In-line metrologies currently used in the semiconductor industry are being challenged by the aggressive pace of device scaling and the adoption of novel device architectures. In defect inspection, conventional bright field techniques will not likely be able to meet defect capture rate requirements beyond the 16 nm node. Electron beam-based inspection is able to meet resolution limits well below this node, but operates at a significantly lower throughput. It, therefore, has become necessary to explore alternative approaches with the potential to meet both resolution and throughput requirements. Critical dimension (CD) metrology, on the other hand, is less challenged by resolution than by the increasingly 3D nature of the information that needs to be collected from modern device structures. It is therefore valuable to explore metrology techniques that are sensitive to spatial variations across the entire volume of the interrogated feature. Through-focus scanning optical microscopy (TSOM) is a novel method that allows conventional optical microscopes to collect dimensional information down to the nanometer level by combining 2D optical images captured at several through-focus positions. This relatively simple technique is inexpensive and has high throughput, making it attractive for a variety of semiconductor metrology applications, such as CD, photomask, overlay, and defect metrologies. In this work, we expand on the analysis of TSOM as a potential technique for defect inspection and study its ability to characterize 3D high aspect ratio (HAR) features. For defect inspection applications, we extend the simulation space well beyond the 11 nm node, based on dense features with CDs ranging from 13 nm to 7 nm. The optical response of a variety of patterned defect modes, sizes, and heights was likewise explored under different polarization and wavelength illumination conditions. Results indicate TSOM has the ability to extract defect signal for most of the cases studied. Work on HAR features focused on exploring 3D sensitivity to features such as bottom CD, sidewall angle, and depth. HAR targets were studied using simulations down to the 11 nm node. Promising results were observed in terms of sensitivity to bottom CD, sidewall angle, and depth.