Modeling of probe-surface interactions in nanotopographic measurements

Abstract

Contact stylus methods remain important tools in surface roughness measurement, but as metrological capability increases there is growing need for better understanding of the complex interactions between a stylus tip and a surface. For example, questions arise about the smallest scales of topographic features that can be described with acceptable uncertainty, or about how to compare results taken with different types of probe. This thesis uses simulation methods to address some aspects of this challenge. A new modelling and simulation program has been developed and used to examine the measuring of the fine structure of the real and simulated surfaces by the stylus method. Although able to scan any arbitrary surface with any arbitrary stylus shape, the majority of the results given here uses idealized stylus shapes and ‘real’ ground steel surfaces. The simulation is not only used to measure the roughness of the surface but also to show the contacts distribution on the tip when scanning a surface. Surface maps of the fine structure of ground steel surfaces were measured by Atomic Force Microscopy (AFM) to ensure high lateral resolution compared to the capability of the target profilometry instruments. The data collected by the AFM were checked for missing data and interpolated by the scanning probe image processor (SPIP) software. Three basic computer generated stylus tips with different shapes have been used: conical, pyramid and spherical shapes. This work proposes and explores in detail the novel concept of “thresholding” as an adjunct to kinematic contact modelling; the tip is incremented downwards 'into' the surface and resulting contact regions (or islands) compared to the position of the initial kinematic contact. Essentially the research questions have been inquiring into the effectiveness of so-called kinematic contact models by modifying them in various ways and judging whether significantly different results arise. Initial evidence shows that examination of the contact patterns as the threshold increases can identify the intensity with which different asperity regions interact with the stylus. In the context of sections of the ground surface with a total height variation in the order of 500 nm to 1 μm, for example, a 5 nm threshold caused little change in contact sizes from the kinematic point, but 50 nm caused them to grow asymmetrically, eventually picking out the major structures of the surface. The simulations have naturally confirmed that the stylus geometry and size can have a significant effect on most roughness parameters of the measured surface in 3D. Therefore the major contribution is an investigation of the inherent (finite probe) distortions during topographic analysis using a stylus-based instrument. The surprising finding which is worthy of greater investigation, is how insensitive to major changes in stylus condition some of the popular parameters are, even when dealing with very fine structure within localized areas of a ground surface. For these reasons, it is concluded that thresholding is not likely to become a major tool in analysis, although it can certainly be argued that it retains some practical value as a diagnostic of the measurement process. This research will ultimately allow better inter-comparison between measurements from different instruments by allowing a ‘software translator’ between them. Short of fully realizing this ambitious aim, the study also contributes to improving uncertainty models for stylus instruments