Reactions at metal-semiconductor interfaces

Examination of the architecture of a semiconductor-based microelectronics device shows that metallic, highly conductive components are an integral part of the miniature circuits. As geometries become increasingly small (e.g. at the sub-micron level) the structure at critical interfaces influences the electrical performance to a greater extent. Accordingly metal-semiconductor junctions have significant technological importance, in addition to any natural scientific interest associated with the bonding of two unlike materials. This article reviews some of our recent work on this topic, with particular emphasis on the reactions which can occur either during fabrication of the interface or upon heating in conjunction with device processing or prolonged service.The simplest system consists of an elemental metal and an elemental semiconductor, silicon being the most important example of the latter. Consideration of phase equilibria indicates that such an interface is thermodynamically unstable: upon heating either a reaction can occur to produce a compound phase (i.e. a silicide), or mutual dissolution of the elements within each other takes place to achieve saturated solid solution compositions. Reference to the appropriate binary phase diagram allows prediction of the result if local equilibrium is achieved. Thus although an atomically abrupt metal-semiconductor interface might be grown under specialized circumstances, this situation can be expected to be unusual and moreover it is not stable to elevated temperatures when atomic mobility and diffusion are rapid.