A post baking apparatus comprising a baking chamber configured to receive a substrate with an exposed photoresist film, a lower heater in the baking chamber under the substrate to heat the exposed photoresist film, an applier applying an electric field or a magnetic field to the exposed photoresist film along a vertical direction, which is substantially perpendicular to an upper surface of the exposed photoresist film, to control diffusions of an acid or a secondary electron, which are generated from the exposed photoresist film, along a horizontal direction, and a controller configured to control an operation of the applier.