Michael T. Stocker

Scatterfield microscopy for extending the limits of image-based optical metrology

We have developed a set of techniques, referred to as scatterfield microscopy, in which the illumination is engineered in combination with appropriately designed metrology targets to extend the limits of image-based optical metrology. Previously we reported results from samples with sub-50-nm-sized features having pitches larger than the conventional Rayleigh resolution criterion, which resulted in images having edge contrast and elements of conventional imaging. In this paper we extend these methods to targets composed of features much denser than the conventional Rayleigh resolution criterion. For these applications, a new approach is presented that uses a combination of zero-order optical response and edge-based imaging. The approach is, however, more general and a more comprehensive set of analyses using theoretical methods is presented. This analysis gives a direct measure of the ultimate size and density of features that can be measured with these optical techniques. We present both experimental results and optical simulations using different electromagnetic scattering packages to evaluate the ultimate sensitivity and extensibility of these techniques.

High-resolution optical overlay metrology

Optical methods are often thought to lose their effectiveness as a metrology tool beyond the Rayleigh criterion. However, using advanced modeling methods, the conventional resolution limitations encountered in well-defined edge-to-edge measurements using edge thresholds do not apply. In fact, in this paper we present evidence that optics can be used to image and measure features as small as 10 nm in dimension, well below the imaging wavelength. To understand the limits of optical methods we have extensively studied both linewidth and overlay metrology applications. Although overlay applications are usually thought to involve pitch or centerline measurements of features from different process levels, some target designs present optical proximity effects which pose a significant challenge. Likewise, line width measurements require determination of the physical edges and geometry which created that profile. Both types of measurements require model-based analysis to accurately evaluate the data and images. In this paper we explore methods to optimize target geometry, optical configurations, structured illumination, and analysis algorithms with applications in both critical dimension and overlay metrology.

Optical critical dimension measurement of silicon grating targets using back focal plane scatterfield microscopy

We demonstrate optical critical dimension measurement of lines in silicon grating targets using back focal plane scatterfield icroscopy. In this technique, angle-resolved diffraction signatures are obtained from grating targets by imaging the back focal plane of a brightfield microscope that has been modified to allow selection of the angular distribution and polarization of the incident illumination. The target line profiles, including critical dimension linewidth and sidewall angle, are extracted using a scatterometry method that compares the diffraction signatures to a library of theoretical signatures. Because we use the zero-order component of the diffraction, the target features need not be resolved in order to obtain the line profile. We extracted line profiles from two series of targets with fixed pitch but varying linewidth: a subresolution 300-nm-pitch series, and a resolved 600-nm-pitch series. Linewidths of 131 nm to 139 nm were obtained, with nanometer-level sensitivity to linewidth, and a linear relationship of linewidth obtained from scatterfield microscopy to linewidth measured by scanning electron microscopy was demonstrated. Conventional images can be easily collected on the same microscope, providing a powerful tool for combining imaging metrology with scatterometry for optical critical dimension measurement.

High-resolution optical metrology

Recent advances in optical imaging techniques have unveiled new possibilities for optical metrology and optical-based process control measurements of features in the 65 nm node and beyond. In this paper we discuss methods and applications that combine illumination engineering and structured targets which enable sensitivity to nanometer scale changes using optical imaging methods. These methods have been investigated using simulation tools and experimental laboratory apparatus. The simulation results have demonstrated substantial sensitivity to nanometer changes in feature geometry. Similar results have now been observed in the laboratory. In this paper we will show simulation data to motivate the use of low numerical aperture and structured illumination optical configurations. We will also present the basic elements and methods which we are now using in the design of an optical tool specifically designed for these types of measurements. Target configurations which enhance the scattered electromagnetic fields will be shown along with experimental verification of the methodology. The simulation and experimental apparatus is used to explore and optimize target geometry, optical configurations, and illumination structure for applications in both critical dimension and overlay metrology.

Evaluation of New In-Chip and Arrayed Line Overlay Target Designs

Two types of overlay targets have been designed and evaluated for the study of optical overlay metrology. They are in-chip and arrayed overlay targets. In-chip targets are three-bar two-level targets designed to be placed in or near the active device area of a chip. They occupy a small area in the range of 5 μm2 to 15 μm2 and have line widths, which are nominally device dimensions. The close proximity of the line features result in strong proximity effects. We have used two well-established theoretical models to simulate and study the effects of proximity on overlay measurements. In this paper, we also present a comparison of optical overlay results with scanning electron microscope measurements. Arrayed targets have also been designed to improve and enhance the optical signal for small critical dimension features. We have also compared theoretical simulations of arrayed targets to experimental results. In these comparisons we observe a significant variation in the location of the best focus image with respect to the features. The through-focus focus-metric we have implemented in the current work to determine the best focus image shows interesting properties with potential applications for line width metrology and process control. Based on simulation results, the focus-metric is sensitive to changes in line width dimensions on the nanometer scale.

Comparison of measured optical image profiles of silicon lines with two different theoretical models

In this paper, we describe a new method for the separation of tool-induced measurement errors and sample-induced measurement errors. We apply the method to standard overlay target configurations. This method is used to separate the effects of the tool and sample errors in the measured optical intensity profiles and to obtain the best estimate of the correct intensity profile for a given sample geometry. This most accurate profile is then compared to calculated profiles from two different theoretical models. We explain the modeling in some detail when it has not been previously published.

New method to enhance overlay tool performance

New methods to enhance and improve algorithm performance and data analysis are being developed at NIST for overlay measurement applications. Both experimental data and improved theoretical optical scattering models have been used for the study. We have identified error sources that arise from (i) the optical cross talk between neighboring lines on an overlay target (ii) the selection of the window size used in the auto-correlation and (iii) the portion of the intensity profile that is used in the overlay calculation (defined as a truncated profile). Further, we suggest methods to optimally minimize these error sources. We also present a relationship between tool-induced shift (TIS) and the asymmetry in the intensity profile.

Metrology for Fuel Cell Manufacturing

The project was divided into three subprojects. The first subproject is Fuel Cell Manufacturing Variability and Its Impact on Performance. The objective was to determine if flow field channel dimensional variability has an impact on fuel cell performance. The second subproject is Non-contact Sensor Evaluation for Bipolar Plate Manufacturing Process Control and Smart Assembly of Fuel Cell Stacks. The objective was to enable cost reduction in the manufacture of fuel cell plates by providing a rapid non-contact measurement system for in-line process control. The third subproject is Optical Scatterfield Metrology for Online Catalyst Coating Inspection of PEM Soft Goods. The objective was to evaluate the suitability of Optical Scatterfield Microscopy as a viable measurement tool for in situ process control of catalyst coatings.

Scatterfield microscopy using back focal plane imaging with an engineered illumination field

We have implemented back focal plane (conoscopic) imaging in an optical microscope that has also been modified to allow selection of the illumination angles and polarization at the sample, and collected back focal plane images of silicon on silicon grating scatterometry targets with varying line widths. Using a slit illumination mask, the zero-order diffraction versus angle for −60° to +60° incident angles at a given polarization was obtained from a single image. By using reference images taken on a flat silicon background, we correct the raw target images for illumination source inhomogeneities and polarization-dependent transmission of the optics, and convert them to reflectance versus angle data for s- and p-polarizations, similar to that obtained from angle-resolved grating scatterometry. As with conventional scatterometry, the target lines need not be resolved for the reflectance signature to show sensitivity to small changes in the grating parameters. For a series of 300 nm pitch targets with line widths from 150 nm to 157 nm, we demonstrate nanometer-level sensitivity to line width with good repeatability, using 546 nm illumination. Additionally, we demonstrate a technique for separating the zero order from higher order diffraction on targets with multiple diffraction orders, allowing collection of both zero and higher order diffraction versus angle from the back focal plane image. As conventional images can be easily collected on the same microscope, the method provides a powerful tool for combining imaging metrology with scatterometry for optical critical dimension measurements in semiconductors.

The limits of image-based optical metrology

An overview of the challenges encountered in imaging device-sized features using optical techniques recently developed in our laboratories is presented in this paper. We have developed a set of techniques we refer to as scatterfield microscopy which allows us to engineer the illumination in combination with appropriately designed metrology targets. The techniques have previously been applied to samples with sub-50 nm sized features having pitches larger than the conventional Rayleigh resolution criterion which results in images having edge contrast and elements of conventional imaging. In this paper we extend these methods to targets composed of features much denser than the conventional Rayleigh resolution criterion. For these applications, a new approach is presented which uses a combination of zero order optical response and edge-based imaging. The approach is, however, more general and a series of analyses based on theoretical methods is presented. This analysis gives a direct measure of the ultimate size and density of features which can be measured with these techniques and addresses what measurement resolution can be obtained. We present several experimental results, optical simulations using different electromagnetic scattering packages, and statistical analyses to evaluate the ultimate sensitivity and extensibility of these techniques.