Advanced semiconductor devices are moving toward 3D geometries due to scaling demands and performance requirements. The non-destructive metrology necessary for process control for high volume manufacturing of 3D structures must be advanced to facilitate their transition from technology development to high volume manufacturing. Optical Critical Dimensions (OCD) techniques based on Mueller Matrix Scatterometry (MMSE) as well as X-Ray Diffraction (XRD) have proved capable of measuring the Si/Si1-xGex few layer superlattices used to fabricate Gate All Around (GAA) transistor device test structures. To understand the limits of MMSE scatterometry and XRD, a four superlattice layer Si/Si1-xGex structure was measured as a film stack, column etched sample, and at two levels of cavity etch. OCD measurement strategies were created which allowed the characterization of relevant test structure dimensions including cavity etch. Simulations were used to explore the ability of X-Ray diffraction to study characterization of the many superlattices predicted for 3D memory.