Imaging surfaces of nano-scale roughness by atomic force microscopy with carbon nanotubes as tips: A comparative study

摘要

Accurate knowledge of the nanoroughness of surfaces is crucial for many applications related to optics, electronics or tribology. Although atomic force microscopy (AFM) can image surfaces with a nanometre spatial resolution, the finite size of standard tips means that pores, pits or grooves with dimensions similar to or smaller than the tip apex will not be accurately imaged. Furthermore, standard tips are made of silicon or silicon nitride and are prone to wear. Mitigation may arise from the availability of AFM tips with a carbon nanotube (CNT) at their foremost end. This study compares the imaging performance of ultrasharp Si tips, CNT AFM tips prepared by a Langmuir-Blodgett (LB) technique, and of CNT AFM tips prepared by a chemical vapour deposition (CVD) technique. The free length of the CNT AFM tips is in the range 80-200 and 600-750 nm, respectively. A polycrystalline niobium film surface is imaged that shows nanoroughness. The measurements demonstrate that CNT AFM tips allow excellent imaging if the scan parameters are adjusted very carefully. Nevertheless, in some cases distortions are found. The measured average grain diameter is 19.9 ± 3.6 nm in the case of a CNT AFM tip made by the LB technique, and 18.0 ± 3.3 nm in the case of a CNT AFM tip made by CVD. In addition to cross-sections of topography images, also the power spectral density (PSD) is analyzed. An empirical approach for the readout of the characteristic length is suggested that involves the first derivative of the decadic logarithm of the PSD.