Simulated SEM images for resolution measurement.

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.

Electron beam-based metrology after CMOS.

The magnitudes of the challenges facing electron-based metrology for post-CMOS technology are reviewed. Directed selfassembly, 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 signal 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.

Metrology for the next generation of semiconductor devices.

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.

Virtual rough samples to test 3D nanometer-scale scanning electron microscopy stereo photogrammetry.

The combination of scanning electron microscopy for high spatial resolution, images from multiple angles to provide 3D information, and commercially available stereo photogrammetry software for 3D reconstruction offers promise for nanometer-scale dimensional metrology in 3D. A method is described to test 3D photogrammetry software by the use of virtual samples-mathematical samples from which simulated images are made for use as inputs to the software under test. The virtual sample is constructed by wrapping a rough skin with any desired power spectral density around a smooth near-trapezoidal line with rounded top corners. Reconstruction is performed with images simulated from different angular viewpoints. The software's reconstructed 3D model is then compared to the known geometry of the virtual sample. Three commercial photogrammetry software packages were tested. Two of them produced results for line height and width that were within close to 1 nm of the correct values. All of the packages exhibited some difficulty in reconstructing details of the surface roughness.

Scanning electron microscope measurement of width and shape of 10nm patterned lines using a JMONSEL-modeled library.

The width and shape of 10nm 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.

Application of the low-loss scanning electron microscope image to integrated circuit technology. Part 1--Applications to accurate dimension measurements.

Scanning electron microscopes (SEMs) are the most extensively used tools for dimensional metrology and defect inspection for integrated circuit technologies with 180 nm and smaller features. Currently, almost all SEMs are designed to collect as many secondary and backscattered electrons as possible. These signals are mainly secondary electrons (SE1, SE2, and SE3) detected with various detection schemes. To facilitate the electron collection, very strong electric and magnetic fields are applied not just in the path of the primary electron beam but to the emerging electrons as well. These new systems provide strong signals, thus better signal-to-noise ratio, and thus resulting in higher throughput than older ones. On the other hand, the use of secondary electrons means that measurement results are much more prone to the detrimental effects of electron beam interactions, sample charging, and sample contamination than measurements with higher-energy backscattered electrons. The use of backscattered electrons, especially low-loss electrons (LLE), can provide better surface sensitivity, edge accuracy, and repeatability, possibly at the expense of measurement speed. This two-part study investigates the benefits and drawbacks of low-loss electron imaging to edge characterization for dimensional metrology and enhancement of fine surface features done through filtration or separation of the generated LLE signal and the use of energy-dependent signals. Part 1 reviews and illustrates the potential for accurate dimensional measurements at low accelerating voltage by LLE, and Part 2 will concentrate on the enhancement of surface features in chemical-mechanically planarized specimens with the use of a novel LLE detector.

Application of the low-loss scanning electron microscope image to integrated circuit technology part II--chemically-mechanically planarized samples.

Chemical-mechanical planarization (CMP) is a process that gives a flat surface on a silicon wafer by removing material from above a chosen level. This flat surface must then be reviewed (typically using a laser) and inspected for scratches and other topographic defects. This inspection has been done using both the atomic force microscope (AFM) and the scanning electron microscope (SEM), each of which has its own advantages and disadvantages. In this study, the low-loss electron (LLE) method in the SEM was applied to CMP samples at close to a right angle to the beam. The LLEs show shallower topographic defects more clearly than it is possible with the secondary electron (SE) imaging method. These images were then calibrated and compared with those obtained using the AFM, showing the value of both methods. It is believed that the next step is to examine such samples at a right angle to the beam in the SEM using the magnetically filtered LLE imaging method.

Image sharpness measurement in the scanning electron-microscope--part III.

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.

Modeling the point-spread function in helium-ion lithography.

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.