The objective of this thesis was to develop a method of distinguishing between alpha and beta phases of tantalum films on steel substrates, using the four-point probe resistivity measurements. While there is a large difference between resistivities of the two Ta phases, the problem is challenging because the measurements are influenced by the low resistivity of steel.
For reference, the resistivity measurements of tantalum coatings (1-20 μm) on silicon and silicon dioxide substrates were conducted first. The measured resistivities were related to the crystallographic phase composition of Ta, which has lower resistivity in bcc form, α-phase, (13.5 to 42 μΩcm), than in the tetragonal form, β-phase, (87 to 157 μΩcm). The phase composition of Ta coatings was determined by X-ray diffraction (XRD) measurements, and the ratios of intensities of α-Ta (110) and β-Ta (002) peaks in XRD spectra were correlated with the measured resistivity values. Resistivities of the films deposited on silicon and silicon dioxide substrates were found to be same within the measurement error. The four-point probe measurements on these substrates were effective in distinguishing between films dominated by either α-Ta or β-Ta phase.
For thin Ta coatings on steel substrates the resistivity measurements are compromised by the "short circuit" effect of the steel, which has low resistivity, comparable to that of the α-Ta phase. The effect of the substrate, which depends on the ratio of the probe spacing to the film thickness, was investigated for thick Ta coatings on steel (up to 0.8 mm), where the shorting effect was negligible, and different probe spacing (0.25 mm to 4 mm). The results are in agreement with the theoretical study of Weller, who found that resistivity measurements on conductive substrates are possible for a film thickness to probe spacing ratio equal or larger than 0.2 [14]. Microscopic four-point probes are suggested for thin films resistivity measurements. The development of microprobes with spacing as small as 500 nm has been recently reported. They will allow measuring the resistivity of films as thin as 2.5 ~trn on conducting substrates.
Ta coatings investigated in this work were deposited by DC magnetron sputtering. The surface of steel substrates was prepared using different methods, which included grinding, polishing, electrochemical cleaning and in-situ sputter etching. The substrate surface morphology and roughness was measured with atomic force microscopy (AFM). The smoothes steel surface was obtained by polishing with a suspension of 50 nm silica particles, which resulted in the surface roughness as low as 5.1 nrn was measured on 30 ~trn x 30 ~trn scan area. Sputter etching in Ar gas resulted in the formation of characteristic cones on steel surface. No influence of various surface preparation methods on the phase of Ta was observed. However, cleaning, especially sputter-etching, was found to enhance film adhesion.
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