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In-situ investigation of the formation of nickel silicides during interaction of single-crystalline and amorphous silicon with nickel
Journal of Alloys and Compounds 319 (2001) 187–195
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In-situ investigation of the formation of nickel silicides during interaction of single-crystalline and amorphous silicon with nickel B. Bokhonov*, M. Korchagin Institute of Solid State Chemistry, Siberian Branch, Russian Academy of Sciences, Kutateladze 18, 630128 Novosibirsk, Russia Received 23 October 2000; accepted 2 January 2001
Abstract In situ investigations showed that the sequence of phase formation during interaction of nickel particles with single crystalline (100) silicon and amorphous silicon corresponds to the following sequence of stages during the annealing of thin-film systems: (a) within a temperature range up to 5008C, the first and prevailing phase formed is Ni 2 Si; and (b) annealing at temperatures above 6008C is accompanied by the formation and epitaxial growth of the NiSi 2 phase. The growth of the nickel disilicide crystalline phase is accompanied by the formation of dislocations both in the nickel disilicide phase and in the silicon phase. The interaction of the amorphous silicon film with nickel particles at temperatures above 6008C leads to the crystallization of several silicide phases: NiSi 2 , NiSi, Ni 3 Si 2 . The formation of silicide phases due to the interaction of nickel particles with silicon during annealing did not confirm the formation of an intermediate amorphous silicide that was observed earlier in thin-film nickel–silicon systems. Irradiation with a beam of accelerated electrons in a microscope leads to an increase of the rate of silicide phase formation and to a decrease of the temperature at which the nickel disilicide phase is formed epitaxially, at least to 4008C. In our opinion, the observed effect can be due to the formation of defects in the structure of single crystalline silicon. 2001 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Transition metal alloys; Amorphous materials; Solid state reactions; Transmission electron microscopy
1. Introduction The formation of phases during interaction of silicon with metals has been the subject of numerous investigations which is explained by the importance of silicide phases in modern technologies and undoubtedly by the scientific interest from the viewpoint of information concerning the formation of intermediate phases in silicide synthesis. Besides, this system can be used as a model solid-phase chemical reaction in order to obtain fundamental knowledge of the nature of the interface. The system silicon–nickel is among the best-studied systems in which the formation of silicide phases occurs. The analysis of literature data showed that at temperatures below 5008C the first phase to be formed is Ni 2 Si. Further on, the sequence of silicide phase formations depends on silicon to nickel ratio. For example, it was demonstrated in studies of silicide phases formation during interactions in thin-film systems such as Si–(Pt, Pd, Ni) [1] that in all cases formation of *Corresponding author. E-mail address: [email protected] (B. Bokhonov).
the Me 2 Si phase occurs at the initial stages of the interaction at the Si–Me interface (Pt 2 Si, Pd 2 Si, Ni 2 Si). According to the results obtained in the present study, an increase of the annealing time and temperature is accompanied by growth of this phase until the silicon is consumed. Then a phase richer in metal starts to grow. It grows until the first phase is completely consumed. Sequentially, a phase that is richer in metal begins to grow. Such phases have been detected up to the final ones. The authors of Ref. [2] report an investigations of the rate of silicide formation and the composition of interaction products when annealing vacuum-deposited thin ˚ on silicon substrates of nickel films (1000–5000 A) different orientation. The annealing temperature was 200– 3258C and the annealing time was 0.5–24 h. Only one silicide phase, Ni 2 Si, was observed on single crystal silicon substrates of (111) and (100) orientation and on polycrystal silicon. Two different phases NiSi and Ni 2 Si were formed in two distinct layers on amorphous (vacuumdeposited) silicon. In all cases the growth rate depended as a square root on time while the activation energy was within the range 1.3–1.6 eV. The observed growth rate depended on the substrate type. On the (100) Si surface, on
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )00902-1
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polycrystalline and amorphous silicon, when annealing for 1 h at 2758C, the growth rates of the silicide phase were ˚ It was rather close to each other and were about 1000 A. demonstrated that the growth rate is about four times lower on the silicon substrate with (111) orientation. It was shown with the help of transmission electron microscopy that silicides growing on different substrates formed different crystalline microstructure. Moreover, it was demonstrated that some reaction between thin Ni film and Si has occurred even during the deposition of Ni on evaporated Si, as indicated by the presence of Ni 2 Si diffraction lines in the X-ray films of as-deposited samples. According to the data reported in Ref. [3], the Ni 2 Si phase is formed due to interaction of the nickel film deposited onto a silicon single crystal an annealing temperature of 200– ˚ Nickel is 3258C. The crystallite size of Ni 2 Si was 600 A. assumed to be the prevailing diffusant in the interacting couple. The formation of the NiSi phase was observed during annealing for a long period of time. The authors of Ref. [4] studied the solid-phase reaction between a deposited Ni film (7% Au) and a Si substrate within the temperature range 250–8008C. It was observed that the sequence of phase formation depended on temperature at which chemical reaction took place. Annealing at 3008C leads to the formation of the Ni 2 Si phase; annealing at 6008C leads to the formation of the NiSi and Ni 2 Si phases. The NiSi 2 phase is formed during annealing at 8008C. These results were explained from the viewpoint of the classical scheme of silicide formation: Ni 2 Si with ortho˚ b53.72 A, ˚ c57.01 rhombic structure (Pnma, a54.99 A, ˚ is the first to be formed at a temperature about 2508C. A) Then, when nickel is completely transformed into Ni 2 Si, the phase NiSi begins to form at the Si–Ni 2 Si interface. Nickel monosilicide under normal conditions is orthorhombic (Pnma space group with unit cell parameters ˚ b53.258 A, ˚ c55.659 A). ˚ The nickel silicides a55.233 A, formed adopt orientations depending on that of the silicon substrate: Ni 2 Si(100) i Si(111) and NiSi (0001) i Si(111). A temperature increase to 8008C leads to the formation of the nickel disilicide according to the nucleation mechanism. Another sequence of silicide phase formation was observed in annialing studies of nickel implanted with silicon at very high doses (more than 4.5310 17 ions / cm 2 ) [5]. Amorphous Si–Ni alloy was formed during implantation of nickel with silicon. Annealing at 200–3008C caused crystallization of the Ni 2 Si phase. Later on, at 300–4008C, the phase Ni 5 Si 2 crystallized. The formation of the Ni 3 Si phase occurred at 400–6008C. It was discovered that the Ni 3 Si silicide exhibits an orientation depending on that of the nickel substrate: Ni 3 Si (100) i Ni(100) and Ni 3 Si (110) i Ni(110). The nickel silicide Ni 3 Si is dissolved in the nickel matrix with the evolution of silicon during prolong annealing at 6008C. A large number of dislocations was observed at the interface between the silicide and the nickel matrix. Besides the investigation of the interaction of crystal
silicon with nickel, several investigators studied also the interaction of nickel with amorphous silicon. For example, the authors of Ref. [6] studied the reaction of nickel with amorphous silicon using isothermal calorimetry and constant-heating calorimetry, transmission electron microscopy, thin-film X-ray diffraction, thermodynamic and kinetic analyses. The ratio of atoms in the multiplayer composition was two nickel atoms per one silicon atom. The formation of amorphous nickel silicide at the interface between nickel and amorphous silicon (20–30 nm) was observed during the preparation of the thin films. The thickness of the amorphous silicide increased insignificantly during isothermal annealing (1878C), then the formation of the crystalline phase Ni 2 Si was observed at the boundary between nickel and the amorphous nickel silicide. Prolonged annealing (at 187–2628C) caused growth of both the amorphous and crystalline nickel silicide. It should be noted that the conclusion concerning the composition and structure of both the amorphous and crystalline nickel silicide have been drown on the basis of energy-dispersive X-ray microanalysis in scanning transmission electron microscope and differential scanning calorimetry. The kinetics of silicide formation in a multilayer system nickel–amorphous silicon was studied in Ref. [7] by means of the differential scanning calorimetry. It was shown that for a silicon to nickel layer thickness ratio 1:1 only the Ni 2 Si silicide is formed. The activation energy of silicide formation is 1.5 eV, as reported by these authors. The observed temperature of the start of silicide formation decreases with decreasing layer thickness. The temperature at which Ni 2 Si formation is observed is a function of layer thickness, with the thinner layers reacting at lower temperatures. Upon mechanical impact, a film composed of ˚ reacted explosively at room very thin layers (,125 A) temperature to form Ni 2 Si. The formation of amorphous silicide during the interaction of metals with silicon was described in Ref. [8]. It was assumed, on the basis of the analysis of experimental data on a two-layer diffusion couple metal-silicon, that the first phase to be formed during the interaction between the solid components should be amorphous and that it reaches critical thickness before the second equilibrium phase starts its formation. The critical thickness of the amorphous phase was calculated in Ref. [8]. Among the examples involving the formation of an amorphous phase are the systems: La–Au, Ni–Zr, Co,Zr, Ni–Hf, Cu–Zr, Ti,Ni, Fe–Zr, Sn–Co, Y– Au, Rh–Si, Ti–Si, Ni–Si. According to the data of this study, the critical thickness is limited by the thickness of the individual layers that are used to prepare the bulk amorphous alloy. The observed critical thickness of the ˚ which is much less amorphous layer can be 10–1000 A than the thickness of the predicted equilibrium crystalline phase. The data on the possibility of the formation of metastable phases at the interface during the formation of intermetallic compounds, including the systems with silicide formation, are described in Ref. [9]. The formation of silicide phases and the diffusion
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processes involved in the interaction of amorphous silicon ˚ with thin nickel layers (1500 A) ˚ was studied films (500 A) in Ref. [10]. The diffusion and interaction were stimulated by the irradiation with 4 MeV Au 21 at room temperature. The diffusion after irradiation was observed to be two orders of magnitude higher in the direction perpendicular to the surface than into the silicon film. At least two intermetallic compounds were observed: Ni 3 Si 2 and Ni 2 Si (the first growing phase). Along with silicide phase formation during annealing in the nickel–silicon system, also the interaction of silicon with the silicide phase Ni 2 Si was studied [11]. It was demonstrated that the NiSi phase was formed by solidphase interaction between the Ni 2 Si film and (100) and (111) silicon. The existence of the hexagonal NiSi modification was discovered. Attention was paid to the anisotropy of the silicide phase formation during the interaction. It was concluded, on the basis of the experimental results obtained, that the formation of NiSi occurs via anisotropic diffusion of nickel atoms through the Ni 2 Si and NiSi layers, the activation energy being 1.7 eV. NiSi layers grow faster on (100) silicon than on (111). Besides, the existence of epitaxially growing hexagonal silicide NiSi (which is stable in very thin layers) was observed at the NiSi–Si interface. Investigations of the effect of annealing for nickel– silicon system at temperatures above 5008C showed that the nickel disilicide NiSi 2 is formed in this case. For example, the authors of Ref. [12] observed the formation of Ni 2 Si due to an interaction of the single-crystal silicon ˚ in substrate (111) with the deposited nickel film (3750 A) a temperature range of 440–8258C. It was demonstrated that the main moving species during the formation of NiSi 2 is the same as during the formation of NiSi or during the formation of Ni 2 Si, namely nickel. A predominant growth of nickel disilicide phase was observed in studies of crystallization kinetics and thermal stability of NiSi 2 which was formed by vapour co-deposition of nickel and silicon on a single-crystal silicon substrate [13]. The co-deposition caused the formation of both crystal and amorphous disilicide. Crystallisation of the NiSi 2 phase was observed at temperatures below 2008C (in some cases, the NiSi phase was formed). The crystallized disilicide was stable till 8008C. The analysis of results of investigations of the sequence of silicide phase formation during the annealing of metal– silicon systems has lead the authors of Ref. [14] to the formulation of a rule as to the first phase to be formed: The first compound nucleated in planar binary reaction couples is the most stable congruently melting compound adjacent to the lowest-temperature eutectic in the bulk equilibrium phase diagram. According to these rule the nickel disilicide NiSi 2 should be formed during interaction between nickel and silicon. As was demonstrated [15], the next phase which formed at the interface between the first phase and the remaining element (Si or metal) is the nearest congruently melting
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compound richer in the unreacted element. If the compounds between the first phase and the remaining element do not melt congruently (such as peritectic or peritectoid phase), the next phase formed is that with the smallest temperature difference between the liquidus curve and the peritectic (or peritectoid) point. The authors of Ref. [16] proposed a scheme of stage sequence in the interaction of nickel and silicon, based on the analysis of literature data and thermodynamic calculations: • • • •
1st step: Ni / amorphous / Si 2nd step: Ni / Ni 2 Si / amorphous / Si→Ni 2 Si / Si 3rd step: Ni 2 Si / NiSi / Si→NiSi / Si 4th step: NiSi / NiSi 2 / Si→NiSi 2 / Si
As one can see from this scheme, it is assumed that amorphous silicide is formed at the initial stages of the interaction, It then crystallized to form the Ni 2 Si phase. So, the analysis of literature data allows us to conclude that the first crystal phase formed during the interaction between nickel and silicon is the Ni 2 Si phase. It is assumed that the formation of this crystal phase is preceded by the formation of an amorphous silicide. Orientation relations between the silicides formed in solid-phase interactions between nickel and silicon substrates were stated in some studies. Besides, an increase in defect concentration in the vicinity of the interface was observed. However, in spite of large number of works dealing with solid-phase interactions in the nickel–silicon system, a shortcoming should be noted which is substantial from our point of view: Practically all studies involved the interacting couple nickel–silicon prepared as a thin-film couple, with a thin layer of metal deposited onto silicon, the latter usually being a single-crystal. In our opinion, this type of system under investigation can bring substantial changes into the structure of the Ni–Si interface which is confirmed by some data available from literature. As we noted above, in some cases the formation of an amorphous silicide was observed after the deposition of metal or silicon. Because of this, the question arises whether the layer of intermediate amorphous silicide is formed during solidphase interaction between silicon and nickel crystallites. So, in spite of the availability of literature data on the formation of silicide phases, several questions remain not clear yet and require additional studies. In our opinion, the most important among these questions are the following ones: 1. What is the sequence of silicide phase formation during the solid-phase chemical reaction in the metal–silicon system? Which phase is the first to be formed? 2. What is the structure of the interface between silicon and the silicide, and between the silicide phases? 3. What defects are formed during the solid-phase interaction of silicon with metal? In which phase are these defects found?
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4. Does the silicon structure (crystalline or amorphous) affect on the sequence of silicide phase formation?
2. Experimental In-situ electron microscopic studies were carried out with a transmission electron microscope JEM-2000 FX II equipped with a special holder for heating EM-SHH4. Single crystalline silicon foils were prepared by chemical etching of single crystal silicon plates of (100) Si orientation with 98% HNO 3 , 40% HF solutions mixed at a ratio of 1:1. ˚ thick amorphous Si film was deposited on A 15000 A NaCl substrates in a vacuum chamber (at a pressure of |2310 27 Torr during evaporation) by electron gun evaporation. Transmission electron microscopy samples were prepared by dissolving the NaCl substrate in de-ionized water and picking the film up on copper grids. Before in-situ experiments, fine nickel particles were deposited onto silicon thin films or single-crystal foils in order to obtain the reaction couple. In some cases, in order to achieve a tighter contact between the film (foil) and particle, metal particles were locally heated by focusing an electron beam with increased intensity on their surface. An electron microscope of the type EF-4 (Karl Zeiss Jena) was used for this purpose. In some cases, in order to perform in situ and ex-situ ˚ thick was experiments, a thin nickel layer 100 to 1000 A deposited onto a thin single-crystal silicon foil by means of thermal vacuum evaporation.
3. Experimental results
3.1. In situ investigation of phase formation due to the interaction of single-crystal (100) silicon foil with nickel particles When studying the solid-phase interaction between nickel particles and a silicon film it was discovered that the temperature at which the reaction front propagation rate (with crystal components used) reaches a value that can be observed with an electron microscope (i.e. over 0.01 mm / s) is 600–7008C. At temperatures below 6008C, the time necessary for the formation of the chemical reaction product phase is more than 1 h. The investigations showed that the composition of phases formed by in-situ annealing depends on reaction temperature. For example, at temperature below 6008C, the formation of the Ni 2 Si phase is observed at the boundary between the nickel particles and the single crystal silicon film (Fig. 1). The reaction front propagates into the single crystal silicon film. The size of the crystal blocks of the formed Ni 2 Si phase is 0.2–0.5 mm and has no strict orientation relation to the single-crystal silicon substrate.
Fig. 1. TEM microphotograph of the Ni 2 Si phase formed on the surface of a single-crystalline silicon film during annealing at temperature up to 6008C.
Annealing of the silicon single-crystal film with nickel particles did not lead to the formation of any phases except Ni 2 Si. If the temperature is increased to 7008C after the Ni 2 Si silicide phase is formed, we observe the formation of the disilicide phase which grows epitaxially into the singlecrystal silicon. If the heating is produced by a highintensity electron beam, we also observe the formation of isles of the disilicide phase in front of the propagating boundary of the growing phase NiSi 2 (Fig. 2). The growth of the NiSi 2 phase is accompanied by the ‘dissolution’ of Ni 2 Si phase and the formation of pores at the Ni 2 Si–NiSi 2 interface. If the annealing of the system nickel particle-silicon is initially carried out at 7008C, only the NiSi 2 phase is observed to have formed and grown during the interaction. The growing nickel disilicide phase is shaped as a square and has a strict orientation relation to the (100) single crystal silicon film, as can be seen in electron diffraction patterns (Fig. 3). The increase in size of the growing NiSi 2 phase is accompanied by the appearance of contrast characteristic of dislocations (Fig. 4). A temperature decrease to 508C causes some decrease of the number of dislocation lines in the nickel disilicide phase. The growth of the disilicide phase is not only accompanied by the formation of dislocations in the NiSi 2 phase. An increase in the size of NiSi 2 crystals causes also the formation of dislocations on the NiSi 2 –Si interface in the silicon phase (Fig. 5). The movement of dislocations in the disilicide and silicon phase during formation of the NiSi 2 phase was
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Fig. 4. TEM microphotograph of the dislocations in NiSi 2 crystals formed during annealing of the system: nickel particle / single crystalline silicon film at 7008C. Fig. 2. Electron microphotograph of the Ni 2 Si–NiSi 2 interface formed during annealing of the Ni 2 Si–Si system at 7008C. Epitaxial growth of the NiSi 2 phase in a single-crystalline silicon foil is observed.
observed directly during our in-situ electron microscopic investigation. It should be noted that we did not observe the formation
of any amorphous phase at the interface between nickel silicide and silicon during the formation of silicide phases when annealing single crystalline silicon with nickel particles deposited onto its surface.
3.2. In situ investigation of phase formation due to the interaction of a single-crystal silicon (100) foil with a thin nickel film In-situ studies of the interaction of (100) silicon with a ˚ thick) deposited onto the thin nickel film (about 20–50 A
Fig. 3. SAD patterns from the Si–NiSi 2 interface formed in the annealing of the system: nickel particle / single-crystalline silicon film at 7008C. The reflections of nickel disilicide phase exhibit a strict orientation relationship to the reflections of the initial single crystal silicon substrate: (100) Si i (100) NiSi 2 and [100] Si i [100] NiSi 2 .
Fig. 5. TEM microphotograph of dislocations formed in the silicon phase near the NiSi 2 –Si interface.
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Fig. 6. TEM microphotograph of crystal (100) silicon with thin deposited nickel particles (film).
Fig. 8. TEM microphotograph of square isles of the NiSi 2 phase formed during annealing at a temperature above 6008C of (100) crystalline silicon with a deposited nickel film.
silicon surface (Fig. 6) showed that the sequence of phase formation during annealing is identical to that observed in the annealing of the system nickel particle-single crystalline silicon foil. Annealing for 15–60 min at temperatures up to 4008C causes the formation of the Ni 2 Si phase (Fig. 7). Annealing at a temperature above 6008C is accompanied by the growth of square isles of the NiSi 2 phase (Fig. 8). The orientation relations of these isles with the single-crystalline silicon foil are as follows: (100) Si i (100) NiSi 2 and [100] Si i [100] NiSi 2 . An increase of annealing time causes a complete disappearance of nickel. The surface (100) of the single-crystalline silicon is completely
covered by the isles of nickel dicilicide crystals. The size of the formed crystallites of the phase after complete consumption of nickel is 20–100 nm. The annealing at 6008C of the system nickel film–single-crystalline silicon foil, in which the crystals of silicide phase Ni 2 Si were formed at low temperature, causes the growth of nickel disilicide phase. The front of disilicide phase crystallization propagates into the silicon single crystal, while the formation of pores is observed at the interface Ni 2 Si– NiSi 2 , until the contact between Ni 2 Si phase and the growing phase is completely destroyed. When investigating phase formation in the system thin nickel film–single-crystalline silicon, we came across the rather interesting experimental fact that the irradiation with a beam of accelerated electrons in the microscope has a substantial effect on the rate of solid-phase interaction and on the sequence of phase formations. For example, during the annealing of the system thin nickel film–single-crystalline silicon at 4008C, the Ni 2 Si phase is formed within several tens minutes, while the annealing of the same system at 4008C with simultaneous irradiation with a 200 kV electron beam causes not only a decrease of the time of silicide phase formation but also leads to the formation of an epitaxially growing NiSi 2 phase (Fig. 9) instead of Ni 2 Si. An increase of NiSi 2 phase formation rate is also observed with annealing at 600–7008C. An increase of the rate of disilicide phase formation during isothermal annealing is observed also in the case when the combination (thin nickel film–single-crystalline silicon foil) was preliminarily irradiated in the electron microscope at low temperatures (1008C), after which it was isothermally annealed at 4008C without irradiation. In this case we also observed prevailing formation of NiSi 2 phase at the irradiated regions.
Fig. 7. TEM microphotograph of the Ni 2 Si phase formed during annealing of (100) silicon crystal with the deposited nickel particles at a temperature up to 4008C.
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Fig. 9. Formation of an epitaxially growing NiSi 2 phase formed during heating (4008C) and irradiation with electron beam of the system thin nickel film / single-crystalline silicon (the formation of isles of the Ni 2 Si phase is observed in the irradiated region).
3.3. In-situ investigation of phase formation due to the interaction of amorphous silicon with nickel particles Electron microscopic studies of structural and morphological characteristics of in-situ interaction of amorphous silicon with nickel particles showed that phases formation is observed to proceed at a noticeable rate within temperature range 600–7008C. Annealing of the amorphous silicon film with the deposited nickel particle leads to the formation of layers composed of the interaction products. The propagation of the reaction front (formation of silicide phases) occurs inside the amorphous silicon film. It was discovered that the sequence of phase formation depends on the method by which the solid-phase interaction is initiated, and it is more complicated than that observed in the interaction of nickel with single-crystalline silicon. For example, if heating of the sample was carried out with the help of a special electron microscopic sample heating holder (in this case, the temperature of the amorphous film is equal to that of the nickel particle on its surface), at least three reaction zones can be noticed between the nickel particle and amorphous silicon film (Fig. 10). According to electron diffraction data, the sequence of phases when approaching the nickel particle is as follows: NiSi 2 , NiSi, Ni 3 Si 2 . The width of the zones of the individual phases formed during the interaction of amorphous silicon with the nickel particle exhibits depends only weakly on annealing time and is 1–2 mm for all the
Fig. 10. Formation of multilayered region structure of the products during interaction of amorphous silicon and nickel particle under isothermal annealing. According to the SAD patterns, the first region (most remote from the nickel particle) corresponds to the formation of the NiSi 2 silicide phase, the second layer corresponds to the formation of the NiSi silicide phase and the third layer corresponds to the formation of the Ni 3 Si 2 silicide phase.
observed phases. It should be noted that we did not observe the formation of any transition amorphous or liquid phases. The interface between the first growing silicide phase and the amorphous silicon, though containing some steps, is rather sharp. If the reaction was initiated by an electron beam (in this case, the particle temperature can be much higher than film temperature), the formation of only two phases was observed during the initial stages of interaction of the nickel particle with the amorphous silicon (Fig. 11). According to electron diffraction data, these are the silicide phases Ni 2 Si and NiSi 2 . As reaction proceeds, the formation of the Ni 3 Si 2 phase is observed at the interface between the two silicide phases Ni 2 Si and NiSi 2 (Fig. 12) that were formed during the initial stages of annealing. The propagation of the reaction front is accompanied by an increase of the size of the Ni 3 Si 2 phase and a simultaneous decrease of the size of the Ni 2 Si phase. It should be noted that the rate of reaction front propagation during the formation of the nickel disilicide on single-crystal substrates is much lower than that during the interaction of nickel particles with amorphous silicon. In spite of the fact that the kinetic characteristics of the reaction front propagation cannot be plotted at present, one
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Fig. 11. Formation of two layers of silicide phases (Ni 2 Si and NiSi 2 ) during electron beam heating of the system amorphous silicon–nickel particle.
can state that the propagation of the reaction front slows down while the size of reaction zone increases.
4. Discussion of results In-situ investigation of phase formation due to the interaction of (100) crystalline silicon with nickel showed
Fig. 12. Formation of the Ni 3 Si 2 phase at the Ni 2 Si–NiSi 2 interface during thermal annealing of amorphous silicon and a nickel particle heated by an electron beam.
that the sequence of phase formation observed during the annealing of this system at different temperatures is the same as that known before. At low temperatures, the first and predominant phase growing during the annealing is Ni 2 Si while at higher temperature (600–7008C) the predominant and the only one phase is the nickel disilicide. Unlike the interaction of crystalline silicon with nickel particles, the reaction between amorphous silicon and nickel particles at 600–7008C leads to the formation of several crystalline silicide phases: NiSi 2 , NiSi, Ni 3 Si 2 . The formation of a larger number of phases due to the interaction of amorphous silicon with nickel particles can have several reasons. It is most probable that the formation of silicide phases is determined both by the rate of reaction front propagation and the rate of nickel atoms diffusion to the interface. At the initial stages, diffusion of nickel may take place at the interface Ni–amorphous Si, and then also at the interfaces NiSi 2 –amorphous Si and Ni–NiSi 2 . In our opinion, the rate of nickel diffusion in amorphous silicon is much faster than the rate of nickel diffusion in crystalline silicon which allows to form a wider diffusion front. Higher rates of reaction front propagation lead to the possibility of several observed silicide phases to crystallise. Nevertheless, we think that additional studies are necessary to find the exact reasons why several silicide phases are formed. A common feature of the formation of silicide phases during annealing of both the crystalline and the amorphous silicon with nickel is preferential diffusion of nickel. This is unambiguously confirmed by the observed reaction front propagation inside the crystalline (or amorphous) silicon during annealing. Another interesting fact discovered during the studies is the formation of dislocations in the epitaxially growing nickel disilicide phase and single crystalline silicon at the NiSi 2 –Si interface. It is well known that the strain at the interface may be due to several factors, including noncorrespondence between the crystalline lattices and the difference in thermal linear expansion coefficients of the substrate material and the growing (depositing) phase. Strains, arising as a result of silicide formation, differ from each other depending on what metal is used to form the silicide phase. It is accepted that the strain arising at the interface during the formation of nickel disilicide is much smaller than that for other metals [17]. The reason of lower strain is the epitaxial growth and the similarity of the lattice parameters of the nickel disilicide and silicon. According to the data obtained by us, in the processes of nickel disilicide formation and epitaxial growth, dislocations are formed in crystal when it reaches the size of about 1 mm. In our opinion, the reason of the observed dimensional effect is the fact that the increase of the size of nickel disilicide crystal is accompanied by anm increase of strain at the Si–NiSi 2 interface. This leads to of plastic deformation causing the formation and movement of dislocations both in silicide phase and in single-crystal silicon matrix. Besides, theoretical considerations of the
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interface energy for coherent isles of the new phase reported in Ref. [18] showed that dimensional effect really exists, in the case when the formation of dislocations at the interface becomes favoured from the energy point of view. Besides, we discovered that the decrease of temperature to 508C leads to some decrease of the number of dislocation lines in the nickel disilicide. This effect may be due to the fact that the thermal linear expansion coefficients of silicides are much higher than those of metals and silicon. For example, the thermal linear expansion coefficient for nickel disilicide is 12.06310 26 C 21 while for silicon it is 3310 26 C 21 [17]. This difference in thermal linear expansion coefficients most likely causes a decrease of strain at the interface and re-distribution of dislocations in the silicide phase. The acceleration of solid-phase chemical reaction accompanied by changes in phase composition in the irradiated samples can be explained by several reasons. It is known that, as a rule, the primary act in the interaction of high-energy particles with the substance is ionization. The probability of formation of point defects, namely, Frenkel pairs, in the case when high-energy particle has a small mass (for example, like electron), is negligibly small in comparison with the probability of formation of secondary electrons. The excitation followed by relaxation of the electron subsystem in semiconductors leads to the formation of long-lived electron defects. Besides, relaxation of electron excitations occurs. Thus, the obtained Frenkel pairs usually exhibit sufficient mobility and easily form different complexes including bi-vacancies, vacancies localized at impurities, dislocation loops, rod-like defects, etc. [19–23]. These are the final products of irradiation and determine the physicochemical properties of the substance. So, the increase of reaction rate can be compared with the change of the concentration of defects in silicon arising during the irradiation with a beam of accelerated electrons. It is natural that the increase in defect concentration in silicon should lead to an acceleration of nickel diffusion in the silicon lattice. It is probable that an increase of the number of defects (acceleration of diffusion processes) also determines the changes of phase composition of reaction products formed during low-temperature annealing of the investigated system (nickel–silicon). Nevertheless, in order to reveal the true reasons of the changes of phase composition of products formed in the interaction of nickel with silicon, additional investigations are necessary.
5. Conclusions In situ investigations have shown that the sequence of phase formation during the interaction of nickel particles with single-crystalline (100) silicon foils and amorphous silicon films can be characterized as follows: Within the temperature range up to 5008C, Ni 2 Si is the first and predominant phase to be formed. The formation and
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epitaxial growth of the NiSi 2 phase is observed during annealing at temperatures above 6008C. The growth of the nickel disilicide crystalline phase is accompanied by the formation of dislocations both in the nickel disilicide and the silicon phase. Unlike the sequence of phase formation during the interaction of crystalline silicon with nickel particles, the interaction of an amorphous silicon film with nickel particles at temperatures above 6008C leads to the crystallization of several phases: NiSi 2 , NiSi, Ni 3 Si 2 . The formation of silicide phases due to the interaction of nickel particles with silicon during annealing did not confirm the formation of an intermediate amorphous silicide which was observed earlier in thin-film nickel–silicon systems. Irradiation with a beam of accelerated electrons in a microscope was found to cause an increase of the rate of silicide phase formation and to a decrease of the formation temperature of the epitaxially growing nickel disilicide phase, at least to 4008C. In our opinion, the observed effect can be connected with the formation of defects in the structure of single-crystalline silicon.
References [1] C. Canali, G. Majni, G. Ottaviani, J. Appl. Phys. 50 (1) (1979) 255. [2] J.O. Olowolafe, M.-A. Nicolet, J.W. Mayer, Thin Solid Films 38 (1976) 143. [3] K.N. Tu, W.K. Chu, J.W. Mayer, Thin Solid Films 25 (1975) 403. [4] D. Mangelinck, P. Gas, A. Grob, B. Pichaud, O. Thomas, J. Appl. Phys. 79 (8) (1996) 4078. [5] Z. Rao, J.S. Williams, A.P. Pogany, D.K. Sood, G.A. Collins, J. Appl. Phys. 77 (8) (1995) 3782. [6] L.A. Clevenger, C.V. Thompson, J. Appl. Phys. 67 (3) (1990) 1325. [7] L.A. Clevenger, C.V. Thompson, R.S. Cammarata, K.N. Tu, Appl. Phys. Lett. 52 (10) (1988) 795. [8] U. Goselle, K.N. Tu, J. Appl. Phys. 66 (6) (1989) 2619. [9] R.W. Bene, J. Appl. Phys. 61 (5) (1987) 1826. [10] J. Desimoni, S. Hagege, A. Traverse, J. Appl. Phys. 77 (8) (1995) 3774. [11] F. d’Heurle, C.S. Peterson, J.E.E. Baglin, S.J. La Placa, C.Y. Wong, J. Appl. Phys. 55 (12) (1984) 4208. [12] F. d’Heurle, S. Peterson, L. Stolt, B. Strizker, J. Appl. Phys. 55 (8) (1982) 5678. [13] I. Grimberg, B.Z. Weiss, J. Appl. Phys. 77 (8) (1995) 3791. [14] R.M. Walster, R.W. Bene, Appl. Phys. Lett. 28 (10) (1976) 624. [15] B.Y. Tsaur, S.S. Lau, J.W. Mayer, M.-A. Nicolet, Appl. Phys. Lett. 38 (11) (1981) 922. [16] B.-J. Lee, J. Mater. Res. 14 (3) (1999) 1002. [17] S.P. Murarka, Silicides for VLSI Applications, Academic Press, Orlando, FL, 1983. [18] H.T. Johnson, L.B. Freund, J. Appl. Phys. 81 (9) (1997) 6081. [19] J.W. Corbett, J.C. Bourgoin, L.J. Cheng, J.C. Corelli, Y.H. Lee, P.M. Mooney, C. Weigel, The status of defect studies in silicon, in: Radiation Effects in Semiconductor, Institute of Physics, London– Bristol, 31 (1977) 1–11. [20] M.D. Matthews, S.J. Ashby, Phil. Mag. 27 (6) (1973) 1313. [21] I.G. Salisbury, M.H. Loretto, Phil. Mag. A 39 (3) (1979) 317. [22] M. Hirata, M. Kiritani, Physica B–C 116 (1983) 616. [23] E. Nes, J. Washburn, J. Appl. Phys. 42 (9) (1971) 3559.
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In-situ investigation of the formation of nickel silicides during interaction of single-crystalline and amorphous silicon with nickel B. Bokhonov*, M. Korchagin Institute of Solid State Chemistry, Siberian Branch, Russian Academy of Sciences, Kutateladze 18, 630128 Novosibirsk, Russia Received 23 October 2000; accepted 2 January 2001
Abstract In situ investigations showed that the sequence of phase formation during interaction of nickel particles with single crystalline (100) silicon and amorphous silicon corresponds to the following sequence of stages during the annealing of thin-film systems: (a) within a temperature range up to 5008C, the first and prevailing phase formed is Ni 2 Si; and (b) annealing at temperatures above 6008C is accompanied by the formation and epitaxial growth of the NiSi 2 phase. The growth of the nickel disilicide crystalline phase is accompanied by the formation of dislocations both in the nickel disilicide phase and in the silicon phase. The interaction of the amorphous silicon film with nickel particles at temperatures above 6008C leads to the crystallization of several silicide phases: NiSi 2 , NiSi, Ni 3 Si 2 . The formation of silicide phases due to the interaction of nickel particles with silicon during annealing did not confirm the formation of an intermediate amorphous silicide that was observed earlier in thin-film nickel–silicon systems. Irradiation with a beam of accelerated electrons in a microscope leads to an increase of the rate of silicide phase formation and to a decrease of the temperature at which the nickel disilicide phase is formed epitaxially, at least to 4008C. In our opinion, the observed effect can be due to the formation of defects in the structure of single crystalline silicon. 2001 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Transition metal alloys; Amorphous materials; Solid state reactions; Transmission electron microscopy
1. Introduction The formation of phases during interaction of silicon with metals has been the subject of numerous investigations which is explained by the importance of silicide phases in modern technologies and undoubtedly by the scientific interest from the viewpoint of information concerning the formation of intermediate phases in silicide synthesis. Besides, this system can be used as a model solid-phase chemical reaction in order to obtain fundamental knowledge of the nature of the interface. The system silicon–nickel is among the best-studied systems in which the formation of silicide phases occurs. The analysis of literature data showed that at temperatures below 5008C the first phase to be formed is Ni 2 Si. Further on, the sequence of silicide phase formations depends on silicon to nickel ratio. For example, it was demonstrated in studies of silicide phases formation during interactions in thin-film systems such as Si–(Pt, Pd, Ni) [1] that in all cases formation of *Corresponding author. E-mail address: [email protected] (B. Bokhonov).
the Me 2 Si phase occurs at the initial stages of the interaction at the Si–Me interface (Pt 2 Si, Pd 2 Si, Ni 2 Si). According to the results obtained in the present study, an increase of the annealing time and temperature is accompanied by growth of this phase until the silicon is consumed. Then a phase richer in metal starts to grow. It grows until the first phase is completely consumed. Sequentially, a phase that is richer in metal begins to grow. Such phases have been detected up to the final ones. The authors of Ref. [2] report an investigations of the rate of silicide formation and the composition of interaction products when annealing vacuum-deposited thin ˚ on silicon substrates of nickel films (1000–5000 A) different orientation. The annealing temperature was 200– 3258C and the annealing time was 0.5–24 h. Only one silicide phase, Ni 2 Si, was observed on single crystal silicon substrates of (111) and (100) orientation and on polycrystal silicon. Two different phases NiSi and Ni 2 Si were formed in two distinct layers on amorphous (vacuumdeposited) silicon. In all cases the growth rate depended as a square root on time while the activation energy was within the range 1.3–1.6 eV. The observed growth rate depended on the substrate type. On the (100) Si surface, on
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )00902-1
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polycrystalline and amorphous silicon, when annealing for 1 h at 2758C, the growth rates of the silicide phase were ˚ It was rather close to each other and were about 1000 A. demonstrated that the growth rate is about four times lower on the silicon substrate with (111) orientation. It was shown with the help of transmission electron microscopy that silicides growing on different substrates formed different crystalline microstructure. Moreover, it was demonstrated that some reaction between thin Ni film and Si has occurred even during the deposition of Ni on evaporated Si, as indicated by the presence of Ni 2 Si diffraction lines in the X-ray films of as-deposited samples. According to the data reported in Ref. [3], the Ni 2 Si phase is formed due to interaction of the nickel film deposited onto a silicon single crystal an annealing temperature of 200– ˚ Nickel is 3258C. The crystallite size of Ni 2 Si was 600 A. assumed to be the prevailing diffusant in the interacting couple. The formation of the NiSi phase was observed during annealing for a long period of time. The authors of Ref. [4] studied the solid-phase reaction between a deposited Ni film (7% Au) and a Si substrate within the temperature range 250–8008C. It was observed that the sequence of phase formation depended on temperature at which chemical reaction took place. Annealing at 3008C leads to the formation of the Ni 2 Si phase; annealing at 6008C leads to the formation of the NiSi and Ni 2 Si phases. The NiSi 2 phase is formed during annealing at 8008C. These results were explained from the viewpoint of the classical scheme of silicide formation: Ni 2 Si with ortho˚ b53.72 A, ˚ c57.01 rhombic structure (Pnma, a54.99 A, ˚ is the first to be formed at a temperature about 2508C. A) Then, when nickel is completely transformed into Ni 2 Si, the phase NiSi begins to form at the Si–Ni 2 Si interface. Nickel monosilicide under normal conditions is orthorhombic (Pnma space group with unit cell parameters ˚ b53.258 A, ˚ c55.659 A). ˚ The nickel silicides a55.233 A, formed adopt orientations depending on that of the silicon substrate: Ni 2 Si(100) i Si(111) and NiSi (0001) i Si(111). A temperature increase to 8008C leads to the formation of the nickel disilicide according to the nucleation mechanism. Another sequence of silicide phase formation was observed in annialing studies of nickel implanted with silicon at very high doses (more than 4.5310 17 ions / cm 2 ) [5]. Amorphous Si–Ni alloy was formed during implantation of nickel with silicon. Annealing at 200–3008C caused crystallization of the Ni 2 Si phase. Later on, at 300–4008C, the phase Ni 5 Si 2 crystallized. The formation of the Ni 3 Si phase occurred at 400–6008C. It was discovered that the Ni 3 Si silicide exhibits an orientation depending on that of the nickel substrate: Ni 3 Si (100) i Ni(100) and Ni 3 Si (110) i Ni(110). The nickel silicide Ni 3 Si is dissolved in the nickel matrix with the evolution of silicon during prolong annealing at 6008C. A large number of dislocations was observed at the interface between the silicide and the nickel matrix. Besides the investigation of the interaction of crystal
silicon with nickel, several investigators studied also the interaction of nickel with amorphous silicon. For example, the authors of Ref. [6] studied the reaction of nickel with amorphous silicon using isothermal calorimetry and constant-heating calorimetry, transmission electron microscopy, thin-film X-ray diffraction, thermodynamic and kinetic analyses. The ratio of atoms in the multiplayer composition was two nickel atoms per one silicon atom. The formation of amorphous nickel silicide at the interface between nickel and amorphous silicon (20–30 nm) was observed during the preparation of the thin films. The thickness of the amorphous silicide increased insignificantly during isothermal annealing (1878C), then the formation of the crystalline phase Ni 2 Si was observed at the boundary between nickel and the amorphous nickel silicide. Prolonged annealing (at 187–2628C) caused growth of both the amorphous and crystalline nickel silicide. It should be noted that the conclusion concerning the composition and structure of both the amorphous and crystalline nickel silicide have been drown on the basis of energy-dispersive X-ray microanalysis in scanning transmission electron microscope and differential scanning calorimetry. The kinetics of silicide formation in a multilayer system nickel–amorphous silicon was studied in Ref. [7] by means of the differential scanning calorimetry. It was shown that for a silicon to nickel layer thickness ratio 1:1 only the Ni 2 Si silicide is formed. The activation energy of silicide formation is 1.5 eV, as reported by these authors. The observed temperature of the start of silicide formation decreases with decreasing layer thickness. The temperature at which Ni 2 Si formation is observed is a function of layer thickness, with the thinner layers reacting at lower temperatures. Upon mechanical impact, a film composed of ˚ reacted explosively at room very thin layers (,125 A) temperature to form Ni 2 Si. The formation of amorphous silicide during the interaction of metals with silicon was described in Ref. [8]. It was assumed, on the basis of the analysis of experimental data on a two-layer diffusion couple metal-silicon, that the first phase to be formed during the interaction between the solid components should be amorphous and that it reaches critical thickness before the second equilibrium phase starts its formation. The critical thickness of the amorphous phase was calculated in Ref. [8]. Among the examples involving the formation of an amorphous phase are the systems: La–Au, Ni–Zr, Co,Zr, Ni–Hf, Cu–Zr, Ti,Ni, Fe–Zr, Sn–Co, Y– Au, Rh–Si, Ti–Si, Ni–Si. According to the data of this study, the critical thickness is limited by the thickness of the individual layers that are used to prepare the bulk amorphous alloy. The observed critical thickness of the ˚ which is much less amorphous layer can be 10–1000 A than the thickness of the predicted equilibrium crystalline phase. The data on the possibility of the formation of metastable phases at the interface during the formation of intermetallic compounds, including the systems with silicide formation, are described in Ref. [9]. The formation of silicide phases and the diffusion
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processes involved in the interaction of amorphous silicon ˚ with thin nickel layers (1500 A) ˚ was studied films (500 A) in Ref. [10]. The diffusion and interaction were stimulated by the irradiation with 4 MeV Au 21 at room temperature. The diffusion after irradiation was observed to be two orders of magnitude higher in the direction perpendicular to the surface than into the silicon film. At least two intermetallic compounds were observed: Ni 3 Si 2 and Ni 2 Si (the first growing phase). Along with silicide phase formation during annealing in the nickel–silicon system, also the interaction of silicon with the silicide phase Ni 2 Si was studied [11]. It was demonstrated that the NiSi phase was formed by solidphase interaction between the Ni 2 Si film and (100) and (111) silicon. The existence of the hexagonal NiSi modification was discovered. Attention was paid to the anisotropy of the silicide phase formation during the interaction. It was concluded, on the basis of the experimental results obtained, that the formation of NiSi occurs via anisotropic diffusion of nickel atoms through the Ni 2 Si and NiSi layers, the activation energy being 1.7 eV. NiSi layers grow faster on (100) silicon than on (111). Besides, the existence of epitaxially growing hexagonal silicide NiSi (which is stable in very thin layers) was observed at the NiSi–Si interface. Investigations of the effect of annealing for nickel– silicon system at temperatures above 5008C showed that the nickel disilicide NiSi 2 is formed in this case. For example, the authors of Ref. [12] observed the formation of Ni 2 Si due to an interaction of the single-crystal silicon ˚ in substrate (111) with the deposited nickel film (3750 A) a temperature range of 440–8258C. It was demonstrated that the main moving species during the formation of NiSi 2 is the same as during the formation of NiSi or during the formation of Ni 2 Si, namely nickel. A predominant growth of nickel disilicide phase was observed in studies of crystallization kinetics and thermal stability of NiSi 2 which was formed by vapour co-deposition of nickel and silicon on a single-crystal silicon substrate [13]. The co-deposition caused the formation of both crystal and amorphous disilicide. Crystallisation of the NiSi 2 phase was observed at temperatures below 2008C (in some cases, the NiSi phase was formed). The crystallized disilicide was stable till 8008C. The analysis of results of investigations of the sequence of silicide phase formation during the annealing of metal– silicon systems has lead the authors of Ref. [14] to the formulation of a rule as to the first phase to be formed: The first compound nucleated in planar binary reaction couples is the most stable congruently melting compound adjacent to the lowest-temperature eutectic in the bulk equilibrium phase diagram. According to these rule the nickel disilicide NiSi 2 should be formed during interaction between nickel and silicon. As was demonstrated [15], the next phase which formed at the interface between the first phase and the remaining element (Si or metal) is the nearest congruently melting
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compound richer in the unreacted element. If the compounds between the first phase and the remaining element do not melt congruently (such as peritectic or peritectoid phase), the next phase formed is that with the smallest temperature difference between the liquidus curve and the peritectic (or peritectoid) point. The authors of Ref. [16] proposed a scheme of stage sequence in the interaction of nickel and silicon, based on the analysis of literature data and thermodynamic calculations: • • • •
1st step: Ni / amorphous / Si 2nd step: Ni / Ni 2 Si / amorphous / Si→Ni 2 Si / Si 3rd step: Ni 2 Si / NiSi / Si→NiSi / Si 4th step: NiSi / NiSi 2 / Si→NiSi 2 / Si
As one can see from this scheme, it is assumed that amorphous silicide is formed at the initial stages of the interaction, It then crystallized to form the Ni 2 Si phase. So, the analysis of literature data allows us to conclude that the first crystal phase formed during the interaction between nickel and silicon is the Ni 2 Si phase. It is assumed that the formation of this crystal phase is preceded by the formation of an amorphous silicide. Orientation relations between the silicides formed in solid-phase interactions between nickel and silicon substrates were stated in some studies. Besides, an increase in defect concentration in the vicinity of the interface was observed. However, in spite of large number of works dealing with solid-phase interactions in the nickel–silicon system, a shortcoming should be noted which is substantial from our point of view: Practically all studies involved the interacting couple nickel–silicon prepared as a thin-film couple, with a thin layer of metal deposited onto silicon, the latter usually being a single-crystal. In our opinion, this type of system under investigation can bring substantial changes into the structure of the Ni–Si interface which is confirmed by some data available from literature. As we noted above, in some cases the formation of an amorphous silicide was observed after the deposition of metal or silicon. Because of this, the question arises whether the layer of intermediate amorphous silicide is formed during solidphase interaction between silicon and nickel crystallites. So, in spite of the availability of literature data on the formation of silicide phases, several questions remain not clear yet and require additional studies. In our opinion, the most important among these questions are the following ones: 1. What is the sequence of silicide phase formation during the solid-phase chemical reaction in the metal–silicon system? Which phase is the first to be formed? 2. What is the structure of the interface between silicon and the silicide, and between the silicide phases? 3. What defects are formed during the solid-phase interaction of silicon with metal? In which phase are these defects found?
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4. Does the silicon structure (crystalline or amorphous) affect on the sequence of silicide phase formation?
2. Experimental In-situ electron microscopic studies were carried out with a transmission electron microscope JEM-2000 FX II equipped with a special holder for heating EM-SHH4. Single crystalline silicon foils were prepared by chemical etching of single crystal silicon plates of (100) Si orientation with 98% HNO 3 , 40% HF solutions mixed at a ratio of 1:1. ˚ thick amorphous Si film was deposited on A 15000 A NaCl substrates in a vacuum chamber (at a pressure of |2310 27 Torr during evaporation) by electron gun evaporation. Transmission electron microscopy samples were prepared by dissolving the NaCl substrate in de-ionized water and picking the film up on copper grids. Before in-situ experiments, fine nickel particles were deposited onto silicon thin films or single-crystal foils in order to obtain the reaction couple. In some cases, in order to achieve a tighter contact between the film (foil) and particle, metal particles were locally heated by focusing an electron beam with increased intensity on their surface. An electron microscope of the type EF-4 (Karl Zeiss Jena) was used for this purpose. In some cases, in order to perform in situ and ex-situ ˚ thick was experiments, a thin nickel layer 100 to 1000 A deposited onto a thin single-crystal silicon foil by means of thermal vacuum evaporation.
3. Experimental results
3.1. In situ investigation of phase formation due to the interaction of single-crystal (100) silicon foil with nickel particles When studying the solid-phase interaction between nickel particles and a silicon film it was discovered that the temperature at which the reaction front propagation rate (with crystal components used) reaches a value that can be observed with an electron microscope (i.e. over 0.01 mm / s) is 600–7008C. At temperatures below 6008C, the time necessary for the formation of the chemical reaction product phase is more than 1 h. The investigations showed that the composition of phases formed by in-situ annealing depends on reaction temperature. For example, at temperature below 6008C, the formation of the Ni 2 Si phase is observed at the boundary between the nickel particles and the single crystal silicon film (Fig. 1). The reaction front propagates into the single crystal silicon film. The size of the crystal blocks of the formed Ni 2 Si phase is 0.2–0.5 mm and has no strict orientation relation to the single-crystal silicon substrate.
Fig. 1. TEM microphotograph of the Ni 2 Si phase formed on the surface of a single-crystalline silicon film during annealing at temperature up to 6008C.
Annealing of the silicon single-crystal film with nickel particles did not lead to the formation of any phases except Ni 2 Si. If the temperature is increased to 7008C after the Ni 2 Si silicide phase is formed, we observe the formation of the disilicide phase which grows epitaxially into the singlecrystal silicon. If the heating is produced by a highintensity electron beam, we also observe the formation of isles of the disilicide phase in front of the propagating boundary of the growing phase NiSi 2 (Fig. 2). The growth of the NiSi 2 phase is accompanied by the ‘dissolution’ of Ni 2 Si phase and the formation of pores at the Ni 2 Si–NiSi 2 interface. If the annealing of the system nickel particle-silicon is initially carried out at 7008C, only the NiSi 2 phase is observed to have formed and grown during the interaction. The growing nickel disilicide phase is shaped as a square and has a strict orientation relation to the (100) single crystal silicon film, as can be seen in electron diffraction patterns (Fig. 3). The increase in size of the growing NiSi 2 phase is accompanied by the appearance of contrast characteristic of dislocations (Fig. 4). A temperature decrease to 508C causes some decrease of the number of dislocation lines in the nickel disilicide phase. The growth of the disilicide phase is not only accompanied by the formation of dislocations in the NiSi 2 phase. An increase in the size of NiSi 2 crystals causes also the formation of dislocations on the NiSi 2 –Si interface in the silicon phase (Fig. 5). The movement of dislocations in the disilicide and silicon phase during formation of the NiSi 2 phase was
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Fig. 4. TEM microphotograph of the dislocations in NiSi 2 crystals formed during annealing of the system: nickel particle / single crystalline silicon film at 7008C. Fig. 2. Electron microphotograph of the Ni 2 Si–NiSi 2 interface formed during annealing of the Ni 2 Si–Si system at 7008C. Epitaxial growth of the NiSi 2 phase in a single-crystalline silicon foil is observed.
observed directly during our in-situ electron microscopic investigation. It should be noted that we did not observe the formation
of any amorphous phase at the interface between nickel silicide and silicon during the formation of silicide phases when annealing single crystalline silicon with nickel particles deposited onto its surface.
3.2. In situ investigation of phase formation due to the interaction of a single-crystal silicon (100) foil with a thin nickel film In-situ studies of the interaction of (100) silicon with a ˚ thick) deposited onto the thin nickel film (about 20–50 A
Fig. 3. SAD patterns from the Si–NiSi 2 interface formed in the annealing of the system: nickel particle / single-crystalline silicon film at 7008C. The reflections of nickel disilicide phase exhibit a strict orientation relationship to the reflections of the initial single crystal silicon substrate: (100) Si i (100) NiSi 2 and [100] Si i [100] NiSi 2 .
Fig. 5. TEM microphotograph of dislocations formed in the silicon phase near the NiSi 2 –Si interface.
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Fig. 6. TEM microphotograph of crystal (100) silicon with thin deposited nickel particles (film).
Fig. 8. TEM microphotograph of square isles of the NiSi 2 phase formed during annealing at a temperature above 6008C of (100) crystalline silicon with a deposited nickel film.
silicon surface (Fig. 6) showed that the sequence of phase formation during annealing is identical to that observed in the annealing of the system nickel particle-single crystalline silicon foil. Annealing for 15–60 min at temperatures up to 4008C causes the formation of the Ni 2 Si phase (Fig. 7). Annealing at a temperature above 6008C is accompanied by the growth of square isles of the NiSi 2 phase (Fig. 8). The orientation relations of these isles with the single-crystalline silicon foil are as follows: (100) Si i (100) NiSi 2 and [100] Si i [100] NiSi 2 . An increase of annealing time causes a complete disappearance of nickel. The surface (100) of the single-crystalline silicon is completely
covered by the isles of nickel dicilicide crystals. The size of the formed crystallites of the phase after complete consumption of nickel is 20–100 nm. The annealing at 6008C of the system nickel film–single-crystalline silicon foil, in which the crystals of silicide phase Ni 2 Si were formed at low temperature, causes the growth of nickel disilicide phase. The front of disilicide phase crystallization propagates into the silicon single crystal, while the formation of pores is observed at the interface Ni 2 Si– NiSi 2 , until the contact between Ni 2 Si phase and the growing phase is completely destroyed. When investigating phase formation in the system thin nickel film–single-crystalline silicon, we came across the rather interesting experimental fact that the irradiation with a beam of accelerated electrons in the microscope has a substantial effect on the rate of solid-phase interaction and on the sequence of phase formations. For example, during the annealing of the system thin nickel film–single-crystalline silicon at 4008C, the Ni 2 Si phase is formed within several tens minutes, while the annealing of the same system at 4008C with simultaneous irradiation with a 200 kV electron beam causes not only a decrease of the time of silicide phase formation but also leads to the formation of an epitaxially growing NiSi 2 phase (Fig. 9) instead of Ni 2 Si. An increase of NiSi 2 phase formation rate is also observed with annealing at 600–7008C. An increase of the rate of disilicide phase formation during isothermal annealing is observed also in the case when the combination (thin nickel film–single-crystalline silicon foil) was preliminarily irradiated in the electron microscope at low temperatures (1008C), after which it was isothermally annealed at 4008C without irradiation. In this case we also observed prevailing formation of NiSi 2 phase at the irradiated regions.
Fig. 7. TEM microphotograph of the Ni 2 Si phase formed during annealing of (100) silicon crystal with the deposited nickel particles at a temperature up to 4008C.
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Fig. 9. Formation of an epitaxially growing NiSi 2 phase formed during heating (4008C) and irradiation with electron beam of the system thin nickel film / single-crystalline silicon (the formation of isles of the Ni 2 Si phase is observed in the irradiated region).
3.3. In-situ investigation of phase formation due to the interaction of amorphous silicon with nickel particles Electron microscopic studies of structural and morphological characteristics of in-situ interaction of amorphous silicon with nickel particles showed that phases formation is observed to proceed at a noticeable rate within temperature range 600–7008C. Annealing of the amorphous silicon film with the deposited nickel particle leads to the formation of layers composed of the interaction products. The propagation of the reaction front (formation of silicide phases) occurs inside the amorphous silicon film. It was discovered that the sequence of phase formation depends on the method by which the solid-phase interaction is initiated, and it is more complicated than that observed in the interaction of nickel with single-crystalline silicon. For example, if heating of the sample was carried out with the help of a special electron microscopic sample heating holder (in this case, the temperature of the amorphous film is equal to that of the nickel particle on its surface), at least three reaction zones can be noticed between the nickel particle and amorphous silicon film (Fig. 10). According to electron diffraction data, the sequence of phases when approaching the nickel particle is as follows: NiSi 2 , NiSi, Ni 3 Si 2 . The width of the zones of the individual phases formed during the interaction of amorphous silicon with the nickel particle exhibits depends only weakly on annealing time and is 1–2 mm for all the
Fig. 10. Formation of multilayered region structure of the products during interaction of amorphous silicon and nickel particle under isothermal annealing. According to the SAD patterns, the first region (most remote from the nickel particle) corresponds to the formation of the NiSi 2 silicide phase, the second layer corresponds to the formation of the NiSi silicide phase and the third layer corresponds to the formation of the Ni 3 Si 2 silicide phase.
observed phases. It should be noted that we did not observe the formation of any transition amorphous or liquid phases. The interface between the first growing silicide phase and the amorphous silicon, though containing some steps, is rather sharp. If the reaction was initiated by an electron beam (in this case, the particle temperature can be much higher than film temperature), the formation of only two phases was observed during the initial stages of interaction of the nickel particle with the amorphous silicon (Fig. 11). According to electron diffraction data, these are the silicide phases Ni 2 Si and NiSi 2 . As reaction proceeds, the formation of the Ni 3 Si 2 phase is observed at the interface between the two silicide phases Ni 2 Si and NiSi 2 (Fig. 12) that were formed during the initial stages of annealing. The propagation of the reaction front is accompanied by an increase of the size of the Ni 3 Si 2 phase and a simultaneous decrease of the size of the Ni 2 Si phase. It should be noted that the rate of reaction front propagation during the formation of the nickel disilicide on single-crystal substrates is much lower than that during the interaction of nickel particles with amorphous silicon. In spite of the fact that the kinetic characteristics of the reaction front propagation cannot be plotted at present, one
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Fig. 11. Formation of two layers of silicide phases (Ni 2 Si and NiSi 2 ) during electron beam heating of the system amorphous silicon–nickel particle.
can state that the propagation of the reaction front slows down while the size of reaction zone increases.
4. Discussion of results In-situ investigation of phase formation due to the interaction of (100) crystalline silicon with nickel showed
Fig. 12. Formation of the Ni 3 Si 2 phase at the Ni 2 Si–NiSi 2 interface during thermal annealing of amorphous silicon and a nickel particle heated by an electron beam.
that the sequence of phase formation observed during the annealing of this system at different temperatures is the same as that known before. At low temperatures, the first and predominant phase growing during the annealing is Ni 2 Si while at higher temperature (600–7008C) the predominant and the only one phase is the nickel disilicide. Unlike the interaction of crystalline silicon with nickel particles, the reaction between amorphous silicon and nickel particles at 600–7008C leads to the formation of several crystalline silicide phases: NiSi 2 , NiSi, Ni 3 Si 2 . The formation of a larger number of phases due to the interaction of amorphous silicon with nickel particles can have several reasons. It is most probable that the formation of silicide phases is determined both by the rate of reaction front propagation and the rate of nickel atoms diffusion to the interface. At the initial stages, diffusion of nickel may take place at the interface Ni–amorphous Si, and then also at the interfaces NiSi 2 –amorphous Si and Ni–NiSi 2 . In our opinion, the rate of nickel diffusion in amorphous silicon is much faster than the rate of nickel diffusion in crystalline silicon which allows to form a wider diffusion front. Higher rates of reaction front propagation lead to the possibility of several observed silicide phases to crystallise. Nevertheless, we think that additional studies are necessary to find the exact reasons why several silicide phases are formed. A common feature of the formation of silicide phases during annealing of both the crystalline and the amorphous silicon with nickel is preferential diffusion of nickel. This is unambiguously confirmed by the observed reaction front propagation inside the crystalline (or amorphous) silicon during annealing. Another interesting fact discovered during the studies is the formation of dislocations in the epitaxially growing nickel disilicide phase and single crystalline silicon at the NiSi 2 –Si interface. It is well known that the strain at the interface may be due to several factors, including noncorrespondence between the crystalline lattices and the difference in thermal linear expansion coefficients of the substrate material and the growing (depositing) phase. Strains, arising as a result of silicide formation, differ from each other depending on what metal is used to form the silicide phase. It is accepted that the strain arising at the interface during the formation of nickel disilicide is much smaller than that for other metals [17]. The reason of lower strain is the epitaxial growth and the similarity of the lattice parameters of the nickel disilicide and silicon. According to the data obtained by us, in the processes of nickel disilicide formation and epitaxial growth, dislocations are formed in crystal when it reaches the size of about 1 mm. In our opinion, the reason of the observed dimensional effect is the fact that the increase of the size of nickel disilicide crystal is accompanied by anm increase of strain at the Si–NiSi 2 interface. This leads to of plastic deformation causing the formation and movement of dislocations both in silicide phase and in single-crystal silicon matrix. Besides, theoretical considerations of the
B. Bokhonov, M. Korchagin / Journal of Alloys and Compounds 319 (2001) 187 – 195
interface energy for coherent isles of the new phase reported in Ref. [18] showed that dimensional effect really exists, in the case when the formation of dislocations at the interface becomes favoured from the energy point of view. Besides, we discovered that the decrease of temperature to 508C leads to some decrease of the number of dislocation lines in the nickel disilicide. This effect may be due to the fact that the thermal linear expansion coefficients of silicides are much higher than those of metals and silicon. For example, the thermal linear expansion coefficient for nickel disilicide is 12.06310 26 C 21 while for silicon it is 3310 26 C 21 [17]. This difference in thermal linear expansion coefficients most likely causes a decrease of strain at the interface and re-distribution of dislocations in the silicide phase. The acceleration of solid-phase chemical reaction accompanied by changes in phase composition in the irradiated samples can be explained by several reasons. It is known that, as a rule, the primary act in the interaction of high-energy particles with the substance is ionization. The probability of formation of point defects, namely, Frenkel pairs, in the case when high-energy particle has a small mass (for example, like electron), is negligibly small in comparison with the probability of formation of secondary electrons. The excitation followed by relaxation of the electron subsystem in semiconductors leads to the formation of long-lived electron defects. Besides, relaxation of electron excitations occurs. Thus, the obtained Frenkel pairs usually exhibit sufficient mobility and easily form different complexes including bi-vacancies, vacancies localized at impurities, dislocation loops, rod-like defects, etc. [19–23]. These are the final products of irradiation and determine the physicochemical properties of the substance. So, the increase of reaction rate can be compared with the change of the concentration of defects in silicon arising during the irradiation with a beam of accelerated electrons. It is natural that the increase in defect concentration in silicon should lead to an acceleration of nickel diffusion in the silicon lattice. It is probable that an increase of the number of defects (acceleration of diffusion processes) also determines the changes of phase composition of reaction products formed during low-temperature annealing of the investigated system (nickel–silicon). Nevertheless, in order to reveal the true reasons of the changes of phase composition of products formed in the interaction of nickel with silicon, additional investigations are necessary.
5. Conclusions In situ investigations have shown that the sequence of phase formation during the interaction of nickel particles with single-crystalline (100) silicon foils and amorphous silicon films can be characterized as follows: Within the temperature range up to 5008C, Ni 2 Si is the first and predominant phase to be formed. The formation and
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epitaxial growth of the NiSi 2 phase is observed during annealing at temperatures above 6008C. The growth of the nickel disilicide crystalline phase is accompanied by the formation of dislocations both in the nickel disilicide and the silicon phase. Unlike the sequence of phase formation during the interaction of crystalline silicon with nickel particles, the interaction of an amorphous silicon film with nickel particles at temperatures above 6008C leads to the crystallization of several phases: NiSi 2 , NiSi, Ni 3 Si 2 . The formation of silicide phases due to the interaction of nickel particles with silicon during annealing did not confirm the formation of an intermediate amorphous silicide which was observed earlier in thin-film nickel–silicon systems. Irradiation with a beam of accelerated electrons in a microscope was found to cause an increase of the rate of silicide phase formation and to a decrease of the formation temperature of the epitaxially growing nickel disilicide phase, at least to 4008C. In our opinion, the observed effect can be connected with the formation of defects in the structure of single-crystalline silicon.
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