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Effect of P addition on the thermal stability and electrical characteristics of NiSi films
Thin Solid Films 518 (2009) 1538–1542
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effect of P addition on the thermal stability and electrical characteristics of NiSi films H.F. Hsu ⁎, C.L. Tsai, H.Y. Chan, T.H. Chen Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC
a r t i c l e
i n f o
Available online 19 September 2009 Keywords: Electroless deposition Silicide Thermal stability
a b s t r a c t Immersion Ni–P deposition is undoubtedly one of the most important catalytic deposition process, due to its simplicity in operation and low equipment cost. In this study, immersion deposited Ni–P films were used to form Ni-silicide films. Ni–P films with a thickness of 100 nm were fabricated by immersing Si(100) substrates in an aqueous deposition solution. Ni-silicide films were then formed by annealing the samples in a furnace at temperatures ranging from 400 °C to 900 °C for 1 h in an argon ambient. Experimental results indicate that a phosphor addition in Ni films increased the transformation temperature of NiSi to NiSi2 to 900 °C. Moreover, the feasibility of enhancing the thermal stability of NiSi by varying the interface energy at the NiSi2/Si interface and the surface energy of a Ni–P–Si capping layer on the NiSi surface is discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Low resistivity metal silicides are extensively used as ohmic contacts, electrodes and interconnects in semiconductor devices. With the increasing miniaturization of the dimensions of integrated devices, nickel self-aligned silicide (SALICIDE) is a promising material for deep submicron devices because nickel monosilicide (NiSi) has a low resistivity (10.5–18 μΩ cm), low silicon consumption and low formation temperature (≧400 °C) [1–4]. In general, the processes of Ni-silicide formation are in a vacuum condition. Ni thin films are usually deposited by electron beam evaporation or sputter deposition. Samples are then annealed by rapid thermal annealing (RTA) or in a furnace to form the nickel silicide phase. Alternatively, electroless deposition has attracted attention because of its high deposition rate, almost no space limitation and low cost. In semiconductor devices, electroless deposition is performed to make ohmic contacts and contact filling in integrated circuits [5]. Therefore, this technique is feasible for producing microelectronic devices. Conventional electroless Ni deposition requires surface activation, in which the substrate is immersed in an aqueous solution containing metal ions (such as Pd2+). Metal (Pd) islands are formed as deposition “seeds” [6], and a reducing agent is required in the deposition solution. Another wet process, i.e. immersion Ni deposition, is simpler than conventional Ni electroless deposition, in which the substrate is simply immersed in a solution containing Ni ions. The deposition mechanism is based on the precipitation of Ni through a galvanic displacement reaction, 2 Ni2+ + Si → 2Ni0 + Si4+ [7]. This method has been adopted in the mass production of fine structures on a silicon
⁎ Corresponding author. Tel.: 886 4 22840500x300; fax: 886 4 22857017. E-mail address: [email protected] (H.F. Hsu). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.058
substrate due to its selective deposition property [8]. However, the deposition rate of immersion deposition is lower than that of conventional electroless deposition. Our previous study shows that the deposition rate can be enhanced by adding a reducing agent, such as sodium hypophosphite, in the deposition solution [9], resulting in the deposition of Ni films with P additions, i.e. Ni–P films. In addition, the thermal instability of NiSi limits its potential application in semiconductor devices because the transformation of NiSi to the high resistivity disilicide (NiSi2) at elevated temperatures can be a concern in the back end process. In recent years, some investigations have addressed the thermal stability of NiSi films. The addition of either Pt or Zr enhances the thermal stability of NiSi [10– 12], while the addition of either Co or Au reduces the thermal stability of NiSi [13]. Because the Ni–P films formed by the immersion deposition can be regarded as a Ni film with P additions, this study investigates the effects of P additions on the thermal stability and electrical characteristics of NiSi films. 2. Experiments N-type Si(100) substrates (P doped with a resistivity of 0.1–1 Ω cm) were used in this study. These substrates were cleaned using a standard RCA procedure [14] and then dipped in a dilute HF solution. Before immersing the substrate in the plating solution, they were pretreated in a pretreatment solution (30% H2O2:36% HCl:H2O = 1:1:4) at 80 °C to form a hydrophilic surface and then rinsed with deionized water. By this pretreatment process, the deposition rate increased and the Ni film became uniform [9]. The plating solution was composed of a mixture of NiSO4 (0.1 mol dm− 3) for supplying nickel ions, NaH2PO2 (0.15 mol dm− 3) for serving as the reducing agent, Na3C6H5O7 (0.2 mol dm− 3) and (NH4)SO4 (0.5 mol dm− 3) for serving as complex and buffer agent. The pre-treated substrates were immersed into the plating solution at 75 °C for 90 s. The pH value of the solution was
H.F. Hsu et al. / Thin Solid Films 518 (2009) 1538–1542
adjusted at 8.6 with 1 M NH4OH. For comparison, a pure Ni film was deposited on Si(100) substrates by electron beam evaporation. The deposition rate was 0.01 nm/s and the pressure in the chamber was 2.7 × 10− 4 Pa. As-deposited samples were annealed in a furnace at temperatures ranging from 400 °C to 900 °C for 1 h in Ar ambient. Field emission scanning electron microscopy (FE-SEM), X-ray diffraction analysis (XRD), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) techniques were used to characterize the as-deposited and the annealed samples. For TEM observation, a JEOL 2000F TEM operating at 200 keV was used. For SEM observation, a JEOL JSM 6700F SEM was used. For XRD analysis, a Rigaku RINT 2000 diffractometer was used. For EDS analysis, a Link ISIS 300 and an Oxford Inca Energy 400 equipped in the TEM and SEM, respectively, were used. Plane-view and cross-sectional TEM samples were prepared by grinding, polishing and ion beam milling. Cross-sectional SEM samples were prepared by cutting. The sheet resistances of the samples were measured using four-point-probe measurement. 3. Results The immersion deposited Ni–P films were formed by chemical reactions, nucleation and growth processes. The films have a thickness of around 100 nm after immersion in the plating solution for 90 s. The EDS analysis of the cross-sectional FE-SEM samples indicated that the P content of the Ni–P films was 20 at.%. From the X-ray diffraction spectrum of an as-deposited Ni–P film, as shown in Fig. 1, a broad peak at 45° was observed. The full width at half maximum (FWHM) of the peak is approximately 6°. According to the Scherrer formula, the grain size is about 1.4 nm.
Fig. 1. XRD spectra of as-deposited and annealed Ni–P(100 nm)/Si(100) samples. The X-ray wavelength used for the measurement is 0.1542 nm.
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For convenience, the two sets of immersion deposited Ni–P (100 nm)/Si(100) and evaporated Ni(100 nm)/Si(100) samples were designated as samples A and B, respectively. As sample A was annealed at 400 °C, polycrystalline NiSi and Ni2P coexisted on the sample, as shown in Fig. 1. When the annealing temperature increased to 500 °C, only the NiSi phase was detected by XRD analysis. The NiSi phase was stable at annealing temperatures up to 800 °C. At 900 °C, polycrystalline NiSi2 peaks were observed. For sample B annealed at temperatures ranging between 400 °C and 700 °C, only peaks corresponding to NiSi were present, as shown in Fig. 2. The transformation of NiSi to NiSi2 occurred at 800 °C. Thus, the above results indicate that adding P enhanced the phase transformation temperature of NiSi to NiSi2. Fig. 3 shows the FE-SEM images of samples A after annealing at various temperatures. When annealing at 400 °C, a film with a two layered structure was observed. According to the cross-sectional TEM analysis [Fig. 4(a)], the grain size of the bottom layer was larger than that of the top one. The bright-field plane-view TEM image also indicated that the fine and coarse grains coexisted [Fig. 4(b)]. According to XRD data, NiSi and Ni2P coexisted in sample A annealed at 400 °C. In Fig. 4(c), the SAED patterns taken from the plane-view TEM image with a strong intensity are attributed to reflections of NiSi (021), while the other ones are attributed to reflections of Ni2P. According to the NiSi(021) dark-field image [Fig. 4(d)], the layer with the large grain size can be identified as NiSi. Thus, the bottom layer is regarded as NiSi, while the top one is Ni2P. Although only XRD reflexes of the NiSi phase were observed when the Ni–P(100 nm)/Si(100) samples A were annealed at 500–800 °C, the films in the cross-sectional SEM images were distinct, as shown in Fig. 3(b)–(e). For the sample annealed at 500 °C, only one layer was observed. By the EDS analysis of the cross-sectional TEM sample (data not shown), the Si:Ni atomic ratios in the surface region and at the center of the film are 1:1.2 and 1:1.02, respectively, both of which are close to the ratio of NiSi. The phosphor content was lower than 1 at.%.
Fig. 2. XRD spectra of as-deposited and annealed pure Ni(100 nm)/Si(100) samples. The X-ray wavelength used for the measurement is 0.1542 nm.
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Fig. 3. FE-SEM cross-sectional images of the Ni–P(100 nm)/Si(100) samples annealed at (a) 400, (b) 500, (c) 600, (d) 700 and (e) 800 °C for 1 h in a furnace.
However, two layered structures were formed again after annealing up to 600 °C. Notably, increasing the annealing temperature to 800 °C, obviously increased the thickness of the bottom layer, while decreasing that of the top layer. Fig. 5 shows the cross-sectional TEM image and the EDS analyses of sample A annealed at 800 °C. No P content at the bottom layer could be detected, and the Si/Ni atomic ratio was 1.07 (the uncertainty is ±4%). Thus, this bottom layer can be defined as the NiSi layer and the solubility of P in the NiSi layer was almost zero. The composition of the upper layer included Ni, Si and P. However, because the thickness of the layer was low, the ratio of Ni:P:Si could not be identified clearly by EDS analysis. Furthermore, the XRD spectra for sample A annealed at 600–800 °C reveal that there is no extra peak except for NiSi peaks, which may be due to the small quantity or an amorphous-like structure of the top layer. Thus, the top layers formed at 600–800 °C annealing are described as Ni–P–Si capping layers in this study. Fig. 6 shows the sheet resistance data of as-deposited and annealed samples A and B. The sheet resistance of samples A and B reduced to a low level after annealing at 400 °C owing to the formation of a polycrystalline NiSi film with low resistivity. The sheet
resistance of sample A for 500 and 800 °C annealing was lower than that for 600 and 700 °C annealing because the thickness of the NiSi layers for 500 and 800 °C was larger than that obtained at other annealing conditions, as shown in Fig. 3. However, upon annealing at 800 °C, the sheet resistance of sample B was significantly higher than that of sample A due to the formation of NiSi2 in the sample B. After annealing at 900 °C, the sheet resistance of sample B increased abruptly due to the thermal breakdown of the silicide film, while that of sample A slightly increased because of the formation of NiSi2. The above results suggest that the NiSi films that formed in Ni–P (100 nm)/Si(100) samples are more stable than those that formed in pure Ni(100 nm)/Si(100) samples. 4. Discussion 4.1. Phase transformation of Ni–P on Si(100) The formation of NiSi has been reported as being diffusion controlled [3]. Nickel atoms diffuse into the Si substrate and a NiSi
H.F. Hsu et al. / Thin Solid Films 518 (2009) 1538–1542
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Fig. 5. (a) Cross-sectional TEM images of immersion deposited Ni–P(100 nm)/Si(100) samples annealed at 800 °C. EDS spectra of (b) the top layer and (c) the bottom layer in (a), respectively.
Fig. 4. (a) Cross-sectional and (b) plan-view bright-field TEM images of the Ni–P(100 nm)/ Si(100) samples annealed at 400 °C. (c) A SAED pattern corresponding to (b). (d) Planeview of NiSi2[210] dark-field TEM image.
phase is formed. In this study, Ni atoms from the Ni–P films diffused into Si substrates at elevated temperatures, causing a change of the composition of Ni–P films. Therefore, when annealing at 400 °C, the P
content of top Ni–P layer increases, which results in the formation of a Ni2P phase. At 500 °C annealing, only one NiSi layer was formed. One explanation may be that most Ni atoms diffused into Si substrate to form a NiSi phase, which causes the P content of Ni–P film to increase abruptly. A P-rich Ni–P compound with liquid phase could possibly be formed [15,16]. Thus, only one NiSi layer was observed. However, the Si–P and Ni–P phase diagrams [16] show that the solubility of P in the Si or Ni increases at elevated temperatures. Thus, the formation of a Ni–P–Si capping layer at 600–800 °C may be due to the increase of solubility of P in the Si or Ni films. However, in order to verify the chemical composition of individual layers, further analytical work with high resolution methods, i.e. AES depth profiles, is necessary. In
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nucleation of the NiSi2 phase in sample A was expected to be close to that in sample B because the solubility of P in NiSi and NiSi2 is limited even though sample A contained P. In the solid state, estimates of interface energy vary between about 0 for epitaxial interfaces to about 2 J/m2 for random interfaces [19]. According to the XRD data in Figs. 1 and 2, polycrystalline and epitaxial NiSi2 films were found in annealed samples A and B, respectively. As is expected, the surface energy of an epitaxial interface is lower than that of a polycrystalline one [10]. Adding P solute in NiSi2 increases σ(NiSi2/Si) while Δσ increases. Thus, the critical activation energy increases due to the increase of Δσ, which can cause the delay of NiSi2 formation. Secondly, the formation of a Ni–P–Si capping layer on the NiSi layer may be another reason for the delay of NiSi2 formation because of the surface energy effect [20]. Many investigations have shown that coating a capping layer affects the thermal stability of NiSi films [21–23]. Fig. 6. Sheet resistance vs. annealing temperature for immersion deposited Ni–P(100 nm)/ Si(100) and pure Ni(100 nm)/Si(100) samples before and after annealing at 400–900 °C for 1 h in a furnace.
our further study, the mechanism of reaction will be investigated by fabricating Ni-silicide films using Ni–P films with various thicknesses and deriving the chemical composition of the individual layers by AES. Table 1 lists the measured and listed XRD positions of NiSi2. According to the Bragg law, all measured interplanar spacings are smaller than the listed ones. The lattice constant of NiSi2 reduced from 0.5406 nm to around 0.538 nm by adding P. As is well known, NiSi2 tends to be epitaxially grown on a silicon substrate because its crystal structure is similar to that of Si; in addition, the lattice constant is only 0.4% less than that of Si. For sample A annealed at 900 °C, the difference between the measured lattice constant of NiSi2 and that of Si increased up to 0.9%. Although the lattice mismatch of NiSi2 with P addition increased slightly, the change may be sufficiently large to form a polycrystalline film on the Si(100) substrate. This phenomenon can be observed as well in the CoSi2/Si(100) system: CoSi2 possesses CaF2 structure with a lattice constant of 0.5365 nm, i.e. smaller than Si by 1.2%. It is known that polycrystalline CoSi2 is formed on the Si(100) substrate by annealing process[3,17].
5. Conclusions This study investigated how the addition of P enhances the thermal stability and electrical characteristics of NiSi. Ni-silicide films were formed by annealing the immersion deposited Ni–P(100 nm)/Si (100) samples. The sheet resistance reduced to low level after annealing at 400 °C owing to the formation of a polycrystalline NiSi film with a Ni2P layer on the surface. At annealing temperatures ranging from 600 to 800 °C, a Ni–P–Si capping layer of the NiSi layer was formed. The addition of P increased the NiSi2 formation temperature to 900 °C, leading to an enhanced thermal stability of NiSi at high temperatures. By the addition of P, the lattice constant of NiSi2 was reduced to 0.538 nm, subsequently causing the formation of polycrystalline NiSi2 films. Thus, the interface energy between NiSi2 and Si substrates was increased and a Ni–P–Si capping layer on the NiSi layer may also affect the NiSi2 formation through the surface energy effect. Acknowledgement The research is supported by the Republic of China National Science Council grant no. NSC 96-2221-E-005-108. References
4.2. Temperature of formation of NiSi2 The P additions in Ni films could affect the temperature of formation of NiSi2 in two ways. The first effect is the decrease of critical activation energy of NiSi2 formation. The transformation of NiSi to NiSi2 is a nucleation-controlled reaction [3]. The critical activation energy for nucleation is proportional to Δσ3/ΔG2, where Δσ and ΔG are an increase of the interfacial energy and a decrease of Gibbs free energy, respectively, during the reaction of NiSi + Si→ NiSi2. The addition of the impurity may decrease the Gibbs free energy and stabilize the silicide [8,13]. Previous studies demonstrated that the addition of a small amount of solute decreases the Gibbs energy and stabilizes the silicide [10,12,13,18]. Pt, Pd and Zr contents enhanced the transformation temperature of NiSi to NiSi2 owing to their extremely low solubility in NiSi2 and the larger solubility in NiSi. In this study, |ΔG| for the
Table 1 Measured and listed XRD peak positions, 2θ, of NiSi2. Measured (Sample A)
Measured (Sample B)
NiSi2
28.72 47.78 56.70
28.56 – –
28.576 47.534 56.405
Listed XRD peak positions of NiSi2 are from JCPDS 65-2974.
[1] F. Deng, R.A. Johnson, P.M. Asbeck, S.S. Lau, W.B. Dubbelday, T. Hsiao, J. Woo, J. Appl. Phys. 81 (1997) 8047. [2] J. Chen, J.P. Colinge, D. Flandre, R. Gillon, J.P. Raskin, D. Vanhoenacker, J. Electrochem. Soc. 144 (1997) 2437. [3] L.J. Chen, Silicide Technology for Integrated Circuits, The Institution of Electrical Engineer, London, 2004. [4] L.J. Chen, JOM 57 (2005) 24. [5] C.H. Ting, H. Paunovic, J. Electrochem. Soc. 136 (1989) 456. [6] C.M. Liu, W.L. Liu, S.H. Hsieh, T.K. Tsai, W.J. Chen, Appl. Surf. Sci. 243 (2005) 259. [7] C.H. Ting, M. Paunovic, J. Electrochem. Soc. 136 (1989) 456. [8] D. Niwa, N. Takano, T. Yamada, T. Osaka, Electrochim. Acta 48 (2003) 1295. [9] H.F. Hsu, C.L. Tsai, C.W. Lee, H.Y. Wu, Thin Solid Films 517 (2009) 4786. [10] D. Mangelinck, J.Y. Dai, J.S. Pan, S.K. Lahiri, Appl. Phys. Lett. 75 (1999) 1736. [11] J.F. Liu, H.B. Chen, J.Y. Feng, J. Cryst. Growth 220 (2000) 488. [12] W. Huang, L.C. Zhang, Y.Z. Gao, H.Y. Jin, Microelectron. Eng. 83 (2006) 345. [13] D. Mangelinck, P. Gas, J.M. Gay, B. Pichaud, O. Thomas, J. Appl. Phys. 84 (1998) 2583. [14] W. Kern, D.A. Puotinen, RCA Rev. 31 (1970) 187. [15] C. Schmetterer, J. Vizdal, H. Ipser, Intermetallics 17 (2009) 826. [16] T.B. Massalski, Binary Alloy Phase Diagrams, ASM International, Ohio, 1990. [17] S.P. Murarka, Silicides for VLSI Application, Academic Press, Orlando, 1983. [18] R.N. Wang, J.Y. Feng, J. Phys., Condens. Matter 15 (2003) 1935. [19] F.M. d'Heurle, J. Mater. Res. 3 (1988) 167. [20] C. Lavoie, C. Detavernier, C. Cabral Jr., F.M. d'Heurle, A.J. Kellock, J. Jordan-Sweet, J.M.E. Harper, Microelectron. Eng. 83 (2006) 2042. [21] Y.W. Ok, C.J. Choi, T.Y. Seong, J. Electrochem. Soc. 150 (2001) G385. [22] C.J. Choi, Y.W. Ok, T.Y. Seong, H.D. Lee, Jpn. J. Appl. Phys. 41 (2002) 1969. [23] C.C. Wu, W.F. Wu, P.Y. Su, L.J. Chen, F.H. Ko, Microelectron. Eng. 84 (2007) 1801.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effect of P addition on the thermal stability and electrical characteristics of NiSi films H.F. Hsu ⁎, C.L. Tsai, H.Y. Chan, T.H. Chen Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC
a r t i c l e
i n f o
Available online 19 September 2009 Keywords: Electroless deposition Silicide Thermal stability
a b s t r a c t Immersion Ni–P deposition is undoubtedly one of the most important catalytic deposition process, due to its simplicity in operation and low equipment cost. In this study, immersion deposited Ni–P films were used to form Ni-silicide films. Ni–P films with a thickness of 100 nm were fabricated by immersing Si(100) substrates in an aqueous deposition solution. Ni-silicide films were then formed by annealing the samples in a furnace at temperatures ranging from 400 °C to 900 °C for 1 h in an argon ambient. Experimental results indicate that a phosphor addition in Ni films increased the transformation temperature of NiSi to NiSi2 to 900 °C. Moreover, the feasibility of enhancing the thermal stability of NiSi by varying the interface energy at the NiSi2/Si interface and the surface energy of a Ni–P–Si capping layer on the NiSi surface is discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Low resistivity metal silicides are extensively used as ohmic contacts, electrodes and interconnects in semiconductor devices. With the increasing miniaturization of the dimensions of integrated devices, nickel self-aligned silicide (SALICIDE) is a promising material for deep submicron devices because nickel monosilicide (NiSi) has a low resistivity (10.5–18 μΩ cm), low silicon consumption and low formation temperature (≧400 °C) [1–4]. In general, the processes of Ni-silicide formation are in a vacuum condition. Ni thin films are usually deposited by electron beam evaporation or sputter deposition. Samples are then annealed by rapid thermal annealing (RTA) or in a furnace to form the nickel silicide phase. Alternatively, electroless deposition has attracted attention because of its high deposition rate, almost no space limitation and low cost. In semiconductor devices, electroless deposition is performed to make ohmic contacts and contact filling in integrated circuits [5]. Therefore, this technique is feasible for producing microelectronic devices. Conventional electroless Ni deposition requires surface activation, in which the substrate is immersed in an aqueous solution containing metal ions (such as Pd2+). Metal (Pd) islands are formed as deposition “seeds” [6], and a reducing agent is required in the deposition solution. Another wet process, i.e. immersion Ni deposition, is simpler than conventional Ni electroless deposition, in which the substrate is simply immersed in a solution containing Ni ions. The deposition mechanism is based on the precipitation of Ni through a galvanic displacement reaction, 2 Ni2+ + Si → 2Ni0 + Si4+ [7]. This method has been adopted in the mass production of fine structures on a silicon
⁎ Corresponding author. Tel.: 886 4 22840500x300; fax: 886 4 22857017. E-mail address: [email protected] (H.F. Hsu). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.058
substrate due to its selective deposition property [8]. However, the deposition rate of immersion deposition is lower than that of conventional electroless deposition. Our previous study shows that the deposition rate can be enhanced by adding a reducing agent, such as sodium hypophosphite, in the deposition solution [9], resulting in the deposition of Ni films with P additions, i.e. Ni–P films. In addition, the thermal instability of NiSi limits its potential application in semiconductor devices because the transformation of NiSi to the high resistivity disilicide (NiSi2) at elevated temperatures can be a concern in the back end process. In recent years, some investigations have addressed the thermal stability of NiSi films. The addition of either Pt or Zr enhances the thermal stability of NiSi [10– 12], while the addition of either Co or Au reduces the thermal stability of NiSi [13]. Because the Ni–P films formed by the immersion deposition can be regarded as a Ni film with P additions, this study investigates the effects of P additions on the thermal stability and electrical characteristics of NiSi films. 2. Experiments N-type Si(100) substrates (P doped with a resistivity of 0.1–1 Ω cm) were used in this study. These substrates were cleaned using a standard RCA procedure [14] and then dipped in a dilute HF solution. Before immersing the substrate in the plating solution, they were pretreated in a pretreatment solution (30% H2O2:36% HCl:H2O = 1:1:4) at 80 °C to form a hydrophilic surface and then rinsed with deionized water. By this pretreatment process, the deposition rate increased and the Ni film became uniform [9]. The plating solution was composed of a mixture of NiSO4 (0.1 mol dm− 3) for supplying nickel ions, NaH2PO2 (0.15 mol dm− 3) for serving as the reducing agent, Na3C6H5O7 (0.2 mol dm− 3) and (NH4)SO4 (0.5 mol dm− 3) for serving as complex and buffer agent. The pre-treated substrates were immersed into the plating solution at 75 °C for 90 s. The pH value of the solution was
H.F. Hsu et al. / Thin Solid Films 518 (2009) 1538–1542
adjusted at 8.6 with 1 M NH4OH. For comparison, a pure Ni film was deposited on Si(100) substrates by electron beam evaporation. The deposition rate was 0.01 nm/s and the pressure in the chamber was 2.7 × 10− 4 Pa. As-deposited samples were annealed in a furnace at temperatures ranging from 400 °C to 900 °C for 1 h in Ar ambient. Field emission scanning electron microscopy (FE-SEM), X-ray diffraction analysis (XRD), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) techniques were used to characterize the as-deposited and the annealed samples. For TEM observation, a JEOL 2000F TEM operating at 200 keV was used. For SEM observation, a JEOL JSM 6700F SEM was used. For XRD analysis, a Rigaku RINT 2000 diffractometer was used. For EDS analysis, a Link ISIS 300 and an Oxford Inca Energy 400 equipped in the TEM and SEM, respectively, were used. Plane-view and cross-sectional TEM samples were prepared by grinding, polishing and ion beam milling. Cross-sectional SEM samples were prepared by cutting. The sheet resistances of the samples were measured using four-point-probe measurement. 3. Results The immersion deposited Ni–P films were formed by chemical reactions, nucleation and growth processes. The films have a thickness of around 100 nm after immersion in the plating solution for 90 s. The EDS analysis of the cross-sectional FE-SEM samples indicated that the P content of the Ni–P films was 20 at.%. From the X-ray diffraction spectrum of an as-deposited Ni–P film, as shown in Fig. 1, a broad peak at 45° was observed. The full width at half maximum (FWHM) of the peak is approximately 6°. According to the Scherrer formula, the grain size is about 1.4 nm.
Fig. 1. XRD spectra of as-deposited and annealed Ni–P(100 nm)/Si(100) samples. The X-ray wavelength used for the measurement is 0.1542 nm.
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For convenience, the two sets of immersion deposited Ni–P (100 nm)/Si(100) and evaporated Ni(100 nm)/Si(100) samples were designated as samples A and B, respectively. As sample A was annealed at 400 °C, polycrystalline NiSi and Ni2P coexisted on the sample, as shown in Fig. 1. When the annealing temperature increased to 500 °C, only the NiSi phase was detected by XRD analysis. The NiSi phase was stable at annealing temperatures up to 800 °C. At 900 °C, polycrystalline NiSi2 peaks were observed. For sample B annealed at temperatures ranging between 400 °C and 700 °C, only peaks corresponding to NiSi were present, as shown in Fig. 2. The transformation of NiSi to NiSi2 occurred at 800 °C. Thus, the above results indicate that adding P enhanced the phase transformation temperature of NiSi to NiSi2. Fig. 3 shows the FE-SEM images of samples A after annealing at various temperatures. When annealing at 400 °C, a film with a two layered structure was observed. According to the cross-sectional TEM analysis [Fig. 4(a)], the grain size of the bottom layer was larger than that of the top one. The bright-field plane-view TEM image also indicated that the fine and coarse grains coexisted [Fig. 4(b)]. According to XRD data, NiSi and Ni2P coexisted in sample A annealed at 400 °C. In Fig. 4(c), the SAED patterns taken from the plane-view TEM image with a strong intensity are attributed to reflections of NiSi (021), while the other ones are attributed to reflections of Ni2P. According to the NiSi(021) dark-field image [Fig. 4(d)], the layer with the large grain size can be identified as NiSi. Thus, the bottom layer is regarded as NiSi, while the top one is Ni2P. Although only XRD reflexes of the NiSi phase were observed when the Ni–P(100 nm)/Si(100) samples A were annealed at 500–800 °C, the films in the cross-sectional SEM images were distinct, as shown in Fig. 3(b)–(e). For the sample annealed at 500 °C, only one layer was observed. By the EDS analysis of the cross-sectional TEM sample (data not shown), the Si:Ni atomic ratios in the surface region and at the center of the film are 1:1.2 and 1:1.02, respectively, both of which are close to the ratio of NiSi. The phosphor content was lower than 1 at.%.
Fig. 2. XRD spectra of as-deposited and annealed pure Ni(100 nm)/Si(100) samples. The X-ray wavelength used for the measurement is 0.1542 nm.
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Fig. 3. FE-SEM cross-sectional images of the Ni–P(100 nm)/Si(100) samples annealed at (a) 400, (b) 500, (c) 600, (d) 700 and (e) 800 °C for 1 h in a furnace.
However, two layered structures were formed again after annealing up to 600 °C. Notably, increasing the annealing temperature to 800 °C, obviously increased the thickness of the bottom layer, while decreasing that of the top layer. Fig. 5 shows the cross-sectional TEM image and the EDS analyses of sample A annealed at 800 °C. No P content at the bottom layer could be detected, and the Si/Ni atomic ratio was 1.07 (the uncertainty is ±4%). Thus, this bottom layer can be defined as the NiSi layer and the solubility of P in the NiSi layer was almost zero. The composition of the upper layer included Ni, Si and P. However, because the thickness of the layer was low, the ratio of Ni:P:Si could not be identified clearly by EDS analysis. Furthermore, the XRD spectra for sample A annealed at 600–800 °C reveal that there is no extra peak except for NiSi peaks, which may be due to the small quantity or an amorphous-like structure of the top layer. Thus, the top layers formed at 600–800 °C annealing are described as Ni–P–Si capping layers in this study. Fig. 6 shows the sheet resistance data of as-deposited and annealed samples A and B. The sheet resistance of samples A and B reduced to a low level after annealing at 400 °C owing to the formation of a polycrystalline NiSi film with low resistivity. The sheet
resistance of sample A for 500 and 800 °C annealing was lower than that for 600 and 700 °C annealing because the thickness of the NiSi layers for 500 and 800 °C was larger than that obtained at other annealing conditions, as shown in Fig. 3. However, upon annealing at 800 °C, the sheet resistance of sample B was significantly higher than that of sample A due to the formation of NiSi2 in the sample B. After annealing at 900 °C, the sheet resistance of sample B increased abruptly due to the thermal breakdown of the silicide film, while that of sample A slightly increased because of the formation of NiSi2. The above results suggest that the NiSi films that formed in Ni–P (100 nm)/Si(100) samples are more stable than those that formed in pure Ni(100 nm)/Si(100) samples. 4. Discussion 4.1. Phase transformation of Ni–P on Si(100) The formation of NiSi has been reported as being diffusion controlled [3]. Nickel atoms diffuse into the Si substrate and a NiSi
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Fig. 5. (a) Cross-sectional TEM images of immersion deposited Ni–P(100 nm)/Si(100) samples annealed at 800 °C. EDS spectra of (b) the top layer and (c) the bottom layer in (a), respectively.
Fig. 4. (a) Cross-sectional and (b) plan-view bright-field TEM images of the Ni–P(100 nm)/ Si(100) samples annealed at 400 °C. (c) A SAED pattern corresponding to (b). (d) Planeview of NiSi2[210] dark-field TEM image.
phase is formed. In this study, Ni atoms from the Ni–P films diffused into Si substrates at elevated temperatures, causing a change of the composition of Ni–P films. Therefore, when annealing at 400 °C, the P
content of top Ni–P layer increases, which results in the formation of a Ni2P phase. At 500 °C annealing, only one NiSi layer was formed. One explanation may be that most Ni atoms diffused into Si substrate to form a NiSi phase, which causes the P content of Ni–P film to increase abruptly. A P-rich Ni–P compound with liquid phase could possibly be formed [15,16]. Thus, only one NiSi layer was observed. However, the Si–P and Ni–P phase diagrams [16] show that the solubility of P in the Si or Ni increases at elevated temperatures. Thus, the formation of a Ni–P–Si capping layer at 600–800 °C may be due to the increase of solubility of P in the Si or Ni films. However, in order to verify the chemical composition of individual layers, further analytical work with high resolution methods, i.e. AES depth profiles, is necessary. In
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nucleation of the NiSi2 phase in sample A was expected to be close to that in sample B because the solubility of P in NiSi and NiSi2 is limited even though sample A contained P. In the solid state, estimates of interface energy vary between about 0 for epitaxial interfaces to about 2 J/m2 for random interfaces [19]. According to the XRD data in Figs. 1 and 2, polycrystalline and epitaxial NiSi2 films were found in annealed samples A and B, respectively. As is expected, the surface energy of an epitaxial interface is lower than that of a polycrystalline one [10]. Adding P solute in NiSi2 increases σ(NiSi2/Si) while Δσ increases. Thus, the critical activation energy increases due to the increase of Δσ, which can cause the delay of NiSi2 formation. Secondly, the formation of a Ni–P–Si capping layer on the NiSi layer may be another reason for the delay of NiSi2 formation because of the surface energy effect [20]. Many investigations have shown that coating a capping layer affects the thermal stability of NiSi films [21–23]. Fig. 6. Sheet resistance vs. annealing temperature for immersion deposited Ni–P(100 nm)/ Si(100) and pure Ni(100 nm)/Si(100) samples before and after annealing at 400–900 °C for 1 h in a furnace.
our further study, the mechanism of reaction will be investigated by fabricating Ni-silicide films using Ni–P films with various thicknesses and deriving the chemical composition of the individual layers by AES. Table 1 lists the measured and listed XRD positions of NiSi2. According to the Bragg law, all measured interplanar spacings are smaller than the listed ones. The lattice constant of NiSi2 reduced from 0.5406 nm to around 0.538 nm by adding P. As is well known, NiSi2 tends to be epitaxially grown on a silicon substrate because its crystal structure is similar to that of Si; in addition, the lattice constant is only 0.4% less than that of Si. For sample A annealed at 900 °C, the difference between the measured lattice constant of NiSi2 and that of Si increased up to 0.9%. Although the lattice mismatch of NiSi2 with P addition increased slightly, the change may be sufficiently large to form a polycrystalline film on the Si(100) substrate. This phenomenon can be observed as well in the CoSi2/Si(100) system: CoSi2 possesses CaF2 structure with a lattice constant of 0.5365 nm, i.e. smaller than Si by 1.2%. It is known that polycrystalline CoSi2 is formed on the Si(100) substrate by annealing process[3,17].
5. Conclusions This study investigated how the addition of P enhances the thermal stability and electrical characteristics of NiSi. Ni-silicide films were formed by annealing the immersion deposited Ni–P(100 nm)/Si (100) samples. The sheet resistance reduced to low level after annealing at 400 °C owing to the formation of a polycrystalline NiSi film with a Ni2P layer on the surface. At annealing temperatures ranging from 600 to 800 °C, a Ni–P–Si capping layer of the NiSi layer was formed. The addition of P increased the NiSi2 formation temperature to 900 °C, leading to an enhanced thermal stability of NiSi at high temperatures. By the addition of P, the lattice constant of NiSi2 was reduced to 0.538 nm, subsequently causing the formation of polycrystalline NiSi2 films. Thus, the interface energy between NiSi2 and Si substrates was increased and a Ni–P–Si capping layer on the NiSi layer may also affect the NiSi2 formation through the surface energy effect. Acknowledgement The research is supported by the Republic of China National Science Council grant no. NSC 96-2221-E-005-108. References
4.2. Temperature of formation of NiSi2 The P additions in Ni films could affect the temperature of formation of NiSi2 in two ways. The first effect is the decrease of critical activation energy of NiSi2 formation. The transformation of NiSi to NiSi2 is a nucleation-controlled reaction [3]. The critical activation energy for nucleation is proportional to Δσ3/ΔG2, where Δσ and ΔG are an increase of the interfacial energy and a decrease of Gibbs free energy, respectively, during the reaction of NiSi + Si→ NiSi2. The addition of the impurity may decrease the Gibbs free energy and stabilize the silicide [8,13]. Previous studies demonstrated that the addition of a small amount of solute decreases the Gibbs energy and stabilizes the silicide [10,12,13,18]. Pt, Pd and Zr contents enhanced the transformation temperature of NiSi to NiSi2 owing to their extremely low solubility in NiSi2 and the larger solubility in NiSi. In this study, |ΔG| for the
Table 1 Measured and listed XRD peak positions, 2θ, of NiSi2. Measured (Sample A)
Measured (Sample B)
NiSi2
28.72 47.78 56.70
28.56 – –
28.576 47.534 56.405
Listed XRD peak positions of NiSi2 are from JCPDS 65-2974.
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