008C for 30±120 min to be 5.5±15 m. The amount of SiO2 loss during silicidation failed to be measured in this study since the thickness of the Ti layer in the Co/Ti/SiO2 sample was as thin as 8 nm and the RTA annealing time was as short as 30 sec. However, the amount of the SiO2 loss seems to be negligible. No evidence for the formation of conducting TiSix was found from the sheet resistance measurement, and XRD and XPS analyses.
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Fig. 4 shows the XRD spectra for the RTA-annealed Hf(10 nm)/SiO2 samples. HfO2 peaks are found for the samples annealed at temperatures higher than 5008C. The same HfO2 peak as is observed here was also found for the Co/Hf/SiO2 samples (Fig. 2). The sheet resistance of the asdeposited Hf ®lm on SiO2 was measured to be as high as a few thousand /&, which suggests that a part of the HfO2 formed during the sputter-deposition of Hf, although no HfO2 peak is found in the XRD spectra for the as-deposited Hf/SiO2 sample. The sheet resistance of the Co/Hf/SiO2 sample annealed at temperatures higher than 5008C approaches that of the SiO2 substrate implying that electrically conducting phases like Hf silicides did not form during silicidation annealing. It should be pointed out that the lowest sheet resistance and the best epitaxial CoSi2 layer was obtained for the Co/Hf/(100)Si sample annealed at 6008C in our previous work [8]. The XRD spectra of the Co/Hf/SiO2 and Co/Ti/SiO2 samples annealed at 8008C are shown in Fig. 5. In the case of Co/Hf/SiO2 no SiO2 peak is found at all, unlike Co/Ti/SiO2. This may be because the top of the SiO2 substrate in contact with the Hf layer was completely decomposed. Since the tendency of oxidation for Hf (Gibbs's free energy for formation, Gf ÿ253.6 kcal moleÿ1) is much stronger than that for Ti (Gf ÿ212.5 kcal moleÿ1), the reaction between Hf and SiO2 occurs more actively than that between Ti and SiO2. The easier decomposition of the SiO2 in contact with Hf can also be explained using a structural concept as follows: SiO2 has a tetrahedral structure in which a Si ion is surrounded by four O ions. A transition metal atom like Ti and Hf is a network modi®er which occupies an interstitial site between matrix atoms [13]. Since the size of the Hf atom (atomic volume 13.6 cm3 moleÿ1) is larger than that of the Ti atom (atomic volume 10.6 cm3 moleÿ1), Hf atoms are more likely to collapse the SiO2 tetrahedral structure
Fig. 5. XRD spectra of (a) Co/Hf and (b) Co/Ti on SiO2 substrate after RTA annealing for 30 s at 8008C.
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Y. Kwon, C. Lee / Materials Chemistry and Physics 62 (2000) 29±34
Fig. 6. AES depth profiles of the Co(28nm)/Ti(8nm)/SiO2 samples (a) asdeposited, and RTA annealed for 30 s at (b) 7008C, (c) 8008C.
resulting in its decomposition. The oxygen atoms coming out of the SiO2 tetrahedra react with Hf atoms to form HfO2. Fig. 6 shows AES depth pro®les of the Co/Ti/SiO2 samples. In the case of as-deposited Co/Ti on SiO2 (Fig. 6a), Ti and O peaks are nearly overlapped although the O peak is positioned slightly on the righthand side of the Ti peak. This suggests that Ti reacted with SiO2 when Ti atoms with high kinetic energies bombarded the SiO2 substrate during their sputter deposition. The high oxygen concentration at the surface of the Co layer seems to be due to the incorporation of the oxygen atoms existing as a residual gas in the sputter chamber. In the case of annealing at 7008C (Fig. 6b) there is little change in the layer structure compared with the as-deposited samples. The Ti and O peak positions are perfectly coincident suggesting that substantial reactions occurred between Ti and SiO2. Since the Ti layer was very thin (8 nm), all the Ti atoms in the Ti layer participated in the reaction with SiO2, although only the SiO2 molecules in the upper side of the SiO2 substrate did participate in the reaction with Ti. The x value of the TiOx in the Co/Ti/SiO2 sample annealed at 6008C is higher than that
Fig. 7. AES depth profiles of the Co(28 nm)/Hf(8 nm)/SiO2 samples (a) as-deposited, and RTA annealed for 30 s at (b) 6008C and (c) 7008C.
in the as-deposited Co/Ti/SiO2 sample. Also some interdiffusion of Ti and Co atoms seems to have occurred at the Co/Ti interface. In the case of Co/Ti/SiO2 annealed at 8008C (Fig. 6c) the resulting layer structure of the sample is quite different from that annealed at 7008C. Co atoms diffused deep into the SiO2 substrate during the annealing at 8008C. As a result of Co diffusion the relative positions of Co and Ti layers are reversed. The tetrahedral structures of the upper side of the SiO2 substrate collapsed and many oxygen atoms released from the SiO2 tetrahedra bonded with Ti atoms, which resulted in the increase of the oxygen concentration in the Ti layer. Thus, the x value of the TiOx in the Ti layer increased from 1 to 2 during annealing at 8008C. The as-deposited Co/Hf/SiO2 sample and the one annealed above 6008C for 30 s are shown in Fig. 7. There is little change after annealing at 6008C (Fig. 7b) except that the Hf (virtually HfOx) layer became thicker and the x value of HfOx increased, but there is a signi®cant change after annealing at 7008C (Fig. 7c). At 7008C Co atoms diffused deep into the SiO2 substrate and the upper side of the SiO2 substrate was collapsed with the result that the composition
Y. Kwon, C. Lee / Materials Chemistry and Physics 62 (2000) 29±34
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Table 1 Sheet resistances of Co/Hf and Co/Ti bilayers on SiO2 ( /&)
Co/Ti/Si2 Co/Hf/Si2
As depo.
3308C
4208C
5008C
5508C
6008C
7008C
15.0 42
9.7 ±
8.7 ±
7.6 8.5
± >10,000
952 >10,000
5,341 >10,000
of the SiO2 changed to SiO1.7. Also it can be seen that the Co and Hf peak positions are reversed after annealing at 7008C. It should be noted that the collapse of the upper side of the SiO2 substrate occurred at 8008C in the Co/Ti/SiO2 sample, compared with 7008C in the Co/Hf/SiO2 sample. Table 1 compares the sheet resistances of Co/Ti/SiO2 with those of Co/Hf/SiO2. In this Table `>10,000' means the sheet resistance value out of the measurement range with the four point probe used in this experiment. In the temperature range below 5008C the sheet resistances for both samples tend to somewhat decrease with increasing the annealing temperature, due to the crystallization and grain growth of the metal ®lms. However, the sheet resistances of Co/Ti/ SiO2 and Co/Hf/SiO2 increase rapidly in the temperature range above 600 and 5008C, respectively. The sheet resistance of the Co/Hf/SiO2 sample at 5508C is in the range of dielectric materials. The rapid increase in the sheet resistance of Co/Ti/SiO2 at 6008C may be owing to the reaction between the Ti layer and the SiO2 substrate and the agglomeration of the Co layer on SiO2. The reaction between the Ti and SiO2 was con®rmed from the XPS results (Fig. 3) earlier. The agglomeration of the Co layer is shown in the AFM results (Fig. 8). Surface roughness of Co/Ti/ SiO2 starts increasing rapidly from 4208C, while that of Co/Hf/SiO2 does from 5008C. The temperature ranges for the rapid increase in the sheet resistances of Co/Ti/SiO2 and Co/Hf/SiO2 agree roughly with those for the rapid increase in the surface roughnesses of those two systems.
However, the high sheet resistances in these temperature ranges do not necessarily mean that the silicidation annealing should not be performed at these temperatures. Since the metals on SiO2 will be removed after silicidation annealing eventually, if only the SiO2 substrate is not collapsed or decomposed, the high resistances are not a big problem. An important thing is the temperature from which the SiO2 substrate starts collapsing, because silicidation annealing should be performed below this critical temperature. The authors reported previously that silicidation annealing should be performed at a temperature higher than 7008C to get epitaxial CoSi2 for Co/Ti/Si but that the optimum silicidation annealing temperature to get epitaxial CoSi2 for Co/Hf/Si is 6008C. Considering the temperature to get epitaxial CoSi2 along with the temperature from which the SiO2 substrate starts collapsing we may conclude that optimum silicidation annealing temperatures for Co/Ti/Si and Co/Hf/Si are 7008C and 6008C, respectively. 4. Conclusions The reaction of the Co/Hf bilayer with SiO2 as well as that of the Co/Ti bilayer with SiO2 during silicidation annealing were investigated. The collapse of the upper side of the SiO2 substrate occurred at 8008C in the Co/Ti/SiO2 sample, while that occurred at 7008C in the Co/Hf/SiO2 sample. The sheet resistance of both samples began to increase rapidly in the temperature range above 5508C. The rapid increase in the sheet resistance of Co/Ti/SiO2 may be owing to the reaction between the Ti layer and the SiO2 substrate and the agglomeration of the Co layer on SiO2. Considering the temperature to get epitaxial CoSi2 along with the temperature from which the SiO2 substrate starts collapsing we may conclude that optimum silicidation annealing temperatures for Co/Ti/ Si and Co/Hf/Si are 700 and 6008C, respectively.
Acknowledgements The authors wish to gratefully acknowledge ®nancial support for the present work by Samsung Electrics.
References Fig. 8. The AFM surface morphology and standard deviation of roughness of (a) Co/Ti/SiO2 and (b) Co/Hf/SiO2 samples as a function of various annealing temperatures.
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Y. Kwon, C. Lee / Materials Chemistry and Physics 62 (2000) 29±34
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