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Characterization of Si-Rich WSix on Si
286
Applied
CHARACTERIZATION
Norifumi
FUJIMURA,
Deportment
OJ Metallurgicul
OF Si-RICH
WSi,
Shoji TACHIBANA Engineering,
Surface
Science 41/42
(1989) 2866289 North-Holland
ON Si
* and Taichiro
College of Engineertng,
IT0
Unroerstt_y of Osuku PreJecture, Mom-Umrmuch~.
Sakar,
Osaku 591, Japan
Received
8 November
1988; accepted
for publication
5 April 1989
The structural change of CVD WSiZ.h by annealing in vacuum and in oxygen atmosphere has been studied. By annealing in vacuum. the first phenomenon, which occurs at 400°C, is crystallization of the amorphous film to a metastable semiconducting hexagonal WSi,. Excess Si in WSi,, has already precipitated at the WSi,/Si interface. The second phenomenon, which occurs at 600 o C, is the phase transformation of the hexagonal structure to the tetragonal one. Above 1000 o C, the precipitated excess Si grows epttaxially on the Si(100) substrate, while a SiO, layer is formed by oxidizing above 700 o C. The film thickness of the SiO, layer increases with the increase of the oxidation temperature. Owing to the increment of the ftlm thickness, the interface between WSiz and Si moves into the Si substrate. Moreover. there are some circular parts crystallized as a tetragonal SiO, (a-cristobalite) of about 5 pm in diameter.
1. lntrbduction
The disilicides have been chosen to pair with heavily doped poly-Si to form polycide gates because of the criteria of low electrical resistivity and high temperature stability. The CVD WSiZ has been widely studied and used as polycide gates in high density field effect transistor (FET) [l-5]. Besides, WSi, formed by other methods such as co-evaporation, sputtering and solid phase reaction has been investigated [6-S]. In this paper, characterization of Si-rich WSi,,, particularly precipitation of excess Si and the oxidation mechanism, has been investigated.
2. Experiments wsi 2.6 films were formed by cold wall CVD directly on p-type Si(100) substrate. WF, and SiH, were used as reaction gases, and these flow rates were 2 and 120 cm3/min, respectively. Helium was used as a dilution gas. The pressure was 40 Pa. The substrate temperature was 350°C. The
* Graduate
school.
0169-4332/89/$03.50 (North-Holland)
University
of Osaka
Prefecture.
((2 Elsevier Science Publishers
B.V
film composition analyzed by Rutherford backscattering was WSi 2,h. The samples were annealed in vacuum of 4 X lo-’ Torr and oxidized in 100% dry 0,. The crystal structure, the electrical resistivity. chemical bonding state were evaluated by using selected area diffraction (SAD) of transmission electron microscopy (TEM), a four-point probe method and X-ray photoelectron spectroscopy (XPS).
3. Results
and discussion
As shown in figs. 1 and lb, the as-deposited structure and crystalWSi 2.6 has an amorphous lizes into hexagonal WSi, by annealing in vacuum at 400°C for 1 h. Then the hexagonal phase begins to transform into the tetragonal one above 500 o C. By annealing at 800” C for 1 h. the phase transformation has completely finished as shown in fig. lc. The depth profile of XPS spectrum shows that the excess Si has precipitated at the by interface between WSi? and the Si substrate annealing at 400°C (fig. 2). The binding energies WSi 2,h are of W 4f,,, and 4f,,, from as-deposited 33.1 and 30.9 eV. respectively. These values do not
N. Fujimura et al. / Characterization
d,:
measured
d-spacings,
d,: d-spacings
dm
d,
4.00 3.39 2.31 2.17 2.00 H-WSi,:
4.00 3.39 2.31 2.17 2.00 hexagonal
from JCPDS
Fig. 1. SAD from the sample annealed
at each temperature
100
\ 50 Depth
150
100 from
Fig. 2. Depth profiles
the
surface
of XPS spectra.
H-WSi,(lOO) H-WSi2(101) H-WSi,(llO) H-WSi2(111) H-WSi,(200) WSiz
287
dm
d,
3.91 3.01 2.21 2.03 1.97 1.61 1.48 T-WSi 2:
3.91 2.97 2.27 2.02 1.96 1.60 1.49 tetragonal
‘T-WSiz(002) T-WSi,(lOl) T-WSi2(110) T-WSi,(l03) T-WSiz(112) T-WSi2(200) T-WSi2(202) WSi,
card.
change depending on the state (amorphous, hexagonal and tetragonal). However, the temperature dependence of resistivity is different between the
0
of Si-rich WSi 1 on Si
200
(nm)
for 1 h: (a) as-deposited,
(b) 400 o C, (c) 800 ’ C.
hexagonal phase and the tetragonal one. The hexagonal WSi, shows a semiconducting resistivity change against temperature and the tetragonal one shows a metallic resistivity change. So the resistivity of the tetragonal WSi, at room temperature, about 45 @ cm, is much lower than that of the hexagonal one. In order to observe the precipitated excess Si, TEM and SAD observation was performed on the sample after removal of the surface WSi, with HNO, : HF : H,O (5 : 3 : 200). By annealing at 1000 o C, the excess Si precipitated at the WSi,/substrate interface grows epitaxially on the Si(100) substrate. Fig. 3 shows the SAD from the sample annealed at 1000” C. There are two kinds of spots. One is from the substrate and another is from the precipitated excess Si (small spots). On the other hand, a SiO, layer is formed by annealing above 700 “C in a 100% dry 0, atmosphere. XPS depth analysis identified that a 830 A thick SiO, layer is formed at the WSi, surface and the WSiJsubstrate interface has moved into the
288
N. Ftqimura
et al. / Churacterrzatmn
of St-nch
WSI I on SI
already precipitated free energy changes tion are:
by annealing at 400°C. The occurring during the oxida-
WSi,+20,=2SiO,+W, AC = 0.8008T - 408.574
(kcal/mol),
at 1273 K (lOOO”C), AC = - 306.632 (kcal/mol)
Fig. 3. SAD
from the sample annealed at 1000°C
for 1 h and
etched WSi 2 with 47 HF/HNO,.
substrate. Fig. 4 shows the binding energies (E,,) of W 4f, Si 2p and 0 Is of the sample oxidized at 1000 o C. The E, from Si 2p shifts from 99.3 (Si in WSi,) to 104.7 eV (SiO,). The E, of W4f: 33.1 (4f,,,) and 31 eV (4f7,2), is the same as for the sample annealed in vacuum. These values shift towards higher values (about 1 eV), and approach the values of W (37 and 33 eV). From this fact, it seems that there are W-W bonds in WSi, near the SiO,/ WSi interface. Excess Si in WSi was h
.
The following oxidation mechanism of Si-rich WSi, is proposed: (1) precipitation of excess Si at the WSi,/substrate interface, (2) oxidation of Si in WSi, at the surface, (3) diffusion of W atoms toward the substrate, (4) bonding of W atoms with Si at the WSiJsubstrate interface. The TEM observation was performed for SiO, on the WSi, layer (fig. 5a). Many domains can be seen in the SiO, layer. Fig. 5b shows the TEM of the WSi, layer on the sample after chemical removal of surface SiO,. These domains in SiO, correspond well to the grains of WSi,. This phenomenon is one piece of evidence of the oxidation of Si in WSi:. Moreover, there are some circular crystallized regions in the SiOz layer. SAD analysis identified the crystal structure as tetragonal SiO, (cY-cristobalite). Amorphous SiOZ consists of
N. Fujimura et al. / Characterization
Fig. 5. TEM and SAD from the sample oxidized
of S-rich
WSi,
dm
d,
3.91 3.98 2.28 2.03 1.91 1.60 1.48 T-WSi,:
3.91 2.91 2.21 2.02 1.96 1.60 1.49 tetragonal
at 1000 o C for 1 h: (a) as-oxidized,
composition tetrahedral SiO,,, , a fundamental unit. As the units are cross linked with bridging oxygens, they cannot move. However, if the bridging oxygen is replaced by fluorine with one valence electron, the units can ensure the binding angle necessary for crystallization. In summary, annealing and oxidation characteristics of CVD WSi,,6 films have been studied. The excess Si has already precipitated when amorphous WSi 2,6 is crystallized as semiconducting hexagonal W Si 2, and grown epitaxially to the substrate. A oxidation mechanism of WSi,, has been proposed. Moreover, circular crystallized regions as cy-cristobalite have been observed in the amorphous SiO, layer.
289
on Si
T-WSi,(002) T-WSi,(lOl) T-WSi,(llO) T-WSi,(l03) T-WSi,(llZ) T-WSi,(200) T-WSi,(202) WSi,
(b) after removal
of surface
SiO, layer.
References [1] A. Akimoto and K. Watanabe, Appl. Phys. Letters 39 (1981) 445. [2] D.L. Brors, J.A. Fair, K.A. Manning and K.C. Saraswat, Solid State Technol. 26 (1983) 183. (3) M. Fukumoto, A. Shinohara, S. Okada and K. Kugimiya, IEEE Trans. Electron Devices ED-31 (1984) 1432. [4] T. Hara, S. Emoto and T. Jinbo, Japan. J. Appl. Phys. 23 (1984) L455. [5] Y. Shioya, T. Itoh, I. Kobayashi and M. Maeda, J. Electrothem. Sot. 133 (1986) 1475. [6] S.P. Murarka, M.H. Read, C.J. Doherty and D.B. Fraser, J. Electrochem. Sot. 129 (1982) 293. (71 F. Nava, B.Z. Weiss, K.Y. Ahn, D.A. Smith and K.N. Tu, J. Appl. Phys. 64 (1988) 354. [E] T. Hara, H. Hayashida and S. Takahashi, J. Electrochem. Sot. 135 (1988) 970.
Applied
CHARACTERIZATION
Norifumi
FUJIMURA,
Deportment
OJ Metallurgicul
OF Si-RICH
WSi,
Shoji TACHIBANA Engineering,
Surface
Science 41/42
(1989) 2866289 North-Holland
ON Si
* and Taichiro
College of Engineertng,
IT0
Unroerstt_y of Osuku PreJecture, Mom-Umrmuch~.
Sakar,
Osaku 591, Japan
Received
8 November
1988; accepted
for publication
5 April 1989
The structural change of CVD WSiZ.h by annealing in vacuum and in oxygen atmosphere has been studied. By annealing in vacuum. the first phenomenon, which occurs at 400°C, is crystallization of the amorphous film to a metastable semiconducting hexagonal WSi,. Excess Si in WSi,, has already precipitated at the WSi,/Si interface. The second phenomenon, which occurs at 600 o C, is the phase transformation of the hexagonal structure to the tetragonal one. Above 1000 o C, the precipitated excess Si grows epttaxially on the Si(100) substrate, while a SiO, layer is formed by oxidizing above 700 o C. The film thickness of the SiO, layer increases with the increase of the oxidation temperature. Owing to the increment of the ftlm thickness, the interface between WSiz and Si moves into the Si substrate. Moreover. there are some circular parts crystallized as a tetragonal SiO, (a-cristobalite) of about 5 pm in diameter.
1. lntrbduction
The disilicides have been chosen to pair with heavily doped poly-Si to form polycide gates because of the criteria of low electrical resistivity and high temperature stability. The CVD WSiZ has been widely studied and used as polycide gates in high density field effect transistor (FET) [l-5]. Besides, WSi, formed by other methods such as co-evaporation, sputtering and solid phase reaction has been investigated [6-S]. In this paper, characterization of Si-rich WSi,,, particularly precipitation of excess Si and the oxidation mechanism, has been investigated.
2. Experiments wsi 2.6 films were formed by cold wall CVD directly on p-type Si(100) substrate. WF, and SiH, were used as reaction gases, and these flow rates were 2 and 120 cm3/min, respectively. Helium was used as a dilution gas. The pressure was 40 Pa. The substrate temperature was 350°C. The
* Graduate
school.
0169-4332/89/$03.50 (North-Holland)
University
of Osaka
Prefecture.
((2 Elsevier Science Publishers
B.V
film composition analyzed by Rutherford backscattering was WSi 2,h. The samples were annealed in vacuum of 4 X lo-’ Torr and oxidized in 100% dry 0,. The crystal structure, the electrical resistivity. chemical bonding state were evaluated by using selected area diffraction (SAD) of transmission electron microscopy (TEM), a four-point probe method and X-ray photoelectron spectroscopy (XPS).
3. Results
and discussion
As shown in figs. 1 and lb, the as-deposited structure and crystalWSi 2.6 has an amorphous lizes into hexagonal WSi, by annealing in vacuum at 400°C for 1 h. Then the hexagonal phase begins to transform into the tetragonal one above 500 o C. By annealing at 800” C for 1 h. the phase transformation has completely finished as shown in fig. lc. The depth profile of XPS spectrum shows that the excess Si has precipitated at the by interface between WSi? and the Si substrate annealing at 400°C (fig. 2). The binding energies WSi 2,h are of W 4f,,, and 4f,,, from as-deposited 33.1 and 30.9 eV. respectively. These values do not
N. Fujimura et al. / Characterization
d,:
measured
d-spacings,
d,: d-spacings
dm
d,
4.00 3.39 2.31 2.17 2.00 H-WSi,:
4.00 3.39 2.31 2.17 2.00 hexagonal
from JCPDS
Fig. 1. SAD from the sample annealed
at each temperature
100
\ 50 Depth
150
100 from
Fig. 2. Depth profiles
the
surface
of XPS spectra.
H-WSi,(lOO) H-WSi2(101) H-WSi,(llO) H-WSi2(111) H-WSi,(200) WSiz
287
dm
d,
3.91 3.01 2.21 2.03 1.97 1.61 1.48 T-WSi 2:
3.91 2.97 2.27 2.02 1.96 1.60 1.49 tetragonal
‘T-WSiz(002) T-WSi,(lOl) T-WSi2(110) T-WSi,(l03) T-WSiz(112) T-WSi2(200) T-WSi2(202) WSi,
card.
change depending on the state (amorphous, hexagonal and tetragonal). However, the temperature dependence of resistivity is different between the
0
of Si-rich WSi 1 on Si
200
(nm)
for 1 h: (a) as-deposited,
(b) 400 o C, (c) 800 ’ C.
hexagonal phase and the tetragonal one. The hexagonal WSi, shows a semiconducting resistivity change against temperature and the tetragonal one shows a metallic resistivity change. So the resistivity of the tetragonal WSi, at room temperature, about 45 @ cm, is much lower than that of the hexagonal one. In order to observe the precipitated excess Si, TEM and SAD observation was performed on the sample after removal of the surface WSi, with HNO, : HF : H,O (5 : 3 : 200). By annealing at 1000 o C, the excess Si precipitated at the WSi,/substrate interface grows epitaxially on the Si(100) substrate. Fig. 3 shows the SAD from the sample annealed at 1000” C. There are two kinds of spots. One is from the substrate and another is from the precipitated excess Si (small spots). On the other hand, a SiO, layer is formed by annealing above 700 “C in a 100% dry 0, atmosphere. XPS depth analysis identified that a 830 A thick SiO, layer is formed at the WSi, surface and the WSiJsubstrate interface has moved into the
288
N. Ftqimura
et al. / Churacterrzatmn
of St-nch
WSI I on SI
already precipitated free energy changes tion are:
by annealing at 400°C. The occurring during the oxida-
WSi,+20,=2SiO,+W, AC = 0.8008T - 408.574
(kcal/mol),
at 1273 K (lOOO”C), AC = - 306.632 (kcal/mol)
Fig. 3. SAD
from the sample annealed at 1000°C
for 1 h and
etched WSi 2 with 47 HF/HNO,.
substrate. Fig. 4 shows the binding energies (E,,) of W 4f, Si 2p and 0 Is of the sample oxidized at 1000 o C. The E, from Si 2p shifts from 99.3 (Si in WSi,) to 104.7 eV (SiO,). The E, of W4f: 33.1 (4f,,,) and 31 eV (4f7,2), is the same as for the sample annealed in vacuum. These values shift towards higher values (about 1 eV), and approach the values of W (37 and 33 eV). From this fact, it seems that there are W-W bonds in WSi, near the SiO,/ WSi interface. Excess Si in WSi was h
.
The following oxidation mechanism of Si-rich WSi, is proposed: (1) precipitation of excess Si at the WSi,/substrate interface, (2) oxidation of Si in WSi, at the surface, (3) diffusion of W atoms toward the substrate, (4) bonding of W atoms with Si at the WSiJsubstrate interface. The TEM observation was performed for SiO, on the WSi, layer (fig. 5a). Many domains can be seen in the SiO, layer. Fig. 5b shows the TEM of the WSi, layer on the sample after chemical removal of surface SiO,. These domains in SiO, correspond well to the grains of WSi,. This phenomenon is one piece of evidence of the oxidation of Si in WSi:. Moreover, there are some circular crystallized regions in the SiOz layer. SAD analysis identified the crystal structure as tetragonal SiO, (cY-cristobalite). Amorphous SiOZ consists of
N. Fujimura et al. / Characterization
Fig. 5. TEM and SAD from the sample oxidized
of S-rich
WSi,
dm
d,
3.91 3.98 2.28 2.03 1.91 1.60 1.48 T-WSi,:
3.91 2.91 2.21 2.02 1.96 1.60 1.49 tetragonal
at 1000 o C for 1 h: (a) as-oxidized,
composition tetrahedral SiO,,, , a fundamental unit. As the units are cross linked with bridging oxygens, they cannot move. However, if the bridging oxygen is replaced by fluorine with one valence electron, the units can ensure the binding angle necessary for crystallization. In summary, annealing and oxidation characteristics of CVD WSi,,6 films have been studied. The excess Si has already precipitated when amorphous WSi 2,6 is crystallized as semiconducting hexagonal W Si 2, and grown epitaxially to the substrate. A oxidation mechanism of WSi,, has been proposed. Moreover, circular crystallized regions as cy-cristobalite have been observed in the amorphous SiO, layer.
289
on Si
T-WSi,(002) T-WSi,(lOl) T-WSi,(llO) T-WSi,(l03) T-WSi,(llZ) T-WSi,(200) T-WSi,(202) WSi,
(b) after removal
of surface
SiO, layer.
References [1] A. Akimoto and K. Watanabe, Appl. Phys. Letters 39 (1981) 445. [2] D.L. Brors, J.A. Fair, K.A. Manning and K.C. Saraswat, Solid State Technol. 26 (1983) 183. (3) M. Fukumoto, A. Shinohara, S. Okada and K. Kugimiya, IEEE Trans. Electron Devices ED-31 (1984) 1432. [4] T. Hara, S. Emoto and T. Jinbo, Japan. J. Appl. Phys. 23 (1984) L455. [5] Y. Shioya, T. Itoh, I. Kobayashi and M. Maeda, J. Electrothem. Sot. 133 (1986) 1475. [6] S.P. Murarka, M.H. Read, C.J. Doherty and D.B. Fraser, J. Electrochem. Sot. 129 (1982) 293. (71 F. Nava, B.Z. Weiss, K.Y. Ahn, D.A. Smith and K.N. Tu, J. Appl. Phys. 64 (1988) 354. [E] T. Hara, H. Hayashida and S. Takahashi, J. Electrochem. Sot. 135 (1988) 970.