A method of forming contact holes includes: forming a first conductive layer and a second conductive layer; forming an insulating layer on the first conductive layer and the second conductive layer; forming a photoresist pattern which exposes first and second etch surfaces of a top surface of the insulating layer; performing a first etching process on the insulating layer at a first etching rate; and performing a second etching process on the insulating layer at a second etching rate which is higher than the first etching rate, after a top surface of the first conductive layer is exposed through the insulating layer. The first etch surface is on the first conductive layer, the second etch surface is on the second conductive layer, and a distance between the second etch surface and the second conductive layer is greater than a distance between the first etch surface and the first conductive layer.