In situ spectroscopic ellipsometry study of TiO2 films deposited by plasma enhanced chemical vapour deposition

TiO2 thin films of 300–400 nm were deposited at low pressure (3 mTorr) and temperature (<150 °C) on silicon substrates using plasma enhanced chemical vapour deposition with different substrate self-bias voltages (Vb). The impact of growth interruptions on the film characteristics was studied by in situ spectroscopic ellipsometry (SE), scanning electron microscopy (SEM) and X-ray diffraction. The interruptions were carried out by stopping the plasma generation and gas injection once the increase of the layer thickness during each deposition step was about 100 nm. Suitable ellipsometric models were built to account for the structural and optical differences among the layers grown at different stages. When no bias is applied or Vb = −10 V, the films deposited with and without interruptions are composed of a dense layer near substrate, an intermediate gradient layer and a top roughness layer. But the growth interruptions result in an increase of the dense layer thickness and a decrease of surface roughness. The discrepancy of the refractive index measured by SE between bottom and upper layers can be reduced by growth interruptions or biasing the substrate. In the case of Vb = −50 V, the film becomes well organized, the top surface appears smoother, and the refractive index can be increased greatly, without significant effect of growth interruptions.