Design and analysis of a miniaturized Atomic Force Microscope scan head

Abstract

An Atomic Force Microscope (AFM) is a type of scanning probe microscope used for nano-scale characterization, topography imaging and manipulation of conducting and insulating samples, at sub-nanometer resolution. Its conventional implementation as a macroscale instrument imposes several measurement and performance constraints such as difficulty in scanning large sized samples, imaging artefacts due to external vibrations and thermal drifts and low bandwidth. At the heart of its bulkiness is the bulky scan head used for positioning the tip and for tip-sample interaction measurement. In this thesis, we propose the design of a miniaturized AFM scan head to address these lacunae. First, a miniature flexure-based design for an AFM scan head that incorporates a mechanism to achieve 3-axis nano- and micro-positioning is proposed. The fine positioner has been designed by employing parallel kinematics approach and achieves a displacement range of ±5µm along X, Y and Z axes, which is comparable to conventional AFM. To achieve in-plane positioning, a lever-based displacement amplifier, with parallelogram-based flexures, has been employed. Out-of-plane positioning has been achieved by using a triangular displacement amplifier design. To decouple the motion between the three axes, a decoupling stage has been designed. The coarse positioning system has been implemented by incorporating an inchworm motor capable of moving in incremental steps of 10 µm to achieve large range of motion in three dimensions (3D). A bridge amplifier and a gripper have been designed for this purpose. The volume of the proposed compact design of the fine positioner is 1.2x1.2x0.3 cm3 and that of the coarse positioner is 9x4x2 cm3 wherein the achievable footprint was assumed to be limited by fabrication constraints of wire electrical discharge machining technology Subsequently, a lumped parameter model has been obtained for the proposed designs and their quasi-static and dynamic characteristics have been analyzed, using Euler Bernoulli Beam theory and Rayleigh’s technique, respectively. A comparison of the derived analytical expressions for the displacement gain and eigen modes with FEM simulations revealed a match within 10%. The bandwidth along Z-axis is about 5 kHz which is much larger than that of a conventional AFM. Finally, a feedback control system has been designed to achieve position control in 3D and tip-sample interaction force control in contact mode. To achieve this, the transfer function of the mechanical system has been extracted from its frequency response and model inversion has been used along with proportional-integral-derivative control to achieve high bandwidth positioning