P Bariani

Critical factors in SEM 3D stereo microscopy

This work addresses dimensional measurements performed with the scanning electron microscope (SEM) using 3D reconstruction of surface topography through stereo-photogrammetry. The paper presents both theoretical and experimental investigations, on the effects of instrumental variables and measurement parameters on reconstruction accuracy. Investigations were performed on a novel sample, specifically developed and implemented for the tests. The description is based on the model function introduced by Piazzesi and adapted for eucentrically tilted stereopairs. Two main classes of influencing factors are recognized: the first one is related to the measurement operation and the instrument set-up; the second concerns the quality of scanned images and represents the major criticality in the application of SEMs for 3D characterizations.

Geometrical modelling of scanning probe microscopes and characterization of errors

Scanning probe microscopes (SPMs) allow quantitative evaluation of surface topography with ultra-high resolution, as a result of accurate actuation combined with the sharpness of tips. SPMs measure sequentially, by scanning surfaces in a raster fashion: topography maps commonly consist of data sets ideally reported in an orthonormal rectilinear Cartesian coordinate system. However, due to scanning errors and measurement distortions, the measurement process is far from the ideal Cartesian condition. The paper addresses geometrical modelling of the scanning system dynamics, presenting a mathematical model which describes the surface metric x-, y- and z- coordinates as a function of the measured x-, y- and z-coordinates respectively. The complete mathematical model provides a relevant contribution to characterization and calibration, and ultimately to traceability, of SPMs, when applied for quantitative characterization.

Fast technique for AFM vertical drift compensation

The paper presents implementation and validation of a method for accurate imaging of three-dimensional surface topographies, developed for AFM metrology but in principle applicable to the whole family of scanning probe microscopes. The method provides correction of the vertical drift, and can be applied to any AFM scanner system, without need for modification of the instrument hardware. Surface correction is performed based on two topographies taken with mutually orthogonal scanning directions. Reconstruction is rapidly achieved through the use of an automatic routine developed for the purpose. The proposed method has been tested by using an optical and a silicon flat and a high precision cylindrical artefact. Examples of successful reconstructions on different samples are also reported within the paper.

Development and analysis of a software tool for stitching three-dimensional surface topography data sets

This paper presents development and application of a new software solution for stitching surface three-dimensional topography data where best matching positions are detected through maximization of a cross-correlation function. Differently from other stitching software tools developed in the past, the one here presented is based not on colour gradients but on matching of physical coordinates. The developed routine fully compensates positioning errors occurring when the measuring instrument is displaced relative to the surface. Qualitative as well as quantitative analyses were carried out in order to verify the applicability of the stitching process. Both synthetic and real scanned surfaces were used for testing. It was demonstrated that misalignments after software compensation are negligible: sub-pixel level inaccuracies, with absolute deviations <0.2%, were in fact verified when stitching two images. The method was developed specifically for atomic force microscopy, but can be effectively applied to any 3D topography measuring instrument.

Increase of maximum detectable slope with optical profilers, through controlled tilting and image processing

Optical methods are of choice in a huge number of applications. In particular, those instruments based on vertical scanning methods provide extremely fast, non-contact characterization of surface topography. However some limitations are present. Among them, maximum detectable slope is limited (generally <30°). Local loss of signal, resulting from this limited detection, originates data files containing void pixels, which eventually provide poor surface characterization. This work presents an original approach to overcome instrumental limitation on the maximum detectable slope. The method presented here is based on a software tool that processes images taken with controlled tilt, and returns a high-quality 3D profile of the sample being investigated. Experimental evidence is given with reference to the case of a Vickers indentation on steel.

Coordinate metrology using scanning probe microscopes

New positioning, probing and measuring strategies in coordinate metrology are needed for the accomplishment of true three-dimensional characterization of microstructures, with uncertainties in the nanometre range. In the present work, the implementation of scanning probe microscopes (SPMs) as systems for coordinate metrology is discussed. A new non-raster measurement approach is proposed, where the probe is moved to sense points along free paths on the sample surface, with no loss of accuracy with respect to traditional raster scanning and scan time reduction. Furthermore, new probes featuring long tips with innovative geometries suitable for coordinate metrology through SPMs are examined and reported.

Modelling and Measurement Uncertainty Estimation for Integrated AFM-CMM Instrument

Abstract This paper describes modelling of an integrated AFM - CMM instrument, its calibration, and estimation of measurement uncertainty. Positioning errors were seen to limit the instrument performance. Software for offline stitching of single AFM scans was developed and verified, which allows compensation of such errors. A geometrical model of the instrument was produced, describing the interaction between AFM and CMM systematic errors. The model parameters were quantified through calibration, and the model used for establishing an optimised measurement procedure for surface mapping. A maximum uncertainty of 0.8% was achieved for the case of surface mapping of 1.2×1.2 mm2 consisting of 49 single AFM scanned areas.