Atomic force microscopy for high resolution sidewall scans

Abstract

Over the last decades, atomic force microscopy (AFM) became one of the most important measuring instruments in various disciplines covering life science, biology, material science, semiconductor industries, and micro- and nanotechnology. Conventional AFM scanning techniques are limited to a 2.5D image acquisition, resulting a simple 2D high-map of the surface. Due to the ongoing miniaturization in semiconductor and nanomanufacturing industries, measurements on the critical dimensions are of growing importance. Additionally, novel measurement tasks as the determination of linewidths or even sidewall roughness arise as need from nanometrology. Especially, the upcoming technology of nanooptics and photonics relies critically on powerful and ultra-precise characterization tools for complex surfaces on the sub-nanoscale. In order to produce suitable optical waveguides and building-blocks, all surface roughness and surface impurities have to be minimized, since the result in severe light diffusion and diffraction. Conventional AFM probes are suitable for 2.5D image acquisitions only and don't allow a correct scanning of complex structures. The pyramidal tip of standard probes cannot scan high aspect ratio structures, which leads to a distorted AFM image and to incorrect trench width and height. Furthermore, a standard tip is not able to contact surfaces with tilt-angles larger than the pyramid's side-angle. In general, this makes sidewall scans impossible at all. To overcome these problems, a novel method for performing sidewall measurements is proposed, based on standard AFM equipment in combination with customized FIB-milled AFM tips. Standard AFM environment is improved with an additional control loop incorporating the torsion of the cantilever as feedback to control the lateral position of the AFM tip.