Tapping AFM measurements artefacts in the acquisition of high-aspect-ratio rectangular nanostructures using dedicated sharp tips

In the application of diffractive optical elements (DOE), the aspect ratio of the surface features is continuously increasing on demand of a larger span of end applications. Measuring this kind of structures is, on the other hand, a non-trivial process, which requires dedicated metrological solution and approaches. When the structures cover a large span of the investigated surface, a compromise between measuring precision and field of investigation has to be met. In this research, a rectangular grating with a pitch of 700 nm, a trench size of 350 nm, a nominal height of 1100 nm, representing an aspect ratio of 3 is measured using an AFM (icon-Pt provided by company Bruker®). The grating structure covers an area of 2 mm x 2 mm and it is included in a specimen with global dimensions of 10 mm x 10 mm. The grating is manufactured on a pure silicon wafer using ion-beam lithography. The AFM is operated in tapping mode using dedicated sharp silicon tips. The research shows that even though the tip nominal geometry fits into the grating geometry, the tip convolution cannot be neglected in the reconstruction of the effective surface topography. Repetitive artefacts are sampled while measuring the grating topography and a deconvolution procedure is finally proposed.