A method for fabricating a semiconductor structure includes forming a carbon masking layer on a semiconductor layer, forming a protective layer on the carbon masking layer. The method further includes forming an opening in the protective layer and the carbon masking layer and processing the semiconductor layer through the opening to form a first processed region in the semiconductor layer. The method further includes enlarging the opening in the carbon masking layer and performing an additional processing step on the semiconductor layer through the enlarged opening to form a second processed region in the semiconductor layer.