M.I. Alayo

On the nitrogen and oxygen incorporation in plasma-enhanced chemical vapor deposition (PECVD) SiOxNy films

Silicon oxynitride films were deposited by plasma-enhanced chemical vapor deposition at low temperatures using nitrous oxide (N2O) and silane (SiH4) as gas precursors. The influence of the N2O/SiH4 flow ratio (varied from 0.25 up to 5) and the thickness of the films on the optical and structural properties of the material was analyzed. The films were characterized by ellipsometry, Fourier-transform infrared spectroscopy, Rutherford backscattering spectroscopy and optical absorption. Two distinct types of material were obtained, silicon dioxide-like oxynitrides SiO2−xNx and silicon-rich oxynitrides SiOxNy (x+y<2). The results demonstrate that in silicon dioxide-like material, the nitrogen concentration can be adequately controlled (within the range 0–15 at.%) with total hydrogen incorporation below 5 at.% and no appreciable SiH bonds. It is also shown that the composition remains uniform through the entire thickness of the films. Furthermore, a linear relation between the refractive index and the nitrogen concentration is observed, which makes this material very attractive for optoelectronic applications. On the other hand, silicon-rich material is similar to amorphous silicon, and presents an increasing concentration of SiH bonds, increasing refractive index and decreasing optical gap, which makes it promising for applications in light-emitting devices.

Thick SiOxNy and SiO2 films obtained by PECVD technique at low temperatures

In this work we present the results on the fabrication of thick silicon oxynitride and dioxide films deposited by conventional r.f. direct plasma enhanced chemical vapor deposition (DPECVD), at temperature as low as 320°C and from (N2O+SiH4) gaseous mixtures. The samples were characterized by profile measurements, ellipsometry measurements, etching rate, Fourier transform infrared spectroscopy (FTIR), and by scanning electron microscopy (SEM). The results show that for appropriate N2O/SiH4 flow ratio and SiH4 flow, it is possible to obtain very thick SiO2 and SiOxNy films (up to ∼10 μm) at high deposition rates (∼3 μm/h) and preserving the compositional and structural properties of similar high quality thin films obtained in a previous work (I. Pereyra, M.I. Alayo, J. Non-Cryst. Solids 212 (1997) 225). These thick SiO2 and SiOxNy films, exhibit a very well controlled refractive index, in a short range between ∼1.43 and ∼1.53, which is very attractive to SiO2/SiOxNy based waveguide fabrication. Besides the large thickness, the results show that the films present an etching rate just twice the thermally grown SiO2 rate, therefore lower than the reported values for PECVD SiO2 by other authors (M.S. Haque, H.A. Naseem, W.D. Brown, J. Electrochem. Soc. 142 (1995) 3864). Also etching experiments were performed using reactive ion etching (RIE) equipment on thick silicon oxynitride film grown onto silicon substrates covered by a thick DPECVD SiO2 buffer layer, in order to simulate a waveguide structure (ridge type) fabrication. The results of these tests show that it is possible to define vertical walls in these thick SiOxNy films, which is very important for ridge type waveguides.

Local structure and bonds of amorphous silicon oxynitride thin films

Abstract This work reports on the local structure and bonds of amorphous silicon oxynitride thin films, deposited by plasma enhanced chemical vapor deposition. The dependence of the structural properties and chemical bonds with the films composition was investigated. The used analytical techniques were X-ray absorption at the Si K-edge and Fourier transform infrared spectroscopies. The coordination numbers, interatomic distances and Debye–Waller disorder factors of the Si first shell and, the bond types and the concentration of hydrogen in the films were obtained. All the analyzed data support the formation of a materials homogeneous network with a random distribution of SiO and SiN bonds. The basic structural element of the network is a tetrahedron with a central Si atom connected to N and O, consistent with a random bonding model. As the nitrogen content in the solid phase decreases the SiON 3 , SiO 2 N 2 tetrahedral units gradually change to SiO 4 , keeping the quantity of SiO 3 N tetrahedrons almost unchanged, approximately 40%. The amount of SiO 4 units is 100% for films with high oxygen content. The nitrogen is preferentially bonded to silicon and hydrogen, while the hydrogen is mostly bonded to nitrogen.

Study of nitrogen-rich silicon oxynitride films obtained by PECVD

Abstract The results of the fabrication and characterization of silicon oxynitride films deposited by the plasma-enhanced chemical vapor deposition (PECVD) technique at low temperature and from N 2 , N 2 O and SiH 4 gaseous mixtures are reported herein. It is shown that high nitrogen concentration films with characteristics close to stoichiometric silicon nitride (Si 3 N 4 ) can be obtained. In previous experiments utilizing N 2 O and SiH 4 as precursor gases, it was demonstrated that precise control of the refractive index for silicon dioxide-like oxynitride SiO x N y ( x + y =2) material in the 1.46 (SiO 2 ) to 1.57 range can be attained. In this study, nitrogen gas (N 2 ) was added to the previously studied gaseous mixture. In this way, it was possible to control the refractive index from 1.46 (SiO 2 ) to ∼2 (Si 3 N 4 ) through the appropriate choice of the deposition parameters. The films were characterized by profilometry, ellipsometry, Rutherford backscattering spectroscopy (RBS) and Fourier transform infrared spectroscopy (FTIR).

Silicon rich silicon oxynitride films for photoluminescence applications

Abstract In this work results on the study of the physical and optical properties of silicon rich SiOxNy thin films are presented. The films were deposited by the plasma enhanced chemical vapor deposition technique at low temperature (≈320 °C) using nitrous oxide and silane as precursor gases. The films were thermally annealed at 750 and 1000 °C, at low pressures (10−2 Pa) and in N2 ambient for different annealing times. The samples were characterized through Fourier-transform infrared spectroscopy, Raman scattering and photoluminescence (PL). Raman spectra for all samples show a band approximately at 480 cm−1, related to amorphous silicon. The spectrum for the sample with the highest silicon content, heat treated at 1000 °C also presents a band at 520 cm−1, related to microcrystalline silicon. Finally, PL experiments showed the presence of visible luminescence for the as-deposited samples in the region between 1.5 and 2 eV, attributed to defects (at higher energies) and to the presence of small variable size amorphous silicon clusters embedded in the dielectric matrix. High temperature annealing substantially decreases the PL intensity, result attributed to increased cluster size and crystallization.

Chemical and morphological properties of amorphous silicon oxynitride films deposited by plasma enhanced chemical vapor deposition

Abstract The deposition of amorphous hydrogenated silicon oxynitride thin films, varying the nitrogen and oxygen content in the solid phase, is reported. The films were deposited by plasma enhanced chemical vapor deposition at different nitrous oxide/silane flow ratios, keeping constant the silane flow and the deposition temperature at 320°C. The composition of the thin films was determined by Rutherford backscattering spectroscopy (RBS) and the morphological properties were investigated by small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The composition data showed that the oxygen content increases and the nitrogen content decreases, inside the films, as the ratio between the nitrous oxide flow and silane flow goes toward larger values. The oxygen ( x ) plus nitrogen ( y ) content in the chemical formula (a-SiO x N y ) is always close to two, suggesting that these atoms share the same atomic positions around the silicon atoms in a local structure similar to SiO 2 . The SAXS results revealed the presence of scatterers with an average radius 〈 R 〉 that varies from small values, like 10 A, up to 100 A. The TEM results showed the formation of particles with a circular cross-section, composed of Si, N and O spread in a matrix with the same elemental composition. These particles have a radius larger than 50 A.

Structural investigation of Si-rich amorphous silicon oxynitride films

Abstract In this work we investigated the structural and chemical properties of amorphous silicon oxynitride thin films, with distinct composition, deposited by plasma enhanced chemical vapor deposition from nitrous oxide and silane gas precursors. The utilized characterization techniques were Rutherford backscattering spectroscopy, X-ray absorption spectroscopy at the Si K-edge, and Fourier transform infrared spectroscopy. The results show a silicon first coordination shell composed of silicon, oxygen and nitrogen atoms, in a proportion that fits their atomic content. The SiSi bonds are found only in samples having silicon content higher than ∼50 at.%. These films also present higher amount of SiH bonds. The film having the highest Si content, which shows evidences of Si clustering, was annealed in vacuum at 550, 750 and 1000 °C. The results demonstrated that the annealed film is chemically stable under heat treatments in vacuum up to 1000 °C. The hydrogen is totally released at 750 °C. The main effect of the annealing process is to increase the segregation of silicon clusters.

Silicon clusters in PECVD silicon-rich SiOxNy

In recent years, the integration of optical and microelectronic devices has been of great interest, motivating studies related to the production of photoluminescent material compatible with present silicon technology. In this context, silicon clusters embedded in a SiO2 matrix are ideal candidates for these applications, mainly due to the intense emission that these new structures present, being promising for the production of devices such as lasers and photodiodes. In this work, we report the fabrication and characterization of silicon-rich SiOxNy deposited by plasma-enhanced chemical vapor deposition (PECVD) technique at low temperatures. These films were annealed at different temperatures, in N2 ambient. Fourier transform infrared spectroscopy (FTIR) and Raman scattering results showed phase segregation and crystallization in the clusters induced by heat treatment. Photoluminescence measurements showed visible emission peaks, compatible with variable size cluster distribution.

Pedestal anti-resonant reflecting optical waveguides

The applicability of anti-resonant reflecting optical waveguides fabricated on silicon substrates has been demonstrated for different optical devices and sensors. In particular, it has been shown that in order to have virtual single-mode operation in ARROWs, smaller constraints are imposed in the thickness and refractive index of the constituent layers than in the case of Total Internal Reflection waveguides. On the other hand, if rib ARROWs are fabricated through Reactive Ion Etching (RIE), high sidewall roughness is observed if metallic mask is used, which leads to undesirable losses. This can be improved if the RIE step is done in the lower layers, leading to rounder but smoother core sidewalls. In this work we present an alternative method for achieving the lateral confinement in ARROW waveguides fabricated with silicon technology. This method consists in doing the RIE step before the core definition so as to have the lower cladding layer and part of the silicon substrate etched away. Pedestal hollow core ARROWs have been proposed and fabricated but in the case of conventional ARROW waveguides this has not been done, to our best knowledge. Simulations results regarding propagation losses are presented for different rib heights and widths and compared to experimental results.

a-SiC:H anti-resonant layer ARROW waveguides

Over the last decades, anti-resonant reflecting optical waveguides (ARROW) have been used in different integrated optics applications. In this type of waveguide, light confinement is partially achieved through an anti-resonant reflection. In this work, the simulation, fabrication and characterization of ARROW waveguides using dielectric films deposited by a plasma-enhanced chemical vapor deposition (PECVD) technique, at low temperatures ( ∼ 300 °C), are presented. Silicon oxynitride (SiO x N y ) films were used as core and second cladding layers and amorphous hydrogenated silicon carbide (a-SiC:H) films as first cladding layer. Furthermore, numerical simulations were performed using homemade routines based on two computational methods: the transfer matrix method (TMM) for the determination of the optimum thickness of the Fabry-Perot layers; and the non-uniform finite difference method (NU-FDM) for 2D design and determination of the maximum width that yields single-mode operation. The utilization of a silicon carbide anti-resonant layer resulted in low optical attenuations, which is due to the high refractive index difference between the core and this layer. Finally, for comparison purposes, optical waveguides using titanium oxide (TiO 2 ) as the first ARROW layer were also fabricated and characterized.