Novel high-resolution sidewall imaging using standard Atomic Force Microscopy equipment: Exceeding surface scanning using customized FIB-milled AFM tips in torsional feedback mode

The Atomic Force Microscope (AFM) represents an essential measuring instrument in various disciplines covering life science, biology, material science, semiconductor industries, and micro- and nanotechnology. However, conventional AFM technology is limited as it is a 2.5D image acquisition technique thus only giving a “view from above”. In semiconductor and nanomanufacturing industries the measurement of linewidths, critical dimensions (CD), and sidewall angle/roughness on the wafer level is one of the most fundamental dimensional nanometrology needs. As technology progresses the critical dimension size and tolerance decrease. Especially, characterizing the sidewall roughness of nanostructured photonic components is one of the key challenges and plays an important role in optimizing the efficiency of nanooptical devices such as waveguides. The main source for loss in waveguides is the sidewall roughness which results in diffuse scattering. Standard pyramidal AFM probes are unable to correctly scan these structures. Firstly, the pyramidal tip of the AFM probe cannot scan high aspect ratio structures in a correct way leading to a distorted AFM image and to incorrect trench width and height. Secondly, the sidewall roughness and angle of the structure cannot be measured at all since the AFM probe is not able to contact the sidewall structure. To overcome these problems, we suggest a novel method for performing sidewall measurements that is based on utilization of standard AFM equipment in combination with customized FIB-milled AFM tips and a control loop incorporating the torsion of the cantilever as feedback to control the lateral position of the AFM tip.