(4de) Electrically Induced Pillar Arrays Formed Using Photocurable Materials

My research has been primarily focused on materials and process development for novel patterning techniques such as imprint lithography and self-assembly. Here, I will present my recent work on pillar arrays formed via electrohydrodynamic instabilities.

Self-assembly based patterning techniques are appealing because of their ability to harness natural phenomena to form useful structures. Recently, a technique has emerged that is capable of forming ordered polymeric pillar arrays. (1-6) Pillars are formed by the amplification of thin film surface instabilities through the application of an electric field normal to the film surface. Experimentally this is achieved by placing a thin film coated substrate within planar proximity of another surface, forming a simple capacitor. Applying an electric field across the gap results in the formation of an array of uniformly sized pillars. Pillars form due to the force imbalance at the film interface where the electric field amplifies film undulations against the restoring forces of gravity and surface tension. Features as small as 140 nm have been created using this method through the use of templates with relief patterns.(1)

To date, this process has only been performed on polymer films, which require a heating step to induce flow prior to pillar formation and a cooling step to lock the columnar structures into place after formation. Here we show that this process can be performed using photocurable monomers at room temperature. This reduces the processing time from hours to seconds due to the lowered film viscosity and the elimination of the heating / cooling cycle required for polymers. The pillar structures are locked into place by irradiating the photocurable solution through a transparent template.

Materials for the pillar formation process must be tailored to meet several processing requirements. First, the material must photocure rapidly to high conversions under ambient conditions. The throughput of the process is impacted by the cure rate while the mechanical stability depends both on the extent of polymerization and the degree of crosslinking. When the template is removed, the photocured polymer must adhere entirely to the substrate and maintain structural integrity. In addition, the monomer solution must form a stable film on a substrate, which involves considerations of both surface energy and volatility.

The advantages and limitations of acrylates, vinyl ethers, and thiol-ene photochemistries for the pillar formation process will be presented. Acrylates and vinyl ethers rapidly form pillars, but appear to be inhibited by ambient species such as oxygen, base, and water. Thiol-ene systems seem to be the most promising materials for pillar arrays due to their insensitivity to atmospheric species and their unique radical step polymerization mechanism, which permits control of certain material properties as a function of conversion.

(1) Schaffer, E.; Thurn-Albrecht, T.; Russell, T. P.; Steiner, U. Nature (London) 2000, 403, 874-877.

(2) Chou, S. Y.; Zhuang, L. Journal of Vacuum Science & Technology, B: Microelectronics and Nanometer Structures 1999, 17, 3197-3202.

(3) Schaffer, E.; Thurn-Albrecht, T.; Russell, T. P.; Steiner, U. Europhysics Letters 2001, 53, 518-524.

(4) Lin, Z.; Kerle, T.; Baker, S. M.; Hoagland, D. A.; Schaffer, E.; Steiner, U.; Russell, T. P. Journal of Chemical Physics 2001, 114, 2377-2381.

(5) Chou, S. Y.; Zhuang, L.; Deshpande, P.; Chen, L.; Sun, X. Polymer Preprints (American Chemical Society, Division of Polymer Chemistry) 2000, 41, 78.

(6) Schaffer, E.; Harkema, S.; Roerdink, M.; Blossey, R.; Steiner, U. Advanced Materials (Weinheim, Germany) 2003, 15, 514-517.