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Polarity asymmetry of electrical characteristics of thin nitrided polyoxides prepared by in-situ multiple rapid thermal processing
Solid-State Electronics Vol. 34, No. 2, pp. 181-184, 1991
0038-1101/91 $3.00 + 0.00 Copyright ~ 1991 Pergamon Press plc
Printed in Great Britain. All rights reserved
POLARITY A S Y M M E T R Y OF ELECTRICAL CHARACTERISTICS OF THIN N I T R I D E D POLYOXIDES P R E P A R E D BY IN-SITU MULTIPLE RAPID T H E R M A L PROCESSING G. Q. Lo, *A. W. CHEUNG, I D. L. KWONG1 and N. S. ALVI2 •Microelectronics Research Center, Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, "IX 78712 and 2Delco Electronics Corporation, Kokomo, IN 46902, U.S.A. (Received 23 April 1990: in revised form 18 June 1990)
Abstract--Thin ( ~ 20 nm) nitrided polyoxide filmshave been grown on n +-polycrystallinesilicon (poly-Si) by in-situ multiple rapid thermal processing including thermal oxidation (RTO) and rapid thermal nitridation (RTN). Effects of nitridation on the electrical properties such as electrical conduction and charge trapping have been studied using both bias polarities. It is found for the first time that after RTN, the resulting polyoxides have an unusual polarity symmetry dependence of electrical conduction and trapping properties, i.e. a higher electrical conduction and electron trapping rate occurred for electrons injected from the top poly-Si electrode than for electrons injected from the bottom poly-Si electrode.
For nonvolatile memory applications, the scaling of device geometries makes thin polyoxides necessary in order to keep a high gate coupling ratio. Device performance is determined by its data retentivity, programming efficiency in terms of leakage current and dielectric strength. However, polyoxides usually exhibit higher leakage current and inferior dielectric strength in comparison with oxides grown on singlecrystalline Si due to the asperities and bumps at the oxide/polycrystalline silicon (poly-Si) interfaces which lead to a localized field enhancement[l-6]. Several techniques have been proposed to improve the quality of interpoly dielectrics, including rapid thermal oxidation (RTO)[7,8] and furnace nitridation of polyoxides[9]. In Ref. [9], a high dielectric breakdown field of 14 MV/cm was reported for thick furnace nitrided polyoxides of 76 nm. However, the bias polarity dependence of electrical characteristics in these nitrided polyoxide films has not been addressed[7-9]. Since polyoxids with symmetrical electrical characteristics may have advantages for some floating-gate nonvolatile memory structure[5], such as the triple poly-Si layers structure[10], the bias polarity dependence of electrical characteristics of these polyoxides needs to be studied. In this letter, thin nitrided polyoxides have been fabricated by in-situ rapid thermal processing (RTP) including RTO and rapid thermal nitridation (RTN) of doped poly-Si. The effects of RTN on the bias polarity dependence of electrical conduction and charge trapping properties are studied. It is found that after RTN, polyoxide films exhibit an unusual polarity asymmetry in electrical conduction and trapping properties. Although RTN increase the dielectric strength of polyoxides, it also introduces a high density of electron traps.
The n+-poly-Si/SiOfln'-poly-Si capacitor structure was used approx in this study. 0.1/zm SiO2 was thermally grown on p-type Si substrates followed by the first poly-Si (~0.35/~m) deposition at 620°C in an LPCVD system. The poly-Si films were then doped by phosphorus ion implantation at 100 keV to a dose of 5 x 10~5cm 2. The dopants were activated by rapid thermal annealing (RTA) at 950°C for 10s in N 2. Subsequent fabrication steps included low temperature oxide (~0.6/~m) deposition and patterning to expose active areas on the poly-Si surface. Polyoxides were then grown by RTO at I I00:C for 60 s in pure 02. Some samples received RTN at 1000°C in pure NH 3 for 10 and 60 s. After the second poly-Si (~0.3/zm) deposition, doping and metal sputtering, both layers were etched using the same mask to form the upper electrode. Metal microalloy using RTA at 450°C for 15 s in forming gas completed the process. Both cross-sectional transmission electron microscopy (TEM) and highfrequency capacitance-voltage (HFCV) characteristics were used to measure the thickness of resulting interpoly dielectrics. The effects of RTN on the electrical conduction of polyoxide for both bias polarities are illustrated in Fig. 1, where a typical set of J - E characteristics of RTN polyoxides is plotted. The noise spikes with the J - E curves are due to small isolated breakdowns which are self-healing during ramp-up of the electric field[4]. As shown in Fig. l(a), when the upper electrode is positively biased ( + Vs, i.e. electrons injected from the bottom poly-Si), the leakage current decreases with RTN time. The polyoxides have a lower onset tunneling field ( < 2 . 5 MV/cm) than SiO: on single-crystalline Si ( ~ 6 MV/cm) because of the localized field enhancement at the tips of the 181
G.Q. Lo et al.
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180
260
340
420
500
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Fig. I. Typical J - E characteristics for (a) + V, and (b) - V, bias (i.e. electrons were injected from bottom and top poly-Si, respectively) in samples with RTN polyoxides of nitridation time for I0 and 60s at 100OC. Polyoxide without RTN is included for comparison. (c) Illustrates the F - N plots of log(Js/E2o,) vs I/Eo~ for RTN polyoxides subjected - Vs bias and the slope of the curves is used to extract the barrier height. asperities[I-6]. The observed similar onset fields for all polyoxides with and without RTN indicate that RTN has no effect on the bottom poly-Si/polyoxide interface roughness. The slopes of J - E curves decrease with increasing applied electric field, indicating the occurrence of electron trapping during charge injection and trapping becomes more severe for
longer RTN. This is because the buildup of negative charge due to electron trapping lowers the injecting field and thus the electron injection. Therefore, the observed leakage current reduction in RTN polyoxides is primarily due to the increased electron trap density in RTN polyoxides. In contrast, different effects of RTN on the conduction behavior of polyoxides are observed when the upper electrode is negatively biased ( - 1 / i.e. electrons injected from the top poly-Si). As shown in Fig. l(b), the leakage current increases with RTN. Therefore, for heavily nitrided polyoxide (1000 C. 60 s), a higher leakage current at a given field is obsreved for V~ than for + V~. This unusual effect of nitridation on polyoxide polarity asymmetry of electrical conduction is observed tbr the first time. The increased leakage current level may indicate the injecting barrier lowering effect due to nitridation. In addition, the decreased onset of conduction from 4 to 2.5 MV/cm suggests that RTN may "roughen" the top poly-Si/polyoxide interface, leading to a localized field enhancement. The decreasing slopes of J E curves with increasing field for RTN polyoxides at higher curernt levels again indicate an increasing electron trapping due to RTN. For both polarities, highly nitrided polyoxide shows a decreased dielectric breakdown field while heavily nitrided samples shown an improved dielectric strength (9- 10 MV/cm) which is even higher than that of polyoxide without RTN. The enhancement of dielectric strength after RTN is consistent with a previous report on thick films (76 nm) in furnace nitridation[9]. Secondary-ion mass spectrometry (SIMS) was performed to RTN polyoxides (not shown). Nitrogen pile-ups are observed at both top and bottom polySi/polyoxide interfaces after RTN. However, the surface nitrogen concentration is several orders higher than that of the bottom polyoxide/poly-Si interface. It is assumed that the nitrogen pile-up lowers the injecting barrier in RTN polyoxides similar to RTN SiO2 on single-crystalline Si[ll]. Therefore, the electrical conduction is expected to increase for both bias polarities. However, since the surface nitrogen concentration is higher than that at the bottom interface, the barrier lowering effect is more pronounced for electron injection occurring at the top interface than at the bottom interface. Therefore, one explanation for the high leakage current in RTN samples for electron injection occurring at the top interface is that nitrogen-induced injecting barrier lowering plays a more dominant role at the top interface than at the bottom interface. However, the reduction of leakage current for electron injection from the bottom poly-Si is mainly due to the electron trapping effect which is evident from the strong decrease of the slopes of J - E curves with increased applied electric field. In order to deduce the barrier height of the top poly-Si/polyoxide interface, J - E curves are replotted as Iog(Js/E:o ,) vs I;Eo~. Based on the F - N tunneling current as
Electrical characteristics of nitrided polyoxides
JsF2o, = A exp( - B,~IEo~), B,~ is the emission factor related to the barrier height. As shown in Fig. I(c), the barrier height values are calculated to 1.92, 1.65 and 1.47eV for polyoxides without RTN, RTN at 1000°C for 10 s and RTN at 1000°C for 60 s, respectively. Obviously, this barrier height lowering is directly related to the nitrogen incorporated at the top interface. However, the strong shift of Iog(Js/E2o~) vs l/Eo~ curves toward lower electric fields indicates there is a field enhancement effect due to nitridation. The reasons for this field enhancement are currently unknown. Similar observation of an unusual polarity dependence of leakage current was reported by Lee and Hu[5]. Polyoxides grown on in-situ doped poly-Si had a higher leakage current when electrons were injected from the top poly-Si, which was speculated to be caused by a high density of small bumps at the top poly-Si/polyoxide interface. Figure 2 compares the change of applied electric field (AEo~) needed to maintain a constant current stress of 100 pA/cm 2 for RTN polyoxides. For both bias polarities, the electron trapping rate increases with RTN. Both Figs 2(a) and (b) show nonsaturated
(o) Without RTN
I.O
trapping behavior, indicating that electron trap generation occurs during electron injection. It can be seen that polyoxide without RTN exhibits a normal polarity asymmetry of electron trapping behavior, i.e. the electron trapping rate is higher when electrons are injected from the bottom poly-Si than when electrons are injected from the top poly-Si[4-6]. This polarity asymmetry is attributed to the relative difference in the degree of roughness between top and bottom poly-Si/polyoxide interfaces, since the rougher bottom interface leads to a smaller conduction area and local current density intensification, and thus, results in a higher electron trapping rate[4-6]. In contrast, RTN polyoxides show an opposite trapping behavior. More importantly, the electron trapping rate is relatively insensitive to RTN when electrons are injected from the top poly-Si. Correlating this with the aforementioned electrical conduction behaviors in RTN polyoxides, it is speculated that the "roughened" top poly-Si/polyoxide interface after RTN is responsible for the observed higher electron trapping rate for electron injection fromk top poly-Si, and the interface becomes rougher as RTN proceeds. Furthermore, the electric field-induced trap generation rate and charge trapping centroid in RTN polyoxides are illustrated in Figs 3(a) and (b), wherein the derivative of AEox with respect to time
to)
............ R T N H~@'C, 10 • ....
0.15
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Fig. 2. Change of applied electric field (AEo~)as a function of stress time for (a) + VI bias and (b) - Vs bias under F-N electron injection of 100~uA/cm2 in samples with RTN polyoxides. Apparently, the charge trapping properties a r e sensitive to nitridation when electrons injected from top poly-Si than when electrons injected from bottom poly-Si.
$0
IO0 Time (s)
150
2011
Fig. 3. (a) Derivative of change of applied electric field (dAEo,/dt) and (b) charge trapping ccntroid as a function o f stress time. RTN increases the electric-field induced trap generation rate and pushes the trapping ccntroid away from the bottom poly-Si/polyoxid¢ interface.
184
G.Q. Lo et al.
(dAEox/dt) and centroids (to bottom polySi/polyoxide interface) are plotted as functions of stress time when electrons are injected from the top poly-Si at 1 0 0 g A / c m 2. As shown in Fig. 3(a), all samples show saturated dAEo~/dt values which indicates finite field-induced trap generation rates[l 2,13]. Obviously, R T N polyoxides not only have a higher nitridation-induced electron trap density, but also have a higher-field-induced electron trap generation rate. This high electron trapping rate could be attributed to incorporated hydrogen-species[14] due to NH3 decomposition during RTN[15]. As proposed by NissanoCohen and Gorczyca[14], the new electron traps result from the rupture of S i - H or S i - O H bonds during high-field electron injection. However, the saturated d A E o , / d t values in polyoxides with R T N times of 10 and 60 s seem to suggest that the amount of Si-H or S i - O H bonds subject to rupture is finite after R T N . In addition, as shown in Fig. 3(b), the charge-trapping--centroids in resulting dielectrics are seen to move away from the bottom polyoxide/polySi interface. Based on the calculation of bi-directional / .V curves' shift (AVg- and AV~ ) during electron injection, trapping centroids have been estimated [x c = T , , x x A V ~ - / ( A V ~ + AVg)]. Normally, the centroids in polyoxides were found to be very close to the bottom interface due to the out-diffusion of phosphorus into polyoxides during oxidation[6]. However, if the incorporated hydrogen species distribute in position away from the bottom interface as R T N proceeds, then the field-induced electron traps would be away from the bottom interface as well. Our results seem to support this assumption. In summary, the dependence of polarity asymmetry of electrical conduction and trapping
properties of R T N polyoxides has been studied. It is found that R T N polyoxides have an unusual polarity asymmetry in both conduction and trapping properties, suggesting that R T N may result in injecting barrier lowering and/or a rougher interface at the top poly-Si/polyoxide interface due to the nitrogen presence. REFERENCES
1. D. J. DiMaria and D. R. Kerr, Appl. Phys. Lett. 27, 505 (1975). 2. R. M. Anderson and D. R. Kerr, J. appl. Phys. 48, 4834 (1977). 3. E. Irene, E. Tierny and D. Dong, J. Electrochem. Sot'. 127, 705 (1980). 4. P. Heimann, S. Muraka and T. Sheng, J. appl. Phys. 53, 6240 (1982). 5. J. Lee and C. Hu, IEEE Trans. Electron Derices ED-35, 1063 (1988). 6. M. S. Liang, N. Radjy, W. Cox and S. Cagnina, Proc. Electrochem. Sot., Fall Meeting, p. 608 (1986). 7. A. Maury, S. C. Kim, A. Manocha, K. H. Oh, D. Kostelnick and S. Shive, IEDM Tech. Dig.. p. 676 (1986). 8. N. S. Alvi, S. K. Lee and D. L. Kwong, IEEE Electron Det'ice EDL-8, 197 (1987). 9. C. F. Chen and C. Y. Wu, Appl. Phys. Left. 48, 165 (1986). 10. J. Drori, S. J.-Larsen, R. Klein, W. Owen, R. Simko and W. Tchon, IEEE Int. Solid-St. Circuits Conll Tech. Dig., p. 148 (1981). 11. D. K. Shih, W. T. Chang, S. K. Lee, Y. H. Ku, D. L. Kwong and S. Lee, Appl. Phys. Lett. 52, 1698 (1988). 12. M. S. Liang and C. Hu, IEDM Tech. Dig., p. 396 ( 1981 ). 13. C. S. Jeng, T. R. Ranganath, C. H Huang, tt. S. Jones and T. L. Chang, IEDM Tech. Dig., p. 388 (1981). 14. Y. Nissan-Cohen and T. Gorczyca, IEEE Electron Derices EDL-9, 287 (1988). 15. T. Hori and lwasaki, IEDM Tech. Dig., p. 570 (1987).
0038-1101/91 $3.00 + 0.00 Copyright ~ 1991 Pergamon Press plc
Printed in Great Britain. All rights reserved
POLARITY A S Y M M E T R Y OF ELECTRICAL CHARACTERISTICS OF THIN N I T R I D E D POLYOXIDES P R E P A R E D BY IN-SITU MULTIPLE RAPID T H E R M A L PROCESSING G. Q. Lo, *A. W. CHEUNG, I D. L. KWONG1 and N. S. ALVI2 •Microelectronics Research Center, Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, "IX 78712 and 2Delco Electronics Corporation, Kokomo, IN 46902, U.S.A. (Received 23 April 1990: in revised form 18 June 1990)
Abstract--Thin ( ~ 20 nm) nitrided polyoxide filmshave been grown on n +-polycrystallinesilicon (poly-Si) by in-situ multiple rapid thermal processing including thermal oxidation (RTO) and rapid thermal nitridation (RTN). Effects of nitridation on the electrical properties such as electrical conduction and charge trapping have been studied using both bias polarities. It is found for the first time that after RTN, the resulting polyoxides have an unusual polarity symmetry dependence of electrical conduction and trapping properties, i.e. a higher electrical conduction and electron trapping rate occurred for electrons injected from the top poly-Si electrode than for electrons injected from the bottom poly-Si electrode.
For nonvolatile memory applications, the scaling of device geometries makes thin polyoxides necessary in order to keep a high gate coupling ratio. Device performance is determined by its data retentivity, programming efficiency in terms of leakage current and dielectric strength. However, polyoxides usually exhibit higher leakage current and inferior dielectric strength in comparison with oxides grown on singlecrystalline Si due to the asperities and bumps at the oxide/polycrystalline silicon (poly-Si) interfaces which lead to a localized field enhancement[l-6]. Several techniques have been proposed to improve the quality of interpoly dielectrics, including rapid thermal oxidation (RTO)[7,8] and furnace nitridation of polyoxides[9]. In Ref. [9], a high dielectric breakdown field of 14 MV/cm was reported for thick furnace nitrided polyoxides of 76 nm. However, the bias polarity dependence of electrical characteristics in these nitrided polyoxide films has not been addressed[7-9]. Since polyoxids with symmetrical electrical characteristics may have advantages for some floating-gate nonvolatile memory structure[5], such as the triple poly-Si layers structure[10], the bias polarity dependence of electrical characteristics of these polyoxides needs to be studied. In this letter, thin nitrided polyoxides have been fabricated by in-situ rapid thermal processing (RTP) including RTO and rapid thermal nitridation (RTN) of doped poly-Si. The effects of RTN on the bias polarity dependence of electrical conduction and charge trapping properties are studied. It is found that after RTN, polyoxide films exhibit an unusual polarity asymmetry in electrical conduction and trapping properties. Although RTN increase the dielectric strength of polyoxides, it also introduces a high density of electron traps.
The n+-poly-Si/SiOfln'-poly-Si capacitor structure was used approx in this study. 0.1/zm SiO2 was thermally grown on p-type Si substrates followed by the first poly-Si (~0.35/~m) deposition at 620°C in an LPCVD system. The poly-Si films were then doped by phosphorus ion implantation at 100 keV to a dose of 5 x 10~5cm 2. The dopants were activated by rapid thermal annealing (RTA) at 950°C for 10s in N 2. Subsequent fabrication steps included low temperature oxide (~0.6/~m) deposition and patterning to expose active areas on the poly-Si surface. Polyoxides were then grown by RTO at I I00:C for 60 s in pure 02. Some samples received RTN at 1000°C in pure NH 3 for 10 and 60 s. After the second poly-Si (~0.3/zm) deposition, doping and metal sputtering, both layers were etched using the same mask to form the upper electrode. Metal microalloy using RTA at 450°C for 15 s in forming gas completed the process. Both cross-sectional transmission electron microscopy (TEM) and highfrequency capacitance-voltage (HFCV) characteristics were used to measure the thickness of resulting interpoly dielectrics. The effects of RTN on the electrical conduction of polyoxide for both bias polarities are illustrated in Fig. 1, where a typical set of J - E characteristics of RTN polyoxides is plotted. The noise spikes with the J - E curves are due to small isolated breakdowns which are self-healing during ramp-up of the electric field[4]. As shown in Fig. l(a), when the upper electrode is positively biased ( + Vs, i.e. electrons injected from the bottom poly-Si), the leakage current decreases with RTN time. The polyoxides have a lower onset tunneling field ( < 2 . 5 MV/cm) than SiO: on single-crystalline Si ( ~ 6 MV/cm) because of the localized field enhancement at the tips of the 181
G.Q. Lo et al.
182 C0) 10" 1
+V s
I0" 2 ,/
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180
260
340
420
500
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Fig. I. Typical J - E characteristics for (a) + V, and (b) - V, bias (i.e. electrons were injected from bottom and top poly-Si, respectively) in samples with RTN polyoxides of nitridation time for I0 and 60s at 100OC. Polyoxide without RTN is included for comparison. (c) Illustrates the F - N plots of log(Js/E2o,) vs I/Eo~ for RTN polyoxides subjected - Vs bias and the slope of the curves is used to extract the barrier height. asperities[I-6]. The observed similar onset fields for all polyoxides with and without RTN indicate that RTN has no effect on the bottom poly-Si/polyoxide interface roughness. The slopes of J - E curves decrease with increasing applied electric field, indicating the occurrence of electron trapping during charge injection and trapping becomes more severe for
longer RTN. This is because the buildup of negative charge due to electron trapping lowers the injecting field and thus the electron injection. Therefore, the observed leakage current reduction in RTN polyoxides is primarily due to the increased electron trap density in RTN polyoxides. In contrast, different effects of RTN on the conduction behavior of polyoxides are observed when the upper electrode is negatively biased ( - 1 / i.e. electrons injected from the top poly-Si). As shown in Fig. l(b), the leakage current increases with RTN. Therefore, for heavily nitrided polyoxide (1000 C. 60 s), a higher leakage current at a given field is obsreved for V~ than for + V~. This unusual effect of nitridation on polyoxide polarity asymmetry of electrical conduction is observed tbr the first time. The increased leakage current level may indicate the injecting barrier lowering effect due to nitridation. In addition, the decreased onset of conduction from 4 to 2.5 MV/cm suggests that RTN may "roughen" the top poly-Si/polyoxide interface, leading to a localized field enhancement. The decreasing slopes of J E curves with increasing field for RTN polyoxides at higher curernt levels again indicate an increasing electron trapping due to RTN. For both polarities, highly nitrided polyoxide shows a decreased dielectric breakdown field while heavily nitrided samples shown an improved dielectric strength (9- 10 MV/cm) which is even higher than that of polyoxide without RTN. The enhancement of dielectric strength after RTN is consistent with a previous report on thick films (76 nm) in furnace nitridation[9]. Secondary-ion mass spectrometry (SIMS) was performed to RTN polyoxides (not shown). Nitrogen pile-ups are observed at both top and bottom polySi/polyoxide interfaces after RTN. However, the surface nitrogen concentration is several orders higher than that of the bottom polyoxide/poly-Si interface. It is assumed that the nitrogen pile-up lowers the injecting barrier in RTN polyoxides similar to RTN SiO2 on single-crystalline Si[ll]. Therefore, the electrical conduction is expected to increase for both bias polarities. However, since the surface nitrogen concentration is higher than that at the bottom interface, the barrier lowering effect is more pronounced for electron injection occurring at the top interface than at the bottom interface. Therefore, one explanation for the high leakage current in RTN samples for electron injection occurring at the top interface is that nitrogen-induced injecting barrier lowering plays a more dominant role at the top interface than at the bottom interface. However, the reduction of leakage current for electron injection from the bottom poly-Si is mainly due to the electron trapping effect which is evident from the strong decrease of the slopes of J - E curves with increased applied electric field. In order to deduce the barrier height of the top poly-Si/polyoxide interface, J - E curves are replotted as Iog(Js/E:o ,) vs I;Eo~. Based on the F - N tunneling current as
Electrical characteristics of nitrided polyoxides
JsF2o, = A exp( - B,~IEo~), B,~ is the emission factor related to the barrier height. As shown in Fig. I(c), the barrier height values are calculated to 1.92, 1.65 and 1.47eV for polyoxides without RTN, RTN at 1000°C for 10 s and RTN at 1000°C for 60 s, respectively. Obviously, this barrier height lowering is directly related to the nitrogen incorporated at the top interface. However, the strong shift of Iog(Js/E2o~) vs l/Eo~ curves toward lower electric fields indicates there is a field enhancement effect due to nitridation. The reasons for this field enhancement are currently unknown. Similar observation of an unusual polarity dependence of leakage current was reported by Lee and Hu[5]. Polyoxides grown on in-situ doped poly-Si had a higher leakage current when electrons were injected from the top poly-Si, which was speculated to be caused by a high density of small bumps at the top poly-Si/polyoxide interface. Figure 2 compares the change of applied electric field (AEo~) needed to maintain a constant current stress of 100 pA/cm 2 for RTN polyoxides. For both bias polarities, the electron trapping rate increases with RTN. Both Figs 2(a) and (b) show nonsaturated
(o) Without RTN
I.O
trapping behavior, indicating that electron trap generation occurs during electron injection. It can be seen that polyoxide without RTN exhibits a normal polarity asymmetry of electron trapping behavior, i.e. the electron trapping rate is higher when electrons are injected from the bottom poly-Si than when electrons are injected from the top poly-Si[4-6]. This polarity asymmetry is attributed to the relative difference in the degree of roughness between top and bottom poly-Si/polyoxide interfaces, since the rougher bottom interface leads to a smaller conduction area and local current density intensification, and thus, results in a higher electron trapping rate[4-6]. In contrast, RTN polyoxides show an opposite trapping behavior. More importantly, the electron trapping rate is relatively insensitive to RTN when electrons are injected from the top poly-Si. Correlating this with the aforementioned electrical conduction behaviors in RTN polyoxides, it is speculated that the "roughened" top poly-Si/polyoxide interface after RTN is responsible for the observed higher electron trapping rate for electron injection fromk top poly-Si, and the interface becomes rougher as RTN proceeds. Furthermore, the electric field-induced trap generation rate and charge trapping centroid in RTN polyoxides are illustrated in Figs 3(a) and (b), wherein the derivative of AEox with respect to time
to)
............ R T N H~@'C, 10 • ....
0.15
RTN I ~ ' C ,
183
(~ •
lO o
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,
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120 Time (s)
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100
//
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Without RTN
..........~............................................................
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200
Time (s)
Fig. 2. Change of applied electric field (AEo~)as a function of stress time for (a) + VI bias and (b) - Vs bias under F-N electron injection of 100~uA/cm2 in samples with RTN polyoxides. Apparently, the charge trapping properties a r e sensitive to nitridation when electrons injected from top poly-Si than when electrons injected from bottom poly-Si.
$0
IO0 Time (s)
150
2011
Fig. 3. (a) Derivative of change of applied electric field (dAEo,/dt) and (b) charge trapping ccntroid as a function o f stress time. RTN increases the electric-field induced trap generation rate and pushes the trapping ccntroid away from the bottom poly-Si/polyoxid¢ interface.
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(dAEox/dt) and centroids (to bottom polySi/polyoxide interface) are plotted as functions of stress time when electrons are injected from the top poly-Si at 1 0 0 g A / c m 2. As shown in Fig. 3(a), all samples show saturated dAEo~/dt values which indicates finite field-induced trap generation rates[l 2,13]. Obviously, R T N polyoxides not only have a higher nitridation-induced electron trap density, but also have a higher-field-induced electron trap generation rate. This high electron trapping rate could be attributed to incorporated hydrogen-species[14] due to NH3 decomposition during RTN[15]. As proposed by NissanoCohen and Gorczyca[14], the new electron traps result from the rupture of S i - H or S i - O H bonds during high-field electron injection. However, the saturated d A E o , / d t values in polyoxides with R T N times of 10 and 60 s seem to suggest that the amount of Si-H or S i - O H bonds subject to rupture is finite after R T N . In addition, as shown in Fig. 3(b), the charge-trapping--centroids in resulting dielectrics are seen to move away from the bottom polyoxide/polySi interface. Based on the calculation of bi-directional / .V curves' shift (AVg- and AV~ ) during electron injection, trapping centroids have been estimated [x c = T , , x x A V ~ - / ( A V ~ + AVg)]. Normally, the centroids in polyoxides were found to be very close to the bottom interface due to the out-diffusion of phosphorus into polyoxides during oxidation[6]. However, if the incorporated hydrogen species distribute in position away from the bottom interface as R T N proceeds, then the field-induced electron traps would be away from the bottom interface as well. Our results seem to support this assumption. In summary, the dependence of polarity asymmetry of electrical conduction and trapping
properties of R T N polyoxides has been studied. It is found that R T N polyoxides have an unusual polarity asymmetry in both conduction and trapping properties, suggesting that R T N may result in injecting barrier lowering and/or a rougher interface at the top poly-Si/polyoxide interface due to the nitrogen presence. REFERENCES
1. D. J. DiMaria and D. R. Kerr, Appl. Phys. Lett. 27, 505 (1975). 2. R. M. Anderson and D. R. Kerr, J. appl. Phys. 48, 4834 (1977). 3. E. Irene, E. Tierny and D. Dong, J. Electrochem. Sot'. 127, 705 (1980). 4. P. Heimann, S. Muraka and T. Sheng, J. appl. Phys. 53, 6240 (1982). 5. J. Lee and C. Hu, IEEE Trans. Electron Derices ED-35, 1063 (1988). 6. M. S. Liang, N. Radjy, W. Cox and S. Cagnina, Proc. Electrochem. Sot., Fall Meeting, p. 608 (1986). 7. A. Maury, S. C. Kim, A. Manocha, K. H. Oh, D. Kostelnick and S. Shive, IEDM Tech. Dig.. p. 676 (1986). 8. N. S. Alvi, S. K. Lee and D. L. Kwong, IEEE Electron Det'ice EDL-8, 197 (1987). 9. C. F. Chen and C. Y. Wu, Appl. Phys. Left. 48, 165 (1986). 10. J. Drori, S. J.-Larsen, R. Klein, W. Owen, R. Simko and W. Tchon, IEEE Int. Solid-St. Circuits Conll Tech. Dig., p. 148 (1981). 11. D. K. Shih, W. T. Chang, S. K. Lee, Y. H. Ku, D. L. Kwong and S. Lee, Appl. Phys. Lett. 52, 1698 (1988). 12. M. S. Liang and C. Hu, IEDM Tech. Dig., p. 396 ( 1981 ). 13. C. S. Jeng, T. R. Ranganath, C. H Huang, tt. S. Jones and T. L. Chang, IEDM Tech. Dig., p. 388 (1981). 14. Y. Nissan-Cohen and T. Gorczyca, IEEE Electron Derices EDL-9, 287 (1988). 15. T. Hori and lwasaki, IEDM Tech. Dig., p. 570 (1987).