Ptychographic imaging ellipsometry with visible and extreme ultraviolet light

The chemical and structural properties of a specimen can often be inferred by determining its complex refractive index. Ellipsometry is the standard method to measure the complex refractive index [1]. In ellipsometry, the complex refractive index is retrieved by acquiring absolute reflectivity changes for different states of the incident light (varying polarization or angle of incidence). While ellipsometry works well for bulk and thin-film specimens without transverse structure, imaging an object with spatially varying refractive index is difficult. Scanning ellipsometry uses a tightly focussed beam which requires accurate knowledge of the beam properties and more complex analysis. Furthermore, the signal is averaged over the spot size, limiting spatial resolution. Other implementations like imaging ellipsometry require an imaging setup with high-NA optics, which lead to measurement errors caused by aberrations and other defects. This becomes especially problematic when decreasing the wavelength in the extreme ultraviolet (EUV) regime, to improve spatial resolution.