Development of photocurable pillar arrays formed via electrohydrodynamic instabilities
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As photolithography approaches both fundamental and economic barriers, interest in alternative patterning technologies has grown. This thesis focuses on two alternative patterning techniques: nanoimprint lithography (NIL) and electric field assisted assembly. NIL is a high resolution, yet inexpensive contact patterning process. Step and Flash Imprint Lithography (SFIL) is a type of NIL that involves pressing a topographically patterned template onto a substrate covered with a small volume of liquid. The liquid fills the voids of the template and is hardened by UV irradiation. Low viscosity liquids are ideal for rapid patterning. An acrylate material formulation was developed to meet the various processing needs of SFIL. Unfortunately, oxygen inhibits the free radical photopolymerization used to cure the acrylate. The effects of oxygen were characterized using a semi-empirical model that relies on rate coefficients measured by real time IR spectroscopy. The model predicts an inhibition period at the beginning of irradiation as radicals are quenched by oxygen. After the oxygen is consumed, the polymerization proceeds rapidly except at the perimeter of the template, which is subject to oxygen diffusion from the ambient. Electric field assisted assembly is another attractive patterning technique that is capable of forming polymeric pillar arrays. Pillars form by the amplification of thin-film surface instabilities through the application of an electric field normal to the film. Work to date on pillars has focused on glassy polymers that are limited by the requirement of heat to modulate the rheological properties. A focus of this thesis is on developing low viscosity materials for the formation of pillars. Low viscosity materials form pillars orders of magnitude faster than high-melt viscosity polymers. The pillars form at room temperature and are hardened by UV irradiation. In addition to developing and characterizing low viscosity materials, the aspect ratio of the pillars was optimized. The aspect ratio of the pillars was increased by physically stretching the pillars through the development of an active gap tool. Methods to improve long range order were also investigated. Electric field assisted assembly and imprint lithography are promising photolithographic alternatives that benefit considerably from the use of low viscosity materials.