Titanium Hafnium Oxide Alloy Films by a Novel Sub-Atomic Layer Sputtering Process for High Index and Graded Index Applic
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Titanium Hafnium Oxide Alloy Films by a Novel Sub-Atomic Layer Sputtering Process for High Index and Graded Index Applications Nobuhiko P. Kobayashi1, R. Ernest Demaray2, Junce Zhang1, Kate J. Norris1, David M. Fryauf1, Juan J. Díaz León1, Amanda Flores1, Ravi Mullapdi3, Tana Arunagiri3, Lai Lu3 1 Baskin School of Engineering, UCSC, Santa Cruz, CA, U.S.A.; Nanostructured Energy Conversion Technology and Research (NECTAR), Advanced Studies Laboratories, University of California Santa Cruz - NASA Ames Research Center, Moffett Field, CA, U.S.A. 2 Antropy Inc. & Demaray LLC, Portola Valley, California, U.S.A. 3 Tango Systems, Inc. San Jose, California, U.S.A. ABSTRACT We studied physical properties of titanium hafnium oxide (TixHf1-xO2) alloy thin films deposited by pulsed DC reactive magnetron sputtering with AC substrate bias. Thin films of two end oxides, hafnium oxides (HfO2) and titanium oxides (TiO2), and their alloys TixHf1-xO2 with a range of compositions deposited with and without the substrate bias were compared to study the dependence of physical properties of the thin films on the substrate bias. Structural, chemical and optical properties of the thin films were analyzed to assess inter-relationship among these properties. Thin films deposited with the AC substrate bias consistently show much higher refractive index and significantly lower optical extinction coefficient than those of thin films deposited without the substrate bias suggesting that characteristic microstructures developed in these thin films are responsible for the differences in the optical properties. INTRODUCTION Oxide alloys such as HfO2 and TiO2 have been extensively studied in the course of developing next-generation dielectric materials to replace prevalent silicon dioxide in electronics such as silicon metal oxide field-effect transistors and dynamic random-access memories [1]. In these applications, reduction of electrical leakage current associated with two types of quantum mechanical tunneling mechanisms, direct tunneling and Fowler-Nordheim tunneling, through dielectric materials is critical. One approach to reduce the direct tunneling is to use dielectric thin films with large physical thickness for given capacitance, which suggests that dielectric materials with high relative permittivity (εr) are preferred. The Fowler-Nordheim tunneling can be reduced by increasing the conduction band offset (δΕc), the difference between the conduction band edge energy of a dielectric material and the Fermi level of a metal electrode at their interface. It is, however, well known that εr and δΕc are inversely related [2,3]. Further challenges arise as HfO2 and TiO2 form various crystalline polymorphs and, as a result, their physical properties vary as they undergo heat treatments. Three types of HfO2 crystalline phases, monoclinic, tetragonal, and cubic, have been identified to be stable in three different temperature ranges at atmospheric pressure [4] while TiO2 exhibits two types of tetragonal polymorphs, rutile and anatase, and one orthorhomb
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