High efficiency photoresist-free lithography of UO 3 patterns from amorphous films of uranyl complexes
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High efficiency photoresist-free lithography of UO3 patterns from amorphous films of uranyl complexes M. Gao and Ross H. Hilla) Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6 (Received 23 April 1997; accepted 20 October 1997)
The solid state photochemistry of uranyl carboxylate complexes is presented with the purpose of developing methods for optical lithography of uranium oxide films. These complexes of the general formula, UO2 (OOCR)2 (R i-C3 H7 , C5 H11 , CH2 C6 H5 , CH2 OC2 H5 , C2 H4 OC2 H5 ), were all photosensitive as thin amorphous films. The primary photochemical reaction for each of these complexes was the extrusion of a CO2 from the ligand and the production of radicals which initiated a chain reaction. The nature of this chain reaction was dependent upon the identity of the organic substituents, R. In some cases the chain reaction required a photochemical step while others were entirely thermal in nature. Of importance are the potentially high quantum yields which can be associated with thermal chain reactions. Some of the systems presented here exhibit quantum yields in excess of 1. This process was shown to be compatible with optical lithography by the patterning of the uranium oxide product on silicon surfaces.
I. INTRODUCTION
In the coming decades materials chemists face the challenge of replacing current lithography methods with methods capable of further reductions of the minimum feature size. The two methods that are considered most likely to supplant the current methods are x-ray1–6 and electron beam lithography.7,8 These methods both have inherently better possible resolution than traditional photolithography due to the small size (wavelength) of the initiating photon or electron.7 Similar to current lithographic methods, x-ray lithography is conducted with a mask. An ideal mask is constructed from an x-ray absorbing material patterned on an x-ray transparent material. Gold is representative of materials that have been utilized as the x-ray absorber.1 Uranium, in contrast, has a higher x-ray cross section than gold which would permit the use of thinner films reducing Fresnal diffraction.1,4 While there are many methods and examples of the deposition of metals9–11 and metal oxides,12–14 there is little literature on the deposition of uranium thin films.15,16 Uranium thin films are thermodynamically, unstable oxidizing in air.17 Films of the oxide are, however, stable.18 The thickness of UO3 films required to stop 90% of incident x-rays at 1 keV can be calculated from the available photoabsorption cross sections19 and density.20 The required thickness is 540 nm which compares with thicknesses of 390 and 260 nm for tantalum and gold, both of which have been used previously for mask materials.6 In this paper we demonstrate the production a)
Address correspondence to this author. J. Mater. Res., Vol. 13, No. 5, May 1998
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of uranium oxide films using
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