Oxide thin films for nanometer scale electron beam lithography
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The exposure response of high resolution oxide resist materials has been examined under high intensity irradiation conditions (~1 x 105 A/cm2) to determine the relationships among film characteristics, exposure requirements, and ultimate resolution, and to explore further the processes responsible for ablative exposure. Amorphous films of A12O3, Y2O3, Sc2O3, 3A12O3 • 2SiO2, and MgO A12O3 were deposited by rf sputtering onto substrates cooled to -196 °C and found to require an exposure dose of approximately 5 x 103 C/cm2 to complete exposure. Amorphous film structure was found to be necessary to achieve rapid removal of material during exposure. Material properties also found to influence irradiation response and help guide the selection of new materials included ionic character, heat of formation, and melting point. Film thickness was found to influence strongly both exposure requirements and resolution, an optimum thickness occurring at approximately 90 nm in amorphous A12O3. The dose requirement in 90 nm thick amorphous A12O3 was determined to be 2.5 x 103 C/cm2, which is two to three orders of magnitude lower than that of oxide films produced by other techniques. Resolution of the rf sputtered oxide films allowed the production of 5.0 nm holes on 8.1 nm centers. A dedicated STEM was used for exposure studies as well as imaging, microdiffraction analysis, and monitoring of the transmitted beam current, and allowed a qualitative model of the exposure process in rf sputtered oxide resists to be developed. I. INTRODUCTION
Exposure of oxide resist materials using high intensity electron beam irradiation has resulted in the ablative removal of material and in the direct formation of lithographic structures as small as 1-2 nm.1"3 Structures on this scale are among the smallest currently obtainable in lithographic systems, surpassing the 10-12 nm limit imposed by the spatial range of secondary electrons generated in conventional high resolution polymeric materials, such as PMMA.4"7 The ablative removal process responsible for exposure in these materials allows oxide resists to be used with no post development processing. This minimizes the number of steps required in a lithographic sequence and eliminates loss of resolution associated with resist development. Applications for controlled production of nanometer scale structures are expanding with the general trend toward device size reduction and improved microstructural control. Two applications where ablative inorganic resist materials have been examined include high resolution resists for lithography and development of electronic devices based on quantum mechanical phenomena,8"10 and high density data storage media for electron beam addressed memory systems.1112 The inherent high resolution advantage which oxide resists have demonstrated has stimulated interest 1058
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J. Mater. Res., Vol. 5, No. 5, May 1990
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in further study of these materials and their behavior under high intensity exposure conditions. A summary
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