Inline metrology of high aspect ratio hole tilt using small-angle x-ray scattering

High aspect ratio (HAR) structures found in 3D NAND memory structures have unique process control challenges. The etch used to fabricate channel holes several microns deep with aspect ratios beyond 50:1 is a particularly challenging process that requires exquisitely accurate and precise control. It is critical to carefully analyze multiple aspects of the etch process, such as hole profile, tilt, uniformity, and quality both during development and production. X-ray critical dimension (XCD) metrology, which is also known as critical dimension small-angle X-ray scattering (CD-SAXS), is a powerful technique that can provide valuable insights on the arrangement, shape, and size of periodic arrays of HAR features. XCD is capable of fast, non-destructive measurements in the cell-area of production wafers, making XCD ideal for in-line metrology. Through several case studies, we will show that XCD can be used to accurately and precisely determine key properties of holes etched into hard mask and multilayer oxide / nitride (ON) film stacks. We show that the measurement of hole tilt can be achieved without the aid of a structural model using a fast tilt methodology that provides sub-nanometer precision. Measurements were performed across several production wafers to determine the etch uniformity and quality. Particular attention was given at the edge of the wafers to account for large variation observed.