Photolithography has been widely used to produce a variety of micropatterns. The selective exposure of ultraviolet (UV) light passed through photomask arrives onto a photoresist laid beneath the photomask. The light induces local crosslinking or degradation of the photoresist molecules, thereby providing patterns after the removal of the uncrosslinked molecules. Although photolithography is still powerful tool in industry and research field, the technique faces several technical limitations, in that only two-dimensional micropatterns may be obtained and size and shape of micropatterns are hard to change because of a low reconfigurability of a conventional photomask. To overcome these limitations, complex photolithography process including alignment process or self-assembled nanobuilding blocks, such as colloids, and block-copolymers, have been used as templates for micropatterns. Such novel approaches partially address the current limitations on photolithography. However, applying reconfiburability to micropatterns with high degree of freedom is still remained challenging problems.
Here, I demonstrate the reaction-diffusion-mediated photolithography using oxygen inhibition in photopolymerization to control the size and shape of microstructures. To overcome the low reconfigurability of micropatterns fabricated from conventional photolithography process, firstly, in chapter 2, reaction-diffusion-mediated photolithography technique was developed using oxygen inhibition process as a driving factor of size and shape controllability. In photopolymerization process, oxygen molecule prevents the polymerizaiton by consuming radicals in polymerization. Although the phenomenon is considered to undesired result in conventional polymerization, it is possible to apply for controlling the morphology of micropatterns. Applying oxygen inhibition in photolithography precisely, three-dimensional microstructures with controlled size and shape were fabricated.
Using the developed technique of reaction-diffusion-mediated photolithography, in chapter 3, I report the overhanging microdisk arrays toward omniphobic surfaces. Overhanging structures have an advantage for omniphobic surfaces, but the structures are hardly achieved by conventional photolithography with complex operation steps. In chapter 3, omniphobic surfaces are created by designing an array of overhanging microdisks on a polymer film through two steps of photolithography. Two distinct edges and the large height of the microdisks relative to their separation ensure the formation of an air mat under the microdisks, providing an omniphobic property. Moreover, the freestanding omniphobic films are transparent and flexible, potentially serving as liquid-repellent surfaces in various applications
For further development of reaction-diffusion-mediated photolithography, development of reconfigurable photomask is required. To applying adjustability, elaborate structure of emulsion droplets in a microfluidic device for adjustable photomask was designed and investigated. In chapter 4, I report the crystalline phases of monodisperse emulsion droplets for adjustable photomasks. Such photomasks were prepared using a microfluidic device in which a flow-focusing junction, side channels, and a reservoir were connected in series. Transparent oil droplets were generated in a dye-containing continuous water phase at the flow-focusing junction. The droplets were then concentrated through the selective removal of the continuous phase using the side channels. This process led to the formation of a regular array of droplets in the reservoir with a configuration that depended on the relative height of the reservoir to the droplet diameter. The droplet arrays were used as a photomask to create hexagonal or square arrays of microdots. Parabolic transmittance profile of the UV light enabled the dot size to be tuned by controlling the UV irradiation time which is otherwise difficult to achieve using conventional photomasks. The combination of droplet size adjustments and the UV irradiation time provided independent control over the dot size and array periodicity to enable the preparation of a series of hexagonal microarrays with a wide spectrum of array parameters using a single microfluidic device.