Optical profilometry is a non-contact form of profilometry that can be used to characterise the surface steps and the roughness of a material. Optical profilometry employs phase-shifting and/or vertical scanning interferometry to resolve the topology of complex 3D structures. The technique marries precision z-axis control with interference-based techniques to resolve features from the angstrom to millimetre scale. The technique lends itself well to die-based measurements for ISO/QA and large area mapping. Profilometry is useful in process control steps such as measuring etch depth and lithography patterns.