Instability Induced by Near-substrate Electric Field
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Instability Induced by Near-substrate Electric Field David Salac and Wei Lu Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109, USA ABSTRACT This paper reports experimental work using an electric field close to a substrate to induce morphological patterns in a thin polymer film. The film was spin-coated onto a glass wafer, and subsequently heated to above its glass transition temperature to allow viscous flow. An electric field was applied by two parallel electrodes spacing 10 mm apart. The initially flat polymer/air interface lost stability and formed islands. The self-assembled islands exhibited a narrow size distribution and demonstrated spatial ordering. We attribute the pattern formation to the minimization of combined interface energy and electrostatic energy. INTRODUCTION Polymeric thin films enable manufacture of a wide variety of novel, nanostructured devices and templates from which other nanofeatured materials can be created. Examples include block co-polymer self-assembly [1], nanoimprint lithography [2], and soft lithography [2]. One promising fabrication technique discovered recently is lithographically induced selfassembly (LISA) [3-6]. LISA is a process that utilizes the concept of electrohydrodynamic (EHD) instabilities of polymers when exposed to an electric field perpendicular to the surface. In instances of LISA, a flat, thin polymer layer was first applied to a substrate using a technique such as spin coating. An upper mask is then set at a fixed distance (a few hundred nanometers) above the film, forming a small air gap. Between the substrate and the mask an electric field perpendicular to the film surface is developed from either image charges [3,4] or external power supply [5,6]. This causes pressure buildup at the polymer/air interface, and subsequent destabilization of the thin film. As a result, columns are formed towards the upper mask. It is also possible to direct this assembly using a topographically structured top mask [6], thus reproducing the mask pattern. Various orderings of columns have been constructed between the substrates and the masks, for example, use of polymer/polymer/air trilayers have yielded structures comprised of cylinders having polymer cores surrounded by another polymer [6]. Removal of the core via selective etching results in an ordered structure comprised of hollow cylinders. Thus, in the LISA process, polymer structures are determined by the topographical patterns on the mask, making the technique well-suited to pattern replication. However, the requirement of close placement of upper mask relative to substrate complicates production of large or curved surfaces. Specifically, the mask surface must be treated to prevent polymer adherence, so that the fine structures induced are not damaged during mask removal. Here, we describe our recent experiment, in which we have used an electric field applied in the plane of the air/polymer interface, instead of perpendicular to the interface. This setup greatly simplifies production,
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