Through-focus scanning optical microscopy (TSOM) is another 'scanning' based method that provides threedimensional information (i.e. the size, shape and location) about micro- and nanometer-scale structures. TSOM, based on a conventional optical microscope, achieves this by acquiring and analyzing a set of optical images collected at various focus positions going through focus (from above-focus to under-focus). The measurement sensitivity is comparable to what is possible with typical light scatterometry, SEM and AFM. One of the unique characteristics of the TSOM method is its ability to separate different dimensional differences (i.e. ability to distinguish, for example, linewidth difference from line height difference), and hence is expected to reduce measurement uncertainty. TSOM holds the promise of high-throughput, through comparative measurement applications for wide variety of application areas with potentially significant savings and yield improvements.