Properties of Titanium Oxide Thin Film Prepared with E-beam Evaporation
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Properties of Titanium Oxide Thin Film Prepared with E-beam Evaporation Ping Hou, Nortel Networks Inc. Lianchao (Richard) Sun*, SOPRA Inc.
ABSTRACT The optical and mechanical properties of TiO2 film prepared by ion-assisted e-beam evaporation have been examined in this research. Spectroscopic ellipsometry analysis revealed an inhomogeneous behavior in both optical property and growth structure, vertically from substrate to the top surface of the film. This phenomenon was further confirmed with the electron microscopic analyses. The effects of deposition rate, chamber pressure, anode voltage and current on the stress of TiO2 films were also investigated and reported. Further study showed that a structural homogeneous film could be obtained through TiO2-SiO2 co-evaporation.
I. INTRODUCTION Titanium oxide (TiO2) thin film has been widely used as optical coatings due to its high index of refraction, generally when high-low index contrast is desired in multiple layer interference coatings. It is well known that structures and properties of TiO2 films are highly dependent on processing techniques1. E-beam evaporation has long been recognized as a practical way of depositing optical thin films. However, preparing a homogenous TiO2 film with this technique is still a challenge2. This is mainly because different microstructures, such as amorphous, anatase, or rutile, could be formed depending on the processing temperatures. In this paper, the stress and structural homogeneity of e-beam evaporated TiO2 films will be examined and discussed. II. EXPERIMENTAL DETAIL 2.1 TiO2 Film Deposition TiO2 film was deposited by the technique of ion-assisted e-beam evaporation. Substrates used for the experiment were 2-inch single-side-polished silicon wafers with (001) orientation. Samples were mounted onto an 8-inch copper holder that rotated at the speed of 32 rpm. To minimize the temperature gradient, the chamber was preheated to 150 °C by a quartz-halogen lamp. A thermal couple close to the sample holder measured the chamber temperature. Two MDC e-guns with 270° beam deflection were installed at the bottom of the vacuum chamber to evaporate TiO2 and SiO2 materials. The source material for evaporating TiO2 was pre-melted solid block from Ti2O3 tablets. Deposition rate was given by averaging the signals of three quartz crystals housed in Mextek bakeable holders. INFICON IC/5 is a close-loop controller that was used to control the evaporation rate and final film thickness. A MARK II gridless ion gun was employed to assist the evaporated deposition process. The advantage of using ion beam is that the parameters, such as incident angle, anode voltage and current, can be independently controlled to a great extent. The input gas for the ion gun was a mixture of 90 % Argon and 10 % oxygen in volume fraction. Although the anode current is a function of filament current and gas flow, only the filament current was adjusted to tune the anode current during the experiment. The gas flow was always fixed at 12 sccm. The ion energy typicall
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