A metrology approach to detect nanoscale asymmetries in structures on a silicon wafer is being introduced through simulation investigations. The simulations were performed based on a rigorous coupled-wave analysis. A structured spot focused on the wafer with a high-numerical aperture () has been scanned over the wafer. Having access to the complex amplitude of the wavefront over the field, both the intensity and the phase profile of the spot have been investigated in the far-field image plane. To show the proof of concept, we considered a 10-nm asymmetry that appears in the radius of the bottom roundings of a trench on the wafer. The results have been compared to the case of using a conventional spot and it has been shown that the structured illumination provides more sensitivity to the presence of asymmetry. In both illumination cases, the phase distribution along the spot was shown to be more sensitive to the changes due to the presence of asymmetry in the wafer.