Chemical bonding structures of silicon oxynitride films grown by ionised N 2 and pure O 2 gas mixtures at low temperature

Introduction

As the geometries of metal oxide semiconductor field effect transistors have been scaled down to nanometre regime, Si oxynitride (SiO_xN_y) film has been considered as a viable candidate for the alternative gate dielectric without sacrificing the productivity required by semiconductor device manufacturers. The main advantages of SiO_xN_y film over traditional SiO₂ gate dielectric include the low leakage current injection, strong immunity of charge trapping and interface state generation caused by hot carrier stressing and the suppression of boron penetration from p+ poly‐Si gates in to underlying channel region.1 Generally, the oxynitridation technique using NH₃, NO or N₂O ambient2^–4 has been widely used to grow SiO_xN_y film. The NH₃ nitridation results in the incorporation of N at the interface between Si and SiO₂, and leads to the improvement of dielectric integrity and reliability. However, the excessive H incorporation into growing film may cause electron traps, which is responsible for the degradation of gate dielectric.5 To avoid such a problem, the reoxidation process at relatively high temperature is required. On the other hand, the nitridation using NO or N₂O ambient is effective in the formation of SiO_xN_y film without the problem related to trap generation associated with H incorporation. However, since the amount of N incorporated into gate dielectric is extremely small (<1 at‐%), the obstruction of B penetration from p+ poly‐Si gates into underlying channel region is insufficient. Recently, Chang et al. demonstrated that rapid thermal annealing in the mixture of N₂ and O₂ is an effective approach for the formation of N enriched SiO_xN_y film. They also showed that the film exhibited low leakage levels, a large charge to breakdown, and low charge trapping, which is applicable for advanced CMOS devices. However, relatively high growth temperature (900°C) is required for the reaction between N₂–O₂ gas mixture and Si, leading to the formation of stable N enriched SiO_xN_y film. Lowering the film processing thermal budget allows for improved process margin and the reduced manufacturing cost. Nevertheless, the attempts to reduce growth temperature for the SiO_xN_y film formed using thermal treatment in the mixture of N₂ and O₂ were not investigated extensively. Especially, the bonding nature of SiO_xN_y film can give a significant effect on its dielectric properties. For example, the dielectric constant of SiO_xN_y layer is proportional to the amount of bonding between Si and N atoms.6 On the other hands, the decrease in the refractive index of SiO_xN_y film caused by excess Si–O bond leads to the degradation of the dielectric properties, which results in increases in the trap density and leakage current.7 Furthermore, N atoms in the form of random bonding to Si atoms minimise the fluctuations of composition, permittivity and bandgap at the interface and improve the reliability of metal oxide semiconductor devices.8 Namely, for further development of SiO_xN_y film, the detailed knowledge about its chemical bonding structure is of importance. In this work, we have fabricated the SiO_xN_y film using thermal oxynitridation of Si in the ionised N₂ and pure O₂ gas mixture at relatively low temperature (600°C) and investigated the effects of process conditions on its chemical structure using in situ X‐ray photoelectron spectroscopy (XPS).

Experimental procedures

An ultrahigh vacuum system comprising both growth and analysis chambers accessible through in‐vacuum sample transfer was used to perform a deposition of SiO_xN_y layers. The base pressure of the chambers was 1·0×10⁻⁹ torr. The sample cut from the p‐type (100) Si wafer was first cleaned using standard RCA cleaning followed by dipping in a diluted HF solution to remove the native SiO₂ which is spontaneously formed during air exposure of a bare Si wafer. Before the deposition of SiO_xN_y film, the thermal annealing was carried out at 1000°C for 5 min in a growth chamber in order to obtain a clean surface, which was confirmed by XPS. The sample was then exposed to gas mixture of ionised N₂ and pure O₂ gases at the temperature of 600°C for 30–180 min. The ionised N₂ gas was introduced by the discharging type AG5000 cold cathode ion gun (VG Microtech, East Grinstead, UK). The mixing ratio of ionised N₂ and pure O₂ gases (denoted as /O₂) varied from 25∶1 to 100∶1. The exposing pressure was maintained at 5·0×10⁻⁶ torr. Finally, for in situ XPS measurements, the sample was transferred to the analysis chamber. For XPS measurements, the authors used VG Microtech CLAM2 electron analyser and the Mg K_α (hv = 1253·6 eV) as a radiation source. The binding energies of photoelectron spectra were calibrated by the position of Au 4f_7/2 core level (84·0 eV). Especially, the authors performed XPS measurements several times for each sample. All XPS spectra showed the almost identical behaviour, implying that measurement error is negligible.

Results and discussion

Figure 1 shows the O 1s, N 1s, and Si 2p core level XPS spectra taken from the films formed using various /O₂ mixing ratio at the growth temperature of 600°C for 30 min. It is apparent that the binding energies of O 1s, N 1s and Si 2p are virtually the same regardless of /O₂ mixing ratio, although the details are somewhat different. For example, the peak positions of O 1s and N 1s are measured to be 532·6 and 397·9 eV respectively. As for Si 2p core level XPS spectra, in addition to dominant bulk Si 2p peak (99·3 eV), there is a shoulder at 102·5 eV, indicated by an arrow. Such a presence of the shoulder directly reveals the formation of SiO_xN_y film in the present samples at relatively low growth temperature (600°C).9^–11 Furthermore, the thickness of SiO_xN_y film was estimated using sampling depth and areal ratio between SiO_xN_y and pure Si peak. Namely, the total sampling depth d of the XPS measurement by a first approximation is given by9 (1) where λ and θ are the electron effective attenuation length and the angle between the photoelectron emission direction and the plane of the sample surface respectively. The λ represents 2·96 nm for the Si 2p photoelectron excited by X‐rays, and θ is 54° for a common XPS configuration.12,13 Based on the sampling depth combined with the areal ratio between SiO_xN_y and pure Si peak, the thickness was approximately calculated to be less than 2 nm.

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Figure 1

a Si 2p, b O 1s and c N 1s core level XPS spectra taken from SiO_xN_y film formed using various /O₂ mixing ratios at growth temperature of 600°C for 30 min

To investigate the detail of the chemical structure of SiO_xN_y film, a Gaussian–Lorentzian function combined with the Shirley method of background was employed to deconvolute N 1s spectrum obtained from the SiO_xN_y film formed using oxynitridation of Si with /O₂ mixing ratio of 25∶1 at the growth temperature of 600°C for 180 min. The convolution clearly resolves that the N 1s spectrum consists of three different Gaussian peaks, as shown in Fig. 2. This indicates that N atoms in the present film have at least three different chemical states owing to its different bonding configurations to adjacent atoms. The peaks are centred at 397·5, 398·0 and 398·8 eV, which are labelled as N‐3, N‐2 and N‐1 respectively. The N‐3 peak corresponds to typical N≡Si₃ bond in Si₃N₄, which are derived from each N atom bound to three Si atoms.3,14,18 The peak location of N‐2 (398·0 eV) is close to the binding energy value reported for N–(SiO_x)₃ bond, implying the dispersion of N atoms within the SiO₂ matrix.13^–18 A relatively weak N‐1 peak at 398·8 eV could be associated with the N = Si₂–O bonding structure.16^–20 It should be noted that unlike SiO_xN_y films formed using N₂O gas or plasma oxyntridation process,2,3 direct bond between N and O atoms, showing 2·0–2·5 eV higher binding energy shift than that in the bulky N species, is not visible in present films. This implies that the oxidation and nitridation processes sequentially occur at Si–Si dangling bond on the Si surface during SiO_xN_y growth using the direct oxynitridation of Si in the ionised N₂ and pure O₂ gas mixture. In other words, as considering a model of oxynitride growth proposed by Saito,21 the spontaneous addition of O atoms between Si atoms caused by the reaction of oxidation species with one of the Si–Si back bonds at SiO₂/Si interface leads to the induction of stress in the remaining Si–Si back bond, followed by the incorporation of N atoms into the unstable back bond.

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Figure 2

Curve fitted N 1s spectra obtained from SiO_xN_y film formed using oxynitridation of Si with /O₂ mixing ratio of 25∶1 at growth temperature of 600°C for 180 min: dot symbols and solid lines represent measured and fitted data respectively

Figure 3a shows the plots of surface concentrations of N‐1, N‐2 and N‐3 bonds in SiO_xN_y films formed using /O₂ mixing ratio of 25∶1 at the temperature of 600°C as a function of growth time. The plots clearly reveal that with increasing growth time, the surface concentrations of N‐3 and N‐2 bonds gradually decrease and increase respectively. However, the change in the surface concentration of N‐1 bond is insignificant regardless of growth time. This indicates that as increasing growth time, N‐3 bonds, which dominate the SiO_xN_y film at the initial growth stage, would break to form N‐2 ones during SiO_xN_y film growth. This is comparable to previous reports.21^–23 In other words, N‐2 bonds are more stable bonding configuration than N‐3 ones as increasing growth time. Furthermore, as shown in Fig. 3b, relative N concentrations in SiO_xN_y film (represented as N/[N+O] or O/[N+O]), calculated from the integrated areas of O 1s and N 1s core level XPS spectra using atomic sensitivity factors (ASF: N 1s = 0·477 and O 1s = 0·711),24 exhibits that the increase in the growth time leads to the increase and decrease in the concentration of N and O atoms respectively. Such a different behaviour of relative concentrations of N and O atoms implies that the ionised N₂ molecules are more reactive than pure O₂ ones even though SiO_xN_y growth temperature is relatively low (600°C). On the other hand, the increase in growth time is effective in the formation of N rich SiO_xN_y film.

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Figure 3

Plots of a surface concentrations of N‐1, N‐2 and N‐3 bonds and b relative N concentrations in SiO_xN_y films formed using /O₂ mixing ratio of 25∶1 at temperature of 600°C as function of growth time

Figure 4 represents growth temperature dependence of the surface concentrations of N‐1, N‐2, and N‐3 bonds in films formed using /O₂ mixing ratio of 25/1 for growth time of 60 min. With increasing growth temperature, the surface concentrations of both N‐3 and N‐2 bonds continuously decrease, while the surface concentration of N‐1 bond increases. This implies that the increase in growth temperature results in the breakdown of N‐3 and N‐2 bonds, followed by the transformation into N‐1 bond. Similar to the plot of relative N concentration in SiO_xN_y film as a function of growth time shown in Fig. 3b, with increasing growth temperature, N/[N+O] and O/[N+O] increase and decrease respectively. However, O 1s, N 1s and Si 2p core level XPS spectra taken from the corresponding SiO_xN_y films (Fig. 5) clearly reveal that the growth temperature of 900°C leads to the significant reduction in the intensities of O 1s, N 1s and a shoulder in Si 2p spectra. This implies that the desorption of N atoms in the nitride related compositions such as Si₃N₄ or SiO_xN_y preferentially occurs at relatively high growth temperature (900°C).

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Figure 4

Plots of a surface concentrations of N‐1, N‐2 and N‐3 bonds and b relative N concentrations in SiO_xN_y films formed using /O₂ mixing ratio of 25∶1 for growth time of 60 min as function of growth temperature

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Figure 5

a Si 2p, b O 1s and c N 1s core level XPS spectra taken from films formed using /O₂ mixing ratio of 25∶1 at growth temperature of 900°C for 30 min

Conclusion

The authors have fabricated the SiO_xN_y films using the direct oxynitridation of Si in the /O₂ mixed gas and investigated the chemical bonding states of these films via XPS. At the initial growth stage at the temperature of 600°C, N≡Si₃ bonds are mainly formed in the SiO_xN_y film. The change of N = Si₂–O bond is insignificant regardless of the growth time. However, with increasing growth time, the surface concentrations of N–(SiO_x)₃ and N≡Si₃ bonds increase and decrease respectively. The absence of N–O bonds indicates nitridation and oxidation processes proceed independently. With increasing growth temperature, both N–(SiO_x)₃ and N≡Si₃ bonds are broken and eventually transformed into N = Si₂–O bonding structure.