Doctoral Degrees (Physics)

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Browsing Doctoral Degrees (Physics) by Author "De Waal, Hendrik Schalk"

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    The effect of diffusion barriers, stress and lateral diffusion on thin-film phase formation
    (Stellenbosch : Stellenbosch University, 1999-11) De Waal, Hendrik Schalk; Pretorius, R.; Lombaard, J. C.; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: Instead of studying phase formation in conventional thin film binary diffusion couples, various unconventional thin film sample structures were used to investigate the influence of certain parameters on phase formation. Iron-silicide formation on Si(lOO) through metal alloy diffusion barriers, Tisilicide formation on stressed Si(lOO) substrates and phase formation in various lateral diffusion couples was studied. A variety of amorphous alloys were used as diffusion barriers through which Fe reacted with Si(100). It was found that the diffusion barrier did not change the effective concentration at the reaction interface enough to cause the formation of the semiconducting βFeSi2 phase instead of EFeSi as first phase. There is much technical interest in growing the semiconducting βFeSi2 phase epitaxially on Si(100) for use in amongst others infra red detectors and light emiting devices. The use of the diffusion barriers did however control the reaction in such a way as to allow the formation of a uniform film of polycrystalline FeSi2 at 800°C. In the case where Fe reacted directly with Si(lOO) without a diffusion barrier the Fe film reacted very non-uniformly with the substrate. The uniform polycrystalline thin film of βFeSi2 was then converted to epitaxial Fe0.5Si[CsCl] using a 0.8J/cm2 laser pulse from an excimer laser. This was the first time that a continuous film of epitaxial Fe0.5Si[CsCI] has been formed on Si(100). The βFeSi2 films with large grain sizes formed the best quality epitaxial Fe0.5Si[CsCI] after laser annealing. The grain size was dependant on the type of diffusion barrier used. The influence of stress on Ti-silicide phase formation was also studied. TiSi2 is commonly used in integrated circuits and with the move to smaller devices and linewidth's stress is playing a larger role. Varying amounts of compressive and tensile stress was induced in Si(100) substrates by depositing different thicknesses of SiO2 or Si3N4 on the backside of the Si wafers. These back side films deformed the wafers causing stress throughout the substrates. The stress in the substrates was determined by measuring the radius of curvature of the samples using a laser technique and then relating the radius of curvature to stress using Stoney's equation. It was found that when Ti reacted with a substrate under compressive stress the Si diffusion process was slowed down so that TiSi2[C49] grew 50% slower than when Ti reacted with substrates under tensile stress. In the samples with the most compressive stress in the substrate the Si diffusion was limited so effectively that the Ti-rich Ti5Si3 phase formed instead of TiSi2. This is explained using the Effective Heat of Formation (EHF) model. Lateral phase formation must take place in order to form some of the complex device structures present in the modem integrated circuit. In this study the nuclear microprobe, electron microscope and optical microscope was used to study phase formation in Ru-Al, Pt-Al and Cr-Si lateral diffusion couples. A typical lateral diffusion couple consisted of an island of one material deposited onto a thin film of another it was found that if the island did not consist of the diffusing species there was no lateral reaction. In the aluminide systems the AI-island geometry always showed lateral reaction. In the Ru-AI system the RU4Al13 phase was the only phase to grow laterally with reaction limited kinetics while in the Pt-Al system two phases namely Pt8Al21 and PtAl2 grew laterally with diffusion limited kinetics. The EHF model is used to explain how diffusion limited growth causes the simultaneous formation of more than one phase. In the Cr-Si system the Si-island geometry grew laterally forming CrSi2. Oxidation made it impossible to do any kinetic measurements. It is shown how lateral diffusion studies can be used to give information on diffusing species and diffusing mechanism. A summary of all the work done on lateral diffusion couples was also compiled in this study.