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Charge relaxation in nitrided and non-nitrided sputtered oxide
Journal of Non-Crystalline Solids 254 (1999) 99±105
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Charge relaxation in nitrided and non-nitrided sputtered oxide E.V. Jelenkovic *, K.Y. Tong Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Room EF501, Hung Hom, Kowloon, Hong Kong, People's Republic of China
Abstract The charge relaxation of nitrided silicon oxide was studied in an MOS structure. Oxynitrides were formed by reactive sputtering in an Ar/O2 /N2 plasma from a silicon oxide target. The nitrogen mixing ratio was varied from 0% to 20%, with the argon mixing ratio kept at 80%. The electrical stress was performed with injection from an aluminium gate and the relaxation of stress-induced charge under ¯oating and gate bias conditions was monitored via mid-gap voltage and interface states density measurement in a time interval from 1 min to about four months. A trend that nitrided oxides experience more relaxation of mid-gap voltage was observed while non-nitrided oxide had the largest relaxation of interface states density in the same time interval. We suggest that relaxation under applied bias due to the stress-induced charge is, in part, due to the slow states. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Sputtered insulators have been considered as appropriate gate insulators in amorphous and polysilicon thin ®lm transistors for display technologies [1,2]. Suyama et al. [3,4] have reported breakdown and interface properties of silicon oxide deposited in argon/oxygen gas mixtures. They suggested it as a dielectric of choice in polysilicon thin ®lm transistor (TFT) structures because of low leakage current and step coverage in ®lms as thin as 7 nm [2,5]. Sputtering can be a truly low (<200°C) temperature process and it was recently demonstrated that fully sputtered TFTs of device grade quality can be obtained at only 125°C [1]. Electrical stability and reliability of sputtered insulators are important issues for applications in electronic devices which were ignored until our
rather recent studies [5,6]. We found that there is a need to optimise sputtering conditions for improved electrical stability of sputtered oxide [5]. Also, sputtered oxide has a remarkable reliability in nonhydrogenated TFTs [5]. In analogy to nitridation of thermal oxide, we have co-sputtered oxides in an Ar/O2 /N2 mixture and related nitrided oxide stability to nitrogen level incorporation [6]. In real applications electronic devices are exposed to periods of stress (noticeably electrical) and periods of relaxation. Therefore the study of relaxation of charges in sputtered oxide is important to obtain a better mode of its reliability and possibly relate it to the sputter deposition process. In this paper we report stability properties and charge relaxation of oxide nitrided by co-sputtering in a nitrogen containing ambient. 2. Experiment
* Corresponding author. Tel.: +852 2766 6134; fax: 852 2362 8439; e-mail: [email protected]
Oxides were sputtered in Ar/O2 /N2 atmosphere on p-type (1 0 0) silicon wafers from a SiO2 target in
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 3 8 0 - 4
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E.V. Jelenkovic, K.Y. Tong / Journal of Non-Crystalline Solids 254 (1999) 99±105
an RF-sputtering system (ANELVA, model SPF332H). Oxide thicknesses were from 22 to 28 nm. A nitrogen mixing ratio N2 /(Ar + O2 + N2 ) was de®ned by (nitrogen ¯ow rate)/(total ¯ow rate) and was varied from 0% to 20%, with the argon mixing ratio constant. Sputtering pressure was 0.3 Pa and sputtering power was about 2 W cmÿ2 . The deposition temperature was 300°C while post-deposition sintering was performed at 900°C in nitrogen for 20 min. This sintering temperature has been found to anneal most eciently sputtering process built-in damage and provide the most stable metal-oxidesemiconductor (MOS) structure with sputtered non-nitrided oxide [7]. After aluminium sputtering at low sputtering power and patterning to obtain MOS capacitor structures, samples were annealed in forming gas at 400°C for 10 min. MOS capacitor area was 1.3 ´ 10ÿ4 cmÿ2 . 2.1. Physical analysis Several observations in oxides nitrided by cosputtering in nitrogen atmosphere are worthwhile mentioning, without going into details. (i) As demonstrated through secondary ion spectroscopy (SIMS) measurements, the presence of nitrogen was observed in all ®lms including non-nitrided samples (Fig. 1). The presence of nitrogen in non-nitrided oxides is attributed to either residual gas in the SIMS analysing chamber or residual nitrogen in the sputtering chamber [8]. However, there is obvious evidence of increased nitrogen content for oxides co-sputtered in nitrogen Ar/O2 /N2 mixture, which is particularly true for the ®lms with 20% nitrogen ¯ow. The areas under SiN and N peaks are located 8 nm from the silicon/oxide interface [8]. (ii) FTIR analysis (not shown) has found that the Si±O stretching mode vibrational peak wave number has a tendency to decrease with increased nitrogen ¯ow rate. This observation is valid for both as deposited and annealed samples [9]. Annealed oxides have larger vibrational peak wave numbers for the Si±O asymmetric stretching mode [9]. (iii) Fig. 1 also shows an increase in oxide etch rate in buered oxide etchant with greater nitrogen ¯ow rate.
Fig. 1. SIMS peak values for N () and SiN (s) for oxynitrides sputtered with dierent mixing ratios after annealing at 900°C; (j) ± normalised etch rate of as deposited oxynitrides. Lines are drawn as guides for the eye. The random error in the data is of the order of the symbols.
(iv) With nitrogen incorporation, leakage current in the oxide in the MOS structure increases [6]. Along with it, more uniform distribution of breakdown events was recorded with a drawback of reduced breakdown voltage [6]. 2.2. Electrical stability and charge relaxation For the purpose of electrical stress and charge relaxation study, only samples with non-nitrided oxide and nitrided with up to 15% N2 mixing ratio were used. (Samples with 20% mixing ratio had a hysteresis in capacitance as a function of voltage (C V ) curves and were not considered either for stability or for charge relaxation studies.) Electrical stress with electron injection from the metal electrode was applied. In all experiments the stress current density was 38 lA cmÿ2 . Stress-induced instability and post-stress relaxation phenomena were monitored through the changes in mid-gap voltage (Vmg ) and interface states density (Dit ) in the time interval ranging from 1 min to about three months. For each stress time a dierent MOS capacitor was used, which makes the obtained
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result more representative. Both monitoring parameters (mid-gap voltage and density of interface states) were extracted from the high-frequency C V measurement. Terman's method was used for interface states calculation [10]. It should be noted that the time between stress removal and C V measurement completion was over 1 min. This time is also true for the test on the study of charge relaxation under gate bias, described in Section 3.2. 3. Results 3.1. Electrical stability Fig. 2 illustrates the mid-gap voltages after 100, 200 and 300 s stress times for four dierent nitrogen ¯ows. It indicates an increase of positive charge in the oxides with longer stress time. However, in the initial period of stress, which is not presented in Fig. 2, all ®lms experienced a shift of the mid-gap voltage to more positive voltages, which can be related to electron charge trapping [6]. With longer stress time trapping-generation of charge in the
Fig. 3. Dependence of interface states on nitrogen mixing ratio for 100 s (), 200 s (s) and 300 s (M) constant current stress time. The random error in the data is 1 ´ 1011 cmÿ2 eVÿ1 .
oxides is gradually overtaken by positive charge generation, which is indicated by the shift of the mid-gap voltage in the opposite direction [6]. For the same stress time intervals Fig. 3 shows that for nitrided oxide obtained under 10% of nitrogen ¯ow, there is a minimum in the interface state density generated. 3.2. Relaxation under the ¯oating gate
Fig. 2. Change in mid-gap voltage with nitrogen mixing ratio for 100 s (), 200 s (s) and 300 s (M) constant current stress time. The random errors in the data is 0.1 V.
To obtain further insight into the charging properties of nitrided and non-nitrided oxides, samples stressed up to 200 s were monitored with time for the relaxation processes. Relaxation process was carried out at room temperature and under ¯oating gate potential. Typical property of nonstressed, stressed and relaxed high-frequency C V curves is illustrated in Fig. 4. It shows that the stressed curve after the relaxation moves towards the original, with obvious stretch decrease in the portion approaching inversion. Relaxed Vmg s and Dit s are presented in Figs. 5 and 6, respectively. Except in oxynitride sputtered with a 5% nitrogen mixing ratio, which shows increased positive charge trapping, in all other oxides there is a trend to de-
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Fig. 4. Behaviour of C±V curve before stress, after stress and after relaxation; relaxation under ¯oating bias.
Fig. 6. Relaxation of interface states generated during the constant current stress with time; during the relaxation gate was kept ¯oating; current stress ± 38 lA cmÿ2 , with injection from aluminium; stress time ± 200 s; nitrogen mixing ratio: (M) ± 0%, (s) ± 5%, () ± 10% and (O) ± 15%. The random error in the data is 1 ´ 1011 cmÿ2 eVÿ1 .
crease the amount of positive charge originally built-in by the stress. It should be noted that in Fig. 6, Dit for all oxynitrides decreases as a function of time. In none of the samples, over the time scale used, was generation of interface states observed after the stress was removed. The oxynitride with the largest net positive charge trapping (15% nitrogen mixing ratio) has the largest relaxation in Vmg . The relative change in Dit is the most signi®cant for non-nitrided oxide and decreases with nitrogen mixing ratio. Such Vmg and Dit trends during the relaxation period dier from the usual observation that insulators with greater numbers of generated interface states also have a larger charge relaxation [12]. 3.3. Relaxation under bias Fig. 5. Relaxation of mid-gap voltage for oxynitrides after constant current stress with time; current stress ± 38 lA cmÿ2 , with injection from aluminium; stress time ± 200 s; nitrogen mixing ratio: (M) ± 0%, (s) ± 5%, () ± 10% and (O) ± 15%. The random error in the data is 0.1 V.
In Section 3.1 it was observed that at certain stress ¯uence a turn-around eect appeared which we ascribed to net positive charge trapping. Also the ¯oating bias relaxation experiment did not provide
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much information about the relaxed positive charge. By the following experiment it is intended to make a dierentiation between positive charge and slow states in sputtered oxynitrides. Namely, there is not a dividing line between oxide traps and slow donor states (the demarcation line is primarily determined by the distance of the defect from the interface and the energy level of the trap [14]). After the stress (samples with 0% and 15% nitrogen mixing ratio), MOS structures were relaxed with the gate ¯oating for about 15 500 min. This relaxations was followed by bias relaxation, as illustrated in Figs. 7 and 8, for mid-gap voltage and interface states density, respectively. Firstly, negative bias was applied, then negative + positive and ®nally negative + positive + negative. As in all previous stress experiments, each set of data in Figs. 7
Fig. 8. Relaxation of generated interface states during constant current stress with biased gate; constant current stress 38 lA cmÿ2 with injection from aluminium; stress time 200 s; nitrogen mixing ratio: () ± 15% and (s) ± 0%; initial refers to mid-gap voltage measured about 1 min after the stress; relaxed refers to 15 500 min relaxation after the stress; relaxation time is 1 h under each positive or negative bias. The random errors in the density of states is 1 ´ 1011 cmÿ2 eVÿ1 .
Fig. 7. Relaxation of mid-gap voltage with biased gate for oxynitrides after constant current stress; current stress ± 38 lA cmÿ2 , with injection from aluminium; stress time ± 200 s; (s) ± 15% nitrogen mixing ratio, unstressed capacitor; () ± 15% nitrogen mixing ratio, stressed capacitor; (d) ± 0% nitrogen mixing ratio, unstressed capacitor; (j) ± 0% nitrogen mixing ratio, stressed capacitor; initial refers to about 1 min after the stress; relaxed refers to 15 500 min relaxation after the stress; relaxation under each positive or negative bias was 1 h. The random errors in the mid-gap voltage is 0.1 V.
and 8 was taken on a separate adjacent MOS capacitor. The bias ®eld was about 2 MV cmÿ1 and each stress±relaxation cycle at certain bias was 1 h long. In the same manner non-stressed structure was biased, and the mid-gap voltage variation with the relaxation time is also illustrated in Fig. 7. For non-stressed capacitors Vmg remains constant for cycled gate relaxation voltage (Fig. 7). But the mid-gap voltage for stressed-¯oating gate relaxed MOS capacitors was shifted backward and forward by cycling the bias. While Vmg shows a visible cycling under the bias relaxation, Dit increases if the number of cycles is increased (Fig. 8). 4. Discussion Silicon oxide nitridation by reactive sputtering was con®rmed by FTIR measurement, SIMS
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analysis, and the increased etch rate of nitrided ®lms (Fig. 1). Of particular interest is that the incorporation of nitrogen in a controllable way can remarkably reduce the generation of interface states (Fig. 3). The improved endurance to interface states generation after electron injection from the gate electrode at the optimum mixing ratio was explained by the creation of Six Ny Ox /Si interface [6,11]. It contains shorter Si±N bonds which reduce the strain gradient at the interface. At even larger nitrogen levels sucient amount of nitrogen is incorporated to form mismatched Si±N bonds which distort and weaken the nearby SiOnetwork. The tendency shown in Fig. 3 was observed for electron injection from silicon as well [6]. As stated in Section 3.1, after prolonged charge injection positive charge (hole) generation overtakes negative charge trapping. No matter what the mechanism of hole transport in amorphous silicon oxide, it is known that the hole motion in such material is dispersive with the transient time in the range of few microseconds to months [12,13]. While we were not able to access the shorter time scales, because of the experimental set-up, we could observe positive charge relaxation even after four months. In the series of experiments on the dependence of hole relaxation on temperature and bias voltage, Lakshmanna and Vengurlekar [13] ruled out detrapping as the transport mechanism due to the mobility of holes. Instead they suggested that thermally activated electrons in the silicon conduction band tunnel into the oxide and neutralize positive charge near the interface Si/SiO2 , which may be applicable to our nitrided and non-nitrided oxides, with the exception of 5% oxynitride. This tunneling-neutralizing process has dierent time constants and is dependent on the distance of the charge from the interface [12,13]. The experiment of charge relaxation under a ¯oating gate (Fig. 7) con®rms that the source of post-stress instability is not processing (sputtering) built-in defects, but due to the defects induced during the electrical stress. We assume that the cycling pattern in Fig. 7 is not due to the newly generated positive charge, because the bias ®eld was only 2 MV cmÿ1 . It is probably due to the
charging of the defects, which appear to be neutral in the relaxed state and positively charged in the excited state. Similar results in thermal or nitrided oxides were reported by Trombetta et al. [15] for room temperature relaxation and by Krisch et al. [12] at larger bias relaxation temperatures. From this Vmg forward±backward change under the bias relaxation we conclude that a certain part of the stress created charge is related to slow states [12,15], which should be due to a permanent damage in the oxynitrides caused by electrical stress. In contrast to this non-cycling eect of interface states in sputtered oxynitrides (Fig. 8), Krisch et al. [12] reported that Dit cycles in the same direction as Vmg for thermally nitrided oxides. It may be explained by the fast relaxation of the interface states in our samples during the period between the removal of stress bias and C V measurement completion (as mentioned in Section 2.2). Therefore, we suggest that relaxation of stressed sputtered oxide and nitrided oxide under bias is a part of the trapped positive charge which is located close to the Si/SiO2 interface and which we attribute to slow states.
5. Conclusion We found that electrical stability of sputtered oxide can be improved by optimisation of the nitrogen content in the sputtering ambient. Nitrided oxides are prone to larger positive charge relaxation than non-nitrided oxides. By bias relaxation stress test we concluded that in both non-nitrided and nitrided sputtered oxide a part of the stress induced charge is situated close to Si/SiO2 interface and can be ascribed to slow states.
Acknowledgements This project is supported by a research grant from the Hong Kong Polytechnic University. SIMS analysis was arranged by Professor Xu Shouding (Institute of Semiconductors, Chinese Academy of Sciences) for which we are grateful.
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References [1] C.S. McCormick, C.E. Weber, J.R. Abelson, A.M. Gates, Appl. Phys. Lett. 70 (1997) 226. [2] N. Yamauchi, R. Reif, Appl. Phys. Lett. 75 (1994) 3235. [3] S. Suyama, A. Okamoto, T. Serikawa, J. Electrochem. Soc. 134 (1987) 2260. [4] S. Suyama, A. Okamoto, T. Serikawa, IEEE Trans. Electron. Dev. ED-34 (1987). [5] E.V. Jelenkovic, K.Y. Tong, Microelectron. Reliab. 37 (1997) 159. [6] E.V. Jelenkovic, K.Y. Tong, Appl. Phys. Lett. 67 (1995) 2693. [7] E.V. Jelenkovic, K.Y. Tong, J. Vac. Sci. Technol. B 15 (1997) 209.
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[8] E.V. Jelenkovic, K.Y. Tong, in: Proc. IEEE Tencon, Hong Kong, 1995, p. 274. [9] E.V. Jelenkovic, PhD thesis, 1996. [10] L.M. Terman, Solid State Electron. 5 (1962) 285. [11] A.B. Joshi, G.O. Lo, D.K. Shih, D.L. Kwong, IEEE Trans. Electron. Dev. ED-39 (1992) 883. [12] K.S. Krisch, B.J. Gross, C.G. Sodini, J. Appl. Phys. 70 (1991) 2185. [13] V. Lakshmanna, A.S. Vengurlekar, J. Appl. Phys. 63 (1988) 4548. [14] W.L. Warren, D.M. Fleetwood, M.R. Shaneyfelt, J.R. Schwank, P.S. Winokur, R.A.B. Devine, D. Mathiot, Appl. Phys. Lett. 64 (1994) 64. [15] L.P. Trombetta, F.J. Figl, R.J. Zeto, J. Appl. Phys. 69 (1991) 2512.
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Charge relaxation in nitrided and non-nitrided sputtered oxide E.V. Jelenkovic *, K.Y. Tong Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Room EF501, Hung Hom, Kowloon, Hong Kong, People's Republic of China
Abstract The charge relaxation of nitrided silicon oxide was studied in an MOS structure. Oxynitrides were formed by reactive sputtering in an Ar/O2 /N2 plasma from a silicon oxide target. The nitrogen mixing ratio was varied from 0% to 20%, with the argon mixing ratio kept at 80%. The electrical stress was performed with injection from an aluminium gate and the relaxation of stress-induced charge under ¯oating and gate bias conditions was monitored via mid-gap voltage and interface states density measurement in a time interval from 1 min to about four months. A trend that nitrided oxides experience more relaxation of mid-gap voltage was observed while non-nitrided oxide had the largest relaxation of interface states density in the same time interval. We suggest that relaxation under applied bias due to the stress-induced charge is, in part, due to the slow states. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Sputtered insulators have been considered as appropriate gate insulators in amorphous and polysilicon thin ®lm transistors for display technologies [1,2]. Suyama et al. [3,4] have reported breakdown and interface properties of silicon oxide deposited in argon/oxygen gas mixtures. They suggested it as a dielectric of choice in polysilicon thin ®lm transistor (TFT) structures because of low leakage current and step coverage in ®lms as thin as 7 nm [2,5]. Sputtering can be a truly low (<200°C) temperature process and it was recently demonstrated that fully sputtered TFTs of device grade quality can be obtained at only 125°C [1]. Electrical stability and reliability of sputtered insulators are important issues for applications in electronic devices which were ignored until our
rather recent studies [5,6]. We found that there is a need to optimise sputtering conditions for improved electrical stability of sputtered oxide [5]. Also, sputtered oxide has a remarkable reliability in nonhydrogenated TFTs [5]. In analogy to nitridation of thermal oxide, we have co-sputtered oxides in an Ar/O2 /N2 mixture and related nitrided oxide stability to nitrogen level incorporation [6]. In real applications electronic devices are exposed to periods of stress (noticeably electrical) and periods of relaxation. Therefore the study of relaxation of charges in sputtered oxide is important to obtain a better mode of its reliability and possibly relate it to the sputter deposition process. In this paper we report stability properties and charge relaxation of oxide nitrided by co-sputtering in a nitrogen containing ambient. 2. Experiment
* Corresponding author. Tel.: +852 2766 6134; fax: 852 2362 8439; e-mail: [email protected]
Oxides were sputtered in Ar/O2 /N2 atmosphere on p-type (1 0 0) silicon wafers from a SiO2 target in
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 3 8 0 - 4
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an RF-sputtering system (ANELVA, model SPF332H). Oxide thicknesses were from 22 to 28 nm. A nitrogen mixing ratio N2 /(Ar + O2 + N2 ) was de®ned by (nitrogen ¯ow rate)/(total ¯ow rate) and was varied from 0% to 20%, with the argon mixing ratio constant. Sputtering pressure was 0.3 Pa and sputtering power was about 2 W cmÿ2 . The deposition temperature was 300°C while post-deposition sintering was performed at 900°C in nitrogen for 20 min. This sintering temperature has been found to anneal most eciently sputtering process built-in damage and provide the most stable metal-oxidesemiconductor (MOS) structure with sputtered non-nitrided oxide [7]. After aluminium sputtering at low sputtering power and patterning to obtain MOS capacitor structures, samples were annealed in forming gas at 400°C for 10 min. MOS capacitor area was 1.3 ´ 10ÿ4 cmÿ2 . 2.1. Physical analysis Several observations in oxides nitrided by cosputtering in nitrogen atmosphere are worthwhile mentioning, without going into details. (i) As demonstrated through secondary ion spectroscopy (SIMS) measurements, the presence of nitrogen was observed in all ®lms including non-nitrided samples (Fig. 1). The presence of nitrogen in non-nitrided oxides is attributed to either residual gas in the SIMS analysing chamber or residual nitrogen in the sputtering chamber [8]. However, there is obvious evidence of increased nitrogen content for oxides co-sputtered in nitrogen Ar/O2 /N2 mixture, which is particularly true for the ®lms with 20% nitrogen ¯ow. The areas under SiN and N peaks are located 8 nm from the silicon/oxide interface [8]. (ii) FTIR analysis (not shown) has found that the Si±O stretching mode vibrational peak wave number has a tendency to decrease with increased nitrogen ¯ow rate. This observation is valid for both as deposited and annealed samples [9]. Annealed oxides have larger vibrational peak wave numbers for the Si±O asymmetric stretching mode [9]. (iii) Fig. 1 also shows an increase in oxide etch rate in buered oxide etchant with greater nitrogen ¯ow rate.
Fig. 1. SIMS peak values for N () and SiN (s) for oxynitrides sputtered with dierent mixing ratios after annealing at 900°C; (j) ± normalised etch rate of as deposited oxynitrides. Lines are drawn as guides for the eye. The random error in the data is of the order of the symbols.
(iv) With nitrogen incorporation, leakage current in the oxide in the MOS structure increases [6]. Along with it, more uniform distribution of breakdown events was recorded with a drawback of reduced breakdown voltage [6]. 2.2. Electrical stability and charge relaxation For the purpose of electrical stress and charge relaxation study, only samples with non-nitrided oxide and nitrided with up to 15% N2 mixing ratio were used. (Samples with 20% mixing ratio had a hysteresis in capacitance as a function of voltage (C V ) curves and were not considered either for stability or for charge relaxation studies.) Electrical stress with electron injection from the metal electrode was applied. In all experiments the stress current density was 38 lA cmÿ2 . Stress-induced instability and post-stress relaxation phenomena were monitored through the changes in mid-gap voltage (Vmg ) and interface states density (Dit ) in the time interval ranging from 1 min to about three months. For each stress time a dierent MOS capacitor was used, which makes the obtained
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101
result more representative. Both monitoring parameters (mid-gap voltage and density of interface states) were extracted from the high-frequency C V measurement. Terman's method was used for interface states calculation [10]. It should be noted that the time between stress removal and C V measurement completion was over 1 min. This time is also true for the test on the study of charge relaxation under gate bias, described in Section 3.2. 3. Results 3.1. Electrical stability Fig. 2 illustrates the mid-gap voltages after 100, 200 and 300 s stress times for four dierent nitrogen ¯ows. It indicates an increase of positive charge in the oxides with longer stress time. However, in the initial period of stress, which is not presented in Fig. 2, all ®lms experienced a shift of the mid-gap voltage to more positive voltages, which can be related to electron charge trapping [6]. With longer stress time trapping-generation of charge in the
Fig. 3. Dependence of interface states on nitrogen mixing ratio for 100 s (), 200 s (s) and 300 s (M) constant current stress time. The random error in the data is 1 ´ 1011 cmÿ2 eVÿ1 .
oxides is gradually overtaken by positive charge generation, which is indicated by the shift of the mid-gap voltage in the opposite direction [6]. For the same stress time intervals Fig. 3 shows that for nitrided oxide obtained under 10% of nitrogen ¯ow, there is a minimum in the interface state density generated. 3.2. Relaxation under the ¯oating gate
Fig. 2. Change in mid-gap voltage with nitrogen mixing ratio for 100 s (), 200 s (s) and 300 s (M) constant current stress time. The random errors in the data is 0.1 V.
To obtain further insight into the charging properties of nitrided and non-nitrided oxides, samples stressed up to 200 s were monitored with time for the relaxation processes. Relaxation process was carried out at room temperature and under ¯oating gate potential. Typical property of nonstressed, stressed and relaxed high-frequency C V curves is illustrated in Fig. 4. It shows that the stressed curve after the relaxation moves towards the original, with obvious stretch decrease in the portion approaching inversion. Relaxed Vmg s and Dit s are presented in Figs. 5 and 6, respectively. Except in oxynitride sputtered with a 5% nitrogen mixing ratio, which shows increased positive charge trapping, in all other oxides there is a trend to de-
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Fig. 4. Behaviour of C±V curve before stress, after stress and after relaxation; relaxation under ¯oating bias.
Fig. 6. Relaxation of interface states generated during the constant current stress with time; during the relaxation gate was kept ¯oating; current stress ± 38 lA cmÿ2 , with injection from aluminium; stress time ± 200 s; nitrogen mixing ratio: (M) ± 0%, (s) ± 5%, () ± 10% and (O) ± 15%. The random error in the data is 1 ´ 1011 cmÿ2 eVÿ1 .
crease the amount of positive charge originally built-in by the stress. It should be noted that in Fig. 6, Dit for all oxynitrides decreases as a function of time. In none of the samples, over the time scale used, was generation of interface states observed after the stress was removed. The oxynitride with the largest net positive charge trapping (15% nitrogen mixing ratio) has the largest relaxation in Vmg . The relative change in Dit is the most signi®cant for non-nitrided oxide and decreases with nitrogen mixing ratio. Such Vmg and Dit trends during the relaxation period dier from the usual observation that insulators with greater numbers of generated interface states also have a larger charge relaxation [12]. 3.3. Relaxation under bias Fig. 5. Relaxation of mid-gap voltage for oxynitrides after constant current stress with time; current stress ± 38 lA cmÿ2 , with injection from aluminium; stress time ± 200 s; nitrogen mixing ratio: (M) ± 0%, (s) ± 5%, () ± 10% and (O) ± 15%. The random error in the data is 0.1 V.
In Section 3.1 it was observed that at certain stress ¯uence a turn-around eect appeared which we ascribed to net positive charge trapping. Also the ¯oating bias relaxation experiment did not provide
E.V. Jelenkovic, K.Y. Tong / Journal of Non-Crystalline Solids 254 (1999) 99±105
103
much information about the relaxed positive charge. By the following experiment it is intended to make a dierentiation between positive charge and slow states in sputtered oxynitrides. Namely, there is not a dividing line between oxide traps and slow donor states (the demarcation line is primarily determined by the distance of the defect from the interface and the energy level of the trap [14]). After the stress (samples with 0% and 15% nitrogen mixing ratio), MOS structures were relaxed with the gate ¯oating for about 15 500 min. This relaxations was followed by bias relaxation, as illustrated in Figs. 7 and 8, for mid-gap voltage and interface states density, respectively. Firstly, negative bias was applied, then negative + positive and ®nally negative + positive + negative. As in all previous stress experiments, each set of data in Figs. 7
Fig. 8. Relaxation of generated interface states during constant current stress with biased gate; constant current stress 38 lA cmÿ2 with injection from aluminium; stress time 200 s; nitrogen mixing ratio: () ± 15% and (s) ± 0%; initial refers to mid-gap voltage measured about 1 min after the stress; relaxed refers to 15 500 min relaxation after the stress; relaxation time is 1 h under each positive or negative bias. The random errors in the density of states is 1 ´ 1011 cmÿ2 eVÿ1 .
Fig. 7. Relaxation of mid-gap voltage with biased gate for oxynitrides after constant current stress; current stress ± 38 lA cmÿ2 , with injection from aluminium; stress time ± 200 s; (s) ± 15% nitrogen mixing ratio, unstressed capacitor; () ± 15% nitrogen mixing ratio, stressed capacitor; (d) ± 0% nitrogen mixing ratio, unstressed capacitor; (j) ± 0% nitrogen mixing ratio, stressed capacitor; initial refers to about 1 min after the stress; relaxed refers to 15 500 min relaxation after the stress; relaxation under each positive or negative bias was 1 h. The random errors in the mid-gap voltage is 0.1 V.
and 8 was taken on a separate adjacent MOS capacitor. The bias ®eld was about 2 MV cmÿ1 and each stress±relaxation cycle at certain bias was 1 h long. In the same manner non-stressed structure was biased, and the mid-gap voltage variation with the relaxation time is also illustrated in Fig. 7. For non-stressed capacitors Vmg remains constant for cycled gate relaxation voltage (Fig. 7). But the mid-gap voltage for stressed-¯oating gate relaxed MOS capacitors was shifted backward and forward by cycling the bias. While Vmg shows a visible cycling under the bias relaxation, Dit increases if the number of cycles is increased (Fig. 8). 4. Discussion Silicon oxide nitridation by reactive sputtering was con®rmed by FTIR measurement, SIMS
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analysis, and the increased etch rate of nitrided ®lms (Fig. 1). Of particular interest is that the incorporation of nitrogen in a controllable way can remarkably reduce the generation of interface states (Fig. 3). The improved endurance to interface states generation after electron injection from the gate electrode at the optimum mixing ratio was explained by the creation of Six Ny Ox /Si interface [6,11]. It contains shorter Si±N bonds which reduce the strain gradient at the interface. At even larger nitrogen levels sucient amount of nitrogen is incorporated to form mismatched Si±N bonds which distort and weaken the nearby SiOnetwork. The tendency shown in Fig. 3 was observed for electron injection from silicon as well [6]. As stated in Section 3.1, after prolonged charge injection positive charge (hole) generation overtakes negative charge trapping. No matter what the mechanism of hole transport in amorphous silicon oxide, it is known that the hole motion in such material is dispersive with the transient time in the range of few microseconds to months [12,13]. While we were not able to access the shorter time scales, because of the experimental set-up, we could observe positive charge relaxation even after four months. In the series of experiments on the dependence of hole relaxation on temperature and bias voltage, Lakshmanna and Vengurlekar [13] ruled out detrapping as the transport mechanism due to the mobility of holes. Instead they suggested that thermally activated electrons in the silicon conduction band tunnel into the oxide and neutralize positive charge near the interface Si/SiO2 , which may be applicable to our nitrided and non-nitrided oxides, with the exception of 5% oxynitride. This tunneling-neutralizing process has dierent time constants and is dependent on the distance of the charge from the interface [12,13]. The experiment of charge relaxation under a ¯oating gate (Fig. 7) con®rms that the source of post-stress instability is not processing (sputtering) built-in defects, but due to the defects induced during the electrical stress. We assume that the cycling pattern in Fig. 7 is not due to the newly generated positive charge, because the bias ®eld was only 2 MV cmÿ1 . It is probably due to the
charging of the defects, which appear to be neutral in the relaxed state and positively charged in the excited state. Similar results in thermal or nitrided oxides were reported by Trombetta et al. [15] for room temperature relaxation and by Krisch et al. [12] at larger bias relaxation temperatures. From this Vmg forward±backward change under the bias relaxation we conclude that a certain part of the stress created charge is related to slow states [12,15], which should be due to a permanent damage in the oxynitrides caused by electrical stress. In contrast to this non-cycling eect of interface states in sputtered oxynitrides (Fig. 8), Krisch et al. [12] reported that Dit cycles in the same direction as Vmg for thermally nitrided oxides. It may be explained by the fast relaxation of the interface states in our samples during the period between the removal of stress bias and C V measurement completion (as mentioned in Section 2.2). Therefore, we suggest that relaxation of stressed sputtered oxide and nitrided oxide under bias is a part of the trapped positive charge which is located close to the Si/SiO2 interface and which we attribute to slow states.
5. Conclusion We found that electrical stability of sputtered oxide can be improved by optimisation of the nitrogen content in the sputtering ambient. Nitrided oxides are prone to larger positive charge relaxation than non-nitrided oxides. By bias relaxation stress test we concluded that in both non-nitrided and nitrided sputtered oxide a part of the stress induced charge is situated close to Si/SiO2 interface and can be ascribed to slow states.
Acknowledgements This project is supported by a research grant from the Hong Kong Polytechnic University. SIMS analysis was arranged by Professor Xu Shouding (Institute of Semiconductors, Chinese Academy of Sciences) for which we are grateful.
E.V. Jelenkovic, K.Y. Tong / Journal of Non-Crystalline Solids 254 (1999) 99±105
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