Conference Paper

High-precision lateral distortion correction in coherence scanning interferometry using an arbitrary surface

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Interference microscopy plays a central role in noncontact strategies for process development and quality control, providing full 3D measurement of surface characteristics that influence the functional behavior of manufactured parts. Here I briefly review the history and principles of this important technique, then concentrate on the details of hardware, software, and applications of interference microscopy using phase-shifting and coherence scanning measurement principles. Recent advances considered here include performance improvements, vibration robustness, full color imaging, accommodation of highly sloped surfaces, correlation to contact methods, transparent film analysis, and international standardization of calibration and specification.
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Height-dependent variations in fringe visibility related to optical coherence in an interference microscope provide a powerful, non-contact sensing mechanism for 3D measurement and surface characterisation. Coherence scanning interferometry extends interferometric techniques to surfaces that are complex in terms of roughness, steps, discontinuities, and structure such as transparent films. Additional benefits include the equivalent of an autofocus at every point in the field of view and suppression of spurious interference from scattered light.
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The manufacturing of flat panel displays requires a number of photomasks for the placement of pixel patterns and supporting transistor arrays. For large area photomasks, dedicated ultra-precision writers have been developed for the production of these chromium patterns on glass or quartz plates. The dimensional tolerances in X and Y for absolute pattern placement on these plates, with areas measured in square meters, are in the range of 200–300 nm (3σ). To verify these photomasks, 2D ultra-precision coordinate measurement machines are used having even tighter tolerance requirements. This paper will present how the world standard metrology tool used for verifying large masks, the Micronic Mydata MMS15000, is calibrated without any other references than the wavelength of the interferometers in an extremely well-controlled temperature environment. This process is called self-calibration and is the only way to calibrate the metrology tool, as no square-meter-sized large area 2D traceable artifact is available. The only parameter that cannot be found using self-calibration is the absolute length scale. To make the MMS15000 traceable, a 1D reference rod, calibrated at a national metrology lab, is used. The reference plates used in the calibration of the MMS15000 may have sizes up to 1 m2 and a weight of 50 kg. Therefore, standard methods for self-calibration on a small scale with exact placements cannot be used in the large area case. A new, more general method had to be developed for the purpose of calibrating the MMS15000. Using this method, it is possible to calibrate the measurement tool down to an uncertainty level of <90 nm (3σ) over an area of (0.8 × 0.8) m2. The method used, which is based on the concept of iteration, does not introduce any more noise than the random noise introduced by the measurements, resulting in the lowest possible noise level that can be achieved by any self-calibration method.
In this paper, the development of a number of material measures , which are designed to allow users to calibrate areal surface texture instruments, is presented. The material measures include grid structures , for the determination of the amplification , linearity and squareness of the x , y and z-­‐axes , and star patterns , for the determination of the lateral period limit. Relatively complex methods of manufacture have been used to produce master artefacts , which are then replicated to produce cost-­‐effective material measures for users. The process chain for the material measures is presented along with the methods used to calibrate them .
For a complete calibration of optical surface topography measuring instruments, which encompass their ability to measure slope and curvature, a determination of their optical transfer function is required. Errors induced by non-linearity of the scales of the instrument can affect their shift invariant properties, which in turn affect their transfer function. The non-linearity can be caused by distortion produced by the quality of the optical setup. A method to develop a correction model that combines a simple model of optical distortion with error separation techniques is discussed. Experimental tests of the method are presented and measurement uncertainties are investigated. (C) 2013 CIRP.
A method of lens distortion correction is proposed in order to improve the measurement accuracy of digital image correlation for two-dimensional displacement measurement. The amounts of lens distortion are evaluated from displacement distributions obtained in a rigid body in-plane translation or rotation test. After detecting the lens distortion, its coefficient is determined using the method of least squares. Then, the corrected displacement distributions are obtained. The effectiveness of the proposed method is demonstrated by applying the correction method to an in-plane translation test and tension tests. The experimental results show that the proposed distortion correction method eliminates the effect of lens distortion from measured displacements.
Image Correlation for Shape, Motion and Deformation Measurements provides a comprehensive overview of data extraction through image analysis. Readers will find and in-depth look into various single- and multi-camera models (2D-DIC and 3D-DIC), two- and three-dimensional computer vision, and volumetric digital image correlation (VDIC). Fundamentals of accurate image matching are described, along with presentations of both new methods for quantitative error estimates in correlation-based motion measurements, and the effect of out-of-plane motion on 2D measurements. Thorough appendices offer descriptions of continuum mechanics formulations, methods for local surface strain estimation and non-linear optimization, as well as terminology in statistics and probability. With equal treatment of computer vision fundamentals and techniques for practical applications, this volume is both a reference for academic and industry-based researchers and engineers, as well as a valuable companion text for appropriate vision-based educational offerings. © Springer Science+Business Media, LLC 2009. All rights reserved.
Modern manufacturing industry is beginning to benefit greatly from the ability to control the three-dimensional, or areal, structure of a surface. To underpin areal surface manufacturing, a traceable measurement infrastructure is required. In this paper, the development of a new traceable instrument for the measurement of areal surface texture is presented. The instrument uses a two-axis coplanar air-bearing slideway to move the measured surface beneath a stylus probe. The motion of the slideway is measured using linear and angular interferometers. The key to the new instrument is a novel probing system incorporating a cylindrical air-bearing guideway and an electromagnetic system to maintain a constant stylus force on the surface. The deflection of the stylus is measured using a differential plane mirror interferometer thereby minimizing the effect of any error motion in the metrology frame. The uncertainties of the instrument are calculated using a Monte Carlo approach and are evaluated to be 5 nm in the z axis and 16 nm in the x and y axes (all at k = 2). The results are given for the instrument and are compared to results from a traceable profile measuring instrument and a coherence scanning interferometer.
In this paper, we present methods for determining the measurement noise and residual flatness of areal surface topography-measuring instruments. The methods are compliant with draft international specification standards on areal surface texture. We first introduce the international standards framework and then present current methods based on averaging and subtraction to isolate the measurement noise and residual flatness from the sample surface topography. These methods are relatively difficult to apply and time consuming in practice. An alternative method is presented based on thresholding and filtering techniques. This method is simple to apply in practice. Traceability and measurement uncertainty are discussed.
A mathematical theory is presented along with some simple resulting procedures that permit an electron beam lithography machine to be calibrated by using it to make multiple observations of an imprecisely defined but stable planar object. The calibration thus obtained yields a degree of accuracy that is approximately equal to the reproducibility of the machine. For this purpose a rigid, movable grid plate is used, with grid points placed at arbitrary but more-or-less evenly distributed, fixed positions on its surface. An e-beam lithography machine with a high precision X-Y stage has been used to measure such a grid plate. Two interferometer beams, nominally parallel to the X- and Y-axes, measure stage displacements. No geometric assumptions are made concerning the stage, other than repeatability. Although the assumptions are very general, it is possible to observe the grid in just three orientations to determine both an inverse distortion function (calibration) and accurate, absolute rectangular coordinates for the grid points, providing that the absolute distance between a pair of points on the grid is known. A calibration program has been written applying the theory to the practical problem of calibrating an e-beam lithography system. Simulations based upon actual e-beam measurements confirm the theory for the practical situation. The paper concludes with suggestions for applictions to three other areas of science and technology: astronomy, optometry, and satellite geodesy.
Over the years many techniques have been developed for accurate measurement of part features without reference to an externally calibrated artefact. This paper presents a partial survey of such methods for dimensional metrology, their ranges of application, and their limits. Finally, the paper attempts to distil the common features of the various methods in the hope that this may provide the basis, or inspiration, for development of “new” methods.
A New 2D Self-Calibration Method with Large Freedom and High-Precision Performance for Imaging Metrology Devices
  • P Ekberg
  • L Mattsson
Ekberg P, Mattsson L. A New 2D Self-Calibration Method with Large Freedom and High-Precision Performance for Imaging Metrology Devices, Proceedings of the 15th International Conference of the European Society for Precision Engineering and Nanotechnology, Elsevier, 2015, 159-160.
Image Correlation for Shape, Motion and Deformation Measurements: Basic Concepts
  • M Sutton
  • J Orteu
  • H Schreier
Sutton M, Orteu J, Schreier H. Image Correlation for Shape, Motion and Deformation Measurements: Basic Concepts, Theory and Applications.
Measurement Noise and Residual Flatness
Measurement Noise and Residual Flatness. Measurement Science and Technology. 2012; 23: 035008.