A method of improving the physical and/or electrical and/or magnetic properties of a thin film material formed on a substrate, wherein the properties of the thin film material are stress-dependent, by selectively applying force to the substrate during the film formation and/or thereafter during the cooling of the film in the case of a film formed at elevated temperature, to impose through the substrate an applied force condition opposing or enhancing the retention of stress in the product film. The method of the invention has particular utility in the formation of ferroelectric thin films which are grown at temperature above the Curie temperature, and which may be placed in tension during the cooling of the film to provide ferroelectric domains with polarization in the plane of the film.