Evaluation of intermittent contact mode AFM probes by HREM and using atomically sharp CeO₂ ridges as tip characterizer

The imaging process of the atomic force microscope (AFM) in contact, noncontact, and intermittent contact mode is still debated after more than a decade of widespread use, in particular when imaged features are approaching atomic dimensions. Several models for the interaction between the tip and the surface have been suggested, but, generally they all need an exact description of the geometry of either the tip, the surface, or both. We present here a tip characterizer with close to reproducible geometry, exactly known angles of all surfaces, and sharp features with close to atomic dimension. It has been tested on three commercial AFM probes and a laboratory-made electron-beam-deposited tip, sharpened by oxygen plasma etching. High-resolution transmission electron microscopy has been used to unambiguously verify the tip shapes down to atomic dimensions, both before and after imaging in intermittent contact mode. The effect on the recorded AFM images is shown of tip shape, tip wear, spallation, and accumulation on the tip of amorphous and crystalline debris. The imaging is shown to be a dynamic event, with a continuously changing tip and occasional catastrophic events that give abrupt changes in imaging conditions. The tips are severely worn down already after scanning a few centimeters, but accumulated amorphous material may still give it imaging capabilities in the nanometer range, even with having a tip radius exceeding 130 nm. Accumulated amorphous material seems to be more important than previously believed. Procedures for tip in situ characterization and reliable imaging are suggested.