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Tech Beat - July 23, 2013

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Editor: Michael Baum
Date created: July 23, 2013
Date Modified: July 23, 2013 
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E.U. and U.S. to Extend Scientific Cooperation on Measurements and Standards

The European Commission's Joint Research Centre (JRC) and the U.S. Department of Commerce’s National Institute of Standards and Technology (NIST) yesterday* agreed to expand their current scientific cooperation to include new areas of research, such as energy, healthcare and clinical measurements, and food safety and nutrition.

 NIST/JRC agreement signing
Congressman Chaka Fattah (D-PA), Under Secretary of Commerce for Standards and Technology and NIST Director Patrick Gallagher, Joint Research Centre (JRC) Director General Dominique Ristori and European Parliament Member Edit Herzog at a signing ceremony July 17, 2013. NIST and the JRC agreed to expand their current scientific cooperation to include new areas of research.

Credit: K. Delak/NIST

High resolution version

Under Secretary of Commerce for Standards and Technology and NIST Director Patrick Gallagher and JRC Director General Dominique Ristori held a signing ceremony during Transatlantic Week, an annual event intended to raise the profile of the transatlantic relationship as well as to foster a dialogue on shared purpose and joint action among U.S. and E.U. policymakers.

NIST and the commission have collaborated on many projects since the signing of the U.S.-EU Agreement on Scientific and Technological Cooperation in 1997. The new Implementing Arrangement expands on previous collaboration and provides joint access to scientific infrastructure, the exchange of scientific and technological information and experts, and support for training scientists, engineers and technical experts. The arrangement is initially for five years and can be extended.

While NIST and the JRC, the commission’s in-house science service, have a history of working together, this overarching agreement replaces individual agreements on each project. It also provides additional focus on shared research priorities, including potential new areas such as security technology and systems, and environment and climate.

Speaking at the signing ceremony, JRC's Ristori said, “The Implementing Arrangement we signed will create an overarching framework for a cross-Atlantic cooperation on standards and measurements in a wide range of areas. It will also serve as a leading example in the process towards setting global standards.”

NIST's Gallagher highlighted current collaborative projects, including foundational research that supports the measurements underpinning manufacturing and standards, and also work developing measurement protocols and standards in fields such as homeland security technology.

“The new agreement affirms our relationship, and we look forward to new and exciting areas of interaction that will ultimately serve to support the relationship between the European Union and the United States,” said Gallagher. “By working together, we can take advantage of each other’s respective strengths to further the science. And ultimately, we will benefit from being on the same page as the science matures and allows us to establish and implement these new technologies.”

Both organizations have the strategic goal to support competitiveness and economic growth, and have cooperated on standardization since 2007. The Implementing Arrangement encompasses 10 areas related to standards and measurements. Environment and climate, energy, transportation and security are high on the collaborative research agenda. In addition to reference materials in a range of areas, the cooperation will include research on civil engineering structures (such as bridges, roads and dams) and emerging information and communication technologies, as well as marine optical radiometry.

As a non-regulatory agency of the U.S. Department of Commerce, NIST promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards and technology in ways that enhance economic security and improve our quality of life. To learn more about NIST, visit www.nist.gov.

For more information on the JRC's research on standards and measurements, visit http://ec.europa.eu/dgs/jrc/downloads/jrc_science_for_standards_reports.pdf. And to learn more about JRC-U.S. collaborations, visit http://ec.europa.eu/dgs/jrc/downloads/jrc_country_leaflet_us_en.pdf.

* Originally issued on July 18, 2013.

Media Contact: Jennifer Huergo, jennifer.huergo@nist.gov, 301-975-6343

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New NIST Standard Reference Material to Help Calibrate Hospital CAT Scanners

Scientists at the National Institute of Standards and Technology (NIST) have developed a new standard reference material (SRM), the first such measurement tool to enable hospitals to link important tissue density measurements made by CAT scans to international standards.

SRM 2088
Standard Reference Material 2088 makes it possible to link CAT scans to the International System of Units for the first time. The SRM - a collection of five blocks of polyurethane foam of different densities, shown here in front of its delivery case - allows scans to be tied to measurement units important for diagnosing lung disease.
Credit: Boutin/NIST
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Computed tomography ("CT" or "CAT") uses computer processing to combine multiple X-ray images into three-dimensional scans that resemble slices of the body. These cross-sections are useful for spotting changes that are difficult to discern from ordinary two-dimensional X-ray images alone, such as the changes in lung tissue that indicate cancer or emphysema. Millions of CAT scans take place every year in the U.S. alone, but over time, the devices' outputs have a tendency to "drift," according to NIST physicist Zachary Levine.

"Scanners are calibrated daily, but over periods of months you still see variations," says Levine. "Some of these come from the physical degradation of the machine, or even from changes made by software upgrades. Doctors "get to know" their own machine's idiosyncrasies and make good diagnoses, but they'd still like to be more certain about what they're seeing, especially when it comes to lung tissue."

Calibrating a CAT scanner is not difficult in principle. All that is needed are physical objects of known density that can be run through the scanner together with a patient so the scan shows both the patient and the reference. Finding an appropriate material was a bit challenging though. Lung tissue is among the lightest in the body, and varies constantly in its density depending on how much air is trapped in it.

A lung tissue SRM must span a range of densities not only for this reason, but for diagnostic purposes as well. While emphysema makes lung tissue grow less dense, in a tumor the density increases. Scientists who work on CAT scans often express this density range as a "CT number," where a change in CT number by a few points can mean the difference between one diagnosis and another.

SRM 2088, a collection of five small blocks that the NIST team fashioned from polyurethane foam of different densities, makes it possible to link scans to the International System of Units (SI) for the first time. Together with a second reference material (SRM 2087) developed last year, SRM 2088 allows CAT scans to be tied to SI units for length, density and mass attenuation coefficient—three characteristics that are particularly important for diagnosing lung diseases effectively.

The NIST team chose polyurethane foam, which Levine says is ideal because it is both inexpensive and available off the shelf in an appropriate range of densities corresponding to the density range of lung tissue. The SRM needed to be accurate to within 10 CT numbers to be useful, and density tests indicate that they are accurate to within 0.15 CT numbers, roughly 100 times more than necessary.

SRM 2088, "Density Standard for Medical Computed Tomography" and SRM 2087, " Dimensional Standard for Medical Computed Tomography" are available from the NIST Standard Reference Materials office at www.nist.gov/srm/.

*Z.H. Levine, H.H. Chen-Mayer, A.L. Pintar and D.S. Sawyer IV. A low-cost density reference phantom for computed tomography. Medical Physics 36, pp. 286-288 (2009).

Media Contact: Chad Boutin, boutin@nist.gov, 301-975-4261

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New NIST Nanoscale Indenter Takes Novel Approach to Measuring Surface Properties

Researchers from the National Institute of Standards and Technology (NIST) and the University of North Carolina have demonstrated a new design for an instrument, a "instrumented nanoscale indenter," that makes sensitive measurements of the mechanical properties of thin films—ranging from auto body coatings to microelectronic devices—and biomaterials. The NIST instrument uses a unique technique for precisely measuring the depth of the indentation in a test surface with no contact of the surface other than the probe tip itself.*

nanoindenter head
Good vibrations: Close-up image shows the tip of the new NIST nanoscale indenter flanked by two tuning forks that provide a stable, noncontact reference relative to the specimen, a piece of single-crystal silicon. Using a pair of tuning forks allows the system to compensate for any tilt.
Credit: Nowakowski/NIST
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Indenters have a long history in materials research. Johan August Brinell devised one of the first versions in 1900. The concept is to drop or ram something hard onto the test material and gauge the material's hardness by the depth of the dent. This is fine for railway steel, but modern technology has brought more challenging measurements: the stiffness of micromechanical sensors used in auto airbags, the hardness of thin coatings on tool bits, the elasticity of thin biological membranes. These require precision measurements of depth in terms of nanometers and force in terms of micronewtons.

Instead of dents in metal, says NIST's Douglas Smith, "We are trying to get the most accurate measurement possible of how far the indenter tip penetrates into the surface of the specimen, and how much force it took to push it in that far. We record this continuously. It's called 'instrumented indentation testing'."

A major challenge, Smith says, is that at the nanoscale you need to know exactly where the surface of the test specimen is relative to the indenter's tip. Some commercial instruments do this by touching the surface with a reference part of the instrument that is a known distance from the tip, but this introduces additional problems. "For example, if you want to look at creep in polymer—which is one thing that our instrument is particularly good at—that reference point itself is going to be creeping into the polymer just under its own contact force. That's an error you don't know and can't correct for," says Smith.

The NIST solution is a touchless surface detector that uses a pair of tiny quartz tuning forks—the sort used to keep time in most wrist watches. When the tuning forks get close to the test surface, the influence of the nearby mass changes their frequency—not much, but enough. The nanoindenter uses that frequency shift to "lock" the position of the indenter mechanism at a fixed distance from the test surface, but without exerting any detectable force on the surface itself.

"The only significant interaction we want is between the indenter and the specimen," says Smith, "or at least, to be constant and not deforming the surface. This is a significant improvement over the commercial instruments."

The NIST nanoindenter can apply forces up to 150 millinewtons, taking readings a thousand times a second, with an uncertainty lower than 2 micronewtons, and while measuring tip penetration up to 10 micrometers to within about 0.4 nanometers. All of this in done in a way that can be traceably calibrated against basic SI units for force and displacement in a routine manner.

The instrument is well suited for high-precision measurements of hardness, elasticity and creep and similar properties for a wide range of materials, including often difficult to measure soft materials such as polymer films, says Smith, but one of its primary uses will be in the development of reference materials that can be used to calibrate other instrumented indenters. "There still are no NIST standard reference materials for this class of instruments because we wanted to have an instrument that was better than the commercial instruments for doing that," Smith explains.

*B.K. Nowakowski, D.T. Smith, S.T. Smith, L.F. Correa and R F. Cook. Development of a precision nanoindentation platform. Review of Scientific Instruments, 84(7), 075110, DOI: 10-1063/1.4811195, (2013). Published online July 18, 2013.

Media Contact: Michael Baum, michael.baum@nist.gov, 301-975-2763

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New Nanoscale Imaging Method Finds Application in Plasmonics

Researchers from the National Institute of Standards and Technology (NIST) and the University of Maryland have shown how to make nanoscale measurements of critical properties of plasmonic nanomaterials—the specially engineered nanostructures that modify the interaction of light and matter for a variety of applications, including sensors, cloaking (invisibility), photovoltaics and therapeutics.

plasmonic resonators
Infrared laser light (purple) from below a sample (blue) excites ring shaped nanoscale plasmonic resonator structures (gold). Hot spots (white) form in the rings' gaps. In these hot spots, infrared absorption is enhanced, allowing for more sensitive chemical recognition. A scanning AFM tip detects the expansion of the underlying material in response to absorption of infrared light.
Credit: NIST
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Their technique is one of the few that allows researchers to make actual physical measurements of these materials at the nanoscale without affecting the nanomaterial's function.

Plasmonic nanomaterials contain specially engineered conducting nanoscale structures that can enhance the interaction between light and an adjacent material, and the shape and size of such nanostructures can be adjusted to tune these interactions. Theoretical calculations are frequently used to understand and predict the optical properties of plasmonic nanomaterials, but few experimental techniques are available to study them in detail. Researchers need to be able to measure the optical properties of individual structures and how each interacts with surrounding materials directly in a way that doesn't affect how the structure functions.

"We want to maximize the sensitivity of these resonator arrays and study their properties," says lead researcher Andrea Centrone. "In order to do that, we needed an experimental technique that we could use to verify theory and to understand the influence of nanofabrication defects that are typically found in real samples. Our technique has the advantage of being extremely sensitive spatially and chemically, and the results are straightforward to interpret."

The research team turned to photothermal induced resonance (PTIR), an emerging chemically specific materials analysis technique, and showed it can be used to image the response of plasmonic nanomaterials excited by infrared (IR) light with nanometer-scale resolution.

The team used PTIR to image the absorbed energy in ring-shaped plasmonic resonators. The nanoscale resonators focus the incoming IR light within the rings' gaps to create "hot spots" where the light absorption is enhanced, which makes for more sensitive chemical identification. For the first time, the researchers precisely quantified the absorption in the hot spots and showed that for the samples under investigation, it is approximately 30 times greater than areas away from the resonators.

The researchers also showed that plasmonic materials can be used to increase the sensitivity of IR and PTIR spectroscopy for chemical analysis by enhancing the local light intensity, and thereby, the spectroscopic signal.

Their work further demonstrated the versatility of PTIR as a measurement tool that allows simultaneous measurement of a nanomaterial's shape, size, and chemical composition—the three characteristics that determine a nanomaterial's properties. Unlike many other methods for probing materials at the nanoscale, PTIR doesn't interfere with the material under investigation; it doesn't require the researcher to have prior knowledge about the material's optical properties or geometry; and it returns data that is more easily interpretable than other techniques that require separating the response of the sample from response of the probe.

For background on PTIR, see the February 2013 NIST Tech Beat story, "NIST Captures Chemical Composition with Nanoscale Resolution" at www.nist.gov/public_affairs/tech-beat/tb20130220.cfm#ptir.

*B. Lahiri, G. Holland, V. Aksyuk and A. Centrone. Nanoscale imaging of plasmonic hot spots and dark modes with the photothermal-induced resonance technique. Nano Letters. June 18, 2013. DOI: 10.1021/nl401284m.

Media Contact: Mark Esser, mark.esser@nist.gov, 301-975-8735

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NIST Shows How to Make a Compact Frequency Comb in Minutes

Laser frequency combs—high-precision tools for measuring different colors of light in an ever-growing range of applications such as advanced atomic clocks, medical diagnostics and astronomy—are not only getting smaller but also much easier to make.

Physicists at the National Institute of Standards and Technology (NIST) can now make the core of a miniature frequency comb in one minute.* Conventional microfabrication techniques, by contrast, may require hours, days or even weeks.

The NIST technique involves laser machining of a quartz rod (a common type of glass) to shape and polish a small, smooth disk within which light can circulate (see video clip). The user controls the size and shape of this optical cavity, or resonator. Its diameter can be varied from about one-fifth of a millimeter to 8 millimeters, and its thickness and curvature can be shaped as well. The quality factor—Q factor, which is a measure of the length of time light circulates inside the cavity without leaking out—equals or exceeds that of cavities made by other methods.

photo showing 4 microcavities
NIST physicists have developed a one-minute process for creating optical microcavities made of fused quartz. The photo shows four cavities with diameters (top to bottom) of 0.36 millimeters (mm), 0.71 mm, 1.2 mm, and 1.5 mm. When excited with laser light, the cavities can be used for many applications, including the generation of frequency combs used to precisely measure different colors of light. Smaller cavities produce wider spacing between the comb teeth (specific colors).
Credit: Del'Haye/NIST
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After machining the quartz, NIST scientists use a small, low-power infrared laser to pump light into it. A primary benefit of the high Q factor is that only a few milliwatts of laser light are required to generate a comb.

"We make a resonator in one minute, and one minute after that we are making a frequency comb," NIST researcher Scott Papp says.

NIST's one-minute method is simple and far less expensive than conventional microfabrication. The system for the NIST process costs about $10,000—most of that for purchase of a carbon dioxide laser used for cutting—compared to between $1 million and $10 million for a microfabrication system that must be used in a cleanroom.

A full-size frequency comb uses high-power, ultrafast lasers and is generally the size of a small table. NIST researchers have been making compact frequency combs for several years and often make cavities out of bulk fused quartz, an inexpensive glass material.**

By confining light in a small space, the optical cavity—which, confusingly enough, is solid—enhances optical intensity and interactions. The comb itself is the light, which starts out as a single color or frequency that through optical processes is transformed to a set of additional shades, each sharply defined and equally spaced on the spectrum. A typical NIST microcomb might have 300 "teeth," or ticks on the ruler, each a slightly different color. A key advantage of microcombs is the ability to tune the spacing between the teeth, as needed, for applications such as calibrating astronomical instruments. The spacing is determined by the size of the cavity; a smaller cavity results in wider spacing between the comb teeth.

Scientists plan to apply for a patent on the machining technique, which could be applied to a variety of other glassy materials. Future NIST research will focus on continuing improvements in comb performance and use of the resonators in other compact applications such as optical frequency standards and low-noise microwave oscillators.

The research is supported, in part, by the Defense Advanced Research Projects Agency and National Aeronautics and Space Administration.

*S.B. Papp, P. Del'Haye and S.A. Diddams. 2013. Mechanical control of a microrod-resonator optical frequency comb. Physical Review X. 3, 031003 (2013). DOI: 10.1103/PhysRevX.3.031003. Published online July 8, 2013.
P. Del'Haye, S.A. Diddams and S.B. Papp. 2013. Laser-Machined Ultra-High-Q Microrod Resonators for Nonlinear Optics. Applied Physics Letters. Posted online June 7, 2013.
**See 2011 NIST Tech Beat article, "Future 'Comb on a Chip': NIST's Compact Frequency Comb Could Go Places," at www.nist.gov/pml/div688/comb-102511.cfm.

Media Contact: Laura Ost, laura.ost@nist.gov, 303-497-4880

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Who Are You? NIST Biometric Publication Provides Two New Ways to Tell Quickly

The National Institute of Standards and Technology (NIST) has issued a new publication that broadens agency security options for Personal Identity Verification (PIV) cards. Biometric Data Specifications for Personal Identity Verification (Special Publication 800-76-2) adds iris images as biometric identifiers and on-card fingerprint comparison as options for the cards.

iris images
These iris images meet the biometric standard. The camera image on the left is run through commercial software to produce the image (right) that can be placed on the PIV card and for other uses such as an e-passport.
Credit: NIST
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A PIV card is a government-issued smart card used by federal employees and contractors to access government facilities and computer networks. The PIV card carries a photo, fingerprint information, personal identification number (PIN) and a cryptographic credential–random computer-generated data that are recognized only by the PIV card–all of which serve to bind the card to the card holder.

To assist agencies seeking stronger security and greater operational flexibility, NIST made several modifications to the previous version of Biometric Data Specification for Personal Identity Verification. Major additions include:

  • On-card comparison of fingerprints for improved privacy. The specifications describe how to place one or two compact fingerprint templates and a recognition algorithm on the card. When the user wants to sign a document digitally or open a secure file, for example, she can place her finger on a reader attached to the keyboard to verify her identity. Currently, employees have to type in a PIN for matching, which is subject to error and misuse.
  • Iris recognition capability for increased security. Standardized compact images of one or both irises (the images are no more than 3 kilobytes each) can be loaded on the PIV card for compact on-card storage and fast reading times. The document provides performance specifications for iris biometrics to assure high accuracy and provides specifications for iris cameras to guide implementers on camera selection. These standards-based elements support interoperability within and across agencies using iris recognition technology.

Agencies may choose to add iris images as an alternate biometric over fingerprints, because, for some users, fingerprint collection can be difficult. At times, the fingerprints are too dry to yield a good image, and lotions, wounds or illness also can make for poor images. Agencies now have the option of using two biometric sources to avoid such circumstances.

Several recent NIST research projects have led to improved technologies for identity management that are included in the updated specification.

"NIST research supports the users of these biometric technologies through its ongoing quantitative research activities," explains Biometric Testing Project Leader Patrick Grother. After applying standard compression algorithms to a large number of iris images and then using these compact images with state-of-the-art recognition algorithms, NIST researchers determined that an iris image compressed to 3KB provides enough detail to accurately recognize an individual's iris.

"This collaboration with industry and the standards community led to the ISO/IEC 19794-6 iris standard published in late 2011. The iris standard can support PIV authentication and other uses, such as e-passports," says Grother. "More importantly, the iris standard ensures that the iris data is interoperable, that is, it can be exchanged easily between cameras and readers from different makers and across the world."

In another project NIST scientists evaluated how quickly irises age. Using two data sets with hundreds of thousands of iris images collected from frequent travelers in airports, they found no significant deterioration in recognition over nearly a decade. This result guides re-enrollment schedules. Particularly, the NIST measurement suggests that irises would meet the PIV requirement that biometric data should be viable over a 12-year period.

NIST is also collaborating with the Department of Homeland Security on a camera certification process to define a repeatable optical laboratory test of a camera's peak imaging capability. This approach follows that used in developing the Federal Bureau of Investigation's "Appendix F" specification for certifying fingerprint scanners.

Biometric Data Specifications for Personal Identity Verification (NIST SP 800-76-2) can be obtained at www.nist.gov/manuscript-publication-search.cfm?pub_id=914224.

Media Contact: Evelyn Brown, evelyn.brown@nist.gov, 301-975-5661

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NIST Releases Updates to Digital Signature Standard

The National Institute of Standards and Technology (NIST) has released a revision to the digital standard used to ensure the integrity of electronic documents, as well as the identity of the signer.

The new document, Federal Information Processing Standard (FIPS) 186-4, concerns what is commonly known as the digital signature standard. First published in 1994 and revised several times since then, the standard provides a means of guaranteeing authenticity in the digital world. It uses complex math operations to encrypt and unscramble “signatures” that are all but impossible to forge. Updates to the standard are still necessary as technology changes.

According to NIST computer scientist Elaine Barker, FIPS 186-4 contains no major revisions, but rather focuses on keeping the standard consistent with other NIST cryptographic guidelines. Other than clarifying a number of terms and correcting typographical errors, most of the changes aim to align the standard with other publications, such as NIST Special Publication 131A, so that all NIST documents offer consistent guidance regarding the use of random number generators.

Another change concerns the use of prime number generators, which requires random initial values for searching for prime numbers. FIPS 186-3 specifically allowed saving these “seeds” only for use as evidence that the generated values were determined in an arbitrary manner; FIPS 186-4 permits saving them for additional purposes, such as the regeneration of the values.

FIPS 186-4 is available at http://csrc.nist.gov/publications/PubsFIPS.html

Media Contact: Chad Boutin, chad.boutin@nist.gov, 301-975-4261

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NIST Chip Measurement Advance Earns 'Oscar of Innovation'

A fundamental advance in measurement capabilities that could save semiconductor manufacturers billions of dollars annually has earned a 2013 R&D 100 Award for its National Institute of Standards and Technology (NIST) inventors.

Called quantitative hybrid metrology (QHM), the new NIST method integrates statistical techniques and measurements made with two or more instruments to rigorously determine the sizes of nanoscale transistor features on semiconductor chips. In one demonstration the final measurement was three times more accurate than the results achieved with any one method alone.*

The NIST inventors of QHM  include physicists Bryan Barnes and Richard Silver and statistician Nien Fan Zhang. Hui Zhou, a NIST research associate, was a member of the development team.

On the basis of a 2007 paper**, the team estimates that a three-fold decrease in measurement uncertainty achievable with their innovation could save manufacturers as much as $7 for each chip they produce.

The public-domain invention does not replace chip makers' standard measurement tools, such as scanning electron microscopes (SEMs) and optical devices known as scatterometers. Rather, the new method enhances these tools and ties them together in novel combinations. This flexibility in measurement strategies minimizes measurement uncertainties and optimizes throughput.

A key element of the NIST method is a model library—a collection of simulated data based on typical chip feature dimensions that are compared to actual measurements, made with an SEM, scatterometer, or by other means.

Before performing the comparison, however, the NIST team employs an elegant statistical treatment—called Bayesian analysis—to incorporate a few key additional measured values from other tools into the library model. This step reduces the uncertainty in the measurements, lowering them by more than a factor of three in some cases.

Several companies and other organizations, including IBM, GlobalFoundries, the University of California Berkeley and Sematech are adopting hybrid metrology. All have made important advances in their implementations of the technique since the inventors first introduced the statistical foundations to quantitative hybrid metrology.

Beyond immediate applications, QHM has the potential to be a cost-effective solution to an imposing challenge facing high-volume semiconductor manufacturers as dimensions decrease below 20 nanometers (nm). By 2019, transistor dimensions are slated to shrink to 10.9 nm. At present there are no known solutions for measuring critical dimensions of that scale on chips, according to the International Technology Roadmap for Semiconductors.

"No single measurement technique can fulfill this critical need that is central to the continued success of the entire semiconductor industry," according to the inventors. "Measurement-tool combination through QHM is uniquely positioned as the best option for process control for circuits with line widths below 10 nm."

To foster the adoption of this technology among semiconductor manufacturers and to facilitate collaboration among measurement instrument makers, this novel technology was placed in the public domain.

For more, see the 2012 NIST Tech Beat story, "NIST 'Hybrid Metrology' Method Could Improve Computer Chips" at www.nist.gov/public_affairs/tech-beat/tb20120905.cfm#hybrid.

The R&D 100 Awards are given annually by R&D Magazine. The winners receive their awards at a special event in November. Details are available at www.rd100awards.com/.

*N.F. Zhang, R.M. Silver, H. Zhou and B.M. Barnes. Improving optical measurement uncertainty with combined multitool metrology using a Bayesian approach. Applied Optics, Vol. 51, No. 25. Sept. 1, 2012. DOI: http://dx.doi.org/10.1364/AO.51.006196.
** B. Bunday et al. Value-added metrology. IEEE Trans. on Semi. Manuf., Vol. 20, No. 3, Aug. 8, 2007. DOI: http://dx.doi.org/10.1109/TSM.2007.901851.

Media Contact: Mark Bello, mark.bello@nist.gov, 301-975-3776

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