Control of AFMs in contact mode

Abstract

The Atomic Force Microscope (AFM) is a high precision surface characterization tool commonly used in Nano-technology, Bio-technology, semiconductors, MEMS, and life sciences' applications. As most versatile systems, AFM offers little guarantees on achieving repeatable satisfactory operation. This is the case as AFMs are not used to perform a single predictable task. AFM systems are feedback regulators, which rely on photodiode detector (PSD) sensing and piezoelectric actuation. The change in probe-surface contact is a disturbance created by scanning across a surface. This disturbance is to be rejected to maintain probe-surface contact and thus allow proper surface characterization. AFM feedback systems are not only required to maintain a nominal PSD output but also guarantee that the control signal used is representative of the rejected disturbance. This is due to the fact that the image of the scanned surface is created from this control voltage. These characteristics impose severe limitations on the system's operation bandwidth, repeatability, and precision. In this effort, the key characteristics and limitations of AFM operation are analyzed. Challenges due to surface variations, plant dynamics, and contact nonlinearity are presented. The closed loop response of AFM systems in single actuator as well as in dual actuator configurations is evaluated. The emphasis is on the underlying structure corresponding to each configuration and not on a particular system tuning. In this regard, the bounds on achievable performance in each configuration are contrasted for operation within the system's overall objectives.