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Titanium disilicide formation by rf plasma enhanced chemical vapor deposition and film properties
Applied Surface Science 206 (2003) 159±166
Titanium disilicide formation by rf plasma enhanced chemical vapor deposition and ®lm properties Osama A. Fouada,*, Masaaki Yamazatoa, Hiromichi Ichinoseb, Masamitsu Naganoa a
Department of Applied Chemistry, Faculty of Science and Engineering, Saga University, Honjo 1, Saga 840-8502, Japan b Saga Ceramics Research Laboratory, 3037-7 Arita, Nishimatsura-gun, Saga 844-0024, Japan Received 18 March 2002; received in revised form 17 June 2002; accepted 22 October 2002
Abstract Titanium disilicide thin ®lms have been deposited on Si(1 0 0) substrate by rf plasma enhanced chemical vapor deposition using TiCl4/H2 gas mixture at different deposition temperatures. At low temperature of 650 8C excessive silicon substrate etching took place and silicide formation could not be con®rmed. While at 700 8C Ti5Si3 was the only detected phase as found by X-ray diffraction (XRD) analysis. As the deposition temperature increased from 750 to 900 8C, the polycrystalline C54-TiSi2 phase deposited. Morphology of the ®lm surface changed noticeably as the deposition temperature increased. At low temperature of 700 8C the ®lm had a ¯ake structure. Increasing the temperature up to 850 8C resulted in a continuous ®lm with smoother grains, while at 900 8C agglomeration of grains took place resulting in coarse grains and discontinuous ®lm. At the optimum experimental conditions it was possible to deposit a homogeneous ®lm with smooth interface and suppressing silicon etching. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Plasma CVD; Titanium silicide; Crystal structure; Interface; Scanning electron microscopy
1. Introduction TiSi2 thin ®lms prepared by self-aligned silicide process (salicide) have been used for fabrication of advanced integrated circuits due to their high temperature stability and low electrical resistivity [1±3]. The salicide process is a multiple step process based on the solid-phase reaction of Ti with Si followed by post-annealing and selective etching. Although this * Corresponding author. Permanent address: CMRDI, P.O. Box 87, Helwan, Cairo, Egypt. Tel.: 81-952-28-8676; fax: 81-952-28-8591. E-mail address: [email protected] (O.A. Fouad).
reaction is thermodynamically spontaneous, some obstacles that make its update application rather dif®cult accompanied the salicide process. It is dif®cult to deposit titanium at the bottom of a high-aspect-ratio contact hole [3]. Incomplete phase transformation from the high resistivity phase (C49) to the low resistivity phase (C54) and agglomeration occur on narrow Si lines. In addition, the relatively poor step coverage with the damage effects by various beams are common problems [4]. Special interest has arisen in fabrication via low pressure chemical vapor deposition (LPCVD) [5±8] to solve the problems of the salicide process. LPCVD process offers the advantages of excellent conformal
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 1 2 1 0 - 2
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step coverage and high throughput. However, it has shortcomings such as relatively high growth temperature, large silicon consumption, and the large grain size of the deposited ®lms [8]. On the other hand, few works have been done on deposition of titanium silicide ®lms using PECVD [3,5,9,10] to improve the ®lm properties and overcome the LPCVD problems. However, it was reported that the deposited ®lm was amorphous and a post-annealing step was required to bring down the resistivity to 20 mO cm [5]. The high resistivity phase (C49) was formed with or without using SiH4 gas [10]. The discontinuity of the deposited ®lm, roughness of the silicide/silicon interface, and the phase transformation from C49 phase to C54 phase are still worsening problems [3,10]. In our previous work we investigated the preparation of the low resistivity C54-TiSi2 phase in one step reaction by PECVD, using TiCl4/H2 gas system without using the hazardous SiH4 gas [11,12]. The best deposition temperature has been addressed to be between 800 and 850 8C. In this temperature range, homogeneous ®lms with smooth interface have been obtained. Attempts to decrease the deposition temperature are done with careful analysis of the deposited ®lms to investigate the possibility of depositing homogeneous ®lm with smooth interface at lower deposition temperature in the present work. The initial stage of the ®lm growth is also investigated to better understand the reaction process. The properties of the as-deposited titanium silicide ®lm: crystal structure, composition and morphology of the surface and interface are also explored. 2. Experimental 2.1. PECVD system The PECVD system is schematically represented in Fig. 1. The reaction tube is a quartz tube of 35 mm inner diameter and 1000 mm length. The tube is sealed from both ends by steel ¯anges. TiCl4 (99.999%) and H2 are introduced to the reaction tube from one end through mass ¯ow controllers and needle valves. The plasma source is an rf generator that operates at 13.56 MHz with a maximum power of 1 kW and couples through matching network to a copper coil
Fig. 1. Schematic representation of rf±PECVD system.
(100 mm length and 5 mm diameter). An electric resistance furnace is used to heat the substrate. The temperature of the substrate is monitored by a Pt/Pt13%Rh thermocouple 5 mm apart from the substrate. The exhausted gases are pumped out from the system using a combination of rotary pump and oil diffusion pump. The pressure is monitored by Penning gauge and capacitance manometer. 2.2. Deposition procedure A detailed description of the deposition procedure is published elsewhere [11]. In brief, titanium silicide ®lms are deposited on a mirror polished n-type Si(1 0 0) substrate. The native oxide layer on Si substrate surface is removed by dipping in HF solution prior to loading into the reaction tube. After loading the substrate, the system is pumped down to a base pressure of 1 10 5 Torr. The substrate is then heated to the desired deposition temperature from 650 to 900 8C. Prior to deposition, the substrate surface is in situ cleaned with hydrogen plasma. Unless otherwise mentioned, the experiments are carried out under the following conditions: the TiCl4 vapor is introduced to the reaction tube at H2/TiCl4 ¯ow rate ratio of 250, the total pressure is adjusted at 0.5 Torr, the deposition time is 10 min, the applied rf power is 100 W. For the sake of comparison some experiments are carried out in absence of H2 and plasma. Films are examined by X-ray diffraction (XRD; Rigaku Rint 700), X-ray photoelectron spectroscopy (XPS; Kratos Axis-HS) and focused ion beam system (FIB; Hitachi FB-2000A). Plasma emission spectra are measured in ultraviolet-visible range by an optical spectrometer (OES; Jobin Yvon HR-320).
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3. Results and discussion When Si substrate is heated at a temperature 800 8C, a rapid reaction with TiCl4 in presence of H2 is observed resulting in formation of titanium silicide ®lms. The physical appearance of the ®lms varied from dark gray to light gray, from metallic shining to dull, from hard to soft and from light to dense depending on the reaction conditions. We ®nd purple and brown deposits on the quartz tube walls near the substrate, under the rf coil and near the outlet end of the reaction tube. These deposits are moisture sensitive and attribute to TiCl3 and TiCl2, respectively [9,13]. 3.1. Crystal structure XRD patterns of titanium silicide ®lms deposit at different deposition temperatures are shown in Fig. 2. At low deposition temperature of 650 8C no titanium silicide peak can be observed. As the temperature increases to 700 8C, two peaks are observed at 2y 41:02 and 51.308, respectively. These two peaks are ascribed to (2 1 1) and (3 1 0) planes of Ti5Si3 phase (JCPDS Card # 08-0041), respectively. As the temperature increases up to 900 8C, the peaks ascribe to C54-TiSi2 (JCPDS Card # 35-0785) phase appear while those of Ti5Si3 phase disappear. A remarkable increase of the intensities of the C54-TiSi2 peaks is observed as the temperature increases from 750 to 850 8C. Then the peak intensities decrease as the deposition temperature increases to 900 8C.
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At the initial stage of growth at low deposition temperature of 700 8C, the growth rate was quite slow probably due to lack of silicon atoms diffusion and/or limit of mobility of adsorbed species (H and TiClx) at such low temperature. This might be the reason for the formation of Ti5Si3 phase. Bouteville et al. [15] have linked the subsilicide phase to H2O/O2 traces in the reactor, suggesting that the presence of surface oxygen may govern this phenomenon. Saito et al. reported that the ®rst phases formed on Si(0 0 1) substrate are the Ti5Si3, Ti5Si4, C49- and C54-TiSi2 in LPCVD [10], while epitaxial C49-TiSi2 is the ®rst phase formed in PECVD using SiH4 gas as silicon precursor [10]. Other authors also reported that C49-TiSi2 is the ®rst formed phase in LPCVD, although some of them employed no SiH4 gas [5]. It seems that the ®rst phase(s) formed at the initial stage of growth is (are) a strong function of both deposition temperature and silicon precursor. While no silicon precursor is used, Ti5Si3 is formed and when SiH4 or other silicon precursors are used C49-TiSi2 phase is formed. This observation is also supported by the results from our previous and present works. We deposited titanium silicide ®lm from the TiCl4/SiCl4/ H2 gas system [13]. The deposited ®lms had the C49 phase in addition to the C54 phase. It is also supported by the fact that the ®lm starts to grow in the form of small grains and the suggestion from Saito et al. [10] that SiH4 contributes largely to the ®lm growth after the initial reaction of TiCl4 with Si. This means that either Ti amount will exceed the atomic ratio (Si/Ti) of TiSi2 in the initial grains and form Ti5Si3 or excessive etching of Si substrate probably takes place and leads to the formation of Ti5Si3 according to the following reaction (1): 5TiCl4 8Si Ti5 Si3 5SiCl4 ; DG1000 K
0:47 kJ=mol
(1)
The above reaction is thermodynamically spontaneous and its DG8 value is negative and near the value of the simple reaction (2) [11] TiCl4 3Si TiSi2 SiCl4 ; DG1000 K Fig. 2. XRD patterns of titanium silicide ®lms deposit at H2/TiCl4 ratio of 250, reaction pressure of 0.5 Torr, rf power of 100 W and at different deposition temperatures.
0:67 kJ=mol
(2)
So, the driving force for both reactions will be the TiCl4 ¯ow. When either SiH4 or H2 is introduced into the reaction equations, the driving force will be either
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the TiCl4/(SiH4) or (H2) ¯ow ratio and/or the reaction pressure (reactions (3) and (4)) TiCl4 Si SiH4 TiSi2 4HCl; DG1000 K 0:37 kJ=mol
(3)
TiCl4 2Si 2H2 TiSi2 4HCl; DG1000 K 6:41 kJ=mol
(4)
Increasing temperature and decreasing pressure now favor the TiSi2 formation [9]. In plasma, H2 decomposed into H atoms as well known and found from optical emission spectra of plasma during deposition. The produced H atoms play a very important role in driving the reaction (4) thermodynamically to give reaction (5), which has negative DG8 value far from reactions (1) and (2) [11]: TiCl4 2Si 4H TiSi2 4HCl; DG1000 K
31:45 kJ=mol
(5)
As the deposition temperature increases from 750 to 850 8C, the deposition rate increases and C54-TiSi2 phase is formed. Higher deposition temperature enhances reaction (5) and the formation of the disilicide phase becomes dominant. The increase of XRD peak intensities of C54-TiSi2 phase might be due to the increase of the ®lm thickness. At high deposition temperature of 900 8C the ®lm surface becomes rough and agglomeration of the grains are observed as will be shown later. The agglomeration results in a discontinuous ®lm and thus weak XRD peaks. 3.2. Composition XPS spectra of the as-deposited ®lms at different deposition temperature and after Ar etching are shown in Fig. 3a and b. The peaks from Ti and Si due to titanium silicide are detected for ®lms deposited at temperature 700 8C at around 454 eV (Ti2p), 100 eV (Si2p) and 150 eV (Si2s), respectively. The ®lm deposited at 650 8C does not exhibit Ti2p peak, suggesting that the silicide phase does not form as con®rmed by XRD analysis. No contamination with Cl for all ®lms since no peak is observed at 198.5 eV due to Cl2p. However, the contamination with carbon (C1s, 284.5 eV) and oxygen (O1s, 530 eV) is observed. As the temperature increases, the intensity of Ti2p peak increases while that of Si2p peak decreases. This is
Fig. 3. XPS spectra of titanium silicide ®lms: (a) as-deposited at different deposition temperatures, and (b) after etching by Ar ion for 5 min.
probably due to enhancement of Ti deposition at higher temperatures. After etching of the ®lms surface by Ar ion (Fig. 3b) the carbon and oxygen peaks disappear while a sharp increase in titanium peak is observed. From the XPS spectra of Ti2p and Si2p (not shown here), no remarkable shift in binding energy is observed either for Si peaks and/or Ti peaks with increasing the deposition temperature, suggesting that all ®lms have the same bulk composition. No peak at 465 eV due to Ti2p3=2 of Ti in oxides is observed ruling out the possibility of a titanium oxide phase. It is clear that the carbon and oxygen exist only on the silicide surface and have no effect on the bulk composition of the ®lm. 3.3. Morphology SEM images of the ®lms deposit at different deposition temperatures are shown in Fig. 4(a±f). At low
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Fig. 4. SEM images of titanium silicide ®lms deposit at different deposition temperatures: (a) 650 8C, (b) 700 8C, (c) 750 8C, (d) 800 8C, (e) 850 8C and (f) 900 8C.
temperature of 650 8C excessive etching of silicon substrate takes place and no silicide deposition can be con®rmed (Fig. 4a). As the temperature increases to 700 8C, a ®lm with ¯ake structure forms (Fig. 4b). As the deposition temperature increases up to 850 8C, ®lms with grain-like structure forms. These grains coalesce together to form continuous ®lm with smoother surface (Fig. 4c±e). The coalescence of these grains promoted the phase transition to the C54-TiSi2 rather than high temperature annealing
used in the conventional salicide process [5]. In some areas cracks in the ®lm are observed (Fig. 4d). This fracturing occurs owing to the difference in the thermal expansion coef®cients for titanium silicide and Si substrate [14]. At high temperature of 900 8C agglomeration of grains takes place resulting in coarse grains and discontinuous ®lm (Fig. 4f). The observed excessive etching of silicon substrate at low temperature can be attributed to that most of Cl produces from the TiCl4 dissociation desorbs in the
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Fig. 5. XRD pattern of TiSi2 ®lm deposit at deposition temperature of 850 8C and deposition time of 5 min in absence of H2 and plasma.
form of SiCl2 or SiCl4 at such low temperature [3,9]. This is also probably the reason of the appreciable amounts of Cl reported by other authors for their ®lms that deposited at relatively low temperatures [5,16]. As the temperature increases to 700 8C, etching of silicon substrate and deposition of silicide ®lm take place concurrently and a ¯ake Ti5Si3 phase ®lm deposits. As the temperature increases from 750 to 900 8C, most of the Cl desorbs in the form of HCl and as a result titanium disilicide ®lms deposit and silicon etching suppresses. Reduction of TiCl4 by Si in absence of H2 and plasma yields the TiSi2 phase as con®rmed by XRD analysis (Fig. 5). However, this reaction is accompanied by excessive silicon substrate etching through the formation of volatile chloride species such as SiCl2 and SiCl4 (Fig. 6). Maury et al. reported that the substrate etching induced an increase in contact resistance and junction leakage current became critical [2]. In a trial to suppress silicon substrate etching and enhance the silicide deposition, many authors introduced either SiH4 or other silicon precursor to the reaction gas mixture [9]. According to our thermodynamic calculations, silicon consumption takes place even in presence of SiH4
Fig. 6. SEM image of TiSi2 ®lm deposit at deposition temperature of 850 8C, deposition time of 5 min in absence of H2 and plasma. Broken arrows implying etch pits at the TiSi2/Si interface.
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165
Fig. 7. SEM image of TiSi2 ®lm deposit at 850 8C deposition temperature, 250 H2/TiCl4 ¯ow ratio, 0.5 Torr and 200 W rf power.
gas (reaction (3)) [11]. Silicon consumption seems to be inevitable in this process, thus ®nding the optimum conditions at which silicon consumption takes place uniformly, homogeneously and is of great interest for device fabrication. Once these conditions are obtained, a homogeneous ®lm with smooth interface could be obtained (Fig. 7) [11]. At this optimum condition (850 8C and 200 W) most of Cl might be desorbed in the form of HCl and etching of silicon substrate is suppressed. The investigation of the deposition of the ®lm at different deposition temperatures suggests that the deposition temperature should be in the range from 750 C < depositiontemperature 850 8C. Below this range of temperature excessive silicon etching takes place without remarkable silicide deposition or with deposition of subsilicide phase and above this range agglomeration of grains takes place. 4. Summary and conclusions Titanium disilicide thin ®lms have been deposited by rf plasma enhanced chemical vapor deposition using TiCl4/H2 as source gases at different deposition temperatures. At temperature of 650 8C excessive
silicon substrate etching takes place and the silicide deposition cannot be con®rmed. At 700 8C Ti5Si3 deposits in the form of ¯ake structure. As the temperature increases from 750 to 900 8C, the low resistivity C54-TiSi2 phase is formed based on X-ray diffraction analysis. At the recommended temperature range 750 8C < deposition temperature 850 8C the ®lms deposit in the form of grains that coalesce together to form continuous ®lm with smoother surface. At high deposition temperature of 900 8C agglomeration of the grains is observed and the ®lm becomes discontinuous. At the optimum deposition condition a homogeneous ®lm with smooth interface can be obtained. At this optimum conditions silicon etching is suppressed and Cl might be desorbed in the form of HCl. By controlling the reaction temperature and/or decomposing hydrogen into H atoms in plasma it is possible to deposit homogeneous titanium silicide ®lm, reducing silicon etching. Acknowledgements The authors would like to thank Dr. I. Usui and Mr. T. Hirai for XPS and SEM measurements.
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References [1] P. Gouy-Pailler, M. Haond, D. Mathiot, M. Gauneau, A. Perio, J.L. Reglolini, Appl. Surf. Sci. 73 (1993) 25. [2] D. Maury, M.L. Rostoll, P. Gayet, J.L. Regolini, J. Vac. Sci. Technol., B 15 (1997) 133. [3] Y. Ohshita, M. Oshida, M. Seki, K. Watanabe, J. Cryst. Growth 193 (1998) 322. [4] R.P. Southwell, M.A. Mendicino, E.G. Seebauer, J. Vac. Sci. Technol., A 14 (1996) 928. [5] J.S. Byun, S.J. Choi, J.J. Kim, J.T. Choi, J. Swenberg, R. Achutharaman, J. Electrochem. Soc. 145 (1998) 3941 (and references therein). È ztuÈrk, Mater. Res. Soc. [6] D.B. Gladden, C.E. Weintraub, M.C. O Symp. Proc. 402 (1996) 295. [7] M.A. Mendicino, R.P. Southwell, E.G. Seebauer, Thin Solid Films 253 (1994) 473.
[8] K. Saito, Y. Higashi, T. Amazawa, Y. Arita, J. Electrochem. Soc. 141 (1994) 1879. [9] G.J. Reynolds, C.B. Cooper III, P.J. Gaczi, J. Appl. Phys. 65 (1989) 3212. [10] K. Saito, Y. Arita, J. Electrochem. Soc. 143 (1996) 3778. [11] O.A. Fouad, M. Yamazto, M. Era, M. Nagano, T. Hirai, I. Usui, J. Cryst. Growth 234 (2002) 440. [12] O.A. Fouad, M. Yamazato, M. Nagano, Appl. Surf. Sci. 195 (2002) 130. [13] M. Nagano, O.A. Fouad, M. Matsui, M. Yamazato, M. Era, in: Proceedings of the 13th Fall Meeting of the Ceramic Society of Japan, Kitakyushu, 2000, p. 20. [14] C.Y. Lee, J. Mater. Synth. Process. 6 (1998) 55. [15] A. Bouteville, A. Royer, J.C. Remy, J. Electrochem. Soc. 134 (1987) 2080. [16] J. Hu, M. Ameen, G. Leusink, D. Webb, J.T. Hillman, Thin Solid Films 308±309 (1997) 589.
Titanium disilicide formation by rf plasma enhanced chemical vapor deposition and ®lm properties Osama A. Fouada,*, Masaaki Yamazatoa, Hiromichi Ichinoseb, Masamitsu Naganoa a
Department of Applied Chemistry, Faculty of Science and Engineering, Saga University, Honjo 1, Saga 840-8502, Japan b Saga Ceramics Research Laboratory, 3037-7 Arita, Nishimatsura-gun, Saga 844-0024, Japan Received 18 March 2002; received in revised form 17 June 2002; accepted 22 October 2002
Abstract Titanium disilicide thin ®lms have been deposited on Si(1 0 0) substrate by rf plasma enhanced chemical vapor deposition using TiCl4/H2 gas mixture at different deposition temperatures. At low temperature of 650 8C excessive silicon substrate etching took place and silicide formation could not be con®rmed. While at 700 8C Ti5Si3 was the only detected phase as found by X-ray diffraction (XRD) analysis. As the deposition temperature increased from 750 to 900 8C, the polycrystalline C54-TiSi2 phase deposited. Morphology of the ®lm surface changed noticeably as the deposition temperature increased. At low temperature of 700 8C the ®lm had a ¯ake structure. Increasing the temperature up to 850 8C resulted in a continuous ®lm with smoother grains, while at 900 8C agglomeration of grains took place resulting in coarse grains and discontinuous ®lm. At the optimum experimental conditions it was possible to deposit a homogeneous ®lm with smooth interface and suppressing silicon etching. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Plasma CVD; Titanium silicide; Crystal structure; Interface; Scanning electron microscopy
1. Introduction TiSi2 thin ®lms prepared by self-aligned silicide process (salicide) have been used for fabrication of advanced integrated circuits due to their high temperature stability and low electrical resistivity [1±3]. The salicide process is a multiple step process based on the solid-phase reaction of Ti with Si followed by post-annealing and selective etching. Although this * Corresponding author. Permanent address: CMRDI, P.O. Box 87, Helwan, Cairo, Egypt. Tel.: 81-952-28-8676; fax: 81-952-28-8591. E-mail address: [email protected] (O.A. Fouad).
reaction is thermodynamically spontaneous, some obstacles that make its update application rather dif®cult accompanied the salicide process. It is dif®cult to deposit titanium at the bottom of a high-aspect-ratio contact hole [3]. Incomplete phase transformation from the high resistivity phase (C49) to the low resistivity phase (C54) and agglomeration occur on narrow Si lines. In addition, the relatively poor step coverage with the damage effects by various beams are common problems [4]. Special interest has arisen in fabrication via low pressure chemical vapor deposition (LPCVD) [5±8] to solve the problems of the salicide process. LPCVD process offers the advantages of excellent conformal
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 1 2 1 0 - 2
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step coverage and high throughput. However, it has shortcomings such as relatively high growth temperature, large silicon consumption, and the large grain size of the deposited ®lms [8]. On the other hand, few works have been done on deposition of titanium silicide ®lms using PECVD [3,5,9,10] to improve the ®lm properties and overcome the LPCVD problems. However, it was reported that the deposited ®lm was amorphous and a post-annealing step was required to bring down the resistivity to 20 mO cm [5]. The high resistivity phase (C49) was formed with or without using SiH4 gas [10]. The discontinuity of the deposited ®lm, roughness of the silicide/silicon interface, and the phase transformation from C49 phase to C54 phase are still worsening problems [3,10]. In our previous work we investigated the preparation of the low resistivity C54-TiSi2 phase in one step reaction by PECVD, using TiCl4/H2 gas system without using the hazardous SiH4 gas [11,12]. The best deposition temperature has been addressed to be between 800 and 850 8C. In this temperature range, homogeneous ®lms with smooth interface have been obtained. Attempts to decrease the deposition temperature are done with careful analysis of the deposited ®lms to investigate the possibility of depositing homogeneous ®lm with smooth interface at lower deposition temperature in the present work. The initial stage of the ®lm growth is also investigated to better understand the reaction process. The properties of the as-deposited titanium silicide ®lm: crystal structure, composition and morphology of the surface and interface are also explored. 2. Experimental 2.1. PECVD system The PECVD system is schematically represented in Fig. 1. The reaction tube is a quartz tube of 35 mm inner diameter and 1000 mm length. The tube is sealed from both ends by steel ¯anges. TiCl4 (99.999%) and H2 are introduced to the reaction tube from one end through mass ¯ow controllers and needle valves. The plasma source is an rf generator that operates at 13.56 MHz with a maximum power of 1 kW and couples through matching network to a copper coil
Fig. 1. Schematic representation of rf±PECVD system.
(100 mm length and 5 mm diameter). An electric resistance furnace is used to heat the substrate. The temperature of the substrate is monitored by a Pt/Pt13%Rh thermocouple 5 mm apart from the substrate. The exhausted gases are pumped out from the system using a combination of rotary pump and oil diffusion pump. The pressure is monitored by Penning gauge and capacitance manometer. 2.2. Deposition procedure A detailed description of the deposition procedure is published elsewhere [11]. In brief, titanium silicide ®lms are deposited on a mirror polished n-type Si(1 0 0) substrate. The native oxide layer on Si substrate surface is removed by dipping in HF solution prior to loading into the reaction tube. After loading the substrate, the system is pumped down to a base pressure of 1 10 5 Torr. The substrate is then heated to the desired deposition temperature from 650 to 900 8C. Prior to deposition, the substrate surface is in situ cleaned with hydrogen plasma. Unless otherwise mentioned, the experiments are carried out under the following conditions: the TiCl4 vapor is introduced to the reaction tube at H2/TiCl4 ¯ow rate ratio of 250, the total pressure is adjusted at 0.5 Torr, the deposition time is 10 min, the applied rf power is 100 W. For the sake of comparison some experiments are carried out in absence of H2 and plasma. Films are examined by X-ray diffraction (XRD; Rigaku Rint 700), X-ray photoelectron spectroscopy (XPS; Kratos Axis-HS) and focused ion beam system (FIB; Hitachi FB-2000A). Plasma emission spectra are measured in ultraviolet-visible range by an optical spectrometer (OES; Jobin Yvon HR-320).
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3. Results and discussion When Si substrate is heated at a temperature 800 8C, a rapid reaction with TiCl4 in presence of H2 is observed resulting in formation of titanium silicide ®lms. The physical appearance of the ®lms varied from dark gray to light gray, from metallic shining to dull, from hard to soft and from light to dense depending on the reaction conditions. We ®nd purple and brown deposits on the quartz tube walls near the substrate, under the rf coil and near the outlet end of the reaction tube. These deposits are moisture sensitive and attribute to TiCl3 and TiCl2, respectively [9,13]. 3.1. Crystal structure XRD patterns of titanium silicide ®lms deposit at different deposition temperatures are shown in Fig. 2. At low deposition temperature of 650 8C no titanium silicide peak can be observed. As the temperature increases to 700 8C, two peaks are observed at 2y 41:02 and 51.308, respectively. These two peaks are ascribed to (2 1 1) and (3 1 0) planes of Ti5Si3 phase (JCPDS Card # 08-0041), respectively. As the temperature increases up to 900 8C, the peaks ascribe to C54-TiSi2 (JCPDS Card # 35-0785) phase appear while those of Ti5Si3 phase disappear. A remarkable increase of the intensities of the C54-TiSi2 peaks is observed as the temperature increases from 750 to 850 8C. Then the peak intensities decrease as the deposition temperature increases to 900 8C.
161
At the initial stage of growth at low deposition temperature of 700 8C, the growth rate was quite slow probably due to lack of silicon atoms diffusion and/or limit of mobility of adsorbed species (H and TiClx) at such low temperature. This might be the reason for the formation of Ti5Si3 phase. Bouteville et al. [15] have linked the subsilicide phase to H2O/O2 traces in the reactor, suggesting that the presence of surface oxygen may govern this phenomenon. Saito et al. reported that the ®rst phases formed on Si(0 0 1) substrate are the Ti5Si3, Ti5Si4, C49- and C54-TiSi2 in LPCVD [10], while epitaxial C49-TiSi2 is the ®rst phase formed in PECVD using SiH4 gas as silicon precursor [10]. Other authors also reported that C49-TiSi2 is the ®rst formed phase in LPCVD, although some of them employed no SiH4 gas [5]. It seems that the ®rst phase(s) formed at the initial stage of growth is (are) a strong function of both deposition temperature and silicon precursor. While no silicon precursor is used, Ti5Si3 is formed and when SiH4 or other silicon precursors are used C49-TiSi2 phase is formed. This observation is also supported by the results from our previous and present works. We deposited titanium silicide ®lm from the TiCl4/SiCl4/ H2 gas system [13]. The deposited ®lms had the C49 phase in addition to the C54 phase. It is also supported by the fact that the ®lm starts to grow in the form of small grains and the suggestion from Saito et al. [10] that SiH4 contributes largely to the ®lm growth after the initial reaction of TiCl4 with Si. This means that either Ti amount will exceed the atomic ratio (Si/Ti) of TiSi2 in the initial grains and form Ti5Si3 or excessive etching of Si substrate probably takes place and leads to the formation of Ti5Si3 according to the following reaction (1): 5TiCl4 8Si Ti5 Si3 5SiCl4 ; DG1000 K
0:47 kJ=mol
(1)
The above reaction is thermodynamically spontaneous and its DG8 value is negative and near the value of the simple reaction (2) [11] TiCl4 3Si TiSi2 SiCl4 ; DG1000 K Fig. 2. XRD patterns of titanium silicide ®lms deposit at H2/TiCl4 ratio of 250, reaction pressure of 0.5 Torr, rf power of 100 W and at different deposition temperatures.
0:67 kJ=mol
(2)
So, the driving force for both reactions will be the TiCl4 ¯ow. When either SiH4 or H2 is introduced into the reaction equations, the driving force will be either
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the TiCl4/(SiH4) or (H2) ¯ow ratio and/or the reaction pressure (reactions (3) and (4)) TiCl4 Si SiH4 TiSi2 4HCl; DG1000 K 0:37 kJ=mol
(3)
TiCl4 2Si 2H2 TiSi2 4HCl; DG1000 K 6:41 kJ=mol
(4)
Increasing temperature and decreasing pressure now favor the TiSi2 formation [9]. In plasma, H2 decomposed into H atoms as well known and found from optical emission spectra of plasma during deposition. The produced H atoms play a very important role in driving the reaction (4) thermodynamically to give reaction (5), which has negative DG8 value far from reactions (1) and (2) [11]: TiCl4 2Si 4H TiSi2 4HCl; DG1000 K
31:45 kJ=mol
(5)
As the deposition temperature increases from 750 to 850 8C, the deposition rate increases and C54-TiSi2 phase is formed. Higher deposition temperature enhances reaction (5) and the formation of the disilicide phase becomes dominant. The increase of XRD peak intensities of C54-TiSi2 phase might be due to the increase of the ®lm thickness. At high deposition temperature of 900 8C the ®lm surface becomes rough and agglomeration of the grains are observed as will be shown later. The agglomeration results in a discontinuous ®lm and thus weak XRD peaks. 3.2. Composition XPS spectra of the as-deposited ®lms at different deposition temperature and after Ar etching are shown in Fig. 3a and b. The peaks from Ti and Si due to titanium silicide are detected for ®lms deposited at temperature 700 8C at around 454 eV (Ti2p), 100 eV (Si2p) and 150 eV (Si2s), respectively. The ®lm deposited at 650 8C does not exhibit Ti2p peak, suggesting that the silicide phase does not form as con®rmed by XRD analysis. No contamination with Cl for all ®lms since no peak is observed at 198.5 eV due to Cl2p. However, the contamination with carbon (C1s, 284.5 eV) and oxygen (O1s, 530 eV) is observed. As the temperature increases, the intensity of Ti2p peak increases while that of Si2p peak decreases. This is
Fig. 3. XPS spectra of titanium silicide ®lms: (a) as-deposited at different deposition temperatures, and (b) after etching by Ar ion for 5 min.
probably due to enhancement of Ti deposition at higher temperatures. After etching of the ®lms surface by Ar ion (Fig. 3b) the carbon and oxygen peaks disappear while a sharp increase in titanium peak is observed. From the XPS spectra of Ti2p and Si2p (not shown here), no remarkable shift in binding energy is observed either for Si peaks and/or Ti peaks with increasing the deposition temperature, suggesting that all ®lms have the same bulk composition. No peak at 465 eV due to Ti2p3=2 of Ti in oxides is observed ruling out the possibility of a titanium oxide phase. It is clear that the carbon and oxygen exist only on the silicide surface and have no effect on the bulk composition of the ®lm. 3.3. Morphology SEM images of the ®lms deposit at different deposition temperatures are shown in Fig. 4(a±f). At low
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Fig. 4. SEM images of titanium silicide ®lms deposit at different deposition temperatures: (a) 650 8C, (b) 700 8C, (c) 750 8C, (d) 800 8C, (e) 850 8C and (f) 900 8C.
temperature of 650 8C excessive etching of silicon substrate takes place and no silicide deposition can be con®rmed (Fig. 4a). As the temperature increases to 700 8C, a ®lm with ¯ake structure forms (Fig. 4b). As the deposition temperature increases up to 850 8C, ®lms with grain-like structure forms. These grains coalesce together to form continuous ®lm with smoother surface (Fig. 4c±e). The coalescence of these grains promoted the phase transition to the C54-TiSi2 rather than high temperature annealing
used in the conventional salicide process [5]. In some areas cracks in the ®lm are observed (Fig. 4d). This fracturing occurs owing to the difference in the thermal expansion coef®cients for titanium silicide and Si substrate [14]. At high temperature of 900 8C agglomeration of grains takes place resulting in coarse grains and discontinuous ®lm (Fig. 4f). The observed excessive etching of silicon substrate at low temperature can be attributed to that most of Cl produces from the TiCl4 dissociation desorbs in the
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Fig. 5. XRD pattern of TiSi2 ®lm deposit at deposition temperature of 850 8C and deposition time of 5 min in absence of H2 and plasma.
form of SiCl2 or SiCl4 at such low temperature [3,9]. This is also probably the reason of the appreciable amounts of Cl reported by other authors for their ®lms that deposited at relatively low temperatures [5,16]. As the temperature increases to 700 8C, etching of silicon substrate and deposition of silicide ®lm take place concurrently and a ¯ake Ti5Si3 phase ®lm deposits. As the temperature increases from 750 to 900 8C, most of the Cl desorbs in the form of HCl and as a result titanium disilicide ®lms deposit and silicon etching suppresses. Reduction of TiCl4 by Si in absence of H2 and plasma yields the TiSi2 phase as con®rmed by XRD analysis (Fig. 5). However, this reaction is accompanied by excessive silicon substrate etching through the formation of volatile chloride species such as SiCl2 and SiCl4 (Fig. 6). Maury et al. reported that the substrate etching induced an increase in contact resistance and junction leakage current became critical [2]. In a trial to suppress silicon substrate etching and enhance the silicide deposition, many authors introduced either SiH4 or other silicon precursor to the reaction gas mixture [9]. According to our thermodynamic calculations, silicon consumption takes place even in presence of SiH4
Fig. 6. SEM image of TiSi2 ®lm deposit at deposition temperature of 850 8C, deposition time of 5 min in absence of H2 and plasma. Broken arrows implying etch pits at the TiSi2/Si interface.
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Fig. 7. SEM image of TiSi2 ®lm deposit at 850 8C deposition temperature, 250 H2/TiCl4 ¯ow ratio, 0.5 Torr and 200 W rf power.
gas (reaction (3)) [11]. Silicon consumption seems to be inevitable in this process, thus ®nding the optimum conditions at which silicon consumption takes place uniformly, homogeneously and is of great interest for device fabrication. Once these conditions are obtained, a homogeneous ®lm with smooth interface could be obtained (Fig. 7) [11]. At this optimum condition (850 8C and 200 W) most of Cl might be desorbed in the form of HCl and etching of silicon substrate is suppressed. The investigation of the deposition of the ®lm at different deposition temperatures suggests that the deposition temperature should be in the range from 750 C < depositiontemperature 850 8C. Below this range of temperature excessive silicon etching takes place without remarkable silicide deposition or with deposition of subsilicide phase and above this range agglomeration of grains takes place. 4. Summary and conclusions Titanium disilicide thin ®lms have been deposited by rf plasma enhanced chemical vapor deposition using TiCl4/H2 as source gases at different deposition temperatures. At temperature of 650 8C excessive
silicon substrate etching takes place and the silicide deposition cannot be con®rmed. At 700 8C Ti5Si3 deposits in the form of ¯ake structure. As the temperature increases from 750 to 900 8C, the low resistivity C54-TiSi2 phase is formed based on X-ray diffraction analysis. At the recommended temperature range 750 8C < deposition temperature 850 8C the ®lms deposit in the form of grains that coalesce together to form continuous ®lm with smoother surface. At high deposition temperature of 900 8C agglomeration of the grains is observed and the ®lm becomes discontinuous. At the optimum deposition condition a homogeneous ®lm with smooth interface can be obtained. At this optimum conditions silicon etching is suppressed and Cl might be desorbed in the form of HCl. By controlling the reaction temperature and/or decomposing hydrogen into H atoms in plasma it is possible to deposit homogeneous titanium silicide ®lm, reducing silicon etching. Acknowledgements The authors would like to thank Dr. I. Usui and Mr. T. Hirai for XPS and SEM measurements.
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