Microwave dielectric behavior of nanocrystalline titanium dioxide thin films

Dielectric behavior of titania thin films, deposited by DC magnetron sputtering on to borosilicate glass substrates, in the microwave region is reported. Deposition in a 100% pure oxygen DC plasma is demonstrated. The nanocrystalline films showed a crystallite size range between 16 and 50 nm. The crystallite size decreased with increase in film thickness between 150 and 700 nm and increase in rate of deposition. Refractive index decreased with increase in percentage of oxygen in the sputter gas. The dielectric constants were measured using the extended cavity perturbation technique at 8.98, 10.01 and 10.98 GHz. The dielectric constant and loss tangent showed a very small decrease with increase in frequency but exhibited a stronger dependence on processing parameters as well as crystallite size. The dielectric constant peaked at a value of 46 (±0.1) at a frequency of 8.98 GHz with 50% O₂ in the plasma, decreasing above and below it. Similarly, it peaked at a value of 46 (±0.1) for a crystallite size of 40 nm decreasing thereafter. Interestingly, the dielectric constant also showed a maximum at a bandgap of 3.36 eV at the same value. In each case the maximum dielectric constant was accompanied by a minimum in the dielectric loss. The variation in the dielectric properties can be correlated with microstructural changes as evidenced by SEM and AFM images.