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Studies of thin oxides grown by rapid thermal processing
Nuclear Instruments and Methods in Physics Research B21 (1987) 629-632 North-Holland, Amsterdam
629
S T U D I E S OF T H I N OXIDES G R O W N BY RAPID T H E R M A L P R O C E S S I N G S. M E H T A i), D . T . H O D U L 2) a n d C.J. R U S S O t) tJ Varian Associates, Extrion Division, Blackburn Industrial Park. Gloucester. MA 01930. USA "bVarian Research Center. 611 Hansen Way, Palo Alto. CA 94303. USA
Thin oxides have been rapidly grown using lamp heating. The growth kinetics show Arrhenius temperature dependence and are lineal- as a function of time. The process is diffusion limited rather than mass transport limited. Thickness uniformity measured to 2 mm of the wafer edge on 100-400 ,~ oxides grown at 1100-1200 C is 4% one sigma for an unoptimized process and can be improved to < 2~ using multistep processes. Some of the non uniformities may be related to substrate dopant, temperature effects and surface irregularities. Derived temperature uniformity is between 2.6 ° and 6 ° C one sigma. Flatband voltages of 0.7 V were obtained for aluminum capacitors with breakdown field strengths of 13 MV/cm for 250 A oxides. RCA cleaning and forming gas pretreatment improved the electrical performance of the oxides.
1. Introduction One of the most active new areas of rapid thermal processing is the use o f oxygen ambients to grow high quality oxides ( < 150 A) for gate dielectrics in VLSI devices [1,2]. In the experiments described below, silicon wafers were subjected to a variety of oxidizing ambients and temperature - time conditions using lamp heating to determine the kinetics and process uniformity of the oxidation. These data have been used to optimize the uniformity. The effects of surface preparation on the electrical characteristics of the oxides were determined using C - V and breakdown measurements; oxide contamination was qualitatively characterized using a bias-temperature stress method.
Oxide thickness measurements made on 100 mm (100) Si wafer (1200 ° C for 5 rain in 207o 02 and argon mixture) revealed an uncorrected average thickness of 287.3 ,~ with a standard deviation of 6.39 ,~. Temperature uniformity across the wafer can also be estimated from thickness uniformity of the oxides. Using linear growth kinetics of fig. 3, the temperature uniformity obtained was 2.6°C one sigma at 1200°C oxidation temperature [1].
Tungsten LampArray
\
H,gh Reflectance Water-Cooled M=trored Chamber Inter=or
L u uuuuuuuu/ u I 2. Oxidation kinetics Oxides were grown using Varian's rapid thermal processor model RTP-800 in a controlled oxygen ambient gas mixture. A schematic diagram of the heating and processing chambers is shown in fig. 1. Detailed description of the system is provided elsewhere [3]. Oxide growth was studied both as a function of time and oxidation temperature. For such studies, a single temperature - time profile as shown in fig. 2 was carried out for oxidation. Oxide thickness was measured by Nanospec and ellipsometry. Fig. 3 shows the oxide thickness as a function of time at the oxidation temperature of 1 1 0 0 ° C (fig. 3a) and oxidation temperature (fig. 3b). It can be seen that the thickness-time relationship is linear in the entire range from 1 to 8 min. This relationship is not linear for very short oxidation times. 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Window ~,~
Pho..... trig
Waler --
d I
.,a,en P,s,on
~t Vacuum
I
/
.,n.
--22--'----:----------------L-----r--i.
__J
L_ . . . . .
[----i iI
1 1
1
Fig. 1. Schematic description of RTP 800 heating and processing chambers. XI. RAPID THERMAL PROCESSING
630
S. Mehta et al.
/ Studies
of thin oxides
12OO11501100A
o IOOOo
w 900n.-
i.< 8OOn~ LO O_ 700uJ t--
WAFER SIZE = 150
or 6 0 0 uJ
15ooc FOR 5 SECS
~500400"
300-
J
,
T
I o
I 5
I IO
I 15
I 20
I 25
I 3o
I 35
TIME
I 40
I 45
I 50
I 55
I 60
I 65
I
7c
(SECONDS)
Fig. 2. A typical RTP 800 temperature-time profile.
600-
b
a 500-
S i < IO0> OXIDATION TEMPERATURE =llOO°C
40C
Si < 1 0 0 > OXIDATION TIME = 3 M I N S . AMBIENT = 2 0 % O z l N ARGON AT 2 0 CU FT (HR.) -I
A
400-
/
/
.=,/
/
°<[ 300-
/
P
/ /
ixl z
300-
_u
- zoo-
I
I--
/
/
J
/
t~ 200E3
/ ~)
0
BOO /
IO0,
O
TIME
9oo
(MINUTES)
,boo
,;oo
,~oo
TEMPERATURE
,;oo (°C)
Fig. 3. (a) Thickness of RTO oxide as a function of oxidation time. (b) Thickness of RTO oxide as a function of oxidation temperature.
3. Effect of gas flow rate and oxidant concentration As seen in fig. 3, oxides were grown in both dilute oxygen (20% 02 in argon) and pure oxygen. An increased rate of oxidation was observed when pure oxygen was chosen as the ambient gas. However, there was no deviation in the linearity of the thickness-time relationship, independent of oxidation temperature and
wafer crystal orientation. N o significant effect of gas flow rate on the oxide growth rate was observed.
4. Uniformity The oxide thickness uniformity was determined by both Nanospec and ellipsometry. Contour maps were
S. Mehta et aL / Studies of thin oxides
631
Table 1 Sample preparation, conditions and properties measured
Sample identity
Temp.-time conditions
(V)
OX-2 BRK-11 HROX-2 BRK-6
300s/1100 ° C 180s/1200 ° C 15X( < ls)/1200 o C 4X15s/1200 o C
-
VFB
Uncompensated oxide thickness
VFB") (V)
Wafer surface preparation
(A)-lo
6.0 1.5 0.7 0.8
-0.4 -0.2 - 0.1
290-10.9% 262.4-5.48% 181-7.16% 145-4.11%
-- 0 . 0 4 b)
none rca + forming gas rca + forming gas rca + forming gas
") + 16 V Stress, 15 min at 200° C. b)+4 V Stress, 15 min at 200°C.
generated using the ellipsometer assuming a refractive index of 1.46 by measuring 45 points on the wafer, to within 2 m m from the edge of the wafer. Using these maps routine oxidation gives uniformities better than 4% one sigma for thickness in the range of 100-400 ,~ when corrected for the random error of the ellipsometer, which at 150 A is approximately _+3 ,~ one sigma. Similar results were obtained for the nanospec on oxides - 250-300 ,~ thick. Since the edge of the wafer preferentially absorbs radiation during the heatup and radiates during the cooldown and steady state, a multicycle t e m p e r a t u r e - t i m e process was devised to optimize the uniformity. In table 1 the sample HROX-2 has the oxide thicker at the edges than at the center; and BRK-11 has the oxide thicker in the center. Sample BRK-6 is an attempt to make the oxides uniform. More work is required for this optimization. Gas flow rates above 5 l / r a i n were also found to effect the uniformity.
UNIFORMITY
~z] . . -.... I0
Capacitance-voltage measurements were performed using a Signatone model S1041 hot chuck system and an E G & G model 410 C - V plotter. 0.75 mm aluminum dots were evaporated onto the oxides and sintered in forming gas at 4 5 0 ° C for 15 min. Flat-band voltages were calculated to be 0.7 V with shifts of < 0.1 V (see table 1) for bias stress of +16 V, at 2 0 0 ° C for 15 minutes. The breakdown strength, was found to be 13 M V / c m or higher for oxides 250 A thick (see fig. 4). As expected the best results were obtained for wafers which received an R C A clean a n d / o r a forming gas pretreatment (20 s. at 1150°C in 6.5% H 2 and argon mixture) prior to the oxidation. The wafers were transported cross country between the R C A clean and oxidation. Therefore, some irregularities in the flatband voltages and shifts have occurred. The oxide quality will only get better if proper surface preparation procedures are followed. These experiments are in progress.
,0 o
w
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~ a I.u
w f__ z w 'o
8-
N " Z
!iI
5. E l e c t r i c a l m e a s u r e m e n t s
re . 1~0
4-
7
I
I0 13 VOLTS
16
19
22
Fig. 4. A histogram showing the breakdown field strength for sample HROX-2; 15 cycles for < I s each of 1200°C. The insert shows the breakdown field strength from the center to 1 cm from the edge of the wafer.
6. C o n c l u s i o n s
We have seen that thin oxides grown in the RTP follow linear kinetics of growth both for dilute as well as concentrated oxidizers. The oxide thickness uniformity measurements indicate good temperature uniformity across the wafer. Electrical measurements made on the oxides indicate that the RTP technique has a potential for producing high quality oxides. The authors would like to thank Mike Titus and Nick Parisi for developing the software for the multicycle process, Dave Barton and Bill Hoef for assistance on the mechanical aspects of the instrument and Juanita XI. RAPID THERMAL PROCESSING
632
S. Mehta et al. / Studies of thin oxides
SonJcs and Maggie Erikson for wafer cleaning and electrical measurements. The authorg also wish to thank Liz Coakley for typing the manuscript.
References [1] S. Mehta, C.J. Russo and D.T. Hodul, to be published in: Proc. of SPIE Conf. Los Angeles (January, 1986).
[2] M. Moslehi, S. Shatas and K. Saraswat, Appl. Phys. Lett. 47 (1985) 1353. [3] D. Aitken, S. Mehta, N. Parisi, C.J. Russo and V. Schwartz, these Proceedings (Ion Implantation Technology, Berkeley, 1986) Nucl. Instr. and Meth. B21 (1987) 622. [4] J. Nulman, J.P. Krusuis, and A. Gat, IEEE Trans. Electron Devices (1985) 205. [5] C.J. Russo, to be published in: Proc. of SPIE Conf. Los Angeles (January, 1986).
629
S T U D I E S OF T H I N OXIDES G R O W N BY RAPID T H E R M A L P R O C E S S I N G S. M E H T A i), D . T . H O D U L 2) a n d C.J. R U S S O t) tJ Varian Associates, Extrion Division, Blackburn Industrial Park. Gloucester. MA 01930. USA "bVarian Research Center. 611 Hansen Way, Palo Alto. CA 94303. USA
Thin oxides have been rapidly grown using lamp heating. The growth kinetics show Arrhenius temperature dependence and are lineal- as a function of time. The process is diffusion limited rather than mass transport limited. Thickness uniformity measured to 2 mm of the wafer edge on 100-400 ,~ oxides grown at 1100-1200 C is 4% one sigma for an unoptimized process and can be improved to < 2~ using multistep processes. Some of the non uniformities may be related to substrate dopant, temperature effects and surface irregularities. Derived temperature uniformity is between 2.6 ° and 6 ° C one sigma. Flatband voltages of 0.7 V were obtained for aluminum capacitors with breakdown field strengths of 13 MV/cm for 250 A oxides. RCA cleaning and forming gas pretreatment improved the electrical performance of the oxides.
1. Introduction One of the most active new areas of rapid thermal processing is the use o f oxygen ambients to grow high quality oxides ( < 150 A) for gate dielectrics in VLSI devices [1,2]. In the experiments described below, silicon wafers were subjected to a variety of oxidizing ambients and temperature - time conditions using lamp heating to determine the kinetics and process uniformity of the oxidation. These data have been used to optimize the uniformity. The effects of surface preparation on the electrical characteristics of the oxides were determined using C - V and breakdown measurements; oxide contamination was qualitatively characterized using a bias-temperature stress method.
Oxide thickness measurements made on 100 mm (100) Si wafer (1200 ° C for 5 rain in 207o 02 and argon mixture) revealed an uncorrected average thickness of 287.3 ,~ with a standard deviation of 6.39 ,~. Temperature uniformity across the wafer can also be estimated from thickness uniformity of the oxides. Using linear growth kinetics of fig. 3, the temperature uniformity obtained was 2.6°C one sigma at 1200°C oxidation temperature [1].
Tungsten LampArray
\
H,gh Reflectance Water-Cooled M=trored Chamber Inter=or
L u uuuuuuuu/ u I 2. Oxidation kinetics Oxides were grown using Varian's rapid thermal processor model RTP-800 in a controlled oxygen ambient gas mixture. A schematic diagram of the heating and processing chambers is shown in fig. 1. Detailed description of the system is provided elsewhere [3]. Oxide growth was studied both as a function of time and oxidation temperature. For such studies, a single temperature - time profile as shown in fig. 2 was carried out for oxidation. Oxide thickness was measured by Nanospec and ellipsometry. Fig. 3 shows the oxide thickness as a function of time at the oxidation temperature of 1 1 0 0 ° C (fig. 3a) and oxidation temperature (fig. 3b). It can be seen that the thickness-time relationship is linear in the entire range from 1 to 8 min. This relationship is not linear for very short oxidation times. 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Window ~,~
Pho..... trig
Waler --
d I
.,a,en P,s,on
~t Vacuum
I
/
.,n.
--22--'----:----------------L-----r--i.
__J
L_ . . . . .
[----i iI
1 1
1
Fig. 1. Schematic description of RTP 800 heating and processing chambers. XI. RAPID THERMAL PROCESSING
630
S. Mehta et al.
/ Studies
of thin oxides
12OO11501100A
o IOOOo
w 900n.-
i.< 8OOn~ LO O_ 700uJ t--
WAFER SIZE = 150
or 6 0 0 uJ
15ooc FOR 5 SECS
~500400"
300-
J
,
T
I o
I 5
I IO
I 15
I 20
I 25
I 3o
I 35
TIME
I 40
I 45
I 50
I 55
I 60
I 65
I
7c
(SECONDS)
Fig. 2. A typical RTP 800 temperature-time profile.
600-
b
a 500-
S i < IO0> OXIDATION TEMPERATURE =llOO°C
40C
Si < 1 0 0 > OXIDATION TIME = 3 M I N S . AMBIENT = 2 0 % O z l N ARGON AT 2 0 CU FT (HR.) -I
A
400-
/
/
.=,/
/
°<[ 300-
/
P
/ /
ixl z
300-
_u
- zoo-
I
I--
/
/
J
/
t~ 200E3
/ ~)
0
BOO /
IO0,
O
TIME
9oo
(MINUTES)
,boo
,;oo
,~oo
TEMPERATURE
,;oo (°C)
Fig. 3. (a) Thickness of RTO oxide as a function of oxidation time. (b) Thickness of RTO oxide as a function of oxidation temperature.
3. Effect of gas flow rate and oxidant concentration As seen in fig. 3, oxides were grown in both dilute oxygen (20% 02 in argon) and pure oxygen. An increased rate of oxidation was observed when pure oxygen was chosen as the ambient gas. However, there was no deviation in the linearity of the thickness-time relationship, independent of oxidation temperature and
wafer crystal orientation. N o significant effect of gas flow rate on the oxide growth rate was observed.
4. Uniformity The oxide thickness uniformity was determined by both Nanospec and ellipsometry. Contour maps were
S. Mehta et aL / Studies of thin oxides
631
Table 1 Sample preparation, conditions and properties measured
Sample identity
Temp.-time conditions
(V)
OX-2 BRK-11 HROX-2 BRK-6
300s/1100 ° C 180s/1200 ° C 15X( < ls)/1200 o C 4X15s/1200 o C
-
VFB
Uncompensated oxide thickness
VFB") (V)
Wafer surface preparation
(A)-lo
6.0 1.5 0.7 0.8
-0.4 -0.2 - 0.1
290-10.9% 262.4-5.48% 181-7.16% 145-4.11%
-- 0 . 0 4 b)
none rca + forming gas rca + forming gas rca + forming gas
") + 16 V Stress, 15 min at 200° C. b)+4 V Stress, 15 min at 200°C.
generated using the ellipsometer assuming a refractive index of 1.46 by measuring 45 points on the wafer, to within 2 m m from the edge of the wafer. Using these maps routine oxidation gives uniformities better than 4% one sigma for thickness in the range of 100-400 ,~ when corrected for the random error of the ellipsometer, which at 150 A is approximately _+3 ,~ one sigma. Similar results were obtained for the nanospec on oxides - 250-300 ,~ thick. Since the edge of the wafer preferentially absorbs radiation during the heatup and radiates during the cooldown and steady state, a multicycle t e m p e r a t u r e - t i m e process was devised to optimize the uniformity. In table 1 the sample HROX-2 has the oxide thicker at the edges than at the center; and BRK-11 has the oxide thicker in the center. Sample BRK-6 is an attempt to make the oxides uniform. More work is required for this optimization. Gas flow rates above 5 l / r a i n were also found to effect the uniformity.
UNIFORMITY
~z] . . -.... I0
Capacitance-voltage measurements were performed using a Signatone model S1041 hot chuck system and an E G & G model 410 C - V plotter. 0.75 mm aluminum dots were evaporated onto the oxides and sintered in forming gas at 4 5 0 ° C for 15 min. Flat-band voltages were calculated to be 0.7 V with shifts of < 0.1 V (see table 1) for bias stress of +16 V, at 2 0 0 ° C for 15 minutes. The breakdown strength, was found to be 13 M V / c m or higher for oxides 250 A thick (see fig. 4). As expected the best results were obtained for wafers which received an R C A clean a n d / o r a forming gas pretreatment (20 s. at 1150°C in 6.5% H 2 and argon mixture) prior to the oxidation. The wafers were transported cross country between the R C A clean and oxidation. Therefore, some irregularities in the flatband voltages and shifts have occurred. The oxide quality will only get better if proper surface preparation procedures are followed. These experiments are in progress.
,0 o
w
"0
~ a I.u
w f__ z w 'o
8-
N " Z
!iI
5. E l e c t r i c a l m e a s u r e m e n t s
re . 1~0
4-
7
I
I0 13 VOLTS
16
19
22
Fig. 4. A histogram showing the breakdown field strength for sample HROX-2; 15 cycles for < I s each of 1200°C. The insert shows the breakdown field strength from the center to 1 cm from the edge of the wafer.
6. C o n c l u s i o n s
We have seen that thin oxides grown in the RTP follow linear kinetics of growth both for dilute as well as concentrated oxidizers. The oxide thickness uniformity measurements indicate good temperature uniformity across the wafer. Electrical measurements made on the oxides indicate that the RTP technique has a potential for producing high quality oxides. The authors would like to thank Mike Titus and Nick Parisi for developing the software for the multicycle process, Dave Barton and Bill Hoef for assistance on the mechanical aspects of the instrument and Juanita XI. RAPID THERMAL PROCESSING
632
S. Mehta et al. / Studies of thin oxides
SonJcs and Maggie Erikson for wafer cleaning and electrical measurements. The authorg also wish to thank Liz Coakley for typing the manuscript.
References [1] S. Mehta, C.J. Russo and D.T. Hodul, to be published in: Proc. of SPIE Conf. Los Angeles (January, 1986).
[2] M. Moslehi, S. Shatas and K. Saraswat, Appl. Phys. Lett. 47 (1985) 1353. [3] D. Aitken, S. Mehta, N. Parisi, C.J. Russo and V. Schwartz, these Proceedings (Ion Implantation Technology, Berkeley, 1986) Nucl. Instr. and Meth. B21 (1987) 622. [4] J. Nulman, J.P. Krusuis, and A. Gat, IEEE Trans. Electron Devices (1985) 205. [5] C.J. Russo, to be published in: Proc. of SPIE Conf. Los Angeles (January, 1986).