A method includes followings operations. A substrate including a first surface and a second surface is provided. The substrate and a transparent film are heated to attach the transparent film on the first surface. A first coefficient of a thermal expansion (CTE) mismatch is between the substrate and the transparent film. The substrate and the transparent film are cooled. A polymeric material is disposed on the second surface. A second CTE mismatch is between the substrate and the polymeric material. The second CTE mismatch is counteracted by the first CTE mismatch.