- Email: [email protected]
Study of the mechanical and structural properties of silicon oxynitride films for optical applications
Journal of Non-Crystalline Solids 352 (2006) 2319–2323 www.elsevier.com/locate/jnoncrysol
Study of the mechanical and structural properties of silicon oxynitride films for optical applications D. Criado *, M.I. Alayo, M.C.A. Fantini, I. Pereyra Escola Politecnica da Universidade de Sao Paulo, Dept. de Engenharia de Sistemas Eletronicos, Av. Prof. Luciano Gualberto 158, trav. 3-Cid. Univ., University of Sa˜o Paulo, CEP 5424-970, CP 61548, Sa˜o Paulo, SP, Brazil Available online 16 May 2006
Abstract In this work, we report studies on the residual stress and structure of silicon oxynitride films deposited by PECVD with nitrogen atomic percent varying from 24 to 55. The stress response to thermal annealing at different temperatures is analyzed and correlated with Rutherford backscattering spectroscopy, ellipsometry, Fourier transform infrared spectroscopy, small-angle X-ray scattering spectroscopy and X-ray near edge absorption spectroscopy at the Si–K edge measurements. The results show that the stress varies from compressive to tensile for the as-deposited samples. The annealing process increased the stress value in samples that had a tensile behavior as-deposited, while decreased its value in samples with compressive stress as-grown. It is observed that the stress shifts with annealing in a way that can be correlated with the volume density of voids, also depending on the composition and structure of the films. Ó 2006 Elsevier B.V. All rights reserved. PACS: 42.70. a; 81.15.Gh; 78.55.Qr; 60.00.00 Keywords: X-ray diffraction; Ellipsometry; Nitride glasses; Oxynitride glasses; Rutherford backscattering; Mechanical, stress relaxation
1. Introduction Silicon oxynitride (SiOxNy) has been considered a promising material for integrated optics applications due to its excellent optical properties, such as low absorption loss in the visible and near infrared wavelength ranges [1,2] and, mostly because of its tunable refractive index over a wide range (1.46–2.0), by simply changing the film chemical composition [3]. This property leads to a high flexibility in the design of optical structures, allowing for instance the fabrication of waveguides with different geometries and optical characteristics [1,4]. Besides this, the mechanical properties of the SiOxNy films are also strongly dependent of the film composition, so it is possible to obtain low mechanical stress material, allowing thus the deposition of thick films, as required by these kind of applications [5]. *
Corresponding author. Tel.: +55 11 30915256; fax: +55 11 3091 5585. E-mail addresses: [email protected] (D. Criado), malayo@ lme.usp.br (M.I. Alayo). 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.03.012
The SiOxNy films production is compatible with current microelectronics manufacture technology, allowing the integration of optical and electronic devices in the same chip, what is an advantage over previous technologies, based on LiNbO3, or III–V materials as GaAs and InP [5,6]. Currently, much effort has been directed to fabricate SiOxNy films with the plasma enhanced chemical vapor deposition technique (PECVD) [2,6–8] at low deposition temperatures (300–400 °C) since this technique permits an accurate control of the atomic concentrations, refractive index, thickness and roughness of the deposited material only by the appropriate choice of the deposition parameters [4,6]. Stoichiometric silicon oxynitride, given by the formula Si2ON2, is a compound governed by the Mott rule [9–11], where each Si atom is fourfold-coordinated with four O and/or N atoms, each O atom (as in stoichiometric SiO2) is coordinated to two Si atoms, and each N atom (as in Si3N4) is coordinated to three Si atoms [10,11]. In the case of non-stoichiometric material (SiOxNy), as for all
D. Criado et al. / Journal of Non-Crystalline Solids 352 (2006) 2319–2323
2. Experimental The SiOxNy films studied in this work were deposited in a standard 13.56 MHz RF PECVD capacitively coupled system described elsewhere [13], from appropriate gaseous mixtures of electronic grade (99.999%) silane (SiH4), nitrous oxide (N2O) and nitrogen (N2). The films were obtained utilizing different nitrogen and nitrous oxide gaseous flows but maintaining the total N2O + N2 flow equal to 75 sccm. The utilized nitrogen flows were 75, 71, 67.5, 64 and 60 sccm. All the samples were deposited at 320 °C, [14] and the RF power density was kept at 500 mW cm 2. The SiH4 flow was fixed at 15 sccm, value high enough to lead to appropriate deposition rates for thick films production [15], but sufficiently low to prevent undesirable gas phase reactions. For these conditions, the deposition rate was between 15 nm/min and 25 nm/min [16]. Hundred nanometer thick SiOxNy films were deposited for RBS and ellipsometry measurements, and 3000 nm ones for SAXS, XANES and residual stress measurements. The films were deposited onto p type, (1 0 0), single crystalline silicon substrates in the 1–10 X cm resistivity range for XANES and ellipsometry measurements, onto ultra dense amorphous carbon for RBS measurements and onto aluminum foil for SAXS measurements. The thickness of the thick samples was determined with a Tencor 500 profilometer while an ellipsometer, having a He–Ne laser at wavelength of 632.8 nm as a light source, was employed in order to obtain the refractive index and the thickness of the thin films. These measurements were performed in different points of the films in order to determine the standard deviation of the refractive index and the thickness along the sample.
The residual stress was calculated with the aid of a system that measures the radius of curvature of the wafer, before and after the film’s deposition, using the laser beam deflection method. The RBS experiments were done at LAMFI/USP, Sa˜o Paulo, using a He+ beam with energy of 2.4 MeV, charge of 30 lC, current of 30 nA and detection angle of 170°. The XANES measurements were conducted at the SXS beamline [17] of the Brazilian Synchrotron Light Laboratory (LNLS, Campinas, Brazil) in the 1835–1855 eV range with 0.2 eV steps using a channel cut InSb (1 1 1) monochromator. The spectra were collected at the Si–K edge with total electron yield (TEY) detection mode. The SAXS experiments were performed in films deposited on aluminum foil substrates. The measurements were made with a rotating anode X-ray genera˚ ) at a 10 kW tor, using CuKa radiation (k = 1.5418 A power and point focus geometry, varying the scattering angle 2h between 0.2° and 3°. The scattered intensity I(q), where the scattering vector q = (4p/k) sin h, was registered in a image plate with counting time of 12 h. The scattering amplitude of the substrate itself was determined before the film’s deposition. The measured intensity was corrected by absorption effects (film and substrate), besides substrate scattering. The annealing treatments were performed in vacuum at temperatures of 650, 850 and 1000 °C. 3. Results In Fig. 1 the RBS results for all as-deposited samples are shown. It is observed that the nitrogen content increases for increasing N2 flow (decreasing N2O flow), varying from 25% up to 54%. It is also observed an increase of the Si
55
Si O N
50 45 40 35 30 25
4
20
N concentration (1E17 atm/cm2)
tetrahedral amorphous non-stoichiometric alloys, two different models can describe its structure, the random bonding (RBM) and the random mixture (RMM) models. The RMM assumes that a-SiOxNy films are formed by a mixture of two phases: SiO2 and Si3N4. The RBM, on the other hand, considers that these materials are constituted by Si– O and Si–N bonds coordinated in five types of tetrahedral: SiOvN4 v where v = 0, 1, 2, 3, 4 [9–11]. In a previous work, the mechanical stress of PECVDSiOxNy films with chemical composition varying from SiO2 to Si3N4 was investigated [12]. In this work, we concentrate our studies on silicon oxynitride films whose chemical composition leads to low mechanical stress and correlate the atomic concentrations with the mechanical, optical and structural properties, obtained by Rutherford backscattering spectroscopy (RBS), ellipsometry, small-angle X-ray scattering spectroscopy (SAXS) and X-ray absorption near edge spectroscopy at the Si–K edge (XANES) measurements, respectively. The films are also annealed at different temperatures and the effect of these heat treatments on the material’s properties is analyzed.
atomic concentration (%)
2320
15 10 5
650°C
60.0
850°C
2 1
0
as deposited
3
60
62.5
65 70 75 N2 flow (sccm)
65.0
67.5
70.0
72.5
75.0
N2flow (sccm) N2O flow Fig. 1. Atomic concentration of oxygen (d), nitrogen (j) and silicon (m) in the films deposited with different N2, N2O and SiH4 gaseous flows. In the inset, the nitrogen atomic concentration for samples as-deposited and annealed at 650 and 850 °C is shown.
D. Criado et al. / Journal of Non-Crystalline Solids 352 (2006) 2319–2323
concentration for increasing N concentration (5%), due to the fact that Si3N4 samples present a higher Si concentration than SiO2 samples. The RBS measurements performed on the samples after annealing at 650 °C and 850 °C indicate that the Si and O atomic concentration remain almost unchanged with the heat treatments, only a slight decrease in the N concentration was observed as shown in the inset of Fig. 1. The volume distribution function (Dv(R)) of the particles or voids was calculated using the software PGC routine, considering a polydisperse system. A good agreement between model and experiment was attained by considering that the films have spherical, non-correlated, electronic in homogeneities of different size that, in accordance to previous works are probably voids [18]. In Fig. 2 we present the volume distribution function for the samples analyzed by
SAXS. These results evidence that the maximum amplitude ˚ and 18 A ˚. for all samples occurs for a radius between 13 A It is also observed that the volume distribution of voids is higher for the two samples that present lower N content. With the increase in N content a decrease in voids volume distribution, accompanied by an increase in voids size, is observed. In Fig. 3 the refractive index for the as-deposited and annealed samples as a function of N concentration is depicted, indicating a linear increase of the refractive index, for increasing N concentration. Also a slight decrease in refractive index for increasing annealing temperature is observed. The normalized XANES spectra at the Si–K edge for all as-deposited samples are shown in Fig. 4(a). The spectra for the Si3N4 standard, deposited by CVD, and thermal SiO2 are also shown for comparison. It is observed that
N2 (sccm) 75 71 67.5 64 15
Absorption (arb.u.)
Dv(R) (arb.u.)
N2O (sccm) 0 4 7.5 11 60
0
5
10
15
20
25
2321
N content (%) 54 43 35 26 25
(a)
SiO2
Si3N4
30
R (Å) 1840
1842
Fig. 2. Volume distribution function of particles or voids as a function of spherical radius for SiOxNy as deposited films.
Refractive index
1.85
as deposited 650 ºC 850 ºC 1000 ºC
1.80 1.75
1846
1848
1850
Energy (eV)
Absorption (arb.u.)
1.90
1844
C.D. o 650 C o 850 C o 1000 C
(b)
N2O:N2 0:75
1.70 1.65 1840
1.60 60.0
67.5
75.0
N2 flow (sccm) Fig. 3. Refractive index as a function of N2 flow for samples as-deposited and annealed at 650, 850 and 1000 °C.
1842
1844
1846
1848
1850
Energy (eV) Fig. 4. XANES spectra at the Si–K absorption edge for (a) as-deposited samples and standard Si3N4 and SiO2 samples and (b) sample deposited with 75 sccm of N2 as-deposited and after annealing at 650, 850 and 1000 °C.
2322
D. Criado et al. / Journal of Non-Crystalline Solids 352 (2006) 2319–2323 3000
as deposited 650 ºC 850 ºC 1000 ºC
2500
Stress (MPa)
2000 1500 1000 500 0 -500 -1000
60.0
62.5
65.0
67.5
70.0
72.5
75.0
N2 flow (sccm) Fig. 5. Mechanical stress results as function of N2 flow for samples asdeposited and annealed at 650, 850 and 1000 °C.
the Si–K absorption edge for the sample produced with the highest N2 flow is very close to the edge of the Si3N4 spectrum. For decreasing N2 flow, the absorption edge shifts towards the edge of the SiO2 spectrum, behavior attributed to the increase in Si–O bonds and the decrease in Si–N bonds. The spectra for the three samples deposited with higher N2 flows exhibit a second structure at an energy value intermediate between the edges of the SiO2 and Si3N4 standards, which can be related with a second absorption edge. This second structure does not shift with N concentration, but its intensity increases when the N2 flow decreases. To illustrate the effect of thermal annealing in Fig. 4(b) the spectra for sample deposited with 75 sccm of N2 (maximum N%), as-grown and after the heat treatments are shown. An absorption edge increases of 0.5 eV after annealing at 1000 °C is observed, the second edge also shifts to higher energy after annealing. Films produced with lower N2 flow do not present any expressive shift in the Si–K edge with the heat treatments. In Fig. 5 the mechanical stress results obtained for samples as-deposited and after annealing are presented as a function of the N2 flow. For samples as-deposited we observe stress values varying from compressive to tensile for increasing N2 flow. After annealing, all the samples varied their stress values but the behavior for each sample is different depending on its composition, those with lower N content, which presented compressive values as-deposited, decreased their stress values up to almost zero, while those with higher N content experimented an expressive increase in their tensile stress value. 4. Discussion The refractive index results, presented in Fig. 3, are compatible with the increase in nitrogen content indicated by
our previous RBS results [19] since Si3N4 material presents a higher refractive index than SiO2 material. The observed decrease in refractive index after annealing is attributed to the observed N concentration decrease with heat treatments. The results observed at the Si–K edge (Fig. 4(a)) suggest that the local structure for samples with lower N-content (SiO2-like oxynitride films) is better represented by a RBM model, this is to say, Si atoms bonded randomly with O and N atoms. Films with higher N content, on the other hand, show two absorption edges, one at lower energies related to Si3N4-like material and other at higher values close to SiO2-like material, suggesting that for higher nitrogen content a two phase material is obtained. This hypothesis is compatible with other author’s results [20] that describe the intermediate stoichiometry material by a combination of RBM and RMM model. The material cannot be described only by RBM due to the instability of the Si2,2 tetrahedron caused by the electronegativity adjustment, which leads to preferential formation of Si1,3 and Si3,1 tetrahedrons (where Sin,m are the numbers of oxygen and nitrogen atoms, respectively). The increase in the absorption edge after the annealing treatments for the sample deposited with 75 sccm of N2 (maximum N%) observed in Fig. 4(b), can be related with the decrease of N content in the material, demonstrated by the RBS results. The increase in the second edge contribution after heat treatments, can be related to enhancement of the SiO2-like material fraction due to the decrease in the N content. The stress results (Fig. 5) can be related with the voids content in these films, since samples with higher N concentration present lower voids content and, after the annealing processes the bonds do not have freedom to relax in the material structure. The films, which presented higher stress stability with the annealing treatments, present higher voids content thus allowing the structure to relax. Other hypothesis is related to the bond arrangements observed by XANES experiments, which suggest that films with higher N content have a RMM structure and, the tendency of the material to separate in two phases with annealing can originate the observed increase in the stress value. On the other hand, samples with lower N content present only one phase and do not shifts with the annealing, indicating a very stable structure. 5. Conclusion In this work morphological and structural studies on SiOxNy films deposited by PECVD technique are performed and compared with mechanical stress measurements in as-deposited and annealed samples. These studies showed that the annealing treatments have great influence in the mechanical stress of the SiOxNy films. It is observed that the stress shifts with the annealing treatments in a way that can be correlated with the volume density of voids in the films.
D. Criado et al. / Journal of Non-Crystalline Solids 352 (2006) 2319–2323
Upon heat treatments samples with higher N concentration presented larger shifts in the silicon absorption edge accompanied by larger changes in nitrogen concentration. The samples with lower nitrogen concentration did not present any appreciable structural neither mechanical changes, thus being very stable structures. Acknowledgements Thanks are due to Brazilian Synchrotron Light Laboratory – LNLS/Brazil and Ion Beam Materials Analyze Laboratory – LAMFI-IF/Brazil for the RBS measurements. The authors are grateful also to Brazilian agencies FAPESP (Number processes: 03/04523-6 and 00/10027-3 and 01/06516-1) and CNPq for financial support. References [1] R.K. Pandey, L.S. Patil, J.P. Bange, D.R. Patil, A.M. Mahajan, D.S. Patil, D.K. Gautam, Opt. Mater. 25 (2004) 1. [2] F. Ay, A. Aydinli, Opt. Mater. 26 (2004) 33. [3] K.C. Mohite, Y.B. Khollan, A.B. Mandale, K.R. Patil, M.G. Takwale, Mater. Lett. 4494 (2003) 1. [4] A. Zhang, K.T. Chan, Appl. Phys. Lett. 83 (13) (2003) 2524. [5] C. Gorecki, Opt. Lasers Eng. 33 (2000) 15.
2323
[6] Y.K. Kim, S.M. Cho, H.Y. Lee, H.D. Yoon, D.H. Yoon, Surf. Coat. Tech. 174&175 (2003) 166. [7] N. Daldosso, G. Dalba, R. Grisenti, L. Dal Negro, L. Pavesi, F. Rocca, F. Priolo, G. Franzo`, D. Pacifici, F. Iacona, Physica E 16 (2003) 321. [8] R.T. Neal, M.D.C. Charlton, G.J. Parker, C.E. Finlayson, M.C. Netti, J.J. Baumberg, Appl. Phys. Lett. 83 (22) (2003) 4598. [9] F. Pinakidou, M. Katsikini, E.C. Paloura, Nucl. Instrum. Meth. Phys. Res. B 200 (2003) 66. [10] V.A. Gritsenko, J.B. Xu, R.W.M. Kwok, Y.H. Ng, I.H. Wilson, Phys. Rev. Lett. 81 (1998) 1054. [11] V.A. Gritsenko, R.W.M. Kwok, Hei. Wong, J.B. Xu, J. Non-Cryst. Solids 297 (2002) 96. [12] M.I. Alayo, D. Criado, L.C.D. Gonc¸alves, I. Pereyra, J. Non-Cryst. Solids 338–340 (2004) 76. [13] M.N.P. Carren˜o, J.P. Bottechia, I. Pereyra, Thin Solid Films 308 (1997) 219. [14] I. Pereyra, M.I. Alayo, J. Non-Cryst. Solids 212 (1997) 225. [15] M.I. Alayo, I. Pereyra, M.N.P. Carren˜o, Thin Solid Films 332 (1998) 40. [16] D. Criado, M.I. Alayo, I. Pereyra, M.C.A. Fantini, Mater. Sci. Eng. B 112 (2004) 123. [17] M. Abbate, F.C. Vincentin, V. Compagnon-Cailhol, M.C. Rocha, H. Tolentino, J. Synchrotron Radiat. 6 (1999) 964. [18] W.L. Scopel, R.R. Cuzinatto, M.H. Tabacniks, M.C.A. Fantini, M.I. Alayo, I. Pereyra, J. Non-Cryst. Solids 288 (2001) 88. [19] D. Criado, I. Pereyra, M.I. Alayo, Mater. Character. 50 (2003) 167. [20] K.-M. Behrens, E.-D. Klinkenberg, J. Finster, K.-H. Meiwes-Broer, Surf. Sci. 402–404 (1998) 729.
Study of the mechanical and structural properties of silicon oxynitride films for optical applications D. Criado *, M.I. Alayo, M.C.A. Fantini, I. Pereyra Escola Politecnica da Universidade de Sao Paulo, Dept. de Engenharia de Sistemas Eletronicos, Av. Prof. Luciano Gualberto 158, trav. 3-Cid. Univ., University of Sa˜o Paulo, CEP 5424-970, CP 61548, Sa˜o Paulo, SP, Brazil Available online 16 May 2006
Abstract In this work, we report studies on the residual stress and structure of silicon oxynitride films deposited by PECVD with nitrogen atomic percent varying from 24 to 55. The stress response to thermal annealing at different temperatures is analyzed and correlated with Rutherford backscattering spectroscopy, ellipsometry, Fourier transform infrared spectroscopy, small-angle X-ray scattering spectroscopy and X-ray near edge absorption spectroscopy at the Si–K edge measurements. The results show that the stress varies from compressive to tensile for the as-deposited samples. The annealing process increased the stress value in samples that had a tensile behavior as-deposited, while decreased its value in samples with compressive stress as-grown. It is observed that the stress shifts with annealing in a way that can be correlated with the volume density of voids, also depending on the composition and structure of the films. Ó 2006 Elsevier B.V. All rights reserved. PACS: 42.70. a; 81.15.Gh; 78.55.Qr; 60.00.00 Keywords: X-ray diffraction; Ellipsometry; Nitride glasses; Oxynitride glasses; Rutherford backscattering; Mechanical, stress relaxation
1. Introduction Silicon oxynitride (SiOxNy) has been considered a promising material for integrated optics applications due to its excellent optical properties, such as low absorption loss in the visible and near infrared wavelength ranges [1,2] and, mostly because of its tunable refractive index over a wide range (1.46–2.0), by simply changing the film chemical composition [3]. This property leads to a high flexibility in the design of optical structures, allowing for instance the fabrication of waveguides with different geometries and optical characteristics [1,4]. Besides this, the mechanical properties of the SiOxNy films are also strongly dependent of the film composition, so it is possible to obtain low mechanical stress material, allowing thus the deposition of thick films, as required by these kind of applications [5]. *
Corresponding author. Tel.: +55 11 30915256; fax: +55 11 3091 5585. E-mail addresses: [email protected] (D. Criado), malayo@ lme.usp.br (M.I. Alayo). 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.03.012
The SiOxNy films production is compatible with current microelectronics manufacture technology, allowing the integration of optical and electronic devices in the same chip, what is an advantage over previous technologies, based on LiNbO3, or III–V materials as GaAs and InP [5,6]. Currently, much effort has been directed to fabricate SiOxNy films with the plasma enhanced chemical vapor deposition technique (PECVD) [2,6–8] at low deposition temperatures (300–400 °C) since this technique permits an accurate control of the atomic concentrations, refractive index, thickness and roughness of the deposited material only by the appropriate choice of the deposition parameters [4,6]. Stoichiometric silicon oxynitride, given by the formula Si2ON2, is a compound governed by the Mott rule [9–11], where each Si atom is fourfold-coordinated with four O and/or N atoms, each O atom (as in stoichiometric SiO2) is coordinated to two Si atoms, and each N atom (as in Si3N4) is coordinated to three Si atoms [10,11]. In the case of non-stoichiometric material (SiOxNy), as for all
D. Criado et al. / Journal of Non-Crystalline Solids 352 (2006) 2319–2323
2. Experimental The SiOxNy films studied in this work were deposited in a standard 13.56 MHz RF PECVD capacitively coupled system described elsewhere [13], from appropriate gaseous mixtures of electronic grade (99.999%) silane (SiH4), nitrous oxide (N2O) and nitrogen (N2). The films were obtained utilizing different nitrogen and nitrous oxide gaseous flows but maintaining the total N2O + N2 flow equal to 75 sccm. The utilized nitrogen flows were 75, 71, 67.5, 64 and 60 sccm. All the samples were deposited at 320 °C, [14] and the RF power density was kept at 500 mW cm 2. The SiH4 flow was fixed at 15 sccm, value high enough to lead to appropriate deposition rates for thick films production [15], but sufficiently low to prevent undesirable gas phase reactions. For these conditions, the deposition rate was between 15 nm/min and 25 nm/min [16]. Hundred nanometer thick SiOxNy films were deposited for RBS and ellipsometry measurements, and 3000 nm ones for SAXS, XANES and residual stress measurements. The films were deposited onto p type, (1 0 0), single crystalline silicon substrates in the 1–10 X cm resistivity range for XANES and ellipsometry measurements, onto ultra dense amorphous carbon for RBS measurements and onto aluminum foil for SAXS measurements. The thickness of the thick samples was determined with a Tencor 500 profilometer while an ellipsometer, having a He–Ne laser at wavelength of 632.8 nm as a light source, was employed in order to obtain the refractive index and the thickness of the thin films. These measurements were performed in different points of the films in order to determine the standard deviation of the refractive index and the thickness along the sample.
The residual stress was calculated with the aid of a system that measures the radius of curvature of the wafer, before and after the film’s deposition, using the laser beam deflection method. The RBS experiments were done at LAMFI/USP, Sa˜o Paulo, using a He+ beam with energy of 2.4 MeV, charge of 30 lC, current of 30 nA and detection angle of 170°. The XANES measurements were conducted at the SXS beamline [17] of the Brazilian Synchrotron Light Laboratory (LNLS, Campinas, Brazil) in the 1835–1855 eV range with 0.2 eV steps using a channel cut InSb (1 1 1) monochromator. The spectra were collected at the Si–K edge with total electron yield (TEY) detection mode. The SAXS experiments were performed in films deposited on aluminum foil substrates. The measurements were made with a rotating anode X-ray genera˚ ) at a 10 kW tor, using CuKa radiation (k = 1.5418 A power and point focus geometry, varying the scattering angle 2h between 0.2° and 3°. The scattered intensity I(q), where the scattering vector q = (4p/k) sin h, was registered in a image plate with counting time of 12 h. The scattering amplitude of the substrate itself was determined before the film’s deposition. The measured intensity was corrected by absorption effects (film and substrate), besides substrate scattering. The annealing treatments were performed in vacuum at temperatures of 650, 850 and 1000 °C. 3. Results In Fig. 1 the RBS results for all as-deposited samples are shown. It is observed that the nitrogen content increases for increasing N2 flow (decreasing N2O flow), varying from 25% up to 54%. It is also observed an increase of the Si
55
Si O N
50 45 40 35 30 25
4
20
N concentration (1E17 atm/cm2)
tetrahedral amorphous non-stoichiometric alloys, two different models can describe its structure, the random bonding (RBM) and the random mixture (RMM) models. The RMM assumes that a-SiOxNy films are formed by a mixture of two phases: SiO2 and Si3N4. The RBM, on the other hand, considers that these materials are constituted by Si– O and Si–N bonds coordinated in five types of tetrahedral: SiOvN4 v where v = 0, 1, 2, 3, 4 [9–11]. In a previous work, the mechanical stress of PECVDSiOxNy films with chemical composition varying from SiO2 to Si3N4 was investigated [12]. In this work, we concentrate our studies on silicon oxynitride films whose chemical composition leads to low mechanical stress and correlate the atomic concentrations with the mechanical, optical and structural properties, obtained by Rutherford backscattering spectroscopy (RBS), ellipsometry, small-angle X-ray scattering spectroscopy (SAXS) and X-ray absorption near edge spectroscopy at the Si–K edge (XANES) measurements, respectively. The films are also annealed at different temperatures and the effect of these heat treatments on the material’s properties is analyzed.
atomic concentration (%)
2320
15 10 5
650°C
60.0
850°C
2 1
0
as deposited
3
60
62.5
65 70 75 N2 flow (sccm)
65.0
67.5
70.0
72.5
75.0
N2flow (sccm) N2O flow Fig. 1. Atomic concentration of oxygen (d), nitrogen (j) and silicon (m) in the films deposited with different N2, N2O and SiH4 gaseous flows. In the inset, the nitrogen atomic concentration for samples as-deposited and annealed at 650 and 850 °C is shown.
D. Criado et al. / Journal of Non-Crystalline Solids 352 (2006) 2319–2323
concentration for increasing N concentration (5%), due to the fact that Si3N4 samples present a higher Si concentration than SiO2 samples. The RBS measurements performed on the samples after annealing at 650 °C and 850 °C indicate that the Si and O atomic concentration remain almost unchanged with the heat treatments, only a slight decrease in the N concentration was observed as shown in the inset of Fig. 1. The volume distribution function (Dv(R)) of the particles or voids was calculated using the software PGC routine, considering a polydisperse system. A good agreement between model and experiment was attained by considering that the films have spherical, non-correlated, electronic in homogeneities of different size that, in accordance to previous works are probably voids [18]. In Fig. 2 we present the volume distribution function for the samples analyzed by
SAXS. These results evidence that the maximum amplitude ˚ and 18 A ˚. for all samples occurs for a radius between 13 A It is also observed that the volume distribution of voids is higher for the two samples that present lower N content. With the increase in N content a decrease in voids volume distribution, accompanied by an increase in voids size, is observed. In Fig. 3 the refractive index for the as-deposited and annealed samples as a function of N concentration is depicted, indicating a linear increase of the refractive index, for increasing N concentration. Also a slight decrease in refractive index for increasing annealing temperature is observed. The normalized XANES spectra at the Si–K edge for all as-deposited samples are shown in Fig. 4(a). The spectra for the Si3N4 standard, deposited by CVD, and thermal SiO2 are also shown for comparison. It is observed that
N2 (sccm) 75 71 67.5 64 15
Absorption (arb.u.)
Dv(R) (arb.u.)
N2O (sccm) 0 4 7.5 11 60
0
5
10
15
20
25
2321
N content (%) 54 43 35 26 25
(a)
SiO2
Si3N4
30
R (Å) 1840
1842
Fig. 2. Volume distribution function of particles or voids as a function of spherical radius for SiOxNy as deposited films.
Refractive index
1.85
as deposited 650 ºC 850 ºC 1000 ºC
1.80 1.75
1846
1848
1850
Energy (eV)
Absorption (arb.u.)
1.90
1844
C.D. o 650 C o 850 C o 1000 C
(b)
N2O:N2 0:75
1.70 1.65 1840
1.60 60.0
67.5
75.0
N2 flow (sccm) Fig. 3. Refractive index as a function of N2 flow for samples as-deposited and annealed at 650, 850 and 1000 °C.
1842
1844
1846
1848
1850
Energy (eV) Fig. 4. XANES spectra at the Si–K absorption edge for (a) as-deposited samples and standard Si3N4 and SiO2 samples and (b) sample deposited with 75 sccm of N2 as-deposited and after annealing at 650, 850 and 1000 °C.
2322
D. Criado et al. / Journal of Non-Crystalline Solids 352 (2006) 2319–2323 3000
as deposited 650 ºC 850 ºC 1000 ºC
2500
Stress (MPa)
2000 1500 1000 500 0 -500 -1000
60.0
62.5
65.0
67.5
70.0
72.5
75.0
N2 flow (sccm) Fig. 5. Mechanical stress results as function of N2 flow for samples asdeposited and annealed at 650, 850 and 1000 °C.
the Si–K absorption edge for the sample produced with the highest N2 flow is very close to the edge of the Si3N4 spectrum. For decreasing N2 flow, the absorption edge shifts towards the edge of the SiO2 spectrum, behavior attributed to the increase in Si–O bonds and the decrease in Si–N bonds. The spectra for the three samples deposited with higher N2 flows exhibit a second structure at an energy value intermediate between the edges of the SiO2 and Si3N4 standards, which can be related with a second absorption edge. This second structure does not shift with N concentration, but its intensity increases when the N2 flow decreases. To illustrate the effect of thermal annealing in Fig. 4(b) the spectra for sample deposited with 75 sccm of N2 (maximum N%), as-grown and after the heat treatments are shown. An absorption edge increases of 0.5 eV after annealing at 1000 °C is observed, the second edge also shifts to higher energy after annealing. Films produced with lower N2 flow do not present any expressive shift in the Si–K edge with the heat treatments. In Fig. 5 the mechanical stress results obtained for samples as-deposited and after annealing are presented as a function of the N2 flow. For samples as-deposited we observe stress values varying from compressive to tensile for increasing N2 flow. After annealing, all the samples varied their stress values but the behavior for each sample is different depending on its composition, those with lower N content, which presented compressive values as-deposited, decreased their stress values up to almost zero, while those with higher N content experimented an expressive increase in their tensile stress value. 4. Discussion The refractive index results, presented in Fig. 3, are compatible with the increase in nitrogen content indicated by
our previous RBS results [19] since Si3N4 material presents a higher refractive index than SiO2 material. The observed decrease in refractive index after annealing is attributed to the observed N concentration decrease with heat treatments. The results observed at the Si–K edge (Fig. 4(a)) suggest that the local structure for samples with lower N-content (SiO2-like oxynitride films) is better represented by a RBM model, this is to say, Si atoms bonded randomly with O and N atoms. Films with higher N content, on the other hand, show two absorption edges, one at lower energies related to Si3N4-like material and other at higher values close to SiO2-like material, suggesting that for higher nitrogen content a two phase material is obtained. This hypothesis is compatible with other author’s results [20] that describe the intermediate stoichiometry material by a combination of RBM and RMM model. The material cannot be described only by RBM due to the instability of the Si2,2 tetrahedron caused by the electronegativity adjustment, which leads to preferential formation of Si1,3 and Si3,1 tetrahedrons (where Sin,m are the numbers of oxygen and nitrogen atoms, respectively). The increase in the absorption edge after the annealing treatments for the sample deposited with 75 sccm of N2 (maximum N%) observed in Fig. 4(b), can be related with the decrease of N content in the material, demonstrated by the RBS results. The increase in the second edge contribution after heat treatments, can be related to enhancement of the SiO2-like material fraction due to the decrease in the N content. The stress results (Fig. 5) can be related with the voids content in these films, since samples with higher N concentration present lower voids content and, after the annealing processes the bonds do not have freedom to relax in the material structure. The films, which presented higher stress stability with the annealing treatments, present higher voids content thus allowing the structure to relax. Other hypothesis is related to the bond arrangements observed by XANES experiments, which suggest that films with higher N content have a RMM structure and, the tendency of the material to separate in two phases with annealing can originate the observed increase in the stress value. On the other hand, samples with lower N content present only one phase and do not shifts with the annealing, indicating a very stable structure. 5. Conclusion In this work morphological and structural studies on SiOxNy films deposited by PECVD technique are performed and compared with mechanical stress measurements in as-deposited and annealed samples. These studies showed that the annealing treatments have great influence in the mechanical stress of the SiOxNy films. It is observed that the stress shifts with the annealing treatments in a way that can be correlated with the volume density of voids in the films.
D. Criado et al. / Journal of Non-Crystalline Solids 352 (2006) 2319–2323
Upon heat treatments samples with higher N concentration presented larger shifts in the silicon absorption edge accompanied by larger changes in nitrogen concentration. The samples with lower nitrogen concentration did not present any appreciable structural neither mechanical changes, thus being very stable structures. Acknowledgements Thanks are due to Brazilian Synchrotron Light Laboratory – LNLS/Brazil and Ion Beam Materials Analyze Laboratory – LAMFI-IF/Brazil for the RBS measurements. The authors are grateful also to Brazilian agencies FAPESP (Number processes: 03/04523-6 and 00/10027-3 and 01/06516-1) and CNPq for financial support. References [1] R.K. Pandey, L.S. Patil, J.P. Bange, D.R. Patil, A.M. Mahajan, D.S. Patil, D.K. Gautam, Opt. Mater. 25 (2004) 1. [2] F. Ay, A. Aydinli, Opt. Mater. 26 (2004) 33. [3] K.C. Mohite, Y.B. Khollan, A.B. Mandale, K.R. Patil, M.G. Takwale, Mater. Lett. 4494 (2003) 1. [4] A. Zhang, K.T. Chan, Appl. Phys. Lett. 83 (13) (2003) 2524. [5] C. Gorecki, Opt. Lasers Eng. 33 (2000) 15.
2323
[6] Y.K. Kim, S.M. Cho, H.Y. Lee, H.D. Yoon, D.H. Yoon, Surf. Coat. Tech. 174&175 (2003) 166. [7] N. Daldosso, G. Dalba, R. Grisenti, L. Dal Negro, L. Pavesi, F. Rocca, F. Priolo, G. Franzo`, D. Pacifici, F. Iacona, Physica E 16 (2003) 321. [8] R.T. Neal, M.D.C. Charlton, G.J. Parker, C.E. Finlayson, M.C. Netti, J.J. Baumberg, Appl. Phys. Lett. 83 (22) (2003) 4598. [9] F. Pinakidou, M. Katsikini, E.C. Paloura, Nucl. Instrum. Meth. Phys. Res. B 200 (2003) 66. [10] V.A. Gritsenko, J.B. Xu, R.W.M. Kwok, Y.H. Ng, I.H. Wilson, Phys. Rev. Lett. 81 (1998) 1054. [11] V.A. Gritsenko, R.W.M. Kwok, Hei. Wong, J.B. Xu, J. Non-Cryst. Solids 297 (2002) 96. [12] M.I. Alayo, D. Criado, L.C.D. Gonc¸alves, I. Pereyra, J. Non-Cryst. Solids 338–340 (2004) 76. [13] M.N.P. Carren˜o, J.P. Bottechia, I. Pereyra, Thin Solid Films 308 (1997) 219. [14] I. Pereyra, M.I. Alayo, J. Non-Cryst. Solids 212 (1997) 225. [15] M.I. Alayo, I. Pereyra, M.N.P. Carren˜o, Thin Solid Films 332 (1998) 40. [16] D. Criado, M.I. Alayo, I. Pereyra, M.C.A. Fantini, Mater. Sci. Eng. B 112 (2004) 123. [17] M. Abbate, F.C. Vincentin, V. Compagnon-Cailhol, M.C. Rocha, H. Tolentino, J. Synchrotron Radiat. 6 (1999) 964. [18] W.L. Scopel, R.R. Cuzinatto, M.H. Tabacniks, M.C.A. Fantini, M.I. Alayo, I. Pereyra, J. Non-Cryst. Solids 288 (2001) 88. [19] D. Criado, I. Pereyra, M.I. Alayo, Mater. Character. 50 (2003) 167. [20] K.-M. Behrens, E.-D. Klinkenberg, J. Finster, K.-H. Meiwes-Broer, Surf. Sci. 402–404 (1998) 729.