Media Contact: The semiconductor industry continues to introduce a dizzying array of manufacturing innovations, leading to smaller and more complex integrated circuits and electronic components. But these breakthroughs pose an obvious challenge: scientists and engineers must invent new ways to measure tiny dimensions and quantities, such as the dimensions of minuscule circuits, ultrathin layers of insulation and the power outputs of lasers. Without such measurements, new products cannot be tested accurately and characterized for commercial distribution. To help American industry shoulder the research and development burdens associated with new measurement technologies, the National Institute of Standards and Technology launched the National Semiconductor Metrology Program in 1994. The program, managed by NIST’s Office of Microelectronics Programs, was designed to meet the most critical measurement needs identified by industry, including those listed in the Semiconductor Industry Association’s National Technology Roadmap for Semiconductors. The following developments represent a sample of the program’s progress:
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Innovative Microscope Ready to Measure Industry SamplesNIST recently commissioned a measuring instrument like no other: a calibrated atomic force microscope that traces its highly accurate, nanometer-scale measurements to the wavelength of light, the international standard of length. The agency plans to use the instrument for a special service for testing nano-engineered grids and other references used to ensure the accuracy of dimensional measurements critical to semiconductor processing. Initial services will focus on tools for pitch (the spacing between lines and spaces or other features on a chip) and step height (the levels on a chip or wafer) measurements. Capabilities for measurements of linewidth (the width of a patterned feature) are under development. The instrument is expected to be the centerpiece of a full-fledged NIST calibration service. The service will address the measurement needs of the growing number of users of atomic force microscopes and other types of scanning probe microscopes, valued for their ability to image and probe surfaces of materials in almost atomic detail. Industrial applications of AFMs, in particular, are multiplying as the scale used to measure the dimensions of integrated-circuit features and other kinds of devices drops from micrometers (millionths of a meter) to nanometers (billionths of a meter) to angstroms (ten-billionths of a meter). The instrument’s measurements in all three dimensions are traceable to the wavelength of light. Over its 50-micrometer range, the instrument achieves a lateral resolution of better than 1.2 nanometers and a vertical resolution down to 0.04 angstrom—significantly finer than one ten-millionth of the diameter of a human hair. For measurements of step height, the uncertainty level is about 0.6 percent of scale. The instrument has been used to evaluate silicon samples with regular, terrace-like features that could serve as references for sub-nanometer measurements of step height. For pitch measurements, an uncertainty level of about 0.1 percent of scale has been achieved. Through refinements of the instrument, NIST researchers aim to reduce measurement uncertainty levels even further and to speed up its performance so that it can accommodate sizable calibration workloads. Technical Information:
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| NIST Enters Negotiations To License Revolutionary
Microcalorimeter NIST is negotiating licenses with companies for commercialization of a revolutionary X-ray microcalorimeter that will be used in semiconductor and other materials-intensive manufacturing. The semiconductor industry is eagerly awaiting the commercialization in order to provide more precise materials analysis. Last March, Mark Melliar-Smith, president and chief executive officer of SEMATECH, called on NIST to get the instrument “into commercialization as soon as they possibly can.” SEMATECH is the semiconductor industry’s manufacturing research consortium, located in Austin, Texas. The new microcalorimeter fits easily onto a commercially available scanning electron microscope, conveniently operates even though the sensor is cooled to near absolute zero and achieves X-ray energy resolution that is at least 10 times better than conventional products. The vastly improved detector system will enable chemical analysis of particles that are difficult or impossible to study with current detectors. It permits the chemical analysis of tiny particles that contaminate silicon wafers during semiconductor fabrication. It also has been used to measure the shift in X-ray energy that occurs due to chemical bonding of one atom to another. The license negotiations involve the X-ray detector’s use in the characterization and analysis of materials by X-rays. The NIST research team is exploring other uses for the detector. One recently announced (see NIST Update, June 8, 1998, on the World Wide Web at http://www.nist.gov/public_affairs/update/upd980608.htm) is the plan to evaluate the microcalorimeter’s role as the detector on a high-resolution mass spectrometer that might help speed up human gene sequencing. Technical Information:
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| New Reliability Testing Technique Saves Months of Time Assessing the reliability of gate oxides (the most critical layer in microelectronic devices) is a critical need for semiconductor manufacturers who want tighter control of their microchips’ electrical parameters in the quest for smaller feature sizes. Unfortunately, fast production schedules and continually changing technology make it difficult to implement long-term reliability stress tests during chip production. So, manufacturers have been forced to rely on estimates of device reliability from highly accelerated stress tests. NIST researcher John Suehle has developed a new technique that enables long-term reliability parameters to be extracted from these very fast stress tests. The net result: the new method can save manufacturers at least an order of magnitude in testing time compared to conventional long-term, time dependent dielectric breakdown (TDDB) tests. Conventional TDDB tests can take up to several months to obtain the statistically significant failure data used to develop the parameters from which product life can be estimated. The Suehle technique uses voltage versus time curves generated during fast stress tests using constant or ramped current. An integration of the curves is performed using a cumulative damage generation model. This not only gives the same TDDB acceleration parameters as those obtained from long-term TDDB life tests but also provides the same statistics on the parameters in a tenth of the time or less. Technical Information:
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| SRMs Improve Silicon Wafer Resistivity Testing The resistivity (electrical resistance of a conductor per unit length) of silicon wafers is a vital concern to the multibillion dollar semiconductor industry. NIST recently has released six new Standard Reference Materials that will enable manufacturers to calibrate resistivity test instruments to 0.3 percent or better, with 95 percent confidence. This is at least a fivefold improvement in the uncertainty of certified value, compared to previous resistivity SRMs. The improved uncertainty significantly surpasses the accuracy and precision requirements for resistivity measurement capability set forth at a previous SEMATECH Workshop on Silicon Materials for Mega-IC applications. The SRMs are 100 millimeters in diameter, approximately 625 micrometers thick, and are intended for the calibration, or performance verification, of four-point probes and eddy current testers. In addition to greatly reduced uncertainty, the new reference standards have better uniformity of resistivity and thickness, and a larger characterized area than previous NIST resistivity SRMs. Unlike previous NIST resistivity SRMs that were certified for value only at the wafer center, the new SRMs provide certified measurements at the center and on circles of 10 millimeter and 20 millimeter diameters for better compatibility with automated resistivity uniformity mapping instruments. The available SRMs are: SRM 2541 at 0.01 ohm centimeter, SRM 2542 at 0.1 ohm centimeter, SRM 2544, at 10 ohm centimeter, SRM 2445 at 25 ohm centimeter, SRM 2546 at 100 ohm centimeter and SRM 2547 at 200 ohm centimeter. A seventh resistivity level in this series, SRM 2543 at 1 ohm centimeter, is expected to be released early fall 1998. To purchase the SRMs, price $708 each, contact the Standard Reference Materials Program, Bldg. 202, Rm. 204, NIST, Gaithersburg, Md. 20899-0001, (301) 975-6776, fax: (301) 948-3730, srminfo@enh.nist.gov. Technical Information:
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| More Accurate Dopant Profiling Possible from Scope Images A theoretical model of the scanning capacitance microscope developed by NIST has been used to extract more accurate two-dimensional dopant profiles from images. Dopant is an impurity added to a semiconductor that gives a specific electrical resistance, enabling the design of integrated circuits for various purposes. Two-dimensional dopant profiling of silicon with 20 nanometer spatial resolution and with 10 percent accuracy has been identified in the National Technology Roadmap for Semiconductors as a critical measurement need for the development of next-generation integrated circuits. Small dopant changes in a device’s active region can lead to large changes in device performance. Scanning capacitance microscopy holds great promise as the technique that can produce the required quantitative dopant profiles. Although this technique has progressed from a research topic to use in the analytical laboratories of many integrated circuit makers, the ability to extract accurate and quantitative dopant information from the technique has been limited. Technical Information:
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| Ultra-Precise Tool for Calibrating Water-Vapor
Detectors Unveiled Manufacturers lose millions of dollars annually because of water contamination of gases used in processing semiconductors. That’s no drop in the bucket for companies struggling to compete in a fierce marketplace. What would appear to be the tiniest drop in a vast sea, just 50 molecules of water per billion molecules (50 parts per billion) of gas, is more than enough to ruin a batch of in-production computer chips. To detect these trace amounts of water vapor, semiconductor manufacturers install elaborate measurement systems (known as hygrometers). These instruments must undergo periodic calibration to ensure proper functioning and accurate readings. In order to quantify a hygrometer’s behavior, a calibration gas containing an accurately known water vapor content must flow through the device. Infusing a completely dry gas with water vapor is a technical nightmare. For example, how dry is the “dry gas” to begin with? And how does one accurately mix this dry gas with a water vapor stream? Flow mixing systems employing multiple flow controllers are required. Each controller has inherent inaccuracies which become significant in total. And there is the constant worry that part of the system could change, resulting in erroneous calibrations. NIST researchers have developed an absolute humidity calibration system, based on the physical properties of water. The new device, known as the low frost-point humidity generator, sidesteps the inaccuracies of mixing dry gas and water. Instead, it produces accurately known water-vapor/gas mixtures by cooling the carrier gas down to a desired temperature (minus 5 to minus 101.6 degrees Celsius) and flowing the gas over a surface of pure ice. The resulting water content of the gas-water mixture is dependent on the vapor pressure of ice, a temperature-specific thermodynamic property. By tightly controlling the temperature of the gas stream and the ice to within 0.002 degrees Celsius, NIST researchers can produce test gases with water vapor contents from 4 parts per thousand to 3.4 parts per billion—1 million times drier—with a precision of better than 0.04 percent. NIST calibration services for trace moisture level hygrometers and humidity generators, using test gas supplied by the new instrument, will commence in early 1999. Technical Information:
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U.S. Department of Commerce July 1998 |
