Abstract
Although coherence scanning interferometry (CSI) is capable of measuring surface topography with sub-nanometre precision, it is well known that the performance of measuring instruments depends strongly on the local tilt and curvature of the sample surface. Based on 3D linear systems theory, however, a recent analysis of fringe generation in CSI provides a method to characterize the performance of surface measuring instruments and offers considerable insight into the origins of these errors. Furthermore, from the measurement of a precision sphere, a process to calibrate and partially correct instruments has been proposed. This paper presents, for the first time, a critical look at the calibration and correction process. Computational techniques are used to investigate the effects of radius error and measurement noise introduced during the calibration process for the measurement of spherical and sinusoidal profiles. Care is taken to illustrate the residual tilt and curvature dependent errors in a manner that will allow users to estimate measurement uncertainty. It is shown that by calibrating the instrument correctly and using appropriate methods to extract phase from the resulting fringes (such as frequency domain analysis), CSI is capable of measuring the topography of surfaces with varying tilt with sub-nanometre accuracy.
Highlights
The function and performance of engineered components or micro and nano-manufactured products is highly dependent on their surface topography [1,2]
The general assumptions and conditions of this study are: 1) The surfaces of the simulated objects meet the Kirchhoff approximation, i.e. the surfaces are slowly varying on the optical scale; 2) The surface geometry is such that the effects of multiple scattering can be neglected; 3) The simulated coherence scanning interferometry (CSI) system is aberration-free, such that the phase transfer function (PTF) of the simulated CSI system is zero; 4) In order to minimize the impact of the algorithm that calculates the surface height from the CSI fringe data, the a priori height information of the nominal surface is used to find the phase of the fringes at the surface
Results and discussion we will first verify the simulation of the CSI signal by comparing with experimental results, where the parameters used in the simulation are based on the instrument configurations
Summary
The function and performance of engineered components or micro and nano-manufactured products is highly dependent on their surface topography [1,2]. Traditional contact stylus techniques that provide profiles of surface topography become insufficient for modern manufacturing where simultaneous areal topographic information, fast signal acquisition and nanometre scale height resolution are desired. Optical metrology techniques, such as coherence scanning interferometry (CSI), phase shifting interferometry, focus variation microscopy and confocal microscopy, have been widely employed in the research and manufacturing environment for conducting surface topography measurement and dimensional micro-scale metrology [3]. High precision surface height can be calculated from the phase and envelope of the interference signal, for example, using envelope detection methods [6] or the frequency domain analysis (FDA) method [7]. Traditional step artefacts that contain two parallel flat surfaces are, not considered sufficient for calibration of CSI systems for measuring complex geometry [12]
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