Oxide thin films for nanometer scale electron beam lithography

The exposure response of high resolution oxide resist materials has been examined under high intensity irradiation conditions (∼1 ⊠ 10⁵A/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 Al₂O₃, Y₂O₃, Sc₂O₃, 3Al₂O₃·2SiO₂, and MgO·Al₂O₃were deposited by rf sputtering onto substrates cooled to –196°C and found to require an exposure dose of approximately 5 ⊠ 10³C/cm²to 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 Al₂O₃. The dose requirement in 90 nm thick amorphous Al₂O₃was determined to be 2.5 ⊠ 10³C/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.