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Oxidation of tantalum silicide thin films in an RF oxygen plasma
Applied Surface Science 70/71 (1993) 479-482 North-Holland
applied surface science
Oxidation of tantalum silicide thin films in an RF oxygen plasma R. G 6 m e z - S a n R o m a n a, R. P 6 r e z - C a s e r o a, j. P e r r i ~ r e b, j . p . E n a r d b a n d J.M. M a r t i n e z - D u a r t a Departamento Fisica Aplicada C-XII, UAM, 28049 Madrid, Spain b GPS Universit~s Paris VI et VII, Tour 23, 2 Place Jussieu, 75251 Paris, France Received 4 August 1992; accepted for publication 20 November 1992
We have investigated the RF plasma oxidation at floating potential of tantalum silicide with a Si:Ta ratio of 2.2 on (100) silicon. The oxidation was carried out in the 500-800°C temperature range. Rutherford backscattering spectrometry and nuclear reaction analysis have been used to study the composition of the samples and to determine the oxide growth kinetics. The oxidation rate was found to be controlled by the diffusion of the oxidizing species through the oxide. The oxidation leads to the growth of a pure silicon oxide film on surface. The growth of pure SiO 2 is postulated to occur by the supply of silicon atoms from both the substrate and the silicide layer. The existence of a diffusion barrier of SiO 2 between the silicon and the silicide avoids the supply of free silicon from the substrate. In this case, after the initial growth of SiO 2, the formation of an oxide layer where both silicon and metal atoms are present was observed.
1. Introduction The properties of silicides have made these materials quite useful for ULSI technology. Tantalum disilicide is a good candidate among the possible silicides for metallization schemes. In this case it is advantageous to growth an insulating layer on silicide. Thermal oxidation has been commonly used to growth this passivating layer [1]. However, this technique shows some disadvantages due to the high temperatures involved (dopants redistribution, defects generation). The decrease of processing temperature is necessary when device dimensions decrease. Plasma oxidation appears as a lower temperature technique to passivate silicides [2].
2. Experimental Thin films of tantalum silicide have been obtained by magnetron sputtering of TaSi z targets on Si(100) substrates. Some of the silicides were also deposited on SiO z (130 nm) substrates. RBS analysis shows the deposition of 200 nm thickness
layers with a Si:Ta ratio of 2.2. The samples were oxidized during several times in the 500-800°C temperature range. Oxidations were carried out in an RF oxygen plasma at floating potential. Plasmas were obtained at 2.5 × 10 -2 mbar oxygen pressure with a capacitively coupled RF (13.6 MHz) electrode and the R F power was 300 W. The composition of the grown oxides has been analyzed by means of Rutherford backscattering spectrometry. Nuclear analysis by direct observation of nuclear reactions ( 1 6 0 (d, p ) 1 7 0 * ) w a s used to determine the oxide growth kinetics.
3. Results The kinetics and temperature dependence of the oxide growth have been investigated. The amount of incorporated oxygen as a function of time at 800°C is shown in fig. 1. The relationship between incorporated oxygen and time is seen to follow X " = Kt where X is the amount of incorporated oxygen atoms in a time t and n provides information about the growth mechanism. The fitting yields a value of n = 1.9 + 8%. This means
0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
R. Gdmez-San Romdn et al. / Oxidation of tantalum silicide
480 100
o
200
TEMPERATURE (°C)
300
8
0
800 700
C9
--~
-
600
500
I
I
~--
400 - -
O
TaSiz s/Si o
:4
lOOi lO
•- ~
%
/ a
5
\\ \
0
\
e
lOO
\\
1
t (rain)
N
Fig. 1. N u m b e r of incorporated oxygen atoms versus time on logarithm scale for the plasma oxidation of TaSi2. 2 / S i at 800°C. The growth kinetics fitted to a pure parabolic law is also shown.
\\',,\\\
10
08
that the oxidation tends to be mostly diffusion limited. In fact, the data fit a X 2 - X i e = B t law where Xia accounts for the initial transient time of oxidation. The parabolic coefficient, B, is 389 (×1012 at ox/cm2) e min - l ) at 800°C and it is roughly the same for plasma oxidation of silicon under the same experimental conditions [2]. The existence of parabolic growth has been observed in the studied temperature range. Fig. 2 shows the oxygen incorporated during a fixed oxidation time (2 hours) at several temperatures in an Arrhenius representation. The activation energy for this process is 0.5 eV that is again the
103/T
30,
0.7 I
0.8 t
0.9 I
1.0 t
(K -~)
same that for silicon plasma oxidation and is much lower than that found for thermal oxidation of silicon (1-2 eV) [3]. The elemental oxide composition has been determined by RBS. Fig. 3 shows the spectra of TaSi2. 2 samples deposited on silicon before and after oxidation at 800°C during 3 hours. The
160
1.2
1.1 t
1.4
Fig. 2. Logarithm of incorporated oxygen atoms versus 1 / T for the plasma oxidation of TaSi2. 2 / S i during a fixed time (2 hours).
Energy (kieV) 0.6 t
1.2
165
Energy (MeV) t TO 1.75 t.8o I
I
I
L85
19o
I
120
25 2
l //
/:.<
°
;'i
"
t
;20 dv ~.~...~_.~ "~ 15
100
.
~ , , :,
8O Si
6o I
0
z
20 51o
",' I
I
i , , 420 440 460 480 380 400 100 150 200 Channel Channel Fig. 3. RBS spectra (2 MeV, O = 165 °, normal incidence) of a sample of TaSi2.2/Si before oxidation (solid line), oxidized at 800°C for 3 h (cross) and of a sample of T a S i 2 . J S i O 2 oxidized at 800°C for 3 h (dots).
R. Gdmez-San Romdn et al. / Oxidation of tantalum silicide
1° f
I a)
Z
0 ~.~ .<
...............
O0
~
Z
0
i
i
i
I
i
I
--
b)
0.5
512 N 0.0 .< 0
481
oxidation of silicides deposited on silicon substrates leads to the growth of SiO z on surface. Diffusion phenomena can be appreciated at the silicon/silicide interface at this temperature. However, the oxidation of silicides deposited on SiO 2 produces the initial growth of silicon oxide layer and, as the oxidation goes on, an oxide layer where both silicon and metal atoms are present is formed. The tantalum enrichment that had been previously observed is greatly enhanced in this case. The oxidation rate are closely related to the oxide composition. In fact, nuclear analysis shows that the amount of oxygen incorporated is greater when silicide is deposited on SiO 2 (358 × 1015 a t / c m 2) than when it is deposited on Si (289 × 1015 at/cm2).
o) I
o
1
2
4. Discussion
3
DEPTH ( x l O ta a t / e r a z) Fig. 4. Normalized composition (CTa + Csi + C o = 1) profiles of tantalum silicon and oxygen obtained from the simulation of the spectra of fig. 3: (a) TaSi2. 2 / S i before oxidation, (b) oxidized at 800°C for 3 h and (c) TaSi2. 2 / S i O 2 oxidized at 800°C for 3 h.
presence of the oxygen signal at its theoretical surface energy position makes evident the growth of an oxide layer on silicide. The silicon signal also appears at its surface position but the tantalum signal is shifted towards lower energies. This indicates that the grown oxide is almost pure SiO v The spectra reveal that this underlying silicide is slightly tantalum enriched (Si:Ta = 2,1) compared with the nonoxidized silicide. The composition of the oxide is quite different when the silicide is deposited on a diffusion barrier. It can be observed in the corresponding spectrum of an oxidized sample deposited on SiO z (fig. 3). The tantalum signal is shifted towards lower energies and increases when energy decreases. It is consistent with the formation of a SiO 2 layer on surface followed by an oxide layer where metal and silicon atoms are present. In a more detailed fashion fig. 4 shows the normalized composition profiles obtained from the simulation of the previous spectra [4]. The
Plasma oxidation of TaSi2. 2 silicides has been investigated in the 500-800°C temperature range. Oxidation kinetics seem to follow a parabolic law indicating that oxidation rate is controlled by diffusion of the oxidizing species through the oxide layer. The oxidation of silicides deposited on Si substrates leads to the formation of almost pure SiO 2 on surface. The growth of free metal oxide could be explained resorting to the formation of volatile tantalum oxides or to the silicide dissociation and diffusion of metal atoms towards the silicon/silicide interface. However, the RBS analysis demonstrates that there is no loss of metal after the oxidation process within the experimental accuracy (3%). On the other hand, no tantalum diffusion has been reported even at higher oxidation temperatures [5]. So, it is necessary to resort to silicon atomic transport. Under this assumption the silicon necessary for the SiO 2 formation could be supplied from the silicide or from the silicon substrate. Both mechanisms exist as can be observed by the slightly tantalum enrichment of the silicide layer and by the diffusion profiles near the silicon/silicide interface. The extent of each one seems to depend on the oxidation temperature. In this way the diffusion of silicon from the substrate is increased with temperature and plays an important role at tempera-
482
R. G6mez-San Romdn et al. / Oxidation of tantalum silicide
tures higher than 600°C. When the oxidation is carried out in presence of a diffusion barrier that avoids the silicon supply from the substrate, only the silicon from the silicide can participate in the formation of SiO2. In fact, silicon oxide is grown during the first steps of oxidation, but a mixture of oxides is grown after some time. This time coincides with the event that Si:Ta ratio in the silicide layer is 2. So, it can be inferred that the silicon necessary to grow the initial SiO 2 layer comes from the silicon excess in the silicide. Once these silicon atoms are depleted (Si:Ta < 2) the silicide oxidation occurs and gives rise to the formation of an oxide mixture. The different oxidation rate related to oxide composition would agree with different oxygen diffusion coefficients through SiO 2 or an oxide mixture. It could explain the differences between the amounts of oxygen incorporated during oxidation of the samples deposited on Si or SiO 2.
5. Conclusions T h e plasma oxidation of T a S i 2 . J S i leads to the formation of a SiO 2 layer on surface in the 500-800°C range. The oxide growth fits parabolic kinetics and seems to be controlled by the diffu-
sion of oxygen through the growing oxide. The dependence of oxide growth on t e m p e r a t u r e follows an Arrhenius law with an activation energy of 0.5 eV. The silicon supply needed for the formation of free metal oxide is provided either by the silicon substrate or by the silicide. In absence of silicon supply from the substrate, once the silicon excess is depleted from the silicide (Si:Ta < 2) an oxide layer in which both cations are present is grown.
Acknowledgement This work has been supported by the CNRS ( G D R 86).
References [1] F.M. D'Heurle, A. Cros, R.D. Frampton and E.A. Irene, Phil. Mag. B 55 (1987) 291. [2] R. P6rez-Casero, J. Perri~re. J.P. Enard, A. Straboni, B. Vuillermoz, A. Climent and J.M. Martlnez-Duart, J. Appl. Phys. 69 (1991) 1407. [3] H. Du, R.E. Tressler, K.E. Spear and C.G. Pantano, J. Electrochem. Soc. 136 (1989) 1527. [4] L.R. Doolittle, Nucl. Instrum. Meth. B 9 (1985) 344. [5] K.C. Saraswat, R.S. Nowicki and J.F. Moulder, Appl. Phys. Lett. 41 (1982) 1127.
applied surface science
Oxidation of tantalum silicide thin films in an RF oxygen plasma R. G 6 m e z - S a n R o m a n a, R. P 6 r e z - C a s e r o a, j. P e r r i ~ r e b, j . p . E n a r d b a n d J.M. M a r t i n e z - D u a r t a Departamento Fisica Aplicada C-XII, UAM, 28049 Madrid, Spain b GPS Universit~s Paris VI et VII, Tour 23, 2 Place Jussieu, 75251 Paris, France Received 4 August 1992; accepted for publication 20 November 1992
We have investigated the RF plasma oxidation at floating potential of tantalum silicide with a Si:Ta ratio of 2.2 on (100) silicon. The oxidation was carried out in the 500-800°C temperature range. Rutherford backscattering spectrometry and nuclear reaction analysis have been used to study the composition of the samples and to determine the oxide growth kinetics. The oxidation rate was found to be controlled by the diffusion of the oxidizing species through the oxide. The oxidation leads to the growth of a pure silicon oxide film on surface. The growth of pure SiO 2 is postulated to occur by the supply of silicon atoms from both the substrate and the silicide layer. The existence of a diffusion barrier of SiO 2 between the silicon and the silicide avoids the supply of free silicon from the substrate. In this case, after the initial growth of SiO 2, the formation of an oxide layer where both silicon and metal atoms are present was observed.
1. Introduction The properties of silicides have made these materials quite useful for ULSI technology. Tantalum disilicide is a good candidate among the possible silicides for metallization schemes. In this case it is advantageous to growth an insulating layer on silicide. Thermal oxidation has been commonly used to growth this passivating layer [1]. However, this technique shows some disadvantages due to the high temperatures involved (dopants redistribution, defects generation). The decrease of processing temperature is necessary when device dimensions decrease. Plasma oxidation appears as a lower temperature technique to passivate silicides [2].
2. Experimental Thin films of tantalum silicide have been obtained by magnetron sputtering of TaSi z targets on Si(100) substrates. Some of the silicides were also deposited on SiO z (130 nm) substrates. RBS analysis shows the deposition of 200 nm thickness
layers with a Si:Ta ratio of 2.2. The samples were oxidized during several times in the 500-800°C temperature range. Oxidations were carried out in an RF oxygen plasma at floating potential. Plasmas were obtained at 2.5 × 10 -2 mbar oxygen pressure with a capacitively coupled RF (13.6 MHz) electrode and the R F power was 300 W. The composition of the grown oxides has been analyzed by means of Rutherford backscattering spectrometry. Nuclear analysis by direct observation of nuclear reactions ( 1 6 0 (d, p ) 1 7 0 * ) w a s used to determine the oxide growth kinetics.
3. Results The kinetics and temperature dependence of the oxide growth have been investigated. The amount of incorporated oxygen as a function of time at 800°C is shown in fig. 1. The relationship between incorporated oxygen and time is seen to follow X " = Kt where X is the amount of incorporated oxygen atoms in a time t and n provides information about the growth mechanism. The fitting yields a value of n = 1.9 + 8%. This means
0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
R. Gdmez-San Romdn et al. / Oxidation of tantalum silicide
480 100
o
200
TEMPERATURE (°C)
300
8
0
800 700
C9
--~
-
600
500
I
I
~--
400 - -
O
TaSiz s/Si o
:4
lOOi lO
•- ~
%
/ a
5
\\ \
0
\
e
lOO
\\
1
t (rain)
N
Fig. 1. N u m b e r of incorporated oxygen atoms versus time on logarithm scale for the plasma oxidation of TaSi2. 2 / S i at 800°C. The growth kinetics fitted to a pure parabolic law is also shown.
\\',,\\\
10
08
that the oxidation tends to be mostly diffusion limited. In fact, the data fit a X 2 - X i e = B t law where Xia accounts for the initial transient time of oxidation. The parabolic coefficient, B, is 389 (×1012 at ox/cm2) e min - l ) at 800°C and it is roughly the same for plasma oxidation of silicon under the same experimental conditions [2]. The existence of parabolic growth has been observed in the studied temperature range. Fig. 2 shows the oxygen incorporated during a fixed oxidation time (2 hours) at several temperatures in an Arrhenius representation. The activation energy for this process is 0.5 eV that is again the
103/T
30,
0.7 I
0.8 t
0.9 I
1.0 t
(K -~)
same that for silicon plasma oxidation and is much lower than that found for thermal oxidation of silicon (1-2 eV) [3]. The elemental oxide composition has been determined by RBS. Fig. 3 shows the spectra of TaSi2. 2 samples deposited on silicon before and after oxidation at 800°C during 3 hours. The
160
1.2
1.1 t
1.4
Fig. 2. Logarithm of incorporated oxygen atoms versus 1 / T for the plasma oxidation of TaSi2. 2 / S i during a fixed time (2 hours).
Energy (kieV) 0.6 t
1.2
165
Energy (MeV) t TO 1.75 t.8o I
I
I
L85
19o
I
120
25 2
l //
/:.<
°
;'i
"
t
;20 dv ~.~...~_.~ "~ 15
100
.
~ , , :,
8O Si
6o I
0
z
20 51o
",' I
I
i , , 420 440 460 480 380 400 100 150 200 Channel Channel Fig. 3. RBS spectra (2 MeV, O = 165 °, normal incidence) of a sample of TaSi2.2/Si before oxidation (solid line), oxidized at 800°C for 3 h (cross) and of a sample of T a S i 2 . J S i O 2 oxidized at 800°C for 3 h (dots).
R. Gdmez-San Romdn et al. / Oxidation of tantalum silicide
1° f
I a)
Z
0 ~.~ .<
...............
O0
~
Z
0
i
i
i
I
i
I
--
b)
0.5
512 N 0.0 .< 0
481
oxidation of silicides deposited on silicon substrates leads to the growth of SiO z on surface. Diffusion phenomena can be appreciated at the silicon/silicide interface at this temperature. However, the oxidation of silicides deposited on SiO 2 produces the initial growth of silicon oxide layer and, as the oxidation goes on, an oxide layer where both silicon and metal atoms are present is formed. The tantalum enrichment that had been previously observed is greatly enhanced in this case. The oxidation rate are closely related to the oxide composition. In fact, nuclear analysis shows that the amount of oxygen incorporated is greater when silicide is deposited on SiO 2 (358 × 1015 a t / c m 2) than when it is deposited on Si (289 × 1015 at/cm2).
o) I
o
1
2
4. Discussion
3
DEPTH ( x l O ta a t / e r a z) Fig. 4. Normalized composition (CTa + Csi + C o = 1) profiles of tantalum silicon and oxygen obtained from the simulation of the spectra of fig. 3: (a) TaSi2. 2 / S i before oxidation, (b) oxidized at 800°C for 3 h and (c) TaSi2. 2 / S i O 2 oxidized at 800°C for 3 h.
presence of the oxygen signal at its theoretical surface energy position makes evident the growth of an oxide layer on silicide. The silicon signal also appears at its surface position but the tantalum signal is shifted towards lower energies. This indicates that the grown oxide is almost pure SiO v The spectra reveal that this underlying silicide is slightly tantalum enriched (Si:Ta = 2,1) compared with the nonoxidized silicide. The composition of the oxide is quite different when the silicide is deposited on a diffusion barrier. It can be observed in the corresponding spectrum of an oxidized sample deposited on SiO z (fig. 3). The tantalum signal is shifted towards lower energies and increases when energy decreases. It is consistent with the formation of a SiO 2 layer on surface followed by an oxide layer where metal and silicon atoms are present. In a more detailed fashion fig. 4 shows the normalized composition profiles obtained from the simulation of the previous spectra [4]. The
Plasma oxidation of TaSi2. 2 silicides has been investigated in the 500-800°C temperature range. Oxidation kinetics seem to follow a parabolic law indicating that oxidation rate is controlled by diffusion of the oxidizing species through the oxide layer. The oxidation of silicides deposited on Si substrates leads to the formation of almost pure SiO 2 on surface. The growth of free metal oxide could be explained resorting to the formation of volatile tantalum oxides or to the silicide dissociation and diffusion of metal atoms towards the silicon/silicide interface. However, the RBS analysis demonstrates that there is no loss of metal after the oxidation process within the experimental accuracy (3%). On the other hand, no tantalum diffusion has been reported even at higher oxidation temperatures [5]. So, it is necessary to resort to silicon atomic transport. Under this assumption the silicon necessary for the SiO 2 formation could be supplied from the silicide or from the silicon substrate. Both mechanisms exist as can be observed by the slightly tantalum enrichment of the silicide layer and by the diffusion profiles near the silicon/silicide interface. The extent of each one seems to depend on the oxidation temperature. In this way the diffusion of silicon from the substrate is increased with temperature and plays an important role at tempera-
482
R. G6mez-San Romdn et al. / Oxidation of tantalum silicide
tures higher than 600°C. When the oxidation is carried out in presence of a diffusion barrier that avoids the silicon supply from the substrate, only the silicon from the silicide can participate in the formation of SiO2. In fact, silicon oxide is grown during the first steps of oxidation, but a mixture of oxides is grown after some time. This time coincides with the event that Si:Ta ratio in the silicide layer is 2. So, it can be inferred that the silicon necessary to grow the initial SiO 2 layer comes from the silicon excess in the silicide. Once these silicon atoms are depleted (Si:Ta < 2) the silicide oxidation occurs and gives rise to the formation of an oxide mixture. The different oxidation rate related to oxide composition would agree with different oxygen diffusion coefficients through SiO 2 or an oxide mixture. It could explain the differences between the amounts of oxygen incorporated during oxidation of the samples deposited on Si or SiO 2.
5. Conclusions T h e plasma oxidation of T a S i 2 . J S i leads to the formation of a SiO 2 layer on surface in the 500-800°C range. The oxide growth fits parabolic kinetics and seems to be controlled by the diffu-
sion of oxygen through the growing oxide. The dependence of oxide growth on t e m p e r a t u r e follows an Arrhenius law with an activation energy of 0.5 eV. The silicon supply needed for the formation of free metal oxide is provided either by the silicon substrate or by the silicide. In absence of silicon supply from the substrate, once the silicon excess is depleted from the silicide (Si:Ta < 2) an oxide layer in which both cations are present is grown.
Acknowledgement This work has been supported by the CNRS ( G D R 86).
References [1] F.M. D'Heurle, A. Cros, R.D. Frampton and E.A. Irene, Phil. Mag. B 55 (1987) 291. [2] R. P6rez-Casero, J. Perri~re. J.P. Enard, A. Straboni, B. Vuillermoz, A. Climent and J.M. Martlnez-Duart, J. Appl. Phys. 69 (1991) 1407. [3] H. Du, R.E. Tressler, K.E. Spear and C.G. Pantano, J. Electrochem. Soc. 136 (1989) 1527. [4] L.R. Doolittle, Nucl. Instrum. Meth. B 9 (1985) 344. [5] K.C. Saraswat, R.S. Nowicki and J.F. Moulder, Appl. Phys. Lett. 41 (1982) 1127.