Development of a high resolution position sensor for atomic force microscopy

Abstract

This work aims to develop a position sensor with nanometric resolution and its implementation in a scanner that performs displacements in a three-dimensional space, responsible for the tip-sample scanning in an atomic force microscope (AFM). The work will be developed in the laboratory of “Atomic Force Microscopy and related techniques” (AFMaRT) under the guidance of professors Mario S Rodrigues and Miguel V Vitorino. ´ All AFMs use piezoelectric elements to perform displacements in 3 dimensions with high spatial resolution. Consequently, the observables of the instrument are the voltages applied to the various piezoelectric elements, later converted to displacement of the sample/tip, after calibrating the scanner movement. Due to hysteresis and creep characteristics of piezoelectric elements this conversion is only approximated and is insufficient for some applications that require greater precision in the knowledge of the sample position relative to the probe. After a previous study of the state of the art, it was decided to develop position sensors using lasers and quadrant photodetectors. Laser-photodetector systems are mainly sensitive to angular variations. To take advantage of this fact, the idea is to focus 3 lasers on 3 reflecting cylinders solidary with the sample. The angles of the surfaces on which the 3 beams lasers are incident, depend on the position of the cylinders with respect to fixed lasers in an inertial reference frame. This system was mounted on a prototype of AFM already existing in the laboratory, requiring the development/adaptation of the existing scanner to facilitate integration of the sensors, which included redrawing the scanner and the sample’s holder. During this work, it was necessary to dimension the optical system, design ways of coupling to the scanner, design electronic circuits that allow reading sensors and characterize and calibrate the sensors.