Electron microscope study of electrically active impurity precipitate defects in silicon

Abstract

A specific defect which was found to electrically degrade junctions in Si transistor devices has been studied using both transmission and scanning electron microscopy, together with electron probe X-ray micro-analysis. It was observed that the defect had a characteristic rod-like morphology, penetrating the [001] Si foils along inclined directions. Both electron diffraction and X-ray micro-analytical data indicated that the rods had a precipitate structure consistent with that of ζ _a -FeSi, (α-lebolite). The rod/matrix orientation relationship was found to be ( 10)FeSi₂∥( 01)Si and (111) FeSi₂∼ ∥(010)Si, each rod axis being a [ 10] FeSi₂ direction. Furthermore, rod defects often occurred with accompanying small subsidiary precipitates thought to be a Cu silicide, which either decorated the rods themselves or formed planar colonies enclosed within irregular dislocation loops attached to the rods. The rod precipitate growth is discussed both with reference to the orientation relationship with the matrix and also with regard to the diffusion characteristics of Fe in Si. A kinetic model is constructed to provide a comparison with Cu silicide precipitate growth. The presence of ζ _a -FeSi₂ precipitate rods in device structures is most undesirable since this phase exhibits metallic conduction properties. Because it is necessary to remove rod defects from transistor devices in order to restore satisfactory electrical properties, the use of the P-diffusion gettering technique is also examined and shown to be effective for this purpose.