Evaluation of the electrical contact area in contact-mode scanning probe microscopy

Abstract

The tunneling current through an atomic force microscopy (AFM) tip is used to evaluate the effective electrical contact area, which exists between tip and sample in contact-AFM electrical measurements. A simple procedure for the evaluation of the effective electrical contact area is described using conductive atomic force microscopy (C-AFM) in combination with a thin dielectric. We characterize the electrical contact area for coated metal and doped-diamond tips operated at low force (<200 nN) in contact mode. In both cases, we observe that only a small fraction (<10 nm2) of the physical contact (∼100 nm2) is effectively contributing to the transport phenomena. Assuming this reduced area is confined to the central area of the physical contact, these results explain the sub-10 nm electrical resolution observed in C-AFM measurements.

Measuring and Understanding Contact Area at the Nanoscale: A Review

“…One investigation [47] showed qualitatively the effect of the materials contribution to resistance. More recently, a method was demonstrated to quantitatively treat the contribution of an oxide using the Fowler-Nordheim tunneling model [48]. Fitting the model to measured data enables the calculation of what the authors call an "electrical contact area."…”

Nanoscaled Electrical Characterization

“…3.3, can be defined by the portion of the tip-sample contact-area having minimum resistance for the current. The size of this conductive spot in C-AFM (≈10 nm 2 ) has been experimentally evaluated [29]. This area can be thought as the conductive spot through which the current is physically flowing.…”

“…The latter is based on the I-V curve measured with C-AFM on a 1.5-nm-thick SiO 2 film grown on Si. From this curve we extract the electrical contact area for commercial platinum (Pt/Ir) tips as well as for our in-house developed doped-diamond tips [29]. The conductive tip is used as a virtual metal electrode scanning in contact onto the sample surface.…”

Handling Geometric Features in Nanoscale Characterization of Charge Injection and Transport in thin Dielectric Films

“…Concerning ultra-thin dielectric layers, three main approaches are used to determine the collection area: (i) Contact area is computed using the Hertz approach, corresponding to the mechanical contact area [13]; (ii) Effective surface is determined fitting the current-voltage experimental curve. This surface depends on few parameters as tip work function, contact force or dielectric thickness and presents a broad range values from 10 nm² to 100 nm² [14]; (iii) Scanning Electron Microscopy observations on the AFM tip after measurements [15]. Additionally, surface roughness needs to be considered to evaluate the current collection area.…”