Magnification Calibration Research Articles

Automated Analysis of Data Mark Microstructure of Optical Discs

Abstract Introduction. By volume of material produced, optical disc manufacturing is the world's large nanotechnology industry, far surpassing semiconductors and magnetic disks. The development of various optical disc media has introduced a variety of challenges, which necessitate viewing the geometrical structure of the bumps, pits, or grooves on the surface of the discs. The Atomic Force Microscope (AFM) has the resolution and sensitivity needed for device analysis, but in the past has been limited by magnification errors, image distortion, data handling, and calibration. Furthermore, the common measurement procedure for AFM images consists of manual dimensional measurements. This introduces error and limits the number of measurements taken at one time. We have developed an automated process, which overcomes these limitations and provides a comprehensive approach to feature measurement, data analysis, calibration, and reporting.

A new way of measuring microscope aberrations

Two-fold astigmatism, three-fold astigmatism and misalignment (coma) can be found from the orientations only of the diffractograms of a set of images with injected beam tilts – typically eight images with tilts of equal magnitude and azimuths 45° apart. The spherical aberration must be determined independently, and the defocus is not measured; however, the measurement required is very much simpler than the measurement of image displacements or induced defocus/astigmatism values, and no calibration of the microscope magnification is needed.

Chemical and Structural Characterization of Ultrathin Dielectric Films Using AEM

ABSTRACTThe structure of ultrathin silicon oxynitride films, used as gate dielectrics in integrated circuits (ICs), is studied using analytical electron microscopy (AEM). Laterally homogeneous blanket films approximately 2 nm in thickness are characterized in cross section using a 300 keV field emission TEM/STEM. High resolution imaging (HRTEM) is used to investigate the accuracy and precision of film thickness measurements and their comparability to other techniques such as secondary ion mass spectrometry, spectroscopic ellipsometry, x-ray reflectivity, x-ray photoelectron spectroscopy, and medium energy ion scattering. A two dimensional magnification calibration scheme that fits a pair of basis vectors to experimental images is presented, and integrated intensity profiles are used to define film boundaries for measurement. These image processing tools simultaneously improve the repeatability of the measurements and remove subjective operator bias from the measurement process.

Quantitation of latent resist images using photon tunneling microscopy

Photon tunneling microscopy (PTM) is an optical microscopy technique that can, by using frustrated total internal reflection, be sensitive to changes in surface topography as small as 1 nm. We have developed a simple calibration technique that generates plots of sample reflectivity as a function of topography. This empirical data is then used to convert a gray-scale image of a specimen into an accurate topographic image. Errors, such as accurate magnification calibration and detector nonlinearity, lead to only small differences between the empirical data and our theoretical prediction. We have used PTM to study the evolution of latent images in a chemically amplified resist, ARCH2, as a function of dose, both after exposure and after postexposure bake, and have obtained good agreement between the topography measured in the PTM with thickness changes determined by ellipsometry from large exposed areas. Comparison of the results of the PTM with those obtained by near-field scanning optical microscopy demonstrate that changes in topography on the order of 3–5 nm are visible in the PTM. The topography that appears to develop in a latent image is a function of feature size. This effect occurs before the lateral resolution limit of the microscope is reached, and is a result of the mechanical constraint arising from unexposed resist material around the exposed features. Techniques such as adjusting the polarization and wavelength of the illumination can improve the sensitivity of the technique to small changes in topography, while in the case of materials that undergo changes in refractive index, contrast can be seen even in the absence of topography.

Calibrating the TEM

Abstract An accurate calibration of magnification, diffraction patterns and the relative rotation between the two is critical in quantitative transmission electron microscopy (TEM). The manufacturers of TEM's supply magnification values for each image magnification step and each value of camera length for their instruments, but these values have been observed to be in error by up to 10%, These errors can occur due to variations in lenses in individual microscopes as well as through power supply variations and aging electronic components. Before attempting quantitative TEM analysis, a series of calibrations of the individual TEU should be undertaken.

Problems associated with the calibration of optical probe based topography instruments

Although non-contacting instruments for the measurement of the three-dimensional (3-D) micro-topography of surfaces are now widely available and are used on a routine basis in many branches of science and engineering, information on many aspects of use from measurement through characterisation to interpretation is still limited. One important area in which clear information is required is calibration. This paper investigates some of the problems that arise during the calibration of the vertical magnification of optical probe-based systems. These problems are related to the choice of data positions as well as the integrity of the circle fitting algorithms. The paper also proposes a new technique based on optical sensors for the calibration of the diameter of the optical probe.

A transmission electron microscope (TEM) calibration standard sample for all magnification, camera constant, and image/diffraction pattern rotation calibrations.

A calibration sample for transmission electron microscopy (TEM) has been developed that performs the three major instrument calibrations for a transmission electron microscope: the image magnification calibration for measurements of images, the camera constant calibration for indexing diffraction patterns, and the image/diffraction pattern rotation calibration for relating crystal directions to features in the image. This offers an improvement over commercially available calibration standards, where up to five different samples are required to perform these three calibrations. The new calibration sample consists of an electron-transparent cross-sectional TEM sample made from a molecular beam epitaxy (MBE)-grown, single-crystal semiconductor wafer. When the calibration structure is viewed in a TEM, it appears as a series of light and dark layers where the layer thicknesses are very accurately known. The calibrated thickness measurements of these light (silicon) and dark (SiGe alloy) layers are based on careful TEM measurements of the [111] lattice spacing of silicon which is visible on the calibration sample itself, and are supported by X-ray diffraction measurements. Furthermore, the layer thickness variation across the entire silicon wafer has been verified to be less than 1%, allowing all samples prepared from the same wafer to have errors in the given layer thickness values of less than 1%. As the sample is a single crystal of silicon, the calibrations requiring electron diffraction information such as the camera constant calibration and the image/diffraction pattern rotation calibration can also be performed easily and unambiguously. One single calibration sample can therefore be used to provide all three of the major TEM instrument calibrations at all magnifications and all camera lengths.

Fabrication issues for the prototype National Institute of Standards and Technology SRM 2090A scanning electron microscope magnification calibration standard

A new National Institute of Standards and Technology scanning electron microscope magnification calibration standard has been fabricated and distributed in production prototype form. The SRM 2090A samples contain structures ranging in pitch from 3000 to 0.2 μm and are useful at both high- and low-accelerating voltages. The design and fabrication of the samples has incorporated many of the improvements suggested through two previous prototype series. This article discusses optimization of the lithographic techniques in the fabrication process. We used electron beam lithography for fabrication because of the 0.1 μm features required on the samples. After liftoff, unexpected linewidth differences were measured among the smallest features. These differences were reduced through modification of the electron beam writing paths. We propose that the resist heating effect caused an axial dependence among features fabricated using the original electron beam writing paths. In addition, the susceptibility of the present prototype design to electron beam proximity effects was analyzed. Proximity effects from the 9 μm range of backscattered electrons have little effect on the pitch structures of the prototype but may impact development of the prototype as a linewidth standard.

Accurate measurements of mean inner potential of crystal wedges using digital electron holograms

The mean inner potential of a solid is a fundamental property of the material and depends on both composition and structure. By using cleaved crystal wedges of known angle, combined with dogital recording of off-axis electron holograms and with theoretical calculations of dynamical effects, the mean inner potential of Si (9.26±0.08 V), MgO (13.01±0.08 V), GaAs (14.53±0.17 V) and PbS (17.19±0.12 V) is measured with high accuracy of about 1%. Dynamical contributions to the phase of the transmitted beam are found by Bloch wave calculations to be less than 5% when the crystal wedges are titled away from zone-axis orientations and from major Kikuchi bands. The accuracy of the present method is a factor of 3 better than previously achieved by reflection high-energy electron diffraction and electron interferometry. The major causes of uncertainty were specimen imperfections and errors in phase measurement and magnification calibration.

Scanning Electron Microscope magnification calibration interlaboratory study

The National Institute of Standards and Technology (NIST) is in the process of developing a new scanning electron microscope (SEM) magnification calibration reference standard useful at both high and low accelerating voltages. This standard will be useful for all applications to which the SEM is currently being used, but it has been specifically tailored to meet many of the particular needs of the semiconductor industry. A small number of test samples with the pattern were prepared on silicon substrates using electron beam lithography at the National Nanofabrication Facility at Cornell University. The structures were patterned in titanium/palladium with maximum nominal pitch of approximately 3000μm scaling down to structures with minimum nominal pitch of 0.4μm. Several of these samples were sent out to a number of university, research, semiconductor and other industrial laboratories in an inter-laboratory study. The purpose of the study was to test the SEM instrumentation and to review the suitability of the sample design.

Interlaboratory Study on the Lithographically Produced Scanning Electron Microscope Magnification Standard Prototype.

NIST is in the process of developing a new scanning electron microscope (SEM) magnification calibration reference standard useful at both high and low accelerating voltages. This standard will be useful for all applications to which the SEM is currently being used, but it has been specifically tailored to meet many of the particular needs of the semiconductor industry. A small number of test samples with the pattern were prepared on silicon substrates using electron beam lithography at the National Nanofabrication Facility at Cornell University. The structures were patterned in titanium/palladium with maximum nominal pitch structures of approximately 3000 μm scaling down to structures with minimum nominal pitch of 0.4 (μm. Eighteen of these samples were sent out to a total of 35 university, research, semiconductor and other industrial laboratories in an interlaboratory study. The purpose of the study was to test the SEM instrumentation and to review the suitability of the sample design. The laboratories were asked to take a series of micrographs at various magnifications and accelerating voltages designed to test several of the aspects of instrument performance related to general SEM operation and metrology. If the instrument in the laboratory was used for metrology, the laboratory was also asked to make specific measurements of the sample. In the first round of the study (representing 18 laboratories), data from 35 instruments from several manufacturers were obtained and the second round yielded information from 14 more instruments. The results of the analysis of the data obtained in this study are presented in this paper.

Use of radial density plots to calibrate image magnification for frozen-hydrated specimens

Accurate magnification calibration for transmission electron microscopy is best achieved with the use of appropriate standards and an objective calibration technique. We have developed a reliable method for calibrating the magnification of images from frozen-hydrated specimens. Invariant features in radial density plots of a standard are compared with the corresponding features in a “defocused” X-ray model of the same standard. Defocused X-ray models were generated to mimic the conditions of cryo-electron microscopy. The technique is demonstrated with polyoma virus, which was used as an internal standard to calibrate micrographs of bovine papilloma virus type 1 and bacteriophage ΓX174. Calibrations of the micrographs were estimated to be accurate to 0.35%–0.5%. Accurate scaling of a three-dimensional structure allows additional calibrations to be made with radial density plots computed from two- or three-dimensional data.

The use of radial density plots to calibrate images of frozen-hydrated specimens

Accurate magnification calibration for transmission electron microscopy is best achieved with appropriate external or internal standards. The use of the icosahedral polyoma virus as a standard in frozen-hydrated preparations was previously described. This method used the known diameter of polyoma (from x-ray measurements) as a reference to calibrate the diameters of other viruses. Measurements were made from circular averages computed from individual particle images or from an average image of many individual particle images. The measured diameters of several different spherical viruses are in good agreement with measurements of the same viruses made with x-ray crystallographic and solution scattering techniques.We found, however, that it is very difficult to accurately measure particle diameters in images taken from frozen-hydrated specimens. This difficulty is attributable to several factors including i) uncertainty about the solvent density level, ii) superposition of density features that occur in two-dimensional projections of three-dimensional objects, iii) uneven outer surfaces of virus particles, iv) characteristically high noise and low contrast levels in micrographs, and v) contrast transfer function (CTF) effects.

Quantitative geometric scanning electron microscopy

Quantitative SEM assessment of the geometry of biological specimens requires attention to 1) statistically valid sampling, 2) dimensional changes during fixation, dehydration, drying, and cutting, 3) artifacts produced by coating, 4) microscope raster and display system field distortion 5) magnification calibration 6) stage tilt calibration and hysteresis, and 7) counting protocols for selecting parts of the final images as data.This analysis is necessary, for example, in measuring the thickness of the liquid layer lining the alveoli in lung from cross-fractured frozen hydrated pulmonary alveoli.Adequate numbers of animals, with samples from each significant organ region must be studied. Sampling may be in a stratified random manner or by guided microdissection to preserve additional structural information.

Precise on-line Measurement of Lattice Vectors

Established methods for measurement of lattice spacings and angles of crystalline materials include x-ray diffraction, microdiffraction and HREM imaging. Structural information from HREM images is normally obtained off-line with the traveling table microscope or by the optical diffractogram technique. We present a new method for precise measurement of lattice vectors from HREM images using an on-line computer connected to the electron microscope. It has already been established that an image of crystalline material can be represented by a finite number of sinusoids. The amplitude and the phase of these sinusoids are affected by the microscope transfer characteristics, which are strongly influenced by the settings of defocus, astigmatism and beam alignment. However, the frequency of each sinusoid is solely a function of overall magnification and periodicities present in the specimen. After proper calibration of the overall magnification, lattice vectors can be measured unambiguously from HREM images.Measurement of lattice vectors is a statistical parameter estimation problem which is similar to amplitude, phase and frequency estimation of sinusoids in 1-dimensional signals as encountered, for example, in radar, sonar and telecommunications. It is important to properly model the observations, the systematic errors and the non-systematic errors. The observations are modelled as a sum of (2-dimensional) sinusoids. In the present study the components of the frequency vector of the sinusoids are the only parameters of interest. Non-systematic errors in recorded electron images are described as white Gaussian noise. The most important systematic error is geometric distortion. Lattice vectors are measured using a two step procedure. First a coarse search is obtained using a Fast Fourier Transform on an image section of interest. Prior to Fourier transformation the image section is multiplied with a window, which gradually falls off to zero at the edges. The user indicates interactively the periodicities of interest by selecting spots in the digital diffractogram. A fine search for each selected frequency is implemented using a bilinear interpolation, which is dependent on the window function. It is possible to refine the estimation even further using a non-linear least squares estimation. The first two steps provide the proper starting values for the numerical minimization (e.g. Gauss-Newton). This third step increases the precision with 30% to the highest theoretically attainable (Cramer and Rao Lower Bound). In the present studies we use a Gatan 622 TV camera attached to the JEM 4000EX electron microscope. Image analysis is implemented on a Micro VAX II computer equipped with a powerful array processor and real time image processing hardware. The typical precision, as defined by the standard deviation of the distribution of measurement errors, is found to be <0.003Å measured on single crystal silicon and <0.02Å measured on small (10-30Å) specimen areas. These values are ×10 times larger than predicted by theory. Furthermore, the measured precision is observed to be independent on signal-to-noise ratio (determined by the number of averaged TV frames). Obviously, the precision is restricted by geometric distortion mainly caused by the TV camera. For this reason, we are replacing the Gatan 622 TV camera with a modern high-grade CCD-based camera system. Such a system not only has negligible geometric distortion, but also high dynamic range (>10,000) and high resolution (1024x1024 pixels). The geometric distortion of the projector lenses can be measured, and corrected through re-sampling of the digitized image.

Single Relay Lens Magnification Effects In Off-Axis Particle Field Holography

Transverse and volume magnification effects are described for the case of a single lens used to relay the object space in off-axis holography of micro-objects. Using a single calibration of the transverse magnification at a mean longitudinal image distance, we derive relationships for transverse and volume magnifications for a given control volume. The validity of simple limiting solutions in relevant physical conditions is also discussed. Using the knowledge of the transverse magnification at two longitudinal locations, we determine a simple solution for the magnification calculation at any third location. The exact solutions for the transverse and volume magnifications are then obtained in a simpler manner.

Low-Voltage Accelerating-Voltage SEM Magnification Standard Prototype

The National Bureau of Standards has had a continuing effort for almost a decade to develop feature-size measurement techniques and the associated linewidth standards for the semiconductor industry. Recently, work began on a scanning electron microscope (SEM) based feature-size measurement program specifically aimed at the development and certification of SEM linewidth standards and the associated techniques for their calibration and use.Primary to the development and use of SEM linewidth measurement standards is the calibration of the magnification of the SEM. The only standard reference material presently available from NBS for calibrating the magnification of the SEM is the NBS SRM 484.1 This standard provides a known pitch between gold lines in a nickel matrix that is traceable to NBS primary standards. This standard has proven useful for many SEM applications and should continue to be useful for some time to come. However, since SRM 484 was developed prior to the recent interest in low accelerating voltage operation for integrated circuit wafer inspection and measurement, this SRM, in its present form, is unsuitable for use in many of the newly introduced SEM measurement instruments.

A metrology electron microscope system

An electron microscope system has been constructed which is capable of calibration-quality dimensional measurements from 0.5 to 150 μm. This system was designed to certify one dimensional calibration artifacts for micrometrology instruments. Examples of these artifacts include stage micrometers for microscope magnification calibration and polystyrene latex for flow-through particle sizing instrumentation.The principle of operation of the instrument is similar to that of an ordinary optical measuring microscope. The electron beam is fixed in position and acts as the reference point. The measured object is translated beneath the electron beam on an electromechanically scanned stage. The displacement of the stage is determined by a commercial optical intei— ferometric measurement system. Therefore the metric of the system is directly traceable to the wavelength standard. One of the electron output signals from the specimen is stored synchronously with the data output from the interferometer by a dedicated minicomputer system. Subsequent analysis of the electron intensity profile is used to determine the measured size of the object.

Further development of a computer-aided image analysis method of quantifying radiolucencies in approximal enamel.

The performance of a new computer-aided image analysis method of detecting and measuring approximal radiolucencies depicted on bitewing radiographs was evaluated. Image magnification and intensity calculations were found to be reproducible and the magnification calibration routines were sufficiently accurate. Randomly selected clinical radiographs and radiographs of extracted teeth were used; serial sectioning of the extracted teeth allowed histological validation of the results for 24 surfaces. Although lesion detection and the measurement of radiolucency depth were reproducible, initial estimates of area were less satisfactory and some radiolucencies visible to the naked eye could not be detected. Modifications to the system software were made and tested. The problems were largely due to the presence of radiopaque islets within radiolucencies and were overcome by the introduction of a ‘peak-merging routine’. This modified software, however, proved to be too sensitive and required adjustments to the threshold of detection which would appear to have remedied this difficulty. Further work employing the developed software with larger numbers of radiographs is justified. The performance of this system to date is promising; particularly when compared to the variability reported for repeated visual diagnoses and estimates of radiolucency size.

Improvements in SPECT technology for cerebral imaging

Advancement in three major areas of SPECT (single photon emission computed tomography) technology have resulted in improved image quality for cerebral studies. In the first area, single-crystal camera electronics, extensive use of microprocessors, custom digital circuitry, an data bus architecture have allowed precise external control of all gantry motions and improved signal processing. The new digital circuitry permits energy, uniformity, and linearity corrections to be an integral part of the processing electronics. Calibration of these correlations is controlled by algorithms stored in the camera's memory. In addition, digital signals can be routed directly to interface circuitry of auxiliary computer systems without analog-to-digital conversion. Look-up tables, downloaded to the interface from the central processing unit (CPU), permit computer-controlled real-time processing of coordinate signals, including truncation, magnification, and spatial calibration. The second area of improved SPECT technology is camera collimation and related imaging techniques. In this area, system resolution has been improved without loss of sensitivity by decreasing the air gap between patient and collimator surface. Rotating the detector in close apposition to the head has required various stratagems to avoid detector-shoulder contact: the selective reduction of camera shielding, the use of long bore collimators, and the 30 degrees angulation of the camera head for slant hole collimation. Since cerebral studies characteristically image high-contrast regions less than 1 cm in size, image quality has been improved by increasing collimator resolution even at the expense of sensitivity. Increased resolution also improved image contrast for studies using 123I-labeled pharmaceuticals with 3% to 4% 124I contamination. Such studied acquired with low energy or medium energy collimation and a window centered on the 159 keV 123I photopeak contain appreciable septal breakthrough signals originating from Compton scatter of high energy photons primarily from 124I. The third area of advancements in technology, multidetector instrumentation, offers the promise of increased sensitivity and resolution. For the dynamic computer-assisted tomograph (DCAT) system, which was especially designed for regional blood flow studies with 133Xe or 127Xe, a count rate of 170,000 counts per microCi/cc for three slices has been achieved. This system consists of four detector banks each with 16 rectangular NaI crystals. An alternative system at Harvard uses an array of 12 moving detectors with focused collimators to acquire a single slice.(ABSTRACT TRUNCATED AT 400 WORDS)