Phase retrieval of periodic patterns

Coherent Diffraction Imaging (CDI) is a promising method for the metrology of lithographic structures due its low cost, ease of implementation, and scalability. CDI can be realized in different ways, but the core process involves illuminating the sample with coherent light, recording the diffracted intensity distribution, and computationally recovering the phase to reconstruct the complex amplitude of the sample. One of the primary challenges in semiconductor metrology is high resolution inspection for lithographic patterns. Although the patterns can be relatively complex, periodic structures are found in many layers of modern devices. Given the traction gained by hybrid bonding for wafer–to–wafer (W2W) or chip–to–wafer (C2W) interconnects, their metrology is becoming important. It is interesting to study and research phase retrieval methods that can potentially be applied to reconstruct the topography of the Cu pads structures that are used for the electrical connections in such scenario. This paper presents a study of phase retrieval algorithms applied to reconstruct the profile of the dishing/protrusions present on each Cu pillar of the array. In particular, we explore a sample model featuring a Cu pad array where the single pillars have a bowl-shaped topography and we investigate, by means of simulations, the accuracy that we can achieve with this approach in the measurement of the depth and shape of the concavity of the surface of the pillars.