1. A method of fabricating a semiconductor device, the method comprising: forming first and second mask structures on respective first and second regions of a feature layer, each of the first and second mask structures comprising a dual mask pattern and an etch mask pattern thereon having an etch selectivity relative to the dual mask pattern; isotropically etching the etch mask pattern of the first and second mask structures to remove the etch mask pattern from the first mask structure while maintaining at least a portion of the etch mask pattern on the second mask structure; forming spacers on opposing sidewalls of the first and second mask structures; and selectively removing the first mask structure from between the spacers in the first region using the portion of the etch mask pattern on the second mask structure as a mask to define a first mask pattern comprising the opposing sidewall spacers with a void therebetween in the first region and a second mask pattern comprising the opposing sidewall spacers with the second mask structure therebetween in the second region.