Superlattice effects and limitations of non-destructive measurement of advanced Si/Si(1-x)Ge(x) superlattice structures using Mueller matrix scatterometry (MMSE) and high-resolution x-ray diffraction (XRD)

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 XxRay Diffraction (XRD) have proved capable of measuring the Si/Si_1-xGe_x few layer superlattices used to fabricate Gate All Around (GAA) transistor device test structures. The limitations of these techniques associated with superlattice device structures needed to be further understood. To understand the limits of MMSE Scatterometry and XRD, a four superlattice layer Si/Si_1-xGe_x structure was measured as a film stack, a column etched sample, and at two levels of cavity etch. XRD was used to determine individual layer thicknesses and compositions of the film stack, and electron microscope data was used to verify initial structure dimensions for OCD models. First, film stack characterization and metrology for both ellipsometry and x-ray characterization were explored. OCD modeling and measurement strategies were evaluated to address the challenge of fitting the superlattice thicknesses in the stack at different etch levels. We demonstrate the characterization of relevant test structure dimensions, including cavity etch using MMSE-Scatterometry. We also demonstrate an alternate technique, x-ray fluorescence for determining cavity etch on the same structures.