Fidelity study in surface measurements in nanometre metrology

Abstract

The object of this Ph.D work is to evaluate fidelity in surface measurements in nanometric metrology for both contact and non-contact methods, namely stylus instruments and scanning tunnelling microscopy. Fidelity is defined, in this thesis, as a measure to which an instrument system reproduces the surface features and thus the parameters of interest. High fidelity measurement has two meanings; less distortion in the measured result and less disturbance to the surface being measured. Interaction at the interface between the probe and the surface is the source of failure to achieve high fidelity. No instrument measures surface topography alone: all instruments measure a convolution of topography and the geometrical and physical interaction of the measured probe and the surface. In the case of a mechanical stylus, factors extraneous to the topography include (a) the shape and size of the stylus, (b) mechanical properties of the stylus and the specimen such as elastic moduli and hardness, (c) frictional force of the sliding pair. and (d) dynamic interaction forces during the sliding. For the scanning tunnelling microscope, factors which affect measurement in addition to topography include the geometry of the tip, the electronic properties of the surface and mechanical deformation due to electrostatic forces and contamination. 'These factors have been investigated in great detail, particularly for the stylus instruments. A specially designed electro-magnetic force actuator has been developed to give a better control on loading force during the experiments. Tracking force effects were evaluated by profiling statistical parameters, and scanning electron microscopy. Friction between a stylus and specimen has been measured for different loading force, sliding speed, material and surface finish. Improvement on dynamic characteristics of a stylus system has been achieved by active damping control. An optimal damping ratio for stylus instruments is found to be within 0.5-0.7. Through the study, the tracking force and traversing speed are found to be the crucial factors to be tackled so that high fidelity measurement can be obtained. A similar investigation has been also made on two home-built scanning tunnelling microscopes to explore the potential applications of STM on nanometric metrology