Control of dynamics-coupling effects in piezo-actuator for high-speed AFM operation

This article addresses the compensation for the dynamics-coupling effects in piezo-actuators used for positioning in atomic force microscopes (AFMs). Piezo-actuators are used to position the AFM probe (relative to the sample) both parallel to the sample surface (x-y-axes) and perpendicular to the sample surface (z-axis). During AFM operation, such as nanofabrication and imaging of soft biological samples, the probe-sample distance (in the z-axis) needs to be precisely controlled to maintain the probe-sample interaction at a desired value; otherwise, large variation of the probe-sample distance results in distortions of the fabricated parts (in nanofabrication) and can cause sample damage (in imaging soft biological samples). In this article, we show that dynamics-coupling from the x-y-axes (the scanning axes) to the z-axis, referred to as x-to-z dynamics-coupling, can generate significant variations in the probe-sample distance when operating AFM at high speed, i.e., when the sample is scanned at high speed. We present an inversion-based approach to compensate for these dynamics-coupling effects. Additionally, for applications where the x-y-axes movement is repetitive (as in AFM scanning operations), an iterative approach is proposed to further reduce the coupling-caused positioning errors. Convergence of the iterative approach is investigated and experimental results show that the coupling-caused errors can be reduced to the noise level using the proposed approach. Thus, the main contribution of this article is the development of an approach to substantially reduce the coupling-caused positioning errors and thereby, enable high-speed high-precision positioning of piezoscanners used in AFMs.