NOVELTY - Preparing thin film involves: coating thin film-forming composition (30) on substrate (20); and heat-treating coated composition under less than 760 Torr. The coating is performed by inkjet printing, spin coating, slit coating, spraying, immersion, roll-to-roll method or nanoimprinting. The heat-treating is performed at 1x 10-6 to less than 760 Torr and 250-450 degrees C. The composition comprises metal oxide precursor, sol stabilizer and solvent. The composition includes solvent having lower and higher boiling points to respectively increase and decrease compact layer thickness of thin film.    USE - For preparing a thin film comprising oxide semiconductor thin film, insulator thin film, and metal thin film useful in an electronic device selected from thin film transistor, liquid crystal display device, organic light-emitting diode display device, electrophoretic display device, memory device, solar cell device and optical film (claimed).    ADVANTAGE - The method prepares thin film including a compact layer having a relatively large thickness of greater than 50-20000 Angstrom and a relatively high refractive index of 1.85-2. The method prepares thin film by which the thickness of a thin film layer having a relatively high refractive index can be adjusted and that is performed at relatively low costs; and can prepare compact layers having various thicknesses.    DETAILED DESCRIPTION - Preparing a thin film, involves: coating a thin film-forming composition (30) on a substrate (20); and heat-treating the coated thin film-forming composition under a pressure less than 760 Torr. The coating is performed by one of inkjet printing, spin coating, slit coating, spraying, immersion, roll-to-roll method, and nanoimprinting. The heat-treating is performed at a pressure of 1x 10-6 to less than 760 Torr and at 250-450 degrees C. The thin film-forming composition comprises a metal oxide precursor, a sol stabilizer, and a solvent. The thin film-forming composition includes one of a solvent having a lower boiling point to increase a thickness of a compact layer of the thin film and a solvent having a higher boiling point to reduce the thickness of the compact layer, the compact layer having a refractive index of 1.85-2. INDEPENDENT CLAIMS are included for the following:    (1) thin film prepared using the method, where the thin film comprises a compact layer having a thickness of greater than 50-20000 Angstrom and a refractive index of 1.85-2, where the thin film further comprises porous layer having a refractive index less than 1.85;    (2) an apparatus for preparing the thin film, comprising: a coating unit to coat the thin film-forming composition on the substrate; and a reduced pressure heat treatment unit to form a thin film on the substrate, where the apparatus further comprises: a buffer unit to perform at least one of a preheating process of the substrate passed through the coating unit and a cooling process of the thin film formed in the reduced pressure heat treatment unit, where the coating unit includes at least one of a substrate-heating member and a pressure reducing member; and    (3) an electronic device comprising the thin film prepared by the method.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view for explaining the method of preparing the thin film.    Substrate (20)    Thin film-forming composition (30)    heat plate (40)    Vacuum chamber (50)    Pressure of heat treatment atmosphere (P)