Characterisation of Complex Multilayer Nanostructures with High Aspect Ratio

Deposition studies of deep vertical dips on semiconductor wafers can create problems at an industrial manufacturing scale, since cross-sectioning requires a lot of time for analysis. The PillarHall test chip is a solution to this problem, because it transforms deep vertical dips with high aspect ratio to similar structures in horizontal direction. A key parameter for deposition studies is the thickness of the horizontal air gap of 100–500 nm in different components. This study addresses the related uncertainties when characterising complex thin-film structures using reflectance measurements of the layer structure. Optical profilometry indicates bending of the upper surface of the PillarHall component which needs to be considered when analysing the reflectometry results. An advanced four-layer model including the effects of bending is presented and tested by fitting the simulated reflectance spectrum to a measured one. Reflectometry results on structural characteristics of the uppermost layer agree with profilometry measurements, supporting the method's reliability. The obtained minimum air gap thickness is 454 nm with an expanded uncertainty of 8 nm at 95% confidence level.