Synthesis, Characterization and Optical Constants of Silicon Oxycarbide

High refractive index glasses are preferred in integrated photonics applications to realize higher integration scale of passive devices. With a refractive index that can be tuned between SiO2 (1.45) and aSiC (3.2), silicon oxycarbide SiOC offers this flexibility. In the present work, silicon oxycarbide thin films from 0.1 – 2.0 μm thickness are synthesized by reactive radio frequency magnetron sputtering a silicon carbide SiC target in a controlled argon and oxygen environment. The refractive index n and material extinction coefficient k of the silicon oxycarbide films are acquired with variable angle spectroscopic ellipsometry over the UV-Vis-NIR wavelength range. Keeping argon and oxygen gases in the constant ratio, the refractive index n is found in the range from 1.41 to 1.93 at 600 nm which is almost linearly dependent on RF power of sputtering. The material extinction coefficient k has been estimated to be less than 10-4 for the deposited silicon oxycarbide films in the visible and near-infrared wavelength regions. Morphological and structural characterizations with SEM and XRD confirms the amorphous phase of the SiOC films.


Introduction
High performance optical waveguide materials must combine low loss with the flexibility to fabricate a variety of passive devices on a single material platform.With a refractive index that can be tuned between silica glass SiO2 (1.45) and amorphous silicon carbide a-SiC (3.2), silicon oxycarbide SiOC offers this flexibility.However, the methods which were used to produce silicon oxycarbide such as chemical vapor deposition (CVD) [1,2,3,4] and sol-gel pyrolysis [5,6,7,8] introduce hydrogen and high material absorption.In the recent years, the authors in [9,10] showed the deposition of SiOC films with RF sputtering technique and discussed physical and structural characteristics of SiOC thin films.In another study by T. Nakai et.al. [11], low refractive-index SiOC films were developed with rf sputtering and exploited for HD DVD rewritable media.However optical properties (n and k) of the SiOC films over a wide wavelength spectrum ranging from UV to near-infrared are not reported.In this work, we demonstrate fabrication of silicon oxycarbide thin films on Si (100) substrate by reactive RF magnetron sputtering of a SiC target in a controlled argon and oxygen gases environment.The SiOC thin films are characterized by variable angle spectroscopic ellipsometry over the broad UV-VIS-NIR wavelength range to obtain optical constants (refractive index and absorption spectra).The motivation to use sputter deposition comes from the fact that it has potential to prepare high quality material with essentially no hydrogen content at room temperature compared to typical deposition techniques such as CVD [3] which introduce hydrogen and high material absorption coefficient α on the order of 10 5 cm -1 .The refractive index n of the deposited SiOC films ranges from 1.41 to 1.93 at 600 nm as a function of RF power level.The extinction coefficient k of all the deposited SiOC films is estimated to be less than 10 -4 above λ = 1000 nm.To the best of our knowledge, it is the first time to report optical properties mainly the refractive index n and extinction coefficient k spectra in a broad wavelength range extending from UV (300 nm) to Near IR (1200 nm).Profilometry and scanning electron microscopy (SEM) were used to determine thickness and analyse morphology of the deposited SiOC films, respectively.The phase of the films was examined with X-ray diffraction (XRD).

Deposition of SiOC Films
Depositions of silicon oxycarbide thin films were carried out by reactive RF magnetron sputtering of a 99.9% pure 3 inches diameter silicon carbide SiC target which was manufactured by Ajaint International.The sputtering chamber was pumped down to better than 10 -4 Pa before the argon and oxygen gas flux.High purity argon Ar and oxygen O2 were used as the plasma and reactive gases in the sputtering chamber during preparation of amorphous silicon oxycarbide films.The depositions were performed on boron doped p-type Si (100) substrate at room temperature.Prior to deposition, substrates were first cleaned in acetone and then isopropanol in ultrasonic cleaner to remove the contaminants and organic matter on Si substrate surface.In order to clean the SiC target surface, fifteen minutes pre-sputtering without reactive gas was done each time before the actual film deposition.To check repeatability and achieve precise control of the sputter process a large number of SiOC samples were produced by setting the recipe: RF power level from 50 to 450 watts, argon to oxygen gas ratio of 47 sccm and deposition time of 15 minutes.Here we report the results of 6 SiOC films which were deposited as function of increasing RF power level.

Results and Discussion
The optical properties of the silicon oxycarbide films grown on Si subtstrate as a function of RF power have been characterized by spectroscopic ellipsometry.The ellipsometric ψ and Δ data acquired over the wide wavelength range from 300 to 1200 nm for each SiOC film have been modelled with a four-layer model comprising surface roughness/Tauc-Lorentz/native-SiO2/Si layers as illustrated in figure 2

Fig. 1 .
Fig. 1.Blue stars show measured thickness and red circles show deposition rate of the deposited SiOC films as a function of RF power.The thickness of the SiOC films was measured with KLA Tencor P-15 Surface Profilometer.To determine the thickness with profilometer, kapton tape was used on cleaned substrates as a mask before deposition.The deposited samples were cleaned in acetone by means of ultrasonic device to eliminate the mask.Due to the good quality adhesion of sputtered SiOC films on Si (100) substrates, no wear off or pin holes in SiOC films were observed after ultrasonication.Figure1shows the thickness and deposition rate of silicon oxycarbide thin films deposited as a function of RF power from 50 to 450 W. The thickness of the SiOC films was determined to be between 100 nm and 2000 nm.The deposition rate increases with RF power that is varied from 50 to 450 W and a large rate of 150 nm/min has been achieved, which suggests that micron size films can be prepared in a fraction of an hour.
. Since ellipsometry is very sensitive to native silica and surface roughness conditions, the 2 nm SiO2 and surface roughness layers were added to improve fit and MSE.The optical constants of Si substrate and SiO2 were used from the WVASE ® database [15].The surface roughness SR layer based on Bruggemann and Mawxell-Garnett effective medium approximation (EMA) [16] composed of 50 % material and 50 % voids is used to simulate the surface roughness of SiOC films.The SR layer is ellipsometry equivalent of AFM and gives peak-tovalley roughness, not the rms roughness.The parameters of the Tauc-Lorentz oscillator described earlier were declared fit in the model shown in figure 2(a) and regression analysis algorithm was run to minimize the global mean squared error (MSE).Figure 2(b) shows an example of fitting of ellipsometric ψ and Δ spectra of the SiOC film deposited at 300 W over the broad wavelength range 300 -1200 nm at one measurement angle 74 o for the sake of brevity.