Real-Time Shape Evolution of Nanoimprinted Polymer Structures during Thermal Annealing Ronald L. Jones,* ,†,§ Tengjiao Hu, † Christopher L. Soles, † Eric K. Lin, † Ronald M. Reano, ‡ Stella W. Pang, ‡ and Diego M. Casa | Polymers DiVision, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, Department of Electrical Engineering and Computer Science, UniVersity of Michigan, Ann Arbor, Michigan 48109, and CMC-CAT, AdVanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439 Received May 12, 2006 ABSTRACT The real-time shape evolution of nanoimprinted polymer patterns is measured as a function of annealing time and temperature using critical dimension small-angle X-ray scattering (CD-SAXS). Periodicity, line width, line height, and sidewall angle are reported with nanometer resolution for parallel line/space patterns in poly(methyl methacrylate) (PMMA) both below and above the bulk glass transition temperature (T G ). Heating these patterns below T G does not produce significant thermal expansion, at least to within the resolution of the measurement. However, above T G the fast rate of loss in pattern size at early times transitions to a reduced rate in longer time regimes. The time-dependent rate of polymer flow from the pattern into the underlying layer, termed pattern “melting”, is consistent with a model of elastic recovery from stresses induced by the molding process. Nanoimprint lithography (NIL) is a low-cost, effective nanofabrication tool for patterning arbitrary structures with critical dimensions (CD) well below 100 nm. 1,2 To date, novel patterns with feature sizes less than 5 nm have been demonstrated. 3,4 The maturation of NIL as a pattering technology is reflected by its inclusion as a potential next- generation lithography in the International Technology Roadmap for Semiconductors. 5 Looking beyond semicon- ductors, the potential of NIL to pattern polymers of arbitrary chemistry and/or architecture promises to provide functional patterns (e.g., patterns that perform an electrical, optical, mechanical, structural, biomedical, etc. task) that can be built directly into nanometer-scale devices. This is generally not possible with state-of-the-art optical lithographies where the polymeric patterns are used as a sacrificial resist. However, the end use of a polymer nanostructure as a device, such as in micro- or nanofluidic channels, organic semiconductors, sensors, photonic devices, and so forth, requires a broad understanding of pattern stability outside of the mold (i.e., “free-standing”), both over time and with varying environ- mental conditions. Pattern stability, for example, is expected to result from a balance of viscosity, internal stresses from the molding process, and surface tension. For feature sizes ,100 nm, stability is anticipated to be a significant issue in light of the reductions in viscosity or glass transition observed both at polymer surfaces and in ultrathin films. 6-9 Under- standing the relative roles of pattern size, shape, environ- mental conditions, and bulk material properties requires precise measurements of shape in polymeric NIL patterns as a function of processing parameters. Here we present data on pattern stability (after removal of the mold) as a function of annealing temperature, both below and above the bulk glass transition temperature (T G ), for parallel line/space patterns imprinted into PMMA. Stability is quantified by measuring the rate of change in the average cross-sectional shape of the lines. Real-time pattern shape is obtained using critical dimension small-angle X-ray scattering (CD-SAXS). 10,11 CD-SAXS is capable of measuring the cross section of periodic patterns ranging from 10 to 500 nm in width; the width of the patterns here was ∼100 nm. The CD-SAXS measurements are nondestructive, with subnanometer precision, and capable of quantifying lines directly supported on a hard Si substrate. We note that electron microscopy techniques, which are also capable of similar studies, are often destructive to polymer films and make a study of stability, especially at high temperatures, challenging. This study is conceptually similar to those of * Corresponding author. E-mail: ronald.jones@nist.gov. † National Institute of Standards and Technology. ‡ University of Michigan. § Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States. | Argonne National Laboratory. NANO LETTERS 2006 Vol. 6, No. 8 1723-1728 10.1021/nl061086i CCC: $33.50 © 2006 American Chemical Society Published on Web 07/18/2006