Technical note: Comparison of film density, stoichiometry, optical and electrical properties of thin metal oxide films produced by reactive d.c. magnetron sputtering and electron beam evaporation techniques

Abstract

Multilayer thin metal oxide films are used in the fabrication of some electro-optic devices. Many of these devices are produced by reactively sputtering metals or by evaporating the metal oxide. To design and engineer such devices, a thorough knowledge of the physical properties of these films is essential. One can then relate these properties to the conditions under which the films were made. Critical parameters in the fabrication process are then identified and the process is not only better controlled but can also be optimized for the end product required. In this study, film thicknesses varied from 20 nm to 360 nm and were determined by a surface profiling technique. The film density and stoichiometry were then determined by Rutherford backscattering measurements. The reactively sputtered metal oxide films were made between base argon pressures of 2.5 and 20 mTorr, oxygen partial pressures of 1% and 65% of the total pressure and direct current magnetron power levels of 2 and 6 kW. Significant enhancements to deposition rates and variation in physical properties were accomplished with non-standard gas flow geometries. Comparable metal oxide films were made with electron beam evaporation. Optical properties of the films were determined from transmission and reflection measurements from 380 nm to 780 nm. The index of refraction and absorption as a function of wavelength were deduced from these data. The index of refraction exhibits a surprising, significant and changing structure with deposition parameters and film thickness. The resistivities of the sputtered films vary greatly with process parameters. The values of film resistivity and index of refraction can be altered in a controlled manner by subsequent in-line substrate heating in an enriched O₂ atmosphere. Ta_xO_y films will be used as typical examples to illustrate the above phenomena.