Mats Ekberg, Per-Uno Skotte, and Tomas Utterbäck, Micronic Laser Systems; Oleg Kishkovich, David Ruede, and John Higley, Extraction Systems; and Vladimir L. Orkin, National Institute of Standards and Technology

The proliferation of photomask laser pattern generators presents new challenges to system designers and manufacturers. Laser pattern generators are susceptible to the influence of airborne molecular contamination (AMC), which affects both chemically amplified resists (CARs) and laser optics. However, while semiconductor wafer lithographers have confronted similar challenges and developed reasonable methods for dealing with AMC, photomask and photomask equipment manufacturers have not acquired comparable experience. Moreover, some AMC issues in the photomask area differ from those in the semiconductor wafer lithography area.

Pattern generators used in the maskmaking industry have shifted from the use of E-beam writing technologies to higher-speed, higher-productivity laser writing technologies. While the latter have appreciably increased the throughput rates at which quality masks can be created, they have also introduced AMC issues that manufacturers and users of mask pattern generators have not previously encountered.

This article addresses the challenge posed by AMC to modern mask-writing tools. It is based on work performed by Micronic Laser Systems (Täby, Sweden) to investigate the sources of AMC and identify suitable methods for minimizing its effects.¹ To accomplish that task, the company teamed with Extraction Systems (Franklin, MA), whose air-filtration, metrology, and analytical technologies had previously been developed for the semiconductor fabrication industry for use in deep ultraviolet (DUV) photolithography applications. The two companies performed a needs analysis, which led to the development of a suitable air-filtration system for DUV mask-writing tools.

First, the article compares the effects of AMC on semiconductor manufacturing and maskmaking. It discusses air-sampling and construction-material analyses that were performed to understand AMC challenges. Then it details the methods that were used to develop an appropriate filtration specification for different classes of contaminants and to fabricate an air-filtration system for a series of laser pattern generators. The article highlights the importance of cooperation between tool designers and AMC experts to maximize process stability and equipment productivity in advanced maskmaking applications.

AMC in IC Fabs and Mask Houses

In 2002 and 2003, Extraction's Airelab group collected and analyzed ambient air samples from more than 30 fabs worldwide.² As shown in Table I, average NH₃ levels were in the 10-ppbv range, half of the reported average a few years earlier. SO₂ levels were often lower than the detection limits of common laboratory capture and analytical methods (<0.1 µg/m³), possibly because of the natural tendency of strong acids to combine with molecular bases. Since significant quantities of ammonia exist in the air, acids can combine with the ammonia to form salts, lowering ambient acid levels. HEPA and ULPA filters used in the air-handling systems in photolithography bays can act as reaction beds to promote the combination of acidic and basic species.

Figure 1: NH3, SO2, and organic levels in a new cleanroom over a 17-month period. While inorganic levels remained relatively steady, condensable organic levels fell.

Levels of condensable organic compounds (compounds with boiling points above 150°C) vary greatly from cleanroom to cleanroom. In fact, tests have shown that in the same cleanroom, condensable organic compound levels can double from one day to the next because of the use of cleaning solvents and unplanned releases of such solvents as PGMEA, which is used in photoresist processing. In addition, new cleanrooms have been shown to have higher levels of condensable organics at start-up than after a period of operation. The data presented in Figure 1 demonstrate that in one fab, organic levels fell by more than four times, from >240 µg/m³ to <60 µg/m³, during its first 17 months of existence. The tendency of organic compound levels in new buildings to decrease over time can be attributed to initially high outgassing from cleanroom construction materials as well as construction and tool-installation activities. Presumably that tendency affects both mask and IC cleanrooms.

Effects of AMC

To understand the effects of AMC on maskmaking applications, it is useful to review the experience of the semiconductor fabrication industry. Leading-edge IC wafer production takes place at 130 nm. At such resolutions, CARs and the optics systems used to print patterns are extremely sensitive to AMC.

While semiconductor fabricators have focused on airborne molecular contaminants such as ammonia and 1-methyl-2-pyrrolidone (NMP), dozens of acids, bases, condensable organics, and dopants—all of which are routinely found in cleanroom environments—threaten DUV processes and equipment, as seen in research performed by Extraction Systems, International Sematech, IMEC, and other organizations.³ Moreover, refractory contaminants, those that land on optics, are not removed effectively. All of these contaminants can be found in ambient air, purge gases, and chemically filtered chamber air surrounding optical components.

Molecular contaminants can alter the physical properties of photoresists. For example, in semiconductor fabrication applications, they can reduce the quality of the lithographic image on the wafer by causing yield-limiting "T-topping." Moreover, minute amounts of condensable organic contaminants can deposit damaging films on 193- and 157-nm optical elements, causing such forms of optics degradation as transmission loss, illumination nonuniformity, and light scattering, which can result in stray light. Contaminants can also shorten optics' service life spans.^4,5

Removing Acids and Organic Compounds. Molecular bases can contaminate CARs. Since certain compounds can react with other compounds at room temperatures to form condensable or nonvolatile products, they must be removed. For example, because strong acids such as HCl and H₂SO₄ can react with ever-present NH₃ to form nonvolatile salts, it is important to filter strong acids. However, while filters can be designed to remove any species, the practical limits of filtration must address airflow requirements, pressure-drop budgets, size limitations, and costs.

The importance of filtering organics is acknowledged among microlithographers and exposure-tool suppliers. A naturally occurring substance for removing organic contaminants is ozone, or "active oxygen," which is formed in air at wavelengths used in exposure tools. Since ozone can remove hydrocarbon films from lens surfaces, toolmakers have adopted strategies based on ozone formation during exposure and are investigating methods for using ozone to maintain the cleanliness of 157-nm optical surfaces. However, while this strategy works well for true hydrocarbons—compounds containing only carbon, hydrogen, and oxygen—it may not be suitable for the removal of all organic species.

Very large organic molecules found in engineered plastics, which can readily condense to form monolayers of thin films, pose a great challenge, especially when they contain not only carbon, hydrogen, and oxygen, but also species such as phosphorus and silicon. In the presence of ozone, such species, which are commonly referred to as refractory compounds not because of their optical nature but because they are difficult to corrode or draw out, can be oxidized to form nonvolatile residues on optical surfaces. Whole classes of compounds used as flame-retardant materials fall into this category. Fortunately they are well known. Hence flame-retardant materials used in flexible air ducts can be manufactured without such compounds, or rigid metal ducting can be used.

The Impact of AMC on Optics. Optical surfaces exposed to potentially harmful species during exposure and idle times must be protected. The absorption of UV radiation by contaminant molecules, especially at shorter wavelengths, can cause the molecules to undergo photodecomposition, resulting in chemical deposition on the optical surfaces of exposure tools.

Figure 2: Absorption spectra of commonly used silicon-containing organics through the range of wavelengths used in IC production fabs.

In an effort to understand the potential for optical degradation, Extraction commissioned the National Institute of Standards and Technology (NIST; Gaithersburg, MD) to analyze chemical species commonly found in the photobay and determine their ability to interact with exposure radiation. In particular, hexamethyldisilazane (HMDS) and some of its derivatives were examined for their potential to absorb radiation through the range of wavelengths used or soon to be used in production tools.

Figure 2 shows the absorption cross sections of compounds from 160 to 240 nm. In the region of the krypton fluoride (KrF) laser (248 nm), all measured absorption cross sections were far less than 10^–20 cm²/molecule. Absorption increased toward the argon fluoride (ArF) laser wavelength of 193 nm, becoming as high as 3 X 10^–18 cm²/molecule in the case of HMDS and one to two orders of magnitude less than that in the case of hexamethyldisiloxane and trimethylmethoxysilane. Near the fluorine (F₂) laser wavelength of 157 nm, the silicon-containing contaminants under investigation exhibited very strong absorption levels exceeding 10^–17 cm²/molecule.

Notably, these high absorption levels are related to chemical composition, not just to the presence of silicon atoms in the contaminant molecules. Thus, the absorption cross sections of siloxane (SiH₄) are at least two orders of magnitude smaller than those of all other silicon compounds over the entire wavelength range. But because they contain silicon, all of these compounds can leave SiO₂ film deposits on optical surfaces that are difficult, if not impossible, to remove without damaging optical coatings. Silicon-containing solvents are handled so that they do not enter cleanroom air. Nevertheless, because they can be spilled or otherwise released inadvertently, filtration systems must be able to remove them from the cleanroom ambient.

AMC Control in Chipmaking versus Maskmaking

As in the semiconductor fabrication industry, the mask-making industry must protect resists and optics against AMC as it shifts to the use of laser pattern generation. Unlike the IC industry, however, mask-making faces unique challenges stemming from the nature of maskmaking processes and equipment.

• Mask geometries are four times larger than wafer geometries. Hence the mask industry is investigating the use of CARs to achieve optical proximity correction and is developing fast, high-resolution atmospheric tools.

• Long write times in maskmaking mean that the coated mask plate may be in the tool exposure chamber for hours rather than a minute, as in chipmaking.

• Whereas the chipmaking industry is relatively mature and accustomed to AMC control in 248-nm-wavelength lithography, maskmakers are still in the process of developing new molecular base materials criteria, operating procedures, and even cleanroom designs to protect CARs.

• In contrast to IC manufacturing, in which wafers are prepared, coated, exposed, and baked in a clustered toolset comprising an exposure and a development tool, lithography processing in the maskmaking industry does not lend itself to the use of clustered exposure tools.

• Resists are most at risk between the exposure and postexposure bake steps. In chipmaking, the wafer may be coated, exposed, and baked at the rate of 100 wafers/ hr. In maskmaking, however, the plate may be coated by a plate manufacturer in one country and then transported to the maskmaker in another country in a purged enclosure. Coating may take place more than a month before exposure, and the exposure process can take hours.

• In wafer processing, resists are mature and optimized.⁶ In maskmaking, however, resists are relatively new and not as well characterized. Consequently, the critical dimension shift that occurs during a 10-ppb ammonia spike is not as well understood in the mask-making as in the chipmaking process.⁷ Thus, rework decisions must be made based on less experience.

Developing Methods to Combat AMC

Understanding the importance of AMC control, Micronic performed a needs analysis to design and build a suitable chemical air-filtration system for use in the Sigma7000 series of 130–100- and 90–65-nm high-resolution DUV laser pattern generators. The systems operate properly only in ultraclean environments that are free of AMC. Therefore, early in the design phase, AMC measurements were taken by Extraction to evaluate the laser generators' as-built operating environments and existing air-filtration systems, and to establish a baseline for characterizing the AMC challenge. Such factors as construction materials, process chemicals, clean dry air (CDA), ambient cleanroom atmosphere, cleaning solvents, and the presence of humans were evaluated. The analysis had four objectives:

• To define a performance specification encompassing acids, bases, and organics.

• To design a chemical air-filter system comprising tool-enclosure and optics purge-gas filtration.

• To devise a method for monitoring AMC in and around the tool during construction.

• To formulate a final AMC qualification procedure for production tools being shipped to customers.

Measurements were taken using a real-time total molecular base (TMB) monitoring system from Extraction. Then trap sampling was performed, followed by ion chromatography (IC) and gas chromatography–mass spectroscopy (GC-MS) analysis.

Figure 3: Comparative data derived from a TMB monitor illustrating how different materials outgas molecular bases.

Total Molecular Base Monitoring. The TMB monitor combines patented condition and conversion technology with chemiluminescence detection to achieve a detection limit of 500 ppt. The monitor's multipoint measurement capability enabled readings at locations such as the tool enclosure, system optics, makeup air (MUA) duct, filter cabinet, and ambient cleanroom. From these measurements, it was possible to determine the total molecular concentration of ammonia, NMP, amines, and other pollutants.⁸ Figure 3 shows how different materials outgas molecular bases.

Figure 4: Output from a GC-MS analysis of a sealant containing silicon, an unacceptable refractory contaminant, that has outgassed large amounts of BHT, a well-known antioxidant added to polymeric materials.

Trap Sampling and Analysis. Trap sampling was used to identify and quantify acids, bases, and organics with concentrations as low as 100 ppt. Acid and base samples were analyzed using IC, while organics were analyzed using a GC-MS system equipped with a mass selective detector and a thermal desorption system. Individual compounds were identified by performing a library search of chromatographic peak positions, which are presented in Figures 4 and 5.

The ability to determine the quantities and types of compounds found in the CDA and nitrogen used in exposure tools helped to specify these gases' cleanliness levels and prevent potentially damaging compounds from contacting lens surfaces.

Figure 5: Output from a GC-MS analysis of polymeric tubing. (Total siloxanes = 0.001 µg/gr/min, and total emissions = 0.1 µg/gr/min.)

Developing a Suitable AMC Air-Filtration System

In response to the AMC challenge faced by the semiconductor fabrication industry, Extraction had already developed an OEM-approved filtration system to protect DUV lithography optics and resist processes. The objective now was to transfer those AMC metrology and filtration technologies to the maskmaking industry, facilitating the development and construction of an effective air-filtration system for the Sigma7000-series laser mask writers.

Air-Filtration System Design. As more experience was gained on early ArF tools, concerns increased about even low levels of specific volatile compounds found in cleanroom air, leading to more-stringent filter requirements. Filters had to be capable of removing not only NH₃, amines, and NMP, which contaminated highly sensitive early photoresists, but also molecular acids and condensable organic compounds.

Protecting lenses from a range of contaminants requires filtration strategies that balance efficiency and longevity against real-world flow and pressure-drop requirements. Extraction's filters use a pleated nonwoven fabric to suspend high-surface-area media, ensuring that contaminants are removed while purified air passes through freely. This design has a two-layer structure to optimize media combinations. The pleated structure maximizes filter area, resulting in higher removal efficiency and a lower pressure drop than older packed-bed designs.

The hybrid filter installed in the Micronic laser pattern generators uses highly activated carbon to remove condensable organic species by means of adsorption (sometimes referred to as chemisorption), a powerful filtration process for nonpolar species such as hydrocarbons. To remove acids and bases, the filter is chemically treated to react with the target species and combine with it to form a nonvolatile compound that is locked into the porous media structure.

The pleated filter's low pressure-drop characteristic allows serial stacking of filter cells, which has several advantages:

• Serial filters can be fitted with sample-collection ports to gather AMC data, enabling real-time monitoring. The data are used to plan filter changes before contaminants break through the final filter and contact photoresists and optics. Nonserial filters, particularly packed-bed designs with a high pressure-drop characteristic, cannot provide advance warning of filter failures.

• Each filter is initially capable of removing >99% of the target contaminants. Multiple serial filters protect against extreme contamination challenges, such as developer spills or leaks.

• Partially spent filters can help to extend the lifetime of fresh filters, resulting in a low cost of operation. In nonserial designs, filters can fail when their removal efficiency drops below 99%. In other words, a 1% contamination breakthrough can be cause for a filter change. In serial designs, filters with removal efficiencies of <99% still protect the next filter downstream against most contaminants.

Figure 6 presents GC-MS scans of condensable organic compounds taken from filter inlet, filter interstack, and filter outlet sampling ports in a production lithography environment. While organic pollutants moved through the filter, they did not break through. This example demonstrates that the use of interstack sample ports to perform periodic monitoring enables exposure-tool users to predict filter performance and plan maintenance events around production requirements.

Figure 6: GC-MS scans of condensable organic compounds taken from (a) filter inlet, (b) filter interstack, and (c) filter outlet sampling ports in a production lithography environment.

Redesigning the Air-Filtration Prototype. A comparison between the preliminary AMC specifications developed by Micronic and the actual values captured during the needs analysis resulted in revised AMC guidelines, influencing the design of a prototype air-filtration system for the laser pattern generators.

Figure 7: Schematic diagram of the climate system of the laser pattern generator.

A schematic diagram of the generator's airflow unit is presented in Figure 7. The unit's filtered-air climate module is based on global laminar airflow. Air from the cleanroom is conveyed through a single-pass MUA filter, and then recirculated climate air is successively passed through the recirculation air-filter unit. The MUA unit isolates the writer's climate air from the cleanroom, while the recirculation filter unit handles possible internal outgassing. Low-outgassing particle filters are the final filters before the interior of the writer chamber. Such filters are tested without the use of aerosols from the synthetic oils dioctylphthalate (DOP) or diethylhexyl sebacate, which can cause prolonged filter outgassing. Compressed CDA and nitrogen gases are purified in separate gas scrubbers (pictured in Figure 8a) before entering the writer chamber.

Figure 8: (a) Gas scrubbers for compressed CDA and nitrogen, and (b) the filtration module of the laser pattern generator. In the MUA unit (left), airflow goes downward. Cleanroom air is taken from the ceiling, where air quality is expected to be the highest. To the right, the filter cartridges are visible.

Designing the laser generators' air-filtration system also included intensive materials selection, which involved research and materials studies using outgassing tests based on GC-MS analyses. As a result of those analyses, the use of plastic and polymeric materials in the writer was minimized. In addition, the differences between surface and bulk outgassing materials were investigated. While surface outgassing materials can be cleaned using organic solvents and can then be baked or vacuum baked, bulk outgassing compounds—including antioxidants such as butylated hydroxytoluene (BHT), plasticizers such as DOP, and fire retardants such as triethylphosphate—can never be rendered clean or turned into low-outgassing materials.

Few lubricants, greases, or elastomers are used in the air-filtration system. When necessary, lubricants based on fluorine oils, such as vacuum greases, are employed. However, mineral oil–based lubricants are excluded. As much as possible, elastomers, including soft or elastic polymeric materials and room-temperature vulcanizing materials, are not permitted to have direct contact with the climate air, CDA, or nitrogen.

Electric wires, a potential source of AMC, have been placed outside the climate airflow unit. The optical system is subject to forced airflow. Critical optical components are connected to a special purge system built with clean components.

Figure 9: Data indicating a very low, stable, TMB concentration for amines of <0.8 ppb at the stage area close to the mask plate.

Installing the Air-Filtration System. When design and prototyping activities were completed, the air-filtration system (pictured in Figure 8b) was integrated with a laser pattern generator. Certification testing was then performed using a TMB monitor and GC-MS analysis. The ability of the air-filtration system to remove AMC is demonstrated in Figure 9, which shows a very low, stable, TMB concentration (<0.8 ppb) of amines at the stage area close to the mask plate.

Conclusion

The joint AMC studies described in this article verified that the maskmaking industry, specifically the sector involved with the development of DUV laser pattern generators, faces AMC challenges comparable to those long understood by the semiconductor fabrication industry.

AMC at parts-per-billion-by-volume levels or below concerns advanced microlithographers because low levels of certain species can contaminate CARs and exposed optical surfaces. Extensive sampling has produced a baseline of chemical contaminants that can be used in filtration systems to ensure that exposed optics contact chemically clean air. Hybrid filters can remove potentially damaging species, such as molecular acids, bases, and condensable organics. Laboratory and field characterization of filter systems will continue as more and more ArF tools are deployed and more data are collected.

Acknowledgments

The authors wish to thank Michael Alexander, Phil Cate, Frank Belanger, and Robert Peterson for air quality and filter test data. They also wish to acknowledge Eric Bergeron, John Sergi, James Mastrobuono, and William Goodwin for new filter media production data.

References

1. M Ekberg, S Paul, and O Kishkovich, "Laser Pattern Generator Challenges in Airborne Molecular Contamination Protection," in Proceedings of Photomask Japan (Bellingham, WA: SPIE, 2003), 318–327.

2. D Ruede, "Airborne Molecular Contamination: The Good, the Bad and the Ugly" (paper presented at Interface 2003, Arch Chemicals Microlithography Symposium, San Diego, September 21–23, 2003).

3. D Ruede, M Ercken, and T Borgers, "The Impact of Airborne Molecular Bases on DUV Photoresists," Solid State Technology 44 (2001): 63–70.

4. T Bloomstein et al., "Contamination Rates of Optical Surfaces at 157 nm: Impurities Outgassed from Construction Materials and from Photoresists," in Proceedings of Optical Microlithography XVI (Bellingham, WA: SPIE, 2003), 650–661.

5. A Grayfer, O Kishkovich, and D Ruede, "Protecting DUV Optics from Airborne Molecular Contamination," Microlithography World 11, no. 1 (2002): 20–24.

6. D Kinkead, A Grayfer, and OP Kishkovich, "Prevention of Optics and Resist Contamination in 300nm Lithography: Improvements in Chemical Air Filtration," in Proceedings of Metrology, Inspection, and Process Control for Microlithography XV (Bellingham, WA: SPIE, 2001), 739–752.

7. JS Hudzik, OP Kishkovich, and JK Higley, "Molecular Contamination Control in Photomask/Reticle Manufacturing Using Chemically Amplified Resists (CAR)—Lessons from Wafer Lithography," in Proceedings of Photomask Japan (Bellingham, WA: SPIE, 2002), 260.

8. OP Kishkovich et al., "Real-Time Methodologies for Monitoring Airborne Molecular Contamination in Modern DUV Photolithography Facilities," in Proceedings of Metrology, Inspection, and Process Control for Microlithography XIII (Bellingham, WA: SPIE, 2001), 348–376.

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Mats Ekberg, PhD, is manager of the physics group in the core development department at Micronic Laser Systems (Täby, Sweden), where he focuses on optics and diffractive optical elements (DOEs). He also works in the areas of resist processing, proximity compensation methods, and AMC. Previously, he was with ABB Corporate Research, where he worked with photoconductivity, DOEs, and high-power lasers. Ekberg has authored or coauthored approximately 20 scientific papers and holds more than 30 Swedish and international patents. He is a member of SPIE and the European Optical Society. He attended Chalmers University of Technology in Göteborg, Sweden, where he received a degree in engineering physics in 1987 and a PhD based on work in design, analysis, and manufacture of computer-generated phase holograms and DOEs manufactured by E-beam and laser lithography. (Ekberg can be reached at +46 8 6385477 or mats.ekberg@micronic.se.)

Per-Uno Skotte is involved with R&D design and the practical applications and installation of pneumatic systems at Micronic Laser Systems. He joined the company in 1995. He began his career working for the Swedish defense department in the area of information-secure computer systems. (Skotte can be reached at +46 8 6385200 or peruno.skotte@ micronic.se.)

Tomas Utterbäck is a senior technology specialist at Micronic Laser Systems. He joined the company in 1990 to build and manage the mechanics group in the core development department. Previously, he was a consultant. In 1987 he received an MS in engineering, with specialization in applied mechanics, from the Royal Institute of Technology in Stockholm. (Utterbäck can be reached at +46 8 6385262 or tomas.utterback@micronic.se.)

Oleg Kishkovich, PhD, is technology director and principal scientist at Extraction Systems (Franklin, MA), where he pursues method development for molecular contamination monitoring and product analysis for chemical air filtration of DUV lithography environments. He has expertise in chemical kinetics, atmospheric chemistry, trace-gas analysis, and magnetic resonance techniques. Kishkovich has presented papers at international conferences on the subject of molecular contamination measurement and control in ultraclean environments and has published papers on real-time monitoring, measurement, and control of molecular contamination. He received an MS in engineering physics from the Moscow Institute of Physics and Technology and a PhD in chemical and molecular physics from the Moscow Institute of Chemical Physics. (Kishkovich can be reached at 508/553-3900, ext. 16, or okishkovich@extraction.com.)

David Ruede is the filter group manager at Extraction Systems. Previously, he was the company's senior manager of OEM sales. Before joining the company, he had acquired more than 15 years of semiconductor industry experience, including in positions at major microlithography and process equipment firms. He received BS and MS degrees in chemistry from Central Connecticut State University in New Britain. (Ruede can be reached at 508/553-3900, ext. 12, or druede@extraction.com.)

John Higley is vp of sales and marketing at Extraction Systems. With more than 19 years of experience in the field of environmental products and services, he has focused on molecular contamination control for the microelectronics industry. Prior to joining Extraction, he was senior vp of GTI. He was a contributing author of the International Sematech document, "Forecast of Airborne Molecular Contamination Limits for the 0.25 Micron High Performance Logic Process." He has also published many papers and lectured extensively. He is a member of the SEMI Standards Task Force on Classification of Molecular Contamination and a senior member of the Institute of Environmental Sciences and Technology. He received a BA in political science from Denison University in Granville, OH. (Higley can be reached at 508/553-3900, ext. 23, or jhigley@extraction.com.)

Vladimir L. Orkin, PhD, is a research chemist at the National Institute of Standards and Technology (NIST; Gaithersburg, MD) and a member of the NASA panel for kinetic and photochemical data evaluation. Since 1987, he has directed a research group at the Institute of Energy Problems of Chemical Physics, Russian Academy of Sciences (RAS), in the area of photochemical properties of industrial chemicals. His research interests are gas kinetics, photochemistry and its technical applications, atmospheric chemistry, and photochemical contamination of lithography optics. Orkin has published papers in international scientific journals and is a frequent presenter at technical conferences. He received an MS from the Moscow Institute of Physics and Technology and a PhD in chemical physics from the Institute of Chemical Physics, RAS, also in Moscow. (Orkin can be reached at vladimir.orkin@nist.gov or 301/975-4418.)