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Microstructure development during the thermal oxidation of silicon in chlorine containing ambients
Journal of Non-Crystalline Solids 49 (1982) 201-207 North-Holland Publishing Company
201
MICROSTRUCTURE DEVELOPMENT DURING THE THERMAL O X I D A T I O N O F S I L I C O N IN C H L O R I N E C O N T A I N I N G A M B I E N T S M.D. M O N K O W S K I *, J.R. M O N K O W S K I **, I.S.T. T S O N G *, J. S T A C H ** and R.E. T R E S S L E R * The Pennsylvania State University, Universi(v Park, PA 16802, USA
During the thermal oxidation of silicon in atmospheres containing oxygen and chlorine or oxygen and hydrogen chloride, an interfacial phase is formed between the silicon and its oxide. In addition, chlorine is incorporated into the oxide near the interface. Upon further oxidation a gas phase develops at the interface producing bubbles under the oxide. Still longer oxidation results in the growth of "stalactite" structures from the roof of the bubbles. We discuss these observations and present a model that accounts for their appearance in terms of the activities and diffusivities of chlorine and oxygen through the oxide.
1. Introduction The properties of silicon oxides thermally grown in the presence of chlorine have been extensively researched [1]. If the oxidation is carried out under the appropriate conditions of chlorine concentration, time and temperature, beneficial effects are realized. These effects include trapping and neutralization of sodium contamination in the oxide, and gettering of metallic impurities and stacking faults from the silicon. It is in order to understand these conditions and to optimize their effects that we study this system. During the course of these investigations it became clear that the appearance and electrical properties are not due simply to the incorporation of chlorine into the growing silica, but rather are the result of the formation of additional phases between the silica and the silicon [2]. The first additional phase to form during the oxidation is condensed at oxidation temperatures, and is believed to be a chlorosiloxane. This phase is responsible for sodium passivation, metallic impurity gettering, and is believed to play a role in stacking fault annihilation. The second phase to form is gaseous, and is believed to be either a chlorosiloxane with a low boiling point or a silicon chloride. This phase is responsible for deleterious effects such as anisotropic etching of the silicon and separation of the oxide from the silicon.
* Department of Materials Science and Engineering. ** Department of Electrical Engineering. 0 0 2 2 - 3 0 9 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d
M.D. Monkowski et a L / Microstructure development
202
2. Model of competitive oxidation This oxidation process has been explained [3] using a model [4] that involves the competition between oxygen and chlorine for reaction with the silicon. The appearance of silica, silicon oxychlorides, or silicon chloride is governed by the relative activities of the oxidants in the oxide film. Oxygen incorporation is limited by diffusion through the oxide, as evidenced by the parabolic growth kinetics of silica, and so its activity falls as we move from the surface of the oxide to the growth interface. Chlorine, on the other hand, diffuses much faster than oxygen, and so its activity remains constant throughout the oxide. The ratio of the acitivities of oxygen to chlorine at the growth interface, therefore, decreases with time. This allows for the formation of phases that would not be stable if exposed to the oxidizing atmosphere, and results in a two layered film that has a chlorine-rich phase sandwiched between silica and silicon. This situation is schematically illustrated in fig. 1. Fig. 2 shows the microstructure of the oxide shortly after the development of the condensed phase. The hillocks which are apparent in the figure appear quite abruptly and can be used as an indicator of phase formation. When the activity of oxygen at the interface is plotted versus the partial pressure of chlorine in the ambient at the appearance of these hillocks, for oxides grown under various conditions, a linear dependence is seen [6], confirming the validity of the model.
[
sio 2
Si
SiO2
CliricI Si
p~os¢
z z O
DEPTH
DEPTH
Fig. I. Schematic of the activities of oxygen and chlorine and the corresponding depth profiles for oxides on silicon, before and after additional phase formation.
M.D. Monkowski et al. / Microstructure development
203
Fig. 2. SEM micrograph of an oxide grown in 10%HCI/O 2 at 1150°C for I h. Sample tilted 65 ° [5].
U p o n continued oxidation, the oxygen activity falls even more so that a gaseous phase becomes stable. Because the gas cannot diffuse out through the already grown silica, it instead lifts the oxide away from the silicon as shown in fig. 3. Further oxidation must then proceed by a vapor transport mechanism. Silicon is etched by chlorine to form a gaseous species, which then diffuses to the silica, where it reacts with oxygen to form additional silica. This results in a microstructure resembling caves with stalactites as shown in fig. 4.
Fig. 3. Cross-sectional SEM micrograph of an oxide grown in 10%HC1/O 2 at 1200°C for 6 h [5].
204
M.D. Monkowski et al. / Microstructure development
Fig. 4. Cross-sectional SEM micrograph of an oxide grown in 10%HCI/O 2 at 1200°C for 9 h.
3. Kinetic aspects of chlorine incorporation Although the model can predict the stability of the phase assemblages, it does not directly predict the kinetics. We therefore analyzed three sets of samples oxidized at 900°C, 950°C and 1000°C for the depth distribution of chlorine. All sets were oxidized for 1, 3, 9, 27, and 81 h in a 6%HCI/O 2 gas mixture. This corresponds to a partial pressure of chlorine of approximately 1.2 × 10-2 atm. The chlorine profiles obtained by secondary ion mass spectrometry (SIMS) are shown in fig. 5. The depths were normalized using ellipsometry to determine the oxide thicknesses and the chlorine concentrations were normalized to the oxygen profile for each sample and calibrated using a standard measured by Rutherford backscattering (RBS). The appearance of hillocks was observed by optical microscopy after 81 h at 950°C, and after 27 h at 1000°C. When the areas under the depth profiles are integrated and plotted versus oxidation time as in fig. 6, we notice an initial rise, followed by a level interval which gives way to a second rise in concentration. This second rise is correlated with the appearance of the condensed phase and is expected according to the model. It is not expected, however, that there be any significant amount of chlorine in the oxides prior to the onset of the phase formation. The explanation of this discrepanccy lies in the kinetics of the process. Because there is a discontinuity in the oxygen activity from the silica to the silicon, the chlorine is given an opportunity to react with the silicon, although it becomes unstable once the growth interface moves forward and the chlorine is left behind in the oxide. This gives rise to the small and decaying profile of chlorine extending from the interface into the silica. Because the chlorine activity remains constant and
M.D. Monkowski et aL / Microstructure development
205
1.8 p')
1.Bt~.
900 °C
o 1.4
6% HCI / 02
~ nO
o o I--.8
<
o o
5 9
.4
81
.2 g
l R
4BEg
l
8gg9
r----
............
-T--
120~R
DEPTH (11 2
1.8
1.B ~ 1 1.4 1.2
9 5 0 °C 6% HCI / 02
~ ..J -r
.6
o
_
o
27
jo.,x1
.2 o~
F-4R~al
Fig. 5.
DEPTH (.~)
G~gZ
12Bgg
M.D. Monkowski et aL / Microstructure development
206 2
--
1.fl
--
1°6
--
1.4
~
IO00°C 3
9
6%HCI / 02
~
27
uA
1.2
~
l
0 ..J '1"O --
--
o9
I
.8
°
_
o .4
.2
s•W-------- - [ fl
4flflfl
OEPTH
(~)
8flflfl
F-- ........ --T--12flflf
Fig. 5. SIMS 35C1 ÷ depth profiles of oxides on silicon. The oxide surface is at the left; the oxide/silicon interface is near the peak of each profile. Oxidation times (h) are given above the profiles. i016
CE o *6 i0 f S o
z
07- 1014 I--Z bA Z 0 i013
I I
I 3
9
I 27
I 81
OXIDATION TIME (hrs)
Fig. 6. Total chlorine concentration of the oxides shown in fig. 5.
M.D. Monkowski et al. / Microstructure development
207
because the instability condition persists, we can expect the total chlorine concentration to reach a value that remains constant with continued oxidation. This is exactly as we see: an initial rise saturates at a constant level at all three temperatures tested. We note however that the period of saturation decreases with increasing temperature so that the second rise m a y join the first and m a y even overtake it. Previous investigations using Rutherford backscattering have shown that the curve is quite linear [7] at 1100°C but at higher temperatures it develops a discontinuous increase [8] whose magnitude increases with temperature. After a sufficient thickness of silica forms, the activity of oxygen at the growth interface falls and we enter a regime where the chlorine-rich phase is no longer unstable so that the total chlorine, as we have seen, is no longer saturated, but rises again. This is represented in fig, 1. Because the oxygen activity is lowest at the growth interface, the chlorine will follow the interface until the gaseous phase becomes stable, at which time the complex microstructure shown in fig. 4 dictates that oxidation proceed by a vapor transport reaction.
4. Conclusions We have shown how t h e r m o d y n a m i c stability, governed by the activities of competing oxidants, together with reaction kinetics, account for the changing chlorine profiles, the growth of additional phases, and the development of microstructural features during the thermal oxidation of silicon. This work was supported by the U.S. A r m y Research Office under Contract D A A G 2 9 - 8 0 - K - 0 0 8 5 and National Science F o u n d a t i o n under G r a n t D M R 7809767.
References [1] J. Monkowski, Sol. St. Tech. (July, 1979) 58; (August 1979) 113. [2] J. Monkowski, J. Stach and R.E. Tressler, J. Electrochem. Soc. 126 (1979) 1129. [3] J.R. Monkowski, J. Stach and R.E. Tressler, in: Proc. 30th Electronic Components Conf., San Francisco (April 28-30, 1980) p. 61. [4] R.A. Rapp, in: Proc. United States-Japanese Seminar, Chemical kinetics of pyrometallurgy, The MIT Endicott House (1976) B-I. [5] J. Monkowski, R.E. Tressler and J. Stach, J. Electrochem. Soc. 125 (1978) 1867. [6] M.D. Monkowski, J. Monkowski, R.E. Tressler and J. Stach, in: Ceramic and ceramic-metal systems, eds., J. Pask and A. Evans (Plenum, New York, 1981) p. 361. [7] I.S.T. Tsong, M.D. Monkowski, J.R. Monkowski, P,D. Miller, C.D. Moak, B.R. Appleton and A.L. Wintenberg, in: The physics of MOS insulators, eds., G. Lucovsky, S.T. Pantelides and F.L. Galeener (Pergamon, New York, 1980) p. 321. [8] A. Rohatgi, S.D. Butler, F.J. Feigl, H.W. Kraner and K.W. Jones, J. Electrochem. Soc. 126 (1979) 143.
201
MICROSTRUCTURE DEVELOPMENT DURING THE THERMAL O X I D A T I O N O F S I L I C O N IN C H L O R I N E C O N T A I N I N G A M B I E N T S M.D. M O N K O W S K I *, J.R. M O N K O W S K I **, I.S.T. T S O N G *, J. S T A C H ** and R.E. T R E S S L E R * The Pennsylvania State University, Universi(v Park, PA 16802, USA
During the thermal oxidation of silicon in atmospheres containing oxygen and chlorine or oxygen and hydrogen chloride, an interfacial phase is formed between the silicon and its oxide. In addition, chlorine is incorporated into the oxide near the interface. Upon further oxidation a gas phase develops at the interface producing bubbles under the oxide. Still longer oxidation results in the growth of "stalactite" structures from the roof of the bubbles. We discuss these observations and present a model that accounts for their appearance in terms of the activities and diffusivities of chlorine and oxygen through the oxide.
1. Introduction The properties of silicon oxides thermally grown in the presence of chlorine have been extensively researched [1]. If the oxidation is carried out under the appropriate conditions of chlorine concentration, time and temperature, beneficial effects are realized. These effects include trapping and neutralization of sodium contamination in the oxide, and gettering of metallic impurities and stacking faults from the silicon. It is in order to understand these conditions and to optimize their effects that we study this system. During the course of these investigations it became clear that the appearance and electrical properties are not due simply to the incorporation of chlorine into the growing silica, but rather are the result of the formation of additional phases between the silica and the silicon [2]. The first additional phase to form during the oxidation is condensed at oxidation temperatures, and is believed to be a chlorosiloxane. This phase is responsible for sodium passivation, metallic impurity gettering, and is believed to play a role in stacking fault annihilation. The second phase to form is gaseous, and is believed to be either a chlorosiloxane with a low boiling point or a silicon chloride. This phase is responsible for deleterious effects such as anisotropic etching of the silicon and separation of the oxide from the silicon.
* Department of Materials Science and Engineering. ** Department of Electrical Engineering. 0 0 2 2 - 3 0 9 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d
M.D. Monkowski et a L / Microstructure development
202
2. Model of competitive oxidation This oxidation process has been explained [3] using a model [4] that involves the competition between oxygen and chlorine for reaction with the silicon. The appearance of silica, silicon oxychlorides, or silicon chloride is governed by the relative activities of the oxidants in the oxide film. Oxygen incorporation is limited by diffusion through the oxide, as evidenced by the parabolic growth kinetics of silica, and so its activity falls as we move from the surface of the oxide to the growth interface. Chlorine, on the other hand, diffuses much faster than oxygen, and so its activity remains constant throughout the oxide. The ratio of the acitivities of oxygen to chlorine at the growth interface, therefore, decreases with time. This allows for the formation of phases that would not be stable if exposed to the oxidizing atmosphere, and results in a two layered film that has a chlorine-rich phase sandwiched between silica and silicon. This situation is schematically illustrated in fig. 1. Fig. 2 shows the microstructure of the oxide shortly after the development of the condensed phase. The hillocks which are apparent in the figure appear quite abruptly and can be used as an indicator of phase formation. When the activity of oxygen at the interface is plotted versus the partial pressure of chlorine in the ambient at the appearance of these hillocks, for oxides grown under various conditions, a linear dependence is seen [6], confirming the validity of the model.
[
sio 2
Si
SiO2
CliricI Si
p~os¢
z z O
DEPTH
DEPTH
Fig. I. Schematic of the activities of oxygen and chlorine and the corresponding depth profiles for oxides on silicon, before and after additional phase formation.
M.D. Monkowski et al. / Microstructure development
203
Fig. 2. SEM micrograph of an oxide grown in 10%HCI/O 2 at 1150°C for I h. Sample tilted 65 ° [5].
U p o n continued oxidation, the oxygen activity falls even more so that a gaseous phase becomes stable. Because the gas cannot diffuse out through the already grown silica, it instead lifts the oxide away from the silicon as shown in fig. 3. Further oxidation must then proceed by a vapor transport mechanism. Silicon is etched by chlorine to form a gaseous species, which then diffuses to the silica, where it reacts with oxygen to form additional silica. This results in a microstructure resembling caves with stalactites as shown in fig. 4.
Fig. 3. Cross-sectional SEM micrograph of an oxide grown in 10%HC1/O 2 at 1200°C for 6 h [5].
204
M.D. Monkowski et al. / Microstructure development
Fig. 4. Cross-sectional SEM micrograph of an oxide grown in 10%HCI/O 2 at 1200°C for 9 h.
3. Kinetic aspects of chlorine incorporation Although the model can predict the stability of the phase assemblages, it does not directly predict the kinetics. We therefore analyzed three sets of samples oxidized at 900°C, 950°C and 1000°C for the depth distribution of chlorine. All sets were oxidized for 1, 3, 9, 27, and 81 h in a 6%HCI/O 2 gas mixture. This corresponds to a partial pressure of chlorine of approximately 1.2 × 10-2 atm. The chlorine profiles obtained by secondary ion mass spectrometry (SIMS) are shown in fig. 5. The depths were normalized using ellipsometry to determine the oxide thicknesses and the chlorine concentrations were normalized to the oxygen profile for each sample and calibrated using a standard measured by Rutherford backscattering (RBS). The appearance of hillocks was observed by optical microscopy after 81 h at 950°C, and after 27 h at 1000°C. When the areas under the depth profiles are integrated and plotted versus oxidation time as in fig. 6, we notice an initial rise, followed by a level interval which gives way to a second rise in concentration. This second rise is correlated with the appearance of the condensed phase and is expected according to the model. It is not expected, however, that there be any significant amount of chlorine in the oxides prior to the onset of the phase formation. The explanation of this discrepanccy lies in the kinetics of the process. Because there is a discontinuity in the oxygen activity from the silica to the silicon, the chlorine is given an opportunity to react with the silicon, although it becomes unstable once the growth interface moves forward and the chlorine is left behind in the oxide. This gives rise to the small and decaying profile of chlorine extending from the interface into the silica. Because the chlorine activity remains constant and
M.D. Monkowski et aL / Microstructure development
205
1.8 p')
1.Bt~.
900 °C
o 1.4
6% HCI / 02
~ nO
o o I--.8
<
o o
5 9
.4
81
.2 g
l R
4BEg
l
8gg9
r----
............
-T--
120~R
DEPTH (11 2
1.8
1.B ~ 1 1.4 1.2
9 5 0 °C 6% HCI / 02
~ ..J -r
.6
o
_
o
27
jo.,x1
.2 o~
F-4R~al
Fig. 5.
DEPTH (.~)
G~gZ
12Bgg
M.D. Monkowski et aL / Microstructure development
206 2
--
1.fl
--
1°6
--
1.4
~
IO00°C 3
9
6%HCI / 02
~
27
uA
1.2
~
l
0 ..J '1"O --
--
o9
I
.8
°
_
o .4
.2
s•W-------- - [ fl
4flflfl
OEPTH
(~)
8flflfl
F-- ........ --T--12flflf
Fig. 5. SIMS 35C1 ÷ depth profiles of oxides on silicon. The oxide surface is at the left; the oxide/silicon interface is near the peak of each profile. Oxidation times (h) are given above the profiles. i016
CE o *6 i0 f S o
z
07- 1014 I--Z bA Z 0 i013
I I
I 3
9
I 27
I 81
OXIDATION TIME (hrs)
Fig. 6. Total chlorine concentration of the oxides shown in fig. 5.
M.D. Monkowski et al. / Microstructure development
207
because the instability condition persists, we can expect the total chlorine concentration to reach a value that remains constant with continued oxidation. This is exactly as we see: an initial rise saturates at a constant level at all three temperatures tested. We note however that the period of saturation decreases with increasing temperature so that the second rise m a y join the first and m a y even overtake it. Previous investigations using Rutherford backscattering have shown that the curve is quite linear [7] at 1100°C but at higher temperatures it develops a discontinuous increase [8] whose magnitude increases with temperature. After a sufficient thickness of silica forms, the activity of oxygen at the growth interface falls and we enter a regime where the chlorine-rich phase is no longer unstable so that the total chlorine, as we have seen, is no longer saturated, but rises again. This is represented in fig, 1. Because the oxygen activity is lowest at the growth interface, the chlorine will follow the interface until the gaseous phase becomes stable, at which time the complex microstructure shown in fig. 4 dictates that oxidation proceed by a vapor transport reaction.
4. Conclusions We have shown how t h e r m o d y n a m i c stability, governed by the activities of competing oxidants, together with reaction kinetics, account for the changing chlorine profiles, the growth of additional phases, and the development of microstructural features during the thermal oxidation of silicon. This work was supported by the U.S. A r m y Research Office under Contract D A A G 2 9 - 8 0 - K - 0 0 8 5 and National Science F o u n d a t i o n under G r a n t D M R 7809767.
References [1] J. Monkowski, Sol. St. Tech. (July, 1979) 58; (August 1979) 113. [2] J. Monkowski, J. Stach and R.E. Tressler, J. Electrochem. Soc. 126 (1979) 1129. [3] J.R. Monkowski, J. Stach and R.E. Tressler, in: Proc. 30th Electronic Components Conf., San Francisco (April 28-30, 1980) p. 61. [4] R.A. Rapp, in: Proc. United States-Japanese Seminar, Chemical kinetics of pyrometallurgy, The MIT Endicott House (1976) B-I. [5] J. Monkowski, R.E. Tressler and J. Stach, J. Electrochem. Soc. 125 (1978) 1867. [6] M.D. Monkowski, J. Monkowski, R.E. Tressler and J. Stach, in: Ceramic and ceramic-metal systems, eds., J. Pask and A. Evans (Plenum, New York, 1981) p. 361. [7] I.S.T. Tsong, M.D. Monkowski, J.R. Monkowski, P,D. Miller, C.D. Moak, B.R. Appleton and A.L. Wintenberg, in: The physics of MOS insulators, eds., G. Lucovsky, S.T. Pantelides and F.L. Galeener (Pergamon, New York, 1980) p. 321. [8] A. Rohatgi, S.D. Butler, F.J. Feigl, H.W. Kraner and K.W. Jones, J. Electrochem. Soc. 126 (1979) 143.