The second buffer layer 114 may be formed of multiple layers. When the second buffer layer 114 is formed of multiple layers, an uppermost layer of the second buffer layer 114 in contact with the second active layer 131 may be formed of a silicon oxide (SiOx) material having a low hydrogen content. For example, the uppermost layer of the second buffer layer 114 may be formed of a silicon dioxide (SiO2) material. For example, as shown in FIG. 4, when the second buffer layer 114 is formed as a double layer, the second buffer layer 114 may include a second lower buffer layer 114a disposed on the first interlayer insulating layer 113 and a second upper buffer layer 114b disposed on the second lower buffer layer 114b. The second upper buffer layer 114b in direct contact with a lower portion of the second active layer 131 may have a relatively lower hydrogen content than the second lower buffer layer 114a. In addition, the second lower buffer layer 114a disposed on the first interlayer insulating layer 113 may have higher insulating properties than the second upper buffer layer 114b. The second upper buffer layer 114b of the second buffer layer 114 in direct contact with the second active layer 131 of the second thin film transistor 130 may be formed of silicon oxide (SiOx). For example, the second upper buffer layer 114b may be formed of silicon dioxide (SiO2). In addition, the second lower buffer layer 114a may be formed of silicon nitride (SiNx). However, the present disclosure is not limited thereto.