Sol-Gel Synthesis of Thick Ta2O5 Films for Photonic Band Gap Materials
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0929-II04-18
Sol-Gel Synthesis of Thick Ta2O5 Films for Photonic Band Gap Materials Nicholas Ndiege1, Tabitha Wilhoite1, Vaidyanathan Subramanian2, Mark Shannon3, and Rich Masel2 1 Chemistry, University of Illinois, Urbana, IL, 61801 2 Chemical & Biomolecular Engineering, University of Illinois, Urbana, IL, 61801 3 University of Illinois, Urbana, IL, 61801 Introduction Insulation for high temperature microelectromechanical systems has become a key area in research due to the development of microreactor systems that can attain wall temperatures in the order of 1200ºC.1, 2 In order to add utility and portability to such systems it is required to thermally isolate parts of composite devices that incorporate such high temperature components. Refractory blocks (e.g. fibrous alumina) of centimeter order of thickness are the best mode of insulation currently available. Photonic crystals have demonstrated a tunabilty such as to exhibit a photonic bandgap in the infrared wavelength.3 This is promising since at temperatures above 600ºC, IR radiation is the dominant mode of heat loss.4, 5 Thus multilayer quarterwave photonic structures can be explored to not only effectively insulate but also to anisotropically expel heat generated by microreactors. The greatest challenge posed in realizing such materials is that photonic crystals with a bandgap in the IR calls for structures made out of dense refractory material with thicknesses ranging from 1-5 µm.3 Ta2O5 is the material of choice due to its high index of refraction, refractory nature and negligible absorbance in the infrared region.4, 6, 7 Current deposition techniques typically attain thicknesses in the order of 1-1.5 µm.7-14 Furthermore, these are achieved by use of high cost systems such as chemical vapour deposition, e-beam deposition etc. Any attempts at thicknesses beyond this range are hampered by very slow growth rates, film buckling, cracking and even catastrophic delamination.9, 15, 16 Solgel synthesis techniques are appealing due to their low cost and flexibility with regards to chemical tuning of the properties of the final material sought. Binder molecules such as ethylcellulose, polyethyleneimine (PEI), polyvinyl pyrollidone (PVP) and polyvinyl butyral (PVB) have been shown to increase film thickness with little or no cracking when incorporated in the sol precursor.17-22 Previous studies have shown PVP to yield dense, 2.4µm thick Ta2O5 films on silicon with negligible cracking. Also observed are changes in film microstructure as the sol ages.23 These films retained their stability even after prolonged heating at high temperature. Despite these discoveries, the mechanisms via which PVP and the corresponding solvent, facilitates the superior quality of oxide films still remains to be understood, hence in this paper, we seek to investigate how PVP interacts with its corresponding solvent and the tantalum complex in the preparation of thick Ta2O5 films. The sol precursors were analyzed with time from the point of synthesis till 12 days later. Chemical probing was
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