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Secondary ion species containing nitrogen atoms from plasma-enhanced chemical vapor deposited silicon oxide films on silicon
Applied Surface Science 254 (2008) 5727–5731
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Secondary ion species containing nitrogen atoms from plasma-enhanced chemical vapor deposited silicon oxide films on silicon Kiyoshi Chiba *, Yoshinori Tsuji Department of Nano Material and Bio Engineering, Tokushima Bunri University, Sanuki, Kagawa 769-2193, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 October 2007 Received in revised form 3 March 2008 Accepted 8 March 2008 Available online 16 March 2008
Secondary ion species from plasma-enhanced chemical vapor deposited (PECVD) SiO2 films have been investigated using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Comparative studies of PECVD SiO2 films prepared using a mixture of SiH4/N2O reaction gas at 400 8C with thermally oxidized SiO2 films grown at 900 8C were carried out in the mid-range mass spectra from 95 to 165 amu. Small amounts of ion species containing nitrogen atoms, including Si2O2N+, Si3O2N+and Si3O3N+, were detected in the SiO2 bulk from the PECVD SiO2 films. Furthermore, large amounts of Si3O2N+ and Si2O3N were found at the interface between silicon and the SiO2 films. Depth analysis showed that the intensity peak shapes of these ion species containing nitrogen atoms at the interface were closely coincident with those of Si3O3+ corrected by subtracting the influence of the SiO2 matrix. The variation in the spectra of these ion species clearly indicates that two types of structures of oxynitride exist for the PECVD SiO2 films in the SiO2 bulk films and at the interface. These are likely produced by the reaction of reactive gas with SiO2 and silicon surfaces where dangling bonds of silicon may exist in the different form. ß 2008 Elsevier B.V. All rights reserved.
PACS: 68.49.Sf 68.55.Jk 73.40.Qv 77.55.+f Keywords: Silicon oxide Nitrogen incorporation Oxynitride Secondary ion species Time-of-flight secondary ion mass spectrometry
1. Introduction Silicon oxynitride films have attracted increased interest recently as ultra-thin dielectric films on silicon for gate oxides in the miniaturization of semiconductor devices [1–3], optical wave guide films for optoelectronic devices [4,5] and gas barrier films to protect against gas diffusion through SiO2 films [6,7]. For fabrication of metal-oxide-semiconductor (MOS) devices, nitrogen is incorporated into SiO2 films using either thermal oxynitridation or annealing, or chemical and physical deposition methods [1,8– 18]. The incorporation of nitrogen into thin SiO2 films improves the electrical and structural characteristics of the oxides. In addition, it results in increased protection against boron penetration and impurities through the gate dielectric, as well as low density of surface states [19–23]. Accumulation of the incorporated nitrogen at the Si–SiO2 interface often occurs and has been reported by a number of authors [1,11,16–18,24]. SiO2 films prepared in N2O gas during plasma processing are particularly attractive because they
* Corresponding author. Tel.: +81 87 894 5111; fax: +81 87 894 4201. E-mail address: [email protected] (K. Chiba). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.03.037
allow for the incorporation of a small but significant amount of nitrogen near the Si–SiO2 interface at low temperatures [1]. The characterization of the incorporated nitrogen in SiO2 films at the nanoscale is key to yield further knowledge of the chemical and structural features of these films and to pursue tailored nitrogen profiles in ultra thin dielectrics. A number of characterization techniques [9,25,26], including Fourier transform infrared (FTIR) spectroscopy [18,27], Rutherford backscattering spectrometry (RBS) [25,27], Auger electron spectroscopy (AES) [9,10,15–17], X-ray photoelectron spectroscopy (XPS) [9,25] and secondary ion mass spectrometry (SIMS) [9,11,12,28–34], have been reported thus far to investigate the behavior of nitrogen atoms incorporated into SiO2 films. Although FTIR provides information on lattice vibrations of the molecular structure, a film thickness exceeding approximately 100 nm is necessary to obtain an adequate signal. In this case, information on atomic layers cannot be obtained, except that of average structures through the films. Among the above methods, SIMS can provide information on several atomic layers of the films with high sensitivity, whereas either the depth resolution or sensitivity is limited for the other methods. SIMS has frequently been applied to the study of depth profiles of nitrogen in SiO2 films, for example using N+ secondary ions with Cs+ primary ions
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[12,14,28–31], SiN+ ions with O2 primary ions [9], and SiN ions with Cs+ primary ions [32]. In these studies, SIMS was used in dynamic mode. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a useful tool to characterize solid structures by detection of secondary ion species emitted from solid materials using pulsed mode of bombardment of primary ions associated with high sensitivity and low surface damage [35]. TOF-SIMS has also been applied to the study of silicon oxynitride films using secondary Si+ matrix ions to cationize neutral molecular and elemental species, and to detect the Si2N+ ion with a Ga+ primary ion [25,33]. These SIMS studies have clarified the depth profile of nitrogen, particularly the accumulation of nitrogen at the Si–SiO2 interface. However, further information on the bonding structure of species in the films as well as at or near the Si–SiO2 interface could not be obtained because the secondary ion species detected in these studies were very small or elementally fragmented mass species. In TOF-SIMS, the identification and understanding of characteristic features in the mid-range of the mass spectra showing the fragmentation patterns is key to relate them to structural features of materials at the nanoscale [36,37]. These spectra show various parts of the fragmented mass species of the SiO2 films which can be rebuilt to original structures. In this paper, we report the characterization of secondary ion species containing nitrogen atoms emitted from PECVD-grown SiO2 films on silicon with TOF-SIMS using the Au3+ primary ion. We used spectra in both positive and negative polarities in the midrange of the mass spectra, and identified ion species of Si2O2N+, Si3ON+, Si3O2N+, Si3O3N+ and Si2O3N in the SiO2 bulk films and at the Si–SiO2 interface.
MM2 molecular mechanics calculations of the Si–O–N structure were carried out using the program package from Molecular Design Limited. 3. Results and discussion Fig. 1 shows the TOF-SIMS spectra of thermally oxidized SiO2 films on silicon in the mass range from 95 to 165 amu. The spectra are presented in the form of a diagram showing both positive (up) and negative (down) polarities [36]. This diagram shows the spectra of fragment ion species of both positive and negative charge ions simultaneously. Various parts of the fragmented pieces of the SiO2 matrix films can be seen in the mid-range mass spectra from 95 to 165 amu with adequate sensitivity. The series of Si3On+ (n = 2–5) and Si2O3+ ion peaks as parts of fragmented pieces of the SiO2 matrix clearly appear from the SiO2 film (sputter time 250 s, 10-nm depth from the surface) in Fig. 1a. They are assigned to Si3O2+, Si3O3+, Si3O4+, Si4O5+ and Si2O3+, respectively [36]. The intensities of the Si3On+ ion species decreased as Si3O4+ > Si3O3+ > Si3O2+. Fig. 1b shows the spectral diagram from the interface between silicon and the SiO2 film (sputter time 620 s). Silicon cluster ion species of Si4+ and Si5+ are observed in Fig. 1b. These silicon cluster ions most likely arise from the silicon substrate surface. Comparing the spectra in Fig. 1b with those in Fig. 1a, the intensities of Si3O+, Si3O2+, Si3O3+, Si3O4+ and Si3O5+ peaks at the Si–SiO2 interface significantly differed from the
2. Experimental Thermally oxidized 25-nm-thick SiO2 films as well as plasmaenhanced CVD-prepared SiO2 films on single-crystal n-type (100) silicon wafers (resistivity of 4–6 V cm) were used in this study. The thermally oxidized SiO2 films were prepared on silicon wafers using a gas mixture of O2 and HCl (10%) at 900 8C after pre-cleaning of the wafer surface with a mixture solution of trimethyl-2hydroxyethylammonium hydroxide (choline) and hydrogen peroxide (H2O2). Plasma-enhanced CVD-prepared SiO2 films were grown from SiH4 and N2O mixtures at a flow rate ratio of 1:18 at 400 8C after pre-cleaning of the wafer with dilute HF and a mixture solution of choline and H2O2 without a post anneal. TOF-SIMS measurements were performed with an ULVAC-PHI TRIFT III TOF-SIMS spectrometer. Sputter etching was accomplished by Ar+ ions at 1 kV and 200 nA rastering over a 1000 mm 1000 mm area. The corresponding etching rate was approximately 0.04 nm/s for the SiO2 films. Spectral data were obtained at an etching time of 50 and 250 s in the SiO2 layer and intervals of 10 s from 450 to 750 s near the Si/SiO2 interface. TOFSIMS analyses were performed in positive and negative polarities using a pulsed Au3+ primary ion beam operated at 22 kV and 2.0 nA. It should be noted that a bias voltage was applied to the sample holder in this instrument of +3 kV and 3 kV for measurements of positive and negative secondary ions, respectively. This gives rise to a difference in the actual acceleration voltage of the primary ion, namely, 19 kV and 25 kV for measurements of positive and negative secondary ions, respectively. The beam was rastered on a 100 mm 100 mm surface area. Secondary ions in the mass range from 0 to 1860 amu were collected for 300 s. No charge compensation using an electron flood gun was required. The working pressure was in the 107 Pa range. Identification of the peaks of the secondary ion species was carried out using the mass of the peaks to an accuracy within 0.01 amu.
Fig. 1. TOF-SIMS spectra diagram from a 25 nm-thick thermally grown SiO2 film on Si showing both positive (up) and negative (down) polarities in the mass range from 95 to 165 amu: (a) 10-nm depth from the surface (sputter time 250 s) and (b) the SiSiO2 interface (sputter time 620 s). Possible fragment ion species are shown. Note that the increase in intensities of Si3O+, Si3O2+ and Si3O3+ ion species is clearly observed at the Si–SiO2 interface.
K. Chiba, Y. Tsuji / Applied Surface Science 254 (2008) 5727–5731
spectra of the SiO2 bulk films. The intensities of the Si3On+ ion species decreased as Si3O2+ > Si3O3+ > Si3O4+. It has been reported that Si3O+, Si3O2+ and Si3O3+ ion species observed at the interface are not as same as those emitted from SiO2 [36]. Fig. 2 shows the TOF-SIMS spectral diagram from PECVD prepared SiO2 films on silicon in the mass range from 95 to 165 amu. The spectra at a 10-nm depth from the surface, that is, in the SiO2 bulk film, is shown in Fig. 2a. Comparing the spectra in
Fig. 2. TOF-SIMS spectra diagram from a 25-nm-thick PECVD grown SiO2 film on Si in the mass range from 95 to 165 amu: (a) 10-nm depth from the surface (sputter time 250 s), (b) the Si–SiO2 interface (sputter time 620 s) and (c) silicon substrate (sputter time 850 s). Small amounts of Si2O2N+, Si3O2N+ and Si3O3N+ ions appear in the spectra from the 10-nm depth surface. These ion species suggest that nitrogen atoms are incorporated in the PECVD grown SiO2 bulk films and the interface. Strong peaks of Si3O2N+ and Si2O3N ions are seen at the Si–SiO2 interface and are marked by an asterisk. The variation in intensity ratios of Si3On1N+ (n = 2–4) to those of Si3On+ are noted. No trace of oxide or oxynitride ion species is observed in the Si substrate.
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Fig. 2a with those in Fig. 1a, the spectral pattern of Si3On+ (n = 2–5) and Si2O3+ ion peaks from PECVD SiO2 films is almost the same as that from the 10-nm depth surface of thermally-oxidized SiO2 films. It is, however, to be noted that very small peaks at masses of 102, 130 and 146 were detected for the PECVD-prepared SiO2 films in Fig. 2a. These peaks were assigned to Si2O2N+, Si3O2N+ and Si3O3N+, respectively. These peaks indicate that nitrogen atoms are incorporated into the SiO2 bulk film. Fig. 2b shows the spectral diagram from the Si–SiO2 interface for PECVD-prepared SiO2 films (sputter time 620 s). The intensities of some peaks at the Si–SiO2 interface significantly differed from the spectra of the SiO2 bulk films. An anomalous increase in the intensity of the peaks at masses of 130 and 118, attributed, respectively, to Si3O2N+ and Si2O3N ion species, was observed at the Si–SiO2 interface. The intensity of the peaks at the mass of 102, assigned to Si2O2N+, increased approximately twice at the interface. The peak at the mass of 114, attributed to Si3ON+ ion species, appeared in the spectra at the Si–SiO2 interface. The intensity ratio between the peaks of Si3O3+ and Si3O2N+ at the interface significantly differed from that at 10-nm depth in the SiO2 bulk film. These spectra suggest that Si2O2N+, Si3ON+, Si3O2N+, Si3O3N+ and Si2O3N ion species observed at the interface are different from those from the SiO2 bulk film. Fig. 2c shows the spectra from the silicon substrate (sputter time 850 s). Only cluster ion species of Si4+ and Si5+ are found in Fig. 2c. This spectrum suggests that residual gas containing nitrogen atoms in the analytical chamber influences the spectra to a negligible extent. The depth profiles of secondary ion species were also examined. Fig. 3 shows the depth profiles of Si3O3+ and Si3O4+ ions for thermally oxidized SiO2 films on silicon. The intensities are normalized to the total positive or negative secondary ion counts, respectively. It is worth noting that an intensity ratio of Si3O3+ to Si3O4+ ions was approximately 0.3–0.4 in the SiO2 bulk films. This ratio suggests the value of the fragment ratio of Si3O3+ to Si3O4+ decomposed from the SiO2 matrix. Considering this ratio of 0.4 as the correction factor for the intensity of the Si3O3+ ion species at the Si–SiO2 interface, the depth profile of the Si3O3+ ion was corrected by subtracting the influence factor of the SiO2 matrix; in other words, these are the values of the intensity of Si3O4+ multiplied by 0.4. The corrected depth profile of the Si3O3+ ion species is also shown in Fig. 3. Sharply increased peaks of Si3O3+ ions, which most likely correspond to a transition region from
Fig. 3. TOF-SIMS depth profiles of Si3O3+ and Si3O4+ of a 25-nm-thick thermally grown SiO2 film on Si. The depth profile of Si3O3+ ions after subtraction of the influence of the SiO2 matrix is also shown. The sputter etching rate is approximately 0.04 nm/s for the SiO2 films. The increase in intensities of the Si3O3+ ion species is clearly observed at the interface region between silicon and the SiO2 film.
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K. Chiba, Y. Tsuji / Applied Surface Science 254 (2008) 5727–5731 Table 1 Comparison between the relative intensities of Si3On1N+ (n = 2–4) and Si2O3N ions normalized to the intensity of the corresponding Si3On+ and Si2O4 ion peaks from the SiO2 film (sputter time 250 s) and the Si–SiO2 interface (sputter time 620 s), respectively Source
SiO2 Si–SiO2 interface a
Fig. 4. TOF-SIMS depth profiles of SiON, Si2O3N and Si3O2N+ ions of 25-nm-thick PECVD grown SiO2 film on Si. The depth profile of Si3O3+ ions after subtraction of the influence of the SiO2 matrix is also presented. The increase in intensities of SiON, Si2O3N and Si3O2N+ ions species is clearly observed at the interface region between silicon and the SiO2 film. The profiles of Si3O2N+ and Si3O3+ after correction of the influence of the matrix are almost the same.
silicon to the SiO2 structures, are clearly observed at the Si–SiO2 interface. Fig. 4 shows the depth profiles of Si3O2N+ and Si2O3N ions for PECVD prepared SiO2 films on silicon. Intensities are normalized to the total positive or negative secondary ion counts, respectively. The depth profiles of SiON and Si3O3+ ions are also presented in Fig. 4. The depth profile of Si3O3+ ions are corrected by the matrix effect in the same manner as performed for the thermally oxidized SiO2 films shown in Fig. 3. The sharply increased peaks of Si3O2N+, Si2O3N, and SiON ions are clearly observed at the Si–SiO2 interface. The intensities of Si3O2N+ and Si3O3+ ions after the correction indicate almost the same profile distribution in this sample. These ion species containing nitrogen atoms at the interface likely arise from accumulated nitrogen at the Si–SiO2 interface for PECVD prepared SiO2 films on silicon. Possible fragment structures corresponding to the parts of fragmented pieces of PECVD-prepared SiO2 films were investigated. The intensity variation of the ion pairs of Si3On+ and Si3On1N+ (n = 2–4) species were analyzed. The intensities of peaks decreased as Si3O4+ > Si3O3+ > Si3O2+ in the SiO2 bulk films as shown in Fig. 2a. On the other hand, intensities of the Si3On+ ion species decreased as Si3O2+ > Si3O3+ > Si3O4+ at the Si–SiO2 interface, as shown in Fig. 2b. It is, however, important to note that the intensity variations of Si3On1N+ (n = 2–4) do not follow these sequences for those in the SiO2 bulk films and the interface. These results suggest that the observed secondary ion species containing nitrogen atoms may not be produced through fragment reaction of Si3On+ with nitrogen atoms on the surface or in the gas phase near the surface, but may correspond to the original respective surface structures. The intensity ratios of Si3On1N+ to Si3On+ ion species (n = 2–4) for those in the SiO2 bulk films and at the Si–SiO2 interface are listed in Table 1. The intensity ratio of Si2O3N to Si2O4 species in negative polarity is also shown in the table. The ratio of Si3O2N+ to Si3O3+ ion species is seen to be the largest among the ion species (n = 2–4) in positive polarity for both the SiO2 bulk film and the Si–SiO2 interface. However, the values for the SiO2 bulk film and the interface are different. These results suggest that ion species of Si3On1N+ (n = 2–4) observed from the SiO2 bulk films and the Si– SiO2 interface may most likely be different in the structure,
Ion species Si3 ONþ Si3 O2 þ
Si3 O2 Nþ Si3 O3 þ
Si3 O3 Nþ Si3 O4 þ
Si2 O3 N Si2 O4
–a 0.44
0.45 1.12
0.08 0.43
–a 1.02
Negligibly small.
whereas the mixed structures may also possibly exist. As previously reported assigned structures [36] for those in the SiO2 bulk films and at the Si–SiO2 interface for thermally-oxidized SiO2 films, Si3O3+ ion species from the SiO2 films and the Si–SiO2 interface are assigned to O–Si–O–Si–O–Si+ and O–Si–Si(O)– Si(O)+, respectively. These structures may lead to the assignment of Si3On1N+ (n = 2–4), Si2O2N+ and Si2O3N species for the SiO2 bulk films and the Si–SiO2 interface; the O–Si(N)–O–Si+, Si–O– Si(N)–O–Si+ and O–Si–O–Si(N)–O–Si+ structures are assigned to the Si3On1N+ (n = 2–4) ion species from the SiO2 bulk films. Fig. 5 shows the assigned structures of ion species containing nitrogen atoms for the Si–SiO2 interface. It is worth noting that the peak at the mass of 134 in negative polarity, attributed to the Si2O4N ion species, appears negligibly in the spectra at the Si–SiO2 interface, whereas Si2O5 ion species in the replacing form of nitrogen atom with oxygen show a strong peak. The possible structure of the Si2O4N ion species may be thought to be O–Si(O)2–Si(N)–O or O–Si(O)–N–Si(O)–O according to the formula. These secondary ion structures may not be sufficiently stable to exist for long or may hardly be formed. The relationship between the fragment structures and stability [38] is a subject for further study. The depth profile indicates the existence of nitrogen in the bulk films and the accumulation of nitrogen at the Si–SiO2 interface for PECVD SiO2 films on silicon. Considering rebuilding of parts of fragmented pieces of fragment ion observed in these spectra, nitrogen atoms are not incorporated physically in the films, but two types of chemical structure of oxynitrides having chemical bonds of silicon with nitrogen may exist in the SiO2 bulk film and at the Si–SiO2 interface. They are likely produced due to the reaction of reactive gas with SiO2 films and silicon surfaces where dangling bonds of silicon atoms may exist in the different form [39–41]. The molecular model of the structures
Fig. 5. TOF-SIMS spectra diagram from a 25-nm-thick PECVD grown SiO2 film on Si in the mass range from 95 to 165 amu at the Si–SiO2 interface. Possible fragment structures corresponding to Si2O2N+, Si3ON+, Si3O2N+, Si2O3N+ and Si2O3N ions are shown.
K. Chiba, Y. Tsuji / Applied Surface Science 254 (2008) 5727–5731
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Acknowledgements We thank Tohoku Semiconductor Co. for supplying silicon oxide films on silicon wafers. We are indebted to Mr. M. Kawamura of Tokushima Bunri University for his help with the TOF-SIMS measurements. References
Fig. 6. Molecular modelling of a part of the structure of silicon oxynitrides in the SiO2 bulk films and at the Si/SiO2 interface. Nitrogen atoms are linked to silicon atoms to form two types of silicon oxynitride structures. These structures were calculated and energetically minimized using the MM2 package.
with the nearest neighbours can be built and is illustrated in Fig. 6. This is calculated and energetically minimized using the MM2 force field. 4. Conclusions Secondary ion species emitted from PECVD prepared SiO2 films on silicon have been characterized with TOF-SIMS using spectral diagrams showing positive and negative ion spectra. Comparative studies of PECVD SiO2 films prepared using a mixture of SiH4/N2O reaction gas at 400 8C with thermally grown SiO2 films have been performed in the mid-range mass spectra from 95 to 165 amu. Small amounts of ion species containing nitrogen atoms, Si2O2N+, Si3O2N+and Si3O3N+, were detected in the SiO2 bulk for PECVD SiO2 films. Additionally, large amounts of Si3O2N+ and Si2O3N were found at the Si–SiO2 interface for PECVD SiO2 films. Depth analysis showed that intensity peak shapes of these ion species at the interface were closely coincident with those of Si3O3+ corrected by subtraction of the influence of the SiO2 matrix. Spectral variations of these ion species, reflecting a pattern of fragmented pieces of material, clearly indicated that two structural types of oxynitride exist for the PECVD SiO2 films, in the SiO2 films and at the interface. Nitrogen atoms do not exist physically incorporated into the films, but rather they bond to silicon atoms to form oxynitrides at the nanoscale. They are likely produced due to the reaction of a reactive gas with SiO2 and silicon surfaces where dangling bonds of silicon atoms exist in the different form.
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Secondary ion species containing nitrogen atoms from plasma-enhanced chemical vapor deposited silicon oxide films on silicon Kiyoshi Chiba *, Yoshinori Tsuji Department of Nano Material and Bio Engineering, Tokushima Bunri University, Sanuki, Kagawa 769-2193, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 October 2007 Received in revised form 3 March 2008 Accepted 8 March 2008 Available online 16 March 2008
Secondary ion species from plasma-enhanced chemical vapor deposited (PECVD) SiO2 films have been investigated using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Comparative studies of PECVD SiO2 films prepared using a mixture of SiH4/N2O reaction gas at 400 8C with thermally oxidized SiO2 films grown at 900 8C were carried out in the mid-range mass spectra from 95 to 165 amu. Small amounts of ion species containing nitrogen atoms, including Si2O2N+, Si3O2N+and Si3O3N+, were detected in the SiO2 bulk from the PECVD SiO2 films. Furthermore, large amounts of Si3O2N+ and Si2O3N were found at the interface between silicon and the SiO2 films. Depth analysis showed that the intensity peak shapes of these ion species containing nitrogen atoms at the interface were closely coincident with those of Si3O3+ corrected by subtracting the influence of the SiO2 matrix. The variation in the spectra of these ion species clearly indicates that two types of structures of oxynitride exist for the PECVD SiO2 films in the SiO2 bulk films and at the interface. These are likely produced by the reaction of reactive gas with SiO2 and silicon surfaces where dangling bonds of silicon may exist in the different form. ß 2008 Elsevier B.V. All rights reserved.
PACS: 68.49.Sf 68.55.Jk 73.40.Qv 77.55.+f Keywords: Silicon oxide Nitrogen incorporation Oxynitride Secondary ion species Time-of-flight secondary ion mass spectrometry
1. Introduction Silicon oxynitride films have attracted increased interest recently as ultra-thin dielectric films on silicon for gate oxides in the miniaturization of semiconductor devices [1–3], optical wave guide films for optoelectronic devices [4,5] and gas barrier films to protect against gas diffusion through SiO2 films [6,7]. For fabrication of metal-oxide-semiconductor (MOS) devices, nitrogen is incorporated into SiO2 films using either thermal oxynitridation or annealing, or chemical and physical deposition methods [1,8– 18]. The incorporation of nitrogen into thin SiO2 films improves the electrical and structural characteristics of the oxides. In addition, it results in increased protection against boron penetration and impurities through the gate dielectric, as well as low density of surface states [19–23]. Accumulation of the incorporated nitrogen at the Si–SiO2 interface often occurs and has been reported by a number of authors [1,11,16–18,24]. SiO2 films prepared in N2O gas during plasma processing are particularly attractive because they
* Corresponding author. Tel.: +81 87 894 5111; fax: +81 87 894 4201. E-mail address: [email protected] (K. Chiba). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.03.037
allow for the incorporation of a small but significant amount of nitrogen near the Si–SiO2 interface at low temperatures [1]. The characterization of the incorporated nitrogen in SiO2 films at the nanoscale is key to yield further knowledge of the chemical and structural features of these films and to pursue tailored nitrogen profiles in ultra thin dielectrics. A number of characterization techniques [9,25,26], including Fourier transform infrared (FTIR) spectroscopy [18,27], Rutherford backscattering spectrometry (RBS) [25,27], Auger electron spectroscopy (AES) [9,10,15–17], X-ray photoelectron spectroscopy (XPS) [9,25] and secondary ion mass spectrometry (SIMS) [9,11,12,28–34], have been reported thus far to investigate the behavior of nitrogen atoms incorporated into SiO2 films. Although FTIR provides information on lattice vibrations of the molecular structure, a film thickness exceeding approximately 100 nm is necessary to obtain an adequate signal. In this case, information on atomic layers cannot be obtained, except that of average structures through the films. Among the above methods, SIMS can provide information on several atomic layers of the films with high sensitivity, whereas either the depth resolution or sensitivity is limited for the other methods. SIMS has frequently been applied to the study of depth profiles of nitrogen in SiO2 films, for example using N+ secondary ions with Cs+ primary ions
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[12,14,28–31], SiN+ ions with O2 primary ions [9], and SiN ions with Cs+ primary ions [32]. In these studies, SIMS was used in dynamic mode. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a useful tool to characterize solid structures by detection of secondary ion species emitted from solid materials using pulsed mode of bombardment of primary ions associated with high sensitivity and low surface damage [35]. TOF-SIMS has also been applied to the study of silicon oxynitride films using secondary Si+ matrix ions to cationize neutral molecular and elemental species, and to detect the Si2N+ ion with a Ga+ primary ion [25,33]. These SIMS studies have clarified the depth profile of nitrogen, particularly the accumulation of nitrogen at the Si–SiO2 interface. However, further information on the bonding structure of species in the films as well as at or near the Si–SiO2 interface could not be obtained because the secondary ion species detected in these studies were very small or elementally fragmented mass species. In TOF-SIMS, the identification and understanding of characteristic features in the mid-range of the mass spectra showing the fragmentation patterns is key to relate them to structural features of materials at the nanoscale [36,37]. These spectra show various parts of the fragmented mass species of the SiO2 films which can be rebuilt to original structures. In this paper, we report the characterization of secondary ion species containing nitrogen atoms emitted from PECVD-grown SiO2 films on silicon with TOF-SIMS using the Au3+ primary ion. We used spectra in both positive and negative polarities in the midrange of the mass spectra, and identified ion species of Si2O2N+, Si3ON+, Si3O2N+, Si3O3N+ and Si2O3N in the SiO2 bulk films and at the Si–SiO2 interface.
MM2 molecular mechanics calculations of the Si–O–N structure were carried out using the program package from Molecular Design Limited. 3. Results and discussion Fig. 1 shows the TOF-SIMS spectra of thermally oxidized SiO2 films on silicon in the mass range from 95 to 165 amu. The spectra are presented in the form of a diagram showing both positive (up) and negative (down) polarities [36]. This diagram shows the spectra of fragment ion species of both positive and negative charge ions simultaneously. Various parts of the fragmented pieces of the SiO2 matrix films can be seen in the mid-range mass spectra from 95 to 165 amu with adequate sensitivity. The series of Si3On+ (n = 2–5) and Si2O3+ ion peaks as parts of fragmented pieces of the SiO2 matrix clearly appear from the SiO2 film (sputter time 250 s, 10-nm depth from the surface) in Fig. 1a. They are assigned to Si3O2+, Si3O3+, Si3O4+, Si4O5+ and Si2O3+, respectively [36]. The intensities of the Si3On+ ion species decreased as Si3O4+ > Si3O3+ > Si3O2+. Fig. 1b shows the spectral diagram from the interface between silicon and the SiO2 film (sputter time 620 s). Silicon cluster ion species of Si4+ and Si5+ are observed in Fig. 1b. These silicon cluster ions most likely arise from the silicon substrate surface. Comparing the spectra in Fig. 1b with those in Fig. 1a, the intensities of Si3O+, Si3O2+, Si3O3+, Si3O4+ and Si3O5+ peaks at the Si–SiO2 interface significantly differed from the
2. Experimental Thermally oxidized 25-nm-thick SiO2 films as well as plasmaenhanced CVD-prepared SiO2 films on single-crystal n-type (100) silicon wafers (resistivity of 4–6 V cm) were used in this study. The thermally oxidized SiO2 films were prepared on silicon wafers using a gas mixture of O2 and HCl (10%) at 900 8C after pre-cleaning of the wafer surface with a mixture solution of trimethyl-2hydroxyethylammonium hydroxide (choline) and hydrogen peroxide (H2O2). Plasma-enhanced CVD-prepared SiO2 films were grown from SiH4 and N2O mixtures at a flow rate ratio of 1:18 at 400 8C after pre-cleaning of the wafer with dilute HF and a mixture solution of choline and H2O2 without a post anneal. TOF-SIMS measurements were performed with an ULVAC-PHI TRIFT III TOF-SIMS spectrometer. Sputter etching was accomplished by Ar+ ions at 1 kV and 200 nA rastering over a 1000 mm 1000 mm area. The corresponding etching rate was approximately 0.04 nm/s for the SiO2 films. Spectral data were obtained at an etching time of 50 and 250 s in the SiO2 layer and intervals of 10 s from 450 to 750 s near the Si/SiO2 interface. TOFSIMS analyses were performed in positive and negative polarities using a pulsed Au3+ primary ion beam operated at 22 kV and 2.0 nA. It should be noted that a bias voltage was applied to the sample holder in this instrument of +3 kV and 3 kV for measurements of positive and negative secondary ions, respectively. This gives rise to a difference in the actual acceleration voltage of the primary ion, namely, 19 kV and 25 kV for measurements of positive and negative secondary ions, respectively. The beam was rastered on a 100 mm 100 mm surface area. Secondary ions in the mass range from 0 to 1860 amu were collected for 300 s. No charge compensation using an electron flood gun was required. The working pressure was in the 107 Pa range. Identification of the peaks of the secondary ion species was carried out using the mass of the peaks to an accuracy within 0.01 amu.
Fig. 1. TOF-SIMS spectra diagram from a 25 nm-thick thermally grown SiO2 film on Si showing both positive (up) and negative (down) polarities in the mass range from 95 to 165 amu: (a) 10-nm depth from the surface (sputter time 250 s) and (b) the SiSiO2 interface (sputter time 620 s). Possible fragment ion species are shown. Note that the increase in intensities of Si3O+, Si3O2+ and Si3O3+ ion species is clearly observed at the Si–SiO2 interface.
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spectra of the SiO2 bulk films. The intensities of the Si3On+ ion species decreased as Si3O2+ > Si3O3+ > Si3O4+. It has been reported that Si3O+, Si3O2+ and Si3O3+ ion species observed at the interface are not as same as those emitted from SiO2 [36]. Fig. 2 shows the TOF-SIMS spectral diagram from PECVD prepared SiO2 films on silicon in the mass range from 95 to 165 amu. The spectra at a 10-nm depth from the surface, that is, in the SiO2 bulk film, is shown in Fig. 2a. Comparing the spectra in
Fig. 2. TOF-SIMS spectra diagram from a 25-nm-thick PECVD grown SiO2 film on Si in the mass range from 95 to 165 amu: (a) 10-nm depth from the surface (sputter time 250 s), (b) the Si–SiO2 interface (sputter time 620 s) and (c) silicon substrate (sputter time 850 s). Small amounts of Si2O2N+, Si3O2N+ and Si3O3N+ ions appear in the spectra from the 10-nm depth surface. These ion species suggest that nitrogen atoms are incorporated in the PECVD grown SiO2 bulk films and the interface. Strong peaks of Si3O2N+ and Si2O3N ions are seen at the Si–SiO2 interface and are marked by an asterisk. The variation in intensity ratios of Si3On1N+ (n = 2–4) to those of Si3On+ are noted. No trace of oxide or oxynitride ion species is observed in the Si substrate.
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Fig. 2a with those in Fig. 1a, the spectral pattern of Si3On+ (n = 2–5) and Si2O3+ ion peaks from PECVD SiO2 films is almost the same as that from the 10-nm depth surface of thermally-oxidized SiO2 films. It is, however, to be noted that very small peaks at masses of 102, 130 and 146 were detected for the PECVD-prepared SiO2 films in Fig. 2a. These peaks were assigned to Si2O2N+, Si3O2N+ and Si3O3N+, respectively. These peaks indicate that nitrogen atoms are incorporated into the SiO2 bulk film. Fig. 2b shows the spectral diagram from the Si–SiO2 interface for PECVD-prepared SiO2 films (sputter time 620 s). The intensities of some peaks at the Si–SiO2 interface significantly differed from the spectra of the SiO2 bulk films. An anomalous increase in the intensity of the peaks at masses of 130 and 118, attributed, respectively, to Si3O2N+ and Si2O3N ion species, was observed at the Si–SiO2 interface. The intensity of the peaks at the mass of 102, assigned to Si2O2N+, increased approximately twice at the interface. The peak at the mass of 114, attributed to Si3ON+ ion species, appeared in the spectra at the Si–SiO2 interface. The intensity ratio between the peaks of Si3O3+ and Si3O2N+ at the interface significantly differed from that at 10-nm depth in the SiO2 bulk film. These spectra suggest that Si2O2N+, Si3ON+, Si3O2N+, Si3O3N+ and Si2O3N ion species observed at the interface are different from those from the SiO2 bulk film. Fig. 2c shows the spectra from the silicon substrate (sputter time 850 s). Only cluster ion species of Si4+ and Si5+ are found in Fig. 2c. This spectrum suggests that residual gas containing nitrogen atoms in the analytical chamber influences the spectra to a negligible extent. The depth profiles of secondary ion species were also examined. Fig. 3 shows the depth profiles of Si3O3+ and Si3O4+ ions for thermally oxidized SiO2 films on silicon. The intensities are normalized to the total positive or negative secondary ion counts, respectively. It is worth noting that an intensity ratio of Si3O3+ to Si3O4+ ions was approximately 0.3–0.4 in the SiO2 bulk films. This ratio suggests the value of the fragment ratio of Si3O3+ to Si3O4+ decomposed from the SiO2 matrix. Considering this ratio of 0.4 as the correction factor for the intensity of the Si3O3+ ion species at the Si–SiO2 interface, the depth profile of the Si3O3+ ion was corrected by subtracting the influence factor of the SiO2 matrix; in other words, these are the values of the intensity of Si3O4+ multiplied by 0.4. The corrected depth profile of the Si3O3+ ion species is also shown in Fig. 3. Sharply increased peaks of Si3O3+ ions, which most likely correspond to a transition region from
Fig. 3. TOF-SIMS depth profiles of Si3O3+ and Si3O4+ of a 25-nm-thick thermally grown SiO2 film on Si. The depth profile of Si3O3+ ions after subtraction of the influence of the SiO2 matrix is also shown. The sputter etching rate is approximately 0.04 nm/s for the SiO2 films. The increase in intensities of the Si3O3+ ion species is clearly observed at the interface region between silicon and the SiO2 film.
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K. Chiba, Y. Tsuji / Applied Surface Science 254 (2008) 5727–5731 Table 1 Comparison between the relative intensities of Si3On1N+ (n = 2–4) and Si2O3N ions normalized to the intensity of the corresponding Si3On+ and Si2O4 ion peaks from the SiO2 film (sputter time 250 s) and the Si–SiO2 interface (sputter time 620 s), respectively Source
SiO2 Si–SiO2 interface a
Fig. 4. TOF-SIMS depth profiles of SiON, Si2O3N and Si3O2N+ ions of 25-nm-thick PECVD grown SiO2 film on Si. The depth profile of Si3O3+ ions after subtraction of the influence of the SiO2 matrix is also presented. The increase in intensities of SiON, Si2O3N and Si3O2N+ ions species is clearly observed at the interface region between silicon and the SiO2 film. The profiles of Si3O2N+ and Si3O3+ after correction of the influence of the matrix are almost the same.
silicon to the SiO2 structures, are clearly observed at the Si–SiO2 interface. Fig. 4 shows the depth profiles of Si3O2N+ and Si2O3N ions for PECVD prepared SiO2 films on silicon. Intensities are normalized to the total positive or negative secondary ion counts, respectively. The depth profiles of SiON and Si3O3+ ions are also presented in Fig. 4. The depth profile of Si3O3+ ions are corrected by the matrix effect in the same manner as performed for the thermally oxidized SiO2 films shown in Fig. 3. The sharply increased peaks of Si3O2N+, Si2O3N, and SiON ions are clearly observed at the Si–SiO2 interface. The intensities of Si3O2N+ and Si3O3+ ions after the correction indicate almost the same profile distribution in this sample. These ion species containing nitrogen atoms at the interface likely arise from accumulated nitrogen at the Si–SiO2 interface for PECVD prepared SiO2 films on silicon. Possible fragment structures corresponding to the parts of fragmented pieces of PECVD-prepared SiO2 films were investigated. The intensity variation of the ion pairs of Si3On+ and Si3On1N+ (n = 2–4) species were analyzed. The intensities of peaks decreased as Si3O4+ > Si3O3+ > Si3O2+ in the SiO2 bulk films as shown in Fig. 2a. On the other hand, intensities of the Si3On+ ion species decreased as Si3O2+ > Si3O3+ > Si3O4+ at the Si–SiO2 interface, as shown in Fig. 2b. It is, however, important to note that the intensity variations of Si3On1N+ (n = 2–4) do not follow these sequences for those in the SiO2 bulk films and the interface. These results suggest that the observed secondary ion species containing nitrogen atoms may not be produced through fragment reaction of Si3On+ with nitrogen atoms on the surface or in the gas phase near the surface, but may correspond to the original respective surface structures. The intensity ratios of Si3On1N+ to Si3On+ ion species (n = 2–4) for those in the SiO2 bulk films and at the Si–SiO2 interface are listed in Table 1. The intensity ratio of Si2O3N to Si2O4 species in negative polarity is also shown in the table. The ratio of Si3O2N+ to Si3O3+ ion species is seen to be the largest among the ion species (n = 2–4) in positive polarity for both the SiO2 bulk film and the Si–SiO2 interface. However, the values for the SiO2 bulk film and the interface are different. These results suggest that ion species of Si3On1N+ (n = 2–4) observed from the SiO2 bulk films and the Si– SiO2 interface may most likely be different in the structure,
Ion species Si3 ONþ Si3 O2 þ
Si3 O2 Nþ Si3 O3 þ
Si3 O3 Nþ Si3 O4 þ
Si2 O3 N Si2 O4
–a 0.44
0.45 1.12
0.08 0.43
–a 1.02
Negligibly small.
whereas the mixed structures may also possibly exist. As previously reported assigned structures [36] for those in the SiO2 bulk films and at the Si–SiO2 interface for thermally-oxidized SiO2 films, Si3O3+ ion species from the SiO2 films and the Si–SiO2 interface are assigned to O–Si–O–Si–O–Si+ and O–Si–Si(O)– Si(O)+, respectively. These structures may lead to the assignment of Si3On1N+ (n = 2–4), Si2O2N+ and Si2O3N species for the SiO2 bulk films and the Si–SiO2 interface; the O–Si(N)–O–Si+, Si–O– Si(N)–O–Si+ and O–Si–O–Si(N)–O–Si+ structures are assigned to the Si3On1N+ (n = 2–4) ion species from the SiO2 bulk films. Fig. 5 shows the assigned structures of ion species containing nitrogen atoms for the Si–SiO2 interface. It is worth noting that the peak at the mass of 134 in negative polarity, attributed to the Si2O4N ion species, appears negligibly in the spectra at the Si–SiO2 interface, whereas Si2O5 ion species in the replacing form of nitrogen atom with oxygen show a strong peak. The possible structure of the Si2O4N ion species may be thought to be O–Si(O)2–Si(N)–O or O–Si(O)–N–Si(O)–O according to the formula. These secondary ion structures may not be sufficiently stable to exist for long or may hardly be formed. The relationship between the fragment structures and stability [38] is a subject for further study. The depth profile indicates the existence of nitrogen in the bulk films and the accumulation of nitrogen at the Si–SiO2 interface for PECVD SiO2 films on silicon. Considering rebuilding of parts of fragmented pieces of fragment ion observed in these spectra, nitrogen atoms are not incorporated physically in the films, but two types of chemical structure of oxynitrides having chemical bonds of silicon with nitrogen may exist in the SiO2 bulk film and at the Si–SiO2 interface. They are likely produced due to the reaction of reactive gas with SiO2 films and silicon surfaces where dangling bonds of silicon atoms may exist in the different form [39–41]. The molecular model of the structures
Fig. 5. TOF-SIMS spectra diagram from a 25-nm-thick PECVD grown SiO2 film on Si in the mass range from 95 to 165 amu at the Si–SiO2 interface. Possible fragment structures corresponding to Si2O2N+, Si3ON+, Si3O2N+, Si2O3N+ and Si2O3N ions are shown.
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Acknowledgements We thank Tohoku Semiconductor Co. for supplying silicon oxide films on silicon wafers. We are indebted to Mr. M. Kawamura of Tokushima Bunri University for his help with the TOF-SIMS measurements. References
Fig. 6. Molecular modelling of a part of the structure of silicon oxynitrides in the SiO2 bulk films and at the Si/SiO2 interface. Nitrogen atoms are linked to silicon atoms to form two types of silicon oxynitride structures. These structures were calculated and energetically minimized using the MM2 package.
with the nearest neighbours can be built and is illustrated in Fig. 6. This is calculated and energetically minimized using the MM2 force field. 4. Conclusions Secondary ion species emitted from PECVD prepared SiO2 films on silicon have been characterized with TOF-SIMS using spectral diagrams showing positive and negative ion spectra. Comparative studies of PECVD SiO2 films prepared using a mixture of SiH4/N2O reaction gas at 400 8C with thermally grown SiO2 films have been performed in the mid-range mass spectra from 95 to 165 amu. Small amounts of ion species containing nitrogen atoms, Si2O2N+, Si3O2N+and Si3O3N+, were detected in the SiO2 bulk for PECVD SiO2 films. Additionally, large amounts of Si3O2N+ and Si2O3N were found at the Si–SiO2 interface for PECVD SiO2 films. Depth analysis showed that intensity peak shapes of these ion species at the interface were closely coincident with those of Si3O3+ corrected by subtraction of the influence of the SiO2 matrix. Spectral variations of these ion species, reflecting a pattern of fragmented pieces of material, clearly indicated that two structural types of oxynitride exist for the PECVD SiO2 films, in the SiO2 films and at the interface. Nitrogen atoms do not exist physically incorporated into the films, but rather they bond to silicon atoms to form oxynitrides at the nanoscale. They are likely produced due to the reaction of a reactive gas with SiO2 and silicon surfaces where dangling bonds of silicon atoms exist in the different form.
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