Effect of substrate overetching and heat treatment of titanium oxide waveguide gratings and thin films on their optical properties

We investigate subwavelength titanium oxide (TiO2) resonance waveguide gratings (RWGs) and TiO2 thin films of thicknesses ~200 nm fabricated by atomic layer deposition (ALD), in both amorphous and crystalline phases on fused silica substrates. The TiO2 RWGs are fabricated by electron beam lithography, reactive ion etching, and ALD. The thin films and RWGs are characterized structurally by x-ray diffraction and scanning electron microscopy. The optical characterization of RWGs and optical constants of TiO2 films are studied by an ellipsometer. RWGs are designed for TE and TM modes in such a way that an overetch effect of the fused silica substrate can be investigated. Various RWG samples are prepared by gradually increasing the overetch depth and subsequently measuring the performance of the RWGs. A close agreement between the calculated and experimentally measured resonance wavelength spectral shifts is obtained; however, the magnitudes of the measured shifts are greater than calculated ones. A parallel study related to the measurement of the refractive indices and remeasuring the optical shifts of RWGs is carried out after a heat treatment of all the samples under study. The RWGs do not reveal significant spectral changes after the heat treatment; this is primarily due to a change in the surface chemistry by the redeposition of the reaction byproducts on the grating lines.