Tungsten oxynitride (W-O-N) thin films were deposited onto silicon (100) and quartz substrates using direct current (DC) sputtering. Composition variations in the W-O-N films were obtained by varying the nitrogen gas flow rate from 0 to 20 sccm, while keeping the total gas flow constant at 40 sccm using 20 sccm of argon with the balance comprised of oxygen. The resulting crystallinity, optical properties, and chemical composition of the DC sputtered W-O-N films were evaluated. All the W-O-N films measured were shown to be amorphous using x-ray diffraction. Spectrophotometry results indicate that the optical parameters, namely, the transmission magnitude and band gap (E g ), are highly dependent on the nitrogen content in the reactive gas mixture. Within the W-O-N system, E g was able to be precisely tailored between 2.9 eV and 1.9 eV, corresponding to fully stoichiometric WO 3 and highly nitrided W-O-N, respectively. Rutherford backscattering spectrometry (RBS) coupled with X-ray photoelectron spectroscopy (XPS) measurements indicate that the composition of the films varies from WO 3 to W-O-N composite oxynitride films.
a b s t r a c tOptical properties, including the index of refraction, extinction coefficient and band gap of 100 nm thick tungsten oxynitride (W-O-N) films are reported. In addition, the Wemple and DiDomenico (WDD) model was used to calculate the dispersion energies and oscillator energies of the films, establishing a correlation among the films' optical, chemical, and physical properties, as a function of nitrogen content. Nitrogen concentration in the W-O-N films was varied by adjusting the nitrogen gas flow rate from 0 to 20 sccm while keeping total gas flow (nitrogen þ oxygen þ argon) constant at 40 sccm. Both the index of refraction (n) and extinction coefficient (k) of W-O-N films demonstrated a high degree of sensitivity to the nitrogen content during deposition. The optical constants of films fabricated without any nitrogen correspond to transparent W-oxide (WO 3 ) where n 550 ¼ 2.1 and k 550 ¼ 0.0. The magnitude of the spectral response for both n and k tends to increase with increasing nitrogen content. Systematic increases of the films' nitrogen content lead to the formation of W-oxide (E g z 3 eV) / W-O-N oxynitride semiconductor (E g z 2 eV) / N-rich W-O-N semi-metal (E g < 2 eV) / WN 2 type metallic transition was evident in dispersion profiles of n and k for W-O-N films with increasing nitrogen content. The corresponding mechanical characteristics, namely hardness (H) and Young's modulus (E), attain a maximum of 4.46 GPa and 98.5 GPa, respectively, at a nitrogen flow rate of 5 sccm, at which point H and E values decrease to attain 3.57 GPa and 72.91 GPa, respectively. The trend observed in H and E values correlate with the W-O and W-N bonds formation in W-O-N along with the interruption of local epitaxy attributed to increasing nitrogen content within the growth chamber. A correlation among the nitrogen content, optical constants and physical properties, along with the associated dispersion model, is presented to account for the optical properties of sputter-deposited W-O-N films. The results demonstrate that tailoring the properties of W-O-N films for desired applications can be achieved by tuning the nitrogen content and chemical composition.
The effect of post-deposition thermal treatment on the optical and mechanical properties of W-ON thin films is reported. W-ON thin films were deposited onto silicon (100) and quartz substrates utilizing direct current (DC) sputtering under the variable flow of reactive gas mixture. Post-deposition annealing was performed at a temperature (T a) of 400 °C under inert (argon) and oxygen (air) atmosphere. For annealing in an inert atmosphere, spectroscopic ellipsometry (SE) revealed changes to the dispersion profiles and magnitudes of the index of refraction (n) and extinction coefficient (k) as a result of thermal treatment. X-ray photoelectron spectroscopy (XPS) measurements revealed the loss ofboth nitrogen and oxygen during the annealing process in an inert atmosphere while the samples remain amorphous as ev idenced in X-ray diffraction measurements. The chemical changes correlate with the changes seen in the optical constants for annealed W-ON films. Furthermore, the physical changes during post thermal treatment probed by calculating the film-density using the Lorentz-Lorenz relation indicate a decrease in density that supports the changesin chemical composition and optical properties. Density values for annealed W-ON films vary in the range ~9-12 g/cm 3 , the low-to-high end of density values are noted for low and high values of nitrogen flow rate, respectively, employed during deposition. Mechanical properties of W-ON films were demonstrated a drastic improvement as a result of post thermal treatment. The hardness values generally were improved but the W-ON films annealed in air showed the significant improvement, up t o ~10 GPa. XRD measurementsindicated the amorphous-to-crystalline transition with m-WO 3 phase as dominant component for samples annealed in an air due to the abundance of oxygen absorbed during annealing.
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