Robert D. Larrabee

Theory and application of a two-layer Hall technique

The electrical characterization of epitaxial layers on substrates of the opposite conductivity type presents serious problems if the p-n junction at the interface has significant leakage current such that it cannot be used to effectively electrically isolate the two regions. In order to meet the need for nondestructively characterizing such structures, a modification of the conventional Hall technique was developed in which the Hall measurements are made simultaneously on both the epitaxial layer and its substrate, the interface impedance is measured, and the interaction between the two regions is modeled and taken into account. This technique can be used to verify those cases in which the perturbing effects of a high-resistivity substrate are negligible, thus justifying conventional measurements on the epitaxial layer. In principle, it can be used to measure the resistivity and Hall coefficient of each layer separately if the assumptions of the model are realized in practice. The use of this technique is discussed and applied to the case of a thin n-type silicon epitaxial layer on: 1) a conducting substrate of indium-doped silicon that had a significant amount of leakage at the interface p-n junction and 2) a high-resistivity silicon substrate that had negligible influence on the measurement of the Hall coefficient of the epitaxial layer.

X-Ray Lithography Mask Metrology: Use of Transmitted Electrons in an SEM for Linewidth Measurement

X-ray masks present a measurement object that is different from most other objects used in semiconductor processing because the support membrane is, by design, x-ray transparent. This characteristic can be used as an advantage in electron beam-based x-ray mask metrology since, depending upon the incident electron beam energies, substrate composition and substrate thickness, the membrane can also be essentially electron transparent. The areas of the mask where the absorber structures are located are essentially x-ray opaque, as well as electron opaque. This paper shows that excellent contrast and signal-to-noise levels can be obtained using the transmitted-electron signal for mask metrology rather than the more commonly collected secondary electron signal. Monte Carlo modeling of the transmitted electron signal was used to support this work in order to determine the optimum detector position and characteristics, as well as in determining the location of the edge in the image profile. The comparison between the data from the theoretically-modeled electron beam interaction and actual experimental data were shown to agree extremely well, particularly with regard to the wall slope characteristics of the structure. Therefore, the theory can be used to identify the location of the edge of the absorber line for linewidth measurement. This work provides one approach to improved x-ray mask linewidth metrology and a more precise edge location algorithm for measurement of feature sizes on x-ray masks in commercial instrumentation. This work also represents an initial step toward the first SEM-based accurate linewidth measurement standard from NIST, as well as providing a viable metrology for linewidth measurement instruments of x-ray masks for the lithography community.

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.

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.

Precision, accuracy, uncertainty and traceability and their application to submicrometer dimensional metrology☆

Abstract The terms in the title of this paper are often used to characterize the quality of any measurement result. However, these terms (particularly accuracy and traceability) are very often confused (and abused) in practice. They often do not have the same meaning to the seller, buyer, and user of metrology instruments. Each of these terms has a very specific meaning and definition and each should be fully understood and quantified properly before being used to convey metrological information for any purpose. This paper summarizes the generally-accepted generic metrological meaning and significance of these terms for the purpose of clarifying any misunderstanding that might otherwise arise between the metrologist and the user of metrological data. These meanings are illustrated by discussing their application to dimensional standards presently available to the semiconductor industry from NIST.

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.

A new approach to accurate X-ray mask measurements in a scanning electron microscope

The authors present the basic concept and some preliminary experimental data on a novel method for measuring critical dimensions on masks used for X-ray lithography. The method uses a scanning electron microscope (SEM) in a transmitted-electron imaging mode and can achieve nanometer precision. Use of this technique in conjunction with measurement algorithms derived from electron-beam interaction modeling may ultimately enable measurements to these masks to be made to nanometer accuracy. Furthermore, since a high contrast image results, this technique lends itself well to automated mask defect recognition and inspection. It is concluded that this method has the potential advantage of avoiding or at least minimizing the basic limitations imposed by the electron-beam interaction effects normally encountered in conventional methods of dimensional metrology in the SEM. >

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.