1. A method for fabricating a semiconductor device comprising: forming a feature of the semiconductor device in a first region of a layer of material on a semiconductor substrate; forming a test structure in a second region of the layer, the test structure being formed concurrently to forming the feature; and determining a dimension of the feature using the test structure, wherein determining the dimension comprises: signaling a source to emit a reference signal, the test structure modifying the reference signal to produce a modulated signal; and estimating the dimension of the feature based on the modulated signal.