The invention provides a method for preventing fencing during process of forming an ABS on a slider substrate includes the steps of: (a) providing a slider substrate having a photo-resist covered thereon, the photo-resist having a plurality of trenches developed thereon, the plurality of trenches collectively defining an air bearing surface pattern, the plurality of trench each having at least a non-vertical sidewall, the at least non-vertical sidewall including a zigzag-shaped edge adjoining the slider substrate; (b) etching the photo-resist by oxygen ions, such that the zigzag-shaped edge is trimmed to be a straight edge and the at least non-vertical sidewall is trimmed to be a vertical sidewall; and (c) etching the slider substrate exposed from the trenches such that a fencing-free air-bearing surface is formed on the slider substrate.