A method of fabricating an integrated circuit device includes forming first and second mask structures on respective first and second regions of a feature layer. Each of the first and second mask structures includes a dual mask pattern and an etch mask pattern thereon having an etch selectivity relative to the dual mask pattern. The etch mask patterns of the first and second mask structures are etched to partially remove the etch mask pattern from the second mask structure. Spacers are formed on opposing sidewalls of the first and second mask structures. The first mask structure is selectively removed from between the spacers in the first region to define a first mask pattern including the opposing sidewall spacers with a void therebetween in the first region, and a second mask pattern including the opposing sidewall spacers with the second mask structure therebetween in the second region.