Influence of Illumination Polarization and Target Structure on Measurement Sensitivity of Through-Focus Scanning Optical Microscopy

Abstract

Unlike the optical information taken from a single in-focus image of general optical microscopy, through-focus scanning optical microscopy (TSOM) involves scanning a target through the focus and capturing of a series of images. These images can be used to conduct three-dimensional inspection and metrology with nanometer-scale lateral and vertical sensitivity. The sensitivity of TSOM strongly depends on many mechanical and optical factors. In this study, how illumination polarization and target structure affect the sensitivity of TSOM is analyzed. Firstly, the complete imaging procedure of the polarized light is investigated. Secondly, through-focus scanning results of different targets with two illumination polarizations are simulated using the finite-difference time-domain method. Thirdly, a few experiments are performed to verify the influence of illumination polarization and target structures on the sensitivity of TSOM. Both the results of the simulation and experiments illustrate an apparent influence of polarization on the sensitivity of inspecting the targets with center asymmetric structures. For enhanced sensitivity, illumination polarization should be perpendicular to the target texture. This conclusion is meaningful to adjust illumination polarization purposefully for different structure characteristics and improve the sensitivity of metrology.2 of 12 thus, the interaction between incident light wave and target surface is complicated, and the spatial distribution of scattered light wave depends on illumination polarization and target structures [6]. TSOM images with different OIRs correspond to different spatial distributions of scattered light [7]. Consequently, the illumination polarization and structure characteristics of the target strongly affect the sensitivity of TSOM [8,9]. To address the lacking systematic analysis of these effects, many works attempt to optimize illumination polarization according to different targets through pre-tests [10,11]. The drawbacks are long preparation time and low inspection efficiency. To understand the light scattering around the target surface and conduct the inspection in the most optimized condition, the effects on TSOM sensitivity brought by illumination polarization and target structure are analyzed and summarized in this paper.The reminder of this article is organized as follows: the complete image-forming procedure, from incidence of illumination to the imaging plane of the charge-coupled device (CCD), is analyzed in the second section. The simulation of the system is presented in the third section. A few experiments are performed in the fourth section to verify the simulation results. The simulation and experimental results are interpreted in the fifth section. A conclusion based on the analyses and results is proposed in the sixth section.

Multiple parametric nanoscale measurements with high sensitivity based on through‐focus scanning optical microscopy

“…Subsequently, the TSOM image can be extracted from these defocusing images and further processed according to the methods by Attota & Kang () and Qu et al . (). The differential TSOM (DTSOM) image is obtained by directly subtracting one TSOM image from the other.…”

Special Issue on Precision Dimensional Measurements

“…The first paper on 3D surface profile measurement is introduced by Qu and his colleagues at Beihang University on the measurement sensitivity of through-focus scanning optical microscopy (TSOM), which involves scanning a target through the focus and capturing of a series of images [1]. It was found that for enhanced sensitivity, illumination polarization should be perpendicular to the target texture.…”