Local order structure of a-SiOxNy:H grown by PECVD

Abstract

In this work we study the structural properties of amorphous oxynitride films (a-SiOxNy), grown by plasma enhanced chemical vapour deposition (PECVD)at 320° C. The films were deposited at different ow ratio of N2O and SiH4. The atomic composition of the samples was determined by means of Rutherford backscattering spectrometry (RBS). The local order structure was studied by X-ray absorption spectroscopy (XAS) and the chemical bondings were investigated by Fourier transform infrared (FTIR) spectroscopy. The results evidence a tetrahedric arrangement of the oxynitride network. The tetrahedrons are similiar to SiO3N for x values between 1.43 and 1.64. For x values higher than 1.64 the tetrahedrons are similar to SiO4.

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Local Order Structure of a-SiO_xN_y:H Grown by PECVD

W.L. Scopel, M.C.A. Fantini,

Instituto de Física, USP

M.I. Alayo and I. Pereyra

Departamento de Engenharia de Sistemas Eletrônicos, EPUSP

13.081-970 São Paulo, SP, Brazil

Received on 23 April, 2001

In this work we study the structural properties of amorphous oxynitride films (a-SiO_xN_y), grown by plasma enhanced chemical vapour deposition (PECVD)at 320ºC. The films were deposited at different ow ratio of N₂O and SiH₄. The atomic composition of the samples was determined by means of Rutherford backscattering spectrometry (RBS). The local order structure was studied by X-ray absorption spectroscopy (XAS) and the chemical bondings were investigated by Fourier transform infrared (FTIR) spectroscopy. The results evidence a tetrahedric arrangement of the oxynitride network. The tetrahedrons are similiar to SiO₃N for x values between 1.43 and 1.64. For x values higher than 1.64 the tetrahedrons are similar to SiO₄.

The Plasma Enhanced Chemical Vapour Deposition (PECVD) method has been extensively applied to obtain silicon based dielectric films. In particular, the use of SiO₂ and SiO_xN_y films, grown by PECVD at low temperature, became very attractive due to the possibility of integrating optical and electrical devices in the same chip[1,2,3]. Moreover, PECVD is a useful technique to deposit thick films, about 4-5 (mm), a common requirement in optical device technologies. The fundamental advantage of this technique is the possibility of controlling the chemical composition, as well as the structural and optical properties of the silicon oxynitride films, by adequately adjusting the deposition parameters[4,5,6]. Our previous works[4,7,8,9] established the experimental conditions to grow a-SiO_xN_y:H films at high deposition rates, having a continuous variation in its refractive index, with controlled thickness, selected chemical bonds and improved morphology. In particular, high quality, low temperature a-SiO₂ films were deposited.

The aim of this work was to determine the local atomic structure around the Si atoms of our good quality a-SiO_xN_y:H films as a function of their chemical composition. The atomic content was determined by Rutherford backscattering spectrometry (RBS); the chemical and structural properties were investigated by extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES) and Fourier transform infrared spectroscopy (FTIR).

The RBS experiments, used to determine the film's composition, were performed at LAMFI/USP, São Paulo, using a He⁺ beam with energy E=1.7 MeV, charge Q=30 mC, current I= 30 nA and detection angle q =170 ^º. The RBS data were analyzed through the RUMP routine[10] and provided the Si, N and O atomic density (atoms.cm^-2).

The analysis of the chemical bonds was performed by means of the Fourier transform infrared (FTIR) spectroscopy in a BioRad FTS-40 spectrometer, on films deposited over polycrystalline silicon wafer, with high resistivity. The films were measured at room temperature in a nitrogen atmosphere, with a resolution of 4 cm^-1. The H content in the films was calculated using the FTIR N-H signal. It decreases from 5 at. % in the most nitrogenated film to zero in films without detectable amount of H.

The XAS (X-ray absorption spectroscopy) experiments were carried out at the SXS beamline[11] of the Synchrotron Light National Laboratory (LNLS, Campinas, Brazil). The measurements were performed in the energy range of (1800-2300) eV, using a double-crystal InSb(111) monochromator. The EXAFS spectra of the a-SiO_xN_y films, a-SiO₂ and b-Si₃N₄ reference samples were recorded at the Si K-edge, at room temperature, being collected in TEY mode, with 1 eV step. Further, more accurate measurements were done in the near absorption edge (XANES) region, in the (1840-1870) eV energy range, with 0.2 eV step.

Table I shows the RBS data, i.e, reveals the concentration ratio [O/Si], x, and [N/Si], y, of different analyzed films. It's clear from this table that the chemical composition is dependent on the deposition conditions. In fact, the [O/Si] concentration ratio varied from 1.43 to 1.85 with the increasing of the N₂O flow in the deposition chamber. On the other hand, the [N/Si] concentration ratio decreased almost 6 times (from 0.57 to 0.12) as the nitrous oxide flow increased. It's important to point out that the [(O+N)/Si] concentration ratio is always close to 2.

The infrared spectra of the a-SiO_xN_y films are shown in Fig. 1. It should be noted that the 2500 to 4000 cm^-1 spectral region was magnified 5 times in order to better appreciate the N-H band. The FTIR data present a continuous shift in the peak position of the Si-O streching vibration mode to lower frequencies, as the amount of O in material matrix decreases. This shift is explained by the presence of the Si-N bonds. Also, the shoulder at around 1250 cm^-1 [,], characteristics of SiO₂, becomes less defined. The 3400 cm^-1 band, corresponding to the stretching vibration mode of the N-H bonds, indicates the presence of N-H bonds in films A, B and C, that have higher nitrogen content. The sample with high oxygen content (sample E) presents a FTIR spectrum very similar to the thermically grown a-SiO₂.

The XANES spectra of amorphous silicon dioxide, polycrystalline silicon nitride and amorphous silicon oxynitride films have been taken at the silicon K edge, as shown in Fig. 2. The spectra of b-Si₃N₄ and a-SiO₂ show no significant difference from those previously reported[,]. From Fig. 2, one can see strong threshold peaks of the a-SiO_xN_y films, located between b-Si₃N₄ and a-SiO₂, closest to a-SiO₂. There is a broad structure between 1860 and 1870 eV, for all XANES spectra, similiar to a-SiO₂.

The XANES spectra of all samples and of a-SiO₂ reference are very similar and confirms the amorphous state of our samples. One can notice the existence of a chemical shift from silicon nitride to silicon dioxide. This shift indicates that the threshold position is dependent on the nature of the atoms which build the tetrahedron environment of Si and is directly related to the electronegativity of the surronding atoms. We can observe that the threshold position of all samples changes continuously but it stays between the values of the silicon nitride and silicon dioxide references (see Fig. 2). This result is consistent with the formation of a fourfold coordinate Si network such that Si is either connected to O and/or N in tetrahedra having the SiO_nN_4-n(n=0,1,2,3 and 4) configurations.

The Fourier transform (F.T.) in the real-space (also referred to R-space) of the k²-weighted oscillation was calculated over the same energy range (1.5 - 9.5 Å) for all samples and references. The back Fourier transform in the reciprocal-space (k-space) corresponding to a given coordination shell was fitted with electronic parameters extracted from the reference samples, whose structure are similar to those being investigated in this work.

The Fourier transform curves obtained for the samples (Fig. 3), can give us some qualitative information about the local order structure. First, one can observe the absence of long-range order (for R values higher than 3 Å). Nevertheless, the short range order is slightly dependent on the chemical composition, as evidenced by the magnitude of the first peak in the Fourier transform (Fig. 3).

The EXAFS data analysis was done considering a two-shell model with partial coordination numbers N_N-O and N_Si-O which provides an average picture of the local arrangement of the N and O atoms in the first coordination shell surrounding the absorbing Si atoms. The EXAFS results provide the coordination number, average interatomic distance and the Debye-Waller factors, as presented in table II.

The results show that the disorder of the films is comparable to that of the reference compounds, being larger for the film with the highest nitrogen content.

The RBS data showed that [(O+N)/Si] concentration ratio is always close to 2. This result suggests that both oxygen and nitrogen atoms occupy the first coordination shell around the Si atoms, keeping the tetrahedral Si coordination. This structural model was confirmed by the fitting of the EXAFS signal, considering that the first Si coordination shell is partially filled with O and N atoms, whose concentration was taken from the RBS data. These results, together with the FTIR data, suggest that the better description of the material is a homogeneous network formed by SiO_nN_4-n tethahedrons.

Acknowledgments

This work was supported by FAPESP/Brazil (process number: 98/09806-6). We acknowlegde Dr. F.C. Vicentin for the XAS measurements at LNLS/Brazil and Dr. M.H. Tabacniks for the RBS measurements.

[18] K. von Klitzing, unpublished.

• [1] U. Hilleringmann and K. Goser, Microelectronic. Eng. 19, 211 (1992).
• [2] M. Tabasky, E.S. Bulat, B. Tweed, and C. Herrick, J. Vac. Sci. Technol. A12, 1244 (1994).
• [3] A. Borghesi, A. Sassella, B. Pivac, and L. Zanotti, Solid State Commun. 100, 657 (1996).
• [4] I. Pereyra, M.I. Alayo, J. Non-Cryst. Solids 212, 225 (1997).
• [5] J. Viard, R. Berjoan, J. Durand, J. Eur. Ceram. Soc. 17, 2001 (1997).
• [6] C.F. Lin, W.T. Tseng, M.S. Feng, Jpn. J. Appl. Phys. Pt. 137, 6364 (1998).
• [7] M.I. Alayo and I. Pereyra, Braz. J. Physics 27A, 146 (1997).
• [8] M.I. Alayo, I. Pereyra, and M.N. Carreńo, Thin solid Films 332, 40 (1998).
• [9] W.L. Scopel, R.R. Cuzinatto, M.H. Tabacniks, M.C.A. Fantini, M.I Alayo, and I. Pereyra, accepted in J. Non-Cryst. Solids, (2001).
• [10] L.R. Doolittle, Nucl. Instrum. Meth. B 9, 344 (1995).
• [11] M. Abbate, F.C. Vicentin, V. Compagnon-Cailhol, M.C. Rocha, and H. Tolentino, J. Synchrotron Radiation. 6, 964-972 (1999).
• [12] P.G. Pai, S.S. Chao, Y. Takagi, and G. Lucovsky, J. Vac. Sci. Technol. A4, 689 (1986).
• [13] H.J. Schiliwinski, U. Schnakenberg, W. Kindbracke, H. Neff, and P. Lange, J. Eletrochem. Soc. 139, 1730 (1992).
• [14] J. Viard, E. Beche, J. Duran, and R. Berjoan, J. Eur. Ceram. Soc. 17, 2039 (1997).
• [15] K.M. Behrens, E.D. Klinkenberg, J. Finster, and K.H. Meiwes-Broer, Surf. Sci.404, 729 (1998).
• [16] K. v. Klitzing, G. Dorda, and M. Pepper, Phys. Rev. Lett. 45, 494 (1980).
• [17] H. Ibach and H. Lüth, Solid-State Physics, (Sringer-Verlag, 1993).
• [19] D.C. Tsui, H. Stoermer, and A.C. Gossard, Phys. Rev. Lett. 48, 1559 (1982).
• [20] H. Stoermer, Proc. 2nd Braz. School of Semicond. Phys., Eds. J.R. Leite and C.E.T. Gonçalves da Silva, p. 539.
• [21] A.H. MacDonald, Braz. J. Phys. 26, 43 (1996).

Publication Dates

• Publication in this collection
  26 Nov 2002
• Date of issue
  June 2002

History

• Received
  23 Apr 2001