Design and control of porosity in oxide thin films grown by PECVD

This work presents a series of results about the synthesis and characterization of porous oxide thin films (thicknesses similar to 300 nm) prepared at low temperature by PECVD. Two different experimental strategies are described. A first one, used for the preparation of porous SiO2 thin films, consists of the use of a polymeric sacrificial layer that is removed during deposition of the oxide thin film. A second one, used for the preparation of TiO2 thin films, relies on the modification of some critical deposition parameters (i.e., temperature, Ar/O-2 ratio in the plasma mixture, etc.). Thin films with a large variation of pore structures as evidenced by Scanning Electron Microscopy (SEM) have been prepared by the two methodologies. The thin films have been characterised by different techniques and some of their properties related with their microstructure assessed by ellipsometry (optical behaviour) or water contact angle measurements (hydrophobic/hydrophilic character). A quartz crystal monitor has been used to measure water vapour adsorption/desorption isotherms in the films. From the shape of these isotherms it is possible to estimate the type of pores existing in the different thin films. Examples of the potential use of these porous thin films as humidity sensors or hydrophilic surfaces are reported.