An Assessment of the National Institute of Standards and Technology Measurement and Standards Laboratories: Fiscal Years 2004-2005 (2005)

Prior to the current assessment period, the Electronics and Electrical Engineering Laboratory (EEEL) had been organized in six divisions and two offices: Electricity Division, Semiconductor Electronics Division, Electromagnetic Technology Division, Radio-Frequency Technology Division, Optoelectronics Division, Magnetic Technology Division, Office of Microelectronics Programs, and Office of Law Enforcement Standards.

In 2003, a major reorganization took place: the Electricity Division and the Electromagnetic Technology Division merged into the new Quantum Electrical Metrology Division, and the Radio-Frequency Technology Division and Magnetic Technology Division merged into the new Electromagnetics Division. In addition, a major budget reduction caused two rounds of reduction in force (RIF) during fiscal years (FY) 2003-2005, with the reduction in the workforce totaling 45 personnel (see Appendix A, Figure A.4, for staffing trends).

The laboratory is now organized in four divisions, as shown in Appendix A, and continues to include the Office of Microelectronics Programs (OMP) and the Office of Law Enforcement Standards (OLES). Its divisions are these:

• Quantum Electrical Metrology (QEM) Division,

• Optoelectronics Division,

• Semiconductor Electronics Division (SED), and

• Electromagnetics Division.

• The overall technical quality of EEEL continues to be very high and innovative. The EEEL has an outstanding staff, a solid history of achievement, and close ties to customers. The projects are generally well aligned with the NIST mission and provide an excellent value for the money to the country and its industrial infrastructure.

• The merging of the former Electricity Division and Electromagnetic Technology Division into the Quantum Electrical Metrology Division has prompted a complete reexamination of all of the new division’s projects and the manner and the extent of their support. The process is not yet complete, but it is already clear that this reexamination will force a more uniform assessment and support framework within the new division. This reexamination has had one immediate benefit in that other agencies (OA) opportunities are now being pursued more generally throughout the division. The Board commends the EEEL management on its prompt actions on realignment to strategic planning objectives.

• The Electronic Kilogram project is the best of its kind. Its work is on the threshold of changing the entire manner in which the International System of Units (SI) and the fundamental constants are defined and realized.

• Of particular note is the recent development of a portable Josephson voltage standard. There are about a dozen commercial Josephson array systems in North America, which allow customers to get the highest possible accuracy in a direct or indirect comparison. The portable standard has improved comparison accuracies by approximately an order of magnitude.

Opportunities for improvement at EEEL include the following:

• The reorganization and downsizing described above prompted two extreme responses among the staff—in some areas it was handled effectively, allowing new research directions to grow, but in other areas it created significant morale issues. On the whole, staff morale is still good, and many view their employment as a rich and exciting opportunity. However, the continuing strain on budgets and staff size are taking a toll and causing some staff members, including some of the best, to consider other career options.

• The equipping of the Advanced Measurement Laboratory (AML) is progressing extremely well. The only concern is whether future NIST budgets will allow this process to continue at a reasonable rate. These new facilities are excellent but very costly, producing a significant impact on the capital equipment funds available for other projects during this period.

• The shrinking budgets in FY 2004 and FY 2005 are threatening EEEL’s ability to maintain its global leadership in a number of areas. Progress is being impeded by the inability to make timely hires, to refill vacancies, and to upgrade or in some cases even maintain research facilities.

• The mission and long-term plan for the Boulder campus of NIST are very unclear, which is affecting both the morale of and progress by the excellent scientists at that location.

• Metrology, though recognized as NIST’s core competency, is being seriously compromised in recent years, particularly through funding competition with the Strategic Focus Areas. The EEEL should undertake a conscientious reexamination of this trend in order to reach a clear decision about the laboratory’s level of commitment to metrology and to develop a strategy for implementing the decision. It is particularly necessary to communicate clearly to the staff where metrology lies on NIST’s priority list.

The technical quality and merit of research and services carried out by EEEL continued at a very high level during this assessment period. Many projects are on the cutting edge of advancing scientific

knowledge, and advance the standards and calibration services that the laboratory is asked to perform. The following discussion indicates some of the projects that stand out for excellence and illustrate the merit of the laboratory’s work in brief.

The work of the Electronic Kilogram project (to define the kilogram by electrical standards) has reached a milestone in realizing a recent electronic determination of the kilogram that has much lower uncertainty but is still consistent with previous results. These NIST results make a major contribution to a new value for Planck’s constant (Mills et al., 2005). The project team is participating in international discussions on redefining the kilogram, based on a consensus definition of Planck’s constant. Such a definition could result in substantially lower uncertainties of almost all of the electrical quantities, including voltage, resistance, and current. The adoption of the electronic kilogram as the mass standard would improve the consistency of the International System of Units and would also provide better determinations of many fundamental physical constants, such as the charge and mass of the electron, that serve the general scientific and technological communities.

The space imaging of the present space observatory, the Submillimeter Common-Use Bolometer Array (SCUBA), is impressive; SCUBA is the second-most-referenced telescope, following the Hubble Space Telescope. The planned SCUBA 2 system, with its array of about 10,000 superconducting transition edge sensors designed and fabricated by the Quantum Sensors project, offers 100 to 1,000 times more spatial resolution or mapping speed, as well as substantial improvements in the low-level detection limit. The new SCUBA 2 system will be installed on the James Clerk Maxwell Telescope in 2006. This is excellent work at the exotic frontiers of measurement.

The DNA fingerprinting project is a good example of how to combine biology with nanoelectronics. The project has good external support, showing the quality of the work. The project is a good example of how basic technology, in this case microfluidics, developed at NIST can be applied to systems that have both commercial and defense applications. While the techniques currently being developed are aimed at DNA fingerprinting, it appears that the core technology will lead to several new biological applications.

A second activity that continues to grow is single-molecule manipulation, which is a real strength of the group that performs this work. As part of this activity, the nanoparticle and nanopore field is becoming more mature and very competitive. This may be an opportunity for NIST to take a leadership role in the metrology of nanopores and nanoparticles. This group is vibrant, and recent personnel additions should enhance its productivity even further.

The work on optical frequency combs is outstanding. The technology is now solid enough that critical applications can be pursued. The group performing this work should consider a collection of possible key applications, analyze their potential, and choose one or two of them to focus on. These should be applications for which the impact of this new technology is substantial and the business vision predicting that impact can be clearly articulated. Once chosen, these applications should be aggressively pursued.

The high-speed Electro-Optic Sampling (EOS) project is at the leading edge and is an outstanding capability with powerful applications in metrology. This project is a collaborative effort between the Electromagnetics and Optoelectronics Divisions. An on-wafer EOS system for phase and waveform time calibrations was developed. The team established calibration methods and uncertainty analyses in coaxial media up to 110 GHz and on-wafer to 200 GHz. This capability is expected to migrate as a fundamental phase and time-domain quantity. It will have significant impact for the optoelectronics as well as the semiconductor industry. In related work, the Electromagnetics Division has developed an accurate method of measuring the characteristic impedance of a transmission line on lossy silicon substrates and on-wafer calibration using this method. The division also developed instrumentation and methods for accurately and completely characterizing multiports and small printed coupled lines.

The accomplishments of the Electromagnetic Properties of Materials project are impressive. They support metrology and technology for bulk, thin-film, liquid, and biological materials. The team remains the best materials measurement center in this arena. It has the broadest and deepest capabilities in this area. This team supports many projects, some of which seem to be specific to one customer. Industrial growth in the life sciences areas is much larger than in electronics. There should be more emphasis on the life sciences aspects of the materials work.

The Electronics and Electrical Engineering Laboratory addresses relevant needs of U.S. industry in the extremely broad area of electrical measurements. Today, this scope includes the characterization of devices for optoelectronics and micro-/nanoelectronics and the associated manufacturing technologies. Responding to the growing breadth of this challenge requires EEEL to carefully prioritize the areas in which to deploy its resources, mainly addressing the most technically demanding and those that have the most metrological impact for the laboratory’s customers. Ideally, there would be a steady procession of new calibration capabilities fed from EEEL’s research that are developed and offered to customers, while others are phased out and given over to second-tier calibration laboratories. For example, a major fraction of the activities of the EEEL Quantum Electrical Metrology Division is directly related to maintaining and disseminating electrical measurement standards for voltage, resistance, current, impedance, power, and energy over extensive ranges and at very low uncertainties. The customers for these services represent manufacturing, power utilities, process control companies, the semiconductor industry, the military, and the aerospace, transportation, and communications sectors. In this way the division has a direct impact over an extremely broad fraction of the total American infrastructure.

The QEM Division’s Electric Power Metrology project is an example of direct impact at a relatively small number of power utilities and equipment manufacturers that in turn has a very broad, positive, secondary impact on every American. The QEM Division’s Voltage Metrology project is directly relevant to a larger number of customers and also indirectly impacts most Americans, although its visibility to them is typically reduced by more layers of measurement traceability.

The EEEL generally does a commendable job of balancing such near-term development with long-range metrology research. A good example on the long-range side is the QEM Division’s Electronic Kilogram, a project with more relevance to the fundamental knowledge base than to near-term industrial applications. It is in a unique position to have profound impact on all of measurement science in every National Metrology Institute in the world, but industry will barely notice these results in the immediate future.

The relevance of EEEL is often enhanced by cross-campus collaboration, such as the linkage between the QEM Division’s Nanoscale Cryoelectronics and Single Electron Tunneling projects. Jointly, this work is addressing one of the fundamental challenges of metrology, the quantum metrology triangle, and the results are being eagerly watched by the international metrology community. The metrology triangle experiment consists of developing a quantum Ohm’s law from three effects: Josephson, single-electron tunneling, and quantum Hall. The completion of the quantum metrology triangle through the Nanoscale Cryoelectronics project’s single-electron counting capacitor will be important for fundamental science and metrology.

An example of interlaboratory collaboration is provided by the work of the EEEL Electromagnetics Division with the Physics Laboratory in exploring a fundamentally new approach to microwave power measurements. A direct comparison system for WR-15 and WR-10 waveguides has been constructed, and new WR-15 and WR-10 calorimeters have been designed. This project area still represents a very

valuable fundamental NIST capability that is needed in the industry. The immediate need is to complete the capabilities in 1.85 mm (67 GHz) and 1.0 mm (110 GHz) connectors to support high-frequency coax already widely deployed in industry. The emerging goals are above 1.0 mm (110 GHz). Many of the existing services at lower frequencies are stable and mature. These should be considered for transfer from NIST to other laboratories, which could free up key resources for the effort at higher frequencies.

Collaboration that is interdivisional, interlaboratory, and even external (to NIST) on projects of relevance to the microelectronics industry are promoted by the EEEL Office of Microelectronics Programs. In spite of its relatively small budget, this office continues to be successful in starting and managing a broad portfolio of NIST programs in support of industry. The collaboration of OMP with the Semiconductor Manufacturing Technology Consortium (SEMATECH) is also effective in generating funding for NIST programs, as highlighted in the NIST work on low-dielectric-constant interconnection metrology using the small-angle X-ray scattering facility.

A project of particular relevance to national defense is the Electromagnetics Division’s application of reverberation chambers to calibrating radio-frequency (RF) field probes to speed the calibration process for the U.S. Army. This Complex Fields project has demonstrated a potential weakness in existing field probe systems that can result in personnel being exposed to excessively high fields without being aware of the hazard. These results should be followed up with both the Army and probe suppliers to improve understanding of this vulnerability and to redesign probe systems to eliminate it.

The Semiconductor Electronics Division of EEEL is conducting an appropriate mix of near-term and long-term research and development (R&D). Currently more than half of its projects are focused on specific items already identified as being of immediate priority by its industrial customers. One example is SED’s Electrical Test-Structure Metrology project. The International Technology Roadmap for Semiconductors (ITRS) has repeatedly stressed the need for improved critical dimension (CD) metrology traceable measurement standards. The development of an adequate CD metrology infrastructure is essential in order to support the technical and manufacturing needs of optical lithography below 100 nm. At the present time, there is no established technique that meets the stated requirements. In response to this need, SED has developed three-dimensional CD standards fabricated in single-crystal silicon with nominal widths as low as 40 nm. In collaboration with NIST’s Manufacturing Engineering Laboratory (MEL) and Information Technology Laboratory (ITL), SED has developed an improved measurement and data analysis procedure to minimize the expanded uncertainty associated with the reference features. The second-generation units have CDs as low as 45 nm and uncertainties, based on statistical analyses performed by personnel of ITL’s Statistical Engineering Division, of between 1.5 nm and 3 nm. In cooperation with MEL and ITL, SEMATECH, and VLSI Standards, Inc., 10 chips containing the improved NIST-calibrated CD reference features were distributed to SEMATECH’s 10 member companies in 2004, along with a Technology-Transfer Report.

The Optoelectronics Division of EEEL is engaged in very relevant work on single-photon devices, including sources and detectors, which are key enablers for advances in quantum technology. The new quantum technology holds great promise for a multitude of new devices and systems, including fully secure optical communications, a current high-priority need. The customer focus and market outreach of this work has resulted in steady growth in terms of projects and calibration income.

The Optoelectronics and Electromagnetics Divisions both had calibration income growth in FY 2004, with that of the Quantum Electrical Metrology Division holding roughly constant. As a whole, EEEL calibration income in FY 2004 recovered from an approximately 10 percent drop in FY 2003, but it is still flat when analyzed over several years. In the growth areas, services are focused on a sufficiently wide customer base so that if there is a downturn in one sector, the revenue income does not suffer greatly. Such growth is an indication of the relevance of this activity to the industry.

The EEEL continues to demonstrate effective and timely delivery of results to its customers. For example, the Farad and Impedance Metrology project of the QEM Division recently responded to a customer’s request to greatly improve the world’s best capacitance accuracies in the range of 20 Hz to 20 kHz. Applying a novel technique related to their primary standard, the Thompson-Lampard capacitor, the division successfully completed this project and now offers these accuracies as a regular calibration service. The improved uncertainties have impact not just for the initial customer but also among a much larger subset of customers who use high-accuracy capacitance bridges or precision capacitance standards. The initial customer had originally sought calibration services abroad but now has improved the specifications and traceability of its products on the basis of the NIST developments. This is an excellent example of the division’s responsiveness to customers’ demands as well as of its success in realizing an effective solution.

Another core capability in which EEEL continues to enhance capability as well as efficiency of dissemination is the QEM Division’s maintenance of the U.S. legal volt. This unit is provided as an internationally consistent, accurate, reproducible, and traceable voltage standard that is readily and continuously available for the national scientific and industrial base. Of particular note is the recent utilization of a portable Josephson voltage standard for direct calibration of customer Josephson voltage systems. This couples NIST’s expertise with hysteretic Josephson junction operation and the novel design of a compact, easily operated Josephson array system specifically designed for transport to customers. There are about a dozen commercial Josephson array systems in North America, which allow customers to get the highest possible accuracy in a direct or indirect comparison. The portable standard has improved comparison accuracies by approximately an order of magnitude. In an effort to improve EEEL efficiency while maintaining a leadership role, this project is playing a key role in a comparison of 10 V Josephson array systems within North America. By using its portable Josephson array system, NIST will act as the pilot laboratory. However, instead of measuring in each loop of the comparison, NIST will only measure directly with four subpilot laboratories, which will then measure with respect to the rest of the participating laboratories. This system promises to reduce the uncertainty of the overall comparison and still reduce the number of measurements required at NIST, enhancing EEEL’s efficiency.

The Electromagnetics Division’s Scattering Parameters project has demonstrated exemplary effectiveness for military customers by delivering updated Six-Port Systems to the Navy, a suite of vector network analyzer software to the Air Force Primary Standards Laboratories, and a 30 MHz attenuator system and capability to the Army Primary Standards Laboratory.

The Electromagnetics Division has also been particularly effective in the Reference Fields and Probes project area, which is providing important support to standards work in the area of test-facility qualification at frequencies above 1 GHz. Qualification of test facilities at frequencies below 1 GHz is well understood in the electromagnetic compatibility community, but significantly less experience is available for frequencies above 1 GHz. The design for the co-conical field generation system being developed for the U.S. Air Force has been completed, and it is intended that a system will be delivered to the Air Force in 2006. This tool will provide a system for rapid, cost-effective probe calibration over the frequency range of 10 MHz to 45 GHz.

In response to the need for improved integrated-circuit CD metrology referred to in the section above on “Relevance,” the Semiconductor Electronics Division’s Single Crystal Critical Dimension Reference Materials project has developed three-dimensional CD standards fabricated in single-crystal silicon with nominal widths as low as 40 nm. As stated, in collaboration with NIST’s MEL and ITL,

SED has developed an improved measurement and data analysis procedure to minimize the expanded uncertainty associated with the reference features. This expanded uncertainty is derived statistically from the calibration function, which enables tracing the atomic-force microscopy (AFM) measurements to the silicon lattice-plane spacing using high-resolution transmission electron microscope (HRTEM) imaging. The previous generation of reference materials, which was delivered in 2001 and used electrical CD as the transfer calibration, had uncertainties of approximately 14 nm. The second-generation units have CDs as low as 45 nm and uncertainties, based on statistical analyses performed by personnel of ITL’s Statistical Engineering Division, of between 1.5 nm and 3 nm. This decrease in uncertainty is of major importance to the end user of these reference features, as demonstrated by a cooperative effort between NIST’s MEL and ITL, SEMATECH, and VLSI Standards, Inc. This collaboration distributed 10 chips containing the improved NIST-calibrated CD reference features to SEMATECH’s 10 member companies in 2004, along with a Technology-Transfer Report. These reference materials were delivered with sub-100 nm CDs and combined uncertainties of less than 5 nm. The improvement in uncertainty resulted from the implementation of a new type of HRTEM-target test structure, the extensive use of SEM inspection to identify targets with superior CD uniformity, and the use of advanced AFM to serve as the transfer metrology.

This year, the Semiconductor Electronics Division again contributed significantly to the effectiveness of EEEL by organizing (with SEMATECH) the 2005 Conference on Characterization and Metrology for ULSI [ultralarge-scale integration] Technology, a biennial event that is the major technical conference in this field for the semiconductor industry and its R&D partners in academia and government.

This type of effectiveness in outreach to industry is also demonstrated by the Optoelectronics Division, which continues to sponsor the Symposium on Fiber Optic Measurements. Although attendance at this event has decreased from its peak, the decrease is far less than one might expect given market conditions, reflecting the importance placed on this meeting by the core community of attendees. As the optical communications market recovers, the symposium will be well positioned as a forum for advanced research in fiber metrology and related fields.

The Optoelectronics Division has also shown its effectiveness by working with the National Institute of Justice on developing optical techniques for the testing and measurement of body armor. While this application is unlikely to lead the Optoelectronics Division into major new frontiers of optical sensing, it is a good example of how the division can apply its fiber-optics expertise to diverse and important applications areas. This application is interesting, challenging, and important.

The Advanced MOS [metal-oxide semiconductor] Device Reliability activity is a very good example of how NIST can impact U.S. industry. With continuing device scaling, the gate dielectric film thickness has decreased to an oxide-equivalent value of 1.1 nm in 2005 and 0.7 nm or less by 2010. This has been identified as a critical front-end technology issue in the Semiconductor Industry Association Technology Roadmap. There is great interest in the semiconductor industry in the development of high-k dielectrics. Another important aspect of high-k gate dielectrics is the development of models to explain dielectric degradation. This technology need has prompted the MOS Device Reliability Program to shift its research focus to study high-k dielectrics with metal gate electrodes. In 2004, the NIST group continued to be successful in making significant contributions to the study of high-k dielectric reliability. Among the recent accomplishments of the group are characterization of the energy dependence of interface traps in hafnium oxide (HfO₂); publication of the new Joint Electron Device Engineering Council standard, JESD-92, for soft breakdown of ultrathin gate oxides; completion of the study of progressive breakdown in small-area ultrathin gate oxides; and study of anomalous threshold voltage roll-up behavior in HfO₂ metal oxide silicon field-effect transistors.

As discussed in the following subsections, lack of laboratory resources in the Electronics and Electrical Engineering Laboratory is cause for serious concern.

Funding for EEEL during the current review cycle has been severely reduced; this restriction is having a detrimental effect on some promising activities. As noted in the October 2004 EEEL Operational Plan, “Continued flat and shrinking budgets are threatening the Laboratory’s ability to maintain its global leadership in a number of areas. Progress is being impeded by the inability to make timely hires, to re-fill vacancies, to upgrade or in some cases even maintain research facilities. This impact is especially felt in fundamental metrology research …” (NIST, EEEL, 2004, p. 2). This same observation has been emphasized and reiterated by the Board for the past several years. The budget issue is, more than ever, severely felt in this review cycle, with two rounds of RIFs implemented.

Since the Board’s previous report, in 2003, EEEL has undergone reorganization in an attempt to better align work groups and to reduce expenses. The shrinking EEEL budget can be expected to have a significant negative impact on U.S. industry. As noted in past reports and reflected in the EEEL Operational Plan, “There are several areas in which NIST is at risk of becoming outdated or losing its world-class status due to lack of funds…. The current scope of the Electromagnetics Division cannot be maintained as is with its current budget. While other agency funding might help fill in the gaps, it also creates a trade-off between sponsor and agency objectives, and is difficult to obtain in those calibration areas core to the NIST mission” (NIST, EEEL, 2004, p. 22). The budget reductions remain an ongoing problem for the division as well as for all of EEEL.

The Board views with concern the emphasis of EEEL shifting from the core mission of metrology to technology-development programs, which appears to be driven in large part by budgetary sources favoring the latter. This change is of particular concern since the current NIST Strategic Plan is uniformly expansionist, and NIST’s budget has very recently increased, in stark contrast to the budget contractions in EEEL; it is exceptionally damaging to technical productivity for the sunset of an initiative to be implicitly communicated by means of its budget or RIFs rather than through a clear strategic plan that allows for the orderly conclusion of work.

Several key activities have been heavily publicized because they are of fundamental importance to electrical metrology. (These include the Capacitance Standard Based on Counting Electrons, Johnson Noise Thermometry, the Arbitrary Waveform Josephson Array Generator, and the Electronic Kilogram.) Each of these activities has experienced a reduction in resources to the extent that, although there may be continued support for the short term, there is concern that their outputs will be substantially reduced. This broad reduction in mission-aligned projects that serve to showcase NIST’s excellence will tarnish NIST’s international reputation and have a chilling effect on the future hiring of top researchers.

The concern with the highest priority involves the inadequate resources assigned for the Nanofabrication (Nanofab) facility. Currently there are three technician slots assigned to this facility.

The Board believes that this number should be at least six for a facility of this size and complexity. Also, there is some concern about using contract labor for process development. While this approach will bring in critical expertise to get unit processes qualified, it will still be important to have in-house process engineering support to maintain and tweak processes. The operating budget is low, but that may be because there are more operating costs covered by NIST upper management than comprehended by the Board.

The second-ranking area of concern is the low level of activity on back-end metrology needs. As back-end dimensions and process control push the limits of current metrology, there is a growing need for new metrology techniques and standards for back-end processing (process steps from contact through completion of the wafer prior to electrical testing). A specific example of how this could be improved would be to better define the objectives of the Cu back-end metrology project. It is important that the back-end projects look far enough out at industry needs to allow the research to stay ahead of industry requirements.

The setting up of the Advanced Measurement Laboratory caused financial fluctuations for other activities. While this situation may have been inevitable, management should be persistent in stressing that such fluctuations are temporary and that important projects closely aligned with the laboratory’s mission will continue to be supported. Such assurances are necessary to keep morale buoyant and to retain key researchers during this turbulent period.

The Board has commented in past years on the proposed RF EM-Field Metrology Laboratory (REML) facility in the Boulder area. The EEEL Operational Plan states: “The Electromagnetics Division facilities are becoming inadequate for the next generation of EMI, EMC, wireless and radar measurements for industry. The EEEL strongly supports an initiative for the construction of the new world class ‘RF EM-Field Metrology Laboratory Facility’ (REML). This would cost approximately $30M and would fund a new large high bay facility with a basic set of metrology-focused electromagnetic (EM) test chambers to address a wide range of topics, including EM compatibility of electronic products, bioeffects of EM fields, international standards affecting commerce, and basic research related to measurement and characterization of EM field quantities” (NIST, EEEL, 2004, p. 23). The Board is not aware of the details of this laboratory that would drive the estimated cost to $30 million, but it fully supports the need for the development the REML facility to replace the deteriorating facilities presently available in Boulder and to enhance the capabilities for this work within the laboratory.

The Board has noted in the past the need for succession planning as key staff members approach retirement. This is noted in the EEEL Operational Plan: “Not only is it difficult to make timely hires, but it is also difficult to provide the appropriate rewards and incentives to maintain excellent staff members. Many areas in EEEL are only 1 or 2 experts ‘deep,’ whom the Laboratory can ill afford to lose” (NIST, EEEL, 2004, p. 22). This issue will only continue to accelerate unless steps are taken to address it.

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