Influence of parasitic capacitances on conductive AFM I-V measurements and approaches for its reduction

Abstract

Using conductive atomic force microscopy (cAFM), I-V characteristics on dot-like areas can be acquired to study current conduction mechanisms or dielectric breakdown statistics on the nanoscale. However, today such I-V measurements exhibit relatively low sensitivity. It is shown that parasitic capacitances Cpar in the pF range resulting from the cantilever of the probe and the probe holder limit the sensitivity of cAFM. This is proven by the evaluation of different voltage sweep rates sr and the analysis of the influence of measurement position on the sample for both, commercially available probes as well as shielded coplanar probes prepared by focused ion beam. Compared to standard probes, shielded probes show decreased displacement currents and nearly negligible transient effects for the I-V characteristics even at high sweep rates up to 10 V/s. In addition, the influence of the measurement position is much less pronounced for the shielded probes. This means, the increase of Cpar (including the probe holder) when measuring in the center of a large sample compared to measuring at the edge of the sample is around 45% for the shielded probes compared to nearly 85% for standard probes. Here, a simple data evaluation procedure is proposed to correct the measured data for the displacement current, which will strongly improve the effective sensitivity of cAFM especially for high sr, which are preferred to decrease electrical stress during the measurement. However, for higher sr, noise increases and must be reduced in future cAFM systems by additional measures.