Current Nanoscience, 2010, 6, 1-11 1 1573-4137/10 $55.00+.00 © 2010 Bentham Science Publishers Ltd. Reliability and Fabrication of Molds for Nanoimprinting R. Scaffaro 1,2,* , E. Shim 2 and H.T. Hahn 2 1 Department of Chemical Engineering and Materials, University of Palermo, Viale delle Scienze, Ed. 6, 90128 Palermo, Italy 2 Multifunctional Composite Laboratory, University of California, Los Angeles, 420 Westwood Plaza, 90095 Los Angeles, CA, USA Abstract: During the last decade there has been a growing attention to nanoscaled materials and to the related technologies to produce them. The problems to overcome in the manufacturing of these kinds of items increase dramatically on decreasing the dimension of the devices. In this sense, the scientific research has been strongly stimulated to try to improve and optimize all the critical issues. One of the most attractive fields in nanomanufacturing is related to nanoimprinting, i.e. to the possibility to transfer a nanoscaled pattern from a mold to another substrate. In this technology, among the others, there are two main critical steps: the preparation of a good mold and the use of a correct releasing agent to reduce the sticking between the mold and the imprinted substrate. In this review paper the authors will describe the most recent advances on the preparation of the mold, including the studies on the releas- ing agents used during the manufacturing. Keywords: Nanoimprinting, resist, nanoLithography, nanopatterns, nanomolds, mold fabrication, reliability. INTRODUCTION Nanotechnology is becoming year by year a major challenge both for scientists and for manufacturers. The need to reproduce nano-sized patterns onto substrates of different nature is revealing critical for several applications ranging from information technol- ogy and electronics to biotechnology and medicine, including vari- ous aspects of material science and physics as well. In the midst of these advancing technologies, it is crucial that various patterns nec- essary for the tasks such as electrodes or structures are fabricated in a reliable and economical manner. Over the past decades, several methods have been developed and proposed to produce nanostructures with the smallest possible size and the highest reproducibility and quality. Aim of this paper s to present an overview of the main nanoimprinting processes focus- ing on the variables and parameters that have been found critical for a high quality final product. In the mid 1990s a new method to produce patterns in the nanometric scale has been proposed by Chou and coworkers [1-5] who transferred, at a nanoscale level, a methodology proposed few years before by Kumar et al. [6] to produce gold patterns on silicon substrates at a microscale. They demonstrated to be able to repro- duce patterns on a sub 10 nm scale on a substrate [5] opening a new phase in the nanotechnology research and application. Few years later, several examples of nanoimprinted substrates obtained using commercial equipments were known [7-10]. The new method was developed to answer the growing demand of a low-cost, high- throughput manufacturing technology. Previously, this was particu- larly difficult to achieve for structures with size below 100 nm. In the nanoimprint lithography process, there are two basic steps. In the first, a mold, bearing the nanopattern, is pressed onto a resist thin layer cast on a substrate. After the hardening of the resist (by cooling, by thermal curing or by UV curing) and the removal of the mold, to get the transferred pattern on the substrate, there is a second step, consisting in an anisotropic etching process, e.g. Reac- tive Ion Etching (RIE), necessary to remove the residual resist in the compressed area. This latter step transfers the thickness contrast pattern into the entire resist. If we exclude the chemical modifica- tion of the resist, this kind of imprinting can be considered more a physical than a chemical method. Differently from other lithography methods, imprint lithogra- phy itself does not use any energetic beam and, therefore, the qual- *Address correspondence to this author at the Department of Chemical Engineering and Material, University of Palermo, Viale delle Scienze, Ed. 6, 90128 Palermo, Italy; Tel: +39-091-23863723; Fax: +39-091-7025020; E-mail: scaffaro@dicpm.unipa.it ity and the resolution of the pattern is not limited by the wave dif- fraction, by the scattering in the resist or by the back scattering from a substrate. Theoretically, the only limit to a perfect patterning in the nanoscale is given by the possibility to prepare an appropriate mold. During the following years, the researchers devoted their inves- tigations on several aspects that can dramatically affect the success- fulness of the imprinting. In particular, among them: minimizing the imprinting size; developing new materials to produce molds and to increase their resistance to be used several times; synthesizing new resists, adhesive layers for the substrate or anti-adherent coat- ing for the mold; investigating new processing conditions; propos- ing descriptive and predictive models for the nanoimprinting proc- ess. Several papers have been written on those topics but, however, only few ones are describing a systematic analysis of the system, putting into evidence the critical variables and studying the process- property relationships. In other words, a manufacturing rather than scientific approach has been adopted to develop rapidly this method, even if some basic questions still remains unanswered. Further, a full prediction of the behaviour of a nanoimprinted structure is really hard to get. As already stated above, the limit for a perfect nanoimprinting is given by the mold and therefore in this paper, we will review the available scientific literature in order to point out the most impor- tant issues that can critically affect the fabrication of molds for nanoimprinting. In particular, we will describe the materials used for the molds, the releasing agents, the methods adopted and proc- essing conditions used. 1. MOLD FABRICATION FOR UV-NIL PROCESS Nanoimprint lithography with ultra violet (UV) curing of the resist, known as Step-and-flash lithography, requires the mold sub- strate to be transparent to the UV light, due to the evident fact that the UV light must have a path to reach the resist in between the mold and the sample substrate. Various types of glass, such as Quartz or borofloat, are widely used as a mold substrate material. The most conventional method of fabricating sub-micro sized pat- terns is using e-beam lithography. However, the biggest drawback of using glass substrate is the fact that they are not conductive, which requires extra fabrication process to deposit a conducting layer, such as metal or indium-tin-oxide, for e-beam lithography. In addition to conventional Quartz and chromium photomask fabrica- tion scheme, there are a number of novel methods which use vari- ous types of electron or ion beam lithography with different kinds of high resolution resists. Some reported using imprinting to fabri-