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 both the first and second mask structures to remove the etch mask pattern from the first mask structure and partially remove the etch mask pattern from the second mask structure such that at least a portion of the etch mask pattern on the second mask structure remains; 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.