Effect of annealing on the structural, optical and surface properties of chromium oxide (Cr2O3) thin films deposited by e-beam evaporation for plasma etching applications

Cr2O3 thin films were prepared by e-beam deposition onto silicon substrates and annealed in a furnace in air ambient at a wide range of temperatures. The effect of annealing on the microstructural, morphological, optical and surface properties were studied. XRD reveals the deposited chromia is amorphous in nature. However, post-deposition annealing at elevated temperatures (400 ºC and above) resulted in a polycrystalline stoichiometric Cr2O3 phase. Surface morphology and film cross-sections with respect to annealing temperature were monitored. SEM showed the amorphous films exhibited a smooth and uniform growth on the entire surface whereas the polycrystalline phases exhibited granular structures. Also, there was a thermally-grown oxide layer at the interface between the silicon substrate and the Cr2O3 film approximately 64 and 190 nm high in the samples annealed at 900 and 1100 °C, respectively. EDX and XPS techniques were employed to investigate the elemental composition and surface chemical analysis. After in-situ argon sputter cleaning, XPS confirmed that the amorphous phases were slightly oxygen-rich whereas the polycrystalline samples contained approximately 60% oxygen and 40% chromium. Optical properties of the as-deposited and annealed samples were characterized by the reflectance spectra and variable angle spectroscopic ellipsometry. The crystalized samples had relatively smaller refractive indices compared to the amorphous samples, which may be due to the increase in crystallinity, as well as the surface morphology and texture of the annealed samples. Finally, the etch robustness of Cr2O3 as a hard mask material was tested in major plasma chemistries. The polycrystalline films were significantly more resistant to plasma etching than the amorphous films. The differences between the etching rates of amorphous and crystalline films were attributed to the increase in the degree of crystallinity of the chromia films as a result of annealing.