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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.

Electrical double layers play a key role in a variety of electrochemical systems. The mean free path of secondary electrons in aqueous solutions is on the order of a nanometer, making them suitable for probing ultrathin electrical double layers at solid-liquid electrolyte interfaces. Employing graphene as an electron-transparent electrode in a two-electrode electrochemical system, we show that the secondary electron yield of the graphene-liquid interface depends on the ionic strength and concentration of the electrolyte and the applied bias at the remote counter electrode. These observations have been related to polarization-induced changes in the potential distribution within the electrical double layer and demonstrate the feasibility of using scanning electron microscopy to examine and map electrified liquid-solid interfaces.

A new and exciting imaging technique being applied to thin films, nanocoatings, nanogels, and nanoparticle analysis is ultra-low accelerating voltage or ultra-low-landing-energy scanning electron microscopy (ULVSEM). Instrument conditions in this mode are different than with typical SEM observation or contemporary low accelerating voltage (LVSEM) imaging. Hence, the images appear far different due to reduced beam penetration. The landing energy of the primary electron beam can be much lower than LVSEM, it can be reduced to far below 500 electron volts (eV), even as low as 10 eV. Thus, the electron beam range and penetration are reduced tremendously with some unavoidable loss of spatial resolution. Surface details are enhanced, contrast might favorably change, and secondary electron (SE) edge enhancement or “blooming” contributing to measurement uncertainty is greatly reduced, potentially allowing for more precise and new measurements once this imaging mode is fully characterized and accurately modeled. High-resolution field-emission electron sources, improved lens, detector designs, and sample biasing all contribute to the ability to image at such low electron landing energies. The techniques of ULVSEM are discussed, and an application example is presented.

The semiconductor industry continues to produce ever smaller devices that are ever more complex in shape and contain ever more types of materials. The ultimate sizes and functionality of these new devices will be affected by fundamental and engineering limits such as heat dissipation, carrier mobility and fault tolerance thresholds. At present, it is unclear which are the best measurement methods needed to evaluate the nanometre-scale features of such devices and how the fundamental limits will affect the required metrology. Here, we review state-of-the-art dimensional metrology methods for integrated circuits, considering the advantages, limitations and potential improvements of the various approaches. We describe how integrated circuit device design and industry requirements will affect lithography options and consequently metrology requirements. We also discuss potentially powerful emerging technologies and highlight measurement problems that at present have no obvious solution.

Three-dimensional (3D) reconstruction of a sample surface from scanning electron microscope (SEM) images taken at two perspectives has been known for decades. Nowadays, there exist several commercially available stereophotogrammetry software packages. For testing these software packages, in this study we used Monte Carlo simulated SEM images of virtual samples. A virtual sample is a model in a computer, and its true dimensions are known exactly, which is impossible for real SEM samples due to measurement uncertainty. The simulated SEM images can be used for algorithm testing, development, and validation. We tested two stereophotogrammetry software packages and compared their reconstructed 3D models with the known geometry of the virtual samples used to create the simulated SEM images. Both packages performed relatively well with simulated SEM images of a sample with a rough surface. However, in a sample containing nearly uniform and therefore low-contrast zones, the height reconstruction error was ≈46%. The present stereophotogrammetry software packages need further improvement before they can be used reliably with SEM images with uniform zones.

In this chapter, sample preparation, image acquisition, and nanoparticle size and shape characterization methods using the scanning electron microscope (SEM) in reflective and transmitted working modes are described. These help in obtaining reliable, highly repeatable results. The best solutions vary case by case and depend on the raw (powdered or suspension) nanoparticle material, on the required measurement uncertainty, and on the performance of the SEM.

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.

Substantial research efforts are under way to optimize the production of composites enhanced by the incorporation of nanomaterial fillers such as multiwall carbon nanotubes (MWCNTs). It is therefore critical to develop robust methods to detect and characterize MWCNTs in nanocomposites to measure product performance and potential risks from release of the MWCNTs. In this study, the effectiveness of X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and scanning electron microscopy (SEM) methods is assessed on a series of MWCNT-epoxy nanocomposites samples. A general trend of positive correlation between signal intensity and MWCNT mass loading was observed by both Raman spectroscopy and XPS. Raman spectroscopy was capable of detecting MWCNTs at the lowest nominal concentration tested (0.01%), while for XPS, MWCNTs were detected down to a threshold of approximately 0.5%. Analysis of the same locations of two nanocomposite samples with nominal MWCNT mass fractions of 0.3% and 1% using Raman chemical imaging and SEM revealed a similar detectability of MWCNT clusters and with the higher mass loading sample having a greater number of MWCNT-rich domains. Overall, these results show a good comparability among the different techniques and therefore provide comprehensive, nondestructive microscopy methods to characterize nanocomposites.

Dimensional measurements from secondary electron (SE) images were compared with those from backscattered electron (BSE) and low-loss electron (LLE) images. With the commonly used 50% threshold criterion, the lines consistently appeared larger in the SE images. As the images were acquired simultaneously by an instrument with the capability to operate detectors for both signals at the same time, the differences cannot be explained by the assumption that contamination or drift between images affected the SE, BSE, or LLE images differently. Simulations with JMONSEL, an electron microscope simulator, indicate that the nanometer-scale differences observed on this sample can be explained by the different convolution effects of a beam with finite size on signals with different symmetry (the SE signal’s characteristic peak versus the BSE or LLE signal’s characteristic step). This effect is too small to explain the >100 nm discrepancies that were observed in earlier work on different samples. Additional modeling indicates that those discrepancies can be explained by the much larger sidewall angles of the earlier samples, coupled with the different response of SE versus BSE/LLE profiles to such wall angles.

Single particle inductively coupled plasma-mass spectrometry (spICP-MS) is an emerging technique capable of simultaneously measuring nanoparticle size and number concentration of metal-containing nanoparticles (NPs) at environmental levels. single particle ICP-MS will become an established measurement method once the metrological quality of the measurement results it produces have been proven incontrovertibly. This Article presents the first validation of spICP-MS capabilities for measuring mean NP size and number size distribution of gold nanoparticles (AuNPs). The validation is achieved by (i) calibration based on the consensus value for particle size derived from six different sizing techniques applied to National Institute of Standards and Technology (NIST) Reference Material (RM) 8013; (ii) comparison with high-resolution scanning electron microscopy (HR-SEM) used as a reference method, which is linked to the International System of Units (SI) through a calibration standard characterized by the NIST metrological atomic force microscope; and (iii) evaluation of the uncertainty associated with the measurement of the mean particle size to enable comparison of the spICP-MS and HR-SEM methods. After establishing HR-SEM and spICP-MS measurement protocols, both methods were used to characterize commercial AuNP suspensions of three different sizes (30, 60, and 100 nm) with four different coatings and surface charge at pH 7. Single particle ICP-MS measurements (corroborated by HR-SEM) revealed the existence of two distinct subpopulations of particles in the number size distributions for four of the 60 nm commercial suspensions, a fact that was not apparent in the measurement results supplied by the vendor using transmission electron microscopy. This finding illustrates the utility of spICP-MS for routine characterization of commercial AuNP suspensions regardless of size or coating.

Nanocellulose is a high value material that has gained increasing attention because of its high strength, stiffness, unique photonic and piezoelectric properties, high stability and uniform structure. Through utilization of a biorefinery concept, nanocellulose can be produced in large volumes from wood at relatively low cost via ionizing radiation processing. Ionizing radiation causes significant break down of the polysaccharide and leads to the production of potentially useful gaseous products such as H2 and CO. The application of radiation processing to the production of nanocellulose from woody and non-wood sources, such as field grasses, bio-refining byproducts, industrial pulp waste, and agricultural surplus materials remains an open field, ripe for innovation and application. Elucidating the mechanisms of the radiolytic decomposition of cellulose and the mass generation of nanocellulose by radiation processing is key to tapping into this source of nanocelluose for the growth of nanocellulostic-product development. More importantly, understanding the structural break-up of the cell walls as a function of radiation exposure is a key goal and only through careful, detailed characterization and dimensional metrology can this be achieved at the level of detail that is needed to further the growth of large scale radiation processing of plant materials. This work is resulting from strong collaborations between NIST and its academic partners who are pursuing the unique demonstration of applied ionizing radiation processing to plant materials as well as the development of manufacturing metrology for novel nanomaterials.

The semiconductor industry is moving toward a half-pitch of 7 nm. The required metrology equipment should be one order of magnitude more accurate than that. Any metrology tool is only as good as it is calibrated. The characterization of metrology systems requires test patterns that are one order of magnitude smaller than the measured features. The test sample was designed in such a way that the distribution of linewidths appears to be random at any location and any magnification. The power spectral density of such pseudo-random test pattern is inherently flat, down to the minimum size of lines. Metrology systems add a cut-off of the spectra at high frequencies; the shape of the cut-off characterizes the system in its entire dynamic range. This method is widely used in optics, and has allowed optical systems to be perfected down to their diffraction limit. There were attempts to use the spectral method to characterize nanometrology systems such as SEMs, but the absence of natural samples with known spatial frequencies was a common problem. Pseudo-random test patterns with linewidths down to 1.5 nm were fabricated. The system characterization includes the imaging of a pseudo-random test sample and image analysis by a developed software to automatically extract the power spectral density and the contrast transfer function of the nanoimaging system.

This paper discusses a two step enhancement technique applicable to noisy Helium Ion Microscope images in which background structures are not easily discernible due to a weak signal. The method is based on a preliminary adaptive histogram equalization, followed by 'slow motion' low-exponent Levy fractional diffusion smoothing. This combined approach is unexpectedly effective, resulting in a companion enhanced image in which background structures are rendered much more visible, and noise is significantly reduced, all with minimal loss of image sharpness. The method also provides useful enhancements of scanning charged-particle microscopy images obtained by composing multiple drift-corrected 'fast scan' frames. The paper includes software routines, written in Interactive Data Language (IDL),(1) that can perform the above image processing tasks.

This paper reports an interlaboratory comparison that evaluated a protocol for measuring and analysing the particle size distribution of discrete, metallic, spheroidal nanoparticles using transmission electron microscopy (TEM). The study was focused on automated image capture and automated particle analysis. NIST RM8012 gold nanoparticles (30 nm nominal diameter) were measured for area-equivalent diameter distributions by eight laboratories. Statistical analysis was used to (1) assess the data quality without using size distribution reference models, (2) determine reference model parameters for different size distribution reference models and non-linear regression fitting methods and (3) assess the measurement uncertainty of a size distribution parameter by using its coefficient of variation. The interlaboratory area-equivalent diameter mean, 27.6 nm ± 2.4 nm (computed based on a normal distribution), was quite similar to the area-equivalent diameter, 27.6 nm, assigned to NIST RM8012. The lognormal reference model was the preferred choice for these particle size distributions as, for all laboratories, its parameters had lower relative standard errors (RSEs) than the other size distribution reference models tested (normal, Weibull and Rosin–Rammler–Bennett). The RSEs for the fitted standard deviations were two orders of magnitude higher than those for the fitted means, suggesting that most of the parameter estimate errors were associated with estimating the breadth of the distributions. The coefficients of variation for the interlaboratory statistics also confirmed the lognormal reference model as the preferred choice. From quasi-linear plots, the typical range for good fits between the model and cumulative number-based distributions was 1.9 fitted standard deviations less than the mean to 2.3 fitted standard deviations above the mean. Automated image capture, automated particle analysis and statistical evaluation of the data and fitting coefficients provide a framework for assessing nanoparticle size distributions using TEM for image acquisition.

The scanning electron microscope (SEM) has gone through a tremendous evolution to become indispensable for many and diverse scientific and industrial applications. The improvements have significantly enriched and augmented the overall SEM performance and have made the instrument far easier to operate. But, the ease of operation also might lead, through operator complacency, to poor results. In addition, the user friendliness has seemingly reduced the need for thorough operator training for using these complex instruments. One might then conclude that the SEM is just a very expensive digital camera or another peripheral device for a computer. Hence, a person using the instrument may be lulled into thinking that all of the potential pitfalls have been eliminated and they believe everything they see on the micrograph is always correct. But, this may not be the case. An earlier paper (Part 1), discussed some of the potential issues related to signal generation in the SEM, instrument calibration, electron beam interactions and the need for physics-based modeling to understand the actual image formation mechanisms. All these were summed together in a discussion of how these issues effect measurements made with the instrument. This second paper discusses another major issue confronting the microscopist: electron-beam-induced specimen contamination. Over the years, NIST has done a great deal of research into the issue of sample contamination and its removal and elimination and some of this work is reviewed and discussed here.

Scanning electron microscopes (SEM) are used extensively in research and advanced manufacturing for materials characterization, metrology and process control. Unfortunately, noise can limit the specimen-specific detail and the information that can be acquired in any SEM micrograph, or measurement made from those data. The majority of SEM measurements are done at low primary electron beam currents and fast imaging mode resulting in rather noisy signals - often too noisy. The amount and the type of the noise and the steps taken to deal with it are critical to the quality and amount of the information gathered. This fifth presentation, in this series of SEM dimensional metrology tutorial papers, discusses some of the various causes of measurement uncertainty in scanned particle beam instruments specifically dealing with signal-to-noise (SNR) and its contribution to measurement imprecision.

We present here a hybrid approach to modeling helium-ion lithography that combines the power and ease-of-use of the Stopping and Range of Ions in Matter (SRIM) software with the results of recent work simulating secondary electron (SE) yield in helium-ion microscopy. This approach traces along SRIM-produced helium-ion trajectories, generating and simulating trajectories for SEs using a Monte Carlo method. We found, both through simulation and experiment, that the spatial distribution of energy deposition in a resist as a function of radial distance from beam incidence, i.e. the point spread function, is not simply a sum of Gauss functions.

A review of the evolution of modeling for accurate dimensional scanning electron microscopy is presented with an emphasis on developments in the Monte Carlo technique for modeling the generation of the electrons used for imaging and measurement. The progress of modeling for accurate metrology is discussed through a schematic technology timeline. In addition, a discussion of a future vision for accurate SEM dimensional metrology and the requirements to achieve it are presented.

In the version of this Review Article originally published, the labelling of the reflected beam in Fig. 4a was incorrect. This has now been corrected in the Review Article.

The magnitudes of the challenges facing electron-based metrology for post-CMOS technology are reviewed. Directed self-assembly, nanophotonics/plasmonics, and resistive switches and selectors are examined as exemplars of important post-CMOS technologies. Materials, devices, and architectures emerging from these technologies pose new metrology requirements: defect detection, possibly subsurface, in soft materials, accurate measurement of size, shape, and roughness of structures for nanophotonic devices, contamination-free measurement of surface-sensitive structures, and identification of subtle structural, chemical, or electronic changes of state associated with switching in non-volatile memory elements. Electron-beam techniques are examined in the light of these emerging requirements. The strong electron-matter interaction provides measurable signals from small sample features, rendering electron-beam methods more suitable than most for nanometer-scale metrology, but as is to be expected, solutions to many of the measurement challenges are yet to be demonstrated. The seeds of possible solutions are identified when they are available.

Harmonized sample pre-treatment is an essential first step in ensuring quality of measurements as regards repeatability, inter-laboratory reproducibility and commutability. The development of standard preparation methods for single-wall carbon nanotube (SWCNT) samples is therefore essential to progress in their investigation and eventual commercialization. Here, descriptions of sample preparation and pre-treatment for the physicochemical characterization of SWCNTs are provided. Analytical methods of these protocols include: scanning electron microscopy (SEM; dry, wet), transmission electron microscopy (TEM; dry, wet), atomic force microscopy (AFM), inductively-coupled plasma mass spectrometry (ICP-MS), neutron activation analysis (NAA), Raman spectroscopy (dry, wet), UV-Vis-NIR absorption and photoluminescence spectroscopy, manometric isothermal gas adsorption and thermogravimetric analysis (TGA). Although sample preparation refers to these specific methods, application to other methods for measurement and characterization of SWCNTs can be envisioned.

All forms of microscopy are being pushed to the limit by nanotechnology. This is especially true for high-resolution scanning electron and helium ion microscopes, which are proving to be extremely useful for nanometer-scale imaging, characterization, and dimensional measurements. Excellent resolution is essential for imaging nanomaterials. Hence, there is a relentless quest to achieve better and better resolution with various electron and ion microscopes and to monitor and maintain these instruments to achieve the best possible performance levels. The ability to resolve fine details with a microscope has greatly improved over the past 20 years. The resolution achievable with the newest scanning electron microscopes (SEMs) can now be at or below 0.4 nm and for the scanning helium ion microscope (HeIM), 0.24 nm has been reported. To put this into perspective, the {111} crystal plane in silicon (Si) has 0.32 nm lattice spacing, so one cubic nanometer in the Si crystal contains only a few atoms. Therefore, an instrument must be performing at the highest level possible to resolve routinely the finest structures. Clearly, the instrument operator must also be functioning at a high level as well to achieve the best resolution and to prove that the instrument is capable of doing the work.

Helium ion microscopy (HeIM) presents a new approach to nanotechnology and nanometrology, which has several potential advantages over the traditional scanning electron microscope (SEM) currently in use in research laboratories and manufacturing facilities across the world. Owing to the very high source brightness, and the shorter wavelength of the helium (He) ions, it is theoretically possible to focus the ion beam into a smaller probe size relative to that of the electron beam of an SEM. Hence, resolution 2 × – 4 × better than that of comparable SEMs is theoretically possible. In an SEM, an electron beam interacts with the sample and an array of signals are generated, collected and imaged. This interaction zone may be quite large depending upon the accelerating voltage and materials involved. Conversely, the helium ion beam interacts with the sample, but it does not have as large an excitation volume and, thus, the image collected is more surface sensitive and can potentially provide sharp images on a wide range of materials. Compared with an SEM, the secondary electron yield is quite high—allowing for imaging at extremely low beam currents and the relatively low mass of the helium ion, in contrast to other ion sources such as gallium, potentially results in minimal damage to the sample. This article reports on some of the preliminary work being done on the HeIM as a research and measurement tool for nanotechnology and nanometrology being done at NIST. SCANNING 30: 457–462, 2008. Published 2008 by Wiley Periodicals, Inc.

Resolution is a key performance metric, which often defines the quality of a scanning electron microscope (SEM). Traditionally, there is the subjective measurement of the distance between two points on special "resolution" samples and there are several computer-based resolution-calculation methods. These computer-based resolution-calculation methods are much more precise than direct measurement, but none of them can currently be considered an objective way of measuring the resolution. The methods are still under development; therefore, objective testing is necessary. One approach to algorithm testing is to use simulated images. Simulated images are very useful for this purpose because they can be well-defined in all parameters unlike the real SEM images. Simulated images can be generated that closely mimic the gold-on-carbon SEM test sample images that usually consist of bright grains on a dark background. Simulation can account for edge effect, roughness of the substrate, different focusing, drift and vibration, and noise. Shapes, positions, and sizes of the grain structures are random. The simulated images can be then used for testing the resolution-calculation methods, especially for finding how the particular properties of SEM images affect the resultant instrument performance and image resolution. To support this testing, NIST has developed and made available a reference set of simulated SEM images generated using the methods described in this article.

The combination of scanning electron microscopy for high spatia l resolution, images from multiple angles to provide 3Dinformation, and commercially available stereo photogrammetry software for 3D reconstruction offers promise fornanometer-scale dimensional metrology in 3D. A method is described to test 3D photogrammetry software by the use ofvirtual samples—mathematical samples from which simulated images are made for use as inputs to the software undertest. The virtual sample is constructed by wrapping a rough skin with any desired power spectral density around a smoothnear-trapezoidal line with rounded top corners. Reconstruction is performed with images simulated from different angularviewpoints. The software’s reconstructed 3D model is then compared to the known geometry of the virtual sample. Threecommercial photogrammetry softwa re packages were tested. Two of them produced results for line height and width thatwere within close to 1 nm of the correct values. All of the p ackages exhibited some difficulty in reconstructing details ofthe surface roughness.Keywords: critical dimension (CD), dimensional metrology, model-based metrology, scanning electron microscopy(SEM), simulation, stereo photogrammetry, surface roughness, virtual sample

This is the fourth part of a series of tutorial papers discussing various causes of measurement uncertainty in scanned particle beam instruments, and some of the solutions researched and developed at NIST and other research institutions. Scanned particle beam instruments especially the scanning electron microscope (SEM) have gone through tremendous evolution to become indispensable tools for many and diverse scientific and industrial applications. These improvements have significantly enhanced their performance and made them far easier to operate. But, the ease of operation has also fostered operator complacency. In addition, the user-friendliness has reduced the apparent need for extensive operator training. Unfortunately, this has led to the idea that the SEM is just another expensive “digital camera” or another peripheral device connected to a computer and that all of the problems in obtaining good quality images and data have been solved. Hence, one using these instruments may be lulled into thinking that all of the potential pitfalls have been fully eliminated and believing that, everything one sees on the micrograph is always correct. But, as described in this and the earlier papers, this may not be the case. Care must always be taken when reliable quantitative data are being sought. The first paper in this series discussed some of the issues related to signal generation in the SEM, including instrument calibration, electron beam-sample interactions and the need for physics-based modeling to understand the actual image formation mechanisms to properly interpret SEM images. The second paper has discussed another major issue confronting the microscopist: specimen contamination and methods to eliminate it. The third paper discussed mechanical vibration and stage drift and some useful solutions to mitigate the problems caused by them, and here, in this the fourth contribution, the issues related to specimen “charging” and its mitigation are discussed relative to dimensional metrology.

The scanning electron microscope (SEM) historically has been used mainly as an image-producing device and, in spite of certain obvious and sometimes serious electronics problems, serves in this function as an acceptable and effective instrument for many applications. Today, the SEM is being used in areas other than imaging. The major area of current use and of concern of this Workshop paper is SEM-based metrology. SEM-based metrology is of great interest to the microelectronics industry, and for this industry, measurement accuracy and repeatability issues have become paramount. This paper presents a short description of some of the electrical properties of different components of an SEM which can seriously affect its performance as a metrology tool. These include the electronics associated with the formation of the electron beam and scanning circuitry, the electron detectors, the video signal chain, and the analog-to-digital (AD) and digital-to-analog (DA) conversion circuits. This discussion relates the possible influences generated by these components to the detected and observed signal. This paper also includes the concept that computer modeling of the SEM image, with Monte Carlo or other techniques, must be done completely in order to be meaningful. Modeling just the electron beam/sample interactions alone only deals with a portion of the issues. Any modeling must include the shortcomings of the real, nonideal measuring tool. By knowing the electrical characteristics and possible imperfections caused by the different parts of an SEM, a more accurate comparison of the measured and calculated signal can be made. One practical example is presented which demonstrates how it is possible to improve the agreement between calculated and measured signals.

Experimental nanotips have shown significant improvement in the resolution performance of a cold field emission scanning electron microscope (SEM). Nanotip electron sources are very sharp electron emitter tips used as a replacement for the conventional tungsten field emission (FE) electron sources. Nanotips offer higher brightness and smaller electron source size. An electron microscope equipped with a nanotip electron gun can provide images with higher spatial resolution and with better signal-to-noise ratio. This could present a considerable advantage over the current SEM electron gun technology if the tips are sufficiently long-lasting and stable for practical use. In this study, an older field-emission critical dimension (CD) SEM was used as an experimental test platform. Substitution of tungsten nanotips for the regular cathodes required modification of the electron gun circuitry and preparation of nanotips that properly fit the electron gun assembly. In addition, this work contains the results of the modeling and theoretical calculation of the electron gun performance for regular and nanotips, the preparation of the SEM including the design and assembly of a measuring system for essential instrument parameters, design and modification of the electron gun control electronics, development of a procedure for tip exchange, and tests of regular emitter, sharp emitter and nanotips. Nanotip fabrication and characterization procedures were also developed. Using a "sharp" tip as an intermediate to the nanotip clearly demonstrated an improvement in the performance of the test SEM. This and the results of the theoretical assessment gave support for the installation of the nanotips as the next step and pointed to potentially even better performance. Images taken with experimental nanotips showed a minimum two-fold improvement in resolution performance than the specification of the test SEM. The stability of the nanotip electron gun was excellent; the tip stayed useful for high-resolution imaging for several hours during many days of tests. The tip lifetime was found to be several months in light use. This paper summarizes the current state of the work and points to future possibilities that will open when electron guns can be designed to take full advantage of the nanotip electron emitters.

Fully automated or semi-automated scanning electron microscopes (SEM) are now commonly used in semiconductor production and other forms of manufacturing. Testing and proving that the instrument is performing at a satisfactory level of sharpness is an important aspect of quality control. The application of Fourier analysis techniques to the analysis of SEM images is a useful methodology for sharpness measurement. In this paper, a statistical measure known as the multivariate kurtosis is proposed as an additional useful measure of the sharpness of SEM images. Kurtosis is designed to be a measure of the degree of departure of a probability distribution. For selected SEM images, the two-dimensional spatial Fourier transforms were computed. Then the bivariate kurtosis of this Fourier transform was calculated as though it were a probability distribution. Kurtosis has the distinct advantage that it is a parametric (i.e., a dimensionless) measure and is sensitive to the presence of the high spatial frequencies necessary for acceptable levels of image sharpness. The applications of this method to SEM metrology will be discussed.

Measurements done with scanning electron microscopes (SEMs) may lose their validity due to contamination and charging caused by the primary electron beam. The sample stage and the electron beam also slightly drift during the course of the measurements. Consequently, it is essential to find out the time limit of valid measurements, that is, the maximum time before the sample or its position changes too much. This paper describes digital time-lapse SEM, a useful tool for investigating the extent and effects of contamination and stage drift. It works with hardware and software that create a stack of sequential images. Later these images can be viewed as a short movie; it is also possible to apply all the image processing and analysis procedures that are otherwise applied to separate, individual images. This method gives a reliable way of measuring the rate of contaminant deposition and of stage and electron beam drift of SEMs, and it could be helpful in controlling these problems.

Traditional Monte Carlo modeling of the electron beam-specimen interactions in a scanning electron microscope (SEM) produces information about electron beam penetration and output signal generation at either a single beam-landing location, or multiple landing positions. If the multiple landings lie on a line, the results can be graphed in a line scan-like format. Monte Carlo results formatted as line scans have proven useful in providing one-dimensional information about the sample (e.g., linewidth). When used this way, this process is called forward line scan modeling. In the present work, the concept of image simulation (or the first step in the inverse modeling of images) is introduced where the forward-modeled line scan data are carried one step further to construct theoretical two-dimensional (2-D) micrographs (i.e., theoretical SEM images) for comparison with similar experimentally obtained micrographs. This provides an ability to mimic and closely match theory and experiment using SEM images. Calculated and/or measured libraries of simulated images can be developed with this technique. The library concept will prove to be very useful in the determination of dimensional and other properties of simple structures, such as integrated circuit parts, where the shape of the features is preferably measured from a single top-down image or a line scan. This paper presents one approach to the generation of 2-D simulated images and presents some suggestions as to their application to critical dimension metrology.

The semiconductor industry has exploited productivity improvements through aggressive feature size reduction for over four decades. While enormous effort has been expended in developing the optical lithography tools to print ever finer features, significant advances have also been required to measure the printed features. In this article we will discuss the current state of the art in the metrology for measuring critical dimensions of printed features for scanning electron microscopy and atomic force microscopy, and describe work at the National Institute of Standards and Technology advancing these tools as well as exploratory work on two new promising techniques, scatterfield microscopy and small angle X-ray scattering. Line width roughness critical dimension and overlay metrology and control are two of the most significant industry needs mentioned in the International Technology Roadmap for Semiconductors (2005). To cite this article: S. Knight et al., C. R. Physique 7 (2006).

In industrial applications, such as automated on-line semiconductor production, there is a growing realization of the need for the development of a procedure for periodic performance testing of the image sharpness of the scanning electron microscope. Two statistical and comparative measures of image sharpness based on the generalized variance and bivariate kurtosis of the spectral density for a two-dimensional stationary process are proposed.

The semiconductor electronics industry places significant demands upon secondary electron imaging to obtain dimensional measurements that are used for process control or failure analysis. Tolerances for measurement uncertainty and repeatability are smaller than the spatial resolution of edges in the scanning electron microscope (SEM) that is used to perform the measurements. Image processing techniques, historically used to identify edge locations, are inadequate under these conditions. An alternative approach, based upon Monte Carlo electron transport modeling to assign edge positions, has been developed. The specimen shape is parameterized, and parameters are iteratively adjusted to produce the best least squares fit to the measured image. Because Monte Carlo simulators are too slow to be used directly in such an iterative calculation, the Monte Carlo technique is used relatively few times to construct a library of results for parameters spanning the process space of interest. A function that interpolates the library then becomes a surrogate that is used to rapidly compute the model function as needed. This procedure has yielded measurement results from top-down SEM images that are in good agreement with cross-section measurements and that have as much as a factor of 3 better same-site repeatability than the more traditional techniques.

Variable-pressure/environmental scanning electron microscopy has been used for successful investigation binary and phase-shifting chromium on quartz optical photomasks. This methodology was also applied to patterned 193 nm photoresist structures. The application of this methodology to semiconductor metrology is new because of the recent availability of variable-pressure scanning electron microscopy (SEM) instrumentation equipped with high-resolution, high-signal, thermally assisted field emission technology in conjunction with large chamber and sample transfer capabilities. The variable-pressure SEM methodology employs a gaseous environment around the sample to help diminish the charge build-up that occurs under irradiation with the electron beam. Although very desirable for the charge reduction in many biological, pharmaceutical, and food applications, this methodology has not been employed for semiconductor photomask or wafer metrology until now. This is a new application of this technology to this area, and it shows great promise in inspection, imaging, and metrology in a charge-free operational mode. For accurate metrology, variable-pressure SEM methodology also affords a path that minimizes, if not eliminates, the need for charge modeling. This paper presents some of the early results in the variable-pressure SEM metrology of photomask and photoresist structures.

This is the fourth part of a series of tutorial papers discussing various causes of measurement uncertainty in scanned particle beam instruments, and some of the solutions researched and developed at NIST and other research institutions. Scanned particle beam instruments, especially the scanning electron microscope (SEM), have gone through tremendous evolution to become indispensable tools for many and diverse scientific and industrial applications. These improvements have significantly enhanced their performance and made them far easier to operate. But, the ease of operation has also fostered operator complacency. In addition, the user-friendliness has reduced the apparent need for extensive operator training. Unfortunately, this has led to the idea that the SEM is just another expensive "digital camera" or another peripheral device connected to a computer and that all of the problems in obtaining good quality images and data have been solved. Hence, one using these instruments may be lulled into thinking that all of the potential pitfalls have been fully eliminated and believing that, everything one sees on the micrograph is always correct. But, as described in this and the earlier papers, this may not be the case. Care must always be taken when reliable quantitative data are being sought. The first paper in this series discussed some of the issues related to signal generation in the SEM, including instrument calibration, electron beam-sample interactions and the need for physics-based modeling to understand the actual image formation mechanisms to properly interpret SEM images. The second paper has discussed another major issue confronting the microscopist: specimen contamination and methods to eliminate it. The third paper discussed mechanical vibration and stage drift and some useful solutions to mitigate the problems caused by them, and here, in this the fourth contribution, the issues related to specimen "charging" and its mitigation are discussed relative to dimensional metrology.

The width and shape of 10 nm to 12 nm wide lithographically patterned SiO2 lines were measured in the scanning electron microscope by fitting the measured intensity vs. position to a physics-based model in which the lines' widths and shapes are parameters. The approximately 32 nm pitch sample was patterned at Intel using a state-of-the-art pitch quartering process. Their narrow widths and asymmetrical shapes are representative of near-future generation transistor gates. These pose a challenge: the narrowness because electrons landing near one edge may scatter out of the other, so that the intensity profile at each edge becomes width-dependent, and the asymmetry because the shape requires more parameters to describe and measure. Modeling was performed by JMONSEL (Java Monte Carlo Simulation of Secondary Electrons), which produces a predicted yield vs. position for a given sample shape and composition. The simulator produces a library of predicted profiles for varying sample geometry. Shape parameter values are adjusted until interpolation of the library with those values best matches the measured image. Profiles thereby determined agreed with those determined by transmission electron microscopy and critical dimension small-angle x-ray scattering to better than 1 nm.

The ability to model the effect of fields due to charges trapped in insulators with floating conductors has been added to JMONSEL (Java Monte Carlo simulator for Secondary Electrons) and applied to a simple photomask metal on glass geometry. These capabilities are prerequisites if model-based scanning electron microscope (SEM) metrology is to be extended beyond wafer to photomask applications, where such samples must frequently be measured. Model-based SEM is an alternative to the customary threshold- or gradient-based approach. It is more demanding inasmuch as it requires a model of the physics of image formation, but the reward is greater accuracy, lower sensitivity to secondary sample characteristics (e.g., edge shape) that affect the intensity, and information about 3D geometrical shape (not simply the width) of the measured features. The prerequisites are ability to measure a signal, such as the SEM image, that is sensitive to changes in the parameters one wishes to measure and a model that describes the relationship between the signal and the parameters. The simulation shows the development of the potential energy barrier to electron escape during an initial transient charging-up phase, accompanied by increasing electron recapture and stabilization of the average yield.

Many advanced manufacturing processes employ scanning electron microscopes (SEM) for on-line critical measurements for process and quality control. This is the third of a series of papers discussing various causes of measurement uncertainty in scanned particle beam instruments, and some of the solutions researched and developed at NIST. Scanned particle beam instruments especially the scanning electron microscope have gone through tremendous evolution to become indispensable tools for many and diverse scientifi c and industrial applications. These improvements have signifi cantly enhanced their performance and made them far easier to operate. But, ease of operation has also fostered operator complacency. In addition, the user-friendliness has reduced the need for extensive operator training. Unfortunately, this has led to the concept that the SEM is just another expensive digital camera or another peripheral device connected to a computer and that all of the issues related to obtaining quality data have been solved. Hence, a person (or company) using these instruments may be lulled into thinking that all of the potential pitfalls have been fully eliminated and they believe everything they see on the micrograph is always correct. But, as described in this and the earlier presentations this may not be the case. The fi rst paper in this series discussed some of the issues related to signal generation in the SEM, including instrument calibration, electron beam-sample interactions and the need for physics-based modelling to understand the actual image formation mechanisms to properly interpret SEM images. The second paper, discussed another major issue confronting the microscopist: specimen contamination and methods of contamination elimination. This third paper, discusses vibration and drift and some useful solutions.

There has been significant interest in hybrid metrology as a novel method for reducing overall measurement uncertainty and optimizing measurement throughput (speed) through rigorous combinations of two or more different measurement techniques into a single result. This approach is essential for advanced 3-D metrology when performing model-based critical dimension measurements. However, a number of fundamental challenges present themselves with regard to consistent noise and measurement uncertainty models across hardware platforms, and the need for a standardized set of model parameters. This is of paramount concern when the various techniques have substantially different models and underlying physics. In this paper we present realistic examples using scanning electron microscopy, atomic force microscopy, and optical critical dimension (CD) methods applied to sub-20 nm dense feature sets. We will show reduced measurement uncertainties using hybrid metrology on 15 nm CD features and evaluate approaches to adapt quantitative hybrid metrology into a high volume manufacturing environment.