Stress and Density of Thin TiO 2 Films Produced by Different Methods
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C. R. OTTERMANN, M. HEMING and K. BANGE SCHOTT GLASWERKE, R & D, P.O. Box 24 80, 55014 Mainz, Germany
ABSTRACT Stress, density and refractive behavior are analyzed for TiO2 layers deposited by reactive evaporation (RE), ion plating (IP), plasma impulse chemical vapour deposition (PICVD) and spin coating (SP) from bending of the substrate and by means of Rutherford Backscattering Spectroscopy (RBS), respectively. Densities between 2.7 gcm-3 and 3.8 gcm3 are obtained depending on coating techniques and coating conditions. Refractive index depends linear on density. A correlation between film stress and density is observed which is independent from coating process, production parameters and crystal structure of the as-deposited films. Stress is tensile at low densities and compressive for films with densities of the order of the crystal phase of anatase (3.84 gcm-3). The phase transition from amorphous to anatase due to annealing induces an increase in stress. Film density determines the transition temperature, and the magnitude of stress increase.
INTRODUCTION Mechanical properties of coatings are still one of the primary challenges in thin film technologies. In the last decade, the complexity of coatings on glass has heavily increased by the extension of applications and the necessity to meet the higher demands of tougher specifications. Appropriate deposition methods and optimized process parameters have to be chosen for each product to obtain coatings of desired quality. Sophisticated optical coatings require an increasing number of layers that increase the total thicknesses. Beside optimized optical behavior of the coatings, other basic properties of the film materials are also important, such as the mechanical behaviour of the films, especially stress which should be below a certain limit to avoid cracking or delamination. Stress is not a fixed film property and can vary with time due to relaxation processes. Also thermal load of the coated parts may have a huge impact on film stress and, therefore, on mechanical stability of the coatings. In this paper, we report stress results on titania single layers deposited by several methods with varying conditions. Relations are discussed between stress and density and correlations are drawn to other properties such as refractive index, morphology and behavior under annealing conditions. Finally, the influence of impurities in TiO 2 is also studied.
187 Mat. Res. Soc. Symp. Proc. Vol. 356 0 1995 Materials Research Society
EXPERIMENTAL DETAILS All experiments were performed on fused silica substrates polished to high quality and flatnesses adequate for interferometric measurements. Before coating, all samples were cleaned using a standard 4-step cleaning process [1]. TiO 2 films with thicknesses of approximately 100 nm were deposited by reactive e-beam evaporation (RE), reactive ion plating (IP) and plasma impulse chemical vapour deposition (PICVD) [2]. Substrate temperature was kept below 300 'C. Films were also produced from alkoxide solutions [3] by means of a spi
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