A method for etching an insulating layer includes: sequentially forming a first gate insulating layer, an amorphous silicon layer, a first interlayer insulating layer, and a second interlayer insulating layer on a substrate; applying a photoresist on the second interlayer insulating layer, and patterning the photoresist through a photo-process; first etching the second interlayer insulating layer and the first interlayer insulating layer until at least a portion of the amorphous silicon layer is exposed by using the patterned photoresist as a mask; second etching the second interlayer insulating layer and the first interlayer insulating layer; third etching the amorphous silicon layer; and fourth etching the first gate insulating layer, wherein an etching gas used in the second etching includes a material having a higher etching selection ratio of the first and second interlayer insulating layers to the amorphous silicon layer than an etching gas used in the first etching.