According to the present embodiment, the polysilicon layer 134 having a relatively small surface roughness may be formed by performing the cleaning process using the hydrofluoric acid 210 and the rinsing process using the hydrogenated deionized water 220 before the crystallization process.

Hereinbefore, the cleaning process, the rinsing process, and the crystallization process for forming the polysilicon layer 134 are described, however, processes for forming the polysilicon layer 134 other than the above-described process may be added, or some of the above-described processes may be omitted. Further, the above-described processes may be performed multiple times. For example, the crystallization process may be performed twice or more.

Referring to FIGS. 7 to 11, a polysilicon pattern 138 may be formed by patterning the polysilicon layer 134. The polysilicon pattern 138 may include a first region R1 and a second region R2 each having a first thickness TH1, and a third region R3 having a second thickness TH2 less than the first thickness TH1. The first region R1 and the second region R2 may respectively overlap a source contact hole and a drain contact hole formed in a subsequent process.