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Co bilayers
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Applied Surface North-Holland
Science
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applied surface science
Growth of epitaxial CoSi, on Si( 100) using Si( lOO)/Ti/Co AndrC Vantomme
.;..
..,.
73 (1993) 117-123
bilayers
I,*, Marc-A. Nicolet
California Institute of Technology 116-81, Pasadena, CA 91125, USA
Gang Bai and David B. Fraser Intel Corporation, Santa-Clara, CA 95052.8126, Received
29 March
1993; accepted
USA
for publication
4 May 1993
Epitaxial CoSi, is grown on SitlOO) by reaction of a Co film deposited on a Si substrate, initially separated by a thin Ti layer. Inversion of the layers and formation of a highly strained CoSi, layer can be obtained by steady-state annealing of these SitlOO)/Ti/Co structures in both reactive (N,, N, + H,, He + H,) and nonreactive (vacuum) ambients. A reactive annealing ambient chemically binds the Ti near the surface as an oxide or nitride layer, on top of an epitaxial CoSi, layer. This bilayer structure remains preserved during a high-temperature treatment. In a nonreactive ambient (vacuum), a similar inversion of Co and Ti is observed, as well as Si outdiffusion to the surface. At intermediate temperatures, vacuum annealing of Si(lOO)/Ti/Co thus results in an epitaxial CoSi, layer on the Si substrate, capped with a layer containing Co, Ti and Si, the composition of which evolves rapidly with annealing time and/or temperature. During high-temperature treatment in vacuum, these layers react further, resulting in a uniform layer of Co u,zsTi, ,sSi2 and epitaxial CoSi,.
1. Introduction Because of their promising applications in very large scale integration technology, epitaxial metal silicides have been the object of extensive investigations during recent years. Among all epitaxial silicides on Si, CoSi, has by far the most attractive properties compatible with VLSI: a low resistivity (N 15 pfl. cm at room temperature, 1 ~0. cm at 4.2 K [l]), a good thermal stability (stable up to > 1000°C for 30 min in the case of surface layers, and > 1100°C for buried layers [2]), . . . Moreover, the small lattice mismatch with Si ( - 1.2% at room temperature, decreasing to -0.35% at 1000°C) is expected to give rise to a good epitaxial growth. Indeed, on Si(lll), epitaxial CoSi, has been formed successfully using a
’ On leave from Instituut voor Kern- en Stralingsfysika, Catholic University of Leuven, Belgium. * Senior Research Assistant, NFWO (National Fund for Scientific Research, Belgium).
0169-4332/93/%06.00
0 1993 - Elsevier
Science
Publishers
number of very different techniques, such as reaction of a pure Co layer deposited by sputtering or evaporation onto Si, codeposition of Co and Si in the exact stoichiometric ratio (molecular beam epitaxy) and high-dose Co implantation into Si (ion beam synthesis). On Si(100) however, the formation of epitaxial CoSi, has not been straightforward. The only success thus far was achieved using high-dose implantation of Co into Si(lOO), resulting in an epitaxial, buried CoSi, layer [3,4]. Recently, Dass et al. [5,6] showed that an epitaxial, fully coherent Si(lOO)/CoSi z structure can be grown by reaction of a Co film deposited on a Si substrate, if the Co and Si are initially separated by a thin Ti layer. Hsia et al. [7] pointed out that when a Co-Ti alloy is deposited on the Si substrate instead of two separate layers, a continuous (Ti free) CoSi, can still be formed. In this case however, the silicide turned out to be polycrystalline. The success of this epitaxial growth is likely due to the reducing nature of Ti
B.V. All rights
reserved
118
A. Vantommr
rt al. / Growth
of epitaxial
on the native oxide of the Si substrate (thus obtaining a clean, oxygen-free Si surface), and to the fact that Co (the diffusing species during CoSi, formation) is slowed down by the Ti layer, thus favoring epitaxial nucleation and growth of CoSi,. Cobalt indeed is the diffusing species for CoSi, formation when Co and Si are in direct contact, but to our knowledge, no experiments have been performed to determine the moving species when an intermediate Ti layer is present. In their experiments, Dass et al. [5] use a N? ambient for the thermal annealing step. They mention that the initially buried titanium moves to the surface where it reacts with nitrogen, thus forming TIN. Whether this surface reaction of Ti is necessary to achieve an inversion of Ti and Co and obtain an epitaxial growth of CoSi, remained an open question. To find an answer, we carried out steady-state annealing experiments of Si(lOO)/Ti/Co structures in various ambients. The inversion of the Co and Ti layers and the subsequent phase formation are studied for the different gaseous annealing ambients used, and are compared to the results obtained by vacuum annealing. The final state of the samples and the crystalline quality of the layers are characterized for all ambients, after annealing at various temperatures.
2. Experimental
Co%,
on Si(1001 using Si~lOOl / Ti / Co hilayrrs
to clearly discern how the reaction evolves with temperature and time. On the other hand, this long annealing time results in a extended exposure of the sample to contaminants in the anncaling ambients. The samples were analyzed by MeV ‘He backscattering spectroscopy for atomic composition and layer thickness, channeling spectroscopy for quality of crystalline structure, double-crystal X-ray rocking curve diffractions for strain and glancing-angle X-ray diffraction for phase dctermination.
3. Results and discussion 3.1. Reactirte ambients Annealing the Si(lOO)/Ti/Co structure in N, at temperatures up to 500°C does not result in any silicide formation detectable with backscattering spectroscopy or X-ray diffraction. However, at 600°C inversion of Co and Ti starts, and it is nearly completed at 700°C. Fig. 1 shows the backscattering spectra of the sample as deposited, and after annealing in NZ at 700 and 9OO”C, respectively. The CoSi ~ (x = 2) layer
procedures
Si(100) wafers were covered with 8.5 A Ti and 230 A Co consecutively, without breaking the vacuum, using sputter deposition in a system with a base pressure of 5 X lo-’ Torr. The reaction of the metals and the silicon was studied upon isochronal (30 min) steady-state annealing in a temperature range between 300 and 900°C. Consequently, the temperature never exceeded a value where CoSi, thin layers dissociate [2]. These annealings were performed in various reactive ambients (N,, N, + 5% Hz and He + 14% H,), as well as in a nonreactive, vacuum ambient (pressure I 5 x lo-’ Torr). The much longer annealing time used in these steady-state experiments, in comparison with rapid thermal processing (30 min versus typically 10 s [5-71) enables us
500
2 ,” >
-
OS
0
7OO’C
deposited annealed
in
N,
+
9OO’C
annealed
I”
N,
400
Tz z
300
E ; Z
200
;
100
0
Energy
(MeV)
Fig. 1. 2.Y MeV “He backscattering spectra of the Si(lOO)/ Ti/Co sample as a function of annealing temperature. using a nitrogen ambient. The scattering angle of the detected particles is 170” and the sample was tilted 50” with respect to the incident beam.
A. Vantomme
(a)
reactive
ambient
(N2, N,+H,, +
0,
He+H,.
et al. / Growth of epitaxial Co%, on Si(lO0) using Si(lOO) / Ti / Co bilayers
(b)
inert
ambient
(vacuum)
contamination)
Fig. 2. Reaction sequence of Si(lOO)/Ti/Co upon steady-state annealing in a reactive (a) or inert ambient (b).
formed after 700°C annealing contains large amounts of oxygen, and is buried underneath a TiO, (y > 2) top layer in which Co is present. The oxygen originates from the trace impurities of the N, annealing ambient. When the sample is annealed at 900°C a TiO, ( y = 3) is formed on top of a CoSi, (x = 2) layer. The interfaces between the layers and the substrate are reasonably sharp, within the depth resolution of backscattering spectroscopy (fig. 1). The CoSi, phase was positively identified by X-ray analysis (the X-ray spectrum is very similar to the one shown in fig. 5, that was obtained after annealing in N/H,). The oxygen peaks near 1 MeV in the backscattering spectra of fig. 1 clearly show that during the 900°C annealing, oxygen is driven out of the silicide layer. It is remarkable that after reaching the sample surface, the Ti only reacts with oxygen, although plenty of nitrogen is available. Within the detection limit of the backscattering technique, no nitrogen is observed in any sample. The reaction schematics of these N,-annealed samples are summarized in fig. 2a. It should be noted that the composition of the cap layer on top of the CoSi, strongly depends on the actual experimental conditions and annealing parameters that are chosen. After 900°C annealing in N,, and using an intermediate Ti layer with a tl$ckness of 60 A [Xl, 85 A (present study) or 100 A [.5], the CoSi, layer was found to be capped with a Co-Ti-Si alloy, TiO, (y = 3) or
119
TiN, respectively. A possible explanation for the presence of Si observed in the top layer in the first case, is that both Co and Si may diffuse through a thin (- 50 A) Ti layer, whereas slightly thicker (- 100 A) Ti layers may form a diffusion barrier for Si, but not for Co 193. A detailed study of how the reaction evolves with varying Ti thickness, will be presented by Dass et al. [lo]. In an attempt to reduce the oxygen present in the annealing system, a small amount (5%) of hydrogen was added to the nitrogen. The use of this N/H, forming gas for the growth of epitaxial ion-beam-synthesized CoSi, layers has proven to prevent oxidation completely during steadystate annealing [2]. After annealing at 3OO”C, no oxidation is observed whereas in pure N,, all metal has oxidized at this temperature. At 500°C inversion of Co and Ti starts. Annealing at higher temperatures results in the formation of 700 A CoSi, on the Si substrate, covered with a 350 A TiO, (y = 3) cap. It should be noted that after the 900°C thermal treatment, the Ti signal of the backscattering spectrum (not shown) has a tail to the low energy side, which we assume to be due to a rough interface between the two thin films. To study the importance of the presence of N,, a He + 14% H, gas was used during annealing. As expected, the larger fraction of H, in the ambient further decreased the amount of metal oxidation, but could still not completely suppress it. The reaction sequence is similar to that of both previous cases: inversion of Co and Ti takes place between 500 and 600°C and results in a Si(lOO)/CoSi,/TiO, structure after high-temperature treatment. In all cases, identification of the CoSi, phase was confirmed by X-ray rocking curve measurements. From these experiments, we conclude that the inversion of Co and Ti and the subsequent formation of CoSi, take place during steady-state annealing independently of the ambient used. In particular, the presence of N,, resulting in the formation of TiN [5], is not required. However, our long exposure of the samples to an impure ambient at high temperatures leads to the oxidation of the Ti instead of its nitridation. It could conceivably be the oxygen that acts as a sink for Ti in the present case.
A. Vuntomme
120
600 n
E
et al. / Growth
-
OS deposited
o
700°C
annealed
in
wcuurr
+ 900°C
annealed
I”
“OC”“rn
of epitaxial
a, .t $ 400 N ‘Z F h =
200
0, Energy
(MA’)
Fig. 3. 3.05 MeV “He backscattering spectra Ti/Co sample as a function of annealing vacuum. The scattering angle of the detected and the sample was tilted hO” with respect beam.
of the Si(lOO)/ temperature in particles is 170 to the incident
3.2. Nonreactirle ambients We thus performed vacuum annealings. Up to 500°C the sample is unaffected. Neither oxidation nor interdiffusion of Co and Ti is observed. After 600°C annealing, Co and Ti have inverted, as was the case after steady-state annealing in a gaseous ambient. Unlike in reactive ambients however, during this vacuum annealing Si diffuses out to the surface, as can be clearly seen from fig. 3. At 700°C for 30 min (fig. 3) the layer sequence is Si in contact with CoSi, and a top layer containing Co, Ti and Si. The composition of this top layer evolves rapidly with the annealing temperature in the range around 700°C. A slightly lower annealing temperature results in a more Co-rich composition, whereas a higher Si fraction is found when the annealing temperature is elevated. On the other hand, changing the annealing time (in a range from 5 to 120 min) hardly affects the structure of this top layer. The annealing time only affects the redistribution of the Co in the layer closest to the substrate, until finally the CoSiz composition is reached. In one particular instance, the sample was clearly subdivided into layers with atomic compositions corresponding to Si( lOO)/CoSi,/Ti/CoSi ?, where the Ti layer
CoSi,
on Si(100) usirq Si(100) / Ti / Co hiluyrrs
contains small amounts of Co and Si (this is the situation shown in figs. 2b and 3). A detailed report of the time/temperature dependence of these structures will be given in ref. [l 11. Further annealing of these samples results in a homogenizftion of all layers. At 900°C finally, a 1200 i 50 A thick homogeneous layer with a stoichiometry Co/Ti/Si equal to 0.78/0.22/2.00 is obtained (fig. 3). X-ray analysis (spectrum not shown) indicates the presence of Co,,,,,Ti,,,sSi,, a phase that has also been observed by Setton and Van der Spiegel [12], in a study of high-temperature behavior of Co-Ti-Si ternaries, when an infinite amount of Si is present. Because of the small amount of this ternary silicide, only a few lines representing this phase are observed. Thus, after annealing at 900°C in vacuum, WC obtain a uniform mixture of Co,,,,sTi,,.,,Siz and CoSi,, the relative amount of which depends on the initial ratio of Co and Ti. A transmission electron microscopy study is being performed in order to get additional information on this structure. These experiments prove that inversion of the Co and Ti film sequence occurs also in a nonreactive ambient. However, three major differences arc found in the case of vacuum annealing: (i) detectable amounts of Si diffuse out to the sample surface. This phenomenon is not observed with gaseous ambients. (ii) Complete inversion of the Co and Ti is not observed in a wide range of annealing time/temperature combinations. (iii) The final state is a homogeneous mixture of CoSi, and ternary silicides, whereas in reactive gaseous ambients, a well defined CoSi, layer exists adjacent to the Si substrate, on top of which Ti (and possibly some Co) is trapped at the sample surface in the form of compounds that prevent it from diffusing and combining with the CoSi, layer below. The reaction sequences for react&e and non-reactive ambients are compared in fig. 2. 3.3. Crystalline quality Backscattering spectrometry in channeling geometry was used to study the crystalline quality of the CoSi, layers. In the case of reactive ambicnts,
121
A. Vantomme et al. / Growth of epitaxial CoSi, on Si(lO0) using Si(100) / Ti/ Co bilayers
no channeling effect is observed in the Ti or in the 0 signals, indicating a polycrystalline structure of the TiO, (y = 3) layer. However, even aftebthe He beam diverged by passing through a 400 A thick polycrystalline layer, a minimum yield of the order of 20% is still found in the underlying CoSi,, proving the highly epitaxial nature of the layer. Fig. 4 shows the backscattering spectra for (100) aligned and random beam incidence of a Si(lOO)/Ti/Co sample annealed at 900°C in N,. xmin is 18% in the Co signal and 20% in the Si signal. This is a minimum yield similar to that found by Dass et al. [5] after removing the polycrystalline TiN cap of their Si(lOO)/CoSi,/TiN structure. The crystalline perfection of that sample is thus somewhat inferior to the present one. This result indicates that oxidation of the sample does not affect the crystalline quality of the silitide. Similar results were observed for all reactive annealing ambients used, with the minimum yields slightly varying with the annealing temperature (e.g. 700°C annealing in N, gives rise to xm,,, = 26%). The single crystalhne nature of these TiO,-capped CoSi? layers was further confirmed by the observation of a CoSi, (400) diffraction peak of an X-ray rocking curve at room temperature. Fig. 5 shows a typical rocking curve obtained after 900°C (the example of a N,/H, ambient is shown). From the angular separation
500
> I
0’
’
’
I 9OO’C
0.6
0.8
1.0
Energy
”
/
4 ”
onneoled
1.2
II in
’
4
N,
1.4
1.6
(MeV)
Fig. 4. 2.0 MeV “He backscattering spectra for (100) and random beam incidence, of a Si(lOO)/Ti/Co annealed at 900°C in Nz.
aligned sample
at
9OO’C
in
N,/H,
10’ 0
t,I,.,.I,,I,l,,,,i,,~.l 45.5
46.0
Bragg
0
00
46.5
angle
47.0
(“)
Fig. 5. (400) X-ray diffraction rocking curve of a Si(lOO)/ Ti/Co sample annealed in N,/H, at 900°C. The arrows indicate the CoSi, (400) peak position far a fully relaxed and a pseudomorphic layer.
of the Si and CoSi, (400) diffraction peaks, a perpendicular strain as large as E ’ = - 2.2% with respect to the Si lattice constant was derived. Comparison with the strain values E’ = - 1.2% for a fully relaxed and E L = - 2.5% [13] for a fully coherent CoSi, layer on Si, indicates that the silicide layers are close to pseudomorphic. Dass et al. 151reported that their CoSi, layers are fully coherent, as no misfit dislocations were observed in transmission electron microscopy studies. Electron microscopy experiments are being performed to estimate the misfit dislocation density in our samples. The large width of the CoSi, (400) diffraction peaks (as compared to the Si peak, fig. 5) further suggests that the silicide lattice contains threading dislocations. Of particular interest are the channeling data of the samples annealed in vacuum. After annealing at 700°C an epitaxial CoSi, layer is formed on the Si substrate, and is capped with a polycrystalline film. For instance, in the case where two CoSi, layers are found to be separated by a Ti layer after annealing at 700°C only the buried layer shows a channeling effect (fig. 6). In the top silicide layer, no difference is found between the random and aligned signals. Also remarkable is that the uniform layer of mixed Co,.,,Ti,.,,Si, and CoSi, obtained after 900°C annealing still shows channeling with a
122
A. Vantomme
500
,
(
et al. / Growth of @axial
,
0
700°C
/ annealed
% t
0
”
0.6
” 0.8
”
”
10
Energy
se
1.2
,
,
in va~uurn
“g
;
44
,a
1.4
0
J
0.6
1.6
(MeV)
Fig. 6. 2.0 MeV 4He backscattering spectra for (100) and random beam incidence, of a Si(lOO)/Ti/Co annealed at 700°C in vacuum.
t
/
aligned sample
1
0.8
1.0
Energy
1.2
1.4
1.6
CoSi,
on SdlOO) using Si(100) / Ti / Co bilayrrs
not the driving force for the inversion of the metal layers observed during annealing of a Si(lOO)/Ti/Co structure. Annealing in various reactive ambients, including exposure to contaminants such as oxygen, all result in the formation of a single crystalline (xmin I 200/o), highly strained CoSi, layer on a Si(100) substrate. This epitaxial heterostructure is capped with Ti, trapped at the sample surface in the form of compounds. Even in the absence of any gaseous ambient, inversion of Co and Ti still takes place at a temperature of about 700°C. However, unlike in the case of reactive gases, the samples are not stable upon high-temperature treatment in vacuum. Annealing at 900°C in vacuum results in a homogeneous mixture of CoSi, and a ternary silicide. The growth of epitaxial CoSi, on Si(100) substrates via solid-state reaction, by Dass et al. [S,lO], Hsia et al. [7,8], Hong et al. [9] and ourselves proves that the excellent lattice match between Si and CoSi, favors the formation of pseudomorphic films in both (111) and (100) directions, as it does for Nisi, [14]. The observation that successful epigrowth of CoSi, along (100) demands special means thus leads to a refined formulation of the peculiarity associated with CoSi, and Si(100): what property of the Si(100) surface is it that prevents epitaxial growth by solid-phase reaction directly between Co and Si?
(MeV)
Fig. 7. 2.0 MeV ‘He backscattering spectra for (100) and random beam incidence, of a Si(lOO)/Ti/Co annealed at 900°C in vacuum.
aligned sample
X m,n = 70% in the Co signal (fig. 7). On the other
hand, no channeling is found in the Ti signal, indicating that the ternary phase is present in a polycrystalline or amorphous form. One explanation is that the Co,,,Ti,,,,Si, grains are distributed in random orientations throughout an epitaxial CoSi, matrix.
Acknowledgements A partial financial support by the Semiconductor Research Corporation (93-SJ-100) is thankfully acknowledged.
References [II A.E. White, K.T. Short, R.C. Dynes, J.P. Garno and J.M. Gibson,
Appl.
Phys. Lett. 50 (1987) 95. H. Pattyn. G. Langouche. K. Maex. J. Vanhellemont, J. Vanacken, H. Vloeberghs and Y. Bruynseraede, Nucl. Instr. Meth. B 45 (1990) 658. S. Petterson and A. Lauwers, [Xl K. Maex. J. Vanhellemont, Appl. Surf. Sci. 53 (1991) 273. [41 R. Jebasinski, S. Mantl, L. Vescan and Ch. Dieker, Appl. Surf. Sci. 53 (1991) 264.
[21 M.F. Wu, A. Vantomme,
4. Summary From presence
and conclusions
these experiments, we conclude that the of nitrogen in the annealing ambient is
A. Vantomme et al. / Growth of epitaxial Co.%, on Si(100) using Si(lO0) / Ti / Co bilayers [5] M.L.A. Dass, D.B. Fraser and C.-S. Wei, Appl. Phys. Lett. 58 (1991) 1308. [6] C.-S. Wei, D.B. Fraser, M.L.A. Dass and T. Brat, in: Proc. 6th Int. IEEE VLSI Multilevel Interconnection Conf., Santa Clara, CA (1989) p. 241. [7] S.L. Hsia, T.Y. Tan, P. Smith and G.E. McGuire, J. Appl. Phys. 70 (1991) 7579. [8] S.L. Hsia, T.Y. Tan, P. Smith and G.E. McGuire, J. Appl. Phys. 72 (1992) 1864. [9] F. Hong, G.A. Rozgonyi and B. Patnaik, Appl. Phys. Lett. 61 (1992) 1519.
123
[lo] M.L.A. Dass, G. Bai and D. Fraser, to be published. [ll] A. Vantomme, M.-A. Nicolet and N.D. Theodore, to be published. [12] M. Setton and J. Van der Spiegel, Appl. Surf. Sci. 38 (1989) 62. [13] G. Bai, M.-A. Nicolet, T. Vreeland, Q. Ye and K.L. Wang, Appl. Phys. Lett. 55 (1989) 1874. [14] R.T. Tung, A.F.J. Levi and J.M. Gibson, J. Vat. Sci. Technol. B 4 (1986) 1435.
;-$+
Applied Surface North-Holland
Science
.“.
:.:..
..:.:
>:..,:\::
,:..::::.:::,
ii:;:;
.”
applied surface science
Growth of epitaxial CoSi, on Si( 100) using Si( lOO)/Ti/Co AndrC Vantomme
.;..
..,.
73 (1993) 117-123
bilayers
I,*, Marc-A. Nicolet
California Institute of Technology 116-81, Pasadena, CA 91125, USA
Gang Bai and David B. Fraser Intel Corporation, Santa-Clara, CA 95052.8126, Received
29 March
1993; accepted
USA
for publication
4 May 1993
Epitaxial CoSi, is grown on SitlOO) by reaction of a Co film deposited on a Si substrate, initially separated by a thin Ti layer. Inversion of the layers and formation of a highly strained CoSi, layer can be obtained by steady-state annealing of these SitlOO)/Ti/Co structures in both reactive (N,, N, + H,, He + H,) and nonreactive (vacuum) ambients. A reactive annealing ambient chemically binds the Ti near the surface as an oxide or nitride layer, on top of an epitaxial CoSi, layer. This bilayer structure remains preserved during a high-temperature treatment. In a nonreactive ambient (vacuum), a similar inversion of Co and Ti is observed, as well as Si outdiffusion to the surface. At intermediate temperatures, vacuum annealing of Si(lOO)/Ti/Co thus results in an epitaxial CoSi, layer on the Si substrate, capped with a layer containing Co, Ti and Si, the composition of which evolves rapidly with annealing time and/or temperature. During high-temperature treatment in vacuum, these layers react further, resulting in a uniform layer of Co u,zsTi, ,sSi2 and epitaxial CoSi,.
1. Introduction Because of their promising applications in very large scale integration technology, epitaxial metal silicides have been the object of extensive investigations during recent years. Among all epitaxial silicides on Si, CoSi, has by far the most attractive properties compatible with VLSI: a low resistivity (N 15 pfl. cm at room temperature, 1 ~0. cm at 4.2 K [l]), a good thermal stability (stable up to > 1000°C for 30 min in the case of surface layers, and > 1100°C for buried layers [2]), . . . Moreover, the small lattice mismatch with Si ( - 1.2% at room temperature, decreasing to -0.35% at 1000°C) is expected to give rise to a good epitaxial growth. Indeed, on Si(lll), epitaxial CoSi, has been formed successfully using a
’ On leave from Instituut voor Kern- en Stralingsfysika, Catholic University of Leuven, Belgium. * Senior Research Assistant, NFWO (National Fund for Scientific Research, Belgium).
0169-4332/93/%06.00
0 1993 - Elsevier
Science
Publishers
number of very different techniques, such as reaction of a pure Co layer deposited by sputtering or evaporation onto Si, codeposition of Co and Si in the exact stoichiometric ratio (molecular beam epitaxy) and high-dose Co implantation into Si (ion beam synthesis). On Si(100) however, the formation of epitaxial CoSi, has not been straightforward. The only success thus far was achieved using high-dose implantation of Co into Si(lOO), resulting in an epitaxial, buried CoSi, layer [3,4]. Recently, Dass et al. [5,6] showed that an epitaxial, fully coherent Si(lOO)/CoSi z structure can be grown by reaction of a Co film deposited on a Si substrate, if the Co and Si are initially separated by a thin Ti layer. Hsia et al. [7] pointed out that when a Co-Ti alloy is deposited on the Si substrate instead of two separate layers, a continuous (Ti free) CoSi, can still be formed. In this case however, the silicide turned out to be polycrystalline. The success of this epitaxial growth is likely due to the reducing nature of Ti
B.V. All rights
reserved
118
A. Vantommr
rt al. / Growth
of epitaxial
on the native oxide of the Si substrate (thus obtaining a clean, oxygen-free Si surface), and to the fact that Co (the diffusing species during CoSi, formation) is slowed down by the Ti layer, thus favoring epitaxial nucleation and growth of CoSi,. Cobalt indeed is the diffusing species for CoSi, formation when Co and Si are in direct contact, but to our knowledge, no experiments have been performed to determine the moving species when an intermediate Ti layer is present. In their experiments, Dass et al. [5] use a N? ambient for the thermal annealing step. They mention that the initially buried titanium moves to the surface where it reacts with nitrogen, thus forming TIN. Whether this surface reaction of Ti is necessary to achieve an inversion of Ti and Co and obtain an epitaxial growth of CoSi, remained an open question. To find an answer, we carried out steady-state annealing experiments of Si(lOO)/Ti/Co structures in various ambients. The inversion of the Co and Ti layers and the subsequent phase formation are studied for the different gaseous annealing ambients used, and are compared to the results obtained by vacuum annealing. The final state of the samples and the crystalline quality of the layers are characterized for all ambients, after annealing at various temperatures.
2. Experimental
Co%,
on Si(1001 using Si~lOOl / Ti / Co hilayrrs
to clearly discern how the reaction evolves with temperature and time. On the other hand, this long annealing time results in a extended exposure of the sample to contaminants in the anncaling ambients. The samples were analyzed by MeV ‘He backscattering spectroscopy for atomic composition and layer thickness, channeling spectroscopy for quality of crystalline structure, double-crystal X-ray rocking curve diffractions for strain and glancing-angle X-ray diffraction for phase dctermination.
3. Results and discussion 3.1. Reactirte ambients Annealing the Si(lOO)/Ti/Co structure in N, at temperatures up to 500°C does not result in any silicide formation detectable with backscattering spectroscopy or X-ray diffraction. However, at 600°C inversion of Co and Ti starts, and it is nearly completed at 700°C. Fig. 1 shows the backscattering spectra of the sample as deposited, and after annealing in NZ at 700 and 9OO”C, respectively. The CoSi ~ (x = 2) layer
procedures
Si(100) wafers were covered with 8.5 A Ti and 230 A Co consecutively, without breaking the vacuum, using sputter deposition in a system with a base pressure of 5 X lo-’ Torr. The reaction of the metals and the silicon was studied upon isochronal (30 min) steady-state annealing in a temperature range between 300 and 900°C. Consequently, the temperature never exceeded a value where CoSi, thin layers dissociate [2]. These annealings were performed in various reactive ambients (N,, N, + 5% Hz and He + 14% H,), as well as in a nonreactive, vacuum ambient (pressure I 5 x lo-’ Torr). The much longer annealing time used in these steady-state experiments, in comparison with rapid thermal processing (30 min versus typically 10 s [5-71) enables us
500
2 ,” >
-
OS
0
7OO’C
deposited annealed
in
N,
+
9OO’C
annealed
I”
N,
400
Tz z
300
E ; Z
200
;
100
0
Energy
(MeV)
Fig. 1. 2.Y MeV “He backscattering spectra of the Si(lOO)/ Ti/Co sample as a function of annealing temperature. using a nitrogen ambient. The scattering angle of the detected particles is 170” and the sample was tilted 50” with respect to the incident beam.
A. Vantomme
(a)
reactive
ambient
(N2, N,+H,, +
0,
He+H,.
et al. / Growth of epitaxial Co%, on Si(lO0) using Si(lOO) / Ti / Co bilayers
(b)
inert
ambient
(vacuum)
contamination)
Fig. 2. Reaction sequence of Si(lOO)/Ti/Co upon steady-state annealing in a reactive (a) or inert ambient (b).
formed after 700°C annealing contains large amounts of oxygen, and is buried underneath a TiO, (y > 2) top layer in which Co is present. The oxygen originates from the trace impurities of the N, annealing ambient. When the sample is annealed at 900°C a TiO, ( y = 3) is formed on top of a CoSi, (x = 2) layer. The interfaces between the layers and the substrate are reasonably sharp, within the depth resolution of backscattering spectroscopy (fig. 1). The CoSi, phase was positively identified by X-ray analysis (the X-ray spectrum is very similar to the one shown in fig. 5, that was obtained after annealing in N/H,). The oxygen peaks near 1 MeV in the backscattering spectra of fig. 1 clearly show that during the 900°C annealing, oxygen is driven out of the silicide layer. It is remarkable that after reaching the sample surface, the Ti only reacts with oxygen, although plenty of nitrogen is available. Within the detection limit of the backscattering technique, no nitrogen is observed in any sample. The reaction schematics of these N,-annealed samples are summarized in fig. 2a. It should be noted that the composition of the cap layer on top of the CoSi, strongly depends on the actual experimental conditions and annealing parameters that are chosen. After 900°C annealing in N,, and using an intermediate Ti layer with a tl$ckness of 60 A [Xl, 85 A (present study) or 100 A [.5], the CoSi, layer was found to be capped with a Co-Ti-Si alloy, TiO, (y = 3) or
119
TiN, respectively. A possible explanation for the presence of Si observed in the top layer in the first case, is that both Co and Si may diffuse through a thin (- 50 A) Ti layer, whereas slightly thicker (- 100 A) Ti layers may form a diffusion barrier for Si, but not for Co 193. A detailed study of how the reaction evolves with varying Ti thickness, will be presented by Dass et al. [lo]. In an attempt to reduce the oxygen present in the annealing system, a small amount (5%) of hydrogen was added to the nitrogen. The use of this N/H, forming gas for the growth of epitaxial ion-beam-synthesized CoSi, layers has proven to prevent oxidation completely during steadystate annealing [2]. After annealing at 3OO”C, no oxidation is observed whereas in pure N,, all metal has oxidized at this temperature. At 500°C inversion of Co and Ti starts. Annealing at higher temperatures results in the formation of 700 A CoSi, on the Si substrate, covered with a 350 A TiO, (y = 3) cap. It should be noted that after the 900°C thermal treatment, the Ti signal of the backscattering spectrum (not shown) has a tail to the low energy side, which we assume to be due to a rough interface between the two thin films. To study the importance of the presence of N,, a He + 14% H, gas was used during annealing. As expected, the larger fraction of H, in the ambient further decreased the amount of metal oxidation, but could still not completely suppress it. The reaction sequence is similar to that of both previous cases: inversion of Co and Ti takes place between 500 and 600°C and results in a Si(lOO)/CoSi,/TiO, structure after high-temperature treatment. In all cases, identification of the CoSi, phase was confirmed by X-ray rocking curve measurements. From these experiments, we conclude that the inversion of Co and Ti and the subsequent formation of CoSi, take place during steady-state annealing independently of the ambient used. In particular, the presence of N,, resulting in the formation of TiN [5], is not required. However, our long exposure of the samples to an impure ambient at high temperatures leads to the oxidation of the Ti instead of its nitridation. It could conceivably be the oxygen that acts as a sink for Ti in the present case.
A. Vuntomme
120
600 n
E
et al. / Growth
-
OS deposited
o
700°C
annealed
in
wcuurr
+ 900°C
annealed
I”
“OC”“rn
of epitaxial
a, .t $ 400 N ‘Z F h =
200
0, Energy
(MA’)
Fig. 3. 3.05 MeV “He backscattering spectra Ti/Co sample as a function of annealing vacuum. The scattering angle of the detected and the sample was tilted hO” with respect beam.
of the Si(lOO)/ temperature in particles is 170 to the incident
3.2. Nonreactirle ambients We thus performed vacuum annealings. Up to 500°C the sample is unaffected. Neither oxidation nor interdiffusion of Co and Ti is observed. After 600°C annealing, Co and Ti have inverted, as was the case after steady-state annealing in a gaseous ambient. Unlike in reactive ambients however, during this vacuum annealing Si diffuses out to the surface, as can be clearly seen from fig. 3. At 700°C for 30 min (fig. 3) the layer sequence is Si in contact with CoSi, and a top layer containing Co, Ti and Si. The composition of this top layer evolves rapidly with the annealing temperature in the range around 700°C. A slightly lower annealing temperature results in a more Co-rich composition, whereas a higher Si fraction is found when the annealing temperature is elevated. On the other hand, changing the annealing time (in a range from 5 to 120 min) hardly affects the structure of this top layer. The annealing time only affects the redistribution of the Co in the layer closest to the substrate, until finally the CoSiz composition is reached. In one particular instance, the sample was clearly subdivided into layers with atomic compositions corresponding to Si( lOO)/CoSi,/Ti/CoSi ?, where the Ti layer
CoSi,
on Si(100) usirq Si(100) / Ti / Co hiluyrrs
contains small amounts of Co and Si (this is the situation shown in figs. 2b and 3). A detailed report of the time/temperature dependence of these structures will be given in ref. [l 11. Further annealing of these samples results in a homogenizftion of all layers. At 900°C finally, a 1200 i 50 A thick homogeneous layer with a stoichiometry Co/Ti/Si equal to 0.78/0.22/2.00 is obtained (fig. 3). X-ray analysis (spectrum not shown) indicates the presence of Co,,,,,Ti,,,sSi,, a phase that has also been observed by Setton and Van der Spiegel [12], in a study of high-temperature behavior of Co-Ti-Si ternaries, when an infinite amount of Si is present. Because of the small amount of this ternary silicide, only a few lines representing this phase are observed. Thus, after annealing at 900°C in vacuum, WC obtain a uniform mixture of Co,,,,sTi,,.,,Siz and CoSi,, the relative amount of which depends on the initial ratio of Co and Ti. A transmission electron microscopy study is being performed in order to get additional information on this structure. These experiments prove that inversion of the Co and Ti film sequence occurs also in a nonreactive ambient. However, three major differences arc found in the case of vacuum annealing: (i) detectable amounts of Si diffuse out to the sample surface. This phenomenon is not observed with gaseous ambients. (ii) Complete inversion of the Co and Ti is not observed in a wide range of annealing time/temperature combinations. (iii) The final state is a homogeneous mixture of CoSi, and ternary silicides, whereas in reactive gaseous ambients, a well defined CoSi, layer exists adjacent to the Si substrate, on top of which Ti (and possibly some Co) is trapped at the sample surface in the form of compounds that prevent it from diffusing and combining with the CoSi, layer below. The reaction sequences for react&e and non-reactive ambients are compared in fig. 2. 3.3. Crystalline quality Backscattering spectrometry in channeling geometry was used to study the crystalline quality of the CoSi, layers. In the case of reactive ambicnts,
121
A. Vantomme et al. / Growth of epitaxial CoSi, on Si(lO0) using Si(100) / Ti/ Co bilayers
no channeling effect is observed in the Ti or in the 0 signals, indicating a polycrystalline structure of the TiO, (y = 3) layer. However, even aftebthe He beam diverged by passing through a 400 A thick polycrystalline layer, a minimum yield of the order of 20% is still found in the underlying CoSi,, proving the highly epitaxial nature of the layer. Fig. 4 shows the backscattering spectra for (100) aligned and random beam incidence of a Si(lOO)/Ti/Co sample annealed at 900°C in N,. xmin is 18% in the Co signal and 20% in the Si signal. This is a minimum yield similar to that found by Dass et al. [5] after removing the polycrystalline TiN cap of their Si(lOO)/CoSi,/TiN structure. The crystalline perfection of that sample is thus somewhat inferior to the present one. This result indicates that oxidation of the sample does not affect the crystalline quality of the silitide. Similar results were observed for all reactive annealing ambients used, with the minimum yields slightly varying with the annealing temperature (e.g. 700°C annealing in N, gives rise to xm,,, = 26%). The single crystalhne nature of these TiO,-capped CoSi? layers was further confirmed by the observation of a CoSi, (400) diffraction peak of an X-ray rocking curve at room temperature. Fig. 5 shows a typical rocking curve obtained after 900°C (the example of a N,/H, ambient is shown). From the angular separation
500
> I
0’
’
’
I 9OO’C
0.6
0.8
1.0
Energy
”
/
4 ”
onneoled
1.2
II in
’
4
N,
1.4
1.6
(MeV)
Fig. 4. 2.0 MeV “He backscattering spectra for (100) and random beam incidence, of a Si(lOO)/Ti/Co annealed at 900°C in Nz.
aligned sample
at
9OO’C
in
N,/H,
10’ 0
t,I,.,.I,,I,l,,,,i,,~.l 45.5
46.0
Bragg
0
00
46.5
angle
47.0
(“)
Fig. 5. (400) X-ray diffraction rocking curve of a Si(lOO)/ Ti/Co sample annealed in N,/H, at 900°C. The arrows indicate the CoSi, (400) peak position far a fully relaxed and a pseudomorphic layer.
of the Si and CoSi, (400) diffraction peaks, a perpendicular strain as large as E ’ = - 2.2% with respect to the Si lattice constant was derived. Comparison with the strain values E’ = - 1.2% for a fully relaxed and E L = - 2.5% [13] for a fully coherent CoSi, layer on Si, indicates that the silicide layers are close to pseudomorphic. Dass et al. 151reported that their CoSi, layers are fully coherent, as no misfit dislocations were observed in transmission electron microscopy studies. Electron microscopy experiments are being performed to estimate the misfit dislocation density in our samples. The large width of the CoSi, (400) diffraction peaks (as compared to the Si peak, fig. 5) further suggests that the silicide lattice contains threading dislocations. Of particular interest are the channeling data of the samples annealed in vacuum. After annealing at 700°C an epitaxial CoSi, layer is formed on the Si substrate, and is capped with a polycrystalline film. For instance, in the case where two CoSi, layers are found to be separated by a Ti layer after annealing at 700°C only the buried layer shows a channeling effect (fig. 6). In the top silicide layer, no difference is found between the random and aligned signals. Also remarkable is that the uniform layer of mixed Co,.,,Ti,.,,Si, and CoSi, obtained after 900°C annealing still shows channeling with a
122
A. Vantomme
500
,
(
et al. / Growth of @axial
,
0
700°C
/ annealed
% t
0
”
0.6
” 0.8
”
”
10
Energy
se
1.2
,
,
in va~uurn
“g
;
44
,a
1.4
0
J
0.6
1.6
(MeV)
Fig. 6. 2.0 MeV 4He backscattering spectra for (100) and random beam incidence, of a Si(lOO)/Ti/Co annealed at 700°C in vacuum.
t
/
aligned sample
1
0.8
1.0
Energy
1.2
1.4
1.6
CoSi,
on SdlOO) using Si(100) / Ti / Co bilayrrs
not the driving force for the inversion of the metal layers observed during annealing of a Si(lOO)/Ti/Co structure. Annealing in various reactive ambients, including exposure to contaminants such as oxygen, all result in the formation of a single crystalline (xmin I 200/o), highly strained CoSi, layer on a Si(100) substrate. This epitaxial heterostructure is capped with Ti, trapped at the sample surface in the form of compounds. Even in the absence of any gaseous ambient, inversion of Co and Ti still takes place at a temperature of about 700°C. However, unlike in the case of reactive gases, the samples are not stable upon high-temperature treatment in vacuum. Annealing at 900°C in vacuum results in a homogeneous mixture of CoSi, and a ternary silicide. The growth of epitaxial CoSi, on Si(100) substrates via solid-state reaction, by Dass et al. [S,lO], Hsia et al. [7,8], Hong et al. [9] and ourselves proves that the excellent lattice match between Si and CoSi, favors the formation of pseudomorphic films in both (111) and (100) directions, as it does for Nisi, [14]. The observation that successful epigrowth of CoSi, along (100) demands special means thus leads to a refined formulation of the peculiarity associated with CoSi, and Si(100): what property of the Si(100) surface is it that prevents epitaxial growth by solid-phase reaction directly between Co and Si?
(MeV)
Fig. 7. 2.0 MeV ‘He backscattering spectra for (100) and random beam incidence, of a Si(lOO)/Ti/Co annealed at 900°C in vacuum.
aligned sample
X m,n = 70% in the Co signal (fig. 7). On the other
hand, no channeling is found in the Ti signal, indicating that the ternary phase is present in a polycrystalline or amorphous form. One explanation is that the Co,,,Ti,,,,Si, grains are distributed in random orientations throughout an epitaxial CoSi, matrix.
Acknowledgements A partial financial support by the Semiconductor Research Corporation (93-SJ-100) is thankfully acknowledged.
References [II A.E. White, K.T. Short, R.C. Dynes, J.P. Garno and J.M. Gibson,
Appl.
Phys. Lett. 50 (1987) 95. H. Pattyn. G. Langouche. K. Maex. J. Vanhellemont, J. Vanacken, H. Vloeberghs and Y. Bruynseraede, Nucl. Instr. Meth. B 45 (1990) 658. S. Petterson and A. Lauwers, [Xl K. Maex. J. Vanhellemont, Appl. Surf. Sci. 53 (1991) 273. [41 R. Jebasinski, S. Mantl, L. Vescan and Ch. Dieker, Appl. Surf. Sci. 53 (1991) 264.
[21 M.F. Wu, A. Vantomme,
4. Summary From presence
and conclusions
these experiments, we conclude that the of nitrogen in the annealing ambient is
A. Vantomme et al. / Growth of epitaxial Co.%, on Si(100) using Si(lO0) / Ti / Co bilayers [5] M.L.A. Dass, D.B. Fraser and C.-S. Wei, Appl. Phys. Lett. 58 (1991) 1308. [6] C.-S. Wei, D.B. Fraser, M.L.A. Dass and T. Brat, in: Proc. 6th Int. IEEE VLSI Multilevel Interconnection Conf., Santa Clara, CA (1989) p. 241. [7] S.L. Hsia, T.Y. Tan, P. Smith and G.E. McGuire, J. Appl. Phys. 70 (1991) 7579. [8] S.L. Hsia, T.Y. Tan, P. Smith and G.E. McGuire, J. Appl. Phys. 72 (1992) 1864. [9] F. Hong, G.A. Rozgonyi and B. Patnaik, Appl. Phys. Lett. 61 (1992) 1519.
123
[lo] M.L.A. Dass, G. Bai and D. Fraser, to be published. [ll] A. Vantomme, M.-A. Nicolet and N.D. Theodore, to be published. [12] M. Setton and J. Van der Spiegel, Appl. Surf. Sci. 38 (1989) 62. [13] G. Bai, M.-A. Nicolet, T. Vreeland, Q. Ye and K.L. Wang, Appl. Phys. Lett. 55 (1989) 1874. [14] R.T. Tung, A.F.J. Levi and J.M. Gibson, J. Vat. Sci. Technol. B 4 (1986) 1435.