Search

Although Scanning Electron Microscopes (SEM) have improved greatly over the last decade the techniques usually employed to measure their performance have not changed significantly in half a century. In particular, describing the imaging performance of an SEM by a single number - its 'resolution' - provides no useful information about its real world imaging capabilities nor about any of the factors that might limit that usefulness of the SEM for tasks such as metrology. The Contrast Transfer Function (CTF) discussed here analyses the way in which the SEM processes signal components of different spatial frequencies. The resultant plot provides information on the noise limited spatial resolution limit, predicts how this will vary with noise level, and provides a powerful general diagnostic capability. This type of measurement, which has become standard practice for transmission electron microscopes, can be performed using the public domain software package IMAGE-J, is rapid, and requires only a specimen offering a broad and flat Fourier spectrum. The capabilities of this approach are demonstrated by a number of examples.

IONiSE is a Monte Carlo simulation which describes the interactions of 5-50 keV energy He+ ions with solids, and predicts the production of ion induced secondary electron (iSE) emission. Its use to determine the most probable implant depth, the maximum ion range, and the effect of straggle are presented. IONiSE has been used to numerically fit literature tabulations of iSE generation from five elements so as to derive excitation energy and mean free path parameters. By employing those parameters in IONiSE the topographic yield variation for iSE as a function of energy and the atomic number of the target has been predicted, and estimates of the individual secondary electron contributions from the incident and backscattered ions have been made. These simulations help to create a foundation for the application and the interpretation of iSE images for metrology.

The physical dimensions of nanoscale objects are an important indicator of their functionality. However, measuring feature size from a SEM image is difficult not only because of fundamental considerations, such as the nature of beam interactions and the information transfer properties of the microscope, but because the magnification of the SEM image from which a measurement will be made is completely uncalibrated and additionally is subject to local distortions and variations. Nano-gauges fabricated by electron beam lithography - one or two dimensional structures on the size scale of the objects of interest - provide a local length standards within the image field from which the relative size of features can be accurately determined. In order to provide an absolute measurement of size the dimensions of the nano-gauge structure must themselves be calibrated against some primary standard. Because there are no convenient standards of appropriate scale available we propose that this can be done using a moire fringe technique to bridge the gap between the nanoscale and common length standards such as ruled diffraction gratings.

With the semiconductor industry moving into the 65nm technology node, the metrology of the critical dimension (CD) becomes an important part for the industry. Metrology relies not only on the precision, but also on the accuracy of the tools like the high performance CD-SEMs. A major area of concern affecting the accuracy of the high performance CD-SEMs is the magnification calibration. The purpose of the research is to address this area of concern by fabrication of magnification calibration artifact by using direct write electron beam lithography. A calibration artifact has been fabricated in negative resist Hydrogen Silsesquioxane (HSQ) onto a silicon substrate, thereby decreasing the contamination on the substrate. The design of the artifact has been corrected for the proximity effects, giving a 2-D dense grid structure with 100nm pitch. Pitch determination using optical metrology tools and the inbuilt laser interferometer in the electron beam lithography tool is being evaluated for making the artifact traceable to some national standards. Once the traceability is achieved, mass production at low cost using Nanoimprint technology is feasible.

A scanning ion microscope (SIM) is analogous to a scanning electron microscope (SEM) but utilizes a beam of helium ions, with energy of 10 to 25 keV , instead of electrons. The SIM potentially offers several advantages for device critical dimension metrology as compared to the more familiar CD-SEM. These include a high brightness source which is sub-nanometer in size, an enhanced secondary electron yield, restricted beam penetration, and superior image contrast and information content. Possible problems include pervasive positive charging, ion implantation, and a lack of detailed experimental and theoretical knowledge about low energy ion interactions with solids. Comparison of line profiles across structures made by electron induced and ion induced secondary electrons show that there are some significant differences between them which arise from the different modes of interaction in the two cases. As a result the algorithms employed for line width determination will require revision in order to produce data which is consistent with CD-SEM data.

It is important to be able to quantify the imaging performance of CD-SEMs for such purposes as verifying the specification, rechecking after a routine maintenance, or for tool matching. To perform tests such as these it is necessary to have both appropriate software for image analysis and suitable test samples. A package of 2-D Fourier transform and analysis software, designed as a plug-in for the shareware IMAGE-Java program, has been developed and is freely available on line. The requirement for a reproducible and well characterized sample has been met by using direct-write electron beam lithography to fabricate suitable Fresnel zone plate structures.

To examine the practical limits and effects of low voltage operation, studies of electron beam lithography (EBL) in the low (few keV) to ultra-low (E < 500eV) energy range, employing commonly used resists such as PMMA was done, and the results were compared to those from conventional high voltage processing. The direct writing was performed at low energies by our homemade scan generator and a Schottky field emission gun scanning electron microscope (SEM), used in cathode-lens mode for ultra-low voltage operation. The exposure characteristics and sensitivity of the system at these energies have been investigated using an advanced Monte Carlo simulation method. Our modeling of the lithographic process showed a significant increase in resolution and process latitude for thinner resists.

Lithographic processes for printing device structures on integrated circuits (ICs) are the fundamental technology behind Moore's law. Next-generation techniques like maskless lithography or ML2 have the advantage that the long, tedious and expensive process of fabricating a unique mask for the manufactured chip is not necessary. However, there are some rather daunting prblems with establishing ML2 as a viable commercial technology. The data rate necessary for ML2 to be competitive in manufacturing is not feasible with technology in the near future. There is also doubt that the competing technologies for the writing mechanisms and corresponding photoresist (or analogous medium) will be able to accurately produce the desired patterns necessary to produce multi-layer semiconductor devices. In this work, we model the maskless printing system from a signal processing point of view, utilizing image processing algorithms and concepts to study the effects of various real-world constraints and their implications for a ML2 system. The ML2 elements are discrete devices, and it is doubtful that their motion can be controlled to the level where a one-for-one element to exposed pixel relationship is allowable. Some level of sub-element resolution can be achieved with gray scale levels, but with the highly integrated manufacturing practices required to achieve massive parallelism, the most effective elements will be simple on-ofrf switches that fire a fixed level of energy at the target medium. Consequently gray-scale level devidces are likely not an option. Another problem with highly integrated manufacturing methods is device uniformity. Consequently, we analyze the redundant scanning array concept (RSA) conceived by Berglund et al. which can defeat many of these problems. We determine some basic equations governing its application and we focus on applying the technique to an array of low-energy electron emitters. Using the results of Monte Carlo simulations on electron beam profiles, we determine an empirical "impulse response" for each emitter and thus determine how each emission manifests itself in the final printed lithographic pattern. We apply methods to determine the best printable image for a variety of RSA geometries, including different levels of redundancy and achieved printer element spacing. We use concepts of total printing error to help quantify the printing quality. Through simulation, we report the effects of dead or missing elements. We also present some error analysis to account for non-ideal array positioning. Ultimately, we believe that printing quality should be the grounds for determining the necessary data rates to support competitive manufacturing with ML2 devices.

The purpose of this paper is to define standard methods for effective and efficient image-based dimensional metrology for microlithography applications in the manufacture of integrated circuits. This paper represents a consensual view of the co-authors, not necessarily in total agreement across all subjects, but in complete agreement on the fundamentals of dimensional metrology in this application. Fundamental expectations in the conventional comparison-based metrology of width are reviewed, with its reliance on calibration and standards, and how it is different from metrology of pitch and image placement. We discuss the wealth of a priori information in an image of a feature on a mask or a wafer. We define the estimates of deviations from these expectations and their applications to effective detection and identification of the measurement errors attributable to the measurement procedure or the metrology tool, as well as to the sample and the process o fits manufacture. Although many individuals and organizations already use such efficient methods, industry-wide standard methods do not exist today. This group of professionals expects that, by placing de facto standard meth-odologies into public domain, we can help reduce waste and risks inherent in a "spontaneous" technology build-out, thereby enabling a seamless proliferation of these methods by equipment vendors and users of dimensional metrology. Progress in this key technology, with the new dimensional metrology capabilities enabled, leads to improved perform-ance and yield of IC products, as well as increased automation and manufacturing efficiency, ensuring the long-term health of our industry.

Phase defects in extreme UV lithography masks made from Mo/Si multilayer thin films can be removed by heating the metal multilayer to produce a localized phase transition. The experimental situation has been simulated using a Monte Carlo method to determine the deposition of energy from the incident electrons, and the resultant elevation of temperature has been found by solving the thermal diffusion equation. The effects of operating parameters such as beam energy, beam current, and beam spot size have been investigated. It is shown that the effect of surface radiation cooling is negligible, and that only a steady state solution needs to be considered.

Electron-beam-stimulated etching has been investigated as a clean, alternative method for nanoscale selective processing. Primarily fluorine-based precursors have been used to etch a variety of technologically relevant materials. Empirical data reveals that with decreasing the electron beam energy increases the material removal rate, however the effective beam width increases. Both of these observations are consistent with the fact that cross-sections for electron-gas scattering increases with decresaing beam energy. Monte Carlo models of the electron-gas and electron-solid interactions have been performed to better udnerstand the fundamentals of the process. Finally, specific application to soft transmission electron microscopy sample preparation is made.

Laser vaporization (LV) is a remarkably versatile technique for the catalytically-aided synthesis of nanomaterials, such as single-wall carbon nanotubes (SWNT). SWNT show remarkable promise for future generations of electronics and structural materials, however their application and commercialization has been hampered by a lack of control over the synthesis process, and low production quantities. Time-resolved in situ spectroscopic investigations of the laser-vaporization SWNT-synthesis process are described which are yielding some of the first direction determinations of carbon nanotube growth mechanisms and rates necessary to evaluate strategies for controllable synthesis and large-scale production. Our measurements indicate that SWNT grow over extended annealing times during the LV process by the conversion of condensed phase nanoparticle feedstock. These measurements were extended to grow carbon nanotubes by CO2- laser-annealing heat treatments of carbon and metal nanoparticle mixtures, offering an alternative synthesis approach to vapor- phase methods. These results present opportunities for scaled-up production of nanomaterials compatible with commercial high- power-laser technology.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Electron-beam tools have been an important part of the semiconductor industry since its beginnings especially in the area of metrology and are a major contributor towards the goals of process control and yield enhancement. However, in order to stay on the Moore's Law curve instrument performance must be improved by a factor of two every two or three years and with the proximity of fundamental physical limits such major improvements in performance become increasingly difficult to obtain. This paper examines the opportunities that exist for continued improvement, and considers what other options are available for tools to replace the CD-SEM and its associates.

Conventional scanning electron microscopes are now close to the limit of their performance for tasks such as the metrology of sub-micron design rule devices. In order to overcome these limits we have designed, and are presently testing, a low voltage point source microscope operated with a nanotip field emitter and without any electron optical lenses. The microscope is designed such that can be operated in the transmission mode as well as in a reflection mode. The ultra-sharp field emitter delivers emission currents of several nanoamps at energies less than 100 eV. The magnification of the object wave is achieved by placing the specimen in the divergent electron beam from the nanotip and observing the object wave using a microchannel plate (MCP) at a great distance from the sample. Images obtained that way are out of focus images. As no lenses are present a special procedure for scaling the magnification has been developed. Since electrons from a point source are highly coherent the out of focus images of the sample are interferograms. Electrons diffracted at an edge of the specimen cause Fresnel fringes in the image plane. An electrically charged holey carbon foil acts in the same way on the electrons as the Youngs double slit experiment and results in an interference pattern consisting of parallel fringes. A comparison between the transmission mode and the perfection mode shows great similarities with respect to the magnification and the interference pattern. An electron gun needed in the transmission mode is the most important difference between the two modes of operation. The experimental results at a reflection of 45 degrees are in good agreement with our simulation. Following our simulations a reflection angle of 90 degrees is most promising for easiest image interpretation.

We show simulation images of projection microscope, which uses parallel beam to the sample, detecting secondary electrons, photoelectrons and mirror electrons using electrons and UV/X-ray sources as probe beam. The results show we can see image for square and dense line patterns without charging and damage in the objects. This research can be used for quantitative analysis for projection microscope using multi beam source to get comprehensive information of many characteristics of objects as well as topographic information.

Conventional scanning electron microscopes are now close to the limit of their performance for tasks such as the metrology of sub-micron design rule devices. In order to overcome these limits we are investigating the use of in-line electron holography for device metrology. The in-line holograms are formed in a point projection microscope using ultra-low energy electrons (50-250eV) emitted from a nano-tip electron source. Holograms in the transmission mode and in the reflection mode of the microscope as well are possible. Since these in-line holograms are equivalent to out of focus micrographs acquired in a transmission electron microscope with a field emission gun we can reconstruct the original wave front by means of Fourier optics. The resolution of the point projection microscope is given by the sharpness of the emitter. We investigate the electric potential of the emitter using off-axis electron holography in a transmission electron microscope and compare the results to simulations obtained by solving the appropriate Laplace equation.

Charging effects on secondary electron (SE) profiles with bias voltage in deep contact holes are investigated. We show first in detail the SE beam profiles for operating conditions such as scanning time, current and landing energy, the brightness of the bottom of the contact hole depends on the charge of SE yield with incident energy. We conclude that we can enhance the contrast of the beam profile by optimizing the applied bias voltage.

The charging of the workpiece in electron beam direct writing processes has been identified as a problem that disturbs the electron beam and causes pattern displacement error. In this paper the electron beam deflection caused by surface charging is evaluated by the SIMION and MATHEMATICA simulation programs. Isolated charged patterns of different geometry are simulated as electrodes with given potentials in SIMION to calculate the extent of beam deflection, while secondary electron emission yield and beam dosage are assigned in MATHEMATICA programming for determination of surface potential and spatial field, from which the beam deflection is then calculated. The simulation results are in good agreement with each other and they are compared with values available in literature. The initial charge content along with the pattern dimensions chosen in simulation are found to be the major factors that determine the extent of beam deflection. The limitation of SIMION simulation on the beam deflection is also discussed.

Charging effects on beam deflection of incident electrons in electron beam lithography are investigated. We show first in detail how the non-unity yield of electron generation in insulator resists leads to local charging accumulation and affects the beam deflection of incident electrons as charging develops. Then the amounts of beam deflection are identified for various operating and resist dimension conditions, and then we conclude that the beam deflection should be avoided for more accurate manufacturing semiconductor devices by the control of charging effects.

The 1999 International Technology Roadmap for Semiconductors describes the critical measurement challenges for all areas of wafer processing in the Metrology Roadmap. The roadmap indicates that the research and development community must advance microscopy, especially scanning electron microscopy, in the near term and simultaneously develop alternate technology for IC generations with sub 100 nm feature sizes. In this paper, the issues such as loss of depth of focus that are facing CD-SEM are described. In addition, some of key challenges for overlay are briefly mentioned.

The performance of scanning electron beam instruments such as CD-SEMs can be defined in terms of parameters such as the beam probe size, the spatial resolution, and the signal to noise ratio of the image. A knowledge of these quantities is important in verifying the fact that an instrument meets its specification, and subsequently for tracking and optimizing its performance during use. Analytical methods based on the power spectrum (2-D Fourier transform analysis) of images are now beginning to be used for these purposes but care must be exercised to ensure reliable and meaningful results. Two new methods are suggested which can offer more detailed information about the microscope performance while avoiding the pitfalls of the simpler technique. Code implementing these tests, written as a plug-in macro for the well known NIH Image program, is available on-line.

Conventional scanning electron microscopes are now close to the limit of their performance for tasks such as the metrology of sub-micron design rule devices. In order to overcome these limits we are investigating the use of in-line electron holography for device metrology. This device will use ultralow energy electrons (50-250eV) emitted from a nano-tip electron source. Out of focus holograms of a mesh were simulated and reconstructed in the transmission mode of the microscope whereas in the reflection mode a sample consisting of only two points was used. In both operation modes of the microscope it is possible to change the distance from the point source (crossover of a lens) to the sample and the distance from the sample to the detector plane such that the magnification is kept constant. Series of simulated holograms consisting of only a few points reveal the distances resulting in the easiest interpretable images. When in-line holography is performed using electrons, the beams are deflected by the electric field between point source and sample. Ray tracing of the electrons performed by solving the appropriate Laplace equation can help to determine an optimum geometry for the microscope.

Secondary electron spectra have been obtained from a variety of materials under vacuum conditions similar to those encountered in a conventional scanning electron microscope. Although secondary emission is restricted to the near surface region of a sample, and so would be expected to be affected by the presence of a film of contamination, it is shown that the modifications to the secondary electron spectrum are generally minor in nature and do not make it impossible to associate the spectrum with the underlying material. Secondary electron spectroscopy may provide a method to add a materials classification category to defect review.

Ultra-low energy (ULV) scanning electron microscopy is performed in the range between 20 eV and 100 eV. By the use of a retarding field cathode lens assembly ULV imaging with a spatial resolution in the nanometer range has been achieved. This performance is close the predicted theoretical value but is limited by astigmatism and signal to noise considerations. The modes of signal formation at ULV are well suited for metrology and this type of operation may offer reduced beam damage and charging artifacts.

The scanning electron microscope (SEM) has many advantages for tasks for as metrology and defect review compared to other competitive technologies, but the level of performance required to meet the specifications proposed for the next generation of devices will raise some significant problems that must be overcome. These include theoretical limits on the spatial resolution, practical limits to performance set by the electron-optical characteristics of the SEM, and the dynamic response of the instrument to signal information. Unwanted artifacts of the electron-solid interaction such as charging and radiation damage must also be considered as potential restrictions to performance.