Optical properties of plasmonic titanium nitride thin films from ultraviolet to mid-infrared wavelengths deposited by pulsed-DC sputtering, thermal and plasma-enhanced atomic layer deposition

We present a comparative study of the optical properties of 50 nm-thick titanium nitride (TiN) films deposited on a silicon substrate by pulsed-DC sputtering, thermal, and plasma-enhanced atomic layer deposition. Silicon was chosen as the optimal material for the complementary metal–oxide–semiconductor (CMOS) integration. Using ellipsometry and Fourier-transform infrared (FTIR) spectroscopy, broadband permittivity of the films was obtained from a series of free-space reflection measurements in the wavelength range of 210 nm to μm (47619–500 cm⁻¹). Our particular focus is on the influence of the deposition method and its process temperature on the TiN plasmonic properties. We used grazing incidence X-ray diffraction, X-ray photoelectron spectroscopy, X-ray reflectometry, atomic force microscopy, and electrical resistivity measurements to analyze the composition of thin films and to elucidate the effect of different deposition parameters on their optical properties. All measurements were carried out at room temperature and the thickness of all film samples was fixed to 50 nm to exclude the influence of these factors on the optical properties. We demonstrate that PEALD-deposited films have smoother surfaces than those deposited by the sputtering technique, while sputtered films have higher densities. In addition, by changing the temperature of both processes, it is possible to influence the stoichiometry and crystal orientation of the layers. Based on this characterization, we found out that TiN thin film deposited by pulsed-DC at 600 °C gives the best plasmonic response (high negative permittivity ε₁ and relatively low losses associated with ε₂, where ε₂ and ε₂ are the real and imaginary parts of permittivity, respectively) among all samples for the entire wavelength range under study. The same film also exhibits the excellent resistivity of 29 µΩ. These results suggest that there is a direct relationship between stoichiometry, structural quality, and oxygen incorporation in TiN films and their plasmonic response.