Thin-film reactions of Al with Co, Cr, Mo, Ta, Ti, and W
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J. W. Mayer Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853 (Received 10 October 1988; accepted 3 March 1989)
The thin-film interactions of Al with refractory metals (Co, Cr, Mo, Ta, Ti, and W) have been investigated. The composition and thickness of the reacted aluminide layers were determined by Rutherford backscattering and phase identification was made by x-ray diffraction. Scanning electron microscopy was used to examine the lateral uniformity. The initial aluminide phases to grow are the Al-rich phases: Co2Al9, Cr2Al13, MoAl,2, TaAl3, TiAl3, and WA112. These are the most Al-rich phases on the phase diagrams. The reaction temperatures varied between 350 and 525 °C.
I. INTRODUCTION Most silicon MOS and bipolar integrated circuits now manufactured use aluminum, or one of its alloys, as metallizations.1 Among the important properties of Al are low resistivity and adhesion to SiO2 and silicate glasses. It is necessary to separate the Al metallization from the Si substrate in order to prevent spiking of the junction at device processing temperatures.2 This problem can be solved by interposing a diffusion barrier between the Al and Si.3 To design an effective diffusion barrier, both the Al/ and Si/barrier interactions must be well understood. Most of the actual or candidate diffusion barriers consist of, or have as a component, a transition metal. In the past decade the systematics of silicide formation have been developed and a general pattern of phase formation has emerged.4"6 The studies of aluminide formation have not yet progressed as far but some general trends are evident. As with metal-silicon binary systems, with metalaluminum binary systems there are usually several intermetallic equilibrium aluminide phases. In thin-film bilayers, usually only one phase is observed to grow at a time, unlike bulk diffusion couples where with sufficient annealing all the equilibrium phases are generally present. It is not yet possible to predict from first principles what aluminide phase will grow initially, though a phenomenological rule has been proposed.7 The reaction temperature, interfacial uniformity, and initial phase are sample preparation dependent. There have been previous investigations of Al/Co, Cr, Mo, Ta, Ti, and W thin-film reactions. There are three references in the literature to Al/Co reactions. 8 " 10 The phase Co 4 Al n was found to grow parabolically in the temperature range 350-400 °C by Howard et al} In later work by Howard et al. ,9 the phase detected was Co2Al9. A marker experiment has also been performed which found that Al was the dominant diffusing species during Co2Al9 formation.10 The interactions of thin-film Al and Cr have been studied, 8 '" 12 and the kinetics determined for Cr2Al13.8'12'13 This phase is referred to as CrAl7 in older references, and J. Mater. Res., Vol. 4, No. 4, Jul/Aug 1989
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it has been recently identified as Cr2Al13. In work by Howard et al.8 a conductance method and Ruth
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