Coherent diffractive imaging simulations for wafer inspection of periodic structures

BackgroundOne of the primary challenges in semiconductor metrology is high-resolution inspection of lithographic patterns. Although the patterns can be relatively complex, periodic structures are found in many layers of modern devices, and their metrology is fundamental. In the context of hybrid bonding, a periodic array of copper pads within a wafer is used for electrical connections, and metrology methods are deployed to characterize their topography. Although conventional methods include, e.g., atomic force microscopy and scatterometry, we discuss coherent diffractive imaging (CDI).AimWe aim to study CDI for the metrology of the copper pads used in hybrid bonding. We aim to demonstrate that CDI is a good option for this problem, especially given the phase sensitivity, which may be advantageous when a sample with slight topographic changes is inspected.ApproachCDI is employed for the metrology of a copper pad array. We model the sample as a reflection function. The impact of the copper pad topography on the data is introduced by a phase term that accounts for the optical path length difference induced by the recessions in the copper pad with respect to the surrounding layer. In addition, standard phase retrieval algorithms are modified to incorporate information on the reflectivity value of the substrate/materials that surround the pad array.ResultsSimulations show that the proposed algorithms can drastically improve imaging. Features are better resolved, and the loss function associated with the proposed methods can decrease up to 2 orders of magnitude with respect to standard phase retrieval methods. This improvement is more substantial for an Airy spot illumination, and it is milder when a structured illumination is employed.ConclusionsWe present a study of phase retrieval algorithms applied to the metrology of copper pad topography for hybrid bonding. We demonstrate that by including a priori information in the update function of the object, a better estimation of the recession of the copper pads can be achieved, leading to an improved metrology by CDI.