Low-Temperature Processing of Sol-Gel Derived Metal Oxide Thin Films using Supercritical Carbon Dioxide Fluid
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1113-F06-27
Low-Temperature Processing of Sol-Gel Derived Metal Oxide Thin Films using Supercritical Carbon Dioxide Fluid Hiroshi Uchida, Kaori Fujioka and Seiichiro Koda Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo, 102-8554, Japan
ABSTRACT We demonstrate a novel technique using supercritical carbon dioxide (scCO2) fluid for lowering processing temperature of sol-gel-derived metal oxide thin films. The film processing was performed in a hot-wall closed vessel filled with scCO2 fluid. The effects of fluid temperature and additives on the sol-gel synthesis reaction under scCO2 fluid were also investigated. Precursor films of titanium dioxide (TiO2) prepared on silicon wafer and silica glass by sol-gel coating using Ti-alkoxide were converted to crystalline TiO2 (anatase) films successfully by treatment in scCO2 without additive agent at a fluid pressure of 15 MPa and at a substrate temperature of above 250oC, which is significantly lower than the processing temperature of conventional sol-gel deposition. Furthermore, additive agents such as water (H2O) and nitrogen-oxygen mixture (N2-O2) promoted the decomposition and crystallization of precursor films in scCO2 fluid to form the crystalline TiO2 (anatase) films at a substrate temperature at as low as 200oC although it also produced surface absorbates consisted of hydroxides on the film surface. The experimental results suggested that the hydrolysis and polymerization reactions of Ti-alkoxide in the precursor films were proceeded by the scCO2 processing to form titanium-oxygen (Ti-O) networks and that byproducts such as alcohols were removed from the resulting films.
INTRODUCTION Low-temperature processing of oxide thin films on thermally-degradable substrates (such as light-metals or polymers) is one of important challenges in order to developing various kinds of innovated electronic /photonic devices for the next-generation. However, conventional techniques, e.g., sol-gel synthesis, chemical vapor deposition, etc., include heat treatment operation at higher temperature for decomposing and crystallizing the precursor materials consisted of mainly organometallic compounds generally, and it can be a critical disadvantage due to thermal damage or interdiffusion to neighboring layers and/or substrates. For manufacturing advanced electronic / photonic devices such as large-scale integrated circuits, nano-/meso-sized materials, inorganic-organic composites, etc., technical breakthrough for the film processing at lower temperature would be required strongly, and then various approaches have been examined in recent researches. For example, there have been many reports concerned to the low-temperature film processing based on photo (UV, laser, etc.)- and ion-beam-assisted techniques. [1-4] They attempted energetic activation for decomposing and crystallizing reactions of precursor materials using photon or ion-beam, however, which still might cause considerable damage to substrates
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