Optical microscope illumination analysis using through-focus scanning optical microscopy

Motion-free TSOM using a deformable mirror

Through-focus scanning optical microscopy (TSOM) is a model-based optical metrology method that involves the scanning of a target through the focus of an optical microscope. Unlike a conventional optical microscope that directly extracts the diffraction-limited optical information from a single in-focus image, the TSOM method extracts nanometer scale sensitive information by matching the target TSOM data/image to reference TSOM data/images that are either experimentally or computationally collected. Therefore, the sensitivity and accuracy of the TSOM method strongly depends on the similarities between the conditions in which the target and reference TSOM images are taken or simulated, especially the lateral instability during through-focus scanning. As a remedy to the lateral instability, we proposed the application of adaptive optics to the through-focus scanning operation and initially developed a closed-loop system with a tip/tilt mirror and a Shack-Hartmann sensor, with which we were able to keep the plane position within peak-to-valley (PV) 33 nm. We then further developed a motion-free TSOM tool reducing the instability down to practically zero by the replacement of the tip/tilt mirror with a deformable mirror that performs through-focus scanning by deforming its mirror surface. The motion-free TSOM tool with a × 50 (NA 0.55) objective lens could provide a scanning range of up to ± 25 µm with a minimum step of 25 nm at a maximum update rate of 4 kHz. The tool was demonstrated to have a recognition accuracy of < 4 nm for critical dimension (CD) values in the range of 60 ∼ 120 nm with a reference TSOM image library generated by a Fourier modal method matching various observations conditions.

Lateral movement and angular illuminating non-uniformity corrected TSOM image using Fourier transform

Through-focus scanning optical microscopy (TSOM) is a high-efficient, low-costed, and nondestructive model-based optical nanoscale method with the capability of measuring semiconductor targets from nanometer to micrometer level. However, some instability issues resulted from lateral movement of the target and angular illuminating non-uniformity during the collection of through-focus (TF) images restrict TSOM's potential applications so that considerable efforts are needed to align optical elements before the collection and correct the experimental TSOM image before differentiating the experimental TSOM image from simulated TSOM image. An improved corrected TSOM method using Fourier transform is herein presented in this paper. First, a series of experimental TF images are collected through scanning the objective of the optical microscopy, and the ideally simulated TF images are obtained by a full-vector formulation. Then, each experimental image is aligned to its corresponding simulated counterpart before constructing the TSOM image. Based on the analysis of precision and repeatability, this method demonstrates its capability to improve the performance of TSOM, and the promising possibilities in application of online and in-machine measurements.

Fidelity test for through-focus or volumetric type of optical imaging methods

Rapid increase in interest and applications of through-focus (TF) or volumetric type of optical imaging in biology and other areas has resulted in the development of several TF image collection methods. Achieving quantitative results from images requires standardization and optimization of image acquisition protocols. Several standardization protocols are available for conventional optical microscopy where a best-focus image is used, but to date, rigorous testing protocols do not exist for TF optical imaging. In this paper, we present a method to determine the fidelity of the TF optical data using the TF scanning optical microscopy images.

Through-focus or volumetric type of optical imaging methods: a review

In recent years, the use of through-focus (TF) or volumetric type of optical imaging has gained momentum in several areas such as biological imaging, microscopy, adaptive optics, material processing, optical data storage, and optical inspection. We provide a review of basic TF optical methods highlighting their design, major unique characteristics, and application space.

Through-focus scanning optical microscopy with the Fourier modal method

We propose a Fourier modal method (FMM) based through-focus scanning optical microscopy (TSOM) featuring sub-nano scale measurement tolerance. TSOM is very recently conceptualized non-destructive optical metrology technique just at the beginning stage of research. Nowadays the reliability and feasibility of TSOM concept is subject to controversy. We experimentally demonstrate stable nano-scale metrology of the FMM-based TSOM for the verification of the TSOM metrology and provide a numerical tool for true nano-meter scale TSOM through devising the FMM based TSOM scheme. By considering the illumination light parameters of incidence angle, polarization, degree of coherence, illumination numerical aperture, and collection numerical apertures in the FMM modeling of TSOM image acquisition, we reach precise agreement between the calculated and experimentally measured TSOM images. The essential elements of the FMM based TSOM for achieving high-level consistency are elucidated.

Enhancing optical microscopy illumination to enable quantitative imaging

There has been an increasing push to derive quantitative measurements using optical microscopes. While several aspects of microscopy have been identified to enhance quantitative imaging, non-uniform angular illumination asymmetry (ANILAS) across the field-of-view is an important factor that has been largely overlooked. Non-uniform ANILAS results in loss of imaging precision and can lead to, for example, less reliability in medical diagnoses. We use ANILAS maps to demonstrate that objective lens design, illumination wavelength and location of the aperture diaphragm are significant factors that contribute to illumination aberrations. To extract the best performance from an optical microscope, the combination of all these factors must be optimized for each objective lens. This requires the capability to optimally align the aperture diaphragm in the axial direction. Such optimization enhances the quantitative imaging accuracy of optical microscopes and can benefit applications in important areas such as biotechnology, optical metrology, and nanotechnology.