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Backscattering analysis of the successive layer structures of titanium silicides
Thin Solid Films, 64 (1979) 439-444 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands
439
B A C K S C A T T E R I N G ANALYSIS O F T H E SUCCESSIVE L A Y E R S T R U C T U R E S OF T I T A N I U M SILICIDES * JER-SHEN MAAt, CHIEN-JUNG LIN AND JUI-HSIANG LIU Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu ( Taiwan ) YUEN-CHUNG LIU Department of Physics, National Tsing Hua University, Hsinchu ( Taiwan ) (Received April 12, 1979; accepted April 27, 1979)
Solid state reactions of titanium thin films with silicon and with SiO 2 were studied using a backscattering technique. For the Ti-Si system layers of ZiSi2, TiSi and TisSi 3 were detected in the temperature range 500-600 °C. For the Ti-SiO2 system layers of TiO~ and Ti s Si a were formed in the temperature range 700-900 °C. At temperatures above 1000 °C the oxygen in the film disappeared and silicon was found to reach the film surface. A surface structure of concentric circular rings was observed.
1. INTRODUCTION
The solid state reaction of titanium thin films with silicon was first studied by Bower and Mayer 1. Only the TiSi 2 phase was observed in the temperature range 600-1000°C. Intermediate phases of titanium silicides have been formed by annealing at lower temperatures and have been analysed by Kato and Nakamura 2 using a Guinier focusing X-ray camera in the Seemann-Bohlin configuration. This work presents the results of a study which used a backscattering technique to analyse the successive layer structure of titanium silicides in the temperature range 500-600 °C. Previous studies 3,4 on the reaction of titanium with SiO z have indicated the formation of TisSi 3 and TiO~ layers at annealing temperatures higher than 700 °C. In this work the temperature range was extended to 1150 °C. Some phenomena, which occurred only at temperatures higher than 1000 °C, are described. 2. EXPERIMENTAL PROCEDURE Titanium films were deposited from an electron gun source onto both polished silicon wafers in the (111) orientation and oxidized silicon wafers. The silicon wafers had been dipped in dilute H F solution prior to deposition. The oxidized wafers were obtained by wet oxidation in a high temperature tube furnace. Annealing experiments were performed in a tantalum furnace inside the vacuum chamber. The * Paper presented at the International Conference on Metallurgical Coatings, San Diego, California, U.S.A., April 23-27, 1979. t Present address: RCA Laboratories, David Sarnoff Research Center, Princeton, N.J. 08540, U.S.A.
440
J.-S. MAA et al.
vacuum during the film deposition and the annealing was about 10-7 Torr. The backscattering analysis was performed in a High Voltage Engineering 3 MeV Van de Graaff accelerator. 3.
EXPERIMENTAL RESULTS
3.1. T i - S i system
Backscattering spectra of the samples were obtained before and after vacuum annealing. The silicon edge and the two sides of the titanium peak in the backscattering spectra of the unannealed samples were almost parallel to each other, as shown by the solid lines in Fig. 1. In the annealed samples the sharp edges of the silicon peak and the low energy side of the titanium peak became tilted and several plateaus appeared on the tilted side of the spectrum, as shown by the broken lines in Fig. 1. The overall picture was similar to two staircases facing each other. The number of plateaus was found to depend on the annealing temperature and on the annealing time. The number of plateaus which appeared on the inclined silicon edge was always equal to the number which appeared on the low energy side of the titanium peak. Each pair of plateaus corresponded to the formation of one silicide phase. From the ratios of the heights of the silicon and titanium plateaus, i.e. from the ratios of the backscattering yields, the relative concentrations of titanium and silicon in each silicide phase were obtained after correcting for stopping powers and for the scattering cross section 5. The TiSi2, TiSi and TisSi 3 phases were identified without too much difficulty. In order to obtain the thickness of each silicide, the peak width at half height of each silicide phase was estimated by careful analysis of the spectrum. Annealing at 500 °C was performed for periods of 30, 60, 120 and 240 min. Silicides were found to grow at the Si-Ti interface in the samples which were annealed for 30 and 60 min, but the plateaus in the backscattering spectrum were too small to allow accurate separation ofTiSi 2 and TiSi. TiSi and TiSi 2 were identified in the samples which were annealed for 120 min (Fig. l(a)). Since no TisSi 3 was detected, a successive layer structure Ti/TiSi/TiSi2/Si was obtained. All the phases TiSi2, TiSi and TisSi 3 were identified in the spectra of the samples which were annealed for 30 min at 550 °C (Fig. l(b)). A structure Ti/TisSi3/TiSi/TiSi2/Si was obtained. The total thickness of the silicides formed by annealing was obtained by summing the thickness of each silicide phase. The growth of the total silicide was found to follow a parabolic law with an activation energy of 2.3 ---0.2 eV. Only the TiSi 2 phase was observed at temperatures of 600 °C and above, which is consistent with previous work. 3.2. T i - S i O 2 system
Titanium reacts with SiO 2 only at temperatures above 700 °C. A layer structure TiOx/TisSi3/SiO2/Si was formed after annealing for 30 min in the temperature range 700-900 °C (Fig. 2(a)). The formation of TisSi3 instead of the silicon-rich TiSi 2 phase may be related to the difficulty of releasing silicon atoms from the S i O 2 film. The thickness of silicide was found to increase with the annealing temperature and with the annealing time. Annealing at temperatures above 1000 °C resulted in some unexpected changes
SUCCESSIVE LAYER STRUCTURES OF T I T A N I U M SILICIDES
441
4-.0
3•0
Tisi j LTiSi
Si ~
'
cJ
+>
2OO (a)
CHANNEL
~ i P z
2C
NUMBER
Ti
LTISSi3
~s #
' ~
•
./I /t'"
"\.
1o
/
,,
i (si)
,do (b)
L__
4OO
~o~
TLSi ~
30
,
'
3OO
Si) (Si) L.
~o
~o
I<~, I+i • (Ti
+
+0
~ ....
CHANNEL NUMBER
Fig. 1. B a c k s c a t t e r i n g spectra of a t i t a n i u m layer 3000 ,~ thick o n silicon: (a) , as d e p o s i t e d ; - - a n n e a l e d at 500 °C for 120 m i n ; (b) , as d e p o s i t e d ; - - , a n n e a l e d at 550 °C for 30 min.
in the layer structure. At 1000 °C silicon atoms were found to reach the surface of the TiO~ film (Fig. 2(b)). At 1100 °C (Fig. 2(c)) the silicon concentration on the surface and inside the film reached a constant value, except near the interface of the silicon substrate where the silicon concentration increased gradually to the value for bulk silicon. At this temperature all the oxygen in the SiOE and the TiO x layers disappeared (i.e. S i O / a n d TiO~ were no longer present). The titanium peak became broader and a step was observed on the low energy side of the peak. This indicated reaction of titanium with the silicon substrate. Similar phenomena were observed at temperatures up to 1150 °C. Figure 3 shows the backscattering spectra after annealing for 5 and 10 min at 1050°C. Oxygen was found to disappear after 10 min of annealing. Optical micrographs were taken from one specific region of a sample which had undergone an annealing sequence. After annealing for 5 min the surface was smooth and no change was detected. After annealing for 10 min many circular structures which had dark centres surrounded by several concentric circular grooves were observed (Fig. 4(a)). The diameters of these circular grooves remained unchanged whilst the number of grooves and the total diameter of the white region increased with increasing annealing time (Figs. 4(a) and 4(b)). No further change was observed
442
J.-S. ~ A
20MeV
4He+
Ti
ii
o
~3
e t al.
r
(a)
f
,
(b) ,r-~,
o
¢'~
I
11 ~
"--'<--L I
(c)
s~
~ I
c ~
Fig. 2. Backscattering spectra of a titanium layer 1000 A thick on oxidized silicon wafers (with 2000 A of SiO2): (a) - - - , as deposited; - - . , annealed at 900 °C for 30 min; ( b ) - -, as deposited; , annealed at 1000 °C for 30 min; ( c ) - - - , as deposited; , annealed at l l 0 0 ° C f o r 30 min. 2o~ ~÷ Ti
0
I
"', i
o
~o
2~o
f
''
"--"
1
"
CHANNEL NUMBER
Fig. 3. Backscattering spectra of a titanium layer I000 A thick on oxidized silicon wafers (with 2000 A of SiO2): "--, as deposited; - - - , annealed at 1050 °C for 5 rain; . . . . . . , annealed at 1050 °C for 10 rain.
when these structures touched each other (Figs. 4(c) and 4(d)). X-ray microanalysis showed that the central region of each ring structure had a higher silicon concentration than the surrounding regions. The density of the ring structure was found to increase with the annealing time and with decreasing SiO 2 thickness (Figs. 4(d)-4(f)). A scanning electron micrograph is shown in Fig. 4(g).
443
SUCCESSIVE LAYER STRUCTURES OF TITANIUM SILICIDES
°
U
,7)
o r,
7:
O
(a)
(b)
(c)
(d)
iii"
(e)
(f)
Fig. 4. Optical micrographs of a titanium layer 1000 A thick on oxidized silicon wafers annealed at 1050 °C for (a) 10 rain (with 2000 A of SiO2); (h) 15 min (with 2000 A of SiO2); (c) 20 rain (with 2000 A ofSiO2); (d) l h (with 2000A of SiO2); (e) 1 h (with 6000 A of SiO2), (f) 1 h (with 8000 A of SiO2). (Magnification, 24 x .) (g) A scanning electron micrograph of the sample shown in Fig. 4(d).
(g)
444
J.-S. MAA e t al.
It is believed that the ring structure is related to the migration of silicon to the surface. Titanium films deposited onto single-crystal quartz were annealed at up to 1100 °C. No ring structure was observed and silicon did not reach the film surface. The disappearance of oxygen can be related to the silicide formation. The aluminium and oxygen impurities in platinum films have been reported 6 to be repelled to the surface after the formation of PtSi. In this work the oxygen may be repelled to the surface after the silicide formation and may be pumped away through the high speed diffusion pump. ACKNOWLEDGMENT
The authors gratefully acknowledge the financial support of the National Science Council of the Republic of China (Taiwan) under contract number NSC65E-0404-03 (01). REFERENCES 1 2 3 4
R . W . BowerandJ. W. Mayer, Appl. Phys. Lett.,20(1972)359. H. Kato and Y. Nakamura, Thin Solid Films, 34 (1976) 135. H. Kr~utle, M.-A. Nieolet and J. W. Mayer, Phys. Status Solidi A, 20 (1973) K33. H. Krfiutle, W. K. Chu, M.-A. Nicolet, J. W. Mayer and K. N. Tu, Proc. Int. Conf. on Applications o f Ion Beams to Metals, Albuquerque, New Mexico, Plenum, New York, 1974, p. 193. 5 W.K. Chu, J. W. Mayer, M.-A. Nicolet, T. M. Buck, G. Amsel and F. Eisen, Thin Solid Films, 17 (1973) 1. 6 J.B. Bindell, J. W. Colby, D. R. Wonsidler, J. M. Poate, D. K. Conley and T. C. Tisone, Thin Solid Films, 37 (1976) 441.
439
B A C K S C A T T E R I N G ANALYSIS O F T H E SUCCESSIVE L A Y E R S T R U C T U R E S OF T I T A N I U M SILICIDES * JER-SHEN MAAt, CHIEN-JUNG LIN AND JUI-HSIANG LIU Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu ( Taiwan ) YUEN-CHUNG LIU Department of Physics, National Tsing Hua University, Hsinchu ( Taiwan ) (Received April 12, 1979; accepted April 27, 1979)
Solid state reactions of titanium thin films with silicon and with SiO 2 were studied using a backscattering technique. For the Ti-Si system layers of ZiSi2, TiSi and TisSi 3 were detected in the temperature range 500-600 °C. For the Ti-SiO2 system layers of TiO~ and Ti s Si a were formed in the temperature range 700-900 °C. At temperatures above 1000 °C the oxygen in the film disappeared and silicon was found to reach the film surface. A surface structure of concentric circular rings was observed.
1. INTRODUCTION
The solid state reaction of titanium thin films with silicon was first studied by Bower and Mayer 1. Only the TiSi 2 phase was observed in the temperature range 600-1000°C. Intermediate phases of titanium silicides have been formed by annealing at lower temperatures and have been analysed by Kato and Nakamura 2 using a Guinier focusing X-ray camera in the Seemann-Bohlin configuration. This work presents the results of a study which used a backscattering technique to analyse the successive layer structure of titanium silicides in the temperature range 500-600 °C. Previous studies 3,4 on the reaction of titanium with SiO z have indicated the formation of TisSi 3 and TiO~ layers at annealing temperatures higher than 700 °C. In this work the temperature range was extended to 1150 °C. Some phenomena, which occurred only at temperatures higher than 1000 °C, are described. 2. EXPERIMENTAL PROCEDURE Titanium films were deposited from an electron gun source onto both polished silicon wafers in the (111) orientation and oxidized silicon wafers. The silicon wafers had been dipped in dilute H F solution prior to deposition. The oxidized wafers were obtained by wet oxidation in a high temperature tube furnace. Annealing experiments were performed in a tantalum furnace inside the vacuum chamber. The * Paper presented at the International Conference on Metallurgical Coatings, San Diego, California, U.S.A., April 23-27, 1979. t Present address: RCA Laboratories, David Sarnoff Research Center, Princeton, N.J. 08540, U.S.A.
440
J.-S. MAA et al.
vacuum during the film deposition and the annealing was about 10-7 Torr. The backscattering analysis was performed in a High Voltage Engineering 3 MeV Van de Graaff accelerator. 3.
EXPERIMENTAL RESULTS
3.1. T i - S i system
Backscattering spectra of the samples were obtained before and after vacuum annealing. The silicon edge and the two sides of the titanium peak in the backscattering spectra of the unannealed samples were almost parallel to each other, as shown by the solid lines in Fig. 1. In the annealed samples the sharp edges of the silicon peak and the low energy side of the titanium peak became tilted and several plateaus appeared on the tilted side of the spectrum, as shown by the broken lines in Fig. 1. The overall picture was similar to two staircases facing each other. The number of plateaus was found to depend on the annealing temperature and on the annealing time. The number of plateaus which appeared on the inclined silicon edge was always equal to the number which appeared on the low energy side of the titanium peak. Each pair of plateaus corresponded to the formation of one silicide phase. From the ratios of the heights of the silicon and titanium plateaus, i.e. from the ratios of the backscattering yields, the relative concentrations of titanium and silicon in each silicide phase were obtained after correcting for stopping powers and for the scattering cross section 5. The TiSi2, TiSi and TisSi 3 phases were identified without too much difficulty. In order to obtain the thickness of each silicide, the peak width at half height of each silicide phase was estimated by careful analysis of the spectrum. Annealing at 500 °C was performed for periods of 30, 60, 120 and 240 min. Silicides were found to grow at the Si-Ti interface in the samples which were annealed for 30 and 60 min, but the plateaus in the backscattering spectrum were too small to allow accurate separation ofTiSi 2 and TiSi. TiSi and TiSi 2 were identified in the samples which were annealed for 120 min (Fig. l(a)). Since no TisSi 3 was detected, a successive layer structure Ti/TiSi/TiSi2/Si was obtained. All the phases TiSi2, TiSi and TisSi 3 were identified in the spectra of the samples which were annealed for 30 min at 550 °C (Fig. l(b)). A structure Ti/TisSi3/TiSi/TiSi2/Si was obtained. The total thickness of the silicides formed by annealing was obtained by summing the thickness of each silicide phase. The growth of the total silicide was found to follow a parabolic law with an activation energy of 2.3 ---0.2 eV. Only the TiSi 2 phase was observed at temperatures of 600 °C and above, which is consistent with previous work. 3.2. T i - S i O 2 system
Titanium reacts with SiO 2 only at temperatures above 700 °C. A layer structure TiOx/TisSi3/SiO2/Si was formed after annealing for 30 min in the temperature range 700-900 °C (Fig. 2(a)). The formation of TisSi3 instead of the silicon-rich TiSi 2 phase may be related to the difficulty of releasing silicon atoms from the S i O 2 film. The thickness of silicide was found to increase with the annealing temperature and with the annealing time. Annealing at temperatures above 1000 °C resulted in some unexpected changes
SUCCESSIVE LAYER STRUCTURES OF T I T A N I U M SILICIDES
441
4-.0
3•0
Tisi j LTiSi
Si ~
'
cJ
+>
2OO (a)
CHANNEL
~ i P z
2C
NUMBER
Ti
LTISSi3
~s #
' ~
•
./I /t'"
"\.
1o
/
,,
i (si)
,do (b)
L__
4OO
~o~
TLSi ~
30
,
'
3OO
Si) (Si) L.
~o
~o
I<~, I+i • (Ti
+
+0
~ ....
CHANNEL NUMBER
Fig. 1. B a c k s c a t t e r i n g spectra of a t i t a n i u m layer 3000 ,~ thick o n silicon: (a) , as d e p o s i t e d ; - - a n n e a l e d at 500 °C for 120 m i n ; (b) , as d e p o s i t e d ; - - , a n n e a l e d at 550 °C for 30 min.
in the layer structure. At 1000 °C silicon atoms were found to reach the surface of the TiO~ film (Fig. 2(b)). At 1100 °C (Fig. 2(c)) the silicon concentration on the surface and inside the film reached a constant value, except near the interface of the silicon substrate where the silicon concentration increased gradually to the value for bulk silicon. At this temperature all the oxygen in the SiOE and the TiO x layers disappeared (i.e. S i O / a n d TiO~ were no longer present). The titanium peak became broader and a step was observed on the low energy side of the peak. This indicated reaction of titanium with the silicon substrate. Similar phenomena were observed at temperatures up to 1150 °C. Figure 3 shows the backscattering spectra after annealing for 5 and 10 min at 1050°C. Oxygen was found to disappear after 10 min of annealing. Optical micrographs were taken from one specific region of a sample which had undergone an annealing sequence. After annealing for 5 min the surface was smooth and no change was detected. After annealing for 10 min many circular structures which had dark centres surrounded by several concentric circular grooves were observed (Fig. 4(a)). The diameters of these circular grooves remained unchanged whilst the number of grooves and the total diameter of the white region increased with increasing annealing time (Figs. 4(a) and 4(b)). No further change was observed
442
J.-S. ~ A
20MeV
4He+
Ti
ii
o
~3
e t al.
r
(a)
f
,
(b) ,r-~,
o
¢'~
I
11 ~
"--'<--L I
(c)
s~
~ I
c ~
Fig. 2. Backscattering spectra of a titanium layer 1000 A thick on oxidized silicon wafers (with 2000 A of SiO2): (a) - - - , as deposited; - - . , annealed at 900 °C for 30 min; ( b ) - -, as deposited; , annealed at 1000 °C for 30 min; ( c ) - - - , as deposited; , annealed at l l 0 0 ° C f o r 30 min. 2o~ ~÷ Ti
0
I
"', i
o
~o
2~o
f
''
"--"
1
"
CHANNEL NUMBER
Fig. 3. Backscattering spectra of a titanium layer I000 A thick on oxidized silicon wafers (with 2000 A of SiO2): "--, as deposited; - - - , annealed at 1050 °C for 5 rain; . . . . . . , annealed at 1050 °C for 10 rain.
when these structures touched each other (Figs. 4(c) and 4(d)). X-ray microanalysis showed that the central region of each ring structure had a higher silicon concentration than the surrounding regions. The density of the ring structure was found to increase with the annealing time and with decreasing SiO 2 thickness (Figs. 4(d)-4(f)). A scanning electron micrograph is shown in Fig. 4(g).
443
SUCCESSIVE LAYER STRUCTURES OF TITANIUM SILICIDES
°
U
,7)
o r,
7:
O
(a)
(b)
(c)
(d)
iii"
(e)
(f)
Fig. 4. Optical micrographs of a titanium layer 1000 A thick on oxidized silicon wafers annealed at 1050 °C for (a) 10 rain (with 2000 A of SiO2); (h) 15 min (with 2000 A of SiO2); (c) 20 rain (with 2000 A ofSiO2); (d) l h (with 2000A of SiO2); (e) 1 h (with 6000 A of SiO2), (f) 1 h (with 8000 A of SiO2). (Magnification, 24 x .) (g) A scanning electron micrograph of the sample shown in Fig. 4(d).
(g)
444
J.-S. MAA e t al.
It is believed that the ring structure is related to the migration of silicon to the surface. Titanium films deposited onto single-crystal quartz were annealed at up to 1100 °C. No ring structure was observed and silicon did not reach the film surface. The disappearance of oxygen can be related to the silicide formation. The aluminium and oxygen impurities in platinum films have been reported 6 to be repelled to the surface after the formation of PtSi. In this work the oxygen may be repelled to the surface after the silicide formation and may be pumped away through the high speed diffusion pump. ACKNOWLEDGMENT
The authors gratefully acknowledge the financial support of the National Science Council of the Republic of China (Taiwan) under contract number NSC65E-0404-03 (01). REFERENCES 1 2 3 4
R . W . BowerandJ. W. Mayer, Appl. Phys. Lett.,20(1972)359. H. Kato and Y. Nakamura, Thin Solid Films, 34 (1976) 135. H. Kr~utle, M.-A. Nieolet and J. W. Mayer, Phys. Status Solidi A, 20 (1973) K33. H. Krfiutle, W. K. Chu, M.-A. Nicolet, J. W. Mayer and K. N. Tu, Proc. Int. Conf. on Applications o f Ion Beams to Metals, Albuquerque, New Mexico, Plenum, New York, 1974, p. 193. 5 W.K. Chu, J. W. Mayer, M.-A. Nicolet, T. M. Buck, G. Amsel and F. Eisen, Thin Solid Films, 17 (1973) 1. 6 J.B. Bindell, J. W. Colby, D. R. Wonsidler, J. M. Poate, D. K. Conley and T. C. Tisone, Thin Solid Films, 37 (1976) 441.