Detecting nanoscale contamination in semiconductor fabrication using through-focus scanning optical microscopy.

This paper reports high-throughput, light-based, through-focus scanning optical microscopy (TSOM) for detecting industrially relevant sub-50 nm tall nanoscale contaminants. Measurement parameter optimization to maximize the TSOM signal using optical simulations made it possible to detect the nanoscale contaminants. Atomic force and scanning electron microscopies were used as reference methods for comparison.

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.

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.

Nondestructive shape process monitoring of three-dimensional high aspect ratio targets using through-focus scanning optical microscopy.

Low-cost, high-throughput and nondestructive metrology of truly three-dimensional (3-D) targets for process control/monitoring is a critically needed enabling technology for high-volume manufacturing (HVM) of nano/micro technologies in multi-disciplinary areas. In particular, a survey of the typically used metrology tools indicates the lack of a tool that truly satisfies the HVM metrology needs of 3-D targets, such as high aspect ratio (HAR) targets. Using HAR targets here we demonstrate that through-focus scanning optical microscopy (TSOM) is a strong contender to fill the gap for 3-D shape metrology. Differential TSOM (D-TSOM) images are extremely sensitive to small and/or dissimilar types of 3-D shape variations. Based on this here we propose a TSOM method that involves creating a database of cross-sectional profiles of the HAR targets along with their respective D-TSOM signals. Using the database, we present a simple-to-use, low-cost, high-throughput and nondestructive process-monitoring method suitable for HVM of truly 3-D targets, which also does not require optical simulations, making its use straightforward and automatable. Even though HAR targets are used for this demonstration, the similar process can be applied to any truly 3-D targets with dimensions ranging from micro-scale to nano-scale. The TSOM method couples the advantage of analyzing truly isolated targets with the ability to simultaneously analyze many targets present in the large field-of-view of a conventional optical microscope.

Optical microscope illumination analysis using through-focus scanning optical microscopy.

Misalignment of the aperture diaphragm present in optical microscopes results in angular illumination asymmetry (ANILAS) at the sample plane. Here we show that through-focus propagation of ANILAS results in a lateral image shift with a focus position. This could lead to substantial errors in quantitative results for optical methods that use through-focus images such as three-dimensional nanoparticle tracking, confocal microscopy, and through-focus scanning optical microscopy (TSOM). A correlation exists between ANILAS and the slant in TSOM images. Hence, the slant in the TSOM image can be used to detect, analyze, and rectify the presence of ANILAS.

Step beyond Kohler illumination analysis for far-field quantitative imaging: angular illumination asymmetry (ANILAS) maps.

For far-field optical imaging of three-dimensional objects and such critical applications as quantitative optical imaging, optical metrology, and optical lithography, it is necessary not only to meet the Kohler illumination condition (i.e. uniform spatial intensity) but also to minimize angular illumination asymmetry (ANILAS) at the sample plane. The presence of ANILAS results in distorted optical images, and most likely in erroneous quantitative measurements. ANILAS results from optical and illumination aberrations, optical misalignment and other problems. We present a detailed procedure to measure and create maps of ANILAS across the field-of-view (FOV). ANILAS maps enable visualization of the state of illumination at the sample plane. Since the presence of ANILAS is detrimental to quantitative measurements, it is important to know the magnitude and type of ANILAS across the FOV before making any attempt to correct it. Here we intentionally create different types of illumination distortions and generate corresponding ANILAS maps, which help us evaluate the state of illumination beyond the Kohler illumination criterion. We expect that the ANILAS maps will be helpful for a wide range of far-field imaging applications.

Volume determination of irregularly-shaped quasi-spherical nanoparticles.

Nanoparticles (NPs) are widely used in diverse application areas, such as medicine, engineering, and cosmetics. The size (or volume) of NPs is one of the most important parameters for their successful application. It is relatively straightforward to determine the volume of regular NPs such as spheres and cubes from a one-dimensional or two-dimensional measurement. However, due to the three-dimensional nature of NPs, it is challenging to determine the proper physical size of many types of regularly and irregularly-shaped quasi-spherical NPs at high-throughput using a single tool. Here, we present a relatively simple method that determines a better volume estimate of NPs by combining measurements from their top-down projection areas and peak heights using two tools. The proposed method is significantly faster and more economical than the electron tomography method. We demonstrate the improved accuracy of the combined method over scanning electron microscopy (SEM) or atomic force microscopy (AFM) alone by using modeling, simulations, and measurements. This study also exposes the existence of inherent measurement biases for both SEM and AFM, which usually produce larger measured diameters with SEM than with AFM. However, in some cases SEM measured diameters appear to have less error compared to AFM measured diameters, especially for widely used IS-NPs such as of gold, and silver. The method provides a much needed, proper high-throughput volumetric measurement method useful for many applications. Graphical Abstract The combined method for volume determination of irregularly-shaped quasi-spherical nanoparticles.

Parameter optimization for through-focus scanning optical microscopy.

It is important to economically and non-destructively analyze three-dimensional (3-D) shapes of nanometer to micrometer scale objects with sub-nanometer measurement resolution for emerging high-volume nanomanufacturing, especially for process control. High-throughput through-focus scanning optical microscopy (TSOM) demonstrates promise for such applications. TSOM uses a conventional optical microscope for 3-D shape metrology by making use of the complete set of through-focus, four-dimensional optical data. However, a systematic study showing the effect of various parameters on the TSOM method is lacking. Here we present the optimization of the basic parameters such as illumination numerical aperture (NA), collection NA, focus step height, digital camera pixel size, illumination polarization, and illumination wavelength to achieve peak performance of the TSOM method.

Feasibility study on 3-D shape analysis of high-aspect-ratio features using through-focus scanning optical microscopy.

In-line metrologies currently used in the semiconductor industry are being challenged by the aggressive pace of device scaling and the adoption of novel device architectures. Metrology and process control of three-dimensional (3-D) high-aspect-ratio (HAR) features are becoming increasingly important and also challenging. In this paper we present a feasibility study of through-focus scanning optical microscopy (TSOM) for 3-D shape analysis of HAR features. TSOM makes use of 3-D optical data collected using a conventional optical microscope for 3-D shape analysis. Simulation results of trenches and holes down to the 11 nm node are presented. The ability of TSOM to analyze an array of HAR features or a single isolated HAR feature is also presented. This allows for the use of targets with area over 100 times smaller than that of conventional gratings, saving valuable real estate on the wafers. Indications are that the sensitivity of TSOM may match or exceed the International Technology Roadmap for Semiconductors (ITRS) measurement requirements for the next several years. Both simulations and preliminary experimental results are presented. The simplicity, lowcost, high throughput, and nanometer scale 3-D shape sensitivity of TSOM make it an attractive inspection and process monitoring solution for nanomanufacturing.