Research from the MSELs Polymers Division was prominently highlighted with the release of the new American Chemical Society (ACS) journal, ACS Nano, which focuses on nanostructures, nanobiotechnology, nanofabrication, methods and tools for nanoscience and nanotechnology, and self- and directed-assembly. The first volume contains an article by Polymers Division researchers* who describe how simple laser diffraction experiments can be used to perceive the levels of residual stress in patterns fabricated by nanoimprint lithography. In this article, the authors discuss ways to measure and control these induced stresses, thereby influencing the stability of these nanoscale structures. Nanoimprint lithography is a high-resolution stamping process where nanoscale patterns in a hard mold, e.g., Si, quartz, or some other hard materials, with nanoscale features, typically fabricated by e-beam lithography, focused ion beam milling or deep UV optical lithography, is pressed into a softer polymeric resist film to leave an impression of the patterns in the mold. These patterns can then be transferred or templated with conventional subtractive or additive nanofabrication processes. Within the community, nanoimprint is quickly becoming the leading next generation lithography because of its fine patterning resolution, low tool cost compared to alternatives with comparable resolution, and high throughput.

Also highlighted in this first volume of ACS Nano is related Polymers Division research on the nanoimprint patterning of semicrystalline poly(ethylene oxide) (PEO) films. The cover art for the second issue features an atomic force microscopy (AFM) image showing the unique morphology of a poly(ethylene oxide) (PEO) spherulite on an imprint patterning substrate where the underlying topology guides the crystal growth.

For further information, see the article or the cover art in ACS Nano.

*Yifu Ding, Hyun Wook Ro, Thomas A. Germer, Jack F. Douglas, Brian C. Okerberg, Alamgir Karim, and Christopher L. Soles, ACS Nano, 1, 84 (2007).

CONTACT: Christopher Soles (MSEL), ext. 8087

One of the major lithographic challenges in manufacturing next generation integrated circuits is to measure of line edge roughness (LER) and linewidth roughness (LWR) as the feature size continues to shrink. Methods capable of measuring pattern shape and size at a nanometer resolution will be needed in the near future, for example, at a feature size of 45 nm the targeted resolution for linewidth roughness metrology is 1.4 nm (3sigma) with a tool precision of 0.28 nm. The prevalent measurement techniques, including scanning electron microscopy (SEM), electrical critical dimension, and atomic force microscopy (AFM) all face challenges in this application of measuring LER/LWR. A transmission small angle x-ray scattering (SAXS) technique has been developed and applied successfully by researchers in MSELs Polymers Division to determine the critical dimensions, including average linewidth, pitch, line height and sidewall angle, in test line gratings with a nanometer resolution (CD-SAXS technique).

Recently, in collaboration with researchers from Intel and the Advanced Photon Source, the Polymers Division researchers have extended the capabilities of their CD-SAXS technique to quantify the LER and LWR. As described in a recent Journal of Applied Physics article, model line grating patterns with controlled LER and LWR were measured using the x ray technique, and their results analyzed with model calculations. The magnitude of LER/LWR deduced from x-ray results (approximately 3 nm) compared favorably with the scanning electron microscopy results obtained from the same samples. An apparent Debye-Waller factor, which can be deduced from the SAXS data without any detailed model-based calculations, was found to be a convenient parameter to quantify the amplitude of LER/LWR.

For further information see the article in Journal of Applied Physics (volume 102, Art. No. 024901, July 15, 2007).

CONTACT: Wen-li Wu (MSEL), ext. 6839

On September 26-28, 2007, MSEL and PL held the third NASA-NIST Workshop on Carbon Nanotube Measurements in Gaithersburg, Maryland along with colleagues from NASA JSC and NASA Langley. Over 70 leading experts - including representatives from Asia, South America and Europe - convened to discuss forefront efforts in the areas of dispersion, purity, and characterization of carbon nanotubes. Recent rapid advances in the development of purification methods to produce high quality carbon nanotubes and the increasingly sophisticated methods to characterize them were presented in lectures and a poster session. In breakout sessions, discussions focused on environmental, health and safety concerns that are driving the need for high quality, well characterized carbon nanotube materials. Community feedback on efforts to develop both documentary standards and standard reference materials provided important input to inform future NIST and NASA activities.

The goal of the NASA-NIST Workshop Series on Carbon Nanotubes is to focus the community's attention on the issues of carbon nanotube quality and characterization. The output of the second meeting formed the basis for the current ISO documentary standards activities in single wall carbon nanotubes.

For more information please visit the workshop website at: http://polymers.nist.gov/Nanotube3/Workshop3.htm

CONTACTS: Kalman Migler (MSEL), ext. 4876, Stephanie Hooker (MSEL), ext. 4326 (Boulder), Angela Hight Walker (PL), ext. 2155

In a forthcoming issue of Macromolecules,[1] MSEL and SEMATECH researchers provide significant insight into the effect of photoacid generator (PAG) size on the deprotection reaction-diffusion front in a model photoresist polymer, by measuring the spatial extent of the deprotection reaction front with nanometer resolution using neutron reflectivity.

In the microelectronics industry, control over the kinetics and spatial extent of the reaction-diffusion processes is necessary to produce functional devices with nanometer scale feature sizes. In current 193 nm photolithography, high resolution imaging is performed by converting an optical image into a chemical latent image within a photoresist via the reaction-diffusion process involving a photoinitiated acid catalyst. The chemical latent image is then developed with an aqueous base solution to produce the final pattern. These imaging materials are commonly known as chemically amplified photoresists because each photo-generated acid can catalyze several hundred reactions as it diffuses through the photoresist material during a post-exposure bake; the effect of a single photon absorption is amplified.

The direct measurement of the reaction-diffusion profile in these experiments provides the insight needed to extend predictive models as well as to select advantageous aspects of this lithographic component. These fundamental measurements are also applicable to improved general understanding of reaction-diffusion processes for applications outside of photolithography for microelectronics fabrication.

For further information, see the article in Macromolecules or visit the Polymers Division website at www.nist.gov/polymers.

[1] Macromolecules 40(5), 1497-1503 (2007)

CONTACT: Vivek M. Prabhu, ext. 3657

Atomic force microscopy (AFM)measurements of directed-assembly of ferromagnetic nanoparticles into one-dimensional mesostructures (1-D)was recently reported[1]. These measurements were enabled using a Fossilized Liquid Assembly (FLA) platform reported previously[2] for characterizing interaction between nanoparticles with minimal perturbation. In this process, the position of nanoparticles chains were instantaneously fixed at the oil interface through photopolymerization. The polymer-coated[3] ferromagnetic colloids (19 nm, 24 nm) were assembled at a crosslinkable oil-water interface under both 8 mT applied magnetic field and zero-field conditions and permanently fixed into 1-D mesoscopic polymer chains of length varying between (1 to 9) micrometers. Using the FLA methodology, the different conditions were systematically investigated and demonstrated that dispersed ferromagnetic colloids possess sufficient dipolar interactions to organize into mesoscopic assemblies with up to 400 nanoparticles. Application of the external magnetic field during assembly enabled the formation of linear micron-sized chains which were aligned in the direction of the applied field. The process was mostly reversible under zero-field conditions. In addition to potentially enabling quantification of dipolar interactions of magnetic nanoparticles important for biological applications such as targeted cancer therapy, this universal methodology is an attractive alternative technique to current practice of use of cryogenic transmission electron microscopy (cryo-TEM) for the visualization of nanoparticle assembly in dispersed organic media.

1 Journal of American Chemical Society, 129(19), 6291 -6297 (2007).

2 "Fossilized Liquid Assembly: Nanomaterials Research Tool", NIST Tech Beat, Oct 12, 2006 ( http://www.nist.gov/public_affairs/techbeat/tb2006_1012.htm ).

3 Materials synthesis collaboration with Jeff Pyun at Chemistry Department, U. Arizona.

CONTACT: Alamgir Karim (MSEL), ext. 6588

From a butterfly,s iridescent wing to a gecko's sticky foot, nature derives extraordinary properties from ordinary materials like wax and keratin. Its secret is hierarchical topology: macroscale structures assembled from microscale components of varying sizes. Borrowing a page from nature's playbook, researchers at the National Institute of Standards and Technology (NIST) have developed a novel platform for the self-assembly of experimental hierarchical surfaces in a fluid. Their work offers diverse industries a new way to generate and measure self-assembly at the nano-scale.

A butterfly's wings shimmer because light plays upon tiny rows of scales, like tiles on a Spanish roof. The gecko sticks to surfaces because its feet are patterned with microscopic hairs, each hair tipped with hundreds of even tinier projections. Beads of water roll off the lotus's leaf because its surface is streaked with microscopic peaks, each with a finer structure, that makes the surface "super hydrophobic." These enhanced properties-other possibilities include super adhesion and low friction-have attracted the attention of design engineers for applications from bioengineered tissues to photonic crystals to submarines that slice through water with minimal drag.

Creating these topologically complex, self-assembled surfaces for study has been a challenge. If the components are mixed on a surface, that substrate affects how they assemble; if mixed in a solvent and dried, the drying process similarly distorts the results. In a recent paper*, the NIST team detailed a much simpler and faster system they dubbed "fossilized liquid assembly" to create experimental models of hierarchical topologies in which the components are allowed to mix and assemble freely in a fluid, and then quickly "frozen" in place for study. The key is the use of solutions of water and a special monomer that polymerizes-links together-when exposed to ultraviolet light. Like an oil-water mixture, the fluid forms liquid interfaces that can be manipulated to create a desired hierarchical structure and then suddenly solidified with a burst of UV light.

Lead researcher and physicist Alamgir Karim estimates that it takes about five minutes to make a sample of self-assembling particles using NIST's approach. Other methods, he notes, not only are more complicated and costly, but also do not allow the structures to form as freely. With the new technique, engineers also will be able to build complex dynamic structures and freeze them into solid form, studying self-assembly under the microscope. "How do you take a snapshot of shampoo in action?" asks physicist Jason Benkoski, first author of the paper. "We can now directly observe these small, mobile, delicate structures."

The researchers also are using the new platform to better understand the fundamental physics behind the formation of hierarchical topology, studying, for example how different forces dominate at different scales of length. Looking ahead, the NIST team plans to build on this study, expanding the technology as a 3D imaging platform.

The work was supported by NIST and a National Research Council Fellowship.

* J.J. Benkoski, H. Hu, and A. Karim. Generation of hierarchical topologies from photocrosslinkable, particle-stabilized emulsions. Macromolecular Rapid Communications. Aug. 2, 2006

Surface chemical heterogeneities can govern the performance and behavior of substrate supported polymer films. From a quality control standpoint, substrate chemical heterogeneities can be defects that drive failure in polymer film products like barrier coatings, electronics packaging, and resist systems. Alternatively, patterned substrate chemical heterogeneities can template overlying film structures for micro- and nano-manufacturing aims. In response to both of these themes, researchers in MSEL's NIST Combinatorial Methods Center (NCMC) located in the Polymers Division have produced a breakthrough method for characterizing the effect of surface chemical heterogeneities on the performance of thin film polymer materials and devices. This new combinatorial platform measures a key, yet under-evaluated, factor in thin film behavior: the "strength" of substrate chemical heterogeneities. The NCMC approach merges soft-lithography and gradient techniques to fabricate a micropatterned library with gradually diminishing chemical contrast. In particular, the test substrate includes a pattern of micrometer-scale lines that exhibits a continuous gradient in surface energy differences against a constant surface energy matrix. The library design includes a means to calibrate the surface energy contrast along the micropattern, e.g., through contact angle measurements. Accordingly, such libraries afford an unparalleled means for quantifying critical phenomenon and regimes of behavior that depend on this critical parameter.

The NCMC team recently demonstrated the power and utility of these combinatorial substrates in a forthcoming article in Soft Matter (January 2007). As a testbed, the team examined surface-heterogeneity driven dewetting of polymer thin films. A polystyrene film was deposited on the gradient micropattern library, and then thermally treated to induce dewetting; subsequently, automated optical microscopy was used to track the dewetted morphology along the test library. This single experiment provided a systematic evaluation of film dewetting over a huge range of surface energy contrast, and showed critical surface energy conditions associated with this behavior. In particular, the gradient library illuminated the minimum chemical heterogeneity strength to induce film failure. Additionally, this combinatorial approach identified substrate pattern conditions needed to produce highly ordered arrays of polymer microdroplets. This library design also has the potential for examining a huge range of surface-driven phenomena, including polymer microphase separation, nanoparticle assembly, adhesion, and cell behavior.

CONTACT: Michael Fasolka (MSEL), ext. 8526

The First International Symposium on Polymer Materials Science was held on December 7 and 8, 2006, at the Polymer Science and Engineering Department of the Kyoto Institute of Technology, Japan between representatives from MSEL's Polymers Division and polymer research institutions in Japan. The research participants from Japan had close prior connections with the Polymers Division, either as guest scientists or as post-doctoral researchers during their scientific careers. The focus of the meeting was to enable the NIST-affiliates and Polymers Division researchers to reinforce existing and identify new collaborative areas that will enrich the polymers research programs of the participating institutions. The symposium talks focused on the latest measurements and advanced research in polymer science including nanoparticles and block copolymer nanotechnology, biopolymeric and combinatorial soft materials science, and nanoimprinting, in addition to classical areas of polymer science such as polymer blends, gels, and nanocomposites. A well-attended poster session also gave an excellent opportunity for post-doctoral researchers and students to present and discuss their key findings.

Key symposium organizers included Alamgir Karim from the Polymers Division, NIST and Qui Tran-Cong from the Department of Polymer Science and Engineering of Kyoto Institute of Technology, Japan, assisted by an international and local organizing committee.

MSEL plans to reciprocate by organizing the Second International Symposium on Polymer Materials Science to be held at NIST.

CONTACT: Alamgir Karim (MSEL), ext. 6588

January 7-12, 2007 marked the biennial event of the Polymers West Gordon Research Conference (www.grc.org) in Ventura, CA. This conference assembles leading polymer scientists from throughout the world for an open exchange of new and exciting polymers related research. This year, researchers from NIST's Polymers Division received several recognitions. Christopher Soles was invited by the organizers to speak on "Stresses and Viscoelastic Effects in Nanoimprint Lithography Patterning." In addition, NIST-NRC Postdoctoral Fellow Derek Patton's contribution, "Statistical Copolymer Brush Composition Gradients via Microchannel Confined Surface-Initiated Photopolymerization," was selected as the Most Outstanding Poster Presentation of the Conference. For this first time this year, an equally competitive Graduate Research Symposium (GRS) for graduate students and postdoctoral associates was held in conjunction with the conference. Yifu Ding, a NIST associate was selected to give one of the competitive invited talks on "Polymer Viscoelasticity and Residual Stress Effects on Nanoimprint Lithography." Rounding out the recognition was NIST-NRC Postdoctoral Fellow Brian Okerberg's contribution, "Crystallization of Poly(ethylene oxide)Patterned by Nanoimprint Lithography," which received the Most Outstanding Poster Presentation at the GRS.

CONTACT: Chad R. Snyder (MSEL), ext. 4526

The NIST Combinatorial Methods Center (NCMC) conducted NCMC-11 Complex Interfaces: Library Design and Performance Measurements at NIST's Gaithersburg, Maryland campus on April 30 and May 1. Chris Stafford (Technical Lead) and Carol Laumeier (Outreach Coordinator) of the Polymers Division organized the Workshop, which brought together more than 88 attendees, including 31 NCMC industry members, 6 academic representatives, and NIST staff and invited guests, to consider key measurement advances and needs for the high-throughput assessment of the structure and performance of complex materials interfaces.

The event included keynote lectures from top experts in mechanics, surfaces, and interfaces, including Nicholas Colaneri (AZ State Flexible Display Center), Teng Li (U. of MD), Krystyn Van Vliet (MIT), David Dillard (Virginia Tech), Zhiqun Lin (Iowa State), and Robert Cook (Ceramics Division, NIST). In addition, a Special Topic Lecture on challenges in combi data management was presented by NCMC member, Michael Smith (Symyx).

NCMC-11 technical sessions examined emerging technology applications that depend on robust complex materials interfaces, and measurement and modeling approaches that can enable high-throughput mechanical testing of such systems, including nanoindentation, buckling methods, parallel fracture techniques, and new combinatorial library fabrication routes. The second day of the Workshop focused on combi-related research efforts in MSEL, and featured a tour of 12 laboratory demonstrations in the NCMC facilities.

NCMC-11 is the latest in the biannual Workshop series conducted by the NCMC as part of its outreach to industry. The next NCMC Workshop will be held November 1-2, 2007, at NIST's Gaithersburg, Maryland campus.

For more information on the NIST Combinatorial Methods Center, please visit the NCMC Web site at www.nist.gov/combi.

CONTACT: Michael Fasolka (MSEL), ext. 8526 or Carol Laumeier (MSEL), ext. 6093

MSEL's Polymers Division showed its continued leadership in quantitative polymer mass spectrometry at the 2007 ASMS in Indianapolis the week of June 4th.

Charles Guttman presented a talk entitled "An Approach To The Quantitative Determination Of Polymer Molecular Mass Distribution By MALDI-TOF-Mass Spectrometry" on the Group's creation of SRMTM 2881 an absolute molecular mass distribution polymer standard. He also presided over a meeting on international polymer standards under the auspices of VAMAS (The Versailles project on Advanced Materials And Standards). At this meeting the protocols for an interlaboratory comparison on mixtures of poly(ethylene oxide)s with different end groups was finalized. Also, the prospects for an affirmative vote on an International Organization for Standardization (ISO) standard for polystyrene mass spectrometry were discussed. This standard, developed by the Polymers Division, has already been accepted by the American Society for Testing and Materials (ASTM) and the Deutsches Institut für Normung (DIN). An affirmative vote by ISO seems likely.

William Wallace gave a presentation entitled "High Temperature MALDI: Mass Spectrometry of Polyethylene as a Function of Sample Temperature" on his recent work developing a method for analyzing polyolefins, polymers that are notoriously difficult to analyze by MS and yet of great commercial importance. He also participated in the ASMS Measurements and Standards Committee meeting where topics ranging from modernization of mass spectrometric nomenclature to data format structures for information exchange were discussed with an eye toward finalizing ASMS endorsements of various proposed standards.

Eun Su Park gave a posted entitled "Degradation of Polymeric Ballistic Materials" with co-authors Kathleen M. Flynn, Gale A. Holmes, Charles M. Guttman, and William E. Wallace, which proved to be very timely as mass spectrometry is increasingly taking center stage in forensics and homeland security matters.

Lastly, Polymers Division alumnus Mark Arnould (Xerox, Rochester, NY) ran the Polymeric Materials Interest Group Meeting which had over 100 attendees.

CONTACT: William Wallace, ext. 5886 (MSEL)

Michael J. Fasolka, Kathryn L. Beers, Christopher M. Stafford, Alamgir Karim, and Eric J. Amis were awarded the Department of Commerce's Silver Medal for Customer Service at the 57th Annual Awards Ceremony for their excellence in transferring NIST-developed technologies to industry, for technical leadership in combinatorial and high-throughput measurement methods for materials research, and for a seminal role in forming a new technical community advancing these revolutionary techniques into the scientific mainstream.

In 2002, the team created the NIST Combinatorial Methods Center (NCMC). Their vigorous research program developed cost-effective techniques and established its researchers as recognized world leaders in combi metrology. The NCMC was designed around a new collaborative legal agreement precluding exchange of proprietary information and assuring research results would be made public immediately, while allowing companies to join with minimum paperwork and delay. NCMC members were invited to assist in developing the research plan as in a typical consortium; however, the NCMC always focused on immediate distribution of research and promotion of the methods. Through bi-annual workshops and extensive web resources (www.nist.gov/combi), members access publications, presentations, instrument designs, device software, and rapid analysis tools. Specific projects on critical problems were launched with cooperation from leading companies.

More information on this event

In a recent Applied Physics Letter* article, MSEL researchers in the Polymers and Materials Reliability Divisions illustrate how Brillouin light scattering (BLS) can be used as a non-contact metrology to characterize the acoustic modes, and thus the elastic properties, of polymeric nanostructures. Specifically, the acoustic modes were characterized in parallel line-space grating patterns with a nominally rectangular cross section. In addition to the typical surface acoustic modes observed in thin polymer films, the phonon spectra from these nanolines display a new mode, lower frequency the Rayleigh surface modes. The dispersion of this new sub-Rayleigh mode was found to scale with the width of the nanoline. Using classic wave theory and finite element analysis, this sub-Rayleigh mode was identified as flexural vibration along the length of the nanolines, except with the bottom of the line fixed by the rigid substrate. An analysis of the phonon spectra for line widths down to 88 nm did not reveal deviations or anisotropies in the elastic constants from their bulk values. However, with this metrology the framework now exist for characterizing size dependent deviations and/or anisotropies in the elastic properties of even smaller nanostructures.

Fabricating mechanically robust polymer structures with nanoscale dimensions is critical for a wide range of emerging technologies, with applications in microelectronics, optical communications and photonics, nano-electro-mechanical systems (NEMS), nanofluidic devices, bio-medical application, etc. However, the rigidity and subsequent stability of these structures is expected to change as the feature sizes approach the characteristic dimensions of the macromolecules. There is uncertainty in how far structural miniaturization schemes can progress before size-dependent material properties compromise performance. Unfortunately there are very few experimental techniques to quantify the mechanical properties of such nanostructured materials. This measurement technique is being developed to address this need.

*R.D. Hartschuh, A. Kisliuk, V. Novikov, A.P. Sokolov, P.R. Heyliger, C.M. Flannery, W.L. Johnson, C.L. Soles, and W. Wu, Appl. Phys. Lett. 87 (2005) 173121.

CONTACT: Christopher Soles, ext. 8087

In a recent issue of Advanced Materials, researchers from MSEL's Polymers and Ceramics Divisions and the University of California at Berkeley reported success in using a nondestructive measurement method to detail three structural properties crucial to making reliable electronic devices with thin films of organic semiconductors. The new capability could help industry clear hurdles responsible for high manufacturing development costs that stand in the way of widespread commercial application of the materials.

Using near-edge x-ray absorption fine-structure spectroscopy (NEXAFS), the team tracked chemical reactions, molecular reordering, and defect formation over a range of processing temperatures. They then evaluated how process-induced changes in thin-film composition and structure affected the movement of charge carriers in organic field effect transistors, devices basic to electronic circuits. With NEXAFS measurements taken over the range from room temperature to 300 °C, the team monitored the conversion of a precursor chemical to an oligothiophene, an organic semiconductor. The molecular organization and composition achieved at 250 °C yielded the highest levels of charge carrier movement and, consequently, maximum electric-current flow. As chemical conversion progressed, the researchers calculated how the molecules arranged themselves on top of an electrical insulator. Top transistor performance corresponded to a vertical alignment of molecules. In addition, they used NEXAFS to determine the angles of chemical bonds and to assess the thickness and uniformity of film coverage, also critical to performance.

For further information see the article in Advanced Materials 17(19), 2340-2344 (2005).

CONTACT: Dean DeLongchamp, ext. 5599