RESEARCH ARTICLE Copyright © 2009 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Holography and Speckle Vol. 5, 1–11, 2009 CoherenceEffectsinApplicationsofFrequencyand TimeDomainFullFieldOpticalCoherence TomographytoOpticalMetrology I. Abdulhalim Department of Electrooptic Engineering, Ben Gurion University of the Negev, P.O. Box 653, Beer Sheva 84105, Israel The interplay of spatial versus temporal coherence in the context of full field optical coherence tomography (OCT) for optical metrology applications is investigated both theoretically and experi- mentally using the Linnik and the Mirau interference microscopes. It is shown that multiple inter- ference effects have a little influence on the thin film thickness measurement when the coherence length is much smaller than the layers thickness. When high numerical apertures used it is possible to rely on the longitudinal spatial coherence and work with narrow spectral band thus improving the measurement accuracy. This is particularly important as it avoids taking the dispersion relations into account. For frequency domain (FD) operation we have shown that using objectives with low numerical aperture (NA) up to 0.3 it is still possible to use FD-OCT with high accuracy. For large NA objectives the use of narrow annular apertures is shown to be possible without sacrificing the axial resolution. In this case one can even use the inverse scattering approach in which a simplified model is built for the multilayered structure and fitting is performed with the measured spectrogram. This is shown to be more sensitive technique than polarized reflectometry. Keywords: 1. INTRODUCTION Optical metrology has been 1 the drive for innovative opti- cal techniques over the years. The need for more and more accurate measurements of thickness, step height and surface profile led to the development of ellipsometry, polarized reflectometry and interferrometry to allow sub- nm accuracy. Modulated spectroscopic and variable angle ellipsometry 2 is capable of measuring sub-Angstrom vari- ations in thickness and allows measuring refractive indices dispersion to accuracy better than 0.0001. With the advent of detector arrays and CCD cameras the ease of build- ing a setup and acquiring data in high speed enhanced the performance of optical metrology techniques. Image processing has played a key role in enhancing optical microscopy as a quantitative measurement tool. In the sil- icon microelectronic industry the optical microscope com- bined with CCD camera, image processing and automatic scanning with nano scale accuracy has acted and still act- ing as a must for any fabrication facility. Among the most prominent applications are in the inspection of overlay misregistration between two layers and the width of a line called critical dimension (CD) which represents the width of the transistor gate. Different optical microscopy modes have been in use for variety of layered structures. 3–5 Interference microscopy 6 7 was used when the layers are transparent to enhance the contrast of the image. Differ- ential interference contrast (DIC) microscopy or Nomarski microscopy is being used for step height and surface profiling. Three dimensional images for lines were also obtained by the KLA-Tencor Linnik microscope system 8 from which CD and overlay were measured with nm accu- racy. Dark field and polarized imaging are used when the edges are scattering, in order to enhance the edge detection. When the era of nano-electronics started, that is with the shrinkage of the design rules to the nano scale 9 (<200 nm) the resolution limit for an optical microscope affected the measurement accuracy of the CD and overlay. This fact has led to the invention of optical spectroscopic scatterometry 10 as a powerful tool to measure line widths down to 20 nm using visible and UV light as well as for overlay measurement. 11 In its simplest configuration, 12–14 optical scatterometry technique measures reflection from zero order gratings as a function of wavelength and uses inverse scattering methodology to extract the lines width with sub-nm accuracy. J. Holography Speckle 2009, Vol. 5, No. 2 1546-900X/2009/5/001/011 doi:10.1166/jhs.2009.1013 1