Superlattice effects and limitations of non-destructive measurement of advanced Si/Si1−xGex superlattice structures using Mueller matrix scatterometry and high-resolution X-ray diffraction

Advanced semiconductor devices are moving toward three-dimensional (3D) geometries due to scaling demands and performance requirements. Non-destructive metrology necessary for process control of 3D structures must be advanced to facilitate their transition from technology development to high-volume manufacturing. Thin film metrology using Mueller matrix spectroscopic ellipsometry (MMSE) and X-ray diffraction (XRD) film metrology, as well as patterned structure metrology using optical critical dimension (OCD) and X-ray fluorescence (XRF) techniques, have proved capable of measuring the Si/Si1−xGex superlattices and gate-all-around transistor test structures. Because these techniques are indirect, their limitations associated with superlattice device structures need to be further understood. To understand these limitations, a four-superlattice layer Si/Si1−xGex structure was measured at four process steps: as an unpatterned film stack, after anisotropic column etch, and at low and high levels of cavity etch. Thin film samples were analyzed with XRD and MMSE, and patterned samples were analyzed using OCD, as well as XRF. A model was developed describing the primary and secondary process effects on the structure. This was evaluated for consistency on datasets collected at different measurement azimuth angles. Square error–based methods were evaluated to quantify OCD model detectability of fit variable step deviations, as well as sensitivity relative to the model to measurement error. OCD and XRF results were compared with reference scanning transmission electron microscopy (STEM) images of nanowire test structure lamellae. Dual-azimuth fit OCD models were found to be within 0.3 nm of the STEM reference mean.