casting solution is substantially parallel to the x direction the expansion portion 112 reaches the slot end by being expanded while having a predetermined inclination angle u03b8 dc with respect to another portion having a constant width in an exemplary embodiment the predetermined inclination angle u03b8 dc may have a range between about 0 degree and about 10 degrees with respect to the x axis and preferably has an angle of less than about 5 degrees for controlling the thickness of the casting solution when the inclination angle u03b8 dc is greater than about 10 degrees the thickness of the casting solution may not be simply controlled in an exemplary embodiment a length l dc of the expansion portion 112 taken along the x axis may be ranged between about 1 millimeter mm and about 3 mm fig3 is a graph illustrating a flow velocity variation of the solution at the peripheral area of the lip of the slot die referring to fig2 and 3 when the expansion portion 112 is provided near the lip end of the slot 111 the flow velocity variation of the casting solution is reduced near the lip end that is when the expansion portion 112 is not provided as shown by the dotted line in fig3 the flow velocity v x of the.