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The impact of F contamination induced by the process on the gate oxide reliability
PII:
Microelectron. Reliab., Vol. 38, No. 2, pp. 255±258, 1998 # 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0026-2714/98 $19.00 + 0.00 S0026-2714(97)00040-1
THE IMPACT OF F CONTAMINATION INDUCED BY THE PROCESS ON THE GATE OXIDE RELIABILITY G. GHIDINI, C. CLEMENTI, D. DRERA and F. MAUGAIN Non Volatile Memory Process Development, Central R&D, SGS-Thomson Microelectronics, via Olivetti 2, 20041 Agrate Brianza, Italy (Received 23 December 1996; in revised form 15 April 1997) AbstractÐThe eects of F contaminants introduced by the CVD WSi2 deposition and diused to the gate oxide interfaces by the thermal treatment performed during the process have been analyzed. High ®eld stresses showed a degradation of the quality of the oxides contaminated by ¯uorine, but decreasing the stress ®eld below a critical value of 10.5 MV/cm no more eect of ¯uorine on the gate oxide reliability was detectable. # 1998 Elsevier Science Ltd.
EXPERIMENTAL
Simple MOS capacitors have been measured on wafers which performed a complete process ¯ow, we have been able to test on the same wafer structures with and without F contamination on gate oxides using as a reference capacitors with a Si3N4 layer over the poly-Si gate to stop ¯uorine diusion. On contaminated oxides large F concentrations have been found of the order of 1 1020 atoms/cm2 by Nuclear Reaction Analysis [1]. Furthermore, TEM cross-sections have shown on F contaminated oxides an increase of the oxide thickness (around 1 nm) regardless of the starting oxide. Apart from the well-known eect of degradation of the trapping characteristics of thin tunnel oxides [2, 3], no clear in¯uence of the presence of ¯uorine on the electrical degradation of gate oxides has been previously reported. We tested by means of constant electric ®eld stress (TDDB) dierent areas, ranging from 1 10ÿ4 up to 1 10ÿ2 cm2, and various oxide thicknesses, ranging from 10 up to 30 nm. Charge trapping evaluation has been performed investigating the leakage voltage evolution after stress at high ®elds. RESULTS
For every oxide thickness a linear dependence of the logarithm of the mean lifetime (t50%) on the electric ®eld has been veri®ed as shown in Fig. 1, where the results obtained for a 20 nm thick oxide are reported as a sample. In the same plot a clear reduction of the mean lifetime and an increase of the ®eld acceleration factor (b) is reported when F contaminants are present, as clearly shown by the increase of the slope of the logarithm of the mean 255
lifetime reported in the ®gure. b is related to the intrinsic dielectric strength and to the charge trapping as will be shown afterwards. Moreover, it has been found that b does not strongly depend on the tested area, and a Poisson distribution ®ts well the experimental data for the investigated area range (Fig. 2). This observation suggests that even when F contamination is present no defect clustering occurs: in fact, in case of clustering, a strong deviation from Poisson statistics is expected. On the other hand, the ®eld acceleration factor b has been found to decrease as the oxide thickness reduces and the eect of the F contamination on b stressed in Fig. 1 practically disappears for the thinnest oxides (Fig. 3). In fact, the b value is related to lifetimes, i.e. to intrinsic oxide breakdown: the breakdown mechanism depends on the charge trapping [4], which is known to strongly decrease with the oxide thickness. Nevertheless, the oxide degradation caused by the presence of ¯uorine still exists since the mean lifetime is signi®cantly reduced, as is evident in Fig. 4 where the ratio between the time to fail of a reference and of an F contaminated sample is reported for dierent oxide thicknesses: in all cases a strong reduction of the mean lifetime is evident. An experimental result con®rming the relationship between ®eld acceleration factor and oxide wear-out can be obtained investigating the leakage voltage evolution at low currents after high ®eld stress as shown in Fig. 5. We de®ne leakage voltage as the gate bias at which a leakage current of 1 nA is detected. Comparing the leakage voltage evolution at dierent stress ®elds it is possible to deduce a ®eld acceleration factor of the charge trapping by plotting at each ®eld the stress time necessary to obtain
256
G. Ghidini et al.
log T50% (sec)
No F β = 2.1 decades/(MV/cm)
B
3
B
2
B
β = 4.1 decades/(MV/cm)
1 0 10.0
10.5
F
11.0
11.5
T50 (F)/ T50 (no F)
1.0
5 4
0.8 0.6
B
0.4 B
0.2
12.0
1.0 100
E stress (MV/cm) ° Gate oxide 200 A Area capacitor = 1E-2 cm 2
200
the same leakage voltage shift shown in Fig. 5 by the dotted line (properly chosen out of the saturation region). This ®eld acceleration factor turns out to be almost the same as that obtained from standard TDDB measurements (Fig. 6). This result holds regardless of the presence of F in the oxide. The agreement is very good except for thicker oxides, for which stress ®elds applied continuously or interrupted in order to measure the leakage can result in very dierent lifetimes (Fig. 2). From the ®eld dependence of the mean lifetime at high ®elds it is also possible to deduce the reliability of the oxide at lower ®elds. From the experiment of which Fig. 1 is an example, we can predict the existence of a critical ®eld (around
Fig. 4. Mean lifetime ratio between F contaminated and reference oxide for dierent dielectric thickness
10.5 MV/cm) below which no ¯uorine related eects should be observed. In fact, the I(t) characteristics of the samples stressed at high ®elds (Fig. 7(a)) show a steeper current decrease corresponding to an enhancement of the negative charge trapping rate in the presence of ¯uorine, while at low ®elds (Fig. 7(b)) the trapping rate becomes more similar to that of the reference oxide. The dierent initial current value in the presence of F contaminants is only related to the oxide thickening already mentioned. The dierent behaviour at low and high ®elds has been investigated using the method described in refs
5 4
DVtest (V)
Fig. 2. Lifetime distribution on dierent tested areas of 20 nm F contaminated gate oxide
F
B B
F
3
B
B
No F
B
No F
B
B
B
2
B B B
B
1 B
B
B
11.00 MV/cm 11.25 MV/cm
B
B
0 1E-1
1E+0
1E+1
1E+2
1E+3
Stress time (sec) 20 nm oxide I test = 1E-9 A
Estress = 11 and 11.25 MV/cm
Fig. 5. Leakage voltage evolution on oxides with and without F contaminants after high ®eld stress at 1 nA for dierent stress ®elds
4.5
F
B
3.5 3.0
B
No F
2.5 2.0
B
β (decades/MV/cm)
6 B
4.0
B
5
10
30
Oxide thickness (nm) Fig. 3. Field acceleration factor with and without F contaminants for dierent oxide thickness
β tapping β TDDB
B
4
B
3
B
2 B B
1 20
B
B
B
1.5 B 1.0
300
° Oxide thickness (A)
Fig. 1. Mean lifetime of 20 nm gate oxide with and with-
β (decades/MV/cm)
B
10
20
30
Oxide thickness (nm) Fig. 6. Field acceleration factors for F contaminated oxides of dierent thickness using standard TDDB and leakage voltage evolution methods
Impact of F contamination on gate oxide reliability
F9 MV/cm No F9 MV
(a) I(t)@ 11.5 MV/cm
1.00E-1
F
2.0E+2
A = 1E-2 cm2
B
1.40E-2
Centroid (A)
No F
1.40E-4
F
A = 1E-4 cm2
1.40E-5 1.00E-6 1.00E-1
1.00E+0
1.00E+1
1.5E+2
1.0E+2
5.0E+1 0.0E+0 1E+0
(b)
1E+1
1E+2
1E+3
1E+4
Stress time (sec)
1.00E-9
I(t)@ 7 MV/cm
No F11 MV F11 MV/cm
No F
1.40E-3
F
TDDBstress at -9 and -11 MV/cm Sensing: 5E-6 A/cm2
A = 1E-3 cm2
Fig. 9. Evolution of the centroid of bulk trapped charge in oxides with and without F contaminants stressed at low or high ®elds
1.32E-10 B
No F 1.32E-11 1.00E-11 7.80E-1
7.80E+0
7.80E+1
1.31E+3
20 nm gate oxide Fig. 7. Current evolution at low and high ®eld stress on 20 nm oxide with and without F contaminants
[5, 6]. While the evolution of the bulk trapped charge is similar at low ®elds, an enhanced charge trapping is found in the presence of F contaminants at high ®elds (Fig. 8). Also, the position of the centroid of trapped charge is dierent at high ®elds (Fig. 9); while at low ®elds it is located in the middle of the oxide moving from the anode to the cathode during the stress, at high ®elds in the presence of F contaminants the trapped charge is located close to the cathode interface, in a completely dierent way with respect to the reference sample. The similar behaviour for F contaminated and reference sample at low ®elds has been ®nally veri®ed with standard TDDB measurements. F9 MV/cm No F9 MV
2.0E-7
No F11 MV F11 MV/cm
Indeed, performing a stress at ®elds lower than 10.5 MV/cm, similar results are obtained showing that the theoretical prediction of the critical ®eld value was correct and the F contaminated oxides have the same mean lifetime as the reference for all ®elds below the critical one (Fig. 10). This threshold ®eld is probably related to the breaking of F related bonds [7], supplying in this way a larger concentration of trapping sites for negative charge, allowing an enhancement of the rate of negative charge trapping, which ®nally causes the premature oxide breakdown. The location of this charge seems to be in the region close to the cathode where a large F concentration has been measured [1].
CONCLUSION
The result of this study on the eect of ¯uorine contamination on the gate oxide quality shows that while a strong degradation is induced at high ®elds, no eects are occurring at low ®elds, in a region where most of the devices are working. A direct correlation between the ®eld acceleration factor and
0.0E+0
No F
-2.0E-7 -4.0E-7
log T50% (sec)
Q (C/cm 2)
257
-6.0E-7 -8.0E-7 -1.0E-6 -1.2E-6 1.4E-6 1E+0
7 6
β = 2.1 decades/(MV/cm)
5 4 3
β = 1.4 decades/(MV/cm)
2 β = 4.1 decades/(MV/cm)
1
1E+1
1E+2
1E+3
Stress time (sec) Sensing: 5E-6 A/cm2 Fig. 8. Evolution of the trapped charge in oxides with and without F contaminants stressed at low or high ®elds
F
0 8.5
9.0
9.5
10.0
10.5 11.0
11.5
12.0
E (MV/cm) A = 1E-2cm 2 Fig. 10. Mean lifetime of 20 nm gate oxide with and without F contaminants for dierent stress ®elds
258
G. Ghidini et al.
the charge build up into the oxide has also been presented.p REFERENCES 1. Ghezzi, P. et al., Semicond. Sci. Technol., 1991, 6, 684. 2. Prendergast, J. et al., IEEE/IRPS Proc., 1995, p. 124.
3. Chaparala, P. et al., Integrated Reliability Workshop Proc., 1995, p. 207. 4. Dumin, D. J. et al., IEEE/IRPS Proc., 1994, p. 143. 5. DiMaria, D. J. et al., J. Appl. Phys., 1989, 65, 2342. 6. Papadas, C. et al., Solid State Electron., 1994, 37, 495. 7. Wright, P. J. and Saraswat, K. C., IEEE Trans. Electron Dev., 1989, 36, 879.
Microelectron. Reliab., Vol. 38, No. 2, pp. 255±258, 1998 # 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0026-2714/98 $19.00 + 0.00 S0026-2714(97)00040-1
THE IMPACT OF F CONTAMINATION INDUCED BY THE PROCESS ON THE GATE OXIDE RELIABILITY G. GHIDINI, C. CLEMENTI, D. DRERA and F. MAUGAIN Non Volatile Memory Process Development, Central R&D, SGS-Thomson Microelectronics, via Olivetti 2, 20041 Agrate Brianza, Italy (Received 23 December 1996; in revised form 15 April 1997) AbstractÐThe eects of F contaminants introduced by the CVD WSi2 deposition and diused to the gate oxide interfaces by the thermal treatment performed during the process have been analyzed. High ®eld stresses showed a degradation of the quality of the oxides contaminated by ¯uorine, but decreasing the stress ®eld below a critical value of 10.5 MV/cm no more eect of ¯uorine on the gate oxide reliability was detectable. # 1998 Elsevier Science Ltd.
EXPERIMENTAL
Simple MOS capacitors have been measured on wafers which performed a complete process ¯ow, we have been able to test on the same wafer structures with and without F contamination on gate oxides using as a reference capacitors with a Si3N4 layer over the poly-Si gate to stop ¯uorine diusion. On contaminated oxides large F concentrations have been found of the order of 1 1020 atoms/cm2 by Nuclear Reaction Analysis [1]. Furthermore, TEM cross-sections have shown on F contaminated oxides an increase of the oxide thickness (around 1 nm) regardless of the starting oxide. Apart from the well-known eect of degradation of the trapping characteristics of thin tunnel oxides [2, 3], no clear in¯uence of the presence of ¯uorine on the electrical degradation of gate oxides has been previously reported. We tested by means of constant electric ®eld stress (TDDB) dierent areas, ranging from 1 10ÿ4 up to 1 10ÿ2 cm2, and various oxide thicknesses, ranging from 10 up to 30 nm. Charge trapping evaluation has been performed investigating the leakage voltage evolution after stress at high ®elds. RESULTS
For every oxide thickness a linear dependence of the logarithm of the mean lifetime (t50%) on the electric ®eld has been veri®ed as shown in Fig. 1, where the results obtained for a 20 nm thick oxide are reported as a sample. In the same plot a clear reduction of the mean lifetime and an increase of the ®eld acceleration factor (b) is reported when F contaminants are present, as clearly shown by the increase of the slope of the logarithm of the mean 255
lifetime reported in the ®gure. b is related to the intrinsic dielectric strength and to the charge trapping as will be shown afterwards. Moreover, it has been found that b does not strongly depend on the tested area, and a Poisson distribution ®ts well the experimental data for the investigated area range (Fig. 2). This observation suggests that even when F contamination is present no defect clustering occurs: in fact, in case of clustering, a strong deviation from Poisson statistics is expected. On the other hand, the ®eld acceleration factor b has been found to decrease as the oxide thickness reduces and the eect of the F contamination on b stressed in Fig. 1 practically disappears for the thinnest oxides (Fig. 3). In fact, the b value is related to lifetimes, i.e. to intrinsic oxide breakdown: the breakdown mechanism depends on the charge trapping [4], which is known to strongly decrease with the oxide thickness. Nevertheless, the oxide degradation caused by the presence of ¯uorine still exists since the mean lifetime is signi®cantly reduced, as is evident in Fig. 4 where the ratio between the time to fail of a reference and of an F contaminated sample is reported for dierent oxide thicknesses: in all cases a strong reduction of the mean lifetime is evident. An experimental result con®rming the relationship between ®eld acceleration factor and oxide wear-out can be obtained investigating the leakage voltage evolution at low currents after high ®eld stress as shown in Fig. 5. We de®ne leakage voltage as the gate bias at which a leakage current of 1 nA is detected. Comparing the leakage voltage evolution at dierent stress ®elds it is possible to deduce a ®eld acceleration factor of the charge trapping by plotting at each ®eld the stress time necessary to obtain
256
G. Ghidini et al.
log T50% (sec)
No F β = 2.1 decades/(MV/cm)
B
3
B
2
B
β = 4.1 decades/(MV/cm)
1 0 10.0
10.5
F
11.0
11.5
T50 (F)/ T50 (no F)
1.0
5 4
0.8 0.6
B
0.4 B
0.2
12.0
1.0 100
E stress (MV/cm) ° Gate oxide 200 A Area capacitor = 1E-2 cm 2
200
the same leakage voltage shift shown in Fig. 5 by the dotted line (properly chosen out of the saturation region). This ®eld acceleration factor turns out to be almost the same as that obtained from standard TDDB measurements (Fig. 6). This result holds regardless of the presence of F in the oxide. The agreement is very good except for thicker oxides, for which stress ®elds applied continuously or interrupted in order to measure the leakage can result in very dierent lifetimes (Fig. 2). From the ®eld dependence of the mean lifetime at high ®elds it is also possible to deduce the reliability of the oxide at lower ®elds. From the experiment of which Fig. 1 is an example, we can predict the existence of a critical ®eld (around
Fig. 4. Mean lifetime ratio between F contaminated and reference oxide for dierent dielectric thickness
10.5 MV/cm) below which no ¯uorine related eects should be observed. In fact, the I(t) characteristics of the samples stressed at high ®elds (Fig. 7(a)) show a steeper current decrease corresponding to an enhancement of the negative charge trapping rate in the presence of ¯uorine, while at low ®elds (Fig. 7(b)) the trapping rate becomes more similar to that of the reference oxide. The dierent initial current value in the presence of F contaminants is only related to the oxide thickening already mentioned. The dierent behaviour at low and high ®elds has been investigated using the method described in refs
5 4
DVtest (V)
Fig. 2. Lifetime distribution on dierent tested areas of 20 nm F contaminated gate oxide
F
B B
F
3
B
B
No F
B
No F
B
B
B
2
B B B
B
1 B
B
B
11.00 MV/cm 11.25 MV/cm
B
B
0 1E-1
1E+0
1E+1
1E+2
1E+3
Stress time (sec) 20 nm oxide I test = 1E-9 A
Estress = 11 and 11.25 MV/cm
Fig. 5. Leakage voltage evolution on oxides with and without F contaminants after high ®eld stress at 1 nA for dierent stress ®elds
4.5
F
B
3.5 3.0
B
No F
2.5 2.0
B
β (decades/MV/cm)
6 B
4.0
B
5
10
30
Oxide thickness (nm) Fig. 3. Field acceleration factor with and without F contaminants for dierent oxide thickness
β tapping β TDDB
B
4
B
3
B
2 B B
1 20
B
B
B
1.5 B 1.0
300
° Oxide thickness (A)
Fig. 1. Mean lifetime of 20 nm gate oxide with and with-
β (decades/MV/cm)
B
10
20
30
Oxide thickness (nm) Fig. 6. Field acceleration factors for F contaminated oxides of dierent thickness using standard TDDB and leakage voltage evolution methods
Impact of F contamination on gate oxide reliability
F9 MV/cm No F9 MV
(a) I(t)@ 11.5 MV/cm
1.00E-1
F
2.0E+2
A = 1E-2 cm2
B
1.40E-2
Centroid (A)
No F
1.40E-4
F
A = 1E-4 cm2
1.40E-5 1.00E-6 1.00E-1
1.00E+0
1.00E+1
1.5E+2
1.0E+2
5.0E+1 0.0E+0 1E+0
(b)
1E+1
1E+2
1E+3
1E+4
Stress time (sec)
1.00E-9
I(t)@ 7 MV/cm
No F11 MV F11 MV/cm
No F
1.40E-3
F
TDDBstress at -9 and -11 MV/cm Sensing: 5E-6 A/cm2
A = 1E-3 cm2
Fig. 9. Evolution of the centroid of bulk trapped charge in oxides with and without F contaminants stressed at low or high ®elds
1.32E-10 B
No F 1.32E-11 1.00E-11 7.80E-1
7.80E+0
7.80E+1
1.31E+3
20 nm gate oxide Fig. 7. Current evolution at low and high ®eld stress on 20 nm oxide with and without F contaminants
[5, 6]. While the evolution of the bulk trapped charge is similar at low ®elds, an enhanced charge trapping is found in the presence of F contaminants at high ®elds (Fig. 8). Also, the position of the centroid of trapped charge is dierent at high ®elds (Fig. 9); while at low ®elds it is located in the middle of the oxide moving from the anode to the cathode during the stress, at high ®elds in the presence of F contaminants the trapped charge is located close to the cathode interface, in a completely dierent way with respect to the reference sample. The similar behaviour for F contaminated and reference sample at low ®elds has been ®nally veri®ed with standard TDDB measurements. F9 MV/cm No F9 MV
2.0E-7
No F11 MV F11 MV/cm
Indeed, performing a stress at ®elds lower than 10.5 MV/cm, similar results are obtained showing that the theoretical prediction of the critical ®eld value was correct and the F contaminated oxides have the same mean lifetime as the reference for all ®elds below the critical one (Fig. 10). This threshold ®eld is probably related to the breaking of F related bonds [7], supplying in this way a larger concentration of trapping sites for negative charge, allowing an enhancement of the rate of negative charge trapping, which ®nally causes the premature oxide breakdown. The location of this charge seems to be in the region close to the cathode where a large F concentration has been measured [1].
CONCLUSION
The result of this study on the eect of ¯uorine contamination on the gate oxide quality shows that while a strong degradation is induced at high ®elds, no eects are occurring at low ®elds, in a region where most of the devices are working. A direct correlation between the ®eld acceleration factor and
0.0E+0
No F
-2.0E-7 -4.0E-7
log T50% (sec)
Q (C/cm 2)
257
-6.0E-7 -8.0E-7 -1.0E-6 -1.2E-6 1.4E-6 1E+0
7 6
β = 2.1 decades/(MV/cm)
5 4 3
β = 1.4 decades/(MV/cm)
2 β = 4.1 decades/(MV/cm)
1
1E+1
1E+2
1E+3
Stress time (sec) Sensing: 5E-6 A/cm2 Fig. 8. Evolution of the trapped charge in oxides with and without F contaminants stressed at low or high ®elds
F
0 8.5
9.0
9.5
10.0
10.5 11.0
11.5
12.0
E (MV/cm) A = 1E-2cm 2 Fig. 10. Mean lifetime of 20 nm gate oxide with and without F contaminants for dierent stress ®elds
258
G. Ghidini et al.
the charge build up into the oxide has also been presented.p REFERENCES 1. Ghezzi, P. et al., Semicond. Sci. Technol., 1991, 6, 684. 2. Prendergast, J. et al., IEEE/IRPS Proc., 1995, p. 124.
3. Chaparala, P. et al., Integrated Reliability Workshop Proc., 1995, p. 207. 4. Dumin, D. J. et al., IEEE/IRPS Proc., 1994, p. 143. 5. DiMaria, D. J. et al., J. Appl. Phys., 1989, 65, 2342. 6. Papadas, C. et al., Solid State Electron., 1994, 37, 495. 7. Wright, P. J. and Saraswat, K. C., IEEE Trans. Electron Dev., 1989, 36, 879.