Selective Preparation of Nickel Silicides and Germanides Using Elemental Multilayers as Reactive Precursors
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Selective Preparation of Nickel Silicides and Germanides Using Elemental Multilayers as Reactive Precursors Jacob M. Jensen, Sochetra Ly, Xavier Kyablue, David C. Johnson Department of Chemistry and Materials Science Institute University of Oregon Eugene, OR 97405
ABSTRACT : Using elemental multilayers with designed composition and ultrathin repeating subunits (λ=20 Ǻ) we have prepared nickel monosilicide and nickel germanide directly without the formation of any intervening phases. When the same synthetic strategy is used to prepare nickel-silicongermanium compounds the first nucleated phase is a ternary phase with the NiAs structure. INTRODUCTION: Reactions at solid-solid interfaces have received considerable attention for several decades.1 From a fundamental perspective, understanding the parameters which control first phase nucleation and subsequent phase formation provide insight into the mechanisms of solid state reactions. Rational design and control of solid-solid interactions is also of significant practical importance for numerous applications, including integrated circuit and photovoltaic device development.1 Transition metal – silicon reactions in particular have been of interest owing to the widespread application of transition metal silicides as diffusion barriers / electrical contacts in microelectronic devices.2,3 Nickel monosilicide has been identified as a promising contact material for next generation integrated circuit technology.4 As in previous technologies, the conventional approach to silicide preparation is to react a thin layer of the transition metal (in this case, Ni) with the exposed Si on a patterned substrate. Perhaps the greatest challenge to preparing NiSi is eliminating unwanted phases from the final contact. In conventional Ni-Si reactions high-resistance Ni2Si is the first nucleated phase.2,5 Multiple-step processes involving alternating etch and anneal steps are typically required to prepare phase pure NiSi contacts. To overcome the difficulty of unwanted phase formation, several alternative strategies which seek to modulate the identity of the first nucleated phase have been developed. Notable among these are coevaporation6 and interlayermediated silicidation.7 The key advantage of using transition metal silicides for contacts is that the conventional preparative process is self-aligning.3 Ni reacts selectively with the Si in the active areas of the device but not with the insulating oxide regions. When the excess Ni is removed, the remaining silicide is found only in the active areas of the device, eliminating the need for additional (expensive) lithographic patterning. Significant effort is currently being directed towards high-performance devices based on Si18 Ge x x rather than Si in active device areas. The ideal process for contact formation in such devices would retain the “self-alignment” of conventional silicidation processes. Reactions between Ni and epitaxial Si1-xGex on Si have been studied9,10, with much effort given to the prevention of Ge segregation an
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