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LPCVD-silicon oxynitride films: low-temperature annealing effects
Vacuum 69 (2003) 385–389
LPCVD-silicon oxynitride films: low-temperature annealing effects S. Alexandrovaa,*, A. Szekeresa, E. Halovab, M. Modreanuc a
Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, Sofia 1784, Bulgaria b Technical University, 8 Kl. Ohridski Blvd., Sofia 1797, Bulgaria c National Institute for Microtechnologies, P.O. Box 38-160, Bucharest 72225, Romania Received 15 March 2002; received in revised form 1 April 2002
Abstract The electrical properties of silicon oxynitride (SiOxNy) films on Si deposited by low-pressure chemical vapour deposition have been studied. The effect of annealing on SiOxNy films on the basis of frequency analysis of capacitance– voltage and conductance–voltage characteristics of metal–SiOxNy–silicon capacitors in the frequency range 1–300 kHz was determined. The post-deposition annealing results in a decrease of the concentrations of the positive dielectric charge and defects in the silicon substrate. The nature of the dielectric charges is discussed. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Silicon oxynitride; Annealing; Charged defect centres
1. Introduction The use of very thin oxides (SiO2) with perfect dielectric and interface parameters is extremely significant as a result of the trend towards size reduction down to several nanometers in modern Si-based technology. At smaller thickness of the oxides substantial problems arise and physical limits are reached, such as, e.g., tunnel currents through the dielectric film [1]. One solution to these problems is the replacement of SiO2 by an alternative insulator with higher dielectric constant so that the physical thickness of the dielectric could be increased. Materials with variable com*Corresponding author. Tel.: +359-2-7144513; fax: +359-29753632. E-mail address: [email protected] (S. Alexandrova).
position and wide variation in their structures and electronic properties, such as silicon oxynitride (SiOxNy) films, offer the solution of a number of currently existing problems in the development of many integrated devices or sensors. Also, these materials offer new possibilities in microelectronics and optoelectronics industry, e.g., passivating coatings [2], thin gate dielectrics [3], membranes and optical wave guides for micro-electro-mechanical systems [4] and micro-opto-mechanical systems. Several investigators [5,6] have recently reported that high-quality SiOxNy films can be produced by the LPCVD process. Much effort has been devoted to improve the electrical properties of these films. By varying the technological conditions in order to get different permittivity, e; it is possible to obtain varying fixed oxide charge in the
0042-207X/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 3 6 3 - 9
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S. Alexandrova et al. / Vacuum 69 (2003) 385–389
dielectric, which, in contrast to thermal SiO2, can also be negative. The aim of this paper is to report our recent study on the effect of a low-temperature anneal on the interface properties of the metal–insulator– silicon (MIS) structures with thin SiOxNy layers grown on silicon. For this purpose the frequency dispersion of the capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics after annealing are studied in detail.
2. Experimental 2.1. Samples Both n-type (1 1 1)Si and p-type (1 0 0)Si wafers of 5–10 O cm resistivity were initially treated with a pre-oxidation wet RCA cleaning in an H2SO4/ H2O2 solution and subsequent etching in diluted HF solution for removal of the native SiO2 films from Si and a final 10 min rinse in flowing deionized H2O immediately before loading in a standard low-pressure chemical vapour deposition (LPCVD) reactor. The amorphous SiOxNy thin films were formed by a chemical reaction of dichlorosilane (SiH2Cl2) with nitrous oxide (N2O) and ammonia (NH3) in a standard LPCVD reactor at a total pressure of 400 mTorr and at a substrate temperature of 8601C. Throughout the deposition process, the gas flow rate of SiH2Cl2 was kept at 40 sccm and the gas flow rates of N2O and NH3 were at a ratio of r ¼ QN2 O =QNH3 ¼ 4:8 and r ¼ 2:4 for the n-type (1 1 1)- and p-type (1 0 0)-oriented Si, respectively. The obtained SiOxNy films used for this study were 160 nm thick. For studying the electrical properties of LPCVD-SiOxNy thin films, after deposition the films were formed into MIS capacitors by vacuum thermal evaporation of about 300 nm circular aluminum dots with areas of 2.8 103 cm2 onto the SiOxNy surface through a metal mask. For making a contact to the Si substrate, evaporated continuous Al films were used. Finally, part of the structures was subjected to a 4501C annealing in dry N2 ambient for 30 min.
2.2. Measurement All the C–V and G–V measurements were performed at room temperature in a frequency range of 1–300 kHz before and after the devices have been subjected to the anneal. A Precision Component Analyser WAYNE KERR 6425 was used for the C–V and G–V measurements. The data reported here are accurate within 75% and are typical for all the samples prepared. The defect concentrations in the structures have been estimated from the C–V and G–V measurements. The so-called fixed oxide charge has been determined from comparison of the measured (at the highest frequency) and ideal theoretical C–V characteristics at the flat-band condition.
3. Results and discussion The C–V characteristics before anneal show large defect density concentrations expressed in a large shift, a high slope of the C–V curves and dispersion in inversion capacitance. This is illustrated in Fig. 1 for n(1 1 1)Si. Characteristic of the measured C–V curves is the absence of hysteresis, which is a proof of low density of slow interface states localized around SiOxNy/Si interface further
Fig. 1. Frequency dispersion of the C–V characteristics of MIS capacitors with LPCVD-SiOxNy on (1 1 1)Si substrate before annealing.
S. Alexandrova et al. / Vacuum 69 (2003) 385–389
inside the dielectric. The results for p(1 0 0)Si substrate are similar except for the sign of the fixed dielectric charge, which in this case is negative. The high-frequency C–V characteristics of the MIS capacitors grown on (1 1 1)Si and (1 0 0)Si after annealing are illustrated in Figs. 2 and 3, respectively. A common feature is the shift of the curves towards more positive voltages, which is an evidence of the decrease of the positive oxide charges. The regular shape of the C–V curves is indicative of a homogeneous distribution of the oxide charges [7]. The C–V characteristics show frequency dispersion. Both sets of C–V curves shifted towards the ideal C–V curve with increasing frequency. This is an indicator of the type of the interface states contributing to the C–V shift. In the depletion region for a given gate voltage, the capacitance dispersion depends on Si orientation. For a (1 1 1)Si substrate a decrease and for (1 0 0)Si an increase of the capacitance with measurement frequency is evident due to the time-dependent response of interface states of different nature. This is further substantiated by the different behaviour of the slope of C–V curves
Fig. 2. Frequency dispersion of the C–V characteristics of MIS capacitors with LPCVD-SiOxNy on (1 1 1)Si substrate after annealing.
387
Fig. 3. Frequency dispersion of the C–V characteristics of MIS capacitors with LPCVD-SiOxNy on (1 0 0)Si substrate after annealing.
for both orientations of the Si substrate. For (1 1 1)Si the slope of the curves towards Vg axis decreases with frequency. For (1 0 0)Si, however, the slope shows no frequency dependence. In accumulation, no appreciable frequency dispersion is evident in this frequency range. Nevertheless, the oxide capacitance was calculated from the MOS capacitance of the low-frequency C–V curve in strong accumulation, to be sure that the series resistance contribution is negligibly small. This capacitance value has been used further throughout the study for estimation of the defect concentrations. The inspection of C–V curves in Figs. 2 and 3 shows the same values of Cmin for different frequencies. In as-deposited films dispersion of Cmin has been registered, as seen in Fig. 1, which was interpreted as due to defect creation in the Si substrate during film deposition [8]. Obviously, these defects have been annealed out in the postdeposition anneal. Sets of G–V characteristics of the MIS capacitors are plotted in Figs. 4 and 5 for the n(1 1 1) and p(1 0 0)Si structures, respectively, for the same frequency interval as in Figs. 2 and 3. The conductance was found to increase with increasing
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Fig. 4. Frequency dispersion of the G–V characteristics of MIS capacitors with LPCVD-SiOxNy on (1 1 1)Si substrate after annealing.
Fig. 5. Frequency dispersion of the G–V characteristics of MIS capacitors with LPCVD-SiOxNy on (1 0 0)Si substrate after annealing.
measurement frequency. The peaks become less sharp and broader with increasing frequency. The peak position of the conductance, similar to the shifts of the C–V curves in Figs. 2 and 3, is shifted towards zero gate voltage with increase in frequency. The comparison of the results in Figs. 4 and 5 reveals the different character of the curves
Fig. 6. Fixed dielectric charges obtained from the flatband voltage values for LPCVD-SiOxNy on p(1 0 0)Si and n(1 1 1)Si substrate at different frequencies before and after annealing.
indicating possibly different defect sites for both orientations. Unlike the well-known thermal SiO2/ Si interface, the deposited SiOxNy film on Si shows no substantial Si orientation dependence of the interface defect concentrations. The data for the fixed dielectric charge before and after annealing are summarized in Fig. 6. It can be seen that the fixed charge decreases with increasing frequency in all cases. The opposite sign of the fixed oxide charge for both Si orientations can be related to differences in the chemical reaction balance due to the twice higher gas flow rate ratio. The positive oxide charges are usually related to O3Si. oxygen-vacancy defects (E0 centres), Si–N bonds (K-centres) and N–H bonds. The E0 centres are well known for the thermal SiO2/Si interface [9], while K-centres and N–H bond have been found in Si3N4 [10] and SiOxNy [11]. The OH-groups are well known as negatively charged electron traps in dielectric films. At higher ratio r; E 0 centres should be dominant in the positive charge mainly because of the higher oxygen content, which makes the SiOxNy structure closer to SiO2. At lower r electron traps related to OH-groups [12] can be suggested as a reason for the observed negative fixed charge.
S. Alexandrova et al. / Vacuum 69 (2003) 385–389
In Fig. 6 a decrease in the positive charge is observed in the n(1 1 1)Si-based structures after annealing, whereas in the p(1 0 0)Si-base structures an increase of the negative charge is found. These negative shifts could be regarded as annealing of positive charges. A consistent explanation of the annealing effect has to take all the defects into account, the most simple being rearrangement of Si–N and N–H bonds.
4. Conclusion From the analysis of the frequency dispersion of the C–V and G–V characteristics of silicon oxynitride films LPCVD on n(1 1 1)- and p(1 0 0)Si substrates, a conclusion was made about the dielectric and interface quality and the possibility to control the sign and the density of the charged defects through choice of the deposition conditions and by additional low-temperature annealing.
389
References [1] Takeda E, Ikuzaki K, Katto H, Dhji Y, Hinode K, Hamada A, Sakuta T, Funatiki T, Sasaki T. Procc IEEE 1993;81:653–7. [2] Lowe A, Powell MJ, Elliott SR. J Appl Phys 1986;59: 1251–4. [3] Iliac BB. Hewlett Packard J 1982;33:25–9. [4] Modreanu M, Cosmin P. Proceedings of International Conference of Semicond-CAS’96, 1996. p. 323–7. [5] Buchanan DA, Batey J, Tierney E. IEEE Electron Dev Lett 1988;EDL-9:576–80. [6] Fountain GG, Rudder RA, Hattangady SV, Markunas RI. J Appl Phys 1988;63:4744–8. [7] Nicollian EH, Brew JR. MOS (metal oxide semiconductor) physics and technology. New York: Wiley, 1982. [8] Szekeres A, Halova E, Alexandrova S, Modreanu M. Proceedings of the International Conference of SemicondCAS, 2001. p. 235–8. [9] Vanheusden K, Stesmans A. Appl Phys Lett 1993;62: 2405–8 and references therein. [10] Lenahan PM, Curry SE. Appl Phys Lett 1990;56:157–60. [11] Hasegawa S, Sakamori S, Futatsudera M, Inokuma T, Kurata Y. J Appl Phys 2001;89:2536–98. [12] Szekeres A, Alexandrova S, Modreanu M. Phys Stat Sol A, 2001;187:493–8.
LPCVD-silicon oxynitride films: low-temperature annealing effects S. Alexandrovaa,*, A. Szekeresa, E. Halovab, M. Modreanuc a
Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, Sofia 1784, Bulgaria b Technical University, 8 Kl. Ohridski Blvd., Sofia 1797, Bulgaria c National Institute for Microtechnologies, P.O. Box 38-160, Bucharest 72225, Romania Received 15 March 2002; received in revised form 1 April 2002
Abstract The electrical properties of silicon oxynitride (SiOxNy) films on Si deposited by low-pressure chemical vapour deposition have been studied. The effect of annealing on SiOxNy films on the basis of frequency analysis of capacitance– voltage and conductance–voltage characteristics of metal–SiOxNy–silicon capacitors in the frequency range 1–300 kHz was determined. The post-deposition annealing results in a decrease of the concentrations of the positive dielectric charge and defects in the silicon substrate. The nature of the dielectric charges is discussed. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Silicon oxynitride; Annealing; Charged defect centres
1. Introduction The use of very thin oxides (SiO2) with perfect dielectric and interface parameters is extremely significant as a result of the trend towards size reduction down to several nanometers in modern Si-based technology. At smaller thickness of the oxides substantial problems arise and physical limits are reached, such as, e.g., tunnel currents through the dielectric film [1]. One solution to these problems is the replacement of SiO2 by an alternative insulator with higher dielectric constant so that the physical thickness of the dielectric could be increased. Materials with variable com*Corresponding author. Tel.: +359-2-7144513; fax: +359-29753632. E-mail address: [email protected] (S. Alexandrova).
position and wide variation in their structures and electronic properties, such as silicon oxynitride (SiOxNy) films, offer the solution of a number of currently existing problems in the development of many integrated devices or sensors. Also, these materials offer new possibilities in microelectronics and optoelectronics industry, e.g., passivating coatings [2], thin gate dielectrics [3], membranes and optical wave guides for micro-electro-mechanical systems [4] and micro-opto-mechanical systems. Several investigators [5,6] have recently reported that high-quality SiOxNy films can be produced by the LPCVD process. Much effort has been devoted to improve the electrical properties of these films. By varying the technological conditions in order to get different permittivity, e; it is possible to obtain varying fixed oxide charge in the
0042-207X/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 3 6 3 - 9
386
S. Alexandrova et al. / Vacuum 69 (2003) 385–389
dielectric, which, in contrast to thermal SiO2, can also be negative. The aim of this paper is to report our recent study on the effect of a low-temperature anneal on the interface properties of the metal–insulator– silicon (MIS) structures with thin SiOxNy layers grown on silicon. For this purpose the frequency dispersion of the capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics after annealing are studied in detail.
2. Experimental 2.1. Samples Both n-type (1 1 1)Si and p-type (1 0 0)Si wafers of 5–10 O cm resistivity were initially treated with a pre-oxidation wet RCA cleaning in an H2SO4/ H2O2 solution and subsequent etching in diluted HF solution for removal of the native SiO2 films from Si and a final 10 min rinse in flowing deionized H2O immediately before loading in a standard low-pressure chemical vapour deposition (LPCVD) reactor. The amorphous SiOxNy thin films were formed by a chemical reaction of dichlorosilane (SiH2Cl2) with nitrous oxide (N2O) and ammonia (NH3) in a standard LPCVD reactor at a total pressure of 400 mTorr and at a substrate temperature of 8601C. Throughout the deposition process, the gas flow rate of SiH2Cl2 was kept at 40 sccm and the gas flow rates of N2O and NH3 were at a ratio of r ¼ QN2 O =QNH3 ¼ 4:8 and r ¼ 2:4 for the n-type (1 1 1)- and p-type (1 0 0)-oriented Si, respectively. The obtained SiOxNy films used for this study were 160 nm thick. For studying the electrical properties of LPCVD-SiOxNy thin films, after deposition the films were formed into MIS capacitors by vacuum thermal evaporation of about 300 nm circular aluminum dots with areas of 2.8 103 cm2 onto the SiOxNy surface through a metal mask. For making a contact to the Si substrate, evaporated continuous Al films were used. Finally, part of the structures was subjected to a 4501C annealing in dry N2 ambient for 30 min.
2.2. Measurement All the C–V and G–V measurements were performed at room temperature in a frequency range of 1–300 kHz before and after the devices have been subjected to the anneal. A Precision Component Analyser WAYNE KERR 6425 was used for the C–V and G–V measurements. The data reported here are accurate within 75% and are typical for all the samples prepared. The defect concentrations in the structures have been estimated from the C–V and G–V measurements. The so-called fixed oxide charge has been determined from comparison of the measured (at the highest frequency) and ideal theoretical C–V characteristics at the flat-band condition.
3. Results and discussion The C–V characteristics before anneal show large defect density concentrations expressed in a large shift, a high slope of the C–V curves and dispersion in inversion capacitance. This is illustrated in Fig. 1 for n(1 1 1)Si. Characteristic of the measured C–V curves is the absence of hysteresis, which is a proof of low density of slow interface states localized around SiOxNy/Si interface further
Fig. 1. Frequency dispersion of the C–V characteristics of MIS capacitors with LPCVD-SiOxNy on (1 1 1)Si substrate before annealing.
S. Alexandrova et al. / Vacuum 69 (2003) 385–389
inside the dielectric. The results for p(1 0 0)Si substrate are similar except for the sign of the fixed dielectric charge, which in this case is negative. The high-frequency C–V characteristics of the MIS capacitors grown on (1 1 1)Si and (1 0 0)Si after annealing are illustrated in Figs. 2 and 3, respectively. A common feature is the shift of the curves towards more positive voltages, which is an evidence of the decrease of the positive oxide charges. The regular shape of the C–V curves is indicative of a homogeneous distribution of the oxide charges [7]. The C–V characteristics show frequency dispersion. Both sets of C–V curves shifted towards the ideal C–V curve with increasing frequency. This is an indicator of the type of the interface states contributing to the C–V shift. In the depletion region for a given gate voltage, the capacitance dispersion depends on Si orientation. For a (1 1 1)Si substrate a decrease and for (1 0 0)Si an increase of the capacitance with measurement frequency is evident due to the time-dependent response of interface states of different nature. This is further substantiated by the different behaviour of the slope of C–V curves
Fig. 2. Frequency dispersion of the C–V characteristics of MIS capacitors with LPCVD-SiOxNy on (1 1 1)Si substrate after annealing.
387
Fig. 3. Frequency dispersion of the C–V characteristics of MIS capacitors with LPCVD-SiOxNy on (1 0 0)Si substrate after annealing.
for both orientations of the Si substrate. For (1 1 1)Si the slope of the curves towards Vg axis decreases with frequency. For (1 0 0)Si, however, the slope shows no frequency dependence. In accumulation, no appreciable frequency dispersion is evident in this frequency range. Nevertheless, the oxide capacitance was calculated from the MOS capacitance of the low-frequency C–V curve in strong accumulation, to be sure that the series resistance contribution is negligibly small. This capacitance value has been used further throughout the study for estimation of the defect concentrations. The inspection of C–V curves in Figs. 2 and 3 shows the same values of Cmin for different frequencies. In as-deposited films dispersion of Cmin has been registered, as seen in Fig. 1, which was interpreted as due to defect creation in the Si substrate during film deposition [8]. Obviously, these defects have been annealed out in the postdeposition anneal. Sets of G–V characteristics of the MIS capacitors are plotted in Figs. 4 and 5 for the n(1 1 1) and p(1 0 0)Si structures, respectively, for the same frequency interval as in Figs. 2 and 3. The conductance was found to increase with increasing
388
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Fig. 4. Frequency dispersion of the G–V characteristics of MIS capacitors with LPCVD-SiOxNy on (1 1 1)Si substrate after annealing.
Fig. 5. Frequency dispersion of the G–V characteristics of MIS capacitors with LPCVD-SiOxNy on (1 0 0)Si substrate after annealing.
measurement frequency. The peaks become less sharp and broader with increasing frequency. The peak position of the conductance, similar to the shifts of the C–V curves in Figs. 2 and 3, is shifted towards zero gate voltage with increase in frequency. The comparison of the results in Figs. 4 and 5 reveals the different character of the curves
Fig. 6. Fixed dielectric charges obtained from the flatband voltage values for LPCVD-SiOxNy on p(1 0 0)Si and n(1 1 1)Si substrate at different frequencies before and after annealing.
indicating possibly different defect sites for both orientations. Unlike the well-known thermal SiO2/ Si interface, the deposited SiOxNy film on Si shows no substantial Si orientation dependence of the interface defect concentrations. The data for the fixed dielectric charge before and after annealing are summarized in Fig. 6. It can be seen that the fixed charge decreases with increasing frequency in all cases. The opposite sign of the fixed oxide charge for both Si orientations can be related to differences in the chemical reaction balance due to the twice higher gas flow rate ratio. The positive oxide charges are usually related to O3Si. oxygen-vacancy defects (E0 centres), Si–N bonds (K-centres) and N–H bonds. The E0 centres are well known for the thermal SiO2/Si interface [9], while K-centres and N–H bond have been found in Si3N4 [10] and SiOxNy [11]. The OH-groups are well known as negatively charged electron traps in dielectric films. At higher ratio r; E 0 centres should be dominant in the positive charge mainly because of the higher oxygen content, which makes the SiOxNy structure closer to SiO2. At lower r electron traps related to OH-groups [12] can be suggested as a reason for the observed negative fixed charge.
S. Alexandrova et al. / Vacuum 69 (2003) 385–389
In Fig. 6 a decrease in the positive charge is observed in the n(1 1 1)Si-based structures after annealing, whereas in the p(1 0 0)Si-base structures an increase of the negative charge is found. These negative shifts could be regarded as annealing of positive charges. A consistent explanation of the annealing effect has to take all the defects into account, the most simple being rearrangement of Si–N and N–H bonds.
4. Conclusion From the analysis of the frequency dispersion of the C–V and G–V characteristics of silicon oxynitride films LPCVD on n(1 1 1)- and p(1 0 0)Si substrates, a conclusion was made about the dielectric and interface quality and the possibility to control the sign and the density of the charged defects through choice of the deposition conditions and by additional low-temperature annealing.
389
References [1] Takeda E, Ikuzaki K, Katto H, Dhji Y, Hinode K, Hamada A, Sakuta T, Funatiki T, Sasaki T. Procc IEEE 1993;81:653–7. [2] Lowe A, Powell MJ, Elliott SR. J Appl Phys 1986;59: 1251–4. [3] Iliac BB. Hewlett Packard J 1982;33:25–9. [4] Modreanu M, Cosmin P. Proceedings of International Conference of Semicond-CAS’96, 1996. p. 323–7. [5] Buchanan DA, Batey J, Tierney E. IEEE Electron Dev Lett 1988;EDL-9:576–80. [6] Fountain GG, Rudder RA, Hattangady SV, Markunas RI. J Appl Phys 1988;63:4744–8. [7] Nicollian EH, Brew JR. MOS (metal oxide semiconductor) physics and technology. New York: Wiley, 1982. [8] Szekeres A, Halova E, Alexandrova S, Modreanu M. Proceedings of the International Conference of SemicondCAS, 2001. p. 235–8. [9] Vanheusden K, Stesmans A. Appl Phys Lett 1993;62: 2405–8 and references therein. [10] Lenahan PM, Curry SE. Appl Phys Lett 1990;56:157–60. [11] Hasegawa S, Sakamori S, Futatsudera M, Inokuma T, Kurata Y. J Appl Phys 2001;89:2536–98. [12] Szekeres A, Alexandrova S, Modreanu M. Phys Stat Sol A, 2001;187:493–8.