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A comparative analysis of silicon dioxide films deposited by ECR-PECVD, TEOS-PECVD and Vapox-APCVD
Journal of Non-Crystalline Solids 352 (2006) 1430–1433 www.elsevier.com/locate/jnoncrysol
A comparative analysis of silicon dioxide films deposited by ECR-PECVD, TEOS-PECVD and Vapox-APCVD A. Pecora b
a,* ,
L. Maiolo a, G. Fortunato a, C. Caligiore
b,1
a CNR-IFN, Via Cineto Romano 42, 00156 Rome, Italy ST Microelectronics, Stradale Primosole 50, 95121 Catania, Italy
Available online 10 March 2006
Abstract In this work, we compared structural and electrical properties of SiO2 films obtained using three different deposition techniques: electron cycoltron resonance-plasma enhanced chemical vapor deposition (ECR-PECVD), tetraethylorthosilicate-plasma enhanced chemical vapor deposition (TEOS-PECVD) and vapor deposited oxide-atmospheric pressure chemical vapor deposition (Vapox-APCVD). Fourier transform infrared spectroscopies (FTIR) were carried out on the as-deposited SiO2 films in order to evaluate the structural properties of the dielectrics. From the infrared absorption bands we also evaluated the evolution of hydrogen content evolution with time at a thermal annealing temperature of 450 C. Electrical characterization was performed on MOS capacitors. In particular, we investigated the interface hydrogen-passivation mechanism at different annealing temperatures and for different annealing times. We also successfully applied the ECR and TEOS oxides as gate insulators on low-temperature polycrystalline silicon thin-film transistors (polysilicon TFTs). 2006 Elsevier B.V. All rights reserved. PACS: 73.40.Qv; 73.61.Ng; 77.55.+f Keywords: Devices; Thin film transistors; Dielectric properties; Relaxation; Electric modulus; Chemical vapor deposition; Plasma deposition; FTIR measurements
1. Introduction There are many techniques used to deposit insulating oxide films, including plasma enhanced chemical vapor depositon (PECVD), atmospheric pressure CVD (APCVD), sputtering and thermal evaporation. However, the systems commonly used to produce high quality SiO2 films are PECVD techniques like radio frequency (RFPECVD) and, electron cyclotron resonance (ECRPECVD) [1,2]. In the case of ECR-PECVD, SiO2 films deposited at the high ion densities (1016 m 3) present in the system allow deposition at very low pressures (<10 mTorr), ideal for removing hydrogen from the grow-
*
1
Corresponding author. Tel.: +39 0641522228; fax: +39 0641522220. E-mail address: [email protected] (A. Pecora). Tel.: +39 0957407696; fax: +39 0957407860.
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.10.030
ing SiO2 films. Moreover, the absence of sample biasing minimizes the damage produced by ion bombardment, resulting in a reduction in the defect density of the growing film [3]. Another interesting PECVD technique to deposit silicon dioxide based films is TEOS, where a liquid source such as tetraethylorthosilicate is used, because of its lower toxicity compared to the more conventional silane precursor. This apparatus enable the growth of highly uniform SiO2 films (2–3%) with the same water content found in the conventional RF-PECVD system, producing conformal films similar to thermal oxide layers [4]. Another technique used to obtain high SiO2 deposition rates and good thickness uniformity on wafers (<4%) is the so-called vapor-deposited-oxide (Vapox) APCVD technique [5]. However this process produces films that are less conformal and exhibit higher water contents. In this work, we comparatively investigated the microstructure and the electrical properties of silicon oxide films deposited by three
A. Pecora et al. / Journal of Non-Crystalline Solids 352 (2006) 1430–1433
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different techniques: ECR-PECVD, TEOS-PECVD and Vapox-APCVD. 2. Experimental ECR-PECVD SiO2 films were deposited at room temperature (RT) by ECR Plasma Source excited by a microwave frequency of 2.45 GHz at a power of 400 W. O2:He:SiH4 reactant gas ratio was 50:25:1 respectively at a working pressure of 3 mTorr with a deposition rate ˚ /min. Undoped TEOS-PECVD films oxide, used in 13 A this work, were deposited at T = 380 C with O2 as carrier gas. TEOS bubbling temperature was maintained at 46 C. Deposition pressure was 9 mTorr obtaining a deposition ˚ /min. Vapox-APCVD films were grown rate of about 370 A at atmospheric pressure, T = 400 C by SiH4:N2O:N2 gas ˚ /min. MOS mixture obtaining a deposition rate 1000 A capacitors were fabricated, depositing 100 nm thick SiO2 film with the three different techniques on epitaxial lightly doped (1014 P/cm3) silicon, 70 lm thick, grown on a highly doped wafer substrate as back channel contact. Finally aluminium dots 0.5 mm diameter were thermally evaporated onto the SiO2. Ramp I–V and quasi-static C–V measurements were performed using a ramp rate of 50 mV/s and 20 mV/s respectively. Polysilicon TFTs were fabricated on oxidized silicon wafers using, as gate insulator, only ECR-PECVD and TEOS oxide, as Vapox films resulted too poor. Phosphorous and Boron doped regions were laser activated to form source–drain contacts for n-channel and p-channel devices respectively. As gate insulator was used 100 nm thick ECR-SiO2 film deposited at RT or TEOS-SiO2 film deposited at 380 C. 3. FTIR characterization Film composition and bonding were evaluated by FTIR over the wave number range 400–4000 cm 1. The main Si– O–Si bands, for all three film type (ECR, TEOS and Vapox), are observed at 1075 cm 1, 810 cm 1, 450 cm 1 corresponding to stretching, bending and rocking motions of oxygen atoms respectively [6] while no Si–H (2100 cm 1) were detected, as shown in Fig. 1. Presence of water in TEOS and Vapox films arise from the analysis of Si–OH broad absorption band between 3000 and 3700 cm 1. As can be seen Vapox film show the greatest presence of O– H bonds. From heat treatment at 450 C for increasing time, performed on Vapox film, we observe the progressive reduction of Si–OH peak. Since conventional PECVD thermal desorption spectrometry (TDS) spectra usually show three peaks related to the presence of water in open pores (peak temperature 370 C), closed pores (peak temperature 490 C) and OH bonded in the network (peak temperature 760 C) [7] water reduction in Vapox is probably mainly related to the desorption of H2O embedded in open pores of the film structure. Lower water content in the deposited ECR-film, is evident from the absence of Si–OH
Fig. 1. FTIR transmittance spectra of ECR, TEOS, Vapox and thermal SiO2 films from the top to the bottom respectively. Each spectrum is displaced vertically for easy observation.
band in Fig. 1 and also confirmed by TDS analysis carried out in previous works [8]. 4. Electrical characterization on MOS structures and polysilicon TFTs From I–V characteristics on MOS capacitors (see Fig. 2), we observed no significant charge trapping, denoted by the current ledge present in the I–V characteristics, up to fields of 6–7 MV/cm and breakdown electric field >8 MV/cm for both ECR and TEOS dielectric films while Vapox films show much poorer electrical characteristics, with charge injection starting for about 2 MV/cm. The interface state density (Nss) was deduced from quasi-static C–V measurements by Khun method [9] and we investigated the effect of different annealing times and temperatures since the Nss of as deposited SiO2 films is very high. We found that the Nss could be reduced down to 2 · 1010 cm 2 eV 1 for ECR and TEOS films after annealing at 450 C for 30 min, while Vapox films have a Nss typically an order of magnitude higher (see Fig. 3). It is well
Fig. 2. Ramp I–V characteristics measured on ECR, TEOS and Vapox MOS capacitors.
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A. Pecora et al. / Journal of Non-Crystalline Solids 352 (2006) 1430–1433
known [10] that in MOS structures Nss reduction can be achieved by H-passivation of Si dangling bonds. By performing thermal treatments in different gas ambient (vacuum, nitrogen and forming gas 5%H2 + N2), on MOS devices with three different oxide films, we did not observe significant differences in Nss reduction. Therefore we conclude that most of the passivating H-atoms come from H-containing species (H, OH, H2O) weakly bonded already present in the film. In addition, we found that to effectively H-passivate the Si dangling bonds present at the interface, it is crucial the use of Aluminum as gate metal in the MOS structure, as clearly demonstrated through a series of annealing experiments performed before and after Al-contacts deposition. On the other hand, some interface state reduction occurs also when performing the thermal annealing prior the Al-contact deposition, as a result of some dangling bond reconstruction and/or passivation at the SiO2/Si interface (Fig. 3). In order to develop a low temperature process, we investigated the Nss for MOS annealed at 200, 350, 450 C for increasing annealing times. In Fig. 4 we report some results for ECR-oxide: as can be expected,
Fig. 3. Nss comparison for the three types of SiO2 films after 30 min thermal annealing at 450 C before Al metal gate deposition (empty symbols) and after Al metal gate deposition (full symbols).
Fig. 5. n and p-channel polycrystalline TFTs transfer characteristics with TEOS and ECR gate oxide annealed at 450 C for 30 min.
by increasing the annealing temperature the Nss is drastically reduced with the minimum Nss value down to 6 · 1010 cm 2 eV 1 for the highest annealing temperature for 30 min. For longer annealing times at 450 C we observed higher Nss values, presumably due to some Si– H bond breaking taking place at the interface [11]. However it can be also noted that by increasing the annealing time up to 12 h we can lower considerably the Nss values, when using low annealing temperatures (200 C), down to 5 · 1011 cm 2 eV 1 (Fig. 4). Similar results were observed in MOS with TEOS oxide, while, due to poorer insulator quality of MOS with Vapox, these experiments were not performed. From TFTs transfer characteristics annealed at 450 C for 30 min (Fig. 5) we obtained for both cases field effect mobility (ln,p) of about 200 cm2/Vs and 120 cm2/Vs for n and p-channel devices respectively. Performing thermal annealing treatment at 200 C for prolonged annealing time (12 h) we can still measure good electrical characteristics with ln = 160 cm2/Vs for ECR-TFTs and ln = 120 cm2/Vs for TEOS-TFTs, while lp = 70 cm2/Vs for both TFTs cases. As confirmed by electrical analysis on MOS structures for the lowest annealing temperature (200 C), the increasing annealing time produces a general improvement of devices performances. These results show that SiO2, deposited by ECR-PECVD and TEOS-PECVD have similar electrical properties and can be successfully applied in low temperature fabrication process of polysilicon TFTs. 5. Conclusions
Fig. 4. The minimum values of Nss vs. time for three different annealing temperatures, deduced from quasi-static characteristics on ECR MOS capacitors.
We compared structural and electrical properties of SiO2 films obtained through three different deposition techniques: ECR-PECVD, TEOS-PECVD and VapoxAPCVD. Very high quality SiO2 films can be obtained by ECR-PECVD and TEOS-PECVD while Vapox-APCVD films exhibited poorer properties. By annealing in different ambients, we demonstrated that H-passivation atoms are already inside the films. Moreover, Nss on MOS capacitors
A. Pecora et al. / Journal of Non-Crystalline Solids 352 (2006) 1430–1433
can be reduced by post-metallization annealing at 450 C and even for lower temperatures (200 C), increasing the annealing time up to 12 h. Polysilicon TFTs have been fabricated with ECR and TEOS gate oxides. From TFTs transfer characteristics, we observed good electrical performances for devices annealed at 450 C for 30 min and still acceptable results for devices annealed at 200 C for prolonged time periods. These results show that SiO2, deposited by ECR at room temperature, followed by very lowtemperature annealing, is suitable for fabrication of polysilicon TFTs on polymer substrates while TEOS oxide can be mainly used in TFTs fabrication-processes where T > 380 C. Vapox film properties are more suitable for applications like intermetal dielectrics, spacers and passivation layers.
1433
References [1] M.J. Thompson, J. Vac. Sci. Technol. B 2 (1984) 827. [2] M.R. Werheimer, M. Moisan, Pure Appl. Chem. 66 (6) (1994). [3] R. Rashid, A.J. Flewitt, J. Robertson, J. Vac. Sci. Technol. A21 (3) (2003) 728. [4] C.S. Pai, C.P. Chang, J. Appl. Phys. 68 (1990) 793. [5] D.A. Glocker, S.I. Shah, Handbook of Thin Film Process Technology, vol. 1, IoP, 1995. [6] D. Della Sala et al., J. Electrochem. Soc. 137 (8) (1990) 2550. [7] N. Hirashita, S. Tokitoh, H. Uchida, Jpn. J. Appl. Phys. 32 (1993) 1787. [8] A. Pecora et al., Microelectron. Reliability 45 (2005). [9] M. Kuhn, Solid-State Electron. 13 (1970) 873. [10] N. Johnson, D. Biegelsen, M. Moyer, Appl. Phys. Lett. 40 (1982). [11] S.T. Pantelides, S.N. Rashkeev, R. Buczko, D.M. Fleetwood, R.D. Schrimpf, IEEE Trans. Nucl. Sci. 47 (2000) 6.
A comparative analysis of silicon dioxide films deposited by ECR-PECVD, TEOS-PECVD and Vapox-APCVD A. Pecora b
a,* ,
L. Maiolo a, G. Fortunato a, C. Caligiore
b,1
a CNR-IFN, Via Cineto Romano 42, 00156 Rome, Italy ST Microelectronics, Stradale Primosole 50, 95121 Catania, Italy
Available online 10 March 2006
Abstract In this work, we compared structural and electrical properties of SiO2 films obtained using three different deposition techniques: electron cycoltron resonance-plasma enhanced chemical vapor deposition (ECR-PECVD), tetraethylorthosilicate-plasma enhanced chemical vapor deposition (TEOS-PECVD) and vapor deposited oxide-atmospheric pressure chemical vapor deposition (Vapox-APCVD). Fourier transform infrared spectroscopies (FTIR) were carried out on the as-deposited SiO2 films in order to evaluate the structural properties of the dielectrics. From the infrared absorption bands we also evaluated the evolution of hydrogen content evolution with time at a thermal annealing temperature of 450 C. Electrical characterization was performed on MOS capacitors. In particular, we investigated the interface hydrogen-passivation mechanism at different annealing temperatures and for different annealing times. We also successfully applied the ECR and TEOS oxides as gate insulators on low-temperature polycrystalline silicon thin-film transistors (polysilicon TFTs). 2006 Elsevier B.V. All rights reserved. PACS: 73.40.Qv; 73.61.Ng; 77.55.+f Keywords: Devices; Thin film transistors; Dielectric properties; Relaxation; Electric modulus; Chemical vapor deposition; Plasma deposition; FTIR measurements
1. Introduction There are many techniques used to deposit insulating oxide films, including plasma enhanced chemical vapor depositon (PECVD), atmospheric pressure CVD (APCVD), sputtering and thermal evaporation. However, the systems commonly used to produce high quality SiO2 films are PECVD techniques like radio frequency (RFPECVD) and, electron cyclotron resonance (ECRPECVD) [1,2]. In the case of ECR-PECVD, SiO2 films deposited at the high ion densities (1016 m 3) present in the system allow deposition at very low pressures (<10 mTorr), ideal for removing hydrogen from the grow-
*
1
Corresponding author. Tel.: +39 0641522228; fax: +39 0641522220. E-mail address: [email protected] (A. Pecora). Tel.: +39 0957407696; fax: +39 0957407860.
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.10.030
ing SiO2 films. Moreover, the absence of sample biasing minimizes the damage produced by ion bombardment, resulting in a reduction in the defect density of the growing film [3]. Another interesting PECVD technique to deposit silicon dioxide based films is TEOS, where a liquid source such as tetraethylorthosilicate is used, because of its lower toxicity compared to the more conventional silane precursor. This apparatus enable the growth of highly uniform SiO2 films (2–3%) with the same water content found in the conventional RF-PECVD system, producing conformal films similar to thermal oxide layers [4]. Another technique used to obtain high SiO2 deposition rates and good thickness uniformity on wafers (<4%) is the so-called vapor-deposited-oxide (Vapox) APCVD technique [5]. However this process produces films that are less conformal and exhibit higher water contents. In this work, we comparatively investigated the microstructure and the electrical properties of silicon oxide films deposited by three
A. Pecora et al. / Journal of Non-Crystalline Solids 352 (2006) 1430–1433
1431
different techniques: ECR-PECVD, TEOS-PECVD and Vapox-APCVD. 2. Experimental ECR-PECVD SiO2 films were deposited at room temperature (RT) by ECR Plasma Source excited by a microwave frequency of 2.45 GHz at a power of 400 W. O2:He:SiH4 reactant gas ratio was 50:25:1 respectively at a working pressure of 3 mTorr with a deposition rate ˚ /min. Undoped TEOS-PECVD films oxide, used in 13 A this work, were deposited at T = 380 C with O2 as carrier gas. TEOS bubbling temperature was maintained at 46 C. Deposition pressure was 9 mTorr obtaining a deposition ˚ /min. Vapox-APCVD films were grown rate of about 370 A at atmospheric pressure, T = 400 C by SiH4:N2O:N2 gas ˚ /min. MOS mixture obtaining a deposition rate 1000 A capacitors were fabricated, depositing 100 nm thick SiO2 film with the three different techniques on epitaxial lightly doped (1014 P/cm3) silicon, 70 lm thick, grown on a highly doped wafer substrate as back channel contact. Finally aluminium dots 0.5 mm diameter were thermally evaporated onto the SiO2. Ramp I–V and quasi-static C–V measurements were performed using a ramp rate of 50 mV/s and 20 mV/s respectively. Polysilicon TFTs were fabricated on oxidized silicon wafers using, as gate insulator, only ECR-PECVD and TEOS oxide, as Vapox films resulted too poor. Phosphorous and Boron doped regions were laser activated to form source–drain contacts for n-channel and p-channel devices respectively. As gate insulator was used 100 nm thick ECR-SiO2 film deposited at RT or TEOS-SiO2 film deposited at 380 C. 3. FTIR characterization Film composition and bonding were evaluated by FTIR over the wave number range 400–4000 cm 1. The main Si– O–Si bands, for all three film type (ECR, TEOS and Vapox), are observed at 1075 cm 1, 810 cm 1, 450 cm 1 corresponding to stretching, bending and rocking motions of oxygen atoms respectively [6] while no Si–H (2100 cm 1) were detected, as shown in Fig. 1. Presence of water in TEOS and Vapox films arise from the analysis of Si–OH broad absorption band between 3000 and 3700 cm 1. As can be seen Vapox film show the greatest presence of O– H bonds. From heat treatment at 450 C for increasing time, performed on Vapox film, we observe the progressive reduction of Si–OH peak. Since conventional PECVD thermal desorption spectrometry (TDS) spectra usually show three peaks related to the presence of water in open pores (peak temperature 370 C), closed pores (peak temperature 490 C) and OH bonded in the network (peak temperature 760 C) [7] water reduction in Vapox is probably mainly related to the desorption of H2O embedded in open pores of the film structure. Lower water content in the deposited ECR-film, is evident from the absence of Si–OH
Fig. 1. FTIR transmittance spectra of ECR, TEOS, Vapox and thermal SiO2 films from the top to the bottom respectively. Each spectrum is displaced vertically for easy observation.
band in Fig. 1 and also confirmed by TDS analysis carried out in previous works [8]. 4. Electrical characterization on MOS structures and polysilicon TFTs From I–V characteristics on MOS capacitors (see Fig. 2), we observed no significant charge trapping, denoted by the current ledge present in the I–V characteristics, up to fields of 6–7 MV/cm and breakdown electric field >8 MV/cm for both ECR and TEOS dielectric films while Vapox films show much poorer electrical characteristics, with charge injection starting for about 2 MV/cm. The interface state density (Nss) was deduced from quasi-static C–V measurements by Khun method [9] and we investigated the effect of different annealing times and temperatures since the Nss of as deposited SiO2 films is very high. We found that the Nss could be reduced down to 2 · 1010 cm 2 eV 1 for ECR and TEOS films after annealing at 450 C for 30 min, while Vapox films have a Nss typically an order of magnitude higher (see Fig. 3). It is well
Fig. 2. Ramp I–V characteristics measured on ECR, TEOS and Vapox MOS capacitors.
1432
A. Pecora et al. / Journal of Non-Crystalline Solids 352 (2006) 1430–1433
known [10] that in MOS structures Nss reduction can be achieved by H-passivation of Si dangling bonds. By performing thermal treatments in different gas ambient (vacuum, nitrogen and forming gas 5%H2 + N2), on MOS devices with three different oxide films, we did not observe significant differences in Nss reduction. Therefore we conclude that most of the passivating H-atoms come from H-containing species (H, OH, H2O) weakly bonded already present in the film. In addition, we found that to effectively H-passivate the Si dangling bonds present at the interface, it is crucial the use of Aluminum as gate metal in the MOS structure, as clearly demonstrated through a series of annealing experiments performed before and after Al-contacts deposition. On the other hand, some interface state reduction occurs also when performing the thermal annealing prior the Al-contact deposition, as a result of some dangling bond reconstruction and/or passivation at the SiO2/Si interface (Fig. 3). In order to develop a low temperature process, we investigated the Nss for MOS annealed at 200, 350, 450 C for increasing annealing times. In Fig. 4 we report some results for ECR-oxide: as can be expected,
Fig. 3. Nss comparison for the three types of SiO2 films after 30 min thermal annealing at 450 C before Al metal gate deposition (empty symbols) and after Al metal gate deposition (full symbols).
Fig. 5. n and p-channel polycrystalline TFTs transfer characteristics with TEOS and ECR gate oxide annealed at 450 C for 30 min.
by increasing the annealing temperature the Nss is drastically reduced with the minimum Nss value down to 6 · 1010 cm 2 eV 1 for the highest annealing temperature for 30 min. For longer annealing times at 450 C we observed higher Nss values, presumably due to some Si– H bond breaking taking place at the interface [11]. However it can be also noted that by increasing the annealing time up to 12 h we can lower considerably the Nss values, when using low annealing temperatures (200 C), down to 5 · 1011 cm 2 eV 1 (Fig. 4). Similar results were observed in MOS with TEOS oxide, while, due to poorer insulator quality of MOS with Vapox, these experiments were not performed. From TFTs transfer characteristics annealed at 450 C for 30 min (Fig. 5) we obtained for both cases field effect mobility (ln,p) of about 200 cm2/Vs and 120 cm2/Vs for n and p-channel devices respectively. Performing thermal annealing treatment at 200 C for prolonged annealing time (12 h) we can still measure good electrical characteristics with ln = 160 cm2/Vs for ECR-TFTs and ln = 120 cm2/Vs for TEOS-TFTs, while lp = 70 cm2/Vs for both TFTs cases. As confirmed by electrical analysis on MOS structures for the lowest annealing temperature (200 C), the increasing annealing time produces a general improvement of devices performances. These results show that SiO2, deposited by ECR-PECVD and TEOS-PECVD have similar electrical properties and can be successfully applied in low temperature fabrication process of polysilicon TFTs. 5. Conclusions
Fig. 4. The minimum values of Nss vs. time for three different annealing temperatures, deduced from quasi-static characteristics on ECR MOS capacitors.
We compared structural and electrical properties of SiO2 films obtained through three different deposition techniques: ECR-PECVD, TEOS-PECVD and VapoxAPCVD. Very high quality SiO2 films can be obtained by ECR-PECVD and TEOS-PECVD while Vapox-APCVD films exhibited poorer properties. By annealing in different ambients, we demonstrated that H-passivation atoms are already inside the films. Moreover, Nss on MOS capacitors
A. Pecora et al. / Journal of Non-Crystalline Solids 352 (2006) 1430–1433
can be reduced by post-metallization annealing at 450 C and even for lower temperatures (200 C), increasing the annealing time up to 12 h. Polysilicon TFTs have been fabricated with ECR and TEOS gate oxides. From TFTs transfer characteristics, we observed good electrical performances for devices annealed at 450 C for 30 min and still acceptable results for devices annealed at 200 C for prolonged time periods. These results show that SiO2, deposited by ECR at room temperature, followed by very lowtemperature annealing, is suitable for fabrication of polysilicon TFTs on polymer substrates while TEOS oxide can be mainly used in TFTs fabrication-processes where T > 380 C. Vapox film properties are more suitable for applications like intermetal dielectrics, spacers and passivation layers.
1433
References [1] M.J. Thompson, J. Vac. Sci. Technol. B 2 (1984) 827. [2] M.R. Werheimer, M. Moisan, Pure Appl. Chem. 66 (6) (1994). [3] R. Rashid, A.J. Flewitt, J. Robertson, J. Vac. Sci. Technol. A21 (3) (2003) 728. [4] C.S. Pai, C.P. Chang, J. Appl. Phys. 68 (1990) 793. [5] D.A. Glocker, S.I. Shah, Handbook of Thin Film Process Technology, vol. 1, IoP, 1995. [6] D. Della Sala et al., J. Electrochem. Soc. 137 (8) (1990) 2550. [7] N. Hirashita, S. Tokitoh, H. Uchida, Jpn. J. Appl. Phys. 32 (1993) 1787. [8] A. Pecora et al., Microelectron. Reliability 45 (2005). [9] M. Kuhn, Solid-State Electron. 13 (1970) 873. [10] N. Johnson, D. Biegelsen, M. Moyer, Appl. Phys. Lett. 40 (1982). [11] S.T. Pantelides, S.N. Rashkeev, R. Buczko, D.M. Fleetwood, R.D. Schrimpf, IEEE Trans. Nucl. Sci. 47 (2000) 6.