continuous series of noncrystalline silicon oxynitride (SiON) solid solutions.1,2 The refractive index of these glasses can be varied continuously according to composition from that of silicon dioxide (nsio2 = 1.45) to that of silicon nitride (nsi3N4 = 2.01).2-7 Silicon oxynitride films may prove valuable to the area of integrated optics because of the versatility afforded by their wide range of refractive index. Only recently has the usefulness of SiON films as dielectric optical waveguides been examined. Zelmon et al.8 report the fabrication of low-loss optical waveguides by thermal nitridation of oxidized silicon. In their results, Zelmon et al. state that the refractive index of the experimental oxynitride films ranges from 1.67 to 1.75, independent of the nitridation time. Thus this method of fabrication appears unable to produce the wide range of refractive index that makes silicon oxynitride a particularly promising material for integrated optic applications.
Studies by Reinberg6 and Nguyen et al.7 have successfully employed a plasma-enhanced chemical vapor deposition (PECVD) process for the deposition of silicon oxynitride films. In our study, the PECVD process is used to deposit SiON waveguide films on fused quartz substrates. The dependence of SiON refractive index, wavelength dispersion, and optical attenuation on film composition is examined for the guided modes of a slab waveguide. Finally, the optical properties of an SiON film deposited on a crystalline quartz substrate are examined.
An ASM/America plasma-enhanced chemical vapor deposition system is used to deposit conformal SiON films on polished fused quartz substrates. In this system, silane (SiH4), nitrous oxide (N2O), and ammonia (NH3) are decomposed in a 450-kHz rf plasma discharge operating at 810 W. The system pressure is 2 Torr, and the operating temperature is 380°C. The N2O:NH3 reactant gas ratio determines the chemical composition of the resulting SiON films. Auger electron spectroscopy and Rutherford backscattering (RBS) are used to establish the elemental composition of the SiON samples. The mole fractions of SiO2 and Si3N4 within the films are then deduced via stoichiometric considerations. These mole fraction values are accurate to within approximately ±0.05 mole fraction absolute.
A prism coupler9 is used to determine the refractive index and thickness of the SiON films using the multimode waveguide technique described by Zernike.10 Values of film index and thickness obtained by this method are accurate to within 1 part in 1000 for refractive index and 1% for thickness.11 By varying the distance between the input and output coupling prisms, the power attenuation per unit distance is calculated for each guided mode.10 All measurements are performed for several wavelengths of input light, namely, λ0 = 0.4880, 0.6328, 0.846, and 1.15 μm.
The wavelength dispersion of SiON refractive index is displayed in Fig. 1 for each experimental waveguide film. Also included in this figure are the SiO2 and Si3N4 reference dispersion curves from previously published data.12,13
Clearly, SiON film refractive index is an increasing function of Si3N4 concentration.2-7 Measurements at four wave-
Fig. 1. Wavelength dispersion curves for various SiO2:Si3N4 mixtures. Also included are the bounding dispersion curves for pure
SiO2 and pure Si3N4, which are taken from published data.
Fig. 2. Experimentally determined film refractive index as a function of mole fraction SiO2 and Si3N4 at λ0 = 0.846 μm. The data are
lengths are not sufficient to determine the exact shapes of the oxynitride dispersion curves, although the shapes of these curves seem to approximate the forms of the pure SiO2 and Si3N4 curves. The samples appear to exhibit normal dispersion characteristics over the range of wavelengths examined in this study, with greatest dispersion occurring at wavelengths <0.7 μm, SiON films with high silicon nitride concentrations experience greater wavelength dispersion than films with lower concentrations. Thus it is seen that the relative amounts of SiO2 and Si3N4 in an oxynitride film affect both the film's refractive index and wavelength dispersion characteristics. This observation is expected for a continuous series of glass compositions.
The SiON refractive index (λ0 = 0.846 μm) is plotted in Fig. 2 as a function of chemical film composition. The abscissa in this plot is the mole fraction of silicon dioxide (xSiO2), which is directly related to the mole fraction of silicon nitride (xsi3N4 = 1 - xsiO2), present within the waveguide films. The published refractive indices of silicon dioxide and silicon nitride are included in this figure. The solid line in Fig. 2 is a linear least-squares fit to the experimental and published data. The data seem to be reasonably well-fitted by such a linear approximation, indicating that the refractive index of an SiON solid solution exhibits a linear dependence on the mole fractions of SiO2 and Si3N4 present. This observation is in accordance with previous work.2-4-5
During two separate depositions using identical fabrication parameters, 61% SiO2/39% Si3N4 films were deposited on separate quartz substrates. Refractive-index measurements on these two samples differed by <0.5%, indicating that the reported refractive index data are reproducible.
Power attenuation measurements for the guided modes in each SiON waveguide film are reported in Table I at the four wavelengths of interest. No single oxynitride film exhibits consistently lower losses than any of the other samples. For simplicity, the loss values of a given mode at a specified wavelength are averaged over the measurements on all the sample films. These averages are reported in the last column of Table I and are presented as an order of magnitude estimate of the optical attenuation in the dielectric films.
The SiON optical loss values reported in Table I are substantially larger than those reported by Zelmon.8 A comparison of SiON film thicknesses for these two studies provides some insight into this discrepancy. Zelmon's oxynitride layers are of the order of 0.1 μm thick and are capable of
Table I. Power Attenuation (dB/cm)
Key: +, mode observed, loss is too high to measure; - , mode not observed.
sustaining only one weakly guided mode. The effective guide index for this mode is slightly greater than the refractive index of the fused quartz substrate. Thus a large fraction of Zelmon's TE0 guided wave energy propagates outside the waveguide film. By contrast, the multimode waveguide films examined in this study are of the order of 1.0 μm thick, resulting in well-confined fundamental modes. As a result, the TE0 attenuation values reported in this paper provide closer examination of the actual SiON film loss properties.
Each SiON film examined in this study exhibits increased power attenuation per unit distance for successively higher-order waveguide modes. A larger fraction of the guided wave's energy travels along the dielectric interfaces and in the bounding media, particularly the substrate, for the successively higher-order modes. The strength of surface scattering and substrate losses is proportional to the intensity of the guided light within these regions. Thus higher-order modes suffer larger scattering losses from rough dielectric interfaces and from scattering centers located within the fused quartz substrate. Given the experimental attenuation data, surface and/or substrate losses in the SiON-fused quartz waveguide structures appear significant.
The extent of the bulk material absorption present in the oxynitride films and in the bounding media is determined over the 0.2-2.0-μm wavelength range using a Cary DX extended range spectrophotometer. The oxynitride films exhibit strong absorption in the 0.2-0.3-μm wavelength range; however, bulk material absorption does not appear to contribute significantly to waveguide loss at wavelengths >0.45 μm yet <2.0 μm.
Refractive-index inhomogeneities produced during the growth of an oxynitride film result in volume or bulk scattering of a guided wave. The simplest bulk scattering process is Rayleigh scattering, which can be caused by geometric and intrinsic material imperfections with dimensions that are small compared to the optical wavelength. A λ0
-4 dependence for the power attenuation is evidence that the waveguide loss mechanism is predominantly Rayleigh scattering.14 The experimental dependence of SiON waveguide attenuation on optical wavelength is shown in Fig. 3 for the lowest-order TE mode. Also included in Fig. 3 is a λ0
-4
reference curve. Despite the large scatter in the experimental data, it may be inferred from the figure that the experimental loss does not exhibit a λ0
-4 functional dependence, indicating that bulk loss in these waveguides is not purely a result of Rayleigh scattering.
A λ0-2 attenuation dependence is predicted analytically
and observed experimentally for scattering from cylindrical centers, such as inclusions and defects, within a waveguide film.15 Included in Fig. 3 is a λ0
-2 reference curve whose functional dependence seems to more closely approximate the experimentally observed SiON waveguide loss behavior. Thus film imperfections such as cylindrical inclusions and defects may account for a significant amount of the bulk scattering observed in the experimental PECVD oxynitride waveguides.
The optical properties of an SiON waveguide film (61% SiO2/39% Si3N4) deposited on an electrooptic crystalline quartz substrate were examined. The oxynitride waveguide fabricated on the crystalline quartz substrate exhibits significantly lower losses than an identical film deposited on a fused quartz substrate. Losses as low as 1.51 dB/cm for the TE0 mode are observed at a free-space wavelength of 1.15 μm in this device. The roughness of the film-substrate interface is reduced in the case of the crystalline quartz sample as a result of a superior substrate polishing technique. The fused quartz substrates received a standard optical polish,
Fig. 3. Power attenuation per unit distance for the TE0 mode in experimental SiON waveguides is plotted as a function of free-space
wavelength, λ0. λ0-2 and λ0
-4 reference curves are also included.
while the crystalline quartz wafer received a surface acoustic wave polish. The superior surface acoustic wave polish results in reduced surface scattering losses, particularly in the higher-order waveguide modes. Thus the planar SiON waveguide fabricated on a crystalline quartz wafer is found to exhibit significantly improved power attenuation characteristics.
To summarize, we have characterized the optical properties of PECVD-grown silicon oxynitride dielectric waveguides as a function of film composition over an extended range of wavelengths. Both the film's refractive index and wavelength dispersion characteristics are shown to be functions of the molar composition. The wide range of SiON film refractive index afforded by the PECVD process should prove valuable to the area of integrated optics. SiON waveguide structures still appear promising despite experimental losses which are greater than desired. Specifically, reduction of surface scattering losses at the film-substrate interface results in significantly improved power attenuation characteristics, as in the case of the SiON-crystalline quartz waveguide structure reported here.
The authors wish to acknowledge R. H. Rediker of the Massachusetts Institute of Technology for advice and encouragement throughout the course of this research.
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