The Mechanics of Nanoimprint Forming
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The Mechanics of Nanoimprint Forming Graham L. W. Cross, Richard M. Langford, Barry S. O’Connell, and John B. Pethica, SFI Trinity Nanoscience Laboratory, Trinity College, Dublin 2, Ireland ABSTRACT Nanoimprint and a number of other related techniques are a collection of surface patterning technologies that involve direct contact of a master template with the target surface. As such, they are governed by the laws of contacting bodies, and the mechanics involved can readily be investigated by existing indentation methods or close variants thereof. Among the many demonstrated applications of nanoimprint, lithographic resist processing has generated considerable interest due to its combination of high resolution with rapid throughput over wide areas. Pattern transfer can be achieved by the application of heat and pressure to the stamp (hot embossing), or solely by the generation of shear stress at the contact (cold forming.) In both cases we have found that elastic and viscoplastic strains are present during the forming process, the former of which can considerably alter the characteristics of the pattern transfer. The use of depth sensing instrumented indentation in conjunction with specially designed stamps and a variety of microscopy techniques has allowed us to isolate, control, and measure many of the stresses and strains directly during the imprint process. Further, in a more standard role, the indenter can be used to characterize the mechanical properties of imprinted structures. In this paper we summarize our experimental findings and conclusions on the role of important factors influencing the fidelity of the imprint process including elastic stresses, plastic deformation mechanisms, complexities in the confined deformation rheology, and choices in the form of applied stress. These are illustrated by a series of idealized experiments ranging from the squeeze flow of prepared coupons to the flat punch indentation of thin films and back extrusion into isolated cavities. A connection between these more localized experiments and the established findings and requirements of applications such as wide area lithography and functional polymer patterning will be made to establish the concept of ”instrumented imprint”. INTRODUCTION Nanoimprint [1] is a mechanical forming technique capable of producing high resolution, high fidelity patterns over wide areas of surfaces and thin films. While originally introduced as an efficient way of patterning lithographic resist for semiconductor processing, its role has been expanded to include patterning of a wide variety of materials for various applications. Mechanical replication has roots in the form of minting and coining processes in metals [2, 3] as well as in the history of sample mimicking down to the finest nanometer scale minutiae by transmission electron microscopy (TEM) replication techniques [4]. As in any forming process, nanoimprint relies on the generation of specific shear stresses in the target sample to induce the plastic flow necessary for accurate produc
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