Speaker
Description
Dynamic Atomic Force Microscopy (dAFM) uses a forced excited oscillating cantilever probe to image sample surface topography. The working principle uses the nonlinear interaction forces between probe and sample and has two operation modes, non-contact or intermittent contact between probe tip and sample surface. One of the most important key characteristics of dAFM is spatial vertical resolution. The resolution, or more generalized sensitivity, is defined as the smallest detectable change of a measured value, limited by noise. Mathematically, sensitivity is often defined as the ratio between noise and responsivity. While the limiting noise is generally considered to be of thermomechanical nature, the responsivity characterizes the cantilever as a resonant sensor. It is defined as the slope of the sensor output as function of the sensor input. In dAFM, input is the sample topography height and output the resonant sensors oscillation amplitude.
Previous works have considered parametric excitation, more precisely parametric resonance, as an approach for improving intermittent AFM. However, sensitivity was not directly part of those investigations. Additionally, the nonlinear limitation in the proposed parametrically excited system was intrinsic and was therefore neither generic nor adjustable.
In this work, we perform new investigations regarding parametric resonance influencing the sensitivity of non-contact AFM. The parametric excitation and resonance limitation is achieved by a feedback circuit with cubic nonlinearity. The study includes different parameters of parametric resonance, as excitation frequency and strength of parametric excitation. Additionally, the influence of more general AFM parameters, like probe characteristics are considered. For this, we investigate the dynamic behavior of MEMS probes in parametric resonance, including the nonlinear probe-sample interaction force, using numerical continuation. In this manner, we calculate frequency responses at different tip-sample distances and amplitude/phase distance curves. Building on this, we present a more generalized formulation of sensitivity in parametrically excited dAFM systems, depending on system and process parameters, to have a global overview of the potential that parametric resonances offer in terms of sensitivity. This is also compared to forced excited dAFM.
