A method of generating a photo mask for use during fabrication of a semiconductor device includes; generating an initial layout design including critical circuit paths and non-critical circuit paths by shielding all gate line patterns used to implement transistors in the critical circuits and non-critical circuits, and thereafter generating a layout design from the initial layout design by selectively un-shielding a non-critical gate line pattern among the gate line patterns used to implement a gate of a non-critical transistor in a non-critical circuit, while retaining the shielding of all critical gate line patterns among the gate line patterns.