William J. Keery

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.

X-Ray Lithography Mask Metrology: Use of Transmitted Electrons in an SEM for Linewidth Measurement

X-ray masks present a measurement object that is different from most other objects used in semiconductor processing because the support membrane is, by design, x-ray transparent. This characteristic can be used as an advantage in electron beam-based x-ray mask metrology since, depending upon the incident electron beam energies, substrate composition and substrate thickness, the membrane can also be essentially electron transparent. The areas of the mask where the absorber structures are located are essentially x-ray opaque, as well as electron opaque. This paper shows that excellent contrast and signal-to-noise levels can be obtained using the transmitted-electron signal for mask metrology rather than the more commonly collected secondary electron signal. Monte Carlo modeling of the transmitted electron signal was used to support this work in order to determine the optimum detector position and characteristics, as well as in determining the location of the edge in the image profile. The comparison between the data from the theoretically-modeled electron beam interaction and actual experimental data were shown to agree extremely well, particularly with regard to the wall slope characteristics of the structure. Therefore, the theory can be used to identify the location of the edge of the absorber line for linewidth measurement. This work provides one approach to improved x-ray mask linewidth metrology and a more precise edge location algorithm for measurement of feature sizes on x-ray masks in commercial instrumentation. This work also represents an initial step toward the first SEM-based accurate linewidth measurement standard from NIST, as well as providing a viable metrology for linewidth measurement instruments of x-ray masks for the lithography community.

Low‐profile high‐efficiency microchannel‐plate detector system for scanning electron microscopy applications

A new design high‐efficiency microchannel‐plate detector and amplification system is described for use in the scanning electron microscope. This complete detector system consists of four basic units: (1) the microchannel‐plate detector; (2) the video amplifier; (3) the high‐voltage power supply; and (4) the control unit. The microchannel‐plate detector system is efficient at both high and low accelerating voltages, and is capable of both secondary electron and backscattered electron detection modes. The size of the actual detector is approximately 3.5 mm in thickness and 25.4 mm in diameter. Thus, use of this detector system permits using almost all the sample chamber to accommodate large specimens with only the loss of the 3.5 mm of working distance. Another feature is that this system also employs a unique video amplifier where there are no active elements at high voltage. The microchannel‐plate detector system enables the investigation of secondary electron induced contrast mechanisms and backscattered...

A new approach to accurate X-ray mask measurements in a scanning electron microscope

The authors present the basic concept and some preliminary experimental data on a novel method for measuring critical dimensions on masks used for X-ray lithography. The method uses a scanning electron microscope (SEM) in a transmitted-electron imaging mode and can achieve nanometer precision. Use of this technique in conjunction with measurement algorithms derived from electron-beam interaction modeling may ultimately enable measurements to these masks to be made to nanometer accuracy. Furthermore, since a high contrast image results, this technique lends itself well to automated mask defect recognition and inspection. It is concluded that this method has the potential advantage of avoiding or at least minimizing the basic limitations imposed by the electron-beam interaction effects normally encountered in conventional methods of dimensional metrology in the SEM. >

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.

Comparison Of Linewidth Measurements On An Sem/Interferometer System And An Optical Linewidth-Measuring Microscope

In the current linewidth-measurement program at the National Bureau of Standards, the primary measurement of micrometer-wide lines on black-chromium artifacts is made with an interferometer located in a scanning electron microscope (SEM). The data output consists of a line-image profile from the electron detector and a fringe pattern from the interferometer. A correlation between edge location and fringe location is made for both line edges to give the linewidth in units of the wavelength of a He-Ne laser. A model has been developed to describe the interaction of the electrons with the material line and thereby relate a threshold value on the SEM image profile to a selected point on the material line. An optical linewidth-measuring microscope is used to transfer the primary measurements to secondary measurement artifacts; these artifacts will be used to transfer the linewidth measurements to the integrated-circuit industry. Linewidth measurements from the SEM/interferometer system and the optical linewidth-measuring microscope are compared, and the level of measurement uncertainty for each system is discussed.

X-ray mask metrology: the development of linewidth standards for x-ray lithography

The calibration of masks used in x-ray lithography has been successfully accomplished in the scanning electron microscopy (SEM) by utilizing the transmitted scanning electron detection technique. This has been made possible because these masks present a measurement subject different from most (if not all) other objects used in semiconductor processing because the support membrane is, by design, x-ray transparent. This characteristic can be used as an advantage in electron beam-based mask metrology since, depending upon the incident electron beam energies, substrate composition and substrate thickness, the membrane can also be essentially electron transparent.

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.

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.