Thickness–concentration–viscosity relationships in spin-coated metalorganic ceria films containing polyvinylpyrrolidone

Zr-doped ceria thin films were prepared by spin-coating metalorganic solutions containing polyvinylpyrrolidone (PVP), which serves as a relaxing agent to relieve the strong mechanical stresses developed during the thermal decomposition step. The precursor solutions were deposited on silicon substrates and subsequently heated in air at 500 °C. The complete decomposition of organics was checked by infrared spectroscopy with attenuated total reflectance. The molar ratio between PVP and total metal ions was varied between zero and one. Furthermore, the effect of using two different PVP molecular weights was also investigated. For the high molecular weight of PVP, crack-free films as high as 150 nm could be obtained compared to 30 nm when no PVP is added. Thickness after spin-coating and after thermal decomposition were determined by profilometry and correlated with the polymer concentration in the precursor solution, showing a linear dependence with PVP concentration in both cases. The main controlling parameter of the final thickness is the viscosity, with similar power law dependencies both before and after thermal treatment, which would indicate that the porosity fraction remains essentially constant in the final films. Furthermore, AFM analysis was used to investigate the flatness and surface porosity of the films after the thermal treatment.