Reactive Deposition of Dielectrics by Ion Beam Assisted E-beam Evaporation
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0983-LL05-01
Reactive Deposition of Dielectrics by Ion Beam Assisted E-beam Evaporation J. R. Nightingale, T. Cornell, P. Samudrala, P. Poloju, L. A. Hornak, and D. Korakakis Lane Department of Computer Science and Electrical Engineering, West Virginia University, PO Box 6109, Morgantown, WV, 26506 ABSTRACT Fabrication of high index contrast waveguide stacks for biosensing and other applications require nanometer scale thickness control. To achieve single mode waveguides, thin dielectric films of thicknesses 100 – 200 nm are required. However, deposition of such films from electron-beam evaporation can be difficult to obtain due to the resulting porosity and poor stoichiometry of the films. An alternate approach is the reactive deposition of the film from a metal source in the presence of oxygen ions. Using spectroscopic ellipsometry, we have shown that greater control over thickness and index of refraction of silicon dioxide depositions can be obtained through reactive depositions as compared to depositions from SiO2 dielectric source material itself. Through Fourier Transform Infrared Spectroscopy (FT-IR), the Si-O in-phase stretching peak at 1078 cm-1 can be traced, allowing us to determine the stoichiometry of the film. The effects of performing depositions of aluminum oxide dielectric source material in the presence of oxygen ions has also been investigated. Through the use of the oxygen ion source, greater control over index of refraction and optical losses has been observed. By controlling ion source parameters, the aluminum oxide films’ index of refraction can be engineered within a range of 1.58 to 1.64, and waveguide losses can be reduced to as low as 2.0 dB/cm. INTRODUCTION For oxide dielectrics to be used in high index contrast waveguide stacks for biosensor structures, thickness control to the sub-nanometer level and refractive index variations of less than one percent are often necessary1. Electron-beam evaporation is a type of physical vapor deposition (PVD) commonly used for depositing these oxides2. In electron-beam evaporations, this degree of control requires a reproducible deposition flux cloud from run to run which is most easily obtained when evaporation material has formed a liquid phase. In contrast for materials that are nearly subliming in nature there will be spikes and dips in deposition flux as the electron beam reaches untouched source material and forces the system to adjust beam current to compensate. Silicon dioxide is one example of the materials discussed above. Using FT-IR to determine stoichiometry and spectroscopic ellipsometry to determine thickness and refractive index, we have found higher quality, more reproducible electron-beam deposited SiO2 films can be achieved through reactive depositions from silicon source material as compared to films from silicon dioxide source material. In general, dielectric thin films from electron-beam evaporation with densities similar to that of the bulk material can be difficult to obtain due to the porosity and poor stoichiometry often seen
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