Combined effects of thickness, grain size and residual stress on the dielectric properties of Ba0.5Sr0.5TiO3 thin films

Abstract

We studied the microstructure and dielectric properties of Ba0.5Sr0.5TiO3 thin films on polycrystalline alumina substrates with film thicknesses in the range 90–400 nm. Upon annealing at 900 °C in a rapid-thermal-annealing furnace the films crystallized in a pure perovskite phase with uniform and dense microstructures consisting predominantly of columnar grains. As the film thickness increased from 90 to 170 nm, the average lateral grain size increased from 45 to 87 nm. In the same manner, the dielectric permittivity of the films increased from 650 to 1250, measured at 100 kHz and room temperature. In the thickness range between 170 and 240 nm, only a slight change of the grain size and the permittivity was observed. But with a further increase of the thickness to 400 nm the permittivity decreased, even though the grain size remained unchanged. A similar trend was also observed in the GHz range. The reduced dielectric permittivity in the thicker films is related to the tensile residual stress, which develops due to the thermal expansion mismatch between the film and the substrate. The measured tensile residual stress was above 300 MPa in the thickness range 90–170 nm, while it partially relaxed at greater thicknesses because of the formation of cracks. As a result, the dielectric permittivity of the films is reduced.