Samuel N. Jones

Experimental test of blind tip reconstruction for scanning probe microscopy

Determination of the tip geometry is a prerequisite to converting the scanning probe microscope (SPM) from a simple imaging instrument to a tool that can perform width measurements accurately. Recently we developed blind reconstruction, a method to characterize the SPM tip shape. In principle this method allows estimation of the tip shape from an image of a tip characterizer sample that need not be known independently. In this work, we compare blind reconstruction results to those obtained by scanning electron microscopy for two diamond stylus profiler tips, one of which has a gentle shape and the other a more complicated profile. Of the two comparisons, the poorer agreement is still better than 30 nm for parts of the tip within a several micrometer neighborhood of the apex. In both cases the differences are comparable to the combined standard uncertainties of the measurements. We estimate uncertainties from five sources, the most significant of which is the repeatability of the stylus profiling instrument. In a separate measurement we determine the geometry of a silicon nitride SPM tip. The measured radius, 4-fold symmetry, included angle, and tilt are all consistent with expectations for such a tip.

Intercomparison of SEM, AFM, and electrical linewidths

Uncertainty in the locations of line edges dominates the uncertainty budget for high quality sub-micrometer linewidth measurements. For microscopic techniques like scanning electron microscopy (SEM) and atomic force microscopy (AFM), the image of the sharp edge is broadened due to the instruments non-ideal response. Localizing the true edge position within its broadened image requires a model for the instrument-sample interaction. Ideal left and right edges are mirror images of one another, so any modeling error in the position assignment will have opposite signs for the two types of edges. Linewidth measurements inherently involves such opposite edges and consequent doubling of model errors. Similar considerations apply to electrical critical dimension (ECD) measurement. Although ECD is a non-imaging technique, one must still model the offset between the position of the physical edge and the effective edge of the conducting part of the line. One approach to estimating the reliability of existing models is to compare result when fundamentally different instruments measure the same line. We have begun a project to perform such an intercomparison, and we report here initial results for SEM, AFM, and ECD measurements of sub-micrometer lines in single crystal Si. Edge positions are determined from SEM images using Monte Carlo tracing of electron trajectories to predict the edge shape.In the AFM, we estimate and correct for tip geometry using tools from mathematical morphology. ECD measurements are corrected for band bending in the neighborhood of the edges.

Application of Transmission Electron Detection to SCALPEL Mask Metrology

Linewidth measurements were performed on a 4X scattering with angular limitation in projection electron lithography (SCALPEL) e-beam lithography mask using the transmitted electron signal in a modified scanning electron microscope. Features as small as 0.24 μm were measured on the mask. The thin membrane mask structure that was used is found to provide sufficient transmitted signal contrast at energies ranging from 10 to 30 keV. The linewidth measurement accuracy is mostly limited by the variations in the material and not the measurement system. It is concluded that the linewidth measurement technique using transmitted electrons is suitable for the potential certification of SCALPEL mask standards.

Metrological Electron Microscope for the Certification of Magnification and Linewidth Artifacts for the Semiconductor Industry

The National Institute of Standards and Technology has, for several years, been developing a metrological electron microscope system traceable to national standards of length. This metrology instrument will certify standards for the calibration of the magnification of scanning electron microscopes (SEM) and for the certification of artifacts for SEM linewidth measurement. These artifacts are not only directed to instruments used in the semiconductor community but will also be useful for the various other applications to which the SEM is currently being used. The SEM-based metrology system now operational at the Institute will be described as well as its design criteria and procedures for its characterization. The design and criteria for a new lithographically produced SEM low-accelerating-voltage magnification standard to be calibrated on this system will also be presented.

Reference Material 8091: New Scanning Electron Microscope Sharpness Standard

All scanning electron microscope-based inspection instruments, whether they are in a laboratory or on the production line, slowly lose their performance and then are no longer capable of providing as good quality, sharp images as before. Loss in image quality also means loss in measurement quality. Loss of performance is related to changes in the electron source, in the alignment of the electron-optical column, astigmatism, and sample and electron optical column contamination. Detecting a loss in image sharpness easily reveals this decrease of performance. Reference Material (RM) 8091 is intended primarily for routinely checking the performance of scanning electron microscopes. RM 8091 is designed for use in conjunction with Fourier analysis software such as the NIST/SPECTEL2 SEM Monitor Program, the NIST Kurtosis program, or the University of Tenesse SMART program. This RM is supplied as a small, approximately 2 mm x 2 mm diced semiconductor chip. RM 8091 is designed to be mounted onto a so-called drop-in wafer for specialized wafer inspection or dimensional metrology scanning electron microscopes or put on a specimen stub for insertion into laboratory scanning electron microscopes. This Reference Material is fully compatible with state-of-the-art integrated circuit technology.

Statistical measure for the sharpness of SEM images

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 useful methodology for sharpness measurement. In this paper, a statistical measure known as the multivariate kurtosis, is proposed as a useful measure of the sharpness of SEM images. Kurtosis is designed to be a measure of the degree of departure of a probability distribution from the Gaussian distribution. It is a function of both the fourth and the second moments 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, and that kurtosis evaluated as a characterization tool. Kurtosis has the distinct advantage that it is a parametric measure and is sensitive to the presence of the high spatial frequencies necessary for acceptable levels of sharpness. The applications of this method to SEM metrology will be discussed.

Report on the NIST low-accelerating-voltage SEM magnification standard interlaboratory study

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 micrometers scaling down to structures with minimum nominal pitch of 0.4 micrometer. 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 results of the analysis of the data obtained in this study are presented in this paper.

Tip characterization for scanning probe microscope width metrology

Determination of the tip shape is an important prerequisite for converting the various scanning probe microscopies from imaging tools into dimensional metrology tools with sufficient accuracy to meet the critical dimension measurement requirements of the semiconductor industry. Determination of the tip shape generally requires that the tip be used to image a “tip characterizer.” Characterizing the characterizer has itself been an obstacle to progress, since most methods require the geometry of the characterizer to be known with uncertainty small compared to the size of the tip. We have recently developed a “blind” reconstruction method that allows the 3-dimensional tip shape to be estimated from an image of an unknown characterizer. This method has heretofore been tested only in simulations. We report here initial results of an experimental test of the technique. We compare the reconstructed profile of a tip to an independently measured profile from a scanning electron microscope (SEM). A relatively large...