A method of improving mask data used in fabrication of a semiconductor device includes, in part, setting a threshold value associated with a defect based on stochastic failure rate of the defect, performing a first optimal proximity correction (OPC) of the mask data using nominal values of mask pattern contours, identifying locations within the first OPC mask data where stochastically determined mask pattern contours may lead to the defect, placing check figures on the identified locations to enable measurement of distances between the stochastically determined mask pattern contours, and performing a second OPC of the first OPC mask data so as to cause the measured distances to be greater than the threshold value.