The subject of the invention is a heat treatment process by flame treatment of at least one thin film deposited on a glass substrate (1) running in the path of at least one flame treatment device comprising at least one burner (2), said treatment being able to increase the degree of crystallization of said at least one thin film and/or to increase the size of the crystallites in said at least one thin film, said process being characterized in that the maximum transient bending “b” is less than 150 mm and respects the following condition: b≦0.9×d where the bending “b” corresponds to the distance, expressed in mm, between the plane of the substrate without heating (P1) and the point of the substrate closest to the plane (P2) passing through the tip (6) of the burner (2) and parallel to the plane of the substrate without heating (P1), “d” corresponds to the distance between the plane of the substrate without heating (P1) and the tip (6) of the burner (2), expressed in mm, the width of the substrate “L” in a direction perpendicular to the run direction (5) being greater than or equal to 1.1 m.