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Electrical properties of thin SiO2 films nitrided in N2O by rapid thermal processing
MicroelectronicEngineering 19 (1992) 657-660 Elsevier
657
Electrical properties of thin SiO2 films nitrided in N20 by rapid thermal processing M. Severi, G. Mattei, L. Dori, P. Maccagnani, G.L. BMdini and G. Pizzochero CNR-Istituto LAMEL, Via Castagnoli 1, 1-40126 Bologna, Italy
Abstract Charge trapping and dielectric wear-out properties of 8 and 30 nm Si02 layers nitrided in the N20 gas using a Rapid Thermal System are evaluated injecting charge at either low (by avalanche technique) or high (by Fowler-Nordheim technique) electric fields. In the experimental conditions studied, the results have pointed out that, compared to a standard silicon dioxide layer, a Si02 film nitrided in the N20 gas exhibits a reduced electron/hole trapping efficiency and, independently of the injection polarity, an improved charge-to-breakdown (QBD) characteristics.
1
Introduction
In the last ten years, the ammonia nitridation of a SiO2 film has been widely studied to improve the electrical properties of the silicon dioxide layer. As already reported in the literature [1, 2], the NH3-nitrided Si02 exhibits, among others, a very high electron trapping efficiency as well as a pronounced electron trap generation rate [3]. To minimize these two drawbacks and improve the oxide hot-electron immunity, the process complexity has been increased introducing a re-oxidation step performed immediately after the nitridation one. As the optimization of the silicon dioxide NH3-nitridation process is quite complicated [2], very recently the rapid thermal nitridation (RTN) of a Si02 film in a N20 environment has been suggested as an one-step process (no subsequent reoxidation is necessary) that would provide a highly reliable thin dielectric films [4]. In this work, we present new experimental results on the low-field (--~ 3MV/cm) charge trapping efficiency of 30 nm N20-nitrided SiOz films. A comparison between the electron and hole trapping of Si02 layers nitrided either in N20 or NH3 is reported, also. Furthermore, experimental data on charge-to-breakdown (QBD) and injection voltage shifts under constant current stress of very thin (8 am) N20-nitrided SiO2 films are shown.
2
Experimental
N-and P-type (100) Si substrates, 0.1 and 10 ft.cm resistivities were used in this study. After a 500 nm field oxide growth and patterning, SiO2 films of different thicknesses were 0167-9317/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved.
658
M. Severi et al. / Electrical properties of thin Si02films nitrided in N20
grown as follows: 1) 30nm thick oxide at T = 1000 °C in a dry 02 environment. After the oxidation, a RTN either in N20 (at T = l l 0 0 ° C for 10 and 25 sec.) or in NH3 (at T=1000°C for 10 sec.) was performed. Some of the NH3-nitrided SiO2 films were in-situ re-oxidized at T=1050°C for 60 sec.; 2) 8 nm thick SiO2 and N~O-nitrided silicon dioxide films were grown following two different procedures, respectively. That is: i) rapid thermal oxidation (RTO), in dry O2 at T=1000°C, up-to a final SiO,. thickness of 8 nm; ii) RTO in dry-O2 up-to a tsio2= 3-5 nm. Then, the N20 in-situ exposure at T = l l 0 0 ° C for 10 or 25 sec of the ultra thin oxide layers completed the dielectric growth (tsiog: 8 nm ). MOS capacitors of different sizes were fabricated using standard A1 and poly-Si gate technologies. The 400 nm thick poly-Si gate was POC13 doped and annealed at T=920°C. To reduce the contact resistance, aluminum (with 1% at. Si) was deposited on both side of the wafers and patterned on the front. The device fabrication process was completed performing a post metal anneal (PMA) in N2 at 400°C for 20 min.
3
Results
and Discussion
The effect on hole and electron trapping produced by two different nitridation processes is summarized in Fig. la and Fig. lb, respectively, where the flat-band voltage shifts (AVFB) are reported as a function of the avalanche injection time.
AI-gate
HH3, 1000'C, lOs
AI-gate
q2 /.~ /
~ ~
....
HH3+02,1050,C,60s
f / >-8 il ,
=h u. >
sioz
I
0
1
200
I
I
I
400 time,
I
600 S
__l
I
800
I
I
1000
NH3.02,1050"C, 605
~.
/
> 4 t~ u. >
! -/I
\MH3, 1000'C, 10s
jJ
\ \
2
-4
--~
f./
SlO2
0 0
800
1600 time,
2400 s
3200
4000
b) F i g u r e 1. Avalanche hole (Fig. la), electron (Fig. lb) injection in Al-gate MOS capacitors with 30 nm nitrided SiO2. Jh= 1.2 × 10 -7 A/cm2; Je-- 3.0 × 10 -s A / c m 2.
Looking at Fig. la, it is important to point out that after the short nitridation time in NH3 at T=1000°C or in N20 at T = l l 0 0 ° C , the hole trapping efficiency of a nitrided layer increases if the SiO2 is exposed to ammonia, and decreases if exposed to N20. Both these results are explained with the occurrence of one or more chemical reactions between nitrogen, oxygen and hydrogen (coming from the ammonia decomposition) atoms with some weak and/or strained bonds in the SIO2, that results in the saturation and/or in the rupture of those bonds. Following this picture, during the ammonia-nitridation in the conditions reported in the experimental section, the hydrogen coming from the
659
M. Severi et al. / Electrical properties of thin SiO 2 films nitrided in N20
NH3 decomposition has enhanced the generation of trivalent silicon defects (which act as efficient hole traps) that has prevailed on their passivation by nitrogen atoms [5]. In the case of the N20-nitridation, being the effect of a thermal treatment opposite to that observed after an an-12 Si-gate neal in N20 (both effects clearly shown in Fig. 2), the lower hole trap density is N2,1100 'C, 25 s -8 the consequence of a very efficient pas> & sivation of silicon-terminated dangling >~ ._~_$102 bonds and of oxygen vacancies by the "~-4 diffusing nitrogen and oxygen atoms [6]. Looking again at Fig.la, it seems I I I l l I I I I 0 that N20 nitridation times longer than 0 400 800 1200 1600 2000 10 sec tend to increase the number of time, s defects able to trap holes. This sugF i g u r e 2. Avalanche hole injection in Sigests the existence of an optimum nigate MOS capacitors with 30nm SiO2: eftridation time (at the studied temperafect of a rapid thermal anneal in N2 and ture) at which the best compromise beN20. tween reduction and generation of new defects would correspond. Figure lb shows the same trends as those observed in Fig. la. That is: i) nitridation in N20 reduces electron trapping as compared to that in the reference silicon oxide film. In this case, as already proposed by Weinberg [7], the reduction of defects able to trap electrons can be associated with the removal of water-related species from the oxide bulk, as an effect of the thermal treatment itself; ii) increasing the nitridation time, more electron traps are generated. This is probably related to the fact that nitrogen can also act as an electron trap once it occupies an oxygen site [1]. In Figs. 3a) and 3b), the Fowler-Nordheim (FN)injection voltage shifts (AVFN) as a function of the injection time are reported. [
1
-Vgate> 0 ;JFN"
0.8 i
m
Z rL
>.
0.1 A / e r a 2
S~
0.6
a
V/fate <
0 ;
JFN -
tox = 8 n m ; N20
10 s
0.1
A/cm'
b)
Poly S i - g a t e
sio~
0.4 NzO I 0 s
02
tox = 8 n m ; Poly S i - g a t e ,
0 0
I
J
5 time
I
,
I
I0 15 [xl0's]
,
20
0
2.5
5
7.5
time
[xl02s]
I0
F i g u r e 3. Injection voltage shifts vs time under electron injection from the substrate (Fig. 3 a ) a n d from the gate (Fig. 3b). Dot area: 7.85 x 10 -7 cmL
660
M. Severi et al. / Electrical properties of thin SiO 2 films nitrided in NeO
The stress was performed at constant current density (J=0.1 A/cm2). Figures 3a and 3b refer to the electron injection from the substrate and from the gate, respectively. As the Fig.3a) shows, compared to a reference SiO2 layer, the nitridation in N20 increases remarkably the QnD value, with a AVF~ at the device breakdown quite comparable. This improvement in the QBD value is attributed to the formation of strong Si-N bonds either in the bulk of the SiO2 film or at the Si- SiO2 interface [8]. For the injection from the gate (see Fig. 3b), at the breakdown the nitrided oxide still exhibits a higher QBD value but a much lower injection voltage shift. While the different QBD values for the two injection polarities (already observed in conventional oxides) may be determined by a different interface roughness that characterizes the two cases, the difference in the A V F N magnitude and slope observed after injection from the gate and from the substrate can be related to the non-uniform nitrogen distribution in the oxide layer, as shown by the Auger Electron Spectroscopy (AES) analysis of the nitrided SiO2 layer. In particular, the AES data have highlighted a nitrogen pile-up (~ 2 at.%) at the Si-SiO2 interface, with little or no nitrogen detected in the bulk of the film and at the outer interface. This asymmetrical nitrogen distribution, consistent with the results reported in [8], would greatly reduce the electron trap generation rate during the injection from the gate.
4
Conclusions
The charge trapping and the dielectric wear-out characteristics of thin (8 and 30 nm) N20-nitrided SiO2 films have been studied using both low and high field charge injection techniques. Both electron and hole trapping under avalanche injection has been found to decrease as compared to pure SiO2. Under stress at constant current (J= 0.1 A/cm2), injecting electrons either from the gate or from the substrate, QBD (AVFN) values much higher (lower or equal) than that observed in conventional oxides have been measured. These results have been explained with the formation of strong Si-N bonds during the N~O-nitridation step. This work gives further support to the idea that a N20-nitrided Si02 film can be a real candidate as a gate dielectric material for the future generation of VLSI devices.
References 1 2 3 4 5 6 7
M. Severi and M. Impronta, Appl. Phys. Lett., 51 (1987) 1702. T. Hori, H. Iwasaki and K. Tsuji, IEEE Trans., ED-36 (1989) 340. D.J. DiMaria and J.H. Stathis, J. Appl. Phys., 70 (1991) 1500. H. Fukuda, T. Arakawa and S. Ohno, Electron. Lett., 26 (1990) 1505. E.E. Dooms, M.M. Heyns and R.F. De Keersmaecker, Proc. of INFOS-89, (1989). M. Aslam and P. Balk, Proc. of INFOS-83, (1983) 103. Z.A. Weinberg, D.R. Young, J.A. Calise, S.A. Cohen, J.C. DeLuca and V.R. Deline, Appl. Phys. Lett., 45 (1984) 1204. 8 M. Yasuda, H. Fukuda, T. Iwabuchi, and S. Ohno, Jpn. J. Appl. Phys., 30 (1991) 3597.
657
Electrical properties of thin SiO2 films nitrided in N20 by rapid thermal processing M. Severi, G. Mattei, L. Dori, P. Maccagnani, G.L. BMdini and G. Pizzochero CNR-Istituto LAMEL, Via Castagnoli 1, 1-40126 Bologna, Italy
Abstract Charge trapping and dielectric wear-out properties of 8 and 30 nm Si02 layers nitrided in the N20 gas using a Rapid Thermal System are evaluated injecting charge at either low (by avalanche technique) or high (by Fowler-Nordheim technique) electric fields. In the experimental conditions studied, the results have pointed out that, compared to a standard silicon dioxide layer, a Si02 film nitrided in the N20 gas exhibits a reduced electron/hole trapping efficiency and, independently of the injection polarity, an improved charge-to-breakdown (QBD) characteristics.
1
Introduction
In the last ten years, the ammonia nitridation of a SiO2 film has been widely studied to improve the electrical properties of the silicon dioxide layer. As already reported in the literature [1, 2], the NH3-nitrided Si02 exhibits, among others, a very high electron trapping efficiency as well as a pronounced electron trap generation rate [3]. To minimize these two drawbacks and improve the oxide hot-electron immunity, the process complexity has been increased introducing a re-oxidation step performed immediately after the nitridation one. As the optimization of the silicon dioxide NH3-nitridation process is quite complicated [2], very recently the rapid thermal nitridation (RTN) of a Si02 film in a N20 environment has been suggested as an one-step process (no subsequent reoxidation is necessary) that would provide a highly reliable thin dielectric films [4]. In this work, we present new experimental results on the low-field (--~ 3MV/cm) charge trapping efficiency of 30 nm N20-nitrided SiOz films. A comparison between the electron and hole trapping of Si02 layers nitrided either in N20 or NH3 is reported, also. Furthermore, experimental data on charge-to-breakdown (QBD) and injection voltage shifts under constant current stress of very thin (8 am) N20-nitrided SiO2 films are shown.
2
Experimental
N-and P-type (100) Si substrates, 0.1 and 10 ft.cm resistivities were used in this study. After a 500 nm field oxide growth and patterning, SiO2 films of different thicknesses were 0167-9317/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved.
658
M. Severi et al. / Electrical properties of thin Si02films nitrided in N20
grown as follows: 1) 30nm thick oxide at T = 1000 °C in a dry 02 environment. After the oxidation, a RTN either in N20 (at T = l l 0 0 ° C for 10 and 25 sec.) or in NH3 (at T=1000°C for 10 sec.) was performed. Some of the NH3-nitrided SiO2 films were in-situ re-oxidized at T=1050°C for 60 sec.; 2) 8 nm thick SiO2 and N~O-nitrided silicon dioxide films were grown following two different procedures, respectively. That is: i) rapid thermal oxidation (RTO), in dry O2 at T=1000°C, up-to a final SiO,. thickness of 8 nm; ii) RTO in dry-O2 up-to a tsio2= 3-5 nm. Then, the N20 in-situ exposure at T = l l 0 0 ° C for 10 or 25 sec of the ultra thin oxide layers completed the dielectric growth (tsiog: 8 nm ). MOS capacitors of different sizes were fabricated using standard A1 and poly-Si gate technologies. The 400 nm thick poly-Si gate was POC13 doped and annealed at T=920°C. To reduce the contact resistance, aluminum (with 1% at. Si) was deposited on both side of the wafers and patterned on the front. The device fabrication process was completed performing a post metal anneal (PMA) in N2 at 400°C for 20 min.
3
Results
and Discussion
The effect on hole and electron trapping produced by two different nitridation processes is summarized in Fig. la and Fig. lb, respectively, where the flat-band voltage shifts (AVFB) are reported as a function of the avalanche injection time.
AI-gate
HH3, 1000'C, lOs
AI-gate
q2 /.~ /
~ ~
....
HH3+02,1050,C,60s
f / >-8 il ,
=h u. >
sioz
I
0
1
200
I
I
I
400 time,
I
600 S
__l
I
800
I
I
1000
NH3.02,1050"C, 605
~.
/
> 4 t~ u. >
! -/I
\MH3, 1000'C, 10s
jJ
\ \
2
-4
--~
f./
SlO2
0 0
800
1600 time,
2400 s
3200
4000
b) F i g u r e 1. Avalanche hole (Fig. la), electron (Fig. lb) injection in Al-gate MOS capacitors with 30 nm nitrided SiO2. Jh= 1.2 × 10 -7 A/cm2; Je-- 3.0 × 10 -s A / c m 2.
Looking at Fig. la, it is important to point out that after the short nitridation time in NH3 at T=1000°C or in N20 at T = l l 0 0 ° C , the hole trapping efficiency of a nitrided layer increases if the SiO2 is exposed to ammonia, and decreases if exposed to N20. Both these results are explained with the occurrence of one or more chemical reactions between nitrogen, oxygen and hydrogen (coming from the ammonia decomposition) atoms with some weak and/or strained bonds in the SIO2, that results in the saturation and/or in the rupture of those bonds. Following this picture, during the ammonia-nitridation in the conditions reported in the experimental section, the hydrogen coming from the
659
M. Severi et al. / Electrical properties of thin SiO 2 films nitrided in N20
NH3 decomposition has enhanced the generation of trivalent silicon defects (which act as efficient hole traps) that has prevailed on their passivation by nitrogen atoms [5]. In the case of the N20-nitridation, being the effect of a thermal treatment opposite to that observed after an an-12 Si-gate neal in N20 (both effects clearly shown in Fig. 2), the lower hole trap density is N2,1100 'C, 25 s -8 the consequence of a very efficient pas> & sivation of silicon-terminated dangling >~ ._~_$102 bonds and of oxygen vacancies by the "~-4 diffusing nitrogen and oxygen atoms [6]. Looking again at Fig.la, it seems I I I l l I I I I 0 that N20 nitridation times longer than 0 400 800 1200 1600 2000 10 sec tend to increase the number of time, s defects able to trap holes. This sugF i g u r e 2. Avalanche hole injection in Sigests the existence of an optimum nigate MOS capacitors with 30nm SiO2: eftridation time (at the studied temperafect of a rapid thermal anneal in N2 and ture) at which the best compromise beN20. tween reduction and generation of new defects would correspond. Figure lb shows the same trends as those observed in Fig. la. That is: i) nitridation in N20 reduces electron trapping as compared to that in the reference silicon oxide film. In this case, as already proposed by Weinberg [7], the reduction of defects able to trap electrons can be associated with the removal of water-related species from the oxide bulk, as an effect of the thermal treatment itself; ii) increasing the nitridation time, more electron traps are generated. This is probably related to the fact that nitrogen can also act as an electron trap once it occupies an oxygen site [1]. In Figs. 3a) and 3b), the Fowler-Nordheim (FN)injection voltage shifts (AVFN) as a function of the injection time are reported. [
1
-Vgate> 0 ;JFN"
0.8 i
m
Z rL
>.
0.1 A / e r a 2
S~
0.6
a
V/fate <
0 ;
JFN -
tox = 8 n m ; N20
10 s
0.1
A/cm'
b)
Poly S i - g a t e
sio~
0.4 NzO I 0 s
02
tox = 8 n m ; Poly S i - g a t e ,
0 0
I
J
5 time
I
,
I
I0 15 [xl0's]
,
20
0
2.5
5
7.5
time
[xl02s]
I0
F i g u r e 3. Injection voltage shifts vs time under electron injection from the substrate (Fig. 3 a ) a n d from the gate (Fig. 3b). Dot area: 7.85 x 10 -7 cmL
660
M. Severi et al. / Electrical properties of thin SiO 2 films nitrided in NeO
The stress was performed at constant current density (J=0.1 A/cm2). Figures 3a and 3b refer to the electron injection from the substrate and from the gate, respectively. As the Fig.3a) shows, compared to a reference SiO2 layer, the nitridation in N20 increases remarkably the QnD value, with a AVF~ at the device breakdown quite comparable. This improvement in the QBD value is attributed to the formation of strong Si-N bonds either in the bulk of the SiO2 film or at the Si- SiO2 interface [8]. For the injection from the gate (see Fig. 3b), at the breakdown the nitrided oxide still exhibits a higher QBD value but a much lower injection voltage shift. While the different QBD values for the two injection polarities (already observed in conventional oxides) may be determined by a different interface roughness that characterizes the two cases, the difference in the A V F N magnitude and slope observed after injection from the gate and from the substrate can be related to the non-uniform nitrogen distribution in the oxide layer, as shown by the Auger Electron Spectroscopy (AES) analysis of the nitrided SiO2 layer. In particular, the AES data have highlighted a nitrogen pile-up (~ 2 at.%) at the Si-SiO2 interface, with little or no nitrogen detected in the bulk of the film and at the outer interface. This asymmetrical nitrogen distribution, consistent with the results reported in [8], would greatly reduce the electron trap generation rate during the injection from the gate.
4
Conclusions
The charge trapping and the dielectric wear-out characteristics of thin (8 and 30 nm) N20-nitrided SiO2 films have been studied using both low and high field charge injection techniques. Both electron and hole trapping under avalanche injection has been found to decrease as compared to pure SiO2. Under stress at constant current (J= 0.1 A/cm2), injecting electrons either from the gate or from the substrate, QBD (AVFN) values much higher (lower or equal) than that observed in conventional oxides have been measured. These results have been explained with the formation of strong Si-N bonds during the N~O-nitridation step. This work gives further support to the idea that a N20-nitrided Si02 film can be a real candidate as a gate dielectric material for the future generation of VLSI devices.
References 1 2 3 4 5 6 7
M. Severi and M. Impronta, Appl. Phys. Lett., 51 (1987) 1702. T. Hori, H. Iwasaki and K. Tsuji, IEEE Trans., ED-36 (1989) 340. D.J. DiMaria and J.H. Stathis, J. Appl. Phys., 70 (1991) 1500. H. Fukuda, T. Arakawa and S. Ohno, Electron. Lett., 26 (1990) 1505. E.E. Dooms, M.M. Heyns and R.F. De Keersmaecker, Proc. of INFOS-89, (1989). M. Aslam and P. Balk, Proc. of INFOS-83, (1983) 103. Z.A. Weinberg, D.R. Young, J.A. Calise, S.A. Cohen, J.C. DeLuca and V.R. Deline, Appl. Phys. Lett., 45 (1984) 1204. 8 M. Yasuda, H. Fukuda, T. Iwabuchi, and S. Ohno, Jpn. J. Appl. Phys., 30 (1991) 3597.