DOI: 10.1002/adfm.200701402 Nanoimprint Lithography and the Role of Viscoelasticity in the Generation of Residual Stress in Model Polystyrene Patterns** By Yifu Ding, Hyun Wook Ro, Kyle J. Alvine, Brian C. Okerberg, Jing Zhou, Jack F. Douglas, Alamgir Karim, and Christopher L. Soles* 1. Introduction The contact printing of polymers using a hard mold is a rapidly developing technique with substantial promise. [1–15] In addition to the potentially low fabrication costs and high throughput, this technique, known as nanoimprint lithography (NIL), can overcome the patterning resolution imposed by the diffraction limits in both traditional and immersion photolithographic methods. The features on the prepatterned mold can be replicated onto the resist materials via two different mechanisms: thermal embossing [1–6,14,15] and UV-initiated crosslinking. [7–15] Thermal-embossing NIL has attracted broad interest because of its capability to directly pattern functional materials for various applications. Com- pared with indirect patterning of a resist, where subsequent additive and subtractive processes are required, this direct patterning can minimize the required number of processing steps. However, the stability of these ‘‘as-imprinted’’ structures then becomes a central concern in the development of this process. [16,17] Although low temperature patterning of glassy polymers has been demonstrated, most thermal-embossing NIL is performed at temperatures above the glass-transition tem- perature (T g ) of the polymer. [18] The patterning resolution is determined by the viscoelastic deformation of thin polymer films under applied pressure. It is well known that the molar mass affects many of the physical properties of a polymer, especially rheological ones. The potential relevance of this variable for the NIL process both during and after pattern replication should thus be apparent. Specifically, a lower molar mass polymer having lower shear viscosity will tend to fill the mold more readily under given imprinting condi- tions. [19] However, a larger molar mass polymer offers better cohesive strength, which is crucial for fabricating stable ultrasmall or high-aspect-ratio structures. [20] Moreover, a relatively low molar mass polymer can also be more prone to dewetting from the substrate and/or the mold than higher molar mass polymers simply for kinetic reasons. Clearly, a proper selection of the polymer molar mass is important to optimizing the NIL process and achieving stable and faithful pattern replication. FULL PAPER [*] Dr. C. L. Soles, Dr. Y. Ding, Dr. H. W. Ro, Dr. K. J. Alvine Dr. B. C. Okerberg, Dr. J. Zhou, Dr. J. F. Douglas, Dr. A. Karim Polymers Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899-8541 (USA) E-mail: csoles@nist.gov [**] This work is partially funded by the NIST Office of Microelectronic Programs. K. J. Alvine and B. C. Okerberg acknowledge the support of the National Research Council NIST Postdoctoral Fellowship Pro- gram. We acknowledge the nanofabrication laboratory of the Center for Nanoscale Science and Technology (CNST) at NIST for providing facilities for the nanoimprint process. This work is an official contri- bution of the National Institute of Standards and Technology; not subject to copyright in the United States. Certain commercial materials and equipment are identified in this paper in order to specify adequately the experimental procedure. In no case does such identi- fication imply recommendation by the National Institute of Standards and Technology nor does it imply that the material or equipment identified is necessarily the best available for this purpose. The error bars presented throughout this manuscript indicate the relative standard uncertainty of the measurement. Understanding polymer deformation during the nanoimprinting process is key to achieving robust polymer nanostructures. Information regarding this process can be extracted from monitoring the decay of the imprinted polymer patterns during thermal annealing. In the present work, the effect of both the molar mass and the imprinting temperature on the pattern decay behavior during thermal annealing is investigated. Previously, it was found that the decay rate is fastest for a highly entangled polymer due to the elastic recovery caused by the residual stress created during the imprinting process. The present paper demonstrates that this residual stress level can be modified through control of the imprinting temperature. These results are contrasted with those for an unentangled polymer over a similar range of imprinting temperatures, where it is found that the pattern decay is controlled by simple Newtonian flow. In particular, the pattern decay is well described by surface-tension-driven viscous flow, and no imprinting-temperature effect is observed during thermal annealing. It is shown that the stability of the film against pattern decay can be optimized for moderately entangled polymer films. This effect is attributed to the competition between the effect of increased viscosity with increasing molar mass and increased residual stresses with entanglements. These observations provide guidance for the optimization of imprinting process in terms of selection of molar mass and processing temperatures. 1854 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2008, 18, 1854–1862