Properties and Applications of Carbon Nanofibers for Atomic Force Microscopy Gemma Rius, Soichiro Matsui, Masaki Tanemura Nagoya Institute of Technology, NITech Nagoya, Japan rius.gemma@nitech.ac.jp Matteo Lorenzoni, Francesc Perez-Murano Institut de Microelectronica de Barcelona, IMB-CNM-CSIC Bellaterra, Spain francesc.perez@imb-cnm.csic.es Abstract—Scanning probe techniques such as atomic force microscopy (AFM) or scanning probe lithography are powerful methods for the investigation and modification of surfaces at the nanometer scale. In these techniques, the tip curvature radius and aspect ratio of the probe play a crucial role. Here, we employ carbon nanofiber (CNF) as the tip apex of AFM probes. We show that CNF-AFM probes provide good performance in terms of both resolution and reliability when operating the AFM in dynamic mode. In addition, the CNF apex is responsible for an enhancement of the field-induced chemical reaction in a specific form of scanning probe lithography, local anodic oxidation (LAO), which allows the fabrication of silicon oxide (SiOx) patterns with sub-10 nm resolution. Keywords—carbon nanofiber; atomic force microscope; nanolithography I. INTRODUCTION When using an atomic force microscope (AFM) the tip is the ultimate sensing interface with the surface of the sample. For instance, in AFM-based topography imaging the tip curvature radius and its aspect ratio play a crucial role to properly resolve tiny features and steep morphologies of the scanned surface. Tip limiting factors manifest as image artifacts, such as line width widening, mainly due to tip convolution. Nonetheless, tip robustness upon mechanical contact and chemical inertness are also important properties of the tip of an AFM probe [1]. In more advanced modes of an AFM such as electrostatic force microscopy, Kelvin probe force microscopy or nanolithography, tip chemical-structural morphology plays a crucial role in addition to the electric field-based dominant tip- surface interaction. Probe engineering by combining tip morphology and coating material allows field confinement, so that higher sensitivities can be obtained. As an example, carbon nanotubes (CNT) coated with a thin ferromagnetic layer is a paradigmatic instance of this possibility applied to magnetic force microscopy (MFM) [2]. As an all-carbon alternative to the CNTs [3], carbon nanofiber (CNF) materials [4] also showed great promise for scanning probe microscopy. Again for MFM, H. Cui et al. [5] demonstrated the capabilities of CNF probes for high resolution imaging. Many fabrication methods for CNF and CNT functionalized probes have been tested; yet, most of them a very time-consuming or have poor reproducibility. We refer to the work done by Tanemura’s group to synthesize and integrate CNF onto AFM probes [4,6]. They have shown fundamental structural and morphological characteristics of resulting CNF materials. Obtained by Ar + irradiation of a carbon source, their CNFs consist of solid amorphous carbon, forming solid cylindrical structures which have diameters of tens of nanometers to a few hundred nanometers, in part depending on the substrate protrusion where they nucleate. CNFs can be doped during their deposition with selected chemical elements, such as Fe, Cu, etc., to tune or enhance CNF properties, such as electrical conduction. Based in this processing method, CNF- AFM probes can be fabricated in small batches, up to 9 items, with control upon CNF length, diameter, orientation, etc. One of the most interesting SPM-related methods is scanning probe lithography (SPL). SPL is an extremely powerful method for miniaturization and investigations at the nanometer scale [7]. Particularly, SPL-based nanofabrication methods present extraordinary performance in terms of both resolution and flexibility. One of these techniques is the local anodic oxidation (LAO) of Si by means of an AFM, so-called oxidation-scanning probe lithography. LAO-AFM is based on the application of an electric field between a conductive tip and a silicon substrate in ambient conditions. Certain level of humidity is required [8,9]. The condensation of water at the tip/surface location produces a water meniscus that provides oxyanions, which are accelerated towards the surface by the electrical field. As a result, silicon oxide (SiOx) patterns with single/double digit nanometer scale resolution can be precisely obtained. The final resolution is dictated primarily by the extension of the water meniscus. LAO-AFM working parameters and experimental conditions (applied voltage, oxidation time, tip-surface separation, tip and surface composition, relative humidity) are the main factors determining the kinetics of LAO-AFM. Additionally, tip morphology and sharpness of its apex are actually main factors affecting minimum feature size and electric field. Therefore, optimization of oxide nanopatterning benefits from tip engineering. Similar to imaging, carbon nanotube (CNT) AFM tips have been employed for SPL [3]. With excellent electronic conduction, mechanical and chemical properties, intrinsic very high aspect ratios and tiny tip radii, both single and multi- walled CNTs showed remarkable performance for LAO-AFM. However, this approach has not been further developed, Proceedings of the 10th IEEE International Conference on Nano/Micro Engineered and Molecular Systems (IEEE-NEMS 2015) Xi’an, China, April 7-11, 2015 978-1-4673-6695-3/15/$31.00 ©2015 IEEE 571