A natural oxide layer NOL may be formed on the amorphous silicon layer 132. The natural oxide layer NOL may be formed when an upper portion of the amorphous silicon layer 132 is exposed to air. When the natural oxide layer NOL remains on the amorphous silicon layer 132, projections each having a relatively large thickness may be formed on a surface of the polysilicon layer by the natural oxide layer NOL in the process of crystallizing the amorphous silicon layer 132 for forming the polysilicon layer.

Then, as illustrated in FIG. 2, the amorphous silicon layer 132 may be cleaned.

The amorphous silicon layer 132 may be cleaned using hydrofluoric acid 210. The hydrofluoric acid 210 may be an aqueous solution in which hydrogen fluoride (HF) is dissolved. For example, the hydrofluoric acid 210 may contain about 0.5% hydrogen fluoride. The natural oxide layer NOL formed on the amorphous silicon layer 132 may be removed by cleaning the amorphous silicon layer 132 with the hydrofluoric acid 210.

In an embodiment, the amorphous silicon layer 132 may be cleaned by the hydrofluoric acid 210 for about 60 seconds to about 120 seconds. When the amorphous silicon layer 132 is cleaned for less than about 60 seconds, the natural oxide layer NOL formed on the amorphous silicon layer 132 may not be sufficiently removed, and grains of the polysilicon layer formed thereafter may not sufficiently grow. Further, when the amorphous silicon layer 132 is cleaned for greater than about 120 seconds, the amorphous silicon layer 132 may be affected by the hydrofluoric acid 210 and grains of the polysilicon layer formed thereafter may burst.