Methods for fabricating a semiconductor device include forming a first layer on an underlying layer, forming a hardmask on the first layer, and patterning holes through the hardmask and first layer. An overhang is formed extending over sides of the holes. A conformal layer is deposited over the overhang and in the holes until the conformal layer closes off the holes to form a void/seam in each hole. The void/seam in each hole is exposed by etching back a top surface. The void/seam in each hole is extended to the underlying layer.