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High-resolution and in situ tem studies of annealing of Ti-Si multilayers
Journal
of the Less-Common
Metals,
140 (1988)
139
- 148
139
HIGH-RESOLUTION AND IN SITU TEM STUDIES OF Ti-Si MULTILAYERS* KAREN
HOLLOWAY
Department CA 94305
and ROBERT
of Materials (U.S.A.)
Science
OF ANNEALING
SINCLAIR
and Engineering,
Stanford
University,
Stanford,
summary The solid state amorphization reaction in sputtered amorphous-siliconpolycrystalline-titanium multilayers has been studied in cross-section by high-resolution transmission electron microscopy (TEM). Heat treatments included rapid thermal annealing and in situ heating in the microscope. The bilayer spacing of the sample is 25.0 nm and the composition is 50at.%Ti5Oat.%Si. Growth of the alloy is strictly planar. The amorphization does not occur preferentially along titanium grain boundaries. The reaction is diffusion controlled with an activation energy of about 2.0 eV atom-‘. When the layers are partially reacted, the amorphous alloy composition is silicon rich, with an estimated Ti:Si atomic ratio of 1:2. dnce all of the elemental silicon is consumed the alloy incorporates more titanium, reaching a composition of about Ti&Si,,. In situ studies reveal that Kirkendall voids form at this point in place of the silicon layers. In situ annealing studies duplicate the microstructure observed in the bulk-annealed samples; from this we conclude that bulk diffusion dominated in the thin TEM foil.
1. Introduction Phase formation in refractory-metal-silicon systems is of current technological interest, since the high stabilities and low resistivities of the metal silicides make them suitable for use in integrated circuit interconnection schemes [l]. The equilibrium titanium disilicide (C54 TiSi,), which melts at 1542 “C and has a resistivity of around 20 @ cm, is a primary candidate for this application [l]. The growing number of reports of solid state amorphization in binary systems has led investigators in this field, which is not traditionally concerned with amorphous alloy formation, to examine the metal-silicon systems more closely for this reaction at relatively low *Paper Los Alamos,
presented at the Conference NM, August 10 - 13,1987.
on Solid
@ Elsevier
State
Amorphizing
Sequoia/Printed
Transformations,
in The Netherlands
140
temperatures. The amorphization of a crystalline-rhodium-amorphoussilicon bilayer was observed while annealing the material in situ in a transmission electron microscope [2]. The formation of an Ni-Si amorphous alloy has been noted using X-ray diffraction [3] and transmission electron microscopy (TEM) [41,and the formation of a Pt-Si amorphous alloy on deposition of platinum onto a single-crystal silicon substrate has also been observed [ 51. Finally, the authors have reported that TEM studies of sputtered crystalline titanium and amorphous silicon multilayers with bilayer spacings of about 10 nm have shown that an amorphous titanium-silicon alloy forms upon rapid thermal annealing at 450 “C - 550 “C!, temperatures below which crystalline tit~ium silicides nucleate 161. Thus, titanium-silicon has been added to the growing list of systems which undergo amorphization by a solid state reaction. We have extended our investigation of the formation of the amorphous Ti-Si alloy to study the kinetics of this reaction with high resolution electron microscopy (HREM) ‘and to observe the reaction mechanism in situ during annealing of a TEM cross-section sample in the microscope. Cross-section TEM is powerful and direct in its ability to reveal the microstruct~e of the multilayers and the physical extent of reaction. The planar nature of the solid state amorphization reaction, which was first reported in Zr-Co multilayers [7], could only be demonstrated by crosssection TEM. The high-resolution instrument allows smaller topological features to be examined, thus allowing a more detailed description of the structure. In situ heating experiments allow the dynamic observation of the reaction in real time. Transient events, such as the nucleation of voids, can be studied in this manner; such events would probably be missed in a sequence of bulk annealed samples. Thus, provided that the results can be shown to be representative of bulk behavior, itz situ studies complement bulk studies in elucidating mechanisms of annealing behavior. Qu~titative kinetic information can potentially be derived; however, the presence of the TEM foil surfaces near the reacting volume and uncertainties in local temperature measurement must be carefully accounted for during the experiment [8]. The in situ results reported here are qualitative.
2. Experiment Titanium-silicon multilayers were sputter deposited in a cryogenically pumped system equipped with a rotating table (described elsewhere [9 1). Altemat~g layers of titanium and silicon were deposited at room temperature onto organically and inorganically cleaned 3 in (100) silicon wafers. The relative thicknesses of titanium and silicon were controlled by the power applied to the targets; the bilayer spacing of the multilayer was controlled by the rotation speed of the table. The overall composition of the film is close to 50 at.% titanium, 50 at.% silicon, as me&ured by Rutherford
141
backscattering spectrometry, and the bilayer spacing is 25.0 nm. Base vacuum pressure was 6 X lo-’ Torr; the argon sputter gas pressure was 2.2 mTorr. The substrate temperature did not rise above 50 - 100 “C during deposition, since the samples spent only a fraction of the time under the targets. The film consisted of 30.5 bilayers, with silicon deposited first and last. A three-inch wafer bearing the multilayer film was scribed into 3/4 inch square coupons. Their bilayer spacings were measured from low-angle X-ray diffraction patterns obtained using a four-circle Picker diffractometer using Cu Ka radiation. The thickness varied by about 10% from the center to the edge of the wafer owing to geometry and variation in sputter deposition rates under the targets. The coupons were rapid-thermal annealed in an AG Associates Heatpulse 610 in flowing argon (purity, 99.998%) for 30 seconds at temperatures ranging from 300 “C to 550 “C. Cross-section samples of annealed and as-deposited multilayers were prepared for TEM analysis by mechanical thinning and ion-beam milling using a low-temperature (2’ < 90 “C) variation of the technique developed by Bravman and Sinclair [lo, 111. A specimen of the asdeposited film was also annealed in situ in the microscope using a Philips PW6592 heating holder equipped with a resistively-heated stage. The temperature was measured with a thermocouple built into the holder and located next to the specimen. The temperatures reported herein are nominal; measured temperature can differ from the true local value typically by about 50 “C [S] . The problem of specimen drift during heating is circumvented by recording the images on video tape at a frame rate of 30 per second. All experiments were performed in an intermediate voltage TEN (a Philips EM430ST operating at 300 kV) with a resolution of 0.22 nm. The cross-sections were aligned perfectly with respect to the beam using the low-angle electron diffraction pattern which arises from the bilayer periodicity of the film.
3. Results and discussion 3.1. High-resolution kinetic study A high-resolution TEM micrograph of the as-deposited Ti-Si multilayer is shown in Fig. 1. The 0.22 nm resolution of the microscope allows imaging of the hexagonal cr-Ti crystalline lattice. A high degree of texture (OliO in the growth direction) allows at least one set of fringes to be imaged in most grains. Grain boandaries can be distinguished by discontinuities of the fringe pattern. The lighter silicon layers have the typical amorphous HREM appearance. At each cr-Ti-a-Si interface, an amorphous interlayer 2.9 nm in thickness has formed. The presence of these interlayers demonstrates that an amorphous Ti-Si alloy has formed on sputter deposition of the layers. Close examination of the amorphous-alloy-crystalline-titanium interface (Fig. 1, insert) reveals that the amorphous alloy does not form preferentially at grain
142
Fig. 1. Cross-section high-resolution TEM micrograph of the unannealed ‘I%-Si multilayer structure. The crystal lattice of the hexagonal o-Ti is resolved in the darker layers; the strong OliO texture of the grains allows this set of fringes (d = 0.26 nm) to be imaged in almost all grains. The silicon (lighter) layers are amorphous. An amorphous Ti-Si alloy 3.0 nm in thickness is clearly seen at the Ti-Si interfaces.
boundaries in the as-deposited structure. Intrusions of amorphous material at the ar-Ti grain boundaries, which would be expected if that were the case, are not observed. Instead, a largely planar alloy layer is formed. An isochronal series of samples was rapid-thermal annealed for 30 s at 300 “C, 350 “C!, 375 “C, 400 “C, 450 “C and 550 “C. At 300 “C, the amorphous interlayer is only 2.4 nm thick, as compared with 2.9 nm in the as-deposited layers. The reason for this apparently anomalous result is unknown; a 20% contraction in the amorphous alloy layer thickness is too large to be due to structural relaxation. Thirty second anneals at 350 “C, 375 “C and 400 “C yielded planar growth of the amorphous alloy layer. Figure 2 shows the microstructures of the multilayer as they evolve with temperature. The width of the interlayers was measured from each micrograph, using the silicon (111) interplanar spacings (0.31 nm) of high-resolution images of the (110) axis of the silicon substrate for an accurate magnification calibration. The interlayer thickness X, alloy growth AX (subtracting the as-deposited alloy width) and the diffusion constant from a (Dt)1’2 relation were calculated for each case (see Table 1). As the solid state amorphization reaction has been shown to be diffusion-controlled in several systems [ 12,131, an activation energy was calculated from these data assuming a t 1’2 growth law. Although data were obtained at only three temperatures, they
Fig. 2. Cross-section high-resolution TEM micrographs of the Ti-Si multilayers which were rapid-thermal annealed in bulk for 30 s at 350 ‘C, 375 “c and 400 “C. Planar growth occurred in each case. TABLE 1 Planar growth of the amorphous Ti-Si alloy
W)
Temperature
Alloy (nm)
350 375 400
3.70 4.47 6.05
width x
Ax (nm)
D (cm2 s-l)
0.80 1.57 3.15
2.13 x lo-l6 8.22 x lo-l6 3.31 x 10-15
fit a linear dependence with t 1’2quite well (Fig. 3). Our preliminary analysis gives an activation energy of about 2.0 eV atom-‘. This value is higher than that previously reported for the Ti-Si reaction at low temperatures. Chambers et al. [14] studied a Ti-Si diffusion couple at 275 “C and 340 “C with X-ray photoelectron spectroscopy (XPS) and derived an activation energy of 1.0 + 0.4 eV atom-‘. However, this value was obtained from the reaction rates at only two temperatures. Remarkably, an activation energy of 2.0 eV is comparable with that measured for the growth of crystalline titanium disilicide at higher temperatures (1.8 +_0.1 eV [15]). The diffusion constants are lo6 times larger than crystalline disilicide formation would be at these temperatures, and are more consistent with the values measured by XPS [14]. Although the reaction rates in the present TEM study were determined using only one annealing time, and the activation energy is derived by assuming Arrhenius behavior at only three temperatures, the measurement of the extent of reaction from TEM micrographs is direct and accurate. The value of about 2.0 eV atom-’ can be given with a fair degree of confidence. A very high pre-exponential factor is derived from the Arrhenius relation (II, = 2.35 cm2 s-l) for the high reaction rates at low temperatures. After rapid thermal annealing at 450 “C for 30 seconds, amorphous TiSi alloy layers 7.5 nm in thickness have formed (Fig. 4). At this temperature,
144
4
1.46
1.50
1.52
1.54
1.56
1.50
1.60
1.62
l/T(lO"K“)
Fig. 3. Arrhenius plot of the planar growth rates of the amorphous Ti-Si alloy in the temperature range of 350 - 400 “C.
the silicon has been completely consumed by the growth of the alloy. A nearly continuous 2.0 - 2.5 nm band of crystalline cr-Ti remains, as is evident from the lattice image in the center of each layer. Rows of Kirkendall voids with an average diameter of 4.4 nm, formed by the interdiffusion process, are present in the place of the amorphous silicon layers. The presence and location of the voids confirms that silicon is the more mobile element in this process, as is the case for the growth of the crystalline titanium silicide [ 161. Such voids also form as a consequence of the amorphization reaction in evaporated Co-Zr multilayers [ 71. A cubic crystalline silicide nucleated in the multilayer films after a 30second anneal to 550 “C. This phase, which was identified from electron diffraction patterns from a corresponding through-foil TEM specimen, is f.c.c. with a 1.3 nm lattice parameter. It has been reported to occur in amorphous co-sputtered equiatomic Ti-Si films after annealing at 650 “C [ 171. 3.2. Composition of the amorphous Ti-Si alloy The composition of the amorphous alloy was estimated from the rate at which the o-titanium and amorphous silicon layers are consumed in the reaction. The atomic ratio in the as-deposited intermixed layers and in the amorphous alloy layers during planar growth is roughly 1:2 Ti:Si. Once the elemental silicon is consumed, as is the case for the sample annealed at 450 “C, the alloy incorporates more titanium, reaching a composition of about Tir)$iSS. This second step in the solid state amorphizing process, which occurs once one of the pure elements is no longer present in the system, has been described in the Co-Zr [ 171 and Ni-Zr [ 181 multilayer systems. In an earlier TEM study of Ti-Si multilayers with a 10 nm bilayer spacing 163, the amorphous alloy composition was reported to be roughly
145
Fig. 4. Cross-section high-resolution TEM micrograph of the Ti-Si multilayers which were rapid-thermal annealed in bulk for 30 s at 450 “C. The amorphous alloy layer is 7.5 nm thick; a 2.0 - 2.5 nm layer of o-Ti remains in the center of each titanium layer. Rows of Kirkendall voids with an average diameter of 4.4 nm are present in the place of the amorphous silicon layers.
equiatomic once titanium was consumed in a 40at.%Ti-GOat.%Si multilayer sample. Also, at 455 “C and 550 “C!,a sample with an overall composition of GOat.%Ti-40at.%Si became completely amorphous. The range of compositions observed in these studies, from Ti,,Si,, to about Ti33Si66, is very different from the narrow range (around Tis&$,,) found in titanium-silicon alloys formed by rapid liquid quenching [19]. The crystallization temperatures of the equiatomic or more silicon-rich alloys are 550 “C or below, but the Ti,,$&, alloy is stable at that temperature [6], indicating greater stability in the more titanium-rich alloy. 3.3. In situ TEM study The amorphization reaction was observed in situ in a cross-section TEM sample heated in the microscope. The sequence and morphology of the reaction observed in situ correspond exactly to those observed in cross-section samples fabricated from bulk-annealed material. This experiment allowed us to follow closely the various stages of the amorphization reaction. Figures 5(a) - (c) show the Ti-Si multilayers at room temperature and at nominal temperatures of about 510 “C and 590 “C respectively. Each of these figures was produced by photographing the videotape on playback using a halfsecond exposure, averaging 15 frames. The crystalline cr-Ti region is
146
Fig. 5. Cross-section TEM video photographs of the Ti-Si multilayers at various stages during the in situ annealing experiment. (a) Before annealing, an initial 3.0 nm amorphous alloy layer is apparent at each interface. The crystalline o-Ti region is distinguished in the darker layers by the various diffracting conditions of the titanium grains. (b) At a nominal temperature of 510 “C, the interface between the a-Si and the amorphous alloy shows a roughened appearance. (c) At a nominal temperature of 590 “C, Kirkendall voids are forming between neighboring layers. A band of crystalline titanium remains in the centers of the layers.
distinguished in the darker layers by the various diffracting conditions of the titanium grains. The amorphous alloy is seen to grow in a planar fashion in the nominal temperature range of 380 to 490 “C. After 18 minutes at 510 “C (Fig. 5(b)), the interface between the amorphous alloy layers and the remaining amorphous silicon had developed a roughness of about 1 nm, giving the interfaces a ‘wavy’ appearance. When the alloy layers had grown until the silicon was completely consumed (15 min at 590 “C!), the interfaces from neighboring layers met and voids formed in the troughs of the ‘waves’ (Fig. 5(c)). A thin, continuous band of crystalline titanium remains in the middle of each layer, as was observed in the bulk study. At temperatures above 600 “C the voids coarsen. At a nominal temperature of 770 “C, crystalline silicide was seen to heterogeneously nucleate in the amorphous alloy at the void surfaces. The nucleation site of the crystalline silicide could be
147
determined only by the in situ experiment, as the crystalline disilicide grows rapidly once nucleated [ 201. In an in situ experiment, care must be taken to consider the proximity of the surfaces of the TEM foil to any point in the material. The kinetics and perhaps the morphology of the reaction may change due to surface diffusion and capillarity effects. However, in this case, the formation of Kirkendall voids in the TEM sample confirms that bulk diffusion is taking place during the course of the in situ experiment. If the proximity of the foil surfaces allowed surface diffusion to dominate, vacancies, or their analogue in amorphous materials, created by the unequal diffusion flux, would migrate to the surfaces of the foil rather than coalesce into voids. However, each reaction stage occurs at a much higher temperature than that observed in bulk, indicating that the kinetics of the reaction is retarded by the presence of the foil surfaces or the local temperature is relatively cool.
4. Conclusions The formation of a Ti-Si amorphous alloy has been studied in multilayers with a bilayer spacing of 25.0 nm and equiatomic composition by annealing in bulk and in situ in the transmission electron microscope. The sequence and morphology of the reaction is the same in both cases. Highresolution TEM shows that the alloy growth is strictly planar, showing no marked intrusions of amorphous material in the a-titanium grain boundaries. The reaction is diffusion controlled, with an activation energy of about 2.0 eV atom- ’ in the temperature range of 350 - 400 “C. The composition of the growing alloy layers was silicon-rich, with an atomic ratio of about 1:2 Ti:Si. When the elemental silicon is consumed, the alloy incorporated more titanium as homogenization took place. At this point, rows of Kirkendall voids formed in the place of the silicon layers. At 550 “C, a cubic crystalline silicide had nucleated. Observation of the amorphization reaction during annealing in the microscope has proven to be very useful in revealing the planar nature of the reaction, the formation of voids when the silicon has been consumed, and the nucleation of the crystalline silicide on the void surfaces.
Acknowledgments The authors would like to acknowledge the use of the facilities of the Materials Science and Engineering Department and the Center for Materials Research at Stanford University. Funding has been provided by the NSFMRL through the Center for Materials Research. We thank Kenneth P. MacWilliams (Stanford University) for assistance with the Heatpulse 610 RTA system in the Center for Integrated Systems at Stanford University. Funding for the Philips EM430 electron microscope was kindly provided by the
148
NSF-MRL program through the Center for Materials Research, Stanford University, the Pew Foundation, the Schools of Engineering, Humanities and Sciences, and Earth Sciences, and the Departments of Electrical Engineering and Materials Science and Engineering at Stanford University.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
S. P. Murarka, in Silicides for VLSI Applications, Academic Press, New York, 1983. S. Herd, K. N. Tu and K. Y. Ahn, Appl. Phys. Lett., 42 (1983) 599. D. M. Vanderwalker, Appl. Phys. Lett., 48 (1986) 707. M. Natan, J. Vat. Sci. Technol. B, 4, (1986) 1404. J. Abelson, K. B. Kim, D. E. Mercer, C. R. Helms, R. Sinclair and T. W. Sigmon, J. Appl. Phys., 68 (1988) 689. K. Holloway and R. Sinclair, J. Appl. Phys., 61 (1987) 1359. H. Schroder, K. Samwer and U. Koster, Phys. Rev. Lett., 51 (1983) 415. R. Sinclair, M. A. Parker and K. B. Kim, Ultramicroscopy, 23 (1987) 383. T. Barbee and D. L. Keith, in E. S. Machlin and T. J. Rowland (eds.), Synthesis and Properties of Metastable Phases, AIME, New York, 1980, p. 93. J. C. Bravman and R. Sinclair, J. Electron Microsc. Tech., I (1984) 53. K. Holloway and R. Sinclair, Mater. Res. Symp. Proc., 77 (1986) 357. E. J. Cotts, W. J. Meng and W. L. Johnson, Phys. Rev. Lett., 57 (1986) 2295. H. U. Krebs and K. Samwer, Europhys. Lett., 2 (1986) 141. S. A. Chambers, D. M. Hill, F. Xu and J. H. Weaver, Phys. Reu. B, 35 (1987) 634. L. S. Hung, J. Gyulai, J. W. Mayer, S. S. Lau and M. A. Nicolet, J. Appl. Phys., 54 (1983) 5076. W. K. Chu, S. S. Lau, J. W. Mayer, J. Muller and K. N. Tu, Thin Solid Films, 25 (1975) 393. R. Beyers and R. Sinclair, J. Appl. Phys., 57 (1985) 5240. W. J. Meng, E. J. Cotts and W. L. Johnson, Mater. Res. Symp. Proc., 77 (1986) 223. D. E. Polk, A. Calka and B. C. Giessen, Acta Metall., 26 (1978) 1097. I. J. M. M. Raaijmakers, A. H. Reader and H. J. W. van Houtum, J. Appl. Phys., 61 (1987) 2527.
of the Less-Common
Metals,
140 (1988)
139
- 148
139
HIGH-RESOLUTION AND IN SITU TEM STUDIES OF Ti-Si MULTILAYERS* KAREN
HOLLOWAY
Department CA 94305
and ROBERT
of Materials (U.S.A.)
Science
OF ANNEALING
SINCLAIR
and Engineering,
Stanford
University,
Stanford,
summary The solid state amorphization reaction in sputtered amorphous-siliconpolycrystalline-titanium multilayers has been studied in cross-section by high-resolution transmission electron microscopy (TEM). Heat treatments included rapid thermal annealing and in situ heating in the microscope. The bilayer spacing of the sample is 25.0 nm and the composition is 50at.%Ti5Oat.%Si. Growth of the alloy is strictly planar. The amorphization does not occur preferentially along titanium grain boundaries. The reaction is diffusion controlled with an activation energy of about 2.0 eV atom-‘. When the layers are partially reacted, the amorphous alloy composition is silicon rich, with an estimated Ti:Si atomic ratio of 1:2. dnce all of the elemental silicon is consumed the alloy incorporates more titanium, reaching a composition of about Ti&Si,,. In situ studies reveal that Kirkendall voids form at this point in place of the silicon layers. In situ annealing studies duplicate the microstructure observed in the bulk-annealed samples; from this we conclude that bulk diffusion dominated in the thin TEM foil.
1. Introduction Phase formation in refractory-metal-silicon systems is of current technological interest, since the high stabilities and low resistivities of the metal silicides make them suitable for use in integrated circuit interconnection schemes [l]. The equilibrium titanium disilicide (C54 TiSi,), which melts at 1542 “C and has a resistivity of around 20 @ cm, is a primary candidate for this application [l]. The growing number of reports of solid state amorphization in binary systems has led investigators in this field, which is not traditionally concerned with amorphous alloy formation, to examine the metal-silicon systems more closely for this reaction at relatively low *Paper Los Alamos,
presented at the Conference NM, August 10 - 13,1987.
on Solid
@ Elsevier
State
Amorphizing
Sequoia/Printed
Transformations,
in The Netherlands
140
temperatures. The amorphization of a crystalline-rhodium-amorphoussilicon bilayer was observed while annealing the material in situ in a transmission electron microscope [2]. The formation of an Ni-Si amorphous alloy has been noted using X-ray diffraction [3] and transmission electron microscopy (TEM) [41,and the formation of a Pt-Si amorphous alloy on deposition of platinum onto a single-crystal silicon substrate has also been observed [ 51. Finally, the authors have reported that TEM studies of sputtered crystalline titanium and amorphous silicon multilayers with bilayer spacings of about 10 nm have shown that an amorphous titanium-silicon alloy forms upon rapid thermal annealing at 450 “C - 550 “C!, temperatures below which crystalline tit~ium silicides nucleate 161. Thus, titanium-silicon has been added to the growing list of systems which undergo amorphization by a solid state reaction. We have extended our investigation of the formation of the amorphous Ti-Si alloy to study the kinetics of this reaction with high resolution electron microscopy (HREM) ‘and to observe the reaction mechanism in situ during annealing of a TEM cross-section sample in the microscope. Cross-section TEM is powerful and direct in its ability to reveal the microstruct~e of the multilayers and the physical extent of reaction. The planar nature of the solid state amorphization reaction, which was first reported in Zr-Co multilayers [7], could only be demonstrated by crosssection TEM. The high-resolution instrument allows smaller topological features to be examined, thus allowing a more detailed description of the structure. In situ heating experiments allow the dynamic observation of the reaction in real time. Transient events, such as the nucleation of voids, can be studied in this manner; such events would probably be missed in a sequence of bulk annealed samples. Thus, provided that the results can be shown to be representative of bulk behavior, itz situ studies complement bulk studies in elucidating mechanisms of annealing behavior. Qu~titative kinetic information can potentially be derived; however, the presence of the TEM foil surfaces near the reacting volume and uncertainties in local temperature measurement must be carefully accounted for during the experiment [8]. The in situ results reported here are qualitative.
2. Experiment Titanium-silicon multilayers were sputter deposited in a cryogenically pumped system equipped with a rotating table (described elsewhere [9 1). Altemat~g layers of titanium and silicon were deposited at room temperature onto organically and inorganically cleaned 3 in (100) silicon wafers. The relative thicknesses of titanium and silicon were controlled by the power applied to the targets; the bilayer spacing of the multilayer was controlled by the rotation speed of the table. The overall composition of the film is close to 50 at.% titanium, 50 at.% silicon, as me&ured by Rutherford
141
backscattering spectrometry, and the bilayer spacing is 25.0 nm. Base vacuum pressure was 6 X lo-’ Torr; the argon sputter gas pressure was 2.2 mTorr. The substrate temperature did not rise above 50 - 100 “C during deposition, since the samples spent only a fraction of the time under the targets. The film consisted of 30.5 bilayers, with silicon deposited first and last. A three-inch wafer bearing the multilayer film was scribed into 3/4 inch square coupons. Their bilayer spacings were measured from low-angle X-ray diffraction patterns obtained using a four-circle Picker diffractometer using Cu Ka radiation. The thickness varied by about 10% from the center to the edge of the wafer owing to geometry and variation in sputter deposition rates under the targets. The coupons were rapid-thermal annealed in an AG Associates Heatpulse 610 in flowing argon (purity, 99.998%) for 30 seconds at temperatures ranging from 300 “C to 550 “C. Cross-section samples of annealed and as-deposited multilayers were prepared for TEM analysis by mechanical thinning and ion-beam milling using a low-temperature (2’ < 90 “C) variation of the technique developed by Bravman and Sinclair [lo, 111. A specimen of the asdeposited film was also annealed in situ in the microscope using a Philips PW6592 heating holder equipped with a resistively-heated stage. The temperature was measured with a thermocouple built into the holder and located next to the specimen. The temperatures reported herein are nominal; measured temperature can differ from the true local value typically by about 50 “C [S] . The problem of specimen drift during heating is circumvented by recording the images on video tape at a frame rate of 30 per second. All experiments were performed in an intermediate voltage TEN (a Philips EM430ST operating at 300 kV) with a resolution of 0.22 nm. The cross-sections were aligned perfectly with respect to the beam using the low-angle electron diffraction pattern which arises from the bilayer periodicity of the film.
3. Results and discussion 3.1. High-resolution kinetic study A high-resolution TEM micrograph of the as-deposited Ti-Si multilayer is shown in Fig. 1. The 0.22 nm resolution of the microscope allows imaging of the hexagonal cr-Ti crystalline lattice. A high degree of texture (OliO in the growth direction) allows at least one set of fringes to be imaged in most grains. Grain boandaries can be distinguished by discontinuities of the fringe pattern. The lighter silicon layers have the typical amorphous HREM appearance. At each cr-Ti-a-Si interface, an amorphous interlayer 2.9 nm in thickness has formed. The presence of these interlayers demonstrates that an amorphous Ti-Si alloy has formed on sputter deposition of the layers. Close examination of the amorphous-alloy-crystalline-titanium interface (Fig. 1, insert) reveals that the amorphous alloy does not form preferentially at grain
142
Fig. 1. Cross-section high-resolution TEM micrograph of the unannealed ‘I%-Si multilayer structure. The crystal lattice of the hexagonal o-Ti is resolved in the darker layers; the strong OliO texture of the grains allows this set of fringes (d = 0.26 nm) to be imaged in almost all grains. The silicon (lighter) layers are amorphous. An amorphous Ti-Si alloy 3.0 nm in thickness is clearly seen at the Ti-Si interfaces.
boundaries in the as-deposited structure. Intrusions of amorphous material at the ar-Ti grain boundaries, which would be expected if that were the case, are not observed. Instead, a largely planar alloy layer is formed. An isochronal series of samples was rapid-thermal annealed for 30 s at 300 “C, 350 “C!, 375 “C, 400 “C, 450 “C and 550 “C. At 300 “C, the amorphous interlayer is only 2.4 nm thick, as compared with 2.9 nm in the as-deposited layers. The reason for this apparently anomalous result is unknown; a 20% contraction in the amorphous alloy layer thickness is too large to be due to structural relaxation. Thirty second anneals at 350 “C, 375 “C and 400 “C yielded planar growth of the amorphous alloy layer. Figure 2 shows the microstructures of the multilayer as they evolve with temperature. The width of the interlayers was measured from each micrograph, using the silicon (111) interplanar spacings (0.31 nm) of high-resolution images of the (110) axis of the silicon substrate for an accurate magnification calibration. The interlayer thickness X, alloy growth AX (subtracting the as-deposited alloy width) and the diffusion constant from a (Dt)1’2 relation were calculated for each case (see Table 1). As the solid state amorphization reaction has been shown to be diffusion-controlled in several systems [ 12,131, an activation energy was calculated from these data assuming a t 1’2 growth law. Although data were obtained at only three temperatures, they
Fig. 2. Cross-section high-resolution TEM micrographs of the Ti-Si multilayers which were rapid-thermal annealed in bulk for 30 s at 350 ‘C, 375 “c and 400 “C. Planar growth occurred in each case. TABLE 1 Planar growth of the amorphous Ti-Si alloy
W)
Temperature
Alloy (nm)
350 375 400
3.70 4.47 6.05
width x
Ax (nm)
D (cm2 s-l)
0.80 1.57 3.15
2.13 x lo-l6 8.22 x lo-l6 3.31 x 10-15
fit a linear dependence with t 1’2quite well (Fig. 3). Our preliminary analysis gives an activation energy of about 2.0 eV atom-‘. This value is higher than that previously reported for the Ti-Si reaction at low temperatures. Chambers et al. [14] studied a Ti-Si diffusion couple at 275 “C and 340 “C with X-ray photoelectron spectroscopy (XPS) and derived an activation energy of 1.0 + 0.4 eV atom-‘. However, this value was obtained from the reaction rates at only two temperatures. Remarkably, an activation energy of 2.0 eV is comparable with that measured for the growth of crystalline titanium disilicide at higher temperatures (1.8 +_0.1 eV [15]). The diffusion constants are lo6 times larger than crystalline disilicide formation would be at these temperatures, and are more consistent with the values measured by XPS [14]. Although the reaction rates in the present TEM study were determined using only one annealing time, and the activation energy is derived by assuming Arrhenius behavior at only three temperatures, the measurement of the extent of reaction from TEM micrographs is direct and accurate. The value of about 2.0 eV atom-’ can be given with a fair degree of confidence. A very high pre-exponential factor is derived from the Arrhenius relation (II, = 2.35 cm2 s-l) for the high reaction rates at low temperatures. After rapid thermal annealing at 450 “C for 30 seconds, amorphous TiSi alloy layers 7.5 nm in thickness have formed (Fig. 4). At this temperature,
144
4
1.46
1.50
1.52
1.54
1.56
1.50
1.60
1.62
l/T(lO"K“)
Fig. 3. Arrhenius plot of the planar growth rates of the amorphous Ti-Si alloy in the temperature range of 350 - 400 “C.
the silicon has been completely consumed by the growth of the alloy. A nearly continuous 2.0 - 2.5 nm band of crystalline cr-Ti remains, as is evident from the lattice image in the center of each layer. Rows of Kirkendall voids with an average diameter of 4.4 nm, formed by the interdiffusion process, are present in the place of the amorphous silicon layers. The presence and location of the voids confirms that silicon is the more mobile element in this process, as is the case for the growth of the crystalline titanium silicide [ 161. Such voids also form as a consequence of the amorphization reaction in evaporated Co-Zr multilayers [ 71. A cubic crystalline silicide nucleated in the multilayer films after a 30second anneal to 550 “C. This phase, which was identified from electron diffraction patterns from a corresponding through-foil TEM specimen, is f.c.c. with a 1.3 nm lattice parameter. It has been reported to occur in amorphous co-sputtered equiatomic Ti-Si films after annealing at 650 “C [ 171. 3.2. Composition of the amorphous Ti-Si alloy The composition of the amorphous alloy was estimated from the rate at which the o-titanium and amorphous silicon layers are consumed in the reaction. The atomic ratio in the as-deposited intermixed layers and in the amorphous alloy layers during planar growth is roughly 1:2 Ti:Si. Once the elemental silicon is consumed, as is the case for the sample annealed at 450 “C, the alloy incorporates more titanium, reaching a composition of about Tir)$iSS. This second step in the solid state amorphizing process, which occurs once one of the pure elements is no longer present in the system, has been described in the Co-Zr [ 171 and Ni-Zr [ 181 multilayer systems. In an earlier TEM study of Ti-Si multilayers with a 10 nm bilayer spacing 163, the amorphous alloy composition was reported to be roughly
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Fig. 4. Cross-section high-resolution TEM micrograph of the Ti-Si multilayers which were rapid-thermal annealed in bulk for 30 s at 450 “C. The amorphous alloy layer is 7.5 nm thick; a 2.0 - 2.5 nm layer of o-Ti remains in the center of each titanium layer. Rows of Kirkendall voids with an average diameter of 4.4 nm are present in the place of the amorphous silicon layers.
equiatomic once titanium was consumed in a 40at.%Ti-GOat.%Si multilayer sample. Also, at 455 “C and 550 “C!,a sample with an overall composition of GOat.%Ti-40at.%Si became completely amorphous. The range of compositions observed in these studies, from Ti,,Si,, to about Ti33Si66, is very different from the narrow range (around Tis&$,,) found in titanium-silicon alloys formed by rapid liquid quenching [19]. The crystallization temperatures of the equiatomic or more silicon-rich alloys are 550 “C or below, but the Ti,,$&, alloy is stable at that temperature [6], indicating greater stability in the more titanium-rich alloy. 3.3. In situ TEM study The amorphization reaction was observed in situ in a cross-section TEM sample heated in the microscope. The sequence and morphology of the reaction observed in situ correspond exactly to those observed in cross-section samples fabricated from bulk-annealed material. This experiment allowed us to follow closely the various stages of the amorphization reaction. Figures 5(a) - (c) show the Ti-Si multilayers at room temperature and at nominal temperatures of about 510 “C and 590 “C respectively. Each of these figures was produced by photographing the videotape on playback using a halfsecond exposure, averaging 15 frames. The crystalline cr-Ti region is
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Fig. 5. Cross-section TEM video photographs of the Ti-Si multilayers at various stages during the in situ annealing experiment. (a) Before annealing, an initial 3.0 nm amorphous alloy layer is apparent at each interface. The crystalline o-Ti region is distinguished in the darker layers by the various diffracting conditions of the titanium grains. (b) At a nominal temperature of 510 “C, the interface between the a-Si and the amorphous alloy shows a roughened appearance. (c) At a nominal temperature of 590 “C, Kirkendall voids are forming between neighboring layers. A band of crystalline titanium remains in the centers of the layers.
distinguished in the darker layers by the various diffracting conditions of the titanium grains. The amorphous alloy is seen to grow in a planar fashion in the nominal temperature range of 380 to 490 “C. After 18 minutes at 510 “C (Fig. 5(b)), the interface between the amorphous alloy layers and the remaining amorphous silicon had developed a roughness of about 1 nm, giving the interfaces a ‘wavy’ appearance. When the alloy layers had grown until the silicon was completely consumed (15 min at 590 “C!), the interfaces from neighboring layers met and voids formed in the troughs of the ‘waves’ (Fig. 5(c)). A thin, continuous band of crystalline titanium remains in the middle of each layer, as was observed in the bulk study. At temperatures above 600 “C the voids coarsen. At a nominal temperature of 770 “C, crystalline silicide was seen to heterogeneously nucleate in the amorphous alloy at the void surfaces. The nucleation site of the crystalline silicide could be
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determined only by the in situ experiment, as the crystalline disilicide grows rapidly once nucleated [ 201. In an in situ experiment, care must be taken to consider the proximity of the surfaces of the TEM foil to any point in the material. The kinetics and perhaps the morphology of the reaction may change due to surface diffusion and capillarity effects. However, in this case, the formation of Kirkendall voids in the TEM sample confirms that bulk diffusion is taking place during the course of the in situ experiment. If the proximity of the foil surfaces allowed surface diffusion to dominate, vacancies, or their analogue in amorphous materials, created by the unequal diffusion flux, would migrate to the surfaces of the foil rather than coalesce into voids. However, each reaction stage occurs at a much higher temperature than that observed in bulk, indicating that the kinetics of the reaction is retarded by the presence of the foil surfaces or the local temperature is relatively cool.
4. Conclusions The formation of a Ti-Si amorphous alloy has been studied in multilayers with a bilayer spacing of 25.0 nm and equiatomic composition by annealing in bulk and in situ in the transmission electron microscope. The sequence and morphology of the reaction is the same in both cases. Highresolution TEM shows that the alloy growth is strictly planar, showing no marked intrusions of amorphous material in the a-titanium grain boundaries. The reaction is diffusion controlled, with an activation energy of about 2.0 eV atom- ’ in the temperature range of 350 - 400 “C. The composition of the growing alloy layers was silicon-rich, with an atomic ratio of about 1:2 Ti:Si. When the elemental silicon is consumed, the alloy incorporated more titanium as homogenization took place. At this point, rows of Kirkendall voids formed in the place of the silicon layers. At 550 “C, a cubic crystalline silicide had nucleated. Observation of the amorphization reaction during annealing in the microscope has proven to be very useful in revealing the planar nature of the reaction, the formation of voids when the silicon has been consumed, and the nucleation of the crystalline silicide on the void surfaces.
Acknowledgments The authors would like to acknowledge the use of the facilities of the Materials Science and Engineering Department and the Center for Materials Research at Stanford University. Funding has been provided by the NSFMRL through the Center for Materials Research. We thank Kenneth P. MacWilliams (Stanford University) for assistance with the Heatpulse 610 RTA system in the Center for Integrated Systems at Stanford University. Funding for the Philips EM430 electron microscope was kindly provided by the
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NSF-MRL program through the Center for Materials Research, Stanford University, the Pew Foundation, the Schools of Engineering, Humanities and Sciences, and Earth Sciences, and the Departments of Electrical Engineering and Materials Science and Engineering at Stanford University.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
S. P. Murarka, in Silicides for VLSI Applications, Academic Press, New York, 1983. S. Herd, K. N. Tu and K. Y. Ahn, Appl. Phys. Lett., 42 (1983) 599. D. M. Vanderwalker, Appl. Phys. Lett., 48 (1986) 707. M. Natan, J. Vat. Sci. Technol. B, 4, (1986) 1404. J. Abelson, K. B. Kim, D. E. Mercer, C. R. Helms, R. Sinclair and T. W. Sigmon, J. Appl. Phys., 68 (1988) 689. K. Holloway and R. Sinclair, J. Appl. Phys., 61 (1987) 1359. H. Schroder, K. Samwer and U. Koster, Phys. Rev. Lett., 51 (1983) 415. R. Sinclair, M. A. Parker and K. B. Kim, Ultramicroscopy, 23 (1987) 383. T. Barbee and D. L. Keith, in E. S. Machlin and T. J. Rowland (eds.), Synthesis and Properties of Metastable Phases, AIME, New York, 1980, p. 93. J. C. Bravman and R. Sinclair, J. Electron Microsc. Tech., I (1984) 53. K. Holloway and R. Sinclair, Mater. Res. Symp. Proc., 77 (1986) 357. E. J. Cotts, W. J. Meng and W. L. Johnson, Phys. Rev. Lett., 57 (1986) 2295. H. U. Krebs and K. Samwer, Europhys. Lett., 2 (1986) 141. S. A. Chambers, D. M. Hill, F. Xu and J. H. Weaver, Phys. Reu. B, 35 (1987) 634. L. S. Hung, J. Gyulai, J. W. Mayer, S. S. Lau and M. A. Nicolet, J. Appl. Phys., 54 (1983) 5076. W. K. Chu, S. S. Lau, J. W. Mayer, J. Muller and K. N. Tu, Thin Solid Films, 25 (1975) 393. R. Beyers and R. Sinclair, J. Appl. Phys., 57 (1985) 5240. W. J. Meng, E. J. Cotts and W. L. Johnson, Mater. Res. Symp. Proc., 77 (1986) 223. D. E. Polk, A. Calka and B. C. Giessen, Acta Metall., 26 (1978) 1097. I. J. M. M. Raaijmakers, A. H. Reader and H. J. W. van Houtum, J. Appl. Phys., 61 (1987) 2527.