Comparative investigation of hydrogen bonding in silicon based PECVD grown dielectrics for optical waveguides
Introduction
In recent years, growing attention has been paid to silicon based dielectrics such as silicon oxides, nitrides, and oxynitrides as potential materials for integrated optics [1], [2], [3], [4], [5]. This attention has been motivated mainly by their promising optical properties such as low absorption losses in the visible and near infrared. Moreover, the dielectric properties of SiO2 and the good chemical inertness and low permeability of Si3N4 can be combined together to obtain silicon oxynitride (SiON) layers with desired properties. The index of refraction of these silicon based amorphous layers can easily be adjusted continuously over a wide range between 1.45 (SiO2) and 2.0 (Si3N4), which comes to be very attractive property that allows fabrication of waveguides with desired characteristics of fiber match and compactness [6], [7]. The growth of these layers can be done by well established standard silicon integrated circuit processing tools, such as plasma enhanced chemical vapor deposition (PECVD) or low pressure chemical vapor deposition (LPCVD) techniques, which is also a key point for low cost mass production [8].
The major problem for integrated optics applications in the CVD grown silicon based layers has been reported to be the incorporation of hydrogen in the form of N–H bonds into the film matrix [9], [10]. Although there has been considerable number of both compositional and device related studies on the above mentioned dielectric films separately, there is a lack of systematic analysis comprising all three silicon based layers [11], [12]. Namely, the dependence of the optical properties on film composition and growth parameters should be established for the whole range of compositions starting from silicon oxide and ending with silicon nitride films. In this study, an attempt is made to establish such a relation, to identify possible drawbacks of the films in the mentioned range and to possibly eliminate them, in a systematic way for the first time. In the following sections the deposition, material characterization, their treatment towards loss minimization, and finally the fabrication and characterization of single-mode waveguides are described.
Section snippets
Experimental
The silicon oxide (SiOx), silicon nitride (SiNx), and silicon oxynitride (SiOxNy or SiON for short) layers were deposited in a parallel-plate type Plasmalab 8510C PECVD reactor. The layers were grown at 250 or 350 °C, 1 Torr pressure at an RF power of 10 W with 13.56 MHz frequency applied to plates of diameter of 24 cm. Silane (2% SiH4/N2) gas flow rate was kept constant at 180 sccm, for all the samples. Nitrous oxide (N2O) was used in the deposition of all the three types of the films with
Refractive index and growth rate characterization
Due to hydrogen and nitrogen incorporation into the film, the stoichiometry of the PECVD grown layers in general deviates from SiO2 and Si3N4 taking the form of SiOx and SiNx, respectively. Moreover, their index of refraction is expected to vary with the growth parameters. The samples analyzed in this section were grown typically on 10 × 20 mm sized silicon substrates. The process parameters mentioned in Section 2 and given in Table 1 were used. The refractive index variation obtained for the SiOx
Conclusions
Silicon oxide, nitride, and oxynitride layers were grown by standard PECVD technique by varying the flow rates of N2O and NH3 precursor gases and keeping that of 2% SiH4/N2 constant at 180 sccm. The refractive index of the layers could be varied between 1.93 and 1.47 by changing the flow rates of the precursor gases.
The compositional properties of these three types of films were investigated via Fourier transform infrared transmission spectroscopy. Special attention was given to the absorption
Acknowledgements
We wish to thank Dr. G.L. Bona (IBM Zurich Research Labs), Dr. A. Driessen and C. Roeloffzen (University of Twente, The Netherlands) and I. Kiyat (Bilkent University) for the useful discussions and help with the loss measurements. This work was supported, in part, by Bilkent University Research Fund (Code: Phys-03-02) and The Scientific and Technical Research Council of Turkey (TUBITAK, project no. 199E006).
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