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tetraethoxysilane plasmas in a helicon reactor
Thin Solid Films 471 (2005) 123 – 127 www.elsevier.com/locate/tsf
Effect of the ion bombardment energy on silicon dioxide films deposited from oxygen/tetraethoxysilane plasmas in a helicon reactor D. Goghero 1, A. Goullet *, G. Borvon, G. Turban Laboratoire des Plasmas et des Couches Minces, IMN-CNRS and Universite´ de Nantes, Nantes Cedex 0344322, France Received 4 July 2002; received in revised form 12 January 2004; accepted 29 April 2004 Available online 24 July 2004
Abstract Silicon dioxide films are deposited on silicon substrates from oxygen/tetraethoxysilane (TEOS) plasmas in a helicon reactor operated at low pressure (5 mtorr). The effect of the negative dc self-bias voltage Vb (0 to 200 V) on structural and electrical bulk properties of SiO2 films is investigated. The structural characterization has been performed using Fourier-transform infrared (FTIR) spectroscopy, spectroscopic ellipsometry and wet etching. Electrical measurements including capacitance – voltage (C – V), current – voltage (I – V) and constant current stressing (CCS) have been carried out on metal – oxide – semiconductor (MOS) capacitors. As soon as a dc self-bias is applied (jVbj z 50 V), a significant enhancement of the oxide quality is observed in terms of macroscopic densification and reduction in the porosity. These modifications in the structural properties of the deposited SiO2 films are correlated with an improvement in the I – V characteristics but C – V and CCS measurements revealed that limiting the substrate bias at 50 V leads to best quality silicon dioxide films. D 2004 Elsevier B.V. All rights reserved. Keywords: Plasma processing and deposition; Ion bombardment; Silicon oxide
1. Introduction At present, high density plasma reactors, in millitorr range, have emerged to deposit good quality silicon dioxide films as isolation interlayer, passivation layers or lithographic masks. One of the most important advantages, related to this kind of reactors, is their ability to control the ion energy, independently of the plasma excitation. Thus, these plasmas, operated at low pressure (1 – 50 mtorr), can be used to deposit SiO2 on silicon substrates using a radiofrequency (rf) bias. The influence of the ion bombardment, applied during the deposition process, can be investigated to optimize the film properties [1– 3]. Highdensity inductively coupled plasma reactors, such as helicon ones, are used in the semiconductor industry because the films can also be deposited at low temperature ( < 100 jC) with regard to other techniques such as parallel-plate rf plasmas (200 – 300 jC) or thermally activated chemical * Corresponding author. Tel.: +33-2-40373964; fax: +33-2-40373959. E-mail address: [email protected] (A. Goullet). 1 Present address: CNR-IMM, Sezione de Catania, stradale Primosole 50, 95121 Catania, Italy. 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.04.065
vapor deposition (>400 jC) [4]. As the bombardment by high-energy ions can induce damage and a significant increase in the substrate temperature due to the energy density deposited on the surface [5], the ion energy is usually limited at 200 eV to modify the properties of deposited SiO2 films [6]. In practice, silicon dioxide PECVD films are usually deposited from silane or tetraethoxysilane (TEOS) respectively mixed with N2O or O2 oxidizing gas [7]. The use of TEOS is preferred because of its safer operating conditions, superior conformity and good stability. It has been reported from experiments in O2/TEOS inductive plasmas that a low TEOS fraction ( < 5%) in the mixture is required to obtain higher quality films [8,9]. In these studies, refractive index, infrared absorption bands and wet etch rates have widely been investigated. However, to our knowledge, results on both the structural and electrical bulk properties in a helicon reactor have not been published by other teams. In this work, the effect of the ion energy on the structural and electrical properties of SiO2 films has been investigated in order to obtain the optimal conditions for the insulator deposition.
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2. Experimental The helicon reactor is represented in Fig. 1. The helicon source is a glass tube surrounded by a helicon antenna, stuck to the tube. These two elements are surrounded themselves by a coil which creates an axial dc magnetic field (about 50 G in the center of the source) to confine the electrons [10]. During deposition, the reactor is operated in inductive mode by means of a 300 W rf power generator and the chamber pressure was kept at 5 mtorr with a total flow rate of 37.5 sccm. The oxygen gas is introduced in the upper part of the reactor (the source), while the TEOS vapour is injected in the diffusion chamber a few centimeters above the substrate. In this study, a 2% TEOS fraction in the O2/TEOS mixture (lower ratios would not be reliable with our TEOS massflow controller) was used to ensure good quality of the deposited silicon oxide films. A second 13.56-MHz rf signal is applied to the substrate holder to generate a self-bias (Vb), which ranged from 0 to 200 V. The corresponding coupled power did not exceed 20 W and the plasma perturbation could be neglected. Assuming a collisionless sheath, the ion energy was ranged from 15 to 215 eV. When no voltage is applied, the ion energy corresponds to Ei = e(Vp Vf), where Vp and Vf are the plasma and floating potential, respectively. Although the samples were not intentionally heated, we measured an increase in the sample temperature when increasing the ion energy. Using a contactless fluoroptometry probe (optical thermometer), we found that, for our experimental conditions, the film surface temperature never exceeded 130 jC.
The 2 in. n-type silicon substrates were cleaned, dipping them in a diluted HF solution followed by a rinse in deionised water for 5 min and finally dried with nitrogen. The in situ evolution of the ellipsometric parameters (w,D) as a function of time (kinetic mode) was monitored with a UV – visible phase modulated ellipsometer (1.5 – 5 eV, ISA Jobin Yvon). Six wavelengths, recorded simultaneously in the UV – visible range over time, were also used to determine the evolution of the film thickness in real time. Moreover, spectroscopic measurements (SE) with typically 70 energy values between 1.5 and 5 eV (step of 0.05 eV) were performed after deposition to determine with higher accuracy the final film thickness and in situ refractive index at 1.96 eV (633 nm). After deposition, the experimental procedure was the following: firstly, Fourier-transform infrared (FTIR) spectra (400 – 4000 cm 1) were recorded with a Nicolet (20 SXC) spectrometer equipped with a broadband HgCdTe detector. The thickness of the samples, measured in real time by ellipsometry, is fixed ( f 500 nm) to avoid thickness effects which appear in the FTIR spectra [11]. Wet chemical etching (p-etch) was carried out at room temperature in a buffered solution (HF/HNO3/H2O = 3:2:60). Indeed, p-etch rate is sensitive to density variations of inorganic SiO2 films. The p-etch rate was determined by SE measurements, as a function of the etching time. The electrical properties of the deposited films have been characterized using Al gate (area: 4.1 10 3 cm2) metal – oxide – semiconductor (MOS) capacitors with a 100 nm thick silicon dioxide film. After deposition, a 30 min post oxidation annealing in nitrogen was performed at 400 jC. Aluminium contact was implemented for C –V and I –V measurements. The aluminium layer was annealed at 400 jC in forming gas for 30 min. The back side of the substrates was also metallized and glued on the sample holder with silver epoxy to decrease as much as possible the influence of the series resistance on the electrical measurements. High-frequency (1 MHz) capacitance – voltage (C – V), current– voltage (I –V) and constant current stressing (CCS) characteristics were measured using HP-4194A and HP-4145B.
3. Structural analysis
Fig. 1. Experimental setup of the helicon reactor. A second radiofrequency (13.56 MHz) generator is used to create a dc self-bias applied to the substrate holder to adjust the energy of the ion bombardment.
To eliminate Si absorption components from the FTIR spectra, the absorption spectrum of a bare silicon (used as background and originated from the same wafer) was subtracted. We checked that the characteristic absorption line at f 610 cm 1 was similar both in the bare silicon and in the sample (silicon oxide film + substrate). All the FTIR spectra exhibit a sloping baseline due to an optical interference effect which has been removed by fitting the experimental curve in the transparent region (between 2000 and 2400 cm 1). The spectra showed the three characteristic (rocking, bending and stretching) SiO2 bands.
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The SiUOUSi stretching band, which is related to the film quality, was further studied and the evolution of the peak position and full-width at half-maximum (FWHM) are shown as a function of Vb in Fig. 2. The position of the stretching peak increases from ms = 1070 cm 1 (Vb = 0 V) to ms = 1078 cm 1 (Vb = 200 V) and the FWHM decreases from 105 (Vb = 0 V) to 84 cm 1(Vb = 200 V). The values for Vb = 200 V are the closest with respect to those a thermal oxide of the same thickness (ms = 1090 cm 1 and FWHM = 78 cm 1). Such a behavior is usually associated to a microscopic relaxation of the silica atomic structure, induced by an increase in the ion energy, and corresponds to an improvement of the oxide quality [3,12]. To further assess this trend, a study of the film structure was done by spectroscopic ellipsometry measurements. The samples have been modeled using a homogeneous layer of silicon oxide deposited on a semiinfinite c-Si substrate. The Bruggeman effective medium approximation [13] was used to take into account the presence of voids. In this hypothesis, the film is a homogeneous mixture of voids and amorphous SiO2 and the following expression is used: X
fi ðei heiÞ=ðei þ 2heiÞ ¼ 0
ð1Þ
i
where hei is the composite dielectric function and ei and fi are the dielectric function and volume fraction respectively for both components. Typical results obtained for silicon oxide deposited films with thickness around 100 nm at Vb = 0 V and Vb = 50 V are shown in Fig. 3. The quality of the ellipsometric angles (D,w) fit is expressed by the mean square error v2 [14]. When no bias is applied, the presence of voids in the SiO2
Fig. 2. Evolution of the SiUOUSi stretching peak position and full-width at half-maximum (FWHM) for 500 nm deposited SiO2 films as a function of Vb (Vb = 0, 50, 100, 200 V).
Fig. 3. Ellipsometric angles (w,D) ranged in the 1.5 – 5 eV deduced from experimental data ( ), a SiO2 layer model (D) and a SiO2 + voids layer model (U). The oxide thickness is about 100 nm in both cases.
atomic structure is clearly evidenced as the v2 value is significantly decreased for a mixture (100 x)% aSiO2 + x% voids in the deposited film (x = 6– 7%). For the biased samples, the best fit is obtained by taking into account a 100% SiO2 layer. By comparison with the unbiased sample, no trace of voids is detected when Vb = 50, 100 and 200 V. The film structural properties have also been investigated by measuring the p-etch rate (Vg) on the same samples. The quality of the deposited films is estimated from the values of the p-etch rate, which are normalized to ˚ /s). The obtained the one of a thermal oxide (Vg c 1.5 A results are reported in Fig. 4 as a function of Vb and are compared with the film refractive index value determined at 633 nm from the spectroscopic (D,w) fit. Both the increase in the refractive index and decrease in the p-etch rate measurements, mainly between Vb = 0 V and Vb = 50 V, suggest that the ion energy is responsible for the densification of the films. This behavior is also correlated with the absence of voids for jVbj z 50 V and confirms a
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Fig. 4. Normalized p-etch rate and refractive index as a function of the applied bias.
reduction in the porosity of the film enhanced by the ion bombardment [3].
4. Electrical measurements Typical high frequency (1 MHz) C – V measurements were performed to determine the influence of Vb on the electrical properties of the silicon oxide films deposited on ntype silicon substrates. The samples were cleaned, dipping them in a diluted HF solution in a similar way to those used for the structural analysis. For these experiments, the oxide thickness is chosen around 100 nm to facilitate the determination of the breakdown voltage (100 V is the maximum applied bias available with the HP4145B apparatus). The flat band voltage (Vfb) and fixed oxide charge densities ( Qf /q) as a function of Vb are presented in Table 1. The Vfb and Qf /q values lie from 1.5 to 4.2 V and from 3 1011 to 1 1012 cm 2, respectively. The negative Vfb values for all samples are due to the trapping of positive charges in the silicon oxide films, which are found to be larger as Vb increases (jVbj z 50 V). The large negative flat band voltage shifts and high oxide fixed charge densities are mainly induced by the higher energy of the ion bombardment.
Fig. 5. Typical I(V) characteristics obtained from 100 nm silicon dioxide films as a function of Vb (Vb = 0, 50, 100, 200 V). The sweep rate is fixed in all cases at 7 V/s.
Typical I– V characteristics are presented in Fig. 5 as a function of Vb. The data were recorded using a staircase voltage with a high sweep rate of 7 V/s to limit charge accumulation and a subsequent degradation of the oxide during acquisition [15]. Nevertheless, we have noted in all cases a plateau likely caused by a charge trapping effect before oxide breakdown. From the I– V curve, we have investigated the leakage current and critical electric field, respectively noted Jlk and Ecr. The value of Ecr is determined for a 1 AA/cm2 current density [16], whereas Jlk is deduced for a 2 MV/cm applied electric field [17]. For these two electrical parameters, the results are averaged from five dots taken on the same wafer. From the criteria above, we have determined
Table 1 Summary of the electrical results obtained from MOS capacitor with 100nm-thick SiO2 films deposited from O2/TEOS plasmas in a helicon reactor Ion energy (eV)
Flat band voltage (V)
Fixed charge density (cm 2)
Leakage current (A/cm2)
Critical electric field (MV/cm)
15 65 115 215
1.5 1.3 3 4.2
3.8 1011 3.8 1011 8.2 1011 1.0 1012
3.0 10 9 1.2 10 9 1.6 10 9 7.6 10 10
3.7 6.5 6.6 7
Fig. 6. Evolution of the gate voltage shift as a function of time under various Vb (Vb = 0, 50, 100, 200 V). The injected current density is maintained to 1 AA/cm2 for each condition.
D. Goghero et al. / Thin Solid Films 471 (2005) 123–127
values of Jlk = 3 10 9 A/cm2 and Ecr = 3.7 MV/cm for the unbiased sample. For Vb = 50 V, Jlk is decreased at 10 9 A/cm2 and Ecr is increased at 6– 7 MV/cm, while no significant evolution is observed beyond. Furthermore, the average breakdown electrical field was respectively estimated between 8 and 9 MV/cm (Vb = 0 V) and 9 and 10 MV/cm (jVbj z 50 V). These results, summarized in Table 1, showed that the electrical properties of the deposited films are improved when a dc self-bias is applied. This behavior can be correlated with the macroscopic densification effect reported in Section 3. The evolution of the gate voltage shift under stress as a function of time for an injected current density of 1 AA/cm2 has also been determined. The results reported in Fig. 6 show an initial hole trapping followed by a stress induced electron trap generation for longer times. This characteristic evolution is no longer observed when the film is submitted to the ion bombardment, where only electron traps are evidenced. We suggest that the initial hole trapping could be related to oxygen-deficient defects [18]. Indeed, a preliminary study, performed in similar experimental conditions, has shown that the primary defect detected from electron spin resonance (ESR), which is attributed to the oxygen-vacancy center, dramatically decreases when applying a 120 V dc self-bias voltage [19]. However, ESR measurements have to be carried out on our samples to valid this assumption. Finally, based on our various electrical results, it must be pointed out that a slight self-bias voltage (Vb = 50 V) is sufficient to obtain a good quality SiO2 deposited layer doubtless limiting a degradation of the interfacial electrical properties during the first steps of the deposition process [20].
5. Conclusion Silicon dioxide films from O2/TEOS plasmas have been deposited in a helicon reactor by varying the applied voltage at the substrate holder down to 200 V. The structural and electrical properties of the deposited SiO2 film are modified by the increase in the ion bombardment energy. As soon as a dc self-bias is applied (jVbj z 50 V), a macroscopic densi-
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fication and a reduction in the porosity have been evidenced into the films. These structural modifications are correlated with an improvement in the I– V measurements but C – V and time-dependent gate voltage under stress measurements revealed that limiting the substrate bias to 50 V is an optimal condition to obtain the best quality SiO2 films. This low ion energy criterion is often required to improve the properties of silicon dioxide films deposited in high density plasma reactors.
References [1] R.G. Amdosca, W.J. Varhue, E. Adams, J. Appl. Phys. 72 (1992) 1126. [2] N. Jiang, B. Agius, M.C. Hugon, J. Olivier, M. Puech, J. Appl. Phys. 76 (1994) 1847. [3] O. Joubert, R. Burke, L. Vallier, C. Martinet, R.A.B. Devine, Appl. Phys. Lett. 62 (1993) 228. [4] K.L. Seaward, J.E. Turner, K. Nauka, A.M.E. Nel, J. Vac. Sci. Technol., B 13 (1995) 118. [5] A. Durandet, O. Joubert, J. Pelletier, M. Pichot, J. Appl. Phys. 67 (1990) 3862. [6] A. Durandet, D.R. McKenzie, J. Appl. Phys. 80 (1996) 4707. [7] G.E. McGuire, Semiconductor Materials and Process Technology Handbook for VLSI and ULSI, Noyes Publications, Park Ridge, New Jersey, 1988. [8] S.C. Deshmukh, E.S. Aydil, J. Vac. Sci. Technol., B 14 (1996) 738. [9] C. Vallee, A. Goullet, F. Nicolazo, A. Granier, G. Turban, J. NonCryst. Solids 272 (2000) 163. [10] A. Granier, F. Nicolazo, C. Vallee, A. Goullet, G. Turban, B. Grolleau, Plasma Sources Sci. Technol. 6 (1997) 147. [11] R. Ossikovski, B. Drevillon, M. Firon, J. Opt. Soc. Am. A 8 (1995) 1797. [12] R.A.B. Devine, Trans. Mater. Res. Soc. Jpn. 8 (1992) 165. [13] D.A.G. Bruggeman, Ann. Phys. 24 (1935) 636. [14] B. Drevillon, J.Y. Parey, M. Stschakovsky, R. Benferhat, Y. Josserand, B. Schlayen, SPIE Proc. 1188 (1989) 174. [15] D.J. DiMaria, R. Ghez, D.W. Dong, J. Appl. Phys. 51 (1980) 4830. [16] M. Mukhopadhayay, S.K. Ray, D.K. Nayak, C.K. Maiti, Appl. Phys. Lett. 68 (1996) 1262. [17] Y.C. Chen, Jpn. J. Appl. Phys. 38 (1999) 4226. [18] B. Senapati, S.K. Samanta, S. Maikap, L.K. Bera, C.K. Maiti, Appl. Phys. Lett. 77 (2000) 1840. [19] F. Templier, L. Vallier, R. Madar, J.C. Oberlin, R.A.B. Devine, Thin Solid Films 241 (1994) 251. [20] D. Landheer, D.X. Xu, Y. Tao, G.I. Sproule, J. Appl. Phys. 77 (1995) 1600.
Effect of the ion bombardment energy on silicon dioxide films deposited from oxygen/tetraethoxysilane plasmas in a helicon reactor D. Goghero 1, A. Goullet *, G. Borvon, G. Turban Laboratoire des Plasmas et des Couches Minces, IMN-CNRS and Universite´ de Nantes, Nantes Cedex 0344322, France Received 4 July 2002; received in revised form 12 January 2004; accepted 29 April 2004 Available online 24 July 2004
Abstract Silicon dioxide films are deposited on silicon substrates from oxygen/tetraethoxysilane (TEOS) plasmas in a helicon reactor operated at low pressure (5 mtorr). The effect of the negative dc self-bias voltage Vb (0 to 200 V) on structural and electrical bulk properties of SiO2 films is investigated. The structural characterization has been performed using Fourier-transform infrared (FTIR) spectroscopy, spectroscopic ellipsometry and wet etching. Electrical measurements including capacitance – voltage (C – V), current – voltage (I – V) and constant current stressing (CCS) have been carried out on metal – oxide – semiconductor (MOS) capacitors. As soon as a dc self-bias is applied (jVbj z 50 V), a significant enhancement of the oxide quality is observed in terms of macroscopic densification and reduction in the porosity. These modifications in the structural properties of the deposited SiO2 films are correlated with an improvement in the I – V characteristics but C – V and CCS measurements revealed that limiting the substrate bias at 50 V leads to best quality silicon dioxide films. D 2004 Elsevier B.V. All rights reserved. Keywords: Plasma processing and deposition; Ion bombardment; Silicon oxide
1. Introduction At present, high density plasma reactors, in millitorr range, have emerged to deposit good quality silicon dioxide films as isolation interlayer, passivation layers or lithographic masks. One of the most important advantages, related to this kind of reactors, is their ability to control the ion energy, independently of the plasma excitation. Thus, these plasmas, operated at low pressure (1 – 50 mtorr), can be used to deposit SiO2 on silicon substrates using a radiofrequency (rf) bias. The influence of the ion bombardment, applied during the deposition process, can be investigated to optimize the film properties [1– 3]. Highdensity inductively coupled plasma reactors, such as helicon ones, are used in the semiconductor industry because the films can also be deposited at low temperature ( < 100 jC) with regard to other techniques such as parallel-plate rf plasmas (200 – 300 jC) or thermally activated chemical * Corresponding author. Tel.: +33-2-40373964; fax: +33-2-40373959. E-mail address: [email protected] (A. Goullet). 1 Present address: CNR-IMM, Sezione de Catania, stradale Primosole 50, 95121 Catania, Italy. 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.04.065
vapor deposition (>400 jC) [4]. As the bombardment by high-energy ions can induce damage and a significant increase in the substrate temperature due to the energy density deposited on the surface [5], the ion energy is usually limited at 200 eV to modify the properties of deposited SiO2 films [6]. In practice, silicon dioxide PECVD films are usually deposited from silane or tetraethoxysilane (TEOS) respectively mixed with N2O or O2 oxidizing gas [7]. The use of TEOS is preferred because of its safer operating conditions, superior conformity and good stability. It has been reported from experiments in O2/TEOS inductive plasmas that a low TEOS fraction ( < 5%) in the mixture is required to obtain higher quality films [8,9]. In these studies, refractive index, infrared absorption bands and wet etch rates have widely been investigated. However, to our knowledge, results on both the structural and electrical bulk properties in a helicon reactor have not been published by other teams. In this work, the effect of the ion energy on the structural and electrical properties of SiO2 films has been investigated in order to obtain the optimal conditions for the insulator deposition.
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2. Experimental The helicon reactor is represented in Fig. 1. The helicon source is a glass tube surrounded by a helicon antenna, stuck to the tube. These two elements are surrounded themselves by a coil which creates an axial dc magnetic field (about 50 G in the center of the source) to confine the electrons [10]. During deposition, the reactor is operated in inductive mode by means of a 300 W rf power generator and the chamber pressure was kept at 5 mtorr with a total flow rate of 37.5 sccm. The oxygen gas is introduced in the upper part of the reactor (the source), while the TEOS vapour is injected in the diffusion chamber a few centimeters above the substrate. In this study, a 2% TEOS fraction in the O2/TEOS mixture (lower ratios would not be reliable with our TEOS massflow controller) was used to ensure good quality of the deposited silicon oxide films. A second 13.56-MHz rf signal is applied to the substrate holder to generate a self-bias (Vb), which ranged from 0 to 200 V. The corresponding coupled power did not exceed 20 W and the plasma perturbation could be neglected. Assuming a collisionless sheath, the ion energy was ranged from 15 to 215 eV. When no voltage is applied, the ion energy corresponds to Ei = e(Vp Vf), where Vp and Vf are the plasma and floating potential, respectively. Although the samples were not intentionally heated, we measured an increase in the sample temperature when increasing the ion energy. Using a contactless fluoroptometry probe (optical thermometer), we found that, for our experimental conditions, the film surface temperature never exceeded 130 jC.
The 2 in. n-type silicon substrates were cleaned, dipping them in a diluted HF solution followed by a rinse in deionised water for 5 min and finally dried with nitrogen. The in situ evolution of the ellipsometric parameters (w,D) as a function of time (kinetic mode) was monitored with a UV – visible phase modulated ellipsometer (1.5 – 5 eV, ISA Jobin Yvon). Six wavelengths, recorded simultaneously in the UV – visible range over time, were also used to determine the evolution of the film thickness in real time. Moreover, spectroscopic measurements (SE) with typically 70 energy values between 1.5 and 5 eV (step of 0.05 eV) were performed after deposition to determine with higher accuracy the final film thickness and in situ refractive index at 1.96 eV (633 nm). After deposition, the experimental procedure was the following: firstly, Fourier-transform infrared (FTIR) spectra (400 – 4000 cm 1) were recorded with a Nicolet (20 SXC) spectrometer equipped with a broadband HgCdTe detector. The thickness of the samples, measured in real time by ellipsometry, is fixed ( f 500 nm) to avoid thickness effects which appear in the FTIR spectra [11]. Wet chemical etching (p-etch) was carried out at room temperature in a buffered solution (HF/HNO3/H2O = 3:2:60). Indeed, p-etch rate is sensitive to density variations of inorganic SiO2 films. The p-etch rate was determined by SE measurements, as a function of the etching time. The electrical properties of the deposited films have been characterized using Al gate (area: 4.1 10 3 cm2) metal – oxide – semiconductor (MOS) capacitors with a 100 nm thick silicon dioxide film. After deposition, a 30 min post oxidation annealing in nitrogen was performed at 400 jC. Aluminium contact was implemented for C –V and I –V measurements. The aluminium layer was annealed at 400 jC in forming gas for 30 min. The back side of the substrates was also metallized and glued on the sample holder with silver epoxy to decrease as much as possible the influence of the series resistance on the electrical measurements. High-frequency (1 MHz) capacitance – voltage (C – V), current– voltage (I –V) and constant current stressing (CCS) characteristics were measured using HP-4194A and HP-4145B.
3. Structural analysis
Fig. 1. Experimental setup of the helicon reactor. A second radiofrequency (13.56 MHz) generator is used to create a dc self-bias applied to the substrate holder to adjust the energy of the ion bombardment.
To eliminate Si absorption components from the FTIR spectra, the absorption spectrum of a bare silicon (used as background and originated from the same wafer) was subtracted. We checked that the characteristic absorption line at f 610 cm 1 was similar both in the bare silicon and in the sample (silicon oxide film + substrate). All the FTIR spectra exhibit a sloping baseline due to an optical interference effect which has been removed by fitting the experimental curve in the transparent region (between 2000 and 2400 cm 1). The spectra showed the three characteristic (rocking, bending and stretching) SiO2 bands.
D. Goghero et al. / Thin Solid Films 471 (2005) 123–127
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The SiUOUSi stretching band, which is related to the film quality, was further studied and the evolution of the peak position and full-width at half-maximum (FWHM) are shown as a function of Vb in Fig. 2. The position of the stretching peak increases from ms = 1070 cm 1 (Vb = 0 V) to ms = 1078 cm 1 (Vb = 200 V) and the FWHM decreases from 105 (Vb = 0 V) to 84 cm 1(Vb = 200 V). The values for Vb = 200 V are the closest with respect to those a thermal oxide of the same thickness (ms = 1090 cm 1 and FWHM = 78 cm 1). Such a behavior is usually associated to a microscopic relaxation of the silica atomic structure, induced by an increase in the ion energy, and corresponds to an improvement of the oxide quality [3,12]. To further assess this trend, a study of the film structure was done by spectroscopic ellipsometry measurements. The samples have been modeled using a homogeneous layer of silicon oxide deposited on a semiinfinite c-Si substrate. The Bruggeman effective medium approximation [13] was used to take into account the presence of voids. In this hypothesis, the film is a homogeneous mixture of voids and amorphous SiO2 and the following expression is used: X
fi ðei heiÞ=ðei þ 2heiÞ ¼ 0
ð1Þ
i
where hei is the composite dielectric function and ei and fi are the dielectric function and volume fraction respectively for both components. Typical results obtained for silicon oxide deposited films with thickness around 100 nm at Vb = 0 V and Vb = 50 V are shown in Fig. 3. The quality of the ellipsometric angles (D,w) fit is expressed by the mean square error v2 [14]. When no bias is applied, the presence of voids in the SiO2
Fig. 2. Evolution of the SiUOUSi stretching peak position and full-width at half-maximum (FWHM) for 500 nm deposited SiO2 films as a function of Vb (Vb = 0, 50, 100, 200 V).
Fig. 3. Ellipsometric angles (w,D) ranged in the 1.5 – 5 eV deduced from experimental data ( ), a SiO2 layer model (D) and a SiO2 + voids layer model (U). The oxide thickness is about 100 nm in both cases.
atomic structure is clearly evidenced as the v2 value is significantly decreased for a mixture (100 x)% aSiO2 + x% voids in the deposited film (x = 6– 7%). For the biased samples, the best fit is obtained by taking into account a 100% SiO2 layer. By comparison with the unbiased sample, no trace of voids is detected when Vb = 50, 100 and 200 V. The film structural properties have also been investigated by measuring the p-etch rate (Vg) on the same samples. The quality of the deposited films is estimated from the values of the p-etch rate, which are normalized to ˚ /s). The obtained the one of a thermal oxide (Vg c 1.5 A results are reported in Fig. 4 as a function of Vb and are compared with the film refractive index value determined at 633 nm from the spectroscopic (D,w) fit. Both the increase in the refractive index and decrease in the p-etch rate measurements, mainly between Vb = 0 V and Vb = 50 V, suggest that the ion energy is responsible for the densification of the films. This behavior is also correlated with the absence of voids for jVbj z 50 V and confirms a
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Fig. 4. Normalized p-etch rate and refractive index as a function of the applied bias.
reduction in the porosity of the film enhanced by the ion bombardment [3].
4. Electrical measurements Typical high frequency (1 MHz) C – V measurements were performed to determine the influence of Vb on the electrical properties of the silicon oxide films deposited on ntype silicon substrates. The samples were cleaned, dipping them in a diluted HF solution in a similar way to those used for the structural analysis. For these experiments, the oxide thickness is chosen around 100 nm to facilitate the determination of the breakdown voltage (100 V is the maximum applied bias available with the HP4145B apparatus). The flat band voltage (Vfb) and fixed oxide charge densities ( Qf /q) as a function of Vb are presented in Table 1. The Vfb and Qf /q values lie from 1.5 to 4.2 V and from 3 1011 to 1 1012 cm 2, respectively. The negative Vfb values for all samples are due to the trapping of positive charges in the silicon oxide films, which are found to be larger as Vb increases (jVbj z 50 V). The large negative flat band voltage shifts and high oxide fixed charge densities are mainly induced by the higher energy of the ion bombardment.
Fig. 5. Typical I(V) characteristics obtained from 100 nm silicon dioxide films as a function of Vb (Vb = 0, 50, 100, 200 V). The sweep rate is fixed in all cases at 7 V/s.
Typical I– V characteristics are presented in Fig. 5 as a function of Vb. The data were recorded using a staircase voltage with a high sweep rate of 7 V/s to limit charge accumulation and a subsequent degradation of the oxide during acquisition [15]. Nevertheless, we have noted in all cases a plateau likely caused by a charge trapping effect before oxide breakdown. From the I– V curve, we have investigated the leakage current and critical electric field, respectively noted Jlk and Ecr. The value of Ecr is determined for a 1 AA/cm2 current density [16], whereas Jlk is deduced for a 2 MV/cm applied electric field [17]. For these two electrical parameters, the results are averaged from five dots taken on the same wafer. From the criteria above, we have determined
Table 1 Summary of the electrical results obtained from MOS capacitor with 100nm-thick SiO2 films deposited from O2/TEOS plasmas in a helicon reactor Ion energy (eV)
Flat band voltage (V)
Fixed charge density (cm 2)
Leakage current (A/cm2)
Critical electric field (MV/cm)
15 65 115 215
1.5 1.3 3 4.2
3.8 1011 3.8 1011 8.2 1011 1.0 1012
3.0 10 9 1.2 10 9 1.6 10 9 7.6 10 10
3.7 6.5 6.6 7
Fig. 6. Evolution of the gate voltage shift as a function of time under various Vb (Vb = 0, 50, 100, 200 V). The injected current density is maintained to 1 AA/cm2 for each condition.
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values of Jlk = 3 10 9 A/cm2 and Ecr = 3.7 MV/cm for the unbiased sample. For Vb = 50 V, Jlk is decreased at 10 9 A/cm2 and Ecr is increased at 6– 7 MV/cm, while no significant evolution is observed beyond. Furthermore, the average breakdown electrical field was respectively estimated between 8 and 9 MV/cm (Vb = 0 V) and 9 and 10 MV/cm (jVbj z 50 V). These results, summarized in Table 1, showed that the electrical properties of the deposited films are improved when a dc self-bias is applied. This behavior can be correlated with the macroscopic densification effect reported in Section 3. The evolution of the gate voltage shift under stress as a function of time for an injected current density of 1 AA/cm2 has also been determined. The results reported in Fig. 6 show an initial hole trapping followed by a stress induced electron trap generation for longer times. This characteristic evolution is no longer observed when the film is submitted to the ion bombardment, where only electron traps are evidenced. We suggest that the initial hole trapping could be related to oxygen-deficient defects [18]. Indeed, a preliminary study, performed in similar experimental conditions, has shown that the primary defect detected from electron spin resonance (ESR), which is attributed to the oxygen-vacancy center, dramatically decreases when applying a 120 V dc self-bias voltage [19]. However, ESR measurements have to be carried out on our samples to valid this assumption. Finally, based on our various electrical results, it must be pointed out that a slight self-bias voltage (Vb = 50 V) is sufficient to obtain a good quality SiO2 deposited layer doubtless limiting a degradation of the interfacial electrical properties during the first steps of the deposition process [20].
5. Conclusion Silicon dioxide films from O2/TEOS plasmas have been deposited in a helicon reactor by varying the applied voltage at the substrate holder down to 200 V. The structural and electrical properties of the deposited SiO2 film are modified by the increase in the ion bombardment energy. As soon as a dc self-bias is applied (jVbj z 50 V), a macroscopic densi-
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fication and a reduction in the porosity have been evidenced into the films. These structural modifications are correlated with an improvement in the I– V measurements but C – V and time-dependent gate voltage under stress measurements revealed that limiting the substrate bias to 50 V is an optimal condition to obtain the best quality SiO2 films. This low ion energy criterion is often required to improve the properties of silicon dioxide films deposited in high density plasma reactors.
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