Tunable Dielectric Properties of CVD-Processed Polymer via Click Chemistry

Recently, the increasing demand for high-end displays has led to the development of flexible displays using non-traditional flexible substrates rather than traditional rigid substrate. Currently, thin-film transistors (TFTs) for flexible devices mainly use inorganic dielectrics such as SiO₂, and Al₂O₃. These inorganic gate dielectrics have critical issues to applicate in flexible devices because of their high temperature process, high-cost production, and limited flexibility. Therefore, the mass production yield inevitably decreases, leading to increase in price of devices. From this point of view, polymer dielectrics have attracted considerable attention as a gate insulator for flexible transistors because of their mechanical nature. Even if polymer gate dielectrics are suitable for future flexible electronics, properties such as dielectric constant, and leakage current, are not suitable for cutting-edge devices. Also, fabrication process for polymers is usually consist of solution process, which has poor step coverage, side effect like coffee-ring effect and pinholes.<br/>poly(p-xylylene) (PPx, commercialized name is parylene) is well known passivation materials for various electronics, which can be fabricated by chemical vapor deposition system that can give pinhole-free conformal film. Conventional parylene family, such as parylene-C, -D, -N, and -AF4 shows low dielectric constant of ~ 3, which is does not meet requirements for future advanced electronic. However, physical, dielectric, and electrical properties of parylene family can be tuned by adjusting functional groups. We reported custom synthesized parylene-ethynyl that shows higher dielectric constant, smoother surface and similar leakage current compared to conventional parylene. This approach is quite intricate as it necessitates design from the monomer.<br/>In this work, click chemistry was employed to further tuning the properties of custom-synthesized parylene-ethynyl for dielectric layer for TFTs. IGZO oxide semiconductors were used as the active layer to demonstrate high-performance hybrid TFTs. Unlike most polymers fabricated through solution processes, parylene's fabrication method relies on the Gorham method CVD system. This system provides a thin, conformal, and pinhole-free film, resulting in a compatible interface with oxide materials. Additionally, the deposition temperature is around room temperature, and no additional high-temperature annealing is required, making it suitable for plastic substrates with low glass transition temperatures, ideal for flexible or stretchable applications. Four types of click reactions were conducted on parylene-ethynyl, and it was confirmed that all of them exhibited click reactions. After click reaction, parylene-ethynyl demonstrated an improved dielectric constant exceeding 4 without compromising film quality. Successful fabrication of IGZO TFTs combined with parylene-ethynyl showcased reliable switching properties, indicating that the combination of parylene-ethynyl with click reaction and IGZO posed no issues at interfaces. The click reaction is anticipated to facilitate easy adjustments to the characteristics of the polymer gate dielectric. Moreover, it opens up the possibility of manufacturing a functional device utilizing the clicked part.