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Proceedings Paper

A novel method for diameter estimation of small opaque objects using Fraunhofer diffraction
Author(s): Khushi Vyas; Kameswara Rao Lolla
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Paper Abstract

A novel optical method is proposed and demonstrated, for real-time dimension estimation of thin opaque cylindrical objects. The methodology relies on free-space Fraunhofer diffraction principle. The central region, of such tailored diffraction pattern obtained under suitable choice of illumination conditions, comprises of a pair of 'equal intensity maxima', whose separation remains constant and independent of the diameter of the diffracting object. An analysis of 'the intensity distribution in this region' reveals the following. At a point symmetrically located between the said maxima, the light intensity varies characteristically with diameter of the diffracting object, exhibiting a relatively stronger intensity modulation under spherical wave illumination than under a plane wave illumination. The analysis reveals further, that the said intensity variation with diameter is controllable by the illumination conditions. Exploiting these 'hitherto unexplored' features, the present communication reports for the first time, a reliable method of estimating diameter of thin opaque cylindrical objects in real-time, with nanometer resolution from single point intensity measurement. Based on the proposed methodology, results of few simulation and experimental investigations carried-out on metallic wires with diameters spanning the range of 5 to 50μm, are presented. The results show that proposed method is well-suited for high resolution on-line monitoring of ultrathin wire diameters, extensively used in micro-mechanics and semiconductor industries, where the conventional diffraction-based methods fail to produce accurate results.

Paper Details

Date Published: 2 September 2010
PDF: 13 pages
Proc. SPIE 7792, Reflection, Scattering, and Diffraction from Surfaces II, 779219 (2 September 2010); doi: 10.1117/12.869591
Show Author Affiliations
Khushi Vyas, Indian Institute of Science (India)
Kameswara Rao Lolla, Indian Institute of Science (India)

Published in SPIE Proceedings Vol. 7792:
Reflection, Scattering, and Diffraction from Surfaces II
Zu-Han Gu; Leonard M. Hanssen, Editor(s)

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