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Interfacial reaction between monosilane and a polycrystalline tantalum substrate
Applied Surface Science 38 (1989) 133-138 North-Holland, Amsterdam
13.,
IlNTERFACL~L ~,EAC-'TION B E T W E E N M O N O S ] t L A N E A N D A POLYC-~YSTALLI~NE T A N T A L U M S U B S T R A T E M. A L A O U I , F. R I N G E I S E N , D. M U L L E R , D. B O L M O N T a n d J.J. K O U L M A N N Laboratoire de Physique et de Spectroscopie Electronique, FST, 4 rue des Frdres LumiJre, 68093 Mulhouse Cedex, France
Received 19 March 1989; accepted for publication 6 April 1989
We report XPS results on the thermal cracking of monosilane on a polycrystalline tantalum suhstrate. Ta substrates were heated in the 20°C-800°C temperature range and exposed to low pressure (10 -s Tort) Sill 4 doses. We followed the Ta/TaSi2 interface growth by recording the Si2p and Ta4f core level shifts. For temperatures below 600°C, results show growth of a thin ( -15 ,~) TaSi 2 film followed by a thin Si overlayer. By annealing at 750°C for 2 min, this Si surface layer intermixes with bulk tantalum. For preparation at higher temperatures (600°C-800°C), grown TaSi2 films get thicker ( - 25 ,~) but the Ta,/TaSi 2 interface is always covered by a thin silicon overlayer. The growth process of the Ta/TaSi 2 interface is limited by the diffusion coefficient of Ta through the TaSi 2 f'dm which acts as a diffusion barrier.
Polycrystalline TaSi2 is an interesting material ha rnieroelectronics d u e to its metallic character. Xts resistivity at r o o m t e m p e r a t u r e (RT) is a b o u t 40 ~ . c m [1]. W e were interested ha its p r e p a r a t i o n t h r o u g h m o n o s i l a n e exposure o f a heated t a n t a l u m substrate. T h e TaffFaSi2 interface is g r o w n b y thermal cracking o f low pressure (10 - s Torr) m o n o s i l a n e o n a polycrystalline t a n t a l u m substrate. U p to n o w only a few p h o t o e m i s s i o n studies w e r e p u b lished o n the S i / q ' a interface [2] (i.e. T a e v a p o r a t e d o n a Si substrate) but, to our knowledge, this p a p e r p r e s e n t s the first e x p e r i m e n t a l w o r k dealing with the inverse T a / S i interface (i.e. Si d e p o s i t e d o n a Ta substrate).
2. E x l m r i m e n ~ T h e experimental p r o c e d u r e has b e e n extensively described in ref. [3]. Briefly, polycrystalline T a foils (8 x 10 ram) were s u b m i t t e d to repetitive (Ar + s p u t t e r i n g - h e a t i n g ) cycles to clean the surface. T h e s a m p l e s are stuoied using X-ray p h o t o e m / s s i o n s p e c t r o s c o p y (XPS). 0169-4332/89/$03.50 © Elsev/er Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Div/slon)
] 34
M. A taoui et al. ,/Reaction between monosilane and a tantalum substrate
Before monosilane exposure the Ta samples were considered "clean" when the XPS peaks for impurities (C, O) were undetectable. After cleaning, the tantalum substrate was exposed to monosilane (Sill4) at a pressure of 10-5-10 -6 Tort for a given time. Monosilane (Air Liquide - Electronic quality) was introduced into the U H V chamber through a calibrated microleak in order to accurately control the pressure during exposure. As the arrival rate for monosilane was unknown we specify the Sill 4 partial pressure times the exposure time (1 L = 10 -6 Tort. s).
3. R e s ~ We submitted a tantalum substrate to increasing monosilane exposures at temperatures in the range 20-800 ° C. Two regimes can be distinguished: (a) 2 0 ° C < T~ < 6 0 0 ° C : We followed, by means of XPS, the evolution of the Si2p and T a 4 f core levels as a function of substrate temperature T~ and monosilane exposures. Fig. 1 shows the evolution of the Si2p peak for a substrate temperature of 400 o C. Even for low exposures we observe a shift toward higher binding energies. For a 104 L exposure, the Si2p structure is fixed at a position corresponding to bulk silicon and further growth is extremely slow. A 750 ° C anneal (2 rain) induces a 0.2-0.3 eV shift toward lower binding energies. This last position corresponds to Si2p in T a S i 2 as confirmed both by results obtained at higher temperatures and by Weaver's work on bulk TaSi2 [4]. The T a 4 f peak (fig. 2) shifts in the same way for increasing Sill 4 exposures pinning at a pos~.tion corresponding to T a 4 f in bulk TaSi 2. However, the T a 4 f signal is asymmetric suggesting a network in which the Ta atoms are not all in the same configuration. A 750 ° C anneal (2 min) causes a 0.2 eV shift toward lower binding energies, corresponding probably to a Ta richer environment. For all temperatures between 20 ° C and 600 ° C, results remain qualitatively the same. The average thicknesses of the TaSi 2 films were estimated from XPS peak intensities and are about 12 A at 3 0 0 ° C and 18 A at 4 0 0 ° C (fig. 5). (b) 6 0 0 ° C < T~ < 8 0 0 ° C : T h e mechanisms occurring in this temperature range seem easier to explain. The layers formed are thicker due to both thermally activated Ta diffusion in TaSi 2 and more efficient Sill4 thermal cracking at higher temperatures. We followed, by means of XPS, the Si2p and T a 4 f core level evolution as a function of monosilane exposure (figs. 3 and 4 respectively). For low exposures ( < 50 L), the Si2p core level binding energy is about 0.6 eV lower than in bulk silicon (fig. 3). In the intermediate 50-1000 L exposure range, its position remains unaffected corresponding to TaSi 2 environment. At higher
M. Alaoui et aL / Reaction between monosilane and a tantalum substrate
I 400 nO
Si 2p Sq Ir,..~2
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135
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.
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I j/-7 28
26
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2,9.
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Bln~lng ~:ner~y (OV) Fig. 1. Si 2p core level s~cU'a after various Sill 4 exposures on a heated ( 4 0 0 ° C ) polycrystalllne T a substrate.
Fig. 2. T a 4 f core level spectra after various SiH 4 exposures on a heated ( z ~ 0 ° C ) polycrystalline T a substrate.
exposures ( > 1000 L), the Si2p peak progressively shffLs to the position related to bulk silicon. We checked the formation of a s/l/con surface layer by recording the relative variations of the $i2p and Ta4f peak intensities. For exposures above 104 L, the $i2p peak grows w/thout any shift while the intensity of the Ta4f peak continuously decreases. To identify this Si surface layer, we exposed it to atomic hydrogen obtained by cracking of He molecules at a hot W filament. By us/rig surface sensitive UPS we detected the characteristic features of the Si-H and Si-H z phases [3]. Th/s Si surface ~.ayer ascertains the b/riding energy of the Si 2p peak as being relevant to that of bulk silicon [3]. The binding energy of the Ta4f core level (fig. 4) increases w/th increasing Sill 4 exposures and a maximal shift of 0.9 eV is obtained for a 100 L exposure correspond/rig to a Ta$i z configuration [4]. At higher exposures, when a silicon surface layers forms, the Ta4f core level shifts about 0.2 eV toward lower binding energy w/thout any corresponding ddsplacement of the Si 2p core level. We conclude that the s~/dde near the surface gels Ta em'/ched, the "ira atoms being prey/deal by the underneath layers. At these temperatures, s/I/con
136
M. A laoui et al. / Reaction between monosilane and a tantalum substrate
Si2p 750
7 5 0 °C
"C
o2~ j
.
"~'~4f
T
I
i
d
I I
I
s.~o~ L,
"N o
Ta
Bin~ing Energy(eV) Fig. 3. Si 2p core level spectra after various Sill 4 exposures on a heated (750°C) polycrystallineTa substrate.
28
26 24 22 20 Binding Energy (Or) Fig. 4. T a 4 f core level spectra after various Sill 4 exposures on a heated ( 7 5 0 ° C ) polycrystalline T a substrate.
clusters and further growth of these Si crystallizes requires silicon being taken away from TaSi2. A similar situation should occur in silicon/metal Schottky barrier formation where an interfacial vitreous phase was evidenced with a composition close to that of the lowest temperature eutectic composition [5,6]. For the T a / S i system, this eutectic composition corresponds to approximately TaSil. s stoicbiometry, which is Ta richer than "~'aSi2 [7]. UPS spectra (not represented) show weakly defined structures wh, h cannot be totally attributed to the silicon surface overtayer [8]. Fig. 5 shows that no layer growth seems to occur in the 0-10 L exposure range. This is a direct consequence of the high reactivity of monosilane toward tantalum or/des. Indeed, it must be emphazised that, due to the general trend of cleaned metals to oxidize, a thin <>ride overlayer always forms aZ high temperature. As a consequence of the active reducing character of silane gas, monos/lane acts as a reducer of tantalum oxides induced by heating of the Ta subsZrate. During Sill 4 exposure under our experimental conditions, the first
214. Alaoui et al. / Reaction between monosilane and a tantabtm substrate
137
loo.ooo.o
o
io
~o 2
~o3
~o4
SIH4 ~POSURE (L)
Fig. 5. Film thicknessas a functionof Sill4 exposure for variousTa substrat¢ temperatures. 10 L are always used in this reduction pr~ .¢ss and the T a / S i H 4 interaction starts in fact above ~ 10 L. The 6 0 0 ° C temperature seems to be a linfit between two stages. For substrate temperatures below 600* C, saturation is ra~idly achieved whereas above 600 o C, thicker films (however lh-nited to - 30 A) can be grown.
4. C o n c i s i o n This study shows that it is possible to grow TaSi 2 f/]rns by direct thermal cracking of monosilane on a hot polyerystallme tantalum substrate. At low temperatures ( < 6 0 0 ° C ) , the grown films have TaSi2 stoichiometry. The thicknesses of these layers are essentially lindted by the diffusion coefficient of Ta through TaSi2 and the topmost surface layer is quite pure siUcon. Growth of th/s Si overlayer is limited by the low stick/rig coefficient of Sill4 on a $i surface [9]. At higher temperatures, the TaSi2 layers get thicker but are always covered by a thin silicon overlayer. Anew, Ta diffusion through TaSi 2 l/hilts the th/ckness of the grown f'flms. In this respect, the TaSi2 layer acts as a barrier toward Ta diffusion. At very high temperatures, or by annealing of layers prepared at lower temperatures, we observe formation of a surface smcide w/th composition different from that of bulk s/l/cide. The growth method described here is linfited as it does not perm/t the growth of thick TaSi2 films. For technological appfications where thicker films are required, other techniques ~ke coevaporation [10], cosputtefing [11] or chemical vapor deposition (CVD) [12] have to be employed.
RefereRee$ [I] M.T. Huang, T.L. Martin, V. Malhotra and J.E. Mahan, J. Vacuum Sci. Teclmol. B 3 (1985) 836.
138
M. Alaoui et al. / Reaction between monosilane an.q a tantalum substrate
[2] T.A. Nguyen Tan, M. Azizan and J. Demen, Surface Sci. 189/190 (1987) 339. [3] D. Muller, F. Ringeisen, D. Bolmont and J.J. Koulmann, J. Non-Cryst. Solids 97&98 (1987) 1411. [4] J.H. We.wer, V.L. Moruzzi and F.A. Schmidt, Phys. Rev. B 23 (1981) 2916. [5] M. Aziz~n, Thesis, Grenoble (1987). [6] R.M. Walser and R.W. B6ne, Appl. Phys. Letter's 28 (1976) 624. [7] W. H0sler, W. Rudnick, H. G0bel, K, Cr~iber, T. Hillmer and A. Mitwalsky, Surface Interface Anal. 12 (1988) 356. [8] M. Alaoui. D. Muller, F. Ringeisen, D. Bolmont and J.J. Koulmann, to be published. [91 S.M. Gates, Surface Sci. 195 (1988) 307. [10l F. Neppl atxd U. Schwabe, IEEE Trans. Electron De'.-z~ ED-29 (1982) 508. [11] K. Hieber and F. Neppl, Thin Solid Films 140 (1986) 131. [121 C. Wieczoreiq Thin Solid Films 126 (1985) 227.
13.,
IlNTERFACL~L ~,EAC-'TION B E T W E E N M O N O S ] t L A N E A N D A POLYC-~YSTALLI~NE T A N T A L U M S U B S T R A T E M. A L A O U I , F. R I N G E I S E N , D. M U L L E R , D. B O L M O N T a n d J.J. K O U L M A N N Laboratoire de Physique et de Spectroscopie Electronique, FST, 4 rue des Frdres LumiJre, 68093 Mulhouse Cedex, France
Received 19 March 1989; accepted for publication 6 April 1989
We report XPS results on the thermal cracking of monosilane on a polycrystalline tantalum suhstrate. Ta substrates were heated in the 20°C-800°C temperature range and exposed to low pressure (10 -s Tort) Sill 4 doses. We followed the Ta/TaSi2 interface growth by recording the Si2p and Ta4f core level shifts. For temperatures below 600°C, results show growth of a thin ( -15 ,~) TaSi 2 film followed by a thin Si overlayer. By annealing at 750°C for 2 min, this Si surface layer intermixes with bulk tantalum. For preparation at higher temperatures (600°C-800°C), grown TaSi2 films get thicker ( - 25 ,~) but the Ta,/TaSi 2 interface is always covered by a thin silicon overlayer. The growth process of the Ta/TaSi 2 interface is limited by the diffusion coefficient of Ta through the TaSi 2 f'dm which acts as a diffusion barrier.
Polycrystalline TaSi2 is an interesting material ha rnieroelectronics d u e to its metallic character. Xts resistivity at r o o m t e m p e r a t u r e (RT) is a b o u t 40 ~ . c m [1]. W e were interested ha its p r e p a r a t i o n t h r o u g h m o n o s i l a n e exposure o f a heated t a n t a l u m substrate. T h e TaffFaSi2 interface is g r o w n b y thermal cracking o f low pressure (10 - s Torr) m o n o s i l a n e o n a polycrystalline t a n t a l u m substrate. U p to n o w only a few p h o t o e m i s s i o n studies w e r e p u b lished o n the S i / q ' a interface [2] (i.e. T a e v a p o r a t e d o n a Si substrate) but, to our knowledge, this p a p e r p r e s e n t s the first e x p e r i m e n t a l w o r k dealing with the inverse T a / S i interface (i.e. Si d e p o s i t e d o n a Ta substrate).
2. E x l m r i m e n ~ T h e experimental p r o c e d u r e has b e e n extensively described in ref. [3]. Briefly, polycrystalline T a foils (8 x 10 ram) were s u b m i t t e d to repetitive (Ar + s p u t t e r i n g - h e a t i n g ) cycles to clean the surface. T h e s a m p l e s are stuoied using X-ray p h o t o e m / s s i o n s p e c t r o s c o p y (XPS). 0169-4332/89/$03.50 © Elsev/er Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Div/slon)
] 34
M. A taoui et al. ,/Reaction between monosilane and a tantalum substrate
Before monosilane exposure the Ta samples were considered "clean" when the XPS peaks for impurities (C, O) were undetectable. After cleaning, the tantalum substrate was exposed to monosilane (Sill4) at a pressure of 10-5-10 -6 Tort for a given time. Monosilane (Air Liquide - Electronic quality) was introduced into the U H V chamber through a calibrated microleak in order to accurately control the pressure during exposure. As the arrival rate for monosilane was unknown we specify the Sill 4 partial pressure times the exposure time (1 L = 10 -6 Tort. s).
3. R e s ~ We submitted a tantalum substrate to increasing monosilane exposures at temperatures in the range 20-800 ° C. Two regimes can be distinguished: (a) 2 0 ° C < T~ < 6 0 0 ° C : We followed, by means of XPS, the evolution of the Si2p and T a 4 f core levels as a function of substrate temperature T~ and monosilane exposures. Fig. 1 shows the evolution of the Si2p peak for a substrate temperature of 400 o C. Even for low exposures we observe a shift toward higher binding energies. For a 104 L exposure, the Si2p structure is fixed at a position corresponding to bulk silicon and further growth is extremely slow. A 750 ° C anneal (2 rain) induces a 0.2-0.3 eV shift toward lower binding energies. This last position corresponds to Si2p in T a S i 2 as confirmed both by results obtained at higher temperatures and by Weaver's work on bulk TaSi2 [4]. The T a 4 f peak (fig. 2) shifts in the same way for increasing Sill 4 exposures pinning at a pos~.tion corresponding to T a 4 f in bulk TaSi 2. However, the T a 4 f signal is asymmetric suggesting a network in which the Ta atoms are not all in the same configuration. A 750 ° C anneal (2 min) causes a 0.2 eV shift toward lower binding energies, corresponding probably to a Ta richer environment. For all temperatures between 20 ° C and 600 ° C, results remain qualitatively the same. The average thicknesses of the TaSi 2 films were estimated from XPS peak intensities and are about 12 A at 3 0 0 ° C and 18 A at 4 0 0 ° C (fig. 5). (b) 6 0 0 ° C < T~ < 8 0 0 ° C : T h e mechanisms occurring in this temperature range seem easier to explain. The layers formed are thicker due to both thermally activated Ta diffusion in TaSi 2 and more efficient Sill4 thermal cracking at higher temperatures. We followed, by means of XPS, the Si2p and T a 4 f core level evolution as a function of monosilane exposure (figs. 3 and 4 respectively). For low exposures ( < 50 L), the Si2p core level binding energy is about 0.6 eV lower than in bulk silicon (fig. 3). In the intermediate 50-1000 L exposure range, its position remains unaffected corresponding to TaSi 2 environment. At higher
M. Alaoui et aL / Reaction between monosilane and a tantalum substrate
I 400 nO
Si 2p Sq Ir,..~2
£
135
a2~c.~g_ "~4,
400 "C
'~1
.,.:. =
.
!
/ ~ol,
I j/-7 28
26
24
2,9.
20
Bln~lng ~:ner~y (OV) Fig. 1. Si 2p core level s~cU'a after various Sill 4 exposures on a heated ( 4 0 0 ° C ) polycrystalllne T a substrate.
Fig. 2. T a 4 f core level spectra after various SiH 4 exposures on a heated ( z ~ 0 ° C ) polycrystalline T a substrate.
exposures ( > 1000 L), the Si2p peak progressively shffLs to the position related to bulk silicon. We checked the formation of a s/l/con surface layer by recording the relative variations of the $i2p and Ta4f peak intensities. For exposures above 104 L, the $i2p peak grows w/thout any shift while the intensity of the Ta4f peak continuously decreases. To identify this Si surface layer, we exposed it to atomic hydrogen obtained by cracking of He molecules at a hot W filament. By us/rig surface sensitive UPS we detected the characteristic features of the Si-H and Si-H z phases [3]. Th/s Si surface ~.ayer ascertains the b/riding energy of the Si 2p peak as being relevant to that of bulk silicon [3]. The binding energy of the Ta4f core level (fig. 4) increases w/th increasing Sill 4 exposures and a maximal shift of 0.9 eV is obtained for a 100 L exposure correspond/rig to a Ta$i z configuration [4]. At higher exposures, when a silicon surface layers forms, the Ta4f core level shifts about 0.2 eV toward lower binding energy w/thout any corresponding ddsplacement of the Si 2p core level. We conclude that the s~/dde near the surface gels Ta em'/ched, the "ira atoms being prey/deal by the underneath layers. At these temperatures, s/I/con
136
M. A laoui et al. / Reaction between monosilane and a tantalum substrate
Si2p 750
7 5 0 °C
"C
o2~ j
.
"~'~4f
T
I
i
d
I I
I
s.~o~ L,
"N o
Ta
Bin~ing Energy(eV) Fig. 3. Si 2p core level spectra after various Sill 4 exposures on a heated (750°C) polycrystallineTa substrate.
28
26 24 22 20 Binding Energy (Or) Fig. 4. T a 4 f core level spectra after various Sill 4 exposures on a heated ( 7 5 0 ° C ) polycrystalline T a substrate.
clusters and further growth of these Si crystallizes requires silicon being taken away from TaSi2. A similar situation should occur in silicon/metal Schottky barrier formation where an interfacial vitreous phase was evidenced with a composition close to that of the lowest temperature eutectic composition [5,6]. For the T a / S i system, this eutectic composition corresponds to approximately TaSil. s stoicbiometry, which is Ta richer than "~'aSi2 [7]. UPS spectra (not represented) show weakly defined structures wh, h cannot be totally attributed to the silicon surface overtayer [8]. Fig. 5 shows that no layer growth seems to occur in the 0-10 L exposure range. This is a direct consequence of the high reactivity of monosilane toward tantalum or/des. Indeed, it must be emphazised that, due to the general trend of cleaned metals to oxidize, a thin <>ride overlayer always forms aZ high temperature. As a consequence of the active reducing character of silane gas, monos/lane acts as a reducer of tantalum oxides induced by heating of the Ta subsZrate. During Sill 4 exposure under our experimental conditions, the first
214. Alaoui et al. / Reaction between monosilane and a tantabtm substrate
137
loo.ooo.o
o
io
~o 2
~o3
~o4
SIH4 ~POSURE (L)
Fig. 5. Film thicknessas a functionof Sill4 exposure for variousTa substrat¢ temperatures. 10 L are always used in this reduction pr~ .¢ss and the T a / S i H 4 interaction starts in fact above ~ 10 L. The 6 0 0 ° C temperature seems to be a linfit between two stages. For substrate temperatures below 600* C, saturation is ra~idly achieved whereas above 600 o C, thicker films (however lh-nited to - 30 A) can be grown.
4. C o n c i s i o n This study shows that it is possible to grow TaSi 2 f/]rns by direct thermal cracking of monosilane on a hot polyerystallme tantalum substrate. At low temperatures ( < 6 0 0 ° C ) , the grown films have TaSi2 stoichiometry. The thicknesses of these layers are essentially lindted by the diffusion coefficient of Ta through TaSi2 and the topmost surface layer is quite pure siUcon. Growth of th/s Si overlayer is limited by the low stick/rig coefficient of Sill4 on a $i surface [9]. At higher temperatures, the TaSi2 layers get thicker but are always covered by a thin silicon overlayer. Anew, Ta diffusion through TaSi 2 l/hilts the th/ckness of the grown f'flms. In this respect, the TaSi2 layer acts as a barrier toward Ta diffusion. At very high temperatures, or by annealing of layers prepared at lower temperatures, we observe formation of a surface smcide w/th composition different from that of bulk s/l/cide. The growth method described here is linfited as it does not perm/t the growth of thick TaSi2 films. For technological appfications where thicker films are required, other techniques ~ke coevaporation [10], cosputtefing [11] or chemical vapor deposition (CVD) [12] have to be employed.
RefereRee$ [I] M.T. Huang, T.L. Martin, V. Malhotra and J.E. Mahan, J. Vacuum Sci. Teclmol. B 3 (1985) 836.
138
M. Alaoui et al. / Reaction between monosilane an.q a tantalum substrate
[2] T.A. Nguyen Tan, M. Azizan and J. Demen, Surface Sci. 189/190 (1987) 339. [3] D. Muller, F. Ringeisen, D. Bolmont and J.J. Koulmann, J. Non-Cryst. Solids 97&98 (1987) 1411. [4] J.H. We.wer, V.L. Moruzzi and F.A. Schmidt, Phys. Rev. B 23 (1981) 2916. [5] M. Aziz~n, Thesis, Grenoble (1987). [6] R.M. Walser and R.W. B6ne, Appl. Phys. Letter's 28 (1976) 624. [7] W. H0sler, W. Rudnick, H. G0bel, K, Cr~iber, T. Hillmer and A. Mitwalsky, Surface Interface Anal. 12 (1988) 356. [8] M. Alaoui. D. Muller, F. Ringeisen, D. Bolmont and J.J. Koulmann, to be published. [91 S.M. Gates, Surface Sci. 195 (1988) 307. [10l F. Neppl atxd U. Schwabe, IEEE Trans. Electron De'.-z~ ED-29 (1982) 508. [11] K. Hieber and F. Neppl, Thin Solid Films 140 (1986) 131. [121 C. Wieczoreiq Thin Solid Films 126 (1985) 227.